One-pot synthesis of CoFe2O4/rGO hybrid hydrogels with 3D networks for high capacity electrochemical energy storage devices

CoFe₂O₄/reduced graphene oxide (CoFe₂O₄/rGO) hydrogel was synthesized in situ via a facile one-pot solvothermal approach. The three-dimensional (3D) network structure consists of well-dispersed CoFe₂O₄ nanoparticles on the surfaces of graphene sheets. As a binder-free electrode material for supercapacitors, the electrochemical properties of the CoFe₂O₄/rGO hybrid hydrogel can be easily adjusted by changing the concentration of the graphene oxide (GO) precursor solution. The results indicate that the hybrid material made using 3.5 mg mL⁻¹ GO solution exhibits an outstanding specific capacitance of 356 F g⁻¹ at 0.5 A g⁻¹, 68% higher than the pure CoFe₂O₄ counterpart (111 F g⁻¹ at 0.5 A g⁻¹), owing to the large specific surface area and good electric conductivity. Additionally, an electrochemical energy storage device based on CoFe₂O₄/rGO and rGO was assembled, which exhibits a high energy density of 17.84 W h kg⁻¹ at a power density of 650 W kg⁻¹ and an excellent cycling stability with 87% capacitance retention at 5 A g⁻¹ after 4000 cycles. This work takes one step further towards the development of 3D hybrid hydrogel supercapacitors and highlights their potential application in energy storage devices.

CoFe₂O₄/reduced graphene oxide (CoFe₂O₄/rGO) hydrogel was synthesized in situ via a facile one-pot solvothermal approach.     

Preparation and electrochemical properties of nanostructured porous spherical NiCo2O4 materials

Porous spherical NiCo₂O₄ powders with a micro–nano structure were prepared by the spray drying method using citric acid as the chelating agent and soluble salts as cobalt and nickel sources. By calcination at 300 °C, spherical and single-phase spinel-type NiCo₂O₄ powders were obtained. The powders with particle sizes of 0.5–3 μm were aggregations of nano-sized grains (about 15–30 nm). The electrical property tests demonstrated that the synthesized NiCo₂O₄ has a specific capacitance of 430.67 F g⁻¹ at a current density of 1 A g⁻¹ and a capacitance retention rate of 100% after 3000 cycles at a current density of 4 A g⁻¹, indicating excellent cycling stability. On assembly into an asymmetric supercapacitor device, a specific capacitance of 37.06 F g⁻¹ and energy density of 13.18 W h kg⁻¹ at a power density of 800 W kg⁻¹ and a current density of 1 A g⁻¹ were demonstrated. The micro–nano structured porous NiCo₂O₄ powders have a larger specific surface area, which can allow the sample to come in full contact with the electrolyte. The nanopore channels are favourable for releasing the lattice distortion stress during the charge–discharge process, maintaining the structural stability of the crystal and improving the cycle life.

Porous NiCo₂O₄ microspheres with a micro–nano structure, prepared by the spray drying method, have excellent cycle stability and 100% capacitance retention.     

Green synthesis of Au decorated CoFe2O4 nanoparticles for catalytic reduction of 4-nitrophenol and dimethylphenylsilane oxidation

Gold nanoparticles (Au NPs) have been widely employed in catalysis. Here, we report on the synthesis and catalytic evaluation of a hybrid material composed of Au NPs deposited at the surface of magnetic cobalt ferrite (CoFe₂O₄). Our reported approach enabled the synthesis of well-defined Au/CoFe₂O₄ NPs. The Au NPs were uniformly deposited at the surface of the support, displayed spherical shape, and were monodisperse in size. Their catalytic performance was investigated towards the reduction of 4-nitrophenol and the selective oxidation of dimethylphenylsilane to dimethylphenylsilanol. The material was active towards both transformations. In addition, the LSPR excitation in Au NPs could be employed to enhance the catalytic performance, which was demonstrated in the 4-nitrophenol reduction. Finally, the magnetic support allowed for the easy recovery and reuse of the Au/CoFe₂O₄ NPs. In this case, our data showed that no significant loss of performance took place even after 10 reaction cycles in the oxidation of dimethylphenylsilane to dimethylphenylsilanol. Overall, our results indicate that Au/CoFe₂O₄ are interesting systems for catalytic applications merging high performances, recovery and re-use, and enhancement of activities under solar light illumination.

We present a cleaner chemical synthesis process of a magnetic recoverable Au/CoFe₂O₄ hybrid nanocomposite catalyst that has remarkable activity in catalytic reduction and oxidation, improved by surface plasmon resonance.     

Controllable synthesis of nanostructured ZnCo2O4 as high-performance anode materials for lithium-ion batteries

Nanostructured ZnCo₂O₄ anode materials for lithium-ion batteries (LIBs) have been successfully prepared by a two-step process, combining facile and concise electrospinning and simple post-treatment techniques. Three different structured ZnCo₂O₄ anodes (nanoparticles, nanotubes and nanowires) can be prepared by simply adjusting the ratio of metallic salt and PVP in the precursor solutions. Charge–discharge tests and cyclic voltammetry (CV) have been conducted to evaluate the lithium storage performances of ZnCo₂O₄ anodes, particularly for ZnCo₂O₄ nanotubes obtained from a weight ratio 2 : 4 of metallic salt and PVP polymer in the precursor solution. Remarkably, ZnCo₂O₄ nanotubes exhibit high specific capacity, good rate property, and long cycling stability. Reversible capacity is still maintained at 1180.8 mA h g⁻¹ after 275 cycles at a current density of 200 mA g⁻¹. In case of rate capability, even after cycling at the 2000 mA g⁻¹ current density, the capacity could recover to 684 mA h g⁻¹. The brilliant electrochemical properties of the ZnCo₂O₄ anodes make them promising anodes for LIBs and other energy storage applications.

