Improved Photocatalytic Activity of TiO2 on the Addition of

Mar 3, 2015 - observed with 2 wt % CuWO4 was about 2.83 times that in the absence of CuWO4. A similar result was also observed with CuO. However, six...
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Improved Photocatalytic Activity of TiO2 on the Addition of CuWO4 Xianqiang Xiong, Haihang Chen, and Yiming Xu* State Key Laboratory of Silicon Materials and Department of Chemistry, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: Various methods that aim to improve the photocatalytic activity of TiO2 have been reported in the literature. Herein, we report that addition of CuWO4 into the aqueous suspension of TiO2 can result in significant enhancement in the rate of phenol degradation. As the amount of CuWO4 increased, the rate of phenol degradation increased and then decreased. A maximum rate of phenol degradation observed with 2 wt % CuWO4 was about 2.83 times that in the absence of CuWO4. A similar result was also observed with CuO. However, six consecutive tests showed that CuWO4/TiO2 was much more stable than CuO/TiO2, due to the very high stability of CuWO4 against photocorrosion. The improved activity of TiO2 is not due to CuWO4 and CuO themselves and also does not match their solubility in aqueous solution. Moreover, for the generation of OH radicals, and for the decomposition of H2O2 in aqueous solution, CuWO4/TiO2 was also more active than TiO2. Through a (photo) electrochemical measurement, a possible mechanism is proposed, involving electron transfer from the irradiated TiO2 to CuWO4 that facilitates the charge separation of TiO2 and consequently accelerates reactions at interfaces. solution.15,16 This recycle behavior of Cu2+ is superior to that of Fe3+ because Fe2+ once formed is hardly reoxidized back by O2 in an acidic solution.16 On the other hand, Cu2+ ions grafted or impregnated onto TiO2, followed by thermal treatment, are also positive to organic degradation.17−24 In this case, the immobilized Cu2+ ions on TiO2 are present in the form of CuO. A study of electron paramagnetic resonance (EPR) spectroscopy shows that the reduced CuO on TiO2 can be reoxidized back to CuO by O2.21,24 In thermodynamics, the electron transfer from the irradiated TiO2 to CuO is possible. The conduction band edge potential for anatase TiO2 in water at pH 0 is −0.12 V versus normal hydrogen electrode (NHE),3 which is more negative than that for CuO (−0.03 V vs NHE).25 This interfacial charge transfer from TiO2 to CuO would inhibit the recombination of ecb− and hvb+ on TiO2 and thus facilitate organic degradation at the solid−liquid interface. Interestingly, a simple mixture of TiO2 and CuO has been also claimed to be more active than TiO2 for the photocatalytic production of H2 in a deaerated methanol−aqueous solution.26 However, there is one report showing that the deposited CuO on TiO2 is not stable against photodissolution, consequently slowing down the photocatalytic generation of H2 under N2.27 Then, a question arises regarding whether CuO/TiO2 is recyclable under air in an aqueous solution. In the application of CuO/TiO2 for water treatment, the leakage of Cu2+ into the aquatic environment would cause secondary contamination and/or take extra charge for subsequent processing. Therefore, searching for a highly

1. INTRODUCTION Semiconductor photocatalysis for environmental remediation has been widely studied.1−5 Although many semiconductors have been investigated, anatase TiO2 is the mostly appropriate not only in its cost, photocatalytic activity, and stability, but also in its band structure. This morphological TiO2 has a conduction band electron (ecb−) and valence hand hole (hvb+) capable of O2 reduction to O2−• and H2O oxidation to HO•, respectively. Since these species have different redox properties, a variety of toxic and recalcitrant pollutants can degrade into CO2 and/or small fragments at normal temperature and pressure only using O2 as an oxidant. However, the overall efficiency of organic degradation achieved so far with the TiO2based materials is still not high enough for practical application.6 This is mainly due to the photogenerated charge carriers of TiO2 that easily recombine to heat, without net chemical reactions with surface adsorbates. To improve the quantum efficiency of TiO2 photocatalysis, much research has been done in the past years, including surface modification with noble metals for acceleration of O2 reduction7,8 and coupling with other semiconductors for enhancement of charge separation.9,10 Obviously, noble metals as cocatalysts are expensive and would not be practical for water treatment. Recently, some Cu-containing TiO2 materials have been claimed to be more active than TiO2 for organic degradation in aerated aqueous solution. On one hand, the addition of Cu2+ ions into the aqueous suspension of TiO2 can result in the increased rate of organic degradation,11−14 ascribed to the faster consumption of ecb− by Cu2+ than that by O2. More impressively, the reduced Cu2+ (normally Cu+) on TiO2 can be reoxidized back to Cu2+ by the dissolved O2 in aqueous © 2015 American Chemical Society

