Bauhinia variegata mediated Co₃O₄ and Co₃O₄@CNT nanomaterials: Electrochemical and antibacterial studies

2.1. Collection of Bauhinia variegata leaves

Fresh leaves of Bauhinia variegate (Figure 1) were collected from Sardar Garh, District Rahim Yar Khan (Punjab), Pakistan, on 05 September 2022. The plant was taxonomically authenticated by the Department of Life Sciences at KFUEIT, Rahim Yar Khan, Pakistan. The harvested leaves were then rinsed thoroughly with distilled water, dried in a shaded area for 7 days, and finally converted into a fine powder by a grinder.

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Figure 1.

Bauhinia variegata (a) plant and (b) its leaves.

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2.2. Preparation of Bauhinia variegate extracts (aqueous & ethanolic)

For this, 10 g of B. variegata powder were mixed with 200 mL of distilled water in a 500 mL beaker and stirred for 2 h at 70°C. The mixture was passed through Whatman filter paper, and the resulting filtrate was collected as the aqueous extract (Figure 2).

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Figure 2.

Synthesis of cobalt oxide NPs.

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The same experimental steps were applied for the preparation of the ethanolic extract of B. variegata leaves by using ethanol in place of water (Figure 2).

2.3. Biosynthesis of Co₃O_4(aq) and Co₃O_4(et) NPs

For this, 0.145 g cobalt(II) nitrate hexahydrate was solubilized in distilled water (50 mL) and added to a beaker containing aqueous extract (100 mL) of B. variegata leaves, followed by stirring for 2 h. The mixture was left to stand at room temperature for 24 h and subsequently filtered using Whatman filter paper. The precipitates obtained were initially rinsed with distilled water, followed by ethanol. Finally, they were dried in an oven at 55°C, followed by their calcination at 400°C for 1 h to leave behind the final product of Co₃O_4(aq) NPs.

The same experimental approach was adopted for the production of Co₃O_4(et) NPs by using the ethanolic extract in place of the aqueous extract of B. variegata leaves. Figure 2 describes the reaction pathways for the biosynthesis of Co₃O_4(aq) and Co₃O_4(et) NPs.

2.4. Preparation of composites of Co₃O_4(aq) and Co₃O_4(et)NCs with CNTs

Co₃O_4(aq)@CNT₁, Co₃O_4(aq)@CNT₂, and Co₃O_4(aq)@CNT₃ NCs were prepared by integration of Co₃O_4(aq) with CNTs in the ratios of 97:3, 94:6, and 91:9, respectively. For this purpose, 0.097, 0.094, and 0.091g of Co₃O_4(aq) were sonicated using the Ultra Sonic Machine (Elmasonic Easy 30 H) for 12 h with 0.003, 0.006, and 0.009g of CNTs, respectively, in three different beakers, each beaker containing 20 mL of water. Then the contents of three beakers were separately poured into three different China dishes and finally placed in an oven at 80°C for drying to leave behind the solid CNT decorated Co₃O_4(aq) NCs, i.e., Co₃O_4(aq)@CNT₁, Co₃O_4(aq)@CNT₂, and Co₃O_4(aq)@CNT₃ NCs, respectively (Figure 3).

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Figure 3.

Synthesis of Co₃O_4(aq) decorated CNT NCs where Co₃O_4(aq)@CNT = Co₃O_4(aq)@CNT₁, Co₃O_4(aq)@CNT₂, and Co₃O_4(aq)@CNT₃; Co₃O_4(et)@CNT = Co₃O_4(et)@CNT₁, Co₃O_4(et)@CNT₂ and Co₃O_4(et)@CNT₃.

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The same procedure was applied for the synthesis of Co₃O_4(et)@CNT₁, Co₃O_4(et)@CNT₂, and Co₃O_4(et)@CNT₃ NCs by sonicating Co₃O_4(et) with CNTs in the ratios of 97:3, 94:6, and 91:9, respectively, for 12 h by using ethanol solvent (in place of water) (Figure 3).

