Hydrothermal synthesis of metal oxide nano- and microparticles in supercritical water

Article abstract of DOI:10.1134/S0036024411030034

Hydrothermal syntheses of nano- and microparticles of metal oxides of two types, LiMO_n (LiCo0₂, LiNiO₂, LiZnO ₂, and LiCuO₂) and MO_n (Ga₂O ₃, CeO₂) were performed under continuous conditions in a tubular reactor with the use of supercritical water. An important role in the synthesis of nanoparticles and the reproducibility of the results was played by the conditions of mixing of supercritical water and precursor solution flows. The morphology and composition of synthesized compounds were studied by scanning electron microscopy and X-ray diffraction. The syntheses of LiCoO₂, LiNiO₂, LiZnO₂, LiCuO₂, Ga₂O ₃, and CeO₂ were most successful.

Full text of DOI:10.1134/S0036024411030034

ISSN 0036ꢀ0244, Russian Journal of Physical Chemistry A, 2011, Vol. 85, No. 3, pp. 377–382. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © V.I. Anikeev, 2011, published in Zhurnal Fizicheskoi Khimii, 2011, Vol. 85, No. 3, pp. 440–445.
CHEMICAL KINETICS
AND CATALYSIS
Hydrothermal Synthesis of Metal Oxide Nanoꢀ and Microparticles
in Supercritical Water
V. I. Anikeev
Boreskov Institute of Catalysis, Siberian Division, Russian Academy of Sciences, Novosibirsk
eꢀmail: anik@catalysis.ru
Received March 29, 2010
Abstract—Hydrothermal syntheses of nanoꢀ and microparticles of metal oxides of two types, LiMOn
(
LiCoO , LiNiO , LiZnO , and LiCuO2) and MO (Ga O , CeO ) were performed under continuous conꢀ
2 2 2 n 2 3 2
ditions in a tubular reactor with the use of supercritical water. An important role in the synthesis of nanoparꢀ
ticles and the reproducibility of the results was played by the conditions of mixing of supercritical water and
precursor solution flows. The morphology and composition of synthesized compounds were studied by scanꢀ
ning electron microscopy and Xꢀray diffraction. The syntheses of LiCoO , LiNiO , LiZnO , LiCuO ,
2
2
2
2
Ga O , and CeO2 were most successful.
2
3
Keywords: hydrothermal synthesis, metal oxide nanoꢀ and microparticles, supercritical water.
DOI: 10.1134/S0036024411030034
INTRODUCTION
Metal and/or metal oxide nanoparticles deposꢀ
ited on a surface are of considerable interest for
applications because of their unique properties
determined by metal morphology, dispersity, and
concentration on the surface. For instance, nanoꢀ
particles with a high specific surface area deposited
on various substrates are used as catalysts of various
chemical transformations. Depending on the
selected materials and supercritical fluid, nanopartiꢀ
cles can be produced by the chemical method, the
method of rapid expansion of supercritical solution
Many methods were suggested for the synthesis
of nanostructured materials, such as nanoparticles,
nanofilms, and nanowires. These methods can be
divided into two main groups: (1) the production of
nanostructures from bulk materials and (2) the proꢀ
duction of nanostructures from molecular level sysꢀ
tems.
Supercritical fluids are an attractive medium for
the synthesis, modification, and formation of
in)organic material nanoparticles [1–4]. Such nanoꢀ
(
(
RESS), gas antiꢀsolvent processes (the GaSR,
structures and materials exhibit unusual properties
different from those of massive materials. Supercritical
fluids are extensively used for the preparation of inorꢀ
ganic material nanoparticles (metals (Pt, Pd, Rh, Au,
Ag, etc.) and their composites, metal oxides and
GASP, SAS, PCA, and SEDS variants), processes of
particle preparation from gasꢀsaturated solutions
TM
(
[
PGSS ) [2], the hydrothermal synthesis method
2, 5], and the method of reverse micelles. The selecꢀ
nitrides (TiO , Cr O , Co N, and Cr N) [3]), tion of a method for synthesizing nanoparticles is
2
2
3
2
2
LiFePO4ꢀtype metal oxide (used as materials for cathꢀ determined by particular purposes.
odes in Li cells) and similar compound nanoparticles
Among the suggested methods for the synthesis of
metal, metal oxide, and metal hydroxide nanopartiꢀ
cles, hydrothermal synthesis methods [5–7] in superꢀ
critical water are most advantageous and promising.
Several main oneꢀ and multistage reactions of precurꢀ
sors and metal salts can be used in the synthesis of parꢀ
ticles in supercritical water, including hydrolysis,
dehydration, thermolysis, reduction, and oxidation
[
4], and for the microencapsulation of inorganic
nanoparticles in polymer coatings.
