Shuang Xi

Synthesis of a nanowire self-assembled hierarchical ZnCo2O4 shell/Ni current collector core as binder-free anodes for high-performance Li-ion batteries

In this paper, ZnCo2O4 nanowires have been grown and self-assembled as hierarchical structures on a 3D conductive Ni foam substrate. Both leaf-like ZnCo2O4 and dandelion-like ZnCo2O4 assemblies were synthesized through a hydrothermal process followed by a post-annealing treatment. It is shown that leaf-like assemblies are directly grown on the substrate while dandelion-like assemblies are adsorbed on the surface of the structures. A possible formation mechanism of ZnCo2O4 hierarchical structures was proposed. It is shown that these nanowires are porous structures which provide much increased specific surface area. Further work was conducted by taking these Ni foam supported ZnCo2O4 structures as binder-free electrodes for Li-ion batteries. Remarkably, the leaf-like ZnCo2O4/Ni foam electrode exhibits greatly improved electrochemical performance with high capacity and excellent cycling stability. A high reversible capacity of 1050 mA h g−1 at the rate of 100 mA g−1 was obtained after 60 cycles. Meanwhile, the electrode showed a high rate of 416 mA g−1 with a high capacity of 850 mA h g−1 even after 50 cycles. Our work demonstrates that this unique nanowire self-assembled ZnCo2O4 hierarchical structure is promising for high-performance electrochemical energy applications.

Growth of Hierarchal Mesoporous NiO Nanosheets on Carbon Cloth as Binder-free Anodes for High-performance Flexible Lithium-ion Batteries

Mesoporous NiO nanosheets were directly grown on three-dimensional (3D) carbon cloth substrate, which can be used as binder-free anode for lithium-ion batteries (LIBs). These mesoporous nanosheets were interconnected with each other and forming a network with interval voids, which give rise to large surface area and efficient buffering of the volume change. The integrated hierarchical electrode maintains all the advantageous features of directly building two-dimensional (2D) nanostructues on 3D conductive substrate, such as short diffusion length, strain relaxation and fast electron transport. As the LIB anode, it presents a high reversible capacity of 892.6 mAh g−1 after 120 cycles at a current density of 100 mA g−1 and 758.1 mAh g−1 at a high charging rate of 700 mA g−1 after 150 cycles. As demonstrated in this work, the hierarchical NiO nanosheets/carbon cloth also shows high flexibility, which can be directly used as the anode to build flexible LIBs. The introduced facile and low-cost method to prepare NiO nanosheets on flexible and conductive carbon cloth substrate is promising for the fabrication of high performance energy storage devices, especially for next-generation wearable electronic devices.

Carbonization-assisted integration of silica nanowires to photoresist-derived three-dimensional carbon microelectrode arrays

We propose a novel technique of integrating silica nanowires to carbon microelectrode arrays on silicon substrates. The silica nanowires were grown on photoresist-derived three-dimensional carbon microelectrode arrays during carbonization of patterned photoresist in a tube furnace at 1000 °C under a gaseous environment of N(2) and H(2) in the presence of Cu catalyst, sputtered initially as a thin layer on the structure surface. Carbonization-assisted nucleation and growth are proposed to extend the Cu-catalyzed vapor-liquid-solid mechanism for the nanowire integration behaviour. The growth of silica nanowires exploits Si from the etched silicon substrate under the Cu particles. It is found that the thickness of the initial Cu coating layer plays an important role as catalyst on the morphology and on the amount of grown silica nanowires. These nanowires have lengths of up to 100 µm and diameters ranging from 50 to 200 nm, with 30 nm Cu film sputtered initially. The study also reveals that the nanowire-integrated microelectrodes significantly enhance the electrochemical performance compared to blank ones. A specific capacitance increase of over 13 times is demonstrated in the electrochemical experiment. The platform can be used to develop large-scale miniaturized devices and systems with increased efficiency for applications in electrochemical, biological and energy-related fields.

Scalable fabrication of carbon-based MEMS/NEMS and their applications: a review

The carbon-based micro/nano electromechanical system (MEMS/NEMS) technique provides a powerful approach to large-scale manufacture of high-aspect-ratio carbon structures for wafer-level processing. The fabricated three-dimensional (3D) carbon structures have the advantages of excellent electrical and electrochemical properties, and superior biocompatibility. In order to improve their performance for applications in micro energy storage devices and microsensors, an increase in the footprint surface area is of great importance. Various approaches have been proposed for fabricating large surface area carbon-based structures, including the integration of nanostructures such as carbon nanotubes (CNTs), graphene, nanowires, nanofilms and nanowrinkles onto 3D structures, which has been proved to be effective and productive. Moreover, by etching the 3D photoresist microstructures through oxygen plasma or modifying the photoresist with specific materials which can be etched in the following pyrolysis process, micro/nano hierarchical carbon structures have been fabricated. These improved structures show excellent performance in various applications, especially in the fields of biological sensors, surface-enhanced Raman scattering, and energy storage devices such as micro-supercapacitors and fuel cells. With the rapid development of microelectronic devices, the carbon-based MEMS/NEMS technique could make more aggressive moves into microelectronics, sensors, miniaturized power systems, etc. In this review, the recent advances in the fabrication of micro/nano hierarchical carbon-based structures are introduced and the technical challenges and future outlook of the carbon-based MEMS/NEMS techniques are also analyzed.