Received: November 1, 2014 Revised: March 3, 2015 Published: March 3, 2015 5946

DOI: 10.1021/jp510974f J. Phys. Chem. C 2015, 119, 5946−5953

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The Journal of Physical Chemistry C

90 °C overnight. Finally, the powders were sintered in air at 500 °C for 3.5 h. This CuWO4 was used throughout this study. Solid was characterized with X-ray diffraction (XRD), N2 adsorption, diffuse reflectance spectroscopy and scanning electron microscope (SEM), (Figure S1, Supporting Information), and the resulting parameters are summarized in Table 1.

photoactive and stable Cu-containing TiO2 is more desirable in this field. Very recently, CuWO4 has been used as a photoanode for water oxidation at an applied potential bias, and this new material looks very stable against photocorrosion.28,29 Then, as a surface modifier of TiO2, CuWO4 might be a good candidate to replace CuO. On the other hand, CuWO4 has a conduction band edge potential (0.20 V vs NHE at pH 0) more positive than that of CuO.25 Then, CuWO4 might have a higher driving force than CuO to scavenge ecb− on TiO2, further increasing the photocatalytic activity of TiO2. It is known that ecb− on CuWO4 can reduce O2 to form H2O2 through a two-electron transfer pathway.25 Accordingly, an appropriate coupling between TiO2 and CuWO4 may also result into notable improvement in the activity for organic degradation, as proposed in Scheme 1.

Table 1. Physical Parameters of the Catalystsa samples

dXRD (nm)

dTEM (nm)

Asp (m2/g)

Vp (cm3/g)

dp (nm)

Eg (eV)

TiO2 CuWO4

13.4 55.3

16.8 ± 3.2 150 ± 50

144 6.3

0.31281 0.03574

81 258

3.20 2.35

a

dXRD, crystallite size estimated by XRD; dTEM, particle size estimated by SEM; Asp, BET surface area; Vp, total pore volume; dp, average pore size; Eg, band gap energy.

Scheme 1. Possible Mechanism for the Enhanced Photocatalytic Activity of CuWO4/TiO2

