Citrate Sol Gel Method Research Articles

Magnetic interactions and spin-wave stiffness constant of In-substituted yttrium iron garnets

Abstract Indium-substituted yttrium iron garnet samples, Y3Fe5−xInxO12 (x = 0–0.7, step 0.1), were prepared using a citrate solgel method. Synchrotron X-ray diffraction (SXRD) measurements combined with the Rietveld refinement technique were used to investigate the crystallization, structure parameters and lattice distortion in the samples. Magnetization and Curie temperature of the samples were measured with the SQUID and VSM equipments. The non-magnetic indium ions were found to reside at the octahedral sites, leading to an increase of the total magnetization and a decrease in the Curie temperature. Molecular field coefficients Naa, Ndd and Nad were determined by fitting the experimental thermomagnetization curves in the framework of a two-sublattice ferrimagnetic model. The stiffness constant of the spin waves, D, in these samples was calculated based on the exchange interaction constants, Jij, and its temperature dependence was derived for the sublattice magetizations. The calculated results for D(T) are in good agreement with the experimentally measured data and are significantly large around room temperature.

Bovine serum albumin templated porous CeO2 to support Au catalyst for benzene oxidation

Bovine serum albumin (BSA)-templated CeO2 was prepared by citrate sol-gel method and used as a support to synthesize Au catalyst for benzene oxidation. It was found that the Au/BSA-CeO2 with 3.2 nm Au nanoparticles (NPs) gave rise to superior catalytic performance for benzene oxidation in which the temperature for 90 % benzene conversion was only 210 °C. In addition, the catalytic activity of Au/BSA-CeO2 remained stable for 140 h of on-stream reaction. Besides, density functional theory calculation demonstrated how the porous structure of CeO2 influenced the oxygen vacancy generation, and the synergetic effects of Au NPs and CeO2 support facilitated the activation of benzene which was adsorbed on Au NPs prior to benzene oxidation. The catalysis of benzene oxidation over Au/CeO2 catalyst follows the dual-site mechanism. BAS-templated CeO2 support possessed hierarchical structure, abundant adsorbed oxygen species and synergic interaction with Au that determined the performance of Au/BSA-CeO2.

Green emitting holmium (Ho) doped yttrium oxide (Y2O3) phosphor for solid state lighting

This paper reports the synthesis of Y2-xHoxO3 phosphor by citrate sol-gel method. Phase purity and morphological studies were performed by X-Ray diffraction (XRD) and scanning electron microscopy (SEM) characterization technique, respectively. Diffuse reflectance (DR) spectrum of the sample was recorded, and the band gap energy was estimated by using Kubelka–Munk (K–M) model. Photoluminescence (PL) studies of the prepared phosphors were carried out in this work. CIE coordinates of the studied phosphor were calculated, and which indicate the green color of the phosphor. The studied phosphor Y2-xHoxO3 exhibits strong green luminescence upon the blue excitation which claims its candidature for solid state lighting applications.

A Comparative Study on Photocatalytic Performance of Perovskite Materials Synthesized from Discarded Rare Earth Magnet with the one from Pure Neodymium Salt

Perovskite-type rare earth orthoferrite (ReFeO3) nanocrystalline were synthesized by using sol-gel citrate method and followed by calcination from oxalate salt Re2(C2O4)3, which composed of Nd (76.5 %), Pr (14.8 %), Dy (5.5 %) and Tb (3.2 %) recovered from discarded neodymium magnets (NdFeB) in hard-disk-drive waste. The synthesized materials were characterized by X-ray diffraction (XRD), field emission scanning electron microscopes (SEM), Brunauer-Emmett-Teller nitrogen adsorption and desorption (BET), Ultraviolet-visible diffuse reflection spectroscopy (UV-Vis DRS). The perovskite NdFeO3 was synthesized under the same conditions from an analytical grade pure salt to be used as a control material. Photocatalytic characteristics of both perovskite ReFeO3 and NdFeO3 were investigated through evaluation of methylene blue (MB) degradation under visible light. The degradation efficiency of MB by the ReFeO3 catalyst was higher than that by NdFeO3 after 3 h of illumination under visible light. The degradation efficiency could reach 99.2 % and the photocatalytic capacity remained after 5 cycles for ReFeO3 catalyst.

