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Development of scanning electrochemical microscopy for the investigation of photocatalysis at semiconductor surfaces
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Fonseca, Sofia Margarida Martins Costa da (2002) Development of scanning electrochemical microscopy for the investigation of photocatalysis at semiconductor surfaces. PhD thesis, University of Warwick.
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WRAP_THESIS_Fonseca_2002.pdf - Submitted Version Download (16Mb) | Preview |
Official URL: http://webcat.warwick.ac.uk/record=b1380097~S1
Abstract
This thesis is concerned with the development and application of scanning
electrochemical microscopy (SECM) to investigate interfacial photoelectrochemical
processes occurring at supported Ti02 surfaces. The new SECM approach, involving
both amperometric and potentiometric electrodes, was used to monitor interfacial
photoprocesses with high spatial resolution.
A new in situ photoelectrochemical approach to chemical actinometry has been
developed and used to determine the light flux through a quartz fibre employed in the
SECM system. In this system an ultramicroelectrode (UME) probe is positioned with
high precision at a known distance close to a TiOrcoated fibre and used to detect
reactants or products of the ongoing photodegradation process. The
microelectrochemical actinometry approach was developed using the well-known liquid
phase potassium ferrioxalate actinometer. The approach involved recording the steadystate
current for Pe(III) reduction at an SECM tip positioned close to the fibre. A step
function in the light flux (off-on) was then applied and the resulting
chronoamperometric behaviour due to the reduction of Fe(III) at the UME was recorded
as a function of tip-fibre separation distance. A theoretical model has been developed to
simulate experimental current-time profiles, which allowed the light flux through the
quartz fibre to be quantified.
An experimental approach to investigating the photoelectrochemical reduction
of oxygen at UV-illuminated Ti02 surfaces, in aqueous aerated and oxygenated
electrolyte solutions, in the absence and presence of hole scavengers, has been
developed. In this new approach the chronoamperometric behaviour for oxygen
reduction was recorded at an UME tip after stepping the light flux at a back-illuminated
Ti02 film on and off The kinetics of the reduction process were interpreted through
various theoretical models proposed in the literature. This experimental approach
demonstrated a significant depletion of the oxygen concentration at the illuminated Ti02
surface, which provides a new insight into the photomineralisation process, showing the
important role of oxygen in controlling the kinetics.
Using an SECM potentiometric approach, the photomineralisation kinetics of a
model organic pollutant, 4-chlorophenol (4-CP), in aerated and oxygenated aqueous
solutions at supported Ti02 films, were quantitatively investigated. A potentiometric
Agi AgCI UMB, positioned at a known distance above the Ti02 film, was used to
monitor directly the cr production from the photomineralisation of 4-CP. A theoretical
model, employing a Langmuir-Hinshelwood type kinetic equation, has been developed
to interpret the kinetics of the photomineralisation process and determine the associated
quantum efficiency. A direct correlation between oxygen consumption at the
illuminated Ti02 surface and cr formation in the photomineralisation process has been
found.
SECM has also been used to monitor photoelectrochemical transfer kinetics at
the Ti02/aqueous interface using a well-known electron scavenger, methyl viologen.
The Ti02 film was in contact with a solution containing methyl viologen dication
(MV2+) as the redox mediator and sodium acetate as the hole scavenger. The
chronoamperometric behaviour for MV2+ reduction was recorded at an UME tip after
stepping the light flux at the Ti02 film from off to on. The rate constants for the
reduction process were obtained through a theoretical model based on zero-order
kinetics. The substrate generation! tip collection mode of the SECM was also used to
detect the MV.+ radical cation produced at the Ti02 surface at the UME tip.
Item Type: | Thesis (PhD) | ||||
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Subjects: | Q Science > QD Chemistry | ||||
Library of Congress Subject Headings (LCSH): | Scanning electrochemical microscopy, Photoelectrochemistry, Photocatalysis, Semiconductors -- Surfaces -- Analysis, Titanium dioxide -- Surfaces -- Analysis | ||||
Official Date: | September 2002 | ||||
Dates: |
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Institution: | University of Warwick | ||||
Theses Department: | Department of Chemistry | ||||
Thesis Type: | PhD | ||||
Publication Status: | Unpublished | ||||
Supervisor(s)/Advisor: | Unwin, Patrick R. ; Kemp, T. J. (Terence James), 1938- | ||||
Sponsors: | Fundação para a Ciência e a Tecnologia (FCT) | ||||
Extent: | vi, 178 leaves | ||||
Language: | eng |
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