Volume 10, Issue 3, 2020, 5648 - 5655 ISSN 2069-5837 Biointerface Research in Applied Chemistry www.BiointerfaceResearch.com https://doi.org/10.33263/BRIAC103.648655 Original Research Article Open Access Journal Received: 14.02.2020 / Revised: 20.03.2020 / Accepted: 21.03.2020 / Published on-line: 29.03.2020 Synthesis and characterization of single phase ZnO nanostructures via solvothermal method: influence of alkaline source 1, 3, * 1, * 1 2 Eric Kwabena Droepenu , Boon Siong Wee , Suk Fun Chin , Kuan Ying Kok , Ebenezer 1,3 Aquisman Asare 1Resource Chemistry Program, Faculty of Resource Science and Technology, Universiti Malaysia Sarawak 94300, Kota Samarahan, Sarawak, Malaysia 2Malaysian Nuclear Agency, Bangi, Kajang, 43000 Selangor, Malaysia 3Graduate School of Nuclear and Allied Sciences, University of Ghana, AE1, Kwabenya-Accra, Ghana *corresponding author e-mail address: kobladodzie01@yahoo.com; swboon@unimas.my | Scopus ID 57194506096 ABSTRACT Single phase ZnO nanostructures were synthesized by simple and low temperature solvothermal process from two different alkaline sources; Potassium hydroxide (KOH) and Sodium hydroxide (NaOH) with zinc acetate dihydrate (Zn(CH3COO)2∙2H2O) as precursor. This facile and rapid synthesis technique achieve high purity of Zinc oxide (ZnO) nanostructures on large scale negating the use of complex and high temperature routes. The synthesized particles were characterized by X-Ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FE-SEM), Transmission Electron Microscopy (TEM), Energy-dispersive X-ray spectroscopy (EDX), Fourier Transform Infrared (FT-IR) Spectroscopy, Ultraviolet Visible (UV-Vis) spectroscopy and Brunauer-Emmett-Teller (BET) analysis. ZnO synthesized using KOH and NaOH exhibit wurtzite hexagonal and flake-like nanostructures with average crystallite size of 11.0 nm and 14.9 nm respectively. Surface area of 59.50 m2/g and 31.43 m2/g were determined for KOH and NaOH sources respectively. The optical absorption spectra of the two samples showed absorption bands of 367.70 and 365.30 nm. The results showed the effect of alkaline sources on the surface morphology, structural and optical properties of ZnO. Keywords: Nanostructures; Solvothermal synthesis; Alkaline source; Precursor; Microscopy; Spectroscopies. 1. INTRODUCTION Technology today endeavor the use of crystalline particles sources used, temperature variation, reaction time for nucleation in nanometric scale with uniform size and shape due to their of the nanoparticles as well as the calcination temperatures [28- numerous fascinating properties including catalytic, electrical and 30]. It should be noted that some of the methods employed in optical; which are desired in many promising applications. Among fabricating these nano-sized particles require complex processes these, ZnO has attracted much attention because of its wide and sophisticated equipment. Solvothermal technique, however, versatility in various fields such as pharmaceutical, electronics, offers many great advantages over other techniques mentioned cosmetics, optical and electrical devices, drug delivery and above. This technique requires simple setup, less expensive environmental remediation [1-6]. equipment, relatively low synthesis temperature to yield a large ZnO has many interesting properties including its wide area of deposition. Also, it allows the microstructural control of band gap (3.37 eV), high bond strength, high thermal stability and the particles produced by altering experimental parameters such as large exciton binding energy (60 meV), which are required in temperature, reaction time, type of solvent, surfactant and electronics, optoelectronics and laser technologies [7-13]. Besides, precursor. Qualities of nanostructures in the manufacture of ZnO also possesses good electrical, optical and antibacterial devices such as sensors, photo diodes, transistors are based on the properties which are pre-requisite in the photoconductors, shape, size and purity of the synthesized nanoparticles. These integrated sensors, electrodes and biomedicine industries [14-18]. properties can also be achieved using simple experimental set-ups Different routes have been used in synthesizing with minimum reaction conditions not necessarily using highly nanocrystalline ZnO powders as well as other nanostructures such complex equipment under very high reaction conditions. It is as hydrothermal, spray pyrolysis, sol-gel, precipitation, microwave against this backdrop that the current study employed two alkaline assisted, thermos-decomposition processes, micro-emulsion, sources (KOH and NaOH) in synthesizing ZnO NPs with Zinc electrodeposition, ultrasonic, microwave-assisted technique and acetate dihydrate precursor in a solvothermal method at relatively chemical vapour deposition [19-27]. All these methods produce lower reaction conditions to achieve high quality ZnO similar nanocrystalline structures based on different conditions. nanostructures. The quality of the ZnO produced was investigated These conditions may arise from the precursor and alkaline using a series of physiochemical characterization tools. 2. MATERIALS AND METHODS 2.1. Materials. Ethanol (C2H5OH). All chemicals were of analytical reagent grade Zinc acetate dihydrate [Zn(CH3COO)2∙2H2O], Potassium from Sigma-Aldrich without further purification. hydroxide (KOH), Sodium hydroxide (NaOH), 95% Absolute Page | 5648 Synthesis and characterization of single phase ZnO nanostructures via solvothermal method: influence of alkaline source 2.2. Synthesis of Zinc oxide (ZnO) Nanoparticles (NPs). analysis was carried out using the method outlined by [32]. The Synthesis of ZnO NPs was based on the solvothermal powdered solid ZnO NPs were coated on a silicon wafer with the technique reported by [31] with slight modification. A weighed help of a carbon tape. Before the FE-SEM imaging, the dry mass of 1.83 ± 0.1 g (0.01 mol) of [Zn(CH3COO)2∙2H2O], [Sigma- powders were mounted on a stub followed by coating with Aldrich, India] was dissolved in 100 mL of absolute ethanol in a Platinum (Pt) for 1 minute using a sputter coater. TEM analysis 250 mL Schott bottle and heated to a temperature of 60 °C with was done using TEM (JEOL 1230, Japan). Powdered ZnO NPs constant stirring using electrical stirring hotplate (Favorit). were first diluted with absolute ethanol (95%) and sonicated with Subsequently, 5.60 ± 0.1 g (0.01 mol) of KOH (VWR Amresco, ultrasonic cleaner (Elma, Germany) for 30 minutes. A 4 µl of the US) was also weighed and dissolved in 100 mL of absolute solution sample was loaded onto a Foamvar film Copper grid ethanol in 250 mL Schott bottle under the same condition as the (FF300-Cu) before being observed under TEM. The purity of the Zinc acetate. After complete dissolution, the alkaline solution ZnO NPs was determined with EDX (JEOL 6390LA, Japan). The (KOH) was slowly drained dropwise from a burette into the ZnO NPs were diluted with 95% Absolute ethanol and sonicated Zn(CH3COO)2∙2H2O solution, maintaining the temperature at 60 with ultrasonic cleaner (Elma, Germany) for 30 minutes. A 4 µl of °C with vigorous stirring for 60 minutes until white precipitate ZnO NPs sample was loaded onto an aluminium plate before was formed. The mixture was allowed to cool at room temperature being analyzed. for 180 minutes before centrifuging with FLETA 5 Multi-Purpose FT-IR (Thermo Scientific Nicolet