Computational study of the structure and electronic properties of Ag and Au doped (TiO2)n clusters (n=2-6)
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Date
2020
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Publisher
University of Namibia
Abstract
The titanium dioxide (TiO2) clusters are aggregates of TiO2 units that are intermediate in size between a molecule and the bulk material. It is known that molecular clusters exhibit unique properties as a function of size, composition, and atomic configuration. The goal of this study is to investigate variation in the structure and electronic properties of Ag and Au doped (TiO2)n as the cluster size (n) increases. It is of interest to determine the size limit where the electronic properties of the doped clusters mimic that of the bulk material.
And the need to shift the absorption wavelength of the (TiO2)n clusters into the visible
region of the electromagnetic spectrum has necessitated investigation into doping with
the Au and Ag atoms.
The results reported in this thesis were obtained using mainly BPW91 variant of the Density
Functional Theory (DFT) approximation. The 6-311+G (d) basis set, and the
Stuttgart-Dresden (SDD) effective core potential were used to represent atomic orbitals.
The structures of the most stable undoped and doped neutral cluster are similar for n = 6
only. In case of the anion, similar structures are observed for n = 3 and n = 6, while they
are different for n = 2, 4 and 5. Vertical electron detachment energies (VEDE) are within
3.40 – 5.87 eV while adiabatic electron detachment energies (AEDE) and electron affinity
(EA) vary from 1.92 to 6.52 eV. The values of these properties increase with the clusters
size. The HOMO-LUMO (HL) gap was used as an approximation to the band gap, and it
ranges between 0.33 - 3.59 eV and 0.19 - 3.27 eV for Ag and Au doped TiO2 clusters,
respectively. Singlet – singlet electronic transitions are predicted to occur at the visible
region (400 – 800 nm) for n = 2, 5 and 6, in both doped systems. Major contributions to
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the molecular orbitals are obtained from FMO. Molecular adsorption was the predominant
type of interaction between water molecules and clusters. Binding energy values of
-1.32 and -1.43 eV were obtained for the more reactive interaction of one water molecule
with Ag and Au doped (TiO2)n clusters (n = 2 - 6), respectively. With the exception of the
HL gap of anionic Au doped (TiO2)n, the properties of the doped clusters investigated in
this study did not converge to the bulk values, for n = 6 cluster length.
Description
A thesis submitted in fulfillment of the requirements for the Degree of Master of Science (Chemistry)
Keywords
Electronic properties, Au dopped