Enhanced photocatalytic performance of CdFe2O4/Al2O3 nanocomposite for dye degradation

In the present work, CdFe2O4/Al2O3 magnetic nanocomposite photocatalyst is successfully synthesized by simple sol-gel auto-combustion method. The role of this sample is studied as a photocatalyst. The influence of Al2O3 concentration with CdFe2O4 on the photocatalytic property is also studied. We have considered three weight percentage of Al2O3, 5%, 10%, and 20% with CdFe2O4. All the samples are characterized with X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) with selected area electron diffraction (SAED), vibrating sample magnetometer (VSM), UV-Visible, and photoluminescence (PL) spectroscopy techniques. The 10% composite sample showed the lower particle size, higher surface area, enhanced porosity, higher saturation magnetization, and considerable band gap as compared to that of 5% and 20% CdFe2O4/Al2O3 as well as bare CdFe2O4 nanoparticles. The photocatalytic activity of the sample is evaluated towards the degradation of the xylenol orange (XO) dye under UV light. The degradation process of the dye is monitored spectrophotometrically. The performance in terms of removal efficiency is studied by varying the contact time, dye concentration and amount of catalyst. Among the three concentrations of Al2O3, the 10% weight concentration of Al2O3 with CdFe2O4 is found to be the optimal concentration and showed the higher degradation rate. After 30 min photocatalytic reaction, the degradation rate is 92.29% for 10% CdFe2O4/Al2O3 and for bare CdFe2O4, it is 85.79%. This work provides a new reference for designing Al2O3-based spinel ferrite nanocomposites and their role in wastewater management.


Introduction
Nowadays, organic dyes and pigments are routinely used in the textiles, paper, plastics, leather, food, and cosmetic industry to color the products (Shindhal et al. 2021). The discharge of unpremediated industrial wastes from the textile industry is a potential source of water pollution (Kadier Ilyas et al. 2021). As these industrial pollutants are highly structure and good magnetic properties (Chiu et al. 2019;Dutta et al. 2022). The application of ferrites as well as mixed metal ferrite composites is extensively studied as a photocatalyst for the degradation of various dyes (Chiu et al. 2019;Dutta et al. 2022;Dippong et al. 2021;Somwanshi et al. 2020;Kirankumar and Sumathi 2020;Borade et al. 2020;Kharisov et al. 2019;Kefeni and Mamba 2020;Mapossa et al. 2021;Viswanathan 2018;Moreno et al. 2021;Salehi et al. 2017). Among the various ferrites, cadmium ferrite (CdFe 2 O 4 ) is one of the important spinel ferrites used in various magneto-optical devices due to its excellent chemical stability (Golsefidi 2017;Sagadevan et al. 2017). The cadmium ferrite is widely used as a photocatalyst in wastewater treatment (Harish et al. 2013;Patil et al. 2018;Shi et al. 2015). The photocatalytic activity of CdFe 2 O 4 nanoparticles has been investigated towards methylene blue (MB) dye under solar light irradiation (Harish et al. 2013;Patil et al. 2018) and for congo red dye under microwave irradiation (Shi et al. 2015). The degradation efficiency depends upon the methodology (Patil et al. 2018), calcination temperature (Harish et al. 2013), and particle size (Harish et al. 2013;Shi et al. 2015).
