Oriented contraction: a facile nonequilibrium heat-treatment approach for fabrication of maghemite fiber-in-tube and tube-in-tube nanostructures

Charge Storage Mechanism in Electrospun Spinel-Structured High-Entropy (Mn0.2 Fe0.2 Co0.2 Ni0.2 Zn0.2 )3 O4 Oxide Nanofibers as Anode Material for Li-Ion Batteries

High-entropy oxides (HEOs) have emerged as promising anode materials for next-generation lithium-ion batteries (LIBs). Among them, spinel HEOs with vacant lattice sites allowing for lithium insertion and diffusion seem particularly attractive. In this work, electrospun oxygen-deficient (Mn,Fe,Co,Ni,Zn) HEO nanofibers are produced under environmentally friendly calcination conditions and evaluated as anode active material in LIBs. A thorough investigation of the material properties and Li+ storage mechanism is carried out by several analytical techniques, including ex situ synchrotron X-ray absorption spectroscopy. The lithiation process is elucidated in terms of lithium insertion, cation migration, and metal-forming conversion reaction. The process is not fully reversible and the reduction of cations to the metallic form is not complete. In particular, iron, cobalt, and nickel, initially present mainly as Fe3+ , Co3+ /Co2+ , and Ni2+ , undergo reduction to Fe0 , Co0 , and Ni0 to different extent (Fe < Co < Ni). Manganese undergoes partial reduction to Mn3+ /Mn2+ and, upon re-oxidation, does not revert to the pristine oxidation state (+4). Zn2+ cations do not electrochemically participate in the conversion reaction, but migrating from tetrahedral to octahedral positions, they facilitate Li-ion transport within lattice channels opened by their migration. Partially reversible crystal phase transitions are observed.

SnO x /graphene anode material with multiple oxidation states for high-performance Li-ion batteries

Tin and its oxides are promising anode materials owing to their high theoretical capacity, rich resource, and environmental benignity. To achieve low cost and green synthesis, a facile synthetic route of SnO _x /graphene composites is proposed, using a simple galvanic replacement method to quickly obtain abundant foamed tin as raw material and ball milling method to realize a mechanochemical reaction between SnO _x (0 ≤ x ≤ 2) and graphene. Under different annealing conditions, the foamed tin is converted to tin oxides with multiple oxidation states (Sn₃O₄, SnO, and SnO₂). These unique components can greatly affect the electrochemical performance of the electrode in LIBs. The as-prepared electrode (SnO _x -300/G) obtained by annealing foamed tin at 300 °C for 4 h and combining SnO _x powders with graphene via ball milling shows great cycling stability, retaining a high capacity of 786 mA h g^-1 at 0.1 A g^-1 after 150 cycles, and its initial Coulombic efficiency can reach 84.03%. Thus, this facile synthesis can provide an environmentally friendly route for commercial production of high-performance energy storage materials.

Structure, Defects, and Magnetism of Electrospun Hematite Nanofibers Silica-Coated by Atomic Layer Deposition

In the last years, hematite has been utilized in a plethora of applications. High aspect-ratio nanohematite and hematite/silica core-shell nanostructures are arousing growing interest for applications exploiting their magnetic properties. Atomic layer deposition (ALD) is utilized here to produce SiO₂-coated α-Fe₂O₃ nanofibers (NFs) through two synthetic routes, viz. electrospinning/calcination/ALD or electrospinning/ALD/calcination. The number of ALD cycles (10-100) modulates the coating thickness, while the chosen route controls the final nanostructure. Porous and partially hollow NFs are produced. Their hierarchical structure and the nature and density of the lattice defects and strain are characterized by combining electron microscopy, diffraction, and spectroscopy techniques. The uncoated hematite NFs mostly have surface-related strain, which is attributed to oxygen vacancies/Fe²⁺ sites. ALD coating causes microstrain release and decrease of surface states. NFs calcined after ALD have extensive bulk strain, which is ascribed to the presence of dislocations throughout the volume of the NF grains. Bulk strain determines the remanent magnetization, whereas both surface and bulk strain influence the coercive field and the thermal behavior across the Morin temperature, including the magnetic memory effect. To the best of the authors' knowledge, the correlation between lattice defects/strain and magnetic properties of SiO₂-coated α-Fe₂O₃ NFs has never been reported before.

