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.

The aim of the paper is to explain the mechanism of zinc oxide (ZnO) nanoparticle (NP) size control, which enables the size control of ZnO NPs obtained in microwave solvothermal synthesis (MSS) within the size range between circa 20 and 120 nm through the control of water content in the solution of zinc acetate in ethylene glycol. Heavy water was used in the tests. The mechanism of ZnO NPs size control was explained, discussed and experimentally verified. The discovery and investigation of this mechanism was possible by tracking the fate of water molecules during the whole synthesis process. All the synthesis products were identified. It was indicated that the MSS of ZnO NPs proceeded through the formation and conversion of intermediates such as Zn(OH)(CHCOO) · xHO. Esters and HO were the by-products of the MSS reaction of ZnO NPs. We justified that the esterification reaction is the decisive stage that is a prerequisite of the formation of ZnO NPs. The following parameters of the obtained ZnO NPs and of the intermediate were determined: pycnometric density, specific surface area, phase purity, average particles size, particles size distribution and chemical composition. The ZnO NPs morphology and structure were determined using scanning electron microscopy.

Zinc oxide nanoparticles (ZnO NPs) were obtained by the microwave solvothermal synthesis (MSS) method. The precursor of the MSS reaction was a solution of hydrated zinc acetate in ethylene glycol with water addition. It was proved that by controlling the water concentration in the precursor it was possible to control the size of ZnO NPs in a programmed manner. The less the water content in the precursor, the smaller the size of ZnO NPs obtained. The obtained NPs with the average particle size ranging from 25 nm to 50 nm were characterised by homogeneous morphology and a narrow distribution of particle sizes. The following parameters of the obtained ZnO NPs were determined: pycnometric density, specific surface area, phase purity, chemical composition, lattice parameters, average particle size, and particle size distribution. The average size of ZnO NPs was determined using Scherrer’s formula, Nanopowder XRD Processor Demo web application, by converting the results of the specific surface area, and TEM tests using the dark field technique. ZnO morphology and structure were determined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The test performed by the X-ray powder diffraction (XRD) confirmed that crystalline ZnO, pure in terms of phase, had been obtained.

Microwave energy has been in use for many applications for more than 50 years, from communication, food processing, and wood drying to chemical reactions and medical therapy. The areas, where microwave technology is applied, include drying, calcination, decomposition, powder synthesis, sintering, and chemical process control. Before the year 2000, microwaves were used to produce ceramics, semiconductors, polymers, and inorganic materials; in next years, some new attempts were made as well. Nowadays, it has been found that microwave sintering can also be applied to sintered powder and ceramics and is more effective than conventional sintering. Particularly interesting is its use for the synthesis of nanomaterials. This review identifies the main sources of microwave generation, the delivery mechanisms of microwave energy, and the typical designs and configurations of microwave devices, as well as the measurement and construction material problems related to microwave technology. We focus our attention on the configurations, materials, optimized geometries, and solvents used for microwave devices, providing examples of products, especially nanoparticles and other nanomaterials. The identified microwave devices are divided into four groups, depending on the scale, the maximum pressure developed, the highest temperature for sintering, or other special multi-functions. The challenges of using microwave energy for the synthesis of nanopowders have been identified as well. The desirable characteristics of microwave reactors in the synthesis of nanostructures, as well as their superiority over conventional synthetic methods, have been presented. We have also provided a review of the commercial and self-designed microwave reactors, digestors, and sintering furnaces for technology for synthesis of nanomaterials and other industries.