## 3 Results
Meanwhile, the band at the wavelength of 788 cm\({}^{-1}\) was due to the Si-O-Si bond stretching, while at 432 cm\({}^{-1}\), it was due to the bending vibration of the same functional group [8; 11]. Hence, the presence of bands at 1066, 788 and 432 cm\({}^{-1}\) confirmed the formation of silica following the acid treatment method using CHA, and is tabulated in Table 2.  
Nanosize 30 was used in this study to determine the average particle size. The graph of the particle size distribution for CHA is shown in Figure 6. Based on the graph, the average particle size distributions were found to be in the range of 500-750, 270-560, 360-740 and 200-410 nm for 500, 600 and 700 \({}^{\circ}\)C, and after acid treatment, respectively. With the increase in temperature, no obvious pattern was observed in the average particle size distributions of the CHA. In contrast, the average particle sizes decreased to 200-410 nm after the acid treatment, which was due to the reaction with sulfuric acid, which caused the particles to become finer and smaller [9; 40]. This occurrence may also be caused by the reaction between sulfuric acid and CHA, which dissolved the bigger-sized elements in the ash and caused the distribution of the average particles to become smaller. The data obtained are also in line with the FESEM images, where the size distribution decreased after the acid treatment. The UV-VIS spectroscopy of CHA samples was performed at room temperature in the spectral region of 220-800 nm in order to determine the optical absorbance of the sample. Figure 7 illustrates the absorption curves of the CHA at 500, 600 and 700 \({}^{\circ}\)C. The absorption region displayed a broad band in the UV region below 350 nm for all of the spectra, except for 500 \({}^{\circ}\)C. Based on the absorbance spectrum, the valence-conduction band transition of the sample could be determined, while its optical band gap could be calculated based on the absorption edge. Basically, the measurement of the optical band gap can be used to describe to the nature of the materials. The principle of the absorbance spectroscopy is based on the Beer-Lambert law, whereby the relation between the absorption coefficient, \(\alpha\) and photon energy (_lvv_) is used to plot a graph so as to find the optical band gap [41]. \[\alpha(\
u)=k\frac{(h\
u-E)^{n}}{h\
u} \tag{1}\]  
\[(\alpha h\
u)=k(h\
u-E)^{n} \tag{2}\]  
Figure 2:0  
Inserting n = \(\frac{1}{2}\) allows for a direct transition, as follows:  
Figure 2:1  
where \(\
u\) is the frequency, \(h\) is the [PERSON]'s constant, k is proportional constant, exponent \(n\) is the type of transition that allowed for the transition (n = 1/2) and E is the optical energy band gap. \begin{table}
\begin{tabular}{c c} \hline \hline
**Wavenumber (cm\({}^{-1}\))** & **Functional Group** \\ \hline
432.98 & Si-O-Si bending vibration \\
774.85 & Si-O-Si stretch vibration \\
1066.15 & Si-O-Si stretching vibration \\
1407.40 & CH\({}_{2}\) strain \\
1628.30 & C=O \\
3300.23 & Si-OH \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Functional group in the CHA after treatment. The optical band graph can be found by extrapolating the linear portion of the optical absorption curve to the x-intercept. From the plotted graph, as in Figure 8, the optical band gaps of the CHA were 3.50, 4.15 and 4.05 eV for 500, 600 and 700 \({}^{\circ}\)C, respectively. After acid treatment, the sample of CHA showed a slightly higher band gap of 4.30 eV in comparison with the other samples. This suggests that the presence of oxide impurities (approximately 50%) in CHA lowers the optical band gap of the samples. Silica is an insulator in nature, with a wide band gap found to be greater than 6 eV by several studies [42; 43; 44]. However, after the acid treatment, there were still up to 8.24% impurities according to the XRF data, and this caused a disturbance in the treated CHA structure and caused the optical band gap value to become lower when compared with pure silica.