1 Introduction

Nanoscale inorganic materials have grown considerable interest because of their unique properties and wide range of applications [1,2,3,4,5]. Among the various inorganic materials, chromium-based compounds have gained importance in many fields such as photosensitizers and photoconductive dielectric, near-infrared lasers, catalysis, ionic conductors, and magnetic materials [6].

A solution casting technique was reported for the preparation of polyvinyl alcohol-potassium chromate composite film, which exhibited excellent dielectric properties within the frequency range from 50 Hz to 30 MHz and expected to be applied as an electrical charge storage device [7]. The electrical studies of pure and Cu-doped Cr2O3 nanostructures indicated the enhancement of dielectric constant, dielectric loss, and conductivity with rising the temperature in the frequency scan. Increasing of dopant concentration also contributed higher dielectric constant at a higher temperature [8]. Ag2CrO4 is a well-known photocatalyst and the photocatalytic activity was highly dependent on the preparation methods [9]. The photocatalytic activity of Ag2CrO4 was boosted by incorporating AgBr, which showed 1.5 times higher activity than pure Ag2CrO4 [10]. In addition, composite of Ag2CrO4 and graphene oxide shows excellent photo-catalytic and dye hydrolysis activity under the visible light irradiation [11]. Dielectric property of strontium chromate exhibited colossal behavior in the dielectric constant value near ambient temperature and relaxor-like dielectric behavior around 730 K with high dielectric constant value [12].

Recent studies on dielectric materials demonstrated the electromechanical properties and pressure-induced capacitive performances [13,14,15]. The capacitive pressure sensor is considered the most promising material due to their simple fabrication process, rapid dielectric response, low dielectric loss, and low power consumption [13]. It was reported that the flexible conducting polymer-based substrate, coated with a dielectric layer, usually improves the capacitive sensor activity [16]. Conductive fillers such as metal oxide nanoparticles, carbon nanotube, grapheme, and polymer (polyvinylidene fluoride, polyvinylpyrrolidone) are applied as the most effective material for dielectric layer for capacitive pressure application [17].

Lead chromate is an important solid material that is used as a yellow pigment employed for industrial purposes [18]. Lead chromate was also reported to be used as a host substance for humidity-sensing resistors and photosensitizers [19]. High permittivity value (~ 800) of PbCrO4 nanorods has been reported at the lower frequency range, originated due to the interfacial polarization, which was formed from the accumulations of the charge species at the surface of nanorods [6]. The PbCrO4 nanorod was also applied as electrode materials for Li-ion batteries with the capacity value of ~ 163 mA.h.g−1 during the discharge process [6].

Graphene-mediated charge transfer process was observed in lead chromate-based nanocrystal for photocatalytic water splitting application [20]. Photocatalytic dye degradation and water splitting performances were also reported in the monoclinic phase of PbCrO4 nanostructure [21, 22]. Efforts have been made to create nanostructured lead chromate with various morphologies. It is well established that the morphology, dimensions, and size of the nanomaterials have a significant impact on their physical and chemical properties [23, 24]. Various synthesis routes have been documented in the literature to prepare of lead chromate with various sizes and shapes. Spherical- and nanotube-shaped lead chromate have been synthesized by the microemulsion method [25, 26], whereas rod-shaped lead chromate was obtained by the hydrothermal [27] and microwave [28] assisted routes. Lead chromate nanowires were synthesized by a reaction of lead nitrate, potassium dichromate in the presence of poly(vinylpyrrolidone) at room temperature [29].

The preparation of lead chromate in the nanosized range has attracted much attention because of their unique properties and applications in many fields. Herein, a complexation-assisted approach was utilized for the synthesis of aniline functionalized nanostructured lead chromate. The structural properties of the synthesized organic–inorganic hybrid material were determined using X-ray diffraction, Raman analysis, and X-ray photoelectron spectroscopy techniques. The dielectric, AC conductivity, electrical impedance, and field-dependent polarization performance of the hybrid material were analyzed in the form of a device. The fabricated lead chromate–based device was also functioned as a voltage generator under applied pressure.

2 Experimental section

2.1 Materials

All the chemicals (aniline, potassium chromate, and lead nitrate) used in this study were analytical grade, Sigma Aldrich, and used without any further purification.