ZnCo₂O₄ nanoparticles, nanotubes, and nanofibers can be controllably prepared by simply tuning the weight ratios of metallic salts and PVP polymer in the precursor solution.     

4-Pentenoyl-isoleucyl-chitosan oligosaccharide and acrylamide functional monomer-dependent hybrid bilayer molecularly imprinted membrane for sensitive electrochemical sensing of bisphenol A

In this work, an electrochemical sensor was designed for trace monitoring of bisphenol A (BPA) by decorating a hybrid bilayer molecularly imprinted membrane (MIM) on a multi-walled carbon nanotube-modified glassy carbon electrode. When BPA in the MIM was eluted, a composite molecularly imprinted electrochemical sensor was constructed. Under optimal conditions, the developed sensor showed two linear relationships between ΔI_p and BPA concentration in the range of 0.04 μM to 8 μM, as well as good selectivity and stability, and was also applied to detect BPA in water samples with desirable recoveries ranging from 92.0% to 107.0%.

A hybrid bilayer molecularly imprinted membrane-dependent electrochemical sensor was developed for bisphenol A assay based on 4-pentenoyl-isoleucyl-chitosan oligosaccharide and acrylamide functional monomers.     

Room-temperature solution synthesis of ZnMn2O4 nanoparticles for advanced electrochemical lithium storage

Taking advantage of synergistic effects, the ternary metal oxides have attracted tremendous interest. Herein, ZnMn₂O₄ nano-particles have been fabricated via a facile one-step approach at room temperature, that of simply mixing ZnO and MnO in KOH aqueous solution without templates. When used as an anode for lithium ion batteries, it delivers the excellent structure stability (1028.9 mA h g⁻¹ at 1.0 A g⁻¹ after 400 cycles). Surprisingly, the low-cost and eco-friendly route provides a novel strategy to synthesize the mixed transition metal oxide electrodes with readily scaled-up production.

ZnMn₂O₄ nanoparticles were fabricated via a low-cost and ecofriendly one-step approach at room temperature. The particles exhibited excellent structure stability and superior lithium storage.

