High Potential of Aerosol-Made 3D Graphene-Based Composites for Enhanced Energy Storage.

Recently, many researchers have developed advanced energy storage and energy conversion systems to address the increased demand for energy resources. The performance of these electrochemical energy storage and conversion devices depends considerably on the properties of their unique electrode materials. Among electrode materials, graphene (GR) has attracted much attention due to its unique properties of high flexibility, a large specific surface area, and superior electric conductivity rates that are well-suited to energy storage systems. Specifically, aerosol-made 3D GR composites are known to be more resistant to compressive forces such as paper balls owing to their stronger and harder compressive tolerance levels and higher and more stable surface areas compared to 2D GR sheets. These unique properties of 3D GR composites result in enhanced electrochemical performances for energy storage systems. This review focuses on recent studies of aerosol-made 3D GR-based composites for energy storage systems such as supercapacitors, lithium-ion batteries, and sodium-ion batteries.

Self-dispersed crumpled graphene balls in oil for friction and wear reduction.

Ultrafine particles are often used as lubricant additives because they are capable of entering tribological contacts to reduce friction and protect surfaces from wear. They tend to be more stable than molecular additives under high thermal and mechanical stresses during rubbing. It is highly desirable for these particles to remain well dispersed in oil without relying on molecular ligands. Borrowing from the analogy that pieces of paper that are crumpled do not readily stick to each other (unlike flat sheets), we expect that ultrafine particles resembling miniaturized crumpled paper balls should self-disperse in oil and could act like nanoscale ball bearings to reduce friction and wear. Here we report the use of crumpled graphene balls as a high-performance additive that can significantly improve the lubrication properties of polyalphaolefin base oil. The tribological performance of crumpled graphene balls is only weakly dependent on their concentration in oil and readily exceeds that of other carbon additives such as graphite, reduced graphene oxide, and carbon black. Notably, polyalphaolefin base oil with only 0.01-0.1 wt % of crumpled graphene balls outperforms a fully formulated commercial lubricant in terms of friction and wear reduction.

Stacking-Controlled Assembly of Cabbage-Like Graphene Microsphere for Charge Storage Applications.

Nanostructured graphene electrodes generally have a low density, which can limit the volumetric performance for energy storage devices. The liquid-phase mild reduction process of graphene oxide sheets is combined with the continuous aerosol densification process to produce high-density graphene agglomerates in the form of microspheres. The produced graphene assembly shows the cabbage-like morphology with a high density of 0.75 g cm-3 . In spite of such high density, the cabbage-like graphene microspheres have narrow-ranged mesopores and a high surface area. The cabbage-like graphene microsphere exhibits both high gravimetric and volumetric energy densities due to the optimized microstructure, which shows a high gravimetric capacitance of 177 F g-1 and volumetric capacitance of 117 F cm-3 in supercapacitors. As a cathode for lithium-ion capacitors, the cabbage-like graphene delivers a reversible capacity of ≈176 mAh g-1 . The stacking-control approach provides a new pathway to control the microstructure of the graphene assembly and corresponding charge storage characteristics for energy storage applications.

Disassembly-Reassembly Approach to RuO2 /Graphene Composites for Ultrahigh Volumetric Capacitance Supercapacitor.

A porous, yet compact, RuO2 /graphene hybrid is successfully prepared by using a disassembly-reassembly strategy, achieving effective and uniform loading of RuO2 nanoparticles inside compact graphene monolith. The disassembly process ensures the uniform loading of RuO2 nanoparticles into graphene monolith, while the reassembly process guarantees a high density yet simultaneously unimpeded ion transport channel in the composite. The resulting RuO2 /graphene hybrid possesses a density of 2.63 g cm-3 , leading to a record high volumetric capacitance of 1485 F cm-3 at the current density of 0.1 A g-1 . When the current density is increased to 20 A g-1 , it remains a high volumetric capacitance of 1188 F cm-3 . More importantly, when the single electrode mass loading is increased to 12 mg cm-2 , it still delivers a high volumetric capacitance of 1415 F cm-3 at the current density of 0.1 A g-1 , demonstrating the promise of this disassembly-reassembly approach to create high volumetric performance materials for energy storage applications.

High-Yield Spreading of Water-Miscible Solvents on Water for Langmuir-Blodgett Assembly.

