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Article

Sequential Synthesis Methodology Yielding Well-Defined Porous 75%SrTiO3/25%NiFe2O4 Nanocomposite

by
Ilyes Baba-Ahmed
1,
Daniel Ghercă
2,†,
Alexandra-Raluca Iordan
3,
Mircea Nicolae Palamaru
3,
Carmen Mita
3,
Rachid Baghdad
4,
Gabriel Ababei
2,
Nicoleta Lupu
2,
Mohamed Amine Benamar
5,
Abdelkader Abderrahmane
6,
Tiberiu Roman
7,
Georgiana Bulai
7,
Liviu Leontie
8 and
Adrian Iulian Borhan
2,3,*,†
1
Laboratory of Fundamental and Applied Physics (FUNDAPL), Physics Department, Sciences Faculty, Saad Dahleb Blida 1 University, BP 270, Blida 09000, Algeria
2
National Institute of Research and Development for Technical Physics, 47 Mangeron Boulevard, 700050 Iasi, Romania
3
Faculty of Chemistry, Alexandru Ioan Cuza University of Iasi, 11 Carol I Boulevard, 700506 Iasi, Romania
4
Synthesis and Catalysis Laboratory, Matter Sciences Faculty, Ibn Khaldoun University of Tiaret, Tiaret 14000, Algeria
5
Laboratory of Fundamental and Applied Physics (FUNDAPL), University Center of Tamenghasset, Amine Elokkal Elhadj Moussa Eg-Akhamouk, BP 10034, Sersouf, Tamanghasset 11000, Algeria
6
Department of Electrical Engineering, Chosun University, 375, Seosuk-dong, Dong-gu, Gwangju 501759, Korea
7
Integrated Center of Environmental Science Studies in the North-Eastern Development Region (CERNESIM), Department of Exact and Natural Sciences, Institute of Interdisciplinary Research, Alexandru Ioan Cuza University of Iasi, 700506 Iasi, Romania
8
Faculty of Physics, Alexandru Ioan Cuza University of Iasi, 11 Carol I Boulevard, 700506 Iasi, Romania
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Nanomaterials 2022, 12(1), 138; https://doi.org/10.3390/nano12010138
Submission received: 9 December 2021 / Revised: 27 December 2021 / Accepted: 28 December 2021 / Published: 31 December 2021
(This article belongs to the Section Nanocomposite Materials)
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Versions Notes

Abstract

:
In this research, we reported on the formation of highly porous foam SrTiO3/NiFe2O4 (100−xSTO/xNFO) heterostructure by joint solid-state and sol-gel auto-combustion techniques. The colloidal assembly process is discussed based on the weight ratio x (x = 0, 25, 50, 75, and 100 wt %) of NiFe2O4 in the 100−xSTO/xNFO system. We proposed a mechanism describing the highly porous framework formation involving the self-assembly of SrTiO3 due to the gelation process of the nickel ferrite. We used a series of spectrophotometric techniques, including powder X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), N2 adsorption isotherms method, UV-visible diffuse reflectance spectra (UV-Vis DRS), vibrating sample magnetometer (VSM), and dielectric measurements, to investigate the structural, morphological, optical, magnetic, and dielectric properties of the synthesized samples. As revealed by FE-SEM analysis and textural characteristics, SrTiO3-NiFe2O4 nanocomposite self-assembled into a porous foam with an internally well-defined porous structure. HRTEM characterization certifies the distinctive crystalline phases obtained and reveals that SrTiO3 and NiFe2O4 nanoparticles were closely connected. The specific magnetization, coercivity, and permittivity values are higher in the 75STO/25NFO heterostructure and do not decrease proportionally to the amount of non-magnetic SrTiO3 present in the composition of samples.

