Minwei Xu

Electrochemical synthesis of leaf-like CuO mesocrystals and their lithium storage properties

We have developed an electrochemical approach for the synthesis of leaf-like CuO mesocrystals. The oriented attachment mechanism is responsible for the formation of CuO mesocrystals. As anode materials for lithium ion batteries, the high reversible capacity and enhanced cycle performance were demonstrated.

Co3O4–carbon nanotube heterostructures with bead-on-string architecture for enhanced lithium storage performance

In this report, alkylcarboxyl group-decorated carbon nanotubes (CNTs) with clustered functionalization patterns are achieved based on a modified Birch reduction in liquid ammonia. By using these functional CNTs (f-CNTs), a new type of Co3O4-CNT heterostructure is prepared via a simple hydrothermal method. SEM and TEM analyses reveal that the as-synthesized Co3O4-CNT heterostructures exhibit bead-on-string architecture, in which the Co3O4 spheres are threaded with CNTs. A possible growth mechanism is proposed to explain the formation of these Co3O4-CNT heterostructures. The electrochemical properties of the Co3O4-CNT heterostructures as anode materials for lithium ion batteries are investigated. The Co3O4-CNT heterostructures display high electrochemical activity, good cycle stability and improved rate performance. Such a large improvement of the electrochemical performance can be related to the robust necklace-like architectures which possess properties such as high mechanical stability, excellent electric conductivity and good strain accommodation.

Electrocatalytic properties of hollow coral-like platinum mesocrystals.

Controlling the morphology and size of Pt nanostructures can provide a great opportunity to improve their electrocatalytic properties and durability toward methanol oxidation. We successfully synthesized well-defined hollow coral-like Pt mesocrystals via a facile replacement reaction, using hierarchically structured Ag nanoparticles as the template. The as-synthesized coral-like Pt mesocrystals had hollow structures composed of porous walls that were 5-7 nm thick, which were constructed from numerous pores (~2 nm) and small (~5 nm) Pt nanoparticle building units. These hollow and porous structures exhibited large electrochemical surface areas. The Pt mesostructures that have single-crystal inherence as well as hollow and porous features possess improved performance with respect to electrocatalytic activity (2.72 mA/cm(2) at peak potential, which is 2.1 times greater than that of Pt black) and durability toward methanol oxidation.

Lithium-assisted exfoliation of pristine graphite for few-layer graphene nanosheets

A lithium-assisted approach has been developed for the exfoliation of pristine graphite, which allows the large-scale preparation of few-layer graphene nanosheets. The process involves an unexpected physical insertion and exfoliation, and the graphene nanosheets prepared by this method reveal undisturbed sp2-hybridized structures. A possible two-step mechanism, which involves the negative charge being trapped around the edges of the graphite layers and a subsequent lithiation process, is proposed to explain the insertion of lithium inside the graphite interlayers. If necessary, the present exfoliation can be repeated and thinner (single or 2–3 layer) graphene can be achieved on a large scale. This simple process provides an efficient process for the exfoliation of pristine graphite, which might promote the future applications of graphene.

Magnetic properties of different CoFe2O4 nanostructures: nanofibers versus nanoparticles

We fabricated CoFe2O4 (CFO) nanofibers (NFs) and nanoparticles (NPs) via electrospinning and the conventional sol–gel process, respectively. Sizes of these nanomaterials can be controlled by adjusting the annealing temperature. Magnetic dynamic behaviour showed differences between them. These differences have been linked to specific size and shape configurations. At room temperature, magnetic properties were size dependent, but the specific shape configuration dominated the magnetic properties at 5 K.

Robust ferromagnetism in Mn-doped MoS2 nanostructures

Layered transition metal dichalcogenides (TMDs) have attracted extensive attention due to their interesting properties originating from an effective honeycomb lattice and strong spin-orbit coupling, and have potential applications in catalysis, lithium batteries, photonic, electronic, and valleytronic devices. Introducing magnetism in the TMDs can lead to the interesting functionalities such as magnetic order and carrier spin polarization with potential applications in spintronics. Here, we demonstrate an effective approach to induce robust ferromagnetism in MoS2 nanostructures by transition metal doping. After doping with a few percent Mn2+, the magnetism of MoS2 nanostructures is enhanced dramatically. Moreover, the magnetic properties are strongly temperature dependent, which is clearly different from the behavior of defect-induced magnetism. Our approach opens up the possibility for tuning the spin and magnetic properties in two-dimensional nanostructures.

