J. Phys. Chem. C 2008, 112, 13405–13409
13405
Twin-Crystal Nature of the Single-Crystal-Like Branched Cu2O Particles Yan-Yun Ma, Zhi-Yuan Jiang,* Qin Kuang, Shu-Hong Zhang, Zhao-Xiong Xie,* Rong-Bin Huang, and Lan-Sun Zheng State Key Laboratory for Physical Chemistry of Solid Surfaces, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen UniVersity, Xiamen 361005, China ReceiVed: NoVember 19, 2007; ReVised Manuscript ReceiVed: June 30, 2008
In recent years, considerable attention has been focused on various polyhedron-based branched particles due to their fundamental significance and widespread potential application. However, knowledge of the original nature and growth mechanisms for these complicated structures is still limited. In this paper, polyhedronbased branched Cu2O microcrystals have been prepared via an electrodeposition process without any surfactant at room temperature. The electron diffraction pattern of the branched Cu2O microcrystals can be indexed as a single crystal, as many other polyhedron-based branched crystals exhibit. By carefully analyzing the intensities of the diffraction spots we, for the first time, conclude that the single-crystal-like Cu2O branched particles are in fact twinning crystals. It could be anticipated that the crystal twinning (or the stacking fault) could be popular in the single-crystal-like branched particles. 1. Introduction The architectural control of nanosized materials with welldefined shape has attracted increasing interest in past decades due to the unique shape-dependent physical and chemical properties of nanomaterials.1 Understanding the growth mechanism and shapeguiding process is essential for predicting and controlling the final architecture of nanoscale building blocks, which is a critical requirement for successful “bottom-up” approaches toward future nanodevices.2 In recent years, considerable attention has been focused on the various polyhedron-based dendrites or branched structures.3-15 For example, PbS star-shaped nanocrystals, multipods and dendrites,4-6 hierarchical mesophase silicates,7 MnO multipods,8-10 flower-shaped Ag2O particles,11 and branched Cu2O crystals12-15 have been observed from nanometer to micrometer scales, and most of these particles were reported to be single crystals. According to the shape and corresponding crystallographic directions of these structures, some nonclassical mechanisms were suggested to interpret the ramified growth of these complicated structures. It was suggested that formation of single-crystalline PbS star-shaped nanocrystals,4-6 flower-shaped Ag2O particles,11 and branched Cu2O crystals15 occurred through a controlled and strengthened preferential growth of various crystal surfaces of the polyhedron-shaped or truncated polyhedron-shaped nuclei. J. Liu and co-workers proposed a self-similar assembly of polyhedral mesophase crystals for multiscale hierarchical structures and morphologies.7 D. Zitoun et al.8 and X. Zhong et al.9 suggested that growth of single-crystalline branched MnO multipods occurred via oriented attachment. Although investigations of such mechanisms have rapidly increased in the literature, there are still a number of unresolved issues on which contradictory explanations have been proposed; in particular, knowledge of the original nature of the branched crystals is still limited. In addition, though much of the literature has reported branched particles with a singlecrystalline nature, the single-crystal branched particles are less stable than perfect single crystals due to the fact that extended surfaces in branched particles considerably increase the surface energy in * To whom correspondence should be addressed. E-mail: zxxie@ xmu.edu.cn.
