Large-Scale Growth and Shape Evolution of Cu2O Cubes Debao Wang,† Maosong Mo,† Dabin Yu,† Liqiang Xu,† Fanqing Li,‡ and Yitai Qian*,†,‡ Department of Chemistry and Structure Research Laboratory, University of Science and Technology of China, Hefei 230026, Anhui, China
CRYSTAL GROWTH & DESIGN 2003 VOL. 3, NO. 5 717-720
Received April 5, 2003
ABSTRACT: Uniform crystalline Cu2O cubes were synthesized in high yield by reducing the copper-citrate complex solution with glucose. A series of shape evolutions of Cu2O particles from the transient species such as multi-pod and star-shaped particles to cubic crystals have been arrested based on TEM and SEM observation. The higher growth rate along 〈111〉 induces the shrinking of the eight {111} faces, while six {100} faces remained to form Cu2O cubes because of their lower growth rate. The experimental results suggest that the building blocks with desired architecture can be selectively synthesized by programming the growth parameters in the initial synthetic scheme. Introduction It is well-known that the shape and size of inorganic nanocrystals have much influence on their widely varying physical properties.1,2 Thus, the synthesis of inorganic nanocrystals of controlled size and shape are of special interest. Although, in the past decade, there has been an increasing number of experiments on the synthesis of novel colloid nanocrystals such as rods,3 cubes,4,5 prisms,6,7 triangles,8 and disks,9 the challenge of synthetically controlling the shape of nano-building blocks has been met with limited success. Understanding the growth history and the shape-guiding process of nanocrystals is technologically important in the shape- and size-controlled synthesis of nanocrystals and will make it possible to program the system to yield the building blocks with a desired shape and/or size.10 As an example, herein, we focus on the preparation and shape evolution of Cu2O cubes. Cuprous oxide (Cu2O) is a metal-oxide p-type semiconductor that has attracted much current interest because of its potential applications in solar energy conversion and catalysis.11,12 Also, Cu2O crystals have been at the center of the research of Bose-Einstein condensation (BEC) of excitons.13 A lot of effort has been devoted to the synthesis of Cu2O thin films,14 nanoparticles,15,16 nanowires,17,18 and whiskers.19 McFadyyen and Matijevic´ reported the synthesis of Cu2O cubic particles.20 Further studies showed that the cubic single crystals of Cu2O could be selectively achieved by using different organic additives.21-23 Recently, Murphy and Gou reported the solution-phase synthesis of highly monodisperse Cu2O cubes with cetyltrimethylammonium bromide as the protecting reagent,24 and the assynthesized Cu2O cubes are made of smaller nanoparticles rather than the single crystals. However, most of the above-mentioned Cu2O cubic crystals belong to the final growth stage. It is meaningful to investigate the morphological growth process of Cu2O cubes, which can provide important information to the fields of crystal growth and design and morphology-controlled synthesis * Corresponding author. Tel/Fax:
[email protected]. † Department of Chemistry. ‡ Structure Research Laboratory.
+86-551-3607402. E-mail:
of Cu2O and other inorganic building blocks. To study the growth history of Cu2O cubes, we reexamine the reaction of glucose and the copper-citrate complex, which makes it possible to arrest the Cu2O crystals in different stages of their growth. Experimental Procedures All chemicals were analytical pure and were used as received. The copper-citrate complex aqueous solution is normally prepared in two parts that are mixed in equal volumes prior to use: solution A containing 0.68 M CuSO4‚ 5H2O and solution B containing 0.74 M sodium citrate and 1.2 M anhydrous sodium carbonate. The reducer solution C contains 1.0 M glucose. In a typical procedure, a mixture of the reaction solution was prepared by mixing 1.0 mL of A, 1.0 mL of B, and 1.4 mL of C in a capped test tube. The solution was further diluted to 20 mL with deionized water, then was aged at 80 °C for a period of time (30 min to 4 h). The colloid solution was centrifuged and washed with deionized water followed by ethanol. The TEM images and electron diffraction (ED) patterns of the samples were taken on a Hitachi 800 transmission electronic microscope. SEM images were obtained on a Hitachi (X-650) scanning electron microanalyzer and a JSM-6700F field emission scanning electron microscope. XRD patterns of the products were recorded by employing a Philips Xpert X-ray diffractometer with CuK-R radiation (λ ) 1.54187 Å).
