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ISSN : 1225-0562(Print)
ISSN : 2287-7258(Online)
Korean Journal of Materials Research Vol.31 No.9 pp.481-487
DOI : https://doi.org/10.3740/MRSK.2021.31.9.481

# Effect of Additive Ammonium Hydroxide on ZnO Particle Properties Synthesized by Facile Glycol Process

Kongsy Phimmavong1, Seok-Hyoung Hong1, Jeong-Hwan Song2
1Department of Materials Engineering, Graduate School of PaiChai University, Daejeon 35345, Republic of Korea
2Department of Materials Science and Engineering, PaiChai University, Daejeon 35345, Republic of Korea
Corresponding author E-Mail : song_jeonghwan@pcu.ac.kr (J.-H. Song, PaiChai Univ.)
August 5, 2021 August 30, 2021 September 1, 2021

## Abstract

ZnO particles are successfully synthesized at 150 °C for 30 min using zinc acetate as the Zn source and 1,4- butanediol as solvent using a relatively facile and convenient glycol process. The effect of ammonium hydroxide amounts on the growth behavior and the morphological evolution of ZnO particles are investigated. The prepared ZnO nanoparticle with hexagonal structure exhibits a quasi-spherical shape with an average crystallite size of approximately 30 nm. It is also demonstrated that the morphology of ZnO particles can be controlled by 1,4-butanediol with an additive of ammonium hydroxide. The morphologies of ZnO particles are changed sequentially from a quasi-spherical shape to a rod-like shape and a hexagonal rod shape with a truncated pyramidal tip, exhibiting preferential growth along the [001] direction with increasing ammonium hydroxide amounts. It is demonstrated that much higher OH− amounts can produce a nano-tip shape grown along the [001] direction at the corners and center of the (001) top polar plane, and a flat hexagonal symmetry shape of the bottom polar plane on ZnO hexagonal prisms. The results indicate that the presence of NH4+ and OH− ions in the solution greatly affects the growth behaviors of ZnO particles. A sharp near-band-edge (NBE) emission peak centered at 383 nm in the UV region and a weak broad peak in the visible region between 450 nm and 700 nm are shown in the PL spectra of the ZnO synthesized using the glycol process, regardless of adding ammonium hydroxide. Although the broad peak of the deep-level-emission (DLE) increases with the addition of ammonium hydroxide, it is suggested that the prominent NBE emission peaks indicate that ZnO nanoparticles with good crystallization are obtained under these conditions.

## 초록

This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

## 1. Introduction

ZnO is a II–VI binary compound semiconductor with a direct wide band gap energy (3.37 eV) and a strong exciton binding energy (60 meV) at room temperature.1.2) ZnO becomes one of the leading contenders in micro-/ nano-optoelectronic industries materials due to these advanced properties such as inexpensive cost, environmentfriendly nontoxicity, thermal stability, high transparency, and material abundance. It is one of the most attractive materials for numerous applications in cosmetics, drug delivery, ultraviolet lasers, field-effect transistors, gas sensors, biosensors, photocatalysts, field-emission displays, solar cell windows, and piezoelectric devices.3-12)

In order to obtain ZnO with appropriate chemical and optoelectronic properties for their intended applications, control of the preferential crystal face or morphology and the particle size take part in a crucial role. Numerous studies have specifically demonstrated the synthesis of ZnO nanostructures with various morphologies including zero-dimensional (spherical-like), one-dimensional (rod, tube, wire), two-dimensional (disk, sheet, hexagon, tower, comb), and multi-dimensional (flower) structures by using various chemical precursors, the aid of surfactants, and different processing methods.3,8,13-18)

The various methods have been performed for the preparation of ZnO including thermal evaporation, chemical vapor deposition, sol–gel method, wet chemical synthesis, precipitation, microwave synthesis, and the hydrothermal method.18-25) The above methods require complex equipment and processes to manufacture the particle. Nevertheless, ZnO particles continue to capture much interest of researchers due to their distinctive and novel morphologies and remain a challenge for fabricating various ZnO micro-/nano-structures by a facile process. The glycol process has proven to be useful in the synthesis of metal oxide nanoparticles because of the low reaction temperature and short reaction time without the need for a mineralizer for processing.26,27) In addition, the shape and size of the obtained particle by the glycol process for synthesizing the anhydrous crystalline materials in a nonaqueous solvent using various glycols can be easily controlled without growth directing agents. The glycol of non-aqueous solvents with multiple functions acts as a stabilizer, reducing agent, and suppresses agglomeration.

