Morphologic characteristics of bismuth and silver particles formed upon the reduction of metal stearates with benzyl alcohol

Article abstract of DOI:10.1134/S0036023607060174

The products of the reduction of individual and mixed bismuth oxohydroxostearate and silver stearate with benzyl alcohol are studied by electron microscopy and X-ray powder diffraction. The reduction of mixed bismuth and silver carboxylates with benzyl alcohol is a promising route to both individual submicronzised bismuth and silver metal powders and to their alloys. Nauka/Interperiodica 2007.

Full text of DOI:10.1134/S0036023607060174

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2007, Vol. 52, No. 6, pp. 922–926. © Pleiades Publishing, Inc., 2007.
Original Russian Text © B.B. Bokhonov, Yu.M. Yukhin, 2007, published in Zhurnal Neorganicheskoi Khimii, 2007, Vol. 52, No. 6, pp. 993–998.
PHYSICAL METHODS
OF INVESTIGATION
Morphologic Characteristics of Bismuth and Silver Particles
Formed upon the Reduction of Metal Stearates
with Benzyl Alcohol
B. B. Bokhonov and Yu. M. Yukhin
Institute of Solid State Chemistry and Mechanochemistry, Russian Academy of Sciences,
Novosibirsk, Russia
Received May 10, 2006
Abstract—The products of the reduction of individual and mixed bismuth oxohydroxostearate and silver stear-
ate with benzyl alcohol are studied by electron microscopy and X-ray powder diffraction. The reduction of
mixed bismuth and silver carboxylates with benzyl alcohol is a promising route to both individual submicron-
zised bismuth and silver metal powders and to their alloys.
DOI: 10.1134/S0036023607060174
Metallic bismuth has several unique properties at 200–250°ë) under an inert atmosphere or in the pres-
which make it attractive for practical applications. Bis- ence of a high-boiling organic solvent [7].
muth is used in nuclear engineering for manufacturing
Nanocrystalline silver also has interesting proper-
polonium, and its alloys are used as high-temperature
ties. Unlike for bismuth, the preparation of colloidal sil-
heat-transfer agents. Alloys of bismuth are used exten-
ver particles is well developed [8].
sively: tellurium alloy is used in refrigerators and ther-
mogenerators; antimony and tellurium alloys are used
The formation of intermetallic compounds or bis-
as thermoelectric cells in solar thermoelectric genera- muth-base alloys upon the reduction of mixtures of var-
tors; chromium, indium, europium or manganese ious metal salts is of the same interest as the manufac-
alloys, as magnetic materials; silver and cesium alloys, turing of metallic bismuth. The silver alloy of bismuth
as photocathodes in photomultipliers; silver and sulfur prepared by electrochemical deposition from alkaline
alloys, in semiconductor devices; and silver and tin solutions [9] is significant for application. Silver and bis-
alloys, as solders [1]. It is believed that bismuth, which muth metal powders are widely used in medicine [10, 11].
has semimetal properties, acquires semiconductor
The phase diagram of the bismuth–silver system is
properties in response to decreasing crystallite size.
a simple eutectic with limited solid solution regions
The magnetoresistance effect observed in single crys-
[
12]. The eutectic is formed at (265.5 ± 2)°ë and con-
tals and thin films also expands the applications of bis-
muth. For example, the range of electrons at room tem-
perature is 100 to 600 nm [2]. These facts have stimu-
lated researchers to search for new methods and to
modify the existing methods for manufacturing nanom-
eter- and submicron-sized bismuth.
tains 95.3 at. % bismuth. The solubility of silver in bis-
muth is infinitesimal; the solubility of bismuth in silver
is 2.615 at. % at 500°ë and 0.83 at. % at the eutectic
temperature.
From mineral acid solutions, bismuth precipitates in
the form of oxohydroxostearate Bi O (OH) St , where
6
4
4
6
Bismuth nanoparticles with sizes of 15 to 35 nm
were prepared by the reduction of microemulsions of
organobismuth compounds with dioctyl ether (a high-
boiling apolar solvent) [3]. Metallic bismuth was
obtained by the reduction of monoclinic bismuth oxide
with supercritical isopropanol [4]. Bismuth nano- and
microparticles were prepared by the reduction of bis-
muth salts with a polyatomic alcohol [5]. Nanocrystal-
St is the stearate anion [13]. Associated metals are effi-
ciently removed in this way. In turn, silver precipitates
in the form of stearate AgSt from these solutions.
EXAFS showed that silver stearate molecules form
dimers Ag(OOC H
) with longer Ag–O bonds,
2n – 1 2
n
forming four-membered rings [14].
Here, we report the results of our investigation of the
line bismuth can also be prepared by the dispersion of morphology of metal particles produced by the reduc-
metallic bismuth in paraffin at 280°ë [6]. We demon- tion of individual and mixed bismuth oxohydroxostear-
strated that powdered bismuth can be prepared via the ate (BiOSt) and silver stearate (AgSt) with benzyl alco-
reduction of its monocarboxylate to metal (by calcining hol solution.
9
22