Results and Discussion It is well-known that the copper-citrate complex (Benedict’s solution) can be reduced by glucose to form Cu2O precipitates, which are widely used in the analytical determination of saccharides.25 The fundamental reaction involved in Benedict’s reaction can be simplified as follows:
Cu(citrate)-(aq) + C5H11O5-CHO(aq) f Cu2O(s) + C5H11O5COOH(aq) The present method is advantageous in that it could provide a time delay of more than 5 min before the reaction takes place, and all reactants can, therefore, be thoroughly mixed. It is expected that the reaction begins and proceeds in a homogeneous environment,
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Figure 1. XRD pattern of the as-synthesized Cu2O cubes.
and more important, it makes it the possible to follow the growth history of Cu2O crystals. Figure 1 shows the XRD pattern of the as-prepared Cu2O cubes. All the diffraction peaks are labeled and can be indexed according to cubic phase Cu2O (JCPDS file No.05-0667), although our samples’ (200) reflection is a little more intense as compared to the standard powder diffraction pattern. The lattice constant was calculated to be a ) 4.258 Å, comparable with that reported in the literature. The TEM and SEM images shown in Figure 2 provide direct information about the size and typical morphologies of the as-synthesized Cu2O particles grown for different period of times. TEM image in Figure 2a reveals that Cu2O particles exhibit a multi-pod-like morphology in the earlier stage of the reaction. The inset of Figure 2a shows the corresponding ED pattern, which can be identified as the [01h 1] zone axis projection of the Cu2O reciprocal lattice. The slighter spot of (100) in the inset of Figure 2a might result from the secondary reflection of (200) in Cu2O crystals. As the reaction proceeds, the multi-pod particles subsequently lose their shape by crystal growth into the spaces between the pods to show star-shaped morphology, most of which exhibit four symmetric horns (Figure 2b). Figure 2c shows the coexistence of the four-horn-shaped particles together with the cube-shaped ones, which suggests that, eventually, the star-shaped particles will develop into Cu2O cubes. These star-shaped particles showing four symmetric horns are a typical appearance of the Cu2O particles. Because TEM micrographs, even of shadowed samples, do not always reveal the true morphologies of the particles, SEM images were recorded to reveal the true structures of the star-shaped Cu2O particles. The SEM image in Figure 2d reveals the true structures of the four symmetric star-shaped particles shown in Figure 2c. It is clear that the four symmetric star-shaped particles under TEM observation are, in fact, three-dimensional eight-pod particles and that these eight-pod particles are developing into Cu2O cubes. It is easy to think that the eight pods exactly match the eight 〈111〉 directions of the cubic lattice of Cu2O. Figure 2e shows the TEM image of Cu2O cubes obtained in the final stage of the reaction. It is visible that the as-synthesized Cu2O cubes have a uniform size and an edge length of about 1.0 µm. SEM images in Figure 2f,g are presented to show the high yield and
Wang et al.
good uniformity of the as-synthesized Cu2O cubes, which is an important feature for connecting the building blocks into devices. It is concluded by Murphy26 that the preferential absorption of molecules and ions in solution to different crystal faces directs the growth of nanoparticles into various shapes by controlling the growth rates along different crystal axes. Wang27 suggested that the shape of an fcc nanocrystal was mainly determined by the ratio of the growth rate in the 〈100〉 to that in the 〈111〉, and cubes bounded by the six {100} planes will be formed when the ratio is relatively lower. Cheon and coworkers10 found that the faster growth on the {111} faces favors the formation of cube-shaped PbS crystals. These views are verified by our experiments in the synthesis of Cu2O cubes. On the basis of the TEM and SEM analysis, the shape evolution of Cu2O crystals ranging from eight-pod particles through star-shaped particles and then to cubes indicates that the formation of Cu2O cubes in this study may also be attributed to the higher growth rate of {111} planes of the Cu2O lattice. Cu2O crystallizes in the cuprite structure, in which each O is surrounded by a tetrahedral of copper ions, and each Cu has two oxygen neighbors. The {111} and {100} surfaces in the Cu2O crystal lattice are different in the surface atom structures and