Ag Composite

Article pubs.acs.org/IECR

One-Pot Room Temperature Synthesis of Cu2O/Ag Composite Nanospheres with Enhanced Visible-Light-Driven Photocatalytic Performance Weixin Zhang,* Xiaoning Yang, Qian Zhu, Kun Wang, Jianbo Lu, Min Chen, and Zeheng Yang* School of Chemistry and Chemical Engineering, Hefei University of Technology and Anhui Key Laboratory of Controllable Chemical Reaction & Material Chemical Engineering, Hefei, Anhui 230009, People’s Republic of China S Supporting Information *

ABSTRACT: Cu2O/Ag composite nanospheres (CNSs) with tunable Ag coverage and optical properties have been prepared based on a one-pot room temperature method by adding AgNO3 solution to fresh Cu2O nanosphere-produced mother solution in various ratios. Ag+ ions can be reduced by the primary Cu2O nanospheres in the acidic solution, and the obtained Ag nanoparticles can be deposited on the surfaces of the Cu2O nanospheres. The photocatalytic activity of Cu2O/Ag CNSs has been evaluated for photodegradation of methyl orange (MO) dye under visible-light irradiation, which demonstrates that Cu2O nanospheres with Ag loading exhibit significantly enhanced photocatalytic activity compared with pure Cu2O counterparts, and their photocatalytic properties depend on the coverage density of Ag nanoparticles. The enhanced photocatalytic activity can be attributed to the deposition of Ag acting as electron sinks to prevent the recombination of the photogenerated electrons and holes, and the plasmon resonances of the Ag nanoparticles generating more electron−hole pairs in the semiconductor. photocatalytic activity will be improved.15 In addition, the plasmon resonance of noble metal nanoparticles (NPs) is expected to enhance absorption of incident photons, which will enhance the photocatalytic efficiency of the semiconductors.16 Therefore, it is postulated that Cu2O nanospheres modified with Ag particles may exhibit an enhanced photocatalytic activity. In the case of the synthesis of Cu2O/Ag CNSs, four typical methods have been reported including the photocatalytic method, the electron beam irradiation method, hydrothermal method and the liquid phase reduction method. For instance, Wang et al.17 reported the synthesis of Cu2O/Ag heterogeneous nanocrystals by depositing Ag nanoparticles onto the surfaces of Cu2O octahedral nanocrystals with edge length of 300−500 nm by the photocatalytic effect of Cu2O under the irradiation of visible light and found that degradation ratio of Pyronine B in the presence of Cu2O/Ag is 74%. Lin et al.18 presented a one-step electron beam irradiation method for the synthesis of Ag/Cu2O nanocomposites with irregular sizes and shapes, which has higher photocatalytic efficiency than that of Cu2O under visible light. The method involved the reduction of Ag+ and Cu2+ ions in the aqueous solution to Ag/Cu2O without adding any reducing reagent just under electron beam (EB) irradiation. Pan et al.19 prepared Ag/Cu2O hybrids with different Ag contents by reducing AgNO3 and Cu(Ac)2 with Na2SO3 via a hydrothermal method. The samples were used as catalysts for degradation of p-nitrophenol in wastewater and degradation ratio of the sample with accretion Ag content of 25% was 98.1% after being irradiated for 180 min. Yang et al.20

1. INTRODUCTION Over the last decades, photocatalysis has attracted special attention as an economic and environmentally safe option for solving energy and pollution problems.1 Since the discovery of photocatalytic splitting of water on a TiO2 electrode in 1972,2 an enormous research effort has been dedicated to the study of semiconductors in the area of photocatalysis. Among various semiconductors showing photocatalytic activity, wide band gap semiconductors such as TiO2, ZnO, etc. bear tremendous hope in helping ease the energy and environment crisis through effective utilization of solar energy based on photovoltaic and water-splitting devices.3,4 Unfortunately, their wide band-gaps do not allow the utilization of visible light, which limit their practical applications. Therefore, it is imperative to develop new kinds of photocatalysts with high activities under visible light in view of the efficient utilization of solar energy. Cu2O is one of the few p-type direct band gap semiconductors with a narrow band gap of 2.17 eV,5 which matches the solar visible spectrum well. With the additional advantages of nontoxicity, low cost and abundance of its starting material, Cu2O has attracted interest as a good candidate material for photocatalytic degradation of organic pollution, such as methyl orange (MO)6,7 and brilliant red X-3B,8 etc. However, previous reports indicated that Cu2O usually exhibited low photocatalytic activity,9,10 and could be deactivated by photocorrosion.11 Ag is well-known for its intense interactions with visible-light via the resonance of the oscillations of the free electrons within the particles,12 and is considered as a relatively cheap noble metal. Photoexcited electrons in the conduction band (CB) of a semiconductor can be transferred to the noble metals, which act as electron sinks due to the Schottky barrier at the metal−semiconductor interface, while the holes can remain on the semiconductor surface.13,14 The recombination of electrons and holes can therefore be prolonged, and the © XXXX American Chemical Society