comparison with that of a single crystal adopting a polyhedron shape. Therefore, the intrinsic properties of ‘single-crystal branched particles’ should be explored. In this paper, we demonstrate for the first time that crystal twinning (or the stacking fault) is responsible for the polyhedron-based branched Cu2O microcrystals based on careful analyses of the crystal structure of Cu2O and selected area electron diffraction (SAED) patterns, which indicate that these single-crystal-like branched particles in fact consist of iso-oriented mosaic structures other than a perfect single crystal. It is anticipated that crystal twinning (or the stacking fault) could be a common phenomenon in single-crystal-like branched particles. Cu2O is an attractive, relatively nontoxic, p-type semiconductor (direct band gap of 2.1 eV) with unique optical and magnetic properties, which makes it a promising material with potential applications in solar energy conversion, catalysis, biosensing, micro/nanoelectronics, and magnetic storage devices.12-14,16,17 In the past several years, many approaches, including solution reduction routes,12,13,16,17 γ-irradiation reduction route,14 electrodeposition,15,18,19 and reductive self-assembly of CuO nanocrystals,20 have been developed to control Cu2O micro/nanocrystals with various shapes. In contrast with many other methods, electrodeposition provides a simple, nonwasteful, and inexpensive way to prepare many kinds of well-shaped structures with high uniformity. Recent publications show the surfactants can be used to control the Cu2O shape evolution during electrochemical growth.15 In this work, the shapes of Cu2O micro/nanoparticles have been successfully controlled by tuning the applied current density without any surfactant at room temperature. The intrinsic nature and growth mechanism of the branched particles have been discussed. 2. Experimental Section Growth of Cu2O particles was carried out in a glass cell equipped with a 10 × 10 mm2 platinum plate as the counter electrode and a platinum wire as the quasi-reference electrode at room temperature. The working electrode was a glassy carbon plate with a surface area of 10 × 10 mm2 exposed to the electrolyte. The electrolytes are 0.01 M CuSO4 aqueous solution without any additive or supporting electrolyte. The glassy carbon
10.1021/jp803459c CCC: $40.75 2008 American Chemical Society Published on Web 08/09/2008
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Figure 1. SEM images of Cu2O nanoparticles with well-controlled shapes synthesized by electrodeposition from 0.01 M CuSO4 aqueous solution with different current densities. (a) 50, (b) 10, (c and d) 0.5, and (e and f) 0.1 mA cm-2. Insets are the corresponding SEM images of typical branched particles with large magnification. (d) Schematic model of cuboctahedron (left-top) and cuboctahedron-based branched Cu2O nanoparticles with various shapes observed (cuboctahedron frames of different orientations were superimposed on the particles). (f) Schematic model of cube (left-top) and cube-based branched Cu2O nanoparticles with various shapes observed (cube frames of different orientations were superimposed on the particles). The deposition time was 30 s in all experiments.
electrode was carefully polished and washed before every experiment. The Cu2O particles were prepared by the electrodeposition method in a galvanostatic mode using a commercial electrochemical workstation (CHI631A). The current densities applied during electrodeposition will be described in the Results and Discussion. It should be pointed out that we do not use the potentiostatic mode because the potential cannot be well controlled in dilute solution without supporting electrolytes. The morphology of the deposited particles at different experimental conditions was characterized by scanning electron microscopy (SEM) (LEO-1530) with an electron accelerating voltage of 20 kV. The structure was further identified by transmission electron microscopy (TEM, FEI Tecnai F30). 3. Results and Discussion Figure 1 shows the morphologies of the electrodeposited particles from 0.01 M CuSO4 aqueous solution with applied current densities from 50 to 0.1 mA · cm-2. Electrodepostion from the CuSO4 aqueous solution at neutral condition produces
Ma et al. Cu2O, which has been reported by many other groups.15,18,19 In the present case, electrodeposited particles were carefully checked by selected area electron diffraction (SAED) and highresolution TEM, confirming that the particles were Cu2O. Figure 1a and 1b shows that deposited particles are a truncated octahedron with a size range from 300 to 400 nm when the applied current density is not less than 10 mA · cm-2. However, when the applied current density was reduced to (or lower than) 0.5 mA · cm-2, the morphology of the particles has a dramatic change. Branched particles with a size of about 800 nm are found instead of the isolated polyhedral particles as shown in Figure 1c. Although various branched particles were observed, the basic shape is cuboctahedron as shown in the left-top of Figure 1d, and different types of branched particles are in fact due to the different orientations of the cuboctahedron. The particle shapes resulting from the different orientation of cubotahedron-based branching are summarized in Figure 1d. We thus defined this kind of particle as a cuboctahedron-based branched particle. When the current density is reduced to 0.1 mA · cm-2, the deposited products are still branched particles. However, the