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C A R B O N 6 8 ( 2 0 1 4 ) 1 – 3 2
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Review
Sulfur-doped porous carbons: Synthesisand applications
0008-6223/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.carbon.2013.11.004
* Corresponding author.E-mail address: [email protected] (W. Kicinski).
Wojciech Kicinski a,*, Mateusz Szala a, Michał Bystrzejewski b
a Department of New Technologies and Chemistry, Military University of Technology, Kaliskiego 2 str., 00-908 Warsaw, Polandb Department of Chemistry, University of Warsaw, Pasteur 1 str., 02-093 Warsaw, Poland
A R T I C L E I N F O A B S T R A C T
Article history:
Received 7 September 2013
Accepted 4 November 2013
Available online 10 November 2013
Heteroatom doping of carbon materials may become the ‘‘Next Big Thing’’ in materials sci-
ence further enhancing research concerning carbon nanostructures. In particular, the S-
doped porous carbons have gained a great deal of attention in the last few years. They
are already proven to be versatile functional materials with a wide range of potential appli-
cations, including heterogeneous catalysis, sorption, as well as in the areas of energy con-
version and storage. To date, a few approaches have been developed to intrinsically blend
sulfur into the carbon matrix. Yet there is still a need to design new porous structures with
controllable porosity and well defined chemical status of sulfur doped into the carbon
matrix. In this review, we summarize recent reports on the preparation of S-doped carbons,
with special emphasis on porous carbons with intrinsically doped sulfur. The effect of S-
doping on the properties determining applications is delineated. Special attention is paid
to differentiate between elemental sulfur impregnation, intercalation, surface functionali-
zation and S bulk doping of porous carbons. To this end, synthesis and applications of S-
impregnated, S-functionalized and S-intercalated carbons are shortly discussed before
the intrinsically S-doped carbons are presented in detail. The importance of the sulfide –
C–S–C– system for the properties of S-doped carbon is stressed. At the very end, Se-doped
carbons are shortly presented as a promising next generation of chalcogen-doped carbon.
� 2013 Elsevier Ltd. All rights reserved.
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 02
2. From carbon–sulfur composites to bulk doping; sulfur as a carbon nanostructures growth promoter . . . . . . . . . . . . . . 03
2.1. Substitutionally doped sp2 carbons and theoretical considerations of S-doping . . . . . . . . . . . . . . . . . . . . . . . . . . 03
2.2. Carbon/sulfur physical composites for superconduc- tors and Li–S batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . 04
2.3. Sulfur-containing carbons as contaminant sorbents, catalyst supports and solid-acids . . . . . . . . . . . . . . . . . . . . 05
2.3.1. Sulfur-enriched carbon for heavy metal decontamination. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 05
2.3.2. Sulfur-enriched carbon as a catalyst support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 06
2.3.3. Sulfonated carbon as solid acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 06
2 C A R B O N 6 8 ( 2 0 1 4 ) 1 – 3 2
2.4. Advantages of bulk S-doping vs. surface S-enrichment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 06
2.5. Sulfur as a carbon nanostructures growth promoter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 07
3. Synthesis of intrinsically S-doped carbons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 07
3.1. Evolution of the S-species during carbonization of sulfur-containing precursors . . . . . . . . . . . . . . . . . . . . . . . . . 07
3.2. Thiophene-based precursors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.3. Other S-containing precursors and S-doped graphene. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.4. S-co-doped carbons: binary or ternary doping with N and/or P. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.4.1. Binary and ternary doped porous carbons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.4.2. S and N dual-doped graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
4. Applications of S-doped carbons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
4.1. Electrochemical applications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.1.1. Heterogeneous catalysis – oxygen reduction reaction (ORR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.1.2. Anodes for Li-ion batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
4.1.3. Cathode for lithium–oxygen battery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
4.1.4. Electrode for supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
4.2. Adsorption. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
4.2.1. H2 storage and CO2 capture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
4.2.2. Adsorption of heavy metals and toxic gases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
4.2.3. Desulfurization of diesel and crude oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
4.2.4. Photoactivity of S-doped carbons – a reactive adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
5. Summary and outlook. Se-doped carbon, S-doped g-C3N4 and carbon dots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Appendix A. Supplementary data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
1. Introduction
The discovery of ordered nanoporous carbons has greatly
whetted researchers’ interest in carbons with controllable,
well defined nanoarchitectures as a new class of functional
materials [1]. Also, it seems like old-fashion activated carbon
is going through a period of ‘‘renaissance’’ owing to its poten-
tial application for supercapacitors, H2 storage, CO2 capture
and photocatalysis [2]. Finally, the discovery of graphene
additionally boosted the effort to fabricate and theoretically
evaluated the sp2 carbons with new nanosized features, e.g.
heteroatom doping. In the face of tremendous research con-
cerning carbonaceous materials, it is somewhat trite to state
that carbon as an element and as a material exhibits extraor-
dinary versatility unmatched by any other known element.
Yet, from all of the column-IV elements of the periodic table,
solely the carbon is able to create sp, sp2 and sp3 bonding,
since only carbon has no inner p-electrons. As a result, car-
bon significantly differs from its closest column-IV counter-
part – Si. Even though it exists in a few allotropic forms, it
is the sp2 hybridized carbon that possesses the exclusive
and unique ability of building macro-scale materials and ob-
jects from nano-sized entities resulting in endless nanotex-
tural arrangements [3,4]. This in turn, gives rise to
extremely varied physicochemical properties (i.e. electrical
conductivity, low density, porosity and chemical persistence).
Although exceptional, carbon is truly ubiquitous; it can be de-
rived from abundant precursors through simple processes.
While considering carbonaceous materials, it is essential
to keep in mind that they are usually built not only of elemen-
tal carbon, but also of a whole range of heteroatoms such as
hydrogen, oxygen, nitrogen, and to lesser extent sulfur, bor-
on, phosphor or halogens (e.g. F, I and Cl), which are generally
present as surface functional groups at the edges of graphene
layers. The unusual properties of carbons, especially the acti-
vated ones, are significantly influenced by the profuse surface
functionalities containing heteroatoms. As a result, when it
comes to applications, the surface chemistry is as important
as the carbon texture. It is essential that the properties of
the carbonaceous material will precisely match the require-
ments of the future applications. A simple way to tune and
control physicochemical properties of carbon materials must
be elaborated in order to utilize them for advanced applica-
tions [5]. There are two basic and usually complementary
ways in which porous carbon properties can be easily con-
trolled: the design of pore architecture and the incorporation
of specific functional groups onto the carbon surface and
within the carbon matrix [6,7]. While tremendous progress
has been made in syntheses of nanoporous carbons with
new morphologies [8], exciting opportunities remain for het-
eroatom-doping of such materials. Heteroatom doping pro-
vides a useful tool for tuning and enhancing carbons’
unique physical and chemical properties. Heteroatom-doped
carbons (HDCs) with controllable structural and chemical
properties have been attracting increasing attention, mainly
due to their versatility as advanced functional materials
[9,10]. In particular, HDCs with well developed porosity and
high surface area exhibit improved performance as materials
for the storage and conversion of electrical energy in fuel
cells, supercapacitors and batteries; CO2 capture and H2 stor-
age; the high-capacity adsorption of specific contaminants in
liquid and gas phases; ‘‘carbon-only catalysts’’ in certain
chemical reactions; and in photocatalysis e.g. as semiconduc-
tors for solar energy harvesting.
C A R B O N 6 8 ( 2 0 1 4 ) 1 – 3 2 3
Up to now, several strategies, including in situ doping and
post-treatment have been proposed to introduce heteroatoms
into the carbon framework [9,10]. The in situ doping route
involves the direct carbonization of heteroatom-containing
carbonizable species (e.g. polymers or ionic liquids). In the
post-treatment procedure, the carbon matrix is subjected to
certain chemical agents (i.e. H2S, NH3, etc.) at a high temper-
ature. Functional groups may be covalently attached (grafted)
onto the carbon surface by a wide range of methodologies [6].
However, if organic substances containing heteroatoms are
used as carbon precursors not only is the surface functional-
ized, but also bulk-doped carbons are obtained without the
need for additional steps. This synthetic path is extensively
employed to alter carbon properties through N-doping. In-
deed, there is extensive research concerning N-doped carbons
[9,10], often extending into binary and ternary co-doping with
B, P and S [11,12]. B-doping has also been widely studied [13].
P-doping remains somewhat less popular and calls for future
research [14,15]. Nevertheless, when it comes to bulk S-dop-
ing of carbons a shortage of research is still evident. A few
examples of S-doped carbons produced by carbonization of
sulfur-rich precursors can be found in the literature [16].
The sulfur present in these carbons is predominantly forming
sulfide bridges (–C–S–C–) due to the inherently reducing car-
bonization conditions. However, a detailed review of S-doped
carbons requires a short introduction concerning the basic
principles of carbon doping and the variety of sulfur enriched
carbon materials.
2. From carbon–sulfur composites to bulkdoping; sulfur as a carbon nanostructures growthpromoter
Doping, through which atoms and molecules interact (cova-
lently or non-covalently) with the surface and bulk is an effi-
cient way for altering carbon properties [5]. Tuning the
electronic structure of materials by heteroatom doping has
been utilized for a long time in the semiconductor industry.
However, in the case of carbon (in particular carbon nanoma-
terials based on sp2-hybridized carbon) the term doping
might be equivocal and include not only atomic substitution
in the carbon crystal lattice, but also intercalation, nanocom-
positions or even surface functionalization. For instance, for
carbon nanotubes three main categories of doping are now
well established, i.e., exohedral, endohedral and inplane dop-
ing. Exohedral doping stands for intercalation, endohedral
doping implies encapsulation and substitutional doping is
an in-plane replacement of carbon in the graphene sheet [17].
2.1. Substitutionally doped sp2 carbons and theoreticalconsiderations of S-doping
Electronic, magnetic, chemical and mechanical properties of lay-
ered sp2 carbons are significantly affected by the introduction of
non-carbonatoms.HDCsmayrevealnewquantumeffects,allow-
ing tailoring of the bandgaps of sp2 type carbon if it is doped in
small concentrations. Doping also induces local curvature in the
sp2 sheets that tends to increase the local reactivity [18]. For
instance, S-doped graphene may be able to chemically bind NO2
and possibly NO and serve as a polluting gas sensor [19]. The
change in the electronic structure caused by NO2 adsorption
and by orbital hybridization is expected to produce a large change
in conductivity, making S-doped graphene useful as a sensor.
For many years researchers have been interested in mod-
ulating graphite properties by doping with heteroatoms, as
well as inserting intercalants (e.g. sulfur [20]) between the lay-
ers of its highly anisotropic structure. Graphite can serve as a
host lattice, whose electronic properties can be tailored by
doping with electron-donating or electron-accepting guest
atoms that reside in the interstitials and can increase the
density of free charge carriers (the p- and n-type doping). As
theoretical considerations suggest, graphite can be substitu-
tionally doped with B, N, O, S, P, Se or Si. The substitutional
doping of carbon atoms in the honeycomb lattice by atoms
with a different number of valence electrons will in general
introduce additional states in the density of states of graphite.
Whether these will be electron-donor states, electron-accep-
tor states or neither of these two, depends on the local bond-
ing arrangements of the heteroatoms. The most popular type
of carbon heteroatom doping is graphite-like substitution of
boron and nitrogen atoms into the carbon lattice [21]. B and
N are the most natural choice for doping since they differ only
by one in their number of valence electrons compared to car-
bon. The B-doped carbons have reduced interlayer spacing
and enhanced stability at high temperature [22]. Boron has
one electron less than carbon atom, and when it replaces car-
bon in the graphene sheets localized states below the Fermi
level (valence band) appear [17]. These states are caused by
the presence of holes in the structure, so that carbon could
be considered as a p-type conductor and is more likely to re-
act with donor-type molecules. A three-coordinated N atom
within the sp2-hybridized carbon network (graphitic/quater-
nary/N-Q nitrogen) induces sharp localized states above the
Fermi level due to the presence of additional electrons. As
for example, nitrogen doping in nanotubes shifts the Fermi
level to the valence bands, making all N-doped carbon nano-
tubes metallic, regardless of their geometry [17]. From the
theoretical point of view, it should also be possible to substi-
tutionally dope oxygen in the graphitic plane; especially since
oxygen has a similar atomic radius to carbon. Calculations
suggest that a bandgap of about 0.5 eV is generated through
the O-doping of graphene, and geometrically the O atom re-
mains in the plane [23]. No magnetic moment was predicted
in O-, and B- or N-doped graphene. Phosphorus atoms, even
though larger than carbon atoms, can also be incorporated
within the carbon nanotube lattice, behaving as an n-type do-
nor, and thereby modifying the electronic properties [24].
Doped carbons exhibiting n-type conduction are more likely
to react with acceptor-type molecules. While B- and N-doped
graphene retain a planar form, the S or P atoms protrude out
of the graphene layer. Sulfur acts as a donor of electrons and
shows different changes at the electronic density of states
when compared to N or B doping. The increase in local reac-
tivity of S-doped carbon comes from the presence of lone
electron pairs brought about by the S-doping.
However, is it possible to substitutionally dope sp2 carbon
structures with sulfur? The introduction of second row atoms
in the sp2 framework is tricky, because they have a larger ra-
dius than carbon and thus cannot maintain a planar struc-
4 C A R B O N 6 8 ( 2 0 1 4 ) 1 – 3 2
ture. Denis has pondered this issue and presented extensive
theoretical studies concerning sulfur substitutional defects
of sp2-carbons [25–27]. He studied the dependence of the band
gap with respect to the dopant concentration. As he stated,
from a thermodynamic standpoint, sulfur doping into gra-
phitic nanostructures is not difficult. Sulfur atoms move out-
wards to the surface of the carbon nanotube and as a result a
larger curvature favors sulfur doping. S-doping modifies the
electronic structure of single walled nanotubes and graphene,
depending on the sulfur content. Pristine graphene behaves
like a zero-band-gap semiconductor and the band gap can
be opened via heteroatom doping. Indeed, theoretical studies
indicated that after S-doping the band gap is opened. The
changes in the electronic structure are due to the sulfur atom
that affects p electrons in the carbon lattice. In the case of
graphene, S-doping (n-type dopant) may induce different ef-
fects: the doped sheet can be a small-bandgap semiconduc-
tor, or it could exhibit enhanced metallic properties in
comparison to the non-doped graphene sheet. Such semicon-
ducting S-doped graphene might, for instance, result in an in-
creased electrode polarization in the case of battery
applications. S-doped graphene may be useful for construct-
ing nanoelectronic devices, since it is possible to vary the
electronic properties of the sheet by adjusting the doping le-
vel of sulfur into the lattice [5,28]. Theoretical calculations
showed that in the S-doped graphene not only the electronic
(Fermi level shifts towards the conduction band), but also the
magnetic properties are significantly modified [29,30]. The re-
sults indicate the charge transfer from the doped sulfur atom
to the graphene. Also, the metallic single-wall carbon nano-
tubes (SWCNTs) can be converted into semiconducting ones
by doping the tube wall with proper S concentrations [31].
S-doping might provide a new route for mass production of
semiconducting SWCNTs. Indeed, recently, the synthesis of
purely semiconducting SWCNTs has been reported [32]. S-
doped SWCNTs were synthesized by arc discharge between
two graphite rods containing up to 2.0 at.% of S. Such S-doped
SWCNTs have been preliminarily applied in the field effect
transistors.
2.2. Carbon/sulfur physical composites for superconduc-tors and Li–S batteries
Unlike nitrogen, the solid state elemental sulfur can be
introduced into carbonaceous materials to form C/S com-
posites. In general, carbonaceous materials are routinely
modified with other elements to build functional carbon/
heteroatom composites [20]. For example, the intake of io-
dine by graphite leads to the formation of homogeneous
graphite–iodine nanocomposites with enhanced structural
disordering and carbon–polyiodide covalent compounds
(C–I3, C–I5) [33].
Recently, sulfur containing carbon materials have at-
tracted a great deal of attention as promising candidates
for high-temperature superconductors [34]. It has been re-
ported that S-doped graphite (graphite–sulfur composites)
exhibit superconducting properties below 35 K [35]. The C–
S composites can be prepared by a reaction between sulfur
vapor and graphite powder, since above 400 �C, sulfur could
be intercalated into graphitic layers [35,36]. However, S-
doped carbon materials go beyond graphitic carbons. In
considering S-doped carbon it should be noted that there
is extensive research concerning diamonds and diamond-
like carbons doped or functionalized with sulfur [37,38].
For example, S-doped diamond films have been successfully
synthesized through various chemical vapor deposition
techniques and the potential of S to serve as a shallow-level
donor in diamond was studied. However, the most versatile
carbon, i.e. sp2-type, is the paramount theme hereafter.
After all, c.a. 90% of all carbon materials used in industry
are graphitic.