Received: July 9, 2014 Revised: September 16, 2014 Accepted: September 30, 2014

A

dx.doi.org/10.1021/ie502737t | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

Article

was used as the internal standard. The concentration of dye methyl orange was also measured on a Shimadzu 2550 UV−vis spectrophotometer by detecting the absorbance of the solution in the wavelength range of 200−800 nm. 2.5. Photocatalytic Activity. The photocatalytic activities of the Cu2O nanospheres and Cu2O/Ag CNSs were evaluated based on photodegradation of methyl orange (MO) in an aqueous solution under visible-light irradiation at a distance of 10 cm. A 500 W xenon lamp was used as the light source. Typically, 0.03 g of the samples was dispersed in 100 mL of MO aqueous solution with different initial concentrations, and then magnetically stirred in the dark for 30 min to reach adsorption equilibrium. At intervals of every 10 min, 3 mL of the dispersion was drawn from the system. After removal of the catalyst by centrifugation, residual MO concentration was determined by detecting the characteristic adsorption peak intensity at 463 nm on a UV−visible spectrometer. The degradation ratio was calculated using the following equation:

synthesized hollow Cu2O/Ag composite nanoframes (Cu2O/ Ag CNFs) with tunable silver content by reduction of Ag+ with sodium citrate in a 14 day old Cu2O-containing mother solution and then a second component (Ag) with a size of about 90−120 nm was deposited onto primary nanomaterials (Cu2O nanoframes of 1−2 μm in size). In this paper, Cu2O/Ag CNSs have been prepared by a facile one-pot room temperature reduction method. Ag+ ions can be reduced by the fresh Cu2O nanospheres in the acidic solution, and Ag nanoparticles can be directly deposited on the Cu2O nanospheres. Ag content on the Cu2O nanospheres can be tuned by controlling the volume of AgNO3 solution, which further influences the photocatalytic performances greatly as well as the optical properties. The photocatalytic activities of Cu2O/Ag CNSs have been investigated by photodegradation of methyl orange (MO) under the irradiation of visible light in comparison with pure Cu2O counterparts.

2. EXPERIMENTAL SECTION 2.1. Materials. All reagents were analytical grade and used without further purification. Silver nitrate (AgNO3, 99.8%) was purchased from Beijing Chemical Reagent Company. Cupric acetate hydrate (Cu(CH3COO)2·H2O) was purchased from Changchun Fine Chemical Company. Hydrazine hydrate (N2H4·H2O) was purchased from Sinopharm Chemical Reagent Co., Ltd., China. Deionized water was used in all of the experiments. 2.2. Preparation of Cu2O Nanospheres. Similar to previous studies,21 Cu2O nanospheres were prepared by reducing Cu(CH3COO)2·H2O with N2H4·H2O. Briefly, 0.159 g of Cu(CH3COO)2·H2O was dissolved in 40 mL of deionized water in a beaker and stirred with a magnetic stirrer to give a clear solution. 8.0 mL of N2H4·H2O (0.1 mol/L) solution was quickly added into the mixture under vigorous stirring, which resulted in an immediate color change from blue to bright yellow. The mixture was further stirred for 30 min to complete the reaction. Then, the precipitates were separated by centrifugation, washed with deionized water and absolute ethanol several times each, and dried in an oven at 60 °C for 4 h. 2.3. Preparation of Cu2O/Ag CNSs. The procedure for the synthesis of Cu2O/Ag CNSs was only one step more than the preparation of Cu2O nanospheres. The AgNO3 (0.05 mol/ L) solution was added dropwise into the Cu2O-containing mother solution and the suspension was stirred for another 30 min before centrifugation. Samples prepared with 0.5, 1 and 2 mL of AgNO3 are referred to as Cu2O/Ag(1) CNSs, Cu2O/ Ag(2) CNSs and Cu2O/Ag(3) CNSs, respectively. 2.4. Catalyst Characterization. The phase purity and the structure of the samples were investigated by X-ray powder diffraction based on a Japan Rigaku D/max-γB X-ray diffraction instrument with Cu Kα radiation (λ = 0.154 178 nm), operated at 40 kV and 80 mA. The morphology and structures of the asprepared samples were analyzed by field-emission scanning electron microscopy (FESEM, FEI Sirion-200) at an accelerating voltage of 10 kV and high-resolution transmission electron microscopy (HRTEM, JEM-2100F) at an accelerating voltage of 200 kV. The composition and the elemental state of the samples were characterized using X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi) with Al Kα radiation as the excitation source. Diffuse reflectance UV−visible spectra within a wavelength range of 200−800 nm were recorded on a Shimadzu 2550 UV−visible spectrometer, and BaSO4 powder

D (%) = (C0 − C)/C0 × 100% = (A 0 − A)/A 0 × 100% (1)

where D represents the degradation ratio, C0 and C are the concentrations of MO in solution at times corresponding to 0 and t moment, and A0 and A are the absorbance of the MO solution at times corresponding to 0 and t at the characteristic adsorption wavelength of 463 nm, respectively. To guarantee the reliability of the experiment, a set of parallel experiments was conducted to estimate the experimental error.

