Electron Transfer Reactions and Flat-Band Potentials of WO3 Colloids

Electron transfer reactions from different electron donors, such as (CH₃)₂ĊOH, ĊOO^-, MV⁺, Eu²⁺, and HȮ₂ radicals, to WO₃ colloidal particles were studied by pulse radiolysis techniques. The optical properties of excess electrons in WO₃ colloids show an increase in absorption at wavelengths above 700 nm, which is consistent with free carrier absorption. The same absorption spectrum was also obtained after steady-state γ-irradiation and after illumination of WO₃ colloidal sols with visible light. Electrons injected into WO₃ colloids are stable in an argon atmosphere but disappear rapidly upon addition of Fe³⁺, O₂, or Fe(CN)₆^3-. In the presence of Cu²⁺ ions and PVA, the equilibrium Cu⁺ ⇄ Cu²⁺ + (e^-)WO₃ is established. The equilibrium concentration of excess electrons in the presence of Cu²⁺ can be exploited to derive the flat-band potential of semiconductor colloids; the values are about 150 mV more negative than those for compact electrodes. The flat-band potential of WO₃ sols becomes slightly more negative with increasing radiation dose or intensity of light. These results are explained by the corresponding changes in the surface charge on the colloids.