ISSN 00360244, Russian Journal of Physical Chemistry A, 2010, Vol. 84, No. 4, pp. 642–647. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © A.F. Dresvyannikov, E.V. Petrova, M.A. Tsyganova, 2010, published in Zhurnal Fizicheskoi Khimii, 2010, Vol. 84, No. 4, pp. 727–732.
PHYSICAL CHEMISTRY OF NANOCLUSTERS
AND NANOMATERIALS
Physical and Chemical Properties
of NanoSized Aluminum Hydroxide and Oxide Particles Obtained
by the Electrochemical Method
A. F. Dresvyannikov, E. V. Petrova, and M. A. Tsyganova
Kazan State Technological University, Kazan, 420111 Russia
email: katrinvv@mail.ru
Received April 28, 2009
Abstract—The structure and properties of nanoparticles of aluminum hydroxides and oxides obtained by
electrochemical, chemical, and combined methods were studied by transmission electron microscopy, Xray
diffraction, thermal analysis, and atomic emission spectroscopy. The influence of synthesis conditions on the
structure and morphology of nanoparticles was studied. It was shown that the effect of an electrochemical
field allows monophasic systems to be obtained with a narrower range of particle sizes than in the case of
chemical deposition.
DOI: 10.1134/S0036024410040217
INTRODUCTION
Nanosized particles of metal oxides are capable of
exhibiting unusual properties due to the features of
individual particles and the conglomerates formed by
these particles, as well as to the nature of the interac
tion between them [1, 2]. Nanodispersed metal oxides
are widely used in industry: aluminum oxide, e.g., is
used as a highly effective catalyst, sorbent, and catalyst
carrier; for creating new functional materials with spe
cial properties (magnetic, electrical, and optical); as
fillers in modifying polymer materials to improve their
qualities (mechanical, physicochemical, chemical,
and so on); and in medicine, cosmetics, and other
fields [1, 3, 4].
There are many different methods for preparing
nanosized aluminum oxides; one of the most promis
ing, however, is the electrochemical method of obtain
ing metal oxides, since the ability to effectively regu
late the parameters of electrochemical process allows
us to obtain nanosized particles with a narrow range of
sizes and control their shape, morphology, and phase
composition [5, 6].
The purpose of this work was to study the effect of
synthesis conditions on the shape, morphology, and
phase composition of nanoparticles of aluminum
hydroxides and oxides obtained by various methods.
EXPERIMENTAL
A coaxial electrochemical reactor, in which
X18H10T steel was served as cathode and A5 alumi
num was used as an anode, was used for aluminum
hydroxide. Electrolysis was performed at different
anodic current densities, with aqueous solutions of
NaCl with different concentrations (chemically pure)
being used as electrolyte (Table 1). In obtaining alumi
num hydroxide by the chemical method, deposition
was performed from solutions of salts of aluminum,
with solutions of sodium and ammonium hydroxide
being used as the nonsolvent. In the combined
method, samples obtained by chemical deposition
were subjected to a constant electric current in a coax
Table 1. Profiles of obtained aluminum hydroxide
No. cNaCl, M j, А/m2 T, min No. cNaCl, M j, А/m2 T, min No.
1
2
3
0.1
0.2
0.5
Electrochemical
166.7 130
4
0.1
166.7 120
5
0.2
166.7
60
6
0.5
83.3
83.3
83.3
150
120
150
7
8
9
10
Salt
No. j, А/m2
Chemical
11
1 M AlCl3
0.2 M Al2(SO4)3 12
0.5 M Al(NO3)3 13
0.5 M Al(NO3)3
Combined
166.7 0.2 M Al2(SO4)3
166.7 0.2 M Al2(SO4)3
166.7 0.5 M Al(NO3)3
Note: In sample 10, 10 M NH4OH was added upon precipitation; 3 M NaOH was added to the other samples.
