Radiative and Non-Radiative Recombination Pathways in Mixed-Phase TiO2 Nanotubes for PEC Water-Splitting

Rambabu Yalavarthi,Ondřej Tomanec,Alberto Naldoni,Hana Kmentová,Radek Zbořil,Patrik Schmuki,Luca Mascaretti,Štěpán Kment

doi:10.3390/catal9020204

Abstract

Anatase and rutile mixed-phase TiO2 with an ideal ratio has been proven to significantly enhance photoelectrochemical (PEC) activity in water-splitting applications due to suppressing the electron–hole recombination. However, the mechanism of this improvement has not been satisfactory described yet. The PEC water oxidation (oxygen evolution) at the interface of TiO2 photoanode and electrolyte solution is determined by the fraction of the photogenerated holes that reach the solution and it is defined as the hole transfer efficiency. The surface and bulk recombination processes in semiconductor photoanodes majorly influence the hole transfer efficiency. In this work, we study the hole transfer process involved in mixed-phase TiO2 nanotube arrays/solution junction using intensity-modulated photocurrent and photovoltage spectroscopy (IMPS and IMVS); then, we correlate the obtained hole transfer rate constants to (photo)electrochemical impedance spectroscopy (PEIS) measurements. The results suggest that the enhanced performance of the TiO2 mixed-phase is due to the improved hole transfer rate across the TiO2/liquid interface as well as to the decrease in the surface trap recombination of the holes.

Highlights

Since the discovery of water photo-oxidation at semiconducting photoanodes, tremendous research has been undertaken to improve the practical efficiency of photoelectrochemical (PEC)water-splitting devices [1,2]
The TiO2 nanotubes annealed at 500 ◦ C (TNT500) show reflections belonging to the sole presence of the anatase phase
The nanotubes annealed at 600 ◦ C (TNT600) have the optimum anatase-to-rutile ratio (82:18) showing improved photocurrent compared to the single anatase phase (TNT500) and to the mixed-phase having a higher rutile content (77%, the sample annealed at 700 °C (TNT700))

Summary

Introduction

Since the discovery of water photo-oxidation at semiconducting photoanodes, tremendous research has been undertaken to improve the practical efficiency of photoelectrochemical (PEC)water-splitting devices [1,2]. In practice, the recombination of the photoinduced electron–hole pairs in the bulk of the materials, as well as at the electrode/electrolyte interface, limits the overall efficiency associated with photoelectrodes for PEC water-splitting [4,5,6,7,8]. Several strategies have been reported to minimize the recombination processes in semiconductor photoelectrodes, such as: increasing conductivity with doping [9,10,11,12], heterojunction formation [7,13,14], modification of photoanode surface with co-catalysts [15,16,17], and coatings of thin conformal layers using atomic layer deposition (ALD) [18,19,20]. This strategy has already been successfully employed for titanium dioxide (TiO2 ), which is one of the most investigated materials in PEC water-splitting [12,21,22,23]

Methods

Results

Conclusion