Investigation on the Synthesis and Chemical Properties of Nanomaterials

Nanoscience is assumed to have capability to change the efficiency of the current electronic and electrical devices. So nanotechnology is being developed to achieve the same. Various kinds of nanomaterials are being prepared such that they show tremendous properties at the nanoscale dimensions. Hence, it is worth studying about the synthesis of nanomaterials. The information that we present in this paper corresponds to a basic review on the synthesis and recent work done on nanomaterials. The distinctness of this review is that, the classification and major types of nanomaterials has been reported. The literature was collected through internet, from various available sources. The synthesis methods that have been discussed are sputtering, hydrothermal, microwave assisted, and electrochemical and chemical reduction methods.

dimension with size of more than 100 nanometers. They are also called as bulky nanomaterials.
Examples include the stack of nanowires/nanorods/nanotubes, etc.

Types of Nanomaterials
Based on the size and properties, nanomaterials can be categorized into different types as shown in figure 2.

Carbon Nanomaterials
The nanomaterial that contain carbon as the one of its major component, is known as carbon based nanomaterial. Fullerenes and carbon nanotubes come under this category. The carbon nanotubes are incorporated in graphene sheets, and then sheets are rolled into a tube. The mechanical strength of these tubes is very much higher than steel. Fullerenes are the allotropes of carbon [2-3].

Metal Nanomaterials
Metal ions of +2 and +3 valency are used as the raw materials to prepare metal based nanoparticles. Preparation can be done through chemical synthesis or photochemical reaction. These metal ions are simplified into metal based nanoparticles, with the help of chemical reductants. The resulting nanoparticles contain large surface areas. Also, it is possible to prepare the new nanoparticles by combining more than two single nanoparticles [2-3].

Semiconductor based Nanomaterials
These kind of nanomaterials are extensively used in catalysis and some electrical applications. Some of the most widely investigated semiconductor nanomaterials are zinc sulphide (ZnS), ZnO, cadmium sulphide (CdS), cadmium selenide (CdSe), etc. They exhibit both metallic and nonmetallic characteristics. Their properties can be finely tuned by using graphene [1].

Nanocomposite Materials
Nanocomposite is a solid substance that possess different phases where at least one phase size should be less than 100 nanometers. Nanocomposites can be further divided into ceramic/metal/polymer matrix composites. These materials has got significant importance in solar cells, and catalysis applications [1]. During the period of earlier decades, development of nanoscience and nanotechnology is exceptionally well, and it should be acknowledged. Distinct nanomaterials having 1-/2-/3-dimension(s) have been developed for the applications such as sensors (both electrical and bio), energy devices (storage and harvesting), catalytic applications, etc. [4][5]. In the technology of sensors and energy devices, nanomaterials of following characteristics are essentially required:  Large porosity  High surface energy  Large surface area (specific surface area) Because, the above properties hold a key role in the chemical processes occurring at surfaces and interfaces. Transition metal oxide based nanomaterials exhibit the following features that are quite useful in the applications of electrochemistry [4].
 Catalytic activity  Easy reinforcement in noble metal nanoparticles  Large abundancy  High energy storage capacity  Non-degradation for long time  Low cost preparation  Less toxic in nature One of the most widely used nanomaterial is zinc oxide (ZnO), for several optical and piezoelectric applications. The bandgap of ZnO nanomaterial is nearly 3.37 eV. Nanodevices fabrication, became effective, with the use of ZnO nanomaterials in the form of spheres, rods, wires, arrays, etc. Because they show good piezoelectric, dielectric, and optical properties. The incorporation of transition metals, in ZnO nanostructures, resulted in tremendous electrical properties that could be useful for devices based on spin [6]. Also, the magnetic properties can be induced and altered with the introduction of cation of magnetic nature. For instance, chromium (Cr) doped ZnO exhibit the good magnetic properties. Cr is identified as one of the suitable modifier forZnO, for following reasons [7-8]:  Stable ferromagnetism in Cr-doped ZnO  Low ionic radius than Zn  Can easily replace Zn position  Paramagnetic nature at high temperatures  Antiferromagnetic nature at low temperatures In this review paper, we present a basic understanding on the different preparation methods of nanomaterials, and the recent work done.

