1 Introduction

Recently, supercapacitors have gained attention due to their long life cycle, high power density, and flexibility as traditional dielectric capacitors and energy units [1, 2]. Researchers are exploring supercapacitors with high energy density and greater electricity storage capability to bridge the gap between batteries and capacitors. Supercapacitors can store energy through non-Faradaic mechanisms in electrical double layer capacitors and electrostatic methods in pseudo-capacitors [3, 4]. Therefore, nanomaterials exhibit superior characteristics over their bulk counterparts [5]. Compared to carbon based nanomaterials, inorganic metal oxides offer opportunities for tailor-made properties [5]. Because of their extremely narrow band gap and low cost, spinel-type metal oxides have drawn more attention than many other metal oxides [6]. The spinel-structured material, with its octahedral and tetrahedral voids, plays a crucial role in photocatalytic and electrochemistry applications [7]. A broad variety of areas utilize ZnAl2O4 nanoparticles due to their exceptional properties. These fields include photocatalysis, sensors, high temperature ceramics, dielectric materials, magnetics, electrode materials, opto-electronics, and dosimetry [8,9,10,11]. Researchers reported the optical band gap of spinel ZnAl2O4, a direct band gap semiconductor, to be between 3.8 and 3.9 eV [12].

Zinc aluminate (ZnAl2O4) exhibits a cubic structure and belongs to the space group Fd-3m. The spinel structure of ZnAl2O4 contains cations Zn2+ and Al3+, which occupy tetrahedral and octahedral sites, respectively. Thermodynamic stability of these cations is compromised by the movement of Zn2+ and Al3+cations between sites, resulting in an inverted spinel with all Zn2+ ions in the octahedral sites [13]. Reports indicate that spinels doped with transition metals excel in electrocatalysis. ZnAl2O4 is a suitable material for electrocatalytic applications due to its high redox activity and stability. Thirumala Rao et al. used hydrothermal technique to synthesize ZnAl2O4 for supercapacitor applications. Spinel ZnAl2O4 exhibits a specific capacitance of 260.4 mAhg−1 and a columbic effectiveness of 98% over 500 cycles [14]. Zinc aluminate with rare earth doping, such as Eu3+, Dy3+, Tb3+, and Ce3+, enhanced their structural, optical and luminescent properties. However, because of the high cost of rare earth elements, transition elements have gained popularity as substitutes. Unlike rare earth elements, transition elements can occupy both octahedral and tetrahedral sites in the crystals due to valence states [15,16,17,18]. Researchers have used a variety of rare earth metal ions and transition metal ions as dopants. Due to its comparable ionic radii to rare earth ions, the ZnAl2O4 host material commonly uses the Cr3+ ion as a dopant [19].

Various synthesis methods were used to prepare ZnAl2O4, including solid-state method [20, 21], hydrothermal synthesis [22, 23], and the sol-gel method [24, 25]. The solid-state synthesis needs the physical mixing of the various precursors, followed by sintering [26]. It also requires high temperature sintering and long hours. It is difficult to control the particle size and maintain homogeneity [27]. The chemical methods are advantageous because of their ease of preparation, high yields, low sintering temperatures and times, homogeneity, and so on. Miron, Grozescy, and Hong et al. [28, 29] reported the hydrothermal synthesis of Cr3+ doped ZnAl2O4, with synthesis times ranging from 16 h to 96 h. Motloung et al. [30] described the sol-gel synthesis of Cr3+ doped ZnAl2O4. Dong et al. [31] synthesized Cr-doped ZnAl2O4 using an electrospinning method. Samvit Menon et al. reported the synthesis of Cr doped ZnAl2O4 using the co-precipitation method [19].

