Microwave-hydrothermal synthesis of ceramic powders

Inorganics, 2014

An efficient microwave-assisted synthesis of TiO 2 :(B) nanorods, using titanium tetraisopropoxide (TTIP), benzyl alcohol as the solvent, together with boric acid and oleic acid as the additive reagents, has been developed. Chemical modification of TTIP by oleic acid was demonstrated as a rational strategy to tune the shape of TiO 2 nanocrystals toward nanorod formation. The differently-shaped TiO 2 :(B) nanocrystals were characterized in detail by transmission electron microscopy (TEM), inductively coupled plasma-atomic emission spectroscopy (ICP-AES), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy and nitrogen absorption-desorption. Oleic acid coordinated on the nanocrystal surface was removed by the reduction of its carboxyl group, and the photocatalytic activity of bare TiO 2 nanocrystals, under visible light irradiation, was also evaluated. The synthesized TiO 2 anatase nanorods exhibited a good photoactivity and completely degraded Rhodamine B solution within three hours.

Journal of Advanced Ceramics, 2012

The present article aims to give a brief overview about the advantages of the hydrothermal crystallization method for the synthesis of advanced ceramics. Emphasis is given, not only on the conventional hydrothermal crystallization, but also on some of its variants; such as ultrasound-assisted, electrochemical-assisted, microwave-assisted and surfactant-assisted hydrothermal methods which open up new opportunities for the synthesis of ceramic materials with novel properties demanded for advanced applications. In the current work the synthesis of barium titanate (BaTiO 3 ), lithium metasilicate (Li 2 SiO 3 ) and sodium-potassium niobate (Na, K)NbO 3 powders are reported as cases of study.

Inorganics, 2015

The synthesis of complex functional inorganic materials, such as oxides, can be successfully performed by using microwave irradiation as the source of heat. To achieve this, different routes and set-ups can be used: microwave-assisted synthesis may proceed in the solid state or in solution, aqueous or not, and the set ups may be as simple and accessible as domestic oven or quite sophisticated laboratory equipment. An obvious advantage of this innovative methodology is the considerable reduction in time-minutes rather than hours or days-and, as a consequence, energy saving. No less important is the fact that the particle growth is inhibited and the broad variety of different microwave or microwave-assisted synthesis techniques opens up opportunities for the preparation of inorganic nanoparticles and nanostructures. In this work, various microwave synthesis techniques have been employed: solid-state microwaves, single-mode microwaves using a TE10p cavity and microwave-assisted hydrothermal synthesis. Relevant examples are presented and discussed.

Novel one-dimensional (1D) ceria nanostructure has been investigated as a promising and practical approach for the reforming of methanol reaction. Size and shape of the ceria nanomaterials are directly involved with the catalytic activities. Several general synthesis routes as including soft and hard template-assemble phenomenon for the preparation of 1D cerium oxide are discussed. This preparation phenomenon is consisting with low cost and ecofriendly. Nanometer-sized 1D structure provides a high-surface area that can interact with methanol and carbon-monoxide reaction. Overall, nanometer-sized structure provides desirable properties, such as easy recovery and regeneration. As a result, the use of 1D cerium has been suitable for catalytic application of reforming. In this paper, we describe the 1D cerium oxide syntheses route and then summarize their properties in the field of CO oxidation and steam reforming of methanol approach.

Journal of Nanoparticle Research, 2012

The exploration of novel synthetic methodologies that control both size and shape of functional nanostructure opens new avenues for the functional application of nanomaterials. Here, we report a new and versatile approach to synthesize SnO 2 nanocrystals (rutile-type structure) using microwave-assisted hydrothermal method. Broad peaks in the X-ray diffraction spectra indicate the nanosized nature of the samples which were indexed as a pure cassiterite tetragonal phase. Chemically and physically adsorbed water was estimated by TGA data and FT-Raman spectra to account for a new broad peak around 560 cm -1 which is related to defective surface modes. In addition, the spherical-like morphology and low dispersed distribution size around 3-5 nm were investigated by HR-TEM and FE-SEM microscopies. Room temperature PL emission presents two broad bands at 438 and 764 nm, indicating the existence of different recombination centers. When the size of the nanospheres decreases, the relative intensity of 513 nm emission increases and the 393 nm one decreases. UV-Visible spectra show substantial changes in the optical absorbance of crystalline SnO 2 nanoparticles while the existence of a small tail points out the presence of localized levels inside the forbidden band gap and supplies the necessary condition for the PL emission.