Various composites such as SiO 2 , TiO 2 , ZnO, Al 2 O 3 , and graphene oxide are used to improve the photocatalytic efficiency of the spinel ferrite nanoparticles (Viswanathan 2018;Moreno et al. 2021;Salehi et al. 2017;. For rhodamine B dye, the enhanced degradation efficiency is found for Fe 3 O 4 @TiO 2 @SiO 2 (94%) and Fe 3 O 4 @TiO 2 (72%) as compared to that of Fe 3 O 4 (11%) (Li et al. 2016). CdFe 2 O 4 /SiO 2 nanocomposites have been synthesized and evaluated towards the applications like immunosensing, as well as a nanocatalyst (Plocek et al. 2003;Nasseri et al. 2020). The photocatalytic activity measurements demonstrated that CdFe 2 O 4 with TiO 2 , graphene oxide, and g-C 3 N 4 exhibits higher photocatalytic activity as compared to that of bare CdFe 2 O 4 (Ragab et al. 2019;Zhang et al. 2015;Shahid 2020;Fang et al. 2020). Nowadays, growing attention has been drawn towards the improvement of nano-alumina particles for progressive engineering and industrial applications. Along with TiO 2 , ZnO, Fe 2 O 3 , Fe 3 O 4 , and many other semiconductor materials, Al 2 O 3 can be employed as photocatalyst for different applications like environment, energy, and chemical synthesis (Kusuma et al. 2020). Al 2 O 3 nanoparticles act as an effective photocatalyst against methylene blue and acid green (Kusuma et al. 2020;Danish et al. 2021). The Al-doped NiFe 2 O 4 nanoparticles exhibit significant biological activity against the human pathogens and a suitable material for biomedical applications . The role of aluminum-doped cobalt ferrite nanocomposites is studied for the photodegradation of methylene blue dye (Abbas et al. 2020). The enhanced degradation efficiency was observed as compared to that of bare cobalt ferrite nanoparticles. Based on these studies (Kusuma et al. 2020;Abbas et al. 2020), we have made an attempt to design the CdFe 2 O 4 /Al 2 O 3 nanocomposite by sol-gel autocombustion method. The photocatalytic activities of the CdFe 2 O 4 /Al 2 O 3 nanocomposite are investigated towards the degradation of xylenol orange dye under UV light irradiation. The results are compared with the photocatalytic activity of bare CdFe 2 O 4 nanoparticles. At room temperature, with the variation in contact time, we have studied the activity for different concentration of xylenol orange dye and catalyst doses.

Experimental procedure
We have synthesized CdFe 2 O 4 nanoparticles and CdFe 2 O 4 /Al 2 O 3 nanocomposite using sol-gel autocombustion method. The CdFe 2 O 4 particles are synthesized by using cadmium nitrate, ferric nitrate, and citric acid as precursors. The precursors are dissolved in distilled water where the molar ratio of citric acid to metal ions is kept fixed at 1:1. The pH of the solution is kept at 7 by dropwise addition of ammonia (named as sol A). Furthermore, the solution evaporated at about 70 • C, then heated at 120 • C to allow a dried gel formation. For the preparation of nanocomposites, the aluminium nitrate and ethanol is used. The ratio of aluminium nitrate, ethanol, and distilled water is fixed at 1:4:7. The acetic acid is added to maintain the pH 2. The solution is stirred at 50 • C for 2 h (named as sol B). Solutions A and B are mixed together and heated at 120 • C. To understand the influence of Al 2 O 3 in the nanocomposite formation and on the catalytic property, we have considered three weight percentages of Al 2 O 3 , 5%, 10%, and 20% with CdFe 2 O 4 . All the prepared samples are annealed at 800 • C for 6 h. In throughout the discussion, to identify these four samples, we have used the notations : P 0 , P 5% , P 10% , P 20% for bare CdFe 2 O 4 , CdFe 2 O 4 /Al 2 O 3 with 5 wt%, 10 wt%, and 20 wt%, respectively. All the samples, P 0 , P 5% , P 10% , and P 20% are characterized by XRD, BET, FTIR, FESEM, EDS, TEM with SAED pattern, VSM, UV-Visible, and PL spectroscopy techniques.