In brief, the XRD patterns of TiO2 and CuWO4 well matched those for anatase TiO2 (PDF no. 65−5714) and triclinic CuWO4 (PDF no. 21−0307), respectively. No peaks due to other phases were observed. Furthermore, TiO2 had a smaller particle size, larger surface area, and band gap energy than those of CuWO4, respectively. Photocatalysis and Analysis. Reactions were carried out in a Pyrex-glass reactor, thermostated at 25 °C. Light source was a high pressure mercury lamp (300 W, Shanghai Yamin) and put in the front of the reactor at a fixed distance of 10 cm. The light intensity of Hg lamp reaching the external surface of the reactor was 3.30 mW/cm2, measured with an irradiance meter (Instruments of Beijing Normal University). Typically, an aqueous suspension of CuWO4 at 1.316 g/L was first prepared and then mixed with the aqeuous suspension of TiO2. Unless stated otherwise, the final concentration of CuWO4, TiO2, and phenol in the working mixture were fixed at 0.020 g/ L, 1.00 g/L, and 0.41 mM, respectively. After the reactor was stirred in the dark for 1 h, it was irradiated with UV light. At given intervals, 2.0 mL of the suspension was withdrawn and filtered through a membrane. Then, the filtrate was analyzed by HPLC (high-performance liquid chromatography) on a Dionex P680 (Apollo C18 reverse column, and 50% CH3OH aqueous solution as an eluent). Hydrogen peroxide was analyzed at 553 nm through the POD-catalyzed oxidation of DPD,31 on an Agilent 8453 UV−visible spectrophotometer. EPR spectra of DMPO−OH adducts were recorded at room temperature on a Bruker A300 spectrometer at X-band equipped with a xenon lamp (100 W). Catalyst stability were examined as follows. A suspension (50 mL) containing 0.21 mM phenol, 1.00 g/L TiO2, and 0.064 mM CuWO4 or CuO was first stirred in the dark for 1 h and then irradiated and analyzed as described above. After each run was completed, a certain amount of phenol stock solution was supplied, so as to renew the initial concentration of phenol at 0.21 mM. After stirring in the dark for 0.5 h, this new suspension was irradiated and analyzed again. Such procedure was repeated six times. Finally, the dissolved Cu2+ ion in the filtrate was detected at 435 nm, through its complex with sodium diethyldithiocarbamate.32 Electrode Fabrication and Measurement. Fluorinated tin oxide (FTO) substrate, purchased from Pilkington Glass (Tec 15, 2.2 mm thick, 12−14 Ω/sq), was first cleaned with ethanol and acetone, followed by rinsing with water, and dried in air. Then, the FTO electrode was coated with a TiO2 gel by doctor blade method, followed by sintering in air for 3 h at 500

Herein, we report that a simple mixture of CuWO4 and TiO2 is not only more active than TiO2 but also very stable against photocorrosion. Experiments were carried out in aqueous solution under UV light by using phenol degradation as a model reaction. The formation of HO• radicals, H2O2, and Cu2+ ions were measured with a spin-trapping EPR and colorimetric method, respectively. Furthermore, the possible interfacial charge transfer between CuWO4 and TiO2 was investigated through a (photo)electrochemical measurement. The open circuit potential technique allows one to monitor the concentration change of the photogenerated electrons on TiO2 on the addition of CuWO4 in the dark, whereas the linear sweep voltammetry at an applied potential bias can provide information on the hole transfer. Finally, a possible mechanism responsible for the enhanced photocatalytic activity of TiO2 on the addition of CuWO4 is discussed.

2. EXPERIMENTAL SECTION Materials. Anatase TiO2, horseradish peroxide (POD), N,N-diethyl-p-phenylenediamine (DPD), and 5,5-dimethyl-1pyrroline-N-oxide (DMPO) were purchased from Sigma− Aldrich and CuO from Aladdin Chemistry Co. Ltd. Other chemicals in analytical grade were purchased from Shanghai Chemicals, Inc., including phenol, and poly(vinyl alcohol) (PVA). All the reagents were used as received without further treatment. CuWO4 was homemade by following the literature procedures.25,30 Briefly, 10 mmol of Na2WO4·2H2O and Cu(NO3)2·3H2O were dissolved in 250 mL of water, followed by heating for 3 h at 85 °C. Then the precipitates were collected, washed thoroughly with distilled water, and dried at 5947

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The Journal of Physical Chemistry C

Figure 1. (A) Phenol degradation in the aerated aqueous suspensions of (a) TiO2, (b) TiO2 + 2 wt % CuWO4, (c) CuWO4, and (d) CuO. (B) Formation of hydroquinone (solid bar) and benzoquinone (open bar). Curve (e) represents phenol photolysis in the absence of catalyst.

