Differing Photo-oxidation Mechanisms: Electron Transfer in Titanium Dioxide vs. Modified Titanium Dioxide.
Abstract: Anatase phase undoped and iron-doped titanium dioxide (TiO2/Fe-TiO2)
nanoparticles were synthesized, characterized, and probed with the objective of testing
their photo activity in aqueous solutions. Fe-TiO2 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 ... read moredoped 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 TiO2 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. TiO2 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-TiO2. The catalyst is more than two times efficient than undoped TiO2 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
Thesis (Ph.D.)--Tufts University, 2013.
Submitted to the Dept. of Chemistry.
Advisor: Mary Jane Shultz.
Committee: Jonathan Kenny, Arthur Utz, and Jonathan Rochford.
Keywords: Chemistry, and Physical chemistry.read less