Photoeletrocatalytic Activity of a Cu2O-Loaded Self-Organized Highly

Dec 11, 2008 - Key Laboratory of Industrial Ecology and Environmental Engineering and State Key Laboratory of Fine Chemicals, School of Environmental ...
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Environ. Sci. Technol. 2009, 43, 858–863

Photoeletrocatalytic Activity of a Cu2O-Loaded Self-Organized Highly Oriented TiO2 Nanotube Array Electrode for 4-Chlorophenol Degradation Y A N G H O U , † X I N Y O N G L I , * ,†,‡ XUEJUN ZOU,† XIE QUAN,† AND G U O H U A C H E N * ,‡ Key Laboratory of Industrial Ecology and Environmental Engineering and State Key Laboratory of Fine Chemicals, School of Environmental and Biological Science and Technology, Dalian University of Technology, Dalian 116024, and Department of Chemical and Biomolecular Engineering, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, People’s Republic of China

Received August 28, 2008. Revised manuscript received November 10, 2008. Accepted November 12, 2008.

Differently sized Cu2O nanoparticles have been assembled photocatalytically on the surface of self-organized highly oriented TiO2 nanotubes obtained by anodization of a Ti sheet in fluoridecontaining electrolytes. X-ray diffraction analysis identifies an anatase structure and fine preferential orientation of 〈101〉 planes. The UV-vis absorption edge of the TiO2 nanotube arrays shift to lower energy after Cu2O loading. The composite array electrode exhibits a higher photovoltage response than the TiO2 powders directly deposited on a Ti sheet. The highest photoconversion efficiencies observed for the Cu2O-loaded electrode are 17.2% and 0.82% under UV light and visible light irradiation, respectively. Especially, the composite array electrode shows a higher efficiency than the nonloaded one for the photoelectrocatalytic decomposition of 4-chlorophenol. The improved photoeletrocatalytic activity of the TiO2/Cu2O composite array electrode is attributed to the synergistic effect of Cu2O nanoparticles and TiO2 nanotube arrays. The Cu2O nanoparticles could enhance the efficiency of photon harvesting and reduce the chances of electron-hole recombination by sending the electrons to the conduction band of TiO2 nanotubes. The accumulated electrons in the conduction band of TiO2 nanotubes would reduce oxygen to form peroxides for enhanced advanced oxidation. The byproducts were identified by highperformance liquid chromatography.

Introduction Over the past few decades, 4-chlorophenol has been widely used in various fields, leading to an increased problem of its disposal and treatment. Compared to the conventional biological treatment processes, photocatalysis has attracted much attention owing to its potential application in purifying * Address correspondence to either author. Phone: +852-23587138(G.C.); +86-411-8470-7733(X.L.). Fax: +852-2358-0054 (G.C.); +86-411-8470-8084 (X.L.). E-mail: [email protected] (G.C.); xinyongli@ hotmail.com (X.L.). † Dalian University of Technology. ‡ The Hong Kong University of Science & Technology. 858

