ARTICLE pubs.acs.org/JPCC
Macroporous V2O5BiVO4 Composites: Effect of Heterojunction on the Behavior of Photogenerated Charges Juan Su,†,‡ Xiao-Xin Zou,†,‡ Guo-Dong Li,*,† Xiao Wei,‡ Chang Yan,† Yu-Ning Wang,† Jun Zhao,† Li-Jing Zhou,† and Jie-Sheng Chen*,‡ †
State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, P. R. China ‡ School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China
bS Supporting Information ABSTRACT: Macroporous V2O5BiVO4 composites with a heterojunction structure have been successfully synthesized under the assistance of colloidal carbon spheres. X-ray diffraction, Raman, and X-ray photoelectron spectroscopies reveal that the as-prepared composites are composed of monoclinic BiVO4 and orthorhombic V2O5. The behavior of photogenerated charges in V2O5, BiVO4, and the V2O5BiVO4 composite have been investigated through surface photovoltage spectroscopy (SPS) and transient photovoltage (TPV) techniques. It is demonstrated that the formation of heterojunction structure in the V2O5BiVO4 composite plays an important role in the kinetic behaviors (including separation, transport, and recombination) of photogenerated charges. The heterojunction greatly increases the separation extent and the lifetime of the photogenerated charges in the composites.
’ INTRODUCTION Composite semiconductor materials with a heterojunction structure have been extensively exploited in many fields such as photocatalysis and solar energy conversion15 because heterojunction dominates the behaviors, including the direction of transport, the distance of separation, and the rate of recombination, of photogenerated charges in semiconductors.6 In composite semiconductor materials, a heterojunction interface is constructed between the semiconductors with matching band potentials, and accordingly a contact electric field is built at this heterojunction interface. Driven by the contact electric field, photogenerated charges can transport from one semiconductor to another, leading to efficient separation of photogenerated electronhole pairs.79 Therefore, rational design of heterojunction semiconductors and study of the heterojunction effect on the behavior of photogenerated charges are important and desired for the exploitation of composite materials with advanced functions. Recently, monoclinic bismuth vanadate (m-BiVO4) has garnered considerable attention as a promising photocatalyst working under visible light.10,11 With a narrow band gap (2.4 eV), this compound has been extensively investigated as an attractive advanced material for the photocatalytic degradation of organic pollutants12,13 and the evolution of photocatalytic O2.1416 Nevertheless, low separation efficiency of electronhole pairs is always the main drawback of m-BiVO4 for application. Coupling m-BiVO4 with another semiconductor with matching r 2011 American Chemical Society
band potentials to form a heterojunction is an effective approach to improve the separation of photogenerated electronhole pairs.68 As a promising BiVO4-based coupled semiconductor, V2O5BiVO4 was reported to exhibit improved photocatalytic performance in comparison with bare BiVO4.17 However, the mechanism for the enhancement of V2O5BiVO4 photocatalytic activity and the nature of the composite material have not been elucidated yet. Herein, we report macroporous V2O5BiVO4 composites prepared under the assistance of colloidal carbon spheres (CCSs). Surface photovoltage spectroscopy (SPS) and transient photovoltage (TPV) techniques have been employed to investigate the effect of heterojunction structure on the behavior of photogenerated charges in these composite materials.
