The Preparation, Characterization And Visible-Light Catalytic Performance Of B And N-doped TiO₂

Titanium dioxide (TiO₂), as a cheap, nontoxic, and highly efficient photocatalyst, has been extensively applied for degradation of organic pollutants, air purification, sterilization, and as a demister. However, because of the wide band gap of titanium dioxide (3.2 eV), only a small UV fraction of solar light (3-5%) can be utilized. Therefore, the most important and challenging issue is to develop efficient visible light sensitive photocatalysts. In this paper, in order to improve the photocatalytic activity of titanium dioxide both under UV and visible light irradiation, nonmetal (B, N) doped titanium dioxide catalysts were studied. The synergistic effects of B and N codoping and the effect factors on activity for B,N-TiO₂ were investigated. The photoelectrochemical method was designed to evaluate the photocurrent response under visible light. The main content and viewpoints are as follow:(1)The B doped TiO₂ was prepared firtly by hydrothermal method using Ti[OCH(CH3)2]4 as a raw materials, and subsequently B,N-TiO₂ photocatalyst was synthesized by nitrogen doping in NH3 at variable temperatures.The effects of the nitrogen doping temperature on the structure and photocatalytic activity of B,N-TiO₂ were investigated. The surface structures of B,N-TiO₂ were investigated based on XPS spectra of B 1s, N 1s, O 1s and Ti 2p. The results suggested the boron and nitrogen could be located in the TiO₂ lattice interstitially or/and substitutionally, while the Ti-O-B-N structure played a vital role in photocatalytic activity under visible light. The photocatalytic activity of B,N-TiO₂ increased with the temperatures of nitrogen doping increased first, and then decreased with the temperatures of nitrogen doping. The optimal nitrogen doping tempera?tur e is 550. Otherwise, higher temperatures of nitrogen doping brought many oxygen vacancies and Ti3+ species, resulting in decrease of photocatalytic activity in visible light.(2) The anatase N-TiO₂ sheets, anatase N-TiO₂ sheets with dominant {001} facets and B,N-TiO₂ nanorods were prepared by a facile hydrothermal method using TiN as a precursor. (a) The anatase N-TiO₂ sheets looked like double-headed arrowhead with the crystal sizes of less than 20 nm in width and approximately 100 nm in length. The photocatalytic activity of this N-TiO₂ sheets are better than N-P25 under both UV-Visible and visible irradiation. (b) The N-TiO₂ nanorods were firstly formed in the hydrothermal synthesis, and then boron was introduced N-TiO₂ nanorodsby a hydrothermal method to prepare B,N-TiO₂ nanorods with the diameter of approximately 50-100 nm and the length of several micrometers. The process of growing of nanorods was observed by SEM and a most probable formation mechanism of the trititanate nanorods was proposed. The B,N-TiO₂ nanorods showed higher photocatalytic activity for methyl orange dye (MO) degradation and more excellent photoelectrochemical property than N-TiO₂ nanorods under visible light irradiation. (c) The anatase N-TiO₂ sheets with dominant {001} facets were bipyramids with side length of ca. 180-200 nm, and thickness of ca. 25-35 nm. The percentage of exposed {001} facets is about 69.7%. Both the particle size and the percentage of exposed {001} facets could be adjusted by different hydrothermal temperature. The photocurrent of anatase N-TiO₂ sheets with dominant {001} facets are 3.2 times than N-P25.(3) The B, N co-doped TiO₂ nanotube arrays were successfully synthesized in an electrolyte containing BF3 via an anodization process on a Ti sheet and subsequently anneal in ammonia ambient. The XRD indicated that they consist of a mixture of major anatase and minor rutile. XPS analysis suggested that there are the Ti-O-B-N moieties at their surface. The photocurrent response measurement and the decomposition of organic pollutant OII indicated that they exhibit more excellent photoelectrochemical properties and photocatalytic activities than that of the N-TNTs and B-TNTs under UV-visible or visible light irradiation. Thus, the excellent photoelectrochemical properties and photocatalytic activities of the BN-TNTs should be attributed to the synergistic effects of B and N co-doping and the mixed phases of anatase and rutile.(4) The TiO₂ nanotube arrays (TNTs) were obtained by Ti anodization on a Ti sheet, CN_x polymers were deposited into the crystallized TNTs by electrodeposition method. The results showed that CN_x can be used to sensitize TNT arrays making them more responsive to the visible light. The photocurrent response of the CN_x modified TNT electodes is 13.2 times higher than that of the plain TNT. The cell efficiency is 4.6% for the as-modified CN_x/TNT. The improved efficiency may be attributed to two major improvements. First, the polymers afford multiple excitons from the absorption of a single photon. Second, the crystalline nature of the nanotubes and the film geometry allow a fast and efficient transfer of the photogenerated electrons from CN_x polymer to the Ti substrate, which compares favorably with the traditional mesoporous film electrodes leading to a much reduced electron-hole recombination and much improved photocurrent and efficiency. This conclusion will help to design more effective visible-light photocatalyst for solar energy conversion.