Geometrical Recognition: A General Route to ... - ACS Publications

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J. Phys. Chem. C 2008, 112, 3272-3282

Geometrical Recognition: A General Route to Shape-Controlled Syntheses of Transitional Metal Chalcogenides, Silicides, and Copper(I) Chloride Nanocrystals Hao-Xu Zhang,†,‡ Qing Peng,‡ and Ya-Dong Li*,‡ Department of Physics and Tsinghua-Foxconn Nanotechnology Research Center, Tsinghua UniVersity, Beijing 100084, People’s Republic of China, and Department of Chemistry, Tsinghua UniVersity, Beijing 100084, People’s Republic of China ReceiVed: October 8, 2007; In Final Form: December 20, 2007

In our former publications (J. Phys. Chem. B 2005, 109, 11585; and J. Phys. Chem. B 2006, 110, 14107), we have proposed the geometrically kinetic competition (GKC) mechanism to be responsible for the silicavapor-pressure-dependent growth of silica-sheathed Fe7S8 and digenite Cu2S nanocrystals. Here, we demonstrate that geometrical recognition (GR), the basis of GKC, can be a general route to shape-controlled syntheses of a host of materials’ nanocrystals. According to the types of the metal (or silicon for CoSi and MnSi) atom sublattices, the experimental results are presented, and the mechanisms for the growth of various shapes of products are discussed in detail. SiO4 clusters have been proposed as the competing opponents to SiO3 clusters for shape-controlled syntheses of MnS, CuCl, Cu2S, CdS, ZnSe, and ZnS nanocrystals.

Introduction Templates have proved very effective for shape-controlled synthesis of nanocrystals in the past decade. For examples, surfactant molecules have been widely used for size- and shapecontrolled syntheses of nanoparticles,1-7 nanorods,3,8,9 nanowires,10-12 nanobelts,13 nanotubes,14 and nanoplatelets;15-17 metal particles,18-26 especially gold nanoparticles,21-26 have proved unique for the growth of long nanowires or nanobelts in various chemical vapor deposition systems via the vaporliquid-solid or vapor-solid mechanism; semi-liquid oxides have been successfully used to control the shapes of a host of materials;27-29 and surface charges have resulted in the formation of ZnO nanohelixes and nanorings,30-32 as well as titanate nanotubes.33,34 Recently, we have focused our attention to how the amorphous sheath material (silica) can dominate the growth behaviors of single crystalline core materials (Fe7S8,35 digenite Cu2S,36 and Bi2S337,38). These materials can be classified into two types according to their interactions with silica species through oxygen-bridged metal-oxygen-silicon (M-O-Si) bonds: (i) Geometrical Recognition (GR): Fe7S8 and Digenite Cu2S. Freely grown Fe7S8 crystals are hexagonal platelets; introducing of silica species into the reaction atmosphere results in the formation of silica sheath on {001} planes and an increase in the aspect ratio of the core crystals; however, when the pressure of silica species is high enough, the core crystals, forced by the silica sheath simultaneously formed around them, turn to grow along the 〈001〉 direction. Such behavior can be wellexplained by the geometrically kinetic competition (GKC) mechanism, which means kinetic competition between GR of triangle units (SiO3 clusters adsorbed on Fe2-Fe3-Fe4 equilateral triangle units of the 4Fe layer) and rectangle units (Si2O5 * To whom correspondence should be addressed. E-mail: ydli@ tsinghua.edu.cn. † Department of Physics and Tsinghua-Foxconn Nanotechnology Research Center, Tsinghua University. ‡ Department of Chemistry, Tsinghua University.

