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MATERIALS AND INTERFACES Preparation and Formation Mechanism of Alumina Hollow Nanospheres via High-Speed Jet Flame Combustion Yanjie Hu,† Chunzhong Li,*,† Feng Gu,† and Jan Ma‡ Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China UniVersity of Science & Technology, Shanghai 200237, China, and School of Materials Science and Engineering, NanYang Technological UniVersity, Blk N4.1, NanYang AVenue, Singapore 639798, Singapore
A simple and effective method for the preparation of hollow alumina nanospheres was established via a high-speed jet flame combustion process. The morphology and structure of the hollow nanospheres were investigated using TEM, HRTEM, SEM, FTIR, XRD, BET, etc. A vapors mixture of C2H5OH and AlCl3 was exhausted at high speed into a flame reactor at 150 m/s and condensed into mesoscale droplets due to the Joule-Thomson cooling effect and the entrainment of cool gases into the expanding high-speed jet. The hollow alumina nanospheres were formed after the hydrolysis of AlCl3 in the H2/air flame at about 1200 °C. The hollow alumina nanosphere was composed of nanocrystallites about 5 nm and the shell thickness was 10-30 nm. The formation mechanism of the hollow alumina nanospheres was investigated, conforming to the one-droplet-to-one-particle theory. Introduction Hollow spheres, owing to their tailored structural, optical, and surface properties, have many potential applications such as photonic crystals, delivery vehicle system, fillers, and catalysts.1,2 So far, various approaches such as templates and sonochemical, hydrothermal, and emulsion combustion methods have been developed for the preparation of hollow spheres of different materials including ceramics, semiconductors, and metals.3,4 Hollow spheres of metal oxide were recently obtained by the emulsion combustion method (ECM) using metal precursors, kerosene, and surfactant.5,6 It has been shown that the structure, size, and composition of the hollow spheres can be altered in a controllable way to tailor various properties over a broad range.6-8 Flame aerosol technology has been employed widely for large-scale manufacture of carbon blacks and ceramic commodities such as SiO2, TiO2, and Al2O3.9-11 Recently, core/ shell TiO2/SiO2 nanoparticles were also synthesized by this method, and it was found that the core/shell TiO2/SiO2 nanocomposites exhibited better optical properties in comparison with pure TiO2 nanoparticles.12 Generally, it is difficult to produce multicomponent materials with homogeneous chemical composition by using a vapor-fed flame reactor because the differences of the chemical reaction, nucleation, and growth rates would lead to nonuniform composition from particle to particle, or even within a single particle.13 Therefore, a flame-assisted liquid droplet-to-particle conversion process has been of interest to produce single- and multicomponent nanoparticles with the * To whom correspondence should be addressed. E-mail: czli@ ecust.edu.cn. Tel.: 86-21-64250949. Fax: 86-21-64250624. † East China University of Science & Technology. ‡ NanYang Technological University.
advantage of starting with precursor solution. The nanoparticles generated by the flame spray pyrolysis have shown high purity, controlled stoichiometry, and crystallinity because the flame could be maintained at temperatures high enough to complete thermal decomposition through intense oxidation.14,15 Transition aluminas are disordered crystalline phases formed through the thermal dehydration of aluminum hydroxides and oxyhydroxides.16,17 These oxides are used as adsorbents and catalysts or catalyst supports in many chemical processes, including the cracking, hydrocracking, and hydrodesulfurization of petroleum feedstocks. The properties of these materials for applications in catalysis and adsorption are determined in large part by textural parameters (i.e., surface area, pore volume, and pore size).18 A mesostructured alumina containing a transition alumina phase has been reported recently,19 but well-expressed alumina mesostructures with hollow interiors have yet not been demonstrated. In this paper, we report a new method for simple and effective synthesis alumina hollow nanospheres via a highspeed jet flame approach using AlCl3 as a precursor. Compared with the conventional wet chemical process, the most important characteristics of flame synthesis lie in the fact that the vapor mixture could condense into microdroplets so that reactions are limited to every droplet or particle. Controlling the reactions, mass transfer, heat transfer, and sintering within every droplet/ particle give many possibilities for the control of the structure, phase composition, and morphology of the prepared particles. Experimental Section Alumina hollow nanospheres were synthesized via the highspeed jet flame approach (shown in Figure 1). Ethanol solution of AlCl3 (A.R.) (150 g/L) was evaporated at 300 °C in a tubular furnace. The air (4 m3/h) preheated to 250 °C was used as carrier gas for the precursor vapor mixture. Vapor mixture of
10.1021/ie070451t CCC: $37.00 © 2007 American Chemical Society Published on Web 10/26/2007
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Figure 1. Schematic setup of the high-speed jet flame approach.
