Enhanced Visible-Light Photoactivity of CuWO4 through a Surface

Apr 28, 2014 - Haihang Chen, Wenhua Leng, and Yiming Xu*. State Key Laboratory of Silicon Materials and Department of Chemistry, Zhejiang University, ...
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Enhanced Visible-Light Photoactivity of CuWO4 through a SurfaceDeposited CuO Haihang Chen, Wenhua Leng, and Yiming Xu* State Key Laboratory of Silicon Materials and Department of Chemistry, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: Several papers have shown that CuWO4 is active under visible light for water oxidation at an applied potential bias and for organic degradation in an aerated aqueous suspension. In this work, we report that the observed reduction of O2 on the irradiated CuWO4 is a multielectron transfer process with the formation of H2O2. More importantly, the surface modification of CuWO4 with 1.8 wt % of CuO can increase the activity by approximately 9 times under UV light and by 5 times under visible light, for phenol degradation in aerated aqueous suspension. The catalyst was prepared by a hydrothermal reaction between Cu(NO3)2 and Na2WO4, followed by thermal treatment at 773 K. High-resolution transmission electron microscopy revealed that triclinic CuWO4 (40 nm) was covered by monoclinic CuO (4 nm). Through a combination of photo- and electrochemical measurement, a plausible mechanism responsible for the activity enhancement is proposed, involving an interfacial electron transfer from CuO to CuWO4 and an interfacial hole transfer from CuWO4 to CuO.

1. INTRODUCTION Semiconductor photocatalysis for environmental remediation has been studied for over 30 years.1−3 It is generally recognized that after band gap excitation a semiconductor would generate electron (ecb−) and hole (hvb+) in its conduction and valence bands, respectively. These charge carriers may recombine to heat or migrate onto the surface, reacting with suitable electron donors and acceptors. For example, the photogenerated ecb− and hvb+ on anatase TiO2 can reduce O2 to O2−• and oxidize H2O/OH− to •OH, respectively. Because of that, anatase TiO2 is the most suitable photocatalyst for environmental use. However, this TiO2 is excited only with UV light, which greatly limits the utilization of sunlight reaching the Earth’s surface. Alternatively, WO3 has been studied as a candidate of visiblelight photocatalyst.4−8 Its hvb+ has a reactivity similar to that of TiO2, so that water oxidation to both •OH and O2 has been observed with the irradiated WO3.7 However, organic degradation in the aerated aqueous suspension of WO3 is very slow in either UV or visible light. This is ascribed to the fact that the photogenerated ecb− on WO3 is not capable of O2 reduction to O2−•. The WO3-photocatalyzed reaction can notably occur only in the presence of Pt, Pd, and Ag novel metals as cocatalysts for O2 reduction4−6 or in the presence of H2O2, Fe3+, and Cu2+ as alternative electron scavengers.7,8 © 2014 American Chemical Society

Obviously, these additives and post treatment would be expensive for practical application. Therefore, development of a highly active, stable, and low-cost photocatalyst is a big challenge in this field. Recently, CuWO4 as a photocatalyst has attracted increasing interest. This semiconductor can harvest light at wavelengths up to 540 nm.9 Under simulated solar light, the CuWO4 film electrodes, fabricated by electrodeposition, spin-coating, reactive cosputtering, and spray pyrolysis, can initiate water splitting to O2 at an applied potential bias.10−15 In comparison to WO3, CuWO4 is more stable against photocorrosion in aqueous solution at neutral pH.13,15 Interestingly, CuWO4 has been also claimed to be active under visible light for the degradation of methanol, chlorophenol, methylene blue, and methyl orange in aerated aqueous suspensions.16,17 Since O2 is the only electron scavenger in these systems, it follows that the irradiated CuWO4 with visible light is capable of O2 reduction. However, the conduction band edge potential (ECB) of CuWO4, which is approximately 0.20−0.44 V vs normal hydrogen electrode (NHE) in aqueous solution at pH 0,10,13−15 Received: March 16, 2014 Revised: April 28, 2014 Published: April 28, 2014 9982

