Development and characterization of nano-multifunctional materials for advanced applications

Abstract

Multifunctional zinc oxide–bismuth ferrite and tin dioxide–bismuth ferrite have been synthesized using a double precipitation technique. The structural formation, chemical composition, morphology and thermal properties were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, thermogravimetric analysis, scanning electron microscopy with energy dispersive spectroscopy and transmission electron microscopy. Temperature-dependent magnetic behaviour of zinc oxide–bismuth ferrite and tin dioxide–bismuth ferrite were studied using a vibrating sample magnetometer in the range of 5 K to 300 K. The results indicate that zinc oxide–bismuth ferrite is a potential candidate for spintronics applications.

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1. Introduction

Multifunctional nanomaterials have attracted huge interest, due to their excellent magnetic, electrical, optical and other properties.^1,2 Due to their characteristics, they are wieldy used in several advanced energy fields, such as: sensors, spintronics, transducers, memory and electronic filed controlled ferromagnetic resonance devices.^3,4 Bismuth ferrite (BiFeO₃) is one of the multifunctional materials that have anti-ferroelectric, anti-ferromagnetism and anti-ferroelastic order in signal phase.⁴ Owing to the functionalities of bismuth ferrite, such as: semi-conductive nature, wide band gap (2.0–2.20 eV), and excitation binding energy (∼60 meV) at room temperature, it has ferro-electromagnetism property, which enhances its applicability in electromagnetic sensors, memory devices and electric energy harvesting in materials sciences.^4,5 In addition, their magnetic moment and magnetoelectric coupling offers several benefits in materials sciences and physics.⁶ However, in order to enhance the applicability of BiFeO₃ in advanced nanotechnology, it can be modified with suitable metal–oxide semiconductor elements for pollutant degradation without any harmful residues.

Zinc oxide (ZnO) is one of the semi-conductive multifunctional materials. It has attracted attention for several potential applications due to its non-toxic nature, significant direct band gap [3.37 eV], high electron mobility [200 cm² V⁻¹ s⁻¹] and high excitation binding energy (60 meV) at room temperature.⁷ Due to its optimistic characteristics, it has been used for application in sensors, piezoelectric devices, optoelectronics, piezoelectric devices, medical, etc.⁸

Tin oxide is another important inorganic-oxide. It is a suitable candidate for a range of applications, such as: sensor devices, transparent conductive coatings for displays, electrodes, light emitting diodes and solar cells, because of its excellent optical and electrical properties.⁹ Thus, ZnO and SnO₂ are excellent choices for positive template for the development of multifunctional nanostructures for future advanced nanotechnology applications in advanced sciences.

In this scientific report, double precipitation method was used for the synthesis of possible nano-multifunctional zinc oxide-bismuth ferrite [(shell) ZnO–BiFeO₃ (core)] and tin dioxide-bismuth ferrite [(shell) SnO₂–BiFeO₃ (core)]. The nano-multifunctional materials were characterized with Fourier transform infrared spectroscopy, X-ray diffraction, thermogravimatric analysis, scanning electron microscope with energy dispersive spectroscopy and transmission electron microscopy and evaluated for their temperature- and magnetic field-dependent applications. The resulting nano-multifunctional materials have great potential for advanced functional applications.

2. Experimental

2.1. Materials

All the reactions processes were carried out at room temperature, under ambient reaction conditions. Bismuth(III) nitrate pentahydrate, iron(III) nitrate nanohydrate, tin(IV) chloride, zinc nitrate, ammonium hydroxide, nitric acid and potassium hydroxide were obtained from Sigma Aldrich Chemicals Company.

2.2. Preparation of oxide nanoparticles

2.2.1. Synthesis of single-phase bismuth ferrite (BiFeO₃). Bismuth ferrite was synthesized by a precipitation technique. Bismuth(III) nitrate pentahydrate (0.01 M) and iron(III) nitrate nanohydrate (0.01 M) were completely dissolved in 100 ml of diluted nitric acid (6.3 ml of HNO₃/93.7 ml of distilled water) solution in a 500 ml beaker under constant stirring condition at room temperature. To this solution, potassium hydroxide was slowly added drop-wise in order to obtain a co-precipitate (Fe³⁺, Bi³⁺ ions) until a brown color precipitate was formed during which the pH was adjusted to ∼9. After starring for 30 minutes, the precipitate formed was washed several times with distilled water in order to remove the K⁺, NO and other unwanted materials and the pH of the filtrate was reduced to 7. The resultant mixture was dried at 120 °C for 120 min. Finally, it was cooled to room temperature. Subsequently, the possible powders were dried and heat-treated at different temperatures.

