Lianxi Zheng

Bio‐Inspired Nacre‐like Composite Films Based on Graphene with Superior Mechanical, Electrical, and Biocompatible Properties

Bio-inspired multifunctional composite films based on reduced poly(vinyl alcohol)/graphene oxide (R-PVA/GO) layers are prepared by a facile solution casting method followed by a reduction procedure. The resulting films with nacre-like, bricks-and-mortar microstructure have excellent mechanical properties, electrical conductivity, and biocompatibility.

Self-powered, visible-light photodetector based on thermally reduced graphene oxide–ZnO (rGO–ZnO) hybrid nanostructure

Here we report a new type of self-powered, visible-light photodetector fabricated from thermally reduced rGO–ZnO hybrid nanostructure. The photocurrent generation of the photodetectors under zero-bias enables hybrid rGO–ZnO devices to work like photovoltaic cells, which could power themselves without electrical power input. The thermal treatment at elevated temperature not only reduces graphene oxide (GO) into reduced graphene oxide (rGO), but also dopes the ZnO nanoparticles with carbon atoms, enabling their visible-light photoresponse capability. The pronounced and fast photocurrent generation was attributed to the efficient charge transfer between the rGO and carbon-doped ZnO nanoparticles, which were in intimate contact. The efficient charge transfer of the rGO–ZnO hybrid nanostructures also indicates that there could be applications in other light energy harvesting devices, including solar cells, sensors and visible-light photocatalysis.

Synergistic effect of hybrid carbon nantube–graphene oxide as a nanofiller in enhancing the mechanical properties of PVA composites

A poly(vinyl alcohol) (PVA) based nanocomposite using fully exfoliated graphene oxide (GO) sheets and multi-walled carbon nanotubes (CNTs) were prepared via a simple procedure. It is confirmed from optical imaging that dispersion of CNTs in the PVA matrix can be significantly improved by adding GO sheets. Molecular dynamics (MD) simulations suggest that the GO–CNT interaction is strong and the complex is thermodynamically favorable over agglomerates of CNTs. The GO–CNT scroll-like structure formed with the hydrophilic outer surface of GO can be well dispersed in water. More important, a synergistic effect arises from the combination of CNT and GO, the GO–CNT/PVA composite films show superior mechanical properties compared to PVA composite films enhanced by GO or CNT alone, not only the tensile strength and Youngs modulus of the composites are significantly improved, but most of the ductility is also retained. The enhanced mechanical properties of the GO–CNT/PVA composite film can be attributed to the fully exploited reinforcement effect from GO and CNT via good dispersion.

Cubic-phase GaN light-emitting diodes

The feasibility of growing device-quality cubic GaN/GaAs(001) films by metal organic chemical vapor deposition has been demonstrated. The optical quality of the GaN films was characterized by room-temperature photoluminescence measurements, which shows a full width at half maximum of 46 meV. The structural quality of the films was investigated by transmission electron microscopy. There are submicron-size grains free from threading dislocations and stacking faults. More importantly, a cubic-phase GaN blue light-emitting diode has been fabricated. The device process, which is very simple and compatible with current GaAs technology, indicates a promising future for the blue light-emitting diode

Three-Dimensional Plasmonic Photoanodes Based on Au-Embedded TiO2 Structures for Enhanced Visible-Light Water Splitting

Plasmon-assisted visible light photocatalysis presents a possible solution for direct solar-to-fuel production. Here we investigate the plasmon-enhanced photocatalytic water splitting using different TiO2/Au electrode structures. Experimental data demonstrates that the Au embedded in TiO2 (Au-in-TiO2) electrode greatly outperforms the Au sitting on TiO2 (Au-on-TiO2) electrode. Numerical simulation shows that the local electric field is very intense in the semiconductor near Au nanoparticles, which causes the enhancement of electron-hole pair generation. A 3D Au-embedded TiO2 structure is thus proposed to further improve the light absorption and photocatalytic performance.

