Efficient photocatalytic degradation of Rhodamine B dye using solar light-driven La˗Mn co-doped Fe2O3 nanoparticles

doi:10.21203/rs.3.rs-1462007/v1

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Research Article

Efficient photocatalytic degradation of Rhodamine B dye using solar light-driven La˗Mn co-doped Fe2O3 nanoparticles

https://doi.org/10.21203/rs.3.rs-1462007/v1

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This work aims to develop a highly efficient solar light-induced photocatalyst based on La˗Mn co-doped Fe₂O₃ nanoparticles. Pure Fe₂O₃ and La˗Mn co-doped Fe₂O₃ nanoparticles were fabricated by a simple co-precipitation method. The photocatalysts were analyzed for their morphological, structural, and magnetic characteristics. The photocatalytic performance of the Fe₂O₃, La_0.1Mn_0.3Fe_1.60O₃, and La_0.2Mn_0.2Fe_1.60O₃ catalysts was assessed by their capability to degrade Rhodamine B (RhB) under solar light illumination. La_0.2Mn_0.2Fe_1.60O₃ displayed exceptional degradation performance, degrading RhB to 91.78% in 240 min, in comparison to La_0.1Mn_0.3Fe_1.60O₃ (71.09%) and pristine Fe₂O₃ (58.21%) under specified reaction conditions [(RhB) = 50 ppm; (catalyst) = 40 mg/L; pH = 7; T = 25 ºC)]. The increased photocatalytic performance of La_0.2Mn_0.2Fe_1.60O₃ was attributed to the large surface area of the catalyst as a result of La˗Mn co-doping. RhB degradation was affected by changing pH, catalytic dosage, dye concentration and temperature. The degradation of RhB was found to be pseudo-1st order kinetics. The photocatalyst material exhibited exceptional stability in four consecutive cyclic runs. The excellent photodegradation potential of La_0.2Mn_0.2Fe_1.6O₃ nanoparticles suggests that the effective eradication of organic pollutants can be achieved by these particles.

Solar-light

Photocatalyst

Degradation

Organic Pollutants

Rhodamine B

Doped Fe2O3

From the past few years, it was found that the economy of the developed countries had been shifted from agriculture to industry due to a huge increase in the world’s population (Zheng, Zhang, Meng, Wang, & Li, 2020). Although industrialization has provided ease in human life at the same time, it also cast a devastating impact on the environment by producing toxic and hazardous chemical species (Payra et al., 2019; Usman et al., 2021). Most of these industries discharge their toxic and stable chemical species into the wastewater, which ultimately damages aquatic life and human health (Lv et al., 2020; Steplin Paul Selvin et al., 2018). The wastewater contaminants are mostly organic dyes that are stable and toxic due to their complex and aromatic structures and are mostly waste-products of the tanneries, paint and textile industries (Ritika, Kaur, Umar, Mehta, & Kansal, 2019). Due to this stable nature, these organic pollutants are considered non-biodegradable. These pollutants cause serious environmental problems and human health issues, including skin and lungs destruction (Ashar et al., 2020; Das & Mahalingam, 2020). Many types of dyes, which are the most common organic pollutants, are produced in different industries and discharged into water bodies without proper treatment and are the major sources of water pollution (Ahmed et al., 2020; Chowdhury, Khan, Kumari, & Hussain, 2019; Khodadadi, Bordbar, & Nasrollahzadeh, 2017; Xu, Sun, Wang, Song, & Wang, 2018).

These water pollutants threaten human and aquatic life when discharged untreated into water bodies. These contaminants must be eradicated from the waste material before discharge into the water body. These contaminants are most commonly removed from wastewater by employing different methods. These methods include some electrochemical and physio-chemical techniques such as oxidation, chemical precipitation, adsorption, and microwave-assisted catalysis (Martínez-López et al., 2019; Mostafa, Elsawy, Darwish, Hussein, & Abdaleem, 2020; Ndagijimana, Liu, Li, Yu, & Wang, 2019; Tang et al., 2020). These methods are very important and advantageous to remove the organic compounds from water, but there are various disadvantages and drawbacks associated with these methods. Due to environmental issues caused by above-mentioned methods, there was a need to develop an effective and eco-friendly technique to treat the harmful organic material (Bibi et al., 2017). The photocatalysis served the requisite properties, non-toxic, cost-effective and commercially easy to handle (Ahmed et al., 2019; Wu et al., 2020). In this technique, a photocatalyst is used to degrade the harmful organic materials in the presence of sunlight (Demirci, Yurddaskal, Dikici, & Sarıoğlu, 2018; Raza et al., 2020). The active sites available on the surface of catalyst are responsible to absorb the sunlight and hence for the creation of active species to degrade harmful organic compounds (Gao et al., 2020; Kumar et al., 2020). For the last few years, metal oxide nanoparticles have been most commonly used as photocatalysts for the removal of toxic organic compounds from sewage water (Duan et al., 2020). These metallic nanoparticles have been fabricated by the use of different methods i.e., hydrothermal, sol-gel, co-precipitation and microwave-assisted techniques (Almeida, Rodembusch, Ferreira, & Caldas de Sousa, 2020; Dinesh, Pramod, & Chakma, 2020; Park & Ahn, 2020; Yousuf et al., 2019; Zare, Namratha, Thakur, & Byrappa, 2019; Zinatloo-Ajabshir, Salehi, & Salavati-Niasari, 2018). Although these fabrication methods are very effective and efficient for the production of nanoparticles, there are still some drawbacks of these methods, including high cost, requiring more energy, and producing harmful by-products (Liu et al., 2019; Rojas & Horcajada, 2020). It is required to choose a comparatively more effective, cheaper and least harmful technique for the fabrication of nanoparticles (Usman et al., 2019).