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2. EXPERIMENTAL SECTION Reagents. Copper(II) nitrate trihydrate and tungstate(VI) sodium dihydrate were purchased from Shanghai Chemicals Inc. and Sinopharm Chemical Reagent Co., Ltd., respectively. Textile dye X3B (Reactive Orange 86) in a purity of 98% was obtained from Jining dye manufacture of China. Other chemicals including phenol were analytical grade and used as received. Synthesis. CuO/CuWO4 was prepared by using a hydrothermal method. Typically, 50 mL of 0.1 M Na2WO4·2H2O was added dropwise to 50 mL of 0.1 M Cu(NO3)2·3H2O. The aqueous solution was adjusted to pH 8.5 with NaOH and stirred further for 1 h. Then the blue-sky suspension was transferred into a stainless autoclave and heated at 170 °C for 20 h. After cooling to room temperature, the particles was collected by centrifuge, washed thoroughly with water, and dried at 60 °C for 10 h. Finally, the solid was sintered in air at 500 °C for 3 h. Three reference samples were prepared as follows. (1) CuO was prepared in a manner similar to that used above, except that Na2WO4·2H2O was not added. (2) CuWO4 was prepared by using a precipitation method.16 First, 0.01 mol of Cu(NO3)2·3H2O was dissolved in 250 mL of water, and the solution pH was adjusted to 5.0 with NaOH or HCl, followed by heating to 60 °C. Then, to this above solution, 0.01 mol of Na2WO4·2H2O was added under vigorous stirring, followed by heating at 90 °C for 3 h. After cooling to room temperature, the particles were collected by centrifuge, washed thoroughly with water, and dried at 60 °C for 10 h. Finally, CuWO4 was sintered in air at 500 °C for 3 h. (3) A simple mixture, denoted as SCuO/CuWO4, was prepared by dispersing 0.018 g of CuO and 1.0 g of CuWO4 in 50 mL of distilled water with vigorous stirring. The suspension was then dried at about 80 °C through a rotary evaporator. Characterization. X-ray powder diffraction (XRD) pattern was recorded on a D/max-2550/PC diffractometer (Rigaku). From the integrated intensity at the 2θ angle of 19°, the average crystallite size of CuWO4 was calculated by using the Scherrer equation. The Brunauer−Emmett−Teller (BET) specific surface area (SBET) and pore volume (Vp) were calculated from the N2 adsorption isotherm, measured at 77 K on a Micromeritics ASAP2020 apparatus. Scanning electron microscope (SEM) measurement was performed on a Hitachi S4800, attached with energy-dispersive X-ray spectroscopy (EDS). The HRTEM image was obtained with a Tecnai G2 electron microscope (FEI, Netherlands). X-ray photoelectron spectroscopy (XPS) was made with a Kratos AXIS UItra DLD spectrometer. Diffuse reflectance spectra were recorded on a Shimadzu UV-2550 with BaSO4 as a reference. Reactions and Analysis. The reactor was made of a Pyrex glass (inner diameter 2.9 cm and height 9.1 cm), thermostated at 25 °C through a recycle system. Light sources were a 300 W high-pressure mercury lamp (Shanghai Mengya) and a 150 W xenon lamp (USHIO) equipped with a 320 and 420 nm cutoff filter, respectively. The distance between the reactor and lamp was fixed at 10 cm for the Hg lamp and at 40 cm for the Xe lamp. The light intensities of Hg and Xe lamps reaching the external surface of the reactor were 4.50 and 5.50 mW/cm2, respectively, measured with an irradiance meter (Instruments of Beijing Normal University). Except stated otherwise, all the experiments were conducted under fixed conditions (1.7 g/L of catalyst, 0.22 mM phenol, 0.066 mM X3B dye, and pH 6.5).