Catalytic activity of synthesized sample is examined by studying degradation of xylenol orange dye under UV light. The luminous intensity of the UV source is 2.40 W/m 2 . We have studied the activity with the variations in concentration of xylenol orange dye and catalyst dose with the variation in contact time. All the reactions are carried out at room temperature. The absorbance is recorded on a double beam UV-Visible spectrophotometer (Shimadzu UV-250) after every 30 min. The dye degradation efficiency (D %) is calculated by using the following equation : where D is the photocatalytic degradation efficiency; C and C o are the concentrations of XO dye at irradiation time of t and t 0 , respectively. The specific surface area (SSA) of the synthesized composite nanoparticles is calculated by 6000/Dρ, where D is crystallite size (nm) and ρ is density of the material (g/cm 3 ). The SSA for CdFe 2 O 4 is 40.10 m 2 /g. The SSA For P 5% , P 10% , and P 20% is 59.21, 63.64, and 41.44 m 2 /g, respectively. For 10% weight percentage, the enhanced SSA is found as compared to that of P 5% , P 20% , and P 0 samples. The increased SSA of P 10% sample may help in bringing the more active sites from the material in contact with the reaction surroundings. Along with SSA, porosity is one of the important factor to enhance the photocatalytic activity. The porosity (P) is calculated by using the formula (1-(ρ B /ρ XRD )), where ρ B is bulk density and ρ XRD is X-ray density. The calculated porosity of CdFe 2 O 4 is 11.16%. With the increase in concentration of Al 2 O 3 from 5% to 10% to 20%, the porosity is 26.82%, 29.45%, and 27.64%, respectively. The enhanced micropores in the nanocomposite and availability of large surface area and diffusion path may help the free radicals (OH · and O − 2 ) to diffuse faster inside the coating. To calculate SSA more accurately, we have performed the BET analysis for CdFe 2 O 4 and CdFe 2 O 4 /Al 2 O 3 (10%) (Fig. 2). The specific surface area of CdFe 2 O 4 and CdFe 2 O 4 /Al 2 O 3 nanocomposite is calculated using the nitrogen adsorptiondesorption isotherm. The specific surface area obtained is 16.729 and 56.859 m 2 /g for CdFe 2 O 4 nanoparticles and 10 wt% CdFe 2 O 4 /Al 2 O 3 (P 10% ) nanocomposites, respectively. The introduction of alumina into cadmium ferrite give rise to increase in specific surface area. The specific surface area calculated from BET is consistent with the XRD analysis. From the above results, we found that among the considered three weight percentages of Al 2 O 3 with CdFe 2 O 4 , the 10% CdFe 2 O 4 /Al 2 O 3 nanocomposite showed smaller particle size and higher specific surface area as compared to that of bare CdFe 2 O 4 . Figure 3 (a-h) shows the surface morphology and the chemical composition of the synthesized nanocomposites (P 5% , P 10% , and P 20% ) along with bare one (P 0 ). The CdFe 2 O 4 ( Fig. 3(a)) sample exhibits a compact arrangement of spherical-shaped particles on the surface. The synthesized nanoparticles are agglomerated due to the presence of magnetic interactions among the particles. The particle size of CdFe 2 O 4 nanoparticles is around 80 nm. The morphology of the CdFe 2 O 4 /Al 2 O 3 nanocomposite is not clearly seen due to the inter-grain agglomeration. With the increase in concentration of Al 2 O 3 , the increase in agglomeration as well as increase in the nanoparticle size is also observed. In presence of Al 2 O 3 with CdFe 2 O 4 for different weight percentage (P 5% , P 10% , and P 20% ), the particle size is around 72 nm, 40 nm, and 60 nm, respectively. The calculated particle sizes are in good agreement with the crystallite size obtained from the XRD analysis. The large interface strain shows the high grain reactivity of the magnetostatic interaction which causes the agglomeration. The weight and atomic percentage of Cd, Fe, O, and Al elements for all the samples is presented via the EDS spectra ( Fig. 3(b, d, f, h)). The atomic ratio of Cd:Fe is nearly close to 1:2 which confirms the formation of the CdFe 2 O 4 nanoparticles. The oxygen to metal ion ratio (O−M, M = Fe, Cd and Al) in all the samples is higher and indicates the oxygen-rich surface behavior of all the samples. The oxygen-rich behavior induces the surface and interface defects in the microstructures. To get more insight about morphology in the composite form, the TEM micrographs along with SAED patterns of P 0 and P 10% the samples are presented in Fig. 4 (a-d). The hexagonal morphology is observed for the prepared P 10% nanoparticles. The particle size of P 10% is 38 nm. The particle size obtained from FESEM and TEM is consistent with the crystallite size calculated from the XRD analysis. SAED pattern of the P 10% sample shown in Fig. 4(c) and (d) revealed the formation of good crystalline material. Indexing of the SAED pattern is done by calculating the d-spacing values from the magnified electron diffraction pattern. The (220) (110) and (113) peaks corresponding to the Al 2 O 3 phase. These values are consistent with the standard values obtained from the XRD analysis. To confirm the formation of spinel ferrite nanostructures, FTIR spectra is recorded. Figure S1 shows the FTIR spectra of CdFe 2 O 4 /Al 2 O 3 nanocomposites along with are CdFe 2 O 4 . The FTIR result confirm the formation of heterogeneous structure consists with CdFe 2 O 4 and Al 2 O 3 .

