Five Branching Growth Patterns in the Cubic Crystal System: a Direct Observation of Cuprous Oxide Microcrystals

Acta Materialia 55 (2007) 2397–2406 www.actamat-journals.com

Five branching growth patterns in the cubic crystal system: A direct observation of cuprous oxide microcrystals

Jiasheng Xu, Dongfeng Xue *

State Key Laboratory of Fine Chemicals, Department of Materials Science and Chemical Engineering, School of Chemical Engineering, Dalian University of Technology, 158 Zhongshan Road, Dalian 116012, China

Received 29 September 2006; received in revised form 13 November 2006; accepted 17 November 2006 Available online 1 February 2007

Abstract

Cuprous oxide (Cu2O) is selected in this paper to elucidate the shape evolution in the cubic crystal system. A wide range of novel cuprous oxide microcrystals (based on the ﬁve branching growth patterns) has been prepared through an ethylenediaminetetraacetic acid tetrasodium salt dihydrate (EDTA) reduction route by employing the EDTA molecule as both chelating reagent and reductant. The morphology of these microcrystals has a strong dependence on the reaction conditions, which implies vast possibilities of designing new crystal morphologies and may result in the development of powerful and economical design strategies to enable future progress. 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Crystal morphology; Branching growth; Cubic crystal system; Cuprous oxide

1. Introduction patterns whose artistic value alone is worth understanding [27]. Crystal growth is a universal phenomenon in the field of A baseline for crystal morphology is the equilibrium materials science and technology. Knowledge of the funda- shape that results from minimizing the anisotropic surface mentals of crystal growth is due entirely to the morpholog- free energy of a crystal under the constraint of constant ical studies of naturally occurring crystals, which slowly led volume. As is well known, the equilibrium shape that cor- to the understanding of the atomistic process of crystal responds to a given c-plot is given by the Wulff construc- growth [1–5]. The recent explosion of research on nano- tion, according to which the shape is the inner convex structured materials depends heavily on the crystal growth hull bounded by planes (Wulff planes) [28–32]. When inor- theory and technology [6–18]. As a synergetic result of the ganic crystals are formed under equilibrium conditions, crystal structure and specific growth conditions, the crystal their crystal habit is determined by the relative order of sur- morphology comprehensively reflects the detailed growth face energies. Fast crystal growth will occur in the direction history and records the variation of growth conditions perpendicular to the face with the highest surface energy, [19–26]. The variously shaped crystals possess both aes- while, during real crystal growth process, branching thetic beauty and scientific attraction; natural gemstones, growth can be created by a diffusion effect. When a crystal for example, possess crystal morphologies and degrees of grows, ions or molecules near its surface are consumed by perfection that have yet to be duplicated by artificial pro- the growing crystal and a concentric diffusion field forms cess, while snowflakes decorate our world with beautiful around the crystal [33]. This makes the apexes of a polyhe- dral crystal, which protrude further into the region of higher concentration, grow faster than the central part of facets, thus forming branches. * Corresponding author. As a p-type semiconductor (direct bandgap 2.17 eV) E-mail address: [email protected] (D. Xue). with unique optical and magnetic properties, cuprous oxide

