Microwave-assisted Hydrothermal Route Research Articles

Inorganic frameworks based on bimetallic nanoparticles encapsulated in hollow MnO2 structures

A novel microwave-assisted hydrothermal route has been demonstrated to allow the synthesis of hollow MnO2 particles with dandelion-like structure. The as-prepared particles show the ability to enclose PtAu bimetallic nanoparticles which good electrocatalytic activity toward the oxidation of glycerol and the reduction of oxygen in alkaline media, in addition to high stability, what we attribute to the MnO2 cage, which is responsible for the confinement of reaction intermediates impeding their diffusion to the bulk electrolyte. Considering the inherent properties of MnO2, such as low acute toxicity to living organisms, biocompatibility, stability in neutral and alkaline media and the ability to enclose other possible different materials, including functionalized nanoparticles, we suggest a broad field of potential application for these structures, such as drug delivery and bioimaging, catalysis, (bio)sensing, etc.

A selective morphosynthetic approach for single crystalline hematite through morphology evolution via microwave assisted hydrothermal synthesis

Abstract Hematite single crystals are one of most promising materials which is able to apply for the field of catalyst industry. Even its prospective applicability, strategies for improving its catalytic performance are still not enough to meet the requirements for industrial application. In the present work, we demonstrate a morphology selective one pot reaction for a hematite single crystal by microwave-assisted hydrothermal synthesis. The microwave-assisted hydrothermal route, taking advance of the exposed crystal facet-based activity of the material, is suggested to crystal facet controlling synthesis of the hematite crystal. The dispersant and capping agents such as sodium carboxymethyl cellulose and N 2 H 4 , respectively, play a very important role in controlling the exposed crystal facet and morphological uniformity for a selective morphology of hematite crystal. In addition, systematically experimental examination of the well-exposed facet is interestingly giving an insight into the crystal growth and morphological evolution of single crystalline hematite from plate to tetrahexagonal bipyramid, octahedral and oblique cubic shape. Physicochemical techniques were used to characterize and evaluate the photoactivity of the material in detail. In here, a well-defined crystal facet is obtained and expected to be applied as an efficient catalysts and its synthetic methodology is applicable to other metal oxide photocatalyst.

Carbon quantum dots originated from chitin nanofibers as a fluorescent chemoprobe for drug sensing

Abstract This paper reports the synthesis of carbon quantum dots (CQDs) from chitin nanofibers (CNFs) as a new carbon source via microwave assisted-hydrothermal route within three minutes. The FL of as-prepared CQDs can be quenched after addition of Cu2+ ions. However, in the presence of d -Penicillamine (DPA), Cu2+ ions prefer to bind to DPA owing to their high affinity toward DPA and subsequently the FL of CQDs is effectively recovered. Therefore, here for the first time the Cu2+ ion modified CQDs (CQDs-Cu2+) was utilized as a FL chemoprobe for quantitative determination of DPA in biological and pharmaceutical samples with satisfactory results.

Microwave-assisted hydrothermal synthesis, temperature quenching and laser-induced heating effect of hexagonal microplate β-NaYF4: Er3+/Yb3+ microcrystals under 1550 nm laser irradiation

Rare earth doped materials can usually achieve high efficiency upconversion luminescence (UCL) under high power density infrared laser excitation, in which process the heat generation cannot be avoided, and then induce temperature dependent UCL quenching. This paper dealt with the temperature sensing property and 1550nm laser irradiation induced thermal effect of Er3+/Yb3+ co-doped NaYF4 microcrystals. The sample was synthesized via a microwave-assisted hydrothermal route, and its crystal structure and microscopic morphology were characterized by means of XRD and FE-SEM. To describe the temperature quenching behaviors for the two typical green emissions of Er3+, a model was proposed and well explained the temperature dependence of two green UCL intensities. Temperature sensing property for NaYF4: Er3+/Yb3+ microcrystals were studied by fluorescence intensity ratio technique, and the temperature sensitivity were also obtained. Furthermore, the 1550nm laser induced thermal effect was studied, and the result showed that the sample temperature increased with increasing excitation power density.

