Submicrostructured Metal Oxides with All Polar

Nov 19, 2008 - In this paper, a general method for the preparation of metal oxides ... and slow down the growth rate of polar planes, resulting in the...
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Syntheses of Nano/Submicrostructured Metal Oxides with All Polar Surfaces Exposed via a Molten Salt Route Tao Xu, Xi Zhou, Zhiyuan Jiang, Qin Kuang, Zhaoxiong Xie,* and Lansun Zheng State Key Laboratory for Physical Chemistry of Solid Surfaces, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen UniVersity, Xiamen 361005, China

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 1 192–196

ReceiVed February 24, 2008; ReVised Manuscript ReceiVed August 29, 2008

ABSTRACT: In this paper, a general method for the preparation of metal oxides with all polar surfaces exposed is presented. The syntheses of the products were carried out with simply performing reactions in a molten salt system, in which cations and anions tend to have strong electrostatic interactions with positive or negative charged polar planes so as to lower the surface energy and slow down the growth rate of polar planes, resulting in the formation of exposed polar surfaces. With this strategy, wurtzite structured ZnO, rocksalt structured MgO, spinel structured Co3O4, and ternary element compound ZnFe2O4 of spinel structure were successfully synthesized with all polar surfaces exposed, which demonstrated a universality of our proposed strategy. Introduction Anisotropy is a basic property of single crystals. Various facets or surfaces have different geometric and electronic structures and dangling bonds, etc., so surface-dependent physical and chemical properties are very important in crystals. For example, different surfaces of Pt crystals exhibit totally different catalytic properties in the catalytic reaction of hexane in excess H2. The Pt(111) and (755) surfaces cause the aromatization reaction to produce benzene, while the Pt(100) surface results in an isomerization or cyclization reaction, and a hydrogenolysis reaction happens on the Pt(10 8 7) surface.1 Recently, Sun et al. developed an electrochemical method to prepare Pt crystallites with high-index crystal planes exposed, exhibiting much higher catalytic activity than that of the most common stable planes.2 Moreover MgO nanosheets with unusual bare (111) polar surfaces display catalytic activity for ClaisenSchmidt condensation of benzaldehyde with acetophenone much higher than that for nanocrystalline MgO samples.3 Controls of the structure of exposed surfaces of materials are therefore able to tailor the electronic, optical, magnetic, and catalytic properties of a functional material.4-7 Although the exposed surfaces can be controlled by cutting or polishing a large crystal mechanically, for crystallites under microscale, their exposed surfaces can hardly be controlled by methods other than the crystal growth. During crystal growth, chemically active crystal planes usually grow fast and finally vanish. The exposed surfaces of a crystal are usually stable and chemically less active crystal planes. Therefore, it remains a great challenge to develop general methods for the controllable growth of unusual exposed crystal surfaces. For the crystals of ionic compounds, the exposed surfaces are usually those with lower surface energy, which are usually neutrally charged. For example, naturally grown NaCl particles from aqueous solution are cubes, which are enclosed by neutrally charged {100} surfaces. To have the polar lattice planes exposed, electrostatic interactions could be an effective way, which lowers the surface energy of polar surfaces.8,9 In recent years, room temperature ionic liquids have been applied as solvents for the syntheses of some metal oxides.10-14 However, less attention was paid to the effects of electrostatic interactions. Recently, we proposed an ionic liquid growth route by which hexagonal ZnO micropyramids enclosed with all polar * E-mail: [email protected].

