Phytochemical-Assisted Green Synthesis of Nickel Oxide Nanoparticles for Application as Electrocatalysts in Oxygen Evolution Reaction

: Electrocatalytic water splitting is a promising solution to resolve the global energy cri-sis. Tuning the morphology and particle size is a crucial aspect in designing a highly efﬁcient nanomaterials-based electrocatalyst for water splitting. Herein, green synthesis of nickel oxide nanoparticles using phytochemicals from three different sources was employed to synthesize nickel oxide nanoparticles (NiO x NPs). Nickel (II) acetate tetrahydrate was reacted in presence of aloe vera leaves extract, papaya peel extract and dragon fruit peel extract, respectively, and the physicochemical properties of the biosynthesized NPs were compared to sodium hydroxide (NaOH)-mediated NiO x . Based on the average particle size calculation from Scherrer’s equation, using X-ray diffractograms and ﬁeld-emission scanning electron microscope analysis revealed that all three biosynthesized NiO x NPs have smaller particle size than that synthesized using the base. Aloe-vera-mediated NiO x NPs exhibited the best electrocatalytic performance with an overpotential of 413 mV at 10 mA cm − 2 and a Tafel slope of 95 mV dec − 1 . Electrochemical surface area (ECSA) measurement and electrochemical impedance spectroscopic analysis veriﬁed that the high surface area, efﬁcient charge-transfer kinetics and higher conductivity of aloe-vera-mediated NiO x NPs contribute to its low overpotential values.


Introduction
One of the greatest challenges of the technological era is the inflating global energy demand as electricity becomes a necessity in every facet of life. The non-renewable nature of fossil-fuel-based energy production has stimulated the pursuit of sustainable energy sources such as solar and wind energy. To overcome the intermittency issues of these renewable energy sources, production of hydrogen fuel from electrocatalytic water splitting is considered a strategic approach. Under a standard environment, 1.23 V of thermodynamic potential is required to drive electrochemical water splitting [1]. Nonetheless, in practical conditions, the sluggish kinetics of oxygen evolution reaction (OER) mandates high overpotential to generate the required current density. Hence, most practical electrolyzers consume more than 1.23 V to drive the desired reaction.
Electrocatalysts are usually employed to attain satisfactory water-splitting efficiency, but by far best electrocatalytic performance was demonstrated by noble-metal-based materials (e.g., RuO 2 and IrO 2 ). The high cost and low abundancy of noble metals are not conducive for large-scale water-splitting applications. Therefore, non-precious transition metal oxides and hydroxides were explored as alternative electrocatalysts. Nickel oxide (NiO x ) is one such promising material for electrocatalytic water splitting due to its high electron transport capacity, good chemical stability and facile method of synthesis. Particularly, in the case of NiO x NPs, the synthetic route adapted influences the electrical and structural properties of the material.
Traditionally, metal oxide nanoparticles are synthesized using a strong base as a reducing agent, followed by reaction with a capping agent or reducing agent. Sometimes, additional organic solvents are required to dissolve these capping agents. Alternatively, a green synthesis approach can be utilized to reduce the usage of unnecessary toxic chemicals [2]. In green synthesis, primary and secondary metabolites of plants are used to aid the synthesis of metal oxide nanoparticles [3]. These metabolites form a coating layer or stabilizing layer on the surface of the metal oxide nanoparticles and prevent intensive aggregation, thus yielding well-defined morphologies and tunable particle size [4]. The choice of plant species and its respective parts play an important role in determining the morphology and particle size of the NPs as they contain distinct phytochemical profiles [5].
One of the major advantages of chemical capping agents is the tunability of the species to yield nanoparticles with different particle size and morphology. For example, in the case of synthetic-polymers-based capping agents, factors such as the nature of polymer (functional group, branching), molecular weight and concentration were found to affect its function as a stabilizer [6]. However, in the context of green synthesis, much work is dedicated to exploring the effect of using a single plant extract formulation. Since each plant species comes with its unique cocktail of phytochemicals, it is useful to compare the effect of using different plant extracts on the overall size and morphology of the nanoparticles. Thus, in this study we have explored three different plant extracts, namely, aloe vera leaves extract, papaya fruit peel extract and dragon fruit peel extract, as capping agents for NiO x NPs.
Aloe vera gel contains mostly water and polysaccharides, such as pectins, cellulose, hemicellulose, glucomannan and acemannan [7,8]. In addition to that, the aloe latex is made up of hydroxyanthracenic derivatives and anthraquinone glycosides, along with emodin. Papaya peel is rich in a variety of vitamins including β-carotene, vitamin B (thiamine, riboflavin, niacin and folate), vitamin C and vitamin E [9]. On the other hand, dragon fruit peel is reported to contain wide compositions of antioxidants such as betasianin, flavonoids, phenols, terpenoids, thiamine, niacin, pyridoxine and cobalamin [10]. The effect of these different phytochemical compositions of extracts on the structural and morphological properties of the NiO x NPs and, consequently, their electrocatalytic activities for OER were studied and reported in detail. Figure 1 shows the structural properties of the synthesized NiO x nanoparticles using different extracts after calcination at 300 • C and 600 • C. As shown in the diffractograms, all the samples exhibited diffraction peaks at 2θ values of 37. 2 [11,12]. These peaks were in agreement with the standard JCPDS card no. 00-004-1159. Significant differences in the peak intensities were evident between samples calcined at 300 • C and 600 • C. At lower calcination temperature, the presence of organic residues from the plant extract used during NiO x synthesis reduced the crystallinity of the nanoparticles. Upon calcination at 600 • C, high purity NiO x nanoparticles were produced, leading to sharp and intense XRD peaks. Figure 1 shows the structural properties of the synthesized NiOx nanoparticles using different extracts after calcination at 300 °C and 600 °C. As shown in the diffractograms, all the samples exhibited diffraction peaks at 2θ values of 37.2, 43.2, 62.8, 75.4 and 79.4° attributed to (101), (012), (104), (113) and (006) crystal planes of cubic NiOx [11,12]. These peaks were in agreement with the standard JCPDS card no. 00-004-1159. Significant differences in the peak intensities were evident between samples calcined at 300 °C and 600 °C. At lower calcination temperature, the presence of organic residues from the plant extract used during NiOx synthesis reduced the crystallinity of the nanoparticles. Upon calcination at 600 °C, high purity NiOx nanoparticles were produced, leading to sharp and intense XRD peaks.  The average crystal sizes (D) were calculated based on the width of the peak due to the (200) planes by using the Scherrer's formula [13]:

