Facile one-pot synthesis of NiCo<Subscript>2</Subscript>O<Subscript>4</Subscript> hollow spheres with controllable number of shells for high-performance supercapacitors

In this work,single-and double-shelled NiCo2O4 hollow spheres have been synthesized in situ by a one-pot solvothermal method assisted by xylose,followed by heat treatment.Employed as supercapacitor electrode materials,the double-shelled NiCo2O4 hollow spheres exhibit a remarkable specific capacitance (1,204.4 F·g-1 at a current density of 2.0 A·g-1) and excellent cycling stability (103.6％ retention after 10,000 cycles at a current density of 10 A·g-1).Such outstanding electrochemical performance can be attributed to their unique internal morphology,which provides a higher surface area with a larger number of active sites available to interact with the electrolyte.The versatility of this method was demonstrated by applying it to other binary metal oxide materials,such as ZnCo2O4,ZnMn2O4,and CoMn2O4.The present study thus illustrates a simple and general strategy for the preparation of binary transition metal oxide hollow spheres with a controllable number of shells.This approach shows great promise for the development of next-generation high-performance electrochemical materials.     

Self-assembly synthesis of LaPO<Subscript>4</Subscript> hierarchical hollow spheres with enhanced photocatalytic CO<Subscript>2</Subscript>-reduction performance

Urchin-like LaPO4 hollow spheres were successfully synthesized by a facile solution route using citric acid (CA) as a structure-directing agent.The size of the three-dimensional (3D) hollow spheres was tuned by changing the concentration of CA.The formation mechanism of the 3D LaPO4 hollow spheres was revealed by studying the time-dependent morphology evolution process.Importantly,compared with monodispersed one-dimensional (1D) LaPO4 nanorods,the 3D LaPO4 hollow spheres self-assembled from nanorods showed a 6.8-fold enhancement in photocatalytic activity for CO2 reduction,which is attributed to the synergistic effect of their hierarchical hollow structure,higher light-harvesting capacity,and faster electron transfer.Our findings provide not only a simple,facile method for the synthesis of hierarchical hollow micro/nanoarchitectures but also an efficient route for enhancing the photocatalytic performance.     

An efficientfficient,controllable and facile two-step synthesis strategy: Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>@RGO composites with various Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> nanoparticles and their supercapacitance properties

An efficient,controllable,and facile two-step synthetic strategy to prepare graphene-based nanocomposites is proposed.A series of Fe3O4-decorated reduced graphene oxide (Fe3O4@RGO) nanocomposites incorporating Fe3O4 nanocrystals of various sizes were prepared by an ethanothermal method using graphene oxide (GO) and monodisperse Fe3O4 nanocrystals with diameters ranging from 4 to 10 nm.The morphologies and microstructures of the as-prepared composites were characterized by X-ray diffraction,Raman spectroscopy,nitrogen adsorption measurements,and transmission electron microscopy.The results show that GO can be reduced to graphene during the ethanothermal process,and that the Fe3O4 nanocrystals are well dispersed on the graphene sheets generated in the process.The analysis of the electrochemical properties of the Fe3O4@RGO materials shows that nanocomposites prepared with Fe3O4 nanocrystals of different sizes exhibit different electrochemical performances.Among all samples,Fe3O4@RGO prepared with Fe3O4 nanocrystals of 6 nm diameter possessed the highest specific capacitance of 481 F/g at 1 A/g,highlighting the excellent capability of this material.This work illustrates a promising route to develop graphene-based nanocomposite materials with a wide range of potential applications.     

Solvothermal synthesis of CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript> hollow spheres

Hollow CoFe₂O₄ spheres consisted of CoFe₂O₄ nanoparticles were synthesized by a facile solvothermal treatment of an ethylene glycol solution of FeCl₃ · 6H₂O, CoCl₂ · 6H₂O, and NaAc at 200 °C in the presence of polyethylene glycol and oleic acid. The products were characterized by powder X-ray diffraction, transmission electron microscopy, selected area electron diffraction, high-resolution transmission microscopy, scanning electron microscopy. The magnetic properties were evaluated using a vibrating sample magnetometer. The probable mechanism of the formation of Hollow CoFe₂O₄ spheres was discussed. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Synthesis and characterization of Cd-doped ZnMn<Subscript>2</Subscript>O<Subscript>4</Subscript> microspheres as supercapacitor electrodes

