Proceedings of September Meeting 2016 (NFM16)
Publication date: 14th June 2016
Titanium dioxide is a widely investigated material for numerous applications, including photoelectrochemical (PEC) water splitting as the way to utilize solar energy for hydrogen production. Nanostructuring of TiO2, in the form of nanocolumn arrays, may lead to substantial improvements of the PEC performance owing to an increase in the surface-to-volume ratio combined with shorter charge transport pathways. On the other hand, too small nanostructures may reside in flat-band condition, having no depletion layer crucial for effective charge carrier separation. Thus, an adjusted size of the nanocolumns having a depletion layer of optimum thickness, occupying a big portion of the column itself, may lead to charge separation taking place in a substantial portion of the nanocolumn volume, increasing the PEC response. Therefore, in this work, we study various types of morphology, crystallinity, oxygen vacancy concentration, and doping level in the TiO2 nanocolumn arrays in order to achieve a depletion layer taking up substantial volume of the nanocolumns. To evaluate these semiconductive properties, we perform electrochemical impedance spectroscopy (EIS) measurements at various voltages in the dark on differently fabricated and modified TiO2 surfaces.
The TiO2-based nanocolumn arrays are prepared by porous-anodic-alumina (PAA)-assisted anodizing[1] of Al/Ti[2] or Al/TiNx precursor layers in an organic-acid-based electrolyte. The Al layer is anodized at 40 V to form PAA; then the Ti or TiNx layer is reanodized to 100 V. This leads to the growth of titania or N-containing titania nanocolumns within the alumina pores. The fabricated nanocolumns are approx. 40 nm wide and 160 nm long. Subsequently, the surrounding PAA is partially or completely etched to vary the extent to which the columns protrude from the alumina overlayer. Additionally, some samples are annealed in ambient atmosphere or vacuum at 600°C to alter the crystallinity and oxygen vacancy concentration.
The EIS measurements are performed at different voltages in borate buffer solution in the dark. The Mott-Schottky analysis reveals an increase in the depletion layer capacitance with decreasing the voltage, which is in agreement with the n-type doping of the TiO2 nanocolumns. Besides, the calculated donor density and flat-band potential vary with the annealing conditions, which allows for tuning the thickness of the depletion layer in order to enhance charge carrier separation during photoelectrochemical water splitting.
Research leading to these results was supported by GAÄŒR grant no. 15-23005Y.
[1] A. Mozalev, M. Sakairi, I. Saeki, H. Takahashi, Electrochim. Acta 2003, 48, 3155.
[2] R. M. Vázquez, R. Calavia, E. Llobet, F. Guirado, J. Hubálek, A. Mozalev, Procedia Engineering 2012, 47, 833.
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