Degradability of methylene orange synthetic dyes of multiferroic NiFe₂O₄–Ba_0.8Sr_0.2TiO₃ composites

Today, the research and application of multiferroics are increasingly developing, attracting many scientists worldwide. Although multiferroic materials have simultaneous existence of ferroelectric (or anti-ferroelectric) and ferromagnetic (or anti-ferromagnetic) properties, especially electromagnetic correlation in materials, the electrical properties can be controlled by magnetic fields, and vice versa.^1–3 Multiferroic composites possess much stronger electromagnetic effects than structural single-phase multiferroics. Thus, they attract many application studies in logical memories, electromagnetic memories, magnetic field sensors, energy harvesters, magnetoelectric resonators, and the read head.^3–5 Besides, multiferroic composites are also considered as potential materials for water pollution treatment, dye discoloration, and decomposition of organic compounds under light ultraviolet, sunlight, or ultrasonic vibration.^6,7

Currently, dyes have properties that resist the effects of detergents, water, and sunlight [methyl orange (MO), Rhodamine B (RhB), Methylene Blue (MB), etc.], so they are increasingly difficult to decompose. Azo dyes such as MO account for over 50% of global dye production, equivalent to 700 000 tons per year. This dye is used in the pharmaceutical, textile, and paper industries. However, the remaining dye discharged into the environment accounts for about 15% of the total.^8–10 MO dye is highly toxic, so it can pollute water sources, pollute the environment, destroy ecosystems, and even cause cancer among humans.^10,11 Therefore, the treatment of water contaminated with MO dyes is considered urgent and necessary. Among the current treatment methods, photocatalysis is the dominant method to treat water contaminated by dyes and organic chemical products.

Photocatalysis is a process where a semiconductor absorbs photons from ultraviolet rays or sunlight, generating electrons and holes that can induce chemical reactions or generate an electric current. To enhance the efficiency of photocatalytic reactions, it is crucial to select materials with a bandgap energy that can absorb ultraviolet light or sunlight and exhibit low electron–hole recombination rates. Recent studies have shown that ABO₃ (BaTiO₃ and SrTiO₃) compounds possess ferroelectric properties and have potential applications in photocatalytic degradation of synthetic dyes as noble metal-free catalysts for oxygen production at low cost.¹² ABO₃ nanoparticles have also been prepared and tested as photocathode catalytic components in microbial fuel cells.¹³ Among these, BaTiO₃ (BTO) is a ferroelectric material with a simple and flexible structure, making it suitable for various applications. One of its notable properties is its ability to decompose organic pollutants, making it environmentally friendly.^14–17 BTO is known to be an n-type semiconductor, with valence band (VB) positions at ∼2.31 eV and conduction band (CB) positions approximately at −0.83 eV. The bandgap energy (E_g) of BTO is typically in the range of 3.0–3.3 eV.¹⁸ 

The photocatalytic reaction efficiency depends on the electrical polarization of the material as improved electrical polarization results in better separation of charge carriers inside the bulk or on the surface of the semiconductor photocatalyst. Therefore, improving the electrical polarization of BTO-based ferroelectric materials is an important issue. Recent studies have doped Sr into the sublattice of BTO to make Ba_1−ySr_yTiO₃ compounds. As previously reported, Ba_1−ySr_yTiO₃ is a ferroelectric perovskite ceramic with a high dielectric constant, exhibiting excellent dielectric properties and good electrical polarization, making it suitable for various applications, such as supercapacitors, ferroelectric memory, piezoelectric sensors, and gas sensors.^19–22 

