Efficient and recyclable Nd³⁺-doped CoFe₂O₄ for boosted visible light-driven photocatalytic degradation of Rhodamine B dye

Abstract

Rare earth metal doping spinel ferrites offer excellent electronic, magnetic, and photocatalytic properties, but they have not been well explored for environmental mitigation. Herein, we report the facile fabrication of novel CoNd_xFe_2−xO₄ (x = 0–0.05) photocatalysts based on Nd³⁺ incorporated into CoFe₂O₄ for the degradation of Rhodamine B under visible light irradiation. The Nd³⁺ dopant considerably increased the specific surface area (35 m² g⁻¹) and enhanced the degradation performance (94.7%) of CoNd_xFe_2−xO₄ catalysts. Nd³⁺-doped CoFe₂O₄ played a role in the formation of radicals, including ˙OH, h⁺, and ˙O₂⁻. With high recyclability and performance, CoNd_0.05Fe_1.95O₄ nanoparticles can be efficient and reusable photocatalysts for degrading organic dyes, including Rhodamine B from wastewaters.

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1. Introduction

Discharging the effluent containing organic dyes into water sources has been recently posing enormous impacts on aquatic ecosystems.¹ The nature of dyes is water-soluble and low biodegradable, accumulating in durable wastewaters.² Consequently, they increase chemical and biochemical oxygen demands, which depletes water-dissolved oxygen, thereby causing the biological disruption of the photosynthetic activities of aquatic microorganisms.³ The conventional methods of wastewater management, such as membrane filtration and coagulation processes, are mismatched with cost-effectiveness and performance.⁴ Therefore, an increasing demand for the development of effective treatment techniques has been addressed. The visible light-driven photocatalytic dye degradation process is proposed as a feasible method with the main advantages of treatment efficiency and technical simplicity.⁵

Over the past years, nanostructured photocatalysts, including but not limited to TiO₂, ZnO, and WO₃, have observed significant growth and have been utilized for various applications in wastewater treatment, electrochemical, energy storage, and sensing.^6,7 Ferrite-based semiconductor photocatalysts are of great interest owing to their unique electronic structure and magnetism.⁸ Among common ferrites, CoFe₂O₄ has a narrow band gap energy of 1.6–2.4 eV, making it capable of harvesting photons from visible light.⁹ Despite such potential, CoFe₂O₄ nanoparticles still show limited photocatalytic activity because rapid recombination between electrons and holes can significantly lower the catalytic performance of CoFe₂O₄.¹⁰ To address this issue, researchers have explored the use of CoFe₂O₄ with various rare earth dopants, such as Dy and Ce, to intercept this recombination and improve photocatalytic efficiency. Chen et al. reported the partial substitution of Fe³⁺ with Dy³⁺ to create a new lattice of CoFe₂O₄.¹¹ These authors observed that Dy³⁺-doped CoFe₂O₄ exhibited good photocatalytic performance against methyl orange (78.7%). Zhu et al. synthesized Ce³⁺-doped CoFe₂O₄ using the hydrothermal method and found an orange II degradation percentage of 98.5% for 60 min.¹² However, only few studies have investigated the introduction of rare earth metals into CoFe₂O₄ and their application in the photocatalytic removal of dyes.

Herein, we incorporated neodymium (Nd) into CoFe₂O₄ to enhance the photocatalytic efficiency of CoNd_xFe_2−xO₄. Nd with an electronic configure ([Xe]4f⁴6s²) is one of the most reactive lanthanides. It is hypothesized that Nd³⁺ substitution into the lattice of CoFe₂O₄ leads possibly to the formation of oxygen vacancies and surface defects, enhancing the electron transfer and hindering the recombination capability of electrons and holes.¹³ Although the synthesis and magnetic properties of CoNd_xFe_2−xO₄ have been reported previously, the photocatalytic performance of this nanomaterial has not yet been investigated. Therefore, this study aims to synthesize and investigate photocatalytic CoNd_xFe_2−xO₄ with different Nd³⁺ doping ratios from 0 to 5% by molar. The structure of the CoNd_xFe_2−xO₄ nanocomposites was analyzed, and their photocatalytic activity was examined under visible light. In addition, the plausible mechanism of Rhodamine B degradation in the presence of CoNd_xFe_2−xO₄/H₂O₂/visible light catalytic system was suggested.