Langmuir-Blodgett (LB) assembly is a classical molecular thin-film processing technique, in which the material is spread onto water surface from a volatile, water-immiscible solvent to create floating monolayers that can be later transferred to solid substrates. LB has also been applied to prepare colloidal thin films with an unparalleled level of microstructural control and thickness, which has enabled the discovery of many exciting collective properties of nanoparticles and the construction of bulk nanostructured materials. To maximize the benefits of LB assembly, the nanoparticles should be well dispersed in both the spreading solvent and on water. This is quite challenging since colloids usually need contrasting surface properties in order to be stable in the water-hating organic solvents and on water surface. In addition, many organic and polymeric nanostructures dissolve in those organic solvents and cannot be processed directly. Using water-liking spreading solvents can avoid this dilemma. However, spreading of water-miscible solvents on water surface is fundamentally challenging due to extensive mixing, which results in significant material loss. Here we report a conceptually simple strategy and a general technique that allows nearly exclusive spreading of such solvents on water surface using electrospray. Since the volume of these aerosolized droplets is reduced by many orders of magnitude, they are readily depleted during the initial spreading step before any significant mixing could occur. The new strategy drastically reduces the burden of material processing prior to assembly and broadens the scope of LB assembly to previously hard-to-process materials. It also avoids the use of toxic volatile organic spreading solvents, improves the reproducibility, and can be readily automated, making LB assembly a more robust tool for colloidal assembly and thin-film fabrication.

Synthesis and characterization of hollow silica particles from tetraethyl orthosilicate and sodium silicate.

We herein introduce an effective method to synthesize hollow silica particles (HSPs) from tetraethyl orthosilicate (TEOS) and sodium silicate (Na2SiO3) as silica sources using a sacrificial template method with a simple modification. The advantage of the method is that it can be applied to synthesize HSPs from not only TEOS but also Na2SiO3 silica sources without changing the method adopted to obtain the sacrificial polymeric templates. Polystyrene particles are adopted as sacrificial templates to synthesize the HSPs, and a conventional dispersion polymerization method is used to synthesis polystyrene particles in an oil medium. Size control of HSPs is enabled by modulation of the polymerization initiator content (2,2'-Azoisobutyronitrile). The particle size, shell thickness, and morphology are analyzed. Light reflection spectra are measured to obtain the light reflection properties of the HSPs. The results indicate that the hollow architecture is the most important factor in determining the light reflection properties of the particles. Such particles are potential candidates for use in light reflectors and heat insulators, as they may reduce energy consumption in heating and cooling applications.

A novel recovery of silicon nanoparticles from a waste silicon sludge.

As the semiconductor and photovoltaic industry undergo rapid growth, a large amount of silicon sludge is generated from the cutting process of silicon ingots. However, it is not effectively recycled. Recovery of nanometer-sized silicon (Si) particles from the sludge has become an important concern because the silicon sludge contains valuable resources including high purity silicon. In the present study, we investigated the novel recovery of Si nanoparticles from waste silicon sludge. The waste silicon sludge also contained surfactant, silicon carbide particles and metallic fragments. After removal of the surfactant by distillation, the Si nanoparticles were recovered by applying controlled ultrasonic waves and centrifugation in series. Metallic impurities in the recovered Si nanoparticles were purified by HCl treatment. The overall maximum yield and purity of the Si nanoparticles were about 80% and 99.7%, respectively.

Aerosol synthesis of cargo-filled graphene nanosacks.

Water microdroplets containing graphene oxide and a second solute are shown to spontaneously segregate into sack-cargo nanostructures upon drying. Analytical modeling and molecular dynamics suggest the sacks form when slow-diffusing graphene oxide preferentially accumulates and adsorbs at the receding air-water interface, followed by capillary collapse. Cargo-filled graphene nanosacks can be nanomanufactured by a simple, continuous, scalable process and are promising for many applications where nanoscale materials should be isolated from the environment or biological tissue.

Preparation and characterization of vanadium-doped ZnO nanoparticles for environmental application.

