TiO2 nanocrystal rods on titanium microwires: growth, vacuum annealing, and photoelectrochemical oxygen evolution

Abstract

Water splitting by photocatalysis, for example employing seawater or wastewater, has the potential to make a substantial contribution to a future hydrogen economy. New types of 3D metal-wire substrates in woven form or brushes for photoelectrochemical processes are desirable to replace flat semiconductor-coated surfaces. In this report, titanium microwires (100 μm diameter; pre-passivated) are employed to produce high surface area photoanodes with a TiO2 (rutile) nanocrystal array coating that was grown hydrothermally. A photo-active electrode is obtained with nanocrystal TiO2 rods of adjustable length from 1.67 μm up to 7.38 μm. Saturation of the photoelectrochemical response occurs at an intermediate length of approx. 3.19 μm with a photocurrent density of approximately 4.4 mA cm−2 at 1.0 V vs. Ag/AgCl (3 M) in aqueous 1 M Na2SO4 under 100 mW cm−2 385 nm LED illumination. The effectiveness of vacuum annealing for enhancing TiO2 photochemical processes is demonstrated. The formation of oxygen in aqueous 1 M Na2SO4 electrolyte media (with/without chloride) is assessed using Clark probe measurements. Co-evolution of chlorine was evident in 1 M KCl. This work demonstrates that microwire photoelectrodes for woven form or brush structures are entirely feasible, and that pre-passivated titanium metal can be readily coated with titania nanocrystals via hydrothermal synthesis.