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(a) Schematic illustration of emulsion droplet deposition, drying and collapse. (b) Stylus profilometry of deposited droplet showing film thickness of ∼130 nm, corresponding to ∼30 monolayers interfacial film thickness. (c) Raman map of G peak intensity illustrating the uniformity of deposited film; 30 × 30 μm image. (d) Low-magnification optical micrographs of deposited droplets on PET showing the sequential passes of emulsion deposition with percolation and formation of densely packed films; scale bars 500 μm. (e) Atomic force micrograph of nanosheet film confirming dense and uniform areal packing of the nanosheets deposited from a single emulsion droplet; scale bar 500 nm, height range 200 nm. (f) Electrical conductivity of the graphene film deposited from emulsion as a function of film thickness showing the scaling attributed to deposition uniformity, which reaches expected bulk-like value. Inset: Scanning electron micrograph of film cross section (false colored) showing the dense-packed nanosheet network; scale bar 1 μm.
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Here, we develop a framework for assembly, understanding, and application of functional emulsions stabilized by few-layer pristine two-dimensional (2D) nanosheets. Liquid-exfoliated graphene and MoS2 are demonstrated to stabilize emulsions at ultralow nanosheet volume fractions, approaching the minimum loading achievable with 2D materials. These na...
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... stability on hydrophobic substrates facilitates preferential evaporation of the solvent capping layer and confers a degree of spatial control to deposition even for manual dropwise deposition. As shown in Figure 2a, water droplets are stable on a substrate until spreading and evaporation of the capping layer of the solvent. The exposed graphene-coated water droplet then forms an unstable three-phase interface with the air (only stable for air-in-water), resulting in deformation, drying, and collapse of the droplet onto the substrate. ...
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... deposited nanosheets are essentially collapsed bilayers of the disordered interfacial films which stabilize the emulsions, enabling verification of the thin-film structure such as by stylus profilometry. Figure 2b shows a representative height profile of a deposited droplet with ∼130 nm thickness. This can be interpreted as a nanosheet network with 50% porosity, that is, ∼60 nm equivalent nanosheet thickness, as a bilayer of the coating, suggesting <30 monolayers interfacial film thickness. ...
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... addition, spectroscopic Raman mapping can be applied to deposited droplets to characterize material quality and uniformity. As shown in Figure 2c, the Raman G-peak intensity map (indicative of local graphene coverage) is extremely uniform across the droplet, confirming mitigation of the coffee ring effect which is prohibitive for deposition of standard depositions as microliter droplets. ...
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... coffee-ring-free deposition of individual droplets suggests that it could be possible to assemble these deposited films into large-area nanosheet networks as solution-processed conductive films. To prepare such films, droplets were deposited manually in a dropwise manner, allowing them to dry before further deposition and repetition until the densepacked networks were assembled, as shown in Figure 2d. These micrographs show areal increasing connectivity of the droplets as well as increasing film thickness as the deposited droplets are overcoated. ...
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... micrographs show areal increasing connectivity of the droplets as well as increasing film thickness as the deposited droplets are overcoated. The structure and thickness of these large-area films is elucidated by atomic force microscopy, as shown in Figure 2d, showing dense-packed nanosheets comparable to deposition by other techniques and allowing measurement of per-pass film thickness. The macroscopic conductivity of these films shown as a function of thickness for sequential deposition passes in Figure 2f. ...
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... structure and thickness of these large-area films is elucidated by atomic force microscopy, as shown in Figure 2d, showing dense-packed nanosheets comparable to deposition by other techniques and allowing measurement of per-pass film thickness. The macroscopic conductivity of these films shown as a function of thickness for sequential deposition passes in Figure 2f. Interestingly, the conductivity only becomes measurable after a certain number of deposition passes where the thickness is around 1 μm, but areal percolation has only just been reached. ...
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Citations
... In the nanofluid system, the PR nanosheets as a 2D nanomaterial with a high aspect ratio can serve as a Pickering stabilizer to form a stable Pickering emulsion for the nanofluid due to their high affinity in the oil-water interface [36]. These nanosheets are expectedly distributed in the fluid-fluid interface, forming a barrier between the nanofluids, assembling side-by-side, and overlapping into multilayers to prevent the coalescence of nanofluids [37]. As a result, the PR nanosheets can act as an emulsion stabilizer and functional filler in the nanofluid system. ...
