Citric Acid Assisted Sol Gel Research Articles

Cobalt-free spinel–layered structurally integrated Li0.8Mn0.64Ni0.183Fe0.091O2 cathodes for lithium-ion batteries

Cobalt-free, Li-rich layered-spinel structurally integrated positive electrode (cathode) materials for lithium-ion batteries (LIBs), with nominal composition 0.6(Li1.2Mn0.56Fe0.08Ni0.16O2)∙0.4(LiFe0.2Mn1.4Ni0.4O4) which can otherwise be written as Li0.8Mn0.64Ni0.183Fe0.091O2 (FeSL) are synthesized via simple citric acid-assisted sol-gel route. Four different final annealing temperatures (550 °C, 650 °C, 750 °C, and 850 °C) were chosen to investigate their influence on electrochemical performance. The obtained composite materials contain spinel (space group Fd3¯m) as well as Li-rich layered phases (space group C2/m) as revealed by X-ray diffraction (XRD), HRTEM and cyclic voltammetry investigations. The excellent electrochemical performance of the composite material could be attributed to the structural stability achieved by the integration of spinel and layered components. Selected synthesized composite materials exhibit high discharge capacities close to 200 mAh g‐1. The FeSL750 shows better capacity retention (92(3)% at the 50th cycle and 82(5)% at the 70th cycle) and rate capability than the FeSL550 and FeSL650 samples. However, the FeSL850 sample shows different charge-discharge behaviour. The charge capacity corresponding to FeSL850 is found to increase up to the 20th cycle, then stabilizes and displays a capacity retention of almost 100 % at the 50th cycle and 91(1)% at the 70th cycle. The electrochemical mechanism of FeSL750 was elucidated via in operando XAS investigations, which reveal electrochemical activity from all transition metals.

A simple sol–gel method for the synthesis of Pt co-catalyzed spinel-type CuFe2O4 for hydrogen production; the role of crystallinity and band gap energy

In this investigation, spinel-type copper ferrites (CuFe2O4) at different calcination temperatures (600–900 °C) were produced using a facile citric acid-assisted sol–gel method. The crystal structure, morphology, optical, and electrical properties of the as synthesized photocatalysts were comprehensively characterized. XRD and UV–vis diffuse reflectance spectra (UV–vis (DRS)) showed that the phase structure and the band gap energy of the copper ferrite are strongly correlated to the applied calcination temperature. TEM results investigated that the copper ferrites calcined at higher temperatures presented large particle sizes and crystallized very well. In addition, the photocatalytic activity was tested for hydrogen production in the presence of methanol, with and without Pt nanoparticles as a cocatalyst. The results indicated that the CuFe2O4 annealed at 900 °C (CuF-900) has higher hydrogen production activity than photocatalysts calcined at lower temperatures in the presence and absence of Pt, which is mainly assigned to the higher crystallinity, and narrower bandgap energy. Moreover, this study explained in more detail the role of Pt nanoparticles in enhancing the photocatalytic activity of CuF-900. This work introduces a new direction of thinking for the tetragonal phase structure as an efficient photocatalyst for hydrogen production.

Optical, magnetic and spin resonance studies of MFe2O4 (M = Mn, Co, Zn) ferrites

MFe2O4 (M = Mn, Co, Zn) ferrites have been prepared using citric acid assisted sol–gel autocombustion technique. Structural, microstructural, optical, magnetic and spin resonance studies have been carried out using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Field emission scanning electron microscope (FESEM), UV–Visible spectroscopy, Vibrating sample magnetometer (VSM) and Electron spin resonance spectroscopy (ESR) respectively. The crystallite size determined using Scherrer method lies between 12 nm and 21 nm. From the UV–Visible absorption spectra the optical band gap energy, refractive index and optical dielectric constant have been estimated. The magnetic parameters have been obtained from the room temperature M−H loop of the ferrite samples. ESR spectra obtained at X-band microwave energy have been fitted using Lorentzian function to determine the spin resonance parameters like gyromagnetic ratio, peak to peak linewidth and resonance field. CoFe2O4 has the largest linewidth of 4874 Oe. The observed broad ESR linewidth of the investigated samples may find suitability as radar absorbing materials for microwave applications.