2.2.2. Zinc oxide (ZnO) nanoparticles. 0.05 M of zinc nitrate aqueous solution was prepared by adding of 14.87 g zinc nitrate to 50 ml of distilled water under the constant stirring condition at ambient temperature for 1 h. Then, the solution pH was increased to 9 by adding ammonium hydroxide drop-by-drop with stirring for 3 h until there was a white color precipitation. After the reaction was completed, the resulting final product was collected by filtration technique. The filtrate was thoroughly washed with distilled water and then dried at 120 °C for 120 min.

2.2.3. Tin dioxide (SnO₂) nanoparticles. SnO₂ was prepared by hydrothermal process. In a typical synthesis, 0.025 M of tin(IV) chloride was dissolved in 100 ml distilled water and stirred for 30 min at ambient temperature and then (10 to 15 ml) ammonium hydroxide was add drop-wise to this solution for the formation of a slurry-like white precipitation. After 20 min of stirring, the resulting solution was washed, several times, with distilled water in order to remove the (chlorine and other ions) unwanted elements. Finally, it was filtered and dried at 120 °C for 120 min.

2.3. Preparation of possible functional nanoparticles by double precipitation technique

2.3.1. Method A.
Tin dioxide–bismuth ferrite (SnO₂–BiFeO₃) nanoparticles. In this process, 0.025 M of tin(IV) chloride was initially dissolved in 100 ml distilled water under stirring condition. Secondly, the 30 ml of bismuth ferrite (3 g) solution was introduced to the above solution. Finally, it was precipitated with ammonium hydroxide solution, pH 9. After 30 min of continuous stirring, the precipitated solution was washed with distilled water in order to remove the chloride ions and the unreached reactants from the precipitation. Then, possible core–shell was filtered, dried and heat-treated at 120 °C for 120 min.

2.3.2. Method B.
Zinc oxide–bismuth ferrite (ZnO–BiFeO₃) nanoparticles. 14.87 g of zinc nitrate and 3 g of bismuth ferrite were dissolved in 50 ml of distilled water under constant stirring condition at ambient temperature for 1 h. Subsequently, ammonium hydroxide solution was added drop-wise, until there was the formation of core–shell precipitation during which, the pH was adjusted to ∼9. Here, the alkaline solution is increases the reaction rate to get a precipitation, since alkaline cations play vital role in the formation of ZnO–BiFeO₃. Furthermore, the core–shell precipitation was filtered and rinsed 3 times with distilled water to adjust the pH from 9 to 7 neutralize and wash-out any unreacted metal ions.¹⁰ Finally, the washed powder was dried at ambient temperature and cooled to ambient temperature. The possible core–shell powders were dried and heat-treated at different temperatures (Scheme 1).

Scheme 1 Schematic diagram for the formation of possible core–shell nano-multifunctional materials.

2.4. Characterizations

The structures of the SnO₂–BiFeO₃, ZnO–BiFeO₃ possible core–shell nanoparticles formed were confirmed with electron microscopes (Scanning and Transmission electron microscopes (SEM and TEM)), X-ray diffraction (XRD) studies, Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA). In addition, their temperature-dependent magnetic properties were determined by vibrating sample magnetometer (VSM). SEM micrographs were recorded using a JEOL JEM 7500F (Tokyo, Japan) scanning electron microscope at 2 keV. Energy dispersive spectroscopy analyses were carried out on a JEOL JEM 7500F SEM. Transmission electron microscopes were recorded on JEM-1200EX, JEOL (Tokyo, Japan). The samples were dispersed in a 1 : 1 methanol and water solution and deposited on a 3 mm copper grid and dried at ambient temperature after removing the excess solution using filter paper. X-ray diffraction measurements were carried out using a Rikagu diffractometer with Cu-K_α radiation and using a scan rate of 0.02° s⁻¹. FTIR absorption spectra were recorded in a Perkin Elmer Model impact 410 (Wisconsin, MI, USA) spectrophotometer in order to identify the functional groups in the multifunctional materials. Thermal properties were determined from the TGA data, using SDT Q600 thermal analyzer (T.A. Instruments-water LLC, Newcastle, DE, USA), at a heating rate of 20 °C min⁻¹ and passing N₂ gas at a flow rate of 100 ml min⁻¹.

