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Surface Plasmon Effect of Cu and Presence of n-p HeteroJunction in Oxide Nanocomposites for Visible Light Photocatalysis Jaya Pal, Anup Kumar Sasmal, Mainak Ganguly, and Tarasankar Pal J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5114812 • Publication Date (Web): 26 Jan 2015 Downloaded from http://pubs.acs.org on February 4, 2015

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Surface Plasmon Effect of Cu and Presence of n-p HeteroJunction in Oxide Nanocomposites for Visible Light Photocatalysis Jaya Pal,a Anup Kumar Sasmal,a Mainak Ganguly,a and Tarasankar Pala* a

Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, India E-mail: [email protected]

Abstract: In this paper, we report the design, synthesis, and characterization of three different composite nanomaterials (Cu-ZnO, Cu-Cu2O-ZnO, and Cu2O-ZnO) with surface plasmon resonance (SPR) effect and/or n-p hetero-junction for visible light photocatalysis. We have accounted for the first time the SPR effect of Cu in photocatalysis for promotion of efficient electron-hole separation that further enhances visible light induced photocatalytic activity. To make the composite efficient we have judiciously introduced cheap and common Cu and Cu2O in ZnO matrix individually and/or co-jointly to make the composites visible light sensitizer. Furthermore, wide band gap barrier of ZnO crosses its UV limit in the composites and spreads over to visible region. By simply varying the complexing agents, here we achieve success to obtain three kinds of highly stable composite nanomaterials with three distinct structures from identical experimental condition. This synthetic strategy offers a radically different approach where oxidation of Cu is inhibited in the matrix and visible light induced photocatalytic performance remains unaltered for months. Interestingly, the combined effect of both Cu and Cu2O in the as-synthesized ternary composite, Cu-Cu2O-ZnO endorses highest photocatalytic activity than the other two composites. This activity attributed to

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extended light absorption, effective transfer of photogenerated carriers and presence of strong SPR effect. Finally, the comparative photocatalytic activity of all the nanocomposites has been accounted from methylene blue (MB) degradation in aqueous solution under visible light irradiation. Introduction Photocatalysis has now become the most promising green technology for implementing the large scale purification of waste water.1,2 Over a few decades, it has been reported that semiconductor based oxide materials have drawn much interest due to solar energy conversion for water decontamination and water splitting reaction.3,4 Among the semiconductor photocatalysts, TiO2 has been extensively investigated in the application field of water splitting and dye degradation reaction.5-9 Now-a-days researchers are more focused for the fabrication of ZnO nanomaterial since it is a contender to TiO2 for its high photocatalytic activity, non toxicity, ease of availability, and low production cost.10-12 Being a well known n-type semiconductor, ZnO has been widely studied in the fields of solar cells, gas sensors, field emission devices and photocatalytic degradation of organic pollutants.13-18 However the large band gap value (3.3 eV) of ZnO limits its utility to the UV region only. So our main focus is to extend the absorption range of ZnO in the visible region and improve its photocatalytic activity. Several attempts have been taken to solve the problem and these are (i) noble metal deposition,19-21 (ii) transition metal ion doping22 and (iii) coupled semiconductor systems,23-28 etc. Among them, coupling of ZnO with a narrow band gap semiconductor, such as Fe2O3, CdS, CuO, Cu2O and WO3 is one of the most promising methods to sensitize the photocatalytic activity of ZnO in visible region and also to suppress the charge carrier recombination rate.2328

Particularly, coupling of p-type semiconductor with n-type semiconductor has received

tremendous importance because an internal electric field is built up when a hetero-junction is


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formed between them. Cu2O is an attractive low cost highly stable p-type semiconductor with narrow band gap energy of 2.2 eV, which has shown excellent activity in the field of solar cells, photocatalysis, sensors and photochemical water splitting.29-32 It is also a good candidate to tune the photoresponse of wide band gap semiconductor. So coupling of n-type ZnO with p-type Cu2O semiconductor is a good strategy to improve the visible light photocatalytic activity where electron transfer from one semiconductor (Cu2O) to other (ZnO) takes place and simultaneously reduce the propensity of charge recombination. To the best of our knowledge, coupling of ZnO and Cu2O is far less explored for visible light driven photocatalysis. So synthesis of Cu2O-ZnO hetero-structure is now a challenging task for visible light photocatalysis. On the other hand, doping of noble metals (Au, Ag, and Cu) in ZnO is believed to be very much helpful to absorb visible light because of their unique surface plasmon resonance19-21,33 (SPR). Because of the SPR excitation, noble metal absorbs the resonant photons to shift the Fermi level to further negative potential and produces hot electrons.34 Thus the noble metal nanoparticles behave as an antenna to extend the excitation wavelength to visible region leading to enhanced photocatalytic activity under visible light irradiation. As Cu is very much less expensive than Au and Ag, so it is our main priority for synthesis of Cu-ZnO nanocomposite as a visible light driven photocatalyst. To the best of our knowledge, only a couple of papers involving photocatalytic activity of Cu-ZnO nanocomposites are in existence.35-38 But a detailed study on the mechanism of SPR induced visible light activity of Cu-ZnO nanocomposite has not been yet established. By simultaneously taking the advantage of both Cu and Cu2O, one Cu-Cu2O-ZnO nanocomposite has been thought to be an ideal material for highly efficient plasmonic photocatalysis. However, there is still no report of enhanced photocatalytic activity of such type of low cost composite nanomaterial. So it is



