Rapid Ag recovery using photocatalytic ZnO nanopowders prepared by solution-combustion method

Abstract

The study of photocatalytic reactions is a recently emerging area out of the field of catalytic chemistry. In general, photocatalytic reaction is an interaction between molecules and surfaces including electron transfer between molecules and catalysts. Semiconductors are usually used for photocatalysts because their carrier concentration can be controlled [1, 2]. Recent studies have mainly concentrated on environmental problems such as purification of contaminated water or air [3, 4]. Semiconductors for photocatalytic reactions should have characteristics such as high optical activity, high optical stability, high sensitivity for UV-visible light and low fabrication cost [2, 5]. Many researchers have paid attention to oxide semiconductors because of their wide band gap energy and ease of handling [6–9]. Nano-sized particles have different physical and chemical properties compared to bulk materials. High catalyst activity may be expected because of their large surface area and different surface properties, such as surface defects, when these nano-sized particles are used as catalysts. The fabrication of nano-sized particles with high stability and dispersibility in solution is essential to obtain high photocatalytic activity [10, 11]. In this paper, synthesis of ZnO was tried to obtain the nano-sized semiconductor particles for photocatalytic application. Two methods are generally used to obtain such ZnO powder. One is vapor method and the other is sol-gel method. In the case of vapor method, the resulting powders contain agglomerates rather than separated particles because it is very difficult to control the reaction conditions during the process. This means that the vapor method is not a satisfactory method to obtain nano-sized ZnO powders. On the other hand, the sol-gel method could provide uniform ZnO powders. However, it is required to control the reaction condition strictly because of the violent hydrolysis reaction in the air during synthetic process. In addition, this method is not attractive because metal alkoxide as a starting source is very expensive. This is why the method is still not commercialized but is being tried only in a small laboratory scale [12, 13]. Park et al. proposed the solution combustion method to synthesize highly pure and fine ceramic powders. Using this method, the heating and evaporation of metal nitrate solution with glycine results in self-firing and generates intense heat by exothermic reaction [14]. This intense heat is used to synthesize the powders. In this study, nano-sized ZnO powders were prepared using this solution combustion method. Their characteristics and photocatalytic efficiency were also investigated. Zn(OH)2 (Junsei, Japan) powder solved in nitric acid was used as the starting material (oxidant), and glycine(H2NCH2COOH, Yakuri pure chemicals co. Ltd, Japan) was used as a fuel, in order to synthesize ZnO powder. The starting material was dissolved in distilled water in a beaker. Then the glycine was added to the starting solution in the beaker. The solution was then heated on a hot plate with stirring. As the distilled water evaporated, the solution became viscous and generated small bubbles. The nitric acid group (NO3 ) reacted with the fuel and intense heat was generated (the temperature rises to 1500–1800 ◦C). This high temperature resulted in a high pressure, which leads to an explosion. ZnO powder was formed by this high temperature and high pressure and was collected in a collector that was placed above the beaker. In this paper, this process is referred to the solution combustion method. In general glycinenitrate process (GNP), oxide powders are synthesized adding the fuel in equilibrium state (fuel/oxidant = 1). However, in this study after calculating the oxidation number by the general GNP method, ZnO powder was prepared by fuel lean state considering the vapor pressure of Zn or ZnO. Five different ZnO powders were prepared with the variation of the fuel/oxidant ratio. X-ray diffractometer (XRD) measurement was performed to examine the crystalline state of the prepared ZnO powder. As shown in Fig. 1, single phase ZnO was obtained by the above method; the ZnO powder with fuel/oxidant ratio of 0.8 showed the highest XRD peaks and hence was assumed to be the most crystalline. As shown in Fig. 2, the photoluminescence spectra of commercial ZnO powder showed two peaks: one is in the vicinity of 500 nm, and the other one is near 400 nm. The peak near 500 nm probably came from the energy transition between O vacancy level and Zn vacancy level as shown in Fig. 3. The UV peak near 400 nm might be from band to band transition. However, ZnO powder synthesized by the solution combustion method showed only one sharp peak near 390 nm that was slightly shifted to UV side. This peak is equivalent to the energy gap of ZnO (∼3.2 eV). These results indicate