2. Experimental

2.1. Synthesis of Nd-doped CoFe₂O₄ nanocomposites

Nd³⁺-doped CoFe₂O₄, other names CoNd_xFe_2−xO₄ (x = 0, 0.01, 0.03, 0.05), could be facilely synthesized as follows. The process involved dissolving 500 mg of urea, 1 mmol of cobalt nitrate, (2−x) mmol of iron(III) nitrate and x mmol of neodymium nitrate in 30 mL of pure H₂O. The liquid was heated to 100 °C under stirring, maintained at this temperature for 6 h, and dried in an oven overnight. The solid was calcined at 500 °C for 4 h to obtain the final CoNd_xFe_2−xO₄ products.

2.2. Photocatalytic experiments

To assess the efficiency of Nd³⁺-doped CoFe₂O₄ nanocomposites in photodegradation reaction, a visible-light source of 30 W provided by LED lamp emitting light within the wavelength range of 400–700 nm filtered by a UV filter, with a rate luminous flux of 2300 lm, and a color rendering index of 80 was used. The light source was fixed 12 cm higher than the RhB solution surface. To begin the experiment, a fixed amount (0.75 g L⁻¹) of the CoNd_xFe_2−xO₄ was added to 0.1 L of 10 mg L⁻¹ Rhodamine B solution. Subsequently, a 30% hydroperoxide solution was slowly dropwised to reach a concentration of 0.1 M. After sorption in the dark was equilibrated for 60 min, the photocatalytic system was exposed to a visible light source for 0–180 min. The concentration of RhB was calibrated using a UV-Vis spectrophotometer at 554 nm. The effect of CoNd_xFe_2−xO₄ dose on RhB degradation efficiency was examined at 0.5, 0.75, and 1 g L⁻¹, while H₂O₂ concentration was tested at 0.05, 0.1, and 0.15 M.

2.3. Recyclability study

After the catalytic experiment was completed, the CoNd_0.05Fe_1.95O₄ catalyst was separated from the reaction and washed with pure water and ethanol to clean the Rhodamine B dye. Afterward, the reused CoNd_0.05Fe_1.95O₄ was dried overnight in an oven and used as a catalyst for further experiments. In detail, 0.75 g L⁻¹ of recycled CoNd_xFe_2−xO₄ and 30% H₂O₂ with a concentration of 0.1 M were added to 0.1 L of 10 mg L⁻¹ Rhodamine B solution. After 60 min of stirring in the dark, the photocatalytic system was exposed to a visible light source for 180 min. This process was repeated until the degradation efficiency against Rhodamine B dye was significantly reduced.

3. Results and discussion

3.1. Characterization

The X-ray diffraction patterns of CoNd_xFe_2−xO₄ (x = 0, 0.01, 0.03, 0.05) nanoparticles are characterized, as shown in Fig. 1a. Diffraction patterns indicated the presence of well-defined peaks at 2θ degrees for the as-synthesized CoFe₂O₄, with lattice planes at 30.2° (220), 35.3° (331), 43.2° (400), 54.8° (422), 57.1° (511), and 62.7° (440) matching the standard pattern of spinel cubic CoFe₂O₄ structure as per JCPDS card no. 03-0864. The presence of ortho-ferrite NdFeO₃ single phase became more prominent with increased Nd³⁺ dopants caused by the substitution of Fe³⁺ by Nd³⁺. Because Nd³⁺ ions have a larger radius than those of Fe³⁺, the atomic substitution in the CoFe₂O₄ lattice is restrained, resulting in the aggregation of Nd³⁺ on the grain boundary to form NdFeO₃.¹⁴ As Fe³⁺ substituted by Nd³⁺ increased, the peak intensity at the (311) plane increased, and a slight shift in the (311) angle as a result of unstable d-spacing was observed in Table S1,† confirming the transfer of Nd³⁺ ions into octahedral and tetrahedral sites to replace Co²⁺/Fe³⁺ ions. CoNd_xFe_2−xO₄ crystallite sizes were calculated based on the Scherrer equation for full width at half maximum of the (311) plane. Table S1† shows an upward crystallite size trend (14.35–29.14 nm) with increasing Nd³⁺ dopant, which suggests that Nd³⁺ substitution might extend unit cells, leading to increased lattice constants.

