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
Sonochemistry is 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, the
sonochemical reaction is occurring in the liquid phase. The products are sometimes
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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 asprepared 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.
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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
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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.
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Figure 1. X-ray diffraction patterns for (a) amorphous Fe20Ni80, (b)
crystalline α-Fe, (c) crystalline γ-Ni, (d) crystalline Fe20Ni80, (e)
crystalline Fe40Ni60, (f) crystalline Fe60Ni40
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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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