’ EXPERIMENTAL SECTION Chemical Reagents. All chemical reagents used in the present experiments were of analytical grade. Glucose and ammonium metavanadate were all purchased from Sinopharm Reagent Chemical Co., Ltd., whereas bismuth nitrate pentahydrate (Bi(NO3)3 3 5H2O) was purchased from Shantou Xilong Chemical Factory Co., Ltd. Received: November 15, 2010 Revised: March 27, 2011 Published: April 05, 2011 8064
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The Journal of Physical Chemistry C Preparation of Colloidal Carbon Spheres. Colloidal carbon spheres (CCSs) were prepared on a large scale by following the reported procedure.18 Glucose (7.2 g) was dissolved in deionized water (40 mL) to form a clear solution, and the solution was loaded in a 50 mL Teflon-sealed autoclave, which was then heated at 180 °C for 10 h. The resulting product was collected after the autoclave was cooled naturally to room temperature. Synthesis of V2O5BiVO4 Heterojunction Semiconductors. To the freshly prepared CCSs were added 60 mL of deionized water, and the mixture was ultrasonicated. The as-formed suspension was divided into two parts of equal volume, to which were added Bi(NO3)3 3 5H2O (1.455 g) and an appropriate amount of NH4VO3, respectively. After stirring for 30 min, the two suspensions were mixed up, and the mixture was subsequently heated at 90 °C for 20 h under continuous stirring. A deep-green precipitate was obtained after centrifugation, and the obtained solid was washed thoroughly with deionized water and dried at 60 °C for 12 h. Finally, the dried sample was calcined at 450 °C for 2 h in air. The asprepared coupled semiconductor material is designated x wt % V2O5BiVO4 (x wt % is the mass percentage of V2O5 in V2O5BiVO4; x = 0, 2.7, 5.3, 14.4, 25.1, and 100). The reference materials, bare BiVO4 and bare V2O5 powders, were mixed physically for the subsequent photocatalytic reaction without further treatment. Bulk BiVO4 was prepared by solidstate reaction according to the procedures described in the lietrature.19 General Characterization. The X-ray diffraction (XRD) patterns of the as-prepared powders were recorded on a Rigaku D/Max 2550 X-ray diffractometer using Cu KR radiation (λ = 1.5418 Å) operated at 200 mA and 40 kV. The X-ray photoelectron spectroscopy (XPS) was performed on an ESCALAB 250 X-ray photoelectron spectrometer with a monochromatized X-ray source (Al KR hυ = 1486.6 eV). The energy scale of the spectrometer was calibrated using Au4f7/2, Cu2p3/2, and Ag3d5/2 peak positions. The standard deviation for the binding energy (BE) values is 0.1 eV. Deconvolution of the XPS curves was conducted by using a fitting procedure based on the summation of Gaussians (a surface analysis software: AVANTAGE 4.30). The Raman spectra were obtained with a Renishaw Raman system model 1000 spectrometer with a 20 mW air-cooled argon ion laser (514.5 nm) as the exciting source (the laser power at the sample position was typically 400 μW with an average spot size of 1 μm in diameter). The scanning electron microscopic (SEM) images were taken on a JEOL JSM 6700F electron microscope, whereas the transmission electron microscopic (TEM) and highresolution TEM (HRTEM) images were obtained on a JEOL JSM-3010 microscope. The concentration of MB was analyzed with a Shimadzu UV-2450 spectrophotometer, whereas the UVvis diffuse reflectance spectra were recorded on a PerkinElmer Lambda 20 UVvis spectrometer. Surface Photovoltage Spectroscopy (SPS) and Transient Photovoltage (TPV) Measurements. The SPS measurement system consisted of a source of monochromatic light, a lock-in amplifier (SR830-DSP) with a light chopper (SR540), a photovoltaic cell, and a computer. A 500 W xenon lamp and a grating monochromator (Omni-λ 500) were combined to provide the monochromatic light. A low chopping frequency of ∼23 Hz was used to obtain stable and intensive signals. The photovoltaic cell was a sandwich-like structure consisting of ITOsampleITO. For TPV measurement, the sample chamber was formed by a platinum wire gauze electrode as the top electrode, a glass substrate covered by ITO as the bottom electrode, and a 10 μm thick mica
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Figure 1. SEM images of (a) colloidal carbon spheres, (b) precursors of 5.3 wt % V2O5BiVO4, (c) 5.3 wt % V2O5BiVO4 with low magnification, and (d) 5.3 wt % V2O5BiVO4 with high magnification.
