Rare earth ions (La³⁺, Nd³⁺) substituted cobalt–strontium spinel ferrites for photocatalytic degradation of textile dyes

Despite being a minor part of the electromagnetic spectrum ranging from 100 to 400 nm, ultraviolet radiation has a significant impact on living things (Diffey 2002). Furthermore, excessive ultraviolet light can have a major impact on plant photosynthesis (Hollósy 2002; Verdaguer et al. 2017). Recently, photocatalytic degradation of dangerous chemical compounds utilizing ultraviolet light and a semiconductor material has gotten a lot of interest. This strategy is a simple way to reduce river and ecosystem pollution (Schneider et al. 2014; Di Mauro et al. 2017). So, for the purpose of water purification, degradation through photocatalysis is the most efficient solution for the elimination of a wide variety of dyes in a wide range of concentrations; hence, this application is beneficial (Mironyuk et al. 2019a, 2019b).

Spinel ferrite is considered as an important study material because of its low band gap and photocatalytic activity in the visible light area for the removal of pollutants (Niu et al. 2015a). Cobalt–Strontium (Co–Sr) spinel ferrite is a unique magnetic substance that has been used in biological, electronic, and recording technology (Bensebaa et al. 2004; Gao et al. 2009; Ahalya et al. 2014). Purity, shape, size, and attractive stability are all important factors in the photocatalytic activity and magnetic properties of ferrite nanoparticles (Haruna et al. 2020). Even though a large particle size of NPs may reduce photocatalytic activity by limiting the surface area of the catalyst, particles having smaller sizes likely to provide improved photocatalytic efficiency due to the high number of active sites (Giannakas et al. 2006). Ponraj et al. (2017) thoroughly discuss in their review article the work performed on the degradation of various dyes by using BiFeO₃ NPs. Further, they examined the surface phenomena of photocatalyst BiFeO₃, surface area (Wei et al. 2012; Soltani & Entezari 2013), and particle size (Gao et al. 2007; Liu et al. 2010; Gao et al. 2014; Niu et al. 2015b) and the way they impact photoactivity in their study on visible light-activated photocatalysts for the degradation of various textile dyes. The arrangement of divalent and trivalent ions in octahedral and tetrahedral locations determines the structure of spinel ferrite. Divalent compounds contain tetrahedral positions in normal spinel; however, in the inverse crystalline phase, the remaining trivalent ions occupy tetrahedral positions and the divalent ions occupy octahedral positions (Goldman 2006). Since several of the characteristics of ferromagnetic materials are reported to be influenced by the presence of impurity, significant research has been conducted that investigate the impact of cation contamination on the spinel ferrite lattice. The magnetic, electrical, and structural characteristics of ferromagnetic materials are altered when limited concentrations of these contaminants are added (Rana & Abbas 2002). Several studies have examined the effects of modifying the spinel crystalline structure of various ferromagnetic materials on their chemical characteristics as well as, as a result, magnetic permeability, their inherent characteristics of magnetization, anisotropy, electrical resistivity, Curie temperature, and magnetic resonance. Co–Sr spinel ferrite nanoparticles with lanthanum and neodymium substitutes have received a lot of attention throughout the area of nanomaterials because of their increased magnetic and electrical characteristics (Melagiriyappa & Jayanna 2009; Ishaque et al. 2010; Zhang & Wen 2010). Rare-earth ions (La³⁺, Nd³⁺) are potential replacements for enhancing the characteristics of spinel ferrites. The rare-earth element's 4f shell is insulated by 5s²5p⁶ and is nearly completely unaffected through the prospective domain of neighboring molecules. The magnetic and electrical transport characteristics of Co–Sr spinel ferrite nanoparticles could be improved via replacing rare-earth elements and existing 4f–3d interactions (Rezlescu et al. 2000). When these are compared to undoped spinel ferrites, structural properties such as magnetization, saturation, coercivity, and retentivity as well as anisotropy, vary significantly (Kumar & Kar 2012).

