1 Introduction

Multiferroic materials are a class of materials that got immense interest owing to the coexistence of electrical and magnetic properties. In addition, multiferroic materials have potential applications in field of spin valves, information storage devices, microelectronic devices spintronic devices, and sensors [1]. Among various multiferroic materials, magneto electric multiferroic are one of the most commonly studied materials in which magnetization can be affected by electric field and vice-versa. A number of single phase magneto electric multiferroic materials have been reported so far, however, most of them are insufficient because of chemical and physical incompatibility between ferroelectric and ferromagnetic ordering [2, 3]. In view of this, Multiferroic BiFeO3 (BFO) nanoparticles are found to be newly emerging material with practical applications in various fields of electrical, magnetic and optical devices [4, 5]. Multiferroic BFO are concurrently subsistence of both the ferroelectric and antiferromagnetic properties at room temperature with tunable structure, Curie (≈1100 K) and Neel (≈643 K) temperature [6]. Multiferroic BFO possesses a perovskite type unit cell, having rhombohedral phase with point group R3c. Its unit cell have a lattice parameter, αrh, of 3.965A˚ and a rhombohedral angle, αrh, of ca. 89.3–89.48 at room temperature [7]. Multiferroic BFO are also known for its intriguing optical properties such as light-induced size changes, anomalous photovoltaic effect, persistent photoconductivity and switchable diode effects [8]. At the nanoscale BFO exhibit tremendous novel physical properties that are different from the bulk counter parts which make them potential candidates for the use in future electronic nano devices [9]. Further, due to its narrow band gap (2.1–2.7 eV) and high chemical stability, it has enormous appliances as photo catalyst for the waste water treatment [10, 11]. A number of reports in the literature have been found for its use as magnetically recoverable visible light photocatalyst for the degradation of industrial effluents [12,13,14,15].

Due to vide spread applications of Multiferroic BFO, number of techniques are used for the preparation of BFO nanoparticles, however, each of them has their own advantages and limitations [10]. Thus, development of Multiferroic BFO nanoparticles with high purity, uniform size, and homogeneous morphology is still in demand. In light of this, here, we are using an auto-combustion technique for the synthesis new BiFeO3-tartaric acid (BFO-T) nanoparticles by using tartaric acid as a chelating agent. We have used the auto combustion technique for the preparation of BFO-T nanoparticles as it is the one of the most promising techniques for the synthesis of numerous high purity nanomaterials. In addition, this technique is found to be straightforward, proficient, cost effective, and swift in nature. To our pleasure, we were successful in preparation of highly pure form of BFO-T nanoparticles where the sample had undergone calcination at different range of temperature such as 150 °C, 450 °C, 550 °C, 650 °C and 750 °C. Further, the extensive study of effect of different range of calcination temperature on purity, surface area, surface morphology, optical, thermal stability, crystalline and local structure of as-synthesized nanoparticles were studied. To the best of our knowledge there is no report in literature where the properties of BFO-T nanoparticles calcinated at different temperature have been discussed. The prepared pure BFO-T nanoparticles exhibited ferromagnetic behaviour, which can be used as magnetic recoverable catalyst in organic synthetic transformations. optical properties showed that these materials can be used as visible light photo catalyst for the degradation of industrial effluents and can be used for potential applications in optoelectronic devices.

2 Experimental

For the synthesis of BFO-T nanoparticles, Bi(NO3)3·5H2O, Fe(NO3)3·9H2O and tartaric acid were taken as 1:1:2 mmol, respectively. Metal precursors, Bi(NO3)3·5H2O and Fe(NO3)3·9H2O (as in 1:1 mmol) were dissolved in distilled water and 3 M HNO3 (as in 1:2 ml), respectively, followed by sonication for 10 min. After this, the two solutions were mixed and undergone sonication for 15 min more. Then chelating agent tartaric acid (2 mmol) was added to the reaction mixture. For homogeneous mixing, the mixture was stirred for 30 min at 800 rpm. The mixture was then heated on hot plate at 100 °C for 30 min. The black powder obtained was then undergone calcination at 150 °C, 450 °C, 550 °C, 650 °C and 750 °C temperature for one hour. During the calcination process, the complexing agent decomposes and the mass of gases generated (like CO2, N2 etc.) leads to the creation of highly crystalline and single phased perovskite BFO-T nanoparticles as shown below:

$$ {\text{Bi}}\left( {{\text{NO}}_{3} } \right)_{3} + {\text{ Fe}}\left( {{\text{NO}}_{3} } \right)_{3} + \, 10{\text{C}}_{4} {\text{H}}_{6} {\text{O}}_{6} + 45/2{\text{O}}_{2}\, {\mathop{\longrightarrow}\limits^{\Delta}}\, {\text{BiFeO}}_{3} + \, 40{\text{CO}}_{2} + {\text{N}}_{2} + 30{\text{H}}_{{2}} {\text{O}} $$

