CRYSTAL GROWTH & DESIGN
Solvothermal Synthesis of ZnO with Various Aspect Ratios Using Organic Solvents Ayudhya,†
Tonto,†
Sirachaya Kunjara Na Parawee Varong Pavarajarn,† and Piyasan Praserthdam*,†
Okorn
2006 VOL. 6, NO. 11 2446-2450
Mekasuwandumrong,*,‡
Center of Excellence on Catalysis and Catalytic Reaction Engineering, Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn UniVersity, Bangkok 10330, Thailand, and Department of Chemical Engineering, Faculty of Engineering and Industrial Technology, Silpakorn UniVersity, Nakorn Pathom 73000, Thailand ReceiVed July 19, 2005; ReVised Manuscript ReceiVed June 25, 2006
ABSTRACT: Nanocrystalline ZnO powders were synthesized by a solvothermal method using various organic solvents as the reaction medium. Products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and selected area electron diffraction (SAED). The aspect ratio of ZnO primary particles depended upon the solvent employed, such that the aspect ratio increased when the solvent with a low dielectric constant was employed. For the use of n-alkanes and aromatic compounds as solvent, ZnO nanorods were obtained. The polar characteristic of the solvent was proposed to be the main factor that affects both nucleation and growth of ZnO nanoparticles and, consequently, determines the aspect ratio of the products. Introduction ZnO is a polar inorganic crystalline material with many applications, mostly as electronic and photonic materials, because of its wide direct band gap of 3.37 eV.1 Potential applications include UV photodetection, transparent electronics, humidity sensor, gas and chemical sensor, microlasers, memory arrays, coatings, catalysts, and biomedical applications.2-6 For these applications, it is preferred that the size and shape of ZnO particles are controlled.7-13 Several approaches to preparing shape-controlled ZnO nanoparticles have been reported.14-16 Vapor-liquid-solid epitaxy is one of the common methods for achieving vertical alignment of ZnO nanowires on lattice-matched substrates.17,18 However, the main limitation of the epitaxial growth methods is that the crystals are attached to a substrate or embedded in a matrix, which limits the range of possible applications.19,20 Other methods for synthesizing one-dimensional ZnO nanostructures include liquid-phase syntheses such as hydrothermal synthesis, solvothermal synthesis, precipitation method, direct deposition in aqueous solution, and colloidal routes. The solution chemical routes provide a promising way for large-scale production and have long been used for ZnO single-crystal synthesis. Among these liquid-phase syntheses, the hydrothermal method is popular because it has many adjustable parameters that allow for control of ZnO particle formation. However, most wet chemical methods fail to produce rods with diameters less than 100 nm.21-29 A solvothermal process is a useful wet chemical route. In solvothermal synthesis, a solvent acts as a reaction medium that allows the relatively high temperature required for crystallization of inorganic materials to be achieved.30 Patzke et al. surveyed publications on the synthesis of oxide nanotubes and nanorods and concluded that solvothermal synthesis is one of the most powerful tools for providing distinct morphologies of nanomaterials.31 There have been many adaptations of this simple process for controlling the size and shape of nanopar* Corresponding author. E-mail:
[email protected] (O.M.). † Chulalongkorn University. ‡ Silpakorn University.
