Without further purification, all of the chemicals used in the studies were obtained from commercial sources and were of analytical quality (99.8%). A simple co-precipitation technique was used to develop Ni doped-Co₃O₄ NPs (Alem et al., 2023a). In this typical synthesis process, 100 mL of distilled water was used to dissolve 0.2 M of cobalt nitrate hexahydrate [Co(NO₃)₂. 6H₂O] and the equivalent mole of nickel (II) nitrate hexahydrate [Ni(NO₃)₂. 6H₂O]. The aforementioned solution of Co(NO₃)₂. 6H₂O and Ni(NO₃)₂. 6H₂O was agitated for 30 min before 0.02 mL (2.12 g) of Na₂CO₃ was added. To create a homogeneous solution, the mixture was agitated and heated at 60°C for 3 h. The as-prepared sample was cleaned with distilled water and dried in a hot air oven for 12 h at 110°C. Lastly, the resulting powder was calcined for 3 h in a muffle furnace at 500°C to produce Ni-doped Co₃O₄ NPs. The same method was used to make both pure Co₃O₄ and Co₃O₄ NPs doped with copper.

Using powder X-ray diffraction (XRD-7000, SHIMADZU) in the (2 θ range of 30^ο–80^ο using Cu-K radiation (=1.54061 = 0.15406 nm), the crystal structure and phase composition of the synthesized nanoparticles were examined. Using a Perkin-Elmer 800, inductively coupled plasma optical emission spectrometry (ICP-OES) analysis was used for the elemental examination. The functional groups of the sample product were analyzed using the Fourier Transform-Infrared Spectrometer (FT-IR 6660-JASCO MODEL). Using the Ultraviolet-Visible Spectrometer [UV-Vis, Lambda 35 (PerkinElmer)] in the wavelength range of 200–800 nm, the optical properties of as-prepared nanoparticles were examined. Scanning electron microscopy was used to study the morphological characteristics and microstructure of (SEM). Also, a TGA/DTA research was used to do a thermal property analysis. The specific surface areas of as-prepared nanoparticles were calculated using the Brunauer-Emmett-Teller (BET Quanta chrome instrument version 11.0).

In order to fabricate electrodes for electrochemical investigations, a sample impregnated with CS (chitosan) was cast onto a 5 mm diameter glassy carbon electrode to create the supercapacitor electrode. The exposed glassy carbon working electrode (GCE) was cleaned extensively in ethanol and ultrapure water, respectively, after being polished with 0.3 and 0.05 m alumina slurry. To create a homogenous suspension, the electrode material (10 mg) was sonicated into 1 mL of 0.5 wt% CS. After that, the 30 L of dispersion was placed on the glassy carbon electrode and allowed to dry in the air. Prior to electrochemical testing, a second 30 L dispersion was applied to the dried electrode surface and allowed to dry naturally. Using electrochemical workstations (Shanghai Chenhua Instrument Co. Ltd. CHI660E) and a 1 M KOH electrolyte solution, all electrochemical tests were performed. Hence, Eq. 1 was used to determine an electrode’s specific capacitance based on CV curves. C s = ∫ I d V m . v . Δ V (1) where, m, I, v , Δ V , C_s, are mass of electrode active material in gram (g), current in A, scan rate in mV/sec, potential window and, the specific capacitance in F/g. Moreover, the energy density in Whkg⁻¹and power density in Wkg⁻¹ of the as-prepared electrodes can be calculated by using Eqs 2, 3 E = 1 2 C s Δ v 2 (2) P = E Δ t 2 (3) where, E, C_s, Δ V, P and Δ t are energy density in Whkg^-1, specific capacitance of the electrode in Fg⁻¹, cells potential window in volt (V), maximum power density in Wkg⁻¹ and discharging time, respectively.

