Synthesis and photoinduced charge-transfer properties of a ZnFe2O4-sensitized TiO2 nanotube array electrode.

TiO2 nanotube arrays sensitized with ZnFe2O4 nano-crystals were successfully fabricated by a two-step process of anodization and a vacuum-assistant impregnation method followed by annealing. The sample was studied by an environmental scanning electron microscope, a transmission electron microscope, energy-dispersive X-ray analysis, and X-ray diffraction to characterize its morphology and chemical composition. Ultraviolet-visible (UV-vis) absorption spectra and a photoelectrochemical measurement approved that the ZnFe2O4 sensitization enhanced the probability of photoinduced charge separation and extended the range of the photoresponse of TiO2 nanotube arrays from the UV to visible region. In addition, the behaviors of photoinduced charge transfer in a TiO2 nanotube array electrode before and after sensitization by ZnFe2O4 nanocrystals were comparatively studied. The photoluminescence of the TiO2 nanotube array electrode became suppressed, and the surface photovoltage responses on the spectrum were significantly enhanced after the introduction of ZnFe2O4 nanocrystals. The transfer dynamics of the photoinduced charges were observed directly by a transient photovoltage measurement, which revealed a fast charge separation at the interface between ZnFe2O4 nanocrystals and TiO2 nanotubes upon light excitation.