High-yield preparation of edge-functionalized and water dispersible few-layers of hexagonal boron nitride (hBN) by direct wet chemical exfoliation.

Owing to many fascinating properties including high thermal and chemical stability, excellent electrical insulation, fire-retardant and antibacterial properties, hexagonal boron nitride (hBN) has emerged as a prominent 2D material for broad applications. However, the production of high quality of hBN by chemical exfoliation from its precursor is still challenging. This paper presents a high-yield (+83%), low-cost and energy-efficient wet chemical exfoliation strategy, which produces few-layers (FL, 3-6 layers) of edge-functionalized (OH) hBN nanosheets with uniform size (486 ± 51 nm). This optimized preparation is established based on a comprehensive investigation on the key exfoliation parameters such as exfoliation temperature, time and amount of the oxidant (potassium permanganate). High quality of FL-hBN was confirmed by various characterization techniques including scanning electron microscopy coupled with energy dispersive X-ray, transmission electron microscopy, Raman, Fourier transform infrared spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy analyses. The outcome of this study paves a promising pathway to effectively produce hBN through a cost-efficient exfoliation approach, which has a significant impact on industrial applications.

Tuning the Multifunctional Surface Chemistry of Reduced Graphene Oxide via Combined Elemental Doping and Chemical Modifications.

The synthesis of graphene materials with multiple surface chemistries and functionalities is critical for further improving their properties and broadening their emerging applications. We present a simple chemical approach to obtain bulk quantities of multifunctionalized reduced graphene oxide (rGO) that combines chemical doping and functionalization using the thiol-ene click reaction. Controllable modulation of chemical multifunctionality was achieved by simultaneous nitrogen doping and gradual chemical reduction of graphene oxide (GO) using ammonia and hydrazine, followed by covalent attachment of amino-terminated thiol molecules using the thiol-ene click reaction. A series of N-doped rGO (N-rGO) precursors with different levels of oxygen groups were synthesized by adjusting the amount of reducing agent (hydrazine), followed by subsequent covalent attachment of cysteamine via the thermal thiol-ene click reaction to yield different ratios of mixed functional groups including N (pyrrolic N, graphitic N, and aminic N), S (thioether S, thiophene S, and S oxides), and O (hydroxyl O, carbonyl O, and carboxyl O) on the reduced GO surface. Detailed XPS analysis confirmed the disappearance of unstable pyridinic N in cys-N-rGO and the reduction degree threshold of N-rGO for effective cysteamine modification to take place. Our study establishes a strong correlation between different reduction degrees of N-rGO with several existing oxygen functional groups and addition of new tunable functionalities including covalently attached nitrogen (amino) and sulfur (C-S-C, C=S, and S-O). This simple and versatile approach provides a valuable contribution for practical designing and synthesis of a broad range of functionalized graphene materials with tailorable functionalities, doping levels, and interfacial properties for potential applications such as polymer composites, supercapacitors, electrocatalysis, adsorption, and sensors.

Multifunctional Binding Chemistry on Modified Graphene Composite for Selective and Highly Efficient Adsorption of Mercury.

Engineering of multifunctional binding chemistry on graphene composites using thiol-ene click reaction for selective and highly efficient adsorption of mercury(II) is demonstrated. Graphene oxide (GO) is used as an initial material for covalent attachment of cysteamine molecules by thiol-ene click reaction on C═C groups to achieve a partially reduced graphene surface with multiple binding chemistry such as O, S, and N. Batch adsorption studies showed remarkable adsorption rate with only 1 mg L-1 dosage of adsorbent used to remove 95% Hg (II) (∼1.5 mg L-1) within 90 min. The high adsorption capacity of 169 ± 19 mg g-1, high selectivity toward Hg in the presence of 30 times higher concentration of competing ions (Cd, Cu, Pb) and high regeneration ability (>97%) for five consecutive adsorption-desorption cycles were achieved. Comparative study with commercial activated carbon using spiked Hg (II) river water confirmed the high performance and potential of this adsorbent for real mercury remediation of environmental and drinking waters.

Magnetic iron oxide nanoparticles decorated graphene for chemoresistive gas sensing: The particle size effects.