Therefore, the present investigation employs the microwave-hydrothermal method to synthesize Cr doped ZnAl2O4 nanoparticles. The influence of Cr3+ on the structural, spectroscopic, and electrochemical properties was investigated to find a suitable material for supercapacitor applications. The Cr doped ZnAl2O4 samples were characterized using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), Fourier transform infrared spectroscopy (FTIR), UV-Vis spectrometer, X-ray photoelectron spectroscopy (XPS), and an electrochemical analyzer, and all the results are explained in detail.

2 Experimental method

The nanopowders of ZnAl2-xCrxO4 (x = 0.0–0.1) were synthesized using the microwave-hydrothermal method. A similar synthesis procedure was reported elsewhere [22, 32, 33]. Analytical-grade chemicals Zn(NO3)2.6H2O, Al(NO3)3.9H2O, and Cr(NO3) were used in the synthesis. For the synthesis of ZnAl2O4, the zinc nitrate and aluminum nitrate were weighed according to the stoichiometric ratio, dissolved in deionized water, and stirred well to obtain a clear solution. A drop wise NaOH solution was added to the clear solution to get a pH of ~11. The precipitate was then transferred into a tetrafluorometoxil (TFM) reaction vessel which was then mounted on the turntable in the Microwave Accelerated Reaction System (Model MARS-5, CEM Corp. Mathews, NC) for microwave treatment at 200 °C for 60 min. It operates at 2.45 GHz with 1200 ± 50 W of power and can attain a maximum pressure of 800 psi and a temperature of 240 °C. After treatment, the product was centrifuged and repeatedly washed it with deionized water until the pH reached neutral, and then filtered it. The powders were dried overnight at 80 °C in an oven. The dried powders were finally sintered at 1250 °C for 4 h in a muffle furnace.

The X-ray diffraction patterns were recorded using X-ray diffractometer (Phillips PANanalytical) that used Cu-Kα radiation (λ = 1.5406), a scanning range of 2θ = 20°–80°, and a scan rate of 1°/min at 40 kV and 30 mA. The field emission scanning electron microscope (FESEM, FEI Quanta 200) in conjunction with energy dispersive spectra (EDS), were used for microstructural study and elemental analysis, respectively. The ImageJ software was used to analyze the microstructure of all the samples, which included grain size distribution, histograms, and average grain size. The Fourier transform infrared (FTIR) spectra were recorded in the mid-IR range of 4000–400 cm–1 at a resolution of 4 cm–1 using Brucker Tensor 27 spectrometer. The absorbance spectra were collected between 200 and 900 nm using a UV-visible spectrophotometer (Shimadzu, UV-1800) at a resolution of 1 nm. A 10 kV and 10 mA Shimadzu ESCA-3400 spectrophotometer (Kyoto, Japan) was used for the monochromatic Mg-Kα X-ray source (1253.6 eV) measurements.

An electrochemical analysis was conducted using the Metrohm Autolab PGSTA302N instrument and NOVA 2.0.2 software. The analysis involved techniques such as cyclic voltammetry (CV) and galvanostatic charge/discharge (GDC), and a three-electrode system was utilized with Ag/AgCl as the reference electrode and Pt as the counter electrode. The working electrodes were prepared using current samples as the active material (80%), activated carbon (10%), and polyvinylidene fluoride (PVDF) (10%), which acts as the cathode. PVDF possesses remarkable chemical resistance to electrodes, a remarkable temperature tolerance, and an organic binder, enabling it to endure a diverse array of weather conditions without succumbing to deterioration. Activated carbon was used as a solvent to enhance the chemically active surface area and minimize the adsorbent between the electrode and electrolyte, thus expanding its effectiveness. Apply a layer of PVDF and activated carbon compound as a thick slurry onto a 1 by 1 centimeter Ni foam substrate. Heat the coated substrate in an oven at 80 °C for 12 h. The fabricated samples are used for supercapacitor applications.

