Ni²⁺doping effect on potassium barium titanate nanoparticles: enhancement optical and dielectric properties

Pure potassium barium titanate KBTO and KBTO doped with nickel (Ni²⁺) were prepared by a modified sol–gel method. The sol–gel method has the advantage that it is relatively simple, has easy stoichiometric composition control, and is low cost. Weighted amounts of the appropriate proportions of high purity barium acetate (Ba(CH₃COO)₂), titanium(IV) isopropoxide (Ti[OCH(CH₃)₂]₄), Potassium nitrate (KNO₃), and nickel nitrate (Ni(NO₃)₂·6H₂O), as a source of Ba²⁺, Ti⁴⁺, K¹⁺, and Ni²⁺ ions respectively, were used as the starting precursors. Acetic acid (HAc)–H₂O mixture, acetyl acetone (AcAc, C₅H₈O₂), and distilled water were adopted as solvents, respectively. For pure KBTO, barium acetate was dissolved in a mixture of acetic acid and water. Potassium nitrate was dissolved in water, while titanium (IV) isopropoxide was dissolved in acetyl acetone. The different solutions were mixed according to the stoichiometric ratio (K: Ba: Ti—0.1: 0.9: 1), the resultant solution was stirred for 1h and dried at 200 °C. While Ni-KBTO was prepared as pure KBTO except nickel nitrate was dissolved into the distilled water and was added to the mixture according to the stoichiometric ratio (K: Ba: Ti: Ni—0.1: 0.9: 1-x: x, respectively), table 1, then followed with stirring and drying.

The dried gel was milled and calcinated at 600 °C for 2 h after calcination the final powder was milled again to carry out the different characterization.

The pure KBTO and KBTO doped with Ni²⁺ were characterized by the x-ray Diffraction (XRD), (XPERT x-ray) with Cu Kα radiation. The surface morphology was characterized using scanning electron microscopy, SEM (JEOL: JSM-7610FPlus). High-resolution transmission electron microscopy (HR-TEM) was carried out on a transmission electron microscope, TEM (FEI -Tecnai T20). The effect of Ni²⁺ on the thermal decomposition of KBTO was surveyed through the thermogravimetric analysis (TGA).The experiments were directed under a flowing atmosphere of the nitrogen at a purge rate of (50 ml min⁻¹). The weight for each sample was between 5–10 mg and was heated from room temperature to 700 °C preserving a programmed heating rate of (10 °C min⁻¹).

The effective tool for evaluating the bandgap of nanoparticles samples by UV–Vis diffuse reflectance spectroscopy (DR unit; JASCO V550) and the optical band gap of semiconductors can be assessed using the Tauc Plot method.

Dielectric measurements were performed using LCR METER IM3536 with a frequency range from 4Hz to 8MHz. The powders were pressed in a special die (10 mm diameter) to form tablets with a thickness of nearly 1.2 mm. Dielectric measurements were carried in the frequency range from 4 Hz to 8 MHz and in the temperature range from 25 °C to 260 °C.

The relative permittivity of materials is given from the equation:

$$\begin{eqnarray}&&\left\{{\varepsilon }^{* }=\varepsilon ^{\prime} +{\rm{i}}\varepsilon ^{\prime\prime} \right\}\end{eqnarray} \tag{ 1 }$$

The dielectric constant calculated from the relation:

$$\begin{eqnarray}&&\left\{\varepsilon ^{\prime} \left(v\right)=\displaystyle \frac{{\rm{C}}^{\prime} \left({\rm{v}}\right){\rm{d}}}{{\varepsilon }_{{\rm{o}}}{\rm{A}}}\right\}\end{eqnarray} \tag{ 2 }$$

Where A is the area of the upper electrode, d is the thickness of the sample, ν is the frequency in Hertz, and ε_o is the permittivity of the free space (8.854 × 10^–12 F m⁻¹). While, the loss tangent (tan (δ)) is given by

$$\begin{eqnarray}&&\tan \left(\delta \right)=\displaystyle \frac{\varepsilon ^{\prime\prime} }{\varepsilon ^{\prime} }\end{eqnarray} \tag{ 3 }$$

The AC conductivity calculated from the relation:

$$\begin{eqnarray}&&\left({\sigma }_{{\rm{ac}}}=\varepsilon ^{\prime\prime} {\varepsilon }_{{\rm{o}}}{\rm{\omega }}\right)\end{eqnarray} \tag{ 4 }$$

Where ε'' is dielectric loss of the sample and ω = 2πν.

Figure 1 presents the XRD patterns of potassium barium titanate doped with Ni²⁺. The XRD pattern shows a hump that indicates the presence of an amorphous phase structure. The amorphous structure appeared due to the low calcinating temperature and it confirms the presence of carbon atoms and water molecules in the structure[20].