3. RESULTS AND DISCUSSION 3.1. Structures and Morphologies of the Samples. The composition and phase purity of the prepared samples were examined by XRD. Figure 1 shows a typical XRD pattern of the

Figure 1. XRD patterns of (a) Cu2O nanospheres, (b) Cu2O/Ag(1) CNSs, (c) Cu2O/Ag(2) CNSs, and (d) Cu2O/Ag(3) CNSs.

as-obtained Cu2O nanospheres and Cu2O/Ag(2) CNSs. All of the diffraction peaks can be well indexed to cubic Cu2O (JCPDS card No. 05-0667). After Ag NPs are deposited on Cu2O, new sharp peaks labeled with asterisks come from cubic Ag (JCPDS card No. 04-0783). No other obvious crystalline impurities can be observed. The X-ray photoelectron spectra have been taken to further confirm the composition and the elemental states in Cu2O/Ag CNSs. The binding energies in the XPS spectra presented in Figure 2 are calibrated by referring that of C 1s (285.0 eV). In Figure 2a, all of the peaks on the curve can be ascribed to Cu, Ag, O and C elements. The presence of C mainly results from B



Article

Figure 2. XPS spectrum of Cu2O/Ag(2) CNSs. (a) XPS full spectrum of the sample, (b) Cu 2p spectrum, (c) Ag 3d spectrum and (d) O 1s spectrum.

the hydrocarbon from the XPS instrument itself.22 Figure 2b shows the XPS spectrum in the Cu 2p3/2 and Cu 2p1/2 binding energy region with a spin orbital separation of 19.75 eV. The Cu 2p3/2 peak is centered at 932.68 eV, which is close to that of Cu2O, and no peaks related with CuO (933.6 eV) are observed.23−25 The high-resolution XPS spectra of Ag deposited on the Cu2O nanospheres in Figure 2c indicate that the Ag 3d5/2 peak appears at a binding energy of 368.68 eV, and the splitting of the 3d doublet is 6.0 eV, which hints that silver is of metallic nature.26 The O 1s region shown in Figure 3d could be fit into two peaks. The main peak (530.78 eV) is attributed to the Cu−O in Cu2O27 whereas the minor peak (531.92 eV) can be ascribed to O adsorbed on the surface of the sample.28,29 The morphology and structures of the as-prepared samples were examined by FESEM. As observed in Figure 3a, spherical

Cu2O nanospheres with an average diameter of 300 nm are distributed homogeneously over a large scale. The highmagnification FESEM image in Figure 3b shows that the Cu2O spheres with rough surfaces are formed through the aggregation of smaller nanocrystals of about 50 nm. The lowmagnification FESEM image of the prepared Cu2O/Ag(2) CNSs in Figure 3c shows that many tiny nanoparticles are deposited on the surfaces of the Cu2O nanospheres. The highmagnification FESEM image in Figure 3d displays the nanoparticles size ranging from 20 to 50 nm with irregular shapes. With increasing volume of AgNO3 solution, the size of Ag particles gradually increases (Figure S1, Supporting Information). The morphology and microstructure of the prepared Cu2O/ Ag CNSs were further characterized with TEM and HRTEM. Figure 4a shows a TEM image of Cu2O/Ag(2) CNSs. The contrast difference of the light color interior and dark color shell indicates the hollow structure of the nanospheres. The corresponding HRTEM image (Figure 4c) is taken from the edge area marked by a circle in Figure 4b, which reveals a distinct set of visible lattice fringes. The fringes with a value of 0.205 nm, 0.231 nm correspond to the (200) and (111) planes of cubic Ag, whereas the fringes with value of 0.305 nm, 0.249 nm are consistent with the (110) and (111) planes of cubic Cu2O. Combined with FESEM images, it can be inferred that the Ag nanoparticles are closely deposited on the surfaces of the Cu2O nanospheres. Energy dispersive X-ray spectroscopy (EDS) for elemental mapping is a useful technique for elemental composition analysis and distribution identification. The TEM-EDS elemental mappings of the as-prepared Cu2O/Ag CNSs in Figure 4d−f shows that the distribution of Ag (Figure 4f) element with the similar contours shown in Figure 4a and the distributions of Cu (Figure 4d) and O (Figure 4e) elements with slightly smaller contours than that further reveal that the Ag element deposits outside the Cu2O nanospheres. In a word,

Figure 3. FESEM images of (a, b) Cu2O nanospheres and (c, d) Cu2O/Ag(2) CNSs. C



Article

Figure 4. TEM image of (a) Cu2O/Ag(2) CNSs, (b and c) HRTEM images of Ag and Cu2O taken from the Cu2O/Ag(2) CNSs. Elemental mapping images of Cu2O/Ag: (d) Cu mapping, (e) O mapping and (f) Ag mapping.

