Preparation And Photocatalytic Properties Of TiO₂ Nanoparticles Doped With Nonmental Elements

Titanium dioxide (TiO₂) has been considered as one of the most promising materials for its application in photocatalysis and photoelectric conversion because of its high chemical stability, corrosion resistance, non-toxicity and low cost etc. However, the wide band gap (3.0-3.2eV, only absorbing the UV light ofλ<387nm) and the easy recombination of photoinduced electrons and holes result in a low efficiency of photocatalysis and seriously limit the practical application of TiO₂. In order to improve the photocatalytic activity or extend the optical absorption to the visible light region, in this dissertation, a sol-gel method was developed to prepare the B-TiO₂, C-TiO₂, N-TiO₂ and C-N-TiO₂ nanopaticles, the dye adsorption was employed to prepare the N-doped TiO₂ modified by Zn porphyrins, and a pyrolysis method using (NH4)2TiF6 as the precursor was introduced to prepare the N-F-TiO₂, respectively. The chemical composition and structural properties of these nonmetals doping TiO₂ nanoparticles were investigated by TG-DTA, XRD, TEM, XPS, FT-IR and UV-Vis spectroscopy, and their photocatalytic activity were studied in detail.Firstly, TiO₂ nanoparticles only doped with B element were prepared by a sol-gel method, and their structure and photocatalytic properties were studied. The results showed that B³⁺ was likely to weave into the interstitial TiO₂ structure, forming the chemical environment like Ti-O-B. The doping of boron ions could efficiently inhibit the grain growth and facilitate the anatase-to-rutile transformation prior to the formation of diboron trioxide phase. When the B amount is high enough to form the diboron trioxide, the doping of boron ions could inhibit the anatase-to-rutile transformation. The photocatalytic activity of the B-doped TiO₂ nanoparticles was evaluated by the photoregeneration of nicotinamide adenine dinucleotide (NADH) under UV-Vis light. All B-doped TiO₂ nanoparticles calcined at 500oC showed higher photocatalytic activity than pure TiO₂ sample due to the increase of band gap and absorption in the UV range. When the molar ratio of B to Ti was 5%, the TiO₂ nanoparticles could photocatalytically reproduce 94% NADH.Secondly, C-N-codoped TiO₂ nanoparticles were prepared by a sol-gel method, and their structure and photocatalytic properties were studied. The results revealed that N atoms could incorporate into the lattice of anatase through substituting the sites of oxygen atoms and induce N2p state, which can narrow the band gap of TiO₂ and enhance the visible light absorption, while most of C atoms could form a mixed layer of deposited active carbon and carbonate species on the surface of TiO₂, which can act the role of photosensitiser like the organic dyes and widen the region of visible light absorption. The doping of C and N atoms could suppress the crystal growth of TiO₂, and the effect of C doping was more pronounced than that of N doping. The photocatalytic activity of resulting C-TiO₂, N-TiO₂ and C-N-TiO₂ samples with different C and N content were evaluated by methylene blue degradation under visible light irradiation. It was found that C-N-TiO₂ nanomaterials exhibited the highest photocatalytic activity, which could be attributed to the synergistic effect of doped C and N atoms.Thirdly, N-F-codoped TiO₂ nanoparticles were prepared by pyrolysis of (NH4)2TiF6, and their structure and photocatalytic properties were studied. The results demonstrated that, with the increase of calcination temperature and time, (NH4)2TiF6 was decomposed into NH4TiOF3 and TiOF2 firstly, and then transformed into anantase N-F-codoped TiO₂. H3BO3 as oxygen source can promote the formation of anantase TiO₂, increase the crystallinity and productivity of TiO₂. N and F atoms replaced oxygen sites of anatase TiO₂, meanwhile, the amount of doping N and F of N-F-TiO₂ prepared by this approach was greater than those prepared by the sol-gel approach. N doping enhanced the visible light absorption, and F doping led to the enhancement of surface acidity and active sites. So, there is the synergetic effect of doped N and F atoms together. The photocatalytic activity of N-F-TiO₂ was evaluated by methylene blue degradation under visible light. It was found that N-F-TiO₂ exhibited higher photocatalytic activity than Degussa P25.Finally, N-TiO₂ nanoparticles sensitized by dye were prepared by direct adsorption of Zn porphyrin onto the surface of N-TiO₂, and the characterization and photocatalytic properties of these TiO₂ nanoparticles were carried out. The results showed the ZnTCPP was chemisorbed on the surface of TiO₂ through a C-O-Ti bond, while the ZnTPP was physically adsorbed. Therefore, it was easier for ZnTCPP to adsorb on the surface of N-TiO₂ than ZnTPP. The photocatalytic activity of N-TiO₂ sensitized by ZnTCPP was evaluated by methylene blue degradation under visible light. It was found that N-TiO₂ sensitized by ZnTCPP exhibited higher photocatalytic activity than N-TiO₂ and undoped TiO₂ sensitized by ZnTCPP due to the synergistic effect of adsorbed dye and doped N atoms.