Presently, the energy density of lithium-ion batteries (LIBs) needs urgently to be improved to meet the market development, which is principally restricted by the current commercial graphite anode materials (≈372 mA h g⁻¹).^1,2 Hence, enormous efforts have been implemented to exploit anode materials with high capacity, such as metal oxide/sulfide,^3–5 phosphide,⁶ Si-based compounds.⁷ In consideration of their nontoxicity, low cost and high energy density, transition metal oxides (TMOs) have attracted extraordinary attention, owing to their high theoretical reversible capacity, which is twice as high as that of graphite.^8–10 Especially for ternary TMOs, they are considered as the most promising candidates for LIBs owing to their higher electrochemical activity and better ion/electron conductivity induced by their synergistic effects, compared with their corresponding single metal oxides.^11,12Among the numerous ternary TMOs, ZnMn₂O₄ are particularly attractive and extensively investigated, as it can offer richer redox reactions and display a relatively lower delithiation potential of 0.5 V than those of single-component oxides vs. Li/Li⁺ (ZnO, 1.2 V and MnO, 1.5 V).^13,14 However, ZnMn₂O₄ will suffer from the unavoidable volume expansion, resulting in the detrimental structural collapse upon long-term lithiation/delithiation process.¹⁵ Generally, preparing the nano-size electrode is an effective approach to mitigate the intrinsical issue, which can enhance the electrochemical performance and stabilize the microstructure. While the achievement of nano-structured materials is heavily depended on the synthetic methods. For instance, flower-like ZnMn₂O₄ is synthesized by solvothermal process, and it displays an initial charge capacity of about 763 mA h g⁻¹ and retains stable performance after 50 cycles.¹⁶ One-dimensional ZnMn₂O₄ nanowires have been fabricated by mixing α-MnO₂ and Zn(CH₃COO)₂ under high temperature calcination at 480 °C in O₂ atmosphere. However, the synthesis procedure introduces the template of precursor of α-MnO₂ nanowires, which are complicated to obtain by hydrothermal approach.¹⁷ Besides, the ball-in-ball hollow microspheres of ZnMn₂O₄ electrode can only be manufactured by liquid phase method with reflux condensation and post-thermal treatment.¹⁸ Unfortunately, although the nanosized ZnMn₂O₄ prepared by the above studies deliver a good electrochemistry property, the operational process is fairly complex and the productivity is relatively low, hindering the further development of ZnMn₂O₄ anodes. Therefore, it is urgent to explore a novel and highly efficient synthetic method to fabricate nanosized ZnMn₂O₄ with improved electrochemical performance.Herein, one-step room-temperature synthesis of ZnMn₂O₄ nano-particles based on facile KOH solution system and low-cost precursor materials has been designed and presented in this investigation. The synthesized ZnMn₂O₄ anode exhibits satisfied electrochemical properties in regard of high reversible capacity and excellent cycling stability. Compared with previously reported ZnMn₂O₄ materials synthesized at elevated temperature, it is facile, up-scalable and high-efficient for our work, which provide a new strategy to realize the large fabrication of other TMOs, such as ZnFe₂O₄, ZnCo₂O₄.The ZnMn₂O₄ nano-particles were synthesized through a facile, low cost route in one-step process, which is depicted in Fig. 1a. In the basic medium of KOH aqueous solution, MnO could be slowly oxidized to Mn₃O₄ (Fig. S3^†) by O₂. Meanwhile, with the presence of the Zn²⁺, the ZnMn₂O₄ could be prepared at same time in oxidization.¹⁹ As displayed in the SEM image of ZnMn₂O₄ materials (Fig. 1b), the nano-sized particles with dimension of 20–30 nm get aggregation into bulk morphology, which is obviously different from the pristine samples (Fig. S1^†) and is also confirmed by TEM (Fig. 1c). Predictably, after mixing the precursors in the solution process, the ZnMn₂O₄ materials are synthesized, which indicate that the approach is feasible and is confirmed by the following measurements of HRTEM and XRD. From the HRTEM image (inset of Fig. 1c), it can be clearly observed the interplanar spacing of 0.271 nm, well matching with the (103) planes of ZnMn₂O₄ phase, indicating the high crystallization of as-prepared ZnMn₂O_4. Moreover, the corresponding SAED pattern (Fig. 1d) is well indexed to the crystal planes of (211), (220), and (312) of ZnMn₂O₄, demonstrating the polycrystalline characters by the clear diffraction rings. The elemental mapping images (Fig. 1e) ascertains that the electrode is consisted of Zn, Mn and O with evenly distributed, and the molar ratio of Zn and Mn is approaching 1 : 2 (Fig. 1f), confirming the formation of ZnMn₂O₄.Open in a separate windowFig. 1(a) The schematic illustration of the fabrication procedure of ZnMn₂O₄; the SEM (b), TEM (c), SAED pattern (d), mapping profile (e) and the EDS spectroscopy pattern (f).The XRD pattern of the ZnMn₂O₄ nanoparticles is shown in Fig. 2a, which can be well indexed into the tetragonal structure of ZnMn₂O₄ with a space group of I4₁/amd (141) (JCPDS no. 24–1133). Meanwhile, as presented in the Raman spectra (Fig. 2b), it exhibits three obvious peaks located at 326, 369 and 671 cm⁻¹, which is consistent with the typical vibration modes of ZnMn₂O₄.²⁰ The peak at 671 cm⁻¹ is corresponding to the oxygen motion in the tetrahedral AO₄ group with A_1g symmetry, and the other two modes are characteristic of the octahedral site (BO₆).²¹ The valence state and chemical composition are analysed by XPS (Fig. 2c and d). The full spectrum displays four elements, Zn, Mn, O, and C in Fig. 2c and S3,^† and the C 1s spectrum (282.8 eV) is assigned to carbonate materials existed in the XPS instrument. The Mn 2p peak can be split into two peaks at 653.4 and 641.7 eV with an energy margin of 11.7 eV, associated to the Mn 2p_1/2 and 2p_3/2, respectively, which confirms the oxidation state of Mn³⁺ in the ZnMn₂O₄ nanoparticles.