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Precise control of versatile microstructure and properties of graphene aerogel via freezing manipulation
Abstract
A deep understanding of the shaping technique is urgently required to precisely tailor the pore structure of a graphene aerogel (GA) in order to fit versatile application backgrounds. In the present study, the microstructure and properties of GA were regulated by freeze-casting using an ice crystal template frozen from −10 °C to −196 °C. The phase field simulation method was applied to probe the microstructural evolution of the graphene–H2O system during freezing. Both the experimental and simulation results suggested that the undercooling degree was fundamental to the nucleation and growth of ice crystals and dominated the derived morphology of GA. The pore size of GA was largely regulated from 240 to 6 μm via decreasing the freezing temperature from −10 °C to −196 °C but with a constant density of 8.3 mg cm⁻³. Rapid freeze casting endowed GA with a refined pore structure and therefore better thermal, electrical, and compressive properties, whereas the GA frozen slowly had superior absorption properties owing to the continuous and tube-like graphene lamellae. The GA frozen at −196 °C exhibited the highest Young's modulus of 327 kPa with similar densities to those reported in the literature. These findings demonstrate the diverse potential applications of GA with regulated pore morphologies and also contribute to cryogenic-induced phase separation methods.
... After sublimating ice crystals, a three-dimensional, highly porous structure can be synthesized. In freeze casting, it is easy to control the pore size of aerogel through the temperature gradient [35]. Additionally, it is ecofriendly. ...
... However, precise equipment is needed to control the temperature gradient finely [36]. Likewise, during the freeze casting, factors such as temperature and freezing speed can be controlled to modify the pore size of the aerogel [35,37]. The available control variables in the freeze-casting method and their impacts on the pore size of aerogel are summarized in Table 2. Table 2. Available control variables and their impacts on pore size in the freeze casting. ...
... The lower the freezing temperature, the smaller the pores [35] Precursor concentration The higher the precursor concentration, the smaller the pores [38] Flake size The larger the graphene flakes, the smaller the pores [39] Suspension viscosity The higher the viscosity, the smaller the pores [39] ...
Aerogels are three-dimensional solid networks with incredibly low densities, high porosity, and large specific surface areas. These aerogels have both nanoscale and macroscopic interior structures. Combined with graphene, the aerogels show improved mechanical strength, electrical conductivity, surface area, and adsorption capacity, making them ideal for various biomedical applications. The graphene aerogel has a high drug-loading capacity due to its large surface area, and the porous structure enables controlled drug release over time. The presence of graphene makes it a suitable material for wound dressings, blood coagulation, and bilirubin adsorption. Additionally, graphene's conductivity can help in the electrical stimulation of cells for improved tissue regeneration, and it is also appropriate for biosensors. In this review, we discuss the preparation and advantages of graphene-based aerogels in wound dressings, drug delivery systems, bone regeneration, and biosensors.
... The microstructure of aerogel, especially the pore size and surface area, can be easily modified by precisely controlling nucleation and growth of ice crystals in different freezing temperature (Fig. 6b). Zhu et al. [53] have demonstrated the impact of freezing temperature onto final aerogel microstructure in wide range of temperature (−10 °C to −196 °C) ( Fig. 7a-e). In the whole temperature range, the obtained aerogels were characterized by a similar density, but differed in morphology. ...
... The freezing temperature and freezing rate influence the nucleation and growth of ice crystals, and thus, have a direct impact onto aerogel microstructure. Decreases of freezing temperature causes smaller average pore size of aerogel [53]. In the FD process, a high number of mesopores fuse together into larger ones, causes higher specific surface area, and pore volume [24]. ...
... The impact of temperature and time of freeze casting method onto aerogel microstructure: a schematic illustration of the ice crystal growth in graphene-based hydrogel; b schematic illustration of the crystallization of Gn under different freezing temperature. Adapted with permission from[53] ...
We summarized the research status of graphene-based aerogels and put forward the challenges and outlook of
graphene-based aerogels dedicated to biomedical usage, especially by the formation of joints with biocompatible metals.
... The microstructure of aerogel, especially the pore size and surface area, can be easily modified by precisely controlling nucleation and growth of ice crystals in different freezing temperature (Fig. 6b). Zhu et al. [53] have demonstrated the impact of freezing temperature onto final aerogel microstructure in wide range of temperature (−10 °C to −196 °C) ( Fig. 7a-e). In the whole temperature range, the obtained aerogels were characterized by a similar density, but differed in morphology. ...
... The freezing temperature and freezing rate influence the nucleation and growth of ice crystals, and thus, have a direct impact onto aerogel microstructure. Decreases of freezing temperature causes smaller average pore size of aerogel [53]. In the FD process, a high number of mesopores fuse together into larger ones, causes higher specific surface area, and pore volume [24]. ...
... The impact of temperature and time of freeze casting method onto aerogel microstructure: a schematic illustration of the ice crystal growth in graphene-based hydrogel; b schematic illustration of the crystallization of Gn under different freezing temperature. Adapted with permission from[53] ...
Graphene-based aerogels (GA) have a high potential in the biomedical engineering field due to high mechanical strength, biocompatibility, high porosity, and adsorption capacity. Thanks to this, they can be used as scaffolds in bone tissue engineering, wound healing, drug delivery and nerve tissue engineering. In this review, a current state of knowledge of graphene (Gn) and graphene oxide (GO) aerogels and their composites used in biomedical application is described in detail. A special focus is paid first on the methods of obtaining highly porous materials by visualizing the precursors and describing main methods of Gn and GO aerogel synthesis. The impact of synthesis parameters onto aerogel microstructure and porosity is discussed according to current knowledge. Subsequent sections deal with aerogels intended to address specific therapeutic demands. Here we discuss the recent methods used to improve Gn and GO aerogels biocompatibility. We explore the various types of GA reported to date and how their architecture impacts their ultimate ability to mimic natural tissue environment. On this basis, we summarized the research status of graphene-based aerogels and put forward the challenges and outlook of graphene-based aerogels dedicated to biomedical usage especially by formation of joints with biocompatible metals.
... However, low-frequency absorbing materials can not meet the current demand. In the past decades, graphene-based microwave absorption materials have been extensively explored [5][6][7]. To solve these issues, various magnetic nanoparticles are introduced into graphene to improve the microwave absorption in low-frequency of graphene-based dielectric materials in an effective way [8][9][10]. ...
... Moreover, impedance matching characteristic ( ) describes the ability of electromagnetic waves to propagate into materials by generating more input impedance ( ) rather than a reflection of the air [41,45]: (6) where constants present the light velocity ( ), is the frequency (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18), indicates the thickness of samples and parameters and represent the permittivity and permeability. In general, the value of in the range of 0.8-1.2 ...
... Moreover, impedance matching characteristic ( ) describes the ability of electromagnetic waves to propagate into materials by generating more input impedance ( ) rather than a reflection of the air [41,45]: (6) where constants present the light velocity ( ), is the frequency (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18), indicates the thickness of samples and parameters and represent the permittivity and permeability. In general, the value of in the range of 0.8-1.2 ...
Graphene-based microwave absorption materials have been profoundly and extensively investigated because of their excellent properties and is potential absorbing material. However, graphene-based absorbing materials about low-frequency remain to be researched and developed. Here we obtain that Reduced Graphene Oxide (RGO) modified with magnetic xNi/yNiO nanoparticles by a simple hydrothermal method, which presents a large reflection loss (RL) in S-band. The electromagnetic parameters, attenuation constants and impedance matching of xNi/yNiO/RGO composite were analyzed to determine the appropriate ratio of magnetic xNi/yNiO nanoparticles and GO. The maximum RL can reach −46.5 dB with a thickness of 3.6 mm at 3.57 GHz with a loading wax ratio of 15 wt %. The probability of falling on EAB is maximized to cover 94.5% of the S-band. Notably, the RL and efficient absorption bandwidth (EAB) of xNi/yNiO/RGO composite material can be adjusted according to the quarter wavelength matching theory. All the results indicate that xNi/yNiO/RGO composite has a great potential of low frequency absorbing material for low frequency absorbing material research on practical.
... It was reported that micropores become denser, and the pore size decreases as the temperature of the cooling source decreases. Enhanced properties of graphene aerogels, including thermal, electrical, and compressive ones, were obtained by decreasing the freezing temperature from −10 °C to −196 °C [17]. ...
... As it can be observed, the modified aerogels show a well-consolidated 3D structure. Lower TFC (higher freeze-casting rate) results in more compact monoliths with small pores and improved mechanical stability in comparison to the corresponding counterparts frozen at higher TFC, which is attributed to the smaller size of ice water crystals induced by the high freezing rate [17]. The effect of preparation parameters on the apparent density and pore volume was further analyzed. ...
... The freeze-casting at low TFC by using liquid nitrogen appears to result in the highest nitrogen content in the modified aerogel, most probably because the smaller ice water crystals formed at such rate do not disrupt the network between dendrimer and rGO formed during the hydrothermal process. Moreover, it was suggested that oversized solvent crystals could damage the GO sheets, as well [17]. ...
This article presents novel poly(amidoamine) (PAMAM) dendrimer-modified with partially-reduced graphene oxide (rGO) aerogels, obtained using the combined solvothermal synthesis-freeze-casting approach. The properties of modified aerogels are investigated with varying synthesis conditions, such as dendrimer generation (G), GO:PAMAM wt. ratio, solvothermal temperature, and freeze-casting rate. Scanning electron microscopy, Fourier Transform Infrared spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy are employed to characterize the aerogels. The results indicate a strong correlation of the synthesis conditions with N content, N/C ratio, and nitrogen contributions in the modified aerogels. Our results show that the best CO2 adsorption performance was exhibited by the aerogels modified with higher generation (G7) dendrimer at low GO:PAMAM ratio as 2:0.1 mg mL−1 and obtained at higher solvothermal temperature and freeze-casting in liquid nitrogen. The enclosed results are indicative of a viable approach to modify graphene aerogels towards improving the CO2 capture.
... The pore wall thickness in the aerogel is negligible, and is not considered in the calculations. The reported pore diameters of freeze-cast aerogels range from 10 to 240 microns, far above the upper limit of the nanofluidic channel (< 100 nm) 39 . Hence, the electrical double layer at the GO surface does not significantly influence the flow 40 . ...
... The average pore diameter rises slowly at low temperatures but increases rapidly near 0°C. This is because along with the increase in freezing temperature, the ice nucleation rate decreases non-linearly, while the growth rate increases initially and then reduces 39 . Figure 2f illustrates the bidirectional freeze-casting method for constructing the planar structure. ...
Materials following Murray’s law are of significant interest due to their unique porous structure and optimal mass transfer ability. However, it is challenging to construct such biomimetic hierarchical channels with perfectly cylindrical pores in synthetic systems following the existing theory. Achieving superior mass transport capacity revealed by Murray’s law in nanostructured materials has thus far remained out of reach. We propose a Universal Murray’s law applicable to a wide range of hierarchical structures, shapes and generalised transfer processes. We experimentally demonstrate optimal flow of various fluids in hierarchically planar and tubular graphene aerogel structures to validate the proposed law. By adjusting the macroscopic pores in such aerogel-based gas sensors, we also show a significantly improved sensor response dynamics. In this work, we provide a solid framework for designing synthetic Murray materials with arbitrarily shaped channels for superior mass transfer capabilities, with future implications in catalysis, sensing and energy applications.
... Built on this conception, it is worth noting that the removal of unnecessary surface functional groups at quasi-2D/1D heterointerfaces combined with ex situ alignment induced approaches may contribute to the formation of ordered symmetric, centrosymmetric, and asymmetric nanoarchitecture patterns [23]. These outcomes are derived from the chemical and physical crosslinking of monodispersed colloids that occur during the manipulation of freezing conditions [222], such as thermal gradients (∆T H and ∆T V ) and directionality (Figure 16a-c) [219,223]. Furthermore, the implementation of external fields, such as electromagnetic [117,224], light irradiation [119], and ultrasound acoustic [118], can induce ex situ alignment directionality and orientation patterns of quasi-2D/1D at liquid-solid interfaces during solidification [19]. ...