Nano, 2012

Various methods for the synthesis of copper nanoparticles employing chemical, physical and biological techniques considering bottom-up and top-down methods synthesis have been studied. The properties of copper nanoparticles depend largely on their synthesis procedures. The results from various investigations performed by di®erent scientists using these methods have been summarized. The applications, characterization techniques, advantages and disadvantages of each synthesis method are also the point of discussion. A detailed study of the results reveals that chemical reduction methods are most suitable for the synthesis of copper nanoparticles. Chemical reduction of copper salts using ascorbic acid (Vitamin C) is a new and green approach in which ascorbic acid is used both as the reduction and capping agent. This approach is the most e®ective and is also economical. Wide applications have been reported in various¯elds, including heat transfer, catalyst production, electronics and medicine at a commercial scale. This process is nontoxic, environment-friendly and economical. The applications, characterization techniques, advantages and disadvantages of each synthesis method have been presented.

CrystEngComm, 2012

In this paper, aggregated CaWO 4 micro-and nanocrystals were synthesized by the co-precipitation method and processed under microwave-assisted hydrothermal/solvothermal conditions (160 C for 30 min). According to the X-ray patterns, all crystals exhibited only the scheelite-type tetragonal structure. The data obtained by the Rietveld refinements revealed that the oxygen atoms occupy different positions in the [WO 4 ] clusters, suggesting the presence of lattice distortions. The crystal shapes as well as its crystallographic orientations were identified by field-emission scanning electron microscopy and high-resolution transmission electron microcopy. Electronic structures of these crystals were evaluated by the first-principles quantum mechanical calculations based on the density functional theory in the B3LYP level. A good correlation was found between the experimental and theoretical Raman and infrared-active modes. A crystal growth mechanism was proposed to explain the morphological evolution. The ultraviolet-visible absorption spectra indicated the existence of intermediary energy levels within the band gap. The highest blue photoluminescence emission, lifetime and quantum yield were observed for the nanocrystals processed in the microwave-assisted solvothermal method.

Journal of Materials Research, 2003

Orthorhombic KNbO3 and NaNbO3 powders were hydrothermally synthesized in KOH and NaOH solutions (6.7–15 M) at 150 and 200 °C. An intermediate hexaniobate species formed first before eventually converting to the perovskite phase. For synthesis in KOH solutions, the stability of the intermediate hexaniobate ion increased with decreasing KOH concentrations and temperatures. This led to significant variations in the induction periods and accounted for the large disparity in the mass of recovered powder for different processing parameters. It is also believed that protons were incorporated in the lattice of the as-synthesized KNbO3 powders as water molecules and hydroxyl ions.

RSC Advances, 2014

A new route for the aqueous phase synthesis of single crystalline, shape-selective, magnetic MnWO 4 nanomaterials on a DNA scaffold has been reported. The synthesis was done by the reaction of MnCl 2 $4H 2 O with Na 2 WO 4 in DNA within five minutes of microwave heating. The process exclusively generates wire-like, flake-like and rice-like morphology just by tuning the DNA to Mn(II) salt and WO 4 2À ion concentration and changing other reaction parameters. The field-cooled (FC) and zero-field-cooled (ZFC) magnetization study reveals that the flake-like structure shows the highest magnetization at 5 K compared to that of the wire-like and rice-like structures. The potential of the shape-selective MnWO 4 nanomaterials has been tested in two different applications, firstly in a catalysis study for the decomposition of toxic KMnO 4 and secondly in electrochemical supercapacitor applications. It was found that the MnWO 4 nanomaterials showed different specific capacitance (SC) values for the various shapes and the order of the SC values is: wire-like > flake-like > rice-like. The highest SC of 34 F g À1 was observed for MnWO 4 having wire-like shape. The yields of the products with uniform shapes have been found to be significantly high and the synthesized materials are stable for more than six months under ambient conditions. The present work will find a new platform for the generation of other mixed oxides using bio-molecules as scaffolds at low temperature and in short time scales. Moreover, the synthesized material might be useful for other potential applications in the fields of catalysis, sensors, energy storage materials and so on.

Frontiers in Chemistry

This study analyzes the mapping of temperature distribution generated by graphene in a glass slide cover after illumination at 808 nm with a good thermal resolution. For this purpose, Er,Yb:NaYF 4 nanoparticles prepared by a microwave-assisted solvothermal method were used as upconversion luminescent nanothermometers. By tuning the basic parameters of the synthesis procedure, such as the time and temperature of reaction and the concentration of ethanol and water, we were able to control the size and the crystalline phase of the nanoparticles, and to have the right conditions to obtain 100% of the β hexagonal phase, the most efficient spectroscopically. We observed that the thermal sensitivity that can be achieved with these particles is a function of the size of the nanoparticles and the crystalline phase in which they crystallize. We believe that, with suitable changes, these nanoparticles might be used in the future to map temperature gradients in living cells while maintaining a good thermal resolution.