The solar light-sensitive metal oxide photocatalysts are very important for the decomposition of water pollutants. These metal oxides include CoO and CuO, which are light-sensitive (Lim, Chua, Lee, & Chi, 2014). Due to their variable oxidation state and active surface area, transition metal oxides have exhibited more vital activities. Iron oxide being transition metal oxide, have attained more consideration since the previous few years because of its outstanding uses in different fields. Iron oxides have been extensively used for photocatalysis (Bouhjar, Derbali, Marı´, & Bessaı's, 2020). The Fe₂O₃ is an effective and advantageous photocatalyst to oxidize H₂O because of its environment-friendly potential. It absorbs a longer range of solar light, hence a more effective photocatalyst (W. Guo, Sun, Lv, Kong, & Wang, 2017). The photocatalytic activity of Fe₂O₃ nanoparticles has been found to be increased when treated with some other metals, more preferentially the transition metals. The Mn being cheaper, easily available, and environment-friendly properties, is the best transition metal used for doping of Fe₂O₃ nanoparticles (Chen et al., 2019). The doping of Fe₂O₃ with Mg also enhances the catalytic degradation of organic pollutants by generating more oxygen vacancies (Sun et al., 2020). The photocatalytic activity of Cr doped Fe₂O₃ nanorods was also found to be enhanced compared to undoped (Popov et al., 2021). The Ca doped Fe₂O₃ nanoparticles being cheaper and least harmful to the environment, were used preferably to degrade the RhB (an organic pollutant). The doping of Fe₂O₃ nanoparticles with Ca enhances electron transfer, so the degradation is increased by many folds (S. Guo et al., 2020). [email protected]₂O₃ nanoparticles have been fabricated by using sol-gel technique. Doping by Ti significantly enhanced the surface area of Fe₂O₃ nanoparticles. It also enhanced reducibility and catalytic activity in oxidation of toluene (Abbas Khaleel, Maliha Parvin, Moahmmed AlTabaji, & Al-zamly, 2018).

Co metal doping of Fe₂O₃ reduces its bandgap, and it also enhances photocatalytic activity because it acts as a trapping center for electrons. Co-doped Fe₂O₃ nano sized particles have shown a good photocatalytic performance to remove the hazardous industrial effluents (R. Suresh et al., 2017). Hydrothermally synthesized Mg-doped CuO/Fe₂O₃ was found to be a good catalyst for phenol degradation. The degradation rate of nanoparticles is enhanced by doping the material with Mg. This increase is due to the fact that OVs number is enhanced due to Mg doping in the catalysts (Mengying Sun, Yu Lei, Hao Cheng, Jianfeng Ma, Yong Qin, Yong Kong, et al., 2020). The chromium metal is used as dopant material because of its certain characteristic properties like ion donor and enhancement of charge carrier concentration in Fe₂O₃ sheets. Mainly the doping of Cr metal onto the iron oxide sheet enhances the conductance and charge transport in these sheets (Feriel Bouhjar, Lotfi Derbali, Bernabe Marı´a, & Bessaıs, 2020). The photocatalytic performance of CuS/Fe₂O₃/Mn₂O₃ nanomaterials was determined by the degradation of ciprofloxacin (CIP) (Yan Huang et al., 2020). By using the magnesium metal as dopant have also enhanced the photocatalytic activity of CuO/Fe₂O₃ to degrade the phenolic compounds (Mengying Sun, Yu Lei, Hao Cheng, Jianfeng Ma, Yong Qin, Kong, et al., 2020).

The doping with Cu enhanced its photoelectrochemical property. The co-doping further enhanced some characteristic properties of the Fe₂O₃. Co-doping of the Fe₂O₃ with Si and Ti has increased its donor concentration. The co-doping of Fe₂O₃ with N and Zn has enhanced its concentration of acceptors and photo response (Reddy et al., 2019). It is observed that the doping of nanoparticles with Mn imparts various tremendous characteristics to them. The enhancement of photocatalytic performance of the Fe₂O₃ nanoparticles was also observed by doping with Mn. This doping has enhanced the photo-induced charges which in turn increased the capacity of the Fe₂O₃ nanoparticles to transfer the charges (N. Wang, Han, Wen, Liu, & Li, 2019). The nanoparticles having Mn doping also undergo agglomeration due to their higher surface energy. The photocatalytic effect was found to be further enhanced by the use of rare-earth metal ions in addition to Mn. These rare-earth metals are responsible for the inhibition of photo-induced electron-hole recombination (Rashmi et al., 2017). These rare earths also enhance the number of active sites which are responsible to degrade the harmful organic compounds by the creation of heterojunctions in the nanoparticles, which ultimately inhibits the electron-hole recombination (Harish, Bhojya Naik, Prashanth kumar, & Viswanath, 2013). The rare-earth metals in combination with Mn are also responsible for changing the intrinsic properties of the nanoparticles due to the unique 4f-3d coupling of electrons. La is more preferable as compared to the other rare-earths due to its distinct properties such as larger size (1.6061 Å), simple electronic spectra, reactivity, paramagnetic nature, higher resistivity (615 Ωm) and lower melting point (920°C). La in combination with Mn have enhanced physical, chemical and catalytic properties of the nanoparticles (Baig et al., 2020; Peng, Fu, Yang, & Ouyang, 2016).

Because of these remarkable properties of cations of lanthanide series, in the present work, we have chosen La in combination with Mn for the co-doping of Fe₂O₃ nanoparticles to enhance the photocatalytic performance of these nanoparticles.