2.5 Electrochemical studies

The electrochemical behavior and specific capacitance of Co₃O₄ NPs and their composites with CNTs were examined through CV and GCD experiments using a three-electrode setup at ambient temperature. The electrode material was prepared by cutting nickel foam into uniform strips, each having a 1 cm width and 2 cm length. The strips were immersed in a solution (freshly prepared) consisting of 0.5 mL of HCl and 100 mL of deionized water. Afterwards, the nickel foam strips were sonicated for 60 min and subsequently dried at 30°C for 24 h. Next, a slurry was prepared in a beaker containing 80% of a test material (synthesized NMs), 15% activated carbon, and 5% binder, and then adding a few drops of N-Methyl-2-Pyrrolidone (NMP). The prepared slurry was brushed onto the surface of nickel foam, and the coated material was left to dry for 24 h at room temperature.

The nickel foam electrodes were coated with a fixed slurry composition, and the active mass was strictly controlled at 5 mg per electrode, verified by weighing before and after coating. This loading is consistent with typical ranges reported in similar electrochemical studies on nickel foam electrodes (2-6 mg cm⁻²), ensuring comparability with literature values [15,16]. The reproducibility of CV and GCD responses further confirms uniform film thickness across all samples.

2.6 Antibacterial studies

In vitro antibacterial activity of the green-synthesized NMs was determined by the disc diffusion method using the small filter paper discs, each having a 6 mm diameter. Nutrient agar served as a culture medium, whereas tetracycline was employed as a reference drug (positive control). The culture plates were incubated at 37°C for a period of 24 h. After that, the zones of inhibition were measured in millimeters by a zone reader [17].

3.1. Structural studies by XRD analysis

XRD is primarily used to identify the crystal structure and the average sizes of the crystalline domains, often referred to as the crystallite size of a material. The synthesized nano-materials were subjected to XRD analysis; the acquired XRD patterns have been presented in Figure 5.

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Figure 5.

(a) XRD patterns of Co₃O₄_(aq), Co₃O₄_(aq)@CNT₁, Co₃O₄_(aq)@CNT₂, and Co₃O₄_(aq)@CNT₃, (b) XRD patterns of Co₃O₄_(et), Co₃O₄_(et)@CNT₁, Co₃O₄_(et)@CNT₂, and Co₃O₄_(et)@CNT₃.

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The XRD analysis revealed peaks located at distinct 2θ angles of 44°, 47°, and 62° in Co₃O_4(aq) and at 42.07°, 47.6°, and 62.68° in Co₃O_4(et). The existence of these three peaks, corresponding to the (311), (400), and (331) planes, respectively, confirms the cubic structures in both these nanostructures. Moreover, the observed XRD patterns of Co₃O_4(aq) and Co₃O_4(et) are very similar to each other (Figure 5) and match the JCPDS card number 74-1656.

The CNTs decorated Co₃O₄ NCs exhibited almost the same diffraction patterns and very similar 2θ values, which corresponded to the identical crystal planes of (220), (222), (311), (400), (422), (511), and (440) (Table 1). However, an additional peak corresponding to (002) crystal plane of CNT structure was also observed at a 2θ value of 24.87°, 23.9°, 23.23°, 23.97°, 23.38°, and 24.46° in Co₃O_4(aq)@CNT₁, Co₃O_4(aq)@CNT₂, Co₃O_4(aq)@CNT₃, Co₃O_4(et)@CNT₁, Co₃O_4(et)@CNT₂, and Co₃O_4(et)@CNT₃, respectively. The observed diffraction pattern was matched to the standard JCPDS card number 76-1802 of cubic structures, and verified the successful formation of CNT decorated Co₃O₄ NCs.

The crystallite sizes of the synthesized NMs were calculated by applying the Debye-Scherrer equation (Eq. 1) [22].

Here, K (0.94) denotes the crystallized factor, β is the full width at half maximum (FWHM), λ represents the X-ray wavelength (0.154 nm), D corresponds to the crystallite size, and θ is the Bragg diffraction angle.

Using the Debye-Scherer equation, the average grain sizes of NMs were found to be 22.54, 13.73, 16.48, 16.82, 20.48, 16.08, 16.42, and 19.47 nm for Co₃O_4(aq), Co₃O_4(aq)@CNT₁, Co₃O_4(aq)@CNT₂, Co₃O_4(aq)@CNT₃, Co₃O_4(et), Co₃O_4(et)@CNT₁, Co₃O_4(et)@CNT₂, and Co₃O_4(et)CNT_3, respectively [23]. Thus, the average crystalline sizes of Co₃O_4(aq) (22.54 nm) and Co₃O_4(et) (20.48 nm) were decreased to 13.73-16.82 nm and 16.08-19.47 nm, respectively, in their respective NCs with CNT. The decreased grain size in an NC is attributed to the frequent existence of oxygen-containing functional groups (e.g., hydroxyl & carboxyl) on the CNTs’ surfaces, which restrict diffusion and suppress the crystallization and growth of Co₃O₄ grains [24,25].

