Modulation of the Morphology of ZnO Nanostructures via Aminolytic

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Modulation of the Morphology of ZnO Nanostructures via Aminolytic Reaction: From Nanorods to Nanosquamas Zhihua Zhang, Shuhua Liu, Shueyin Chow, and Ming-Yong Han* Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, and DiVision of Bioengineering, National UniVersity of Singapore, Singapore 117576 ReceiVed February 6, 2006. In Final Form: April 24, 2006 Various diversified morphology-modulated ZnO nanostructures including nanorods, nanotetrahedrons, nanofans, nanodumbbells, and nanosquamas have been successfully prepared via an effective aminolytic reaction of zinc carboxylates with oleylamine in noncoordinating and coordinating solvents. Their shape- and structural defect-dependent optical properties have been investigated as well. Highly crystalline defect-free nanotetrahedrons/nanorods have a sharp band-edge emission, and highly defective nanodumbbells/nanosquamas show a very broad deep-trap emission, resulting from the radiative recombination of electrons with holes in singly ionized oxygen vacancies.

Introduction ZnO nanostructures have attracted a great amount of attention because of their unique optical/electronic properties and novel applications in optoelectronics, piezoelectricity, energy conversion, field emission, catalysis, and sensing.1 Currently, a catalytically activated vapor-phase synthesis has been widely employed to achieve great morphology control of various diversified ZnO nanostructures, including nanorods, nanowires, nanotubes, nanobelts, nanoribbons, nanorings, nanocombs, and nanotetrapods.2 With good potential for scale-up production, a hydrolytically seeded aqueous-phase synthesis has also been investigated intensively, using simple zinc salts as starting materials in the presence of bases (methenamine, hexamethylenetetramine, or formamide).3-6 However, because of the low morphological diversity and production yield of ZnO nanostructures, a few organic-phase synthetic approaches have been further developed recently. For example, solvothermal/hydrolytic approaches have been used to prepare uniform ZnO nanodots and nanorods in low-boiling-point tetrahydrofuran, xylene, or alcohols with the use of zinc salts or organometallic zinc precursors.7-9 More recently, nonhydrolytic approaches have been explored to form complex morphologies of transition-metal * Corresponding author. E-mail: [email protected]; biehanmy@ nus.edu.sg. (1) (a) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897. (b) Kind, H.; Yan, H.; Messer, B.; Law, M.; Yang, P. AdV. Mater. 2002, 14, 158. (c) Beek, W. J. E.; Wienk, M. M.; Janssen, R. A. J. AdV. Mater. 2004, 16, 1009. (d) Kniep, B. L.; Ressler, T.; Rabis, A.; Girgsdies, F.; Baenitz, M.; Steglich, F.; Schlogl, R. Angew. Chem., Int. Ed. 2004, 43, 112. (e) Banerjee, D.; Jo, S. H.; Ren, Z. F. AdV. Mater. 2004, 16, 2028. (2) (a) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (b) Lao, J. Y.; Wen, J. G.; Ren, Z. F. Nano Lett. 2002, 2, 1287. (c) Yan, H.; He, R.; Pham, J.; Yang, P. AdV. Mater. 2003, 15, 402. (d) Lao, J. Y.; Huang, J. Y.; Wang, D. Z.; Ren, Z. F. Nano Lett. 2003, 3, 235. (e) Kong, X. Y.; Ding, Y.; Yang, R.; Wang Z. L. Science 2004, 303, 1348. (3) (a) Vayssieres, L.; Keis, K.; Lindquist, S. E.; Hagfeldt, A. J. Phys. Chem. B 2001, 105, 3350. (b) Vayssieres, L. AdV. Mater. 2003, 15, 464. (4) Greene, L. E.; Law, M.; Goldberger, J.; Kim, F.; Johnson, J. C.; Zhang, Y.; Saykally, R. J.; Yang, P. Angew. Chem., Int. Ed. 2003, 42, 3031. (5) (a) Tian, Z. R.; Voigt, J. A.; Liu, J.; Mckenzie, B.; Mcdermott, M. J. J. Am. Chem. Soc. 2002, 124, 12954. (b) Tian, Z. R.; Voigt, J. A.; Liu, J.; Mckenzie, B.; Mcdermott, M. J.; Rodriguez, M. A.; Konishi, H.; Xu, H. Nat. Mater. 2003, 2, 821. (6) (a) Yu, H. D.; Zhang, Z. P.; Han, M. Y,; Hao, X. T.; Zhu, F. R. J. Am. Chem. Soc. 2005, 127, 2378. (b) Zhang, Z. P.; Yu, H. D.; Shao, X. Q.; Han, M. Y. Chem.sEur. J. 2005, 11, 3149.