1. Introduction

Nanomaterials such as nanoparticles, nanowires, nanotubes, nanorods, and quantum dots present unique optoelectronic, electrical, magnetic, and mechanical properties [1]. The development of new strategies to assemble these nanomaterials into patterned heterostructures with multiple functionalities and tailored physical properties is an urgent need for application in nanotechnological devices. The heterostructures offer attractive new possibilities for device applications due to the controlled integration of complementary nano-components, which exhibit synergistic effects that combine multiple functionalities in one structure [2,3]. The rapid advances in materials synthesis methods have made it possible to synthesize nanomaterials with well-defined sizes, shapes, structures, and morphologies. Although many heterostructure [4,5] systems have been reported, very few articles have shown the effect of morphology on the physical parameters [6]. Zheng et al. [1] explain that the assembling process of nanomaterials “is critically dependent on the nanostructure morphologies including domain patterns and shapes as well as structures and properties of the interfaces”. Starting from this concept, we present, in this study, how the colloidal assembly of NiFe2O4 nanoparticles can induce structural and morphological changes in a SrTiO3 matrix, that presents a superposition of cubic and tetragonal phases, depending on the weight ratio x of NiFe2O4. Moreover, the colloidal assembly of the attachment of the NiFe2O4 particles in the presence of seed SrTiO3 particles by using the sol-gel auto-combustion method leads to an open and porous foam morphology. The system SrTiO3/NiFe2O4 is known mostly for its capability as a visible-light-driven photocatalyst [7,8]. SrTiO3 (STO) is a prototypical cubic perovskite oxide with unique physical properties, such as wide bandgap, high dielectric constant, thermal stability, and large permittivity [9,10,11]. Besides, SrTiO3 is known as versatile powder support for the formation of heterostructures with specific properties for technological applications [12,13,14,15,16], because SrTiO3, as an intrinsic paraelectric, is able to stabilize ferroelectric order via substrate-induced strain, cation doping, 18O isotope substitutions, and defect engineering [17]. Hence, to induce ferromagnetic properties into SrTiO3, some researchers reported doping of STO using iron (Fe) [18], cobalt (Co) [19], manganese (Mn) [11] or by using non-magnetic sp-additives [12]. Recently, a ceramic composite of STO and nickel ferrite NiFe2O4 (NFO) has been reported to show ferroelectric, ferromagnetic, and magneto-dielectric properties [13]. NFO is a well-known nanomaterial with high Curie temperature, high chemical, and structural stability [14], having useful magnetic and electrical properties [8]. NFO is used in a wide range of applications including, electrical memory and switching devices [15], high-performance lithium-ion batteries [16], photocatalysis [20], and cancer therapy [21,22,23]. On the other hand, porous materials [24,25] have been obtained in the past decade [26,27] by various methods such as self-assembly of primary nanoparticles [28], combustion [29,30], smelting reaction [31], hydrothermal [32]. According to the literature [33], we tried here to extend the sol-gel auto-combustion method for the synthesis of porous heterostructure foams due to the possibility of controlling the chemical composition, homogeneity, morphology, shape, and phase composition of the materials. Unique and interesting features of SrTiO3/NiFe2O4 binary composite have gained currently great attention from many research studies [8,34]. Yongmei Xia et al. [34] showed from TEM images of SrTiO3/NiFe2O4 nanocomposites, that NiFe2O4 particles are uniformly dispersed onto the SrTiO3 without any accumulation, proving that the prepared nanocomposite is a uniform mixture of SrTiO3 and NiFe2O4 nanoparticles [34]. Moreover, in order to obtain hierarchical SrTiO3/NiFe2O4 composite nanostructures with an excellent light response, Panpan Jing et al. [8] used single-spinneret electrospinning and a side-by-side-spinneret electrospinning technique. In fact, the main advantage of these techniques is high productivity, but the formation of an uneven distribution of fiber diameter is difficult to avoid, while in the present study the uniformity of such nanocomposite is considered beneficial for our experimental ability to design useful photocatalytic properties in a rational way.
The results presented in this work aim to highlight a new topic and findings on the formation of the porous foam SrTiO3/NiFe2O4 (namely 100−xSTO/xNFO), heterostructure by joint solid-state reaction and sol-gel auto-combustion technique. The mechanism of colloidal assembly is discussed in the present study based on the weight ratio x (x = 0, 25, 50, 75, and 100 wt %) of NiFe2O4 in the 100−xSTO/xNFO system. The rational synthesis presented in this study, with emphasis on the colloidal assembly of the attachment of the NiFe2O4 particles in the presence of SrTiO3, represents a step forward in reaching multifunctional properties. These advances are at the core of progress in photocatalytic applications, including water remediation (dyes and pharmaceutical drugs degradation) and especially in water splitting (photocatalytic generation of H2). Thus, synthesized porous 75%SrTiO3/25%NiFe2O4 nanocomposite materials offer flexibility for integrating multiple functionalities such as catalytic activity, adsorption capacity, photocatalytic activity, and magnetic properties that make them attractive for applications in photocatalysis.

2. Materials and Methods

2.1. Porous Heterostructure Foams 100−xSTO/xNFO Multistep Synthesis Method

Analytical grade precursors have been used to synthesize our materials, such as strontium carbonate (SrCO3, 99%, UCB chemicals; Brussels, Belgium), titanium dioxide (TiO2, 99%, Loba Feinchemie; Fischamend, Austria), nickel (II) nitrate (Ni (NO3)2·6H2O, 97%, Sigma Aldrich; Steinheim am Albuch, Baden-Württemberg, Germany), iron (III) nitrate (Fe (NO3)3·9H2O, 98%] Sigma Aldrich; Steinheim am Albuch, Baden-Württemberg, Germany), glycine (NH2CH2COOH, 99.7%, Merck; Steinheim am Albuch, Baden-Württemberg, Germany). First, the solid-state technique was used for the fabrication of the pristine SrTiO3 (STO) nanoparticles by mixing strontium carbonate and titanium dioxide with a molar ratio of 1:1. To ensure high homogeneity, the mixture was ground in an agate mortar for 6 h after adding a few drops of ethanol. Thereafter, the powder was calcined in air for 9 h at a temperature of 1000 °C. Pristine NiFe2O4 (NFO) were synthesized via a sol-gel auto-combustion method by mixing nickel nitrate, iron nitrate and glycine with a molar ration of 1:2:1 in 10 mL total volume of distilled water. The solution was agitated at speed of 300 rpm and temperature of 80 °C for 60 min until gelation occurs. Then, the as-prepared gel was heated on a sand bath for 6 h with a temperature-increasing step of 50 °C up to 350 °C, where the auto-ignition started suddenly, quickly, and violently with the flame. The typical experimental procedure used for the synthesis method is given as follows: first, the pristine synthetized STO powder is mixed with nickel nitrate, iron nitrate, and glycine with a molar ration of 1:2:1, in 10 mL of distilled water. Thereafter, gelation was achieved after 30 min under magnetic agitation of the solution at a speed of 300 rpm and temperature of 80 °C, followed by auto-ignition of the as-prepared gel in a sand bath for 6 h with a temperature increasing step of 50 °C up to 350 °C. In the end, the powder was kept for 9 h at a temperature of 1100 °C to obtain a stabilized structure. Thus, the newly heterostructures, designated as of 100−xSTO/xNFO, were obtained at a different mass percentage of NiFe2O4. Otherwise, to get insight over system design, primordially we consider the structural, magnetic, and optical properties translated in terms of morphology.

