## 1 Introduction
### Processes in the Synthesis of Silica Nano-Particles  
To process agriculture waste products into SNPs consisting of specific forms, sizes, dimensions, nature, and structures, several processing methods are used and are generally categorized as top-down (physical strategy) or the bottom-up (chemical approach), as two commonly used methods for producing silica particles. The top-down strategy is characterized by the use of externally controlled tools or processes that are used to cut, mill, and shape materials into the desired shapes and sizes. Conversely, the bottom-up technique, a preferred strategy for creating SNPs, consists of processes in which molecular components arrange themselves into more complex assemblies via atom-by-atom, molecule-by-molecule, or cluster-by-cluster approaches [32]. The main driving force behind the bottom-up approach is the reduction in Gibbs free energy, \(\Delta\)G, and the materials thus produced are close to their equilibrium state. An approach where both of these techniques are employed is known as a hybrid approach, such as lithography. The top-down or physical method is characterized by employing unique size reduction techniques to reduce the dimension of the original size [33]. These methods depend on bulk material removal, etching, or division, as well as bulk process downsizing, i.e., to produce the necessary attrition framework attributes [34]. The milling process is an example of a top-down strategy, in which bulk materials are broken down to achieve small particles [35]. Ball milling is a shear-force-dominating process that uses impact and attrition to decrease particle size. It employs a revolving shell to generate centrifugal force and metallic balls (typically Zirconia or steel balls) as the grinding medium [36]. This method is generally extremely cost-effective, with good product purity, low-temperature reaction, controllability, convenience, and repeatability [37]. However, due to the production of structural faults in the generated material, the top-down synthesis approach requires additional modification. This is a serious constraint since the material structure substantially influences the surface chemistry and other physical properties of nanoparticles. Furthermore, the resulting material may have a high level of \"dust\" from flying particles. Despite these major disadvantages, this approach is thought to be suitable for large-scale production since it allows for deposition across a vast substrate and does not require chemical purification [38]. Chemical aggregation of SNPs via atom-to-atom or molecule-to-molecule processes is part of the bottom-up strategy [39]. Since it provides an exact particle size control, this approach is preferred for nanoparticle elaboration. Furthermore, bottom-up procedures are more adaptable and are based on the self-assembly principle, thus, this strategy has resulted in a wide range of nanoparticle production methods [38]. The high yield purity of this approach has considerable benefits [40]. Chemical vapor deposition (CVD) is a method for condensing vapor-phase chemicals into solid-phase materials [41], generally for greater resistance, erosion protection, and a combination of the two used for many applications [42]. CVD is widely used for producing large-scale and high-purity fused silica glass [43]. In order to synthesize SiO\({}_{2}\), a precursor is injected into an H\({}_{2}\)/O\({}_{2}\) flame. The subsequent steps include chemical transformation from precursor to oxide, the nucleation of oxide vapors, the collisions of oxide particles, the condensation of oxide vapor onto oxide particles, and the sintering of oxide particles. However, it is not easy to control the concentration of each precursor during the synthesis of multi-component particles. Halide compounds, such as silicon tetrachloride (SiCI\({}_{4}\)), are commonly used as a starter for the CVD production of silica glass in the industry. However, this approach necessitates high temperatures and has a slower rate of development.