[email protected] (P.P.);
ticles, such as the emulsion/surfactant assisted method,32 and changes in precursors and reaction conditions.33 In the present work, the solvothermal process was employed to synthesize ZnO nanoparticles. Effects of properties of the solvent on the aspect ratio and crystallization mechanism of ZnO were investigated. Experimental Section Zinc oxide was prepared by using zinc acetate (Zn(CH3COO)2; 99.99%, Aldrich) as the precursor. The precursor was suspended in 100 mL of solvent in a test tube. The test tube was then placed in a 300 mL autoclave. An additional 30 mL of the solvent was added in the gap between the test tube and autoclave wall. After the autoclave was completely purged with nitrogen, the autoclave was heated to the desired reaction temperature, in the range of 250-300 °C, at a rate of 2.5 °C min-1. Autogenous pressure during the reaction gradually increased as the temperature was raised. The system was kept at the reaction temperature for 2 h, after which time the autoclave was cooled to room temperature. The resulting powder in the test tube was collected and repeatedly washed with methanol by centrifugation. The products were then dried in air. Four types of organic solvents were used in the experiments, i.e., alcohols, glycols, n-alkanes, and aromatic compounds. The alcohols were 1-butanol (99.4%, Ajax Finechem), 1-hexanol (98%, Aldrich), 1-octanol (99%, Aldrich), and 1-decanol (99%, Aldrich). Glycols used included 1,3-propanediol (98%, Aldrich), 1,4-butanediol (99%, Aldrich), 1,5-pentanediol (99%, Merck), and 1,6-hexanediol (99%, Aldrich). Alkanes were n-hexane (99%, Merck), n-octane (99%, Carlo Erba), and n-decane (98%, Fluka Chemika). Aromatic solvents were benzene (99.7%, Merck), toluene (99.5%, Ajax Finechem), o-xylene (97%, Aldrich), and ethylbenzene (99.5%, Carlo Erba). All solvents were used as received without further purification. The products were characterized by powder X-ray diffraction (XRD; SEIMENS D5000, using Cu-KR radiation at 30 kV and 30 mA with a Ni filter), scanning electron microscopy (SEM; JEOL JSM6400, with an accelerating voltage of 15 and 20 kV) and transmission electron microscopy (TEM; JEOL JEM-2010, with an accelerating voltage of 80 and 200 kV).
Results In this work, the solvothermal synthesis of ZnO in alcohols or glycols was conducted at 250 °C, where the decomposition of the precursor to form ZnO was completed.34 On the other hand, our preliminary studies have suggested that the reaction
10.1021/cg050345z CCC: $33.50 © 2006 American Chemical Society Published on Web 09/28/2006
Solvothermal Synthesis of ZnO with Various Aspect Ratios
Crystal Growth & Design, Vol. 6, No. 11, 2006 2447
Figure 1. XRD patterns of ZnO powders synthesized in (a) 1-hexanol, (b) 1,6-hexanediol, (c) n-hexane, and (d) benzene.
Figure 2. SEM micrographs of ZnO particles synthesized via the solvothermal process in (a) 1,3-propanediol, (b) 1,4-butanediol, (c) 1,5-pentanediol, and (d) 1,6-hexanediol. Insets in the images are the corresponding TEM micrographs. (e) Sample of the SAED pattern of the synthesized ZnO.
in either n-alkanes or aromatic compounds at 250 °C was not complete to result in ZnO formation. Therefore, the reaction temperature was raised to 300 °C for these two types of solvent. Figure 1 shows the XRD patterns of products synthesized in various groups of organic solvents. All samples were ZnO in the hexagonal phase (wurtzite structure)34-36 without contamination from other crystalline phases. A similar intensity ratio of diffraction peaks indicates that ZnO crystals grow along the same lattice direction, regardless of the solvent used.37 Morphologies of the products synthesized in various organic solvents are shown in Figures 2-4. It can be clearly observed that the size and shape of ZnO nanoparticles generally depend on the type of solvent employed. The products synthesized in glycols (Figure 2) consisted of polyhedral crystals with the lowest aspect ratios (length-to-diameter ratio), whereas those synthesized in alcohols (Figure 3) had moderate aspect ratios. Finally, the products obtained using n-alkanes or aromatic compounds as solvent (Figure 4) were ZnO nanorods with extremely high aspect ratios. The results from the selected area
electron diffraction (SAED) analysis revealed that all products were the collection of ZnO single crystals. It is interesting to note that the morphology of ZnO nanoparticles synthesized in alcohols strongly depended on the chain length of the alcohol molecule (see Figure 3). The aspect ratio of the products increased when a long-chain alcohol, such as 1-decanol, was used. A lesser effect from solvent chain length was observed on products obtained in glycols. For n-alkanes and aromatic compounds, this effect was unnoticeable. Sizes of products synthesized in alcohols and glycols, as well as the physical properties of the solvent, are summarized in Table 1. The particle size was determined from TEM micrographs of particles, using image analysis software. Similar data for the rodlike products synthesized in n-alkanes or aromatic compounds are shown in Table 2. It should be noted that the length of the synthesized ZnO nanorods was in a very wide range, from submicrometer to that longer than 10 µm, which was too large to be captured via the TEM technique. Therefore, the size determination was done on
2448 Crystal Growth & Design, Vol. 6, No. 11, 2006
Kunjara Na Ayudhya et al.