We report a synthesis of magnetic nanoparticles chemically immobilized onto reduced graphene oxide sheets (referred to as rGO-Fe3O4 NPs) as a gas and vapor sensing platform with precisely designed particle size of 5, 10 and 20 nm to explore their influence of particle size on sensing performance. The rGO-Fe3O4 NP sensors have been investigated their responses to different gases and volatile organic compounds (VOCs) at part-per-million (ppm) levels. Results show that the Fe3O4 NPs with smaller size (5 and 10 nm) on the rGO surface led to a lower sensitivity, while particles of a size of 20 nm have a significant enhancement of sensitivity compared to the bare rGO sensor. The rGO-Fe3O4 NP20 sensor can detect trace amounts of NO2 gas and ethanol vapor at the 1 ppm and is highly selective to the NO2 and ethanol among other tested gases and VOCs, respectively. The particle size causes different distribution behaviour of NPs over rGO surface and interspaced between them, which results in deceased or increased the surface interactions between gas and graphene. The NPs themselves contained different defects level and the charge depletion layer that affect their adsorption gas/vapor molecules, which are explained for different sensing responses.

The hydrothermal processing of iron oxides from bacterial biofilm waste as new nanomaterials for broad applications.

Iron oxides and their hydroxides have been studied and analysed with properties of their mutual transformations under different hydrothermal conditions being indicated. Amorphous bacteria nanowires produced from biofilm waste were investigated under the influence of pH at a fixed duration (20 h) and reaction temperature (200 °C). The morphology, structure, and particle size of the transformation of hematite (α-Fe2O3) was obtained and characterised with SEM, XRD, FTIR, and particle sizer. The optimal conditions for the complete conversion of amorphous iron oxide nanowires to crystalline α-Fe2O3 is under acidic conditions where the pH is 1. The flower-like α-Fe2O3 structures have photocatalytic activity and adsorbent properties for heavy metal ions. This one-pot synthesis approach to produce α-Fe2O3 at a low cost would be greatly applicable to the recycling process of biofilm waste in order to benefit the environment.

From Graphene Oxide to Reduced Graphene Oxide: Impact on the Physiochemical and Mechanical Properties of Graphene-Cement Composites.

Graphene materials have been extensively explored and successfully used to improve performances of cement composites. These formulations were mainly optimized based on different dosages of graphene additives, but with lack of understanding of how other parameters such as surface chemistry, size, charge, and defects of graphene structures could impact the physiochemical and mechanical properties of the final material. This paper presents the first experimental study to evaluate the influence of oxygen functional groups of graphene and defectiveness of graphene structures on the axial tension and compression properties of graphene-cement mortar composites. A series of reduced graphene oxide (rGO) samples with different levels of oxygen groups (high, mild, and low) were prepared by the reduction of graphene oxide (GO) using different concentrations of hydrazine (wt %, 0.1, 0.15, 0.2, 0.3, and 0.4%) and different reduction times (5, 10, 15, 30, and 60 min) and were added to cement mortar composites at an optimal dosage of 0.1%. A series of characterization methods including scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, thermogravimetric analysis, and Fourier transform infrared spectroscopy were performed to determine the distribution and mixing of the prepared rGO in the cement matrix and were correlated with the observed mechanical properties of rGO-cement mortar composites. The measurement of the axial tension and compression properties revealed that the oxygen level of rGO additives has a significant influence on the mechanical properties of cement composites. An addition of 0.1% rGO prepared by 15 min reduction and 0.2% (wt %) hydrazine with mild level of oxygen groups resulted in a maximum enhancement of 45.0 and 83.7%, respectively, in the 28-day tensile and compressive strengths in comparison with the plain cement mortar and were higher compared to the composite prepared with GO (37.5 and 77.7%, respectively). These results indicate that there is a strong influence of the level of oxygen groups and crystallinity of graphene structures on the physiochemical and mechanical properties. The influence of these two parameters are interconnected and their careful balancing is required to provide an optimum level of oxygen groups on rGO sheets to ensure that there is sufficient bonding between the calcium silicate hydrate (C-S-H) components in the cement matrix and minimum level of defects and higher crystallinity of graphene structures, which will improve the mechanical properties of the composite. Finding the optimized balance between these two parameters is required to formulate graphene cement composites with the highest performance.

