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Title: A study of photo-induced wettability changes at metal oxide semiconductor surfaces
Author: Denison, Kieth Royston
Awarding Body: Lancaster University
Current Institution: Lancaster University
Date of Award: 2011
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Colloidal semiconductor photocatalytic materials such as Ti02• α-Fe203 and Sn02 have been used in laboratory-scale pollution abatement systems, reducing I oxidising both organic and inorganic impurities / toxins to chemically useful and / or biologically harmless compounds. For Ti02 there are potentially two distinct photoinduced phenomena: the first is the photocatalytic redox phenomena described above. The second involves photolytically induced high wettability or superhydrophilicity. The longer the surface is illuminated with IN light, the smaller the contact angle formed between the air - water - TiO2 interfaces until, in some cases the angle approaches zero. We have studied Photo-induced SuperHydrophilicity (PISH) on mesoporous TiO2(M-Ti02) films produced from reverse micellar solutions of titanium isopropoxide. M-Ti02 films have been deposited on quartz by spin coating and shown to clearly exhibit phot(}-induced wettability when irradiated with ultra-band gap light. Using a new continuous contact angle measurement technique. the time dependence of the photo-induced wettability changes on the film have been evaluated through the spreading of a sessile water drop. Results obtained at 100% RH, 293 K showed that drops on M-Ti02 exhibited a photo-induced "stick-slip" behaviour. The thermodynamic driving force for this photo-induced stick-slip has been attributed to the departure of the system from capillary equilibrium as the equilibrium contact angle of the drop, θo, decreased with illumination time. Based on this model, a simple theoretical description of photo-induced stick-slip has been derived and used to calculate a value of the potential energy barrier opposing the onset of movement of the triple line, U = 6.26 X 10-6 J m-1. This is the first time that U has been quantified for a surface with photo-induced wenability. Micro gravimetric studies of M-Ti02 layers in humid atmospheres show that, especially at RH > 95%, illumination of the M-Ti02 surface leads to a photo-induced watcr adsorption I condensation at the semiconductor / gas interface. Studies of this photo-induced water condensation on M-Ti02 coated crystals as a function of crystal age / experiment run number indicates that there are two possible modes of condensation - in-pore / in-layer condensation and condensation over the outer surface of the layer.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available