2.2 Synthesis of organic molecule stabilized lead chromate

For the current experiment, 1.5 mL of aniline was added in 10 mL of methyl alcohol in a conical flask. To the diluted aniline, an aqueous lead nitrate solution (7 mL with the concentration of 1 × 10−1 M) was added and subsequently Pb(II)-aniline complex was formed in the form of a precipitation. Five milliliters of potassium chromate solution (0.1 M) was added to the above precipitation that resulted the formation of yellow-colored aniline functionalized lead chromate. The solid product was filtered, dried under vacuum, and characterized using microscopic, optical, and surface analysis techniques. The electrical properties of the synthesized lead chromate were investigated and measured in the format of a device.

2.3 Device fabrication

The aniline-stabilized lead chromate nanoparticle-based device (PCO) was fabricated according to the following protocol. Initially, a slurry was made with the synthesized material and chloroform and deposited the slurry by using drop and dry method on an indium tin oxide–coated polyethylene terephthalate (PET) of thickness (~ 100 µm). The coated PET film was oven dried (50°C) for 30 min and then gold electrode was printed on the top side of the PET film using physical vapor deposition technique (EMSCOPE SC 500).

2.4 Material characterization and device measurement

The microscopic characterization was performed using transmission electron microscope (JEOL, JEM-2100) technique. The structural analysis was investigated by X-ray diffraction method (Philips PAN-analytical X’pert diffractometer) using Cu-Kα radiation source. Raman spectrum was recorded using Jobin–Yvon T64000 spectrometer under 514.5 nm excitation of an argon ion laser source. X-ray photoelectron spectroscopy, XPS, was measured using VG MultiLab 2000 system with a monochromatic X-ray source.

Temperature (30–120°C) and frequency (100 Hz–1 MHz) dependence dielectric capacitance, impedance, and AC-conductivity studies were performed using HP 4284A LCR meter interface to a computer-controlled EC1A environmental chamber. Electric field–dependent polarization (P-E) loops were collected using a Sawyer–Tower circuit where the samples were subjected to a triangular wave with a frequency of 10 Hz. Pressure-induced capacitance and voltage were measured on a flexible substrate of PCO-based device by using a force gauge meter (ZTA-500N, IMADA Corp.) and simultaneously monitored the capacitance performance on two-terminal LCR meter (HIOKI-3536), under the varying frequency condition. The contact area of ~ 1.0 cm2 was maintained between the gauge meter and the PET film (PCO) during the measurement and the exposed area was shielded by an insulating tape to avoid any electrical short circuit. The variation of pressure-dependent (0–45 kPa) open circuit voltage (Voc) output was recorded by a digital oscilloscope (Tektronix-TBS 1000C, USA).

3 Result and discussion

Figure 1 shows the transmission electron microscope (TEM) image of the lead chromate rods accompanied with irregular-shaped nanoparticles. The regular arrangement of diffraction in the SAED image (inset) revealed the crystalline phase of the lead chromate system. Figure 2A displays the X-ray diffraction pattern of lead chromate, recorded within the range (2θ) from 15 to 80°. The diffraction pattern was indexed according to monoclinic lattice symmetry with the space group of P21/n. The lattice constant values are a = 7.12 Å, b = 7.43 Å, c = 6.79 Å, and α = γ=90°, and β=102.4° (ICDD: 01–073-2059), with the unit cell volume of ~ 350.8 Å3. The unit cell structure is represented in Fig. 2B, belongs to monazite-type of structure, interconnected with a chain of alternating polyhedra (PbO9) and tetrahedra (CrO4)-units via edge and corner sharing network [30,31,32,33]. The PbO9 unit with eight shorter Pb–O bonds (in the range of ~ 2.53–2.80 Å) and one longer Pb–O bond (~ 3.08 Å) create a distorted type of structure. The CrO4 unit is slightly distorted and the bond lengths are in the range of ~ 1.65–1.67 Å [30, 31].