iS10, US) and UV-Vis Centrifuge at 4000 rpm for 30 minutes. The precipitate was (UV-1800 SHIMADZU UV Spectrophotometer) was used to filtered using 0.45 μm Whatman filter membrane, washed twice determine the surface functional groups and the optical property of with acetone and then with deionized water, dried at room the sample. The FT-IR sample preparation involved mixing the temperature and finally ground in a powdery form for powdered ZnO with Potassium bromide (KBr) in the ratio of 1: 19 characterization. The same process was repeated for the NaOH [33]. The sample was then placed in the metal hole, pressed until (VWR Amresco, US) where 4.0 ± 0.1 g (0.01 mol) was weighed the sample compressed inside the hole, and analyzed using FT-IR. into 100 mL of 95% absolute ethanol under the same reaction In the case of UV-Vis, the spectral range of 4000–400 cm-1 with conditions. resolution of 4 cm-1 was used. The optical property of the samples 2.3. Characterization of synthesized ZnO. was determined by measuring their maximum absorbance using The synthesized samples were characterized using X-ray UV–vis spectrophotometry. The ZnO-NPs were dispersed in Diffraction, XRD, (Xpert Pro MPD PW3040/60) for their crystal ethanol and sonicated for 10 minutes before it was used for the structure and crystallite size. Diffraction patterns from the XRD measurement. analysis in Figure 1 (a & b) were obtained using X-ray In addition, Brunauer-Emmett-Teller (BET) diffractometer with Cu-Kα radiation of 40 kV and 30 mA with (Quantachrome, US Autosorb iQ,version 2.01) was used for the step size of 0.017°. The morphology of ZnO NPs was determined surface area determination. Approximately, 0.3 g of ZnO NPs using Field Emission Scanning Electron Microscopy (Carl Zeiss powder was degassed at 175 °C for 2 hours [34] in a flowing N2 GeminiSEM 500) with acceleration voltage of 10.0 kV, the gas. The N2 absorption-desorption isotherms of the samples were working distance of 11.6 mm and a chamber pressure of 40 Pa. then be measured. Sample preparation for FE-SEM, TEM, EDX, FT-IR and UV-vis 3. RESULTS 3.1. Structural Properties. The average crystallite size of ZnO prepared using KOH 3.1.1. XRD Results. determined from the Panalytical X’Pert Pro MPD, diffraction peak The XRD pattern of the ZnO prepared by the solvothermal was found to be 11.0 ± 1.3 nm whereas the sample synthesized process at 60°C for 180 minutes from KOH and NaOH alkaline using NaOH was 14.9 ± 1.2 nm. When these results were source is shown in Figure 1 (a & b). All detectable peaks can be compared to a study by Ramachandra et al., the crystallite size indexed to ZnO wurtzite structure with ICSD Number (ICSD: 98- determined in their study was 43 nm and 45 nm using 000-9346) and PDF Number (Experimental and calculated powder hydrothermal method at a reaction temperature of 150 oC and diffraction data) of 36-1451 and 01-074-0534 respectively. The calcination temperature of 600 oC [38]. This is evident that, patterns are broadened due to the nanosize of the ZnO crystals despite the simplicity and moderate conditions employed in this [35,31,36]. Based on the XRD diffraction patterns, all synthesized study, smaller crystallite sizes could be produced. According to ZnO NPs were identified as being 100 % pure ZnO. The average [39], a crystallite size of 21.59 nm and 36.89 nm can be obtained crystallite size of the samples was calculated using Scherer’s for ZnO from KOH and NaOH, respectively in a sol-gel method. formula [37]. Their reaction time and drying temperature were maintained at 2 (1) hours and 70 oC. In another study by [24], an average crystallite size of 33 nm was recorded in a solvothermal method using Zinc where; acetate