is more positive than the one-electron reduction potential of O2 (−0.05 V vs NHE). Then, in thermodynamics, the one-electron reduction of O2 by ecb− on CuWO4 should not be allowed. Considering that the value of ECB for CuWO4 is less positive than the standard potentials for the two- and four-electron reduction of O2 (which are 0.68 and 1.23 V vs NHE, respectively), it is highly possible that the observed reduction of O2 on the irradiated CuWO4 results from a multielectron transfer process. This reduction process occurring on the irradiated CuWO4 is unclear at present. On the other hand, it is desirable to improve the photocatalytic activity of CuWO4. A general strategy is to retard recombination of the charge carriers through surface modification. Very recently, Kang and co-workers have found that deposition of the n-type CuWO4 thin layer on the top of the p-type CuO film electrode, followed by sintering at 500 °C, can improve the charge separation efficiency at the p−n junction, but the photocurrent of water oxidation is greatly reduced due to the increased resistance of the ITO electrode after thermal treatment.18 From the literature survey, the values of ECB for CuO in aqueous solution at pH 0 vary from −0.38 to +0.01 V vs NHE.19−23 Since these values of ECB for CuO are more negative than that of CuWO4, it is highly possible that deposition of CuO on CuWO4 may result in electron transfer from the irradiated CuO to CuWO4, followed by a multielectron reduction of O2 on CuWO4. Such an interfacial charge transfer between CuO and CuWO4 would improve the efficiency of charge separation and consequently accelerate water oxidation and organic degradation at the solid−liquid interface (Scheme 1). This possible pathway has not been demonstrated yet in the literature. Scheme 1. Possible Mechanism for the Enhanced Photocatalytic Activity of CuO/CuWO4

In this work, we have prepared samples of CuO/CuWO4, CuWO4, and CuO, followed by measurement of their photocatalytic and (photo)electrochemical performance. Solids were characterized with several techniques, including highresolution transmission electron microscopy (HRTEM). Phenol and textile dye X3B were used as model substrates (Figure S1 of the Supporting Information), and their degradation was carried out in aerated aqueous solution under light illumination at wavelengths longer than 320 or 420 nm. Results show that CuO particles (4 ± 1 nm) at 1.8 wt % are deposited onto CuWO4 (40 ± 6 nm) and that under UV and visible light the photocatalytic activities of CuO/CuWO4 are approximately 9 and 5 times higher than those of CuWO4, respectively. A possible mechanism responsible for the enhanced activity of CuO/CuWO4 is discussed in the text. 9983

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Figure 1. (A) XRD patterns of (a) CuO/CuWO4, (b) CuWO4, and (c) CuO. (B) HRTEM images of CuO/CuWO4.

Figure 2. Photodegradation of (A) phenol under UV light and (B) X3B under visible light, measured in the aqueous suspensions of (a) CuO/ CuWO4, (b) CuWO4, (c) S-CuO/CuWO4, and (d) CuO. The curve (e) represents the experiments in the absence of catalyst.

The aqueous suspension containing necessary components was stirred in the dark for 2 h and then irradiated with a Hg or Xe lamp. At given intervals, small aliquots were withdrawn by a microsyringe and filtered through a membrane (0.22 μm pore diameter). Phenol was analyzed by high-performance liquid chromatography (HPLC) on a DIONEX Ultimate 3000 (Apollo C18 reverse column and 50% CH3OH aqueous solution as an eluent). Textile dye X3B was analyzed from its absorbance at 533 nm on an Agilent 8451 spectrometer. H2O2 was quantified at 551 nm through the peroxidase-catalyzed oxidation of N,N-diethyl-1,4-phenolenediammonium.24 Electrode Preparation and Measurement. First, 0.75 g of poly(vinyl alcohol) and 0.80 g of the unsintered CuWO4 or CuO were dispersed in 10 mL of water, and the suspension was heated at 95 °C under vigorous stirring. Then, the mixture was spread over an indium-doped tin oxide (ITO) conducting glass by the doctor blade method, followed by sintering in air at 500 °C for 3 h. To examine the effect of CuO, the above CuWO4 electrode was soaked in an aqueous solution of 0.1 M Cu(NO3)2 for 30 min. Then the electrode was taken out from the solution and sintered in air at 400 °C for 30 min and denoted as the CuO/CuWO4 electrode. (Photo)electrochemical measurement was carried out with a three-electrode cell on a CHI660A Electrochemical Station (Chenghua, Shanghai). A saturated calomel electrode (SCE) was used as the reference electrode, a platinum wire as the counter electrode, and a catalyst film as the working electrode. The electrolyte was 0.5 M NaClO4, and the solution pH was adapted to 6.65 with NaOH or HClO4. The electrode was placed at 10 cm from the fiber optics source, and a 0.28 cm2