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highly desirable to develop a simple and cost effective strategy for fabrication of visiblelight-sensitive photocatalysts. In this article, we have synthesized three different types of composite nanomaterials (Cu ZnO, Cu-Cu2O-ZnO and Cu2O-ZnO) only varying the complexing agents under modified hydrothermal (MHT) condition and examine their photocatalytic performance towards methylene blue (MB) degradation under visible light irradiation. Here we have explored the mechanism of the photocatalysis in brief with a view to enriching the formation mechanism. We have also explored the mechanism of surface plasmon induced visible light photocatalytic activity of Cu-ZnO nanocomposites in brief for the first time. In case of Cu2O-ZnO nanocomposite, the enhanced photocatalytic activity is due to the formation of n-p heterojunction between two semiconductors which improves the separation of charge carriers and suppress their recombination. Under the combined action of Cu and Cu2O, the as-synthesized ternary nanocomposite Cu-Cu2O-ZnO exhibits highest photocatalytic activity than the other two composites which has been explained by the effective transfer of photogenerated carriers and presence of strong SPR effect of Cu. Experimental Section Synthesis of Cu-ZnO, Cu-Cu2O-ZnO, Cu2O-ZnO Nanocomposites. In a typical synthesis, 100 µL of ethanolamine (ETA), ammonia (NH3), and pyridine (Py) were added separately into a mixture of 2 mL 0.05 M Cu(CH3COO)2.H2O (CA) and 2 mL 0.05 M Zn(CH3COO)2.2H2O (ZA) solution under constant stirring. Then 2 mL of 0.1 M glucose solution was added with a successive addition of 4 mL of 0.1 M NaOH solution. Then the mixtures were transferred to screw capped test tubes (10.5 cm in length and 1.5 cm in diameter) followed by heat treatment (80-90 ˚C) using a 100 W tungsten bulb for 2 h under modified hydrothermal39,40 (MHT) condition (Scheme 1). The as-obtained products were collected by centrifugation and carefully washed; first with distilled water and then with


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absolute ethanol and dried in vacuum. When we used ETA as complexing agent, we obtained flower like Cu-ZnO nanocomposite. From the reaction mixture with ammonia as complexing agent, we obtained pop-corn like Cu-Cu2O-ZnO nanocomposite. When we used Py instead of ETA/ ammonia then we obtained spherical Cu2O-ZnO nanocomposite. Photocatalytic Activity. In a typical process, 0.01 g of as-synthesized Cu- ZnO, Cu-Cu2O-ZnO, and Cu2O-ZnO photocatalysts were dispersed separately in 50 mL of 2 × 10−5 M aqueous solution of MB and the mixtures were kept in the dark for 1 h to establish the adsorption−desorption equilibrium. Then the solution was exposed under visible light irradiation (in front of 100 W bulb, under water coated condition) with continuous stirring. In a typical measurement, 3 mL of the reaction mixture was taken out at different times and centrifuged to avoid the light scattering due to the interference from the suspended catalyst particles. Then absorption spectra of the supernatant solution were recorded using a UV− visible spectrophotometer. Photoelectrochemical Measurements. Photoelectrochemical measurement was carried out in the three-electrode system with CHI660E electrochemical workstation in which platinum wire was used as counter electrode, Ag/AgCl as the reference electrode and 0.1 M Na2SO4 as an electrolytic solution. To prepare working electrode, FTO (Fluorine-doped tin oxide) glass was ultrasonically cleaned first with distilled water and then with acetone. Then we prepared a thin film of sample on FTO glass by drop-casting of a homogeneous ink of sample dispersed in ethylene glycol controlling the area of 0.5 × 0.5 cm2. A 300 W Xe lamp with a UV cut filter (λ > 420 nm) was used as the light source. The distance between the working electrode and the light source was maintained 15 cm.



