From the journal:

Photomaterials & Devices

Facile Sonochemical Fabrication of Ag₂O–NiO Nanocomposite with Improved Photocatalytic Activity and Electrochemical Sensing Efficiency

Graphical Abstract

1. Introduction

Due to their non-biodegradable behavior, which causes a variety of illnesses that affect the entire biota, the discharge of specific industrial pollutants, such as drugs, paint, textiles, leather, food, and many other industries, into the aquatic environment has made control, monitoring, and removal of these toxic materials crucial. Organic effluents need to be eliminated in order to create a safe and healthy environment [1]. In recent decades, photocatalysis, which seeks to develop an inexpensive, eco-friendly catalyst with sufficient mineralization capacity, has grown in popularity, particularly for addressing energy and pollution problems [2]. Since the band gaps of the various semiconductors typically forbid it, the mineralization process has comparatively few applications. Therefore, it is necessary to modify the semiconductors to develop new photocatalysts with high activity under UV radiation. Many metal oxides, including TiO₂, Co₃O₄, Ag₂O, and NiO, have been utilized in recent decades to photochemically mineralize hazardous compounds [3,4,5,6,7,8,9,10]. However, each of these photocatalysts has serious disadvantages, including (i) decreased light transmittance, (ii) aggregation of nanoparticles, and (iii) decreased surface area. Therefore, to increase the photocatalytic efficiency, more innovative synthesis is required.

Due to their chronic toxicity and non-biodegradability, organic dye pollution causes a number of health problems in humans, primarily affecting the liver, brain, and central nervous system [11]. As a result, dye-containing wastewater must be treated immediately before being discharged into the environment.

Fast blue is a synthetic dye that is frequently used in biological staining, especially in histology and textiles [12]. It is well known for its vivid color and ability to bind to a variety of materials, like many other azo dyes. However, because Fast blue and related dyes are persistent in soil and water systems, they can pose environmental problems, making their degradation a noteworthy concern. Because of their poisonous and carcinogenic breakdown products, improper disposal of colors like Fast Blue can contaminate water, harm aquatic life, and possibly even impact humans. Therefore, reducing their environmental impact requires the development of efficient degradation techniques.

Researchers are studying the broad field of nanoscience from a variety of technological angles. Since nanocomposite (NC) materials combine special properties, researchers have focused on them during the last few decades. Advances in new technologies to solve energy-related problems are currently focusing more on supercapacitors [13,14]. Because they combine the properties of their constituent materials, nanocomposites are extensively studied in the domains of energy and catalysis. Large-scale applications are currently drawing more attention to the development of high-band-gap (Eg) semiconductors with distinctive features, such as TiO₂, ZnO, NiO, and SnO₂ [15,16]. Metal oxide (MO) semiconductors are widely employed in various applications, including gas-sensing devices, photocatalysis, hybrid solar cells, emission control, UV shields, LEDs, temperature-control paints, piezoelectric devices, and supercapacitors [17,18,19,20,21,22,23,24]. Semiconducting materials need to be used more widely to restore our damaged atmosphere and develop technology sustainably. In the current era of electronics-based electronic base technologies, the development of sophisticated electronic features that are both advantageous and entertaining is essential [25]. The supercapacitor is an efficient energy-storage component used in next-generation energy-storage systems for high-power applications [26].

Metal nanostructures have been the focus of a lot of research recently. In particular, the combination of metal nanoparticles (MNPs) and oxide systems has garnered interest for its potential across a range of domains, including transformations, photocatalysis, biological research, sensing, and catalysis in various supercapacitors [27,28]. NiO is an important metal oxide with many applications due to its beneficial properties, including fuel cell composite anodes, solar cells, gas sensing, electrochromic coatings, dye-sensitized photocathodes, smart windows, energy storage systems, and supercapacitors, among others [29,30]. Nickel oxide (NiO) is a semiconductor material with noteworthy optical, electrical, and catalytic capabilities as well as a large band gap. High theoretical capacitance (2584 F/g for NiO), affordability, environmental compatibility, superior chemical and thermal stability, and widespread availability are just a few of the benefits that make nickel-based systems attractive options for electrode materials. However, NiO's pseudocapacitive property is caused by poor intrinsic conductivity, delayed reaction kinetics, and a limited number of electrochemically active sites, largely on the surface. Therefore, investigating methods to improve NiO's conductivity through nanocomposite (NC) structures may offer more profound understanding of its functional potential.