3 Results and discussion

3.1 X-ray Diffractograms (XRD)

The X-ray diffractograms of ZnAl2-xCrxO4 (x = 0.0–0.10) samples are shown in Fig. 1. All the diffraction peak positions were compared to JCPDF card no: 74-1138 and all the peaks were matched with spinel ZnAl2O4. The characteristic peak (hkl:311) suggests that all the samples exhibit cubic structure with the space group Fd-3m. No impurity phases were observed in all the samples. The intensity of hkl: 311 increased with Cr doping for x = 0.02 and decreases for x = 0.04. For samples with x > 0.04, there is an increase in intensity. The variation in the intensity of the peak with respect to doping is that Cr ions are incorporated nonhomogeneously into the host ZnAl2O4 matrix due to the difference in the ionic radius and chemical oxidation states [34]. It is obvious that Cr3+ions enter the crystal lattice of ZnAl2O4. In general, Cr3+ions occupy octahedral sites because they replace Al3+ ions in the ZnAl2O4 host, however, depending on the Cr doping concentration, synthesis circumstances, sintering temperature, and particle size, Cr may occupy both tetrahedral and octahedral sites [31]. Using the conventional Archimedes method, the bulk densities (db) were calculated for all of the samples. The strain (ε) and an average crystallite size (<D > ) are determined from the Williamson-Hall plot, as illustrated in Fig. 2. Table 1 shows the average crystallite size, lattice strain, lattice parameter (a), unit cell volume (V), X-ray density (dx), bulk density (db) and percent of porosity (%P).

Fig. 1
figure 1

X-ray diffraction patterns of ZnAl2-xCrxO4 (x = 0.0 to 0.10)

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Fig. 2
figure 2

Williamson-Hall plots for Cr doped ZnAl2O4

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Table 1 Structural parameters: lattice constant (a), unit cell volume (V), X-ray density (dx), bulk density (db), percentage of porosity (%P), crystallite size < D > , lattice strain (ε) and grain size
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Figure 3 shows the variation of lattice constant (a) in response to Cr doping (x). With Cr doping, a is increased from 8.085 Å (x = 0.0) to 8.128 Å (x = 0.04) and then decreased to 8.104 Å (x = 0.10). The substitution the smaller ionic radii of Al3+ (0.53 Å) with higher ionic radii of Cr3+ (0.63 Å) results in increase in the lattice constant (a) [28, 34]. For higher substitution (x > 0.04) of Cr3+ ions for Al3+ ions at octahedral sites and Zn2+ (ionic radii of 0.74 Å) ions at tetrahedral sites, the drop in a occurs [30]. The unit cell volume (V) varies in the same manner as the lattice constant, suggests that Cr3+ is soluble in the ZnAl2O4 lattice. The X-ray density (dx) of samples varied non linearly with Cr doping, as the density of the sample depends on the molar masses of Zn (65.4 g/mol), Al (27 g/mol), and Cr (51.996 g/mol) [35]. It is observed from Table 1 that the bulk density (db) varied non-linearly with x. Variations in the percentage of porosity (%P) with doping concentration is may be due to defects. The rise in lattice strain (ε) could be attributed to the generation of oxygen vacancies on the nanoparticle surfaces [36].

Fig. 3
figure 3

The Variations in the lattice constant (a) with respect to Cr doping (x)

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3.2 Field emission scanning electron microscopy (FESEM)

Figure 4 displays FESEM images of ZnAl2-xCrxO4 (where x ranges from 0.0 to 0.10) samples. The images exhibit uneven morphology and irregular particles sizes distribution. The average grain size (<Dg > ) was determined using Image J software. The grain size distribution histograms are presented in Fig. 5. The average grain size (<Dg > ) values are in a range of 84 nm (x = 0.0) to 198 nm (x = 0.10) and the results are tabulated in Table 1. It is observed that the average grain size increased with Cr doping. The increase in grain size may be due to the presence of Van Der Waals force, coulomb force, or chemical bond cooperation between the nanoparticles [37]. Figure 6 represents the energy-dispersive X-ray spectroscopy (EDS) of the samples, revealing the presence of Zn, Al, Cr, and O elements. This observation suggests that the prepared samples are devoid of any impurities.

