Abstract:
Photocatalysts are of importance as they have a wide range of applications such as degradation of organic compounds and dyes, self-cleaning surfaces, anti-bacterial action, and fuel generation via water splitting and carbon dioxide reduction. Complex metal oxides such as molybdates and tungstates have previously shown photocatalytic properties, with potential benefits to the current energy economy and organic waste disposal means. In this work we explored the following series of Zn1-xCoxMoO4 and Zn1-xCoxWO4 with a focus on how the doping of cobalt (Co) affects the structure and physical properties of these oxide systems. Materials were prepared via standard solid-state synthesis and characterised via X-ray diffraction, Rietveld refinements, Infrared spectroscopy, UV-Vis spectroscopy, Scanning Electron Microscopy, and Energy Dispersive Spectroscopy. X-ray diffraction and Rietveld refinements have shown that the molybdate series undergoes a phase transition between x = 0.3 – 0.4 Co doping, from a triclinic system to a monoclinic system. These results have also been supported by Infrared spectroscopy and UV-Vis spectroscopy of these materials. The tungstate series was found to have no phase transition with cobalt doping. This was again confirmed by Infrared spectroscopy and UV-Vis spectroscopy. Solid evidence was found showing that the fully doped tungstate compound (CoWO4) does exist in a monoclinic system. This was shown in both the Rietveld refinements for the powder and single crystal materials. In literature, CoWO4 has α, β, and γ angles all equalling 90°, with a, b, and c having different values1. This would normally result in an orthorhombic structure, but CoWO4 is labelled as having a monoclinic structure in space group P2/c. ZnWO4 is known to exist in the monoclinic system, therefore the lack of evidence for phase transition between ZnWO4 to CoWO4 shows that CoWO4 stays in the monoclinic system and not the orthorhombic system. Single crystal X-ray diffraction results confirm that finding. It was found via UV-Vis spectroscopy that the doping of Co into both systems played a key role in altering the optical properties of the materials. It allowed all the doped materials to strongly absorb in the visible range of the spectrum. This could have further implications for the photocatalytic activity of these materials. Energy Dispersive spectroscopy has shown that the powder materials for both series contain the right composition throughout the series. However, the single crystals were shown to have less than ideal Zn and Co composition, therefore a reassessment of the reaction scheme needs to be done in order to obtain single crystal of the right composition.