Graphene Oxide: A New Carrier for Slow Release of Plant Micronutrients.

The environmental problems and low efficiency associated with conventional fertilizers provides an impetus to develop advanced fertilizers with slower release and better performances. Here, we report of development of a new carrier platform based on graphene oxide (GO) sheets that can provide a high loading of plant micronutrients with controllable slow release. To prove this concept, two micronutrients, zinc (Zn) and copper (Cu), were used to load on GO sheets and hence formulate GO-based micronutrients fertilizer. The chemical composition and successful loading of both nutrients on GO sheets were confirmed by X-ray photoelectron spectroscopy, thermogravimetric analysis, and X-ray diffraction (XRD). The prepared Zn-graphene oxide (Zn-GO) and Cu-graphene oxide (Cu-GO) fertilizers showed a biphasic dissolution behavior compared to that of commercial zinc sulfate and copper sulfate fertilizer granules, displaying desirable fast and slow micronutrient release. A visualization method and chemical analysis were used to assess the release and diffusion of Cu and Zn in soil from GO-based fertilizers compared with commercial soluble fertilizers to demonstrate the advantages of GO carriers and show their capability to be used as a generic platform for macro- and micronutrients delivery. A pot trial demonstrated that Zn and Cu uptake by wheat was higher when using GO-based fertilizers compared to that when using standard zinc or copper salts. This is the first report on the agronomic performance of GO-based slow-release fertilizer.

Graphene-Borate as an Efficient Fire Retardant for Cellulosic Materials with Multiple and Synergetic Modes of Action.

To address high fire risks of flamable cellulosic materials, that can trigger easy combustion, flame propagation, and release of toxic gases, we report a new fire-retardant approach using synergetic actions combining unique properties of reduced graphene oxide (rGO) and hydrated-sodium metaborates (SMB). The single-step treatment of cellulosic materials by a composite suspension of rGO/SMB was developed to create a barrier layer on sawdust surface providing highly effective fire retardant protection with multiple modes of action. These performances are designed considering synergy between properties of hydrated-SMB crystals working as chemical heat-sink to slow down the thermal degradation of the cellulosic particles and gas impermeable rGO layers that prevents access of oxygen and the release of toxic volatiles. The rGO outer layer also creates a thermal and physical barrier by donating carbon between the flame and unburnt wood particles. The fire-retardant performance of developed graphene-borate composite and mechanism of fire protection are demonstrated by testing of different forms of cellulosic materials such as pine sawdust, particle-board, and fiber-based structures. Results revealed their outstanding self-extinguishing behavior with significant resistance to release of toxic and flammable volatiles suggesting rGO/SMB to be suitable alternative to the conventional toxic halogenated flame-retardant materials.

Facile Adhesion-Tuning of Superhydrophobic Surfaces between "Lotus" and "Petal" Effect and Their Influence on Icing and Deicing Properties.

Adhesion behavior of superhydrophobic (SH) surfaces is an active research field related to various engineering applications in controlled microdroplet transportation, self-cleaning, deicing, biochemical separation, tissue engineering, and water harvesting. Herein, we report a facile approach to control droplet adhesion, bouncing and rolling on properties of SH surfaces by tuning their air-gap and roughness-height by altering the concentrations of poly dimethyl-siloxane (PDMS). The optimal use of PDMS (4-16 wt %) in a dual-scale (nano- and microparticles) composite enables control of the specific surface area (SSA), pore volume, and roughness of matrices that result in a well-controlled adhesion between water droplets and SH surfaces. The sliding angles of these surfaces were tuned to be varied between 2 ± 1 and 87 ± 2°, which are attributed to the transformation of the contact type between droplet and surface from "point contact" to "area contact". We further explored the effectiveness of these low and high adhesive SH surfaces in icing and deicing actions, which provides a new insight into design highly efficient and low-cost ice-release surface for cold temperature applications. Low adhesion (lotus effect) surface with higher pore-volume exhibited relatively excellent ice-release properties with significant icing delay ability principally attributed to the large air gap in the coating matrix than SH matrix with high adhesion (petal effect).

