Cu(II) as a General Cocatalyst for Improved Visible-Light

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Cu(II) as a General Cocatalyst for Improved Visible-Light Photocatalytic Performance of Photosensitive Ag-Based Compounds Ping Wang,† Yang Xia,† Panpan Wu,† Xuefei Wang,† Huogen Yu,*,†,‡ and Jiaguo Yu§ †

Department of Chemistry, School of Science, Wuhan University of Technology, Wuhan 430070, People’s Republic of China State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan 430070, People’s Republic of China § State Key Laboratory of Advanced Technology for Material Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, People’s Republic of China ‡

S Supporting Information *

ABSTRACT: Usually, cocatalyst modification of photocatalysts is an efficient approach to enhance the photocatalytic performance by promoting effective separation of photogenerated electrons and holes. It is highly required to explore new and effective cocatalysts to further enhance the photocatalytic performance of photocatalytic materials. In the present work, Cu(II) cocatalyst was successfully loaded on the surface of various Ag-based compounds (such as AgCl, Ag3PO4, AgBr, AgI, Ag2CO3, and Ag2O) by a simple impregnation route, and their photocatalytic activity of Cu(II)/Ag-based photocatalysts was evaluated by the photocatalytic decolorization of methyl orange and photocatalytic decomposition of phenol solution under visible-light illumination. As one of the typical photosensitive Ag-based compounds, the photocatalytic activity of AgCl could be greatly improved by optimizing the amount of Cu(II) cocatalyst, and the highest photocatalytic performance of the resulted Cu(II)/AgCl was higher than that of the unmodified AgCl by a factor of 2.1. Significantly, the Cu(II) was demonstrated to be a general and effective cocatalyst to improve the visible-light photocatalytic performance of other various photosensitive Agbased compounds (such as AgBr, AgI, Ag3PO4, Ag2CO3, and Ag2O) in addition to the AgCl photocatalyst. Based on the present results, it is proposed that the Cu(II) cocatalyst functions as electron scavengers to quickly capture photogenerated electrons from the excited photocatalysts and then works as reduction active sites to reduce O2 effectively, resulting in an effective separation of photogenerated electrons and holes. Compared with the expensive noble metal cocatalyst (such as Pt, Au, and Pd), the present promising Cu(II) cocatalyst can be considered to be one of the perfect cocatalysts for the smart preparation of various highly efficient photocatalysts in view of its abundance and low cost. and AgBr/Bi2WO621) have been widely developed and were demonstrated to show a higher photocatalytic performance than the single-component Ag-based photocatalysts. In recent years, we have also developed various Ag-based composite photocatalysts such as AgCl/rGO,22 AgCl/H2WO4,17 Ag/ Ag(I)-TiO2,23 and Ag2O/Bi2WO6.24 The general principle for the improved performance of composite photocatalysts is that different components have different electronic energy levels, which can promote the effective separation of photogenerated charges by their internal electric field driving force formed between their interfaces. On the other hand, cocatalyst modification has also been demonstrated to be an effective strategy to enhance photocatalytic performance. For example, noble metals (such as Pt, Pd, and Au) are effective cocatalysts to improve photocatalytic performance of various photocatalytic materials such as TiO2 and WO3.25−28 The noble metal cocatalysts usually function as reduction active sites to quickly capture photogenerated electrons from the semi-

1. INTRODUCTION To utilize visible light, more and more researchers focus on the development of highly efficient visible-light-driven photocatalysts in a wide region of solar spectrum as visible-light photocatalysis is a well-known technique that shows great potential for the control of aqueous organic contaminates or air pollutants.1−3 Recently, Ag-based compounds have been demonstrated to be a new family of visible-light photocatalytic materials such as AgCl,4−6 AgBr,7,8 AgI,9,10 Ag3PO4,11,12 Ag2CO3,13,14 and Ag2O.15,16 Compared with the well-known N-doped TiO2 visible-light photocatalyst, the Ag-based photocatalytic materials usually shows an obviously higher photocatalytic activity for the effective decomposition of various organic species.10,17 Although the real mechanism for the high photocatalytic efficiency of Ag-based photocatalytic materials is still unknown, however, it is very important and highly required to further enhance the photocatalytic activity of Ag-based photocatalytic materials in view of their potential applications. To further enhance their photocatalytic performance, one of the effective methods is to construct composite photocatalysts of Ag-based substances. Recently, various Ag-based composite photocatalysts (such as AgBr/TiO2,18 AgCl(AgBr)/rGO,19,20 © 2014 American Chemical Society

