Microwave Assisted Sol Gel Route Research Articles

Magneto-transport and magneto-dielectric anomalies in Co doped iron oxide thin films – Microwave assisted sol-gel route

Use of microwave (MW) energy as an alternate to high temperature treatment during material synthesis is gaining much attention as it offers the advantages of being faster and economical than other heating methods. A comprehensive study of structure, magnetization, and polarization behaviour of Cobalt (Co) doped Fe3O4 thin films, prepared by means of microwave assisted sol–gel method is performed. Co doped (0-10 wt%) sols are prepared after irradiating them at MW power of 1000 W. Phase analysis is confirmed using XRD where Fe3O4 phase persisted with Co concentrations of 0-6 wt%. Higher concentration results in the appearance of cobalt oxide phase. Magnetic and transport properties are strongly affected by the presence of Co ions. Co doped Fe3O4 films show anisotropic behaviour with high saturation magnetization obtained for 6 wt% Co doped Fe3O4 thin films, i.e. ∼ 125 emu/g. Verwey transition is observed for thin films with low dopant concentration while no sharp transition in magnetization behaviour is observed for higher dopant concentration. High frequency anomaly in dielectric analysis is observed at high frequencies for Co doped Fe3O4 thin films. Negative values of magneto-dielectric coefficient show coupling of magnetic and dielectric behaviour at room temperature.

Microwave-assisted sol-gel synthesis of CeO2–NiO nanocomposite based NO2 gas sensor for selective detection at lower operating temperature

The progress in the development of gas sensors has considerably grown using some novel nanomaterials of metal, metal oxide and composite. In the current study, we intended and evaluated the properties of nanomaterials like CeO2, NiO, and CeO2–NiO composite and its application as NO2 gas sensor. Sensing of low concentration of NO2 gas at optimum functional temperature was succeeded using CeO2–NiO nanocomposites (NCs) film. The working temperature ranges in between 100 and 225 ​°C. Highly crystalline nanomaterials (CeO2, NiO and CeO2–NiO) have been prepared by applying microwave-assisted sol-gel route. The as-prepared nanomaterials are characterized for their structure, size, morphology and constitution by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray analysis. XRD studies of nanoparticles reveal the formation of nanoscale CeO2 and NiO with crystallite size 26, 23 ​nm, respectively. Both are having a face centered cubic structure. The nanocomposite (NC) Ce:Ni ​= ​60:40 has crystallite size of 13 ​nm. XRD study of NCs shows assimilation of Ni metal into the ceria and proves physical similarities of two phases. It can be observed from SEM that prepared NC has a porous surface which enables more surface active sites for adsorbing oxygen. The optical properties are measured with the help of UV–Vis. Spectroscopy. Optical band gaps of 3.19, 3.41 and 2.9 ​eV were observed for CeO2, NiO nanoparticles (NPs) and CeO2–NiO NC, respectively. Gas sensing properties state that the NC material shows a higher gas response % of 67.34% for NO2 gas (25 ​ppm) at comparatively low operating temperature (125 ​°C). It gives response time as (∼28 ​s) and the recovery (∼54 ​s). NiO incorporation in CeO2 results in a decline of operating temperature of NC and improves the sensing features.

Oxidative degradation of erythromycin using calcium carbonate under UV and solar light irradiation: Condition optimized by response surface methodology

In the present investigation, CaCO3 (nano-calcite) was prepared using a microwave-assisted sol-gel route and photocatalyst efficiency was investigated by degrading antibiotic, Erythromycin (Ery). The prepared sample was characterized by XRD, SEM, EDX, FTIR and UV–vis techniques. The process variables such as Ery concentration, irradiation time and catalyst dose were optimized by RSM (response surface methodology) techniques. The Ery degradation was achieved up to 93% (6 h sunlight) and 73% (2 h UV irradiation) at Ery concentration 30 mg/L and catalyst load 20 mg. The water quality parameters (COD, BOD and TOC) were also evaluated pre- and post-treatment and the COD, BOD and TOC values were reduced up to 78.5, 77.6, 64.5 (%), respectively. The reusability/recyclability of CaCO3 was also performed and up to 3 cycles the activity did not change, which reduced slightly for 4–9 cycles slightly at each run. The results indicate that CaCO3 is an efficient catalyst for the degradation of Ery under sunlight irradiation, which could be used for Ery degradation in effluents.