Figure 2. (A) Apparent rate constant of phenol degradation (kobs), measured in the aerated aqueous suspension of TiO2 (1.00 g/L) under UV light, and (B) concentration of Cu2+ ions dissolved in the aqueous solution of phenol (0.41 mM), measured in the dark and in the absence of TiO2. Copper compound added into the suspension was (a) CuWO4 and (b) CuO.

the time profiles of phenol degradation fit well to the pseudofirst-order rate equation. These observations indicate that the reactive species responsible for phenol degradation remain constant in their concentrations, so that the observed kinetics is first order in phenol. Since O2 is required for phenol degradation (Figure S2, Supporting Information), it follows that CuWO4 is probably recyclable during the reaction process. Moreover, by means of HPLC, and by comparison to a standard sample, hydroquinone and benzoquinone were identified as the major intermediates or products of phenol degradation (Figure 1B). In accordance with the formation rates of these intermediates, CuWO4/TiO2 also showed a higher activity than TiO2, in agreement with that observed from phenol degradation. It implies that the degradation pathway of phenol in the irradiated aqueous suspension of TiO2 is not notably changed on the addition of CuWO4. Figure 2A shows the effect of CuWO4 loading on the apparent rate constant of phenol degradation. In all cases, TiO2 was fixed at 1.00 g/L. As the amount of CuWO4 increased, the rate of phenol degradation increased and then decreased. A maximum rate of phenol degradation was observed at 65 μM or 2.0 wt % of CuWO4 and was approximately 2.83 times that measured in the absence of CuWO4. Interestingly, when CuWO4 was replaced with CuO, a similar result was also obtained. At a Cu loading lower than 200 μM, the activity of CuO/TiO2 was almost the same as that of CuWO4/TiO2. In this case, CuO alone was also nearly not photoactive for phenol

°C. The gel was prepared by dispersing 0.8 wt % TiO2 in the aqueous solution of 2.9 wt % PVA under magnetic stirring. Finally, the TiO2 films with an exposed area of 1 × 1 cm were used as working electrodes. The (photo)electrochemical measurements were carried out on a CHI660E Electrochemical Station (Chenghua, Shanghai), using a saturated calomel electrode (SCE) as a reference electrode and a platinum gauze as a counter electrode. The supporting electrolyte was 0.5 M NaClO4 at pH 6.65, adjusted by NaOH or HClO4. The working electrode was illuminated with a 500 W Xe lamp from the electrode/electrolyte side through a quartz window.

3. RESULTS AND DISCUSSION Photoactivity. In this study, all the experiments were carried out under UV light at wavelengths longer than 320 nm. Under these conditions, phenol photolysis and its dark adsorption on TiO2 in aqueous solution were both negligible. Then, the observed rate of phenol degradation can be used as a measure of the catalyst activity. Figure 1A shows the result of phenol degradation in aerated aqueous solution. At first glance, the photocatalytic degradation of phenol over TiO2 was significantly faster in the presence of CuWO4 than that in the absence of CuWO4. Control test with CuWO4 alone showed a very slow degradation of phenol under UV light. Since the amount of TiO2 in the reactor was fixed, the increased rate of phenol degradation on the addition of CuWO4 is surely due to the improved photocatalytic activity of TiO2. Impressively, all 5948

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The Journal of Physical Chemistry C degradation (Figure 1A). Since Cu2+ ions in aqueous solution have a positive effect on the TiO2-photocatalyzed reaction,11−16 one may wonder whether the observed effect of solid CuWO4 and CuO are simply due to the effect of their dissolved cupric species. With this in mind, separate experiments with CuO and CuWO4 suspended in the aqueous solution of phenol were performed, and the result is shown in Figure 2B. In this case, TiO2 was not used in the working suspension because Cu2+ ions could adsorb onto TiO2 in aqueous solution. As the loading of CuO or CuWO4 increased, the concentration of Cu2+ ions dissolved in aqueous phase increased toward saturation. This trend in the solid solubility as a function of Cu loading does not match that in the activity for the TiO2photocatlyzed reaction. Moreover, at a Cu loading higher than 35 μM, the solubility of CuO in aqueous solution was always higher than that of CuWO4. At a solid loading of 65 μM, the solubility of CuO was about 2 times that of CuWO4. However, at this Cu loading, CuO and CuWO4 had a similar effect on the photocatalytic degradation of phenol over TiO2. In accordance with these data, the observed positive effect of CuWO4 and CuO on the photocatalytic activity of TiO2 are not simply due to the effect of their dissolved cupric species, which will be further discussed below. Furthermore, CuO and CuWO4 have a color of black and mustard yellow, respectively. Then, the observed decay in the catalyst activity after reaching a maximum (Figure 2A) is probably due to the excess CuO and CuWO4 that absorb and reduce the number of photons reaching TiO2, consequently slowing down the TiO2-photocatalyzed reaction. Photostability. A recycling test was carried out with a mixture of TiO2 with 65 μM CuWO4 or CuO in an aerated aqueous suspension, and the result is shown in Figure 3. During