3.2. FTIR spectroscopy

FTIR spectroscopic analysis of the synthesized NMs was carried out within a spectral range of 400-4000 cm^-1 to determine the vibrational frequencies of their functional groups, and the obtained spectra have been displayed in Figures 5 and 6, respectively.

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Figure 6.

FTIR spectra of Co₃O_4(aq), Co₃O_4(aq)@CNT₁, Co₃O_4(aq)@CNT₂, and Co₃O_4(aq)@CNT₃ NPs derived from aqueous extract of B. variegata.

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All the synthesized NMs displayed two distinct vibrations in the ranges of 561-578 (v₁) and 660-666 (v₂) cm^-1, which were attributed to the stretching vibrations of the Co–O bonds, and thus, verify the formation of Co₃O₄ nanostructures. The v₁ band corresponds to Co³⁺–O vibrations in octahedral coordination, while the v₂ band is associated with Co²⁺–O vibrations in tetrahedral coordination [26]. FTIR spectra of Co₃O_4(aq), Co₃O_4(aq)@CNT₁, Co₃O_4(aq)@CNT₂, and Co₃O_4(aq)@CNT₃ NPs derived from aqueous extract of B. variegata, demonstrated a very broad peak at 3409-3486 cm⁻¹ (Figure 6) indicating the presence of hydroxyl groups of water molecules either in the form of moisture [27] or interstitial water molecules [28]. The existence of hydroxyl groups was further verified by the appearance of Co-OH vibrations at 837-843 cm⁻¹ in these nanostructures. Among the ethanolic extract-derived NMs, only Co₃O_4(et) exhibited a broad signal at 3551 cm⁻¹ for O-H and a strong intensity band at 843 cm⁻¹ for Co-OH; both these bands were absent in Co₃O_4(et)@CNT₁, Co₃O_4(et)@CNT₂, and Co₃O_4(et)@CNT₃ (Figure 7). A peak at 1113-1130 cm⁻¹ was owed to the presence of the C-O bond [29], and that at 1383-1386 cm⁻¹ was attributed to the C-OH stretching of the phenolic hydroxyl group [30], whereas C=C stretching vibrations were disclosed at 1613-1635 cm⁻¹ [31]. These peaks indicate the existence of an organic coating upon the surfaces of NPs, which originated from the aqueous and ethanolic extracts of B. variegata during the nano-synthetic process.

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Figure 7.

FTIR spectra of Co₃O_4(et), Co₃O_4(et)@CNT₁, Co₃O_4(et)@CNT₂, and Co₃O_4(et)@CNT₃ NMs derived from the ethanolic extract of B. variegata.

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A comparison between the FTIR data of all NMs (Figures 6 and 7) clarifies that Co₃O_4(aq) and Co₃O_4(eq) are richer in FTIR peaks as compared to their respective NCs with CNTs. These additional peaks at 790, 931, 1237, 1706, and 2046 cm⁻¹ in Co₃O_4(aq) and 725 cm⁻¹ in Co₃O_4(et) represent the presence of various organic moieties from plants. Thus, Co₃O_4(aq) synthesized with aqueous extract of B. variegata was richer in organic biomaterial as compared to all other biosynthesized NPs. However, integration of CNT with Co₃O₄ results in the disappearance of many organic peaks, thus changing the complex spectral pattern of Co₃O_4(aq) and Co₃O_4(et) into fewer simple spectral peaks in their respective CNT decorated NCs.

3.3. UV-Visible spectroscopy

In the case of NPs, a uniform distribution would result in narrow and distinct absorption peaks in the UV-Vis spectra. The synthesized NMs were subjected to UV-Visible spectroscopy in a spectral range of 200 to 400 nm; the obtained spectra have been shown in Figure 8.

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Figure 8.

UV-Visible spectra of NMs synthesized with (a) aqueous and (b) ethanolic extracts of B. variegata.