In this paper, we demonstrate the effects of Cd-doping ZnMn₂O₄ on structural and electrochemical performance. Cd-doped ZnMn₂O₄ spheres with diameters of about 2 μm were successfully synthesized by a facile hydrothermal method at 200 °C for 18 h. The fabricated Cd-doped ZnMn₂O₄ samples were characterized by X-ray diffraction, scanning electron microscopy, Brunauer Emmett Teller surface area analyzer and X-ray photoelectron spectroscopy. The electrochemical performance was investigated by cyclic voltammetry and electrochemical impedance spectrometry. The experimental results show that the synthesized spherical Cd-doped ZnMn₂O₄ exhibit far better rate capability and cyclic stability than that of pure spinel porous ZnMn₂O₄ microspheres. The result of cyclic voltammetry measurement indicates that the obtained Cd-doped ZnMn₂O₄ microspheres exhibited the high specific capacitance of 364 Fg^?1 at 2 mV/s.     

Nanowires assembled from MnCo<Subscript>2</Subscript>O<Subscript>4</Subscript>@C nanoparticles for water splitting and all-solid-state supercapacitor

The rational design of earth-abundant catalysts with excellent water splitting activities is important to obtain clean fuels for sustainable energy devices. In this study, mixed transition metal oxide nanoparticles encapsulated in nitrogendoped carbon (denoted as AB₂O₄@NC) were developed using a one-pot protocol, wherein a metal–organic complex was adopted as the precursor. As a proof of concept, MnCo₂O₄@NC was used as an electrocatalyst for water oxidation, and demonstrated an outstanding electrocatalytic activity with low overpotential to achieve a current density of 10 mA·cm^?1 (η ₁₀ = 287 mV), small Tafel slope (55 mV·dec^?1), and high stability (96% retention after 20 h). The excellent electrochemical performance benefited from the synergistic effects of the MnCo₂O₄ nanoparticles and nitrogen-doped carbon, as well as the assembled mesoporous nanowire structure. Finally, a highly stable all-solid-state supercapacitor based on MnCo₂O₄@NC was demonstrated (1.5% decay after 10,000 cycles).

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Facile synthesis and characterization of MnO<Subscript>2</Subscript> nanomaterials as supercapacitor electrode materials

MnO₂ nanomaterials are synthesized via calcinations in air at various temperatures. Amorphous MnO₂ masses appear between 100 and 300 °C and nanorods form above 400 °C. Transmission and scanning electron microscopy are used to observe the geometries of each material, with further structural analyses conducted using X-ray photoelectron spectroscopy, X-ray diffraction, and BET method. The electrochemical properties are investigated through galvanostatic charge/discharge cycling, electrochemical impedance spectra, and cyclic voltammetry within a three-electrode test cell filled with 1 mol L^?1 Na₂SO₄ solution. The slightly asymmetric galvanostatic cycling curves suggest that the reversibility of the Faradaic reactions are imperfect, requiring a larger time to charge than discharge. The specific capacitances of each sample are calculated and trends are identified, proving that the samples synthesized at higher temperatures exhibit poorer electrochemical behaviors. The highest calculated specific capacitance is 175 F g^?1 by the sample calcinated at 400 °C. However, the lower temperature samples exhibit more favorable geometric properties and higher overall average specific capacitances. For future research, it is suggested that surface modifications such as a carbon coating could be used in conjunction with the MnO₂ nanorods to reach the electrochemical properties required by contemporary industrial applications.     

Controllable synthesis of triangular Ni(HCO<Subscript>3</Subscript>)<Subscript>2</Subscript> nanosheets for supercapacitor

Triangular Ni(HCO₃)₂ nanosheets were synthesized via a template-free solvothermal method. The phase transition and formation mechanism were explored systematically. Further investigation indicated that the reaction time and pH have significant effects on the morphology and size distribution of the triangular Ni(HCO₃)₂ nanosheets. More interestingly, the resulting product had an ultra-thin structure and high specific surface area, which can effectively accelerate the charge transport during charge–discharge processes. As a result, the triangular Ni(HCO₃)₂ nanosheets not only exhibited high specific capacitance (1,797 F·g^-1 at 5 A·g^-1 and 1,060 F·g^-1 at 50 A·g^-1), but also showed excellent cycling stability with a high current density (~80% capacitance retention after 5,000 cycles at the current density of 20 A·g^-1).

