Properties of Cu(II) and Ni(II) sulfides prepared by coprecipitation in aqueous solution

Among semiconductors, metal sulfides are very interesting materials that ma y be used for solar energy conversion arrangements. We prepared a series of CuxNi1-xS powders, where x = 0.05, 0.20, 0.50, and 0.80, by spontaneous pr ecipitation. Various stoichiometries of Cu(NO3)(2), Ni(NO3)(2), and (NH4)(2 )S solutions at 25 degrees C and pH 2.50, at conditions in which the aqueou s solutions were supersaturated with respect to CuS and NiS, yielded precip itates corresponding to intermediate mixed sulfides. The appearance of new peaks in the powder X-ray diffraction spectra suggested the formation of ne w phases. Measurements of the resistance of the bulk solids as a function o f temperature for the preparations with x = 0.05, 0.20, and 0.50 showed a d ecrease with increasing temperature. A hysteresis was found upon thermal cy cling of the sulfide preparations, possibly due to changes in the solid sta te that affect the energy bands of the semiconductors. The Cu and Ni sulfid e preparations with x:= 0.80 showed metallic behavior. All preparations wer e highly charged when suspended in aqueous media, and the electrical charge was found to depend strongly on the activity of copper ion in solutions eq uilibrated with the solid phase. Finally, current potential curves of elect rodes prepared from the precipitated sulfides under constant illumination g ave values for the open circuit potential as high as 0.868 V, with 0.119 mA for the photocurrent and 0.58 for the field factor. These values are encou raging for the potential application of these materials in cells for the co nversion of solar into electric energy.