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UV-Visible spectra had shown absorption peaks at 300 and 335 nm in Co₃O_4(aq), which were slightly shifted to 302 and 338 nm in Co₃O_4(aq)@CNT₁, Co₃O_4(aq)@CNT₂, and Co₃O_4(aq)@CNT₃ (Figure 8). Co₃O_4(et) displayed absorptions at 244 and 298 nm, which were shifted to 301 and 298 nm in Co₃O_4(et)@CNT₁, Co₃O_4(et)@CNT₂, and Co₃O_4(et)@CNT₃ (Figure 8). The peaks were sharper and more intense in the samples synthesized with aqueous extracts compared to those with ethanolic extracts of B. variegata. The sharp peaks indicate the stability of the synthesized NMs, while the higher intensity suggests a higher yield of the NMs prepared in the presence of aqueous extracts [32].

The band gaps of NMs were considered by utilizing the Tauc equation (Eq. 2).

Here, v is the energy of a photon, $\alpha$ is the coefficient of absorption, h is Planck’s constant, transition (indirect) is n =1/2, and direct shift is n = 2; the band gap of energy is denoted by Eg, A represents a proportionality constant [33]. The band gaps of Co₃O_4(aq), Co₃O_4(aq)@CNT₁, Co₃O_4(aq)@CNT₂, Co₃O_4(aq)@CNT₃, Co₃O_4(et), Co₃O_4(et)@CNT₁, Co₃O_4(et)@CNT₂, and Co₃O_4(et)@CNT₃ were found to be 4.78, 4.9, 4.74, 5.15, 5.1, 5.34, 5.21, and 5.36 eV, respectively (Figure 9). The energy band gaps are higher in ethanolic extract derived NMs i.e., Co₃O_4(et), Co₃O_4(et)@CNT₁, Co₃O_4(et)@CNT₂, and Co₃O_4(et)@CNT₃ as compared to ethanolic extract derived counterparts i.e., Co₃O_4(aq), Co₃O_4(aq)@CNT₁, Co₃O_4(aq)@CNT₂ & Co₃O_4(aq)@CNT₃, respectively. It has been well established that higher band gap materials contribute substantially to the advancement of modern technologies across various sectors, offering improved efficiency, reliability, and performance in demanding applications ranging from optoelectronics and solar energy to aerospace and power electronics [34].

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Figure 9.

Band gaps of (a) Co₃O_4(aq), (b) Co₃O_4(aq)@CNT₁, (c) Co₃O_4(aq)@CNT₂, (d) Co₃O_4(aq)@CNT_3, (e) Co₃O_4(et), (f) Co₃O_4(et)@CNT₁, (g) Co₃O_4(et)@CNT₂, & (h) Co₃O_4(et)@CNT₃.

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3.4. SEM

The SEM images were used to study the morphologies of the synthesized NMs; the obtained photographs have been shown in Figure 10. The Co₃O_4(aq) NPs contain irregular agglomerated structures with pores (dark region) (Figure 10a), whereas Co₃O_4(et)NPs display larger clusters, suggesting higher aggregation or sintering (Figure 10b). The agglomeration indicates the strong interparticle interactions.

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Figure 10.

SEM image of (a) Co₃O_4(aq), (b) Co₃O_4(et), (c) Co₃O_4(aq)@CNT₁, (d) Co₃O_4(aq)@CNT₂, (e) Co₃O_4(aq)@CNT₃, (f) Co₃O_4(et)@CNT₁, (g) Co₃O_4(et)@CNT₂, and (h) Co₃O_4(et)@CNT₃.

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The SEM graphs of the CNT-decorated NCs have shown well-uniform particles with a narrow size distribution. They exhibit smooth surface morphologies, a feature consistent with observations reported in earlier studies [34]. Co₃O_4(aq)@CNT₁, Co₃O_4(aq)@CNT, and Co₃O_4(aq)@CNT₃ (Figures 10c-e) form stable hollow spherical structures composed of interconnected NPs. The Co₃O₄ NPs were found to self-assemble into a sieve-like arrangement, adhering to the surface and giving rise to internal cavities and porous architectures [35]. The encapsulation of the NPs within the nanotubes exhibits a dendrite morphology. The SEM graphs of Co₃O_4(et)@CNT₁, Co₃O_4(et)@CNT₂, and Co₃O_4(et)@CNT₃ (Figures 10f-h) demonstrate the grain-like morphology, where CNTs are visibly decorated with NPs and uniformly distributed within the Co₃O₄ matrix. Since Co₃O₄ alone exhibits a limited conductivity, the incorporation of well-distributed CNTs facilitates a 3D electron transport pathway, thereby improving the interconnection between Co₃O₄ particles and boosting the electrical performance of the composite [36].

