Tiehu Li

Electromagnetic wave absorbing properties of amorphous carbon nanotubes.

Amorphous carbon nanotubes (ACNTs) with diameters in the range of 7–50 nm were used as absorber materials for electromagnetic waves. The electromagnetic wave absorbing composite films were prepared by a dip-coating method using a uniform mixture of rare earth lanthanum nitrate doped ACNTs and polyvinyl chloride (PVC). The microstructures of ACNTs and ACNT/PVC composites were characterized using transmission electron microscope and X-ray diffraction, and their electromagnetic wave absorbing properties were measured using a vector-network analyzer. The experimental results indicated that the electromagnetic wave absorbing properties of ACNTs are superior to multi-walled CNTs, and greatly improved by doping 6 wt% lanthanum nitrate. The reflection loss (R) value of a lanthanum nitrate doped ACNT/PVC composite was −25.02 dB at 14.44 GHz, and the frequency bandwidth corresponding to the reflector loss at −10 dB was up to 5.8 GHz within the frequency range of 2–18 GHz.

Electrochemistry of Selenium with Sodium and Lithium: Kinetics and Reaction Mechanism

There are economic and environmental advantages by replacing Li with Na in energy storage. However, sluggishness in the charge/discharge reaction and low capacity are among the major obstacles to development of high-power sodium-ion batteries. Among the electrode materials recently developed for sodium-ion batteries, selenium shows considerable promise because of its high capacity and good cycling ability. Herein, we have investigated the mechanism and kinetics of both sodiation and lithiation reactions with selenium nanotubes, using in situ transmission electron microscopy. Sodiation of a selenium nanotube exhibits a three-step reaction mechanism: (1) the selenium single crystal transforms into an amorphous phase Na0.5Se; (2) the Na0.5Se amorphous phase crystallizes to form a polycrystalline Na2Se2 phase; and (3) Na2Se2 transforms into the Na2Se phase. Under similar conditions, the lithiation of Se exhibits a one-step reaction mechanism, with phase transformation from single-crystalline Se to a Li2Se. Intriguingly, sodiation kinetics is generally about 4-5 times faster than that of lithiation, and the kinetics during the different stages of sodiation is different. Na-based intermediate phases are found to have improved electronic and ionic conductivity compared to those of Li compounds by first-principles density functional theory calculations.

A three-dimensional MnO2/graphene hybrid as a binder-free supercapacitor electrode

Highly aligned manganese dioxide (MnO2) nanowall arrays electrodeposited onto Ti sheets are used as substrates to grow graphene (GR) through chemical vapor deposition (CVD), thus forming a three-dimensional (3D) MnO2/GR hybrid composite. Furthermore, a 3D MnO2/GR hybrid with different structures and properties has been prepared at different temperatures. The as-prepared hybrid materials could be directly used as supercapacitor electrodes without any binder and conductive additive, and fully maintain the high conductivity and high specific area of GR, and large pseudocapacitance of MnO2 nanowall arrays. In aqueous electrolytes, the hybrids show a high specific capacitance of ∼326.33 F g−1 with good cycling stability at the scan rate of 200 mV s−1 and high energy density of 23.68 W h kg−1 while maintaining high power density of 7270 W kg−1. The preparation method provides a novel method to fabricate 3D graphene-based composite materials, and the as obtained hybrid electrode demonstrates its potential applications in supercapacitors.

Supported Tetrahedral Oxo-Sn Catalyst: Single Site, Two Modes of Catalysis

Mild calcination in ozone of a (POSS)-Sn-(POSS) complex grafted on silica generated a heterogenized catalyst that mostly retained the tetrahedral coordination of its homogeneous precursor, as evidenced by spectroscopic characterizations using EXAFS, NMR, UV-vis, and DRIFT. The Sn centers are accessible and uniform and can be quantified by stoichiometric pyridine poisoning. This Sn-catalyst is active in hydride transfer reactions as a typical solid Lewis acid. However, the Sn centers can also create Brønsted acidity with alcohol by binding the alcohol strongly as alkoxide and transferring the hydroxyl H to the neighboring Sn-O-Si bond. The resulting acidic silanol is active in epoxide ring opening and acetalization reactions.

Oxidation behaviour of matrix-modified carbon-carbon composites at high temperature

Abstract The approach of matrix modification has been used to increase the high-temperature oxidation resistance of carbon-carbon composites. A distinct effect has been obtained, and the oxidation law of matrix-modified composites at high temperature has been investigated. Initially, the change in weight loss with oxidation time for matrix-modified carbon-carbon composites at constant temperature was studied. The results show that the oxidation process can be divided into three stages: a linear stage (1) with the weight loss increasing from 0 to 60%, an exponential stage (2) with weight losses between 60% and 80%, and a linear stage (3) with weight losses from 80% to 100%. Also, the variation of weight loss with oxidation temperature for matrix-modified carbon-carbon composites at constant oxidation time was measured. The results showed that this variation could be divided into three stages: a constant weight stage from ambient temperature to the temperature of initial weight loss, an exponential stage from the commencement of initial weight loss to a residual weight of about 70%, and a linear stage as the residual weight falls from 70% to 0. A mathematical theory for the three stages has been derived and tested. This treatment may prove useful for improving the high-temperature oxidation resistance of carbon-carbon composites and their properties in service.

