Coordination Polyhedra: A Probable Basic Growth Unit in Solution for

Article pubs.acs.org/crystal

Coordination Polyhedra: A Probable Basic Growth Unit in Solution for the Crystal Growth of Inorganic Nonmetallic Nanomaterials? Jianwei Nai,† Jinliang Wu,† Lin Guo,*,† and Shihe Yang*,†,‡ †

School of Chemistry and Environment, Beihang University, Beijing 100191, China Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China

‡

S Supporting Information *

ABSTRACT: Learning from the classical crystallization mode and the conventional oriented attachment mode, we demonstrate another understanding of the crystal growth of inorganic nonmetallic nanomaterials in solution from the perspective of coordination polyhedra. A family of β-Ni(OH)2 hourglass-like nanostructures is controllably synthesized and chosen to illustrate this understanding, in which the coordination polyhedra of Ni(OH)64− are supposed to serve as the basic growth unit to grow these crystals in solution. According to this “coordination polyhedra growth unit” mode, a probable crystal growth mechanism featuring two-stage oriented attachment is put forth. In addition, with this deliberate mode, a series of anisotropic features as well as interesting structural patterns of the as-prepared β-Ni(OH)2 nanocrystals have also been successfully explained. The nanocrystal growth mechanism proposed in this paper may be general; for example, it might reflect the actual circumstances of crystallization of certain inorganic nonmetallic nanocrystals in solution.

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INTRODUCTION New properties can be acquired from materials at the nanoscale and, equally important, these properties change with their sizes or shapes.1 Therefore, tailoring the shapes and sizes of nanostructures to tune materials properties is regarded as a grand challenge that must be met from the viewpoint of applications in nanotechnology. Solution-based procedures for the preparation of nanostructures (a “bottom-up” approach) are considered very promising because of their potential to allow for precise control over the morphology of nanoparticles and thus tune their properties.2−4 In the past few years, increasing interest as well as unremitting efforts have been put into the research of controllably synthesizing a variety of fascinating nanomaterials via wet-chemical approaches, which is documented in several review articles.5−11 According to the literature, a good understanding of the crystal growth mode in a solution is undoubtedly vital and helpful for researchers to improve the engineering of a desired nanostructure with specific properties.12 This is even more important and necessary in the fabrication of inorganic nonmetallic nanocrystals since they contain more than one chemical element resulting in a more complicated crystallographic structure and are more sensitive to the growth condition. Basically, there are two popular growth modes for crystallization of monocrystals in solution. The classical crystallization (CC) mode13 starts from atoms, ions, or molecules as basic growth units, forming clusters, some of which eventually reach the size of the critical crystalline nucleus and grow further via ion-by-ion attachment in terms of unit cell replication, leading to a plane-by-plane growth of a single crystal. This CC mechanism might be a helpful tool for comprehending the bulk materials but might not qualify for © 2012 American Chemical Society

interpreting some detected experimental phenomena from the preparation of nanoscale inorganic nonmetallic materials by self-assembly or self-aggregation pathways. Moreover, the universal applicability of the CC mechanism was already questioned a few years ago.2 In contrast to the ion-based classical crystallization theory, a new crystal growth mechanism, the so-called “oriented attachment” (OA) mode, was first presented by Penn and Banfield.14−16 They regard the “primary nanoparticles” as the basic unit for the growth of a single crystal. Since then, the OA mechanism was further developed and introduced in numerous systems of inorganic nonmetallic nanocrystal recently, such as PbSe,17−19 ZnM (M = S, Te),20−22 TiO2,23−25 CeO2,26,27 NiSe,28 CuO,29,30 Y2O3,31 SnTe,32 goethite,33 and manganese oxide;34 it was also applied in the impressive investigations of inorganic nonmetallic mesocrystals by Cölfen and his coworkers.35−41 This mechanism holds that the bigger particles are grown from small primary nanoparticles through OA. The spontaneous self-organization of these adjacent nanoparticles would result in sharing a common crystallographic orientation, followed by docking of these particles at a planar interface.14 It seems that the OA mechanism could be easily understood and accepted in the explanation of the growth of simple selfassembly low-dimensional nanostructures, such as nanorods,17,21,26 nanowires,22,32 and nanoneedles.24 However, when it comes to some sophisticated high-dimensional structures with unique features,27−30,34,42 the OA mechanism could only provide a probable stacking approach to explain how Received: February 18, 2012 Revised: March 23, 2012 Published: April 2, 2012 2653

dx.doi.org/10.1021/cg300239c | Cryst. Growth Des. 2012, 12, 2653−2661

Crystal Growth & Design

Article

ethanol and dropped onto a carbon film supported on a copper grid for the drying process in air.