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Results and Discussion X-Ray Diffraction Analysis. To understand the phase purity and crystallinity of the as-synthesized composite nanomaterials, X-ray diffraction (XRD) experiments were carried out and the patterns are displayed in Figure 1. Figure 1a illustrates the XRD pattern of Cu-ZnO composite nanomaterial. The diffraction peaks at 2θ = 43.4˚, 50.4˚, and 74.2˚ correspond to (111), (200), and (220) planes for Cu (JCPDS file No. 85-1326) and the other peaks at 2θ = 31.8˚, 34.6˚, 36.3˚, 47.6˚, 56.6˚, 62.8˚, 66.3˚, 67.9˚, and 69.1 correspond to (100), (002), (101), (102), (110), (103), (200), (112), and (201) for ZnO (JCPDS file No. 80-0075). Figure 1b shows the XRD pattern of Cu-Cu2O-ZnO composite nanomaterial and all the diffraction peaks can be well indexed to the standard diffraction patterns of Cu (JCPDS file No. 85-1326), Cu2O (JCPDS file No. 05-0667), and ZnO (JCPDS file No. 80-0075). Figure 1c displays the XRD pattern of Cu2O-ZnO composite nanomaterial and all the diffraction peaks indicate the presence of pure Cu2O (JCPDS file No. 05-0667) and ZnO (JCPDS file No. 80-0075). DRS Study. Figure 1d shows comparative DRS spectra of ZnO, Cu2O, Cu, Cu-ZnO, Cu-Cu2O-ZnO and Cu2O-ZnO. Figure 1d(i) shows a strong absorption below 380 nm which corresponds to the band gap absorption of ZnO nanoparticle [synthesized hydrothermally from a mixture of ZA and ETA (Figure S1a and d, Supporting Information)].26 In the spectral range of 300 to 400 nm, there is no apparent change of the absorption peak of ZnO in Cu-ZnO, Cu-Cu2O-ZnO, and Cu2O-ZnO nanocomposites and these values are also comparable with pure ZnO nanoparticle [Figure 1d(ii)-(iv)]. Figure 1d(v) depicts a broad absorption spectrum centered at 560 nm which is due to the surface plasmon resonance (SPR) absorption of Cu nanoparticle (synthesized by a reported method41). The surface plasmon band of Cu in Cu-ZnO nanocomposite [Figure 1d(ii)] is red-shifted by about 10 nm relative to pure Cu nanoparticle.


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The Fermi energy level of copper is higher than ZnO due to its lower work function value (4.7 eV) than ZnO (5.2-5.3 eV) which causes transfer of electron from Cu to ZnO during the formation of metal-semiconductor junction to establish a constant Fermi energy level42. As a result, this decrease in electron density of Cu is responsible for band broading and red shift of surface plasmon band. As shown in Figure 1d(iv), the absorption spectrum of Cu2O-ZnO shows a broad absorption band from 600 nm to the UV region, which can be assigned to the characteristic absorption of Cu2O and this value is also comparable with pure spherical Cu2O nanoparticle (synthesized by a reported method2) [Figure 1d(vi)].26 However Cu-Cu2O-ZnO nanocomposite [Figure 1d(iii)] exhibits broad absorption spectrum in the visible region of 400-700 nm which covers band gap absorption of both Cu2O and Cu nanoparticles. XPS Analysis. Chemical state and surface atomic composition of the composite nanomaterials have been authenticated by XPS analysis and the results are showed in Figure 2. Figure 2a-c displays Cu2p spectra of Cu-ZnO, Cu-Cu2O-ZnO, and Cu2O-ZnO nanocomposites, respectively. In all the three cases high resolution XPS spectrum of Cu2p depicts two major peaks at 932.6 and 952.6 eV, respectively, corresponding to Cu2p3/2 and Cu2p1/2 which is in good agreement with the Cu2O/Cu [Cu(I)/Cu(0)].43 So it is difficult to distinguish the presence of Cu2O and/or Cu metal in the above three composite nanomaterials by the Cu2p XPS spectrum as the binding energies of Cu and Cu2O are very close. In order to confirm the presence of Cu and/or Cu2O, Cu L3 VV Auger spectra of the samples have also been performed. Figure 2d,f illustrates the Cu Auger spectra of metallic Cu and Cu2O at 918.0 and 916.0 eV, respectively, arising from Cu-ZnO and Cu2O-ZnO nanocomposites which coincide with the literature value.44 From the Auger spectrum of Cu (Figure 2e), it is also confirmed that the ternary nanocomposite consists of both Cu and Cu2O. A few shakeup satellite peaks are also observed in case of Cu-ZnO, Cu-Cu2O-ZnO at 940.4 and 943.1 eV, which suggest that a thin



