Transparent Nanotubular TiO2 Photoanodes Grown Directly on FTO Substrates

Abstract

This work describes the preparation of transparent TiO2 nanotube (TNT) arrays on fluorine-doped tin oxide (FTO) substrates. An optimized electrolyte composition (0.2 mol dm−3 NH4F and 4 mol dm−3 H2O in ethylene glycol) was used for the anodization of Ti films with different thicknesses (from 100 to 1300 nm) sputtered on the FTO glass substrates. For Ti thicknesses 600 nm and higher, anodization resulted in the formation of TNT arrays with an outer nanotube diameter around 180 nm and a wall thickness around 45 nm, while for anodized Ti thicknesses of 100 nm, the produced nanotubes were not well defined. The transmittance in the visible region (λ = 500 nm) varied from 90% for the thinnest TNT array to 65% for the thickest TNT array. For the fabrication of transparent TNT arrays by anodization, the optimal Ti thickness on FTO was around 1000 nm. Such fabricated TNT arrays with a length of 2500 nm exhibit stable photocurrent densities in aqueous electrolytes (~300 µA cm−2 at potential 0.5 V vs. Ag/AgCl). The stability of the photocurrent response and a sufficient transparency (≥65%) enables the use of transparent TNT arrays in photoelectrochemical applications when the illumination from the support/semiconductor interface is a necessary condition and the transmitted light can be used for another purpose (photocathode or photochemical reaction in the electrolyte).

TiO2 Nanotubes on Transparent Substrates: Control of Film Microstructure and Photoelectrochemical Water Splitting Performance

“…Recently, the anodization of titanium thin films deposited on glass substrates via different Physical Vapor Deposition methods [18,25], especially magnetron sputtering, have proven to be a promising route to achieve devices with high stability and performance [16,[24][25][26][27][28][29].…”

“…For instance, highly ordered TiO 2 nanotube arrays are typically grown by electrochemical anodization of titanium foils [23]. However, fully transparent photoelectrodes or photonic devices are highly desirable in many applications including photovoltaics and PEC cells [18,24]. For instance, TNT reaching transparency higher than 65% and producing stable photocurrent can be employed in PEC tandem cells, in which the illumination from the support-semiconductor interface is a required condition, enabling the efficient excitation of both photoanode and photocathode materials [24].…”

The Anodization of Thin Titania Layers as a Facile Process towards Semitransparent and Ordered Electrode Material

“…As already discussed above the substrate temperature during the deposition is considered as one of the most important parameter that should be adjusted to achieve a good adhesion of Ti films to the substrate. However, the works done by Buettner et al [ 16 ], Pausova et al [ 1 ] and Krysa et al [ 33 ] have shown that not only the substrate temperature, but also the morphology of the deposited Ti films play a major role in good adhesion of TNTs layer. This section discusses the impact of the deposited Ti thickness on the geometrical features of anodic semitransparent TNTs.…”

“…This indicates a significant improvement in the absorption capacity towards the visible range of the semitransparent material after CdS quantum dots decoration. Moreover, a good transmittance, about 90%, in the visible range was achieved by Paušová et al [ 1 ] after the annealing of TNTs grown on FTO at 500 °C.…”

“…The depletion of fossil-based resources and the continuous warming of the climate have prompted human beings to attach considerable interest to the use of renewable energy resources [ 1 ]. Among the various renewable energy sources, the energy from sun that hits the earth in one hour is sufficient to meet the energy demand of the world in a year [ 2 ].…”