USING ULTRASOUND RADIATION FOR THE PREPARATION OF AMORPHOUS NANOPHASE METALS, ALLOYS AND OXIDES

A Gedanken

Department of Chemistry, Bar-Ilan University, Ramat-Gan, Israel,

52900.

Introduction

In the last few years nanotechnology became a topic of great interest to scientists specializing in a large variety of fields. The interest is perhaps best expressed in President Clinton’s speech on Jan. 2000, on announcing the new national initiative on nanotechnology. Clinton said “….the ability to manipulate matter at the atomic and molecular level. Materials with ten times the strength of steel and only a small fraction of the weight---shrinking all the information housed in the Library of Congress into a device the size of a sugar cube---detecting cancerous tumors when they are only a few cells in size. Some of our research goals may take 20 or more years to achieve….”. This is a short explanation for the great interest in nanotechnology. Our role in this field is developing new methodologies for the fabrication of nanomaterials. My group has used three novel techniques for the fabrication of nanomaterials: sonochemistry, microwave heating, and sonoelectrochemistry. Of these three methods I decided to concentrate only on Sonochemistry. The method and the materials we have prepared will be described in my lecture and a short introduction is presented in this manuscript. Since sonochemistry was the most popular technique in my laboratory, and most of the nanomaterials were prepared by this technique, the method will be explained in great detail.

Physical and Chemical Principles of the Preparation Methods

Sonochemistryis the research area in which molecules undergo chemical reaction due to the application of powerful ultrasound radiation (20 KHz-10 MHz) [1]. The physical phenomenon responsible for the sonochemical process is acoustic cavitation. Various theories explain why a 20 KHz radiation can rupture chemical bonds. All theories agree that the main events in sonochemistry are the a) formation b) growth and c) implosive collapse of a cavity. Following one of the theories, the hot spot mechanism, very high temperatures (5000-25000 K) [2] are obtained upon the collapse of the bubble. Since this collapse occurs in less than a nanosecond [2, 3], very high cooling rates, in excess of 1011 K/sec, are obtained . This high cooling rate hinders the organization and crystallization of the products. For this reason, in all cases dealing with volatile precursors, where gas phase reactions are predominant, amorphous nanoparticles are obtained. While the explanation for the creation of amorphous products is well understood, the reason for the nanostructured products is not clear. One explanation is that the fast kinetics does not permit the growing of the nuclei. If on the other hand the precursor is a non-volatile compound, the reaction occurs in a 200 nm ring surrounding the collapsing bubble [4]. In this case, thesonochemical reaction is occurring in the liquid phase. The products are sometimes nanoamorphous particles and in the other cases nanocrystalline. This depends on the temperature in the ring region where the reaction takes place. The temperature in this ring is lower than inside the collapsing bubble, but higher than the temperature of the bulk. Suslick has estimated the temperature in the ring region as 1900 0 C [4]. The control over the particle size was demonstrated in three cases for reaction gas phase as well as liquid phase reactions [5-7].

Preparation of nanoamorphous metals, alloys and metal oxides

The sonication of Fe(CO)5 either as a neat liquid or in its solution in decalin or decane under argon yields nanosized amorphous iron particles. The amorphicity of the products is determined in our measurements in the following way. We conduct PXRD (no peaks), electron diffraction (diffused ring pattern), TEM (no crystalline edges), and DSC (exothermic peak yielding the crystallization temperature), and TGA (no weight loss at the crystallization temperature) measurements. Subsequent to the DSC results, we heat our product to the crystallization temperature and remeasure the PXRD. These are a battery of experiments that we carry our to confirm that our as-prepared material is amorphous. In our first step in the field [5], we have demonstrated that by changing the concentration of the Fe(CO)5 in the decalin solution we can control the particle size. A more dilute solution, yields smaller particles. This is not deduced directly from the TEM measurements since the nanoparticles are highly aggregated due to magnetic forces operating between them.