MORPHOLOGIC CHARACTERISTICS OF BISMUTH AND SILVER PARTICLES
923
The choice of benzyl alcohol was dictated by the washed with ethanol and dried in air. The X-ray powder
following reasons. First, alcohols are widely used for diffraction analysis of the products was carried out on a
the reduction of metals from their salts. Second, the DRON-3 diffractometer using Cuä radiation with a
α
boiling temperature of benzyl alcohol (205°ë) is higher counter speed of 0.5 deg/min and I = 1000. Differential
than the temperatures at which bismuth oxohydrox- scanning calorimetry (DSC) curves were recorded on a
ostearate and silver stearate are stable. This also should DIFSI-550 instrument at a heating rate of 10 K/min.
favor the decomposition of the test compounds. In addi- Electron micrographs were recorded using a JEM-
tion, benzyl alcohol keeps newly formed metal parti- 2000FXII transmission electron microscope with an
cles from oxidation and agglomeration.
accelerating voltage of 200 kV. Bismuth and silver in
liquid and solid phases were determined using atomic
absorption on a Saturn 2M spectrophotometer. Before
the determination, the synthesis products were treated
EXPERIMENTAL
Synthesis of bismuth(III) oxohydroxostearate. To with nitric acid (1 : 1) for 30 min.
distilled water (1.2 L) heated to ~80°ë, stearic acid
(
61.0 g) and a solution of NaOH (8.0 g) in water (0.150 L)
RESULTS AND DISCUSSION
were added under stirring. To the resulting mixture, an
aqueous solution (0.137 L) of bismuth perchlorate con-
In the X-ray diffraction patterns of the solid prod-
taining 333 g/L bismuth was poured gradually under ucts produced upon exposure of silver stearate crystals
stirring. The synthesis was performed in Teflon vessels to bismuth perchlorate solution (Fig. 1), there are virtu-
equipped with stirrers. The mixture was stirred for ally no substantial changes in the positions of basal
3
0 min. After 30 min of settling, the precipitate was fil- reflections while the bismuth concentration in the prod-
tered off in vacuum, washed on a filter with distilled ucts is not higher than 10 at. %; extra basal reflections
water, and dried in air. The stock solutions of bismuth appear only when the bismuth ion concentration in the
perchlorate (1000 g/L bismuth) were prepared by dis- mixture reaches 20 at. %.
solving high purity grade bismuth oxide (os. ch. 13–3,
Silver stearate undergoes several phase transitions
Russia) in 7.0 M perchloric acid. Solutions with lower
upon heating [15]. At 122°ë, AgSt transforms to a
bismuth concentrations were prepared by diluting the
high-temperature phase; at 147°ë, the transition to the
stock solution with distilled water.
liquid-crystal state occurs. The DSC curves of the prod-
Synthesis of silver stearate. To distilled water (1.0 L) ucts obtained by the treatment of AgSt crystals by bis-
heated to ~80°ë, added were stearic acid (48.5 g), a muth perchlorate solution (Fig. 2) show that the phase-
solution of NaOH (7.1 g) in distilled water (120 mL), transition temperatures increase with increasing bis-
1
.3 M HNO (10 mL), and (in increments) a solution of muth concentration in the batch to 10%. Until 10 at. %,
3
silver nitrate (29.6 g) in water (120 mL). The mixture reflections from individual bismuth oxohydroxostear-
was stirred for 30 min. After 30 min of settling, the pre- ate do not appear in the X-ray diffraction patterns of
cipitate was filtered off, washed with water, and dried in AgSt crystals exposed to bismuth perchlorate solution
air. When required, the products were washed with eth- and thermal features intrinsic to BiOSt are absent from
anol and dried in air in order to free bismuth oxohy- the DTA curves. Therefore, we can suggest that mixed
droxostearate and silver stearate from unreacted stearic silver stearate and bismuth oxohydroxostearate are
acid.
formed at these stages.
Both the synthesized individualAgSt and BiOSt and
The morphologic investigation of silver stearate crys-
their mechanical mixtures in various proportions were tals exposed to bismuth perchlorate solution shows that
reduced with benzyl alcohol. In addition, AgSt + BiOSt AgSt crystals retain their habit even after the AgSt +
compositions were prepared by exposure of silver 10 mol % BiOSt composition is formed. In the AgSt +
stearate crystals to bismuth perchlorate solution.
20 mol % BiOSt composition, a bismuth oxohydrox-
ostearate phase is observed as individual particles both
on the lateral faces and at the basal surface of AgSt
crystals (Fig. 3a). This character of exchange reactions,
with the product phase mainly formed on the lateral
faces of the crystal, is due to the anisotropic layered
structure of metal carboxylates.
Bismuth nitrate solutions can also be used in the
synthesis of bismuth oxohydroxostearate and its mix-
tures with silver, but care should be taken to avoid the
contamination of the products with bismuth oxohy-
droxonitrate, which results from hydrolysis. The reduc-
tion of bismuth oxohydroxostearate, silver stearate, and
their mixtures was performed as follows: to benzyl
Our investigation shows that the particle morphol-
alcohol (50 mg), the chosen compound (5 g) was added, ogy resulting from the collective reduction of bismuth
and the mixture was heated on a sand bath at oxohydroxostearate and silver stearate with benzyl
1
50−205°ë for 2 h. The resulting metal powders were alcohol differs substantially from the particle morphol-
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 6 2007

