**2. Classification of nanoparticles**

Nanoparticles may be metallic, non-metallic [1], anthropogenic, engineered, organic, or inorganic as outlined in **Figure 1**. Metallic nanoparticles include copper,

#### **Figure 1.** *Schematics on the classifications of nanoparticles.*


#### **Table 1.**

*Some nanoparticles and their respective features.*

*Diverse Synthesis and Characterization Techniques of Nanoparticles DOI: http://dx.doi.org/10.5772/intechopen.94453*

magnesium, zinc, gold, titanium, silver etc.; while non-metallic nanoparticles include silica, carbon nanotubes etc. Anthropogenic NPs are by-products obtained from industrial produce while engineered nanoparticles are directly obtained from manufacturing processes.

Some of the nanoparticles and their features [2, 4] have been summarized in **Table 1**.

### **3. Synthesis techniques of nanoparticles**

The techniques applied in synthesizing nanoparticles greatly influence their morphology, size, structure, and performance. The electrochemical, physiochemical, optical, and electrical features of the nanoparticles are also affected. In some occasions, nanoparticles are coated so as to retain their features after precipitating out of suspensions. The synthesis methods for nanoparticles are broadly divided into top-down and bottom-up approaches [4].

#### **3.1 Top-down approach**

Top-down method is a destructive method that breaks down large molecules into smaller parts before converting into the relevant nanoparticles. This approach involves some decomposition strategies like chemical vapor deposition (CVD), milling process, and physical vapor deposition (PVD). Milling is used to extract nanoparticles from coconut shells with the crystallite size reducing with increasing time. Nanoparticles of iron oxide, carbon, dichalcogenides, cobalt (III) oxide have been produced using this method.

#### **3.2 Bottom-up approach**

This approach involves the formation of nanoparticles from simple materials in a build-up manner. It is environmentally friendly, less poisonous, feasible, and of low cost. The materials used are usually Reduction and sedimentation processes like green synthesis, bio-chemical, spin coating, sol–gel etc. adopt this approach. Nanoparticles of titanium dioxide, gold, bismuth have been synthesized via this approach. The reaction chain for the production of gold nanoparticle has been illustrated in **Figure 2** [5].

Synthesizing nanoparticles could also involve chemical or biological processes [1]. Some chemical synthesis techniques of nanoparticles include sol–gel method, wet chemical synthesis, hydrothermal method, thermal decomposition, microwave method etc. [2]; while the biological means involve enzymes, microorganisms, plant extracts, and fungi.

#### **3.3 Chemical methods**

Some chemical methods adopted in synthesizing nanoparticles include sol gel, precipitation, hydrothermal, thermal decomposition, solvothermal, vapor synthesis etc. [6, 7]. Sol–gel method is an easy means of producing nanostructures by homogenously mixing precursors in a solvent to form a gel material which is then heated to produce the required nanoparticle. It begins from preparing a sol which undergoes gelation process to solvent removal. Wet chemical/precipitation method is a fast and easy process for synthesizing large scale nanoparticles. Hydrothermal method utilizes high pressure and temperature to power heterogeneous reactions under aqueous solvents like water. The kind of pressure, pH, and temperature


applied affects the features of the synthesized nanoparticles. Such nanoparticles are suitable for biotechnological use because of their hydrophilic surface nature [8]. Thermal decomposition involves oxidizing a solid material in optimal temperature. Solvothermal method uses a solvent to produce various materials like polymers, semiconductors, or metals at moderate or high pressure [9]. It produces novel and stable nanoparticles with controlled thicknesses and temperature. To synthesize nanodots; the cationic source is dissolved in suitable solvent alongside a surfactant which stabilizes the growth rate. Cadmium selenide, zinc oxide, zinc selenide are producible using this method and can be applied in magnetic and biotech industries [10]. In vapor synthesis, gaseous molecules chemically react to produce a phase which condenses and leads to particle growth. The higher the temperature, the faster the particles are formed. Different means of inducing homogenous nucleation include condensing inert gases, vaporizing a supersaturated material using a pulsed laser, generating a spark discharge by charging electrodes, sputtering the material with unreactive gaseous ions; or through some chemical methods like chemical vapor deposition, photothermal method, flame synthesis, or spray pyrolysis [11]. This method suitably yields nanoparticles of titania, carbon, and silica. Flame synthesis is commonly used to commercially produce silica, carbon black, optical fiber, and titania [12]. Particles produced by converting gases in furnace reactors or hot walls are usually very pure, although it produces agglomerated particles.