Synthesis of heterostructured catalyst coupling MOF derived Fe2O3 with TiO2 for enhanced photocatalytic activity in anti-inflammatory drugs mixture degradation

A new synthesis method to prepare the heterostructured catalyst MIL-101(Fe) derived Fe2O3 with TiO2 via microwave-assisted sol-gel route followed by pyrolysis was carried out. The mixed oxide catalyst was characterized by high-resolution transmission electron (HRTEM) and scanning electron microscopy (SEM), X-ray diffraction (XRD), UV–Vis DRS spectroscopy, N2 physisorption, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), photoluminescence (PL), and zetametry. The photocatalytic activity was evaluated in the degradation of the nonsteroidal anti-inflammatory drugs (NSAIDs), ibuprofen (IBP), and naproxen (NPX) in a mixture (10 mg/L/each one) at pH 3, using 1 g/L of the catalyst under visible light. The synthesized Fe2O3/TiO2 showed higher surface area, smaller average crystallite size, absorption in the visible range, and narrower band gap than TiO2, which enhanced the photocatalytic performance of TiO2 under visible radiation, allowing 91 and 100% degradation of IBP and NPX within 240 and 15 min of reaction, respectively. Moreover, 79.0% of mineralization of the mixture of NSAIDs in 240 min was achieved using the derived Fe2O3/TiO2 showing good reusability and stability even after three cycles of usage. In the photocatalytic process, holes were the dominant species for NSAIDs degradation.

Microwave-assisted sol–gel synthesis of TiO2-mixed metal oxide nanocatalyst for degradation of organic pollutant

Abstract Titanium dioxide (TiO2) is the most effective photocatalysts for low-cost degradation of organic pollutant; however, the wide band gap and the high recombination rate of the charge carriers are drawbacks that hinders it practical application. In this study, TiO2 and titanium mixed metal oxides ternary (V/Ag/TiO2) nanocatalyst was synthesized through a microwave-assisted sol–gel route using Ti(C4H9O)4, NH4VO3, and AgNO3 as precursors. The XRD analysis of the synthesized TiO2 and V/Ag/TiO2 depicts lattice fringes for rutile and anatase crystalline phases. Raman spectra indicate the formation of a mesoporous multiphase sample mixture of rutile and anatase phases. The spectrum shift to the visible light region was demonstrated by the UV-visible spectroscopy analysis. Diffuse reflectance spectroscopy (DRS) reveals a reduced band gap of 2.9 eV for TiO2 and 2.65 eV for V/Ag/TiO2. Brunauer–Emmett–Teller (BET) indicates a large surface area of 92.8 and 84.8 m2 g−1 for TiO2 and V/Ag/TiO2, respectively. Nitrogen adsorption–desorption isotherm exhibits type IV isotherm, signifying the presence of the mesoporous structure. SEM portrays a cluster of rod-like aggregate particles, while the HRTEM analysis illustrates nanoparticles of rod-like cylindrical shape with a homogeneous size diameter. The synthesized nanocatalyst demonstrated a significant photocatalytic ability in the degradation of methyl orange (MO) and methylene blue (MB). V/Ag/TiO2 shows higher activity in the visible region. Thus, the present report suggests efficient, suitable, and economical microwave-assisted sol–gel techniques to yield V/Ag/TiO2 nanocatalysts with harnessed photocatalytic performance for the degradation of toxic organic pollutants in the presence of visible light irradiation.

