EER and Photocurrent Transient Investigation of the Photoassisted Evolution of Hydrogen at an Electroplatinized TiO2 Single Crystal: Implications to Water Splitting at Suspensions
In order to go further into the mechanisms of water splitting at platinized powder suspensions, the photoelectrochemical behavior of both bare and electroplatinized single crystal in and saturated electrolytes has been studied by electrolyte electroreflectance (EER) and photocurrent transient techniques. EER is shown to be an interesting tool to follow the potential distribution changes occurring at the interface during photo‐ and electrochemical reactions. In particular, reversible shifts of the flatband potential, due to local pH changes associated with evolution, could be detected for the first time by EER. Direct experimental evidence has shown that equilibrium of OH− and H+ ions between the electrolyte and the surface is reached in times no longer than a few seconds. The combined information obtained from EER and photocurrent transient experiments with the platinized single crystal, strongly suggests that the ∼1 V Schottky barrier existing at the microjunctions drastically collapses when the electrolyte in the interface with the semiconductor becomes saturated. Evidence is also shown that either dissolved or evolved can be photo‐oxidized with holes trapped at Pt islands. This constitutes an important source of surface recombination, responsible to a great extent for the low water splitting efficiency at platinized suspensions. Finally, it can be inferred from our experimental results that, in colloidal systems, band asymmetry between bare and platinized zones is not a sine qua non condition for water splitting to be a feasible process. Nevertheless, the barrier height diminution at the contact upon interaction of Pt with seems to be decisive for to evolve with an acceptable rate at Pt islands. On the other hand, Pt plays an important role as a catalyst by reducing the overpotential for evolution, which can occur in spite of the small energy difference between the redox level and the conduction bandedge.