Facile green synthesis of silver doped NiO nanoparticles using aloe vera latex for efficient energy storage and photocatalytic applications

Abstract

Herein, we report the biosynthesis of pure NiO and NiO nanoparticles doped with Silver (Ag@NiO NPs) 2, 4, 6, and 8 mol% from aloe vera extract by solution combustion method at 400 °C and calcined at 500 °C for 3 h. By utilizing silver-doped NiO nanoparticles synthesized with Aloe Vera latex, which not only enhances the material's properties but also promotes environmentally friendly fabrication methods. The morphological, structural elemental compositions were analysed through SEM, HRTEM, SAED, XRD and EDAX. The band gap was determined as 2.48, 2.57, 2.58, 2.60, and 2.62 eV for pure NiO, 2–8% Silver doped NiO using Kubelka-Munk plot. The photocatalytic potential of the 6 % Ag doped NiO has been explored by assessing their effectiveness in degrading fast blue (FB) dye, demonstrating significant activity at 605 nm. Remarkably, with 120 min of UV radiation, the FB dye reaches an impressive photodegradation rate of 98 %, making the dye nearly colorless. The green synthesized NPs were tested in 1 M KOH to investigate their supercapacitor performance as an effective material for electrode. The Cyclic Voltammetry (CV), the Galvanostatic charge-discharge (GCD), and the Electrochemical impedance spectroscopy (EIS) studies were conducted to determine the materials' electrochemical activity. The GCD study for 6 mol% Ag@NiO in a 3-electrode system shows a capacitance of 535 F g⁻¹ at a current density of 1 Ag^-1. 6 mol% Ag@NiO modified electrode shows excellent long-term stability, retention of more than 92 % of its initial capacitance after the operation of 2000 cycles. The important outcome of this work lies in multifunctional application of the as-synthesized materials, demonstrating their effectiveness for both efficient energy storage and improved photocatalytic performance, paving the way for sustainable and multifunctional devices.

1. Introduction

Nanometals are widely used in industrial applications due to their higher melting temperatures, greater specific surface areas, optical characteristics, mechanical strengths, and magnetizations [1,2]. NiO and other metal oxide nanoparticles have gained increasing industrial and technological attention. Mechanical hardness, thermal stability, and chemical passivity [3,4] are linked to optical, magnetic, electrical, and catalytic capabilities, which attract attention. Common metal oxide such as NiO can be generated via sol-gel thermal breakdown of freshly synthesized nickel hydroxide at 300 °C. NiO can make electrochromic coatings for plastics and textiles, nanowires and nanofibers, alloys and catalysts, and lithium-ion micro-batteries with nickel oxide cathodes [5]. It also works as a catalyst, antiferromagnetic layers, active optical filters, alkaline battery cathodes, and formaldehyde, CO, and H sensors. Thermal evaporation of nickel oxide thin films has been used for energy-saving transparent heat mirror multilayer systems [6,7]. Optical properties make NiO nanoparticles interesting, they absorb UV radiation yet are transparent to visible light. NiO's unique properties and applications in transparent electronics, chemical sensors, and spin electronics have prompted extensive research [8]. Chemical coprecipitation, sol-gel, chemical vapor deposition, thermal decomposition, hydrothermal synthesis, solid-state reactions, spray pyrolysis, the vapor-liquid-solid method, and microemulsion precipitation have been used with ZnO, TiO₂, and SiO₂ [9]. NiO is used as ferrites, ceramic pigments, glasses, glazes, and electro-rheological ceramics including thermistors and varistors. Transparent optoelectronic electrodes were deposited using gold-doped nickel oxide layers [10].

TiO₂, NiO, ZnO, and SiO₂ are examples of transparent semiconducting nanomaterials that excel where other materials have failed [11,12]. The applications of metal oxide nanoparticles range from architectural windows, solar cells, polar electrodes, photovoltaic devices to sensors. Nano-NiO photocathodes are self-cleaning, catalytic, and dye-sensitive [13]. Glass, textiles, and cement are just some of the places where self-cleaning coatings might save labour costs. This possibility is partially realized in European self-cleaning paint [14]. Coatings designed to clean themselves are either hydrophobic or hydrophilic. These two layers disinfect themselves by rolling or sheeting away dirty water. The chemical bonds that are present the dirt and the hydrophilic coating breaks down when exposed to sunlight or UV light. NiO nanostructures may be synthesized and manipulated via evaporation, sputtering, electro-deposition, thermal decomposition, pulsed laser deposition (PLD), laser ablation in liquid (PLAL), and sol-gel processes [15]. The sol-gel method is attractive for commercial use because it is straightforward to implement, and relatively inexpensive products with a high degree of purity [16]. Aloe Vera latex is could be potentially used for biogenic synthesis of nanoparticles to create symmetric devices (such as Symmetric Supercapacitor) from silver-doped NiO nanoparticles that have uniform charge distribution and improved conductivity, which makes them useful for energy storage applications. By enabling greater charge separation, asymmetric devices (such as Asymmetric Hybrid Capacitor), on the other hand, take advantage of the special qualities of the nanoparticles to provide a potential difference that increases the effectiveness of photocatalytic reactions. By taking advantage of the advantages of the materials as-fabricated, both device architectures can be customized to maximize performance for their specific applications.

Metal oxide nanostructures, such as NiO and ZnO have shown to improve solar energy harvesting in active layers of the organic solar cells (OSCs) because of their ability to transfer charge carriers. Elements like P, N, As, Li, Sb, and Ag are all used as doping agents. Since Ag ions are simple link matrices, their sensitivity to surface states in nanomaterials becomes increasingly important as their size decreases. Compared to other methods, sono-chemical production results in smaller truncated nanorods, making it a popular choice for designing the novel materials with a wide range of properties [17]. Chemical coprecipitation through the sol-gel method yields nanocomposite materials with high degrees of homogeneity, purity, small average particle size, narrow size distribution, and an abundance of nanorods. The formation of nanometal oxides by a variety of physical and chemical processes, including sol-gel, precipitation, hydrothermal, combustion and thermal breakdown has been the subject of recent articles. It is becoming more and more significant to produce excellent morphological features of nanosized metal oxide. A novel green synthesis approach has evolved towards the synthesis of nanomaterials with desired features and great stability, and this approach is attracting the interest of many researchers [18]. The use of plant derivatives in the green route synthetic method of metal oxide nanoparticles has been substantial studied over the past 20 years. Plants can be used to synthesize a variety of environmentally friendly metal oxide nanoparticles, including CeO₂, Fe-doped MgO, and Eu-doped ZnO [19]. A variety of industries, including medicine, therapy, sustainable and green energy, and other industrial products, could use the bio-inspired nanoparticles. It is noticed that the use of metal oxide nanoparticles derived from plants will aid in the early detection and effective treatment of a variety of diseases.