Metals and their oxides in the form of nanopartiꢀ
cles having chemical, thermal, optical, magnetic, and
other properties different from those of their massive
analogues are extensively used in catalysis, medicine,
electronics, etc. The applicability range of nanopartiꢀ
cles in many respects depends not only on their propꢀ
erties, such as size, structure, and morphology, but
also on the method used for their preparation. The
nanosize of a material is determined by one of its
(
as a rule, in the presence of hydrogen). In the region
of critical water parameters, the dissociation of water
+
–
and, therefore, the concentrations of
H and OH
dimensions (diameter or thickness), which change ions increase. As a result, the hydrothermal synthesis
from 1 to 200 nm. of metal oxide nanoparticles from metal salts is perꢀ
377

378
ANIKEEV
3
1
6
9
5
10
8
4
7
2
Fig. 1. Unit with a tubular reactor for the hydrothermal synthesis of nanoparticles:
pump; , syringe pump; , tubular reactor; , mixer; , furnace with a boiling sand layer;
and 10, vessel for synthesized products.
1
,
2
8
, vessels for water and reagents;
3, piston
4
5
6
7
, refrigerator; , back pressure regulator;
9
formed in supercritical water in twoꢀstage hydrolysis boiling sand layer
7, refrigerator 8, mixer of flows 6,
and dehydration reactions, manometers and pressure gauges.
All the syntheses of metal oxide nanoparticles were
performed using approximately the same procedure.
The initial mother liquor was prepared by dissolving
OH–
→
M(OH) + n
B ,
n/2)H O.
–
MB +
n
n
n
M(OH)n → MOn/2 + (
2
equiꢀ or nonequimolar amounts of first metal (M1
)
Hydrothermal transformations of metal salts
nitrates and sulfates) in supercritical water were perꢀ
formed to synthesize Fe O , Co O4, NiO, TiO , ZrO ,
CeO , LiCoO , CoFe O , NiFe O4, etc. particles [5–
]. Hydrothermal method is simple to use and scale,
hydrothermal reactions are performed in reactorsꢀ
autoclaves, and the properties and size of particles can
be controlled. At the same time, one of important
shortcomings of this method is ineffective control over
the aggregation of nanoparticles.
nitrate, sulfate, or acetate and another salt of the secꢀ
ond metal (M2). The mother liquor (flow 1) was
introduced by the syringe pump into mixer 6 in a
ratio of 1 : 10 or 2 : 10 with respect to supercritical
water (flow 2) continuously supplied to the same mixꢀ
ture by the piston pump. Transformations were perꢀ
formed at temperatures and pressures close to the critꢀ
ical parameters of mixtures containing more than 95%
water (temperature above 371°C and pressure above
(
2
3
3
2
2
2
2
2
4
2
9
230 atm). The products of the interaction of salts with
supercritical water passed from the reactor into a heat
exchanger and, through in back pressure regulator,
into an accumulating vessel.
This work is concerned with the hydrothermal synꢀ
thesis of LiM O and M O metal oxide nanoparticles
x
y
m
n
with the use of supercritical water under continuous
conditions. The procedures for performing experiꢀ
Depending on the size and properties of crystals
mental studies are described and the results obtained formed, hydrothermal synthesis products were often a
for the synthesized samples by scanning electron mixture of nonprecipitating particles in water. Metal
microscopy (HRTEM) and Xꢀray diffraction (XRD) oxide particles were isolated from solutions for analyzꢀ
are presented.
ing the solid phase by solution centrifugation or evapꢀ
oration followed by solid phase drying.
An important role in the hydrothermal synthesis of
metal oxide nanoparticles is played by the principle
and device for mixing mother liquor and supercritical
water flows. On the one hand, it was shown [8, 10] that
EXPERIMENTAL
Unit and Experimental Procedures
A scheme of the unit used for the hydrothermal uniform mixing of two flows in the shortest contact
synthesis of metal oxide nanoparticles is shown in time possible provides good reproducibility of the
Fig. 1. The unit includes a tubular flow reactor conꢀ results and a narrow particleꢀsize distribution in
sisting of two parts, each 3 m long, with an inside repeated experiments. On the other hand, the kinetic
diameter of 1.75 mm (the volume of the reactor is then peculiarities of the reaction itself, the properties of
3
7
.2 and 14.4 cm ), one piston pump
3, syringe pump
4,
particles formed, the ratio between flows, slow transꢀ
back pressure regulator
9, electrical furnace with a port of particles from the zone of mixing of flows into
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HYDROTHERMAL SYNTHESIS OF METAL OXIDE NANOꢀ AND MICROPARTICLES
379
500 nm
2 µm
(а)
(b)
Fig. 2. HRTEM images of LiCoO2 particles obtained in hydrothermal synthesis.
the reactor result in the agglomeration of particles,
their agglomeration on the surface of the mixer and
reactor, and, eventually, blocking of reactor tubes. The
creation of a universal device for mixing or a reactor
for the hydrothermal synthesis of a broad class of
nanoparticles is, likely, not the main goal, and studies
are developing in the direction of creating mixing
devices for particular classes of reactions.