clusters adsorbed on Fe2-Fe3-Fe4-Fe5 rectangle units parallel to the 〈001〉 direction) on the metal atom sublattice. Similar growth behavior also happens on silica-sheathed Cu2S nanocrystals. (ii) Strongest Metal-Oxide Bonds: Bi2S3. Silica-sheathed single crystalline Bi2S3 nanowires rather than microcrystals have been obtained on silicon substrates under extraordinary hightemperature (650 °C), when the vapor pressure of BiCl3 precursor is tripled.38 Therefore we have attributed the synthesis of nanowires to selective adsorption of silica species on Bi(1) sites, which can provide the strongest Bi-O bonds, rather than the intrinsic growth behavior of Bi2S3 crystals. In comparison, the former has provided an efficient way for both the formation (adsorption of silica species) and growth (connection between neighboring silica clusters) of stable (at least three bonds with the core crystal) silica sheath, while the later cannot, that is, it come into play only when the former fails. Thus, we must answer the substantial question: “Can GR be applied to other compounds?” Here, we demonstrate GR to be a general route to shape-controlled synthesis of transition metal sulfides (MnS, CoS, NiS, ZnS, CdS), selenides (CoSe, NiSe, ZnSe), silicides (MnSi, CoSi), and copper(I) chloride nanocrystals. Since the interactions between silica sheath and the compounds depend only on the metal (or Si in silicides) sublattice, we have classified the compounds into five types as shown in Figure 1a-e. We have also drawn the four types of silica clusters, as SiO3, SiO4, Si2O5, and Si2O6+ (larger clusters than Si2O5), that are supposed to be responsible for the GR mechanism, in Figure 1f, so that the readers can easily compare them with the metal (or Si) sublattice and have better understanding on the topics of this manuscript. Our discussions will focus on the materials with metal sublattices of face-centered cubic (fcc), simple cubic, or the two-layered hexagonal structure, as shown in Figure 1a,b,d. SiO3 and SiO4 clusters are supposed to be the main competing opponents for the formation of silica sheaths on different planes of these materials.

10.1021/jp709832m CCC: $40.75 © 2008 American Chemical Society Published on Web 02/12/2008

Geometrical Recognition

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Figure 1. Crystal structures of (a) MnS, space group Fm3m (225); CuCl, ZnS, and ZnSe, space group F4h3m (216); (b) digenite Cu2S, space group Fm3m (225); (c) hexagonal NiS, CoS, NiSe, and CoSe, space group P63/mmc (194); (d) hexagonal ZnS, ZnSe, and CdS, space group P63mc (186); and (e) CoSi and MnSi, space group P213 (198). In a-d, only the metal atoms are shown, while in e, only Si atoms are shown. (f) Supposed SimOn clusters that may facilitate the formation of silica sheaths on specific planes of the materials’ crystals.

Experimental Section Silica species in the vapor phase were obtained from the reaction between SiCl4 and water. SiCl4 was supplied with a mixture of silicon (200 mesh) and CuCl powder, except in the cases of MnS, CoSi, and CoS, where it was supplied by the reaction between MnCl2 (or CoCl2) vapor and silicon powder (or wafers). Water was carried from pinholes into the reaction atmosphere by argon flow, except in the cases of MnS and CoSi, where it was mainly supplied by adsorbed water on silicon powder and the quartz tubes. Metal elements came from their chlorides or chlorides hydrates, which were pretreated up to 240 °C in vacuum to remove water before use, except Zn, which came from small zinc balls covered by ZnCl2 powder. Powders

of sulfur, selenium, and silicon were used as nonmetal sources. All of the chemicals are analytical grade. The arrangements of the chemical reagents for the experiments were illustrated in Scheme 1. And the experimental details were as following. CdS, ZnS, ZnSe, CoSe - Scheme 1a. In a typical synthesis, the system was first evacuated, and then the furnace temperature was elevated to 240 °C (120 °C for ZnCl2) and kept for 20 min to remove the adsorbed water on chlorides powders. Then argon was filled to atmospheric pressure and an argon flow was kept to the end of the experiment. In the following 30 min, the furnace temperature was elevated to a set value (650 °C for CdS and ZnSe; 700 °C for CoSe; and 750 °C for ZnS) and was

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Figure 2. SEM images of (a) MnS 〈111〉 nanowires and hexagonal platelets; (b) MnS mushrooms with 〈111〉 nanowires stalks and hexagonal caps; (c) MnS crosses extending along either 〈111〉 or 〈110〉 directions; and (d) MnS 〈110〉 nanowires.