The particle sizes of the NMs were determined using ImageJ software and were found to be 48.86, 44.06, 54.65, 45.75, 31.88, 23,63, 22.81, and 24.34 nm for Co₃O_4(aq), Co₃O_4(et), Co₃O_4(aq)@CNT₁, Co₃O_4(aq)@CNT₂, Co₃O_4(aq)@CNT₃, Co₃O_4(et)@CNT₁, Co₃O_4(et)@CNT₂, and Co₃O_4(et)@CNT₃, respectively.

3.5. TGA and DSC

TGA and DSC of the materials are generally performed to evaluate their thermal decomposition [37], stability [38,39], and kinetic parameters [40]. We have calculated the weight loss percentages and enthalpies of our synthesized samples by TGA/DSC analyses within a temperature range of 25-1000°C, and the obtained curves are shown in Figures 11(a,b). The results indicated cumulative weight losses of 52.7% and 58.06% in Co₃O_4(aq) and Co₃O_4(et), respectively. The initial weight loss of 8.12% and 6.21% observed in Co₃O_4(aq) and Co₃O_4(et), occurring between 23-101.62°C and 27-89.25°C, respectively, was attributed to endothermic loss of moisture or interstitial water molecules. A subsequent weight loss occurred between 301-924.38°C and 306-939.03°C in Co₃O_4(aq) and Co₃O_4(et), respectively was primarily attributed to the endothermic decomposition of materials and exothermic loss of organic moieties. TGA-DSC analysis of Co₃O_4(aq) and Co₃O_4(et) revealed weight loss of 52.7 and 58.06% with an enthalpy of 4068.4 and 2913.4 J/g, respectively. Substances with higher enthalpy possess greater stored energy since they must absorb more heat, which makes them more reactive but less stable. Conversely, lower enthalpy values correspond to reduced reactivity and enhanced stability. The comparatively lower enthalpy of Co₃O_4(et) NPs indicates that they occupy a more stable energy state than Co₃O_4(aq) NPs, suggesting enhanced thermodynamic stability of the ethanolic extract–derived NPs.

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Figure 11.

TGA/DSC curves of (a) Co₃O_{4 (aq)} and (b) Co₃O_4(et) NPs.

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3.6. Electrochemical performance testing

3.6.1. CV

Figure 12 displays the CV curves recorded within the potential range of 0-0.5 V at scan rates of 10-50 mV/s. The NMs exhibited two separate redox pairs, associated with the oxidation and reduction of the NPs, indicating pseudocapacitive behavior originating from Faradaic reactions. The CV curves of the NMs showed minor shifts in their reduction peak potentials, varying between 0.1 and 0.2 V in Co₃O_4(aq)@CNT₁ and Co₃O_4(et)@CNT₂ and 0.2 and 0.3 V in the remaining NMs, i.e., Co₃O_4(aq), Co₃O_4(aq)@CNT₂, Co₃O_4(aq)@CNT₃, Co₃O_4(et), Co₃O_4(et)@CNT₁, and Co₃O_4(et)@CNT₃. Additionally, the oxidation peaks of Co₃O₄ and their GO-decorated NMs exhibited similar trends within the 0.2-0.45 V range. Importantly, all CV curves displayed redox peaks, indicating that the charge-discharge processes in the NMs are reversible. This behavior enhances catalytic activity and facilitates better access to the active sites. Owing to their large surface area and improved porosity, the NCs showed higher peak currents and lower peak potentials.

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Figure 12.

Cyclic voltammograms of (a) Co₃O_4(aq), (b) Co₃O_4(aq)@CNT₁, (c) Co₃O_4(aq)@CNT₂, (d) Co₃O_4(aq)@CNT₃, (e) Co₃O_4(et), (f) Co₃O_4(et)@CNT₁, (g) Co₃O_4(et)@CNT₂, and (h) Co₃O_4(et)@CNT₃.

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3.6.2. GCD behavior

To further highlight the electrochemical behavior of Co₃O₄ NPs and their CNT NCs, GCD tests were performed at the current densities of 1-5 A/g (Figure 13). The GCD curves displayed a nearly triangular and highly symmetric shape, suggesting excellent electrochemical reversibility of the Co₃O₄ electrode. Moreover, as the current density increased, the specific capacitance gradually decreased, while no voltage drop was observed during polarity reversal, demonstrating the ideal capacitive behavior of the Co₃O_4(aq)NPs. The charge-discharge curves did not exhibit the typical behavior of a pure electric double-layer capacitor, but instead indicated pseudocapacitive characteristics. This observation was consistent with the findings obtained from the CV curves.

