Wettability Of Graphene Research Articles

Temporospatial Control of Graphene Wettability

The reversible migration of adatoms along a basal plane of graphene under electrical bias is experimentally demonstrated. Single-layer graphene oxide with partial oxygen adatom coverage is utilized for this demonstration. The intensity ratio of G and G' Raman modes is used to determine the oxygen adatoms migration. Finally, reversible wettability property of graphene due to oxygen adatom migration is demonstrated.

Ultra-wetting graphene-based membrane

Carbon nanomaterials such as graphene and its derivatives based membranes in real applications are still far from reality due to their hydrophobic nature and limitations in their fabrication process. Here, we have devised a simple and feasible fabrication method to bring the high end nanocarbon based material to real downstream applications. In order to achieve this objective, the wettability of graphene was initially increased to an ultra-wetting level by incorporating amine and carboxyl functionality onto the graphene. The amine and carboxylated graphene is then covalently attached to a polymer matrix to fabricate a water filtration membrane. The characterization of the modified supported graphene-based membrane indicates that significantly higher hydrophilicity than previously expected is achieved, with the water contact angle reduced to zero. The ultra-wetting graphene increases the water permeability of the membrane by 126% without any changes in the selectivity. Based on our findings, we conclude that the ultra-wetting graphene will be an ideal material for new generation water filtration membranes.

Electro-mechanical performance of polyurethane dielectric elastomer flexible micro-actuator composite modified with titanium dioxide-graphene hybrid fillers

Interface interactions between nanoparticles and polymer matrix have crucial effects on microstructures and properties. We demonstrate that polyurethane dielectric elastomer filled with titanium dioxide functionalized graphene, fabricated as a flexible micro-actuator, displays evident electric stimulus response and electric field induced strain. The hydrophilicity and wettability of graphene can be greatly enhanced by decorating nano-TiO2 onto graphene, which causes the improved interface interactions between fillers and polymers. Higher dielectric constant, less loss modulus, elevated breakdown strength, and better electric stimulation responses of the novel titanium dioxide functionalized graphene–polyurethane dielectric elastomer composite are achieved due to enhanced filler–matrix interface. The maximum electric induced thickness strain of this titanium dioxide functionalized graphene–polyurethane dielectric elastomer composite is 72.4% at 38.7V/μm, which is 1.8 times of the electric induced strain of graphene–polyurethane composite.

Wettability of Graphene-Coated Surface: Free Energy Investigations Using Molecular Dynamics Simulation

A clear understanding of the wettability of graphene and graphene-coated surfaces is of critical importance for the practical applications of graphene. The present study provides microscopic and thermodynamic perspectives into the wettability of graphene-coated surfaces by molecular dynamics simulations along with free energy calculations utilizing the umbrella sampling. The water droplet adhesion process on graphene-coated surface was characterized by the change in surface area, mean force, and free energy of the droplet. The thermodynamic landscape analysis reveals that the different contributions to the free energies from different underlying substrates induce different entropic resistances from graphene, which leads to the similarity in wettability for graphene-coated silicon and hydroxylated silicon dioxide substrates.

Wetting and motion behaviors of water droplet on graphene under thermal-electric coupling field

Wetting dynamics and motion behaviors of a water droplet on graphene are characterized under the electric-thermal coupling field using classical molecular dynamics simulation method. The water droplet on graphene can be driven by the temperature gradient, while the moving direction is dependent on the electric field intensity. Concretely, the water droplet on graphene moves from the low temperature region to the high temperature region for the relatively weak electric field intensity. The motion acceleration increases with the electric field intensity on graphene, whereas the moving direction switches when the electric field intensity increases up to a threshold. The essence is the change from hydrophilic to hydrophobic for the water droplet on graphene at a threshold of the electric field intensity. Moreover, the driven force of the water droplet caused by the overall oscillation of graphene has important influence on the motion behaviors. The results are helpful to control the wettability of graphene and further develop the graphene-based fluidic nanodevices.

Elucidation of the wettability of graphene through a multi-length-scale investigation approach

Multi-length-scale approach was employed to attempt to reach univocal conclusions around the wettability of graphene exposed to environmental conditions.