2.1. Chemicals

The chemicals required for the fabrication of La˗Mn co-doped Fe₂O₃ nanoparticles are listed here. Fe (NO₃)₃. 9H₂O (Merck, 98%), Mn (NO₃)₂.4H₂O (Merck 98%), La (NO₃)₃, (Merck 98%), deionized water, sodium hydroxide [NaOH, Sigma, 98%], were utilized as precursors and reagents for this research work. The RhB an organic dye utilized during this work, was procured from Almadina chemicals. All these precursors and chemicals which were utilized in this research work were in highly purified form and were used as received from the supplier.

2.2. Fabrication of Fe₂O₃ nanoparticles

To fabricate the Fe₂O₃ nanoparticles, 20.2 g of Fe (NO₃)₃. 9H₂O was dissolved in 250 ml of deionized water to prepare a 0.2M iron solution. Fe₂O₃ nanoparticles were synthesized by using the chemical co-precipitation route. During this particular procedure, Fe (NO₃)₃.9H₂O solution was stirred on the hotplate for 15 minutes at 60 ^oC temperature to get a homogeneous solution. At the start of this experiment pH of the solution was found to be 4, and the color was orange-yellow. Then solution pH was increased from 4 to 11 by the addition of 1M sodium hydroxide solution and was maintained at 11. After the maintenance of pH, the solution became blackish-green. Stirring was continued for 4hrs at 80 ^oC. The solution was cooled, and precipitates were separated. The washing of these precipitates was carried out by deionized water many times till the pH became neutral. These precipitates were then dried in an oven at 150–200 ºC and annealed at 450 ºC for 4hrs in a furnace to get the final product.

2.3. Preparation of La˗Mn co-doped Fe₂O₃ nanoparticles

For the preparation of La˗Mn co-doped Fe₂O₃ nanoparticles, different molar concentrations of solutions were made such as ferric nitrate (0.16M), manganese nitrate (0.03M, 0.02M) and lanthanum nitrate (0.01M, 0.02M) were made by the addition of specific quantities of these salts in water separately. Briefly, 16.6 g of ferric nitrate was added in 250 ml of deionized water and was stirred for 15 minutes at 60 ^oC to get a homogeneous solution. 0.02M solution of manganese nitrate was added to the above solution at the same temperature with constant stirring for 15 minutes. After this, 0.02M solution of lanthanum nitrate was added drop wise while it was also stirred continuously. At the start of this experiment pH of the solution was found to be 4, and the color was orange-yellow. Then solution pH was increased from 4 to 11 by the addition of 1M sodium hydroxide solution and was maintained at 11. Stirring was continued for 4hrs at 80 ºC. Then the solution was cooled, and precipitates were separated. The washing of these precipitates was carried out by deionized water many times till the pH became neutral. The precipitates were then dried in an oven at 150–200 ºC and annealed at 450 ºC for 4hrs in a furnace to get the final product. The schematic representation for the synthesis of La˗Mn co-doped Fe₂O₃ nanoparticles is presented in Fig. 1.

2.4. Characterization

The morphology and particle size of undoped Fe₂O₃ and La˗Mn co-doped Fe₂O₃ nanoparticles was examined by using Scanning Electron Microscope (SEM, Hitachi SX-650). Similarly, the TEM is used for the visualization and determination of morphological properties of nanoparticles. The FTIR spectrometer (Nikolet Nexus-470) was used for taking the spectra of the fabricated doped and undoped nanoparticles in the 400–3000 cm^− 1 wavelength range. The X-ray analysis of the prepared nanoparticles was performed to determine the nanoparticles' purity and crystallinity. The Dynamic light scattering analysis was performed using NANO ZS Malvern Zetasizer for the measurement of zeta potential and particle size distribution. The magnetic properties of as fabricated particles were determined by the use of a magnetic field of -15000 to + 15000 Oe strength. The nitrogen adsorption-desorption on the surface of nanoparticles and subsequently plotting a BET isotherm was utilized to measure the surface area of as fabricated particles. This analysis was performed by using Micrometrics Physisorption Analyzer (ASAP 2020). The UV spectrophotometer (Perkin Elmer, USA) was used for the measurement of photocatalytic performance of the as-prepared nanoparticles in the region of 200–700 nm.

2.5. Photocatalytic activity

The photocatalytic efficiency of pure Fe₂O₃ and La˗Mn co-doped Fe₂O₃ nanoparticles was assessed by the degradation of RhB dye under solar light illumination. In a conventional degradation procedure, 40 mg/L of catalyst was mixed with 100 mL of RhB (50 ppm) in ultrapure water. To achieve the adsorption-desorption equilibrium, the resulting mixture were put in the dark and continuously agitated for 30 minutes. Following that, the mixture was illuminated with solar light, and 1 mL sample was taken at fixed time intervals and centrifuged at 5000 rpm to remove the residual catalyst particles. A UV-Vis spectrophotometer was used to check the concentration of RhB dye (Eq. 1).

$$\text{D}\text{e}\text{g}\text{r}\text{a}\text{d}\text{a}\text{t}\text{i}\text{o}\text{n} \left(\text{%}\right)= \frac{\left({\text{C}}_{\text{O}}-\text{C}\right)}{{\text{C}}_{\text{O}}} \times 100 \left(1\right)$$

C_O = Initial concentration of RhB dye, C = Dye concentration at time t.

The photocatalytic effect of the La_0.2Mn_0.2Fe_1.6O₃ nanoparticles was further studied at different experimental conditions.

3.1. SEM analysis

The shape and particle size of undoped Fe₂O₃ and La˗Mn co-doped Fe₂O₃ nanoparticles was examined by using SEM analysis. The SEM micrographs of Fe₂O₃ nanoparticles have shown that the fabricated nanoparticles appeared as semi-spherical shaped with small aggregations in clusters and different sizes as shown in Fig. 2a.

