Load-Independent Friction: MoO3 Nanocrystal Lubricants

The microstrain-accompanied structural phase transition from h-MoO3 to α-MoO3 investigated by in situ X-ray diffraction

In situ X-ray diffraction indicates that the structural phase transition from h-MoO₃ to α-MoO₃ is a first-order transition with a phase transition temperature range of 378.5-443.1 °C. The linear coefficients of thermal expansion of h-MoO₃ are strongly anisotropic, that is, α_a=b = 72.87 × 10^-6 K^-1 and α_c = -19.44 × 10^-6 K^-1. In the h-MoO₃ phase, water molecules are located at the (0 0 0.25) site inside the MoO₆ octahedra tunnel that is formed by six MoO₆ corner-sharing octahedron zigzag chains. With increasing temperature, the release of water molecules from the octahedra tunnel causes the octahedra chains to shrink and the octahedra tunnel to expand. When the phase transition occurs, the anomalous expansion of the MoO₆ octahedra tunnel ruptures the Mo-O₂ bonds, forming individual MoO₆ octahedron zigzag chains that then share corners to generate octahedron layers in the ⟨100⟩_α direction. The octahedron layers are bonded by van der Waals interactions in the ⟨010⟩_α direction, crystalizing into the α-MoO₃ structure.

Method of surface energy investigation by lateral AFM: application to control growth mechanism of nanostructured NiFe films

A new method for the specific surface energy investigation based on a combination of the force spectroscopy and the method of nanofriction study using atomic force microscopy was proposed. It was shown that air humidity does not affect the results of investigation by the proposed method as opposed to the previously used methods. Therefore, the method has high accuracy and repeatability in air without use of climate chambers and liquid cells. The proposed method has a high local resolution and is suitable for investigation of the specific surface energy of individual nanograins or fixed nanoparticles. The achievements described in the paper demonstrate one of the method capabilities, which is to control the growth mechanism of thin magnetic films. The conditions for the transition of the growth mechanism of thin Ni₈₀Fe₂₀ films from island to layer-by-layer obtained via electrolyte deposition have been determined using the proposed method and the purpose made probes with Ni coating.

Thickness-Insensitive Properties of α-MoO3 Nanosheets by Weak Interlayer Coupling

van der Waals (vdW) materials have shown unique electrical and optical properties depending on the thickness due to strong interlayer interaction and symmetry breaking at the monolayer level. In contrast, the study of electrical and tribological properties and their thickness-insensitivity of van der Waals oxides are lacking due to difficulties in the fabrication of high quality two-dimensional oxides and the investigation of nanoscale properties. Here we investigated various tribological and electrical properties, such as, friction, adhesion, work function, tunnel current, and dielectric constant, of the single-crystal α-MoO₃ nanosheets epitaxially grown on graphite by using atomic force microscopy. The friction of atomically smooth MoO₃ is rapidly saturated within a few layers. The thickness insensitivity of friction is due to very weak mechanical interlayer interaction. Similarly, work function (4.73 eV for 2 layers (hereafter denoted as L)) and dielectric constant (6 for 2L and 10.5-11 for >3L) of MoO₃ in MoO₃ showed thickness insensitivity due to weak interlayer coupling. Tunnel current measurements by conductive atomic force microscopy showed that even 2L MoO₃ of 1.4 nm is resistant to tunneling with a high dielectric strength of 14 MV/cm. The thickness-indifferent electrical properties of high dielectric constant and tunnel resistance by weak interlayer coupling and high crystallinity show a promise in the use of MoO₃ nanosheets for nanodevice applications.

Molybdenum Oxides - From Fundamentals to Functionality

The properties and applications of molybdenum oxides are reviewed in depth. Molybdenum is found in various oxide stoichiometries, which have been employed for different high-value research and commercial applications. The great chemical and physical characteristics of molybdenum oxides make them versatile and highly tunable for incorporation in optical, electronic, catalytic, bio, and energy systems. Variations in the oxidation states allow manipulation of the crystal structure, morphology, oxygen vacancies, and dopants, to control and engineer electronic states. Despite this overwhelming functionality and potential, a definitive resource on molybdenum oxide is still unavailable. The aim here is to provide such a resource, while presenting an insightful outlook into future prospective applications for molybdenum oxides.