In addition to their many scientific and commercial applications, silver nanoparticles are increasingly attracting attention for their unique physical, chemical, and biological properties, including high electrical conductivity [31,32]. Furthermore, studies have demonstrated the exceptional catalytic capabilities of Ag nanostructures in reducing organic dyes [33]. Ag-NPs have been extensively studied as a catalyst in industrial applications due to their efficiency. and cost [34]. Studies on Ag NPs enhancing the electrochemical stability of electrode materials have also been reported [35]. The earlier study looked at how Ag loading affected the supercapacitor performance of graphene-based nanocomposites [36]. It would be interesting to study the electrochemical and catalytic properties of Ag-incorporated NiO systems.

Ag-NiO (silver-nickel oxide) composites have attracted attention for their improved photocatalytic and sensor capabilities compared to their individual components, which is why we have selected them for the photocatalytic and sensor applications in this study. By combining their own advantages, Ag and NiO both increase the total photocatalytic and sensor efficiency [37,38].

NiO is a p-type semiconductor that mainly permits light absorption in the ultraviolet spectrum. Its band gap is between around 3.4 and 4.0 eV. However, the high rate of electron-hole pair recombination reduces its photocatalytic efficacy. Ag nanoparticles can be successfully added to NiO to lower the band gap and improve the performance of the composite. By extending light absorption into the visible spectrum, Ag's surface plasmon resonance (SPR) effect enhances photocatalytic activity in the presence of sunlight or other visible light. Furthermore, the Ag-NiO combination creates a Schottky junction between the metal Ag and the semiconductor NiO, which facilitates electron flow from NiO to Ag and increases photocatalytic efficiency even more. By preventing electron-hole pair recombination, this increases photocatalytic efficiency by enabling more electrons to take part in redox processes. The synergistic interaction between Ag and NiO has led to the effective degradation of organic pollutants, such as dyes and phenols, by Ag-NiO composites. This contact makes it easier for reactive oxygen species (ROS) to form, including superoxide anions (O₂⁻) and hydroxyl radicals (•OH), which are essential for the breakdown of pollutants [39].

Because of the combination of Ag's high electrical conductivity and NiO's high pseudocapacitive behavior, the Ag-NiO composite shows potential as a sensor material [40]. This hybrid arrangement improves electrochemical performance by leveraging the strengths of both components. Because of redox reactions at the material's surface, NiO is renowned for having exceptional pseudocapacitive qualities. Reversible Faradaic processes involving charge transfer can occur in NiO, resulting in high specific capacitance. Ag nanoparticles improve the composite's conductivity by providing efficient electron pathways that increase the rates of charge storage and discharge. The Ag-NiO composite's specific capacitance is increased by the addition of Ag nanoparticles. Ag nanoparticles successfully increase the electrical conductivity of the composite, improving electrochemical performance, whereas NiO alone has low intrinsic electrical conductivity, which can limit its performance in supercapacitors. Ag reduces internal resistance (equivalent series resistance, or ESR) by acting as a conductive network that speeds up electron movement during charging and discharging. NiO and Ag work in concert to increase charge storage capacity [41]. Faster ion/electron transit and improved use of active materials are the results of the composite structure's large surface area, high electrochemical activity of NiO, and superior conductivity of Ag [41].

