Juergen Biener

High Surface Area, sp2-Cross-Linked Three-Dimensional Graphene Monoliths

Developing three-dimensional (3D) graphene assemblies with properties similar to those individual graphene sheets is a promising strategy for graphene-based electrodes. Typically, the synthesis of 3D graphene assemblies relies on van der Waals forces for holding the graphene sheets together, resulting in bulk properties that do not reflect those reported for individual graphene sheets. Here, we report the use of sol-gel chemistry to introduce chemical bonding between the graphene sheets and control the bulk properties of graphene-based aerogels. Adjusting synthetic parameters allows a wide range of control over surface area, pore volume, and pore size, as well as the nature of the chemical cross-links (sp(2) vs sp(3)). The bulk properties of the graphene-based aerogels represent a significant step toward realizing the properties of individual graphene sheets in a 3D assembly with surface areas approaching the theoretical value of an individual sheet.

Structural Optimization of 3D Porous Electrodes for High-Rate Performance Lithium Ion Batteries

Much progress has recently been made in the development of active materials, electrode morphologies and electrolytes for lithium ion batteries. Well-defined studies on size effects of the three-dimensional (3D) electrode architecture, however, remain to be rare due to the lack of suitable material platforms where the critical length scales (such as pore size and thickness of the active material) can be freely and deterministically adjusted over a wide range without affecting the overall 3D morphology of the electrode. Here, we report on a systematic study on length scale effects on the electrochemical performance of model 3D np-Au/TiO2 core/shell electrodes. Bulk nanoporous gold provides deterministic control over the pore size and is used as a monolithic metallic scaffold and current collector. Extremely uniform and conformal TiO2 films of controlled thickness were deposited on the current collector by employing atomic layer deposition (ALD). Our experiments demonstrate profound performance improvements by matching the Li(+) diffusivity in the electrolyte and the solid state through adjusting pore size and thickness of the active coating which, for 200 μm thick porous electrodes, requires the presence of 100 nm pores. Decreasing the thickness of the TiO2 coating generally improves the power performance of the electrode by reducing the Li(+) diffusion pathway, enhancing the Li(+) solid solubility, and minimizing the voltage drop across the electrode/electrolyte interface. With the use of the optimized electrode morphology, supercapacitor-like power performance with lithium-ion-battery energy densities was realized. Our results provide the much-needed fundamental insight for the rational design of the 3D architecture of lithium ion battery electrodes with improved power performance.

Dynamic restructuring drives catalytic activity on nanoporous gold-silver alloy catalysts

Bimetallic, nanostructured materials hold promise for improving catalyst activity and selectivity, yet little is known about the dynamic compositional and structural changes that these systems undergo during pretreatment that leads to efficient catalyst function. Here we use ozone-activated silver-gold alloys in the form of nanoporous gold as a case study to demonstrate the dynamic behaviour of bimetallic systems during activation to produce a functioning catalyst. We show that it is these dynamic changes that give rise to the observed catalytic activity. Advanced in situ electron microscopy and X-ray photoelectron spectroscopy are used to demonstrate that major restructuring and compositional changes occur along the path to catalytic function for selective alcohol oxidation. Transient kinetic measurements correlate the restructuring to three types of oxygen on the surface. The direct influence of changes in surface silver concentration and restructuring at the nanoscale on oxidation activity is demonstrated. Our results demonstrate that characterization of these dynamic changes is necessary to unlock the full potential of bimetallic catalytic materials.

Effect of surface chemistry on the stability of gold nanostructures.

Understanding the role of surface chemistry in the stability of nanostructured noble-metal materials is important for many technological applications but experimentally difficult to access and thus little understood. To develop a fundamental understanding of the effect of surface chemistry on both the formation and stabilization of self-organized gold nanostructures, we performed a series of controlled-environment annealing experiments on nanoporous gold (np-Au) and ion-bombarded Au(111) single-crystal surfaces. The annealing experiments on np-Au in ambient ozone were carried out to study the effect of adsorbed oxygen under dynamic conditions, whereas the ion-bombarded Au single-crystal surfaces were used as a model system to obtain atomic-scale information. Our results show that adsorbed oxygen stabilizes nanoscale gold structures at low temperatures whereas oxygen-induced mobilization of Au surface atoms seems to accelerate the coarsening under dynamic equilibrium conditions at higher temperatures.

