Analytical grade chemicals, Bi(NO₃)₂∙6H₂O (99% Merck, India) and iron nitrate Fe(NO₃)₃∙9H₂O were taken as starting materials for the synthesis of Bi₂Fe₄O₉ ceramic by the chemical route_. The stoichiometric amount of bismuth nitrates was dissolved in de-ionized distilled water along with appropriate amount of glycine (C₂H₅(NO₂), 99% Merck, India) as per equivalent to the metal ions. The resulting reaction mixtures were heated on a hot plate magnetic stirrer and maintaining the temperature 70˚C - 80˚C for evaporation of excesses of water. A fluffy mass obtained in the ignition process which burnt with a sooty flame. The obtained fluffy mass was crushed into fine powder with the help of mortar and pestle. The resultant dry powder was calcined at 600˚C for 6 h in the electrical muffle furnace and made pellets (13.0 mm diameters, 1.6 mm thicknesses) using 2 wt% polyvinyl alcohol (PVA) as a binder. These pellets were sintered at 800˚C for 6 h and further carried out different types of physiochemical characterizations such as XRD, SEM, TEM, EDX, Cyclic voltammetry, and Photocatalytic activity.

A Congo Red (CR) dye was selected as the model pollutant in the adsorption study. Initially, a 500 mg/L of CR stock solution was prepared, and experimental samples of respective concentrations (5, 10, 20, 30, 40, and 50 mg/L) were prepared by serial dilution technique. A standard curve of CR was prepared at 497 nm by using a UV-Vis spectrophotometer (Universal Bio, India). The batch adsorption study was performed in 100 mL Erlenmeyer flask by adding absorbent into 50 mL CR solution and shaken at 120 rpm and 30˚C ± 1˚C. The effect of process time (0.5 - 10 min), CR concentration (5 - 50 mg/L), and adsorbent dosage (0.1 - 1.0 g/L) were studied against the removal of CR. The samples were taken at regular time intervals and then centrifuged at 5000 rpm for 5 to measure the final CR concentration. The removal efficiency (%) and adsorption capacity (q_e (mg/g)) of CR dye was measured by the following equations.

Removal efficiency of CR dye = C i n − C o u t C i n × 100 (1)

Adsorption capacity (q_e) = C i n − C o u t M × V (2)

where C_in and C_out represent the inlet and outlet concentration of CR dye (mg/L), respectively. V is the volume of CR solution (L) and M is the mass of adsorbent (mg/g).

The crystalline phase of sintered material was identified by an X-rays Diffractometer (Rigakuminiflex 600, Japan) at a scan rate of 3˚/min in 2θ range of 20˚ - 80˚ employing CuK_α radiation of wavelength, λ = 1.54059 Å. The morphology of the fractured surface of sintered Bi₂Fe₄O₉ was characterized by scanning electron microscope (ZEISS, model EVO–18 research; Germany).The Bright field TEM images were obtained with the help transmission electron microscopy (TEM, FEI Tecnai-20G²) with an accelerating voltage of 200 kV. For dielectric measurements, the pellets were polished and painted by silver paste; then dried at 300˚C for 3 h to form electrodes for the respective characterizations. Electrical measurement data were taken by LCR meter (PSM1735-NumtriQ, Newton 4^th Ltd UK) by varying the temperature and frequency. Optical band gap energy of Bi₂Fe₄O₉ nanoparticle was evaluated from the observed diffuse reflectance spectra. Reflectance spectra recorded with the help of balanced deuterium-halogen source (DH 2000 BAL, Ocean Optics) with 210 - 2500 nm spectral range.

Cyclic voltammetry and electrochemical impedance spectroscopy of all the samples have been performed by Versa STAT 3. Corresponding details have been studied in our earlier research work [15] [16].

The electrode preparation methods have been discussed in our earlier paper [17] [18].

Fabrication of the electrode was done by making a dispersion of 5 mg of prepared material, 2-propanol of 0.2 mL, Nafion of 12 µL and drop casting 80 µL on carbon paper of 1 cm² area. Silver glue was used to connect the current collector (copper wire) with the substrate (carbon paper).

