ZnO nanoparticles with controlled shapes and sizes prepared using a simple polyol synthesis

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1 Superlattices and Microstructures 43 (2008) ZnO nanoparticles with controlled shapes and sizes prepared using a simple polyol synthesis Sul Lee a, Sunho Jeong a, Dongjo Kim a, Sookhyun Hwang b, Minhyon Jeon b, Jooho Moon a, a Department of Materials Science and Engineering, Yonsei University, Seoul , South Korea b Department of Nano Systems Engineering, Inje University, Gimhae, Gyongnam, , South Korea Received 25 November 2007; received in revised form 3 January 2008; accepted 3 January 2008 Available online 12 February 2008 Abstract Nanocrystalline zinc oxide (ZnO) particles with controlled shapes and sizes were prepared at 180 C by a simple polyol method. The amount of water and the method of addition played an important role in determining the characteristics of the synthesized particles. Rod-shaped ZnO particles with major axis lengths of 114 nm were obtained by heating the precursor solution, while equiaxial particles with average diameters of 24 nm were prepared by injecting water into hot precursor solution. Increasing the amount of water added to the precursor solution enlarged the aspect ratio of the rod-shaped particles and increased the particle size of the equiaxial particles due to enhanced hydrolysis and condensation of the Zn ion complex. c 2008 Elsevier Ltd. All rights reserved. Keywords: Zinc oxide; Nanoparticles; Particle synthesis; Polyol process; Shape control 1. Introduction Zinc oxide (ZnO) is a direct wide band gap semiconductor (E g = 3.37 ev) which makes it an excellent candidate for use in UV-light-emitting diodes (LEDs), lasers, and transparent transistors [1,2]. In addition, ZnO is used for various applications, such as in catalysts, gas sensors, varistors, and actuators [3]. In particular, the conduction band of ZnO is primarily composed of large, metal 4s orbitals spread out spatially with isotropic shapes, such that Corresponding author. Tel.: ; fax: address: jmoon@yonsei.ac.kr (J. Moon) /$ - see front matter c 2008 Elsevier Ltd. All rights reserved. doi: /j.spmi

2 S. Lee et al. / Superlattices and Microstructures 43 (2008) direct overlap among the neighboring, metal orbitals is possible [4]. The unique properties of the conduction band led to recent interest in ZnO as a channel material for thin film transistors (TFTs) and, thus, as an alternative to conventional Si-based materials and organic semiconducting materials. Most of the current high-performance TFTs involve either lowtemperature polycrystalline Si (LTPS) or hydrogenated amorphous silicon (a:si-h). However, both of these Si-based thin films are difficult to fabricate using solution processes, such as spin coating and ink-jet printing, which enable the economical production of TFTs. On the other hand, organic semiconductors, such as α,ω-dihexylquaterthiophene (DH4T) and poly- 3-hexylthiophene (P3HT), are suitable for solution-based processing of transistors, but their mobility values are still inadequately low ( cm 2 V 1 s 1 ) for application in high-speed devices and they are even unstable in atmospheric conditions. ZnO-based transistors can exhibit high device performance with a mobility value reaching up to 80 cm 2 V 1 s 1 [5]. In general, ion-beam sputtering, RF sputtering, and pulsed laser deposition have been used to form ZnO channels. However, such vacuum deposition techniques are not suitable for low-cost production of large-area-display devices and flexible electronics. Therefore, there have been significant attempts to produce a high-quality ZnO channel layer using solution-based processes. It is essential to develop functional inks to prepare solutionprocessed ZnO thin films at lower temperatures. A ZnO functional ink can be either a dispersion of ZnO nanoparticles or a chemical solution of metal salt/alkoxide. For particle-based ZnO inks, the particles should be not only sufficiently small for low-temperature particle sintering, but also well dispersed to form a dense, defect-free ZnO layer using simple, solution-based coating or printing processes [6,7]. Furthermore, the characteristics, such as particle size and shape, of the ZnO particles used also play an important role in controlling the performance of TFT devices since these characteristics can determine the number of grain boundaries between the source and drain electrodes. In this regard, a particle-based functional ink requires well-controlled ZnO nanoparticles. Various techniques have been employed to synthesize ZnO nanoparticles, including sol gel methods, hydrothermal synthesis, chemical vapor deposition, and thermal oxidation [8 11]. We have developed herein a facile polyol method in which well-controlled ZnO nanoparticles are reproducibly prepared. Our method offers several advantages as compared to other ZnO polyol syntheses [12 14]. Highly crystalline ZnO nanoparticles free of aggregation are obtained at 180 C. The shape of the ZnO nanoparticles can be controlled to yield either spheres or rods by adjusting the method used for the addition of water to the synthesis. 2. Materials and methods 2.1. Synthesis of ZnO particles by heating the precursor solution Polyvinylpyrrolidone (PVP, M w = 10,000 g/mol, Aldrich), dissolved in diethyleneglycol (DEG, 99.9%, Aldrich), was used to prevent agglomeration of the synthesized particles. PVP, zinc acetate dihydrate (Zn(CH 3 COO) 2 2H 2 O, Aldrich), and deionized (DI) water, used as an initiator for hydration and condensation at room temperature, were put into a reactor fitted with a reflux condenser. The solution was stirred vigorously, and this was followed by heating the precursor solution to a temperature of 180 C, at a constant heating rate of 7.5 C/min. The zinc acetate was completely dissolved when the solution was heated over 100 C. Nucleation of the zinc oxide seemed to start at around 150 C as determined visually by a rapid change in turbidity. The reaction continued for 30 min at 180 C, after which the system was cooled down

