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Fe-doped Graphene Nanosheet as an Adsorption Platform of Harmful Gas

Molecules (CO, CO2, SO2 and H2S), and the co-adsorption in O2 environments

Article  in  Applied Surface Science · January 2018

DOI: 10.1016/j.apsusc.2017.08.216

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1

Fe-doped Graphene Nanosheet as an

Adsorption Platform of Harmful Gas

Molecules (CO, CO2, SO2 and H2S), and

the co-adsorption in O2 environments

Diego Cortés-Arriagada1,*

, Nery Villegas-Escobar2, Daniela E. Ortega

2

1Programa Institucional de Fomento a la Investigación, Desarrollo e Innovación.

Universidad Tecnológica Metropolitana. Ignacio Valdivieso 2409, P.O. Box 8940577, San

Joaquín, Santiago, Chile. *E-mail address: dcortes@utem.cl

2Laboratorio de Química Teórica Computacional, Facultad de Química, Pontificia

Universidad Católica de Chile, Av. Vicuña Mackenna 4860, Macul, Santiago, Chile.

Abstract. The adsorption of pollutant gases (CO, CO2, SO2 and H2S) onto Fe-doped

graphene nanosheets (FeG) is studied on the basis of density functional theory calculations

at the PBE/Def2-SVP level of theory. The most stable adsorption configurations, binding

characteristics, electronic properties and stability at room temperature of the FeGGas

interactions is fully analyzed. The gas molecules are chemisorbed onto FeG with adsorption

energies in the range of 0.54 to 1.8 eV, with an enhanced adsorption strength compared to

intrinsic graphene. The stability of the FeG-Gas interactions is dominated by Lewis-acid-

base interactions, and its strength is sorted as SO2>CO>H2S>CO2. The adsorption stability

2

is also retained at room temperature (300 K). Due to the strong interaction of SO2, CO, and

H2S, FeG could catalyze or activate these gas molecules, suggesting the possibility of FeG

as a catalyst substrate. The electron acceptor/donor character of CO, CO2, SO2 and H2S

molecules when adsorbed onto FeG causes charge transfer processes that are responsible

for the change in conductance of FeG; thus, the response of the HOMO-LUMO gap of FeG

under gas adsorption could be useful for sensing applications. Furthermore, the analysis of

the co-adsorption in O2 environments shows that the CO2 interaction turns unstable onto

FeG, while the sensing response towards H2S is suppressed. Finally, these results give new

insights into the emerging applications of Fe-doped graphene in gas capture/filtration

devices, solid-state gas sensors or as a catalyst substrate.

Keywords: Fe-doped graphene; adsorption; gas pollutants; DFT calculations; co-

adsorption; gas sensors

1. Introduction

The emission of gaseous pollutants in the atmosphere, industrial and house

environments are of a great concern due to the risk that these pollutants exert[1]. Although

the natural production of most atmospheric pollutants is much higher than artificial and

industrial production, the problem of the latter is that it usually occurs in a much localized

way, so that in places close to the emission source concentrations can be very high[2].

Carbon dioxide (CO2) capture is of interest due to its environmental and economic

relevance. Likewise, carbon monoxide (CO) and hydrogen sulfide (H2S) are considered as

suffocating agents for humans, even in reduced concentrations; as well as sulfur oxides

(SOX), which has a negative contribution to the environment and human health[1, 2].

3

These gases are also recognized to contribute to the global warming through the greenhouse

effect, cause acid rain and photochemical smog, and several respiratory diseases due to its

chronic exposure[3-8]. Therefore the sensing of these gases to avoid chronic exposure is of

a great interest, in addition to the development of materials allowing its collection and

capture before the release of burning or industrial gases to the environment.

An alternative of environmental monitoring to this problem is the emerging

application of graphene as gas sensing or capture material due to its low cost, low power

consumption and high surface area[9, 10]. Graphene is highly stable, causes low

contamination and bind gas molecules by intermolecular interactions to its surface,

reaching high molecular adsorption and storage[9-13]. For instance, the adsorption abilities

of graphene towards toxic gaseous species (such as CO2, CO, NO2, NH3) have been well

described[9, 11, 14-19]. In this regard, most of the recent developments are focused on

materials enhancing the adsorption stability of adsorbates onto graphene. In this sense, the

local reactivity of graphene can be tailored via doping, which creates more reactive

adsorption sites[20-26]. Metals such as Cu, Ag, Au, Ti, Cr, Mn and Pd have been

theoretically considered as dopants in graphene to enhance its adsorption, storage capacity

and sensing properties towards CO, CO2, NO2, NO, H2S and other harmful molecules[27-

31]. Moreover, doping of graphene oxide has been also reported to form stable and

excellent sorbents for gas collection and filtration, even with low interference of O2

molecules[32]. Thus, the application of metal-doped graphene for gas collection/sensing

applications is expected to emerge as these materials will be experimentally available.