In our work, the silver oxide-nickel oxide nanocomposite SO-NO NC was prepared by the sonochemical technique with Tulsi leaf extract. The composite material is analyzed using various spectral methods. XRD of SO-NO NC shows crystallites of 19.80 nm in size. The SEM shows face-centered cubic and an average particle size of roughly 19.80 nm. According to the DRS spectra, SO-NO NC has an energy band gap value of 2.95 eV. Fast blue dye photodegradation was used to assess the NC Photocatalytic Activity; the results Shows under UV-Vis Light irradiation, maximum absorption observed at 620 nm with 71.45% degradation. In cyclic-voltammetric investigations, the SO-NO NC is also employed as an electrode. It displays the surface redox mechanism of Ni²⁺ to Ni³⁺, which manifested anodic oxidation potential of -0.22 V and cathodic reduction potentials of -0.38 and -0.09 V at 10 mV/s s^-1 in 0.1 M KOH solution. The SO-NO NC effectively identified glyphosate by displaying a redox peak in the CVs spectrum. This implies that the electrode can be used as a sensor electrode.

2. Materials and Methods

2.1. Materials

Sigma -Aldrich uses analytical reagent (A.R.) to extract all of the chemicals. These include NaOH, Fast Blue, 0.1 M KOH, silver nitrate (AgNO₃), nickel nitrate (Ni(NO₃)₂·6H₂O), and double-distilled water. Three electrode systems were used for the CV studies: prepared NC was used as the working electrode, platinum was used as the counter electrode, and silver-silver chloride was used as the reference electrode.

2.2. Methods

2.2.1. Synthetic method for the preparation of SO-NO NC

Scheme-1. Schematic diagram for the Synthesis of SO-NO NC

Using major precursors of 1.1 ratio of nickel nitrate hexahydrate (Ni(NO₃)₂•6H₂O) and 1 M silver nitrate (AgNO₃), the sonication method (ultrasound radiation method) was used to synthesize SO-NO NC. The solution was then gently stirred after 1 M sodium hydroxide (NaOH) was added dropwise to the mixture above. A microprocessor-controlled 13 mm-diameter ultrasonic probe made of premium titanium alloy (model PRO-550, 20 kHz, 500 W) operating at 20% amplitude, 20 kHz frequency, and potentially dissipating 100 W of power was used to expose this solution to high-intensity ultrasound radiation for two hours at 45°C [42]. When the reaction was finished, a brown precipitate was created. The solution was centrifuged to remove excess NaOH, and the precipitate that resulted was cleaned with distilled water and ethanol. To Create brown SO-NO NC; the precipitate was calcined for three hours at 400°C after being dried for one hour at 100°C in a hot air oven (Scheme-1).

2.3. Characterization

PXRD was performed for the structural analysis using a Shimadzu X-ray diffractometer (CuKa, 1.541 Å) with a scan rate of two degrees per minute. We can ascertain whether functional groups are present in the produced materials by using a Shimadzu (IR Affinity-1S) FTIR spectrometer in the 4000-400 cm⁻¹ area. Using a JEOL JEM-2100 (accelerating voltage up to 200 kV, LaB6 filament) and a transmission electron microscope (TEM), the three-dimensional morphology, polycrystallinity, and intersectional distance were evaluated. Using a Shimadzu UV-Vis spectrophotometer model 2600 with a 200–800 nm range, the necessary diffuse reflectance spectrum (DRS) measurements were performed. Electrochemical impedance tests were conducted using a three-electrode setup and a CH Instruments Electrochemical Analyzer (Model 608E).

2.4. Photocatalytic Studies

The photocatalytic degradation of FB dye under UV light exposure was investigated using SO-NO NC. After dispersing 25 mg of photocatalyst in 250 mL of a 20 ppm dye solution, the mixture was exposed to UV radiation under standard conditions. The research was conducted in a Pyrex glass beaker. After magnetically stirring the reaction mixture, a 400-W mercury vapor lamp (Hg lamp) with a wavelength of 250 nm was used as the UV light source. The SO-NO NC catalyst was transferred into a circular glass reactor. The resulting mixture spent 120 minutes in an oxygen atmosphere exposed to UV radiation. After 5 mL of the sample solution was extracted every 15 minutes throughout this period, the UV-visible range of 200–800 nm was measured. The photocatalytic activity of SO-NO NC was assessed by degrading an aqueous solution of FB [43]. The percentage of degradation was examined using eq-1 [43].

