Introduction

Environmental pollution has increased in recent years due to the production of many pollutants such as organic dyes in various industrial sectors, such as paper, leather, fertilizer, food, cosmetics, pharmaceuticals, and textile industries, which is an important source of specified environmental pollution [1]. Organic dyes have become an important part of industrial wastewater due to their extensive application and large-scale production [2]. Wastewater must be treated before reuse is possible [3]. Photocatalytic water purification is used due to its advantages such as excellent performance, performance at ambient pressure and temperature, low cost, and non-production of secondary waste, which makes it safe for the environment [4].

Materials with special nanostructures have attracted a lot of attention in the field of materials science due to their unique physical and chemical properties, which are distinctly different from normal materials, among which bismuth ferrite nanoparticles can be mentioned [5, 6]. BFO is regarded as a potential visible-light photo-catalyst for the degradation of organic pollutants, due to its high photosensitivity at narrow band gaps with high excitation binding energy, non-toxicity, chemical stability, low cost, and multiferroic properties at room temperature [7, 8]. Since BFO possesses a narrow band gap energy, it is responsive to a wide spectrum of solar or visible light [9,10,11,12,13,14,15,16]. Besides, its multiferroic behavior is a contributing factor to the recyclability of a photocatalyst [17]. The investigation of the origin of magnetism in BFO could reveal interesting insights into its magnetic phenomena [18]. BFO exhibits enhanced magnetic properties through the suppression of its cycloidal AFM spin structures and the formation of AFM/FM core/shell structures [19, 20]. The magnetic property that occurs due to one of these origins would also influence other properties of BFO. On the other hand, BFO also exhibits visible-light-driven photocatalytic activity that is currently being explored for applications such as water splitting for hydrogen production and degradation of organic pollutants [21, 22]. The rare earth element dysprosium (Dy) is chosen as a substituent because it could enhance the magnetic property by suppressing the cycloidal spin structure in the host BFO [23]. Furthermore, its optical properties and 4f shell electronic configuration is expected to enhance visible light absorption and photocatalytic properties [24].Various methods including the sol–gel method, hydrothermal methods, solid state reaction, co-precipitation synthesis, rapid liquid-phase sintering method, pulsed laser deposition, electro-spinning and magnetron sputtering have been utilized to obtain BFO nanoparticles with determined sizes. Among these techniques, the sol–gel synthesis method is widely used for the synthesis of BFO, because it allows obtaining pure phases and controlling the size and distribution of particles. Also, by comparing the sol–gel method with other BFO synthesis techniques, we see that the sol–gel method is often considered a superior choice due to the many advantages it offers including the ability to produce nanostructured materials with high purity and good homogeneity, as well as the ability to control the size and shape of the resulting particles. The sol–gel method also allows the incorporation of impurities and other additives that can significantly change its physical and chemical properties [25, 26]. P4R is poisonous and carcinogenic aromatic ring structures and azo functional groups (–N=N–) [27]. Therefore, P4R is likely to lead to many adverse health effects such as allergies, neurobehavioral toxicities, and cancers if it is excessively consumed. The molecular structure of P4R is shown in Fig. S1. [28]. To guarantee food safety, the dosages of P4R have been severely restricted by laws and regulations. For example, the joint FAO/WHO Expert Committee on Food Additives (JECFA) has set the acceptable daily intake (ADI) for P4R as 4.0 and mg/kg bw/day [29].

The main purpose and advantage of this study is the photocatalytic degradation of P4R dye in the presence of BFO nanoparticles synthesized by the sol–gel method to find other means to speed up the efficiency of the photocatalytic process and remove pollutants from the environment. Therefore, the effect of different parameters such as the type of light irradiation, the amount of H2O2, the amount of pH and the amount of catalyst including BFO nanoparticles and BFO doped with dysprosium were also investigated to optimize the process for maximum degradation of P4R using UV–Vis spectroscopy. Among the optimal cases, the doping of BFO with rare earth metals through the creation of electron–hole trapping centers in the photocatalyst network has a great effect on the efficiency of the photocatalytic process, which is a vital method to improve the photocatalytic properties of semiconductors. Introducing doping by rare earth metals can improve the photocatalytic efficiency of BFO by reducing the particle size and increasing its surface area. In the continuation of this study, the optical properties and photocatalytic activity of pure BFO nanoparticles and BFO doped with dysprosium were investigated.

