INTRODUCTION:

A nano-scale object is one billionth of a metre in size, i.e. 1nm = 10-9m. If one of the dimensions of a material is in the nanoscale, it is referred to as a nanomaterial (preferably less than 100 nm). Nanotechnology is a technology that is driven by nanomaterials and connects various branches of study (physics, chemistry and materials sciences). Nanotechnology's entire goal is to develop, modify, and create nonmaterials as well as their front-end applications. Aside from achieving application, nanotechnology's main goal is to develop and comprehend fundamental physical and chemical properties, as well as the phenomena of nonmaterials and nanostructures at the atomic level. We can even generate intriguing morphologies of these nanomaterials, such as nanodots, nanowires, nanobelts, nanospheres, and so on, thanks to a specific synthesis technique. As a result, the difference between bulk and nanomaterials has an impact on attributes such as magnetism, conductivity, and superconductivity, and is difficult to investigate.^1,2,3 Apart from confinement-based nanostructures, metallic and semiconductor nanomaterials can also be classified based on their electrical and optical properties. When the diameter of nanocrystals reaches the Bohr's radius regime, the energy levels increase to the point where the difference exceeds the thermal energy, i.e. KT, and this energy difference is much larger in semiconductors than in metals or Van der Waals molecular crystals at ambient temperature. In semiconducting crystals, numerous types of confinements can be examined depending on the form and size of crystallites. Because semiconducting nanocrystals have an observable energy gap, the study of semiconducting nanostructures is more explorable and intriguing. The ability to be semiconducting is determined by the materials used.⁴ Metal oxides are insulating, conducting, semi-conducting, and other materials. Metal oxides are a class of materials that is undergoing substantial research due to its wide range of applications. Microelectronic circuits, condensers, piezoelectric devices, solar cells, fuel cells, Photocatalysis, touch screens, Flat panel Displays, DSSCs, and other electronic and opto-electronic devices are all covered. Fe2O3, SnO2, TiO2, and ZnO are all oxides of iron.

Investigation:

Metal oxides such as Fe3O4, Al2O3, ZnO, CaO, and others abound in our Earth's crust, and even pure elements oxidise in the presence of oxygen and water to create oxides. Metal-Oxides are chemical compounds that contain one oxygen anion in the -2 oxidation state and another element in their formula.⁵ Due to their vast spectrum of features, metal oxides have occupied substantial space in many disciplines of material science, physics, and chemistry. Metal oxides play an important role in our daily lives because of their various qualities. From touch screens on our phones to aluminium foil for food packaging, metal oxides are present in practically every aspect of our lives.

One-dimensional, two-dimensional, and three-dimensional nanostructures can be made in a variety of ways. Top-down and Bottom-up strategies have previously been defined for the current study. Hydrothermal approach, CVD (Chemical Vapour Deposition), sol-gel, precipitation route, sputtering, lithographic techniques, and others are some of the effective ways for oxide nanostructures creation. Dimensionality, shape, crystal structure, degenerate or non-degenerate level, growth process, and many other attributes of nanostructures are influenced by the synthesis technique and growth conditions. Several characterization approaches are developed and used to investigate a certain property of the object.^6-10

Chemical vapour deposition (CVD) is a strong process for creating nanostructures in thin films using a variety of growth conditions. This approach is particularly beneficial for producing high-quality thin and thick layer coatings on hard surfaces while maintaining homogeneous film hardness. CVD deposits a consistent thickness coating on any substrate, regardless of its shape or size. CVD also produces material with a high degree of purity and consistency. CVD is utilised in large-scale manufacture of thin film coatings of corrosion-resistant materials, heat-resistant materials, and layers for microelectronics, among other things.¹¹

Figure 1.1: Chamberlin of two-layer thermal CVD

A reaction chamber is included in a conventional CVD system. The deposition material is held at a high temperature, while the substrate over which the thin layer will be created is kept at a low temperature. To transport the vapours from the source to the substrate, inert gas or flowing gas is introduced. In this research, we used CVD to make Zn2SnO4, Ba doped SnO2, SnO2, nanowires, and nanorods. We used a two-zone split furnace to do this. Figure 1.1 shows a visual illustration of nanowire synthesis using CVD in a split furnace.^13-17

