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Carbon quantum dots embedded with mesoporous hematite nanospheres asefficient visible light-active photocatalysts†
Byong Yong Yu and Seung-Yeop Kwak*
Received 31st December 2011, Accepted 13th February 2012
DOI: 10.1039/c2jm16931b
Carbon quantum dots (CQDs) and mesoporous hematite (a-Fe2O3) complex photocatalysts were
successfully prepared using a facile solvent-thermal process in an aqueous solution. Mesostructured a-
Fe2O3 clusters with a high surface area and a porous framework were an important consideration in the
design of the photocatalysts because such structures enhance the absorption of photons and promote
the decomposition of organic pollutants. More significantly, the CQDs in this catalyst play a pivotal
role in improving the photocatalytic activity under visible light irradiation. The nanocomposites were
characterized using X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, X-ray
diffraction, energy-dispersive spectroscopy, field-emission scanning electron microscopy, transmission
electron microscopy, dynamic light scattering methods, ultraviolet-visible spectroscopy, and other
techniques. The results confirmed the formation of CQD/mesoporous a-Fe2O3 hybrid clusters with
a uniform size (about 700 nm), three-dimensional spherical morphologies, a large internal surface area
(up to 187 m2 g�1), and a wormhole-like mesopore structure. Moreover, these novel composite catalysts
displayed a continuous absorption band in the visible region. Photocatalytic studies of the CQD-
embedded mesoporous a-Fe2O3 showed excellent photocatalytic efficiency (up to 97% capacity
retention after three cycles) toward the degradation of organic compounds in aqueous media under
visible light irradiation. The relationship between the physicochemical properties and the
photocatalytic performance in our system is described and discussed on the basis of the results.
Introduction
Environmental contaminants in water or air pose a serious threat
to public health and safety, and they have attracted considerable
attention from a range of research groups. The use of photo-
sensitive semiconductor-based materials as a green technology
Department of Materials Science and Engineering, Seoul NationalUniversity, 599 Gwanak-ro, Gwanak-gu, Seoul, 151-744, Korea. E-mail:[email protected]; Fax: +82-2-880-1748; Tel: +82-2-880-8365
† Electronic supplementary information (ESI) available: Table S1:FWHM values of the main diffraction peaks and the crystallite size formesoporous hematite with the respective diffraction planes; Fig. S1:chemical structure of CQD; Fig. S2: CQDs optical image in waterilluminated under (a) white (left; CQDs in water, right; water) and (b)UV light (left; CQDs in water, right; water); Fig. S3: photograph ofmesoporous magnetite and hematite powders; Fig. S4: FT-IR spectrumof (a) MH, (b) CQD, and (C) CQD/MH; Fig. S5: FE-SEM images of(a) MH and (b) CQD/MH (a more detailed view); Fig. S6: TEMimages of CQD/MH hybrid clusters; Fig. S7: the intensityauto-correlation function (ACF), G2(s) for CQD/MH sample in DLS;Fig. S8: UV-visible spectra of the MH and CQD/MH; Fig. S9:absorption spectra of MB solution taken at different photocatalyticdegradation times using (a) MH, (b) MH + H2O2 and (c) CQD/MH;Fig. S10: decolorization profiles of MB aqueous solution with visiblelight irradiation in the presence of the CQD/MH + H2O2; andFig. S11: schematic illustration of possible catalytic mechanism forCQD/MH under visible light. See DOI: 10.1039/c2jm16931b
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shows promise for the development of environmentally benign
and economically feasible catalysts through their efficiency and
broad applicability.1–3 Among the common semiconductors,
titanium dioxide (TiO2) displays outstanding properties as
a photosensitive catalyst, such as long-term chemical stability,
nontoxicity, strong oxidizing activity, and good photostability,
that make TiO2 ideal for the treatment of organic pollutants in
water and contaminated air.4,5 These properties are attributed to
the large band gap of 3.2 eV for the anatase form and the
frequent recombination of photogenerated electron–hole pairs.
To date, TiO2 is a benchmark material for photocatalytic reac-
tions and has been intensively investigated. Unfortunately, the
wide band gap restricts the excitation of TiO2 semiconductors to
high-energy ultraviolet (UV) radiation of wavelengths less than
388 nm. UV radiation accounts for less than 5% of the entire
solar spectrum, and 45% of the solar spectral energy is contained
in the visible light.6,7 To improve the efficient utilization of solar
light, visible light-responsive novel photocatalysts with narrow
band gaps must be developed in which the absorption wave-
length range is extended into the visible region. Such efforts are
an active research area for environmental remediation
applications.3
Hematite (a-Fe2O3), the most stable form of iron oxide under
ambient conditions, is an attractive material with potential
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applications in catalysis, magnetic storage, chemical sensors,
water treatment, drug delivery, and lithium ion electrode mate-
rials, due to its good stability, environmental friendliness, high
theoretical capacity, and resistance to corrosion.8–12 Hematite
has a relatively narrow band gap of 2.2 eV and can be activated
by illumination with visible region in a solar spectrum. It has,
therefore, attracted interest for use in solar cells and photo-
catalysts, particularly for its visible light response and superb
photochemical stability.13 With the development of nanotech-
nology, substantial attention has been focused on architecture
and composition control toward particular applications. Meso-
structured and mesoporous materials are useful in applications
that rely on surface or interface properties, such as a high
surface-to-volume ratio, particular morphologies (including
shape and multi-dimensionality), well-defined topologies, and
pore diameters appropriate for particular applications (2–5 nm
in size).14,15 Efforts have focused on the development of tech-
niques for producing mesoporous carbon, silica and titania-
based materials to achieve superior catalytic performance and
utilization efficiencies.16–18 A previous report described the
preparation of well-defined highly crystalline spherical meso-
porous TiO2 with a large internal surface area and continuous
pore channels using a simple sol–gel approach in aqueous solu-
tions. The photocatalytic activity of these particles was found to
be better than that of nonporous commercial TiO2.19 In this
background and given the scientific value of these materials, the
fabrication of mesoporous hematite (MH) for use as a novel
photocatalyst has been intensively pursued. Mesostructured
hematite should be even more effective in the contexts of pho-
tocatalysis and adsorption because of its large specific surface
area and porous framework.