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XRD spectra indicate that explicit chemical reactions and coexistent are detected between BaO, TiO₂, K₂O, and NiO₂ during the sol-gel proceedings for KBTO and Ni-KBTO nanoparticles. The formation of decrepitating diffraction intensity peaks after the calcination process at 600 °C is a directory of the uncompleted long range order in the internal structure of these nanoparticles [21, 22]. These peaks were indexed to tetragonal BaTiO₃ and Monoclinic BaTi₂O₅.

As obviously seen in SEM images of KBTO (figures 2(a) and 3 Ni²⁺ co-doped KBTO (figure 2(b)), after the calcinating process at 600 °C, the surface exhibited a highly connected, aggregates nanoparticles, and dense structure with various particle nanosized shapes of KBTO and 3 Ni-KBTO linkages into the surface. From figure 2 seems the densification process of KBTO and 3 Ni-KBTO nanocomposites might be influenced by the initial composition and the high activity of the substitution of K¹⁺ and Ni²⁺ within the BTO framework [23].

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Both surface structures and grains nano-size of the prepared nanocomposites are containing nano-spheres and nano-rods shapes, as shown in the SEM images (figures 2(c), (d)). Doping with Ni²⁺ ions brought distinct variation in the smoothness of surface morphology of KBTO. The higher magnification of SEM images shows that the KBTO (figures 2(c) and 3) Ni-KBTO (figure 2(d)) nanocomposites were composed of nano-crystals with 20–45 nm and 80–120 nm in length for rod shape.

TEM images of KBTO and doped with 3 Ni²⁺nano-powders calcined at 600 °C, are given in figure 3. Figure 3 appears that the KBTO and 3Ni-KBTO nanopowders consist of nano-crystallites with various diameters, and the presence of aggregated particles. This result confirms the broad and irregularity XRD peaks of KBTO and 3Ni-KBTO nanopowders (figure 1). These mostly KBTO nano-crystallites aggregate in large particles and Ni²⁺doped samples form fine nanoparticles. The observed lattice fringes for KBTO and 3 Ni-KBTO samples using the HR-TEM give a well nanocrystalline structure (figures 3(c), (d)). Also, the observation for figures 3(c), (d) had been exhibiting the lattice planes of KBTO with interplanar distance 0.353 nm, and 0.470 nm for 3 Ni-KBTO nanostructures, which confirm the semi-crystalline nature of these nanostructures. It can be observed a difference in the interplanar distance between the KBTO and doped sample, indicates the lower crystallinity (relative amorphous structure, more lattice distortions) in the local structure for the 3Ni-KBTO.

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Thermogravimetry is described as a technique where the mass of the material is continuously verified as it is cooled or heated at a constant rate in an appropriate environment[24]. The widest application of thermogravimetric analysis including determining purity and thermal stability of both primary and secondary standards, determination of the composition of alloys and mixtures, and the direct application to analytical problems [24]. Thermogravimetry is regularly used for determining the drying ranges of precipitates. However, the dynamic nature of the technique must be considered when interpreting the best drying temperature. TGA studies may be done isothermally or non-isothermally[25]. If the heating rate is taken as zero then the technique becomes isothermal. This would mean that temperature is kept constant and mass variations are verified as a function in the time. When the heating rate is not equal to zero the technique is non-isothermal, mass changes are verified as a function of temperature[24].

In the present work, thermogravimetric analysis (TGA) was performed on a TGA (LINSEIS STA PT1600, Germany).

Figure 4 shows the TGA results obtained for the Ni-KBTO powders. The TG curves show a total weight loss of the powders till 700 °C of approximately 30, 28, 21, and 14% for 0, 1, 2, and 3 Ni-KBTO samples, respectively. These results indicate the enhancement of thermal stability of the nanocomposites by NiO addition. The thermal events showed in the results are the impurities, crystallization water, or adsorbed water releasing (from heating start to about 496, 490, 482, and 392 °C) followed by degradation event, which has a sharp loss rate. The degradation plots revealed also that impurities such as H₂O and other oxides remaining from the preparation were present in an amount of 21, 11, 9, and 2% for 0, 1, 2, and 3 Ni-KBTO samples, respectively. The results are in agreement with that detected by Maček Kržmanc et al [26].

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3.5.1. Diffuse reflectance and bandgap estimation

One of the most significant optical modulation techniques is diffuse reflectance (DR) spectroscopy. The method is appealing because of its nondestructive nature, simplicity, and relatively crisp features detected at room temperature. It has recently been widely used in semiconductor quantum wells and bulk materials to explore critical point transitions, damage patterns, and subsurface inhomogeneities [27].