3 Characterization

The crystal structure of prepared magnetic nanoparticles catalyst was investigated by X‐ray diffraction (XRD) using a Panlytical XPERTPRO (NPD) X‐ray diffractometer with Cu Kα radiation (λ = 0.154 nm), in the 2θ range 10°–80°. Fourier transform IR (FT‐IR) spectroscopy was carried out by using Agilent Cary 600 in the range 400–4000 cm−1. The morphology was studied using transmission electron microscopy (TEM). The sample was prepared by dispersion of nanoparticles in ethanol, which were deposited on a carbon‐coated Cu grid, and after drying, analysis was performed using a Hitachi S7500 instrument. The magnetic properties were measured using a vibrating sample magnetometer (VSM, Princeton Applied Research model155) at room temperature with a maximum magnetic field range of + 1 T to − 1 T. Thermogravimetric analysis (TGA) was performed on the catalyst using an EXSTAR6000TG/DTA 6300 instrument in the temperature range 50 − 700 °C at a heating rate of 10 °C/min under nitrogen atmosphere. Surface area and pore characteristics were characterized using Quanta Chrome Nova-1000 surface analyzer instrument under liquid nitrogen temperature with vacuum degassing at 110 °C for 3 h. The specific surface area was calculated using the BET equation. Raman spectra were recorded on Renishaw Invia Raman Microscope equipped with argon laser (514 nm) as source of excitation.

4 Result and Discussion

4.1 3.1 Structural and morphology analysis

The Fourier Transformed Infrared (FT-IR) spectra of BFO-T nanoparticles were recorded at different calcinated temperatures as shown in Fig. 1.

Fig. 1
figure 1

Comparison study of Fourier transform infrared (FT-IR) spectra of BFO-T nanoparticles calcinated at 150 °C, 450 °C, 550 °C, and 650 °C

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In a case of BFO-T nanoparticles calcinated at 150 °C, a wide peak in the range from 3000–3400 cm−1 was observed which was ascribed to the OH stretching mode of the water molecules. However, with increase in the calcination temperature from 150 °C to 650 °C, it was found that, the intensity of this peak goes on decreasing from the surface of samples, indicating the removal of the water molecules from the sample. Further, BFO-T nanoparticles with calcination temperatures 150 °C, 450 °C and 550 °C showed the presence of peaks in the range from 1628–1672 cm−1 of symmetric bending vibration of C=O, 1375–1440 cm−1 of symmetric bending vibration of C–H bond and 1275–1282 cm−1 of stretching vibration of C–O of tartaric acid precursor [16]. The existence of these peaks was due to the fact that during auto combustion, a huge amount of energy was released in the form of gases forming a partially crystalline form of product, consisting of organic chelating agents and metal residual precursors, respectively. From this observation, we found that more thermal treatment is required to obtain pure crystalline form of the BFO-T nanoparticles. So when sample was further calcinated at higher temperature i.e. 650 °C, no such kinds of peaks have been observed or we can say that there was no tartaric acid remained in the sample [17]. In addition, BFO-T nanoparticles at 650 °C showed strong peaks at 433 cm−1, 517 cm−1 and 819 cm−1,which were ascribed to the stretching and bending vibrations of Bi–O and Fe–O bonds [16]. Moreover, during calcination at higher temperature (650 °C), impurities were released as CO2 and H2O from the reaction mixture.

To characterize the stable single phase of materials, XRD spectra of BFO-T nanoparticles was recorded when calcinated at a different range of temperatures i.e.150 °C, 450 °C, 550 °C, 650 °C and 750 °C as shown in Fig. 2. The XRD spectrum of BFO-T nanoparticles at low temperature i.e. 150 °C showed no characteristic peak, indicating the amorphous nature of the material. However, the main diffraction peaks of BFO-T nanoparticles started appearing at 450 °C along with secondary minor impurity phases like Bi2Fe4O9 and Bi36Fe24O57 corresponding to 2ϴ = 30.35°, 47.01°, 48.32° and 27.81°, 32.80°, 35.60o, 48.51°, 55.28°, 56.23° and 57.77°, respectively. The appearance of secondary impurity phases was due to the comparable oxygen affinity of both Fe and Bi ions. On further increasing temperature around 550 °C, the intensity of secondary phases decreases along with the sharpening of major diffraction peaks of BFO-T nanoparticles.

