Qiran Cai

Metal-Catalyst-Free Growth of Single-Walled Carbon Nanotubes on Substrates

In this communication, we have demonstrated that SiO(2) nanoparticles can be generated by simply scratching the quartz or silicon wafer with a SiO(2) layer and confirmed it to be active for the growth of SWNTs for the first time. Furthermore, the SWNTs from SiO(2) has a much narrower size distribution. This may open a way to control the diameter of the SWNTs. More importantly, our work has found a series of oxides including Al(2)O(3), TiO(2), and rare earth oxides to be active for SWNT growth as well. These findings not only provide an alternative new type of catalysts for the growth of SWNTs but also give more insight into the role of the catalysts and a deeper understanding of the growth mechanism of SWNTs. The effective catalysts and catalytic activity for SWNT growth seem to be more size-dependent than the catalysts. Long oriented SWNTs generated from these catalysts enable us to rule out the relationship between the catalysts and the structures of the SWNTs. Thus controlled growth of SWNTs including the diameter and chirality is expected to be eventually realized.

Molecule-level g-C3N4 coordinated transition metals as a new class of electrocatalysts for oxygen electrode reactions

Organometallic complexes with metal-nitrogen/carbon (M-N/C) coordination are the most important alternatives to precious metal catalysts for oxygen reduction and evolution reactions (ORR and OER) in energy conversion devices. Here, we designed and developed a range of molecule-level graphitic carbon nitride (g-C3N4) coordinated transition metals (M-C3N4) as a new generation of M-N/C catalysts for these oxygen electrode reactions. As a proof-of-concept example, we conducted theoretical evaluation and experimental validation on a cobalt-C3N4 catalyst with a desired molecular configuration, which possesses comparable electrocatalytic activity to that of precious metal benchmarks for the ORR and OER in alkaline media. The correlation of experimental and computational results confirms that this high activity originates from the precise M-N2 coordination in the g-C3N4 matrix. Moreover, the reversible ORR/OER activity trend for a wide variety of M-C3N4 complexes has been constructed to provide guidance for the molecular design of this promising class of catalysts.

Mechanical properties of atomically thin boron nitride and the role of interlayer interactions

Atomically thin boron nitride (BN) nanosheets are important two-dimensional nanomaterials with many unique properties distinct from those of graphene, but investigation into their mechanical properties remains incomplete. Here we report that high-quality single-crystalline mono- and few-layer BN nanosheets are one of the strongest electrically insulating materials. More intriguingly, few-layer BN shows mechanical behaviours quite different from those of few-layer graphene under indentation. In striking contrast to graphene, whose strength decreases by more than 30% when the number of layers increases from 1 to 8, the mechanical strength of BN nanosheets is not sensitive to increasing thickness. We attribute this difference to the distinct interlayer interactions and hence sliding tendencies in these two materials under indentation. The significantly better interlayer integrity of BN nanosheets makes them a more attractive candidate than graphene for several applications, for example, as mechanical reinforcements.

Boron nitride nanosheets as improved and reusable substrates for gold nanoparticles enabled surface enhanced Raman spectroscopy

Atomically thin boron nitride (BN) nanosheets have been found to be excellent substrates for noble metal particles enabled surface enhanced Raman spectroscopy (SERS), thanks to their good adsorption of aromatic molecules, high thermal stability and weak Raman scattering. Faceted gold (Au) nanoparticles have been synthesized on BN nanosheets using a simple but controllable and reproducible sputtering and annealing method. The size and density of the Au particles can be controlled by sputtering time, current and annealing temperature etc. Under the same sputtering and annealing conditions, the Au particles on BN of different thicknesses show various sizes because the surface diffusion coefficients of Au depend on the thickness of BN. Intriguingly, decorated with similar morphology and distribution of Au particles, BN nanosheets exhibit better Raman enhancements than silicon substrates as well as bulk BN crystals. Additionally, BN nanosheets show no noticeable SERS signal and hence cause no interference to the Raman signal of the analyte. The Au/BN substrates can be reused by heating in air to remove the adsorbed analyte without loss of SERS enhancement.

Boron Nitride Nanosheets Improve Sensitivity and Reusability of Surface-Enhanced Raman Spectroscopy.

Surface enhanced Raman spectroscopy (SERS) is a useful multidisciplinary analytic technique. However, it is still a challenge to produce SERS substrates that are highly sensitive, reproducible, stable, reusable, and scalable. Herein, we demonstrate that atomically thin boron nitride (BN) nanosheets have many unique and desirable properties to help solve this challenge. The synergic effect of the atomic thickness, high flexibility, stronger surface adsorption capability, electrical insulation, impermeability, high thermal and chemical stability of BN nanosheets can increase the Raman sensitivity by up to two orders, and in the meantime attain long-term stability and extraordinary reusability not achievable by other materials. These advances will greatly facilitate the wider use of SERS in many fields.