The photocatalytic properties of hematite under visible light
irradiation have been demonstrated in the context of the
photodecomposition of toxic pollutants, water splitting, and
semiconductor electrode application.20,21 Despite these reports,
hematite itself does not show a high catalytic activity compared
to TiO2 because of its low charge carrier mobility and the fast
recombination or back-reaction of the generated electron–hole
pairs.22,23 Recently, complexes of metal oxides and nanospecies
as supporting materials for photocatalyst systems have been
tested in an attempt to improve the photocatalytic activity of
metal oxides.24–26 Carbon nanoparticles present a new class of
carbon-related materials that are nontoxic, biocompatible, and
eco-friendly relative to fluorescent semiconductor nanocrystals
(quantum dots).27 For example, Lee et al. described the design of
TiO2 or SiO2/carbon quantum dot (CQD) complex photo-
catalysts that harness the full solar spectrum.28 Jaroniec et al.
recently described the fabrication of carbon self-doped TiO2
sheets for enhanced visible light photocatalytic activity.29 Wang
and Fu have developed magnetically separable zinc ferrite
(ZnFe2O4)–graphene composite photocatalysts that display high
photocatalytic performances under visible light.30 Liu and co-
workers reported the facile preparation of hematite/CQD
nanocomposites with a quasi-cube morphology and a single
crystal structure around 600 nm in size. They investigated the
effective photocatalytic activity toward the degradation of gas
phase benzene and methanol under visible light irradiation.31
More recently, Wang et al. prepared monoclinic bismuth vana-
date (BiVO4) with a band gap of 2.4 eV and a graphene coupling
8346 | J. Mater. Chem., 2012, 22, 8345–8353
photocatalyst via a one-step hydrothermal method. They evalu-
ated the visible light-driven photocatalytic performance of these
particles.32 To the best of our knowledge, the photocatalytic use
of mesostructured hematite and carbonaceous material hybrids
toward organic molecular degradation under visible light has not
been previously reported.
Herein, we describe several studies of new, efficient and stable
visible light-responsive photocatalysts, in which highly dispersed
CQDs are incorporated within the framework of MH particles.
The MH offers a robust template for the incorporation of
quantum-confined carbon sensitizers. The nanocomposites
exhibited open morphologies with a large internal surface area
and a mesoporous structure. The structures are unusual and are
potentially useful for enhancing the photocatalytic activity
toward organic pollutants and toxic water pollutants under
visible light irradiation. The chemical composition of the
composites was examined by characterizing the crystal structure,
morphology, and other properties of the prepared photocatalysts
using a variety of techniques, including X-ray photoelectron
spectroscopy, X-ray diffraction, energy-dispersive spectroscopy,
field-emission scanning electron microscopy, transmission elec-
tron microscopy, dynamic light scattering method, UV-visible
spectroscopy, and nitrogen adsorption–desorption studies. The
photocatalytic activity of these catalysts was investigated by
measuring the degradation of methylene blue (MB) as a test
substance without and with low concentrations of hydrogen
peroxide. A reasonable model mechanism underlying the cata-
lytic degradation at these hybrid clusters is proposed.
Experimental
Materials
Chemically pure iron(III) chloride hexahydrate (FeCl3$6H2O),
anhydrous sodium acetate (CH3COONa), ethylene glycol
(99.8%), and L-ascorbic acid were purchased from Sigma
Aldrich, and were used without further purification. The water
used throughout this work in all syntheses and tests was distilled
and deionized.
Preparation of CQD-embedded MH photocatalysts
CQDs were synthesized according to the method reported by Liu
and co-workers.33 A typical procedure was carried out as follows.
First, 1.1 g of L-ascorbic acid as a carbon source were dissolved in
deionized water (25 mL), and anhydrous ethanol (25 mL) was
added and stirred vigorously for at least 2 h to give a transparent
solution. Next, 25 mL of the suspension was transferred to
a Teflon-lined stainless-steel autoclave and maintained at 180 �C.After a reaction time of 4 h, the autoclave was cooled to room
temperature. The dark brown product, which settled at the
bottom of the autoclave, was extracted with dichloromethane,
and the water-phase solution was dialyzed to remove impurities.