Discussion and Analysis
This section deals with the different preparation methods of nanomaterials of different kinds. Nanomaterials based on iridium metal, are quite useful in electrical/optical/catalytic applications. Hence, it is worth to have basic understanding on the preparation techniques (majority techniques are shown in figure 3) of Iridium based nanomaterials.

Preparation through Sputtering
Sputtering is one of the deposition technique that can be done physically i.e. sputtering is a physical vapor deposition technique. Under the subject of an external and alternating electric field, the charged ions can be accelerated in a specified direction, through sputtering. When these charged ions collide with the target surface, atomization of surface takes place. Then the substrate surface will be sputtered with the neutral atoms, and hence, nano (particles or films) materials are said to be formed. Through this method, nanomaterials of relatively high melting points can be produced. Also, nanocomposites can produced with the usage of multiple sputtering targets. Moreover, the narrow size distribution of nanoparticles can be achieved through this method. Niwa and other researchers, realized an electrode made of Iridium/carbon nanocomposite [9]. It has been identified that the Iridium nanoparticles, which are dispersed on carbon nanofilm, exhibit a size of nearly 2 nanometers. Hence, the surface morphology can be altered/controlled with sputtering method.

Electrochemical Preparation
This method is based on redox reaction. Under the application of electric potential, the dissolved cations (metallic) get discharged, and become crystals on the cathode. With the control of voltage and current, the physical properties of nanomaterials can be disciplined. Through this method, single nanocrystalline materials can be produced on the single crystalline substrates. Using the iridium chloride (IrCl 3 -solution type) electrolyte, Iridium nanoparticles are electrochemically deposited. Deposition has been done on the film of carbon nanotube [10]. On average, the size of nanoparticle is approximately around 100 to 180 nanometers [10].

Hydrothermal Preparation
Under the elevated high temperature and pressure conditions, aqueous preparation of nanomaterials is generally referred as hydrothermal method. The preparation is done in a separate closed system where environment pollution is avoided. But still, observing the nanocrystals is quite difficult. Because, the reactions will not stop at high pressure. Nanoparticles based on Iridium, have been prepared through hydrothermal synthesis, in hydroxyethyl piperazineethanesulphonic acid. The temperature condition is 180 °C [11].

Fig. 3. Different Preparation Techniques of Metal Nanoparticles 2.4 Microwave assisted Preparation
It is one of the rapid technique to produce materials of nanoscale. In the span of very few minutes, nanomaterials can be prepared effectively and efficiently. The frequency (electromagnetic) ranges from 300 megahertz to 300 gigahertz. When the source of microwave is switched off, the reactions are stopped immediately. In the presence of argon atmosphere, iridium nanoparticles have been prepared through microwave assisted method.
Ir 6 (CO) 16 and 1-butyl-3methylimidazoliumtetraflouroborate have been used as the precursor and reaction medium respectively. Uniformly distributed nanoparticles can be produced through this method.

Preparation through Chemical Reduction
It is one of the common technique to prepare the noble metal based nanoparticles. It is basically referred as a one-step method. Because, the reactants are chemically reduced in a single reactor, and still the desired compound can be achieved. No advance steps/procedure are/is required to separate and purify the intermediate substances. This method helps to produce the desired product in short time. Also, this method involves faster reaction rate. In one of the work [12], iridium nanoparticles of size between 2.25 and 4.25 nanometers have been prepared by reducing chemically the lithium triethylborohydride. The following chemical substances have been utilized:  Precursor: H 2 IrCl 6  Media: tetrahydrofuran  Stabilizer: octadecanethiol The as prepared nanoparticles were found to be crystalline in nature, and exhibit face centered cubic packing. The preparation has been done under ambient atmosphere. In another work [13], iridium nanoparticles of size nearly 3 nanometers have been produced. The method of preparation is chemical reduction. Precursor used is IrCl 3ethylene glycol. The reduction was very mild, for obtaining the Iridium nanoparticles.