Comparison of photocatalytic activity of αFe2O3-TiO2/P on the removal of pollutants on liquid and gaseous phase

The photocatalytic activity of mixed oxide α-Fe2O3/TiO2 doped with P (αFe-Ti/P) was evaluated on diuron degradation in aqueous solution and NOx oxidation under UV and visible light irradiation. The microwave-assisted sol-gel route was used to prepare the αFe-Ti/P catalyst. The TiO2 doped with P (Ti/P), α-Fe2O3/TiO2 (αFe-Ti) and TiO2 (Ti) were also synthesized under similar conditions as reference catalysts. The samples were characterized by different instrumental techniques (XRD, UV–VIS with DRS, PL, N2 adsorption, pHPZC, XPS, HRTEM, and FTIR). The photocatalytic activity on diuron degradation in aqueous media under UV and visible light followed the tendency Ti/P > αFe-Ti/P > αFe-Ti > Ti. The characteristics of the catalysts that enhanced photocatalytic reaction in liquid media were the low value of point of zero charge associated with Brønsted surface acid sites, narrow band-gap, small crystallite size, reduced e-/h+ recombination rate, and large surface area attributed to the P incorporation on the TiO2. On the other hand, the NOx oxidation on the gas-phase reaction under UV and visible light followed the trend αFe-Ti > Ti > Ti/P > αFe-Ti/P. These results could be related to the assemble of αFe2O3 nanoparticles with TiO2 to form αFe2O3/TiO2 heterostructure that extended the optical absorption in the visible range and increased the basicity of the prepared TiO2 based material (Lewis basic surface sites).

Microwave assisted tuning of optical and magnetic properties of zinc oxide nanorods—efficient antibacterial and photocatalytic agent

Microwave radiations play an important role in chemical reactions for reducing overall time and cost. The method has many advantages over conventional heating methods like phase purity, high crystallinity, enhanced reaction selectivity, narrow size distribution, energy saving, and high product yield. Microwave-assisted sol–gel route is used to prepare zinc oxide (ZnO) nanorods. X-ray diffraction results show peaks corresponding to Zn (OH)2 and ZnO at low microwave (MW) powers, i.e., at 136, 264, and 440 W. Whereas, increase in MW powers to 616 and 800 W results in dissociation of Zn (OH)2 to ZnO thus forming single-phase ZnO with wurtzite structure. The presence of absorption bands at 461 and 496 cm−1, in FTIR analysis, match to stretching modes of ZnO. Nanorods with diameter ~90 nm and with length of ~210 nm are observed in SEM micrographs at MW power of 800 W. Increased values of transmission (i.e., from 80.98 to 87.49%) are observed with the increase of MW powers from 136 to 800 W. Direct energy band gap of ZnO nanorods is observed in the range of 3.109–3.274 eV. Ferromagnetic behavior instead of diamagnetic nature of ZnO nanorods arises due to bound magnetic polarons (BMP) effect. At 800 W ZnO nanorods show high saturation magnetization (i.e., 0.104 emu/g). Antimicrobial activity is tested against B. Subtilis and E. Coli with 19 mm and 17 mm zone of inhibition, respectively. ZnO nanorods are found efficient photocatalyst for degradation of methylene blue (MB). It is worth to describe that combined optical and magnetic properties’ tuning is studied in this research work.

Coupled heterogeneous photocatalysis using a P-TiO2-αFe2O3 catalyst and K2S2O8 for the efficient degradation of a sulfonamide mixture

