Effects of MoS2 Layer Thickness on Its Photochemically Driven Oxidative Dissolution

Photoactivated defect engineering and nanostructure functionalization of MoS2via a photochemical Fenton process

Molybdenum disulfide (MoS2) is a promising 2D material for (photo)catalysis. However, its performance in (photo)catalytic applications is usually limited by a small amount of catalytically active defects. Here, we developed a novel large-scale, rapid, green, low-cost photoetching technique to transform multilayer MoS2 into a few-layer MoS2 with high defect density and simultaneous spatial functionalization of MoS2 with magnetic nanostructures using a photo-driven Fenton reaction. The photoetching process and resulting nanostructures were characterized by optical microscopy, atomic force microscopy, photoluminescence, and Raman spectroscopy. We elucidated the reaction mechanism driven by the Fenton reaction in which photogenerated charge carriers in MoS2 play a dual role: reducing Fe3+ and Cu2+ ions and generating hydrogen peroxide (H2O2) from water and dissolved O2. In this Fenton reaction, Fe2+ ions react with H2O2 to generate hydroxyl (˙OH) radicals, oxidizing MoS2 and forming metal oxide nanostructures at the reaction sites. This dual pathway, triggered by MoS2 photon absorption even at low-intensity illumination, ensures in situ generation of Fenton reactants (Fe2+ and H2O2), generating ˙OH, to achieve on-demand thinning and functionalization of MoS2 in a single step. Electron paramagnetic resonance spectroscopy confirmed the generation of ˙OH radicals as the main reactive oxygen species. This photochemical approach enables the photo-driven creation and growth of defects from submicrometer regions up to a dozen micrometers, both at native defects and predefined defective region seeds, by photochemical processing of MoS2 in FeCl3 and CuSO4 solutions. The presence of metal oxide nanostructures on MoS2 was verified using magnetic force microscopy, scanning electron microscopy with elemental mapping by energy dispersive X-ray spectroscopy and Raman spectroscopy. The simultaneous photoetching and metal oxide deposition improves the catalytic performance of MoS2 in the electrical hydrogen evolution reaction, evidenced by a potential shift from -0.7 V (graphite electrode) to -0.47 V (MoS2 sample photoetched in FeCl3 solution under a halogen lamp illumination) at a current density of 10 mA cm-2.

Augmented antibacterial performance of MoS2-integrated lignin-polyaniline composites through near-infrared stimulated photothermal and peroxidase-like activities

Photothermal synergistic antibacterial therapy has emerged as a compelling approach for addressing antibiotic-resistant infections. In this study, lignosulfonate (LS) served dual roles as both a scaffolding template and a functional modifier in the fabrication of the lignin-polyaniline composite (LS-PANI). Subsequently, molybdenum disulfide (MoS2) was integrated into LS-PANI through physical adsorption, resulting in the formation of MoS2@LS-PANI. The bactericidal assays demonstrated that MoS2@LS-PANI, at a concentration of 400 μg/mL, achieved a sterilization rate of 99.9 % against Escherichia coli and Staphylococcus aureus when exposed to near-infrared (NIR) radiation (808 nm, 1.8 W/cm2) for only 5 min in the presence of H2O2. Moreover, MoS2@LS-PANI disrupts bacterial membranes through physical contact while converting NIR energy into local heat and enhancing peroxidase-like activity, synergistically amplifying oxidative stress for effective pathogen elimination. The low-cost, facile synthesis and eco-friendly nature of MoS2@LS-PANI underscore its potential as an innovative approach in the development of lignin-derived inorganic nanocomposites for the highly efficient eradication of pathogenic bacteria.

Degradation of molybdenum disulfide through cascade reactions with hydrogen peroxide in aqueous system

Transformation is a crucial process determining the lifespan and risk of MoS2 nanomaterial during usage and after disposal. This study revealed the degradation of MoS2 in the presence of H2O2 using experimental and computational methods. Experimental results showed that MoS2 nanosheets were degraded by 45.1 % after 72-h incubation with H2O2. MoS2 decomposed H2O2 into various reactive oxygen species, among which ·O2- played a dominant role breaking MoS2 into fragments with defects and holes. Mo (IV) in MoS2 catalyzed ·O2- formation through electron transfer towards H2O2. Additionally, electrons generated from cleavage of O-O in H2O2 initiated the O2 reduction to generate ·O2-. The interaction of MoS2 with ·O2- yielded soluble MoO42- and SO42-, and 22.4 % of Mo on residual MoS2 was in the form of Mo (VI) after 72-h incubation. Density function theory calculations elucidated that·O2- is more potent than ·OH in adsorbing on MoS2 ( -2.25 eV vs. -0.14 eV) to initiate reaction. The reaction occurred preferentially from Mo and adjacent S atoms, which transferred 1.07 electrons toward ·O2- to induce O-O cleavage and formation of O-M and O-S bonds. The obtained finding on MoS2 degradation is fundamental for promoting sustainable applications and risk assessment of MoS2-based nanomaterials.

