Synthesis of size-quantized metal sulfides of Pb–Cd and Zn–Cd bimetallic systems in stearate Langmuir–Blodgett films

Abstract

Synthesis of Pb–Cd and Zn–Cd bimetallic sulfides in stearate Langmuir–Blodgett (LB) matrices has been investigated using repeated cycles of sulfidation (S) by exposing a metal stearate LB film to H₂S gas and the intercalation (I) of metal ions into the LB matrix by immersing the film into aqueous metal acetate. In a serial synthetic mode, one metal sulfide was synthesized in advance, and the metal sulfide LB composite film was subjected to the I–S cycles to synthesize the other sulfide. When CdS was pre-synthesized in both the PbS–CdS and ZnS–CdS systems, two sulfide phases coexisted in the stearate LB matrix, and the amount and size of each metal sulfide was controlled independently by the number of I–S cycles in the preparation of each sulfide. In the reverse sequence, on the other hand, the pre-formed PbS and ZnS were dissolved and CdS was formed during the immersion in aqueous Cd(CH₃COO)₂. These results of the serial synthesis are reasonable because the replacement reaction, NS + Cd²⁺ → CdS + N²⁺ (N = Pb, Zn), is thermodynamically favorable. When mixed metal stearate LB films of Zn and Cd were sulfurized, size-quantized solid solution phases of Zn_1−yCd_yS (0 < y < 1) were formed in the LB matrix, and the composition y could be controlled by that of each metal ion in the LB films.