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One-step template-free synthesis of ZnWO<Subscript>4</Subscript> hollow clusters

ZnWO₄ hollow clusters made up of nanorods were successfully prepared through a tripotassium citrate assisted hydrothermal process at 180 °C. The hollow clusters’ diameter was about 400 nm, and these clusters were made up of nanorods with a diameter of about 10 nm and a length of about 50 nm. X-ray power diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were used to characterize the structure and morphology of the synthesized products. Based on experiments, the growth of these hollow clusters followed an aggregation-Ostwald ripening process. The photocatalytic activities for aqueous Rhodamine B of samples were investigated, and it was seen that ZnWO₄ hollow clusters exhibited a strong photocatalytic activity.     

Rambutan-like hierarchically heterostructured CeO<Subscript>2</Subscript>-CuO hollow microspheres: Facile hydrothermal synthesis and applications

Hierarchically heterostructured hollow spheres are of great interest for a wide range of applications owing to their unique structural features and properties.However,the fabrication of well-defined hollow spheres with highly specific morphology for mixed transition metal oxides on a large scale remains challenging.In this work,uniform rambutan-like heterostructured CeO2-CuO hollow microspheres with numerous copper-ceria interfacial sites and nanorods and nanoparticles as building blocks are prepared via a facile hydrothermal method followed by calcination.Importantly,this approach can be readily scaled up and is applicable to the synthesis of various CuO-based mixed metal oxide complex hollow spheres.The as-prepared CeO2-CuO hollow rambutans exhibit superior performance both as electrode materials for supercapacitors and as Cu-based catalysts for the Rochow reaction,mainly due to the small primary nanopartide constituents,high surface area,and formation of numerous interior heterostructures.     

Rational synthesis of carbon shell coated polyaniline/MoS<Subscript>2</Subscript> monolayer composites for high-performance supercapacitors

Conducting polymers generally show high specific capacitance but suffer from poor rate capability and rapid capacitance decay, which greatly limits their practical applications in supercapacitor electrodes. To this end, many studies have focused on improving the overall capacitive performance by synthesizing nanostructured conducting polymers or by depositing a range of coatings to increase the active surface area exposed to the electrolyte and enhance the charge transport efficiency and structural stability. Despite this, simultaneously achieving high specific capacitance, good rate performance, and long cycle life remains a considerable challenge. Among the various two-dimensional (2D) layered materials, octahedral (1T) phase molybdenum disulfide (MoS₂) nanosheets have high electrical conductivity, large specific surface areas, and unique surface chemical characteristics, making them an interesting substrate for the controlled growth of nanostructured conducting polymers. This paper reports the rational synthesis of carbon shell-coated polyaniline (PANI) grown on 1T MoS₂ monolayers (MoS₂/PANI@C). The composite electrode comprised of MoS₂/PANI@C with a ~3 nm carbon shell exhibited a remarkable specific capacitance of up to 678 F·g^–1 (1 mV·s^–1), superior capacity retention of 80% after 10,000 cycles and good rate performance (81% at 10 mV·s^–1) due to the multiple synergic effects between the PANI nanostructure and 1T MoS₂ substrates as well as protection by the uniform thin carbon shell. These properties are comparable to the best overall capacitive performance achieved for conducting polymers-based supercapacitor electrodes reported thus far.

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Polymer-coated CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript> nanoassemblies as biocompatible magnetic nanocarriers for anticancer drug delivery

In this work, we report the preparation of cobalt ferrite nanoparticles (CFNPs) coated with hydrophilic polymers guar gum, gum arabic and poly (methacrylic acid) as magnetic nanocarriers and study their conjugation with doxorubicin for the drug release under applied magnetic field. The effect of polymer coating on structural properties is studied using Fourier transform infrared spectroscopy and powder X-ray diffractometry (XRD). The XRD analysis revealed that the polymer coating on the as-synthesized CFNPs has no influence on their crystallite size and it remains between 18 nm and 19 nm. The characteristic morphology, topography and the evidences of polymer coating over the CFNPs are investigated using scanning electron microscopy, atomic force microscopy and thermogravimetric analysis, respectively. Vibrating sample magnetometry revealed the ferromagnetic nature of uncoated CFNPs with a significant saturation magnetization ～77.2 emu g^?1. The dynamic light scattering measurements are also performed to determine the size of uncoated and coated CFNPs. UV–Visible spectroscopy demonstrated a profound loading (70–75%) of doxorubicin onto the magnetic nanocarriers and the efficient release of drug in the presence of external applied magnetic field. In vitro cytotoxic studies confirmed the cytocompatibility mode of coated CFNPs against Chinese Hamster Ovary and Huh-7 cell line, while 0.2 mg mL^?1 dose of drug-loaded magnetic nanocarriers inhibited the cell viability of Huh-7 up to 60%. These results strongly encourage the utilization of biocompatible magnetic nanocarriers in targeted drug delivery territory.     