(Cu2O) is a promising material with potential applications was adjusted by adding NaOH solution when required. in solar energy conversion, magnetic storage devices, and Prior to the reaction, the mixture was sonicated in an catalysis [34–37]. Cuprous oxide possesses a cubic crystal ultrasonic water bath for 10 min. The bluish solution symmetry, and it has long been known that this solid com- was then transferred into a Teflon-lined stainless steel pound can be prepared into cubic, cuboctahedral and octa- autoclave, which was filled to 80% of its capacity hedral crystals in solution media [38,39]. Many recent (80 ml). The autoclave was heated to 80–150 C for efforts have been devoted to the shape-controlled synthesis 6–24 h (detailed experimental conditions are shown in of Cu2O micro- and nanocrystals, such as electrodeposit- Table 1), and then allowed to cool to room temperature ion, thermal relaxation, sonochemical methods, vacuum naturally. The red products (as shown in the inset of evaporation and the liquid-phase reduction of a metal salt Fig. 6b) were collected and washed several times with dis- [40–47]. Herein, a novel reduction route has been demon- tilled water and absolute ethanol, respectively. The final strated by employing the reductant ethylenediaminetetra- products were dried at 50 C (more than 5 h) for further acetic acid tetrasodium salt dihydrate (EDTA salt, characterizations. The phase and crystallographic struc- C10H12N2Na4O8 Æ 2H2O). ture of the as-prepared samples were determined by pow- The stability of the aqueous solution can be improved by EDTA chelating with Cu(II) cations (stability constant of Cu(II)–EDTA = 1018.8 at 20–25 C), restraining precipi- Table 1 tation in the alkaline environment. Self-oxidation–reduc- A brief summary of representative experiments in this work tion reactions in the autoclave possibly take place under No. EDTA/Cu [Cu2+] pH Temperature Time Results hydrothermal conditions at elevated temperatures. In the molar ratio (C) (h) current synthetic system, EDTA serves as both chelating 1 1 0.05 8 110 6 Fig. 7a1, a2 reagent and reductant, which is different from the previ- 2 1 0.05 8 110 12 Fig. 7a3, a4, Fig. 10i ously reported reductants for the synthesis of Cu2O (such 3 1 0.05 9 110 6 Fig. 7b1, b2 as glucose, formic acid, acetic acid, NaBH4 and DMF). 4 1 0.05 9 110 12 Fig. 7b3 Usually, organic acid can act not only as an acid, but also 5 1 0.05 9 110 24 Fig. 7b4 a reducing agent, a facility that has been widely investi- 6 1 0.05 9 120 6 Fig. 7c1, c2 gated in previous work [16,39]. At the same time, the deg- 7 1 0.05 9 120 12 Fig. 7c3 8 1 0.05 9 120 24 Fig. 7c4 radation (oxidation) of EDTA has been thoroughly 9 1 0.10 9 110 12 Fig. 7d1–d4 investigated in the fields of biochemistry and environmen- 10 1 0.10 9 120 24 Fig. 7e1, e2 tal chemistry, showing the final degradation products of 11 1 0.10 9 130 24 Fig. 7e3, e4 EDTA to be CO2 and H2O [48–51]. EDTA should be a reducing agent in the current synthetic system, since 12 1.5 0.05 8 110 12 Fig. 8a1, a2, c1, Fig. 10iii Cu2O microcrystals have been found at the end of the 13 1.5 0.05 8 110 24 Fig. 8a3, a4 experimental procedures. 14 1.5 0.05 9 110 12 Fig. 8b1, b2 Choi et al. [40–42] reported an electrochemical growth 15 1.5 0.05 9 110 24 Fig. 8b3, b4 16 1.5 0.10 10 110 24 Fig. 8c2–c4 route to obtain Cu2O microcrystals with a vast array of architectures, which indicates that Cu O is generally 17 1.5 0.10 11 110 12 Fig. 8d1, d2 2 18 1.5 0.10 11 110 24 Fig. 8d3, d4 selected as a model to investigate the shape evolutions in 19 1.5 0.10 12 110 12 Fig. 8e1, e2 the cubic crystal system. Usually, Cu2O architectures are 20 1.5 0.10 12 110 24 Fig. 8e3, e4, based on 6-, 8- or 12-pod branching growth patterns. In Fig. 10ii the current synthetic system, five branching growth pat- 21 1.5 0.10 13 120 12 Fig. 8f1, f2 terns have been proposed, which are the full range of 22 1.5 0.10 13 120 24 Fig. 8f3, f4 branching growth patterns for the cubic crystal system. 