Electric contributions to magnetic force microscopy response from graphene and MoS2 nanosheets

Magnetic force microscopy (MFM) signals have recently been detected from whole pieces of mechanically exfoliated graphene and molybdenum disulfide (MoS2) nanosheets, and magnetism of the two nanomaterials was claimed based on these observations. However, non-magnetic interactions or artefacts are commonly associated with MFM signals, which make the interpretation of MFM signals not straightforward. A systematic investigation has been done to examine possible sources of the MFM signals from graphene and MoS2 nanosheets and whether the MFM signals can be correlated with magnetism. It is found that the MFM signals have significant non-magnetic contributions due to capacitive and electrostatic interactions between the nanosheets and conductive cantilever tip, as demonstrated by electric force microscopy and scanning Kevin probe microscopy analyses. In addition, the MFM signals of graphene and MoS2 nanosheets are not responsive to reversed magnetic field of the magnetic cantilever tip. Therefore, the observed MFM response is mainly from electric artefacts and not compelling enough to correlate with magnetism of graphene and MoS2 nanosheets.

In-situ and tunable nitrogen-doping of MoS2 nanosheets.

Molybdenum disulfide (MoS2) nanosheets have unique physical and chemical properties, which make it a perfect candidate for next generation electronic and energy storage applications. Herein, we show the successful synthesis of nitrogen-doped MoS2 nanosheets by a simple, effective and large-scale approach. MoS2 nanosheets synthesised by this method show a porous structure formed by curled and overlapped nanosheets with well-defined edges. Analysis of the nanosheets shows that they have an enlarged interlayer distance and high specific surface area. X-ray photoelectron spectroscopy analysis shows the nanosheets have Mo-N bond indicating successful nitrogen doping. The nitrogen content of the product can be modulated by adjusting the ratio of starting materials easily within the range from ca. 5.8 to 7.6 at%.

Nanopatterning and electrical tuning of MoS2 layers with a subnanometer helium ion beam

We report subnanometer modification enabled by an ultrafine helium ion beam. By adjusting ion dose and the beam profile, structural defects were controllably introduced in a few-layer molybdenum disulfide (MoS2) sample and its stoichiometry was modified by preferential sputtering of sulfur at a few-nanometer scale. Localized tuning of the resistivity of MoS2 was demonstrated and semiconducting, metallic-like, or insulating material was obtained by irradiation with different doses of He(+). Amorphous MoSx with metallic behavior has been demonstrated for the first time. Fabrication of MoS2 nanostructures with 7 nm dimensions and pristine crystal structure was also achieved. The damage at the edges of these nanostructures was typically confined to within 1 nm. Nanoribbons with widths as small as 1 nm were reproducibly fabricated. This nanoscale modification technique is a generalized approach that can be applied to various two-dimensional (2D) materials to produce a new range of 2D metamaterials.

Study on the influence of li-grease EP and AW performance by exfoliated MoS2 nanosheet additive

Tribological behavior of de-agglomerated and exfoliated active molybdenum disulfide (MoS2) nanosheets as additives in lithium based grease was investigated. MoS2 nanosheets were prepared by mechano-chemical process in a planetary ball mill with selective organic molecules particularly lecithin (a source of phosphorus (exfoliating/stabilizing agent)) along with antiwear additives (ZDDP). Tribological evolutions show significant influence of MoS2 nanosheets on the friction coefficient, WSD (wear scar diameter) and extreme pressure properties of lithium based grease. The elemental composition analysis of the wear track shows the presence of Mo, S, and P on the surface protective layer, revealing the formation of a tribofilm containing MoS2 nanosheets along with phosphorus based moieties.

Toward design of synergistically active carbon-based catalysts for electrocatalytic hydrogen evolution

Replacement of precious Pt catalyst with cost-effective alternatives would be significantly beneficial for hydrogen production via electrocatalytic hydrogen evolution reaction (HER). All candidates thus far are exclusively metallic catalysts, which suffer inherent corrosion and oxidation susceptibility during acidic proton-exchange membrane electrolysis. Herein, based on theoretical predictions, we designed and synthesized nitrogen (N) and phosphorus (P) dual-doped graphene as a nonmetallic electrocatalyst for sustainable and efficient hydrogen production. The N and P heteroatoms could coactivate the adjacent C atom in the graphene matrix by affecting its valence orbital energy levels to induce a synergistically enhanced reactivity toward HER. As a result, the dual-doped graphene showed higher electrocatalytic HER activity than single-doped ones and comparable performance to some of the traditional metallic catalysts.

Hydrogen evolution by a metal-free electrocatalyst

Electrocatalytic reduction of water to molecular hydrogen via the hydrogen evolution reaction may provide a sustainable energy supply for the future, but its commercial application is hampered by the use of precious platinum catalysts. All alternatives to platinum thus far are based on nonprecious metals, and, to our knowledge, there is no report about a catalyst for electrocatalytic hydrogen evolution beyond metals. Here we couple graphitic-carbon nitride with nitrogen-doped graphene to produce a metal-free hybrid catalyst, which shows an unexpected hydrogen evolution reaction activity with comparable overpotential and Tafel slope to some of well-developed metallic catalysts. Experimental observations in combination with density functional theory calculations reveal that its unusual electrocatalytic properties originate from an intrinsic chemical and electronic coupling that synergistically promotes the proton adsorption and reduction kinetics.

Study and exploration on inorganic nanomaterials as additive in lubricant application.

 In this thesis, the application of planetary ball milling for the efficient production of nanomaterials is systematically studied. Three inorganic materials: calcium carbonate (CaCO3), molybdenum disulphide (MoS2) and hexagonal-boron nitride (h-BN) are chosen as model systems.

Advanced N-doped mesoporous molybdenum disulfide nanosheets and the enhanced lithium-ion storage performance

With the increasing interest in two-dimensional van der Waals materials, molybdenum disulfide (MoS2) has emerged as a promising material for electronic and energy storage devices. It suffers from poor cycling stability and low rate capability when used as an anode in lithium ion batteries. Here, N-doped MoS2 nanosheets with 2-8 atomic layers, increased interlayer distance, mesoporous structure and high surface area synthesised by a simple sol-gel method show an enhanced lithium storage performance, delivering a high reversible capacity (998.0 mA h g-1, 50 mA g-1), high rate performance (610 mA h g-1, 2 A g-1), and excellent cycling stability. The excellent lithium storage performance of the MoS2 nanosheets might be due to the better electrical and ionic conductivity and improved lithium ion diffusion which are related to their structural characteristics and high concentration N doping. The possible mechanism of the improved performance is proposed and discussed.