A Facile Synthesis of Bi 2 O 3 /CoFe 2 O 4 Nanocomposite with Improved Synergistic Photocatalytic Potential for Dye Degradation

: Semiconductor-based photocatalysis is a probable approach to overcoming many pollution problems and eradicating toxic organic materials from wastewater. This research endeavor aimed to explore the synergistic potential of different semiconductor nanocomposites for photocatalytic degradation of organic pollutants in contaminated water. A facile hydrothermal approach was employed to synthesize bismuth oxide and cobalt ferrite nanoparticles from their precursors— bismuth nitrate pentahydrate, ferric chloride hexahydrate and cobalt chloride hexahydrate—with various concentrations and conditions to optimize the product. Subsequently, nanocomposites of bismuth oxide and cobalt ferrite were prepared by solid-state mixing in varying concentrations followed by calcination. UV/visible diffuse reﬂectance spectroscopy, X-ray diffraction, scanning electron microscopy and elemental dispersive X-ray spectroscopic techniques have corroborated the successful synthesis of nanocomposites. The energy gaps of bismuth oxide and cobalt ferrite nanocomposites were computed in the range of 1.58–1.62 eV by Tauc plots. These nanocomposite materials were ascertained for photocatalytic potential to degrade methyl orange organic dye in water. A nanocomposite with equiquantic proportions has shown the best photocatalytic degradation activity, which may be attributed to the type-II band conﬁguration and a synergistic effect, because Bi 2 O 3 acts as an electron sink. This synergism has reduced the cogent band gap, hindered electron hole recombination and increased electron hole availabilities for photodegradation reactions, thus ensuing an efﬁcient photodegradation co-work of Bi 2 O 3 /CoFe 2 O 4 nanocomposites.


Introduction
Water pollution is a major environmental problem worldwide. Wastewater contains numerous hazardous and toxic substances such as dyes, resins, heavy metals, phenolic compounds, pesticides and herbicides etc. [1][2][3]. Dye pollutants are carcinogenic and mutagenic; they can lead to central nervous system dysfunction and may cause morbidity in human as well as in aquatic life [4,5]. Conventional wastewater treatment pathways are not effective because of the contumacious nature of synthetic dyes and the high salinity of wastewater [6,7]. Ozonation and chlorination are also quite incapable because of their high operating costs [8,9]. The conventional physical methods, such as adsorption via activated carbon, ion exchange through synthetic adsorbent resins, reverse osmosis, ultrafiltration, coagulation by chemical agents etc., have been employed for the removal of toxic dyes from water [10,11]. These procedures are successful for transferring organic pollutants from water to another form, thereby producing secondary pollution which needs more treatment for removal of solid wastes and regeneration of the adsorbent and, thus, raises the cost of the process [12][13][14]. Among these contaminants, dyes, phenolics and pesticides have been of major concern because of their harmfulness to the environment. Advanced Among numerous semiconductors, Bi 2 O 3 and CoFe 2 O 4 are potent candidates for photoactive nanocomposite preparations due to their distinctive structures and physical aspects, such as band gap, thermal stability, high oxygen ion conductivity and high refractive index. However, bare Bi 2 O 3 and CoFe 2 O 4 show less photo catalytic activity, owing to photo corrosion, rapid charge carrier recombination and structural conversions [30][31][32][33]. Different methods can be utilized to increase the photocatalytic activity of Bi 2 O 3 and CoFe 2 O 4 , such as doping of transition metal oxides or rare earth metals and formation of nanocomposites in order to sensitize the photocatalyst to photo energy and to keep electron hole pairs separate. One of the most efficient ways to resolve this issue is to prepare nanocomposites of compatible semiconductor materials with a synergistic effect that will help charge migration [34][35][36][37][38][39][40][41][42][43][44][45][46][47].
The present study has explored the compatibility of Bi 2 O 3 and CoFe 2 O 4 for the preparation of photoactive nanocomposites with good photodegradation potential against methyl orange dye. To our knowledge, no earlier research has been reported in the literature regarding the synthesis of Bi 2 O 3 /CoFe 2 O 4 nanocomposites and their dye degradation potential.