Fig. 1 (a) XRD patterns, (b) FTIR spectra, (c) N₂ adsorption–desorption isotherm curves, (d, e) SEM images, (f, g) TEM images, and (h, i) particle size distribution histograms of (1) CoFe₂O₄, (2) CoNd_0.01Fe_1.99O₄, (3) CoNd_0.03Fe_1.97O₄ and (4) CoNd_0.05Fe_1.95O₄ catalysts.

The surface chemistry of Nd-doped CoFe₂O₄ can be examined using the FT-IR spectra, as illustrated in Fig. 1b. The two transmittance bands ν₁ and ν₂ shown in Table S1† reflect tetrahedral and octahedral metal (Fe, Nd)-oxygen bonds, respectively. Absorption bands shifting increasingly to the high-frequency band were attributable to altering lattice parameters and ionic redistribution as a result of the incorporation of rare earth-like Nd³⁺ into the spinel CoFe₂O₄ structure.¹⁵ The optical characteristics and Tauc plots of (αhν)² versus photon energy (hν) of CoNd_xFe_2−xO₄ can be evaluated using UV-Vis DRS spectra, as depicted in Fig. S1.† It is noticeable that the bandgap energy (E_g) decreased (1.57–1.35 eV) as the Nd³⁺ doping (0–5%) increased, confirming the change in the electric structure of CoNd_xFe_2−xO₄ a result of the bandgap renormalization effect.¹⁶ Specifically, a dynamic screening of Coulomb repulsion may decrease the electronic bandgap of the CoNd_xFe_2−xO₄ semiconductor as it partially cancels the Moss–Burstein shift.¹⁷ The result contradicted an enhanced bandgap caused by the quantum confinement effect as reported previously.¹⁸ Moreover, magnetic properties of CoNd_xFe_2−xO₄ via vibrating sample magnetometer showed that magnetic hysteresis loops were smaller and saturation magnetization values decreased from 47 to 29 emu g⁻¹ with higher Nd³⁺ doping in CoFe₂O₄ (Fig. S2†). The magnetic results herein agree with the previous report,¹⁹ which can be normally due to the lower paramagnetic moment of substituted Nd³⁺ ions than ferromagnetic Fe³⁺ ions.

Fig. 1c shows N₂ adsorption/desorption isotherm plots of CoFe₂O₄ and CoNd_0.05Fe_1.95O₄ with the characteristics of Type II (nonporous or macroporous) based on IUPAC classification. CoFe₂O₄. However, a minor hysteresis loop (H₂) of CoNd_0.05Fe_1.95O₄ rather than CoFe₂O₄ was observed, indicating a disordered, well-undefined and defected interface of CoNd_0.05Fe_1.95O₄.²⁰ Incorporating Nd³⁺ ions into CoFe₂O₄ lattice caused the redistribution of Co²⁺/Fe³⁺ cations and defected crystal lattice.²¹ The specific surface area of CoNd_0.05Fe_1.95O₄ was 35.0 m² g⁻¹ compared with 12.7 m² g⁻¹ for CoFe₂O₄ (Table S2†). The total pore volume of CoNd_0.05Fe_1.95O₄ (0.114 cm³ g⁻¹) was also higher than that of CoFe₂O₄ (0.099 cm³ g⁻¹), suggesting that Nd³⁺ substitution improved the porosity of the origin spinel ferrite. It was interpreted that doping of neodymium elements might optimize the surface structure of cobalt ferrite, leading to a decrease in grain size with an enhanced porous structure and surface area. The substitution of other rare earth metals (e.g., Ce³⁺, La³⁺, and Y³⁺) into CoFe₂O₄ was also found to significantly increase the surface area (4.39–6.95 m² g⁻¹) of rare earth-modified CoFe₂O₄ compared with bare CoFe₂O₄ (2.13 m² g⁻¹) as reported by Gao et al.²² With higher area and defected surface, CoNd_0.05Fe_1.95O₄ was expected to have more active sites for photocatalytic performance.