The bimetallic doping of these Fe₂O₃ nanoparticles with La and Mn in different compositions affected the size of particles to a measurable extent. The morphology of the doped nanoparticles was not affected significantly due to this bimetallic doping. As the size of dopant metal ion Mn^+ 2 is not much different from parent Fe^+ 3 ions, so its doping does not affect the size of doped nanoparticles greatly. The size of the La^+ 3 is much bigger (1.6061 Å) as compared to Mn²⁺ (0.83 Å) and Fe³⁺ (0.63 Å) ions present in parent nanoparticles. These larger La ions induce lattice imperfections and also lattice strain. Based on this reason the particle size of as fabricated doped nanoparticles was found to be reduced by increasing the molar concentration of lanthanum ions. The micrographs of the doped nanoparticles at different concentrations of Mn and La metals were taken.

Figure 2b shows the micrograph of La_0.1Mn_0.3Fe_1.6O₃, and reduction in size of doped particles was observed in comparison to the undoped Fe₂O₃ nanoparticles. Similarly, Fig. 2c shows the micrograph of La_0.2Mn_0.2Fe_1.6O₃ nanoparticles. These micrographs have shown a reduction in the size of the particles that take place by increasing the dopant La metal ions. Due to this reduction in the sizes of as fabricated doped nanoparticles, the active surface of the particles is increased, so the catalytic performance of these particles is also enhanced (Ahmed, Usman, Wang, et al., 2021).

3.2. TEM analysis

TEM analysis was performed to visualize and determine the morphological properties of nanoparticles as presented in Fig. 3. The doping of Fe₂O₃ nanoparticles has resulted in aggregation of the particles, which in turn produced inter-particle mesopores resulting in enhancement of the surface area. It was observed from TEM micrographs of Fe₂O₃ given in Fig. 3a that the average size as calculated by Nanomeasure software was found to be 48.137 nm, while the doping of Fe₂O₃ with La and Mn reduces the size of these particles. Figure 3b shows the micrograph of La_0.1Mn_0.3Fe_1.6O₃ nanoparticles, size of these particles was found to be 46.923 nm. Similarly, Fig. 3c shows the micrograph of La_0.2Mn_0.2Fe_1.6O₃ nanoparticles, and also their size was estimated to be 42.861 nm. The morphology of all the samples was found to be semi-spherical shaped, which is indication of the fact that morphology of particles is not significantly changed, but only the size of the particle is affected.

3.3. FTIR analysis

The FTIR spectra of the prepared nanoparticles (undoped and doped) were taken to determine structural parameters. The spectra of nanoparticles have shown two absorption peaks in 400–3000 cm^− 1 region for metal-oxygen stretching vibrations. Out of these two prominent absorption peaks, one observed in the low frequency region (400–500 cm^− 1) for octahedral M-O stretching vibrations. Similarly, the peak at a higher frequency region (500–600 cm^− 1) appeared because of tetrahedral stretching vibrations (Ahmed, Usman, Wang, et al., 2021). The FTIR spectra for all three types of nanoparticles were taken and are represented in Fig. 4.

The spectra for Fe₂O₃, La_0.1Mn_0.3Fe_1.6O₃ and La_0.2Mn_0.2Fe_1.6O₃ nanoparticles have shown that there is a small shift of absorption peak to the higher frequency region by the increase of concentration of dopant metal La. It is because of enhancement of large-sized La metal ions concentration, a decrease in particle size is observed, causing a decrease in bond length, which in turn causes the increase in the absorption frequency of the stretching bands. Due to this fact, a change in the position of the absorption band takes place, as shown in Fig. 4.

3.4. XRD analysis

The XRD analysis of the undoped Fe₂O₃ and La˗Mn co-doped Fe₂O₃ nanoparticles was performed for the determination of purity and crystallinity of the nanoparticles. The analysis of these nanoparticles had confirmed the hexagonal geometry of the synthesized nanoparticles. The XRD results are shown in Fig. 5. According to these results, there are sharp and intense diffraction peaks at 2θ values, 24.13º (012), 33.15º (104), 35.61º (110), 40.85º (113), 49.47º (024), 54.08º (116), 57.58º (018), 62.44º (214), 63.98º (300), 72.26º (019), and 75.42º (220) respectively. These all the values of the peaks were compared with a standard pattern of Fe₂O₃ having hexagonal geometry. These XRD results found to be greatly comparable to the standard reference (JCPDS card no. 33–0664), which confirmed the synthesis of Fe₂O₃ (Ma & Chen, 2018). As there were no other irrelevant peaks observed in this result, which is the confirmation for the purity of as-synthesized nanoparticles. It was also found that the decrease in intensity of the peaks was observed for the La and Mn-doped nanoparticles. The peaks were also shifted due to this doping, and the shifting was increased by increasing the La ions concentration.

The crystal size of the synthesized Fe₂O₃, La_0.1Mn_0.3Fe_1.6O₃ and La_0.2Mn_0.2Fe_1.6O₃ nanoparticles was calculated by using Debye Scherrer equation and measured to be 59.143, 58.237 and 51.948 nm, respectively.

These results have shown that the size of crystals is decreased by increasing the concentration of dopant metal La. This decrease in size of the doped nanoparticles is explained by the fact that the insertion of large-sized (1.6061 Å) La³⁺ ions into the small-sized (0.63 Å) Iron Fe³⁺ ions caused an increase in constant lattice value, which in turn decreases the crystallite size (Ahmad, Shah, Ashiq, & Khan, 2016).

