Oxide Semiconductors for Solar-to-Chemical Energy Conversion

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Authors: Mohammad A. Alim, Tadeusz Bak
Abstract: The purpose of this work is to consider the basic concepts on the present state of understanding of photocatalytic energy conversion using oxide semiconductors. This work also considers the approaches in derivation of theoretical models that allow explanation of the effect of properties on the performance of oxide-based photocatalysts in photocatalytic water oxidation. In this work we show that the performance of photocatalytic systems must be considered in terms of a range of the key performance-related properties (KPPs) that, in addition to the band gap, include the concentration of surface active sites, charge transport and Fermi level. Taking into account that all these KPPs are related to defect disorder, defect engineering may be applied in processing oxide semiconductors with optimal properties that are required to exhibit maximised performance in solar-to-chemical energy conversion.
Authors: Yi Yang, Ying Chih Pu, Yat Li, Jin Zhang
Abstract: Titanium dioxide (TiO2) has been widely used as photoanodes in photoelectrochemical (PEC) water splitting. However, the typically high density of bandgap trap states results in fast charge carrier recombination and poor electrical conductivity, and thereby weak PEC performance. Rational creation of oxygen vacancy (Vo) in TiO2 has been demonstrated as an effective method to modify the electronic and optical properties, as well as improved PEC performance. Different strategies have been developed to fabricate oxygen deficient TiO2 photoanodes, such as hydrogen treatment, thermal annealing, electrochemical reduction, flame reduction, and chemical reduction. In conjunction with oxygen vacancy creation, doping of TiO2 with elements further enhances the PEC activity by introducing other bandgap states. Various techniques, including ultrafast laser spectroscopy, have been employed to probe the chemical nature and associated charge carrier dynamics of the bandgap states.
Authors: Siti Nur Farhana Mohd Nasir, Mohd Yusof Sulaiman, Mehdi Ebadi, Norasikin Ahmad Ludin, Mohd Adib Ibrahim, Mohd Asri Mat-Teridi
Abstract: One of the great challenges facing the society today is replacing fossil fuels by renewable energy sources. Hydrogen from non-carbon sources is considered one of the viable potentials to help alleviate reliance upon fossil fuels for energy and addressing the environmental problems. Photoelectrochemical water splitting was brought to attention since the pioneering work in 1972. Since then, numerous metal oxide photocatalysts have been investigated to enhance the overall water splitting performance. Up to now, Bismuth vanadate, BiVO4 has emerged as the most promising photocatalyst in the construction of photoelectrochemical cell utilizing sunlight and water, the most abundant resources on earth. In this review, the principles, critical factors influencing the efficient BiVO4-based photoanode properties such as the crystal and electronic properties are discussed. Subsequently, the methods synthesis and research efforts adopted to develop efficient and active BiVO4 photoanode are presented.
Authors: Željko Pastuovic, Mihail Ionescu, Ettore Vittone, Ivana Capan
Abstract: Accelerator-based nuclear techniques are an important tool for the modification and characterization of surfaces in general, down to a depth of around one micrometer. For oxide semiconductors used in solar energy conversion, the surface plays a critical role in facilitating the use of solar photon energy to obtain hydrogen via spontaneous water oxidation. For such a process, the required surface properties are complex and include specific chemical composition, as well as the defect composition, and both of these characteristics may be augmented using accelerator-based nuclear techniques. The targeted modification of surfaces makes use of ion implantation for changing the chemical composition, and ion irradiation for changing the defect structure. The defect formation is a very complex process, and in this work we placed more emphasis on this aspect. We attempted to present the defect formation under the irradiation of ion beams at the two extremes: formation of extensive and large-scale cluster defects; and formation of small-scale point defects. In addition, we review the main characterization techniques based on ion beams, with examples from work carried out on semiconductors and oxide semiconductors.

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