Results
This study aimed to develop different nanocomposites of bismuth oxide and cobalt ferrite semiconductor materials that work synergistically to improve photocatalytic activity for efficient photodegradation of methyl orange dye. As-prepared nanocomposite materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopic techniques while their energy gaps were gauged by UV/Vis diffuse reflectance spectroscopy (DRS) via Tauc plots. Photodegradation experiment results were employed to study photocatalytic efficiency.

Phase Analysis
XRD patterns of prepared samples were scanned in the range of 10 • to 70 • by Xray diffractometer (D8 advance, BRUKER, Camarillo, CA, USA) with Cu-Kα radiation (k = 1.5406 Å) at room temperature. All the peaks in Figure 1a were found to be consistent with the available standard JCPDS 01-077-0426 and can be indexed to a single-phase cubic CoFe 2 O 4 structure. Reflection (220), exhibited at 30.12 • exclusively, depends on the Co 2+ cations occupying the cubic sites. No additional and intermediate phases were observed within the sensitivity of the experimental measurements. The crystallite size of as-prepared CoFe 2 O 4 particles was computed by the classical Scherrer formula [48]. Dhkl = kλ/βcosθ, where Dhkl is the crystallite size derived from the (311) peak of the XRD spectrum, k the sphere shape factor (0.89), θ the angle of diffraction, β the difference between the full width at half-maximum (FWHM) of the sample peak and the standard SiO 2 used to calibrate the intrinsic width associated with the instruments and λ the X-ray wavelength (1.5406 Å). The obtained crystallite size of as-prepared CoFe 2 O 4 particles was about 26.6 nm ( Table 1). The XRD pattern of Bi 2 O 3 (Figure 1e) was in agreement with the available standard JCPDS 00-022-0515. The crystal system of Bi 2 O 3 was a tetragonal form which is a metastable two-dimensional superstructure of β-Bi 2 O 3 [49]. Diffraction lines of Bi 2 O 3 patterns have indicated that the powder has a good degree of crystallinity ( Figure 1e). The crystallite size of Bi 2 O 3 was computed from the full width at half-maximum intensity of the (201) X-ray line diffraction and was found to be approximately 32 nm.
With the increase in Bi 2 O 3 content in Bi 2 O 3 /CoFe 2 O 4 nanocomposites, the intensity of the (201) peak at 27.42 • , which is identified as the main peak of the Bi 2 O 3 phase, gradually increased, whereas that of the (311) peak at 35.16 • , inherent from the CoFe 2 O 4 phase, was decreased. It was evident from Figure 1b-d that individual peaks of Bi 2 O 3 and CoFe 2 O 4 varied directly with their proportions in as-prepared nanocomposite materials, thus depicting no chemical reaction between the constituent chemicals. Furthermore, the purity of the samples is also apparent from the lack of any extra peak in the diffraction patterns of all prepared photocatalysts.
varied directly with their proportions in as-prepared nanocomposite materials, thus depicting no chemical reaction between the constituent chemicals. Furthermore, the purity of the samples is also apparent from the lack of any extra peak in the diffraction patterns of all prepared photocatalysts.

Elemental Analysis
EDX analysis was performed by a VEGA3 machine (TESCAN, Kohoutovice, Czech Republic) to determine the elemental composition of prepared samples. Each element showed its specific peaks in the graph, signifying its quantified presence in the sample. All peaks corresponded to CoFe2O4 with no extra peak showing the purity of the sample. The spectrum depicted signals of cobalt, iron and oxygen with 17.43%, 30.48% and 52.09% by weight, respectively. The composition, determined by energy-dispersive spectroscopy, showed the stoichiometry of CoFe2O4. Since the atomic numbers of cobalt and iron are closer, the ratio of the X-ray intensities from these elements generally associated with their composition. The intensity ratio of Co:Fe was about 1:2, as determined by the EDX. In this case, X-ray intensities do not need to be compensated for absorption and fluorescence effects, as these particles are smaller than the free path required for X-ray transmission through solids, i.e., 100 nm. The EDX pattern of Bi2O3 has shown no extra peak, which