Study of structural and optical properties of Barium Orthosilicate nanopowder

Alkaline earth metal-silicate nanopowder are preferred due to their enhanced structural and optical properties than traditional sulphides having advantages such as easy synthesis, low sintering temperature, better durability and high chemical stability. In present work, Barium orthosilicate (Ba2SiO4) nanopowder has been synthesized by citric acid assisted sol–gel technique using nitrates of metal salt and tetraethyl orthosilicate (TEOS) as a precursor. To study the properties of such phosphor various characterization techniques have been used like XRD, FTIR, UV–Vis, and PL, etc. The powder XRD crystallographic analysis confirms the purity of the phase with crystalline and homogeneous nature of the sample. The famous Debye-Scherrer equation has been used to evaluate the average crystallite size. The structure of the sample has been designated orthorhombic phase with Pmcn space group. The FTIR spectroscopy set forth the molecular fingerprints of the constituents functional group present in synthesized powder. Tauc’s plot derived from UV visible spectroscopy data gives the energy gap nearly 5.89 eV. The prominent photoluminescence (PL) emission peak appears at 466 nm using the excitation wavelength of the NUV region (295 nm). Thus, these outcomes convey the highly crystalline Ba2SiO4 nanopowder with sufficient blue luminescence without any rare earth dopant that would be potentially applicable in display devices.

Oxygen vacancies engineering of Fe doped LaCoO3 perovskite catalysts for efficient H2S selective oxidation

Fe-doped LaCoO3 perovskite catalysts synthesized via a facile citric acid-assisted sol-gel route have been applied for the H2S selective oxidation. The as-synthesized LaFexCo1−xO3 perovskites exhibit a randomly macroporous structure formed by the accumulation of nanoparticles. Characterization studies reveal that appropriate Fe substitution effectively enhances the formation of oxygen vacancies, and thus improves the lattice oxygen mobility and H2S adsorbability, respectively. The obtained LaFe0.4Co0.6O3 exhibited remarkable catalytic activity, with the H2S conversion and sulfur yield of 100% at 190 °C. A combined approach of in-situ FT-IR and in-situ Raman experiments disclosed that the plausible reaction pathway of H2S selective oxidation over LaFexCo1−xO3 perovskites follows the Mars-van Krevelen mechanism. The density functional theory calculations revealed that Fe-doping improves the formation of oxygen vacancy and enhances the adsorption of H2S. This study offers new insights for the design of highly efficient perovskites for the selective oxidation of H2S.

Quaternary bioactive glass-derived powders presenting submicrometric particles and antimicrobial activity

The biomedical engineering advances in the last years have been rising demand for multifunctional biomaterials. Bioactive glass (BG) submicron particles are potential candidates for the formulation of composites with improved dispersion and homogeneity between the constituents. This work presents the preparation of SiO2–Na2O–CaO–P2O5 glass-derived powders composed of particles with homogenous shapes and sizes between 300 and 500 nm. Two types of synthesis were employed for the preparation of the BG powders, the melt-quenching method, and a citric acid-assisted sol-gel route at a low citric acid concentration (0.005 mol L−1). The morphology of the particles was achieved by a low-energy process using a ball mill. These powders were characterized for their structure and surface area and evaluated for in vitro mineralization and antibacterial behavior. X-ray diffraction (XRD) analysis revealed different crystalline silicate phases in the sol-gel-derived powder and confirmed the amorphous structure of the melt-quenching-derived one. The surface of the particles was covered by hydroxycarbonate-apatite (HCA) after five days in simulated body fluid (SBF). The antibacterial activity against Staphylococcus aureus was higher for the sol-gel-derived powder, showing inhibition >99% of the bacteria growth in 24 h for all concentrations studied. These BG-based powders present a set of characteristics useful for the formulation of multifunctional composites for orthopedic applications.

Co-Doped LaFeO3 Gas Sensor for Fast Low-Power Acetone Detection

Co-doped LaFe1−xCoxO3 (x = 0, 0.1, 0.2, 0.4, 0.6, 0.9) nanomaterial films were prepared via a citric acidassisted sol–gel method and characterized by various techniques. Compared with pure LaFeO3, the optimum operating temperature of the LaFe0.9Co0.1O3 gas sensor dropped to 110 °C from 140 °C. The response value to 100 ppm acetone was 35.89, with a fast response–recovery time of 14/9 s. The enhanced gas sensitivity may be attributed to the higher number of surface oxygen vacancies and larger specific surface area of LaFe0.9Co0.1O3. Doping with Co ions improved the moisture resistance of the LaFeO3 sensor at 18%–54% relative humidity. Appropriate doping of Co3+ into LaFeO3 nanofibers is a promising method for designing and fabricating an acetone sensor with high gas-sensing performance.