oxide nanostructures in high-boiling-point organic solvents with the use of transition-metal carboxylates.10,11 Some success has been achieved in the preparation of various diversified ZnO nanostructures,12 and we are expecting to fine tune their anisotropic morphologies and understand their kinetically driven formation/growth mechanism for creating new optical properties. In this article, various diversified morphology-modulated ZnO nanostructures including nanorods, nanotetrahedrons, nanofans, nanodumbbells, and nanosquamas have been successfully prepared via a controllable aminolytic reaction of zinc carboxylates with oleylamine in noncoordinating and coordinating solvents. It has been observed that the optical properties of the obtained ZnO nanostructures are dependent on their shapes and crystal quality/structural defect levels. Because nanosized ZnO can be effectively produced from bulk materials (zinc oxide, zinc carbonate, or zinc hydroxide), this will also offer a great (7) (a) Monge, M.; Kahn, M. L.; Maisonnat, A.; Chaudret, B. Angew. Chem., Int. Ed. 2003, 42, 5321. (b) Kahn, M. L.; Monge, M.; Snoeck, E.; Maisonnat, A.; Chaudret, B. Small 2005, 1, 221. (c) Kahn, M. L.; Monge, M.; Colliere, V.; Senocq, F.; Maisonnat, A.; Chaudret, B. AdV. Funct. Mater. 2005, 15, 458. (8) (a) Pacholski, C.; Kornowski, A.; Weller, H. Angew. Chem., Int. Ed. 2002, 41, 1188. (b) Harnack, O.; Pacholski, C.; Weller, H.; Yasuda, A.; Wessels, J. M. Nano Lett. 2003, 3, 1097. (9) (a) Guo, L,; Ji, Y. L.; Xu, H.; Simon, P.; Wu, Z. J. Am. Chem. Soc. 2002, 124, 14864. (b) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2003, 125, 4430. (c) Mo, M.; Yu, J. C.; Zhang, L.; Li, S. K. A. AdV. Mater. 2005, 17, 756. (10) (a) Jun, Y. W.; Casula, M. F.; Sim, J. H.; Kim, S. Y.; Cheon, J.; Alivisatos, A. P. J. Am. Chem. Soc. 2003, 125, 15981. (b) Jana, N. R.; Chen Y.; Peng, X. Chem. Mater. 2004, 16, 3931. (c) Sun, S. H.; Zeng, H.; Robinson, D. B.; Raoux, S.; Rice, P. M.; Wang, S. X.; Li, G. X. J. Am. Chem. Soc. 2004, 126, 273. (d) Park, J.; Kang, E.; Son, S. U.; Park, H. M.; Lee, M. K.; Kim, J.; Kim, K. W.; Noh, H. J.; Park, J. H.; Bae, C. J.; Park, J. G.; Hyeon, T. AdV. Mater. 2005, 17, 429. (e) Zeng, H.; Rice, P. M.; Wang, S. X.; Sun, S. H. J. Am. Chem. Soc. 2004, 126, 11458. (f) Park, J.; Lee, E.; Hwang, N. M.; Kang, M. S.; Kim, S. C. Hwang, Y.; Park, J. G.; Noh, H. J.; Kini, J. Y.; Park, J. H. Hyeon, T. Angew. Chem., Int. Ed. 2005, 44, 2872. (g) Liu, Q.; Lu, W.; Ma, A.; Tang, J.; Lin, J.; Fang, J. J. Am. Chem. Soc. 2005, 127, 5276. (h) Seo, J. W.; Jun, Y. W.; Ko, S. J.; Cheon, J. J. Phys. Chem. B 2005, 109, 5389. (11) Zhang, Z. H.; Zhong, X. H.; Liu, S. H.; Li, D. F.; Han, M. Y. Angew. Chem., Int. Ed. 2005, 44, 3466. (12) (a) Cozzoil, P. D.; Curri, M. L.; Agostiano, A.; Leo, G.; Lomascolo, M. J. Phys. Chem. B 2003, 107, 4756. (b) Yin, M.; Gu, Y.; Kuskovsky, I. L.; Andelman, T.; Zhou, Y.; Neumark, G. F.; O’Brien, S. J. Am Chem. Soc. 2004, 126, 6206. (c) Cozzoli, P. D.; Kornowski, A.; Weller, H. J. Phys. Chem. B 2005, 109, 2638. (d) Andelman, T.; Gong, Y.; Polking, M.; Yin, M.; Kuskovsky, I.; Neumark, G.; O’Brien, S. J. Phys. Chem. B 2005, 109, 14314. (e) Zhong, X.; Knoll, W. Chem. Commun. 2005, 1158. (f) Park, J.; Kang, E.; Son, S. U.; Park, H. M.; Lee, M. K.; Kim, J.; Kim, K. W.; Noh, H. J.; Park, J. H.; Bae, C. J.; Park, J. G.; Hyeon, T. AdV. Mater. 2005, 17, 429. (g) Chen, Y. F.; Kim, M.; Lian, G.; Johnson, M. B.; Peng, X. G. J. Am Chem. Soc. 2005, 127, 13331.

