Photocatalytic activation of TiO2 under visible light using Acid Red 44
Introduction
The semiconductor TiO2 frequently used as a photocatalyst and numerous applications have been described. This material is of interest because it can split water into hydrogen and oxygen under irradiation by light and is currently of interest for generating clean energy, such as hydrogen generation [1] and solar cells [2]. The mechanism of water-splitting is a red-ox reaction involving the generation of electrons and positive charges, and can be applied to clean technology such as the decomposition of organic compounds [3]. Photocatalysis has many merits in terms of the removal of toxic organic compounds, waste water treatment, and clean-up of polluted air. Organic compounds are not fully converted to minerals by conventional technology. However, organic compounds can be completely decomposed to H2O and CO2 by photocatalysis and no secondary pollutants are generated. Conventional technology for the removal of pollutants requires a suitable concentration of pollutant, but photocatalysis can be used over a broad range of concentration. In addition, TiO2 photocatalyst is nontoxic, inexpensive, more stable than other photocatalyst in ambient conditions and can be recycled [4], [5]. For these reasons, TiO2 would be ideal for use in clean technology.
The TiO2 photocatalyst mainly absorbs UV light of which the wavelength is lower than 400 nm, and catalyzes the decomposition of organic compounds by a inducing red-ox reaction [6]. However, the amount of UV in solar light is less than 5%. To enhance the activity of photoefficiency, activation in the visible light region is required and a number of studies have been published for the purpose. One example is the use of a Ru complex as a sensitizer [7], [8], [9]. This sensitizer absorbs visible light leading to the excitation of an electron. The excited electron of the sensitizer is then transferred to TiO2 after which, the TiO2 has an induced photoactivation under visible light [9]. However, the sensitizer is also an organic compound and is decomposed by photocatalysis. In the case of the Ru complex, the synthesized dye is too expensive to be discharge and, as a result, it is difficult to apply to clean technology.
In this research, we used a conventional dye as a sensitizer with adjustment of the pH environment. A TiO2 photocatalyst was activated under the visible light using Acid Red 44 (C10H7N=NC10H3(SO3Na)2OH). The application of this system was examined by a study of decomposition of phenol, a toxic industrial chemical that is frequently discharged into water.
Section snippets
Materials
Conventional Degussa P-25; TiO2 powder with an average diameter of 30 nm was used as received. Acid Red 44 was purchased from Aldrich (content ∼80%) and was used as a photosensitizer without further purification. Dye solutions were prepared by dissolving the dye in deionized water (18 MΩ cm, Barnstead). The pH of the solution was adjusted with NaOH (SIGMA) and HCl (JUNSEI), after the addition of the catalyst. Phenol (MERCK) was used as a model toxic organic material.
EFISPS method in solution
For certification of activation
Results
The band gap energy of TiO2 is known to be 3.0–3.2 eV [10]. After TiO2 absorbs UV, the electrons of the balance band are excited to the conduction band. Dye-sensitized TiO2 can excite electrons with visible light that has lower energy than UV. The dye absorbs visible light and the electrons of the dye are excited from the HOMO to the LUMO state. The excited electron of the dye is transferred to the conduction band of TiO2 [2], [11]. This dye-sensitization mechanism investigated in this study
Effect of the pH of the solution on the adsorption of dye to TiO2 surface
Acid Red 44 is an acidic dye and is adsorbed to TiO2. This adsorption is due to electrostatic forces. Desorption of the dye also occurred when the pH of solution was adjusted to 10.5. The rate of adsorption and desorption was very rapid and most of the adsorbed dye was desorbed within 2 h. These adsorption and desorption mechanisms can be attributed to a combination of chemical reactions. The reactions are shown below:
Conclusions
We conclude that the adsorption of a dye onto the TiO2 surface is an important factor in the photosensitization by a dye under visible light. In addition, differences in the photocatalytic activation mechanism under visible irradiated conditions with that of UV irradiated condition are proposed. This concept was applied to the decomposition of phenol, a toxic chemical that is used in industry and discharged into steams of waste water. These results show that a conventional dye can be used for
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