When Fewer Photons Do More: A Comparative O₂Photoadsorption Study on Vapor-Deposited TiO₂and ZrO₂Nanocrystal Ensembles

The charge separation properties of TiO2 and ZrO2 nanocrystal powders which were produced by metal organic chemical vapor phase deposition were compared by means of O-2 photoadsorption. Prior to the UV excitation experiments, we carefully explored the structure and morphology as well as the spectroscopic properties of the dehydroxylated nanoparticles in controlled gas atmospheres using UV diffuse reflectance, FT-IR, and electron paramagnetic resonance spectroscopy. On the basis of this information, we utilized the photogeneration of surface-adsorbed O-2(-) radicals and their detection by EPR. spectroscopy to quantify the concentration of separated charges and to relate these figures to the number of oxide particles that were exposed to known numbers of UV photons with defined energies. The results presented in this study reveal that upon band-gap excitation, ZrO2 nanoparticles are by 2 orders of magnitude more efficient in oxygen-assisted charge separation than TiO2. The dramatically diminished charge separation yield in TiO2 is attributed to the existence of deep trap states which persistently stabilize paramagnetic holes and at the same time act as recombination centers. Their absence in ZrO2 nanocrystals gives rise to a comparable performance in terms of photoadsorbed O-2 and essentially compensates for the significantly wider optical band gap in relation to TiO2.