Zinc oxide nanostructures synthesized

Zinc Oxide Nanostructures Synthesized by Oxidization of Zinc

Certificate

This is to certify that the thesis entitled, Zinc Oxide Nanostructures Synthesized by Oxidation of Zinc submitted by Manish Kumar (10604026) and Shakti Swarup Sahu (10604041) in partial fulfillment of the requirements for the award of Bachelor of Technology Degree in Metallurgical & Materials Engineering to the National Institute Of Technology, Rourkela (Deemed University) is an authentic work carried out by them under my supervision and guidance. To the best of my knowledge, the matter embodied in the thesis has not been submitted to any other University/Institute for the award of any Degree or Diploma.

Acknowledgment

We welcome this opportunity to express our heartfelt gratitude and regards to our project guide Prof. S. N. Alam, Department of Metallurgical & Materials Engineering, National Institute of Technology, Rourkela for his superb guidance. We would like to thank him for assigning us the project Zinc Oxide Nanostructures Synthesized by Oxidation of Zinc. Without his guidance and support we would not have been able to take the necessary steps required for the completion of this phase of our project. His motivation and help during the period of work has been invaluable in the completion of this work. He always bestowed parental care upon us and evinced keen interest in solving our problems. An erudite teacher, a magnificent person and a strict disciplinarian, we consider ourselves fortunate to have worked under his supervision.

We wish to record our gratitude to our project coordinators Prof. A. K. Panda and Prof. M. Kumar for helping us at each and every step in conducting this report. We would like to thank them for their stimulating and innovative ideas for implementing this project. We would also like to express our sincere gratitude towards our Head of the Department, Prof. B. B.Verma. We are highly grateful to Prof. B. B. Verma, Head of the department, Metallurgical & Materials Engineering, NIT Rourkela, for providing necessary facilities during the course of the work. We also take this opportunity to thank our parents and family members for constantly supporting us and encouraging us during our study. We also would like to thank all our friends for their help and support.

We are also thankful to Prof. D. Chaira for permitting us to access the SEM. We would like to thank Mr. Rajesh Patnayak for helping us in analyzing the samples in SEM. We would like to thank Mr. Uday Sahu for helping in carrying out the x-ray analysis of our samples. We would also like to thank Mr. S. Hembram and all other staff of the heat treatment lab of our department in helping us during heat treatment of the samples. We wish to place our deep sense of thanks to all who are directly or indirectly associated with our project for their cooperation and critical suggestions during our experimental work.

Abstract

Zinc oxide (ZnO) is a unique material that exhibits semiconducting as well as piezoelectric properties. Oxidation of pure metallic Zinc at low temperatures of 500-600oC for 2 h has resulted in a range of nanostructures of ZnO on the surface of metallic Zinc. These unique nanostructures clearly demonstrate that ZnO has a very rich family of nanostructures. These nanostructures could have applications in a wide range of areas like optoelectronics, sensors, transducers and biomedical sciences. In this article the various nanostructures of ZnO grown by the low temperature oxidation of metallic Zn has been reported. The structure and morphology of ZnO formed by oxidation of high purity metallic Zn at high temperatures (700-1000oC) was also analyzed. Oxidation of metallic Zn at higher temperatures of 700-1000oC for 2 h has resulted in a ZnO having a completely different needle like morphology. The change in the structure of ZnO formed on the surface of the metallic Zn as it was oxidized at different temperatures from 300 to 1000oC for 2 h is reported here. Energy dispersive x-ray spectroscopy and x-ray diffraction were performed in order to determine the composition of the nanostructures and the needle like structures.

1.0 Introduction

Zinc oxide (ZnO), a representative of IIVI semiconductor compounds, is a very versatile and important material. ZnO has a unique position among semiconducting oxides due to its piezoelectric and transparent conducting properties. It has a high electrical conductivity and optical transmittance in the visible region. These properties make it an ideal candidate for applications like transparent conducting electrodes in flat panel displays and window layers in solar cells [1-4]. ZnO has a wide band gap (3.37 eV) and a large exciton binding energy (60 meV) and exhibits many potential applications in areas such as laser diodes, solar cells, gas sensors, optoelectronic devices. A wide band gap has many benefits like enabling high temperature and power operations, reducing electronic noise, making sustenance in large electric fields possible and raising breakdown voltages. By proper alloying with MgO or CdO, the band gap can be tuned in the range of 3-4 eV. ZnO nanostructures have an active role to play in nano devices like nano gas sensors because the huge surface area enhances the gas sensing properties of the sensors. Apart from this, bio-safe characteristics of ZnO make it very attractive for biomedical applications. A method for economical mass production of ZnO nanostructures would therefore be very useful. This is why to study the synthesis of ZnO nanostructures and understanding the ZnO nanostructures is of great interest [5-7].