... Built on this conception, it is worth noting that the removal of unnecessary surface functional groups at quasi-2D/1D heterointerfaces combined with ex situ alignment induced approaches may contribute to the formation of ordered symmetric, centrosymmetric, and asymmetric nanoarchitecture pa erns [23]. These outcomes are derived from the chemical and physical crosslinking of monodispersed colloids that occur during the manipulation of freezing conditions [222], such as thermal gradients (ΔTH and ΔTV) and directionality (Figure 16a-c) [219,223]. Furthermore, the implementation of external fields, such as electromagnetic [117,224], light irradiation [119], and ultrasound acoustic [118], can induce ex situ alignment directionality and orientation pa erns of quasi-2D/1D at liquid-solid interfaces during solidification [19]. ...
In the pursuit of advanced functional materials, the role of low-dimensional van der Waals (vdW) heterointerfaces has recently ignited noteworthy scientific interest, particularly in assemblies that incorporate quasi-2D graphene and quasi-1D nanocellulose derivatives. The growing interest predominantly stems from the potential to fabricate distinct genres of quasi-2D/1D nanoarchitecture governed by vdW forces. Despite the possibilities, the inherent properties of these nanoscale entities are limited by in-plane covalent bonding and the existence of dangling π-bonds, constraints that inhibit emergent behavior at heterointerfaces. An innovative response to these limitations proposes a mechanism that binds multilayered quasi-2D nanosheets with quasi-1D nanochains, capitalizing on out-of-plane non-covalent interactions. The approach facilitates the generation of dangling bond-free iso-surfaces and promotes the functionalization of multilayered materials with exceptional properties. However, a gap still persists in understanding transition and alignment mechanisms in disordered multilayered structures, despite the extensive exploration of monolayer and asymmetric bilayer arrangements. In this perspective, we comprehensively review the sophisticated aspects of multidimensional vdW heterointerfaces composed of quasi-2D/1D graphene and nanocellulose derivatives. Further, we discuss the profound impacts of anisotropy nature and geometric configurations, including in-plane and out-of-plane dynamics on multiscale vdW heterointerfaces. Ultimately, we shed light on the emerging prospects and challenges linked to constructing advanced functional materials in the burgeoning domain of quasi-3D nanoarchitecture.
... Particular drying methods must be applied to replace the pore liquid with air while maintaining the solid network. Supercritical CO 2 drying [27][28][29][30] (SD) and freeze-drying [31][32][33][34] (FD) are two commonly used dying methods. In the SD method, the liquid solvent in the hydrogel is replaced with the supercritical CO 2 (above CO 2 critical temperature 31°C and pressure 7.3 MPa). ...
... The energy absorption capacity of GA FD samples was not as high as that of the GA SD sample, but it should mention that this does not mean that all kinds of GA FD samples have a low energy absorption capacity. If the freezing temperature and drying rate are tuned properly, the random nanoscale porous structures may also be obtained for the GA FD samples [31]. Generally speaking, as the freezing temperature decreases, the crystallization period gets shorter, and the average pore size of GA becomes smaller. ...
The experimental investigation of the dynamic mechanical behavior of graphene aerogel is still lacking due to its low impendence. The present work, therefore, reports on the preliminary experimental characterization of the energy absorption characteristics of graphene aerogel by using the split Hopkinson pressure bar emphasis on the influence of the drying method. The graphene aerogels were synthesized by the sol-gel method and dried, either by supercritical CO2 drying (SD) or by freeze-drying methods (FD). It was observed that under dynamic uniaxial compression, the SD samples exhibited a negative Poisson's ratio throughout gradual compression. However, FD samples failed by radial shattering without this auxetic behavior. The energy dissipation ratios of SD samples increased from 41% to 73% as expected with the specimen thickness increasing from 3mm to 12mm, being overall higher in comparison with FD samples which rises from 35% to 43%. SD graphene aerogels have a large number of random pores (∼50nm), which is beneficial for absorbing the kinetic energy through plastic deformation and pore walls’ collapse. By contrast, the FD graphene aerogels’ pore walls buckle readily under the impact, and fail due to their ordered porous structure at the micron scale (∼1μm), which impairs their energy absorption capability.
... The electrical conductivity values for the different aerogels have been reported to range from 0.135 to 980 SÁm À1 ( Figure 3A and Table S1). 4,6,28,35,[43][44][45][46][47][48][49][50][51][52][53][54] Remarkably, the electrical conductivity of the electrothermally reduced rGO aerogel, achieved in just 30 s, surpasses that of most reported aerogels that were furnace-reduced at 1000 C for 2 h, such as the Cu-Al 2 O 3 /rGO aerogel ( Figure 3B). This highlights the high effectiveness of direct Joule-heating in enhancing the conductivity of nanocarbon aerogels. ...
Ultra‐high‐temperature flash Joule‐heating of organometallic precursor‐embedded reduced graphene oxide (rGO) aerogel represents a highly efficient approach for the ultrafast production of nanocatalysts, while such a methodology has been scarcely applied to 3D nanocarbon‐based aerogel monoliths. Herein, we demonstrate the rapid synthesis of MoO2 nanoparticles within the aerogel matrix via a 1‐s high‐temperature flash Joule‐heating process (~1700°C), resulting in the formation of the hybrid MoO2@rGO aerogel with uniformly distributed nanoparticles. Nitrogen adsorption/desorption analysis indicates discernible internal microstructural disparities attributed to the additional 1‐s flash‐heating and the substantial generation of MoO2 nanoparticles. This aerogel exhibits exceptional catalytic functionality, achieving up to 99.8% efficiency in converting dibenzothiophene to dibenzothiophene sulfone. Density functional theory calculations provide insights into the catalytic mechanism, revealing that the Mo center shows accumulated electron density contributed from the electron‐rich graphene substrate. This electron density enhancement significantly enhances the catalytic activity, enabling deep desulfurization. The proposed flash nanocatalyst synthesis approach presented here can be extended to fabricate multimetallic nanocatalysts and high‐entropy alloys within the cylindrical aerogel entity, exhibiting great potential for applications in industry‐relevant flow chemistry, electrochemistry, industrial catalysis, and beyond.
... Because during the freezing of hydrogel graphene sheets are compressed by ice crystals and stacked together forming the cells and walls of rGOA [18,19], the process gives an unique possibility to control the porous structure through the regulation of the growth pattern of ice crystals. [20][21][22]. ...
Efficient adjusting of reduced graphene oxide aerogels properties requires information about experimental factor-aerogel property relationship. In this work, the reduced graphene oxide aerogels surface and textural functionalities in relation to precursor concentration, gelation time and hydrogel freezing temperature were studied in detail, with the use of dynamic adsorption method of gaseous organic probes and experimental design. The precursor concentration and the hydrogel freezing temperature have the strongest influence on textural properties - a negative correlation with apparent surface area was observed. The highest value of 229.36 m2 g−1 was obtained for samples synthesized at the lowest concentration of precursor (2 mg mL−1) and hydrogel freezing temperature (−196 °C). Low precursor concentration promote formation of more hydrophobic aerogels. All aerogels display tendencies for dispersive, dipole-type and electron donor interactions. Moreover, a repulsion of electron lone pairs was observed, as well as shape-based selectivity (originating from porosity and surface roughness) in gas-solid adsorption process. Analysis of the free surface energy revealed that the maximum value (193.21 mJ m−2) is obtained at 7.2 mg mL−1 precursor concentration, − 104 °C hydrogel freezing temperature and 23 h gelation time. Presented findings can translate directly into reduced graphene oxide aerogels tailored for specific applications such as adsorption or catalysis.
... These unique materials remain without a standardized technique for characterization or modeling of thermal properties. Review of the existing literature determines that optical non-invasive probing is underdeveloped but is the most promising type of characterization to determine GA's fundamental properties without compromising it with embedded materials or alterations of the macroporous structure that make most characterization techniques more accurate but give little insight into the intrinsic properties of GA derived from only rGO [17,18]. Reliable characterization of GA is crucial to identifying plausibility in applications, specifically characterizing its thermal conductivity for potential use as thermal insulation of spacecraft vehicles or usage as a semiconducting material in electronics for extreme applications [2,19,20]. ...
Graphene aerogel (GA) is a promising material for thermal management applications across many fields due to its lightweight and thermally insulative properties. However, standard values for important thermal properties, such as thermal conductivity, remain elusive due to the lack of reliable characterization techniques for highly porous materials. Comparative infrared thermal microscopy (CITM) is an attractive technique to obtain thermal conductance values of porous materials like GA, due to its non-invasive character, which requires no probing of, or contact with, the often-delicate structures and frameworks. In this study, we improve upon CITM by utilizing a higher resolution imaging setup and reducing the need for pore-filling coating of the sample (previously used to adjust for emissivity). This upgraded setup, verified by characterizing porous silica aerogel, allows for a more accurate confirmation of the fundamental thermal conductivity value of GA while still accounting for the thermal resistance at material boundaries. Using this improved method, we measure a thermal conductivity below 0.036 W/m\dotK for commercial GA using multiple reference materials. These measurements demonstrate the impact of higher resolution thermal imaging to improve accuracy in low density, highly porous materials characterization. This study also reports thermal conductivity for much lower density (less than 15 mg/cm^3) GA than previously published studies while maintaining the robustness of the CITM technique.
... The aerogel porosity parameters, including the size, shape, and distribution of solvent ice crystals, can be further tailored by the freeze-casting method [11]. Since the resulting pores mirror the frozen solvent crystals [12], the thermal, electrical, compressive, and sorption properties could be enhanced by adjusting the cooling rate, e.g., freeze-casting in liquid nitrogen [13]. 2 of 12 Another typical approach to improve the properties of GO sorbents is based on exploiting the surface chemistry of GO to attach amine-containing molecules, e.g., ethylenediamine [14]. Recently, poly(amidoamine) (PAMAM) dendrimers have gained research interest for CO 2 capture as they exhibit the advantage of a higher number of functional amine end groups that are available for cross-linking [15][16][17]. ...
Innovative dendrimer-modified graphene oxide (GO) aerogels are reported by employing generation 3.0 poly(amidoamine) (PAMAM) dendrimer and a combined synthesis approach based on the hydrothermal method and freeze-casting followed by lyophilization. The properties of modified aerogels were investigated with the dendrimer concentration and the addition of carbon nanotubes (CNTs) in varying ratios. Aerogel properties were evaluated via scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The obtained results indicated a strong correlation of the N content with the PAMAM/CNT ratio, where optimum values were revealed. The CO2 adsorption performance on the modified aerogels increased with the concentration of the dendrimer at an appropriate PAMAM/CNT ratio, reaching the value of 2.23 mmol g−1 at PAMAM/CNT ratio of 0.6/0.12 (mg mL−1). The reported results confirm that CNTs could be exploited to improve the functionalization/reduction degree in PAMAM-modified GO aerogels for CO2 capture.
... As an important class of electromagnetic wave absorbing materials, a dielectric absorbent, although it does not have magnetic loss capability and cannot theoretically achieve as excellent impedance matching as a magnetic absorbent, it still meets the requirements for wide efficient absorption bandwidth or low reflection loss (RL) to a great extent, not to mention it has better high-temperature resistance and oxidation resistance than the magnetic absorbent. [1][2][3] Porous structural design of carbon materials is an efficient approach to deal with impedance mismatching caused by the high conductivity of carbon networks. [4][5][6][7][8][9][10][11] The inherent void space validates the dispersion in the matrix when it acts as an absorbent, which is another way to decrease the filler ratio/cost. ...
Porous carbon frameworks functionalized with multi-scale particles are an important approach in highly efficient microwave absorbent design since impedance matching and polarization/conductivity loss are well balanced. The absorbent with a versatile structure that is not easily deformed within application is still in need. In this work, open-cell porous graphene (PG) functionalized with [Formula: see text] nanorods is synthesized by the solvothermal method, creating a high specific surface area structure. Complex electromagnetic parameters indicate that the introduction of [Formula: see text] nanorods improves the microwave absorption (MA) performance by optimizing the impedance matching and enhancing the polarization relaxation due to the large numbers of PG@[Formula: see text] interfaces. Polarization loss plays a dominant role in microwave attenuation. In addition, the designed clathrate-like structure is anticipated to further attenuate the MA wave by multiple reflections and scatterings. Interface polarization and dipole polarization can be identified with frequencies in Cole–Cole plotting. The optimized PG@[Formula: see text] composite with a thickness of 1.92 mm exhibits a minimum reflection loss of −55.10 dB at 14.29 GHz with a low filler ratio of 10 wt. %. This work is heuristic in the absorbent structure design and is helpful in enhancing and identification of polarization relaxation loss.