Depending on the texture of the sp2 carbons a variety of
carbon-guest nanocomposites can be obtained. The ability
to precisely control carbon texture by nanocasting (hard
and soft templating) gave a new impetus to research pursu-
ing C/S nanocomposites. Even though C/S composites have
been utilized for many years as heavy metal sorbents, lots
of attention has recently been paid to C/S composites as
cathodes for Li–S batteries. Carbon–sulfur composites as
the cathode of rechargeable Li–S batteries have shown out-
standing electrochemical performance for high power de-
vices. Sulfur constitutes a promising cathode material
with a theoretical specific capacity of 1672 mAh/g. Insertion
of sulfur into porous carbons allows assuaging of the poor
electrical conductivity of sulfur. A number of carbon–sulfur
nanocomposites as Li–S cathodes have been proposed, in
which sulfur is impregnated into: porous hollow carbon
spheres [39,40]; ordered mesoporous carbons [41,42]; hollow
carbon fibers [43]; disordered carbon nanotubes [44]; multi-
walled carbon nanotubes [45]; graphene [46–48] or hierarchi-
cal porous carbon [49]. Sulfur incorporation into porous car-
bon is usually realized by heating the sulfur/carbon
physical mixture at around 155 �C under inert gas. At these
conditions elemental sulfur (S8) becomes liquid and has the
lowest viscosity, so that the liquid can infuse into the por-
ous structure. Yet heat treatment at higher temperatures,
e.g. 500 �C can break down the S8 molecule to S6 or S2
and enables a strong bonding between sulfur and carbon
(chemisorption of S) by possible sulfur intercalation into
the layered graphitic cluster [44]. Surprisingly, the C/S com-
posites may contain up to 90 wt.% of sulfur when graphene
constitutes the carbon component [47]. Graphene can enve-
lope sulfur particles and form a highly conductive network.
Such graphene-wrapped sulfur particles utilized as
rechargeable Li–S battery cathodes have shown a high
capacity and cycling stability [48].
As noted above, the carbon/elemental sulfur composites
are conventionally obtained by impregnation, which re-
quires the molecular mobility of a guest phase (liquid/va-
por). However, ordered mesoporous carbon infiltrated with
solid sulfur was recently obtained by stirring an aqueous
mixture of both [50]. The facile infiltration is an unusual
example of a porous solid which can be completely infil-
trated by another solid phase at room temperature and
the infiltration can reach even 50 wt.%. The proposed prep-
aration method allowed coating of the inner carbon surface
with a thin layer of sulfur that maintains the channel
porosity and accessibility of the sulfur sites. The enhanced
solid–solid ‘‘wetting’’ or adhesion is due to the hydrophobic-
ity of both sulfur and carbon.
C A R B O N 6 8 ( 2 0 1 4 ) 1 – 3 2 5
2.3. Sulfur-containing carbons as contaminant sorbents,catalyst supports and solid-acids
There are a number of methodologies which can be used to
modify the surface of carbon nanostructures and nanoporous
carbons with sulfur for their application as sorbents. The abil-
ity of carbon materials for the sulfur surface-binding has been
known and utilized for many years [51]. Lots of carbon sulfu-
rization mechanisms have been proposed and it was deter-
mined that sulfur occurs in different chemical states on the
carbon surface [51–53]. Covalent sulfur complexes created
on the carbon surface are relatively stable and cannot be ex-
tracted with solvents or completely decomposed by heat
treatment in vacuum up to 1000 �C. It was shown that an
appreciable amount of sulfur can be retained by the sulfu-
rized carbon even after heating the materials to 1200 �C [53].
However, the sulfur complexes can be completely removed
(and evolved as hydrogen sulfide) by heat treatment in hydro-
gen at 700–800 �C [54]. The introduction of sulfur surface com-
plexes onto activated carbons is usually carried out in the
atmosphere of various sulfurizing agents; among others the
following S-sources are the most common choices: H2S (prob-
ably the most important one) [55–57], elemental S [58,59], CS2
and dimethyl disulfide [60,61], SO2 [62–64] and finally Na2S
and polysulfide [65,66]. Sulfurization results in the formation
of surface complexes such as sulfide and hydrosulfide groups,
C–S, S–S, C@S, or S@O bond-containing groups, S8 rings or S2
and S6 chains, short linear chain sulfur allotropes, and sulfur
deposits or thiophene, sulfoxide or sulfone groups [67].
Enrichment with elemental sulfur is usually performed at
higher temperatures (300–400 �C) and as a result some portion
of the elemental sulfur also reacts with the carbon matrix to
form organic sulfur [58]. Eventually, elemental sulfur, organic
sulfur, and sulfates are formed on porous carbon adsorbents
during sulfur impregnation.
2.3.1. Sulfur-enriched carbon for heavy metaldecontaminationDue to the relatively non-polar characteristic of porous car-
bons (e.g. activated carbon) they are less effective in removing
metal species from aqueous solution than in removing organ-
ic compounds. The introduction of heteroatom complexes on
the carbon surface increases the surface polarity and addi-
tionally makes the ion exchange reaction between the heavy
metal cation and negatively charged surface groups possible.
Since adsorption capacity and rate are influenced by the
chemical nature of the carbon surface, the adsorption of hea-
vy metals can be significantly improved by introducing oxy-
gen, nitrogen or sulfur functionalities onto the material. The
literature contains a profusion of studies showing that
impregnation of sulfur onto a virgin activated carbon greatly
enhances its overall performance in Hg capture [58]. S-en-
riched activated carbons are also much more effective than
un-impregnated carbons for removing mercury from a vapor
phase.
While physisorption is a main mechanism that deter-
mines the adsorption capacity of a virgin carbon, it is believed
that chemisorption controls the adsorption ability of sulfur-
impregnated carbons. Sulfurized porous carbons have been
tested as adsorbents of a large number of metal chemical spe-
cies (e.g. Hg(II), Cd(II), Pb(II), Cu(II), Zn(II), Cr(VI)) from aqueous
solutions [54]. The chemical interaction between mercury and
sulfur often leads to the formation of HgS, a less harmful
form of mercury due to its insolubility and very low volatility.
Also, the C/S physical nanocomposites show excellent prop-
erties for the recovery of a wide range of noble metals (Pd,
Ru, Au) from very diluted aqueous solutions [50]. These metal
species have a high affinity for sulfur, as often demonstrated
by their natural occurrence as metal sulfides (i.e. Ag2S, Cu2S,
CuS, PbS, ZnS, HgS). According to the Pearson theory, the
strong affinity of sulfur for heavy metals origins from a soft
acid–soft base interaction; in fact sulfur is a soft base and
heavy metals are soft acids. During an acid–base reaction,
hard acid prefers to coordinate with hard base and soft acid
with soft base. For example, positively charged metallic spe-
cies are soft acids and, as a rule, interactions of Cd2+ or
Cd(OH)+ with sulfur groups are favored [68,69]. Such affinity
is also routinely employed for anchoring metal nanoparticles
on carbon nanotubes. On the other hand, sulfur’s strong affin-
ity for heavy metals may also have some detrimental effects –
it often causes transition metal catalyst poisoning [70,71].
Both elemental and covalently bond sulfur are active
adsorption sites for heavy metals like Hg or HgCl2. However,
the adsorption of Hg(II) from aqueous solution is highly
dependent on the method used for introducing sulfur in acti-
vated carbon. For example, treatment of activated carbons
with SO2 at 700 �C leads to development of organic aromatic
sulfide – thiophene with the ability to efficiently bind Hg
[63]. The SO2 gas as a sulfurizing agent was also successfully
used in adsorbent modification at ambient temperature. The
replacement of oxygen functionalities by sulfur on the sur-
face of activated carbon results in an increase in the adsorp-
tion efficiency against Cd2+ [62]. The SO2 and H2S modified
porous carbons also exhibited a great increase in the adsorp-
tion of Pb2+ from aqueous solution [64].
If the carbon material is subjected to the H2S treatment the
sulfur content and stability generally increase with the in-
crease of temperature and the temperature conditions deter-
mine the form of sulfur groups created on the surface. The
H2S sulfurization process is associated with the decomposi-
tion of pre-existing surface functionalities, which create ac-
tive sites for sulfur bonding [55]. At temperatures below
600 �C, sulfurization is likely to occur through the addition
of H2S onto unsaturated active sites, while at higher temper-
atures direct reaction between H2S and the carbon occurs
[55,72]. Vidic and co-workers observed the effective mercury
uptake capacity for carbon derived from H2S sulfurization at
elevated temperatures up to 800 �C [72]. Above 800 �C the di-
rect chemical reaction between carbon and H2S, and possible
incorporation of sulfur into the graphitic structure occurred.
As a result, sulfur impregnated at 800 �C is strongly bonded
to the carbon surface. This may be due to the formation of or-
ganic sulfur. Formation of very stable sulfur species at 800 �Cwas also reported by Sugawara et al. [73]. According to them,
thiophene is the possible structure of organic sulfur products
deposited on the carbon surface at high temperature. The sul-
fur content of sulfur-impregnated carbon increases with time
of the H2S treatment at 800 �C. Also, the presence of H2S dur-
ing the cooling process increases the sulfur content. This in
6 C A R B O N 6 8 ( 2 0 1 4 ) 1 – 3 2
turn causes an increase in the adsorption capacity of heavy
metals (Pb2+ and Zn2+) from aqueous solution [74]. Vidic and
co-workers also studied the impregnation of sulfur onto acti-
vated carbon fibers through H2S oxidation catalyzed by the
sorbent surface. This method resulted in sulfur impregnated
mainly as elemental S [56]. They also compared the adsorp-
tion capacity of mercury onto activated carbon impregnated
with S through two different methods: (i) reaction with
elemental sulfur at 600 �C, and (ii) oxidation of H2S at 150 �C[75]. The former method was found to be much more effective
in producing sorbents of higher capacity. In addition, sulfur-
containing carbon fibers and carbon nanotubes have been
extensively studied and tested for the recovery of heavy met-
als (i.e. Cd) and both inorganic and organic forms of mercury
[76,77]. Sulfur-containing carbon fibers showed selective sorp-
tion properties due to the presence of sulfide and hydrosulfide
groups and C–S–C, C–S–S–C bridging groups.
Generally, if activated carbon is impregnated by elemental
sulfur, the impregnation temperature dictates the predomi-
nant form of sulfur allotropes [78,79]. As a consequence it is
the actual form of sulfur rather than the total sulfur content
that is a crucial parameter governing the chemisorption pro-
cess. Stronger bonding between sulfur and carbon surface
produced by impregnation at high temperatures prevents sul-
fur agglomerating and clogging the carbon pores.
2.3.2. Sulfur-enriched carbon as a catalyst supportThe sintering resistance of metal particles is essential for the
development of high performance catalysts for fuel cell elec-
trodes. Nanoporous carbons and carbon nanostructures (i.e.
ordered mesoporous carbons, carbon aerogels and nano-
tubes) functionalization or modification with sulfur has been
proposed as an efficient strategy towards stabilization of
nanometer-sized noble metal particles (e.g. Pt, Pd) against sin-
tering [80–84]. The strong platinum–carbon support interac-
tion between the sulfur atoms and the surface atoms of the
loaded Pt nanoparticles play an important role in stabilization
of the Pt particles against the Ostwald ripening [80,82]. The Pt
catalyst supported on the S-containing carbon exhibited
superior sintering-resistant behavior at high temperature, as
compared to Pt particles supported on sulfur-free porous car-
bons [80].
Sulfur species are often used as the capping agents for me-
tal particles. By utilizing the high affinity of sulfur towards
heavy metals, the thiol- and thiophene-sidewall functional-
ized carbon nanotubes have been often self-assembled and
anchored on bare gold surfaces [85,86]. In the same way, thi-
olated nanotubes can be uniformly covered with Au
nanoparticles.
2.3.3. Sulfonated carbon as solid acidsAmong the S-enriched porous carbons, sulfonated materials
are of a particular interest and they have found many practi-
cal applications. In order to introduce –SO3H group onto the
carbon surface, concentrated H2SO4 is routinely used. Liquid
phase treatment of activated carbon with sulfuric acid at
higher temperatures (150 �C) results in a range of sulfur con-
taining surface groups, where the thiol (–SH) and sulfonic acid
(–SO3H) groups are the predominant ones [87,88]. Such car-
bons exhibit catalytic activity in certain chemical reactions
[89,90]. Activated carbon thermally treated with sulfuric acid
containing thiol and sulfonic acid groups are also beneficial
for improved pollutant removal. Using the XPS analysis, Ter-
zyk was the first one to illustrate in detail the chemical groups
of activated carbon treated with fuming H2SO4 [87,88]. The
modification with fuming sulfuric acid leads not only to the
creation of sulfuric surface groups but also to the oxidation
of the carbon surface. Four different forms of sulfur were
determined, i.e. –SH group bonded to the phenol rings, the
sulfur bonded to the carbon in the structure of C–S–C and
R–S–S–OR, the groups R2S@O, SO32� ion or R–SO2–R and also
SO42� ion, RO2–S–S–R and R–SO3H. Some sulfur functional-
ities, similar to those found in coals, were also present.
Porous carbons functionalized with –SO3H have been
extensively studied as environment-friendly solid acid cata-
lysts. A solid acid catalyst that can keep its high activity
and stability is necessary if low-cost bio-waste is utilized for
biodiesel synthesis, because the reaction medium contains
a large amount of water [91–93]. Such catalysts eliminate
the need for liquid acids and they are fully reusable. Grafting
sulfonates as acidic sites revealed high performance in cata-
lyzing the transesterification reaction in biomass conversion.
This approach is advantageous not only for biodiesel produc-
tion but also production of a range of chemicals through acid-
catalyzed reactions. The active solid acids can be prepared by
the sulfonation of various carbonaceous materials. For in-
stance, sulfonation of incompletely carbonized D-glucose
with fuming sulfuric acid at 423 K results in amorphous car-
bon consisting of small polycyclic aromatic sheets with a high
density of –SO3H [94]. Owing to the strong acidity of –SO3H
groups, the carbon material exhibited good catalytic perfor-
mance as a solid catalyst for the esterification of higher fatty
acids. The sulfonic groups were also introduced onto the or-
dered mesoporous carbon by contacting the carbon with the
vapor of fuming H2SO4 inside an autoclave [95]. Under opti-
mal conditions, carbon with an extremely high surface area
and acid sites surface concentration of 1.3 mmol/g was
achieved, which served as a selective catalyst for condensa-
tion reactions.
Even though H2SO4 is the most common carbon sulfonat-
ing agent, other methods have also been presented, e.g. mes-
oporous carbon was functionalized with benzenesulfonic acid
by in situ radical polymerization of 4-styrenesulfonate and
isoamyl nitrite [83]. Wang et al. utilized covalent attachment
of sulfonic acid-containing aryl radicals produced by homoge-
neous reduction of 4-benzene-diazoniumsulfonate using
hypophosphorous acid (H3PO2) on the carbon surface. They
obtained sulfonated ordered mesoporous carbon with an acid
density of 1.93 mmol H+/g and high activity for acid catalyzed
esterification and condensation reactions [96]. Such solid acid
exhibited high stability and could be used for at least five
times without obvious loss of activity. Also, using this same
approach, sulfonated graphene was obtained and used as a
water-tolerant solid acid catalyst [97].
2.4. Advantages of bulk S-doping vs. surface S-enrichment
In spite of all the above mentioned advantages of S-function-
alized and elemental S-impregnated porous carbons, the sur-
C A R B O N 6 8 ( 2 0 1 4 ) 1 – 3 2 7
face sulfurization and sulfonation always causes a substan-
tial decrease of surface area and serious distortion of the por-
ous structure (e.g. blockage of most active micropores). In S-
impregnated porous carbons, the S-based functionalities are
not chemically anchored to the carbon matrix, hence poten-
tially mobile (leachable) [94,98]. Sulfur leaching from the
adsorbents during the adsorption processes is a serious prob-
lem leading to lower adsorption capacity. The leaching might
preclude access to the interior porosity during sorption pro-
cesses or electrochemical reactions. Moreover, elemental sul-
fur is hydrophobic, making wettability an issue when dealing
with aqueous environments. In the case of solid acids, the
disadvantage of C–SO3H is the quick degradation of SO3H
group during the catalyzed reactions [94]. Highly porous car-
bon materials with sulfur groups homogeneously distributed
throughout the matrix and permanently fused into the back-
bone of the carbon framework may allow circumventing
these disadvantages.
2.5. Sulfur as a carbon nanostructures growth promoter
Sulfur and S-containing compounds have been proposed as
growth promoters in the synthesis of carbon nanofibers and
nanotubes [99–103]. Sulfur-based growth promoters such as
thiophene or transition metal sulfides can be utilized to in-
crease the yield and quality of nano-filamentous carbon. In
addition, sulfur-containing compounds can be used to control
the diameter as well as the number of walls of carbon nano-
tubes. For example, it was shown that the addition of thio-
phene to the reaction mixture improves not only the
density but also the purity of SWCNTs film grown on silica
substrate [103]. Sulfur makes the growth processes more
favorable for the nucleation of aligned SWCNTs and the thio-
phene causes a slight decrease in the average nanotube diam-
eter. Moreover, through the addition of S-containing
compounds, the selective production of semiconducting
SWCNTs is facilitated.