Hierarchically interconnected porous MnxCo3-xO4 spinels for Low-temperature catalytic reduction of NO by CO

Developing efficient catalysts for low reaction-temperature catalytic reduction of NO by CO (CO–DeNOx) is desirable but very challenging. Here, we report that hierarchically interconnected porous (HIP) MnxCo3-xO4 spinels, synthesized by a facile citric acid-assisted sol–gel method, can act as highly efficient catalysts for CO–DeNOx. The obtained Mn0.3Co2.7O4 displayed a much-enhanced catalytic performance with 87% NO removal at 100 °C and a wide active-temperature window (100–400 °C). Its superior activity stems from the following reasons: (i) the high-valence of Mn3+, Mn4+, and Co3+ species in the Co–O–Mn structure enables the high catalytic activity and the effective redox-cycling of the Co3+ and Co2+ adsorption sites; (ii) the HIP structure can significantly enhance gas diffusion. This work offers an avenue to design metal oxide catalysts for CO–DeNOx by effectively controlling the doping metal concentration and oxidation states of the active components.

Improvement of structural and electrochemical properties of NMC layered cathode material by combined doping and coating

Prospective cathode materials, pristine and Mg-/Zr-modified LiNi0.33Mn0.33Co0.33O2 (NMC333), are successfully synthesized through a synthetic method including a citric acid-assisted sol-gel processing followed by drying and calcination at different temperatures. The combined results from structural, chemical, and morphological investigations reveal that the modified NMC, where the modification is obtained by doping and a uniform coating layer with an optimal thickness, show a better-organized layered structure and improved interfacial properties compared to the pristine NMC. The modified NMC material shows a remarkable improvement in terms of capacity retention, especially when setting the charge cut-off voltage at 4.2 V, which results in improved cycling stability after more than 200 cycles with capacity retention of around 95%. In order to shed light on the kinetics of redox processes, and its impact on charge/discharge behavior, electrochemical impedance spectroscopy is carried out at different states of charge during oxidation and reduction for both pristine and modified NMC-based electrodes. The promising results obtained with this synthesis open possibilities for performance improvements of intercalating layered oxides by structure doping and surface enhancement.

Effect of microcosmic regulation of unit cells bonding on microwave absorption properties of perovskite structure GdFeO3

GdFe1-xNixO3 material with perovskite structure was prepared using organic monohydrated citric acid-assisted sol–gel self-propagating combustion method to obtain high-efficiency microwave absorption material with thin thickness and wide effective absorption band. The influence of the introduction of Ni on dielectric properties and permeability was further studied. Experimental results show that with the introduction of 10% Ni, the lowest reflection absorption loss value of -49.5 dB is obtained, but the absorption bandwidth is only 0.89 GHz. When Ni is 15%, the lowest reflection absorption loss value of -38.8 dB is obtained, and the effective bandwidth (< -10 dB) is 5.32 GHz. The structure of GdFe1-xNixO3 perovskite was analyzed to explore structural changes in the process of Ni replacing Fe. The influence of strong interaction force, residual magnetization, and natural resonance of ceramic nanoparticles on microwave absorption performance was explored. This work provides a reliable candidate material for the synthesis of electromagnetic attenuation ceramic materials with excellent properties.

