R. Galeazzi

Effects of morphology, surface area, and defect content on the photocatalytic dye degradation performance of ZnO nanostructures

ZnO nanostructures of different morphologies were fabricated through ultrasound-assisted hydrolysis of zinc acetate at room temperature, by controlling the pH of the reaction mixture. It has been observed that the pH of the reaction solution affects both the morphology and defect content of the nanostructures. To study the effects of morphology and other parameters like specific surface area, defect content, and surface contamination on photocatalytic activity, both the as-grown and air-annealed nanostructures were tested for methylene blue (MB) degradation under UV light. While all the above mentioned parameters have been seen to affect the photocatalytic performance of ZnO nanostructures, specific surface area, defect content, and carbon contamination at the surface have been seen to be the most important parameters, and should be controlled for their application in photocatalysis. Therefore, for photocatalytic applications of ZnO nanostructures, not only their morphology or the specific surface area are important, but care should be taken to control their defect contents and surface contaminants.

Structural properties of Zn-ZnO core-shell microspheres grown by hot-filament CVD technique

We report the hot-filament chemical vapor deposition (HFCVD) growth of Zn-ZnO core-shell microspheres in the temperature range of 350-650°C only using ZnO pellets as raw material. The samples were characterized by scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD) techniques. SEM micrographs showed the presence of solid microspheres and a Zn-ZnO layer in all samples. The observed heterogeneous morphology on each sample suggested two different growth mechanisms. On the one hand, solid microspheres were formed by means of gas phase nucleation of Zn atoms. The Zn-ZnO layer was formed on the substrate as result of surface reactions. It is possible that Zn microspheres condensed during the natural cooling of the HFCVD reactor as they were observed on the Zn-ZnO layer.

Physicochemical conditions for ZnO films deposited by microwave chemical bath deposition

Physicochemical analysis was carried out to obtain the species distribution diagrams (SDDs) for the deposition of ZnO films as a function of OH− ion concentration ([OH−]) in the reaction solution. The study of SDDs predicts nucleation and ZnO film growth by means of the dominant species at a given pH value. To confirm this, a series of experiments were made varying the [OH−] in the reaction solution and keeping the others parameters constant. Structured zinc oxide (ZnO) films were obtained on glass substrates by microwave chemical bath deposition (MWCBD). Structural, optical and morphological ZnO film properties were investigated as a function of [OH−]. X-Ray diffraction technique (XRD) measurements show multiple diffraction peaks, indicating the polycrystalline nature of ZnO films. Scanning Electron Microscopy (SEM) images of ZnO structures showed morphological changes with the variation of [OH−]. The stoichiometry of the structures changed as the [OH−] was varied in solution. From Raman spectra, it was observed that the [OH−] of the reaction mixture strongly affects the crystal quality of ZnO structures. A reaction pathway for the synthesis of ZnO structures based on our results is proposed. Experimental results are consistent with the physical–chemical analysis.

Influence of variation in the concentration of ammonium hydroxide on the size of ZnO crystal obtained by Microwave Chemical Bath Deposition

Films of good crystalline quality of ZnO were successfully prepared using the microwave chemical bath deposition method at a temperature of 80 °C. Concentration of the basic precursor was varied systematically in order to obtain different degrees of acidity in the precursor solutions. Increasing the pH causes an increase in yield. This increase is reflected on the thickness of the deposit. The results of atomic force microscopy (AFM) show an increase in particle size with increasing pH in agreement with the results obtained by profilometry.