Synthesis and Characterization of Zinc Oxide Nanoparticles with Small Particle Size Distribution

Solvothermal synthesis has shown to have a great potential to synthesize Zinc Oxide nanoparticles (ZnO NPs) with less than 10 nm size. In this study, we present a rapid synthesis of ZnO NPs in which ZnO NPs with more uniform shape and highly dispersed were synthesized using zinc acetate dihydrate (Zn(CH3COO)2 2H2O) and potassium hydroxide (KOH) as a precursor and absolute ethanol as solvent via solvothermal method. Few techniques were exploited to characterize synthesized ZnO NPs including X-ray diffraction (XRD), transmission electron microscope (TEM), Brunauer-Emmett-Teller (BET), energy-dispersive X-ray spectroscopy (EDX), fourier transform infrared (FT-IR) spectroscopy, and ultraviolet visible (UV-Vis) spectroscopy. Synthesized ZnO NPs that were prepared via solvothermal synthesis method at 60 °C for 3 hours exhibited a wurtzite structure with a crystalline size of 10.08 nm and particle size of 7.4 ± 1.2 nm. The UV-vis absorption spectrum has shown peak at 357 nm indicate the presence of ZnO NPs. Hence, better quality with uniform size ZnO NPs can be easily synthesized with reduced amount of time via solvothermal synthesis method rather than using other complicated and lengthy synthesis methods.


Introduction
High demands of nanomaterials have produced enormous applications in global industries.Due to high demand as NPs based products, various types of engineered nanoparticles (ENPs) are synthesized for myriad of applications. 1These days, ZnO NPs have become a promising candidates and gained more attention especially in nanomedicine and nano-semiconductors. [2][3][4] ZnO NPs exhibit wurtzite crystal structure that has been widely used in industries due to its unique optoelectric properties. 5nO NPs are among of various semiconductivity materials with a distinctive electronic and photonic wurtzite semiconductor with a wide direct band gap (3.37eV) and high exciton binding energy (60 meV) at room temperature. 68][9] Heiligtag et al. 10 stated that smaller size of NPs provide a better protection of skin against UV damage.Besides, high optical absorption UVA and UVB in ZnO NPs are also beneficial in antimicrobial products in nanomedicine as nowadays various nanomaterials development have been applied to improve drugs and other medicine. 11Among other MO NPs, Salem et al. 12 stated that ZnO NPs are the most recommended for antibacterial agent.Hence, the increase productions of consumer products eventually increase the productions of ZnO NPs.Heiligtag et al. 10 has also stated that the potential applications of ZnO NPs make them one of a primary focus in NPs research.Naveed Ul Haq et al. 13 also described that ZnO NPs is one of the cheap materials that this causes the extensive productions in industries.Morphologically, ZnO NPs is an attractive compound that possess thermal and chemical stability. 14ZnO NPs are made into various shapes and sizes depending on the use of NPs in industries including textile, energy, food, cosmetics, and medicines and other characteristics that make them attractive for broad range of application. 15arious synthesis methods of ZnO NPs were developed into different size and forms in order to be used in commercial products.This includes sol-gel method, precipitation, microwave assisted, and thermal oxidation. 4owever, these methods are considered complicated as they involve multiple steps procedures, lengthy reaction period, and toxic solvent and reactants might be used for synthesis. 