Tan Xing

Toward Design of Synergistically Active Carbon-Based Catalysts for Electrocatalytic Hydrogen Evolution

Replacement of precious Pt catalyst with cost-effective alternatives would be significantly beneficial for hydrogen production via electrocatalytic hydrogen evolution reaction (HER). All candidates thus far are exclusively metallic catalysts, which suffer inherent corrosion and oxidation susceptibility during acidic proton-exchange membrane electrolysis. Herein, based on theoretical predictions, we designed and synthesized nitrogen (N) and phosphorus (P) dual-doped graphene as a nonmetallic electrocatalyst for sustainable and efficient hydrogen production. The N and P heteroatoms could coactivate the adjacent C atom in the graphene matrix by affecting its valence orbital energy levels to induce a synergistically enhanced reactivity toward HER. As a result, the dual-doped graphene showed higher electrocatalytic HER activity than single-doped ones and comparable performance to some of the traditional metallic catalysts.

Phosphorus–carbon nanocomposite anodes for lithium-ion and sodium-ion batteries

With the expected theoretical capacity of 2596 mA h g−1, phosphorus is considered to be the highest capacity anode material for sodium-ion batteries and one of the most attractive anode materials for lithium-ion systems. This work presents a comprehensive study of phosphorus–carbon nanocomposite anodes for both lithium-ion and sodium-ion batteries. The composite electrodes are able to display high initial capacities of approximately 1700 and 1300 mA h g−1 in lithium and sodium half-cells, respectively, when the cells are tested within a larger potential windows of 2.0–0.01 V vs. Li/Li+ and Na/Na+. The level of demonstrated capacity is underpinned by the storage mechanism, based on the transformation of phosphorus to Li3P phase for lithium cells and an incomplete transformation to Na3P phase for sodium cells. The capacity deteriorates upon cycling, which is shown to originate from disintegration of electrodes and their delamination from current collectors by post-cycling ex situ electron microscopy. Stable cyclic performance at the level of ∼700 and ∼350–400 mA h g−1 can be achieved if the potential windows are restricted to 2.0–0.67 V vs. Li/Li+ for lithium and 2–0.33 vs. Na/Na+ for sodium half-cells. The results are critically discussed in light of existing literature reports.

Dielectric screening in atomically thin boron nitride nanosheets

Two-dimensional (2D) hexagonal boron nitride (BN) nanosheets are excellent dielectric substrate for graphene, molybdenum disulfide, and many other 2D nanomaterial-based electronic and photonic devices. To optimize the performance of these 2D devices, it is essential to understand the dielectric screening properties of BN nanosheets as a function of the thickness. Here, electric force microscopy along with theoretical calculations based on both state-of-the-art first-principles calculations with van der Waals interactions under consideration, and nonlinear Thomas-Fermi theory models are used to investigate the dielectric screening in high-quality BN nanosheets of different thicknesses. It is found that atomically thin BN nanosheets are less effective in electric field screening, but the screening capability of BN shows a relatively weak dependence on the layer thickness.

Ball milling: a green mechanochemical approach for synthesis of nitrogen doped carbon nanoparticles

Technological and scientific challenges coupled with environmental considerations have attracted a search for robust, green and energy-efficient synthesis and processing routes for advanced functional nanomaterials. In this article, we demonstrate a high-energy ball milling technique for large-scale synthesis of nitrogen doped carbon nanoparticles, which can be used as an electro-catalyst for oxygen reduction reactions after a structural refinement with controlled thermal annealing. The resulting carbon nanoparticles exhibited competitive catalytic activity (5.2 mA cm(-2) kinetic-limiting current density compared with 7.6 mA cm(-2) on Pt/C reference) and excellent methanol tolerance compared to a commercial Pt/C catalyst. The proposed synthesis route by ball milling and annealing is an effective process for carbon nanoparticle production and efficient nitrogen doping, providing a large-scale production method for the development of highly efficient and practical electrocatalysts.