Microwave-Assisted Hydrothermal Synthesis of CuS Nanoplate Films on Conductive Substrates as Efficient Counter Electrodes for Liquid-Junction Quantum Dot-Sensitized Solar Cells

CuS nanoplate films were synthesized on FTO conducting glass substrates via a facile microwave-assisted hydrothermal (MHT) route and applied directly as a low-cost and high-efficiency counter electrode (CE) for liquid-junction quantum dot-sensitized solar cells (QDSSC). The two-dimensional structure of nanoplate provide high surface area for more active catalytic sites and easy accessibility for polysulfide electrolyte, which leads to the remarkable decrease in charge transfer resistance and the enhancement of catalytic activity of CE. As a result, the liquid-junction QDSSC assembled with the prepared CuS-2 CE achieves maximum power conversion efficiency (PCE) of 4.97%, which is superior than that of CuS fabricated by conventional chemical bath deposition method based cell (PCE 4.06%) and the 100 nm thin sputtered Pt/FTO CE based cell (PCE 3.08%). Furthermore, the prepared CuS exhibits highly chemical stability in polysulfide electrolyte, resulting in reproducible performance. These results show the potential application of MHT synthetic method in preparing metal chalcogenide CE for high-performance liquid-junction QDSSC in the future.

Microwave-assisted hydrothermal synthesis and adsorption properties of carbon nanofibers for methamphetamine removal from aqueous solution using a response surface methodology

Carbon nanofibers have been synthesized by simple facile microwave-assisted hydrothermal route and applied as adsorbent for the fast adsorption of methamphetamine. The topological property of CNF was analyzed using XRD, SEM, TEM, and nitrogen adsorption–desorption instrumental techniques. The significance of the independent variables and their interactions were tested by the analysis of variance (ANOVA) and t-test statistics. Influential parameters are optimized using BBD implemented with RSM, and the optimized value of pH, adsorbent dose and temperature was found to be 8.0, 0.1g and 298K, respectively. Under optimal conditions, the removal efficiency of methamphetamine was found to be 55.25mg/g.

Electrical behavior of cerium dioxide films exposed to different gases atmospheres

Here we present an easy-reproducible microwave-assisted hydrothermal route for preparing pure nanocrystalline CeO2 films. The produced materials were characterized using a wide range of techniques (X-ray diffraction, field emission gun scanning electron microscopy, Raman spectroscopy) to understand the synthesis dependent changes in crystallographic structure, and crystallite size. Raman and X-ray diffraction techniques revealed that the films were free of secondary phases and that they crystallize in the cubic structure. The observed hydrodynamic particle size larger than the crystallite size confirms the aggregation phenomenon. Gas sensing measurements have been carried out to rationalize the type and number of surface adsorbed groups and overall nanostructure. Electrical conductance variations, owing to gases adsorption onto semiconductor oxide films surfaces, were observed in this work. Chemiresistive CeO2 film properties depend on the intergranular barrier heights and width.

Adsorption of toxic carbamate pesticide oxamyl from liquid phase by newly synthesized and characterized graphene quantum dots nanomaterials

Graphene quantum dots have been synthesized using the microwave-assisted hydrothermal route. The surface textural and morphological structure of synthesized adsorbent i.e. graphene quantum dots was analyzed using various analytical techniques such as X-ray diffraction, Transmission electron Microscopy, Atomic Force Microscopy and N2 adsorption-desorption instrumental techniques. The application of graphene quantum dots as an adsorbent for the removal of noxious pesticide compound i.e. oxamyl from aqueous solutions was well investigated and elucidated. The impact of several effective parameters such as effect of agitation speed, pH, adsorbent dose, contact time, temperature and initial concentration on sorption efficiency was studied and optimized using batch adsorption experiments. The optimized pH for maximum oxamyl adsorption was found to be 8.0 and for the maximum adsorption rates the adsorbent dose of 0.6g was found to be optimum to carry out the adsorption with in less than 25min of contact time. From the results obtained, it is clear that for all contact times, an increase in oxamyl concentration resulted in increase in the percent oxamyl removal. The adsorption equilibrium and kinetic data were well fitted and found to be in good agreement with the Langmuir isotherm and pseudo-second-order kinetic model.

Acetone gas sensor based on α-Ag2WO4 nanorods obtained via a microwave-assisted hydrothermal route

This manuscript addresses the synthesis of one-dimensional (1D) α-Ag2WO4 nanorod-like structures via a microwave-assisted hydrothermal method to be used as an acetone gas sensor. The nanorods showed excellent gas-sensing performance, evidenced by their sensor response and repeatability, as well as a good range of detection (0.5–20.0 ppm). The manuscript also proposes an easy and efficient way of obtaining 1-D α-Ag2WO4 nanorods that exhibit remarkable acetone sensing properties.