the small particles could coalesce to the final as-synthesized crystal but could not fully interpret why the crystal would present those unique shapes and structures by self-assembly. Hence, the necessity of both a full investigation of this new growth mechanism and a more comprehensive understanding of the crystal growth mode in the solution-based system for the nanocrystals is beyond doubt. Although one material can exhibit diverse crystal morphologies, the shape of the inorganic nonmetallic crystal is often related to its intrinsic crystallographic structure. Coordination polyhedron, anion-coordinated in most cases, has been generally accepted as the crystal structural unit and has taken the place of the unit cell to represent the crystallographic structure of inorganic nonmetals43−48 due to its particular structural feature. This special advantage of the coordination polyhedron has inspired us insomuch that we deem it possible and reasonable to conceive the coordination polyhedron as the basic growth unit in solution. Such a so-called “coordination polyhedra growth unit” (CPGU) mode, although systematically studied by Zhong et al. in bulk inorganic nonmetallic crystals,49 has hitherto received little attention in nanoscale materials.15,50 In this paper, β-Ni(OH)2 is chosen to demonstrate the concept of the CPGU mode in view of its typical crystallographic structure (CdI2-type structure) in inorganic nonmetallic materials. Besides, it is a further investigation of our previous work.51 As an example, a family of β-Ni(OH)2 hourglass nanostructures is controllably synthesized in the presence of surfactant. A probable growth mechanism based on two-stage OA is proposed for interpreting the growth process of the nickel hydroxide crystal. The Ni(OH)6 4− coordination polyhedron is assumed to serve as the basic growth unit to fabricate crystals with unique anisotropic features in solution. This modified OA mechanism provides a unique understanding of crystal growth, which might conform to the actual circumstances of crystallization of other inorganic nonmetallic nanocrystals.

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RESULTS AND DISCUSSION Characterization Investigations and Discussion. Figure 1 illustrates the crystallographic structural characteristics of the

Figure 1. XRD patterns of the as-prepared samples.

as-prepared products investigated by XRD analysis. All of the diffraction peaks of each sample can be indexed as β-phase hexagonal nickel hydroxide (space group: P3̅m1) and are in agreement with the standard values (JCPDS 14-0117). No peaks due to other impurities such as α-Ni(OH)2 were observed in the XRD patterns, which authenticate the high purity of all three samples. The relative intensity of (101̅1)/ (0001) increases obviously from sample 1 to sample 3, indicating a different evolution of the (1011̅ ) plane among the crystals of the three samples. Typical SEM images of sample 1, 2, and 3 are shown in Figure 2. Figure 2a is a panorama of the sample 1 with hexagonal nanoplates morphology which we would prefer to treat as a stunted hexagonal hourglass-like nanostructure in this paper (the reason is explained in the growth mechanism section). The average side length of the hexagon and the height of the hourglass can be observed in Figure 2b,c, which are 200 and 60 nm, respectively. Figure 2d shows the overview of the sample 2. An obvious type of concave polyhedron with a centrosymmetric hourglass-like structure is obtained. A particular morphology is evident with 280/120 nm in average long/short side length of the top/bottom truncated triangle, and with 270 nm in height. The magnified images shown in Figure 2e,f provide clear top and side views of the mesosoma hourglass structure. Well-developed hourglass nanocrystal, which is elongated compared to sample 2, is also obtained in sample 3 and shown in Figure 2g−i. The average side length of the top/bottom triangle is 400 nm and the height is elongated to be about 500 nm. Obviously, each β-Ni(OH)2 concave polyhedron could be seen as two truncated pyramids joining together with in a central-symmetry manner. The side faces can be divided into two shapes-isosceles triangles (A) and trapezoids (B) which are presented alternately. Interestingly, each triangle could match to a corresponding trapezoid to form a larger quasi-isosceles triangle with a curve in the middle (Figure 2i). In addition, some polyhedrons might have been