Results and discussion
The magnetic field (H) versus magnetization (M) curve at room temperature is recorded with the field strength −15 to 15 KOe. Figure 5 presents the hysteresis loop curve for all the samples P 0 , P 5% , P 10% , and P 20% . The characteristics show that CdFe 2 O 4 (P 0 ) is superparamagnetic in nature. This nature is also confirmed from the lower values of saturation magnetization (M s ), remanant magnetization (M r ), and coercive field (H c (Zhang et al. 2012;Tang et al. 2006). The enhanced magnetism is observed in nanocomposite as compared to bare nanoparticles (Zhang et al. 2012;Tang et al. 2006). The optical absorption is relevant to the electronic structure formation which is one of the important key factor in demonstrating the photocatalytic activity. UV-Visible absorbance spectra of the prepared nanocomposite along with the bare CdFe 2 O 4 nanoparticles are presented in Fig. 6(a). From the nature of the spectra, all synthesized samples indicate that the photo-absorption is from UV to visible region. This presents the possibility of high photocatalytic activity of the nanocomposite under UV light. CdFe 2 O 4 /Al 2 O 3 nanocomposite shows the lower absorbance as compared to that of CdFe 2 O 4 nanoparticles. The estimated band gap value of CdFe 2 O 4 is 1.93 eV. The band gap of bulk CdFe 2 O 4 is 1.80 eV (Harish et al. 2013). The CdFe 2 O 4 nanoparticles obtained with various synthesis techniques showed the band gap in the range of 1.46−2.23 eV (Harish et al. 2013;Patil et al. 2018). The effect of crystallite size on band gap is due to the quantum size effect and surface effects. In nanocomposite, the calculated band gap of P 5% is 2.02 eV, P 10% composite is 2.32 eV, and for P 20% composite, it is 2.56 eV. In nanocomposite, the band gap increases with the reduction in crystallite size. These results are consistent with the blue shift observed in the spectra of nanocomposite as compared to CdFe 2 O 4 . The increase in the band gap is attributed to the formation of a more stable complex due to the interaction between CdFe 2 O 4 and Al 2 O 3 .
Room temperature PL spectroscopy gives the information about the band gap with the relative active position of . 6 (a) Absorption spectra of CdFe 2 O 4 /Al 2 O 3 nanocomposite (P 5% , P 10% , and P 20% ) along with CdFe 2 O 4 (P 0 ) nanoparticles. The band gap for P 0 , P 5% , P 10% , and P 20% is presented in the inset.