branching pattern changes from the cuboctahedron-based one to a cube-based one with a size of about 500 nm, as shown in Figure 1e and 1f. The cube-based branching is defined as 8 branched particles along the 8 vertexes of a cube, as shown in the schematic model in the left-top of Figure 1f. In Figure 1f, the particle shapes resulting from the different orientations of cube-based branching are also summarized. By carefully analyzing the branched particles observed, all branched patterns observed in our experiments can be classified as cuboctahedron-based type and cube-based type. The results imply that the crystal structure of Cu2O determines the branching styles. Many previous studies claimed a single-crystal mode for such kinds of branched crystals.11,13 However, it is difficult to explain it from energetic point of view as a perfect polyhedral particle has a smaller surface area than that of a branched particle. By carefully checking the branched particles, we may find there is obvious mirror symmetry between two neighboring branches. The result indicates that there could be a gap in the middle of two neighboring branches and provides a clue that crystal twinning could be the most possible origin of such gaps for formation of branching. Cu2O crystal is a cubic structure, belonging to the Pn3jm space group. When the coordinate origin of the unit cell is shifted to a copper atom, the crystal structure can be drawn as Figure 2a. In this crystal structure, the copper atoms can be viewed as the cubic close-packed (fcc) structure and the oxygen atom seems to occupy 1/4 tetrahedral interstice positions of the copper atoms. The structure results in the possible crystal twinning at either the {100} or {110} plane, as shown in Figure 2b and 2c. While the oxygen atoms shift from their normal positions to the dot circle position (an equivalent position) as shown in Figure 2b, taking the {100} as the mirror plane, stacking faults occur at this (100) plane. Such stacking faults result in a twinning plane along this (100) plane. Similarly, stacking faults may occur on a (110) plane, as shown in Figure 2c. It should be pointed out that the crystal twinning at the {110} or {100} planes of a cubic crystal structure is merohedral twinning, which does not result in changes of the orientation of reciprocal lattices from a symmetric point of view, and thus, no additional diffraction spot can be observed because of the crystal twinning. However, for the twinned particles, the phase angle of the scattering beam from every single crystallite is different from others as the atomic coordinates of some atoms in the cell are different, which would result in the change of
Single-Crystal-Like Branched Cu2O Particles
J. Phys. Chem. C, Vol. 112, No. 35, 2008 13407
Figure 2. Crystal structure and possible twinning plane of the cubic Cu2O. (a) Crystal structure of the cubic Cu2O when the coordination origin is shifted to a copper atom. (b) Possible twinning plane at {100}. (c) Possible twinning plane at {110}.
Figure 3. TEM images and their corresponding SAED images of Cu2O micro/nanoparticles of different shapes: (a) a Cu2O single particle and its SAED from the [001] zone axis, (b) a cube-based branched particle and its SAED from the [001] zone axis, (c) a cuboctahedron-based branched particle and its SAED from the [001] zone axis, (d) the same branched particle as that of c and its corresponding SAED pattern from the [013] zone axis.
the diffraction intensities because of the coherence of the diffraction beams from different crystallites. To obtain deep insight into the nature of the branched particles, the SAED experiments were carefully carried out. Figure 3 shows the TEM images and their corresponding SAED images of Cu2O microparticles with typical shapes, such as perfect cube particle and cube-based and cuboctahedron-based branched particles. All electron diffraction (ED) spots are aligned as two-dimensional arrays, which can be indexed as the diffractions from a single-crystal cubic Cu2O phase as shown in Figure 3. However, some interesting phenomena are found when investigating the reflection intensities carefully. As can be seen from Figure 3a, the intensities of ED spots taken from the perfect single particle are approximate uniformity for the diffraction of the [001] zone axis. For diffraction spots of the same zone axis recorded from cube-based or cuboctahedronbased branched particles (Figure 3b and 3c), however, the intensities are markedly different from those from the perfect cube particle, a single crystal. The intensities of the diffraction spots with indices hkl of mixed even and odd numbers, such as [110], [130], etc., are much weaker than those with all even or all odd hkl values, such as [200], [220], etc. Such a diffraction property is similar to the kinematically forbidden reflection rule of a face-centered structure in which the diffraction spots with indices hkl of mixed even and odd numbers are systematic extinction. By rotating particle to the [013] zone axis, the intensities of diffraction spots have the same rule in comparison with the diffractions from the [001] zone axis (Figure 3d). Apparently, the abnormal intensities of diffraction spots imply that the branched particles are not perfect single crystals. The crystal twinning (or the stacking fault) could be the most probable origin of such change of diffraction intensities. As discussed above from the crystal structure of Cu2O, the crystal twinning may occur along the {100} or {110} planes.
