A Critical Assessment of the Specific Role of Microwave Irradiation in the Synthesis of ZnO Micro- and Nanostructured Materials

A Review of Microwave Synthesis of Zinc Oxide Nanomaterials: Reactants, Process Parameters and Morphoslogies

Zinc oxide (ZnO) is a multifunctional material due to its exceptional physicochemical properties and broad usefulness. The special properties resulting from the reduction of the material size from the macro scale to the nano scale has made the application of ZnO nanomaterials (ZnO NMs) more popular in numerous consumer products. In recent years, particular attention has been drawn to the development of various methods of ZnO NMs synthesis, which above all meet the requirements of the green chemistry approach. The application of the microwave heating technology when obtaining ZnO NMs enables the development of new methods of syntheses, which are characterised by, among others, the possibility to control the properties, repeatability, reproducibility, short synthesis duration, low price, purity, and fulfilment of the eco-friendly approach criterion. The dynamic development of materials engineering is the reason why it is necessary to obtain ZnO NMs with strictly defined properties. The present review aims to discuss the state of the art regarding the microwave synthesis of undoped and doped ZnO NMs. The first part of the review presents the properties of ZnO and new applications of ZnO NMs. Subsequently, the properties of microwave heating are discussed and compared with conventional heating and areas of application are presented. The final part of the paper presents reactants, parameters of processes, and the morphology of products, with a division of the microwave synthesis of ZnO NMs into three primary groups, namely hydrothermal, solvothermal, and hybrid methods.

Synthesis of Cr3+-doped TiO2 nanoparticles: characterization and evaluation of their visible photocatalytic performance and stability

Cr³⁺-doped TiO₂ nanoparticles (Ti-Cr) were synthesized by microwave-assisted sol-gel method. The Ti-Cr catalyst was characterized by X-ray diffraction, ultraviolet-visible diffuse reflectance spectroscopy, N₂ adsorption-desorption analysis, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, photoluminescence spectroscopy, X-ray photoelectron spectroscopy (XPS) and zetametry. The anatase mesoporous Ti-Cr material exhibited a specific surface area of 54.5 m²/g. XPS analysis confirmed the proper substitution of Ti⁴⁺ cations by Cr³⁺ cations in the TiO₂ matrix. The particle size was of average size of 17 nm for the undoped TiO₂ but only 9.5 nm for Ti-Cr. The Cr atoms promoted the formation of hydroxyl radicals and modified the surface adsorptive properties of TiO₂ due to the increase in surface acidity of the material. The photocatalytic evaluation demonstrated that the Ti-Cr catalyst completely degraded (4-chloro-2-methylphenoxy) acetic acid under visible light irradiation, while undoped TiO₂ and P25 allowed 45.7% and 31.1%, respectively. The rate of degradation remained 52% after three cycles of catalyst reuse. The higher visible light photocatalytic activity of Ti-Cr was attributed to the beneficial effect of Cr³⁺ ions on the TiO₂ surface creating defects within the TiO₂ crystal lattice, which can act as charge-trapping sites, reducing the electron-hole recombination process.

Specific microwave effect on Sn- and Ti-MFI zeolite synthesis

A specific microwave effect, demonstrated by the differences on Sn- and Ti-MFI zeolite preparation and characterization, provides a new direction for investigating microwave effects.

Greener and Sustainable Trends in Synthesis of Organics and Nanomaterials

Trends in greener and sustainable process development during the past 25 years are abridged involving the use of alternate energy inputs (mechanochemistry, ultrasound- or microwave irradiation), photochemistry, and greener reaction media as applied to synthesis of organics and nanomaterials. In the organic synthesis arena, examples comprise assembly of heterocyclic compounds, coupling and a variety of other name reactions catalyzed by basic water or recyclable magnetic nanocatalysts. Generation of nanoparticles benefits from the biomimetic approaches where vitamins, sugars, and plant polyphenols, including agricultural waste residues, can serve as reducing and capping agents. Metal nanocatalysts (Pd, Au, Ag, Ni, Ru, Ce, Cu, etc.) immobilized on biodegradable supports such as cellulose and chitosan, or on recyclable magnetic ferrites via ligands, namely dopamine or glutathione, are receiving special attention. These strategic approaches attempt to address most of the Green Chemistry Principles while producing functional chemicals with utmost level of waste minimization.

Microwave assisted synthesis of mesoporous NiCo2O4 nanosheets as electrode material for advanced flexible supercapacitors

Mesoporous nickel cobaltite (NiCo₂O₄) nanosheets are synthesized using a cost effective, ultra fast and environmentally friendly microwave assisted heating method followed by a post-calcination process of the as-prepared precursors.

Unraveling the mysteries of microwave chemistry using silicon carbide reactor technology

In the past few years, the use of microwave energy to heat chemical reactions has become an increasingly popular theme in the scientific community. This nonclassical heating technique has slowly progressed from a laboratory curiosity to an established method commonly used both in academia and in industry. Because of its efficiency, microwave heating dramatically reduces reaction times (from days and hours to minutes and seconds) and improves product purities or material properties among other advantages. Since the early days of microwave chemistry, researchers have observed rate-accelerations and, in some cases, altered product distributions as compared with reactions carried out using classical oil-bath heating. As a result, researchers have speculated that so-called specific or nonthermal microwave effects could be responsible for these differences. Much of the debate has centered on the question of whether the electromagnetic field can exert a direct influence on a chemical transformation outside of the simple macroscopic change in bulk reaction temperature. In 2009, our group developed a relatively simple "trick" that allows us to rapidly evaluate whether an observed effect seen in a microwave-assisted reaction results from a purely thermal phenomenon, or involves specific or nonthermal microwave effects. We use a microwave reaction vessel made from silicon carbide (SiC) ceramic. Because of its high microwave absorptivity, the vessel shields its contents from the electromagnetic field. As a result, we can easily mimic a conventionally heated autoclave experiment inside a microwave reactor under carefully controlled reaction conditions. The switch from an almost microwave transparent glass (Pyrex) to a strongly microwave absorbing SiC reaction vial under otherwise identical reaction conditions (temperature profiles, pressure, stirring speed) then allows us to carefully evaluate the influence of the electromagnetic field on the particular chemical transformation. Over the past five years we have subjected a wide variety of chemical transformations, including organic reactions, preparations of inorganic nanoparticles, and the hydrolysis of proteins, to the "SiC test." In nearly all of the studied examples, we obtained identical results from reactions carried out in Pyrex vials and those carried out in SiC vials. The data obtained from these investigations confirm that in the overwhelming majority of cases a bulk temperature phenomenon drives the enhancements in microwave chemistry and that the electromagnetic field has no direct influence on the reaction pathway.

Microwave-assisted synthesis of CdSe quantum dots: can the electromagnetic field influence the formation and quality of the resulting nanocrystals?

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.