Quantum size effect on surface photovoltage spectra:: Alpha-Fe2O3 nanocrystals on the surface of monodispersed silica microsphere

A new method for the preparation of Fe2O3 nanoparticle/SiO2 microsphere composites is described, in which fine alpha-Fe2O3 nanocrystals were prepared by forced hydrolysis of FeCl3 aqueous solution. The structure and optical spectra of these alpha-Fe2O3 nanocrystals have been studied. Their visible optical absorption can be enhanced by their adsorptions on the surface Of SiO2 microspheres and thereafter simple packing of these microspheres to the aggregated structures. The size-dependent photogenerated surface photovoltage spectra (SPS) of these composites were studied, and quantum confinement effects of the SPS properties were observed. The transport of photoinduced charges between nanocrystals with intrinsic electronic nature of confined states accounts for this phenomenon. These results are helpful in understanding the relationship among d-d transition and charge-transfer transition in transition metal oxides and find applications in photovoltaic devices.