3.1.5. XRD Study

X-ray diffraction is an important technique used to determine the structure and composition of synthesized materials. Figure 5A shows XRD patterns of Fe3O4, PANI, and PANI/Fe3O4 composites before adsorption of BB3. The characteristic diffraction peaks appeared at 2θ = 24.04◦, 33.06◦, 35.6◦, 49.3◦, 53.9◦, and 62.7◦ in the XRD spectrum of Fe3O4, which indicates spinel cubic crystals of Fe3O4. The formation of a strong peak at 33.06◦ indicates the formation of Fe3O4. These peaks were matched with the standard cards on powder diffraction files-2 (PDF 89-598) and have close agreemen<sup>t</sup> [71]. After adsorption of BB3, the intensities of diffraction peaks decrease due to interactions between dye and Fe3O4 (Figure 5B) [72].

**Figure 5.** XRD of Fe3O4, PANI, and PANI/ Fe3O4 (**A**) before and (**B**) after adsorption of BB3.

XRD spectrum (Figure 5A) of PANI shows its amorphous nature. No apparent change is observed in the spectrum of PANI after adsorption of BB3 (Figure 5B). Deshpande et al. [73] have reported a PANI film with amorphous shape. One can observe the presence of Fe3O4 in the PANI matrix due to diffraction peaks in the XRD spectrum of PANI/Fe3O4, but the intensities of these peaks are smaller than those in the spectrum of pure Fe3O4 particles, showing interaction between Fe3O4 and PANI. Obviously, the crystanality in the composites arises due to the presence of Fe3O4 particles. After adsorption of BB3 the peaks in the XRD spectrum of the composites simply disappeared. These observations indicate the strong overlaying layer of the dye on the surface of composites, thereby blunting the XRD peaks that were observed before adsorption of the dye [74].

### 3.1.6. Surface Area Analysis

Surface area analysis has a major role in the adsorption phenomenon. The surface areas of Fe3O4, PANI, and PANI/Fe3O4 composites before and after adsorption of BB3 were determined by adsorption–desorption of nitrogen gas through Brunauer–Emmett–Teller (BET) method (Figure 6) [75]. The obtained results are summarized in Table 1, which show that the surface areas of Fe3O4, PANI, and PANI/Fe3O4 composites before adsorption of BB3 are 65.818, 70.263, and 99.759 m<sup>2</sup>/g, respectively (Figure 6A). After adsorption of BB3, the surface areas of Fe3O4, PANI, and PANI/Fe3O4 composites decreased to 46.608, 46.698, and 53.196 m<sup>2</sup>/g, respectively (Figure 6B). The decrease in surface areas of Fe3O4, PANI, and PANI/Fe3O4 composites after adsorption of dye confirms that PANI/Fe3O4 composites can adsorb comparatively more dye than Fe3O4 and PANI. These results correlate to those obtained through SEM, XRD, EDX, and FTIR.

**Figure 6.** Surface area analysis of (**A**) Fe3O4, PANI, and PANI/ Fe3O4 before and (**B**) after adsorption of BB3.

**Table 1.** Surface area and Barrett, Joyner, and Halenda (BJH) para meters of Fe3O4, PANI, and PANI/Fe3O4 composites before and after adsorption of BB3 dye.


Beside surface area, BET calculation can also be applied to determine the pore volume and average pore diameter, as shown in Table 1.
