Optical, Structural and Morphological Studies of ZnO Nanoparticles Synthesized using Terminalia catappa Leaf Extract

This research aims to synthesize zinc oxide nanoparticles using Terminalia catappa leaf extract. The synthesis method used Terminalia catappa leaves as a reductor. Synthesized ZnO nanoparticles were characterized using UV-vis, XRD, SEM, and TEM. UV-vis spectroscopy of samples showed peak absorption in the 300-380 nm range. The results of XRD characterization show that the ZnO nanoparticles crystal structure is hexagonal wurtzite, where the highest peak of reflection occurs at the reflection angle 2θ=36.24 of the crystal plan (101). SEM characterization results refer to flower-shaped morphology with a size distribution between 392-423 nm. These findings reveal morphology insight into the photocatalytic of ZnO nano-flowers for the environment.


Introduction
ZnO nanoparticles have band gap energy (3.37 eV) and high binding energy (60 MeV) [1], and this material is widely used as an electronics device, photocatalytic [2], cosmetic industry, and potential for antibacterial [3].The growth morphology of ZnO nanoparticles is unique and depends on the method of its growth [4].Chemical and physical methods are widely used for the synthesis of ZnO nanoparticles.Both methods are performed at high temperatures and pressure, with toxic chemicals, and are expensive [5].
This study carried out a green synthesis of ZnO nanoparticles using Terminalia catappa leaf extract.The advantage of this method is that it is environmentally friendly, and the process is simple.Research on the synthesis of ZnO nanoparticles using plants such as seaweed extract [6], citrus extract [7], red clover flowers [8], and other leaf plants [9] have been investigated.In this study, we used Terminalia catappa leaves as a reductor agent.The flavonoid and phenolic compounds can reduce metal ions to nanoparticles [10,11].
This study used Terminalia catappa leaves as reductants to synthesize ZnO nanoparticles.The concentration ratio of zinc precursor solutions with leaf extracts was 1:5 by varying the heating temperatures of 60,70 and 80°C.Using UV-vis, XRD, SEM, and TEM, ZnO powder nanoparticles investigated the optical, structural, and morphology properties.

Extract Preparation
Terminalia catappa leaves were extracted using the Sari et al. procedure with modification [18].A total of 5 grams of powdered Terminalia catappa leaves that have been dried at a temperature of 100°C for 90 minutes is added to 200 mL of boiling water, resulting in a 50 mL solution of extracts from Terminalia catappa.Then, the solution is heated to 80°C for 10 minutes on the hot plate using a magnetic stirrer and filtered with Whatman paper.The obtained solution is stored in a dark bottle and placed in a refrigerator at a temperature of 4°C.The resulting solution is used in zinc oxide synthesis.

Biosynthesis of ZnO
The synthesis of ZnO nanoparticles was prepared with the ratio between the extract of Terminalia catappa leaves and 0.05 M solution of zinc nitrate hexahydrate (ZNH) precursor ratio of 1:5, followed by a stirring process.Then, drop the NaOH solution until it reaches pH 12.The synthesized solution is mixed until homogeneously using magnetic stirer.After that, the solution is heated on a hot plate at 60, 70, and 80 °C for an hour to produce a white filtrate and then separated using a centrifuge at 4000 rpm for 10 minutes.Finally, the ZnO nanoparticle powder is dried using hot plates at 150 °C.

