Effects of pH on the Structure and Optical Property of Molybdenum Disulfide Nanocrystals Synthesized by Hydrothermal Method

Molybdenum disulphide (MoS2) is emerging as one of the most attractive two-dimensional (2D) materials amongst the transition metal dichalcogenides (TMDs) group. MoS2 exhibits a thickness-dependent band gap that changes from indirect band gap of ~1.3 eV for bulk MoS2 to direct band gap of ~1.9 eV in monolayer form. Such indirect-to-direct gap transition due to quantum confi nement results in giant enhancement in photoluminescence quantum yield. Recent study demonstrated that the co-existence of 2H- and 1T-MoS2 synthesized by hydrothermal method exhibited superior electronic conductivity to those with 2H semiconducting phase. Metallic 1T phase of MoS2 is attracting much attention due to its high electronic conductivity and potential applications in supercapacitors, thermoelectric energy harvesting, and memristors. In this paper, we investigated a hybrid structure between 2H and 1T of MoS2 by a hydrothermal process and investigated its optical property via photoluminescence spectra with changes in pH values. Our results indicated that photoluminescence occurred in the visible light region and the band gap was determined to be ~1.96–2.50 eV. In addition, pH of the precursor solution strongly affected the fi nal structure of MoS2 nanocrystals, which in turn, affected their photoluminescence property wherein pH = 4–5 was identifi ed as optimum for 2H–1T-MoS2 synthesis.