Are Graphitic Surfaces Hydrophobic?

Fast Outer-Sphere Electron Transfer and High Specific Capacitance at Covalently Modified Carbon Electrodes

Carbon electrodes typically display sluggish electron transfer kinetics due to the adsorption of adventitious molecules that effectively insulate the surface. Here, we describe a method for rendering graphitic carbon electrodes permanently hydrophilic by functionalization with 4-(diazonium)benzenesulfonic acid. In aqueous electrolytes, these hydrophilic carbon electrodes exhibit metal-like specific capacitance (∼40 μF/cm2) as measured by cyclic voltammetry, suggesting a change in the double-layer structure at the carbon surface. Additionally, the modified electrodes show fast charge transfer kinetics to outer-sphere one-electron redox couples such as ferro-/ferricyanide as well as improved electron transfer kinetics in alkaline aqueous redox flow batteries.

Interfacial Water Is Separated from a Hydrophobic Silica Surface by a Gap of 1.2 nm

The interaction of liquid water with hydrophobic surfaces is ubiquitous in life and technology. Yet, the molecular structure of interfacial liquid water on these surfaces is not known. By using a 3D atomic force microscope, we characterize with angstrom resolution the structure of interfacial liquid water on hydrophobic and hydrophilic silica surfaces. The combination of 3D AFM images and molecular dynamics simulations reveals that next to a hydrophobic silica surface, there is a 1.2 nm region characterized by a very low density of water. In contrast, the 3D AFM images obtained of a hydrophilic silica surface reveal the presence of hydration layers next to the surface. The gap observed on hydrophobic silica surfaces is filled with two-to-three layers of straight-chain alkanes. We developed a 2D Ising model that explains the formation of a continuous hydrocarbon layer on hydrophobic silica surfaces.

Ultrahigh-Flux Water Nanopumps Generated by Asymmetric Terahertz Absorption

Controlling active transport of water through membrane channels is essential for advanced nanofluidic devices. Despite advancements in water nanopump design using techniques like short-range invasion and subnanometer-level control, challenges remain facilely and remotely realizing massive waters active transport. Herein, using molecular dynamic simulations, we propose an ultrahigh-flux nanopump, powered by frequency-specific terahertz stimulation, capable of unidirectionally transporting massive water through asymmetric-wettability membrane channels at room temperature without any external pressure. The key physics behind this terahertz-powered water nanopump is revealed to be the energy flow resulting from the asymmetric optical absorption of water.

Toward a Comprehensive Understanding of the Anomalously Small Contact Angle of Surface Nanobubbles

Experimental studies have demonstrated that the gas phase contact angle (CA) of a surface nanobubble (SNB) is much smaller than that of a macroscopic gas bubble. This reduced CA plays a crucial role in prolonging the lifetime of SNBs by lowering the bubble pressure and preventing gas molecules from dissolving in the surrounding liquids. Despite extensive efforts to explain the anomalously small CA, a consensus about the underlying reasons is yet to be reached. In this study, we conducted experimental investigations to explore the influence of gas molecules adsorbed at the solid-liquid interface on the CA of SNBs created through the solvent exchange (SE) method and temperature difference (TD). Interestingly, no significant change is observed in the CA of SNBs on highly oriented pyrolytic graphite (HOPG) surfaces. Even for nanobubbles on micro/nano pancakes, the CA only exhibited a slight reduction compared to SNBs on bare HOPG surfaces. These findings suggest that gas adsorption at the immersed solid surface may not be the primary factor contributing to the small CA of the SNBs. Furthermore, the CA of SNBs formed on polystyrene (PS) and octadecyltrichlorosilane (OTS) substrates was also investigated, and a considerable increase in CA was observed. In addition, the effects of other factors including impurity, electric double layer (EDL) line tension, and pinning force upon the CA of SNBs were discussed, and a comprehensive model about multiple factors affecting the CA of SNBs was proposed, which is helpful for understanding the abnormally small CA and the stability of SNBs.

Flexible Bifunctional Electrode for Alkaline Water Splitting with Long-Term Stability

Progress in electrochemical water-splitting devices as future renewable and clean energy systems requires the development of electrodes composed of efficient and earth-abundant bifunctional electrocatalysts. This study reveals a novel flexible and bifunctional electrode (NiO@CNTR) by hybridizing macroscopically assembled carbon nanotube ribbons (CNTRs) and atmospheric plasma-synthesized NiO quantum dots (QDs) with varied loadings to demonstrate bifunctional electrocatalytic activity for stable and efficient overall water-splitting (OWS) applications. Comparative studies on the effect of different electrolytes, e.g., acid and alkaline, reveal a strong preference for alkaline electrolytes for the developed NiO@CNTR electrode, suggesting its bifunctionality for both HER and OER activities. Our proposed NiO@CNTR electrode demonstrates significantly enhanced overall catalytic performance in a two-electrode alkaline electrolyzer cell configuration by assembling the same electrode materials as both the anode and the cathode, with a remarkable long-standing stability retaining ∼100% of the initial current after a 100 h long OWS run, which is attributed to the "synergistic coupling" between NiO QD catalysts and the CNTR matrix. Interestingly, the developed electrode exhibits a cell potential (E10) of only 1.81 V with significantly low NiO QD loading (83 μg/cm2) compared to other catalyst loading values reported in the literature. This study demonstrates a potential class of carbon-based electrodes with single-metal-based bifunctional catalysts that opens up a cost-effective and large-scale pathway for further development of catalysts and their loading engineering suitable for alkaline-based OWS applications and green hydrogen generation.

Model Approach to Thermal Conductivity in Hybrid Graphene-Polymer Nanocomposites

The thermal conductivity of epoxy nanocomposites filled with self-assembled hybrid nanoparticles composed of multilayered graphene nanoplatelets and anatase nanoparticles was described using an analytical model based on the effective medium approximation with a reasonable amount of input data. The proposed effective thickness approach allowed for the simplification of the thermal conductivity simulations in hybrid graphene@anatase TiO2 nanosheets by including the phenomenological thermal boundary resistance. The sensitivity of the modeled thermal conductivity to the geometrical and material parameters of filling particles and the host polymer matrix, filler's mass concentration, self-assembling degree, and Kapitza thermal boundary resistances at emerging interfaces was numerically evaluated. A fair agreement of the calculated and measured room-temperature thermal conductivity was obtained.

Effect of Graphite Aging on Its Wetting Properties and Surface Blocking by Gaseous Nanodomains

Early works considered basal planes of highly ordered pyrolytic graphite (HOPG) as hydrophobic, relatively inert materials with low electrocatalytic activity due to nonpolar sp2 carbon. On the contrary, a freshly prepared HOPG surface exhibits intrinsically mildly hydrophilic properties, with a low contact angle of water, which increases after exposure to an ambient atmosphere. This process, called aging, ascribed to adsorption of airborne hydrocarbons, is reportedly accompanied by strong decay of electron transfer kinetics, the mechanism of which is not yet fully understood. Examining both freshly prepared and aged basal plane HOPG immersed in water by PeakForce quantitative nanomechanical imaging, we have found that aged HOPG is occupied by ambient gaseous nanodomains, the existence of which is explained by incomplete wetting. They cover up to 60% of the immersed surface and their incidence is in direct relation with graphite aging time. In contrast with aged graphite, gaseous nanodomains were absent on the freshly stripped HOPG surface. It can be concluded that ambient gaseous nanodomains can prevent aged basal plane HOPG from contact with aqueous media and may thus affect processes at the solid-liquid interface.