EXPERIMENTAL SECTION

Chemicals. Nickel(II) 2,4-pentanedionate (Ni(acac)2, 95%) was purchased from Alfa-Aesar reagent company. Cetyltrimethylammonium bromide (CTAB) and hydrazine monohydrate (N2H4·H2O, 80%) were both obtained from Tianjin Fine Chemical Co. Ltd. All chemicals were of analytical grade and used without further purification. Synthesis of β-Ni(OH)2 Nanocrystals. In a typical procedure, 0.051 g of Ni(acac)2 and 0.779 g CTAB were first dissolved into 50 mL of deionized water at room temperature, yielding a light green transparent solution. After 30 min of vigorous agitation, a certain amount of N2H4·H2O, that is, 0.5, 1.5, and 3.0 mL for sample 1, sample 2, and sample 3 respectively, was introduced dropwise into the mixture, turning the color of solution slowly to light bluish-purple. With another 30 min of stirring, the solution was heated to 80 °C for 5 h. At last, green precipitate was obtained and rinsed with ethanol and deionized water several times. Sample Characterization. The structures and compositions of the as-prepared products were characterized by X-ray powder diffraction (XRD) using a Rigaku Dmax2200 X-ray diffractometer with Cu Kα radiation (λ = 1.5416 Å). The XRD specimens were prepared by means of flattening the powder on the small slides. The morphologies of the synthesized samples were studied by a Hitachi S4800 cold-field emission scanning electron microscope (CFESEM) with the samples obtained from the thick suspension dropping on the silicon slice. Transmission electron microscopy (TEM) and highresolution TEM (HRTEM) investigations were carried out by a JEOL JEM-2100F microscope. The as-grown samples were dispersed in 2654



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Figure 3. (a) TEM and (b) HRTEM images of sample 1; (c, d) TEM images of sample 2 and 3, respectively. All the images are the top view of the samples; the inset of each image is the SAED pattern of the whole corresponding crystal, showing a typical hexagonal set of diffraction spots from the view of the [0001] direction.

Figure 4 illustrates the TEM images of the typical side view of these three samples. From panels a, c, and e of Figure 4, it can be observed that the pyramidal faces of this hourglass crystal family are rough. The details of the zigzag-like architecture can be obviously seen from the magnified TEM images in Figure 4b,d,f, which seem like a lamella-by-lamella construction (further proved by SEM images in Figure S1, Supporting Information). These “lamellas” (some are marked by black arrows) can be distinguished effortlessly by their boundary silhouettes at the edge of the crystal, while difficult since the adjacent faces of the “lamellas” would fuse when it comes to the inside of the crystal. However, the crystallographic fusion seems not to be perfect. Some planar defects, such as stacking faults and dislocations, can be identified in the vicinity of the fusing regions (Figure 4b,d,f). Further investigation of the surface pattern of the selected typical regions (f, g, and h) in Figure 4e could lead to a conclusion that the roughness of the pyramidal faces would transform gradually from clearly dentatelike structure at the center of the crystal to relatively smooth and flat structure at the end of the crystal, which can be readily seen from the different edge profiles of the crystal (Figure 4f− h). Besides, few defects but more continuous lattice fringes can be observed at the end of the crystal (Figure 4h), which might be attributed to a changing growth environment in the solution as the formation of the crystal. Growth Processes and Mechanistic Study. To better understand the formation of these unique hourglass-like structures, the effects of some experimental parameters are discussed. Basically, introducing a high concentration of hydrazine hydrate (80%) here is to provide sufficient hydroxyls to induce the chemical reactions in the solution as follows:

Figure 2. (a, d, g) Overview; (b, e, h) top view; (c, f, i) side view of the samples 1, 2, and 3, respectively.