Vanadium-doped ZnO nanoparticles (ZnO:V) were prepared via flame spray pyrolysis (FSP) from a mixed aqueous solution of zinc hydroxide and vanadyl (IV) acetylacetonate. The morphological, structural and optical properties of the ZnO:V photocatalyst were characterized via transmission electron microscopy (TEM), X-ray powder diffraction (XRD), and UV-visible diffused reflection spectrum (DRS). The photocatalytic activity of ZnO:V was evaluated via photocatalytic degradation of methylene blue (MB). The results showed that the hexagonal wurtzite-structured ZnO:V nanoparticles were successfully synthesized via FSP. The morphology of the as-prepared nanoparticles was polyhedral and non-hollow. The average diameter of ZnO:V, which was calculated from BET result, was 11.7 nm when the molar ratio of V/Zn was 0.1. The maximum decomposition of MB by the ZnO:V nanoparticles was 99.4% after 180 min under UV irradiation, whereas the decomposition of MB by the pure ZnO nanoparticles was 96.6%.

Preparation of nanoporous SiO2 particles and their application in drug release control.

Nanoporous SiO2 particles which have different pore size and volume were prepared from a colloidal mixture of nano-sized silica particles by a spray heating method. The prepared nanoporous SiO2 particles were employed as a drug carrier to investigate the release behaviors of methylene blue (MB) as a model drug for a selected period of time. The concentration of released MB from the porous particles was measured by a UV-Vis spectroscopy with respect to time. The release of MB from the porous particles was maintained for 400 hours and the maximum amount of the released MB was 0.8 mg at 1.56 cm3/g of pore volume. As pore volume of the nanoporous particles increased, the release rate of MB increased.

Hollow Graphene as an Expansion-Inhibiting Electrical Interconnector for Silicon Electrodes in Lithium-Ion Batteries.

Huge volume changes of silicon particles upon alloying and dealloying reactions with lithium are a major reason for the poor cycle performance of silicon-based anodes for lithium-ion batteries. To suppress dimensional changes of silicon is a key strategy in attempts to improve the electrochemical performance of silicon-based anodes. Here, we demonstrate that a conductive agent can be exploited to offset the mechanical strain imposed on silicon electrodes caused by volume expansion of silicon associated with lithiation. Hollow graphene particles as a conductive agent inhibit volume expansion by absorbing the swelling of silicon upon lithiation through flattening the free voids surrounded by the graphene shell. As a result, silicon electrodes with hollow graphene showed a height expansion of 20.4% after full lithiation with a capacity retention of 69% after 200 cycles, while the silicon electrode with conventional carbon black showed an expansion of 76.8% under the same conditions with a capacity retention of 38%. Some of the deflated hollow graphene returns to its initial shape on delithiation due to the mechanical flexibility of the graphene shell layer. Such a robust microstructure of a silicon electrode incorporating hollow graphene that serves as both an expansion inhibitor and a conductive agent greatly improves capacity retention compared with silicon electrodes with the conventionally used carbon black.

K2Ti6O13 Nanoparticle-Loaded Porous rGO Crumples for Supercapacitors.

One-dimensional alkali metal titanates containing potassium, sodium, and lithium are of great concern owing to their high ion mobility and high specific surface area. When those titanates are combined with conductive materials such as graphene, carbon nanotube, and carbon nanofiber, they are able to be employed as efficient electrode materials for supercapacitors. Potassium hexa-titanate (K2Ti6O13, KTO), in particular, has shown superior electrochemical properties compared to other alkali metal titanates because of their large lattice parameters induced by the large radius of potassium ions. Here, we present porous rGO crumples (PGC) decorated with KTO nanoparticles (NPs) for application to supercapacitors. The KTO NP/PGC composites were synthesized by aerosol spray pyrolysis and post-heat treatment. KTO NPs less than 10 nm in diameter were loaded onto PGCs ranging from 3 to 5 µm. Enhanced porous structure of the composites was obtained by the activation of rGO by adding an excessive amount of KOH to the composites. The KTO NP/PGC composite electrodes fabricated at the GO/KOH/TiO2 ratio of 1:3:0.25 showed the highest performance (275 F g-1) in capacitance with different KOH concentrations and cycling stability (83%) after 2000 cycles at a current density of 1 A g-1.

Self-Healing Microcapsule-Thickened Oil Barrier Coatings.