The morphology and inherent structure of CoFe₂O₄ and CoNd_0.05Fe_1.95O₄ are examined by SEM/TEM images, as illustrated in Fig. 1d–g. CoFe₂O₄ and CoNd_0.05Fe_1.95O₄ exhibit nanospherical nanoparticles, with a slight degree of clustering, which was highly commensurate with the morphology of Nd³⁺ doped CoFe₂O₄ as published previously.^21,23 A minor decrease in average particle size from 35 nm (CoFe₂O₄) to 30 nm (CoNd_0.05Fe_1.95O₄) is shown in Fig. 1h and g. This trend can be explained that Nd³⁺ incorporation might intercept the crystal growth of CoFe₂O₄ during combustion synthesis. Fig. S3† illustrates the chemical composition of nanocomposites with three major elements (11.44% Co, 26.94% Fe, and 61.62% O) for CoFe₂O₄ and four major elements (13.41% Co, 21.34% Fe, 64.59% O, and 0.66% Nd) for CoNd_0.05Fe_1.95O₄. Otherwise, no unusual peaks in the EDX spectra suggest that the samples are highly pure.

3.2. Photocatalytic study

The catalytic photodegradation of Rhodamine B dye as a test substance in the presence of CoNd_xFe_2−xO₄ (x = 0, 0.01, 0.03, and 0.05) nanoparticles was conducted, as depicted in Fig. 2a. As depicted in Table S3,† the Rhodamine B removal percentage was almost insufficient (12.9–29.4%) in the absence of either the CoFe₂O₄ catalyst or H₂O₂ oxidant under visible light. The reaction rates were also slow (0.7 × 10⁻³–1.4 × 10⁻³ min⁻¹) in these cases, indicating that Rhodamine B dye could be removed owing to adsorption onto CoFe₂O₄. When both 0.1 M H₂O₂ and CoFe₂O₄ catalysts were added under visible light conditions, 71.7% of Rhodamine B dye was removed by CoFe₂O₄. It is therefore believed that the catalyst, oxidant, and visible light source played a necessary role in the photocatalytic degradation of Rhodamine B dye. With increasing ratios of Nd³⁺ doping (0–0.05), the catalysts exhibited improved catalytic activity for the degradation of Rhodamine B dye (68.2–94.7%). The highest degradation efficiency and kinetic rate were observed under a CoNd_0.05Fe_1.95O₄/H₂O₂/visible light system at 94.7% for 180 min and 5.3 × 10⁻³ min⁻¹, respectively. With a higher doping of Nd³⁺ (x = 0.01), however, the CoNd_0.1Fe_1.9O₄ showed lower catalytic activity, with 85.1% of RhB degradation efficiency. This finding could, therefore, be because of the contribution of 5% Nd³⁺ substituted in CoFe₂O₄ lattice that is enough for accelerating electron transfer and prevention of electron–hole recombination.

Fig. 2 (a) Rhodamine B degradation in the presence of (1) H₂O₂ only, (2) CoFe₂O₄ catalyst only, (3) H₂O₂ + CoFe₂O₄, (4) H₂O₂ + CoNd_0.01Fe_1.99O₄, (5) H₂O₂ + CoNd_0.03Fe_1.97O₄, (6) H₂O₂ + CoNd_0.05Fe_1.95O₄, and (7) H₂O₂ + CoNd_0.1Fe_1.9O₄; (b) comparative reduction efficiencies of chemical oxygen demand index among various catalysts; (c) impact of CoNd_0.05Fe_1.95O₄ loadings (0.5, 0.75, and 1 g L⁻¹); (d) impact of H₂O₂ concentrations (0.05, 0.1, and 0.15 M).

