CRYSTAL GROWTH & DESIGN
Synthesis of Single-Crystalline Hollow Octahedral NiO Xi Wang,†,‡ Lingjie Yu,†,‡ Peng Hu,†,‡ and Fangli Yuan*,† State Key Laboratory of Multi-phase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100080, P. R. China, and Graduate UniVersity of the Chinese Academy of Sciences, Beijing 100049, P. R. China
2007 VOL. 7, NO. 12 2415–2418
ReceiVed December 29, 2006; ReVised Manuscript ReceiVed August 3, 2007
ABSTRACT: We have successfully fabricated single-crystalline hollow NiO crystals with well-defined octahedral morphology. With detailed investigations by using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and thermogravimetric analysis, we propose a growth mechanism of the hollow octahedra. It is the first report that hollow octahedral materials can be synthesized via a template-assisted carbothermal method. During the process, carbon templates appear to be necessary for the formation of single-crystalline hollow NiO octahedra. The proposed facile strategy can be extended to synthesize other polyhedral materials such as metal oxides (Co3O4, etc.).
1. Introduction The ability to tune the structure, size, and shape of inorganic materials is an important goal in current material synthesis, and there has been increasing interest in the controlled synthesis of inorganic micro- and nanostructures with hollow interiors in recent years, mainly owing to their potential applications in catalysts, adsorbents, sensors, photonic crystals, drug-delivery carriers, biomedical diagnosis agents, lightweight fillers, acoustic insulators, and chemical reactors.1–6 Among the numerous approaches for obtaining these materials, template-assisted synthesis is an effective method, in which hard templates, such as sacrificial polymeric core supports, removable metal substrates or organic molecules, and soft templates, such as micelles in emulsions or ionic liquids, have been utilized.7–10 However, it should be mentioned that most nanoproducts prepared through the use of removable templates are polycrystalline and are limited only to spherical morphology. Recently, some efforts have been devoted to the synthesis of inorganic hollow structures with nonspherical morphologies. For example, Co3O4 nanoboxes have been synthesized through surfactant-templated fabrication.11 Furthermore, octahedral silica nanocages have been prepared by a facile one-step approach, which is based on mediated deposition of silica on simultaneously generated salt quasi-templates.12 In addition, single-crystalline octahedral Cu2O nanocages were prepared through a one-pot catalytic solution process.13 However, the controllable synthesis of singlecrystalline hollow structures of other inorganic materials with well-defined nonspherical morphologies is still much needed. As a very useful template, carbon has wide applications in nanomaterial synthesis. For example, Ga2O3 and GaN hollow spheres were synthesized through calcination of carbon templates.14 Furthermore, hollow nickel microspheres covered with oriented carbon nanotubes (CNTs) were synthesized via chemical vapor deposition at 800 °C.15 In addition, Pt hollow spheres were produced by spontaneous combustion of carbon templates.16 As a p-type semiconductor with unique optical and magnetic properties, NiO is a promising material with potential applications in anodic electrochromism and smart windows.17,18 * Corresponding author. Phone: +86-10-82627058. Fax: +86-10-62561822. E-mail:
[email protected]. † Institute of Process Engineering. ‡ Graduate University of the Chinese Academy of Sciences.
Recently, NiO hollow spheres have been successfully produced through the thermal decomposition route.19 However, to the best of the authors’ knowledge, there have been no reports on the synthesis of nonspherical NiO hollow materials. In this paper, we report that single-crystalline hollow NiO crystals with welldefined octahedral morphology were synthesized by using a template-assisted carbothermal method.
2. Experimental Procedures 2.1. Synthesis. In a typical experiment, NiCl2 · 6H2O was added in 10 mL of distilled water to form a 0.05–0.2 M solution. Then 0.2–0.5 g of as-prepared carbon spheres, which were synthesized according to the literature,20 were dispersed in the above NiCl2 solution. After ultrasonic treatment for 40 min, the resulting suspension was aged for 24 h and then oven-dried at 60 °C. Finally, the oven-dried powders were heated at 460 °C in a furnace in air for 1–2.5 h and kept in the furnace until cooling to room temperature. 2.2. Characterization. The phase characterization of the products was done by X-ray powder diffraction (XRD, Philips X′Pert PRO MPD) with Cu KR as the radiation source (λ ) 1.5418 Å) through the 2θrange from 10 to 90 deg and operated at 40 kV and 30 mA. The morphology of the particles was done by both field-emission scanning electron microscopy (SEM, JSM-6700F) and transmission electron microscopy (TEM, JEOL JEM-2010). The detailed morphology and structural characterization were done by high resolution transmission electron microscope (HRTEM) and coupled with selected-area electron diffraction (SAED) detector.