Friction Force Microscopy Analysis of Self-Adaptive W-S-C Coatings: Nanoscale Friction and Wear

Transition metal dichalcogenides (TMD) are increasingly popular due to unique structural and mechanical properties. They belong, together with graphene and similar 2D materials, to a small family of solid lubricants with potential to produce ultralow friction state. At the macroscale, low friction stems from the ability to form well-oriented films on the sliding surface (typically up to 10 nm thick), with the TMD basal planes aligned parallel to the surface. In this study, we quantitatively evaluate tribological properties of three sputtered tungsten-sulfur-carbon (W-S-C) coatings at a nanoscale using friction force microscopy. In particular, we investigate possible formation of well-ordered tungsten disulfide (WS2) layers on the coating surface. The coefficient of friction decreased with increasing load independently of coating composition or mechanical properties. In contrast, hard coatings with high tungsten carbide content were more resistant to wear. We successfully identified a WS2 tribolayer at the sliding interface, which peeled off as ultrathin flakes and attached to AFM tip. Nanoscale tribological behavior of WSC coatings replicates deviation of Amonton's law observed in macroscale testing and strongly suggests that the tribolayer is formed almost immediately after the start of sliding.

Direct conversion of multilayer molybdenum trioxide to nanorods as multifunctional electrodes in lithium-ion batteries

In this study we prepared molybdenum trioxide (MoO3) nanorods having average lengths of 0.5-1.5 μm and widths of approximately 100-200 nm through a one-step mechanical break-down process involving favorable fracturing along the crystal direction. We controlled the dimensions of the as-prepared nanorods by applying various imposing times (15-90 min). The nanorods prepared over a reaction time of 90 min were, on average, much shorter and narrower relative to those obtained over 30 min. Evaluations of lithium-ion storage properties revealed that the electrochemical performance of these nanorods was much better than that of bulk materials. As cathodes, the nanorods could deliver a high specific capacity (>315 mA h g(-1)) with losses of less than 2% in the first cycle at a rate of 30 mA g(-1); as anodes, the specific capacity was 800 mA h g(-1) at a rate of 50 mA g(-1). Relative to α-MoO3 microparticles, these nanorods displayed significantly enhanced lithium-ion storage properties with higher reversible capacities and better rate performance, presumably because their much shorter diffusion lengths and higher specific surface areas allowed more-efficient insertion/deinsertion of lithium ions during the charge/discharge process. Accordingly, enhanced physical and/or chemical properties can be obtained through appropriate nanostructuring of materials.

Two stage chemical bath deposition of MoO3 nanorod films

h-MoO₃ nanorods are deposited on a FTO substrate by a two step chemical bath deposition method. The first stage of the deposition is the nucleation step followed by growth at the second stage.

Tuning electronic and magnetic properties of MoO3 sheets by cutting, hydrogenation, and external strain: a computational investigation

Density functional theory computations were performed to examine the electronic and magnetic properties of MoO3 two-dimensional (2D) nanosheets and their derived one-dimensional (1D) nanoribbons (NRs). The pristine 2D MoO3 sheet is a nonmagnetic semiconductor with an indirect band gap, but can be transformed to a magnetic metal when the surface O atoms are saturated by H. Depending on the cutting pattern, the pristine 1D NRs can be indirect band gap nonmagnetic semiconductors, magnetic semiconductors or magnetic metals. The fully hydrogenated NRs are metallic, while the edge-passivated NRs possess the nonmagnetic semiconducting feature, but with narrower band gap values compared to the pristine NRs. Both the 2D monolayer MoO3 sheet and the 1D nanoribbons maintain the semiconducting behaviors when exerting axial strain. These findings provide a simple and effective route to tune the magnetic and electronic properties of MoO3 nanostructures in a wide range and also facilitate the design of MoO3-based nanodevices.

Molybdenum Trioxide Nanostructures: The Evolution from Helical Nanosheets to Crosslike Nanoflowers to Nanobelts

MoO(3) nanostructures with different morphologies, such as helical nanosheets, crosslike nanoflowers, and nanobelts, have been synthesized on a large scale by an environmentally friendly chemical route. The evolution process from helical nanosheets to crosslike nanoflowers to nanobelts is observed for the first time. The influences of reaction time and the molar ratio of molybdenum and H(2)O(2) on the morphologies of MoO(3) nanostructures have been investigated. The synthetic process is environmentally friendly and may be extended to synthesize nanostructures of other metal (W, Ti, and Cr) oxides.

