Effect of Solution pH on the Synthesis of Two-Dimensional Molybdenum–Tungsten Sulfide Nanostructures

Abstract

Compared with binary transition-metal dichalcogenides (TMDCs), ternary alloys or heterojunctions have more abundant properties. The spin-coating method has been widely used in the synthesis of TMDCs. However, there is still a lack of research on the effect of acidity and alkalinity of precursor solutions on the epitaxial growth of TMDCs. It is found that the Mo x W1–x S2 ternary alloy is formed when an acidic solution of (NH4)2MoO4 and (NH4)2WO4 is used for spin-coating, while the MoS2/WS2 lateral heterojunction with a sharp boundary is formed when an alkaline solution of Na2MoO4, Na2WO4, and NaOH is used. According to the calculation, under alkaline conditions, sulfur atoms preferentially bind to MoO4 2– due to lower energy and finally form a lateral heterojunction. However, under acidic and neutral conditions, MoO4 2– and WO4 2– in the precursor solution will undergo a complex reaction, respectively. Because there is no significant difference in the binding energy of the reaction between the sulfur atom and two complexes, a ternary alloy will be formed. The controllable synthesis of different types of TMDCs can be achieved by regulating acidity and alkalinity.