Porous TiO2 adsorbed with squaraine dye as visible-light-responsive photocatalyst

The present paper reports the development of TiO2 photocatalysts adsorbed with squaraine dyes for the utilization of visible light. We synthesized 1,3-bis(3,3-dimethyl-2-methylene indolenine)squaraine (ISQ) and its derivatives having two COOH (ISQ-(COOH)(2)), COOH and COOC4H9 (ISQ-COOHCOOBu) and two COOC4H9 groups (ISQ-(COOBu)(2)) at both ends. We used porous TiO2 which was synthesized by a sol-gel method and possessed a specific surface area of 229 m(2) g(-1). ISQ without an anchoring group of COOH hardly adsorbed and the number of the adsorbed ISQ-(COOH)(2) or ISQ-COOHCOOBu per gram of TiO2 was estimated to be (5.53 +/- 0.74) x 10-5 or (5.93 +/- 0.68) x 10(-5) mol g(-1), respectively. Photocatalytic degradation of methyl orange (MO) was performed on TiO2 adsorbed with ISQ-(COOH) 2 or ISQ-COOHCOOBu in the range of (0.12-5.4) x 10(-5) or (0.15-6.2) x 10(-5) mol g(-1). The degradation rate increased drastically with an increase in the adsorbed amounts of these dyes less than 1.7 x 10-5 mol g 1 but then increased gradually up to ca. 6 x 10(-5) mol g(-1). The electrochemical measurements revealed that the photogenerated electron was transferred from these dyes to the conduction band of TiO2. No MO was degraded on TiO2 adsorbed with ISQ-(COOH)(2) in the nitrogen atmosphere under VL irradiation. We proposed the degradation mechanism of MO on TiO2 adsorbed with ISQ-(COOH)(2) or ISQ-COOHCOOBu.