Where, C₀ and C_e are the dye concentration with time t seconds.

Further, C/Co values were also investigated by the eq.2.

Where, k - first order rate constant. The first order kinetics was linear connection between log C/C_o and K.

2.5. Preparation of the working electrode

To manufacture the working electrode, graphite powder, prepared nanocomposite, and binder (PTFE) were combined in an agitated mortar in a 70:15:15% ratio. A nickel mesh was affixed with an active substance to improve electrical conductivity, and it was squeezed for 6 minutes at 25 MPa. The constructed electrode was immersed in a 0.1 N KOH solution for 20 minutes to increase the electrolytic mobility [44].

3. Results and Discussion

3.1. XRD Spectral analysis

Figure 1. XRD spectral data of SO-NO NC.

Fig. 1 displays the features of the structure of the SO:NO nanocomposite XRD pattern. The significant peaks at 2θ values of 37.34°, 38.10°, 43.35°, 44.35°, 62.91°, 64.58°, 75.50°, 77.58°, and 79.47° show the evolution of the SO:NO nanocomposite. The standard JCPDS card No. 89–3722 is met by the typical diffraction pattern peak values of 38.10°, 44.35°, 64.58°, and 79.47° with the matching miller indices (hkl) values of (111), (200), (220), (311), and (222). It also possesses a face-centered cubic crystal structure (FCC). Another JCPDS card No. 78–0643 is used to compare the distinctive peaks at 43.35°, 62.91°, 75.50°, and 77.58°, which correspond to the miller indices planes of (200), (220), (311), and (311) of the face-centered cubic NiO crystal system. Because it is a monoxide of NiO with a confined character of 3d electrons, it generates the low -intensity peak at 38.19° with the plane (111) in the XRD pattern. To support the surface plasmon resonance (SPR) effect of Ag, the XRD pattern shows the principal characteristic peak of the (111) plane of NiO, which developed at 38.10° with low intensity due to the restricted behavior of 3d electrons in NiO as a monoxide [45,46]. The typical Scherer formula was used to calculate the average crystallite size of the produced Ag/NiO nanocomposite [47]. The average crystallite size of SO:NO nanocomposite is ~ 19.80 nm.

3.2. SEM Analysis

Figure 2. SEM and EDAX spectra of SO:NO NC.

Physical characteristics, surface morphology, and Ag distribution over the NiO surface were all examined using SEM analysis. As seen in Fig.2(a), SEM micrographs of the SO:NO NC reveal agglomerated quasi-spherical nanoparticles with a rough and porous surface morphology. The interconnected granular structure and uniform dispersion of Ag₂O and NiO phases indicate the formation of a heterojunction, which is expected to enhance charge transfer and photocatalytic activity.

The EDAX method was used to determine which chemicals were present in a sample. Fig.2(b) confirms the existence of Ag (33.65%) in the modified NiO nanoparticle together with the essential elements Ni (37.94%) and O (28.41%). The existence of various signals that correspond to these elements completely supports the presence of Ag throughout the surface of the NiO host structure. The existence of the matching Ag is indicated by the elemental mapping for Ni, O, C, and Ag in Fig.2(b) [48].

3.3. TEM Analysis

Figure 3. (a-b) TEM images (c) HR-TEM (d) SAED and (e) EDAX spectra of SO-NO NC.

Figure 4. Elemental mapping of SO-NO NC.