A nanoscopic approach to studying evolution in graphene wettability

The equilibrium state of graphene surfaces exposed to ambient conditions is of significant importance for applications from electronics to anti-corrosion layers and from a fundamental perspective. The environmental exposure influences macroscopic properties of the graphene surface, as recent studies on wettability reported. However, these studies are controversial for two reasons: firstly, time dependency of graphene wettability complicates comparison of results from different groups. Secondly it’s inherently difficult to understand the underlying physical phenomena by means of macroscopic measurements alone. Here we study the evolution of the wettability of graphene monolayers on copper by exposing samples to controlled ambient conditions. Static contact angle measurements reveal that the graphene undergoes a transition in its wettability from slightly hydrophilic to hydrophobic over timescales of the order of hours. To gain further insight, we apply Fourier transform infrared spectroscopy and a recently developed dynamic atomic force microscopy-based force spectroscopy to probe the sample surface at the nanoscale. This novel nanoscopic approach allows us to demonstrate that the transition in wettability of graphene is induced by the adsorption of water molecules from the environment in combination with hydrocarbon contamination. Furthermore, our nanoscale observations challenge the wetting transparency theory of graphene after exposure to the environment.

Microwetting of Supported Graphene on Hydrophobic Surfaces Revealed by Polymerized Interfacial Femtodroplets

Understanding the wettability of graphene is the crucial step toward the design and control of graphene-based surface in contact with liquids. In this work, the static microwettability of a supported single layer graphene (SLG) immersed in water or alcoholic aqueous solutions is revealed by the morphological characterization of the polymerized interfacial femtoliter droplets. As expected, the contact angle of the femtoliter droplets on the SLG in water is in between that on the underlying silanized silicon and that on graphite (HOPG). However, the wettability of femtoliter droplets on the SLG demonstrates a unique dependence on the compositions of the surrounding liquid medium: Their contact angle on SLG becomes much larger than that on both graphite and on silanized silicon, once short-chain alcohol molecules are present in the surrounding medium. To account for this finding, we hypothesize two scenarios to rationalize the effect of alcohol on the microwettability on SLG. The understanding elucidated in this study may allow for improved control of the interaction between graphene and the surrounding liquid environment and facilitate applications in which graphene is in contact with liquids, such as in microfluidics and in lab-on-chip systems.

Graphene/Ionic liquid composite films and ion exchange.

Wettability of graphene is adjusted by the formation of various ionic surfaces combining ionic liquid (IL) self-assembly with ion exchange. The functionalized ILs were designed and synthesized with the goal of obtaining adjustable wettability. The wettability of the graphene surface bearing various anions was measured systematically. The effect of solvent systems on ion exchange ratios on the graphene surface has also been investigated. Meanwhile, the mechanical properties of the graphene/IL composite films were investigated on a nanometer scale. The elasticity and adhesion behavior of the thin film was determined with respected to the indentation deformation by colloid probe nanoindentation method. The results indicate that anions played an important role in determining graphene/IL composite film properties. In addition, surface wetting and mechanics can be quantitatively determined according to the counter-anions on the surface. This study might suggest an alternate way for quantity detection of surface ions by surface force.

Preparation of Graphene Fibers

Graphene owns intriguing properties in electronic, thermal, and mechanic with unique two-dimension (2D) monolayer structure. The new member of carbon family has not only attracted great attentions in scientific research, but also showed great potential for many applications. Graphene sheets can self-assemble into macroscopic structures, which is conducive to further explore the value of graphene material in application, and also the future major trends.This dissertation introduces the preparation of graphene fiber by graphene oxide solution as spinning dope. The graphene oxide is selected to solve the poor wettability of graphene hindering its direct wet spinning. The oxygen-containing groups on graphene oxide surface make it hydrophilic. The graphene fibers were prepared in the following steps: Firstly assemble the graphene oxide into graphene oxide fibers by wet spinning; then turning the graphene oxide fibers into graphene fibers by thermal reduction. To observe the graphene oxide immersing in the coagulation bath at different times by SEM and the assembly mechanism of macroscopic graphene oxide fibers could be studied. In this work, the conductivity of the graphene fibers have been measured using the four-point method. Single mast mechanics test system was used for the tensile strength of graphene fibers. So the key process factors in graphene fibers preparation were explored and optimized, including the concentration of wet spinning dope, the composition of the coagulation bath, the change in diameter of the needles, injection rate and collection rate, etc. In summary, graphene oxide fibers were assembled by wet-spinning of the graphene oxide dope. And by the thermal reduction graphene fibers were prepared successfully. After studying the assembly mechanism of graphene fibers, the better raw materials and the more proper preparation conditions were chosen to make the fibers which have the superior performances.

Wettability of graphene nanoribbon/single-walled carbon nanotube hybrid film

The wettability of a GNR/SWNT hybrid film is modified when the surface part of the SWNT film is unzipped by a Zn/HCl sputtering process.