3 332 S. Lee et al. / Superlattices and Microstructures 43 (2008) Table 1 Reaction conditions used for synthesizing ZnO nanoparticles Addition method Precursor heating Water injection Sample ID Zinc acetate (mol L 1 ) PVP (mol L 1 ) Total water (mol L 1 ) Hydration ratio Figure ID a a/4d/5b c c b/4b a a/6b c to room temperature. The particles were separated from the liquid by centrifugation and then washed repeatedly with methanol. The detailed conditions used for the synthesis are described in Table Synthesis of ZnO particles by injecting water into hot precursor solution In this method, zinc acetate dihydrate was added to the PVP dissolved in DEG at 25 C, and this was followed by heating of the precursor solution to 180 C. Since the zinc acetate dissolved gradually, no nucleation or growth occurred during the heating. However, when the temperature of the precursor solution reached 180 C, nucleation started due to the water contained in the zinc acetate dihydrate. DI water was injected into hot zinc acetate-dissolved DEG solution using a syringe pump (KDS200, KD Scientific) at a constant rate of 1 ml/s. Because the amount of dropwise added water was much less than the volume of the reactant solution, the overall reaction temperature remained at 180 C. Upon injection of the water, the solution became milky. The reaction was continued for 30 min and then cooled down to room temperature. The particle recovery procedures are identical to those described in the previous section. The specific conditions of synthesis are also shown in Table Characterization of ZnO particles The synthesized ZnO nanoparticles were characterized by scanning electron microscopy (SEM, JEOL JSM-6700F). The mean particle size and corresponding standard deviation of the ZnO particles were determined by image analyses of SEM micrographs. X-ray diffraction (XRD) patterns of ZnO nanoparticles were recorded using an X-ray diffractometer (Rigaku, D/max-Rint 2000) with Cu Kα radiation. 3. Results and discussion 3.1. Influence of the method of addition of water on particle morphology The shapes of the ZnO particles obtained were markedly different depending on whether water was added to the precursor solution at room temperature or water was injected into hot precursor solution. Fig. 1 shows the ZnO particles synthesized using the same reaction conditions, except for the method of adding water. The presence of water prior to heating the precursor solution resulted in rod-shaped particles (Fig. 1a). Average major axis and minor axis lengths determined