Fig. 2
figure 2

XRD analysis of BFO-T nanoparticles calcinated at (a) 150 °C, (b) 450 °C, (c) 550 °C,(d) 650 °C, (e)750 °C

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When the sample was further calcinated at 650 °C, the main diffraction peaks of BFO-T nanoparticles were observed at 22.33°, 31.57°, 32.04°, 38.87°, 39.37°, 45.66°, 51.33°, 51.69°, 56.25°, 56.81° and 57.12° corresponding to (012), (104), (110), (006), (202), (024), (116), (122), (214), (018) and (300) Miller indices, respectively. All the major peaks accurately harmonized with the JCPDS Card no. 86–1518 [6]. The above results indicating that sample BFO-T nanoparticles prepared at 650 °C have more sharpened diffraction peaks along with the exact exclusion of secondary phases. It can also be observed that the majority of the peaks of BFO-T nanoparticles belong to distorted rhombohedral structure of BiFeO3 nanoparticles. From the above studies it was concluded that upon increasing calcination temperature, pure form of the BFO-T nanoparticles was obtained [18, 19], i.e. pure perovskite form of BFO-T nanoparticles was obtained [1]. This result was also in concord as discussed earlier in FT-IR analysis. We also compared the diffraction peaks of BFO-T nanoparticles calcinated at 750 °C with the sample calcinated at 650 °C. We found that new impurity peaks was observed at 2ϴ = 14.67°, 28.83°, 29.61°, 33.62°, 47.03° and 49.30°, which may be attributed to the formation of Bi2O3, Bi3Fe4O9 [1] and Bi2Fe4O4 [20]. Thus, indicated that 650 °C was the optimum calcination temperature for the formation of single phase perovskite form of BFO-T nanoparticles. At this temperature, all the secondary phases were removed entirely and pure form of single phase BFO-T nanoparticles was generated. Further, the average particle size of BFO-T nanoparticles were calculated using Scherrer’s formula, D = kλ/β cos ϴ, where λ is the wavelength, β is the corrected diffraction line full-width at half maximum, k is a constant (having a value of 0.94) and ϴ is Bragg's angle [21], by using different planes like (012), (104) and (110). The calculated average size for BFO-T nanoparticles calcinated at different temperatures 150 °C, 450 °C, 550 °C, 650 °C and 750 °C was found to be 6.0 nm, 13.26 nm, 14.35 nm, 30.0 nm and 45.2 nm, respectively as shown in Table 1. The increase in average particles size with increasing calcination temperature was ascribed due to the formation of cluster and owing to the particle collision [22]. Further, the enhancement in diffraction peaks intensity and diffraction peak sharpness with an increase in temperature was responsible for increase in particle size of BFO-T nanoparticles at higher temperature [23]. In addition to that as there were different types of impurity phases were formed as discussed above, their amount of formation be calculated by XRD and have been illustrated in Table 2.

Table 1 Average particle size calculated by XRD of BFO-T nanoparticles prepared at different temperature
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Table 2 Amount of different phases calculated from XRD of BFO-T nanoparticles prepared at different temperature
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To further investigate the particle size and morphology of the prepared BFO-T nanoparticles, transmission electron microscopy (TEM) was performed using carbon coated copper grid as shown in Fig. 3.

Fig. 3
figure 3

TEM image of BFO-T nanoparticles calcinated at a 150 °C and b 650 °C

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The grain size obtained by TEM analysis (20 nm and 42 nm for calcination temperature of 150 °C and 650 °C respectively) was little bit large as compared to the average size calculated by XRD analysis (6.0 nm and 30 nm for calcination temperature 150 °C and 650 °C respectively). This was due to the fact that grain contains sub-grains, which were separated by low-angle grain boundaries [24]. Furthermore, no agglomeration of particles was observed as the particles were well dispersed before the TEM analysis [23], thus large size was observed in TEM studies.

The investigation of surface area was carried out by using BET studies. The results obtained from BET analysis of BFO-T nanoparticles calcinated at the different range of temperature has been shown in Table 3 and Fig. 4.