Large magnetocaloric effect at low magnetic field in Ni50―xCoxMn35In15 ribbons

Recently, the magnetocaloric effect (MCE) of Ni-Mn-based Heusler alloys has attracted more attention due to its metamagnetic shape-memory effect. However, most MCE investigations focused on the bulk alloys developed at high magnetic field (>2 T). In this paper, by employing the melt-spinning technique, a large MCE at low magnetic field was found in a ribbon of Ni50−xCoxMn35In15. The magnetic entropy change ΔS reaches the maximum value of 5.35 J/kg K at 1 T field in the Ni49CoMn35In15 ribbon near room temperature, and its corresponding temperature span arrives 24 K. The refrigerant capacity is enhanced with the increase of the Co substitution. Such a large MCE at low magnetic field in the Ni50−x CoxMn35In15 ribbon with a wide temperature range provides a potential candidate for room temperature magnetic refrigeration.

Giant spontaneous exchange bias triggered by crossover of superspin glass in Sb-doped Ni50Mn38Ga12 Heusler alloys

A spontaneous exchange bias (SEB) discovered by Wang et al. [Phys. Rev. Lett. 106 (2011) 077203.] after zero-field cooling (ZFC) has attracted recent attention due to its interesting physics. In this letter, we report a giant SEB tuned by Sb-doping in Ni50Mn38Ga12-xSbx Heusler alloys. Such an SEB was switched on below the blocking temperature of approximately 50 K. The maximum exchange bias HE can arrive at 2930 Oe in a Ni50Mn38Ga10Sb2 sample after ZFC to 2 K. Further studies showed that this SEB was attributable to interaction of superspin glass (SSG) and antiferromagnetic matix, which was triggered by the crossover of SSG from canonical spin glass to a cluster spin glass. Our results not only explain the underlying physics of SEB, but also provide a way to tune and control the SEB performance.

A magnetocaloric effect arising from a ferromagnetic transition in the martensitic state in Heusler alloy of Ni50Mn36Sb8Ga6

We report a magnetocaloric effect with nearly zero thermal hysteresis in Heusler alloy of Ni50Mn36Sb8Ga6 around room temperature due to a ferromagnetic transition in martensitic state (FTMS). The refrigerant capacity value of this ferromagnetic transition in the vicinity of onset temperature TCM can arrive at 163.8 J/kg, much higher than that of first-order magnetically martensitic transition in the same alloy. Such a magnetocaloric effect in FTMS is attributed to a second-order (or a weak first-order) transition from an antiferromagnetic phase with modulated 7 M orthorhombic structure to a ferromagnetic phase with modulated 4O orthorhombic structure.

Functionalization of carbon nanotubes via Birch reduction chemistry for selective loading of CuO nanosheets

Mesoporous CuO nanosheets threaded with carbon nanotubes (CNTs) were synthesized via a simple solution-phase method, where Birch reduction chemistry was introduced for the functionalization of CNTs. Selective loading of CuO nanosheets was achieved due to the in situ nucleation and growth of CuO near the functional bands. Finally, discrete CuO nanosheets distributed along the axis direction of CNTs were revealed. The obtained CuO nanosheets exhibited the features of extremely thin thickness, mesoporous structure and high surface area. More importantly, the CNTs pass through the inner part of the CuO nanosheets, which ensures enhanced conductivity for electron transportation. When used as anode materials for lithium ion batteries, these CuO/CNT hybrids exhibited much lower charge transfer resistance, in contrast to pure CuO nanosheets. The construction of the necklace-like CuO/CNT hybrids can largely enhance the kinetic performance of the CuO anodes.