We know that the properties of ZnO during processing strongly depend on the synthesis method and conditions. This study reports a glycol process for the facile and convenient synthesis of highly crystallized ZnO particles at a low reaction temperature without equipment such as an autoclave using 1,4-butanediol as a glycol solvent. The influence of additive ammonium hydroxide on the structural, morphological, and optical characteristics of the synthesized ZnO particles was studied systematically.

## 2. Experimental Procedure

The ZnO particles were successfully synthesized by a glycol process using a glycol solvent, as our previous study described.27) In this study, zinc acetate dihydrate [Zn(CH3COO)2·2H2O] and 1,4-butanediol (C4H10O2) were used as a Zn source and as a non-aqueous solvent. The ammonium hydroxide was used as an additive with various amounts for controlling the morphology of ZnO particles that were obtained by the glycol process. 0.05 mol of zinc acetate dihydrate was dissolved in 100 mL of 1,4- butanediol. The solution was transferred into a refluxing system of a 1,000 mL 3-necked round-bottom glass flask with a heating mantle. Then, to investigate the effect on the phase and morphology of the obtained particles, the different amount of ammonium hydroxide was added into the above-formed solution under vigorous stirring at room temperature for 30 min. The varied amounts of ammonium hydroxide present in the precursor solution were 5 mL, 10 mL, 20 mL, and 30 mL. Afterward, the glycol reactions were performed under a constant stirring rate at a low temperature of 150 °C and in a very short refluxing time of 30 min. The solution gradually changed to a milky color. A white precursor was precipitated in the solution. After the glycol reaction, the glass flask was cooled to room temperature. The precipitated particles were washed repeatedly at least three times using centrifugation and redispersion in ethanol, and then dried at 100 °C in a dry oven for one day. The obtained particles were labeled as ZnO-0, ZnO-5, ZnO-10, ZnO-20, and ZnO-30 from varying amounts of ammonium hydroxide of 0 mL, 5 mL, 10 mL, 20 mL, and 30 mL, respectively.

The dried particles for crystal structure and crystallization were analyzed using X-ray diffraction (XRD-D1w, Cu Kα, 30 kV–30 mA, Shimadzu, Japan). The scanning was performed in the 2θ range from 10° to 80° with a scan speed of 2°/min. The morphology and size of the synthesized ZnO particles were observed using field emission scanning electron microscopy (FE-SEM, S-4800, Hitachi, Japan) and transmission electron microscopy (TEM, JEM- 2100F, JEOL, Japan). The diffuse reflectance spectroscopy (DRS) and photoluminescence (PL) spectra of the ZnO particles were characterized using an ultraviolet-visible (UV–Vis) spectrometer (JASCO V550, Japan) and a He– Cd laser with an excitation wavelength of 325 nm (LabRAM, Horiba, Japan) at room temperature.

## 3. Results and Discussion

The XRD patterns of each particle synthesized by the glycol process with various amounts of ammonium hydroxide at 150 °C for 30 min are shown in Fig. 1. The XRD peaks of all the prepared particles can be indexed to a hexagonal wurtzite structure of ZnO regardless of adding ammonium hydroxide, and the entire pattern matches the reference peaks reported on JCPDS Card No. 36-1451. No characteristic peak of other impurities and residues could be observed in all the ZnO particles, which indicates that the synthesized particles have high crystal phase purity. Upon increasing the amount of ammonium hydroxide, the diffraction peaks of the ZnO became more intense and narrower, reflecting growth in the crystallite size of the particle. It was reported that the addition of ammonium hydroxide improves the stability and homogeneity of the solution by controlling hydrolysis and nucleation.28) An increased basic by the release of more OH ions into the solution accelerates hydrolysis and nucleation, and provides a favorable environment for quickly grown ZnO particles. As such, the higher diffraction peak intensity exhibited by adding ammonium hydroxide could be attributed to the sufficient availability of OH to form the ZnO.

