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Title: The photochemistry and photophysics of colloidal dipsersions of zinc sulphide and cadmium sulphide.
Author: Doherty, Stephen.
ISNI:       0000 0001 3427 5131
Awarding Body: University of East Anglia
Current Institution: University of East Anglia
Date of Award: 1992
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The potential use of colloidal semiconductor systems regarding solar energy conversion and preparative organic chemistry has long been recognised. However there have been few reports concerning the characterisation of semiconductor systems with a view to identifying the factors which affect semiconductor photocatalysis. This thesis identifies the main criteria which control energy ttansfer from the semiconductor excited state to an acceptor molecule. 1bese factors are: degradation (the semiconductor must be stable under photolysis conditions). thermodynamics (charge ttansfer from the semiconductor excited state to the acceptor ground state must be exothermic). kinetics (long lived semiconductor excited states favour energy ttansfer) and physical (charge transfer generally requires intimate contact between the donor/acceptor species). In order to satisfy these conditions ZnS. CdS and ZnS/CdS semiconductors have been prepared in iPAJwa~. AOT/heptane!water and water. Together. these systems provide three distinct phases for additive dissolution (i.e. water. iPA, heptane). UV/VIS absorption spectroscopy has been used to detennine the absorption onset positions of the colloids. From these measurements it has been found that the CdS, ZnS and CdSlLnS systems have absorption onset energies ranging from 2.53 to 4.96 eV which are dependent on the semiconductor particle sizes. Importantly, co-colloidal CdSlLnS systems have been prepared which have tuneable absorption onset energies (2.53 to 4.96 eV). Furthermore. the AOT !heptane/water systems show indefinite stability to dark ageing. Photolysis experiments (l = 254 nm) have revealed that semiconductor photodegradation shows a marked oxygen dependence and can be inhibited by the presence of charge scavengers (e.g. S2-. isopropanol) illustrating the importance of surface reactions. These results led to the development of semiconductor systems which had photodegradation quantum yields of zero. Time-resolved and steady state measurements have proven that semiconductors luminesce with high quantum yields (~ > 0.1), via an "allowed" process, over nanosecond time scales. The luminescence excitation spectra show the characteristic semiconductor absorption profile. The semiconductors give a broad emission band (1.91 to 3.4 eV) which is Stokes shifted from the absorption profile (by up to 0.9 eV). The importance of surface sites has been demonstrated and the main non-radiative (e.g. M-aqua and M-S042-) and radiative centres (M2+, S2-) have been identified. It has been found that luminescence quenching by additives is a powerful indicator for energy ttansfer processes. The results from iP A quenching experiments led to the photochemistry of CdS and ZnS in iP Nwaaer being investigated. Upon illumination of both of these systems (A = 254 nm; a wavelength at which iPA does not absorb) acetone was produced with a concomitant reduction in the iPA concentration (measured using IH and 13C NMR). Finally, new models have been proposed for the "exciton" absorption, photodegradation and photoluminescence of conoidal semiconductor systems
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Physical chemistry