Microwave-assisted Hydrothermal Treatment Research Articles

Histidine-derived nontoxic nitrogen-doped carbon dots for sensing and bioimaging applications.

Nitrogen-doped (N-doped) photoluminescent carbon dots (CDs) were prepared by a one-pot microwave-assisted hydrothermal treatment using histidine as the sole carbon source in the absence of acid, alkali, or metal ions. With a diameter of 2-5 nm, the synthesized CDs had apparent lattice fringes and exhibited an excitation-dependent photoluminescent behavior. The CDs were highly yielded, well-dispersed in aqueous solution, and showed high photostability in the solutions of a wide range of pH and salinity. They were used as probes to identify the presence of Fe(3+) ions with a detection limit of 10 nM. With confirmed nontoxicity, these CDs could enter the cancer cells, indicating a practical potential for cellular imaging and labeling.

Total oxidation of lean propane over α-Fe2O3 using microwaves as an energy source

A combined process of catalytic oxidation and microwave heating for the treatment of lean propane-air mixture, and “green” hematite synthesis were described in this work. The study demonstrated that removal and oxidation efficiencies of propane over α-Fe2O3 were above 99.9 % after a short microwave radiation time (from 9 min) and they were strongly dependent on the applied microwave power. Comparing to conventional heating, the catalytic performance of hematite was enhanced by lowering the operation temperature under microwave conditions. Nanocrystalline α-Fe2O3 catalyst with mesoporous structure was successfully prepared by a microwave-assisted hydrothermal treatment of iron hydroxide in an aqueous solution without any alkali additive, and characterized by XRD, TEM, TPR-H2, FTIR and N2 adsorption–desorption. The microwave-induced heating over hematite catalyst appears to be a very effective alternative method for air treatment to the destruction and removal of short chain hydrocarbons.

Effects of Microwave Processing on the Properties of Nickel Oxide/Zirconia/Ceria Composites

The effect of a microwave-assisted hydrothermal (MWH) treatment on the structural, thermal, and electrical properties of NiO/ZrO2:8 mol%Y2O3(YSZ)/CeO2 (60/20/20 wt.%) composite was investigated. Powders were synthesized by a hydroxide coprecipitation-impregnation technique, in which Ni and Ce oxides were coprecipitaed in a suspension containing YSZ. Simultaneous thermogravimetry and differential thermal analysis revealed that MWH treatment promotes the crystallization of coprecipitated phases. X-ray diffraction analysis was used for phase identification and the calculated lattice parameters indicated the formation of YSZ:CeO2 solid solution during sintering. Impedance spectroscopy measurements showed that the electrical properties of the composite samples were not significantly affected by the MHW.

Long-range and short-range structures of cube-like shape SrTiO3 powders: microwave-assisted hydrothermal synthesis and photocatalytic activity

We report herein a detailed study on the influence of microwave-assisted hydrothermal (MAH) treatment time on both long and short range structures around Ti atoms of SrTiO3 powders. Few studies have been carried out on short-order structural properties as well as the relationship between the local order and the SrTiO3 photocatalytic properties. We use X-ray diffraction to determine the long-range structure, while the local environment around the Ti atom is probed with X-ray absorption spectroscopy and the vibration frequencies are investigated by Raman spectroscopy. The faster crystallization of SrTiO3 powders provided by the MAH system resulted in large distortions of Ti-O bond lengths which remain unchanged even for a longer MAH treatment time. Despite the long-range structure being associated with ideal TiO6 clusters, X-ray absorption spectroscopy measurements identified the presence of undercoordinated TiO5 clusters. Compared with the reference bulk SrTiO3, the hierarchical SrTiO3 cube-like shape showed enhanced photocatalytic activity, which was associated with the presence of these TiO5 clusters. Field emission scanning electron microscopy (FE-SEM) revealed that the superstructures based on a cube-like shape are formed by an assembly process, becoming well defined as a function of MAH treatment time.