... As a result, the conductivity of the MnO x phase can be signicantly boosted by the PrGO networks. 51 In addition, the wrapped MnO x phase is amorphous, as conrmed by the selected area electron diffraction (SAED) patterns (the inset of Fig. 1h). Therefore, this material is called PrGO-boosted a-MnO x microspheres (PrGO-MnO x ). ...
Amorphous manganese oxides (a-MnOx) are widely considered promising materials systems to fabricate cathodes in aqueous zinc ion batteries (AZIBs). However, the Zn-storage mechanism of a-MnOx is still not understood, and...
... Using the ice crystal template as an example, GO or rGO is a popular building block; the GO or rGO dispersion is freeze-casted, and the obtained monolith is freezedried to remove the ice template, producing an intact 3D graphene material. Zhu et al. 195 regulated the microstructure and properties of 3D graphene aerogel (GA) using the freeze casting method, and the GA's pore size was effectively regulated from 240 to 6 μm by decreasing the temperature from −10 to −196°C. The GA frozen at −196°C exhibits the highest Young's modulus (327 kPa) with a density similar to those reported in the literature. ...
Porous flow fields distribute fuel and oxygen for the electrochemical reactions of proton exchange membrane (PEM) fuel cells through their pore network instead of conventional flow channels. This type of flow fields has showed great promises in enhancing reactant supply, heat removal, and electrical conduction, reducing the concentration performance loss and improving operational stability for fuel cells. This review presents the research and development progress of porous flow fields with insights for next-generation PEM fuel cells of high power density (e.g., ∼9.0 kW L-1). Materials, fabrication methods, fundamentals, and fuel cell performance associated with porous flow fields are discussed in depth. Major challenges are described and explained, along with several future directions, including separated gas/liquid flow configurations, integrated porous structure, full morphology modeling, data-driven methods, and artificial intelligence-assisted design/optimization.
... Mesoporous carbon materials have been obtained via a variety of methods including templating, which may produce ordered mesopores with narrow pore size distribution. 10,11 Recently, we established a templating process that involves metal carbide (LaC 2 , CaC 2 , YC 2 ) intermediates to form graphitic carbon. 12 −14 At temperatures above 500°C, carbon is formed by passing steam and acetylene gas over the metal carbide. ...
Two-dimensional mesoporous hexagonal carbon sheets (MHCSs) have been prepared via a chemical vapor deposition method employing mesoporous Mg(OH) 2 hexagonal sheets as the template and acetylene gas as the carbon precursor. MHCSs with porosity in the micropore−mesopore range have a high specific surface area of 1785 m 2 ·g −1. The hierarchical microporous− mesoporous pore structure enables rapid ion transport across the hexagonal carbon sheets, resulting in superior electrochemical performance. The MHCS electrodes showed a maximum specific capacitance of 162 F·g −1 at 5 mV s −1 using the electrolyte 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI). MHCS symmetric coin cells exhibited a maximum energy density of 67 Wh·kg −1 at 0.5 A·g −1 and a maximum power density of 14.97 kW· kg −1 at 10 A·g −1 .
... One potential strategy is to tailor graphene sheets to form three-dimensional (3D) structures, 16,17 such as foams, 18 hydrogels, 19,20 and networks. 21 Various 3D carbon nanosheets have been prepared through different methods, including (1) incorporating carbon materials between the nanosheets, such as carbon nanotubes, 22 (2) carbonizing and activating biomass, 23,24 (3) and using metal compounds as templates. ...
Hierarchical porous carbon nanosheet (HPCN) materials of different thicknesses were fabricated on Mg(OH) 2 substrates utilizing a one-step chemical vapor deposition (CVD) approach. The templated carbon nanosheets are closely packed hierarchical nanostructures possessing high surface areas varying from 1323 to 1978 m 2 ·g −1. Symmetrical electrochemical double-layer capacitors (EDLCs) were constructed using the HPCN and demonstrated a high specific capacitance of 205 F·g −1 at 5 mV/s using 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl)imide (EMIM-TFSI) as an electrolyte. The energy density was 86 Wh·kg −1 , and a power density of up to 16.57 kW·kg −1 was observed. After 5000 charge−discharge cycles at 10 A·g −1 , the supercapacitor retained 90% of its initial capacity. This illustrates the great stability of the templated mesoporous carbon sheets for supercapacitors.
... Therefore, controllable construction of the pore size of aerogel fibers implicates the improved thermal insulating performance. Previous reports have shown that combining nanoparticles with a polymer solution or changing the freezing temperature could regulate the pore size of aerogels [25][26][27][28]. However, these methods require sophisticated equipment, and the agglomeration of nanoparticles hinders their applicability. ...
Application of aerogel fibers in thermal insulating garments have sparked a substantial interest. However, achieving a high porosity and low thermal conductivity for aerogel fibers remain challenging, despite the innovative designs of porous structure. Herein, we fabricated lightweight and super-thermal insulating polyimide (PI) aerogel fibers via freeze-spinning by using polyvinyl alcohol (PVA) as a pore regulator. The high affinity of PVA with water enables it to accelerate the ice crystal nucleation, adjust pore formation, and construct a controllable porous structure of PI aerogel fiber. The as-fabricated PI aerogel fiber has a considerable reduced pore size, high porosity (95.6%), improved flexibility and mechanical strength, and can be woven into fabrics. The PI aerogel fabric exhibits low thermal conductivity and excellent thermal insulation in a wide range of temperature (from − 196 to 300 °C). Furthermore, the PI aerogel fabrics can be easily functionalized to expand their applications, such as in intelligent temperature regulation and photothermal conversion. These results demonstrate that the aerogel fibers/fabric are promising materials for next-generation textile materials for personal thermal management.
... This mixture is then freeze dried to form a Functionalised Graphene Aerogel (FGA). By heating FGA in a microwave an Ultra-light Graphene Aerogel [21,23] is formed (Fig. 2). ...
Graphene is novel & invigorating nanocarbon material that has drawn in both scholastic and industrial interest in last 10-15 years. It is by and large widely investigated as a result of its extraordinary properties including, superior mechanical and non-corrosive property, amazingly high surface area, great optical translucency, outstanding electrical and heat carrying properties and so forth. More importantly, these properties can be tailored to some extent by developing different grades of derivatives of Graphene. In this report, a various routes used to produce Graphene & Graphene-based materials are reviewed, detailed review of recent progress in application of these materials for desalination of water and purification of air is presented. Various attributes of Graphene & its derivatives which can revolutionise the air filtration & water desalination processes are underlined. Graphene Aerogels and doped sheets prove to be efficient alternates for air filtration & pollutant removal applications with relatively high specific surface area of around 1300 m 2 /g. These doped derivatives show improved adsorption efficiency around 55-65%, which is studied to be rising with energy inputs like potential difference. Graphene and its derivatives-based water filtration is proving to be extremely efficient than the conventional water filtration & desalination mechanisms. Reverse Osmosis (RO), one of the widely used conventional methods of desalination comes with a large expense of money and energy but yields low volumes of filtered water. About 85-95% of input water gets wasted in the RO mechanism, yielding only 5-15% of usable water. The conventional Solar desalination is more efficient as compared to RO, but even that can be improved by 70-90% [45-47] by incorporating Graphene and its derivatives as part of a volumetric system, light-absorbing layer and filtration layer in Solar Desalination process. The graphene-based membranes because of their high permeability and excellent ion repellency can be used for desalination applications with a removal efficiency of 33-100% depending on the pore size and the applied pressure. The Capacitive Deionization method of desalination can also be a very efficient alternative to the RO due to its minimal energy requirement of only 0.1-2.03 kWh/m 3.
... Figure 4a-h show C1s and N1s XPS spectra of different 3D N-GA(x). The C1s peaks can be fitted to five typical peaks at 284.8, 285.9, 286.9, 288.7 and 291.3 eV, corresponding to C-C, C-O, C-N, O-C=O and C=C, respectively [31]. In addition, the nitrogen-containing groups in the 3D N-GA(x) samples were confirmed by the high-resolution N1s spectra. ...
Three-dimensional nitrogen-doped graphene aerogels (3D N-GAs) were prepared by ultrasonic stripping, hydrothermal reduction and freeze drying. Those N-GAs exhibited high-specific surface area, high nitrogen doping amount, and 3D porous network structure. Soft electro-active ionic polymer actuators were developed for the first time using this 3D N-GA soft electrode. The developed soft actuator exhibited large peak-to-peak displacement of 11.8 mm (3 V and 0.1 Hz) and high air working durability for 93.8% after 6 h cycles. These successful demonstrations elucidated the wide potential of 3D N-GA soft actuators for the next-generation soft robotic devices.
... More recently, Wang et al [17] developed mechanically strong, super elastic rGO aerogel by using bidirectional freeze-casting method. Several parameters, such as flake size, concentration, cooling rate, and mould shape have been found to influence the final microstructure and properties of the formed aerogels [18][19][20]. Moreover, the GO's carbon to oxygen (C/O) ratio during the freeze-casting can also influence such microstructure [16]. ...
The controlled assembly of 2D materials into well-defined 3D architectures is a potential route to realise the unique thermal, electrical and mechanical properties on the macroscale. The traditional freeze-casting route for processing such aerogels is generally restricted to aqueously dispersed flakes, typically of graphene oxide (GO), which brings restrictions in its electrical properties. It is known that graphene oxide can help to stabilise graphene nanoplatelets (GNP) in a colloidal dispersion. Hence, we report a versatile aqueous processing route that uses this ability to produce rGO-GNP composites into lamellar aerogels via unidirectional freeze-casting. In order to optimise the properites of the aerogel, GO-GNP dispersions were partially reduced by L-ascorbic acid prior to freeze-casting for tuning the carbon and oxygen (C/O) ratio. The aerogels were heat treated afterwards to fully reduce the GO. The chemical reduction time was found to control the microstructure of the resultings aeorgels and tune their properties. An optimal partial reduction time of 35 mins led to an aerogel with compressive modulus of 0.51 ± 0.06 Mpa at a density of 23.2 ± 0.7 mg/cm3 and an electrical conductivity of 42.3 S/m at a density of 20.8 ± 0.8 mg/cm3 was achieved with partial reduction of 60 mins.
... absent and a new peak centered at 26.0°emerged. The decrease in interplanar spacing and intensity indicated that the π-π interaction of GONRs dominated in the network of GONRs-A (Zhu et al., 2020), and PAO chains were inserted into GONRs-A to form PAO/GONRs-A, further illustrating the successful combination of GONRs-A and PAO. ...
Assembling graphene oxide nanoribbons (GONRs) into three-dimensional (3D) materials with controllable and desired structure is an effective way to expand their structural features and enable their practical applications. In this work, an ultralight 3D porous amidoxime functionalized graphene oxide nanoribbons aerogel (PAO/GONRs-A) was prepared via solvothermal polymerization method using acrylonitrile as monomer and GONRs as solid matrices for selective separation of uranium(VI) from water samples. The PAO/GONRs-A possessed a high nitrogen content (13.5%), low density (8.5 mg cm⁻³), and large specific surface area (494.9 m² g⁻¹), and presented an excellent high adsorption capacity of uranium, with a maximum capacity of 2.475 mmol g⁻¹ at a pH of 4.5, and maximum uranium-selectivity of 65.23% at a pH of 3.0. The results of adsorption experiments showed that U(VI) adsorption on PAO/GONRs-A was a pH-dependent, spontaneous and endothermic process, which was better fitted to the pseudo-second-order kinetic model and Langmuir isotherm model. Both X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations revealed that U(VI) adsorption on PAO/GONRs-A mainly did rely on the amidoxime groups anchored on the aerogel while UO2(PAO)2(H2O)3 was dominant after interaction of uranyl with PAO/GONRs-A. Therefore, as a candidate adsorbent, PAO/GONRs-A has a high potential for the removal of uranium from aqueous solutions.
... −3 / W mK in ambient condition [6]. Pore-size and defect density are important parameters that govern the thermal conductivity in these systems [9,10]. While these 3D networks have comparatively better mechanical properties than silica aerogels, they lack thermal stability. ...
