Dye-sensitizing effect of TiOPc thin film on n-TiO₂ (001) surface

Dye-sensitization processes for the (001) surface of n-type titanium oxide (n-TiO₂) single crystal coated with vacuum-deposited titanylphthalocyanine (TiOPc) were investigated by means of photoelectrochemical measurements and X-ray and ultraviolet photoelectron spectroscopy (XPS, UPS). The electrode of n-TiO₂ coated with TiOPc exhibited two photooxidation current bands in an aqueous electrolyte including hydroquinone (H₂Q), due to direct excitation of n-TiO₂ below 420 nm and electron transfer from TiOPc to n-TiO₂ under the Q-band absorption of TiOPc at 600-800 nm, respectively. The former photocurrent process due to n-TiO₂ excitation was remarkably decreased by presence of surface states, which originated from defects of oxygen induced by Ar ion bombardment of the n-TiO₂ surface. The photooxidation process sensitized by the latter was evaluated from the determined energy levels of electronic bands for n-TiO₂ and TiOPc. The LUMO level of TiOPc was located 0.3 eV above that of n-TiO₂, which enabled electrons to transfer from TiOPc to n-TiO₂. The excited state of TiOPc was not quenched by the surface states of n-TiO₂ but was efficiently transferred to n-TiO₂ at the n-TiO₂/TiOPc interface.