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water by mineralizing organic compounds. Titanium dioxide (TiO2) is found to be an efficient photocatalyst because it is photostable, nontoxic, low-cost, and readily available. It has been widely used in photovoltaic cells (1), photocatalysis (2), sensors (3), etc. Recently, there have been extensive studies on nanotubular materials because of their exceptional physical properties and wide range of applications (4). Titania nanotubes of different geometrical shapes and microstructures in powder forms have been fabricated using techniques such as sol-gel synthesis (5), freeze-drying (6), electrodeposition (7), and chemical treatments of fine titania particles (8). It has been shown that a simple anodization process of a Ti sheet in fluorinated electrolytes can form self-organized highly ordered TiO2 nanotube arrays (9), and such a material has displayed superior performance when employed as a photoanode (10). The self-organized one-dimensional nanotube creates a better opportunity to harvest sunlight more efficiently than the randomly oriented nanoparticles or the nanotubes prepared by the sol-gel process. Paulose et al. (11) reported that electron transport in the nanotube arrays was superior to that of the nanoparticulate systems. The highly ordered titania nanotube array structure permits vectorial charge transfer from the solution to the conductive substrate, thereby reducing the losses incurred by charge-hopping across nanoparticle grain boundaries (12). The enhanced electronic transport allows for the use of thicker films to improve light absorption, without sacrificing electrical properties per increased charge carrier recombination (13), thus improving the absorption of low-energy photons in the visible region. The early results by Crimes et al. (14) show that the highly ordered nanotube array architecture is promising for photoelectrochemical applications. However, as is known, the band gap of TiO2 (3.0-3.2 eV) limits the absorption of sunlight to the ultraviolet region of the solar spectrum. To enhance the efficiency in the visible range, considerable efforts have been taken involving dye sensitization (1) or combination with narrow-band-gap semiconductor films (15). Yasomanee and co-workers prepared TiO2/Cu2O composite films and found good activity for the photoelectrolysis of water under UV-vis light irradiation (16). Siripala et al. (17) reported the deposition of TiO2 onto a Cu2O semiconductor grown by a simple electrochemical method. The resulting TiO2/Cu2O composites were found to be highly efficient for photoelectrocatalysis under visible light. Recently, Zhang et al. (18) prepared Cu2Oloaded TiO2 catalysts that showed strong absorption of visible light and high activities for degradation of methylene blue in water under photoirradiation at wavelengths longer than 400 nm. However, no report has been found so far regarding a Cu2O-loaded TiO2 nanotube array electrode and its photoelectrocatalytic characteristics. In the present paper, we report a novel loading of Cu2O nanoparticles onto a TiO2 electrode with a highly ordered vertically oriented nanotube array prepared by using photocatalytic reduction. The simple and green process makes this method a highly attractive one among all the existing methods for the preparation of TiO2/Cu2O nanocomposites (18). Its activity was investigated for the photoelectrocatalytic decomposition of 4-chlorophenol under both UV and visible light irradiation. The correlation of the catalytic performance with the structural properties and the promoting effect of the Cu2O modifications are discussed on the basis of the data of a systematic characterization. 10.1021/es802420u CCC: $40.75

 2009 American Chemical Society

Published on Web 12/11/2008

FIGURE 1. Top surface ESEM view of the prepared Cu2O-nanoparticle-loaded TiO2 nanotube array electrode: (a) TiO2 nanotube arrays and (b) TiO2/Cu2O composite arrays. (c) Cross-sectional view of the TiO2 nanotube array structure. The inset shows the images of the TiO2 nanotube electrode at high magnification. (d) Corresponding EDX pattern of the TiO2/Cu2O composite arrays.

Experimental Section Preparation of Cu2O-Loaded TiO2 Nanotube Arrays. Titanium sheets (0.25 mm thick, 99.7% purity), hydrofluoric acid (48%), and ethyl alcohol (99.9%) were purchased from Aldrich Corp. (Milwaukee, WI). Prior to anodization, the Ti foils used in this study were ultrasonically cleaned with alcohol and cold distilled water for 3 and 20 min, respectively. Then they were chemically etched by immersing them in a mixture of HF and HNO3 acids (HF:HNO3:H2O ) 1:4:5 in volume) (19, 20) for 30 s, followed by rinsing with deionized water. Anodization was performed in a two-electrode configuration with titanium foil as the working electrode and platinum foil as the counter electrode under constant potential at room temperature. The anodizing voltage varied from 0 to 20 V with an increasing rate of 100 mV s-1 and was kept at 20 V for 30 min (21). The electrolyte was 0.2 wt % HF in water. After anodic oxidation, the samples were annealed at 773 K in oxygen for 1 h with heating and cooling rates of 2 K min-1 to convert the amorphous phase to the crystalline one. Cuprous oxide was loaded onto the surface of TiO2 by reducing the Cu2+ ions