Finally, a yellow aqueous solution containing CQDs was
obtained. In a typical preparation, the mesoporous magnetite
(Fe3O4) nanospheres were prepared via a solvothermal self-
assembly process in ethylene glycol. Two grams of anhydrous
sodium acetate were dissolved in ethylene glycol (30 mL), and the
mixture was stirred vigorously at room temperature to give
a transparent solution. Subsequently, 1.0 g of iron(III) chloride
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hexahydrate was added slowly to the solution. This mixture was
then vigorously stirred at 50 �C for least 2 h to form a homoge-
neous solution. The solution was transferred to a Teflon-lined
stainless-steel autoclave, which was then sealed and maintained
at 198 �C for 6 h. The solvothermal reaction produced a dark
black precipitate, which was collected by magnetic separation
and washed with deionized water and ethanol to effectively
remove excess organic matter. The as-synthesized particles were
mesoporous magnetite spheres that were oxidized to MH (a-
Fe2O3) by heat treatment at 600 �C for 2 h.34 Subsequently, MH
(0.1 g) was added to 5 mL of CQD solution with stirring for 10
min and dried in a vacuum oven at 80 �C for 12 h to give the
CQD/MH hybrid clusters.
Fig. 1 FT-IR spectra of CQD.
Characterization
Fourier-transform infrared (FT-IR) spectra of the samples
palletized with KBr powder were collected over the range 4000–
400 cm�1 with a spectral resolution of 4 cm�1 using a Thermo
Scientific Nicolet iS10 IR spectrophotometer. Power wide-angle
X-ray diffraction (WXRD) patterns of the nanoparticles in the
MH clusters were collected at room temperature using a MAC/
Sci MXP 18XHF-22SRA diffractometer equipped with graphite
monochromatized Cu Ka radiation (l ¼ 1.541 �A, 50 kV, and
100 mA) as the X-ray source. Data acquisition was performed
over the 2q angular range from 20� to 80� with a scanning speed
of 5� min�1. The average diameter of the crystals was calculated
according to the Debye–Scherrer formula,35 D ¼ Kl/bcos q,
where D is the crystallite size, K (¼ 0.89) is a constant related to
the shape of the crystal, l is the wavelength of the radiation
employed, b is the peak width (full width at half maximum,
FWHM) in radians, and q is the Bragg diffraction angle. The
FWHM values of the main diffraction peaks were obtained by
means of the X’Pert HighScore Plus software package. XPS was
performed using a KRATOS AXIS photoelectron spectrometer
at a background pressure of about 1 � 10�9 Torr using Mg Ka
X-rays as the excitation source (1253.6 eV). All binding energies
were calibrated by assuming that the binding energy of the C 1s
peak was 284.6 eV. The morphology of the material was exam-
ined using a Carl Zeiss SUPRA 55VP FE-SEM under an applied
voltage of 3.0 kV. The FE-SEM was equipped with an EDS
detector for elemental analysis. Standard and high-resolution
transmission electron microscopy (TEM and HR-TEM, respec-
tively) was performed using a JEOL JEM-2000EXII instrument
operated at an accelerated voltage of 200 kV. The average
particle size according to the size distribution of the CQD/MH
composite clusters was measured using DLS methods with
a Photal DLS-7000 spectrophotometer equipped with a Photal
GC-1000 digital autocorrelator (Otsuka Electronics Co., Ltd.).
In this procedure, the wavelength (l) of the argon laser was 488
nm, and the scattering angle was 90� with respect to the incident
beam. All samples were finely ground and dispersed ultrasoni-
cally in deionized water. The intensity-average and number-
average particle size distributions were analyzed using the
conventional CONTIN algorithm to estimate the diameter of the
particles. These experiments were carried out at room tempera-
ture, and each experiment was repeated at least twice. Nitrogen
(N2) adsorption–desorption isotherms were collected at 77 K on
a Micromeritics ASAP 2000 apparatus. All samples were
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degassed overnight at 100 �C for 24 h under vacuum prior to
collecting the measurements. The specific surface area was esti-
mated by applying the Brunauer–Emmett–Teller (BET) method
to the adsorption data, and the pore-size distribution was
determined using the Barrett–Joyner–Halenda (BJH) method
applied to the nitrogen desorption branch of the isotherm. The
specific surface area and average pore size of the sample were
calculated using the Micromeritics density functional theory
(DFT) plus software package. UV-visible spectroscopy was
performed with a Lambda 25 instrument manufactured by Per-
kin Elmer.
Photocatalytic performance under visible light irradiation
The photocatalytic activity of CQD/MH was evaluated accord-
ing to the decomposition of methylene blue (MB) (Sigma
Aldrich) as a model contaminant under visible light irradiation.
Experiments were conducted at ambient temperature as follows.
A 0.05 g CQD/MH sample as the photocatalyst was added to
100 mL of anMB solution of concentration 20 mg L�1 in a quartz
vessel. Prior to illuminating the sample, the reaction mixture was
stirred for 30 min in the dark to obtain a good dispersion and
permit equilibration of the adsorption–desorption processes
between the catalyst surface and MB. After adding 1.0 mL of
a 34.5% hydrogen peroxide (H2O2) solution as an oxidant to the
above reaction mixture, the lamp was turned on. A commercial
400 W halogen spotlight (the spectrum below 400 nm was
removed using a cutoff filter) was used for visible light illumi-
nation. The light source was positioned 15 cm from the reaction
solution. The absorption of the MB aqueous solution was
monitored spectrophotometrically at lmax ¼ 664 nm during the
photodegradation process. It should be noted that the reported
data are the average values of three separate runs. In addition to
investigating the recyclability of CQD/MH, after each catalytic
run, the sample was washed and dried to permit subsequent
photoreaction cycles.