To offer a solution for low heat transfer and solar energy-storage performance of conventional phase-change fluids, we designed a direct absorption solar energy collecting system based on n-octadecane/phosphorene (PR) nanosheet phase-change nanofluids for capturing solar energy with high energy conversion efficiency and rapid heat response and transfer. The phase-change nanofluids were fabricated through homogenously dispersing PR nanosheets in the n-octadecane aqueous emulsion. The resultant nanofluids exhibit good dispersion stability and suitable viscosity under an optimal emulsifier content of 9.0 wt % and a homogenization rate of 16,000 rpm. Although the nanofluids show a relatively lower latent-heat capacity compared to pristine n-octadecane emulsion, their thermal response and heat transfer were enhanced significantly due to the introduction of PR nanosheets, leading to an increase in thermal conductivity by 98.59% for the nanofluids containing 1.0 wt % PR nanosheets. More importantly, the n-octadecane/PR nanosheet nanofluids exhibit an excellent optical absorption capacity for solar photothermal conversion and thermal energy storage. The nanofluid containing 1.0 wt % phosphorene nanosheets presents a relative thermal storage capacity of 152% by taking pristine n-octadecane emulsion as a control. The nanofluids also show good working stability for the long-term use in solar photothermal conversion and thermal energy storage. The phase-change nanofluids developed in this study exhibit great application potential in solar energy capture and photothermal energy utilization thanks to their enhanced thermal conductivity, high energy-storage density, suitable viscosity, thermal reliability, and excellent photothermal conversion and energy-storage capability.
... 14 Exploiting this benefit, Ogilvie et al. fabricated conductive networks from GO or MoS 2 nanosheets (at volume fractions as low as 10 −5 )stabilized water-in-oil Pickering emulsions. 15 Pickering emulsions stabilized by 2D polymer nanosheets have also been explored for the delivery of oil-soluble pharmaceuticals. 6 Clearly, increasing the diversity of 2D materials used to prepare Pickering emulsions will open new applications. ...
A variety of two-dimensional (2D) nanomaterials, including graphene oxide and clays, are known to stabilize Pickering emulsions to fabricate structures for functions in sensors, catalysts, and encapsulation. We introduce here a novel Pickering emulsion using self-assembled amphiphilic triblock oligoglycine as the emulsifier. Peptide amphiphiles are more responsive to environmental changes (e.g., pH, temperature, and ionic strength) than inorganic 2D materials, which have a chemically rigid, in-plane structure. Noncovalent forces between the peptide molecules change with the environment, thereby imparting responsiveness. We provide new evidence that the biantennary oligoglycine, Gly4-NH-C10H20-NH-Gly4, self-assembles into 2D platelet structures, denoted as tectomers, in solution at a neutral buffered pH using small-angle X-ray scattering and molecular dynamics simulations. The molecules are stacked in the platelets with a linear conformation, rather than in a U-shape. We discovered that the lamellar oligoglycine platelets adsorbed at an oil/water interface and stabilized oil-in-water emulsions. This is the first report of 2D oligoglycine platelets being used as a Pickering stabilizer. The emulsions showed a strong pH response in an acidic environment. Thus, upon reducing the pH, the protonation of the terminal amino groups of the oligoglycine induced disassembly of the lamellar structure due to repulsive electrostatic forces, leading to emulsion destabilization. To demonstrate the application of the material, we show that a model active ingredient, β-carotene, in the oil is released upon decreasing the pH. Interestingly, in pH 9 buffer, the morphology of the oil droplets evolved over time, as the oligoglycine stabilizer created progressively a thicker interfacial layer. This demonstration opens a new route to use self-assembled synthetic peptide amphiphiles to stabilize Pickering emulsions, which can be significant for biomedical and pharmaceutical applications.
The emulsion method is a typical way for the preparation of nanomaterials, however, it usually requires a lot of energy and surfactants to form and stabilize of emulsions. A critical factor limiting its application is to control the morphology of the material by modulating the size of the emulsion droplets. In this work, a novel solid-water–oil (S-W-O) system based on the integration of metastable emulsion and salt template method was proposed, and the functionalized silica nanosheets were prepared by the one-pot method. The size of the prepared nanosheets is relatively uniform, with a length of 800 to 1200 nm and a width of about 200 nm. Subsequently, a large number of epoxy group sites were introduced into the nanosheets relying on distillation precipitation polymerization, and the polyol groups were successfully loaded. Finally, the polyol-functionalized nanosheet adsorbent (NS@GMA-NMDG) was obtained and applied for the rapid removal of boron. NS@GMA-NMDG shows favorable adsorption performance for boric acid in solution, with a maximum adsorption capacity of about 35.49 mg g−1 based on the Langmuir model fitting and a fast adsorption rate of about 30 min to reach 96.3% of the adsorption equilibrium concentration. Remarkably, the investigation of the formation mechanism of nanosheets reveals that salt plays an important role in the S-W-O system. This study not only puts forward a novel method of designing and synthesizing nanosheet adsorbents, but also the application of surfactant-free and low-energy emulsification reflects the concepts of energy saving and clean production, which provides fresh inspirations for the development of new environmentally friendly adsorbents.