Physically, ZnO has the appearance of a white powder and is referred to as Zinc white or zincite. As far as solubility is concerned, its almost insoluble in water and alcohol but its readily soluble in acids like HCl even though its tarnished in their presence. The mineral form of ZnO takes up a red or yellow colour instead of the usual white due to the presence of impurities like Manganese and other elements. Crystalline form of ZnO is thermochromic in nature and due to this it changes from white to yellow on heating and vice versa on cooling. ZnO displays amphoteric properties and its also referred to as a II-VI semiconductor because Zinc belongs to the second group and Oxygen belongs to the sixth group of the periodic table [1-13].

ZnO has three crystal forms: the hexagonal wurtzite, the cubic zinc blende and the cubic rock salt which is rarely observed. The wurtzite structure is most commonly used as it has the highest stability under normal working conditions. Hexagonal and zinc blende structures dont display inversion symmetry and it is properties like these that are responsible for the piezoelectricity and pyroelectricity of ZnO. ZnO is considered to be a soft material with a reading of 4.5 on the Mohs hardness scale. Its elastic constants are not that impressive but ZnO has got very good thermal properties like high melting point, high thermal capacity and conductivity and a low coefficient of thermal expansion which makes it suitable for use as a ceramic [1-17].

Recently, various synthesis techniques have been developed to fabricate ZnO nanocrystals with different shapes, which include metal-catalyzed vapour-liquid-solid (VLS) growth, physical vapour deposition (PVD), chemical vapour deposition (CVD), metal-organic vapour-phase epitaxy, metal-organic CVD, template-assisted growth and oxidation method [8-10]. Many of these techniques however are complicated needing a high temperature for synthesis and also have a long reaction time. Apart from this, these techniques also utilize toxic components and require expensive experimental setup. The oxidation method on the other hand is a simple low-cost technique and very commonly used for the synthesis of ZnO nanostructures. ZnO nanostructures have been successfully synthesized by oxidizing Zn foils at 700C in air without the presence of a catalyst and carrier gas [36]. Sekar and co-workers [9] reported that ZnO nanowires were grown on Si (100) substrates by oxidation of metallic Zn powder at 600C. The synthesized ZnO materials have been found to have rod like, belt like, wire like and needle like morphologies. This route for synthesis of ZnO is economical as it uses the oxygen from the air [14-17, 21, 25].

Here an economical and environment friendly route has been used for the synthesis of ZnO on Zn substrate by oxidation of pure metallic Zn in a muffle furnace. Different morphologies of ZnO both nanostructured and microstructured have been achieved by us by controlling the oxidation temperature and the holding time. Nanostructures such as nanowires, nanorods, nanobelts and nanoneedles were seen on the metallic Zn substrate.

2.0 Literature Survey

2.1 Properties of Zinc oxide

ZnO is a versatile material that exhibits semiconducting, piezoelectric as well as pyroelectric properties. ZnO is a material that has a diverse range of nanostructures, whose configurations are far richer than any other known nanomaterials including carbon nanotubes [2, 4, 6, 11].

2.1.1 Physical properties

ZnO has a stable wurtzite structure with a = 0.325 nm and c = 0.521 nm [3]. The wurtzite structure has an ABAB hexagonal close packing (HCP) structure. The structure of ZnO consists of alternating planes composed of O2- and Zn2+ ions which are tetrahedrally co-ordinated and stacked along the c-axis on an alternate basis (Fig. 2.1). The Zn2+ and O2- ions create a normal dipole moment and instant polarization which results in a diversification of surface energy. The tetrahedral coordination in ZnO results in noncentral symmetric structure and consequently results in piezoelectricity and pyroelectricity in ZnO. Another important characteristic of ZnO is its polar surfaces. The most common polar surface is the basal plane. Extensive surface reconstructions are a means of obtaining a stable structure but ZnO - (0001) are exceptions. They are atomically flat, stable and without reconstruction. Efforts to understand the superior stability of the ZnO (0001) polar surfaces are at the forefront of research in todays surface physics. The other two most commonly observed facets for ZnO are {2 i i 0} and {01 i 0}, which are non-polar surfaces and have lower energy than the {0001} facets [2, 12, 29, 30, 32, 49, 14, 20].