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layer of CuO is present on the surface of the composites due to surface oxidation of Cu and Cu2O.2 Whereas in case of Cu2O-ZnO, satellite peaks are absent which confirms that the sample contains Cu(I) rather than Cu(II). Figure 2g-i shows another two main peaks of Zn2p3/2 and Zn2p1/2 at 1021.7 and 1044.8 eV which ascribe the presence of ZnO in Cu-ZnO, Cu-Cu2O-ZnO, and Cu2O-ZnO, respectively.45 The XPS spectra of Cu2O, Cu, and ZnO nanoparticles are represented in Figure S2, Supporting Information. Figure S2a, Supporting Information shows two main peaks of Cu2p3/2 and Cu2p1/2 at 932.3 and 952.3 eV for Cu2O nanoparticle. The peak at binding energy at 932.2 and 952.2 eV represents the XPS spectrum of Cu2p3/2 and Cu2p1/2 for Cu nanoparticle (Figure S2b, Supporting Information). Figure S2c, Supporting Information represents the XPS spectrum of Zn2p3/2 and Zn2p1/2 at 1021.4 and 1044.5 eV for ZnO nanoparticle. On analysing the XPS spectra of the Cu2p and Zn2p in the composite and individual nanomaterials, it is found that there is a slight shift in the binding energy of Cu2p and Zn2p in the composite nanomaterials comparing to pure Cu2O, Cu, and ZnO nanomaterials. This shift of binding energy values is ascribed to the redistribution of electric charge after formation of the hetero-junction or metal semiconductor junction in the composite nanomaterials.46-48 FESEM, TEM, and HRTEM Analysis. FESEM analysis has been performed to find out the shape and morphology of as-synthesized different composite nanomaterials. Figure 3a-f shows the representative FESEM images of Cu-ZnO, Cu-Cu2O-ZnO, and Cu2O-ZnO nanocomposites obtained only by varying the complexing agents while other reaction conditions remain unaltered. The morphology as well as composition of the composite nanomaterials is found to be drastically different depending on the use of different complexing agents. Figure 3a and b shows the overview of Cu-ZnO nanocomposite with flower like morphology. These FESEM images display that each flower is comprised of sword like nanopetals with a length of about 200 nm and 100 nm of width.


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More interestingly, these petals are originated from a common central zone. Figure 3c and d represents the pop-corn like architecture of Cu-Cu2O-ZnO nanocomposites with a diameter of 800 nm. FESEM images in Figure 3e and f represent sphere (diameter 700-800 nm) like morphology of Cu2O-ZnO nanocomposites with rough surface. To understand the morphology of as-synthesized composite nanomaterials, the products were further executed with TEM analysis (Figure 4). TEM image (Figure 4a) reveals that Cu-ZnO nanocomposite bear flower like morphology comprised of sword like nanopetals. TEM image in Figure 4b depicts pop-corn like morphology of Cu-Cu2O-ZnO nanocomposite. Figure 4c displays sphere like architecture of Cu2O-ZnO nanocomposite with rough surface, in accord with the FESEM images. Figure 4d-f shows the ring like SAED patterns of Cu-ZnO, Cu-Cu2O-ZnO, and Cu2O-ZnO nanocomposites. These results indicate that all the three nanocomposites are polycrystalline in nature. HRTEM image (Figure 5a) shows that Cu-ZnO nanocomposite is well crystallized with fringe spacing values of 0.26 and 0.209 nm which coincide with the interplanar distance of (002) planes of wurtzite phase of ZnO and (111) planes of Cu, respectively. HRTEM image (Figure 5b) of pop-corn like Cu-Cu2O-ZnO demonstrates three fringe spacing values, one is 0.26 nm which matches with the lattice distance of (002) crystal planes of ZnO and another two are 0.209 and 0.30 nm for (111) and (110) crystal planes of Cu and Cu2O respectively. The fringe spacing values of Cu2O-ZnO, in correspondence to the HRTEM image in Figure 5c, are 0.26 and 0.30 nm which are in good agreement with the (002) and (110) crystal planes of ZnO and Cu2O, respectively. The above results support the formation of composite nanomaterials with hetero-junction and metal-semiconductor junction. The compositions of all three composite nanomaterials were further ascertained by energy dispersive X-ray spectroscopic (EDX) analysis (Figure S3, Supporting Information) and area



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mapping (Figure S4, Supporting Information). From each area mapping, it is observed that all three elements such as Cu, Zn, and O are homogeneously disseminated over the whole matrix. EDX analysis suggests that the atomic percentage values of Cu and Zn are in close proximity with the ideal values of 1:1 in Cu-ZnO, 3:1 in Cu-Cu2O-ZnO, and 2:1 in Cu2OZnO. Formation Mechanism. Here we demonstrate a simple, facile, surfactant free and complexing agent assisted synthetic protocol to interpret the precisely controlled synthesis of three different kinds of composite nanomaterials (Cu-ZnO, Cu-Cu2O-ZnO, and Cu2O-ZnO) with distinct shapes (Scheme 1). Although the exact mechanism for the evolution of three different types of composite nanomaterials is still uncertain but the role of complexing agents, presence of zinc acetate, and MHT condition are undoubtedly significant presumably due to altered pH condition of the reaction mixture. Here the synthesis procedures of the composite nanomaterials involve two steps. In all the cases Cu(CH3COO)2.H2O [CA] and Zn(CH3COO)2.2H2O [ZA] are employed as metal ion precursor salts. First, three different types of complexing agents such as ethanolamine, ammonia and pyridine are employed separately to the mixture of Cu+2 and Zn+2 containing solutions. The reagents are chosen considering somewhat stronger affinity of both the metal ions for N-donor. Only in ETA case a white precipitate is obtained for Zn(OH)2 formation leaving aside transparent blue solutions for the other two cases. In the second step, we introduced glucose/NaOH as reducing agent for all the reaction mixtures and obtained different reduced forms of Cu(II) under MHT condition. Pure forms of Cu-ZnO and Cu2O-ZnO nacomposites are obtained when ETA and Py, respectively are used as complexing agents. In the remaining case, a ternary composite i.e., Cu-Cu2O-ZnO is produced only when ammonia is added as complexing agent keeping all other experimental conditions unaltered.