spot was irradiated through the quartz window, with a 150 W Xe lamp (GY-12, Tianjin Dongkang) equipped with a 420 or 570 nm cutoff filter. The current−potential curve was recorded in the region from −0.3 to +1.2 V vs SCE, with a scan rate of 20 mV/s. The photocurrent was measured at −0.15 V vs SCE, by using a chronoamperometry method. The Mott−Schottky (M−S) plot was made through measurement of the impedance as a function of potentials at 1 kHz on an electrochemical workstation (273A Potentiostat plus 5210 lock-phase amplifier). For ease of comparison with literature reports, the results in this study will be presented against the normal hydrogen electrode (NHE), by following the equation, E (vs NHE) = E (vs SCE) + 0.24 V.

3. RESULTS AND DISCUSSION Characterization. Figure 1A shows the XRD patterns of the samples. The diffraction peaks of CuO/CuWO4 were similar to those of CuWO4 and were indexed to triclinic CuWO4 (PDF # 21-0307). The undetectable CuO in CuO/ CuWO4 by XRD is due to low content or small particle size of CuO. By using the Scherrer equation, the average crystallite size of CuWO4 was estimated to be 30 nm in CuO/CuWO4 and 105 nm in CuWO4. A similar result was also obtained from the SEM images, where the average grain size of CuWO4 was 40 ± 6 nm in CuO/CuWO4 and 130 ± 20 nm in CuWO4, respectively (Figure S2, Supporting Information). It is possible that the growth of CuWO4 is inhibited by CuO present in CuO/CuWO4. Element analysis by EDS showed that the atomic ratios of Cu to W for CuO/CuWO4 and CuWO4 were 1.86 and 1.05, 9984

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(kobs) of X3B degradation are listed in Table 1. In this case, the red-colored X3B was not stable against visible light. Then its degradation could occur through self-bleaching, semiconductor photocatalysis, and dye sensitization.25 Moreover, the dark adsorption of X3B on the catalyst in aqueous solution was not negligible. Before light illumination, the amounts of X3B adsorbed (qe) on CuO/CuWO4, CuWO4, and CuO were measured to be 2.4, 1.0, and 12.8 μmol·g−1, respectively. In general, the rate of dye degradation is proportional to its concentration on the catalyst surface.26 Then the observed rate constant of dye degradation (kobs) needs to be normalized with qe. These values of kobs/qe for CuO/CuWO4, CuWO4, and CuO were calculated to be 2.44 × 10−3, 0.19 × 10−3, and 0.05 × 10−3 g·μmol−1·min−1, respectively. According to this specific rate of X3B degradation, the activity of CuO/CuWO4 is approximately 15 times higher than that of CuWO4, which is almost twice the activity enhancement observed from phenol degradation. This discrepancy between the two model substrates is ascribed to the reasoning that enrichment of X3B on the catalyst surface not only increases its degradation rate but also improves the efficiency of charge separation further increasing the rate of X3B degradation. For the catalyst activity assessment, colorless phenol would be better than organic dye as model substrates.16,17 Therefore, we can conclude that CuO/CuWO4 is much more active than CuWO4 under either UV or visible light for organic degradation in aerated aqueous solution. Furthermore, a physical mixture of CuWO4 and CuO was also tested as a photocatalyst. This catalyst, S-CuO/CuWO4, was active for organic degradation under UV or visible light. However, its activity was much lower than that of CuO/ CuWO4 and only slightly higher than that of CuWO4 (Table 1). Since CuO/CuWO4 and S-CuO/CuWO4 contain a similar amount of CuO (1.8 wt %), it follows that the former CuO has a larger effect than the latter on the activity enhancement of CuWO4. Such an outstanding effect of CuO in CuO/CuWO4 might be due to its ultimate contact with CuWO4 (Figure 1B). This would facilitate the charge transfer between CuO and CuWO4 and consequently accelerate surface reactions, which will be demonstrated below. Production of H2O2. The above result suggests that O2 is reducible on the irradiated CuWO4. To understand the fate of O2, the formation of H2O2 was examined. Figure 3 shows the results of H2O2 generation in an aerated aqueous suspension

