ARTICLE pubs.acs.org/JPCC
Hierarchically Grown Urchinlike CdS@ZnO and CdS@Al2O3 Heteroarrays for Efficient Visible-Light-Driven Photocatalytic Hydrogen Generation Dipankar Barpuzary,†,‡ Ziyauddin Khan,†,‡ Natarajan Vinothkumar,§ Mahuya De,§ and Mohammad Qureshi*,† †
Department of Chemistry and §Department of Chemical Engineering, Indian Institute of Technology, Guwahati, Guwahati 39, Assam, India
bS Supporting Information ABSTRACT:
Nanourchin-shaped narrow-band-gap semiconductor photocatalysts with high surface area combined with good crystallinity result in effective photocatalysis. In this work, the impregnating growth of 1D CdS nanowires onto Al2O3 and ZnO templates as cores generates novel urchinlike morphology of CdS@oxide photocatalysts. The CdS@Al2O3 and CdS@ZnO nanourchins explicitly show a major role in enhanced hydrogen generation with apparent quantum yields (AQY) of 11% and 15%, respectively. Mechanistically, the template-based CdS can influence the photocatalytic activity in two ways: (i) direct well-dispersed growth of CdS onto the oxide core, leading to a high surface area for enhanced light absorption, and (ii) charge transfer from the conduction band of highly crystalline CdS to that of the oxide, which facilitate efficient charge separation for hydrogen production. Following these two mechanisms, a simple, low-cost, and environmentally friendly hydrothermal strategy is employed to synthesize novel nanourchin-shaped CdS-based heteroarrays. This new morphology stimulates the surface area per unit volume of the photocatalyst and exhibits promising application for photocatalytic water splitting.
(NPs),3 nanorods/nanoribbons,47 nanowires (NWs),810 and nanotubes1114 having interesting surface morphologies. The photocatalytic water splitting efficiency of a semiconductor can be enhanced many times by addition of cocatalyst or by stoichiometric doping of metal. When light is irradiated, photoinduced electrons and holes are generated in the conduction band (CB) and valence band (VB) of the semiconductor, respectively. These electrons and holes then migrate to the surface and are involved in the redox reaction with water, thereby producing H2 and oxygen (O2), similar to electrolysis. But in the doped photocatalysts, generation of recombination sites between photogenerated electrons and holes is more or less inevitable.15 The discrete energy level formed by the dopants reduces the migration of holes. Here we introduce the idea to generate H2 by designing a new coreshell-based hierarchical heterosystem to
1. INTRODUCTION Semiconductor-based nanomaterials having a suitable band gap for efficient photocatalytic hydrogen (H2) production is one of the critical needs to generate an environmentally clean alternative resource.1 At present, H2 is being generated from hydrocarbons such as fossil fuels or biomass and water. Steam reforming process is also used to release H2 from natural gases, but the unwanted CO2 emission along with it cannot be avoided. Hydrogen can be generated by water splitting via electrolysis method by use of electrolyzers. However, this method is not suitable for generating H2 because of the requirement to harvest renewable sources of electricity such as solar- or wind-powered generators to drive the electrolyzers; electrolysis is also expensive, which may eventually prove to be an impediment to largescale technological applications. Instead, a promising method is the direct implementation of solar energy to generate large-scale hydrogen from photochemical water splitting by use of a powdered photocatalyst.2 Researchers are exploring many photocatalytic systems with various shapes and sizes, for example, nanoparticles r 2011 American Chemical Society
Received: August 4, 2011 Revised: December 1, 2011 Published: December 18, 2011 150
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obtain suitably enhanced surface area and mode of contact for water splitting. The combination of semiconductors, developed by adopting a coreshell strategy, can lead to less crystal defects or high crystallinity. The main concern behind a metal doping or cocatalyst addition is to substantially promote the separation of photoinduced electrons and holes on the photocatalyst surface. The same concept of effective charge separation can be attained by coupling a visible-light-driven semiconductor (CdS) to a suitable wide-band gap metal oxide semiconductor (ZnO, TiO2) such that the charge transfer can occur from the CB of CdS to that of the metal oxide. The present work manifests the production of H2 by use of CdS@Al2O3 and CdS@ZnO heterostructures (hereafter, CdS@oxide) having urchinlike morphology. In a recent paper, our research group has demonstrated the synthesis of 1D CdS NWs on ZnO and Al2O3 templates.16 In essence, the surface area and band gap properties of the nanocomposites successfully induce their photocatalytic activity. Essentially, these nanostructured inorganic metal chalcogenidemetal oxide heteroarrays have unveiled exceptionally high rates of photocatalytic H2 generation from water. The present report mainly highlights the cost-effective hydrothermal method for high rates of H2 production even in the absence of expensive noble metals. This reduces the total cost of H2 production efficiently.