Figure 3. SEM micrographs of ZnO particles synthesized via the solvothermal process in (a) 1-butanol, (b) 1-hexanol, (c) 1-octanol, and (d) 1-decanol. Insets in the images are the corresponding TEM micrographs. (e) Sample of the SAED pattern of the synthesized ZnO.
Discussion
Figure 4. Micrographs of ZnO particles synthesized via the solvothermal process in (a) n-hexane, (b,c) n-decane, (d) benzene, and (e) ethylbenzene. Insets in the images are the SEM images in lower magnification.
SEM micrographs instead. The samples were sonicated to separate the agglomerated rods, in the same fashion as for the preparation for TEM analysis. A sample of the SEM image of the sonicated sample is shown in Figure 5. However, it has been reported that sonication can break ZnO nanorods.38 Thus, it we were not certain whether the shorter rods observed in Figure 5 were as-synthesized particles or parts of long rods that were broken off by sonication. On the other hand, visual observation from the SEM micrographs of samples without sonication (see the inset in Figure 4) could not clearly determine the length of individual rod, because several rods were bundled together. Nevertheless, it was indicated that the rods generally had a very high aspect ratio, as high as 100. Because of this inconclusive observation, data for length and aspect ratio in Table 2 were omitted, but it was reasonable to claim that the products synthesized in either n-alkanes or aromatic compounds had much higher aspect ratios, at least 1 order of magnitude higher than those obtained in alcohols or glycols.
For the solvothermal synthesis in alcohols or glycols, the nucleation as well as the continuing growth of ZnO crystals are similar to those that occur in the hydrothermal process because the solvent also consists of a hydroxyl group, which is a key factor for ZnO nucleus formation in the hydrothermal method.39-42 However, solvothermal reaction in a medium that lacks a hydroxyl group, such as n-alkanes and aromatic compounds, has been rarely reported. A plausible mechanism for the results in this work is discussed as follows. It has been reported that anhydrous zinc acetate, which was used as a precursor in this work, can undergo decomposition and form ZnO nuclei within zinc acetate particles without forming any intermediates.34,43,44 Nevertheless, the thermal stability of zinc acetate has been reported to depend on its interaction with the surrounding solvent.34,45 It was confirmed by the results in this work that a higher temperature was required to synthesize ZnO in n-alkanes or aromatic compounds than in alcohols or glycols. This observation was attributed to the different physicochemical properties of solvents. One of the dominant factors was the dielectric constant of the solvent. The dissociation of ionic bonding as well as the weakening of electrostatic interactions within crystals has been generally acknowledged to be favored in the medium with a high dielectric constant. The decomposition of zinc acetate has also been reported to be enhanced by increasing the content of water, which has a very high dielectric constant, in the system.43 Therefore, within low-dielectric-constant solvents such as nalkanes or aromatic compounds, high temperatures were required for overcoming relatively strong attractions within zinc acetate crystal to triggering the decomposition of zinc acetate to form ZnO nuclei. Effects of solvents on the growth of ZnO crystals have also been reported in the literature.39,46 Nevertheless, unlike the discussion in previous research, a variety of solvents were employed in this work. It was easy to find which type of solvent (e.g., alcohols, glycols, n-alkanes, and aromatic compounds) greatly influenced the growth of ZnO crystals. The chain length of the solvent molecule also affected the crystal growth. However, the effect of chain length was apparent only in alcohols (see Figure 3). For glycols, n-alkanes, and aromatic
Solvothermal Synthesis of ZnO with Various Aspect Ratios
Crystal Growth & Design, Vol. 6, No. 11, 2006 2449
Table 1. Physical Features of the Anisotropic ZnO Synthesized via a Solvothermal Process Using Alcohols and Glycols as Solvents diameter (nm) solvent
b.