Vapor-Transportation Preparation and Reversible Lithium Intercalation/Deintercalation of α-MoO3 Microrods

We report on the preparation and electrochemical application of rechargeable lithium-ion batteries of alpha-MoO3 microrods. A simple and efficient vapor-transportation approach was developed to yield large-scale alpha-MoO3 microrods. The as-prepared products were present in long, uniform, rodlike structures with a diameter of approximately 2 approximately 6 microm, and the proportion of the rod morphology was about 95% according to the analysis of scanning electron microscopy (SEM). The electrochemical lithium intercalation/deintercalation characteristic of the as-prepared microrods was investigated by cyclic voltammetry and a galvanostatic charge-discharge method. The results showed that the alpha-MoO3 microrods exhibited high capacity (225 mAh g(-1)) and excellent cycling reversibility, and are thus promising cathode candidates in advanced rechargeable lithium-ion batteries. The correlation between the specific structural features of the microrods and their superior electrode performance is discussed in detail, revealing that the unique rodlike structure plays an important role in optimizing the electrochemical performance of the electrode.

Novel Multilamellar Mesostructured Molybdenum Oxide Nanofibers and Nanobelts: Synthesis and Characterization

One-dimensional molybdenum oxide nanostructures with layered mesostructures were prepared directly from commercial bulk MoO3 crystals by a surfactant-templated hydrothermal process. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, infrared spectra, and thermal analyses have been used to characterize the obtained molybdenum oxide nanomaterials. By use of cetyltrimethylammonium bromide as the structure-directing template, novel molybdenum oxide nanofibers with triple interlayer distances of 2.84, 2.66, and 2.46 nm have been obtained. The nanofibers have diameters of 20-100 nm and length up to 20 microm. The growth of multilamellar molybdenum oxide nanofibers can be interpreted by the combination of surfactant/inorganic self-assembly process and host/guest intercalation chemistry. On the basis of the X-ray diffraction and infrared results, a possible arrangement of surfactant in the interlayer space of molybdenum oxide by bilayer micelles with different tilt angles has been proposed. In addition, the thermal stability of surfactant has been improved by intercalation. Moreover, molybdenum oxide nanobelts with two kinds of interlayered structures were also produced in the presence of n-alkylamines (n = 12, 14, 16, and 18) following a similar method, these nanobelts show length up to more than 10 microm, width ranging between 200 and 600 microm, and width-to-thickness ratios of about 3-12. A linear relationship is observed between the interlayer distance and the number of carbon atoms in n-alkyl chains.

Superlubricity of graphite

Using a home-built frictional force microscope that is able to detect forces in three dimensions with a lateral force resolution down to 15 pN, we have studied the energy dissipation between a tungsten tip sliding over a graphite surface in dry contact. By measuring atomic-scale friction as a function of the rotational angle between two contacting bodies, we show that the origin of the ultralow friction of graphite lies in the incommensurability between rotated graphite layers, an effect proposed under the name of "superlubricity" [Phys. Rev. B 41, 11 837 (1990)]].

Complex alpha-MoO(3) nanostructures with external bonding capacity for self-assembly

Through manipulating crystal growth directions, we devised a versatile synthetic method to fabricate complex alpha-MoO(3) nanostructures with external bonding capacity for self-organization. Using four-armed forklike alpha-MoO(3) as nanobuilding blocks, we assembled more complex crystal morphologies, such as centrally holed nanorods, tridents, and paintbrushes. With prolonged ultrasonic treatments, pristine forklike alpha-MoO(3) crystals can be turned into less armed nanostructures, giving away the secondary arms (width < 100 nm) at the same time. On the other hand, the resultant alpha-MoO(3) itself can act as a template to produce shaped TiO(2) and other nanocrystals. Square- and horseshoe-shaped nanocrystals of anatase TiO(2) are left undissolved after removing alpha-MoO(3) templates in basic medium.