Enhanced thrombolytic effect induced by acoustic cavitation generated from nitrogen-doped annealed nanodiamond particles

In biomedical research, ultrasonic cavitation, especially inertial cavitation (IC) has attracted extensive attentions due to its ability to induce mechanical, chemical and thermal effects. Like ultrasound contrast agent (UCA) microbubbles or droplets, acoustic cavitation can be effectively triggered beyond a certain pressure threshold through the interaction between ultrasound and nucleation particles, leading to an enhanced thrombolytic effect. As a newly developed nanocarbon material, nitrogen-doped annealed nanodiamond (N-AND) has shown promising catalytic performance. To further explore its effects on ultrasonic cavitation, N-AND was synthesized at the temperature of 1000 °C. After systematic material characterization, the potential of N-AND to induce enhanced IC activity was assessed for the first time by using passive cavitation detection (PCD). Based on experiments performed at varied material suspension concentration and cycle number, N-AND demonstrated a strong capability to generate significant cavitation characteristics, indicating the formation of stable bubbles from the surface of the materials. Furthermore, N-AND was applied in the in vitro thrombolysis experiments to verify its contribution to ultrasound thrombolysis. The influence of surface hydrophobicity on the cavitation potentials of ND and N-AND was innovatively discussed in combination with the theory of mote-induced nucleation. It is found that the cavitation stability of N-AND was better than that of the commercial UCA microbubbles. This study would provide better understanding of the potential of novel carbonous nanomaterials as cavitation nuclei and is expected to provide guidance for their future biomedical and industrial applications.

Microscale Electrochemical Corrosion of Uranium Oxide Particles

Understanding the corrosion of spent nuclear fuel is important for the development of long-term storage solutions. However, the risk of radiation contamination presents challenges for experimental analysis. Adapted from the system for analysis at the liquid-vacuum interface (SALVI), we developed a miniaturized uranium oxide (UO2)-attached working electrode (WE) to reduce contamination risk. To protect UO2 particles in a miniatured electrochemical cell, a thin layer of Nafion was formed on the surface. Atomic force microscopy (AFM) shows a dense layer of UO2 particles and indicates their participation in electrochemical reactions. Particles remain intact on the electrode surface with slight redistribution. X-ray photoelectron spectroscopy (XPS) reveals a difference in the distribution of U(IV), U(V), and U(VI) between pristine and corroded UO2 electrodes. The presence of U(V)/U(VI) on the corroded electrode surface demonstrates that electrochemically driven UO2 oxidation can be studied using these cells. Our observations of U(V) in the micro-electrode due to the selective semi-permeability of Nafion suggest that interfacial water plays a key role, potentially simulating a water-lean scenario in fuel storage conditions. This novel approach offers analytical reproducibility, design flexibility, a small footprint, and a low irradiation dose, while separating the α-effect. This approach provides a valuable microscale electrochemical platform for spent fuel corrosion studies with minimal radiological materials and the potential for diverse configurations.

Evaluating fundamental biochar properties in relation to water holding capacity

Biochar products that hold and release water within a stable carbonised porous structure provide many opportunities for climate mitigation and a range of applications such as for soil amendments. Biochar that are produced from various organic feedstocks by pyrolysis can provide multiple co-benefits to soil including improving soil health and productivity, pH buffering, contaminant control, nutrient storage, and release, however, there are also risks for biochar application in soils. This study evaluated fundamental biochar properties that influence Water Holding Capacity (WHC) of biochar products and provides recommendations for testing and optimising biochar products prior to soil applications. A total of 21 biochar samples (locally sourced, commercially available, and standard biochars) were characterised for particle properties, salinity, pH and ash content, porosity, and surface area (with N₂ as adsorbate), surface SEM imaging, and several water testing methods. Biochar products with mixed particle size, irregular shapes, and hydrophilic properties were able to rapidly store relatively large volumes of water (up to 400% wt.). In contrast, relatively less water (as low as 78% wt.) was taken up by small-sized biochar products with smooth surfaces, along with hydrophobic biochars that were identified by the water drop penetration test (rather than contact angle test). Water was stored mostly in interpore spaces (between biochar particles) although intra-pore spaces (meso-pore and micropore scale) were also significant for some biochars. The type of organic feedstock did not appear to directly affect water holding, although further work is needed to evaluate mesopore scale processes and pyrolytic conditions that could influence the biochemical and hydrological behaviour of biochar. Biochars with high salinity, and carbon structures that are not alkaline pose potential risks when used as soil amendments.

Hydrocarbons in the Meniscus: Effects on Conductive Atomic Force Microscopy

It is commonly accepted that during conductive atomic force microscopy (CAFM) measurement in ambient, a liquid meniscus can form between the tip and the sample. Such a liquid bridge, normally assumed to be composed of water, is a major factor in analyzing and understanding CAFM results. Here, we show that the adsorption of adventitious hydrocarbons from the air to a surface can greatly affect CAFM data both in imaging mode and in local spectroscopy (current-voltage or I-V curves). We propose a model to explain the phenomena whereby hydrocarbon contaminates contribute to the composition of the liquid bridge between the tip and the sample.

Precise quantification of nanoparticle surface free energy via colloidal probe atomic force microscopy

Interfacial interactions of nanoparticles (NPs) in colloids are greatly influenced by the NP surface free energy (SFE). Due to the intrinsic physical and chemical heterogeneity of the NP surface, measuring SFE is nontrivial. The use of direct force measurement methods, such as colloidal probe atomic force microscopy (CP-AFM), have been proven to be effective for the determination of SFE on relatively smooth surfaces, but fail to provide reliable measurements for rough surfaces generated by NPs. Here, we developed a reliable approach to determine the SFE of NPs by adopting Persson's contact theory to include the effect of surface roughness on the measurements in CP-AFM experiments. We obtain the SFE for a range of materials varying in surface roughness and surface chemistry. The reliability of the proposed method is verified by the SFE determination of polystyrene. Subsequently, the SFE of bare and functionalized silica, graphene oxide, and reduced graphene oxide were quantified and validity of the results was demonstrated. The presented method unlocks the potential of CP-AFM as a robust and reliable method of the SFE determination of nanoparticles with a heterogeneous surface, which is challenging to obtain with conventionally implemented experimental techniques.

