Hee Dong Jang

Crumpled Graphene-Encapsulated Si Nanoparticles for Lithium Ion Battery Anodes

Submicrometer-sized capsules made of Si nanoparticles wrapped by crumpled graphene shells were made by a rapid, one-step capillary-driven assembly route in aerosol droplets. Aqueous dispersion of micrometer-sized graphene oxide (GO) sheets and Si nanoparticles were nebulized to form aerosol droplets, which were passed through a preheated tube furnace. Evaporation-induced capillary force wrapped graphene (a.k.a., reduced GO) sheets around the Si particles, and heavily crumpled the shell. The folds and wrinkles in the crumpled graphene coating can accommodate the volume expansion of Si upon lithiation without fracture, and thus help to protect Si nanoparticles from excessive deposition of the insulating solid electrolyte interphase. Compared to the native Si particles, the composite capsules have greatly improved performance as Li ion battery anodes in terms of capacity, cycling stability, and Coulombic efficiency.

Compression and Aggregation-Resistant Particles of Crumpled Soft Sheets

Unlike flat sheets, crumpled paper balls have both high free volume and high compressive strength, and can tightly pack without significantly reducing the area of accessible surface. Such properties would be highly desirable for sheet-like materials such as graphene, since they tend to aggregate in solution and restack in the solid state, making their properties highly dependent on the material processing history. Here we report the synthesis of crumpled graphene balls by capillary compression in rapidly evaporating aerosol droplets. The crumpled particles are stabilized by locally folded, π-π stacked ridges as a result of plastic deformation, and do not unfold or collapse during common processing steps. In addition, they are remarkably aggregation-resistant in either solution or solid state, and remain largely intact and redispersible after chemical treatments, wet processing, annealing, and even pelletizing at high pressure. For example, upon compression at 55 MPa, the regular flat graphene sheets turn into nondispersible chunks with drastically reduced surface area by 84%, while the crumpled graphene particles can still maintain 45% of their original surface area and remain readily dispersible in common solvents. Therefore, crumpled particles could help to standardize graphene-based materials by delivering more stable properties such as high surface area and solution processability regardless of material processing history. This should greatly benefit applications using bulk quantities of graphene, such as in energy storage or conversion devices. As a proof of concept, we demonstrate that microbial fuel electrodes modified by the crumpled particles indeed outperform those modified with their flat counterparts.

Effect of Sheet Morphology on the Scalability of Graphene-Based Ultracapacitors

Graphene is considered a promising ultracapacitor material toward high power and energy density because of its high conductivity and high surface area without pore tortuosity. However, the two-dimensional (2D) sheets tend to aggregate during the electrode fabrication process and align perpendicular to the flow direction of electrons and ions, which can reduce the available surface area and limit the electron and ion transport. This makes it hard to achieve scalable device performance as the loading level of the active material increases. Here, we report a strategy to solve these problems by transforming the 2D graphene sheet into a crumpled paper ball structure. Compared to flat or wrinkled sheets, the crumpled graphene balls can deliver much higher specific capacitance and better rate performance. More importantly, devices made with crumpled graphene balls are significantly less dependent on the electrode mass loading. Performance of graphene-based ultracapacitors can be further enhanced by using flat graphene sheets as the binder for the crumpled graphene balls, thus eliminating the need for less active binder materials.

A glucose biosensor based on TiO2–Graphene composite

A novel glucose biosensor was developed based on the adsorption of glucose oxidase at a TiO(2)-Graphene (GR) nanocomposite electrode. A TiO(2)-GR composite was synthesized from a colloidal mixture of TiO(2) nanoparticles and graphene oxide (GO) nanosheets by an aerosol assisted self-assembly (AASA). The particle morphology of all TiO(2)-GR composites was spherical in shape. It was observed that micron-sized TiO(2) particles were encapsulated by GR nanosheets and that the degree of encapsulation was proportional to the ratio of GO/TiO(2). The amperometric response of the glucose biosensor fabricated by the TiO(2)-GR composite was linear against a concentration of glucose ranging from 0 to 8mM at -0.6 V. The highest sensitivity was noted at about 6.2 μA/mMcm(2). The as prepared glucose biosensor based on the TiO(2)-GR composite showed higher catalytic performance for glucose redox than a pure TiO(2) and GR biosensor.

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.

Oil absorbing graphene capsules by capillary molding

Oil absorbing graphene capsules are synthesized by capillary molding of graphene oxide (GO) sheets against a polystyrene bead template in evaporating aerosol droplets, followed by simultaneous reduction of GO and decomposition of the polymer template during ultrasonic spray pyrolysis.

3D label-free prostate specific antigen (PSA) immunosensor based on graphene-gold composites.

Highly sensitive and label-free detection of the prostate specific antigen (PSA) remains a challenge in the diagnosis of prostate cancer. Here, a novel three-dimensional (3D) electrochemical immunosensor capable of sensitive and label-free detection of PSA is reported. This unique immunosensor is equipped with a highly conductive graphene (GR)-based gold (Au) composite modified electrode. The GR-based Au composite is prepared using aerosol spray pyrolysis and the morphology of the composite is the shape of a crumpled GR ball decorated with Au nanoparticles. Unlike the previous research, this novel 3D immunosensor functions very well over a broad linear range of 0-10 ng/mL with a low detection limit of 0.59 ng/mL; furthermore, it exhibits a significantly increased electron transfer and high sensitivity toward PSA. The highest rate of current change with respect to the PSA concentration is 5 μA/(ng/mL). Satisfactory selectivity, reproducibility, and stability of the 3D immunosensor are also exhibited.

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

Significance Aggregation is a major problem for ultrafine particle additives in lubricant oil because it reduces the effective particle concentrations, prevents particles from entering the contact area of working surfaces, and leads to unstable tribological performance. Molecular ligands can help the particles to disperse, but they tend to degrade under the harsh tribological conditions. Therefore, self-dispersed particles without the need for surfactant are highly desirable. We report here, for the first time to our knowledge, such type of ultrafine particles made of crumpled, paper-ball–like graphene, which indeed can self-disperse in lubricant oil, and exhibit stable and superior tribological performances. 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.

One Step Synthesis of Pt-nanoparticles-Laden Graphene Crumples By Aerosol Spray Pyrolysis and Evaluation of Their Electrocatalytic Activity

Pt-nanoparticles-laden graphene (GR) crumples were directly synthesized from a colloidal mixture of aqueous chloroplatinic acid (H2PtCl6) and graphene oxide (GO) sheets via aerosol spray pyrolysis (ASP). Effects of Pt content in the Pt/GR composite and temperature of heating zone on the particle morphology, diffraction pattern, and specific surface area were investigated. The morphology of Pt/GR was the shape of a crumpled sheet of paper and the average size of the composite was around 1.3 μm in diameter. As Pt content increased from 2 to 20 wt%, higher numbers of Pt nanoparticles are observed on the GR at higher Pt content and the specific surface area of the composite also increased from 122 to 146 m2/g. Also, the intensity of the GR peak decreased, but that of the Pt peak increased. As temperature increased from 500°C to 900°C, an increase of the particle size of Pt due to sintering was observed. Electrocatalytic application of the Pt/GR composites was examined through methanol oxidation reaction. The 20 wt% Pt/GR synthesized at 900°C showed higher performance on methanol oxidation than a commercial 20 wt% Pt/carbon black catalyst. Copyright 2013 American Association for Aerosol Research

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.