respectively. The latter is in agreement with the formula of CuWO4. The former suggests that there is approximately 1.8 wt % of CuO in CuO/CuWO4. Additional analysis by XPS showed that the atomic ratio of Cu to W in CuO/CuWO4 was 2.8, higher than that obtained with EDS. Since XPS has a detection depth shallower than that of EDS, it follows that CuO is located on the external surface of CuWO4. The HRTEM image of CuO/CuWO4 (Figure 1B) showed that CuO particles (4 ± 1 nm) were located indeed on the surface of CuWO4 crystals (40 ± 6 nm) and that there was an intimate contact between CuO and CuWO4. In that image, the (1̅10), and (100) facets of triclinic CuWO4 and the (2̅02) facet of monoclinic CuO were all visible. Measurement of the N2 adsorption isotherm in Figure S3 (Supporting Information) showed that the BET surface area of CuO/CuWO4 (17.3 m2/g) was larger than that of CuWO4 (2.4 ± 1.9 m2/g). Moreover, CuO/CuWO4 had a stronger absorption toward visible light than CuWO4, due to black CuO present in the sample (Figure S4A, Supporting Information). These observations indicate that the tiny particles of CuO are highly dispersed onto the large crystals of CuWO4. Organic Degradation. Figure 2A shows the results of phenol degradation in aerated aqueous solution under UV light. First, in the absence of catalyst, the direct photolysis of phenol was negligible. Second, in the presence of catalyst, phenol degradation was observed. However, the rate of phenol degradation increased in the order of CuO/CuWO4 > CuWO4 > CuO. The intermediate analysis by HPLC showed that there was no reaction between phenol and catalyst in the dark and that the dark adsorption of phenol on the catalyst in aqueous solution was also negligible (less than 3%). Since the solid is the only light-absorbing species in the mixture (Figures S1 and S4, Supporting Information), it follows that the observed phenol degradation is initiated by semiconductor photocatalysis. Third, the time profiles of phenol degradation were well fitted with the pseudo-first-order rate equation. The resulting rate constants (kobs) of phenol degradation are tabulated in Table 1. Since O2 was required for phenol Table 1. Apparent Rate Constants of Phenol and X3B Degradation in Aqueous Solution samples

kobs (UV) (10−3 min−1)

kobs (vis) (10−4 min−1)

kobs (X3B, vis) (10−3 min−1)

CuO/CuWO4 CuWO4 CuO S-CuO/CuWO4

2.10 0.21 0 0.31

3.90 0.66 0 0.55

5.85 0.19 0.67 0.37

degradation, this observation indicates that O2 can react with ecb− on the irradiated catalysts, as reported early with CuWO4.11,12 Once O2 is consumed, it would be immediately supplied from air, so that the rate of phenol degradation is firstorder in phenol. Forth, under visible light, a similar result of phenol degradation was also observed with those catalysts (Table 1). Since the observed reaction surely belongs to semiconductor photocatalysis, the value of kobs for phenol degradation could be used as a measure of the relative activity among the catalysts. Accordingly, the photocatalytic activity of CuO/CuWO4, as compared to that of CuWO4, is increased by 9.0 times under UV light and 4.9 times under visible light, respectively. Figure 2B shows the results of X3B degradation in aerated aqueous solution under visible light. The relevant rate constants