Molecular Probing of the Stress Activation Volume in Vapor Phase Lubricated Friction

When two solid objects slide over each other, friction results from the interactions between the asperities of the (invariably rough) surfaces. Lubrication happens when viscous lubricants separate the two surfaces and carry the load such that solid-on-solid contacts are avoided. Yet, even small amounts of low-viscosity lubricants can still significantly lower friction through a process called boundary lubrication. Understanding the origin of the boundary lubricating effect is hampered by challenges in measuring the interfacial properties of lubricants directly between the two surfaces. Here, we use rigidochromic fluorescent probe molecules to measure precisely what happens on a molecular scale during vapor-phase boundary lubrication of a polymer bead-on-glass interface. The probe molecules have a longer fluorescence lifetime in a confined environment, which allows one to measure the area of real contact between rough surfaces and infer the shear stress at the lubricated interfaces. The latter is shown to be proportional to the inverse of the local interfacial free volume determined using the measured fluorescence lifetime. The free volume can then be used in an Eyring-type model as the stress activation volume, allowing to collapse the data of stress as a function of sliding velocity and partial pressure of the vapor phase lubricant. This shows directly that as more boundary lubricant is applied, larger clusters of lubricant molecules become involved in the shear process thereby lowering the friction.

Study on the surface wetting mechanism of bituminous coal based on the microscopic molecular structure

The chemical wet dust removal method is one of the hot methods for coal dust control, and the key to its success lies in whether the surface of coal dust can be well wetted or not. Nowadays, the wetting mechanism of the coal dust surface is understudied, limiting the further application of chemical wet dust removal. Thus, the exploration of the wetting mechanism based on the microscopic molecular structure characteristics of the coal surface provides a new solution to improve the wet dust removal efficiency. Herein, the bituminous coal collected from 3 groups of coal seams in the Pingdingshan mining area was used as the object of study to reveal the microscopic wetting mechanism. Proximate analysis, nuclear magnetic resonance carbon spectroscopy (¹³C NMR) and X-ray photoelectron spectroscopy (XPS) can well distinguish the microstructural information of the coal surface, enabling building the molecular structure models of three groups of coal. Joint contact angle experiments were conducted to explore the influencing factors between the molecular structure of coal dust and its wettability. Molecular simulation techniques, combined with indoor experiments, were used to explore the essential causes of the differences in the wetting mechanisms of bituminous coal dust. The results showed that the composition and structure of carbon and oxygen elements on the coal surface has a significant effect on the wettability of coal dust. The higher the relative content of aromatic carbon and oxygen elements, the better the wettability of the coal surface. An opposite trend occurred for the aliphatic carbon. The difference in wettability of coal surfaces mainly stems from the ability of hydrophilic functional groups on coal surfaces to form hydrogen bonds with water molecules. The aromatic hydrocarbon structure has a much greater ability to adsorb water molecules than the aliphatic hydrocarbon structure. The research results can provide scientific guidance for the design of efficient and environmentally friendly dust suppressants to realize clean coal production in mines.

The Surface Energy of Hydrogenated and Fluorinated Graphene

The surface energy of graphene and its chemical derivatives governs fundamental interfacial interactions like molecular assembly, wetting, and doping. However, quantifying the surface energy of supported two-dimensional (2D) materials, such as graphene, is difficult because (1) they are so thin that electrostatic interactions emanating from the underlying substrate are not completely screened, (2) the contribution from the monolayer is sensitive to its exact chemical state, and (3) the adsorption of airborne contaminants, as well as contaminants introduced during transfer processing, screens the electrostatic interactions from the monolayer and underlying substrate, changing the determined surface energy. Here, we determine the polar and dispersive surface energy of bare, fluorinated, and hydrogenated graphene through contact angle measurements with water and diiodomethane. We accounted for many contributing factors, including substrate surface energies and combating adsorption of airborne contaminants. Hydrogenating graphene raises its polar surface energy with little effect on its dispersive surface energy. Fluorinating graphene lowers its dispersive surface energy with a substrate-dependent effect on its polar surface energy. These results unravel how changing the chemical structure of graphene modifies its surface energy, with applications for hybrid nanomaterials, bioadhesion, biosensing, and thin-film assembly.

Interfacial Liquid Water on Graphite, Graphene, and 2D Materials

The optical, electronic, and mechanical properties of graphite, few-layer, and two-dimensional (2D) materials have prompted a considerable number of applications. Biosensing, energy storage, and water desalination illustrate applications that require a molecular-scale understanding of the interfacial water structure on 2D materials. This review introduces the most recent experimental and theoretical advances on the structure of interfacial liquid water on graphite-like and 2D materials surfaces. On pristine conditions, atomic-scale resolution experiments revealed the existence of 1-3 hydration layers. Those layers were separated by ∼0.3 nm. The experimental data were supported by molecular dynamics simulations. However, under standard working conditions, atomic-scale resolution experiments revealed the presence of 2-3 hydrocarbon layers. Those layers were separated by ∼0.5 nm. Linear alkanes were the dominant molecular specie within the hydrocarbon layers. Paradoxically, the interface of an aged 2D material surface immersed in water does not have water molecules on its vicinity. Free-energy considerations favored the replacement of water by alkanes.

Feasibility of Carbon Nanoparticle Coatings as Protective Barriers for Copper─Wetting Assessment

Copper is extensively used in a wide range of industrial and daily-life applications, varying from heat exchangers to electrical wiring. Although it is protected from oxidation by its native oxide layer, when subjected to harsh environmental conditions─such as in coastal regions─this metal can rapidly degrade. Therefore, in this study, we analyze the potential use of carbon nanoparticle coatings as protective barriers due to their intrinsic hydrophobic wetting behavior. The nanocarbon coatings were produced via electrophoretic deposition on Cu platelets and characterized via scanning electron microscopy, confocal laser scanning microscopy, and sessile drop test; the latter being the primary focus since it provides insights into the wetting behavior of the produced coatings. Among the measured coatings, graphite flakes, graphene oxide, and carbon nanotube (CNT) coatings showed superhydrophobic behavior. Based on their wetting behavior, and specifically for electrical applications, CNT coatings showed the most promising results since these coatings do not significantly impact the substrate's electrical conductivity. Although CNT agglomerates do not affect the wetting behavior of the attained coatings, the coating's thickness plays an important role. Therefore, to completely coat the substrate, the CNT coating should be sufficiently thick─above approximately 1 μm.

Optimising water holding capacity and hydrophobicity of biochar for soil amendment - A review

Biochar is a product of the thermal treatment of biomass, and it can be used for enhancing soil health and productivity, soil carbon sequestration, absorbance of pollutants from water and soil, and promoting environmental sustainability. Extensive research has been done on applications of biochar to enhance the Water Holding Capacity (WHC) of biochar amended soil. However, a comprehensive road map of biochar optimised for enhanced WHC, and reduced hydrophobicity is not yet published. This review is the first to provide not only quantitative information on the impacts of biochar properties in WHC and hydrophobicity, but also a road map to optimise biochar for enhanced WHC when applied as a soil amendment. The review shows that straw or grass-derived biochar (at 500-600 °C) increases the WHC of soil if applied at 1 to 3 % in the soil. It is clear from the review that soil of varying texture requires different particle sizes of biochar to enhance the WHC and reduce hydrophobicity. Furthermore, the review concludes that ageing biochar for at least a year with enhanced oxidation is recommended for improving the WHC and reducing hydrophobicity compared to using biochar immediately after production. Additionally, while producing biochar a residence time of 1 to 2 h is recommended to reduce the biochar's hydrophobicity. Finally, a road map for optimising biochar is presented as a schematic that can be a resource for making decisions during biochar production for soil amendment.

