Efficient Visible Light Photocatalytic Removal of NO with BiOBr

ARTICLE pubs.acs.org/JPCC

Efficient Visible Light Photocatalytic Removal of NO with BiOBr-Graphene Nanocomposites Zhihui Ai,*,†,§ Wingkei Ho,‡ and Shuncheng Lee§ †

Key Laboratory of Pesticide & Chemical Biology of Ministry of Education of College of Chemistry, Central China Normal University, Wuhan 430079, People's Republic of China ‡ Nano and Advanced Materials Institute Limited, Hong Kong § Department of Civil and Structural Engineering, Research Center for Environmental Technology and Management, The Hong Kong Polytechnic University, Hong Kong, People's Republic of China

bS Supporting Information ABSTRACT: In this study, we demonstrate that bismuth oxybromide and graphene nanocomposites (BGCs) exhibit superior performance on photocatalytic removal of gaseous nitrogen monoxide (NO) to pure BiOBr under visible light irradiation (λ > 420 nm). The photocatalytic NO removal rate constant of BGCs was 2 times that of pure BiOBr. The BGCs were prepared by a facile solvothermal route with using graphene oxide (GO), bismuth nitrite, and cetyltrimethyl ammonium bromide (CTAB) as the precursors. During the synthesis, both of the reduction of GO and the formation of BiOBr nanocrystals were achieved simultaneously. On the basis of the characterization results, we attributed the enhanced photocatalytic activity of the BGCs nanocomposites to more effective charge transportations and separations arisen from the strong chemical bonding between BiOBr and graphene, not to their light absorption extension in the visible region and higher surface area.

’ INTRODUCTION Nitrogen monoxide (NO) is one of the most common gaseous pollutants.1 Although traditional techniques such as physical adsorption, biofiltration, and thermal catalysis methods can remove NO from industrial emission, they are not economically feasible for the removal of NO at parts per billion (ppb) levels in indoor air.2 TiO2 photocatalysis has gained considerable attention in view of solar energy conversion and environmental cleaning, especially for the purification of air pollutants at low concentrations.2 However, its relatively wide band gap of 3.2 eV limits its application in the visible light region (400 nm < λ < 750 nm), which accounts for 43% of the incoming solar energy.2,3 In view of the better utilization of solar light, nontitania-based photocatalysts have been recently developed and utilized for environmental cleaning.4
8 As a new visible light-responding nontitania-based photocatalyst, BiOBr has attracted increasing interest recently.2,5
8 For instance, our group developed a general one-pot nonaqueous sol
gel method to prepare BiOX (X = Cl, Br, I) microspheres with bismuth nitrate and the corresponding potassium halide as the precursors8 and also demonstrated the nonaqueous sol
gel synthesized BiOBr microspheres synthesized with using bismuth nitrate and cetyltrimethyl ammonium bromide (CTAB) as the precursors exhibited superior photocatalytic activity to the chemical precipitation synthesized counterpart and Degussa TiO2 P25 on the degradation of NO under visible light irradiation (λ > 420 nm).2 r 2011 American Chemical Society

Various carbon materials, including activated carbon, carbon black, and graphite, as well as carbon nanotubes (CNTs), are found to play important roles in heterogeneous catalysis as either catalysts or catalyst supports.9 Graphene, a new carbon material with a monolayer of sp2-hybridized carbon atoms in a dense honeycomb crystal structure, has generated great interest both in the fields of fundamental research and industrial applications including digital displays, capacitors, solar cells, and field-effect transistors.9
16 Graphene could act as superior conductive channels for charge carriers because of its faster carrier mobility (1.200 00 cm2 3 V
1 3 S
1) than metal materials. Moreover, graphene's unique physical and electronic properties together with its abundant sources make it promising as the next-generation carbon material in catalysis.12
22 Recently, graphene hybrids with metal oxides, metals, and polymers have been developed for various applications.23
27 It was reported that graphene modification could improve the performance of TiO2 in solar cells and photocatalysis.4,28
30 For example, Liang's group prepared graphene/TiO2 nanocrystals by directly growing TiO2 nanocrystals on GO sheets followed with a subsequent hydrothermal reduction in water and ethanol. They found that the resulting graphene/TiO2 nanocrystals hybrid had superior photocatalytic Received: July 17, 2011 Revised: November 4, 2011 Published: November 27, 2011 25330