X-ray diffraction pattern of Bi₂Fe₄O₉ polycrystalline ceramic sintered at 800˚C for 8 h is shown in Figure 1 which confirms the single phase formation of the materials. All the diffraction peaks are correctly matched with JCPDS card no. 20-0836 which clearly reveals the absence of secondary phases in the synthesized materials. The orthorhombic structure of Bi₂Fe₄O₉ ceramic was fabricated by two formula units per unit cell. These formula units consist of evenly distributed FeO₆ octahedrons and FeO₄ tetrahedrons with a lower packing density than that of corresponding closed packed structures. The Bi³⁺ ions are surrounded by eight oxygen ions with mutually orthogonal shorter BiO₃ and longer BiO₅ units The most intense peaks of Bi₂Fe₄O₉ ceramic corresponding to the planes (211), (220), (240), (133), (310), (420), (213), (530) are observed in the XRD pattern. All the characteristic peaks of XRD data were indexed on the basis of a cubic unit cell similar to Bi₂Fe₄O₉. The lattice parameter and unit cell volume of Bi₂Fe₄O₉ ceramic were determined by using “Cel” software having lattice parameter a = 7.97 Å, b = 8.43 Å, c = 6.01 Å. The crystallite size (D) of Bi₂Fe₄O₉ ceramic was calculated with the help of Debye-Scherer’s formula as given by the following expression.

D = k λ / β cos θ (3)

where, k is a dimensionless shape factor, with a value taken as 0.90, λ is the wavelength of X-ray,θ is the diffraction angle (Bragg angle), andβ is the peak width of the diffraction peak at half-maxima (FWHM) and their value is taken as in radian. The β is corrected value of diffraction peak due to instrumental broadening for crystallite size with reference to standard silicon wafer sample. The average crystallite size of Bi₂Fe₄O₉ ceramic calculated by Debye–Scherer’s formula and found to be 37.71 nm.

Bright field TEM images of Bi₂Fe₄O₉ ceramic sintered at 800˚C for 8 h is depicts in Figure 2(a) which reveals the aggregation of the particles. The average particles size of Bi₂Fe₄O₉ ceramic was found to be 102.58 nm. Bright spot pattern observed in selected area diffraction (SAED) patterns for Bi₂Fe₄O₉ ceramic shown in Figure 2(b), emerging through the corresponding planes (4 4 4), (3 1 0), (4 4 0) and (3 2 1), respectively confirmed crystalline nature of the material. Figure 2(c) exhibits high resolution TEM image displaying inter-planar (d) spacing for Bi₂Fe₄O₉ ceramic. The d-spacing value observed for the Bi₂Fe₄O₉ ceramic is d = 4.157 Å with the corresponding plane (2 1 1). The entire diffraction pattern obtained in the SAED pattern and HR-TEM image are good agreements with the JCPDS card number 46-0416 of the XRD results.

Microstructure feature of the grains of Bi₂Fe₄O₉ceramic was observed in the SEM analysis on the scale 200 nm which is shown in the Figure 3. It is revealed from the figure that homogenous grain was distributed of the materials. The average grain size of Bi₂Fe₄O₉ ceramic was found to be 100 ± 20 nm, measured with the help of Image-J software.

The variation of dielectric constant (ε) with frequency (Hz) and few selected temperatures of Bi₂Fe₄O₉ ceramic are shown in Figure 4. The value of ε decreases with increase of frequency as clear from the figure and remains constant at higher frequency range. This value at lower frequency region may be arises due

to space charge polarization in between semi-conducting grains and insulating grain boundaries and constant value of ε due to rapid periodic reversal of electric at higher frequency regions. That is why no charge accumulation occurs at interface in higher frequency and results lower value of ε. The value of dielectric constant of the material is found to be 645.45 at 20 Hz and 373 K.

Plots of dielectric loss (tanδ) with frequency at different temperatures of Bi₂Fe₄O₉ ceramic is shown in Figure 4. It is clearly revealed from the figure that the same trends in tanδ with frequency is observed as the dielectric constant up to 100 kHz and after which above 100 kHz, the value of dielectric loss exactly merged to each other over all measured temperatures. The value of tanδ for Bi₂Fe₄O₉ ceramic was found to be less than 1.0 at 1 kHz at all measured temperatures. The higher value of dielectric loss of Bi₂Fe₄O₉ ceramic may be due to space charge polarization.

Figure 5 shows the variation of relative permittivity with temperature at some particular frequencies of Bi₂Fe₄O₉ ceramic. The below figure shows that dielectric constant first increases and attain maxima and then decreases with increase of temperature. The increment of ε with decreasing frequency may be due to reduction of bonding force in between the atoms which enhance the polarization of the materials.

The value of dielectric constant for Bi₂Fe₄O₉ at 1 kHz, 10 kHz and 100 kHz were found to be 209, 138, and 88 respectively. The plot of Dielectric loss (tanδ) with temperature at 1 Hz, 10 Hz and 100 kHz is shown in Figure 5. The dielectric loss first increases and then decreases with increasing temperature. The increase in the tanδ with temperature may be regarded as oxygen vacancy present in the BiFeO₃ ceramic during sintering process. The value of tanδ for Bi₂Fe₄O₉ was found to be 0.87, 0.33 and 0.14 at 1 kHz, 10 kHz and 100 kHz, respectively.

