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

Abstract

TiO 2 nanoparticles obtained by an improved polymerization-induced colloid aggregation (Im-PICA) method were sensitized by three squaraine dyes to construct dye/TiO 2 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/TiO 2 composites were assessed in terms of the degradation rate of methylene blue (MB) under visible-light irradiation. The ISQ/TiO 2 composites achieved best enhancement of photocatalytic activity under 150 min of visible-light irradiation. When ISQ/TiO 2 nanocomposites were applied for different photocatalytic degradation cycling times, the degradation rate still reached 68%, which reflects the fact that SQ/TiO 2 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 TiO 2 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/TiO 2 and c-BSQ/TiO 2 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.

Metal–Organic Framework Photosensitized TiO2 Co‐catalyst: A Facile Strategy to Achieve a High Efficiency Photocatalytic System

“…Plots of log (ahu)v ersus log (hu)w ere adopted to obtain the value of n,w hich was found to be 2, corresponding to the slope of the tangent. [39] Calculated from the intercepts of tangents as illustrated in Figure 1b Ultraviolet photoelectron spectroscopy( UPS) and valance band X-ray photoelectron spectroscopy (VBXPS) of the samples were studied to further understand the band structureo fA DA-Cd ( Figure 2). The Fermi energy (E Fermi )a nd the secondary cutoff region are defined through the linear extrapolation method, [40,41] and the work function (f)i sc alculated to be 5.52 eV through Equation (1):…”

“…The value of n indicates the transition nature of a material ( n = for indirect transitions and n =2 for direct transitions). Plots of log (α hυ ) versus log ( hυ ) were adopted to obtain the value of n , which was found to be 2, corresponding to the slope of the tangent . Calculated from the intercepts of tangents as illustrated in Figure b, the band gap values are 3.20, 3.09, and 3.14 eV for TiO 2 , ADA‐Cd and ADA‐Cd/TiO 2 , respectively.…”

Tuning the Photophysical Properties of Symmetric Squarylium Dyes: Investigation on the Halogen Modulation Effects

“…Evident red‐shifts in absorption and emission maximum of these SQDs were observed as the halogen atomic number increased . To further compare the electronic structures of these SQDs, DFT calculations at the B3LYP/6‐31G** level were carried out . The HOMOs for all SQDs are π orbitals mainly localized on the central cyclobutene cores, while π* orbitals of the LUMO are primarily distributed over the peripheral groups (Figure S31 and Table S3).…”

“…[24] To further compare the electronic structureso f these SQDs, DFT calculations at the B3LYP/6-31G** level were carried out. [25] The HOMOs for all SQDs are p orbitals mainly localized on the central cyclobutene cores, while p*o rbitals of the LUMO are primarily distributed over the peripheral groups ( Figure S31 and Ta ble S3 Figure S31). These results illustrate that the introduction of halogen atom can modulate the photophysical properties of squarylium dyes.…”

Controllable interlayer space effects of layered potassium triniobate nanoflakes on enhanced pH dependent adsorption-photocatalysis behaviors

“…Decomposition of pollutants is a popular application for photocatalytic technologies and a challenging mission 24 – 26 . TiO 2 -based catalyst was frequently adopted to photodegrade pollutant 27 – 29 . But the limited efficiency restricts its development.…”