Figure 3. FTIR spectrum of the alumina hollow nanospheres.
Figure 2. XRD pattern of the alumina nanospheres. Figure 4. TG-DTA plot of the alumina nanospheres.
C2H5OH and AlCl3 was exhausted at high speed into a flame reactor at 150 m/s and condensed into mesoscale droplets. The flow rates of H2 in the second ring and air in the third ring were 0.8 and 2.0 m3/h, respectively. The hollow alumina nanospheres were formed after the hydrolysis of AlCl3 in the H2/air flame at about 1200 °C. The products were collected by a baghouse filter and HCl was absorbed by H2O in an absorber. Transmission electron micrograph (TEM) images were taken with a TEM-1200EXII transmission electron microscope. SEM images were obtained with a JSM-6360LV emission scanning electron microscope. The X-ray diffraction (XRD) patterns of the samples were measured by using a Rigaku D/max 2550 VB/ PC diffractometer with Cu KR radiation (λ ) 0.15418 nm). The Fourier transform infrared (FTIR) spectra of the samples were collected using a Nicolet Magna-550 infrared spectrometer. Thermogravimetric analysis (TG-DTA) was carried out on a TGA/SDTA/851e thermal analyzer. The BET surface areas and textures of the samples were by N2 adsorption isotherm (77 K) measurements, performed by the static volumetric method using an ASAP2010. Results and Discussion (1) Crystal Structures. The XRD pattern of the hollow alumina nanospheres is shown in Figure 2, indicating that the product has very poor crystalline nature. Major peaks are identical to those of γ-Al2O3 (JCPDS Card, File No. 1-1308). These peaks at scattering angles (2θ) of 38.5, 45.6, and 67.0° correspond to the reflections from the 222, 111, and 211
Figure 5. Nitrogen adsorption isotherm at 77 K for as-synthesized nanospheres.
crystal planes, respectively, of the γ-Al2O3 phase. The poor crystallinity of the samples originated from the short residence time inside the high-temperature flame. Besides the strong diffraction peaks, a broad diffraction peaking at 23° is also detected, indicating there are some amorphous materials in the product.20 Figure 3 shows the FTIR spectrum of the assynthesized hollow alumina nanospheres. The adsorption peak belonging to Al-O stretch appears at 611 cm-1. The peaks at 1374 and 1429 cm-1 belong to the flexural vibrations of
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Figure 6. (a, b) SEM and (c, d) TEM images of as-synthesized samples.
Figure 7. (a, c, d, f) HRTEM images of as-synthesized sample: (a) general view; (c) surface of the hollow sphere; (d) shell; (f) small hollow spheres aggregated on the outer shell. (b) EDS pattern. (e) ED pattern.