Figure 3. Formation of H2O2 under visible light in the presence of 0.22 mM phenol in the aerated aqueous suspensions of (a) CuWO4, (b) CuO/CuWO4, (c) CuO, and (d) S-CuO/CuWO4. 9985

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Figure 4. (A) Mott−Schottky plots of (a) CuO and (b) CuWO4 measured at 1 kHz in 0.5 M NaClO4 at pH 4.56. (B) Flat band potentials obtained at different pH.

CuWO4 , consequently increasing the rates of organic degradation and H2O2 generation. This proposal of the interfacial charge transfer between the two semiconductors will be proven below. Band Edge Potentials. To verify the mechanism, the conduction band edge potentials (ECB) and the valence band edge potentials (EVB) of CuO and CuWO4 are needed. In general, the flat band potential (Efb) of n-type semiconductor is close to its ECB, while for the p-type semiconductor, Efb is close to EVB. Then, according to the equation of Eg = EVB − ECB, the values of EVB for the n-type semiconductor and ECB for the ptype semiconductor can be calculated. In this work, the values of Eg for CuO and CuWO4, estimated through a Tauc plot (Figure S4B, Supporting Information), were 1.41 and 2.35 eV, respectively. These values of Eg are in the range of those reported in the literature (1.35−1.79 eV for CuO and 2.10− 2.45 eV for CuWO4).10−23 In the following, the values of Efb for CuO and CuWO4 will be determined through the M−S plot. Figure 4A shows the M−S plots, measured with the CuO and CuWO4 film electrodes in 0.5 M NaClO4 at pH 4.56. First, the plots showed a negative and positive slope for CuO and CuWO4, respectively. This observation indicates that CuO and CuWO4 belong to p-type and n-type semiconductors, respectively, as reported in the literature.12 Second, the plot intercept with the potential axis corresponded to the value of Efb. However, the value of Efb was pH-dependent (Figure 4B), due to H2O and OH− groups adsorbed on the electrode surface. The linear relationship between Efb and pH gave slopes of −56 and −59 mV/pH for CuO and CuWO4, respectively, in good agreement with the Nernstian equation. Then, for CuO and CuWO4 electrodes in aqueous solution at pH 0, the values of Efb were calculated to be +1.26 and +0.30 V vs NHE, respectively. However, the value of Efb is not exactly equal to ECB or EVB. The relationship between them is described by eqs 1 and 2,30,31 where ND is the effective charge density, NC the effective density of states in the conduction band of CuWO4 (1.46 × 1022 cm−3),32 NV the effective density of states in the valence band of CuO (1.1 × 1019 cm−3),33 k the Boltzmann constant, and T the absolute temperature. The value of ND can be calculated from the slope of the linear M−S plot (Figure 4A), assuming that the dielectric constants for CuO and CuWO4 are 10.26 and 83, respectively.19,34 The calculated values of ND for CuO and CuWO4 were 9.7 × 1020 and 2.98 × 1020 cm−3, respectively, similar to those reported for CuO (9.0 × 1020 cm−3)23 and for crystalline CuWO4 (4.68 × 1019−2.7 × 1021