23 2 0.05 9 100 12 Fig. 9a1, a2 In particular, two kinds of high symmetric 24-pod branch- 24 2 0.05 10 100 12 Fig. 9a3, a4 ing growth patterns (i.e. 2 · 12-pod and 4 · 6-pod branch- 25 2 0.05 11 110 12 Fig. 9b1–b3, ing growth patterns) have not been reported previously in Fig. 10v any synthetic systems. 26 2 0.05 11 110 24 Fig. 9b4 27 2 0.10 12 110 12 Fig. 9c1, c2 28 2 0.10 12 110 24 Fig. 9c3, c4 2. Experimental 29 2 0.10 13 120 12 Fig. 9d1, d2 30 2 0.10 13 120 24 Fig. 9d3, d4 All reagents in the current experiments were of analy- 31 2 0.10 14 130 12 Fig. 9e1, e2 tical grade (purchased from Shanghai Chemical Industrial 32 2 0.10 14 130 24 Fig. 9e3, e4, Fig. 10iv Co.) and were used as-received without further purifica- tion. Solutions of Cu(II)–EDTA2 were prepared by dis- A series of experiments was also conducted in the present work. When the temperature was lower that 80 C or pH < 8, the self-oxidation–reduction solving equimolar quantities of EDTA and Cu(NO3)2 in reaction could not take place. When the temperature was higher than deionized water or stoichiometric ratios of EDTA/Cu(II) 150 C, some particles with irregular morphologies were observed. These (fixed as 1.5 and 2, respectively). The pH of the solutions data are therefore not included in the above table. J. Xu, D. Xue / Acta Materialia 55 (2007) 2397–2406 2399 der X-ray diffraction (XRD; D/Max 2400, Rigaku, by a Under isothermal conditions, a small crystal in equilib- diffractometer equipped with graphite monochromatized rium with its melt, solution or vapor takes on a so-called Cu Ka radiation) in 2h angles ranging from 20 to 80. equilibrium shape in order to minimize its interfacial free The morphology and size of these crystals were character- energy: Z ized by scanning electron microscopy (SEM), equipped with an energy-dispersive X-ray spectrometer (JSM- XXS ¼ cðn^ÞdA ð1Þ 5600LV, JEOL). A subject to the constraint of constant crystal volume. The quantity cðn^Þ is the anisotropic interfacial free energy (ex- 3. Results and discussion cess Kramers potential) per unit area, n^ is a unit vector perpendicular to the interface, A is the area of crystal interface The cube is a highly symmetrical body, possessing 23 (surface) and the integral is over the interface (surface) of elements of symmetry (a center, nine planes and 13 axes). the entire equilibrium shape. According to Gibbs–Wulff’s An octahedron has the same 23 elements of symmetry; theorem, therefore, despite the difference in outward appearance, c c c there is a definite crystallographic relationship between 1 ¼ 2 ¼ 3 ¼¼constant ð2Þ these two forms. Fig. 1 indicates the shape evolution from h1 h2 h3 the cubic to the octahedral form, and vice versa, by a pro- where cn is the surface tension of crystal face n and hn is the gressive and symmetrical removal of the involved corners. distance of that face from the Wulff’s point in the crystal. The intermediate solid forms (truncated cube, truncated Higher surface tension faces tend to grow along their nor- octahedron and cuboctahedron) are three of the 13 Archi- mal direction and eventually disappear from the final medean semi-regular solids which are called combination appearance. For the cubic phase, h{111} = d{111}/2; forms (i.e. combinations of a cube and an octahedron). h{100} = d{100}/2; h{110} = d{110} (where d is the interplanar The Euler relationship is useful for calculating the number distance), a sequence of c{111} < c{100} < c{110} can be easily of faces (F), edges (E) and corners (C) of a simple con- deduced from the distances between these three faces and nected polyhedron: E = F + C 2. This relationship states the central Wulff’s point [29]. that the number of edges is two less than the sum of the Cuprous oxide (Cu2O) crystallizes in the cuprite struc- number of faces and corners (see Table 2). ture. It belongs to an isometric system and its space