CuO/TiO2 for the photocatalytic degradation of phenol, which would be very important for practical use for water treatment. To understand the different stabilities of CuO/TiO2 and CuWO4/TiO2, separate experiments with 1.3 mM CuO or CuWO4 suspended in an aerated aqueous solution were performed, and the result is summarized in Table 2. In this case, Table 2. Dissolved Cu2+ Ions (μM) in Aqueous Suspensionsa samples

in the dark

under UV light

CuO + H2O CuO + phenol CuWO4 + H2O CuWO4 + phenol

1.8 10.0 1.4 5.5

23.5 43.6 5.7 7.0

a

Suspensions were stirred in the dark or under UV light for 12 h. Conditions: CuO, 1.26 mM; CuWO4, 1.28 mM; and phenol, 0.85 mM.

each component at a high concentration was used to ensure detection of the dissolved Cu2+ ions in the aqueous phase. In the dark, the dissolution of CuO and CuWO4 in water were rather weak. However, in the presence of phenol, the solid solubility became notably increased, ascribed to formation of a surface complex that facilitates the dissolution of Cu(II) ions into the aqueous phase. Under UV light, the solid solubility were further increased, whatever phenol was present or not in the suspension. These observations indicate that both CuO and CuWO4 suffer from a photoassisted dissolution. But, such photodissolution of CuWO4 was rather weak, as compared to that of CuO. This may explain why CuWO4/TiO2 is much more stable than CuO/TiO2 during the photocatalytic degradation of phenol in aqueous solution (Figure 3). The exact reason for the (photo)solubility difference between CuO and CuWO4 is not known. But the electrostatic interaction between Cu2+ and WO42− seems stronger than that between Cu2+ and O2−. Then, the surface reactions of CuWO4 with H2O and phenol would be weaker than those of CuO. As a result, the lower solubility of CuWO4 in aqueous solution than that of CuO has been observed either in the dark or under UV light. Recall that when the Cu loading is lower than 65 μM, the photocatalytic activity of TiO2 is proportional to the amount of CuO or CuWO4 in the suspension (Figure 2A). Due to serious photodissolution, the net amount of CuO in the suspension of 65 μM CuO/TiO2 would decrease from one run to another during the recycling test (Figure 3). As a result, the photocatalytic activity of CuO/TiO2 began to decrease with the time. On the other hand, the observed activity decay of CuO/TiO2 with the number of the run implies that besides the effect of Cu2+ ions, solid CuO and/or CuWO4 also play an important role in the TiO2-photocatalyzed reactions. Formation of Oxygen Reactive Species. During the photocatalytic process of TiO2, various reactive species have been observed, including O2−•, HO•, and H2O2.33−38 Then, the formation of these reactive species over TiO2 may change on the addition of CuWO4. Figure 4 shows the result of HO• formation both in aqueous suspension and on the surface of TiO2, detected with a DMPO spin-trapping EPR.36 Under UV light, a quartet signal characteristic of the DMPO−HO• adduct was observed. This signal intensity was notably enhanced on the addition of CuWO4. According to the maximum signal intensity at 40 s, the activity of CuWO 4 /TiO 2 was approximately 1.4 times that of TiO2. Control experiment with CuWO4 alone showed a negligible signal of the DMPO−