Controllable generation of interfacial gas structures on the graphite surface by substrate hydrophobicity and gas oversaturation in water

Spherical nanobubbles and flat micropancakes are two typical states of gas aggregation on solid-liquid surfaces. Micropancakes, which are quasi-two-dimensional gaseous structures, are often produced accompanied by surface nanobubbles. Compared with surface nanobubbles, the intrinsic properties of micropancakes are barely understood due to the challenge of the highly efficient preparation and characterization of such structures. The hydrophobicity of the substrate and gas saturation of solvents are two crucial factors for the nucleation and stability of interfacial gas domains. Herein, we investigated the synergistic effect of the surface hydrophobicity and gas saturation on the generation of interfacial gas structures. Different surface hydrophobicities were achieved by the aging process of highly oriented pyrolytic graphite (HOPG). The results indicated that higher surface hydrophobicity and gas oversaturation could create surface nanobubbles and micropancakes with higher efficiency. Strong surface hydrophobicity could promote nanobubble nucleation and higher gas saturation would induce bigger nanobubbles. Degassed experiments could remove most of these structures and prove that they are actually gaseous domains. Finally, we draw a region diagram to describe the formation conditions of nanobubbles, micropancakes based on observations. These results would be very helpful for further understanding the formation of interfacial gas structures on the hydrophobic surface under different gas saturation.

Evaluation of Biocompatibility of 316 L Stainless Steels Coated with TiN, TiCN, and Ti-DLC Films

In this study, TiN, TiCN, and Ti-diamond-like carbon (Ti-DLC) films were coated on 316 L stainless steel (AISI 316 L) substrate surface by physical vapor deposition. The biocompatibility of the three films (TiN, TiCN, and Ti-DLC) and three metals (AISI 316 L, Ti, and Cu) was compared on the basis of the differences in the surface morphology, water contact angle measurements, CCK-8 experiment results, and flow cytometry test findings. The biocompatibility of the TiN and TiCN films is similar to that of AISI 316 L, which has good biocompatibility. However, the biocompatibility of the Ti-DLC films is relatively poor, which is mainly due to the inferior hydrophobicity and large amount of sp2 phases. The presence of TiC nanoclusters on the surface of the Ti-DLC film aggravates the inferior biocompatibility. Compared to the positive Cu control group, the Ti-DLC film had a higher cell proliferation rate and lower cell apoptosis rate. Although the Ti-DLC film inhibited cell survival to a certain extent, it did not show obvious cytotoxicity. TiN and TiCN displayed excellent performance in promoting cell proliferation and reducing cytotoxicity; thus, TiN and TiCN can be considered good orthodontic materials, whereas Ti-DLC films require further improvement.

Interfacial Micropancakes: Gas or Contaminations?

Micropancake, a flat domain with micrometer-scale lateral size and a few nanometer thickness, is usually accompanied by the generation of interfacial nanobubbles at the liquid/solid surfaces. Unlike the nanobubbles, micropancakes are difficult to be produced efficiently, impeding further investigations of their mysterious properties. Very recently, An et al. even argued that the previously observed micropancakes were most likely the contaminate, not the gas layers. Herein, to reveal the nature of micropancakes with solid evidence, we presented the in situ characterization of micropancakes at a highly oriented pyrolytic graphite (HOPG) surface produced by the ethanol-water exchange or gas-supersaturated water. By washing with deeply degassed water (DW), the dissolution of those micropancakes was clearly observed, indicating that they may very well be composed of gas. In addition, the analysis of the force measurements showed the intrinsic differences between those gaseous micropancakes and the insoluble organic films. The data and results supported the interpretation that the real existence of gas micropancakes at liquid/solid surfaces.

Organic contaminants and atmospheric nitrogen at the graphene-water interface: a simulation study

Ordered nanoscale patterns have been observed by atomic force microscopy at graphene-water and graphite-water interfaces. The two dominant explanations for these patterns are that (i) they consist of self-assembled organic contaminants or (ii) they are dense layers formed from atmospheric gases (especially nitrogen). Here we apply molecular dynamics simulations to study the behavior of dinitrogen and possible organic contaminants at the graphene-water interface. Despite the high concentration of N₂ in ambient air, we find that its expected occupancy at the graphene-water interface is quite low. Although dense (disordered) aggregates of dinitrogen have been observed in previous simulations, our results suggest that they are stable only in the presence of supersaturated aqueous N₂ solutions and dissipate rapidly when they coexist with nitrogen gas near atmospheric pressure. On the other hand, although heavy alkanes are present at only trace concentrations (micrograms per cubic meter) in typical indoor air, we predict that such concentrations can be sufficient to form ordered monolayers that cover the graphene-water interface. For octadecane, grand canonical Monte Carlo suggests nucleation and growth of monolayers above an ambient concentration near 6 μg m^-3, which is less than some literature values for indoor air. The thermodynamics of the formation of these alkane monolayers includes contributions from the hydration free-energy (unfavorable), the free-energy of adsorption to the graphene-water interface (highly favorable), and integration into the alkane monolayer phase (highly favorable). Furthermore, the peak-to-peak distances in AFM force profiles perpendicular to the interface (0.43-0.53 nm), agree with the distances calculated in simulations for overlayers of alkane-like molecules, but not for molecules such as N₂, water, or aromatics. Taken together, these results suggest that ordered domains observed on graphene, graphite, and other hydrophobic materials in water are consistent with alkane-like molecules occupying the interface.

Towards Recycling of LLZO Solid Electrolyte Exemplarily Performed on LFP/LLZO/LTO Cells

All-solid-state lithium ion batteries (ASS-LIBs) are promising due to their safety and higher energy density as compared to that of conventional LIBs. Over the next few decades, tremendous amounts of spent ASS-LIBs will reach the end of their cycle life and would require recycling in order to address the waste management issue along with reduced exploitation of rare elements. So far, only very limited studies have been conducted on recycling of ASS-LIBS. Herein, we investigate the recycling of the Li₇ La₃ Zr₂ O₁₂ (LLZO) solid-state electrolyte in a LiFePO₄ /LLZO/Li₄ Ti₅ O₁₂ system using a hydrometallurgical approach. Our results show that different concentration of the leaching solutions can significantly influence the final product of the recycling process. However, it was possible to recover relatively pure La₂ O₃ and ZrO₂ to re-synthesize the cubic LLZO phase, whose high purity was confirmed by XRD measurements.

