Nitriding

DOI: 10.1002/smll.200500218

Boron Nitride Nanocages Synthesized by aModerate Thermochemical Approach**

Yi Pan, Kaifu Huo, Yemin Hu, Jijiang Fu, Yinong Lu,Zhendong Dai, Zheng Hu,* and Yi Chen

The discovery of carbon fullerenes[1] and nanotubes[2] hasled to the important research area of novel carbon materi-als, which combines interesting science and new technolo-gies.[3] This has also stimulated the increased study of analo-gous nanostructures of various inorganic compounds.[4]

Among them, materials with a hollow, sphere-like nanomor-phology have attracted particular interest due to the likeli-

[*] Y. Pan, K. Huo, Y. Hu, J. Fu, Prof. Z. Hu, Y. ChenKey Laboratory of Mesoscopic Chemistry of MOE andJiangsu Provincial Laboratory for NanotechnologyDepartment of Chemistry, Nanjing UniversityNanjing 210093 (China)Fax: (+86)25-8368-6251E-mail: zhenghu@nju.edu.cn

Y. LuCollege of Materials Science and EngineeringNanjing University of Technology, Nanjing 210009 (China)

Z. DaiDepartment of Mechanical EngineeringNanjing University of Aeronaut & AstronautNanjing 210016 (China)

[**] This work was financially supported by the “863” project(no. 2003 AA302150), NSFC (nos. 20471028 and 50302004), aHigh-Tech Project of Jiangsu Province (BG203029), as well as theChina Postdoctoral Science Foundation (2004036013) and Jiang-su Planned Projects for Postdoctoral Research funds.

Supporting information for this article is available on the WWWunder http://www.small-journal.com or from the author.

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hood of finding novel properties and applications.[5–7] For ex-ample, owing to their hollow structure, low density, andlarge surface area, they can serve as extremely small con-tainers for encapsulation, delivery of drugs, and develop-ment of artificial cells.[5,8, 9] In particular, some inorganicnanocages, such as WS2 and MoS2, have been confirmed tohave superior tribological properties and have, therefore,been identified as strong candidates for tribological applica-tions, such as solid-state lubricants, lubricating additives,and so on.[10–12] These materials are characterized by a lay-ered structure with weak interlayer forces (van der Waals in-teractions) that allow easy, low-strength shearing.[10] Hexag-onal boron nitride (h-BN), which has a similar honeycomb,layered structure, is a favored member due to its wide-bandgap semiconducting properties, high melting point, highmechanical strength, hardness, corrosion resistance, oxida-tion resistance, and outstanding thermal and electrical prop-erties.[13] In addition to the intense studies on one-dimen-sional BN nanostructures,[14–16] BN nanocages have been at-tracting increasing attention. Although great progress hasbeen achieved in theoretical aspects of their structures andproperties,[17,18] experimental progress is still limited, al-though BN nanocages have been found as by-products, es-pecially in violent syntheses such as arc discharge and laserablation.[19,20] Very recently, Zhu et al. reported a high-tem-perature (1750 8C) synthesis of BN nanocages with diame-ters below 200 nm by using a homemade B–N–O precur-sor.[21] Micrometer or sub-micrometer, hollow BN particleshave also been prepared by some solution methods.[22–24]

However, to date, the controllable synthesis of BN nanocag-es of less than 100 nm is still difficult. Indeed, the prepara-tion of hollow spheres with diameters of less than 100 nmoffers significant synthetic challenges.[8] In recent years, wehave developed a moderate chemical route for synthesizingBN nanotubes[25] and nanowires[26] simply by nitriding Fe–Bnanoparticles under different conditions. In this contribu-tion, BN nanocages of less than 80 nm in size are synthe-sized in quantity at only 750 8C by extending this new route.The product shows rather promising solid-state lubricatingproperties, and a synthesis mechanism is also proposed.Figure 1 shows the TEM images of the Fe–B particles

and the as-prepared product just after the nitriding reaction.It can be seen that the Fe–B particles are monodispersewith a size of about 20–150 nm, and that the as-preparedproduct is composed of many aggregated BN nanocagesranging from 20 to 50 nm in size with a shell thickness ofabout 6 nm as well as some Fe–B-encapsulated BN nanopar-ticles, as discussed later. It is also seen that there are somehollow or bubble-like BN nanostructures surrounding andclinging to the surface of a few relatively big Fe–B particles.The as-prepared product was further analyzed by high-