Theoretical studies based on the Density Functional Theory (DFT) framework have

shown the excellent sorption properties of Fe-embedded graphene (FeG) for a wide class of

4

air and water pollutants, significantly enhancing the adsorption with respect to intrinsic

graphene through strong Lewis-acid-base interactions[25, 26, 33-36]. Among the

advantages of iron as dopant are its low cost, low environmental toxicity compared to noble

metals, and its high acceptor character, making it an excellent candidate in terms of

improving the sensitivity of graphene at environmental levels. In addition, iron is bonded to

graphene with high binding energies (7.0 eV) and high diffusion barriers (6.8 eV)[37,

38], then forming high stable adsorbents. In this sense, synthesized FeG through aberration-

corrected transmission electron microscopy technique shows high stability at the air and

resistance to oxidants and corrosive species, where the dopants are able to disperse and

bind to defective graphene with low cluster formation[39]. Additionally, the bandgap of

graphene is opened by Fe-doping, which turns it useful for sensing applications[25]; for

example, the CO2 detection onto FeG nanoribbons has been proved[40, 41]. Furthermore,

we recently have theoretically studied the adsorption and sensing properties of FeG toward

nitrogen oxides and formaldehyde[25, 26], indicating that FeG is highly sensitive to these

gas molecules even in the presence of oxygen.

Taking into consideration that FeG emerges as a promising material for adsorption,

filtration, collection and/or sensing of harmful gas molecules, a DFT study was performed

to study the gas adsorption of harmful gas molecules (CO, CO2, SO2 and H2S) as target

gases onto FeG nanosheets, characterizing also the role of O2 interference in the adsorption

mechanism. The FeG-Gas systems were characterized from its geometrical, energetic,

electronic and binding properties. Molecular dynamics studies were performed to analyze

the FeG-Gas interaction stability at ambient conditions, and the adsorption stability was

also characterized in aerobic conditions. As a reference, the gas adsorption was also studied

5

onto pristine graphene. Through this study, FeG is suggested to enhance the gas adsorption

process of toxic gaseous pollutants with negative effects on human health and on the

environment.

2. Computational Methodology

All the calculations were performed at the DFT level in the ORCA 3.0.3

program[42]. The PBE functional[43] was implemented in combination with the all-

electron Def2-SVP basis sets[44-46]. The PBE method was selected due to its wide use in

sorption studies related to the adsorption of small gas molecules onto doped graphene[29,

33, 47-52]. Dispersion corrections for energies and gradients were obtained through the

pair-wise DFT-D3 method, including also the BeckeJohnson damping function to avoid

repulsive interatomic forces at short distances[53, 54]. The dispersion correction (Edisp) is

added to the SCF-PBE energies (ESCFPBE), and the corrected total energies are expressed as

a sum of electronic and dispersion contributions: EPBED3=ESCFPBE+Edisp. Vibrational

frequency calculations were also performed for all the molecular systems; only positive

frequencies were associated with all the vibrational modes in all the systems, indicating that

they correspond to stable energy minimums. The binding nature was analyzed through the

Natural Bond Orbital (NBO) method in the NBO 6.0 program[55]. Wavefunction analyses

were also performed in Multiwfn[56]. The stability of the adsorption configurations was

analyzed in terms of their adsorption energies (Eads):

vdWadsorbateFeGadsorbateFeGads EEEEE (1)

where, EFeG, Eadsorbate and EFeG-adsorbate are the total energies of FeG, adsorbate and FeG-

adsorbate systems, respectively; hence, the more positive values of Eads, the more stable the

6

FeG-adsorbate systems are. EvdW is the dispersion contribution to Eads, which is obtained as

EvdW=Edisp(adsorbent)+Edisp(adsorbate)Edisp(adsorbentadsorbate), where Edisp(i) are the DFT-D3

dispersion corrections of the isolated fragments and the FeG-adsorbate system. Thus,

adsorption energies are decomposed into the sum of electronic and dispersion contributions.

Basis set superposition errors were corrected with the geometrical counterpoise method

gCP[57].

Ab-initio molecular dynamic calculations were performed in order to insure for the

stability of the FeG-gas interactions at room temperature (300 K). Trajectories were

obtained with the ADMP (atom density matrix propagation) method[58-60] via the Verlet

velocity algorithm[61], which include propagation of both the nuclear centers and electron

density, thus, giving an adequate behavior of the chemical bonding under kinetic energy.

This tool has allowed to us analyze the adsorption and chemical stability of pollutants onto

graphene based materials at room conditions[25, 26, 62, 63]. ADMP calculations were

performed in the Gaussian09 program[64], using as input the optimized structures in the

ORCA program. The potential was determined "on-the-fly" at the PBE/6-31G(d) level of

theory with a time step (t) of 0.1 fs. A total time 2.0 ps was used for statistic analyzes (this

is 20000 conformations). Temperature (T) was set to 300 K and controlled by velocity

scaling.

Graphene nanosheets are modeled as a finite carbon cluster (G: C94H24), where the

dangling bonds are saturated with hydrogen atoms. FeG was built by replacing one carbon

atom in the graphene model (FeG: FeC93H24). The dopant concentration in FeG was

retained below 6% to be in agreement with synthesized graphene nanosheets as those with

Si, N and B[65-69]. Note that the selection of these surface models was based on well

7

converged adsorption energies as the surface size increases. Finally, FeG was modeled as a

closed-shell system in its ground state as obtained from previous DFT studies, where higher

spin states are at least 0.2 eV above the closed-shell ground state[37, 38, 63].

