Transition Met Chem (2010) 35:689–694 DOI 10.1007/s11243-010-9380-z

Synthesis and characterization of ZnO-doped cupric oxides and evaluation of their photocatalytic performance under visible light Dongfang Zhang

Received: 18 March 2010 / Accepted: 21 May 2010 / Published online: 11 June 2010 Ó Springer Science+Business Media B.V. 2010

Abstract ZnO–CuO binary oxide photocatalysts were synthesized by the liquid phase coprecipitation method. The catalysts were characterized by X-ray diffraction, transmission electron microscopy and UV–vis spectroscopy. The photocatalytic activity of the ZnO–CuO nanocomposites was estimated on the basis of decoloration of methyl orange dye under visible light. The effects of parameters such as calcining temperature, amount of catalyst and pH on the photocatalytic degradation efficiency of methyl orange solutions were investigated in detail. The maximum photocatalytic activity was obtained on ZnO–CuO nanocomposites with a calcining temperature of 350 °C, using a catalyst amount of 0.056 g/L and a pH of 7.5. The visible lightdriven capability of ZnO–CuO nanocomposites is much better than that of commercially available TiO2 photocatalysts under comparable conditions.

Introduction Photocatalytic purification of water has many advantages over the conventional procedures like electrochemical oxidation, chlorination and oxidation by KMnO4–H2O2 [1]. At present, photocatalytic destruction of organic pollutants is mainly performed using TiO2 semiconductor oxide, which is viewed as an ideal material for such photocatalytic processes [2–4]. However, this traditional photocatalyst requires UV light irradiation due to its large band-gap energy (Eg = 3.2 eV). Therefore, the utility of solar energy

D. Zhang (&) College of Science, Huazhong Agricultural University, Wuhan 430070, People’s Republic of China e-mail: [email protected]

is extremely low, and this drawback severely limits its practical applications. Widespread use of TiO2 is not very cost-effective for large-scale wastewater remediation because of the low quantum yield under irradiation in the visible region and the rapid recombination that occurs in relation to the photoinduced electrons and holes [5]. In this context, development of new visible light-driven photocatalysts is a priority. For this purpose, the usage of coupled semiconductors with nanometer sizes in the field of environmental cleaning has been the subject of recent research [6–8]. Through the aforementioned approaches, the efficiency of the TiO2-photocatalyzed process can be improved, since these alternatives permit the enhancement of charge separation in the photocarrier generation process as well as the ability to extend the high reactivity to irradiation in the visible light range, thereby allowing use of the main part of the solar spectrum [9, 10]. The nanoparticles of ZnO have been broadly used in the field of photocatalytic degradation [11–13], and ZnO is considered to be a suitable alternative to TiO2 due to its photodegradation mechanism that has been proven to be similar to that of TiO2. However, it is photoactive in the near-UV domain, therefore only 3% of solar light can be utilized. As a p-type semiconductor with narrow band gap (Eg = 1.7 eV), cupric oxide (CuO) has found use in a wide range of applications. Nanocrystalline CuO exhibits optical and electronic properties that differ from those of bulk CuO and has been shown to enhance catalysis [14, 15]. Moreover, both the conduction and valence bands of CuO lie above those of ZnO, which thermodynamically favors the transfer of excited electrons and holes between them. The separation of carriers in different semiconductors effectively inhibits the recombination of electron–hole pairs and promotes the quantum efficiency of ZnO. Moreover, the absorption of visible light by CuO greatly extends the

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wavelength range of ZnO–CuO heterostructures, further enhancing the efficiency of solar energy usage. It is thus expected that the photocatalytic efficiency of ZnO will be improved considerably through preparation of nanoscale coupled semiconductors incorporating CuO. In this work, coupled semiconductors composed of nanoscale ZnO–CuO composites have been prepared via the alcohol aqueous–based chemical precipitation method, and the crystal structure, morphology, absorption range and photocatalytic performance of the ZnO–CuO compounds were examined. The photocatalytic activities of ZnO–CuO binary oxides under visible illumination were evaluated by bleaching a basic organic dye, namely methyl orange (MO), as a test reaction. It was found that, compared with commercial TiO2 (Degussa P25), the photocatalytic properties of ZnO–CuO compounds under visible light were significantly better. The photocatalytic mechanism has been studied. The results showed that the coupled semiconductor formed by ZnO and metal oxides such as CuO can reduce the band gap, extend the absorption range into the visible light region, promote electron–hole pair separation under irradiation and, consequently, achieve higher photocatalytic activity.