Materials and Solutions
Solutions of precursors were prepared from pure
anhydrous LiNO3
,
·
CoSO · 7H O, Fe(NO ) · 3H O,
4
4H O, Ni(NO3)3
2
3 3
2
6H O,
3 3
Ni(CH COO)2
3
·
Cu(CH COO) · H O, ZnSO · 7H O, Ga(NO ) ·
3
2
2
2
2
4
2
8
H O, and Ce(NO ) · 6H O salts without additional
2 3 3 2
purification and doubly distilled water. The hydrotherꢀ
mal syntheses of (expected) oxides of two metals
LiMOx (LiCoO , LiFeO , LiNiO , LiZnO , and
2
2
2
2
In performing hydrothermal syntheses of metal
oxide nanoparticles in supercritical water, the author
encountered the phenomenon of fast formation of
metal oxide nanoparticles (Cd O and CuO2) directly
LiCuO2), one of which was Li, was performed under
continuous conditions in a flow reactor. The M O
y
x
(
Ga O , Cd O , and Ce Ox) compounds were syntheꢀ
y x y x y
m
n
sized in supercritical water using a similar procedure.
The initial mother liquors for the synthesis of these
compounds were prepared by the solution of one or
two salts in water. A mother liquor was introduced into
the reactor (flow 1) through a mixer, in which it was
mixed with supercritical water (flow 2).
in the zone of mixing of flows followed by their
agglomeration and blocking of reactor tubes. Clearly,
additional studies are necessary for synthesizing such
compounds for the creation of mixing conditions for
continuous synthesis of metal oxide nanoparticles in a
tubular flow reactor. For this reason, the results
obtained in synthesizing these compounds are not
reported in this work.
RESULTS AND DISCUSSION
Hydrothermal Syntheses of LiMOx Metal Oxide
Nanoparticles
Sample Analysis Methods
The synthesis of LiCoOx
thesis of the LiCoO2 compound was performed using
.1 M aqueous solutions of lithium nitrate LiNO3 and
. The hydrothermal synꢀ
The structure, phase, and elemental composition
of the samples were analyzed by the HRTEM and
XRD methods. Several aqueous products were anaꢀ
lyzed using UV spectroscopy. The Xꢀray diffraction
patterns were recorded on an HZGꢀ4 (Germany) difꢀ
0
cobalt sulfate CoSO4. The temperature and pressure of
hydrothermal synthesis were 382–397°C and 226–
30 atm. The HRTEM images of particles (hydrotherꢀ
mal synthesis products) are shown in Fig. 2. We see
that LiCoO2 particles are round and have a size of
500 nm. These particles are surrounded by a coat
consisting of products formed after solvent evaporaꢀ
2
fractometer using Cu
K radiation. The Xꢀray patterns
α
were analyzed using the PDF base of Xꢀray powder
data.
~
The transmission spectra of solutions containing tion. Large (~500 nm) particles form agglomerates
nanoparticles were recorded on a UVꢀ2501 PC (Shiꢀ consisting of 3–4 particles, smaller particles form
madzu) spectrophotometer with an ISRꢀ240 A diffuse agglomerates of 10 and more particles.
reflectance accessory. The spectra were recorded with
respect to a reflectance standard (BaSO4) over the freꢀ formed using a 0.1 M aqueous solution of lithium
The synthesis of LiFeOx. The synthesis was perꢀ
–1
quency range 11000–54000 cm . Samples in the nitrate LiNO3 and a 0.1 M aqueous solution of iron
form of solutions were placed into quartz cells with nitrate Fe(NO3)3. The temperature and pressure were
optical paths of 1, 2, and 10 mm. The data were repreꢀ 376°C and 230 atm. Reaction products were dark red,
sented in the optical density
nates.
D
–wave number coordiꢀ they precipitated with time. The Xꢀray pattern of dry
samples did not substantiate the presence of a LiFeOx
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380
ANIKEEV
50 nm
10 nm
(а)
(b)
200 nm
(c)
(d)
Fig. 3. HRTEM images of (a, b, c) synthesized LiNiO2 sample and (d) the structure of interatomic interplanar distances.
compound. Reaction products only contained the was the same as for the layered phase. The conclusion
α
ꢀFe O crystalline phase and an amorphous phase.
can be drawn that the layered phase is LiNiO2 (a high
degree of the replacement of nickel by lithium),
whereas small contrast particles with a low degree of
replacement are the NiO phase. This was substantiꢀ
ated by the Xꢀraty diffraction data.