3. Results and discussion

3.1 Adsorption energies and geometry

The most stable adsorption configurations of CO, CO2, H2S and SO2 onto pristine

and Fe-doped graphene are displayed in Fig. 1. The adsorption energies and contribution of

dispersion forces are in Table 1.

Table 1. Adsorption energies (Eads) and contribution of dispersion forces (EvdW) of the G-

adsorbate and FeG-adsorbate systems. Energies are in eV.

system Eads EvdW

with pristine graphene (G)

G-CO 0.08 0.15

G-CO2 0.11 0.19

G-H2S 0.15 0.19

G-SO2 0.28 0.29

with Fe-doped graphene (FeG)

FeG-CO 1.60 0.12

FeG-CO2 0.54 0.20

FeG-H2S 1.19 0.18

FeG-SO2 1.80 0.26

Firstly, the interaction of the gas molecules onto intrinsic graphene was studied for

comparison purposes. Fig. 1b shows that all the gas molecules are adsorbed onto pristine

graphene at distances of 3.0-3.2 Å, and with low adsorption energies ranging from 0.08 to

8

0.28 eV (Table 1). The interaction in these cases is entirely due to dispersion interactions as

noted from the EvdW values, where the electronic contribution is repulsive (Eads<EvdW).

Consequently, the interaction is also accompanied by a low electron transfer of the order of

±110-2

|e|. The low adsorption energies indicate that adsorption of CO, CO2, H2S and SO2

onto intrinsic graphene is lowly stable at room temperature. At next, it is studied the

interaction of the gas molecules onto FeG. The attention was focused in the most stable

adsorption configurations (Fig. 1c). All the FeG-Gas interactions take place by chemical

bonding between the gas molecule and dopant atom, with adsorption energies ranging from

0.54 to 1.80 eV. Dispersion energies only contribute up to 0.26 eV to the adsorption

energies. With respect to intrinsic graphene, the adsorption energies increase in 1.52, 0.43,

1.04 and 1.52 eV CO, CO2, H2S and SO2, respectively, when FeG is implemented as a gas

adsorbent; thus, the adsorption strength is enhanced in at least 390% compared to graphene.

Fig. 1. Optimized molecular structures of: a) the isolated gas molecules; b) gas molecules

adsorbed onto pristine graphene (G); and (c) gas molecules adsorbed onto FeG. Distances

are in angstroms (Å) and angles in degrees (°). Color code: White (H); grey (C); red (O);

yellow (S); and orange (Fe).

9

In the case of the FeG-CO system, CO acquires an almost perpendicular adsorption

configuration onto FeG by means of C-O bonding (C-end configuration) that is similar as

obtained onto Al, Co, Cu, Ag and Au-modified graphene[20, 29, 47, 70]. This adsorption

configuration is explained because of CO has a single 2p lone pair orbital according to its

Lewis structure, which is parallel with respect to the C-O bonding. The C-end configuration

has an adsorption energy of 1.60 eV and intermolecular Fe-C bond length of dFe-C=1.88 Å.

Due to the interaction, the C-O bond distance is slightly elongated from 1.14 to 1.17 Å with

respect to the free CO molecule (Fig. 1a). Note that the adsorption configuration in the O-

end mode was found to be 1 eV less stable that the C-end configuration. The adsorption

energy of the FeG-CO system is comparable with those reported of 1.38 and 1.45 eV in the

same C-end configuration[38, 71]; note that these comparisons are on the basis of DFT

calculations with the PW91 and PBE functionals without dispersion corrections, explaining

the low differences in stability. On the other hand, the FeG-CO2 system has an low Eads

value of 0.54 eV, which is 1 eV lower compared to the CO adsorption; the latter indicates

that CO is preferably adsorbed than CO2. In this case, CO2 is absorbed by Fe-C and Fe-O

bonds of 2.16 and 2.00 Å, respectively. Additionally, the O-C-O angle is decreased from

180° to 154°, while the interacting C-O bond of the CO2 molecule is elongated from 0.07 Å

with respect to the free adsorbate.

Likewise, H2S and SO2 are also chemisorbed onto FeG. In the case of the FeG-H2S

system, H2S binds to FeG in a top configuration through Fe-S bonding since the sulfur

atom has a single lone pair in the H2S molecule. The FeG-H2S system shows an adsorption

energy of 1.19 eV, with a Fe-S bond length of dFe-S=2.31 Å; the interaction almost does not

affect the interatomic distances in the H2S molecule. This adsorption configuration is

10

similar as found by Zhang and co-workers with Eads1.02 eV from PBE calculations

without dispersion corrections[33]; they also proposed FeG as a catalytic support for the

H2S dissociation[33]. Finally, the FeG-SO2 system shows an adsorption energy of 1.80 eV,

where the SO2 molecule is chemisorbed in a parallel conformation onto the dopant atom at

Fe-O and Fe-S distances of dFe-O=1.92 and dFe-S=2.47 Å, respectively. In this case, the

interacting S-O bond is elongated in 0.12 Å with respect to the free SO2 molecule.

