Interaction of Diethyldithiocarbamate with n-Type Cadmium Sulfide and Cadmium Selenide: Efficient Photoelectrochemical Oxidation to the Disulfide and Flat-Band Potential of the Semiconductor as a Function of Adsorbate Concentration

Article abstract of DOI:10.1021/ja00273a004

The behavior of sodium diethyldithiocarbamate, Na, at n-type semiconducting CdX (X = S, Se) in CH3CN/0.2 M NaClO4 has been studied.The Et2NCS2^- interacts strongly with the CdX surface and shifts the flat-band potential, E_FB, up to 1.0 V more negative with increasing Et2NCS2^- concentration.The concentration dependence of the shift in E_FB has been studied in the range 0-0.2 M, with 0.01 M Et2NCS2^- being sufficient to shift E_FB the maximum amount.The shift in E_FB is due to excess negative charge on the CdX surface due to the presence of adsorbed dithio carbamate.The shift in E_FB is assumed to be proportional to Et2NCS2^- coverage.A plot of E_FB with change in bulk concentration of Et2NCS2^- can be modeled by using Langmuir adsorption isotherms.Adsorption data for two bis(dithiocarbamates), Na⁺2->2 and Na⁺2->2, show that maximum shifts of E_FB are obtained at lower solution concentrations than for Et2NCS2^-.The data show that the equilibrium constant for dithiocarbamate binding is somewhat greater (by a factor of 2) for CdS than CdSe.The value of E_FB in the presence of 0.2 M Et2NCS2^- measured by interfacial capacitance accords well with the electrode potential corresponding to onset of photoelectrochemical oxidation upon illumination with light of energy greater than the band gap, E_g, of the semiconductor.High current efficiency (at least 98percent) can be maintained to large extent conversion (70percent) in the photoelectrochemical oxidation of Et2NCS2^- to 2 at either illuminated CdS or CdSe.Oxidation of Et2NCS2^- can be effected at an electrode potential significantly more negative than <*> of Et2NCS2^-/2, showing that visible light can be used to drive the oxidation in an uphill sence.Compared to a Pt anode, the CdX (X = S, Se) photoanodes allow a voltage savings of the order of 1.0 V.The photoanodes are durable and show constant output of at least 10 mA/cm2 for greater than 48 h.