16,17Prominent methods usually have undergone approximately 24 hours of reaction time to yield NPs products.For instance, Zarei et al. 18 has synthesized ZnO NPs via sol-gel method in which more than 12 hours and high calcined temperature were required to produce ZnO NPs.Other than that, ZnO NPs synthesis via room-temperature method was also lengthy, as at least 4 days of synthesis was needed to prepare ZnO NPs. 19Due to high cost and maintenance as well as lengthy preparation time to set up expensive equipment for synthesis, solvothermal synthesis was developed to synthesize NPs.Previous studies have carried out NPs synthesis via solvothermal method thus this study was carried out in order to support the method with some modifications mainly the use of absolute ethanol as a solvent.
According to Li et al., 20 solvothermal process is defined as performing chemical reactions in solvents under specific temperature.Matei et al. 21also stated that solvothermal synthesis can be easily performed under controlled condition as ZnO NPs can be synthesized into different morphologies depending on the reaction conditions.Bai et al. 22 has stated that solvothermal method has the ability in enhancing the dispersity of ZnO NPs.Besides, this process has been widely used specially to synthesize metal oxides NPs since it is more reliable, affordable, and undergo simpler process of synthesis. 23Wu et al. 17 also described that solvothermal synthesis method is free from using surfactant in which this offers a better potential for environmental friendly synthesis that can be produce in large quantities.Besides, solvothermal synthesis is considered one of the most promising approach to synthesize NPs. 20In solvothermal synthesis, organic solvent mainly alcohol such as ethanol as being used by previous researchers to synthesize ZnO NPs. 4,23,24urthermore, NPs characterization using XRD, TEM, BET, EDX, FT-IR spectroscopy, and UV-Vis spectroscopy are fundamental steps especially for examining NPs surface properties and functionality.It is important to characterize NPs in order to determine the behaviour of NPs for further study such as toxicological studies. 17Due to extensive usage of ZnO NPs, ZnO NPs were prepared via solvothermal synthesis by using zinc acetate dihydrate (Zn(CH 3 COO) 2 • 2H 2 O) as a zinc source and potassium hydroxide (KOH) as a precursor which dissolved in organic solvent mainly ethanol as only short reaction period is required for the synthesis.
Ethanol was used for ZnO NPs synthesis as it has hydroxyl group that interact better with NPs as well as increase solubility to allow more interaction between parti-cles and capping molecules. 25This method has utilized the organic solvent mainly ethanol which generally has low boiling point and generate high pressure that are conducive to obtain a better product crystallization. 26The utilization of absolute ethanol as a solvent has proven to be monodisperse size and shape of ZnO NPs via solvothermal methods.Wang et al. 26 has also explained that the presence of smaller surface tension of ethanol as compared to other alcohol has effectively contribute to inhibit the oxidation of powders thus uniform spherical ZnO NPs were formed.The aim of this research are to synthesize spherical ZnO NPs with less than 10 nm size by using zinc acetate dihydrate and potassium hydroxide with a absolute ethanol as solvent via solvothermal method and to characterize synthesized ZnO NPs using few techniques including XRD, TEM, BET, EDX, FT-IR, and UV-Vis spectroscopy.