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So to understand the formation mechanism of different composite nanomaterials, some additional experiments have also been performed. In absence of ZA, when all the three reaction mixtures are subjected to MHT reaction keeping all other experimental conditions unaltered, only one type of product i.e., Cu-Cu2O nanocomposite is obtained (Figure S5, Supporting Information). Under MHT condition, single precursor compound ZA upon treatment with only ETA or ammonia or Py always evolves exclusively ZnO (Figure S1, Supporting Information). Without incorporation of any complexing agents, if we treat the mixture of CA and ZA with glucose/NaOH, then we obtain Cu2O-ZnO nacomposites (Figure S6a, Supporting Information). Upon addition of glucose/NaOH, the as-synthesized Cu2O is partially reduced to Cu(0) and ultimately gives Cu-Cu2O (Figure S6b, Supporting Information) under MHT reaction condition. In general CA, in turn Cu+2 ion in the reaction mixture easily can produce Cu(OH)2 at higher pH condition and upon its dehydration, CuO is formed under MHT for always in the successive stage. This is redundant under the present experimental condition in presence of a reducing agent. So from the above additional experiments, two important conclusions emerge out for variable composites formation: one is the presence of ZA solution in the reaction mixture and latter is the variation of different complexing agents. Being an amphoteric oxide, ZnO is thought to play a dominant role to achieve variable composite formation as it provides [Zn(OH)4]2-/ Na2[ZnO2] depending on the concentration of a strong base. ZnO + H2O + NaOH → [Zn(OH)4]2-/ Na2[ZnO2] For this reason in situ produce ZnO (in absence of any complexing agent) lowers the pH of the reaction mixture and eventually makes glucose a somewhat weaker reducing agent and that is why we obtained Cu2O. Similar result is also obtained in presence of Py, as it is weak base. But in presence of ETA, pH of the reaction mixture increases which makes glucose a stronger reducing agent as a result of which we obtain Cu(0). Whereas in presence of NH3,



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the pH of the solution is maintained in such a way that we obtain a mixture of two reduced forms of Cu(II) i.e., Cu(0) and Cu2O. So glucose slowly reduces Cu(II) complex under MHT condition either straight forwardly to Cu(0) or to Cu2O or a mixture of both Cu and Cu2O. Cu2O presumably being the intermediate oxidation state for the evolution of Cu(0) from Cu(II). Catalytic Activity. Though organic dyes have innumerable applications in our daily commodities, but because of their acute toxicity, long persistence, and nonbiodegradable nature, they are hazardous to our environment.2 Now-a-days, visible-light driven photocatalysis is a promising method among all other processes (e.g., adsorption tactics, Fenton-like reactions, and biological degradation) for employing the large-scale purification of waste water through mineralization of different organic pollutants.49-52,2 To explore the visible light photocatalytic activity of Cu-ZnO, CuCu2O-ZnO and Cu2O-ZnO nanocomposites, the photocatalytic degradation of MB is chosen as a model reaction. For the photocatalytic experiments, 0.01 g of differently shaped Cu-ZnO, Cu-Cu2O-ZnO, and Cu2O-ZnO nanocomposites were dispersed in 50 mL of 2 × 10−5 M MB solution. Before the light illumination, we kept the mixture in the dark condition for 1 h to establish adsorption−desorption equilibrium on the surface of the catalyst. Then the mixture was exposed to visible light (in front of 100 W bulb, under water coated condition) under stirring and 3 mL of the mixture was withdrawn at different time interval and centrifuged immediately to remove the catalyst. The specific absorbance spectra of the supernatant were recorded by using an UV−visible spectrophotometer. Figure 6a-c shows the extent of the photodegradation of MB solution with respect to time using Cu-ZnO, Cu-Cu2O-ZnO and Cu2O-ZnO nanocomposites as catalyst. Cu-ZnO, CuCu2O-ZnO and Cu2O-ZnO photocatalysts are able to degrade MB solution completely within 200, 140 and 180 min, respectively in presence of visible light. So, the order of the


