Min Of Microwave Irradiation Research Articles

Facile microwave fabrication of CdS nanobubbles with highly efficient photocatalytic performances

A new fabrication method of CdS nanobubbles with high photocatalytic activity has been explored under ambient conditions using SiO2 hard templates and microwave irradiation. The one-step Stober method has been used to produce SiO2 core nanospheres, onto which CdS shells have been readily and efficiently coated by 10 min microwave irradiation. The acid etching of the SiO2 cores has produced monodispersive CdS nanobubbles with outer diameters of 300, 370, and 400 nm with a uniform shell thickness of 11 nm. The CdS nanobubbles have been found to photocatalyze the degradation of rhodamine B efficiently under Xe-lamp irradiation via simultaneous reaction pathways of N-deethylation and conjugated-ring attack. It has been found that the degradation rate increases linearly with the concentration increase of the nanocatalyst but decreases nonlinearly with increasing nanobubble diameters. The pseudo-first-order degradation rate constant via the nanobubbles (0.081 min−1) was found to be about 20 times larger than that via the SiO2@CdS core@shell nanocomposites (0.0044 min−1) due to the nanoreactor confinement effect leading to a decrease in the activation energy of the reaction.

Rapid microwave assisted synthesis of graphene sheets supporting CdS nanocrystals for highly efficient visible light photocatalyst

We have developed a facile, fast and scalable microwave irradiation (MWI) method for the synthesis of CdS nanocrystals dispersed on graphene sheets. The reduction of graphene oxide (GO) into graphene takes place in ethylene glycol within 5 min of MWI. The method indicates the simultaneous reduction of GO, and a homogeneous distribution of CdS nanocrystals on the graphene nanosheets (GNs) was formed. Moreover, visible photocatalytic activity of the composites was tested using rhodamine B as the model contaminant. Compared with the corresponding bare GNs and CdS, the GNs-CdS nanocomposites display distinctly enhanced photocatalytic activities. The investigation gave a promise to the development of original yet highly efficient graphene based photocatalysts that utilise visible light as an energy source.

Preparation of zinc oxide coatings by using newly designed metal–organic complexes of Zn: Effect of molecular structure of the precursor and surfactant over the crystallization, growth and luminescence

Abstract We report large scale deposition of tapered zinc oxide (ZnO) nanorods on Si(1 0 0) substrate by using newly designed metal–organic complex of zinc (Zn) as the precursor, and microwave irradiation assisted chemical synthesis as a process. The coatings are uniform and high density ZnO nanorods (∼1.5 μm length) grow over the entire area (625 mm2) of the substrate within 1–5 min of microwave irradiation. ZnO coatings obtained by solution phase deposition yield strong UV emission. Variation of the molecular structure/molecular weight of the precursors and surfactants influence the crystallinity, morphology, and optical properties of ZnO coatings. The precursors in addition with the surfactant and the solvent are widely used to obtain desired coating on any substrate. The growth mechanism and the schematics of the growth process of ZnO coatings on Si(1 0 0) are discussed.

Microwave assisted synthesis of bis and tris(ω-bromoacetophenones): versatile precursors for novel bis(imidazo[1,2-a]pyridines), bis(imidazo[1,2-a]pyrimidines) and their tris-analogs