spacer as the electron isolator. The sample was excited with a laser radiation pulse (wavelength of 532 nm and pulse width of 5 ns) from a third-harmonic Nd:YAG laser (Polaris, New Wave Research, Inc.). The intensity of the pulse was regulated with a neutral gray filter and determined by EM500 SIGLE-CHANNEL JOULEMETER (Molectron, Inc.). The TPV signals were registered by a 500 MHz digital phosphor oscilloscope (TDS 5054, Tektronix) with a preamplifier. Photocatalytic Activity. The photocatalytic activity was assessed in aqueous solution in a water-cooled quartz cylindrical cell. Generally, the reaction mixture in the cell was maintained at about 20 °C by a continuous flow of water and was illuminated with an external light source. The visible-light source was a 500 W Xe lamp (main output >400 nm), with a glass optical filter used to cut off the short wavelength part (λ < 420 nm). The as-prepared semiconductor material (0.3 g) was mixed with an aqueous solution of methylene blue (MB) (300 mL, 1 105 mol/L), followed by magnetic stirring in the dark for at least 2 h to establish an adsorption/desorption equilibrium of MB on the particle surface of the material. The reaction system was then subject to visible-light irradiation. Each reaction cycle lasted for about 3 h during which oxygen was bubbled through the solution. At given irradiation time intervals, a series of aqueous solution samples (3 mL) were collected and separated from the suspended catalyst particles for analysis. The concentration of the MB was determined on a UVvis spectrophotometer by monitoring its characteristic absorption at 665 nm.
’ RESULTS AND DISCUSSION Synthesis of V2O5BiVO4 Composite Materials. All the macroporous V2O5BiVO4 heterojunction samples have been prepared under the assistance of colloidal carbon spheres (CCSs) (Figure 1a). In general, the formation of macroporous structures for the composites is through two steps (Figure S1, Supporting Information). First, an intermediate precipitates on the surface of the CCSs after the reflux reaction (Figure 1b), and subsequently the V2O5BiVO4 composite material with the macroporous structure is obtained after removing the CCSs through calcination (Figure 1c, d). From the SEM images it is seen that the macroporous structures are formed through the interconnection of 8065
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Figure 2. Powder X-ray diffraction patterns of (a) 5.3 wt % V2O5BiVO4, (b) 14.4 wt % V2O5BiVO4, and (c) 25.1 wt % V2O5BiVO4.
Figure 3. Raman spectra of (a) bulk BiVO4, (b) 0 wt % V2O5BiVO4, (c) 5.3 wt % V2O5BiVO4, (d) 14.4 wt % V2O5BiVO4, (e) 25.1 wt % V2O5BiVO4, and (f) 100 wt % V2O5BiVO4.
irregular nanoparticles. These particles remain intact after sonication for 30 min, suggesting that the particles in the material are tightly bound together and the macroporous structure of the composite is rather robust. General Characterization. Figure 2 shows the XRD patterns of three as-prepared V2O5BiVO4 composite samples with various V2O5/(V2O5 þ BiVO4) weight ratios. It is seen that the characteristic peaks appearing in each of the three XRD patterns can be well indexed on the basis of monoclinic BiVO4 (JCPDS No. 14-0688) and orthorhombic V2O5 (JCPDS No. 72433), suggesting that in each sample both BiVO4 and V2O5 with a good crystallinity have been formed. As the amount of V2O5 increases from 5.3 wt % to 25.1 wt %, the diffraction peaks of orthorhombic V2O5 are gradually intensified. Figure 3 presents the Raman spectra of the as-prepared V2O5BiVO4 samples in the 1001100 cm1 region. The typical Raman bands of both monoclinic BiVO4 and orthorhombic V2O5 are observed in the spectra of the V2O5BiVO4 materials. The bands at 820, 712, 367, 324, and 210 cm1 correspond to the typical vibrations of monoclinic BiVO4. The bands at 820 and 712 cm1 are assigned to the typical symmetric and antisymmetric stretching modes of VO bonds, whereas those at 367 and 324 cm1 are attributed to the typical symmetric