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photodegradation ability of different nanocomposites towards MB degradation can be summarized as follows: Cu-Cu2O-ZnO (b) > Cu2O-ZnO (c) > Cu-ZnO (a). BET measurement shows that the surface areas of the differently shaped nanocomposites follow the sequence of Cu-ZnO > Cu-Cu2O-ZnO > Cu2O-ZnO (Figure S7, Supporting Information). Considering the BET results, it can be concluded that the surface area of these composite nanomaterials is not the supreme criteria to judge catalytic efficiency of the materials. Hetero-junction might supersede surface area and that is currently observed in case of Cu-Cu2O-ZnO. We have also performed the same photocatalytic reaction by using ZnO and Cu2O for comparative study. The MB dye shows no noticeable degradation in presence of ZnO catalyst under the same experimental condition (Figure S8a, Supporting Information). Whereas Cu2O nanoparticle has poor photodegradation ability than the other three nanocomposites and it requires 7 h for complete degradation of MB (Figure S8b, Supporting Information). However the photocatalytic efficiency of our as-synthesized composite nanomaterials has been found to be superior to that of neat ZnO and Cu2O. Again the photocatalytic performances of the composite nanomaterials were further compared with the physical mixtures of corresponding metal or metal oxides following a weight ratio of 1:1 and 1:1:1. It was observed that all the physical mixtures show poor photocatalytic activity (Figure S9, Supporting Information) and comparable to neat ZnO and Cu2O components. It is worth pointing out that the photocatalytic performance of Cu-ZnO, Cu-Cu2O-ZnO, and Cu2O-ZnO is superior to that of the corresponding physical mixtures, indicating the formation of composite nanomaterials (cannot be a mixture) with hetero-junction. The existing of n-p hetero-junction and surface plasmon effect of Cu actually facilitates the electron-hole separation with suppression of charge recombination. The plots lnA vs. time (min) for different catalysts display straight line having negative slopes (Figure 6d). The values of rate constant for Cu-ZnO, Cu-Cu2O-ZnO, and Cu2O-ZnO (0.01g) are 0.0142, 0.0185, and 0.0161 min-1 respectively.



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On the basis of the above analysis, we proposed three mechanisms for visible light driven photocatalytic activity of Cu-ZnO, Cu-Cu2O-ZnO, and Cu2O-ZnO nanocomposites which are presented in Scheme 2. ZnO is an n-type semiconductor having wide band gap value (3.3 eV) and large excitation energy (60 MeV) and thus the use of ZnO is limited to the UV region only. So in presence of visible light, the photocatalytic efficiency of ZnO is negligible. However, due to narrow band value (2.2 eV), Cu2O becomes active under visible light. But the efficiency of Cu2O nanoparticle as visible light photocatalyst is not so promising due to its low band gap value which causes easy recombination of the photogenerated electron (e-) and hole (h+). In case of Cu-ZnO nanocomposite, from literature we know that, the Fermi level of Cu is higher than ZnO because of its lower work function (φ) value42. When Cu and ZnO come into electronic contact, electron will transfer from Cu Fermi level to ZnO Fermi level until thermodynamic equilibrium is established. As a result, conduction band (CB) bending is induced and a Schottky barrier is produced at the interface between metal and semiconductor. During visible light irradiation, photoexcited state of Cu stays high up than the CB of ZnO due to SPR and produces hot electrons.33 These hot electrons will flow from SPR band of Cu to the CB of ZnO, leaving behind hole in metal (Scheme 2a). So in Cu-ZnO nanocomposite, two important factors which play crucial role to make ZnO as visibile light active photocatalyst. First one is the absorption of visible light and eventually surface plasmon generation by Cu nanoparticle and the second one is the manifestation of higher SPR band position of Cu than the CB of ZnO. Then the electron in CB of ZnO reacts with dissolve oxygen and form superoxide radical anions (O2-.) which again react with water and produce hydro peroxyl radicals (HO2.), oxydol (H2O2), and hydroxyl radicals (OH.) and these products are responsible for photodegradation of MB dye.