We investigate thermal transport in three-dimensional graphene aerogel networks at elevated temperatures. The aerogels are solution-processed from graphene-oxide flakes using amine-based linkers and then partially reduced to impart stability in the chemical structure at elevated temperatures. Thermal conductivity of the system is estimated using steady-state electrothermal technique in vacuum in the temperature interval from 30 to 200 °C. The thermal conductivity value is κ ∼ 0.2 W/mK at room temperature, and is found to be weakly dependent on temperature across the entire temperature interval. To examine the microscopic origin of this stable response, the thermal conductivity estimates are complemented with insights from temperature-dependent transient electrothermal response. We show that the temperature stable thermal insulation behaviour observed in this system can be attributed to two factors: point-defect scattering at the flake level from the remnant oxygen-functionalities which dominates over Umklapp scattering processes, and another contribution that arises from interfacial thermal resistance between flakes. The partial reduction thus achieves a delicate balance between imparting chemical stability while also retaining the dominance of point-defect phonon scattering, where the latter contributes to temperature stable thermal conductivity.
... absent and a new peak centered at 26.0°e merged. The decrease in interplanar spacing and intensity indicated that the π-π interaction of GONRs dominated in the network of GONRs-A [46]. Compared to GONRs-A, a new dispersion peak between 11.2 and 21.5°appeared, suggesting the amorphous polymer chains were anchored on GONRs-A to form PAFP/GONRs-A [47], further illustrating the successful combination of GONRs-A and polymer. ...
An ultralight three-dimensional porous network phosphonic acid functionalized polymer/graphene oxide nanoribbons aerogel (PAFP/GONRs-A) was prepared using trimethylolpropane trimethacrylate (TRIM) and vinylphosphonic acid (VPA) as monomers via solvothermal polymerization method for thorium capture from aqueous solutions. The synthesized aerogel presented a low density (10.6 mg cm⁻³), large specific surface area (433.2 m² g⁻¹) and high phosphorus content (18.2%). The adsorption process of Th(IV) on PAFP/GONRs-A was pH-dependent, spontaneous and endothermic, and well described by the pseudo-second-order kinetic and Langmuir isotherm model. Under optimal experimental conditions (10 mg of adsorbent dosage, 50 mL of solution volume, 240 min of contact time, 298 K of temperature), PAFP/GONRs-A presented an excellent high adsorption capacity of thorium, with maximum capacity of 457.9 mg g⁻¹ at a pH of 3.0, and maximum thorium-selectivity of 87.1% at a pH of 2.0. The values of ΔG for Th(IV) adsorption on PAFP/GONRs-A were calculated to be −21.78, –23.32, −24.87 and −26.41 kJ mol⁻¹ at 283, 298, 313 and 328 K, respectively. Density functional theory (DFT) calculations and X-ray photoelectron spectroscopy (XPS) revealed that Th(IV) ions were fixed in aerogel by coordinating with the P = O groups of PAFP/GONRs-A, and both 1:2 ratio of Th(IV) with the P = O ligands on the same graft chain and 1:4 ratio of Th(IV) with P = O ligands on two distinct graft chains could jointly contribute to the adsorption of Th(IV) on PAFP/GONRs-A. This work offers a facile approach for synthesizing the phosphonic acid functionalized graphene oxide nanoribbons aerogel and demonstrates that PAFP/GONRs-A has high potential as a candidate adsorbent for the capture of thorium from aqueous solution.
... After 3 h, the graphene hydrogel was placed in deionized water for 3-4 times [32]. The GA was produced through freeze drying [33,34]. To apply the electrical field, a piece of copper was connected to the GA by a conductive paste. ...
Fine particles pose a dangerous threat to our environment and human health. Although a series of air filters have been developed, to achieve high removal efficiency while maintaining a low pressure drop is still a challenging task. Herein, we introduce an additional force into action – the space charge when designing the filter to capture the particulate matters (PMs). The corona discharge drives the PMs toward graphene aerogel filter under the imposed electrostatic force. The prepared filter displays a very high PM removal efficiency and low pressure drop under an extremely high concentration (>10000 μg/m³) of both non-oily particles (>99.9%) and oily particles (>99.6%). In addition, the filter also has the advantages of being active, reusable, and flame-retardant. This charged graphene aerogel filter (CGAF) is very stable and can withstand 10 times washing and 5-min burning.
... An ice template process, called ice-segregation-induced selfassembly (ISISA), [27][28][29] is considered to be the easiest method for constructing 3D graphene aerogels. In the process of synthesizing a GAH, when the system is about to reach the gel point, the precursor is immediately frozen in liquid nitrogen or in a refrigerator and ice crystals formed during the freezing process are used as templates. ...
Graphene aerogels (GAs) were synthesized via a one-step hydrothermal method. Generally, the pore shape and diameter of GAs are difficult to control or the preparation process is complicated, requiring a multi-step operation. Herein, a soft-template one-step hydrothermal synthesis process was proposed to produce GAs with controllable pore sizes. Cyclohexane and n-butanol were added to a graphene oxide suspension to form a uniform aqueous dispersion under emulsification by sodium lauryl sulfate. The reduction process may have occurred around the organic droplets during the hydrothermal reaction, and a large number of organic droplets became countless physical barriers inside the hydrogel. In the later freeze-drying and high-temperature calcination procedures, the droplets evaporated to form a rich pore structure. Compared to the conventional templating method, the organic template was volatilized during the drying process such that no additional process for removing the template was required. In addition, GAs prepared by the template method possessed lower density (2.66 mg cm⁻³) and better compression performance and, as an adsorbent material, absorbed organic matter and petroleum from wastewater more efficiently than GAs obtained by the traditional one-step hydrothermal method; Q for n-hexane reached 116, and Q for xylene reached 147; also, the GAs prepared by the soft template method can absorb all crude oil in water samples within 30 s.
Stemming from the unique in-plane honeycomb lattice structure and the sp 2 hybridized carbon atoms bonded by exceptionally strong carbon–carbon bonds, graphene exhibits remarkable anisotropic electrical, mechanical, and thermal properties. To maximize the utilization of graphene's in-plane properties, pre-constructed and aligned structures, such as oriented aerogels, films, and fibers, have been designed. The unique combination of aligned structure, high surface area, excellent electrical conductivity, mechanical stability, thermal conductivity, and porous nature of highly aligned graphene aerogels allows for tailored and enhanced performance in specific directions, enabling advancements in diverse fields. This review provides a comprehensive overview of recent advances in highly aligned graphene aerogels and their composites. It highlights the fabrication methods of aligned graphene aerogels and the optimization of alignment which can be estimated both qualitatively and quantitatively. The oriented scaffolds endow graphene aerogels and their composites with anisotropic properties, showing enhanced electrical, mechanical, and thermal properties along the alignment at the sacrifice of the perpendicular direction. This review showcases remarkable properties and applications of aligned graphene aerogels and their composites, such as their suitability for electronics, environmental applications, thermal management, and energy storage. Challenges and potential opportunities are proposed to offer new insights into prospects of this material.
The controlled conformational changes of planar graphene nanosheets are of great importance to the realization of their practical applications. Despite substantial effort in the area, the controlled folding of two-dimensional (2D) graphene sheets into one-dimensional (1D) structures still remains a significant challenge. Here, for the first time, we report an ice crystal guided folding strategy to fabricate 1D folded graphene nanobelts (FGBs), where the formation and growth of ice crystals in a confined space function to guide the folding of 2D graphene oxide (GO) nanosheets into 1D nanobelts (i.e. folded graphene oxide belts, FGOBs), which were subsequently converted to FGBs after annealing. Thin aqueous GO containing films were obtained by blowing air through a GO dispersion in the presence of a surfactant, polyoxypropylenediamine (D400), resulting in a foam containing uniform air bubbles. Subsequent shock cooling of the foam using liquid nitrogen resulted in the facile fabrication of FGOBs. This technique provides a general approach to encapsulate catalytic nanomaterials such as Fe3O4 nanorods, TiO2 and Co3O4 nanoparticles into the folded graphene structure for practical applications such as Li-ion batteries.
Inorganic aerogels with low density, high porosity, large specific surface area, and superior mechanical properties are excellent candidate materials in fields such as thermal management, energy, catalysis, and biomedical applications. A comprehensive overview of existing elastic inorganic aerogels is provided, covering their structural units, preparation methods, mechanical performances, and applications. Meanwhile, based on the constituent building blocks and microstructures, a detailed analysis of the mechanical properties and guidelines for elastic design of aerogels is presented. Concluding with a succinct summary of prospective developmental direction, this review deliberates on the challenges and potential opportunities of elastic inorganic aerogels, with the intent of providing a versatile platform for designing new types of elastic inorganic aerogels for various applications.
Three-dimensional (3D) graphene is a promising active component for various engineering fields, but its performance is limited by the hidebound electrical conductivity levels and hindered electrical transport. Here we present a novel approach based on interlayer engineering, in which graphene oxide (GO) nanosheets are covalently functionalized with varied molecular lengths of diamine molecules. This has led to the creation of an unprecedented class of 3D graphene with highly adjustable electronic properties. Theoretical calculations and experimental results demonstrate that ethylenediamine, with its small diameter acting as a molecular bridge for facilitating electron transport, has the potential to significantly improve the electrical conductivity of 3D graphene. In contrast, butylene diamine, with its larger diameter, has a reverse effect due to the enlarged spacing of the graphene interlayers, resulting in conductive degradation. More importantly, the moderate conductive level of 3D graphene can be achieved by combining the interlayer spacing expansion effect and the π-electronic donor ability of aromatic amines. The resulting 3D graphene exhibits highly tunable electronic properties, which can be easily adjusted in a wide range of 2.56–6.61 S·cm−1 compared to pristine GO foam (4.20 S·cm−1). This opens up new possibilities for its use as an active material in a piezoresistive sensor, as it offers remarkable monitoring abilities.
A high-temperature accelerometer plays an important role for ensuring normal operation of equipment in aerospace, such as monitoring and identifying abnormal vibrations of aircraft engines. Phase transitions of piezoelectric crystals, mechanical failure and current leakage of piezoresistive/capacitive materials are the prominent inherent limitations of present high-temperature accelerometers working continuously above 973 K. With the rapid development of aerospace, it is a great challenge to develop a new type of vibration sensor to meet the crucial demands at high temperature. Here we report a high-temperature accelerometer working with a contact resistance mechanism. Based on the improved graphene aerogel (GA) prepared by a modulated treatment process, the accelerometer can operate continuously and stably at 1073 K and intermittently at 1273 K. The developed sensor is lightweight (sensitive element <5 mg) and has high sensitivity (an order of magnitude higher than MEMS accelerometers) and wide frequency response range (up to 5 kHz at 1073 K) with marked stability, repeatability and low nonlinearity error (<1%). These merits are attributed to the excellent and stable mechanical properties of the improved GA in the range of 299-1073 K. The accelerometer could be a promising candidate for high-temperature vibration sensing in space stations, planetary rovers and others.
Aerogel materials with anisotropic nature have attracted increasing interest because of fascinating, potential applications arising from their novel functional capabilities. However, the strategy to achieve highly anisotropic structure of aerogel...
Thermally conductive polymer nanocomposites are enticing candidates for not only thermal managements in electronics but also functional components in emerging thermal energy storage and conversion systems and intelligent devices. A high thermal conductivity (k) depends largely on the ordered assembly of high-k fillers in the composites. In the past decades, various templating assembly techniques have been developed to rationally construct nanoscale fillers into three-dimensional (3D) interconnected structures, further improving the k of composites compared to conventional methods. Herein, recent advances are summarized in developing thermally conductive polymer composites based on self-templating, sacrificial templating, foam-templating, ice-templating and template-directed chemical vapor deposition techniques. These unique templating methods to fabricate 3D interconnected fillers in the form of segregated, cellular, lamellar, and radially aligned structures are reviewed, and their correlations to the k of composites are thoroughly probed. Moreover, multiscale structural design strategies combined with different templating methods to further improve the k of composites are highlighted. This review offers a constructive guidance to fabricate next-generation thermally conductive polymer composites for diverse thermal energy applications.