under visible light. In this study, phenol was used as a hole scavenger, to accelerate H2O2 production and to consume •OH radicals as well. With each catalyst, H2O2 was detectable. However, the rate of H2O2 generation was greatly dependent on the catalyst. According to the initial rate of H2 O2 production, the relative activity among the catalysts increased in the order of CuO/CuWO4 > CuWO4 ≥ S-CuO/CuWO4 > CuO, the trend of which was nearly the same as that observed from organic degradation (Figure 2). These observations suggest that the reduction of O2 by ecb− on CuWO4 is a twoelectron transfer process, as proposed in Scheme 1. Moreover, the maximum concentrations of H2O2 obtained from CuWO4 and CuO/CuWO4 were 0.56 and 3.56 μM, respectively. After that, the concentration of H2O2 declined with time. This observation suggests that H2O2 has participated in some reactions. For example, H2O2 may react with ecb− on CuWO4 to form •OH or H2O. The standard redox potentials for the H2O2/•OH and H2O2/H2O couples are 0.72 and 1.78 V vs NHE, respectively, all of which are more positive than the conduction band edge of CuWO4 (0.20−0.44 V vs NHE at pH 0).10,13−15 In general, the photocatalytic activity of a semiconductor, such as TiO2, increases with the larger surface area and the enhanced crystallinity.27−29 In the present case, CuO/CuWO4 has a higher surface area than CuWO4, but the crystallinity of CuWO4 in CuO/CuWO4 is lower than that of CuWO4 (Figure 1A). After the apparent rate constant of organic degradation in Table 1 is normalized with the BET surface area of the catalyst, it still follows that CuO/CuWO4 is more active than CuWO4. Note that this specific rate of organic degradation is useful only as a reference because the real surface area of the solid dispersed in aqueous solution would be different from the BET surface area measured by N2 adsorption in a solid−gas phase. Apart from the surface area and crystallinity, the optical property of photocatalyst is also important. Due to the presence of CuO, CuO/CuWO4 has a stronger absorption toward visible light than CuWO4 (Figure S3A, Supporting Information). However, CuO is located on the external surface of CuWO4 (Figure 1B), and its effect on the intrinsic absorptivity of CuWO4 is unlikely. Since CuO is nearly not active for phenol degradation and S-CuO/CuWO4 is slightly more active than CuWO4 (Table 1), we consider that the observed higher activity of CuO/CuWO4 than that of CuWO4 would be attributed to the intimate contact between CuO and CuWO4. If this hypothesis is operative (Scheme 1), CuO particles in CuO/ CuWO4 would harvest extra photons and transfer their ecb− to 9986

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Figure 5. (A) LSV curves for (a) CuWO4, (b) CuO, and (c) CuO/CuWO4 in the dark (dotted lines) and under visible light with a 570 nm cutoff filter (solid lines). (B) Photocurrent at +0.09 V vs NHE under visible light with a 570 nm cutoff filter. All the experiments were carried out in 0.5 M NaClO4 at pH 6.65 under N2 and in the presence of 0.22 mM phenol.

cm−3).10,12,35 Then, for the CuO electrode in aqueous solution at pH 0, the values of ECB and EVB were calculated to be −0.03 and +1.38 V vs NHE, respectively. For the CuWO4 electrode in aqueous solution at pH 0, the calculated values of ECB and EVB were +0.20 and +2.55 V vs NHE, respectively. According to those data, the electron transfer from CuO to CuWO4 and the hole transfer from CuWO4 to CuO are both thermodynamically possible. This hypothesis will be examined below through a linear sweep voltammetry (LSV). ECB = Efb + kT ln(NC/ND)

(1)

E VB = Efb + kT ln(NV /ND)

(2)

result, the efficiency of charge separation is improved, and the photocurrent is enhanced, as compared to those of the CuO film electrode. The second experiment was to examine the hole transfer from CuWO4 to CuO. In this case, a 420 nm cutoff filter was used to excite both CuWO4 and CuO, and no organic substrates were added to the electrolyte solution. The result is shown in Figure 6. As the applied potential swept from +0.64 to