To assess the amount of oxygen necessary to oxidize the organic matter in aquatic wastewaters, the chemical oxygen demand (COD) index was measured. The results of the photocatalytic study of CoNd_xFe_2−xO₄ nanocomposites on the removal of Rhodamine B dye are shown in Fig. 2b. The COD percentage obtained after 360 min is as follows: CoFe₂O₄ (71.7%) < CoNd_0.01Fe_1.99O₄ (76.5%) < CoNd_0.03Fe_1.97O₄ (81.2%) < CoNd_0.05Fe_1.95O₄ (89.5%). The best catalyst, CoNd_0.05Fe_1.95O₄, was able to significantly reduce the COD value of the Rhodamine B wastewater sample from 394.7 mg L⁻¹ to 41.0 mg L⁻¹. This result demonstrates an efficient Rhodamine B mineralization of CoNd_0.05Fe_1.95O₄ into less toxic compounds, such as CO₂, H₂O, and N₂.^24,25 Several photocatalytic degradation pathways for the mineralization of Rhodamine B could be deethylation, ring opening, and degradation of aryl chromophore.²⁶ Overall, our results indicated a good degree of RhB dye mineralization catalyzed by CoNd_0.05Fe_1.95O₄, which can make dyes-containing wastewater cleaner and safer.

Several factors (e.g., catalyst loading and H₂O₂ concentration) can influence the visible light photocatalytic degradation reaction of Rhodamine B. Fig. 2c shows that the optimal amount of CoNd_0.05Fe_1.95O₄ catalyst was 0.75 g L⁻¹, which led to a 93.4% removal efficiency of the Rhodamine B dye. Furthermore, the influence of H₂O₂ concentration was studied, with oxidant concentrations ranging from 0.05 to 0.15 M. Fig. 2d depicts that 0.1 M H₂O₂ concentration was found to be optimal, as this oxidant provided a sufficient amount of reactive oxygen radicals for the degradation of dyes.

Scavenging experiments can be performed to comprehend the significance of radicals, such as ˙OH and ˙O₂⁻, under CoNd_0.05Fe_1.95O₄/H₂O₂/visible-light system. Fig. 3a illustrates that the Rhodamine B degradation efficiencies of Rhodamine B dye over CoNd_0.05Fe_1.95O₄ catalyst were considerably reduced (49.8–61.6%) as one of the scavengers for trapping radicals, e.g., ascorbic acid (˙O₂⁻), ethylenediaminetetraacetic acid (h⁺), and isopropyl alcohol (˙OH), was added into the reaction. Table S4† shows a comparison of degradation efficiencies and pseudo-first order kinetic rates (k₁) as follows: EDTA (49.8%, 2.7 × 10⁻³ min⁻¹) < isopropyl alcohol (52.1%, 2.9 × 10⁻³ min⁻¹) < ascorbic acid (61.6%, 3.4 × 10⁻³ min⁻¹) < no scavengers (93.7%, 5.3 × 10⁻³ min⁻¹). Because the degradation efficiency did not differ significantly under various scavengers, radical species, including ˙OH, ˙O₂⁻, and h⁺, might take main responsibility for the degradation of Rhodamine B.

Fig. 3 (a) Rhodamine B removal efficiencies under CoNd_0.05Fe_1.95O₄/H₂O₂/visible light photocatalytic system, (1) no scavenger was added, (2) ascorbic acid was added, (3) ethylenediaminetetraacetic acid was added, and (4) isopropyl alcohol was added (4); (b and c) plausible Rhodamine B degradation mechanisms using CoNd_0.05Fe_1.95O₄/H₂O₂/visible-light system; and (d) recyclability study for CoNd_0.05Fe_1.95O₄ catalyst.

