Spontaneous Decoration of Ultrasmall Pt Nanoparticles on Size-Separated MoS2 Nanosheets.

Liquid exfoliation can be considered as a viable approach for the scalable production of 2D materials due to its various benefits, although the polydispersity in the obtained nanosheet size hinders their straightforward incorporation. Size-separation can help alleviate these concerns, however a correlation between nanosheet size and property needs to be established to bring about size-specific applicability. Herein, size-selected aqueous nanosheet dispersions have been obtained via centrifugation-based protocols, and their chemical activity in the spontaneous reduction of chloroplatinic acid is investigated. Growth of ultrasmall Pt nanoparticles was achieved on nanosheet surfaces without a need for reducing agents, and stark differences in the nanoparticle coverage were observed as a function of nanosheet size. Defects in the nanosheets were probed via Raman spectroscopy, and correlated to the observed size-activity. Additionally, the effect of reaction temperature during synthesis was investigated. The electrochemical activity of the ultrasmall Pt nanoparticle decorated MoS2 nanosheets was evaluated for the hydrogen evolution reaction, and enhancement in performance was observed with nanosheet size, and nanoparticle decoration density. These findings shine light on the significance of nanosheet size in controlling spontaneous reduction reactions, and provide a deeper insight to intrinsic properties of liquid exfoliated nanosheets.

Additive-free Aqueous Dispersions of Two-Dimensional Materials with Glial Cell Compatibility and Enzymatic Degradability.

Water-dispersible two-dimensional (2D) materials are desirable for diverse applications. Aqueous dispersions make processing safer and greener and enable evaluation of these materials on biological and environmental fronts. To evaluate the effects of 2D materials with biological systems, obtaining dispersions without additives is critical and has been a challenge. Herein, a method was developed for obtaining additive-free aqueous dispersions of 2D materials like transition metal dichalcogenides and hexagonal boron nitride (h-BN). The nanosheet dispersions were investigated through spectroscopic and microscopic methods, along with the role of size on stability. The aqueous media enabled investigations on cytocompatibility and enzymatic degradation of molybdenum disulphide (MoS2 ) and h-BN. Cytocompatibility with mixed glial cells was observed up to concentrations of 100 µg mL-1 , suggesting their plausible usage in bioelectronics. Besides, biodegradation using human myeloperoxidase (hMPO) mediated catalysis was investigated through Raman spectroscopy and electron microscopy. The findings suggested that additive-free 2H-MoS2 and h-BN were degradable by hMPO, with 2H-phase exhibiting better resistance to degradation than the 1T-phase, while h-BN exhibited slower degradation. The findings pave a path for incorporating 2D materials in the burgeoning field of transient bioelectronics.

Solution-based electrostatic self-assembly route for obtaining graphene-transition metal dichalcogenide heterostructures.

Heterostructures of 2D materials have provided splendid insights into fundamental phenomena and are also promising for numerous applications. However, the protocols for obtaining them remain highly specific and lack scalability. Herein, the demonstrated protocol employs surfactant-assisted exfoliation and centrifugation-based size-selection of nanosheets for synthesizing heterostructures through electrostatic self-assembly.

Spontaneous formation of gold nanoparticles on MoS2 nanosheets and its impact on solution-processed optoelectronic devices.

Understanding size-dependent properties of 2D materials is crucial for their optimized performance when incorporated through solution routes. In this work, the chemical nature of MoS2 as a function of nanosheet size is investigated through the spontaneous reduction of chloroauric acid. Microscopy studies suggest higher gold nanoparticle decoration density in smaller nanosheet sizes, resulting from higher extent of reduction. Further corroboration through surface-enhanced Raman scattering using the gold-decorated MoS2 nanosheets as substrates exhibited an enhancement factor of 1.55 × 106 for smaller nanosheets which is 7-fold higher as compared to larger nanosheets. These plasmonic-semiconductor hybrids are utilized for photodetection, where decoration is found to impact the photoresponse of smaller nanosheets the most, and is optimized to achieve responsivity of 367.5 mAW-1 and response times of ∼17 ms. The simplistic modification via solution routes and its impact on optoelectronic properties provides an enabling platform for 2D materials-based applications.

Highly concentrated and stabilizer-free transition-metal dichalcogenide dispersions in low-boiling point solvent for flexible electronics.

Liquid-phase exfoliation has provided an efficient and scalable route to obtain dispersions of layered materials. Dispersions in low-boiling solvents facilitate the ease of processing; however, the challenge of obtaining them at high concentrations still prevails. Herein, the use of 2-butanone (B.P. 80 °C) as an effective solvent for the exfoliation of transition-metal dichalcogenides is reported for the first time. Among these, MoS2 was studied in detail to maximize the dispersion concentrations, reaching values up to 5.5 mg ml-1 without the use of any stabilizer. This exceptional efficiency of 2-butanone to exfoliate and stabilize the dispersions at high concentrations enabled the size separation of nanosheets by liquid cascade centrifugation. Extensive characterization by spectroscopic and microscopic techniques revealed the efficacy of the proposed process in separating mono- and few-layers. To showcase the utility of this low-boiling point solvent, a flexible photodetector was fabricated by spray-coating the dispersions on a polyethylene terephthalate substrate. The device exhibited a fast response time (<50 ms) and 80% retention in responsivity after 1000 flexing cycles. The present study suggests that molecular interactions between the solvent and nanosheet could play a critical role in the achievement of high efficiencies and provide an additional aspect to consider in solvent selection, along with the Hansen solubility parameters.