4 S. Lee et al. / Superlattices and Microstructures 43 (2008) Fig. 1. SEM images of ZnO nanoparticles prepared by (a) heating the precursor solution (Sample 2) and (b) water injection into hot precursor solution (Sample 5). Fig. 2. XRD diffraction patterns of synthesized ZnO particles: (a) rod-shaped particles (Sample 2) and (b) equiaxial particles (Sample 5). by image analysis were 114 ± 54 nm and 31 ± 10 nm, respectively, with an aspect ratio of 3.6. On the other hand, rather aggregated, equiaxial ZnO particles were observed when water was injected into hot precursor solution (Fig. 1b). The particles exhibited a mixture of relatively spherical and slightly elongated shapes, in which the average particle size was 34 ± 9 nm. Fig. 2 shows the XRD patterns for ZnO nanoparticles synthesized using different methods of adding water. The diffraction peaks were well matched with hexagonal-wurtzite-structure ZnO peaks. This indicates that the synthesized ZnO particles have good crystallinity and are of high purity, regardless of the addition method used. Also by XRD analysis, the preferred c-axis oriented growth is confirmed more for the rod-shaped ZnO particles compared to the equiaxial particles. The relative XRD peak intensity ratio of I 002 /I 100 for the rod-shaped ZnO particles is 1.12, while it is 0.85 for the equiaxial particles. In this polyol synthesis of ZnO, water plays an important role in determining the particle morphology since water can induce hydrolysis and condensation reactions of the Zn precursor. In the case of particles obtained by heating the precursor solution, water is present in the reaction

5 334 S. Lee et al. / Superlattices and Microstructures 43 (2008) Fig. 3. Different schemes of the particle growth depending on the method of addition of water: (a) precursor heating method and (b) water injection method. medium together with Zn ions, such that nucleation and growth would start before the solution temperature reaches 180 C. It was observed that the solution became milky-white at 150 C. Under these conditions, it is expected that nucleation events are sluggish, and the number of nuclei is small due to the lower reaction temperature. Upon reaching 180 C, the additional heat supply causes further nucleation and its growth. Because heterogeneous nucleation is preferred over homogeneous nucleation, it is thought that Zn ions in the solvent tend to condense on the surfaces of pre-existing nuclei, rather than forming new nuclei [15]. In the wurtzite structure of ZnO, the growth rates of each crystallographic plane differ somewhat according to the crystal orientation. When the growth process is relatively slow, there is a greater chance for the particles to grow into rods, because the material deposits on the most stable spots on the surface by which the overall surface energy can be minimized. The c-axis is known to be the direction for fast growth in wurtzite ZnO since the (001) plane has the lowest surface energy [14,16,17]. Our XRD analysis also supports that rod-shaped ZnO particles have a tendency to grow in the c-axis direction. In contrast, water injected into hot precursor solution maintained at 180 C induces a short burst of homogeneous nucleation. Upon injection of water, most of the Zn ions undergo rapid hydrolysis and condensation, forming numerous nuclei followed by fast particle growth. The presence of many nuclei prevents preferential growth along the c-axis due to a limited amount of Zn ions available for monomeric addition onto the pre-existing, growing particles. Under these conditions, the ZnO grows into relatively equiaxial particles. Unstable particles may undergo aggregation due to an increased possibility of inter-particle collisions. We obtained ZnO particles free of aggregation when a higher amount of PVP was added (results not shown here). PVP is a water-soluble polymer, consisting of N-vinyl pyrrolidone monomer that can form a coordinate bond with the Zn ions on the growing particles. This leads to PVP being strongly adsorbed on the surface of particles, preventing the nanoparticles from aggregating. Fig. 3 illustrates the different particle growth mechanisms that occur depending on the method of water addition. The concentration of zinc acetate also influences the particle characteristics, such as particle size and aspect ratio, as shown in Fig. 4. When ZnO particles were synthesized by the method

6 S. Lee et al. / Superlattices and Microstructures 43 (2008) Fig. 4. SEM images of synthesized ZnO particles as a function of concentration of zinc acetate, both by the water injection method: (a) 0.1 mol L 1, (b) 0.4 mol L 1, and by the precursor heating method: (c) 0.1 mol L 1, (d) 0.4 mol L 1. of injecting water into hot precursor solution, the particle morphology did not vary greatly with the concentration of zinc acetate. Relatively spherical, small particles were obtained at a concentration of 0.1 mol L 1, compared to larger, equiaxial particles with a slight elongation at 0.4 mol L 1. Reducing the reaction concentration of zinc acetate decreases the number of nuclei formed by homogeneous nucleation, resulting in rapid growth of spherical and less aggregated particles. In contrast, the concentration of the reactant influences, to a great extent, the shape of particles synthesized by heating the water-added precursor solution. Particles obtained at a concentration of 0.1 mol L 1 were severely aggregated, and the aspect ratio markedly increased with decrease in the concentration. Due to the limited amount of Zn precursors, it is believed that nucleation occurs predominantly over growth during heating of the precursor solution and after the reaction temperature reaches 180 C. The lack of a continuous supply of the growing material prevents preferential growth on the pre-existing particles Influence of hydration ratio on particle morphology The hydration ratio (h) is defined as the ratio of the molar concentration of total water (DI water + hydrated water) to zinc acetate. Poul et al. reported that the various morphologies of ZnO can be controlled by the hydration ratio, and the particle sizes tend to become smaller with decreasing hydration ratio [14]. In our polyol method, ZnO particles were synthesized by both the precursor heating method and the water injection method with varying hydration ratios from 4 to 8. Fig. 5 shows the SEM images of ZnO particles synthesized at different