^22,23Open in a separate windowFig. 2The results of XRD (a), Raman spectra (b), XPS full spectrum (c) and the high-resolution spectrum of Mn 2p (d).To evaluate the lithium storage performance of ZnMn₂O₄, various electrochemical measurements were conducted as anodes in the coin cells. Cyclic voltammetry (CV) test at a rate of 0.1 mV s⁻¹ in potential range of 0.01–3.0 V is employed to explore the lithium-ion reaction mechanism of ZnMn₂O₄, as shown in Fig. 3a. During the initial cathodic sweep, the first occurred peak at 1.08 V is consistent to the reduction from Mn³⁺ to Mn²⁺, and the following peak located at 0.75 V is ascribed to the electrolyte decomposition accompanied with the formation of the solid electrolyte interphase (SEI), respectively, which disappears in the subsequent scans. Meanwhile, the peak at 0.41 V is assigned to the irreversible reduction reaction of Mn²⁺ and Zn²⁺ into Zn⁰ and Mn⁰ embedded in amorphous Li₂O matrix. While the peak at low potential (about 0.08 V) is corresponding to the formation of Li–Zn alloy (Fig. S6^†), which is consistent with the earlier CV profiles.¹⁶ In the corresponding anodic scan, two oxidation peaks at approximately 1.2 and 1.5 V are associated to the reversible oxidation of Zn⁰ and Mn⁰, and formation of ZnO and MnO, respectively.^24,25 In the following scans, the reduction peak for conversion reaction shifts from 0.41 V to 0.49 V, ascribing to the small overpotentials induced by smaller particles.²² Furthermore, the following CV profiles almost completely superimposed for each other, suggesting the excellent reversibility and stability of ZnMn₂O₄ anode.Open in a separate windowFig. 3(a) The CV curves of ZnMn₂O₄ electrode at a scan rate of 0.1 mV s⁻¹ between 0.01 V and 3 V. (b) Charge–discharge profiles for the first cycle at a current of 0.1 A g⁻¹, (c) the rate property at different rates, the cycling performance at 0.1 A g⁻¹ (d) and 1 A g⁻¹ (e) for all the samples.The first discharge–charge curves of ZnMn₂O₄ nano-particles (abb. ZMO) and precursor materials (ZnO and MnO) are shown in Fig. 3b, recorded at 0.1 A g⁻¹ in the potential range of 0.01–3.0 V. The as-prepared ZnMn₂O₄ shows a voltage plateau at 0.43 V, which is in accordance with the CV profiles. While it displays an initial charge capacity of 865.6 mA h g⁻¹ with an acceptable coulombic efficiency (70.1%), higher than those of the ZnO (851.6 mA h g⁻¹) and MnO (509.8 mA h g⁻¹). The rate capability of ZnMn₂O₄ is demonstrated in Fig. 3c. The ZnMn₂O₄ anode delivers 863.4, 781.9, 676, 622.7 and 470 mA h g⁻¹ at rate increasing from low current to ultrahigh (0.1, 0.2, 0.5, 1.0 and 2.0 A g⁻¹). When returned back to 0.1 A g⁻¹, it immediately recovered to 555.6 mA h g⁻¹, much higher than that of pristine ZnO and MnO samples, indicating their advanced kinetic properties. The cycling performance of ZnMn₂O₄ is also superior compared with pristine ZnO and MnO. It is noteworthy that ZnMn₂O₄ shows the reversible capacity of 884.5 mA h g⁻¹ after 50 cycles at rate of 0.1 A g⁻¹ (Fig. 3d). Furthermore, it maintains a reversible capacity of 1028.9 mA h g⁻¹ after 400 cycles at rate of 1 A g⁻¹, equivalent to capacity retention of 82.08%. Interestingly, it suffers from capacity loss during the initial dozens of cycles, but thereafter, it begins to increase significantly, which is normally found in other TMO-material anodes. This phenomenon is generally attributed to the polymeric gel-like film with continuous and reversible formation during the kinetic activation process of anode.^18,26The EIS profiles of the three samples are Nyquist plots, fitted by the corresponding equivalent circuit as displayed inset of Fig. 4a. The charge-transfer resistance is noted as R_ct and Li⁺ diffusion coefficient is calculated as followed:^27,28D = (R²T²)/(2A²n⁴F⁴C²σ²)1Herein, R, T, A, n, F, and C are attributed to the gas constant, absolute temperature, area of electrode, electrons number, Faraday constant, and concentration of Li⁺, respectively. Furthermore, σ represents the Warburg impedance coefficient, which is associated with Z′:Z′= R_s + R_ct + σω^−1/22Where the R_s and ω are ohmic resistance and angular frequency, respectively.²³ The R_ct value of ZnMn₂O₄ (8.61 Ω) is lower than that of ZnO (66.69 Ω) and MnO (12.33 Ω) and the D value is higher than that of others (D_ZMO = 2.87 × 10⁻¹², D_ZnO = 8.07 × 10⁻¹³, D_MnO = 1.28 × 10⁻¹²), which is consistent with the analysis of the cycling and rate performance. This enhanced inherent kinetics can be assigned to the shortened distance for Li-ion diffusion, resulting from the characteristic of nanosized particles. While the ternary oxide can facilitate electron transport in contrast to the individual oxide induced by the synergistic effect.Open in a separate windowFig. 4(a) The EIS spectra of all samples and the equivalent electrical circuit (inset), (b) the liner fitting of Z′ vs. ω^−1/2 in the low-frequency and the value of σ (inset).In summary, nano-particles ZnMn₂O₄ with high yields were synthesized by a facile solution reaction route without template at room temperature. Unlike other materials for amorphous state, the as-prepared ZnMn₂O₄ exhibits strong crystalline with a nanostructure morphology. Electrochemical measurements demonstrate that the as-prepared ZnMn₂O₄ exhibits remarkable reversible capacity and cycling stability, superior to those of pristine materials. The facile strategy opens up a new approach to fabricate ternary or multiphase transition metal oxides at room temperature, which can promote the development and keep highly promising to scale-up of transition metal oxides as anodes for LIBs.     

