pubs.acs.org/crystalPublished on Web 12/11/2009r 2009 American Chemical Society

DOI: 10.1021/cg901148q

2010, Vol. 1029–31

Ostwald Ripening Growth of Silicon Nitride Nanoplates

Weiyou Yang,*,† Fengmei Gao,† Guodong Wei,† and Linan An‡

†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 formationof the plates involves three basic steps: formation and aggregation of the nanoparticles, grain coalescence within selective areas, andgrowth of one coarsened grain at the expense of the rest of the nanoparticles via an Oswald ripening process assisted by the orientedattachment mechanism. The obtained nanoplates exhibit an extremely high aspect ratio with an ultrathin thickness, flat surface, andperfect crystal structure, and they could be utilized as substrates for constructing nanodevices.

Synthesis of low-dimensional nanomaterials with differentmorphologies has attracted great recent interest, since the proper-ties and applications of the materials strongly depend on theirshapes.1 Among different types of low-dimensional nanomater-ials such as nanowires, nanobelts, and nanotubes, nanoplatesare usually characterized by extremely high anisotropy with anultrathin thickness andare considered as excellentbuildingblocksfor constructing nanodevices and other applications.2

Tremendous efforts have been devoted to fabricate nanoplatessuch as metals,3 oxides,4 sulfides,5 and other compounds.6 Thegrowth mechanism of nanoplates can be classified into fivecategories: (i) confined growth of nanoparticles with the assistantof surfactant capping agents or polymer,7 (ii) seed inducedgrowth of nanoplates,8 (iii) aggregation of nanoparticles intocrystalline nanoplates,9 (iv) assembly of existing smaller triangu-lar nanoplates into a bigger one,10 and (v) self-repair of the nano-pores in porous nanoframes.11 These nanoplates are usually inquasi-circular, triangular, square, and hexagonal shapes, with thesize ranging from tens to hundreds of nanometers.

Silicon nitride (Si3N4) is a well-known wide-band gap semi-conductorwith excellent thermomechanical properties and chemi-cal stability, useful for high-temperature and/or short-wavelengthapplications.12 While a variety of Si3N4 nanostructures has beensynthesized,13 Si3N4 nanoplates have not been reported yet. Inthis communication, we report the growth of single-crystallineSi3N4 nanoplates via anOstwald ripeningmechanismby catalyst-assisted pyrolysis of polymeric precursors. The present workprovides a new method for the fabrication of Si3N4 nanoplates,which could be expanded to the other materials system by usingvarious polymer precursors. The obtainednanoplates can be usedas substrates for constructing nanodevices.

Si3N4 nanoplates were synthesized by pyrolyzing polyalumi-nasilazane in the presence of a catalyst. The precursor wasobtained by reaction of 95 wt % polyureasilazane with 5 wt %aluminum isopropoxide.14 The obtained liquid polymer wassolidified by heat-treatment at 260 �C for 0.5 h in N2 and thencrushed into a fine powder by high-energy ball milling for 24 hwith 3 wt%FeCl2 powder (99.9%) as additive. Then the powdermixtures were placed in an alumina crucible (99%) andpyrolyzedin a conventional furnace with a graphite resistance under ultra-high purity N2 (99.99%) of 0.1 MPa with a flowing rate of200 sccm. To investigate the morphology evaluation of theSi3N4 nanoplates, the powder mixture was heated to the desiredtemperature 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

transmission electron microscopy (HRTEM, JEOL-2010F,JEOL, Japan) at 200 kV.

Figure 1 is TEM images of the synthesized products obtainedat the pyrolysis time of 2 h. The products are Si3N4 nanoplateswith a size up to severalmicrometers. The plates are very thinwithan extremely flat surface and are highly transparent to electronseven when the plates are overlapped (Figure 1b). It is interestingto see that the plates take various shapes, such as tetragonal,pentagonal, and hexagonal, which is quite different from thecases of previous works, where the nanoplates were usuallyformed in a single shape such as a triangle, a quasi-circle, or ahexagon.

In order to understand the growth mechanism of the nano-plates, the intermediate products at different pyrolysis times areobtained and examined using transmission electron microscopyand high resolution transmission electron microscopy. Figure 2shows the typical morphology of the product obtained at thepyrolysis time of 0.5 h. It is seen that there are many nanostruc-tures with sizes ranging from hundreds of nanometers to severalmicrometers. Closer examinations reveal that these nanostruc-tures consisted of a large amount of smaller nanoparticles, whichaggregated together to form the irregularly shaped nanostruc-tures with truncated edges and rough surfaces (Figure 2a).Further examination of the nanostructures (Figures 2b-d) re-veals that the packing densities of the nanoparticles in differentnanostructures are quite different. Parts e and f ofFigure 2 are theTEM images at higher magnification corresponding to themarked areas of “I” and “II” in Figure 2d, respectively. Bothimages further confirm that the nanostructures are composed oftiny nanoparticles which are sized in several to tens of nano-meters. The inset picture inFigure 2e is a typical SAEDpattern ofthe nanostructures, which is identical over the whole sample,indicating the polycrystalline nature of the nanostructures. Theresults suggest that, at this stage, the nanostructures are justassemblies of individual nanoparticles rather than in single-crystal forms.