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Figure 13.

GCD of (a) Co₃O_4(aq), (b) Co₃O_4(aq)@CNT₁, (c) Co₃O_4(aq)@CNT₂, (d) Co₃O_4(aq)@CNT₃, (e) Co₃O_4(et), (f) Co₃O_4(et)@CNT₁, (g) Co₃O_4(et)@CNT₂, and (h) Co₃O_4(et)@CNT₃.

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The specific capacitance of the investigated materials was also calculated in a potential range of −0.5 to 0.4V in 2 M KOH at a current density of 1-5 Ag⁻¹, from their charge–discharge curves. The specific capacitance vales for Co₃O_4(aq), Co₃O_4(aq)@CNT₁, Co₃O_4(aq)@CNT₂, Co₃O_4(aq)@CNT₃, Co₃O_4(et), Co₃O_4(et)@CNT₁, Co₃O_4(et)@CNT₂, and Co₃O_4(et)@CNT₃ electrodes were found to be 39.5, 378.3, 365, 22.06, 513.8, 175.15, 425.7, and 989.25 F g⁻¹, respectively. Clearly, the incorporation of CNT markedly enhanced the specific capacitance of the pure Co₃O_4(et) electrode. The specific capacitance of Co₃O_4(et) and its NCs, Co₃O_4(et)@CNT₁, Co₃O_4(et)@CNT₂, and Co₃O_4(et)@CNT₃, was significantly higher (175.15-989.25 Fg⁻¹) as compared to that of (22.06-378.3 Fg⁻¹) in Co₃O_4(aq) and its NCs, Co₃O_4(aq)@CNT₁, Co₃O_4(aq)@CNT₂, and Co₃O_4(aq)@CNT₃. The results thus elaborate that the ethanolic extract of B. variegata leaves is best for the production of SCs. The highest value of specific capacitance (989.25 F g⁻¹) was exhibited by Co₃O_4(et)@CNT₃, demonstrating it is an excellent material as an SC. It is worth mentioning that higher capacitance is exhibited by SCs as compared to conventional capacitors; it is due to the greater surface areas of the former electrodes (SCs). Moreover, SCs can deliver higher power than batteries, which is mainly due to the fundamental difference in their energy storage mechanisms. In SCs, charge storage takes place at the electrode surfaces rather than within the bulk material, while in batteries, the storage process predominantly occurs throughout the bulk volume [41].

The integration of renewable resources into energy storage systems has the potential to promote a greener and more sustainable energy infrastructure [42]. From our Bauhinia variegate mediated Co₃O₄ NPs and their CNTs decorated NCs, the two ethanolic extract derived NMs, i.e., Co₃O_4(et)@CNT₂ and Co₃O_4(et)@CNT₃, exhibited exceptional electrochemical performance, with specific capacitances of 425., and 989.25 F g⁻¹, respectively, compared to the previously reported CNT-decorated Co₃O₄ nanostructures. Tao et al. (2015) reported that Co₃O₄ coated on multiwalled CNTs, prepared by a simple chemical deposition method, exhibited a maximum specific capacitance of 273 F g⁻¹ at a charge–discharge current density of 0.5 A g⁻¹ [43]. Shan and Gao (2007) investigated the SC potential of multi-walled CNT/Co₃O₄ NCs and found their maximum specific capacitance of 200.98 F g⁻¹, compared to 90.1 F g⁻¹ for pure multi-walled CNTs [44]. Ke et al. (2015) reported that functionalized CNT-decorated Co₃O₄ nanostructures exhibited a capacitance of 559 F g⁻¹, approximately 3.5 times higher than that of pristine Co₃O₄ [45]. Our Co₃O₄(et)@CNT₃ electrode delivered 989.25 F g⁻¹, placing it among the upper tier of Co₃O₄-CNT reports. It outperforms many earlier Co₃O₄/CNT coatings that typically showed ∼200–400 F g⁻¹ at comparable or milder conditions, and it matches or exceeds more advanced composites such as supercritical-CO₂-mediated CNT/Co₃O₄ (950 F g⁻¹ at 1 A g⁻¹) [46] while approaching the current state-of-the-art oxygen-vacancy-rich Co₃O₄-NSs/CNTs (1280 F g⁻¹ at 1 A g⁻¹) [47]. Given that practical gravimetric capacitances are far below the theoretical 3560 F g⁻¹ limit for Co₃O₄, the present value indicates an efficient Co₃O₄-CNT interface and percolating electron pathways, and thus represents a notable advance relative to most Co₃O₄-CNT benchmarks [48].