Received: October 21, 2013 Revised: April 10, 2014 Published: April 14, 2014 8891

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Cu(II)/AgCl (X). Cu(II)/AgCl photocatalyst was prepared by an impregnation technique. Typically, 0.5 g of AgCl powder was dispersed into 32 mL of Cu(NO3)2 solution under stirring. After being stirred at room temperature for 60 min, the resultant powder was recovered by filtration, rinsing, and drying to obtain Cu(II)-loaded AgCl photocatalysts. Before evaluation of the photocatalytic performance, the resulting powder was pretreated by visible light under an identical experimental condition as the AgCl sample. To investigate the effect of Cu(NO3)2 concentration on the microstructures and photocatalytic performance of AgCl photocatalyst, the concentration of Cu(NO3)2 solution was controlled to be in the range of 0.005−0.5 M. To simplify the sample name, the resulting product will be referred to as Cu(II)/AgCl (X) (X = 0.005, 0.01, 0.05, 0.1 and 0.5 M), with X representing the molar concentration of Cu(NO3)2 solution. Cu(II)/AgCl (X-H). To investigate the effect of Cu(II) cocatalyst on the photocatalytic activity of Cu(II)/AgCl, the Cu(II) cocatalyst was removed by dispersing 0.05 g of Cu(II)/ AgCl (0.05 M) photocatalyst into a 20 mL of hydrochloric acid solution (1 mol L−1) at room temperature for 60 min. After filtration, washing with distilled water and drying, the resulting product was referred to as Cu(II)/AgCl (0.05 M-H). 2.3. Preparation of Other Cu(II)/Ag-Based Compound (AgBr, AgI, Ag3PO4, Ag2CO3, Ag2O) Photocatalysts. Other Ag-based photocatalysts (such as AgBr, AgI, Ag3PO4, Ag2CO3, and Ag2O) were first prepared by a simple precipitation reaction similar to AgCl particles. Typically, AgNO3 solution was added with drop by drop into KBr, KI, Na2HPO3, Na2CO3, and KOH solutions under vigorous stirring, respectively. The resulting AgBr, AgI, Ag3PO4, Ag2CO3, and Ag2O particles were then modified by Cu(II) cocatalyst under an identical experimental condition as Cu(II)/AgCl (0.05 M), and the corresponding produces could be referred to as Cu(II)/AgBr (0.05 M), Cu(II)/AgI (0.05 M), Cu(II)/Ag3PO4 (0.05 M), Cu(II)/Ag2CO3 (0.05 M), and Cu(II)/Ag2O (0.05 M), respectively. 2.4. Characterization. X-ray diffraction (XRD) patterns were performed by a D/MAX-RBX-ray diffractometer (Rigaku, Japan). Morphological analysis was obtained on a Quanta x50 (USA-FEI) field emission scanning electron microscope (FESEM). UV−vis absorption spectra were measured using a UV−vis spectrophotometer (UV-2550, SHIMADZU, Japan). X-ray photoelectron spectroscopy (XPS) data were determined by a KRATOA XSAM800 XPS system with Mg Kα source. All the binding energy was referenced to the C 1S peak at 184.8 eV for the surface adventitious carbon. 2.5. Photocatalytic Activity. The photocatalytic performance of various samples was evaluated via the photocatalytic decolorization of MO and photocatalytic decomposition of phenol solution at room temperature.34,35 Typically, 50 mg of the photocatalyst was dispersed into 10 mL of MO (20 mg L−1) or phenol solution (10 mg L−1) in a disk with a diameter of ca. 5 cm. The mixed suspension was placed in dark for 2 h to reach the adsorption−desorption equilibrium between the photocatalyst and the solution. Under room conditions, the disk was exposed to 350 W Xe lamp equipped with a UV cutoff filter (λ > 400 nm), and the illumination intensity was ca. 40 mW cm−2. At certain time intervals, the reaction solution was centrifuged to measure the concentration of the solution. Owing to a low concentration of the used MO and phenol solutions, the corresponding photocatalytic decolorization can be regarded to be a pseudo-first-order reaction. Therefore, their