Results and discussion
The FT-IR spectra were used to determine the functional groups
present on the surfaces of the CQD. As shown in Fig. 1, a broad
J. Mater. Chem., 2012, 22, 8345–8353 | 8347
Fig. 3 X-Ray diffraction patterns of (a) CQD/MH and (b) mesoporous
magnetite, and the JCPDS patterns for standard hematite and magnetite.
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absorption band was observed over the range 3000–3600 cm�1,
associated with the stretching modes of the hydroxy (–OH)
group. Other absorption bands were observed due to sp2 and sp3
C–H stretching modes at 3072 cm�1 and 2985 cm�1, respectively,
and an alkene (C]C) stretching mode at 1634 cm�1. Many
intense and sharp bands were observed in the range 1024–1153
cm�1, which corresponded to asymmetric and symmetric C–O–C
stretching vibrations and secondary C–OH stretching modes.
These results indicate that the surfaces of the CQDs were
partially oxidized, and the surface hydrophilic groups could
stabilize the CQDs in aqueous media. Structural models for the
carbogenic dots have been proposed,36,37 and are illustrated in
Fig. S1 (see ESI†). The present CQDs were freely dispersed in
water, yielding a transparent solution without the need for
further ultrasonic dispersion. Fig. S2† shows an optical image of
the CQDs illuminated under white light and UV light (365 nm),
respectively. The bright blue photoluminescence of the CQDs
was strong enough to be easily seen with the naked eye (see
Fig. S2(b) in the ESI†). Fig. 2 shows a TEM image of the CQDs,
revealing that the tiny CQDs were spherical and monodisperse
with a diameter of about 3 nm. Fig. 2(b) and (c) show HR-TEM
micrographs and fast Fourier transform (FFT) diffractograms of
selected areas in the CQD sample. The CQDs were characterized
as having a single interplanar distance of about 0.32 nm, which
was consistent with the lattice spacing of the (0 0 2) planes of
graphitic carbon.38
WXRD studies were used to determine the phase purity,
crystal structure, and average diameter of the primary nano-
crystals (subunits) in the mesoporous magnetite and hematite
clusters. Fig. 3 shows the WXRD patterns for mesoporous
hematite (Fig. 3(a)) and magnetite (Fig. 3(b)) powders, as well as
the Joint Committee on Power Diffraction Standards (JCPDS)
patterns for standard hematite (JCPDS #33-0664, a ¼ 5.035 �A)
and magnetite (JCPDS #19-0629, a ¼ 8.396 �A).39 After thermal
treatment at 600 �C for 2 h, the as-prepared magnetite (Fe3O4)
sample was completely transformed into hematite (a-Fe2O3) with
a dark reddish brown color, as can be seen in Fig. S3 in the ESI†.
The mesoporous hematite clusters produced several relatively
strong and well-resolved reflection peaks in the 2q region of 20–
80�, and the positions and relative intensities of all peaks agreed
well with those derived from the standard JCPDS patterns (as
Fig. 2 (a) TEM image of CQDs, (b) HR-TEM image of CQDs (scale
bar: 2 nm), and (c) corresponding fast Fourier transform (FFT) spot
diagram.
8348 | J. Mater. Chem., 2012, 22, 8345–8353
shown in Fig. 3(a)). These results supported that the resultant
material had a pure-phase hematite crystal structure. No peaks
corresponding to any other phases were observed. For example,
magnetite and maghemite (g-Fe2O3), have been detected. The
diffraction peaks corresponding to the (0 1 2), (1 0 4), (1 1 0), (1 1
3), (0 2 4), (1 1 6), (1 1 2), (2 1 4), (3 0 0), (1 0 10), and (2 2 0) planes
provided clear evidence for a pure rhombohedral structure (space
group: R3c, a and b ¼ 5.035 �A, c ¼ 13.748 �A).40 The prominent
diffraction peaks in the WXRD patterns of the MH powder
revealed their high crystallinity. The WXRD results were used to
determine the crystallite domain sizes of the hematite nano-
crystals in the mesoporous clusters based on the FWHM values
of the (0 1 2), (1 0 4), (1 1 0), (1 1 3), (0 2 4), (1 1 6), (2 1 4), and (3 0
0) diffraction peaks. The Debye–Scherrer formalism was applied,
giving an average crystal size of 23.39 � 0.58 nm (see Table S1 in
the ESI†). These results agreed within experimental error with
the TEM findings.
XPS was used to study the components and surface properties
of the CQD/MH sample. Fig. 4 shows several regions of the XPS
spectra of the CQD/MH sample. Wide survey scans (Fig. 4(a))
identified the presence of iron (Fe 2p), carbon (C 1s), and oxygen
(O 1s). The Fe 2p spectrum in Fig. 4(b) was fitted to two peaks at
711.4 eV and 724.5 eV (with a spin energy separation of 13.1 eV
due to the spin–orbit coupling) corresponding to the 2p3/2 and
2p1/2 spin–orbital components, respectively. These peaks
confirmed the presence of hematite.41 XPS analysis of the C 1s
spectrum (Fig. 4(c)) revealed the surface functional groups on the
CQDs, among which a main peak at 284.3 eV was attributed to
the C–C bond with sp2 orbital. The deconvoluted XPS peaks of
the C 1s were centered at the binding energies 285.7 eV, 286.9 eV,
288.4 eV, and 289.8 eV and were assigned to the sp3 hybridized
carbons, C–O–C, C]O, and C–OH, respectively. The
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Fig. 4 Wide (a) and narrow scan (b, c and d) XPS patterns for CQD/
MH.
