Dye-Sensitizing Effect of TiOPc Thin Film on n-TiO₂ (001) Surface

Abstract

Dye-sensitization processes for the (001) surface of n-type titanium oxide (n-TiO 2 ) 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-TiO 2 coated with TiOPc exhibited two photooxidation current bands in an aqueous electrolyte including hydroquinone (H 2 Q), due to direct excitation of n-TiO 2 below 420 nm and electron transfer from TiOPc to n-TiO 2 under the Q-band absorption of TiOPc at 600−800 nm, respectively. The former photocurrent process due to n-TiO 2 excitation was remarkably decreased by presence of surface states, which originated from defects of oxygen induced by Ar ion bombardment of the n-TiO 2 surface. The photooxidation process sensitized by the latter was evaluated from the determined energy levels of electronic bands for n-TiO 2 and TiOPc. The LUMO level of TiOPc was located 0.3 eV above that of n-TiO 2, which enabled electrons to transfer from TiOPc to n-TiO 2 . The excited state of TiOPc was not quenched by the surface states of n-TiO 2 but was efficiently transferred to n-TiO 2 at the n-TiO 2 /TiOPc interface.