Figure 4. Schematic illustration of cubic Cu2O unit cell in different situations: (a) crystal cell of the normal cubic Cu2O, where the coordination origin is shifted to a copper atom, (b) one of the mirror structures of the crystal cell, (c) the result of the crystal cell when the two Cu2O crystals with mirror symmetry pack together vertically along the 〈100〉 direction, where the dot circles represent additional positions generated by projecting the crystal cell, (d) a normal fcc crystal structure.
As an example, Figure 4a and 4b shows the crystal cell in a normal cubic Cu2O crystal and one of its mirror structures, respectively. When the two Cu2O crystals with mirror symmetry pack together vertically (i.e., twinning occurs), an equivalent fcc (face-centered cubic) lattice can be deduced if projecting along the 〈100〉 direction of the crystals, as shown in Figure 4c, where the dot circles represent additional positions generated by projecting the crystal cell. In Figure 4d, the equivalent fcc crystal structure is shown if considering the additional positions
13408 J. Phys. Chem. C, Vol. 112, No. 35, 2008
Figure 5. (a) TEM image of a cube-based particle along the [001] axis; the inset is the corresponding high-magnification TEM image of the white-square area. (b) Corresponding HRTEM image of the branching area marked by the arrow in the inset of a. The normal crystal lattice projected from [001] axis with a lattice spacing of 0.30 nm was shown on a small bulge attached on the particle as marked by the arrow in b. In comparison with the normal lattice, a centered lattice was clearly observed with the periodicity decreased to 0.21 nm in the part of the branching area.
as real atoms. For such a twinned crystal, the electron diffraction of the twinned particle will have similarity with the equivalent fcc crystals, where all of the reflections that are kinematically forbidden in the fcc structure will also be weak in the twinned crystal. As a result, the observed abnormal diffraction intensities can be well explained by the crystal twinning. To obtain direct information of the crystal twinning in the branched particles, high-resolution TEM (HRTEM) observations were also carried out. Figure 5a and 5b shows the TEM image of a cube-based particle and the HRTEM image of the branching area along the [001] axis, respectively. Although we did not directly observe the twinned boundaries, some information about the crystal twinning can be deduced by comparing HRTEM image of a small bulge attached on the particles and the branching area. It can be found that the normal crystal lattice projected from [001] axis with a lattice spacing of 0.30 nm (corresponding to the (110) lattice plane) was observed on a small bulge attached on the particle as marked by the arrow in Figure 5b. However, in the part of the branching area, the periodicity clearly decreases to 0.21 nm and a centered lattice in comparison with the normal lattice is clearly observed, which can be considered as the face-centered lattice of the normal cube Cu2O lattice. Such a face-centered-like lattice demonstrates the occurrence of crystal twinning between the upper and lower branches, as discussed and shown in Figure 4, which also can well explain the modulation of diffraction intensities. The occurrence of twinning could be very complicated. According to the above experimental facts, it can be found that the branching growth only takes place when the applied current density is extremely low. It provides some clues for the branching growth from the view of the supply of the reduced Cu2O intermediates. Figure 6 shows a schematic diagram of the concentration of reduced Cu2O intermediates at the electrode surface versus the deposition time. When a reduction current is applied on the electrode, the Cu2+ ions are first reduced to Cu2O intermediates that could be Cu2O clusters or oligomers. The concentration of such Cu2O intermediates at the interface then increases until nucleation of Cu2O crystal occurs. It is well known that a supersaturation condition should be first reached when the nucleation process starts, and therefore, the concentration of such Cu2O intermediates should drop during the nucleation process, as shown in Figure 6. After nucleation, the supply of the Cu2O intermediates results in growth of Cu2O particles. When the applied current density is high (such as 10
Ma et al.
