Sonochemical hydrolysis of Ga3+ ions: Synthesis of scroll-like cylindrical nanoparticles of gallium oxide hydroxide

Article abstract of DOI:10.1021/ja9835584

The sonochemical reaction of an aqueous solution of GaCl₃ led to the formation of gallium oxide hydroxide rolled up in a scroll-like layered structure to give cylinders 80-120 nm in diameter and 500-600 nm in length. Small amounts of metallic Ga were incorporated with these tubes. A mechanism for this reaction has been suggested where the reaction takes place in a shell surrounding the collapsing bubble.

Full text of DOI:10.1021/ja9835584

4196
J. Am. Chem. Soc. 1999, 121, 4196-4199
Sonochemical Hydrolysis of Ga3+ Ions: Synthesis of Scroll-like
Cylindrical Nanoparticles of Gallium Oxide Hydroxide
S. Avivi,† Y. Mastai,‡ G. Hodes,*,‡ and A. Gedanken*,†
Contribution from the Department of Chemistry, Bar-Ilan UniVersity, Ramat-Gan 52900, Israel, and
Department of Materials and Interfaces, Weizmann Institute of Science, RehoVot 76100, Israel
ReceiVed October 8, 1998
Abstract: The sonochemical reaction of an aqueous solution of GaCl3 led to the formation of gallium oxide
hydroxide rolled up in a scroll-like layered structure to give cylinders 80-120 nm in diameter and 500-600
nm in length. Small amounts of metallic Ga were incorporated with these tubes. A mechanism for this reaction
has been suggested where the reaction takes place in a shell surrounding the collapsing bubble.
Introduction
amorphous cobalt from Co(CO)3NO16 and its solution in Decalin
or decane. Our group has synthesized nanophase amorphous
nickel by sonication of a Decalin solution of Ni(CO)4.17 Other
metallic nanoparticles that we have prepared using ultrasound
radiation include copper18 and palladium.19 The ultrasound-
initiated formation of metal particles in aqueous solution is
reviewed by Grieser.20 He reports in particular on the preparation
of metallic colloids such as Ag, Au, Tl, Pt, and Pd.
Little effort appears to have been devoted to sonochemical
formation of metallic gallium or to its oxide hydroxide in general
and to nanoparticles of these compounds in particular. There
have been, however, a few attempts to document the preparation
of gallium nanoparticles. They include homogeneous reduction
with alkalides and electrides,5 the use of inert gas condensation,21
and evaporation-condensation in ultrahigh vacuum.22 The major
product in our sonochemical reaction is gallium oxide hydroxide,
also known as gallium oxy hydrate or monohydrate. It has been
prepared by heating either R-Ga2O3 or δ-Ga2O3 in a wet
atmosphere.23 It can also be prepared by heating gallium metal
with water in an autoclave at 200 °C or by dehydration of
gallium trihydroxide at 100 °C.24 Gallium oxide hydroxide has
an orthoromobic crystal structure (Pbnm).
The synthesis of metallic nanoparticles has been extensively
studied, and various methods have been employed for this
purpose. Rieke and co-workers have reduced salts of active
metals in ethers or hydrocarbon solvents either heterogeneously
with alkali metals or homogeneously with radical anions of
aromatic compounds.1-3 Reductions using alkalides and elec-
trides in aprotic solvents,4,5 reductions with alkali metal orga-
noborohydride solutions, such as NaB(Et)3H,6 and reductions
with BH4- for Co, Ni, Au, Ag, and Pt7 have all yielded metallic
nanoparticles. Other methods include pyrolysis of various
precursors,8 evaporation9 and sputtering10 of metals, and sol-
gel processes.11 Among the more recent methods are controlled
chemical12 and electrochemical13 reductions.
Sonochemical reactions arise from acoustic cavitation phe-
nomena: the formation, growth, and collapse of bubbles in a
liquid medium.14 The extremely high temperatures (>5000 K),
pressures (>20 MPa), and cooling rates (>1010 K/s) attained
during acoustic cavitation lead to many unique properties in
the irradiated solution.15 Suslick and co-workers have prepared
nanophase amorphous F,15 from the sonication of both pure Fe-
(CO)515 and Fe(CO)5 solution in Decalin.15b They also prepared
In this study, we have sonicated an aqueous solution of GaCl3
and have obtained rolled-up tubular particles consisting of
gallium oxide hydroxide containing a small amount of metallic
Ga. The formation of the rolled-cigar-shaped layers will be
† Bar-Ilan University.
‡ Weizmann Institute of Science.
(1) Rieke, R. D. Science 1989, 24116, 1260.
(2) Rieke, R. D.; Hudnall, P. M. J. Am. Chem. Soc. 1972, 94, 7178.
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1973, 269.
(4) Tsai, K.-L.; Dye, J. L. J. Am. Chem. Soc. 1991, 113, 1650.
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Engl. 1990, 29, 273.
(15) (a) Suslick, K. S.; Choe, S. B.; Cichowlas, A. A.; Grinstaff, M. W.
Nature 1991, 353, 414. (b) Grinstaff, M. W.; Cichowlas, A. A.; Choe, S.
B.; Suslick, K. S. Ultrasonics 1992, 30, 168.
(16) Suslick, K. S.; Fang, M.; Hyeon, T.; Cichowlas, A. A. In Molecularly
Designed Nanostructured Materials; Gonsalves, K. E., Chow, G. M., Xiao,
T. O., Cammarata, R. C., Eds.; MRS Symposium Proceedings 351; Materials
Research Society: Pittsburgh, PA, 1994; pp 443-448.
(17) Koltypin, Yu.; Katabi, G.; Prozorov, R.; Gedanken, A. J. Non. Cryst.
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(19) Dhas, N, A.; Gedanken, A. J. Mater. Chem. 1998, 8, 445.
(20) Grieser, F. In Semiconductor Nanoclusters; Kamat, P. V., Meisel,
D. Eds.; Elsevier Science: Amsterdam, The Netherlands, 1996; p 67.
(21) Laur, St.; Ehrhardt, H.; Zimmer, H. G.; Wolf, H.; Wichert, Th.
Mater. Sci. Forum 1996, 225, 197.
(22) Nisoli, M.; Stagira, S.; De Silvestri, S.; Stella, A.; Tognini, P.
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(23) Harms, G. J.; Gunsser, W. Ber. Bunsen-Ges. Phys. Chem. 1986,
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10.1021/ja9835584 CCC: $18.00 © 1999 American Chemical Society
Published on Web 04/20/1999