Tuning Array Morphology for High‐Strength Carbon‐Nanotube Fibers

Vertically aligned carbon-nanotube arrays are synthesized by chemical vapor deposition. Carbon-nanotube fibers are directly spun from the obtained nanotube arrays and then tested mechanically. A strong correlation between the array morphologies and the mechanical properties of the fibers is observed: well-aligned arrays yield fibers with much higher performance, while wavy and entangled arrays give poor fiber properties. More importantly, such array morphologies could be controlled by introducing hydrogen or oxygen during the nanotube synthesis. By simply switching the growth condition from 150 ppm oxygen addition to 2% hydrogen addition, the nanotube array changes from the wavy morphology to the well-aligned morphology, and correspondingly the tensile strength of the resultant fibers could be increased by 4.5 times, from 0.29 GPa for the fibers spun from the oxygen-assistance-grown nanotube arrays to 1.3 GPa for the fibers spun from the hydrogen-assistance-grown nanotube arrays. The detailed effects of hydrogen and oxygen on the nanotube growth, especially on the growth rate and the array spinnability, are extensively studied. The formation mechanism of the different morphologies of the nanotube arrays and the failure mechanism of the nanotube fibers are also discussed in detail.

Reduction of threading defects in GaN grown on vicinal SiC(0001) by molecular-beam epitaxy

We observe a significant reduction of threading dislocations in GaN grown on vicinal substrates of SiC(0001). Using scanning tunneling microscopy, we find films grown on vicinal substrates maintain the surface misorientation of the substrate and display terraces with straight edges. On top of the terraces there is no spiral mound, which is the main feature found for films grown on singular substrates. Transmission electron microscopy studies confirm that threading screw dislocations are reduced by two orders of magnitude while edge dislocations are reduced by one order.

The applications of carbon nanotubes and graphene in advanced rechargeable lithium batteries

Advanced rechargeable lithium batteries are desired energy storage devices for electric vehicles. These batteries require their electrodes to have high electrical and thermal conductivity, an appropriate high specific surface area, an outstanding hierarchical architecture, high thermal and chemical stability and to be relatively low cost and environmentally benign. Carbon nanotubes (CNTs) and graphene are two candidate materials that could meet these requirements, and thus have been widely studied. The present paper reviews the applications of CNTs and graphene in batteries, with an emphasis on the particular roles (such as conductive, active, flexible and supporting roles) they play in advanced lithium batteries. We will summarize the unique advantages of CNTs and graphene in battery applications, update the most recent progress, and compare the prospects and challenges of CNTs and graphene for future full utilization in energy storage applications. The effects and mechanisms of heteroatoms doping, the distribution of pore sizes, different architectures (anchored, sandwich-like and wrapped hybrid architecture) are discussed in detail.

Rayleigh-instability-driven simultaneous morphological and compositional transformation from Co nanowires to CoO octahedra

Arrays of regularly distributed CoO nano-octahedra are obtained by annealing Co nanowires at high temperatures. Both the size and the separation distance of the nano-octahedra can be controlled by tuning the annealing temperature. These self-assembled linear arrays of CoO nanocrystals result from the synergetic combination of the morphological transformation due to the intrinsic Rayleigh instability and the phase transformation due to the cobalt oxidation.

Three-Dimensional Porous LiFePO4: Design, Architectures and High Performance for Lithium Ion Batteries

The olivine-structured lithium ion phosphate (LiFePO4) is one of the most competitive candidates of cathode materials for the sustainable lithium ion battery (LIB) systems. However, the major drawback of olivine-structured LiFePO4 is the poor intrinsic electronic and lithium ion conductivities arising from the lack of mixed valency and the one- dimensional lithium ion diffusion, which influence its high electrochemical performance, especially high rate capability. Nano-structured LiFePO4 materials offer a potential solution to enhance surface-to-volume ratio and reduce transport length for mobile charges, but they have high interfacial energy, aggregate easily and need more agglutinant in electrode, which seriously impact the electrochemical performance and practical applications of LiFePO4. Furthermore they con- tinue to experience limitations as energy and power requirements escalate with the evolution of technology. Recently, three-dimensional (3D) porous LiFePO4 architectures have been widely designed and studied. This has led to increased in- terest in the development of cathode materials and processing capabilities necessary to enable high-performance, next- generation LIB system that can deliver large amounts of energy at high rates. In this review, we focus on 3D porous LiFePO4 architectures for high power LIBs, summarize and discuss its structure, synthesis, electrochemical behaviors, mechanism, and the problems encountered in its application. The major goal is to highlight the recent progress of 3D po- rous LiFePO4 architectures with high rate capability, high energy density and application.