Diffuse reflectance was employed in studying the optical properties of KBTO nanoparticles in the wavelength range 200–2500 nm as shown in figure 5(a).The diffuse reflectance of all samples increases with wavelength increase till the saturation point, at about 795 nm. Then, it tacks a decreasing path till 2500 nm wavelength. There is a convergence in the DR values where the increase or decrease does not indicate a clear effect of the addition of nickel. Also, the percentage of nickel is small, and therefore its effect on the interference will be limited.

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From the diffuse reflectance, the Kubelka-Munk ((R_∞)) can be calculated from the next formula [28]:

$$\begin{eqnarray}&&F\left({R}_{\infty }\right)={\left(1-{R}_{\infty }\right)}^{2}/2{R}_{\infty }=C{\left(h\nu -{E}_{g}\right)}^{n}/h\nu \end{eqnarray} \tag{ 5 }$$

where F(R_∞) gives the Kubelka-Munk formula,R_∞ is the diffuse reflectance ratio between sample and reference, C is constant, hυ photon energy, E_g gives optical band gap, and n is an index which takes the value ½ (direct allowed), 2 (indirect allowed), 3 (direct forbidden), and 3/2 (indirect forbidden) count on the style of the electronic transition affect the reflection. So, the type and the value of the bandgap can be determined from the Tauc plot defined as: [29]

$$\begin{eqnarray}&&\alpha h\nu =C{\left(h\nu -{E}_{g}\right)}^{n}\end{eqnarray} \tag{ 6 }$$

Where C is the proportionality constant.

The bandgap type and value can be determined from the Tauc plot by extrapolating the linear part to zero or from the first derivative of the absorbance regarding photon energy and uncovering the maxima at the lower energy in the derivative spectra. Figure 5(b) shows the plotting of (αhʋ)^1/2 versus (hʋ) to obtain the type and the value of the gap transition. Indirect energy gap types are valid in current samples with values of 2.77, 2.49, 2.39, and 2.3eV for 0, 1, 2, and 3 KBTO, respectively.

It is noticed that with the increase of NiO content the indirect transition gap decreases as presented in figure 5(c). The optical band gap is influenced by the existence of localized electronic states in the specimen [30].

Lead-free (ABO₃-type) perovskite materials based on BaTiO₃ have achieved some interest where the ABO₃-type perovskite materials with physical properties are controlled via A and B site cations and have an octahedral structure [31]. The valence and conduction band of ABO₃-perovskite are collected of d orbital of the transition metal ions and the O-2p orbital. Also, their bandgap relates to the energy difference between their valence and conduction bands [31, 32].

Newly, many studies have proven that doping by transition metal ions in the B-site (such Ni²⁺, Fe³⁺, Zn²⁺, and Co²⁺) can powerfully decrease the band gap of perovskite materials, thus progress the absorption efficiency of light. It was found that the doping of BaTiO₃ by Ni²⁺ ion leads to the decrease of bandgap [33].

This behavior was interpreted through the doping mediation mechanisms[31]. Many studies have verified that in BaTiO₃, the conduction and valence bands are created throughO-2p orbital and Ti-3d orbital, respectively [34]. On the other hand, the conduction and valence bands are in NiO primarily generated from Ni-3d orbital and O-2p orbital, respectively [35]. The lowering the conduction band energy, the larger the electronegativity of ions, [34], electronegativity of Ti = 1.54, and Ni = 1.91 [34]. In K(1-x)BaTiO₃-xNiO nanoparticles, as the electronegativity of the (Ni²⁺) ion is larger than that of the (Ti⁴⁺) ion, and the Ni-3d orbital is lower than Ti-3d orbital in energy, the energy band gap of doped KBaTiO₃ materials forced into narrowing [31]. Also, the addition of Ni²⁺ was introduced at the B-site of KBaTiO₃ as a substituting of Ti⁴⁺. As the two ions have a different radius, oxygen vacancy defects will be created and distort the formed lattice which in turn will alter the KBaTiO₃ energy band [36]. The results indicate that the E_g of KBaTiO₃ can be tuned by adjusting the doping level of NiO.

3.5.2. Refractive index and optical dielectric constant

The material refractive index is known to minimize with energy gap. Some of the relations that shows this relation are [37, 38]:

Moss relation:

$$\begin{eqnarray}&&{n}^{4}{E}_{g}=95\,{\rm{eV}}\end{eqnarray} \tag{ 7 }$$

Where n and E_g are respectively the refractive index and energy gap.

Vandamme relation:

$$\begin{eqnarray}&&{n}^{2}=1+{\left(A/\left({E}_{g}+B\right)\right)}^{2}\end{eqnarray} \tag{ 8 }$$

Where A is the hydrogen ionization energy and B is a constant defines the difference among the UV resonance energy and bandgap energy.

Kumar and Singh relation:

$$\begin{eqnarray}&&n=K{E}_{g}^{C}\end{eqnarray} \tag{ 9 }$$

Where K and C are constants.