3. Results and discussion

3.1. FTIR study

Fig. 1 shows the FTIR spectrum of BiFeO₃, ZnO, SnO₂, ZnO–BiFeO₃ and SnO–BiFeO₃ nanoparticles, representative of their perovskite type vibration. In Fig. 1A, the absorption band observed at 850 cm⁻¹ indicates the symmetric stretching of Bi–O/Fe–O bonds in BiFeO₃.^11,12 The strong absorption band at 544 cm⁻¹ is attributed to Fe–O stretching of the BiFeO₃ group.¹³ The band that appeared at 1365 cm⁻¹ is attributed to the strong vibration of NO₃⁻.¹⁴ The spectra of ZnO showed vital peaks at 452 cm⁻¹ and 644 cm⁻¹, which indicates Zn–O stretching mode.¹⁵ The bands at 3372 cm⁻¹, 1306 cm⁻¹, 477 cm⁻¹ are correlated to O–H, C=O vibration modes.¹⁶ The SnO₂ spectra (Fig. 1B) show absorption peaks at 497.67 and 610 cm⁻¹ and are attributed to the stretching vibration of O–Sn–O and Sn–O.¹⁷

Fig. 1 FTIR spectra of (A) BiFeO₃, ZnO and ZnO–BiFeO₃. (B) SnO₂ and SnO₂–BiFeO₃ nano-functional materials.

Similar bands are observed in the case of ZnO–BiFeO₃ and SnO₂–BiFeO₃, but the bands appeared to have shifted positions and are attributed to the core–shell formation. The spectrum of ZnO–BiFeO₃ (Fig. 1A) shows a characteristic peaks at around 857 cm⁻¹ and 697 cm⁻¹ and are assigned to the vibration frequencies of Bi-, Fe–O in the Zn–O lattice. The main absorption band at 462 cm⁻¹ is attributed to Zn–O/Fe–O stretching of ZnO–BiFeO₃ group. SnO₂–BiFeO₃ spectra (Fig. 1B) show absorption peaks at 466 cm⁻¹ (O–Sn–O, Fe⁻), 589 cm⁻¹ (Fe⁻), 677 cm⁻¹ (Sn–O) and around 846 cm⁻¹ (Bi–O/Fe–O) are attributed to the vibration of SnO₂–BiFeO₃. Furthermore, these results are supporting XRD studies and nanostructure morphologies of pure ZnO, SnO₂ and BiFeO₃, and their nanocomposites.

3.2. SEM-EDS and TEM analyses

In order to obtain detailed information on the possible nano-multifunctional structure and morphology of the BiFeO₃, ZnO, SnO₂, ZnO–BiFeO₃ and SnO–BiFeO₃ nanoparticles synthesized, SEM and TEM studies were carried out. The SEM images of the nano-energy materials developed are presented in Fig. 2. Fig. 2A explains, the BiFeO₃ multifunctional nanoparticles have nano-flower structure with few spherical particles. From Fig. 2B, it is evident from the SEM morphology that the ZnO nanoparticles are rods-like shape with excellent alignment, whereas ZnO–BiFeO₃ multifunctional nanomaterials (Fig. 2AB) shows BiFeO₃ nano-flowers are highly curved with ZnO rods. Fig. 2C shows the SEM image of SnO₂ nanoparticles developed which look like small spherical clusters on the surface. Fig. 2AC illustrates the image of the SnO₂–BiFeO₃ core–shell nanoparticles prepared. The resulting images explain the fact that BiFeO₃ nanoparticles are covered with spherical SnO₂ clusters nanoparticles.

Fig. 2 SEM image of nano-functional materials.

