Effect of different hole scavengers on the photoelectrochemical properties and photocatalytic hydrogen evolution performance of pristine and Pt-decorated TiO2 nanotubes

TiO2 nanotubes have been investigated in photoelectrochemistry and photocatalysis for more than a decade. However, up to now, a systematic investigation of different hole scavengers is still lacking. Here we investigate the effect of the most relevant sacrificial hole scavengers on the photoelectrochemical properties and photocatalytic H-2 evolution performance of pristine and Pt-decorated anodic TiO2 nanotubes. We examine methanol, isopropanol, ethylene glycol, EDTA-Na-2, as well as Na2SO3 and H2O2 and find that the incident photocurrent conversion efficiency (IPCE) of the TiO2 nanotubes in 0.1 M Na2SO4 electrolytes increases by 1.8-3.1 times, depending on the used hole scavenger. The efficiency increases in the sequence H2O2 < Na2SO3 < isopropanol < methanol < ethylene glycol < EDTA-Na-2. In presence of any hole scavenger, for nanotubes in the length-range of 2-10 mu m, the photocurrent spectra and the IPCE magnitude are independent of the tube length. The photocurrent onset potential (optical flatband potential) is significantly affected by the different type of scavengers, in line with their red-ox potential. Under open circuit conditions (photocatalytic conditions), organic hole scavengers lead to a 10.0-28.8 times higher H2 production by TiO2 nanotubes than the scavenger-free case, with a sequence MeOH > i-PrOH > EDTA-Na-2 > EG, while little or even detrimental effects of Na2SO3 and H2O2 are observed (note, this inactivity is in contrast to photoelectrochemical H-2 generation). These results are compared to results obtained for TiO2 particles, and discussed in terms of various concepts in the literature. (C) 2019 Published by Elsevier Ltd.