3. Results and Discussion Figure 1 shows the structural characteristics of as-synthesized samples investigated by XRD. The diffraction peaks typical to NiO are clearly observed (line d), which agree with those of standard NiO of cubic structure (JCPDS Card No. 47-1049). A representative field-emission scanning electron microscopy (FESEM) image of NiO crystals obtained after calcination at 460 °C for 1.5 h is shown in Figure 2A, and it suggests that the products exhibit regular octahedral shape. It is also found that the size of as-synthesized NiO with uniform octahedral structures is about 800 nm. The well-defined octahedral morphology with a cubic symmetry is characteristic of cubic-structured NiO crystals bound by eight {111} planes (Figure 2B). From the TEM image (Figure 2C), one can see that the center of the individual particle is brighter than the edge, which shows that the synthesized NiO powder has hollow structure. The SAED pattern corresponding to individual hollow octahedron is shown
10.1021/cg060957z CCC: $37.00 2007 American Chemical Society Published on Web 11/03/2007
2416 Crystal Growth & Design, Vol. 7, No. 12, 2007
Wang et al.
Figure 1. Powder XRD patterns of products synthesized with different calcination times: (a) 10 min, (b) 20 min, (c) 40 min, and (d) 1.5 h. Figure 3. (A-D) SEM images of cracked NiO octahedra with hollow interiors.
Figure 2. (A) SEM image of hollow NiO octahedra obtained after calcination for 1.5 h. (B) Schematic illustration of an octahedral NiO crystal. (C) TEM image of NiO octahedra with hollow interiors. (D) HRTEM image and the SAED pattern (inset image) of the product at the right-hand of panel C (marked as arrow).
in the inset of Figure 2D, which shows that all of the hollow octahedra are nearly single-crystalline as indicated by the clear diffraction spots. An HRTEM image of the edge area of such a hollow octahedron is shown in Figure 2D. The crystallinity within a crystallite block is high, and perfect diffraction spots are shown in the inset image of Figure 2D, which presents experimental evidence that as-synthesized NiO octahedra are single-crystalline. The lattice fringe of d220 was measured at about 0.15 nm, which is in good agreement with the facecentered cubic NiO (JCPDS Card No. 47-1049). Figure 3 shows high-magnification FESEM images of individual NiO particles with a broken hole, which further confirms that the NiO octahedra are indeed hollow. These holes may serve as exchange channels for chemical constituents inside and outside the octahedra when employing them as nanoreactors or nanocontainers. Interestingly, the hole on cracked octahedra generally lies at or near their apexes. It is well-known that the weak points in the octahedral structures are at their apexes and arris as shown in Figure 2B. The growth process of NiO hollow octahedra was investigated in detail at different reaction times. Figure 1 displays a series of evolutional XRD patterns of the samples investigated. The solid products formed after 10 min of calcinations are a mixture of NiCl2 and NiO. With a longer reaction time of 20–40 min,
Figure 4. (A) TEM image and the SAED pattern (inset image) of products obtained after calcination of 10 min; SEM images of the products obtained after different calcinaton times: (B) 20 min, (C) 40 min, and (D) 1.5 h.
the intensity of NiCl2 peaks weakened, whereas that of the corresponding ones of NiO intensified since NiCl2 gradually transformed into NiO. EDX analysis also testified that the products obtained at different calcination times (10–40 min) are composed of C, O, Ni, Cl (Figure S1, Supporting Information). Then this mixture is converted into a pure NiO phase over a calcination time span of 1.5 h. Corresponding to the above phase evolution, the timedependent crystal morphology of the samples was reported in Figure 4. At a short reaction time of 10 min, TEM images show that quasi-spherical solid particles with rugged surfaces are obtained (Figure 4A). The phase of these crystallites is also confirmed in the polycrystalline ED rings (inset, Figure 4A). It is interesting to note that some particles are partially inclined to transform into octahedron-like frames at this reaction time (Figure S2, Supporting Information). SEM images of Figure 4B illustrate the morphology of the product formed at an intermediate reaction time corresponding to the formation of the NiO and NiCl2 mixture (20 min; also refer to Figure 1b). It is noted that quasi-octahedral crystals were obtained. Quite interestingly, surfaces of these quasi-octahedral products are not particularly smooth, suggesting that they might be made from
Synthesis of Single-Crystalline Hollow Octahedral NiO
