Synthesis of MnO2 Nano materials

Manganese oxide nanowires

            A nanowire is a nanostructure, with the diameter of the order of a nanometre (10−9 meters). Many different types of nanowires exist, including superconducting, metallic, semiconducting and insulating (Boston et al., 2014). Manganese is a chemical element with symbol Mn and atomic number 25. It is not found as a free element in nature; it is often found in minerals in combination with iron. Manganese is a metal with important industrial metal alloy uses, particularly in stainless steels (Meija et al., 2016).

            Nanomaterial, having a dimension scale 1-100 nm, have received increasing interest owing not only to their fundamental scientific significance but also to the potential applications that derive from their fascinating electrical, magnetic and catalytic properties (Clapsaddle et al., 2004). Compared to bulk active electrode materials, the corresponding nanomaterial possess more excellent electrochemical activity, such as higher capacities, larger surface areas, and lower current densities, thereby, nanomaterial have wildly potential application in electrochemistry field. Manganese oxides, including MnO, MnO2 and Mn3O4, are intriguing composites and have been used in wastewater treatment, catalysis, sensors, super-capacitors, and alkaline and rechargeable batteries. Particularly, MnO and MnO2 nanomaterial have attracted great interest as anode materials in lithium-ion batteries (LIBs) for their high theoretical capacity, low cost, environmental benignity, and special properties (Kung et al., 1989).

            Nowadays, Nano metal oxide is one of the auspicious materials for scientists. The applications of nano metal oxide particles are increasing day by day. The most commonly used transition metal oxides are titanium, manganese oxide, zinc oxide, cobalt oxide, nickel oxide, iron oxide etc. nanotechnology is the most growing field of this century seeking the attentions towards the growth and applications of nanomaterial.  Important aspect of the nanotechnology is to prepare the material at Nano scale and utilize them for the better purposes due the unique properties of nanoparticles as compared to their bulk material. Increased surface to the volume ratio is effective in new researches in surface based science. Due to decrease in size the number of properties is pronounced for the manganese oxide i.e. statistical and quantum mechanical effect. The quantum size effect is more prominent due to which the properties of nanoparticles change very rapidly with reduction of size. The macro dimensions do not come into play in that size but dominates with e Nano size range. Their properties rapidly change at nano level as compared to their respective macro level and bulk matter and enhances their uses. Due to this some Martials become transparent like copper. The materials like platinum become important for catalysis. Gold is turned to be liquid even at room temperature and the semiconductor materials become the best conductors such as silicon. The inert materials at room temperature become the potential chemical catalysis when turned into the Nano scale such as gold. Many more other fascinations are associated with materials at Nano level (Burda et al., 2005).

1.4. Synthesis of MnO2 Nano materials

            Due to various applications in the fields of optical, electronics and mechanical devices MnO2 is synthesized according to their requirement. Plenty of methods have been reported for the fabrication of metal oxide nano particles but the control of particles is most alarming issue in this sense. So it is very important to modify the methodology to make it very much cost effective as well as environment and chemical friendly. The fabrication of nanoparticles is very much important as they exhibit many different properties as compared to their bulk matter such as high surface to volume ratio. Chemical purity and high crystallinity makes these nanoparticles very much applicable in different fields. It has been reported that the physical and chemical properties are relative to its stoichiometry as well as particle size and shape. MnO2, Mn3O4 nanoparticles are potentially used in various fields such as electrodes catalysis sensors and in optoelectronics (Li et al., 1997).

1.5. Synthesis method of MnO2

            It is important to have a safe and environmental healthy process for the fabrication of metal oxide nanoparticles. Due to this reason it is highly recommended challenge to have a unique and healthy process. Many methods have been reported since. These are divided into two main categories i.e. gas-solid and liquid-solid nature methods. However the development of the metal oxide nanoparticles and their production enhanced the significance and importance of their synthesis and utilization. This is due to the fact that these metal oxides have unique properties at nano level as compared to their bulk matter. For this purpose both chemical and physical process are used to fabricate nanoparticles by researcher. Supported metal oxide and mixed metal oxide are usually prepared by the following methods (Li et al., 1997).

