Using light, X-rays and electrons for evaluation of the nanostructure of layered materials

Componential and molecular-weight-dependent effects of natural organic matter on the colloidal behavior, transformation, and toxicity of MoS2 nanoflakes

The potential widespread applications in water processing have rendered the necessity for investigations of the fate and hazard of molybdenum disulfide (MoS2) nanosheets. Herein, it was found that humic acid (HA) had better performances toward stabilizing pure 2H phase MoS2 and chemical-exfoliated MoS2 (ce-MoS2) in electrolyte solutions than fulvic acid (FA), and molecular weight (MW)-dependent manners were disclosed due to steric repulsions. Compared with darkness, the extent to which the aggregation and sedimentation of ce-MoS2 facilitated by visible light irradiation was greater in the presence of HA and FA fractions, likely due to the introduction of stronger plasmonic dipole-dipole interaction and Van der Waals attraction forces. HA-triggered structural disintegration of nanosheets was performed after irradiation and it was observed to be more significant with the increase in MWs, whereas the MW-dependent dissolution of MoS2 caused by FA was much quicker than that by HA owing to the higher generation of singlet oxygen. Moreover, FA lowered the bioavailability of MoS2 and relieved its toxicity to zebrafish more effectively than HA. Our findings boost the insights into the effects of organic molecules on the fates and hazards of MoS2, providing guidance for the MoS2-based nanotechnological development on environment.

Molecular stabilization of chemically exfoliated bare MnPS3 layers

Transition metal chalcogenophosphates of general formula MPX₃ have attracted recent interest in the field of 2D materials due to the possibility of tuning their properties upon reaching the 2D limit. Several works address this challenge by dry mechanical exfoliation. However, only a few of them use a scalable approach. In this work, we apply a general chemical protocol to exfoliate MnPS₃. The method employs in the first step chemical intercalation and liquid phase exfoliation and in the second step the addition of molecules used as capping agents on the inorganic layers. Therefore, molecules of different nature prompt the quality of the exfoliated material and its stabilization in an aqueous solution, opening the possibility of using these functionalized layers in several fields. Here we illustrate this possibility in electrochemistry. Thus, we show that when polyethylenimine is used as the capping agent, it is possible to reach a compromise between the stability of high quality MnPS₃ flakes in aqueous suspension and their optimum performance as an electrocatalytic system for HER activity.

Sulfur vacancies affect the environmental fate, corona formation, and microalgae toxicity of molybdenum disulfide nanoflakes

Sulfur vacancy (SV) defects have been engineered in two-dimensional (2D) transition metal dichalcogenides (TMDs) for high performance applications in various fields involving environmental protection. Understanding the influence of SVs on the environmental fate and toxicity of TMDs is critical for evaluating their risk. Our work discovered that SVs (with S/Mo ratios of 1.65 and 1.32) reduced the dispersibility and promoted aggregation of 2H phase molybdenum disulfide (2H-MoS₂, a hot TMD) in aqueous solution. The generation capability of •O₂^- and •OH was increased and the dissolution of 2H-MoS₂ was significantly accelerated after SVs formation. Different with pristine form, S-vacant 2H-MoS₂ preferentially harvested proteins (i.e., forming protein corona) involved in antioxidation, photosynthetic electron transport, and the cytoskeleton structure of microalgae. These proteins contain a higher relative number of thiol groups, which exhibited stronger affinity to S-vacant than pristine 2H-MoS₂, as elucidated by density functional theory calculations. Notably, SVs aggravated algal growth inhibition, oxidative damage, photosynthetic efficiency and cell membrane permeability reduction induced by 2H-MoS₂ due to increased free radical yield and the specific binding of functional proteins. Our findings provide insights into the roles of SVs on the risk of MoS₂ while highlighting the importance of rational design for TMDs application.

A reversible and stable doping technique to invert the carrier polarity of MoTe2

Two-dimensional (2D) materials can be implemented in several functional devices for future optoelectronics and electronics applications. Remarkably, recent research on p-n diodes by stacking 2D materials in heterostructures or homostructures (out of plane) has been carried out extensively with novel designs that are impossible with conventional bulk semiconductor materials. However, the insight of a lateral p-n diode through a single nanoflake based on 2D material needs attention to facilitate the miniaturization of device architectures with efficient performance. Here, we have established a physical carrier-type inversion technique to invert the polarity of MoTe₂-based field-effect transistors (FETs) with deep ultraviolet (DUV) doping in (oxygen) O₂and (nitrogen) N₂gas environments. A p-type MoTe₂nanoflake transformed its polarity to n-type when irradiated under DUV illumination in an N₂gaseous atmosphere, and it returned to its original state once irradiated in an O₂gaseous environment. Further, Kelvin probe force microscopy (KPFM) measurements were employed to support our findings, where the value of the work function changed from ∼4.8 and ∼4.5 eV when p-type MoTe₂inverted to the n-type, respectively. Also, using this approach, an in-plane homogeneous p-n junction was formed and achieved a diode rectifying ratio (I_f/I_r) up to ∼3.8 × 10⁴. This effective approach for carrier-type inversion may play an important role in the advancement of functional devices.

Site-Selective Oxidation of Monolayered Liquid-Exfoliated WS2 by Shielding the Basal Plane through Adsorption of a Facial Amphiphile

In recent years, various functionalization strategies for transition‐metal dichalcogenides have been explored to tailor the properties of materials and to provide anchor points for the fabrication of hybrid structures. Herein, new insights into the role of the surfactant in functionalization reactions are described. Using the spontaneous reaction of WS₂ with chloroauric acid as a model reaction, the regioselective formation of gold nanoparticles on WS₂ is shown to be heavily dependent on the surfactant employed. A simple model is developed to explain the role of the chosen surfactant in this heterogeneous functionalization reaction. The surfactant coverage is identified as the crucial element that governs the dominant reaction pathway and therefore can severely alter the reaction outcome. This study shows the general importance of the surfactant choice and how detrimental or beneficial a certain surfactant can be to the desired functionalization.

Dissolved Oxygen and Visible Light Irradiation Drive the Structural Alterations and Phytotoxicity Mitigation of Single-Layer Molybdenum Disulfide

Understanding environmental fate is a prerequisite for the safe application of nanoparticles. However, the fundamental persistence and environmental transformation of single-layer molybdenum disulfide (SLMoS₂, a 2D nanosheet attracting substantial attention in various fields) remain largely unknown. The present work found that the dissolution of SLMoS₂ was pH and dissolved oxygen dependent and that alterations in phase composition significantly occur under visible light irradiation. The 1T phase was preferentially oxidized to yield soluble species (MoO₄^2- and SO₄^2-), and the 2H phase remained as a residual. The transformed SLMoS₂ exhibited a ribbon-like and multilayered structure and low colloidal stability due to the loss of surface charge. Dissolved oxygen competitively captured the electrons of SLMoS₂ to generate superoxide radicals and accelerated the dissolution of nanosheets. Compared to pristine 1T-phase SLMoS₂, the transformed 2H-phase SLMoS₂ could not easily enter algal cells and induced a low developmental inhibition, oxidative stress, plasmolysis, photosynthetic toxicity and metabolic perturbation. The downregulation of amino acids and upregulation of unsaturated fatty acids contributed to the higher toxicity of 1T-phase SLMoS₂. The dissolved ions did not induce apparent phytotoxicity. The connections between environmental transformation (phase change and ion release) and phytotoxicity provide insights into the safe design and evaluation of 2D nanomaterials.