Ba_1−ySr_yTiO₃ is essentially a perovskite-based solid solution of BTO and SrTiO₃ (STO), and factors such as the Ba/Sr ratio, phase purity, and particle size directly affect the characteristic properties of Ba_1−ySr_yTiO₃ materials.^21–26 As the concentration of Sr substituting for Ba gradually increases, the Curie temperature of the material is gradually reduced.^27–29 Among these, Ba_0.8Sr_0.2TiO₃ (BSTO) is considered typical. The crystal structure of BSTO exhibits high flexibility, allowing for sensitive response to external stress under conditions near room temperature or below 120 °C.^25,30 It has also shown that the energy band structure and CB and VB positions of BSTO are similar to those of BaTiO₃.^22,30,31 In 2018, Yuan et al.³⁰ demonstrated that under the influence of ultrasonic vibration for 120 min, Ba_1−ySr_yTiO₃ nanowires could completely decompose the MO dyes. As the Ba_1−ySr_yTiO₃ crystal is excited by ultraviolet light or mechanical stress, electrons on the CB can be transferred to the solution and reduce O₂ dissolved in water to generate •O²⁻ radicals. At the same time, the holes on the VB will oxidize OH⁻ to generate •OH⁻ radicals.³⁰ Furthermore, Kanamadi et al.²⁵ demonstrated that the composites formed by the combination of ferrite spinel and BSTO exhibit excellent magnetic and dielectric properties. As excited with reasonable energy, at the same time, two types of charge carriers can appear in the material, specifically electrons and holes. In addition, on the octahedral sites, the electron exchange between Fe²⁺ and Fe³⁺ ions contributes to the local displacement in the direction of the external electric field and the dielectric polarization in ferromagnetic materials.²⁵ Besides, MFe₂O₄ (M = Fe, Ni, Co) ferrite spinels are semiconductors with various advanced properties, such as good chemical stability, good magnetism, narrow bandgap, and visible light operation with high potential electrical performance.^31–33 Dang et al.¹⁵ successfully decomposed MB synthetic dyes using a CoFe₂O₄–BaTiO₃ composite. With a photocatalyst concentration of 0.4 g/l and under the effect of UV light, the decomposition efficiency reached 72% after irradiation for 180 min.¹⁵ Among magnetic semiconductors, NiFe₂O₄ (NFO) is considered a typical compound. NFO has an inverse spinel structure and exhibits properties such as high permeability, low anisotropy, large resistivity, and high piezomagnetism.^34–37 It is a chemically stable and photocatalytic semiconductor with a narrow bandgap, corresponding to the CB and VB positions at approximately −0.6 and 1.28 eV, respectively.^38,39 However, due to the rapid recombination of photoelectron–hole pairs, the photocatalytic potential of NFO is limited. The successful combination of NFO with BSTO to create NFO/BSTO multiferroic nanocomposites with strong photocatalytic ability is considered an innovative idea. Changing the NFO content in the composites can result in positive changes in the physical properties of NFO/BSTO multiferroics. These results have significance for the development of future research-oriented applications of multi-materials in the field of water pollution treatment contaminated with dyes and organic compounds by photocatalysis and for the recovery of photocatalyst materials using the magnetic field.

Multiferroic composites with component formula xNFO/(1 − x)BSTO (NFO/BSTO) were synthesized using raw chemicals, including high purity (≥ 99.9%) BaCO₃, SrCO₃, TiO₂, NiO, and Fe₂O₃ powders. Initially, the particulate composites with BSTO ferroelectric and NFO ferromagnetic phases were fabricated separately using a Spex-8000D machine. The respective raw chemical powder mixtures of BSTO and NFO phases were ground and mixed separately in air for 10 h at 875 rpm. The powder mixtures obtained after grinding and mixing were then pressed into pellets under a pressure of 7000 kg/cm². The pellets corresponding to each phase of the materials were sintered separately in the air at 1250 and 1000 °C to successfully fabricate BSTO and NFO samples, respectively. Subsequently, the NFO and BSTO samples were subjected to high-energy ball milling for 10 h in the air, resulting in the formation of fine powders as the final products.