Chahar et al. (2021) used the citrate precursor approach to make Co_xZn_1−xFe₂O₄ nanoferrites with varying concentrations, x = 0.00, 0.01, 0.02, 0.03, 0.04, and 0.05. It was reported that the degradation efficiency of methylene blue reached 77% for x = 0.05, compared to the minimum of 65% for x = 0.00, in 60 min during exposure to sunlight. Vosoughifar & Kimiay (2016) reported the synthesis of CuFe_2−xNd_xO₄ photocatalyst through a sol–gel auto-combustion process and examined that methyl orange was degraded by 65% when exposed to UV light for 80 min. Naik et al. (2020) reported the fabrication of CoFe₂O₄ nanoparticles by sol gel route and also investigated that after 150 min of radiation, MB and EB had degraded 97 and 88%, respectively, due to low bandgap properties of prepared ferrites. Dhiman & Singhal (2019) synthesized CoRE_0.02Fe_1.98O₄ (RE = Eu, Gd, and Dy) spinel ferrites via a simple sol–gel auto-combustion method. For the composition CoEu_0.02Fe_1.98O₄, the maximum degradation of more than 90% was examined for both SO and RBY dyes, respectively, during 100 min of sunlight exposure. Abdo & El-Daly (2021) prepared Co_0.5Cu_0.5Sm_xFe_2−xO₄ (x = 0.00, 0.03, 0.06, 0.09, 0.12, and 0.15) nanoferrites by the citrate combustion approach. The nanoferrites demonstrated a degradation efficiency of 94.36% for RhB dye within 270 min at a concentration of x = 0.15, compared to Co_0.5Cu_0.5Fe₂O₄, which was 21.94%.

Co-precipitation (Maaz et al. 2009), hydrothermal method (Yan et al. 2015), and solid-state method (Deraz & Hessien 2009) are some of the methods used to make magnetic nanoparticles. The sol–gel approach, on the other hand, allows for good size control as well as a broad selection of reaction conditions for more modification (Zhang et al. 2015). The potential of sol–gel synthesis of oxide materials to produce nanoparticles exhibiting a stable pattern of crystalline solids, crystallite dimensions, and substantial surface area (Zhang et al. 2015) has piqued attention, in addition to its cheap cost and simple method.

The two dyes tested in this research, Congo red and rhodamine B, are azo dyes that are widespread in wastewater from industries and are very hazardous to human beings, causing skin irritation, nausea, blood clots, and asthma (Yu et al. 2009; Tavakoli-Azar et al. 2020). As a result, researchers have a significant task and aim to eliminate or reduce such toxins from the environment.

The goal of this study is to use the sol–gel process to synthesize pure, and rare-earth ions (La³⁺, Nd³⁺) substituted Co–Sr spinel ferrites with general formula Co_0.5Sr_0.5Fe₂O₄, Co_0.5Sr_0.5La_0.06Fe_1.94O₄ and Co_0.5Sr_0.5Nd_0.06Fe_1.94O₄ and to investigate its potential to degrade dyes in industrial water. Further, this study described the synthesis, characterization, and degradation efficiency of Congo red (CR) and rhodamine B (RhB) dyes.