Fig. 1
figure 1

TEM image of lead chromate particles. The inset figure shows the selected area electron diffraction pattern

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Fig. 2
figure 2

A The X-ray diffraction pattern of PbCrO4 system (black line) recorded within the range (2θ) from 15 to 80° and indexed according to the monoclinic lattice symmetry (ICDD: 01–073-2059). B Schematic view of unit cell structure of PbCrO4 projected along a-axis. The structure consists of interconnected polyhedra (PbO9) and tetrahedral (CrO4) units in the network

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The various vibrational modes of lead chromate nanoparticles were investigated using Raman spectroscopy technique within the ranges from 20 to 2000 cm−1, Fig. 3A. The spectrum was de-convoluted in two different regions from 20 to 430 cm−1 and from 600 to 2000 cm−1, illustrated in Fig. 3B and C, respectively. In the low-frequency region (20–430 cm−1) six Raman active modes (ν1 to ν6) were identified, positioned at ν1 ~ 69 cm−1, ν2 ~ 92 cm−1, ν3 ~ 138 cm−1, ν4 ~ 183 cm−1, ν5 ~ 332 cm−1, and ν6 ~ 376 cm−1. The external vibrational modes (ν1–ν4) below 300 cm−1 originated due to the translation and rotational motion of lead-oxygen (Pb–O) network.23 The internal modes (> 300 cm−1) are related to the bending and stretching vibration of CrO4 tetrahedral unit [34, 35]. The ν5and ν6 modes, positioned at 338.4 cm−1 and 376.3 cm−1, respectively, correspond to the deformation of Cr–O bond inside the CrO4 tetrahedral unit. An intense peak ~ 834 cm−1 (ν7) represents the symmetrical stretching vibration of Cr–O–Cr bridging for poly-chromate (CrO4) unit. The existence of off-stoichiometric CrOx component was also identified in the spectrum with a diffuse type of mode ~ 949 cm−1 (ν8), developed during the synthesis process [35, 36]. This mode revealed the presence of mixed valence state of chromium ion (Cr6+/Cr3+). The C–N and C–C stretching modes at 1213 cm−1 (ν9) and 1658 cm−1 (ν10), respectively, are observed in the spectrum due to the presence of organic matrix [37].

Fig. 3
figure 3

A Raman spectra of the lead chromate system, measured within the range from 20 to 2000 cm−1. The internal (CrO4) and external (PbO9) vibrational modes were identified (ν1 to ν10) in the spectrum. B, C The deconvoluted (blue color) Raman spectra with Raman active modes are illustrated within the ranges from 20 to 430 cm−1 and 600 to 2000 cm−1, respectively

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X-ray photoelectron spectroscopy (XPS) analysis was used to extract the oxidation states of aniline-stabilized nanostructured lead chromate system. The survey spectrum, Fig. 4A, exhibited the elemental peaks of Pb, Cr, O, N, and C. Nitrogen and carbon peaks are generated from the organic components. Figure 4B displays the high-resolution XPS-spectrum of core-level Pb 4f with the spin–orbit splitting components 4f5/2 and 4f7/2, positioned at 143.0 eV and 138.2 eV, respectively, associated with the Pb2+ oxidation state [38]. The high resolution Cr 2p core level spectrum, after deconvolution using Gaussian-Lorentizan curve fitting, Fig. 4C, shows two prominent peaks located at 588.1 eV and 578.8 eV, correspond to 2p1//2 and 2p3//2, respectively, for Cr6+ oxidation state of oxyanion CrO42− unit and supported the formation of lead chromate (PbCrO4) [38, 39]. However, Cr 2p3//2 also associated with an additional diffuse peak at 576.3 eV represents the off-stoichiometric component of CrOx, which indicates the presence of mixed oxidation state of chromium (Cr6+/Cr3+) in the system [40, 41]. The O 1 s XPS spectrum, Fig. 4D, shows a prominent peak positioned at 529.2 eV with a diffused hump at 530.4 eV, which correspond to the lattice oxygen and oxygen vacancy, respectively, in the lead chromate system [12]. The high-resolution C 1 s spectrum, Fig. 4E, revealed the presence of C–N and C–C interaction at 285.8 eV and 284.5 eV, respectively, originated from the organic component of the hybrid system.

