Abstract:
Among all nanoparticle types, TiO2 (anatase) has proven to be the most promising one, primarily because of its superhydrophilic properties and effective antibacterial actions. The photo-induced self-cleaning characteristic can be demonstrated by the hydrophilicity of TiO2 nanoparticles. However, the lack of photosensitivity to visible light (>390 nm) reduces the scope for utilizing TiO2 in photocatalytic processes. Therefore, most studies regarding TiO2 nanoparticles have focused to broaden its spectral sensitivity through band gap manipulation for effective photocatalysis under visible light. Transition metal dopants have been used to decrease the band gap energy of TiO2 for visible light-responsive photocatalytic purposes. Fe3+ is a good dopant candidate for producing visible light-sensitive nanoparticles owing to its capability to decrease the band gap energy and enhance electron/hole trapping. In this study, nanocrystalline undoped and Fe-doped TiO2 (Ti1-xFexO2, x = 0.01 to 0.04) was synthesized by the sol-gel method. The prepared samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and ultraviolet and visible spectroscopy (UV–vis) to study their structure, morphology, and optical properties. A photocatalytic dye degradation test and a hydrophilic conversion test were carried out to compare the photocatalytic activity and hydrophilicity of the samples under UV and visible light. The obtained results demonstrated the self-cleaning characteristic of undoped and Fe-doped TiO2 nanoparticles. According to the characterizations and comparisons of the prepared samples, particle size and band gap reduction were observed due to an increase in Fe concentration in Fe-doped TiO2, which yielded a red shift in the absorption band edge. Fe0.03-doped TiO2 (average particle size 21.3 nm, crystallite size 11.97 nm, and band gap 2.37 eV) showed the optimum photocatalytic activity (60% MB degradation) and super-hydrophilicity (water droplet contact angle 9°) under visible light radiation. This optimum result was possible because the band gap reduction of Fe-doped TiO2 reached an optimum level in Fe0.03-doped TiO2 so that more (⦁OH) radicals and (O_2^-) groups were produced by achieving the optimum number of electron- and hole-trapping sites and the lowest rate of electron-hole recombination. Further increase of Fe content (Fe0.04-doped TiO2) increased the electron-hole recombination center, hence an increment of the rate of recombination which yielded lower photocatalytic reaction and hydrophilicity. Therefore, Fe0.03-doped TiO2 was found to be the optimum doping content that can effectively be used as self-cleaning coating material.