Figure 3. Stability test for phenol degradation over TiO2 (1.00 g/L) in the presence of 65 μM CuWO4 (solid symbols) or CuO (open symbols).

six runs, the rate of phenol degradation over CuWO4/TiO2 remained nearly unchanged. After the fifth run, the catalyst activity was decreased only by 4%. However, when the same reaction was performed with CuO/TiO2, the rate of phenol degradation decreased from one run to another. At the sixth run, the activity of CuO/TiO2 was about 55% that at the first run. Such activity loss of CuO/TiO2 was very serious, as compared to that of CuWO4/TiO2. Moreover, for the degradation of 45 μmol phenol, CuO/TiO2 took about 16 h, also longer than did CuWO4/TiO2 (12 h). These observations clearly indicate that CuWO4/TiO2 is much more stable than 5949

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Figure 4. (A) EPR spectra of the DMPO−HO• adducts recorded in the dark or under UV light for 60 s, and (B) time profile of the maximum signal intensity around 3500 G, measured in the aerated aqueous suspensions of (a) TiO2, (b) TiO2 + 4.6 wt % CuWO4, and (c) CuWO4.

Figure 5. (A) Production of H2O2 in the aerated aqueous suspensions of (a) 1.00 g/L TiO2, (b) 1.00 g/L TiO2 + 0.020 g/L CuWO4, and (c) 1.00 g/ L CuWO4, measured under UV light in the presence of 0.43 mM phenol. (B) Dark adsorption and photocomposition of H2O2 (0.1 mM). Curve (d) represents the direct photolysis of H2O2 in the absence of catalyst.

HO• adduct. The signal decay with time after reaching a maximum is due to decomposition of the DMPO−HO• adduct on the solid surface.37 In general, HO• radicals can result from water oxidation by hvb+, and from O2 reduction by ecb−, followed by H2O2 reduction. Since the EPR experiments were performed in aerated aqueous solution, the observed formation of HO• radicals is ascribed to both H2O oxidation and O2 reduction. Moreover, the enhanced formation of HO• radicals on the addition of CuWO4 was also verified with tert-butyl alcohol (TBA) as a HO• quencher. In the presence of 0.1 M TBA, the rate constants of phenol degradation obtained with TiO2 and CuWO4/TiO2 were decreased by 31 and 44%, respectively (Figure S3, Supporting Information). A larger decrease in the reaction rate implies a higher concentration of HO• involved. These observations indicate that CuWO4/TiO2 is also more active than TiO2 for the photocatalytic production of HO•, in agreement with that observed from phenol degradation. Figure 5A shows the result of H2O2 formation in aerated aqueous suspension. In this case, phenol was used as hole scavengers, so as to accelerate O2 reduction to H2O2. As the irradiation time increased, the concentration of H2O2 in the irradiated suspension of TiO2 increased toward saturation. However, this concentration limit of H2O2 was reduced by approximately 50% on the addition of 2 wt % CuWO4, which is not expected. In the irradiated suspension of CuWO4 alone, H2O2 was also detected, but its concentration was lower than

those from TiO2 and CuWO4/TiO2. Moreover, the concentration of H2O2 measured with CuWO4 began to decrease with time after reaching a maximum. It is highly possible that H2O2 once formed has participated some reactions. To verify the above hypothesis, separate experiments were carried out by using H2O2 as a reacting substrate, and the result is shown in Figure 5B. In the dark, H2O2 in aqueous solution strongly adsorbed onto TiO2, due to formation of a peroxide complex with the surface Ti(VI) sites of TiO2.38 This adsorption of H2O2 on TiO2 only changed a little on the addition of CuWO4. Under UV light, the concentration of H2O2 in the aqueous suspension of TiO2 decreased with time, which is almost entirely ascribed to the TiO2-photocatalyzed decomposition of H2O2. However, in the presence of 2 wt % CuWO4, the rate of H2O2 photodecomposition was increased by 1240%, from 2.64 μM/min to 35.4 μM/min. Such rate increase of H2O2 photodecomposition is only little due to the CuWO4-photocatalyed reaction (0.004 μM/min). These observations indicate that CuWO4/TiO2 is much more active than TiO2 for the photocatalytic decomposition of H2O2. Recall that the initial rate of H2O2 formation over TiO2 on the addition of CuWO4 was decreased only by 43%, from 0.455 to 0.26 μM/min (Figure 5A). Then, the observed 1240% increase in the rate of H2O2 decomposition (Figure 5B) implies that CuWO4/TiO2 would also be more active than TiO2 for the photocatalytic generation of H2O2. Since the reaction was carried out in the presence of excess hole scavenger, the 5950