Low-viscosity oils could potentially act as self-healing barrier coatings because they can readily flow and reconnect to heal minor damage. For the same reason, however, they typically do not form stable coatings on metal surfaces. Increasing viscosity helps to stabilize the oil coating, but it also slows down the healing process. Here, we report a strategy for creating highly stable oil coatings on metal surfaces without sacrificing their remarkable self-healing properties. Low-viscosity oil films can be immobilized on metal surfaces using lightweight microcapsules as thickeners, which form a dynamic network to prevent the creep of the coating. When the coating is scratched, oil around the opening can rapidly flow to cover the exposed area, reconnecting the particle network. Use of these coatings as anticorrosion barriers is demonstrated. The coatings can be easily applied on metal surfaces, including those with complex geometries, both in air or under water, and remain stable even in turbulent water. They can protect metal in corrosive environments for extended periods of time and can self-heal repeatedly when scratched at the same spot. Such a strategy may offer effective mitigation of the dangerous localized corrosion aggravated by minor imperfections or damage in protective coatings, which are typically hard to prevent or detect, but can drastically degrade metal properties.

Effect of reaction temperatures and media on crystal structure of colloidal nanocrystals synthesized from an aerosol flow system.

The CdSe and CdTe nanocrystals were successfully synthesized from the ultrasonic-assisted aerosol flow system using a surfactant/solvent system of oleic acid (OA)/trioctylphosphine (TOP)/trioctylphosphine oxide (TOPO)/octadecene (ODE). A comprehensive examination on the control of the CdSe and CdTe crystal structures and optical properties was performed as functions of reaction temperatures and amount of reaction media in the aerosol flow system. CdSe nanocrystals with spherical shape and having zinc blende structure were produced. It was found that the crystal structure and shape of CdSe are not affected by reaction temperature. However, it was found that reaction temperature significantly affects the crystal structure and morphology of CdTe. Zero-dimensional CdTe nanocrystals with spherical shape and having zinc blende structure were obtained at the temperature higher than 300 degrees C. Three-dimensional tetrapod-shaped CdTe nanocrystals having wurtzite structure were obtained at the temperature lower than 250 degrees C. Furthermore, the anisotropic growth was observed to be enhanced as increasing the amount of OA used at the temperature of 250 degrees C.

Pore size-controlled synthesis and characterization of nanostructured silica particles.

Silica nano-materials with meso- and macroporosity are of great interest due to their variety of potential applications. For the application as a catalytic supporter, nanostructured spherical silica particles having both mesopores and macropores were prepared by using an aerosol templating method with colloidal mixtures of polystyrene latex (PSL) particles and silica nanoparticles. The as-prepared particles showed bimodal size distribution consisting of mesopores ranging 2-20 nm and macropores ranging 60-160 nm. As the PSL size decreased, mesopore size increased due to a reduction in packing rates of primary SiO(2) nanoparticles composing the walls of nanostructured porous particles. With an increment of the weight ratio of PSL/SiO(2), mesopores size increased but mesopore volume decreased due to the broken structure of particles and reduction in packing rates. Mesopores disappeared when the furnace temperature was 900 degrees C. The residuals of organic template were detected when furnace temperature and flow rate of carrier gas were below 600 degrees C and above 3 l/min, respectively.

Fabrication of porous nanostructured TiO2 particles by an aerosol templating method.

An aerosol templating method was applied to fabricate the spherical nanostructured TiO2 particles containing both mesopores and macropores using two kinds of colloidal mixture such as polystyrene latex (PSL) particles and TiO2 nanoparticles (P25), and PSL and a titanium hydroxybislactato (TC315). As the weight ratio of PSL/P25 increased from 0 to 1.30, morphology of the as-prepared particles changed from mesoporous particles to particles containing mesopores and macropores. As the furnace temperature decreased from 800 to 600 degrees C at the fixed process conditions, the increase of mesopore volume and specific surface area were determined. The TiO2 particles fabricated from a mixture of TC315 and PSL were composed of lots of mesopores and a few macropores. The width of UV-absorption spectra of the porous particles synthesized from two colloidal mixtures decreased a little with respect to the increase of the weight ratio. The complete decomposition of p-xylene of 97.9 ppm was accomplished within 2 h under the illumination of UV-light by all the porous particles.

Microbial synthesis of magnetite and Mn-substituted magnetite nanoparticles: influence of bacteria and incubation temperature.

