Effect of composition of few-layered transition metal dichalcogenide nanosheets on separation mechanism of hydrogen selective membranes

Abstract

The chemical composition of two-dimensional transition metal dichalcogenides (TMDs) affects the stacking of the nanosheets and changes the gas separation behavior of the stacked layer; however, this phenomenon has not been discussed in detail so far. In this work, a simple, environmental-friendly aqueous exfoliation of TMDs is applied using tannic acid (TA) for membrane synthesis. To efficiently delaminate bulk TMD, TA not only serves as a negatively charged surfactant but also adsorbs to the delaminated nanosheets of TMD owing to its amphiphilic nature. The exfoliated nanosheets and prepared membranes were characterized using spectroscopic and microscopic techniques. The microstructure of WSe2, MoSe2, and MoS2 membranes were explored using positron annihilation spectroscopy and X-ray diffraction to correlate membrane microstructure and interlayer spacing with the gas transport mechanism. The H2/CO2 separation performance of the resultant membranes surpasses the Robeson's upper bound. On comparison of WSe2 with MoSe2, replacing W with Mo changes the dominative gas transport mechanism of the membrane from molecular sieving to Knudsen diffusion. However, a change in the chalcogen atom will not affect the gas transport mechanism. Our findings are an important step towards the industrial application of membranes, such as the refinement of H2 and removal of CO2.