Fig. 4
figure 4

A XPS survey spectrum of the lead chromate system and high-resolution de-convoluted spectrum of B Pb 4f, C Cr 2p, D O 1 s, and E C 1 s

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The frequency (100 Hz to 1 MHz) dependent dielectric constant (εʹ) of the PCO-based device was measured under different temperature conditions (30–120°C), displayed in Fig. 5A. The dielectric constant value was increased with decreasing frequency, and at 30°C the value reached ~ 372 at 100 Hz. At 120°C, a significant improvement of the dielectric constant value was achieved toward low-frequency region and reached to ~ 2400 at 100 Hz. The high value of dielectric constant (ε′) toward low frequency can be explained in term of space charge polarization and orientation polarization of dipoles [42, 43]. The contribution from orientation polarization of PbCrO4 was originated from the dipolar type of interaction in the chromium-oxyanion unit [39]. The strong covalent bond between the O-2p and Cr-3d influenced the net dipolar polarization in the structure. The presence of mixed oxidation state of Cr6+/Cr3+ facilitates the formation of oxygen vacancy and defect site in chromium-oxyanion unit. The electrons generated during the formation of oxygen vacancies are trapped at defect sites, which caused a space charge polarization contribution. In addition, the accumulation of these space charges at the interface between organic (aniline) and inorganic (PbCrO4) systems amplifies the polarization to the high dielectric constant value in the low-frequency region [44]. With rising temperature, the space charge polarization process is predominated over the dipolar interaction in the PCO-based device. At high-frequency condition, the induced polarizations are not able to track the frequency variation and that caused a decrease in the dielectric constant value. The dielectric loss (tan δ) curves for selected temperatures, 30, 80, and 120°C, Fig. 5A, inset, are almost similar in behavior to the obtained dielectric constant and maintained a low loss (tan δ < 0.9) over the entire frequency and temperature ranges. The εʹ curves were fitted according to the modified Cole–Cole relaxation model (Eq. S2, Supplementary Information) at selected temperatures, 30°C (a), 80°C (b), and 120°C (c), for the entire frequency range. The fitted curves exhibited an increase of dielectric relaxation strength (\({\varepsilon }_{s}/{\varepsilon }_{\infty })\) in the order of ~ 6.5, compared between (a), at 30°C and (c) at 120°C, Fig. 5A. The fitted parameters, from the relaxation model, are listed in Table S1, Supplementary Information. The relaxation time (τ) and conduction parameter (S) decrease with increasing temperature, which revealed the space-charge polarization is thermally activated process via hopping assisted mechanism [2, 5, 45]. The activation energy of the dielectric relaxation was studied in terms of electrical modulus (M*) spectra, extracted from dielectric constant data, using the relation \({M}^{*}= {M}^{\prime}+j{M}^{{\prime}{\prime}}=1/{\varepsilon }^{*}\) where Mʹ (real) and M″ (imaginary) are components of the electric modulus and ε* is the complex part of the dielectric constant. Variations of M′ and M′′ versus frequency under different temperature conditions (30–120°C) are shown in Fig. 5B and 5C, respectively. The Mʹ curves, Fig. 5B, display a step-like pattern and the Mʹ values decreased with decreasing frequency in the presence of increasing temperature. The step-like pattern is an indication of the relaxation process in the system, which is transformed to a peak in the corresponding M′′ plots with respect to the frequency, Fig. 5C. These peaks are shifted toward high-frequency region with increasing the temperature, indicating long range charge carrier mobility in the material [46]. With increasing temperature, the asymmetric nature of the curves and the maximum value of M′′ peaks are diminished, which indicate a non-Debye type distribution of relaxation time [45, 46]. The non-Debye relaxation process in the imaginary part of the electric modulus spectra (M′′) versus frequency, Fig. 5C, was investigated by using modified Kohlrausch, Williams, and Watts (KWW) relation (Eq. S5, Supplementary Information). The modified KWW relation was applied for M′′ curves at selected temperature conditions (30, 80, and 120°C) and the fitted parameters are listed in Table S1, Supplementary Information. The values of β parameters increase with raising temperature. The broad M′′ peaks and the asymmetric nature were due to the distribution of relaxation time. The long-range hopping conduction of charge carrier was appeared below the M′′max, whereas above M′′max hopping conduction was restricted to the short-range interaction. Figure 5D exhibits the graphical representation of M′′/M′′max as a function of \(f/{f}_{max}\) for the selected temperatures, 30, 80, and 120°C, where fmax is the maximum peak frequency of M′′ curves. The figure shows that all peaks, for different temperatures, are merged, which revealed the same relaxation mechanism for the origin of these peaks. The figure shows that all peaks, for different temperatures, are merged into a single curve, which revealed the relaxation time distribution is temperature independent and that indicates the conduction and carrier transport process are same irrespective of the temperature condition. As a result, the hopping conduction process might be responsible for the relaxation [47]. However, toward higher frequency region the non-overlapping curves were observed, originated from long-range conductivity, and non-exponential short-range conductivity relaxation [48]. The peak frequency (fmax) as function of temperature (1/T) is plotted and fitted with the Arrhenius equation, \({f}_{max}^{M^{{\prime}{\prime}}}= {f}_{0 }\mathrm{exp}({-E}_{a}/{K}_{b}T)\), Fig. 5E. The activation energy (Ea) ~ 0.16 eV is extracted from the slope of a linear fit in the Arrhenius relation. The value of fitted activation energy (0.16 eV) is related to the relaxation process of single ion oxygen vacancy \(({V}_{0}^{\cdot })\) near the boundary region. The presence of mixed oxidation state of chromium (Cr6+/Cr3+) facilitates the formation of oxygen vacancies in the system and contributes free electron in the conduction band. The oxygen vacancies act as donor level and lowering the energy barrier for polaronic type of hopping conduction process [47].