Literature on Nanomaterials/Nanoparticles
Recently [14], oxide of composition Li 1.2 Mn 0.56 Co 0.12 Ni 0.12 O 2 , has been prepared with distinct morphology. It means that the material synthesized is in the form of micro-sphere/rod, nanoplate and nanoparticle. It was found that the electrochemical properties were completely dependent on the morphology. Also, it was identified that microrods and nanoparticles exhibit high and low electrochemical properties respectively.
The high electrochemical performance of microrods was may be due to the Lithium active ion sites originated from porosity of the structures. Also, the contact area between the electrolyte and electrode is very large. Cobalt quantum dots exhibit good electrocatalyticfeatures. In one of the work [15], cobalt nanocomposite has been prepared. The dimensional size is around 3.2 nanometers. It exhibit the properties that are shown in table 1. Overpotential 0.37 V Further, the nanocomposite also exhibit large dispersion, enhanced electron transfer mechanism and low dimension. Therefore, cobalt quantum dots distributed on reduced graphene oxide possess exceptional electrocatalytic properties. Noble and stable metals like gold, silver, iridium, platinum, etc. possess tremendous properties like electrical/optical/mechanical/magnetic properties. Especially at the nanoscale, they are said to be stable, active catalysts, and exhibit corrosion resistance [16][17][18][19][20][21][22]. As per the existing literature, the metal based nanoparticles can be used as the catalysts. The chemical characteristics of Iridium nanoparticles is mainly based on composition, and surface morphology. It was identified that, the surface enhanced Raman scattering was studied well for Iridium nanochains than Iridium nanoparticles [23][24]. Nanomaterials based on transition metals and noble metals show versatile chemical properties. Mudusu and others [25], have developed the Tinsulphide (SnS) nanomaterials with 1dimension. The developed SnS nanostructures high single crystalline nature, and exhibit growth (1 0 0) plane direction. These nanostructures may be useful for making SnS based photovoltaics, batteries, etc. The SnS nanowires preparation is briefly shown, as follows:  Catalyst: Sapphire, modified by gold  Substrate: Silicon  Technique: chemical vapour deposition  Temperature range: 500-950 °C  Growth Mechanism: Vapor → Liquid → Solid Metal oxide nanoparticles of hollow spheres, have been prepared for the devices of electrochemical energy storage [26][27][28][29]. The nanoparticles include nickel oxide (NiO), tungsten oxide (WO 3 ), titanium oxide (TiO 2 ), etc. The size of NiO nanomaterials is nearly 28 nanometers. Through hydrothermal method, cobalt oxide (Co 3 O 4 ) quantum dots have been prepared [30]. The crystallite size of these quantum dots is approximately 2 nanometers. Transition metal based nanostructures are found to be alternate candidates for platinum and other noble metal catalysts. On the titanium foil, Co 3 O 4 nanomaterial is fully grown. It exhibit rod structure with a sharp tip. On average, the diameter of the rods is 400 nanometers and length is around 10 micrometers. These properties were altered when Co 3 O 4 was doped with zinc (Zn +2 ). From the existing literature, we found that, nanoparticles of size < 10 nanometers can be produced through hydrothermal method. But still, chemical reduction is an effective method to synthesize nanoparticles. Because, the electrochemical properties are high for the nanomaterials prepared through chemical reduction. Cobalt quantum dots have been prepared by dispersing them in graphene nano sheets. The size of the prepared cobalt quantum dots is 3.2 nanometers, on average [15]. In one of the recent work [6], Cr doped ZnO nanomaterials have been prepared through hydrothermal route. The XRD analysis indicated that materials exhibit Wurtzite hexagonal structure. The space group of the structure is identified as P36mc. The magnetization values (x) at 27 °C and -263 °C are given in table 2. The magnitude of field (Coercive field (C)) required to make magnetization zero, and the values of remanant magnetization (M r ) is shown  table 3 and table 4 respectively.

Conclusions
Based on the review that has been done in this work, the following conclusions and recommendations have been made:  For electrochemical applications, nanomaterials based on transition metals are quite useful.  Very low size (< 10 nm) nanomaterials can be prepared through the process of sputtering.  Chemical reduction is mostly used to prepare noble metal based nanomaterials.