Green Synthesis of Zinc Oxide Nanoparticles Using Hyptis Leaf Extract and Activated Carbon-Based Zinc Oxide Composite of Supercapacitor Applications

Objective: To prepare the zinc oxide nanoparticles using the extract of Hyptis suaveolens leaf by adopting the green synthesis methods. The prepared ZnO nanoparticles along with the activated carbon nanocomposite electrode will be used to fabricate supercapacitor cell. Methods/statistical analysis: Nanoparticles of ZnO were prepared using the green synthesis method. The structural and molecular characterizations were carried out using conventional techniques. The particle size of nanoparticles along with its elemental composition was determined using TEM-SAED and its optical transparency was studied. Finally, the nanocomposite of ZnO and activated carbon is subjected to the electrochemical impedance spectroscopy for understanding its supercapacitor properties. Findings: Hyptis suaveolens leaf extract was used in the preparation of ZnO nanoparticles and TEM confirms that the particle size was in the range of 10 and 200 nm. An absorption peak at 376 nm was observed from the UV spectroscopy, and the sample is transparent in the visible region. The capacitance value of the prepared sample is estimated to be 184.8 F/cm2 at 5 mV/sec. Applications/improvements: Nanocomposites of ZnO and activated carbon with different mass ratios (1:1 to 1:3) were subjected to electrochemical impedance measurement, and from the estimated capacitance value (184.8 F/cm2) it was found to be a potential candidate for the supercapacitor application.


Introduction
Metal oxides (MO) have strong photocatalytic and photo oxidizing properties due to the presence of many active sites and fabrication of hydroxial radicals on their surface. [1][2][3] During the past decade, various physical and chemical techniques used in the synthesis of metal oxide nanostructures have been developed, including for the purpose of assessing thermal decomposition, nanoparticles. This green approach has emerged to be a cost-effective alternative to the conventional synthesis methods. Currently, the use of green organic compounds extracted from leaves, stem, flowers, fruits, or seeds as reductants have been reported by several researchers. [12][13][14][15][16][17][18][19][20] Apart from the plant extracts, pure organic compounds obtained from green products such as andrographolide, 21 Carboxylic curdlan, 22 Ellagic acid, and geraniin 23 have also been used for producing reducing agents that help in the immediate reduction of metal ions in nanostructured metal oxides. Among different nanostructured metal oxides, ZnO-NPs attracts tremendous attention because of its excellent properties like wide direct band gap of 3.37 eV at room temperature and high excitation binding energy of 60 meV. [24][25][26] It has a wide range of applications, including in plastics, 27 sensors, 28 solar cells, 29 cosmetics, 30 sunscreens, 31 and diagnostics. 32 Also, due to its low toxicity, ZnO has been listed 33 as '"Generally Recognized as Safe'" (GRAS) by the U.S. Food and Drug Administration (21,CFR 182,8991). Zinc oxide is a familiar energy storage device having high energy density of 650 A/g. Even though it has disadvantages such as formation of dendrite growth during consecutive cycling, which leads to decrease in life cycle, it is a promising electrode material for supercapacitor. The plant Hyptis suaveolens (L.) Poit belongs to the family Lamiaceae is native of Tropical America. The different parts of H. suaveolens plant, like the bark, seed, and leaves, are used to cure swellings, abscesses, haemorrhoids, and improve memory.
Sharma Prince 34 reported that the H. suaveolens contains many diverse phytochemicals like sterols, phenols, saponins, terpenes, alkaloids, and flavonoids and secondary metabolites like suaveolic acid, suaveolol, methyl suaveolate, β-sitosterol, ursolic acid, rosamarinic acid, methyl rosmarinate, oleanoic acid, 1,19 a-dihydroxyurs-2(3),12-dien-28-oic acid, 3β-hydroxy lup-12-en-28oic acid, urs-12-en-3β-ol-27-oic acid, and 3β-hydroxyl lup-20(29)-en-27-oic acid. Therefore, H. suaveolens leaf extract was used as a reducing agent in the phytomediated synthesis of ZnO nanoparticles. Hence, the present work aims to develop the green synthesis of ZnO nanoparticle using H. suaveolens leaf extract based on the green chemistry approach. This approach has advantages such as low-cost precursor and a product that is high in purity and of more quantity. It does not require costly equipment and does not use any intermediate substance (any base groups); additional advantages include its rapidity and environment-friendly approach.

Preparation of Hyptis suaveolens Leaf Extract
Hyptis suaveolens were collected from Arumbavur, Perambalur District, India. They were cleaned with tripledeionized water and cut into small pieces. Five grams of the fresh leaf pieces were extracted with 100 ml deionized water in a reflux condenser for 30 min. After 30 min, the extract was cooled to room temperature (approximately 30 o C). Then the solution was filtered through Whatman filter paper, and the resultant filtrate was refrigerated for further use.

Eco-friendly Synthesis of Zinc Oxide Nanoparticles
Zinc nitrate was acquired from Sigma-Aldrich. The freshly prepared mixture of leaf extract and 0.01 M Zinc nitrate solution in the ratio of 1:4 was added in a conical flask and stirred at room temperature for 4 h. The resultant mass was dried in hot air oven at 110°C. The yellow-coloured paste thus obtained was finally calcined at 400°C to get fine crystals of ZnO-NPs and stored in air-free containers for further characterization. Synthetic route of formation of Hyptis-mediated Zinc oxide nanoparticle is shown in Figure 1.