Stable anode performance of an Sb–carbon nanocomposite in lithium-ion batteries and the effect of ball milling mode in the course of its preparation

Materials that alloy with lithium (Si, Ge, Sn, Sb, and P) are considered as alternatives to graphitic anodes in lithium-ion batteries. Their practical use is precluded by large volume changes (200–370%) during cycling. Embedding nanoparticles into carbon is being investigated as a way to tackle that, and ball milling is emerging as a technique to prepare nanocomposites with enhanced capacity and cyclic stability. Using Sb as a model system, we investigate the preparation of Sb–carbon nanocomposites using a reconfigurable ball mill. Four distinctive milling modes are compared. The structure of the composites varies depending on the mode. Frequent strong ball impacts are required for the optimal electrochemical performance of the nanocomposite. An outstanding stable capacity of 550 mA h g−1 for 250 cycles at a current rate of 230 mA g−1 is demonstrated in a thin electrode (1 mg cm−2) and a capacity of ∼400 mA h g−1 can be retained at 1.15 A g−1. Some capacity fade is observed in a thicker electrode (2.5 mg cm−2), i.e. the performance is sensitive to mass loading. The electrochemical stability originates from the nanocomposite structure containing Sb nanoparticles (5–15 nm) dispersed in a carbon component.

Boron Nitride Nanosheets for Metal Protection

Although the high impermeability of graphene makes it an excellent barrier to inhibit metal oxidation and corrosion, graphene can form a galvanic cell with the underlying metal that promotes corrosion of the metal in the long term. Boron nitride (BN) nanosheets which have a similar impermeability could be a better choice as protective barrier, because they are more thermally and chemically stable than graphene and, more importantly, do not cause galvanic corrosion due to their electrical insulation. In this study, the performance of commercially available BN nanosheets grown by chemical vapor deposition as a protective coating on metal has been investigated. The heating of the copper foil covered with the BN nanosheet at 250 °C in air over 100 h results in dramatically less oxidation than the bare copper foil heated for 2 h under the same conditions. The electrochemical analyses reveal that the BN nanosheet coating can increase open circuit potential and possibly reduce oxidation of the underlying copper foil in sodium chloride solution. These results indicate that BN nanosheets are a good candidate for oxidation and corrosion protection, although conductive atomic force microscopy analyses show that the effectiveness of the protection relies on the quality of BN nanosheets.

Mechanically activated catalyst mixing for high-yield boron nitride nanotube growth

Boron nitride nanotubes (BNNTs) have many fascinating properties and a wide range of applications. An improved ball milling method has been developed for high-yield BNNT synthesis, in which metal nitrate, such as Fe(NO3)3, and amorphous boron powder are milled together to prepare a more effective precursor. The heating of the precursor in nitrogen-containing gas produces a high density of BNNTs with controlled structures. The chemical bonding and structure of the synthesized BNNTs are precisely probed by near-edge X-ray absorption fine structure spectroscopy. The higher efficiency of the precursor containing milling-activated catalyst is revealed by thermogravimetric analyses. Detailed X-ray diffraction and X-ray photoelectron spectroscopy investigations disclose that during ball milling the Fe(NO3)3 decomposes to Fe which greatly accelerates the nitriding reaction and therefore increases the yield of BNNTs. This improved synthesis method brings the large-scale production and application of BNNTs one step closer.

Gas Protection of Two-Dimensional Nanomaterials from High-Energy Impacts

Two-dimensional (2D) materials can be produced using ball milling with the help of liquid surfactants or solid exfoliation agents, as ball milling of bulk precursor materials usually produces nanosized particles because of high-energy impacts. Post-milling treatment is thus needed to purify the nanosheets. We show here that nanosheets of graphene, BN, and MoS2 can be produced by ball milling of their bulk crystals in the presence of ammonia or a hydrocarbon ethylene gas and the obtained nanosheets remain flat and maintain their single-crystalline structure with low defects density even after a long period of time; post-milling treatment is not needed. This study does not just demonstrate production of nanosheets using ball milling, but reveals surprising indestructible behaviour of 2D nanomaterials in ammonia or hydrocarbon gas under the high-energy impacts; in other milling atmospheres such as air, nitrogen or argon the same milling treatment produces nanosized particles. A systematic study reveals chemisorption of ammonia and hydrocarbon gases and chemical reactions occurring at defect sites, which heal the defects by saturating the dangling bonds. Density functional theory was used to understand the mechanism of mechanochemical reactions. Ball milling in ammonia or hydrocarbon is promising for mass-production of pure nanosheets.

High-resolution x-ray absorption studies of core excitons in hexagonal boron nitride

In the K-shell excitation spectroscopy studies of hexagonal boron nitride, a sharp π* resonance at 192.0 eV in the B K-edge region due to the B 1s–π* transition has been widely observed and accepted for three decades. However, our high-resolution near-edge x-ray absorption fine-structure studies disclose that this characteristic exciton peak actually consists of an asymmetric double-peak structure. Notable differences have been revealed in the polarization character of the two split peaks. These fine structures are explained by the coupling of core excitons to lattice vibrations with the inclusion of Jahn-Teller effect which breaks down Franck-Condon principle.