The lattice constants (a and c) of a unit cell for all the ZnO particles were calculated from peaks (100) and (002) in their XRD patterns using the following relation formula.

$1 d hkl 2 = 4 3 [ h 2 +hk+k 2 a 2 ] + ( 1 c ) 2$

In an ideal hexagonal close-packed (hcp) structure of ZnO, the ratio of the lattice constant (c/a) is 1.6330. All the ZnO particles showed a significantly smaller c/a ratio in the range of 1.6029 - 1.6037 as compared to that of bulk ZnO, which might indicate the presence of oxygen vacancies and/or zinc vacancies. In addition, the volumes (V) of the unit cell and the length (L) of the Zn–O bond in all the ZnO particles were estimated by means of the relation. Where, u is the positional parameter for the hexagonal structure,

V = 0.866 × a2 × c

The calculated magnitudes by the volume of the unit cell and the internal parameter values did not noticeably change as the amount of the ammonium hydroxide varied.

The crystallite sizes of the ZnO particle were calculated from the higher intensity diffraction peak corresponding to the three major peaks using Scherrer’s equation.

where D is the average crystalline size, k (=0.89) is Scherrer’s constant, λ (=0.15406 nm) is the wavelength of Cu, β is the full-width at half–maximum (FWHM), and θ is the diffraction angle of the center of the peak. The average crystallite sizes of the synthesized ZnO with added ammonium hydroxide of 0 mL, 5 mL, 10 mL, 20 mL, and 30 mL are 26.3 nm, 28.7 nm, 33.5 nm, 38.1 nm, and 36.9 nm, respectively. All the above-mentioned calculated structural parameters are shown in Table 1.29,30)

FE-SEM images of each particle synthesized at 150°C for 30 min using the glycol process when the amount of ammonium hydroxide increased from 0 mL to 30 mL are presented in Fig. 2. Our previous study showed that in the case of excluding the addition of ammonium hydroxide, the obtained ZnO nanoparticles were synthesized the morphology of the quasi-sphere and the size of approximately 30 nm as shown in Fig. 2(a).27) When the added amount of ammonium hydroxide is 5 mL, the ZnO nanorods begin to grow along the c-axis preferentially and have a diameter of around 40 nm and a length of about 100 nm as shown in Fig. 2(b). In the early growth stage, ZnO is grown along the [001] direction due to the polar of the (001) basal plane and the lowest surface energy of the (002) facet, resulting in a rod-like shape. In adding a 10 mL amount of the ammonium hydroxide, the morphology and size of ZnO nanoparticles were changed from a rod-like shape to a hexagonal rod with a truncated pyramidal tip, and increased to about 60 nm–80 nm in diameter and 100 nm - 120 nm in length, respectively as shown in Fig. 2(c). Fig. 2(d) shows that the ZnO particles are composed of a number of irregular hexagonal rods with non-uniform tip-shape nanostructures. The size of these particles increased to 100 nm - 150 nm in diameter and 150 nm - 200 nm in length as the addition of ammonium hydroxide provided more NH4+ ions and OH ions. It can be seen that the growth rates, which oriented along the (101) and (100) planes, are relatively slow compared to the preferential growth from the (001) plane.31) Furthermore, when the ammonium hydroxide was added to a sufficient amount of 30 mL, the shape of the ZnO particles changed to a hexagonal prism with a nano-tip of one crystal plane and a flat hexagonal symmetry of the other crystal plane. The size of these particles slightly increased to approximately 150 nm in diameter and 250 nm in length as shown in Fig. 2(e).

When the amount of ammonium hydroxide is sufficient, the Zn2+ ions react with OH, and NH4+ ions lead to the formation of Zn(OH)2 or $Zn ( OH ) 4 2-$ ions, and [Zn(NH3)4]2+ ions.

The growth of ZnO is crystallized as a result of the hydrolysis of [Zn(NH3)4]2+ complex ions and OH ions, and the hydroxide-oxide conversion of $Zn ( OH ) 4 2-$ ions.