Structural Properties
From Table 1, it could be seen that the average crystallite size of the nanoparticles synthesized with only NaOH was 15.9 nm, whereas aloe vera, papaya and dragon fruit extracts was 11.5 nm, 8.1 nm and 10.2 nm, respectively. The reduced crystallite size of NiO x nanoparticles synthesized in presence of plant extracts emphasizes the role of phytochemicals that are able to chelate with metal ions, thus acting as stabilizers or capping agents. Figure 2 illustrates the plausible mechanism for formation of NiO x nanoparticles in this study. Upon dissolution, nickel acetate tetrahydrate dissociated to release Ni 2+ ions which then combined with hydroxide ions from the base to form Ni(OH) 2 . The continuous stirring under a heated environment enabled Ni(OH) 2 to thermally decompose into NiO nuclei [14]. The phytochemicals present in each plant extract served as capping ligands which formed interactions with the nanoparticles' surface, leading to steric hindrance and providing eventual stability to the nanocomposite. Hence, the presence of biomass-derived extracts controlled the particle size and minimized agglomeration of the nanoparticles.

Morphological Properties
The morphological properties of the synthesized NiOx nanoparticles are shown in Figure 3. From the images, it is clear that the NiOx nanoparticles were spherical-shaped, densely packed and intercalated with each other. Dragon fruit and papaya-extract-mediated NiOx NPs depict well-defined nanostructures with fewer intercalations. The particle size distribution for the NiOx NPs synthesized without extract ranged between 40 and 60 nm, whereas plant-extract-mediated NiOx NPs were between 20 and 30 nm in size. The trend in particle size coincided with the trend in crystallite size derived from the XRD analysis. For all the samples, the particle size revealed in morphological analysis was