Highly conductive proton-conducting electrolyte with a low sintering temperature for electrochemical ammonia synthesis

BaCe0·7Zr0.1Gd0.2O3-δ (BCZG) powder is synthesized by a citrate sol-gel method, and different amounts of Li2CO3 are introduced to lower the sintering temperature. The densification temperature of BCZG ceramic is decreased drastically to 1250 °C by using Li2CO3 as sintering aid. BCZG with 2.5 wt% of Li2CO3 (BCZG-2.5L) can not only remarkably promote the sintering process of BCZG but also enhance its electrical conductivity. The total ionic conductivity of BCZG-2.5L attains to 1.9 × 10−2 S cm−1 at 600 °C in a wet H2 atmosphere. Ammonia synthesis at atmospheric pressure is conducted on (2K, 10Fe)/Ni-BCZG | BCZG-2.5L | Ni-BCZG electrolytic cell with an applied voltage of 0.2–1.6 V at a temperature of 450–600 °C. The highest NH3 formation rate of 1.87 × 10−10 mol s−1 cm−2 and the highest current efficiency of 0.53% is achieved at 500 °C with an applied voltage of 0.8 V.

EPR and Optical Properties of Green Emitting Mn Activated Sr2ZnSi2O7 Phosphors Prepared by Sol–Gel Method

A series of Mn2+ activated Sr2ZnSi2O7 phosphors with variable Mn2+ ions concentration were prepared via citrate sol–gel method. The systematic analysis of structure and luminescent properties were carried out by recording x-ray diffraction patterns, scanning electron micrographs, excitation and emission spectra along with electron paramagnetic resonance (EPR) spectra. The optimization of Mn2+ ion concentration in the synthesized phosphors was done by analyzing emission spectra. The Mn2+ activated Sr2ZnSi2O7 phosphors exhibit green emission under 263 nm excitation. The EPR spectra exhibit sextet hyperfine lines with resonance at g ≈ 2.0 indicating tetrahedral symmetry and covalent bonding between Mn2+ ions and O2− ligand ions bound to it. All the aforesaid results support the suitability of Mn2+ activated Sr2ZnSi2O7 phosphors as visible green emitters in optoelectronic display devices.

Analysis of varying Gd3+ concentrations on the structure and optical properties of ZnAl2O4:0.1% Eu3+; x% Gd3+ (0 ≤ x ≤ 1.2) synthesized via citrate sol–gel method

We report the preparation of 0.1% Eu3+; x% Gd3+ (0 ≤ x ≤ 1.2) ZnAl2O4 phosphors using the citrate sol–gel method. X-ray diffraction (XRD) data revealed that all annealed samples consisted of the single phase of cubic ZnAl2O4 structure. The scanning electron microscopic (SEM) images indicated pronounced effect of dual doping on the surface morphology of the phosphor. The estimated crystal sizes estimated by the XRD was confirmed by the high-resolution transmission electron microscopy (HR-TEM) to be approximately around 20 nm. The photoluminescence (PL) spectroscopy results revealed four distinct emission peaks located at 393, 400, 578 and 618 nm. The peaks at around 393 and 400 nm were attributed to the defect levels located at different positions on the host material (ZnAl2O4). The emission peak at 578 and 618 nm were attributed to the 5D0 → 7F1 and 2D0 → 7F2 characteristic transition within the Eu3+ ions. The International Commission on Illumination (CIE) colour coordinates revealed that the emission colour was not influenced by varying the concentration of Gd3+.

Peculiarities of the microwave properties of hard-soft functional composites SrTb0.01Tm0.01Fe11.98O19-AFe2O4 (A = Co, Ni, Zn, Cu, or Mn).