and Triethanolamine (TEA) media. In their study, the K = shape factor = 0.89 TEA/Zn2+ solution was autoclaved at 150 oC for 18 hours. √ (2) 3.1.2. FE-SEM and TEM Results. the peak broadening after removing the instrumental broadening. The FE-SEM and TEM images of ZnO nanoparticles β(FWHM) is the full width half maximum of the diffraction peak and prepared from KOH and NaOH alkaline sources are shown in β0 is the correction factor for instrumental broadening (0.07 o2θ). Figure 2 (a & b) and Figure 2 (c & d) respectively. The FE-SEM Page | 5649 Eric Kwabena Droepenu, Boon Siong Wee, Suk Fun Chin, Kuan Ying Kok, Ebenezer Aquisman Asare and TEM images of the two samples show agglomeration/aggregation of the particles. FE-SEM and TEM images of ZnO NPs synthesized from KOH source (Figure 2 a & c) confirm the formation of homogeneous hexagonal wurtzite structures. Figure 1. XRD diffractogram of ZnO nanoparticles synthesized using (a) KOH and (b) NaOH. Figure 2. FE-SEM images of synthesized ZnO NPs with (a) KOH and (b) The sample synthesized using KOH displayed morphology NaOH, alkaline source and TEM images of synthesized ZnO NPs with (c) similar to the nanostructures obtained by [24], when zinc acetate KOH and (d) NaOH alkaline source. and Triethanolamine (TEA) were used in a solvothermal method autoclaved at a temperature of 150 oC for 18 hours. Samples 3.1.3. FT-IR Results. shown in Figure 2a have a high surface area due to their small The FT-IR spectra of ZnO NPs synthesized from KOH and particle size, rendering them appropriate for catalytic applications NaOH are shown in Figure 3. The characteristic band of wurtzite [40]. ZnO in the two samples occurred at peaks of 420 cm -1 and 422 cm- 1 When NaOH was used under the same experimental respectively. This agrees with [51-53,33], who estimated the -1 conditions, the morphology changed to flake-like nanostructures range to be between 400-500 cm . as depicted in Figure 2 (b). Its corresponding TEM image, Figure 2 (d) also shows flake-like nanostructures. For nanocrystalline ZnO powder synthesized by sol-gel method, flake-like nanostructures were fabricated at a lower concentration as Polyvinylpyrrolidone (PVP) was used as a capping agent when Zn(ZnCH3COO)2∙2H2O was reacted with NaOH and calcined at a temperature of 600 oC [41]. In other study by Gopal and Kamila, ZnO flake-like nanostructures were obtained from a reaction of zinc nitrate and NaOH using precipitation technique at a calcination temperature of between 400-600 oC [42]. Apart from Gopal and Kamila, other studies that fabricated flake-like nanostructures using different synthesis techniques include [43- Figure 3. FT-IR spectra of ZnO NPs synthesized from KOH and NaOH 46]. In all these studies, complicated methods, expensive alkaline source. equipment with very critical reaction conditions were used as compared to this study. Table 1 below gives a summary of the Both samples show broad absorption bands in the peak -1 conditions and the particle size synthesized by the different range of 3500-3700 and 1400-1600 cm (in the case of ZnO from studies. NaOH) corresponding to O-H stretching and bending of hydroxyl Page | 5650 Synthesis and characterization of single phase ZnO nanostructures via solvothermal method: influence of alkaline source group [54-56]. But in the case of ZnO synthesized from KOH, were of high purity. Similar findings were also reported in peaks within the 1400-1600 cm-1 range diminishes. It can also be previous studies by [62,49,31]. observed that asymmetric and symmetric C=O stretching of zinc acetate is intense in ZnO synthesized from NaOH than that of KOH at peak range of 1640 and 1500 cm-1 [41]. This might be the effect of difference in pH which results in low rate of transformation thereby introducing some carboxylate (-COO-) functional group [57]. The –C – H bending in alkane is much more intense in the ZnO synthesized from NaOH than the one from KOH which occurs at 1400 cm-1. 