3.7. Antimicrobial evaluation

Antimicrobial agents are highly important for the treatment of pathogenic diseases, the control of foodborne infections [49], and food preservation [50], and numerous metal oxide NPs have demonstrated effectiveness in these applications [51]. The B. variegata mediated NMs were tested for their antibacterial potential against Bacillus subtilis (Gram-positive bacterium) by the disc diffusion method, whereas tetracycline was used as a standard antibiotic drug. The zones of inhibition (ZOI) were measured in mm; the obtained results have been displayed in Figure 14 and summarized in Table 2.

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Figure 14.

Petri plates showing the inhibition zones on small filter paper discs of Co₃O_4(aq) (na), Co₃O_4(aq)@CNT₁ (na1), Co₃O_4(aq)@CNT₂ (na2), Co₃O_4(aq)@CNT₃ (na3), Co₃O_4(et) (ne), Co₃O_4(et)@CNT₁ (ne1), Co₃O_4(et)@CNT₂ (ne2), Co₃O_4(et)@CNT₃ (ne3) and tetracycline-standard drug (sd).

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All the synthesized NMs, i.e., Co₃O_4(aq),Co₃O_4(aq)@CNT₁, Co₃O_4(aq)@CNT₂, & Co₃O_4(aq)@CNT₃, Co₃O_4(et), Co₃O_4(et)@CNT₁, Co₃O_4(et)@CNT₂, and Co₃O_4(et)@CNT₃ had shown significant activities against B. subtilis. However, it is worth noting that the antibacterial activities of Co₃O_4(aq) or Co₃O_4(et) were increased in their respective CNTs decorated NCs (Table 2). Co₃O_4(aq)was totally inactive but its NCs i.e., Co₃O_4(aq)@CNT₁, Co₃O_4(aq)@CNT₂ & Co₃O_4(aq)@CNT₃ had shown 9.3-11.4 mm ZOIs. Also, the antibacterial activity of Co₃O_4(et) (11.3±0.14 mm) was slightly increased to 12.3-13.5mm in its corresponding NCs, i.e., Co₃O_4(et)@CNT₁, Co₃O_4(et)@CNT₂, and Co₃O_4(et)@CNT₃. However, all the NMs exhibited lower activity as compared to the standard drug, tetracycline (21.6±0.19 mm).

3.8. Comparison of the existing study with the early reported studies

Our study utilizes aqueous and ethanolic extracts of B. variegata leaves as reducing, capping, and stabilizing agents for the sustainable synthesis of Co₃O₄ NPs. Then, Co₃O₄ decorated CNTs NCs were produced to synergistically integrate the electrochemical/biological properties of the biosynthesized Co₃O₄ NPs and CNTs. Moreover, this study was performed in the presence of water and ethanol solvents, which are both environmentally friendly.

The green synthesis of Co₃O₄ NPs was earlier reported with numerous plant extracts, including Aspalathus linearis, Azadiracta indica, Colotropis giganta, Colotropis procera, Euphorbia heterophylla, Gingko, Helianthus annus, Hibiscus Rosa-sinensis, Manihot esculenta crantz, and Moringa oleifera, and investigated their numerous structural features and applications, as summarized in Table 3. However, there were still no reports on the green synthesis of Co₃O₄ NPs in the presence of Bauhinia variegata leaves as reducing, capping, and stabilizing agent, which is the target of the current study. We evaluated the electrochemical potential of the synthesized NMs by CV and GCD and they were found suitable for battery applications due to their reversible charging and discharging behavior. Co₃O_4et)@CNT₃ displayed the highest specific capacitance (989.25 F g⁻¹), showcasing its exceptional potential as a SC material. All the synthesized NMs except Co₃O_4(aq) have shown significant antibacterial potential (ZOI = 9-13 mm) as compared to tetracycline (ZOI = 21 mm) against Bacillus subtilis (Gram-positive) by the disc diffusion method.