Backgroundα-Bromination of the side chain of aromatic ketones using NBS in the presence of p-toluenesulfonic acid (p-TsOH) in acetonitrile is very common. However, regioselective bromination of bis and tris(ω-bromoacetophenones) with NBS in the presence of p-TsOH in acetonitrile under microwave irradiation is quite novel. The bis- and tris(ω-bromoacetophenones) are used in synthesis of bis and tris(heterocycles). bis(heterocycles) have received a great deal of attention, because many biologically active natural and synthetic products have molecular symmetry. The use of the pressurized microwave irradiation is very advantageous to many syntheses and provide a large rate enhancement.ResultsBis and tris(ω-bromoacetophenones) were obtained as single monobrominated derivatives in a shorter time than the conventional conditions. The results clearly demonstrate the better reactivity and selectivity of NBS/p-TsOH/CH3CN as a brominating mixture under microwave conditions. The reaction of bis and tris(ω-bromoacetophenone) with 2-aminopyridine and 2-aminopyrimidine proceeded smoothly in a mixture of anhydrous ethanol and DMF under reflux or using 300 W/105°C/ 20 min microwave irradiation conditions to afford the corresponding bis(imidazo[1,2-a]pyridine), bis(imidazo[1,2-a]pyrimidine) and tris(imidazo[1,2-a]pyridine) derivatives in moderate to excellent yields. The carbonyl analogue of the targeted bis(imidazopyridines) could be synthesized by the reaction of N,N-dimethyl-N'-(pyridin-2-yl)formimidamide with bis(ω-bromoacetophenone) in refluxing ethanol. The structures of the newly synthesized compounds were confirmed by their spectral data as well as their elemental analyses.ConclusionIn conclusion, selective α-bromination of bis- and tris(acetophenones) has been accomplished efficiently utilizing NBS/p-TsOH/CH3CN under microwave irradiation. In addition, a facile synthesis of novel series of bis- and tris(imidazopyridine) and bis(imidazopyrimidine) derivatives.

Low temperature and rapid deposition of ZnO nanorods on Si(100) substrate with tunable optical emissions

This research article describes the large scale fabrication of ZnO nanorods of various shapes on Si(100) substrate, by using metalorganic precursor of Zn in solutions with microwave as the source of energy. This is a low temperature, environmental friendly and rapid thin film deposition process, where ZnO nanorods (1–3 μm length) were grown only in 1–5 min of microwave irradiation. All as-synthesized nanorods are of single crystalline grown along the <0001> crystallographic direction. The coated nanorods were found to be highly dense having a thickness of ∼1–3 μm over the entire area 20 mm × 20 mm of the substrate. The ZnO thin film comprising of nanorods exhibits good adhesion with the substrate. A possible mechanism for the initial nucleation and growth of ZnO is discussed. A cross over from a strong visible light emission to an enhanced UV emission is observed, when the nature of the surfactants are varied from polymeric to ionic and nonionic. The position of the chromaticity coordinates in yellow region of the color space gives an impression of white light generation from these coatings by exciting with a blue laser.

Microwave-assisted synthesis of silver nanoparticles using benzo-18-crown-6 as reducing and stabilizing agent

Benzo-18-crown-6 is employed to work as both reducing and stabilizing reagent in the reaction for synthesis of silver nanoparticles. Silver nanoparticles are analyzed using transmission electron microscope and UV–visible spectroscopic technique. The silver nanoparticles prepared in this way are uniform and stable, which can be stored at refrigerator for 5 months. Appearance of surface plasmon band at 420 nm indicated the formation of silver nanoparticles. Highly monodispersed stable silver nanoparticles were obtained within 3 min of microwave irradiation. Through transmission electron microscopy, silver nanoparticles were observed to be spherical.

Microwave-Mediated Reduction of Disulfide Bridges with Supported (Tris(2-carboxyethyl)phosphine) as Resin-Bound Reducing Agent

We report on the synthesis and use of a new supported reagent consisting in tris(2-carboxyethyl)phosphine (TCEP) immobilized on hydrophilic PEG based resin beads. Used in conjunction with a 5 min microwave (MW) irradiation, "supported TCEP" reduced disulfide bridges in free thiols in peptides having two or more cysteine residues. Separation of reaction products from reducing agent was easily performed by simple filtration.

