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Breakdown of the anodized nanostructured anatase for photovoltaic devices: the effect of electrolyte composition on preparation of large surfaces of nanotubes

The insight in recent investigations of titanium dioxide show this material is maybe the most extensively studied semiconducting material with application in the area of photovoltaics (PV), sensors, water splitting, etc. PV titania is active in generation, transport and recombination of photo-induced electron-hole pairs under illumination, thereof, it is worth considering upgrades of its key parameters. It is already well known that recombination consequences may be reduced by texturization of the surfaces, where the discovery of 1D materials was shown as beneficial. TiO2 nanotubes (TNT) were first time successfully electro-chemically prepared by Zwilling [1]. Here we focus on the electrochemical field assisted growth and etching over intermediate phase to yield ordered hollow 1D nanostructures [2]. Having previously optimised the electrical conditions and cell geometry, here we focus on the electrolyte contribution, specifically water content which can influence the equilibrium of the several competing process responsible for the nanotubes formation: titania formation, TNT formation over intermediate titanium hexafluoride and etching of the mentioned phases. Higher water content increases electrolyte conductivity, consequently allowing faster oxidation of Ti and subsequently faster etching. Higher water content also contributes to the dissolution of the only water-soluble phase in the system ; the titanium hexafluoride. At moderate water content increase, these processes facilitate the initial titania protective layer formation (titania prevents short circuits) and TNT growth (better charge transfer) as well as diminish the scram of residual titanium oxide or hydroxide phases (better charge transfer, better transparency, post-treatment not necessary). Highest water content promotes excessive oxidation/etching where first TNTs with expanded tip (crown-like TNTs), and then wormholes arise. Similarly, disproportioning in dissolution/oxidation/etching occurs at the lowest water contents resulting in less perfect NT. (Micro) Structural, electrical and spectroscopic characterisation of NTs and electrolytes enabled identification of mechanisms and bottlenecks in the course of the anodization. Thereof we can prepare larger homogeneous crack-free TNT surfaces.

TiO2, anodization, 1D nanotubes, photocatalysis

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