TEM was used to further investigate the SO-NO NC's microstructure and detailed morphologies. The TEM images in Fig. 3(a) and 3(b) demonstrate that the particles' morphology was uniform, intermingled with one another, and displayed spherical-like features. The average particle size found in the TEM scans was between 19 and 20 nm. The XRD results and the computed particle sizes agreed well. The high-resolution (HR-TEM displayed spherica-like features. The average particle size found in the TEM scans was between 19 and 20 nm image in Fig. 3c shows discrete lattice fringes with a spacing of d = 0.23 and d = 0.36 nm, which correspond to the (012) lattice plane of NiO and the (311) lattice plane of metallic Ag, respectively. The strong interfacial contact between the Ag and NiO nanoparticles and the effective synthesis of the SO-NO NC was amply demonstrated by the results. Fig. 3d shows an example selected-area electron diffraction (SAED) pattern that further demonstrates the polycrystalline mixed-phase structure of SO-NO NC. The SAED pattern and HR-TEM images verified the formation of Ag and NiO composites, which was in strong agreement with the XRD findings.

Elemental mapping analysis, Fig. 4, further supported the EDX data. The compositional map shows that the Ag, Ni, O, and O, Ni, Ag species are evenly distributed over the matrix surface. The fine distribution of active species on the material surface certainly has a substantial impact on the catalytic or electrochemical activity.

3.4. DRS Analysis

Figure 5. UV and Energy bandgap spectra of SO-NO NC

SO-NO NC absorbance in the UV region is displayed in Fig. 5. Using equation (3), the energy band gap was determined.

According to the UV spectra, the SO-NONC has a band gap value of 2.95 eV, which is in line with earlier research on the material [49,50]. Fig.5. These characteristics were discovered to be brought about by the interfacial electrical contact between Ag and NiO in SO-NONC via O 2p, which lowers the band gap by raising the valence band potential through electron-electron repulsion [49,50].

3.5. Photocatalytic degradation of Fast Blue dye

Figure 6. (a) PC degradation of FB dye under UV light irradiation (b) % of degradation v/s Time

An anionic organic dye was used to determine the SO-NO NC's photocatalytic activity. The SO-NO NCPCA was conducted for the photodegradation of FB dye during a time period of 0 to 90 minutes. In the UV region, the highest absorption was recorded at 620 nm Fig.6(a)[51]. The degradation of the FB dye under UV radiation is confirmed by Fig.6(a), which clearly shows that the absorption peak value decreases as the time period increases. The above data clearly indicates the catalyst role in the degradation of FB dye.

The percentage of degradation increases from 0 to 90 minutes of UV exposure, as seen in Fig. 6(b). Under UV light, the FB dye photodegradation rate increased to 71.45%. This reveals unequivocally that SO-NO NC has greater photocatalytic activity as a catalyst in the degradation of FB dye when exposed to UV light.

3.6. Kinetic studies

Figure 7. (a) Kinetic studies of SO-NO NC (b) The plot of pseudo first-order kinetics of SO-NONC (c) Half-life period

50 mL of dye solution with an initial concentration of 2.2 × 10^-3 mol. L^-1 were employed in the kinetics study. After that, the thin layer was regularly exposed to radiation at ambient temperature before coming into touch with it. The pseudo-first -order equation was utilized to quantify the SO-NO NC kinetics effect [52].

After 90 minutes of exposure to UV light, the SO-NO NC PCA gradually improved. NiO had a significantly lower C/Co value than Ag-doped NiO, according to the C/Co equilibrium values, suggesting that the addition of Ag dopants improved photocatalytic performance (Fig. 7(a)). According to previous studies, there is a correlation between decreased optical band gap energy and increased photocatalytic activity [51,52]. In this study, the band gap lowering enhanced the electrical connection between the dye molecules and NiO. between the dye molecules and NiO was enhanced by the band gap lowering. Charge transfer from the catalyst to the dye molecules' Fermi level was made easier by a smaller band gap, which improved charge transfer from the valence band to the conduction band. As a result, the increase in dye molecule adsorption on the catalyst's surface raised the degrading efficiency [52]. The results show that adding Ag dopant to NiO thin films significantly impacts the photocatalytic degradation of FB dye. photocatalytic degradation of FB dye is significantly impacted by the addition of Ag dopant to NiO thin films.