Aerogels are special porous materials with low thermal conductivity, light weight, high energy absorption rate and large surface area, which have been applied in many fields. However, controlling the aerogel microstructure remains an academic challenge. Herein, by employing graphene oxide (GO) as the aerogel skeleton and utilizing poly(vinyl alcohol) (PVA) to regulate the ice crystal growth, we elucidate the relationships between the physicochemical properties of GO/PVA aerogel precursors and the nucleation and growth of ice crystals by using an ice-templating method. We demonstrate that due to the hydrogen bond formed between PVA and water molecules, resulting in the initial crystallization temperature being reduced from −12.60 °C (GO/PVA-0.01) to −16.21 °C (GO/PVA-0.1). Meanwhile, the strong hydrogen bond between PVA and GO limits the diffusion of water molecules, thereby inhibiting the growth of ice crystals, decreasing the pore size of the GO/PVA aerogel from 9.96 nm (GO/PVA-0.01) to 7.19 nm (GO/PVA-0.3), and thus the compressive strength of the aerogel increases from 0.045 MPa to 0.13 MPa. Overall, the finding of this study can be extended to other aerogel precursors, and exhibit important scientific value and practical significance for the preparation of aerogel materials with highly controllable structures and performances.
Graphene is an attractive material for many applications due to excellent inherent properties such as lower density, high mechanical strength, higher thermal conductivity, etc. However, it has been used as an additive material due to size limitation. To overcome the size limitation without affecting the inherent properties of 2D graphene sheets, researchers have developed graphene aerogel (GA) by different synthesis techniques. GA has become an emerging light-weight structure in the group of aerogel materials because of various applications. In this review article, the mechanical properties of GA have been discussed by considering the structural parameters of GA, such as pore size, graphene sheet alignments, and wall thickness, because the mechanical performance of the GA is an important criterion prior to its application in any field. It has also been highlighted that the structural parameters can be altered during synthesis to achieve desired mechanical properties. Applications of GA have been thoroughly discussed in the field of polymer composites and how GA-polymer structures can help to generate a cleaner environment by separating spilled oil from water and removing organic pollutants from water. In addition, potential of GA for various other applications with suitable nano material additives have been highlighted.
Carboxymethyl cellulose/ graphene composite aerogel beads (CMC/GAs) were prepared by the easily scaling-up method, i.e., wet spinning- environmental pressure drying method. The influences of the type and concentration of coagulating bath on the formation of aerogel beads were discussed, and the forming mechanism was analyzed. The CMC/GAs was characterized through SEM, XRD, FI-IR, Raman, XPS, electronic universal testing machine and other methods. The CMC/GAs-30 has an average particle size and a mean pore diameter of 3.83 mm and 82 μm, respectively. The analysis results indicated that the adsorption mechanisms of CMC/GAs on methylene blue (MB) are mainly through the electrostatic interaction. The adsorption process conforms to the Langmuir model (R² = 0.9964) and pseudo-second-order kinetic model (R² is higher than 0.99). When the particle size of CMC/GAs-30 decreases, the equilibrium adsorption capacity for MB increases. Under the experimental conditions explored, the Langmuir maximum adsorption capacity of CMC/GAs-30 for MB is 222.72 mg.g⁻¹. The CMC/GAs-30 show good recycle performance in MB adsorption. The removal rate of MB from water by CMC/GAs-30 remained at about 90% after 30-times adsorption- regeneration cycles.
Supercritical CO2 drying and freeze-drying are two standard drying methods for fabricating aerogels. We conducted a comprehensive experimental study for clarifying the influence of the drying method on the properties of graphene aerogel, and further discussed its deformation mechanism via characterization and simulation results. The results showed that the specific modulus of two drying methods prepared graphene aerogels is similar. The supercritical CO2 dried sample had a higher compression strength due to the plastic collapse of the mesopore walls. In the freeze-drying process without thermal baffle, the reformed honeycomb geometric microstructure can be divided into three regions according to the growth of ice crystals: vertical, transition, and gradient regions. Therefore, the freeze-dried graphene aerogels exhibited a nonlinear superelastic behavior due to the random out-of-plane buckling of the thicker pore walls. In addition, two cellular structures were developed by mimicing the microstructure of two kinds of graphene aerogels for numerical analysis. Simulation results highlighted the decisive role of geometric nonlinearity in the deformation mechanism.
Flexible sensors are of great interests for future applications in the next-generation intelligent lifestyles. However, available fabrication methods exhibit shortages such as tedious preparation process and non-environmentally friendly matrix, while the obtained sensors usually possess poor ability to discriminate stress from different directions. Here, we propose a highly sensitive aligned graphene/sodium alginate (SA) aerogel sensor with anisotropic properties fabricated through a facile unidirectional freezing method. The resulted orientated nanostructure combined with designed SA/Ca²⁺ network endows the aerogel with desired anisotropic properties and enhanced mechanical properties. The anisotropic performance data of resistance, strain sensitivity and compressive strength between the two directions reach up to 57, 3 and 1.5 times respectively. Moreover, the obtained sensors can be used in expression recognition and real-time health monitoring owing to the excellent sensitivity and desired anisotropic conductive response ability. This work is a step toward bio-based high-performance anisotropic strain sensors, which opens up new opportunities for the design and fabrication of advanced intelligent electronic devices for future lifestyles.
Three-dimensional (3D) graphene oxide/MXene (GO/MX) composite aerogels with characters of lightweight, porous structure, and superior elasticity have attracted great attention in electromagnetic shielding, oil-water separation, and chemical catalysis, especially smart electronic sensors. However, due to the lack of adequate interaction force between GO and MX nanosheets, it is challenging to construct a high strength and long-range ordered GO/MX aerogel, resulting in inferior mechanical properties and poor conductivity. Here, a facile gelation strategy is developed to form long-range ordered GO/MX aerogel by ethylenediamine (EDA) induced self-assembly of MX and GO liquid crystals (LCs). In this system, EDA plays dual roles of inducing ordered self-assembly of GO LCs nanosheets and strengthening interaction between GO and MX nanosheets. As a result, the obtained GO/MX aerogel with long-range ordered microstructure presents an outstanding mechanical elasticity of 9.5 kPa and a high conductivity of 0.99 S m⁻¹. Especially, the assembled GO/MX-based sensor has an extremely low detection limit of ∼0.2% strain, prominent physiological signal capture capability, and excellent durability over 10000 compression/release cycles. This work paves the way for the potential applications of GO/MX composite aerogel in real-time human motion monitor, artificial skins, and flexible smart devices.
Aerogels are ultralight porous materials whose matrix structure can be formed by interlinking 880 nm long M13 phage particles. In theory, changing the phage properties would alter the aerogel matrix, but attempting this using the current production system leads to heterogeneous lengths. A phagemid system that yields a narrow length distribution that can be tuned in 0.3 nm increments from 50 to 2500 nm is designed and, independently, the persistence length varies from 14 to 68 nm by mutating the coat protein. A robotic workflow that automates each step from DNA construction to aerogel synthesis is used to build 1200 aerogels. This is applied to compare Ni–MnOx cathodes built using different matrixes, revealing a pareto‐optimal relationship between performance metrics. This work demonstrates the application of genetic engineering to create “tuning knobs” to sweep through material parameter space; in this case, toward creating a physically strong and high‐capacity battery. An engineered phagemid system generates M13 phage with tunable length and stiffness. Changing the phage properties changes the electrochemical and mechanical properties of metal nanofoams, used as cathodes in Li ion batteries. This is applied to scan the properties of cathodes to identify those with high capacity and strength, as a step towards structural batteries.
Ionic conductive hydrogels have recently been increasingly studied due to their broad applications in sensing and flexible devices. Nevertheless, it is still a challenge to simply develop an ionic conductive hydrogel with satisfying comprehensive performance. Herein, ionic conductive hydrogels have been crosslinked by carboxymethyl cellulose and phytic acid via a simple one-pot approach to address these challenges. The unique double crosslinked microstructure ensures that the hydrogel has favourable mechanical performance, resilience (93%, similar to natural resilin), and recovery (20 min, after 7 cycles at 300%) along with less residual strain (6.7%, after 20 successive cycles at 125%). The hydrogel also exhibits outstanding ionic conductivity (6.0 S/m). The combined mechanical performance and ionic conductivity of the prepared hydrogel results in its remarkable performance when used in sensors. The hydrogel-based sensor displays superior sensitivity (GF of 2.86, at a strain of 600%), stability and durability towards both tensile and compressive deformation. In practical applications, the sensor demonstrates a broad strain window to detect both large and very small human activities, showing the excellent potential of this hydrogel in sensing and flexible devices. The approach in this work has also been optimized to potentially allow for large-area, low-cost fabrication.
An three-dimensional (3D) porous structure graphene oxide nanoribbons (GONRs) aerogel has been prepared via hydrothermal method to overcome the challenges of solid–liquid separation for powdered carbon-based nanomaterials. GONRs aerogel showed low density, good mechanical strength and easy separation from water. Uranium(VI) and thorium(IV) adsorption by GONRs aerogel was investigated by batch experiments, demonstrating their strongly pH-dependent, spontaneous and endothermic adsorption processes. GONRs aerogel exhibited the maximum U(VI)- and Th(IV)-uptake capacity (430.6 and 380.4 mg g−1, respectively) due to its large specific area (597.4 m2 g−1) and abundant oxygen-containing groups. This work suggests that GONRs aerogel has great potential for treatment of uranium and thorium-containing effluents.
Reasonable design of hybrid graphene aerogels with low cost, high specific surface area (SSA), and excellent mechanical properties is of great significance for future large-scale energy storage applications. Herein, we report a composite aerogel of biomass carbon/reduced graphene oxide (rGO)/nanocellulose (CNF) by a one-step self-assembly method. In the hybrid aerogel, biomass carbon particles with low cost and high SSA are assembled into the framework of rGO effectively preventing the restacking of rGO nanosheets and resulting in high SSA and high conductivity. Negatively charged nanocellulose fibers act as the binder between biomass carbon particles and rGO sheets, facilitating the formation of a strengthening network. As a result, the hybrid aerogel with a high content of biomass carbon (76% mass ratio) demonstrates excellent mechanical strength (240 kPa), high SSA (1007.9 m² g⁻¹), and efficient MnO2 deposition capability (33.9 mg cm⁻²). Employing the composite aerogel as electrodes to fabricate a self-supporting supercapacitor, an admirable capacitive performance (4.8 F cm⁻²) was achieved. This work provides an effective method to fabricate low cost hybrid graphene aerogel materials with excellent performances for wide applications.
The freeze casting process has been widely used for fabricating aerogels due to its versatile and environmentally friendly nature. This process offers a variety of tools to tailor the entire micropore morphology of the final product in a monolithic fashion through manipulation of the freezing kinetics and precursor suspension chemistry. However, aerogels with nonmonolithic micropore morphologies, having pores of various sizes located in certain regions of the aerogels, are highly desired by certain applications such as controlled drug-delivery, bone tissue engineering, extracellular simulation, selective liquid sorption, immobilized catalysts, and separators. Furthermore, aerogels composed of micropores with predesigned size, shape, and location can open up a new paradigm in aerogel design and lead to new applications. In this study, a general manufacturing approach is developed to control the size, shape, and location of the pores on the aerogel surface by applying a precise control on the local thermal conductivity of the substrate used in a unidirectional freeze casting process. With our method, we created patterned low and high thermal conductivity regions on the substrate by depositing patterned photoresist polymer features. The photoresist polymer has a much lower thermal conductivity, which resulted in lower cooling/freezing rates compared to the silicon substrate. Patterned thermal conductivity created a designed temperature profile yielding to local regions with faster and slower freezing rates. Essentially, we fabricated aerogels whose micropore morphology on their surface was a replica of the patterned substrates in terms of size and location of the micropores. Using the same substrates, we further showed the possibility of 3D printed aerogels with precisely controlled, surface micropore morphologies. To the best of our knowledge, this is the first study that reports aerogels having micropore morphologies (e.g., size, shape, and location) that are precisely controlled through locally controlled thermal conductivity of the substrates.
