(Invited) Self-Activation of TiO2 Nanotubes for Photocatalytic H2 Generation

Abstract

TiO2 and particularly its anatase form, is one of the most widely studied oxide semiconductors for photocatalytic production of H2 from aqueous solutions.In anatase, the conduction band is situated at an energetic position that allows the transfer of photogenerated electrons to aqueous media and thus the conversion of H+ into H2. Moreover, TiO2 provides an almost unique high resistance against chemical and photocorrosion. On the other hand, photocatalytic H2 production from aqueous electrolytes shows very slow kinetics on all TiO2 surfaces. Therefore, typically decoration of titania with co-catalysts is used to accelerate the H2 evolution kinetics. Most effective co-catalysts are costly noble metals, such as Pt, Pd, or Au. Economically it is of great interest to develop means and approaches to reduce the use of such costly co-catalyst. In the presentation, we discuss efficient noble-metal-free approaches for catalyzing photocatalytic hydrogen generation. A particularly interesting approach is provided by defined defect engineering of anatase. Specific shallow Ti3+ states can promote the generation of photocatalytic H2. Most spectacular is that these catalytic sites can be formed by photoinduced reduction. In other words, under suitable illumination conditions it is possible to create, in situ, energetically suitable Ti3+ states that then act as an intrinsic co-catalyst for photocatalytic H2 evolution. As a result, both co-catalytic formation of active sites and photocatalytic H2 formation is observed. I.e. with increasing illumination time, a light‐induced self‐amplification of the photocatalytic H2 production rate occurs. The effect is characterized by electron paramagnetic resonance (EPR) spectroscopy, reflectivity, and photoelectrochemical techniques. The origin of the self‐activation and amplification behavior observed for anatase nanoparticles and anodically grown self-organized TiO2 nanotube structures will be discussed in the presentation.