Controllable Nanoparticle Aggregation through a Superhydrophobic Laser-Induced Graphene Dynamic System for Surface-Enhanced Raman Scattering Detection

Surface-enhanced Raman scattering (SERS) is widely used for low-concentration molecular detection; however, challenges related to detection uniformity and repeatability are bottlenecks for practical application, especially as regards ultrasensitive detection. Here, through the coupling of bionics and fluid mechanics, a lotus-leaf effect and rose-petal effect (LLE-RPE)-integrated superhydrophobic chip is facilely developed using laser-induced graphene (LIG) fabricated on a polyimide film. Dense and uniform aggregation of gold nanoparticles (AuNPs) in droplets is realized through a constant contact angle (CCA) evaporation mode in the dynamic enrichment process, facilitating reliable ultrasensitive detection. The detection chip consists of two components: an LLE zone containing an ethanol-treated LIG superhydrophobic surface with a low-adhesive property, which functions as an AuNP-controllable aggregation zone, and an RPE zone containing an as-fabricated LIG superhydrophobic surface with water-solution pinning ability, which functions as a droplet solvent evaporation and a AuNP blending zone. AuNPs realize uniform aggregation during rolling on the LLE zone, and then get immobilized on the RPE zone to complete evaporation of the solvent, followed by Raman detection. Here, based on dense and uniform AuNP aggregation, the detection system achieves high-efficiency (242 s/18 μL) and ultralow-concentration (10^-17 M) detection of a target analyte (rhodamine 6G). The proposed system constitutes a simple approach toward high-performance detection for chemical analysis, environmental monitoring, biological analysis, and medical diagnosis.

Highly Selective Metal-Free Electrochemical Production of Hydrogen Peroxide on Functionalized Vertical Graphene Edges

Electrochemical generation of hydrogen peroxide (H₂ O₂ ) is an attractive alternative to the energy-intensive anthraquinone oxidation process. Metal-free carbon-based materials such as graphene show great promise as efficient electrocatalysts in alkaline media. In particular, the graphene edges possess superior electrochemical properties than the basal plane. However, identification and enhancement of the catalytically active sites at the edges remain challenging. Furthermore, control of surface wettability to enhance gas diffusion and promote the performance in bulk electrolysis is largely unexplored. Here, a metal-free edge-rich vertical graphene catalyst is synthesized and exhibits a superior performance for H₂ O₂ production, with a high onset potential (0.8 V versus reversible hydrogen electrode (RHE) at 0.1 mA cm^-2 ) and 100% Faradaic efficiency at various potentials. By tailoring the oxygen-containing functional groups using various techniques of electrochemical oxidation, thermal annealing and oxygen plasma post-treatment, the edge-bound in-plane ether-type (COC) groups are revealed to account for the superior catalytic performance. To manipulate the surface wettability, a simple vacuum-based method is developed to effectively induce material hydrophobicity by accelerating hydrocarbon adsorption. The increased hydrophobicity greatly enhances gas transfer without compromising the Faradaic efficiency, enabling a H₂ O₂ productivity of 1767 mmol g_catalyst ^-1 h^-1 at 0.4 V versus RHE.

Polyurethane Sponge with a Modified Specific Surface for Repeatable Oil-Water Separation

Oil spill accidents contaminate the oceanic environment and cause economic distress, and they continue to occur. Many methods have been developed to restore waters contaminated with spilled oil. However, still most commercially available methods are not environmentally or economically sustainable solutions. Therefore, there is a need for the development of sustainable materials with running water treatment capabilities. In recent years, a polyurethane (PU) sponge-based adsorbent has been reported as an oil-water separation and reusable adsorbent. This is because the porous 3D structure of the PU sponge provides a large surface area. However, as the PU sponge has a carboxyl group and an amino group, it exhibits hydrophilicity, so surface modification is essential for oil-water separation. Therefore, to modify the surface of PU to have hydrophobic/oleophilic properties, a hydrophobic/oleophilic adsorbent (HOA) was prepared using graphite and polydimethylsiloxane. On the basis of this, a PU sponge, a porous material, was used to manufacture an adsorbent that can be used in a sustainable and environmentally friendly way. The prepared HOA can selectively adsorb water or oil and can be reused. Furthermore, continuous oil-water separation is possible through a simple flow of fluid. Therefore, it is confirmed that the studied HOA can have great potential for ocean restoration in the future as an adsorbent that mitigates the disadvantages of the currently commercialized method.

Hydrophilic, Clean Graphene for Cell Culture and Cryo-EM Imaging

The wettability of graphene is critical for numerous applications but is very sensitive to its surface cleanness. Herein, by clarifying the impact of intrinsic contamination, i.e., amorphous carbon, which is formed on the graphene surface during the high-temperature chemical vapor deposition (CVD) process, the hydrophilic nature of clean graphene grown on single-crystal Cu(111) substrate was confirmed by both experimental and theoretical studies, with an average water contact angle of ∼23°. Furthermore, the wettability of as-transferred graphene was proven to be highly dependent on its intrinsic cleanness, because of which the hydrophilic, clean graphene exhibited improved performance when utilized for cell culture and cryoelectron microscopy imaging. This work not only validates the intrinsic hydrophilic nature of graphene but also provides a new insight in developing advanced bioapplications using CVD-grown clean graphene films.

Direct Chemical Vapor Deposition Synthesis of Porous Single-Layer Graphene Membranes with High Gas Permeances and Selectivities

Single-layer graphene containing molecular-sized in-plane pores is regarded as a promising membrane material for high-performance gas separations due to its atomic thickness and low gas transport resistance. However, typical etching-based pore generation methods cannot decouple pore nucleation and pore growth, resulting in a trade-off between high areal pore density and high selectivity. In contrast, intrinsic pores in graphene formed during chemical vapor deposition are not created by etching. Therefore, intrinsically porous graphene can exhibit high pore density while maintaining its gas selectivity. In this work, the density of intrinsic graphene pores is systematically controlled for the first time, while appropriate pore sizes for gas sieving are precisely maintained. As a result, single-layer graphene membranes with the highest H₂ /CH₄ separation performances recorded to date (H₂ permeance > 4000 GPU and H₂ /CH₄ selectivity > 2000) are fabricated by manipulating growth temperature, precursor concentration, and non-covalent decoration of the graphene surface. Moreover, it is identified that nanoscale molecular fouling of the graphene surface during gas separation where graphene pores are partially blocked by hydrocarbon contaminants under experimental conditions, controls both selectivity and temperature dependent permeance. Overall, the direct synthesis of porous single-layer graphene exploits its tremendous potential as high-performance gas-sieving membranes.

Modern Carbon-Based Materials for Adsorptive Removal of Organic and Inorganic Pollutants from Water and Wastewater

Currently, a serious threat for living organisms and human life in particular, is water contamination with persistent organic and inorganic pollutants. To date, several techniques have been adopted to remove/treat organics and toxic contaminants. Adsorption is one of the most effective and economical methods for this purpose. Generally, porous materials are considered as appropriate adsorbents for water purification. Conventional adsorbents such as activated carbons have a limited possibility of surface modification (texture and functionality), and their adsorption capacity is difficult to control. Therefore, despite the significant progress achieved in the development of the systems for water remediation, there is still a need for novel adsorptive materials with tunable functional characteristics. This review addresses the new trends in the development of new adsorbent materials. Herein, modern carbon-based materials, such as graphene, oxidized carbon, carbon nanotubes, biomass-derived carbonaceous matrices-biochars as well as their composites with metal-organic frameworks (MOFs) and MOF-derived highly-ordered carbons are considered as advanced adsorbents for removal of hazardous organics from drinking water, process water, and leachate. The review is focused on the preparation and modification of these next-generation carbon-based adsorbents and analysis of their adsorption performance including possible adsorption mechanisms. Simultaneously, some weak points of modern carbon-based adsorbents are analyzed as well as the routes to conquer them. For instance, for removal of large quantities of pollutants, the combination of adsorption and other methods, like sedimentation may be recommended. A number of efficient strategies for further enhancing the adsorption performance of the carbon-based adsorbents, in particular, integrating approaches and further rational functionalization, including composing these adsorbents (of two or even three types) can be recommended. The cost reduction and efficient regeneration must also be in the focus of future research endeavors. The targeted optimization of the discussed carbon-based adsorbents associated with detailed studies of the adsorption process, especially, for multicomponent adsorbate solution, will pave a bright avenue for efficient water remediation.