(b) Photoluminescence spectra of CdFe 2 O 4 /Al 2 O 3 nanocomposites (P 5% , P 10% , and P 20% ) along with CdFe 2 O 4 (P 0 ) nanoparticles. The excitation wavelength of 250 nm is used to obtain the emission spectra sub band gap and defect states of the metal oxide semiconductors and it is an important tool for investigating the electronic and optical properties of the semiconducting materials. Figure 6( to 10% to 20%, PL intensity of the spectra increases. After the excitation, three major peaks are observed at 275, 366, and 469 nm. At 366 nm wavelength, a UV emission peak is observed, which is ascribed to the surface exciton recombination of oxygen cation vacancies, a major characteristic of the nanostructured materials owing to their large surface area. The observed UV band occurs basically as a result of the exciton electron-hole (e − -h + ) recombination in the interaction process which is synonymous with near band edge emission. The defects arise from the donor levels near the conduction band edge of the oxide. An increase in Al 2 O 3 concentration in the matrix of cadmium ferrite, the PL intensity also increases; it may be due to the increase in defect due to the addition of Al 2 O 3 . Moreover, PL emission intensity affected by the dopant concentration and the cadmium ferrite nanoparticles find promising applications in photocatalyst. The photocatalytic activity is effectively dependent on the particle size and surface area. It has been reported that the nanocatalyst have high surface area which exhibits higher photocatalytic activity compared to their counterparts having lower surface area. The lower particle size also supports the easy transition of electrons from the valence to conduction band, thereby generating electron-hole pairs when it exposed to UV-Visible radiation. The generated electrons and holes further interact with dissolved oxygen and water to produce highly reactive free radical species capable of degrading the XO dye (Zhu 2011;Chen et al. 2017).
To understand the photocatalytic activity of CdFe 2 O 4 /Al 2 O 3 nanocomposite towards XO dye, we have initiated all the reactions on CdFe 2 O 4 nanoparticles under UV light. The photocatalytic degradation for the prepared samples is carried out on the basis of change in color. The effect of different initial concentration of catalyst, dosage of XO dye, and contact time is studied under UV light. To start with, the degradation ratio is examined by varying the concentration of XO dye from 0.5 to 5 mg/L. The contact time for the reaction is varied from 30 to 180 min. Under UV irradiation and with the variation in contact time, in Fig. 7(a), we have presented the concentration dependent degradation ratio of CdFe 2 O 4 catalyst towards XO dye. With the increase in concentration, the degradation ratio decreases. It is observed that, with the variation in contact time from 30 to 90 min, the rapid degradation takes place. From 90 to 180 min contact time, the rate of degradation is very slow. For 0.5 mg/L concentration of XO dye, the degradation efficiency is 70.90%. With the increase in concentration from 0.5 to 1 mg/L, degradation ratio increases to 85.79%. Further at 5 mg/L concentration of XO dye, the degradation ratio decreases to 76.06%. In fact, the increase of the concentration of XO dye leads to decrease in the number of photons adsorbed at the surface of a photocatalyst. Molecules of XO dye absorb more light and the excitation of photocatalyst particles by photons is reduced. Consequently, photodegradation efficiency decreases at higher concentration. From Fig. 7(a), it is found that among the considered concentrations, at 0.5 g of catalyst dose, the 1 mg/L concentration of XO dye is the optimum concentration for the degradation of XO dye. Furthermore, by selecting 1 mg/L dose of XO dye, the effects of different initial concentrations of CdFe 2 O 4 on the degradation ratio are examined. We have considered three concentrations, 0.3 g, 0.5 g, and 0.7 g of