While thiophene can enhance the synthesis of SWCNTs,
the FeS promoter increases synthesis selectivity towards dou-
ble-wall carbon nanotubes (DWCNTs) and significantly
decreasing the yield of SWCNTs. In particular conditions,
the presence of sulfur not only promotes the formation of
DWCNTs but also widens the diameter of SWCNTs [104–
107]. There are two possible ways through which sulfur influ-
ences the DWCNT growth. One is that sulfur may increase the
reactivity of the iron catalysts by forming the Fe–S eutectic or
by selectively poisoning the surface of the catalyst [107]. In-
deed, S-containing compounds were used to modify the
graphitization catalysts and slow the graphitization down in
order to obtain thin-walled porous graphite [108]. However,
it is more likely that the formation of DWCNTs or increased
diameter of SWCNTs from the action of the sulfur additives
is predominantly attributed to the increased size of catalyst
particles due to enhanced agglomeration of the Fe catalyst
[107]. As a result, the type of CNTs, namely SWCNTs or
DWCNTs, can be simply controlled by adjusting the concen-
tration of the FeS promoter with respect to the Fe catalyst.
Generally, it can be stated that sulfur influences nanotube
growth by blocking active sites on the catalyst particle, lower-
ing the melting point of the catalyst, and/or interacting with
the growing nanotube hence stabilizing the growing structure
[109–111]. The appropriate amount of sulfur can also help to
prevent the buildup of amorphous carbon and thus the poi-
soning of the catalyst particles.
To sum up the influence of sulfur on synthesis of graphitic
carbons, it is worth mentioning, that sulfur has been also re-
ported as a graphitization catalyst. Studies on the structural
changes of the heat treated sulfur-containing cokes indicated
the sulfur-enhanced graphitization [112].
3. Synthesis of intrinsically S-doped carbons
Carbon bulk doping alters not only the surface chemistry, and
thus adsorption, electrochemical and catalytic properties, but
also modifies all the fundamental physicochemical proper-
ties, i.e. induces semiconductivity, unusual magnetic behav-
ior or increased catalytic photoactivity [5]. Sulfur-doped
carbons are usually based on sulfur-rich precursors and can
be produced via pyrolysis. In fact, thermolysis of heteroatom
rich precursors (usually under the flow of inert gas) has been
applied routinely for the synthesis of doped carbonaceous
materials, e.g. by using ionic liquids. Liquid salts with dicyan-
amide anions are an especially versatile precursor for
N-doped carbons, and for co-doping with S [10,113]. For appli-
cations concerning sorption and energy storage/conversion
devices, S-doped carbons with controllable and well devel-
oped porosity are desired. This is reached through a variety
of well-established methods including nanocasting [6–8] and
activation [16].
3.1. Evolution of the S-species during carbonization ofsulfur-containing precursors
Carbonaceous materials are routinely produced from organic
precursors through a series of carbonization/graphitization
processes. Such processes constitute transformation of or-
ganic substrates to graphitic materials, progressing through
a range of carbonaceous intermediates with increasing level
of aromatization [114]. The transformation includes chemical
reactions and phase change from the organic precursors to
solid carbon. The chemical and physical transformations
(progressing polycondensation and polymerization into less-
volatile and non-fusible products) are governed by the proper-
ties of the organic substrates, which also define the final
structure of the carbon. Gradual removal of heterogenous
atoms (release of CO2, CO, NO2, NO, SO2, H2S, CS2, HCN, N2,
etc.) is an inevitable stage of the carbonization and graphiti-
zation, which lead to more ordered, pure sp2 type carbon.
The chemical transitions of organic, sulfur-containing
moieties during thermolysis and their final state after the
high temperature treatment is a complex issue. An interest-
ing insight into the sulfur state and transformation within
carbonaceous materials was presented by Kelemen et al.
[115,116]. Even though their research concerned long-lasting
transitions from organic to carbonaceous biomass, the results
give perspective on the synthetic S-doped carbons (prepared
via much more rapid pyrolysis). The chemical pathways for
sulfur and nitrogen transformations during coalification (nat-
Table 1 – Sulfur precursors and characteristics of the S-doped carbons.
Sulfur precursors Amount of S-doping Identified S moieties (mainly byXPS)
Textural parameters, Ref. No.
2-Thiophenemethanol(polymerized to polythiophene)
4.4–7.2 wt.% Not specified SBET = 1400–1930 m2/g, Vp = 1.68–2.14 cm3/g [98]
0.9–6.1 wt.% Sulphide –C–S–C– and sulphone–C–S(O)2–C– groups. Sulphonegroups represent only 5% of thetotal S-content
SBET = 340–840 m2/g, Vp = 0.56–1.6 cm3/g [120]
1.3–6.6 wt.% Mainly thiophenic sulfurincorporated into graphenesheets (C–S bonds)
SBET = 729–1627 m2/g, Vp = 0.60–0.90 cm3/g [122]
2.5–11.8 wt.% (depends of theactivation conditions)
Sulphide –C–S–C– and sulphone–C–S(O)2–C– groups (bridgesbetween adjacent aromaticrings)
SBET = 1430–3010 m2/g, Vp = 0.69–1.75 cm3/g [16,121,181]
Tetra(thiophene-2-ylmethoxy)silane
11.5–16.5 wt.% (but only 2.3 aftercarbonization at 1100 �C)
C–S and S–S bonds between twoaromatic rings; S in the aromaticconfiguration involved indelocalized p-electron systems
SBET = 680–870 m2/g, Vp = 0.66–1.26 cm3/g [123]
Microporous poly(1,3,5-tris(thienyl)benzene) network(PTTB)
5.6–23 wt.% depending on thecarbonization temperature
Sulfur directly incorporated intothe carbon backbone; S–S bondsbetween aromatic rings, S–Sbonds neighboring carbon atomsand/or C@S double bonds
SBET = 599–711 m2/g, Vmic = 0.13–0.24 cm3/g [125]
Thiophene Atomic ratio of C/S = 31.9 or 28.0 – [126]Up to 3.4 at.% Two distinct forms of S: �35% as
thiophenic –C–S–C–, and �65%as sulfate –C–SO4–C– orsulfonate –C–SO3–C– groups
[127]
2.6–5.5 at.% (17.2 beforeactivation)
Oxidized S moieties SBET = 579–1567 m2/g [183]
3,4-Ethylenedioxythiophene(EDOT) polymerized to poly(3,4-ethylene dioxythiophene),(PEDOT)
2.6 wt.% (in the Si/S-dopedcarbon composite)
– SBET = 59 m2/g (of the Si/S-dopedcarbon composite) [128]
PEDOT and poly-(styrenesulfonate), (PSS)
2.66 wt.% S2- in (–C–S–C–) and S4+ in (–C–SO3–C–)
[130]
Macroporouspoly(divinylbenzene), (PDVB)sulfonated with H2SO4
mole (%) of S/C = 0.64% beforeand 0.83% after activation (EDX)
– SBET = 770–2420 m2/g, Vp = 0.41–1.43 cm3/g [131,132]
Poly(4-styrene sulfonic acid co-maleic acid), sodium salt and/orpoly(sodium 4-styrenesulfonate)
4.6–11.1 (EDX)0.5–6.7 (XPS)0.2–7.0 (RFX), wt.%, [133,134],3.3–5.5 wt.% [135], 2.6–3.1 at.%(EDX) [136]
Majority of sulfur exists as R–SH,also as C–S–C/R–S2–OR (sulfides/thioethers), R2–S@O (sulfoxides),R–SO2–R (sulfones) R–SO3H(sulfonic acids)
SBET = 334–1449 m2/g, Vp = 0.26–1.08 cm3/g [133–135]. SBET = 700–847 m2/g, Vp = 0.59–0.61 cm3/g[136]
8C
AR
BO
N6
8(2
01
4)
1–
32
4,4 0-Thioldiphenol (TDP) Up to 1.5 wt.% Mainly in the form of stablearomatic sulfide, (sulfoxide andsulfone species appeared aftertreatment with H2O2)
SBET = 619–732 m2/g, Vp = 0.57–0.80 cm3/g [139]
H2S/graphene oxide 1.2–1.7 at.% Thiophene-S and oxidizedsulfur. S–C bonds are mainlypresent at the defect sites ofgraphene
[141]
2.8 at.% (XPS) or3.0 at.% (EDX)
R–SH groups, C–S–C/R–S2–ORsulfides/thiophenes andthioethers, R2–S@O sulfoxides;also elemental sulfur and/orpolysulfides
SBET = 196 m2/g, Vp = 0.25 cm3/g[142]
H2S, SO2, CS2/Staudenmaier,Hummers, and Hofmanngraphene oxide
3.69–11.99 wt.% depending of thetemperature, S-source and thetype of the GO used
S in the S6+ state, whichindicates the presence of –SO3H.Also, sulfur exists inside thelattice of the graphene
[146]
CS2/graphene 0.59–1.98 wt.% Only thiophene-type sulfurbonding
[173]
Benzyl disulfide (BDS) 1.45–1.5 wt.% (XPS) Mostly as sulphide –C–S–C– andonly small fraction as oxidizedsulfur groups –C–SOx–C–
SBET = 1099–1261 m2/g, Vp = 1.38–1.62 cm3/g [137]
1.30–1.53 wt.% –C–S–C–, and oxidized sulfurgroups (–C–SOx–C– (x = 2–4)) suchas sulfate or sulfonate
SBET = 435–440 m2/g [143]
p-Toluenesulfonic acid (TSA) 2.3 wt.% - SBET = 1030 m2/g [138]1.9 at.% –C–S–C– and –C–SOx–C– (x = 2–4),
sulfate or sulfonate[144]
1-Butyl-3-methylimidazoliumhydrosulfate [Bmim][HSO4]
1.88% (XPS) –C–S–C–, sulfate (–C–SO4–C–) orsulfonate (–C–SO3–C–)
SBET = 928 m2/g [145]
Sublimed sulfur/starch 1.95 at.% (XPS) S in thiol, thiophene, –S–S–,SO2
2– and SO32–, elemental S
SBET = 433 m2/g [184]
Sulfur/graphite 17.84 wt.%,9.64 wt.% (from XPS)
C–S bonds at the edges ofgraphene sheets. The C–S–C atthe edges can be oxidized to C–SO2–C
SBET = 143.7 m2/g, Vp = 0.07 cm3/g[174]
K2SO4 or Na2S2O3/glucose Up to 16 wt.% for K2SO4; up to30 wt.% for Na2S2O3
S-atoms hybridized into the sp2
aromatic carbon in thiophoneform, also small amount ofoxidized sulfur (S–O bonds)
SBET up to 3250 m2/g [165]
Hexane in the presence ofelemental S (growth promoter)
0.6 at.% S doped into graphene’s latticemainly forms linearnanodomains
[147]
CA
RB
ON
68
(2
01
4)
1–
32
9
10 C A R B O N 6 8 ( 2 0 1 4 ) 1 – 3 2
ural formation of coal) was elucidated by comparing the
chemical forms of unaltered peats, lignites, coals and pyro-
lyzed peats [115,116].
As known, fossil fuels are rich in nitrogen-containing
compounds of biogenic origin. Similarly to nitrogen, or-
ganic sulfur may be derived directly from sulfur-contain-
ing biomolecules, yet most of the sulfur is incorporated
into organic matter during diagenesis (conversion of sedi-
ment to rock-like phase) from H2S, which is produced by
bacteria (sulfate reduction) [115,116]. H2S reacts either
with iron to form pyrite or with organic matter to form
organic sulfur species. As a result, the aliphatic and aro-
matic sulfur forms are present in coals with the relative
amount of aromatic sulfur increasing as the coal rank in-
creases. The natural conversion of aliphatic sulfides into
more stable aromatic forms competes with their loss as
the coal rank increases. If not converted to an aromatic
state, aliphatic sulfur is lost as SO2 and H2S. Elemental
S, pyrite, sulfides, thiophene, and sulfates have been de-
tected in a variety of coals, asphaltenes, carbon blacks
and crude oils [117–119].
Aliphatic sulfur is the predominant organic sulfur form
in unaltered peat, followed by SO3 groups, whereas the level
of aromatic sulfur is relatively low. In peats subjected to
moderate pyrolysis aliphatic sulfur is presented mostly as
mercapto („C–SH) and disulfide species. Mercapto species
are more stable than disulfides and may be created from
disulfides as a result of breaking the weak disulfide bond,
followed by hydrogen abstraction. In general, thiol, thioe-
ther, and disulfide are among the sulfur functional groups
most readily lost as H2S during pyrolysis of fossil fuels
(peat) [119]. Kelemen et al. showed that the level of aro-
matic (tiofenic) sulfur increases as the severity of peat pyro-
lysis increases [115,116]. It happens through the selective
loss of disulfide („C–S–S–C„), sulfide („C–S–C„), and SO3
groups and transformation of other aliphatic sulfur forms.
In addition, the non-aromatic sulfur forms in coal are more
prone to ambient oxidation. As a result, organic sulfur ex-
ists almost exclusively in aromatic forms when peats are
pyrolyzed to a higher extent and the level of aromatic sul-
fur increases as the weight percentage of elemental carbon
increases (higher level of carbonization).
Research concerning high-temperature treatment of fossil
fuels indicates that aromatic sulfur is the most stable one
against thermolysis. Such sulfur motifs may withstand high
temperature treatment of organic compounds, allowing syn-
thesis of highly carbonized, graphitic materials with substan-
tial amount of permanently infused sulfur.
Fig. 1 – Synthesis of sulfur-doped heterocarbon from 2-thiophe
copyright � 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinh
3.2. Thiophene-based precursors
From a few precursors, which have been used to synthesize
intrinsically S-doped carbons, tiophene and thiophen-based
compounds, especially the commercially available 2-thioph-
enemethanols are the most common choices [16,98] (Table 1).
They are sufficiently stable to survive carbonization condi-
tions. The chemical polymerization of 2-thiophenemethanol
is usually induced by FeCl3 leading to the thermostable poly-
thiophene. In such polythiophene-derived carbons the incor-
porated sulfur predominantly forms sulphides –C–S–C– due to
carbonization reducing conditions. It was Shin et al. who first
applied a linear polymerization of 2-thiophenemethanol for
the preparation of high surface area, sulfur-doped mesopor-
ous carbons by a nanocasting method using ordered meso-
porous silica as a hard template [98]. An acetone solution of
2-thiophenemethanol was infused into the template and
the polymerization was carried out at 100 �C. This raw mate-
rial was subjected to carbonization at elevated temperatures
(700–900 �C and above), as outlined in Fig. 1. The mesoporous
carbon product contained 4–7 wt.% sulfur, its pore volume ex-
ceeded 2 cm3/g and the surface area was up to 1930 m2/g.
Ordered mesoporous silica and polythiophene were also
employed to obtain porous SiO2/S-doped carbon nanocom-
posite [120]. Such a composite, which combines the structural
properties of silica (uniform ordered mesopores) with charac-
teristics of carbon materials (good electrical conductivity) and
additionally enriched with heteroatoms, might find a range of
advanced applications. The composites made up of the S-
doped carbon layers coating the pores of two types of meso-
structured silica were prepared via the oxidative polymeriza-
tion of 2-thiophenemethanol inside the silica pores followed
by carbonization. The resulting silica/S-doped carbon com-
posites consisted of 20.6–26.6 wt.% of carbonaceous matter
containing up to 6.1 wt.% of S. However, the amount of S de-
creased sharply as the carbonization temperature was raised
from 500 to 800 �C. The SiO2/carbon composites exhibited
high surface area, large pore volume and a well-ordered mes-
oporosity. Removal of the silica gives rise to ordered mesopor-
ous carbons exhibiting a large amount of sulfur (3.9 wt.%).
Similarly to results presented elsewhere [16,121], the carbon-
ized polythiophene contained two types of S groups, i.e. –C–S–
C– sulphide, and –C–S(O)2–C– sulphone, which acted as
bridges between the adjacent aromatic rings (Fig. 2). The sul-
phone bridges represented only c.a. 5% of the total sulfur con-
tent, indicating that most of the sulfur present in the
carbonized polythiophene (13.8 wt.%) is in the form of sul-
phide bridges.
nemethanol. [Reprinted with permission from reference 98;
eim.]
S
S
S
O H
OH
SO
S
O
O
O
OH
S
O
Fig. 2 – Structure of S-doped carbon (also after additional
activation) obtained from polymerized 2-thiophenemethanol
[16,98,120–122].
C A R B O N 6 8 ( 2 0 1 4 ) 1 – 3 2 11
Besidesorderedsilicas,zeolitewasalsoappliedasahardtem-
plate to obtain structurally ordered, sulfur-doped carbon materi-
als characterized by enhanced microporosity [122]. A zeolite
sample was impregnated with a solution of 2-thiophenemetha-
nol, which was then polymerized and eventually pyrolysed. S-
doped microporous carbons with sulfur content in the range of
1.3–6.6 wt.% and large surface area of up to 1630 m2/g were ob-
tained. Insuch carbons sulfurwas mainly in the thiophenic state
as S atoms incorporated into the graphene sheets.
