Xingyan Xu

Two-dimensional ultrathin ZnCo2O4 nanosheets: general formation and lithium storage application

Two-dimensional (2D) multicomponent transition-metal oxide nanosheets are the most promising candidate in low-cost and eco-friendly energy storage/conversion applications. Their surface-enhanced properties and synergic effects are fascinating, yet still underdeveloped. Here, we first report the high-quality ultrathin 2D nanosheets of ZnCo2O4 synthesized on a large scale via microwave-assisted liquid-phase growth coupled with a post annealing procedure. The well-defined and freestanding nanosheets exhibit a micron-sized planar area and ultrathin thickness, suggesting a high surface atom ratio with an unique surface and electronic structure, thus facilitating the charge transfer to enhance the overall performances in electrochemical reaction. When used as anode materials for lithium ion batteries, the ultrathin ZnCo2O4 nanosheets exhibit a high reversible lithium storage capacity of 930–980 mA h g−1 at 200 mA g−1 current density in 200 cycles with an excellent cycling stability and good high-rate capability. Even more importantly, we have extended the facile method for the formation of other analogue nanosheets including binary and ternary transition metal oxides (NiO, Co3O4, NiCo2O4, and CuCo2O4) and make a possibility in exploring more unique properties and promising commercial applications.

LiNi1/3Co1/3Mn1/3O2 hollow nano-micro hierarchical microspheres with enhanced performances as cathodes for lithium-ion batteries

LiNi1/3Co1/3Mn1/3O2 hollow nano-micro hierarchical microspheres (NCM-HS) are synthesized using MnCO3 both as a self-template and Mn source. The hollow microspheres with diameters of about 1 μm have walls about 250 nm thick, which are composed of approximately 100 nm primary nanoparticles. NCM-HS cathodes have an initial discharge capacity of 212 mA h g−1 at 0.1 C between 2.5 and 4.5 V. After 40 charge–discharge cycles, the capacity retention at 0.1 C is 85.1%. At higher rates, the reversible capacities of the NCM-HS cathodes are 208.9 (0.5 C), 204.8 (1 C), 180.7 (2 C), 155.7 (5 C) and 135.9 mA h g−1 (10 C). The high performances can be attributed to the distinctive hollow microspherical structures with the 100 nm building blocks, which could effectively reduce the path of Li ion diffusion, increase the contact area between electrodes and electrolyte and buffer the volume changes during the Li ion intercalation/deintercalation processes.

Large-scale cubic InN nanocrystals by a combined solution- and vapor-phase method under silica confinement.

Large-scale cubic InN nanocrystals were synthesized by a combined solution- and vapor-phase method under silica confinement. Nearly monodisperse cubic InN nanocrystals with uniform spherical shape were dispersed stably in various organic solvents after removal of the silica shells. The average size of InN nanocrystals is 5.7 ± 0.6 nm. Powder X-ray diffraction results indicate that the InN nanocrystals are of high crystallinity with a cubic phase. X-ray photoelectron spectroscopy and energy-dispersive spectroscopy confirm that the nanocrystals are composed of In and N elements. The InN nanocrystals exhibit infrared photoluminescence at room temperature, with a peak energy of ~0.62 eV, which is smaller than that of high-quality wurtzite InN (~0.65-0.7 eV) and is in agreement with theoretical calculations. The small emission peak energy of InN nanocrystals, as compared to other low-cost solution or vapor methods, reveals the superior crystalline quality of our samples, with low or negligible defect density. This work will significantly promote InN-based applications in IR optoelectronic device and biology.

Hierarchical LiMn2O4 Hollow Cubes with Exposed {111} Planes as High-Power Cathodes for Lithium-Ion Batteries

Hierarchical LiMn2O4 hollow cubes with exposed {111} planes have been synthesized using cube-shaped MnCO3 precursors, which are fabricated through a facile co-precipitation reaction. Without surface modification, the as-prepared LiMn2O4 exhibits excellent cyclability and superior rate capability. Surprisingly, even over 70% of primal discharge capacity can be maintained for up to 1000 cycles at 50 C, and with only about 72 s of discharge time the as-prepared materials can deliver initial discharge capacity of 96.5 mA h g(-1). What is more, the materials have 98.4% and 90.7% capacity retentions for up to 100 cycles at 5 C under the temperatures of 25 and 60 °C, respectively. The superior electrochemical performance can be attributed to the unique hierarchical and interior hollow structure, exposed {111} planes, and high-quality crystallinity.

Template-assisted synthesis of ordered single crystal InN nanowires

Aligned InN nanowires were synthesized by a template-assisted two-step method. The as-synthesized aligned InN nanowires were of single crystal structure and exhibited good electrical properties with a carrier mobility of 93.8 cm2 V−1 s−1 and a strong photoluminescence emission peak at 740 nm at room-temperature. The alignment characteristic makes it more convenient to integrate the nanowires into nanodevices. The two-step method provides an effective and low-cost road for ordered nitride nanowires in a controllable way.

Scalable 2D Mesoporous Silicon Nanosheets for High-Performance Lithium-Ion Battery Anode

Constructing unique mesoporous 2D Si nanostructures to shorten the lithium-ion diffusion pathway, facilitate interfacial charge transfer, and enlarge the electrode-electrolyte interface offers exciting opportunities in future high-performance lithium-ion batteries. However, simultaneous realization of 2D and mesoporous structures for Si material is quite difficult due to its non-van der Waals structure. Here, the coexistence of both mesoporous and 2D ultrathin nanosheets in the Si anodes and considerably high surface area (381.6 m2 g-1 ) are successfully achieved by a scalable and cost-efficient method. After being encapsulated with the homogeneous carbon layer, the Si/C nanocomposite anodes achieve outstanding reversible capacity, high cycle stability, and excellent rate capability. In particular, the reversible capacity reaches 1072.2 mA h g-1 at 4 A g-1 even after 500 cycles. The obvious enhancements can be attributed to the synergistic effect between the unique 2D mesoporous nanostructure and carbon capsulation. Furthermore, full-cell evaluations indicate that the unique Si/C nanostructures have a great potential in the next-generation lithium-ion battery. These findings not only greatly improve the electrochemical performances of Si anode, but also shine some light on designing the unique nanomaterials for various energy devices.

A general synthetic strategy to monolayer graphene

The emergence and establishment of new techniques for material fabrication are of fundamental importance in the development of materials science. Thus, we herein report a general synthetic strategy for the preparation of monolayer graphene. This novel synthetic method is based on the direct solid-state pyrolytic conversion of a sodium carboxylate, such as sodium gluconate or sodium citrate, into monolayer graphene in the presence of Na2CO3. In addition, gram-scale quantities of the graphene product can be readily prepared in several minutes. Analysis using Raman spectroscopy and atomic force microscopy clearly demonstrates that the pyrolytic graphene is composed of a monolayer with an average thickness of ∼0.50 nm. Thus, the present pyrolytic conversion can overcome the issue of the low monolayer contents (i.e., 1 wt.%–12 wt.%) obtained using exfoliation methods in addition to the low yields of chemical vapor deposition methods. We expect that this novel technique may be suitable for application in the preparation of monolayer graphene materials for batteries, supercapacitors, catalysts, and sensors.