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high-resolution O 1s spectrum is shown in Fig. 4(d). A broad
peak between 528 eV and 535 eV could be deconvoluted into four
peaks. An Fe–O signal appeared at 530.3 eV, whereas the other
peaks were oxygens in the functional groups of partially oxidized
CQDs, corresponding to C]O at 531.8 eV, HO–C]O at 532.9
eV, and C–O–C at 534.3 eV.42 Fig. S4† shows the FT-IR spectra
of MH, CQD, and the CQD/MH composite. In the CQD/MH
sample (Fig. S4(c) in the ESI†), the specific adsorption peaks of
the CQD particles were observed, as shown in Fig. 1, and the
strong absorption peaks at 578 cm�1 and 480 cm�1 were attrib-
uted to Fe–O vibrations, in agreement with those observed for
the hematite particles. These results indicated that the CQD
particles were successfully incorporated into the MH clusters.
FE-SEM was used to examine the surface structures and
morphology of the mesoporous hematite (MH) and CQD/MH
Fig. 5 FE-SEM images of particles in (a and b) MH and (c and d) CQD/
MH.
This journal is ª The Royal Society of Chemistry 2012
hybrid clusters. As shown in Fig. 5(a) and (b), uniform mono-
disperse magnetite spheres were obtained by a solvothermal self-
assembly process. The average diameter of the hematite spheres
was around 700 nm. Fig. 5(c) and (d) show FE-SEM images of
CQD/MH nanocomposites. The spherical hybrid clusters were
apparently well-dispersed and were 700 nm in size. Detailed FE-
SEM images at high magnification (as shown in Fig. S5(a) in the
ESI†) showed that the MH clusters formed through the packing
of numerous adjacent primary subunits about 20 nm in size,
which produced the mesoporous structure. In contrast, the CQD/
MH sample displayed a spherical morphology with a relatively
smooth surface (see Fig. S5(b) in the ESI†). The chemical
composition of the CQD/MH was determined using EDS
measurements. As shown in Fig. 6, EDS elemental mapping
clearly reveals that the elements Fe, O, and C were evenly
distributed throughout the CQD/MH composites. It is also
obvious that a heterogeneous junction between CQD and MH
has been derived by the present means owing to the intimate
contact between CQD and MH. The EDS spectrum of CQD/
MH, as shown in Fig. 7, indicated the presence of Fe, O, and C as
the major chemical components with atomic ratios of 35.16%,
53.29%, and 10.83%, respectively, over the entire region of the
prepared sample. The EDS results from different regions were
consistent within experimental error, confirming the composi-
tional uniformity of the resulting materials. TEM and HR-TEM
were used to further confirm the particle size and formation of
the CQD/MH nanocomposite structures. Representative TEM
micrographs of the CQD/MH clusters at low and high magnifi-
cation were present, as shown in Fig. 8(a) and (b), respectively.
As can be seen in the images, each CQD/MH cluster was
composed of a large number of primary nanoparticles with sizes
less than 20 nm. This result was consistent with that calculated
based on WXRD results, and included the disordered, worm-
hole-like framework of a mesostructured cluster. The mesopores
were formed by the adjacent crystallized primary particles in the
interior of the hybrid clusters. As clearly observed in Fig. 8(b),
a large number of CQDs was highly distributed over the pore
walls of the MH. The tiny dark particles marked with yellow
circles in this image were considered to be CQDs. This confirms
Fig. 6 EDS element mapping data of (a) all displayed elements, (b) Fe,
(c) O and (d) C elements throughout the CQD/MH.
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Fig. 7 EDS spectra of CQD/MH.
Fig. 8 Representative (a) TEM images of CQD/MH composite and (b)
HR-TEM image of the boxed region in image (a).
Fig. 9 DLS histograms showing CQD/MH particle-size distribution.
Fig. 10 Nitrogen adsorption (-)–desorption (:) isotherms for (a) MH
and (b) CQD/MH composites, and the corresponding pore-size distri-
bution curve (inset).
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that CQDs about 3 nm in size were embodied within the meso-
porous network of the hematite. Intimate contact between the
CQDs and the hematite subunits can lead to more efficient
electron transfer between the two components and can improve
the charge separation and visible light photocatalytic activity.
Fig. S6† shows a TEM image of CQD/MH nanocomposites in
which highly dispersed particles 700 nm in diameter may be
observed.
These results were consistent with the FE-SEM image results.
In addition, DLS analysis supported these TEM results. Fig. 9
shows the measured average size and size distribution of CQD/
MH particles. The correlation functions were analyzed using the
constrained regularization method to determine the distribution
decay rates (see Fig. S7 in the ESI†). The CQD/MH sample could
be dispersed in water simply by shaking, and the CQD/MH
aqueous dispersion was stable for more than 1 h without
precipitation. The stability of the solution further adds to the
advantages of the prepared materials for applications in water
purification and catalysis. The DLS results show that the size
distribution of the CQD/MH hybrid clusters derived from
aqueous dispersions was relatively narrow, with a mean size of
763 � 121 nm, in agreement with the FE-SEM and TEM
observations.
