Development of scanning electrochemical microscopy for the investigation of photocatalysis at semiconductor surfaces
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.