Fig. 2. Relaxed potential energy profile (in relative energy, Erel) as a function of the FeG-

adsorbate distances (in parenthesis) in the FeG-adsorbate systems.

Fig. 2 shows the relaxed potential energy surface as a function of the intermolecular

FeG-Gas bond distance. It is clear that the adsorption/desorption process occurs without an

energy barrier in all the cases (the only energy barrier is the adsorption energy). This

property is required for the efficient recovery of the adsorption platform after adsorption,

where thermal annealing could be easily implemented to allow the recovery of the material

without its chemical degradation due to the high dopant-graphene binding energy. For

instance, Shimoyama and Baba have reported the efficient recovery (in 84%) of

synthesized P-doped graphene after thiophene adsorption by means of thermal treatment at

11

1073 K without the thermal degradation of the adsorbent[72]; graphene based sensors have

also shown their surface reactivation at temperatures above 470 K[73].

In order to ensure the applicability of FeG for sensing or gas capture at room

temperature, the stability of the FeG-Gas interactions was studied at 300 K by means of ab-

initio molecular dynamics. The propagation of the bond distances was characterized

thought the radial pair distribution function gab(r) (Fig. 3), which determines the

distribution of distances between pairs of atoms along the overall trajectory. Fig. 3a shows

the intermolecular FeGGas distances, where the chemisorption is noted to be retained

along all the trajectory, thus neither diffusion nor desorption over/from FeG occurs. The

intermolecular distances for the FeG-CO and FeG-H2S systems are in a range dFe-

C=1.762.0 Å and dFe-S=2.122.54 Å, respectively, which are consistent with those

obtained in the ground state (1.88 Å and 2.31 Å for CO and H2S, respectively). While the

bidentated interaction of the FeG-CO2 ground state is retained under dynamic conditions,

the bidentated FeG-SO2 interaction turns into a monodentated after 1.3 fs. For FeG-CO2 the

intermolecular distances are in the range of dFe-O=1.83-2.03 and dFe-C=1.84-2.38 Å. For the

FeG-SO2 system, the range of distance was found to be of dFe-O1=1.72-2.03 Å (similar to

the 1.92 Å in the ground state); since the sulfur atom breaks the bond with the dopant after

1.3 ps, two main distributions of distances were obtained: dFe-S=2.16-2.56 Å and dFe-S=2.94-

3.20 Å. Otherwise, Fig. 3b contains the intramolecular bond distances of the adsorbed

gases, which are consistent with those obtained in the respective ground state and no

chemical transformations are observed. It is worth noting the S-H bond distances in the

FeG-H2S system are quite similar along the trajectory as expected since the hydrogen atoms

do not interact with the Fe atom. Finally, Fig. 3c depicts the intramolecular Fe-C distances

12

in FeG, which are retained in the range of dFe-S=1.70-2.00 Å, ensuring the stability of the

doped nanoadsorbent under gas adsorption at 300 K proposed adsorbant. These results

indicate that neither the absorbent sheet nor the pollutant suffer large structural deformation

that would prevent the usage of FeG surfaces for sensing or gas capture.

Fig. 3. Radial pair distribution function [gab(r)] of: (a) Intermolecular Fe-Gas distances, (b)

intramolecular distances in the gas molecules, and (c) intramolecular Fe-C distance in FeG.

20000 conformations per system were used for statistics.

13

In summary, our results indicate that FeG performs as a promising adsorbent

platform towards CO, CO2, H2S and SO2 molecules, with a best adsorption strength

compared to intrinsic graphene. In addition, the adsorption performance of FeG appears to

be good in comparison with other metal doped graphene surfaces. For comparison

purposes, Table 2 shows the adsorption energies for the interaction of gas molecules onto

another metal-doped graphene; these adsorption energies were obtained by different DFT

functionals. In this regard, the adsorption energy reached by CO and CO2 onto FeG is

considered good by comparison with those reached onto another metal-doped graphene,

which reach adsorption energies of 0.4-1.4 eV (CO) and 0.1-0.2 eV (CO2) (Eads values at

the PBE level of theory). For instance, the adsorption stability of CO onto FeG is

comparable to that reached onto CuG (1.30 eV) and AuG (1.37 eV) with the PBE

functional. In the case of H2S, its adsorption stability onto FeG is 0.2-0.5 eV higher than

onto PtG, SiG and CaG (at the PBE level of theory). However, compared to Co, Sn and Ti-

doped graphene the adsorption strength of H2S onto FeG is slightly weaker. In the case of

the SO2 adsorption, the adsorption stability is enhanced in at least 0.7 eV onto FeG with

respect to CoG and PtG. Finally, Due to the strong interaction of SO2, CO, and H2S, FeG

could catalyze or activate these gas molecules, suggesting the possibility of FeG as a

catalyst substrate.

14

Table 2. Comparison of adsorption energies (in eV) for the adsorption of CO, CO2, H2S

and SO2 onto metal doped graphene and computed at the DFT level of theory.