Experimental

Materials

The materials used:

  • Bismuth nitrate pentahydrate [Bi (NO3)3⋅5H2O] and dysprosium nitrate pentahydrate [Dy (NO3)3 5H2O] from Chem-Lab NV (Belgium),

  • Iron nitrate nonahydrate [Fe (NO3)3·9H2O] from Scharlab S.L. (Spain),

  • Tartaric acid (C4H6O6) from LOBA CHEMIE PVT. LTD. (India) and Ponceau 4R (C20H11N2Na3O10S3) from Merck (Germany).

Ammonia (NH3), hydrochloric acid (HCl), sodium hydroxide (NaOH), nitric acid (HNO3), and hydrogen peroxide (H2O2) were purchased from Merck too. Aqueous solutions were used with double distilled water.

Instruments

A UV–Vis spectrophotometer (Varian Cary 50 Bio, Australia) was used to measure the absorbance of P4R dye. The XRD pattern was performed using an X-ray diffractometer (Bruker D8 ADVANCE, Germany). Morphology and the particle size of BFO and Dy-doped BFO nanoparticles were used by field-emission scanning electron microscopy (FE-SEM, TESCAN MIRA3, Czech Republic). The local structure of the nanoparticles was obtained with Fourier-transform infrared spectroscopy (FT-IR, Bruker Optik GmbH TENSOR 27, Germany). The optical properties were calculated with the help of diffuse reflectance spectra (DRS, PerkinElmer, LAMBDA 365 UV–Vis spectrophotometer, United States). From oven (Pars Azma Co K.J 55, Iran) and an electric muffle furnace (SCI FINETECH FTMF706/706P, Korea) were utilized in the synthesis of BFO and Dy-doped BFO. A mercury vapor lamp (250 W) was utilized for the photocatalytic activity of BFO and Bi1−xDyxFeO3 (x = 0.02, 0.04, and 0.06) nanoparticles for the P4R removal. The pH measurements were used using a pH meter (Metrohm 780, Switzerland), and stirring of the solution was performed with a magnetic stirrer (Heidolph Hei-Standard, Germany).

Synthesis of BFO and Dy-doped BFO nanoparticles

The typical sol–gel method was used to synthesis of BFO and Bi1−xDyxFeO3, x = 0.02, 0.04, and 0.06 nanoparticles and photocatalyst was prepared with the tartaric acid assisted. In this method, an appropriate amount of Bi(NO3)3·5H2O, Fe(NO3)3·9H2O, and Dy (NO3)3·5H2O were added into a solution containing 1 mL of HNO3 and 20 mL of distilled water [30, 31]. The mixture was stirred for 1 h at 60 °C and 750 rpm to completely dissolve all materials, after the addition of tartaric acid and ethylene glycol. Following, the mixture was stirred at room temperature for 2 h and a lemon-color suspension was found. The suspension was continuously stirred, and NH3 1 mol L−1 was added dropwise to the suspension to adjust the pH. The XRD results showed that the best BFO and Dy-doped BFO nanoparticles were obtained at pH 6. The suspension was heated with a thermal program up to 200 °C in the oven. The obtained powder was calcined in a furnace for 3 h with a thermal program to 400 °C. For removal impurities in of phases such as Bi2Fe4O9 and Bi25FeO39, the powder was filtered and washed several times with HNO3 0.10 mol L−1 and deionized water. In the end, the powder for the second time, was heated in a furnace at 650 °C for 4 h.