To modify the structural, electrical and optical characteristics of the parent NPs, the doping procedure involves introducing particular transition metals or impurities into the NMs' crystal structure [20]. Doping requires only one or more of the atoms present at certain locations in the crystal lattice to replace by the dopant atom, which makes it a challenging process to do given that the NPs' crystal lattice system must stay intact [21]. According to recent literature, the most common metals used to dope NiO NPs are copper (Cu), cobalt (Co), manganese (Mn), and Zinc (Zn) and Iron (Fe) [22,23]. NiO NPs doped with Cu were synthesized by Varunkumar et al., they found that by increasing the dopant concentration from 0 to 4% improved the optical band gap quality from 3.26 to 3.67 eV [20]. Additionally, Lee et al. produced NiO NPs doped with Co and reported that the doping procedure improved the NiO NPs' electrical conductivity and interfacial properties [24]. According to these findings, doping technique for NiO NPs is a great way to enhance their characteristic features. Silver has emerged as one of the important elements that has been emerged as a candidate as dopant for NiO due to some of its promising features such as larger ion size, high solubility and smaller orbital energy [25]. Apart from that, Ag ions can be used as dopants in NiO-NPs which serves as acceptor impurity due to their superior properties and have capabilities of easy substitution [26,27].

In last few years, there has been an increasing demand to synthesize novel materials towards the extensive removal of dyes that are harmful from water sources. To address these challenges, NiO has emerged as one of the most promising candidates in the recent past for the removal of methylene blue and other prominent dyes. Recently, many studies have been reported on the on the synthesis of Silver doped metal oxide NPs for photocatalytic activity towards removing the organic dyes from aqueous solutions. Among the Ag doped metal oxide nanoparticles reported for Photocatalytical activity are Ag-doped ZrO₂ nanoparticles, Silver doped ZnO NPs, Silver doped CuO nanoparticles, Silver doped TiO₂, Silver doped ZnO NPs, and Silver doped WO₃ NPs [28,29]. Considering the importance of NiO in Photocatalytical activity as well as role of Ag as a dopant to enhance the Photocatalytical activity of metal oxide nanoparticles, in this study we propose to obtain Ag doped NiO NPs for the improved Photocatalytical degradation.

Very recently, NiO NPs have been synthesized with different structural features such as porous nanofibers, nanoflowers, hollow nanospheres, nanotubes and nanoflakes, as a material for electrodes in supercapacitor applications. However, these NiO NPs have drawbacks like poor electrical conductivity, electrochemical features and poor cyclic stability. Apart from that the transportation of electrons and ions in the electrode has limitations due to poor conducting channels. Hence there is a need to electrical conductivity enhancement and cycle life of the NiO NPs as electrodes for supercapacitor applications. One such technique to improve the electrical conductivity in the electrode materials, it can be combined conductive fillers, like conductive polymers, carbon materials and doping with metals are highly feasible. Recent literature has reported that, manganese dioxide doped with Ag electrode films can prominently enhance the conductivity, as well as improving the performance of the supercapacitor [27]. The Ag doping is found to accelerate the transportation of electrons that leads to improved conductivity, and hence improves the performance of electrode material and supercapacitor. The incorporation of Ag nanoparticles can increase the electronic conductivity for the electrode materials as well as provides unique morphological features by maintaining the low internal resistance [28].

The present study aims to explore the innovative realm of green combustion as a means to synthesize Silver doped NiO NPs, capitalizing on the remarkable properties of aloe vera gel. Comprehensive characterization of the synthesized nanoparticles encompasses an evaluation of their structural and morphological attributes. Fig. 1 shows the graphical abstract of our research. Moreover, the photocatalytic capabilities of Silver doped NiO were enhanced through UV light exposure, facilitating a more efficient decolorization of fast blue dye. Preliminary investigations were also conducted to assess the suitability of Silver doped NiO as an electrode for energy storage applications. The Silver doped NiO NPs synthesized in the present investigation could be used as a multifunctional material for the Photocatalytic organic dyes’ degradation and electrodes for supercapacitor applications.

Fig. 1.

The graphical abstract of our thesis.

2. Experimental

2.1. Synthesis of pure NiO NPs

10 ml of 1 M [Ni (NO₃)₂.6H₂O] (Sigma Aldrich, purity ≥98.0 %) and 1 ml of Aloe Vera gel extract were mixed using a magnetic stirrer approximately for 10–15 min. The mixture was then shifted to a muffle furnace heated at 400 ± 10 °C. The mixture was allowed to boil until it becomes a clear gel. The white foamy substance developing through the gel which fills the container. The reaction starts from the interior and swiftly spread as a flame, leaving a porous white powder. NiO NPs were produced in a self-replicating technique that remained stable under extreme heat. The whole reaction mechanism takes less than 5 min to complete. Upon further calcination at 500 °C for 3 h, the end products were obtained and are used investigate morphological, structural, electrochemical and dye degradation analyses.

2.2. Synthesis of silver doped NiO NPs (2, 4, 6 and 8 mol%)

The redox combination of 1M silver nitrate (AgNO₃) (sigma Aldrich, purity ≥99.0 %), 1M nickel nitrate hexahydrate (Ni (NO₃)₂.6H₂O) (sigma Aldrich, purity ≥98.0 %), and 1 ml aloe vera gel extract was stirred for ∼10–15 min using a magnetic stirrer. (The leaves of the Aloe vera plant were gathered, cleaned with distilled water, and then sliced transversely into pieces in order to extract the mucilaginous fluid from the centre of the leaf. The leaf's interior, gel-like pulp was separated with a spoon, chopped, and homogenized in a mixer after the thick skin was carefully peeled off with a vegetable peeler). The crucible is filled with all of the precursors and heated to 400 ± 10 °C. The reaction mixture bubbled, releasing a clear gel that quickly burned throughout the entire volume to leave a very porous, brown powder. To get the final product, the brown powder was further calcined at 500 °C for 3 h.