3.1.4. UV-Vis Results The UV-Vis absorption spectra of ZnO NPs synthesized using NaOH and KOH respectively are illustrated in Figure 4 (a & b). The absorption spectra of the synthesized samples were acquired using UV-Vis spectrophotometer in the wavelength region of 300-400 nm. The spectrum reveals a strong characteristic absorption band of ZnO at wavelength of 367.70 nm and 365.30 nm for the KOH and NaOH alkaline source respectively. The absorption depends on factors such as band gap, oxygen deficiency, size and structure of the nanoparticles, surface roughness and impurity centers [58]. The good absorption characteristics of the ZnO-NPs in the UV region proves its suitability in applications such as sunscreen protectors or antiseptic ointments [59-60]. Figure 5. EDX spectra of ZnO NPs synthesized using (a) KOH and (b) NaOH. 3.1.6. BET Results Figure 6 (A and B) shows the BET analysis isotherms of ZnO NPs synthesized using KOH and NaOH. Figure 4. Optical absorption Spectra of ZnO NPs synthesized using (a) KOH and (b) NaOH. 3.1.5. EDX Results. EDX spectroscopy was used to provide elemental analysis Figure 6. Nitrogen (N2) adsorption- desorption isotherms of ZnO NPs of the particles and is displayed in the spectra in Figure 5 (a & b). synthesized using (A) KOH and (B) NaOH alkaline sources. The spectra show the presence of two main elements, namely Zn and O in the proportion of 75.2%: 24.8% and The specific surface area was also determined to be 59.50 2 2 73.9%:26.1% for samples A and B respectively. When these data m /g and 31.43 m /g for the zinc oxide NPs synthesized from were compared to the theoretical value of Zn and O (80.3:19.7) by KOH and NaOH respectively. This suggests that sample A shows [61], it can be concluded that the synthesized ZnO nanostructures a large surface area as compared to sample B. Furthermore, the Page | 5651 Eric Kwabena Droepenu, Boon Siong Wee, Suk Fun Chin, Kuan Ying Kok, Ebenezer Aquisman Asare average particle size calculated from BET data for sample A due where; is the average particle size determined by BET, ρ is to its spherical shape was 16.5 nm based on the equation; the theoretical density of the sample which was 6.11 g cm–3, and (3) SABET is the obtained surface area [34,63]. Table 1. Characteristics of ZnO nanostructures synthesized by different studies. Precursor Zinc and Alkali salt Synthesis condition Properties Reference o Zn(CH3COO)2∙2H2O + KOH Reaction temp: 60 C Ave. crystallite size: 11.0 Current study Reaction time: 3 h Drying temp: room temp. Zn(CH3COO)2∙2H2O + NaOH Same conditions as above. Ave. crystallite size: 14.9 o Zn(CH3COO)2 + NaOH Reaction temp: 25 C Particle size: 97-174 nm [42] o Drying: 100 C 4 h Ave. crystallite size: 23.04 nm o Calcined: 400 C 2 h Reaction time: 7 h Synthesis time: 8 h Zn(NO3)2 + NaOH Synthesis condition same as the above Particle size: 52-93 nm Ave. crystallite size: 19 nm Synthesis condition same as the above Zn(SO4)2 + NaOH Particle size: 49-179 nm Ave. crystallite size: 37 nm o Zn(CH3COO)2 + (TEA) + NaOH Reaction temp: 60 C Particle size: 48 ± 7 nm [22] o Drying: 60 C overnight Ave. crystallite size: 23 ± 2 nm o Calcined: 150 C 18 h o Reaction temp: 150 C Pore diameter size: 9-12 nm [38] Zn(NO3)2∙6H2O + NaOH Reaction time: 7 h Ave. crystallite size: 45 & 43 nm o Calcined: 600 C 1 h Zn(CH3COO)2∙2H2O + Reaction time: 1 h Ave. crystallite size: 45, 48, 49, & 56 nm [41] o Polyvinylpyrrolidone (PVP) + Drying: 60 C for different