Elemental Analysis
EDX analysis was performed by a VEGA3 machine (TESCAN, Kohoutovice, Czech Republic) to determine the elemental composition of prepared samples. Each element showed its specific peaks in the graph, signifying its quantified presence in the sample. All peaks corresponded to CoFe 2 O 4 with no extra peak showing the purity of the sample. The spectrum depicted signals of cobalt, iron and oxygen with 17.43%, 30.48% and 52.09% by weight, respectively. The composition, determined by energy-dispersive spectroscopy, showed the stoichiometry of CoFe 2 O 4 . Since the atomic numbers of cobalt and iron are closer, the ratio of the X-ray intensities from these elements generally associated with their composition. The intensity ratio of Co:Fe was about 1:2, as determined by the EDX. In this case, X-ray intensities do not need to be compensated for absorption and fluorescence effects, as these particles are smaller than the free path required for X-ray transmission through solids, i.e., 100 nm. The EDX pattern of Bi 2 O 3 has shown no extra peak, which indicates that the sample was pure. The spectrum has shown signals of O and Bi with 23.06% and 76.94% by weight, respectively ( Table 2). Qualitative and quantitative EDX results from the nanocomposites agreed with asprepared compositions, which indicates that no element was lost due to any volatile product formation by heating during nanocomposite calcination.  indicates that the sample was pure. The spectrum has shown signals of O and Bi with 23.06% and 76.94% by weight, respectively ( Table 2). Qualitative and quantitative EDX results from the nanocomposites agreed with asprepared compositions, which indicates that no element was lost due to any volatile product formation by heating during nanocomposite calcination.

Morphological Analysis
A field-emission scanning electron microscope (VEGA3 TESCAN) was used for morphological studies of as-prepared Bi2O3, CoFe2O4 and their nanocomposites.  Scanning electron micrographs of as-prepared nanocomposites have shown irregular granular microstructures with slight agglomeration despite the dipolar interaction between the particles. For compositional uniformity from particle to particle, it was demonstrated that both Bi2O3 and CoFe2O4 phases were positioned on the grain surface side-byside. Hence, the prepared Bi2O3/CoFe2O4 nanocomposites were considered to be a tightly contacting heterojunction structure between Bi2O3 and CoFe2O4, formed in nano-size level. SEM images of the Bi2O3/CoFe2O4 nanocomposites have shown no significant change in the morphology of the constituent nanoparticles. However, it was established that as Bi2O3 contents were increased, the grains in the nanocomposite matrix became larger and more Scanning electron micrographs of as-prepared nanocomposites have shown irregular granular microstructures with slight agglomeration despite the dipolar interaction between the particles. For compositional uniformity from particle to particle, it was demonstrated that both Bi 2 O 3 and CoFe 2 O 4 phases were positioned on the grain surface side-by-side. Hence, the prepared Bi 2 O 3 /CoFe 2 O 4 nanocomposites were considered to be a tightly contacting heterojunction structure between Bi 2 O 3 and CoFe 2 O 4 , formed in nano-size level. SEM images of the Bi 2 O 3 /CoFe 2 O 4 nanocomposites have shown no significant change in the morphology of the constituent nanoparticles. However, it was established that as Bi 2 O 3 contents were increased, the grains in the nanocomposite matrix became larger and more agglomerated, while the shape of the grains was not changed (Figure 2c-e). SEM micrographs have shown that the average particle size of prepared nanoparticles and their composites was below 100 nm, which was also in agreement with those computed from the X-ray powder diffraction peaks.

