Dynamic Article LinksC<Journal ofMaterials Chemistry

Cite this: J. Mater. Chem., 2012, 22, 8345

www.rsc.org/materials PAPER

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 / Journal Homepage / Table of Contents for this issue

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:sykwak@snu.ac.kr; 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

This journal is ª The Royal Society of Chemistry 2012

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

J. Mater. Chem., 2012, 22, 8345–8353 | 8345

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

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

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

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

This journal is ª The Royal Society of Chemistry 2012

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.

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

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

This journal is ª The Royal Society of Chemistry 2012

Fig. 4 Wide (a) and narrow scan (b, c and d) XPS patterns for CQD/

MH.

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

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.

J. Mater. Chem., 2012, 22, 8345–8353 | 8349

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).

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

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

This journal is ª The Royal Society of Chemistry 2012

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.

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

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

J. Mater. Chem., 2012, 22, 8345–8353 | 8351

Fig. 12 The photocatalytic degradation of the MB aqueous solutions

using CQD/MH + H2O2 for three cycles.

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

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