Growth of nanocrystalline MoO3 on Au(111) studied by in situ scanning tunneling microscopy

The growth of nanocrystalline MoO3 islands on Au(111) using physical vapor deposition of Mo has been studied by scanning tunneling microscopy and low energy electron diffraction. The growth conditions affect the shape and distribution of the MoO3 nanostructures, providing a means of preparing materials with different percentages of edge sites that may have different chemical and physical properties than atoms in the interior of the nanostructures. MoO3 islands were prepared by physical vapor deposition of Mo and subsequent oxidation by NO2 exposure at temperatures between 450 K and 600 K. They exhibit a crystalline structure with a c(4 x 2) periodicity relative to unreconstructed Au(111). While the atomic-scale structure is identical to that of MoO3 islands prepared by chemical vapor deposition, we demonstrate that the distribution of MoO3 islands on the Au(111) surface reflects the distribution of Mo clusters prior to oxidation although the growth of MoO3 involves long-range mass transport via volatile MoO3 precursor species. The island morphology is kinetically controlled at 450 K, whereas an equilibrium shape is approached at higher preparation temperatures or after prolonged annealing at the elevated temperature. Mo deposition at or above 525 K leads to the formation of a Mo-Au surface alloy as indicated by the observation of embedded MoO3 islands after oxidation by NO2. Au vacancy islands, formed when Mo and Au dealloy to produce vacancies, are observed for these growth conditions.

Structure of incommensurate gold sulfide monolayer on Au(111)

We develop an atomic-scale model for an ordered incommensurate gold sulfide (AuS) adlayer which has previously been demonstrated to exist on the Au(111) surface, following sulfur deposition and annealing to 450 K. Our model reproduces experimental scanning tunneling microscopy images. Using state-of-the-art Wannier-function-based techniques, we analyze the nature of bonding in this structure and provide an interpretation of the unusual stoichiometry of the gold sulfide layer. The proposed structure and its chemistry have implications for related S-Au interfaces, as in those involved in self-assembled monolayers of thiols on Au substrates.

Novel synthesis of two-dimensional TiS2 nanocrystallites on Au(111)

We describe a novel approach to synthesize two-dimensional nanocrystalline TiS2 islands on Au111. Ti is deposited by physical vapor deposition at room temperature on AuS-covered Au111 surfaces. Subsequent annealing to temperatures between 670 K and 800 K leads to the formation of single-layer, triangular TiS2 islands. These TiS2 nanocrystallites reflect the structure of bulk TiS2, and are composed of S-Ti-S stacking units with hexagonally close-packed layers of sulfur atoms and titanium occupying the octahedral sites in between. The lattice constant of the hexagonal unit cell is 3.45 A. A superlattice with a repeat distance of 17.3 A results from the coincidence of five TiS2 units with six Au atoms and is observed in scanning tunneling microscopy and low energy electron diffraction. The triangular shape of the islands indicates a preference for one of the two possible edge terminations. The observation of two island orientations rotated by 60 degrees with respect to each other can be attributed to the formation of twin-related TiS2 domains. The population of the two different island orientations changes during annealing at 800 K indicating a thermodynamic preference for one of the possible stacking sequences.

In Situ Real-Time Radiographic Study of Thin Film Formation Inside Rotating Hollow Spheres

Hollow spheres with uniform coatings on the inner surface have applications in optical devices, time- or site-controlled drug release, heat storage devices, and target fabrication for inertial confinement fusion experiments. The fabrication of uniform coatings, which is often critical for the application performance, requires precise understanding and control over the coating process and its parameters. Here, we report on in situ real-time radiography experiments that provide critical spatiotemporal information about the distribution of fluids inside hollow spheres during uniaxial rotation. Image analysis and computer fluid dynamics simulations were used to explore the effect of liquid viscosity and rotational velocity on the film uniformity. The data were then used to demonstrate the fabrication of uniform sol-gel chemistry derived porous polymer films inside 2 mm inner diameter diamond shells.

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.

Synthesis and Functionalization of 3D Nano-graphene Materials: Graphene Aerogels and Graphene Macro Assemblies

Efforts to assemble graphene into three-dimensional monolithic structures have been hampered by the high cost and poor processability of graphene. Additionally, most reported graphene assemblies are held together through physical interactions (e.g., van der Waals forces) rather than chemical bonds, which limit their mechanical strength and conductivity. This video method details recently developed strategies to fabricate mass-producible, graphene-based bulk materials derived from either polymer foams or single layer graphene oxide. These materials consist primarily of individual graphene sheets connected through covalently bound carbon linkers. They maintain the favorable properties of graphene such as high surface area and high electrical and thermal conductivity, combined with tunable pore morphology and exceptional mechanical strength and elasticity. This flexible synthetic method can be extended to the fabrication of polymer/carbon nanotube (CNT) and polymer/graphene oxide (GO) composite materials. Furthermore, additional post-synthetic functionalization with anthraquinone is described, which enables a dramatic increase in charge storage performance in supercapacitor applications.