In a typical sol–gel method, components such as chlorides, carbonates, and nitrates are employed; however, only nitrates are utilized in this work. To synthesize an undoped Co_0.5Sr_0.5Fe₂O₄ (x = 0.00) and two doped samples with a cubic-spinel structure, Sr(NO₃)₂ purchased from DAEJUNG, citric acid C₆H₈O₇, Co(NO₃)₂·6H₂O purchased from UNI-CHEM, Fe(NO₃)₃·9H₂O, La(NO₃)₃·6H₂O, and N₃NdO₉·6H₂O (99.9%) purchased from Merck-Germany and ammonia are utilized. Nitrates are dissolved in deionized water to prepare solutions of desired samples based on stoichiometric calculations. Lanthanum and neodymium are doped at a concentration of x = 0.06 to synthesize samples with the chemical compositions Co_0.5Sr_0.5La_0.06Fe_1.94O₄, and Co_0.5Sr_0.5Nd_0.06Fe_1.94O₄. To form a homogeneous mixture, the samples' solutions were prepared, and a magnetic stirrer was employed for constant stirring for about 10–20 min. After mixing well and preparing appropriate amounts of solution in three separate beakers, they were placed on magnetic hotplates. To keep the pH between 7 and 8, (35%) ammonia with a molarity of 51.67 mol/L was added randomly, drop by drop. The deionized water is evaporated from the solutions by heating below 80 °C and, once it evaporates, a dark brown wet gel is formed. It was then placed in an electrical thermostatic oven for 6 h approximately at 240 °C. To get loose powder, an agate mortar and pestle are employed. To achieve the final products, the fine powder is sintered in the furnace for 9 h at 950 °C. Schematic representation for the synthesis of Co_0.5Sr_0.5RE_xFe_2-xO₄ (x = 0.00 and 0.06) is shown in Figure 1”.

The XRD patterns of prepared spinel ferrites were accumulated over the range from 20° to 70° of 2value by XRD analysis using a Bruker D Z Phaser (PW 1830) diffractometer. The microstructures are revealed using a VEGA3 TESCAN scanning electron microscope. The optical bandgap and degradation of CR and RhB dyes for pure and rare-earth (La, Nd) doped Co–Sr spinel ferrites were determined using UV visible spectroscopy (AE-S90-2D).

Here, initial absorbance and final absorbance, respectively.

Here, Scherer's constant is shown as k = 0.9, the wavelength is shown as ⁠, full-width half maximum (rad) is shown as and Brag's angle is given as (Kokare et al. 2018).

In this case, θ denotes the peak's highest diffraction angle.

Here ‘D’ denotes for the crystalline size. Table 1 shows that the lattice parameter, cell volume, and average crystallite size all increase as lanthanum and neodymium ions are substituted. The findings for these structural properties are higher than those for pure Co–Sr spinel ferrite because of the formation of expansions and tensions inside the spinel structure. Additionally, the doping of La³⁺ and Nd³⁺ results in different values as a consequence of the higher ionic radii of lanthanum and neodymium ions over iron ions (Sharma et al. 2017). At the octahedral positions, La³⁺ and Nd³⁺ ions overlap with iron ions, which could have distorted the structure at the interstitial positions. As a result, the substitution of RE's metal ions may potentially affect structural properties like average crystallite size and lattice parameter. The values of lattice strain, micro-strain, and dislocation density decrease with the doping of lanthanum and neodymium, confirming that genuine spinel structure remains after the substitution of RE's metal ions (Yousaf et al. 2020).

Pure and rare-earth metal ions (La, Nd)-substituted Co_0.5Sr_0.5RE_xFe_2−xO₄ (x = 0.00, and 0.06) nanoferrites were prepared using an inexpensive sol–gel approach. Powder XRD analysis demonstrates the emergence of single-phase face-centered cubic-spinel structures. Since REs have wider ionic radii than iron elements, the substitution of these cations alters the lattice parameter and crystalline size. By substituting REs³⁺, the average crystallite size and optical energy bandgap values varied from 31 to 36 nm and from 2.91 to 2.52 eV, respectively. SEM analysis allowed for the identification of the microstructures of the synthesized ferrites. Inhomogeneous grains with average particle diameters ranging from 0.4 to 0.75m were seen in the SEM pictures of the prepared spinel ferrites. The substitution of La³⁺ and Nd³⁺ ions enhanced the degradation activity of the Co–Sr spinel ferrite samples. In this study, we achieved a significant improvement in the photocatalytic activity of CR and RhB dyes, attaining 90 and 85%, respectively. Amazingly, the nano-ferrite sample Co_0.5Sr_0.5Nd_0.06Fe_1.94O₄ exhibited the highest percentages of CR and RhB degradation over the duration of 60 min because it had the lowest energy gap, and it can be used in treatments to clean up environmental pollutants effectively (Table 3).

This work is acknowledged by HEC-funded project NRPU#10408.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.