Fig. 5
figure 5

A Variation of real part of the dielectric constant (εʹ) with respect to frequency from 100 Hz to 1 MHz at different temperatures (30°C, 40°C, 60°C, 80°C, 100°C, and 120°C), for PCO-based device, fitted according to the modified Cole–Cole relation. The inset figure shows the tan (δ) curves of the device for selected temperatures. B, C Real (Mʹ) and imaginary (Mʹʹ) part of the electric modulus of PCO-based device, respectively. (Selected M″ curves were fitted according to KWW model.) D Normalized electric modulus M″/M″max versus frequency for selected temperatures (30°C, 80°C, and 120°C) and E Arrhenius fitting of activation energy extracted from the modulus spectra

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The real part of electrical impedance (Z′) of PCO-based device was recorded as a function of frequency (100 Hz–1 MHz) under varying temperature conditions (30–120°C), Fig. 6A (log–log scale). High value of Z′ at lower frequency suggested an increase in the electrical conductivity of the device. The value of Z′ decreased gradually with increasing frequency and temperature that indicate negative temperature coefficient resistance behavior of device. A linear relationship was established, with the slope value of ~  − 0.92, when log (Z′) was plotted as a function of log (f) at 30°C over the entire frequency range (Fig. S1, Supplementary Information). However, the log (Z′) pattern deviates from the linearity with rising temperature and a plateau like feature was formed toward the lower range of frequencies at 120°C, figure not shown, originated from the space charge conduction process. A Bode phase (θ) plot versus frequency, Fig. 6B, displayed a higher dispersion in phase angle below 10 kHz, indicates a deviation from the ideal capacitive behavior (θ=-90°). Shifting of Bode plots with temperature at lower frequency region suggested the resistive component (θ=-90°) is originated from particle boundaries (organic–inorganic interface) that contribute to the conduction process [49, 50]. Figure 6C and D represents the Nyquist plot of the impedance data for selected temperatures 30 and 80°C and 100 and 120°C, respectively. The Nyquist plot was applied to extract the contribution of particle and particle interface for the electrical impedance. The plots show a single semi-circular arc from an overlapping contribution of the individual components of lead chromate particles and the corresponding interfaces. The experimental Nyquist plots were fitted (red and black) with an equivalent electrical circuit model over the entire frequency range, Fig. S2 (Supplementary Information). The detail of the fitting parameters and the electrical components (R1, R2, Q1, and Q2) are listed in Table S2. The decrease in the value of resistive components (R1 and R2) and an increment in the capacitive components (Q1 and Q2) were observed with rising the temperature. The thermally activated conduction process and the localized hopping of charge carrier control the overall impedance behavior of the device.