Table 3 Surface area and pore volume analysis of samples of BFO-T nanoparticles prepared at different temperature
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Fig. 4
figure 4

Surface area analysis of BFO-T nanoparticles calcinated at a 150 °C, b 450 °C, c 550 °C, d 650 °C

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As expected, the surface areas of BFO-T nanoparticles were reduced with increase in calcination temperature. This was due to the increase in particle size with increase in calcination temperature as calculated by XRD analysis. Thus, the result obtained for BFO-T nanoparticles from BET analysis were consistent with the XRD analysis. It was observed that calcinated at higher temperature leads to decrease in the number of nitrates molecules which resulted in increment in the size of BFO-T nanoparticles. Moreover, at highest temperature 650 °C, the BFO-T nanoparticles are in the most crystalline form, thus, have the smallest surface area [25].

5 Analysis of optical properties

In order to examine the optical properties, UV–vis analysis of synthesized BFO-T nanoparticles calcination at different temperature i.e. 150 °C, 450 °C, 550 °C and 650 °C was carried out as shown in Fig. 5. The Tauc’s equation was used for the estimation of the band gap, (αhν)n = B(hν-Eg), where α, h, ν, Eg, and B are the absorption coefficient, Plank constant, light frequency, band gap energy and a constant respectively. For BiFeO3, n = 2, which indicates the direct band gap. The band gap energy was calculated by extrapolating the linear portion of (αhν)2 vs hν to the point α = 0 [14].

Fig. 5
figure 5

UV–Vis spectra of BFO-T nanoparticles calcinated at a 150 °C, b 450 °C, c 550 °C, and d 650 °C

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The value of the band gap for the samples prepared at 150 °C, 450 °C, 550 °C, and 650 °C was 2.56 eV, 2.40 eV, 2.26 eV and 2.20 eV, respectively Fig. 6. It has been observed that the value of band gap decreases with increase in calcinated temperature. This was due to the increase in average particle size of BFO-T nanoparticles at high temperature, which resulted in “quantum size effect”. This “quantum size effect” leads to decrease in the Eg values with increase in particle size of the nanoparticles [26, 27]. In addition, the obtained least band gap i.e. 2.20 eV for BFO-T nanoparticles calcinated at 650 °C temperature was in agreement with the previous literature values [13, 14]. This value also demonstrated that the prepared BFO-T nanoparticles are suitable for performing as a photocatalyst for the degradation of harmful compounds in the range of visible light.

Fig. 6
figure 6

Band gap values of BFO-T nanoparticles calcinated at a 150 °C, b 450 °C, c 550 °C, and d 650 °C

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6 Analysis of magnetic properties

Further, we also studied the magnetic behavior of the prepared BFO-T nanoparticles by VSM technique. For this, the study of magnetic hysteresis M (H) of samples calcinated at the different range of temperatures was carried out at room temperature over a magnetic field of ± 1 T as shown in Fig. 7. All the prepared samples of BFO-T nanoparticles showed narrow hysteresis cycle which indicated their soft magnetic character. It is also well known that the magnetic character possessed by the BFO nanoparticles were due to the existence of Fe3+ and excess of O vacancies [1]. The values of saturation magnetization (Ms) for BFO-T nanoparticles synthesized at different calcination temperature are 0.32 emu/g, 5.72 emu/g, 5.56 emu/g and 0.36 emu/g for 150 °C, 450 °C, 550 °C and 650 °C, respectively. The low saturation magnetization value of the sample calcinated at 150 °C was might be due to the little bit formation of Fe–O, O–Fe–O bonds as also revealed by the FT-IR analysis. Further, increase in the Ms value was observed at high calcinated temperature i.e. at 450 °C and 550 °C. This abrupt increase in the Ms value was due to complete formation of Fe–O and O–Fe–O bonds in the prepared BFO-T nanoparticles. However, at 650 °C, again decrease in the magnetization value was observed which was due to the increase in the bond length of Bi-Fe bond and bond angle of Fe–O–Fe, O–Fe–O bonds [28].

Fig. 7
figure 7

VSM of BFO-T nanoparticles calcinated at 150 °C, 450 °C, 550 °C and 650 °C

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Another reason for a decrease in the value of magnetic saturation with increasing calcination temperature was due to the fact that some grains at higher temperature do not possess a spontaneous magnetic moment and therefore they were not capable to contribute in the magnetization [16]. The value of Ms i.e. 0.36 emu/g clearly confirmed the ferromagnetic character of pure form of BFO-T nanoparticles obtained at 650 °C [29]. Further, the observed value of magnetization saturation was enough to separate the BFO-T nanoparticles from the reaction mixture by applying strong magnet, which further enhanced its use as a magnetically separable catalyst in the organic reactions [30].