disturbed by an external force and broke at the central concave position (marked by arrows in Figure 2d,g). Another common feature of these three samples observed in SEM images is that the top/bottom faces are smoother than the side faces. The structures of the as-prepared samples were further characterized by TEM. Typical overhead views of the three samples are shown in Figure 3a,c,d, respectively. The selected area electron diffraction (SAED) patterns share the same set of diffraction spots of single crystal structure with the electron beam along the [0001] direction, that is, the c axis of βNi(OH)2. It exhibits that the top/bottom faces of this crystal family are parallel to the (0001) plane, and the crystal is elongated and becomes higher along this fixed c axis from sample 1 to 3. A high-resolution TEM image in Figure 3b is obtained from a selected square area in Figure 3a. Clear lattice fringes of 0.27 nm between two adjacent planes is in good accordance with the d value of the (101̅0) plane. These TEM experimental data could lead us to believe that the pyramidal faces (side faces) of the crystals have something to do with the crystallographic pyramidal planes which are {101̅1} and {1̅011}. Hence, we infer that the significant increasing of the area of pyramidal faces from sample 1 to 3 (Figure 2) might account for the changes of the relative intensity of (101̅1)/ (0001) in the XRD patterns (Figure 1), which implies there might be a correlation between the enlarging of the pyramidal faces and the evolution of the crystallographic pyramidal planes. Some analogical inferences have been put forward in the investigations of ZnO with resemblant hourglass-like architecture.52,53 2655

Ni 2 + + n N2H4 ·H 2O ⇌ [Ni(N2H 2)n ]2 + + nH 2O

(1)

N2H4 + H 2O ⇌ N2H5+ + OH−

(2)



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coordinates with Ni2+ ion to avoid rapid nucleating at room temperature in the case of NaOH/KOH which is well-known. Moreover, increasing the amount of hydrazine hydrate gradually (from sample 1 to 3) favors a higher initial supersaturation of the solution (SS). This would induce the generation of a larger number of NCOs in the solution, which would be beneficial for the formation of numerous larger particles at the beginning of the crystal growth. These produced larger particles show poor growth anisotropy when they superimpose on the crystal interface, leading to a relatively rapid growth rate along the c axis of nickel hydroxide. Therefore, the more hydrazine hydrate initially added, the longer (along the c axis) crystals can be obtained. The role that the surfactant CTAB played on the formation of the hourglass-like structure was checked by preparing the samples in the absence of CTAB. It turned out that a similar morphology was readily obtained. However, the crystal was not well-grown with shorter height along the c axis as well as nonuniform size (Figure S2, Supporting Information), which was also observed in our previous work.51 It indicates that CTAB could be eliminated from the contribution to the formation of hourglass-like structure, but it could narrow the size distribution and lead to a well-grown crystal of nickel hydroxide. To obtain a further understanding of the anisotropic growth mechanism of hourglass-like particles, we collected reaction products at different growth stages of the sample 3 procedure for TEM investigation (Figure 5). At an early reaction stage (5 min), small Ni(OH)2 nanocrystals with about 3−4 nm in diameter and with the same (101̅0) lattice fringes but in various lattice direction (white arrows) were formed and aggregated together (Figure 5a and its inset). As the time reached 15 min, the aggregate particle became larger, that is, from 45 nm (inset of Figure 5) to about 100 nm (Figure 5b) in diameter. Three

Figure 4. Typical TEM images of sample 1 (a), sample 2 (c), and sample 3 (e) from the side view. Selected areas from (a), (c), and (e) are magnified and shown in (b), (d), and (f−h), respectively. The lamellar-like structure is highlighted by black arrows in (b), (d), and (f), while the edge profiles of the crystal in (f−h) are delineated by black dotted lines. The insets are HRTEM images of some typical regions in which planar defects can be detected (delineated by white lines) in (b), (d), and (f), while relatively perfect fusion and continuous lattice fringes can be observed in (h).

[Ni(N2H4)n ]2 + + 6OH− ⇌ Ni(OH)6 4 − + n N2H4

(3)

Herein, one Ni2+ ion is easily coordinated with six OH− ligands to constitute the intermediate complex, Ni(OH)64− coordination octahedron (NCO). The construction of these NCOs is similar to that of the structural units, Ni-O6 octahedron54 of the crystal. Then, NCOs would dehydrate into Ni(OH) 2 precipitates.55 Hence, the hydrazine hydrate can not only achieve the facilely formation of abundant NCOs but also