2.2. Porous Heterostructure 100−xSTO/xNFO Characterization

Phase identification and structural properties of starting nanomaterials calcined at 1000 °C and composite ceramics, obtained at 1100 °C, were determined by X-ray diffraction (Shimadzu LabX 6000; (Tokyo, Japan); diffractometer using Cu-Kα radiation (λ = 1.5406 Å)). The intensity and voltage of the X-ray source were set at 40 mA and 40 kV, respectively. The samples were scanned in reflection mode in the range 20–80° in 2θ with a step increment of 0.02° per step and a time per step of 0.2 s. Next, the morphology of the samples was studied using FE-SEM analysis, carried out on a Carl Zeiss NEON 40EsB; (Jena, Germany) with thermal Schottky field emission and accelerated Ga ions column. The FE-SEM micrographs were collected at different acceleration voltage and magnifications (1.8 kV, 50 kX 20 kV, 200 kX; 1.8 kV, 100 kX; and 5 kV, 150 kX). High-resolution electron micrographs of the calcined samples were taken by using a transmission electron microscope equipped with energy-dispersive X-ray spectroscopy (EDS) (Carl Zeiss LIBRA 200 MC UHR-TEM); (Jena, Germany) (at magnification of 700 kX and accelerating voltage HV of 200 kV. The samples for high resolution transmission electron microscopy (HR-TEM) were prepared by ultrasonically dispersing the powder in isopropanol and allowing a drop of this to dry on a carbon-coated copper grid. The specific surface area and the pore size of the calcined samples were measured by gas adsorption–desorption isotherms method, using nitrogen as adsorbate at 78 K on a Quantachrome Nova 2200 analyzer and the samples being degassed for 4 h at 250 °C before testing. UV-Vis absorption spectrum of the 75STO/25NFO nanocomposite in the wavelength range of 200–1100 nm was investigated by using a Shimadzu UV-Vis spectrophotometer. The energy of the bandgap ( E g ) was calculated using Tauc’s method [35]. The variation of magnetic properties of the composite ceramics was studied by hysteresis loop obtained using a vibrating sample magnetometer (VSM, Princeton/Lakeshore M3900, Lake Shore Cryotronics, Westerville, OH, USA)), at room temperature under a magnetic field in the range of ±10 kOe. Dielectric properties of the calcined samples, in a wide temperature range (20–200 °C) and for the frequency 2 ÷ 2 × 106 Hz, were carried out on the Ag-electrodes applied to the polished faces of the pelletized samples by using an Agilent E4980A Precision LCR Meter (Santa Clara, CA, USA). The cylindrical pellets with a diameter of 12 mm and 1.5 mm thickness were obtained at 140 MPa by using a Hand Press (Carver Inc., Model 4350.L, Wabash, IN, USA) and thermal treatment of 3 h at 700 °C.