Together with the polar surfaces due to atomic terminations, ZnO exhibits a large scale of novel structures that can be grown by varying the growth rates along these directions. One of the most important factors determining the morphology involves the relative surface activities of various growth facets under given conditions. Macroscopically, a crystal has different kinetic parameters for different crystal planes, which are emphasized under controlled growth conditions. Thus, after an initial period of nucleation and incubation, a crystallite will commonly develop into a 3D object with well-defined, low index crystallographic faces. Fig. 2.2(a-c) as reported by Zhong Lin Wang [2] shows a few typical growth morphologies of 1-dimensional nanostructures for ZnO. The morphology as shown in Fig. 2.2(d) [2] is dominated by the polar surfaces, which can be grown by introducing planar defects parallel to the polar surfaces. Planar defects and twins are observed occasionally parallel to the (0001) plane, but dislocations are rarely seen [1, 2, 11, 14].

It is observed that when the dimensions of a semiconducting material keep decreasing i.e. from macro to micro to nano scale, some of the physical properties of that material undergo changes. This phenomenon is known as quantum size effect [4].

2.1.2 Electrical properties

Zinc oxide (ZnO) has a wide direct band gap (3.37 eV) and a relative large excitation binding energy (60 meV). Owing to its unique properties wide band gap and large excitation binding energy, ZnO has the potential to be applied chemical sensors, solar cells, luminescence devices etc. ZnO nanowires are also reported to have behaved like a n-type semiconductor due to the presence of interstitial defects and vacancies [4, 6, 11, 14, 20]. The major roadblock of ZnO for widespread applications in electronics and photonics is its problems with p-type doping. Quite a few p-type doping efforts have been reported like Ga and N co-doping method which resulted in the formation of low resistivity (0.5 Ocm) p-type ZnO thin films. Successful p-type doping for ZnO nanostructures will give a boost to their future prospects in nanoscale electronics and in the field of optoelectronics. P-type and n-type ZnO nanowires can serve as p-n junction diodes and LEDs. Field effect transistors (FETs) fabricated from them can be used to make complementary logic circuits [4, 20, 30, 32, 49, 50].

2.1.3 Optical Properties

Intrinsic optical properties of ZnO nanostructures are the focus of studies right now as they can be used for implementing photonic devices. As a part of these studies, Photoluminescence (PL) spectra of ZnO nanostructures have been widely reported. Excitonic emissions have been observed from the PL spectra of ZnO nanorods. It is shown that if we confine the quantum size then it can greatly enhance the exciton binding energy but an interesting observation is that the green emission intensity increases with a decrease in the diameter of the nanowires. This is because of the larger surface-to-volume ratio of thinner nanowires which favours a higher level of defects and surface recombination. Red luminescence band has also been reported for which doubly ionized oxygen vacancies are considered responsible. Quantum confinement was also observed to be responsible in causing a blue shift in the near UV emission peak in the ZnO nanobelts. Some other fields of application include optical fibers, surface acoustic wave devices, solar cells etc [4,8].

2.2 Applications

ZnO has got a wide array of applications. The reactivity of ZnO makes it ideal to be used as a precursor for obtaining other compounds of Zn. ZnO has proved to be a boon for materials science as it has got a combination of unique properties like UV absorption, anti microbial properties, steady thermal and optical properties. Be it ceramics, lubricants, ointments, adhesives or the rubber industry ZnO has got a very significant contribution [39, 43-49].

2.2.1 Rubber manufacture

The rubber industry is responsible for consuming about 50% of the ZnO thats produced globally. ZnO along with stearic acid are a must for activating the process of vulcanization in rubber manufacture. Rubber curing becomes faster and more controlled when a combination of these two substances is used. Its also a very important additive in car tyres. It helps in improving their thermal conductivity which helps the tyres to dissipate heat quickly when they are in motion thereby increasing their life span [49].