10.1021/la060351c CCC: $33.50 © 2006 American Chemical Society Published on Web 05/28/2006

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opportunity for industrial production with the potential use of naturally occurring zincite (zinc oxide) or smithsonite (zinc carbonate). Experimental Section Materials. Zinc oxide (99.99%), zinc carbonate (99.9%), zinc hydroxide (99.9%), acetic acid (99%), hexanoic acid (98%), octanoic acid (98%), oleic acid (technical grade), diphenyl ether (99%), dioctyl ether (99%), 1-octadecene (technical grade), trioctylphosphine (TOP, technical grade), trioctylphosphine oxide (TOPO, 99%), oleyl alcohol (technical grade), 1,2-hexadecanediol (technical grade), and oleylamine (technical grade) were used as received from Aldrich. HPLCgrade solvents including hexane, toluene, and ethanol were purchased from J. T. Baker. Zinc oxide (zinc carbonate or zinc hydroxide) was used to prepare zinc alkylcarboxylates for converting bulk zinc oxide into nanosized zinc oxide. X-ray diffraction patterns of as-prepared ZnO nanocrystals were consistent with that of wurtzite-structured ZnO (Supporting Information). Preparation of ZnO Nanorods in Noncoordinating Solvents. Zinc oxide (1 mmol) was first reacted at 110 °C with acetic acid (3 mmol) in dioctyl ether (3 mL) for 30 min followed by drying/ degassing under reduced pressure at 120 °C for 1 h and further purging three times with an argon flow. The resulting turbid suspension of zinc acetate became a clear solution quickly at 120 °C after injecting a certain amount of oleylamine (0.6 mL, 2 mmol) into the reaction system. After quickly heating the reaction system to 240 °C, the mixed clear solution gradually turned to a milky white suspension in a few minutes. After 30 min, the reaction system was cooled to room temperature, and ethanol was added to precipitate the ZnO product completely. The white precipitate was retrieved by centrifugation and then washed several times with a mixed solvent of toluene and ethanol. Diphenyl ether or octadecene was also chosen as a coordinating solvent to prepare ZnO nanorods with the identical synthetic procedure. With the use of 10-50 mmol of zinc oxide, a scale-up production is easily achieved in a high-temperature injectionfree fashion. (Oleylamine was injected at a temperature of 120 °C, which is much lower than the aminolytic reaction temperature of 240 °C for the effective formation of ZnO nanomaterials.) Preparation of ZnO Nanotetrahedrons in Coordinating Solvents. Zinc oxide (1 mmol) was first reacted at 110 °C with acetic acid (3 mmol) in TOP (3 mL) until a clear solution was formed after 30 min. The obtained solution was dried/degassed under reduced pressure at 120 °C for 1 h and further purged three times with an argon flow. After injecting 2 mmol of oleylamine, the mixed solution was quickly heated to 290 °C, and a creamy turbid solution was formed after 30 min. The remaining procedure is identical to that described above for the preparation of ZnO nanorods. An increased amount of oleylamine (3 or 6 mmol) was then used to modulate the morphologies of ZnO nanostructures at 320 °C while other conditions remained the same. TOPO or oleic acid was also used as a strong coordinating solvent to prepare ZnO following the identical synthetic procedure. Preparation of ZnO Nanosquamas from Zinc Carboxylates. Zinc oxide (1 mmol) was first reacted at 320 °C with oleic acid (3 mmol) for 30 min in TOP (3 mL) until a clear, colorless solution was formed. The obtained solution was dried/degassed at 120 °C for 1 h and then purged three times with an argon flow. After injecting 2 mmol of oleylamine, the mixed solution was quickly heated to 320 °C, and a creamy turbid solution was formed after 30 min. The remaining procedure is identical to that described above for the preparation of ZnO nanorods and nanotetrahedrons. Different alkylcarboxylic acids were also used to form zinc carboxylates at certain temperatures (3 mmol of hexanoic acid at 210 °C, 3 mmol of octanoic acid at 280 °C for 30 min) to produce morphologycontrolled ZnO nanostructures. Characterization. Powder X-ray diffraction (XRD) patterns were recorded by a Siemens D5005 X-ray powder diffractometer. Transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM) images were collected on a JEOL 1010 and Philips 3010 operating at acceleration voltages of 100 and 300 kV, respectively.