Preparation of Hyptis suaveolens Activated Carbon
H. suaveolens leaves were first washed several times with double-distilled water to remove dust, soil, and suspended impurities. The material was ground using pestle and mortar to turn it into fine particles. About 100 g of powdered fine materials was mixed with 50 ml of concentrated sulphuric acid and kept at room temperature for 1 day. The dried material was washed with double-distilled water for removing excess of acid. Finally, material was dried in 110°C for 14 h to remove moisture and kept in closed bottle.

Preparation of AC-ZnO Electrode
Activated carbon was mixed with ZnO in three different ratios (1:3, 1:2, 1:1) by using a paste made out of mixing n-methylpyrrolidone with polyvinylidiene fluoride. After that, AC-ZnO composite paste was applied with a brush to pre-weighted stainless steel current collector and dried at 25°C. Electrochemical experiments were carried out by electrochemical impedance spectroscopic (EIS). AC impedance measurements were performed in the frequency range of 0.01 Hz to 100 KHz with amplitude of 5 mV peak-to-peak using AC signal.

UV-Vis Spectral Analysis
UV-Vis spectroscopy is an effective tool for analysing the formation of metal nanoparticles in aqueous solution. UV-Vis spectrum of the H. suaveolens leaf extract, H. suaveolens, Zinc Nitrate, and biosynthesized ZnO nanoparticles is shown in Figure 2. The SPR band of ZnO-NPs at 376 nm confirms the formation. The SPR absorption is due to the oscillation of the free conduction band electrons excited by the incident electromagnetic radiation. This type of resonance is observed when the wavelength of the incident light far exceeds the diameter of particles. The sharp peak indicates the uniform size of the synthesized particles. This is in agreement with the reported studies on the biosynthesis of zinc oxide nanoparticles (absorption peak at 374 nm) using various plant extracts, 35,36 which confirmed the presence of ZnO nanoparticles.

SEM and EDX Studies
Scanning electron microscope (SEM) analysis of the Hyptis-mediated nanoparticles predicts the shape, size, and morphology of the biosynthesised nanoparticles. It is evident from Figure 4a that synthesised nanoparticles have Wurtzite hexagonal shape with particle size in the range of 10 to 70 nm. The EDX spectrum of biosynthesised ZnO-NPs as shown in Figure 4b confirms the elemental composition of ZnO nanoparticles having 49 % Zn and 51 % O and no other signals present reveal the high purity of synthesised ZnO-NPs without impurities.

HR-TEM Analysis
The HR-TEM analysis of biosynthesised ZnO-NPs explains the size and morphology of ZnO-NPs. Figure  5 shows the TEM image of biosynthesized ZnO-NPs. The image reveals hexagonal (Wurtzite) shape of ZnO-NPs with the particle size in the range of 10 to 200 nm, which is in accordance with XRD data. The agglomeration of ZnO-NPs is due to high surface energy because of its synthesis in aqueous medium and also possibly due to densification resulting in narrow space between particles. [36][37][38] The selected area (electron) diffraction (SAED) pattern suggested that the particles are highly crystalline in nature and also display an arrangement of rings containing spots, indicating that nanoparticles have a bigger grain size, uniform shape, and are polycrystalline in nature.

XRD Studies
The XRD analysis of the Hyptis-mediated Zinc oxide nanoparticles reveals the crystallographic structure,   From the XRD pattern, high intensity peak at 101 plane is assigned to be the characteristic peak of ZnO-NPs. No other diffraction peaks are identified, suggesting the purity of the synthesized ZnO-NPs.

Electrochemical Impedance Spectroscopy (EIS)
Electrochemical impedance spectroscopy (EIS) is an essential tool to investigate the impedance differences in various ratios of AC-ZnO (1:3, 1:2, and 1:3) electrode. Figure 7 reveals the Nyquist plots of the AC-ZnO with two different ratios [1:3, 1:2] in 0.1 M Na 2 SO 4 . A distinctive semicircle has been observed in AC-ZnO (1:3 and 1:2), suggesting a single charge-transfer process taking place among the working material and electrolyte. The charge transfer resistance and specific capacitance values of AC-ZnO electrodes are given in Table 1. The charge transfer resistance increased with an increase in the content of ZnO in AC-ZnO composite mixture. Impedance diagrams were obtained for AC-ZnO (1:1) electrode as working electrode in 0.1 M Na 2 SO 4 ( Figure 8). A linear part is obtained at low frequency and a semicircle in the high-frequency region. The capacitance value increases at low frequencies due to a large number of ions moving which cause a decrease in the bulk resistance of the capacitor. The semicircle results from the parallel combination of resistance and capacitance, and the linear region is due to Warburg impedance. In the low-frequency region, the linear region leans more towards imaginary axis, and this indicates good capacitive behaviour. From the figures it can be seen that the diameter of the semicircle (1:1 AC-ZnO) decreased compared to the other two compositions (2:1 and 3:1 AC-ZnO), which suggests an increase in the available large surface area on AC-ZnO (1:1 ratio) and less restriction in the electron transfer process. Hence, AC-ZnO composite electrode is an excellent supercapacitor in energy storage devices.

Conclusion
In the present work, we report the green synthesis of ZnO-NPs from Zinc nitrate and H. suaveolens leaf extract. The biosynthesised ZnO-NPs was found to have Wurtzite hexagonal structure with particle size in the range of 20 to 200 nm. X-ray diffraction analysis revealed the distinctive facets (100, 002, 101, 102, 110, 103, 112, 201, and 202 planes) of ZnO-NPs. The prepared AC-ZnO composite was shown to be an excellent choice for material with good conductivity with 0.1 M Na 2 SO 4 as the electrolytes for its application in an energy storage device such as a supercapacitor.