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

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 W 1– x S 2 ternary alloy is formed when an acidic solution of (NH 4 ) 2 MoO 4 and (NH 4 ) 2 WO 4 is used for spin-coating, while the MoS 2 /WS 2 lateral heterojunction with a sharp boundary is formed when an alkaline solution of Na 2 MoO 4, Na 2 WO 4, and NaOH is used. According to the calculation, under alkaline conditions, sulfur atoms preferentially bind to MoO 4 2– due to lower energy and finally form a lateral heterojunction. However, under acidic and neutral conditions, MoO 4 2– and WO 4 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.