9
24
BOKHONOV, YUKHIN
I
1
2
3
0
5
10
15
20
25
30
35
40
45
50
θ, deg
2
Fig. 1. X-ray powder diffraction patterns for (1) silver stearate and (2, 3) its mixtures with bismuth oxohydroxostearate. The bismuth
concentration (at. %) is (2) 10 and (3) 20.
ogy formed upon the reduction of the individual com-
pounds.
1
The reduction of silver stearate with benzyl alcohol
yields silver particles with sizes of 0.2 to 0.5 µm (Fig. 3b).
Individual bismuth oxohydroxostearate yields spheri-
cal bismuth metal particles with sizes of 0.1 to 0.5 µm
1
22°C 147°C
2
(Fig. 3c).
For the bismuth–silver alloy crystallizing upon the
1
23.6°C
150°C
3
4
reduction with benzyl alcohol of a mechanical mixture
of silver stearates and bismuth oxohydroxostearate or
for the compositions obtained by the exchange reac-
tion, the morphologic features differ significantly from
the individual metal particles that are formed upon the
reduction of metal carboxylates. In the AgSt + BiOSt
composition obtained by the exchange reaction in
which the bismuth oxohydroxostearate concentration is
as low as 0.25 mol %, the reduction dramatically
changes the morphology of the alloy: spherical aggre-
gates with sizes up to 1 µm consisting of fine (~0.1 µm)
silver and bismuth particles appear (Fig. 3d). Alloy par-
ticles with this morphology are formed upon the reduc-
tion of mixed bismuth oxohydroxostearate and silver
0
1
24°C
1
52°C
1
24.5°C
1
53.3°C
5
0
75
100
125
150
175
T, °C
Fig. 2. DSC traces for (1) silver stearate and (2–4) its mix-
tures with bismuth oxohydroxostearate. The bismuth con-
centration (at. %) is (2) 2.5, (3) 5, and (4) 10.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 6 2007