SCHEME 1: Scheme of the Apparatus and the Arrangements of the Chemical Reagents for the Syntheses of the Underlined Materials

kept for 20 min to fulfill the synthesis. At last the furnace was allowed to naturally cool down to room temperature. The

positions of the reagents were adjusted according to their equivalent vapor pressures. Here, ZnCl2 powder was used to



Figure 3. HRTEM characterization of MnS nanocrystals. (a) A MnS hexagonal platelet and (b) its HRTEM image. (c) A MnS mushroom, and the HRTEM images of its (d) stalk and (e) cap; (f and g) higher resolution images of its head part before and after rotating 40° rightward out of paper, respectively. (h) A MnS cross and (i) its higher resolution image; the inset of h shows the higher resolution image (rotated to fit in the space) of the part highlighted by the arrow; (j) the HRTEM image of the cross, recorded on the regions highlighted by the rectangles in (i), the left half corresponds with the 〈111〉 wire, and the right half corresponds to the 〈11h0〉 wire; (k) element distribution across the 〈110〉 wire in (i); (l) element distribution across the two neighboring 〈111〉 wires in (i), the solid and dashed black curves are the simulation results of the nanowire with equilateral hexagonal and round shapes, respectively.

prevent the zinc balls from direct exposure to sulfur or selenium vapor, so that a stable zinc vapor pressure was guaranteed.

NiS, NiSe - Scheme 1b. When the reagents were arranged according to Scheme 1a, the obtained samples (prepared in July),


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Figure 4. (a-e) SEM images of CuCl crystals prepared under increasing supply of silica. (f) Higher resolution image of e. The insets are higher resolution images of the corresponding images. (g) TEM image of a CuCl platelet; and (h) the corresponding SAED (selected area electron diffraction) pattern.

which were supposed to be silica-sheathed NiS nanocrystals, quickly adsorbed water from the atmosphere, and in a few minutes, water began to flow from the silicon wafer. The phenomenon cannot be attributed to freely grown or surfacesulfurized NiCl2 crystals, because such products colleted further away at the cold side only slowly dissolved in water. So we deduce the phenomenon to be caused by silica-sheathed NiCl2 platelets, which suffered some tension when cooled down from 700 °C to room temperature. The tension may have resulted in pealing of the NiCl2 crystals and may have made them easier to adsorb water. The arrangement of chemical reagents like in Scheme 1b has well-resolved the problem. MnS, CoSi - Scheme 1c. Manganese is so akin to oxygen that silicon powder must be used, and the furnace temperature must be kept at 580 °C for 1-3 h before it was elevated to the set value of 720 °C. In the case of CoSi, the furnace temperature was 700 °C, and sulfur was not used. CoS - Scheme 1d. Both CoCl2 and NiCl2 are highly reactive with silicon wafers above 700 °C, but the former is much less reactive with sulfur vapor than the latter. So Al2O3 wafer was used to avoid the too fast growth of CoS microcrystals, and a long reaction path was designed. The tiny holes were designed

to suck some of the liquid CoCl2 out of the two small quartz tubes and to control the CoCl2 vapor pressures around the two silicon pieces. The furnace temperature was 750 °C. CuCl - Scheme 1e. The arrangement guaranteed efficient use of SiCl4 and greatly reduced the side reaction between CuCl and water. The furnace temperature was 500 °C. Characterization. The samples were characterized by a Sirion 200 scanning electron microscope (SEM), a Tecnai G2 F20 high-resolution transmission electron microscope (HRTEM), a Hitachi Model H-800 TEM, and a Rigaka D/Max2500PC X-ray diffractometer (XRD) with Cu KR radiation (λ ) 1.5418 Å). Results and Discussions MnS I - SEM. Figure 2a shows bundles of parallel nanowires decorated with thin hexagonal platelets of up to 20 µm in width. Obviously, the nanowires should grow along the 〈111〉 direction; and the platelets should grow just perpendicularly to the nanowires. Figure 2b shows mushrooms with nanowires stalks and platelets caps. Although these platelets still have the hexagonal symmetry as those shown in Figure

