Fabrication of stable substoichiometric WOx films with high SERS sensitivity by thermal treatment

Generally, annealing-induced surface oxygen vacancy defects in oxides are easily healed by air. Here, stable Magnéli phase WOx films were fabricated by thermal treatment of WO3 films in vacuum at 500 °C for 4 h. Aboundant electronic levels, derived from W5+-singly charged oxygen vacancy (Vo+) defect complexes, were detected at the surface of the WOx films, which indicates the formation of stable oxygen-defect phases (Magnéli phases). The concentration of surface oxygen vacancy depends on both annealing energy and the oxygen concentration gradient between the surface and the bulk of films. The reduced WO2.88 films exhibited greatly enhanced surface-enhanced Raman scattering (SERS) performance which was even comparable to noble metals without “hot spots”. The detection limit of Rhodamine 6G was decreased from 10−1 to 10−7 M. The outstanding SERS performance was mainly attributed to defect-related levels providing additional and efficient exciton and charge-transfer resonances in the system of molecule and oxide. Combining experimental results with theoretical study, this work demonstrates that the substoichiometric WOx induced by vacuum annealing shows good stability and it is indeed a potential candidate for SERS detection. •Stable substoichiometric WO3-x films as SERS substrates with high sensitivity were fabricated by vacuum annealing.•The detection limit of R6G for the prepared WO3-x reached 10−7 M.•A high density of electronic states related to oxygen vacancy defects was detected at the surface of WO3−x films.•The defect states provide efficient charge transfer routes, leading to outstanding SERS activity.•First-principle simulations indicate that oxygen vacancy in monoclinic WO3 compared with rutile TiO2 is more stable.