photocatalytically using CuSO4 · 5H2O as a Cu precursor and methanol solution as a hole scavenger. The photoreduction of aqueous Cu2+ ions was carried out under slightly acidic conditions at pH 6 adjusted by the addition of phosphate buffer solution. First, the anodized samples were soaked in 0.05 M CuSO4 solution for 24 h before being gently rinsed with DI water and dried in a N2 stream. Then the samples were exposed to UV irradiation of λ ) 365 nm using a 300 W high-pressure mercury lamp (Southern New England Ultraviolet Co.) with an intensity of 1.4 mW cm-2. The irradiation time was about 40 min, sufficient to induce photocatalytic reduction of Cu2+ to Cu+ in the form of nanoparticles (Supporting Information, Figure S1). Finally, the obtained composites were dried under vacuum at a temperature of 333 K for 2 h. Characterization. Characterization of the TiO2/Cu2O composite array electrode included environmental scanning electron microscopy (ESEM), X-ray diffraction (XRD), energydispersive X-ray (EDX) analysis, UV-vis diffuse reflectance spectroscopy (DRS), fluorescence spectroscopy (FL), and VOL. 43, NO. 3, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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surface photovoltage spectroscopy (SPS) (see the Supporting Information for characterization). Photoelectrochemical Measurements. Photocurrents were measured using a potentiostat (France Radiometer Ltd.) in a standard three-electrode configuration with the Cu2Oloaded self-organized TiO2 nanotube array electrode as a photoanode, Pt foil as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. A 300 W high-pressure mercury lamp (Southern New England Ultraviolet Lighting Co.) was used as the UV light source, providing a light intensity of 1.4 mW cm-2. A 500 W Xe lamp (Phillips 500 W) served as the visible light source. The light was passed through a glass filter, which allowed wavelengths between 450 and 550 nm to be incident on the photoanode at a measured intensity of 33 mW cm-2. Photoelectrocatalytic Degradation of 4-Chlorophenol (4-CP). The photoelectrocatalytic oxidation of 4-CP was carried out in a single photoelectrochemical compartment. All the experiments were performed with magnetic stirring, using 0.01 M sodium sulfate as the electrolyte. The initial concentration of the 4-CP aqueous solution was 20 mg L-1 during the experiment. Other detailed information about the experimental procedure and the product analyses is available in the Supporting Information, Figure S2.

FIGURE 2. UV-vis absorption spectrum of the TiO2 nanotube array electrode and Cu2O-loaded TiO2 nanotube array electrode.

Results and Discussion Characterization of Photocatalysts. As shown in Figure 1a, the morphologies of the TiO2 nanotube arrays were examined using ESEM. The electrochemical etching of titanium foil in a fluoride medium produces an ordered array of hollow TiO2 tubes. The details on the mechanism of formation of the TiO2 tubular array structure on a titanium substrate are described elsewhere (22, 23). Figure 1a is a low-magnification SEM image of the as-synthesized TiO2 nanotube arrays, which reveals a regularly arranged pore structure of the film. These pores have a uniform size distribution around 80 nm in diameter and 550 nm in length (Figure 1c). The highmagnification SEM image of the sample shows the tubular structure more clearly (inset of Figure 1a). The well-ordered pore structure also exists on the surface of the Cu2O-loaded TiO2 nanotube array film. Some Cu2O nanoparticles (marked with arrows) have been deposited into self-organized highly oriented TiO2 nanotubes, and the deposition process does not damage the ordered TiO2 nanotube array structure (Figure 1b). It is apparent that, on the surface of tubular structures, Cu2O nanoparticles with a diameter of approximately 18 nm are deposited. The success of the preparation of Cu2Onanoparticle-loaded TiO2 nanotube arrays was further demonstrated by the elemental signature in the EDX spectrum (Figure 1d). The EDX spectrum exhibits O, Cu, and Ti peaks, showing the presence of copper element in the bulk of the TiO2 nanotube arrays and the atomic ratio of the Cu is about 1.76%. Annealing at 773 K converted the TiO2 nanotubes from the amorphous state to the anatase state with a fine preferential growth of the self-organized highly oriented TiO2 nanotube arrays in the 〈101〉 direction. Details of the XRD results can be found in the Supporting Information, Figure S3. No Cu or CuO phase was found in the composite. There are two peaks with 2θ values of 36.52° and 42.44°, corresponding to 〈111〉 and 〈200〉 crystal planes of pure Cu2O, respectively. These results are in good agreement with the Cu2O powder obtained from the International Center of Diffraction Data card reflections (JCPDS 05-667 and JCPDS 05-661). The UV-vis absorption spectra of TiO2 nanotube arrays and Cu2O-nanoparticle-loaded TiO2 nanotube layers are given in Figure 2. Their band gap absorption edges were around 387 and 505 nm, and that of the TiO2/Cu2O composite arrays had shifted into the visible spectrum (Supporting 860

