The Nature of Surface States on Chemically Modified TiO2 Electrodes

Two important parameters which strongly influence the flatband potential of a semiconductor in electrolytic solutions are the electron affinity of the semiconductor and the net charge on the surface. An attempt to modify these parameters by chemical derivatization of the surface of the semiconductor will be described. When the surface of TiO 2 is modified with alkylsilanes, impedance and photocurrent measurements indicate that surface states are introduced at the semiconductor‐electrolyte interface. These states have broad Gaussian distribution, with a maximum near the bottom of the conduction band. The states act as effective recombination centers for light‐generated minority carriers. When the alkylsilane is derivatized with a phosphonate group, the Gaussian distribution of the states is retained but with reduced width, and the maximum is shifted considerably to more positive potentials. These states do not act as recombination centers. An attempt also was made to change the electron affinity of the surface by fluorination. The only measured result was an increase in surface recombination of minority carriers, which resulted in a decrease in the anodic photocurrent.