Enhanced upconversion of NaYF_4:Er^3+/Yb^3+ phosphors prepared via the rapid microwave-assisted hydrothermal route at low temperature: phase and morphology control

Monophasic NaYF4:Er(3+)/Yb(3+) crystals were synthesized via the microwave-assisted hydrothermal route at 180°C. Microwave heating during the hydrothermal process substantially reduces the duration of reaction for the formation of cubic-NaYF4:Er(3+)/Yb(3+) nanocrystals from 6 h to 30 min. As the duration of the reaction increases, cubic-NaYF4:Er(3+)/Yb(3+) nanocrystals are transformed to uniform hexagonal-NaYF4:Er(3+)/Yb(3+) microprisms because of the enhanced reaction kinetics. Bright upconverted emission from the NaYF4:Er(3+)/Yb(3+) crystal, obtained by the efficient two-photon excitation, is related to crystal structure and morphology. The hexagonal microprisms exhibit better upconversion and are employed in security applications.

Novel ozone gas sensor based on ZnO nanostructures grown by the microwave-assisted hydrothermal route

Good quality ZnO nanostructures were obtained by the microwave-assisted hydrothermal synthesis, at low reaction temperatures, using zinc acetate as the starting precursor. X-ray diffraction confirmed the crystallinity of the ZnO nanostructures, which resulted free of impurities. Field emission gun scanning electron microscopy analysis revealed that the ZnO nanostructures crystallized at 120°C were more homogeneous and had a constant diameter along the entire wire length, exhibiting an ideal defect density that favors the gas sensing response. A new ozone gas sensor based on these nanostructures was evaluated at low exposure times (15s) by recording the change in the film resistance. The ZnO nanostructures showed good sensitivity even at low ozone concentration (100ppb), and fast response and short recovery time at 200°C, demonstrating great potential for a variety of applications. Two main effects were observed: the first one is intrinsic to that of the sample, while the second is a consequence of the surface and interface complex cluster defects, which produce extrinsic defects.

Improvement in the Grain Growth of Plasma-Treated Nano-Sized ZnO Films and Their Characterization.

The well-aligned ZnO nanorods were rapidly grown on an indium tin oxide (ITO)-coated glass substrate using Al-doped ZnO (AZO) thin film as seed layer by the microwave-assisted hydrothermal chemical route. The optimal growth conditions for the well-aligned ZnO nanorods were obtained by modulating H2 plasma pretreatment time for the seed layer and synthesis time for ZnO nanorods. The H2 plasma effect of the seed layer on the alignment, growth rate and crysallinity of ZnO nanods is also demonstrated. The synthesized ZnO nanorods were annealed in atmosphere of N2, O2 and H2 + N2 mixed gas to improve the related physical characteristics, the ZnO nanorods on grapheme/ITO substrate were also investigated. The results show that the alignment and growth rate of ZnO nanorods depends on the physical characteristics and roughness of the seed layer, which can be improved by H2 plasma pretreatment. The average growth rate of ZnO nanorods synthesized by microwave hydrothermal technique is about 2.2 μm/hr which significantly superior to other conventional techniques. After the appropriate N2 annealing treatment, good quality and well-aligned ZnO nanorods, which are single crystal with stacking defects and pyramid or candle shape, were obtained. A fundamental model of the effect of H2 plasma pretreatment on the surface of seed layer and the growth of ZnO nanorods using a microwave-assisted hydrothermal chemical route is also described.

Nanocrystalline BaSnO₃ as an Alternative Gas Sensor Material: Surface Reactivity and High Sensitivity to SO₂.

Nanocrystalline perovskite-type BaSnO3 was obtained via microwave-assisted hydrothermal route followed by annealing at variable temperature. The samples composition and microstructure were characterized. Particle size of 18–23 nm was unaffected by heat treatment at 275–700 °C. Materials DC-conduction was measured at variable temperature and oxygen concentration. Barium stannate exhibited n-type semiconductor behavior at 150–450 °C with activation energy being dependent on the materials annealing temperature. Predominant ionosorbed oxygen species types were estimated. They were shown to change from molecular to atomic species on increasing temperature. Comparative test of sensor response to various inorganic target gases was performed using nanocrystalline SnO2-based sensors as reference ones. Despite one order of magnitude smaller surface area, BaSnO3 displayed higher sensitivity to SO2 in comparison with SnO2. DRIFT spectroscopy revealed distinct interaction routes of the oxides surfaces with SO2. Barium-promoted sulfate formation favoring target molecules oxidation was found responsible for the increased BaSnO3 sensitivity to ppm-range concentrations of SO2 in air.