3. Results

3.1. Microstructural Characterization

The crystal structure and crystallinity of nanopowders, as pristine SrTiO3 and NiFe2O4, as well as 75STO/25NFO porous-foam nanocomposite, were analyzed using X-ray diffraction analysis performed at room temperature in the 20–80° (2θ degree) range (Figure 1).
The diffraction peaks can be indexed to ICSD no. 98-009-1899 for SrTiO3 and to ICSD no. 98-016-5448 for NiFe2O4, obtained via a sol-gel auto-combustion method without any other impurity or secondary phases. The pristine SrTiO3 obtained by solid-state reaction presents a cubic structure and a Sr-rich Ruddlesden-Popper (RP) phase [36,37]. After combining simultaneously, the above two synthetic methodologies, 75STO/25NFO porous-foam nanocomposite was obtained as presented in Figure 1b. The diffraction peak of both, perovskite and inverse spinel, respectively are tracked together in the powder XRD pattern of the obtained foam. Thus, the sequential synthesis methodology addressed in this study allows the formation of a novel porous foam nanocomposite 75STO/25NFO. Compared with the cubic perovskite and the inverse spinel-type structures, the diffraction lines of both SrTiO3 and NiFe2O4 phases are present in the XRD pattern of the nanocomposites (Figure S1). The traditional crystallographic approach for structure determination (Rietveld refinement) was sufficient to confirm the crystal structure of the nanocomposites (Figures S2–S7). Rietveld analysis showed that the rutile TiO2 is present as a secondary phase because SrTiO3 is locally nonstoichiometric having Sr-rich RP phase. Using the normalized reference intensity ratio method (RIR), the percentage of rutile TiO2 phase in the 75STO/25NFO porous-foam nanocomposite was determined to be about 2%.
According to data output from Rietveld refinement, as presented in Table 1, the crystallite size is 45 nm for NiFe2O4 and 49 nm for SrTiO3, respectively. In the case of composite, the Rietveld refinement was made by indexing the present phases, and their crystallite sizes were calculated by decomposing each corresponding diffraction peak, calculating FWHM. Therefore, the crystallite size values for NFO and STO were found to be 55 and 50 nm, respectively.
From the FE-SEM analysis of the 75STO/25NFO porous-foam nanocomposite, the hierarchically porous material was formed by joint solid-state and sol-gel auto-combustion techniques. Hierarchical porosity is quite desirable for adsorption and photocatalytic processes and thus 75STO/25NFO as visible-light-driven porous-foam nanocomposite with high contact surface area, high storage volume, ready mass transport, and a well-controlled porosity, which grant to this material a very high adsorption capacity and a high photocatalytic efficiency.
The pore formation mechanism in 75STO/25NFO is shown in Figure 2. Particle interaction is dependent on sol-gel auto-combustion, which plays a key role in determining the morphology of the porous nanocomposite. Herein, a probable mechanism of pore formation in a highly porous framework is proposed and involves the self-assembly of SrTiO3 via the gelation process of the nickel nitrate, iron nitrate, and glycine. In the first step, polyhedral SrTiO3 nanoparticles were synthesized via the solid-state reaction of strontium carbonate and titanium dioxide with a molar ratio of 1:1 and thermally treated in the air for 9 h at a temperature of 1000 ºC. Next, these polyhedral SrTiO3 nanoparticles are used as a template to form a 3D hierarchical xerogel composed of nickel and iron glycinate at a temperature of 80 ºC for 60 min. Finally, the highly porous framework was converted to porous-foam 75STO/25NFO nanocomposite on a sand bath for 6 h with a temperature-increasing step of 50 °C until auto-ignition occurs.
To understand the morphological evolution of the 75STO/25NFO nanocomposite into a hierarchical porous-foam structure, composition-dependent experiments were performed.
Figure 3 depicts the FE-SEM images of the SrTiO3 template obtained from the solid-state reaction, pure NiFe2O4 via a sol-gel auto-combustion, and its composition-dependent morphological evolution following different weights ratio x = 0, 50, 75, and 100 wt % of NiFe2O4 in the 100−xSTO/xNFO system. Histograms, inserted in Figure 3, show the particle size distributions of the calcined starting materials and the 50STO/50NFO and the 25STO/75NFO. It is clear that with the NiFe2O4 increasing content in system, the average particle size decreases, due to the smaller particle size of the ferrite.
As seen in Figure 3b, the template consists of highly nonuniform polyhedral nanoparticles. When these polyhedral nanoparticles were reacted with different amounts of aqueous Ni2+ and Fe3+ solutions (concerning composition stoichiometry) at room temperature in the presence of glycine, they form nonhomogeneous frameworks. The framework becomes more and more disordered as the amount of NiFe2O4 increases, while highly porous-foam nanocomposite is obtained at the 3:1 mass ratio for SrTiO3:NiFe2O4 (Figure 2a–d), thereby indicating the optimal quantity of the reactants. Moreover, the selection of glycine, as fuel and chelating agent, in this synthesis was made according to its characteristic combustion temperature, because it affects the size of the crystallites, the structural stabilization, and the morphology. Glycine ignites at low temperatures, but the exothermic reaction is strong and violent with a large amount of gas released and high enthalpy leading to an increase in crystallites along with a good formation and high purity of the spinel-type structure. Our additional experiments reveal that when the concentration of the aqueous Ni2+ and Fe3+ glycinate was increased, the porous-foam-like structure partially collapsed to accommodate the formation of a nonhomogeneous system mixture of the two components.
To observe in more detail the morphology nature and lattice structure of 75STO/25NFO, both transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) imaging were performed. Figure 4a–d reveals the interconnected NiFe2O4 nanoparticles with polyhedral SrTiO3 nanoparticles leading to porous nature of 75STO/25NFO nanocomposite, as well as another interesting feature of small NiFe2O4 particle attachments indicated by the yellow arrows present on the surface of the SrTiO3. Furthermore, it can be observed from Figure 4d that the optimum 3:1 ratio of components forms bulk chains rather than a core-shell type structure. The TEM energy dispersive spectroscopy (TEM-EDS) elemental mapping images of 75STO/25NFO nanocomposite reveals the uniform distribution of Sr, Ti, and O atoms in the nanocomposite (Figure S8) while Fe and Ni atoms appear attached and among to the surface and interconnected respectively to the perovskite phase.
Figure 5 shows the nitrogen adsorption–desorption isotherms measurements of the composites and the corresponding average pore size distributions. It is found that all isotherms are of type IV with a H3 narrow hysteresis curve according to the IUPAC classification, characteristic of mesoporous materials (pores size in the range of 2.0–50.0 nm) [38]. The type of hysteresis loop provides information about the shape and connectivity of the inner pores [39], which plays an important role in the rate of gas adsorption. In our materials, H3 type hysteresis loops are associated with a porous structure that has pores with irregular size and shapes [40]. For all analyzed samples a gradual increase of the amount of N2 adsorbed from relatively small values of relative pressure is observed. However, the sudden increase in the amount of N2 adsorbed at low values of relative pressure, P/P0, proves that the samples present relatively large specific surface areas. Moreover, hysteresis loop at high relative pressures can be found, indicating the presence of porous structure, especially for the 75STO/25NFO nanocomposite. The pore size distribution curve of 75STO/25NFO shows a monomodal distribution with average pore diameters of between 13.97 and 15.86 nm (calculated from BJH desorption and adsorption isotherms, respectively) [41]. The 50STO/50NFO exhibits a linear pore distribution with average pore diameter between 8.23 nm and 7.44 nm and the 25STO/75NFO composite shows a narrow pore-size distribution with pore size of about 7.5 nm. Moreover, t-Plot micropore volume [42] is negative for all composites, meaning the samples do not contain micropores. The calculated textural characteristics of the samples, by using BET, BJH and t-plots methods, are summarized in Table 2. As expected, 75STO/25NFO nanocomposite has the highest SBET and Langmuir surface area, of 52 and 75 m2 g−1, respectively, as well as pore volume, suggesting that meso-pores make the largest contribution to the surface area. In this way, meso-porosity is produced in 75STO/25NFO composite leading to an ordered porous structure, as confirmed by FE-SEM and HR-TEM. 50STO/50NFO and 25STO/75NFO composites showed smaller SBET and pore volume, suggesting the entry of the nitrogen gas molecules was partially restricted [43]. Nearly double values of pore volume and diameter (0.148 nm and 15.86 nm, respectively) and larger size of specific surface area suggest that 75STO/25NFO nanocomposite can provide more active sites and adsorb more reactive species in further photocatalytic experiments.
Figure 6 shows the UV-Vis absorbance spectra of the 75STO/25NFO nanocomposite as a hierarchical porous-foam structure to reveal the optical properties. It is very well studied in the literature that the absorption band of pure SrTiO3 sharply drops at about 388 nm whereas that of the pure NiFe2O4 continuously extends to the visible-light region (300 nm < λ < 1000 nm). This means that SrTiO3 only has a response to the UV light but the pure NiFe2O4 can respond to both the UV and visible light. Using Tauc’s relation: ((αhν)2 = A(hν-Eg)), the bandgap of 1.5 eV is determined by extrapolating the linear portion of the curve to the energy axis as shown in Figure 6 (inset). We can note that the 75STO/25NFO nanocomposite with a hierarchical porous-foam structure has extended absorption spectra into the visible light region. Interestingly, it is noteworthy that the characteristic absorption peak of NiFe2O4 at about 750 nm still excites in that of synthesized hierarchical porous-foam and is certified further to the earlier reports about the similar composite nanoparticles and from this point of view, the heterojunctions of SrTiO3/NiFe2O4 bring up the complementarity that should exist between the two components (Table 3). Note that the study of these nanomaterials is a complex one, and here is presented only the first stage, in which we wanted to highlight an extremely interesting phenomenon of colloidal attachment of two reference nanomaterials in current technological and environmental applications.
Room temperature hysteresis loops of our pure and composite materials are represented in Figure 7. Indeed, as shown in Figure 7d the recorded room temperature hysteresis loops reveal that the pure SrTiO3 has no magnetism; however, pristine NiFe2O4 nanoparticles show a ferromagnetic behavior with the saturation magnetization (MS) of about 45 emu g−1 (Figure 7c).
Considering the presence of the non-magnetic SrTiO3, the M S values for the synthesized composites with different ratios of NiFe2O4 from 25% up to 75% (Figure 7b), are lower than that of the pure spinel phase and they are between 25 and 39 emu g−1 (see Table 4). It is interesting to note that the magnetization does not decrease proportionally to the amount of non-magnetic SrTiO3 present in the composition of samples (Figure 7b). Moreover, the highest coercivity value is determined for the 75STO/25NFO sample, maybe due to the high magneto crystalline anisotropy, emphasizing the same trend described vide-supra. The mentioned phenomenon needs more experimental work corroborated with theoretical simulation to unequivocally assess the magnetic properties of the as-prepared nanocomposites in hierarchical assembly as well in nonhomogeneous form.