Graphene Oxide-Assisted Liquid Phase Exfoliation of Graphite into Graphene for Highly Conductive Film and Electromechanical Sensors.

Here, we report a new method to prepare graphene from graphite by the liquid phase exfoliation process with sonication using graphene oxide (GO) as a dispersant. It was found that GO nanosheets act a as surfactant to the mediated exfoliation of graphite into a GO-adsorbed graphene complex in the aqueous solution, from which graphene was separated by an additional process. The preparation of isolated graphene from a single to a few layers is routinely achieved with an exfoliation yield of up to higher than 40% from the initial graphite material. The prepared graphene sheets showed a high quality (C/O ∼ 21.5), low defect (ID/IG ∼ 0.12), and high conductivity (6.2 × 10(4) S/m). Moreover, the large lateral size ranging from 5 to 10 µm of graphene, which is believed to be due to the shielding effect of GO avoiding damage under ultrasonic jets and cavitation formed by the sonication process. The thin graphene film prepared by the spray-coating technique showed a sheet resistance of 668 Ω/sq with a transmittance of 80% at 550 nm after annealing at 350 °C for 3 h. The transparent electrode was even greater with the resistance only 66.02 Ω when graphene is deposited on an interdigitated electrode (1 mm gap). Finally, a flexible sensor based on a graphene spray-coating polydimethylsiloxane (PDMS) is demonstrated showing excellent performance working under human touch pressure (<10 kPa). The graphene prepared by this method has some distinct properties showing it as a promising material for applications in electronics including thin film coatings, transparent electrodes, wearable electronics, human monitoring sensors, and RFID tags.

Robust Superhydrophobic Graphene-Based Composite Coatings with Self-Cleaning and Corrosion Barrier Properties.

Superhydrophobic surfaces for self-cleaning applications often suffer from mechanical instability and do not function well after abrasion/scratching. To address this problem, we present a method to prepare graphene-based superhydrophobic composite coatings with robust mechanical strength, self-cleaning, and barrier properties. A suspension has been formulated that contains a mixture of reduced graphene oxide (rGO) and diatomaceous earth (DE) modified with polydimethylsiloxane (PDMS) that can be applied on any surface using common coating methods such as spraying, brush painting, and dip coating. Inclusion of TiO2 nanoparticles to the formulation shows further increase in water contact angle (WCA) from 159 ± 2° to 170 ± 2° due to the structural improvement with hierarchical surface roughness. Mechanical stability and durability of the coatings has been achieved by using a commercial adhesive to bond the superhydrophobic "paint" to various substrates. Excellent retention of superhydrophobicity was observed even after sandpaper abrasion and crosscut scratching. A potentiodynamic polarization study revealed excellent corrosion resistance (96.78%) properties, and an acid was used to provide further insight into coating barrier properties. The ease of application and remarkable properties of this graphene-based composite coating show considerable potential for broad application as a self-cleaning and protective layer.

Graphene-Diatom Silica Aerogels for Efficient Removal of Mercury Ions from Water.

A simple synthetic approach for the preparation of graphene-diatom silica composites in the form of self-assembled aerogels with three-dimensional networks from natural graphite and diatomite rocks is demonstrated for the first time. Their adsorption performance for the removal of mercury from water was studied as a function of contact time, solution pH, and mercury concentration to optimize the reaction conditions. The adsorption isotherm of mercury fitted well with the Langmuir model, representing a very high adsorption capacity of >500 mg of mercury/g of adsorbent. The prepared aerogels exhibited outstanding adsorption performance for the removal of mercury from water, which is significant for environmental applications.

Graphene Aerogels Decorated with α-FeOOH Nanoparticles for Efficient Adsorption of Arsenic from Contaminated Waters.