1. Synthesis of Zinc Oxide Nanoparticles
Zinc oxide nanoparticles was synthesized using solvothermal synthesis process from modified published procedures. 24Briefly, 1.48 g of Zn(CH 3 COO) 2 • 2H 2 O (Sigma-Aldrich, India) was dissolved in 63 ml of absolute ethanol (HmBG Chemicals) in a 250 ml Schott bottle and was heated under 60 °C with constant stirring.0.74 g of KOH (VWR Amresco, US) was also dissolved separately in 33 ml of absolute ethanol in 100 ml Schott bottle under same condition of Zn(CH 3 COO) 2 • 2H 2 O.After both solutions have dissolved completely, dropwise, KOH was added into Zn(CH 3 COO) 2 • 2H 2 O slowly under 60 °C with vigorous stirring.The mixture solution was left for 3 hours until the reaction was completed.A white precipitate (ZnO) was formed and collected by centrifugation at 4000 rpm for 10 minutes, washed with acetone twice and ultrapure water once to remove all the impurities.The obtained product was then dried at room temperature and ground to form powder.

Characterization of ZnO Nanoparticles
Different techniques were used to characterize the synthesized ZnO NPs.Crystal structure and primary crystal size was characterized using XRD (Xpert Pro Diffractometer, Netherlands).The XRD pattern was obtained using X-ray diffractometer with Cu-Kα radiation of 40 kV and 30 mA with step size of 0.017°.
Other than that, the morphological features especially the size and the shape of ZnO NPs were determined using TEM (JOEL 1230, Japan).Basically, copper grid was prepared by applying fomvar coating on the copper grid.ZnO NPs were diluted with ethanol and sonicated with ultrasonic cleaner (Elma, Germany) for 30 minutes.Then, 4 µl of ZnO NPs sample was loaded onto the coated copper grid before being observed under TEM.Shamhariet al.: Synthesis and Characterization of Zinc Oxide ... Besides, Brunauer-Emmett-Teller (BET) (Quantachrome, US) was used to analyse the surface area of the synthesized ZnO NPs.About 0.3 g of ZnO NPs powder were placed in the tube and was allowed to degas at 175 °C for 2 hours as referred to Zhou et al. 27 in flowing nitrogen.The N 2 absorption-desorption isotherms of samples were then be measured.Energy-dispersive X-ray spectroscopy (EDX) was also being used for ZnO NPs characterization.EDX (JOEL 6390LA, Japan) was used in order to determine the purity of synthesized ZnO NPs.Meanwhile, Fourier transform infrared spectroscopy (FT-IR) was used in order to obtain the surface functional group that was present in ZnO NPs.ZnO NPs powder was mixed with potassium bromide (KBr) with ratio of 1: 19. 28The sample was then placed in the metal hole, pressed until the sample compressed inside the hole, and analysed using FT-IR (Thermo scientific Nicolet iS10, US).Ultraviolet visible spectroscopy (UV-vis) (Perkin Elmer Lamda 25) was also used in order to determine the optical absorption spectra of ZnO NPs that was dispersed in water.

1. X-Ray Diffraction (XRD)
XRD pattern of synthesized ZnO NPs is shown in Figure 1.Based on the XRD pattern, synthesized ZnO NPs has high purity of wurtzite crystalline structure as the diffraction peak is seen to be intense and narrower.This result was also being compared with the given standard XRD pattern of ZnO (JCPDS 36-1451) for confirmation purpose.The peak shown is broad which indicates that the particles is smaller which were also described in previous literature. 22,26Apart from that, as mentioned by Tagreed et al. 29 the average crystalline structure (D) was calculated according to Debye-Scherrer's formula: Where 0.89 refers to Scherrer's constant, is λ a wavelength of X-rays, θ refers to Bragg diffraction angle, and d is full width at half maximum (FWHM) of diffraction peak.The most intense diffraction was chosen which is <010> and the crystalline size of synthesized ZnO NPs was determined to be 10.08 nm.Besides, the percentage of zinc content from the synthesized ZnO NPs via the XRD analysis, which revealed that there is 99% of zinc without any other elements being detected as shown in Table 1.From this obtained data, it shows that synthesized ZnO NPs were determined to be of high purity.

2. Transmission Electron Microscope (TEM)
Physical characterization of NPs is commonly characterized using transmission electron microscope (TEM).Phoohinkong et al. 30 stated that TEM was carried out in order to obtain high accuracy of the actual particle size and shape pattern.This shows that TEM is one of the reliable tools for NPs characterization.The morphological feature of synthesized ZnO NPs was determined using TEM as shown in Figure 2. Size of less than 10 nm was obtained by using solvothermal synthesis method.About 200 particles were chosen randomly and measured.The calculated mean size of synthesized ZnO NPs was determined to be 7.4 ± 1.2 nm.TEM particle distribution result in Figure 2 also confirmed that a narrow size distribution of ZnO NPs can be obtained via solvothermal synthesis   method.Based on the particle size distribution in the figure, most of the NPs measured were determined to be 6, 7, and 8 nm which attribute to 23%, 29.5% and 25%, respectively.The biggest size of ZnO NPs measured is about 9 nm which only attribute to 10.5%.Besides, synthesized ZnO NPs that was observed under TEM showed a homogenous shape that seem to be near hexagonal or nanosphere that was also described from previous study in which solvothermal synthesis was being conducted by Zak et al. 23 which obtained 50 nm ZnO NPs.The synthesized ZnO NPs observed under TEM also correlates with the XRD patterns that reveal high purity of wurtzite crystalline structure of ZnO NPs.Therefore, this shows that solvothermal synthesis can be used to obtain a better image that proves the presence of less than 10 nm of synthesized ZnO NPs with high dispersity.

