Investigation of Electron Behavior in Nano‐TiO₂ Photocatalysis by Using In Situ Open‐Circuit Voltage and Photoconductivity Measurements

Abstract

The in situ open-circuit voltages (Voc ) and the in situ photoconductivities have been measured to study electron behavior in photocatalysis and its effect on the photocatalytic oxidation of methanol. It was observed that electron injection to the conduction band (CB) of TiO2 under light illumination during photocatalysis includes two sources: from the valence band (VB) of TiO2 and from the methanol molecule. The electron injection from methanol to TiO2 is slower than that directly from the VB, which indicates that the adsorption mode of methanol on the TiO2 surface can change between dark and illuminated states. The electron injection from methanol to the CB of TiO2 leads to the upshift of the Fermi level of electrons in TiO2 , which is the thermodynamic driving force of photocatalytic oxidation. It was also found that the charge state of nano-TiO2 is continuously changing during photocatalysis as electrons are injected from methanol to TiO2 . Combined with the apparent Langmuir-Hinshelwood kinetic model, the relation between photocatalytic kinetics and electrons in the TiO2 CB was developed and verified experimentally. The photocatalytic rate constant is the variation of the Fermi level with time, based on which a new method was developed to calculate the photocatalytic kinetic rate constant by monitoring the change of Voc with time during photocatalysis.

Intrinsic intermediate gap states of TiO2 materials and their roles in charge carrier kinetics

“…Photoexcitation and electrochemical reduction can inject non-thermal excess electrons in TiO 2 [170,171]. Excess holes are mainly created by photo-excitation for n-type semi-conductors, such as TiO 2 , ZnO, and other oxide materials, which are seldom studied as compared to the electrons [172,173]. Excess electrons and holes can be delocalized at the CB and the VB extended states, self-localized at regular lattice sites, or localized around defect sites.…”

Thermodynamic and kinetic analysis of heterogeneous photocatalysis for semiconductor systems

“…Recently, we studied the thermodynamic driving force of photocatalytic oxidation of methanol by nano-TiO 2 under UV light irradiation. 52 As shown in Fig. 3, the nano-TiO 2 changes from thermal equilibrium state to non-equilibrium state due to the excess electron production in the CB, as indicated by the change of Fermi level of electrons.…”

“…As an example, the electron injection from methanol to TiO 2 can increase the electron Fermi level from 0.52 to 0.65 eV. 52 The 0.13 eV difference of Fermi level forms the thermodynamic driving force of photocatalytic oxidation of methanol.…”

Thermal coupled photoconductivity as a tool to understand the photothermal catalytic reduction of CO2

“…They suggested that the transient photoconductivity of a nanocrystalline TiO2 film in vacuum was governed by the thermal re-lease of electrons from the trap states in the bandgap. Liu et al [12][13][14] successfully employed in situ temperature-dependent photoconductivity to understand the photocatalytic degradation of gaseous formic acid in air. They revealed that the electron transfer was determined by the interface transfer of electrons to O2 rather than by the transport.…”