  • Vapor condensation method
  • Spray Pyrolysis
  • Precipitation method
  • Co-precipitation method
  • Reverse micro-emulsion/micelle method
  • Sol-gel method

Figure 1.3: Fabrication of Nanomaterial

            Sol-gel method is used to fabricate the nanoparticles of manganese oxide. In this process sol is prepared and then it is followed by the gel. Starting materials are used mostly inorganic precursors and colloidal dispersions. However the number of precursors can be used in sol gel method. The two key points are reported for the sol-gel technique irrespective of the simplicity. One is the formation of gel due to the condensation of material and results in the form of three dimensional networks. Other one is the effect of parameters on the gel and the particles that are effective against both the processes. Sol-gel method has this property of the control due to which it is distinguished from other methods. These parameters which require control are solvent, precursors, water content, precursor’s concentration, acid base content and the physical conditions including temperature. These parameters have greater effect on the formed gel as well as the properties of the fabricated nanoparticles (Okuhara et al., 2001).

            The mixture is first exposed to some source or air to remove the solvent present. This process is called aging. Aging is also important parameters effective against properties. Continuous hydrolysis as well as condensation process during the aging according to scherer. Some other parameters such as synerecis and coarsening are also reported during the formation as well as aging process. Synerecis is the process in which shrinkage of gel occurs due to which solvent is excluded. While precipitation as well as dissolution is termed as coarsening. These above mentioned properties are very much effective against the formation of nanoparticles as well as their properties after the formation of gel. Another parameter that’s effects the sol-gel method is the dry condition. Conventionally the evaporating drying is used in which gel is heated by placing it in an oven and induced capillary is concerned with the interface of liquid and solid inside a pore. Porous networks are disturbed during the capillary action process in drying. However the material formed during this process in dry form has low capability of use in the catalysis. This material formed is termed as xerogel. There are many techniques to reduce this effect and prepare the xerogel which has good surface area and porous volume. Most important method is kistler technique. The supercritical drying is used by kistler to reduce the effects due to drying. The material formed by this technique is called aerogel. This has the high surface area and porous volumetric morphology with low density. Generally drying is an aging process and material in this drying process undergoes physical and chemical changes in all aspects (Ward et al., 1995).

Figure 1.4: Schematic diagram of Sol-Gel Method

1.6. Top-Down Methods

            Mechanical method is used to fabricate the nanoparticles of metals because they are very much low cost. The simplest is ball milling technique. The grinding medium’s energy is first transferred to the medium used for reduction which is done mechanical attrition the most important phenomena are the compaction and consolidation. During these processes a material is constructed by combining the nanoparticles. Best example is the formation of metal alloys. These methods are termed as top-down approach and used an industrial method. Thermal processes are also used for this purpose I which heat is transfused by some means to fabricate nanoparticles. For example nano thread materials are fabricated by using electroplating technique. The energy may be due to heat, electricity or other form like solar energy. The carbon nanotubes are formulated first time by high energy method by arc discharge (Arole et al., 2014).

1.7. Bottom-up Methods

            In such type of methods, the nanoparticles are formed by the atoms and molecules. For example chemical vapor deposition method in which reaction is done over a catalyst or some other pre template surface is used to fabricate nano materials. The carbon nanotubes are synthesized very economically by chemical vapor deposition method. In the presence of Fe, Ni or Co catalysis the gases are allowed to pass through precursors such as aceltyne, methane or carbon the nanotubes are formed when decomposition of material occurs. One of the important, method is the atomic layer method in which the materials in the form of layers are deposited over a thin sheet or slab. This method is important because it can deposit material irrespective of size. Some other industrial methods include MOCVD and “molecular beam epitaxy”. Numerous methods are reported in literature which process like bottom-up method such as liquid phase method. (Arole et al., 2014).

Figure 1.5: Top down and bottoms up approach

1.8. Objectives

Following are some important aims of the present study.

  • To obtain and characterize MnO2 nanowires by Sol-gel method.
  • To study the optical properties of MnO2 nanowires.
  • To study the surface morphology of the MnO2 nanowires.
  • To study the structural properties of MnO2 nanowires.

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