Strongly Enhancing Photocatalytic Activity of TiO2 Thin Films by Multi-Heterojunction Technique

Hsyi-En Cheng,Chi-Hsiu Hung,Ing-Song Yu,Zu-Po Yang

doi:10.3390/catal8100440

Abstract

The photocatalysts of immobilized TiO2 film suffer from high carrier recombination loss when compared to its powder form. Although the TiO2 with rutile-anatase mixed phases has higher carrier separation efficiency than those with pure anatase or rutile phase, the single junction of anatase/rutile cannot avoid the recombination of separated carriers at the interface. In this study, we propose a TiO2/SnO2/Ni multi-heterojunction structure which incorporates both Schottky contact and staggered band alignment to reduce the carrier recombination loss. The low carrier recombination rate of TiO2 film in TiO2/SnO2/Ni multi-heterojunction structure was verified by its low photoluminescence intensity. The faster degradation of methylene blue for TiO2/SnO2/Ni multi-junctions than for the other fabricated structures, which means that the TiO2 films grown on the SnO2/Ni/Ti coated glass have a much higher photocatalytic activity than those grown on the blank glass, SnO2-coated and Ni/Ti-coated glasses, demonstrated its higher performance of photogenerated carrier separation.

Highlights

Titanium dioxide, TiO2, is an inexpensive, non-toxic and chemically stable material
As the deposition temperature is raised to 350 ◦ C, the structure of TiO2 films grown on the blank glass and SnO2 coated glass become a mixture of anatase and rutile, but the structure of TiO2 films grown on the Ni/Ti coated glass become an almost pure rutile
Ta underlying are still under investigation, this phenomenon indicates a high carrier recombination occurring in our layers that the photocatalytic activity of TiO2 films decreases when the film structure changes from pure anatase to a mixture of anatase and rutile [38]

Summary

Introduction

TiO2 , is an inexpensive, non-toxic and chemically stable material. The problem with the immobilized TiO2 films is the high carrier recombination rate of TiO2, which results in a low effective thickness for converting photon energy into chemical energy. In our previous study [38], we grew TiO2 films on Ni, Ta, and Ti coated glass substrates and found that the TiO2 films on the Ni-coated substrate performs the highest photocatalytic activity, followed by on the Ti-coated substrate, and on the Ta-coated substrate, the same as the sequence of their electron work function of Ni ~5.04–5.35 eV, Ti 4.33 eV, and Ta 4.00–4.15 eV [39] This is because the high work function metals like platinum, gold, silver or nickel can attract the photogenerated electrons from TiO2 as they come into contact with TiO2 , leading to a decrease of carrier recombination loss. TiO2 /SnO2 /Ni, the photocatalytic activity of TiO2 films has been highly improved

Results and Discussion

ResidualTiO

Despite the same features of TiO

Schematic

Experimental

Conclusions