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and antisymmetric bending modes of the vanadate anion. The external mode appears at 210 cm1.20,21 In addition, the orthorhombic V2O5 shows the typical bands at 143 and 995 cm1 associated with the skeleton bending vibration and the vanadyl mode, respectively.22,23 As seen from the spectra, with an increase of V2O5 content from 0 to 25.1 wt %, the intensities of the typical vibration bands for the orthorhombic V2O5 are increased, whereas in the meantime, the intensities of the typical bands for the monoclinic BiVO4 are decreased accordingly. In comparison with bulk BiVO4, the VO bond stretching mode of BiVO4 in the V2O5BiVO4 samples shifts from 827 to 820 cm1, but no change of the band position has been observed with the increase of V2O5 content. The red-shift is due to the quantum confinement effect of the BiVO4 samples rather than to the coupling of V2O5 and BiVO4.24 The chemical states of the V2O5BiVO4 heterojunction semiconductors are further revealed through XPS analysis. As shown in the overall XPS spectra of the samples (Figure S2, Supporting Information), only the characteristic peaks of Bi, V, O, and C elements were detected. The observed peak of C1s at around 284.6 eV is attributed to the signal from carbon in the instrument.25 Although the raw material NH4VO3 contains a nitrogen element, no XPS peaks of N1s were detected at around 400 eV, indicating that no nitrogen is present in the V2O5BiVO4 composite product. The peaks located at 164.7 and 159.4 eV are assigned to the Bi4f5/2 and Bi4f7/2 (Figure 4a), respectively, confirming that the bismuth species in V2O5BiVO4 are Bi3þ cations. The very weak peaks located at about 161.2 and 166.5 eV are attributable to the presence of a very small amount of Bi4þ in the materials. It is possible that a small proportion of Bi3þ, especially on the surface of the microcrystals, has been oxidized to Bi4þ during calcination in air.26 The intensities of the two weak peaks decrease with the decrease of BiVO4 content. All the samples show similar XPS signals for O1s at 530.2 eV, which correspond to the O2 anions in both BiVO4 and V2O5 crystallites (Figure 4b). Although there are two kinds of crystal lattice oxygens in the V2O5BiVO4 composite, no difference in XPS peak position of O1s is observed as their chemical environments are similar. Because of spinorbital splitting, the V2p signal shows a doublet corresponding to V2p1/2 and V2p3/2 (Figure 4c). For monoclinic BiVO4 and orthorhombic V2O5, the V2p1/2 and V2p3/2 peaks related to V5þ are located at different positions due to the different chemical environments of the V5þ cations in the two materials. The V5þ species in BiVO4 and in V2O5 are designated as V1 and V2, respectively. V1 gives rise to V2p1/2 and V2p3/2 peaks at 524.3 and 517.1 eV, and the peaks at 525.3 and 518.0 eV are assigned to V2.27,28 As the content of V2O5 increases from 0 to 100 wt %, the V2p1/2 peak shifts from 524.3 to 525.3 eV, and the V2p3/2 peak shifts from 517.1 to 518.0 eV. The V2p peaks of the asprepared composite samples are deconvoluted into two components corresponding to V1 and V2 (Figure 4df). Quantitative analysis shows that the ratios of V1 and V2 are consistent with the ratio of V2O5 and BiVO4 in the composites (Table S1, Supporting Information). Therefore, the V2p1/2 and V2p3/2 peaks of the asprepared composite samples originated from V1 in BiVO4 and V2 in V2O5, and the peak shift of V2p is caused by the variation of V2O5 content in the V2O5BiVO4 heterojunction samples. The optical properties of the as-prepared samples are probed by UVvis diffuse reflectance spectroscopy (Figure 5). The bandgaps of V2O5 and BiVO4 are estimated to be 2.26 and 2.48 eV, respectively, on the basis of the UVvis spectra of the samples. The optical absorptions of the V2O5BiVO4 composites start at about 550 nm, corresponding to the absorption edge of V2O5. 8066
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Figure 4. XPS spectra of (a) Bi4f, (b) O1s, and (c) V2p for V2O5BiVO4 with different V2O5 contents. (d)(f) V2p for V2O5BiVO4 with different V2O5 contents after deconvolution.