Self-Reconstructed Formation of a One-Dimensional Hierarchical Porous Nanostructure Assembled by Ultrathin TiO2 Nanobelts for Fast and Stable Lithium Storage

Owing to their unique structural advantages, TiO₂ hierarchical nanostructures assembled by low-dimensional (LD) building blocks have been extensively used in the energy-storage/-conversion field. However, it is still a big challenge to produce such advanced structures by current synthetic techniques because of the harsh conditions needed to generate primary LD subunits. Herein, a novel one-dimensional (1D) TiO₂ hierarchical porous fibrous nanostructure constructed by TiO₂ nanobelts is synthesized by combining a room-temperature aqueous solution growth mechanism with the electrospinning technology. The nanobelt-constructed 1D hierarchical nanoarchitecture is evolves directly from the amorphous TiO₂/SiO₂ composite fibers in alkaline solutions at ambient conditions without any catalyst and other reactant. Benefiting from the unique structural features such as 1D nanoscale building blocks, large surface area, and numerous interconnected pores, as well as mixed phase anatase-TiO₂(B), the optimum 1D TiO₂ hierarchical porous nanostructure shows a remarkable high-rate performance when tested as an anode material for lithium-ion batteries (107 mA h g^-1 at ∼10 A g^-1) and can be used in a hybrid lithium-ion supercapacitor with very stable lithium-storage performance (a capacity retention of ∼80% after 3000 cycles at 2 A g^-1). The current work presents a scalable and cost-effective method for the synthesis of advanced TiO₂ hierarchical materials for high-power and stable energy-storage/-conversion devices.

Tubular Micro/Nanomotors: Propulsion Mechanisms, Fabrication Techniques and Applications

Micro/nanomotors are self-propelled machines that can convert various energy sources into autonomous movement. With the great advances of nanotechnology, Micro/Nanomotors of various geometries have been designed and fabricated over the past few decades. Among them, the tubular Micro/Nanomotors have a unique morphology of hollow structures, which enable them to possess a strong driving force and easy surface functionalization. They are promising for environmental and biomedical applications, ranging from water remediation, sensing to active drug delivery and precise surgery. This article gives a comprehensive and clear review of tubular Micro/Nanomotors, including propulsion mechanisms, fabrication techniques and applications. In the end, we also put forward some realistic problems and speculate about corresponding methods to improve existing tubular Micro/Nanomotors.

Intricate Hollow Structures: Controlled Synthesis and Applications in Energy Storage and Conversion

Intricate hollow structures garner tremendous interest due to their aesthetic beauty, unique structural features, fascinating physicochemical properties, and widespread applications. Here, the recent advances in the controlled synthesis are discussed, as well as applications of intricate hollow structures with regard to energy storage and conversion. The synthetic strategies toward complex multishelled hollow structures are classified into six categories, including well-established hard- and soft-templating methods, as well as newly emerging approaches based on selective etching of "soft@hard" particles, Ostwald ripening, ion exchange, and thermally induced mass relocation. Strategies for constructing structures beyond multishelled hollow structures, such as bubble-within-bubble, tube-in-tube, and wire-in-tube structures, are also covered. Niche applications of intricate hollow structures in lithium-ion batteries, Li-S batteries, supercapacitors, Li-O₂ batteries, dye-sensitized solar cells, photocatalysis, and fuel cells are discussed in detail. Some perspectives on the future research and development of intricate hollow structures are also provided.

Crystalline-phase-induced formation of fibre-in-tube TiO2–SnO2 fibres for a humidity sensor

We provide a general crystalline-phase-induced strategy for the fabrication of fibre-in-tube TiO₂–SnO₂ materials for a high-sensitivity humidity sensor.

Electrospun nanofibers for cancer diagnosis and therapy

The advent of nanotechnology has provided unprecedented opportunities for nanomedicine. Electrospun nanofibers have some astounding features such as high loading capacity, extremely large surface area and porosity, high encapsulation efficiency, ease of modification, combination of diverse therapies, low cost and great benefits. These remarkable structure-dependent properties have far reaching application potential in cancer diagnosis and therapy such as ultra-sensitive sensing systems for point-of-care cancer detection, targeted cancer cell capture, and functional and smart anticancer drug delivery systems. This review summarizes the principal mechanism of electrospun nanofibers and a variety of modified electrospun nanofibers, illustrates their application in biosensors for cancer detection, and enumerates their application in implantable drug delivery for cancer therapy.

