Comparative Study of Zinc Oxide Nanostructures Synthesized by Oxidization of Zinc Foil and Zinc Powder

Physical properties 13

Mechanical properties 14

Electronic properties 15

Optical properties 15

This and other lattice symmetry properties result in the piezoelectricity of hexagonal ZnO and zincblende, and the pyroelectricity of hexagonal ZnO. The structure of ZnO can be simply described as a number of alternating planes composed of tetrahedrally coordinated O2- and Zn2+ ions, located alternately along the c-axis [23-27]. The high heat capacity and thermal conductivity, low thermal expansion, and high melting temperature of ZnO are beneficial for ceramics. Among the tetrahedrally bonded semiconductors, ZnO has been reported to have the highest piezoelectric tensor or at least one of comparable to that of GaN and AlN.

The ZnO bandgap can be further tuned to ~3-4 eV by alloying with magnesium oxide or cadmium oxide. ZnO is a wide-bandgap semiconductor that exhibits luminescent properties in the near-ultraviolet and visible region. The emission properties of ZnO nanoparticles in the visible range are largely dependent on their synthetic method, as they can be attributed to surface defects.

It is shown that if we limit the quantum size it can significantly increase the exciton binding energy, but an interesting observation is that the green emission intensity increases with a decrease in the diameter of the nanowires. It was also observed that quantum confinement was responsible for causing a blue shift in the near UV emission peak in the ZnO nanobelts.

Figure 2.1 Crystal structure of the ZnO (the of tetrahedrally coordinated O 2- and Zn 2+ ions)

Rubber manufacture 17

The applications of zinc oxide powder are numerous, and the most important ones are summarized below. For materials science applications, zinc oxide has a high refractive index, good thermal, bonding, antibacterial and UV protection properties.

Medical uses 17

Zinc oxide is a component of cigarette filters for removing selected components from tobacco smoke. A filter consisting of charcoal impregnated with zinc oxide and iron oxide removes significant amounts of HCN and H2S from tobacco smoke without affecting its taste [31].

Food additives 18

Anti-corrosive coatings 18

Electronic applications 18

The sensor detects hydrogen concentrations down to 10 parts per million at room temperature, while there is no response to oxygen. As field-effect transistors, they may not even need a p-n junction, thus avoiding the p-type doping problem of ZnO. Piezoelectricity in ZnO-coated textile fibers has been shown capable of “self-powered nanosystems” with daily mechanical stress generated by wind or body movements [ 30 ].

Zinc oxide is used in zinc-carbon dry cells, zinc-silver oxide batteries, nickel-cadmium oxide batteries, and even secondary batteries. In fuel cells, zinc oxide is used as an electrode material, a cathode material, and as a fuel element.

Synthesis of ZnO

Physical vapor deposition (PVD): 20

Zinc oxide crystallizes in three forms: hexagonal wurtzite, cubic zinc blende, and the rarely observed cubic rock salt. ZnO has a relatively large direct band gap of ~3.3 eV and a relatively large excitation binding energy (60 meV) compared to the thermal energy (26 meV) at room temperature. Advantages associated with a large bandgap include higher breakdown voltages, the ability to sustain large electric fields, lower electronic noise, and high-temperature, high-power operation.

Physical vapor deposition (PVD) is a variety of vacuum deposition and is a general term used to describe any of a variety of methods to deposit thin films by condensing a vaporized form of material onto various surfaces (eg, in semiconductor wafers) . Cathodic Arc Deposition: In which a high-power arc discharged into the target material explodes some into highly ionized vapor. Electron Beam Physical Vapor Deposition: In which the material to be deposited is heated to a high vapor pressure by electron bombardment in "high" vacuum.

Evaporative deposition: where the material to be deposited is heated to a high vapor pressure by electrical resistive heating in "low" vacuum. Pulsed laser deposition: where a high-powered laser ablates material from the target into a vapor. Sputter deposition: where a glow plasma discharge (usually localized around the "target" by a magnet) bombards the material and spews some of it away as a vapor.

Chemical vapor deposition (CVD) is a chemical process used to produce high-purity, high-performance solid materials.

Experimental Instruments

Scanning Electron Microscope 25

Differential Scanning Calorimetry 28

Zn (foil or powder) shows a color change from initial white to gray after cooling, indicating the formation of ZnO nanorods. The surface of the as-received zinc contains 100% zinc with no trace of zinc oxide on the surface, as confirmed by EDX analysis. The structure in the case of zinc foil has become finer and the presence of ZnO can be found by EDX studies.

While in the case of oxidized zinc dust stored under similar conditions, EDX analysis confirmed the presence of ZnO in certain regions. 4.6(a-b) shows that at 600oC the Zn surface is completely filled with ZnO nanostructures. The entire surface of the zinc has oxidized and the oxide is very uniform and adheres to the surface.

4.7(a-b) shows that zinc oxidation at 700oC no longer shows any ZnO nanostructure. There is a gradual increase in the size of the ZnO structures with further increase in temperature. Here the zinc oxide that has grown on the surface is somewhat smoother and appears to be highly adherent to the surface of the metal Zn substrate.

The simple technique of oxidation of metallic Zn foils in air has been demonstrated as a very economical and effective technique for the synthesis of ZnO. In contrast to Zn foils, Zn powder only showed formation of ZnO nanostructures at a relatively higher temperature of 600oC or higher. It is found that the growth of ZnO structures increases with increasing temperature of oxidation of the Zn foils.

The SEM observation shows that needle or rod-like structures of ZnO can be formed at temperatures above 700oC in the case of Zn films. The needle or rod-like structure of ZnO was found to grow very uniformly in size and shape and was very close to the Zn foil surface at oxidation temperatures ranging from 700-800oC compared to the various nano-structured ZnO that grew less densely. and appeared more dispersed and non-uniform in size and structure. The formation of ZnO was only confirmed at temperatures above 300oC in the case of Zn films.

Comparing the results of thermogravimetric and SEM analysis indicates that oxidation takes place only above 200oC, and ZnO structures can be observed at oxidation temperatures above 300oC in the case of Zn foils. 3] Dapeng Wu, Zhengyu Bai, Kai Jiang, Temperature-induced hierarchical growth of ZnO microcrystals, Materials Letters.

Fig 3.1: Schematic view of heating process

X-Ray Diffraction 28