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

Abstract

The production of exfoliated MoS2 via lithium intercalation has been widely used to prepare 1T polytype dominated MoS2 (1T-MoS2) monolayers. These metallic single layers hold promise as high-performance electrodes for various electrochemical applications as well as for direct conversion to the semiconducting 2H polytype (2H-MoS2), a material of significant interest for next-generation electronics and optoelectronics. The performance in these applications is largely determined by defects introduced during processing. In this work, we systematically investigate the degradation rate and products resulting from the aging of aqueous MoS2 dispersions, obtained by chemical exfoliation under a variety of common processing conditions. Depending on the size and number of defects initially present in the material, the resulting MoS2 is found to have a surprisingly short half-life of only 2 to 6 days under alkaline conditions and exposure to both light and air. By aging samples under various environments and analyzing by UV–vis, Raman, Fourier-transform infrared, and X-ray photoelectron spectroscopies, we demonstrate that light-induced generation of superoxide by MoS2 is largely responsible for its oxidation (both 1T and 2H polytypes are affected equally). This process is accelerated under alkaline conditions because of the solubility of MoO3, which we suggest would otherwise passivate defects and edge sites. The soluble molybdates are found to be a strong indicator of oxidation, which can easily be followed by measuring the absorption at 209 nm using UV–vis spectroscopy. We hope that this work will help guide researchers in minimizing or controlling degradation, which will accelerate the successful use of MoS2 in applications.

Intrinsic Capacitance of Molybdenum Disulfide

“…The XPS results also indicate the presence of MoO 2 and MoO 3 which comprise 5.72% of the bulk MoS 2 powder but are also formed during processing when the material is exposed to light and oxygen via a mechanism recently reported by us . The untreated samples showed 22.4% MoO x content, which was reduced to 17.1% MoO x content following heat treatment at 150 °C and further reduced to 3.61% MoO x content at 300 °C.…”

“…The XPS results also indicate the presence of MoO 2 and MoO 3 which comprise 5.72% of the bulk MoS 2 powder but are also formed during processing when the material is exposed to light and oxygen via a mechanism recently reported by us. 37 The untreated samples showed 22.4% MoO x content, which was reduced to 17.1% MoO x content following heat treatment at 150 °C and further reduced to 3.61% MoO x content at 300 °C. Acid flocculation was suspected to have caused the increase in MoO x content, which was verified through XPS of MoS 2 nanosheets before and after acid treatment and can be found in the Supporting Information (Figure S3).…”

“…This oxygen reduction was found to cause a significant and irreversible increase in capacitance over the entire potential window likely caused by the reactive oxygen species generated (for example, H 2 O 2 ). The reactive oxygen species would then interact with the MoS 2 , forming either soluble molybdates and high activity edge sites or possibly pseudocapacitive MoO 3 . Because of this, a voltage window of −0.3 to 0.5 V was used to avoid damaging the films.…”

Unveiling the abnormal capacity rising mechanism of MoS2 anode during long-term cycling for sodium-ion batteries

“…According to Michael et al , MoS 2 is more likely to be present in the form of MoO 4 2+ rather than MoO 3 when oxidized. 48 In the ex situ XPS, we can clearly see a peak of Mo at 235.5 eV, again proving that some MoS 2 was oxidized during the cycle. Curiously, in the ex situ XPS for the samples discharging to 0.9 V and 0.5 V and charging to 1.5 V, we can see a clear peak at 228.0 eV, showing the existence of Mo, which is not consistent with the previous paper.…”

Phase-Dependent MoS2 Nanoflowers for Light-Driven Antibacterial Application

“…The superior light-driven antibacterial activity of 1T-MoS 2 NFs compared to 2H-MoS 2 NFs may be due to phase differences between the metallic and semiconducting phases. Previous reports demonstrated that metallic 1T-MoS 2 NFs produce photoelectrons under light irradiation by a photoelectronic effect. − The mechanism of the light-driven antibacterial activity of 1T-MoS 2 NFs appears to be that, with light irradiation, photoelectrons generated by metallic 1T-MoS 2 NFs further reacted with oxygen to generate superoxide ( • O 2 – ), as shown in Figure . Under light irradiation, the semiconducting 2H-MoS 2 NFs formed photoinduced electron–hole pairs.…”