Zhang et al. XRD and TEM samples were prepared by dropping a toluene solution of ZnO nanocrystals on silicon (111) wafers and Formvar/carboncoated copper grids, respectively, followed by drying them at room temperature. UV-vis and PL spectra were obtained on a Shimadzu UV-1601 spectrometer and an RF-5301 PC fluorometer, respectively.

Results and Discussion Preparation of ZnO Nanorods in Noncoordinating Solvents. Typically, 1 mmol of zinc oxide was first reacted with 3 mmol of acetic acid in dioctyl ether (3 mL) to form a turbid suspension of zinc acetate, which was further dissolved by the addition of 2 mmol of oleylamine at 120 °C. After quickly heating the mixed solution to 240 °C, a turbid solution was formed again after a few minutes. (It was slow when 1 mmol of oleylamine was used.) As shown in Figure 1A, uniform ZnO nanorods of ∼20 nm diameter and ∼700 nm length were formed effectively with a production yield of ∼85% after 30 min. Alternatively, diphenyl ether or octadecene was also used as a noncoordinating solvent to prepare uniform ZnO nanorods at 240 °C. Their wurtzite structure was determined by a powder XRD pattern (Supporting Information) and the HR-TEM image of a single nanorod in Figure 1B. The lattice spacing (0.26 nm) between adjacent lattice planes is consistent with the interplanar distance of the (002) planes of ZnO, confirming the epitaxial growth along the c axis of ZnO in the 〈001〉 direction. After the complete reaction of acetic acid with ZnO in dioctyl ether at 110 °C (less than the 118 °C boiling point of acetic acid), the excess acetic acid was removed effectively under reduced pressure at 120 °C for 1 h. The resulting pure zinc acetate (1 mmol) was then reacted with freshly injected oleylamine (2 mmol) at 240 °C to produce ZnO nanorods. After 30 min, a small amount of the obtained reaction mixture was dispersed in 0.5 mL of CDCl3 for NMR studies. As shown in the 1H NMR spectrum of the reaction residue (Supporting Information), there is a new strong peak with a chemical shift of 5.56 ppm due to the formation of amide bonds (-CO-NH-), confirming an aminolytic reaction of zinc acetate with oleylamine to form oleyl acetamide.11 The neighboring peak with a chemical shift of 5.33 ppm is attributed to the protons from CdC bonds in oleylamine. A schematic aminolytic reaction for producing ZnO through the condensation of -Zn-OH intermediates is proposed as follows. Scheme 1. Aminolytic Reaction between Zinc Acetate and a Primary Amine

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Figure 1. Morphology-controlled formation of rodlike and tetrahedral ZnO nanocrystals in coordinating and noncoordinating solvents, respectively, with the use of freshly prepared zinc acetate (1 mmol) and oleylamine (2 mmol). (A) TEM and (B) HRTEM images of ZnO nanorods prepared at 260 °C in a noncoordinating solvent, dioctyl ether. (C) TEM and (D) HRTEM images of ZnO nanotetrahedrons prepared at 290 °C in a coordinating solvent, TOP.