Nowadays, flexible conductive materials (FCMs) have made progress in pursuing multi-performance with high conductivity, high sensitivity and stable conductive application in deformation. However, it remains a challenge to design FCMs with relatively stable conductivity under small strain but high sensitivity at large strain. Here, we report a conductive composite based on carboxylic styrene butadiene rubber (XSBR) and Ti3C2Tx MXenes using sodium alginate (SA) as dispersing agent. It owns a percolation threshold of 5.11 wt%, high electrical conductivity of 5.99 S/m and high stretchability up to 270%. The composite keeps relatively stable conductivity of no less than 4.11 S/m within 60% strain where its relative resistance change is only 35% while it shows high sensitivity with gauge factor of 11.35 beyond 120% strain. The XSBR/MXene/SA composite can be applied in sensors area which may require relatively stable and high conductivity under small strain and simultaneously could monitor action with large strain.
Humans rely increasingly on sensors to address grand challenges and to improve quality of life in the era of digitalization and big data. For ubiquitous sensing, flexible sensors are developed to overcome the limitations of conventional rigid counterparts. Despite rapid advancement in bench-side research over the last decade, the market adoption of flexible sensors remains limited. To ease and to expedite their deployment, here, we identify bottlenecks hindering the maturation of flexible sensors and propose promising solutions. We first analyze challenges in achieving satisfactory sensing performance for real-world applications and then summarize issues in compatible sensor-biology interfaces, followed by brief discussions on powering and connecting sensor networks. Issues en route to commercialization and for sustainable growth of the sector are also analyzed, highlighting environmental concerns and emphasizing nontechnical issues such as business, regulatory, and ethical considerations. Additionally, we look at future intelligent flexible sensors. In proposing a comprehensive roadmap, we hope to steer research efforts towards common goals and to guide coordinated development strategies from disparate communities. Through such collaborative efforts, scientific breakthroughs can be made sooner and capitalized for the betterment of humanity.
The direct 3D printing of ultralight architectures with ultralow-concentration 2D nanomaterial inks is necessary yet challenging. Here, we describe an emulsion-based ink for direct printing using 2D nanomaterials, i.e., MXene and graphene oxide (GO). The electrostatic interactions between the ligands in the oil phase and the 2D nanomaterials in the aqueous phase help form sheet-like surfactants at the interface. The interactions between the anchored ligands among different droplets dictate the rheological characteristics of inks, enabling a gel-like behavior ideally suitable for 3D printing at ultralow concentrations of 2D nanomaterials. The 3D printed foams possess lightweight structures with densities of 2.8 mg cm-3 (GO-based) and 4.1 mg cm-3 (MXene-based), and the latter integrates outstanding electrical conductivity, electromagnetic shielding performance, and thermal insulation comparable to air. This work describes a general approach for direct-printing ultralight porous structures that take advantage of the inherent properties of 2D building blocks.
Scalable production of high-quality graphene nanosheets is urgently needed to realize its practical applications. Here, we show that electrochemical dual-electrode exfoliation via alternating current can effectively produce high-yield solution-processable graphene. The selected molecule, tetrabutylammonium hydrogen sulfate, has the ability to diminish anodic oxidation damage to structures while also allowing for simultaneous intercalation of anion and cation species. Additionally, repeated intercalation of cations and anions with alternating current accelerates the ion intercalation and exfoliation process, leading to highly efficient graphene production with minimum destruction. After precisely controlling the intercalation process, high-yield graphene (88.6%) with uniform thickness distribution (77%, 1–3 layers) was achieved. Meanwhile, as-prepared graphene exhibits outstanding electrical conductivity (>6 × 10⁴ S m⁻¹), a considerably high C/O ratio of > 47.2, and a very low defect density (ID/IG < 0.045). Remarkably, the up-scaled manufacturing technique not only achieves a record high production rate (over 80 g h⁻¹), but also improves upon the best-reported graphene quality and exfoliation efficiency (by > 60%). This rapid and scalable method will facilitate the widespread manufacture of high-quality graphene and open up a myriad of potential uses.