conductors and then to reduce oxygen effectively, resulting in a highly efficient separation of photogenerated charges. However, the previous results about the cocatalysts for oxygen reduction are mainly focused on the noble metal cocatalysts loaded on the surface of semiconductor oxides or sulfides, and seldom has investigation about the cocatalyst modification been explored to improve the photocatalytic performance of Ag-based photocatalytic materials.25−28 Moreover, in addition to the wellknown and expensive noble metal cocatalysts, it is very interesting and important to develop new, low-cost, and general cocatalyst to improve the photocatalytic efficiency of photocatalysts. In recent years, some transition metal ions such as Cu(II) and Fe(III) (or their oxides) have been demonstrated to be an effective cocatalyst as oxygen-reduction active sites to enhance the photocatalytic performance of TiO 2 , WO 3 , and SrTiO3.29−32 More recently, Fe(III) cocatalyst was loaded on the AgBr particle surface to form Fe(III)/AgBr with an improved photocatalytic performance.33 In the present study, Cu(II) as a cocatalyst was successfully loaded on the surface of various Ag-based photocatalysts (such as AgCl, Ag3PO4, AgBr, Ag2CO3, AgI, and Ag2O) by an impregnation method, and the photocatalytic activity of Cu(II)/Ag-based compounds was evaluated by the photocatalytic decolorization of methyl orange (MO) and photocatalytic decomposition of phenol solution under visible light illumination. It was demonstrated that the Cu(II) could work as a general cocatalyst to greatly improve the photocatalytic performance of various Ag-based photocatalysts. To the best of our knowledge, this is the first report about the Cu(II) as a general cocatalyst to enhance the photocatalytic activity of various Ag-based compounds. Compared with the expensive noble metal cocatalysts (e.g., Pt, Au, Pd), the present promising Cu(II) cocatalyst can be considered to be one of the perfect cocatalysts for the smart design and synthesis of various highly efficient photocatalysts in view of its abundance, low cost, and its simple impregnation technology.

2. EXPERIMENTAL SECTION All the reagents (analytical grade) were supplied by Shanghai Chemical Reagent Ltd. (P. R. China) and used as received without further purification. 2.1. Preparation of AgCl Particles. AgCl particles were synthesized by a facile precipitation reaction between Ag+ and Cl− ions in the solution. The starting aqueous solutions of AgNO3 solution (0.1 mol L−1) and KCl solution (0.1 mol L−1) were first prepared. In a typical synthesis, a 34.9 mL of AgNO3 solution (0.1 mol L−1) was added with drop by drop into a 34.9 mL of KCl solution (0.1 mol L−1) under vigorous magnetic stirring. After being stirred for 60 min, the resultant AgCl samples were filtrated, rinsed, and dried at room temperature. 2.2. Preparation of AgCl, Cu(II)/AgCl (X), and Cu(II)/ AgCl (X−H) Photocatalysts. AgCl. In view of the photoinduced instability of pure AgCl, the as-prepared AgCl particles was dispersed into 10 mL of MO solution (20 mg/L) and was then irradiated for 30 min under visible light to produce Ag/ AgCl. A 350 W xenon lamp with a UV-cutoff filter (λ > 400 nm) was used as a visible-light source, and the illumination intensity was ca. 40 mW cm−2. To simplify the sample name, the resulting sample was referred to as AgCl. For comparison, the Pt/AgCl (1 wt %) photocatalyst was also prepared by a similar photocatalytic reduction method with the addition of H2PtCl6 solution. 8892

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decomposition kinetics can be showed as follows: ln(c/c0) = −kt, where c0 and c are the precursor concentrations at initial state and after irradiation for t min, respectively, while k is the apparent rate constant.34,35

3. RESULTS AND DISCUSSION 3.1. Microstructures of Cu(II)/AgCl Photocatalyst. The XRD patterns of AgCl before and after Cu(II) cocatalyst modification are shown in Figure 1. It was obvious that before

Figure 2. FESEM images of (a) AgCl, (b) Cu(II)/AgCl (0.05 M), (c) Cu(II)/AgCl (0.5 M), and (d) EDX of sample c.

Figure 1. XRD patterns of (a) AgCl, (b) Cu(II)/AgCl (0.005 M), (c) Cu(II)/AgCl (0.01 M), (d) Cu(II)/AgCl (0.05 M), (e) Cu(II)/AgCl (0.1 M), and (f) Cu(II)/AgCl (0.5 M).

