Distorted 2H/1T MoS2 nanostructures with improved field emission and sodium-ion battery performance

Abstract

The present investigation demonstrates a facile and scalable synthesis strategy for the preparation of 2H and 1T phases of MoS₂ using a solid-state method. Our findings reveal that MoS₂ exhibits a distorted octahedral coordination structure, formed owing to the thiourea concentration. The structural transitions from the semiconducting 2H phase to the metallic 1T phase have been confirmed by XRD and Raman spectroscopy. The resulting MoS₂ (1T) can provide more active sites by forming an ultrathin monolayer with a thickness of a few nanometers, enhancing the electrical conductivity. Field electron emission (FEE) behavior was investigated at a base pressure of 1 × 10^-8 mbar. The utmost emission current density i.e. 820 μA cm^-2 was observed for the 1T MoS₂ emitter at an applied field of 4.56 V μm^-1, in comparison to 2H MoS₂, which showed 542 μA cm^-2 at 4.85 V μm^-1. Considering the good electrical conductivity and layered ultrathin sheet structure, 1T MoS₂ has been explored as an anode for SIBs. Interestingly, it delivered a reversible capacity of 870 mA h g^-1 at 100 mA g^-1, and good cycling performance at 200 mA g^-1 with 81% retention after 300 cycles. By integrating MoS₂ electrodes with glyme electrolytes, a synergistic effect is achieved, leading to stable electrochemical performance of SIBs. The existence of an active 1T-MoS₂ basal plane compared to 2H-MoS₂ is responsible for the noticeable field electron emission and SIB application. It is noteworthy that MoS₂ without any modifications (such as doping, carbon integration or composite formation) showed excellent performance in both applications. This paper underscores MoS₂'s versatility and promising avenues for revolutionizing electronic devices and energy storage technologies.

Fig. 1. (a) XRD patterns and (b) Raman spectra of 2H MoS₂ (MS-H) and 1T MoS₂ (MS-T).

Fig. 2. FESEM images of MoS₂ samples: (a and b) MS-H and (c and d) MS-T.

Fig. 3. (a) TEM image of the MS-H sample, (b) HR-TEM image of lattice fringe spacing and the SAED pattern (inset in (b)), and (c–e) STEM elemental mapping of the MoS₂ sample: (c) greyscale image, (d) Mo-Kα, and (e) S-Kα of MS-T.

Fig. 4. (a) TEM image of the MS-T sample, (b) HR-TEM image of lattice fringe spacing and the SAED pattern (inset in (b)), and (c–e) STEM elemental mapping of the MoS₂ sample: (c) greyscale image, (d) Mo-Kα, and (e) S-Kα of MS-T.

Fig. 5. Schematic illustration of the formation of 2H and 1T MoS₂.

Fig. 6. (a) Plot of field emission current density as a function of the applied electric field for the as-synthesized MS-H and MS-T emitters. (b) Corresponding Fowler–Nordheim (F–N) plots. (c and d) Long-term emission current stability plots of MS-H (c) and MS-T (d). The insets show typical field emission images recorded during the measurements.

Fig. 7. Electrochemical properties: (a) the rate performance of all samples at different current densities, (b) cycling stability, (c) the discharge–charge profiles of MS-T at different cycles, (d) cyclic voltammetry curves of MS-T at a scan rate of 0.1 mV s⁻¹, (e) Nyquist plots of MS-H and MS-T, and (f) the relationship between Z′ and ω^−1/2 in the low-frequency region.