Arsenic (As) is the world's most hazardous chemical found in drinking water of many countries; therefore, there is an urgent need for the development of low-cost adsorbents for its removal. Here, we report a highly versatile and synthetic route for the preparation of a three-dimensional (3D) graphene-iron oxide nanoparticle aerogel composite for the efficient removal of As from contaminated water. This unique three-dimensional (3D) interconnected network was prepared from natural graphite rocks with a simple reaction, without the use of harsh chemicals, which combines with the exfoliation of graphene oxide (GO) sheets via the reduction of ferrous ion to form a graphene aerogel composite decorated with iron oxide nanoparticles. The prepared adsorbent showed outstanding absorption performance for the removal of As from contaminated water, because of its high surface-to-volume ratio and characteristic pore network in the 3D architecture. The performed case study using real drinking water contaminated with As under batch conditions showed successful removal of arsenic to the concentration recommended by the World Health Organisation (WHO).

Dynamic performance of duolayers at the air/water interface. 2. Mechanistic insights from all-atom simulations.

The novel duolayer system, comprising a monolayer of ethylene glycol monooctadecyl ether (C18E1) and the water-soluble polymer poly(vinylpyrrolidone) (PVP), has been shown to resist forces such as wind stress to a greater degree than the C18E1 monolayer alone. This paper reports all-atom molecular dynamics simulations comparing the monolayer (C18E1 alone) and duolayer systems under an applied force parallel to the air/water interface. The simulations show that, due to the presence of PVP at the interface, the duolayer film exhibits an increase in chain tilt, ordering, and density, as well as a lower lateral velocity compared to the monolayer. These results provide a molecular rationale for the improved performance of the duolayer system under wind conditions, as well as an atomic-level explanation for the observed efficacy of the duolayer system as an evaporation suppressant, which may serve as a useful guide for future development for thin films where resistance to external perturbation is desirable.

Dynamic performance of duolayers at the air/water interface. 1. Experimental analysis.

Understanding, and improving, the behavior of thin surface films under exposure to externally applied forces is important for applications such as mimicking biological membranes, water evaporation mitigation, and recovery of oil spills. This paper demonstrates that the incorporation of a water-soluble polymer into the surface film composition, i.e., formation of a three-duolayer system, shows improved performance under an applied dynamic stress, with an evaporation saving of 84% observed after 16 h, compared to 74% for the insoluble three-monolayer alone. Canal viscometry and spreading rate experiments, performed using the same conditions, demonstrated an increased surface viscosity and faster spreading rate for the three-duolayer system, likely contributing to the observed improvement in dynamic performance. Brewster angle microscopy and dye-tagged polymers were used to visualize the system and demonstrated that the duolayer and monolayer system both form a homogeneous film of uniform, single-molecule thickness, with the excess material compacting into small floating reservoirs on the surface. It was also observed that both components have to be applied to the water surface together in order to achieve improved performance under dynamic conditions. These findings have important implications for the use of surface films in various applications where resistance to external disturbance is required.

Molecular interactions behind the synergistic effect in mixed monolayers of 1-octadecanol and ethylene glycol monooctadecyl ether.

Mixed monolayers of 1-octadecanol (C18OH) and ethylene glycol monooctadecyl ether (C18E1) were studied to assess their evaporation suppressing performance. An unexpected increase in performance and stability was found around the 0.5:0.5 bicomponent mixture and has been ascribed to a synergistic effect of the monolayers. Molecular dynamics simulations have attributed this to an additional hydrogen bonding interaction between the monolayer and water, due to the exposed ether oxygen of C18E1 in the mixed system compared to the same ether oxygen in the pure C18E1 system. This interaction is maximized around the 0.5:0.5 ratio due to the particular interfacial geometry associated with this mixture.

Foamability of aqueous suspensions of fine graphite and quartz particles with a triblock copolymer.

The foamability of a triblock copolymer solution was strongly influenced by the presence of particles. The stability of the foam was evaluated by measuring the foam volume, the drainage of water and particles, and the bubble size as a function of time. The higher stability of foams produced with hydrophilic quartz particles, compared with hydrophobic graphite particles, was related to the presence of quartz aggregates in the lamellae and Plateau borders reducing water drainage, and therefore thin film rupture and bubble coalescence. The copolymer adsorbed slightly more on the hydrophobic graphite, causing the particles to disperse and drain, with the water out of the foam, whereas the hydrophilic quartz aggregated and remain in the lamellar and Plateau borders of the foam.