2.3. Photocatalytic experiment

Photocatalytic activities were investigated using a Shimadzu UV–visible spectrophotometer, model 2600. Photocatalytic experiments on pure NiO, 2–8% Silver doped NiO NPs degraded Fast Blue (FB) under UV light. Dyes were diluted into a 250-mL stock solution at 10, 15, 20, and 25 parts per million. In a circular glass reactor, catalyst varying from 20 to 60 mg was introduced into the aqueous solution containing the dye. The reaction mixture containing the dye was magnetically stirred under a mercury vapor 400-W Hg lamp that emits 254 nm light. Pyrex beakers were used for mixing the solution. Further, the reaction mixture was exposed to UV radiation in open air, 5 mL samples were taken every 15 min. The UV–visible spectrophotometer (UV 26000- Shimadzu) was used to investigate the UV–visible features of the prepared nanocomposites in 200–800 nm wavelength range.

2.4. Preparation of nickel mesh electrode

To make a nickel working electrode, 75 % NiO NPs, 15 % graphite powder was added, and 10 % PTFE solution was used as a binder. A nickel mesh was fastened to the resultant sheet. For a reliable electrical connection, the coated electrodes (active materials) were pressed against the nickel mesh at a pressure of 20 MPa. Teflon tape was used to isolate the electrode and the wire from the back, and the final product 2 mm × 1 mm in size was prepared. Soaking the electrolyte for 30 min in a 0.1 N HCL solution improved the interaction between active material and the electrolyte.

2.5. Characterizations

The structural features of the prepared nanocomposites were examined using PXRD-Shimadzu X-ray diffractometer (CuKα radiation, 1.541 Å) with 2θ values ranging from 10° to 80°. The chemical groups exist in the synthesized samples were investigated using FT-IR spectroscopy (PerkinElmer Spectrum-1000) in the wave number range 4000–400 cm⁻¹. The surface morphology of the nanocomposites and elemental composition (EDAX) of the nanocomposites were examined using a Hitachi tabletop scanning electron microscope (TM-3000). The 3D surface morphology, crystalline structure, and interplanar spacing were analysed using TEM (JEOL, JEM-2100) at an accelerating voltage of 200 kV with a LaB₆ filament. DRS studies were conducted to determine the energy band gap in the 200–800 nm range, and the data was analysed using the Kubelka-Munk function (Shimadzu UV–Vis model 2600). A three-electrode configuration was used to investigate the electrochemical impedance studies with nickel mesh as a working electrode, platinum wire as a counter electrode, and Ag/AgCl electrode as a reference electrode. The electrochemical experiments were performed with Electrochemical Analyzer (CH608E). Further, the EIS and Bode measurements were done in the frequency range 1 Hz to 1 MHz at an Alternate Current having amplitude of 5 mV.

3. Results and discussions

3.1. PXRD analysis

To understand the structural features and phases of pure NiO, 2–8% Silver doped NiO NPs, PXRD was utilized in addition to the conventional way of analysis (Fig. 2). The characteristic peaks correspond to pure NiO (JCPDS card no. 47–1049) [18] at 2θ values of 37.2, 43.3, 62.9, 75.4, and 79.4 confirms the face cantered cubic structure of NiO NPs. These characteristic peaks correspond to NiO NPs are assigned with index values of (111), (200), (220), (311), and (222). Further, it can be noticed in spectra [a, b, c, and d], the doping of Ag into NiO results into splitting of NiO characteristic peaks indicating the presence of secondary phase corresponds to Ag [26,27]. The splitting of NiO NPs characteristic peaks becomes more pronounced with the increasing concentration of Silver in NiO NPs. The characteristic peaks corresponds to silver were observed at 38.26°, 44.47°, 64.71°, and 77.74° indicate the FCC structure of Ag NPs corresponds to indexed values of (111), (200), (220) and (311) crystal planes correspond to the JCPDS card No. 87–0719 [26]. Further, the intensity of the characteristic NiO peaks decreases with the increasing concentrations of Ag, the XRD spectra reported in this work is in well agreement with the previously reported literature on Ag doped NiO NPs [[26], [27], [28]].

Fig. 2.

PXRD spectra of pure NiO, [a) 2 %, b) 4 %, c) 6 %, and d) 8 % Ag doped NiO nanoparticles].

Furthermore, the crystallite size of Silver doped NiO NPs was calculated using Scherer equation (1) [29].

$$D=\frac{\mathrm{k}\lambda }{\mathrm{\beta }\text{Cos}\mathrm{\theta }}------\left(1\right)$$

where θ-Bragg's angle, β- FWHM (in radians), and λ- X-ray wavelength (1.5406 Å). The average particle size of the Silver doped NiO nanoparticles were approximately ∼20 nm obtained from Scherrer's equation.

The following relation (2) was used to analyze the lattice parameters:

$$a=d_{hkl}\sqrt{h^2+k^2+l^2}----\left(2\right)$$

where the Miller indices are represented by h, k, and l and the interplanar spacing between the planes as d. The X-ray diffraction peaks were used to compute the d-spacing values (Table 1) using the following relation (3):

$$d_{hkl}=\frac{\lambda }{2\sin \theta }-----$$ (3)

Table-1.

PXRD parameters analysis for pure NiO and 2–8% Silver doped NiO nanomaterials.

2Ɵ	Sin2Ɵ	h2 + k2 +l2	h2 + k2 +l2	h k l	FWHM	D (nm)	d (Ao)	a (Ao)	δ (10 16)	V (Ao)3
37.2	0.101735	1	3	1 1 1	0.388	19.39797	2.414101	4.181346	0.265759	73.10519
43.3	0.136114	1.3379	4.01376	2 0 0	0.388	19.02315	2.087084	4.181346	0.276334	73.10519
62.9	0.272228	2.6758	8.02754	2 2 0	0.388	17.46031	1.47579	4.181346	0.328017	73.10519
75.4	0.373965	3.6758	11.0276	3 1 1	0.388	16.19397	1.259143	4.181346	0.381323	73.10519
79.4	0.408024	4.0106	12.0319	2 2 2	0.388	15.7473	1.205446	4.181346	0.403263	73.10519

The Williamson–Smallman relation (4) was used to calculate the dislocation density.

$$\delta =\frac{1}{D^2}----\left(4\right)$$

The following relation (5) is used to calculate a unit cell volume.

$$V=a^2-----\left(5\right)$$

Fig. 3, displays the XRD characteristic peak of Ag NPs with the standard (retrieved data). It has been noticed that, the major XRD peaks of Ag located at (111), (200), (220) and (311) of the experimental directly fitted with the standard peaks at the bottom. This is a direct way of for the confirmation of pure Ag NPs formation in the absence of any secondary phase impurities. The retrieved peak directly aligns with the peaks of the experimental Ag NPs, so it could be possible to conclude, as the synthesized Ag NPs is well formed. As can be depicted in Fig. 3 (a), the experimental and the standard is fitted with the JCPDS card number of 87–0719 [30]. Similarly, the experimental NiO NPs was directly fitted with the standard, as can be presented in Fig. 3(b). The miller indices value of NiO was (111), (200), (220), (311) and (222), which is a path way that indicates for the formation of pure NiO NPs, as JCPDS card number also confirms (at the bottom with the standard) [31].