In order to obtain S-doped carbons with extremely large
surface areas, chemical activation with KOH of the polythio-
phene was utilized [16,121]. The carbons obtained exhibited
very large surface areas of up to 3010 m2/g and high pore vol-
umes of up to 1.75 cm3/g (such extremely high surface area is
surprising, taking into account that graphene has a theoretical
surface area of 2630 m2/g). As the authors noticed, the polythi-
ophene polymer obtained with FeCl3 as an oxidant contains
28 wt.% sulfur, a content that is significantly lower than that
of the unsubstituted polythiophene (38 wt.%). The lower sulfur
content in the polymer is due to the presence of oxygen
(11 wt.%), which arises from the oxidative polymerization pro-
cess. Samples activated at 600 �C contained an extremely large
amount of sulfur (11.8 wt.%) – one of the largest sulfur contents
ever reported for a highly activated carbon. The sulfur present
in these carbons forms only two types of functional groups (i.e.
sulphide –C–S–C– and sulphone –C–SO2–C–). The oxidation
SS
S
S
S
1
Fig. 3 – Monomers (1 and 2) used for the synthesis of the micro
thienyl)-9,90-spirobifluorene (1), 1,3,5-tris(2-thienyl)benzene) (2)
reactions taking place during the chemical activation gener-
ated a variety of oxygen functional groups with total oxygen
content up to 20 wt.%. The chemical activation of polythio-
phene induced the formation of a large number of sulphone
groups, yet the contribution of sulfone bridges increased with
the decrease of the activation temperature.
A very elegant and versatile temple-assisted synthesis of
S-doped porous carbon was proposed by Bottger-Hiller et al.
[123]. They employed a thiophene-based twin monomer and
carried out the twin polymerization on silica templates to ob-
tain sulfur-doped carbon materials with hierarchical pore
structure. The starting material was tetra(thiophene-2-
ylmethoxy)silane, a thiophene-based twin monomer, which
can be eventually converted into microporous sulphur-doped
carbon. After adding an acid catalyst, the twin monomers re-
act by means of twin polymerization to form a polymer–SiO2
nanocomposite. During the formation of the composite mate-
rial, the organic compound can be considered as a template
for the inorganic component and vice versa. The obtained
S-doped carbons were decorated with 12–17% of sulfur. No
elementary S was found in the S-doped carbon samples and
the majority of sulfur species were carbon bonded, i.e. C–S
bonds and S–S bonds between two aromatic rings (S atoms
permanently bonded to aromatic carbon atoms). The authors
assumed that by using the twin polymerization it is possible
to easily adjust the parameters controlling the shapes of the
final products, such as the diameter and the shell thickness
of S-doped carbon hollow spheres. Thus a variety of sulfur-
doped carbon materials with a tailored pore texture are avail-
able via this unique procedure.
Starting from a sulfur containing microporous poly(1,3,5-
tris(thienyl)benzene) (PTTB) polymer network (SBET = 1060 m2/
g, Vmic = 0.26 cm3/g) [124], Schmidt et al. obtained S-doped
carbon with sulfur content of up to 23 m%, but with specific
surface areas lower than the precursor polymer [125]. The
cross-linked structure of the precursor network was the key
to the formation of the solid-state S-doped carbon material.
Comparison of the PTTB-network and its monomer 1,3,5-
tris(thienyl)benzene (Fig. 3) showed that the monomeric
structure evaporates completely upon thermal treatment in
an inert gas (boiling point: 310 �C), while the crosslinked
microporous network yields high residual masses of 68% at
temperatures as high as 900 �C. Besides the high amount of
S atoms due to the thienyl units, the microporosity of the
PTTB represents a further advantage, inducing intrinsic
porosity in the carbonaceous materials.
S
S
S
S
S
2 3
porous polythiophene networks (3); 2,2 0,7,7 0-tetrakis(2-
and poly(1,3,5-tris(thienyl)-benzene), PTTB (3) [124,125].
S
OO
S
OO
n SO3 n_
Fig. 4 – Structure of 3,4-ethylenedioxythiophene (EDOT),
poly(3,4-ethylenedioxythiophene) (PEDOT) and polystyrene
sulfonate (PPS).
12 C A R B O N 6 8 ( 2 0 1 4 ) 1 – 3 2
The cross-linked PTTB structure hindered complete ther-
mal decomposition and partially suppressed the elimination
of sulfur atoms due to the lack of available protons neighbor-
ing the sulfur atoms, as the thienyl units are known to
decompose via the elimination of H2S [125]. Upon increasing
the temperature of synthesis, the sulfur atoms were directly
incorporated into the forming carbon backbone. Carbon-
bound sulfur atoms were detected as: S–S bonds between
two aromatic rings, other S–S bonds neighboring carbon
atoms and/or C@S double bonds. The microporous morphol-
ogy of the precursor network was maintained in the sulfur-
doped carbon with SBET of 599–711 m2/g and micropore vol-
ume of 0.13–0.24 cm3/g.
Unlike thiophene based polymers, which can be directly
subjected to carbonization, synthesis of S-doped carbons
from thiophene itself requires a completely different syn-
thetic approach. In such cases, methods appropriate for vola-
tile species, e.g. CVD or flame synthesis are more appropriate.
For instance, an amorphous carbonaceous material described
as CxS (x = 28.0 or 31.9) was synthesized by CVD method from
thiophene or thiophene with added chlorine at 800 �C [126]. It
was observed that the presence of chlorine significantly af-
fected the final carbon composition, i.e. the sulfur content in-
creased and the hydrogen content decreased. The values of
the C/S atomic ratio in the carbonaceous materials synthe-
sized from thiophene or thiophene with chlorine were 31.9
and 28.0, respectively. These results indicate that chlorine
promotes the elimination of hydrogen as HCl and prevents
the elimination of sulfur in heat-treated thiophene.
Very recently a flame synthesis was applied to produce
sulfur-containing carbon soot by directly burning flammable
thiophene without any solvent or metal source [127]. The
flame synthesis provides not only a rapid single step low-cost
approach, but also a very efficient and scalable methodology
for sulfur-containing carbons with good control of the sulfur
content. The sulfur content was found to vary from 1.0 to
3.4 at.%. Sulfur was found to be incorporated into the carbon
in two distinct forms, �35% as –C–S–C– and �65% in the form
of sulfate (–C–SO4–C–) or sulfonate (–C–SO3–C–). The obtained
carbon soot was amorphous with the interlayer distance of
the (002) plane increasing after the addition of sulfur from
3.354 A expected for graphite to 3.616 A, as the diameter of
the sulfur atom is larger than that of carbon.
Besides polythiophene [16,98,120–122] other, different thio-
phene-based polymers were also employed to obtain S-doped
carbons. For instance 3,4-ethylenedioxythiophene (EDOT) can
be polymerized into conducting poly(3,4-ethylenedioxythio-
phene) PEDOT (Fig. 4) and subsequently carbonized, giving rise
to S-doped carbon [128]. This precursor was used to produce a
porous Si/S-doped carbon nanocomposite using the post-car-
bonization method. First, the Si/S-doped polymer composite
was prepared by dispersing porous Si in the aqueous solution
of EDOT, followed by its chemical polymerization using Fe2(-
SO4)3/(NH4)2S2O8 as a catalyst. The Si/S-doped carbon composite
was prepared by carbonizing the Si/PEDOT composite. The as-
prepared Si composite was homogeneously coated with highly
disordered S-doped carbon, with 2.6 wt.% of S in the composite.
The conducting polymer PEDOT is often composited with
poly(styrenesulfonate), PSS (Fig. 4), which is used as a
charge-balancing dopant and dispersing agent during poly-
merization to yield a PEDOT:PSS composite. The PEDOT/PED-
OT:PSS mixtures have been used as cathode materials in
lithium-ion batteries [129]. Since both PEDOT and PSS contain
one sulfur atom per repeat unit, they are promising precursor
for S-doped carbon. The Si/S-doped carbon composite was
prepared by carbonizing the Si/PEDOT:PSS composite [130].
The Si content in the Si/S-doped C composite was determined
to be 84.7 wt.%. A 2.66 wt.% of sulfur was doped in the matrix
of the amorphous carbon of the Si/C composite. The sulfur
doping state in the carbon matrix of the Si/C composite was
determined as S2� (–C–S–C–) and S4+ (–C–SO3–C–) respectively.
3.3. Other S-containing precursors and S-doped graphene
As shown, polymers with thiophenic configurations are capa-
ble of retaining a substantial amount of sulfur after high tem-
perature pyrolysis. Nonetheless, polymers sulfonated with
sulfuric acid have been also employed as S-doped carbon pre-
cursors. Macro/meso/microporous S-doped carbon monoliths
with high surface areas of up to 2420 m2/g have been prepared
from sulfonated poly(divinylbenzene), PDVB monolith net-
works, followed by activation with CO2 [131,132]. The starting
macroporous PDVB was sulfonated with H2SO4 at 150 �C. After
carbonization the obtained macroporous carbon monoliths
were subjected to activation. Both the carbonized and the
additionally activated materials contained sulfur. The per-
centages of S/C in the carbon material before and after activa-
tion were 0.64% and 0.83%, respectively (in atomic percent;
EDX). Since the ratio of S/C increased after activation, it was
assumed that carbon is more prone to be pyrolyzed than sul-
fur by activation with CO2.
Bandosz et al. also used polymers containing sulfonic
groups to obtain carbonaceous materials enriched with sulfur
[133–136]. By carbonizing poly(4-styrene sulfonic acid co-
maleic acid), sodium salt and/or poly(sodium 4-styrene sulfo-
nate) they obtained two series of polymer-based S-doped car-
bons with different contents of sulfur and surface area of up
to 1450 m2/g. The surface chemistry of the carbons was tuned
by additional oxidization either by heating in air or by chem-
ical treatment. A significant amount of sulfur (5–11 wt.%) was
detected on the carbon surface. Sulfur in R–S configuration
and sulfonic groups (R–SO3) were detected. Yet not only poly-
meric species containing covalently bonded sulfur are utilized
to produce intrinsically S-doped carbons. For instance, S-
doped ordered mesoporous carbon has been prepared using
p-toluenesulfonic acid (TSA) or organic sulfides: benzyl disul-
fide (BDS) and 4,4 0-thioldiphenol (TDP) [137–139], using hard
or soft templating methods. Utilizing sucrose and BDS as
C A R B O N 6 8 ( 2 0 1 4 ) 1 – 3 2 13
the carbon and sulfur sources, ordered porous carbons with
different sulfur content of up to 1.5 wt.% (predominantly as
sulfide groups (–C–S–C–)) were obtained [137]. Using TSA as
a sulfur and carbon source, a high surface area carbon was
prepared and probed as a Pt catalyst support [138]. A strong
metal-support interaction between Pt nanoparticles and S
atoms incorporated into the ordered mesoporous framework
(support) significantly improved the stability and catalytic
activity of Pt particles.
As stated above, in order to obtain S-doped carbons with
ordered uniform porosity, hard templating has usually been
used [98,120,122]. Nonetheless, Zhao et al. obtained mono-
liths of sulfur containing mesoporous carbon via aqueous
self-assembly synthesis (soft templating) using 4,4 0-thioldi-
phenol (TDP) as a sulfur source [139]. TDP and resorcinol were
employed as the carbon/sulfur precursors and the copolymer
F127 as the structure directing agent. The resultant S-contain-
ing organic precursors were carbonized at 650 �C. These
materials exhibited a uniform pore size (6–13 nm), relatively
large surface areas (620–730 m2/g) and a controllable sulfur
content of up to 1.5 wt.% in the form of stable aromatic sul-
fides. Further treatment of these materials with H2O2 allowed
alteration of the aromatic sulfide, sulfoxide, and sulfone moi-
eties ratios. The sulfur in the form of aromatic sulfide ex-
pressed outstanding stability under pyrolysis at 1000 �C.
Extensive research has already shown that graphene oxide
(GO) constitutes a versatile precursor for S-doped carbons
with a variety of textural features (e.g. developed porosity).
The synthesis of S-doped carbons utilizing GO usually follows
the same scheme. First, the graphene oxide is produced in the
form of graphene nanosheets by the oxidation of graphite
powder using the well established Hummers’ method [140].
Then it is subjected to the direct thermal annealing with S-
containing precursors. For clarity, it should be recalled that
graphene oxide does not retain a stacked structure of graphite
oxide, but the material is exfoliated into monolayers or few-
Fig. 5 – Schematic illustration of the fabrication of S-doped graph
with the aid of a cationic surfactant CTAB; (3–2) thermal anneali
removal of silica by HF or NaOH solution. [Adapted with permiss
GmbH & Co. KGaA, Weinheim.] (A colour version of this figure c
layered stacks. The surface functionality greatly weakens
the platelet–platelet interactions [140]. A variety of thermal
and mechanical methods can be used to exfoliate graphite
oxide to graphene oxide and sonicating or stirring in water
are the most common ones.
The direct thermal annealing, which is routinely used to
obtain heteroatom-doped graphene, usually results in an irre-
versible stacking of graphene (due to the strong p-interac-
tions) and low specific surface area. Nonetheless, S-doped
graphene with very high surface area was obtained via a ther-
mal reaction between graphene oxide and H2S on the basis of
ultra-thin graphene oxide/porous silica sheets [141]. In addi-
tion, it was shown for the first time that S can be doped into
graphene sheets in the form of thiophene-like S. The ultra-
thin sandwich-like GO/porous silica sheets were fabricated
via the hydrolysis of tetraethyl orthosilicate (TEOS) on the sur-
face of graphene oxide, Fig. 5 [141]. The unique porous silica
layer favored the transport of the gas source to the surface
of GO and effectively prevented the irreversible re-aggrega-
tion of graphene during the S-doping process at high temper-
ature. The S-doping of different binding configurations
occurred only at the edges or on the defects of the graphene.
At higher temperature (>700 �C) sulfur was mainly doped in
the thiophene form. The same synthetic route, i.e. high tem-
perature treatment of graphite oxide with H2S was used by
Bandosz et al. and highly hydrophobic S-doped carbon
with surface area of c.a. 200 m2/g was produced [142]. At
high temperature the oxygen functionalities of the graphene
layers decompose and react with H2S forming sulfur groups.
About 3 at.% of sulfur was introduced into the GO (mainly
as the thiophenic groups and sulfur in aromatic rings),
however, elemental sulfur and/or polysulfides were also
detected.
Sulfur-doped graphene was also obtained by directly
annealing commercial graphene oxide and benzyl disulfide
[143]. Sulfur was doped into the carbon in two distinct forms:
ene: (1) and (2) hydrolysis of TEOS around the surface of GO
ng of GO-silica sheets in H2S at 500, 700 or 900 �C; and (4–2)
ion from reference 141; copyright � 2012 WILEY-VCH Verlag
an be viewed online.)
Table 2 – Precursors and characteristics of the S-co-doped carbons.
Heteroatom precursors Amount of heteroatoms Identified heteroatom moieties Textural parameters, Ref. No.
