3.1.3. FTIR Spectroscopy

FTIR spectroscopy is used to study and identify organic, polymeric, and in some cases inorganic materials. Figure 3A shows FTIR spectra of Fe3O4, PANI, and PANI/Fe3O4 composites before adsorption of BB3. The details of FTIR signals associated with different types of vibrations are summarized in Table S2 of the Supplementary information.

A characteristic absorption band is observed at 554.8 cm<sup>−</sup><sup>1</sup> due to the stretching vibration of Fe–O bonds in the Fe3O4 spectrum. In an early study, stretching vibrations of Fe–O bonds were reported at 560 cm<sup>−</sup><sup>1</sup> [63]. This shift in the Fe–O band towards lower frequency in the present study may be due to the presence of DBSA in the Fe3O4 particles. Peaks at 1133.6 and 1534.6 cm<sup>−</sup><sup>1</sup> correspond to CH2 bending modes of DBSA. Similarly, a weak peak at 3494.3 cm<sup>−</sup><sup>1</sup> is because of –OH stretching attached to the Fe3O4 surface and shows close resemblance to the already reported work [64]. Another weak band at 1734.7 cm<sup>−</sup><sup>1</sup> is assigned to residual NH4OH, as already reported elsewhere [65]. The peak at 554.8 cm<sup>−</sup><sup>1</sup> is due to stretching vibrations of Fe–O disappearing and a new peak at 539.5 cm<sup>−</sup><sup>1</sup> appearing, showing BB3 dye adsorbtion onto Fe3O4, as shown in Figure 3B. This is because of the interaction of oxygen present in the dye structure with Fe of Fe3O4. The appearance of more intense peaks at 1224.6 and 1365.7 in Figure 3B is also attributed to the adsorption of BB3 [66].

FTIR spectrum of PANI is also shown in Figure 3A. Peaks at 1568 cm<sup>−</sup><sup>1</sup> and 1466 cm<sup>−</sup><sup>1</sup> are due to C=C and C=N stretching vibrations of benzoinoid and quinoid rings, respectively. Phang and Kuramoto have reported the C=C and C=N stretching vibrations of PANI at 1572 and 1497 cm<sup>−</sup>1, respectively [54]. The bands at 1307.6 cm<sup>−</sup><sup>1</sup> are due to C–N•+ stretching of secondary aromatic amine of PANI doped with protic acid. The peak at 670.1 cm<sup>−</sup><sup>1</sup> shows out-of-plane bending vibrations of the C–H bond. The peak at 1017.9 cm<sup>−</sup><sup>1</sup> is assigned to –SO3H group of DBSA bonded to nitrogen of PANI. The bands at 1133.7 and 829.2 cm<sup>−</sup><sup>1</sup> are assigned to the aromatic C–H bending in-plane and

out-of-plane deformation of C–H. The peaks at 2844.6, 2931.6, and 3249.9 cm<sup>−</sup><sup>1</sup> are assigned to N–H stretching vibrations of secondary amines. In the early research, such peaks appeared in the range of 3000–3500 cm<sup>−</sup><sup>1</sup> [67]. The shifting towards the low frequency range in the present work may be due to the presence of DBSA. After adsorption of BB3 dye, all these peaks shift towards high frequency, with a decrease in the intensity of peaks at 2844.6 and 2931.6 cm<sup>−</sup>1, as shown in the Figure 3B [50].

**Figure 3.** FTIR spectra of (**A**) Fe3O4, PANI, and PANI/Fe3O4 before and (**B**) after adsorption of BB3.

All these peaks appeared in the FTIR spectra of PANI/Fe3O4 composites, with a slight shift towards low frequency, as shown in Figure 3A. The shifting of absorption bands towards low frequency shows the existence of physical forces between PANI and Fe3O4. The band at 3249.9 cm<sup>−</sup><sup>1</sup> in the FTIR spectrum of PANI is replaced by a broad absorption plateau in the FTIR spectrum of PANI/Fe3O4 composites. The appearance of a very small peak at 539.5 cm<sup>−</sup>1, due to Fe–O bond stretching, shows the presence of Fe3O4 in the composite [68]. The absorption bands in the FTIR spectrum of PANI/Fe3O4 shift towards low frequency after adsorption of BB3, as was also observed in the spectra of PANI and Fe3O4, but the peaks are more intense in the former case, as shown in Figure 3B.