Investigation for the synthesis of hierarchical Co<Subscript>3</Subscript>O<Subscript>4</Subscript>@MnO<Subscript>2</Subscript> nanoarrays materials and their application for supercapacitor

We designed and fabricated hierarchical Co₃O₄@MnO₂ nanoarrays directly grown on nickel foam by hydrothermal and calcination methods. After the investigation of growth mechanism, we found that the deposition of MnO₂ was based on the self-decomposition of KMnO₄ and the reducibility of Co₃O₄ during the hydrothermal process. Thanks to the hierarchical structure, the obtained electrode exhibited excellent capacitive performance in supercapacitor. It delivered 21.72 F cm^?2 at a current density of 5 mA cm^?2 and retained ~94 % capacitance of initial value after 5000 cycles.     

Fabrication of flexible reduced graphene oxide/Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> hollow nanospheres based on-chip micro-supercapacitors for integrated photodetecting applications

Micro-supercapacitors (MSCs) as important on-chip micropower sources have attracted considerable attention because of their unique and advantageous design for optimized maximum functionality within a minimized sized chip and excellent mechanical flexibility/stability in miniaturized portable electronic device applications. In this work, we report a novel, high-performance flexible integrated on-chip MSC based on hybrid nanostructures of reduced graphene oxide/Fe₂O₃ hollow nanospheres using a microelectronic photo-lithography technology combined with plasma etching technique. The unique structural design for on-chip MSCs enables high-performance enhancements compared with graphene-only devices, exhibiting high specific capacitances of 11.57 F·cm^-3 at a scan rate of 200 mV·s^-1 and excellent rate capability and robust cycling stability with capacitance retention of 92.08% after 32,000 charge/discharge cycles. Moreover, the on-chip MSCs exhibit superior flexibility and outstanding stability even after repetition of charge/discharge cycles under different bending states. As-fabricated highly flexible on-chip MSCs can be easily integrated with CdS nanowire-based photodetectors to form a highly compacted photodetecting system, exhibiting comparable performance to devices driven by conventional external energy storage units.

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One-step hydrothermal synthesis of ZnFe<Subscript>2</Subscript>O<Subscript>4</Subscript> nano-octahedrons as a high capacity anode material for Li-ion batteries

Binary transition metal oxides are considered as promising anode materials for lithium-ion batteries (LIB), because they can effectively overcome the drawbacks of simple oxides. Here, a one-step hydrothermal method is described for the synthesis of regular ZnFe₂O₄ octahedrons about 200 nm in size at a low temperature without further annealing being required. The ZnFe₂O₄ octahedrons were characterized by powder X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy. The electrochemical performance of the ZnFe₂O₄ octahedrons was examined in terms of cyclic voltammetry and discharge/charge profiles. The ZnFe₂O₄ octahedrons exhibit a high capacity of 910 mA·h/g at 60 mA/g between 0.01 and 3.0 V after 80 cycles. They also deliver a reversible specific capacity of 730 mA·h/g even after 300 cycles at 1000 mA/g, a much better performance than those in previous reports. A set of reactions involved in the discharge/charge processes are proposed on the basis of ex situ high-resolution transmission electron microscopy (HRTEM) images and selected area electron diffraction (SAED) patterns of the electrode materials. The insights obtained will be of benefit in the design of future anode materials for lithium ion batteries.      

Electronic Structure and Magnetic Properties of New Rare-earth Half-metallic Materials AcFe₂O₄ and ThFe₂O₄: Ab Initio Investigation

Electronic structure and magnetism of the rare-earth metals Ac and Th doped Fe₃O₄ Fe_1-xRe_xFe_2-yRe_yO₄(Re=Ac, Th; x=0, 0.5, 1; y=0, 0.5, 1.0, 1.5, 2.0) are investigated by first-principle calculations. AcFe₂O₄, FeAc₂O₄ and ThFe₂O₄ are found to be II B-type half-metals. The large bonding-antibonding splitting is believed to be the origin of the gap for AcFe₂O₄, FeAc₂O₄ and ThFe₂O₄, resulting in a net magnetic moment of 9.0μ_B, 4.0μ_B and 8.1μ_B, respectively, compared with 4.0μ_B of Fe₃O₄. Also, the conductance of AcFe₂O₄ and ThFe₂O₄ are both slightly larger than that of Fe3O4. It can be predicted that the new rare-earth half-metals AcFe₂O₄ and ThFe₂O₄ have wider application ground in spin electronic devices due to their larger magnetoresistance and higher conductivity than that of Fe₃O₄. The half-metallic feature can be maintained up to the lattice contraction of 8%, 3% and 4% for Fe₃O₄, AcFe₂O₄ and ThFe₂O₄, respectively.     

