3.1.1. SEM Study

The surface morphology of Fe3O4, PANI, and PANI/Fe3O4 composites were studied with scanning electron microscopy. The SEM image (Figure 1a) shows that Fe3O4 consists of finite spherical shape with average particle size of 0.25 μm, which tends to form aggregates. It is somewhat porous in texture and becomes rough after adsorption of BB3 (Figure 1b). The adsorption of dye on the surface of Fe3O4 decreases its porosity, as reported elsewhere [51]. Shreepathi and Holze reported fibrous morphology of PANI prepared in di fferent concentrations of DBSA [52]. The SEM image of PANI synthesized in this work shows cauliflower-like surface morphology, which after adsorption of dye changes into clusters of small ball-like structures, shown in Figure 1c,d. The SEM image of PANI/Fe3O4 depicts surface characteristics of both PANI and Fe3O4. Close observation of the composite morphology indicates adherence of Fe3O4 particles on the surface of PANI. The average size of composite particles was 0.28 μm. The development of magnetic micro and nanoparticle composites with PANI has been reported to greatly enhance adsorption characteristics of the hybrid materials [53–56].

### 3.1.2. UV-Vis Spectroscopic Study

UV-Vis spectroscopy is widely used for studying optical properties of materials. UV-Visible spectra of Fe3O4, PANI, and PANI/Fe3O4 composites were recorded in ethanol and chloroform. Figure 2A shows the UV-Vis spectra of Fe3O4, PANI, and PANI/Fe3O4 composites before adsorption of BB3. In Fe3O4 spectrum, the band at 441.9 is due to the surface plasmon resonance e ffect (SPR). The surface plasmon resonance phenomenon occurs due to interactions between incident radiations and valence electrons of the metal atom in Fe3O4 and causes the valence electron of the metal to oscillate with the frequency of the electromagnetic source [57]. The other band at 570.7 nm arises due to the presence of DBSA moieties in the synthesized magnetic oxide particles, as reported earlier [58].

**Figure 1.** SEM images of Fe2O3, PANI, and PANI/Fe2O3 composites before (**<sup>a</sup>**,**c**,**<sup>e</sup>**) and after (**b**,**d**,**f**) adsorption of BB3.

In the spectrum of PANI, the band at 325–338 nm is due to π-π\* transitions of the benzenoid ring and the band at 660–680 nm is attributed to excitation of the quinoid ring [59]. The spectrum of PANI/Fe3O4 composites shows a small band at 441 nm due to doping of benzenoid amine with Fe3O4 particles, while the band at 770 nm is due to the change from polaron to bipolaron state, suggesting interactions between PANI and Fe3O4 materials, which is in close resemblance to the already reported results [60,61].

Figure 2B shows the UV-Vis spectra of Fe3O4, PANI, and PANI/Fe3O4 composites after adsorption of BB3, respectively. The appearance of absorption band at 647–651nm in all the spectra clearly indicates the adsorption of BB3 on Fe3O4, PANI, and PANI/Fe3O4 composites. As reported previously, BB3 gives a strong absorption band at 654 nm [62]. This absorption band is more intense in the spectrum of the composites as compared to the spectra of PANI and Fe3O4. The enhancement in the intensity of the absorption band of the composite around 650 nm shows strong interactions and adsorption capability of PANI/Fe3O4 composites towards BB3 as compared to pristine PANI and Fe3O4.

**Figure 2.** UV-Vis spectra of Fe3O4, PANI, and PANI/Fe3O4 composites (**A**) before and (**B**) after adsorption of BB3. The inset (A) shows the spectrum of PANI/Fe3O4 in the long wavelength region.
