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

: The behavior of sodium diethyldithiocarbamate, Na[Et₂NCS₂], at n-type semiconducting CdX (X = S, Se) in CH₃CN/0.2 M NaC10₄ has been studied. The Et₂NCS_{2 -} interacts strongly with the CdX surface and shifts the flat-band potential, E_FB, up to 1.0 V more negative with increasing Et₂NCS_{2 -} concentration. The concentration dependence of the shift in efbhas been studied in the range 0-0.2 M, with 0.01 M Et₂NCS_{2 -} 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 dithiocarbamate. The shift in efbis assumed to be proportional to Et₂NCS_{2 -} coverage. A plot of change of E_FBwith change in bulk concentration of Et₂NCS_{2 -} can be modeled by using Langmuir adsorption isotherms. Adsorption data for two bis(dithiocarbamates), Na_{+ 2}[CH₂N(Et)CS_{2 -}]₂and Na_{+ 2}[CH₂CH₂CH₂N(Me)CS_{2 -}]₂, show that maximum shifts of efbare obtained at lower solution concentrations than for Et₂NCS_{2 -}. 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 Et₂NCS_{2 -} 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 98%) can be maintained to large extent conversion (70%) in the photoelectrochemical oxidation of Et₂NCS_{2 -} to [Et₂NC(S)S]₂at either illuminated CdS or CdSe. Oxidation of Et₂NCS_{2 -} can be effected at an electrode potential significantly more negative than E_o’of Et₂NCS_{2 -}/[Et₂NC(S)S]₂, showing that visible light can be used to drive the oxidation in an uphill sense. 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/cm₂for greater than 48 h.