Brunauer-Emmett-Teller (BET)
BET was carried out in order to determine the specific surface area for three different sized of ZnO NPs by N 2 adsorption temperature of 77 K. Figure 3 shows nitrogen (N 2 ) adsorption-desorption isotherms of ZnO NPs obtained from BET analysis (Quantachrome, US). Figure 3 also shows a typical type IV adsorption obtained from synthesized ZnO NPs.The isotherm relative was observed to be relative flat and similar result was also obtained by Zhou et al. 27 .Zhou et al. 27 has described that the adsorption iso-therm was completely under superposition which usually occurs in micropores.
The specific surface area was also determined to be 101.32m 2 /g.Similar finding was also found from previous literature by Bian et al. 24 which obtained 105 m 2 /g for 4 nm ZnO NPs as measured by TEM.This shows that smaller NPs attribute to high surface area.Furthermore, the average particle can also be calculated from BET data.Since the shape of ZnO NPs was determined to be in spherical shape, average particle size can be calculated based on the equation D BET = 6000/ ρ•S w in which D BET is the average particle size, ρ is the theoretical density of the sample which was 6.11 g cm -3 , and S w is the obtained surface area as referred to Zhou et al. 27 and Ghasemzadeh et al. 31 .Table 2 summarised the BET results of ZnO NPs.The mean particle size of ZnO NPs obtained from BET (9.7 nm) is in agreement with the particle size obtained from TEM (7.4 nm) and XRD (10.8 nm).Thus, this confirms that the particle size of synthesized ZnO NPs was in nanoscale which is approximately 10 nm.

4. Energy-dispersive X-ray Spectroscopy (EDX)
The purity of ZnO NPs was determined via the EDX analysis.Figure 4 shows the EDX spectrum of ZnO NPs.EDX was used in order to determine the element composition that present in the samples.Result revealed that the  ).This result has confirmed that the ZnO NPs has high purity.Similar finding was also found in previous studies by Brintha and Ajitha 32 that obtained the mass percentage of Zn and O were 73.9% and 26.1 %, respectively.Hasnidawi et al. 33 has stated that the theoretical expected mass percent of Zn and O were 80.3% and 19.7%.Thus, the EDX result revealed that the synthesized ZnO NPs were of high purity, which contain high Zn and O element composition.

5. Fourier Transform Infrared Spectroscopy (FT-IR)
FT-IR was performed in order to study and determine the functional groups of synthesized ZnO NPs.Fig-ure 5 showed the FT-IR spectrum of synthesized ZnO NPs that were obtained from solvothermal synthesis procedure.FT-IR spectrum analysis was done by referring to Yang et al. 28 where a broad absorption band was observed at 414 cm -1 that attribute to Zn-O stretching vibration.In previous studies regarding ZnO NPs they were also able to observe FT-IR spectrum with the band around 400 cm -1 . 23,28,34ll the observed peaks were referred from previous literatures in order to confirm the findings.Similar findings were also found from previous studies related to ZnO NPs synthesis and characterization.The peaks at 1339 and 1556 cm -1 were symmetric and asymmetric O-C-O stretching vibration of adsorbed carbonate anion respectively.Meanwhile, the peaks at 1047 cm -1 that indicate the lattice vibration of carbonate generated absorption peaks.Besides, hydroxyl group stretching can be seen at the absorption peak of 3417 cm -1 .
Apart from that, peaks of 1402 cm -1 and 1339 cm -1 indicate the presence of Zn(CH 3 COO) 2 •2H 2 O that associate with CH 3 bending modes similar with result obtained by previous literature. 17Many ZnO NPs has been made using different types of synthesis method, however the obtained FT-IR spectrum regarding ZnO NPs synthesis have shown similarities. 35,36Therefore, FT-IR result has shown to be high purity of synthesized ZnO NPs.Wu et al. 17 has stated that this technique provides information about surface functional group that are present on surface that give a useful description of surface speciation.