2.2.2 Medical uses

A mixture of ZnO with 0.5% Fe2O3 is known as Calamine and is used in manufacturing Calamine lotions. Fine particles of ZnO have anti microbial and deodorizing qualities and hence they are used for packaging purposes. These properties along with its ability of neutralizing acids makes it ideal for use in antiseptic creams, healing creams etc. they are also an important component of toothpastes and dental prosthetics. Due to its ability to absorb ultraviolet light, ZnO is also used in sunscreens and sunblocks to prevent sunburns. ZnO is also used in the making of cigarette filters as it helps in removing harmful substances like H2S and HCN without affecting the flavour [49,50].

2.2.3 Food additives

ZnO is usually added to food products as a source of Zn which is considered to be a necessary nutrient as it helps in the performance of various physiological activities like growth and proper functioning of the sexual organs. ZnO is also added to fodder as a Zn supplement for livestock. Zinc oxide is used to manufacture zinc gluconate which is nowadays found in cold prevention lozenges [49,50].

2.2.4 Anti corrosive coatings

Zinc oxide is an excellent inhibitor of fungi, mildew and mould [50]. Paints which have ZnO are usually used as anti corrosive coatings for various metals like galvanized Zn which is especially hard to protect as it makes organic coatings brittle and unsuitable for adhesion [49]. ZnO paints on the other hand maintain their flexibility and adhesive properties for years on end. The UV blocking abilities of ZnO also play an important role in improving the resilience of the paint. Unlike some lead pigments that are available commercially, ZnO is impervious to the effect of sulphur compounds that are present in the atmosphere [43, 47].

2.2.5 Electronic applications

The wide band gap of ZnO enables it to be used for the purpose of making LEDs and laser diodes. Transparent thin-film transistors (TTFT) can also be produced with ZnO. Nanorod sensors made of ZnO are devices that detect fluctuations in electric current passing through the wires as a result of the adsorption of gas molecules. The nanorods can be made partial towards H2 gas by sputtering Pd clusters on the surface which help in dissociating the H2 molecules thereby increasing the sensitivity of the nanorods. H2 concentrations up to 10 ppm can be detected at room temperature without any response to O2 [27,49].

Properties like biocompatibility have enabled ZnO to be used as a biomimic material to modify and immobilize biomolecules. As Field-effect transistors (FETs), they can function without even a p-n junction thus bypassing doping problems. This is why some FETs use nanorods made of ZnO as conducting channels. ZnO is also used in making Zn-C dry cells, Ni-Cd oxide batteries, Zn-Ag oxide batteries and also some secondary batteries. Another major area of ZnO application is fuel cell where its used to make various parts like the electrodes and sometimes also as the fuel. It can also act as a photo catalyst in places like the solar cell [49, 50].

2.3 Synthesis

Keeping in view the extensive uses of ZnO, various types of synthesis techniques have been formulated over the years. ZnO nanostructures can be synthesized by several methods such as sputtering technique, vapour deposition, pulsed laser deposition (PLD), metal organic chemical vapour deposition (MOCVD) and oxidation. Some other methods are vapour liquid-solid process and anodization. Unfortunately, many of the stated techniques are problematic, needing high temperature for synthesis, and having long reaction time, utilize toxic components and involve costly equipments. The oxidation method is a simple, low-cost and most commonly used technique for the synthesis of ZnO nanostructures. However, the deposition of zinc oxide by vapour deposition has seen increased research activity over the past several years as the need for high quality zinc oxide thin films has increased. A key advantage of chemical vapour deposition is its ability to easily incorporate dopants into films [17-26, 33-38].

2.3.1 Proposed Method for ZnO Synthesis

Keeping in view the various aspects of ZnO synthesis, we have reached the conclusion that the simplest, cheapest and most effective way to synthesize ZnO is by oxidation method. Pure metallic Zn was oxidized in air in a muffle furnace at various temperatures ranging from 300oC to 1000oC for 2 h and the ZnO nanostructures and the microstructures formed on the Zn substrate were studied. The temperature at which the ZnO structures changed from nano to micro was also tracked.