Microwave absorption properties of rare metal-doped multi-walled carbon nanotube/polyvinyl chloride composites

Using rare metal nitrate-doped multi-walled carbon nanotubes as the absorber and polyvinyl chloride as the matrix, the microwave electromagnetic and absorbing properties of multi-walled carbon nanotube/polyvinyl chloride composites were studied. The complex permittivity of the composites doped with different rare metal nitrate decreased in the frequency region of 8.2–12.4 GHz. The minimum reflection loss of rare metal nitrate-doped multi-walled carbon nanotube/polyvinyl chloride composites decreased and shifted slightly to the higher frequency region, and the absorption bandwidth (<−10 dB or >90%) increased in the frequency range of 8–18 GHz compared to multi-walled carbon nanotube/polyvinyl chloride composites. The reflection loss (<−10 dB) of 0.2 wt% La(NO3)3-doped multi-walled carbon nanotube/polyvinyl chloride composites is the widest from the absorption bandwidth (maximum is 5.12 GHz).

Expanded graphite embedded with aluminum nanoparticles as superior thermal conductivity anodes for high-performance lithium-ion batteries

The development of high capacity and long-life lithium-ion batteries is a long-term pursuing and under a close scrutiny. Most of the researches have been focused on exploring electrode materials and structures with high store capability of lithium ions and at the same time with a good electrical conductivity. Thermal conductivity of an electrode material will also have significant impacts on boosting battery capacity and prolonging battery lifetime, which is, however, underestimated. Here, we present the development of an expanded graphite embedded with Al metal nanoparticles (EG-MNPs-Al) synthesized by an oxidation-expansion process. The synthesized EG-MNPs-Al material exhibited a typical hierarchical structure with embedded Al metal nanoparticles into the interspaces of expanded graphite. The parallel thermal conductivity was up to 11.6 W·m−1·K−1 with a bulk density of 453 kg·m−3 at room temperature, a 150% improvement compared to expanded graphite (4.6 W·m−1·K−1) owing to the existence of Al metal nanoparticles. The first reversible capacity of EG-MNPs-Al as anode material for lithium ion battery was 480 mAh·g−1 at a current density of 100 mA·g−1, and retained 84% capacity after 300 cycles. The improved cycling stability and system security of lithium ion batteries is attributed to the excellent thermal conductivity of the EG-MNPs-Al anodes.

Three-Dimensional Graphene/MnO2 Nanowalls Hybrid for High-Efficiency Electrochemical Supercapacitors

In this paper, a facile method is designed to fabricate three-dimensional (3D) graphene (GR)/manganese dioxide (MnO2) nanowall electrode material. The 3D GR/MnO2 hybrid is prepared by a combination...

Supported cobalt oxide nanocrystals: morphology control and catalytic performance for styrene oxidation

Metal oxide nanoparticles with controlled size and morphology seem extremely important due to the strong correlation between these parameters and their catalytic properties. Herein supported cobalt oxide nanoparticles with controlled size and different morphologies were successfully synthesized via a facile hydrothermal synthetic route under mild conditions. The size and morphology of the products can be readily tuned by tuning process parameters such as hydrothermal time and hydrothermal temperature. As synthesized supported cobalt oxide nanoparticles, namely Co3O4-150, Co3O4-550 and Co3O4-650, were quite active in the catalytic oxidation of styrene with H2O2 as a green oxidant. However, due to the size and morphology differences, these supported cobalt oxide nanoparticles exhibited different activities and selectivities. Co3O4-550 resulted in higher activity while Co3O4-650 exhibited relatively higher selectivity to epoxide. Moreover, these cobalt catalysts could be recycled, indicating this to be a promising method of fabricating supported cobalt oxide catalysts for the oxidation of olefins in large-scale processes.

Amine-functionalized siloxane oligomer facilitated synthesis of subnanometer colloidal Au particles

Amine-functionalized siloxane oligomers were synthesized and used successfully to prepare colloidal Au particles smaller than 1 nm. Using NMR to follow the interaction of Au(THT)Cl with the functionalized siloxane, it was determined that the amine ligands displaced the THT ligand effectively. By comparison with other functionalized siloxane oligomers/compound, parameters such as density of ligating groups, oligomer steric barrier and reduction rates were found to be essential for the formation and stability of subnanometer Au particles. Without further treatment, the formed Au particles were active catalysts for the reduction of p-nitrobenzaldehyde by triethylsilane, forming an imine as the major coupling product. Deposition of the Au colloids onto silica, followed by thermal treatment to remove the organic groups resulted in subnanometer Au on silica, indicating this to be a promising method of fabricating subnanometer supported Au catalyst.