By employing Cu-K radiation (=1.54061) and XRD at a doping level of 0.05 M of Ni and Cu in the (2 θ range from 30^ο to 80^ο, the crystal structures and phase purity of Co₃O₄ and Ni/Cu- doped Co₃O₄ NPs were studied. Figure 1 displays the XRD patterns of un-doped and Ni/Cu-doped Co₃O₄ NPs samples with varied levels of doping. As can be observed, the samples contain XRD patterns that are similar and in good agreement with the standard JCPDS No. 78-1970 cards. (220), (311) (400), and (511) and can be used to denote the diffraction peaks at 28.18°, 37.13°, 46.14°, and 59.93°, respectively (Ramesh et al., 2019). The XRD spectra show no diffraction peaks from the Co₃O₄ species, suggesting that the Co atom is most likely substituted for Ni/Cu during the doping process. The XRD patterns’ features show that Ni and Cu doping has no effect on the crystal structure of Co₃O₄ (Yadav et al., 2017).

The crystallite size (D) of the as-prepared NPs were estimated using the Scherer’s Eq. 4. D = K λ β Cos θ (4) where, D, K, λ , β , and θ is crystallite size (nm), Scherer constant (0.94), wavelength of the X-ray sources (0.15406 nm), characteristic diffraction peaks corresponding to the full width at half maximum (FWHM) in radians, Bragg diffraction angle, respectively (Li et al., 2020). The estimated crystal size of Co₃O_4, 0.05 M Ni, and 0.05 M Cu doped Co₃O₄ nanoparticles were 35.74, 34.72, and 34.70 nm (Table 1). This shows that crystallite size was decreased with Ni and Cu doping. The decrease in crystallite size might be the ionic radii difference (Ni⁺² = 0.7 Å, Cu = ⁺2 0.7 Å and Co⁺³ = 0.63 Å) (Wei et al., 2019). The inter-planar spacing (d) of prepared samples were calculated according to Eq. 5.   d h k l = a h 2 + k 2 + l 2 (5) where,   d hkl indicates the inter-planar spacing or distance between lattice planes a, is lattice constant, and h, k, and l denotes to miller indices of peaks (311).

According to ICP-OES, the Cu content in 0.01 M Cu-Co₃O₄, 0.02 M Cu-Co₃O_4, 0.03 M Cu-Co₃O_4, 0.04 M Cu-Co₃O₄ and 0.05 M Cu-Co₃O₄ are 0.034, 0.052, 0.061, 0.073 and 0.085 at% (Table 2), correspondingly. Moreover, rendering to ICP-OES, the Cu content in 0.01 M Ni-Co₃O₄, 0.02 M Ni-Co₃O_4, 0.03 M Ni-Co₃O_4, 0.04 M Ni-Co₃O₄ and 0.05 M Ni-Co₃O₄ are 0.029, 0.036, 0.045, 0.058 and 0.067 at% (Table 3), correspondingly. The above results consist with the given content of Cu and Ni, indicating that the corresponding amount of Cu and Ni has been incorporated into cobalt oxide.

To confirm the functional groups that are present in the samples, the FTIR analysis of pure cobalt oxide and cobalt oxide nanoparticles that have been doped with Ni and Cu was conducted. Using an FTIR 6600 spectrometer, FTIR spectroscopy of pure and Ni and Cu-doped cobalt oxide nanoparticles was carried out in the absorption range of 4,000–400 cm⁻¹ wave number. The FTIR spectra of Co₃O₄ and Cu-doped Co₃O₄ NPs at various doping levels are depicted in Figure 2. Due to the as-prepared samples collecting moisture from the air, the broad absorption bands around 3,496 and 1,623 cm⁻¹ are attributed to the stretching and bending vibrations of water molecules (O-H). Furthermore, the stretching vibrations of NO₃ ⁻ caused by the precursor cobalt nitrate hexahydrate were identified as the absorption peaks at 1,383 cm⁻¹ (Bai and Yang, 2021). Nanoparticles vibrate in the C-O stretching mode in the band at 1,114 cm⁻¹ (Ali et al., 2021b). Hence, CO₃ ²⁻-anion may have a distinctive peak at 830 cm⁻¹. It is possible to attribute the two absorption bands at 618 cm⁻¹ were the stretching vibrations of metal-oxygen (Co-O or Cu-O) in spinel oxide Co₃O₄ NPs. The Co⁺²- O vibrations in the tetrahedral site of the Co₃O₄ NPs lattice are measured at 627 cm⁻¹, while the Co⁺³-O vibrations are measured at 517 cm⁻¹. The FTIR band of Co₃O₄ and Cu-doped Co₃O₄ NPs thus confirmed the production of M-O or M-O-M and O-H (Yang et al., 2020). The peak intensity and function groups for Ni and Cu doped Co₃O₄ increased as Ni and Cu concentration was added, improving the functionality of the nanoparticles. As a result, the FTIR bands of Co₃O₄ and Ni or Cu- Co₃O₄ NPs confirmed the formation of M-O and M-O-M (M= Ni, Cu or Co) and O-H (Worku et al., 2021f). The FTIR spectra seen in this investigation are compatible with the literature that has been previously reported (Jirátová et al., 2019).

