Heterogeneous photocatalysis using photoexcited TiO2 substrates (either in dispersed
suspensions or in thin film form) is shown to be a versatile method for preparing Se- or
metal selenide (MSe)-modified TiO2 surfaces. The mechanistic aspects underlying this novel
preparative route are addressed using chronopotentiometry and linear sweep photovoltammetry. The photovoltammograms of Se/TiO2 composite electrodes exhibit unusual “bipolar”
behavior, i.e., both photocathodic and photoanodic enhancement of current flow on the same
trace, depending on the imposed potential bias. This interesting photoelectrochemical
behavior is rationalized on the basis of the electronic properties of the Se/TiO2 junction. Ex
situ characterization of the composite semiconductor samples was also carried out. Finally,
an application of CdSe/TiO2 samples for environmental remediation of CrVI is demonstrated.
This paper describes the indirect photocatalytic reduction of four metal ions: Cd 2+ , Zn 2+ , Mn 2+ , and Tl + using formate radical anions (CO 2 •-) generated in UV-irradiated aqueous TiO 2 suspensions. Trends in the reactivity of these four metal ions are compared with corresponding rate constants reported in the literature for the homogeneous reactions of these metal ions with CO 2 •in aqueous media. In both cases, the reaction rates follow the same order: Cd 2+ > Tl + > Mn 2+ > Zn 2+ . Using Tl + as a model metal ion, a simple kinetics scheme is developed for the indirect reduction route, and the predictions from this model are shown to be in excellent agreement with experimental data.
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