An in-vitro evaluation of microwave irradiation and denture cleansers on disinfection and destaining of denture base resin

PurposeDenture cleansing tablets have been used effectively against stains and Candida when soaked for longer time yet complete sterilization against broad range of bacteria is questionable. Microwave sterilization has been shown effective against bacteria and fungus both but not against stains. Our study aims to comparatively evaluate the efficacy of microwave irradiation and denture cleanser on the disinfection and destaining of stained denture. Materials and methodsNinety-nine acrylic sheets were stained with Acacia catechu and lime and sterilized in UV chamber. Three samples served as negative control and remaining 96 samples were divided in groups A, B, C and D according to contaminating microorganism; three bacteria and one fungus. According to time of irradiation, groups were subdivided into subgroups-1 (2 min), 2(4 min), 3(6 min) and 4 (denture cleanser for 8 h). Microwave irradiation was done either in presence of water or denture cleansing solution. Following procedure samples were scored for staining and then inoculated for 24 h. Then after broth were checked for OD. One sample from each subgroup was inoculated for cfu count. ResultsEffective disinfection for all stained samples achieved with microwave for 6 min in presence of water or denture cleansing solution. Denture cleanser for 8 h was more effective for Candida albicans. 100% stain removal was found with irradiation for 6 min in presence of denture cleanser. Conclusion6 min microwave irradiation in presence of denture cleansing solution could achieve complete sterilization even for spore and membrane forming bacteria. Simultaneously it completely destained all the samples.

Application of microwave-irradiated manganese dioxide in the removal of polychlorinated biphenyls from soil contaminated by capacitor oil

The removal of polychlorinated biphenyls (PCBs) from soil contaminated with capacitor oil, using microwave (MW)-irradiated manganese dioxide (MnO2), was examined under different conditions. The effects of different types of MnO2 added as oxidant, as well as the initial amount of water, MnO2, and sulphuric acid solution, were also investigated. The removal efficiencies for dichlorobiphenyls, trichlorobiphenyls, tetrachlorobiphenyls, pentachlorobiphenyls, hexachlorobiphenyls, heptachlorobiphenyls, and octachlorobiphenyls were approximately 95.9%, 82.5%, 52.0%, 71.6%, 62.5%, 28.6%, and 16.1%, respectively, by 800 W MW irradiation for 45 min with the assistance of 0.1 g δ-MnO2 and 0.2 mL water in 1.0 g severely PCB-contaminated soil (∑PCB=1560.82 mg/kg); meanwhile, the concentrations of Mn2+ ions detected were from 10.6±1.9 mg/kg at 0 min to 108.2±7.8 mg/kg after 45 min MW irradiation, indicating that MnO2 acted as not only a MW absorber but also an oxidizer. Removal efficiencies of PCBs from contaminated soil increased with increasing the amounts of water and MnO2 added. The type of MnO2 also affected the removal of PCBs, following an order of . The addition of low concentration of sulphuric acid (such as 1.0 mol/L) solution was favourable for the removal of low chloro-substituted PCBs, but the addition of more than 1.0 mol/L sulphuric acid reduced the removal of all PCBs. The pronounced removal of PCBs from contaminated soil in a short treatment time indicates that MW irradiation with the assistance of MnO2 is an efficient and promising technology for the remediation of PCB-contaminated soil.

Methylene Blue–Cu2O Reaction Made Easy in Acidic Medium

Truncated Cu2O cubes with well-defined morphology have been successfully synthesized using a stable Cu(II)–EDTA precursor and glucose as the reducing agent in alkaline conditions under 5 min of microwave irradiation. The truncated cubic Cu2O particles as a solid powder were characterized by different physical methods. Curiously enough, upon the addition of Cu2O particles, the blue-colored methylene blue (MB) in aqueous acidic (pH ≈ 1.0) solution successively bleaches to colorless leuco-MB (LMB). The redox reaction generates a colorless solution which easily reverts back to blue in air. The reaction has been quantitatively monitored by UV–vis spectrophotometry. The reversible color change, i.e., oscillation between a blue MB solution and colorless LMB solution happens to be a periodic phenomenon for more than 50 cycles and is reproducibly demonstrated as a simple “clock reaction”. Acidic (pH ≈ 3.0) conditions favor zeroth-order kinetics for the underlying redox phenomena. Dilute H2SO4 has been proven to be the best choice to provide a passive reaction medium, and the undisturbed reaction mixture showed oscillatory behavior even after one month.