and 40 mA. A scan rate of 0.1°/s was applied to record the powder XRD patterns for 2θ in the range of 2070°. The morphology of the photocatalyst samples was investigated by scanning electron microscopy (SEM) on a LEO 1430vp instrument operated at 1015 kV. The transmission electron microscopy (TEM) measurements were conducted on a JEOL JEM 2100 microscope working at 200 kV. UVvis diffuse reflectance spectra (DRS) were obtained with a Shimadzu 2550 UVvis spectrophotometer equipped with an integrating sphere attachment ISR 240A and BaSO4 powder as the internal standard. BET surface areas were analyzed by nitrogen (N2) adsorption at liquid N2 temperature with a Beckman-Coulter SA 3100 nitrogen adsorption apparatus. All the samples were degassed at 150 °C for 2 h prior to the N2 adsorption measurements. 2.3. Evaluation of Photocatalytic Activity. The photocatalytic H2 production reactions using CdS NW and CdS@oxide photocatalysts were carried out in an outer irradiation-type airtight photoreactor. It is well-known that CdS may undergo photocorrosion and photocatalytic dissolution. In order to avoid photocorrosion, H2 production experiments were performed under N2 atmosphere in the presence of sacrificial reagents. In each experiment, 100 mg of the catalyst was dispersed by a magnetic stirrer in 50 mL of deionized water containing both 0.25 M Na2SO3 and 0.35 M Na2S as sacrificial agents inside the reactor. The reaction mixture was purged with N2 gas at a flow rate of 50 mL/min for 30 min to remove any oxygen dissolved. The experimental conditions and reactor setup showed no H2 evolution in the absence of either irradiation or photocatalyst, that is, prior to the photocatalysis. A 500-W Phoenix tungsten halogen lamp, fixed at a distance of 15 cm from the reactor, was used as the light source. The choice of light source was based on the spectral characteristics of the emitted light. The emission profile of the lamp was recorded on an Ocean Optics USB4000 spectrometer in the wavelength range 1951126 nm. Figure S1 in Supporting Information shows the emission spectrum of the lamp. The light spectrum shows negligible emission in the ultraviolet range. Therefore, a cutoff filter was used with the lamp to eliminate the UV component of light from reaching the photocatalysts. The temperature was maintained at 25 ( 5 °C for all experiments by constant cold water circulation around the photoreactor. The water circulation also eliminated the probability of evaporation of water molecules. Each reaction mixture was irradiated for 1 h. The released gas was allowed to pass through KOH dissolved water (to remove any SO2 gas if released) and then through moisture and oxygen traps before analysis. The H2 gas produced was then analyzed directly by a gas chromatograph (GC, Varian CP 3800 TCD detector, Carbosieve S II column, and 99% N2 as carrier gas at a flow rate of 20 mL 3 min1). A schematic of the photochemical reactor setup is shown in Figure S2 of Supporting Information. 2.4. Evaluation of Apparent Quantum Yield. The apparent quantum yield (AQY), defined for H2 production from water by photocatalysis, was estimated from the following equation.