Morphological Properties
The morphological properties of the synthesized NiO x nanoparticles are shown in Figure 3. From the images, it is clear that the NiO x nanoparticles were sphericalshaped, densely packed and intercalated with each other. Dragon fruit and papaya-extractmediated NiO x NPs depict well-defined nanostructures with fewer intercalations. The particle size distribution for the NiO x NPs synthesized without extract ranged between 40 and 60 nm, whereas plant-extract-mediated NiO x NPs were between 20 and 30 nm in size. The trend in particle size coincided with the trend in crystallite size derived from the XRD analysis. For all the samples, the particle size revealed in morphological analysis was higher than average crystallite size estimated from XRD analysis, hence indicating that the single NiO x particle is made up of few crystallites.

Morphological Properties
The morphological properties of the synthesized NiOx nanoparticles are shown in Figure 3. From the images, it is clear that the NiOx nanoparticles were spherical-shaped, densely packed and intercalated with each other. Dragon fruit and papaya-extract-mediated NiOx NPs depict well-defined nanostructures with fewer intercalations. The particle size distribution for the NiOx NPs synthesized without extract ranged between 40 and 60 nm, whereas plant-extract-mediated NiOx NPs were between 20 and 30 nm in size. The trend in particle size coincided with the trend in crystallite size derived from the XRD analysis. For all the samples, the particle size revealed in morphological analysis was higher than average crystallite size estimated from XRD analysis, hence indicating that the single NiOx particle is made up of few crystallites.  The elemental composition of the nanoparticles was verified from EDX analysis. As shown in Figure 4, the choice of plant extract influenced the composition of nickel and oxygen in the samples. NiO x NPs synthesized using aloe vera were more oxygen deficient compared to papaya and dragon fruit. Aloe-vera-mediated nickel oxide also exhibited the highest amount of residual carbon, despite calcining the samples at 600 • C. It is deduced that this could be due to the more viscous nature of the aloe vera extract, which adheres to the surface of the nanoparticles during synthesis. The absence of residual carbon originating from the phytochemicals was evident in dragon-fruit-and papaya-extract-mediated NiO x NPs, which could explain the occurrence of fewer intercalations, as shown in the FESEM images.

Optical Properties
The absorption spectrum of the NiO x nanoparticles, as shown in Figure 5a, was recorded at room temperature within the range of 200-800 nm. There was a peak observed at around 300-350 nm for all synthesized nanopowders, which can be assigned to the exciton transitions [15]. The band gap energies were estimated by extrapolation using Tauc plots, and the energy curves are shown in Figure 5. It was seen that the band gap of NiO x nanoparticle synthesis in different samples were ranging from 3.09 to 3.22 eV and the band gap was slightly increased for plant-extract-assisted synthesized nanoparticles. Particularly, the decrease in particle size, as well as the increase in band gap energy of the as-prepared nanoparticles, signified the size quantization effects [16]. When particle size reached the nanosized scale, the overlapping of adjacent energy levels minimized and, subsequently, the width of energy band widened [17,18]. that this could be due to the more viscous nature of the aloe vera extract, which adheres to the surface of the nanoparticles during synthesis. The absence of residual carbon originating from the phytochemicals was evident in dragon-fruit-and papaya-extract-mediated NiOx NPs, which could explain the occurrence of fewer intercalations, as shown in the FESEM images.  nanoparticle synthesis in different samples were ranging from 3.09 to 3.22 eV and the band gap was slightly increased for plant-extract-assisted synthesized nanoparticles. Particularly, the decrease in particle size, as well as the increase in band gap energy of the asprepared nanoparticles, signified the size quantization effects [16]. When particle size reached the nanosized scale, the overlapping of adjacent energy levels minimized and, subsequently, the width of energy band widened [17,18].

