## 7 Characterization of Calcium Carbonate and Oxide Particles  
The characterization of calcium carbonate NPs (CCNPs) can be done by all the sophisticated instruments but especially by FTIR, XRD, scanning electron microscopy-electron diffraction spectroscopy (SEM-EDS), TEM, and Raman spectroscopy. The characterization of CCNPs plays an important role in the confirmation of synthesis and identification of polymorphs. Fourier transform infrared spectroscopy plays an important role in the identification of functional groups in the synthesized CCNPs. A typical FTIR spectrum of a CCNPs is shown in Figure 10. The FTIR analysis of CaCO\({}_{3}\) reveals that there is a strong peak for carbonate ions due to vibration peaks v1, symmetric stretching; v2, out-of-plane bending; v3, doubly degenerate planar asymmetric stretching; and v4, doubly degenerate planar bending. The spectrum shows vibrational bands at 1456.3, 876.7, 712.7, and 409.8 cm\({}^{-1}\) indicates plane bending vibration of carbonate [87]. The FTIR analysis confirmed that the CaCO\({}_{3}\) nanopowder had the characteristic peak of the carbonate group. A sharp peak at 876.7 cm\({}^{-1}\) confirmed that the CaCO\({}_{3}\) nanopowder obtained from the cockleshell was calcite.  
Similarly, Raman spectroscopy also helps in the identification of molecules and functional group identification. The most important technique is the SEM which helps in the differentiation among the three polymorphs of CCNPs otherwise all these three polymorphs will look alike. The TEM also helps in the size determination of the particles. Based on the external morphology these polymorphs can be easily recognized as nano calcite. They are rhombohedral in shape, aragonite is needle-shaped and the vaterite is either spherical or flower shape. The typical field emission scanning electron microscope images of nano calcite, aragonite, and vaterite are shown in Figure 11a-d. The rhombohedral shaped particles confirm the calcite phases whose surface is smooth with sharp edges and corners [88].  
Figure 11b is showing the aragonite phase, which is rod-shaped. The vaterite phase in Figure 11c,d is showing flower and spherical shape. The EDS attached with SEM helps in the confirmation of the formation of CaCO\({}_{3}\) along with its purity. The EDS also reveals the purity of the CCNPs by giving a molecular percentage of elements mainly C, O, and Ca in the elemental spectra. While the TEM image in Figure 12A reveals a rhombohedral shaped micron particle whose morphology is similar to the SEM images in Figure 12A [88].  
Figure 10: Fourier transform infrared spectra of calcium carbonate nanoparticles adapted from [PERSON] et al., 2010.  
Figure 11: FESEM of (**a**) nano calcite; (**b**) aragonite; (**c**) spherical vaterite; (**d**) floral vaterite [89].  
Figure 12: Transmission electron microscopic images of (**A**) calcium carbonate of microparticles and (**B**) aragonite nanorods adapted from ([PERSON] et al., 2012).  
While Figure 12B is showing nanorods whose diameter in width is below 50 nm and the length is above 200 nm. The rod-shaped particles indicate the aragonite phase of Ca NPs [88].  
X-ray diffraction (XRD) analysis helps in the confirmation of mineralogy and phase determination of the CCNPs. A typical XRD pattern is shown in Figure 13, which exhibits characteristics peaks of aragonite at 20 values of 26.34\({}^{\circ}\), 33.24\({}^{\circ}\), 45.98\({}^{\circ}\), 33.24\({}^{\circ}\), 45.98\({}^{\circ}\), and 26.3\({}^{\circ}\), which correlate with (hkl) indices of (111), (012), (221), and (021), (0.12), and (221), respectively [23].