The photochemistry and photophysics of colloidal dipsersions of zinc sulphide and cadmium sulphide.
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