Next, the NFO and BSTO powders were weighed according to the molar ratio of x = 0, 0.1, 0.2, 0.3, and 0.4 to create the xNFO/(1 − x)BSTO (NFO/BSTO) composites denoted as BSTO, BSNF1, BSNF2, BSNF3, and BSNF4, respectively. After weighing, the powder mixture was thoroughly mixed and then pressed into pellets under a pressure of 7000 kg/cm². These pellets were then incubated for 6 h at 800 °C to form solid pellets of the same size. The structure of all samples was analyzed using x-ray diffraction (XRD) patterns obtained from an Equinox 5000 device from Thermo Fisher Scientific, using Cu-K_α radiation (λ = 1.54056 Å) with a voltage of 40 kV and a current of 40 mA. An acquisition time of 45 min per sample and 2θ step size of 0.015° were used. Using field emission scanning electron microscopy (FE-SEM) S4800 equipment (Hitachi), the morphology and particle size of the samples were determined. The composition of chemical elements and the distribution of substances in samples were analyzed by Aztec Energy dispersive spectroscopy (EDS) equipment from Oxford Instruments (UK). The characteristic ferromagnetic and ferroelectric properties of the samples were evaluated through the M(H) and P(E) hysteresis curves, which were measured on a vibrating sample magnetometer and a Precision LC II model 609 system at room temperature, respectively. The typical optical properties of the samples were specifically determined based on the analysis of the UV–Vis spectrum carried out on the Cary 5000 UV–Vis–near-infrared spectrophotometer instrument. MO solution was used to evaluate the photocatalytic performance of NFO/BSTO nanocomposites. Accordingly, 0.2 g of powder from each NFO/BSTO sample was stirred in 1000 ml of MO (0.2 mg/l, PH = 7) solution in the dark for 60 min to ensure equilibrium. After that, an ultraviolet lamp with a luminous flux intensity of 22 mJ/cm² and a wavelength of 254 nm (model 30E, OAI, USA) was used to stimulate the photocatalytic reaction at different irradiation times.

SEM images present the morphology and size of the particles in all samples. The SEM images of two representative samples of BSTO and BSNF4 are shown in Figs. 2(a) and 2(b), respectively. Based on the results of determining the particle size in various regions of the samples, the average value and particle size distribution were constructed and are presented representatively in the insets of Figs. 2(a) and 2(b). We can see that the particle sizes of the particles in the samples are distributed over a quite wide range with an average value of ∼69 nm. Because the sample conducts electricity poorly, the grain boundaries are unclear, and clumps between the particles occur. Besides, the particle size in the sample did not change with the sample’s composition, as well as the content of NFO. However, the particle size determined from SEM images is significantly larger than the crystallite size, suggesting that each particle contains more than one crystal.

The energy dispersive x-ray (EDX) spectra demonstrate the presence of Ba, Sr, Ti, Ni, O, and Fe elements. These elements are all derived from the original raw chemicals, and no other foreign elements appear. This is demonstrated by the EDX spectra of two representative samples of BSTO and BSNF4, as presented in Figs. 2(c) and 2(d). The atomic percentages of chemical elements in the samples are quite consistent with the expected sample composition. Besides, chemical elements such as Ba, Sr, Ti, O, Fe, and Ni are also distributed quite uniformly as desired in the samples. This is shown in the EDX mappings for two representative samples of BSTO and BSNF4 in Figs. 3 and 4, respectively.

Figure 5 depicts the hysteresis M(H) curves for all samples at room temperature. It shows that all the samples exhibit ferromagnetic characteristics except the BSTO sample, which is a paramagnetic compound with very low value of magnetization. However, the magnetization of the NFO/BSTO composites increases rapidly with NFO concentration. For NFO content increased in the range corresponding to x = 0.1–0.4, the value of the saturation magnetization (M_s) increases in the range of 4.7–17.5 emu/g, and the value of the residual magnetization (M_r) increases in the range of 0.36–1.78 emu/g, see the inset in Fig. 5 and Table I. These can be explained by the vital contribution of the magnetic moments of the ferromagnetic phase of NFO. The results of this study are considered to be relatively consistent with the results of previous studies on this family of multiferroics.^6,7,42–44 Improving the magnetization of NFO/BSTO composites makes them more useful for recovery and reuse by using external magnetic fields.¹⁵ 