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The Journal of Physical Chemistry C activity to other TiO2 materials on the degradation of rhodamine B.31 Chen et al. demonstrated that GO/semiconductor composites could show different properties by using a tunable semiconductor conductivity type of GO.29,30 Zhang's group demonstrated that a chemically bonded P25-graphene photocatalyst exhibited enhanced activity on the photodegradation of methylene blue, compared to the bare P25 and P25-CNTs with the same carbon content.32 Graphene/TiO2 thin film composites could have inactive bacteria in solar light irradiation.33 Recently, graphene modified with gold nanoparticles have been reported to display an excellent visible-light photocatalytic performance to degrade dyes in water.26 In this work, we demonstrate that a facile solvothermal route synthesized bismuth oxybromide and graphene nanocomposites (BGCs) exhibit a superior performance on photocatalytic removal of gaseous NO to pure BiOBr under visible light irradiation (λ > 420 nm) for the first time. We systematically characterize the resulting BGCs and discuss the reasons for the enhanced photocatalytic activity of the BGC composites in detail.

’ EXPERIMENTAL SECTION Sample Preparation. Graphite oxide (GO) was first prepared using the modified Staudenmaier method.34,35 In a typical procedure, 20 mL of HNO3 (70%) and 36 mL of H2SO4 (98%) were mixed and stirred for 15 min in a 500 mL reaction flask immersed in an ice bath. Then 2 g of graphite was added into the above concentrated acid mixture and stirred for 20 min. The ice bath was then removed, and 22 g of KClO3 was added slowly into the solution. The reaction mixture was stirred continuously for 48 h at room temperature. On completion of the reaction, the mixture was added to excess distilled water, washed with 5% HCl solution, and then repeatedly washed with distilled water until the pH of the filtrate was neutral. To remove unexfoliated GO, the prepared GO was dispersed in distilled water with a concentration of 0.6 g/L and sonicated for 1 h. The dispersion was then centrifuged at 4000 rpm for 30 min, and the resultant homogeneous yellow-brown dispersion was then filtered through a 200 nm pore size polytetrafluoroethylene (PTFE) membrane. The exfoliated GO in the filtrates was then collected and dried in a vacuum oven at 40 °C overnight to get sample in powder form for further use. The BiOBr and graphene composites (BGCs) were synthesized via a facile solvothermal method using bismuth nitrite, cetyltrimethyl ammonium bromide (CTAB), and exfoliated GO as the precursor, as illustrated by Scheme S1 (Supporting Information, SI). In a typical synthesis, 0.1 mmol of Bi(NO3)3 3 5H2O was added into 35 mL of ethylene glycol (EG) containing stoichiometric amounts of CTAB. The mixture was stirred for 0.5 h at room temperature, and then 35 mL of ethanol solution containing appropriate amount of as-prepared GO was added into the suspension under magnetic stirring for 60 min. The resulting suspension was then transferred into a 100 mL Teflon-lined stainless autoclave. The autoclave was allowed to be heated at 180 °C for 12 h under autogenous pressure and then air-cooled to room temperature. The resulting precipitates were collected and washed with ethanol and deionized water thoroughly and dried at 50 °C in air. The obtained BiOBr-graphene samples with molar ratios of Bi and graphene of 10:1, 20:1, 50:1, and 100:1 were denoted as BGC-10, BGC-20, BGC-50, and