Figure 5. TEM and HRTEM images taken from the reaction products of the sample 3 procedure when heating to 80 °C at (a) 5 min, (b−e) 15 min, (f) 30 min, (g, h) 45 min, and (i) 1.5 h. Panels (a) and (c−e) are the HRTEM images of the selected region in the TEM of the inset of (a) and (b), respectively. The various lattice directions are marked by arrows, while some typical “bigger particles” are delineated by white wireframes in (c−e), and each white dotted line reveals the lattice junction between two neighboring particles. 2656



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Figure 6. Schematic illustration of the formation of monocrystalline hourglass-like nanostructures via the two-stage oriented attachment mechanism. Please note that the hydrogen atoms in all the schematic illustrations provided in this paper are omitted for clarity. Abbreviations used: FOA, firststage oriented attachment; SOA, secondary-stage oriented attachment; SS, supersaturation of the solution; NCO, Ni(OH)64− coordination octahedron; LDGU, larger-dimensional growth unit; MDGU, medium-dimensional growth unit; SDGU, small-dimensional growth unit and CES, center-to-ends stacking.

NCO stacking. Hence, we hypothesize qualitatively herein that there could be three types of complexes appearing in the each reaction procedure of the three samples: larger-dimensional growth unit (LDGU), medium-dimensional growth unit (MDGU), and small-dimensional growth unit (SDGU). The LDGU appeared at the initial stage of the reaction should consist of a larger number of NCO as the SS is higher at this stage; the MDGU should consist of a medium number of NCO as the SS will decease at the medium-term stage of the reaction; then the SDGU should consist of few NCOs when the SS is very low at the last stage of the reaction. It is important to mention that only LDGUs, we infer, could nucleate to be crystalline grains in the reaction system reported here, which might correspond to the 3−4 nm particles observed in Figure 5a; MDSUs and SDGUs might present to be an amorphous complex because of their relatively small size-scale, so could not be clearly distinguished by TEM. In each stage of the reaction, numerous NCOs would serve as the basic building blocks to stack and bond together into the crystallographic register to form corresponding dimensional secondary growth units (LDGUs, MDGUs, or SDGUs). We define this process as the first-stage OA. And then these diverse dimensional secondary building blocks would aggregate, align, and interconnect along a common crystallographic orientation, following the stacking sequence of LDGUs−MDGUs−SDGUs to achieve participation in the evolution of the crystal growth. We define this process as the secondary-stage OA, which is similar with the conventional OA. In this stage, we believe that the OA growth follows a center-to-end stacking (CES) mode (Figure S3, Supporting Information): the later-generated growth units would stack on or below (along the ⟨0001⟩ direction) the previous-generated growth units, forming a final architecture with LDGUs locating in the center and the SDGUs locating in the ends. Of course, this CES growth mode also

regions were selected to be magnified and are shown in Figure 5c−e. It is observed that most of the nanocrystals were enlarged to bigger ones (delineated by white wireframes) with the same (101̅0) lattice fringes as identified before. It indicates that bigger crystals tend to be formed through the coalescence of smaller crystals though there might be initial mismatching of lattice fringes between some two adjacent smaller particles. When the reaction time was extended to 30 min, the aggregated particle could grow even larger with about 150 nm in diameter (Figure 5f). Compared with the diameter at 30 min, the particle obtained from 45 min changed little but with a much better contrast (Figure 5g), indicating that (i) small and dispersive crystals assembled and coalesced to form big crystals in a continuous manner; (ii) the thickness of the crystal increased, which agrees with the 80 nm thickness crystal observed in Figure 5h. As the time went to 1.5 h, both the contrast and the size of the crystal were further enhanced (Figure 5i). Then the crystals would continue to grow to the size shown in Figure 2g−i at 2.5 h and would not change any longer. The remaining 2.5 h of the reaction time should be a typical aging procedure for the crystal growth, during which Ostwald ripening would be involved. This observed time-dependent evolution of the crystal accompanied by the microstructural features (e.g., gap, “dimples”, and “creases”) retained in the final crystals (Figure 4) provides evidence that the final crystal might result from particle-by-particle aggregation growth rather than classical atom-by-atom addition.29,56 According to our experimental results, a probable growth scenario (Figure 6) based on a two-stage OA mechanism is propounded here via employing the CPGU model, in which NCO is regarded as the basic growth unit in this reaction system. These NCOs ought to orient assemble to form some certain complex, with a well-ordered internal structure, to stack on the growth interface of the crystal rather than one-by-one 2657



