Enhanced photoelectrochemical-response in highly ordered TiO2 nanotube-arrays anodized in boric acid containing electrolyte

We examine the photoelectrochemical properties of highly ordered titanium dioxide nanotube-array photoanodes, fabricated by anodization of titanium in a nitric acid/hydrofluoric acid electrolyte, with and without the addition of boric acid. Under UV–Vis illumination the photocurrent densities achieved with TiO2 nanotube-arrays fabricated in the H3BO3–HNO3–HF electrolyte are a factor of seven greater than the TiO2 nanotube-array samples obtained in the commonly used HNO3–HF electrolyte, indicating the ability to control the photoelectrochemical response of the highly ordered nanotube arrays by tailoring the electrolyte composition. For 560 nm long boric-acid fabricated nanotube arrays, a photoconversion efficiency of 7.9% is achieved upon a 320–400 nm illumination at an intensity of 98 mW/cm2, with hydrogen generated by water photoelectrolysis at the power-time normalized rate of 1708-μmol/h W (42 ml/h W). The resulting nanotube-arrays demonstrate excellent photocorrosion stability, with no detectable degradation in photoconversion properties over 6 months of testing. While the titania bandgap is not suitable for high visible spectrum efficiencies, the high photoconversion efficiency achieved under UV illumination indicates the suitability of the highly ordered nanotube-array architecture for hydrogen generation by water photoelectrolysis.