The expansion of the k app values in FB deterioration is proportionate to the Ag dopant level. This implies that Ag boosts the photocatalytic activity in the NiO lattice and hastens breakdown. This decrease in the band gap is the cause of this increase in the rate constant [43]. In providing more electron and/or hole trapping sites, the addition of Ag ions accelerates the breakdown of FB and enhances the production of reactive oxidative species. These results advance our knowledge of the fundamental chemistry behind the photocatalytic destruction of organic contaminants through the use of doped semiconductor materials.

SO-NO NC catalysts' photocatalytic performance is evaluated by measuring the degradation of FB dye. The pseudo-first-order kinetics of the degradation process were shown as a straight line in Fig. 7(b), a plot of C/Co vs. time. Based on the graph's slope, a rate constant (k¹) of 0.01367 min⁻¹ (R² = 0.9705) for FB dye was determined.

As the irradiation period increases, the concentration of FB dyes (C/Co) clearly decreases, as seen in Fig. 7(c). The FB dye concentrations declined from 1.0 to 0.28 after 90 minutes of photoirradiation. As an SO-NO NC, the half-life was 43.25.

3.7. Mechanism

Figure 8. Mechanism of degradation of Fast Blue dye

The FB dye was degraded by a series of chemical processes known as photocatalysis, induced by light. This process emphasizes the dye molecules close to the light, which causes them to become excited and generate pairs of electrons on the SO-NO NC photocatalyst surface, creating highly active species. Electrons and holes are involved in the oxidation and reduction processes. Superoxide radicals are produced when holes oxidize water molecules, while hydroxyl radicals are produced when electrons decrease molecular oxygen. The hydroxyl and superoxide radicals generated in the FB dye destroyed its chemical bonds [56]. The probable reaction route of the NiO photocatalyst-mediated FB dye degradation is shown in equations 4 to 10.

The addition of benzoquinone considerably reduced the degradation efficiency, according to scavenger experiments, suggesting that superoxide radicals (•O₂⁻) constitute the main active species. The discernible decrease in activity upon the addition of EDTA further supports the presence of photogenerated holes (h⁺). On the other hand, isopropanol had a moderate effect, indicating that hydroxyl radicals (•OH) play a secondary role. These findings show that the Ag₂O–NiO nanocomposite has various reactive species-driven multifunctional photocatalytic activity.

SO:NO NC + hν → SO-NO NC (e_CB^- + h_VB⁺) …… (4)

H₂O + H⁺ → OH^∗ + H⁺…… (5)

OH^- + h⁺ → OH^∗ (hydroxyl radical) …… (6)

O₂ + e^- → O₂^∗- (superoxide anion) …… (7)

O₂^∗- + H⁺→ OH^∗ …… (8)

FB + OH^∗ → Oxidation Product …… (9)

FB + h⁺ → Oxidation of FB …… (10)

3.8. Electrochemical Sensing Behavior of the SO-NO NC/CPE for Glyphosate

Figure 9. (a) CVs of the constructed electrode and how the scan rate varies. (b) Calibration curve (c) CVs studies of the constructed electrode with and without the glyphosate analyte. (d) CVs studies of the constructed electrode and its variation in the Glyphosate concentration at 10 mVs-1.

It is now feasible to examine their electrocatalytic reaction due to SO-NO NC intriguing properties regarding the electrochemical sensing of Glyphosate on SO-NO NC/Carbon paste electrode (CPE). The potential range of 0.6 V to -0.6 V and the cyclic voltammograms of 1 µM glyphosate on the bare CPE are displayed in Fig.9 at a scan rate of 10 mVs^-1.