The contact time of dye solution with adsorbent is adversely affects the dye removal efficiency [19]. It is helpful to analyze the kinetics of dye removal. The effect of process time with respect to Congo Red (CR) removal (%) by Bi₂Fe₄O₉ under the following conditions (pH: 7.0, CR concentration: 20 mg/L, adsorbent dose, 0.1 g/L, temperature: 30˚C ± 1.0˚C, and 120 rpm) is shown in Figure 6. The removal efficiency of CR dye was very fast up to 1.5 min and thereafter reached to the state of equilibrium after a very short contact time (5.0 min). Initially, 93.5% of CR removal efficiency was obtained in 1.5, which further increased with process time and reached 99.3% in 5 min. The obtained result reveals that the material offers very rapid adsorption of CR dye on Bi₂Fe₄O₉. The effective adsorption of CR dye by Bi₂Fe₄O₉ was endorsed to its higher surface area and more available surface active sites for adsorption of Cr dye.

Effect of process time on CR removal efficiency by Bi₂Fe₄O₉ (pH: 7.0, CR concentration: 20 mg/L, adsorbent dose: 0.1 g/L: temperature: 30˚C ± 1.0˚C, and 120 rpm): The influence of adsorbent dose (0.1 - 1.0 g/L) against CR removal (%) by Bi₂Fe₄O₉ under following conditions (pH: 7.0, CR concentration: 20 mg/L, process time: 5.0 min, temperature: 30˚C ± 1.0˚C, and 120 rpm) is shown in Figure 7. A gradually increasing trend of CR dye removal efficiency with increasing adsorbent dose was obtained. The low amount of adsorbent provides less adsorption site at constant CR dye concentration, resulting in low removal efficiency was found. Whereas, large adsorbent dosage leads to an increase in active surface area and adsorption site at constant CR dye concentration, and thus increased CR dye removal efficiency. The CR removal efficiency of 25.4% was found with an adsorbent dose of 0.1 g/L, which was further increased to 97% with 0.8 g/L of adsorbent dose. After that, the CR removal efficiency was not much improved with the adsorbent dose and found to be 99% with 1.0 g/L of adsorbent dose. It was probably due to about all adsorbate (i.e., CR dye) was adsorbed onto the active sites of the adsorbent (Bi₂Fe₄O₉). The effect of adsorbent dose (i.e., polydopamine-kaolin and polydopamine-reduced graphene oxide-kaolin) on the

removal of Methylene Blue (MB) was studied and high removal efficiency of MB was obtained at high adsorbent dose due to availability of high active surface area and adsorption site [19].

Figure 8 depicts Effect of adsorbent dose on CR removal efficiency by Bi₂Fe₄O₉ (pH: 7.0, CR concentration: 20 mg/L, process time: 5.0 min, temperature: 30˚C ± 1.0˚C, and 120 rpm). The effect of initial CR concentration (5 - 50 mg/L) in removal efficiency (%) of CR dye by Bi₂Fe₄O₉ under following conditions (pH: 7.0, adsorbent dose: 0.1 g/L, process time: 5.0 min, temperature: 30˚C ± 1.0˚C, and 120 rpm) is shown in Figure 8. It can be perceived from the figure that the adsorption of CR dye on Bi₂Fe₄O₉ was decreased with increasing initial concentration of CR. Initially, CR removal efficiency was gradually decreased with increasing CR up to 20 mg/L, and thereafter the CR removal efficiency was decreased very sharply with increasing CR concentration. The highest CR removal efficiency of 97.8% was obtained at the lowest CR concentration of 20 mg/L, whereas the lowest CR removal efficiency of 42.5% was obtained at the highest CR concentration of 50 mg/L. The high concentration of CR dye could lead to the quick saturation of active site; hence the removal efficiency decreased at high CR concentration. The adsorption of methylene blue (MB) by graphene oxide modified kaolin was studied at various MB concentrations (5 - 40 mg/L), and the lowest removal efficiency of MB was obtained at the highest concentration due to saturation of active sites [20].

The conductivity behavior of Bi₂Fe₄O₉ material shows photoconductivity and optical activity for the improvement of their properties. Figure 9 shows the frequency-dependent conductivity behavior at completely different temperatures. The electrical transport properties of the materials derived from AC conductivity and therefore the total conductivity of the materials measured through Almond-West power law that was calculated by the following Equation (3). Frequency

dependent of AC conductivity for Bi₂Fe₄O₉ ceramic was determine by Jonscher’s power law as given by using Equation (4).