conc. of PVP o NaOH Calcined: 600 C 1 h Particle size: 150 nm Zn(CH3COO)2∙2H2O + N2H4 Irradiation: 15 min 510W [43] 10 min 680 W o Drying: 100 C 2 h Needle shaped structure Zn(NO3)2∙6H2O + NaOH + NH3 Irradiation: 15 min 150 W Flower shaped structures o Drying: 100 C 2 h Particle diameter: 50-150 nm o Calcination: 600 C 3 h Zn(CH3COO)2∙2H2O + CTAB + Particle size: 23.7-88.8 nm o NaOH Reaction temp: 25, 35, 55 & 75 C Crystallite size: 23.7, 82.5, 69.6 & 88.8 [45] nm o Zn(CH3COO)2∙2H2O + NaOH Reaction temp: 90 C Crystallite size: 75 & 54 nm [46] Reaction time: 2 h Zn(CH3COO)2∙2H2O + (CTAB) + Reaction time: 50 min Particle diameter: 10-30 nm o NaOH Reaction temp: 25 C Particle length: 150-250 nm [47] o Drying: 60 C 2h Zn(CH3COO)2∙2H2O + KOH Reaction time: 3 h Particle size: 7.4 ± 1.2 nm [31] o Reaction temp: 60 C Crystallite size: 10.08 nm Drying: room temp. Zn(CH3COO)2∙2H2O + NaOH Reaction time: 1-3 h [48] o Conc. ratio (1:1, 1:2, 1:4, 1:8) Reaction temp: 120 C Crystallite size: 300 nm to 10 μm o Drying: 100 C Zn(CH3COO)2∙2H2O + NaOH Reaction temp: room temp Crystallite size: 20-36 nm [49] o Drying temp: 100 C 5 h Particle size: 18-36 nm o Calcination: 250 C 3 h Zn(NO3)2 + NaOH Crystallite size: 36.89 and 21.59 nm [39] Particle size: 17-25 nm Reaction time: 2 h + 24 h o Drying: 70 C for several hours Crystallite size: 21.59 nm Zn(NO3)2 + KOH Particle size: 30-50 nm Zn(CH3COO)2∙2H2O + NaOH Crystallite/Particle size: 23.04 nm/97- [28] o Reaction Temp: 25 C 174 nm Zn(NO3)2 + NaOH Reaction time: 7 h Crystallite/Particle size: 19.00 nm/52-93 o Drying: 100 C for 4 H nm o Zn(SO4)2 + NaOH Calcination: 400 C for 2 h Crystallite/Particle size: 37.00 nm/49- Synthesis time: 8 h 179 nm o Zn(CH3COO)2∙2H2O + C2H4(OH)2 Reaction temp: 70 C Particle size: 25-50 nm [50] Microwave radiation: 600 W, 2.45 GHz Crystallite size: 23-48 nm o Reaction time: 25 min 220 C Drying: freeze drying Page | 5652 Synthesis and characterization of single phase ZnO nanostructures via solvothermal method: influence of alkaline source 4. CONCLUSIONS Single phase ZnO nanostructures were successfully m2/g was determined for ZnO NPs obtained from KOH and NaOH synthesized by a simple and low temperature solvothermal process sources respectively. The optical absorption spectra of the two from two different alkaline sources (KOH and NaOH) with zinc samples showed absorption bands of 367.70 and 365.30 nm as an acetate dehydrate (Zn(CH3COO)2∙2H2O) as the precursor. The indication for a pure ZnO NPs. EDX has also proven the purity of synthesis technique achieved a high purity ZnO nanostructures synthesized samples to contain high Zn and O element with wurtzite hexagonal and flake-like nanostructures of average composition. The study showed that the type of alkaline source crystallite size of 11.0 nm and 14.9 nm for the two alkaline source used has a significant effect the surface morphology, structural respectively. The average surface area of 59.50 m2/g and 31.43 and optical properties of ZnO NPs. 5. REFERENCES 1. 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Imran, K.; Khan, S.; Nongjai, R.; Ahmed, H.; Khan, W. synthesis of benzo[b][1,5]diazepines Cogent Chemistry 2015, 1, Structural and optical properties of gel-combustion synthesized https://doi.org/10.1080/23312009.2015.1095060. 6. ACKNOWLEDGEMENTS The authors acknowledge the contribution of colleagues from Faculty of Resource Science and Technology (FRST), Geochemistry Laboratory and Analytical Laboratory, Universiti Malaysia Sarawak. This research was supported by Universiti Malaysia Sarawak, Tun Openg Chair, with Research Grant Code: F07/TOC/1738/2018. © 2020 by the authors. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). Page | 5655