One of the parameters to comprehend the activity of our provided material is the impact of scan rate. One of the parameters to comprehend the activity of our provided material is the impact of scan rate. Fig. 9(a) showed the impact of scan rates ranging from 10 to 50 mVs⁻¹.. The data shows that the anodic peak's peak current (Ip) rises with varying scans speeds. The deposited SO-NO NC improved stability causes cathodic and anodic peak currents to rise as the number of scan rates increases. As the number of cycles grows, the redox potential peak's location remains constant, suggesting that the SO-NO NC thickness has no effect on the redox process. The graph plotted between peak current and the square root of scan rates with a correlated diffusion coefficient of 2.671×10^-4 and sensitivity of 0.0000148 (µM) + 0.0000078 µAnM^-1cm^-2 (Fig. 9(b)).  The acquired data show that the oxidation of SO-NO NC results in a linear increase in peak potential with increasing scan rates.

Glyphosate's remarkable redox behavior demonstrated that, when applied to the electrode surface, SO-NO NC had a high surface -to-volume ratio and outstanding electrocatalytic activity. Nevertheless, a significant increase in the current response and two separate redox peaks at an anodic oxidation potential of -0.22 V and a cathodic reduction potential of -0.38 and -0.09 V are seen upon the addition of 1 µM glyphosate (Fig. 9(c)).These results showed that the electrochemical properties of the modified electrode are significantly enhanced by the Tulasi leaf-assisted SO-NO NC. The accelerated electron transfer process, wide accessible surface area, and increased surface energy of the CP electrode may be responsible for the improved electrochemical characteristics of the modified electrode, as demonstrated by the SO-NO NC [53]. In cyclic voltammograms, glyphosate typically exhibits significant redox properties, depending on the electrolyte and electrode material. Glyphosate readily oxidizes (loses an electron) at a negative potential to create the glyphosate ion. After undergoing a highly reversible redox reaction during the reverse scan, it can be reduced back to metallic glyphosate at the same potential, which is typically shown by a discernible peak in the voltammogram.

SO-NO NC electrocatalytic activity measured by the CVs method because to its high current sensitivity and resolution. As shown in Fig.9(d), the CVs Measurements in 0.1 M KOH with varying concentrations of glyphosate (1 µM to 5 µM) also showed that the green generated SO-NO NC performed well electrochemically for glyphosate detection. The findings showed that the anodic oxidation current increased gradually as the concentration of glyphosate in the KOH solution increased, indicating that the modified electrode's electrocatalytic response for glyphosate sensing is highly concentration-sensitive [54]. This type of immediate electrode response indicates that the material (SO-NO NC) efficiently stimulates glyphosate redox activity to produce the characteristic voltammetric signals.

3.9. Impedance Spectra

Figure 10. Impedance Spectra of SO-NO NC (inset: circuit)

The Nyquist plot of the SO-NO NC impedance data is shown in Figure 10. The x- and y-axes, respectively, indicate the imaginary and real components. Electrical properties such as the charge transfer resistance, double-layer capacitance, and diffusion processes at the material. An interface can be found using the characteristics and shape of the Nyquist plot. According to frequency, the Nyquist plot can be separated into two sections: (i) a low-frequency region, represented by a straight line, that displays the electrode capacitance; and (ii) a high-frequency zone, represented by a semicircle that displays charge transfer at the electrode–electrolyte interface. Charge-transfer resistance (RCt), capacitance (Cdl), and equivalent circuit fit data can be used to immediately determine the diameter of the semicircle arc on the real axis. The remarkably low RCt and Cdl of SO-NO NC are determined to be 42.5°C and 0.000142 F, respectively. This suggests that the charge-transfer mechanism of the SO-NO NC is highly efficient, enabling rapid and effective energy transfer. SO-NO NC's low resistance demonstrates high conductivity and effective charge transfer between its components. To ensure the SO-NO NC system operates as efficiently as possible, more research into the Rct involved in the sensing activity is necessary [55]. [55].

3.10. Amperometry Studies

Figure 11. (a) Amperometric i-t curves of electrode (SO-NO NC) sensing the glyphosate concentration range 1 to 5 M at 10 mV/s scan rate and (b) calibration plot of the current response against glyphosate concentration.

