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Synthesis of black TiO2
The sodium borohydride (NaBH4) is a commonly used reducing reagent, due to reducing Ti4+ to Ti3+, and to produce in situ active H2 at room temperature, that reduces the white TiO2 into black TiO2. During the NaBH4 reduction process, boron oxide species are produced due to their insolubility in the ethanol, and it can be easily washed out by HCl solution to remove the surface impurities and expose the colour centres on the surface of the catalyst significantly increase the visible light absorption. The degradation efficiency increases 9 times after washing with HCl solution.
FIGURE 1 The heterojunction between white TiO2 and colored TiO2.
The principle of the semiconductors photocatalysis consists of the following components: photon absorption, carriers separation, carrier diffusion simultaneously with carrier transport, catalytic efficiency and mass transfer of reactants and products.
⇨ Photon absorption: if the semiconductor has energy equal or greater than Eg and consequent excitation of electrons (e−) to the CB leaving positively charged vacancies, holes (h+), in the VB (Figure 1)
⇨ Carriers separation: the heterojunction between nanoparticles can better band gap arrangement, to improve the separation of photo-generated charge carriers (Figure 1), which is advantageous of improving the photocatalytic performance. The oxygen vacancy defects and Ti3+ centres on the surface of TiO2 favour the separation of charge carriers (electrons and holes) and can trap the hole.
⇨ Carrier diffusion simultaneously with carrier transport. The photo-generated electrons can initiate the reduction processes, including O2 reduction to superoxides, H2 generation, and CO2 reduction to methane, methanol, or formaldehyde. The electron transfer is significant for the knowledge of the fundamental concepts of photocatalytic processes and to have an opinion about design and industrialisation of the photocatalytic process.
⇨ The transfer of electron/hole pairs to the interface initiates the redox reaction. The lifetime of the photo-generated charge carrier determines the efficiency of photocatalytic processes. Hence, increasing the efficiency of charge separation/transport in semiconductor nanoparticles is one of the major problems in photocatalysis to be addressed by the black TiO2.
⇨ The presence of oxygen vacancies in TiO2 can efficiently extend the visible light absorption range of titania because the localised oxygen vacancy states are located at 0.75 to 1.18 eV below the conduction band of TiO2 (Figure 1). The hydroxyl radicals (HO•) can be formed when hydroxyl anions (HO-), and adsorbed water trap the holes, which are capable of degrading the organic pollutants in wastewater.
Argon-assisted thermal treatment of sol-gel TiO2 for photocatalytic degradation of pesticides and aquatic toxicity evaluation on Chlorella Vulgaris growth
Argon-assisted thermal treatment of sol-gel TiO2 for photocatalytic degradation of pesticides and aquatic toxicity evaluation on Chlorella Vulgaris growth
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