Electrocatalytic Properties for Oxygen Evolution Reaction (OER)
The electrocatalytic activity of the synthesized NiO x NPs towards OER was explored by performing linear scanning voltammetry (LSV) in an aqueous solution of 1.0 M KOH. Generally, the oxygen evolution reaction in alkaline medium is represented as follows [19][20][21]: The four-electron reaction mechanism can be described sequentially via the following equations: where OH*, O* and OOH* indicate the species adsorbed to the active site of the catalyst. Based on the polarization curves shown in Figure 6a, to attain the current density of 10 mA cm −2 , aloe-vera-and papaya-extract-mediated NiO x NPs exhibited the overpotential values of 416 mV and 433 mV, respectively, which are lower than NaOH-based NiO x , which records 474 mV. Interestingly, dragon-fruit-extract-mediated NiO x showed the overpotential of 501 mV, the highest overpotential compared to the previous three samples. As listed in Table 2, the onset potential, which indicates the voltage at which current begins to rise, was lowest for aloe-vera-mediated NPs and highest for dragon fruit. → * + (3) * + → * + + (4) * + → * + (5) * + → + 2 + where OH*, O* and OOH* indicate the species adsorbed to the active site of the catalyst. Based on the polarization curves shown in Figure 6a, to attain the current density of 10 mA cm −2 , aloe-vera-and papaya-extract-mediated NiOx NPs exhibited the overpotential values of 416 mV and 433 mV, respectively, which are lower than NaOH-based NiOx, which records 474 mV. Interestingly, dragon-fruit-extract-mediated NiOx showed the overpotential of 501 mV, the highest overpotential compared to the previous three samples. As listed in Table 2, the onset potential, which indicates the voltage at which current begins to rise, was lowest for aloe-vera-mediated NPs and highest for dragon fruit.   The plausible reaction mechanism of NiO x for OER is outlined as follows: NiOOH Throughout the electrocatalytic process, Ni 2+ ions that can be present on surface of the NiO x NPs will undergo oxidation to form Ni 3+ ions which then aid to catalyze the oxidation of OH − ions, leading to the formation of water [22].
To further comprehend the reaction kinetics of the nickel-oxide-based electrocatalysts, the Tafel plot was plotted (Figure 6c), and the linear section of Tafel slope was derived from the following equation: where η is the overpotential, b is the Tafel slope and j refers to current density. NaOH-, aloe-vera-, papaya-and dragon-fruit-mediated NiO x recorded slope values of 191, 95, 122 and 224 mV/decade, respectively. The Tafel slope values indicate the rate-limiting step which is attributed to the reaction with first electron transfer. Thus, the lower Tafel slope value of aloe-vera-mediated NiO x NPs translates to more efficient reaction kinetics for OER. One of the proposed explanations for the superior electrocatalytic performance of aloe-vera-mediated NiO x NPs can be related to the relatively high oxygen deficiency in the samples, as evidenced by EDX measurements. The non-stoichiometric composition most likely led to the presence of surface defects, which have been found to enhance the electrocatalytic performance of metal oxides for OER [23,24].
Another key factor that governs the performance of electrocatalyst is the surface area available as active sites for catalytic activities. In OER, electrochemically active surface area (ECSA) is used as the parameter to assess the specific catalytic activity of an electrocatalyst, and it can be derived from electrochemical double-layer capacitance (C dl ). To calculate C dl values, the samples were subjected to cyclic voltammetry (CV) at different scan rates in the non-Faradaic segment where the voltage was scanned from 1.46 to 1.56 V vs. RHE (Figure 7). The double-layer capacitance was then derived based on the gradient of linear plot of ∆J = (J a − J c ) (where J a and J c are anodic and cathodic current density) at 1.52 V vs. RHE as a function of the scan rate [25]. The ECSA was then calculated from the double-layer capacitance based on the following equation: where C s is the specific capacitance which is equated to 0.040 mF cm −2 based on the conventional value for metal electrode in KOH solution. The roughness factor (R f ) can then be evaluated by dividing ECSA by the electrode area. Table 3 summarizes the ECSA and roughness factor for all the samples. Papaya-mediated NiO x NPs exhibit the highest value of ECSA and R f of 5.50 mFcm −2 and 1936, respectively, thus indicating that it has the highest catalytically active surface area. This could be the boosting factor that supports the low overpotential value recorded by papaya-mediated NiO x NPs despite its high Tafel slope. The variation of active surface area of the electrocatalysts prepared using different plant extracts further emphasizes the role of phytochemicals in fine-tuning the morphology and size distribution of nanoparticles during the synthesis process. of ECSA and Rf of 5.50 mFcm −2 and 1936, respectively, thus indicating that it has the highest catalytically active surface area. This could be the boosting factor that supports the low overpotential value recorded by papaya-mediated NiOx NPs despite its high Tafel slope. The variation of active surface area of the electrocatalysts prepared using different plant extracts further emphasizes the role of phytochemicals in fine-tuning the morphology and size distribution of nanoparticles during the synthesis process.  