As an example for the preparation of an alloy we will present the synthesis of iron/nickel. Fe-Ni alloy was prepared by ultrasonic irradiation of the solution of Fe(CO)5 and Ni(CO)4 in decalin at 273 K , under 1 to 1.5 atm of argon, with a high intensity ultrasonic probe (Sonics & Materials , Model VC-600, 0.5 inch, Ti horn, 20Kz, 100W/cm2). After three hours of irradiation a black powder was obtained which was centrifuged , and washed with dry pentane inside the glove box. Fig. 1 shows [8] the XRD pattern for the amorphous Fe20Ni80 as well as the heated samples of -Fe, -Ni, and various compositions of Fe-Ni alloy, namely Fe20Ni80, Fe40Ni60 and Fe60Ni40. The X-ray diffractogram of the amorphous solid does not show any sharp diffraction patterns characteristic of crystalline phases except for a broad peak centered around a 2 value of 44.6o. After the heat treatment under argon (extra pure, <10 ppm O2) for 3 h (to induce crystallization), lines characteristic of -Ni (fcc phase) starts appearing . Lack of any feature at 2 values of 65.2 and 82.3o ( = 1.5418ֵ) which are the expected positions of bcc -Fe (200 and 300) confirm that the Fe and Ni atoms have become alloyed, rather than forming separate grains. One can see that even for the higher concentration of Fe in Fe-Ni alloy ( Fe60Ni40), the phase is still fcc of -Ni and not bcc of -Fe. Varying the initial precursor concentration in solution can control the composition of these alloy particles. We have noticed that the sonochemical efficiency for the decomposition is less for Fe(CO)5 than for Ni(CO)4 and so is necessary to take an initial excess of Fe(CO)5 to give a final required alloy composition. The poor reactivity of Fe(CO)5 over Ni(CO)4 can be traced to its very low vapor pressure when compared to Ni(CO)4. For example, at 293 K, the vapor pressure of Ni(CO)4 is 332 Torr where as it is only 21 Torr for Fe(CO)5 at the same temperature.

The preparation of pure amorphous Fe2O3 [9] is similar to that of amorphous Fe. In short, neat Fe(CO)5 (Aldrich) or its solution in decalin (Fluka) whose concentrations were 4M, 1M, 0.25M were irradiated with a high intensity ultrasonic horn (Ti-horn, 20 KHz) under 1.5 atm of air at 0oC for 3hrs.

Amorphous Mn2O3 and Cr2O3 were prepared [10] by the sonication of KMnO4, and (NH4)2Cr2O7 respectively, in aqueous medium at room temperature. The corresponding particle sizes were 50 and 200 nm.

The most interesting transition-metal oxide prepared in our group [11] was the blue colored Mo2O5·2H2O. It was obtained by sonicating a slurry of Mo(CO)6 in decalin at room temperature under ambient air.

Coating Nanosized particles on ceramic surfaces

We have published about 10 papers on the use of ultrasound radiation for the deposition of various amorphous and crystalline nanoparticles on the surface of submicron spheres of silica, alumina, and zirconia. This was done by synthesizing the ceramic spheres by conventional techniques (like the Stobber method for silica particles). The spheres were then introduced in the sonication bath, mixed with the solution of the precursor and the ultrasonic radiation was passed through the solution for a predetermined time. In this way we were able to deposit on the surface of the ceramic sphere nanoparticles of metals (Ni and Co for example), metal oxides (Fe2O3, Mo2O5), rare earth oxides (Eu2O3, Tb2O3), semiconductors(CdS, ZnS) and Mo2C. A picture presenting a bare silica sphere and a coated silica sphere is shown in Figure 3. The coating of the silica spheres is by nanocrystalline silver [12]. Ultrasound irradiation of a slurry of silica submicrospheres, silver nitrate, and ammonia in an aqueous medium for 90 minutes under an atmosphere of argon to hydrogen (95:5), yielded a silver-silica nanocomposite. By controlling the atmospheric and reaction conditions, we could achieve the deposition of metallic silver on the surface of the silica spheres. The role that the ultrasound radiation plays in the deposition procedure will be discussed.

Sonochemical Synthesis of Mesoporous Materials

In the last two years we are concentrating our sonochemical efforts in the synthesis of mesoporous (MSP) materials. We have developed a sonochemical method to prepare MSP silica [13], MSP titania [14], MSP YSZ (Yittria stabilized zirconia) and other MSP materials. The sonochemical products were shown to have thicker walls than those synthesized by the conventional methods. We have shown that the products are more hydrothermally stable than the sol-gel products. Our MSP titania has the highest surface area reported.

In addition we have used sonochemistry not only to synthesize the MSP materials but also for the insertion of amorphous nanoparticles into the mesopores. We have deposited Mo2O5 into the mesopores of MCM-41 (MSP silica)[15], and amorphous Fe2O3 [16] in the mesopores of MSP titania. Five physical methods were used to prove that the amorphous nanoparticles are indeed anchored on the inner walls of the channels. The amount of Mo2O5 that was inserted in the mesopores is 45% by weight. An attempt to increase this amount showed the excess is deposited outside the mesopores.

Controlling the Shape of the Nanomaterials

Producing nanorods, nanotubes, nanorings at wish have become a major issue in Materials Science. Although sonochemistry does not provide any new insight into the fabrication of special structure, it is worth mentioning that many of the important structures have been prepared also sonochemically. For example we have reported on the synthesis of titania nanotubes [17], inorganic fullerenes (see Figure 2) [18], nanorods [19], and other unique structures. If sonochemistry is compared to the other methods by which these structures have been prepared, it is clear that the advantage is in the faster reaction time and the lower temperature that the technique requires.