111 100

(iii) R= 0.87 Cuboctahedron 111 111 100 100

(ii) R = 0.70 (iv) R = 1.15 Truncated cube Truncated octahedron

111 100

(i) R ≤ 0.58 (v) R ≥ 1.73 Cube Octahedron

Fig. 1. Five branching growth patterns of cuprous oxide (Cu2O) microcrystals under the current synthetic conditions. Blue circles represent activated corners: (i) 8-pod branching growth along the Æ111æ direction, based on 8 corners of the cube; (ii) 24-pod branching growth, based on 12 · 2 corners of the truncated cube; (iii) 12-pod branching growth along the Æ110æ direction, based on 12 corners of the cuboctahedron; (iv) 24-pod branching growth, based on 6 · 4 corners of the truncated octahedron; (v) 6-pod branching growth along the Æ100æ direction, based on 6 corners of the octahedron. R is deﬁned as the growth rate along the Æ100æ direction to that along the Æ111æ direction. (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.) 2400 J. Xu, D. Xue / Acta Materialia 55 (2007) 2397–2406

Table 2 Properties of some regular and semi-regular forms found in the crystalline state Form Faces Edges Corners Edges at a corner Elements of symmetry Center Planes Axes Regular solids Hexahedron (cube) 6 12 8 3 Yes 9 13 Octahedron 8 12 6 4 Yes 9 13 Semi-regular solids Cuboctahedron 14 24 12 4 Yes 9 13 Truncated cube 14 36 24 3 Yes 9 13 Truncated octahedron 14 36 24 3 Yes 9 13 The number of corners is indicated in bold, and corresponds to the ﬁve branching growth patterns (Fig. 1).

Cu+

Cu+ O2–

O2–

b a b

Fig. 2. Crystal structure of cuprous oxide (Cu2O). Blue lines represent the cell edge. (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.) c

Fig. 3. Projection of cuprous oxide (Cu2O) crystal structure along the symmetry is expressed as Pn3m (Figs. 2–4). The coordina- a-axis, i.e. the {100} plane lies in the plane of the paper. Blue lines tion of the copper atoms is twice that of the oxygen atoms represent the cell edge. (For interpretation of the references to color in this in the unit cell. The structure can be described as a cubic figure legend, the reader is referred to the web version of this article.) close packing of copper atoms. According to Gibbs– Wulff ’s theorem (Eq. (2)), for the cubic crystallographic system, both the {111} and {100} faces can be easily maintained in the final appearance. The surface composed of {111} faces exhibits threefold rotational axes at the center of triangular faces and fourfold axes at the apexes; the surface consisting of {100} faces possesses threefold axes at the apexes (Fig. 5). The crystallographic structure and Cu+ chemical composition of the as-prepared samples have been confirmed by XRD measurements (Fig. 6, JCPDS O2– card No. 05-0667) and EDX analysis (atomic ratio, Cu:O 2:1). Five branching growth patterns of Cu2O microcrystals under the current synthetic conditions are shown in Fig. 1; R is defined as the growth rate along the c Æ100æ direction to that along the Æ111æ direction [52]. Blue circles1 represent activated corners (the number of corners corresponds to that of the regular and semi-regular forms, as shown in Table 2). a b The Cu2O microcrystals were prepared through a novel solution-phase route that is successfully proposed in this work. In a typical experimental procedure, the Cu2O micro- Fig. 4. Projection of cuprous oxide (Cu2O) crystal structure along the Æ111æ direction, i.e. the {111} plane lies in the plane of the paper. Blue lines represent the cell edge. (For interpretation of the references to color 1 For interpretation of the references in colour in figures, the reader is in this figure legend, the reader is referred to the web version of this referred to the Web version of this article. article.) J. Xu, D. Xue / Acta Materialia 55 (2007) 2397–2406 2401