6. UV-Vis Absorption Spectrum
UV-Vis spectroscopy was also performed to further confirm the formation of ZnO NPs.The absorption spectrum of synthesized ZnO NPs was shown in Figure 6.The UV-Vis measurement was performed after the ZnO NPs was dispersed in ultrapure water.The absorption peak was observed at 357 nm, which attribute to the intrinsic band-gap of Zn-O absorption.Similar result of absorption band that represent ZnO NPs was also obtained from previous research in which the range of ab-sorption band were from 355 to 380 nm as summarised in Table 3. 4,23,24,34,37 These supporting data confirm the presence of ZnO NPs as the absorption band obtained are similar.Wang et al. 26 also obtained similar findings which deduced that the obtained peak showed a better UV absorption for ZnO NPs.
Furthermore, the absorption peak of ZnO NPs also confirmed the properties of ZnO NPs, which is known for UV protections in sunscreens products. 38

5. Absolute Ethanol as a Solvent
From the obtained TEM result as shown in Figure 2, it shows that solvent also plays an important role for ZnO NPs synthesis.This includes the physico-chemical properties of ZnO NPs in terms of size and shape.The utilization of absolute ethanol as a solvent has formed a highly dispersed small ZnO NPs with uniform shape and size that was determined to be less than 10 nm as expected.Similar finding also described the formation of spherical shape of ZnO when ethanol was being used as a solvent. 14This study has shown that the absolute ethanol is one of the suitable solvent to be used to synthesize ZnO NPs via solvothermal synthesis method.Wang et al. 26 has found that different alcohol gives significant effect on ZnO NPs in terms of their morphology.Previous studies have also used other alcohol for ZnO NPs synthesis however, different forms of ZnO NPs were produced such as rod, flower-shaped, tubes, and etc. 22 Therefore, eminent production of ZnO NPs with uniform spherical shape with high dispersity can be easily obtained by utilizing ethanol as a solvent for solvothermal synthesis method.Other benefit of using absolute ethanol would be the short period of synthesis process needed to produce less than 10 nm of ZnO NPs with uniform size.

Conclusion
ZnO NPs with less than 10 nm (7.4 nm) was successfully prepared by using zinc acetate dihydrate and potassium hydroxide via the solvothermal synthesis process.The utilization of absolute ethanol as a solvent was able to produce uniform shape and better dispersity of ZnO NPs.Synthesized ZnO NPs were also able to be confirmed by various characterization techniques including XRD, TEM, FT-IR, and UV-vis spectroscopy.XRD has revealed a wurtzite crystalline structure of ZnO NPs where by physical characterization of ZnO NPs was determined by using TEM and size less than 10 nm of ZnO NPs was obtained.BET revealed that the synthesized ZnO NPs has high surface area that correlate with the particle size obtained from TEM. EDX has proven the purity of synthesized ZnO NPs which contain high Zn and O element composition.FT-IR and UV-vis spectroscopy also showed the absorbance spectrum that indicates the presence of ZnO NPs.Therefore, solvothermal synthesis method is one of the most suitable methods to obtain a better quality of ZnO NPs.This study also presents a potential effective method to prepare ZnO NPs within shorter time with smaller particle size distribution.

Acknowledgement
Authors sincerely thank MOHE grant FRGS/ ST01(01)/1208/2014(09) for the funding, Faculty of Resource Science and Technology (FRST) for providing the necessary facilities to carry out this research.Authors also want to thank Mdm Ting Woei for the TEM image results.

Figure 2 .
Figure 2. ZnO NPs image under transmission electron microscope (TEM) and particle size distribution of ZnO NPs.

Table 1 .
Pattern list of ZnO NPs obtained from XRD.

Table 2 .
BET results of synthesized ZnO NPs in comparison with other analytical techniques.

Table 3 .
UV vis absorption peak of ZnO from previous literatures.