642
Salt
T, min
30
30
30
PHYSICAL AND CHEMICAL PROPERTIES OF NANOSIZED ALUMINUM HYDROXIDE
(а)
100 nm
643
100 nm
(b)
Fig. 1. Electron microscope image of nanoparticles of aluminum hydroxide obtained by (a) electrochemical and (b) combined
methods. Magnification (a) 105000; (b) 55000.
ial electrolytic cell; OPTA was used as anode, while
steel X18H10T served as a cathode. For crystalliza
tion, the precipitate was kept in the mother solution
and then filtered and dried at 363–383 K. To obtain
the oxides, the corresponding hydroxides were cal
cined at 823 K.
Xray analysis was performed on a D8 ADVANCE
diffractometer (Bruker) employing monochromatic
CuKα radiation in the step scan mode. The distance
between the planes of the diffraction reflexes was cal
culated automatically by the EVA program. Crystal
line phases were identified by comparing the experi
mental values of the interplanar distances and relative
intensities with the control values.
Thermal studies were carried out with use of an
STA 409 PC Luxx synchronous thermoanalyzer (sam
ples were heated at temperatures of 298 to 1273 K at a
rate of 10 K/min). Water content in the samples was
calculated from the results of thermogravimetric anal
ysis by the formula
n ( H 2 O ) = ω ( H 2 O )/100M H2 O , mol/g ,
where ω(H2O) is the total mass loss after heating to a
temperature of 1273 K, %; n(H2O) is the amount of
water in the sample, mol/g; and M H2 O is the molecular
weight of water, g/mol.
The size of the particles in the synthesized samples
was determined by transmission electron microscopy
(TEM) using an EMMA4 microscope microana
lyzer.
The series of studies to determine the percentage of
impurities of iron, chromium, arsenic, bismuth,
cobalt, manganese, stibium in oxides and hydroxides
of aluminum was performed on an ICAP 6000 atomic
emission spectrometer.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
RESULTS AND DISCUSSION
Our studies of hydroxides and oxides of aluminum
by means of transmission electron microscopy
revealed the dependence of particle size on the man
ner and mode in which they were obtained (Fig. 1,
Table 1). Samples of aluminum hydroxide 1–6
obtained by the electrochemical method are highly
refined particles ~50 nm in size inclined to aggrega
tion, against which there are clearly visible aggregates
of ≥150–200 nm and larger ones with thicknesses of
>1 μm. In hydroxide samples 7–10 obtained by tradi
tional chemical precipitation, the particle size distri
bution depends on the concentration of the initial
solution of aluminum salt. In the case of samples
obtained from concentrated solutions, we observe par
ticles more varied in shape and size at submicron and
micron ranges in the micrographs (needles, elongated
plates, barlike, and granular in small quantities). Dilu
tion of the solution promotes the formation of a super
fine phase, against which nanoscale particles are
clearly seen. For sample 8, rounded and slightly fac
eted particles with dimensions of 20–30 nm, needle
like particles measuring ~10 × 120 nm, and single
elongated platelike units forming a package of thin
shifted plates sizes of ~0.2 × 0.6 μm were detected.
Finegrained particles measuring 7–10 nm, dark
teardropshaped particles of 10–50 nm, and scaly
aggregates with sizes ranging from 0.3 to 1 μm were
found during study of sample 9.
Increasing the concentration of solutions of alumi
num (III) and precipitating agent promotes the for
mation of particles with blurred contours, ranging in
size from 50 to 250 nm, 80 × 500 nm packets of plates
displaced relative to one another, and large aggregates
measuring 0.2 × 3.5 μm.
Increasing the concentration of aluminum salt in
the solution from which deposition takes place, along
with increasing the concentration of precipitant solu
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DRESVYANNIKOV et al.
(а)
100 nm
85 nm
(b)
Fig. 2. Electron microscope image of nanoparticles of aluminum oxide, obtained by (a) electrochemical and (b) combined meth
ods. Magnification (a) 113000; (b) 98000.
tion, thus contributes to the formation of larger parti
cles due to aggregation.