Diffuse Reflectance Spectroscopy and Tauc Plots
DRS spectra were employed to construct Tauc plots of the prepared catalysts. Tauc plots were plotted between photon energy and (αhν) 2 to calculate band gaps. Bi 2 O 3 has shown a single hump at 521 nm. Extrapolation of the linear region of the Tauc plot (inset Figure 3a) has furnished a band gap of 2.55 eV for hydrothermally prepared Bi 2 O 3 . CoFe 2 O 4 has an intrinsic black color, so it has shown absorption in a wide range of wavelengths [50]. The absorption spectra of cobalt ferrite exhibited a single hump at 751 nm. As cobalt ferrite is a direct semiconductor, the band gap value was 1.63 eV, computed from the Tauc plot (inset Figure 3b). The band gaps of the nanocomposites, namely NC-1, NC-2 and NC-3, were calculated as 1.62, 1.58 and 1.61 eV, respectively, by Tauc plots (inset Figure 3c- agglomerated, while the shape of the grains was not changed (Figure 2c-e). SEM micrographs have shown that the average particle size of prepared nanoparticles and their composites was below 100 nm, which was also in agreement with those computed from the X-ray powder diffraction peaks.

Diffuse Reflectance Spectroscopy and Tauc Plots
DRS spectra were employed to construct Tauc plots of the prepared catalysts. Tauc plots were plotted between photon energy and (αhν) 2 to calculate band gaps. Bi2O3 has shown a single hump at 521 nm. Extrapolation of the linear region of the Tauc plot (inset Figure 3a) has furnished a band gap of 2.55 eV for hydrothermally prepared Bi2O3. CoFe2O4 has an intrinsic black color, so it has shown absorption in a wide range of wavelengths [50]. The absorption spectra of cobalt ferrite exhibited a single hump at 751 nm. As cobalt ferrite is a direct semiconductor, the band gap value was 1.63 eV, computed from the Tauc plot (inset Figure 3b). The band gaps of the nanocomposites, namely NC-1, NC-2 and NC-3, were calculated as 1.62, 1.58 and 1.61 eV, respectively, by Tauc plots (inset Figure 3c-e). For the Bi2O3/CoFe2O4 nanocomposites, absorbance reduced as the loading of Bi2O3 increased. However, those values of absorbance could be useful in predicting the relative amount of CoFe2O4 and Bi2O3 in the synthesized composites by using the Beer-Lambert law.

