Jianbing Niu

Effect of Microstructure of Nitrogen-Doped Graphene on Oxygen Reduction Activity in Fuel Cells

The development of fuel cells as clean-energy technologies is largely limited by the prohibitive cost of the noble-metal catalysts needed for catalyzing the oxygen reduction reaction (ORR) in fuel cells. A fundamental understanding of catalyst design principle that links material structures to the catalytic activity can accelerate the search for highly active and abundant nonmetal catalysts to replace platinum. Here, we present a first-principles study of ORR on nitrogen-doped graphene in acidic environment. We demonstrate that the ORR activity primarily correlates to charge and spin densities of the graphene. The nitrogen doping and defects introduce high positive spin and/or charge densities that facilitate the ORR on graphene surface. The identified active sites are closely related to doping cluster size and dopant-defect interactions. Generally speaking, a large doping cluster size (number of N atoms >2) reduces the number of catalytic active sites per N atom. In combination with N clustering, Stone-Wales defects can strongly promote ORR. For four-electron transfer, the effective reversible potential ranges from 1.04 to 1.15 V/SHE, depending on the defects and cluster size. The catalytic properties of graphene could be optimized by introducing small N clusters in combination with material defects.

Rationally designed graphene-nanotube 3D architectures with a seamless nodal junction for efficient energy conversion and storage

Seamlessly joint graphene-nanotube 3D architectures were created by one-step CVD for efficient energy conversion and storage. One-dimensional (1D) carbon nanotubes (CNTs) and 2D single-atomic layer graphene have superior thermal, electrical, and mechanical properties. However, these nanomaterials exhibit poor out-of-plane properties due to the weak van der Waals interaction in the transverse direction between graphitic layers. Recent theoretical studies indicate that rationally designed 3D architectures could have desirable out-of-plane properties while maintaining in-plane properties by growing CNTs and graphene into 3D architectures with a seamless nodal junction. However, the experimental realization of seamlessly-bonded architectures remains a challenge. We developed a strategy of creating 3D graphene-CNT hollow fibers with radially aligned CNTs (RACNTs) seamlessly sheathed by a cylindrical graphene layer through a one-step chemical vapor deposition using an anodized aluminum wire template. By controlling the aluminum wire diameter and anodization time, the length of the RACNTs and diameter of the graphene hollow fiber can be tuned, enabling efficient energy conversion and storage. These fibers, with a controllable surface area, meso-/micropores, and superior electrical properties, are excellent electrode materials for all-solid-state wire-shaped supercapacitors with poly(vinyl alcohol)/H2SO4 as the electrolyte and binder, exhibiting a surface-specific capacitance of 89.4 mF/cm2 and length-specific capacitance up to 23.9 mF/cm, — one to four times the corresponding record-high capacities reported for other fiber-like supercapacitors. Dye-sensitized solar cells, fabricated using the fiber as a counter electrode, showed a power conversion efficiency of 6.8% and outperformed their counterparts with an expensive Pt wire counter electrode by a factor of 2.5. These novel fiber-shaped graphene-RACNT energy conversion and storage devices are so flexible they can be woven into fabrics as power sources.

Dynamic enhancement in adhesion forces of microparticles on substrates.

We report a dynamically induced enhancement in interfacial adhesion between microsized particles and substrates under dry and humid conditions. The adhesion force of soft (polystyrene) and hard (SiO2 and Al2O3) microparticles on soft (polystyrene) and hard (fused silica and sapphire) substrates was measured by using an atomic force microscope with retraction (z-piezo) speed ranging over 4 orders of magnitude. The adhesion is strongly enhanced by the dynamic effect. When the retraction speed varies from 0.02 to 156 μm/s, the adhesion force increases by 10% to 50% in dry nitrogen while it increases by 15% to 70% in humid air. Among the material systems tested, the soft-soft contact systems exhibit the smallest dynamic effect while the hard-hard contacts show the largest enhancement. A dynamic model was developed to predict this dynamic effect, which agrees well with the experimental results. The influence of dynamic factors related to the adhesion enhancement, such as particle inertia, viscoelastic deformations, and crack propagation, was discussed to understand the dynamic enhancement mechanisms.

