Sean Li

Interfacial characteristics of a carbon nanotube–polystyrene composite system

The performance of a composite material system is critically controlled by the interfacial characteristics of the reinforcement and the matrix material. Here we report a study on the interfacial characteristics of a carbon nanotube (CNT)-reinforced polystyrene (PS) composite system through molecular mechanics simulations and elasticity calculations. In the absence of atomic bonding between the reinforcement and the matrix material, it is found that the nonbond interactions consists of electrostatic and van der Waals interaction, deformation induced by these forces, as well as stress/deformation arising from mismatch in the coefficients of thermal expansion. All of these contribute to the interfacial stress transfer ability, the critical parameter controlling material performance. Results of a CNT pullout simulation suggests that the interfacial shear stress of the CNT–PS system is about 160 MPa, significantly higher than most carbon fiber reinforced polymer composite systems.

Al doped graphene: A promising material for hydrogen storage at room temperature

A promising material for hydrogen storage at room temperature–Al doped graphene is proposed theoretically by using density functional theory calculation. Hydrogen storage capacity of 5.13 wt % is predicted at T=300 K and P=0.1 GPa with an adsorption energy Eb=−0.260 eV/H2. This is close to the target specified by U.S. Department of Energy with a storage capacity of 6 wt % and a binding energy of −0.2 to −0.4 eV/H2 at ambient temperature and modest pressure for commercial applications. It is believed that the doped Al alters the electronic structures of both C and H2. The bands of H2 overlapping with those of Al and C simultaneously are the underlying mechanism of hydrogen storage capacity enhancement.

Phosphorus-Based Alloy Materials for Advanced Potassium-Ion Battery Anode

Potassium-ion batteries (PIBs) are interesting as one of the alternative metal-ion battery systems to lithium-ion batteries (LIBs) due to the abundance and low cost of potassium. We have herein investigated Sn4P3/C composite as a novel anode material for PIBs. The electrode delivered a reversible capacity of 384.8 mA h g-1 at 50 mA g-1 and a good rate capability of 221.9 mA h g-1, even at 1 A g-1. Its electrochemical performance is better than any anode material reported so far for PIBs. It was also found that the Sn4P3/C electrode displays a discharge potential plateau of 0.1 V in PIBs, slightly higher than for sodium-ion batteries (SIBs) (0.01 V), and well above the plating potential of metal. This diminishes the formation of dendrites during cycling, and thus Sn4P3 is a relatively safe anode material, especially for application in large-scale energy storage, where large amounts of electrode materials are used. Furthermore, a possible reaction mechanism of the Sn4P3/C composite as PIB anode is proposed. This work may open up a new avenue for further development of alloy-based anodes with high capacity and long cycle life for PIBs.

Size-Dependent Raman Red Shifts of Semiconductor Nanocrystals

The size effects on Raman red shifts in low-dimensional semiconductor nanocrystals are investigated by considering the size-dependent root-mean-square average amplitude associated with the thermal vibration of atoms. The lower limit of vibrational frequency was obtained by matching the calculation results of Raman red shifts with the experimental data of Si, InP, CdSe, CdS0.65Se0.35, ZnO, CeO2, as well as SnO2 nanocrystals. The results indicate the following: (1) the Raman frequency decreases as the nanocrystal size decreases in both narrow and wide bandgap semiconductors; (2) the influence of crystal size on the Raman frequency of nanoparticles is more pronounced than that of nanowires and thin films; and (3) the Raman red shift is ascribed to the size-induced phonon confinement effect and surface relaxation. This model may provide new insights into the fundamental understanding of the underlying mechanism behind the Raman red shifts.

Size dependence of Zn 2p 3/2 binding energy in nanocrystalline ZnO

The experimental results in size dependence of electronic structure and optical band gap show that the nanocrystalline ZnO has two binding states which energies are lower than that of the bulk ZnO. This size dependence of binding states is associated with the number of broken bonds of the individual Zn ion, which could be modified by size reduction in nanoscale. Varying the number of broken bonds on the surface and the underneath layer might result in possible complications in the electronic structure of the nanocrystalline ZnO, thus giving rise to different optical properties rather than those relating to quantum size effects.