3.0 Experimental

Zn having purity 99.9 % was obtained from RFCL Limited (RANKEM), India. The oxidation of metallic Zn was performed by heating the Zn at temperatures 300, 400, 500, 600, 700, 800, 900 and 1000C for 2 h in a muffle furnace to obtain ZnO nanostructures on the surface of Zn. Zn was oxidized in a silica crucible. The atmosphere in the muffle furnace used for oxidation was air. Oxygen present in air was used for oxidizing Zn. The morphology of ZnO that was formed on metallic Zn was analyzed using a JEOL JSM-6480LV scanning electron microscope (SEM). In order to identify the composition of the nanostructures formed at lower oxidizing temperatures and the needle like structures formed at higher oxidizing temperatures, energy dispersive x-ray spectroscopy (EDX) and x-ray diffraction (XRD) were performed. The crystallographic structure of ZnO was analyzed with a Philips Pananalytical X' Pert x-ray diffractometer using Cu Ka ?radiation (?? = 0.15406 nm). The JEOL JSM-6480LV scanning electron microscope was attached with a high angle and ultra-thin window type energy dispersive x-ray spectroscopy detector. An Oxford Instruments energy dispersive x-ray spectroscopy analyzer was attached. An INCAPenta FET- X3 with a powerful Si (Li) detector having a 30 mm2 detecting crystal was used.

3.1 Scanning Electron Microscope

Thescanning electron microscope(SEM) is a type ofelectron microscopethat helps in forming an image of the sample surface by scanning. The electrons in the beam interact with the atoms in the surface to generate signals that shed valuable light properties like composition, topography and electrical conductivity. The so called signals produced by an SEM include secondary and back-scattered electrons, characteristic x-rays, specimen current and light (due to cathodoluminescence). All SEMs usually have the ability to detect secondary electrons but its highly unlikely that a single SEM will have the ability to detect all the signals mentioned above. SEM helps in obtaining high resolution images of specimens ranging in size from those visible to the naked eye to those which are just a few nanometers in size [28,41,43].

In most of the applications, the data collected is over a pre selected area of the sample surface and following this, a 2D image is generated that shows the various spatial variations. Conventional SEMs with a magnification range of 20X-30000X with a spatial resolution of 50-100 nm can scan areas which vary from 1 cm to 5 m in width. SEMs also have the ability to analyse particular points as can be seen during EDX operations which help in determining the chemical composition of the sample concerned [41].

Secondary electrons and backscattered electrons are most commonly used for the purpose of imaging samples: secondary electrons depict morphology and topography of the samples while backscattered electrons depict contrasts in composition in multiphase samples.X-rays are produced when incident electrons collide inelastically with the electrons of the atoms in the sample. As the excited electrons get back to their previous positions, they emit x-rays which is the characteristic property of a material. SEM analysis is said to be a non destructive analysis as the bombardment of electrons do not cause any damage to the samples in any way whatsoever [41].

Essential components of all SEMs include the following:

  • Electron Source ("Gun")
  • Electron Lenses
  • Sample Stage
  • Detectors for all signals of interest
  • Display / Data output devices
  • Infrastructure Requirements:
    1. Power Supply
    2. Vacuum System
    3. Cooling system
    4. Vibration-free floor
    5. Room free of ambient magnetic and electric fields[41]

Sample preparation is not really a problem in case of SEM analysis as the only requirement for a sample to undergo SEM analysis is that it should be conductive. It also allows a large amount of the sample to be focused at once as it has a large depth of field. The high resolution images obtained also help in analyzing the spatial features closely. A combination of all these features has made the SEM one of the most popular instruments in the world on materials science [44].

3.2 Energy Dispersive X-Ray Spectroscopy

Energy dispersive x-ray analysis, also known as EDS, EDX or EDAX, is a technique used to identify the elemental composition of a sample. During EDS, a sample is exposed to an electron beam inside a scanning electron microscope (SEM). These electrons collide with the electrons within the sample, causing some of them to be knocked out of their orbits. The vacated positions are filled by higher energy electrons which emit x-rays in the process. By analyzing the emitted x-rays, the elemental composition of the sample can be determined. EDS is a very handy tool for performing the constitutional analysis of any kind of material [40].