Highly sensitive broadband binary photoresponse in gateless epitaxial graphene on 4H-SiC

Due to weak light-matter interaction, standard chemical vapor deposition (CVD)/exfoliated single-layer graphene-based photodetectors show low photoresponsivity (on the order of mA/W). However, epitaxial graphene (EG) offers a more viable approach for obtaining devices with good photoresponsivity. EG on 4H-SiC also hosts an interfacial buffer layer (IBL), which is the source of electron carriers applicable to quantum optoelectronic devices. We utilize these properties to demonstrate a gate-free, planar EG/4H-SiC-based device that enables us to observe the positive photoresponse for (405-532) nm and negative photoresponse for (632-980) nm laser excitation. The broadband binary photoresponse mainly originates from the energy band alignment of the IBL/EG interface and the highly sensitive work function of the EG. We find that the photoresponsivity of the device is > 10 A/W under 405 nm of power density 7.96 mW/cm² at 1 V applied bias, which is three orders of magnitude greater than the obtained values of CVD/exfoliated graphene and higher than the required value for practical applications. These results path the way for selective light-triggered logic devices based on EG and can open a new window for broadband photodetection.

Structural properties of deprotonated naphthenic acids immersed in water in pristine and hydroxylated carbon nanopores from molecular perspectives

We use molecular dynamic simulations to study the structural properties of deprotonated cyclohexanoic acid (DCHA) and heptanoic acid (DHA) immersed in water in pristine and hydroxylated carbon nanopores (PACNs and HACNs) in relation to NA removal by activated carbons (ACs). In PACNs, both NAs can aggregate on the pore surface by depleting water molecules, while water molecules accumulate in the area where there is no NA aggregation. The hydrophobic tails of NAs are generally in the interface water region (IWR), while the hydrophilic head groups prefer to be hydrated by water and form pairing with Na⁺ ions outside the IWR. The linear carbon tails of DHA tend to be parallel to the pore surface, while a slightly inclined configuration of the carbon ring in DCHA is observed. In HACNs, water forms a predominant interface layer by forming hydrogen bonding with -OH groups, while the hydrophobic tails of NAs are driven away from the surface. Both NAs have a more perpendicular orientation close to the pore surface with their hydrophilic head groups forming hydrogen bonding with -OH groups. The strong water film greatly reduces hydrophobic interactions as well as decreases the available adsorption sites for NAs.

Reversible Lectin Binding to Glycan-Functionalized Graphene

The monolayer character of two-dimensional materials predestines them for application as active layers of sensors. However, their inherent high sensitivity is always accompanied by a low selectivity. Chemical functionalization of two-dimensional materials has emerged as a promising way to overcome the selectivity issues. Here, we demonstrate efficient graphene functionalization with carbohydrate ligands—chitooligomers, which bind proteins of the lectin family with high selectivity. Successful grafting of a chitooligomer library was thoroughly characterized, and glycan binding to wheat germ agglutinin was studied by a series of methods. The results demonstrate that the protein quaternary structure remains intact after binding to the functionalized graphene, and that the lectin can be liberated from the surface by the addition of a binding competitor. The chemoenzymatic assay with a horseradish peroxidase conjugate also confirmed the intact catalytic properties of the enzyme. The present approach thus paves the way towards graphene-based sensors for carbohydrate–lectin binding.

Recent Progress in the Synthesis of MoS2 Thin Films for Sensing, Photovoltaic and Plasmonic Applications: A Review

In the surge of recent successes of 2D materials following the rise of graphene, molybdenum disulfide (2D-MoS2) has been attracting growing attention from both fundamental and applications viewpoints, owing to the combination of its unique nanoscale properties. For instance, the bandgap of 2D-MoS2, which changes from direct (in the bulk form) to indirect for ultrathin films (few layers), offers new prospects for various applications in optoelectronics. In this review, we present the latest scientific advances in the field of synthesis and characterization of 2D-MoS2 films while highlighting some of their applications in energy harvesting, gas sensing, and plasmonic devices. A survey of the physical and chemical processing routes of 2D-MoS2 is presented first, followed by a detailed description and listing of the most relevant characterization techniques used to study the MoS2 nanomaterial as well as theoretical simulations of its interesting optical properties. Finally, the challenges related to the synthesis of high quality and fairly controllable MoS2 thin films are discussed along with their integration into novel functional devices.

Hydrocarbon contamination in angström-scale channels

Nonspecific molecular adsorption such as airborne contamination occurs on most surfaces including those of 2D materials and alters their properties. While surface contamination is studied using a plethora of techniques, the effect of contamination on confined systems such as nanochannels/pores leading to their clogging is still lacking. We report a systematic investigation of hydrocarbon adsorption in angstrom (Å) slit channels of varying heights. Hexane is chosen to mimic the hydrocarbon contamination and the clogging of the Å-channels is evaluated via a helium gas flow measurement. The level of hexane adsorption, in other words, the degree of clogging depends on the size difference between the channels and hexane. A dynamic transition of the clogging and revival process is shown in sub-2 nm thin channels. Long-term storage and stability of our Å-channels are demonstrated here for up to three years, alleviating the contamination and unclogging the channels using thermal treatment. This study highlights the importance of the nanochannels' stability and demonstrates the self-cleansing nature of sub-2 nm thin channels enabling a robust platform for molecular transport and separation studies. We provide a method to assess the cleanliness of nanoporous membranes, which is vital for the practical applications of nanofluidics in various fields such as molecular sensing, separation and power generation.

Predicting Gas Separation through Graphene Nanopore Ensembles with Realistic Pore Size Distributions

The development of nanoporous single-layer graphene membranes for gas separation has prompted increasing theoretical investigations of gas transport through graphene nanopores. However, computer simulations and theories that predict gas permeances through individual graphene nanopores are not suitable to describe experimental results, because a realistic graphene membrane contains a large number of nanopores of diverse sizes and shapes. With this need in mind, here, we generate nanopore ensembles in silico by etching carbon atoms away from pristine graphene with different etching times, using a kinetic Monte Carlo algorithm developed by our group for the isomer cataloging problem of graphene nanopores. The permeances of H₂, CO₂, and CH₄ through each nanopore in the ensembles are predicted using transition state theory based on classical all-atomistic force fields. Our findings show that the total gas permeance through a nanopore ensemble is dominated by a small fraction of large nanopores with low energy barriers of pore crossing. We also quantitatively predict the increase of the gas permeances and the decrease of the selectivities between the gases as functions of the etching time of graphene. Furthermore, by fitting the theoretically predicted selectivities to the experimental ones reported in the literature, we show that nanopores in graphene effectively expand as the temperature of permeation measurement increases. We propose that this nanopore "expansion" is due to the desorption of contaminants that partially clog the graphene nanopores. In general, our study highlights the effects of the pore size and shape distributions of a graphene nanopore ensemble on its gas separation properties and calls into attention the potential effect of pore-clogging contamination in experiments.