The large specific surface areas generated by mesopores
promoted the chemical activity.43 The BET specific surface area,
pore-size distribution, and pore volume for the pure MH and
CQD/MH hybrid clusters were estimated from their respective
nitrogen adsorption–desorption isotherms to demonstrate their
8350 | J. Mater. Chem., 2012, 22, 8345–8353
potential utility as catalytic materials, as shown in Fig. 10. The
physisorption measurements were essentially type-IV isotherms
with H2-type hysteresis loops associated with the capillary and
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Fig. 11 (a) Change in the absorption of theMB solutions in the presence
of the CQD/MH + H2O2 and (b) comparison of photocatalytic activity
for MB with different catalysts: (-) CQD/MH without light, (O) MH,
(:) MH + H2O2, (B) CQD/MH, and (C) CQD/MH + H2O2.
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wormhole-like pores. Such hysteresis typically indicates the
presence of mesopores, according to IUPAC.44 A large hysteresis
loop between the adsorption and desorption isotherms over a P/
P0 range of 0.56–0.89, which is characteristic of highly porous
materials, confirmed the formation of mesopores in the clusters.
The BET specific surface areas of the pure MH and CQD/MH
were as high as 203 m2 g�1 and 187 m2 g�1, respectively, and the
total pore volumes were 0.45 cm3 g�1 and 0.39 cm3 g�1. The large
BET surface area and porosity strongly indicated that the CQD/
MH nanocomposites were mesoporous in structure. The pore
size distributions were estimated using the BJH method, as
shown in the insets of Fig. 10. The relatively narrow pore-size
distribution and the average pore sizes of the MH and CQD/MH
were found to be 16.6 nm and 12.5 nm, respectively. The BET
results also showed that the mesoporous channels remained
open. The open mesostructured hematite created a large internal
surface area that offered a high number of active adsorption sites
and photocatalytic reaction centers. This material may provide
an enhanced photocatalytic activity.45,46 The interconnected
wormhole-like porous structures facilitated access to the surface
for the adsorption of the organic pollutants. The photocatalyst
could thereby decompose any adsorbed substances more readily
and effectively. Fig. S8† shows a comparison of the UV-visible
absorption spectra of the MH and CQD/MH samples.
Compared with MH, the CQD-embedded MH sample revealed
a higher intensity overall visible light absorbance spectrum, and
the absorption edge at 570 nm was red-shifted. These features
confirmed that the carbon nanoparticles in the CQD/MH
nanocomposite were successfully introduced into the MH
framework. These results suggested that the CQD/MH hybrid
clusters would provide effective photoabsorption for visible
light-driven applications.
With the aim of investigating the visible light photocatalytic
performance of the CQD/MH nanocomposites toward the
degradation of organic pollutants, we selected methylene blue
(MB) as a model contaminant for photocatalytic decolorization.
Prior to visible light irradiation, the reaction solution was
magnetically stirred in the dark for 30 min to equilibrate the
adsorption and desorption processes. Fig. 11(a) shows the
changes in the absorption spectra of an MB aqueous solution
exposed to visible light over time in the presence of the CQD/MH
and hydrogen peroxide (H2O2) sample. The MB spectrum
revealed a major absorption band at 664 nm. Under visible light
illumination, the absorption peaks dropped rapidly; times, for
the MH, MH + H2O2 and CQD/MH solutions, are shown in
Fig. S9†. Fig. 11(b) compares the photocatalytic activities of the
MH, MH + H2O2, CQD/MH and CQD/MH + H2O2 samples.
The change disappeared after 60 min of irradiation, indicating
that the chromophoric structure of MB was destroyed. The
absorption intensities declined dramatically, indicating that the
MB underwent photocatalytic decomposition. As can be seen in
Fig. S10†, the color of the dispersion nearly spectra of MB
solutions at different photocatalytic degradations in the
absorption spectra of the MB aqueous solution indicates the
change in the MB concentration. The degradation efficiency,
de%, was calculated using the following equation: de%¼ (C0–Cf)/
C0 � 100, where C0 and Cf represent the initial and final MB
concentrations, respectively. Under dark conditions (without
visible light), no appreciable degradation of the MB solution was
This journal is ª The Royal Society of Chemistry 2012
observed after 90 min in the presence of the CQD/MH
composites. After visible light irradiation for 90 min, the
degradation efficiency of MB was found to be only 56.2% when
assisted with MH + H2O2. In contrast, the CQD/MH + H2O2
coupling system led to a remarkable increase in the degradation
efficiency of 97.3%. Pure mesoporous hematite is not an efficient
photocatalyst; therefore, CQDs were shown to play a crucial role
in enhancing the photocatalytic activity of the CQD/MH nano-
composites. The excellent capacity of the CQD/MH was attrib-
uted to the mesoporous architecture with small pores (12.5 nm)
and a large surface area (up to 187 m2 g�1), good dispersity, fast
electron transfer, and good scattering properties. The narrow
mesopores increased the number of collisions between the fluid
and the pore walls, thereby reducing the mean free path. This was
supported by the fact that in the present system, the MB aqueous
solution diffuses rapidly into MH.47,48 The large surface area and
porous structure not only provide more abundant active sites for
degrading the MB molecules, but also effectively promote the
efficient separation of electron–hole (e�–h+) pairs. Fast electron
transfer between CQDs and MH can increase the quantum effi-
ciency. The above results are summarized in Fig. S11†, which
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Fig. 12 The photocatalytic degradation of the MB aqueous solutions
using CQD/MH + H2O2 for three cycles.
