Hydrogen treatment has been demonstrated as a successful method for modifying optical and electronic properties of metal oxides.  For example, the color of TiO2 can be changed from white to black using hydrogen treatment, indicating significant changes in optical and electronic properties that have important implications for various applications including light energy conversion.  However, the chemical nature of the bandgap states responsible for the increased visible absorption is not yet well understood.  We have recently carried out a systematic 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 paramagnetic resonance spectroscopy (ESR), time-resolved fluorescence (TRF), and femtosecond transient absorption (TA) spectroscopy in order to explain what constitutes the strong visible absorption.  The combined ESR, TRF, and TA data suggest that the presence of a localized mid-bandgap oxygen vacancy (VO) occupied by a lone electron is responsible for the visible absorption of the material.  The data further indicate that hydrogen treatment leads to quenching of the fluorescence and highly efficient charge carrier recombination from the VO state following excitation with visible light.  With ultraviolet excitation, however, the charge carrier recombination of the H:TiO2 NWs exhibits a larger component of a slow decay compared to untreated TiO2, which is correlated with enhanced photoelectrochemical performance.  The results have provided deeper insight into the chemical nature and photophysicalrelated properties of bandgap states in chemically-modified TiO2 nanomaterials.

Understanding the nature of bandgap states in hydrogen-treated TiO<sub>2</sub> nanostructures

246th National Meeting (2013)

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Understanding the nature of bandgap states in hydrogen-treated TiO2 nanostructures

Understanding the nature of bandgap states in hydrogen-treated TiO₂ nanostructures