DOI: 10.1021/cg901148q
Ostwald Ripening Growth of Silicon Nitride Nanoplates Weiyou Yang,*,† Fengmei Gao,† Guodong Wei,† and Linan An‡
2010, Vol. 10 29–31
†
Institute of Materials, Ningbo University of Technology, Ningbo 315016, P.R. China, and Advanced Materials Processing and Analysis Center, University of Central Florida, Orlando, Florida 32816
‡
Received September 18, 2009; Revised Manuscript Received November 28, 2009
ABSTRACT: In this paper, we have demonstrated the Oswald ripening growth of single-crystalline Si3N4 nanoplates. The formation of the plates involves three basic steps: formation and aggregation of the nanoparticles, grain coalescence within selective areas, and growth of one coarsened grain at the expense of the rest of the nanoparticles via an Oswald ripening process assisted by the oriented attachment mechanism. The obtained nanoplates exhibit an extremely high aspect ratio with an ultrathin thickness, flat surface, and perfect crystal structure, and they could be utilized as substrates for constructing nanodevices. Synthesis of low-dimensional nanomaterials with different morphologies has attracted great recent interest, since the properties and applications of the materials strongly depend on their shapes.1 Among different types of low-dimensional nanomaterials such as nanowires, nanobelts, and nanotubes, nanoplates are usually characterized by extremely high anisotropy with an ultrathin thickness and are considered as excellent building blocks for constructing nanodevices and other applications.2 Tremendous efforts have been devoted to fabricate nanoplates such as metals,3 oxides,4 sulfides,5 and other compounds.6 The growth mechanism of nanoplates can be classified into five categories: (i) confined growth of nanoparticles with the assistant of surfactant capping agents or polymer,7 (ii) seed induced growth of nanoplates,8 (iii) aggregation of nanoparticles into crystalline nanoplates,9 (iv) assembly of existing smaller triangular nanoplates into a bigger one,10 and (v) self-repair of the nanopores in porous nanoframes.11 These nanoplates are usually in quasi-circular, triangular, square, and hexagonal shapes, with the size ranging from tens to hundreds of nanometers. Silicon nitride (Si3N4) is a well-known wide-band gap semiconductor with excellent thermomechanical properties and chemical stability, useful for high-temperature and/or short-wavelength applications.12 While a variety of Si3N4 nanostructures has been synthesized,13 Si3N4 nanoplates have not been reported yet. In this communication, we report the growth of single-crystalline Si3N4 nanoplates via an Ostwald ripening mechanism by catalystassisted pyrolysis of polymeric precursors. The present work provides a new method for the fabrication of Si3N4 nanoplates, which could be expanded to the other materials system by using various polymer precursors. The obtained nanoplates can be used as substrates for constructing nanodevices. Si3N4 nanoplates were synthesized by pyrolyzing polyaluminasilazane in the presence of a catalyst. The precursor was obtained by reaction of 95 wt % polyureasilazane with 5 wt % aluminum isopropoxide.14 The obtained liquid polymer was solidified by heat-treatment at 260 °C for 0.5 h in N2 and then crushed into a fine powder by high-energy ball milling for 24 h with 3 wt % FeCl2 powder (99.9%) as additive. Then the powder mixtures were placed in an alumina crucible (99%) and pyrolyzed in a conventional furnace with a graphite resistance under ultrahigh purity N2 (99.99%) of 0.1 MPa with a flowing rate of 200 sccm. To investigate the morphology evaluation of the Si3N4 nanoplates, the powder mixture was heated to the desired temperature of 1450 °C and kept there for 0.5, 1, and 2 h, respectively, followed by furnace-cool to ambient temperature. The resultant samples were then investigated by high-resolution *Corresponding author. E-mail:
[email protected]. r 2009 American Chemical Society
