(M,N) codoping (M = Nb or Ta) and CoO nanoparticle decoration of TiO2 nanotubes: synergistic enhancement of visible photoelectrochemical water splitting

The (M,N) codoping of TiO2 and the formation of heterojunctions by association with co-catalyst nanoparticles are two approaches that can improve the efficiency of TiO2 used as photoanodes for PEC water splitting. In this study, reactive pulsed laser deposition (PLD) of CoO nanoparticles on the surface of (M,N)-codoped TiO2 nanotubes has been used to combine these two methods. The (M,N) codoping reduced the band gap of TiO2 nanotubes and the subsequent nanodecoration significantly increased the photocurrent production under simulated sunlight and visible light conditions. The most significant improvement was obtained for (Nb,N) TiO2-NTs decorated with CoO-NPs, which showed a factor of 3 increase, and a significant contribution from the visible part of the spectrum, which accounts for 40% of the photocurrent generated in this sample. Our results suggest that this enhancement is due to a synergistic effect between the (M,N) codoping of TiO2-NTs and the presence of CoO-NPs through the formation of a high density of local triple phase interface (TPI) (n-TiO2/p-CoO/electrolyte). A modification of the band energy structure at these TPIs allows better extraction of charge carriers generated by visible light in (M,N) TiO2-NTs thanks to the presence of CoO nanoparticles.