Breakdown of the anodized nanostructured anatase for photovoltaic devices: The effect of water content in the electrolyte on preparation of large surfaces of nanotubes

Many researchers are heavily involved in attempts to optimise the course of the electrochemical anodization for preparing nanostructured anatase thin-films. Particularly challenging is to prepare vertically aligned arrays of ordered nanotubes of anatase for application in the photovoltaic (PV) systems which require preparing larger homogeneous crack-free surfaces without the need for pre-or post-treatments. Having previously optimised the anodization cell geometry and electrical settings, here we focus on the electrolyte contribution during the anodization, specifically water content. It was elucidated that the water content can influence the equilibrium and the mechanisms behind several competing processes responsible for the formation of nanotubes (NT): (i) titania formation, (ii) NT formation over intermediate titanium hexafluoride, and (iii) etching of the mentioned phases. We employed broad structural, microstructural, electrochemical, and spectroscopic characterisation tools to shed more light on the development of the system with respect to water presence. Namely, water presence built up conductivity of the electrolyte, and consequently allowed faster oxidation of Ti and subsequently faster etching. Water content increase also facilitated the dissolution of the titanium hexafluoride which is the only water-soluble phase in the system. At moderate water content, the synergy of these processes facilitated: (I) forming the initial titania protective layer (titania prevented short circuits and thus eliminated the need for pre-deposition of protective layers), (II) growth of NT (upgraded the charge transfer), and (III) diminishing the re-sidual nanoformations at the surface of residual titanium oxide or hydroxide phases (fewer nanoformations at surface and fewer residues increased charge transfer and apparent transparency and thus eliminated the need for post-treatment). The highest water content promoted excessive oxidation/etching where first NT with expanded tip were formed (crown-like NT), and then wormholes arose. Similarly, disproportioning in dissolution/oxida-tion/etching occurred at the lowest water contents resulted in less perfect NTs. The bottlenecks of the whole anodization process were identified proving the feasibility of the compositionally novel preparing procedure for obtaining perfect NT. Specifically for the solar cell application, 12% of water in the electrolyte will facilitate growth of the large-scale crack-free TNTs, with other parameters constant. In the course of the investigation, it was evidenced electrolyte can be utilised as a useful tool for controlling anatase to rutile occurrence and vertical distribution.