Transmission electron microscopy (TEM), as an efficient and widely used characterization means of heterojunction,2931 has been used to prove the formation of heterojunctions in the V2O5BiVO4 composite material (Figure 6). The low magnification TEM image of the 14.4 wt % V2O5BiVO4 composite reveals that BiVO4 and V2O5 nanoparticles of the composite are tightly bound together (Figure 6a), whereas the high-resolution transmission electron microscopy (HRTEM) images of the different parts in Figure 6a clearly show (Figure 6be) interfaces of monoclinic BiVO4 and orthorhombic V2O5 with corresponding d-spacings. The fringes of d = 0.226 nm match the (211) crystallographic planes of monoclinic BiVO4, while the fringes of d = 0.437 nm and d = 0.340 nm match the (010) and (101) crystallographic planes of orthorhombic V2O5 nanoparticles, respectively. It is therefore concluded that the heterojunctions of BiVO4 and V2O5 are rather popular in the composite sample.
Effect of Heterojunction on the Behavior of Photogenerated Charges. To elucidate the effect of heterojunction on the
behavior of photogenerated charges, surface photovoltage spectroscopy (SPS)3234 and transient photovoltage (TPV) techniques are employed, and the kinetic behaviors of the photogenerated charges in the as-prepared semiconductor materials are revealed.3537 The morphologies and surface areas for all the V2O5BiVO4 samples (Figure S3, Table S2, Supporting Information) are similar, and therefore, the effect of morphology and surface area on the behavior of photogenerated charges for these samples is negligible.38 The signal of surface photovoltage (SPV) is attributed to the change of surface potential barriers before and after light illumination. The SPV amplitude as a function of the incident wavelength reveals the light-responsive wavelength range and the separation extent of the photogenerated charges in the 8067
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semiconductor materials.39,40 Figure 7 shows the SPV spectra of the samples. An apparent SPV response ranging from 300 to 525 nm is observed for bare BiVO4 (0 wt % V2O5BiVO4) and bulk BiVO4, whereas a quite weak SPV response is observed for bare V2O5. In addition, the SPV response region of the V2O5BiVO4
composite is not widened by the V2O5 coupling, indicating that the photogenerated electronhole pairs in bare V2O5 cannot be separated effectively.41 Compared to the bulk BiVO4, bare BiVO4 (0 wt % V2O5BiVO4) exhibits a distinctly stronger SPV response, indicating that the photogenerated charges are more effectively
Figure 5. UVvis diffuse reflectance spectra of the as-prepared samples.
Figure 7. SPV spectra of bulk BiVO4 and the as-prepared V2O5BiVO4 samples.
Figure 6. (a) TEM and (b)(e) HRTEM images of the V2O5BiVO4 composite. The HRTEM images correspond to the parts marked in (a). 8068
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one positive photovoltage response is observed, and the lifetime of the response is rather short, corresponding to the low separation efficiency of the photogenerated charges in bare V2O5 observed by SPV spectroscopy (Figure 7). In contrast, the bare BiVO4 gives rise to a negative TPV response with a significantly longer lifetime, corresponding to the higher separation efficiency of the photogenerated charges than that of V2O5. Differing from bare V2O5 and BiVO4, the V2O5BiVO4 coupled semiconductor shows an initial negative response in the TPV spectrum, and this response is reversed to positive at 7 106 s. The intensities of both the positive and negative TPV signals are stronger than those of the bare BiVO4 and V2O5, whereas the lifetime of the TPV response for the composite is longer than those of the two bare materials. These TPV features of V2O5BiVO4 can be explained on the basis of formation of a contact electric field (Figure 8b) at the interface of V2O5BiVO4 composite. As shown in Figure 8b, the conduction band (CB) and valence band (VB) potentials of BiVO4 and V2O5 at the point of zero charge can be calculated by the following empirical equation.45,46 EVB ¼ X Ee þ 0:5Eg
Figure 8. (a) Transient photovoltage (TPV) responses of bare BiVO4, bare V2O5, and 5.3 wt % V2O5BiVO4. (b) Schematic diagram for electronhole separation at the interface of the V2O5BiVO4 material.