Cu(II) cocatalyst modification all the diffraction peaks of the sample (Figure 1a) could be attributed to AgCl phase (JCPDS file: 31-1238). After modification by Cu(II) in Cu(NO3)2 aqueous solution, the resulting Cu(II)/AgCl samples (Figure 1b−f) showed a similar diffraction peak as the unmodified AgCl, indicating that the AgCl phase could be well maintained after the Cu(II) cocatalyst modification owing to a lowtemperature impregnation process. Moreover, it should be noted that no typical diffraction peaks belonging to Cu-like compounds were observed due to a room-temperature modification process and the limited amount of the Cu(II) cocatalyst in the Cu(II)/AgCl composites (shown below). In addition, owing to the photosensitive and instable properties of the AgCl particles under light irradiation, a small amount of metallic Ag (JCPDS file: 65-2871) could be easily detected (inset in Figure 1), in good agreement with the previous studies.4 Figure 2 shows typical FESEM images of AgCl particles with different amount of Cu(II) cocatalyst modification. For the unmodified AgCl, the sample was composed of many homogeneous particles with a size of ca. 1 μm (Figure 2a). As for the Cu(II)/AgCl photocatalyst (Figure 2b,c), it was clear that no obvious change about the morphology and size of the Cu(II)/AgCl particles could be found due to its lowtemperature preparation and limited Cu(II) cocatalyst. However, compared with the unmodified AgCl sample, the EDX result (Figure 2d) suggested that the Cu(II) cocatalyst had been successfully loaded on the AgCl surface, and the amount of Cu(II) cocatalyst is about 0.6 at. %. The successful modification of Cu(II) cocatalyst on the AgCl surface could be further demonstrated by XPS results (Figure 3). It was found that both of the AgCl samples before and after Cu(II) cocatalyst modification showed the XPS peaks of Ag, Cl, O, and C elements. The Ag element was mainly from the AgCl phase and metallic Ag based on the XRD results, while the C element came from the surface adventitious carbon. As the preparation of AgCl was performed in the water, the O element with a low XPS intensity was possibly from the adsorbed water or oxygen. In addition, compared with the unmodified AgCl

Figure 3. XPS spectra of the (a) AgCl and (b) Cu(II)/AgCl (0.05 M).

sample, new XPS peaks of Cu in the Cu(II)/AgCl could be clearly found (inset of Figure 3). The binding energy values of the Cu 2p3/2 and Cu 2p1/2 are 932.6 and 952.7 eV,36 respectively, suggesting that the Cu cocatalyst is in the 2+ state. According to previous studies, the Cu(II) was in an amorphous CuO-like structure.37,38 The amount of the Cu(II) cocatalyst was calculated to be ca. 0.88 at. % according to XPS results. The UV−vis spectra of the AgCl samples with different amount of Cu(II) cocatalyst modification are shown in Figure 4. It was clear that the AgCl sample (Figure 4a) also showed obvious visible light absorption in the 400−800 nm region in addition to its band gap absorption at ca. 400 nm. Considering the instability of AgCl particles, the visible-light absorption of

Figure 4. UV−vis spectra of (a) AgCl, (b) Cu(II)/AgCl (0.005 M), (c) Cu(II)/AgCl (0.05 M), and (d) Cu(II)/AgCl (0.1 M). 8893

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the Cu(II) concentration was controlled to be 0.05 M, the Cu(II)/AgCl (0.05 M) (Figure 5d) showed the highest photocatalytic activity (k = 4.2 × 10−2 min−1), a value larger than that of the AgCl by a factor of 2.1. Although the Cu(II)/ AgCl samples exhibited a decreased photocatalytic activity with further increase of the Cu(II) concentration, they still represented a higher photocatalytic performance than the pure AgCl photocatalyst. The present experimental results were in good agreement with other cocatalyst-modified photocatalysts (such as Cu(II)/TiO239 and Fe(III)/AgBr33). On the other hand, compared with the well-known Pt-modified AgCl (Pt/AgCl) photocatalyst, the prepared Cu(II)/AgCl samples could show a slightly higher photocatalytic performance by optimizing the Cu(II) cocatalyst. Therefore, in addition to the improved photocatalytic performance, the Cu(II) cocatalyst with its abundance and low cost should have more advantages for the potential various applications. To investigate the performance stability of Cu(II)-modified AgCl, we repeated the photocatalytic activity for AgCl and Cu(II)/AgCl (0.05 M) for five times, and their results are shown in Figure 6A,B. It is found that both of the AgCl and Cu(II)/AgCl can effectively decolorize MO solution and preserve a steady and highly efficient photocatalytic performance during repeated tests. Moreover, after loading Cu(II) cocatalyst, the resulting Cu(II)/AgCl clearly exhibits a higher photocatalytic activity than the AgCl sample. In view of a comparable morphology and phase structure of AgCl before and after Cu(II) modification, it is very clear that the Cu(II) can work as an effective cocatalyst to greatly enhance the photocatalytic performance of AgCl photocatalyst. In addition, further experiments suggested that the Cu(II)/AgCl composite photocatalyst also showed a more effective photocatalytic recycle ability for the decomposition of phenol solution than the AgCl, as shown in Figure 6C. Therefore, the above results strongly demonstrated that the photocatalytic performance of