2. EXPERIMENTAL SECTION 2.1. Material Preparation. All the chemicals were purchased from Merck and used as received without any further purification. Freestanding CdS NWs and CdS@oxide nanourchins were synthesized as reported.16 In a typical procedure, Al(NO3)3 3 9H2O (1.5 g, 3.9 mmol) was dissolved in 10 mL of water, followed by the addition of aquous NH3 (to attain pH ∼8), thioglycolic acid (1 mL, 9 mmol) under vigorous stirring, and Cd(CH3COO)2.2H2O (2.39 g, 9 mmol). The reaction mixture was stirred for 30 min at room temperature and then kept in an electronic oven at 140 °C for 48 h in a Teflon-lined stainless steel autoclave. Products were washed with warm ethanol and dried in vacuum. ZnO-templated growth of CdS is carried out on ex situ-generated ZnO, followed by the same procedure adopted in the case of CdS@Al2O3. Freestanding CdS NWs were grown hydrothermally by mixing thioglycolic acid and Cd(CH3COO)2 3 2H2O in appropriate mole ratio in the absence of any template. CdS-free ZnO NPs were also synthesized by adopting the procedure reported by Becheri et al.17 First ZnCl2 (5.5 g, 40 mmol) was dissolved in 200 mL of distilled water at 90 °C, and 16 mL of 5 M NaOH aquous solution was then added dropwise with gentle stirring over a time period of 10 min. ZnO NPs were separated from the supernatant dispersion by sedimentation method (the supernatant solution was discarded) and the remaining suspension was washed with distilled water until NaCl was completely removed (confirmed by AgNO3 solution test). The purified particles were treated with 2-propanol in ultrasonic bath for 10 min at room temperature, collected by centrifugation, and finally maintained at 250 °C for 5 h in a Teflon-lined stainless steel autoclave to obtain the ZnO nanoparticles. The well-directed growth process of CdS NWs into alumina particles was in situ, whereas that into ZnO was ex situ. The ex situ generation of ZnO was introduced to avoid the formation of zinc sulfide under in situ reaction conditions. 2.2. Material Characterization. Powder X-ray diffraction (XRD) patterns were obtained by using Bruker D8 Advanced X-ray diffractometer with Cu Kα irradiation (λ = 1.54 Å) at 40 kV
AQY ¼ ¼
number of reacted electrons 100 number of incident photons 2 number of H2 molecules evolved in 1 hour 100 number of incident photons in 1 hour
The light flux incident on the reactor was evaluated by use of a digital Lutron LX101 lx meter. Details about AQY calculations are given in Supporting Information. This estimated AQY is 151
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Figure 1. Powder X-ray diffraction patterns for (a) CdS nanowires, (b) ZnO nanoparticles, (c) CdS@ZnO, and (d) CdS@Al2O3.
Figure 2. Diffuse reflectance absorption spectra for CdS NW, CdS@Al2O3, and CdS@ZnO photocatalysts. (Inset) Tauc’s plot of (αhν)2 versus photon energy for optical band-gap calculations.
lower than the real quantum yield because the number of absorbed photons is usually smaller than the number of incident photons. Also, the determination of real number of photons absorbed by the photocatalyst in a dispersed system, where light scattering comes into play, is a challenging task.
current synthetic conditions at a temperature of 140 °C without calcination. 3.2. UVVis Diffuse Reflectance Absorption Spectra. The UVvis diffuse reflectance absorption spectra of CdS NW, CdS@Al2O3, and CdS@ZnO are shown in Figure 2, where normalized absorptions of the photocatalysts are plotted against wavelength. All these spectra are recorded in the wavelength range of 325750 nm. Interestingly, all these photocatalysts show almost the same absorption profile with a steep absorption edge in 480520 nm wavelength range, which corresponds to the band gap energy for the CdS NWs. As can be seen, there are two sharp absorption steps for CdS@ZnO photocatalyst; one at ca. 380 nm is for ZnO and the other at ca. 512 nm is for CdS. The same type of absorption curve is also expected for CdS@Al2O3; however, only one step at ca. 496 nm for CdS is observed. This is because of the high band-gap energy or low wavelength value (λ < 200 nm) for the Al2O3 template. For all the systems, the completely diminished absorption curves after 570 nm indicate the absence of impurity energy level transitions. Tauc’s relationship for optical band-gap calculations for the photocatalysts is described as
