טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentHossam Haick
SubjectPhoto-Induced Phenomena in Mono-Molecular Films
Chemisorbed on Micro-Domains at the Vicinity of
Titanium Dioxide
DepartmentDepartment of Chemical Engineering
Supervisor Full Professor Paz Yaron


Abstract

Titanium dioxide is a well-known photocatalyst for water and air treatment.  The photocatalystic destruction of organics include oxidation by hydroxyl radicals formed when photogenerated holes interact with adsorbed water and by oxygen radicals formed when photogenerated electrons interact with adsorbed oxygen.

            Structures comprised of photoactive titanium dioxide and inert adsorbents may enhance the overall performance of the photocatalyst by concentrating molecules, which do not adsorb on TiO2, at the vicinity of the photocatalyst.  In this research, it was shown that photogenerated oxidizing species, formed on titanium dioxide well-defined microdomains, are capable of inducing, within minutes, the mineralization of an octadecyltrichlorosilane (OTS) monolayer anchored to inert silicon (or aluminum) domains, even when these molecules are located as far as 20 mm away from the titanium dioxide microzones.  The remote photodegradation effect is not due to direct oxidation by charge carriers, although such a mechanism may exist on the titanium dioxide domains, but is due to out-diffusion of oxidizing species, probably on the surface.  Unlike the OTS molecules on silicon (or aluminum), no decrease in the amount of octadecanethiol (ODT) molecules chemisorbed on same-size inert microdomains of gold (or platinum) could be observed upon exposure by UV light, despite the similarity between the ODT and OTS.

Gold or platinum nanodomains, loaded onto titanium dioxide powder are known to have a beneficial effect on photocatalysis.  In contrast, the present study, which aimed at longer-range effects, have revealed that the presence of a metal in the vicinity of the photocatalyst gives rise to a complex behavior, where, under certain conditions, not only that the presence of a metal does not increase photoefficiency, but in fact reduces it significantly.  This effect depends to large extent on the size of the metal domains, as well as the humidity and the type of metal.   It was also found that the presence of metals in the vicinity of the photocatalyst could improve significantly the adsorption of SAMs on the TiO2 domains.  This effect, which occurs on titanium dioxide but not on silicon, depends to large extent on the size of the metal islands, as well as on the pretreatment prior to chemisorption.