Electrochemical impedance spectroscopy (EIS) is a powerful technique that allows us to obtain insights on electrode-electrolyte interface dynamics. The EIS measurements for all the samples were performed with a frequency range from 0.1 Hz to 100 kHz, and the resulting Nyquist plots comprising of semicircles are presented in Figure 8. The starting point of the semicircle signifies the internal resistance (also known as solution resistance (R S )) of the catalyst, whereas the diameter of the semicircle denotes chargetransfer resistance between the electrode-electrolyte interfaces (R CT ) [26]. In terms of equivalent circuit, the depressed semicircle is represented by R S connected in series to the capacitor (Q DL ) and R CT in a parallel arrangement, as shown in Figure 8. The following equation evaluates impedance values according to the described equivalent circuit [27]: (13) Electrochemical impedance spectroscopy (EIS) is a powerful technique that allows us to obtain insights on electrode-electrolyte interface dynamics. The EIS measurements for all the samples were performed with a frequency range from 0.1 Hz to 100 kHz, and the resulting Nyquist plots comprising of semicircles are presented in Figure 8. The starting point of the semicircle signifies the internal resistance (also known as solution resistance (RS)) of the catalyst, whereas the diameter of the semicircle denotes charge-transfer resistance between the electrode-electrolyte interfaces (RCT) [26]. In terms of equivalent circuit, the depressed semicircle is represented by RS connected in series to the capacitor (QDL) and RCT in a parallel arrangement, as shown in Figure 8. The following equation evaluates impedance values according to the described equivalent circuit [27]: The equation was used to fit the Nyquist plots and the solution resistance, chargetransfer resistance and double-layer capacitance based on the equivalent circuit fitting are tabulated in Table 4. Typically, charge-transfer resistance (RCT) is considered the most significant parameter as it dictates the extent of electronic transfers between the reactant and electrocatalyst surface. A lower RCT value also indicates higher conductivity of the electrocatalyst material which favors electron mobility during the electrocatalytic reaction [28]. In this study, aloe-vera-mediated NiOx NPs showed the lowest solution resistance and charge-transfer resistance. This can be attributed to better conductivity of the NiOx NPs due to its higher oxygen deficiency compared to other green-synthesized nanoparticles. The equation was used to fit the Nyquist plots and the solution resistance, chargetransfer resistance and double-layer capacitance based on the equivalent circuit fitting are tabulated in Table 4. Typically, charge-transfer resistance (R CT ) is considered the most significant parameter as it dictates the extent of electronic transfers between the reactant and electrocatalyst surface. A lower R CT value also indicates higher conductivity of the electrocatalyst material which favors electron mobility during the electrocatalytic reaction [28]. In this study, aloe-vera-mediated NiO x NPs showed the lowest solution resistance and charge-transfer resistance. This can be attributed to better conductivity of the NiO x NPs due to its higher oxygen deficiency compared to other green-synthesized nanoparticles. Therefore, it is evident that efficient charge-transfer kinetics and improved conductivity is one of the crucial factors for the higher electrocatalytic performance of aloe-vera-mediated NiO x NPs compared to its dragon fruit and papaya counterparts. To evaluate the stability of aloe-vera-mediated NiO x NPs, chronoamperometry analysis was performed for 24 h at an overpotential of 415 mV. Within the first 2 h, it was observed that there was a gradual increase in current density. This is attributed to the activation of nickel oxide into nickel oxyhydroxide (NiOOH) during the initial part of the analysis [22]. Higher conductivity of NiOOH compared to NiO rendered the gradual increase in current density. As the chronoamperometry was extended for 24 h, the current density remained stable without any significant decay.
The stability of the final product was further verified by subjecting the NiO x -modified glassy carbon electrode to cyclic voltammetry (CV) analysis for 500 continuous cycles, which were carried out between 1.2 and 1.8 V at a scan rate of 100 mVs −1 . As shown in Figure 9, after 500 CV cycles, the overpotential of aloe-vera-mediated NiO x NPs increased from 416 mV to 428 mV with a difference of only 0.12 mV at the current density of 10 mA cm −2 . This observation validates the high electrocatalytic stability of the greensynthesized nickel oxide nanoparticle for OER activity. To evaluate the stability of aloe-vera-mediated NiOx NPs, chronoamperometry analysis was performed for 24 h at an overpotential of 415 mV. Within the first 2 h, it was observed that there was a gradual increase in current density. This is attributed to the activation of nickel oxide into nickel oxyhydroxide (NiOOH) during the initial part of the analysis [22]. Higher conductivity of NiOOH compared to NiO rendered the gradual increase in current density. As the chronoamperometry was extended for 24 h, the current density remained stable without any significant decay.
The stability of the final product was further verified by subjecting the NiOx-modified glassy carbon electrode to cyclic voltammetry (CV) analysis for 500 continuous cycles, which were carried out between 1.2 and 1.8 V at a scan rate of 100 mVs −1 . As shown in Figure 9, after 500 CV cycles, the overpotential of aloe-vera-mediated NiOx NPs increased from 416 mV to 428 mV with a difference of only 0.12 mV at the current density of 10 mA cm −2 . This observation validates the high electrocatalytic stability of the green-synthesized nickel oxide nanoparticle for OER activity.