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involves the stacking of secondary growth units along other crystallographic directions, for example, the direction. Eventually, a series of hourglass-like monocrystalline crystal can be obtained via this two-stage OA mechanism as shown in Figure 6. On the basis of the analysis above, some similarities and differences between these two stages of OA can be summarized as follows for further understanding this crystal growing mechanism. The similarities are as follows: (i) The driven forces which could orient assemble the growth units into the crystallographic register are the Brownian motion and van der Waals attraction; (ii) the outcome of the OA process is some complex or particles with oriented and well-organized internal structure. Because the building blocks in the secondarystage OA are larger than the ones in the first-stage OA, some phenomena might arise in the secondary-stage OA but not in the first-stage OA: (i) apart from the OA, the process might also involve densification, intra-aggregate reactivity, and ripening;41 (ii) although some degree of “error-correction” could restructure the initial-mismatched building blocks into the crystallographic register, the defects that arise from imperfect OA are still expected.14,57 These defects were also verified by our TEM results (Figure 4b,d,f). Additionally, it is worth noting that (i) although the existence of the three types of growth unit are hypothesized and some can hardly be distinguished through TEM (MDGUs and SDGUs), this hypothesis is proposed on the basis of the probable changes of the SS. Besides, it could be inferred that the low velocity of the LDGUs diffusing at the crystal interface would cause a surface structure of pyramidal faces with a rough (lamella-like) pattern and some defects at the center of the crystal, while the surface with a relatively flat pattern and continuous lattice fringes would appear at the two ends of the crystal as the stacking units are SDGUs. The putative rough-toflat change of surface pattern and the change of the quantity of defects are consistent with the TEM observed result in Figure 4, which thus demonstrates the reasonableness of both the hypothesis of three types of secondary growth unit and the CES mode. (ii) The shape of the secondary growth unit is portrayed as triangular prism in this paper in Figure 6, because it should reflect the intrinsic trigonal crystallographic structure of nickel hydroxide since it is comprised of the oriented stacking of NCOs. Moreover, this shape could facilitate long-range crystallographic coherence via more efficient packing and epitaxial matching. Interestingly, three main distinct features of the series of hourglass nanostructures could be also observed as centralsymmetry, changes in the shape of crystal surface, and the concave crystal somatotype. It is not difficult to understand these features from the perspective of the CPGU model, while it might be inscrutable from the viewpoint of CC or conventional OA mechanism. It is believed that the intrinsic crystallographic structure of βNi(OH)2 plus the special CES growth mode could be responsible for the centrosymmetric structure that hourglasslike crystals present. To understand this, one should start from the analysis of the crystallographic structure of β-Ni(OH)2 (Figure 7a): it could be viewed as the repeating of unit cells on one hand; on the other hand, it tends to be described as the stacking of NCO layers since the oxygen packing is ABAB58 (CdI2-type structure). In one layer, NCOs are bonding together with edge-sharing. The crystallographic oriented bonding can be achieved by ion-covalent binding within the layers, while along the c axis by van de Waals forces.59 According to the

Figure 7. Schematic illustration of the intrinsic crystallographic structure of β-Ni(OH)2. (a) The spatial relationship between the single unit cell and stacking NCOs; (b) an existing relative slide between two adjacent NCO layers with the (0001) plane as the sliding plane (the oxygen atoms within the pink region belong to a single unit cell); (c) top view of the path in which NCOs slide along the a (a1, a2, or a3) axis; (d) NCO rotates around the c axis, forming a structural spiral (the NCO in pink zone is the central one). The upper and the lower inset show the single NCO in (b) and (c), respectively.

