Extension of the (photo)electrochemical working range of WO3 electrodes to pH 8 by ALD coverage with TiO2

WO₃ electrodes (monoclinic) were covered with thin (up to 50 nm) layers of TiO₂ by ALD in order to extend the range of chemical stability and operation in photoelectrochemical cells from the acid medium to pH 8 where WO₃ would dissolve. As deposited ALD TiO₂ layers were amorphous; annealing in air (500 °C, 1 h) resulted in the formation of anatase. The TiO₂ layers were found to be chemically stable at pH 8. WO₃ capped with a 20 nm thick TiO₂ layer in contact with electrolytes of pH 8 led to: i) a decrease of the dark dissolution rate from 252 nm·h⁻¹ to 0.58 nm·h⁻¹, ii) a decrease of only 25 % of the photocurrent, iii) an extension of the “lifetime” of photocurrents from 3 to 25 hours, and iv) a decrease of the Faradaic efficiency of photocorrosion by a factor of 13, i.e. from 0.8 to 0.064. The lesser decrease of photocurrents with TiO₂ capping layers is ascribed to a favourable relative position of valence band energies of the two semiconductors in contact, allowing easier passage of photogenerated holes from the absorber material (WO₃) to the capping layer and represents a proof of concept as to the use of capping layers for the suppression of dissolution and photocorrosion.