Perovskite structure (ABO3) is a highly attractive ternary oxide crystal form seen in multiferroic materials such as BiFeO3, PbTiO3, BiMnO3, and others. It has A and B cations with distinct atomic radii bonded with anion oxygen in its chemical formula.[18-20] In general, an atom with the letter ‘A' is larger than one with the letter ‘B.' In a unit cell, there is only one formula unit. Cation B has six-fold coordination in octahedrons of anions, while cation A has twelve-fold cubooctahedral coordination. The divergence from the ideal cubic Perovskites structure to other lower symmetry deformed structures is caused by a slight change in the coordination number due to relative ionic radii of cations.

Figure 1.2: Crystal structure of Perovskites unit cell and BO6 octahedra

Measurements and Analysis:

SnO2, ASnO3, and A2SnO4 are examples of popular binary and ternary stannates, where A can be Zn, Cd, Ba, or Sr, resulting in a family of materials such as ZnSnO3, BaSnO3, Cd2SnO4, and so on. SnO2, commonly known as stannic oxide, is a versatile multifunctional metal oxide with a broad band gap that is utilised as a Transparent Conducting Oxide (TCO) electrode in DSSCs, gas sensors, and as an anode for lithium ion batteries. The fact that SnO2 is transparent to over 90% and has a resistivity of 6.1 x 10-3-cm makes it a popular stannates compound. SnO2 is a multifunctional oxide with a highly dispersive conduction band and a large band gap, making it a highly efficient material for high electrical conductivity (about 108S/cm) and optical transparency (> 95%).

Several types of SnO2 nanostructure morphologies have been reported for gas sensing and photo-catalyst applications, including nanowires, nanobelts, and nanoparticles. SnO2 is accidentally an n-type semiconductor due to impurities or flaws. As a result, defects play an essential role in influencing electrical characteristics, as they enhance carrier concentration (1019–1020 cm3) in most metal oxides. Furthermore, oxygen vacancy sites contribute to the transport mechanism by delivering carrier electrons. As a result, the conductivity of metal oxides is enhanced by oxygen vacancy combined with defect richness. During the synthesis of stannates, oxygen vacancies are generated.

Synthesis Strategies for Nanostructures:

Nanostructures can be created using a number of different techniques. Based on the principle involved, different synthesising processes fall into two categories: Bottoms-up Synthesis: All bottom-up synthesis procedures begin with a gaseous or liquid starting ingredient. These are further divided into two categories. Thermal evaporation, e-beam evaporation, sputtering, and pulsed laser deposition are examples of physical processes that involve the vapour phase.

Chemical Methods is Chemical vapour deposition (CVD), Pyrolysis, Sol-gel, Co-precipitation, Electrochemical deposition, and other methods involving the deposition of the reaction species' vapour phase. Top-down Synthesis is another method of synthesis uses a solid-state starting material, such as lithography, solid-state method, and ball milling, and among others. In order to research synthesis parameters based on anoxide nanostructures, synthesis methods such as CVD, sol-gel and co-precipitation, spray pyrolysis, and a few others are the best methodologies.

Methodology: Doped zinc stannate:

Through carbothermal reduction of binary oxide as precursors via CVD in a base pressure of 1mTorr, pure phase of Zinc stannate polycrystalline nanowires without SnO2 impurity was achieved. Temperature and Ar-flow rate were changed during the synthesis, and the effect of pulsing the Ar flow rate on the morphology was investigated. X-ray diffraction, FESEM, EDS, HRTEM, Raman, and CL measurements were used to investigate the behaviour of ternary stannates synthesised at two temperatures. There is a close link between the creation of oxygen vacancies and the incorporation of dopants. Magnetic dopants increase the concentration of oxygen vacancies while providing d-electron magnetism. In this chapter, a detailed experimental examination of Zinc Stannate nanorods generated by chemical vapour transfer (CVD), such as Zn2SnO4 (ZTO) and Mn-doped ZTO (ZTO: Mn), is carried out. It is investigated how Mn influences shape, structural characteristics, chemical composition, and electrical behaviour. This chapter has been broken into two parts for this purpose. Part A deals with virgin ZTO, while Part B is about Mn doped ZTO.^21-24

