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

Abstract

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 m2 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) × 10-5 or (5.93 ± 0.68) × 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) × 10-5 or (0.15–6.2) × 10-5 mol g−1. The degradation rate increased drastically with an increase in the adsorbed amounts of these dyes less than 1.7 × 10-5 mol g−1 but then increased gradually up to ca. 6 × 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.