separated for the latter material. The improved separation of photogenerated charges is due to the smaller crystal size of the as-prepared bare BiVO4 (0 wt % V2O5BiVO4) (Table S2, Supporting Information). The small crystal size shortens the transfer path of photogenerated charges from bulk to surface and thus promotes the separation efficiency of the charges.42 On the other hand, the SPV response intensity increases as the V2O5 content in the V2O5 BiVO4 composite increases from 0 wt % to 25.1 wt %, passing through a maximum at 5.3 wt %. The increased SPV intensity indicates that the introduction of a suitable amount of V2O5 is beneficial to the separation of photogenerated electronhole pairs in BiVO4.43 Nevertheless, if an excessive amount of V2O5 is involved in the coupled V2O5BiVO4 composite system, the light absorption by BiVO4 is reduced as more V2O5 nanoparticles prevent the light from reaching the BiVO4 particles, and as a result the SPV signal is weakened in comparison with that for the 5.3 wt % V2O5BiVO4 sample. TPV spectroscopy has been further used to elucidate the dynamic property of photogenerated charges in the V2O5 BiVO4 composite material.35 From the transient spectra, the moving direction of the photogenerated charges can be deduced from the signs of the TPV responses.36,37 A positive sign implies that positive charges accumulate on the sample surface at the top electrode under irradiation, whereas a negative sign is attributed to negative charge accumulation on the same sample surface.6,44 Figure 8a presents the TPV spectra of bare BiVO4, bare V2O5, and the 5.3 wt % V2O5BiVO4 composite. For bare V2O5, only
where EVB is the VB edge potential; X is the electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms (X values for BiVO4 and V2O5 are ca. 6.035 and 6.100 eV,47,48 respectively); Ee is the energy of free electrons on the hydrogen scale (4.5 eV); and Eg is the band gap energy of the semiconductor. The ECB value can be obtained by ECB = EVB Eg. The CB edge potential of BiVO4 (0.29 eV) is more negative than that of V2O5 (0.47 eV), and thus a difference of band potentials exists between the two materials. This potential difference induces a contact electric field at the interface of the V2O5BiVO4 composite. When electrons and holes are photogenerated in BiVO4, the photogenerated electrons migrate to the surface of BiVO4, and a negative TPV response is observed, just as in the case of bare BiVO4. Afterward, a part of the photogenerated electrons in BiVO4 is driven by the contact electric field, causing electrons to be enriched at the interface of the V2O5BiVO4 composite. Due to the fact that the VB potentials for BiVO4 and V2O5 are very close, the photogenerated holes on the valence band of BiVO4 do not move to that of V2O5, and as a result the holes can stay on the surface of BiVO4, reversing the TPV response from negative to positive. The separate enrichment of electrons and holes also explains the origin of improved separation and lifetime of photogenerated charges in the V2O5BiVO4 composite. In general, a photocatalytic process of semiconductor materials involves the formation of photogenerated electrons in the conduction band and holes in the valence band, and the subsequent chemical reactions with the surrounding media after the photogenerated charges move to the particle surface. It is obvious that the photogeneration and separation of the electronhole pairs in a semiconductor are the key factors to influence a photocatalytic reaction.9,49 Therefore, the photocatalytic performance can effectively reflect the separation efficiency of the photogenerated electron hole pairs and the lifetime of the charge carriers. The photocatalytic performances of the as-prepared photocatalysts were evaluated by testing the decoloration of methylene blue (MB) under visible-light irradiation. Prior to irradiation, the photocatalytic reaction systems were magnetically stirred in the dark for at least 2 h, and through this treatment the adsorption/ 8069
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Figure 9. Comparison of photocatalytic performance of the samples by testing the decoloration of methylene blue (MB) under visible-light irradiation.
desorption equilibrium of MB on the particle surface of the catalyst was reached. The MB concentration measured after this treatment was used as the initial concentration to evaluate the concentration variation, and it is reasonable to conclude that MB adsorption on the surface of the catalyst had no contribution to the concentration variation of MB during the photocatalytic process. Furthermore, the reaction system was oxygen-saturated and continuously stirred. In this way, the MB molecules could be prevented from being reduced to colorless leuco form (LMB).50 Therefore, the observed photobleaching in the photolysis process should be mainly due to the oxidative photodegradation of the dye molecules. Figure 9 displays the time course of the decrease in dye concentration with the irradiation of the visible light (λ > 420 nm). A control experiment in the absence of photocatalyst demonstrates that no significant change in MB concentration occurs. The first set of MB concentration data was collected at about 20 min of irradiation, and we see that the concentrations for the systems with different catalysts vary. Especially, the MB concentration (about 67% of the initial concentration) at this irradiation time for the system containing the 5.3 wt % V2O5BiVO4 catalyst is distinctly lower than those for the systems containing other catalysts. As shown in Figure 9a, the decrease of MB concentration in the presence of bulk BiVO4 is small (only about 54%) after visible irradiation for 180 min.