Deposition of photocatalytically active TiO2 films by inkjet printing of TiO2 nanoparticle suspensions obtained from microwave-assisted hydrothermal synthesis

In this paper, we present an inkjet printing approach suited for the deposition of photocatalytically active, transparent titanium oxide coatings from an aqueous, colloidal suspension. We used a bottom-up approach in which a microwave-assisted hydrothermal treatment of titanium propoxide aqueous solutions in the presence of ethylenediaminetetraacetic acid and triethanolamine was used to create suspensions containing titania nanoparticles. Different inkjet printing set-ups, electromagnetic and piezoelectric driven, were tested to deposit the inks on glass substrates. The presence of preformed titania nanoparticles was expected to make it possible to reduce the heating temperature necessary to obtain the functionality of photocatalysis which can widen the application range of the approach to heat-sensitive substrates. We investigated the crystallinity and size of the obtained nanoparticles by electron microscopy and dynamic light scattering. The rheological properties of the suspensions were evaluated against the relevant criteria for inkjet printing and the jettability was analyzed. The photocatalytic activity of the obtained layers was analyzed by following the decomposition of a methylene blue solution under UV illumination. The influence of the heat treatment temperature on the film roughness, thickness and photocatalytic activity was studied. Good photocatalytic performance was achieved for heat treatments at temperatures as low as 150 °C, introducing the possibility of using this approach for heat-sensitive substrates.

Microwave-assisted hydrothermal degradation of fructose and glucose in subcritical water

Abstract The microwave-assisted hydrothermal treatment of fructose and glucose is studied under subcritical conditions, in a temperature window between 180 °C and 250 °C. The suitability of microwave heating as well as the impact of the reaction variables temperature and time on the extent of the substrate conversion and on the product composition are elucidated. With rate constants of 3.6*10−3 s−1 (fructose) and 5.2*10−4 s−1 (glucose) at 220 °C the decomposition of fructose took place about eight times faster than that of glucose. 5-HMF was found to be the major decomposition product for both monosaccharides, with maximum yields of 47% 5-HMF (for fructose) and 30% (for glucose) at a maximum selectivity of 65% and 50%, respectively. All other liquid monomeric products like levulinic acid, furfural and trihydroxy benzene are formed at cumulative yields below 16%.

Microwave-assisted rapid synthesis of anatase TiO2 nanocrystals with exposed {0 0 1} facets

Anatase TiO2 nanocrystals with tunable percentage of reactive {001} facets were rapidly synthesized by a microwave-assisted hydrothermal treatment of the mixed solution of tetrabutyl titanate (20g), HF solution (3ml) and additional water (0–21g) at 200°C for 30min. The resulted sample is denoted as Wx, where x represents the volumes of additional water. The photocatalysts were characterized by X-ray diffraction, transmission electron microscopy, nitrogen adsorption–desorption isotherms and X-ray photoelectron spectroscopy. The photocatalytic activity of the photocatalyst was evaluated by degradation of brilliant red X3B (X3B), an anionic dye, and by a photoluminescence technique using coumarin as a probe molecule. With increasing the amount of additional water from 0 to 21ml, the shapes of TiO2 nanocrystals evolve from nanosheets to truncated octahedral bipyramids, resulting in a steady decrease in the percentage of exposed {001} facets (from 71% to 23%). The photocatalytic activity of the resulted surface fluorinated TiO2 nanocrystals increases first and then decrease with increase in the amount of water, and W3 sample with exposed 60% of {001} facets shows the highest photocatalytic activity. However, for the surface clean TiO2 samples by washing with NaOH solution, W9 with 51% of {001} facets shows the highest photocatalytic activity. Our experimental results reflect that both crystal planes and surface chemistry play very important roles on the photocatalytic activity of anatase TiO2 nanocrystals.