transmission electron microscopy (HRTEM, JEOL-2010F, JEOL, Japan) at 200 kV. Figure 1 is TEM images of the synthesized products obtained at the pyrolysis time of 2 h. The products are Si3N4 nanoplates with a size up to several micrometers. The plates are very thin with an extremely flat surface and are highly transparent to electrons even when the plates are overlapped (Figure 1b). It is interesting to see that the plates take various shapes, such as tetragonal, pentagonal, and hexagonal, which is quite different from the cases of previous works, where the nanoplates were usually formed in a single shape such as a triangle, a quasi-circle, or a hexagon. In order to understand the growth mechanism of the nanoplates, the intermediate products at different pyrolysis times are obtained and examined using transmission electron microscopy and high resolution transmission electron microscopy. Figure 2 shows the typical morphology of the product obtained at the pyrolysis time of 0.5 h. It is seen that there are many nanostructures with sizes ranging from hundreds of nanometers to several micrometers. Closer examinations reveal that these nanostructures consisted of a large amount of smaller nanoparticles, which aggregated together to form the irregularly shaped nanostructures with truncated edges and rough surfaces (Figure 2a). Further examination of the nanostructures (Figures 2b-d) reveals that the packing densities of the nanoparticles in different nanostructures are quite different. Parts e and f of Figure 2 are the TEM images at higher magnification corresponding to the marked areas of “I” and “II” in Figure 2d, respectively. Both images further confirm that the nanostructures are composed of tiny nanoparticles which are sized in several to tens of nanometers. The inset picture in Figure 2e is a typical SAED pattern of the nanostructures, which is identical over the whole sample, indicating the polycrystalline nature of the nanostructures. The results suggest that, at this stage, the nanostructures are just assemblies of individual nanoparticles rather than in singlecrystal forms. Figure 3a shows a typical TEM image of a nanostructure obtained at the pyrolysis time of 1 h. The morphology of this one is obviously different from those in Figure 2. The nanostructure exhibits a gradient packing density crossover the sample. Figure 3b is the representative EDS spectrum of the nanostructures, revealing that the nanostructures consisted of Si and N elements only (Cu coming from the copper grid used to support the sample for TEM observation). The atomic ratio of Si to N, within the experimental limit, is close to 4:3, suggesting the nanostructure is Si3N4. Figure 3c and the bottom-right inset picture are the respective SAED patterns recorded from the marked areas of “I” and “II” in Figure 3a. The patterns reveal Published on Web 12/11/2009
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Figure 3. (a) Representative TEM image of the pyrolysis products with a holding time of 1 h. (b) Typical EDS spectrum of the obtained product. Part c and the inset are respective SAED patterns recorded from the marked areas of “I” and “II” in part a. (d) HRTEM image showing the oriented attachment within partial nanoparticles.
Figure 1. Typical types of the obtained Si3N4 nanoplates with various shapes.
Figure 2. (a-d) Typical TEM images of the pyrolysis products with a holding time of 0.5 h. (e-f) Magnified TEM images corresponding to the marked areas of “I” and “II” in part d, suggesting the aggregated forms are composed of tiny nanoparticles. The inset picture in part e discloses the polycrystalline nature of the assemblies.
that area “I” is single-crystalline hexagonal R-Si3N4 while area “II” is polycrystalline in nature. These results clearly suggest that the aggregated nanoparticles start to transfer to a single crystal with a longer holing time. The transition starts from one end (or area) of the sample and then grows into the whole sample, implying that the growth of the nanostructure should follow the Ostwald ripening process. Figure 3d is a HRTEM image between two nanoparticles, suggesting that the oriented attachment among the nanoparticles could happen during the growth of Si3N4 nanoplates. However, even Ostwald ripening and oriented attachment happened in various areas and different stages; the former one should be dominant for the nanoplate growth, since oriented attachments only take place within partial nanoparticles, as disclosed by HRTEM observations. Figure 4a shows a typical TEM image of the products obtained at the pyrolysis time of 2 h. As compared to the samples with
Figure 4. (a) Typical TEM image of the pyrolysis products with a holding time of 2 h. (b) Representative SAED pattern recorded from the nanoplate. (c) Corresponding HRTEM image of the nanoplate. (d) Schematic model for Oswald ripening growth of the Si3N4 nanoplate.
