Visible light photocatalytic activity for hydrogen production from water–methanol mixtures of open-framework V-doped mixed-valence titanium phosphate

Abstract

Titanium dioxide is the most widely used photocatalyst for hydrogen production from water. Its main limitation consists in the lack of photocatalytic activity under visible light irradiation. One strategy to overcome this problem consists in using as photocatalyst Ti3+-self-doped TiO2 that presents an absorption in the visible range. In this context, we report the synthesis, characterization as semiconductor and photocatalytic activity under visible light or solar light irradiation of open-framework V-doped, mixed-valence titanium phosphate. These materials are characterized by an intense absorption spanning all the visible wavelength range caused by the presence of Ti3+ centers in equimolar ratio respect Ti4+. This long wavelength absorption band allows the photocatalytic production of hydrogen under visible light irradiation. Suitable V-doping provides an additional charge separation level in the intra band-gap space that results in an enhancement of initial hydrogen production rate of a factor 5, reaching values of 47μmol/H2h−1gTi−1, in the absence of any noble metal as co-catalyst. Transient electron absorption spectroscopy has allowed to detect the state of charge separation in this phosphate that behave differently upon excitation in the UV or in the visible regions and whose intensity increases upon V-doping, reaching a maximum response at 0.5wt.% in V. The flat band potential of the conduction band, measured by photocurrent as a function of the bias voltage, was estimated to be −0.10V vs. NHE and the charge carrier density determined by impedance spectroscopy was 2×1020 carriers/cm3.