Fig. 4
figure 4

FESEM images for ZnAl2-xCrxO4 (x = 0.0 to 0.10) samples

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Fig. 5
figure 5

Grain size distribution (histograms) for ZnAl2-xCrxO4 (x = 0.0 to 0.10)

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Fig. 6
figure 6

EDS images for ZnAl2-xCrxO4 (x = 0.0 to 0.10)

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3.3 The FTIR spectroscopy

The FTIR spectra of Cr-doped ZnAl2O4 in the range of 400–4000 cm−1 are displayed in Fig. 7. The peaks corresponding to the wavenumbers of around 3311–3325 cm−1 and 1619–1639 cm−1 for all samples are associated with the stretching and bending vibrations of the hydroxyl groups of absorbed water [25, 26, 38]. The N-H bending vibrational modes were observed in a wavenumber range of 1473 to 1483 cm−1 [39]. Three absorption bands were identified at approximately 649 cm−1, 544 cm−1, and 497 cm−1 for x = 0.0. The formation of the spinel cubic phase is indicated by the presence of these three peaks. The two prominent bands at 649 cm−1 and 544 cm−1 represent the symmetric stretching vibrational modes of octahedral Al3+ ions caused by Al-O symmetric stretching. On the other hand, the band at 497 cm−1 corresponds to the symmetric stretching vibrations of tetrahedral ZnO4 [40, 41]. Unlike the absorption bands seen at 497 cm−1 and 544 cm−1, the intensity at 649 cm−1 is influenced by the doping parameter x. The observed change in intensity indicates the incorporation of Cr3+ ions into the spinel lattice, where they substitute Al3+ ions. The absorption bands below 497 cm−1 are attributed to the stretching modes of Zn-O [42, 43]. The band at 809 cm−1 is attributed to the symmetric stretching of octahedral Al3+. The absorption at 516 cm−1 is indicative of the asymmetrical stretching mode of octahedral Al3+. The presence of the 411 cm−1 and 431 cm−1 bands may be attributed to the tetrahedral Al3+ [44,45,46].

Fig. 7
figure 7

The FTIR spectrum of ZnAl2-xCrxO4 (x = 0.0–0.10)

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3.4 UV-Vis absorbance spectra

Figure 8 displays the ultraviolet-visible (UV-Vis) absorbance spectra of ZnAl2-xCrxO4 (where x ranges from 0.0 to 0.10) nanoparticles recorded in within the wavelength range of 200 nm to 800 nm. The absorption edge in the lower wavelength range (230–279 nm) may be caused by the lattice distortion resulting from the introduction of Cr at Al3+ sites [47, 48]. The absorption band at 230–279 nm may be due to the charge transfer transitions of the Cr ion [48]. But such an absorption band is absent for x = 0.0 [49] but clearly visible for Cr doped samples. The samples exhibit high absorbance at wavelengths 273 nm and 371 nm, which fall between the UV and visible region. According to the literature, the absorbance in the sample may be affected by several factors, such as impurity concentrations, thickness of the sample, dopant concentration, the band gap energy, the oxygen packing density, the molar volume density of the sample, and the size of the particle [50]. The different band gaps of all the samples were determined by employing tauc’s plot and applying Equation [10, 51].

$$\left(\alpha h\nu \right)={\left(h\nu -{E}_{g}\right)}^{n}$$
Fig. 8
figure 8

UV-Vis absorbance spectra for x = 0.0–0.10 samples

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For direct band gap semiconductors, the optical band gap energy is denoted by Eg, the energy of an incident photon may be denoted by hv, the coefficient of absorption is denoted by α(ν), the frequency of vibration is denoted by ν, and the value of n is equal to ½. The optical band gap Eg can be determined by extrapolating a straight line in the tauc plot of hν vs (αhν)2.

