Rongliang He

Local structure and photocatalytic property of sol-gel synthesized ZnO doped with transition metal oxides

ZnO nanoparticles doped with up to 5 at% of Co and Mn were prepared using a co-precipitation method. The location of dopant ions and the effect of doping on the photocatalytic activity were investigated. The crystal structure of nanoparticles and local atomic arrangements around dopant ions were analyzed by X-ray absorption spectroscopy. The results showed that the Co ions substituted the Zn ions in the ZnO wurtzite phase structure and induced lattice shrinkage, while Mn ions were not completely incorporated in the crystal lattice. The photocatalytic activity under simulated sunlight was characterized by the decomposition of Rhodamine B dye molecules. It was revealed that Co-doping strongly reduced the photocatalytic activity but Mn-doping showed a weaker effect on the reduction of the photoactivity.

Reduction of the photocatalytic activity of ZnO nanoparticles for UV protection applications

The detrimental effects of UV radiation are having a significant impact on our life and environment. The development of effective UV shielding agents is therefore of great importance to our society. ZnO nanoparticles are considered to be one of the most effective UV blocking agents. However, the development of ZnO-based UV shielding products is currently hindered due to the adverse effects of the inherent photocatalytic activity exhibited by ZnO. This paper reports our recent study on the possibility of reducing the photoactivity of ZnO nanoparticles via surface modification and impurity doping. It was found that the photoactivity was drastically reduced by SiO2-coatings that were applied to ZnO quantum dots using the Stober method and a microemulsion technique. The effect of transition metal doping on the photoactivity was also studied using mechanochemical processing and a co-precipitation method. Cobalt doping reduced the photoactivity, while manganese doping led to mixed results, possibly due to the difference in the location of dopant ions derived from the difference in the synthesis methods.

Synthesis of high surface area amorphous tin-zinc oxides by a sol-gel method

A new method to synthesize conducting oxide nanoparticles with low photocatalytic activity was investigated. Initially, the preparation of amorphous ZnO-SnO2 solid solution nanoparticles was studied using a sol-gel technique. It was found that X-ray amorphous nanopowders with low photocatalytic activity were produced when the precipitates were heat treated below 500 °C. However, FT-IR data showed that the sample may not be an oxide semiconductor. A mixture of ZnO and SnO2 crystalline nanoparticles was also produced at 800 °C and found to have much reduced photoactivity than commercial ZnO nanoparticles having a similar specific surface area.

Room Temperature Synthesis of ZnO Quantum Dots by Polyol Methods

Zinc oxide is an important semiconducting material having a broad range of applications including transparent conductive oxides (Hosono 2007), ultraviolet (UV) light absorbers (Becheri et al. 2008) and photocatalysts (Beydoun et al. 1999). ZnO nanoparticles exhibit quantum size effects when the particle size is smaller than the exciton-Bohr diameter of ~ 8 nm. In the past, many methods to synthesize ZnO quantum dots were investigated, (Zhang et al. 2010; Xiong et al. 2005; Tang et al. 2009; Tsuzuki & McCormick, 2001). Among them, wet chemical synthesis methods, such as sol-gel, hydrothermal and solvothermal methods, are widely used to obtain free ZnO quantum dots that are detached from substrates. However, this approach normally requires surfactants or capping agents to limit the growth of particles below the exciton-Bohr diameter. This causes some drawbacks such as the necessity to have additional steps in the production process to remove the surfactants from nanoparticles and increased chance of contamination. In addition, wet chemical synthesis of ZnO normally requires high reaction temperatures above 100 oC. For example, Tang et al. showed that mono-dispersed ZnO quantum dots can be synthesised in triethylene glycol and demonstrated their promising applications in cell labelling, but the reaction required a temperature as high as 280 oC and stearate acid as a surfactant (Tang et al. 2009). When the high temperatures were not used, the reaction takes a considerable time. Xiong et al. synthesised ZnO quantum dots in triethylene glycol using LiOH and zinc acetate and, although the morphology and optical properties of ZnO was controlled by the molar ratio between LiOH and zinc acetate, this reaction required 30 days to complete at the interface between the air and the polyol solution (Xiong et al. 2011).