System CO CO2 H2S SO2

SiG 0.58(LDA)[74]

0.17(PBE)[48]

0.06(PBE)[48] 0.94(LDA)[75]

FeG 1.46(B3LYP)[34]

1.38(PW91)[38]

1.92(LDA)[75]

CoG 0.94(B3LYP)[34]

0.62(PBE)[47]

1.80(LDA)[75] 1.07(PBE)[47]

CaG 0.66(LDA)[75]

SnG 1.43(PBE)[49] 1.78(PBE)[49]

TiG 0.45(PBE)[50] 2.35(PBE)[49]

2.49(PBE-D)[51]

3.20(PBE)[50]

CuG 1.30(PBE)[29]

1.71(PWC)[76]

0.22(M06-L)[77]

AgG 1.01(PBE)[29]

AuG 1.37(PBE)[29]

PtG 1.29(B3LYP)[34] 0.09(PBE)[52] 1.02(PBE)[33] 0.85(B3LYP)[78]

1.06(PBE)[52]

PdG 1.05(LDA)[31]

0.92(B3LYP)[34]

RuG 1.22(B3LYP)[34]

NiG 1.02(B3LYP)[34]

3.2 Bonding nature

The NBO analysis of the donor-acceptor interactions was implemented to

characterize the nature of the FeG-Gas bonding (Fig. 4). It is important noting FeG can

behave as a Lewis acid or base, according to the different adsorbates. In this regard, Fe

atom ([Ar]4s23d

6) develops a sd

2 hybridization to bind with the carbon atoms in graphene

with one monovacancy[25, 63]. After FeG is formed, Fe atom retains two high occupied 3d

orbitals able to interact with the acceptor orbitals of the adsorbates (behaving as a Lewis

base); and, there are three *Fe-C and one *Fe-C antibonding orbitals, which are able to

interact with donor orbitals in the adsorbates (behaving as a Lewis acid)[25, 63].

15

Fig. 4. Natural bond orbitals (NBOs) associated with the donor-acceptor (bonding-

antibonding) interactions in the different adsorption configurations of the FeG-Gas

systems; isosurface values of 0.15 a.u. Arrows indicate the donoracceptor direction.

Color code: White (H), grey (C); red (O); yellow (S), orange (Fe).

We found that the FeG-Gas interaction is mainly dominated by a Lewis-acid-base

mechanism. First of all, FeG behaves as a Lewis acid in the FeG-CO system, where the

high occupied lone pair of the carbon atom of CO (with electron occupation occ1.6e) acts

as donor for all the antibonding *Fe-C and *Fe-C orbitals in FeG (Fig. 4). In this case,

the *Fe-C orbital is highly occupied (occ0.6e) compared to the unoccupied *Fe-C

orbitals, causing a steric repulsion that limits the interaction strength. With respect to the

FeG-CO2 system, it is necessary pointing-out that the CO2 molecule has a linear molecular

geometry (O-C-O=180°) in its isolated state due to the two C-O bonds according to its

more stable Lewis structure. When CO2 interacts with FeG, the O-C-O angle decreases to

154°, resulting in a destabilization of the CO2 structure; in consequence, C-O bonds break,

16

and the carbon atom results with two lone vacant 2p orbitals with single occupancies

(occ0.9e). As noted in Fig. 4, the 3dz2 orbital of Fe (occ=1.7e) acts as donor for one

carbon lone vacant 2p orbital in CO2, which must result in a lower stability compared to the

FeG-CO system. Additionally, in a back-bonding process, one oxygen lone pair in CO2

(occ=1.6e) acts as a donor for the *Fe-C orbital in FeG (occ=0.6e), increasing the binding

instability due to steric repulsive interactions, and decreasing the adsorption strength with

respect to the FeG-CO system. The latter also increases the C-O bond in 0.07 Å.

Finally, we analyze the sulfur containing gases and their interactions with FeG. In

the case of the free H2S molecule, sulfur atom has a non-bonding 2p lone pair (occ1.7e);

when H2S is chemisorbed, the 2p lone pair delocalizes toward one *Fe-C bond of FeG,

but the interaction strength is decreased when the same sulfur lone pair acts as a donor for

the *Fe-C bond in the adsorbent. Otherwise, like in the CO2 case, the FeG-SO2 interaction

breaks the S-O bonds in the SO2 molecule, which results in a single occupied 2pz orbital

for the S atom. As noted in the Fig. 4, the sulfur 2pz orbital mixes with the iron 3dz2 lone

pair orbital in FeG, which is doubly occupied as noted above. The latter results in a strong

Fe-S bond (occ=1.5e), but also in a relatively high occupied *Fe-S bond (occ=0.8e).

Consequently, the oxygen 2p lone pair orbitals of SO2 act as donors for the *Fe-S orbital,

establishing a three-center bond.