Figure 3a shows a typical TEM image of a nanostructureobtained at the pyrolysis time of 1 h. The morphology of this oneis obviously different from those in Figure 2. The nanostructureexhibits a gradient packing density crossover the sample.Figure 3b is the representative EDS spectrum of the nanostruc-tures, revealing that the nanostructures consisted of Si and Nelements only (Cu coming from the copper grid used to supportthe sample for TEM observation). The atomic ratio of Si to N,within the experimental limit, is close to 4:3, suggesting thenanostructure is Si3N4. Figure 3c and the bottom-right insetpicture are the respective SAED patterns recorded from themarked areas of “I” and “II” in Figure 3a. The patterns reveal*Corresponding author. E-mail: weiyouyang@tsinghua.org.cn.

30 Crystal Growth & Design, Vol. 10, No. 1, 2010

that area “I” is single-crystalline hexagonal R-Si3N4 while area“II” is polycrystalline in nature. These results clearly suggest thatthe aggregated nanoparticles start to transfer to a single crystalwith a longer holing time. The transition starts from one end (orarea) of the sample and then grows into the whole sample,implying that the growth of the nanostructure should followthe Ostwald ripening process. Figure 3d is a HRTEM imagebetween two nanoparticles, suggesting that the oriented attach-ment among the nanoparticles could happenduring the growthofSi3N4 nanoplates. However, even Ostwald ripening and orientedattachment happened in various areas and different stages; theformer one should be dominant for the nanoplate growth, sinceorientedattachments only takeplacewithinpartial nanoparticles,as disclosed by HRTEM observations.

Figure 4a shows a typical TEM image of the products obtainedat the pyrolysis time of 2 h. As compared to the samples with

shorter holding times, the aggregated nanoparticles have beencompletely converted into a nanoplate with a well-defined shape.Figure 4b is the SAEDpattern of the plate, which is identical overthe entire plate, indicating its single-crystalline nature. Thecorresponding HRTEM image of the nanoplates is given inFigure 4c, revealing that the nanoplates possess a perfect crys-tal structure with few structural defects such as dislocationsand stacking faults. The lattice fringe spacings of 0.67 and0.56 nm agree well with the (100) and (001) planes of bulkR-Si3N4, where a = 0.77541 nm and c= 0.56217 nm (JCPDSCard No. 41-0360).

Based on the above observations and analysis, a simple modelbased on anOstwald ripening process is proposed for the growthof nanoplates, as shown in Figure 4d. The process consists ofthree basic steps: formation and aggregation of the nanoparticles,grain coalescence in selective areas, and growth of one coarsenedgrain at the expense of the rest of the nanoparticles via anOswaldripeningmechanism. The formation of the Si3N4 nanoparticles inthe first stage likely follows a typical vapor-solid (VS) process inwhich the starting polymer precursor was first converted into

Figure 1. Typical types of the obtained Si3N4 nanoplates withvarious shapes.

Figure 3. (a) Representative TEM image of the pyrolysis productswith a holding time of 1 h. (b) Typical EDS spectrumof the obtainedproduct. Part c and the inset are respective SAEDpatterns recordedfrom the marked areas of “I” and “II” in part a. (d) HRTEM imageshowing the oriented attachment within partial nanoparticles.

Figure 2. (a-d) Typical TEM images of the pyrolysis products witha holding time of 0.5 h. (e-f)MagnifiedTEM images correspondingto the marked areas of “I” and “II” in part d, suggesting theaggregated forms are composed of tiny nanoparticles. The insetpicture in part e discloses the polycrystalline nature of the assem-blies.

Figure 4. (a) Typical TEM image of the pyrolysis products with aholding time of 2 h. (b)Representative SAEDpattern recorded fromthe nanoplate. (c) Corresponding HRTEM image of the nanoplate.(d) Schematic model for Oswald ripening growth of the Si3N4

nanoplate.

Communication Crystal Growth & Design, Vol. 10, No. 1, 2010 31

amorphous SiAlCNs with a small amount of O.14 Such a smallamount of oxygen could lead to the release of CO and SiO fromthe amorphous SiAlCNs, which could react with N2 in theenvironment to form Si3N4 via the reaction of 3SiO þ 3CO þ2N2 f Si3N4 þ 3CO2.