The refractive indices of all the prepared samples were calculated by these three attempts.

Figure 5(d) shows the change of the refractive index with NiO content calculated by different attempts for indirect transition cases. It is obvious that there are small differences between n values calculated by different methods and this may be due to the difference in mathematical treating and different approximations methods used by every attempt. Also, the refractive index increases slightly with increasing NiO content as a result of bandgap energy decrease.

The material dielectric constant is linked to the refractive index [37] by:

$$\begin{eqnarray}&&{\varepsilon }_{\infty }={n}^{2}\end{eqnarray} \tag{ 10 }$$

The refractive index can be calculated from the above exponential relation and plotted in figure 5(e). It was noted that the dielectric constant change behavior with NiO content is similar to the behavior of the refractive index.

Figure 6 displays the dispersion behavior of the dielectric constant (ε') at different temperatures, where the dielectric constant decreases monotonically with increasing frequency. According to figure 6, the dielectric constant attains a huge value, more than six orders, at the low-frequency zone, while it reaches the least value of 30 to 80 at the high frequency. This divergence in the dielectric value over the total frequency extent can be assigned to the different polarization mechanisms, which take place inside the samples under the effect of the external field, especially the interfacial polarization, electrode polarization, and orientational polarization[39–41]. At the low extent of frequency, the electrode polarization beside the interfacial polarization is the most dominant effect where the free charge carriers have sufficient time to transfer and agglomerate at the electrodes. The interfacial polarization takes place as a result of the agglomeration of the charge carriers around the interfacial barrier between the constituents of the polycrystalline composites samples this effect is known as a Maxwell-Wagner's model[42, 43]. The ferroelectric barium titanate co-doped with potassium and nickel ions calcinated at low temperature characterized by the presence of multi phases with a higher concentration of oxygen vacancies, these oxygen vacancies are released from the replacement of the tetravalent ions (Ti⁴⁺⁾ by divalent (Ni²⁺) ions. Such constituents increase the effect of the electrode polarization and the interfacial polarization, thus increase the total dielectric constant. At low frequency, these constituents increase the direct conductivity contribution in the dielectric constant value. At the elevated frequencies, the dielectric constant (ε') decreases due to the disappearance of the different polarization mechanisms effect [44, 45]. This effect is released due to the effective dipole moment at low frequency can't catch up with the polarity variation of the applied field at high frequency [46]. According to figure 6, the dielectric constant shows temperature dependence. The temperature dependence of the dielectric constant at different frequencies is represented in figure 7.

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The dielectric constant at different frequency increases as the temperature increase up to a certain temperature then decreases with further increase in temperature. This temperature is known as a Curie temperature (Tc), it represents a transition temperature where the materials are transferred from the poled state to the un-poled state. Above this temperature the ferroelectric material obeys the Curie Weiss Law;

$$\begin{eqnarray}&&\varepsilon ^{\prime} =\displaystyle \frac{C}{T-{T}_{c}}\end{eqnarray} \tag{ 11 }$$

Where T_C is the transition temperature (Curie temperature) and C is the Curie constant. Figure 7 refers to the Curie temperature decreases as introducing Ni ions into the Potassium Barium titanate structure. This behavior can be assigned to the electronegativity of Ni²⁺ is higher than the electronegativity of Ti⁴⁺ and so the bond length (Ni-O) is less than the bond length(Ti-O) results in a decrease in the dipolemoment arm (distance between positive and negative charge). Thus, the polarity of nickel doped potassium barium titanate is less than the polarity of potassium barium titanate [4, 47]. The Curie constant obtained from the curve (1/ε'vs T) in the temperature range higher than the Curie temperature, Figure 8. The Curie constant are listed in table 2.

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Figure 9 presents the frequency dependent of loss tangent (tan (δ) versus ν) at different temperatures. A loss peak is observed in the loss tangent curve in all samples. The frequency at the maximum value of the peak is defined as the peak frequency (ν_max) and it is used to get the relaxation time (t = 1/2πν_max). Figure 9 shows that the peak is moved to a higher frequency as the temperature increases to the Curie temperature and then moved to a low frequency with a further increase. This behavior proves that samples are thermally activated below the Curie temperature.

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Figure 10 shows the experimental conductivity at several temperatures, it shows two distinct areas. The first area represents DC conductivity where it shows a steady behavior over the low frequency area. While at the second area the conductivity increases with raising frequency it represents AC conductivity. The temperature dependence of the conductivity shows that the conductivity increases with increasing temperature below the Curie temperature and after this transition temperature the conductivity decreases with increasing temperature. This behavior proves that the ferroelectric samples are thermally activated below the transition temperature and after this temperature, the ferroelectric samples behave like metallic materials.

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