crystals were obtained by the hydrothermal treatment of Cu(II)–EDTA2 solution in an alkaline environment. Sub- sequent experiments suggest that the morphology of the products has a strong dependence on the reaction conditions, such as the molar ratio of EDTA/Cu(II), reagent concentration, pH of the initial solution, and reaction time and temperature. When the molar ratio of EDTA/Cu(II) = 1, Cu2O crystal branching growth takes place along all orientations of Æ111æ, which results in the formation of 8-pod branching growth (type (i), Fig. 1). Some representative SEM images of Cu2O microcrystals are displayed in Fig. 7.InFig. 7a–c, the edges of a hopper (skeletal) crystal extend outwards from its core with a tiny cubic center leav- ing step-like faces, i.e. a family of {100} between these Fig. 5. Three-dimensional view of cube symmetry axes (the regular cube has six twofold axes, four threefold axes and three fourfold axes), which edges. As a result, eight small cubes are developed respec- were modeled with SHAPE V 7.1 (2002 by Shape Software, Kingsport tively from the octa-pods, giving rise to a set of crystal USA). aggregates (eight cubes per aggregate, as shown in Fig. 7a, columns 3 and 4); this is known as self-similar growth. The void spaces created within cube assembly are due to the rapid growth of the outer crystal planes (Fig. 7b and c). Generally speaking, an increase in temperature improves the reaction kinetics and results in an increase in reaction rate (or degree of branching). It should be noted that self-oxidation–reduction reactions can take place under hydrothermal conditions in an alkaline environment but not in an acidic environment (Table 1). As a result, the pH of the initial solution drastically affects the rate of the self-oxidation–reduction. That is to say, the reaction rate is pH dependent: a pH increase would induce an increase in the rate of self-oxidation–reduction, which would consequently result in an increase in the degree of branching growth in the current synthetic system. By further increasing the driving force (i.e. under high concentration, pH or reaction temperature; detailed experimental conditions are shown in Table 1), the degree of branching is effectively increased; flower-like crystals composed of eight identical tower-like horns are observed (Fig. 7d and e). More complexity can be introduced to the architecture of Cu2O crystals when the molar ratio of EDTA/Cu(II) is fixed at 1.5 with different concentrations, pH of the initial solution, and reaction time and temperature (Table 1). Fig. 8a–e shows Cu2O crystals with a cuboctahedral branching growth pattern, crystal branching growth taking place along 12 Æ110æ directions (Fig. 1, type (iii)). By systematically varying the experimental conditions, such as the concentration, pH of the initial solution, reaction temperature and time, a wide range of morphologies can be observed based on the type (ii) and type (iii) branching growth patterns. This strategy mimics the growth of snow crystals in nature, which lead to thousands of unique crystal morphologies. When developing snow crystals fall from the sky, they pass through many atmospheric regions of Fig. 6. (a) Typical XRD pattern of the as-prepared products, which varying humidity and temperature. As each region has dif- corresponds well to the standard diffraction pattern of cuprous oxide ferent preferences for different branching growth patterns, (JCPDS card No. 05-0667). The solid curve is our experimental XRD pattern; the dashed curve is the simulated one. (b) Energy-dispersive X-ray great complexity is created in their final morphologies [27]. spectra of the as-prepared products. The inset indicates the red color of the Various novel and exquisite patterns are generated when as-prepared cuprous oxide. crystals are forced to develop by using branches that point 2402 J. Xu, D. Xue / Acta Materialia 55 (2007) 2397–2406

Fig. 7. SEM images of novel Cu2O microcrystal morphologies, which display systematically varying degrees of the 8-pod branching growth (type (i), Fig. 1). Detailed experimental conditions are shown in Table 1, scale bar = 1 lm. to the 12 apexes of a cuboctahedral crystal. This observa- starlike particles (hexapod-shaped microcrystals) are tion implies vast possibilities of designing new crystal mor- observed in Fig. 9a and b. It is clear that the six legs of phologies by precisely tuning the growth conditions. these architectures grow from the six corners of the octa- Interestingly, when the pH reaches 13, type (ii) branching hedral crystal (Æ100æ direction, type (v) branching growth growth becomes attainable (as shown in Fig. 8f), which is pattern). Fig. 9c–e shows type (iv) branching patterns of the 2 · 12-pod branching growth pattern that point to the architecture with a fascinating structure (4 · 6-pod branch- 24 apexes of a truncated cube crystal (Fig. 1). Although ing growth). By manipulating synthetic conditions (Table 1) the morphologies shown in Fig. 8 appear to be complex, that are directly responsible for the systematic change in they are based on type (ii) and type (iii) branching growth the degree of branching, various elegant Cu2O architec- patterns and can be systematically controlled. For example, tures can be observed, which are the 4 · 6-pod branching a higher concentration, a higher pH and a higher tempera- growth patterns that point to the 24 apexes of a truncated ture favor the formation of complex branching growth pat- octahedron crystal (Fig. 1). terns, and can increase the degree of branching growth. All these cuprous oxide architectures (Figs. 7–10) dis- We have also shown that different Cu2O architectures play the five branching growth patterns as the degree of can be formed through different branching growth pat- branching is independently controlled via precisely terns, and grown with different orientations (when the manipulating synthetic parameters. In the current solu- molar ratio of EDTA/Cu(II) = 2, type (v) and type (iv) tion-phase route, EDTA serves as both chelating reagent branching growth patterns become attainable). For example, and reductant, and performs multiple roles. The existence J. Xu, D. Xue / Acta Materialia 55 (2007) 2397–2406 2403