Figure 5. Nitrogen adsorption−desorption isotherm and corresponding pore size distributions (inset) of (a) Cu2O nanospheres, (b)Cu2O/Ag(1) CNSs, (c)Cu2O/Ag(2) CNSs and (d)Cu2O/Ag(3) CNSs.

respectively. Cu+ ions have higher diffusion rate than Ag+ ions, which results in relatively fast outward movement of Cu+ ions, subsequent evacuation of nanospheres interior, and thus the formation of hollow structures. During the process, a classic “Kirkendall Effect” occurs, which is commonly recognized as the formation mechanism of some hollow structures.31 Ag nanoparticles were gradually deposited on the surface of Cu2O nanospheres while interior Cu2O were continually consumed with the extension of the reaction time. Simultaneously, the unseparated Cu2O nanospheres in the mother solution became hollow inside as the crystallites or amorphous domains in the inner cores gradually dissolved and recrystallized in the outer shells due to Ostwald ripening.21,32 Thus, the interior of the nanospheres was gradually evacuated

the mapping results clearly demonstrate homogeneous coating of Ag nanoparticles on the surface of the Cu2O nanospheres. The synthesis mechanism for Cu2O/Ag CNSs is discussed as the following. Because the standard reduction potential of the Cu2+/Cu2O pair (0.203 V) is lower than that of the Ag+/Ag pair (0.799 V), Cu2O on the surface of the nanospheres is initially oxidized by Ag+ ions in the solution, leading to the formation of a layer of Ag deposited on the surface. The reaction can be described as follows:30 Cu 2O + 2Ag + + 2H+ → 2Cu 2 + + 2Ag + H 2O

(2)

In this reaction, Cu+ ions tend to diffuse outward and, meanwhile, Ag+ ions have a tendency to move inward. The concentration gradient of Cu+ ions and Ag+ ions serves as the driving force for their outward and inward diffusion, D



Article

linear calibration curve presented in Figure S2 (Supporting Information) has a correlation coefficient of 0.9998, indicating a good linear relation between the absorption peak intensity and the concentration of MO aqueous solution. Prior to the irradiation, the adsorption in the dark was conducted so as to reach adsorption equilibrium between the photocatalysts and MO. The adsorption equilibrium could be reached at 30 min (Figure S3, Supporting Information), and the corresponding adsorption ratios of Cu2O, Cu2O/Ag(1), Cu2O/Ag(2) and Cu2O/Ag(3) are 3.51%, 3.88%, 4.26% and 6.51%, respectively, which is agreement with their BET surface areas. Figure 7 plots the variation of the relative concentration of MO aqueous solution (the initial concentration C0 = 10 mg/L)

and led to the formation of Cu2O/Ag CNSs with more hollow structures. H+ ions come from the Cu2O-containing mother solution, which has a pH value of about 4.8. After the reaction, the color of the solution is slightly blue, which implies the generation of Cu2+. Because the amount of Cu2O is much higher than that of Ag+, only a small amount of Cu2O participate in the reaction and a limited amount of Ag nanoparticles deposit on the surface of the unreacted Cu2O nanospheres. Nitrogen sorption experiments were undertaken to characterize the specific surface area and porosity of the Cu2O nanospheres and Cu2O/Ag CNSs. The results in Figure 5 show that all the samples have a type-IV isotherm with a hysteresis loop, indicating their mesoporous structure.33 The insets show that they have a broad distribution of pore sizes. To make a clear comparison between four samples, Table 1 summarized the data of their specific surface area, pore size and pore volume. Table 1. Summary of the Specific Surface Area, Average Pore Size and Pore Volume of Cu2O Nanospheres and Cu2O/Ag CNSs sample

specific surface area (m2/g)

average pore size (nm)

pore volume (cm3/g)

Cu2O Cu2O/Ag(1) Cu2O/Ag(2) Cu2O/Ag(3)

9.502 11.209 13.565 15.259

3.220 3.212 3.216 4.053

0.053 0.069 0.070 0.088

Figure 7. Time-dependent degradation ratios of MO aqueous solution (10 mg/L) in the presence of different catalysts and under exposure to visible light, respectively: (a) Cu2O nanospheres, (b) Cu2O/Ag(1) CNSs, (c) Cu2O/Ag(2) CNSs and (d) Cu2O/Ag(3) CNSs. Each data point and error bar represents the mean and the standard deviations, respectively, of independent quadruplicates.