Density functional theory calculations on the CO catalytic oxidation on Al-embedded graphene

The oxidation of CO molecules on Al-embedded graphene has been investigated by using the first principles calculations. Both Eley–Rideal (ER) and Langmuir–Hinshelwood (LH) oxidation mechanisms are considered. In the ER mechanism, an O2 molecule is first adsorbed and activated on Al-embedded graphene before a CO molecule approaches, the energy barrier for the primary step (CO + O2 → OOCO) is 0.79 eV. In the LH mechanism, O2 and CO molecules are firstly co-adsorbed on Al-embedded graphene, the energy barrier for the rate limiting step (CO + O2 → OOCO) is only 0.32 eV, much lower than that of ER mechanism, which indicates that LH mechanism is more favourable for CO oxidation on Al-embedded graphene. Hirshfeld charge analysis shows that the embedded Al atom would modify the charge distributions of co-adsorbed O2 and CO molecules. The charge transfer from O2 to CO molecule through the embedded Al atom plays an important role for the CO oxidation along the LH mechanism. Our result shows that the low cost Al-embedded graphene is an efficient catalyst for CO oxidation at room temperature.

Highly porous reticular tin–cobalt oxide composite thin film anodes for lithium ion batteries

A tin–cobalt oxide film with a 3-dimensional (3D) reticular structure has been prepared by electrostatic spray deposition (ESD). X-Ray diffraction (XRD) and transmission electron microscopy (TEM) make it clear that the film is amorphous. X-Ray photoemission spectroscopy (XPS) indicates that the 3D grid is composed of SnO2 and CoO. As an anode for the lithium ion battery, the film has 1240.2 mAh/g of initial discharge capacity, shows a gradually increasing capacity after the first cycle, and has 845 mAh/g of discharge capacity up to the 50th cycle. The high porosity of the 3D reticular structure can provide more reaction sites on the electrode surface and accommodate volume variation of Sn particles during Li alloying–dealloying. The CoO in the complex composite film plays an important role, allowing the formation of a polymeric gel-like film to improve its electrochemical performance, preventing Sn aggregation during charging and discharging. Such a composite film can be used as an anode for lithium ion batteries with higher energy densities.

An extended 'quantum confinement' theory: surface-coordination imperfection modifies the entire band structure of a nanosolid

With the miniaturization of a solid, quantum and interface effects become increasingly important. As a result, the band structure of a nanometric semiconductor changes: the band gap expands, the core level shifts, the bandwidth revises, and the sublevel separation within a band increases. Unfortunately, such a thorough change goes beyond the scope of currently available models such as the ‘quantum confinement’ theory. A consistent understanding of the factors dominating the band-structure change is highly desirable. Here we present a new approach for the size-induced unusual change by adding the effect of surface-coordination deficiency-induced bond contraction to the convention of an extended solid of which the Hamiltonian contains the intraatomic trapping interaction and the interatomic binding interaction. Agreement between modelling predictions and the observed size dependency in the photoluminescence of Si oxides and some nanometric III–V and II–VI semiconductors, and in the core-level shift of Cu–O nanosolids has been reached. Results indicate that the spontaneous contraction of chemical bonds at a surface and the rise in the surface-to-volume ratio with reducing particle size are responsible for the unusual change of the band structure of a nanosolid.

Surface Engineering and Design Strategy for Surface-Amorphized TiO2@Graphene Hybrids for High Power Li-Ion Battery Electrodes

Surface amorphization provides unprecedented opportunities for altering and tuning material properties. Surface‐amorphized TiO2@graphene synthesized using a designed low temperature‐phase transformation technique exhibits significantly improved rate capability compared to well‐crystallized TiO2@graphene and bare TiO2 electrodes. These improvements facilitates lithium‐ion transport in both insertion and extraction processes and enhance electrolyte absorption capability.

Atomic Interface Engineering and Electric-Field Effect in Ultrathin Bi2MoO6 Nanosheets for Superior Lithium Ion Storage

Ultrathin 2D materials can offer promising opportunities for exploring advanced energy storage systems, with satisfactory electrochemical performance. Engineering atomic interfaces by stacking 2D crystals holds huge potential for tuning material properties at the atomic level, owing to the strong layer-layer interactions, enabling unprecedented physical properties. In this work, atomically thin Bi2 MoO6 sheets are acquired that exhibit remarkable high-rate cycling performance in Li-ion batteries, which can be ascribed to the interlayer coupling effect, as well as the 2D configuration and intrinsic structural stability. The unbalanced charge distribution occurs within the crystal and induces built-in electric fields, significantly boosting lithium ion transfer dynamics, while the extra charge transport channels generated on the open surfaces further promote charge transport. The in situ synchrotron X-ray powder diffraction results confirm the materials excellent structural stability. This work provides some insights for designing high-performance electrode materials for energy storage by manipulating the interface interaction and electronic structure.