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FIGURE 3. SPS responses of (a) TiO2 nanotube arrays and (b) Cu2O-nanoparticle-loaded TiO2 nanotube arrays. Information, Figure S4). In the UV light region, the absorption intensity of the Cu2O-loaded sample was increased, which revealed that the Cu2O-loaded TiO2 nanotube arrays were more sensitive to UV light than the nonloaded nanotube arrays. Therefore, a better photoelectrochemical capability of the Cu2O-loaded electrode under UV light is assumed. The Cu2O-loaded TiO2 nanotube arrays were characterized by FL spectra (Supporting Information, Figure S5). The FL intensity of nonloaded nanotubes was much higher than that of Cu2O-loaded nanotubes. The reduction of the FL intensity might indicate an increase in the charge separation (24). The experimental results demonstrated that the separation ratio of photogenerated electron-hole pairs was quite sensitive to the loading of Cu2O. The SPS method is a well-established noncontact technique for the characterization of semiconductors, which relies on analyzing illumination-induced changes in the surface voltage (25, 26). Figure 3 shows the SPS spectra of TiO2/Cu2O composite arrays and TiO2 nanotube arrays. It can be seen that TiO2 has a broad absorption band from 300 to 400 nm due to the transition of the O2- antibonding orbital to the lowest empty orbital of Ti4+ (27, 28), and the stronger photovoltage intensity of TiO2/Cu2O composite arrays indicates higher charge separation efficiency and longer excitation lifetimes as compared with those of TiO2 nanotube arrays in the UV region. Also, there is absorption in the visible region, and TiO2/Cu2O composite arrays obviously exhibit a red shift. This phenomenon could be analyzed by the theory of density of states (29). The band gap of TiO2/Cu2O was

FIGURE 5. Process of photocatalytic degradation of 4-CP under UV light illumination (I0 ) 1.4 mW cm-2, 0.2 V vs SCE bias potential).

FIGURE 4. (a) Variation of photocurrent density vs bias potential (vs SCE) and (b) photoconversion efficiency as a function of applied potential (vs SCE) in 0.01 M Na2SO4 solution for the Cu2O-loaded TiO2 nanotube array electrode and TiO2 nanotube array electrode under UV light (1.4 mW cm-2) irradiation. reduced and the separation efficiency of the electron and hole was increased by Cu2O modification. Photoelectrochemical Measurement. The photocurrent densities as a function of applied potential of the Cu2Oloaded TiO2 nanotube array electrode and TiO2 nanotube array electrode, in a 0.01 M Na2SO4 solution, under highpressure mercury lamp irradiation, 1.4 mW cm-2, are presented in Figure 4a. The dark current of TiO2/Ti is always near zero under the specified conditions. Practical photocurrent densities of the Cu2O-loaded TiO2 nanotube arrays dramatically increased with the bias potential when the bias potential exceeded 0.2 V, but the photocurrent densities of the TiO2 nanotube arrays were weak. The photocurrent density of the TiO2/Cu2O nanotube array electrode was more than 1.5 times the value of the nonloaded nanotube array electrode. There is a high probability that the photogenerated electron-hole pairs will recombine at the TiO2/Ti electrode surface. On the other hand, the TiO2/Cu2O composite array electrode is a heterojunction in the UV light absorption wavelength range (200-400 nm), and the recombination of photogenerated charge carriers is expected to be minimal. The photoconversion efficiency, η, is calculated as (30) η (%) ) [(total power output electrical power input)/light power input] × 100 )[jp(Erev ° -|Eapp|) × 100]/I0 where jp is the photocurrent density (mW cm-2), jpErev° is the total power output, jp|Eapp| is the electrical power input, and