3.2. Comparative Analysis of the Dielectric Properties

The evolution with the frequency of the real and imaginary parts of permittivity for the starting material and the composites prepared is comparatively presented in Figure 8a–h. For a better understanding of the extrinsic permittivity of the composite materials, SrTiO3 was prepared under the same conditions as the composites for the dielectric analysis. The frequency dependence of real and imaginary part of permittivity of pure pristine SrTiO3 ceramic at few temperatures are shown in Figure 8a,e, showing a different behavior compared to that of composites. At low frequency, the sample presents a strong decay and at high frequency (105–106) Hz, where all the extrinsic phenomena were canceled, the permittivity tends to its intrinsic values ~1100.
On the other hand, the real part of permittivity vs. frequency indicates rather similar dielectric behaviors for all composites. The frequency dispersion is higher with increasing temperature and this increase is enhanced at lower frequencies. The 75STO/25NFO composite shows higher permittivity at low frequencies, increasing with temperature. For example, at f = 1 kHz, the permittivity assumes values of about 457 at 25 °C and 1072 at 120 °C for the 75STO/25NFO, while for the 25STO/75NFO composite, of about 131 and 710 at the same temperatures, respectively. Imaginary parts of permittivity (Figure 8f–h) show a strong decrease with frequency from 2 × 104 to ~2 for 75STO/25NFO and 3 × 104 to 10 for 25STO/75NFO composites, respectively, at the frequency between 20 Hz to 106 Hz and 120 °C. Both composites show a low-frequency decay of the real and imaginary part of permittivity, which strongly increases with temperature. However, apart from the linear variation observed in the case of 50STO/50NFO composite, the two materials appear to be temperature independent with increasing frequency and temperature. While for 50STO/50NFO composite, these extrinsic mechanisms seem to be canceled for frequency higher than 105 Hz, in case of the 75STO/25NFO and 25STO/75NFO composites, they continue to exist in all investigated frequency range. The observed differences show that extrinsic contributions are slightly different in these types of composites. The extrinsic contributions are due to Maxwell–Wagner relaxations and dc-conductivity caused by the slow-charged species activated at low frequencies and high temperatures [46]. The Maxwell–Wagner relaxation is related to inhomogeneities and interfaces [47], and it seems that the better dielectric characteristics obtained for slightly temperature-dependent 75STO/25NFO composite are a consequence of different microstructural properties, e.g., hierarchical porous-foam morphology by particle attachment. At high frequency (around 105 Hz), where the charge defect-associated relaxations are no longer active, the permittivity tends to its intrinsic value [48,49]. For this frequency, at 120 °C, values around ~ 320 for 75STO/25NFO (independent with temperature), ~250 for 75STO/25NFO (temperature-dependent) and ~190 for 25STO/75NFO (temperature dependent), are found. The dielectric losses at high frequencies (1 MHz) (Figure 8l–n) seem to be independent of the composite type, temperature-dependent and are in the range of 0.02–0.18 for temperatures between 20 and 200 °C.
The temperature dependence of permittivity shows some differences for the composites according to the composition (Figure 8i–k). A broad permittivity maximum can be detected in the range of about 120–140 °C for all composites and it originates from a relaxor character or a reduction of ferroelectric character [50,51]. The shape and the position of the permittivity maximum are affected by an extrinsic dielectric effect that tends to cover the intrinsic ferroelectric behavior in the composites and the permittivity decreases with temperature and frequency. The shape of the permittivity maximum might be explained by local compositional variations and morphological differences [52].