surfaces were prepared, and surface-dependent luminescence was observed on the ZnO micropyramids.8,15 In this paper, we demonstrate a general molten salt route to the syntheses of nano/ submicro-structured metal oxides with bare polar surfaces that are usually not exposed under common crystal growth conditions. By the molten salt method, polar surfaces of well shaped morphologies have been successfully controlled for metal oxides with hexagonal wurtzite, cubic NaCl type, and cubic spinel structures. It is expected that metal oxides with unusual polar surfaces exposed might exhibit some distinguishing properties. Experimental Section Materials. LiNO3 (analytic reagent, AR, g97%) was commercially obtained from Shanghai Hengxin Chemical Reagent Co. Ltd. LiCl · H2O (chemically pure, CP, g97%) was obtained from Guangzhou Chemical Reagent Factory. Zn(CH3COO)2 · 2H2O (AR, g97%) and ZnSO4 · 7H2O (AR, g99.5%) were obtained from Shanghai Chemical Reagent Company. Mg(NO3)2 · 6H2O (AR, g99%), Co(NO3)2 · 6H2O (AR, g99%), and Fe2(SO4)3 (AR, g99%) were obtained from Sinopharm Chemical Reagent Co. Ltd. All the above reagents were used as received. Synthesis of ZnO Hexagonal Micropyramids. The ZnO hexagonal micropyramids were prepared by simply decomposing zinc acetate in molten lithium nitrate at 400 °C. Typically, zinc acetate (1 mmol) was mixed with lithium nitrate (0.1 mol), and the mixture was settled in an alumina crucible. Then the crucible was put into a muffle oven that was already heated to 400 °C. After being heated at 400 °C for 30 min, the crucible was taken out of the muffle oven and cooled to room temperature. Deionized water was used to wash away the lithium nitrate, and white powder (about 60 mg) was obtained as the target product. Synthesis of MgO Submicro-octahedron. The MgO submicrooctahedron was prepared by decomposing magnesium nitrate in molten lithium nitrate at 400 °C. Magnesium nitrate (1 mmol) was mixed with lithium nitrate (0.1 mol) in an alumina crucible, and the crucible was put into a muffle oven at 400 °C and kept for 30 min. Then the crucible was taken out and cooled to room temperature. Deionized water was used to wash away the lithium nitrate, and white powder (about 30 mg) was collected as the target product. Synthesis of Co3O4 Submicro-octahedron. The Co3O4 submicrooctahedron was prepared by decomposing cobaltous nitrate (1 mmol) in molten lithium nitrate (0.1 mol) at 400 °C for about 30 min. The reactant cooled to room temperature was washed by deionized water to remove soluble components. Black powder (about 60 mg) was collected as the product. Synthesis of ZnFe2O4 Micro-octahedron. Zinc sulfate septahydrate (1 mmol) and ferric sulfate (1 mmol) were well mixed with lithium chloride (0.1 mol) in an alumina crucible. Then the crucible was put into a muffle oven that was heated to 800 °C and kept for about 1 h.

10.1021/cg8002096 CCC: $40.75  2009 American Chemical Society Published on Web 11/19/2008

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Figure 1. (a) Schematic model of a ZnO hexagonal micropyramid. (b) SEM image of the as-prepared ZnO products. (c) XRD pattern of the as-prepared ZnO products. (d, e) TEM images of a single ZnO crystallite, the corresponding SAED patterns (top right insets) taken along the [21j1j0] and [3j032] zone axes, and their corresponding models (middle right insets) projected along the same zone axes, respectively. (f) ZnO products prepared without the assistance of molten salt. After being taken out of the muffle oven and cooled to room temperature, the reactant was washed by deionized water to remove the soluble components. A dark-red product (about 150 mg) was obtained. Characterization. X-ray diffraction (XRD) patterns were taken on a Panalytical X’pert PRO diffractometer using Cu KR radiation. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) including selected-area electron diffraction (SAED) studies were performed on a LEO-1530 field emission scanning electron microscope and a TECNAI F-30 high-resolution transmission electron microscope with the acceleration voltage of 300 kV, respectively.

Results and Discussion It is well-known that many ionic compounds can be described as a number of planes composed of cations and anions stacked alternately. The positively charged cations and negatively charged anion planes result in a normal dipole moment and spontaneous polarization along specific axes in these compounds. During crystal growth, the polar surfaces usually appear as growing surfaces because of their high surface energy and exhibit small facets or even disappear. As a result, polar surfaces are generally energetically unfavorable to be the exposed surfaces. To tune the growth behavior of such crystals, changing the surface energy should be effective. In molten salt solvents, strong electrostatic interactions between the ions of the molten salt and the polar surfaces should occur, and surface energies of the polar planes may decrease greatly, resulting in a relatively slow growth rate for these polar planes and thus formation of exposed polar surfaces. The molten salt adopted for our strategy is LiNO3 or LiCl, as it has a relatively low melting point and can be easily removed (washing with water). To demonstrate our strategy, ZnO, a successfully controlled product in our previous work, was first adopted as a target product.8 As reported in many previous papers, the {0001} and {101j1} are polar planes in wurtzite ZnO.9,16 A hexagonal pyramid is formed if all the surfaces are enclosed by these polar planes.8,17 In such a hexagonal pyramid, the base surface is the {0001} and the six side surfaces are {101j1} surfaces, as shown