Phosphorous-doped Ti-Fe mixed oxide (P-TiO2-αFe2O3) catalysts were prepared by the microwave-assisted sol-gel route and characterized using XRD, SEM, N2 physisorption, UV–vis diffuse reflectance, FTIR, and XPS. P-TiO2-αFe2O3 was evaluated during the degradation of a sulfonamide mixture (5 mg/L, each) under visible light. The photocatalytic process was optimized with a face-centered central composite design. Under optimal conditions (0.5 wt% of αFe2O3, pH 10, and 0.75 g/L of catalyst loading), the sulfate radical advanced oxidation process was carried out using 5 mM K2S2O8 (PS). P doping shifted the light absorption of P-TiO2-αFe2O3 in the visible light range owing to substitutional doping, while the coupling of P-TiO2 with α-Fe2O3 enhanced the absorption in the visible range, which resulted in an increase in the lifetime of the charge carriers and in a superior photoactivity of the P-TiO2-αFe2O3 catalyst in comparison to that of TiO2. The mineralization yield of the sulfonamides (SNs) mixture was enhanced in the presence of an electron acceptor (SO4−), allowing nearly 69 % within 300 min with the P-TiO2-αFe2O3/PS system, while P-TiO2-αFe2O3 and K2S2O8 oxidation achieved only 27 % and 21 %, respectively. The biodegradability index was 0.48 using the P-TiO2-αFe2O3/PS system, indicating a less toxic effluent than the original compounds. Recycling tests demonstrated that P-TiO2-αFe2O3 exhibits good stability in activating PS for SNs degradation during three cycles. Two main intermediates (pyrimidine and hydroquinone) and their hydroxylated re-arrangements were detected during the degradation of the SNs by the coupled process. Oxalic, oxamic, sulfonic, and acetic acids were also identified as by-products from the degradation of the SNs.

Microwave-assisted synthesis and critical analysis for YBa2Cu3O6+δ nanoparticles

A new cost effective scheme of a microwave-assisted sol–gel route followed by a short annealing time is proposed to synthesize YBCO nanoparticles (NPs) of various sizes. The advanced techniques of synchrotron radiation x-ray diffraction (SRXRD) and electron spin resonance (ESR) are used to analyze the size effects on their magnetic/superconducting properties. The major interesting finding is that the size of YBCO NPs could confine the amount of oxygen content and consequently change the superconducting transition temperature (TC) of YBCO NPs. The ESR result demonstrates a sensitive probe to characterize surface defects in the oxygen-deficient YBCO NPs.

Improvements in photostability and sensing properties of EuVO4 nanoparticles by microwave-assisted sol–gel route for detection of H2O2 vapors

The inorganic thin-film fluorescence probe, with the advantages of long life span and high sensitivity, has attracted much attention in explosive detection. The poor ultraviolet absorption and lack of an aromatic ring make peroxide-based explosives very hard to detect. As the signature compound of peroxide-based explosives, H2O2 vapor detection became more and more important. Rare-earth-doped vanadate is considered to be a suitable material for the detection of hydrogen peroxide due to the quenching of its fluorescence in the presence of hydrogen peroxide. Its only drawback lies on its poor photostability. In this work, we have developed a new synthesis to limit the photobleaching of EuVO4-based films. We propose herein an original protocol using microwave-assisted sol–gel route. The photostability of luminescent EuVO4 has been achieved and the sensitivity has been greatly increased. The detection limit for H2O2 vapor is as low as 100 parts per billion (ppb). The characterization of these nanoparticles by photoelectron spectroscopy clearly correlates their sensitivity to H2O2 to the surface chemistry.

Synthesis of KLa(WO4)2:Ho3+/Yb3+ Phosphors via Microwave-Assisted Sol-Gel Route and their Upconversion Photoluminescence Properties

Synthesis of KLa(WO4)2:Ho3+/Yb3+ Phosphors via Microwave-Assisted Sol-Gel Route and their Upconversion Photoluminescence Properties

Upconversion Properties of Y2(MoO4)3:Er3+/Yb3+ Particles Synthesized via Cyclic Microwave-Assisted Sol-Gel Route

Upconversion Properties of Y2(MoO4)3:Er3+/Yb3+ Particles Synthesized via Cyclic Microwave-Assisted Sol-Gel Route

Magnetic and magnetization properties of iron oxide thin films by microwave assisted sol–gel route