The discovery of peculiar quasi-liquid layers on ice surfaces marks a major breakthrough in ice-related sciences, as the facile tuning of the reactions and morphologies of substances in contact with these layers make ice-assisted chemistry a low-cost, environmentally benign, and ubiquitous methodology for the synthesis of nanomaterials with improved functionality. Ice-templated synthesis of porous materials offers the appealing features of rapid self-organization and remarkable property changes arising from confinement effects and affords materials that have found a diverse range of applications such as batteries, supercapacitors, and gas separation. Moreover, much attention has been drawn to the acceleration of chemical reactions and transformations on the ice surface due to the effects of freeze concentration, fast self-diffusion of surface water, and modulated surface potential energy. Some of these results are related to the accumulation of inorganic contaminants in glaciers and the blockage of natural gas pipelines. As an emerging theme in nanomaterial design, the dimension-controlled synthesis of hybrid materials with unprecedentedly enhanced properties on ice surfaces has attracted much interest. However, a deep understanding of quasi-liquid layer characteristics (and hence, the development of cutting-edge analytical technologies with high surface sensitivity) is required to achieve the current goal of ice-assisted chemistry, namely the preparation of tailor-made materials with the desired properties.
Water can freeze into diverse ice polymorphs depending on the external conditions such as temperature (T) and pressure (P). Herein, molecular dynamics simulations show evidence of a high-density orthorhombic phase, termed ice χ, forming spontaneously from liquid water at room temperature under high-pressure and high external electric field. Using free-energy computations based on the Einstein molecule approach, we show that ice χ is an additional phase introduced to the state-of-the-art T–P phase diagram. The χ phase is the most stable structure in the high-pressure/low-temperature region, located between ice II and ice VI, and next to ice V exhibiting two triple points at 6.06 kbar/131.23 K and 9.45 kbar/144.24 K, respectively. A possible explanation for the missing ice phase in the T–P phase diagram is that ice χ is a rare polarized ferroelectric phase, whose nucleation/growth occurs only under very high electric fields.
Materials combining lightweight, robust mechanical performances, and multifunctionality are highly desirable for engineering applications. Graphene aerogels have emerged as attractive candidates. Despite recent progresses, the bottleneck remains how to simultaneously achieve both strength and resilience. While multiscale architecture designs may offer a possible route, the difficulty lies in the lack of design guidelines and how to experimentally achieve the necessary structure control over multiple length scales. The latter is even more challenging when manufacturing scalability is taken into account. The Thalia dealbata stem is a naturally porous material that is lightweight, strong, and resilient, owing to its architecture with three-dimensional (3D) interconnected lamellar layers. Inspired by such, we assemble graphene oxide (GO) sheets into a similar architecture using a bidirectional freezing technique. Subsequent freeze-drying and thermal reduction results in graphene aerogels with highly tunable 3D architectures, consequently an unusual combination of strength and resilience. With their additional electrical conductivity, these graphene aerogels are potentially useful for mechanically switchable electronics. Beyond such, our study establishes bidirectional freezing as a general method to achieve multiscale architectural control in a scalable manner that can be extended to many other material systems.
Ultralight graphene/cellulose nanocrystal (CNC) hybrid aerogels (GCHAs) were prepared via an integration strategy of a two-step reduction process and ambient drying technology. Such a preparation strategy causes the as-prepared GCHAs to circumvent an issue of structural collapse induced by shrinkage in an ordinary ambient drying process. GCHAs have three-dimensional large cellular structures with CNC-sandwiched graphene flakes. Incorporation of stiff rod-like CNCs in a flexible graphene aerogel (GA) with hydrophobicity realized the multifunction of hybrid GA materials. With the increase of CNCs loading, the modulus of GCHAs enhanced obviously. The mechanical strength of GCHAs could be controlled by adjusting the CNC loading. The sorption experiments of both organic solvents and water on the GCHAs were carried out and GCHAs presented good amphiphilic absorption capacity. The work provides a promising amphiphilic graphene-based aerogel material with tunable mechanical strength by a facile, economic and environmentally-friendly approach, giving it high potential for use in waste water treatment and pressure sensitive materials.
Aerogels can be fabricated from pristine graphene exfoliated nanosheets using freeze gelation with nonaqueous solvents and no heat treatment or reduction stage. Solvents are selected that disperse pristine graphene with a melting point above room temperature but with a high vapor pressure above the solid at room temperature, enabling sublimation (freeze drying) under ambient conditions.
Significance
We describe the first, to our knowledge, integrated dynamic DNA nanotechnology and 2D material electronics to overcome current limitations for the detection of DNA single-nucleotide polymorphism (SNP). Electrical detection of DNA has been advancing rapidly to achieve high specificity, sensitivity, and portability. However, the actual implementation of DNA detection is still in infancy because of low specificity, especially for analytically optimal and practically useful length of target DNA strands. Most of the research to date has focused on the enhancement of the sensitivity of DNA biosensors, whereas the specificity problem has remained unsolved. The low specificity is primarily attributed to the nonspecific binding during hybridization of the probe and the target DNA. Here, we have addressed these limitations by designing a functional prototype of electrical biosensors for SNP detection.
New graphene aerogels can be fabricated by vacuum/air drying, and because of the mechanical robustness of graphene aerogels, shape-memory polymer/graphene hybrid foams can be fabricated by a simple infiltration-air-drying crosslinking method. Due to the superelasticity, high strength, and good electrical conductivity of the as-prepared graphene aerogels, the shape-memory hybrid foams exhibit excellent thermotropical and electrical shape-memory properties, outperforming previously reported shape-memory polymer foams.
Graphene is a two-dimensional material that offers a unique combination of low density, exceptional mechanical properties, large surface area and excellent electrical conductivity. Recent progress has produced bulk 3D assemblies of graphene, such as graphene aerogels, but they possess purely stochastic porous networks, which limit their performance compared with the potential of an engineered architecture. Here we report the fabrication of periodic graphene aerogel microlattices, possessing an engineered architecture via a 3D printing technique known as direct ink writing. The 3D printed graphene aerogels are lightweight, highly conductive and exhibit supercompressibility (up to 90% compressive strain). Moreover, the Young's moduli of the 3D printed graphene aerogels show an order of magnitude improvement over bulk graphene materials with comparable geometric density and possess large surface areas. Adapting the 3D printing technique to graphene aerogels realizes the possibility of fabricating a myriad of complex aerogel architectures for a broad range of applications.
Three-dimensional (3D) graphene aerogel-supported SnO2 (SGA) nanoparticles (NPs) are presented by a one-pot solvothermal treatment of graphene oxide in the presence of SnCl4 followed by freeze drying. The size of SnO2 nanoparticles on the graphene aerogel is as small as 5-10 nm with uniform distribution. The 3D SnO2-graphene aerogel exhibits interconnected macroporous networks which will be beneficial for gas detection. The SGA shows excellent response and selectivity towards NO2 when other common gases are present at room temperature (RT). The gas sensing responses of the resulting nanocomposites demonstrate that macroporosity in the SGA significantly improves the response and recovery time compared with the 2D SnO2-graphene nanocomposite. The sensing mechanism and the reasons for the better response are also discussed.
It is a challenge to fabricate graphene bulk materials with properties arising from the nature of individual graphene sheets, and which assemble into monolithic three-dimensional structures. Here we report the scalable self-assembly of randomly oriented graphene sheets into additive-free, essentially homogenous graphene sponge materials that provide a combination of both cork-like and rubber-like properties. These graphene sponges, with densities similar to air, display Poisson's ratios in all directions that are near-zero and largely strain-independent during reversible compression to giant strains. And at the same time, they function as enthalpic rubbers, which can recover up to 98% compression in air and 90% in liquids, and operate between -196 and 900 °C. Furthermore, these sponges provide reversible liquid absorption for hundreds of cycles and then discharge it within seconds, while still providing an effective near-zero Poisson's ratio.
A composite cathode material consisting of (010) facet-orientated LiFePO4 nanoplatelets wrapped in nitrogen-doped graphene aerogel is reported. Such a composite possesses a 3D porous structure with a BET surface area as high as 199.3 m2•g-1. In this composite, the nitrogen-doped graphene aerogel combined with its interconnected pore networks provide pathways for rapid electron transfer and ion transport, while the thin LFP nanoplatelets with large (010) surface area enhance the active sites and shorten the Li+ diffusion distances. As a result, a high rate capability (78 mAh•g−1 at 100 C) as well as a long life cycling stability (89% capacity retention over 1000 cycles at 10 C) are achieved.
At moderate temperatures (≤ 70°C), thermal reduction of graphene oxide is inefficient and after its synthesis the material enters in a metastable state. Here, first-principles and statistical calculations are used to investigate both the low-temperature processes leading to decomposition of graphene oxide and the role of ageing on the structure and stability of this material. Our study shows that the key factor underlying the stability of graphene oxide is the tendency of the oxygen functionalities to agglomerate and form highly oxidized domains surrounded by areas of pristine graphene. Within the agglomerates of functional groups, the primary decomposition reactions are hindered by both geometrical and energetic factors. The number of reacting sites is reduced by the occurrence of local order in the oxidized domains, and due to the close packing of the oxygen functionalities, the decomposition reactions become - on average - endothermic by more than 0.6 eV.
Mechanically strong and electrically conductive graphene aerogels can be prepared by either supercritical drying or freeze drying of hydrogel precursors synthesized from reduction of graphene oxide with L-ascorbic acid, and the resulting graphene aerogels possess the specific capacitance of 128 F g−1 with superior rate performance.
In this paper, we develop a new ionic polymer–metal composite (IPMC) by replacing a typical platinum or gold electrode with a multi-walled carbon nanotube (MWNT)–graphene based electrode. A solvent of MWNT and graphene is formed on both sides of the ionic polymer membranes as electrodes by means of spray coating and baking. Then, the ionic liquid process is performed for actuating in air. The four kinds of IPMC samples with different MWNT–graphene ratios are fabricated with the same solid Nafion film. Experimental results show that the IPMC with a pure MWNT based electrode exhibits higher displacement compared to the conventional IPMC with a platinum electrode. Also, the increment of the ratio of graphene to the MWNT–graphene electrode decreases the resultant displacement but increases the fundamental natural frequency of the polymer actuator.
Keeping Electrolytes in Porous Electrodes
Electrochemical capacitors (ECs) can rapidly charge and discharge, but generally store less energy per unit volume than batteries. One approach for improving on the EC electrodes made from porous carbon materials is to use materials such as chemically converted graphene (CCG, or reduced graphene oxide), in which intrinsic corrugation of the sheets should maintain high surface areas. In many cases, however, these materials do not pack into compact electrodes, and any ECs containing them have low energy densities. Yang et al. (p. 534 ) now show that capillary compression of gels of CCG containing both a volatile and nonvolatile electrolyte produced electrodes with a high packing density. The intersheet spacing creates a continuous ion network and leads to high energy densities in prototype ECs.
Graphene-based three-dimensional porous macrostructures are believed of great importance in various applications, e.g. supercapacitors, photovoltaic cells, sensors and high-efficiency sorbents. However, to precisely control the microstructures and properties of this material to meet different application requirements in industrial practice remains challenging. We herein propose a facile and highly effective strategy for large-range tailoring the porous architecture and its properties by a modified freeze casting process. The pore sizes and wall thicknesses of the porous graphene can be gradually tuned by 80 times (from 10 to 800 μm) and 4000 times (from 20 nm to 80 μm), respectively. The property experiences the changing from hydrophilic to hydrophobic, with the Young's Modulus varying by 15 times. The fundamental principle of the porous microstructure evolution is discussed in detail. Our results demonstrate a very convenient and general protocol to finely tailor the structure and further benefit the various applications of porous graphene.
A sensitive and selective field-effect transistor (FET) biosensor is demonstrated using vertically-oriented graphene (VG) sheets labeled with gold nanoparticle (NP)-antibody conjugates. VG sheets are directly grown on the sensor electrode using a plasma-enhanced chemical vapor deposition (PECVD) method and function as the sensing channel. The protein detection is accomplished through measuring changes in the electrical signal from the FET sensor upon the antibody-antigen binding. The novel biosensor with unique graphene morphology shows high sensitivity (down to ~2 ng/ml or 13 pM) and selectivity towards specific proteins. The PECVD growth of VG presents a one-step and reliable approach to prepare graphene-based electronic biosensors.
Many applications proposed for graphene require multiple sheets be assembled into a monolithic structure. The ability to maintain structural integrity upon large deformation is essential to ensure a macroscopic material which functions reliably. However, it has remained a great challenge to achieve high elasticity in three-dimensional graphene networks. Here we report that the marriage of graphene chemistry with ice physics can lead to the formation of ultralight and superelastic graphene-based cellular monoliths. Mimicking the hierarchical structure of natural cork, the resulting materials can sustain their structural integrity under a load of >50,000 times their own weight and can rapidly recover from >80% compression. The unique biomimetic hierarchical structure also provides this new class of elastomers with exceptionally high energy absorption capability and good electrical conductivity. The successful synthesis of such fascinating materials paves the way to explore the application of graphene in a self-supporting, structurally adaptive and 3D macroscopic form.