resolution transmission electron microscopy (HRTEM), asshown in Figure 2. Figure 2a indicates that the polyhedralnanocages are crystalline h-BN nanostructures characterizedby the 0.34-nm spacing between neighboring lattice fringes,which corresponds to the d0002 spacing of h-BN. This is fur-ther confirmed by the corresponding energy dispersive X-ray (EDX) spectroscopy result (Figure 2d), where B and Nsignals exist in addition to the weak Cu and O signals due

to the TEM grid and oxygen absorption contamination, re-spectively. Figure 2b shows Fe–B-encapsulated BN nanopar-ticles, another typical nanostructure in the product. The cor-responding EDX result in Figure 2e clearly reveals the Fesignal besides the B and N signals from the outer shell. Fur-thermore, there is a typical kind of nanocage intimately con-nected to the surface of the Fe–B “catalyst”, as shown inFigure 2c, which might be a BN nanocage, just formed, andcould give us a hint about the growth mechanism.According to the above characterization results, we can

speculate that the BN nanocages are formed as follows(shown schematically in Figure 3a). When NH3/N2 is intro-duced at 750 8C, NH3 is dissociated onto the Fe–B catalystsurface where it forms N atoms, which then combine with Batoms from the Fe–B nanoparticle to form BN species onthe catalyst surface. With the consumption of the surface Batoms, the inner B atoms of the catalyst diffuse outwards tothe particle surface due to the concentration gradient, andthe h-BN shell gradually grows inward around the Fe–B par-

Figure 1. Typical TEM images. a) Fe–B particles; b) the as-preparedproduct: hollow BN nanocages (1); Fe–B-encapsulated BN particles(2); BN nanocages clinging to the Fe–B surface (3); Fe–B particles (4).

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ticle (step 2 in Figure 3a). As this growth continues, with in-creasing shell thickness, the inner diameter of the shell de-creases and strain energy consequently builds up in both theBN shell wall and the Fe–B particle. When the BN shellreaches a critical thickness of around 6 nm, the Fe–B core isexpelled from the BN shell by a strain-induced ejection(step 3 in Figure 3a). The dewetting of the Fe–B nanoparti-cle from the BN cage wall[27] further pushes the Fe–B parti-cle away, and the small opening of the BN nanocage con-nected to the Fe–B particle is sealed by newly grown BNlayers. Then, a fresh BN shell is formed around the expelledFe–B particles and a new nanocage is similarly formed(step 4 in Figure 3a). This growth process is convincinglysupported by the in situ HRTEM observations;[27] it can beseen that the BN nanocages produced by this mechanismshould have a similar size to the corresponding catalyst par-ticles, which is the case for most of the catalyst particleswith sizes of around 50 nm. It is noticeable that some cata-lyst particles with relatively large sizes of about 80 nm ormore are also surrounded by some much smaller BN nano-cages, as shown in Figures 1b and 2c. For these nanocages,the growth mechanism should be principally the same as theaforementioned one for small catalyst particles, but maydiffer in some details. The suggested process is schematicallyshown in Figure 3b. When the thickness of the inward-grow-ing BN reaches a certain degree, due to the relative largesize of the inner Fe–B particle, its expulsion would crackthe shell into several pieces, which would then form the cor-responding nanocages with relatively small sizes due to thedewetting of BN from Fe–B (steps 2 and 3 in Figure 3b).

This procedure then repeats itself to form a group of BNnanocages surrounding the Fe–B catalyst (step 4 in Fig-ure 3b). The growth mechanisms of different BN morpholo-gies, including nanowires, nanotubes, periodic spindle-unitnanotubes, and nanocages, prepared by nitriding Fe–Bunder different conditions, as well as the relationship be-tween them, are discussed in detail in the Supporting Infor-mation.The above results indicate that BN nanocages and some

nanocapsules can successfully be prepared by nitriding Fe–Bnanoparticles. In order to obtain pure nanocages, we puri-fied the as-prepared product by treatment with dilute nitricacid to dissolve the Fe–B catalyst. The TEM image and X-ray diffraction (XRD) pattern of the purified product areshown in Figure 4. It can be seen that the Fe–B catalyst hasbeen effectively removed, and rather pure BN nanocagesare obtained, as confirmed by the typical XRD peaks corre-sponding to h-BN. The dark gray, as-prepared product be-comes light gray after purification.In light of the superior tribological properties of WS2

and MoS2 nanocages,[10–12] we were naturally curious about

the tribological properties of these BN nanocages as theyhave an analogous layered structure. Figure 5a shows a

Figure 2. HRTEM images of the typical structures in the products. a) aBN nanocage; b) a Fe–B-encapsulated BN nanoparticle; c) a BNnanocage clinging to the surface of Fe–B particle; d) the energy dis-persive X-ray (EDX) spectrum corresponding to the BN nanocage in(a); e) the EDX spectrum corresponding to the Fe–B-encapsulated BNnanoparticle in (b).

