# Extraction and Synthesis of Silicon Nanoparticles (SiNPs) from Sugarcane Bagsase Ash: A Mini-Review
## 2 Extraction of Silica
### Sol-Gel Process  
The sol-gel procedure is a wet chemical approach traditionally known as chemical solution deposition. It involves, several steps, namely, hydrolysis and polycondensation, gelation, aging, densification, and crystallization. The processing of inorganic ceramic and glass materials was first conducted as early as the mid-1800s by [PERSON] and [PERSON]'s studies on silica gels [32]. The process mainly involves alkaline extraction, followed by acid precipitation.  
Sugarcane bagasse is cleaned and made moisture-free by drying. Additionally, studies have reported that acid pre-treatment is one of the effective ways to remove of ionic impurities from sugarcane bagasse [33].  
Therefore, 1 M \(HCl\) is used for pre-treatment shown in Figure 5, in a water bath and heated at 75 \({}^{\circ}\)C. The sample is subsequently filtered numerous times to discard metallic ions and further dried in an oven. The as-obtained sugarcane bagasse ash is then immersed in 1 M \(NaOH\) and refluxed for an hour at 90 \({}^{\circ}\)C to produce sodium silicates solution from the sugarcane bagasse, as shown in the following equation [33; 34].  
\[SiO_{2}\ (s)+2 NaOH_{(aq)}\to Na_{2}SiO_{3(aq)}+H_{2}O_{(aq)} \tag{15}\] \[Na_{2}SiO_{3(aq)}+2 HCl_{(aq)}\to SiO_{2(gel)}+2 NaCl_{(aq)}+H_{2}O(I) \tag{16}\]  
The key findings of the investigation were the observations made during the hydrolysis of tetraethyl orthosilicate (TEOS), \(Si(OC_{2}H_{5})\), under acidic media yielded \(SiO_{2}\) in the form of a \"glass-like\" material [35]. [PERSON] and co-workers did another breakthrough study where they highlighted the potential for achieving desired and efficiently high levels of homogeneity in colloidal gels. They used the sol-gel method in the 1950s and 1960s to synthesize a large number of unique ceramic oxides of novel chemical make-up, such as Al, Si, Ti, Zr, and others, which could not be achieved by other ceramic methods [35].  
The high purity silica xerogels were prepared from bagasse ash to form sodium silicate by the sol-gel method. Subsequently, the mixture was treated with acid for gelating formation, whereby condensation reaction, the siloxane linkage between surface silanol groups as shown in the following reaction mechanism [36];  
\[Si(OH)_{4}+HO-Si(OH)_{3}\rightarrow(OH)_{3}Si-O-Si(OH)_{3}+H_{2}O \tag{17}\]  
Figure 5: Sol gel method for silica preparation.  
The precipitation procedure with the pH was maintained at >7 [35]. The sodium silicate solution with the hydrochloric acid process can be shown as:  
\[Na_{2}O.xSiO_{2}+2 HCl\ \to xSiO_{2}\downarrow+2 NaCl+H_{2}O \tag{18}\]  
Moreover, purification with demineralized water was found to be successful for the purity as high as 99 wt%. The structural properties indicated that the as-prepared silica xerogels were amorphous in nature [37].  
Triumphs and Challenges  
The chemical methods, however, showed that there is room to manipulate the SiNPs by fine-tuning the synthesis and achieve the desired properties via surface modification and controllable particle size distribution of Si Quantum Dots. However, the question of purity always arises from residual side products. The high yield and desirable uniform properties have attracted researchers worldwide to produce Si QDs using electrochemical techniques under certain conditions from porous silicon. The shape and size distribution, as well as particle size control are still a major challenge for this technique [38; 39].  
Several methods have been utilized by many researchers to study the production of nanoparticles from various feedstock. Basically, the sol-gel method is most preferred for this current study as it allows for altering of the materials from the nano-level to achieve desirable properties. Silicon nanoparticles possess high physical characteristics, outstanding opto-electronic properties (i.e., photoluminescence), active surface state, and biocompatibility, making them an ideal material and attractive to be utilized in solar cell fabrication [40; 41].