Swanee J. Shin

Ultralow Density, Monolithic WS2, MoS2, and MoS2/Graphene Aerogels

We describe the synthesis and characterization of monolithic, ultralow density WS2 and MoS2 aerogels, as well as a high surface area MoS2/graphene hybrid aerogel. The monolithic WS2 and MoS2 aerogels are prepared via thermal decomposition of freeze-dried ammonium thio-molybdate (ATM) and ammonium thio-tungstate (ATT) solutions, respectively. The densities of the pure dichalcogenide aerogels represent 0.4% and 0.5% of full density MoS2 and WS2, respectively, and can be tailored by simply changing the initial ATM or ATT concentrations. Similar processing in the presence of the graphene aerogel results in a hybrid structure with MoS2 sheets conformally coating the graphene scaffold. This layered motif produces a ∼50 wt % MoS2 aerogel with BET surface area of ∼700 m(2)/g and an electrical conductivity of 112 S/m. The MoS2/graphene aerogel shows promising results as a hydrogen evolution reaction catalyst with low onset potential (∼100 mV) and high current density (100 mA/cm(2) at 260 mV).

Synthesis and characterization of highly crystalline graphene aerogels.

Aerogels are used in a broad range of scientific and industrial applications due to their large surface areas, ultrafine pore sizes, and extremely low densities. Recently, a large number of reports have described graphene aerogels based on the reduction of graphene oxide (GO). Though these GO-based aerogels represent a considerable advance relative to traditional carbon aerogels, they remain significantly inferior to individual graphene sheets due to their poor crystallinity. Here, we report a straightforward method to synthesize highly crystalline GO-based graphene aerogels via high-temperature processing common in commercial graphite production. The crystallization of the graphene aerogels versus annealing temperature is characterized using Raman and X-ray absorption spectroscopy, X-ray diffraction, and electron microscopy. Nitrogen porosimetry shows that the highly crystalline graphene macrostructure maintains a high surface area and ultrafine pore size. Because of their enhanced crystallinity, these graphene aerogels exhibit a ∼ 200 °C improvement in oxidation temperature and an order of magnitude increase in electrical conductivity.

Super-Compressibility of Ultralow-Density Nanoporous Silica

Porosity generally embrittles ceramics, and low-density nanoporous oxides typically exhibit very brittle behavior. In contrast to such expectations, we find that an effective fracture strain of nanoporous silica increases with increasing porosity. At ultralow relative densities of <0.5%,[1] nanoporous monoliths start exhibiting super-compressible deformation with large effective fracture strains of >50%. We attribute such a super-compressible behavior to consequences of an increase in the average aspect ratio of ligaments with decreasing monolith density. These results have important implications for designing novel supercompressive materials and for understanding observations of super-compressibility for other low-density nanoporous systems such as carbon-nanotube-based nanofoams. Understanding effects of porosity on mechanical properties of solids has been a subject of numerous previous investigations, driven by their important technological implications. Indeed, most brittle structural materials, such as masonry materials, ceramics, and bones, are to some extent porous, with the size of pores and/or ligaments often being at the nanoscale. Porosity of different materials covers a very wide range, from zero (i.e., full density solids) to >99% for aerogels (AGs). The AGs are representative materials for the limiting case of low-density/high-porosity systems with submicron uniformity. They are sol-gel-derived solids made from nanoscale ligaments randomly interconnected into a macroscopic three-dimensional structure with open-cell porosity tunable up to ∼99.95%.[2] Numerous previous studies[2] have focused on conventional silica AGs with densities above ∼50 mg cm−3, first made by Kistler a number of decades ago.[3] Ligaments in these AGs are made of amorphous SiO2 with variable surface hydroxylation. Successful synthesis of ultralow-density[4] silica AGs has also been reported.[5–7] Ultralow-density nanofoams are currently of interest for thermonuclear fusion energy applications as scaffolds for condensed hydrogen fuel layers in fusion targets.[8] They are also attractive materials for solid-state targets for ultrabright x-ray lasers,[9] energy absorbing structures,[10] compliant electrical contacts,[11] and electromechanical devices.[12] Poor mechanical properties of nanofoams limit their use in these applications.

Ultra‐strong and Low‐Density Nanotubular Bulk Materials with Tunable Feature Sizes

The synthesis of ultralow-density (>5 mg/cm(3) ) bulk materials with interconnected nanotubular morphology and deterministic, fully tunable feature size, composition, and density is presented. A thin-walled nanotubular design realized by employing templating based on atomic layer deposition makes the material about 10 times stronger and stiffer than aerogels of the same density.