Mn-Ni-Co-O Spinel Oxides towards Oxygen Reduction Reaction in Alkaline Medium: Mn0.5Ni0.5Co2O4/C Synergism and Cooperation

Mn-doped spinel oxides MnxNi1−xCo2O4 (x = 0, 0.3, 0.5, 0.7, and 1) were synthesized using the citric acid-assisted sol–gel method. The Mn0.5Ni0.5Co2O4 (x = 0.5) supported on carbon nanosheets, Mn0.5Ni0.5Co2O4/C, was also prepared using the same method employing NaCl and glucose as a template and carbon source, respectively, followed by pyrolysis under an inert atmosphere. The electrocatalytic oxygen reduction reaction (ORR) activity was performed in alkaline media. Cyclic voltammetry (CV) was used to investigate the oxygen reduction performance of MnxNi1−xCo2O4 (x = 0, 0.3, 0.5, 0.7, and 1), and Mn0.5Ni0.5Co2O4 was found to be the best-performing electrocatalyst. Upon supporting the Mn0.5Ni0.5Co2O4 on a carbon sheet, the electrocatalytic activity was significantly enhanced owing to its large surface area and the improved charge transfer brought about by the carbon support. Rotating disk electrode studies show that the ORR electrocatalytic activity of Mn0.5Ni0.5Co2O4/C proceeds via a four-electron pathway. Mn0.5Ni0.5Co2O4/C was found to possess E1/2(V) = 0.856, a current density of 5.54 mA cm−2, and a current loss of approximately 0.11% after 405 voltammetric scan cycles. This study suggests that the interesting electrocatalytic performance of multimetallic transition metal oxides can be further enhanced by supporting them on conductive carbon materials, which improve charge transfer and provide a more active surface area.

Comprehensive study of structural, physical, and spectroscopic properties of Co-Ni substituted BaMg2Fe16O27 W-type hexaferrites

Cobalt and nickel substituted BaMg2Fe16O27 W-type hexaferrite nanoparticles were synthesized via citric acid-assisted sol–gel auto-combustion technique. XRD results revealed the single-phase W-type hexagonal structure of these nanomaterials. The decrease in lattice parameters and unit cell volume is accredited to smaller ionic radii of substituted ions as compared to that of host ions. The variations in Raman spectra as a function of substitution content (x, y) are ascribed to the induction of strain in the unit cell. The PL spectra of synthesized samples revealed all emissions invisible (red) regions with PL intensity near 661 nm. Furthermore, the semiconducting nature of synthesized materials has been assessed based on bandgap energy Eg = 1.875 eV. FTIR investigations endorsed the successful settlement of substituents at tetrahedral and octahedral sites. SEM images exhibit non-homogenous hexagonal platelet-shaped particles. The research outcomes suggest the promising applications of synthesized compounds with suitable cationic substitution for microwave devices.

Improving the Electrochemical Performance of Na3V2(PO4)3 Cathode in Na-Ion Batteries by Si-Doping

High-performance cathode materials are highly desirable for the realization of commercial Na-ion batteries. Sodium super ion conductor (NASICON)-type Na3V2(PO4)3 is one of the most promising cathode materials. In the present study, an improvement in the kinetics of the redox reactions and consequently, the electrochemical performance of Na3V2(PO4)3 cathode have been demonstrated by the partial substitution of Si for P. Nano-sized powders of Na3+xV2(PO4)3–x(SiO4)x (x = 0, 0.1, 0.2 and 0.4) are synthesized using the citric acid-assisted sol-gel route. A thin carbon layer of ∼8–16 nm thickness is formed on the calcined particles of the active material. The vanadium present in Na3V2(PO4)3 is shown to retain the trivalent state after the Si substitution. Coin cells are fabricated using an organic electrolyte, carbon nanotubes decorated Na3+xV2(PO4)3–x(SiO4)x as a cathode, and Na as an anode. Cyclic voltammetry profiles acquired on the half-cells confirmed (1) improved reversibility of the redox reactions, (2) lower polarization losses, and (3) superior sodium-ion diffusivity in the Si-substituted compounds. As a result, cells with these electrodes exhibit an enhanced electrochemical performance with a high specific capacity, excellent rate capability, and stable cycling performance. The electroactive material with Na3.1V2(PO4)2.9(SiO4)0.1 composition exhibits ∼21% higher specific capacity than that with Na3V2(PO4)3 at 5C charging/discharging rates, furnishing a Coulombic efficiency of >99%. The expansion of the crystal volume and the presence of excess Na-ion for ionic conduction are primarily responsible for the observed superior performance. The exceptional capacity of 85 mA h/g at 1C charging/discharging rates and superior cycling life achieved in Na3.1V2(PO4)2.9(SiO4)0.1/carbon nanotubes show that the strategy of expanding the crystal lattice through appropriate doping is an effective way to develop Na3V2(PO4)3 cathode for Na-ion batteries.