3.5. DLS analysis

The DLS technique is utilized to measure the zeta potential and particle size distribution. The results plotted in Fig. 6 have shown a regular reduction in average size of particles due to the insertion of Mn and La ions in different compositions. The size of as fabricated Fe₂O₃ particles measured by DLS is 68.31 nm, as presented in Fig. 6a. Similarly, the size of the particles is reduced after bimetallic doping in a composition of La_0.1Mn_0.3Fe_1.6O₃. In this case, the average size is 67.42 nm, shown in Fig. 6b. The average size of nanoparticles is further decreased to 57.28 nm when the concentration of dopant metal La is further increased in La_0.2Mn_0.2Fe_1.6O_3, as presented in Fig. 6c.

3.6. Magnetic properties

The magnetic properties of the nanoparticles were measured at standard conditions of temperature and pressure with a field strength of -15000 to + 15000 Oe. The M-H curves are used for the measurement of various magnetic parameters. The magnetic hysteresis curves for Fe₂O₃ nanoparticles and La˗Mn co-doped Fe₂O₃ nanoparticles with different dopant concentrations La_0.1Mn_0.3Fe_1.6O₃ and La_0.2Mn_0.2Fe_1.6O₃ are given in Fig. 7. These M-H curves are used for the measurement of various magnetic properties i.e., Ms, Hc, Mr and squareness ratio. The values for Fe₂O₃ nanoparticles and La˗Mn co-doped Fe₂O₃ nanoparticles as measured by using the M-H curves are given in Table 1.

These magnetic properties of as-synthesized undoped and doped nanoparticles are greatly influenced by various factors such as surface charge, cation stoichiometry, cation distribution, size of particles and the method of fabrication used (Sharifi, Shokrollahi, & Amiri, 2012). The results presented in Table 1 shows that the value of saturation magnetization (Ms) is decreased for the doped nanoparticles compared to undoped particles. The values for Fe₂O₃ nanoparticles is 6.17 emu/g while for La_0.1Mn_0.3Fe_1.6O₃ particles 3.73 emu/g and also for La_0.2Mn_0.2Fe_1.6O₃ is 2.89 emu/g. It is observed that the values are further decreased by increasing the concentration of the La^+ 3 ion. The values of remanent magnetization also reduced similarly, Fe₂O₃ nanoparticles have Mr 1.15 emu/g, which is decreased to 0.68 emu/g for La_0.1Mn_0.3Fe_1.6O₃ particles and was further reduced by increasing the dopant ions concentration up to 0.52 emu/g for La_0.2Mn_0.2Fe_1.6O₃. The values for coercivity and squareness ratio are also affected in a similar way, but there is very little change for these parameters.

Table 1

Room temperature measurement of saturation magnetization (M_s), remanent magnetization (M_r), coercivity (H_c), and squareness ratio (M_r/M_s) of nanoparticles.
Composition	Saturation magnetization, M_s (emu/g)	Remanent magnetization, M_r (emu/g)	Coercivity, H_c (Oe)	Squareness ratio, M_r/M_s
Fe₂O₃	6.17	1.15	756.72	0.186
La_0.1Mn_0.3Fe_1.6O₃	3.73	0.68	756.69	0.182
La_0.2Mn_0.2Fe_1.6O₃	2.89	0.52	756.67	0.180

The decrease in the values of magnetic parameters is considered due to the insertion of La^+ 3 ion into the Fe₂O₃ nanoparticle sublayers, which decreases the crystallinity and uniform morphology of the undoped Fe₂O₃ nanoparticle. These decreasing trends of magnetic parameters are also explained based on cation occupancy on the lattice sites, magnetic moment and the ionic radius. These above results are the basis for the separation of doped and undoped nanoparticles, and thus these materials act as efficient photocatalysts in aqueous environment.

3.7. BET analysis

When BET analysis was performed for Fe₂O₃ nanoparticles, it was found that a type IV isotherm was formed, as shown in Fig. 8a. The surface area of Fe₂O₃ nanoparticles as calculated was 77.93 m²/g. Similarly, the surface area of the La_0.1Mn_0.3Fe_1.6O₃ and also for La_0.2Mn_0.2Fe_1.6O₃ was also measured. It was observed that the surface area of La_0.1Mn_0.3Fe_1.6O₃ was found to be 80.23 m²/g, as shown in Fig. 8b. Further analysis of the La_0.2Mn_0.2Fe_1.6O₃ nanoparticles has shown that by changing the molar concentrations of the dopant metals, the surface properties of these nanoparticles also changed, given in Fig. 8c. Also, for La_0.2Mn_0.2Fe_1.6O₃ was found to be 87.45 m²/g.

From the above results, it is concluded that the surface area of the La˗Mn co-doped Fe₂O₃ nanoparticles is increased by increasing the dopant concentrations. Due to this fact, the interaction between gas molecules and the nanomaterials is increased, which in turn also increases the adsorption-desorption (Zhou et al., 2016). This increased interaction between gas and nanomaterials will enhance the gaseous diffusion into the inner layers of the material having active sites (T. Wang et al., 2018). This whole process caused an enhancement in the photocatalysis of doped nanoparticles. This increased surface area is also beneficial to degrade the harmful organic compound.

3.8. Photocatalytic activity

The UV spectrophotometer is used for the measurement of photocatalytic performance of the as-prepared Fe₂O₃ nanoparticles and La˗Mn co-doped Fe₂O₃ nanoparticles. This experiment was performed by calculating the degradation of RhB dye under sunlight. The dye solution was prepared and kept in dark to avoid any disturbance before applying catalyst. The dye solution and the catalyst solution were then mixed in the dark and kept for some time to attain the equilibrium. This mixture was irradiated by keeping under sunlight.