Universal roles of hydrogen in electrochemical performance of graphene: high rate capacity and atomistic origins.

Atomic hydrogen exists ubiquitously in graphene materials made by chemical methods. Yet determining the effect of hydrogen on the electrochemical performance of graphene remains a significant challenge. Here we report the experimental observations of high rate capacity in hydrogen-treated 3-dimensional (3D) graphene nanofoam electrodes for lithium ion batteries. Structural and electronic characterization suggests that defect sites and hydrogen play synergistic roles in disrupting sp2 graphene to facilitate fast lithium transport and reversible surface binding, as evidenced by the fast charge-transfer kinetics and increased capacitive contribution in hydrogen-treated 3D graphene. In concert with experiments, multiscale calculations reveal that defect complexes in graphene are prerequisite for low-temperature hydrogenation, and that the hydrogenation of defective or functionalized sites at strained domain boundaries plays a beneficial role in improving rate capacity by opening gaps to facilitate easier Li penetration. Additional reversible capacity is provided by enhanced lithium binding near hydrogen-terminated edge sites. These findings provide qualitative insights in helping the design of graphene-based materials for high-power electrodes.

Nanoporous Cu–C composites based on carbon-nanotube aerogels

Current synthesis methods of nanoporous Cu–C composites offer limited control of the material composition, structure, and properties, particularly for large Cu loadings of ≳20 wt%. Here, we describe two related approaches to realize novel nanoporous Cu–C composites based on the templating of recently developed carbon-nanotube aerogels (CNT-CAs). Our first approach involves the trapping of Cu nanoparticles while CNT-CAs undergo gelation. This method yields nanofoams with relatively high densities of ≳65 mg cm−3 for Cu loadings of ≳10 wt%. Our second approach overcomes this limitation by filling the pores of undoped CNT-CA monoliths with an aqueous solution of CuSO4 followed by (i) freeze-drying to remove water and (ii) thermal decomposition of CuSO4. With this approach, we demonstrate Cu–C composites with a C matrix density of ∼25 mg cm−3 and Cu loadings of up to 70 wt%. These versatile methods could be extended to fabricate other nanoporous metal–carbon composite materials geared for specific applications.

Atomic layer deposition-derived ultra-low-density composite bulk materials with deterministic density and composition.

A universal approach for on-demand development of monolithic metal oxide composite bulk materials with air-like densities (<5 mg/cm(3)) is reported. The materials are fabricated by atomic layer deposition of titania (TiO2) or zinc oxide (ZnO) using the nanoscale architecture of 1 mg/cm(3) SiO2 aerogels formed by self-organization as a blueprint. This approach provides deterministic control over density and composition without affecting the nanoscale architecture of the composite material that is otherwise very difficult to achieve. We found that these materials provide laser-to-X-ray conversion efficiencies of up to 5.3%, which is the highest conversion efficiency yet obtained from any foam-based target, thus opening the door to a new generation of highly efficient laser-induced nanosecond scale multi-keV X-ray sources.

Deterministic control over high-Z doping of polydicyclopentadiene-based aerogel coatings.

We report on simple and efficient routes to dope polydicyclopentadiene (PDCPD)-based aerogels and their coatings with high-Z tracer elements. Initially, direct halogenation of PDCPD wet gels and aerogels with elemental iodine or bromine was studied. Although several pathways were identified that allowed doping of PDCPD aerogels by direct addition of bromine or iodine to the unsaturated polymer backbone, they all provided limited control over the amount and uniformity of doping, especially at very low dopant concentrations. Deterministic control over the doping level in polymeric aerogels and aerogel coatings was then achieved by developing a copolymerization approach with iodine and tin containing comonomers. Our results highlight the versatility of the ring-opening metathesis polymerization (ROMP)-based copolymerization approach in terms of functionalization and doping of low density polymeric aerogels and their coatings.

Xenon doping of glow discharge polymer by ion implantation

We demonstrate controlled doping of a glow discharge polymer by implantation with 500 keV Xe ions at room temperature. The Xe retention exhibits a threshold behavior, with a threshold dose of ∼2 × 1014 cm−2. Doping is accompanied by irradiation-induced changes in the polymer composition, including gradual H loss and a more complex non-monotonic behavior of the O concentration. The matrix composition saturates at C0.77H0.22O0.01 for Xe doses above ∼5 × 1014 cm−2 and up to the maximum dose studied (5 × 1015 cm−2). The retention mechanism is attributed to the modification of the polymer from a chain-like to clustered ring structure. The dopant profile and the elemental composition of the implanted polymer exhibit good stability upon thermal annealing up to 305 °C.