Gold Aerogel Monoliths with Tunable Ultralow Densities.

Herein we report the fabrication of ultralight gold aerogel monoliths with tunable densities and pore structures. Gold nanowires are prepared at the gram scale by substrate-assisted growth with uniform size, ultrathin diameters, high purity, and a high aspect ratio. Freeze-casting of suspensions of these nanowires produces free-standing, monolithic aerogels with tunable densities from 6 to 23 mg/cm3, which to the best of our knowledge represents the lowest density monolithic gold material. We also demonstrate that the pore geometries created during freeze-casting can be systematically tuned across multiple length scales by the selection of different solvents and excipients in the feedstock suspension. The mechanical behavior of porous materials depends on relative density and pore architectures.

Elucidating the mechanism of MgB2 initial hydrogenation via a combined experimental-theoretical study.

Mg(BH4)2 is a promising solid-state hydrogen storage material, releasing 14.9 wt% hydrogen upon conversion to MgB2. Although several dehydrogenation pathways have been proposed, the hydrogenation process is less well understood. Here, we present a joint experimental-theoretical study that elucidates the key atomistic mechanisms associated with the initial stages of hydrogen uptake within MgB2. Fourier transform infrared, X-ray absorption, and X-ray emission spectroscopies are integrated with spectroscopic simulations to show that hydrogenation can initially proceed via direct conversion of MgB2 to Mg(BH4)2 complexes. The associated energy landscape is mapped by combining ab initio calculations with barriers extracted from the experimental uptake curves, from which a kinetic model is constructed. The results from the kinetic model suggest that initial hydrogenation takes place via a multi-step process: molecular H2 dissociation, likely at Mg-terminated MgB2 surfaces, is followed by migration of atomic hydrogen to defective boron sites, where the formation of stable B-H bonds ultimately leads to the direct creation of Mg(BH4)2 complexes without persistent BxHy intermediates. Implications for understanding the chemical, structural, and electronic changes upon hydrogenation of MgB2 are discussed.

Covalent attachment of diamondoid phosphonic acid dichlorides to tungsten oxide surfaces.

Diamondoids (nanometer-sized diamond-like hydrocarbons) are a novel class of carbon nanomaterials that exhibit negative electron affinity (NEA) and strong electron-phonon scattering. Surface-bound diamondoid monolayers exhibit monochromatic photoemission, a unique property that makes them ideal electron sources for electron-beam lithography and high-resolution electron microscopy. However, these applications are limited by the stability of the chemical bonding of diamondoids on surfaces. Here we demonstrate the stable covalent attachment of diamantane phosphonic dichloride on tungsten/tungsten oxide surfaces. X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared (FTIR) spectroscopy revealed that diamondoid-functionalized tungsten oxide films were stable up to 300-350 °C, a substantial improvement over conventional diamondoid thiolate monolayers on gold, which dissociate at 100-200 °C. Extreme ultraviolet (EUV) light stimulated photoemission from these diamondoid phosphonate monolayers exhibited a characteristic monochromatic NEA peak with 0.2 eV full width at half-maximum (fwhm) at room temperature, showing that the unique monochromatization property of diamondoids remained intact after attachment. Our results demonstrate that phosphonic dichloride functionality is a promising approach for forming stable diamondoid monolayers for elevated temperature and high-current applications such as electron emission and coatings in micro/nano electromechanical systems (MEMS/NEMS).

All-day passive radiative cooling using common salts.