The degradation rate of dye was monitored by taking the samples from time to time and measuring the concentration by using a UV spectrophotometer. The decrease in dye concentration was detected with the increase in time, and ultimately, the dye concentration has almost vanished after 240 minutes. It is due to the fact that the nanoparticles acted as catalysts to degrade the dye molecules. These results of RhB degradation have shown 58.21% degradation by using Fe₂O₃ as represented in Fig. 9a. After the doping of Fe₂O₃ with Mn and La in the composition of La_0.1Mn_0.3Fe_1.6O_3, the degradation of dye was found to be increased to 71.09%, which is much higher than undoped nanoparticles, as represented in Fig. 9b. The results have shown that with the increase in concentration of dopant metal La, the percentage degradation of the RhB was also increased, as shown in Fig. 9c. It is evident that when the La concentration is increased to La_0.2Mn_0.2Fe_1.6O₃ dye degradation was also increased up to 91.78%.

Based on the above discussion, it is concluded that degradation rate is increased by doping and is further increased by increasing the composition of large-sized La^+ 3 cation due to reduction in size and increase in surface area of the doped nanoparticles. The photocatalytic activity of La_0.2Mn_0.2Fe_1.6O₃ nanoparticles is further studied under different experimental conditions.

3.8.1. Effect of pH

As it is evident from the above discussion that degradation of RhB is best carried out by La_0.2Mn_0.2Fe_1.6O₃ nanoparticles. Further studies were also carried out to explore the effect of other factors on degradation process. Amongst these factors the pH of solution is considered as a very important and most noticeable dynamic factor to affect the degradation. This factor is responsible for the development of charge on catalyst surface, either positive or negative. Due to this reason of charge development, the adsorption rate of dye on the catalyst surface is also changed. In addition to the charge development, the size and aggregation of the catalyst is also affected by the change of pH, which in turn influences the degradation of dye (Ahmed, Usman, Yu, Gao, et al., 2021). In this experiment, a fixed amount of catalyst (40 mg/L) and that of dye (50 ppm) is used at a fixed temperature of 25 ºC, and the pH of the solution is varied from 4 to 10. The pH of different solutions is maintained between 4 to 10 by the addition of hydrochloric acid and sodium hydroxide.

These dye solutions having different pH values were then exposed to sunlight, and degradation was calculated. To determine the effect of pH on degradation, the Zeta potential was used for the measurement of surface charge of the catalyst, as shown in Fig. 10. The point of zero potential (also known as IEP) was measured from Zeta potential values. The point of zero potential for La_0.2Mn_0.2Fe_1.6O₃ nanoparticle was found to be 8.46, as shown in Fig. 10. The catalyst surface attains positive charge when the solution's pH was decreased below 8.46, and the surface became negative by increasing the pH above 8.46.

As RhB is a cationic dye which means its attachment and also degradation, is favored at the pH value above 8.46 because above this point, the catalytic surface becomes negative. This negative surface attracts the cationic dye; hence degradation of dye is increased. Similarly, when the pH of the solution is decreased, a positive charge on the catalytic surface will be developed. This positive charge ultimately repels the cationic dye hence the degradation of dye is decreased with the decrease in pH of the solution. Due to this reason, the minimum degradation value of 43.3% was observed at pH 4. An increase in degradation of dye was observed with the enhancement of pH, and a maximum degradation of 89.34% was observed at pH 10 as represented in Fig. 11 and also in Table 2.

The kinetics of degradation of RhB dye have shown pseudo-first-order reaction as observed for different initial concentrations of dye solution. It is evident from above results that rate constant is increased from 0.0025 to 0.0092 min^− 1 by varying the pH 4 to 10. Similarly, the regression coefficient values also increase from 0.9802 to 0.9960, which is in good agreement with the degradation percentage and are also according to the Langmuir Hinshelwood model of kinetics for pseudo first order.

Table 2

The pseudo-first-order kinetic parameters for RhB degradation under different pH conditions using La_0.2Mn_0.2Fe_1.6O₃ catalysts.
Catalyst	pH	Degradation (%)	k (min^− 1)	R²
	4	43.30	0.0025	0.9802
	5	54.11	0.0031	0.9901
La_0.2Mn_0.2Fe_1.6O₃	6	60.58	0.0038	0.9830
	7	67.22	0.0043	0.9824
	8	72.47	0.0052	0.9915
	9	79.43	0.0064	0.9968
	10	89.34	0.0092	0.9960

3.8.2. Effect of catalytic dosage

The degradation rate was also significantly affected with the change in catalytic dosage. The catalytic dosage is also an important factor for the degradation of dyes because it also causes a change in surface area. The rate of reaction is influenced by this increase of surface area. The effect of photocatalyst La_0.2Mn_0.2Fe_1.6O₃ on photodegradation of RhB was observed using the different amounts 10, 20, 30, 40 and 50 mg/L while keeping other factors constant at the dye concentration 50 ppm, temperature 25 ºC and the pH at 7 as shown in Fig. 12 and Table 3. These experimental results have shown that the dye degradation is increased as the catalytic dosage is enhanced by 10 to 50 mg/L. The minimum degradation of dye was found at a lower catalytic dosage of 10 mg/L, which was 60.49 and went on increasing by an increase of catalytic dosage up to 50 mg/L, which shows maximum photodegradation efficiency of the photocatalyst La_0.2Mn_0.2Fe_1.6O₃ as 90.07% at 25 ºC, 50 ppm dye concentration and pH of the solution was maintained at 7.

This increase in photodegradation is due to an increase of surface area which in turn increases the active sites, due to this reason the rate of degradation is also increased by increasing catalytic amount. There will be an increased absorption of photons with the increase of surface area, which further increases the interfacial interaction among catalytic surface and dye molecules (Ahmed, Usman, Yu, Shen, & Cong, 2021).