AgCl sample (Figure 4a) could be ascribed to its localized surface plasmon resonance (LSPR) of metallic Ag nanoparticles, in good agreement with the above XRD results (Figure 1). After modified by Cu(II) cocatalyst, the resulting Cu(II)/AgCl samples exhibited a similar UV−vis absorption spectra due to a very limited amount of Cu(II) cocatalyst loaded on its surface. Moreover, the above results further suggested that the morphology and phase structure of AgCl could be well maintained by the Cu(II) loading progress. Therefore, the different photocatalytic activity of the AgCl and Cu(II)/AgCl samples was only caused by the loading of very limited Cu(II) cocatalyst. 3.2. Photocatalytic Performance of Cu(II)/AgCl Photocatalysts. The photocatalytic activities of the Cu(II)/AgCl photocatalysts were first evaluated by the photocatalytic decolorization of MO solution, as shown in Figure 5. It was

Figure 5. Rate constant (k) for various photocatalysts: (a) AgCl, (b) Cu(II)/AgCl (0.005 M), (c) Cu(II)/AgCl (0.01 M), (d) Cu(II)/AgCl (0.05 M), (e) Cu(II)/AgCl (0.1 M), (f) Cu(II)/AgCl (0.5 M), and (g) Pt/AgCl (1 wt %).

clear that the unmodified AgCl photocatalyst (Figure 5a) showed a comparable visible-light activity, and the resultant k is 2.0 × 10−2 min−1. After modification by Cu(II) cocatalyst, all the resultant Cu(II)/AgCl samples (Figure 5b−f) exhibited a remarkably higher photocatalytic performance. Especially, when

Figure 6. Cycling runs of (A) AgCl and (B) Cu(II)/AgCl (0.05 M) for the photodegradation of MO. (C) Cycling runs of AgCl and Cu(II)/AgCl (0.05 M) for the photodegradation of phenol. 8894