Fig. 3.

Rietveld refinement of (a) Ag and (b) NiO NPs.

3.2. SEM-EDAX analysis

Fig. 4 shows the SEM micrographs of pure NiO and NiO NPs doped with 2 %, 4 %, 6 %, and 8 % Ag, obtained through green combustion technique. The SEM micrographs both pure NiO and Silver doped NiOreveals an irregular flaky morphology. Further, the SEM images also indicate powders are highly porous in nature with pronounced agglomeration, cracks and large voids, which are common characteristic features associated with combustion synthesis.

Fig. 4.

SEM micrograph images of pure NiO (A) and 2–8% Silver doped NiO NPs respectively (B to E).

The EDAX spectra of pure NiO and 2–8% Silver doped NiO NPs are represented in Fig. 5. The elemental analysis of the Silver doped NiO NPs was performed using EDS (Fig. 5), and the microanalysis technique which identifies the constituent elements in the nanocomposite. The EDAX spectra of all the samples indicates the absence of impurity peaks which confirms the purity of synthesized nanocomposites. The elemental analysis indicates the presence of Ni, Ag, C, and O atoms and revealed their uniform distribution in the nanocomposite. This even dispersion indicates that the material has homogeneity, with well-dispersed composition of Ni, Ag, O, and C in the Silver doped NiO NPs.

Fig. 5.

EDAX spectra with elemental analysis of bare NiO and 2–8% Ag doped NiO NPs (A to E) and Elemental mapping of the Ni, Ag, O, and C atoms respectively (F to I) present in Ag doped NiO NPs.

3.3. TEM analysis

The morphological features of Silver doped NiO NPs were further investigated through TEM analysis, HRTEM, and SAED techniques. The TEM image of 6 % Silver doped NiOnanoparticles synthesized as shown in Fig. 6(a) indicates non-spherical shapes with a combination of asymmetric geometries and irregular shapes. The presence of nanoparticles having dimensions 20 nm or smaller shows the significance of combustion synthesis method in to minimize the particle agglomeration. The size of grains plays a significant role in the enhancement of electrochemical reactivity and ion diffusion, which ultimately leads to improved energy storage capabilities. Furthermore, the smaller particle size facilitates efficient light absorption and promotes better dispersion and contact with reactants, leading to enhanced photocatalytic performance for applications like water splitting, pollutant degradation, or hydrogen production. The HRTEM image in Fig. 6(b) supports the crystalline structure and crystallinity of the Silver doped NiO NPs, and the planes in the SAED pattern (Fig. 6(c)) align with the cubic phase of Silver doped NiO nanoparticles produces these diffraction rings with coordinates (111), (200), (220), (311), and (222). Fig. 6(d) represents the histogram of 6 % Silver doped NiO nanoparticles, which confirms an average crystallite size of nearly 20 nm. The HRTEM analysis in Fig. 6(e) indicates a detailed variation in the distribution of morphology with enhanced lattice fringes, IFFT patterns, and the IFFT profile representing the d-spacing value for a particular crystal plane.

Fig. 6.

(a) TEM micrograph (b) HRTEM image (c) SAED pattern (d) Histogram (e) Profile of IFFT with d-spacing distance of 6 % Ag doped NiO NPs.

Fig. 7(f) shows HRTEM images for undoped and silver-doped NiO samples. The pure NiO exhibits a homogenous and relatively smooth surface morphology, indicating a uniform distribution of the NiO particles. As the silver doping increases (2 %–8 %), noticeable alterations in particle shape and distribution are evident. The 2 % and 4 % Ag-doped samples maintain a relatively similar morphology to the pure NiO, with slight modifications and an increase in particle size. However, at higher doping levels (6 % and 8 %), the particles appear to agglomerate more significantly, leading to larger clusters. This trend suggests that Ag doping influences particle growth and distribution, which could have implications for the material's electronic and catalytic properties.

Fig. 7.

f. HRTEM for pure and doped samples.

3.4. Diffusion reflectance spectroscopy

Experiments using UV–Vis diffuse reflectance spectroscopy (Fig. 8(a)) can be used to understand the optical energy band gap, reflectance, or absorbance [19,20]. The photoexcitation of electrons in the valence band causes an increase in E_g. The absorption coefficient was measured by Schuster−Kubelka-Munk (SKM) equations described below (Eqs. (6) and (7)).

$$F\left(R\right)=\frac{{\left(1-R\right)}^2}{2R}------\left(6\right)$$

$$F\left(R\right)\mathrm{h}\upsilon =A\left(\mathrm{h}\upsilon -E_g\right)------$$ (7)

Kubelka-Munk function is represented by F(R), where R – sample's absolute reflectance, h $\mathrm{\upsilon }$ – is the photon energy. The energy band gap that results have values ranging from 2.48 to 2.62 eV for bare NiO, and 2–8% Ag doped NiO NPs (Fig. 6(b)), these values were obtained by the extrapolation. A linear fit to [F(R) hν]² = 0 and the value for n = 2 reveals (as shown in Fig. 8(b)) the existence of direct band gap in the nanocomposite [32].

Fig. 8.

(a) DRS and Energy band spectra of pure NiO, 2–8% Silver doped NiO NPs.

3.5. BET analysis

The specific surface area as well as pore size distribution of 6 % Ag doped NiO NPs were systematically investigated through N₂ adsorption−desorption isotherm analysis (Fig. 9). Before the analysis, the nanocomposite samples were subjected to degassing at 130 °C for 6 h to ensure optimal conditions. The obtained curves exhibited a Type IV isotherm alongside an H3 hysteresis loop, characterized by a sharp condensation occurring at P/P₀ = 0.5–1, indicating that the Ag doped NiO NPs are predominately mesoporous in nature with irregular, long, slit-like narrow pores. The BET specific surface area (S_BET) of Ag doped NiO NPs, derived from the isotherm data was found to be 148.6 m². g⁻¹. By employing the Barrett-Joyner-Halenda (BJH) technique, the average pore width was calculated as 7.6 nm. These results indicates that the Silver doped NiO NPs possesses ample active sites, making it well-suitable as a catalyst for the efficient degradation of organic pollutants.

Fig. 9.

The adsorption/desorption isotherm of N₂ for the combustion synthesis of 6 % Ag doped NiO NPs.