Exploring carbon particle type and plasma treatment to improve electrochemical properties of stencil-printed carbon electrodes

Stencil-printing conductive carbon inks has revolutionized the development of inexpensive, disposable and portable electrochemical sensors. However, stencil-printed carbon electrodes (SPCEs) typically suffer from poor electrochemical properties. While many surface pretreatments and modifications have been tested to improve the electrochemical activity of SPCEs, the bulk composition of the inks used for printing has been largely ignored. Recent studies of other carbon composite electrode materials show significant evidence that the conductive carbon particle component is strongly related to electrochemical performance. However, such a study has not been carried out with SPCEs. In this work, we perform a systematic characterization of SPCEs made with different carbon particle types including graphite particles, glassy carbon microparticles and carbon black. The relationship between carbon particle characteristics including particle size, particle purity, and particle morphology as well as particle mass loading on the fabrication and electrochemical properties of SPCEs is studied. SPCEs were plasma treated for surface activation and the electrochemical properties of both untreated and plasma treated SPCEs are also compared. SPCEs displayed distinct analytical utilities characterized through solvent window and double layer capacitance. Cyclic voltammetry (CV) of several standard redox probes, FcTMA+, ferri/ferrocyanide, and pAP was used to establish the effects of carbon particle type and plasma treatment on electron transfer kinetics of SPCEs. CV of the biologically relevant molecules uric acid, NADH and dopamine was employed to further illustrate the differences in sensing and fouling characteristics of SPCEs fabricated with different carbon particle types. SEM imaging revealed significant differences in the SPCE surface microstructures. This systematic study demonstrates that the electrochemical properties of SPCEs can be tuned and significantly improved through careful selection of carbon particle type and plasma cleaning with a goal toward the development of better performing electrochemical point-of-need sensors.

Hydrophobic Modification on the Surface of SiO2 Nanoparticle: Wettability Control

Good control of the morphology, particle size, and wettability of silica nanoparticles is of increasing importance to their use in a variety of fields. Here, we propose a strategy to tune the surface wettability of nanosilica by changing the dosage of a chemical modifier. A series of measurements, including scanning electron microscopy (SEM), laser scatting technique, Fourier transform infrared (FTIR) spectroscopy, thermogravimetry, and surface hydroxyl number and water contact angle measurement, were conducted to verify the surface chemistry and wettability of these nanoparticles. Through controlled chemical modification, the contact angle of the treated nanoparticles increases from 34.7 to 155° with increasing amount of dichlorodimethylsilane (DCDMS) within a molar ratio (MR) between DCDMS and nanoparticles of 5.17. The number of hydroxyl groups covered on the particle surface decreases gradually from 1.79 to 0.47, and the surface grafting rate could reach 73.7%. As the addition of dichlorodimethylsilane equals MR 5.17, the contact angle reaches the maximum value of 155°, which displays excellent superhydrophobicity. After surpassing the point of MR 5.17, the contact angle does not increase but starts to decrease, ultimately remaining stable at 135°. It can be concluded that the surface wettability of nano-SiO₂ particles can be precisely modulated by varying the amounts of the modifier. Furthermore, the modulating mechanism of the process occurring on the surface of SiO₂ particles has been investigated at the molecular level.

Computer simulations of the interaction between SARS-CoV-2 spike glycoprotein and different surfaces

A prominent feature of coronaviruses is the presence of a large glycoprotein spike protruding from a lipidic membrane. This glycoprotein spike determines the interaction of coronaviruses with the environment and the host. In this paper, we perform all atomic molecular dynamics simulations of the interaction between the SARS-CoV-2 trimeric glycoprotein spike and surfaces of materials. We considered a material with high hydrogen bonding capacity (cellulose) and a material capable of strong hydrophobic interactions (graphite). Initially, the spike adsorbs to both surfaces through essentially the same residues belonging to the receptor binding subunit of its three monomers. Adsorption onto cellulose stabilizes in this configuration, with the help of a large number of hydrogen bonds developed between cellulose and the three receptor-binding domains of the glycoprotein spike. In the case of adsorption onto graphite, the initial adsorption configuration is not stable and the surface induces a substantial deformation of the glycoprotein spike with a large number of adsorbed residues not pertaining to the binding subunits of the spike monomers.

Reversible Switching of Charge Transfer at the Graphene-Mica Interface with Intercalating Molecules

Understanding and controlling charge transfer through molecular nanostructures at interfaces is of paramount importance, particularly for electronic devices but also for contact electrification or in bioelectronics. We investigate the influence of intercalation and exchange of molecularly thin layers of small molecules (water, ethanol, 2-propanol, and acetone) on charge transfer at the well-defined interface between an insulator (muscovite mica) and a conductor (graphene). Raman spectroscopy is used to probe the charge carriers in graphene. While a molecular layer of water blocks charge transfer between mica and graphene, a layer of the organic molecules allows for it. The exchange of molecular water layers with ethanol layers switches the charge transfer very efficiently from off to on and back. We propose a charge transfer model between occupied mica trap states and electronic states of graphene, offset by the electrostatic potentials produced by the molecular dipole layers, as supported by molecular dynamics simulations. Our work demonstrates how intercalation of molecules of volatile liquids can reversibly affect charge transfer at interfaces. This implies its strong impact on the function of hybrid inorganic-organic electronic devices in different ambients and potential applications, including sensors and actuators.

Graphite Felt Modified by Atomic Layer Deposition with TiO2 Nanocoating Exhibits Super-Hydrophilicity, Low Charge-Transform Resistance, and High Electrochemical Activity

Graphite felt (GF) is a multi-functional material and is widely used as electrodes of electrochemical devices for energy and environmental applications. However, due to the inherent hydrophobicity of graphite felt, it must be hydrophilically pretreated to obtain good electrochemical activity. Metal oxides coating is one of the feasible methods to modify the surface of GF, and in order to ensure that the metal oxides have a better conductivity for obtaining higher electrochemical activity, a subsequent H₂ heat-treatment process is usually adopted. In this study, atomic layer deposition (ALD) is used to deposit TiO₂ nanocoating on graphite felt (GF) for surface modification without any H₂ thermal post-treatment. The results show that the ALD-TiO₂-modified GF (ALD-TiO₂/GF) owns excellent hydrophilicity. Moreover, the ALD-TiO₂/GF exhibits excellent electrochemical properties of low equivalent series resistance (R_s), low charge-transfer resistance (R_ct), and high electrochemical activity. It demonstrates that ALD is an applicable technique for modifying the GF surface. In addition, it can be reasonably imagined that not only TiO₂ film can effectively modify the GF surface, but also other metal oxides grown by ALD with nanoscale-thickness can also obtain the same benefits. We anticipate this work to be a starting point for modifying GF surface by using ALD with metal oxides nanocoating.