Zn and O atoms of the hexagonal ZnO structure are stacked alternately along the c-axis. It is clearly seen that the ZnO particles can be grown by suppressing the preferential growth along the [001] direction due to the adherence of more NH4+ ions onto the Zn2+-terminated and O2−-terminated polar faces of ZnO, and allowing the lateral growth in the <010> direction.28,31-33) The (001) bottom plane of the ZnO hexagonal structure has a wellfaceted and flat hexagonal symmetry due to the adherence of NH4+ ions onto the O2−-terminated face. Furthermore, the top plane (001) consisting of tetrahedral Zn2+-terminated faces adheres to many more OH and NH4+ ions due to the addition of sufficient ammonium hydroxide amounts. This results in the top polar plane of ZnO hexagonal prisms with a nano-tip because of the different growth velocity of ZnO formation from [Zn(NH3)4]2+ complex ions and $Zn ( OH ) 4 2-$ ions under these conditions.

The TEM images of the ZnO particles synthesized by the glycol condition with the addition of 30 mL ammonium hydroxide at 150 °C for 30 min are shown in Fig. 3. The ZnO particles of the hexagonal prismatic with a nano-tip are well dispersed with a relatively narrow size distribution. Fig. 3(b) is the HRTEM image and corresponding selected area electron diffraction (SAED) pattern of the ZnO particle. This is a relatively regular hexagon plane, and the inset SAED pattern can be indexed as a hexagonal diffraction pattern along the [001] axis and also proves the wurtzite structure of the single crystal.

Fig. 4 depicts the UV-Vis diffused reflectance spectra (DRS) of the ZnO particles synthesized by glycol process with various amounts of ammonium hydroxide at 150 °C for 30 min. All the ZnO particles observe the reflectance spectra threshold in the UV region of about 380 nm to 400 nm. It was found that strong absorption suggests as ZnO particles potentially efficient materials for UV lightdriven applications. The reflectance of the obtained ZnO particles by adding 5ml ammonium hydroxide shows the highest reflectance of ~ 98 % over the entire visible spectrum. This is in contrast to the other ZnO particles that are decreased of incident light in the visible region with an increasing amount of ammonium hydroxide. The band gap energy was calculated using the following equation.

where K is the Kubelka-Munk absorption coefficient, hv is the photon energy, and R is the reflectance. The band gap energy is converted into [K·hν]2 vs. photon energy as shown in the inset of Fig. 4, and the band gap energy (Eg) of the ZnO particle was calculated by the extrapolation of the linear portion of the plot corresponding to [K·hν]2 = 0. In our previous study, the Eg of the ZnO nanoparticle that was synthesized by the glycol process without ammonium hydroxide was found to be 3.25 eV.27) The calculated Eg of ZnO increased to 3.27 eV and 3.28 eV, and the amounts of ammonium hydroxide were 5 mL - 10 mL, and 20 mL - 30 mL, respectively. The level of optical absorption strongly depends on particle size. The optical absorption edge slightly shifted toward a longer wavelength, which may be attributed to the increase in particle size as the amount of ammonium hydroxide increases as shown in the FE-SEM results. The size of the particles influences the ability of ZnO particles to absorb UV light. The band gap energies of the prepared ZnO particles are smaller than the known Eg of bulk ZnO (3.37 eV), which could relate to the optical absorption of structural defect levels within the ZnO crystal lattice. Some literature confirmed that the existence of structural defects of the ZnO particles with varying morphologies induced visible-light absorption to occur due to isolated states in the forbidden band gap of ZnO.34,35)