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oxygen. In fact, the multielectron reduction of oxygen was also widely existed in the Fe(III)/TiO2,31,41 Fe(III)/AgBr,33 and Cu(II)/TiO238 system. Owing to the rapid transfer and effective separation of photogenerated charges by the Cu(II) cocatalyst, the resulting Cu(II)/AgCl samples indicated significantly enhanced photocatalytic performance. Moreover, in addition to the LSPR absorption of Ag nanoparticles, a possible direct-interfacial-charge transfer from the valence band of AgCl to the Cu(II) cocatalyst also contributes to the improved photocatalytic performance of Cu(II)/AgCl (Figure 7b). To further demonstrate the significant effect of Cu(II) cocatalyst on the photocatalytic performance of AgCl photocatalyst, the prepared Cu(II)/AgCl (0.05 M) was dispersed into HCl solution to remove the Cu(II) cocatalyst, and the resulting Cu(II)/AgCl (0.05 M-H) was analyzed by EDX and XPS (Figure 8). EDX (Figure 8b) and XPS (Figure 8c) results clearly showed that no Cu element could be detected after the treatment of Cu(II)/AgCl (0.05 M) in HCl solution, indicating the successful removal of Cu(II) cocatalyst from the surface of Cu(II)/AgCl (0.05 M). When the resulting Cu(II)/AgCl (0.05 M-H) was used as a new photocatalyst to decompose the MO solution under an identical experimental conditions, it was interesting to find that the Cu(II)/AgCl (0.05 M-H) showed an obviously decreased photocatalytic activity (k = 2.4 × 10−2 min−1) compared with the Cu(II)/AgCl (0.05 M) (Figure 8d). Therefore, the above results further strongly demonstrated that Cu(II) could work as an effective cocatalyst to promote the rapid separation of photogenerated electrons and holes, leading to an enhanced visible-light activity of Cu(II)/AgCl photocatalysts. 3.4. Photocatalytic Performance and Photocatalytic Mechanism of Cu(II)/Ag-Based Photocatalysts. For an effective cocatalyst of photocatalytic materials, it is very vital and interesting to consider that whether the Cu(II) cocatalyst can become a general cocatalyst to enhance the photocatalytic performance of different photocatalytic materials. In addition to AgCl photocatalyst, in this study, other Ag-based photosensitive photocatalysts such as AgBr, AgI, Ag3PO4, Ag2CO3, and Ag2O were also prepared by a facile precipitation method, and their surfaces were loaded with Cu(II) cocatalyst under an identical experimental conditions as the Cu(II)/AgCl. The photocatalytic experimental results indicated that, compared with their corresponding unmodified samples, all the Cu(II) cocatalyst loaded Ag-based photocatalysts exhibited an obviously enhanced photocatalytic performances (Figure 9). In fact, the different photocatalytic activities of unmodified Agbased photocatalysts were caused by their different semiconductor materials with different crystal structures, while their improved photocatalytic performance could only contribute to the Cu(II) cocatalyst loaded on their surface. Therefore, the above results definitely demonstrated that Cu(II) cocatalyst was a new, effective, and general cocatalyst to greatly improve the photocatalytic performance of Ag-based photocatalytic materials. Moreover, the low-temperature impregnation method of Cu(II) cocatalyst is a very simple, mild, and environmentalfriendly route, which may provide some new insights for the preparation and various potential applications of high-performance photocatalysts. To further investigate the possible mechanism of Cu(II) cocatalyst loaded Ag-based photocatalysts, the band structures of AgBr, AgI, Ag3PO4, Ag2CO3, and Ag2O are first investigated. It is well-known that all the Ag-based photocatalysts can

AgCl photocatalyst could be greatly improved by the Cu(II) cocatalyst via a simple impregnation method. To further evaluate the stability of Cu(II)/AgCl photocatalyst before and after visible-light illumination, the samples of Cu(II)/AgCl (0.05 M) before and after 5 time runs (Figure 6B) were tested by EDX, and the corresponding results are shown in Figure S1. It was found that the amount of Cu(II) ions was about 0.46 and 0.38 at. % for the as-prepared Cu(II)/ AgCl and 5-time repeated Cu(II)/AgCl, respectively. Therefore, the Cu(II) in Cu(II)/AgCl photocatalyst can be regarded to be stable in the present study (pH = 6.5). In addition, the particle size of AgCl phase showed no change before and after 5-time repeated reactions, suggesting the stable structure of Cu(II)/AgCl photocatalyst. 3.3. Photocatalytic Mechanism of Cu(II)/AgCl Photocatalyst. The above results highlight an important function of Cu(II) as an efficient cocatalyst for the enhanced photocatalytic performance of AgCl photocatalyst. To investigate the improved photocatalytic performance of AgCl by Cu(II) cocatalyst, a proposed visible-light photocatalytic mechanism of Cu(II)/AgCl is shown in Figure 7. Considering a wide band

Figure 7. Schematic diagram illuminating the photocatalytic mechanism: (a) AgCl and (b) Cu(II)/AgCl.

gap of AgCl (ca. 3.25 eV), the visible light absorption in AgCl sample can be ascribed to the plasmon resonance absorption of Ag nanoparticles which is formed by the partial decomposition of AgCl.4 For the AgCl photocatalyst (Figure 7a), the plasmoninduced electrons of Ag nanoparticles can transfer to the conduction band (CB) of AgCl to reduce oxygen in view of a more negative potential of the CB of AgCl (ca. −0.06 V)17 than the one-electron oxygen reduction (O2 + H+ + e− = HO2, −0.046 V vs SHE),40 while the plasmon-induced holes are located on the Ag nanoparticle surface to oxidize organic substances with a high efficiency. After the Cu(II) cocatalyst is loaded on the AgCl surface, the resulting Cu(II)/AgCl shows an obviously different mechanism for the electron transfer (Figure 7b). Considering a more positive potential (0.16 V, vs SHE)40 of Cu2+/Cu+ than the one-electron reduction of O2 (O2/HO2, −0.046 V vs SHE),40 it is possible that the Cu(II) cocatalyst transfers photoexcited electrons from the AgCl CB to oxygen by multielectron reduction strategies (O2 + 2H+ + 2e− = H2O2(aq), +0.682 V vs SHE; O2 + 4H+ + 4e− = 2H2O(aq), +1.23 V vs SHE).40 In this case, the Cu(II) cocatalyst first transforms into Cu(I) by accepting a photogenerated electron from the CB of AgCl. In fact, the Cu(I) ions are unstable and can easily turn into Cu(II) via the oxygen reduction under an ambient condition (4Cu+ + 4H+ + O2 → 4Cu2+ + 2H2O or 4Cu+ + 2H2O + O2 → 4Cu2+ + 4OH−);40 namely, the Cu(II) can be easily restored by the Cu(I) oxidation in the present of 8895