3.3 Electronic properties

To get more insights into the FeG-Gas interactions, relevant electronic properties

were analyzed such as the charge transfer, electron density difference, eigenvalues of

frontier orbitals, and density of states (DOS). In the first place, the charge in the adsorbed

17

molecule (qAD) is used as a measure of the charge transfer; this is a positive value of

indicates electron transfer in the FeGdirection. In this regard, CO2 and SO2 act as

acceptor molecules, gaining electrons and forming 0.02 and 0.12 holes per molecule in

FeG, respectively. Conversely, CO and H2S lose electrons, introducing 0.13 and 0.26

electrons per molecule in FeG. In this regard, Song and co-workers have experimentally

reported that chemisorbed H2S transfers electrons to synthesized SnO2/graphene based

sensors, resulting in an improved sensing response by decreasing the electrical

resistance[79]. Zhang and coworkers have also reported the donor character of H2S

towards FeG through PBE calculations[33, 80]. Therefore, the charge transfer is expected

to induce the larger changes in the FeG conductivity as a result of the charge doping[27].

Note that in most of the cases the amount of charge transfer is underestimated by

theoretical calculations because of this value depends on the number of atoms in the

graphene layer[17, 81], where graphene models containing more than 1000 carbon atoms

appear to be reliable to obtain accurate values of electron transfer[17, 81]. The analysis of

charge transfer is in agreement with the plot the electron density difference (r) (Fig. 4).

In the case of FeG-CO2 (Fig. 5b) and FeG-CO2 systems (Fig. 5d), electron density

accumulates in the 2p orbitals of oxygen and sulfur atoms of the adsorbates. Conversely, in

the case of FeG-CO (Fig. 5a) and FeG-H2S systems (Fig. 5c), the transferred electron

density is mainly retained in the chemical bond without significant polarization, and the 2p

lone pairs of the adsorbate suffer an electron depletion; while, the accumulation of electron

density is noted in the carbon atoms surrounding the dopant in FeG. In all the cases, the

dopant atom suffers outflow of electron density under molecular adsorption; the analysis of

atomic charges shows that Fe atom loses 0.3|e| under gas adsorption.

18

Table 3. Mulliken charge of the adsorbate after adsorption (in |e|); eigenvalues of the

HOMO and LUMO levels (HOMO and LUMO), and the HOMO-LUMO energy gap (HL) of

the FeG-adsorbate systems compared to their isolated fragments. Energies are in eV.

system qAD HOMO LUMO HL

FeG -4.27 -3.73 0.55

CO -8.79 -1.88 6.91

CO2 -8.77 -0.23 8.54

H2S -6.16 -0.43 5.73

SO2 -7.58 -4.40 3.18

FeG-CO 0.13 -4.49 -3.40 1.09

FeG-CO2 -0.02 -4.47 -3.49 0.99

FeG-H2S 0.26 -4.25 -3.24 1.02

FeG-SO2 -0.12 -4.52 -3.95 0.57

Fig. 5. Electron density difference [(r)] obtained by adsorption of CO (a), CO2 (b), H2S

(c) and SO2 (d) onto FeG. (r)=AB(r)-A(r)-B(r), where AB(r) is the electron density of

the AB system (adsorbate-adsorbent), and A(r) and B(r) are the electron density of each

fragment. Outflow and accumulation of electron density are displayed with sky-blue and

yellow colors, respectively. Isosurface value of 0.003 e/Bohr3.

19

For a better understanding of the electronic properties of the FeG-Gas, the frontier

molecular orbitals (HOMO and LUMO) were analyzed (Table 3). Due to the weak and

long-range interaction of the gas molecules onto intrinsic graphene, a negligible effect onto

the conductance properties of graphene is expected under gas adsorption. However, FeG

behaves as a semiconductor or a semimetallic material with respect to graphene[82], and its

strong chemical interaction with gas molecules could produce additional changes in its

electronic structure as noted from the charge transfer analysis. Indeed, the HOMO-LUMO

energy gap values (HL) of the FeG-CO, FeG-CO2 and FeG-H2S are increased in at least

0.54 eV with respect to the free adsorbent as observed in Table 3. Because of the HOMO

level of the gas molecules is far away from the HOMO level of FeG (-4.27 eV), the

increase in the HL parameter is mainly governed by destabilization of the LUMO level in

up to 0.5 eV with respect to the free adsorbent. In this regard, the partial DOS plots of

these systems (Fig. 6) clearly show that the frontier energy levels are mainly affected by

hybridization of the unoccupied orbitals of the gas molecules in the conduction band of

FeG. Taking into account that the bandgap value is directly proportional to the conductance

(HL/kT, where k is the Boltzmann constant and T the temperature), these results suggest

that FeG is a promising sensor material for gas sensing because of its conductance

decreases under gas adsorption. On the other hand, the FeG-SO2 system shows a slight

increase in the HL value with respect to the isolated FeG, which is only of 0.02 eV. The

latter is due to that the unoccupied 2* orbital of SO2 (at -4.40 eV) is lower in energy than

the HOMO of FeG (at -4.27 eV); then, the LUMO of the FeG-SO2 system is mainly a 2*

orbital with a low hybridization with the occupied 3d states coming from FeG. Therefore,

these results indicate that the electronic structure of FeG is slightly sensitive to the

20

adsorption of SO2, and it could be considered as an inefficient material for SO2 sensing.