3.6. XPS analysis

The XPS technique in this study is employed to identify the chemical composition of pure and doped (6 % Silver doped) NiO nanoparticles. The survey spectra shown in Fig. 10(a) confirms the presence of elements like as Ni, O 1s, and C 1s, identified by their characteristic photoelectron peaks at binding energy ranges of 853–878 eV, 530–532.2 eV, and 284–289 eV respectively. The elemental chemical states can be obtained through highly resolved spectra, which provides insight into their electrochemical behaviour. Fig. 10(b) indicates the presence of Ni, O 1s, Ag, and C 1s, that can be identified by their corresponding photoelectron peaks at the binding energy values respectively in the range of 853–878 eV, 530–532.2 eV, 360–375 eV, and 284–289 eV. The Ni 2p spectra represented in Fig. 10(c) shows the deconvolution into four distinct peaks. The peaks at 854 eV and 872 eV corresponds to 2p_3/2 and 2p_1/2 orbital splitting of Ni²⁺ species, while the peaks at 855 eV and 873 eV represents the 2p_3/2 and 2p_1/2 orbitals of Ni³⁺ metal ions [33]. In Fig. 10(d), the peak between 360 and 375 eV represents the Ag⁺ species in the 3d orbital [34]. The O 1s spectra shown in Fig. 10(e) indicates binding energy peak corresponds to 528 eV, which represents the metal carbonate or C-O bonding in the metal oxide nanoparticles. These observations from XPS analysis delivers important understandings about the chemical states of different elements present in the nanocomposite and their electrochemical attributes.

Fig. 10.

XPS analysis (a) pure NiO, 6 % Ag doped NiO NPs and the binding energy values of (c) Ni, (d) Ag, and (e) O.

3.7. Photocatalytic activity

Photocatalytic degradation of Fast Blue (FB) dye under direct UV light for NiO and NiO NPs doped with 2 %, 4 %, 6 %, and 8 % Ag was analysed through photodegradation mechanism in an aqueous solution. Prior to the irradiation of UV-rays, all the solutions were stirred magnetically in the dark light for 120 min to reach the adsorption/desorption equilibrium state between the photocatalyst and the dye. The 20 ppm, 250 mL aqueous solution of FB and 60 mg of photocatalysts in a 176.6 cm² circular glass reactor were used to conduct the photocatalytic experiments to analyze the degradation mechanism. A Mercury vapor lamp of 125 W was used as the UV light source, and the mixture was continuously stirred with a magnetic stirrer during the experimental process. The stock dye solution with concentrations of 10, 15, 20, and 25 ppm in 250 mL of dye solution were prepared as described earlier [35]. For each dye aqueous solution, a catalyst of 20–60 mg was introduced in a circular glass reactor. The reaction mixtures were exposed to UV radiation for 120 min in open air, and 5 mL samples were collected at during the time interval of 15-min. The UV irradiation was applied directly to the reaction mixture from the top in open air conditions while maintaining the distance of UV-source at 21 cm. The entire degradation mechanism was monitored in the wavelength range 200–800 nm with a Shimadzu UV–Vis spectrophotometer at room temperature [36,37].

The photocatalytic activity of pure NiO, 2–8% Silver doped NiO nanoparticles were analysed using a cationic dye named Fast Blue FB (Fig. 11(a–e). The wavelengths of 612, 608, 604, 605, and 606 nm are most effectively absorbed by FB. Ag doping in NiO nanoparticles is important as it can increase the photocatalytic activity for the breakdown of dyes when exposed to UV light. Ag doping enhances the availability of reactive sites, encourages effective light absorption, creates synergistic effects with NiO, and stops charge recombination. Together, these elements enhance Silver doped NiO nanoparticles' ability to degrade Fast Blue dye, which makes them attractive options for a range of environmental remediation uses.

Fig. 11.

(a–e) Spectral absorbance of pure NiO, 2–8% Silver doped NiO photocatalyst for the degradation of FB dye upon UV irradiation.

NiO nanoparticles with 2–8% Ag doping were subjected to ultraviolet radiation for 120 min. As can be seen in Fig. 12(a) and (b), they were divided as follows: 62.9 %, 79.5 %, 83.45 %, 98 %, and 95.5 %. These studies show that the synthesis process, size of the crystal, its shape, as well as recombination of electron-hole pairs greatly influence the photocatalytic degradation of FB and AOR dyes [[38], [39], [40]]. The percentage degradation of dye was estimated using Equation (8).

$$\%\text{degradation}=\frac{{\mathrm{C}}_{\mathrm{o}}-{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{C}}_{\mathrm{e}}}\times 100-----\left(8\right)$$

where, C₀ -initial dye concentration and C_e -dye concentration after adsorption at time t seconds.

Fig. 12.

(a & b) % decolorization and C/C₀ vs time of Pure NiO, 2–8% Silver doped NiO photocatalyst for the decolorization of FB under UV light illumination.

The C/Co values were obtained by using the following equation (9).

$$\log \left(\frac{C}{C_o}\right)=-Kt----\left(9\right)$$

Where C and Co reflects the concentrations of the dye at the actual testing time and at t = 0 min respectively, whereas k is the first-order kinematics rate constant. The first-order kinetics mechanism was verified by the numerical results, which indicates a linear relationship between log(C/Co) and k. As seen in Table 2, the value of slope k in case of FB under UV light was determined for pure NiO as well as 2–8% Silver doped NiOby the methods reported earlier [[41], [42], [43]]. The scan rate is lower for pure NiO and rises with increasing Ag concentration in Silver doped NiOsamples. Finally, it reaches maximum when Ag concentration is 6 %.

Table 2.

Comparison of the photocatalytic performance of bare NiO NPs and NiO NPs doped with Ag.

Sample	UV light
	Absorbance (a.u)	% Degradation	Scan rate K min−1
Pure NiO	612	62.9	0.008192
2 % Ag@NiO	608	79.5	0.012192
4 % Ag@NiO	604	83.45	0.012372
6 % Ag@NiO	605	98	0.02678
8 % Ag@NiO	606	95.5	0.028488

Fig. 12(b) displays the variation of (C/Co) vs time for pure NiO as well as 2–8% Silver doped NiO photocatalysts, demonstrating the decolouration of FB under the UV light illumination. In other words, the rate of decline increased over time, reaching a maximum after 120 min [44,45].