Fig. 8. SEM images of novel Cu2O microcrystal morphologies. (a–e) 12-pod branching growth along the Æ110æ direction (type (iii), Fig. 1). (f) 24-pod branching growth (type (ii), Fig. 1). Detailed experimental conditions are shown in Table 1. Scale bar = 1 lm.

of Cu(II)–EDTA complex can restrain the precipitation trast, if the reduction rate is too fast (when the temper- of Cu(II) cations in the alkaline environment ature is higher than 150 C, Table 1), these seeds with (Fig. 11a), which can make the Cu2O precipitate slowly defects can evolve into other structures (irregular parti- and homogeneously from the solution during the crystal- cles) instead of well-defined morphologies. It is worth lization process. Due to the slow reaction process, both noting that self-oxidation–reduction reactions can take nucleation and growth turn into a kinetic control; the place under hydrothermal conditions in an alkaline envi- seeds with stacking faults form at the initial nucleation ronment but not in an acidic environment (Table 1). As stage and then grow into various architectures that are a result, the pH of the solution drastically affects the rate derived from the five branching growth patterns. In con- of the self-oxidation–reduction. In the current synthetic 2404 J. Xu, D. Xue / Acta Materialia 55 (2007) 2397–2406

Fig. 9. SEM images of novel Cu2O microcrystal morphologies. (a, b) 6-pod branching growth along the Æ100æ direction (type (v), Fig. 1). (c–e) 24-pod branching growth (type (iv), Fig. 1). Detailed experimental conditions are shown in Table 1. Scale bar = 1 lm.

system, when a molar ratio of EDTA/Cu(II) = 1 is the intrinsic surface energy of {100} faces of cubic Cu2O, selected, the precipitation of Cu(II) cations in the alka- containing Cu or O only, is higher than that of the line environment will occur if we add more NaOH solu- {111} faces, which contain mixed Cu/O (Figs. 2–4). EDTA tion (in order to have a high pH value). We therefore selectively stabilizes the {100} faces since it interacts design the synthetic conditions carefully to avoid precip- strongly with the charged {100} faces rather than the itation before the hydrothermal process. For example, uncharged {111} faces. Therefore, the cubic shape of the pH value is increased gradually while increasing the Cu2O can be obtained in a lower amount of EDTA amount of EDTA (Table 1). (EDTA/Cu(II) = 1, Fig. 7). When the amount of EDTA The color of EDTA–Cu(II) solution is blue. The optical is increased (EDTA/Cu(II) = 2), the effect of selective color of the Cu(II)–EDTA solutions (before or after hydro- adsorption of EDTA molecules on the different crystallo- thermal process) is shown in Fig. 11, from which we can see graphic planes is significantly eliminated, and octahedral- that there will be more residual EDTA in solution at the shaped Cu2O can be obtained (Fig. 9). In addition, the end of the reaction if more EDTA is added before the amount of EDTA will decrease as the reaction proceeds hydrothermal process. At the same time, with the self- (EDTA also acts as a reductant in the current synthetic sys- oxidation–reduction proceeding, the amount of EDTA will tem, and is decomposed during the Cu2O crystallization decrease gradually. The residual EDTA will selectively sta- process); the residual EDTA will selectively stabilize the bilize the faces of Cu2O during the ensuing growing pro- {100} faces at the end of the synthetic procedure. There- cess, which is also an important factor affecting the fore, small area of {100} faces can be maintained in the morphology of these novel architectures. It is known that final Cu2O architectures, which is different at the initial J. Xu, D. Xue / Acta Materialia 55 (2007) 2397–2406 2405