The PL spectra of the ZnO particles synthesized by the glycol process with various amounts of ammonium hydroxide at 150 °C for 30 min are presented in Fig. 5(a). All the PL spectra were measured under the same excitation density for direct comparisons. There are two dominant emission peaks for all ZnO particles. One is the strong PL narrow peaks centered at approximately 383 nm, which corresponds to the near-band-edge (NBE) emission of typical ZnO at room temperature with a band gap of 3.24 eV and is also in harmony with the reflectance spectra in Fig. 4. This NBE peak could be attributed to the exciton recombination of electrons at the bottom of the conduction band and holes at the uppermost of the valence band. Another broad emission from 450 nm to 700 nm related to the visible regions also appears. The deep-level-emission (DLE) is generally comprised of green and orange emission peaks, which are attributed to the presence of some intrinsic or extrinsic defect states within the ZnO lattice such as interstitial zinc and oxygen, electronic transitions from an interstitial Zn to a Zn vacancy, extrinsic impurities, and the singly and doubly charged oxygen vacancy.35) To further analyze the intensity variation of the DLE, all the PL spectra have been normalized to the intensity of NBE, as shown in Fig. 5(b). The normalized intensity of DLE shows an increase tendency with a specified amount of ammonium hydroxide, which indicates an increasing density of defects in ZnO particles with the larger amount of ammonium hydroxide. In our results, the strong UV-NBE should be attributed to high purity with crystallization of ZnO particles.

## 4. Conclusions

In summary, ZnO nanoparticles with a hexagonal structure were successfully synthesized by the glycol process at 150 ºC for 30 min regardless of adding ammonium hydroxide. The ZnO particles of the quasi-spherical shape have a crystallite size of approximately 30 nm without adding ammonium hydroxide. With an increasing ammonium hydroxide amount, the morphologies of ZnO particles are grown dramatically preferentially along the [001] direction from a rod-like shape to a hexagonal rod with a truncated pyramidal tip. By adding a sufficient amount of ammonium hydroxide, the amounts of NH4+ and OH ions increase, as does their adherence onto the polar plane of ZnO. As a result, the morphology of ZnO particles obtained with an increase of 30 mL of ammonium hydroxide develops into hexagonal prisms with a nanotip shape that is grown along the [001] direction at the corners and center of the top polar plane and a flat hexagonal symmetry shape of the bottom polar plane. These results demonstrated that the ZnO nanorods have a diameter of around 40 nm and a length of about 100 nm; the ZnO hexagonal rods with a truncated pyramidal tip have a diameter of about 60 nm - 80 nm and a length of 100 nm - 120 nm; and the ZnO hexagonal prisms with a nano-tip are approximately 150 nm in diameter and 250 nm in length. The variation of ammonium hydroxide amounts does not significantly change the structural parameters of the obtained ZnO particles. In the PL measurement, all the particles exhibit a strong exciton UV-NBE with a weak DLE peak related to defects. This suggests that the ZnO nanoparticles have good crystallization and optical properties under these conditions. Based on the results, it is demonstrated that the size and morphology of ZnO particles can be controlled by simply changing the ammonium hydroxide amount via the glycol process.

## Acknowledgment

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2018R1D1A1 B07050432).

## Figure

XRD patterns of the ZnO particles synthesized by the glycol process with various amounts of ammonium hydroxide at 150°C for 30 min: (a) 0 mL, (b) 5 mL, (c) 10 mL, (d) 20 mL, and (e) 30 mL.

FE-SEM images of the ZnO particles synthesized by the glycol process with various amounts of ammonium hydroxide at 150°C for 30 min: (a) 0 mL, (b) 5 mL, (c) 10 mL, (d) 20 mL, and (e) 30 mL.

(a) TEM images of the ZnO particles synthesized by the glycol condition with 30 mL ammonium hydroxide at 150°C for 30 min, (b) SAED pattern and HRTEM image.

UV-Vis DR spectra of the ZnO particles synthesized by the glycol process with various amounts of ammonium hydroxide at 150°C for 30 min: (a) 5 mL, (b) 10 mL, (c) 20 mL, (d) 30 mL; and a plot of (K × hυ)2 vs. energy () of the ZnO for the band gap determination (inset).

(a) PL spectra of the ZnO particles synthesized by the glycol process with various amounts of ammonium hydroxide at 150°C for 30 min, (b) the normalized PL spectra of samples, in which all the spectra are normalized to the intensity of NBE.

## Table

Structural parameters of the ZnO nanoparticles synthesized by the glycol process with various amounts of ammonium hydroxide.

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