Preparation and application of wood-supported piezocatalyst with high efficiency and stability via partial hydrolysis of wood cellulose and hemicellulose with Lewis acid

The conveniently recoverable piezocatalyst with self-floating and stable performance has drawn wide attention. Herein, MoS2 was anchored on 1-cm-square eucalyptus wood blocks via a facile hydrothermal/solvothermal process to fabricate two floating piezocatalysts, i.e., MoS2/unpretreated wood (MUW) and MoS2/pretreated wood (MPW). FeCl3 solution was used as a Lewis acid to pretreat the wood through partial hydrolyzing cellulose and hemicellulose for an purpose to creat rich micropores for MoS2 loading in the wood and to form MoFe heterojunction. The piezocatalytic properties and performance of the prepared wood were systematically studied. The scanning electron microscopy confirms MoS2 was anchored on wood surface. The macroscopic photos show that MoS2 penetrated through the MPW interior whereas it was only loaded on the wood surface layer. The X-ray photoelectron spectroscopy reveals the shift of Mo 3d and S 2p, verifying the heterojunction formation of MPW. The Fourier transform infrared spectra prove the partial hydrolysis of wood matrix. In comparison to MUW, MPW had excellent piezocatalytic property, wide pH adaptability, convenient recyclability and high stability. Sildenafil and Cr6+ ions could be completely removed in 20 and 15 min, respectively, by MPW. Contrastly, the removal efficiency of sildenafil and Cr6+ by MUW was 78.6 % and 68.3 % in 20 and 15 min, respectively. After five cycles of use, the removal ratio of sildenafil was 62.4 % by MUW and 90.5 % by MPW in 20 min. The mineralization efficiency of sildenafil reached 99.2 % in 30 min by MPW, and various types of N/S-containing intermediates were effectively degraded. The electron spin resonance characterization and active species scavenging experiments displayed that e- and •O2- were major active species responsible for Cr6+ piezoreduction by MUW and MPW, while •O2- and •OH were the dominant species accounting for sildenafil degradation by MUW and MPW, respectively. And •OH was not generated in the MUW piezocatalysis process. MPW had higher piezoelectric current and lower resistance at the electron transfer interface than MUW. Conclusively, this study paves a new pathway for preparing new floating piezocatalysts with easy recyclability and high stability from biomass for wastewater treatment.

The colloidal stability of molybdenum disulfide nanosheets in different natural surface waters: Combined effects of water chemistry and light irradiation

With the increasing production and application, more molybdenum disulfide (MoS2) nanosheets could be released into environment. The aggregation and dispersion of MoS2 nanosheets profoundly impact their transport and transformation in the aquatic environment. However, the colloidal stability of MoS2 remains largely unknown in natural surface waters. This study investigated the colloidal stability of MoS2 nanosheets in six natural surface waters affected by both light irradiation and water chemistry. Compared to that of the pristine MoS2 nanosheets, the colloidal stability of MoS2 photoaged in ultrapure water declined. Light irradiation induced the formation of Mo-O bonds, the release of SO42- species, and the decrease in 1T/2H ratio, which reduced negative charge and enhanced hydrophobicity. However, the colloidal stability of MoS2 photoaged in natural surface waters was increased relative to that in ultrapure water not only for the smaller extent of photochemical transformation but more importantly the surface modification by water chemistry. Furthermore, the colloidal stability of MoS2 photoaged in natural surface waters followed the order of sea water > lake water > river water. The abundant cations (e.g., Ca2+ and Mg2+) in sea water facilitated the covalent grafting (S-C bonds) of more dissolved organic matter (DOM) on MoS2 via charge screening and cation bridging, thus inducing stronger electrostatic repulsion and steric effect to stabilize nanosheets. The crucial role of the covalent grafting of DOM was further confirmed by the positive correlation between the critical coagulation concentration values and S-C ratios (R2 = 0.82, p < 0.05). Our results highlighted the dominant role of water chemistry than light irradiation in dictating the colloidal stability of MoS2 photoaged in natural surface waters, which provided new insight into the environmental behavior of MoS2 in aquatic environment.