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BGC-100, respectively. The pure BiOBr counterpart was synthesized without adding as-prepared graphene oxide. Photocatalytic NO Removal Experiment. The photocatalytic experiments for the removal of NO in air were performed at ambient temperature in a continuous flow reactor. The volume of the rectangular reactor which was made of stainless steel and covered with Saint-Glass was 4.5 L (10  30  15 cm (H  L  W)). One sample dish containing 0.1 g of catalyst powder was placed in the middle of the reactor. A 300 W commercial tungsten halogen lamp (General Electric) was used as the simulated solar-light source. The lamp was vertically placed outside the reactor above the sample dish. The lamp was vertically placed outside the reactor above the sample dish, and a glass filter was placed to remove light below 420 nm. Four minifans were fixed around the lamp to avoid the temperature rise of the flow system. The catalyst samples were prepared by coating an aqueous suspension of our sample onto the dish with a diameter of 12.0 cm. The dishes containing the photocatalyst were pretreated at 60 °C for water evaporation and then cooled to room temperature. NO gas was selected as the target pollutant for the photocatalytic degradation at ambient temperature. The NO gas was acquired from a compressed gas cylinder at a concentration of 48 ppm NO (N2 balance, BOC gas) with a traceable National Institute of Stands and Technology (NIST) standard. The initial concentration of NO was diluted to about 400 ppb by the air stream supplied by a zero air generator (Thermo Environmental Inc., model 111). The desired humidity level of the NO flow was controlled at 70% (2100 ppmv) by passing the zero air streams through a humidification chamber. The gas streams were premixed completely by a gas blender, and the flow rate was controlled at 4 L/min by a mass flow controller. After the adsorption
desorption equilibrium among water vapor, gases, and photocatalysts was achieved in an hr, the lamp was turned on. The concentration of NO was continuously measured by a chemiluminescence NO analyzer (Thermo Environmental Instruments Inc., model 42c), which monitors NO and NO2 with a sampling rate of 0.7 L/min. The reaction of NO with air was ignorable when performing a control experiment with or without light in the absence of photocatalyst. The production of NO2 in the outlet stream was also ignorable (only 1
2 ppb). The removal efficiency (η) of NO was calculated as eq 1: ηð%Þ ¼ ð1
C=C0 Þ  100%

ð1Þ

where C and C0 were the concentrations of NO in the outlet stream and the feeding stream, respectively.

’ RESULTS AND DISCUSSION All of the power X-ray diffraction (XRD) peaks of BiOBr and BGCs could be indexed to a tetragonal phase of BiOBr with lattice constants of a = 3.915, c = 8.076 (JCPDS Card No. 732061) (Figure 1a).2,8 However, we could not observe the characteristic diffraction peaks from graphene in the synthesized BGCs samples, which may be attributed to the overlapping between the main peak of graphene at 26.6° and the (011) peak of tetragonal BiOBr at 25.2°.36 Similar phenomenon was also observed for graphene/TiO2 composites.28
33 Typical TEM images reveal that BiOBr nanoplates with hundreds of nanometers in width are randomly dispersed on the 2D graphene sheets (Figure 1b).28
30 The lattice spacing between adjacent lattice planes of the nanoplates is about 2.77 Å, corresponding to the 25331

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Figure 1. XRD patterns of the resulting BiOBr and BGC samples (a), typical TEM image (b), and HRTEM images (c) of the resulting BGC-50.

(110) lattice plane of tetragonal BiOBr (Figure 1c). Moreover, we found that the edges of the prepared graphene sheets always fold back, allowing for a cross sectional view of the sheets in HRTEM images. Therefore, the number of layers in graphene sheets can be determined by simply observing the folding at the edge. Typically, HRTEM images show that the as-prepared graphene is of two to several layers (Figure S1, SI). The surface element composition of pure BiOBr and the BGC-50 sample was studied by X-ray photoelectron spectroscopy (XPS) (Figure 2a
e). The survey XPS spectra reveal that both the samples are composed of elements of Bi, O, Br, and C (Figure 2a). Two strong peaks at 158.7 and 164.5 eV in the highresolution spectra are assigned to Bi 4f7/2 and Bi 4f5/2, respectively, which is characteristic of Bi3+ in BiOBr (Figure 2b).3 A slight Bi3+ XPS peak shift (∼0.3 eV) to low binding energy in BGC-50 can be ascribed to a slight surface charging effect because of polarization change of the BiOBr nanoplates, which might be associated with strong chemical bonding between BiOBr and graphene. The Br 3d peaks are associated with a