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shows a proposed degradation mechanism by which the organic
pollutants decompose on the CQD/MH surface under visible
light illumination. The electron–hole pairs were generated on the
hematite (a-Fe2O3) surface upon visible light illumination, fol-
lowed by instant transfer of the photogenerated electrons to the
conducting network of the CQDs via a percolation mecha-
nism.24,49 The CQDs then activated the adsorbed oxygen (O2) to
produce superoxide radical anions (_O2�). The holes then reacted
with water to form active hydroxy radicals (_OH) that subse-
quently degraded the organic pollutants.30 Visible light irradia-
tion over 90 min in air resulted in a photodegradation efficiency
toward MB of 87.7% in the presence of CQD/MH. This effi-
ciency was much lower than that observed (97.3%) in the pres-
ence of the low-concentration hydrogen peroxide oxidant. In this
case, the significantly higher photoactivity was ascribed to the
remarkable dual function as a visible light-responsive electro-
chemical degrader of MB and the generator of strong oxidant
hydroxyl radicals via the decomposition of hydrogen peroxide
under visible light irradiation.23 The dual functionality may
explain why the photodegradation efficiency of MB reached 72%
within 5 min. The efficient visible light-activated photocatalyst
was also very stable, which is important from a practical
perspective. The stability of the CQD/MH clusters was further
investigated by performing recycling experiments using the
CQD/MH samples. As shown in Fig. 12, the performance of
a recycled CQD/MH + H2O2 sample was measured under
identical conditions over three recycle tests. The photocatalytic
activity of the CQD/MH remained nearly unchanged (first cycle:
97.2%, second cycle: 97.0%, and third cycle: 96.6%), which
indicated that the prepared CQD/MH hybrid clusters displayed
a high stability and an efficient photoactivity during degradation
of the organic pollutants under visible light irradiation.
Conclusions
This study describes the facile and straightforward preparation
of CQD/MH hybrid clusters. The photocatalytic performance of
material under visible light illumination was characterized. A
series of experiments unambiguously confirmed that the CQD/
MH composites were highly monodisperse and spherical in
morphology, had a large specific surface area of 187 m2 g�1, and
8352 | J. Mater. Chem., 2012, 22, 8345–8353
a three-dimensional wormhole-like pore structure. The novel
composite catalysts also displayed a continuous absorption band
in the visible region. We proposed a reasonable model by which
photocatalysis proceeded under visible light irradiation, based on
an analysis of the results. It should be pointed out that the CQD/
MH coupling system exhibited a high photocatalytic activity,
excellent recyclability and durability properties, and a good
efficiency (up to 97%). The specific efficiency of the materials was
attributed to the combined effects of the mesoporous structure of
the hematite and the strong conjugated network structure of the
CQDs. The CQD/MH photocatalyst described here provides
a new approach to the design of high-performance photo-
catalysts as green materials and has tremendous potential for
practical use in the removal of organic pollutants and toxic water
pollutants.
Acknowledgements
This research was supported by Basic Science Research Program
through the National Research Foundation of Korea (NRF)
funded by the Ministry of Education, Science and Technology
(R11-2005-065).
Notes and references
1 M. R. Hoffmann, S. T. Martin, W. Y. Choi and D. W. Bahnemann,Chem. Rev., 1995, 95, 69.
2 X. B. Chen and S. S. Mao, Chem. Rev., 2007, 7, 2891.3 H. Zhang, G. Chen and D. W. Bahnemann, J. Mater. Chem., 2009,19, 5089.
4 Z. H. Zhang, Y. Yuan, G. Y. Shi, Y. J. Fang, L. H. Liang, H. C. Dingand L. T. Jin, Environ. Sci. Technol., 2007, 41, 6259.
5 S. Kohtani, M. Tomohiro, K. Tokumura and R. Nakagaki, Appl.Catal., B, 2005, 58, 265.
6 R. Asahi, T. Morikawa, T. Ohwaki and Y. Taga, Science, 2001, 293,269.
7 C. H. An, S. Peng and Y. G. Sun, Adv. Mater., 2010, 22, 2570.8 Z. Wu, K. Yu, S. Zhang and Y. J. Xie, J. Phys. Chem. C, 2008, 112,11307.
9 O. Shekhah, W. Ranke, A. Schule, G. Kolios and R. Schlogl, Angew.Chem., Int. Ed., 2003, 42, 5760.
10 P. Li, D. E. Miser, S. Rabiei, R. T. Yadav and M. R. Hajaligol, Appl.Catal., B, 2003, 43, 151.
11 R. C. Wu, J. H. Qu and Y. S. Chen, Water Res., 2005, 39, 630.12 J. Chen, L. Xu, W. Li and X. Gou, Adv. Mater., 2005, 17, 582.13 Z. Zhang, M. F. Hossain and T. Takahashi, Mater. Lett., 2010, 64,
435.14 Y. Ren, A. R. Armstrong, F. Jiao and P. G. Bruce, J. Am. Chem. Soc.,
2010, 132, 996.15 IUPAC Manual of Symbols and Terminology, Appendix 2, Part 1,
Colloid and Surface Chemistry, Pure Appl. Chem., 1972, 31, 578.16 J. Liu, S. Z. Qiao, Q. H. Hu and G. Q. Lu, Small, 2011, 7, 425.17 S. Araki, H. Doi, Y. Sano, S. Tanaka and Y. Miyake, J. Colloid
Interface Sci., 2009, 339, 382.18 G. Li, E. T. Kang, K. G. Neoh and X. Yang, Langmuir, 2009, 25,
4361.19 D. S. Kim, S. J. Han and S.-Y. Kwak, J. Colloid Interface Sci., 2007,
316, 85.20 M. A. Gondal, A. Hameed, Z. H. Yamani and A. Suwaiyan, Appl.
