Insight into the structural and electronic nature of chemically exfoliated molybdenum disulfide nanosheets in aqueous dispersions

Abstract

Chemical exfoliation of molybdenum disulfide (2H-MoS) for preparing high-yield single-layer sheets has attracted considerable attention in recent years. However, the stability and nature of the resulting nanosheets are poorly understood. Storing the dispersion in ambient air brings about the reoxidation of the nanosheets, releasing their residual negative charges into the environment. The reoxidation facilitates lateral fractures and destabilizes the dispersion. In-plane X-ray diffraction of the nanosheets indicates that they have a 1T structure with a 2D √3 × 1 rectangular cell as the intrinsic structure for chemically exfoliated MoS. We found that the 1T structure was preserved after reoxidation upon aging the dispersions in air, suggesting the formation of metastable neutral MoS. The changes in the chemical nature of the nanosheets can be monitored by X-ray diffraction of the restacked nanosheets. The restacked nanosheets, obtained by drying the freshly prepared dispersion, exhibited an expanded bilayer hydrate structure, accommodating Li ions. On the other hand, dried samples from the aged dispersions were substantially composed of a deintercalated phase and the bilayer hydrate. Upon prolonged aging, the former phase became predominant with total disappearance of the latter. This evolution suggests that the reoxidation occurred sheet by sheet with a direct restoration of the original oxidation states of the nanosheets, whereas the oxidation states of the nanosheets can be discrete at 4+ and (4 - δ)+.

Chemical Insights into the Rapid, Light-Induced Auto-Oxidation of Molybdenum Disulfide Aqueous Dispersions

“…Continued degradation was observed in alkaline (LiOH, pH ≈ 11.5) samples, while precipitation of MoS 2 nanosheets (Figure S1 inset) was found in neutral (Milli-Q) cases with the supernatant displaying a light-yellow color, which later on turned blue. The precipitation phenomenon was observed and studied by Nurdiwijayanto et al It is also worth pointing out that strong absorption in the UV region was detected in the blue supernatant (Figure S1). Simply by preparing the fresh sample at a more diluted concentration, the precipitation effect can be reduced.…”

“…In 2015, Knirsch et al reported the observation of MoO 2 and MoO 3 formed in 1T-MoS 2 dispersion over the course of one to two months, coupled with the phase restoration from 1T to 2H . More recently, Nurdiwijayanto et al reported that 1T-MoS 2 sheets aggregated and restacked in aged aqueous dispersions where they considered the effect of Li contents and gaseous environment (oxygenated vs. oxygen-free dispersion) . According to their study, the resulting aggregates remained in the 1T polymorph but aggregated because of a continuous reduction in the magnitude of the zeta potential which destabilized the dispersions.…”

Monolayer Attachment of Metallic MoS2 on Restacked Titania Nanosheets for Efficient Photocatalytic Hydrogen Generation

“…The presence of 1T-phase MoS 2 on the restacked Ti 0.87 O 2 0.52− was confirmed by electron diffraction (ED) analysis (Figure 3c), as indicated by in-plane diffraction rings of 11 and 32 based on a plausible 2D √3 × 1 superstructure. 43,44 In contrast, high-resolution TEM only showed the lattice fringes from the restacked Ti 0.87 O 2 0.52− with an average interlayer spacing of ∼1 nm. We performed observations over 20 different locations and did not detect any restacked MoS 2 nanosheets, which should show a smaller spacing of ∼0.6 nm.…”

“…Exfoliation was achieved by treating the lithiated product with water under ultrasonication and then centrifuging at 2000 rpm to remove the unexfoliated crystals. The successful formation of the 1T-type MoS 2 nanosheets was confirmed as reported previously …”

“…The successful formation of the 1T-type MoS 2 nanosheets was confirmed as reported previously. 43 Monolayer Modification with MoS 2 . Attachment of 1T-phase MoS 2 nanosheets onto restacked-Ti 0.87 O 2 0.52− was conducted via a two-step flocculation process.…”