Structural, optical and other spectral studies of transition metal Ti4+-doped Zn-Cd oxide nanomaterials

Titanium ([Formula: see text])-doped nanoparticles of the type [Formula: see text] [[Formula: see text], [Formula: see text]] are reported in this study. The samples were synthesized by citric acid assisted sol–gel auto combustion (SGAC) method. The samples are characterized by X-ray diffraction (XRD), Raman, Field emission scanning electron microscopy (FESEM), Energy dispersive analysis of X-rays (EDAX) and Fourier transform infra-red (FTIR) techniques for structural studies. Further, for optical properties, UV-Vis technique has been used. In addition, samples were studied for dielectric properties. Room-temperature XRD data study reveals the sample formation with wurtzite hexagonal structure exhibiting space group [Formula: see text]mc also confirmed from Rietveld refinement of XRD data. Raman spectra displays characteristic active phonon modes in pristine [Formula: see text] and doped [Formula: see text]. UV-Vis diffused reflectance spectroscopy analysis infer bandgap values of 3.14 and 3.12 eV for [Formula: see text] and [Formula: see text], respectively. The dielectric studies confirmed high dielectric constant for [Formula: see text] compared to pristine [Formula: see text]. A non-Debye character with spread of relaxation times was witnessed from impedance study.

An efficient far-red emitting Ba2LaNbO6:Mn4+ nanophosphor for forensic latent fingerprint detection and horticulture lighting applications

Due to their unique optical properties like narrow emission bands, the applicability of fluorescent nanomaterials have recently been extended to various fields such as forensic science and urban farming. Herein, we report a novel fluorescent material of Mn4+ ions doped Ba2LaNbO6 (BLN:Mn4+) with nanorod-like morphology for latent fingerprint detection as well as plant growth light-emitting diodes (LEDs) through a facile citric acid-assisted sol-gel route. The crystallization, elemental composition and oxidation states present in the BLN:Mn4+ nanophosphors are confirmed by the X-ray diffraction and X-ray photoelectron spectroscopy analyses. The photoluminescence spectra reveal a promising red emission band at 685 nm wavelength region under both 351 and 510 nm excitations, which is the required far-red emission for phototropism in plants. Furthermore, the optimized BLN:0.25Mn4+ nanophosphors exhibited all the features required for three levels of latent fingerprint detection on various nonporous surfaces under 365 nm illumination. The fluctuation of the red value on pixel profile confirms the good affinity of BLN:0.25Mn4+ nanophosphors with the fingerprint ridges. Because of their excellent luminescent properties and brilliant performance in the latent fingerprint detection process, the synthesized BLN:Mn4+ nanophosphors can be considered as a promising fluorescent material for the fabrication of plant growth LEDs and latent fingerprint applications.

Structure design and modulation of dual-wavelength sensitive upconversion luminescence in RE2MoO6:Er3+/Yb3+ materials

A series of RE2MoO6: 2%Er3+, 15%Yb3+ (RE = Y, Pr, Nd, Sm, Eu, Gd, Dy, and Lu) nanoparticles were prepared by citric acid-assisted sol–gel method. The upconversion luminescence (UCL) properties of block-shaped Gd2MoO6: 2%Er3+, 15%Yb3+ were the best among the materials obtained. In addition, the UCL properties of the investigated systems by two strategies could be further improved. Modulation of the crystal structure could be achieved by the substitution of rare earth ions in matrix lattice. The UC emission intensity of Gd2MoO6 doped with Y3+ (40%) was enhanced about 2.27 (2.52) more times than that of Y3+-absent samples excited at 980 nm (1550 nm). Secondly, dual wavelength was used as the excitation wavelength simultaneously: The green UCL intensity of RE2MoO6: 2%Er3+, 15%Yb3+ was almost not changed, while the red UCL intensity was enhanced compared to the sum of the red UCL intensities excited by 980 nm and 1550 nm, respectively. The UCL mechanism excited by dual wavelength was investigated in detail, in which the two co-operative excitation processes were presented.