It is known that the optical properties of semiconductors are recognized as important factors to evaluate their photocatalytic activity. Figure 6 shows the typical UV−vis absorption spectra

with the irradiation time based on its UV−vis spectral changes (Figure S4, Supporting Information). After 110 min of visiblelight illumination, the MO degradation ratio over Cu2O, Cu2O/ Ag(1), Cu2O/Ag(2) and Cu2O/Ag(3) can reach 72.13%, 86.56%, 96.11% and 76.10%, respectively. For comparison, a blank experiment was conducted under visible-light irradiation in the absence of any photocatalyst. The photolysis ratio of MO is merely 1.90% (Figure S5, Supporting Information), manifesting that photocatalysts play a dominant role in the decomposition of MO molecules. The result shows that Cu2O/ Ag(2) CNSs possess the highest photocatalytic activity than other catalysts. For a detailed analysis of the photocatalytic degradation kinetics of the MO aqueous solution in our experiments, we apply the pseudo-first-order model as expressed by eq 3, which is generally used for the photocatalytic degradation process if the initial concentration of the pollutant is low.37

Figure 6. UV−vis absorption spectra of (a) Cu2O nanospheres, (b) Cu2O/Ag(1) CNSs, (c) Cu2O/Ag(2) CNSs and (d) Cu2O/Ag(3) CNSs.

for Cu2O nanospheres and Cu2O/Ag CNSs. Intrinsic band gap absorption can be observed for the Cu2O samples in the 400 to 500 nm range without a sharp absorption peak, for the “direct forbidden gap” nature of Cu2O.34 Compared with the Cu2O nanospheres, the Cu2O/Ag CNSs have another broad absorption peak at 500−800 nm. This absorption peak should be attributed to the surface plasmon resonance (SPR) effect of spatially confined electrons in metallic Ag nanoparticles.35,36 3.2. Photocatalytic of the Samples. The pure Cu2O and Cu2O/Ag were evaluated as photocatalysts for the photodegradation of MO dye under visible-light irradiation. The

ln(C0/C) = kt

(3)

where C0 and C are the concentrations of dye in solution at times corresponding to 0 and t, respectively, and k is the pseudo-first-order rate constant. Figure 8 shows the photocatalytic degradation kinetics of MO in solution on the basis of the data plotted in Figure 7. The rate constants obtained from the regression lines in Figure 8 are listed in Table 2. It shows more clearly that with the Cu2O/Ag (2) CNSs as the catalyst, the MO degradation ratio is 96.11% and the pseudo-first-order E



Article

Figure 9. Time-dependent degradation ratios of MO aqueous solution at different initial concentrations in the presence of Cu2O/Ag(2) CNSs: (a) 5 mg/L, (b) 10 mg/L, (c) 20 mg/L, (d) 30 mg/L and (e) 40 mg/L. Each data point and error bar represents the mean and the standard deviations, respectively, of independent quadruplicates.

Figure 8. Photoatalytic degradation kinetics of MO solution in the presence of different catalysts: (a) Cu2O nanoshperes, (b) Cu2O/ Ag(1) CNSs, (c) Cu2O/Ag(2) CNSs, and (d) Cu2O/Ag(3) CNSs.

Table 2. Summary of the Photocatalytic Activity of the Catalysts sample

degradation ratio (%)

k (min−1)

R2

Cu2O Cu2O/Ag(1) Cu2O/Ag(2) Cu2O/Ag(3)

72.13 86.56 96.11 76.10

0.0118 0.0180 0.0274 0.0136

0.9995 0.9995 0.9984 0.9992

dye ions, thereby reducing the number of active sites for ·OH radicals, which is an extremely strong oxidant for the mineralization of organic chemicals.42 To examine the durability of Cu2O/Ag(2) NCSs for water treatment, their life assessment was performed. The corresponding results are presented in Figure 10. All the experiments

rate constant k is 0.0274 min−1, which are higher than those under the other three circumstances. The enhanced photocatalytic property of the Cu2O/Ag CNSs can be ascribed to the surface plasmon resonance and the electron sink effect of Ag nanoparticles. It was postulated that the photoexcited electrons in the conduction band of Cu2O nanospheres can transfer to Ag nanoparticles, which act as electron sinks due to the Schottky barrier at the metal− semiconductor interfaces,14 leading to a drastic improvement in photocatalytic activity over an extended wavelength range.38 The charge separation, in turn, prevents the recombination of electrons and holes and thus enhances the photocatalytic activity of Cu2O. The photoexcited plasmonic energy in the Ag nanoparticles is transferred to Cu2O, generating more electron−hole pairs in the Cu2O, which also is beneficial to improve the photocatalytic activity.39 The decreased photocatalytic effect of Cu2O/Ag(3) compared to Cu2O/Ag(2) might be because the number of active sites capturing the photoexcited electron is decreased with an aggregation of Ag particles. Moreover, excessive Ag can shield the visible-light absorption by Cu2O, deteriorating the photon utilizing efficiency.40 To further test the photocatalytic activity of Cu2O/Ag(2) CNSs, the initial concentration of MO aqueous solution was set as 5, 10, 20, 30 and 40 mg/L. The corresponding results are presented in Figure 9, obtained from the UV−vis spectral changes of MO aqueous solution. After 110 min of visible-light illumination, the corresponding degradation ratios of MO are 95.10%, 96.11%, 83.04%, 75.34% and 61.46%, respectively. The degradation ratio first increases and then decreases with increasing MO concentration, which can be explained in the following. With increasing dye concentration, more dye molecules are adsorbed on the surface of the catalyst, and the highest catalytic activity can be achieved when the adsorption sites are saturated.41 However, as the dye concentration increased further, more active site positions are occupied by