Figure 6. Schematic diagram of the interface concentration of Cu2O intermediates versus the deposition time with different current densities: (a) 10, (b) 0.5, and (c) 0.1 mA cm-2. The corresponding SEM images of Cu2O products are also shown.
mA cm-2), the concentration of the Cu2O intermediates at the interface should reach the supersaturation condition quickly and then drops when the nucleation process occurs. However, the concentration of the Cu2O intermediates at the interface may not drop too much because sufficient supply of the reduced Cu2O intermediates is possible when the reduction current is high. Under this condition, growth of the crystallites is not disturbed, and then single-crystal particles are obtained (Figure 6a). When the reduction current density is low (e.g., 0.5 mA cm-2), the nucleation process results in a rapid decrease of Cu2O intermediates concentration at the interface and even exhaustion of the reduced Cu2O intermediates temporary. Then the Cu2O intermediates concentration increases slightly and keeps stable finally during growth of Cu2O crystallites (as shown in the schematic diagram of Figure 6b and 6c). We think such a great fluctuation of the reduced Cu2O intermediates at the interfaces during the nucleation process may be the origin of twinning. Very possibly, crystal twinning occurs on the surface of newly formed nuclei at very low current density, and then branched particles grow. Supposing crystal twinning occurs on all six {001} surfaces of a nucleus and then normal crystal growth continues independently on the six new crystallites, cube-based branched particles will form. Similarly, crystal twinning occurs on all 12 {011} surfaces of a nucleus, and the succeeding crystal growth will result in cuboctahedron-based branching. As the twinning may occur on different crystal surfaces if the degree of the fluctuation of the reduced Cu2O varies, cuboctahedronbased branching takes place at a current density of 0.5 mA cm-2 and cube-based branching at 0.1 mA cm-2. To obtain evidence of twinning growth, particles deposited at the early stage were observed. Figure 7 shows the particles obtained by decreasing the deposition time. These particles are clearly the rudiments of the branched particle. There already exist some boundaries between two adjacent connected crystallites in these small particles, where the boundaries could be highly energetic area and very possibly resulted from twinning. Such a deposition-time-dependent experiment supports the existence of a twinning nucleus. Twinning defects, whose structure and origin have been studied for more than two centuries, are a special kind of grain boundary that occurs commonly in many different materials.21,22 Recently, high-resolution electron microscopy studies have given direct evidence about the existence of the multiple twin nuclei in the tetrahedron-based branched particles such as ZnO tetrapod nanostructures.23-25 However, the other polyhedron-based
Single-Crystal-Like Branched Cu2O Particles
J. Phys. Chem. C, Vol. 112, No. 35, 2008 13409 Scientific Project of Fujian Province of China (Grant No. 2005HZ01-3), and NCET from the Ministry of Education of China. References and Notes
Figure 7. SEM images of Cu2O particles grown at an earlier stage of the deposition process: (a) 2 and (b) 5 s of deposition time. Other experimental conditions are the same as that for preparing cubocahedron-based branched particles.
branched particles have been thought to be single crystal due to their single-crystal-like diffraction pattern. The present study shows that twinning planes are also possible to be presented in the case of those twinned crystals sharing the same diffraction pattern of a single crystal, which would have deceived the observer. The result also indicates that crystal twinning may be common nature for all polyhedron-based branched particles. In summary, the structures of cuboctahedron-based branched and cube-based branched Cu2O particles have been carefully investigated by SEM and SAED. The SAED images of branched particles show that the diffractions are similar to those from fcc crystals, which provide credible evidence for the multipletwinning nature of the polyhedron-based branched particles. This multiple-twinning nature may be popular for all well-shaped polyhedral branched particles. It is our hope that the present study could deepen the understanding of the origin and mechanism of growth of branched particles and stimulate new efforts in controlling growth of other patterned crystals with important applications in various fields. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grant Nos. 20725310, 20721001, 20673085, and 20671078), the National Basic Research Program of China (Grant No. 2007CB815303), Key
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