Synthesis of Gallium Oxide Hydroxide Nanoparticles
J. Am. Chem. Soc., Vol. 121, No. 17, 1999 4197
Figure 2. XRD patterns for the as-prepared material. The asterisks
point to the diffractions of the metallic gallium.
Figure 1. DSC spectrum of the sonication product. The heating rate
was 0.5 °C/min.
material density of 5 g/cm3, the minimum surface area is
calculated to be 12 m2/g. This leads to the conclusion that the
layered structure enables molecules such as N2 to penetrate into
the structure. The actual density, measured from the weight and
volume of a sample of the powder, was 0.5 g/cm3, an order of
magnitude smaller than the literature value. Even allowing for
increased volume due to packing geometry, it is clear that the
tubes are substantially hollow.
In previous experiments, we demonstrated that we could
control the particle size by varying the concentration of the
precursor in the solvent, decane or Decalin. When the concen-
tration was lowered, the particle size of the product became
smaller.25,26 We have repeated this method for GaCl3 and
sonicated a solution which is 10 times more dilute, namely 0.1
g of GaCl3 in 50 mL of water (0.011 M). Under these conditions,
the nanoparticle diameter was reduced to 80 nm (from 120 nm)
and the average length slightly reduced to 520 nm (from 560
nm).
explained as originating from the sonochemical reaction occur-
ring on the circumference of the collapsing bubble.
Experimental Section
A solution of 1 g of GaCl3 in 50 mL of distilled water (0.114 M)
was exposed to high-intensity (100 W/cm2) 20-kHz ultrasound radiation
for 6 h at room temperature by direct immersion of a titanium horn
(Vibracell, horn diameter 1.13 cm) under argon at a pressure of 1.5
atm. The titanium horn was inserted to a depth of 1 cm in the solution.
The resulting solid product was washed 4 times in distilled water and
dried under vacuum.
Powder X-ray diffraction (XRD) was measured using a Rigaku
diffractometer, model 2028. Differential scanning calorimetry (DSC)
of the dried product was carried out using a Mettler DSC-30 instrument
with a heating rate of 0.5 °C/min. The morphology of the product was
determined by transmission electron microscopy (JEOL-TEM 100SX).
The surface area was measured by the BET nitrogen gas adsorption
method using a Micromeritics Gemini instrument.
As a control experiment, we heated a 1 M aqueous solution
of GaCl3 to 300 °C in a high-pressure cell, without sonication.
No noticeable powder formation occurred after the solution was
heated for 4 h.
Results
The DSC spectrum, shown in Figure 1, displays an endot-
hermic peak at 29.2 °C. This is attributed to metallic gallium.
Although the melting point of gallium is somewhat higher (29.75
°C), a depression of the melting point of this magnitude could
occur if Ga is in the form of very small particles. The measured
∆H of fusion is 2.1 cal/g. Since the literature value for gallium
is 191 cal/g, this indicates that only a small proportion (ca. 1%)
of the material is metallic Ga. The amount of metallic gallium
is dependent on the irradiation time: irradiating the starting
solution for 90 min resulted in a metallic Ga DSC signal which
was an order of magnitude smaller than that in Figure 1, which
was obtained after 6 h of sonication.
All the peaks in the XRD pattern, shown in Figure 2, can be
assigned to R-GaO(OH) (JCPDS card no. 6180). The GaO(OH)
has an orthorhombic crystal structure (Pbmn) which is similar
to that of diaspore, R-aluminum oxide hydroxide.23
A TEM image of the powder is given in Figure 3. The
particles are cylindrical, with a length of about 560 nm and a
diameter of 120 nm. To determine whether the cylinder is solid
or hollow, higher resolution TEM microscopy was carried out.
In Figure 4 we present an HRTEM image of a particle whose
center is defoliated from the layered structure. At least three
exposed edges of three parallel inner layers are seen. The
electron diffraction pattern of the sonication product is shown
in the inset to Figure 4 and shows that the as-prepared material
is crystalline.
Discussion
Unlike the sonication of carbonyls, which resulted in nanophase
amorphous products,15-17 the sonication of GaCl3 yields crystal-
line nanophase products. The difference between the two
reactions can be understood on the basis of the existence of
two regions of sonochemical reactivity, as postulated by Suslick
et al.: the inside of the collapsing bubble and the interface
between the bubble and the liquid.27 In the sonication of the
transition-metal carbonyls, the reaction takes place inside the
collapsing bubble, and the amorphous product is a result of the
high cooling rates (>1010 K/s) obtained during this collapse.
In addition to the reaction in the gas phase that takes place within
the collapsing bubble, a reaction occurs in the thin liquid layer
immediately surrounding the collapsing cavity.27 The amount
of GaCl3 or any hydrolyzed Ga species will be very low inside
the collapsing cavity because of their relatively low vapor
pressure, and little product is expected to occur inside the
bubbles. Similar ideas have been formulated by Henglein and
co-workers28 after studying the sonolysis of a nonvolatile solute,
(25) . Cao, X.; Koltypin, Yu.; Kataby, G.; Prozorov, R.; Gedanken, A.
J. Mater. Res. 1995, 10, 2952.
(26) Cao, X.; Koltypin, Yu.; Prozorov, R.; Kataby, G.; Gedanken, A. J.
Mater. Chem. 1997, 7, 2447.
(27) Suslick, K. S.; Hammerton, D. A.; Cline, R. E. J. Am. Chem. Soc.
1986, 108, 5641.
(28) Gutierrez, M.; Henglein, A.; Fischer, Ch.-H. Int. J. Radiat. Biol.
1986, 50, 313.
The surface area of these cylindrical particles was measured
by the BET method and was found to be 61 m2/g. Assuming a