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In case of Cu2O-ZnO nanocomposite, when p-type Cu2O comes into contact with n-type ZnO, then there is a formation of n-p hetero-junction at their interface.26 Upon visible light irradiation valence band (VB) electrons of Cu2O become excited and move to the conduction band (CB) of Cu2O with simultaneous generation of hole in the VB. But the ZnO remains inactive as it can only absorb UV light. Due to favourable band alignment, photogenerated electrons can easily be transferred from CB of Cu2O to CB of ZnO which can effectively suppress the charge recombination process in Cu2O and thus the catalyst activity is improved (Scheme 2c). After the separation of electron and hole, the excited electron in CB of ZnO reacts with dissolve oxygen and form superoxide radical anions (O2-.) which again react with water and produce hydro peroxyl radicals (HO2.), oxydol (H2O2), and hydroxyl radicals (OḢ.). These radicals are responsible for photodegradation of MB dye. In case of Cu-Cu2O-ZnO nanocomposites, when Cu, Cu2O, and ZnO are in contact then all the three Fermi levels are equilibrated by transferring electrons from the material with lower work function to the other material with higher work functional value and thus a Schottky barrier is produced at the interfaces of metal and semiconductors. Here we proposed a possible mechanism for highest catalytic activity of Cu-Cu2O-ZnO nanocomposites on the basis of plasmon-mediated charge injection and formation of favourable n-p hetero-junction for the first time. Due to formation of favourable n-p hetero-junction in between p-type Cu2O and n-type ZnO, upon visible light irradiation electrons will migrate from CB of Cu2O to the CB of ZnO leaving behind hole in the VB of Cu2O which can effectively inhibit the charge recombination process in Cu2O (Scheme 2b). Meanwhile, because of SPR excitation, photoexcited state of Cu is higher than the CB of both Cu2O and ZnO and produces hot electrons. These hot electrons will flow from SPR band of Cu to the CB of ZnO by two different pathways. First one is the direct electron transfer from SPR band of Cu to the CB of ZnO and the latter is electron transfer from SPR band of Cu to the CB of ZnO via Cu2O



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(Scheme 2b). In latter case, first electron will migrate to the CB of Cu2O and then to avoid the charge recombination, electron will quickly move to the CB of ZnO. The presence of huge number of electrons in the CB of ZnO facilitates the catalytic efficiency of Cu-Cu2OZnO nanocomposites. After the separation of electron and hole, the excited huge number of electron in CB of ZnO reacts with dissolve oxygen and then further with water and produce different radicals (like other two cases) which are responsible for photodegradation of MB dye. Photoelectrochemical measurement technique can further support the presence of n-p heterojunction in our composite nanomaterials. Hence photocurrent response was studied to investigate the separation of photogenerated electron-hole pairs where suppression of charge recombination becomes evident. Figure 7a-e represents the photocurrent response of ZnO, Cu2O, Cu-ZnO, Cu2O-ZnO, and Cu-Cu2O-ZnO, respectively at a 0 V bias vs. Ag/AgCl reference electrode with a pulse of 50 s under intermittent Xe lamp (λ > 420 nm) irradiation. The photocurrent densities of the as-synthesized composite nanomaterials as well as pure ZnO, Cu2O follow the order of Cu-Cu2O-ZnO > Cu2O-ZnO > Cu-ZnO > Cu2O > ZnO which also corroborates with the order of catalytic efficiency of all the samples. The photocurrent density of all the three composite nanomaterials is significantly higher than that of Cu2O and ZnO. The results suggest that n-p hetero-junction and/or surface plasmon effect of Cu enhance the separation of photogenerated electron-hole which greatly facilitates their photocatalytic activity.53-55 After completion of the photocatalytic experiments, the catalysts were removed from the reaction mixture by centrifugation and washed thoroughly with distilled water and ethanol. To verify the stability of the photocatalysts, the photocatalytic experiment was further performed with the used catalysts and it is observed that the catalysts are reusable even after 4th consecutive cycles. The XRD pattern (Figure S10, Supporting Information), FESEM


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(Figure S11a-c, Supporting Information) and TEM (Figure S11d-f, Supporting Information) images of the used and dried catalysts authenticate that the composition and morphology of the catalysts remained unaltered even after the catalysis which suggests that these catalysts are very much stable for practical application. It is worth mentioning that the rate of photodegradation of MB by the two Cu containing composite nanomaterials remain unaltered even after three months. This observation helps us to conclude that the thin layer of CuO on the surface of Cu(0) (authenticated from XPS analysis, Figure 2a,b) inhibits further oxidation of Cu in the composite matrix, enabling protection of the composition. Conclusion Tuning of band gap energy of a semiconductor is now a coveted route to engender visible light activity. Recent endeavour in this field is the introduction of a hetero-junction in oxide based semiconductors. Then, hetero-junction indirectly paves the band gap of UV active materials for successful visible light photocatalysis. The report is an account of new material syntheses with three different complexing agents under MHT condition. Here the well known oxide, ZnO is obtained in situ, controls the pH of the reaction medium and eventually becomes a part and parcel of the constituent of the three composites Cu-ZnO, Cu-Cu2O-ZnO, and Cu2O-ZnO bearing variable shapes. Here rational coupling of Cu or Cu2O or both with ZnO enhanced the visible light induced photocatalytic activity towards MB degradation than the individual components. Finally the as prepared Cu-Cu2O-ZnO has been identified as the best suited cheap material for visible light photocatalysis due to the enlarged light absorption region, electron-hole separation at n-p hetero-junction and the presence of strong SPR induced effect of copper. Acknowledgements The authors are thankful to the UGC, DST, BRNS and CSIR New Delhi for financial assistance and IIT Kharagpur for research facilities.



