Synthesis and size modulation of MoS2 quantum dots by pulsed laser ablation in liquid for viable hydrogen generation

Abstract

In the present work, MoS2 quantum dots (QDs) were synthesized by chemical-free, single step photo-exfoliation of a solid MoS2 target using pulsed laser ablation in distilled water. MoS2 quantum dots (QDs) with average sizes of ∼4, 2.9, and 6.1 nm were synthesized by ablating an MoS2 target for ablation durations of 5, 10, and 20 min at a fixed laser energy of 40 mJ. Furthermore, quantum dots with average sizes of ∼2.9, 3.6, and 4.0 nm were also synthesized at laser energies of 10, 20, and 40 mJ, respectively, for a fixed ablation duration of 5 min. The quantum dots resulted in luminescence in the visible region. The as-synthesized colloidal solution of MoS2 quantum dots in distilled water showed excitation wavelength-dependent luminescence shifted to longer wavelength by varying excitation wavelength from 290 to 390 nm exhibiting the effect of wide size distribution. Energy dispersive x-ray spectroscopy, selected area electron diffraction pattern, and zeta potential analysis demonstrated the formation of stoichiometric, highly crystalline, and stable MoS2 quantum dots. Raman spectra of the samples showed two sharp and intense Raman active modes A1g and E2g1 of the MoS2 crystal, indicating crystalline MoS2 quantum dot formation. As an electrocatalytic activity, MoS2 quantum dots exhibited a high rate of hydrogen generation with a minimum Tafel slope of ∼57 mV/dec. High surface area with a large number of active edges makes MoS2 QDs an active catalyst for hydrogen production.