I0 is the power density of incident light (mW cm-2). Erev° is the standard reversible potential, which is 1.23 V/NHE, and the applied potential is Eapp ) Emeas - Eaoc, where Emeas is the electrode potential (vs Ag/AgCl) of the working electrode at which the photocurrent was measured under illumination and Eaoc is the electrode potential (vs Ag/AgCl) of the same working electrode under open circuit conditions, under the same illumination, and in the same electrolyte. The voltage at which the photocurrent becomes zero was taken as Eaoc. As shown in Figure 4a, the dark current was very low. Figure 4b shows the total percent photoconversion efficiency (ηphoto(total), %) as a function of the applied potential (Eapp (V) vs SCE) for the Cu2O-loaded TiO2 nanotube array electrode and TiO2 nanotube array electrode under high-pressure mercury lamp irradiation. A maximum photoconversion efficiency of 17.2% was observed at an applied potential of 0.1 V vs SCE for the Cu2O-loaded TiO2 nanotube array sample, while it was 13.31% for the TiO2/Ti nanotube arrays at 0.2 V vs SCE. The photoelectrochemical response of the TiO2/Cu2O composite array electrode and TiO2 nanotube array electrode were also investigated using linear-sweep photovoltammetry with 0.1 Hz chopped 500 W Xe lamp irradiation (33 mW cm-2) in 0.01 M Na2SO4 solution. The value of the photocurrent density for the TiO2/Cu2O composite array electrode at 0.4 V versus SCE corresponds to 0.26 mA cm-2, whereas the photocurrent density for the TiO2/Ti nanotube array electrode corresponds to 0.051 mA cm-2 at the same measured potential. It is evident that the value of the photocurrent density of the TiO2/Cu2O composite array electrode was approximately 5.1 times the value for the nonloaded electrode (Supporting Information, Figure S6a). Accordingly, the photoconversion efficiency of the TiO2/Cu2O composite array electrode could be up to 5.87 times higher than that of the TiO2 nanotube array electrode (Supporting Information, Figure S6b). Photoelectrocatalytic Degradation of 4-CP. The electrochemical (EC), direct photolytic (DP), photocatalytic (PC), and photoelectrocatalytic (PEC) processes of 4-CP in aqueous solutions were performed on the TiO2/Cu2O composite array electrode under UV light illumination (Figure 5). The electrochemical process of 4-CP in 120 min was obviously much slower than the DP, PC, and PEC processes, and 45.2% of 4-CP was degraded in a direct photolytic process without any photocatalyst. Compared to direct UV irradiation, the TiO2/Cu2O composite array electrode (without a bias supply) degraded 79.8% of the 4-CP during the same time, implying that the TiO2/Cu2O composite array electrode exhibited photocatalysis. The photoelectrocatalytic process was the VOL. 43, NO. 3, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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fastest among these processes, indicating that the applied bias potential inhibited the recombination of the photogenerated hole/electron pairs effectively and prolonged the photogenerated the life of charge carriers. In the photoelectrocatalytic experiments, the degradation of 4-CP accords with pseudo-first-order kinetics. The kinetic constants of 4-CP oxidation in the EC, DP, PC, and PEC processes were 0.00441, 0.02392, 0.14173, and 0.54793 min-1, respectively, under UV light irradiation (Supporting Information, Table S1). Under UV light irradiation, in 120 min, nearly 100% of 4-CP was degraded by the TiO2/Cu2O composite array electrode in the photoelectrocatalytic process, while 79.78% of 4-CP was degraded by the nonloaded electrode (Supporting Information, Figure S7). As shown in Figure 2, a shift of the band gap absorption edge into the visible light spectrum was observed for the TiO2/Cu2O composite array electrode; thus, the visible photoelectrocatalytic capability of the TiO2/Cu2O composite array electrode was investigated under Xe lamp illumination (33 mW cm-2). Figure S8 of the Supporting Information shows the 4-CP concentration versus reaction time on the TiO2/ Cu2O composite array electrode in the EC, DP, PC, and PEC processes under visible light irradiation. 4-CP was not degraded in the electrochemical process in the whole period investigated. Direct photolysis as a control experiment can be considered in the photoelectrocatalytic experiment, and the degradation efficiency of 4-CP reached 6.3%. The photocatalysis had an effect on the degradation of 4-CP, and the degradation efficiency of 4-CP was 12.1%. When both electrochemical and photocatalytic processes were simultaneously applied, 41.1% 4-CP removal was obtained. This value is 29% higher than the summation of the electrochemical and photocatalytic processes acting alone. The corresponding kinetic constants and regression coefficients of 4-CP degradation by four processes are given in the Supporting Information, Table S2. The data show that the kinetic constants of 4-CP oxidation on the TiO2/Cu2O composite array electrode (0.00122, 0.00149, 0.00265, and 0.05206 min-1 under Xe lamp illumination, respectively). Such a synergetic effect between photo- and electrodegradation has been recognized by Yang et al. (31) and Vinodgopal et al. (32). In the meantime, under Xe lamp illumination (33 mW cm-2), in 120 min, 41.1% of 4-CP was removed by the TiO2/ Cu2O composite array electrode in the photoelectrocatalytic process, while 29.8% of 4-CP was removed by the nonloaded electrode (Supporting Information, Figure S9). Above all, when the TiO2/Cu2O composite array electrode is irradiated by UV or visible light, electrons are transferred from the conduction band of Cu2O to that of TiO2. Thus, electron-hole pairs are effectively separated, which leads to the photocatalytic activity of the TiO2/Cu2O composite array electrode being superior to that of the TiO2 nanotube array electrode. Photocatalytic Mechanism Discussion. On the basis of the principle of sensitizing large-band-gap semiconductors with short-band-gap semiconductors, efforts have been made to employ short-band-gap semiconductors (e.g., CdS (33, 34), PbS (35, 36), Bi2S3 (37), CdSe (38), InP (39) and Cu2O (20)) as sensitizers to extend the photoresponse of TiO2 into the visible region (Supporting Information, Figure S10). Since the conduction band of TiO2 lies more positive than the Cu2O conduction band, electron injection is expected from the photoexcited Cu2O nanoparticles into the TiO2 conduction band, whereas holes can accumulate in the valence band of Cu2O to form hole centers, which can be consumed by participating in oxidation. Accumulated electrons in the conduction band of TiO2 can be transferred to oxygen adsorbed on the surface of TiO2 to form O2- or O22-, which combines with H+ to form H2O2. Hydrogen peroxide (H2O2) 862