Nanoscale, 2012

Microwave-assisted syntheses of colloidal nanocrystals (NCs), in particular CdSe quantum dots (QDs), have gained considerable attention due to unique opportunities provided by microwave dielectric heating. The extensive use of microwave heating and the frequently suggested specific microwave effects, however, pose questions about the role of the electromagnetic field in both the formation and quality of the produced QDs. In this work a one-pot protocol for the tunable synthesis of monodisperse colloidal CdSe NCs using microwave dielectric heating under carefully controlled conditions is introduced. CdSe QDs are fabricated using selenium dioxide as a selenium precursor, 1-octadecene as a solvent and reducing agent, cadmium alkyl carboxylates or alkyl phosphonates as cadmium sources, 1,2-hexadecanediol to stabilize the cadmium complex and oleic acid to stabilize the resulting CdSe QDs. Utilizing the possibilities of microwave heating technology in combination with accurate online temperature control the influence of different reaction parameters such as reaction temperature, ramp and hold times, and the timing and duration of oleic acid addition have been carefully investigated. Optimum results were obtained by performing the reaction at 240 C applying a 5 min ramp time, 2 min hold time before oleic acid addition, 90 s for oleic acid addition, and a 5 min hold time after oleic acid addition (8.5 min overall holding at 240 C). By using different cadmium complexes in the microwave protocol CdSe QDs with a narrow size distribution can be obtained in different sizes ranging from 0.5-4 nm by simply changing the cadmium source. The QDs were characterized by TEM, HRTEM, UV-Vis, and photoluminescence methods and the size distribution was monitored by SAXS. Control experiments involving conventional conductive heating under otherwise identical conditions ensuring the same heating and cooling profiles, stirring rates, and reactor geometries demonstrate that the electromagnetic field has no influence on the generated CdSe QDs. The resulting CdSe NCs prepared using either conductive or microwave dielectric heating exhibited the same primary crystallite size, shape, quantum yield and size distribution regardless of the heating mode.

Journal of Materials Science: Materials in Electronics, 2021

The enhancement of microwave absorbing properties in dysprosium ion (Dy 3? )substituted nickel-zinc ferrites (Ni 0.5 Zn 0.5 Dy x Fe 2-x O 4 ; x = 0.00, 0.01, 0.03, 0.05, 0.07 and 0.09) has been investigated in this work. The ferrite powders were synthesized by microwave-hydrothermal method and then powders were densified at 900 °C for 40 min using microwave furnace. The samples' structural and morphological properties were studied using X-ray diffraction and scanning electron microscopy (SEM), respectively. The structural result confirms the spinel phase under low Dy 3? content, like the pure Ni-Zn ferrite, while a secondary phase of DyFeO 3 appears after the content of Dy 3? exceeds a certain limit (x [ 0.07). Morphological analysis from the SEM images reveals the formation of spherical grains of the samples. DC resistivity of the samples has been measured using two-probe method. Magnetic hysteresis data confirm the soft magnetic nature of the samples. The vector network analyzer results show that adjusting the content of Dy 3? is significant in changing the magneto-dielectric properties and microwave absorption capacity of the materials. The composition x = 0.07 sample showed a reflection coefficient of -33.24 dB at the frequency and bandwidth of 10.31 GHz and 2.59 GHz for an absorber thickness of 2.5 mm for losses less than -10 dB. This acquired result indicates that the investigated samples could be used as a microwave absorber application in X-band.

MATERIAIS 2022

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Advanced Science, 2022

The pillars of Green Chemistry necessitate the development of new chemical methodologies and processes that can benefit chemical synthesis in terms of energy efficiency, conservation of resources, product selectivity, operational simplicity and, crucially, health, safety, and environmental impact. Implementation of green principles whenever possible can spur the growth of benign scientific technologies by considering environmental, economical, and societal sustainability in parallel. These principles seem especially important in the context of the manufacture of materials for sustainable energy and environmental applications. In this review, the production of energy conversion materials is taken as an exemplar, by examining the recent growth in the energy-efficient synthesis of thermoelectric nanomaterials for use in devices for thermal energy harvesting. Specifically, "soft chemistry" techniques such as solution-based, solvothermal, microwave-assisted, and mechanochemical (ball-milling) methods as viable and sustainable alternatives to processes performed at high temperature and/or pressure are focused. How some of these new approaches are also considered to thermoelectric materials fabrication can influence the properties and performance of the nanomaterials so-produced and the prospects of developing such techniques further.