Chemicals
Nickel acetate tetrahydrate (Ni(CH3COO)2.4H2O), NaOH and KOH were purchased from Sigma Aldrich, Hamburg, Germany. Deionized (DI) water was used to prepare all types of solution. All the other chemicals were of analytical reagent grade and used without any further purification.

Chemicals
Nickel acetate tetrahydrate (Ni(CH 3 COO) 2 .4H 2 O), NaOH and KOH were purchased from Sigma Aldrich, Hamburg, Germany. Deionized (DI) water was used to prepare all types of solution. All the other chemicals were of analytical reagent grade and used without any further purification.

Preparation of Plant Extract
The NiO x nanoparticles were synthesized using various plant extracts such as aloe vera leaf extract, dragon fruit peel extract and papaya peel extract. NiO x NPs without extract were also synthesized. Different plant samples were collected from the local market at Hentian Kajang, Selangor, Malaysia. In order to prepare the plant extract, at first, 50 g of different plant samples were washed with DI water, cut into small pieces and were taken in three different 500 mL glass beakers and 250 mL DI water was added to all the beakers. Then the mixtures were heated at 60-70 • C for 30 min with continuous stirring. After heating, the extracts were filtered with vacuum filter and the filtrates were collected in three different 500 mL reagent bottles.

Synthesis of NiO x Nanoparticle
The NiO x nanoparticles were synthesized using the green synthesis technique. In this technique, at first, 30 mL (1 M) nickel acetate solutions were heated in four different 250 mL glass beakers at 60-70 • C temperature with continuous stirring. Then, 30 mL DI water was added to the first beaker (without extract) and 30 mL of three different plant extracts to the other three beakers was added in the precursor solution with constant heating (60-70 • C temperature) and stirring. Then, 1 M NaOH solution was added, dropwise, into the reaction mixture with vigorous stirring and heating for 2 h. After completion of the reaction, the mixtures were washed with DI water several times and filtered using Whatman 1 filter paper. Finally, the synthesized NPs were dried in an oven at 100 • C overnight, calcined at 300 • C and 600 • C for 6 h, powdered using mortar-pastel and stored in the glass vial for further analysis. Figure 10 represents the schematic view of green synthesis process of NiO x NPs.

Synthesis of NiOx Nanoparticle
The NiOx nanoparticles were synthesized using the green synthesis technique. In this technique, at first, 30mL (1 M) nickel acetate solutions were heated in four different 250 mL glass beakers at 60-70 °C temperature with continuous stirring. Then, 30 mL DI water was added to the first beaker (without extract) and 30 mL of three different plant extracts to the other three beakers was added in the precursor solution with constant heating (60-70 °C temperature) and stirring. Then, 1 M NaOH solution was added, dropwise, into the reaction mixture with vigorous stirring and heating for 2 h. After completion of the reaction, the mixtures were washed with DI water several times and filtered using Whatman 1 filter paper. Finally, the synthesized NPs were dried in an oven at 100 °C overnight, calcined at 300 °C and 600 °C for 6 h, powdered using mortar-pastel and stored in the glass vial for further analysis. Figure 10 represents the schematic view of green synthesis process of NiOx NPs.