(Photo)electrochemical Measurement. The first experiment was to examine the possible electron transfer from CuO to CuWO4. Since the spectral onset of CuWO4 was located at wavelengths shorter than 540 nm, a 570 nm cutoff filter was used to only excite CuO in CuO/CuWO4. Figure 5A shows the LSV curves of CuO, CuWO4, and CuO/CuWO4 electrodes, measured in 0.5 M NaClO4 at pH 6.65 under N2 and in the presence of phenol. With the CuWO4 electrode, no photocurrent was observed, as the applied potential swept from −0.06 to +0.64 V vs NHE. The LSV curves obtained in the dark and under visible light nearly overlapped. With the CuO electrode, because of the p-type semiconductor, a cathodic photocurrent was observed, similar to that reported by Kang and co-workers.18 With the CuO/CuWO4 electrode, not only a cathodic photocurrent was observed but also it was notably larger than that obtained with the CuO electrode. Moreover, at an applied potential bias at +0.09 V vs NHE, the photocurrent increased in the order of CuO/CuWO4 > CuO > CuWO4 (Figure 5B). Recall that the CuO/CuWO4 film electrode was prepared by immersing the above-used CuWO4 electrode in 0.1 M Cu(NO3)2, followed by sintering at 400 °C. Then, the results for CuWO4 and CuO/CuWO4 in Figure 5 are comparable because the two electrodes would have the same amount and thickness of the CuWO4 film. Similarly, the CuO film in the CuO/CuWO4 electrode would be very thin, and the amount of CuO would be much lower than the pure CuO film electrode. This was confirmed by a SEM image (Figure S5, Supporting Information). The average thickness of the CuO/CuWO4 film was 1.5 μm, thinner than that of the CuO film (4.3 μm). Therefore, we can conclude that there is an interfacial electron transfer from the irradiated CuO to CuWO4 (Scheme 1). As a

Figure 6. LSV curves of (a) CuWO4, (b) CuO, and (c) CuO/CuWO4, obtained in the dark (dotted line) and under visible light with a 420 nm cutoff filter in 0.5 M NaClO4 at pH 6.65.

+1.44 V vs NHE, the photocurrent of the CuO/CuWO4 electrode was always much larger than the sum of the photocurrents of individual CuO and CuWO4 electrodes. Since the observed photocurrent only originates from water oxidation, this observation indicates that there is an interfacial hole transfer from CuWO4 to CuO (Scheme 1). Since ecb− and hvb+ are formed in a pair, the electron transfer from CuO to CuWO4 and the hole transfer from CuWO4 to CuO would incorporate each other. Such synergism between electron and hole transfers would improve the efficiency of charge separation for both CuWO4 and CuO. As a result, the two-electron reduction of O2 on the CuWO4 site and the oxidation of the organic substrate on the CuO site are both accelerated (Scheme 1).

4. CONCLUSIONS We have shown that surface modification of n-type CuWO4 with a little amount of p-type CuO can result in a significant enhancement in the activity for organic degradation in an 9987

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aerated aqueous solution under either UV or visible light. Through a (photo)electrochemical method, both the processes of the electron transfer from CuO to CuWO4 and the hole transfer from CuWO4 to CuO have been demonstrated. Because of this, the efficiency of charge separation is improved, and organic degradation is accelerated. However, the reduction of O2 by ecb− on CuWO4 is a two-electron transfer process, similar to that occurring on the irradiated Bi2WO6.36 This may limit application of CuWO4 photocatalysis for organic degradation because the two-electron reduction of O2 would be slower than the one-electron reduction of O2. Nevertheless, this composite of CuO/CuWO4 might be useful as a visiblelight photocatalyst for water oxidation at an applied potential bias. In this regard, the CuWO4-based photocatalyst is still worthy of being further studied for improvement of its activity.

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ASSOCIATED CONTENT

S Supporting Information *

SEM images, N2 adsorption isotherms, UV−vis diffuse reflectance spectra, Tauc plots, and substrate absorption spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86 571 87952410. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the 973 program of China (No. 2011CB936003) and NSFC (No. 21377110).

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