CdFe 2 O 4 . Figure 7(b) clearly shows that, for 0.5 g dose of catalyst, the XO dye degrades rapidly as compared to 0.3 g and 0.7 g dose of CdFe 2 O 4 catalyst. Based on all the test results obtained for CdFe 2 O 4 , further, we have investigated the removal efficiency of CdFe 2 O 4 /Al 2 O 3 . To understand the reaction mechanism, we have considered all the optimized parameters from the earlier reactions. From Fig. 8, it is seen that there is no obvious degradation for XO dye without UV light, indicating that light is necessary to initiate the photocatalysis. Using UV light irradiation with CdFe 2 O 4 photocatalyst, the XO dye is degraded upto 85.79% within 180 min. The CdFe 2 O 4 /Al 2 O 3 composite with 5%, 10%, and 20% composition of Al 2 O 3 with CdFe 2 O 4 , the degradation efficiency is 89.20%, 92.29%, and 75.14%, respectively, for 0.5 g catalyst dose. It is seen that, the P 10% nanocomposite shows a higher degradation rate as compared to that of P 5% , P 20% as well as P 0 nanoparticles. The difference of photocatalytic activity between the three concentrations can be ascribed to the change in crystallite size as well as band gap of the composite nanoparticles. The number of electrons reaching the conduction band from the valance band increases, i.e., the electron density in the conduction band is relatively higher for P 10% than P 5% and P 20% . Consequently, the number of holes in the valence band also increases. Both the electrons and holes interact with surface bound H 2 O or OH − to produce OH · radicals. This favors the formation of more OH · free radicals, which are main active species in the photocatalytic degradation process. At 10 wt% concentration of Al 2 O 3 in CdFe 2 O 4 enhances its ability to act as an electron acceptor and transfer channel to facilitate the separation and migration of photogenerated e − . It is also seen that oxygen vacancies are playing a vital role in enhancing the photocatalytic activity of metal oxide semiconductor photocatalysts (Dutta et al. 2022). From EDS pattern, we can observe presence of oxygen in all the four samples. The order of oxygen vacancies in the samples

C/Co
Contact Time (min) Dark P 0 P 5% P 10% P 20% Fig. 8 The photocatalytic degradation of XO dye with CdFe 2 O 4 (P 0 ) and CdFe 2 O 4 /Al 2 O 3 catalysts for 5%, 10%, and 20% concentration of Al 2 O 3 with 1 mg/L concentration of XO dye and 0.5 g dose of catalyst are P 10% > P 5% > P 0 > P 20% . The degradation efficiency of P 10% nanocomposite is higher as compared to that of P 0 , P 5% and P 20% . At 20%, the higher concentration of oxygen vacancies can act as traps for charge carriers. This results in decrease in photocatalytic efficiency. The lower degradation efficiency of P 20% sample is found as compare to that of P 10% , P 5% , and P 0 . Figure 9(a) and (b) present the change in the absorbance of XO dye solution in the presence of P 0 and P 10% photocatalyst under UV light irradiation. It is observed that, the absorption peak at λ = 590 nm diminished gradually after with time. The absorbance of P 10% catalyst is higher than the P 0 . The increased absorbance in composite sample than the bare CdFe 2 O 4 indicates that there is coupling between CdFe 2 O 4 and Al 2 O 3 which results an increased ability to capture photo-induced electrons in comparison with bare CdFe 2 O 4 . The absorbance decreases with increase in contact time from 30 to 180 min, which indicates the degradation takes place gradually. In the UV region small broad bands are observed which is due to the UV light irradiation during the degradation. No new absorption band appearing in the visible region reflects the complete decolorization of XO dye during the reaction. The pattern of decolorization with contact time of the photocatalyst with XO dye is presented in Fig. 9(c) and (d) for P 0 and P 10% samples. The degradation of the XO dye with time is directly reflected by the color change (decolorization) from deep violet to a clear solution.