The EDS patterns of the precipitate synthesized SnO₂, ZnO and BiFeO₃, and double precipitate synthesized ZnO–BiFeO₃ and SnO₂–BiFeO₃ possible core–shell nanocomposites are presented in Fig. 3. The EDS spectra of pure ZnO (Fig. 3B) and SnO₂ (Fig. 3C) have accounted for elements of Sn, Zn, O, respectively presence and have not shown any foreign metals or elements presence, and thus pure composition formation at nano-scale has been witnessed and thus pure ZnO and SnO₂ compositions have formed with stoichiometric end products. It is clearly seen that BiFeO₃ mainly contains bismuth, iron and oxygen elements (Fig. 3A), whereas ZnO–BiFeO₃ (Fig. 3AB), SnO₂–BiFeO₃ (Fig. 3AC) have additional (Zn, Sn) elements in addition to Bi, Fe and O, which are clearly observed in the EDS spectrum of possible core–shell nanofunctional materials respectively.

Fig. 3 EDS images of (A) BiFeO₃, (B) ZnO, (C) SnO₂, (AB) ZnO–BiFeO₃ and (AC) SnO₂–BiFeO₃.

The TEM images of typical ZnO as shell material, SnO₂ as shell material and BiFeO₃ as core material, ZnO (shell)–BiFeO₃ (core), SnO₂ (shell)–BiFeO₃ (core), sample are shown in Fig. 4. Fig. 4A–C show the TEM image of BiFeO₃, ZnO and SnO₂ samples, developed by precipitation technique. It can be seen that the developed nanocrystalline are spherical in shape and are highly agglomerated with a diameter of 15 ± 9 nm, 30 nm, and 3 ± 1 nm, respectively. At higher magnification, the inter-planar d-spacing of 0.397 nm, 0.246 nm and 0.319 nm are clearly visible in Fig. 4A–C, corresponding to the (012), (110) and (001 or 110) planes of BiFeO₃, ZnO and SnO₂, respectively, which is in agreement with previously reported values.^18–20 Fig. 4AB and BC shows the different shapes of ZnO–BiFeO₃ and SnO₂–BiFeO₃ nanoparticles, which show that BiFeO₃ nanoparticles are covered with ZnO and SnO₂. These studies clearly indicate that co-precipitation route supports the formation of well-defined core–shell structures of multifunctional materials, which enhances their applicability in energy, materials sciences and medical applications.

Fig. 4 TEM images of nano-functional materials.

3.3. Thermal characterization of nano-functional materials

The TGA spectra of BiFeO₃, ZnO, SnO₂, SnO₂–BiFeO₃ and ZnO–BiFeO₃ are shown in Fig. 5A. In the case of developed nanoparticles, high rate of weight loss has occurred in the range of 75 °C to 100 °C due to the evaporation of the water in the samples. TGA analysis of the precipitated ZnO and BiFeO₃ samples exhibited (Fig. 5Aa) significant degradation at 317.6 °C with greater mass losses: 37.7% (ZnO) and 5.49% (BiFeO₃), whereas the weight loss observed for ZnO–BiFeO₃ at same temperature is 23.03% which is due to the volatilization and combustible organic species present in the sample. However, this study indicates that ZnO added nano-multifunctional material have higher weight loss when compared BiFeO₃ and less weight loss than ZnO. This is due to the semi-conductive behaviour of ZnO which may cause the changes in weight loss that fallows this order: ZnO(37.7%) > ZnO–BiFeO₃(23.03%) > BiFeO₃(5.49%). Similarly, this phenomenon was observed in the case of SnO₂, SnO₂–BiFeO₃ and BiFeO₃. As shown in Fig. 5Bb, the weight loss of the nano-materials follows the order: SnO₂(8.93%) > SnO₂–BiFeO₃(8.04%) > BiFeO₃ (5.49%) occurred at 317.6 °C. For SnO₂, SnO₂–BiFeO₃, shows a comparatively very low weight loss at 317.6 °C, which was due to the partial decomposition of the SnO₂ nanoparticles.²¹

Fig. 5 A) TGA curves of: (a) BiFeO₃, ZnO, ZnO–BiFeO₃ and (b) BiFeO₃, SnO₂, SnO₂–BiFeO₃; (B) XRD patterns of: (I) ZnO, ZnO–BiFeO₃ and (II) SnO₂, SnO₂–BiFeO₃.

