A novel Ag-alumina hybrid surface-enhanced Raman spectroscopy (SERS) platform has been designed for the spectroscopic detection of surface reactions in the steady state. Single crystalline and faceted silver (Ag) nanoparticles with strong light scattering were prepared in large quantity, which enables their reproducible self-assembly into large scale monolayers of Raman sensor arrays by the Langmuir-Blodgett technique. The close packed sensor film contains high density of sub-nm gaps between sharp edges of Ag nanoparticles, which created large local electromagnetic fields that serve as "hot spots" for SERS enhancement. The SERS substrate was then coated with a thin layer of alumina by atomic layer deposition to prevent charge transfer between Ag and the reaction system. The photocatalytic water splitting reaction on a monolayer of anatase TiO2 nanoplates decorated with Pt co-catalyst nanoparticles was employed as a model reaction system. Reaction intermediates of water photo-oxidation were observed at the TiO2/solution interface under UV irradiation. The surface-enhanced Raman vibrations corresponding to peroxo, hydroperoxo and hydroxo surface intermediate species were observed on the TiO2 surface, suggesting that the photo-oxidation of water on these anatase TiO2 nanosheets may be initiated by a nucleophilic attack mechanism. [Figure not available: see fulltext.] © 2014 Tsinghua University Press and Springer-Verlag Berlin Heidelberg.