Figure 10. Cycling runs in the photocatalytic degradation of MO aqueous solution (10 mg/L) in the presence of Cu2O/Ag(2) CNSs under visible-light irradiation.

were conducted under the same conditions (initial MO concentration, 10 mg/L; visible-light irradiation time, 110 min). After each run, the catalysts were washed sequentially with deionized water and absolute ethanol and then dried in air. As observed, the MO degradation ratio varied from 97.74% of the first run to 83.43% of the fifth run. The results indicate that Cu2O/Ag(2) NCSs exhibit good stability as photocatalysts. To consider the potential feasibility of the Cu2O/Ag NCSs for photocatalytic water treatment on a large scale, scaled amounts of Cu2O/Ag(2) NCSs were prepared by increasing the volume of the solution and keeping the concentration unchanged. Figure S6a (Supporting Information) shows the FESEM image of the sample obtained by increasing the volume of the solution by 30 times in a 2 L glass beaker. It can be seen that a large portion of the product still keeps the similar morphology. Figure S6b (Supporting Information) presents the photocatalytic performance of the sample obtained by scaled up experiment. The degradation ratio of MO (10 mg/L) can still F



Article

(3) Li, Y.; Fang, X.; Koshizaki, N.; Sasaki, T.; Li, L.; Gao, S. Y.; Shimizu, Y.; Bando, Y.; Golberg, D. Periodic TiO2 nanorod arrays with hexagonal nonclose-packed arrangements: Excellent field emitters by parameter optimization. Adv. Funct. Mater. 2009, 19, 2467−2473. (4) Qiu, Y.; Yan, K.; Deng, H.; Yang, S. H. Secondary branching and nitrogen doping of ZnO nanotetrapods: Building a highly active network for photoelectrochemical water splitting. Nano Lett. 2011, 12, 407−413. (5) Lu, C. H.; Qi, L. M.; Yang, J. H.; Wang, X. Y.; Zhang, D. Y.; Xie, J. L.; Ma, J. M. One-pot synthesis of octahedral Cu2O nanocages via a catalytic solution route. Adv. Mater. 2005, 17, 2562−2567. (6) Zheng, Z.; Huang, B. B.; Wang, Z. Y.; Guo, M.; Qin, X. Y.; Zhang, X. Y.; Wang, P.; Dai, Y. Crystal faces of Cu2O and their stabilities in photocatalytic reactions. J. Phys. Chem. C 2009, 113, 14448−14453. (7) Zhang, Y.; Deng, B.; Zhang, T. R.; Gao, D. M.; Xu, A. W. Shape effects of Cu2O polyhedral microcrystals on photocatalytic activity. J. Phys. Chem. C 2010, 114, 5073−5079. (8) Ma, L. L.; Li, J. L.; Sun, H. Z.; Qiu, M. Q.; Wang, J. B.; Chen, J. Y.; Yu, Y. Self-assembled Cu2O flowerlike architecture: Polyol synthesis, photocatalytic activity and stability under simulated solar light. Mater. Res. Bull. 2010, 45, 961−968. (9) Hu, C. C.; Nian, J. N.; Teng, H. Electrodeposited p-type Cu2O as photocatalyst for H2 evolution from water reduction in the presence of WO3. Sol. Energy Mater. Sol. Cells 2008, 92, 1071−1076. (10) Siripala, W.; Ivanovskaya, A.; Jaramillo, T. F.; Baeck, S. H.; McFarland, E. W. A Cu2O/TiO2 heterojunction thin film cathode for photoelectrocatalysis. Sol. Energy Mater. Sol. Cells 2003, 77, 229−237. (11) Huang, L.; Peng, F.; Wang, H.; Yu, H.; Li, Z. Preparation and characterization of Cu2O/TiO2 nano-nano heterostructure photocatalysts. Catal. Commun. 2009, 10, 1839−1843. (12) Peng, S. J.; Li, L. L.; Tan, H. T.; Wu, Y. Z.; Cai, R.; Yu, H.; Huang, X.; Zhu, P. N.; Ramakrishna, S.; Srinivasan, M.; Yan, Q. Y. Monodispersed Ag nanoparticles loaded on the PVP-assisted synthetic Bi2O2CO3 microspheres with enhanced photocatalytic and supercapacitive performances. J. Mater. Chem. A 2013, 1, 7630−7638. (13) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Environmental applications of semiconductor photocatalysis. Chem. Rev. 1995, 95, 69−96. (14) Li, X. Z.; Li, F. B. Study of Au/Au3+-TiO2 photocatalysts toward visible photooxidation for water and wastewater treatment. Environ. Sci. Technol. 2001, 35, 2381−2387. (15) Pan, Y. L.; Deng, S. Z.; Polavarapu, L.; Gao, N. Y.; Yuan, P. Y.; Xu, C. H. Plasmon-enhanced photocatalytic properties of Cu2O nanowire−Au nanoparticle assemblies. Langmuir 2012, 28, 12304− 12310. (16) Schaadt, D. M.; Feng, B.; Yu, E. T. Enhanced semiconductor optical absorption via surface plasmon excitation in metal nanoparticles. Appl. Phys. Lett. 2005, 86, 063106. (17) Wang, Z. H.; Zhao, S. P.; Zhu, S. Y.; Sun, Y. L.; Fang, M. Photocatalytic synthesis of M/Cu2O (M= Ag, Au) heterogeneous nanocrystals and their photocatalytic properties. CrystEngComm 2011, 13, 2262−2267. (18) Lin, X. F.; Zhou, R. M.; Zhang, J. Q.; Fei, S. T. A novel one-step electron beam irradiation method for synthesis of Ag/Cu2O nanocomposites. Appl. Surf. Sci. 2009, 256, 889−893. (19) Pan, L.; Li, L.; Chen, Y. H. Synthesis of Ag/Cu2O hybrids and their photocatalytic degradation treatment of p-nitrophenol. Micro Nano Lett. 2011, 6, 1019−1022. (20) Yang, L. H.; Lv, J.; Sui, Y. M.; Fu, W. Y.; Zhou, X. M.; Ma, J. W.; Su, S.; Zhang, W. J.; Lv, P.; Wu, D.; Mu, Y. N.; Yang, H. B. Fabrication of Cu2O/Ag composite nanoframes as surface-enhanced Raman scattering substrates in a successive one-pot procedure. CrystEngComm 2014, 16, 2298−2304. (21) Zhang, L.; Wang, H. Cuprous oxide nanoshells with geometrically tunable optical properties. ACS Nano 2011, 5, 3257− 3267.