Sustainable approach to tandem catalysis: Expedient access to quinoxalines and pyrido[2,3-b]pyrazines from α-hydroxyketones via microwave-induced [(NH4)6Mo7O24·4H2O – PEG 300] polar paste catalyst system

Abstract [(NH 4 ) 6 Mo 7 O 24 ·4H 2 O–PEG 300] is introduced as a polar paste catalyst system for tandem synthesis of quinoxalines and pyrido[2,3- b ]pyrazines under open-vessel focused microwave irradiation. Low conversions were obtained when catalyst or PEG was used individually. Accordingly, a convenient combination of catalyst and PEG mostly led to quantitative yield of products within 15 min microwave irradiation with good turnover frequency values (11–20 h −1 ) taking a tandem process into consideration. The salient features of this environmentally benign method are fast conversions, high product selectivity and the use of a low-cost, readily available, nontoxic, catalyst and medium.

Effects of combination of steam explosion and microwave irradiation (SE–MI) pretreatment on enzymatic hydrolysis, sugar yields and structural properties of corn stover

A novel process named combination of steam explosion and microwave irradiation (SE–MI) method was investigated and compared with steam explosion (SE) method for pretreating corn stover under 540W microwave power and 3min microwave irradiation time. Both of the two pretreatment methods were operated at 170–210°C for 3–15min. Results showed that compared to SE process, SE–MI process enhanced the enzymatic hydrolysis yields of both glucose and xylose, and slightly increased the total sugar yield. The maximum glucose yield, xylose yield and total sugar yield of the process were 57.4%, 17.8% and 75.2% (corresponding to 100% maximum total sugar in feedstock), respectively, obtained at 200°C for 5min. SE–MI pretreatment showed attractive advantage in inhibiting the increase of biomass crystallinity. The crystallinity following SE–MI pretreatment was 19% lower than that following SE pretreatment at 190°C for 5min.

Effects of microwave power and microwave irradiation time on pretreatment efficiency and characteristics of corn stover using combination of steam explosion and microwave irradiation (SE–MI) pretreatment

The effects of microwave power and microwave irradiation time on pretreatment efficiency and characteristics of corn stover were investigated based on a new process named combination of steam explosion and microwave irradiation (SE–MI) pretreatment. Results showed that with microwave power and microwave irradiation time increasing, glucose and xylose that released into hydrolyzate, as well as enzymatic hydrolysis yields and sugar yields of glucose and xylose were all slightly increased after SE–MI pretreatment. The maximum sugar yield was 72.1g per 100g glucose and xylose in feedstock, achieved at 540W microwave power and 5min microwave irradiation time. XRD analysis showed that the crystallinity of biomass was 15.6–19.9% lower for SE–MI pretreatment with microwave effect than that without microwave effect. However, low microwave power and short microwave irradiation time were favorable for SE–MI pretreatment considering energy consumption.

Comparison of Accelerated Methods for the Extraction of Phenolic Compounds from Different Vine-Shoot Cultivars

Most research on the extraction of high-priced compounds from vineyard/wine byproducts has traditionally been focused on grape seeds and skins as raw materials. Vine-shoots can represent an additional source to those materials, the characteristics of which could depend on the cultivar. A comparative study of hydroalcoholic extracts from 18 different vineyard cultivars obtained by superheated liquid extraction (SHLE), microwave-assisted extraction (MAE), and ultrasound-assisted extraction (USAE) is here presented. The optimal working conditions for each type of extraction have been investigated by using multivariate experimental designs to maximize the yield of total phenolic compounds, measured by the Folin-Ciocalteu method, and control hydroxymethylfurfural because of the organoleptic properties of furanic derivatives and toxicity at given levels. The best values found for the influential variables on each extraction method were 80% (v/v) aqueous ethanol at pH 3, 180 °C, and 60 min for SHLE; 140 W and 5 min microwave irradiation for MAE; and 280 W, 50% duty cycle, and 7.5 min extraction for USAE. SHLE reported better extraction efficiencies as compared to the other two approaches, supporting the utility of SHLE for scaling-up the process. The extracts were dried in a rotary evaporator, reconstituted in 5 mL of methanol, and finally subjected to liquid-liquid extraction with n-hexane to remove nonpolar compounds that could complicate chromatographic separation. The methanolic fractions were analyzed by both LC-DAD and LC-TOF/MS, and the differences in composition according to the extraction conditions were studied. Compounds usually present in commercial wood extracts (mainly benzoic and hydroxycinnamic acids and aldehydes) were detected in vine-shoot extracts.