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Figure 8. (a) FESEM image, (b) EDX, and (c) XPS spectra of the Cu(II)/AgCl (0.05 M-H). (d) Photocatalytic degradation of MO for Cu(II)/AgCl (0.05 M) and Cu(II)/AgCl (0.05 M-H).

Figure 9. Photocatalytic performance of various Ag-based photocatalysts: (a) AgBr, (a′) Cu(II)/AgBr (0.05 M), (b) AgI, (b′) Cu(II)/ AgI (0.05 M), (c) Ag3PO4, (c′) Cu(II)/Ag3PO4 (0.05 M), (d) Ag2CO3, (d′) Cu(II)/Ag2CO3 (0.05 M), and (e) Ag2O, (e′) Cu(II)/ Ag2O (0.05 M).

Figure 10. Schematic diagram showing the general photocatalytic mechanism of Cu(II)/Ag-based photocatalysts.

strongly absorb visible light due to their small band gaps. According to the previous studies, the band gaps of AgBr, AgI, Ag3PO4, Ag2CO3, and Ag2O, are ca. 2.60,42 2.80,10 2.90,43 2.50,13 and 1.4 eV,16 respectively. In this case, under visible light illumination, the photogenerated electrons are excited from the VB to the CB of photocatalysts and are then transferred to the Cu(II) cocatalyst to reduce oxygen with a high efficiency (Figure 10). In addition, owing to the photoinduced instability of the Ag-based photocatalysts, the plasmon-induced electrons of Ag nanoparticles formed from the decomposition of Ag-based photocatalysts can also be injected into the Cu(II) cocatalyst. Therefore, the Cu(II) cocatalyst works as an efficient active site to quickly transfer the photogenerated electrons from the CB or metallic Ag nanoparticles to the adsorbed oxygen, resulting a highly efficient oxidation reaction of organic substances by the photogenerated holes.

4. CONCLUSION In summary, Cu(II) cocatalyst was found to function as a new, effective, and general cocatalyst to significantly promote the photocatalytic performance of various Ag-based photosensitive photocatalysts such as AgCl, Ag3PO4, AgBr, Ag2CO3, AgI, and Ag2O. It is believed that the Cu(II) cocatalyst functions as an electron sink to quickly capture photogenerated electrons from the photocatalysts and then acts as a reduction active site to reduce oxygen effectively, resulting in a highly efficient separation of photogenerated charges and improved photocatalytic activity. Compared with the expensive noble metal cocatalyst (such as Pt, Au, and Pd), the present abundant and low-cost Cu(II) cocatalyst can be regarded as one of the ideal cocatalysts for the various potential applications of highperformance photocatalytic materials. In addition, the simple, mild, and environmental-friendly impregnation method of Cu(II) cocatalyst is possibly widely applied to prepare new cocatalyst-modified photocatalysts. 8896

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ASSOCIATED CONTENT

S Supporting Information *

FESEM images and EDX of the as-prepared and 5-time repeated Cu(II)/AgCl. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone 0086-27-87871029, Fax 0086-27-87879468; e-mail [email protected] (H.Y.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the National Natural Science Foundation of China (61274129, 21277107, and 51208396) and the 973 Program (2013CB632402). This work was also financially supported by program for new century excellent talents in university (NCET-13-0944), Wuhan Youth Chenguang Program of Science and Technology (2014070404010207), the Project-sponsored by SRF for ROCS, SEM, and Fundamental Research Funds for the Central Universities (Grants 2013-1a-039 and 2013-1a-036). We thank the FGM group at Wuhan University of Technology for assistance with XRD and SEM measurements.

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