Abstract
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 TiO2 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 co-existence of various oxides, for example: TiO2 in the form of anatase, TiO and Ti3O5 phases in the TNAs under study pointing that previous well-established mechanisms of the TNA growth were to certain degree incomplete.
Highlights
Photoelectrochemical (PEC) water splitting is one of the most favorable approaches for H2 production as a clean energy vector of the future
We present an improved two-step anodization process for the fabrication of well-aligned, highly-ordered and smooth-mouth TiO2 nanotube arrays on Ti substrate with detailed investigations of morphological and crystalline structures of synthesized nanostructures at each fabrication steps
Highly-ordered, smooth-mouth TiO2 nanotube arrays on Ti substrate were synthesized and their morphological and crystalline structures after each step of fabrication procedure were studied in details
Summary
Photoelectrochemical (PEC) water splitting is one of the most favorable approaches for H2 production as a clean energy vector of the future. The reason for this effect is that the walls of the tubes become thinned at their top ends, so that they cannot support their own weight or withstand capillary forces when drying They collapse and bundle in the form of nanograss which could close the mouths of nanotubes and in turn decrease the effective surface area of TNA, avert charge carrier direction to the substrate and passivate active sites for chemical reactions. To overcome this problem, several effective approaches towards production of highly ordered and uniform TiO2 nanostructures have been developed[15,16,17,18,19].
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