Abstract
Abstract We present an alternative synthesis strategy for developing nanocrystalline (Ga1−xZnx)(N1−xOx) semiconductors known to be very efficient photoabsorbers. In a first step we produce mixtures of highly crystalline β-Ga2O3 and wurtzite-type ZnO nanoparticles by chemical vapor synthesis. (Ga1−xZnx)(N1−xOx) nanoparticles of wurtzite structure are then formed by reaction of these precursor materials with ammonia. Microstructure as well as composition (zinc loss) changes with nitridation time: band gap energy, crystallite size and crystallinity increase, while defect density decreases with increasing nitridation time. Crystallite growth results in a corresponding decrease in specific surface area. In the UV regime photocatalytic activity for overall water splitting can be monitored for samples both before and after nitridation. We find a significantly lower photocatalytic activity in the nitrided samples, even though the crystallinity is significantly higher and the defect density is significantly lower after nitridation. Both properties should have led to a lower probability for charge carrier recombination, and, consequently, to a higher photocatalytic activity.
Highlights
The development of semiconductors that split water photocatalytically under visible-light irradiation is a promising path to the efficient conversion of solar energy
Powders synthesized from the gas phase exhibit higher crystallinity and higher specific surface areas compared to materials obtained by conventional synthesis methods
In a second step the parent ZnO–Ga2O3 nanoparticle mixtures were nitrided under a flow of ammonia for 0.17 h to 10 h to obtain the desired (Ga1−xZnx)(N1−xOx) wurtzite phase
Summary
The development of semiconductors that split water photocatalytically under visible-light irradiation is a promising path to the efficient conversion of solar energy. Two oxynitrides have been found to be active for full overall water splitting: (Zn1+xGe)(N2Ox) [3] and (Ga1−xZnx)(N1−xOx) with RhO2 and Cr as cocatalyst first prepared by Maeda et al [4, 5]. Nanoparticles dispersed in gases as in CVS provide an alternative facile route: by adjusting process parameters such as temperature, system pressure and precursor evaporation rate, we are able to tune the characteristics of the materials, such as specific surface area, crystallinity and particle size, as reported for pure β-Ga2O3 [13]. The goal of the work reported here is to find alternative synthesis routes and a deeper understanding of how the nitridation process influences the properties of CVS nanoparticles and their photocatalytic activities
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