Surface photovoltage phase spectroscopy study of the photo-induced charge carrier properties of TiO2 nanotube arrays

By using the surface photovoltage （SPV） technique based on a lock-in amplifier, surface states located 3.1 eV below the conduction band of TiO2 have been detected in TiO2 nanotube arrays prepared by anodization of titanium foil in fluoride-based ethylene glycol solution. The photo-induced charge transportation behavior of TiO2 nanotube arrays was also studied by quali- tatively analyzing their SPV phase spectra measured under different external bias. When a negative bias was applied, carriers excited from surface states have the same transportation properties as those excited from the valence band; in contrast, when a positive bias was applied, these two kinds of photo-excited carriers exhibit different transportation behavior.