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Supporting Information Details about chemicals, analytical instruments and BET surface area measurements. FESEM images, XRD patterns, XPS analysis, EDX and area mapping analysis, and UV-vis spectra for MB degradation by using pure ZnO, Cu2O, and physical mixtures. XRD, FESEM and TEM analysis of the used photocatalysts after 4th catalytic cycles. This information is available free of charge via the Internet at http://pubs.acs.org.


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Figure, and Scheme

y t i s n e t n I

b

**

a

* **

(

)

c

*

20

40

*

*

* ***

*

*

* ***

*

*

* ***

θ

60

(i) ZnO (ii) Cu@ZNO (iii) Cu@Cu2O@ZnO (iV) Cu2O@ZnO (V) Cu (vi) Cu2O

d Absorbance (a.u.)

*

. u . a

**

O OOOO 2222 n uu Z CC

*

2222

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80

300

450

600

750

Wavelength (nm)

Figure 1. Curves a-c show the XRD patterns of Cu-ZnO, Cu-Cu2O-ZnO, and Cu2O-ZnO nanocomposites, respectively. Curve d shows the DRS spectrum of (i) ZnO, (ii) Cu-ZnO, (iii) Cu-Cu2O-ZnO, (iv) Cu2O-ZnO, (v) Cu, and (vi) Cu2O nanoparticles, respectively.


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Cu 2 eV

. u . a

c

y t i s n e t n I

(

(

y t i s n e t n I

Cu2O

Cu2p1/2

Cu2p3/2

)

)

. u . a

b

f e d

a )

960

912

916

(

Zn2p1/2

)

Zn2p3/2

(

y t i s n e t n I

1035

(

924

)

i h g

V e

y g r e n E g n i d n i B

1020

920

V e

. u . a

(

y g r e n E c i t e n i K

945

V e

930

y g r e n E g n i d n i B

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


)

1050

Figure 2. Curves a-c show the Cu2p spectrum of Cu-ZnO, Cu-Cu2O-ZnO, and Cu2O-ZnO nanocomposites, respectively. Curves d-f show the corresponding Cu L3VV Auger spectra of Cu-ZnO, Cu-Cu2O-ZnO, and Cu2O-ZnO nanocomposites, respectively. Curves g-i show the Zn2p spectrum of Cu-ZnO, Cu-Cu2O-ZnO, and Cu2O-ZnO nanocomposites, respectively.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. Low and high magnification FESEM images of (a and b) flower like Cu-ZnO, (c and d) pop-corn like Cu-Cu2O-ZnO, and (e and f) spherical Cu2O-ZnO nanocomposites, respectively.


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Figure 4. TEM images of (a) flower like Cu-ZnO, (b) pop-corn like Cu-Cu2O-ZnO, and (c) spherical Cu2O-ZnO nanocomposites, respectively. SAED patterns of (d) flower like CuZnO, (e) pop-corn like Cu-Cu2O-ZnO, and (f) spherical Cu2O-ZnO nanocomposites, respectively.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5. HRTEM images of (a) flower like Cu-ZnO, (b) pop-corn like Cu-Cu2O-ZnO, and (c) spherical Cu2O-ZnO nanocomposites, respectively.


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a

b Absorbance (a.u.)

Absorbance (a.u.)

1.2

Initial After adsorption 20 min

0.6 200 min

1.0 Initial After adsorption 20 min

0.5

140 min

0.0

0.0 400

500

600

700

800

400

500

Wavelength (nm) 1.2

600

700

800

Wavelength (nm)

d

c

Cu - ZnO (1) Cu - Cu2O - ZnO (2) Cu2O - ZnO (3)

0 Initial After adsorption 30 min

ln A

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6

-1

180 min

-2

-3

0.0 400

500

600

700

800

2 0

Wavelength (nm)

100

1 3 200

Time (min)

Figure 6. Photodegradation of methylene blue solution in presence of visible light by using 0.01g of (a) flower like Cu-ZnO, (b) pop-corn like Cu-Cu2O-ZnO, and (c) spherical Cu2OZnO nanocomposites, respectively. (d) Corresponding lnA vs time plot.



on

50 sec

off

1 µA/cm2

on

e d

Current density

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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c b off

a Time

Figure 7. Photocurrent response of (a) ZnO, (b) Cu2O, (c) Cu-ZnO, (d) Cu2O-ZnO, and (e) Cu-Cu2O-ZnO.


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Scheme 1. Schematic representation of morphologycally different Cu-ZnO, Cu-Cu2O-ZnO, and Cu2O-ZnO nanocomposites obtained only by varying the complexing agents under MHT condition for 2h.



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Scheme 2. Schematic representation of proposed charge seperation and dye degradation mechanism of (a) Cu-ZnO, (b) Cu-Cu2O-ZnO, and (c) Cu2O-ZnO nanocomposites under visible light irradiation.


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