in Figure 1a. Figure 1b shows the general morphology of the product by decomposing zinc acetate in molten lithium nitrate, which demonstrates that hexagonal micropyramids predominate in the product. The base size and the height of the hexagonal micropyramids are in the range of 1-2 µm. The six side surfaces and the base of the hexagonal micropyramids are very smooth. The XRD pattern of the as-prepared products can be well indexed as wurtzite ZnO (JCPDS 36-1451), as shown in Figure 1c, indicating the successful preparation of the pyramidal structure of wurtzite ZnO. To verify the existence of the polar surfaces surrounding the ZnO micropyramids, TEM and SAED studies were carried out. Figure 1d shows the TEM image and its corresponding SAED pattern (the top right inset in Figure 1d) taken along the [21j1j0] zone axis. The angle between the side and base surfaces is measured to be 62°, corresponding well to the angle obtained from the pyramidal ZnO model projected along the [21j1j0] direction as shown in the inset of Figure 1d. By rotatation of the same crystallite to the [3j032] zone axis, the TEM image of the ZnO micropyramid also corresponds well to the model projected along the same direction as shown in Figure 1e. In this figure, the corresponding SAED pattern and the ZnO pyramid model are also shown. As a result, we conclude that the as-prepared ZnO hexagonal micropyramids are enclosed by polar planes of {0001} and {101j1}. As a comparison, the SEM image of the product by the thermodecomposition of zinc acetate in the absence of LiNO3 molten salt is shown in Figure 1f. No pyramidal structures can be found in the product. Instead, rod shaped products with nonpolar {101j0} surfaces are obtained. To demonstrate the universality of our proposed strategy, rocksalt structured MgO was chosen as the second demonstration. MgO has been used as catalyst or catalyst support in many aspects, and it has been found that the polar {111} planes of MgO exhibit high catalytic activity superior to other systems for the Claisen-Schmidt condensation of benzaldehyde and acetophenone.3 The rocksalt structure is a relatively simple structure, where anions are face-cubic-closest (fcc) packed and

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Figure 2. Schematic model of (a) a MgO unit cell where the polar plane is shown with a dashed line and (b) the MgO octahedron. (c) XRD pattern of the as-prepared products decomposed from Mg(NO3)2 in molten LiNO3. (d) SEM image of the MgO product and a magnified image of a single MgO octahedron (inset). (e) TEM image of a single MgO octahedral particle, its SAED pattern (top right inset) viewed along the [1j11] direction, and a model projected along the same direction (middle right inset). (f) TEM image of the same MgO octahedral particle, its SAED pattern (top right inset) viewed along the [001] direction, and a model projected along the same direction (middle right inset).

the cations occupy all the octahedral interstices composed by the anions. For the ionic crystals with rocksalt structure, they usually grow into a cubic structure,18,19 where {100} surfaces with neutral electric charge are exposed. The {111} surfaces, however, are either positively or negatively charged, because the rocksalt structure can be described as a number of alternating planes composed of the closest packed anions and cations along {111} directions. The alternate packing of O2- and Mg2+ ions along {111} directions results in a normal dipole moment and polar surfaces for the {111} planes (Figure 2a).20 Theoretically, an octahedron is made up with all these polar planes of {111} exposed, as shown in the schematic model of rocksalt MgO in Figure 2b. To prepare the MgO crystallites with all polar {111} surfaces, molten salt strategy is then introduced. Figure 2c shows the XRD pattern of the products decomposed from Mg(NO3)2 in molten LiNO3, where all the peaks can be indexed as the rocksalt structured MgO (JCPDS 045-0946). The SEM image (Figure 2d) shows that the dominant morphology of the asprepared products is octahedron. The inset in Figure 2d gives a clear view of the MgO octahedron. To characterize the exposed surfaces of the MgO octahedron, TEM, HRTEM, and SAED observations were carried out. Figure 2e shows the TEM image viewed along the [1j11] zone axis and its corresponding SAED pattern (top-right inset in Figure 2e). The octahedral model (right inset in Figure 2e) is drawn by projecting the octahedron along the [1j11] direction, which shows a good agreement with the TEM image. Figure 2f is the TEM image viewed along the [001] zone axis at the same crystallite, and its corresponding SAED

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Figure 3. Schematic model of (a) a Co3O4 unit cell where the polar plane is shown with a dashed line and (b) Co3O4 octahedron. (c) XRD pattern of the as-prepared product decomposed from Co(NO3)2 in molten LiNO3. (d) SEM image of the Co3O4 product and a magnified image of a single octahedron. (e) TEM image of a single Co3O4 particle, its SAED pattern taken along the [1j12] direction (top right inset), and a model (middle right inset) viewed along the same direction. (f) TEM image of the same Co3O4 particle, its SAED pattern (top right inset) taken along the [1j11] direction, and a model (middle right inset) viewed along the same direction.