In samples of aluminum hydroxide 11–13,
obtained by means of a combination of chemical
methods (using dilute solutions of aluminum salts and
precipitant) and electrochemical effects, a depen
dence was observed for the size of the obtained parti
cles on the nature of the initial salt. The deposition of
aluminum sulfate solutions of different concentrations
(samples 11 and 12) promoted the formation of nanos
cale particles of needle shape with an average size of
10 × 70 nm, finegrained particles measuring 10–20 nm,
and clear faceted aggregates of submicron size (~0.3 ×
0.75 μm). The use of aluminum nitrate (Sample 13) as
the initial solution led to more aggregated particles of
rectangular shape ≤1 μm thick (linear dimensions var
ied over the range 200–500 nm) and microaggregates
of isometric habitus nanoparticles with an average size
of ~50 nm. In the micrographs, there is also evidence
of an amorphous structure that forms clusters at the
edges of plates and isolated clusters with sizes of up to
300 nm.
It should be noted that during the synthesis of alu
minum hydroxide by electrochemical method,
increasing the current density and concentration of
the electrolyte solution led to the growth of microag
gregates formed by nanoparticles.
Hydroxides of aluminum obtained by chemical
deposition show the greatest disposition toward aggre
gation of particles; this decreases with subsequent
treatment in solution by an electric current (a com
bined method of acquisition).
During our study of oxides of aluminum obtained
from the respective hydroxides, we observed a clearer
size distribution of particles, depending on the method
and mode of acquisition (Table 2, Fig. 2).
Fine aluminum oxide exists in the Xray amor
phous state over a wide temperature range (as high as
1073–1273 K) [7, 8]. After reaching the specified tem
peratures and the corresponding increase in particle
size, metastable phase γAl2O3 is often formed, with
stable crystalline phase Al2O3 finally forming as a rule
at 1373–1473 K. A major task in the synthesis and
application of aluminium oxides is studying the factors
that increase the stability of metastable states. To date,
there are two basic concepts of conservation of oxides
in a metastable state [6, 8, 9]: the first involves the
dimensional (dispersion) factor; the second, in addi
tion to dispersion, takes into account an additional
stabilizing factor—the presence and conservation of
water in the crystal structure of oxides up to high tem
peratures.
The results from our study of the phase composi
tion of hydroxides obtained by the electrochemical
method reveal that samples 1–6 are a mechanical mix
ture of boehmite and bayerite (Fig. 3).
The peaks with interplanar distances (d/n) 4.71,
4.37, 3.20 and 2.22, 1.72 Å observed in the crystallo
gram belong to bayerite [6, 8, 9]. In addition to the
bayerite phase, the boehmite phase is present to one
degree or another in all of the studied samples, as
shown by the reflexes (d/n) 6.13, 3.20, 2.36, and 1.86 Å
[6, 8, 9]. Xray diffraction analysis of the samples syn
thesized by chemical deposition from solutions of alu
minum salts showed less homogeneous phase compo
sition, combining the simultaneous presence of two,
three, or more phases, but the additional effect of the
electric field (a combined method of acquisition)
allows us to obtain monophasic systems (Table 3).
The ratio of integrated intensities of the peaks of
boehmite and bayerite corresponds to the quantitative
ratio of these phases, measured by the ratio of the inte
grated intensities of the peak of boehmite with d/n =
6.2 Å and the sum of the integrated intensities of the
peaks of bayerite with d/n = 4.7 and 4.4 Å. It was found
that among samples 1–3, synthesized at current den
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PHYSICAL AND CHEMICAL PROPERTIES OF NANOSIZED ALUMINUM HYDROXIDE
645
Table 2. Results from transmission electron microscopy of aluminium oxide upon different methods of acquisition
No.
Characteristics of particles
Electrochemical
1
• needleshaped particles ≤10 nm wide and up to 100 nm long (prone to aggregation);
• particles of finegrained structure (grain size, ≥10 nm);
• aggregates formed by particles as small as 60 nm (form close to weakly faceted, hexagonal).
2
• needleshaped particles ≤10 nm wide and up to 100 nm long,
• particles of finegrained structure (grain size ≤10 nm);
• barshaped aggregates measuring ~0.7 × 2.5 μm.
3
• fine particles (~50 nm);
• larger particles (up to 500 nm);
• aggregates (5–10 μm).