4. Conclusions

Sequential synthesis methodology addressed in this study enables the successful fabrication of 75STO/25NFO as porous-foam in nature by joint solid-state and sol-gel auto-combustion technique. The diffraction peak of both, perovskite and inverse spinel, respectively, were tracked together in the powder XRD pattern of the obtained composite. The 75STO/25NFO composite exhibits a powerful visible light response with a band gap energy of 1.505 eV and a value of the room temperature permittivity of 457. In addition, 75STO/25NFO composite shows higher permittivity at low frequencies, which is proportional to the temperature increase. Morphological analyses performed by FE-SEM and HRTEM reveal that the consolidation of particles occurs by exact stacking of NiFe2O4 particles along crystal facets of SrTiO3. 75STO/25NFO nanocomposite showed the highest SBET and Langmuir surface area values, of 52 and 75 m2 g−1, respectively, as well as pore volume, which can suggest that meso-pores make the largest contribution to the surface area. The values of the specific magnetization increased from 25.4 to 45.1 emu g−1 when the ratio x in 100−xSTO/xNFO increases from 25 to 100. The coercivity showed the highest value of 214 Oe for the 75STO/25NFO sample. The nanocomposite can be considered as the famous heterojunction connection by sequential synthesis methodology addressed in this study with emphasis on the colloidal assembly of the attachment of the NiFe2O4 particles in the presence of SrTiO3.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/nano12010138/s1, Figure S1. The superimposed XRD patterns of perovskite phases (SrTiO3) synthesized with different ratios of spinel phase (NiFe2O4); Figure S2. The Rietveld refinement of the 75STO/25NFO. Black solid line is the best fits to the experimental data, the blue line represents the differences between the experimental and calculated data while the vertical markers represent the Bragg peaks positions in blue for Cubic perovskite phase, in red for Cubic spinel phase and in green for Tetragonal TiO2 phase; Figure S3. Le Bail refinement from laboratory XRD patterns at RT The Rietveld refinement of SrTiO3 nanoparticles. Black solid line is the best fits to the experimental data, the blue line represents the differences between the experimental and calculated data while the vertical markers represent the Bragg peaks positions in blue for Cubic perovskite phase and in red for Ruddlesden–Popper phase; Figure S4. The Rietveld refinement of SrTiO3 with 25% of NiFe2O4. Black solid line is the best fits to the experimental data, the blue line represents the differences between the experimental and calculated data while the vertical markers represent the Bragg peaks positions in blue for Cubic perovskite phase and in red for Cubic spinel phase and in green for Tetragonal TiO2 phase; Figure S5. The Rietveld refinement of SrTiO3 with 50% of NiFe2O4. Black solid line is the best fits to the experimental data, the blue line represents the differences between the experimental and calculated data while the vertical markers represent the Bragg peaks positions in blue for Cubic perovskite phase, in green for Tetragonal perovskite phase and in red Cubic spinel phase; Figure S6. The Rietveld refinement of SrTiO3 with 75% of NiFe2O4. Black solid line is the best fits to the experimental data, the blue line represents the differences between the experimental and calculated data while the vertical markers represent the Bragg peaks positions in blue for Cubic perovskite phase, in green for Tetragonal perovskite phase and in red Cubic spinel phase; Figure S7. The Rietveld refinement of NiFe2O4 nanoparticles. Black solid line is the best fits to the experimental data, the blue line represents the differences between the experimental and calculated data while the vertical markers represent the Bragg peaks positions in blue for Cubic spinel phase; Figure S8. TEM dark field image of NiFe2O4 particle attachment on SrTiO3 polyhedral nanoparticles (a) and the corresponding elemental mapping for all elements Fe, Ni, Sr, Ti, O (b) Sr, Ti (c) and Fe, Ni (d).

Author Contributions

Conceptualization, D.G., A.-R.I., M.N.P., N.L., L.L. and A.I.B.; Data curation, G.A., N.L. and A.A.; Formal analysis, I.B.-A., R.B., M.A.B. and T.R.; Investigation, I.B.-A., D.G., C.M., R.B., G.A., N.L., M.A.B., T.R. and G.B.; Methodology, D.G., M.N.P. and A.I.B.; Project administration, A.-R.I. and A.I.B.; Resources, A.I.B., A.-R.I. and M.N.P.; Supervision, A.-R.I. and L.L.; Validation, I.B.-A., T.R., G.B. and L.L.; Writing—original draft, I.B.-A., A.I.B., D.G., C.M. and T.R.; Writing—review & editing, N.L, A.A., D.G., A.-R.I. and A.I.B. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by a grant of the Ministry of Research, Innovation and Digitization, CNCS/CCCDI–UEFISCDI, project number PN-III-P1-1.1-TE-2019-2037, within PNCDI III and by a grant of the Ministry of Research, Innovation and Digitization, CNCS/CCCDI–UEFISCDI, project number PN-III-P2-2.1-PED-2019-4215, within PNCDI III.