15 The FeCl2 was used as the catalyst topromote the formation of the vapor phases, which played a keyrole in the formation of nanoparticles in the early growth stageof the nanoplates. The reason for the formation of platelikenanoparticle aggregations instead of nanowires and nanobeltsis not clear at present. The aggregation of the nanoparticles couldbe driven by the van der Waals force.16 The nanoparticles aresintered together to form larger sized grains, which could occur atdifferent degrees within various areas due to the oriented attach-ment 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 ofthe rest of the grains via Oswald ripening, driven by the surfaceenergy.18 The growth of the plate could be attributed to thecooperationofOswald ripening growth and anOAmechanism indifferent areas and stages, leading to the final formation of asingle-crystal one. The flat and smooth surface and edge ofthe nanoplate is likely due to the surface diffusion, which couldbe favored by the high pyrolysis temperature and long holdingtime used in the current synthesis process. The different shapesof the obtained nanoplates (Figures 1 and 4a) are owing to theoriginal shape formed by the randomdeposition and aggregationof the nanoparticles at the early stage.

In summary, we report the synthesis of Si3N4 nanoplates viacatalyst-assisted pyrolysis of polymeric precursors. The growthmechanism of the nanoplates is ascribed to an Ostward ripeningprocess, in which Si3N4 nanoparticles are first formed due to thedecomposition of the precursor and aggregated into large sizednanostructures. The grain coalescence occurs in the areas wherethe neighboring nanoparticles have either a preferentially orien-ted connection and/or a higher packing density. The coarsenedgrain then grows at the expense of smaller ones, assisted by theoriented attachment mechanism, and results in the formation ofthe single-crystal Si3N4 nanoplates. The obtained nanoplatesexhibit an extremely high aspect ratio with a flat surface andperfect crystal structure, and they could be utilized as substratesfor constructing nanodevices.

Acknowledgment. The authors are thankful for support fromthe National Natural Science Foundation of China (NSFC,Grant Nos. 50602025 and 50872058), the International Coopera-tion Project of the Ningbo Municipal Government (Grant No.2008B10044), and theNatural ScienceFoundation of theNingboMunicipal Government (Grant No. 2009A610035).

References

(1) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. Rev.2005, 105, 1025.

(2) (a) Sun, Y.; Xia, Y. Adv. Mater. 2003, 15, 695. (b) Yun, Y.; Park, G.;Ah, C.; Park, H.; Yun, W.; Ha, D. Appl. Phys. Lett. 2005, 87, 233110.(c) Sun, Y.; Wiederrecht, G. Small 2007, 3, 1964. (d) Deckert-Gaudig,T.; Deckert, V. Small 2009, 5, 432.

(3) (a) Chen, S.; Carroll, D.Nano Lett. 2002, 2, 1003. (b) Zhang, Q.; Ge,J.; Pham, T.; Goebl, J.; Hu, Y.; Lu, Z.; Yin, Y. Angew. Chem., Int. Ed.2009, 48, 3516. (c) Xiong, Y.; McLellan, J.; Chen, J.; Yin, Y.; Li, Z.;Xia, Y. J. Am. Chem. Soc. 2005, 127, 17118.

(4) (a) Jing, Z.; Zhan, J. Adv. Mater. 2008, 20, 4547. (b) Cao, Y. J. Am.Chem. Soc. 2004, 126, 7456. (c) Peng, C.; Richard-Plouet, M.; Tsai,M.; Lee, C.; Chiu, H.; Petit, P.; Sheu, H.; Lefrant, S.; Brohan, L.Cryst.Growth Des. 2008, 8, 3555. (d) Si, R.; Zhang, Y.; You, L.; Yan, C.Angew. Chem., Int. Ed. 2005, 44, 3256.

(5) (a) Seo, J.; Jang, J.; Park, S.; Kim, C.; Park, B.; Cheon, J. Adv.Mater. 2008, 20, 4269. (b) Park, K.; Jang, K.; Son, S. Angew. Chem.,Int. Ed. 2006, 45, 4608.