One-Dimensional RuO2/Mn2O3 Hollow Architectures as Efficient Bifunctional Catalysts for Lithium-Oxygen Batteries

Rational design and massive production of bifunctional catalysts with fast oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) kinetics are critical to the realization of highly efficient lithium-oxygen (Li-O2) batteries. Here, we first exploit two types of double-walled RuO2 and Mn2O3 composite fibers, i.e., (i) phase separated RuO2/Mn2O3 fiber-in-tube (RM-FIT) and (ii) multicomposite RuO2/Mn2O3 tube-in-tube (RM-TIT), by controlling ramping rate during electrospinning process. Both RM-FIT and RM-TIT exhibited excellent bifunctional electrocatalytic activities in alkaline media. The air electrodes using RM-FIT and RM-TIT showed enhanced overpotential characteristics and stable cyclability over 100 cycles in the Li-O2 cells, demonstrating high potential as efficient OER and ORR catalysts.

Design and Synthesis of Bubble-Nanorod-Structured Fe2O3-Carbon Nanofibers as Advanced Anode Material for Li-Ion Batteries

A structure denoted as a "bubble-nanorod composite" is synthesized by introducing the Kirkendall effect into the electrospinning method. Bubble-nanorod-structured Fe2O3-C composite nanofibers, which are composed of nanosized hollow Fe2O3 spheres uniformly dispersed in an amorphous carbon matrix, are synthesized as the target material. Post-treatment of the electrospun precursor nanofibers at 500 °C under 10% H2/Ar mixture gas atmosphere produces amorphous FeOx-carbon composite nanofibers. Post-treatment of the FeOx-carbon composite nanofibers at 300 °C under air atmosphere produces the bubble-nanorod-structured Fe2O3-C composite nanofibers. The solid Fe nanocrystals formed by the reduction of FeOx are converted into hollow Fe2O3 nanospheres during the further heating process by the well-known Kirkendall diffusion process. The discharge capacities of the bubble-nanorod-structured Fe2O3-C composite nanofibers and hollow bare Fe2O3 nanofibers for the 300th cycles at a current density of 1.0 A g(-1) are 812 and 285 mA h g(-1), respectively, and their capacity retentions measured from the second cycle are 84 and 24%, respectively. The hollow nanospheres accommodate the volume change that occurs during cycling. The unique structure of the bubble-nanorod-structured Fe2O3-C composite nanofibers results in their superior electrochemical properties by improving the structural stability during long-term cycling.

Fabrication of spinel one-dimensional architectures by single-spinneret electrospinning for energy storage applications

A facile and general method is developed to fabricate one-dimensional (1D) spinel composite oxides with complex architectures by using a facile single-spinneret electrospinning technique. It is found that precursor polymers and heating rates could control the structures of the products, such as 1D solid, nanotube and tube-in-tubes structures. Especially, the tube-in-tube structures have been successfully fabricated for various mixed metal oxide, including CoMn2O4, NiCo2O4, CoFe2O4, NiMn2O4 and ZnMn2O4. Benefiting from the unique structure features, the tube-in-tube hollow nanostructures possess superior electrochemical performances in asymmetric supercapacitors and Li-O2 batteries.

A comprehensive review on synthesis methods for transition-metal oxide nanostructures

Recent developments of transition-metal oxide nanostructures with designed shape and dimensionality, including various synthesis methods and applications, are presented.

Multi-shelled hollow micro-/nanostructures

Recent advances in multi-shelled hollow micro-/nanostructures were reviewed, and the correlation between their geometric properties and specific performance was highlighted.

Mechanistic insights into formation of SnO₂ nanotubes: asynchronous decomposition of poly(vinylpyrrolidone) in electrospun fibers during calcining process

The formation mechanism of SnO2 nanotubes (NTs) fabricated by generic electrospinning and calcining was revealed by systematically investigating the structural evolution of calcined fibers, product composition, and released volatile byproducts. The structural evolution of the fibers proceeded sequentially from dense fiber to wire-in-tube to nanotube. This remarkable structural evolution indicated a disparate thermal decomposition of poly(vinylpyrrolidone) (PVP) in the interior and the surface of the fibers. PVP on the surface of the outer fibers decomposed completely at a lower temperature (<340 °C), due to exposure to oxygen, and SnO2 crystallized and formed a shell on the fiber. Interior PVP of the fiber was prone to loss of side substituents due to the oxygen-deficient decomposition, leaving only the carbon main chain. The rest of the Sn crystallized when the pores formed resulting from the aggregation of SnO2 nanocrystals in the shell. The residual carbon chain did not decompose completely at temperatures less than 550 °C. We proposed a PVP-assisted Ostwald ripening mechanism for the formation of SnO2 NTs. This work directs the fabrication of diverse nanostructure metal oxide by generic electrospinning method.