binding energy of 68.20 eV (Figure 2c).7 Meanwhile, the high resolution of O 1s XPS spectra (Figure 2d) can be fitted by two peaks at binding energies of 530.0 and 531.6 eV, which are respectively arisen from oxygen in BiOBr and other components such as OH, H2O, and carbonate species adsorbed on the surface of catalysts.2 The hydroxyl/carbonate content in the BGC-50 composite is lower than that in BiOBr, indicating less hydroxyl/ carbonate groups from the chemisorption of water and/or other molecules on the surface of BGCs. The high-resolution XPS spectra for the C 1s region for BiOBr and BGC-50 composites can be fitted by three peaks at binding energies of 288.4, 286.3, and 284.5 eV, respectively (Figure 2e). The peak at 284.5 eV is ascribed to the typical carbon
carbon (C—C) bonds in aromatic networks for graphene and the adventitious carbon on the surface of the sample. The deconvoluted peaks centered at the binding energies of 286.3 and 288.4 eV were assigned to the C— O and CdO bonds, respectively.17,18,33 Comparing the two spectra, we found that their peaks around 284.5 eV (C—C) were similar; however, the peak at 288.4 eV (CdO) in BGC-50 25332

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Figure 2. XPS spectra of the as-prepared BiOBr and BGC-50: (a) survey, (b) Bi 4f, (c) Br 3d, (d) O 1s, and (e) C 1s.

composites almost disappeared. This difference suggests the domination of the C—C bond and the almost vanishing of the carbonate species (CdO) on the surface of BGC-50, consistent with the result of O 1s XPS spectra in Figure 2d. The atomic ratios of Bi, O, Br, and C were calculated to be 100:110.2:87:18.5 and 100:113.6:89.1:23.8 for pure BiOBr and BGC-50, respectively. Obviously, the surface carbon enrichment by 28.6% in BGC-50 could be attributed to the presence of graphene. Raman spectroscopy is a useful nondestructive tool to characterize carbonaceous materials, particularly for distinguishing ordered and disordered carbon structures.17,37,38 It is well-known

that the characteristics of ordered carbon materials in Raman spectra are the G band at ∼1580 cm
1, which is generally assigned to the E2g phonon of sp2 bonds of carbon atoms. As a breathing mode of point phonons of A1g symmetry, the D band at ∼1350 cm
1 is attributed to local defects and disorders, particularly the defects located at the edges of graphene and graphite platelets.17,38 Raman spectra of the as-prepared BiOBr and BGCs (Figure S2, SI) reveal that the characteristic peaks of graphene at around 1574 cm
1 can be clearly observed for all BGC samples, but absent for pure BiOBr, confirming the existence of ordered carbon in BGC samples.39 The G band at 1574 cm
1 could be 25333

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Figure 3. (a) Plots of the decrease in NO concentration vs irradiation time in the presence of BiOBr, BGCs, and a mixture of BiOBr and graphene (molar ratio 20:1) under visible light (λ > 420 nm); (b) dependence of ln(C/C0) on irradiation time under visible light irradiation (λ > 420 nm).

assigned to the characteristic ordered carbon resulting from the in-plane vibration of sp2 carbon atoms in a two-dimensional hexagonal lattice.37 In addition, the peak around 1240 cm
1 was also observed for all samples, which might be ascribed to the Eg22g mode of sp3 bonds of amorphous carbon on the surface of the samples.40 This is in good agreement with the XPS results. However, the D band at around 1350 cm
1 was not observed. This phenomenon is common for graphene without enough structural defects corresponding to D peak in Raman spectrum.41 Compared with the reported G band position (∼1580 cm
1) of the pristine graphene-like carbon, the 6 cm
1 shift of the G band peak position to lower wavenumbers could be assigned to the presence of both single- and multilayer graphene sheets and/or an intensive chemical interaction between graphene and BiOBr in BGC nanocomposites driven by their charge transfer.38,42
44 The above characterizations reveal that both the reduction of GO and the formation of BiOBr nanocrystals were achieved simultaneously during the synthesis in this study. We utilized the resulting BGCs to photocatalytically remove gaseous NO under visible light irradiation (λ > 420 nm) and compared their performances with pure BiOBr and the mixture