Characterization of Materials
The morphological and structural properties of synthesized NiOx nanoparticles were examined using various characterization techniques such as X-ray diffraction (XRD), absorption spectra (UV-Vis) and field-emission scanning electron microscopy (FESEM), along with energy dispersive X-ray (EDX). The XRD patterns were taken in the 2θ ranging from 10° to 80° using BRUKER aXS-D8 Advance Cu-Ka diffractometer (Bruker, MA, USA). The FESEM model LEO 1450 Vp was used for investigating the surface morphology, as well as grain size and growth of the NiOx nanoparticles. The optoelectronic properties of the synthesized nanocatalyst, such as the optical transmittance, absorbance and

Characterization of Materials
The morphological and structural properties of synthesized NiO x nanoparticles were examined using various characterization techniques such as X-ray diffraction (XRD), absorption spectra (UV-Vis) and field-emission scanning electron microscopy (FESEM), along with energy dispersive X-ray (EDX). The XRD patterns were taken in the 2θ ranging from 10 • to 80 • using BRUKER aXS-D8 Advance Cu-Ka diffractometer (Bruker, MA, USA). The FESEM model LEO 1450 Vp was used for investigating the surface morphology, as well as grain size and growth of the NiO x nanoparticles. The optoelectronic properties of the synthesized nanocatalyst, such as the optical transmittance, absorbance and optical band gap, were calculated using UV-Vis spectrometer Perkin Elmer Instruments Lambda35 (PerkinElmer, Waltham, MA, USA).

Electrochemical Characterization
Electrochemical characterization was performed using Metrohm Autolab workstation (Metrohm, Herisau, Switzerland) in a three-electrode setup in which platinum electrode and silver/silver chloride were used as the counter electrode and reference electrode, respectively. The NiO x NPs were drop casted onto glassy carbon electrode with an active area of 0.07 cm 2 to function as the working electrode. A 5 mg amount of the NiO x NPs was mixed with 250 µL ethanol, 250 µL deionized water and 50 µL of Nafion (5 wt%). The dispersion was sonicated for an hour and 10 µL of the dispersion was drop casted on top of the glassy carbon electrode surface followed by drying. Linear sweep voltammetry between 0 and 1 V with scan rate of 5 mV s −1 was performed to evaluate the oxygen evolution reaction. The measured potential vs. Ag/AgCl was converted to a reversible hydrogen electrode (RHE) based on the following equation: Catalysts 2021, 11, 1523 14 of 15 where E 0 Ag/AgCl is 0.1976 V at 25 • C and E Ag/AgCl is measured potential vs. Ag/AgCl. The overpotential is calculated by measuring the difference between measured potential vs. RHE at current density of 10 mA cm −2 and the standard value of 1.23 V.

Conclusions
Utilization of different plant extracts as capping agents affects the structural and morphological properties of NiO x NPs. FESEM results show that NiO x NPs synthesized without extract had a particle size between 40 and 60 nm, whereas plant-extract-mediated NiO x were between 20 and 30 nm. The reduction in particle size upon introducing plant extracts was further supported by the shifting of the band gap to a higher value based on optical characterization. EDX analysis reveal that aloe vera, papaya and dragon fruit extract yielded NiO x NPs with different oxygen and nitrogen stoichiometry. The deficiency of oxygen is higher in aloe vera-mediated NiO x NPs, which might elevate its conductivity and thus lead to lower charge-transfer resistance in EIS analysis. ECSA calculation based on cyclic voltammetry indicated that the highest electrocatalytic surface area was possessed by papaya-extract-mediated NiO x NPs, followed by aloe vera and dragon fruit extract. Overall, the best electrocatalytic performance is attributed to aloe vera-extract-mediated NiO x NPs, which only required 433 mV overpotential to reach 10 mAcm −2 with a Tafel slope of 95 mVdec −1 .