7 Analysis of thermal properties

For the determination of chemical stability of as synthesized nanoparticles, thermal gravimetric analysis (TGA) of BFO-T nanoparticles prepared by calcination at 150 °C and 650 °C was carried out as shown in Fig. 8.

Fig. 8
figure 8

TGA analysis of BFO-T nanoparticles calcinated at 150 °C, and 650 °C

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It has been found that the initial weight loss of 0.12% from 0–264 °C ascribed to the loss of surface hydroxyl group from the surface of nanoparticles. This loss was also well matched with the FT-IR analysis demonstrated earlier. On further increasing the temperature from 264–402 °C, weight loss observed was 0.15%, which verified that reaction starts occurring at this temperature range and leads to the formation of BFO-T nanoparticles. A small weight loss (0.09%) was also observed in the range of 402–584 °C, which may be attributed to the decomposition of organic impurities before 584 °C [23]. Further decrease in weight loss (above 584 °C) was due to the decomposition of nitrate species. It was also observed that, with increasing calcination temperature the stability of BFO-T nanoparticles increases, as weight percentage left for the samples calcinated at 150 °C and 650 °C was 87.79 and 99.66, respectively. The reason for higher weight percentage left with increasing temperature from 150 °C to 650 °C could be ascribed to the formation of stable single phase of BFO-T nanoparticles which was demonstrated by XRD results earlier.

8 Analysis of crystalline and local structure

For the investigation of crystalline and local structure of as synthesized nanoparticles Raman spectroscopy was used. Raman spectrum of BFO-T nanoparticles calcinated at 650 °C (as it was observed as the final optimized calcination temperature for the formation of BFO-T nanoparticles as proved by XRD results)) was recorded as shown in Fig. 9.

Fig. 9
figure 9

Raman Spectra of BFO-T nanoparticles synthesized by Tartaric acid as chelating agent and calcinated at 650 °C

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According to the symmetry consideration, the 13 Raman active modes (4A1 + 9E) for BFO having distorted rhombohedral structure with R3c space group should observed [31]. In our case, three intense peaks at 131 cm−1, 167 cm−1, 210 cm−1 and a weak intensity peak at 474 cm−1 was observed which were assigned for 4A1 modes. The peaks around 112 cm−1, 115 cm−1, 259 cm−1, 272 cm−1, 351 cm−1, 361 cm−1, 457 cm−1, 471 cm−1 and 538 cm−1 were assigned for the 9E modes. The appearance of modes above 200 cm−1 was due to the internal vibrations of FeO6 octahedra and modes below 200 cm−1 were caused by the Bi atoms of the perovskite layer which corresponds to a rigid layer [32]. The above results were in good agreement with the XRD studies and previous reports, thus, confirming the perovskite structure of BFO-T nanoparticles [33].

9 Conclusion

In conclusion, we have synthesized the single phase highly pure BFO-T nanoparticles by auto-combustion route via tartaric acid as a chelating agent. The adopted methodology is simple, cost effective, and efficient. FT-IR studies confirmed the presence of Bi-O and Fe–O bonds in the prepared BFO-T nanoparticles. Effect of different calcination temperature (150°, 450 °C, 550 °C, 650 °C and 750 °C) on various properties of prepared BFO-T nanoparticles had been explored. XRD analysis of prepared BFO-T nanoparticles confirmed that calcination temperature 650 °C is the optimum calcination temperature to obtain the highly pure BFO-T nanoparticles. The particle size of as synthesized nanoparticles was studied by XRD and TEM techniques followed the same trend that particle size increased with increase in calcination temperature. Raman spectroscopy also confirmed the perovskite structure of the prepared pure BFO-T nanoparticles. VSM study confirmed its ferromagnetic character, which can be used as magnetic recoverable catalyst in organic synthetic transformations. In optical band gap study, the value of band was decreased with increase in calcination temperature; this observation was owing to “quantum size effect”. This effect leads to decrease in the Eg values with increase in particle size of the nanoparticles as in our case particle size increased with increase in calcination temperature. Further, this studied showed that these can be used as visible light photo catalyst for the degradation of industrial effluents. In addition, pure phase BFO-T nanoparticles having ferromagnetic and optical properties in visible region may also be used for potential applications in optoelectronic devices.