pattern (top-right in Figure 2f) is also presented. Along this direction, the MgO octahedron appears to be a square, which is also in good accordance with the MgO model (right in Figure 2f) projected along the [001] direction. In the TEM image (Figure 2f), both diagonals can be observed clearly, which divide the square into four parts corresponding to the four {111} surfaces of the octahedron. From the TEM and SAED results above, it can be concluded that the octahedral MgO particles are all enclosed by the polar {111} surfaces. The third example for demonstrating our molten salt strategy is the preparation of spinel Co3O4 microcrystals with all polar surfaces exposed. Spinel cobalt oxide (Co3O4) is an important functional material because of its vast applications in pigments, catalysis, sensors, electrochemistry, magnetism, and energy storage.21-24 The spinel structure is much more complicated than rocksalt or wurtzite structure. In the spinel Co3O4, O2anions take the fcc packing, and eight of such fcc O2- cubes form a unit cell. In the unit cell, there are 32 tetrahedral holes and 16 octahedral holes. Co2+ cations occupy only 1/8 tetrahedral holes and Co3+ cations occupy half of the octahedral holes.Normally,Co3O4 crystalsendupwithacubicmorphology,25,26 where {100} surfaces are the exposed surfaces. While in such a complicated structure, {111} surfaces are also polar planes as shown in Figure 3a, and octahedron should be the morphology of the Co3O4 with all the polar surfaces exposed (Figure 3b). By decomposition of cobaltous nitrate in molten lithium nitrate at 400 °C, it can be found the the XRD pattern (Figure 3c) of the products shows the successful preparation of spinel Co3O4 (JCPDS 042-1467), and no additional peak due to the

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molten salts of either LiNO3 or LiCl. To further demonstrate the effect of the molten salt on the structure of the surface, we have carried out the decomposition of all the precursors in the absence of the molten salt. As expected, we could not get the particles with all polar surfaces exposed (see the Supporting Information) in the absence of the molten salt. As the molten salts we used are very simple inorganic ionic compounds, the electrostatic interactions should be dominant. Therefore, our proposed method could be a universal method for the syntheses of metal oxides with exposed polar surfaces. Conclusions

Figure 4. (a) XRD pattern of the as-prepared product from the precursor of ZnSO4 and Fe2(SO4)3, (b) SEM image of the product, (c) TEM image of a single ZnFe2O4 particle, its SAED pattern taken along the [1j12] direction (top right inset), and a model (middle right inset) viewed along the same direction. (d) TEM image of the same ZnFe2O4 particle, its SAED pattern (top right inset) taken along the [1j11] direction, and a model (middle right inset) viewed along the same direction.

impurities can be found in the XRD pattern. The product consists of octahedral particles as shown in Figure 3d. A magnified image (Figure 3d, inset) was presented to give a clear view of the Co3O4 octahedron. Figure 3e is the TEM image of a single Co3O4 octahedron particle taken along the [1j12] direction and its corresponding SAED pattern (top right inset in Figure 3e). According to the orientation provided by the SAED pattern, the model octahedron (right inset in Figure 3e) of Co3O4 is projected along the [1j12] direction, which shows rectangle shape the same as the TEM image with long edges parallel to {111} planes and the short edges parallel to {110} planes. By rotating the same particle to the [1j11] zone axis, the TEM image also fits well with the model (Figure 3f). As a result, the Co3O4 octahedral particle consists of eight polar {111} surfaces. By far, binary compounds were successfully controlled using our strategy with all polar planes exposed. Can the proposed strategy be applied to polyatomic compounds? Herein, a ternary compound, ZnFe2O4, was used as a demonstration. ZnFe2O4 is also spinel structured which was widely studied and possesses a lot of promising applications and properties in magnetism, electrics, and sensors.27-33 Figure 4a is the XRD pattern of the product from the reaction of ZnSO4 and Fe2(SO4)3 in molten LiCl, which can be indexed as ZnFe2O4 (JCPDS 073-1963). From the SEM image of the product, it can be seen the particles are octahedra and transformative octahedra. The transformative octahedron can be attributed to the lengthening of an octahedron along {110} directions (right of Figure 4b). In Figure 4c and 4d, TEM images show a single ZnFe2O4 particle with transformative octahedral shape projected along the [1j12] and [1j11] directions, respectively. The SAED patterns are also presented in the insets. As can be seen from the schematic models projected along the same directions, the TEM images of the transformative octahedron fit well with the schematic models with all polar {111} surfaces exposed. Above experimental results have demonstrated that the polar surfaces can be controlled by growing the metal oxides in simple

In conclusion, we demonstrated a molten salt route to the syntheses of metal oxides crystallites with all polar planes exposed. The growth of the exposed polar surfaces is due to the strong interaction between ions in molten salt and the polar planes, which lowers the surface energy of the polar surfaces. With this strategy, a series of products with all polar surfaces exposed, including wurtzite ZnO, rocksalt structured MgO, spinel structured Co3O4, and the ternary spinel structured ZnFe2O4, were successfully synthesized. All these results indicate that this method can be extended to synthesize other kinds of metal oxides with polar planes exposed. We believe the exposed polar surfaces with high surface energy will bring novel properties. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grant Nos. 20725310, 20721001, 20673085, and J0630429), the National Basic Research Program of China (Grant No. 2007CB815303, 2009CB939804), and Key Scientific Project of Fujian Province of China (Grant No. 2005HZ01-3). Supporting Information Available: The SEM observations of the products by decomposition of the precursors in the absence of molten salt. This material is available free of charge via the Internet at http:// pubs.acs.org.

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