Thermal stability,electrochemical and structural characterization of hydrothermally synthesised cobalt ferrite (CoFe2O4)

Monophasic nano-crystalline CoFe₂O₄ (CFO) nanoparticles of high purity have been synthesised through a low temperature hydrothermal route, which does not involve hazardous chemicals, or conditions. The easy, green procedure involves a hydrothermal treatment at 135 °C of an aqueous suspension of the oxalate salts of the precursors. No further purification or annealing procedure was necessary to obtain the crystalline nano-structured oxide. The nanoparticles were characterized structurally and chemically by powder X-ray diffraction (PXRD), Inductively Coupled Plasma Spectrometry (ICP-MS) and Scanning Electron Microscopy (SEM), thus confirming the successful synthesis of the CoFe₂O₄ particles with the expected crystal phase and stoichiometry and an almost complete inverse spinel structure. From the nanoparticles pellets were pressed to investigate the electronic conduction properties using electrochemical impedance spectroscopy (EIS). At low temperatures, the conductivity measurements reveal a semiconducting behavior originating from hopping between Co sites and a total conductivity dominated by the grain boundary contribution. At higher temperatures (T > 400 °C) a metallic–insulator transition occurs, which is attributed to additional hopping of electrons between the Fe sites.

Monophasic nano-crystalline CoFe₂O₄ (CFO) nanoparticles of high purity have been synthesised through a low temperature hydrothermal route, which does not involve hazardous chemicals, or conditions.     

Development of an Fe3O4@Cu silicate based sensing platform for the electrochemical sensing of dopamine

Abnormal levels of dopamine (DA) in body fluids is an indication of serious health issues, hence development of highly sensitive platforms for the precise detection of DA is highly essential. Herein, we demonstrate an Fe₃O₄@Cu silicate based electrochemical sensing platform for the detection of DA. Morphology and BET analysis shows the formation of ∼320 nm sized sea urchin-like Fe₃O₄@Cu silicate core–shell nanostructures with a 174.5 m² g⁻¹ surface area. Compared to Fe₃O₄ and Fe₃O₄@SiO₂, the Fe₃O₄@Cu silicate urchins delivered enhanced performance towards the electrochemical sensing of DA in neutral pH. The Fe₃O₄@Cu silicate sensor has a 1.37 μA μM⁻¹ cm⁻² sensitivity, 100–700 μM linear range and 3.2 μM limit of detection (LOD). In addition, the proposed Fe₃O₄@Cu silicate DA sensor also has good stability, selectivity, reproducibility and repeatability. The presence of Cu in Fe₃O₄@Cu silicate and the negatively charged surface of the Cu silicate shell play a vital role in achieving high selectivity and sensitivity during DA sensing. The current investigation not only represents the development of a highly selective DA sensor but also directs towards the possibility for the fabrication of other Cu silicate based core–shell nanostructures for the precise detection of DA.

Abnormal levels of dopamine (DA) in body fluids is an indication of serious health issues, hence development of highly sensitive platforms for the precise detection of DA is highly essential.     

Magnetic CoFe2O4 nanoparticles supported on graphene oxide (CoFe2O4/GO) with high catalytic activity for peroxymonosulfate activation and degradation of rhodamine B

Herein, we report the preparation of magnetic CoFe₂O₄ nanoparticles and CoFe₂O₄/graphene oxide (GO) hybrids and evaluate their catalytic activity as heterogeneous peroxymonosulfate (PMS) activators for the decomposition of rhodamine B. The surface morphologies and structures of both CoFe₂O₄ nanoparticles and CoFe₂O₄/GO hybrids were investigated by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR) and nitrogen adsorption–desorption isotherms. The magnetic properties of the samples were assessed using a SQUID magnetometer at 298 K. Catalytic oxidation experiments demonstrated that CoFe₂O₄/GO hybrids exhibited much better catalytic activity than CoFe₂O₄ nanoparticles or CoFe₂O₄/reduced graphene oxide (rGO) hybrids, suggesting that GO plays an important role in CoFe₂O₄/GO hybrids in the decomposition of rhodamine B. The influence of various reaction conditions such as temperature, concentration of PMS, pH and decomposition time of rhodamine B over the CoFe₂O₄/GO catalyst were investigated and optimized. The rhodamine B degradation process was found to fit a pseudo-first order kinetics model. The catalyst could be easily separated from the reaction mixture by applying an external magnet. In particular, the as-prepared CoFe₂O₄/GO hybrid exhibited good reusability and stability in successive degradation experiments in PMS solution.

Herein, we report the preparation of magnetic CoFe₂O₄ nanoparticles and CoFe₂O₄/graphene oxide (GO) hybrids and evaluate their catalytic activity as heterogeneous peroxymonosulfate (PMS) activators for the decomposition of rhodamine B.     

A facile heterogeneous system for persulfate activation by CuFe2O4 under LED light irradiation

In this study, the removal performance for rhodamine B (RB) by persulfate (PS) activated by the CuFe₂O₄ catalyst in a heterogeneous catalytic system under LED light irradiation was investigated. The effect of vital experimental factors, including initial solution pH, CuFe₂O₄ dosage, PS concentration, co-existing anion and initial RB concentration on the removal of RB was systematically studied. The removal of RB was in accordance with the pseudo first-order reaction kinetics. Over 96% of 20 mg L⁻¹ RB was removed in 60 min using 0.5 g L⁻¹ CuFe₂O₄ catalyst and 0.2 mM PS at neutral pH. In addition, free radical quenching experiments and electron spin resonance (EPR) experiments were performed, which demonstrated the dominant role of sulfate radical, photogenerated holes and superoxide radical in the CuFe₂O₄/PS/LED system. The morphology and physicochemical properties of the catalyst were characterized by XRD, SEM-EDS, TEM, N₂ adsorption–desorption isotherm, UV-vis DRS, and XPS measurements. Moreover, 18.23% and 38.79% total organic carbon (TOC) removal efficiency was reached in 30 min and 60 min, respectively. The catalyst revealed good performance during the reusability experiments with limited iron and copper leaching. Eventually, the major intermediates in the reaction were detected by GC/MS, and the possible photocatalytic pathway for the degradation of RB in the CuFe₂O₄/PS/LED system was proposed. The results suggest that the CuFe₂O₄/PS/LED system has good application for further wastewater treatment.