Experimental Analytically, pure copper (II) sulfate pentahydrate (Alfa Aesar, 99%), zinc nitrate hexahydrate (Alfa Aesar, 99%) and sodium hydroxide were used without further purification. Polyethylene glycol (PEG, Aldrich, 98%) was used as reaction medium for synthesizing the composite nanoparticles. Water used in the experiments was purified by using an Ultrapure Lonex Cartridge Millipore System (Q-GardÒ, TM Quantum IX). Preparation of the nanocomposites is as follows: CuSO4
5H2O (2.50 g, 0.01 mol) and Zn(NO3)2
6H2O (11.90 g, 0.04 mol) were dissolved in distilled water (400 mL) under stirring to form a homogeneous solution. NaOH solution (200 mL), obtained by mixing *10 mL of polyethylene glycol (PEG, Mw = 6,000) with NaOH (4.20 g) in distilled water, was added dropwise to the solution of CuSO4 plus Zn(NO3)2. After 2.0 h, the resultant precipitate was filtered off and washed several times with water and absolute ethanol. During the process of washing, the organic and inorganic impurities were removed totally. The obtained precipitate was dried in a vacuum at 70 °C for 20 h. Then, the as-prepared precursor was calcined at 250, 350 or 450 °C. The resulting composites are labeled as samples A, B and C, respectively. Crystallographic information such as the structure, composition and defect content of the phases was established with powder X-ray diffraction (XRD, Shimadzu XRD-6000, Cu Ka radiation, k = 0.15418 nm) in the

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region 2h = 10–90° with a step size of 0.04°. The size and morphology of the samples were characterized by transmission electron microscopy. The observation and recording of images were performed with a HITACHI H7650 transmission electron microscope at 80 kV and a Gatan 832 CCD camera. UV–vis absorption spectra were obtained in the region of 300–800 nm on a Scanning UV–vis-NIR spectrophotometer (Varian Cary 500), using a diffuse reflectance mode for the powder form of the catalysts. The spectrophotometer was equipped with an integrating sphere assembly, and pure BaSO4 was used as a reflectance standard in the UV–vis absorbance experiments. Artificial light photocatalytic activity tests were conducted in a quartz photoreactor with a cylindrical configuration using aqueous methyl orange as the model pollutant. A halogen tungsten lamp (380 W, k: 200– 800 nm, k \ 400 nm was cut off by an UV filter) was positioned inside the reactor to simulate solar light, of which average light intensity was about 6 mW/cm2. A measured amount of photocatalyst powder (0.03–0.07 g) was added to 100 mL of aqueous methyl orange (0.025 g/L). Before commencing the photocatalytic degradation, the suspension was magnetically stirred in the dark for about 30 min to establish a methyl orange adsorption/desorption equilibrium. During the irradiation experiments, samples of 5 mL were withdrawn from the suspension at appropriate intervals and immediately centrifuged at 3,000 r/min for 10 min to remove solids. The concentration of methyl orange after illumination was monitored at wavelength of 464 nm using an UV–vis spectrophotometer (Spectrumlab 22). The degradation rate (D) of the methyl orange was determined by the formula: D = (A0 - At)/A0, where A0 and At represent the initial absorbance and the absorbance after degradation time td of the methyl orange, respectively.

Results and discussion Figure 1 shows the XRD pattern of the as-prepared ZnO– CuO composites. According to the data in the standard card, all the characteristic peaks can be assigned to the Bragg reflections of monoclinic cupric oxide (JCPDS 895896) and the hexagonal wurtzite structure zinc oxide (JCPDS 36-1451, a = 0.326 nm, c = 0.522 nm). No other diffraction peaks of impurities or new crystal phases were detected. In addition, the corresponding lattice parameters for the ZnO component in the coupled system deviate from the standard values. This suggests that the two oxides (CuO and ZnO) combine together primarily through intra-grain coupling instead of intergranular coupling. This intragranular location of both phases could provide an intimate