Wetting and Interfacial Properties of Water on the Defective Graphene

The wettability of defective graphene and its relation with the interfacial property of water have not been well tackled. By conducting molecular dynamics simulations on the interactions between water and graphene with some vacancy defects, we show that the contact angle is more sensitive to the Stone–Wales (SW) and single-vacancy (SV) defects than the double-vacancy (DV) defects. Density profiles indicate that the graphene sheet induces the interfacial water to change from a liquid into an ordered three-layer structure, which restricts the self-diffusion of water molecules seriously. Moreover, the effect of defects on the ordered liquid layer has also been discussed. Our results are helpful for controlling the wettability of graphene and developing the applications of graphene in nanodevices.

Wettability of Graphene

Graphene, an atomically thin two-dimensional material, has received significant attention due to its extraordinary electronic, optical, and mechanical properties. Studies focused on understanding the wettability of graphene for thermo-fluidic and surface-coating applications, however, have been sparse. Meanwhile, wettability results reported in literature via static contact angle measurement experiments have been contradictory and highlight the lack of clear understanding of the underlying physics that dictates wetting behavior. In this work, dynamic contact angle measurements and detailed graphene surface characterizations were performed to demonstrate that the defects present in CVD grown and transferred graphene coatings result in unusually high contact angle hysteresis (16-37°) on these otherwise smooth surfaces. Hence, understanding the effect of the underlying substrate based on static contact angle measurements as reported in literature is insufficient. The advancing contact angle measurements on mono-, bi-, and trilayer graphene sheets on copper, thermally grown silica (SiO2), and glass substrates were observed to be independent of the number of layers of graphene and in good agreement with corresponding molecular dynamics simulations and theoretical calculations. Irrespective of the number of graphene layers, the advancing contact angle values were also in good agreement with the advancing contact angle on highly ordered pyrolytic graphite (HOPG), reaffirming the negligible effect of the underlying substrate. These results suggest that the advancing contact angle is a true representation of a graphene-coated surface while the receding contact angle is significantly influenced by intrinsic defects introduced during the growth and transfer processes. These observations, where the underlying substrates do not affect the wettability of graphene coatings, is shown to be due to the large interlayer spacing resulting from the loose interlamellar coupling between the graphene sheet and the underlying substrate. The fundamental insights on graphene-water interactions reported in this study is an important step towards developing graphene-assisted surface coatings for heat transfer and microfluidics devices.

Robust adhesion of flower-like few-layer graphene nanoclusters

Nanostructured surface possessing ultrahigh adhesion like “gecko foot” or “rose petal” can offer more opportunities for bionic application. We grow flower-like few-layer graphene on silicon nanocone arrays to form graphene nanoclusters, showing robust adhesion. Their contact angle (CA) is 164° with a hysteresis CA of 155° and adhesive force for a 5 μL water droplet is about 254 μN that is far larger than present reported results. We bring experimental evidences that this great adhesion depends on large-area plentiful edges of graphene nanosheets tuned by conical nanostructure and intrinsic wetting features of graphene. Such new hierarchical few-layer graphene nanostructure provides a feasible strategy to understand the ultra-adhesive mechanism of the “gecko effect” or “rose effect” and enhance the wettability of graphene for many practical applications.

Atomistic Prediction of Nanomaterials: Introduction to Molecular Dynamics Simulation and a Case Study of Graphene Wettability

This article discusses about case study of molecular dynamics (MD) simulation to predict the free energy of a graphene-water system based on the nanoscale perspective.This work can also be considered as a demonstration on the applications of MD in nanomaterials and nanostructure research.