Herein, we investigated the correlation between the chemical composition, microstructure, and microwave properties of composites based on lightly Tb/Tm-doped Sr-hexaferrites (SrTb0.01Tm0.01Fe11.98O19) and spinel ferrites (AFe2O4, A = Co, Ni, Zn, Cu, or Mn), which were fabricated by a one-pot citrate sol–gel method. Powder XRD patterns of products confirmed the presence of pure hexaferrite and spinel phases. Microstructural analysis was performed based on SEM images. The average grain size for each phase in the prepared composites was calculated. Comprehensive investigations of dielectric properties (real (ε′) and imaginary parts (ε′′) of permittivity, dielectric loss tangent (tan(δ)), and AC conductivity) were performed in the 1–3 × 106 Hz frequency range at 20–120 °C. Frequency dependency of microwave properties were investigated using the coaxial method in frequency range of 2–18 GHz. The non-linear behavior of the main microwave properties with a change in composition may be due to the influence of the soft magnetic phase. It was found that Mn- and Ni-spinel ferrites achieved the strongest electromagnetic absorption. This may be due to differences in the structures of the electron shell and the radii of the A-site ions in the spinel phase. It was discovered that the ionic polarization transformed into the dipole polarization.

Sol-gel citrate synthesized Zn doped MgFe2O4 nanocrystals: A promising supercapacitor electrode material

Herein present work, we are reporting sol–gel citrate method for synthesis of MgFe2O4 nanocrystals doped with different concentrations (0, 2, 3 and 4 wt%) of zinc (Zn). The as-synthesized material is used as an electrode material for supercapacitor which demonstrated large specific capacitance, high energy density and cyclic stability. Experimental results revealed the highest specific capacitance of 484.6 Fg−1 and energy density 10.8 WhKg−1 at the current density 1 mAcm−2 using as-synthesized 2 wt% Zn doped MgFe2O4. These values demonstrates the best performance of the Zn doped MgFe2O4 nanocrystals as the electrodes in the supercapacitor application. Such excellent electrochemical properties are attributed to the intrinsic characteristics of Zn doped MgFe2O4 material including its crystal structure, morphology, high temperature stability and surface area.

A highly active Ni catalyst supported on Mg-substituted LaAlO3 for carbon dioxide reforming of methane

The pristine and Mg-substituted LaAlO3 perovskites were synthesized by the citrate sol-gel method and employed as supports for Ni catalysts in CO2 reforming of methane. The Mg/La molar ratio of 2/8 was determined as the optimal amount of Mg substitution in Ni/La1-xMgxAlO3-δ catalysts. Compared with the Ni/LaAlO3 catalyst, the Ni/La0.8Mg0.2AlO2.9 catalyst exhibited enhanced initial activity and improved stability. Structural characterizations including N2 sorption (N2 sorption), X-ray diffraction (XRD), transmission electron microscopy (TEM), energy-dispersive X-ray spectrometry (EDX) elemental mapping, thermogravimetry-differential scanning calorimetry (TG-DSC), CO2 temperature-programmed desorption (CO2-TPD), X-ray photoelectron spectroscopy (XPS) and H2 temperature-programmed reduction (H2-TPR) were performed to elucidate the effect of Mg partial substitution. It is revealed that much smaller Ni nanoparticles were formed with Mg partial substitution, which may explain the enhanced reforming activity. The improved stability over the Ni/La0.8Mg0.2AlO2.9 catalyst was rationalized with better anti-coke and anti-sintering properties, which was closely related with surface basicity, oxygen vacancies and metal-support interactions. The present work demonstrates that perovskites with appropriate surface modification can serve as novel supports for the preparation of highly active catalysts.