All samples exhibited two distinct absorption bands at 330–375 nm and 530–560 nm. There is a possibility that the absorption bands between 330 and 375 nm are caused by transitions from O-Al to tetrahedral sites (4A2 → 4T1). Absorption bands in the range of 530 to 560 nm are produced as a result of transitions (4A2 → 4T1) involving O-Al in octahedral sites [52]. This is because the ZnAl2O4 lattice contains Cr3+ spin permitted d - d transitions, which are responsible for these transitions [19, 53]. Figure 9 displays the tauc plots for ZnAl2-xCrxO4 (x = 0.0 -0.1). For x = 0.0, the band gap energy (Eg) is 3.51 eV, which is lower than the band gap of bulk ZnAl2O4 (3.8 eV) [12]. This difference may be owing to the quantum confinement limit [54]. With Cr3+ doping, the bandgap energy Eg decreased from 3.51 eV (x = 0.0) to 3.39 eV (x = 0.10) by exhibiting a red shift. The red shift is brought about by the sp-d interaction that takes place between band electrons and local electrons in the d-shell of the substituted cation. These interactions decrease the conduction band potential and increase the valence band potential, which ultimately results in a reduction in the band gap [37, 55].

Fig. 9
figure 9

Tauc plots for x = 0.0–0.10

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3.5 X-ray photoelectron spectra (XPS)

The X-ray photoelectron (XPS) wide spectrum of ZnAl2-xCrxO4 (x = 0.06) is shown in Fig. 10a. The Cr 2p core level spectra for x = 0.06 can be seen in Fig. 10b. The Cr 2p spectrum is split into two main peaks, which are Cr 2p3/2 and Cr 2p1/2. These peaks have binding energies 575.77 eV (Cr3+) and 584.39 eV (Cr3+), respectively. In the octahedron, the Cr 2p3/2 peak corresponds to Cr ions, whereas in the tetrahedron, at Cr 2p1/2, there are only a few Cr ions. The core spectrum of Zn 2p can be seen in Fig. 10c. The two major peaks at 1047.19 eV and 1024.01 eV, are ascribed to Zn 2p1/2 and Zn 2p3/2, respectively. In addition to these peaks, two peaks at 1014.65 eV and 1040.35 eV is present. The difference in energy between Zn 2p3/2 and Zn 2p1/2 is 23.18 eV, which confirms that Zn2+ is present in all samples. Figure 10d shows the core level Al 2p spectrum for x = 0.06. The binding energy at 76.66 eV confirms the presence of Al3+ in sample x = 0.06. The O 1 s spectra is deconvoluted and exhibits two peaks at binding energies 533.45 eV and 537.10 eV (shown in Fig. 10e). The peak at 533.45 eV is due to the oxygen vacancies present in the ZnAl2O4 lattice, and the peak at 537.01 eV is due to adsorbed oxygen on the surface of the sample [54].

Fig. 10
figure 10

XPS spectra for samples x = 0.06 (a) wide spectra (b) Cr 2p (c) Zn 2p (d) Al 2p (e) O 1 s

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3.6 Electrochemical properties

The cyclic voltammetry (CV) studies for x = 0.0–0.10 obtained in a 1 M KOH aqueous solution at 5 mV/s scan rate in the potential window of –0.1 V to 0.4 V are displayed in Fig. 11. An observed elevation in the scanning rate demonstrates a subtle shift in the potential values of the oxidation-reduction peaks [56]. The reduction peaks for all samples, ranging from x = 0.0 to x = 0.10, were obtained by conducting reverse/cathodic scans from 0.2 to 0.25 V. Conversely, the oxidation peaks were collected by doing forward/anodic scans from 0.3 to 0.35 V. The cyclic voltammetry (CV) curves provide convincing evidence of the Faradaic capacitive behavior [57], as a distinct pair of redox peaks is prominently observed for samples with x = 0.0 to 0.1. The observed variations in redox peaks in both the anode and cathode directions could potentially be attributed to the delayed migration of electrolyte ions, specifically OH- ions [58]. Figure 12 depicts the Galvanostatic Charge-Discharge (GCD) curve for all the samples.