(6) (a) Xu, L.; Zhan, J.; Hu, J.; Bando, Y.; Yuan, X.; Sekiguchi, T.;Mitome, M.; Golberg, D. Adv. Mater. 2007, 19, 2141. (b) Aagesen,M.; Johnson, E.; Sorensen, C.; Mariager, S.; Feidenhans, R.; Spiecker,E.; Nygrd, J.; Lindelof, P. Nat. Nanotechnol. 2007, 2, 761. (c) Wang,W.; Poudel, B.; Yang, J.; Wang, D.; Ren, Z. J. Am. Chem. Soc. 2005,127, 13792. (d) Shi, W.; Zhou, L.; Song, S.; Yang, J.; Zhang, H. Adv.Mater. 2008, 20, 1892. (e) Garje, S.; Eisler, D.; Ritch, J.; Afzaal, M.;O'Brien, P.; Chivers, T. J.Am.Chem. Soc. 2006, 128, 3120. (f) Guo,Y.;Lee, J.; Maier, J. Adv. Mater. 2005, 17, 2815. (g) Zhang, Y.;Sun, X.; Si, R.; You, L.; Yan, C. J. Am. Chem. Soc. 2005, 127,3260. (h) Du, Y.; Zhang, Y.; Sun, L.; Yan, C. J. Am. Chem. Soc. 2009,131, 3162.

(7) Wang, L.; Chen, X.; Zhan, J.; Chai, Y.; Yang, C.; Xu, L.; Zhuang,W.; Jing, B. J. Phys. Chem. B 2005, 109, 3189.

(8) (a) Jin, R. C.; Cao, Y. W.; Mirkin, C. A.; Kelly, K. L.; Hao, E.;Schatz, G. C.; Zheng, J. G. Science 2001, 294, 1901. (b) Jiang, X.;Zeng, Q.; Yu, A. Nanotechnology 2006, 17, 4929. (c) Umar, A.;Oyama, M. Cryst. Growth Des. 2006, 6, 818. (d) Lim, B.; Wang, J.;Camargo, P.; Jiang, M.; Kim, M.; Xia, Y. Nano Lett. 2008, 8, 2535.

(9) Shankar, S.; Rai, A.; Ankamwar, B.; Singh, A.; Ahmad,A.; Sastry,M. Nat. Mater. 2004, 3, 482.

(10) Jin, R.; Cao, Y.; Hao, E.; M�etraux, G.; Schatz, G.; Mirkin, C.Nature 2003, 425, 487.

(11) Yang, S.; Wang, Y.; Wang, Q.; Zhang, R.; Yang, Z.; Guo, Y.;Ding, B. Cryst. Growth Des. 2007, 7, 2258.

(12) (a) Govila, R. K. J. Mater. Sci. 1985, 20, 4345. (b) Ziegler, G.;Heinrich, J.; W€otting, C. J. Mater. Sci. 1987, 22, 3041. (c) Zanatta,A. R.; Nunes, L. A. O.Appl. Phys. Lett. 1998, 72, 3127. (d)Munakata,F.; Matsuo, K.; Furuya, K.; Akimune, Y. J.; Ishikawa, I. Appl. Phys.Lett. 1999, 74, 3498.

(13) (a) Han, W. Q.; Fan, S. S.; Li, Q. Q.; Gu, B. L.; Zhang, X. B.; Yu,D. P.Appl. Phys. Lett. 1997, 71, 2271. (b) Yin, L.W.; Bando, Y.; Zhu,Y. C.; Li, Y. B.Appl. Phys. Lett. 2003, 83, 3584. (c) Yang, W. Y.; Xie,Z. P.; Miao, H. Z.; Zhang, L. G.; Ji, H.; An, L. N. J. Am. Ceram. Soc.2005, 88, 466. (d) Yang, W. Y.; Xie, Z. P.; Li, J. J.; Miao, H. Z.; Zhang,L. G.; An, L. N. J. Am. Ceram. Soc. 2005, 88, 1647. (e) Yang, W.;Cheng, X.; Wang, H.; Xie, Z.; Xing, F.; An, L. Cryst. Growth Des.2008, 8, 3921. (f) Yang, W.; Gao, F.; Wang, H.; Xie, Z.; An, L. Cryst.Growth Des. 2008, 8, 2606.

(14) Dhamne, A.; Xu, W.; Fookes, B.; Fan, Y.; Zhang, L.; Burton, S.;Hu, J.; Ford, J.; An, L. J. Am. Ceram. Soc. 2005, 88, 2415.

(15) (a) Zhang, A. C.; Cannon,W. R. J. Am. Ceram. Soc. 1984, 67, 691.(b) Gao, F.; Yang, W.; Fan, Y.; An, L. J. Solid State Chem. 2008, 181,211.

(16) (a) Biggs, S.; Mulvaney, P.; Zukoski, C.; Grieser, F. J. Am. Chem.Soc. 1994, 116, 9150. (b) Pei, L.;Mori, K.; Adachi,M.Langmuir 2004,20, 7837.

(17) Yang, W. Y.; Xie, Z. P.; Miao, H. Z.; Zhang, L. G.; An, L. N.J. Phys. Chem. B 2006, 110, 3969.

(18) Ratke, L.; Voorhees, P. W. Growth and Coarsening: OstwaldRipening in Material Processing; Springer: 2002; pp 117-118.