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Figure 6. (A) Time profiles of open circuit potentials (OCP), measured with the TiO2 film electrode under N2 in 0.5 M NaClO4. After UV light was off, 500 μL of 1.316 g/L CuWO4 was added. (B) OCP decay of the TiO2 electrode (a) in the absence and (b) presence of CuWO4.

CuWO4 toward ecb− on TiO2 because the two film electrodes are not exactly the same. A possible hole transfer from TiO2 to CuWO4 was examined with a linear sweep voltammetry (LSV). Figure 7 shows the

observed production of H2O2 would only result from the reduction of O2 by ecb− on TiO2 or on CuWO4/TiO2. Then, the rate of H2O2 formation would represent the rate of O2 reduction. That is, CuWO4/TiO2 would be more active than TiO2 for the photocatalytic reduction of O2, the trend in agreement with that for the photocatalytic degradation of phenol (Figure 1A). Moreover, the photocatalytic reduction of H2O2 over TiO2 or over CuWO4/TiO2 would generate HO• radicals, consequently making some contribution to the observed signal of the DMPO−HO• adduct in Figure 4. Possible Mechanism. To understand the mechanism for the enhanced photoactivity of TiO2 on the addition of CuWO4, the possible electron and hole transfer between TiO2 and CuWO4 were investigated through a (photo)electrochemical measurement. Figure 6A shows the time profiles of an open circuit potential (OCP), measured with the TiO2 film electrode in 0.5 M NaClO4 under N2. At the first step, the electrode was illuminated with UV light to generate the electrons on TiO2. When the potential was stable, the light was blocked off (the second step). Immediately, the number of electrons on the TiO2 electrode began to decrease, due to the recombination with trapped holes and/or due to reactions with electron acceptors such as solution oxidants. At the third step, the electrode was illuminated again to reach a stable state. Then, the light was blocked off. At this moment, CuWO4 was quickly added to the electrolyte (the fourth step), so as to capture the electrons accumulated on the TiO2 electrode. In general, the electron density on the TiO2 electrode exponentially increases with the potential. Thus, a fast decay rate of OCP for a TiO2 electrode after illumination would suggest a rapid interfacial electron transfer. For this purpose, the data for the electron decay in Figure 6A was replotted in Figure 6B. Obviously, with the same electrode, the OCP decay after illumination was faster in the presence of CuWO4 than that in the absence of CuWO4. Since no other electron acceptors were introduced into the system, the faster decay in OCP suggests that there is an electron transfer from TiO2 to CuWO4. This result was also inferred from phenol degradation in a deaerated aqueous solution under UV light (Figure S2, Supporting Information). Phenol degradation over TiO2 under N2 was negligible, but the reaction became faster on the addition of CuWO4. A similar result of OCP was also obtained with CuO. The OCP decay of the TiO2 film electrode became faster on the addition of CuO (Figure S4, Supporting Information). It means that there is also an electron transfer from the irradiated TiO2 to CuO. However, it is difficult to evaluate the relative activities of CuO and

Figure 7. Linear sweep voltammetry of TiO2 film electrode in the dark (dotted lines) or under UV light (solid curves) (a) without CuWO4 and (b) with 500 μL of 1.316 g/L CuWO4 added. Experiments were carried out under N2 in 0.5 M NaClO4 at pH 6.86 with a scan rate of 20 mV/s.