Microwave-assisted methanolysis of green coffee oil

Optimisation of a microwave-assisted methanolysis was performed to obtain cafestol and kahweol directly from green coffee oil (Coffea arabica). A two-factor (the methanolysis period and temperature), three-level, factorial experimental design (3(2)) was adopted. The methanolysis procedure was performed under microwave irradiation, using closed vessel and accurate fast responding internal fibre-optic temperature probe. The effects on the responses were measured by HPLC. After 3 min of microwave irradiation (hold time) at 100°C, with 500 mg of green coffee oil, a yield higher than 99% was obtained. The yield of this reaction is 26% after 2h when working under conventional heating. The methods described in the literature lead to long reaction times, poor yields and formation of side products. The microwave-assisted technique proved to be faster, avoided undesired side products and gave better conversion, when compared to conventional heating process.

Low-Thermal-Budget Solution Processing of Thin Films of Zinc Ferrite and Other Complex Oxides

ABSTRACTFurther miniaturization of magnetic and electronic devices demands thin films of advanced nanomaterials with unique properties. Spinel ferrites have been studied extensively owing to their interesting magnetic and electrical properties coupled with stability against oxidation. Being an important ferrospinel, zinc ferrite has wide applications in the biological (MRI) and electronics (RF-CMOS) arenas. The performance of an oxide like ZnFe2O4depends on stoichiometry (defect structure), and technological applications require thin films of high density, low porosity and controlled microstructure, which depend on the preparation process. While there are many methods for the synthesis of polycrystalline ZnFe2O4powder, few methods exist for the deposition of its thin films, where prolonged processing at elevated temperature is not required. We report a novel, microwave-assisted, low temperature (&lt;100°C) deposition process that is conducted in the liquid medium, developed for obtaining high quality, polycrystalline ZnFe2O4thin films on technologically important substrates like Si(100). An environment-friendly solvent (ethanol) and non-hazardous oxide precursors (β-diketonates of Zn and Fe in 1:2 molar ratio), forming a solution together, is subjected to irradiation in a domestic microwave oven (2.45 GHz) for a few minutes, leading to reactions which result in the deposition of ZnFe2O4films on Si (100) substrates suspended in the solution. Selected surfactants added to the reactant solution in optimum concentration can be used to control film microstructure. The nominal temperature of the irradiated solution, i.e., film deposition temperature, seldom exceeds 100°C, thus sharply lowering the thermal budget. Surface roughness and uniformity of large area depositions (50x50 mm2) are controlled by tweaking the concentration of the mother solution. Thickness of the films thus grown on Si (100) within 5 min of microwave irradiation can be as high as several microns. The present process, not requiring a vacuum system, carries a very low thermal budget and, together with a proper choice of solvents, is compatible with CMOS integration. This novel solution-based process for depositing highly resistive, adherent, smooth ferrimagnetic films on Si (100) is promising to RF engineers for the fabrication of passive circuit components. It is readily extended to a wide variety of functional oxide films.

Synthesis and Characterization of CeO2-SiO2Nanoparticles by Microwave-Assisted Irradiation Method for Photocatalytic Oxidation of Methylene Blue Dye

CeO2-SiO2nanoparticles were synthesized for the first time by a facile microwave-assisted irradiation process. The effect of irradiation time of microwave was studied. The materials were characterized by N2adsorption, XRD, UV-vis/DR, and TEM. All solids showed mesoporous textures with high surface areas, relatively small pore size diameters, and large pore volume. The X-ray diffraction results indicated that the as-synthesized nanoparticles exhibited cubic CeO2without impurities and amorphous silica. The transmission electron microscopy (TEM) images revealed that the particle size of CeO2-SiO2nanoparticles, which were prepared by microwave method for 30 min irradiation times, was around 8 nm. The photocatalytic activities were evaluated by the decomposition of methylene blue dye under UV light irradiations. The results showed that the irradiation under the microwave produced CeO2-SiO2nanoparticles, which have the best crystallinity under a shorter irradiation time. This indicates that the introduction of the microwave really can save energy and time with faster kinetics of crystallization. The sample prepared by 30 min microwave irradiation time exhibited the highest photocatalytic activity. The photocatalytic activity of CeO2-SiO2nanoparticles, which were prepared by 30 min irradiation times was found to have better performance than commercial reference P25.