Boron Nitride Nanosheet-Veiled Gold Nanoparticles for Surface-Enhanced Raman Scattering

Atomically thin boron nitride (BN) nanosheets have many properties desirable for surface-enhanced Raman spectroscopy (SERS). BN nanosheets have a strong surface adsorption capability toward airborne hydrocarbon and aromatic molecules. For maximized adsorption area and hence SERS sensitivity, atomically thin BN nanosheet-covered gold nanoparticles have been prepared for the first time. When placed on top of metal nanoparticles, atomically thin BN nanosheets closely follow their contours so that the plasmonic hot spots are retained. Electrically insulating BN nanosheets also act as a barrier layer to eliminate metal-induced disturbances in SERS. Moreover, the SERS substrates veiled by BN nanosheets show an outstanding reusability in the long term. As a result, the sensitivity, reproducibility, and reusability of SERS substrates can be greatly improved. We also demonstrate that large BN nanosheets produced by chemical vapor deposition can be used to scale up the proposed SERS substrate for practical applications.

In situ prepared V2O5/graphene hybrid as a superior cathode material for lithium-ion batteries

Developing synthetic methods for graphene based cathode materials, with low cost and in an environmentally friendly way, is necessary for industrial production. Although the precursor of graphene is abundant on the earth, the most common precursor of graphene is graphene oxide (GO), and it needs many steps and reagents for transformation to graphite. The traditional approach for the synthesis of GO needs many chemicals, thus leading to a high cost for production and potentially great amounts of damage to the environment. In this study, we develop a simple wet ball-milling method to construct a V2O5/graphene hybrid structure in which nanometre-sized V2O5 particles/aggregates are well embedded and uniformly dispersed into the crumpled and flexible graphene sheets generated by in situ conversion of bulk graphite. The combination of V2O5 nanoparticles/aggregates and in situ graphene leads the hybrid to exhibit a markedly enhanced discharge capacity, excellent rate capability, and good cycling stability. This study suggests that nanostructured metal oxide electrodes integrated with graphene can address the poor cycling issues of electrode materials that suffer from low electronic and ionic conductivities. This simple wet ball-milling method can potentially be used to prepare various graphene based hybrid electrodes for large scale energy storage applications.

Neuron-Inspired Interpenetrative Network Composed of Cobalt–Phosphorus-Derived Nanoparticles Embedded within Porous Carbon Nanotubes for Efficient Hydrogen Production

The ongoing search for cheap and efficient hydrogen evolution reaction (HER) electrocatalysts to replace currently used catalysts based on Pt or its alloys has been considered as an prevalent strategy to produce renewable and clean hydrogen energy. Herein, inspired by the neuron structure in biological systems, we demonstrate a novel fabrication strategy via a simple two-step method for the synthesis of a neuronlike interpenetrative nanocomposite network of Co-P embedded in porous carbon nanotubes (NIN-Co-P/PCNTs). It is found that the interpenetrative network provides a natural transport path to accelerate the hydrogen production process. The embedded-type structure improves the utilization ratio of Co-P and the hollow, tubelike, and porous structure of PCNTs further promote charge and reactant transport. These factors allow the as-prepared NIN-Co-P/PCNTs to achieve a onset potential low to 43 mV, a Tafel slope as small as 40 mV/decade, an excellent stability, and a high turnover frequency value of 3.2 s(-1) at η = 0.2 V in acidic conditions. These encouraging properties derived from the neuronlike interpenetrative network structure might offer new inspiration for the preparation of more nanocomposites for applications in other catalytic and optoelectronic field.

Asymmetric electric field screening in van der Waals heterostructures

A long-standing challenge facing the combination of two-dimensional crystals into heterojunction is the unknown effect of mixing layer of different electronic properties (semiconductors, metals, insulators) on the screening features of the fabricated device platforms including their functionality. Here we use a compelling set of theoretical and experimental techniques to elucidate the intrinsic dielectric screening properties of heterostructures formed by MoS2 and graphene layers. We experimentally observed an asymmetric field screening effect relative to the polarization of the applied gate bias into the surface. Surprisingly, such behavior allows selection of the electronic states that screen external fields, and it can be enhanced with increasing of the number of layers of the semiconducting MoS2 rather than the semi-metal. This work not only provides unique insights on the screening properties of a vast amount of heterojunction fabricated so far, but also uncovers the great potential of controlling a fundamental property for device applications.Charge density reorganization at the interface between 2D materials may lead to electric field screening. Here, the authors investigate the dielectric screening properties of MoS2/graphene van der Waals heterostructures and identify an asymmetric electric response under different directions of the applied electric field.

Rigorous and Accurate Contrast Spectroscopy for Ultimate Thickness Determination of Micrometer-Sized Graphene on Gold and Molecular Sensing

The thickness of graphene films can be accurately determined by optical contrast spectroscopy. However, this becomes challenging and complicated when the flake size reduces to the micrometer scale, where the contrast spectrum is sensitively dependent on the polarization and incident angle of light. Here, we report accurate measurement of the optical contrast spectra of micrometer-sized few-layer graphene flakes on Au substrate. Using a high-resolution optical microscopy with a 100× magnification objective, we accurately determined the layer numbers of flakes as small as one micrometer in lateral size. We developed a theoretical model to accurately take into account the appropriate contribution of light incident at various angles and polarizations, which matched the experimental results extremely well. Furthermore, we demonstrate that the optical contrast spectroscopy is highly sensitive to detect the adsorption of submonolayer airborne hydrocarbon molecules, which can reveal whether graphene is contaminated. Though the technique was demonstrated on graphene, it can be readily generalized to many other two-dimensional materials, which opens new avenues for developing miniaturized and ultrasensitive label-free molecular sensors.