In this study, the removal performance for rhodamine B (RB) by persulfate (PS) activated by the CuFe₂O₄ catalyst in a heterogeneous catalytic system under LED light irradiation was investigated.     

Pulsed laser deposited CoFe2O4 thin films as supercapacitor electrodes

The influence of the substrate temperature on pulsed laser deposited (PLD) CoFe₂O₄ thin films for supercapacitor electrodes was thoroughly investigated. X-ray diffractometry and Raman spectroscopic analyses confirmed the formation of CoFe₂O₄ phase for films deposited at a substrate temperature of 450 °C. Topography and surface smoothness was measured using atomic force microscopy. We observed that the films deposited at room temperature showed improved electrochemical performance and supercapacitive properties compared to those of films deposited at 450 °C. Specific capacitances of about 777.4 F g⁻¹ and 258.5 F g⁻¹ were obtained for electrodes deposited at RT and 450 °C, respectively, at 0.5 mA cm⁻² current density. The CoFe₂O₄ films deposited at room temperature exhibited an excellent power density (3277 W kg⁻¹) and energy density (17 W h kg⁻¹). Using electrochemical impedance spectroscopy, the series resistance and charge transfer resistance were found to be 1.1 Ω and 1.5 Ω, respectively. The cyclic stability was increased up to 125% after 1500 cycles due to the increasing electroactive surface of CoFe₂O₄ along with the fast electron and ion transport at the surface.

Cobalt ferrite thin films were grown by PLD at different temperatures as an electrode material for supercapacitors. The films deposited at room temperature exhibited the best power density (3277 W kg⁻¹) and energy density (17 W h kg⁻¹) values.     

A hydrothermal synthesis of Ru-doped LiMn1.5Ni0.5O4 cathode materials for enhanced electrochemical performance

An Ru-doped spinel-structured LiNi_0.5Mn_1.5O₄ (LNMO) cathode has been prepared via a simple hydrothermal synthesis method. The as-prepared cathode is characterized via Fourier transform infrared (FTIR) spectroscopy, powder X-ray diffraction (XRD), scanning electron microscopy (SEM), laser particle size distribution analysis, X-ray photoelectron spectroscopy (XPS) and electrochemistry performance tests. The FTIR spectroscopy and XRD analyses show that the Ru-doped LNMO has a good crystallinity with a disordered Fd$\bar{3}$m space group structure. The disordered structure in the cathode increased and the Li_xNi_1−xO impurity phase decreased when Ru addition increased. SEM shows that all samples are octahedral particles with homogeneous sizes distribution, and the particle size analysis shows that the Ru-doped samples have smaller particle size. XPS confirms the existence of Ru ions in the sample, and reveals that the Ru induce to part of Mn⁴⁺ transfers to Mn³⁺ in the LNMO. The electrochemical property indicated that the Ru-doped cathode exhibits better electrochemical properties in terms of discharge capacity, cycle stability and rate performance. At a current density of 50 mA g⁻¹, the discharge specific capacity of the Ru-4 sample is 140 mA h g⁻¹, which is much higher than that of the other samples. It can be seen from the rate capacity curves that the Ru-doped samples exhibit high discharge specific capacity, particularly at high current density.

An Ru-doped spinel-structured LiNi_0.5Mn_1.5O₄ (LNMO) cathode has been prepared via a simple hydrothermal synthesis method.     

N-doped graphene/CoFe2O4 catalysts for the selective catalytic reduction of NOx by NH3

In this paper, CoFe₂O₄/graphene catalysts and N-doped graphene/CoFe₂O₄ (CoFe₂O₄/graphene-_N) catalysts were prepared using the hydrothermal crystallization method for the selective catalytic reduction of NO_x by NH₃. The results of the test showed that CoFe₂O₄/graphene catalysts exhibited the best denitrification activity when the loading was at 4% and the conversion rate of NO_x reached 99% at 250–300 °C. CoFe₂O₄/graphene-_N catalysts presented a better denitrification activity at low temperature than CoFe₂O₄/graphene catalysts, and the conversion rate of NO_x reached more than 95% at 200–300 °C. The intrinsic mechanism of CoFe₂O₄/graphene-_N catalysts in promoting SCR activity was preliminarily explored. The physicochemical properties of the samples were characterized using XRD, TEM, N₂ adsorption, XPS, NH₃-TPD, and H₂-TPR. The results indicated that nitrogen doping can improve the dispersion of CoFe₂O₄, and it also increased the acidic sites and the redox performance conducive to improving the denitrification activity of the catalysts. In addition, CoFe₂O₄/graphene-_N catalysts demonstrated a better resistance to water and sulfur than CoFe₂O₄/graphene catalysts.

N-doped graphene/CoFe₂O₄ presented better denitrification activity than CoFe₂O₄/graphene due to the more uniform distribution of CoFe₂O₄ and acidic sites etc.     