4198 J. Am. Chem. Soc., Vol. 121, No. 17, 1999
AViVi et al.
Figure 3. TEM image of the as-prepared material. (a, left) Magnification 13 000×, scale 1.04 cm ) 800 nm. (b, right) Magnification 36 000×,
scale 1.1 cm ) 300 nm.
bubble interface. The formation of GaO(OH) can occur by
reaction 1a or 1b; both Ga species are present in solution.
Ga3+(aq) + 2H2O(l) T GaO(OH)(s) + 3H+
(1a)
(aq)
Ga(OH)4 (aq) f GaO(OH)(s) + OH-(aq) + H2O(l) (1b)
-
The observation that the pH of the solution changes from
2.37 prior to the sonication to 1.61 at the end of the reaction
implies that reaction 1a is the dominant pathway. This reaction
is endothermic,29 with an equilibrium constant at 50 °C of ca.
10-3. The high local temperature in the shell surrounding the
collapsing bubble constitutes the driving force for the formation
of GaO(OH).
We cannot rule out the possibility that GaO(OH) is a product
of reaction between H2O2, a known product of the sonication
of water,20 and zero-valent Ga.
Ga(s) + H2O2(aq) f GaO(OH)(s) + H
(2)
We now consider the mechanism by which the cigar-like
layers are formed and rolled-up. While R-GaO(OH) is not the
usual van der Waals layered compound encountered in nanotube
formation, it does form a type of layered structure. Figure 5
shows a projection of the (001) plane of the diaspore structure,
rotated somewhat to show three layers of this plane. The Ga
atoms are situated at the centers of the tetrahedra with O atoms
(in black) at the vertexes. (The H atoms are not shown for the
sake of clarity.) From this figure, the most likely direction for
bending to occur is for the top (100) plane (the plane with one
side common to the top of the (001) plane in Figure 5 and lying
perpendicular to the plane of the paper) to bend. Since the radius
Figure 4. TEM image of the as-prepared material at a higher resolution
than in Figure 3. Magnification 100 000×, scale 1 cm ) 100 nm.
Inset: Electron diffraction of the as-prepared material.
such as acetate anion. According to our proposed mechanism,
the formation of the oxide hydroxide occurs at the solution-
(29) Craig, H. R.; Tyree, S. Y. Inorg. Chem. 1969, 8, 591.