7 336 S. Lee et al. / Superlattices and Microstructures 43 (2008) Fig. 5. SEM images of rod-shaped particles (precursor heating method) as a function of hydration ratio: (a) h = 4 (Sample 1), (b) h = 6 (Sample 2), and (c) h = 8 (Sample 3). hydration ratios by heating the precursor solution. Rod-shaped ZnO particles were obtained regardless of the hydration ratio. Observations via microscopy revealed significant differences in the morphological characteristics. When the hydration ratio was increased from 4 to 6, the particles were elongated. At h = 4, the average length of the major axis was 100 nm, and the minor axis was 32 nm. At h = 6, these lengths changed to 114 nm and 31 nm, respectively. The length in the major axis direction was greater, while the length in the minor axis direction remained nearly constant; the aspect ratio of the rod-shaped ZnO particles increased from 3.2 to 3.6. However, in the case of h = 8, the major and minor axes increased to 117 nm and

8 S. Lee et al. / Superlattices and Microstructures 43 (2008) Fig. 6. SEM images of equiaxial particles (water injection method) as a function of hydration ratio: (a) h = 4 (Sample 6), (b) h = 6 (Sample 7), and (c) h = 8 (Sample 8). 39 nm, respectively, decreasing the aspect ratio to 3.1. Fig. 6 shows the effect of hydration ratio on particle synthesis by the water injection method. Varying the hydration ratio from 4 to 8 did not change the particle morphology to a great extent. The particle diameter became larger, increasing from 24 nm to 32 nm, and showed a slight deviation from equiaxial growth with increasing hydration ratio. Water can act both as a reactant for and a product of the hydrolysis and condensation reaction. Therefore, the hydration ratio can affect the hydrolysis and condensation rate of the Zn precursor in solution, by which ZnO can nucleate and grow into a distinct particle morphology. Acetate ions

9 338 S. Lee et al. / Superlattices and Microstructures 43 (2008) Fig. 7. Schematic showing the hydration and condensation of the Zn ion complex and the mechanism of particle growth for rod-shaped ZnO particles as a function of the hydration ratio. (CH 3 COO ) can strongly chelate with Zn 2+ ions, such that zinc acetate cannot be completely dissociated in solution, but rather exists as Zn(CH 3 COO) 2(aq). Upon the introduction of water, Zn(CH 3 COO) 2(aq) can be hydrolyzed, forming Zn(OH)(CH 3 COO) as an intermediate: Zn (CH 3 COO) 2 + H 2 O Zn(OH) (CH 3 COO) + CH 3 COOH. Condensation between the hydrolyzed Zn(CH 3 COO)(OH) species can occur to yield a Zn O Zn bond by leaving behind either H 2 O or CH 3 COOH: Zn (CH 3 COO) 2 + Zn(OH) (CH 3 COO) Zn 2 (O) (CH 3 COO) 2 + CH 3 COOH Zn(OH) (CH 3 COO) + Zn(OH) (CH 3 COO) Zn 2 (O) (CH 3 COO) 2 + H 2 O. ZnO is nucleated via successive hydrolysis and condensation reactions. Once the nucleation is accomplished, the particles can grow by addition of the intermediate species. This model of additive growth requires the hydrolysis of the Zn precursor complex on the surface of the growing particles. Using the precursor heating method at low hydration ratio, the hydrolysis of the surface group becomes the rate-limiting step, hindering preferential growth in the c-axis direction. However, at h = 6 sufficient water is present to hydrolyze the Zn complexes on the (001) plane (lowest surface energy), and the ZnO particles grow into a rod-like shape at a moderate speed of growth. But, it would be possible at high hydration ratios that the Zn complexes on the surface of other planes, such as the (100) plane, can be hydrolyzed. Under such conditions, the ZnO can grow in both directions, resulting in shorter and thicker rod-shaped particles with a reduced aspect ratio. Fig. 7 schematically depicts the growth mechanism leading to rod-shaped ZnO particles as a function of the hydration ratio. Rapid injection of water into hot precursor solution induces a short burst of nucleation regardless of the hydration ratio. The same number of initial nuclei will form even at different hydration ratios, but at a constant concentration of 0.1 M zinc acetate. The hydrolysis and condensation reactions required for particle growth are affected by the hydration ratio. Enhanced growth is expected with increase in the hydration ratio. A burst of many nuclei prevents the particles from growing anisotropically at a relatively low concentration of zinc acetate.

