In-situ analysis of the TiO2 / water interface via NAP-XPS is demonstrated in both a lab based system (NAP-cell configuration) and synchrotron endstation (backfill configuration). Ultra-thin wetting layers (UTWL) of liquid water (~10 nm) are formed on a rutile TiO2 surface with minimal contamination present in addition to unique insight during the growth of the liquid films as indicated via NAP-XPS, in-situ sample temperature and background vapour pressure monitoring. Chemical changes at the solid / liquid interface are also demonstrated via healing of Ti3+ surface defect states. Photon depth profiling of the as grown liquid layers indicate that the formed films are ultra-thin (~10 nm) and likely to be continuous in nature. This work demonstrates a novel and flexible approach for studying the solid / liquid interface via NAP-XPS which is readily integrated with any form of NAP-XPS system, thereby making a critical interface of study available to a wide audience of researchers for use in operando electrochemical and photocatalytic research