Preparation of ZnO Nanotetrahedrons in Coordinating Solvents. Zinc acetate is not soluble in a noncoordinating solvent such as dioctyl ether; on the contrary, zinc acetate can dissolve in a coordinating solvent such as trioctylphosphine (TOP). As in dioctyl ether (3 mL), no observable reaction of zinc acetate (1 mmol) occurred in TOP (3 mL) after 2 h at high temperature (>280 °C). When adding 2 mmol of oleylamine, a milky solution was quickly formed in a few minutes at a temperature of >280 °C. As shown in Figure 1C, uniform tetrahedral ZnO nanocrystals with a base length of ∼35 nm and two equal side lengths of ∼60 nm were produced at 290 °C after 30 min (production yield > 70%). Correspondingly, elongated ZnO nanotetrahedrons with a base length of ∼25 nm and two equal leg lengths of ∼80 nm were formed at 320 °C after 30 min (production yield ≈ 90%), as revealed in Figure 2A. Similar to that of ZnO nanorods described above, the HR-TEM images in Figures 1D and 2B reveal the anisotropic growth of ZnO nanotetrahedrons along the c axis of ZnO in the 〈001〉 direction. When more oleylamine (3 mmol) was used, some nanofans consisting of two or more nanotetrahedrons (Figure 2C and D) were also formed at 320 °C. It seems that in the presence of more oleylamine the sharpest tips (with a high surface energy) from the resulting nanotetrahedrons were much more reactive than other tips, edges, or planes and thus there was a tendency for two or more tetrahedral nanoparticles to aggregate to form the bi- or multiarmed nanostructures. However, it is more likely that the bi- or multiarmed nanostructures were formed because of the spontaneous polarization of ZnO that has been employed to explain the formation of bi- or multiarmed ZnO crystals in the literature.13 When 6 mmol of oleylamine was used, only dumbbell-like nanostructures of ∼130 nm length and ∼70 nm width were produced as shown in Figure 2E. The largely increased amount (13) (a) Wang, B. G.; Shi, E. W.; Zhong, W. Z. Cryst. Res. Technol. 1998, 33, 937. (b) Sounart, T. L.; Liu, J.; Voigt, J. A.; Hsu, J. W. P.; Spoerke, E. D.; Tian, Z.; Jiang, Y. B. AdV. Funct. Mater. 2006, 16, 335. (c) Wang, Z. L. Mater. Today 2004, 26.

of oleylamine can greatly accelerate the aminolytic reaction rate to form a large number of very small ZnO particles quickly in the early reaction stage; these were further oriented and aggregated to form small nanoparticle-built nanostructures (nanodumbbells), as revealed by the HR-TEM image in Figure 2F. The oriented aggregation-based growth/attachment of small nanoparticles was very effective in forming the highly defective crystalline nanostructures,8 which can generate a very strong deep-trap emission (very bright yellow fluorescence when excited with a UV lamp, to be discussed below). Preparation of ZnO Nanosquamas from Zinc Alkylcarboxylates. In addition to zinc acetate, other zinc carboxylates with different alkyl chain lengths (prepared by dissolving 1 mmol of ZnO with 3 mmol of alkylcarboxylic acids) can be used to tune the morphologies of ZnO nanostructures in TOP. Like zinc acetate, a zinc carboxylate with a shorter alkyl chain (hexanoic acid/C5H11COOH) can also produce multiarmed ZnO nanostructures consisting of many nanotetrahedrons (Figure 3A). However, a zinc carboxylate with a longer alkyl chain (octanoic acid/C7H15COOH or oleic acid/C17H33COOH) can produce squama-like nanostructures (∼80 nm for octanoic acid in Figure 3B and ∼150 nm for oleic acid in Figure 3C). This is due to the significantly enhanced steric effects from the very bulky alkyl chains and corresponding reduced reactivity to oleylamine; consequently, a relatively lower production yield of ∼70% was achieved after 30 min of reaction. The resulting squamous nanostructures of ∼150 nm diameter have multilayered structures with layers of ∼5 nm (each step), as shown in Figure 3C. Their HR-TEM images in Figure 4 show the preferential growth direction of ZnO nanosquamas in the 〈001〉 direction, indicating the strong coordinating and steric effects in the 〈110〉 direction by the bulky alkyl chains that hinder the penetration of oleylamine to react with carbonyl groups on the nanocrystal surface. Discussion of the Morphology-Modulated Growth of ZnO Nanostructures. From the above investigations, it is seen that oleylamine first served as a coordinating ligand to dissolve zinc