Figure 3. Schematics of the strain-induced growth process of BNnanocages on the Fe–B catalyst. a) For small Fe–B catalyst particleswith sizes of around 50 nm: The Fe–B nanoparticle (1); the Fe–Bnanoparticle is encapsulated by the h-BN shell (2); the Fe–B core isexpelled by a strain-induced ejection (3); a BN nanocage is formedand the Fe–B nanoparticle repeats the cycle (4,5). b) For Fe–B nano-particles with relatively large sizes of about 80 nm or more: A largeFe–B nanoparticle (1); the Fe–B nanoparticle is encapsulated by theh-BN shell (2); the increasing strain cracks the BN shell, and the BNpieces tend to form nanocages (3); more BN nanocages are formedby repeating this procedure (4).

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schematic illustration of the experimental setup that wasused in the present work to evaluate the tribological proper-ties of the BN nanocages. The steel ball is linked to a two-dimensional mechanical controller and sensor driven by acomputer program. Both loading and friction data could becollected automatically. The friction coefficients (m) of asteel ball on a pure silicon wafer and on a silicon wafer withBN nanocages deposited on it under different loads areshown in Figure 5b. The results indicate clearly that the BNnanocages reduce the friction by nearly 50%. This suggeststhat these BN nanocages might be promising candidates fortribological applications. Owing to the high melting point,corrosion resistance, and oxidation resistance of boron ni-tride, BN nanocages may serve as lubricants under extremeconditions.In summary, BN nanocages less than 80 nm in size have

been successfully synthesized in quantity simply by nitridingFe–B nanoparticles with an NH3/N2 mixture at only 750 8C.The purified BN nanocages show rather promising solid-state lubricating behavior. The synthesis mechanism is alsoproposed. The low-temperature synthesis and superior prop-erties of these BN nanocages are both scientifically interest-ing and technologically important.

Experimental Section

The synthesis procedure is similar to our previous one forsynthesizing BN nanotubes or nanowires,[25,26] but is carried outat a much lower temperature. In brief, high-boron-content Fe–Bnanoparticles were first prepared by a solid-state chemical reac-tion,[28] and were then employed as the “catalyst” in this synthe-sis. The Fe–B catalyst was placed in a corundum crucible in thecentral zone of a cylindrical corundum reactor. After the chamberwas evacuated and flushed with Ar several times to removeoxygen and moisture, the reactor was heated to 750 8C underargon. A mixture of NH3/N2 (4 mol% NH3) with a flow rate of50 sccm was then passed through the chamber for 2 h. The reac-tor was then cooled down under Ar and a dark gray, sponge-likeproduct was obtained. The product was purified by treatmentwith dilute nitric acid at 60 8C with vigorous stirring for 48 h todissolve the metal impurity, followed by centrifugal separation.TEM (JEOL-JEM-1005), HRTEM (JEM-2010), EDX (Noran VantageDS), as well as XRD (Philips X’pert Pro X-ray diffractometer) wereemployed to characterize the product. Comparative tribologicalexperiments were tested on Si(111) wafers. A 10-mg sample ofthe BN nanocage was dispersed in 50 mL of ethanol by ultrason-ic treatment, and 2.5 mL of this microemulsion was dried drop-wise on the Si wafer, about 1 cm2 in area, under infrared irradia-tion. Thus, about 0.5 mg of the BN nanocages was deposited onthe Si surface, forming a BN nanocage layer of several microme-ters. The friction coefficients of a steel ball on a pure Si waferand on a Si wafer with BN deposited on it under different loadswere collected for comparison.

Supporting information: The growth mechanisms of differentBN nanostructures by nitriding Fe–B “catalyst” particles underdifferent conditions, the relationship between them, and thechemical reactions involved are discussed.

Figure 4. TEM image of a) the purified BN product and b) the corre-sponding XRD pattern.

Figure 5. a) An illustration of the experimental setup for the frictiontest. The vertical load (N) and the horizontal velocity (v) could beadjusted by computational methods. b) The friction coefficients (m)of a steel ball on a pure silicon wafer and on a silicon wafer with BNnanocages deposited on it under different loads.

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Keywords:boron · lubricants · nanostructures · nitrides ·thermochemical synthesis

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Received: July 3, 2005Published online on October 11, 2005

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