Effects of microwave condition on the formation and characteristics of TiO2 submicron-sized beads and its use in all-plastic flexible dye-sensitized solar cells

Carbon and/or nitrogen doped TiO2 mesoporous beads have been prepared using hexamethylenetetramine as the dopant source in a microwave-assisted hydrothermal synthetic route. It is found that the hydrothermal time has pronounced effects on the crystallinity, grain size, and carbon and/or nitrogen incorporation into the TiO2 lattices. The carbon and/or nitrogen doping creates interband localized states above the valence band of TiO2 and therefore enhances the optical response of TiO2 in the visible light region. Selected carbon and/or nitrogen doped TiO2 mesoporous beads are made into flexible dye-sensitized solar cells (FDSSCs), which are analyzed by solar simulator, electrochemical impedance spectroscopy, intensity modulated photocurrent/photovoltage spectroscopy, and incident photon-to electron conversion efficiency. The photoelectrode consisting of the doped TiO2 mesoporous beads shows improved FDSSCs due to the better electron injection and collection efficiencies. The electron lifetime is also relatively high, indicating that the doping does not introduce recombination. Overall speaking, both improved optical and electronic properties are demonstrated and discussed in depth.

Li2SiO3 fast microwave-assisted hydrothermal synthesis and evaluation of its water vapor and CO2 absorption properties

A series of lithium metasilicate (Li2SiO3) powder materials has been successfully synthesized by the microwave-assisted hydrothermal route using lithium hydroxide and tetraethyl-orthosilicate-derived sol precursors. Ceramic powders were obtained under hydrothermal conditions of autogenous pressure in the presence of a nonionic surfactant. The production of pure and well-crystallized Li2SiO3 using very short reaction times at low temperatures was shown by X-ray diffraction, scanning electron microscopy, and N2 adsorption-desorption analyses. Synthesized Li2SiO3 particles were nanocrystalline and exhibited different morphologies and specific surface areas depending on the synthesis conditions. Additionally, the capability of selected Li2SiO3 samples to absorb H2O and CO2 was evaluated via thermogravimetric analyses by varying the temperature, carrier gas, and water vapor concentration. Li2SiO3 particles exhibited interesting textural and morphological characteristics that make them suitable for use as a CO2 absorbent and which suggest that they also have the potential to be used in other applications.

Citric acid-mediated microwave-assisted hydrothermal synthesis and luminescence property of NaSm(MoO4)2 submicro-crystals

Uniform and well-crystallized NaSm(MoO4)2 submicro-crystals were successfully synthesized via a facile microwave-assisted hydrothermal route using citric acid as surfactant. The as-prepared products were characterized by powder X-ray diffractometer, scanning electron microscope, and photoluminescence (PL) spectrometer. The results show that the amount of citric acid and pH value play important roles on the crystallinity, morphology and size of the NaSm(MoO4)2 products. The possible growth process of the products was proposed based on the time-dependent shape evolution, which experiences a fast crystallization–dissolution–recrystallization growth process. The PL properties of the NaSm(MoO4)2 crystals with different morphologies excited by near-ultraviolet light (404 nm) were also investigated. The PL intensity of the annealed submicro-particles is higher than that of uncalcined submicro-particles and nanosheets. The results implies that the annealed NaSm(MoO4)2 submicro-particles have potential application in the field of optoelectronic materials in the future.

Comparative study on upconversion luminescence and temperature sensing of α- and β-NaYF4:Yb3+/Er3+ nano-/micro-crystals derived from a microwave-assisted hydrothermal route

Yb3+/Er3+ co-doped α- and β-phase NaYF4 nano-/micro-crystals were prepared through a microwave-assisted hydrothermal route. The crystal structure and microscopic morphology of the samples were characterized by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). Frequency upconverted emissions from the two thermally coupled excited state 2H11/2 and 4S3/2 levels of Er3+ in both phases of phosphors were recorded at temperatures ranging from room temperature to 573K under 980nm infrared laser excitation. The time scanning upconversion spectra were investigated in detail to reveal the thermal effect induced by laser irradiation and the luminescent thermal stability of the two phases NaYF4 polycrystals. Comparison of the upconversion luminescence and the sensitivity between the two phases NaYF4 polycrystals indicated that β-phase NaYF4 won much stronger luminescent intensity, better luminescent thermal stability, and higher temperature sensitivity.