Data Availability Statement

The data presented in this study are available from the corresponding author on request.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Powder XRD diffractograms of (a) pure cubic spinel phase NiFe2O4 synthesized by sol-gel auto-combustion method, (b) 75STO/25NFO porous-foam nanocomposite formed by joint solid-state and sol-gel auto-combustion techniques and annealed at 1100 °C, and (c) pristine cubic perovskite phase SrTiO3 synthesized by solid-state technique by mixing strontium carbonate and titanium dioxide with a molar ratio of 1:1 and thermal treated for 9 h at 1000 °C.
Figure 1. Powder XRD diffractograms of (a) pure cubic spinel phase NiFe2O4 synthesized by sol-gel auto-combustion method, (b) 75STO/25NFO porous-foam nanocomposite formed by joint solid-state and sol-gel auto-combustion techniques and annealed at 1100 °C, and (c) pristine cubic perovskite phase SrTiO3 synthesized by solid-state technique by mixing strontium carbonate and titanium dioxide with a molar ratio of 1:1 and thermal treated for 9 h at 1000 °C.
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Figure 2. FE-SEM image of the porous structure (a) [scale 1 µm, electron high tension EHT = 5 KV and MAG = 10 kX], (b) [scale 400 nm, electron high tension EHT = 1.8 KV and MAG = 25 kX], (c) [scale 400 nm, electron high tension EHT = 5 kV and MAG = 30 kX], and (d) [scale 400 nm, electron high tension EHT = 5 KV and MAG = 45 kX], formed from the auto-combustion of hybrid 3D hierarchical xerogel composed of SrTiO3 nanoparticles, nickel, and iron glycinate. Histogram (b-inserted) showing the particle size distribution of the 75STO/25NFO [SD: ±27 nm].
Figure 2. FE-SEM image of the porous structure (a) [scale 1 µm, electron high tension EHT = 5 KV and MAG = 10 kX], (b) [scale 400 nm, electron high tension EHT = 1.8 KV and MAG = 25 kX], (c) [scale 400 nm, electron high tension EHT = 5 kV and MAG = 30 kX], and (d) [scale 400 nm, electron high tension EHT = 5 KV and MAG = 45 kX], formed from the auto-combustion of hybrid 3D hierarchical xerogel composed of SrTiO3 nanoparticles, nickel, and iron glycinate. Histogram (b-inserted) showing the particle size distribution of the 75STO/25NFO [SD: ±27 nm].
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Figure 3. FE-SEM images of (a) pristine NiFe2O4 [scale 400 nm, electron high tension EHT = 50 KV and MAG = 35 kX], (b) pristine SrTiO3 [scale 400 nm, electron high tension EHT = 20 KV and MAG = 20 kX], (c) 50STO/50NFO [scale 400 nm, electron high tension EHT = 20 KV and MAG = 25 kX], and (d) 25STO/75NFO [scale 400 nm, electron high tension EHT = 20 KV and MAG = 35 kX] composites. Histograms (inserted) showing the particle size distribution of the starting materials and the two composites by using the ImageJ software. The mean value of the particle size and the errors (standard deviation (SD) were calculated by a Gaussian distribution applied to the histograms obtained.
Figure 3. FE-SEM images of (a) pristine NiFe2O4 [scale 400 nm, electron high tension EHT = 50 KV and MAG = 35 kX], (b) pristine SrTiO3 [scale 400 nm, electron high tension EHT = 20 KV and MAG = 20 kX], (c) 50STO/50NFO [scale 400 nm, electron high tension EHT = 20 KV and MAG = 25 kX], and (d) 25STO/75NFO [scale 400 nm, electron high tension EHT = 20 KV and MAG = 35 kX] composites. Histograms (inserted) showing the particle size distribution of the starting materials and the two composites by using the ImageJ software. The mean value of the particle size and the errors (standard deviation (SD) were calculated by a Gaussian distribution applied to the histograms obtained.
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Figure 4. TEM image of the 75STO/25NFO structure [scale 50nm] (a), HR-TEM image of the optimum holey interconnected NiFe2O4 nanoparticles with polyhedral SrTiO3 nanoparticles [scale 20 nm] (b–d).
Figure 4. TEM image of the 75STO/25NFO structure [scale 50nm] (a), HR-TEM image of the optimum holey interconnected NiFe2O4 nanoparticles with polyhedral SrTiO3 nanoparticles [scale 20 nm] (b–d).
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Figure 5. Nitrogen adsorption isotherms of the nanocomposites (left), and the corresponding average pore size distributions (right).
Figure 5. Nitrogen adsorption isotherms of the nanocomposites (left), and the corresponding average pore size distributions (right).
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Figure 6. UV-vis DRS of 75STO/25NFO nanocomposite as hierarchical porous-foam structure and (Inset) corresponding Tauc plot for bandgap determination.
Figure 6. UV-vis DRS of 75STO/25NFO nanocomposite as hierarchical porous-foam structure and (Inset) corresponding Tauc plot for bandgap determination.