3.3 X-Ray Diffraction

X-ray diffraction (XRD) is a non-destructive type of analytical technique which provides valuable insight about the lattice structure of a crystalline substance like unit cell dimensions, bond angles etc. XRD is based on the principle of constructive interference of x-rays and the sample concerned which should be crystalline. The x-rays which are generated by a CRT are filtered, collimated and then directed towards the sample. The interaction that follows produces constructive interference based on Braggs law which relates wavelength of the incident radiations to the diffraction angle and lattice spacing. The x-rays that are obtained on diffraction are counted after proper processing. By altering the various parameters like the geometry of incident rays and the orientation of the detector and crystal, we obtain all possible diffraction directions of the lattice [31, 42].

Single-crystal diffractometers use either 3 circle or 4 circle goniometers. A goniometer usually refers to an instrument that helps in measuring angles or in rotating objects by precise angles. Fig. 3.3 shows a 4 circle goniometer. Each circle refers to one of the four angles (2?,?,f, andO) that relate the incident ray, crystal lattice and the detector to each other. Samples are first attached to brass pins. Then they are set up on the goniometer heads. By adjusting the X, Y and Z orthogonal directions we can adjust the centering of the crystal within the x-ray beam [42].

4.0 Results and Discussion

Nanostructures have attracted attention of researchers for their many important technological applications. The synthesis of nanostructures is a growing area of research. The synthesis of nanostructures by a low cost process would be of great technological importance. The ZnO nanostructures were synthesized by oxidizing pure metallic Zn. Pure metallic Zinc was heated at various temperatures ranging from 300 to 1000C for 2 h. It should be noted that Zinc has a melting point of 419.5C whereas its boiling point is 907C. The choice of the range of oxidizing temperature was made in order to find out the change in the morphology of ZnO that was formed on Zn substrate. The aim was to track the change in morphology from nanostructured ZnO to microstructured ZnO as the oxidation temperature was increased. The holding time at all heat treating temperatures was either thirty 0.5 h or 2 h. Fig. 4.1(a) is the SEM image of the surface of pure metallic Zn. The result of EDX analysis in Fig. 4.1(b) shows that the composition of the sample is 100 at. % Zn.

Fig. 4.2 shows that at 300oC we find that there is no trace of formation of ZnO on the Zinc surface. The EDX result also confirms that the surface is 100 % pure Zn. From this result we can conclude that oxidation of Zn and formation of ZnO does not start at 300oC even when the pure Zn was held at this temperature for 2 h. The surface shows some changes as compared to the surface of the as-received Zinc. Fig. 4.3(a-b) shows that at 400oC the surface of Zn contains some ZnO structures. Fig. 4.4(a-b) shows that when Zn was held for 500oC for 2 h, the SEM images of the surface showed the formation of ZnO nanostructures. The composition of these nanostructures is also almost that of ZnO (21.59 at. % O and 24.51 at. % Zn). Fig. 4.5(a-b) shows that at 600oC the surface of Zn is completely filled with nanostructures of ZnO. There is a wide range of nanostructures of ZnO on the Zn surface. Fig. 4.6(a-b) shows that the oxidation of Zinc at 700oC does not show any nanostructure of ZnO anymore. There is a gradual increase in the size of the ZnO structures with further increase in temperature. Fig 4.6(c) shows the EDX analysis of the ZnO microstructure formed on the Zn surface by heat treating at 700oC for 2 h shows that the at. % of Zn (43.57 %) and O (56.43 %) are almost identical suggesting that highly stoichiometric ZnO is formed at this temperature.

