THE PHOTOCHEMISTRY AND PHOTOPHYSICS OF COLLOIDAL DISPERSIONS OF ZINC SULPHIDE AND CADMIUM SULPHIDE

Available from UMI in association with The British Library.; 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 transfer from the semiconductor excited state to an acceptor molecule. These factors are: degradation (the semiconductor must be stable under photolysis conditions), thermodynamics (charge transfer from the semiconductor excited state to the acceptor ground state must be exothermic), kinetics (long lived semiconductor excited states favour energy transfer) 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 iPA/water, 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 determine the absorption onset positions of the colloids. From these measurements it has been found that the CdS, ZnS and CdS/ZnS systems have absorption onset energies ranging from 2.53 to 4.96 eV which are dependent on the semiconductor particle sizes. Importantly, co-colloidal CdS/ZnS 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 {dollar}(lambda{dollar} = 254 nm) have revealed that semiconductor photodegradation shows a marked oxygen dependence and can be inhibited by the presence of charge scavengers (e.g. S{dollar}sp{lcub}2-{rcub}{dollar}, 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 ({dollar}phi >{dollar} 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-SO{dollar}sb4sp{lcub}2-{rcub}{dollar}) and radiative centres (M{dollar}sp{lcub}2+{rcub}{dollar}, S{dollar}sp{lcub}2-{rcub}{dollar}) have been identified. It has been found that luminescence quenching by additives is a powerful indicator for energy transfer processes. The results from iPA quenching experiments led to the photochemistry of CdS and ZnS in iPA/water being investigated. Upon illumination of both of these systems.({dollar}lambda{dollar} = 254 nm; a wavelength at which iPA does not absorb) acetone was produced with a concomitant reduction in the iPA concentration (measured using {dollar}sp1{dollar}H and {dollar}sp{lcub}13{rcub}{dollar}C NMR).; Finally, new models have been proposed for the "exciton" absorption, photodegradation and photoluminescence of colloidal semiconductor systems.