Geometrical Recognition 2a, some of them are more like truncated equilateral triangles; therefore, they should be silica-sheathed rather than freely grown crystals. Figure 2c shows a bundle of crosses, which was expected when neither of the two modes in GKC can dominate the growth. So by controlling the amount of silicon powder and the settle time at 580 °C, we have obtained all typical products predicted by the GKC mechanism. MnS II - TEM. Hexagonal platelets are confirmed to grow perpendicularly to the 〈111〉 direction by HRTEM results shown in Figure 3a,b. HRTEM images recorded on the stalk (Figure 3d) and cap (Figure 3e) of the MnS mushroom shown in Figure 3c confirm the single-crystal nature of it. By rotating the mushroom clockwise by 40° out of the page, we can find that the cap is a truncated equilateral triangle, and all of the sides belong to {211} planes. To absolutely determine the planes competing with {111} planes in GKC, we have also investigated a cross in detail. As revealed by Figure 3j, which is composed of two HRTEM images recorded on the neighboring [111] and [11h0] nanowires respectively, the growth of the single crystalline cross is dominated by silica sheaths formed on the two opposite {211} planes. The slight rotation of about 1.6° between the left and the right half of Figure 3j can be attributed to self-avoiding of collision between the nanowires. The distributions of Mn, S, Si, and O elements across the [11h0] nanowire and the two[111] nanowires are shown in Figure 3k and 3l, respectively. Obviously, the [11h0] nanowire should be precisely called a nanobelt. And simulations indicate that the sections of the[111] nanowires are closer to hexagonal rather than round. So both of the HRTEM results about the mushroom and the cross demonstrate {211} planes to be the competing opponents to {111} planes in GKC. The products shown in Figure 2d have been identified as 〈110〉 nanobelts whose growth is dominated by silica sheaths on the two opposite {211} planes. MnS III - SiO4 on {211} planes. We had previously proposed Si2O5 clusters as the candidate to compete with SiO3 clusters in GKC systems.35 However, in the case of MnS, on one hand, {100} and {110} planes, which are geometrically possible for the adsorption of Si2O5 clusters (see Figure 1a and 1f), have hailed to compete with {111} planes. On the other hand, larger SimOn clusters seem to be unsuitable candidates now that Si2O5 may come into play. So we have to consider the SiO4 cluster. Now let’s turn to the surface structure {211} planes shown in Figure 1a. The three rightmost rows of atoms lie in the same {111} plane, providing suitable positions such as Mn1, Mn2, and Mn3 to form three bonds with a SiO4 cluster. The left tail of the SiO4 cluster may stretch leftward and anchor on Mn5 or Mn6 at the fourth row. The atoms, Mn2, Mn3, Mn5, and Mn6, lie in the same {100} plane. The angle between the neighboring {111} and {100} planes is 125.3°. We deduce such cooperation between the {111} and {100} planes has facilitated the passivation of {211} planes with SiO4 clusters. MnS IV - Features of SiO4. Obviously the SiO4 cluster has two interesting features for silica sheath formation: fast and instable. On one hand, both SiO4 and Si2O5 clusters form four M-O bonds with the core crystal, but the former is composed of less atoms. So if there are proper positions for both of them to adsorb on the core, the former will win in kinetic competition. On the other hand, all of the bonds of Si and O atoms of an adsorbed SiO4 cluster are saturated, which means at least one M-O bond has to be broken sooner or later so that a larger cluster can be formed to stabilize the sheath. In comparison, the bonds of both adsorbed SiO3 and Si2O5 clusters are unsaturated, which means easier growth and stabilization of the sheath. The former feature should have dominated the


