Mo-doped TiO2 photoanodes using [Ti4Mo2O8(OEt)10]2 bimetallic oxo cages as a single source precursor

Abstract

Photoelectrochemical solar water splitting is a promising and sustainable technology for producing solar fuels such as clean hydrogen from water. A widely studied photoanode semiconductor for this application is TiO₂, but it suffers from a large band gap (3.2 eV) and fast recombination of electrons and holes. Herein, we present a novel, facile and rapid strategy to develop Mo-doped TiO₂ (Mo:TiO₂) mixed anatase–rutile photoanodes using [Ti₄Mo₂O₈(OEt)₁₀]₂ bimetallic oxo cages as a single source precursor. These cages dissolved in tetrahydrofuran deposit by spray pyrolysis at 150 °C forming films with hierarchical porosity on the micrometer and nanometer scale. XPS, EDXS and UV-Vis spectroscopy reveal Mo atoms evaporate during annealing in air at temperatures 650–800 °C, contributing to the formation of nanostructures and porosity. XPS depth profiling, XRD, EDXS, Raman, and electron paramagnetic resonance indicate that the remaining Mo atoms are well spread and incorporated in the TiO₂ lattice, at interstitial or substitutional sites of the rutile or anatase phases depending on the annealing temperature. Photocurrent measurements show that Mo:TiO₂ photoanodes optimized at 700 °C outperform a TiO₂ photoanode prepared in a similar manner by a factor of two at 1.23 V_RHE. Finally, UV-Vis spectroscopy, conduction and valence band calculations, and incident-to-photon efficiency measurements show these Mo:TiO₂ photoanodes possess a narrower band gap than TiO₂ and higher efficiency in the visible light range (5% at 400 nm). These outcomes open a new avenue in the exploitation of titanium oxo cages and advance the development of photoelectrodes for water splitting and energy applications.