3.4. X-ray diffraction

The structural analyses of the possible core–shell nano-multifunctional materials synthesized were carried out by using powder X-ray diffraction, which is a suitable technique for identifying the structural crystalline nature of the nano-multifunctional materials synthesized. The XRD patterns of ZnO, SnO₂, ZnO–BiFeO₃, SnO₂–BiFeO₃ nanomaterials are shown in Fig. 5B. The XRD pattern are shows well intensified peaks of possible core–shell nanoparticles developed. We have identified the pure ZnO (JCPDS card no. 71-2494), SnO₂ (JCPDS card no. 41-1445) and BiFeO₃ (JCPDS card no 86-1518), and nanocomposites by using WinXPow software. It is evident that the intensities of clean peaks of ZnO, SnO₂, and BiFeO₃ are slightly modified in nanocomposite powders due to formation of possible core–shell structured nanopowders as supported by TEM pictures. Moreover, Bragg angel of pure peaks are slightly (with-in 0.002θ) shifted towards higher angles than their pures. We have compared and justified by JCPDS standard files. In Fig. 5BI and II, the sharp peaks identified at: 2θ = 32.35° and 57.47°, are the vital characteristics of BiFeO₃ crystal planes (110) and (300), respectively.^21,22 The other diffraction peaks (ZnO, SnO₂) are highly significant to the formation of ZnO–BiFeO₃ and SnO₂–BiFeO₃ materials^19,23,24

3.5. VSM studies

The main emphasis of these possible core–shell nanocomposites developments is to understand magnetic properties with two distinct semiconductive shells materials in forming possible core–shell structured nanocomposite powders. Accordingly, we have investigated nanocomposites magnetic properties and since semiconductive SnO₂ and ZnO are feeble to show magnetic nature when compared to BiFeO₃. In order to investigate the nature of the ferromagnetism character in ZnO–BiFeO₃ and SnO₂–BiFeO₃, magnetic measurements were carried out on wide range of temperature, in the range of between 5–300 K. Fig. 6 shows the temperature-dependence of magnetizations for ZnO–BiFeO₃, and SnO₂–BiFeO₃, which exhibited a saturation magnetization against sufficiently external magnetic field. The zero-field-cooling magnetization of ZnO–BiFeO₃ sample (Fig. 6a) shows a broad peak feature around 5 K with decreasing order until 50 K (the so-called blocking temperature at 50K) and decreased thereafter from 100 K to 300 K. However, the field-cooled magnetization exhibited similar behaviour as zero-field-cooled magnetizations above blocking temperature. However, fluctuation in the magnetization of SnO₂–BiFeO₃ (Fig. 6b), arising from weak moment of the sample and interfacial effects between core BFO and semiconductive shell was observed. The magnetization of ZnO–BiFeO₃ is higher than SnO₂–BiFeO₃, which can make it to be a better magneto-electric interactive candidate.

Fig. 6 M–H curves of (a) ZnO–BiFeO₃ and (b) SnO₂–BiFeO₃.

4. Conclusion

In this scientific paper, zinc oxide–bismuth ferrite and tin dioxide–bismuth ferrite possible nano-multifunctional materials were successfully prepared by double precipitation technique. The nano-multifunctional materials were characterized by spectral, thermal techniques and the temperature-dependent magnetic behavior was studied. Heat-treated nano-multifunctional materials are highly crystalline, as supported by powder XRD studies. The zinc oxide–bismuth ferrite nano-multifunctional materials synthesized, exhibited good temperature-dependent magnetic behaviour for possible spintronic and energy applications.

Acknowledgements

FONDECYT and CONICYT, Chile is greatly acknowledged for their financial support to this investigation with the Fondecyt Postdoctoral Project no. 3130748 (KVP) and Fondecyt Regular Project no. 1110583 (KR) grants. KVP greatly acknowledged to the PIA-Project, Centro de Investigación de Polímeros Avanzados, CIPA, CONICYT Regional, GORE BIO-BIO, R08C1002.

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