Anodically Grown TiO2 Nanotube Arrays Used As Substrates for Thermal Dewetting of Pt Show Significantly Enhanced Photocatalytic H2 Generation with Minimal Co-Catalyst Amounts

Abstract

TiO2 nanostructures have recently attracted much attention due to their wide range of applications, such as photocatalysis, photovoltaics (dye-sensitized solar cell, photo-electrochemistry) and gas sensing. Particularly, electrochemically formed TiO2 nanotube arrays were shown to exhibit improved photocatalytic activity compared to other TiO2 nanostructures, this being ascribed to their directional electron transport and orthogonal charge carrier separation.1 The advantage of an electrochemical growth of the TiO2 nanotube layers is not only the simplicity of the process, but also the possibility of controlling the tube geometry (length, diameter, wall thickness) by selecting suitable anodization parameters (applied voltage, anodization time, temperature, fluoride concentration). Nevertheless, TiO2 alone is not effective in photocatalytic H2 generation: it requires the presence of noble metal co-catalysts (Pt, Au, Pd).2,3 Among the various noble metals, Pt is the most effective co-catalyst: it enables an efficient electron transfer at the semiconductor interface by providing a favorable solid state junction to TiO2 and it enhances the hydrogen recombination reaction. There are several well explored methods to decorate Pt onto TiO2 nanotubes, mostly leading to a decoration of the full tube length with nanoparticles (the most frequently used up to date is photodeposition). With our contribution we show a different approach: we decorate only the outermost surface of anodic TiO2 nanotubes with very thin Pt films (nominally few nm-thick) by using a simple plasma sputtering technique.4 Then, by a controlled thermal treatment, the Pt films are dewetted at the mouth of the tubes into arrays of Pt nanoparticles that are as small as of few nm. This site-selective deposition (only at the tubes top) allows for minimizing the amounts of costly noble metal co-catalyst, while at the same time leads to a significantly enhanced photocatalytic H2 generation compared to tubes that carry along their full length a classic Pt nanoparticle decoration. [1] K. Lee, A. Mazare, P. Schmuki, Chem. Rev. 2014, 114, 9385–9454.[2] N. T. Nguyen, J. Yoo, M. Altomare, P. Schmuki, Chem. Commun. 2014, 50, 9653–9656.[3] N. T. Nguyen, M. Altomare, J. Yoo, P. Schmuki, Adv. Mater. 2015, 27, 3208–3215.[4] N. T. Nguyen, M. Altomare, J. E. Yoo, N. Taccardi, P. Schmuki, Adv. Energy Mater. 2015, doi: 10.1002/aenm.201501926. Figure 1