3. RESULTS AND DISCUSSION 3.1. Powder X-ray Diffraction Analysis. XRD patterns for CdS NW, ZnO NP, CdS@Al2O3, and CdS@ZnO are shown in Figure 1. The crystal planes (100), (002), (101), (102), (110), (103), (112), (201), and (203) for CdS in the diffractogram (Figure 1a) can be indexed to the formation of crystalline hexagonal CdS (hex-CdS) with cell constants a = 0.4136 nm and c = 0.6713 nm (JCPDC reference 06-0314). No diffraction peaks from the other crystalline forms are detected. Interestingly, the same diffraction peaks for CdS can also be seen in the diffractograms for CdS@Al2O3 and CdS@ ZnO photocatalysts. This reveals that the structure and crystallinity of freestanding CdS is retained even after it is grown onto the oxide templates to produce CdS@oxide nanourchins. The calculated crystallite size of freestanding CdS NWs, by use of the DebyeScherrer equation from full width at half-maximum (fwhm) values of the hex-CdS planes, is found to be 28 nm. Figure 1b represents the diffractogram for the formation of ZnO NPs; the crystal planes (100), (002), (101), (102), (110), (103), (200), (112), and (201) for ZnO are indexed to the hexagonal phase of ZnO with cell constants a = 0.3249 nm and c = 0.5205 nm (JCPDC reference 05-0664). The CdS NWs we have synthesized are purely of hexagonal phase. Matsumura et al.18 had reported the advantage of hex-CdS over bulk-phase c-CdS toward higher H2 production rate and higher photoefficiency. In general, an easy and effective process to synthesize hex-CdS is calcination. Increased calcination temperature favors good crystallinity of CdS. But calcination at high temperature decreases the surface area of the photocatalyst. Moreover, high temperature increases the possibility of formation of CdO from CdS species, wherein CdO acts as a photocatalytic activity inhibitor because of its more positive CB position than the redox potential of H+/H2.19 Thus, the XRD results clearly point out that CdS NWs can be easily synthesized and grown onto the metal oxide cores under the
ðαhνÞ1=n ¼ Cðhν Eg Þ
ð1Þ
where α is the absorption coefficient of the semiconductor at a certain value of wavelength λ, h is Planck’s constant, C is the proportionality constant, ν is the frequency of light, Eg is the band gap energy, and n = 1/2 and 2 for direct and indirect transition mode materials, respectively. The absorption coefficient is estimated from the equation 1 It 1 ð2Þ ¼ A log e α ¼ ln t t I0 where A, t, It, and I0 represent the absorbance, thickness of the photocatalyst film, intensity of transmitted light, and intensity of incident light, respectively. The inset to Figure 2 shows Tauc’s plot for the CdS systems, where (αhν)2 is plotted against the photon energy (hν) and the band gap energies are estimated from the extrapolated lines as shown. The band gaps obtained 152
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Figure 3. (a) Transmission electron microscopy, (b) selected area energy diffraction pattern, and (c) energy-dispersive X-ray spectroscopy of CdS nanowires.
Figure 4. (ac) Scanning electron microscopic images of CdS@Al2O3 nanourchins at different magnifications. (d) Energy-dispersive X-ray analysis of CdS@Al2O3 nanourchins.
from Tauc’s plot are found to be 2.45 eV for free CdS NWs, 2.48 eV for CdS@Al2O3, and 2.41 eV for CdS@ZnO. 3.3. Material Morphology. A representative TEM image of the freestanding CdS NWs synthesized by the hydrothermal method is shown in Figure 3a. As can be seen from Figure 3b, the selected area energy diffraction (SAED) pattern clearly reveals the crystallinity of pure hex-CdS. Electron-dispersive X-ray spectroscopy (EDX) of the CdS NWs shown in Figure 3c confirms the presence of Cd and S only. More confirmation for the homogeneous impregnating growth of CdS NRs over the metal oxide templates to generate an urchinlike shape is carried out by SEM imaging of CdS@oxide. Figure 4ac represents the morphology of CdS@Al2O3 nanourchins. From Figure 4a it can be seen that the CdS NWs exhibit organized urchinlike architectures on the scale of 110 μm. It is interesting that each nanourchin is made up of many CdS NWs aligned in a radial way (Figure 4b). Figure 4d shows the EDX analysis of CdS@Al2O3 nanourchin and indicates the presence of Cd, S, Al, and O in the sample. The SEM image of CdS@ZnO photocatalyst, represented in Figure 5a, confirms the
formation of nanourchinlike structure of CdS@ZnO. Figure 5b shows the EDX pattern of CdS@ZnO that indicates the presence of Cd, S, Zn, and O in the sample. The urchinlike morphology of CdS@ZnO is further supported by the TEM image in Figure 5c, where the star-shaped particle represents the CdS NWs grown over ZnO NP and the rodlike structure near the star are partially overlapped bare CdS NWs. The sample for the TEM image shown in Figure 5c was prepared from the supernatant of the CdS@ZnO dispersion having smaller size in order to see the clear structure of the heteroarrays. A similar kind of star-shaped TEM image has been reported for CdS@ZnO in our previous work.16 Thus, the SEM and TEM images of the photocatalysts demonstrate the urchinlike heteroarrays by growing CdS NWs onto a wide-band-gap semiconductor core of choice. 3.4. BET Surface Area Analyses. Figure 6 shows N2 adsorptiondesorption isotherm and corresponding curves of the pore size distribution (inset) for hierarchical CdS@ZnO and CdS@ Al2O3 along with CdS NWs. According to the Brunauer DemingDemingTeller (BDDT) classification, the majority of physisorption isotherms can be grouped into six types.20 All 153
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Figure 5. (a) Scanning electron microscopic, (b) energy-dispersive X-ray spectroscopic, and (c) transmission electron microscopic images for CdS@ZnO nanourchins.