formula of oxygen packing, a relative sliding, rather than inversion symmetry, should exist between hydroxyls of two adjacent layers to allow a minimization of electrostatic interactions. The (0001) plane could be visualized as the structural sliding plane (Figure 7b). Nevertheless, these NCO layers can still maintain the trigonal crystallographic structure since the relative sliding of two adjacent layers is along the a axis (a1, a2, or a3) (Figure 7c). Therefore, the sliding path of NCO rotates around the c axis which, naturally, could be regarded as the structural screw axis of the β-Ni(OH)2 crystal and is shown in Figure 7d (more detailed illustration can be found in Figure S4, Supporting Information). As the oriented attachment would not change the intrinsic crystallographic structure, the stacking of diverse dimensional secondary growth units could be still conceived as the stacking of the primary NCO growth units. Hence, the intrinsic structural spiral with the characteristic of central symmetry (Figure 7d) acting in concert with the CES growth mode would bring up the crystal morphology of all the three samples to be centrosymmetric, which could simultaneously diminish the total energy of the system. The changes in the shape of the crystal surface might be the most interesting phenomenon in this family of hourglass-like nanostructures. This phenomenon could be attributed to the variation of area ratios of pyramidal faces which is a result of the characteristics of crystal growth. When the initial SS is relatively higher (sample 3), it is beneficial for the formation of numerous 2658



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trapezoid-shape morphology (Figure 8a). The shape of the top/bottom faces, as a result, would show equilateral triangle because of the area ratio of B/A increases to a certain extent. When the initial SS is lower (sample 1), relatively fewer LDGUs would form, leading to a slow growth rate along the c axis. Consequently, the two types of pyramidal faces would be both restrained in growing and show nearly no difference in their areas (A/B ≈ 1), favoring the hexagonal shape of the top/ bottom faces. Hence, the formation of the morphology of the crystal in sample 2 should be easily understood based on the discussion above. And this discussion is just in concert with the interpretation of the influence of hydrazine hydrate on the crystal growing along the c axis. The evolution of the shape of the top/bottom faces of this crystal family can be diagrammatized as Figure 8b. In addition, describing the morphology of sample 1 as hexagonal hourglass nanostructures with stunted size would be more appropriate than hexagonal nanoplates to perfect the integrality of the hourglass-family’s membership. Another distinct feature of the whole crystal is concave rather than convex, which is particularly obvious in sample 2 or 3. Although there has been considerable work exploring the formation mechanism of crystals with both convex (doublepyramidal or ellipsoidal) structure30,60,61 and concave (dumbbell-like or hourglass-like) structure,52,53,62−65 it is still a challenge so far for researchers to clearly explain the exact formation process of either of these two opposite crystal structures or to provide the transition mechanism and energy calculation between them. In previous reports, researchers described the formation of dumbbell-like or hourglass-like crystals; take ZnO, for example, as a combining of two semicrystals to form a twinned crystal by the assistance of some certain adsorbed ions63 or surfactants.53,64,65 There are two significant features of the crystal obtained through this proposed formation mechanism: (i) the crystal is polar and the conjunct plane is one of the polar plane; (ii) there is an obvious gap or crack running throughout the middle of the crystal. However, neither of the two features mentioned above for the ZnO crystal can be in accordance with the feature of the β-Ni(OH)2 crystal we obtained. Therefore, an alternative interpretation for formation of the β-Ni(OH)2 concave structure is proposed here. At the very beginning of the reaction, high SS could easily give rise to the formation of the LDGUs, whose poor stacking anisotropy would lead to small differences between the growth rate of the ⟨0001⟩ and other directions. When the LDGUs turn to the MDGUs and SDGUs as the SS is reduced, the gradual smaller growth units would enhance the stacking anisotropy and thus the growth rate between the direction of ⟨0001⟩ and others would be distinct. This is conductive to the expanding of the top/bottom faces as they are perpendicular to the ⟨0001⟩ direction whose growth rate is generally considered as the lowest one among the crystallographic directions of nickel hydroxide. As a result, the growth beginning would be smaller than the growth end, facilitating a consequence of concave polyhedron via CES mode. According to the discussion above, the successful explanation of the concave structure again confirm the reasonableness of the hypothesis of three types of secondary growth units. Additionally, the difference in roughness between the top/ bottom faces and the pyramidal faces of the crystal could also be easily understood by the CPGU model. If one divides eight faces of NCO into two types, then the two faces that are

LDGUs. These LDGUs show poor growth anisotropy when they superimpose on the crystal interface, leading to a relatively rapid growth rate along the c axis of nickel hydroxide and, therefore, creating the opportunity for the evolution of the crystallographic pyramidal plane families, which are R{1011̅ } and r{1̅011}. From the inset of Figure 8a, it is observed that the

Figure 8. Schematic illustration of the formation of various surface shapes of the hourglass-like nanocrystals. (a) Seven typical NCO layers are selected to model the final crystal shape of sample 3, and there should be n (n > 0) layers of NCOs that exist (not shown here) between every two of these seven NCO layers. The pyramidal faces (A) that correspond to the evolution of r planes (represented by purple lines) would eventually form triangle-shape morphology, while the alternate faces (B) that correspond to the evolution of R planes (represented by red lines) would present trapezoid-shape morphology. The inset of (a) shows the spatial relationship between the stacking NCOs and the R/r planes. (b) The changes of the top/bottom face shape of the three samples as the area ratio of two kinds of pyramidal faces (B/A) change.

