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.