Calcium Alginate as an Active Device Component for Light-Triggered Degradation of 2D MoS2-Based Transient Electronics

Transient electronics technology has enabled the programmed disintegration of functional devices, paving the way for environmentally sustainable management of electronic wastes as well as facilitating the exploration of novel device concepts. While a variety of inorganic and/or organic materials have been employed as media to introduce transient characteristics in electronic devices, they have been mainly limited to function as passive device components. Herein, we report that calcium (Ca) alginate, a natural biopolymer, exhibits multifunctionalities of introducing light-triggered transient characteristics as well as constituting active components in electronic devices integrated with two-dimensional (2D) molybdenum disulfide (MoS2) layers. Ca2+ ions-based alginate electrolyte films are prepared through hydrolysis reactions and are subsequently incorporated with riboflavin, a natural photosensitizer, for the light-driven dissolution of 2D MoS2 layers. The alginate films exhibit strain-sensitive triboelectricity, confirming the presence of abundant mobile Ca2+ ions, which enables them to be active components of 2D MoS2 field-effect transistors (FETs) functioning as electrolyte top-gates. The alginate-integrated 2D MoS2 FETs display intriguing transient characteristics of spontaneous degradation upon ultraviolet-to-visible light illumination as well as water exposure. Such transient characteristics are demonstrated even in ambient conditions with natural sunlight, highlighting the versatility of the developed approach. This study emphasizes a relatively unexplored aspect of combining naturally abundant polymers with emerging near atom-thickness semiconductors toward realizing unconventional and transformative device functionalities.

Molybdenum Disulfide-Supported Cuprous Oxide Nanocomposite for Near-Infrared-I Light-Responsive Synergistic Antibacterial Therapy

Drug-resistant bacterial infections pose a serious threat to human health; thus, there is an increasingly growing demand for nonantibiotic strategies to overcome drug resistance in bacterial infections. Mild photothermal therapy (PTT), as an attractive antibacterial strategy, shows great potential application due to its good biocompatibility and ability to circumvent drug resistance. However, its efficiency is limited by the heat resistance of bacteria. Herein, Cu2O@MoS2, a nanocomposite, was constructed by the in situ growth of Cu2O nanoparticles (NPs) on the surface of MoS2 nanosheets, which provided a controllable photothermal therapeutic effect of MoS2 and the intrinsic catalytic properties of Cu2O NPs, achieving a synergistic effect to eradicate multidrug-resistant bacteria. Transcriptome sequencing (RNA-seq) results revealed that the antibacterial process was related to disrupting the membrane transport system, phosphorelay signal transduction system, oxidative stress response system, as well as the heat response system. Animal experiments indicated that Cu2O@MoS2 could effectively treat wounds infected with methicillin-resistant Staphylococcus aureus. In addition, satisfactory biocompatibility made Cu2O@MoS2 a promising antibacterial agent. Overall, our results highlight the Cu2O@MoS2 nanocomposite as a promising solution to combating resistant bacteria without inducing the evolution of antimicrobial resistance.

Photooxidative Behavior of Polystyrene Nanocomposites Filled with Two-Dimensional Molybdenum Disulfide

This study aimed to investigate how an ultralow content of a molybdenum disulfide (MoS₂) two-dimensional particle affects the photodegradation mechanism of polystyrene (PS). Here, an accelerated weathering study was presented on neat polystyrene and its nanocomposites produced with 0.001, 0.002, 0.003 and 0.005 wt% of molybdenum disulfide (MoS₂) exposed for various irradiation intervals (up to 8 weeks). The polymer photo-transformations were monitored using size exclusion chromatography (SEC), infrared spectroscopy (FTIR), and UV-Vis spectroscopy. The FTIR and UV/Vis results indicate that the PS degradation mechanism was not altered by the presence of MoS₂ particles; however, the degradation reactions were slowed down at higher MoS₂ contents (>0.003%). The SEC results proved the stabilizer effect due to MoS₂ particles, where M¯n, M¯w, and M¯w/M¯n values after 8 weeks were less modified when compared with the neat PS results. The MoS₂ acted as a UV stabilizer, and these two-dimensional particles acted by deactivating the free radicals generated by the PS matrix, even considering the low amount of the filler (<0.005 wt%).

In Situ Tracking of Crystal-Surface-Dependent Cu2O Nanoparticle Dissolution in an Aqueous Environment

Metal-oxide-based nanoparticles (MONPs) such as Cu₂O NPs have attracted growing attention, but the potential discharges of MONPs have raised considerable concern of their environmental fate including their dissolution behavior. The impacts of morphology on MONP dissolution are largely uncertain due to the lack of in situ tracking techniques. In this study, we combined a series of in situ technologies including liquid-cell transmission electron microscopy and fluorescence probes to reveal the in situ dissolution process of Cu₂O NPs in freshwater. Our results suggest that cubic Cu₂O NPs exhibit a higher dissolution quantity compared with spherical NPs of the same surface area. The difference was mainly related to the crystal surface, while other factors such as particle size or aggregation status showed minor effects. Importantly, we demonstrated the simultaneous growth of new small NPs and the dissolution of pristine Cu₂O NPs during the dissolution of Cu₂O NPs. Cubic Cu₂O NPs became much less soluble under O₂-limited conditions, suggesting that O₂ concentration largely affected the dependence of dissolution on the NP morphology. Our findings highlight the potential application of in situ techniques to track the environmental fates of MONPs, which would provide important information for assessing the ecological risks of engineered NPs.