2.3. FTIR spectra
The FTIR spectra of RP, AP and SP were performed using KBr pellets. The samples and KBr pellets were mixed at a ratio of 1/100, and then ground into powder to form a transparent sheet. The FTIR spectra were recorded using NICOLET-380 spectrometer at ambient temperature and the wave numbers ranged from 4000 to 400 cm−1.