A facile one-pot green synthesis of gold nanoparticle-graphene-PEDOT:PSS nanocomposite for selective electrochemical detection of dopamine

A facile one-pot and green method was developed to prepare a nanocomposite of gold nanoparticle (AuNP), graphene (GP) and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). Graphene was first electro-exfoliated in a polystyrene sulfonate solution, followed by a one-step simultaneous in situ formation of gold nanoparticle and PEDOT. The as-synthesized aqueous dispersion of AuNP-GP-PEDOT:PSS was thereafter used to modify the glassy carbon electrode (GCE). For the first time, the quaternary composite between AuNP, GP, PEDOT and PSS was used for selective determination of dopamine (DA) and uric acid (UA) in the presence of ascorbic acid (AA). In comparison to a bare GCE, the nanocomposite electrode shows considerably higher electrocatalytic activities toward the oxidation of DA and UA due to a synergistic effect between AuNP, GP, PEDOT and PSS. Using differential pulse voltammetry (DPV), selective determination of DA and UA in the presence of AA could be achieved with a peak potential separation of 110 mV between DA and UA. The sensor exhibits wide linear responses for DA and UA in the ranges of 1 nM to 300 μM and 10 μM to 1 mM with detection limits (S/N = 3) of 100 pM and 10 μM, respectively. Furthermore, the proposed sensor was also successfully used to determine DA in a real pharmaceutical injection sample as well as DA and UA in human serum with satisfactory recovery results.

A facile one-pot green synthesis of gold nanoparticle-graphene-PEDOT:PSS nanocomposite was successfully demonstrated.     

A facile synthesis of molecularly imprinted polymers and their properties as electrochemical sensors for ethyl carbamate analysis

New molecularly imprinted polymers (MIPs), which exhibit specific recognition of ethyl carbamate (EC) have been synthesized and studied. In this process, EC was the template molecule and β-cyclodextrin derivatives were employed as functional monomers in the molecular imprinting technique (MIT). An EC molecularly imprinted sensor (EC-MIS) was prepared by using MIT surface modification. The EC-MIS was characterized by cyclic voltammetry, electrochemical impedance spectroscopy and differential pulse voltammetry. EC detection performance, binding parameters and dynamics mechanism were investigated. The result showed that the synthetic route designed was appropriate and that new MIP and EC-MIS were successfully prepared. The EC-MIS exhibited a good molecular recognition of EC. A linear relationship between current and EC concentration was observed using cyclic voltammetry and the detection limit was 5.86 μg L⁻¹. The binding constant (K = 4.75 × 10⁶ L mol⁻¹) between EC and the EC-MIS, as well as, the number of binding sites (n = 1.48) has been determined. The EC-MIS recognition mechanism for the EC is a two-step process. The sensor was applied for the determination of EC in Chinese yellow wines, and the results were in good agreement with the gas chromatography-mass spectrometry (GC-MS) method.

An ethyl carbamate (EC) molecularly imprinted sensor (EC-MIS) has been prepared. The molecular recognition properties of EC were investigated, the binding parameters determined, and the dynamic mechanism of EC-MIS recognizing EC explored.     

A nickel nanoparticle engineered CoFe2O4@GO–Kryptofix 22 composite: a green and retrievable catalytic system for the synthesis of 1,4-benzodiazepines in water

A composite of Ni nanoparticles incorporated in Kryptofix 22 conjugated magnetic nano-graphene oxide, CoFe₂O₄@GO–K 22·Ni, was synthesized via the grafting of Kryptofix 22 moieties on the magnetic nano-graphene oxide surface, followed by reaction of the nanocomposite with nickel nitrate. The Kryptofix 22 host material unit cavities can stabilize the Ni nanoparticles effectively and prevent their aggregation and separation from the surface. Characterization of the catalysts by FT-IR, FE-SEM, TGA, ICP, EDX, XRD, VSM and BET aided understanding the catalyst structure and morphology. This catalyst was efficiently applied for the synthesis of 1,4-benzodiazepine derivatives. The main advantages of the method are mild reaction conditions, inexpensive catalyst, it is environmentally benign, has high to excellent yields and shorter reaction times. This organometallic catalyst can be easily separated from a reaction mixture and was successfully examined for six runs with a slight loss of catalytic activity.

In this study, a competent and efficient methodology for the synthesis of benzodiazepine over magnetically retrievable novel CoFe₂O₄@GO–K 22 anchored Ni is reported.     

Poly(pyrrole-co-o-toluidine) wrapped CoFe2O4/R(GO–OXSWCNTs) ternary composite material for Ga3+ sensing ability