Catal., A, 2004, 268, 159.21 J. Bandara, U. Klehm and J. Kiwi, Appl. Catal., B, 2007, 76, 73.22 J. K. Leland and A. J. Bard, J. Phys. Chem., 1987, 91, 5076.23 O. Akhavan and R. Azimirad, Appl. Catal., A, 2009, 369, 77.24 Y. Yao, G. Li, S. Ciston, R. M. Lueptow and K. A. Gray, Environ.
Sci. Technol., 2008, 42, 4952.25 A. Nakajima, T. Kobayashi, T. Isobe and S.Matsushita,Mater. Lett.,
2011, 65, 3051.26 Z. Zhang, M. F. Hossain, T. Miyazaki and T. Takahashi, Environ.
Sci. Technol., 2010, 44, 4741.
This journal is ª The Royal Society of Chemistry 2012
Publ
ishe
d on
16
Mar
ch 2
012.
Dow
nloa
ded
by S
eoul
Nat
iona
l Uni
vers
ity o
n 13
/08/
2013
04:
17:1
0.
View Article Online
27 Z. H. Kang, E. B. Wang, B. E. Mao, Z. M. Su, L. Gao, S. Y. Lian andL. Xu, J. Am. Chem. Soc., 2005, 127, 6534.
28 H. Li, X. He, Z. Kang, H. Huang, Y. Liu, J. Liu, S. Lian,C. H. A. Tsang, X. Yang and S. –T. Lee, Angew. Chem., Int. Ed.,2010, 49, 4430.
29 J. Yu, G. Dai, Q. Xiang and M. Jaroniec, J. Mater. Chem., 2011, 21,1049.
30 Y. Fu and X. Wang, Ind. Eng. Chem. Res., 2011, 50, 7210.31 H. Zhang, H. Ming, S. Lian, H. Huang, H. Li, L. Zhang, Y. Liu,
Z. Kang and S.-T. Lee, Dalton Trans., 2011, 40, 10822.32 Y. Fu, X. Sun and X. Wang, Mater. Chem. Phys., 2011, 131, 325.33 B. Zhang, C.-Y. Liu and Y. Liu, Eur. J. Inorg. Chem., 2010, 4411.34 H.-J. Kim, K.-I. Choi, A. Pan, I.-D. Kim, H.-R. Kim, K.-M. Kim,
C. W. Na, G. Cao and J.-H. Lee, J. Mater. Chem., 2011, 21,6549.
35 B. D. Cullity, Elements of X-Ray Diffraction, Addison-Wesley,London, 2nd edn, 1978, pp. 81–99.
36 N. Lu, D. Yin, Z. Li and J. Yang, J. Phys. Chem. C, 2011, 115, 11991.37 D. W. Lee, V. L. De Los Santos, J. W. Seo, L. L. Felix,
D. A. Bustamante, J. M. Cole and C. H. W. Barnes, J. Phys. Chem.B, 2010, 114, 5723.
38 Z. H. Kang, E. B. Wang, B. D. Mao, Z. M. Su, L. Chen and L. Xu,Nanotechnology, 2005, 16, 1192.
This journal is ª The Royal Society of Chemistry 2012
39 Joint Committee on Powder Diffraction Standards (JCPDS) PowderDiffraction File (PDF), International Centre for Diffraction Data,Newton Square, PA, 2004.
40 I. Tamiolakis, I. N. Lykakis, A. P. Katsoulidis, M. Stratakis andG. S. Armatas, Chem. Mater., 2011, 23, 4204.
41 B. T. Hang, I. Watanabe, T. Doi, S. Okada and J.-I. Yamaki, J. PowerSources, 2006, 161, 1281.
42 J. Shao, J. Zhang, J. Jiang, G. Zhou and M. Qu, Electrochim. Acta,2011, 56, 7005.
43 B. Y. Yu and S.-Y. Kwak, J. Mater. Chem., 2010, 20, 8320.44 S. Lowell, J. E. Shields, M. A. Thomas and M. Thommes,
Characterization of Porous Solids and Powders; Surface Area, PoreSize and Density, Kluwer Academic Publishers, Boston, 4th edn,2004, pp. 101–126.
45 D. S. Kim and S.-Y. Kwak, Appl. Catal., A, 2007, 323, 110.46 Z. Guo, C. Shao, M. Zhang, J. Mu, Z. Zhang, P. Zhang, B. Chen and
Y. Liu, J. Mater. Chem., 2011, 21, 12083.47 G.-S. Li, D.-Q. Zhang and J. Yu, Environ. Sci. Technol., 2009, 43,
7079.48 C. Yu, X. Dong, L. Guo, J. Li, F. Qin, L. Zhang, J. Shi and D. Yan, J.
Phys. Chem. C, 2008, 112, 13378.49 S.-L. Chou, J.-Z. Wang, D. Wexler, K. Konstantinov, C. Zhong,
H.-K. Liu and S.-K. Dou, J. Mater. Chem., 2010, 20, 2092.
J. Mater. Chem., 2012, 22, 8345–8353 | 8353