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The higher photocatalytic activity of bare BiVO4 (0 wt % V2O5BiVO4) proves the SPV observation that size miniaturization effectively promotes the separation efficiency of the photogenerated electronhole pairs. Furthermore, the combination of a small amount of V2O5 (5.3%) over BiVO4 leads to a remarkable increase of MB degradation from 73% to 92%. The photocatalytic activity of the 5.3% V2O5/BiVO4 composite photocatalyst is also distinctly superior to that of the physical mixture of V2O5 and BiVO4 with the V2O5 mass percentage of 5.3% (66%) (Figure 9a). This enhancement in photocatalytic performance for the V2O5BiVO4 composite material is considered to arise from the presence of heterojunction structure between the two components (V2O5 and BiVO4) of the coupled semiconductor.9,43 As shown in Figure 9b, the photocatalytic activity of the coupled V2O5BiVO4 material increases and then decreases after reaching a maximum value as the proportion of V2O5 increases from 0 wt % to 25.1 wt %. The 5.3 wt % V2O5 BiVO4 is found to exhibit the highest photocatalytic activity, and about 92% of MB is photodegraded from the aqueous solution after visible-light irradiation for 180 min. The photocatalytic performance observation is consistent with the SPS result; that is, the presence of distinctly excessive V2O5 in the V2O5BiVO4 composite shields light for the BiVO4 component. To demonstrate that the degradation of MB is realized through photocatalysis rather than through photosensitization of MB, a cutoff filter (λ > 600 nm) which can remove the light with a wavelength shorter than 600 nm was used to ensure that only MB was excited by light during the photocatalysis process. MB absorbs visible light with a wavelength longer than 600 nm (the absorption peak is at about 665 nm), whereas the photocatalyst we used does not absorb the visible light in this region. The experimental result shows that under this circumstance there is no change in the MB concentration after 180 min light illumination, clearly indicating that MB photosensitization does not contribute to the degradation of the dye molecules.
’ CONCLUSIONS We have successfully prepared macroporous V2O5BiVO4 composite materials under the assistance of colloidal carbon spheres. Surface photovoltage and transient photovoltage measurements demonstrate that the combination of V2O5 and BiVO4 exerts considerable influence on the behavior of photogenerated charges in the as-prepared semiconductor materials. The V2O5 coupling results in the formation of V2O5BiVO4 heterojunctions which greatly increase the separation extent and lifetime of the photogenerated charges. Consequently, the V2O5BiVO4 composite materials exhibit enhanced photocatalytic performance. Our results show that heterojunction formation from different semiconductors plays an important role in determining the physical features of photogenerated charges, and the photocatalytic properties of the composite materials are closely related with the heterojunctions. The surface photovoltage and the transient photovoltage techniques can be used to elucidate the photoelectric processes of the heterojunction structures effectively, whereas a better understanding of photoelectric behavior is very conducive to exploitation of composite semiconductor materials with advanced functions. ’ ASSOCIATED CONTENT