Characterizations
Biosynthesized samples are prepared using a ratio of 1:5 with heating temperatures of 60, 70, and 80°C characterized using a UV-Vis spectrophotometer.The sample has a high absorbance value chosen for structural and morphological characterization.For structural characterization, XRD is used, and for morphology, SEM and TEM are used.Terminalia catappa leaf extract, in this case acts as a reductant.Terminalia catappa leaf extract produces hydroxyl, assisted by NaOH solution, which can bind to the Zn 2+ ion cluster.Figure 2 shows UV-vis spectrum samples of ZnO nanoparticles heated at 60, 70, and 80°C.This UV-Vis characterization analysis is to determine the presence of nanoparticles formed.The wavelength used in the UV-Vis spectrophotometer characterization was 200-800 nm.The UV-Vis spectrum indicates that the highest absorption is in the ultraviolet range at 300-380 nm wavelengths.This result show formation of ZnO nanoparticles in the UV region [12].UV-vis absorption spectrum of ZnO nanoparticle samples at temperatures of 60, 70, and 80°C in Figure 2 shows that the ZnO nanoparticles of UV-Vis absorption peaks at 347 nm (Eg = 3.57 eV), 342 nm (Eg = 3.62 eV), and 351 nm (Eg = 3.53 eV), respectively.Shorter wavelength peaks of ZnO nanoparticles typically result in smaller particle sizes [13].This wavelength shift occurs due to the increased crystallinity of nanoparticles.Absorption ZnO nanoparticles decrease with increasing temperature due to the reduction in particle size when the precursors reduce, have been reported by reference [14,15].ZnO samples with a synthesis heating temperature of 60 o C are the samples with the highest absorbance intensity selected for XRD, SEM and TEM characterization.Samples heated at 60°C were chosen for XRD, SEM and TEM characterization because from the UV-vis spectrum results the difference in energy gap values was not significant for samples with temperatures of 70°C and 80°C, respectively.Figure 3 shows the result of the characterization of ZnO nanoparticles in sample (60°C) by including the intensity and angle of reflection 2θ using Match 3! software.The results of the diffraction pattern on the graph show that there are peaks at different diffraction angles, namely 31.75°(100), 34.37° (002), 36.24°(101), 47.55° (102); 56.66° (110); 62.80° (013); and 67.95° (112).The peaks obtained from the results of this characterization have matched using the Joint Committee on Power Diffraction Standard (JCPDS) database No. 96-901-1663, which explains that the ZnO sample has a hexagonal wurtzite structure [6].The XRD diffraction peak shows the highest and very sharp at a reflection angle of 2θ = 36.24°,the same as the results reported by [16].

Results and discussion
Figure 4(a) shows an image of the unique ZnO nanoparticles in the form of nano-flowers, consisting of five petals and one regular crown-shaped center.The shape is similar to the Hoya Serpens flower ornamental plant, whose petal arrangement is like a star [Fig.4 (b)].The morphological form of ZnO produced by synthesis using leaf extracts Terminalia catappa is the same as the morphology of syntheses in the literature [6], and also has been reported of three-dimensional zinc oxide nanoflowers [17,18].The size of the ZnO nanoparticles using the IC Measure software is 392-423 nm.There is an increased agglomeration of crystal size, proving that Terminalia catappa extract is a reduction agent in particle size growth.The TEM images show that smaller ZnO nanoparticles were formed at heating temperatures of 60°C on a scale of 50 nm (Figure 5a) and 20 nm (Figure 5b).The shape of the particles is like a flower, as other reports have [17,18].TEM images of ZnO nanoparticles also agreed with SEM characterization results.It is seen clearly that the particle size in the nanoscale and the agglomeration keep their flower shape.

Conclusion
In this work, it can be concluded that Terminalia catappa leaf extract can be used as a reductor to synthesize ZnO nanoparticles.UV-vis spectroscopy characterization indicates that the highest absorbance of all samples occurs at the wavelength interval of 300-380 nm.The XRD spectrum analysis explained that the ZnO crystal structure is hexagonal wurtzite, while SEM analyses confirmed nanoflower morphology.From the analysis of the optical, structural, and morphological properties of ZnO nanoparticles, this study can be applied to photocatalytics.The novelty of this study is that no ZnO nanoparticle synthesis has ever been carried out using Terminalia cattapa leaf extract mediated by varying the heating temperature.Terminalia catappa extract is expected to be a natural capping agent in synthesizing flower-like ZnO nanoparticles.The bandgap energy of samples with different temperature heating was changed from 3.53 to 3.62 eV.The use of Terminalia cattapa leaf extract can be used as a medium as well as a green synthesis method of ZnO nanoparticles.

Figure 1 (
a) shows the colour of the Terminalia catappa leaf extract, which was initially like cooking oil changed to dark yellow and cloudy, as shown in Figure1(b).A colour change in a synthesis process indicates that a reaction has occurred in the solution by the excitation of the surface of the nanoparticles.ZnO nanoparticles are transparent in the visible range and adsorbed in the UV region, which implies the UV ability to stimulate ZnO.According to the literature[1], when the precursor is reduced, it causes a reduction in ZnO particle size.

Figure 1 .
Figure 1.Changes in color when: a) before adding NaOH and b) after adding NaOH, heating and stirring.

Figure 2 .
Figure 2. Optical absorbance of all sample ZnO nanoparticles.

Figure 4 .
Figure 4. SEM image of ZnO nanoparticles sample at temperature of 60°C with magnification (a) 10.000x and (b) 30.000x

Figure 5 .
Figure 5. TEM images of ZnO nanoparticles sample at temperature of 60°C (a) 50 nm and (b) 20 nm.