Environmental Chemistry Letters, 2020

Abatement of emissions of greenhouse gases such as methane and carbon dioxide is crucial to reduce global warming. For that, dry reforming of methane allows to convert methane and carbon dioxide into useful synthesis gas, named 'syngas', a gas mixture rich in hydrogen and carbon monoxide. However, this process requires high temperatures of about 900 °C to activate methane and carbon dioxide because dry reforming of methane reaction is highly endothermic. Therefore, a solid catalyst with appropriate thermal properties is needed for the reaction. As a consequence, efficient heating of the reactor is required to control heat transfer and optimize energy consumption. Microwave-assisted dry reforming of methane thus appears as a promising alternative to conventional heating. Here we review the recent research on microwave-assisted dry reforming of methane. We present thermodynamical aspects of the dry reforming of methane, and basics of microwave heating and apparatus. We analyse reformers that use microwave heating. Catalysts used in a microwave-assisted reformer are presented and compared with reactors using conventional heating. Finally, the energy balance is discussed.

Processes

Zinc oxide (ZnO) has been widely studied over the last decade for its remarkable properties in optoelectronic and photovoltaic devices because of its high electron mobility and excitonic properties. It has probably the broadest range of nanostructured forms that are also easy and cheap to synthesize using a wide variety of methods. The volume of recent work on ZnO nanostructures and their devices can potentially overshadow significant developments in the field. Therefore, there is a need for a concise description of the most recent advances in the field. In this review, we focus on the effect of ZnO nanostructure morphologies on the performance of ZnO-based solar cells sensitized using methylammonium lead iodide perovskite. We present an exhaustive discussion of the synthesis routes for different morphologies of the ZnO nanostructure, ways of controlling the morphology, and the impact of morphology on the photoconversion efficiency of a given perovskite solar cell (PSC). We find tha...

Nanoscale, 2014

Nanocrystalline titania are a robust candidate for various functional applications owing to its non-toxicity, cheap availability, ease of preparation and exceptional photochemical as well as thermal stability. The uniqueness in each lattice structure of titania leads to multifaceted physico-chemical and opto-electronic properties, which yield different functionalities and thus influence their performances in various green energy applications. The high temperature treatment for crystallizing titania triggers inevitable particle growth and the destruction of delicate nanostructural features. Thus, the preparation of crystalline titania with tunable phase/particle size/morphology at low to moderate temperatures using a solution-based approach has paved the way for further exciting areas of research. In this focused review, titania synthesis from hydrothermal/solvothermal method, conventional sol-gel method and sol-gel-assisted method via ultrasonication, photoillumination and ILs, thermolysis and microemulsion routes are discussed. These wet chemical methods have broader visibility, since multiple reaction parameters, such as precursor chemistry, surfactants, chelating agents, solvents, mineralizer, pH of the solution, aging time, reaction temperature/ time, inorganic electrolytes, can be easily manipulated to tune the final physical structure. This review sheds light on the stabilization/phase transformation pathways of titania polymorphs like anatase, rutile, brookite and TiO 2 (B) under a variety of reaction conditions. The driving force for crystallization arising from complex species in solution coupled with pH of the solution and ion species facilitating the orientation of octahedral resulting in a crystalline phase are reviewed in detail. In addition to titanium halide/alkoxide, the nucleation of titania from other precursors like peroxo and layered titanates are also discussed. The nonaqueous route and ball milling-induced titania transformation is briefly outlined; moreover, the lacunae in understanding the concepts and future prospects in this exciting field are suggested.

International Journal of Photoenergy, 2012

CeO 2-SiO 2 nanoparticles 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 N 2 adsorption, 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 CeO 2 without impurities and amorphous silica. The transmission electron microscopy (TEM) images revealed that the particle size of CeO 2-SiO 2 nanoparticles, 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 CeO 2-SiO 2 nanoparticles, 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 CeO 2-SiO 2 nanoparticles, which were prepared by 30 min irradiation times was found to have better performance than commercial reference P25.