Fig. 11. Optical color of the Cu(II)–EDTA solutions, obtained before or after the hydrothermal process. (a) Before the hydrothermal process; (b) filtered solution after the hydrothermal process, with an initial molar ratio of EDTA/Cu(II) = 2 selected; (c) filtered solution after the hydrothermal process, with an initial molar ratio of EDTA/Cu(II) = 1.5 selected; (d) filtered solution after the hydrothermal process, with an initial molar ratio of EDTA/Cu(II) = 1 selected.

4. Conclusions

The shape of crystals grown under or near equilibrium conditions is governed mainly by surface free energies. As a result, crystals grown under these conditions generally have simple shapes with well-developed facets that can achieve a minimum surface energy. As the system is driven farther from equilibrium, surface kinetics and bulk transport of material and heat play a major role in determining crystal shapes. This often results in complicated growth patterns, which are not necessarily the most stable in terms of surface energy (i.e. branching growth). Cuprous oxide is typically selected to elucidate the shape evolution in the cubic crystal system. In this paper, a wide range of novel cuprous oxide architectures (based on the five branching growth patterns) has been prepared through an EDTA reduction route. On the basis of our growth experiments, it is clear that the branching growth is extremely sensitive to the chemical environment, which in turn determines the final crystal organiza- tion and morphology. Among the parameters that influence the morphologies of Cu2O, the molar ratio of EDTA/Cu(II) plays the major role in determining the pattern of Cu2O branching growth. Other parameters (the reagent concentration, pH of the initial solution, and reaction temperature and time) are directly responsible for the systematic change in the Fig. 10. Large-scale observation of five branching growth patterns, which degree of branching. This novel soft-solution approach pro- show the uniformity of microcrystal morphologies. (i–v) branching growth vides a general route to the synthesis of cuprous oxide archi- patterns are the same as in Fig. 1. Detailed experimental conditions are tectures with unique morphologies. Furthermore, this new shown in Table 1. The shape distribution of the Cu2O microcrystals is strategy (by using EDTA as both chelating reagent and based on the corresponding SEM images. reductant) may be extended to other materials in the cubic crystal system. nucleation stage (Fig. 8). On the basis of our growth exper- Acknowledgements iments, it is clear that the selective adsorption of EDTA molecules on different crystallographic planes of Cu2O This work was supported by Program for New Century crystal plays a major role in determining the branching Excellent Talents in University (NCET #05-0278), the growth patterns. National Natural Science Foundation of China (NSFC 2406 J. Xu, D. Xue / Acta Materialia 55 (2007) 2397–2406

