Preparation Of N-doped TiO₂ Heterojunctions And Their Photocatalytic Properties

Titanium dioxide (TiO₂) has great potential for use in the field of photocatalysis due to its chemical stability, low cost, and nontoxicity. However, the applications of TiO₂ are hampered by its relatively large band gap (3.0³.2 eV), which makes it work mainly under ultra-violet light irradiation. Recently, Extensive research has shown that the optically responsive range of TiO₂ can be effectively broadened by nonmetal doping especially nitrogen doping method, leading to high photocatalytic activity under visible light illumination. On the other hand, heterojunctions composed of two or more TiO₂ phases benefit from photogenerated charge separation, which enhances the photocatalytic efficency as a result of a low recombination rate of electron–hole pairs. The aim of this thesis is focused on incorporation of nitrogen doping with heterojunction into TiO₂ so as to acquire new type of photocatalysts with higher activities than comercial TiO₂ Degussa P25.Nitrogen doped TiO₂ heterojunctions with different phase contents have been prepared by a solvothermal method, using tetra-n-butyl titanate (Ti(OnC₄H₉)₄, TBT) and hydrazine hydrate as the starting materials. The photocatalytic properties of the samples have been evaluated by photodegradation of methyl orange in solution. The samples have been characterized with X-ray powder diffraction (XRD), Ramman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), energy dispersive spectroscopy (EDS), nitrogen adsorption-desorption isotherms, and diffuse absorbance ultraviolet–visible (UV–Vis) spectroscopy techniques. The results show that:(1) Controlled syntheses for anatase TiO₂/brookite TiO₂ heterojunctions (A-TiO₂/B-TiO₂) can be achieved at 180²00°C for 36⁴8 h, using TiO₂ sol and hydrazine hydrate (40⁸0%) as the starting materials.(2) Controlled syntheses for anatase TiO2/brookite TiO₂/rutile TiO₂ heterojunctions (A-TiO₂/B-TiO₂/R-TiO₂) can be accomplished at 180²00°C for 36⁴8 h, using TiO₂ gel and hydrazine hydrate (40⁸0%) as the starting materials.(3) The SEM and TEM images show that A-TiO2/B-TiO2 heterojunctions are composed of A-TiO₂ one-dimentional structures and B-TiO₂ nanoparticles, and A-TiO₂/B-TiO₂/R-TiO₂ heterojunctions are made up of A-TiO₂ one-dimentional structures, B-TiO₂ nanoparticles, and R-TiO₂ plates.(4) EDS, FTIR, and XPS results reveal that nitrogen element has been incorporated into TiO2 matrices in the forms of interstitial and substitutional states.(5) Diffuse absorbance UV–Vis spectra show that the formation of heterojunctions can change the energy band-gaps of TiO₂ samples, and the band gaps of A-TiO₂/B-TiO₂/R-TiO₂ samples are lower than those of A-TiO₂/B-TiO₂ samples probably due to the lower band gap of R-TiO₂ as compared to A- and B-TiO2.(6) The photocatalytic properties of the samples have been evaluated by decomposition of methyl orange in solution under UV and simulated visible light irradiations. It is found that all the heterojunctions containing either two or three polymorphs of TiO₂ have relatively higher photocatalytic efficencies than that of Degussa P25. Degradation ratios of 88.8% and 59.3% for methyl orange under UV light for 20 min and simulated visible light for 3 h, respectively, have been acquired with NT40/40 photocatalyst, whereas 83.5% and 75.0% ratios correspond to NT-36 photocatalyst evaluated under otherwise identical conditions.