Microwave-assisted hydrothermal synthesis of marigold-like ZnIn2S4 microspheres and their visible light photocatalytic activity

Marigold-like ZnIn2S4 microspheres were synthesized by a microwave-assisted hydrothermal method with the temperature ranging from 80 to 195°C. X-ray diffraction, X-ray photoelectron spectroscopy, nitrogen sorption analysis, UV–visible spectroscopy, scanning electron microscopy and transmission electron microscopy were used to characterize the products. It was found that the crystallographic structure and optical property of the products synthesized at different temperatures were almost the same. The degradation of methyl orange (MO) under the visible light irradiation has been used as a probe reaction to investigate the photocatalytic activity of as-prepared ZnIn2S4, which shows that the ZnIn2S4 sample synthesized at 195°C shows the best photocatalytic activity for MO degradation. In addition, the photocatalytic activities of all the samples prepared by the microwave-assisted hydrothermal method are better than those prepared by a normal hydrothermal method, which could be attributed to the formation of more defect sites during the microwave-assisted hydrothermal treatment.

Mesoporous RuO 2 for the next generation supercapacitors with an ultrahigh power density

The 3D mesoporous, well crystalline RuO 2 film prepared via the evaporation-induced self-assembled method (EISA) successfully demonstrates the extremely high power performances (e.g., excellent capacitive behavior at 10,000 mV s −1, ultrahigh-frequency capacitive responses (the absence of a knee point in the Nyquist plot), and 2.6 MW kg −1 with an acceptable energy density of 4.6 Wh kg −1). These excellent capacitive performances were identified by means of voltammetric and electrochemical impedance spectroscopic (EIS) analyses. The mesoporous (with mean pore spacing of 18.1 nm) and crystalline nature of this film was characterized by means of the field emission scanning electron microscopy (FE-SEM), Brunaur–Emmett–Teller (BET) method, small-angle X-ray scattering (SAXRS), high-resolution transmission electron microscopy (HR-TEM), electron diffraction (ED), and X-ray diffraction (XRD) analyses. This mesoporous, well crystalline RuO 2 film constrains the redox transition to the superficial region meanwhile the tailored mesoporous structure increases the electrochemically active centers, promotes the penetration of electrolytes, provides the “proton reservoirs”, and enhances the rate of electron transport simultaneously for the ultrahigh power application. The specific capacitance of this mesoporous RuO 2 can be enhanced from 84 to 185 F g −1 after the microwave-assisted hydrothermal treatment.

Rapid Synthesis of Titania Nanowires by Microwave-Assisted Hydrothermal Treatments

Titania nanowires in the anatase phase were successfully synthesized using microwave-assisted hydrothermal methods. To accelerate the fabrication procedure of nanowires, an assisted microwave power was applied to the traditional hydrothermal apparatus. Titanium dioxide nanoparticles (ST01, 100% anatase) were utilized as the starting material, and various parameters (including heating temperature, reaction time, radiation power, and the concentration of raw materials) were discussed in the present study. All of the products were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and Brunauer−Emmett−Teller (BET) surface area analysis. Transmission electron microscopy (TEM) investigations revealed that these nanowires have a diameter of ca. 80−150 nm, with a length ranging from several micrometers to tens of micrometers. Relatively low energy consumption was observed in manufacturing the titania nanowires under the conditions of 350 W of power at 210 °C for only 2 h (and then calci...

Catalytic Method for N-Methyl-4-pyridone Synthesis in the Presence of ZnAl2O4

A composite oxide ZnAl2O4 was prepared by microwave-assisted hydrothermal treatment, a precursor mixture of hydroxides obtained by precipitation of aluminium and zinc nitrates. Characterization by TEM, XRD and textural studies shows that ZnAl2O4 is nanosized and is a micro/mesoporous material with large a surface area (140 m2/g). The gas phase catalytic methylation of 4-hydroxypyridine in the presence of the ZnAl2O4 catalyst was performed in a continuous process at atmospheric pressure in the temperature range of 240–360 °C. A mixture of O- and N-alkylated products, namely 4-methoxypyridine and N-methyl-4-pyridone were obtained. The alkylation of 4-hydroxypyridine with methanol at 345 °C offered 87.6% selectivity towards N-methyl-4-pyridone with about 89% 4-hydroxypyridine conversion.