σ A C = σ D C + A ω s (3)

σ A C = A ω s (4)

where, σ_DC is the frequency-independent conductivity dominant at the lower frequency region. A is the temperature-dependent pre-exponential factor and sis the frequency exponent parameters (0 > s > 1), dependent on both of materials and temperature and frequency independent, ω is angular frequency (ω = 2πf). The value of frequency exponent parameters(s) obtained from the slope of the curve by linear square fitting and found to be −0.23, −0.12, 0.25 and 0.42 at 313, 333, 353 and 373 K temperatures. It is observed that the value of s for Bi₂Fe₄O₉ ceramic almost equal to 1 and increases with increase of temperatures. The increase in s value with temperatures indicates the presence of hopping charge conduction mechanism in the Bi₂Fe₄O₉ ceramic.

The activation energy for the AC conductivity, E_ac of Bi₂Fe₄O₉ ceramic sintered at 800˚C for 8 h, can be calculated from the slope of logσ_AC vs. inverse of temperature curve is depicts in the figure. It is observed from the figure that the value of conductivity linearly increases with increases of temperature. The AC conductivity was strongly correlated with the conduction mechanism in the materials over DC conductivity which occurred at higher temperature side due to thermally activated process.

The temperature-dependent of AC conductivity can be measured by Arrhenius equation and it calculated with the help of following expressions:

σ = σ 0 exp ( − E a k T ) (5)

where, σ is the conductivity of the materials, σ₀ is the pre-exponential factor, E_a is the activation energy for the thermally activated process, T is the absolute temperature and k is the Boltzmann constant.

The activation energy of Bi₂Fe₄O₉ sample measured by using linear fitting of Figure 10 and observed to be 0.042, 0.039 and 0.037 eV at 1 kHz, 10 kHz, and 100 kHz, respectively. From above observed data, it is found that the activation energy values (E_a) slightly increases with decreasing the frequency which may be lead to be thermally activated process. The lesser value of E_a with regarding to higher frequency may be cause of hopping charge mechanism which increases the conduction mechanism of the materials [5].

Diffuse reflectance spectra of Bi₂Fe₄O₉ nanoparticle Figure 11 showing strong light absorption in the UV-Visible region. However, it has also strong optical absorption extending upto NIR region. Similarly, reported optical band gap value (E_g) of Bi₂Fe₄O₉ nanoparticle is 1.29 eV [21] [22], that may vary depending upon

the size of nanoparticle prepared and corresponding synthesis process. Similarly variation in techniques adopted for measurement of optical band gap, also have significant effect on the measured band gap value. In the present experiment we have adopted Kubelka-Munk theory for evaluation of optical band gap value of Bi₂Fe₄O₉ nanoparticle, from recorded diffuse reflectance spectra.

F ( R ) = α S = ( 1 − R ) 2 2 R (7)

F(R): Kubelka-Munk function;

α: absorption coefficient;

R: reflectance;

S: scattering factor.

Equation (7) describes that Kubelka-Munk function is proportional to absorption coefficient.

Optical band gap can be calculated from the plot of diffuse reflectance versus wavelength, where onset of linear increase in curve determines the corresponding band gap energy. But this method of calculating band gap energy produces slightly inaccurate value, hence it is mandatory to report the type of transition (direct/indirect) during evaluation.

Absorption edge analysis was applied to find the type of transition and to determine the optical band gap represented in Equation (8).

α h ν = k ∗ ( h ν − E g ) 1 / n (8)

k: energy independent constant;

E_g: Optical band gap.

The exponent indicates the type of transition where, value of n = 2 and 1/2 for direct and indirect allowed transitions respectively. Since F(R) is proportional to α, modified form of Equation (2) can be represented as:

F ( R ) ∗ h ν = k ∗ ( h ν − E g ) 1 / n (9)

The exponent n in Equation (3) can be determined from the plot of (F(R)∙hν)ⁿ vs. hν. The best fit to the straight line near the onset of absorption, observed for n = 1/2 recommending indirect allowed transition for Bi₂Fe₄O₉ nanoparticle.

From the plot of (F(R) * hν)^1/2 vs. hν, the band gap value of prepared Bi₂Fe₄O₉ nanoparticle is calculated to be 1.83 eV, from the extrapolation of linear region of curve.

It can be seen from Figure 12 that Bi₂Fe₄O₉ has redox peaks at 0.11 V/0.08 V. This shows that the material is pseudo capacitive in nature. The maximum currents generated during the CV are 4.9 A/g and 5.1 A/g for oxidation and reduction, respectively. The calculated specific capacitance for the electrode is 215 F/g. This signifies that the material can also be used as an electrode in electrochemical supercapacitors.

From Figure 13, the equivalent series resistance is found to be 17.1 Ω and charge transfer resistance is 11.5 Ω. Due to minimum charge transfer resistance it can be used in supercapacitors as electrodes (as conducting in nature).