Jibin Pu

Highly hydrophobic and adhesive performance of graphene films

Graphene films with high hydrophobic and adhesive performance were fabricated via two simple steps: chemical exfoliation of natural flake graphite following redox, and film formation by suction filtration without any chemical modification. Irregularly stacked multilayer graphene nanosheets comprised the microstructure, whereas folding and agglomeration of graphene nanoflakes with few layers comprised the nanostructure. The films also showed remarkable surface wettability and reversible transition from hydrophobicity to hydrophilicity via periodic alternation of ultraviolet irradiation and air storage. Based on Wenzels theory and adsorption dynamics, an optimum mechanism is proposed for the surface wettability behavior. On the one hand, the film microstructure and nanostructure enhance the graphene surface hydrophobicity. On the other hand, the capillary force is maximized by the nanostructure such that water fills the grooves of the rough solid surface. This result is a strong interaction between water and the film surface giving highly adhesive property to the films. The highly hydrophobic and adhesive performance of the graphene films could be useful in the device and biomaterials application.

Novel DLC/ionic liquid/graphene nanocomposite coatings towards high-vacuum related space applications

It is currently a challenge for space tribology to develop a long lifetime and high bearing capacity lubricant meeting the requirements of space applications. Herein, we dispersed graphene into ionic liquid, prepared novel composite coatings of diamond-like carbon (DLC)/ionic liquid (IL)/graphene with different graphene concentrations, and investigated its space performance under high vacuum and space radiation conditions. IL/graphene nanofluids with different concentrations were examined by Fourier transform infrared spectroscopy (FTIR). Furthermore, IL/graphene nanofluids after friction tests were investigated by X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HRTEM). The results showed that the graphene concentration would obviously affect the spatial tribology performance of the composite coatings. Because the excess graphene in the IL would tend to form irreversible agglomerates, leading to reduction of the effective graphene dose, an optimum graphene concentration (0.075 mg ml−1) in IL for the composite coatings was required to exhibit the lowest friction coefficient, the highest bearing capacity and the strongest anti-irradiation in a simulated space environment. In addition, XPS spectra further confirmed that the formation of a fluorinated oil-containing carbon-rich tribofilm between the friction pairs further ensured the good antifriction and wear resistance performance of DLC/IL/graphene.

Synergistic Effect of Hybrid Carbon Nanotube–Graphene Oxide as Nanoadditive Enhancing the Frictional Properties of Ionic Liquids in High Vacuum

A remarkable synergetic effect between the graphene oxide (GO) layers and multiwalled carbon nanotubes (MWCNTs) in improving friction and wear on sliding diamond-like carbon (DLC) surfaces under high vacuum condition (10(-5) Pa) and low or high applied load is demonstrated. In tests with sliding DLC surfaces, ionic liquid solution that contains small amounts of GO and MWCNTs exhibited the lowest specific friction coefficient and wear rate under all of the sliding conditions. Optical microscope images of the wear scar of a steel ball showed that GO/MWCNT composites exhibited higher antiwear capability than individual MWCNTs and GO did. Transmission electron microscopy images of nanoadditives after friction testing showed that MWCNTs support the GO layers like pillars and prevent assembly between the GO layers. Their synergistic effect considerably enhances IL-GO/MWCNT composites.

Interface Architecture for Superthick Carbon-Based Films toward Low Internal Stress and Ultrahigh Load-Bearing Capacity

Superthick diamond-like carbon (DLC) films [(Six-DLC/Siy-DLC)n/DLC] were deposited on 304 stainless steel substrates by using a plane hollow cathode plasma-enhanced chemical vapor deposition method. The structure was investigated by scanning electron microscopy and transmission electron microscopy. Chemical bonding was examined by Raman, Auger electron, and X-ray photoelectron spectroscopy techniques. Mechanical and tribological properties were evaluated using nanoindentation, scratch, interferometry, and reciprocating-sliding friction testing. The results showed that implantation of a silicon ion into the substrate and the architecture of the tensile stress/compressive stress structure decreased the residual stress to almost 0, resulting in deposition of (Six-DLC/Siy-DLC)n/DLC films with a thickness of more than 50 μm. The hardness of the film ranged from 9 to 23 GPa, and the adhesion strength ranged from 4.6 to 57 N depending on the thickness of the film. Friction coefficients were determined in three tested environments, namely, air, water, and oil. Friction coefficients were typically below 0.24 and as low as 0.02 in a water environment. The as-prepared superthick films also showed an ultrahigh load-bearing capacity, and no failure was detected in the reciprocating wear test with contact pressure higher than 3.2 GPa. Reasons for the ultrahigh load-bearing capacity are proposed in combination with the finite-element method.

Controlled water adhesion and electrowetting of conducting hydrophobic graphene/carbon nanotubes composite films on engineering materials

Based on the microbumps of graphene nanosheets and the nanostructure of carbon nanotubes (CNTs), novel graphene/CNTs composite films with hierarchical micro- and nanoscale surface roughness were successfully fabricated by simply spraying the mixed acetone dispersion of graphene nanosheets and CNTs onto stainless steel substrates. The as-prepared composite films exhibited controlled surface hydrophobic, adhesive and electrowetting properties via altering the film surface structure and surface energy. Among them the composite film with a 1:5 mass ratio of graphene to CNTs showed high hydrophobicity and conductivity, low water adhesion and contact angle sensitivity to the external electric field, which would help to resolve the surface electrostatic problems and unstable hydrophobicity under applied potential that exist in many conventional insulating hydrophobic materials, and could be useful in some application fields.