Exploration of Mn incorporated CeO2 nanoflakes with meso- and macropores for the effective simultaneous catalytic oxidation of carbon monoxide and propane

Highly porous ceria nanoflakes are prepared by a facile citric acid assisted sol–gel method. A Mn incorporated solid solution of CeO2 was also efficiently developed without disturbing the nanoflake morphology of pure CeO2. The formation of fluorite CeO2 with crystallite size ranging from 5–8 nm is evidently revealed from XRD analysis. The introduction of Mn to CeO2 resulted in the formation of smaller crystallites of CeO2 with reduced particle size and lattice parameters. SEM images displayed the flake like nature of CeO2 and Mn/CeO2 with meso and macro pores. TEM images confirm the pore size reduction of CeO2 with Mn doping which is already indicated from BET-BJH-surface area and pore-volume analysis. The presence of a large amount of oxygen vacancies in CeO2 upon Mn introduction is indicated from the drastic reduction in the peak intensities in the Raman spectrum. XPS results revealed that Ce exists in its +4 oxidation state whereas Mn exists in its +3 oxidation state, both of which are favorable for oxidation reactions. The catalytic performance of the prepared nanoflakes is investigated in the simultaneous oxidation of CO and C3H8. Mn incorporated CeO2 showed 100% oxidation of CO from 300 °C onwards and 84% hydrocarbon oxidation is observed even at a high gas flow rate of 500 ml/min at 400 °C over 0.5 g of the catalyst.

Effect of pH on material properties of ZnAl2O4:Cr3+ nano particles prepared by sol–gel method

ZnAl2O4 nanoparticles (NPs) doped with Cr3+ (1.0 mol%) were prepared by citric acid assisted sol–gel method at pH values of 3, 5, 7, 9, 11 and 13 with the use of ammonia as pH controller. Analysis of x-ray diffraction (XRD) spectra show that the synthesized ZnAl2O4 NPs are cubic structured and the pH did not affect the structure significantly. The average crystallite size increases with an increase in pH. The photoluminescence (PL) intensity was found to be minimum and maximum for the sample synthesized at pH 3 and pH 13, respectively. The PL emission intensity was found to depend on the synthesis pH conditions. The scanning electron microscope (SEM) revealed that the morphology of ZnAl2O4:Cr3+ NPs changed with the pH values of the solution. Ultraviolet–visible (UV–Vis) measurements show that reflectance is maximum in UV and visible region for sample synthesized under acidic pH of 5. The energy band gap energy was tuned from 3.10 to 3.88 eV with increase in pH value from 3 to 13. It was observed that a strong alkaline nature of the solution enhances the structural and optical properties of ZnAl2O4:Cr3+ nanostructures.

Investigation of the formation process of highly porous α-Al2O3 via citric acid-assisted sol-gel synthesis

Macroporous α-alumina can be obtained by polymer-induced phase separation during the sol-gel process based on dissolved aluminum salts. Polyethylene oxide (PEO) acts as phase separating agent, propylene oxide as proton scavenger. Calcination at 1200 °C yields the thermodynamically stable α-Al2O3. It usually exhibits little to no porosity, with internal surface areas below 5 m2/g. We report on the successful establishment of citric acid as a novel phase separating agent in this process. Macropores ranging from 115 nm up to several μm have been successfully introduced in α-Al2O3. Compared to the PEO-route, BET surface areas doubled, going up to 12 m2/g. The materials prepared via the new method also exhibit a significantly different microstructural habitus.13C-NMR studies and TG/DTA measurements revealed a complete incorporation of citric acid into the gel network. This implies a change in phase separation mechanism, giving rise to a more particulate structure and hence the formation of larger macropores.

Electrochemical Lithium Extraction and Insertion Process of Sol-Gel Synthesized LiMnPO4 via Two-Phase Mechanism

Olivine-type LiMnPO4 was synthesized via a citric-acid assisted sol-gel method. The structural evolution of the as-prepared material during calcination and the crystallization of LiMnPO4 was investigated by XRD measurements. A reaction mechanism including two intermediate phases was derived for the synthesis. Furthermore, the electrochemical Li extraction and insertion processes were studied by in situ/ex situ XRD and MAS NMR measurements. The phase transition from LiMnPO4 to MnPO4 and vice versa proceeds via a two-phase mechanism. The 31P MAS NMR spectra of MnPO4, which forms upon delithiation as confirmed by XRD patterns, shows a very broad signal. Possible explanations for the large width of this signal are given and evaluated.