Effect of Dye Structure on Optical Properties and Photocatalytic Behaviors of Squaraine-Sensitized TiO2 Nanocomposites

TiO2 nanoparticles obtained by an improved polymerization-induced colloid aggregation (Im-PICA) method were sensitized by three squaraine dyes to construct dye/TiO2 nanocomposite photocatalysts (1:3 mass ratios) with visible-light responses by a facile hydrothermal method. Scanning electron microscopy (SEM) and transmission electron miscroscopy (TEM) were employed to visually observe the surface morphologies of the samples. We studied the visible-light response using UV-vis diffuse reflectance spectroscopy and optical band gap calculations using the Tauc plot equation. The dyes exhibit excellent thermal stability, which is ascribed to the decomposition temperature reaching 300 C based on thermogravimetric differential thermogravimetric (TG-DTG) analysis. The photocatalytic activities of the as-prepared SQ/TiO2 composites were assessed in terms of the degradation rate of methylene blue (MB) under visible-light irradiation. The ISQ/TiO2 composites achieved best enhancement of photocatalytic activity under 150 mm of visible-light irradiation. When ISQ/TiO2 nanocomposites were applied for different photocatalytic degradation cycling times, the degradation rate still reached 68%, which reflects the fact that SQ/TiO2 composites have a high photostability in the photocatalytic process. When taking the degradation rate and photoresponse into consideration, the combination mode between the ISQ dye and TiO2 that obtained the optimal degradation rate tended to be a hydrogen bond, which exhibited a stronger binding force than the simple physical absorption of m-ISQ/TiO2 and c-BSQ/TiO2 based on the FT-IR analysis. Using total organic carbon (TOC) test and liquid chromatography (LC) results, we verified the degree of mineralization of the MB molecule. Based on the fluorescence spectra, the highest fluorescence quantum yield of ISQ dye (0.124) was far greater than those of c-BSQ (0.036) and m-ISQ (0.02). The molecular structures of the squaraine dyes and the electron distributions of their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) states were given by geometrical optimization with Gaussian 98 at the B3LYP/6-311+G(d,p) level. Compared with the optical band gap calculated using the Tauc plot equation, we suggest that its energy was slightly less than the band gap due to the absorbed photon. This might be related to the localized central electronic, phonon, and local energy levels between the conduction band (CB) and the valence band (VB). The value of the optical band gap calculated from UV diffuse reflection spectroscopy (DRS) becomes less than the theoretical calculated energy band gap.