The resulting i-t curve (potential range of 0.2 V to 0.8 V) is displayed in Fig. 11. The generated SO-NO NC was then used in the amperometric investigations. The current response rises in tandem with the glyphosate concentration and swiftly stabilizes. These results indicate that the produced electrode responds to the glyphosate reaction with a strong and timely amperometric signal. The calibration curve in Fig. 11(a) shows how the oxidation and reduction peak currents varied with the glyphosate level. It was discovered that the SO-NO NC electrode's lowest limit of detection was 1×10^-3 µM. The diffusion-controlled activity shown within the concentration range can be verified using the linear fitting generated in the calibration curve Y=0.000631 µM + 0.000042 µA nM^-1 cm^-2. The addition of different glyphosate concentrations results in a linear increase of peak potential, as shown by the linear regression curve of R²=0.9965 (Fig. 11(b)).

3.11. Stability and Reproducibility

Figure 12. (a) CVs of 1 μM glyphosate with sweep rate 10 mVs-1 at SO-NO NC electrode for 10 cycles. (b) Reproducibility studies.

As shown in Fig. 12(a), the stability tests of the SO-NO NC electrode were conducted by performing CV scans for 10 cycles in 0.1 M KOH and 1 μM glyphosate. During the cycle repeat, it was found that the peak potential (Epa) and peak current (Ipa) remained relatively consistent. The formula = Ipn / Ip1, where Ipn and Ip1 stand for the peak current levels during the nth and first cycles, respectively, is used to determine the electrode disintegration percentage. This results in a number of 96.12%, demonstrating the SO-NO NC electrode's remarkable stability and ability to function even after ten cycles.

Reproducibility is a crucial parameter for evaluating sensing equipment' efficacy. Reproducibility in electrochemical sensors refers to the sensor's ability to produce consistent, accurate data when the same electrode is tested again under identical conditions. Reproducibility is a crucial aspect of sensor performance in applications where precise and consistent data are essential, such as industrial process control, medical diagnostics, and environmental monitoring. With a relative standard deviation (RSD) of barely 1.6%, the response currents for the different SO-NO NC graphite electrodes were remarkably precise and reproducible.

Table-3: Comparison of the several metal oxide NPs' specific capacities that have been published in the literature.

4. Conclusion:

In the present research study, we have successfully synthesized SO-NO NC by the sonochemical technique with Tulsi leaf extract. The composite material is analyzed using various spectrum methods. The XRD of SO-NO NC crystallites is shown to be about 19.80 nm in size on average. A face-centered cubic shape and an average particle size of around 19.80 nm are shown in the SEM image. The Ag diffraction spots often match the face-centered cubic structure, according to the TEM pictures, but NiO exhibits diffraction rings and spots that are compatible with its cubic structure. According to the DRS spectra, SO-NO NC has an energy band gap value of 2.95 eV. The Fast Blue (CR) dye photodegradation was used to assess the NC Photocatalytic Activity (PCA); the results indicate that under UV irradiation, the maximum absorption was observed at 620 nm, with 71.45% degradation over time. In cyclic voltammetric (CV) investigations, the SO-NO NC is also employed as an electrode. It displays the surface redox mechanism of Ni²⁺ to Ni³⁺, which manifested at -0.38 and -0.09 V (cathodic) and -0.22 V (anodic) at 10 mV/s in 0.1 M KOH solution. The SO-NO NC effectively identified glyphosate by displaying a redox peak in the CVs spectrum. This implies that the electrode can be used as a sensor electrode.

Author Contributions

S.T.M: Supervisor, Paper writing, Problem-solving, data validation, Review and Editing. C.R.R: Problem define, corrections, validation, Supervisor, Review and Editing

Data Availability

The datasets used in this study are available from the corresponding author on reasonable request

Funding Statement

The authors received no financial support for the research, authorship, and/or publication of this article.

Conflicts of interest statement

The authors declare no conflicts of interest.