Controlled synthesis of nanoporous tin oxide layers with various pore diameters and their photoelectrochemical properties

Porous anodic tin oxide films with various pore diameters were synthesized by one-step anodization of Sn foil in 1 M NaOH followed by 2 h of annealing at 200 °C. When the potential of 2 V was applied during anodization, oxide layers with ultra-small nanochannels having diameter slightly above 10 nm were obtained, and an average distance between neighboring pores was ∼20 nm. On the contrary, applying the potential of 4 V resulted in the formation of anodic films with much wider channels (∼40 nm in diameter) and larger pore spacing (∼55 nm). The composition of as formed samples was examined by XRD and XPS measurements. The as obtained anodic films were amorphous even after annealing. However, the greater Sn2+ ion content was observed for the sample anodized at 4 V. In consequence, the porous oxide layer grown at the higher potential exhibited a red-shifted absorption edge as well as the lower optical band-gap in comparison to the sample synthesized at 2 V. This fact was ascribed to a higher content of Sn2+ defects which are mainly responsible for a significant enhancement of photoelectrochemical activity of the material in the visible range. However, photoelectrochemical activity in the visible range was noticeable for anodic tin oxide films independently of the anodizing potential. We believe that such kind of self-supported electrodes can be promising candidates for photocatalytic and photoelectrochemical applications.