Radiative cooling materials underperform compared to their theoretical potential due to parasitic heating from contact with ambient air. Solutions to this problem can be expensive or complex to fabricate. Here, a potentially inexpensive, simply fabricated material that improves cooling performance by reducing parasitic heating was created using naturally abundant salts. NaCl and KCl are not typically considered for radiative cooling because of their high hygroscopicity and low mechanical strength; however, these compounds are highly infrared-transparent and can be fabricated into aerogel-like foam structures to provide thermally insulating properties. The salt foams, described herein, scattered (reflected) visible light, transmitted infrared radiation, and provided thermal insulation. They were packaged into mechanical supporting panels to avoid physical disruption and the nanostructure was stabilized to moisture by adding an anti-caking agent. The panels were able to keep an underlying surface below ambient temperature for a full 24 hour cycle and reduced parasitic heating rate by more than half (compared to an uncovered surface). The panels were able to cool a variety of underlying surfaces, even highly absorbing surfaces that are normally well above ambient temperature during the day. This work demonstrates an affordable, easily produced, electricity-free cooling technology with potential to be manufactured for large-scale practical applications.

Submicrosecond Aggregation during Detonation Synthesis of Nanodiamond.

Detonation nanodiamond (DND) is known to form aggregates that significantly reduce their unique nanoscale properties and require postprocessing to separate. How and when DND aggregates is an important question that has not been answered experimentally and could provide the foundation for approaches to limit aggregation. To answer this question, time-resolved small-angle X-ray scattering was performed during the detonation of high-explosives that are expected to condense particulates in the diamond, graphite, and liquid regions of the carbon phase diagram. DND aggregation into low fractal dimension structures could be observed as early as 0.1 µs, along with a separate scattering population also observed from an explosive that produces primarily graphitic products. A counterexample is the case of a high-explosive that produces nano-onions, where no hierarchical scattering was observed for at least 10 µs behind the detonation front. These results suggest that DND aggregation occurs on time scales comparable to particle formation.

Ordering in bio-inorganic hybrid nanomaterials probed by in situ scanning transmission X-ray microscopy.

Phospholipid bilayer coated Si nanowires are one-dimensional (1D) composites that provide versatile bio-nanoelectronic functionality via incorporation of a wide variety of biomolecules into the phospholipid matrix. The physiochemical behaviour of the phospholipid bilayer is strongly dependent on its structure and, as a consequence, substantial modelling and experimental efforts have been directed at the structural characterization of supported bilayers and unsupported phospholipid vesicles; nonetheless, the experimental studies conducted to date have exclusively involved volume-averaged techniques, which do not allow for the assignment of spatially resolved structural variations that could critically impact the performance of the 1D phospholipid-Si NW composites. In this manuscript, we use scanning transmission X-ray microscopy (STXM) to probe bond orientation and bilayer thickness as a function of position with a spatial resolution of ∼30 nm for Δ9-cis 1,2-dioleoyl-sn-glycero-3-phosphocholine layers prepared Si NWs. When coupled with small angle X-ray scattering measurements, the STXM data reveal structural motifs of the Si NWs that give rise to multi-bilayer formation and enable assignment of the orientation of specific bonds known to affect the order and rigidity of phospholipid bilayers.

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 sp(2) 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.

Potential-induced electronic structure changes in supercapacitor electrodes observed by in operando soft X-ray spectroscopy.

The dynamic physiochemical response of a functioning graphene-based aerogel supercapacitor is monitored in operando by soft X-ray spectroscopy and interpreted through ab initio atomistic simulations. Unanticipated changes in the electronic structure of the electrode as a function of applied voltage bias indicate structural modifications across multiple length scales via independent pseudocapacitive and electric double layer charge storage channels.

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.

X-ray absorption spectroscopy for the structural investigation of self-assembled-monolayer-directed mineralization.

Self-assembled monolayers (SAMs) of organothiol molecules prepared on noble metal substrates are known to exert considerable influence over biomineral nucleation and growth and, as such, offer model templates for investigation of the processes of directed biomineralization. Identifying the structural evolution of SAM/crystal systems is essential for a more comprehensive understanding of the mechanisms by which organic monolayers mediate mineral growth. X-ray absorption spectroscopy (XAS) provides the attractive ability to study SAM structure at critical stages throughout the processes of crystallization in SAM/mineral systems. Here, we discuss important theoretical and experimental considerations for designing and implementing XAS studies of SAM/mineral systems.