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
Optical characteristics are of great concern in photocatalytic applications, and photochemists have therefore gathered a good understanding of this approach to reveal their photochemical processes [61, 62]. These are based on the common law of Beer–Lambert and the basic principles of light [63]. These methods include information about the absorption, reflectance, luminescence, and phosphorescence of nanomaterials. Metallic and semiconductor NPs have various colours and are thus ideally suited for photo‐related applications. To understand each application's primary mechanism, it is often essential to see the importance of absorption and reflectance of these materials. UV–vis and photoluminescence are the most common optical devices used to study the optical properties of NP materials (PL, Null Ellipsometer). The Diffuse reflectance spectroscopy (DRS) UV/vis is a fully designed optical absorption, transmission, and reflection measurement unit. The first two are extra, while DRS is mostly a unique technique for the samples sold. It is imperative to use the method for measuring NP band gaps as well as other NPs. MMT, LaFeO3, and LaFeO3/MMT nanocomposite synthesis and differences in their absorption of electromagnetic radiation by UV–vis DRS to identify their optical characteristics were studied [64]. In the case of nanocomposites, a significant red shift was observed compared to pristine MMT and LaFeO3 NPs. Instead of a broad absorption band from 400 to 620 nm, LaFeO3 and LaFeO3/MMT revealed a reduction in their band gap. These NPs are significant for photocatalysis by solar light [64]. To investigate the optical properties of photoactive NPs and other nanomaterials and UV, PL considers useful technologies. This technique gives further information on the absorption or emission potential of the materials and their effects on the picture's overall excitement period. It thus provides valuable details about the charging hybridization and half‐life of the exciting material on their conducting bands for all photo‐ and image applications.
1.5 Physicochemical Properties of NPs
Different physicochemical features such as the large surface are discussed; as previously mentioned, mechanically robust, optically active, and chemically reactive NPs are unique and ideal for multiple uses. Some of its essential properties are discussed in the following.
1.5.1 Mechanical and