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Figure 7. Room temperature hysteresis loops of the (a) 75STO/25NFO nanostructure, (b) all composites synthesized with different spinel phase percentage together with inset graph showing the magnetization and coercivity variation with spinel phase ratios, (c) pristine NiFe2O4 nanoparticles with inset graph showing the loop at near-zero magnetic field, and (d) pristine SrTiO3 nanoparticles.
Figure 7. Room temperature hysteresis loops of the (a) 75STO/25NFO nanostructure, (b) all composites synthesized with different spinel phase percentage together with inset graph showing the magnetization and coercivity variation with spinel phase ratios, (c) pristine NiFe2O4 nanoparticles with inset graph showing the loop at near-zero magnetic field, and (d) pristine SrTiO3 nanoparticles.
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Figure 8. Dielectric dispersion at few temperatures of the materials: (a,e) real and imaginary parts of permittivity of the SrTiO3; and (b–d) real part of permittivity and (f–h) imaginary part of permittivity of 100−xSTO/xNFO composites. Temperature dependence of the dielectric characteristics at a few selected frequencies for 100−xSTO/xNFO composites: (i–k) permittivity; (l–n) tangent loss.
Figure 8. Dielectric dispersion at few temperatures of the materials: (a,e) real and imaginary parts of permittivity of the SrTiO3; and (b–d) real part of permittivity and (f–h) imaginary part of permittivity of 100−xSTO/xNFO composites. Temperature dependence of the dielectric characteristics at a few selected frequencies for 100−xSTO/xNFO composites: (i–k) permittivity; (l–n) tangent loss.
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Table
Table 1. Determined Cell Parameters after Rietveld refinement.
Table 1. Determined Cell Parameters after Rietveld refinement.
SampleSpace GroupDirect Cell Parameters (Å)Direct Cell Volume (Å3)Reliability Factors
SrTiO3P m -3 ma = b = c = 3.9034(3)59.476(3)0.796/0.899
NiFe2O4F d -3 ma = b = c = 8.3402(3)580.141(4)1.79/3.97
TiO2P 42/m n ma = b = 4.5912(2)
c = 2.9697(6)		62.601(4)5.01/13.2
Table
Table 2. Textural characteristic of the obtained nanocomposites.
Table 2. Textural characteristic of the obtained nanocomposites.
SampleBET
Surface Area
(m2 g−1)		Langmuir Surface Area
(m2 g−1)		Volume of Pores (Adsorption)
(cm3 g−1)		Volume of Pores (Desorption)
(cm3 g−1)		Average Pore Width (Adsorption)
(nm)		Average Pore Width (Desorption)
(nm)
75STO/25NFO52 ± 175 ± 10.148 ± 0.0040.146 ± 0.00415.86 ± 0.9113.97 ± 0.78
50STO/50NFO39 ± 159 ± 30.082 ± 0.0010.074± 0.0038.30 ± 0.777.44 ± 0.97
25STO/75NFO25 ± 236 ± 10.078 ± 0.0020.074 ± 0.0067.76 ± 1.127.36 ± 0.64
Table
Table 3. Bandgap values of NiFe2O4, SrTiO3 and SrTiO3/NiFe2O4 composite thereof.
Table 3. Bandgap values of NiFe2O4, SrTiO3 and SrTiO3/NiFe2O4 composite thereof.
CompoundCompositionBand Gap Values (eV)References
SrTiO3pristine3.20[7]
SrTiO3pristine3.15[34]
NiFe2O4pristine1.70[44]
NiFe2O4pristine1.82[34]
NiFe2O4pristine1.40[45]
γ-Fe2O3@NiFe2O450% γ-Fe2O3
50% NiFe2O4		1.38[45]
SrTiO3/NiFe2O415% SrTiO3
85% NiFe2O4		1.42[34]
75STO/25NFO75% SrTiO3
25% NiFe2O4		1.50This Study
Table
Table 4. Magnetic properties of NiFe2O4, SrTiO3, and SrTiO3/NiFe2O4 composites thereof.
Table 4. Magnetic properties of NiFe2O4, SrTiO3, and SrTiO3/NiFe2O4 composites thereof.
SampleSpecific Magnetization
(emu g−1)		Remanent Magnetization
(emu g−1)		Coercive Field
(Oe)		References
SrTiO3---This Study
75SrTiO3/25NiFe2O425.48.2214This Study
50SrTiO3/50NiFe2O433.210.5191This Study
25SrTiO3/75NiFe2O439.110.2119This Study
NiFe2O445.17.172This Study
SrTiO3/NiFe2O4 porous nanotubes 10n.an.a[8]
SrTiO3/NiFe2O4 nanoparticle-in nanotubes18n.an.a[8]
NiFe2O440n.an.a[34]
85% NiFe2O4
15% SrTiO3		23.3n.an.a[34]
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Baba-Ahmed, I.; Ghercă, D.; Iordan, A.-R.; Palamaru, M.N.; Mita, C.; Baghdad, R.; Ababei, G.; Lupu, N.; Benamar, M.A.; Abderrahmane, A.; et al. Sequential Synthesis Methodology Yielding Well-Defined Porous 75%SrTiO3/25%NiFe2O4 Nanocomposite. Nanomaterials 2022, 12, 138. https://doi.org/10.3390/nano12010138

AMA Style

Baba-Ahmed I, Ghercă D, Iordan A-R, Palamaru MN, Mita C, Baghdad R, Ababei G, Lupu N, Benamar MA, Abderrahmane A, et al. Sequential Synthesis Methodology Yielding Well-Defined Porous 75%SrTiO3/25%NiFe2O4 Nanocomposite. Nanomaterials. 2022; 12(1):138. https://doi.org/10.3390/nano12010138

Chicago/Turabian Style

Baba-Ahmed, Ilyes, Daniel Ghercă, Alexandra-Raluca Iordan, Mircea Nicolae Palamaru, Carmen Mita, Rachid Baghdad, Gabriel Ababei, Nicoleta Lupu, Mohamed Amine Benamar, Abdelkader Abderrahmane, and et al. 2022. "Sequential Synthesis Methodology Yielding Well-Defined Porous 75%SrTiO3/25%NiFe2O4 Nanocomposite" Nanomaterials 12, no. 1: 138. https://doi.org/10.3390/nano12010138

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