Figure 6. Nitrogen adsorptiondesorption isotherms and pore-size distribution plots for CdS NWs, CdS@Al2O3, and CdS@ZnO photocatalysts. (Inset) Plot of pore area versus pore diameter. Figure 7. BrunauerEmmettTeller surface area plot and amounts of H2 generated from CdS bulk, CdS nanowire (NW), CdS@Al2O3, and CdS@ZnO photocatalysts. (Inset) Percentage apparent quantum yields for CdS NW, CdS@Al2O3, and CdS@ZnO photocatalysts.
the present samples display typical type IV isotherms and type H3 hysteresis loops, which are the characteristic feature of mesopores. As can be seen (inset, Figure 6), CdS NWs, CdS@Al2O3, and CdS@ZnO have similar isotherms with almost the same pore diameter, which indicates that the surface morphology of CdS NWs remains similar even after its growth on ZnO and Al2O3. The BET surface areas for CdS NWs, CdS@Al2O3, and CdS@ZnO are shown in Figure 7. The BET surface areas for CdS@Al2O3 (87 m2 3 g1) and CdS@ZnO (56 m2 3 g1) nanourchins are increased in contrast to that of CdS NW (30 m2 3 g1), which is attributed to the formation of urchinlike morphology of CdS@oxide. 3.5. Photocatalytic Hydrogen Generation. Photocatalytic H2 production rates for the designed photocatalysts CdS NWs, CdS@Al2O3, and CdS@ZnO under visible-light irradiation in the presence of sacrificial agents are shown in Figure 7. From the figure, the H2 evolution rates are determined to be 530, 724, and 1008 μmol/h for CdS NWs, CdS@Al2O3, and CdS@ZnO, respectively. The inset to Figure 7 represents the percentage AQY values for the photocatalysts; AQY for CdS NWs, CdS@Al2O3, and CdS@ZnO are found to be 8%, 11%, and 15%, respectively. As can be seen from Table 1, CdS@Al2O3 and CdS@ZnO hierarchical structures show higher AQY than most of the previously reported CdS-based photocatalysts from the literature.
The enhanced H2 production rates for crystalline CdS@Al2O3 and CdS@ZnO in contrast to CdS NWs are due to the urchinlike morphology with enhanced BET surface area. It is worth noting that CdS@ZnO has a high rate of H2 production in spite of having lower BET surface area than CdS@Al2O3. This increased hydrogen production rate is attributed to the facile intersystem charge transfer in the case of CdS@ZnO. The proposed mechanism of charge transfer in the case of CdS@ZnO is shown in the schematic of Figure 8. The band positions of CdS and ZnO2729 clearly elucidate the feasibility of intersystem charge transfer in CdS@ZnO nanourchin from CdS to ZnO due to the more negative potentials of the CB and VB edges of CdS than those of ZnO. Furthermore, the nanourchin design of CdS@ZnO constructs an intimate contact between CdS and ZnO, which is crucial to promote the intersystem electron transfer between them. The visible light irradiation generates electronhole pairs in CdS, but no charges are generated in ZnO, which increases the probability of the transferred electrons being accumulated in the CB of ZnO. These electrons are then utilized for the reduction of H+ ions to form atomic H, which finally generates H2 gas. At the same time, the lower positive potential of VB of CdS with respect 154
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Table 1. AQY Values of CdS-Based Photocatalysts for Hydrogen Generation under Visible Light Irradiation photocatalyst
synthetic method
AQYa (%)
cocatalyst RuO
7
CdSMn
hydrothermal
Cd1xZnxS (x = 0.2)
thermal sulfurization
10.23
1
Pt/CdS NWs
chemical deposition
3.9
1
CdS/ZnO
two-step precipitation
Pt
3.2
21
SrS/CdS
precipitation
Pt
5.83
22
CdS/ZnS
H2S thermal sulfurization
10.2
CdS/zirconiumtitanium phosphate
5.84
CdS/graphene nanosheet CdS/MWCNT
solvothermal hydrothermal
Pt
CdS/KNbO3
ion adsorption, precipitation
NiO
22.5 (420 nm) 2.16 (420 nm)
Ni/NiO/KNbO3/CdS
a
ref 1
1 23 24 25
8.8
1
4.4
26
CdS@Al2O3 nanourchin
hydrothermal
11
present work
CdS@ZnO nanourchin
hydrothermal
15
present work
The value of AQY varies with the evaluation method and the system of interest.