The results demonstrate that NiO is a powerful photocatalyst for removing FB dye using UV radiation. To get excited to the conduction band energy level (CB), the positively charged holes (h⁺) must have an energy that is more than or equal to the energy of the band gap (Eg). More number of free electrons and holes were created by relocating electrons (e⁻) from the valence band (VB) to Silver doped NiO NPs [46]. Photogenerated holes in Ag-doped NiO's valence band (VB) oxidise the dye, or adsorbed water molecules create hydroxyl radicals (OH∗). From molecular oxygen, photogenerated electrons may easily reach the surface and create anion radicals (O₂∗). OH∗, O₂∗, and photogenerated holes acts as photocatalytic radicals. Extremely reactive O₂∗ and OH∗ oxidise the adsorbed dye molecules [[47], [48], [49]]. As can be shown in Fig. 13, that the photo-excited active electrons and holes additionally damage the UV-sensitive dye molecules. Decolorization mechanism of NiO: Ag occurs via the following phases described in equations 10–17:

$$\text{NiO}+\mathrm{h}\nu -\to \text{NiO}({\mathrm{e}}_{\text{cb}}^{-}+{\mathrm{h}}_{\text{vb}}^{+})-----\left(10\right)$$

$$\text{Ag}+\left({\mathrm{e}}_{\text{cb}}^{-}\right)-\to \text{Ag}\left(\text{electron}\text{trapping}\right)-----\left(11\right)$$

$$\text{Ag}+{\mathrm{O}}_2-\to \text{Ag}+O^{2⦁-}\left(\text{electron}\text{transfer}\right)-----\left(12\right)$$

$$O^{2⦁-}+{\mathrm{H}}^{+}-\to {OH}^{⦁}-----\left(13\right)$$

$${OH}^{⦁}+{\mathrm{H}}^{+}+{\mathrm{e}}_{\text{cb}}^{-}-\to {\mathrm{H}}_2\mathrm{O}2-----\left(14\right)$$

$${\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{e}}_{\text{cb}}^{-}-\to {OH}^{⦁}+{OH}^{-}-----\left(15\right)$$

$${\mathrm{h}}_{\text{vb}}^{+}+{\mathrm{H}}_2\mathrm{O}-\to {\mathrm{H}}^{+}-\to {OH}^{⦁}-----\left(16\right)$$

$${\mathrm{h}}_{\text{vb}}^{+}+{OH}^{-}-\to {OH}^{⦁}-----\left(17\right)$$

Fig. 13.

The schematic dye degradation mechanism for Ag doped NiO Nanocomposite photocatalyst for FB dye under UV light.

The synthesized sample is expected not only be able to breakdown the dye, but also to reusable and photostable. Using a constant 20 ppm of FB dyes during 5 cycles, we tested the recyclability of pure NiO, as well as 2–8% Silver doped NiO samples under UV and sunshine [50,51]. The photocatalyst was further washed with water and dried so that it could be used again after the initial cycle. As can be seen in Fig. 14(a–e), the proportion of decolorization does not change much across cycles, indicating that it is photostable. Since nanoparticles were utilized in the experiment, the photocatalyst could be easily recovered between cycles by employing a magnet [52].

Fig. 14.

(a–e) Recycling ability of pure NiO, 2–8% Silver doped NiO photocatalyst for the degradation of FB dye with UV light irradiation.

Scavenging analysis was performed to determine the efficiency with which highly charged free radicals may remove colour from FB dye when combined with pure NiO, as well as 2–8% Silver doped NiO photocatalysts when subjected to UV radiation. In Fig. 15 (a), we observe that NiO is employed in a photo decolouration research of FB under UV light with 3 different scavengers: AgNO₃, ethylene-diamine tetra acetic acid (EDTA) and ethanol (58.1 %, 54.1 %, and 51.2 %, respectively). Fig. 15 (b) depicts the results of the photo decolouration results of Silver Nitrate, ethanol, and ethylenediamine tetra acetic acid (EDTA) using UV light on FB (2 % Ag-doped NiO). Using Silver Nitrate, ethanol, and ethylenediamine tetra acetic acid (EDTA), Fig. 15(c) shows the results of a photo decolouration investigation with UV light on FB. The photodecolouration examination of FB (NiO + 6 % Ag) under UV light reveals 94.2 %, 90.25 %, and 87.65 %, respectively, as indicated in Fig. 15 (d). The photodecolouration examination of FB (8 % Ag@NiO) under UV light reveals 91.24 %, 87.77 %, and 84.2 %, respectively, as shown in Fig. 15 (e). The order of AgNO₃, ethanol, and ethylene-diamine tetra acetic acid (EDTA) was followed by various elimination kinetic rates by the addition of organic scavengers. Hence, it can be ascertained that, in comparison to holes and electrons and the radicals of superoxide radicals, hydroxyl play an important role in the photocatalytic degradation of FB dye. This was done only for demonstration purposes. All these studies demonstrate superior photocatalytic behaviour of 6 % Silver doped NiO as a catalyst for FB degradation when exposed to UV light. Hence, Silver doped NiO NPs are effective for waste water treatment.

Fig. 15.

(a–e) Scavenger of pure NiO, 2–8% Silver doped NiO photocatalyst for the degradation of FB dye with UV light irradiation.

3.8. Electrochemical performance as a supercapacitor

The charge-discharge properties, electrode reversibility and charge efficacy were analysed using CV curves. The CV curves of nanomaterial electrodes containing either pure NiO or NiO with 2–8% Ag (Fig. 14) for a range of scan rates (5–10 mV/s) is shown in Fig. 16 against Ag/AgCl, CVs were cycled between −0.6 V and 0.6 V. These results compare with the redox behaviour of NiO NPs electrodes doped with 2–8%Ag, are crucial to the current inquiry. Notably, both the cathodic and anodic redox peaks were seen in the CV curves underscore the pivotal role of pseudo capacitance in the electrochemical reaction [53,54]. The electrode with additive fillers demonstrates the higher capacity [55] since Ni⁰ can be directly oxidised to Ni²⁺ in this process. The peak current intensity can be obtained for a reversible process [eq. (18)] using Randles-Sevcik [56] technique as mentioned below.

$$\text{ip}=2.69\times {10}^5\times n^{3/2}\times A\times D^{1/2}\times C_0\times v^{1/2}----\left(18\right)$$

where variables n, A, D, v, and C₀ represents - number of electrons that are transferred during reaction kinetics; size of electrode; rate of diffusion; scanning rate; and initial concentration of the electrolyte, respectively. C₀ in the above expression can be evaluated using the following relation (19)

$${\mathrm{C}}_0=\frac{\rho }{M}----\left(19\right)$$

Fig. 16.

CV of pure NiO, 2–8% Silver doped NiO electrodes vs Ag-AgCl in 1M KOH solution.

