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Abstract: Zinc oxide can be called a multifunctional material thanks to its unique physical and chemical properties. The first part of this paper presents the most important methods of preparation of ZnO divided into metallurgical and chemical methods. In materials science, zinc oxide is classified as a semiconductor in group II-VI, whose covalence is on the boundary between ionic and covalent semiconductors. The variety of structures of nanometric zinc oxide means that ZnO can be classified among new materials with potential applications in many fields of nanotechnology. One-dimensional structures make up the largest group, including nanorods , -needles , -helixes, -springs and -rings , -ribbons , -tubes -belts , -wires and -combs .

In this review, the methods of synthesis, modification and application of zinc oxide will be discussed. The zinc oxide occurs in a very rich variety of structures and offers a wide range of properties. Methods of Synthesis of Nano- and Micrometric Zinc Oxide 2. This process was developed by Samuel Wetherill, and takes place in a furnace in which the first layer consists of a coal bed, lit by the heat remaining from the previous charge.

Above this bed is a second layer in the form of zinc ore mixed with coal. The immediate reaction of the zinc vapour with oxygen from the air produces ZnO. The particles of zinc oxide are transported via a cooling duct and are collected at a bag filter station. The product consists of agglomerates with an average particle size ranging from 0. 1 to a few micrometres .

The ZnO particles are mainly of spheroidal shape. This has led to the development of a great variety of techniques for synthesizing the compound. The mechanochemical method was proposed by Ao et al. ZnO with an average crystallite size of 21 nm. The milling process was carried out for 6 h, producing ZnCO3 as the zinc oxide precursor.

C produced ZnO with a hexagonal structure. Tests showed that the size of the ZnO crystallites depends on the milling time and calcination temperature. C produced nanocrystallites of ZnO with an average size of 26 nm. This confirmed the important role played by zinc chloride in preventing agglomeration of the nanoparticles. The study also aimed to examine the effects of oxalic acid as an organic PCA, and different milling times, on the crystal structure, average particle size and morphology of ZnO nanopowders. The method involves fast and spontaneous reduction of a solution of zinc salt using a reducing agent, to limit the growth of particles with specified dimensions, followed by precipitation of a precursor of ZnO from the solution. Zinc oxide has also been precipitated from aqueous solutions of zinc chloride and zinc acetate .

Controlled parameters in this process included the concentration of the reagents, the rate of addition of substrates, and the reaction temperature. Zinc oxide was produced with a monomodal particle size distribution and high surface area. A controlled precipitation method was also used by Hong et al. The resulting precipitate was calcined by two different methods.

The single step process with the large scale production without unwanted impurities is desirable for the cost-effective preparation of ZnO nanparticles. In order to reduce the agglomeration among the smaller particles, the starch molecule which contains many O-H functional groups and could bind surface of nanoparticles in initial nucleation stage, was used. Another process of controlled precipitation of zinc oxide was carried out by Wang et al. The ZnO obtained had a narrow range of particle sizes, from 9 to 20 nm. XRD analysis showed both the precursor and the ZnO itself to have a wurtzite structure exclusively. The particle size was affected by temperature, calcination time, flow rate and concentration of the supply phase. In a report of Jia et al.

From a fundamental point of view, these findings provide new insights into the growth of ZnO crystals and arm researchers with potential strategies for the controllable synthesis of ZnO in liquid media. In processes of synthesis of nanopowders based on precipitation, it is increasingly common for surfactants to be used to control the growth of particles. The presence of these compounds affects not only nucleation and particle growth, but also coagulation and flocculation of the particles. The surfactant method involves chelation of the metal cations of the precursor by surfactants in an aqueous environment. C to remove residues of the surfactant.

The product was highly crystalline ZnO with a wurtzite structure and with small, well-dispersed spherical nanoparticles in size of 50 nm. The presence of the surfactant was found to affect both the shape and size of the resulting ZnO particles. Figure 2 shows the effect of SDS on the structure of the ZnO crystal. This changes in properties and extends its range of applications. ZnO powder by sol-gel method from zinc acetate dihydrate, oxalic acid, using ethanol as solvent. The prepared zinc oxide has a hexagonal wurtzite structure with the particles of a spherically shaped. The sol-gel method was also used to obtain nanocrystalline zinc oxide by Ristić et al.

High-filling, unifrom, ordered ZnO nanotubes have been successully prepared by sol-gel method into ultrathin AAO membrane. Integrating the ultrathin AAO membranes with the sol-gel technique may help to fabricate high-quality 1D nanomaterials and to extend its application as a template for nanostructures growth. C and left for several days. An example of a hydrothermal reaction is the synthesis of zinc oxide as proposed by Chen et al. In the autoclave hydrothermal heating takes place at a programmed temperature for a set time, followed by cooling.

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