Photoelectrochemical study of La2NiO4 synthesized using citrate sol gel method—application for hydrogen photo-production

The semi conducting properties of La2NiO4 synthesized through sol gel method are studied for the first time using photoelectrochemistry. The X-ray diffraction (XRD) shows that the phase is formed at 900 °C in agreement with the thermal analysis (TG-TDA/DSC). The SEM micrographs exhibit spherical and uniform grains with agglomerated nature, confirmed through laser granulometry. The direct optical gap (1.31 eV) comes from the crystal field splitting of Ni2+ octahedrally coordinated. La2NiO4 is chemically stable in the pH range (5–14) and anodic potentials at pH ~ 12 give rise to surface oxidation of La2NiO4 in the diffusion plateau (0.35–0.6 V). The electrochemical oxygen insertion, studied using chrono-amperometry, is slow with a diffusion coefficient of ~ 2.5 × 10−18 cm2 s−1 at 500 mV. The Mott–Schottky characteristic plotted in alkaline solution (NaOH 0.1 M) indicates p-type conductivity due to oxygen over-stoichiometry well-known in this class of compound. A flat band potential of 0.1 VSCE and hole concentration of 1019 cm−3 were determined from the capacitance measurement. Therefore, the oxide has been successfully tested for the H2 evolution under visible light irradiation. The best activity occurs at pH ~ 13 in presence of S2O32− as reducing agent with H2 liberation rate of 23.6 μmol mn−1 (g catalyst)−1 under visible light flux of 29 mW cm−2.

The catalytic oxidation performance of toluene over the Ce-Mn-Ox catalysts: Effect of synthetic routes

Ce-Mn-Ox catalysts were synthesized by impregnation (CM-IM), co-precipitation (CM-CP), citrate sol-gel (CM-SG) and hydrothermal (CM-HT) methods. The synthesis methods exhibited a great impact on the physicochemical properties of catalysts, resulting in different catalytic activity. The catalytic oxidation activity of toluene followed the sequence: CM-HT (T50: 234 °C; T90: 246 °C) > CM-SG (T50: 242 °C; T90: 249 °C) > CM-CP (T50: 243 °C; T90: 259 °C) > CM-IM (T50: 251 °C; T90: 261 °C), which was consistent with the sequence of surface relative percentage of Cov, Ce3+, Mn3+, Oα and r values. Among them, CM-HT showed the best catalytic oxidation performance of toluene due to more structural defects, oxygen vacancies, surface adsorption oxygen, normalized conversion rate and other active species. In addition, CM-HT catalysts showed reliable water resistance and good durability, implying the potential industrial application.

Photoluminescence and EPR studies of narrowband ultraviolet B emitting trivalent gadolinium activated CaLaAl3O7 phosphors

We report the synthesis of Gd activated CaLaAl3O7 phosphor by the citrate sol-gel method. Phase purity and morphology of samples have been investigated by X-Ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The Photoluminescence (PL) spectra of the present series of Gd3+ activated phosphor samples showed an intense emission peak at 313 nm owing to the P67/2→S87/2 transition. In Gd3+ activated CaLaAl3O7 phosphor, the electron paramagnetic resonance (EPR) spectra show resonance signals at the effective g-values of 6.98, 4.65, 2.91, and 2.15.

Fe-doped mayenite electride composite with 2D reduced Graphene Oxide: As a non-platinum based, highly durable electrocatalyst for Oxygen Reduction Reaction

Since the last decades, non-precious metal catalysts (NPMC), especially iron based electrocatalysts show sufficient activity, potentially applicant in oxygen reduction reaction (ORR), however they only withstand considerable current densities at low operating potentials. On the other hand iron based electrocatalysts are not stable at elevated cathode potentials, which is essential for high energy competence, and its remains difficult to deal. Therefore, via this research a simple approach is demonstrated that allows synthesis of nanosize Fe-doped mayenite electride, [Ca24Al28O64]4+·(e−)4 (can also write as, C12A7−xFex:e−, where doping level, x = 1) (thereafter, Fe-doped C12A7:e−), consist of abundantly available elements with gram level powder material production, based on simple citrate sol-gel method. The maximum achieved conductivity of this first time synthesized Fe-doped C12A7:e− composite materials was 249 S/cm. Consequently, Fe-doped C12A7:e− composite is cost-effective, more active and highly durable precious-metal free electrocatalyst, with 1.03 V onset potential, 0.89 V (RHE) half-wave potential, and ~5.9 mA/cm2 current density, which is higher than benchmark 20% Pt/C (5.65 mA/cm2, and 0.84 V). The Fe-doped C12A7:e− has also higher selectivity for desired 4e− pathway, and more stable than 20 wt% Pt/C electrode with higher immunity towards methanol poisoning. Fe-doped C12A7:e− loses was almost zero of its original activity after passing 11 h compared to the absence of methanol case, indicates that to introduce methanol has almost negligible consequence for ORR performance, which makes it highly desirable, precious-metal free electrocatalyst in ORR. This is primarily described due to coexistence of Fe-doped C12A7:e− related active sites with reduced graphene oxide (rGO) with pyridinic-nitrogen, and their strong coupling consequence along their porous morphology textures. These textures assist rapid diffusion of molecules to catalyst active sites quickly. In real system maximum power densities reached to 243 and 275 mW/cm2 for Pt/C and Fe-doped C12A7:e− composite, respectively.