Facile synthesis and properties of hierarchical double-walled copper silicate hollow nanofibers assembled by nanotubes

The hierarchical assembly of multilevel, nonspherical hollow structures remains a considerable challenge. Here, we report a facile approach for synthesizing copper silicate hollow nanofibers with an ultrasmall nanotube-assembled, double-walled structure. The as-prepared hollow fibers possess a tailored complex wall structure, high length-to-diameter ratio, good structural stability, and a high surface area, and they exhibit excellent performance as an easily recycled adsorbent for wastewater treatment and as an ideal support for noble metal catalysts. In addition, this strategy can be extended as a general approach to prepare other double-walled, hollow, fibrous silica-templated materials.

In situ assembly of well-dispersed gold nanoparticles on hierarchical double-walled nickel silicate hollow nanofibers as an efficient and reusable hydrogenation catalyst

A facile route to fabricate ultrafine AuNPs immobilized on double-walled nickel silicate hollow nanofibers assembled by nanosheets has been developed.

Wire-in-tube structure fabricated by single capillary electrospinning via nanoscale Kirkendall effect: the case of nickel-zinc ferrite

Wire-in-tube structures have previously been prepared using an electrospinning method by means of tuning hydrolysis/alcoholysis of a precursor solution. Nickel-zinc ferrite (Ni0.5Zn0.5Fe2O4) nanowire-in-nanotubes have been prepared as a demonstration. The detailed nanoscale characterization, formation process and magnetic properties of Ni0.5Zn0.5Fe2O4 nanowire-in-nanotubes has been studied comprehensively. The average diameters of the outer tubes and inner wires of Ni0.5Zn0.5Fe2O4 nanowire-in-nanotubes are around 120 nm and 42 nm, respectively. Each fully calcined individual nanowire-in-nanotube, either the outer-tube or the inner-wire, is composed of Ni0.5Zn0.5Fe2O4 monocrystallites stacked along the longitudinal direction with random orientation. The process of calcining electrospun polymer composite nanofibres can be viewed as a morphologically template nucleation and precursor diffusion process. This allows the nitrates precursor to diffuse toward the surface of the nanofibres while the oxides (decomposed from hydroxides and nitrates) products diffuse to the core region of the nanofibres; the amorphous nanofibres transforming thereby into crystalline nanowire-in-nanotubes. In addition, the magnetic properties of the Ni0.5Zn0.5Fe2O4 nanowire-in-nanotubes were also examined. It is believed that this nanowire-in-nanotube (sometimes called core-shell) structure, with its uniform size and well-controlled orientation of the long nanowire-in-nanotubes, is particularly attractive for use in the field of nano-fluidic devices and nano-energy harvesting devices.

Electro-spinning/netting: A strategy for the fabrication of three-dimensional polymer nano-fiber/nets

Since 2006, a rapid development has been achieved in a subject area, so called electro-spinning/netting (ESN), which comprises the conventional electrospinning process and a unique electro-netting process. Electro-netting overcomes the bottleneck problem of electrospinning technique and provides a versatile method for generating spider-web-like nano-nets with ultrafine fiber diameter less than 20 nm. Nano-nets, supported by the conventional electrospun nanofibers in the nano-fiber/nets (NFN) membranes, exhibit numerious attractive characteristics such as extremely small diameter, high porosity, and Steiner tree network geometry, which make NFN membranes optimal candidates for many significant applications. The progress made during the last few years in the field of ESN is highlighted in this review, with particular emphasis on results obtained in the author's research units. After a brief description of the development of the electrospinning and ESN techniques, several fundamental properties of NFN nanomaterials are addressed. Subsequently, the used polymers and the state-of-the-art strategies for the controllable fabrication of NFN membranes are highlighted in terms of the ESN process. Additionally, we highlight some potential applications associated with the remarkable features of NFN nanostructure. Our discussion is concluded with some personal perspectives on the future development in which this wonderful technique could be pursued.

