Differing Photo-oxidation Mechanisms: Electron Transfer in Titanium Dioxide vs. Modified Titanium Dioxide.

摘要

Anatase phase undoped and iron-doped titanium dioxide (TiO2/Fe-TiO 2) nanoparticles were synthesized, characterized, and probed with the objective of testing their photo activity in aqueous solutions. Fe-TiO 2 results indicate iron activates molecular oxygen in the oxidation of methanol with 0.5% Fe-TiO2 increasing photo efficiency by nearly three times that of undoped TiO2. Fe-TiO2 doped with 1.0 % Fe:Ti increase efficiency by a factor of two. An interband state has been identified due to an Fe*O2 adduct at 1.48 eV. In the absence of oxygen, the Fe3+/2+ reduction band becomes an electron hole recombination site.;Characterization was conducted using analytical, wet lab, and theoretical techniques. The morphology, band gap, and particle sizes were confirmed using Raman and UV-vis spectroscopies. Raman peaks occurring at 144,197,399,514, and 627 cm-1 were due to anatase Eg, Eg, B1g, A1g, and Eg respectively and identified as anatase phase. Further fitting of the Raman spectra allowed for the comparison of peak areas which allow analysis of exposed facets of the crystal. Upon examination, the (101) and (001) faces of the anatase crystal do not change with the doping of the particle. The UV-vis spectrum shows a shift toward the visible from particles doped with iron. Metal doping increases absorption of light to longer wavelengths. For undoped TiO2 the band gap occurs at 340 nm (3.64 eV); the band gap for both 0.5% and 1.0% Fe-doped particles are overlapped at 345nm (3.59 eV). A differentiation of the UV-vis spectrum coupled with the Brus method allowed for the calculation of the radius of the particle given the electron and hole effective masses. Particle radius was calculated as 0.86 nm for undoped TiO2 and 0.90 nm for Fe-doped particles.;The inclusion of both Fe and Au extended absorption into the near visible spectrum however; catalytic reactions with visible photons have not been shown to give a significant boost to the increase of photo activity. UV excited electrons account for the bulk of the increase in photoreactions specifically concerning Fe-TiO2 photo kinetic experiments. Iron doped TiO 2 photo catalyst were consistently more photo reactive than undoped TiO2 in the oxidation of methanol. TiO2 doped at 0.5% was almost three times more efficient while 1.0% was two time more efficient. The decrease in efficiency between 0.5% and 1.0% may possibly be an effect of a stabilization of charges on the surface of the particle. Iron adsorbs on the (001) face in oxygen vacancies. With a single iron atom on the (001) surface an electron can be transferred to an oxygen scavenger. With more than one iron atom the vacancy is stabilized and oxygen atoms are not attracted decreasing the activity of the catalyst.;Photo catalytic experiments were conducted in both oxygen and nitrogen saturated environments revealing a difference in mechanism between methanol oxidized in the presence of Fe-TiO2 and TiO2. TiO 2 either reduces water or stores the electron for reduction purposes. The newly identified interband state at +1.48 eV acts as an efficient electron-hole recombination site in the absence of oxygen for Fe-TiO2. Nitrogen purging greatly reduces methanol oxidation to formaldehyde leading to a quenching of the reaction. This interband state was located by removing fluorescence from the Raman spectra using a log normal.;Finally, gold was photo deposited onto the surface of 0.5% Fe-TiO 2. The catalyst is more than two times efficient than undoped TiO 2 but does not surpass the initial efficiency of iron doping by itself. This may be due to the overall deactivation of the catalyst which has been observed in the literature. However, gold doping does not completely quench the reaction nor does it block oxygen from reaching iron on the surface of the nanoparticle. This is information that may be used in further doping of Fe-TiO2.