The kinetics of degradation of RhB dye have shown pseudo-first-order reaction as studied under different catalytic dosages. The results have shown that the value of the rate constant is increased by 0.0037 to 0.0092 min^− 1 as the catalytic dosage is increased by 10 to 50 mg/L. Similarly, the values of the regression coefficient also increase from 0.9830 to 0.9961, which is in good agreement with the degradation percentage and are also according to the Langmuir Hinshelwood model of kinetics for pseudo first order.

Table 3

The pseudo-first-order kinetic parameters for RhB degradation at different catalyst dosage using La_0.2Mn_0.2Fe_1.6O₃ catalysts.
Catalyst	Catalyst dosage (mg/L)	Degradation (%)	k (min^− 1)	R²
La_0.2Mn_0.2Fe_1.6O₃	10	60.49	0.0037	0.9830
	20	67.13	0.0042	0.9824
	30	72.57	0.0051	0.9915
	40	79.12	0.0064	0.9968
	50	90.07	0.0092	0.9961

3.8.3. Effect of dye concentration

The rate for the degradation of organic dyes is also greatly affected by the change in dye’s concentration. The experiments have shown an increase in rate of reaction by increasing the concentration of dye. The effect of photocatalyst La_0.2Mn_0.2Fe_1.6O₃ on photodegradation of RhB was observed by using the various dye concentrations, i.e., 10, 20, 30, 40, 50 and 60 ppm while keeping other factors constant at the catalytic dosage 40 mg/L, temperature 25 ºC and the pH at 7 as shown in Fig. 13 and Table 4.

The results have shown an increase in photocatalytic performance of La_0.2Mn_0.2Fe_1.6O₃ from 46.19–89.82% as the dye concentration was increased from 10 to 50 ppm while keeping other factors constant as 25 ºC temperature, 40 mg/L catalytic dosage and pH 7. The results have also shown that by further increasing the dye amount the rate of degradation is reduced. It is because the active sites are available to a particular limit. But when amount of dye is further increased, there are not sufficient active sites available for the attachment of molecules (Ahmed et al., 2020). Therefore, the decrease in degradation of dye is observed, presented in Fig. 13.

The kinetics of degradation of RhB dye have shown pseudo-first-order reaction as studied under different amounts of dye. These results have shown that rate constant is increased from 0.0023 to 0.0093 min^− 1 with the increase of dye concentration by 10 to 50 ppm. Similarly, regression coefficient is also increase from 0.9664 to 0.9974 min^− 1, which is in good agreement with the degradation percentage and are also according to the Langmuir Hinshelwood model of kinetics for pseudo first order.

Table 4

The pseudo-first-order kinetic parameters for RhB degradation at different initial dye concentrations using La_0.2Mn_0.2Fe_1.6O₃ catalysts.
Catalyst	Dye Conc. (ppm)	Degradation (%)	k (min^− 1)	R²
La_0.2Mn_0.2Fe_1.6O₃	10	46.19	0.0023	0.9664
	20	63.92	0.0039	0.9912
	30	72.13	0.0053	0.9947
	40	79.98	0.0064	0.9972
	50	89.82	0.0093	0.9974
	60	86.42	0.0078	0.9940

3.8.4. Effect of temperature

The rate of photodegradation of organic pollutants is affected by the change in temperature because the degradation of dyes is either endothermic or exothermic reactions. As it is one of the major factors, so the dye degradation is much affected by changing the temperature. The photodegradation of RhB is an endothermic reaction and is favored by the increase in temperature [21].

As it was found that the temperature has a very small effect on the degradation of dye, there is still an optimum temperature at which degradation of dye is maximum. For RhB, by using photocatalyst La_0.2Mn_0.2Fe_1.6O₃ the experiment was performed at different temperatures (25, 35, 45 and 55 ºC) by keeping other factors, catalytic dosage (40 mg/L), pH 7 and concentration of dye (50 ppm) constant. The experimental results shown in Fig. 14 and Table 5 reveal that dye was degraded maximum up to 94.87% at 45 ºC. These are the optimum conditions for this degradation experiment.

Table 5

The pseudo-first-order kinetic parameters for RhB degradation under different temperature conditions using La_0.2Mn_0.2Fe_1.6O₃ catalysts.
Catalyst	Temperature (^oC)	Degradation (%)	k (min^− 1)	R²
La_0.2Mn_0.2Fe_1.6O₃	25	85.19	0.0079	0.9955
	35	88.23	0.0089	0.9935
	45	94.87	0.0121	0.9879
	55	90.52	0.0098	0.9869

3.9. Degradation mechanism of dye

The results of dye degradation by using La_0.2Mn_0.2Fe_1.6O₃ as photocatalyst have shown that the dye is being converted into simple harmless components by the action of sunlight. The most probable mechanism to degrade the dyes is shown in Fig. 15. As the sunlight irradiate on the catalytic surface it causes the excitation of electron to produce an electron (e⁻) hole (h⁺) pair which further degrade the organic dye. The mechanism shows that the excited electrons produced in the conduction band are responsible for the production of super oxide anion radical (O₂^•−) which is further converted into hydroperoxyl radical (^•OOH) by the reaction with water. In the presence of sunlight this radical rearrange itself to form H₂O₂. The hydrogen peroxide thus produced reacts with oxygen to produce ^•OH [21].

Similarly in the valance band, a reaction between water molecules and holes also takes place and convert it into ^•OH. These hydroxyl radicals then reacted with organic dye (RhB) and oxidize it into simple harmless substances i.e., CO₂ and H₂O. The results have shown that the synthesized La_0.2Mn_0.2Fe_1.6O₃ nanoparticles have great potential to degrade the organic pollutants.