the aggregation of smaller particles. And the existence of some quasi-spherical particles also indicates that the transformation of quasi-spheres into octahedra was not fully finished. After 40 min, the phase of NiO nanocrystallites has been significantly increased in the products, whereas the octahedral shape of NiO crystals has been largely established (Figure 4C). Noting that the surface of products becomes relatively smooth, it is believed that recrystallization takes place by influence of carbothermal reaction (between NiO and carbon). It is further noted that some particles are cracked, indicating the formation of hollow octahedra. Over the next 50 min of reaction (at 1.5 h), hollowing and recrystallization take place with gradual evacuation of carbon templates from the interior, through which a central space is created (Figure 3). Notably, the number of cracked NiO octahedra gradually increases when the calcination time is further prolonged. From the above complementary XRD/EDX/SEM/TEM analyses, it can be understood that the formation process of singlecrystalline NiO hollow octahedra is composed of the following three consecutive steps: (i) production of quasi-spherical particles, (ii) formation of quasi-octahedral and octahedral particles from the preformed NiO crystallites, and (iii) evacuation of central carbon templates and perfection of singlecrystalline NiO hollow nanooctahedra through recrystallization by influence of the carbothermal reaction. In addition, we are not the first to observe the generation of single-crystalline polyhedral nanostructures from polycrystalline particles. For example, Zeng et al. have demonstrated that comparatively good yields of single-crystalline hollow Cu2O nanocubes could be obtained from primary polycrystalline particles.21 To investigate the role of carbon templates on the formation of octahedral structure, NiCl2 as starting materials without adding carbon templates were calcined. As an example, 5 g of pure NiCl2 · 6H2O was heated at 460 °C for 1.5 h, and some of crystalline NiO was detected in products by XRD; however, octahedral particles have not been found in products by SEM. The morphology of products was irregular crystal aggregations (shown in Figure S3, Supporting Information). So, it is believed that the carbon plays an important role in forming NiO octahedra with hollow structures. TGA and DSC data also give evidence that carbon plays an important role in the formation of NiO octahedra. DSC results of samples are shown in Figure 5A. From the curve of sample C1 (carbon templates, trace a), we can see two broad exothermic peaks at 325 and 460 °C, which are similar to that of sample C2 (oven-dried composite products, trace b) at 345 and 445 °C, separately. In other words, the exothermic peaks can be assigned into two groups. One of the groups includes peaks of trace a at 325 °C and trace b at 345 °C, which can be attributed to further dehydration and densification of carbon spheres. The other group includes peaks of trace a at 460 °C and trace b at 445 °C, which can be attributed to the burning of the carbon spheres.14 The rest of the narrow and sharp exothermic peak of trace b for sample C2 at 400 °C can be attributed to the forming of NiO. TGA results of samples are shown in Figure 5B. For sample C1 (trace a), there are two major zones of mass loss, with a total mass loss of about 96%, while for sample C2 (trace b), there are three major zones of mass loss, with a total mass loss of ∼75%. From 350 to 430 °C, there is ∼20% mass loss in trace a attributed to the further dehydration and densification of carbon spheres. In comparison, there is ∼35% mass loss in trace b from 350 to 430 °C, so that about 15% of the mass loss gap could be attributed to the transformation of NiCl2 into NiO,
Crystal Growth & Design, Vol. 7, No. 12, 2007 2417
Figure 5. (A) DSC curves of samples: (a) sample C1 (carbon templates), (b) sample C2 (oven-dried composite spheres); (B) TGA curves of samples: (a) sample C1, (b) sample C2.
which corresponds to the exothermic peak at 400 °C in the DSC curve of sample C2. Interestingly, when the temperature is higher than 450 °C, trace b becomes relatively smooth, which might be caused by oxidation of carbon found deep within the core. However, for trace a, it is still in sharp mass loss, and the final weight of carbon templates was measured to be ∼4%. From the above discussion, we can conclude that the carbon not only acts as template but also promotes the formation of NiO octahedra. Carbothermal reactions have been used to synthesize singlecrystalline nanomaterials with different morphologies.22,23 For example, single-crystalline ZnO hexagonal cones and nanocablealligned ZnS tetrapod nanocrystals were prepared by a carbothermal reaction.24,25 In addition, Si3N4 nanowires were synthesized via carbothermal reduction of carbonaceous silica xerogels.26 Moreover, well-crystallized GaN and VN nanoparticles were obtained by using a-C3N3.69 as both the carbothermal reduction agent and the nitridizing agent.27 In our case, it is reasonable that carbon can offer a weak reductive environment to recrystallize quasi-octahedral and octahedral particles and allow single-crystalline NiO octahedra to form. In addition, other metallic oxides (such as Co3O4) with welldefined polyhedral hollow morphology can also be synthesized by this method. That is, the proposed facile strategy can be extended to other metallic oxide materials with well-defined polyhedral structures.