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of BiOBr and graphene. Figure 3a shows the variation of NO concentration against irradiation time in the presence of different samples. Under visible light irradiation, BGC-100, BGC-50, and BGC-20 could remove 35.4%, 40.3%, and 38.7% of NO in 40 min, respectively, whereas only 29.5% of NO was removed with pure BiOBr. This comparison suggests that the introduction of graphene could efficiently enhance the photocatalytic performance of BiOBr. The photoactivity of the BGCs was found to be dependent on the ratio of BiOBr and graphene. Obviously, BGC50 showed the highest NO removal efficiency (40.3%). When an excess amount of graphene sheets was present, for example, BGC-10, the photocatalytic performance (28.5%) was even slightly lower than that of pure BiOBr. This phenomenon could be explained as follows. As graphene is highly transparent in the optical range,45 excess graphene would decrease the photocatalytic activity not by obstructing the absorbing incident light and/ or scattering more photons in the nanocomposites, but by reducing the amount of active photocatalyst (BiOBr) because pure graphene itself is not active under visible light and serving as a charge carrier recombination center like excess gold nanoparticles.46 As also shown in Figure 3a, the activity of samples showed a slight decline tendency with the increased irradiation time, because some intermediates (HNO2 and HNO3) would generate during the photocatalysis process on the surface of the catalysts.2 Moreover, multiple runs of photodegradation experiments showed that BGC-50 was not significantly deactivated during the photocatalytic oxidation of NO (Figure S3, SI), suggesting that the BGC samples could be promising for air purification. The kinetics of the NO removal reaction were found to fit a pseudofirst-order reaction at low NO concentrations: ln(C/C0) = k, where k is the initial apparent rate constant.2 As shown in Figure 3b and Table 1, the initial removal constants were calculated to be 0.080, 0.132, 0.151, 0.138, and 0.061 min
1 for BiOBr, BGC-100, BGC-50, BGC-20, and BGC-10, respectively. With increasing the amount of graphene, the NO removal constant of BGCs first increased and then decreased. Among them, BGC-50 possessed the highest removal constant, which was about 2 times that of BiOBr. Interestingly, the physical mixture of graphene and pure BiOBr exhibited much lower photocatalytic activity than the BGCs, even slightly lower than pure BiOBr, indicating that the enhanced photocatalytic performance of BGCs was attributed to the strong chemical bonding between graphene and BiOBr. Generally, the overall photocatalytic activity of a semiconductor is mainly related to the photoabsorption ability in the available light energy region, the ability of the photocatalysts to adsorb target pollutants, and the separation and transporting rate of the photogenerated electrons and holes in the catalysts. To find out the reason for enhanced visible light photocatalytic activity of BGCs, the UV
vis diffuse reflectance spectrometer (DRS) was first used to determine the band gap energy of the synthesized samples (Figure 4a). Compared to pure BiOBr, the BGCs exhibit decreased spectra in UV and stronger visible light absorption in the range of 550
800 nm (inset of Figure 4a). Moreover, the red-shifts of the adsorption edge were observed in BGC-20 and BGC-10. Assuming the material to be an indirect semiconductor, plots of the (ahν)1/2 (α is the absorption coefficient, and hν is the photon energy) versus the energy of absorbed light afford the band gaps of as-prepared samples. The band gap energies of BiOBr, BGC-100, BGC-50, BGC-20, and BGC-10 calculated by the transformed Kubelka
Munk function 25334

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are 2.35, 2.34, 2.34, 2.29, and 2.28 eV, respectively47 (Table 1). Although BGC-100 and BGC-50 possess similar band gap energies with pure BiOBr, they displayed significantly higher activities on the removal of NO than pure BiOBr. Meanwhile, BGC-20 and BGC-10 with narrower band gaps showed lower photocatalytic activity on NO removal under visible light than other two BGCs. We therefore believe the band gap narrowing with the introduction of graphene does not contribute to the photocatalytic performance enhancement of BGC nanocomposites. This may be related to the relative narrow band gap (2.35 eV) of BiOBr. Nitrogen sorption was then used to measure the surface areas of the BiOBr, BGCs, and the mixture samples (Figure S4, SI). It was found that all Brunauer
Emmett
Teller (BET) surface areas of BGCs were higher than that (8.2 m2/g) of pure BiOBr, and the surface area (15.4 m2/g) of the mixture of graphene and BiOBr was close to that (16.3 m2/g) of BGC-20, revealing that the higher photocatalytic activity of BGC-20 than the counterpart mixture could not be attributed to the surface area enhancement. To further find out the effect of surface areas on their photocatalytic activities, we normalized the initial rate constants (k) with the surface areas to obtain normalized rate constants (k0 ) and found that that the order of the normalized rates was the same as that of the original ones (Table 1). So we could conclude that the enhanced surface areas are not the major factor to enhance photocatalytic activity of BGC nanocomposites. During the photocatalytic process, the charge transfer and recombination are two competitive reaction pathways, so it is Table 1. Initial Rate Constant k (min
1), BET Surface Area (m2 3 g
1), Normalized Initial Rate Constant with BET Surface Area k0 (g 3 min
1 3 m
2), and Band Gap Energy (eV) of the Samples BET band gap

surface area sample

k (min
1)