Freeze casting is a promising technique to fabricate porous materials with complex pore shapes and component geometries. This review is aimed to elaborate the fundamental principles of the porous microstructure evolution and critical factors that influence the fundamental physics involved in freeze casting of particulate suspensions. The discussion separately analyses homogeneous and directional freeze casting for both aqueous and non-aqueous systems. The effects of additives, freezing conditions, suspension solids loading and particle size on pore shape, size and morphology evolution are discussed. Special techniques based on modified freeze casting, such as freeze tape casting, double sided freeze casting and field directed freeze casting, are also included.
We report the assembly of graphene oxide (G-O) building blocks into a vertical and radially aligned structure by a bidirectional freeze-casting approach. The crystallization of water to ice assembles the G-O sheets into a structure, a G-O aerogel whose local structure mimics turbine blades. The centimeter-scale radiating structure in this aerogel has many channels whose width increases with distance from the center. This was achieved by controlling the formation of the ice crystals in the aqueous G-O dispersion that grew radially in the shape of lamellae during freezing. Because the shape and size of ice crystals is influenced by the G-O sheets, different additives (ethanol, cellulose nanofibers, and chitosan) that can form hydrogen bonds with H2O were tested and found to affect the interaction between the G-O and formation of ice crystals, producing ice crystals with different shapes. A G-O/chitosan aerogel with a spiral pattern was also obtained. After chemical reduction of G-O, our aerogel exhibited elasticity and absorption capacity superior to that of graphene aerogels with “traditional” pore structures made by conventional freeze-casting. This methodology can be expanded to many other configurations and should widen the use of G-O (and reduced G-O and “graphenic”) aerogels.
Low-density metal foams have many potential applications in electronics, energy storage, catalytic supports, fuel cells, sensors and medical devices. Here, we report a new method for fabricating ultralight, conductive silver aerogel monoliths with predictable densities using silver nanowires. Silver nanowire building blocks were prepared by polyol synthesis and purified by selective precipitation. Silver aerogels were produced by freeze-casting nanowire aqueous suspensions followed by thermal sintering to weld the nanowire junctions. As-prepared silver aerogels have unique anisotropic microporous structures, with density precisely controlled by the nanowire concentration, down to 4.8 mg/cm3 and electrical conductivity up to 51,000 S/m. Mechanical studies show silver nanowire aerogels exhibit “elastic stiffening” behavior with Young’s modulus up to 16,800 Pa.
A ceramic/graphene metamaterial (GCM) with microstructure-derived superelasticity and structural robustness is achieved by designing hierarchical honeycomb microstructures, which are composited with two brittle constituents (graphene and ceramic) assembled into multi-nanolayer cellular walls. Attributed to well coupled strengthening effect between graphene framework and nanolayers of Al2O3 ceramic (NAC), GCM demonstrates a sequence of multifunctional properties simultaneously, such as flyweight density, 80% reversible compressibility, high fatigue resistance, high electrical conductivity and excellent thermal insulation/flaming retardant performance simultaneously. Considerable size effects of ceramic nanolayers on mechanical properties are revealed in this ceramics based metamaterials. The designed hierarchical honeycomb graphene with a 4th dimensional control of ceramic nanolayers on new ways to scalable fabrication of advanced multifunctional ceramic composites with controllable design, suggesting a great potential in applications of flexible conductor, shock/vibration absorber, thermal shock barrier, thermal insulation/flaming retardant skin and porous microwave absorbing coating.
Lightweight, superelastic foams that resist creep and fatigue over a broad temperature range are being developed as structural and functional materials for use in numerous diverse applications. Unfortunately, conventional foams display superelasticity degradation, undergo considerable creep, show fatigue under repeated usage, or fracture over large strains, particularly under significant temperature variations. We report that graphene-coated single-walled carbon nanotube (SWCNT) aerogels remain superelastic, and resist fatigue and creep over a broad temperature range of -100–500 °C. The microstructure of these ultralow density (≈ 14 mg/mL; corresponding volume fraction ≈ 9 × 10-3) aerogels is comprised of a three-dimensional network of randomly oriented SWCNTs with junctions between SWCNTs coated with 2–5 layers of ≈ 3 nm long graphene nanoplatelets. Compressive stress (σ) versus compressive strain (ε) curves show that the aerogels fully recover their shapes even when strained by at least 80% over -100–300 °C and 20% at 500 °C, while the Young’s modulus remains similar over the temperature (-100–500 °C) and strain rate ε ̇ (0.01–0.16 1/s) ranges. We suggest that under compression, the graphene layers hinder free rotation and irreversible sliding of the SWCNTS about the junctions, leading to bending of the graphene layers, while the struts form new junctions stabilized via van der Waals interactions. When the compressive load is removed, the bent graphene layers provide a restoring force that breaks the junctions created during compression, accounting for full aerogel shape recovery, albeit with hysteresis. The storage (E′) and loss (E″) moduli measured in the linear regime show ultralow damping ratio (tan δ = E″/E′) ≈ 0.02, and these viscoelastic properties remain constant over three decades of frequencies (0.628–628 rad/s) and across -100–500 °C. The low loss in these aerogels is corroborated by exceptional fatigue resistance for 2,000 (5 × 105) cycles at ε = 60% (1%) from -100–300 °C (-100–500 °C) and creep resistance at least under σ = 20 kPa, for a minimum of 1 minutes from -100–500 °C. Furthermore, these aerogels retain their exceptional creep resistance under the same creep test conditions but for much longer time of 30 min at all tested temperatures except at 500 °C where they show small creep ε of ≈ 0.7% with ≈ 0.8% residual ε. The emergent thermomechanical stability of these aerogels that arises from, in part, microscopic deformations of the graphene-coated junctions, motivate junction modification as a means to control the mechanical properties of CNT foams in general. Furthermore, the temperature-invariant mechanical properties of these aerogels combined with their facile fabrication method, which is readily applicable to other nanotube foams, make this class of aerogels a strong alternative to conventional foams, particularly in environments with large temperature variations.
In this paper porous graphene oxide (GO) foams were prepared from freeze-drying method. Compressive mechanical properties of GO foams with different density were investigated by uniaxial compression experiments and finite element (FE) simulation. GO foam exhibited excellent elasticity, which recovered to its original length even after 300 cycles. The structural evolution during the compression was revealed by FE simulation.
Phase change materials (PCMs) are of interest in many applications which may require shape-stabilization. In this study, a graphene oxide aerogel (GOxA) reinforced paraffin PCM composite is developed, effectively reinforcing and shape-stabilizing the paraffin.
The molecular and diffraction characterizations suggest that the GOxA network potentially affects paraffin's crystallization. The mechanical characterizations using durometer and nanoindentation show that the composite is 3∼7× harder than pure paraffin and maintains significant strength even above paraffin's melting temperature. Moreover, the composite is much less strain-rate sensitive than paraffin. The reinforcement via GOxA is much beyond the prediction by the rule-of-mixture, implying a strong GOxA-paraffin interfacial bonding.
To our best knowledge, this is the first study on the mechanical behavior of paraffin and GOxA-PCM composite, providing critical insights into their behavior. Additionally, the relationship between the hardness and durometer index first-ever developed here will enable the quantitative durometer testing on materials for many other applications at different ambient conditions due to its versatility and simplicity.
Dispersion and spatial distribution of graphene sheets play crucial roles in tailoring mechanical and functional properties of their polymer composites. Anisotropic graphene aerogels (AGAs) with highly aligned graphene networks are prepared by a directional-freezing followed by freeze-drying process and exhibit different microstructures and performances along the axial (freezing direction) and radial (perpendicular to the axial direction) directions. Thermal annealing at 1300 oC significantly enhances the quality of both AGAs and conventional graphene aerogels (GAs). The aligned graphene/epoxy composites show highly anisotropic mechanical and electrical properties and excellent electromagnetic interference (EMI) shielding efficiencies at very low graphene loadings. Compared to the epoxy composite with 0.8 wt% thermally annealed GAs (TGAs) with an EMI shielding effectiveness of 27 dB, the aligned graphene/epoxy composite with 0.8 wt% thermally treated AGAs (TAGAs) has an enhanced EMI shielding effectiveness of 32 dB along the radial direction with a slightly decreased shielding effectiveness of 25 dB along the axial direction. With 0.2 wt% of TAGA, its epoxy composite exhibits a shielding effectiveness of 25 dB along the radial direction, which meets the requirement of ~20 dB for practical EMI shielding applications.
Paper is an attractively assembled form of materials and has accompanied our daily life almost everywhere. Two-dimensional layered materials, especially graphene, have unique intrinsic structures to be exploited for smart architecture of macroscopic papers that are offering many newly emerging applications. Research advances in graphene based papers in the past few years have created a new category of composite materials. This review aims at offering an up-to-date comprehensive summary of graphene-supported papers, with the emphasis on smart assembly and purpose-driven specific functionalization for their critical applications associated with sensing, environmental and energy technologies. The contents of this review are based on a balance combination of our own studies and selected research studies done by worldwide academic groups. We first give a brief introduction to graphene as a versatile building block and to the current status of research studies on graphene papers. This is followed by addressing some crucial methods of how to prepare graphene papers. We then summarize multiple possibilities of functionalizing graphene papers, membranes or films. Finally, we evaluate some key applications of graphene papers in the areas of chemical/electrochemical sensors, biomimetics and energy storage devices, just before leading to our concluding remarks and perspectives.
Graphene aerogels (GAs) with a highly aligned, porous structure are prepared using a novel unidirectional freeze casting method, followed by thermal reduction. The unique graphene orientation in a preferred direction is achieved due to the large temperature gradient generated during freeze casting, in which graphene oxide (GO) sheets are expelled by the rapidly advancing ice front to assemble between the aligned ice crystals. The resulting unidirectional GAs (UGAs) possess ultralow densities, high porosities, and large surface areas, as well as excellent electrical conductivities. The solid UGA/epoxy composites fabricated by vacuum-assisted infiltration of liquid epoxy present an extremely low percolation threshold of 0.007 vol %, which is the lowest value for all graphene/polymer composites reported in the literature. Besides, the anisotropic structure of UGAs gives rise to significant anisotropic electrical conductivities of UGA/epoxy composites, a potentially useful attribute for many important applications. A new analytical model is formulated on the basis of the interparticle distance concept to explain the percolation behaviors of composites with aligned anisotropic nanofillers. The prediction agrees well with experimental data, and the model validates the importance of aspect ratio and orientation state of nanofillers in controlling the percolation threshold of composites.
It is a challenge to fabricate graphene aerogels (GA) using natural drying (ND) technique to replace the high-cost, low-production, and scale-limited freeze drying (FD) or supercritical drying (SD) techniques. Here we design a novel strategy to produce GA with ND technique. By bionically strengthening the sample initial framework stiffness and effectively reducing the solvent evaporation capillary pressure, no noticeable volume shrinkage and structure crack occur during the whole ND process. The as-formed GA exhibit superior properties than those of FD or SD GA, such as ultralarge reversible compressibility (99%) and tunable Poisson's Ratio behaviors (-0.30<v<0.46). This low-cost, high-efficient, and easy-processing approach paves the way to large-scale and scale-up commercial production of GA and provides a promising strategy for Poisson's Ratio-oriented design of graphene metamaterials for a variety of applications.
3D cellular graphene films with open porosity, high electrical conductivity, and good tensile strength, can be synthesized by a method combining freeze-casting and filtration. The resulting supercapacitors based on 3D porous reduced graphene oxide (RGO) film exhibit extremely high specific power densities and high energy densities. The fabrication process provides an effective means for controlling the pore size, electronic conductivity, and loading mass of the electrode materials, toward devices with high energy-storage performance.
Base-induced graphene oxide (GO) liquid crystals form a highly ordered texture. This microstructure can be inherited to the graphene foams prepared by hydrothermal reduction, showing long-range ordered microstructure of graphene sheets in 3D. This provides an insightful understanding into the supramolecular chemistry of GO sheets.