One-step hydrothermal synthesis of Li<Subscript>2</Subscript>FeSiO<Subscript>4</Subscript>/C composites as lithium-ion battery cathode materials

Li₂FeSiO₄/C composites were one-step synthesized under hydrothermal conditions at 200 °C for 72 h using glucose as carbon source. By adjusting the quantity of added glucose, we obtained varied Li₂FeSiO₄/C composites with different size and morphology. A series of electrochemical tests demonstrate that the Li₂FeSiO₄/C nanoparticles with diameters about 20 nm have higher discharge capacity, and slower capacity fading in comparison with Li₂FeSiO₄ and other Li₂FeSiO₄/C composites. Li₂FeSiO₄/C nanoparticles deliver a discharge capacity of 136 mAh g⁻¹ at 0.2 C, and after 100 cycles, the discharge capacity remains 96.1%. Furthermore, Li₂FeSiO₄/C nanoparticles also exhibit an excellent rate capability with a capacity of about 80 mAh g⁻¹ at 10 C.     

One-step thermal synthesis of binary manganese iron cyclotetraphosphate MnFeP<Subscript>4</Subscript>O<Subscript>12</Subscript>

The manganese iron cyclotetraphosphate (MnFeP₄O₁₂) was synthesized through one-step thermal synthesis at 700 °C using the mixing of manganese and iron metals and phosphoric acid in the presence of water–acetone media. Both FTIR and XRD results indicate the cyclotetraphosphate (P₄O₁₂ ⁴⁻) structure and a pure monoclinic phase with space group C2/c (Z = 4). The morphology and crystallite size for the MnFeP₄O₁₂ obtained from SEM data and X-ray line broadening show non-uniform particles and 30 ± 9 nm, respectively. The magnetic study of the synthesized MnFeP₄O₁₂ shows superparamagnetic behavior, which is important for specific application. Some physical properties of the synthesized MnFeP₄O₁₂ powder presented for the first time are comparable with those from individual M₂P₄O₁₂ (M = Mn and Fe) and a binary metal compound as CoFeP₄O₁₂.     

Synthesis of carbon nanotube,graphene, CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript>, and NiFe<Subscript>2</Subscript>O<Subscript>4</Subscript> polypyrrole nanocomposites and study their microwave absorption

In this experimental work, different conductive polymer nanocomposites were synthesized using polypyrrole as conductive polymer and CoFe₂O₄, NiFe₂O₄, CNT and graphene as fillers. X-ray diffraction pattern was used to study the crystallinity of the products and it was found CoFe₂O₄, NiFe₂O₄, CNT, and graphene were successfully embedded in the polymer matrix. To further approve the synthesis of the nanocomposites, energy dispersive X-ray spectroscopy was served. Surface groups of the synthesized nanocomposites were studied by Fourier transform infrared and Raman spectroscopy. The morphology of the products was examined by scanning electron microscopy and transmission electron microscopy. It was found the fillers were successfully embedded in the polymer matrix and they were in nanometer scales. To investigate the magnetic properties and conductivity of the polymer nanocomposites, alternating gradient force magnetometer and four-point probe were used, respectively. Finally, the microwave absorption properties of the polymer nanocomposites were studied and it was found the fillers have different effects on the polymer microwave absorption value.     

Room temperature magnetic exchange coupling in multiferroic BaTiO<Subscript>3</Subscript>/CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript> magnetoelectric superlattice

Multiferroic BaTiO₃/CoFe₂O₄ superlattice films are deposited by laser molecular beam epitaxy. The film growth modes are studied by in situ reflection high energy electron diffraction and the film structures are revealed by high resolution transmission electron microscopy study. Ferroelectric switching behavior was studied by piezoresponse force microscopy, and it shows that good ferroelectricity was retained in the superlattice. Such a multiferroic superlattice also shows a magnetic exchange coupling under room temperature. Detailed analysis reveals that different growth modes and the substrate strain effect may be responsible for the magnetic exchange coupling.