The optical characteristics of pure and Cu-doped Co₃O₄ NPs at various dopant concentrations was investigated via UV-Vis spectroscopy. The UV-Vis absorbance bands of Co₃O₄ and Cu-doped Co₃O₄ NPs are depicted in Figure 3. The various absorption bands in the wavelength range between 250 and 500 nm are visible in the optical characteristics of Co₃O₄ NPs. Co₃O₄ was found to exhibit absorption peaks at 273 nm (Figure 3A). The properties of the Co₃O₄ band at 273 nm are a sign that it was produced via the co-precipitation technique from cobalt nitrate hexahydrate. For Co₃O₄ with Cu doping the matching absorption peak was shifted to 327.40 nm and 235 nm for Figures 3B, C, respectively. According to absorption spectra, as Cu-dopant concentration rises, the peak changes to the positive wavelength. The presence of Cu-impurities may have caused many occupied localized states to be introduced, which in turn changed the absorption band characteristics of the nanoparticles as they were being created (Jirátová et al., 2019). Moreover, the absorption spectrum of Ni-doped Co₃O₄ were shown in Figures 3D–F. The absorption band shifted from 273 nm to 302 nm for 0.01M Ni-Co₃O₄, 353 nm for 0.03 M Ni- Co₃O₄, and 384 nm for 0.05 M Ni-Co₃O₄ NPs.

The energy band gap of pure Co₃O₄ and Cu-doped Co₃O₄ NPs can were estimated from plot (α h v )² to photon energy ( h v ). The extrapolation of the linear region of these plots of (α h v )to photon energy ( h v ) from Tauc plot was used obtain the energy band gaps of Co₃O₄ NPs via Eq. 6 (Zhu et al., 2012).   α h υ n = A hv − Eg (6) where, α is the absorption coefficient, hυ is the photon energy and Eg is the band gap of the synthesized nanoparticle, n denotes the electronic transition value which depends on the electronic transition (n =2 then the transition is direct allowed, n =1/2 indirect allowed transition, and n= 3/2, n= 3 for direct forbidden and indirect forbidden transition, respectively) (Rani et al., 2017). The band gap energy of as-prepared materials were shown in Table 4.

Moreover, the estimated energy band gap values of Co₃O₄ and Ni-Co₃O₄ NPs were shown in Table 5.

SEM was used to examine the morphologies of pure Co₃O₄ and Ni and Cu- Co₃O₄ NPs. Figure 4 displays the morphologies of Co₃O₄, Ni, and Cu- Co₃O₄ NPs at various magnifications. The Co₃O₄ NPs in the SEM pictures at 20 µm magnifications are aggregated and have porous, rocky-like features (Figure 4A). The Co₃O₄ Ni- Co₃O₄ SEM pictures demonstrate homogeneous particle distribution. As Ni concentration is added, the particles’ morphology changes to nano-structured particles with irregular shapes and small aggregated porous particles (Figure 4B). The SEM analysis of Cu-doped Co₃O₄ NPs at various magnifications is displayed in Figure 4C. The examination revealed small, uniform-sized, spherical, and aggregated nanoparticles that are suitable for use in supercapacitors and consistent with earlier studies (Chang et al., 2021).