Photocatalytic experiment

The photocatalytic activity of the BFO and Dy-doped BFO nanoparticles were evaluated in the aqueous solution of P4R under different conditions of darkness, sunlight, and UV light irradiation. At first, calibration steps were performed to determine the P4R initial concentration that acquired 20 mg L−1. Then 300 mL of P4R dye with a concentration of 20 mg L−1 was prepared and transferred to a 600 mL beaker. Next, H2O2 and the nano catalysts were added into the beaker comprising 300 mL P4R solution. The pH of P4R aqueous solutions was adjusted at different amounts using HCl and NaOH 1 mol L−1. Then, the beaker was placed inside the photoreactor and the solution was stirred at 750 rpm. The lamp was turned on and every 15 min, 3 mL of the sample was pipette for 240 min.

This handmade reactor was composed of a UV lamp, two fans, and a magnetic stirrer. The distance between the lamp and the starter was 18 cm. The handmade reactor had dimensions of 50 cm width × 75 cm length × 40 cm height and two places on top of the reactor for sampling. In the following, the degradation of P4R dye was investigated by measuring the absorption of the P4R dye aqueous solution at λmax = 508 nm using a UV–Vis spectrophotometer. The degradation efficiency of the P4R dye was determined by the following equation (Eq. 1) [32]:

$${\text{Degradation\,}}{\mkern 1mu} {\text{efficiency}}\,\left( \% \right) = \left( {{\text{C}}_{0} - {\text{C}}_{t} } \right)/{\text{C}}_{0} \times 100$$
(1)

In the above formula, C0 and Ct are P4R concentrations (mg L−1) at the initial concentration of the dye (t = 0 min) and at irradiation times (t = 0–240 min).

Results and discussions

Characterization of nanomaterials

Nano photocatalyst powders were characterized by means of X-ray diffraction (XRD), FE‑SEM analysis, Fourier transform infrared spectroscopy (FTIR), as well as their optical properties.

XRD analysis

The X-ray diffraction (XRD) pattern of Bi1−xDyxFeO3 nanoparticles (NPs) is shown in Fig. 1a. To investigate the purity of Bi1−xDyxFeO3 NPs and estimate possible impurity phases, X’pert software was used. Investigations indicate that Bi1−xDyxFeO3 NPs are almost single-phase. However, a small number of impurity phases, including Bi2Fe4O9 and Bi2O3 existed as indicated in Fig. 1a with symbols (+) and (*). With substituting Dy ion at the Bi site two phenomena including merging doublet peaks and shifting peaks occur in XRD patterns as shown in Fig. 1a. Both phenomena are due to the difference in ion radii of Bi and Dy.

Fig. 1
figure 1

a XRD pattern of Bi1−xDyxFeO3 nanoparticles, inset: merging and shifting in doublet major peak, and b the Rietveld refinement for Bi1−xDyxFeO3 NPs

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It is known that BFO crystalizes in a rhombohedral structure with the R3c space group. To determine the space group of other samples and structural parameters, the Fullproof software was used. For instance, Fig. 1b indicates the Rietveld refinement for Bi1−xDyxFeO3 NPs. Moreover, the lattice constants (a, b, and c) and unit cell volume (V) of Bi1−xDyxFeO3 NPs were calculated and listed in Table 1. The results show that V decreases with increasing Dy content. This Result always occurs in BFO by replacing rare earth ions (Dy) for the Bi site corresponding to the difference between ion radii of Bi and Dy [33,34,35,36].

Table 1 The lattice parameters (a, b, and c), unit cell volume (V), mean particle diameter (DSEM) and energy gaps of Bi1−xDyxFeO3 nanoparticles
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FE‑SEM analysis

The FE-SEM images of Bi1−xDyxFeO3 NPs are shown in Fig. S2. By comparing images can be concluded that with increasing Dy substitution the particles are more homogenous. The insets exhibit the size particle distribution histogram of Bi1−xDyxFeO3 NPs. The mean particle diameter (DSEM) of Bi1−xDyxFeO3 NPs was calculated using the log–normal function [37,38,39]. The DSEM of our samples were listed in Table 1.

To study the elemental composition of Bi1−xDyxFeO3 NPs, energy dispersive spectroscopy (EDS) characterization was used. The existence of all elemental stoichiometric composition is confirmed as shown in Fig. S3 that the synthesized sample contains the expected amounts of Bi, Fe, Dy, and O.