Applications

The amorphous nanomaterials are used mostly for catalysis, since it was already reported that amorphous nanoparticles are superior catalysts to the corresponding nanocrystalline materials[20]. We have used our amorphous nanoparticles as catalysts for the oxidation of hydrocarbons [16, 21]. The oxidation of cyclohexane is conducted under oxygen with the addition of isobutyraldehyde, and small amounts of acetic acid.

Recently, we have used MSP Fe2O3 as a catalyst for this reaction and obtained 36% conversion to cyclohexanone and cyclohexanol with a very high selectivity. This conversion is achieved at a pressure of 1 atmosphere oxygen and 700 C.

Another application of the nanomaterials is their use as electrode building materials in rechargeable Li batteries [22].

Finally, with Indian collaborators from the CGCRI of Calcutta, we are applying for a patent on the fabrication of rare-earth doped optical fibers. The first stage in the fabrication of these fibers are involved with the sonochemical deposition of nanosized rare-earth oxide on the surface of submicron silica spheres.

References

[1]. T. S. Mason Sonochemistry(Royal Society of Chemistry, 1990)

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b) B.P. Barber, S. J. Putterman, Nature, 352, 414 (1991).

[3]. K. S. Suslick, S-B. Choe, A. A.Cichowlas, M. W. Grinstaff, Nature, 353, 414 (1991).

[4]. K. S. Suslick, R. E. Cline, Hammerton, J. Amer. Chem. Soc. 108, 5641 (1986).

[5]. X. Cao, Yu. Koltypin, G. Kataby, A. Gedanken, J. Mater. Res. 10, 2952 (1995).

[6]. X. Cao, Yu. Koltypin, R. Prozorov, G. Kataby, A. Gedanken, J. Mater. Chem. 7, 2447 (1997).

[7]. S. Avivi, M. Mastai, G. Hodes, A. Gedanken, J. Amer. Chem. Soc. 121, 4196 (1999).

[8]. K.V.P.M. Shafi, A. Gedanken, R.B. Goldfarb and I. Felner. J. of Appl. Phys. 81, 6901 (1997).

[9]. X. Cao, R. Prozorov, Yu. Koltypin,G. Kataby and A. Gedanken. J. Mater. Research12, 402 (1997).

[10]. N. A. Dhas and A. Gedanken. Chem. Mater. 9, 3159 (1997).

[11]. N.A. Dhas and A. Gedanken. J. Phys. Chem. 101, 9495 (1997).

[12]. Vilas G. Pol, D. N. Srivastava, O. Palchik, A. Gedanken, Langmuir 18, 3352 (2002).

[13].X. Tang, S. Liu, Y. Wang, W. Huang Yu. Koltypin. A. Gedanken, Chem. Comm. 2119 (2000).

[14].Y. Wang, X. Tang, L. Yin, W. Huang, A. Gedanken Advanced Mater. 12, 1137 (2000).

[15]. M. V. Landau, L. Vradman and M. Herskowitz, Y. Koltypin and A. Gedanken, J. of Catalysis201, 22 (2001).

[16].Nina Perkas, Yanqin Wang, Yuri Koltypin, Aharon Gedanken, S. Chandrasekaran Chem. Comm. 988 (2001).

[17]. Yingchun Zhu, Hongliang Li, Yuri Koltypin, Yaron Rosenfeld Hacohen, Aharon Gedanken Chem. Comm. 2616 (2001).

[18]. S. Avivi, Y. Mastai, and A. Gedanken. J. Amer. Chem. Soc. 122, 4331(2000).

[19]. R. Vijayakumar, Yu. Koltypin, X. N. Xu, Y. Yeshurun, A. Gedanken, I. Felner J. Appl. Phys. 89, 6324 (2001).

[20]. K. S. Suslick, T. W. Hyeon, M. M. Fang, Chem. Mater. 8, 2172-2179 (1996).

[21]. V. Kesavan, Sivanand, P. S. S. Chandrasekaran, Yu. Koltypin, A. Gedanken Angew. Chem. Int. Ed. 38, 3521 (1999).

[22]. J. Zhu, Z. Lu, T. S. Aruna, D. Aurbach, A. Gedanken, Chem. Mater. 12, 2557(2000).

Figure 1. X-ray diffraction patterns for (a) amorphous Fe20Ni80, (b) crystalline α-Fe, (c) crystalline γ-Ni, (d) crystallineFe20Ni80, (e) crystalline Fe40Ni60, (f) crystalline Fe60Ni40

Figure 2. TEM image of the as-prepared powder from the sonication of an aqueous solution of TlCl3. Scale: 1cm=30nm

Figure 3. TEM image of silver nanoparticles deposited on silica spheres

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