TGA/DTA was used to examine the thermal characteristics of Co₃O₄ Ni and Cu-doped Co₃O₄ NPs. At the temperature range of 25°C–950°C at a heating rate of 20°C per minute in an environment of air, TGA and DTA spectra have been observed. The TGA/DTA curve of Co₃O₄ and Cu-doped Co₃O₄ NPs is shown in Figure 5. The first weight loss of 0.95 mg in the pure Co₃O₄ TGA curve happens between 25°C and 200°C, which indicates that water molecules begin to evaporate from the sample. In the temperature range of 200°C–300°C, the TGA-curve showed the second weight loss of 1.62 mg. By eliminating the organic components, it is showing that complete breakdown occurs, resulting in the creation of Co₃O₄ Np. Hence, the lack of weight loss over 300°C verifies the composite’s full breakdown and crystallization. The overall TGA data reveal that up to 300°C, a total loss of 32.12% occurs (Figure 5C). The endothermic peak at 90 °C that is seen from the DTA curve of pure Co₃O₄ may be the result of the sample losing absorbed water (dehydration). Corresponding to this, the full breakdown of the cobalt precursor and the crystallization of Co₃O₄ spinel correlate to an extreme exothermic peak of 285°C between temperature ranges of 200°C–300°C. The first weight loss of 1.55 mg in the Cu-doped Co₃O₄ TGA curve from room temperature to 150°C and its related endothermic peak at 100 °C may be caused by the loss of physically absorbed water (Figure 5B). At the temperature range of 150°C–200°C, the second weight loss of 1.05 mg was noted. It might be connected to the constant de-nitration of nitrates produced by cobalt nitrate precursor and melting of the precursor. Between 300°C and 400°C, a third weight loss of 0.18 mg was noticed, which might be attributed to the precursor’s complete disintegration as a result of the removal of its organic constituents. The graph is linear and straight after 400°C, showing that no more weight loss takes place. The elimination of organic molecules from the material, which indicates the creation of Co₃O₄ crystals, is likely the cause of the two exothermic peaks that arise at 298°C and 348°C. The average weight loss for this analysis from 25°C to 950°C was 32.12% for Co₃O₄ and 34.7% for Cu- Co₃O₄ NPs, respectively. As a result, the comparison of the TGA and DTA results with the earlier findings was successful. The TGA/DTA curve for 0.05 M Ni-doped Co₃O₄ was recorded between 25°C and 950°C. The TGA curve demonstrates that the material undergoes two stages of heat breakdown. The TGA/DTA analysis of 0.05 M Ni-doped Co₃O₄ is displayed in Figure 5A. The elimination of absorbed water causes the first weight loss at a temperature of between 25°C and 200°C (dehydration). The fact that the second weight loss of 1.19 mg occurred in the 200°C–300°C temperature range shows that complete thermal breakdown can occur, resulting in the creation of Co₃O₄. The endothermic peak at 76°C on the DTA curve can be the result of precursors losing water (water removal from sample). The loss of organic molecules from the sample results in an exothermic peak at 286°C, leaving only the Co₃O₄ spinel. Between 100°C and 300°C, the TGA displays a considerable weight loss. In the process of losing weight, important gaseous products like water and nitrogen dioxide are emitted. Weight loss at 350°C supports the reported results for the conversion of [Co(NO₃)₂. 6H₂O] into Co₃O₄ (Geng et al., 2017).

To determine the total surface areas, pore diameters, and pore volumes of the materials as-prepared, BET was carried out in nitrogen gas at 77.35 K. Table 6 displays the overall surface area, pore volume, and pore diameter of Co₃O₄, Ni- Co₃O₄, and Cu-doped Co₃O₄ NPs. The dopant in Co₃O₄ caused a significant increase in surface area, as evidenced by the data. The increased contribution of the metal ions as extra nucleation sites during precipitation may be linked to the doped materials’ larger surface areas. Thus, the BET analysis revealed that the Co₃O₄ NPs with 0.05 M Ni and Cu doping had the highest surface areas, allowing for ion transfer and diffusion via the faradaic process (Molavi and Sheikhi, 2018). Doping impurities in Co₃O₄ NPs causes the BET surface to increase as a result. This shows that if dopants are added, with different atomic sizes than mother crystal, then the crystal will involve more imperfection. Such imperfections cause more surface roughness and might increase the specific surface area.