FT‑IR spectrum

In Fig. S4, the FTIR spectra, which corresponded to the samples prepared in the presence of dysprosium, were recorded within the range of 400–4000 cm−1. The FT-IR spectrum shows that the characteristics peak in the range of 400–600 cm−1 which can be ascribed to the Fe–O stretching and bending vibrations [40, 41]. These vibrational states are specifications of the octahedral FeO6 groups on the perovskite structure which imply the formation of BFO [42]. The broad bands that are positioned in the range of 3400 and 1630 cm−1 correspond to the ν(O–H) stretching and δ(O–H) bending vibrations of the hydroxyl groups [43, 44].

Optical properties

To study the band gap energy of Bi1−xDyxFeO3 NPs used DRS analysis. Using the Kubelka–Munk (K–M) formula, F(R) = (1 − R)2/2R, where R is the reflectance of the Bi1−xDyxFeO3 NPs [45]. Then, to determine the Eg of Bi1−xDyxFeO3 NPs, the curve of (F(R) × hν)2 versus hν was plotted. For instance, the curve of Eg for Bi1−xDyxFeO3 NPs were shown in Fig. 2. The Eg value for Bi1−xDyxFeO3 NPs were given in Table 1. It is clear that the band gap energy changes negligibly with Dy content. In general, the band gap of a compound can be depending on particle size, shape, and porosity and crystal distortion [15, 36, 46, 47]. In BFO with low doping levels noticeable changes in band gap do not observe corresponding to above mentioned factors. This behavior agrees with the literature [48, 49].

Fig. 2
figure 2

(F(R) × hυ)2 versus photon energy (hυ) for Bi1−xDyxFeO3

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Photodegradation of P4R dye

The photocatalytic activity of pure BFO and Dy-doped BFO nanoparticles was investigated to degrade P4R dye under a dark state, sunlight, and UV light irradiation. It can be seen that the BFO nanoparticles produced by doping Dy show higher photocatalytic activity under UV-light irradiation [50]. In the following, to increase the efficiency of photocatalytic activity, the effect of various operation parameters, such as BFO dosage, initial PH of the dye, presence, and dosage of H2O2, and type of light irradiation, was studied and optimized as a function of time.

Effect of irradiation type on the removal efficiency of P4R dye

The photocatalytic activity of BFO nanoparticles was measured in three modes: dark, direct sunlight, and direct UV light. In the dark state, it has the least amount of degradation, then in the direct sunlight mode, and most of the degrading is in the UV light radiation mode, Fig. 3. UV light helps to produce more hydroxyl radicals (OH·), but sunlight has only 3 to 4% of ultraviolet rays and the amount of degradation is much less than UV light [51].

Fig. 3
figure 3

The percentage of degradation of P4R in a dark state, sunlight, and UV light irradiations. Experimental conditions: concentration of 20 mg L−1 of P4R, 0.50 mL H2O2, 0.10 g of the pure BFO nanoparticles and pH 2 nanoparticles and pH 2

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Effect of H2O2

Apart from its photocatalytic property, BFO also exhibits prominent photo -Fenton-like catalysis ability. The degradation of P4R is carried out in the absence and presence of H2O2 with different amounts including (0.25, 0.50, 0.75, and 1 mL) at pH 2 under UV light irradiation for 240 min. It can be seen from Fig. 4a. That the best value of H2O2 was 0.50 mL. The percentage of degradation of P4R in the presence of 0.50 mL H2O2 was slightly increased compared to other quantities, and the absence and presence of 0.50 mL H2O2 were 2.09% and 98.92% Fig. 4b. Compared with the reaction systems without H2O2, the introduction of H2O2 remarkably enhances the degradation percentage of the dye. This result is mainly due to the photo -Fenton-like reaction mechanism. The H2O2 is an electron scavenger that can produce ·OH radicals. Therefore, it increases the separation of the hole and the electron, which enhances the degradation efficiency [52]. With the addition of 0.50 mL of H2O2, the photocatalytic activity for the degradation of P4R dye increases due to the higher generation of ·OH radicals and dye adsorption on the surface of BFO nanoparticles (Eq. 2). At the highest amount of H2O2, it decreased the rate of the dye reaction due to competition with P4R for the available ·OH radicals (Eqs. 3 and 4)[53].