Photodegradation Activity
The degradation of methyl orange dye was studied by logging changes in the absorption value of a charged dye solution at its λ max ≈ 462 nm for different hourly time intervals of a light irradiation experiment (Table 3). Figure 4 shows degradation spectra of methyl orange for each prepared catalyst along with a blank sample. The photodegradation activities of all prepared nanocatalysts were plotted in terms of percentage degradation versus passage of time ( Figure 5). NC-1 (1:3 Bi 2 O 3 /CoFe 2 O 4 ) has also shown good degradation  (Figure 6). These results have shown that all prepared nanocomposites exhibited good photodegradation activities as compared to bare materials. The blank with no nanocatalyst has exhibited a minimal decrease, i.e., about 2%, in absorbance when subjected to light irradiation for the period of 5 h witnessed in Figure 4. This shows an evident synergism phenomenon operating between the constituents of the nanocomposites. The degradation of methyl orange dye was studied by logging changes in the absorption value of a charged dye solution at its λmax ≈ 462 nm for different hourly time intervals of a light irradiation experiment (Table 3). Figure 4 shows degradation spectra of methyl orange for each prepared catalyst along with a blank sample. The photodegradation activities of all prepared nanocatalysts were plotted in terms of percentage degradation versus passage of time ( Figure 5). NC-1 (1:3 Bi2O3/CoFe2O4) has also shown good degradation results. It degraded up to 82% of the dye in 5 h of light exposure. NC-2 (1:1 Bi2O3/CoFe2O4) has shown the best degradation results. It degraded up to 92% of the dye in 5 h of the light irradiation experiment, as evidenced from the associated absorbance spectra taken during photocatalytic bleaching. NC-3 (3:1 Bi2O3/CoFe2O4) has shown moderate degradation results. It degraded up to 78% of the dye in 5 h of light illumination ( Figure 6). These results have shown that all prepared nanocomposites exhibited good photodegradation activities as compared to bare materials. The blank with no nanocatalyst has exhibited a minimal decrease, i.e., about 2%, in absorbance when subjected to light irradiation for the period of 5 h witnessed in Figure 4. This shows an evident synergism phenomenon operating between the constituents of the nanocomposites.   Photodegradation efficiency of all prepared catalysts against methyl orange was calculated by the formula: where Ao is absorbance at time 0 min and At is absorbance after time "t" min of the photodegradation experiment. Ao and At were recorded at λmax of the dye [51]. Figure 7 shows the comparison of the percentage efficiency of all prepared catalysts against methyl orange. The highest efficiency was calculated for NC-2, which is due to the optimum constitution of Bi2O3 and CoFe2O4 (1:1) as well as the effective migration of exciton pairs in the type-II band configuration between the constituent materials, resulting in the suppression of charge recombination. where Ao is absorbance at time 0 min and At is absorbance after time "t" min of the photodegradation experiment. Ao and At were recorded at λmax of the dye [51]. Figure 7 shows the comparison of the percentage efficiency of all prepared catalysts against methyl orange. The highest efficiency was calculated for NC-2, which is due to the optimum constitution of Bi2O3 and CoFe2O4 (1:1) as well as the effective migration of exciton pairs in the Photodegradation efficiency of all prepared catalysts against methyl orange was calculated by the formula: where A o is absorbance at time 0 min and A t is absorbance after time "t" min of the photodegradation experiment. A o and A t were recorded at λ max of the dye [51]. Figure 7 shows the comparison of the percentage efficiency of all prepared catalysts against methyl orange. The highest efficiency was calculated for NC-2, which is due to the optimum

Proposed Photodegradation Scheme
According to the aforementioned results, a speculative photocatalytic mechanism based on bibliographic data is proposed in Figure 8 [39,[52][53][54]. Under continuous light irradiation, electrons become excited from valence bands of CoFe2O4 and Bi2O3 to their conduction bands, meanwhile producing holes in their respective valence bands. As both the valence band and conduction band of the CoFe2O4 are lower in energy than those of Bi2O3, cobalt ferrite acts as a light energy trapper. Subsequently, these electrons and holes are rearranged and enriched into the conduction band of Bi2O3 and the valence band of CoFe2O4, respectively. A band-to-band transition bridge is established between Bi2O3 and cobalt ferrite in which the energy levels are mismatched between each other. Since the conduction band potential of CoFe2O4 is more negative than that of Bi2O3, excited electrons are constantly shifted to the conduction band of Bi2O3. Meanwhile, as the valence band of CoFe2O4 is less positive than that of the Bi2O3, holes in the VB of Bi2O3 will transfer continuously to the VB of cobalt ferrite. This synergism will reduce the cogent band gap, hinder electron hole recombination and will increase electron hole availabilities for photodegradation reactions, thus ensuring efficient photodegradation co-work of Bi2O3/CoFe2O4 nanocomposites.