### **3.4 Biological methods**

Biological or biosynthesis of nanoparticles is an environmentally-friendly, green, and non-toxic method involving microorganisms [13–15]. Nanoparticles of iron oxide, silver, nickel oxide, copper oxide, zinc ferrite have been synthesized using this method [16–22]. The location of the nanoparticle determines the point of synthesis; whether intracellular or extracellular [1]. Intracellular production of nanoparticles uses enzymes to move ions into the cells of microbes and produces smaller sized nanoparticles in the organism. Extracellular synthesis does not involve *Diverse Synthesis and Characterization Techniques of Nanoparticles DOI: http://dx.doi.org/10.5772/intechopen.94453*

**Figure 3.**

*Diverse bio-development synthesis of nanoparticles and their application areas.*

cell components and yields nanoparticles outside the cell, uses fungi with large secretory organs. Microbes like fungi and bacteria are responsible for controlling the synthesis process. Microorganisms are immensely used to produce nanoparticles because of their economical, non-poisonous nature, and detoxification of heavy metal power. Phytonanotechnology is compatible with biological systems, available source materials, high stability, and entails synthesizing nanoparticles from plants [23]. Changes in the pH level of plants alter their binding strength, morphology, and the number of metallic ions available during the synthesis. The different sources, synthesis methods, and areas of application of nanoparticles have been represented in **Figure 3** [23]. Biogenic means of producing nanoparticles are green and cheap; with the involvement of fungi, waste materials, and bacteria [5].

#### **3.5 Mechanical methods**

Nanoparticles can also be synthesized by mechanical methods like mechanical alloying, milling, and mechanochemical processes [24]. Milling method regenerates interfacial chemical operations at low temperatures. Mechanochemical technique involves continuous welding operations that adequately select milling materials and minimize agglomerations. For effective production; the stoichiometry of source materials, thermal treatment, paths for reaction to occur, and milling conditions would be carefully considered. Nanoparticles of oxides, iron, nickel, silver, cobalt can be synthesized using these methods.

#### **4. Characterization methods for nanoparticles**

Properties of nanoparticles like shape, size, surface morphology, crystalline nature, light absorption etc. need to be completely described using relevant characterization techniques [2]. Some of the methods used to characterize nanoparticles [4] include:

#### **4.1 Morphological features**

The morphology of nanoparticles greatly influence the properties exhibited by nanoparticles. Microscopy methods applied on nanoparticles are usually electron microscopy or scanning probe microscopy. Scanning electron microscope (SEM) gives nanoscale and surface information of the dispersion and morphology of nanoparticles. Microscopy techniques are destructive and used for single-particle measurements. Transmission electron microscopy (TEM) uses transmittance of electrons to provide bulk information at high and low magnifications. Optical microscopic technique is not useful for nanoparticles because the size of nanoparticles is smaller than light diffraction limit. Coupling spectroscopic techniques to electron microscopes would enable elemental studies to be carried out.

#### **4.2 Optical studies**

Optical methods reveal reflectance, transmittance, photochemical, and luminescence features of nanoparticles. Spectroscopy uses the interaction of particles with electromagnetic radiation to determine the shape, concentration, and size of nanoparticles. Spectroscopic techniques like infrared, ultraviolet–visible, photoluminescence (PL), UV/vis-diffuse reflectance spectrometer (DRS), and magnetic resonance methods are applied to nanoparticles. DRS is specially used to determine the band gap energy of nanoparticles. PL studies reveal the effect of emissivity and absorptivity on the excitation of photons, half-life, and recombining effects of the charges. The sizes of nanoparticles affect their optical features and make it useful in bioimaging devices [4].

#### **4.3 Structural analysis**

The structure of nanoparticles gives details about the kind of bond existing between the atoms and the features of the bulk material. Some of the structural techniques used on nanoparticles include BET, X-ray diffractometry (XRD), IR etc. XRD describes the phase, particle size, type of NP, and crystal nature of the nanoparticles.

#### **4.4 Elemental studies**

The elemental composition of nanoparticles can be determined using energy dispersive X-ray spectroscopy (EDX), XPS, Raman, FT-IR etc. EDX details the elemental components of bulk particles. Better contrast is obtainable when the obtained spectra are compared with a computer generated model. XPS is a very sensitive spectroscopic method used to obtain the exact compositional ratio of the elements, their bonding nature, depth profile analysis. Raman and FTIR techniques use vibrational methods to show functionalized peaks and particle information.

#### **4.5 Size estimation**

Sizes of nanoparticles can be estimated using scanning electron microscope, transmission electron microscope, X-ray diffractometer, atomic force microscope etc. The sizes of the nanoparticles are obtained using size distribution profiles and give more precise results when used alongside digital models. The surface area can be estimated using BET via adsorption and desorption processes.