MORPHOLOGIC CHARACTERISTICS OF BISMUTH AND SILVER PARTICLES
925
AGST + BI
SM0185 200 kV
AGST
SM0143 200 kV
(
a)
(b)
X1000
100 µm ISSC
X30K
1 µm ISSC
BIST BI 2A
SM00 1 200 kV
AGST + BIST MIX
SM0193 200 kV
(
c)
(d)
X40K
100 nm 04060
X30K
1 µm ISSC
Fig. 3. Electron micrographs of (a) silver stearate single crystals containing 20 at. % bismuth, (b) silver metal particles, (c) bismuth
metal particles, and (c) a mixture of the last two produced by the reduction of stearates in benzyl alcohol.
stearate over a wide concentration series, and they do increasing temperature to 60°ë, the bismuth concentra-
not change until the bismuth oxohydroxostearate con- tion increases to 1.4 g/L.
centration reaches ~98 mol % in the AgSt + BiOSt mix-
It is notable that the amount of benzyl alcohol taken
ture. When mechanical mixtures of AgSt and BiOSt
to reduce the metal stearates is substantially smaller
powders are reduced with benzyl alcohol, alloy parti-
than required by stoichiometry: the ligand with a long
cles also crystallize in spherical aggregates of fine sil-
methylene chain also enhances the reduction of these
ver and bismuth particles. In our opinion, the morpho-
compounds. This is proven the fact that the thermolysis
logic similarity of the silver–bismuth alloy produced by
of silver stearate and bismuth oxohydroxostearate in
the reduction of chemically synthesized mixed bismuth
vacuum or air produces metallic silver and bismuth.
oxohydroxostearate and silver stearate and physical
mixtures of these stearates, proves the liquid-phase
X-ray diffraction unambiguously confirms that sil-
character of the reduction. When bismuth oxohydrox- ver and bismuth metal particles are the products of the
ostearate and silver stearate are treated with benzyl reduction of AgSt + BiOSt compositions (Fig. 4). The
alcohol at 20°ë, the bismuth and silver concentration in X-ray diffraction patterns of the reduction products
the alcohol is 0.82 and 0.05 g/L, respectively; with only show reflections from silver and bismuth metals;
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 6 2007

9
26
BOKHONOV, YUKHIN
I
1
2
3
0
2
0
30
40
50
60
θ, deg
2
Fig. 4. X-ray powder diffraction patterns for silver–bismuth alloy particles produced by the reduction of stearates in benzyl alcohol.
The bismuth concentration (at. %) is (1) 1.58, (2) 34.5, and (3) 94.0.
the intensity of these reflections is affected by the Bi :
Ag proportion in the metal stearate batch.
7. Y. Yukhin, B. Bokhonov, V. Evseenko, et al., Proceed-
ings of the 10th APAM Topical Seminar and the 3rd Con-
ference “Materials of Siberia; Nanoscience and Tech-
nology”, Novosibirsk, 2003 (Novosibirsk, 2003), p. 151
(2003).
8. Handbuch der präparativen anorganischen Chemie,
Ed. by G. Brauer (Ferdinand Enre, Stuttgart, 1975; Mir,
Moscow, 1985).
Our investigation allows us to conclude that the
reduction of mixtures of bismuth and silver carboxy-
lates is a promising route to submicron-sized bismuth–
silver alloys.
9
. I. Krastev, T. Valkova, and A. Zielonka, J. Appl. Electro-
chem. 34, 79 (2004).
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