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Bothenhancing the excitons’ lifetime and ingeniously controllingthe spatial charge transfer are the key to the realization of efficientlyphotocatalytic and artificially photosynthetic devices. Nanostructuredmetal/metal-oxide interfaces often exhibit improved energy conversionefficiency. Understanding the surface potential changes of nano-objectsunder light illumination is crucial in photoelectrochemical cells.Under ultraviolet (UV) illumination, here, we directly observed thecharge separation phenomena at the Au-nanoparticle/TiO2-nanotube interfaces by using Kelvin probe force microscopy. Thesurface potential maps of TiO2nanotubes with and withoutAu nanoparticles were compared on the effect of different substrates.We observed that in a steady state, approximately 0.3 electron perAu particle of about 4 nm in diameter is effectively charged and consequentlyscreens the surface potential of the underlying TiO2nanotubes.Our observations should help design improved photoelectrochemicaldevices for energy conversion applications. [ABSTRACT FROM AUTHOR]