(m2 3 g
1)

k0 (g 3 min
1 3 m
2)

energy (eV)

BiOBr

0.080

8.2

0.0098

2.35

BGC-100

0.132

11.5

0.0115

2.34

BGC-50

0.151

12.4

0.0121

2.34

BGC-20 BGC10

0.138 0.061

16.3 18.9

0.0085 0.0032

2.29 2.28

important to suppress the recombination rate and accelerate charge transfer to produce more hydroxyl radicals, which are directly beneficial for the oxidation of pollutants. To investigate the efficiency of charge carrier trapping, immigration, and transfer in the as-prepared BiOBr and BGCs, we measured their photoluminescence (PL) emission spectra (Figure 4b). As shown in Figure 4b, a broad PL emission spectrum was observed for pure BiOBr, which could be attributed to the radiative recombination process of self-trapped excitations.48,49 This implies that most of the charges quickly recombine in BiOBr to produce PL emission. In contrast, the PL intensity of the BGCs was significantly reduced, indicating that the electron
hole recombination on the surface of catalysts is largely inhibited to generate more photoelectrons and holes to participate in the photocatalytic reactions. We therefore conclude that the enhanced photocatalytic activity of BGCs is mainly attributed to more effective charge transportations and separations arisen from the strong chemical bonding of BiOBr and graphene. Theoretically, graphene sheets with 100% sp2-hybridized carbon atoms have a high electrical conductivity in storing and shuttling electrons.9,23 When graphene is combined with other materials, electrons would flow from one material to the other (from the higher to lower Fermi level) to align the Fermi energy levels at the interface of two materials.30,50,51 In view of the higher work function (
4.5 eV) of graphene than that (
3.19 eV) of BiOBr, electrons would flow from BiOBr into graphene to adjust the Fermi energy levels,19,29,30 leading to the formation of a Schottky barrier at the BiOBr-graphene interface. This Schottky barrier could capture electrons from BiOBr to graphene and prevent their back flowing to the BiOBr30 and thus effectively separate the photogenerated electron
hole pairs.21,30,51 On the basis of the above results and analysis, we propose a possible mechanism to explain the superior performances of BGCs to remove NO under visible light irradiation as follows (Scheme 1). Under visible light irradiation, electrons could be excited from the valence band (VB) to the conduction band (CB), leaving holes at the valence band of BiOBr (process a, eq 2).
BGCs þ visible light f hþ VB þ eCB

ð2Þ

These holes would react with surface adsorbed OH
and water to produce 3 OH radicals for NO oxidation (process b,

Figure 4. (a) Plots of (αhυ)1/2 vs photon energy (hυ) with the inset of UV
vis diffuse reflectance absorption spectra, and (b) PL spectra of BiOBr and BGCs (room temperature, a Ni:yttrium
aluminum
garnet laser, 325 nm). 25335

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’ CONCLUSIONS In summary, we have presented a facile and rapid solvothermal method to prepare bismuth oxybromide and graphene nanocomposites (BGCs). During the synthesis, both the reduction of GO and the formation of BiOBr nanocrystals were achieved simultaneously. The resulting BGCs showed a superior performance on the photocatalytic removal of gaseous nitrogen monoxide (NO) to pure BiOBr under visible light irradiation (λ > 420 nm). The photocatalytic NO removal rate constant of BGCs was two times that of pure BiOBr. On the basis of the characterization results, we attributed the enhanced photocatalytic activity of the BGCs nanocomposites to more effective charge transportation and separations arisen from the strong chemical bonding between BiOBr and graphene, not to their light absorption extension in the visible region and higher surface area.