Figures 6(a)–6(e) show the ferroelectric hysteresis P(E) loops of NFO/BSTO composites measured at room temperature in a maximum external voltage of 1 kV at a frequency of 50 Hz. It is clear that these P(E) hysteresis loops exhibit ferroelectric characteristics. As the NFO concentration increases, these P(E) curves are widened and the value of the electrical polarization increases significantly. For composites with high NFO content (x ≥ 0.3), there is an attenuation of the polarization in high electric field, which may be due to the much lower resistance of the NFO phase than the BSTO phase and could be related to the energy loss due to leakage currents in these composites.^45–48 The ferroelectricity of NFO/BSTO composites is improved by increasing the value of the residual polarization (P_r), the saturation polarization (P_s), and the coercive field (E_c), as shown in Fig. 6(f). Here, the P_r value increased more than five times from 0.012 to 0.064 μC/cm², the P_r value increased more than three times from 0.044 to 0.16 μC/cm², and the E_c value increased nearly two times from 2.3 to 4.5 kV/cm for the NFO increasing from x = 0 to x = 0.4, respectively. Interestingly, the P_r and P_s values obtained for the BSNF4 sample (x = 0.4) are five and three times higher than those of the pure BSTO compound, respectively. Table I also lists the values of the characteristic parameters of NFO/BSTO multiferroic nanocomposites, including M_s, M_r, H_c, P_s, P_r, and E_c. It shows that composites with x = 0.1–0.4 are multiferroics with rather small H_c (below 100 Oe). The addition of the NFO phase not only improves the ferromagnetic properties but also improves the ferroelectric properties in the composites. Figure 7(a) shows the UV–Vis absorption spectra of NFO/BSTO composites. It implies that the absorption coefficient tends to increase gradually and the absorption region of the samples expands toward the longer wavelengths. Accordingly, their bandgap energy (E_g) values can be estimated as shown in Fig. 7(b). The E_g value is strongly reduced from 3.05 to 2.76 eV, corresponding to an increase in x from x = 0 to x = 0.4.

The value of E_g for NFO/BSTO is reduced corresponding to the gradual increase in the NFO content in the component from x = 0 to x = 0.4, which can be explained as follows: Ni(II) and Fe(III) ions co-exist in NFO/BSTO composites. The electrons in the d subshell of Ni²⁺ and Fe³⁺ are distributed across the t_2g and e_g orbitals. The energy level of the VB in NFO is higher than that of BSTO, while the value of the CB energy in NFO is lower than that of BSTO. As a result, a new energy level of E_g is formed. Internal electric fields have been created in ferroelectric materials; in the presence of ferromagnetism, transition metal 3d states in the Fermi energy band appear, reducing the band-gap and allowing for easier capture of energy phonons.^6,33 Under irradiation by ultraviolet light, the energy required for electron transmission was reduced, and the maximum absorption of NFO/BSTO composites shifted toward a longer wavelength than that of BSTO. This effect is possible because the electrons can either move from the VB to the characteristic energy band of Ni(II) and Fe(III) or they can be transferred from the typical energy band of Ni(II) and Fe(III) to the CB.⁶ The enhancement in the catalytic ability of NFO/BSTO composites may be closely related to their electrical polarization, which is an important factor in promoting their photocatalytic ability. Reducing the electron–hole recombination and narrowing the bandgap are essential to increase the photocatalytic efficiency. These results are considered reasonable when compared with previous research findings, indicating that NFO/BSTO composites are more promising materials for photocatalytic applications.³³ 

To investigate the photocatalytic behavior of NFO/BSTO composites as a function of NFO content, we conducted a study where synthetic methyl orange (MO) dye was exposed to ultraviolet (UV) light. By monitoring the degradation of MO in water in the presence of the NFO/BSTO composite under UV irradiation, the photocatalytic properties of NFO/BSTO were systematically investigated. Figure 8 presents the absorption spectra of MO dye over different UV irradiation times for two typical samples, BSTO [Fig. 8(a)] and BSTO2 [Fig. 8(b)], at room temperature. Similar to other photocatalysts, the intensity of the MO absorption peak decreases with increasing irradiation time. We found that the photocatalytic degradation of MO followed the order BSTO < BSNF1 < BSNF2 < BSNF3 < BSNF4.

The photocatalytic mechanisms that can account for the obtained results of the photocatalysis of NFO/BSTO have been described schematically in Fig. 11. The presence of NFO in the NFO/BSTO composite plays a crucial role in improving the photocatalytic activity. The process of MO decomposition by the photocatalyst under UV radiation can be summarized as follows: Under the effect of ultraviolet light, electrons are excited with enough energy and move to the conduction band from the valence band. Electron and hole pairs will be formed after this process. The generated electrons and holes are transferred to the surface by excluding the recombined electron–hole pairs. On the surface of the semiconductor, superoxide anion (•O²⁻) and hydroxyl (•OH⁻) radicals will be formed by the reaction between electrons and holes with oxygen and water in the air.^6,7,15 In addition, the electrical polarization of the NFO/BSTO composite may also contribute to the improved photocatalytic activity by preventing electron–hole recombination and promoting the formation of oxidizing agents. Overall, the proposed mechanism suggests that the NFO/BSTO composite can effectively utilize UV light to generate active species that decompose organic pollutants into safe materials, which can have important applications in environmental remediation and pollution control.