bS
Supporting Information. Schematic representation for the synthesis of V2O5BiVO4 composites, overall XPS spectra,
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The Journal of Physical Chemistry C supplementary SEM images, quantitative analysis of deconvoluted XPS spectra, and BET surface areas of bulk BiVO4 and V2O5BiVO4 samples. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was financially supported by the National Basic Research Program of China (2007CB613303) and the National Natural Science Foundation of China (20731003 and 21071060). We thank Mingyi Guo and Chunguang Li for the TEM measurement. ’ REFERENCES (1) Wang, P.; Huang, B.; Qin, X.; Zhang, X.; Dai, Y.; Wei, J.; Whangbo, M. H. Angew. Chem., Int. Ed. 2008, 47, 7931–7933. (2) Yang, D.; Liu, H.; Zheng, Z.; Yuan, Y.; Zhao, J.; Waclawik, E. R.; Ke, X.; Zhu, H. J. Am. Chem. Soc. 2009, 131, 17885–17893. (3) Shankar, K.; Mor, G. K.; Prakasam, H. E.; Varghese, O. K.; Grimes, C. A. Langmuir 2007, 23, 12445–12449. (4) Kamat, P. V. J. Phys. Chem. C 2007, 111, 2834–2860. (5) Kuang, Q.; Lao, C. S.; Zhou, L.; Liu, Y. Z.; Xie, Z. X.; Zheng, L. S.; Wang, Z. L. J. Phys. Chem. C 2008, 112, 11539–11544. (6) Wei, X.; Xie, T. F.; Xu, D.; Zhao, Q. D.; Pang, S.; Wang, D. J. Nanotechnology 2008, 19, 275707. (7) Kim, H. G.; Borse, P. H.; Choi, W. Y.; Lee, J. S. Angew. Chem., Int. Ed. 2005, 44, 4585–4589. (8) Tada, H.; Mitsul, T.; Kiyonaga, T.; Akita, T.; Tanaka, K. Nat. Mater. 2006, 5, 782–786. (9) Lin, X.; Xing, J.; Wang, W.; Shan, Z.; Xu, F.; Huang, F. J. Phys. Chem. C 2007, 11, 18288–18293. (10) Xi, G. C.; Ye, J. H. Chem. Commun. 2010, 46, 1893–1895. (11) Zhou, B.; Zhao, X.; Liu, H.; Qu, J.; Huang, C. P. Appl. Catal., B: Environ. 2010, 99, 214–221. (12) Sun, S.; Wang, W.; Zhou, L.; Xu, H. Ind. Eng. Chem. Res. 2009, 48, 1735–1739. (13) Yin, W.; Wang, W.; Shang, M.; Zhou, L.; Sun, S.; Wang, L. Eur. J. Inorg. Chem. 2009, 4379–4384. (14) Sayama, K.; Nomura, A.; Arai, T.; Sugita, T.; Abe, R.; Yanagida, M.; Oi, T.; Iwasaki, Y.; Abe, Y.; Sugihara, H. J. Phys. Chem. B 2006, 110, 11352–11360. (15) Kudo, A. Int. J. Hydrogen Energy 2006, 31, 197–202. (16) Yu, J.; Kudo., A. Adv. Funct. Mater. 2006, 16, 2163–2169. (17) Jiang, H.; Nagai, M.; Kobayashi, K. J. Alloys Compd. 2009, 479, 821–827. (18) Sun, X. M.; Li, Y. D. Angew. Chem., Int. Ed. 2004, 43, 597–601. (19) Gotic, M.; Music, S.; Ivanda, M.; Soufek, M.; Popovic, S. J. Mol. Struct. 2005, 744747, 535–540. (20) Li, G. S.; Zhang, D. Q.; Yu, J. C. Chem. Mater. 2008, 20, 3983–3992. (21) Zhang, A.; Zhang, J. Mater. Lett. 2009, 63, 1939–1942. (22) Wu, Z. L.; Kim, H. S.; Stair, P. C.; Rugmini, S.; Jackson, S. D. J. Phys. Chem. B 2005, 109, 2793–2800. (23) Ramana, C. V.; Smith, R. J.; Hussian, O. M.; Massot, M.; Julien, C. M. Surf. Interface Anal. 2005, 37, 406–411. (24) Adu, K. W.; Gutierrez, H. R.; Eklund, P. C. Vib. Spectrosc. 2006, 42, 165–172. (25) Wang, H.; Wu, Z.; Liu, Y. J. Phys. Chem. C 2009, 113, 13317–13324. (26) Liu, H.; Nakamura, R.; Nakato, Y. J. Electrochem. Soc. 2005, 152, G856–G861.
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