Figure 5. HRTEM image of a Cu2S platelet. The insets show the HRTEM image and the FFT (fast Fourier transformation) pattern.

growth behavior here, although the later one may have manifested itself at the corners of 〈111〉 nanowires (for details, see part I of Supporting Information), where an intrinsic competition between neighboring {211} sheaths also exists. CuCl - Porous Nanosheets Grow Perpendicularly to 〈111〉 Direction. CuCl has the same fcc metal sublattice with MnS, and the lattice unit is just slightly longer by 3.4% than that of MnS. So it is of no surprise that silica can be used to efficiently control the shapes of CuCl nanocrystals. In fact, increasing contents of silica in the products have transformed the original microcrystals (Figure 4a, no silica) to nanosheets (Figure 4b), and finally to nanowire grids (Figure 4e,f). The hexagonal symmetry of the nanowire grids indicates that the nanosheets grow perpendicularly to the 〈111〉 direction. TEM results (Figure 4g,h) also support the conclusion. Interestingly the side planes of the porous platelet in Figure 4g belong either to {110} or to {211} planes. Moreover, all of the parts that have nearly equilateral hexagonal shapes, which is characteristic for freely grown crystals, have {110} side planes; while those parts with {211} side planes take the shapes obviously different from equilateral hexagonal. So just as the case of MnS, {211} planes are the next preferred for silica sheath formation other than {111} planes. Cu2S - {211} Rather Than {110}. We had previously thought the {110} planes to be the opponents to {111} planes in silica-sheathed Cu2S nanocrystals.36 But the results of MnS and CuCl remind us to reexamine the TEM samples of Cu2S products. As shown in Figure 5, all side planes of the highly curved nanobelt are {211} planes. Similar to the case of MnS and CuCl, {211} planes of the simple cubic Cu sublattice of Cu2S crystals can also be viewed as composed of atoms from alternative {111} and {100} planes, as shown in Figure 1b, where Cu1, Cu2, and Cu3 lie in the same {111} plane, and Cu2, Cu3, and Cu4 lie in the same {100} plane. The only difference between the {211} planes of fcc and simple cubic lattices is the absence of the equivalent rows containing Cu4 in the former. The lattice units of CuCl and Cu2S differ only slightly by 2.9%, and the cuprous ions are all 4-coordinated along the 〈111〉 directions with either Cl- or S2-; however, our efforts to synthesis 〈111〉 nanowires have failed in the former


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Figure 6. SEM and TEM characterization of CoS (a-d), NiS (e-h), CoSe (i and j), and NiSe (k-o) crystals. (a) SEM image of CoS microcrystals; (b) SEM image of silica-sheathed CoS hexagonal platelets; (c) TEM and (d) HRTEM images of a CoS platelet. (e) SEM image of NiS microcrystals; (f) SEM image of silica-sheathed, highly porous NiS nanosheets; (g) TEM and (h) HRTEM images of a porous NiS nanosheet. (i and j) SEM images of CoSe hexagonal platelets prepared under lower and higher supply of silica, respectively. (k and l) SEM images of NiSe crystals prepared under lower and higher supply of silica, respectively; (m) high-resolution image of a long wire; (n) TEM and (o) HRTEM images of a NiSe hexagonal platelet. All the insets are either higher resolution images or FFT patterns of the corresponding images.