Effect of synthesis conditions on the pore structure and degree of heteroatom insertion in Zr-doped SBA-15 silica-based materials prepared by classical or microwave-assisted hydrothermal treatment

Zirconium doped mesoporous SBA-15 materials with initial Si/Zr ratios of 10 and 20 have been prepared under microwave-hydrothermal conditions using a non-ionic triblock copolymer surfactant as structure-directing agent, as well as zirconyl chloride and zirconium propoxide as two zirconium sources. The effect of a short (5 min) and long (1.5 h) sol ageing time on the degree of heteroatom incorporation was tested. Changes in the mesopore size and arrangement were monitored with the aid of powder X-ray diffraction (XRD) and gas nitrogen adsorption/desorption at 77 K. The degree of Zr incorporation into the silica framework of the final product was inferred from the analysis by energy-dispersive X-ray spectroscopy and compared with the starting value based on the amount of zirconium added to the reaction mixture. In the case of samples with the lowest initial theoretical molar ratio, the synthesis conditions did not affect the degree of heteroatom incorporation into the silica framework and all zirconium could be introduced under the conditions applied. The structure of materials was strongly dependent on the conditions used, and a more regular pore arrangement was obtained for longer maturation stages. The classical hydrothermal synthesis did not result in a significantly higher efficiency in the heteroatom insertion, though microwave-assisted treatment yielded materials with thicker pore walls. Further increase in the amount of zirconium added to the initial gel did not contribute to a greater heteroatom loading in the final product under microwave conditions.

Hydrothermal and microwave-assisted synthesis of nanocrystalline ZnO photocatalysts

Abstract Novel methods of highly dispersed ZnO powder synthesis based on microwave heating of salt mixtures as well as low temperature microwave-assisted hydrothermal treatment of zinc hydroxide were developed. It was shown that the obtained powders possess high photocatalytic activity in a model reaction of methyl orange photodegradation.

Microwave-assisted hydrothermal synthesis and characterizations of PrF 3 hollow nanoparticles

PrF 3 hollow nanoparticles were synthesized by the microwave-assisted heating hydrothermal treatment of the corresponding colloidal rare earth fluorides precipitates. The products were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The TEM imaging showed that the PrF 3 nanoparticles prepared in this way had hollow sphere-like morphology with the mean particle size of about 31 nm. XRD showed that the PrF 3 hollow nanoparticles had high purity and high crystallinity.

Rapid synthesis of oriented silicalite-1 membranes by microwave-assisted hydrothermal treatment

The objective of this work was to evaluate the potential of microwave (MW) heating for the rapid synthesis of thin silicalite-1 membranes by secondary growth from microwave-derived silicalite-1 seeds. The MW seed suspension was deposited by slip-casting on commercial α-Al 2O 3 supports and zeolite membranes were obtained after a synthesis time as short as 30–150 min, by MW-assisted secondary growth at 393–453 K. The membrane thickness varied typically from 200 nm to a few μm, with glassy-like or columnar morphologies, respectively, depending on the synthesis conditions. The morphology, thickness, homogeneity, crystal preferential orientation (CPO) and single gas permeation properties of the silicalite-1 membranes have been studied in relation to the synthesis parameters. The highest single gas permeances and medium n/ i-butane selectivity were obtained with the membrane prepared at the lowest temperature (120 min at 403 K), although a higher synthesis temperature (433 K) was needed to get a higher n/ i-butane selectivity (40–50). An oblique crystallographic preferred orientation (101-CPO) was found for this attractive membrane whose N 2 permeance reached 1.5 × 10 −6 mol m −2 s −1 Pa −1 at 294 K. A very low ideal selectivity was obtained for the membrane prepared at 453 K for 120 min, which is more clearly c-oriented ( c-axis CPO), and in which defects were detected.