Fig. 6
figure 6

A Frequency and temperature-dependent real part of the impedance (Z′) spectra (log–log scale) and B Bode phase diagram of PCO based device. C, D Nyquist plot of the electrical impedance (Z′ vs. − Z′′) for selected temperatures (30, 80, 100, 120°C) and fitted according to the equivalent circuit models

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Figure 7A shows the AC-conductivity (in log–log scale) of the PCO-based device as a function of frequency (100 Hz–1 MHz) under different temperature conditions (30, 80, and 120°C). At 100 Hz, the AC-conductivity values are ~ 2.5 × 10−8 S/m, 1.1 × 10−7 S/m 1.1 × 10−6 S/m, for 30, 80, and 120°C, respectively. The overall frequency variation of the device obeys the Jonscher’s power law, \({\sigma }_{ac}= {\sigma }_{dc}+A\left(T\right){\omega }^{S}.\) The term σdc is frequency-independent DC part of the conductivity, and A and S determine the polarizability and conduction mechanism, respectively. Various theoretical models have been proposed to study the conductivity mechanism of the charge carriers under variable temperature and frequency conditions and the value of “S” parameters varies between 0 and 1 [46, 51]. The conductivity curves were fitted (green line, Fig. 7A) according to the Jonscher’s power law over the entire frequency range and the extracted “S” parameter is plotted in Fig. 7B. The value of S decreases with increasing the temperature, suggesting a hopping conduction mechanism of polaronic charge carriers in PCO-based device. The activation energies (Ea) of the polaronic charge carrier in AC-conductivity (\({\sigma }_{AC}\)) are calculated for 100 Hz and 10 kHz, Fig. 7C, by using Arrhenius equation, \({\sigma }_{AC}\left(T\right)={\sigma }_{0}\mathrm{exp}\left(-{E}_{a}/{K}_{B}T\right),\) where \({\sigma }_{0}\) is pre-exponential factor. The calculated activation energy values at 100 Hz and 10 kHz are ~ 0.17 eV and ~ 0.13 eV, respectively. The values are in accordance with the activation energy of the charge carrier calculated from the modulus spectra. The value of Ea suggested a single-ion oxygen vacancy mediated hopping conduction process in the lead chromate system.

Fig. 7
figure 7

A AC conductivity of the device for selected temperatures (30, 80, and 120°C), fitted according to the Jonscher’s power law (green line). B Variation of S parameter with temperature and C the activation energy (Ea) of AC conductivity at 100 Hz and 10 kHz, extracted from the Arrhenius fitting (red line) versus inverse of temperature (1/T)

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Figure 8A exhibits the polarization behavior of the lead chromate-based device as a function of electric field under different field conditions (1, 2, 4, and 6 kV/mm) at 10 Hz. The polarization value of the device increased with applied electric field and attained a maximum polarization value (Pmax) ~ 0.38 μC/cm2 at 6 kV/mm, Fig. 8A, inset. A dipolar type of interaction of chromium-oxyanion [CrO4]2− unit was responsible for polarization hysteresis behavior of the device. The space-charge contribution was attributed for unsaturated pattern of the hysteresis loop with respect to applied field. The polarizability of the covalent bond between the O-2p and Cr-3d was increased with increasing applied electric field and attained maximum polarization. A fatigue-free polarization was observed in the device for 103 cycles at ± 4 kV/mm, Fig. 8B, and maintained the stable Pmax value ~ 0.26 μC/cm2 over the switching cycles, Fig. 8C. However, polarization fatigue was observed at higher field condition (± 6 kV/mm), which was associated with the loss of Pmax value of the device upon repeated switching of the electric field, Fig. 8D. The polarization fatigue was originated due to the creation of oxygen vacancy and the vacancy-induced charge trapping at the lead chromate lattice [52, 53]. The redistribution of the charge species during the first quarter of the cycles causes the polarization fatigue and the reduction of Pmax value was noticed from ~ 0.38 to ~ 0.25 μC/cm2, Fig. 8E. The stability of the polarization value was noticed with further increasing of the cycles. Similar behavior in polarization hysteresis loop has been reported in other lead- and bismuth oxide–based thin films [54, 55]. Asymmetric distribution of oxygen vacancies at the lattice creates a conducting path that caused an electrical breakdown, Fig. 8F, and the device sustained for a period of 62 s under an applied field 6.3 kV/mm.

