Structural and optical studies of BiFeO₃@SiO₂ core/shell nanoparticles

Bismuth ferrite nanoparticles were synthesized using sol-gel combustion method. Analytical grade (purity, 99.99 %) Bi(NO₃)₃·6H₂O and Fe(NO₃)·9H₂O were used as precursors. First, bismuth nitrate was dissolved in 1 N nitric acid at 60 °C and iron nitrate was added to the solution and then constantly stirred at 60 °C to obtain a sol. Citric acid was added to it in 2:1 molar ratio and stirred vigorously at 60 °C for 2 h. The light brownish solution was kept on the hot plate at 60 °C (without stirring) for several hours to obtain a thick gel. The gel was transferred to a hot air oven maintained at temperature 220 °C. After a few hours, the gel was observed to be completely dried and auto ignited releasing large amount of gases. The dark brownish powder was collected and subsequently ground and calcined at 500 °C in air for 3 h.

The BFO nanoparticles (NPs) were coated with SiO₂ by the Stöber method [15]. First, 1.5 g of BFO NPs was dispersed in 700 ml of ethanol by sonication. The mixture was kept on the magnetic stirrer and 10 ml of NH₃ solution (25 wt%) was then added drop wise. This was followed by the addition of tetraethyl orthosilicate (Si(C₂H₅O)₄) with concentration of 2% at constant stirring for 24 h. The SiO₂ coated BFO particles were allowed to sediment due to gravity and the excess silica as supernatant. The excess silica was removed and the coated BFO was washed with DI water and ethanol for four times. The BFO@SiO₂ NPs were dried in the oven at 200 °C for 2 h.

The crystal structure of BFO and BFO@SiO₂ samples were characterized by using x-ray diffractometer (XRD) (PANalyticalX'pert PRO) with Cu K_α radiations. The morphology of the samples was observed using a high resolution transmission electron microscopy (HRTEM) (FEI-FP-5022/22Tecnai G2 20 S-TWIN) and a field emission scanning electron microscopy (FESEM) (Nova Nano SEM-450). An FTIR spectrometer (JASCO-FT/IR 4100) was used for studying the vibrational properties. The optical properties were studied from the absorption spectrum of a UV–vis spectrophotometer (SCHIMAZE-2450).

Figure 1(a) shows the XRD pattern of the BFO NPs. The diffraction peaks are observed to be sharp and demonstrate a highly rhombohedral oriented crystal structure according to the JCPDS file no. 86-1518. An impurity phase of Bi₂₅FeO₄₀ is also found with a very small intensity. In the angle range of 31–32°, a partial overlap of doubly split peaks (1 0 4) and (1 1 1) of BFO is observed. This shows the coexistence of small percentage of orthorhombic phase with the parent rhombohedral phase [16]. In figure 1(b), the XRD pattern of BFO@SiO₂ NPs exhibits a combination of both BFO and SiO₂ phases. The intensity of peaks for BFO phase decreases after SiO₂ coating. Moreover, a broad peak centered at 23° is observed. This confirms the coating of amorphous SiO₂ on the surface of BFO NPs.

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3.2.1. SEM study

Figure 2(a) displays typical FESEM image of BFO NPs. The image of BFO exhibits agglomerated NPs with a size distribution of 70–150 nm. The average particle size has been determined to be 100 nm with image J software (shown in figure 2(b)). The morphology of SiO₂ coated BFO particles are shown in figure 2(c). As shown in figure 2(d), the particle size is observed in the range of 90–190 nm after SiO₂ coating (with an average size of 113 nm). The increase in the particle size confirms SiO₂ shell formation.

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3.2.2. TEM study

In order to obtain detail information on as-prepared core/shell BFO@SiO₂ NPs, bright field HRTEM micrographs were recorded. Figure 3(a) shows the typical morphology of the BFO particles. The particles are formed as hexagonal and cubic shapes. This could be due to the calcination temperature, that results in fraction of particles to be cubic i.e. the orthorhombic phase (mentioned in XRD analysis). The size of the particles is estimated from the TEM image. It has been found to be 100–120 nm, which is close to the value obtained from FESEM. Figure 3(b) shows a typical polycrystalline structure without any amorphous region at particle surface. The inter-planar spacing is calculated to be about 0.28 nm which is shown in inset to figure 3(b). As the (1 1 0) surface has the lowest surface energy, the structure prefers to grow along [1 1 0] direction [17].

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Figure 4(a) displays the TEM image of BFO@SiO₂ NPs. The TEM image reveals the formation of well crystalline BFO NPs with the amorphous SiO₂ being deposited on the particles. The particles have amorphous shell and crystalline core structure as confirmed by HRTEM image (figure 4(b)). The inter-planar distance measured from the adjacent lattice fringes has been found to be about 0.28 nm. This indicates that the crystallinity of BFO cores persists after SiO₂ coating.