The simple synthesis of ultralow-density (≈2.32 mg cm-3) 3D reduced graphene oxide (rGO) aerogels that exhibit high electrical conductivity and excellent compressibility are described herein. Aerogels are synthesized using a combined hydrothermal and thermal annealing method in which hexamethylenetetramine is employed as a reducer, nitrogen source, and graphene dispersion stabilizer. The N-binding configurations of rGO aerogels increase dramatically, as evidenced by the change in pyridinic-N/quaternary-N ratio. The conductivity of this graphene aerogel is ≈11.74 S m-1 at zero strain, whereas the conductivity at a compressive strain of ≈80% is ≈704.23 S m-1, which is the largest electrical conductivity reported so far in any 3D sponge-like low-density carbon material. In addition, the aerogel has excellent hydrophobicity (with a water contact angle of 137.4°) as well as selective absorption for organic solvents and oils. The compressive modulus (94.5 kPa; ρ ≈ 2.32 mg cm-3) of the rGO aerogel is higher than that of other carbon-based aerogels. The physical and chemical properties (such as high conductivity, elasticity, high surface area, open pore structure, and chemical stability) of the aerogel suggest that it is a viable candidate for the use in energy storage, electrodes for fuel cells, photocatalysis, environmental protection, energy absorption, and sensing applications.
The preparation of graphitic oxide by methods described in the literature is time consuming and hazardous. A rapid, relatively safe method has been developed for preparing graphitic oxide from graphite in what is essentially an anhydrous mixture of sulfuric acid, sodium nitrate and potassium permanganate.
3D interconnected graphene aerogels (GAs) are prepared through one-step chemical reduction and rational assembly of graphene oxide (GO) sheets, so that the difficulties to uniformly disperse the individual graphene sheets in the polymer matrices are avoided. Apart from ultralow density, high porosity, high electrical conductivity and excellent compressibility, the resulting GAs possess a cellular architecture with a high degree of alignment when the graphene content is above a threshold, ~0.5 wt%. The composites prepared by infiltrating GA with epoxy resin present excellent electrical conductivities, together with high mechanical properties and fracture toughness. The unusual anisotropic structure gives rise to ~44% and ~113% higher electrical conductivity and fracture toughness of the composites, respectively, in the alignment direction than that transverse to it.
Functionalized graphene aerogel with high porosity and hydrophobicity is prepared by surface modification of self-assembled graphene oxide aerogels. Fluorinated functional groups are introduced into the surface of three-dimensionally macroporous graphene aerogel through a one-step solution immersion method. Successful fabrication and surface modification of graphene aerogel are confirmed by various techniques. The functionalized graphene aerogel represents superior physical features, including low density (bulk density of 14.4 mg cm−3), high porosity (>87%), mechanical stability (supports at least 2600 times its own weight), and hydrophobicity (contact angle of 144°). By combining the structural features and hydrophobic surface property, the functionalized graphene aerogel not only exhibits excellent absorption performance for various types of oils and organic solvents (capacity up to 11,200% of its weight), but also shows a remarkable regeneration capability (no obvious change in absorption capacity), making them an ideal candidate for eliminating spilled oils and other toxic organic pollutants.
Amine-functionalized graphene aerogel was prepared through gelation and in situ reduction of GO by using polyethylenimine, and subsequent freeze-drying of the resultant graphene hydrogel. Having a large specific surface area, continuous pore structures, and active chemical adsorbing sites, the graphene aerogel exhibited better properties in adsorbing formaldehyde.
Single layer graphene and graphene oxide feature useful and occasionally unique properties by virtue of their 2-dimensional structure. Given that there is a strong correlation between graphene architecture and its conductive, mechanical, chemical and sorptive properties, which lead to useful technologies, the ability to systematically deform graphene into 3-dimensional structures, therefore provides a controllable, scalable route towards tailoring such properties in the final system. However, the advent of chemical methods to control graphene architecture is still coming to fruition and requires focused attention. The flexibility of the graphene system and the direct and indirect methods available to induce morphology changes of graphene sheets are first discussed in this review. Focus is then given towards chemical reactions that influence the shape of pre-synthesized graphene and graphene oxide sheets, from which a toolbox can be extrapolated and used in controlling the spatial arrangement of graphene sheets within composite materials and ultimately tailoring graphene-based device performance. Finally, the properties of 3-dimensionally controlled graphene-based systems are highlighted for their use as batteries, strengthening additives, gas or liquid sorbents, chemical reactor platforms and supercapacitors.
Polymer-based materials with high electrical conductivity are of considerable interest because of their wide range of applications. The construction of a 3D, compactly interconnected graphene network can offer a huge increase in the electrical conductivity of polymer composites. However, it is still a great challenge to achieve desirable 3D architectures in the polymer matrix. Here, highly conductive polymer nanocomposites with 3D compactly interconnected graphene networks are obtained using a self-assembly process. Polystyrene (PS) and ethylene vinyl acetate (EVA) are used as polymer matrixes. The obtained PS composite film with 4.8 vol% graphene shows a high electrical conductivity of 1083.3 S/m, which is superior to that of the graphene composite prepared by a solvent mixing method. The electrical conductivity of the composites is closely related to the compact contact between graphene sheets in the 3D structures and the high reduction level of graphene sheets. The obtained EVA composite films with the 3D graphene structure not only show high electrical conductivity but also exhibit high flexibility. Importantly, the method to fabricate 3D graphene structures in polymer matrix is facile, green, low-cost, and scalable, providing a universal route for the rational design and engineering of highly conductive polymer composites.
Graphene aerogel materials have attracted increasing attention owing to their large specific surface area, high conductivity and electronic interactions. Here, we report for the first time a novel strategy for the synthesis of nitrogen-doped activated graphene aerogel/gold nanoparticles (N-doped AGA/GNs). First, the mixture of graphite oxide, 2,4,6-trihydroxybenzaldehyde, urea and potassium hydroxide was dispersed in water and subsequently heated to form a graphene oxide hydrogel. Then, the hydrogel was dried by freeze-drying and reduced by thermal annealing in an Ar/H2 environment in sequence. Finally, GNs were adsorbed on the surface of the N-doped AGA. The resulting N-doped AGA/GNs offers excellent electronic conductivity (2.8 × 10(3) S m(-1)), specific surface area (1258 m(2) g(-1)), well-defined 3D hierarchical porous structure and apparent heterogeneous electron transfer rate constant (40.78 ± 0.15 cm s(-1)), which are notably better than that of previous graphene aerogel materials. Moreover, the N-doped AGA/GNs was used as a new sensing material for the electrochemical detection of hydroquinone (HQ) and o-dihydroxybenzene (DHB). Owing to the greatly enhanced electron transfer and mass transport, the sensor displays ultrasensitive electrochemical response to HQ and DHB. Its differential pulse voltammetric peak current linearly increases with the increase of HQ and DHB in the range of 5.0 × 10(-8) to 1.8 × 10(-4) M for HQ and 1 × 10(-8) to 2.0 × 10(-4) M for DHB. The detection limit is 1.5 × 10(-8) M for HQ and 3.3 × 10(-9) M for DHB (S/N = 3). This method provides the advantage of sensitivity, repeatability and stability compared with other HQ and DHB sensors. The sensor has been successfully applied to detection of HQ and DHB in real water samples with the spiked recovery in the range of 96.8-103.2%. The study also provides a promising approach for the fabrication of various graphene aerogel materials with improved electrochemical performances, which can be potentially applied in biosensors, electrocatalysis, and energy storage/conversion devices.
We report three-dimensional (3D) nanoporous graphene with preserved 2D electronic properties, tunable pore sizes, and high electron mobility for electronic applications. The complex 3D network comprised of interconnected graphene retains a 2D coherent electron system of massless Dirac fermions. The transport properties of the nanoporous graphene show a semiconducting behavior and strong pore-size dependence, together with unique angular independence. The free-standing, large-scale nanoporous graphene with 2D electronic properties and high electron mobility holds great promise for practical applications in 3D electronic devices.
We report three-dimensional (3D) nanoporous graphene with preserved 2D electronic properties, tunable pore sizes, and high electron mobility for electronic applications. The complex 3D network comprised of interconnected graphene retains a 2D coherent electron system of massless Dirac fermions. The transport properties of the nanoporous graphene show a semiconducting behavior and strong pore-size dependence, together with unique angular independence. The free-standing, large-scale nanoporous graphene with 2D electronic properties and high electron mobility holds great promise for practical applications in 3D electronic devices.
In this paper, a Pt/reduced graphene oxide (Pt/RGO) modified glassy carbon electrode was prepared for the detection of dopamine (DA) and uric acid (UA) in the presence of high concentration of ascorbic acid (AA). The electrochemical behavior of the Pt/RGO modified electrode was characterized by cyclic voltammetry and differential pulse voltammetry, which showed good performance toward individual detection of DA and UA and even their simultaneous detection in the presence of 1.0 mM AA. Evidently, the electro-oxidation peak currents displayed linear relationship with the associated DA and UA concentrations in the range of 10.0-170.0 μM and 10.0-130.0 μM, respectively, with the detection limits of 0.25 μM for DA and 0.45 μM for UA at three folds of the signal-to-noise ratio. The good performance of the Pt/RGO modified electrode provided a promising alternative in routine sensing applications.
Nucleation and growth in unary and binary systems is investigated in the framework of the phase-field theory. Evaluating the model parameters from the interfacial free energy and interface thickness, a quantitative agreement is found with computer simulations and experiments on the ice–water system. The critical undercoolings predicted for a simple binary system are close to experiment. Phase-field simulations for isotropic and anisotropic systems show that due to the interacting diffusion fields the Avrami–Kolmogorov exponent varies with transformed fraction and initial concentration.
Carbon nanotube (CNT) and graphene constitute the two most exotic classes of functional carbon materials representing one-dimensional (1D) and two-dimensional (2D) nanostructures. With an intention of combining them together in to a single structure, in this communication, we report on vertically aligned, interconnected graphene arrays grown on arrays of CNTs.
There is enormous interest in the use of graphene-based materials for energy storage. This article discusses the progress that has been accomplished in the development of chemical, electrochemical, and electrical energy storage systems using graphene. We summarize the theoretical and experimental work on graphene-based hydrogenstorage systems, lithium batteries, and supercapacitors. Even though the research on the use of graphene for energy storage began very recently, the explosive growth of the research conducted in this area makes this minireview timely.
Three-dimensional self-assembled graphene hydrogels (SGHs) have been fabricated by chemical reduction of graphene oxide (GO) with sodium ascorbate. The SGHs were characterized by scanning electron microscopy, rheological tests, electrical conductivity measurements, X-ray photoelectron spectroscopy, X-ray diffraction, and Raman spectroscopy. Results indicate that the reduction of GO promotes the assembly of graphene sheets. The SGHs are electrically conductive (1 S·m−1) and mechanically strong and exhibit excellent electrochemical performance. In 1 mol·L−1 aqueous solution of H2SO4, the specific capacitance of SGHs was measured to be about 240 F·g−1 at a discharge current density of 1.2 A·g−1.
We reported a new type of graphene aerogel-nickel foam (GA@NF) hybrid material prepared through a facile two-step approach and explored its energy storage application as binder-free supercapacitor electrode. By simple freeze-drying and the subsequent thermal annealing of graphene oxide hydrogel-NF hybrid precursor, three-dimensional graphene aerogels with high mass, hierarchical porosity and high conductivity were deposited on NF framework. The resulting binder-free GA@NF electrode exhibited satisfactory double-layer capacitive behavior with high rate capability, good electrochemical cyclic stability and a high specific capacitance of 366 F g-1 at the current density of 2 A g-1. The versatility of this approach was further verified by the successful preparation of 3D graphene/carbon nanotube hybrid aerogel-NF as supercapacitor electrode, also with improved electrochemical performance. With advantageous features, such a facile and versatile fabrication technique shows great promise in the preparation of various types of carbon-metal hybrid electrodes.
Raman spectra are reported from single crystals of graphite and other graphite materials. Single crystals of graphite show one single line at 1575 cm−1. For the other materials like stress‐annealed pyrolitic graphite, commercial graphites, activated charcoal, lampblack, and vitreous carbon another line is detected at 1355 cm−1. The Raman intensity of this band is inversely proportional to the crystallite size and is caused by a breakdown of the k‐selection rule. The intensity of this band allows an estimate of the crystallite size in the surface layer of any carbon sample. Two in‐plane force constants are calculated from the frequencies.