Fig. 8
figure 8

A Polarization–electric field (PE) hysteresis loop of PCO-based device, measured at 10 Hz. B, C PE loop under the applied field of ± 4 kV/mm for 103 cycles and the stability of Pmax value of the device, respectively. D Hysteresis loop pattern at ± 6 kV/mm for 10.3 cycles. E Variation of Pmax value with switching cycles and F electrical breakdown strength (kV/mm) of the device as function of time (s)

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The fabricated device was tested for pressure-induced electrical performances. Under zero pressure condition the device exhibited an open circuit voltage (Voc) ~ 0.8 μV that reached to ~ 0.4 mV under external pressure of 45 kPa, Fig. 9A. The increment of the voltage output, under various external pressure conditions, was originated due to the dislocation of positive and negative charge centers that polarized the device. The induced polarization increases the internal electric field that produced output voltage in the device [56, 57]. The output voltage retains the stable value for 10 s and then increased with further application of the pressure, indicated by an upward arrow. Drop of output voltage to the initial level was noticed when pressure was released, indicated by a downward arrow. The retention of the output voltage performance of the device was recorded under two different pressure conditions (25 kPa and 45 kPa) for 103 s, Fig. 9B. The device maintained stable output voltage (Voc) ~ 18 μV and ~ 0.4 mV at 25 and 45 kPa, respectively, and performed as a voltage generator under applied pressure conditions. The capacitive performance of the device was measured within the frequency range from 200 Hz to 20 kHz under varying pressure condition, Fig. 9C. Increase of capacitance value was observed with applied pressure and reached to a maximum value of ~ 270 pF at 200 Hz, under 45 kPa. The decrease of capacitance value of the device was observed with increasing frequency. Change of capacitive ratio (ΔC/C0) as a function of applied pressure under different frequency conditions (5, 10, and 15 kHz) is illustrated in Fig. 9D. The ΔC/C0 variation is more significant toward high frequency (15 kHz) and the curve shows a nonlinear pattern with rising the pressure. The pressure sensitivity, S = (ΔC/C0)/P, of the device was calculated from the slope (green solid line) of the capacitive ratio (ΔC/C0) under different pressure conditions. The pressure sensitivities 0.011, 0.08, and 0.05 kPa−1 were obtained at 15, 10, and 5 kHz, respectively, under the pressure range from 15 to 45 kPa. The device showed stable capacitive signal under higher frequency condition, resulted from the low dielectric loss factor, which is similar to the other reported nanocomposite systems [17, 58,59,60].

Fig. 9
figure 9

A Open-circuit voltage response of the PCO-based device under different external pressure (loading and unloading) conditions. B Stability of the output voltage of the device under loading and unloading of pressures (25 and 45 kPa), for 103 s. C Capacitance versus frequency curves under applied pressure conditions. D Pressure sensitivity (S) limit of the device under different frequency (5, 10, and 15 kHz) conditions

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4 Conclusion

Organic molecule–stabilized monoclinic lead chromate was prepared by applying a complexation mediated route using lead nitrate and potassium dichromate precursors. Synthesized lead chromate with monazite type of structure is interconnected with a chain of alternating polyhedra (PbO9) and tetrahedra (CrO4) units via edge and corner sharing network. X-ray photoelectron spectroscopy analysis of the synthesized material confirmed the presence of Pb2+ and the mixed oxidation states of Cr6+/Cr3+. The dielectric constant value of lead chromate based device was achieved 372 at 30°C and increased to 2400 at 120°C under 100 Hz frequency condition. Toward low-frequency region, the high value of dielectric constant was due to increased charge carrier that dominated the space charge polarization contribution in the device. Electrical modulus spectra revealed broad and asymmetric relaxation peaks, which supported non-Debye type of behavior in the system. Single-ion oxygen vacancy-mediated hopping conduction of polaronic charge carriers controls the AC-conductivity of the device. Field-driven polarization hysteresis behavior was originated from the oxygen vacancies and the interaction between chromium-oxyanion unit [CrO4]2− of the PbCrO4 system. The polarization behavior of the device increased with electric field and was attained a maximum value ~ 0.38 μC/cm2 at ± 6 kV/mm. A fatigue-free polarization in the device was observed under an applied field value of ± 4 kV/mm and maintained the stable maximum polarization value ~ 0.26 μC/cm2 for 103 cycles. The electrical breakdown to the device was observed at 6.3 kV/mm due to asymmetric distribution of oxygen vacancies that created a conducting path. The lead chromate-based device also performed as a voltage generator under applied pressure condition.