Fig. 4 Schematic illustration of the proposed mechanism for the formation of the hexagonal Zn structures.
These results for the early growth stage provide strong
evidence that oriented attachment is an important mechanism
underlying the formation of single crystalline Zn structures.8
When a Si wafer without an Al layer was used as the
substrate in a control experiment, crystal structures did not
form on the substrate surface after immersing the substrate in
aqueous ammonia solution at 25 1C for 6 h. Because zinc
cannot be reduced under these conditions, an aluminium layer
was chosen and used as the reductant. Replacement reactions,
in which the more reactive component is consumed without
any external electron source, have been demonstrated to
provide a general and effective method for preparing metallic
nanostructures.9 We propose that the following reactions
occur in the system comprising an aqueous ammonia solution
and an Al layer:
In summary, we report a novel method for covering a
substrate with highly-oriented single crystalline hexagonal zinc
structures under atmospheric pressure and room temperature.
On the basis of the results, we propose a mechanism for the
spontaneous growth of such highly-oriented hexagonal single
crystalline Zn structures. The present method should be
applicable to the synthesis of other single crystalline metal
structures and provides an alternative method for the synthesis
of highly-oriented metal nanostructures.
This work was supported by grants from the second phase
BK21 program of the Ministry of Education of Korea and the
Acceleration Research Program (2009–0079077) of the Korea
Science and Engineering Foundation (KOSEF).
Notes and references
+
NH3 + H2O - NH4 + OHÀ
(1)
(2)
1 (a) J. P. Novak, L. C. Brousseau, F. W. Vance, R. C. Johnson,
B. I. Lemon, J. T. Hupp and D. L. Feldheim, J. Am. Chem. Soc.,
2000, 122, 12029; (b) F. Kim, J. H. Song and P. Yang, J. Am.
Chem. Soc., 2002, 124, 14316; (c) T. S. Ahmadi, Z. L. Wang,
T. C. Green, A. Henglein and M. A. El-Sayed, Science, 1996, 272,
1924; (d) X. Teng, D. Black, N. J. Watkins, Y. Gao and H. Yang,
Nano Lett., 2003, 3, 261; (e) S. Chen and Y. Yang, J. Am. Chem.
Soc., 2002, 124, 5280; (f) P. V. Kamat, J. Phys. Chem. B, 2002, 106,
7729; (g) T. A. Taton, C. A. Mirkin and R. L. Letsinger, Science,
2000, 289, 1757; (h) P. M. Tessier, O. D. Velev, A. T. Kalambur,
J. F. Rabolt, A. M. Lenhoff and E. W. Kaler, J. Am. Chem. Soc.,
2000, 122, 9554; (i) S. Nie and S. R. Emory, Science, 1997, 275,
1102; (j) S. Yugang and X. Younan, Science, 2002, 298, 2176.
2 S. Xu, Y. Ding, Y. Wei, H. Fang, Y. Shen, A. K. Sood, D. L. Polla
and Z. L. Wang, J. Am. Chem. Soc., 2009, 131, 6670.
3 B. Liu and H. C. Zeng, Langmuir, 2004, 20, 4196.
Zn2+ + 4NH3 - Zn(NH3)4
2+
2+
2Al(s) + 3Zn(NH3)4 + 8OHÀ
À
- 3Zn(s) + 2Al(OH)4 + 12NH3
(3)
The proposed mechanism of formation of the hexagonal Zn
structures is illustrated in Fig. 4. The standard reduction
potentials of an Al(OH)4À/Al pair and a Zn(NH3)42+/Zn pair
are À2.33 V and À1.04 V, respectively,10 which indicates that
reaction (3) is spontaneous in aqueous media, and does not
require any external electron source. Zn nanoparticles are
formed by Zn2+ reduction processes near the Al layer through
reaction (3). The Zn nanoparticles then diffuse on the surface
and become attached to other Zn nanoparticles. Jiggling of
these nanoparticles via Brownian motion enables adjacent
particles to rotate to find the lowest-energy configuration,
resulting in a coherent particle–particle interface.11 As a
result of the oriented attachment of nanoparticles, hexagonal
nanostructures form on the substrate surface. The driving
force of this spontaneous oriented attachment is the elimination
4 E. Deiss, F. Holzer and O. Hass, Electrochim. Acta, 2002, 47, 3995.
5 J. G. Wang, M. L. Tian, N. Kumar and T. E. Mallouk, Nano Lett.,
2005, 5, 1247.
6 J. P. Heremans, C. M. Thrush, D. T. Morelli and M. C. Wu, Phys.
Rev. Lett., 2003, 91, 076804.
7 Y. J. Chen, B. Chi, H. Z. Zhang, H. Chen and Y. Chen, Mater.
Lett., 2007, 61, 144.
8 R. L. Penn and J. F. Banfield, Science, 1998, 281, 969.
9 (a) W. Lin, T. H. Warren, R. G. Nuzzo and G. S. Girolami, J. Am.
Chem. Soc., 1993, 115, 11644; (b) L. A. Poter, Jr., H. C. Choi,
A. E. Ribbe and J. M. Buriak, Nano Lett., 2002, 2, 1067.
10 A. J. Bard, R. Parsons and J. Jordan, in Standard Potentials in
Aqueous Solution, M. Dekker, New York, 1985.
of high-energy surfaces.11,12 As the reaction proceeds, the
2+
Zn(NH3)4
concentration, the amount of Al metal and
11 J. F. Banfield, S. A. Welch, H. Zhang, T. T. Ebert and R. L. Penn,
Science, 2000, 289, 751.
12 A. P. Alivisatos, Science, 2000, 289, 736.
13 W. Ostwald Lehrbuch der Allgemeinen Chemie, Engelmann,
Leipzig, Germany, 1896, vol. 2, part 1.
the number of Zn nanoparticles decrease. As time progresses,
the hexagonal crystals formed via oriented attachment are
transformed into regular and smooth hexagonal plates (Fig. 2)
through the coarsening process known as Ostwald ripening.13
ꢀc
This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 6053–6055 | 6055