Nanocrystalline Ga-Zn Oxynitride Materials: Minimized Defect Density for Improved Photocatalytic Activity?

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 beta-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.