4
• fine particles (~50 nm);
• aggregates of isometric (~0.5 μm) to elongated (~1.5 to 5 μm).
5
• needleshaped particles ≤10 nm wide and up to 100 nm long (prone to aggregation);
• aggregates ~3.5 × 4 μm wide and ~4.5 × 6 μm long.
6
• fine particles (~50 nm);
• particle measuring ≥300 nm.
Chemical
7
• aggregates of particles measuring ~70 × 100 nm;
• clusters of platetype particles measuring 200 × 400 nm.
8
• pseudorhombic aggregates formed by plates measuring ~350 × 400 nm;
• pseudotube structure aggregates 0.25 μm wide and ~1.5 μm long.
9
• particles measuring 10–30 nm;
• lamellar particles of quasirectangular shape with sizes ranging from ~50 × 100 to 100 × 150 nm;
• pseudotube aggregates of semicurtail tubes with diameters of 5–10 nm and lengths of ~200 nm;
• finegrained particles with a grain sizes up to 10 nm.
10
• isometric fine particles with sizes up to 50 nm arranged in a unique chain of arbitrary shape and tending to form
aggregates of sizes up to 500 nm;
• translucent platetype particles having no definite shape, with sizes of 400 to 600 nm.
Combined
11
• isometric particles measuring up to 50 nm (prone to aggregation);
• platetype particles measuring 400–600 nm.
12
• thin, isometric nanoparticles measuring 10–20 nm and forming microaggregates;
• particles measuring 20–30 nm, clearly defined at edges of aggregates;
• multilayer large formations (up to several microns) and individual particles of arbitrary shape and thickness;
some particles form bends along the edges (characteristic of squamous morphostructures).
13
• elongated platelike particles with sizes varying from 100 × 250–300 × 600 nm;
• wedgeshaped objects of irregular thickness with linear dimensions: base ~450–600 nm, height ~800–900 nm;
• amorphous substance forming clusters of varying thickness and linear dimensions (up to 1.2 μm); isolated iso
metric particles that form these clusters measuring 50–70 nm.
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3
2
1
10
20
30
40
50
60
70
80
2θ, deg
Fig. 3. Typical diffraction patterns of samples of aluminum hydroxide obtained by (1) electrochemical, (2) chemical, and (3)
combined methods.
sity j = 166.7 A/m2, a reduction in the proportion of
boehmite with increasing electrolyte concentration is
observed; a similar dependence is characteristic for
samples 4–6, synthesized at lower current density j =
83.3 A/m2. In general, lowering the current density by
a factor of 2 promotes the formation of large amounts
of boehmite phase.
High current density and low electrolyte concen
tration is to a greater extent more conducive to the for
mation of boehmite phase: when the concentration of
electrolyte is increased, the proportion of bayerite
phase rises. Reducing the density while simulta
neously increasing the electrolyte concentration leads
to a drop in the proportion of boehmite.
Phase transformations of crystalline hydroxides
according to their heating characteristics are deter
mined by features of the two main stages: the removal
of hydroxyl groups and the transition of the crystal
structure of the hydroxide in the crystal structure of
the oxide. At a heating rate of ~10 K/min, the above
Table 3. Results from studying the phase composition of
aluminum hydroxide
Method of acquisition
Electrochemical
Chemical
Combined
Phase
Boehmite + bayerite
Boehmite + bayerite + gibbsite +
nordstrand
Bayerite
Note: RFA of oxides revealed only the presence of γAl2O3 phase.
stages are combined; thus, from one to three or four
endothermic effects accompanied by mass loss heating
are observed in the curves from differential thermal
analysis of the studied hydroxides. Mass loss (in recal
culation to H2O) in different temperature ranges was
calculated for quantitative interpretation of the results
from differential thermal analysis of the studied sam
ples. These data are listed in Table 4.
According to [8–12], the low temperature endot
hermic effect of the dehydration of aluminum hydrox
ides is due to the removal of physically bounded water.