Thiazolium salts: 3-methyl-thiazol-3-ium-dicyanamide (thia-DCA), and3-(cyanomethyl)-thiazol-3-ium-bromide (CN-thia)
S = 6–7, N = 7.8–8.75 (carbonizationat 1000 �C). After templating:S = 2.17–3.84, N = 5.19–6.17 (wt.%)
S in cyclic carbon structures in anaromatic environment (thiophenicsulfur, C@S double bonds). N aspyridinic (N-6), pyrrolic (N-5) andgraphitic (quaternary N-Q)
SBET = 1174 m2/g (thia-DCA) and1195 m2/g (CN-thia) [113]
1-Ethyl-3-methyl imidazoliumthiocyanate; 1-ethyl-3-methyl-imidazolium dicyanamide; 1-butyl-3-methyl-pyridinium dicyanamide
S = 5.8–6, N = 10.8–12.7 (wt.%) S bonded within nitrogen-aromaticrings; in aromatic carbon–sulfurmotifs in the form of thiophenicsulfur species or C@S double bonds.Also a small amount of oxidizedsulfur. N as N-6, N-5, N-Q andoxidized nitrogen species
SBET = 900–980 m2/g [150]
Cysteine S = 2.74, N = 2.7 (wt.%) S in two distinct forms: –C–S–C– (8%)and –C–S(O)x–C– (x = 2–4) - sulfate/sulfonate (92%). N as: N-Q, N-6 andpyridinic oxide
SBET = 180.5 m2/g [151]
Human hair (keratin containingcysteine)
S = 0.2–1.3, N = 2.6–3.1 (at.% by XPS) Majority of S as thiophenic –C–S–C–,also some oxidized sulfur species. Nas N-6, N-5, N-Q and pyridinic N-oxide (N-X)
SBET = 849–897 m2/g, Vp � 0.5 cm3/g[152]
S-(2-thienyl)-L-cysteine (TC) or L-cysteine
Depends of the precursor: S = 3–12,N �4–5 (wt.%, after HTC); S = 2.9–7.2,N = 3.6–4.5 (wt.%, pyrolysis at 900 �C)
By varying the sulfur and nitrogensource the binding state of S and Ncan be tuned from pending groupssuch as thiols, sulfonates or amidesto structural thiophenic (aromaticC–S–C) or N-6 and N-Q units
SBET = 281 for TC, 440 for L-cysteine;m2/g (after pyrolysis at 550 �C) [153]
S-(2-thienyl)-L-cysteine or 2-thienylcarboxaldehyde (TCA)
S = 0.74–1.0, N = 4.3–5 (wt.%) S in thiophenic (aromatic) C–S–Cstate, N as N-6 and N-Q (afterpyrolysis at 900 �C)
SBET = 224.5 and 321 m2/g, Vp = 0.27and 0.48 cm3/g (depends on theprecursor) [154]
Cysteine, dicyandiamide (N- and S-precursor), phosphoric acid (P-precursor)
S = 1.38, N = 6.66, P = 5.98 (wt.%) S as –C–S–C– and –C–S(O)x–C–, N asN-6, N-5, N-Q, and pyridinic-oxide; Pmainly bonded with the metals,small amount of P (<1 at.%) doped inthe carbon lattice
SBET = 334.5 m2/g [155]
Poly(cyclotri-phosphazene-co-4,4 0-sulfonyldiphenol)
S = 1.6, N = 2.0, P = 4.4 (wt.%, EDX) Not specified SBET = 1140 m2/g, Vp = 0.9 cm3/g [156]
Thiourea S = 3–8, N = 8.5–17 (wt.%, by XPS) S0, S4+, and S2�, N as pyridine-likenitrogen and N-Q
SBET = 81–850 m2/g [157]
S = 1.5–3.5, N = 4.2–15.3 (wt.%) Thiophene like S, nitrogen as N-6, N-5 and N-Q
SBET = 298–1017 m2/g [163]
FeSO4Æ7H2O (S-doping), cyanamide(N-doping)
S = 0.6, N = 0.9 (at.%, carbonizationat 1050 �C)
Mainly thiophene-like S and alsothiolate (or thiocyanide)-like sulfur,N as N-6, N-5 and N-Q
[158]
14
CA
RB
ON
68
(2
01
4)
1–
32
Thiophene/acetonitrile S = 0.7–1.6 (cabron prepared using100% thiophene had 3.6% S), N = 7.6–8.3 (at.%, XPS)
S–C species, sulfate species,elemental sulfur; N as N-6, N-Q andpyridinic-N-O
SBET = 150–240 m2/g [102]
Polyacrylonitrile (PAN) and sulfuricacid
Atomic ratio of S/C = 0.0021, N/C = 0.209
S as sulfide –C–S–C–, sulfate –C–SO4–C– or sulfonate –C–SO3–C–. N as N-Qand in amine-groups
[159]
Pyrrole and sulphuric acid S = 1.7–2.6, N = 4.8–10 (at.%) Thiophene-like sulfur, C–S–Nbinding motifs, –C–S(O2)–C–, N as N-6 and N-Q
SBET = 978–1021 m2/g, Vp = 1.1–1.2 cm3/g [162]
Poly(4-styrenesulfonic acid-co-maleic acid) sodium salt; phosphoricacid
S = 0.2–6.0, P = 0.04–1.2 (wt.%) [160];(S = 0.3–3.6, P = 1.9–3.9, by XPS inat.%) [161]
S in R–SH configuration, also C–S–C/R–S2–OR (sulfides/thioethers), R2–S@O (sulfoxides), R–SO2–R(sulfones), R–SO3H (sulfonic acids). Pas phosphates or pyrophosphates,metaphosphates and P2O5
SBET = 684–1854 m2/g, Vp = 0.5–1.28 cm3/g [160]; SBET = 1308 m2/g,Vp = 1.06 cm3/g [161]
Benzyl disulfide (BDS) andmelamine as S and N precursors,respectively
S = 2.0, N = 4.5 (at.% by XPS) N and S atoms doped into thegraphene framework as C–N–C (N-5,N-6, N-Q) and C–S–C (thiophene-type) species. S can only formdouble bond with C, the doping sitesfor S are on the edges of graphenelayers
SBET = 157–220 m2/g [148]
Thiophene and pyrimidine – Pyrrolic/graphitic N-dominantstructures
SBET = 640 m2/g [149]
NH4SCN/graphene oxide S = 4.1–18.4, N = 4.0–12.3 (wt.%) S as C–Sx–C (x = 1 or 2) bonds andconjugated –C@S– bonds(thiophene-type). Minority (>10%) asoxidized (–SOx–) and reduced (–SH)groups. N as N-6 and N-Q
SBET = 220 m2/g [164]
LiNO3 + Na2S2O3/glucose S up to 13.7, N up to 8 (wt.%) S atoms bonded to the sp2 carbonnetwork as thiophone-S, smallamount of oxidized sulfur (S–Obonds). N as N-6, N-5 and N-Q
SBET up to 2792 m2/g [165]
Diphenyl disulfide/N-dopedgraphene
– Majority of S as –C–S–C– [192]
CA
RB
ON
68
(2
01
4)
1–
32
15
S
N
N
N
N+
_ S
NN
+
_Br
Fig. 6 – Molecular structures of 3-methyl-thiazol-3-ium-
dicyanamide (thia-DCA; N: 33.71, S: 19.29, wt.%) and 3-
(cyanomethyl)-thiazol-3-ium-bromide (CN-thia; N: 13.66, S:
15.64, wt.%) [113].
16 C A R B O N 6 8 ( 2 0 1 4 ) 1 – 3 2
as sulfide (–C–S–C–) and as oxidized sulfur groups (–C–SOx–C–
(x = 2–4)) such as sulfate or sulfonate. However, the C–S bonds
occur predominantly at the edge or at the defect sites. The
contents and bonding configurations of sulfur in these S–
graphenes can be adjusted by varying the mass ratios of GO
and BDS and the annealing temperature. The oxidized sulfur
groups (sulfate/sulfonate) could be transformed into sulfide
groups at higher annealing temperatures. The formation of
a C–S bond was attributed to the direct reaction between oxy-
gen-containing groups in GO (e.g., carbonyl, carboxylic, and
lactone) and BDS.
Starting from graphite oxide derived through the Hum-
mers method, S-doped graphene was also obtained using p-
toluenesulfonic acid as a sulfur source. 1.9 at.% of sulfur
was successfully doped into the graphene framework as
thiophene-like –C–S–C– and –C–SOx–C– (x = 2–4), sulfate or
sulfonate [144]. Synthesis of S-doped carbon/graphene nano-
composites has also been reported recently [145]. S-doped
porous carbon hybridized with graphene was obtained utiliz-
ing an ionic liquid serving as both the stabilizer for graphene
(preventing the aggregation of graphene sheets) and the med-
ium in which the synthesis occurred. The D-glucose and GO
were employed as the starting materials and 1-butyl-3-meth-
ylimidazolium hydrosulfate [Bmim][HSO4] as the solvent. As
noticed, S preferentially combined with the porous carbon
(not with the graphene). The S-doped porous carbon was uni-
formly distributed on both sides of the graphene sheets. S-
doping increased the number of nanopores and the interlayer
distance of d002 in the porous carbon component.
It was recently discovered, that the extent of S doping into
graphene depends more on the type of graphite oxide than on
the type of S-containing gas utilized for the synthesis [146].
Three graphite oxide types, obtained by the Staudenmaier,
Hummers, and Hofmann methods were used to obtain S-
doped graphene via the thermal exfoliation of the GO in
SO2, H2S, or CS2 atmospheres. It was observed that sulfur
doped Hummers GO possessed the highest S content. Higher
incorporation of S was also observed for samples exfoliated in
atmospheres with low valence sulfur compounds (H2S, and
CS2). The results clearly indicated that the three types of
GOs are very different materials. Besides S-doped graphene
obtained from graphite oxide, monolayered S-doped graph-
ene has been also synthesized using the CVD method [147].
A solution of elemental S in hexane was used as a carbon pre-
cursor, however, the level of doping was very low – 0.6 at.%.
3.4. S-co-doped carbons: binary or ternary doping with Nand/or P
Multiple doping is a versatile synthetic approach for new car-
bon materials and takes the tuning of carbon properties one
step further in comparison to one-type-only heteroatom dop-
ing. In particular, nitrogen is reckoned as a peerless carbon
dopant, thus the N-doping is by far the most ubiquitous when
compared with other heteroatoms [9,10]. Complementing
nitrogen as a dopant, sulfur, boron or phosphorous are receiv-
ing increasing attention in carbon materials research. N-dop-
ing is preferential when it comes to tuning of electronic
properties of the carbon material, whereas sulfur, due to its
larger size, has been used for applications where its easily
polarizable electron pairs (and thus chemical reactivity) are
of interest. So far, only a few reports on carbon materials that
are simultaneously co-doped with sulfur, nitrogen and other
atoms have been reported and further research is expected,
since such co-doped carbons exhibit a range of unique prop-
erties. For instance, a synergistic effect of combined sulfur
and nitrogen-doping in the catalysis of the oxygen reduction
reaction (ORR) was presented as a significant discovery for fu-
ture research concerning the improvement of heterogeneous,
metal-free, carbon-based catalysts [148,149]. The precursors
and some characteristics of the S and N (or P)-co-doped por-
ous carbons are collectively presented in Table 2. As can be
seen, in such carbon materials nitrogen occurs predomi-
nantly as pyridinic (N-6), pyrrolic (N-5), graphitic (quaternary
N-Q) and pyridinic N-oxide (N-X); Table 2.
3.4.1. Binary and ternary doped porous carbonsCarbon materials intrinsically co-doped with nitrogen and
sulfur were successfully synthesized by annealing of nitrile-
functionalised thiazolium salts: 3-methyl-thiazol-3-ium-dicy-
anamide (thia-DCA) and 3-(cyanomethyl)-thiazol-3-ium-bro-
mide (CN-thia) [113]. In these salts S occurs in cation in the
tiophenic configurations (Fig. 6). An exceptionally high degree
of S-doping was achieved in the obtained carbons, even after
pyrolysis at 1000 �C.
The amount of S and N doping was easily adjusted by
choosing the appropriate pyrolysis temperature (an increase
of the pyrolysis temperature decreased the heteroatom con-
tent). The obtained materials exhibited an aromatic graph-
ite-like carbon backbone with high degrees of heteroatom
doping of about 6–8 wt.% of both S and N (after pyrolysis at
1000 �C). Such high degree of doping is difficult to achieve
by other approaches with low-molecular-weight precursors.
Nitrogen was fully infused into the graphitic framework
(mainly as an N-Q). It was observed that when the precursors
were heated to higher temperatures, more nitrogen was
firmly bound into the carbon backbone as stable graphitic
nitrogen at the expense of less stable pyridinic/pyrrolic moie-
ties. Sulfur was also bound within the carbon backbone form-
ing cyclic carbon structures in an aromatic environment, such
as thiophenic sulfur, or C@S double bonds. The high degree of
heteroatom doping altered the microstructure of the graphitic
domains (increased interlayer distances). If silica nanoparti-
cles were applied as a template, highly porous S/N-co-doped
carbon materials were produced.
Antonietti et al. utilized sulfur containing ionic liquid (IL)
1-ethyl-3-methyl imidazolium thiocyanate as a precursor for
the synthesis of S–N-co-doped porous carbons [150]. Mixtures
NNN
N
N
+_S
N
+_
N+
N+
N
C H3
N
N
N
_
(a) (b) (c)
Fig. 7 – Structure of 1-ethyl-3-methyl imidazolium thiocyanate (a), 1-ethyl-3-methyl-imidazolium dicyanamide (b) and 1-
butyl-3-methyl-pyridinium dicyanamide (c) [150].
C A R B O N 6 8 ( 2 0 1 4 ) 1 – 3 2 17
with other ILs (1-ethyl-3-methyl-imidazolium dicyanamide
and 1-butyl-3-methyl-pyridinium dicyanamide) containing
structural nitrogen (pyridinium, pyrrolidinium or imidazoli-
um), and cyano functionalities (Fig. 7), enabled tuning of the
S-content. Additional salt templating allowed the synthesis
of N–S-doped carbons with surface areas of up to 980 m2/g.
The isothiocyanate counterion – due to its high polarizability
– enables structural sulfur fixation at comparably low temper-
atures by cycloaddition reactions with sp2-compounds. When
compared to the non-porous N–S-doped carbons, the sulfur
content of the salt templated carbons increased by a factor
of three. The authors stressed the fact that the composition
of the porous carbons was strongly related to their specific
surface area – they concluded that sulfur prefers more ther-
modynamically stable surface termination sites, while it is
thermodynamically unfavorable to be incorporated within
the bulk of a carbon.
A series of papers presented synthesis of S/N-co-doped
carbons derived from amino acids and biomass [151–155].
Renewable and cheap biomass is a promising feedstock for
synthesis of heteroatom doped porous carbons. By selecting
a suitable biomass as a precursor a variety of hetereatoms
can be co-doped in the final carbonaceous material. Choi
et al. synthesized S–N-co-doped graphitic carbons via pyroly-
sis of composites composed of iron chloride, cobalt chloride
and different amino acids (alanine, cysteine, glycine, niacine
and valine) [151]. In particular, in the case of cysteine (C3H7-
NO2S), N and S dual-doped carbon with sulfur doping level
of 2.74 wt.% was synthesized and most of the sulfur occurred
in the sulfate or sulfonate state. Nitrogen was infused into the
S/N-carbon framework mainly as a N-Q. The S/N-co-doped
carbon had a sponge-like structure with pores of around
10 nm. The sponge-like structure of the carbon was induced
by the doping, which caused deformation of the carbon struc-
ture (open edge sites, high curvature). A relatively large
amount of cysteine can be found in keratin – fibrous protein
creating human hair. Cysteine constitutes more than
NH2
O
OH
SHS
S
NH2
Fig. 8 – L-Cysteine, S-(2-thienyl)-L-cysteine (TC) and 2-thienyl ca
hydrothermal synthesis of nitrogen and sulfur dual doped carb
precursor.
10 wt.% of the hair. Such biomass has been recently used to
prepare N/S-dual doped microporous carbons using glucose
and the hair as precursors via hydrothermal carbonization
(HTC) and KOH activation [152]. A sulfur content of up to 1.3
and nitrogen content of up to 3.1 at.% was reached.
A similar procedure, a one-pot HTC towards dual nitrogen/
sulfur doped materials based on D-(+)-glucose, L-cysteine and
S-(2-thienyl)-L-cysteine (TC) was also recently presented [153].
Cysteine and its derivative, thienyl-cysteine, were chosen as
the S and N source (Fig. 8). The carbon materials produced
possessed morphology of discrete microspheres with broad
size distribution (1–15 lm). A nitrogen content of about
5 wt.% and a sulfur content of 3–12 wt.% were obtained, and
the binding state of sulfur and nitrogen was easily tuned
(from pending groups such as thiols, or amides, to more
structurally-infused thiophenic or pyridinic units) by varying
the sulfur and nitrogen source. After pyrolysis at 900 �C, the
resulting S/N-doped carbons showed almost three times
higher specific conductivity than the corresponding undoped
sample (made from pure glucose), as well as a highly in-
creased interlayer distance of buckled carbon sheets.
The material obtained with thienyl-cysteine exhibited
much higher thermal stability with respect to sulfur loss than
the one obtained with cysteine. For TC a remarkably high S-
content of more than 7 wt.% was retained even after heat
treatment at 900 �C, showing the importance of the sulfur
binding state in the as-synthesized carbon materials. While
cysteine results in pending functional groups (aliphatic thiols,
disulfides or thioethers), TC results in the incorporation of
sulfur into the carbon framework at early stages of the reac-
tion. For TC most of the surface sulfur was aromatically
bound already after hydrothermal synthesis at 180 �C. After
pyrolysis at 550 �C, 100% of the surface S was bound in a thio-
phenic state. As the pyrolysis temperature increased the
nitrogen binding state moved from less stable, pending
groups towards more stable structural motifs. As a result,
samples treated at 900 �C contained only quaternary N within
O
OHS
O
rboxaldehyde (TCA). Substrates used as S-sources in
on [153,154]. L-Cysteine is the least effective S-doping
18 C A R B O N 6 8 ( 2 0 1 4 ) 1 – 3 2
the graphitic sheets or pyridinic N at the edges of the carbon
framework. This same approach was utilized to obtain N/S-
dual doped carbon aerogels [154]. D-glucose was used as a car-
bon source and TC, 2-thienyl-carboxaldehyde (TCA), L-cys-
teine and lyophilized albumin powder were used as carbon
and S/N precursors. By changing the sulfur source, distinct
carbon morphologies and varying doping levels of sulfur were
obtained. Two different S-containing carbon aerogels with N-
doping level of c.a. 5 wt.% and sulfur-doping level from 1 wt.%
(using TCA) to 4 wt.% (using TC) were obtained. An additional
pyrolysis step was used to further tune the carbon aerogel
conductivity and heteroatom binding state. Carbon aerogels
obtained only from albumin contained a minor amount of
sulfur (arising from the cysteine moieties of ovalbumin). As
previously observed, cysteine as the S-source did not result
in stable, structurally bonded sulfur, since the aliphatic sulfur
groups were easily lost upon heat treatment [153]. After pyro-
lysis at 900 �C, practically all surface sulfur in the carbon aero-
gels was bound in an aromatic fashion.
