Figure 1.3 Illustration of synthesis of nanoparticles: (a) top‐down method and (b) bottom‐up method.

      Source: From Wang and Xia [39]. © 2004, American Chemical Society.

1.4 NPs and Characterization

For the analysis of other physicochemical properties of NPs, different methods of characterization have been used, such as XRD, X‐ray photoelectron spectroscopy (XPS), infrared (IR), SEM, transmission electron microscopy (TEM), and Brunauer–Emmett–Teller (BET). Advanced methods are applied for the analysis of the particles.

1.4.1 Morphological Characterization

      The morphological characteristics of NPs are still of great importance as morphology still affects most of the NP properties. Various characterization techniques exist for morphological research, but microscopic methods exist, such as polarized optical microscopy (POM), SEM, and TEM.

      1.4.1.1 SEM Technique

      The SEM technique is based on the electron scanning principle and provides all the nanoscale NP data available. This technique is used to study the morphology of their nanomaterials and the dispersion of NPs in the bulk or matrix. This technique [15] showed the distribution of SWNTs in polymer matrix poly(butylene) terephthalate (PBT) and nylon‐6. The morphological characteristics of ZnO‐modified metal–organic frameworks (MOFs) were studied using the SEM technique, which indicates the dispersion of ZnO‐NPs and MOFs' morphologies under different reaction conditions [43].

      1.4.1.2 TEM Technique

It is based on the electron transfer principle, so that it can provide descriptions of the bulk content from very low to greater magnification. In addition, it is commonly used for the analysis of different morphologies of the Au‐NPs [44]. TEM also provides essential information about two‐ or more layer materials; the quadruple hollow shell structure of Co₃O₄ NPs is observed by TEM, for instance. In Li‐ion batteries such as the anode, these NPs have proven themselves to be exceptionally efficient. The porous multi‐shell structure induces shorter Li⁺ diffusion path lengths with ample annulled space for buffer volume expansion, good cycling efficiency, higher speed capacity, and essential capacity [45].

1.4.2 Structural Characteristics

      The structural characteristics of the structure and function of the bonding materials are of primary importance for studying. It gives details about the bulk properties of the subject material. XRD, energy‐dispersive X‐ray (EDX) spectroscopy, XPS, IR, Raman spectroscopy, BET, and Zieta size analyser are the techniques used to study the structural properties of NPs.

      1.4.2.1 XRD

      One of the essential characterization techniques is to reveal the structural properties of NPs. The crystalline phase of NPs is provided with sufficient data. It also provides a rough image of the particle size through the Debye–Scherrer [8] formula. In the identification of single and multiphase NP [46] schemes, this approach worked well. However, in smaller NPs with a size smaller than hundreds of atoms, the acquisition and accurate measurement of structural and other parameters may be difficult. Besides, the XRD diffractogram can be affected by NPs with different interatomic lengths having more amorphous characteristics. To obtain accurate data, the diffractograms of bimetallic NPs must be contrasted with those of the corresponding monometallic NPs and their physical mixtures in this case. The best way to make a substantial difference is to measure the simulated bimetallic NP structural model with the spectra of XRD [47] observed.

      1.4.2.2 Energy‐Dispersive X‐ray (EDX)

      To understand the elementary composition with a rough idea of per cent weight, a usually fixed field emission scanning electron microscopy (FE‐SEM) or TEM system is commonly used. The electron beam centred on a single NP through SEM or TEM through the software functions to obtain the insight knowledge under observation from the NP. NP consists of constituent elements and, by irradiating electron beams, each of these releases X‐ray energy characteristics. The real X‐ray intensity is directly proportional to the explicit part of the particle's concentration. Researchers in preparatory materials commonly use this technique to help SEM and other processes to validate their components [48]. The elemental composition of ultra–sonochemically synthesized BiVO₄ NPs in pseudo‐flower form [49] was calculated using the EDX technique. Similarly, a similar approach was used to perform the indispensable confirmation and graphene impregnation of Ln₂O₃/graphene heterostructure NPs, which showed C, Ln, and O as contributing elements synthesized by the traditional hydrothermal method [50].

      1.4.2.3 XPS

It is a surface‐sensitive tool and can be used to consider the overall composition and the compositional variance with in‐depth profiling studies. XPS is based on the basic principles of spectroscopy. The typical XPS spectrum consists of the number of electrons on the Y‐axis plot versus the X‐axis electrons' binding energy (eV). Each element has its fingerprint value for energy binding and thus gives a particular set of XPS peaks. Corresponding peaks, such as 1s, 2s, 2p, and 3s, come from the electronic configuration [51]. To research the dispersion of Boron NPs (10 nm size) during functionalization with polyethylene glycol (PEG), a depth profile analysis was given with Ar⁺ ions at 1.4 keV and 20 nm. It has been shown that the concentration of NPs increases from 2% to 5% with depth. This offered strong evidence that within the bulk of functionalized PEG, boron NPs are effectively dissolved [52]. In a related analysis, core–shell Au/Ag showed similar behaviour through XPS scope profiling [53].

      1.4.2.4 FT‐IR and Raman Spectroscopies

      The vibration characterization of NPs is typically studied by FT‐IR and Raman spectroscopies. These techniques are the most evolved and feasible compared to other simple analytical methods. The critical range for NPs is the fingerprint region, which provides the details for the material signature. In one sample, Pt‐NP (1.7 nm) functionalization and its interaction with the alumina substrate were analysed using FT‐IR and XPS techniques. FT‐IR confirms the functionality as it showed signature vibrational peaks of carboxylated C–O 2033 cm⁻¹, in addition to a broader O–H peak of 3280 cm⁻¹, respectively [54, 55]. Because of its signal‐enhancing capability via SPR phenomenon, recently improved surface‐enhanced Raman spectroscopy (SERS) is emerging as a vibrational conforming tool [56, 57].

1.4.3 Particle Size and Surface Area Characterization

      Various techniques can calculate the size and surface area of the NPs. These include dispersing SEM, TEM, XRD, AFM, and dynamic light scattering (DLS). It is possible to increase the particle size of SEM, TEM, XRD, and AFM, but the zeta theoretical analyser/DLS should be used to find the NP size at a weak stage. In one study, DLS was used to analyse silica NP size differences while consuming serum proteins. With the acquisition of the protein layer, the findings showed that the size increased. However, in the case of aggregation and hydrophilicity, DLS might prove incapable of accurate measurement, so in that case, we should focus on the high‐resolution technique of differential centrifugal sedimentation [58–60].

1.4.4 Optical Characterizations

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