Inorganics, 2015

This Special Issue on "Inorganic Syntheses Assisted by Microwave Heating" represents one of the few fully dedicated issues on inorganic microwave synthesis published by any international scientific journal and it features five papers and one review article. Microwave irradiation has been used for many years to accelerate chemical reactions. The very first application of 2.45 GHz microwaves in processing began with heating and cooking food in kitchens all over the world. However, the use of microwaves in analytical laboratories began later, starting with drying organic and inorganic samples in order to determine the amount of moisture [1] and later it was used for the dissolution or decomposition of not only biological but also geological materials [2-5]. Komarneni and Roy pioneered the use of microwaves in the liquid state for synthesis and published a seminal paper in Materials Letters in 1985 [6,7]. Unfortunately, the historical significance of this paper entitled "Titania gel spheres by a new sol-gel process" was not recognized by many as the title of this paper did not itself refer to microwaves. Nevertheless, the abstract clearly states that microwaves led to fast reaction as follows and we quote: "A new technique is devised for the preparation of solution derived titania gel spheres. The gel sphere preparation process involves the dispersal of the polymerized titanium ethoxide solution into a two-phase liquid suspension in kerosene followed by heating in a microwave oven. This is a simple and fast method for the preparation of xerogel spheres with diameter in the range of tens of micrometers which may be useful in coating technology by plasma-spraying". Gedye et al., 1986 [8] and Gigure et al., 1986 [9] may have been the first to report on the use of microwaves for organic syntheses in 1986. These two papers are generally recognized as pioneering

Materials Research Bulletin, 2021

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RSC Advances, 2017

Barium fluoride (BaF 2) nanoparticles (NPs) with different sizes were produced through a hydrothermal microwave method (HTMW). The microstructural and electronic properties of the synthesized BaF 2 NPs were investigated using X-ray powder diffraction combined with the Rietveld refinement method and Williamson-Hall formalism, X-ray photoemission spectroscopy (XPS) and scanning electron microscopy (SEM). From the Rietveld method, we have found that the lattice parameter of BaF 2 NPs is smaller than that observed for BaF 2 in its crystal bulk form. These results demonstrated that the lattice parameter shows dependence on size of particle, increasing for larger particles, reducing strain-surface effects. XPS analyses showed that no other elements were present in the material. Photoluminescence (PL) studies in the vacuum ultraviolet (VUV) and visible (VIS) range were also performed to investigate the luminescence properties. The PL results showed a slight shift in the self-trapped exciton (STE) edge for samples with higher particle sizes. In addition, the band gap energy (E g) was found to be around 10.5 eV for all samples. The observed lattice contraction/expansion was in concordance with the bond-order-length strength (BOLS) correlation mechanism. Therefore, we concluded that this behavior was purely due to surface stress as a result of particle size decreasing.

Critical Reviews in Solid State and Materials Sciences, 2018

Zinc Oxide is an important and multi-purpose material in various industries due to its particular chemical and physical properties. Discovering a cheap, fast, clean, safe, and easy to use method, to synthesize this oxide nanoparticle has attracted a lot of attention in recent applications. The unique properties of the microwave and its special heating capabilities have yielded desirable outcomes by combining different synthesis methods. In the recent years, the vast majority of studies focus on the microwave-assisted synthesis of zinc oxide nanoparticles. This review article attempts to go over the recent advancements on the synthesis of zinc oxide nanoparticles with the aid of microwave, different morphologies and applications obtained by this method. Various microwave-assisted synthesis methods are classified, including the solution-based methods such as hydrothermal, solgel, and combustion methods. Morphology of the nanoparticles can affect the properties, and subsequently, applications of these nanoparticles. On the other hand, there is great diversity of morphological and synthesis conditions of zinc oxide nanoparticles. Thus, categorizing the synthesis techniques and providing features of them, facilitates the selection of appropriate method for designing new hierarchical structures with potential properties for future applications. Also it is endeavored to focus on the formation mechanisms of these methods. Finally, the various morphologies obtained under microwave radiation and their formation mechanisms are discussed and the effective factors in the synthesis are analyzed and presented. The potential and suitable fields of development and progress in the future studies are also proposed.

International Journal of Energy Research, 2015

In this study, a two-step process involving a sol-gel and microwave-assisted hydrothermal (MH) techniques has been used to synthesize a novel TiO2 structure, namely, TiO2 mesoporous beads. Various hydrothermal synthesis parameters were investigated to study their effects on the resulting beads. It was found that within the processing conditions used, the TiO2 beads have similar sizes (~325 nm) and contain uniform-sized anatase TiO2 nanoparticles (<10 nm), which are single crystals. Crystalline TiO2 beads are obtained in as short as 15 min at a low temperature of only 140 °C without any postprocess calcination. Less surface oxygen vacancies was obtained at a higher MH temperature. Compare to the commercial P-25 TiO2 powders and conventional hydrothermal synthesized TiO2 beads, MH synthesized TiO2 beads exhibit much higher values of specific surface areas, pore volumes, and porosities. The use of TiO2 beads as photocatalyst and photoelectrode for dye-sensitized solar cells has also been investigated.