NCOs are exposing its faces at R{101̅1} planes, while corners at r{10̅ 11} planes. With this similar spatial relationship between the octahedra and the pyramidal planes families, the CdI2 has been authenticated that the stacking of octahedra along the direction is relatively unstable compared to the direction.49 By analogy, the stacking of the NCOs along the direction should be more stable and then grow faster than the direction since the crystallographic structure of Ni(OH)2 is similar with CdI2. As the crystal growing, the secondary growth unit would follow the growth manner of the NCOs, so the pyramidal faces (A) that correspond to the evolution of r planes would gradually decrease and form triangle-shape morphology at the end of the crystal growth; while the alternate faces (B) that correspond to the evolution of R planes would be remained as a larger area and present to be 2659



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parallel to the (0001) plane can be defined as OF1 and other six as OF2 (Figure S5, Supporting Information). Thereupon, the top/bottom faces of the crystal could be the assembly of OF1 of the bonding NCOs in the top/bottom layer, revealing the nature of smooth (Figure 2b,e,h), while the pyramidal faces could be regarded as the integral path of the OF1−OF2, showing a zigzag surface structure (Figure 4). More detailed schematic illustrations of the model of hourglass-like structure represented by NCOs could be found in Figure S6, Supporting Information.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (L.G.); [email protected] (S.Y.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (No. 2010CB934700), the National Natural Science Foundation of China (20973019 and 50725208), and the Research Grants Council of Hong Kong (GRF No. 604809).

CONCLUSIONS

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In summary, the CPGU mode is successfully employed for the understanding of the crystal growth mechanism in the solution in the case of the β-Ni(OH)2 hourglass-like nanostructure. According to this unique mode, the NCO is regarded as the basic growth unit here, and the stacking of NCOs could form a diverse dimensional complex which, we presumed, would serve as the secondary growth units for the assembled packing to fabricate the final nanocrystals via a two-stage OA process. Besides, this hypothetical model perfectly explains a series of unique features of the hourglass structure: the shape changes in the top/bottom faces as well as the different roughness among the faces of the crystals, which might be difficult to satisfactorily interpret the anisotropic structures relying on CC or conventional OA mechanism alone. In addition, some common crystallographic phenomena of other inorganic nonmetallic materials can be easily understood from the perspective of this CPGU mode. Take ZnO for an example: if one conceives Zn(OH)42− tetrahedra as the basic growth unit66 to fabricate the final ZnO crystal, then the reported phenomenon that the growth rate of (0001) is higher than the one of (0001̅)67,68 can be understood as follows: driven by the electrostatic interactions, more of these Zn(OH)42− tetrahedra growth units would tend to stack on the positive-polar (0001) facet but fewer on the negative-polar (0001)̅ facet. This report is important in that the CPGU mode we highlighted in this paper could build a bridge between the CC mode and the conventional OA mechanism for crystal growth. This can be understood from the perspective of the special structural characteristic of coordination polyhedra: its component elements are the atoms/ions that serve as the basic growth unit in the CC mode, while a certain quantity of coordination polyhedra could stack to form the so-called “primary particles” which are regarded as the basic growth unit in the conventional OA mechanism. Hence, the fact that one material could present various morphologies can be also easily understood since the stacking manner and stability of coordination polyhedra are determined by both internal (intrinsic crystallographic structure) and external (temperature, pressure, pH, adsorption, etc.) factors. All in all, this understanding of crystal growth might be more practical for comprehending the crystallization in the solution-based system and might be applicable in other certain inorganic nonmetallic materials to the control of nanocrystal growth.

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Coordination Polyhedra: A Probable Basic Growth Unit in Solution for

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