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Type of Document Dissertation Author Subramanian, Vaidyanathan Author's Email Address vsubrama@nd.edu,ravi_v_subramanian@yahoo.com,ravivaidyanathan@yahoo.com URN etd-04122004-122926 Title Photoelectorochemical and Photocatalytic Aspects of Semiconductor-Metal Nanocomposites Degree Doctor of Philosophy Department Chemical Engineering Advisory Committee
Advisor Name Title Gary H.Bernstein Committee Chair Agnes E. Ostafin Committee Member Davide A. Hill Committee Member Eduardo E. Wolf Committee Member Mark A. Stadtherr Committee Member Prashant.V.Kamat Committee Member Keywords
- catalysts
- reactor
- nanocomposites
- colloids
- films
- surface characterization
- electrochemistry
- films
- nanoparticles
- photocatalysis
- TiO2
- semiconductor
- metal
Date of Defense 2004-01-16 Availability unrestricted Abstract Semiconductor and metal nanoparticles display unique size dependent photophysical,electrochemical, photocatalytic and optical properties. The presence of metal facilitates
electron transfer from photoexcited semiconductor to the surroundings and decreases
recombination rate between the electrons-holes in the semiconductor.
The focus of this work has been the synthesis and characterization of novel
semiconductor – metal composites in films and solutions. Four areas of importance in
these nanocomposites have been addressed: Photoelectrochemical properties of
composite films, Fermi-level equilibration between semiconductor-metal, dynamic nature
of semiconductor-metal interface and role of metal in promoting photocatalytic activity
of the semiconductor.
Photoelectrochemical performance Deposition of metals on semiconductor films
leads to increased light to energy conversion efficiency and demonstrates beneficial
photoelectrochemical properties such as higher photocurrent and photovoltage compared
to plain semiconductor. A TiO2 – Au film shows ~25% energy efficiency compared to <5
% for the semiconductor alone. The photocurrent using a TiO2-Au film is 3-5 times
higher than a plain TiO2 film.
Fermi level equilibration Photoexcited semiconductor in contact with the metal
undergoes charge equilibration with metal which alters the energetics of the composite by
shifting the Fermi level to more negative potentials. Furthermore, a size dependent
negative shift in the Fermi level is also observed in a TiO2 – Au composite. Greater
negative shifts in Fermi level occurs with smaller size Au nanoparticles.
Dynamic nature of interfacial process The semiconductor-metal interface is dynamic
in nature and demonstrates photomediated chemical transformations. For example, metal
nanoparticles undergo change in oxidation state when a TiO2-Au composite is
photoexcited in the presence of oxygen. Such transformations diminish the beneficial
photoelectrochemical properties of a TiO2-metal film following prolonged
photoillumination.
Photocatalytic properties Colloidal suspension of TiO2-Au Nanocomposites
demonstrates greater photocatalytic activity than TiO2 alone. Presence of a metal- metal
ion mixture enhances the catalytic activity of the composite semiconductor.
Phototoillumination of TiO2 containing an optimal ratio of Au(0)/Au(III) lead to a ~ 40 %
enhancement in the photocatalytic oxidation efficiency of thiocyanate – a probe specie –
compared to TiO2 alone. Similarly, the reduction of fullerene is catalyzed in the presence
of Au nanoparticles of different sizes.
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