bonding as well as the possibility of chemical reactions. In the earlier stage of the reaction, once the Cu2O nuclei are formed, new reactants are continuously arriving at the site. The adsorption and de-adsorption of the citrate ions on the different planes of Cu2O nuclei may kinetically favor the preferential crystal growth along eight 〈111〉 directions. As a result, continuous growth of Cu2O in the 〈111〉 direction leads to the formation of eight-pod particles. If projected or shadowed on the (001) face, the eight-pod particles will exhibit a four-symmetric crosslike or star-shaped morphology (Figure 2a,b). In the following stage, the open spaces between the pods are gradually filled by the crystal growth of possibly the {110} and {100} planes, and in this way, the multi-pod particles gradually lost their shape (Figure 2c,d). Eventually, the {111} facets were eliminated because of their higher growth rate, and the {100} facets remained because they have the lower growth rate. A cube enclosed with six {100} planes is then obtained (Figure 2e). The growth scheme of Cu2O structures is summarized in Figure 3. When in combination with the unit cell of Cu2O, the cube is composed of six square faces, each of which intersects one of the crystallographic axes and is parallel to the other two. If the synthesis of Cu2O particles was conducted by aging the reaction solution in boiling water just as that used in the analytical determination of saccharides,25 the growth rate was sharply increased, and the relative growth rate difference of different surfaces completely diminished. After a burst of nucleation, the precipitate is quenched, and these primary particles may aggregate into larger ones (Figure 4a). As the aging time was prolonged, spherical particles were obtained (Figure 4b). This process of crystal growth and morphology evolution has been described in terms of Ostwald ripening,28 which involves the growth of larger particles at the expense of the smaller ones driven by the tendency of
Growth and Shape Evolution of Cu2O Cubes
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Figure 2. TEM and SEM images of Cu2O particles prepared for different periods of time: (a) 40 min; (b) 1 h; (c and d) 1.5 h; and (e-g) 4 h. Inset (a) is the corresponding ED pattern.
Figure 3. Schematic representation of the mode for the growth of Cu2O cubes.
the solid phase in the systems to adjust itself to achieve a minimum total surface free energy. Further studies are necessary to understand the exact formation mechanism of Cu2O cubes. Conclusion In summary, with glucose as a reductant, cuprous oxide cubes in high quantity were synthesized successfully. On the basis of TEM and SEM analysis, we revealed critical factors for determining the architec-
Figure 4. TEM images of Cu2O particles prepared at 100 °C for (a) 2 min and (b) 10 min.
tural features of Cu2O crystals. We concluded that the {100} cubes resulted from the higher growth rate in the 〈111〉. These Cu2O cubes may find applications in the fields of constructing solar energy conversion devices and studying spontaneous Bose coherence of excitons and polaritons. These synthetic and mechanistic studies
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may be the basis for controlling the geometries of a wide range of novel nano/micro building blocks, and more importantly, for the success of bottom-up approaches toward future devices. Acknowledgment. Financial support from the National Natural Science Fund of China and the 973 Projects of China are appreciated. D.W. acknowledges Drs. Y. Y. Peng and M. W. Shao for their helpful discussions. References (1) Alivisatos, A. P. Science 1996, 271, 933-937. (2) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257-264. (3) Peng, X.; Manna, L.; Yang, W.; Wickham, J.; Scher, E.; Kadavanish, A.; Alivisato, A. P. Nature 2000, 404, 59-61. (4) Sun, Y.; Xia, Y. Science 2002, 298, 2176-2170. (5) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; ElSayed, M. A. Science 1996, 272, 1924-1925. (6) Jin, R.; Cao, Y.; Mirkin, C. A.; Kelly, K. L.; Schatz, G. C.; Zheng, J. G. Science 2001, 294, 1901-1903. (7) Pastoriza-Santos, I.; Liz-Marzan, L. M. Nano Lett. 2002, 2, 903-905. (8) Yang, J.; Fendler, J. H. J. Phys. Chem. 1995, 99, 55055511. (9) Maillard, M.; Giorgio, S.; Pileni, M.-P. J. Phys. Chem. B 2003, 107, 2466-2470. (10) Lee, S.-M.; Cho, S.-N.; Cheon, J. Adv. Mater. 2003, 15, 441444. (11) Musa, A. O.; Akomolafe, T.; Carter, M. J. Sol. Energy Mater. Sol. Cells 1998, 51, 305-316.
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