3.10. Reusability of catalyst

Since the catalyst's stability and durability are crucial factors to consider when considering large-scale industrial applications, four degradation tests were conducted under specific reaction conditions, as shown in Fig. 16. After each cyclic run, the La_0.2Mn_0.2Fe_1.6O₃ catalyst was extracted from the aqueous solution using a magnet, thoroughly washed with deionized water, and dried at 70°C for reuse in the next cycle. When the catalyst is used repeatedly, the degrading efficiency of RhB falls steadily from 91.78 to 85.08%. Photocatalyst aggregation, metal ion leaching, and catalytic site degradation as a result of photocorrosion all contribute to the ultimate loss of catalytic potential. The catalyst used in degradation tests might be recovered in the presence of a high magnetic field. As a result, the reusability experiment anticipates and supports the future use of the La_0.2Mn_0.2Fe_1.6O₃ catalyst for photocatalytic wastewater treatment.

The Fe₂O₃ and La˗Mn co-doped Fe₂O₃ nanoparticles having a composition of La_0.1Mn_0.3Fe_1.6O₃ and La_0.2Mn_0.2Fe_1.6O₃ were prepared by simple chemical precipitation technique. The SEM and TEM micrographs have revealed the average size of particles is 48.137, 46.923 and 42.861nm, respectively, and the morphology of the nanoparticles is semi-spherical shaped. These characteristics, along with phase purity and crystalline nature of the nanoparticles, were further confirmed by FTIR and XRD. The zeta potential value for La_0.2Mn_0.2Fe_1.6O₃ is 8.46. The saturation magnetization is 6.17, 3.73 and 2.89emu/g, respectively. Similarly, the Mr value is 1.15, 0.68 and 0.52 emu/g, respectively. It is observed that doping with Mn and La significantly reduced the particle size, which in turn enhanced the surface area of these synthesized particles. The surface area of undoped and doped particles which was determined by BET is 77.93, 80.23 and 87.45 m²/g, respectively.

The UV spectrophotometer was used for the measurement of photocatalytic performance of nanoparticles. It was observed from the results that 91.78% RhB dye was degraded in 240 minutes by doping with Mn and La metal ions. Degradation of RhB dye was increased from 43.30 to 89.34% with the increase of pH from 4 to 10 and increased by changing catalytic dosage, dye concentration and temperature. This excellent photodegradation effect of as-synthesized La_0.2Mn_0.2Fe_1.6O₃ nanoparticles revealed that these can be used more effectively for the removal of organic pollutants from sewage water

Acknowledgements

We acknowledge to the Institute of Chemistry, The Islamia University of Bahawalpur and Department of Chemistry, The Government Sadiq College Women University Bahawalpur for providing necessary research facilities.

Author Contribution

Sobia maqbool conducted experimental work, data analysis and wrote manuscript. Adeel Ahmadand Arif Mukhtar help in characterizing data. Muhammad Jamshaidhelp in experimental work. Prof. Dr. Aziz Ur Rehman and Dr. Saima Anjum help out in formulating ideas and research work. All authors read and approved the final manuscript.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Ethics approval and consent to participate Not applicable.

Consent for publication Not applicable.

Data availability Not applicable.

Competing interests The authors declare no competing interests.

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Editorial decision: Major Revision
26 May, 2022
Reviews received at journal
14 Apr, 2022
Reviewers invited by journal
14 Apr, 2022
Editor invited by journal
13 Apr, 2022
Editor assigned by journal
07 Apr, 2022
First submitted to journal
17 Mar, 2022

You are reading this latest preprint version

Efficient photocatalytic degradation of Rhodamine B dye using solar light-driven La˗Mn co-doped Fe2O3 nanoparticles

Status:

Version 1

Abstract

Figures

1. Introduction

2. Experimental

2.1. Chemicals

2.2. Fabrication of Fe₂O₃ nanoparticles

2.3. Preparation of La˗Mn co-doped Fe₂O₃ nanoparticles

2.4. Characterization

2.5. Photocatalytic activity

3. Result And Discussion

3.1. SEM analysis

3.2. TEM analysis

3.3. FTIR analysis

3.4. XRD analysis

3.5. DLS analysis

3.6. Magnetic properties

3.7. BET analysis

3.8. Photocatalytic activity

3.8.1. Effect of pH

3.8.2. Effect of catalytic dosage

3.8.3. Effect of dye concentration

3.8.4. Effect of temperature

3.9. Degradation mechanism of dye

3.10. Reusability of catalyst

4. Conclusion

Declarations

References

Status:

Version 1

Efficient photocatalytic degradation of Rhodamine B dye using solar light-driven La˗Mn co-doped Fe2O3 nanoparticles

Status:

Version 1

Abstract

Figures

1. Introduction

2. Experimental

2.1. Chemicals

2.2. Fabrication of Fe2O3 nanoparticles

2.3. Preparation of La˗Mn co-doped Fe2O3 nanoparticles

2.4. Characterization

2.5. Photocatalytic activity

3. Result And Discussion

3.1. SEM analysis

3.2. TEM analysis

3.3. FTIR analysis

3.4. XRD analysis

3.5. DLS analysis

3.6. Magnetic properties

3.7. BET analysis

3.8. Photocatalytic activity

3.8.1. Effect of pH

3.8.2. Effect of catalytic dosage

3.8.3. Effect of dye concentration

3.8.4. Effect of temperature

3.9. Degradation mechanism of dye

3.10. Reusability of catalyst

4. Conclusion

Declarations

References

Status:

Version 1

2.2. Fabrication of Fe₂O₃ nanoparticles

2.3. Preparation of La˗Mn co-doped Fe₂O₃ nanoparticles