Sung Mi Jung

A facile route for 3D aerogels from nanostructured 1D and 2D materials

Aerogels have numerous applications due to their high surface area and low densities. However, creating aerogels from a large variety of materials has remained an outstanding challenge. Here, we report a new methodology to enable aerogel production with a wide range of materials. The method is based on the assembly of anisotropic nano-objects (one-dimensional (1D) nanotubes, nanowires, or two-dimensional (2D) nanosheets) into a cross-linking network from their colloidal suspensions at the transition from the semi-dilute to the isotropic concentrated regime. The resultant aerogels have highly porous and ultrafine three-dimensional (3D) networks consisting of 1D (Ag, Si, MnO2, single-walled carbon nanotubes (SWNTs)) and 2D materials (MoS2, graphene, h-BN) with high surface areas, low densities, and high electrical conductivities. This method opens up a facile route for aerogel production with a wide variety of materials and tremendous opportunities for bio-scaffold, energy storage, thermoelectric, catalysis, and hydrogen storage applications.

A facile methodology for the production of in situ inorganic nanowire hydrogels/aerogels.

Creating inorganic nanowire hydrogels/aerogels using various materials and inexpensive means remains an outstanding challenge despite their importance for many applications. Here, we present a facile methodology to enable highly porous inorganic nanowire hydrogel/aerogel production on a large scale and at low cost. The hydrogels/aerogels are obtained from in situ hydrothermal synthesis of one-dimensional (1D) nanowires that directly form a cross-linking network during the synthesis process. Such a method not only offers great simplicity but also allows the interconnecting nanowires to have much longer length. The longer length offers aerogels with remarkable porosity and surface area extremely low densities (as low as 2.9 mg/cm(3)), are mechanically robust, and can have superelasticity by tuning the synthesis conditions. The nanowires in the hydrogels/aerogels serve both as structural support and active sites, for example, for catalysis or absorption. In this work, we have found that the as-grown hydrogels can be used directly as water filters to remove pollutants such as heavy metal ions and toxic organic contents. Our studies indicate that this method for nanowire hydrogels/aerogels production is not only economical but greatly augmented their applications in environmental, catalysis, sensing, absorption, energy storage, and beyond.

Chemical sensors for sensing gas adsorbed on the inner surface of carbon nanotube channels

The authors have developed gas sensors that operate when gas flowing through carbon nanotube (CNT) channels is adsorbed on the inner surface of the channels. CNTs fabricated on anodic alumina membranes were used. The CNTs are well ordered and connected in parallel, forming parallel channels. The sensors are highly responsive to NH3 and NO2 molecules and the response times are relatively short. They are completely recovered within 10min when a dc voltage of 10V is applied for 2min. The fabrication processes are relatively simple and do not require special techniques such as e-beam lithography.

Fabrication of clean carbon nanotube field emitters

We have developed a method for fabricating a good carbon nanotube (CNT) field emitter using electrophoretic deposition and fissure formation techniques. A thin film of CNTs was deposited on a Ti substrate, by electrophoresis, from an aqueous mixture of CNT and detergent, dried naturally, and then pressed on the surface to make it smooth. By firing, it was changed to numerous microsized islands. As a result very clean CNT tips were generated from the fissures formed. The film showed good field emission properties. Our method affords a process of simple steps and does not need any postdeposition or activation process for field emission.

Anodic aluminum oxide membrane bonded on a silicon wafer for carbon nanotube field emitter arrays

We have developed a method to bond a very thin anodic aluminum oxide membrane (400nm thick) on a Si wafer. Furthermore, we were able to fabricate well-ordered carbon nanotube (CNT) arrays on the membrane at a very high temperature—above 1000°C—without deformation. The CNT arrays fabricated at 800°C exhibited long-term stability and uniform emission. Their current density was higher than 1mA∕cm2; such a density might be required for flat panel displays. When the tip of the CNTs was modified from an open shape to a closed shape by exposure to acetylene gas, the turn-on voltage decreased significantly and the enhancement factor increased significantly.

The hierarchical porosity of a three-dimensional graphene electrode for binder-free and high performance supercapacitors

In this study, we report a binder-free supercapacitor including a unique graphene electrode of hierarchical porosity composed of the submicrometer porosity (∼20 μm) of 3D graphene self-assembled by nanopores (∼2.5 nm) of graphene flakes. The hierarchical 3D porosity of the graphene electrode was prepared by a facile and scalable approach based on acid activation and freeze-drying. Remarkably, we found that this unique graphene-based electrode, when used in a supercapacitor, shows improved performance compared with a reported graphene electrode; these improvements include high specific capacitance (∼442 F g−1), fast rate charging/discharging, and excellent cycle stability (95% after 1600 cycle numbers), which were due to (1) the creation of a high specific surface area and a diffusion path, thus promoting ion transport capability, and (2) the additional pseudocapacitor due to the controlled oxygen amount of functionalization on the graphene surface. The resultant device shows an energy density of 51.9 W h kg−1 and a power density of 467.8 W kg−1. This study introduces a new concept for the binder-free, cost-effective and high performance of a graphene-based supercapacitor, which could pave the way for practical applications in frontier energy devices.

Sculpting carbon bonds for allotropic transformation through solid-state re-engineering of –sp 2 carbon

Carbon forms one of natures strongest chemical bonds; its allotropes having provided some of the most exciting scientific discoveries in recent times. The possibility of inter-allotropic transformations/hybridization of carbon is hence a topic of immense fundamental and technological interest. Such modifications usually require extreme conditions (high temperature, pressure and/or high-energy irradiations), and are usually not well controlled. Here we demonstrate inter-allotropic transformations/hybridizations of specific types that appear uniformly across large-area carbon networks, using moderate alternating voltage pulses. By controlling the pulse magnitude, small-diameter single-walled carbon nanotubes can be transformed predominantly into larger-diameter single-walled carbon nanotubes, multi-walled carbon nanotubes of different morphologies, multi-layered graphene nanoribbons or structures with sp(3) bonds. This re-engineering of carbon bonds evolves via a coalescence-induced reconfiguration of sp(2) hybridization, terminates with negligible introduction of defects and demonstrates remarkable reproducibility. This reflects a potential step forward for large-scale engineering of nanocarbon allotropes and their junctions.

Efficient lithium storage from modified vertically aligned carbon nanotubes with open-ends

Herein, we report the use of vertically aligned carbon nanotubes (VA-CNTs) with controlled structure and morphology as an anode material for lithium-ion batteries. The tailored surface structure and open-end morphology of VA-CNTs made by ion milling and transfer processes increases the accessibility of Li ions, and allows Li ions to diffuse inside as well as on the surface of the CNTs through the generated surface defects, leading to the significantly improved lithium ion storage capacity compared to as-grown close-end VA-CNTs. The irreversible discharge capacity of the modified VA-CNTs anode reaches up to 2350 mA h g−1 at 2 C in the first cycle and the reversible capacity is in the range of 1200–557 mA h g−1 for the 2nd–20th cycles. The second insertion capacity of the modified CNTs electrode was 4 times higher than that of the as-grown CNTs and 3.2 times higher than that of a previously reported CNTs electrode for a commercial graphite device.