2418 Crystal Growth & Design, Vol. 7, No. 12, 2007
4. Conclusions A simple template-assisted carbothermal method to prepare single crystalline metal oxide with interior spaces has been reported. It has been revealed that carbon templates play a key role in forming of single-crystalline hollow structure. Using this synthetic strategy, other metallic oxides polyhedra (such as Co3O4) with hollow interiors can also be obtained. On the basis of the current findings, it has been demonstrated that nanostructured polyhedrons of functional materials with desired interiors can be synthesized by using a template-assisted carbothermal method. Acknowledgment. This work was supported financially by the National Natural Science Foundation of China (No. 50574083). Supporting Information Available: Detailed morphology of synthesized products. Figure S1 reveals the EDX patterns of products shown in Figure 4A-C of the manuscript. Figure S2 reveals the products obtained after calcination of 10 min shown in Figure 4A of the manuscript. Figure S3 shows the morphology of products obtained by calcination of pure NiCl2 · 6H2O (460 °C, 1.5 h). Figure S4 illustrates the detailed surface structure of colloidal carbon spheres. This material is available free of charge via the Internet at http://pubs.acs.org.
References (1) Caruso, F.; Caruso, R. A.; Möhwald, H. Science 1998, 282, 1111. (2) Sun, Y.; Xia, Y. Science 2002, 298, 2176. (3) Yin, Y.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorja, G. A.; Alivisatos, A. P. Science 2004, 304, 711. (4) Yang, H. G.; Zeng, H. C. Angew. Chem., Int. Ed. 2004, 43, 5930.
Wang et al. (5) Yang, J.; Qi, L.; Lu, C.; Ma, J.; Cheng, H. Angew. Chem., Int. Ed. 2005, 44, 598. (6) Chen, J.; Saeki, F.; Wiley, B. J.; Cang, H.; Cobb, M. J.; Li, Z. Y.; Au, L.; Zhang, H.; Kimmey, M. B.; Li, X.; Xia, Y. Nano Lett. 2005, 5, 473. (7) Dinsmore, A. D.; Hsu, M. F.; Nikolaides, M. G.; Marquez, M.; Bausch, A. R.; Weitz, D. A. Science 2002, 298, 1006. (8) Nakashima, T.; Kimizuka, N. J. Am. Chem. Soc. 2003, 125, 6386. (9) Goldberger, J.; He, R.; Zhang, Y.; Lee, S.; Yan, H.; Choi, H.-J.; Yang, P. Nature 2003, 422, 599. (10) Rosei, F.; Schunack, M.; Jiang, P.; Gourdon, A.; Loegsgaard, E.; Stensgaard, I.; Joachim, C.; Besenbacher, F. Science 2002, 296, 328. (11) Tao, H.; Chen, D.; Jiao, X.; Wang, Y. AdV. Mater. 2006, 18, 1078. (12) Lou, X.; Yuan, C.; Zhang, Q.; Archer, L. A. Angew.Chem., Int. Ed. 2006, 45, 3825. (13) Lu, C.; Qi, L.; Yang, J.; Wang, X.; Zhang, D.; Xie, J.; Ma, J. AdV. Mater. 2005, 17, 2562. (14) Sun, X.; Li, Y. Angew. Chem., Int. Ed. 2004, 43, 3827. (15) Han, M.; Zhang, W.; Gao, C.; Liang, Y.; Xu, Z. Carbon 2006, 44, 211. (16) Yang, R.; Li, H.; Qiu, X. P.; Chen, L. Q. Chem. Eur. J. 2006, 12, 4083. (17) Zeng, H.; Li, J.; Liu, J.; Wang, Z.; Sun, S. Nature 2002, 420, 395. (18) Tarascon, J. M.; Armand, M. Nature 2001, 414, 359. (19) Wang, Y.; Zhu, Q.; Zhang, H. Chem. Commun. 2005, 5231. (20) Sun, X.; Li, Y. Angew. Chem., Int. Ed. 2004, 43, 597. (21) Teo, J. J.; Chang, Y.; Zeng, H. C. Langmuir 2006, 22, 7369. (22) Gundiah, G.; Govindaraj, A.; Rao, C. N. R. Chem. Phys. Lett. 2002, 351, 189. (23) Lao, J. Y.; Wen, J. G.; Ren, Z. F. Nano Lett. 2002, 2, 1287. (24) Zhu, Y.; Bando, Y.; Xue, D.; Golberg, D. J. Am. Chem. Soc. 2003, 125, 16196. (25) Jie, J.; Wang, G.; Han, X.; Fang, J.; Xu, B.; Yu, Q.; Liao, Y.; Li, F.; Hou, J. J. Cryst. Growth 2004, 267, 223. (26) Wang, F.; Jin, G.; Guo, X. J. Phys. Chem. B 2006, 110, 14546. (27) Zhao, H.; Lei, M.; Yang, X.; Jian, J.; Chen, X. J. Am. Chem. Soc. 2005, 127, 15722.
CG060957Z