Proposed Photodegradation Scheme
According to the aforementioned results, a speculative photocatalytic mechanism based on bibliographic data is proposed in Figure 8 [39,[52][53][54]. Under continuous light irradiation, electrons become excited from valence bands of CoFe 2 O 4 and Bi 2 O 3 to their conduction bands, meanwhile producing holes in their respective valence bands. As both the valence band and conduction band of the CoFe 2 O 4 are lower in energy than those of Bi 2 O 3 , cobalt ferrite acts as a light energy trapper. Subsequently, these electrons and holes are rearranged and enriched into the conduction band of Bi 2 O 3 and the valence band of CoFe 2 O 4 , respectively. A band-to-band transition bridge is established between Bi 2 O 3 and cobalt ferrite in which the energy levels are mismatched between each other. Since the conduction band potential of CoFe 2 O 4 is more negative than that of Bi 2 O 3 , excited electrons are constantly shifted to the conduction band of Bi 2 O 3 . Meanwhile, as the valence band of CoFe 2 O 4 is less positive than that of the Bi 2 O 3 , holes in the VB of Bi 2 O 3 will transfer continuously to the VB of cobalt ferrite. This synergism will reduce the cogent band gap, hinder electron hole recombination and will increase electron hole availabilities for photodegradation reactions, thus ensuring efficient photodegradation co-work of Bi 2 O 3 /CoFe 2 O 4 nanocomposites.
These holes in the VB of cobalt ferrite will react to H 2 O/OH − and produce highly reactive OH • radicals to start the process of degradation. Simultaneously, electrons gathered in the conduction band of Bi 2 O 3 react with adsorbed oxygen and form oxidants such as superoxide ions. These active species such as OH • and O 2 •− radicals react to degrade molecules of the dye into H 2 O, CO 2 or other small molecular products. Chemical equations of this speculative photocatalytic mechanism based on bibliographic data are illustrated as:   [14,55,56]. The VB edge of Bi 2 O 3 is lower than that of CoFe 2 O 4 ; hence, this system is a type-II heterojunction [57,58]. Since these heterojunctions are tightly bounded at the nanoscale, the hole transfers through the junction are efficacious. Furthermore, the CoFe 2 O 4, working as the main photocatalyst, is found side-by-side in the Bi 2 O 3 /CoFe 2 O 4 nanocomposite. Hence, formation of a heterojunction structure does not shield the active sites of catalysts. These holes in the VB of cobalt ferrite will react to H2O/OH − and produce highly reactive OH • radicals to start the process of degradation. Simultaneously, electrons gathered in the conduction band of Bi2O3 react with adsorbed oxygen and form oxidants such as superoxide ions. These active species such as OH • and O2 •− radicals react to degrade molecules of the dye into H2O, CO2 or other small molecular products. Chemical equations of this speculative photocatalytic mechanism based on bibliographic data are illustrated as: CoFe2O4 Dye + O2 •− + OH • → CO2 + H2O + other by-product The Bi2O3/CoFe2O4 heterojunction designed in this study is not similar to the type-I heterojunction structure. It is considered for the Bi2O3/CoFe2O4 that CoFe2O4 plays its role as the main photocatalyst, while the Bi2O3 works as a photosensitizer [14,55,56]. The VB edge of Bi2O3 is lower than that of CoFe2O4; hence, this system is a type-II heterojunction [57,58]. Since these heterojunctions are tightly bounded at the nanoscale, the hole transfers through the junction are efficacious. Furthermore, the CoFe2O4, working as the main photocatalyst, is found side-by-side in the Bi2O3/CoFe2O4 nanocomposite. Hence, formation of a heterojunction structure does not shield the active sites of catalysts.

Materials and Methods
All the materials, viz., bismuth (III) nitrate pentahydrate, cobalt chloride hexahydrate, ferric chloride hexahydrate, polyvinyl pyrrolidone (MW 40,000), sodium hydroxide, nitric acid and glycerol were purchased from Daejung, Korea and used without further purification. Ethanol and deionized water were used for washing and preparation of solutions. Various concentrations of precursor solution and conditions of reactions were explored for material synthesis based on reported methods. However, the following optimized methods proved to avoid agglomeration of individual products and composites.

