(Invited) Black TiO2 Nanotubes As Functional Supports and Electrodes

Abstract

TiO2 nanomaterials have over the last 30 years attracted tremendous scientific and technological interest. A main research direction for TiO2 in functional applications is its use in photocatalysis, e.g. for the direct splitting of water into H2 and O2 to generate the potential fuel of the future, hydrogen. In order to achieve a maximum reaction rate, high surface area substrates, usually nanoparticles are used either suspended or compacted to a photoelectrode. Over the past decades, various 1D and highly defined TiO2 morphologies were explored for the replacement of nanoparticle networks and were found in many cases superior to nanoparticles. However, in every case the kinetics of H2 generation is sluggish from a plain TiO2 surface and noble metal cocatalysts are needed to reach accelerated reaction rates. Only recently it has been shown that TiO2 can be intrinsically activated by generation of suitable surface defects. Such material of a “gray” or “black” color can be generated with various treatments of many titania polymorphs and morphologies. The presentation will focus on highly ordered nanotube arrays of TiO2 and discuss most recent progress in synthesis and modification for photocatalytic and photoelectrochemical H2 generation. Of special interest is the formation of black TiO2 nanotubes by various reductive treatments. Black titania produced by high pressure hydrogenation has been demonstrated to enable noble-metal-free photocatalytic H2 generation. TEM, EPR, and XPS investigations as well as photoelectrochemistry are used to characterize different degrees of surface hydrogenation, surface termination, electrical conductivity, and structural defects in the differently reduced titania nanotubes. These investigations find that the formation of intrinsic photocatalytic hydrogen evolution activity is induced by optimized and specific defect formation.