Facile formation of black titania films using an atmospheric-pressure plasma jet

A rapid atmospheric-pressure pulsed helium/hydrogen plasma jet method for conversion of TiO2 films into defective, black TiO2 is demonstrated. This method can reduce nanoporous anatase films with fine spatial control, allowing precise 2D patterning of pre-deposited films under ambient conditions. The resulting modified TiO2 films are stable in air and show enhanced photocatalytic activity with respect to the as-deposited films. From photoelectrochemical current measurements, the plasma-treated anatase TiO2 films supported on FTO-glass showed a six-fold increase in photocurrent density under both sub- and super-bandgap illumination compared to untreated anatase films. The changes in optical absorption are mainly due to introduced mid-gap states, with negligible overall band gap narrowing. Thorough characterisation of these black TiO2 materials using X-ray photoelectron, Raman and UV-VIS spectroscopy showed strong evidence for the presence of Ti3+ states in the reduced films. There is a clear relationship between the Ti3+ content of the plasma-treated TiO2 and the enhanced photocurrent, although increased Ti3+ content leads to a decrease in photocurrent, which we ascribe to an increase in the number of electron-hole recombination centres.