Iron oxide thin films have attracted worldwide attraction due to their technological importance in magnetic tunnel junctions, recording media, as a catalyst etc. However, the use of microwave energy for obtaining crystallinity and phase transition in the form of thin films has long being neglected. We here report preparation of iron oxide thin films using microwave energy instead of conventional heating methods during sol–gel synthesis. The solution was subjected to microwave radiations, for sol synthesis, with variation in microwave power. The maximum power used in this study is 800 W with variation in power as 136, 264, 440, 616 and 800 W. These are among the lowest powers applied for synthesis of iron oxide sol. XRD, FTIR and VSM results indicate that use of microwaves can lead to enhanced crystallinity and phase stability, thus enhancing the magnetic properties with variation in power of the incident microwaves. At low power of microwaves mixed phases of iron oxide exist whereas with increased power up to 616 W pure maghemite phase was obtained. Further increasing the microwave power to 800 W, results in formation of pure magnetite phase. FTIR bands at 477 and 550 cm−1 indicate formation of pure magnetite phase. The films exhibit ferromagnetic behaviour with saturation magnetization of 453 emu/cm3. Verwey transition, determined using low temperature electrical properties, reveals sharp reduction in resistivity at a temperature of 125 K.

Effect of multi-walled carbon nanotubes on the electrochemical performance of LiVPO4F cathode material for rechargeable lithium-ion batteries

A novel network cathode material for lithium-ion batteries, pristine and multi-walled carbon nanotubes modified LiVPO4F composites have been successfully synthesized via a microwave assisted sol–gel route. The crystalline structure and morphology of as-synthesized samples are investigated systematically by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Their electrochemical performances are also studied and compared in detail. The achieved results indicate that the multi-walled carbon nanotube modified LiVPO4F sample has excellent C-rate performance, and delivered discharge capacities of 137.6, 131.2, 120.5 and 108.9mAhg−1 at 0.5, 1, 2 and 5C in the voltage range of 3.0–4.5V at room temperature, respectively. Moreover, the capacity retention is 96.2% with the discharge capacity of 132.4mAhg−1 at 0.5C after 100cycles. Electrochemical impedance spectroscopy results reveal that the charge-transfer resistance of multi-walled carbon nanotube modified LiVPO4F is much smaller than that of the pristine LiVPO4F. The outstanding features are desirable and enable the multi-walled carbon nanotube modified LiVPO4F to be an excellent cathode material for lithium-ion batteries.

Synthesis and Characterization of Cu Doped LiCoO<sub>2</sub> Cathode Material for Lithium Batteries Using Microwave Assisted Sol-Gel Synthesis

LiCoO2 is the most studied cathode material for lithium batteries. The doping effect gives a better cycle life in such materials. Apart from the doping effect, the preparation technique also plays an important role. Presently, the layer structured Cu doped LiCoO2 cathode material has been prepared via microwave assisted sol gel route; better cycle life and capacity retention have been attained. It was found that this method could reduce the synthesis time to 30 minutes. The espousal of the microwave method in synthesis could develop a highly efficient, low cost process for synthesis. The surface morphology of the material has been observed using SEM and it is inhomogeneous in nature. The capacity retention is higher than that of pure LiCoO2 material. Compositional analysis was made through EDX. The Cu doped material has a voltage plateau about 4.0V which is obtained from the cyclic voltammetry.

Mesoporous α-Fe2O3 membranes as proton conductors: Synthesis by microwave-assisted sol–gel route and effect of their textural characteristics on water uptake and proton conductivity

This paper reports novel inorganic proton exchange electrolytes based on mesoporous α-Fe2O3 ceramic membranes. The unsupported mesoporous hematite proton ceramic membranes with high specific surface areas, high pore volumes, and narrow pore size distributions have been synthesized from a hydrolytic ferric oxide polymer prepared by a microwave-assisted sol–gel route. The effect of their textural characteristics on water uptake and proton conductivity has been studied.Microwave heating allows us to obtain homogeneous hydrosols at short times (2s) meanwhile conventional heating gives rise to inhomogeneities in the hydrosol independently of heating times. Electron diffraction TEM observations show that the xerogels calcined at 200°C are polycrystalline in nature and correspond to α-Fe2O3 in accordance with the two characteristic vibrations in hematite observed by FTIR spectroscopy. According to EMF measurements, proton transport is observed in these ceramic membranes and shows an Arrhenius dependence on temperature for all relative humidities studied. A sigmoidal dependence of the water uptake and the proton conductivity with the RH at a constant temperature was observed with the greatest increase detected between 58% and 81% RH. The pore volume and the average pore size of the α-Fe2O3 mesoporous ceramic membranes seem to be the main factors which influence the water uptake and consequently the proton conductivity in the studied ranges. The membranes with the largest pore volume, and the largest pore size have the highest water uptake and in turn the highest values of proton conductivity in the whole range of relative humidities (RHs). According to the activation energy values, proton migration in this kind of materials could be dominated by the Grotthuss mechanism in the whole range of RH.

