## 3 Characterization
### Structure and Chemical Composition  
XRD is frequently applied to examine the crystalline structure and phase of BC and BCPs. [PERSON] et al. [50] compared the XRD spectrum before and after catalytic degradation. They found that the crystal structure of the g-MoS\({}_{2}\)/PGBC remains unchanged. [PERSON] et al. [63] used XRD to investigate the BC (macroalgae) and BC-TiO\({}_{2}\) composite. XRD spectra showed that the peak located at 26.9\({}^{\circ}\) and 43\({}^{\circ}\) corresponded to the (002) plane of crystalline carbon, associating with the graphite-like structures. Furthermore, the graphitic peaks declined with the increase of TiO\({}_{2}\) loading, indicating the good intercalation of TiO\({}_{2}\) nanoparticles between BC layers.  
FT-IR spectra can be applied to distinguish the functional groups and the degree of carbonization of BC. The band absorbance at 710 cm\({}^{-1}\) was a characteristic feature of BC [63]. The peak between 875-1035 cm\({}^{-1}\) was associated with C-O and OH- stretching vibration. The absorption band was found to shift to higher wavenumber with the increase of TiO\({}_{2}\), which indicated that the T-OC- bond changed the optical property and charge transfer. [PERSON] et al. [73] used FT-IR spectra to investigate the molecular component and the degree of carbonization of BC (cellulose). The peaks at 3450, 1750, 1635 and 1040 cm\({}^{-1}\) can be assigned to the stretching vibration of the -OH, C=O and C-O-C, respectively. It was stated that the cellulose skeleton was converted to the graphitic structure of amorphous carbon nanofibers during the pyrolysis process.  
XPS measurement is applied to analyze the composition and surface chemistry of BC and BCPs. More functional groups of BC at binding energies of 291.2 eV and 531.8 eV associated with carbonates  
Figure 3: Graphical overview of BC and BCPs characterization.  
and C=O were observed than in the case of pure lignin [62]. It was found that the pyrolysis process can cause an increase in the surface activity of carbon functional groups present on the surface of BC. Moreover, binding energies of 284.8 eV and 288.6 eV were due to the adventitious elemental carbon and C-O bonds, respectively, indicating the presence of Ti-O-C structure [74].  
Raman spectroscopy can be used to characterize BC-based nanostructures. [PERSON] et al. [75] demonstrated the D band at 1351 cm\({}^{-1}\) and G band at 1587 cm\({}^{-1}\) was characteristic bands of graphite carbon. The intensity ratio (I\({}_{\text{D}}\)/I\({}_{\text{G}}\)) reflected the crystalline and graphitization of BC [38]. The I\({}_{\text{D}}\)/I\({}_{\text{G}}\) value of C/Fe\({}_{3}\)O\({}_{4}\)/Bi\({}_{2}\)O\({}_{3}\) and C/Fe\({}_{3}\)O\({}_{4}\) were 0.84 and 0.82, respectively, indicating that the graphitization of BC made the BCPs from disorder to order, which was beneficial for electronic conduction. Lower I\({}_{\text{D}}\)/I\({}_{\text{G}}\) means higher graphitization. The BCPs fabricated via ultrasound route displayed a slightly lower I\({}_{\text{D}}\)/I\({}_{\text{G}}\) value (0.37) compared to pure BC (0.48). It was stated the slight change of graphitization might be caused by the reconstruction of structural defects within the sp\({}^{2}\) carbon network that arose upon the ultrasound-assisted synthesis method [41].  
NMR is another spectroscopic technique commonly used to in-depth investigate the fine structure of BC. The degree of carbonization and stability of BC pyrolyzed at different temperatures and heating rates can be compared using the NMR spectroscopy [76; 77]. Moreover, it can provide details on the functional groups and hydrocarbons [78]. Moreover, it was considered that NMR was more suitable for analyzing BC containing C compounds than functional groups [79]. Hence, researchers commonly used both NMR and FT-IR to examine the structure and functional groups.