2
3
The synthesis of LiNiO2. The synthesis was perꢀ
formed using 0.1 M solutions of lithium nitrate LiNO3
and nickel acetate Ni(CH COO)2. The HRTEM
image of the synthesized desired compound is shown
in Fig. 3. The compound had the form of layered plates
3
The synthesis of LiZnO2. The synthesis was perꢀ
formed using 0.1 M solutions of lithium nitrate LiNO3
and zinc sulfate ZnSO4. The TEM image of the synꢀ
thesized LiZnO2 compound is shown in Fig. 4. The
product contained structures of two types: plates
~
50 nm in size and ~2–3 nm thick. For this structure,
interatomic interplanar distances were (in nm) 0.245 s,
.210 w, 0.150 vs, 0.130 m, and 0.122 m (Fig. 3d); they
0
corresponded to the LiNiO2 compound. The sample
also contained more contrast 3–5 nm particles, for
which interatomic interplanar distance (0.246 nm)
(
characteristic size ~10 nm
×
~1 m) and aggregates of
μ
isometric crystals with size no more than 10 nm. The
plates were unstable, structure disordering occurred
under the action of an HRTEM beam of electrons.
The energy dispersive Xꢀray (EDX) spectra give an
intense Zn line for the plates.
The synthesis of LiCuO2. A mother liquor (0.1 M)
was prepared by dissolving lithium nitrate LiNO3 and
copper acetate Cu(CH COO)2 in water. According to
3
the scanning electron microscopy data, the syntheꢀ
sized sample was a polycrystalline mixture of well facꢀ
eted crystals, which had cubic symmetry and were
fairly uniform as concerns their habitus (Fig. 5). Their
shapes and sizes were: isomorphic (cubes),
prolate (parallelepipeds), 50 150 nm, and flattened
plates), 50 100 nm. Crystals formed characteristics
≈
50 nm;
≈
×
(
×
multibranch dendrites. An amorphous molecular layer
with a molecular thickness formed on the surface of
crystals under the action of an HRTEM electron
beam, as was characteristic of crystalline lithium comꢀ
pounds.
1 µm
Fig. 4. Image of the synthesized LiZnO2 sample.
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HYDROTHERMAL SYNTHESIS OF METAL OXIDE NANOꢀ AND MICROPARTICLES
381
200 nm
10 nm
(а)
(b)
(c)
Fig. 5. Images of a polycrystalline mixture of LiCuO2 crystals obtained in hydrothermal synthesis.
10 nm
(а)
(b)
Fig. 6. HRTEM images of γꢀGa O3 in combination with a not identified crystalline phase.
2
20 nm
200 nm
(а)
(b)
Fig. 7. HRTEM images of CeO2 crystals formed under hydrothermal conditions in supercritical water.
The calculated interatomic interplanar distances
were (nm) 0.282 w, 0.257 m, 0.230 m, 0.175 m, 0.150 m,
and 0.140 s (Fig. 5). They correspond to lithium
cuprite LiCuO2 and CuO. The Xꢀray data also subꢀ
stantiate the presence of a substantial amount of CuO.
The synthesis of Ga O . Ga O3 was synthesized
2 3 2
from a 0.1 M solution of Ga(NO ) · 8H O in water.
3 3
2
The temperature and pressure of the reaction were
65–384°C and 235 atm. The flow rate of water
flow 2) was 10 ml/min, and the flow rate of the
reagent (flow 1) was 2 ml/min. The hydrothermal
reaction gave products in the form of a homogeneous
darkꢀcolored solution, which was evaporated to obtain
the solid phase. According to the HRTEM data, the
synthesized compound consisted of highꢀdispersity
3
(
The Hydrothermal Syntheses
of Metal Oxide M O Nanoparticles
y
x
(
2–5 nm) crystals (Fig. 6). It contained separate
The hydrothermal synthesis of Ga O and CeO2
metal oxides was performed under continuous condiꢀ
tions in a flow reactor as described above.
2
3
blocks of pseudomorphic crystals up to 100 nm in size.
The calculated interatomic interplanar distances were
(nm) 0.295 w, 0.250 vs, 0.230 m, 0.205 m, 0.160 m,
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382
ANIKEEV
and 0.145 s (Fig. 6b). The Xꢀray diffraction data and by the Russian Foundation for Basic Research
showed that the main phase in the synthesized comꢀ (project no. 00282).
pound was ꢀGa O in combination with a not identiꢀ
γ
2 3
fied crystalline phase.
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(
1
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isometric crystals, whose size was ~100 nm (Fig. 7).
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(
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ACKNOWLEDGMENTS
50
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No. 3 2011