10 S. Lee et al. / Superlattices and Microstructures 43 (2008) Therefore, the resulting particles are relatively equiaxially shaped, and their size becomes larger with increasing h. 4. Summary We have demonstrated the synthesis of ZnO nanoparticles with controlled morphology and size by a simple polyol method. The method of addition of water, the hydration ratio, and the concentration of zinc acetate play important roles in determining the characteristics of the synthesized particles. Injection of water into hot precursor solution resulted in equiaxial ZnO particles due to a short burst of nucleation and rapid particle growth. On the other hand, rodshaped particles were produced when the aqueous precursor solution was heated to the reaction temperature because of preferential heterogeneous nucleation and growth on the pre-existing particles. The particles obtained at 180 C were crystalline ZnO with a wurtzite crystal structure as determined by XRD analysis. The rod-shaped particles had an average major axis length of 114±54 nm and minor axis length of 31±10 nm, whereas the equiaxial particles had an average diameter of 34 ± 9 nm. Both the hydration ratio and the concentration of zinc acetate affect the hydrolysis and condensation rates for the Zn ion complex, by which ZnO particles with different morphologies and sizes could be prepared. Acknowledgements This work was supported by the Korea Science and Engineering Foundation (KOSEF) through the National Research Lab. Program funded by the Ministry of Science and Technology (No. R0A ). It was also partially supported by LG Philips LCD and the Second Stage of Brain Korea 21 Project in References [1] D.C. Look, B. Claftin, Phys. Status Solidi B 241 (2004) 231. [2] D.C. Look, D.C. Reynolds, C.W. Litton, R.L. Jones, D.B. Eason, G. Cantwell, Appl. Phys. Lett. 81 (2002) [3] N.A. Anderson, X. Ai, T.Q. Lian, J. Phys. Chem. B 107 (2003) [4] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, H. Hosono, Nature 432 (2004) 488. [5] K. Nomura, H. Ohta, K. Ueda, T. Kamiya, M. Hirano, H. Hosono, Science 300 (2003) [6] B. Sun, H. Sirringhaus, Nano Lett. 5 (2005) [7] S. Lee, S. Jeong, D. Kim, B.K. Park, J. Moon, Superlattices Microstruct. 42 (2007) 361. [8] Y.Q. Huang, M.D. Liu, Y.K. Zeng, C.R. Li, D.L. Xia, S.B. Liu, Mater. Sci. Eng. B 86 (2001) 232. [9] W.J. Li, E.W. Shi, M.Y. Tian, W.Z. Zhong, Z.W. Yin, J. Mater. Res. 14 (1999) [10] B.J. Chen, X.W. Sun, C.X. Xu, B.K. Tay, Physica E 21 (2004) 103. [11] W. Gao, Z.W. Li, R. Harikisun, S. Chang, Mater. Lett. 57 (2003) [12] C. Feldmann, H. Jungk, Angew. Chem.-Int. Edit. 40 (2001) 359. [13] C. Feldmann, Scr. Mater. 44 (2001) [14] L. Poul, S. Ammar, N. Jouini, F. Fievet, F. Villain, Solid State Sci. 3 (2001) 31. [15] R.T. DeHoff, Thermodynamics in Materials Science, 2nd eds., CRC Press, [16] X.M. Liu, Y.C. Zhou, J. Cryst. Growth 270 (2004) 527. [17] R.C. Hoffmann, S. Jia, L.P.H. Jeurgens, J. Bill, F. Aldinger, Mater. Sci. Eng. C 26 (2006) 41.