reach up to 90.46%, indicating the potential engineering applications.

4. CONCLUSIONS In summary, a facile one-pot room temperature method has been presented to prepare Cu2O/Ag CNSs with tunable coverage of Ag by adding AgNO3 to fresh Cu2O nanosphereproduced mother solution in various ratios. The primary Cu2O nanospheres can reduce Ag+ ions in the acidic solution directly, and the obtained Ag nanoparticles can be deposited on the surfaces of the Cu2O nanospheres. The photocatalytic tests indicate that the obtained Cu2O/Ag NCSs exhibit superior photocatalytic activity under visible-light irradiation to bare Cu2O nanospheres, which results from the enhanced light absorption (by Plasmon resonance) and electron sink effect by Ag nanoparticles. The effects of different coverage of Ag and various initial concentrations of MO aqueous solution on catalyst have been investigated, indicating that the Cu2O/Ag(2) NCSs have the highest degradation ratio of MO at 10 mg/L. Furthermore, the Cu2O/Ag catalysts are found to exhibit good durability, with a MO degradation ratio in life-cycle performance varying from 97.74% in the first run to 83.43% in the fifth run. The catalysts, which have the virtues of facile synthesis, excellent photocatalytic activity and good reuse stability, are expected to be promising in wastewater treatment.

■

ASSOCIATED CONTENT

S Supporting Information *

FESEM images of Cu2O/Ag(1) and Cu2O/Ag(3) CNSs, linear calibration curve for a series of standard MO aqueous solutions, adsorption ratio of MO aqueous solution in the presence of different catalysts in the dark before photocatalysis, timedependent UV−vis absorption spectra of MO aqueous solution in the presence of the photocatlysts under exposure to visible light, degradation ratio of MO aqueous solution in the absence of catalyst under exposure to visible light and FESEM image and photocatalytic performance of the sample obtained by scaled up experiment. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Authors

*Weixin Zhang. Tel.: 86-551-62901450. Fax: 86-551-62901450. E-mail: [email protected]. *Zeheng Yang. Tel.: 86-551-62901450. Fax: 86-551-62901450. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors are grateful for the financial support of the National Natural Science Foundation of China (NSFC Grants 20976033, 21176054 and 21271058), the Fundamental Research Funds for the Central Universities (2010HGZY0012) and the Education Department of Anhui Provincial Government (TD200702).