Figure 1.3: Crystal structure of Zn₂SnO₄

Perovskite ZnSnO₃ and cubic spinel Zn₂SnO₄ are two distinct crystal forms of zinc stannate with differing Zn:Sn ratios. The metastable phase ZnSnO₃ is generated at moderate temperatures (300-500°C), but the inverse Cubic spinel Zn₂SnO₄ phase is formed at high temperatures (over 600°C). The phase formation is temperature dependent, since the needed activation energy for stable phase formation increases as the temperature rises above 500°C. In unusual type inverse spinel Zn2SnO4 Zn2+ can occupy both octahedral as well as tetrahedral voids in cubic close packing of oxide anions while Sn4+ occupy only octahedral voids.^25-26 This flexibility of flipping of Zn2+ in both sites makes its tuneable for various dopants and hence modifies its properties accordingly. There are 8 formula units in unit cell and total 56 atoms that are 32 anions and 24 cations. The crystal structure of zinc stannate is shown in figure 1.3. Zn2SnO4 has lattice parameter a = 8.5Å and space group Fd-3m.

Results and Analysis:

In order to make Zn₂SnO₄ nanowires, In a 3:2 ratio, binary oxides ZnO and SnO₂ are used as precursors. As a result, these binary oxides have a high breakdown temperature. ZnO and SnO₂ powders are employed in the manufacture of Zn₂SnO₄ nanowires, and graphite is used as a reducing agent. Precursors were combined and placed in an alumina boat, which was then placed into the furnace at a low vacuum of 10-3 Torr. Si substrate is covered with a thin layer of (5nm and 2nm) The Au catalyst layer was positioned vertically, 11cm away from the boat. Carbothermal reduction is a straightforward, low-cost method for producing high-quality metal oxide nanostructures. In this case, graphite is used as a reducing agent to raise the precursor's vapour pressure.

In comparison to thermal evaporation methods, nanostructures can be generated at a controlled lower temperature. The Ellingham Diagram is used to reduce carbotherms. SnO2 has a melting point of 1350°C and a vapour pressure of 3.6 X 10-5 Pa at 1250°C, while carbothermal reduction reduces the vapour pressure of SnO2 to 234.4 Pa at 680°C, which is sufficient to generate crystalline structures. Similarly, the decreasing temperature of ZnO at the cross section is 950°C. As a result, the most dense, thickest, and longest nanowires are produced when synthesised at 900°C. The growth of 1-D nanowires can be explained using the VLS/VSS theory; in this case, no growth is observed without Au catalyst, implying that the VLS mechanism can be used to explain the growth. The contrast between tip and wire depicts tip growth, and the average thickness of nanowires is less than the average diameter variation (50-120 nm) of catalyst, implying that the VLS mechanism can be used to explain the growth.

CONCLUSION:

By optimising CVD growth conditions, inverted cubic spinel Zn2SnO4 nanowires free of precursor phase impurity were successfully produced. Temperature plays an important impact in determining phase purity, crystallite size, and morphology, as well as affecting luminous qualities and electrical properties. The size of the liquid alloy droplet, its stability, and the supersaturation condition are all affected by increasing the carrier gas flow rate. As a dopant, the transition element ‘Mn’ has a considerable effect on the host matrix of Zinc Stannate Zn2SnO4. These findings shed light on nanostructure defect engineering for altering magnetic and electrical properties.

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Received on 22.09.2021 Accepted on 28.12.2021

Int. J. Tech. 2021; 11(2):44-48.

DOI: 10.52711/2231-3915.2021.00006