Microbial synthesis of magnetite and metal (Co, Cr, Ni)-substituted magnetites has only recently been reported. The objective of this study was to examine the influence of Mn ion on the microbial synthesis of magnetite nanoparticles. The reductive biotransformation of an akaganeite (beta-FeOOH) or a Mn-substituted (2-20 mol%) akaganeite (Fe(1-x)Mn(x)OOH) by Shewanella loiha (PV-4, 25 degrees C) and Thermoanaerobacter ethanolicus (TOR-39, 60 degrees C) was investigated under anaerobic conditions at circumneutral pH (pH = 7-8). Both bacteria formed magnetite nanoparticles using akaganeite as a magnetite precursor. By comparison of iron minerals formed by PV-4 and TOR-39 using Mn-mixed akaganeite as the precursor, it was shown that PV-4 formed siderite (FeCO3), green rust [Fe2+Fe3+(OH)16CO3 x 4H2O], and magnetite at 25 degrees C, whereas TOR-39 formed mainly nm-sized magnetite at 60 degrees C. The presence of Mn in the magnetite formed by TOR-39 was revealed by energy dispersive X-ray analysis (EDX) is indicative of Mn substitution into magnetite crystals. EDX analysis of iron minerals formed by PV-4 showed that Mn was preferentially concentrated in the siderite and green rust. These results demonstrate that coprecipitated/sorbed Mn induced microbial formation of siderite and green rust by PV-4 at 25 degrees C, but the synthesis of Mn-substituted magnetite nanoparticles proceeded by TOR-39 at 60 degrees C. These results indicate that the bacteria have the ability to synthesize magnetite and Mn-substituted magnetite nano-crystals. Microbially facilitated synthesis of magnetite and metal-substituted magnetites at near ambient temperatures may expand the possible use of specialized ferromagnetic nano-particles.

A swelling-suppressed Si/SiOx nanosphere lithium storage material fabricated by graphene envelopment.

A swelling-suppressed, Si nanocrystals-embedded SiOx nanospheres lithium storage material was prepared by graphene envelopment. The free void spaces formed between the graphene envelope and Si/SiOx nanospheres effectively accommodated the volume changes of Si/SiOx nanospheres during cycling, which significantly suppresses the swelling behavior and improves the capacity retention up to 200 cycles.

Hierarchical networks of redox-active reduced crumpled graphene oxide and functionalized few-walled carbon nanotubes for rapid electrochemical energy storage.

Crumpled graphene is known to have a strong aggregation-resistive property due to its unique 3D morphology, providing a promising solution to prevent the restacking issue of graphene based electrode materials. Here, we demonstrate the utilization of redox-active oxygen functional groups on the partially reduced crumpled graphene oxide (r-CGO) for electrochemical energy storage applications. To effectively utilize the surface redox reactions of the functional groups, hierarchical networks of electrodes including r-CGO and functionalized few-walled carbon nanotubes (f-FWNTs) are assembled via a vacuum-filtration process, resulting in a 3D porous structure. These composite electrodes are employed as positive electrodes in Li-cells, delivering high gravimetric capacities of up to ∼170 mA h g(-1) with significantly enhanced rate-capability compared to the electrodes consisting of conventional 2D reduced graphene oxide and f-FWNTs. These results highlight the importance of microstructure design coupled with oxygen chemistry control, to maximize the surface redox reactions on functionalized graphene based electrodes.

One-Step Formation of Silicon-Graphene Composites from Silicon Sludge Waste and Graphene Oxide via Aerosol Process for Lithium Ion Batteries.

Over 40% of high-purity silicon (Si) is consumed as sludge waste consisting of Si, silicon carbide (SiC) particles and metal impurities from the fragments of cutting wire mixed in ethylene glycol based cutting fluid during Si wafer slicing in semiconductor fabrication. Recovery of Si from the waste Si sludge has been a great concern because Si particles are promising high-capacity anode materials for Li ion batteries. In this study, we report a novel one-step aerosol process that not only extracts Si particles but also generates Si-graphene (GR) composites from the colloidal mixture of waste Si sludge and graphene oxide (GO) at the same time by ultrasonic atomization-assisted spray pyrolysis. This process supports many advantages such as eco-friendly, low-energy, rapid, and simple method for forming Si-GR composite. The morphology of the as-formed Si-GR composites looked like a crumpled paper ball and the average size of the composites varied from 0.6 to 0.8 µm with variation of the process variables. The electrochemical performance was then conducted with the Si-GR composites for Lithium Ion Batteries (LIBs). The Si-GR composites exhibited very high performance as Li ion battery anodes in terms of capacity, cycling stability, and Coulombic efficiency.