BET surface area data and photocatalytic activity measurements. This photocatalytic process shows a way to both solar energy harvesting and water electrolysis into a semiconductor photoelectrode. The primary requirement for exhibiting good photocatalytic performance by a heterosystem-based semiconductor photocatalyst are 3-fold: (i) good crystallinity to retard charge recombination, (ii) high surface area by size optimization into nanodimension, and (iii) higher contact area of semiconductor surface and electrolyte responsible for hole scavenging. To achieve these requirements we have chosen the nanourchin model, which also has the ability to cause less coverage of CdS over the templates. The use of wide-band-gap templates has shown higher activity for H2 production in the highly crystalline nanourchins. In the case of CdS@Al2O3 the enhanced photoactivity is solely due to its higher surface area, whereas the same in CdS@ZnO is based on two facts: first, the enhanced surface area, and second, the charge-transfer process between CdS and ZnO, resulting in effective physical separation of the photogenerated electrons and holes.
Figure 8. Schematic of band structures for CdS and ZnO and electron transfer in the CdS@ZnO nanourchin.
to ZnO restricts the photogenerated holes to remain in the VB of CdS itself. The sacrificial agents consume these holes in the VB rapidly, preventing the electronhole pair recombination. It is noteworthy that the formation of nanourchin approach is different than the conventional coreshell structures, because in coreshell structures the accessibility of the core surface is prevented due to its complete coverage by the shell. The uniqueness of the urchinlike structures lies in the sense that although CdS NWs are impregnated on the core oxide layers, still the surface of the core is accessible for the reduction of water molecules.
4. CONCLUSIONS Highly crystalline novel 1D CdS NWs with average diameter around 2530 nm have been prepared as template-free and over Al2O3and ZnO templates by a hydrothermal route, and their efficient photocatalytic H2 production from water splitting is evaluated in the presence of sacrificial agents. With the help of powder XRD, it is shown that the crystallinity and hexagonalphase purity of freestanding CdS NWs are retained even after they are grown on Al2O3 and ZnO cores. It is shown from the UVvis absorption curves that CdS@Al2O3 and CdS@ZnO exhibit almost the same band-gap energies corresponding to freestanding CdS NWs. The specific morphology of these heteroarrays has developed high surface area, which in turn enhances the number of surface reaction sites for photocatalytic water splitting. The AQY of 8% for CdS NWs has been enhanced up to 11% and 15% by growing hierarchically over Al2O3 and ZnO, respectively. This interpretation is further supported by the
’ ASSOCIATED CONTENT
bS
Supporting Information. Two figures, showing emission spectrum of the tungsten halogen lamp and schematic of photochemical experimental setup, and additional text giving the derivation of AQY from the lamp intensity and emission profile. This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail
[email protected]; tel +91-361-2582320; fax +91-3612582349. Author Contributions ‡
These authors contributed equally to this work.
’ ACKNOWLEDGMENT We acknowledge the Department of Science and Technology (DST), India, for financial assistance via Grant SR/S1/IC-25/2009 and Department of Atomic Energy, BRNS Project 2010/37P/11/ 155
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BRNS. Infrastructure and instrumental facilities from Indian Institute of Technology, Guwahati, are acknowledged.
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