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contact between both oxide phases and improve the efficiency of semiconductor coupling effects. Figure 2 shows TEM images of the ZnO–CuO composites. It is evident that the crystallinity of the product is not very good and the product partly exists in amorphous phase when calcined at 250 °C. The quality of crystallinity of the product is enhanced as the calcining temperature increases from 350 to 450 °C, and the particle size increases remarkably, which will lead to a reduction of the specific surface area. However, it is hard to assess the particle shape in detail due to the aggregation of the particles when the calcining temperature is 450 °C. According to the magnification order, the average size of ZnO–CuO particles is concluded to be 30–100 nm. The UV–vis absorption spectra of the ZnO–CuO nanoparticles obtained under different calcining temperatures are shown in Fig. 3. From the graph, it can be seen that the maximal absorption emerges at about 392 nm as the temperature is 250 °C. The character of the peak in the absorption spectra also hints that the nanocrystallites that constitute the composite are nearly monodispersed in nature. The peak position of maximum absorbance is gradually shifted to longer wavelength as the calcining temperature is elevated. As the calcining temperature exceeds 350 °C, the sample shows a broader absorption peaking at about 426 nm, and the long absorption tail enters the visible region, which implies that the ability for visible light absorption is improved to a certain degree. This observation can be ascribed to the excellent crystallinity and larger particle size at the higher temperature, which are both helpful to expand the spectral response range into the visible. Thus, the enhanced crystallinity of the composites may facilitate electron migration from bulk to surface and so impair recombination with photoinduced holes, leading to

Fig. 2 TEM images of ZnO–CuO nanocomposite obtained at different temperatures: Sample A (250 °C); Sample B (350 °C); Sample C (450 °C)

higher quantum efficiency. In addition, the enhanced interaction between CuO and ZnO after calcination could also increase the absorbance of visible light, which responsible for the higher photocatalytic activity in the visible region. However, further increase in the calcination temperature from 350 to 450 °C has little improvement on the crystallinity, but does cause an abrupt decrease in

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surface area, which could account for the abrupt decrease in photocatalytic activity, since a higher surface area or mesoporous structure favors the adsorption of reactant molecules and the light absorbance to generate more holes, as discussed below. The calcining temperature has a large influence on the photocatalytic activity and stability of these materials. As displayed in Fig. 4, the photocatalytic efficiency of the ZnO–CuO composites obtained at 350 °C (sample B) is the best among all the samples. This is probably due to the ZnO–CuO composite being most crystalline when calcined at 350 °C. Besides, the average size of crystals is smaller, so the resultant product has a large specific surface area for reaction between the ZnO–CuO composite and methyl orange. The photocatalytic activity is lower for calcining

temperatures lower than 350 °C, since the crystallinity of the products is not so good, especially the amorphous phase. However, the photocatalytic activity reduces as calcining temperature is increased above 350 °C, since the high temperature can cause particle agglomeration and destruct part of the crystal structure or even induce collapse of the porous structure. The higher quality of photocatalytic activity of sample B also can be attributed to its optical properties, as discussed above. To determine the effect of quantity of catalyst, the methyl orange concentration was fixed and the amount of catalyst was varied from 0.03 to 0.07 g/L. Figure 5 shows the effect of catalyst amount (sample B). It is observed that the degradation efficiency (td = 45 min) depends on the quantity of catalyst. However, the maximum degradation efficiency appears as the amount of catalyst reaches a value of 0.056 g/L, and thereafter it starts to drop gradually. The photocatalytic destruction of other organics also exhibits a similar dependency on catalyst amount [16]. This can be explained on the basis that total active surface area is dependent on initial solute concentration, and availability of more active sites on the catalyst surface is enhanced with increasing amount of catalyst [17]. However, higher levels of catalyst result in an increase in turbidity of the suspension, which blocks light from reaching the particles, and hence the effective photoactivated volume of the suspension decreases. Therefore, it can be inferred that the cooperation of light and catalyst is very important for the photodegradation process, and higher levels of catalyst may not be useful in view of both aggregation and reduced irradiation due to light scattering. Figure 6 shows the role of the pH of suspensions containing sample B on the degradation efficiency of methyl

