## 3 Results and Discussion  
### Biochar Characterization  
The surface morphology and particle size of the pristine and ball-milled biochar samples were characterized using SEM. Figure 1a shows the SEM images of the raw and biochar samples obtained at various pyrolysis temperatures of 400, 500, 600, and 700 \({}^{\circ}\)C (RB 400, RB 500, RB 600, and RB 700). The surface morphologies of the raw and biochar samples show honeycomb-like pores and a layered structure. As the pyrolysis temperature increases, the structure of the biochar shrinks. Furthermore, owing to their thin walls, the structures appear fragile and cracked. Figure 1b presents the SEM images of the biochar sample RB600 milled for 30, 60, 120, 180, and 240 min. After ball milling, the honeycomb-like pores and layered structure of the pristine biochar change to irregular particles. The particle size decreases with increased milling time. DLS measurements were also performed to investigate the particle size distribution. As shown in Figure 1c, the particle size decreases with increased milling time, and after 180 min, the particle size is approximately 350 nm. This result is consistent with the SEM image shown in Figure 1b. XRD was performed in order to characterize the crystalline structures. Figure 2a displays the XRD patterns of the biochar samples at different pyrolysis temperatures (400, 500, 600, and 700 \({}^{\circ}\)C). As shown in Figure 2a, the biochar samples' XRD spectra show two broad peaks centered at 2\(\theta\) = 23\({}^{\circ}\) and 43\({}^{\circ}\), which correspond to the (002) and (100) planes of graphitic carbon materials, respectively. No crystalline structure is detected; that is, there is no sharp peak, implying that the biochar structures are completely amorphous. Figure 2b shows the XRD patterns of the RB600 biochar ball milled for different times (30, 60, 120, 180, and 240 min). The XRD spectra of all biochar samples show similar trends, as shown in Figure 2b, with two broad peaks at 2\(\theta\)\(\approx\) 23\({}^{\circ}\) and 43\({}^{\circ}\). Raman spectroscopy is an important characterization technique for highly disordered carbonaceous materials, such as biochar, owing to its sensitivity to molecular structures with a short-range order. Figure 3 shows the deconvoluted and fitted Raman peaks of the biochar samples at different pyrolysis temperatures and milling times; the black circle and blue line are the original raw data and fitted data, respectively. The D and G bands are depicted in red and green, respectively. As shown in Figure 3a, the Raman spectra of the biochar sample exhibit two broad bands at approximately 1330 and 1580 cm\({}^{-1}\), which are defined as the D and G peaks, respectively. The D band at 1330 cm\({}^{-1}\) is related to the disordered and defective structure of the biochar [32], and the G band at 1580 cm\({}^{-1}\) arises from the in-plane vibrations of the sp\({}^{2}\) graphitic structure of the biochar [33]. The broad D and G bands are deconvoluted into two components. The intensity ratio, \(I_{\text{D}}/I_{\text{G}}\), and full width at half maximum (FWHM) are used to determine the defect density and degree of crystallinity [34], respectively, which can indicate the quality of the biochar. A low \(I_{\text{D}}/I_{\text{G}}\) ratio and narrow FWHM suggest a high biochar quality. As the pyrolysis temperature increases from 400 \({}^{\circ}\)C to 700 \({}^{\circ}\)C, the \(I_{\text{D}}/I_{\text{G}}\) ratio decreases (2.70, 2.05, 1.76, 1.72, 1.73, 1.74, 1.75, 1.76, 1.77,  
and 1.82) (Figure 3a). This result indicates that the disorder decreases and graphitization increases with increased pyrolysis temperature. The FWHM of the biochar becomes narrower with increased pyrolysis temperature, indicating that the crystalline quality of the biochar increases with increased pyrolysis temperature. Further, the \(I_{D}\)/\(I_{C}\) ratio slightly decreases (1.81, 1.75, 1.66, 1.68, 1.71, and 1.77) as the milling time increases (0, 30, 60, 120, 180, and 240 min, respectively).