Tailoring diffraction-induced light distribution toward controllable fabrication of suspended C-MEMS

A simple and controllable method is proposed to fabricate suspended three-dimensional carbon microelectromechanical systems (C-MEMS) structures by tailoring diffraction-induced light distribution in photolithography process. An optical model is set up and the corresponding affecting parameters are analyzed to interpret and predict the formation of suspended structures based on Fresnel diffraction theory. It is identified that mask pattern dimensions, gap distance between the photomask and photoresist, and the exposure time are critical to the final suspended structures, which have also been verified through experimental demonstrations. The fabricated biocompatible suspended C-MEMS structures could find wide applications in electrochemical and biological areas.

Growth of nano-wrinkles on photoresist-derived carbon microelectrode array

A novel fabrication method for integrating nano-wrinkles to carbon microelectromechanical systems (C-MEMS) posts on a silicon substrate is proposed in this paper. In the fabrication of carbon microelectrode array, SU-8, a negative photoresist is patterned by photolithography and subsequently pyrolysed at high temperatures in an oxygen-free environment. In this process, the SU-8 posts are slowly converted to the desired carbon posts with properties resembling those of glassy carbon, and the carbon posts have shrinkage in both vertical and radial directions. As we deposit a thin layer (nanoscale order) of carbon before pyrolysis, nano-wrinkle patterns can be generated with the shrinking of the photoresist due to a large difference in the elastic moduli as well as high compressive stress. The nano-wrinkles generated in carbon films can greatly enlarge specific surface area, which significantly enhances the electrochemical performance compared to the blank posts. Since the carbon nano-wrinkles have the same nature as the carbon posts, the stability and biological compatibility of nano-wrinkles integrated microelectrodes are greatly enhanced.

Growth of highly bright-white silica nanowires as diffusive reflection coating in LED lighting

Large quantities of silica nanowires were synthesized through thermal treatment of silicon wafer in the atmosphere of N(2)/H(2)(5%) under 1200 °C with Cu as catalyst. These nanowires grew to form a natural bright-white mat, which showed highly diffusive reflectivity over the UV-visible range, with more than 60% at the whole range and up to 88% at 350 nm. The utilization of silica nanowires in diffusive coating on the reflector cup of LED is demonstrated, which shows greatly improved light distribution comparing with the specular reflector cup. It is expected that these nanowires can be promising coating material for optoelectronic applications.

Metal-catalyst-free synthesis and characterization of single-crystalline silicon oxynitride nanowires

Large quantities of single-crystal silicon oxynitride nanowires with high N concentration have been synthesized directly on silicon substrate at 1200°C without using any metal catalyst. The diameter of these ternary nanowires is ranging from 10 to 180nm with log-normal distribution, and the length of these nanowires varies from a few hundreds of micrometers to several millimeters. A vapor-solidmechanism was proposed to explain the growth of the nanowires. These nanowires are grown to forma disorderedmat with an ultrabright white nonspecular appearance. The mat demonstrates highly diffusive reflectivity with the optical reflectivity of around 80% over the whole visible wavelength, which is comparable to the most brilliant white beetle scales found in nature. The whiteness might be resulted from the strong multiscattering of a large fraction of incident light on the disordered nanowire mat. These ultra-bright white nanowires could form as reflecting surface to meet the stringent requirements of bright-white lightemitting-diode lighting for higher optical efficiency. They can also find applications in diverse fields such as sensors, cosmetics, paints, and tooth whitening.

Concentration gradient induced morphology evolution of silica nanostructure growth on photoresist-derived carbon micropatterns

The evolution of silica nanostructure morphology induced by local Si vapor source concentration gradient has been investigated by a smart design of experiments. Silica nanostructure or their assemblies with different morphologies are obtained on photoresist-derived three-dimensional carbon microelectrode array. At a temperature of 1,000°C, rope-, feather-, and octopus-like nanowire assemblies can be obtained along with the Si vapor source concentration gradient flow. While at 950°C, stringlike assemblies, bamboo-like nanostructures with large joints, and hollow structures with smaller sizes can be obtained along with the Si vapor source concentration gradient flow. Both vapor–liquid-solid and vapor-quasiliquid-solid growth mechanisms have been applied to explain the diverse morphologies involving branching, connecting, and batch growth behaviors. The present approach offers a potential method for precise design and controlled synthesis of nanostructures with different features.

Stress-Driven and Carbon-Assisted Growth of

An integration strategy is devised for a reliable, scalable, and catalyst-free assembly of SiOxNy nanowires in photoresist-derived carbon microelectrodes. The approach involved UV photolithography process of SU8 photoresist, followed by high-temperature carbonization, and was versatile in yielding various 3-D micro-nano integrated carbon microelectrode arrays (CMEAs). The morphology of the SiOxNy nanowires and the nanowire-integrated CMEA was characterized by scanning electron microscopy and high-resolution transmission electron microscopy. The chemical composition of the SiOxNy nanowires was confirmed by energy-dispersive X-ray and X-ray photoelectron spectroscopy. A synergetic growth mechanism is proposed based on our experimental observations, in which both carbon-assisted and stress-driven mechanisms were identified to interpret the formation of SiOxNy nanowires. Further study revealed that the nanowire-integrated 3-D CMEA showed improved electrical and electrochemical performances than the blank ones, demonstrating potential applications in electrochemical, biological, and energy-related fields. Meanwhile, the developed method represents a low-cost and easy way to mass production of SiOxNy nanowire-integrated CMEA.