and succeeded in the later. Therefore, if the anions only have limited effects, the reasonable conclusion would be that the Cu4 rows have help to reduce the tension for the adsorption of SiO4 clusters on {211} planes; that is, SiO4 clusters are bond to the Cu1, Cu2, Cu3, and Cu4 sites to passivate the {211} planes of Cu2S. CoS, NiS, CoSe, NiSe - Hexagonal Platelets. For CoS, the products are hexagonal platelets (Figure 6b,c) growing perpendicularly to the c axis (Figure 6d) rather than microcrystals (Figure 6a) with no preferred growth direction, when silica sheaths are formed. For NiS, the silica-sheathed products are highly porous nanosheets (Figure 6f,g) with hexagonal symmetry (Figure 6h). As shown in the inset of Figure 6g, all of the overall straight parts have frequently zigzag sides of {100} planes, so we deduce that {100} planes with rectangle units may be the opponents of {001} planes with equilateral triangle units for silica sheath growth. For CoSe (Figure 6i,j), silica sheaths have helped to increase the aspect ratio of the hexagonal platelets. But further increase of the supply of SiCl4 had resulted in the appearance of the more stable, silica-sheathed CoSi platelets. In the cases of NiSe, silica-sheathed equilateral hexagonal

platelets (Figure 6k and the inset) or long nanobelts (Figure 6l,m, and the insets), which are obtained under higher pressure of silica species in gas phase, are always accompanied with freely grown microcrystals with no preferred growth direction. The intrinsic hexagonal symmetry of the long nanobelt shown in Figure 6m and its insets demonstrates the dominant role of GR on {001} planes of NiSe crystals. So as a summary, GR on {001} planes with equilateral triangle units of side lengths varying between 3.370 Å and 3.660 Å (Figure 1c) has dominated the growth of silica-sheathed nanocrystals of these materials. Hexagonal CdS - Nanowires Grow Along the c Axis. Figure 7a,b,c shows SEM images of freely grown CdS products and CdS crystals grown under lower and higher supply of silica species, respectively. The freely grown products are aggregates of microcrystals that show no preferred growth direction, and the silica-sheathed products are, according to the SEM and HRTEM results (Figure 7c,d), bundles of highly curved nanowires growing along the c axis. Figure 8b shows the initial stage for the growth of the nanowires. The base crystals from which the tiny tips initialize have rough surfaces instead of smooth surfaces that are expected for the products passivated by silica


Figure 7. SEM images of (a) CdS microcrystals without silica sheath, (b) the sample showing the initial stage for CdS nanowires to grow from the microcrystals, and (c) silica-sheathed CdS nanowires. Inset of c is the higher resolution image of the nanowires. (d) TEM image of a nanowire. The insets of d show the HRTEM image and the corresponding FFT pattern.

sheaths on {001} planes. So it should be larger clusters than SiO3 that have dominated the sheath formation. Hexagonal ZnSe, ZnS - Stars with Radiation Points Extending Along the c Axis. As for ZnSe, introduction of silica sheaths have converted the freely grown microcrystals (Figure 8a) to star-shaped crystals (Figure 8b-d), which are composed of truncated equilateral triangle microplatelets as the cores and bundles of zigzag nanowires as the radiation points. XRD patterns (Figure 8e) reveal that the structural transformation from fcc to the mixture of fcc (as evidenced by the fcc (400) deflection) and hexagonal has accompanied with the shape control process. One typical set of HRTEM results are shown in Figure 8f and its insets, which are obtained by rotating the zigzag nanowire so that the central straight part seems to be longest. All of the HRTEM results reveal that the straight parts of the zigzag nanowires extend along the c axis. Similar