The Molar mass and density of pure NiO and 2–8% Ag doped NiO nanoparticles were signified by M and ρ. A couple of strong redox peaks observed in the CV curves of Ag doped NiO that are associated with the Faradaic redox reactions in the materials. These redox reactions are associated with possible surface redox reactions of Ni⁰ and Ni²⁺ in the CV curves reveals the pseudocapacitive behaviour of the electrode. The redox reaction mechanism associate with the Ni and Ag can be represented as mentioned in equations (20) and (21).

$$NiorAg---->{Ni}^{+}or{Ag}^{+}+2e^{-}or{1e}^{-}Oxidation----\left(20\right)$$

$${Ni}^{+}or{Ag}^{+}+2e^{-}or{1e}^{-}---->NiorAgReduction----\left(21\right)$$

We can observe from the CV curves that, the shapes of CV curves don't change much with the scan rates, which indicates an excellent reversible kinetics of Ag doped NiO electrodes. Further, the Ag doping in NiO results into more pronounced redox peaks, when the metal nanoparticles are incorporated onto the oxide surface the interfacial contacts between them may lead to the formation of structural defects, that act as trap centres within the oxide band gaps and hence improves the diffusion kinematics at the surface. The redox peaks of Ag@NiO shows the oxidation of Ag.

The peak cathodic current intensity (ip) varies linearly with the square root of the scan rate (v^1/2) as shown in Fig. 17. Electrode reactions in pure NiO, as well as 2–8% Silver doped NiO electrodes shows a linear relation between ip and v^1/2, suggesting that hydrogen diffusion is a limiting factor. Based on our findings, the 6 % Silver doped NiO electrode exhibited the highest proton transfer coefficient. Table 3 displays the computed proton diffusion coefficients for the fitted line (Fig. 17 and Eq. (12)) for pure NiO, as well as 2–8% Silver doped NiO electrode materials.

Fig. 17.

The dependence of peak cathodic current (i_p) on square root of scan rate (v^1/2) for bare NiO and 2–8 % Silver doped NiO electrodes.

Table 3.

The calculated proton diffusion coefficients for pure NiO, and 2–8 % Silver doped NiO electrode.

Electrode	Proton diffusion coefficient (D) cm2 s−1
Pure NiO	7.005 × 10−5
2 % Ag @ NiO	1.795 × 10−4
4 % Ag @ NiO	2.168 × 10−4
6 % Ag @ NiO	3.488 × 10−4
8 % Ag @ NiO	1.976 × 10−4

The electrochemical impedance spectroscopy (EIS) of pure NiO, as well as 2–8% Silver doped NiO electrodes are represented in Fig. 18 (a). Warburg resistance in the EIS spectra is associated with slope at low frequencies, whereas the charge transfer resistance is indicated at higher frequencies [57]. The noticeable decrease in the electrode resistance indicates the presence Ag dopant in NiO NPs, and this decrease is maximum in case of 6 wt% Ag doped NiO NPs due to excellent electrochemical reactions at the electrode surface. These results indicates that the electrode with 6 % Silver doped NiO enhances the charge transfer process and its efficiency at the electrode surface. Hence, the inclusion of Ag leads to reduced resistance thereby improving charge transfer as well as the capacitance of electrode. The EIS parameters extracted from the Nyquist plot using equivalent fitted circuit (Inset of Fig. 18 (a)) are mentioned in Table 4.

Fig. 18.

(a) Nyquist plot with fitted circuit (Inset) and (b) Bode plot of pure NiO, 2–8% Silver doped NiO electrodes vs Ag-AgCl in 1M KOH solution.

Table 4.

EIS data of pure NiO, 2–8% Silver doped NiO electrodes.

Electrode Sample	Resistance at the Solution (RS) (Ω)	Charge transfer resistance (RCt) (Ω)	Double layer Capacitance (Cdl) (F)
Pure NiO	14.76	154.65	1.78 × 10−5
2 % Ag @ NiO	10.34	98.56	9.56 × 10−5
4 % Ag @ NiO	7.56	54.76	3.89 × 10−4
6 % Ag @ NiO	4.72	32.63	8.56 × 10−4
8 % Ag @ NiO	8.23	68.87	0.62 × 10−4

An ordinate (y-axis) represents the phase shift Φ, and the abscissa (x-axis) of the Bode plot (Fig. 18(b)) is a logarithmic scale of the frequency. The primary strength of this graph is the clarity with which all data is presented. The phase shift peak in this spectrum may be traced back to a capacitor connected in parallel with a resistor, a crucial circuit in EIS. For a 6 % Silver doped NiO electrode, the phase angle is seen to be −54.5°, which is similar to that of an ideal capacitor (−90°) [58].

One important characteristic that shows how quickly the electrode charges or discharges is the time constant (τ). The duration needed for the electrode to charge or discharge to 63 % of its entire capacity is represented by the time constant τ. The Bode plot is used to determine it, and the frequency at which the phase angle (θ) is −45° is used. This frequency represents the electrode's characteristic because the resistive and capacitive contributions are equivalent at this range. The time constant of electrode discharge as a function of frequency is shown in relation-22 [59].

$$\mathrm{\tau }=\frac{1}{{2\pi \mathrm{f}}_{\mathrm{c}}}----\left(22\right)$$

ceristic frequency and τ is the time constant, the time constant equals 0.1 s, suggesting that the electrode can charge or discharge quickly. Ag usually improves redox kinetics and conductivity, which speeds up the charging and discharging processes and lowers the time constant. For applications such as supercapacitors, a faster electrochemical response is indicated by a lower time constant.

Fig. 19, Fig. 20 depict the GCD curves of the first 5 and 2000th cycles for pure NiO, as well as 2–8% Silver doped NiO electrodes at a constant current density of 5 Ag^-1, these electrochemical performances of electrodes were measured in comparison to Ag/AgCl for 2000 cycles of operation in a potential window ranging from 0 to 0.85 V. These experiments were conducted to regulate the stability of the electrodes. The electrode with 6 % Silver doped NiO has the best cycling stability when compared to the other electrodes.

Fig. 19.

First 5 cycles of GCD curve of pure NiO, 2–8% Silver doped NiO electrodes vs Ag-AgCl in 1M KOH solution.

Fig. 20.

3000th cycles of GCD curve of pure NiO, 2–8% Silver doped NiO electrodes vs Ag-AgCl in 1M KOH solution.

Fig. 21 shows the GCD curves for pure NiO, as well as 2–8% Silver doped NiO nanomaterial electrodes with specific surface area of the electrodes approximately 92.5 m²/g at varying current densities.