In order for us to see the efficiency when using NFO/BSTO composites as photocatalysts to decompose MO synthetic dyes, statistical results of the photocatalytic processing ability of similar materials^{6,7,15,30,32,33,47,49,50} are shown in Table II. It shows that the efficiency of MO dye treatment by using NFO/BSTO composites under ultraviolet light is compared to that reported for various materials. In 2018, Lassoued et al.³² succeeded in decomposing MO synthetic dyes by using NFO.³² The MO solution was photocatalytic and decomposed at 10 mg/l. The solution has a concentration of the photocatalyst of 1.0 g/l. Using visible light with a wavelength in the region from 350 to 800 nm emitted by a tungsten filament lamp, the decomposition rate of MO was achieved at about 83.2% after irradiation for 140 min.³² Besides, with the same concentration of the photocatalyst and MO synthetic dye, Yu et al.⁵⁰ used the photocatalyst 0.4BaTiO₃/0.6CuO to degrade ∼90% of MO synthetic dyes in 90 min under light from a UV source with a power of 200 W and immersed in an ultrasonic cleaner.⁵⁰ Especially when performing MO dye degradation under UV light, corresponding to a lower concentration of the photocatalyst, Mishra et al. succeeded in decomposing MO synthetic dyes by using CoFe₂O₄/Fe₃O₄ nanoparticles.³³ The concentration of the catalyst used was 0.2 g/l, and the decomposition rate of MO reached about 93% under ultraviolet light for 300 min.^8,38 Eskandari et al.⁶ used the CoFe₂O₄/SrTiO₃ photocatalyst with a concentration of 0.4 g/l to successfully decompose the azo dye solution under the light generated by the UV light source and visible light source. The decomposition rate of the azo dyes solution reached ∼70% after irradiation of 180 min.⁶ The results confirm that the NFO/BSTO composite is a new photocatalyst that effectively decomposes the organic pollutant MO under UV light, using lower concentration than or equal photocatalyst concentration to those in previous publications.

NiFe₂O₄/Ba_0.8Sr_0.2TiO₃ multiferroic composites were successfully synthesized by combining high-energy ball milling with heat treatment. XRD measurements and analysis confirmed the presence of both NFO and BSTO phases in the NFO/BSTO composites. The electrical polarization and magnetization of the composites were found to be significantly improved with increasing NFO content. In addition, the ability to catalyze activity by the UV light of NFO/BSTO composites was shown to be considerably higher than with pure BSTO.

The MO degradation of NFO/BSTO composites reached ∼80% after being illuminated with UV radiation for 70 min. These composites have the advantage of being easily recovered by using an external magnetic field, representing a simple recovery method. In addition, the significant reduction in the bandgap energy and the enhancement in the apparent first-order rate constant (k_p) have demonstrated the role of the NFO ferromagnetic phase in these composites. These suggest that NFO/BSTO composites have promising potential in degrading synthetic organic dyes and treating contaminated water by multiferroic composites under ultraviolet light.

This research was funded by the Institute of Materials Science, Vietnam Academy of Science and Technology, under Grant No. CS.10/21-22.

The authors have no conflicts to disclose.

Dao Son Lam: Conceptualization (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Project administration (equal); Writing – original draft (equal); Writing – review & editing (equal). Nguyen Minh Hieu: Formal analysis (equal); Investigation (equal). Dang Duc Dung: Formal analysis (equal); Investigation (equal). Dinh Chi Linh: Formal analysis (equal); Investigation (equal). Nguyen Ngoc Tung: Investigation (equal). Nguyen Thi Viet Chinh: Formal analysis (equal); Investigation (equal). Ngo Thu Huong: Investigation (equal); Methodology (equal); Writing – review & editing (equal). Tran Dang Thanh: Conceptualization (equal); Investigation (equal); Supervision (equal); Writing – original draft (equal); Writing – review & editing (equal).

The data that support the findings of this study are available within the article.