Probing the nature of bandgap states in hydrogen-treated TiO₂ nanowires

Hydrogen treatment of TiO₂ has been demonstrated to significantly alter its optical properties, including substantially enhanced visible light absorption that has important implications for various applications. The chemical nature of the bandgap states responsible for the increased visible absorption is not yet well understood. This work reports a detailed study of the structural, optical, electronic, and ultrafast properties of hydrogen-treated TiO₂ (H:TiO₂) nanowires (NWs) using a combination of experimental techniques including high-resolution transmission electron microscopy (HRTEM), electron spin resonance spectroscopy (ESR), time-resolved fluorescence (TRF), and femtosecond transient absorption (TA) spectroscopy in order to explain the origin of the strong visible absorption. The combined TEM, ESR, TRF, and TA data suggest that the presence of a localized mid-bandgap oxygen vacancy (V_O) occupied by a lone electron in an antibonding orbital situated at a surface site is likely responsible for the visible absorption of the material. The data further indicate that while untreated TiO₂ NWs are fluorescent, the hydrogen treatment leads to quenching of the fluorescence and highly efficient charge carrier recombination from the V_O state following excitation with visible light. With UV excitation, however, the charge carrier recombination of the H:TiO₂ NWs exhibits a larger component of a slow decay compared to that of untreated TiO₂, which is correlated with enhanced photoelectrochemical performance. Both the treated and untreated samples exhibit a fast decay that dominates the TA signals, which is likely caused by a high density of surface trap states. A simple model is proposed to explain all the key optical and dynamic features observed. The results have provided deeper insight into the chemical nature and photophysical properties of bandgap states in chemically modified TiO₂ nanomaterials.