LSV curve of the TiO2 film electrode, measured in 0.5 M NaClO4 under N2. As the applied potential swept from 0 to +1.0 V vs NHE, the dark current of the TiO2 electrode was very weak and almost not changed on the addition of CuWO4. Under UV light, the photocurrent of the TiO2 electrode was greatly increased, due to water oxidation by hvb+ of TiO2. However, in the presence of 65 μM CuWO4, the photocurrent of the TiO2 electrode was decreased by approximately 25%. Since no increase in the photocurrent was observed, it follows that there is no hole transfer from TiO2 to CuWO4. The valence band edge potentials for TiO2 and CuWO4 in water at pH 0 are 3.08 and 2.55 V versus NHE, respectively.25 Then, the lack of the hole transfer between TiO2 and CuWO4 is probably due to the poor overlapping between their frontier orbitals. The decreased photocurrent of the TiO2 electrode on the addition of CuWO4 is ascribed to the cutoff filter effect of CuWO4 that reduces the light intensity reaching TiO2. The above result indicates that there is only an interfacial electron transfer from the irradiated TiO2 to CuWO4 or CuO. Since the kinetics of phenol degradation in aerated aqueous suspension is first order in phenol (Figure 1A), it follows that 5951

DOI: 10.1021/jp510974f J. Phys. Chem. C 2015, 119, 5946−5953

Article

The Journal of Physical Chemistry C

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the transferred electrons on CuWO4 are capable of reacting with O2 to form H2O2,25 as proposed in Scheme 1. This would result in improvement in the efficiency of the charge separation of TiO2 and, consequently, in the rate of phenol degradation. Recall that the conduction band edge potential of CuWO4 is 0.23 V more positive than that of CuO.25 Then, CuWO4 would be a better electron scavenger of TiO2 than CuO. However, in practice, the same loading of CuWO4 and CuO resulted in a similar effect on the activity enhancement of TiO2 in the first run (Figures 2 and 3). This discrepancy might be due to the fact that CuO has a higher solubility in aqueous solution than CuWO4. Since Cu2+ ions can also capture ecb− on TiO2, this would compensate the lower activity of CuO, as compared to that of CuWO4. Therefore, CuO and CuWO4 at the same loading are similar in their effect on the photocatalytic activity of TiO2 for phenol degradation in aqueous suspension.

4. CONCLUSIONS In this work, we have demonstrated a simple and efficient approach to improve the photocatalytic activity of TiO2 through the addition of CuWO4. Although CuO is also active, for a long-term use, CuWO4 is obviously better than CuO. During the recycling test, CuWO4/TiO2 is much more stable than CuO/TiO2 for phenol degradation in aerated aqueous solution. Through a series of studies, the improved activity of TiO2 on the addition of CuWO4 is mainly ascribed to the electron transfer from TiO 2 to CuWO 4 that inhibits recombination of the photogenerated charge carriers and, consequently, accelerates reactions at the solid−liquid interface of O2 reduction, H2O2 generation, HO• formation, and phenol degradation. This work would be useful for application in the fields of pollutant treatment, water splitting, and photoelectrochemical cell.

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ASSOCIATED CONTENT

S Supporting Information *

XRD patterns, N2 adsorption-desorption isotherm, diffuse reflectance spectra, Tauc plots, SEM photos, time profiles for phenol degradation under N2 or in the presence of tert butyl alcohol, and OCP experiment with CuO. 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. Fax: +86-571-87951895. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Basic Research Program of China (No. 2011CB936003) and NSFC (No. 21377110). Xiong expressed sincere thanks to Dr. Leng for his kind instruction and discussion with the OCP technique.

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REFERENCES

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DOI: 10.1021/jp510974f J. Phys. Chem. C 2015, 119, 5946−5953

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DOI: 10.1021/jp510974f J. Phys. Chem. C 2015, 119, 5946−5953