Zn has a metallic lusture but after heating, the lusture of metallic Zn is lost and it starts to have a white coloured layer on it which is the typical colour of ZnO. The morphology of ZnO that was synthesized on Zn was investigated by using a SEM. The SEM observation shows that rod like or needle like structures of ZnO whose thickness reduced with height are formed at temperatures between 700-1000oC. The ZnO that has been grown on the surface of the metallic Zn at temperatures between 700-1000oC seems to have a very smooth surface and also seems to be highly adherent to the surface of the metallic Zn substrate. The ZnO rods formed at these temperatures have been found to be mostly triangular in shape and have different heights. Its thickness reduced gradually from the base to the tip. The thickness of the base of the ZnO structures formed on Zn surface after oxidation at 900oC for 0.5 h was less than 1 mm and there were a large majority of the ZnO structures having thickness much less than 1 mm. Fig. 4.8(a-b) shows that their heights were relatively large and some of the structures were almost 10 mm long. On the other hand, Fig 4.9(a-b) shows that the thickness of the base of the ZnO structures formed on the Zn surface after oxidation at 900oC for 2 h was less than 1 mm but a majority of them were thicker than the ZnO structures formed at 900oC for 0.5 h. This was possibly due to the longer period of holding time during oxidation which allowed the ZnO structures to grow in thickness. Their heights were comparatively much lower in the range of 3-4 mm. The ZnO structures formed by oxidation at 900oC for 0.5 h reduced in thickness along its height much faster as compared to the ZnO structures formed by oxidation at 900oC for 2 h. The ZnO structures that have been formed at 900oC for 2 h comparatively reduced in thickens along its height much more gradually. It can be concluded that holding Zn sample for a longer period of time at a particular temperature led to ZnO structures having larger cross sectional area and relatively shorter heights. The ZnO structures that have been formed at 900oC for 2 h were faceted and triangular in shape and seemed to be more needle like or rod like whereas the ZnO structures that have been formed at 900oC for 0.5 h were not faceted and were also not triangular and seemed to have a more leaf like thinner structure.

Fig 4.9(a-b): SEM images distinctly showing the triangular morphology of ZnO microstructures obtained on holding Zn at 900C for 2 h. (c) EDX of the oxide microstructures.

Fig 4.9(c) shows the EDX analysis which confirmed that the at. % of oxygen in the ZnO nanostructure is 52 % and the at. % of Zn is 48 %. Nearly stoichiometric ZnO cloud can be synthesized by oxidation of Zn in air. The formation of nearly stoichiometric ZnO on glass substrates at high sputtering pressures has been reported [42]. ZnO nanostructures could be synthesized by oxidizing pure metallic Zn for 2 h at temperatures between 400-600C. At oxidizing temperatures above 700oC needle like or rod like microstructures could be seen. Clearly, the size of ZnO depends on the oxidation temperature, i.e., higher the oxidation temperature, the larger the size of the ZnO structures. Higher temperatures lead to more vigorous oxidation and results in mostly columnar needle or rod like structures which are faceted whereas oxidation at lower temperatures results in nanostructured ZnO. At oxidation temperatures ranging from 400-600oC nanostructured ZnO having different morphology was formed on the Zn substrate.

Fig. 4.10 consists of the XRD plot which shows that all the peaks of ZnO could be traced in the XRD profile of the sample formed by heat treating Zn at 700oC for 2 h. Formation of ZnO could not be traced from the XRD analysis at 300oC and 400oC although SEM image showed the formation of ZnO at 400oC. Possibly due to the low intensity of the peaks of the ZnO formed at 400oC as they had a relatively small size and also due to the lesser amount of ZnO formed at this temperature they could not be traced. So we can conclude that ZnO structures could be formed on the Zn substrates only at temperatures above 300oC.

When metallic Zn was heat treated to 1000oC for 2 h Zn started to boil and vapourized as its boiling point is 907C. As the silica crucible in which Zn was oxidized was not covered all the Zn vapour possibly went out of the vessel and only a small amount of the Zn vapour condensed and solidified on the metallic Zn substrate. This resulted in lesser amount of Zn reacting with the oxygen and forming ZnO on the Zn substrate. This is the reason why in Fig. 4.11(a-b) we see only a few ZnO structures.

5.0 Conclusion

In summary, this simple technique of oxidation of metallic Zn in air has been demonstrated as a very economical and effective technique for the synthesis of ZnO nanostructures on metallic Zn. The ZnO nanostructures were successfully synthesized by oxidizing pure metallic Zn in the temperature range 400-600oC. The structure and size of ZnO nanostructures strongly depends on the oxidizing temperatures. This is an economical route for the synthesis of ZnO on a pure metallic Zn substrate. It is found that the size of ZnO structures increases with increasing temperature of oxidation.

The scanning electron microscopy observation shows that needle like or rod like structures of ZnO could be formed at temperatures above 700oC. Above the oxidizing temperature of 700oC the needle like or rod like structures of ZnO were very uniform in shape and size. The Zn surface was covered densely with the ZnO structures at oxidation temperatures above 700oC. On the other hand at oxidizing temperatures between 400-600oC the nanostructured ZnO were nonuniform in size and structure and were less dense and more scattered on the metallic Zn surface.

6.0 References

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