Synthesis of Gallium Oxide Hydroxide Nanoparticles
J. Am. Chem. Soc., Vol. 121, No. 17, 1999 4199
case, a spherical structure might be expected. However,
consideration of the crystal structure in Figure 5 leads to the
conclusion that a spherical structure is not likely to form, in
contrast to the observed tubular structure, which could do so as
explained above. For either mechanism for more dilute solutions,
there will be less GaCl3 at the bubble-liquid interface, thereby
explaining the smaller particles formed. The sonohydrolyses of
InCl3, TlCl3, and AlCl3 haVe all led to cylindrical-shaped
products similar to that obserVed for GaOOH. This sonochemi-
cal technique, therefore, appears to be generally applicable for
obtaining such cylindrical structures.
Conclusions
The sonication of an aqueous solution of GaCl3 resulted in
the formation of GaO(OH) with a small amount (about 1%) of
metallic gallium enclosed inside GaO(OH). In the absence of
the ultrasonic radiation, no reaction occurred. The formation
of GaO(OH) has been attributed to the high local temperature
in the neighborhood of the collapsing bubble. The products were
formed as cigar-shaped particles having a diameter of about
100 nm. An explanation for the elongated layered structure of
GaO(OH) based on cavitation collapse occurring at the solid-
liquid boundary has been suggested. The particle size was found
to decrease with decreasing concentration of GaCl3 due to a
reduced amount of reactant at the bubble-liquid interface.
Figure 5. Projection of the (001) plane of the diaspore structure. The
Ga is in the center of the tetrahedra, and the O atoms are at the vertexes.
of curvature of the (external) surface is very large on the scale
of a bond length, the strain inherent in this bending need not
necessarily be very large. The [001] axis will then form the
axial direction of the tubes seen in the TEM micrographs, and
the main surface of the tubes will be formed by the (100) plane.
The formation of GaO(OH) is related to the collapse of the
bubble, GaO(OH) and is formed as a layer surrounding the
collapsing bubble. Suslick and Hammerton have calculated that
the diameter of the bubble just before collapse is about 300
µm.30 The liquid zone surrounding the cavity is estimated to
extend to about 200 nm from the bubble. The temperature in
the liquid zone has been measured to be 1900 K.27 The GaO-
(OH) may then be formed around the collapsing bubble. In this
Acknowledgment. We thank Abraham Herman for editorial
assistance. This work was supported, in part, by the Israel
Ministry of Science, Contract Nos. 5839-2-96 and 8461-1-98.
This research was financially supported by a NEDO Interna-
tional Joint Research Grant.
(30) Suslick, K. S.; Hammerton, D. A. IEEE Trans. Sonics Ultrason.
1986, SU-33, 143.
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