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FTIR spectroscopy is used to reveal the functional groups of BFO and BFO@SiO₂ NPs. Figure 5 displays the spectra of samples with indication (arrow mark) of the presence of functional groups at a particular wave number. The metal oxide bands in the range of 400–600 cm⁻¹ usually attribute to the Fe–O stretching and bending vibrations in octahedral FeO₆ unit, which confirm the formation of perovskite structure in the samples [18]. The absorption peaks at 444 [19] and 552 cm⁻¹ [20] correspond to the bending vibration and the Fe–O stretching modes in the FeO₆ octahedral site respectively. The coating of SiO₂ NPs can be confirmed from the observed O–Si–O bands at 474 cm⁻¹ and 1111 cm⁻¹ [21]. This suggests a strong interaction between the BFO core and SiO₂ shell at the interface.

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To investigate the optical properties of BFO and BFO@SiO₂ NPs, the UV–vis diffused reflectance spectra were recorded at room temperature. The diffused reflectance of the NPs was measured with a UV–vis spectrometer with an integrating sphere attached (ISR 2200). The measurement was performed in the wavelength range of 200–900 nm using barium sulphate as a reference. The Kubelka–Munk function, F(R), also known as optical absorption, was calculated from the diffused reflectance (R) of the NPs by the relation F(R)  =  (1  −  R²)/(2R). Absorption spectra of the BFO and BFO@SiO₂ are shown in figure 6(a). The UV–vis spectra demonstrate that the BFO@SiO₂ NPs can absorb significantly more light in the 400–510 nm region than BFO, but the optical absorption in the wavelength region of 510–650 nm exhibits lower value than BFO.

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Figure 6(b) displays the curve between (F(R)hν)² and (hν) in the energy range of 1.5–4.5 eV. A shoulder centered at 1.92 eV is observed, which can be assigned an Fe³⁺ crystal field excitation (6A_1g  →  4T_2g) [22, 23]. These excitations are forbidden formally because they change the total spin from S  =  5/2 to S  =  3/2 of Fe³⁺ ion. However, spin orbit coupling relaxes the spin selection rule and gives rise to these transitions [24, 25]. Thereafter, the absorption increases significantly above 2 eV, with small shoulders centered at 2.5 eV and near 3.4 eV. The transition band centered at ~2.5 eV may be assigned to the Fe₁3d–Fe₂3d intersite electron transfer, whereas the other band around ~3.4 eV, may be associated with the interatomic O2p-Fe3d transition. A noticeable shift in these transition bands clearly indicates the changes in FeO₆ local environment in case of BFO@SiO₂ NPs. Due to some limitations of the UV–vis spectrometer used in this work, the absorption below 200 nm could not be recorded. This prohibited us to estimate the energy band gap of SiO₂ (>5 eV).

To analyse the effect of associated transitions, the energy band gap of BFO and BFO@SiO₂ NPs has been determined from the spectra using Tauc's law:

$$\begin{eqnarray}&&\alpha h\nu =A{{\left(h\nu -\,{{E}_{\text{g}}}\right)}^{n}}\end{eqnarray} \tag{ 1 }$$

where α  =  F(R), hν is the photon energy, A is a constant, E_g is the optical energy band gap. The exponent n represents the type of optical transitions between valence and conduction bands. In case of direct allowed band transition, n  =  1/2, while indirect allowed transition is expected for n  =  2 [26]. Figures 6(c) and (d) display the plots between (F(R)hν)²  −  (hν) for the direct allowed transition and (F(R)hν)^1/2  −  (hν) for indirect allowed transition, in the energy range of 1.5–2.4 eV.

The band gap is determined from an intercept on X-axis, made by extrapolating a straight line. The value of the band gap is found to be 2.0 eV (figure 6(c)), which is consistent with those reported for BFO nanostructures (1.8–2.5 eV) [27, 28]. The band gap for the sample BFO@SiO₂ is 2.1 eV. Considering indirect transitions (figure 6(d)), the E_g values are somewhat lower as compared to the corresponding E_g values, obtained from direct transitions. The values obtained for E_g are presented in table 1. The table also shows the values of E_g obtained from the absorption edge (using E_g  =  1240/λ) of figure 6(a). As per the direct transitions, the values obtained for E_g are nearly matching with that of calculated values using the above relation.

In literature, we find a lot of contradicting arguments in predicting BFO as direct/indirect band gap semiconductor from optical absorption data. To name a few, Lin et al [29] reported the direct band gap of BFO thin films to be 2.3 eV. Based on the lack of the characteristic shape of the (αhν)^1/2 versus hν plot, they have ruled out the possibility of indirect band transition. However, Mc Donnnell et al [30] have observed simultaneous presence of direct and indirect transitions for BFO experimentally and computationally.