Ternary S/N/P-co-doped carbons have also been synthe-
sized recently [155]. Phosphorus and sulfur were addition-
ally doped into N-doped carbon obtained by the pyrolysis
of dicyandiamide on metal seeds (Co and Fe), and phospho-
ric acid and cysteine were used as P and S sources [155]. An
additional doping of P and S into N-doped carbon resulted
in nanotube- and sponge-like structures with severely wrin-
kled and curved morphologies and with many open edge
sites. The morphological modifications, which resulted from
the large atomic sizes of P and S compared to that of C (the
covalent radii are 107 pm for P and 105 pm for S, but only
73 pm for C), induced an increase in the surface area. Addi-
tional doping of P and/or S creates defect sites and hinders
the formation of sp2-bonds between carbon atoms, result-
ing in lower graphitization of the carbon structure. In par-
ticular, additional S-doping induces an amorphous
structure. The ternary doping also increased the portion of
pyridinic-N sites in the carbon materials. In the ternary
N/S/P-doped carbon, N occured in the carbon lattice as
pyridinic, pyrrolic, graphitic, and pyridinic-oxide nitrogen,
S was forming –C–S–C–/–C–S(O)x–C– groups, while P was
rarely present in the carbon lattice and predominantly
formed metal phosphides [155]. The ternary S/N/P-co-doped
porous carbons can also be obtained by carbonization and
activation of polyphosphazenes [156]. Poly(cyclotriphospha-
zene-co-4,4 0-sulfonyldiphenol) (PZS) constitutes a typical
cross-linked polymer (synthesized through polycondensa-
tion of co-monomers hexachlorocyclotriphosphazene and
4,40-sulfonyldiphenol) that forms carbon materials during
carbonization. P, N and S-containing porous carbon nano-
spheres having high surface area (1140 m2/g) were fabri-
cated by synthesizing polyphosphazene nanospheres and
carbonizing them with NaOH as the activating agent.
Tsubota et al. studied thiourea ((NH2)2CS) polymerized with
formaldehyde as a precursor for S/N-doped porous carbon
[157]. The percentages of nitrogen and sulfur in the thiourea-
derived carbons were about 5–20 wt.% and 3–8 wt.%, respec-
tively. The status of the nitrogen in the carbonaceous materials
was determined as pyridine-like nitrogen at the edges of the
graphitic structure and N-Q in the graphitic-layered structure.
Sulfur was determined to be in the S0, S4+ and S2� states.
Interestingly, Chung et al. recently observed that cyana-
mide assists the incorporation of sulfur from the iron sulfate
precursor into the carbon matrix [158]. They obtained the N/
S-dual-doped carbon by mixing cyanamide and FeSO4 with
pre-oxidized carbon (pretreated in 70% HNO3) followed by
high-temperature treatment. Cyanamide was found to sup-
press the evolution of the sulfur from the ferrous sulfate
(FeSO4Æ7H2O) to SO2 during heat-treatment, resulting in the
incorporation of sulfur into the carbon material. The sulfur
content in the material increased with the temperature,
which was contradictory to the change of N content with
the temperature.
As known, sulfur is an effective carbon growth promoter
[99–101]. In fact, it was found that thiophene increases the
yield of N-doped carbon CNx [102]. Pyrolysis of acetonitrile
over Fe/MgO in the presence of thiophene resulted in N-
and S-containing carbon nanostructures (CNx – fibers and
capsules). Sulfur was incorporated into the CNx material as
both adsorbed elemental sulfur and its oxides or in configura-
tions of S–C bonds. Two types of sulfur in the CNx-thiophene
derived carbon were observed – the one that was easily des-
orbed from the surface and sulfur that was permanently
incorporated into the carbon matrix.
An amorphous carbon doped with sulfur was prepared by
heat-treatment of a mixture of polyacrylonitrile (PAN) and
sulfuric acid [159]. The incorporated sulfur (mainly as C–S–
C) caused an increase in the size of graphite crystallites, the
interlayer distance, and developed microporosity in the final
carbon. It was observed that additional heat treatment
caused the removal of amine nitrogen and/or transformed it
into conjugated nitrogen. Sulfur existed at the surface of the
prepared carbon and also in the bulk as a dopant. Besides –
C–S–C– form, some of the sulfur occurred in the state of sul-
fate (–C–SO4–C–) or sulfonate (–C–SO3–C–).
Bandosz et al. obtained P and S-doped, highly porous car-
bons using poly(4-styrenesulfonic acid-co-maleic acid) so-
dium salt as a precursor [160,161]. The powdered polymer
was carbonized and then activated with H3PO4 (and addition-
ally with CO2) [160]. Since the activation increases the content
of surface oxygen, the doped S and P elements have been
identified as oxygen containing functionalities, i.e. S–O, O@P
and O–P groups. As known, P usually bonds within the carbon
in the forms of phosphates. Indeed, it was determined that
phosphorus groups do not exist without bonds with oxygen.
The H3PO4 treatment incorporated phosphorus mainly as
the phosphates/pyrophosphates moieties, along with small
quantities of metaphosphates and P2O5. While phosphates/
pyrophosphates can be incorporated in the carbon matrix,
P2O5 exists as particles distributed on the carbon surface. Sul-
fur was determined as R–SH groups, C–S–C/R–S2–OR in sulfides
and thioethers, R2–S@O in sulfoxides, R–SO2–R in sulfones and
R–SO3H in sulfonic acids, respectively. The CO2 activation de-
creased the amount of the labile groups such as sulfoxides
and sulfones, due to the high temperature of this treatment.
Unlike thiophenic sulfur, phosphates, pyrophosphates, meta-
phosphates and P2O5 particles are bulky species and they can-
not exist in very small pores, less than 1 nm in diameter. It was
suggested that the incorporation of S- and P-containing groups
in the carbon matrix decreases the energy band gap. This kind
of S, P-doped carbon can be considered as a pseudo-semicon-
N+
H
HH
H
S
NH2 NH2
C NS_
NH
NH2 NH2
HSCN*
NH3 CS2 H2S+ ++
432 K 443 K
Fig. 10 – Thermal behavior of NH4SCN at different heating
stages. Upon heating to 432 K NH4SCN isomerizes to thiourea
and further decomposes to NH3, H2S, CS2 and residual
guanidine thiocyanate at 443 K. [Adapted from reference 164;
copyright 2013, with permission from Elsevier.]
C A R B O N 6 8 ( 2 0 1 4 ) 1 – 3 2 19
ductor on whose surface, upon irradiation, electrons and holes
might be formed. As a result the dual doped porous carbons
exhibited significant photoactivity.
A series of S–N-co-doped ordered carbon materials were
also obtained using hard templating methods [162,163]. Nitro-
gen and sulfur co-doped ordered mesoporous carbons with
high specific surface areas and large pore volumes were ob-
tained using the pyrrole as the carbon and nitrogen precursor
and sulfuric acid as the catalyst and sulfur source [162]. It was
observed that higher temperatures favor the incorporation of
sulfur into the carbon framework. Liu et al. described three-
dimensional (3D) S–N-co-doped carbon foams with hierarchi-
cal pore structures obtained from sucrose and thiourea
(where thiourea was used as both: N and S source) [163].
The thermally unstable thiourea was responsible for the
foam-structure formation.
3.4.2. S and N dual-doped grapheneMore recently, researchers started to study the unique proper-
ties of dual N, and S doped graphene [148,149,164]. Liang et al.
proposed N–S dual-doped graphene with regular mesopores
(10–40 nm) as a high-performance metal-free ORR catalyst
[148]. Mesoporous dual-doped carbon was prepared through
a one-step doping process of graphene oxide. Colloidal silica
was used as a template to enhance porosity. Melamine and
benzyl disulfide were selected as N and S precursors respec-
Fig. 9 – Fabrication of N/S dual-doped mesoporous graphene
nanosheets from GO. GO was dispersed in water and
colloidal SiO2 (12 nm) was added to the suspension. The
resulting mixture was evaporated. The obtained solid was
crushed into a powder in a mortar with melamin and BDS.
The mixture was then heated at 900 �C to obtain N-S-doped
carbon/SiO2. The sample was finally washed with HF acid to
remove the silica. [Reprinted with permission from
reference 148; copyright � 2012 WILEY-VCH Verlag GmbH &
Co. KGaA, Weinheim.]
tively. The doping was carried out by pyrolyzing the mela-
mine/BDS/GO/SiO2 mixture (Fig. 9).
The creation of mesopores is of special importance, since
they are supposed to facilitate the diffusion of reactants in
the ORR processes in fuel cells. The design of a hierarchical
pore structure serves as an efficient strategy for improving
the activity of various ORR catalysts [163]. Also, Guan et al.
investigated S and N co-doped graphene as the catalyst for
the ORR [149]. A few-layered, doped graphene oxide was syn-
thesized by using pyrimidine and thiophene as N and S pre-
cursors, respectively. The carbon obtained was characterized
by a high surface area of 640 m2/g.
As shown above, doping of graphene with heteroatoms
usually demands high temperature treatment. However, a
low temperature hydrothermal synthesis (180 �C) of nitrogen
and sulfur co-doped three-dimensional graphene frameworks
employing GO and ammonium thiocyanate (NH4SCN) as the
precursors was presented recently [164]. Upon hydrothermal
treatment, NH4SCN undergoes decomposition into highly
reactive N/S-rich species, such as NH3, H2S, and CS2, which
react with the oxygen-containing defective sites of GO
(Fig. 10). The monolithic, doped graphene obtained just after
hydrothermal treatment was further freeze-dried to yield a
low density carbonaceous aerogel. The prepared N/S-doped
carbons show high surface areas and very large weight content
of N and S, up to 12.3 and 18.4 wt.%, respectively. Four different
sulfur moieties were determined. The majority of the sulfur
occurred in C–Sx–C (x = 1 or 2) bonds and conjugated –C@S–
bonds (thiophene-type). The other two minor states (constitut-
ing only 10%) were assigned to the oxidized (–SOx–) and
reduced (–SH) sulfur moieties, which are expected to occur at
the edges of graphene. N was determined as N-6 and N-Q.
Also Antonietti et al. utilized inorganic N- or S-containing
salts to produce highly porous N-, S- or N/S-co-doped carbons
and graphenes [165]. The process was similar to a black pow-
der reaction, where nitrate or sulfate salts were reduced from
their highest to lowest oxidation states and N or S were
hybridized into the carbon sp2 framework (Fig. 11). The eutec-
tic mixture of LiCl/KCl was used as the solvent to dilute the
mixture of nitrate, sulfate or thiosulfate salts (LiNO3, K2SO4
or Na2S2O3) and glucose. High concentration of S, of up to
30 wt.% was observed for the samples prepared from glu-
cose/Na2S2O3 mixture. With the molten salt route, the control
Fig. 11 – Schematic illustration for the formation of (a) N- and (b) S-doped porous carbon sheets through the reaction of
glucose and nitrate (NO3�) or sulfate (SO4
2�) anions in eutectic molten LiCl/KCl. By using a mixture of both salts, S/N-co-doped
carbon materials can be obtained. Na2S2O3 may also be used as the S-source. [Reprinted with permission from reference 165;
copyright � 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.]
20 C A R B O N 6 8 ( 2 0 1 4 ) 1 – 3 2
of doping level as well as the porosity can be easily achieved
through the adjustment of the glucose/reactive salt ratios and
the synthesis temperature. The highest surface area of
3250 m2/g was observed for an S-doped sample prepared in
glucose/Na2S2O3@LiCl/KCl mixture at 900 �C.
Analysis of the above presented research concerning syn-
thesis and structure of S-doped graphitic carbon enables us to
determine what kind of sulfur motifs should be the most
common in such modified carbon materials (Fig. 12). For in-
stance, sulfide in particular, but also sulfone and sulfoxide,
are often determined as prevalent sulfur motifs in the lattice
of S-doped carbon (Fig. 2, Tables 1 and 2). Depending on the
synthesis conditions some groups are favored and others
are eliminated. Sulfides are predominant owing to their ther-
mal stability and reductive conditions of carbonization. Thio-
lic, sulfonic acid, thioether and disulfide groups (polysulfides
were also detected [142]), on the other hand, are rather ther-
mally unstable. Sulfone and sulfoxide may be created at oxi-
S
S O
O
O SH
SSSH
S
S
SO3H
S SOH
S SO O
Fig. 12 – Scheme of possible configurations of sulfur
incorporated into the graphitic carbon framework (e.g.
sulfide (two different types: in-plane and out-of-plane),
sulfone, sulfoxide, sulfonic acid, thiol, disulfide and sulfide
bridges). Note that sulfur occupies edge sites.
dizing conditions, for instance during activation. However,
the most profound conclusion is that sulfur forms only a dou-
ble bond with carbon and as a result the doping sites for S are
on the edges or defects of graphene layers [25,141,143,148]. In
fact, it was recently noticed by Antonietti et al. that the com-
position of the S-doped highly porous carbon is strongly re-
lated to its specific surface area, since sulfur occupies the
periphery of graphitic layers (favorably at surface termination
sites) [150].
4. Applications of S-doped carbons
In S-doped carbons the sulfur species play a significant role in
modifying the properties, especially when the S atom is
bonded with benzene rings or shares a conjugated planar sys-
tem with delocalized p-electrons (e.g. thiophenic S) [162]. Sul-
fur-doped, porous carbons are attractive functional materials
for application in energy conversion/storage and sorption as
metal-free, electrocatalysts for fuel cells [166], anode materi-
als in Li-ion batteries [126,128,130,145], electrodes in electro-
chemical capacitors [131,136,139,152,162,165], cathode
materials in lithium–oxygen batteries [144], for H2 storage
and CO2 capture [121,122] and as adsorbents of specific con-
taminants from liquid phase [98] or toxic gases [133,134].
From the whole range of above mentioned applications, the
use of carbon as a metal-free catalyst is especially attractive
(in fact, very recently Paraknowitsch and Thomas presented
an excellent review concerning energy applications of N, B,
S and P doped carbons [167]). Precious metals and metal oxi-
des are routinely used as catalysts. To overcome numerous
drawbacks associated with metal-based catalysts (i.e. high
cost, susceptibility to gas poisoning) heteroatom-doped car-
bons could be used as cheap, stable and environmentally-
friendly catalysts [166–168]. The applications of S-doped and
co-doped carbons and the advantages of doping are collec-
tively presented in Tables S1 and S2.
C A R B O N 6 8 ( 2 0 1 4 ) 1 – 3 2 21
4.1. Electrochemical applications
As Rolison aptly stated, porous carbons are workhorse mate-
rials in the field of electrochemistry [81]; they truly are [4].
After a rush of research into the electrochemical applications
of ordered and hierarchical nanoporous carbons, research
into the heteroatom doping of porous carbons now seems to
be taking off. Successful applications of the S-doped, and S/
N and/or P-co-doped carbon materials in electrochemistry re-
quire the manipulation of the appropriate heteroatom-con-
taining groups on a suitably designed porous architecture
allowing for efficient transport of reactants and electrons.
The carbon texture and the nature of sulfur bonding in the
carbon framework are crucial factors for getting good electro-
chemical and catalytic activity.
4.1.1. Heterogeneous catalysis – oxygen reduction reaction(ORR)Fuel cells can directly convert chemical energy into electricity
with a high efficiency by oxidizing fuel (e.g. H2 or CH3–OH) at
an anode and reducing O2 at a cathode to produce H2O as
waste (and CO2 if methanol is used). So far, the proton ex-
change membrane (PEM) and direct methanol (DMFC) fuel
cells remain more expensive than the internal combustion
engine. The high cost of the Pt-catalysts and sluggish kinetics
for the ORR are the major hindrances to commercial imple-
mentation of low-temperature fuel cells. To address this prob-
lem, metal-free, carbon-based electrocatalysts for ORR are
under intense study. They are potentially cheap, show high
catalytic activity and selectivity, and good stability [166,168].
Heteroatom-doped nanostructured carbons are especially
interesting because they possess enhanced porosity and their
electronic structure can be easily tailored by the introduction
of heteroatoms into the sp2-hybridized carbon matrix.
Research concerning the origin of ORR activity in N-, B-, or
P-doped carbon materials such as N-doped carbon nanotubes
(CNTs), P-doped graphite layers and B-doped CNTs indicates
that breaking the electroneutrality of graphitic materials by
doping with elements, which have larger (N) or smaller (P, B)
electronegativity than carbon, might be an important factor
for promoting the ORR activity [169–171]. Theoretical studies
have established that the presence of dopants in the carbon
framework creates positively charged sites, which are favor-
able for the side-on O2 adsorption. This parallel diatomic
sorption could effectively weaken the O–O bonding and facil-
itate the direct reduction of oxygen to OH� (H2O in acidic
solution) via a four-electron (4e) process, hence enhancing
ORR activity [169]. The possibility that the dopant atom in
the carbon framework can serve as an active site for ORR de-
pends on both: its charge density and spin density. Further
considerations indicated that in the case of S-doping, the spin
density plays presumably more important role [143,148,171].
The number of carbon atoms with large positive spin or
charge density should contribute to the catalytic activity
[148,171].