Fig. 21.

GCD curves of pure NiO, 2–8% Silver doped NiO electrodes for various current densities.

Charge/Discharge Mechanism.

NiO structure electrons are extracted while charging, oxidizing Ni²⁺ ions to Ni³⁺. Hydroxide ions (OH⁻) from the electrolyte are involved in this process as represented in euation-23:

$$NiO+{OH}^{-}---->NiOOH+e^{-}----\left(23\right)$$

Improved rate of charge transfer at the electrode surface is ensured by Ag doping, which accelerates electron mobility and enhances electrode conductivity overall.

Discharge mechanism.

NiOOH transforms back into NiO and Ni³⁺ ions and are reduced back to Ni²⁺ during discharge process as shown in the equation-24.

$$NiOOH+e^{-}---->NiO+{OH}^{-}----\left(24\right)$$

Ag's presence assistances in the preservation of a stable structure and inhibit degradation, improving cycle stability and lowering resistance.

The nature of charge-discharge curves indicates the resemblance of that of a pseudo-capacitor rather like a standard electric double-layer capacitor. Higher current densities in the sample facilitates for higher energy storage, since larger surface area is available along the current-potential axis. The electrode capacitance can be analysed using equation mentioned below (25) [[60], [61], [62], [63], [64]].

$$\mathrm{C}=\mathrm{i}.\Delta \mathrm{t}/\mathrm{m}\Delta \mathrm{V}-------\left(25\right)$$

where, the applied current is i, the potential range is ΔV, time of a discharge cycle is Δt and mass of pure NiO, as well as 2–8% Silver doped NiO NPs is represented by m. Specific capacitance values of pure NiO, as well as 2–8% Silver doped NiO NPs were calculated and found to be 185, 198, 223, 259 and 208 Fg^-1, respectively, indicates improved capacitance for 6 % Ag doped NiO electrode. After the operation of 2000 cycles in 1M KOH, the 6 % Ag doped NiO NPs electrode retains more than 90 % of its initial capacitance.

Fig. 22 illustrates the long-term stability of pure NiO as well as 2 %, 4 %, 6 %, and 8 % Silver doped NiO electrodes at a current density of 5 A g⁻¹ for 3000 cycles of charge-discharge operations. After the completion of 1250 cycles of charge-discharge, the voltage corresponds to pure NiO electrode gradually decreases, whereas the voltage of the 6 % Ag-doped electrode starts to drop after 1750 cycles of charge-discharge and stabilizes at the end of 2000 cycles. Further, after the operation of 2000 cycles, the Silver doped NiO electrodes retain more than 92 % of their initial capacity, whereas 6 % Silver doped NiO electrodes retains the maximum. These results suggests that the 6 % Ag-doped electrode likely restored higher capacity, leading to better electrochemical performance and energy storage.

Fig. 22.

Dependence of potential as a function of cycles of operation at a constant current density of 5 Ag^-1 for pure NiO, 2–8% Silver doped NiO electrodes.

Ag doping of NiO facilitates the increase in electrical conductivity of the electrodes. Although NiO is naturally insulating, the addition of Ag dopants can create more conducting channels in the material, improving electron transport during charge and discharge. Ag doping also improves the NiO NPs redox-active cites on their surface, which can cause pseudocapacitive behaviour. This indicates that extra charge storage occurs beyond the double-layer capacitance due to Faradaic redox processes at the electrode-electrolyte interface [65]. Faster charge/discharge rates and higher power density may result from improved electrolyte accessibility and improved ion diffusion kinetics [61,66]. Ag doping enhanced the efficiency of ion adsorption/desorption processes during cycling by providing an improved pore structure and enhanced surface area.

Fig. 23 shows the dependence of specific capacitance on current density, it is noticed that the specific capacitance values decreased with an increase in the current density. Lastly, by adding more electron routes, silver (Ag) doping in NiO increases the electrode's conductivity and improves the redox reaction kinetics. Ag can also increase the surface area of NiO and the electrode's capacity to store charge by raising the number of electroactive sites. Moreover, Ag nanoparticles work as catalysts to lower the overpotential needed for the redox process. Recently, specific capacitance of different metal doped metal oxides has been reported for supercapacitors are shown in Table 5.

Fig. 23.

The dependence of Specific capacitance as a function of current density for various electrodes.

Table 5.

Comparison between the specific capacitance of other metal dopant metal oxides.

Type of metal doped metal oxides	Specific capacitance (F/g)
Zr-doped ZnO	518 [65]
Sr-doped ZnO	698 [66]
Cd-doped ZnO	627 [67]
Ag-doped NiO	535 [Present work]

4. Conclusions

Green combustion approach using Aloe vera plant extract towards the synthesis of both pure NiO and Silver doped NiO nanoparticles with different doping concentrations of Ag was successfully employed. The XRD (X-ray diffraction) pattern indicates a crystallite size (D) range for the prepared nanoparticles about 20 nm. Bandgap of NiO nanoparticles reduces from 2.48 to 2.62 eV when Ag dopant concentrations increase from 2 to 8 mol%. The SEM micrographs indicate flaky structure with significant porosity. The 6 % Silver doped NiO NPs demonstrated the best photodegradation efficiency. When exposed to UV light for 2 h, pure NiO and 6 % Silver doped NiO shows FB degradation of 62.9 % and 98.0 %, respectively. The specific capacitance of 6 % Silver doped NiO NPs electrode was found to be to be 535 Fg^-1 at a constant current density of 1 Ag^-1. 6 % Silver doped NiO NPs electrode was found to retain more than 92 % of the initial capacitance after 2000 cycles of charging-discharging, indicates the excellent stability of the material as a supercapacitor electrode. Considering the low-cost green combustion synthesis, excellent material features such as photocatalytic degradation of FB dyes and electrodes for electrochemical supercapacitor, these Silver doped NiO NPs could be potential candidates for multifunctional applications.

CRediT authorship contribution statement

S. Alfadhli: Writing – original draft, Methodology, Investigation, Funding acquisition. Syed Khasim: Writing – review & editing, Writing – original draft, Formal analysis, Data curation, Conceptualization. A.A.A. Darwish: Resources, Investigation, Conceptualization. Khoulod Al-nahdi: Software, Project administration, Methodology. Mervat Abdelkader: Validation, Supervision, Software, Project administration. Raghad Gamal: Supervision, Project administration, Methodology. Taymour A. Hamdalla: Writing – original draft, Validation, Formal analysis, Data curation.

Data availability

The Authors confirm that the all the data related to the research is enclosed in the manuscript.

Declaration of competing interest

The author declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.