Synthesis of Bi 2 O 3 Nanoparticles
Bismuth nitrate pentahydrate (0.97 g) was dissolved in 10 mL of 1.12 M HNO 3 along with vigorous stirring in order to avoid hydrolyzation of Bi 3+ ions. Subsequently, 0.072 g of polyvinylpyrrolidone (PVP) was added as surfactant into the above mixture and stirred by manual stirring for about 15-20 min. Later, 4 M NaOH solution was gradually added with constant stirring until the pH of the solution turned basic (pH = 11) and white precipitates appeared. After stirring for 10 min, the resultant suspension was placed into a Teflon-lined autoclave (KH100, Guangzhou Aolantec, China) at 90 • C for 1 h. The suspension was allowed to cool at room temperature and its white color turned to yellow. The resultant yellow precipitates were centrifuged in a centrifuge (Hermle, Germany) and washed with distilled water/ethanol several times. The resultant product was dried in a vacuum oven (Memmert, Germany) for 2 h at 80 • C. This furnished yellow, solid particles of bismuth oxide which were ground into a fine powder. This powder was further calcined at 400 • C for 4 h [42,43].

Synthesis of CoFe 2 O 4 Nanoparticles
Magnetic cobalt ferrite nanoparticles were prepared via a simple, one-pot hydrothermal process. An aqueous solution containing 4 mmol FeCl 3 ·6H 2 O was mixed with 0.078 M glycerol (surfactant) under constant stirring. Afterward, 2 mmol CoCl 2 ·6H 2 O solution was added slowly into the above mixture to prepare a uniform solution. Highly alkaline conditions (pH = 13) of the reaction mixture were maintained by dropwise addition of 6 M NaOH solution. This resulted in a brownish-black precipitate mixture. Subsequently, the reaction mixture was transferred into a Teflon-lined autoclave and kept in an oven for 6 h at 200 • C. The resultant suspension was centrifuged and washed with distilled water several times. Later, the precipitates were dried at 100 • C for 6 h in a vacuum oven. The resultant cobalt ferrite product was ground into a powder [44,45].

Synthesis of Bi 2 O 3 /CoFe 2 O 4 Nanocomposites
A physical method was opted for in order to prepare the nanocomposites. This involved simple, solid-state mixing of already synthesized bismuth oxide and cobalt ferrite nanoparticles, followed by 3 h grinding via pestle and mortar. Resultant mixtures were calcined at 400 • C for 4 h. Three nanocomposites with different proportions of CoFe 2 O 4 and Bi 2 O 3 were prepared (Table 4).

Degradation Experiment
Photodegradation studies were carried out against methyl orange dye to explore the photocatalytic properties of as-prepared nanocomposites. A 300 W xenon arc lamp was used as the light source for degradation studies and to irradiate beakers charged with dye stock solution and catalyst. A stock solution of 30 ppm methyl orange dye was prepared in distilled water. For each photodegradation run, 50 mL stock solution was poured into beakers and 10 mg of each prepared catalyst nanocomposite was added into these separately. These solutions were kept in the dark with constant stirring for 2 h to attain adsorption-desorption equilibrium. Afterward, samples were kept in a photoreactor chamber and light was shone on the system. After different predetermined intervals of time, 5 mL of each solution was stripped out and centrifuged for 5 min at 10,000 rpm to let the catalyst settle down. The solutions obtained were examined by UV-visible spectrophotometer (UV/Vis Lambda 365, Perkin Elmer, Akron, OH, USA) to ascertain the residual dye contents in the solution in order to study the photocatalytic activity and the level of methyl orange degradation [46,47]. 92% for the degradation of methyl orange dye. The superior photocatalytic performance of the nanocomposite may be credited to its type-II band configuration, which results in the suppression of charge recombination because Bi 2 O 3 acts as an electron sink. As per our research, no prior study has been reported in the literature regarding the synthesis of a Bi 2 O 3 /CoFe 2 O 4 nanocomposite and its dye degradation potential. Therefore, good photocatalytic degradation activities, demonstrated by these nanocomposites, for contaminated aqueous solutions make these materials a potent candidate to develop robust photocatalysts for practical applications in water decontamination technology. The Bi 2 O 3 /CoFe 2 O 4 is environmentally friendly, the synthesis is simple, and, if it were scaled up, the manufacturing cost would be low. This also leaves scope for researchers to further explore its photocatalytic activities against other dye/drug pollutants and its water splitting potential.