The stability of a photocatalyst is an important parameter which decides the effectivity of the photocatalyst and its practical applicability. In order to test the stability of the photocatalyst, recyclability experiment is carried out over 10% CdFe 2 O 4 /Al 2 O 3 nanocomposite along with bare CdFe 2 O 4 for four successive cycles. After the first cycle run, the filtered sample is annealed and used for the next cyclic run. The results of recyclability are presented in Fig. 10(a). Obviously, the rate of XO dye degradation decreases after each cycle run. The decrease in the degradation efficiency is attributed to the adsorption of intermediate products onto the surface of photocatalyst. The intermediate products block some available reaction cites and affect the photocatalytic degradation efficiency of the photocatalyst. Due to catalyst waste during the washing, after consecutive four cycles, the photocatalytic efficiency of the catalysts decreases to 86.39% for P 10% and 80.04% for P 0 . As mentioned in above, the degradation efficiency is higher for P 10% than P 0 nanoparticles. A small decrease in the degradation activity is observed at the forth cycle, indicating good photocatalytic stability of the P 10% photocatalyst. The as-prepared CdFe 2 O 4 /Al 2 O 3 nanocomposite can be used as ideal catalyst in practical applications. The XRD pattern after catalytic reaction of the CdFe 2 O 4 /Al 2 O 3 with P 10% after and before degradation of XO dye is shown in Fig. 10(b). We have analyzed all the samples. Here, we have presented XRD pattern of P 10% sample. The results of dried reusable sample revealed that the nature of the XRD pattern of P 10% sample shows the similar nature after degradation process. The intensity of the XRD peaks increases after degradation of XO dye as compared to fresh sample. The increase in intensity of the XRD peaks after the degradation reflects the increase in crystallite size of the catalyst. The crystallite size of CdFe 2 O 4 is 87.53 nm. For CdFe 2 O 4 /Al 2 O 3 (P 10% ), the crystallite size is 41.59 nm. These results strongly indicate that, the prepared nanoparticles are stable and recoverable as catalysts under UV radiation. From the XRD study, it is revealed that the structure of the magnetic catalyst is stable and not altered after reusing. The results of photocatalytic reaction for XO dye on CdFe 2 O 4 nanoparticles and composites P 5% , P 10% , and P 20% is further fitted to understand the controlling mechanism of the degradation process (Fig. 11). The linear form of first-order kinetic model can be expressed by : where C t and C 0 is the concentration of XO dye at different time and at initial time t 0 . k is the pseudo firstorder rate constant. Figure 11 shows the logarithmic plot of the concentration of XO dye with P 0 , P 5% , P 10% , and P 20% photocatalysts. The determined values of k with their respective linear regression coefficients (R 2 ) are presented in Fig. 11. The k value of XO removal with CdFe 2 O 4 /Al 2 O 3 (0.0016 min −1 ) is two times higher than bare CdFe 2 O 4 (0.0012 min −1 ) which indicates that the introduction of Al 2 O 3 enhances the photocatalytic performance of CdFe 2 O 4 . The R 2 values are close to unity, which confirms the adherence of the photocatalytic degradation of XO to the first order and the model is highly compatible. To achieve the optimal performance of the photocatalyst, it is crucial to control the concentration of XO in the 10 wt% CdFe 2 O 4 /Al 2 O 3 composite. The initial dye concentration has a prominent function in the quantity of dye adsorbed and the efficiency of dye removal. The statistical tool is used to compare the experimental and modeling results. The response surface methodology (RSM) analysis is performed using Design Expert software (Salehi et al. 2017;Chaker et al. 2021;Tetteh et al. 2021). Figure 12(a) and (b) show the contour three-dimensional (3D) plot of predicted percentage degradation efficiency as a function of (a) contact time and concentration, (b) contact time and catalyst dosage. With the increase in contact time, the repulsive force is reduced and consequently the XO molecules are produced on the surface of the catalyst. As shown in Fig. 12(a), when the concentration of XO dye increases to 1 mg/L, the molecules of XO are presents as a neutral form. Thus, a repulsive force is created between the XO molecules and the catalyst surface which results in the decrease in photo-degradation. As a result, the creation of both (e − /h + ) pairs and the radicals decreases at higher concentration and therefore saturation is observed. Figure 12(b) shows the interaction effects between contact time and the catalyst dosage. With variation in contact time, the catalyst dose between 0.3 and 0.7 g created the best response. At 0.5 g catalyst dose, additionally, more active sites are produced, followed by more ·OH or O 2 . radicals and more (e − /h + ) pairs on the photocatalyst surface. Thus, the improvement of photodegradation efficiency is obtained in abovementioned values. In other words, the higher dosage (0.7 g) of catalyst, the aggregation process may occur for composite nanoparticles which reduces the effective surface area. Hence, beyond the higher limit of the mentioned ranges, the degradation efficiency decreases. The red point in the 3D plot shows the higher degradation efficiency of the catalyst give a maximum yield of 95% with variation in contact time, 0.5 g catalyst dose, and 1 mg/L concentration of XO dye.
