Study On Electronic Structure And Photocatalytic Activity Of Mo/N Co-doped Nano-TiO₂

TiO2 is one of extensively studied semiconductor photocatalyst, and it has been received extensive attention, because of its advantages of good chemical stability, high catalytic activity, high oxidation resistance, no pollution and low cost.But it only absorb UV light in the solar spectrum because of its wide band gap, so it hinder the effective utilization of the light of the sun, and the application prospect is also restricted. Based on the progress of studying on Ti O2 photocatalytic materials at home and abroad, it can be seen that both the theoretical and experimental work show the doping method can modify the energy band structure of Ti O2,and the visible light response can be improved obviously. In the cation doping Ti O2, the cation ion could play a central role to produce a large number of defects which reduce the photogenerated carrier separation. Compared to cation doping, anionic nonmetals role as recombination centers might be minimized. But researchers have found that the content of dopant would decrease during the annealing process, this major drawback limits the incorporation of nonmetallic anions for doping Ti O2, thus reducing the photoactivity under visible radiation. Anion and cation co-doping Ti O2 can overcome two disadvantages of its mono doped and exhibit better photocatalytic activity, but the synergetic mechanism of co-doping is not clear so far. So excited by this idea, this paper uses the theoretical and experimental methods to explain the synergistic effect of the metal(Mo) and nonmetal(N) ions codoped Ti O2, the specific contents are as follows:(1) We introduce some basic knowledge of the theoretical calculation, computational tools and calculation steps, and we used the density functional theory which based on the ultrasoft pseudopotential spin polarized plane wave method to calculate the electronic structure of intrinsic and doped rutile Ti O2. It is analyzed that Mo/N co-doped rutile Ti O2 is a typical direct band gap semiconductor, the band gap has been significantly narrowed compared with the intrinsic titanium dioxide. This means that the visible light absorption can be enhanced by non metal(N) and metal elements(Mo) co-doped rutile Ti O2 in photocatalytic process, so as to achieve better photocatalytic activity.(2) we prepared titanium dioxide nanowires, nitrogen monodoped titanium dioxide nanowires,molybdenum monodoped titanium dioxide nanowires, molybdenum and nitrogen co-doped titanium dioxide nanowires using the hydrothermal method.The results show that the crystal structure of the undoped and doped Ti O2 are the rutile phase,and the morphology are the nanorod. UV-Vis diffuse reflectance absorption showed that the absorption band edge of the samples have different redshift after doping, in which molybdenum and nitrogen co-doped titanium dioxide nanowires the redshift is greatest and the sample has the best light absorption characteristics. Fluorescence spectra also indicated that the intensity fluorescence of the co-doped sample is weakest. This means the electron-hole recombination probability minimum, which creates favorable conditions for the photocatalytic process. XPS characterization showed that Mo 3d binding energy of molybdenum and nitrogen codoped Ti O2 nanowires samples were at 232.4 e V and 235.4 e V. They are corresponsive to Mo 3d3/2 and Mo 3d5/2 peak position, respectively. It means molybdenum in the form of Mo6+ existed in the Ti O2. And N 1s binding energy at 397.9 e V of β-N peak indicated the presence of Ti- N bond in the sample, which can be explained a part of the O atoms are substituted by N atoms in Ti O2. This indicates that N atoms were successfully doped into the Ti O2 lattice in the form of N3- ion. The impurity energy level formed in the bottom of the conduction band and the top of the valence band and narrowed the band gap of Ti O2, thereby broadening and enhancing the absorption of visible light. We know the highest photocatalytic activity sample is the molybdenum and nitrogen co-doped titanium dioxide nanowires by degradation of MB with all kinds of samples.(3) We prepared hollow sphere titanium dioxide by the method of hydrothermal synthesis and studied the related changes in the morphology and crystal type after adding ammonium molybdate. The XRD and Raman results show that the sample which is not modified are of rutile and anatase titanium dioxide phase common existence.When adding different amount of ammonium molybdate, rutile phase disappears and only exsit the anatase phase.But the morphology of unmodified and modified samples did not change and always existed in spherical form. UV-Vis diffuse reflectance absorption show: when adding 0.03 g ammonium molybdate to Ti O2, the redshift degree is the greatest, the absorption edge is extended to about 500 nm. According to the characterization of XPS: we know the two obviously Mo 3d3/2 and Mo 3d5/2 in Mo3 d and β-N peak at about 397.9 e V in the N1 s indicated that Mo and N elements have been doped into the Ti O2 lattice and existed in the form of the Mo6+ and N3- ions, the ratio of Mo and N atom doped into the Ti O2 lattice is relatively high, this is the first time that adding a precursor can dop non metallic elements and metal elements in titanium dioxide simultaneously.