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can react with superoxide radical anion (O2•-), reducing to hydroxyl radicals (OH•), which could further mineralize organic compounds to end products (H2O and CO2). The processes that follow the band gap excitation of Cu2O are presented in eqs 1-6. Cu2O + hν f Cu2O(e + h) f Cu2O + hν

(1)

Cu2O(e) + TiO2 f Cu2O + TiO2(e)

(2)

Cu2O(h) + H2O(red) f H+ + OH•(ox)

(3)

+

2TiO2(e) + 2H + O2 f H2O2 •-

•

-

H2O2 + O2 f OH + OH + O2 •

OH + 4-CP f degradation products

(4) (5) (6)

In a typical photocatalytic degradation of organic contaminants, the photocatalytic activity and excellent catalytic stability of the TiO2/Cu2O composite arrays are higher than those of the TiO2 nanotube arrays under UV or visible light illumination. Intermediate Analysis for Photoelectrocatalytic Degradation of 4-Chlorophenol. Figure S11 of the Supporting Information shows HPLC graphs of the 4-CP solution at different irradiation times. By comparison of the retention time with the standard, hydroquinone (HQ), benzoquinone (BQ), and hydroxyhydroquinone (HHQ) were found to be the main intermediates of the photoelectrocatalytic 4-CP degradation with the Cu2O-loaded TiO2 nanotube array electrode by HPLC measurement. The concentrations of 4-CP and TOC during photoelectrocatalytic degradation are illustrated in Figure S12, Supporting Information. In 120 min, 4-CP was almost totally removed, whereas 44% of TOC was still present in the solution. The phenomenon illustrated that the mineralization efficiency was less than the degradation efficiency. The difference between the mineralization and the degradation processes implied that there were transient organic intermediates present in the photocelectroatalytic system. Therefore, a prolonged illumination time would be reasonable for complete mineralization. Stability of the TiO2/Cu2O Composite Array Electrode. The stability of the Cu2O-loaded TiO2 nanotube array electrode was investigated by repeating 10 times the photoelectrocatalysis degradation of 4-CP under UV light illumination, as shown in the Supporting Information, Figure S13. The TiO2/Cu2O composite array electrode was cleaned with ultrasonication after each experiment, and the results of the 10 repeated experiments for photoelectrocatalytic degradation of 4-CP in 120 min showed that the photoelectrocatalytic degradation efficiency was rather stable and remained above 82%.

Acknowledgments This work was supported financially by the National Nature Science Foundation of China (Grant Nos. 20677007 and 20837001), the National High Technology Research and Development Program of China (863 Program) (Grant No. 2007AA061402), the Major State Basic Research Development Program of China (973 Program) (Grant No. 2007CB613302), and the Ph.D. Program Foundation of the Ministry of Education of China (Grant No. 20070141060).

Note Added after ASAP Publication There was a change made to the affiliations in the version of this paper published ASAP December 11, 2008; the revised version published ASAP December 24, 2008.

Supporting Information Available Figures showing the XRD pattern and band gap graph of the TiO2/Cu2O composite electrode, FL characterization, variation of the photocurrent density and photoconversion

efficiency vs bias potential on TiO2 and TiO2/Cu2O under Xe lamp irradiation, process of PC degradation of 4-CP under Xe lamp illumination, PEC degradation of 4-CP on TiO2 and TiO2/Cu2O under UV and visible light irradiation, PC mechanism graph, and TOC, HPLC, and stability of the TiO2/Cu2O composite electrode and tables showing the kinetic constants and regression coefficients of 4-CP degradation by the TiO2/ Cu2O composite array electrode under UV and visible light irradiation. This information is available free of charge via the Internet at http://pubs.acs.org.

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