hydroxyl groups. The peaks around 1627 and 3421 cm-1 correspond to the stretch and flexural vibrations of water of hydration.21,22 (2) Thermal Properties. TG/DTA was applied to investigate the thermal properties of the samples. From the TG curve in Figure 4, it can be seen that the samples began to lose their weight at the beginning of the heating process, corresponding to the endothermal peak blow 100 °C of DTA curve. This is mainly because of the evaporation of the water absorbed on the surface of the spheres. A relatively remarkable mass loss between 100 and 350 °C indicated in the TG curve can be attributed to the removal of the hydroxyl groups of molecular water in Al(OH)3. The endothermal peak starting from 400 °C can be attributed to the loss of -OH.23 The nitrogen adsorptiondesorption isotherms plot for the as-synthesized samples are shown in Figure 5. The lower BET surface area of about 16 m2/g partly could be due to the thicker framework wall. In addition, the N2 adsorption isotherms further indicate that there exist many uniform pores. The average pore diameter is
calculated in the range of 2-4 nm according to the BJH method.24 (3) Morphology and Microstructures. Figure 6 shows the SEM and TEM images of the as-synthesized hollow alumina nanospheres. As shown in Figures 6a and 6b, the sample consists of well-spherical structures and the size of the hollow spheres is about 50-200 nm. It can also be found that some alumina particles aggregated on the surface of the spheres. The clear contrast observed in Figures 6c and 6d between the dark edge and the relatively bright center is evidence for their hollow nature. The hollow spheres have thin walls whose thickness appears to be relatively uniform (Figure 6c). And the shell thickness was in the range of 10-30 nm. HRTEM image of a typical hollow alumina nanosphere is shown in Figure 7. The EDS spectrum is given in Figure 7b, which indicates the sample is mainly composed of Al and O. The peaks of Cu and C come from the TEM mesh. From the HRTEM image shown in Figures 7c and 7d, it is evident that the shell is constructed of polycrystalline particles about 5 nm and the
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Figure 8. Illustration of the formation process of the alumina hollow nanospheres.
thickness is about 10 nm. These results agree well with the electron diffraction (ED) pattern obtained by focusing an electron beam on a single sphere (Figure 7e). Figure 7f shows the TEM image of one small hollow sphere, which attaches to the shell of a bigger sphere. The diameter of this small sphere is no more than 20 nm and the shell thickness is about only 5 nm. (4) Formation Mechanism. Figure 8 illustrates the formation process of the hollow nanospheres. When the vapors mixture of C2H5OH and AlCl3 was exhausted from the central tube of the burner into the flame reactor at very high speed, the JouleThomson cooling effect and the entrainment of cool gases into the expanding high-speed jet will be present, which would lead to the decrease of gas temperature. Once the temperature of the gas mixture is lower than the condensation temperature of C2H5OH and AlCl3, the vapor mixture of C2H5OH and AlCl3will condense into mesoscale droplets. And the diameter of droplets can be determined by the speed and temperature of the gas mixture.25-27 In the H2/air annular flame, every droplet acts as a microreactor during the reaction individually. On the droplet surface, the combustion of ethanol and the hydrolysis of AlCl3 can occur simultaneously, which would lead to the formation of Al(OH)3. Finally, each droplet changes to one hollow structure at high temperature. The formation mechanism of the hollow nanospheres conforms to the one-droplet-to-oneparticle (ODOP) theory.28,29 Higher concentration and faster reaction rate are conducive to the formation of hollow nanospheres.30,31 Coagulation and breakdown of the droplets and particles can be negligible if the mist concentration is not too high and the preparation condition is controlled carefully. Nanoparticles with very high compositional homogeneity and size uniformity can be achieved even if the particle composition is complex. Conclusions Alumina hollow nanospheres have been synthesized by a simple and effective method via a high-speed jet flame combustion process. The hollow nanosphere was composed of nanocrystallites about 5 nm and a shell thickness about 5-30 nm. The formation mechanism of the alumina hollow spheres conforms to the ODOP theory. The simple flame approach and cheap precursors have made the process appealing for the preparation of insulating and lightweight filler materials as well as catalyst carriers. Acknowledgment This work was supported by the National High Technology Research and Development Program of China (2006AA03Z358), the National Natural Science Foundation of China (20236020, 20706015), the Special Project for Key Laboratories in Shanghai (06DZ22008), and the Special Project for Nanotechnology of Shanghai (0752nm010£-0552nm001, 0652nm034).
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ReceiVed for reView March 28, 2007 ReVised manuscript receiVed September 9, 2007 Accepted September 11, 2007 IE070451T