■

REFERENCES

(1) Fox, M. A.; Dulay, M. T. Heterogeneous photocatalysis. Chem. Rev. 1993, 93, 341−357. (2) Fujishima, A. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972, 238, 37−38. G



Article

(22) Zhou, M. H.; Yu, J. G. Preparation and enhanced daylightinduced photocatalytic activity of C, N, S-tridoped titanium dioxide powders. J. Hazard. Mater. 2008, 152, 1229−1236. (23) McIntyre, N. S.; Cook, M. G. X-ray photoelectron studies on some oxides and hydroxides of cobalt, nickel, and copper. Anal. Chem. 1975, 47, 2208−2213. (24) Chusuei, C. C.; Brookshier, M. A.; Goodman, D. W. Correlation of relative X-ray photoelectron spectroscopy shake-up intensity with CuO particle size. Langmuir 1999, 15, 2806−2808. (25) Wu, C. K.; Yin, M.; O’Brien, S.; Koberstein, J. T. Quantitative analysis of copper oxide nanoparticle composition and structure by Xray photoelectron spectroscopy. Chem. Mater. 2006, 18, 6054−6058. (26) Stathatos, E.; Lianos, P. Photocatalytically deposited silver nanoparticles on mesoporous TiO2 films. Langmuir 2000, 16, 2398− 2400. (27) Teo, J. J.; Chang, Y.; Zeng, H. C. Fabrications of hollow nanocubes of Cu2O and Cu via reductive self-assembly of CuO nanocrystals. Langmuir 2006, 22, 7369−7377. (28) Chen, M.; Diao, G.; Zhou, X. Formation of nanospheres of cuprous oxide with a bridge linker of p-sulfonated calyx[8] arene host. Nanotechnology 2007, 18, 275606. (29) Yang, M.; Zhu, J. J. Spherical hollow assembly composed of Cu2O nanoparticles. J. Cryst. Growth 2003, 256, 134−138. (30) Wang, J.; Cui, F. L.; Chu, S. B.; Jin, X. Q.; Pu, J.; Wang, Z. H. In situ growth of noble-Metal nanoparticles on Cu2O nanocubes for surface-enhanced raman scattering detection. ChemPlusChem 2014, 79, 684−689. (31) Yin, Y. D.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai, G. A.; Alivisatos, A. P. Formation of hollow nanocrystals through the nanoscale Kirkendall effect. Science 2004, 304, 711−714. (32) Zhang, L.; Wang, H. Interior structural tailoring of Cu2O shellin-shell nanostructures through multi-step Ostwald ripening. J. Phys. Chem. C 2011, 115, 18479−18485. (33) Sing, K. S. W. Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity (Recommendations 1984). Pure Appl. Chem. 1985, 57, 603− 619. (34) Zhang, Y.; Deng, B.; Zhang, T. R.; Gao, D. M.; Xu, A. W. Shape effects of Cu2O polyhedral microcrystals on photocatalytic activity. J. Phys. Chem. C 2010, 114, 5073−5079. (35) Jiang, H. Y.; Dai, H. X.; Meng, X.; Zhang, L.; Deng, J. G.; Liu, Y. X.; Au, C. T. Hydrothermal fabrication and visible-light-driven photocatalytic properties of bismuth vanadate with multiple morphologies and/or porous structures for Methyl Orange degradation. J. Environ. Sci. 2012, 24, 449−457. (36) Li, T.; Zhou, W.; Wang, J. Q.; Qu, Y.; Tian, C. G.; Pan, K.; Tian, G. H.; Fu, H. G. Confinement effect on Ag clusters in the channels of well-ordered mesoporous TiO2 and their enhanced photocatalytic performance. ChemCatChem 2013, 5, 1354−1358. (37) Chen, R. G.; Bi, J. H.; Wu, L.; Li, Z. H.; Fu, X. Z. Orthorhombic Bi2GeO5 nanobelts: Synthesis, characterization, and photocatalytic properties. Cryst. Growth Des. 2009, 9, 1775−1779. (38) Xiong, J. Y.; Li, Z.; Chen, J.; Zhang, S. Q.; Wang, L. Z.; Dou, S. X. Facile synthesis of highly efficient one-dimensional plasmonic photocatalysts through Ag@Cu2O core−shell heteronanowires. ACS Appl. Mater. Interfaces 2014, 6, 15716−15725. (39) Li, J. T.; Cushing, S. K.; Bright, J.; Meng, F. K.; Senty, T. R.; Zheng, P.; Bristow, A. D.; Wu, N. Q. Ag@Cu2O core-shell nanoparticles as visible-light plasmonic photocatalysts. ACS Catal. 2013, 3, 47−51. (40) Lin, D. D.; Wu, H.; Zhang, R.; Pan, W. Enhanced photocatalysis of electrospun Ag- ZnO heterostructured nanofibers. Chem. Mater. 2009, 21, 3479−3484. (41) Yang, Z. H.; Yang, Y. M.; Zhu, X.; Chen, G. D.; Zhang, W. X. An outward coating route to CuO/MnO2 nanorod array films and their efficient catalytic oxidation of acid fuchsin dye. Ind. Eng. Chem. Res. 2014, 53, 9608−9615.

(42) Zhou, B.; Liu, Z. G.; Wang, H. X.; Yang, Y. Q.; Su, W. H. Experimental study on photocatalytic activity of Cu2O/Cu nanocomposites under visible light. Catal. Lett. 2009, 132, 75−80.

H


Ag Composite

Recommend Documents