Scheme 1. Schematic Illustration of the Visible Light Photocatalytic Enhancement of BGCs Nanocomposites

’ ASSOCIATED CONTENT eq 3). H2 O þ

hþ VB

f 3 OH þ H

þ

ð3Þ

As the top of the valence band (EVB) (above 3.19 V vs normal hydrogen electrode (NHE)) of BiOBr is more positive than the standard half-cell reduction potential (Eo) (NO2/NO, 1.03 V vs NHE), Eo (HNO2/NO, 0.99 V vs NHE), and Eo (HNO3/NO, 0.94 V vs NHE), the VB holes could also oxidize NO directly (process c, eq 4). hþ VB þ NO f NOx

ð4Þ

There are two fates for the excited electrons on the conduction band of BiOBr: to react with O2 to produce superoxides (eq 5), hydrogen peroxides (eq 6), and 3 OH radicals (eq 7, process d) and to recombine with the holes in the VB (process e).
e
CB þ O2 f 3 O2

3 O2




ð5Þ

þ 2Hþ þ e
CB f H2 O2

H2 O2 þ

e
CB

f 3 OH þ OH




ð6Þ ð7Þ

The strong chemical bonding of BiOBr and graphene would result in the transportation of photogenerated electrons from the conduction band of BiOBr to graphene (process f). Meanwhile, as stated previously, the Schottky barrier formed at the BiOBrgraphene interface could prevent the reverse transportation of photoelectrons from graphene to the conduction band of BiOBr. The recombination of photoelectrons and holes on the surface of BiOBr was thus inhibited. These more effectively separated electrons and holes could then produce more reactive radicals ( 3 OH and 3 O2
, etc.) to oxidize NO (eqs 5
7). The photocatalytic removal of gaseous NO has been proposed to involve reactions displayed in eqs 8
11, in which NO reacted with reactive species to produce HNO2 and HNO3.2 NO þ 2 3 OH f NO2 þ H2 O

ð8Þ

NO2 þ 3 OH f NO3
þ H

ð9Þ

NO þ NO2 þ H2 O f 2HNO2

ð10Þ

NOx þ 3 O2
f NO3


ð11Þ

bS

Supporting Information. Chemicals; characterizations; HRTEM of the as-prepared graphene; Raman spectra; the stability of BiOBr microspheres in multiple runs of degradation of NO over BGC-50 vs prolonged irradiation time; and N2 adsorption
desorption isotherms of the synthesized BiOBr and BGCs. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone/Fax: +86-27-6786 7535.

’ ACKNOWLEDGMENT This work was supported by National Science Foundation of China (Grants 20977039, 21073069, 21173093, and 91023010), Program for Distinguished Young Scientist of Hubei Province (Grant 2009CDA014), Program for Innovation Team of Hubei Province (Grant 2009CDA048), Self-Determine Research Funds of CCNU from the Colleges' Basic Research and Operation of MOE (Grants CCNU09A02014 and CCNU09C01009), National Basic Research Program of China (973 Program) (Grant 2007CB613301), and Materials Science of the State Ethnic Affairs Commission & Ministry of Education, Hubei province (Grants CHCL11001). ’ REFERENCES (1) Li, F. B.; Li, X. Z.; Ao, C. H.; Hou, M. F.; Lee, S. C. Appl. Catal., B 2004, 54, 275. (2) Ai, Z. H.; Ho, W. K.; Lee, S. C.; Zhang, L. Z. Environ. Sci. Technol. 2009, 43, 4143. (3) Gaya, U. I.; Abdullah, A. H. J. Photochem. Photobiol., C 2008, 9, 1. (4) Ng, Y. H.; Iwase, A.; Kudo, A.; Amal, R. J. Phys. Chem. Lett. 2010, 1, 2607. (5) Deng, Z. T.; Chen, D.; Peng, B.; Tang, F. Q. Cryst. Growth Des. 2008, 8, 2995. (6) Huang, W. L.; Zhu, Q. S. J. Comput. Chem. 2009, 30, 183. (7) Zhang, J.; Shi, F. J.; Lin, J.; Chen, D. F.; Gao, J. M.; Huang, Z. X.; Ding, X. X.; Tang, C. C. Chem. Mater. 2008, 20, 2937. (8) Zhang, X.; Ai, Z. H.; Jia, F. L.; Zhang, L. Z. J. Phys. Chem. C 2008, 112, 747. (9) Geim, A. K. Science 2009, 324, 1530. 25336

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