Dynamic Adhesion Forces between Microparticles and Substrates in Water

The interactions between micrometer-sized particles and substrates in aqueous environment are fundamental to numerous natural phenomena and industrial processes. Here we report a dynamically induced enhancement in adhesion interactions between microparticles and substrates immerged in water, air, and hexane. The dynamic adhesion force was measured by pulling microsized spheres off various substrate (hydrophilic/hydrophobic) surfaces at different retracting velocities. It was observed that when the pull-off velocity varies from 0.02 to 1500 μm/s, there is 100-200% increase in adhesion force in water while it has a 100% increase in nitrogen and hexane. The dynamic adhesion enhancement reduces with increasing effective contact angle defined by the average cosine of wetting angles of the substrates and the particles, and approaches the values measured in dry nitrogen and hexane as the effective contact angle is larger than 90(o). A dynamic model was developed to predict the adhesion forces resulting from this dynamic effect, and the predictions correlate well with the experimental results. The stronger dynamic adhesion enhancement in water is mainly attributed to electrical double layers and the restructuring of water in the contact area between particles and substrates.

Heating induced microstructural changes in graphene/Cu nanocomposites

Dynamic heating experiments on graphene/Cu nanocomposites by in situ scanning electron microscopy were conducted to observe the evolution of the morphology and size of the Cu nanoparticles. Microstructural characterization showed that the graphene/Cu nanocomposites system consists of graphene sheets decorated with Cu-based nanoparticles with different chemistries (Cu, Cu2O), shapes (cube, rod, triangle, etc) and sizes. Evidence of neck evolution, coalescence, sublimation and Ostwald ripening were observed. Interestingly, some of the events occurred at the edges of the graphene sheets. The quantitative data of necking evolution deviates from the classical continuum theory indicating that intrinsic faceting and the shape of the nanoparticles played an important role in the necking process. This was supported by molecular dynamics simulations. Experimental data of liquid-spherical nanoparticles on graphene suggested that Cu did not wet graphene. Based on sublimation experiments and surface stability, we propose that graphene decorated with Cu nanoparticles enclosed by {1?1?1} facets are the most stable nanocomposite at high temperatures. The growth mechanism of nanoparticles on graphene is discussed.

Growth mechanisms and mechanical properties of 3D carbon nanotube-graphene junctions: molecular dynamic simulations†

The growth process of carbon nanotube (CNT)–graphene 3D junctions on copper templates with nano-holes was simulated with classical molecular dynamic (MD) simulation. The CNT, graphene and their seamlessly C–C bonded junction can form simultaneously on the templates without catalysts. There are two mechanisms of junction formation: (i) CNT growth over the holes that are smaller than 3 nm, and (ii) CNT growth inside the holes that are larger than 3 nm. The tensile strengths of the as-grown C–C junctions, as well as the junctions embedded with metal nanoparticles (catalysts), were determined by a quantum mechanics MD simulation method. Metal nanoparticles as catalysts remaining in the junctions significantly reduce the fracture strength and fracture energy, making them brittle and weak. Among the junctions, the seamlessly C–C bonded junctions show the highest tensile strength and fracture energy due to their unique structure. This work provides a theoretical basis and route for synthesizing high-quality single-layer CNT–graphene nanostructures.

Template-directed growth and mechanical properties of carbon nanotube–graphene junctions with nano-fillets: molecular dynamic simulation

The template-directed growth process of 3D carbon nanotube–graphene junctions with nano-fillets was simulated with classical molecular dynamic (MD) simulation. The carbon nanotube, graphene and their seamlessly C–C bonded junctions were formed simultaneously on amorphous alumina templates without catalysts. The C–C bonded junctions with various fillet angles were made, and the molecular structures and tensile strength of the “as-grown” C–C junctions were determined by the MD method. Among these junctions, the 135° fillet shows the most stability and improved mechanical strength, which is consistent with the experimental observations. While the fillet is a common technique that is widely used in large-scale engineering structures to reduce the stress concentration, here the nano-fillet enhances both the stability and mechanical properties of carbon nanotube–graphene junctions. This work provides a theoretical base for synthesizing high-quality carbon nanotube–graphene nanostructures via template methods.