Evolution of rough-surface geometry and crystalline structures of aligned TiO 2 nanotubes for photoelectrochemical water splitting OPEN (original) (raw)

Nowadays, increasing awareness of environment and fossil fuels protection stimulates intensive research on clean and renewable sources of energy. Production of hydrogen from water through solar-driven splitting reactions is one of the most promising approaches in the field of photoelectrochemistry (PEC). In this work we have fabricated well-aligned, highly-ordered, smooth-mouth TiO 2 nanotube arrays (TNAs) in a two-step anodization process of titanium foil, which were then used as photoelectrodes for PEC water splitting. It demonstrates for the first time correspondence between non-linear component characteristics of multiscale rough surface and crystalline structure of annealed TNAs measured at various fabrication stages and their photoelectrochemical response. The as-anodized TNAs with isotropic surface (deduced from AFM and SEM images) and largest figure of merit (according to their PEC performance) were annealed at 450 °C in air. Scale-invariant descriptors of the surface structure of the deposits involved: fractal dimension, corner frequency, roughness, size of nanostructures and their dominant habits. Moreover, X-ray diffraction data processed using the Rietveld method confirmed coexistence of various oxides, for example: TiO 2 in the form of anatase, TiO and Ti 3 O 5 phases in the TNAs under study pointing that previous well-established mechanisms of the TNA growth were to certain degree incomplete. Photoelectrochemical (PEC) water splitting is one of the most favorable approaches for H 2 production as a clean energy vector of the future. Since the work by Fujishima and Honda in 1972 1 , increasing research has been carried out towards this issue using electrodes made of various materials, e.g. semiconductors. Unfortunately, their practical application has encountered a number of technical complications. Among the metal oxides that has been taken into consideration, titanium dioxide (TiO 2) is found to be promising in PEC water splitting 2-9 due to its appropriate band-gap structure, superior chemical and optical stability and low cost. In particular, TiO 2 nanotube arrays prepared in anodization processes have numerous advantages over TiO 2 nanoparticle films resulting from facile preparation procedure, high surface-to-volume ratio for contact with the electrolyte, large light harvesting efficiency improved by light scattering into tubular morphology, and high electron mobility induced by their unidirectional channel 10,11. Some strategies such as doping or semiconductor heterocoupling were used for modification of TiO 2 nanotube arrays to be activated under visible light 12-14 .