In this study, we report a novel ternary conductive hybrid material with high stability, conductivity, and excellent electrochemical Ga³⁺ sensing ability. Ternary poly(pyrrole-co-o-toluidine)/CoFe₂O₄/reduced graphene oxide–oxidized single-wall carbon nanotube nanocomposites in the form of P(Py-co-OT)/CF/R(GO–OXSWCNTs) NCs have been synthesized through an in situ chemical polymerization method via a facile three-step approach. Single phase CoFe₂O₄ (CF) nanoparticles (NPs) were synthesized using an egg white method, while reduced graphene oxide–oxidized single-wall carbon nanotubes R(GO–OXSWCNTs) were prepared via co-reduction of graphene oxide along with oxidized SWCNTs flowed by coating CF and R(GO–OXSWCNTs) with a poly(pyrrole-co-o-toluidine) matrix P(Py-co-OT) copolymer. The results of X-ray diffraction spectroscopy (XRD), Fourier-transform infrared spectroscopy (FTIR) and Raman indicated that the P(Py-co-OT)/CF/R(GO–OXSWCNTs) NCs were effectively synthesized with strong interactions among the constituents. The thermal stability of P(Py-co-OT)/CF/R(GO–OXSWCNTs) NCs is considerably enhanced in the composite format. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM) demonstrated that CF and R(GO–OXSWCNTs) were well coated by P(Py-co-OT). The electrical conductivity study showed that P(Py-co-OT) and R(GO–OXSWCNTs) might significantly improve the conductivity and the electrochemical performance of the CF. A Ga³⁺ ion selective electrochemical sensor was fabricated by coating a glassy carbon electrode (GCE) with synthesized P(Py-co-OT)/CF/R(GO–OXSWCNTs) NCs by using 5% Nafion binder. The slope of the calibration curve was used to calculate the sensor''s analytical parameters, such as sensitivity (13.0569 μA μM⁻¹ cm⁻²), detection limit (96.27 ± 4.81 pM), quantification limit (43.523 pM), response time, reproducibility, large linear dynamic range, and linearity. The validation of the P(Py-co-OT)/CF/R(GO–OXSWCNTs) NCs/GCE sensor probe was investigated by a standard addition method (recovery) in the presence of various environmental samples and satisfying results were obtained.

A ternary P(Py-co-OT)/CF/R(GO–OXSWCNTs) nanocomposite has been fabricated as a novel conductive hybrid material with high stability and excellent electrochemical Ga³⁺ sensing ability.     

Enhancing the thermoelectric power factor of nanostructured ZnCo2O4 by Bi substitution

Bi_xZnCo_2−xO₄ (0 ≤ x ≤ 0.2) nanoparticles with different x values have been prepared by the sol–gel method; the structural, morphological, thermal and thermoelectric properties of the prepared nanomaterials are investigated. XRD analysis confirms that Bi is completely dissolved in the ZnCo₂O₄ lattice till the x values of ≤0.1 and the secondary phase of Bi₂O₃ is formed at higher x value (x > 0.1). The synthesized nanomaterials are densified and the thermoelectric properties are studied as a function of temperature. The electrical resistivity of the Bi_xZnCo_2−xO₄ decreased with x value and it fell to 4 × 10⁻² Ω m for the sample with x value ≤ 0.1. The Seebeck coefficient value increased with the increase of Bi substitution till the x value of 0.1 and decreased for the sample with higher Bi content (x ≤ 0.2) as the resistivity of the sample increased due to secondary phase formation. With the optimum Seebeck coefficient and electrical resistivity, Bi_0.1ZnCo_1.9O₄ shows the high-power factor (α²σ_{550 K}) of 2.3 μW K⁻² m⁻¹ and figure of merit of 9.5 × 10⁻⁴ at 668 K respectively, compared with other samples. The experimental results reveal that Bi substitution at the Co site is a promising approach to improve the thermoelectric properties of ZnCo₂O₄.

Nanostructuring and Bi substitution have considerably increased the thermoelectric power factor and ZT of Bi_xZnCo_2−xO₄; Bi_1.9ZnCo_1.9O₄ shows a higher power factor than that of other Bi substituted samples.     

CoFe2O4 nanoparticles decorated MoS2-reduced graphene oxide nanocomposite for improved microwave absorption and shielding performance

Magnetic CoFe₂O₄ nanoparticles decorated onto the surface of a MoS₂-reduced graphene oxide (MoS₂-rGO/CoFe₂O₄) nanocomposite were synthesized by a simple two-step hydrothermal method. The electromagnetic (EM) wave absorption performance and electromagnetic interference (EMI) shielding effectiveness of the materials were examined in the frequency range of 8.0–12.0 GHz (X-band). The MoS₂-rGO/CoFe₂O₄ nanocomposite was characterized by various tools such as X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy. High-resolution transmission electron microscopy results confirmed the decoration of magnetic nanoparticles onto the surface of the MoS₂-rGO nanocomposite with a diameter of 8–12 nm. The multiple interfacial polarization, moderate impedance matching, and defect dipole polarization improve the dielectric and magnetic loss of the materials, which leads to strong attenuation loss ability of incident EM energy within the shield. The pure MoS₂-rGO nanocomposite represents total shielding effectiveness (SE_T ∼16.52 dB), while the MoS₂-rGO/CoFe₂O₄ nanocomposite exhibits total shielding effectiveness (SE_T ∼19.26 dB) over the entire frequency range. It may be explained that the magnetic nanoparticles (CoFe₂O₄) serve as excellent conductive and magnetic fillers with a large surface area, leading to the migration of charge carriers at multi-interfaces.

Magnetic CoFe₂O₄ nanoparticles decorated onto the surface of a MoS₂-reduced graphene oxide (MoS₂-rGO/CoFe₂O₄) nanocomposite were synthesized by a simple two-step hydrothermal method.