Sulfur and carbon have close Pauling electronegativities
(2.58 and 2.55, respectively) and S-doped carbon materials
have rarely been investigated for ORR comparing to the ubiq-
uitous N-doped carbons. Yet doping with elements that have
similar electronegativity to carbon, such as sulfur or even
selenium, can also unveil enhanced catalytic activity. In fact,
preliminary results indicate that such chalcogen-doped car-
bons hold great potential for replacement of the Pt-based cat-
alysts [143,172]. The C–S bond is not as polarized as the C–N
bond (negligible charge transfer between S and C), so the cat-
alytic pathway based on a d+ adjacent carbon atom proposed
for N-doping is unlikely to occur for S-doping. Yang et al. sug-
gested that the change in atomic charge distribution for the S-
doped graphene is relatively small, not only because S and C
have similar electroneutrality, but also because the C–S bonds
predominantly form at the edge of the defect sites [143]. As a
result, spin density should be more important in determining
the catalytic active sites, compared to atomic charge density.
Indeed, through density functional theory (DFT) studies,
Liang et al. observed enhanced spin density of carbon atoms
caused by additional S-doping into N-doped graphene [148].
The catalytic behavior of S-doped carbons was mainly af-
fected by the mismatch of the outermost orbitals of S and C
atoms [148].
S-doping improves not only catalytic performance, but
also the selectivity of oxygen reduction towards the 4e–pro-
cess (i.e. direct reduction of O2 to H2O) [127,137,141–
143,164,173]. S-doped carbons are also highly selective with
respect to the fuel [141,142,174]. The nature of heteroatom
bonding within the carbon framework is a crucial factor for
getting good catalytic activity. Sulfur in the thiophenic (–C–
S–C–) configuration is presumably the active site for promot-
ing ORR [127,137,143,173]. The presence of thiophenic sulfur
improves the overall electrocatalytic activity of the doped car-
bon in both basic and acidic media. Enhancement of the ORR
efficiency could be caused by lone pairs of S, which may con-
tribute to interaction with O2. Sulfur also induces strain and
defects in the carbon matrix, which facilitates charge locali-
zation for favorable O2 chemisorption [5,127]. The S-doped
carbon catalysts are also more stable than the commercial
Pt-C catalysts [137,174]. In addition, Park et al. suggested that
S-doping of carbon (in the thiophenic fashion) may constitute
a more effective method for the catalysis of ORR than N-dop-
ing [173]. Yang et al. reported that S-doped graphene exhibits
higher electrocatalytic activity than the corresponding un-
doped graphene and the C–S bond should be an important
catalytic active site [143]. In fact, Mullen et al. proved, that
sulfur can indeed be fully doped into graphene sheets pre-
dominantly in the form of thiophene-S [141].
Soon enough it was also realized that ORR activity of het-
eroatom doped carbon-based catalysts can be further en-
hanced by co-doping with another element. It was observed
that dual doping with nitrogen and sulfur increases the car-
bon’s ORR activity in acidic or alkaline media, in comparison
to N-only doped carbons [102,148,149,151,154,163–165,175].
Moreover the N/S-co-doped carbons constitute not only a very
promising substitute for expensive Pt/C catalysts, but they
also have great potential in other electrochemical applica-
tions such as capacitors [152,157,162,165].
Liang et al. proposed N/S dual-doped mesoporous graph-
ene as a high-performance ORR electrocatalyst to replace Pt/
C [148]. It was stated that enhanced mesoporosity facilitates
the diffusion of reactants in the ORR process and synergistic
effects originating from multiple-element doping were ob-
served. The dual-doped carbon catalysts exhibited excellent
Fig. 13 – Spin and charge density of theoretical graphene
network (gray) dual-doped by N (black) and S (white). C1 has very
high spin density, C2 and C3 have high positive charge density,
and C4 and C5 have moderately high positive spin densities.
[Reprintedwithpermissionfromreference148; copyright �2012
WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.]
22 C A R B O N 6 8 ( 2 0 1 4 ) 1 – 3 2
ORR performance comparable to that of commercial Pt/C and
significantly better than that of graphene catalysts doped so-
lely with S or solely with N atoms. The DFT calculations re-
vealed that the synergistic performance enhancement
results from the redistribution of spin and charge densities
provided by the dual N/S-doping, which leads to a large num-
ber of carbon atom active sites. Since, as the authors as-
sumed, S can only form a double bond with C, the doping
site for S is on the edge of graphene clusters. In fact, very re-
cently Baek et al. showed that sulfur atoms and sulfur oxides
(O@S@O) doped at the edges of the graphene nanoplatlets
strongly promote the electrocatalytic activity and the elec-
tronic spin density, in addition to charge density, plays a
key role in the high ORR activity [174]. The charge transfer
on N-only-doped graphene is explained by the different elec-
tronegativities of N and C, so the ORR activity mainly origi-
nates from the surrounding C atoms with high positive
charge density. For the S-only-doped graphene, charge trans-
fer could originate from the mismatch of the two elements’
orbitals [148]. The most significant change in the sulfur’s elec-
tron distribution is the loss of an electron on the 3s orbital
due to its embedding into the carbon framework with sp2
hybridization. As a result, the S atom is positively charged
and hence might act as the catalytic center for ORR. When
the graphene cluster is N/S dual-doped, non-paired electrons
are subsequently introduced and the atomic charge/spin den-
sity is greatly modified [148]. The model of dual doped graph-
ene with the largest number of active carbon atoms is
presented in Fig. 13. If S and N are simultaneously incorpo-
rated into graphene, maximum spin density (0.43) is achieved.
A similar synergistic effect of sulfur and nitrogen was ob-
served in few-layered graphene, which exhibited catalytic
activity superior to that of mono-N-doped carbon [149]. The
dual-doped catalyst also showed an efficient 4e-dominant
ORR process and excellent methanol tolerance. The 3D N/S-
co-doped graphene frameworks also manifested excellent
catalytic behavior mainly with the 4e-transfer pathway when
they were applied as metal-free catalysts for ORR in an alka-
line condition [164]. For the solely N-doped samples, only two
electron transfer mechanisms were observed. In addition to
high activity, the 3D N/S-co-doped graphenes also exhibited
excellent selectivity towards ORR with a good tolerance to
crossover effects. Such synergistic effects of combined S/N-
doping were also observed by comparing solely nitrogen-
doped aerogels with nitrogen- and sulfur-doped carbon aero-
gels [154]. S is a large atom causing structural defects in the
carbon crystal lattice, which in turn results in more edge-ac-
tive sites. These sites facilitate charge localization and O2
sorption. In addition, S has large, polarizable vaccant d-orbi-
tals (sulfur groups are usually soft nucleophiles); lone pairs
of sulfur can interact with molecules in the surrounding elec-
trolyte. As proved, even a small amount of S-doping
(0.74 wt.%) is sufficient to generate profound effects on elec-
trocatalytic activity. A synergistic mechanism between nitro-
gen and sulfur dopants was proposed for the S/N-doped
carbon aerogels [154], whereby N directly or indirectly (via
the adjacent carbon atom) aids O2 dissociation and S facili-
tates proton transfer.
Synergistic effects of S and N-co-doping were also ob-
served in the transition metal–nitrogen–carbon catalysts.
Transition metal–nitrogen–carbon catalysts are considered
to be one of the best non-precious metal candidates for
substituting Pt in polymer electrolyte fuel cells. As presented
by Chung et al. [158], the combined effect of nitrogen and sul-
fur incorporation into the carbon accounts for the high activ-
ity of the non-precious metal–nitrogen–carbon oxygen
reduction catalysts. Thiophenic sulfur was found to be
responsible for the improved activity (two p-electrons that
can interact with the p band of the graphene layer and strong
affinity towards metal atoms).
Liu et al. studied the influence of enhanced porosity on the
activity of N/S-doped carbons for ORR. The hierarchical 3D S–
N-co-doped porous carbons exhibited better catalytic activity,
longer-term stability and higher methanol tolerance than a
commercial Pt/C catalyst in alkaline media [163]. Such excel-
lent performance was attributed to the synergistic effect,
which included sites of high activity provided by heteroat-
oms, excellent reactant transport caused by hierarchical
pores and high electron transfer rate provided by continuous
3D networks.
Additional S-doping into the N-doped carbons can cause
activity nearly 2.5 times higher than that of solely N-doped
carbon [155]. Yet not only binary S/N- but also ternary P/S/
N-doping results in enhanced activity for ORR [155]. The bin-
ary and ternary doping of P and/or S in N-doped carbon pro-
duces many edge sites and a high surface curvature, and
also increases the portion of pyridinic-N sites in the carbon
materials (presumably the most active N-doping phase in
ORRs). The increase in activity obtained via the additional
doping of P and S into N-doped carbons results from the en-
hanced asymmetry of the atomic spin or charge density,
which provides the O2-sorption sites. Nonetheless, in the real
systems, each dopant has a variety of different forms in the
carbon lattice [102] and for this reason the origin of the en-
hanced asymmetry of the atomic spin/charge density by the
additional P and/or S-doping remained unclear [155]. Interest-
ingly, due to the synergetic effect of S and N co-doping, doped
carbon nanotubes also show an improved catalytic perfor-
mance for ORR in both acidic and alkaline solutions [175].
Highly porous, active catalysts for the ORR (in acid media)
were also prepared via pyrolysis of cobalt-tetrame-
C A R B O N 6 8 ( 2 0 1 4 ) 1 – 3 2 23
thoxyphenylporphyrin or chloroiron-tetramethoxyphenyl-
porphyrin in the presence of iron oxalate with or without sul-
fur [176,177]. The addition of sulfur resulted in graphitic
material having extended layers and smaller graphitic do-
mains, which presented a particularly suitable matrix for
highly active catalytic centers. In the absence of sulfur, iron
carbide (Fe3C) was formed and induced graphitization, which
erased the active centers. This could be avoided by the addi-
tion of sulfur because iron monosulphide (FeS) was formed
instead of FexC [108]. The results indicated a higher electro-
chemical performance of the S-containing catalysts.
4.1.2. Anodes for Li-ion batteriesSince the commercialization of rechargeable lithium-ion bat-
teries in the early 1990s, a wide range of anode materials have
been studied. In today’s commercial Li-ion batteries, graphite
is the most commonly used anode material. Nonetheless, two
main issues, i.e. the storage capacity and the rate perfor-
mance, call for further improvement in order to achieve better
performance. Various strategies have been proposed to im-
prove the high-rate capabilities of carbon materials, e.g. design
of a hierarchical porous structure to facilitate Li+ transport. An-
other very promising method is the doping of carbons with
heteroatoms, which could enhance the electronic conductivity
as well as the lithium electroactivity [126,159]. For instance, S-
doped porous carbon hybridized with graphene has recently
been probed [145]. The enhanced electrochemical properties
of such materials were explained by the S-doping and the spe-
cific microstructure. The hierarchical porous structure facili-
tates absorption of Li+ and also serves as a reservoir for
storage of Li+ while the S-doping increases the electrochemical
activity of the porous carbon by increasing the number of
nanopores and the interlayer distance of graphene sheets.
S–N-co-doped amorphous carbon has also been used as Li-
ion battery anode [159]. It was shown that the existence of
these heteroatoms in amorphous carbon can enhance its
electrochemical properties. The doped sulfur favorably in-
creases the charge capacity in close correlation with an in-
crease in the size of graphite crystallites, the interlayer
distance, and the number of micropores. S-doping can also
increase the content of the co-dopant – N, which in turn en-
ables more electrons to be introduced into the layers of graph-
ite, i.e. more lithium to be intercalated. However, the fact that
only one form of the incorporated sulfur, i.e. sulfide –C–S–C–,
most probably contributes to an increased charge capacity
[159] is of special importance.
Recently, a nanocomposite made of high surface area Si
coated with S-doped amorphous carbon has been proposed as
an anode material for Li-ion cells [128,130]. The Si/S-doped-car-
bon composite electrodes exhibited higher initial coulombic
efficiency and better cycling performance than the bare nano-
Si anode. It is suspected that the –C–S–C– form of sulfur in
the amorphous carbon anode can increase the charge capacity
of the cells, which may have a favorable effect on improving the
electrochemical performance of the Si/C composite.
4.1.3. Cathode for lithium–oxygen batteryFor another secondary battery type, e.g. lithium–oxygen, an
increased charge performance was also noticed when an S-
doped carbonaceous material was utilized as a cathode
[144]. The discharge product (Li2O2) on the S-doped graphene
was better crystallized than that on pristine graphene, sug-
gesting that the nucleation of Li2O2 is promoted due to the
strong interaction between carbon and the intermediate
products after the sulfur doping. This means that the dis-
charge and charge properties of batteries based on S-doped
graphene are significantly different from those based on un-
doped graphene. The formation of Li2O2 nanorods during dis-
charge was due to S-doping, however the overall role of sulfur
was not clear. Nonetheless, the results obtained offer an
attractive direction for designing cathode materials via car-
bon S-doping to tailor the morphology of Li2O2, hence improv-
ing the performance of lithium–oxygen batteries.
4.1.4. Electrode for supercapacitorsBesides batteries, S-doped carbon has also been proposed as
an electrode for supercapacitors, which are getting growing
attention for complementing or replacing batteries in porta-
ble electronics and hybrid vehicles owing to their large power
density, moderate energy density, and longer cycle-life. Due
to their electrical conductivity, high specific surface area
and chemical inertness, porous carbons are the most promis-
ing electrode materials for supercapacitors. Based on the
charge storage mechanism, supercapacitors can be divided
into the electrochemical double layer capacitors (EDLCs) and
pseudo-capacitors.
EDLCs are based on electrostatic interactions, i.e. the elec-
tric charge is accumulated on an electric double-layer of the
polarized electrode. In EDLCs the energy storage depends on
the charge uptake at the electrolyte/electrode interfacial re-
gions. This is why the EDLCs require electrodes with high sur-
face area and pores adapted to the size of ions. To this end,
activated carbons are the materials of choice for EDLCs. For
better capacitive characteristics, additional larger pores
(meso- and macropores, which facilitate the diffusion of ions
into the micropores) are beneficial [131].
In pseudo-capacitors, the energy is stored through revers-
ible redox reactions of the electroactive species. Heteroatom
doping could induce additional pseudo-capacitance via a
revisable redox reaction and improve the wettability of the
electrodes, consequently improving the performance of the
supercapacitor [152]. In fact, it was shown that N-doped car-
bon electrodes exhibit a superior capacitance to those of un-
doped carbons [178]. Furthermore, Tsubota et al. [157] noticed
that the addition of sulfur (with nitrogen) to the carbon mate-
rials could positively affect the capacitance value. However,
the effect of sulfur and the effect of the co-doping of sulfur
with nitrogen were unclear.
The first demonstration of a monolithic electrode of hier-
archical, S-doped macro/meso/microporous carbons for
EDLCs was presented by Hasegawa et al. [131]. Unfortunately,
the role of sulfur was not specified. Zhao et al. also applied S-
doped mesoporous carbon monoliths as capacitor electrodes
[139]. Carbons with S doped in the form of stable aromatic
sulfides were additionally treated with H2O2 to modulate the
ratio of aromatic sulfide, sulfoxide, and sulfone groups. The
resultant S-doped mesoporous carbon exhibited superior per-
formance as supercapacitor electrodes and several effects
enhancing the performance were postulated (Fig. 14). Firstly,
higher electron density could be located at the surface be-
Fig. 14 – A proposed electrochemical performance mechanism on S-containing mesoporous carbon. [Reprinted from
reference 139; copyright 2012, with permission from Elsevier.] (A colour version of this figure can be viewed online.)
24 C A R B O N 6 8 ( 2 0 1 4 ) 1 – 3 2
cause of a synergistic activation of conjugated carbon in com-
bination with the electron-rich sulfur. Under an applied elec-
tric field, an enhanced electric dipole moment enlarged the
total polarization (P) of the medium. According to equation
(1) (Fig. 14), the electrolyte dielectric constant (er) multiples
with (P), resulting in a superior normalized capacitance.
Therefore, the increase in capacitance can be attributed to a
larger er and the charge transfer process is facilitated by fur-
ther polarization of the surface. It is the presence of aromatic
sulfide itself, rather than its concentration, that modifies the
carbon surface. Secondly, a series of red-ox faradic reactions
are carried out on the S-doped carbon at the additional sul-
fone and sulfoxide species. As illustrated in (2) and (3)
(Fig. 14), pseudo-capacitive properties are related with oxi-
dized sulfur species and the capacitance was proportional
to the S-content. In summary, the introduction of n-type dop-
ant S provides a more polarized surface as well as reversible
pseudo-sites and thus results in superior performance [139].
Bandosz et al. further confirmed the good capacitive
behavior of sulfur doped carbon (and its composites with
graphene) reaching capacitance of up to 178 F/g
[136,179,180]. The good performance was linked to the pres-
ence of sulfur (in the state of organic sulfides) in the small
pores, which brings a positive charge on carbon atoms and
thus increases the attraction to anions, and to the sulfones
and sulfoxides in larger pores, which enhance pseudocapaci-
tive performance. In addition oxygen groups increase the car-
bons’ wettability and mesopores facilitate ion transport.