However, it is important to note that molecular dynamic trajectories showed that the

adsorption configuration in the ground state of the FeG-SO2 system is changed at room

temperature; thus, its three-center bond is displaced towards a single bond interaction. In

this adsorption conformation, we observed that the LUMO of SO2 is extra stabilized and

hybridizes with the HOMO orbital of FeG due to 3d sates; the latter causes a decrease in

the HL value of 0.3 eV. This is in agreement with the SO2 adsorption onto Ti and Al-

doped graphene, where the SO2 mixes with the 3d sates of the metal dopant near to the

Fermi level of modified graphene[50, 83]; as well as the bandgap of Pt-doped graphene is

also decreased under SO2 adsorption[78]. Consequently, the adsorption of SO2 onto FeG

could be recognized by an increase in the conductance of FeG at room temperature.

21

Fig. 6. Partial density of states (DOS) plots of the FeG-Gas systems. Blue line corresponds

to the states of Fe in FeG; red line corresponds to the states of gas molecule. The vertical

green line indicates the position of the HOMO level.

3.3. Adsorption in O2 environments

Although gas pollutants can be present in oxygen-free environments as those in

industrial waste gases, the influence of O2 molecules during the capture of gas molecules

onto FeG could take into account when an aerobic environment is considered. In this

regard, O2 reaches an adsorption energy of 1.6 eV when adsorbed onto FeG[25, 38]; our

22

computations show a value of 1.68 eV, which is near or high that the adsorption of CO,

CO2, H2S and SO2. Therefore, could have a probability of poisoning of the adsorption

platform in oxygen environments, avoiding the sensing and adsorption properties of FeG.

However, it necessary noting that resistive graphene sensors have remained with high

selectivity and sensitivity for gas sensing even in a background of N2 and O2 gases[84].

Considering that O2 is a zero dipole moment molecule (without charge polarization), it is

reasonable to expect adsorption of polarized molecules such as CO, H2S and SO2 (dipole

moments of 0.32, 1.35 and 1.70 Debye) will be favored in a first step by charge-controlled

interactions. Even, CO2 breaks its linear geometry to an angular conformation when

approaches to FeG, showing a dipole moment of 0.61 eV. These statements support that

polarized molecules are preferably adsorbed in a first step rather than O2, even when

differences in its adsorption energies emerge. Despite the latter, experimental studies of

graphene sensors for formaldehyde detection (and based on ZnO and In2O3 substrates)

show that adsorption and detection take place in O2 environments by co-adsorption

mechanisms[85-88]. This is the sensing properties remain by detection of different

intermediate compounds that are formed in a co-adsorption regime[85-88]. To get further

insights into this last point, the co-adsorption of gas molecules onto FeG in the presence of

one O2 molecule (in its triplet state) as an approach was explored; in other words, the most

stable adsorption configurations for the gas-O2-FeG systems were computed, where the gas

molecule and O2 are adsorbed on the same dopant site. The co-adsorption configurations

are displayed in Fig. 7, and their properties are in Table 4.

23

Fig. 7. Co-adsorption configurations of CO, CO2, SO2 and H2S onto FeG in the presence of

O2. Distances are in angstroms (Å) and angles in degrees (°). Color code: White (H); grey

(C); red (O); yellow (S); orange (Fe).

Table 4. Properties of the co-adsorption configurations: Adsorption energies (Eads), charge

of the adsorbate (qAD) and O2 molecule (qO2), and HOMO-LUMO energy gap in and -

channels (HL).

system Eads Eads-af qAD qO2 HL- HL-

FeG-O2-CO 2.11 0.41 0.20 -0.18 0.85 0.42

FeG-O2-CO2 1.34 -0.35 0.09 -0.21 0.93 0.39

FeG-O2-H2S 1.87 0.18 0.18 -0.31 0.83 0.63

FeG-O2-SO2 1.96 0.27 -0.15 -0.14 0.80 0.47

FeG-O2 1.69 - - -0.34 0.87 0.66

Assuming the interaction with one O2 molecule, Fig. 7 shows that the gas molecules

bind to FeG in presence of O2 in a similar way as obtained in the free oxygen states. For

comparison purposes, we obtained that O2 is adsorbed in a parallel configuration (side-on)

onto isolated FeG, without dissociation and adsorption energy of 1.69 eV, which is in

agreement with previous reports[38]. Besides, hybridization between the occupied 3d

orbitals of Fe with the 2* orbitals of O2 causes the charge transfer in the FeGO2

24

direction (qO2=-0.34|e|), which elongates the OO bond in 0.14 Å due to the occupation of

its antibonding 2* orbitals. The strong interaction is characterized by a strong coupling of

the -orbitals of FeG and O2 near to the HOMO and LUMO levels of the FeGO2 system

as noted in the partial DOS plot (Fig. 8). The latter because the unoccupied -orbitals of O2

are available as acceptors, this is the single occupied 2* orbitals.