The role of radicals (˙OH, ˙O₂⁻, and h⁺) on Rhodamine B degradation catalyzed by CoNd_0.05Fe_1.95O₄ under visible light is elucidated herein, as demonstrated in Fig. 3b. Initially, incorporating Nd³⁺ into CoFe₂O₄ changes the electronic density and metal–oxygen bonding energy in CoFe₂O₄ crystal lattice, thereby exerting the formation of oxygen vacancies and defected surface on CoNd_0.05Fe_1.95O₄ photocatalyst.²³ Partially filled f-orbitals of Nd³⁺ ions (Nd³⁺/Nd²⁺, E⁰ = −0.40 V vs. NHE) enable visible light-photoexcited electrons to transfer easily from the valence band of CoFe₂O₄ to the Nd³⁺ doping energy level in NdFeO₃.²⁷ Consequently, Nd³⁺ f-orbitals might hinder the photogenerated e^–/h⁺ pair recombination, thereby improving degradation performance. Fig. 3c illustrates the interaction between e^– and O₂ to create ˙O₂⁻ and between e^– and H₂O₂ to create ˙OH. Holes (h⁺) interact with H₂O/OH⁻ to form ˙OH. It is suggested that ˙OH and ˙O₂⁻ species reacted with Rhodamine B to degrade into mediators, fragments and final products, such as H₂O and CO₂. To assess the stability of CoNd_0.05Fe_1.95O₄, we examined Rhodamine B degradation efficiency after the photocatalytic cycle. As shown in Fig. 3d, CoNd_0.05Fe_1.95O₄ can be reused several times, and the final cycle showed a degradation efficiency of 78%, suggesting that this catalyst had high stability. To check the crystalline structure of the CoNd_0.05Fe_1.95O₄ catalyst after use, XRD patterns were examined, as depicted in Fig. S4.† The primary peaks at (220), (311), (400), (422), (511), (440), and (533) were still maintained, suggesting that the CoNd_0.05Fe_1.95O₄ structure was stable after the recycling process.

The photocatalytic efficiency of CoNd_0.05Fe_1.95O₄ can be compared with other catalysts. As depicted in Table 1, CoNd_0.05Fe_1.95O₄ in this study demonstrated higher dye degradation performance (94.7%) than other photocatalysts. Moreover, this reaction was conducted under mild conditions, i.e., a visible light source instead of a UV light condition, as reported in previous studies. This comparison suggests that the CoNd_0.05Fe_1.95O₄ could be a competitive photocatalyst for degrading hazardous dyes, such as Rhodamine B, in wastewater under visible light condition.

Table 1 Comparative data on the Rhodamine B degradation efficiency of various photocatalysts

No.	Nanocomposite	Light source	H (%)	Ref.
1	CoNd0.05Fe1.95O4	Visible light	94.7	This work
2	ZnFe2O4-50%@ZnO	Visible light	79	28
3	NiFe2O4@HAp-Sn^2+	Visible light	84.4	29
4	NiFe2O4	Visible light	90	30
5	ZnFe@CuS	Visible light	93	31
6	ZnFe2O4	Visible light	94	30
7	MIL-101(Cr)/RGO/ZnFe2O4	Visible light	94	24
8	ZnFe2O4/graphene oxide	Visible light	94	32
9	Ni0.5Zn0.5Fe2O4	Visible light	98	30
10	Zn-doped Fe3O4	UV light	97	33
11	NiFe2O4	UV light	84	34
12	AuSe QDs@Cs2Fe2O4	UV light	99.2	35
13	Mg0.4Zn(0.6−xx)CaxFe2O4	UV light	99.5	36
14	Chitin biochar-based ZnFe2O4	Solar light	100	37

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4. Conclusion

Novel CoNd_xFe_2−xO₄ catalysts with various amounts of Nd³⁺ dopant were synthesized and characterized. Nd³⁺ substitution into CoFe₂O₄ lattice changed its electronic and magnetic structure. Bandgaps and saturation magnetization decreased as the Nd³⁺ doping increased. Moreover, CoNd_0.05Fe_1.95O₄ obtained small particle sizes and a large surface area (35 m² g⁻¹). The highest degradation efficiency was obtained (94.7%) at 0.75 g L⁻¹ of CoNd_0.05Fe_1.95O₄ and 0.1 M H₂O₂ under visible light irradiation. The main radical species, such as ˙OH, h⁺, and ˙O₂⁻, were found to influence the mechanism of Rhodamine B dye degradation. Additionally, the catalyst demonstrated good stability and recyclability. These results suggest that CoNd_0.05Fe_1.95O₄ nanoparticles can be an efficient and reusable photocatalyst for degrading organic dyes in water.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This research is funded by Thai Nguyen University of Education under grant number TNUE-2022-05.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ra00971h

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