J. Phys. Chem. C, Vol. 112, No. 9, 2008 3279 phenomena are also observed for ZnS (Figure 9) except that each bundle has been replaced by single tip and no evidence can be found for the existence of fcc-structured products in the stars. CdS, ZnSe, ZnS - SiO4 or Si2O5 Based Sheath Formation. According to the metal sublattice structures of hexagonal Cds, ZnSe, and ZnS shown in Figure 1d, SiO4 clusters can adsorb on Cd3-Cd4-Cd7-Cd8 units, where Cd3 and Cd4 form two nearly equilateral triangles (the angle between them is 140.4°) with Cd7 or Cd8. In the meanwhile, nearly square units, such as Cd1-Cd2-Cd3-Cd4 and Cd3-Cd4-Cd5-Cd6, can be easily identified on the side planes of the hexagonal prism of Cd sublattice. Zigzag chains of such neighboring units form lines extending along the c axis. Suppose a Si2O5 cluster has adsorbed on Cd1-Cd2-Cd3-Cd4; the dangling bonds of the Si atoms are expected to form bonds with one or both of the Cd5 and Cd6 atoms rather than the Cd7 atom through oxygen bridges, because the angle between the Cd1-Cd2-Cd3-Cd4 and the Cd3-Cd4-Cd5-Cd6 units is 109°, while Cd7 lies much closer to the plane of the Cd1-Cd2-Cd3-Cd4 unit. Thus, on the basis of a Si2O5 cluster adsorbed on one of the two neighboring units, a larger cluster, either Si2O6 or Si3O7, may be formed to passivate the (100) plane. So now both SiO4 and Si2O5 clusters can result in silica sheaths formed on {100} planes, so that the cores have to grow along the c axis unless the SiO3 cluster can come into play. In comparison, SiO4-based sheath is kinetically more favorable than Si2O5-based sheath, so the former may dominate when the pressure of silica species is at a relatively low level, while both of them may cooperate when the pressure is elevated. Such discussions are also proper for ZnSe and ZnS. ZnS, ZnSe - Structural Transformation. Structural transformation of ZnS crystals has been extensively studied. Although bulk ZnS crystals transform from the stable sphalerite (fcc) to metastable wurtzite (hexagonal) only above 1020 °C,39 wurtzite ZnS nanowires doped with Mn2+, Co2+, or Cu2+ had been prepared under 750 °C by chemical vapor deposition.40 Moreover, Huang and Banfield41 had demonstrated that wurtzite ZnS could be obtained from sphalerite at 225 °C in hydrothermal conditions, and these wurtzite crystals stopped their growth when the diameter of the sphalerite-wurtzite interface reached ∼22 nm. So both the doping effects and the size dependent behavior can cause the structural transformation of ZnS products and the similar ZnSe products in our experiments. Since the phase transformation is size-dependent, we can reasonably identify the truncated triangle ZnSe microplatelets cores as fcc structured. The shape of the cores indicates that they are silica-sheathed products. ZnSe, MnS - Possible Mechanism for Triangles. All of the ZnSe platelets (Figure 8c) and most of the MnS platelets (Figure 2b and Figure 3g) have the common truncated equilateral triangle shape, when the side planes are also covered with silica sheaths. Obviously, such behavior cannot be explained unless there are interactions among the sheaths formed on different planes. We had previously proposed a model (as shown in Scheme 3 in ref 36) to attribute the formation of Cu2S nanosheets to the positive feedback between the imbalance of silica sheath coverage on different planes and the net mass diffusion associated with the imbalance. The model should also be valid to the formation of MnS, CuCl, ZnSe, and CoSi, MnSi (as will be introduced later) nanoplatelets growing perpendicularly to the 〈111〉 directions. As for the truncated equilateral triangles, we point out that the mass diffusion path here is narrow and long, just contrary to the case of nanosheets, where


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Figure 8. SEM images of (a) ZnSe microcrystals without silica sheath and (b) silica-sheathed ZnSe stars. (c and d) Higher resolution images of a ZnSe star. (e) XRD patterns of the microcrystals (curve a) and stars (curve b). (f) TEM image of a nanowire. The insets show the HRTEM image and the corresponding FFT pattern.