Study of the Conductivity and Tribological Performance of Ionic Liquid and Lithium Greases

Ionic liquid (IL) lubricating greases were prepared using 1-hexyl-3-methylimidazolium tetrafluoroborate and 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl) amide as base oil and polytetrafluoroethylene (PTFE) as thickener, respectively. Three kinds of lithium greases were also prepared using lithium ILs ([Li(PAG)]X) as base oil and PTFE as thickener. 1-Ethyl-3-methyl imidazolium hexafluorophosphate as an additive was added to the PAG grease, which was prepared using polyalkylene glycol monobutyl ether (PAG) as base oil and PTFE as thickener. The conductivities and tribological properties of the prepared lubricating greases were investigated in detail. Scanning electron microscopy and X-ray photoelectron spectroscopy were employed to explore the friction and wear mechanism. The results showed that the IL and lithium lubricating greases have conductivities and excellent tribological properties. Especially, IL greases have the highest conductivity. The excellent tribological properties are attributed to the formation of boundary films consisting of both tribo-chemical reaction films and physical absorption films, while high conductivities are attributed to the intrinsic electric fields of the ILs.

Preparation and characterization of ultrathin dual-layer ionic liquid lubrication film assembled on silica surfaces

A novel ultrathin dual-layer film, which contained both bonded and mobile phases in ionic liquids (ILs) layer, was fabricated successfully on a silicon substrate modified by a self-assembled monolayer (SAM). The formation and surface properties of the films were analyzed using ellipsometer, water contact angle meter, attenuated total reflectance Fourier transform infrared spectroscopy, multi-functional X-ray photoelectron spectroscopy, and atomic force microscope. Meanwhile, the adhesive and nanotribological behaviors of the films were evaluated by a homemade colloidal probe. A ball-on-plate tribometer was used to evaluate the microtribological performances of the films. Compared with the single-layer ILs film deposited directly on the silicon surface, the as-prepared dual-layer film shows the improved tribological properties, which is attributed to the special chemical structure and outstanding physical properties of the dual-layer film, i.e., the strong adhesion between bonded phase of ILs and silicon substrate via the chemical bonding with SAM, the interlinked hydrogen bonds among the molecules, and two-phase structure composed of steady bonded phase with load-carrying capacity and flowable mobile phase with self-replenishment property.

Supercapacitors based on graphene nanosheets using different non-aqueous electrolytes

In this study, the electrochemical properties of graphene nanosheet (GNS) electrodes are evaluated in depth by cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) and electrochemical impedance spectroscopy (EIS) techniques in [Et4N]BF4/acetonitrile electrolyte, [BPy]BF4/acetonitrile electrolyte, [BMIM]BF4/acetonitrile electrolyte and [P4,4,4,4]BF4/acetonitrile electrolyte, respectively. The electrochemical results exhibit that GNSs show good supercapacitive properties in these aforementioned four non-aqueous electrolytes, especially in [Et4N]BF4/acetonitrile electrolyte. It is also observed that the rate performance and the specific capacitance of GNS electrode increase in the order of [P4,4,4,4]BF4/acetonitrile < [BMIM]BF4/acetonitrile ≈ [BPy]BF4/acetonitrile < [Et4N]BF4/acetonitrile in these four non-aqueous electrolytes. The reasons are attributed to the difference of the relative ionic size and the discrepancy in the functional group among these four non-aqueous electrolytes, which result in the differences of equivalent series resistance, charge transfer resistance, and rate performance. In addition, the GNS electrode shows excellent stability in these four non-aqueous electrolytes after 1500 repeating charge–discharge cycles. These results may provide valuable information to explore new electrolytes and illustrate the exciting potential for high performance supercapacitors based on GNSs.

Corrosion and tribocorrosion performance of multilayer diamond-like carbon film in NaCl solution

The anticorrosion and tribocorrosion properties of a multilayer diamond-like carbon (DLC) film were systematically investigated in NaCl solution. Electrochemical measurements suggest that the corrosion performance of the multilayer DLC film is superior to those of the substrate and single layer DLC film in NaCl solution, which is attributed to the successively multilayered structure with a well-bonded interface and the formation of Si oxides. An extremely high Warburg impedance value, higher than 107 Ω cm2, of the multilayer DLC film has been observed. Tribocorrosion tests show that the multilayer DLC film presents lower wear rate in NaCl solution, with the substrate and single layer DLC film as comparisons. We demonstrate that the multilayer DLC film is an excellent protective material for improving both corrosion and wear performance of the substrate.

Precise Positioning of Lubricant on a Surface Using the Local Anodic Oxide Method

This letter describes a new method for the precise positioning of a lubricant on a surface. The nanometer-sized patterns are first fabricated using the atomic force microscopy (AFM)-based local anodic oxide (LAO) method. Multiply alkylated cyclopentanes (MACs) serving as lubricant matrix layers on nanopatterns of silicon dioxide are fabricated. By controlling the velocity of pull-off and solution conditions, we selectively immobilize MACs on the patterned areas using the dip-coating method. In our study, AFM is used for both fabrication and characterization. AFM-LAO allows the fabricated patterns to be altered in situ without the need to change masks or repeat the entire fabrication process. Furthermore, the nanotribological characterization of lubricant matrix layers on the nanopatterns was investigated with a colloidal probe.