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The larger molecules are disintegrated into smaller units by a destructive process, and these units are transformed into useful NPs, for example, grinding/milling, CVDs, physical vapour deposition, etc. [29]. This approach is generally used to synthesize NPs from coconut shells (CSs). The milling method is used, whereby raw CS powders were finely milled using ceramic balls and a well‐known planetary mill at different time intervals. Via other characterization techniques, the influence of the milling period on the overall size of the NPs is shown.

Furthermore, as time increases, the size of the NP crystallite (Scherrer equation) decreases. In this process, it was also found that the brownish colour faded away with the increment of each hour because of the reduced size of the NPs [30]. Various characterization techniques demonstrated the effect of milling time on the overall size of the NPs. The synthesis of spherical magnetite NPs from natural iron oxide (Fe₂O₃) ore was shown in the presence of organic oleic acid by a destructive top‐down method with a particle size ranging from 20 to 50 nm [31]. To synthesize spherical particles of colloidal carbon using a control scale, a primary top‐down route was used. The synthesis technique was based on the continuous chemical adsorption of polyoxometalates on the carbon interfacial surface. Adsorption has transformed black carbon aggregates into relatively smaller spherical particles with a high dispersion capacity and a narrow distribution of size [32]. Microstructures have found that the quantity of carbon particles is lower during the sonication period. Combined grinding and top‐down sonication techniques synthesized a sequence of transition metal dichalcogenide nanodots (TMD‐NDs) from their crystallites. Every TMD‐ND with a size of less than 10 nm shows excellent dispersion and is demonstrated by the narrow distribution of the measure [33]. Highly photoactive Co₃O₄ NPs have recently been produced by top‐down laser fragmentation, i.e. a top‐down process with an average size of 5.8 ± 1.1 nm. Powerful laser irradiations produce well‐uniformed NPs with adequate oxygen vacancy [34].

Figure 1.2 The synthetic models of NPs: top‐down and bottom‐up approach.

Source: Modified from Iravani [29].