Figure 9. SEM images of (a) ZnS submicrocrystals without silica sheath and (b) silica-sheathed ZnS stars. (c) XRD patterns of the submicrocrystals (curve a) and stars (curve b). (d and e) TEM images of some ZnS stars. (f) The HRTEM image recorded on the tip of the upper branch of the ZnS star shown in e and the corresponding FFT pattern.

the path is wide and short. Taking the MnS platelet shown in Figure 3g for example, the diffusion path for the two opposite {111} planes is about 100 nm wide (the shortest side) and 20 nm long, while when the same path is serving for the triangle shape, it is about 20 nm wide and 100 nm long. Therefore, for the {211} side planes, the opposite planes may be too far away, so that the most favored would be those at meta-positions, and the products would adopt equilateral triangle-based structures. As for the important issue of how long the diffusion path can be to still keep its function of positive feedback, it must depend on both the materials and the experimental conditions, but in our experiments on CuCl crystals (Figure S4 in ESI), when the critical condition is reached so that all of the products are

platelets, the thickness of the platelets varies between 0.5 and approximately 1.5 µm. Another thing to be mentioned is the nearly equilateral hexagonal sections of the MnS 〈111〉 nanowires shown in Figure 3l. At first sight, it seems to contradict with the truncated equilateral triangle platelets. But we emphasize that the nanowires were prepared under higher pressure of silica species. CoSi, MnSi - Hexagonal Platelets. As shown in Figure 10, silica sheath can be used to transform CoSi microparticles into equilateral hexagonal platelets, which grow perpendicularly to the 〈111〉 direction, and whose porosity is controlled by the pressure of silica species. Since silicon is more akin to oxygen than cobalt, Si sublattice in CoSi crystal should be responsible



Figure 10. SEM images of CoSi (a) microcrystals, (b) hexagonal platelets, and (c) highly porous platelets prepared under increasing supply of silica species. Inset of c is the higher resolution image of two porous platelets. (d and e) TEM and HRTEM images of a porous platelet. Inset of e is the corresponding FFT pattern.

for silica sheath formation, especially when the pressure of silica species is low. The structures of Si sublattice and its (111) plane are drawn in Figure 1e. The (111) plane is composed of two layers. Obviously, layer 2 can provide proper sites for silica sheath formation through the adsorption of SiO3 clusters. As for the MnSi truncated equilateral triangle platelet growing perpendicular to the 〈111〉 direction (Figure S5 in ESI), it is one of the byproducts when the pressure of MnCl2 vapor has reached a high level while the sulfur vapor pressure is still low. Conclusions Shape controlled syntheses have been achieved through silica sheaths formed on specific planes of the crystals of a host of materials. The selectivity on the planes can be well explained or predicted by GR of SiO3, SiO4, Si2O5, or larger SimOn clusters to the metal (or Si for CoSi and MnSi) surface lattices of the planes. Moreover, evidence of GKC phenomenon has been observed on all of the materials, except CoS and CoSe, giving firm support to the conclusion that GR is a general route to shape-controlled syntheses of all materials discussed in this paper, as well as Fe7S8 that was reported previously. In addition, SiO4 clusters, which possess the interesting intrinsic characteristics of fast and unstable for silica sheath formation, have been proposed as the competing opponents to SiO3 clusters for shape control on crystals of MnS and CuCl with fcc metal sublattices, Cu2S with simple cubic metal sublattices, and CdS, ZnSe, and ZnS with two-layered hexagonal metal sublattices. Acknowledgment. We are thankful for financial supports from NSFC (90606006), the Foundation for the Author of National Excellent Doctoral Dissertation of China, the State Key Project of Fundamental Research for Nanoscience and Nanotechnology (2006CB932300), and the Key Grant Project of Chinese Ministry of Education (No. 306020). Supporting Information Available: Table of the eight types of acute angles between a [111] line in a (11-2) plane and the common lines of the plane with some other planes of MnS, TEM images showing curves on the sides of two 〈111〉 MnS nanowires, higher resolution SEM images of the highly porous CuCl nanosheets, SEM images of the two CuCl samples where (a) 60% and (b) 100% of the products are platelets (PDF), TEM

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