Nanoscale Profiling of 2D Surface Hydrophobicity Recognition of Environmental Media via AFM Measurements In Situ

The surface hydrophobicity and its heterogeneity are essential physicochemical properties of functional materials and environmental media, which directly influence many critical processes, such as the adsorption capacity of absorbents, water/fertilizer retention of soil and oil-water separation performances of membranes. The conventional method to characterize the surface hydrophobicity is based on the water/air/oil contact angle, which could only analyze the macroscale local hydrophobicity of the surfaces. Until now, it is impossible to profile two-dimensional surface hydrophobicity recognition in the nanoscale. Here we utilized an atomic force microscopy (AFM)-based chemical force spectroscopy to measure the topography and the local adhesion forces in the nanoscale. A novel approach is established to exploit adhesion forces to extract the hydrophobic attractions, enabling mapping of the surface hydrophobicity of environmental media in the nanoscale, which was validated by studying synthetic self-assembled monolayers of known composition. The new method was then applied to directly measure the hydrophobicity of porous biochar particles, to profile two-dimensional nanoscale hydrophobicity images of graphene oxide, and to observe the in situ variations of the graphite surface hydrophobicity in the adsorption process of benzylamine, which cannot be monitored by the conventional methods. The advantages of direct observations of the surface hydrophobicity recognition from a single AFM image dynamically and quantitatively may provide an in-depth insight into the surface hydrophobicity in the nanoscale.

Origin of Ubiquitous Stripes at the Graphite-Water Interface

The investigation of solid-liquid interfaces is pivotal for understanding processes like wetting, corrosion, and mineral dissolution and growth. The graphite-water interface constitutes a prime example for studying the water structure at a seemingly hydrophobic surface. Surprisingly, in a large number of atomic force microscopy (AFM) experiments, well-ordered stripes have been observed at the graphite-water interface. Although many groups have reported on the observation of stripes at this interface, fundamental properties and, in particular, the origin of the stripes are still under debate. Proposed origins include contamination, interplanar stacking of graphene layers, formation of methanol-water nanostructures, and adsorption of nitrogen molecules. Especially, the latter interpretation has received considerable attention because of its potential impact on explaining the long-range nature of the hydrophobic interaction. In this study, we demonstrate that these stripes readily form when using standard plastic syringes to insert the water into the AFM instrument. In contrast, when clean glass syringes are used instead, no such stripes form even though nitrogen was present. We, therefore, conclude that contaminations from the plastic syringe rather than nitrogen constitute the origin of the stripes we observe. We provide high-resolution AFM data that reveal detailed structural insights into the arrangement of the stripes. The rich variability of our data suggests that the stripes might be composed of several different chemical species. Still, we cannot rule out that the stripes observed in the literature might originate from other sources; our study offers a rather straightforward explanation for the origin of the stripes. In the view of these results, we propose to carefully reconsider former assignments.

Nitrogen-Doped Unusually Superwetting, Thermally Insulating, and Elastic Graphene Aerogel for Efficient Solar Steam Generation

By removing the oxygen-containing functional groups, thermal treatment in inert gas has been widely reported to improve the hydrophobicity of carbon materials. However, this work reports a contrary phenomenon for the nitrogen-doped graphene aerogel (NGA). As the temperature of thermal treatment increases from 200 to 1000 °C, NGA becomes more and more hydrophilic and the superwetting property remains for weeks in air. To uncover this unusual phenomenon, the effect of nitrogen doping is studied through both experiment and MD simulations. The effects of air exposure and air humidity are further investigated in detail to illustrate the whole physical picture clearly. The superwetting behavior is attributed to the preferential adsorption of water molecules to the nitrogen-doped sites, which significantly inhibits airborne hydrocarbon adsorption. In combination with the excellent properties including mechanical elasticity, high light absorption, and good thermal insulation, an efficient photothermal and solar steam generation performance is demonstrated by using NGA-600 as the photothermal material, presenting a high energy conversion efficiency of 86.2% and good recycling stability.

Control of Protein Conformation and Orientation on Graphene

Graphene-based biosensors have attracted considerable attention due to their advantages of label-free detection and high sensitivity. Many such biosensors utilize noncovalent van der Waals force to attach proteins onto graphene surface while preserving graphene's high conductivity. Maintaining the protein structure without denaturation/substantial conformational change and controlling proper protein orientation on the graphene surface are critical for biosensing applications of these biosensors fabricated with proteins on graphene. Based on the knowledge we obtained from our previous experimental study and computer modeling of amino acid residual level interactions between graphene and peptides, here we systemically redesigned an important protein for better conformational stability and desirable orientation on graphene. In this paper, immunoglobulin G (IgG) antibody-binding domain of protein G (protein GB1) was studied to demonstrate how we can preserve the protein native structure and control the protein orientation on graphene surface by redesigning protein mutants. Various experimental tools including sum frequency generation vibrational spectroscopy, attenuated total refection-Fourier transform infrared spectroscopy, fluorescence spectroscopy, and circular dichroism spectroscopy were used to study the protein GB1 structure on graphene, supplemented by molecular dynamics simulations. By carefully designing the protein GB1 mutant, we can avoid strong unfavorable interactions between protein and graphene to preserve protein conformation and to enable the protein to adopt a preferred orientation. The methodology developed in this study is general and can be applied to study different proteins on graphene and beyond. With the knowledge obtained from this research, one could apply this method to optimize protein function on surfaces (e.g., to enhance biosensor sensitivity).

The Role of Functionalization in the Applications of Carbon Materials: An Overview

The carbon-based materials (CbMs) refer to a class of substances in which the carbon atoms can assume different hybridization states (sp1, sp2, sp3) leading to different allotropic structures -. In these substances, the carbon atoms can form robust covalent bonds with other carbon atoms or with a vast class of metallic and non-metallic elements, giving rise to an enormous number of compounds from small molecules to long chains to solids. This is one of the reasons why the carbon chemistry is at the basis of the organic chemistry and the biochemistry from which life on earth was born. In this context, the surface chemistry assumes a substantial role dictating the physical and chemical properties of the carbon-based materials. Different functionalities are obtained by bonding carbon atoms with heteroatoms (mainly oxygen, nitrogen, sulfur) determining a certain reactivity of the compound which otherwise is rather weak. This holds for classic materials such as the diamond, the graphite, the carbon black and the porous carbon but functionalization is widely applied also to the carbon nanostructures which came at play mainly in the last two decades. As a matter of fact, nowadays, in addition to fabrication of nano and porous structures, the functionalization of CbMs is at the basis of a number of applications as catalysis, energy conversion, sensing, biomedicine, adsorption etc. This work is dedicated to the modification of the surface chemistry reviewing the different approaches also considering the different macro and nano allotropic forms of carbon.