In a co-adsorption regime, O2 is mainly adsorbed in an O-end configuration

(binding through a single Fe-O bond), excepting for H2S. CO and SO2 are co-adsorbed with

similar bond lengths as in the free O2 case; while, the intermolecular bond length is

increased in 0.4 Å for CO2 and H2S in the presence of O2, suggesting the decrease in the

adsorption strength. Indeed, the adsorption strength of the gas adsorption is sorted as

CO>SO2>H2S>CO2 as determined from the adsorption energies in Table 4, which range

from 2.11 to 1.34 eV. Otherwise, it was considered the adsorption energy of gas molecules

onto FeG but after O2 uptake, where the adsorption energy (Eads-af) is obtained as:

Eads-af=EadsEads(FeG-O2) (2)

where Eads is the adsorption energy of the whole FeGO2Gas system, and Eads(FeG-O2) is the

adsorption energy of O2 onto FeG. In this framework, the gas adsorption after the O2 uptake

is favorable only for CO, H2S and SO2 with Eads-af values of 0.41, 0.18 and 0.27 eV.

Conversely, the CO2 co-adsorption is an unstable process after the O2 binding in the

adsorption site. The latter is explained because of some of preparation energy is required

for the change between side-on and end-on configurations of the adsorbed O2. Therefore,

the CO2 adsorption is expected to be unstable in an oxygen environment. Additionally, note

that co-adsorption of CO2 is almost 0.8 eV less stable than co-adsorption of CO, while the

25

co-adsorption of H2S and SO2 is almost similar. These results suggest that FeG is a

promising candidate for CO oxidation in the presence of CO/O2 mixtures.

With respect to the electronic properties, O2 acts as an acceptor molecule onto FeG,

introducing 0.34 holes/molecule into the adsorbent. The latter causes the decrease in the

conductance of FeG, which is characterized by an increase of the HOMO-LUMO energy

gap; the HL index reaches values of 0.87 and 0.66 eV in the alpha and beta channels,

respectively (keep in mind that O2 causes the spin polarization of the system). Under co-

adsorption of CO, CO2 and SO2, the qO2 parameter (Table 4) indicates that O2 remains as an

acceptor molecule, but the amount of introduced holes in FeG is decreased, this is the

amount of transferred electrons to the 2* orbitals of O2 is decreased. The latter must result

in an increased conductance with respect to the FeG-O2 system as a result of the decrease in

the strong hybridization between the FeG and O2 states near to HOMO and LUMO level in

the -channel. Indeed, the HL values of the beta channels decrease in up to 0.27 eV with

respect to the FeG-O2 system as a result of the stabilization of the LUMO level. From the

DOS plots (Fig. 8), it is observed that the co-adsorbed gas molecules change the way as O2

hybridizes with FeG in the -channel of the conduction band due to the change from the

end-on to the side-on configuration. In this way, the amount of transferred electrons to the

2* orbitals of O2 is decreased and the LUMO level is stabilized. Conversely, the amount

of transferred electrons to O2 is not decreased by the adsorption of H2S, mainly due to the

low acidic character of H2S compared to CO, CO2 and SO2. Additionally, the larger Fe-S

bond distance in the co-adsorption configuration (2.68 Å) does not favor the complete

change from the end-on to the side-on configuration of O2. The latter results in a low

response of the bandgap of and -channels. Indeed, the partial DOS plot of the FeG-O2-

26

H2S system shows that H2S almost does not change the electronic structure compared to the

FeGO2 system, neither - nor -channels. Therefore, the sensing response remains high

for CO, CO2 and SO2, but the weak interaction of CO2 with FeG in O2 environments could

be difficult to reach an chemisorption state; in addition, the sensing response of FeG

towards H2S is suppressed in O2 environments.

Fig. 8. Partial density of states (DOS) plots of the FeG-Gas systems. Blue line corresponds

to the states of Fe in FeG; red line corresponds to the states of the adsorbed gas molecule;

orange line corresponds to the states of O2. The vertical dotted line indicates the position of

the HOMO level.

27

4. Conclusions

The implementation of Fe-modified graphene nanosheet as a nanoadsorbent towards

harmful gas pollutants (CO, CO2, SO2 and H2S) was characterized through a detailed DFT

study. It was found that these gas pollutants are chemisorbed onto FeG with adsorption

energies in the range of 0.54 to 1.80 eV, improving the adsorption strength in at least 390%

compared to those onto intrinsic graphene (on the range of 0.08 to 0.28 eV). Analyses of

the chemical binding indicated that the stability of the FeG-Gas interactions is dominated

by Lewis-acid-base interactions; the chemisorption remain strong at room temperature (300

K) as determined from molecular dynamics trajectories. The acceptor/donor character of

CO, CO2, SO2 and H2S molecules when adsorbed onto FeG causes charge transfer

processes that are responsible for the change in conductance of FeG; thus, the response of

the HOMO-LUMO gap of the FeG system under gas adsorption is expected to be useful for

sensing applications. On the other hand, it was also explored the effect of O2 molecules (co-

adsorption) on the adsorption process of CO, CO2, SO2 and H2S. In these cases, it was

found that the CO2 adsorption turns unstable in the presence of O2; while, the response of

the electronic properties of FeG towards H2S is suppressed in the presence of O2.

Therefore, these results give new insights into the emerging new applications of Fe-doped

graphene in gas capture/filtration devices or solid-state gas sensors.

Acknowledgments

Powered@NLHPC: This research was partially supported by the supercomputing

infrastructure of the NLHPC (ECM-02). N.V-E and D.E.O acknowledge the Ph.D.

fellowship from CONICYT.

28

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