The photocatalytic efficiency of the optimal sample is investigated based on the experimental design using response surface methodology (shown in supporting information along with Table S2). The statistical significance (p < 0.05) of the model and validity is tested by using analysis of variance (ANOVA) ( Table S2). The additional information such as the fisher variation ratio (F-value) adequate precision, coefficient of variance (CV), probability value (Prob>F), and lack of fit are also obtained as shown in Table S2. In our case, the ANOVA gives an F-value equal to 3.42. The model is significant if the tabulated F-value is Fig. 11 First-order kinetic plot of XO dye degradation for CdFe 2 O 4 / Al 2 O 3 -P 5% , P 10% , and P 20% composites along with CdFe 2 O 4 -P 0 nanoparticles. Error bars represent standard deviation of each sample smaller than the calculated value. The model significance threshold is indicated by the p-value probability under 5% and 1% significance levels. A model is considered as significant if the p-value is less than 0.05 or even better less than 0.01. The p-value of our model is in between 0.01 and 0.05. Therefore, our model is significant at 5% and 1% significance level. According to the regression results and the ANOVA analysis, it is found that the model is significant and adequate with the experimental results. The photocatalytic performance has a inverse relation with the pollutant concentration, which is in good agreement with the earlier obtained results (Chaker et al. 2021;Tetteh et al. 2021). The obtained statistical analysis is coherent with our experimental degradation characteristics. Overall, theoretically as well as experimentally it is found that CdFe 2 O 4 /Al 2 O 3 shows enhanced photocatalytic performance as compared to that of bare CdFe 2 O 4 .

Conclusions
In this study, CdFe 2 O 4 /Al 2 O 3 nanocomposite is synthesized via sol-gel auto-combustion method. The photocatalytic activity of the samples is evaluated by the xylenol orange (XO) dye degradation under UV light. The influence of Al 2 O 3 concentration with CdFe 2 O 4 on the catalytic property is also studied. We have considered three weight percentages of Al 2 O 3 with CdFe 2 O 4 as 5%, 10%, and 20%. The enhanced photocatalytic activity of CdFe 2 O 4 /Al 2 O 3 nanocomposite is compared with bare CdFe 2 O 4 nanoparticles. All the samples are characterized with XRD, BET, FTIR, FESEM, TEM, VSM, UV-Visible, and PL spectroscopy techniques. The particle size of bare CdFe 2 O 4 is 83.12 nm. In nanocomposite (P 10% ) form, the crystallite size decreases from 83.12 to 38.68 nm. The band gap of P 0 , P 5% , P 10% , and P 20% is 1.93 eV, 2.02 eV, 2.32 eV, and 2.56 eV, respectively. The variation in contact time, catalyst dose, and concentration of XO dye is studied. The photocatalytic tests confirmed that the XO dye degraded rapidly in presence of composite as compared to that of CdFe 2 O 4 . The reduced particle size, higher saturation magnetization, increased SSA, and higher porosity enhance the catalytic properties of the P 10% nanocomposite. Among these three concentrations, the CdFe 2 O 4 /Al 2 O 3 with 10 wt% concentration shows enhanced catalytic degradation ratio towards degradation of XO dye under UV light. Recycle tests confirmed that the CdFe 2 O 4 /Al 2 O 3 catalyst is more stable and ascertained that the composite catalyst is well suited for practical applications. This study represents an important advance in the synthesis of novel nanocomposite for the photocatalytic degradation of organic pollutant under UV light irradiation for industrial effluent. Additionally, this gives scope to the researchers to continue investigating its water splitting capability and photocatalytic activity against additional dye contaminants.
Author contribution ASV conducted the experimental work and drafted the initial manuscript. MDD supervised the work equally. DRT and AVB equally contributed in catalytic experiments. All authors contributed to the final manuscript.
Funding This work was supported by the Council of Scientific and Industrial Research (CSIR), New Delhi, India, for the project EMR-II-03/1429/18.

Data availability
The data used to support the findings of this study are included within the article.

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Conflict of interest
The authors declare no competing interests.