Correlation between crystal structure parameters with magnetic and dielectric parameters of Cu-doped barium hexaferrite

Copper (Cu) doped barium hexaferrite BaFe12−xCuxO19, (x = 0.0–2.0) were synthesized by the modified citrate sol-gel method. The structural, magnetic and electric properties have been investigated. All samples are crystallized to M-type hexagonal structure with P63/mmc space group. Lattice parameters are found to be maximum for x = 1.0, which has been correlated with the physical properties (magnetic & dielectric) of the material. The particle size distribution (found to be nm order) and surface morphology were analyzed by the FESEM (Field Emission Scanning Electron Microscopy) technique. The magnetic hysteresis (M-H) loops have been analyzed by employing the “Law of approach to saturation” and, important magnetic parameters are obtained. The saturation magnetization and magnetocrystalline anisotropy constants are found to be maximum for x = 1.0 sample, but the coercive field is found to a minimum for the same sample. The modified Debye equation has been employed to understand the impedance response to the frequency. The activation energy was obtained by using the Arrhenius equation. Hence, a correlation between crystal structure parameters and physical (magnetic and dielectric) properties of nano-crystalline Cu-doped barium hexaferrite has been observed. Finally, a correlation between crystal structure parameters and magnetic properties, as well as electrical properties, has been proposed for the Cu doped BHF.

Acetone sensing characteristics of Fe2O3/In2O3 nanocomposite

Fe2O3 and In2O3 nanoparticles were prepared by a citric sol-gel method and mixed subsequently to form the (x)Fe2O3/(1 − x)In2O3 (x = 0, 0.05, 0.10, 0.15, 0.20, 0.50, 1) nanocomposites. XRD confirmed the separate phases of cubic In2O3 and orthorhombic Fe2O3 in the composites. At the prime working temperature of 200 °C, the 0.15Fe2O3/0.85In2O3 nanocomposite-based sensor showed the best acetone sensing performance among all sensors, including the highest gas response, sensitivity, selectivity and stability. Although the mean grain size was slightly increased upon adding Fe2O3 nanoparticles in larger sizes, which leaded to smaller BET surface area, the electron transfer from Fe2O3 to In2O3 across the heterostructure interface increased the electron concentration in the surface region of In2O3, which resulted in more adsorbed oxygen species on the surface of In2O3 and favored the subsequent acetone sensing process.

Raman Characterization of polycrystalline barium hexaferrite nanoparticles: SERS of nanoparticles in powder form

Raman Characterization of Materials in powder form has always remained challenging because of low signal to noise ratio. We, herein, propose a method to make use of Surface Enhanced Raman Scattering (SERS) technique to improve the usefulness of Raman study of powder samples containing particles of sub wavelength size. As a case study, we have successfully applied the method to barium hexaferrite (BHF) synthesized by the modified citrate sol-gel method. The crystal structure and surface morphology of the prepared BHF powder were characterized by X-ray diffraction (XRD) and scanning electron microscopy. As many as 19 Raman peaks in a single spectrum with unpolarized laser were observed in the present work, against the commonly observed (in literature) 10–12 peaks for BHF. This report, as it is believed, will enable the research community to probe further into the distortions in crystal structure through Raman studies.