Microwave coagulation using a perfusion microwave electrode: Preliminary experimental study using ex vivo and in vivo liver.

To assess the coagulation capability of a perfusion microwave electrode (PME) as a key component of microwave coagulation therapy, a preliminary experimental study was performed using ex vivo and in vivo livers. For a microwave electrode, a PME was employed. Using a PME, saline was passed through the electrode and injected continuously into the target tissue. Using an ex vivo bovine liver, the range of tissue coagulation was measured for various volumes of infused saline and microwave outputs. Using an in vivo porcine liver, the efficiency of coagulation by a PME was compared with that of radiofrequency ablation (RFA) using a cool-tip needle. In an ex vivo bovine liver, the range of tissue coagulation increased as the flow rate of saline increased. In the in vivo porcine liver, the range of coagulation was similar to that found in the ex vivo bovine liver. With a PME under conditions of a microwave output of 80 W, a flow rate of 3 ml/min and irradiation time of 5 min, the range of coagulation was 44.8±2.8 mm [maximum vertical diameter: (a)] x 31.2±2.4 mm [maximum transverse diameter: (b)]. The range of RFA (cool-tip needle) at 12 min was 46.0±2.0 mm (a) x 30.2±2.0 mm (b). With only 5 min of microwave irradiation, the use of a PME enabled induction of the same range of coagulation that was obtainable by RFA for 12 min. In comparison with microwave coagulation without saline infusion, the use of a PME made it possible to extend the range of tissue coagulation to a range equal to that of RFA in a short time. Microwave coagulation using a PME may be one of the suitable tissue coagulation systems for local ablation treatment.

Rapid one-step synthesis, characterization and functionalization of silica coated gold nanoparticles

This paper describes a rapid, simple and one-step method for preparing silica coated gold (Au@SiO 2) nanoparticles with fine tunable silica shell thickness and surface functionalization of the prepared particles with different groups. Monodispersed Au nanoparticles with a mean particle size of 16 nm were prepared by citrate reduction method. Silica coating was carried out by mixing the as prepared Au solution, tetraethoxysilane (TEOS) and ammonia followed by microwave (MW) irradiation. Although there are several ways of coating Au nanoparticles with silica in the literature, each of these needs pre-coating step as well as long reaction duration. The present method is especially useful for giving the opportunity to cover the colloidal Au particles with uniform silica shell within very short time and forgoes the use of a silane coupling agent or pre-coating step before silica coating. Au@SiO 2 nanoparticles with wide range of silica shell thickness (5–105 nm) were prepared within 5 min of MW irradiation by changing the concentration of TEOS only. The size uniformity and monodispersity were found to be better compared to the particles prepared by conventional methods, which were confirmed by dynamic light scattering and transmission electron microscopic techniques. The prepared Au@SiO 2 nanoparticles were further functionalized with amino, carboxylate, alkyl groups to facilitate the rapid translation of the nanoparticles to a wide range of end applications. The functional groups were identified by XPS, and zeta potential measurements.

Highly Efficient Microwave‐Assisted Substitution of β‐Dicarbonyl Compounds with Secondary Alcohols Using Fluoroboric Acid as the Catalyst

AbstractThe microwave‐assisted substitution of β‐dicarbonyl compounds with secondary alcohols has been achieved efficiently, using very cheap fluoroboric acid (HBF4) as the catalyst. For various β‐dicarbonyl compounds and a series of secondary alcohols, the direct substitution gives high yields after only 5 min of microwave irradiation.