While studying the polythiophene-derived activated car-
bons as electrodes for EDLCs Yushin et al. concluded that sul-
fide bridges presented in the precursor (Fig. 2) decrease the
shrinkage of the smallest micropores during the carboniza-
tion process (reduce the content of the smallest bottleneck
micropores) and allow for the enhanced ion transport within
the electrodes without inducing unwanted redox reactions
[181]. Such S-doped carbon electrodes showed capacitance
of up to 200 F/g in neutral aqueous electrolytes.
The synergistic effect of multi-S/N/O-doping on the
enhancement of pseudo-capacitance has also been demon-
strated recently, where heteroatom species play multiple
roles [152,165]. N-doping improves the conductivity of the car-
bon skeleton and the N-containing groups can induce extra
pseudo-capacitance due to fast and fully reversible faradaic
redox reactions [152]. The hydrophilic heteroatom species
promote the wettability of the carbon surface. The introduc-
tion of an electron-rich sulfur provides a more polarized sur-
face resulting in superior performance [139]. Due to the
synergistic effect of multi-doping, the capacitance could
reach a very high value of 264 F/g in KOH electrolyte. S–N-
doped mesoporous ordered carbons also exhibited enhanced
double-layer capacitance performance due to their improved
surface activity and conductivity compared with undoped
porous carbon. Despite having a much lower specific surface
area, the specific capacitance of the co-doped carbon reached
180 F/g, an improvement of 40% over that of the undoped car-
bon [162].
4.2. Adsorption
Whereas for heterogeneous catalysis or Li-ion batteries mes-
oporous materials are regarded as very promising candidates,
for gas sorption applications the microporous materials exhi-
bit the best performance. Activated carbons have been rou-
tinely applied for gas storage/capture or removal of
poisonous gases. It was recently observed that bulk hetero-
atom doping of microporous and activated carbons can sig-
nificantly modify and enhance their adsorption properties.
4.2.1. H2 storage and CO2 capturePorous media are able to store H2 by physisorption and nar-
row micropores (0.6–0.7 nm) are the most efficient for this
storage. Large surface areas and high pore volumes are also
necessary to ensure high H2 uptake. Carbon materials with
enhanced microporosity, ultra high surface area and large
pore volume fit these requirements perfectly. Activation is
routinely used to produce carbons with such characteristics,
yet the nature of carbon precursors plays a crucial role in
determining the final carbon properties. For instance, some
precursors enable the doping of heteroatoms onto the acti-
vated carbons. Sevilla et al. proposed S-doped activated car-
bons with surface area of up to 3000 m2/g for H2 storage
[121]. The H2 storage capacity of these carbons was up to
5.71 wt.% (77 K, 20 bar) with an estimated maximum hydro-
gen uptake of 6.64 wt.%. Nevertheless, it was concluded that
sulfur does not have any particular influence on the hydrogen
storage capacity.
On the other hand, Xia et al. proved that sulfur incorpo-
rated into porous carbons is beneficial for the H2 uptake
capacity [122]. They successfully utilized S-doped, ordered
microporous carbons as the H2 and CO2 stores. The S-doped
carbons exhibited isosteric heat of H2 adsorption of up to
9.2 kJ/mol and a high H2 uptake density of 0.0143 mmol/m2
(77 K, 20 bar) – one of the highest ever observed for nanopor-
ous carbons (at that moment). They also showed a high CO2
C A R B O N 6 8 ( 2 0 1 4 ) 1 – 3 2 25
adsorption energy of up to 59 kJ/mol, one of the highest
among any porous solids. It was observed that the presence
of sulfur in the form of a C–S bond in porous carbon enhances
H2 uptake capacity due to the increased polarity of the carbon
framework and therefore the interaction between H2 mole-
cules and the polar surface. This observation is supported
by theoretical work, which previously suggested that incorpo-
ration of heteroatoms into the carbon network can effectively
improve hydrogen storage ability [182]. The S-doped carbons
indeed exhibited higher specific H2 uptake per surface area
than the S-free sample. Moreover, S-doped carbons exhibited
higher CO2 uptake capacity than the S-free counterparts. The
S groups play a paramount role in the initial interaction when
CO2 is adsorbed on the carbon surface, possibly due to the for-
mation of strong acid–base interaction between the acidic
CO2 molecules and basic S–C functionals, and also because
of the strong pole–pole interactions between the large quad-
rupole moment of CO2 molecules and polar sites associated
to S-functionalities [122]. In addition, the initial isosteric heat
of CO2 physisorption increased with the S-content in the car-
bon further indicating the importance of S-functionalities for
CO2 uptake. Nonetheless, Seema et al. [183] presented S-
doped porous carbons, which (in the field of CO2 adsorption
capacity) outranked the S-doped carbons obtained by Xia
et al. [122]. The material displayed CO2 adsorption capacity
of 4.5 mmol/g (298 K, 1 atm) and good CO2 adsorption selec-
tivity over N2, CH4 and H2. The microporosity and oxidized
sulfur content were found to be the determining factors for
the CO2 capture. The CO2 adsorption did not show a correla-
tion with the S content but with the content of the di- (or
tri/tetra-oxidized) S moieties, which are likely to strongly
interact with CO2.
Zheng et al. reported that S-doped monodispersed carbon
microspheres with diameter of about 2–3 lm obtained by HTC
using elemental S and starch exhibited an enhanced H2 stor-
age capacity (3.3 wt.%, 77 K and 0.7 MPa) in comparison with
the sulfur-free carbon spheres (0.98 wt.%) [184]. High H2 stor-
age capacity was also reported for heteroatom multi-doped
(S/N/P) porous carbon nanospheres [156]. A gravimetric H2 up-
take up to 2.7 wt.% (77 K, 1 atm.) was reported.
4.2.2. Adsorption of heavy metals and toxic gasesSulfur doped carbon materials have shown beneficial proper-
ties for the selective adsorption of heavy metals, poisonous
gases, as well as for the desulfurization of crude oil. For in-
stance, Shin et al. prepared high surface area, S-enriched
mesoporous carbons, with good thermal and hydrothermal
stability and excellent mercury sorption performance over a
wide pH range of 1.0–12.8 (much broader than possible for
thiol-based functionality and silica-based supports) [98].
These S-doped carbons were found to have a higher satura-
tion binding capacity and faster sorption kinetics for mercury
adsorption from aqueous media than commercially available
sulfur impregnated activated carbons. These new class of sor-
bents can be applied under conditions where other sorbents
fail, such as high temperatures or extreme pH.
Activated carbon modified with sulfur exhibits unusually
high activity for oxidation reactions [133–135,161,185]. Ban-
dosz et al. studied polymer-based, S-enriched carbons oxi-
dized either by heating in air or by chemical treatment for
ammonia or arsine removal [133,134]. It was found that both
oxygen-containing and sulfur-containing groups enhance
NH3 adsorption. In particular, sulfonic groups play a predom-
inant role in this process (NH3 retention). In the presence of
superoxide anions, they are converted into sulfates that react
with NH3 to form ammonium sulfates. In the case of AsH3,
the hydrophilic surface of S-containing carbons, which facili-
tates adsorption of water even in dry conditions, enhances ar-
sine removal. It was found that oxygen- and S-containing
groups participate in arsine oxidation to arsenic tri- and pent-
oxide and/or in the formation of arsenic sulfides.
4.2.3. Desulfurization of diesel and crude oilBeside heavy metal sorption, S-containing porous carbons
have also been applied to fuel desulfurization. Bandosz
et al. presented extensive research devoted to this problem
[135,160,161,185,186] and highlighted the importance of dis-
tinguishing between structural (–C–S–C–) and pending (thiol/
sulfonate) sulfur in the selective adsorption capacities of S-
containing carbons. They investigated adsorption of dibenzo-
thiophenes from diesel fuel using polymer-derived, S-doped
carbons [135,160] and S-enriched carbons generated via a high
temperature H2S reduction of oxgen-containing groups
[185,186]. Both types of porous carbons displayed improved
adsorption of dibenzothiophenes. It was shown that not only
surface oxygen but also sulfur incorporated into the carbon
matrix has a positive effect on the separation of the sulfur-
containing compounds from fuel. Sulfur increases the capac-
ity and the selectivity of this kind of adsorption. S-doped acti-
vated carbons exhibited strong oxidizing capabilities and
were able to convert dibenzothiophenes to their sulfoxides
and sulfones (sulfur introduced to the carbon matrix activates
oxygen and leads to the formation of superoxygen ions)
[135,185]. Interestingly, it was noticed that thiophenic sulfur,
which can be easily incorporated into the walls of very small
pores (<1 nm), increases the adsorption capacity while selec-
tivity is positively affected by the sulfonic acid present in lar-
ger pores [160]. Incorporation of S atoms into the aromatic
rings of the carbon matrix increases the ability to attract dib-
enzothiophenes via dispersive interactions (sulfur–sulfur
bridges) [185,186]. Sulfur and sulfur–oxygen groups present
in larger pores enhance the amount of adsorbed dibenzothi-
ophenes via specific acid–base and polar interactions, since
heteroatoms increase the polarity of the carbon surface.
S and P dual-enriched carbons have also been probed for
desulfurization of diesel fuels [160,161]. The presence of phos-
phorus bonded in the forms of phosphates, pyrophosphates
and P2O5, increases the capacity and selectivity for removal of
dibenzothiophenes, since the oxygen containing phosphorus
groups increase the surface polarity. The acidic environment
also enhances the adsorption performance via acid–base inter-
actions, in which the acidity of the carbons can be due to the
sulfur, oxygen and phosphorus-containing functional groups.
4.2.4. Photoactivity of S-doped carbons – a reactiveadsorptionIn spite of the fact that activated carbons are used predomi-
nantly as adsorbents, their application in the field of photoac-
tivity is getting rapidly growing attention recently. The ability
of carbon materials to interact with UV light and to generate
26 C A R B O N 6 8 ( 2 0 1 4 ) 1 – 3 2
highly reactive species capable of promoting the photooxida-
tion reactions was recently reported [2]. Indeed, UV irradia-
tion of activated carbons can start the photooxidation of
pre-adsorbed organic compounds [187]. Photoelectrochemical
applications (i.e. solar energy conversion) of carbonaceous
materials have now become feasible. As Bandosz stated, cou-
pling the adsorption properties of activated carbons with the
solar energy utilization towards oxidation can open a new
path for reactive adsorption, separation and more generally,
for photocatalysis. She showed that S-doped activated car-
bons are better catalysts for methylene blue oxidation under
visible light than the semiconducting TiO2 [188]. Moreover, di-
rect proof of visible light photoactivity of S- and S/P-co-doped
activated carbon for oxidation of dibenzothiophenes has been
presented [161,189]. Besides the dibenzothiophenes’ oxidia-
tion to sulfoxides, sulfones and sulfonic acid, the cleavage
of the C–C bonds in aromatic rings was also detected. The
photoactivity is linked to the effect of sulfur, which besides
decreasing the energy gap, also increases the ability of carbon
to adsorb oxygen [189]. Indeed, theoretical calculations indi-
cated that the incorporation of certain heteroatoms decreases
the band gap and thus increases the catalytic reactivity of car-
bon toward activation of oxygen [190]. Another important fac-
tor is the affinity of carbons to retain water, which is the
source of active radicals formed upon absorption of photons.
When S-doped carbon is exposed to irradiation, electrons and
holes are formed. In the photoactivity mechanism, sulfur not
only decreases the band gap, but its incorporation to the car-
bon matrix also brings the positive charge to the carbon
atoms and thus creates more centers for oxygen reduction.
For this process the sulfur in thiophenic configuration was
indicated as the most significant. Electrons formed as a result
of photoactivity reduce adsorbed oxygen to active superoxy-
gen ions O2� and the holes contribute to the formation of
OH radicals (assuming that water is retained in the pores).
The inherent carbon properties can further enhance these
processes, i.e. the conductivity of the graphitic domains im-
proves the charge transfer and the black surface of carbon in-
creases the efficiency of light absorption.
All the discussed examples prove that S-doping signifi-
cantly improves porous carbon performance in a range of
applications. However, even though not discussed, it must
be remembered that oxygen is always present on the porous
carbon surface and it might strongly influence the effects of
S-doping as noted in Refs. [136,142] and [152].
5. Summary and outlook. Se-doped carbon, S-doped g-C3N4 and carbon dots
Even though still fledgling, research concerning S-doped car-
bons is worthy of further effort. New, unusual precursors
(e.g. tetrathiafulvalene [191]) can be studied; however, the bio-
mass seems to be the most promising choice for now. Also,
progress in the chemistry of ionic liquids may provide new,
efficient precursors for the porous, S-doped carbons, while
doping of graphene will certainly remain a very important
synthetic approach.
Graphitic carbon may revolutionize future technology, yet
advanced applications require rigorous tuning of the conduc-
tivity and bandgap, which in traditional silicon-based elec-
tronics is achieved by doping techniques. As already proven,
heteroatom doping of the sp2-hybridized carbon alters the
structure, chemical reactivity, surface energy, electronic and
mechanical properties. S-doping in the graphitic plane results
in the incorporation of localized states and bandgap opening
[5,25,27,189]. Doping carbon with sulfur, an element with
slightly different electronegativity, can induce bond polariza-
tion, but the size of S atoms may have an even more profound
effect. Indeed, S doped into the graphitic carbon sticks out
from the graphitic plane inducing significant lattice distor-
tions, which may cause high electroactivity of S-doped car-
bons, a phenomenon that cannot be elucidated only by the
electronegativity difference.
Carbon S-doping has kindled curiosity regarding other do-
pants that could effectively tailor spin/atomic charge density
distribution, and may also enhance the ORR catalytic activity.
The next natural choice was selenium. So far, only three pa-
pers dealing with such doping can be found [143,172,192],
and two more describing carbon Se-surface functionalization
[193,194]. As a result there is a plenty of room for more study,
and further endeavor in this direction would be welcomed. As
a next chalcogen element, Se is quite similar to S. They have
similar electronegativity (2.55 for Se and 2.58 for S); selenoph-
ene (C4H4Se) is very much like thiophene (C4H4S) – both of
them are planar and the bond lengths and angles of C–Se–C
in C4H4Se are almost equal to those of C–S–C in C4H4S [172].
Nonetheless, Se has larger atomic size and higher polarizabil-
ity. As preliminary results suggest, Se-doped carbons have
great potential as metal free catalysts for ORR [143]. This is
very interesting, since Se has the same electronegativity as
carbon. Yet Se can effectively tailor spin density or atomic
charge density distribution in graphene, hence enhancing
the ORR catalytic activity. Se, with much larger atomic size
than carbon, may cause high strains at the graphitic edges,
which may facilitate charge localization and O2 chemisorp-
tion. Se-doped graphene was synthesized utilizing diphenyl
diselenide and Se occurred as C–Se–C groups in the obtained
carbons (selenophene structures at the plane edges) [143,172].
Also, both S and Se were additionally decorated onto the N-
doped graphene–CNT self-assembly as promoters of ORR
[192]. The chalcogens were doped in the carbon lattice
predominantly as –C–S–C– and –C–Se–C– groups. Additional
Se-doping significantly improved ORR activity with a high
methanol tolerance and long-term stability in acid media
compared to Pt/C. Se-doping may induce formation of a
p-conjugation system, producing highly effective electron
transfer [172]. Se has high polarizability and lone selenium
pairs can easily interact with molecules in the surrounding
electrolyte compared with other heteroatoms. These advan-
tages, derived from the relatively high atomic number of sele-
nium, may be among the dominant reasons why the ORR
activity of the Se-doped carbon materials with a mere
1 wt.% of Se was comparable to that of N-doped carbon mate-
rials with 4–8 wt.% of N [172]. Doping of graphitic materials
with large size atoms might constitute a promising strategy
towards new carbon-based electrocatalysts and it may be
more effective than doping with N, P or even S.
Utilizing thiomalic or thioglycolic acid, S-doped fluores-
cent carbon quantum dots have been obtained for the first
C A R B O N 6 8 ( 2 0 1 4 ) 1 – 3 2 27
time recently [195,196]. N/S co-doped carbon dots with strong
fluorescent emission resulting from the synergy effect of the
dual-doping have also been presented [197]. Due to their
biocompatiblity, S and S/N-doped carbon dots have a great
potential in bioimaging, while owing to their wide band gap
they can be useful for the fabrication of solar cell devices.
Finally, aside from carbons, other graphitic-like structures
can be subjected to S-doping. Graphitic C3N4 (g-C3N4), a poly-
meric semiconductor, has received a great deal of attention
due to its peculiar electronic properties and catalytic activity
for water reduction to produce H2 [198,199]. S-doping into the
g-C3N4 causes modification of the electronic band structures
and enhances photoactivity for H2O splitting under visible
light [200]. S-doping into graphitic structures may eventually
allow adaptation of artificial photosynthesis to meet the
world’s ever-growing energy demand.
Acknowledgements
This work was supported by the National Centre for Research
and Development (Poland) through the project LIDER 527/L-4/
2012 ‘‘New mobile nanocomposites of high corrosion resistance
for removal of aromatic compounds and heavy metal ions’’.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.car-
bon.2013. 11.004.
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