Effects of In3+ doping concentration on the structural, morphological and photoluminescence properties of x% In3+:MgAl2O4 nanophosphor prepared via citrate sol-gel method

A series of nanocrystalline (0 ≤ x ≤ 1.25%) x% In3+:MgAl2O4 (IMAO) phosphor powders have been successfully prepared via citrate sol-gel technique. The x-ray diffraction (XRD) results indicated that the IMAO consists of the cubic phase and average crystallite sizes was found to be 9 nm. Scanning electron microscopy (SEM) revealed that the In3+ concentration influences the phosphor morphology. Ultraviolet-visible (UV–vis) results showed that the reflectance and absorption of the IMAO depends on the In3+ concentration. The prepared samples exhibited the emission at 395 nm, which is attributed to the defects within the host material. The optimum emission intensity was observed at x = 0.3% In3+. The critical distance (Rc) calculation suggested that the concentration quenching occurs due to the multipole-multipole interaction. The Commission Internationale de l’Elcairage (CIE) diagram revealed that varying the In3+ concentration does not influence the emission colour.

Nanostructured zinc aluminates: A promising material for cool roof coating

Nanostructured zinc aluminate pigments were synthesized using a citrate sol–gel method. All the samples were annealed at 600 °C, 700 °C, 800 °C, and 1100 °C for 6 h, respectively. The as-synthesized samples were characterized by X-ray diffraction (XRD), fourier transform infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FE-SEM), UV–Vis–NIR spectroscopy and CIE L*a*b* color analysis. The phase purity and structural analysis of the zinc aluminates were confirmed using X-ray diffraction and Rietveld refinement. The morphological and microchemical analysis was done using FE-SEM, TEM and EDX. The particle size and reflectance properties are found to be sensitive to the annealing temperature. The samples exhibited a total solar reflectance of about 85%. An attempt was also made to compare the total solar reflectance of the synthesized zinc aluminates with that of commercially available TiO2 by applying over a concrete cement surface.

Sol-gel derived, Na/K-doped Li4SiO4-based CO2 sorbents with fast kinetics at high temperature

Li4SiO4 is considered as one of the most potential high-temperature sorbent materials for CO2 capture from the flue gas of fossil fuel-fired power plants. However, it usually suffers from slow kinetics at low CO2 concentrations. In this work, a series of Na- and K-doped Li4SiO4-based sorbents with Na/Si (or K/Si) atomic ratios of 0.05, 0.1 and 0.2 were prepared by a citrate sol-gel method. The prepared materials were characterized by various techniques, which revealed the presence of Li3NaSiO4 (for Na-doped sorbents) and LiKCO3 (for K-doped sorbents) on the surface of Li4SiO4 particles. All Na- and K-doped Li4SiO4 showed greatly enhanced CO2 sorption rates and capacities in 15% CO2 as compared to the heteroatom-free Li4SiO4, and in particular, the Na-doped sorbents had the fastest rates. The improved performance of K-doped Li4SiO4 resulted from the formation of eutectic Li2CO3-K2CO3 molten carbonates during CO2 sorption, while for Na-doped Li4SiO4, both the carbonation of Li3NaSiO4 and the presence of eutectic Li2CO3-Na2CO3 molten salts contributed to the improvement. In addition, both Na- and K-doped Li4SiO4 exhibited incomplete regeneration during cyclic operation, but the latter had better stability. In this context, we deliberately prepared a Na,K-codoped Li4SiO4-based sorbent with a Na:K:Si atomic ratio of 0.1:0.1:1, which showed fast kinetics and good cyclic stability. During 30 consecutive sorption-regeneration cycles (10 min of sorption in 15% CO2 at 650 °C, 5 min of regeneration in pure N2 at 750 °C), the Na,K-codoped Li4SiO4 presented a stable CO2 capture capacity of 0.22 gCO2/gsorbent after the first 10 cycles.