Phys. Scr. 100 (2025) 085992 https://doi.org/10.1088/1402-4896/adfa55 PAPER Condensed-layer-free MoS 2 nanoflakes synthesized by hydrothermal: a morphology, structure and optic-driven route toward electrochemical application L V C Hau 1 , N V Quan 1 , Tien Dai Nguyen 2,3,∗ , Thi Bich Vu 2 , Hoang V Le 4 , Long V Le 5 , T T K Chi 5 , L T H Phong 5 , Nguyen Duy Thien 1 and Tien–Thanh Nguyen 5,∗ 1 Faculty of Physics, VNU-University of Sciences, 334 Nguyen Trai, Hanoi, 100000, Vietnam 2 Institute of Theoretical and Applied Research, Duy Tan University, Hanoi, 100000, Vietnam 3 Faculty of Environmental and Natural Sciences, Duy Tan University, Da Nang, 550000, Vietnam 4 Institute of Science and Technology, TNU-University of Sciences, Thai Nguyen, Vietnam 5 Institute of Materials Science, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Caugiay, Hanoi, Vietnam ∗ Authors to whom any correspondence should be addressed. E-mail: nguyentiendai@duytan.edu.vn and ntthanh@ims.vast.ac.vn Keywords: hydrothermal synthesis, MoS 2 nanoflake, electrocatalytic, electrochemical water-splitting Supplementary material for this article is available online Abstract We report on the synthesis of MoS 2 nanoflakes by the hydrothermal method. The condensed 2H-MoS 2 phase layer beneath the nanoflakes formed at temperatures ranging from 180 to 270 °C, with an average thickness of 7.46 to 10.55 nm. The morphological, structural, and optical characteristics of MoS 2 nanoflakes depend on the synthesis temperature, and the compound exhibits an indirect (direct) band gap semiconductor behavior. Based on these findings, we investigated the potential applications of MoS 2 nanoflakes in electrochemical water-splitting, utilising a 0.5 M H 2 SO 4 electrolyte. The MoS 2 nanoflake electrodes exhibited a high current density of 85 mA cm -2 at –770 mV (versus RHE), η 10 = 254 mV, a low Tafel slope of 57 mV dec -1 . This approach may be suitable for morphology, structure, and optic- driven strategies in designing efficient MoS 2 -based electrocatalysts. 1. Introductions Recently, MoS 2 nanostructures have been attracting much significant attention due to their promising applications in electrochemical water splitting [1–8], hydrogen evolution reaction (HER) [9–16], photocatalysis [17, 18], light-emitting diode [19, 20], photodetector [21–23], sensor [24–26], and supercapacitor [27, 28]. Among these, MoS 2 nanoflakes (2D) have demonstrated remarkable electrocatalytic activity for sustainable HER kinetic, exhibiting a low overpotential of approximately 90 mV (at 10 mA cm -2 ), a small Tafel slope of about 40–60 mV dec -1 , and a low onset potential (∼0.08 mV) close to 0 mV (versus RHE), comparable to Pt-based material in acidic media [1, 5, 6, 29, 30]. The HER kinetics of MoS 2 electrocatalysts in alkaline media are generally inferior to those in acidic environments due to the high activation energy barrier associated with the water dissociation step [31]. MoS 2 nanoflakes consist of multiple stacked S–Mo–S layers, which provide numerous active sites, including Mo/S edge sites, S basal planes, S vacancy defects, and armchair edges, all of which play crucial roles in electrocatalytic performance and HER activity [9, 10, 17, 32–35]. The multilayer stacking of S–Mo–S structures interacts via weak van der Waals forces [34, 36], leading to intriguing physicochemical properties associated with phase transitions such as 2H→3R, 2H→1T, and 1T→1T′ [34, 37, 38]. MoS 2 nanoflakes exhibit high carrier mobility, excellent electrochemical stability, tunable band gaps, robust thermal stability, and pseudo-quantum confinement effects [21, 25, 39–41]. Among the various synthesis techniques, wet chemical methods offer practical advantages for fabricating MoS 2 nanostructures due to their mild reaction conditions, low-temperature growth, flexibility in substrate selection, and cost-effectiveness. Typically, the successful vertical growth of MoS 2 nanoflakes requires a pre- RECEIVED 21 April 2025 REVISED 6 August 2025 ACCEPTED FOR PUBLICATION 11 August 2025 PUBLISHED 22 August 2025 © 2025 IOP Publishing Ltd. All rights, including for text and data mining, AI training, and similar technologies, are reserved.