p.49
1-butanol 1-hexanol 1-octanol 1-decanol 1,3-propanediol 1,4-butanediol 1,5-pentanediol 1,6-hexanediol a
(°C)
dielectric
117 156 196 231 214 230 242 252
constant34
17.24 13.03 10.30 8.10 35.10 31.90 26.20 25.86
average
deviationa
107 91 84 81 42 69 56 62
22.4 14.8 14.6 20.8 8.3 13.1 8.6 11.0
aspect ratio
average
deviationa
average
deviationa
184 264 343 455 55 139 96 94
31.1 64.0 109.9 223.0 13.6 35.2 24.7 28.7
1.72 2.90 4.08 5.62 1.32 2.05 1.89 1.72
0.27 0.66 1.58 1.89 0.26 0.41 0.45 0.51
Population standard deviation.
Table 2. Physical Features of the Rodlike ZnO Synthesized via a Solvothermal Process Using n-alkanes and Aromatic Compounds as Solvents diameter (nm) solvent
b. p.49 (°C)
dielectric constant34
average
deviationa
n-hexane n-octane n-decane benzene toluene o-xylene ethylbenzene
69 126 174 80 111 144 136
1.89 1.95 1.99 2.28 2.38 2.56 2.45
59 67 48 73 78 63 43
15.1 13.0 13.7 14.9 25.9 23.7 12.2
a
length (nm)
Population standard deviation.
Figure 5. SEM image of ZnO synthesized in ethylbenzene after sonication in ethanol for 15 min.
sequently, the growth of the crystal was less confined to the boiling droplet of solvent in the reaction system and resulted in a crystal with a flowerlike morphology. The results from this work contrasted the observation by Zhang et al.39 It was found in this work that the type of solvent affected the shape of the ZnO primary particle. No such shape amalgamation was observed, regardless of the solvents employed. The effects of solvents on ZnO nanoparticle synthesis, i.e., both nucleus formation and crystal growth, were indeed complex. Other properties of the solvent, such as viscosity,48 saturated vapor pressure,39 and molecular structure leading to the steric hindrance effect,48 have been proposed to be key parameters controlling nanoparticle formation. Nevertheless, according to the systematic investigation in this work, no general conclusion could be drawn on these properties of the solvents. However, as shown in Tables 1 and 2, it is noticeable that the dielectric property of the solvents is significantly correlated with the shape of ZnO nanocrystals. The higher the dielectric constant of the solvent, the lower the aspect ratio of the product. Although parts of the mechanism have been discussed above, detailed characterizations are still needed for investigating the actual mechanism of ZnO nanoparticle formation by the solvothermal synthesis. Conclusion
compounds, the chain length had a small or unnoticeable effect on the morphology of the ZnO products. For ZnO nanocrystals, the growth of the crystal is preferentially in the c-axis. However, it has been reported that the (0001) facet of the crystal, which is the slightly positively charged Zn surface, can be adsorbed by negatively charged chemical species.18,37,40,41,46,47 The adsorbed molecules, therefore, retard the growth of the crystal in that direction. The interface-solvent interaction can be applied to justify the results in this work as well. Glycols, which consist of two hydroxyl groups at both ends, could effectively attach to the (0001) surface of ZnO crystals and results in nonpreferential growth of the crystals, as shown in Figure 2. On the contrary, for nonpolar species such as n-alkanes and aromatic compounds, which were not compatible with such electrostatic interaction with the (0001) surface, ZnO crystals grew preferentially in the c-direction to form nanorods (Figure 4). For alcohols, in which the polarity decreases with the chain length of the molecule, ZnO particles with high aspect ratios were obtained only when long-chained alcohol, e.g., octanol or decanol, was employed. A similar observation for ZnO synthesized in alcohols was also reported by Cheng and Samulski.46 Zhang et al.39 have reported that the polarity of the solvent affects not only the nucleus formation and preferential direction of crystal growth but also the amalgamation of crystals. Polarity compatibility between the reactant and the solvent led to homogeneous dispersion of the reactant in the mixture. Con-
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