$${\text{H}}_{2} {\text{O}}_{2} + {\text{BFO}}\left( {{\text{e}}^{ - } } \right) \to \cdot {\text{OH}} + {\text{OH}}^{ - } + {\text{BFO}}$$
(2)
$${\text{H}}_{{2}} {\text{O}}_{{2}} +{ \cdot } {\text{OH }} \to {\text{ HO}}_{{2}}{ \cdot } + {\text{H}}_{{2}} {\text{O}}$$
(3)
$${\text{HO}}_{{2}}^{ \cdot } +{ \cdot } {\text{OH }} \to {\text{ H}}_{{2}} {\text{O }} + {\text{ O}}_{{2}}$$
(4)
Fig. 4
figure 4

a The rate of percentage of degradation of P4R at different H2O2 values (0, 0.25, 0.50, 0.75, and 1 mL) and b The photocatalytic degradation process of P4R in the absence and presence of H2O2. Experimental conditions: concentration of 20 mg L−1 of P4R, 0.10 g of the pure BFO nanoparticles, under UV light irradiation and pH 2

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Effect of the BFO nanoparticles dosage

Catalyst and H2O2 together affect degradation and are not important factors in degradation separately. To optimize the amount of the nanoparticles, different dosages (0, 0.05, 0.10, 0.15, and 0.20 g) were tested Fig. 5. The presence of a catalyst is significant and influential in the rate of P4R degradation. In the absence of BFO, the amount of photocatalytic degradation was very negligible. It was found that 0.10 g of BFO is the optimum dosage for the degradation of P4R. With the increase of catalyst dosage to 0.10 g, the availability of more active sites on the catalyst surface increases. In the amounts of 0.15 and 0.20 g of catalyst, particle aggregation reduces the photocatalytic degradation activity by reducing the specific surface area probability [54,55,56]. When the catalyst and H2O2 are added, the degradation efficiency in the dye solution increases to 98.92% within 240 min. Therefore, we conclude that the photo -Fenton process plays a significant role in the production of ⋅OH radicals.

Fig. 5
figure 5

The rate of percentage of degradation of P4R at different catalyst dosages (0, 0.05, 0.10, 0.15, and 0.20 g). Experimental conditions: concentration of 20 mg L−1 of P4R, 0.50 mL H2O2, under UV light irradiation and pH

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Investigation of initial pH of P4R

pH is one of the main parameters, the effect of which usually goes back to the existing functional groups on the catalyst and the pollutant. The results of the study to determine the percentage of P4R dye degradation are shown in Fig. 6. Experimental findings showed that changes in pH increase the efficiency of the process. In acidic solutions, the positive charge (H +) on the catalyst surface increases with decreasing pH and activates the catalyst surface. As a result, negative contaminant charges tend to be more adsorbed on the catalyst surface, and the adsorption rate and dye degradation are increased. However, with increasing pH, the presence of a negative charge (−OH) on the catalyst increases and causes repulsion between the catalyst and the pollutant, and the amount of degradation decreases. P4R dye degradation in the presence of an acidic environment and a catalyst is very significant [57,58,59]. Therefore, at pH 2, the maximum amount of P4R dye degradation is observed. Degradation does not occur at natural pH and alkaline pH.