Luminescence Properties of Ba2Mg ( BO3 ) 2 : Eu2 +  Red Phosphors Synthesized by a Microwave-Assisted Sol-Gel Route

Fine-sized red-emitting phosphors were synthesized via a microwave-assisted sol–gel route. The crystallization process, structure, and morphology of the as-synthesized phosphor were characterized by thermogravimetry–differential thermal analysis, X-ray diffraction, and scanning electron microscopy analysis, respectively. The luminescence properties were characterized by diffuse reflection spectra (UV-visible) and photoluminescence spectra. The as-prepared phosphors had good excitation characteristics in the region of 320–450 nm, which perfectly matched the emission wavelength of near-UV light emitting diodes. Intense red emission peaking at 608 nm was obtained upon 365 nm excitation with the chromaticity coordinates of (0.585, 0.402). The energy transfer among the nearest-neighbor ions resulted in the concentration quenching of phosphor, and the critical distance was determined to be about 17.32 Å.

Hierarchical porous silica membranes with dispersed Pt nanoparticles

Multifunctional silica membranes with a hierarchical porosity and containing dispersed Pt nanoparticles were prepared by an original one pot microwave-assisted sol–gel route. These membranes exhibit three porosity levels: interconnected micropores (<2 nm) in silica walls, isolated ordered mesopores (∼4 nm), and isolated macropores (∼70 nm). They were directly coated on tubular macroporous alumina supports without any intermediate mesoporous layer contrary to conventional membrane architectures. The isolated macropores and mesopores enable to increase the membrane permeability whereas the interconnected microporosity defines the membrane cut-off. The catalytic Pt nanoparticles (∼4 nm) mainly hosted in the mesoporous volume, are stabilised against undesirable agglomeration under working conditions. A first series of multifunctional membranes were prepared, in which preferential adsorption of hydrocarbon gases and efficient propene oxidation were evidenced. In order to avoid any possible interconnection between macropores, several strategies were investigated, which prevented sol infiltration in the macroporous support during the deposition process. These original multifunctional membranes are potentially attractive for gas separation and catalytic reactor applications.

Synthesis and characterization of La0.8Sr0.2Co0.5Fe0.5O3±δ nanopowders by microwave assisted sol–gel route

Nano-crystalline La0.8Sr0.2Co0.5Fe0.5O3±δ powder has been successfully synthesized by microwave assisted sol–gel (MWSG) method. The decomposition and crystallization behavior of the gel-precursor was studied by Fourier transform infrared spectroscopy (FT-IR) and X-ray diffraction (XRD) analysis. From the result of FT-IR and X-ray diffraction patterns, it is found that a perovskite La0.8Sr0.2Co0.5Fe0.5O3±δ was formed by irradiating the precursor at 700 W for 3 min, but the well-crystalline perovskite La0.8Sr0.2Co0.5Fe0.5O3±δ was obtained at 700 W for 35 min. Morphological and specific area analysis of the powder were done by transmission electron microscopy (TEM), scanning electron microscope (SEM) and Brunauer–Emmett–Teller (BET). The surface areas measured was 38.9 m2/g and the grain size was ∼23 nm. Electrochemical properties of pure LSCF cathode on YSZ electrolyte at intermediate temperatures were investigated by using AC impedance analyzer, which shows a low area specific resistance (0.077 Ω cm2 at 1073 K and 0.672 Ω cm2 at 953 K). Moreover, the synthesis period of 20 h usually observed for conventional heating mode is reduced to a few minutes. Thus, the MWSG method is proved to be a novel, extremely facile, time-saving and energy-efficient route to synthesize LSCF powders.