shorter holding times, the aggregated nanoparticles have been completely converted into a nanoplate with a well-defined shape. Figure 4b is the SAED pattern of the plate, which is identical over the entire plate, indicating its single-crystalline nature. The corresponding HRTEM image of the nanoplates is given in Figure 4c, revealing that the nanoplates possess a perfect crystal structure with few structural defects such as dislocations and stacking faults. The lattice fringe spacings of 0.67 and 0.56 nm agree well with the (100) and (001) planes of bulk R-Si3N4, where a = 0.77541 nm and c = 0.56217 nm (JCPDS Card No. 41-0360). Based on the above observations and analysis, a simple model based on an Ostwald ripening process is proposed for the growth of nanoplates, as shown in Figure 4d. The process consists of three basic steps: formation and aggregation of the nanoparticles, grain coalescence in selective areas, and growth of one coarsened grain at the expense of the rest of the nanoparticles via an Oswald ripening mechanism. The formation of the Si3N4 nanoparticles in the first stage likely follows a typical vapor-solid (VS) process in which the starting polymer precursor was first converted into
Communication
Crystal Growth & Design, Vol. 10, No. 1, 2010
amorphous SiAlCNs with a small amount of O.14 Such a small amount of oxygen could lead to the release of CO and SiO from the amorphous SiAlCNs, which could react with N2 in the environment to form Si3N4 via the reaction of 3SiO þ 3CO þ 2N2 f Si3N4 þ 3CO2.15 The FeCl2 was used as the catalyst to promote the formation of the vapor phases, which played a key role in the formation of nanoparticles in the early growth stage of the nanoplates. The reason for the formation of platelike nanoparticle aggregations instead of nanowires and nanobelts is not clear at present. The aggregation of the nanoparticles could be driven by the van der Waals force.16 The nanoparticles are sintered together to form larger sized grains, which could occur at different degrees within various areas due to the oriented attachment mechanism (OA)7,9,11,17 and/or different packing densities. One of these grains then acts as a seed and grows at the expense of the rest of the grains via Oswald ripening, driven by the surface energy.18 The growth of the plate could be attributed to the cooperation of Oswald ripening growth and an OA mechanism in different areas and stages, leading to the final formation of a single-crystal one. The flat and smooth surface and edge of the nanoplate is likely due to the surface diffusion, which could be favored by the high pyrolysis temperature and long holding time used in the current synthesis process. The different shapes of the obtained nanoplates (Figures 1 and 4a) are owing to the original shape formed by the random deposition and aggregation of the nanoparticles at the early stage. In summary, we report the synthesis of Si3N4 nanoplates via catalyst-assisted pyrolysis of polymeric precursors. The growth mechanism of the nanoplates is ascribed to an Ostward ripening process, in which Si3N4 nanoparticles are first formed due to the decomposition of the precursor and aggregated into large sized nanostructures. The grain coalescence occurs in the areas where the neighboring nanoparticles have either a preferentially oriented connection and/or a higher packing density. The coarsened grain then grows at the expense of smaller ones, assisted by the oriented attachment mechanism, and results in the formation of the single-crystal Si3N4 nanoplates. The obtained nanoplates exhibit an extremely high aspect ratio with a flat surface and perfect crystal structure, and they could be utilized as substrates for constructing nanodevices. Acknowledgment. The authors are thankful for support from the National Natural Science Foundation of China (NSFC, Grant Nos. 50602025 and 50872058), the International Cooperation Project of the Ningbo Municipal Government (Grant No. 2008B10044), and the Natural Science Foundation of the Ningbo Municipal Government (Grant No. 2009A610035).
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