Fig. 6
figure 6

The percentage of P4R degradation at different pH levels (2, 4, 6, 8, and 10). Experimental conditions: concentration of 20 mg L−1 of P4R, 0.50 mL H2O2, 0.10 g of the pure BFO nanoparticles, and under UV light irradiation

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Kinetic study

The photodegradation kinetics of P4R in the presence of Bi1−xDyxFeO3 nanoparticles were investigated using the Langmuir–Hinshelwood model by determining the pseudo-first-order rate constant for the photodegradation (Tables 2 and 3). For this purpose, the pseudo-first-order rate constant for photodegradation of P4R was determined based on the nonlinear (Eq. 5) [60,61,62,63]:

Table 2 Kinetic study of photodegradation and the percentage of P4R degradation with BFO nanoparticle in 240 min in different conditions
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Table 3 Kinetic study and the duration of the P4R degradation process in the presence of 0.10 g Bi1−xDyxFeO3 (x = 0, 0.00, 0, 0.02, 0.04 and 0.06) nanoparticles under optimal condition
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$$A=x\times {e}^{-kt}+ E$$
(5)

The units of the rate constant (k) of pseudo-order unit (min−1), X is the amplitude of the process, and E is the endpoint, both of them have the same units as the measured quantity A. The photodegradation rate constant (k) for a period of 90 min in the presence of the Bi1−xDyxFeO3 were 0.01031, 0.02312, 0.01830, and 0.01346 Cm−1 for x = 0.00, 0.02, 0.04, and 0.06. P4R degradation percentages of 94.67, 87.69, and 79.69 for x = 0.02, 0.04, and 0.06 in Bi1−xDyxFeO3 nanoparticles were obtained for 90 min under the optimal conditions shown in (Fig. 7) instead of 98.92 for 240 min in undoped BFO. Photocatalytic kinetics curve of the Dy-doped BFO (Dy 0.00, 0.02, 0.04, 0.06) nanoparticles it is shown in Fig. S5. The results showed that the rate of the P4R degradation reaction increases significantly with the Dy dopants in the BFO lattice structure.

Fig. 7
figure 7

The rate of percentage of P4R photocatalytic degradation using Dy-doped BFO nanoparticles with values of 0.02, 0.04, and 0.06. Experimental conditions: concentration of 20 mg L−1 of P4R, 0.50 mL H2O2, 0.10 g of the Dy-doped BFO nanoparticles, under UV light irradiation and pH 2

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Conclusion

Two photocatalysts, including BFO and Dy-doped BFO with different values of Dy 0.02, 0.04, and 0.06, were synthesized using a chemical method for P4R degradation under UV light exposure. The morphology and structure of the photocatalysts were evaluated using various techniques to investigate the success of their preparation. Different factors, such as irradiation type, H2O2 values, photocatalyst amount, and pH, were selected as influential factors in the photodegradation of P4R. These factors were optimized for each photocatalyst to obtain the best degradation conditions. Under optimum degradation conditions, the percentages of P4R degradation for each photocatalyst were studied, indicating the percentages of P4R degradation using BFO for 240 min 98.92%, using BFO Dy-doped (Dy 2%), (Dy 4%) and (Dy 6%) for 90 min 94.67%, 87.69% and 79.69% (Fig. 8). Besides, the Langmuir–Hinshelwood model kinetics were selected to evaluate and determine the rate constant of the photodegradation procedure in the presence of BFO and Dy-doped BFO. The rate constants showed that the highest P4R degradation rate was displayed using Dy-doped BFO (Dy 2%). In other words, the P4R degradation rate was significantly increased by increasing Dy (III) ions as a dopant in the BFO lattice through forming trapping sites in the BFO structure as suitable electron acceptor species and improving the stability of electron–hole pairs and lives. Also, the pore size of the BFO nanoparticles decreases with an increasing percentage of dysprosium doped into the BFO structure, leading to an increase in selectivity for the penetration of present molecules in the sample solution, such as contaminants into the BFO lattice. An essential advantage of BFO and Dy-BFO is their single-step and straightforward synthesis, which accelerates the synthesis of photocatalyst.

Fig. 8
figure 8

UV–Vis absorption spectra of the P4R dye degradation process under experimental conditions: concentration of 20 mg L−1 of P4R, 0.50 mL H2O2, 0.10 g of the Dy-doped BFO nanoparticles, under UV light irradiation and pH 2 at λmax = 508 nm in the presence of a un-doped BFO in 240 min and b Dy-doped BFO (Dy 2%) in 90 min

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