Copper(Ⅱ) Phthalocyanine Tetrasulfonate Sensitized Nanocrystalline Titania Photocatalyst: In-situ Synthesis And Photocatalysis Under Visible Light

In recent years, semiconductor photocatalysis has become a promising technology in the following areas such as water purification, air cleaning, disinfection, antibacterial, self-cleaning and water decomposition. In this paper, the progress of the research on semiconductor photocatalysis was summarized and reviewed, the copper(II) phthalocyanine tetrasulfonate (CuPcTs)-TiO₂ system was selected as research subject in order to develop a novel dye-sensitized photocatalyst of enhanced visible-light activity. In-situ dye sensitization was developed to improve the links between TiO₂ and CuPcTs, and intensify the sensitizing effect.CuPcTs sensitized nanocrystalline TiO₂ photocatalyst (CuPcTs/TiO₂) was synthesized in situ employing sonochemistry or hydrothermal method respectively, with titanium tetrachloride as titanium source and triethanolamine as complexing ligands at low temperature in a short time. The as-prepared samples were characterized by X-ray diffraction (XRD), Transmission electron microscopy (TEM) and UV-Vis diffuse reflectance spectroscopy (DRS). The photocatalytic activity of the products was evaluated by the photocatalytic decomposition of methyl orange(MO) or 4-cholophenol(4-CP) under visible light (λ>450nm).TiO₂ samples prepared by sonochemistry or hydrothermal method were anatase structured and nanometric (50-60 nm and 20-30nm, respectively). CuPcTs could extend effectively the light absorption of TiO₂ into visible light region and enhance its photocatalytic activity under visible light via cooperating with TiO₂. CuPcTs/TiO₂ composite photocatalyst synthesized in situ had more phtocatalytic activity than CuPcTs/P25 prepared by adsorbtion, which could be explained as follows: First, CuPcTs was adsorbed on the surface of monodispersed TiO₂ sol particles, these sol particles were then agglomerated to produce secondary particles under ultrasound irradiation or hydrothermal circumstance, and a part of CuPcTs was left within the secondary particles. Consequently, a interpenetrated structure in favor of electron injection from excited CuPcTs to the conduction band of TiO₂ was formed between CuPcTs and TiO₂.Photocatalytic degradation process of MO with CuPcTs/TiO2 as photocatalyst under visible light irradiation was investigated by UV-Vis spectra. The results showed that the azo bonds of MO molecule were broken up to form aromatic ring intermediate (N, N - dimethyl - 4 -nitrobenzenamine and p - aminobenzenesulfonate) under the attack of free hydroxyl radicals. Then, the oxidization process took place to form the chain compounds. Finally, the chain compounds were mineralized to inorganic molecules such as CO2 and H2O.The content of CuPcTs was an important factor affecting the photocatalytic activity of CuPcTs/TiO2. There appeared to be an optimal CuPcTs content at which the CuPcTs/TiO2 sample showed the most photocatalytic activity. At lower content below the optimal value, photoreactivity increased with an increasing CuPcTs content. However, above the optimal value , most of CuPcTs aggregated on the surfaces of TiO2 particles, the excited state was quenched in aggregated CuPcTs complexes and the quantum yield for conduction electron production decreased significantly, on the other hand, the active surface sites declined greatly, so the observed photoreactivity decreased.Heat treatment could change the surface states and adsorbed substances of CuPcTs/TiO2 particles and optimize the photoreactivity. There was an optimal temperature range below which the photoreactivity increased with an increasing heat treatment temperature because the adsorbed organics and a minor part of CuPcTs desorbed and vaporized and the surface structure was optimized. However, above the optimal temperature range, further increase of temperature would lead to inchmeal oxidization of the CuPcTs species and decline of photocatalytic activity of the samples.The experiment on photocatalytic stability indicated that CuPcTs/TiO2 composite photocatalyst was stable, as the oxidized state of CuPcTs could be reduced by methyl orange or other intermediate substances, which promoted the regeneration of CuPcTs dye.