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Figure 2. Morphology-modulated growth of various single- or multiarmed ZnO nanostructures at 320 °C in a TOP solution consisting of a certain amount of oleylamine (2, 3, and 6 mmol) and 1 mmol of freshly prepared zinc acetate, respectively. (A) TEM and (B) HRTEM images of the elongated ZnO nanotetrahedrons; (C) TEM and (D) HRTEM images of a single or multi-armed ZnO nanostructures; (E) TEM and (F) HRTEM images of dumbbell-like ZnO nanostructures.

acetate in a noncoordinating solvent at 120 °C, and it further participated in the effective production of ZnO nanorods at 240 °C. At temperatures much lower than 180 °C, oleylamine was used only to dissolve thermally stable zinc acetate in dioctyl ether; at temperatures higher than 180 °C, oleylamine was slightly reactive to thermally stable zinc acetate in dioctyl ether, and a very small number of ZnO nanorods were produced slowly during 2 h of reaction. With the increase in temperature to 240 °C, zinc acetate was still not very reactive to oleylamine although the aminolytic reaction was accelerated, resulting in a relatively small number of seed nuclei in the early nucleation stage. A high perparticle monomer concentration was then maintained for the favored elongated growth of 1D ZnO nanorods.14 This is a typical phenomenon for wurtzite-structured materials, which is caused by the more reactive {001} faces with a higher surface energy/ atomic density than for the other faces. The rapid 1D growth of

long nanorods in the 〈001〉 direction indicates that the highdensity carbonyl groups from acetate are bound on the (001) surface of ZnO (with a high atomic density) to facilitate the fast aminolytic reaction for the anisotropic extension of 1D nanorods, and then oleylamine should be more likely to bind on the surfaces perpendicular to the c-axis direction. For the growth of ZnO nanostructures in a coordinating solvent (TOP) rather than dioctyl ether, their well-defined anisotropic morphologies were transformed from rodlike to tetrahedral ZnO nanostructures. In addition to oleylamine, the simultaneous use (14) Furthermore, a largely increased amount of oleylamine (6 mmol) or an increased temperature (290 °C, limited by the boiling point of dioctyl ether) dramatically enhanced the aminolytic reaction rate for generating a largely increased number of seed nuclei at the nucleation stage. Consequently, a high per-particle concentration of zinc precursors was not maintained for the favored growth of nanorods after 30 min of reaction, and the resulting rapidly reduced monomer concentration subsequently led to the formation of nanodots.

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Figure 3. TEM images of various ZnO nanosquamas prepared in TOP from different zinc alkylcarboxylates including (A) hexanoic acid, (B) octanoic acid, and (C) oleic acid.

of a large number of TOP molecules can synergistically reduce the reactivity of zinc acetate because of their ability to coordinate zinc and the steric hindrance effect of the periphery of ligands. As a result, a higher aminolytic temperature (320 °C) was needed for the effective formation of ZnO nanostructures. The resulting isosceles nanotetrahedrons have both a flat base parallel with a crystallographic (001) plane and identical edges, but the edge/ side surfaces have no crystallographically defined planes because the length ratio of the leg and the base can be changed under different synthetic conditions, as shown in the TEM images in Figures 1C and 2A. Because oleylamine is a much stronger ligand than TOP as examined from the dissolution studies of zinc acetate,15 it is believed that the oleylamine capping agent should play a major role in the kinetically controlled morphology evolution of various architectures from nanorods to nanotetrahedrons. With increased reaction time, TOP as a coordinating solvent was not consumed; however, oleylamine as an aminolytic reagent was consumed gradually. In the early nucleation stage, more oleylamine molecules were bound to newly born small nuclei; with the increase in reaction time for the continuous consumption of oleylamine, the continuously reduced number of oleylamine molecules may gradually weaken the capping/coordinating capabilities to the surface of the resulting nanocrystals perpendicular to the c-axis direction and further enhance the growth rate in the 〈110〉 direction slowly. Because of the preferential anisotropic growth of wurtzite-structured ZnO in the 〈001〉 (15) Zinc acetate is not soluble in a noncoordinating solvent such as dioctyl ether at room or elevated temperature. In the presence of the widely used longchain functionalized alkanes including oleylamine and trioctylphosphine, their coordination abilities can be reflected by the solubilities of zinc acetate in dioctyl ether (3 mL) at 120 °C. Oleylamine (1 mmol) took only a very short time (