Microwave-assisted hydrothermal synthesis and characterization of ZnO nanorods

For the purpose of this study, the nanorods of zinc oxide were synthesized by rapid microwave-assisted hydrothermal route. The microstructure and surface morphology of the sensitized nanorods were characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM). XRD results indicate that synthesized ZnO nanorods have wurtzite phase. The calculated value of the particle size using Debye Scherrer formula and Williamson Hall plot was found to be 20–28nm and 35.3nm, respectively. Low uniformity distribution of rod-like morphology (60–80nm in diameter and average length about 250nm) are seen in TEM micrographs. The optical parameters of the prepared ZnO nanorods have been calculated using Kubeleka–Munk approach for the UV–vis diffuse reflectance spectrum. It is found that the direct transition optical band gap of the studied sample is 3.17eV. The direct current electrical conductivity (σ) was increased from 6.7×10-8 to 3×10-7Ω-1cm-1 with increasing the temperature (T) in the range (300–425K). The obtained variation of σ with T refers that the conductivity mechanism is controlled by thermally activated process.

Fast synthesis and electrochemical performance of hollow NiCo2O4 flowerlike microstructures

Hollow NiCo2O4 flowerlike microstructures with good electrochemical performance were successfully prepared through a fast microwave-assisted hydrothermal route and sequential pyrolysis.

Dual Ce4+/Fe3+ Redox Phenomena into Nanocrystalline Ce1–xFexO2–x/2 Solid Solution

The Ce1–xFexO2–x/2 solid solution was synthesized using a microwave-assisted hydrothermal route. The solubility limit corresponds to a Fe (x) content equal to 0.15. Our previous works showed also that isolated Fe3+ distorted octahedral sites and Fe3+ clusters are randomly distributed into the ceria network. Thermogravimetric analysis (TGA) under Ar/5%H2 revealed a higher reduction rate for Fe-substituted ceria with changes of slope and the appearance of pseudoplateaus around 400 and 550 °C. The first Fe K-edge X-ray absorption near-edge spectroscopy (XANES) spectra recorded at several temperatures up to 700 °C shift gradually to lower energies as the temperature increases, with the formation of metallic iron starting at 550 °C, as confirmed by X-ray diffraction analysis. Moreover, on the basis of the principal component factor analysis, three K-edge structures associated with three oxidation states of iron (Fe3+, Fe2+, and Fe0) have been distinguished during this temperature-programmed reduction. The second component attributed to Fe2+ is the most intense one around 500 °C. Beyond this temperature, the coexistence of the three oxidation states Fe3+/Fe2+/Fe0 has to be mentioned up to 650 °C, the temperature at which metallic iron is mainly stabilized. Ce LIII-edge and Fe K-edge XANES spectra recorded in quick extended X-ray absorption fine structure mode and at various temperatures show the progressive reduction of Ce4+ and Fe3+ starting beyond 250 °C, which is in good agreement with the change of slope observed on the TGA curves. The Ce LIII-edge XANES spectrum of pure CeO2 recorded at 700 °C under reducing atmosphere exhibits much less Ce3+ stabilized into the fluorite network than Fe-substituted ceria. Fe3+ ions randomly distributed into ceria strongly contribute to enhancing the Ce4+ reducibility properties. The Ce4+ reduction seems to appear at lower temperatures and is slower than the Fe3+ reduction, whose speed decreases around 400 °C. At this temperature, the Fe2+ and Fe3+ components dominate and the Ce3+ content remains high. However, a plateau with the stabilization of Ce4+/Ce3+ mixed valences around 500 °C appears where the Fe2+ contribution is also a maximum and the Fe reduction is slower. Because of the large content of either Ce3+ and Fe2+, a charge-transfer equilibrium Ce4+ + Fe2+ ⇔ Ce3+ + Fe3+ may occur at this temperature at which a change of slope on TGA curve also is observed with the appearance of a plateau at 550 °C associated with the Fe0 demixtion phenomenon. Then, after reaching the maximum Fe2+ rate, the Ce4+ reduction rate increases again up to 700 °C, the temperature at which all Fe atoms have been transformed into metallic iron.