Fig. 11
figure 11

Cyclic voltammetry (CV) curves for x = 0.0–0.10 recorded in 1 M KOH aqueous solution with 5 mV/s rate of scan

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Fig. 12
figure 12

Galvanostatic charge-discharge (GCD) curved for all the samples, x = 0.0–0.10

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The potential versus time curves exhibit the pseudocapacitive characteristics of the electrode. Table 2 shows the values of specific capacitance (Csp), energy density (Ed), and power density (Pd) at a scan rate of 5 mV/s for all the samples. The variation of Csp, Ed, and Pd with doping concentration, x, is shown in Fig. 13. It is observed from Fig. 13a that Csp increased with doping concentration, x. A similar variation is observed for Ed with x. It is observed that Csp is 70.286 F/g, for x = 0.0 and 143.671 F/g for x = 0.10. The Cr3+ doped samples exhibit greater Csp as compared to ZnAl2O4 (x = 0.0) which may be due to the larger electrochemical surface area. Mukhtiar Hussain et al. reported a specific capacitance of 1194.69 Fg−1 for Nd-doped FeAl2O4 nanoparticles subjected to a current density of 1.0 Ag−1 [59]. Salma Aman et al. synthesized SrAl2O4 using the hydrothermal route. It is reported that SrAl2O4 exhibited a large specific capacitance of 737 Fg−1 at 1 Ag−1 [60]. The NiCo2O4 prepared using oxalic acid showed a specific capacitance of 1254 Fg−1 at 2 Ag−1 [61]. The ZnAl2O4 electrode material showed a specific capacity of 255.4 mAhg−1 at the current density of 1 Ag−1 elsewhere [25]. There is a linear variation in the energy density from x = 0.0 (2.439 Wh/kg) to x = 0.10 (4.987 Wh/kg). The power density increased from 15.790 W/kg (x = 0.0) to 58.546 W/kg (x = 0.08) and decreased to 8.537 W/kg at x = 0.10.

Table 2 The data of specific capacitance (Csp), energy density (Ed) and power density (Pd) for x = 0.0–0.10
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Fig. 13
figure 13

The variation of specific-capacitance (Csp), Energy density (Ed) and power density (Pd) with respect to dopant x

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4 Conclusions

The Cr doped ZnAl2O4 samples were prepared using microwave hydrothermal method at 200 °C/60 min. The cubic structure of the spinel was confirmed by XRD. The lattice constant (a) increased from 8.085 Å (x = 0.0) to 8.128 Å (x = 0.04) and decreased to 8.104 Å (x = 0.10) with Cr doping. The changes in lattice constant, a, is explained on the basis of ionic radii. The differing molar masses of Zn (65.4 g/mol), Al (27 g/mol), and Cr (51.996 g/mol) explain the difference in dx with respect to x. The average grain size is estimated to be between 84 nm (x = 0.0) and 198 nm (x = 0.10). Three absorption bands were detected at about 649 cm–1, 544 cm–1, and 497 cm–1 for sample x = 0.0, indicating the formation of the spinel cubic phase. The band gap energy for x = 0.0 is 3.51 eV, which is less than the band gap of bulk ZnAl2O4 (3.8 eV). The energy band gap (Eg) decreased from 3.51 eV (x = 0.0) to 3.39 eV (x = 0.10) with doping. The XPS confirms the presence of Cr, Al, Zn and O in respective valence states. The O 1 s spectra confirms the oxygen defects in the doped samples. The maximum specific capacitance (Csp) of 143.671 F/g, is obtained for x = 0.1. The Cr3+-doped samples have higher Csp than ZnAl2O4 (x = 0.0) due to their larger electrochemical surface area. Therefore, the chromium doped ZnAl2O4 may be useful for energy storage supercapacitors.