Biomimetic Graphene Surfaces with Superhydrophobicity and Iridescence

Triggered by the fantastic functions and bright appearance of biological systems in nature, enormous efforts have been devoted to biomimetic fabrication. For instance, butterfly wings and red rose petals have attracted increasing attention due to their excellent water repellency and splendid structural color. Consequently, colorful superhydrophobic surfaces have become a hot topic with significance in both fundamental research and practical applications. Previous studies have shown that hierarchical micro-/nanostructures on biosurfaces play a critical role in the multifunctional acquisition. On the one hand, the highly rough textures trap a wealth of air bubbles at the interface preventing a water droplet from spreading; thus, the surfaces exhibit a high water contact angle (CA>1508). Along this line, a variety of water-repellent surfaces with multiscale structures have been achieved by classical “top-down” and “bottomup” approaches. On the other hand, as inspired by many natural species that use structural color as a warning or protection, the surface microstructures are not randomly distributed but rigidly arranged in periodic micro-patterns, and therefore triggered light diffraction and scattering contribute to the brilliant appearance. However, due to technical challenges in the fabrication of uniform and well-defined nanostructures in the long-range order, most of the superhydrophobic surfaces hardly show any structural color. To date, only a few attempts to such multifunctional surfaces have been realized. For instance, Gu et al. fabricated colloidal photonic crystal films with both structural color and superhydrophobicity at the cost of long time and high temperature. Jiang et al. reported multicolor superhydrophobic coatings that depend on metal ions for the appearance of color; in their study, individual samples exhibited a single color. Wu et al. fabricated superhydrophobic surfaces with iridescence by employing multiple procedures including interference, surface modification, and chemical plating. Consequently, a facile and convenient approach for surfaces with superhydrophobicity and iridescent structural color is in urgent need. Unlike the classical slippery superhydrophobic surface represented by the famous self-cleaning lotus leaf, rose petals possess a sticky superhydrophobicity, exhibiting both a high water contact angle (CA>1508) and strong adhesion. Because of the adhesive force, flowers are able to maintain a fresh appearance, as a water droplet cannot roll off effortlessly but stays stably on the surface without any movement. To date, by tailoring the chemical composition, the geometrical structure, or interfacial capillary and van der Waals forces, superhydrophobic surfaces with high adhesion were successfully prepared. Due to their representativeness in wetting behavior and their research value in various realms, such as liquid transportation in microfluidic systems and biomedical applications, high-adhesion surperhydrophobic surfaces become increasingly important. Recently, graphene has attracted much attention due to its unique single atom-layer structure, which contributes to its wide applications in nanoelectronics, sensing devices, energy storage, and even tissue engineering. For example, graphene has proven to be a promising biocompatible scaffold that could accelerate the specific differentiation of human mesenchymal stem cells (hMSCs) into bone cells. In this case, the cell adhesion on the graphene surface plays a critical role in the long-term differentiation; therefore, a precise control of the surface wettability of graphene becomes a significant issue. Generally, superhydrophobic graphene substrates could be fabricated by using an irregular stack of graphene oxides (GO) prepared by chemical oxidation of graphite. In this procedure, to reduce the surface energy, the hydrophilic oxygen-containing groups on the GO surface have to be removed beforehand. To the best of our knowledge, modulation of the wetting property of graphene by micro-/nanostructuring and simultaneous control of chemical composition have not been realized. Moreover, despite the pioneering biomimetic fabrications of periodic micro-/nanostructures based on a wide range of materials, a bioinspired graphene surface with properties comparable to natural surfaces has not been reported yet. [a] J.-N. Wang, R.-Q. Shao, Dr. Y.-L. Zhang, L. Guo, Prof. H.-B. Sun State Key Laboratory on Integrated Optoelectronics College of Electronic Science and Engineering Jilin University 2699 Qianjin Street, Changchun 130012 (P. R. China) Fax: (+86)431-85168281 E-mail : yonglaizhang@jlu.edu.cn hbsun@jlu.edu.cn [b] H.-B. Jiang, D.-X. Lu, Prof. H.-B. Sun College of Physics Jilin University 2699 Qianjin Street, Changchun 130012 (P. R. China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201100882.

Wetting and energetics in nanoparticle etching of graphene

Molten metallic nanoparticles have recently been used to construct graphene nanostructures with crystallographic edges. The mechanism by which this happens, however, remains unclear. Here, we present a simple model that explains how a droplet can etch graphene. Two factors possibly contribute to this process: a difference between the equilibrium wettability of graphene and the substrate that supports it, or the large surface energy associated with the graphene edge. We calculate the etching velocities due to either of these factors and make testable predictions for evaluating the significance of each in graphene etching. This model is general and can be applied to other materials systems as well. As an example, we show how our model can be used to extend a current theory of droplet motion on binary semiconductor surfaces.

Surface-Energy Engineering of Graphene

Contact angle goniometry is conducted for epitaxial graphene on SiC. Although only a single layer of epitaxial graphene exists on SiC, the contact angle drastically changes from 69 degrees on SiC substrates to 92 degrees on graphene. It is found that there is no thickness dependence of the contact angle from the measurements of single-, bi-, and multilayer graphene and highly ordered pyrolytic graphite (HOPG). After graphene is treated with oxygen plasma, the level of damage is investigated by Raman spectroscopy and the correlation between the level of disorder and wettability is reported. By using a low-power oxygen plasma treatment, the wettability of graphene is improved without additional damage, which can solve the adhesion issues involved in the fabrication of graphene devices.