#20471012), the Foundation for the Author of National [22] Xu J, Xue D. J Phys Chem B 2005;109:17157. Excellent Doctoral Dissertation of China (FANEDD [23] Xu J, Xue D. J Phys Chem B 2006;110:7750. #200322), the Research Fund for the Doctoral Program [24] Xu J, Xue D. J Phys Chem B 2006;110:11232. [25] Xu J, Xue D. J Phys Chem B 2006;110:17400. of Higher Education (RFDP #20040141004) and the Scien- [26] Xu J, Xue D. J Phys Chem Solids 2006;67:1427. tific Research Foundation for the Returned Overseas Chi- [27] Nakaya U. Snow crystals: natural and artificial. Cambridge, nese Scholars, State Education Ministry. MA: Harvard University Press; 1954. [28] Sekerka RF. Cryst Res Technol 2005;40:291. References [29] Wulff G. Z Krystallogr 1901;34:449. [30] Xu D, Xue D, Ratajczak H. J Mol Struct 2005;740:37. [1] Byrappa K, Ohachi T. Crystal growth technology. Norwich, [31] Xu D, Xue D. Physica B 2005;370:84. NY: William Andrew Publishing; 2002. [32] Xu D, Xue D. J Cryst Growth 2006;286:108. [2] Sunagawa I. Crystals growth, morphology and perfection. Cam- [33] Kudora T, Irisawa T, Ookawa A. J Cryst Growth 1977;42:41. bridge: Cambridge University Press; 2005. [34] Snoke D. Science 2002;298:1368. [3] Mann S, Ozin GA. Nature 1996;382:313. [35] Mittiga A, Salza E, Sarto F, Tucci M, Vasanthi R. Appl Phys Lett [4] Co¨lfen H, Antonietti M. Angew Chem Int Ed 2005;44:5576. 2006;88:163502. [5] Tian ZR, Liu J, Voigt JA, Mckenzie B, Xu H. Angew Chem Int Ed [36] Laskowski R, Blaha P, Schwarz K. Phys Rev B 2003;67:075102. 2003;42:414. [37] Gou L, Murphy CJ. Nano Lett 2003;3:231. [6] Breen TL, Tien J, Oliver SRJ, Hadzic T, Whitesides GM. Science [38] McFadyen P, Matijevic´ E. J Colloid Interface Sci 1973;44:95. 1998;280:2089. [39] Chang Y, Zeng HC. Cryst Growth Des 2004;4:273. [7] Sun Y, Xia Y. Science 2002;298:2176. [40] Siegfried MJ, Choi KS. Adv Mater 2004;16:1743. [8] Manna L, Milliron DJ, Meisel A, Scher EC, Alivisatos AP. Nat [41] Siegfried MJ, Choi KS. Angew Chem Int Ed 2005;44:3218. Mater 2003;2:382. [42] Siegfried MJ, Choi KS. J Am Chem Soc 2006;128:10356. [9] Xiang J, Lu W, Hu Y, Wu Y, Yan H, Lieber CM. Nature [43] Wang D, Mo M, Yu D, Xu L, Li F, Qian Y. Cryst Growth Des 2006;441:489. 2003;3:717. [10] Rosei F. J Phys Condens Mater 2004;16:S1393. [44] Lu C, Qi L, Yang J, Wang X, Zhang D, Xie J, et al. Adv Mater [11] Barth JV, Costantini G, Kern K. Nature 2005;437:671. 2005;17:2562. [12] Moriarty P. Rep Prog Phys 2001;64:297. [45] Yin M, Wu CK, Lou Y, Burda C, Koberstein JT, Zhu Y, et al. Nano [13] Wang X, Zhuang J, Peng Q, Li Y. Nature 2005;437:121. Lett 2005;127:9506. [14] Yu SH, Co¨lfen H, Tauer K, Antonietti M. Nat Mater 2005;4:51. [46] Kumar RV, Mastai Y, Diamant Y, Gedanken A. J Mater Chem [15] Lu L, Shen YF, Chen XH, Qian LH, Lu K. Science 2004;304:422. 2001;11:1209. [16] Sun X, Zhang YW, Si R, Yan CH. Small 2005;1:1081. [47] Ram S, Mitra C. Mater Sci Eng A 2001;304:805. [17] Gleiter H. Acta Mater 2000;48:1. [48] Adak S, Bhattacharyya DK, Mazumder A, Bandyopadhyay U, [18] Chu LW, Hsiue GH, Lin IN. Acta Mater 2006;54:1617. Banerjee RK. Biochem 1995;34:12998. [19] Zhu Y, Bando Y, Xue D, Golberg D. Adv Mater 2004;16:831. [49] Rodriguez JB, Mutis A, Yeber MC, Freer J, Baeza J, Mansilla HD. [20] Zhu Y, Bando Y, Xue D, Golberg D. J Am Chem Soc Water Sci Tech 1999;40:267. 2003;125:16196. [50] Park EH, Jung J, Chung HH. Chemosphere 2006;64:432. [21] Zhu Y, Bando Y, Xue D, Xu F, Golberg D. J Am Chem Soc [51] Bissey LL, Smith JL, Watts RJ. Water Res 2006;40:2477. 2003;125:14226. [52] Wang ZL. J Phys Chem B 2000;104:1153.