It is also known that the process of bayerite dehydra
tion is characterized by the presence of endothermic
effects with maxima at 373, 513, 573, and 768 K. We
may consequently assume that for all the samples, the
first endothermic effect in the 298–466 K range is
caused by the removal of physically bounded water and
the dehydration of bayerite. The second endothermic
peak at 550 K is caused by bayerite dehydration and
the formation of two phases: boehmite and lowtem
perature aluminum oxide (ηAl2O3). The high tem
perature endothermic effect in the 584–973 K range
characterizes the removal of water from the boeh
mite’s structure and the formation of γAl2O3.
The endothermic effects at 383, 583, and 773 K,
due to the removal of physically adsorbed water, the
dehydration of trihydroxide, and the crystallization of
lowtemperature aluminium oxide, respectively, are
characteristic for the thermochemical transformations
of aluminium oxide obtained by calcination of chem
ically precipitated gibbsite. Similar endothermic
effects are also observed for sample 9, in which Xray
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Table 4. Results from thermogravimetric determination of mass loss (Δmi , wt %) in different temperature ranges for sam
ples of aluminum hydroxide
No.
Т1, К
Т1max , К
Δm1
Т2, К
Т2max, К
Δm2
Т3, К
Т3max, К
Δm3
n(H2O),
mol/g
1
2
3
4
5
6
7
8
9
10
11
298–496
298–466
298–466
298–465
298–465
298–458
298–425
298–493
298–422
298–480
298–498
351
353
377
363
369
370
360
358
339
401
360
18.9
7.68
15.99
13.54
11.15
11.92
4.01
20.82
0.27
72.45
15.67
496–606
466–607
466–594
465–584
465–584
458–600
425–510
493–628
422–513
480–628
498–627
549
555
550
543
564
550
–
555
492
480
559
11.25
12.81
10.59
7.43
15.10
15.99
3.08
13.99
8.26
3.86
15.11
606–810
607–791
594–780
584–779
584–779
600–766
510–616
628–1263
513–639
–
627–954
696
680
657
664
674
633
558
680
570
–
–
10.01
8.05
7.36
8.93
5.59
5.31
22.46
10.92
17.48
–
6.80
0.0383
0.0175
0.0211
0.0183
0.0192
0.0211
0.0208
0.0285
0.0174
0.0431
0.0267
Note: For samples 9 and 11 in the range of T4 value 639–865 and 955–1271 K (T4max = 657 and 1138 K) value Δm4 = 5.95 and 0.11 wt %,
respectively.
Table 5. Contents of chemical elements (wt %) in test sample 2 of aluminum hydroxide and oxide according to the atomic
emission spectroscopy
Sample
Aluminum hydroxide
Aluminum oxide
Bi
Co
Cr
Fe
Mn
Sb
0.120
0.020
–
0.002
0.004
0.150
0.003
0.040
0.002
0.013
0.007
0.01
diffraction analysis revealed the presence of gibbsite
phase, and a fourth peak with a maximum at 492 K was
caused by the removal of physically bounded water and
the dehydration of bayerite [8–12].
We also evaluated the chemical composition of the
obtained aluminum hydroxide and oxide for the pres
ence of impurities. The quantitative content of impu
rities of iron, chromium, arsenic, bismuth, cobalt,
manganese (Table 5), antimony was determined by
atomic emission spectroscopy for sample 2.
CONCLUSIONS
Analysis of the results of our studies demonstrates
the greater effectiveness of the electrochemical
method as compared to traditional chemical deposi
tion in acquisition nanosized particles of aluminum
hydroxide and oxide. It was established that control
ling the parameters of the electrochemical processes
makes it possible to obtain nanosized particles of alu
minum hydroxide and oxide with sizes of 10–300 nm,
while the size of particles obtained by the chemical
method, range from 50 nm to 3.5 μm. Using Xray
phase and thermal analysis, it was revealed that the
samples of aluminum hydroxide and oxide are in fact
a biphasic system, and changing the process parame
ters (e.g., current density and concentration of elec
trolyte solution) allows the ratio of phases to be
changed in one direction or another. The last is very
important in acquisition aluminum oxide nanopowder
with a particular crystal structure.
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