In situ synthesis of porous Fe3O4/C microbelts and their enhanced electrochemical performance for lithium-ion batteries

The porous Fe3O4/C microbelts (FCBs) were synthesized by simple electrospinning method, combined with carbonization of the precursor microbelts at high temperature in N2 flow. Compared with α-Fe2O3 microbelt, the FCBs show better cyclic performance. The high capacity of 710 mA h g(-1) is still maintained after 50 cycles. The excellent performance of FCBs in lithium-ion batteries can be attributed to the highly stable porous belt structure of FCBs and to the substantial structure carbon matrix surrounded Fe3O4 nanoparticles. The synthesis method is simple, cheap, and green and could be extended to fabricate other carbon composites.

Oppositely charged twin-head electrospray: a general strategy for building Janus particles with controlled structures

Because of their unique heterostructure characteristics and anisotropic surface properties, Janus particles have gained growing interest in a number of novel applications. For the first time we demonstrate a facile, but versatile and general strategy for large-scale building of Janus particles with controlled structures and chemical composition pairs by an oppositely charged twin-head electrospray. In this protocol, two different droplets electrosprayed respectively from two tip-to-tip nozzles at high voltages of opposite polarities, after solvent evaporation and precursor gelation, collide with each other and coagulate into one Janus particle because of the Coulombic attractive forces. The as-electrosprayed droplets show different transient phase states at collision depending on the kinetic parameters such as the chemical compositions of precursors, humidity, concentration of solvent vapour, etc. Thus the resultant Janus particles have various morphologies and structures controlled by the transient phase state of the eletrosprayed droplets as well as the post-heat-treatment parameters. As examples, we demonstrate here the controlled fabrication of metal oxide-metal oxide and metal oxide-metal sulphide Janus particles with solid snowman-like, hollow-bowl snowman-like, and pot-like structures. Because of their unique heterostructure and novel morphology characteristics, the as-prepared Janus particles, despite a polydispersity in size and inhomogeneity in morphology, have some important potential applications including photocatalytic hydrogen production, environment remediation, and nanomotors.

Facile preparation of magnetic γ-Fe₂O₃/TiO₂ Janus hollow bowls with efficient visible-light photocatalytic activities by asymmetric shrinkage

In this paper, on the basis of a simple side-by-side co-electrospray procedure with a subsequent non-equilibrium calcination process, we have for the first time developed an asymmetric shrinkage approach for the fabrication of magnetic γ-Fe(2)O(3)/TiO(2) Janus hollow bowls (JHBs) by constructing a precursor solution pair with different gelation rates during the solvents evaporation process. The formation mechanisms of the bowl-shapes as well as the hollow interiors are proposed and confirmed. The as-obtained γ-Fe(2)O(3)/TiO(2) JHBs have a transition layer of Fe(3+)-doped-TiO(2) between the γ-Fe(2)O(3) and TiO(2) phases, and show an efficient visible-light photocatalytic activity and convenient magnetic separation for water purification because of the unique structure and morphology as well as the fine magnetic properties. Moreover, the method reported here can be readily extended to the fabrication of other bi-, tri- and multi-component metal oxides hollow particles with asymmetric shapes. Due to the interesting bowl-shaped hollow nanostructure, the as-prepared γ-Fe(2)O(3)/TiO(2) JHBs are expected to have a number of applications that involve drug delivery, micro-/nano-motors, microcontainers, microreactors, sensors, and so forth.

Formation of three-dimensional urchin-like α-Fe₂O₃ structure and its field-emission application

A three-dimensional urchin-like α-Fe(2)O(3) microstructure is formed via a simple, template-free, and one-step thermal oxidation of Fe spheres in an air atmosphere at temperatures in the range of 300-450 °C. The urchin-like α-Fe(2)O(3) microstructure consists of crystalline α-Fe(2)O(3) nanoflakes grown perpendicularly on the surface of the sphere, a shell layer of α-Fe(2)O(3)/Fe(3)O(4), and an Fe core. During the oxidation process, the nanoflakes germinate and grow from cracks in the oxidation layer on the surface. The length of the nanoflakes increases with oxidation time. The tip diameters of the nanoflakes are in ranges of 10-20 nm at 300 °C, 20-30 nm at 350 °C, and 40-60 nm at 400 °C; the length can reach up to a few micrometers. The field-emission characteristics of the samples are experimentally studied and simulated. The results show that the urchin-like emitter has a low turn-on field of 2.8 V/μm, high field-enhancement factor of 4313, excellent emission uniformity of over 4 cm(2), and good emission stability during a 24 h test.