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The dosage of photocatalyst (g/l) Fig. 5 Photocatalytic activity of ZnO–CuO nanocomposite (sample B) with different dose for the degradation of methyl orange

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orange (td = 50 min). The degradation efficiency reaches a maximum at a pH of *7.5. This is not surprising since the adsorption of organics onto the catalyst surface is affected by its surface charge, which is related to the pH. The variation in pH entails an alteration in the properties of the catalyst–liquid interface [18], primarily associated with the acid–base equilibrium of the adsorbed hydroxyl groups. Under light irradiation, the electrons in the valence band will be excited to the conduction band to form holes. The surface of the photocatalyst will be negatively charged under the weakly basic environment, which is favorable to the transfer of holes to the surface, where the hole can react with electron donors such as OH- or H2O to generate highly oxidizing OH
radicals. Therefore, the formation of hydroxyl radical from OH- is enhanced at higher pH. For comparison, the photocatalytic activities of sample B and commercially available TiO2 photocatalyst (Degussa P25) were examined without oxygen bubbling. The degradation of aqueous methyl orange (100 mL, 0.025 g/L) was performed under optimized conditions (catalyst level 0.056 g/L; pH 7.5), and the results are illustrated in Fig. 7. As is evident, Degussa P25 exhibits a comparatively low visible light photocatalytic activity for aqueous methyl orange degradation, which is partly induced by the band structure of Degussa P25. The degradation efficiency of the prepared ZnO–CuO composites is significantly higher than the former, which can be explained as follows. First, a synergetic effect exists in the ZnO–CuO nanocomposites, resulting in the enhancement of charge separation in the photocarriers (holes and electrons) at the surface of the ZnO crystal lattice. This is expected since CuO is a typical p-type semiconductor, whose composition with ZnO leads to a discontinuity in the local band structure at the heterojunction interface and thus cause an enrichment of

Fig. 7 Comparison for the photocatalytic activities of ZnO–CuO nanocomposite with Degussa P25 photocatalyst

photogenerated electrons on the surface of ZnO. Moreover, since the valence band of ZnO is lower than that of CuO, the ZnO–CuO heterojunctions formed in the binary oxides will promote the photogenerated holes in zinc oxide to be transferred to the upper lying valence bands of CuO. Therefore, the recombination rate of photoinduced electron–hole pairs is reduced, and more holes are available to promote photocatalytic reactions. However, it should be recognized that there are many unknowns, including the morphology of the CuO inclusions and their effect on electron/hole creation and recombination rates. The latter effect may account for the eventual decrease in activity that accompanies increasing visible light absorption. Another consideration is the adsorption of the reactant molecules at the surface of the nanosize ZnO–CuO particles, which plays an important role in the methyl orange degradation. This aspect deserves further research. In general, the degradation efficiency of n-type semiconductors, such as TiO2 and ZnO, should be very sensitive to their exposure to oxygen, since oxygen molecules act as an electron scavenger to trap and separate electrons from the positive holes, which helps reduce the chance of electron–hole pair recombination. If O2 is present to capture the electrons, the decoloration of methyl orange will ensue. However, since CuO is a p-type semiconductor, the acceptor impurity will increase due to the presence of adsorbed oxygen on the surface, which will reduce the recombination probability between the electrons and holes. Therefore, the photocatalytic activity of the as-prepared ZnO–CuO composites is not so highly dependent on the exposure to oxygen, and the methyl orange can be completely decomposed. These ZnO–CuO composites offer an ability to destroy most dyes, which is much higher than the Degussa P25 photocatalyst.

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Conclusions Novel ZnO–CuO photocatalytic materials have been prepared via coprecipitation procedure. These nanocomposites exhibit distinctive optical properties, which expands their absorption edge into the visible light region. The rate of photodegradation increases with increase in catalyst amount, up to an optimum value. The optimum degradation is obtained in weakly basic solution. These ZnO–CuO composites show superior visible light photocatalytic activity to degrade aqueous methyl orange, which is higher than that of a commercially available TiO2 photocatalyst (Degussa P25). Acknowledgments This work was supported by the Fundamental Research Funds for the Central Universities and Huazhong Agricultural University Scientific & Technological Self-innovation Foundation (2009QC016).

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