On the nature of decrease in the particles size of nanostructured nickel oxide obtained by the sol-gel method in the presence of surfactants

Article abstract of DOI:10.1134/S1070363210060137

A processes of formation of nanostructured powders of nickel oxide by annealing in the temperature range of 200-700°C of the nickel hydroxide obtained by the sol-gel method at 80°C from solutions of nickel nitrate by precipitation with alkali in the presence of surfactant AF-12 (polyethylene oxide alkylphenyl ether) was investigated. The formation of nanostructured powders of nickel oxide in the presence of a surfactant reduces the size of nanoparticles to 20-25 nm, which is 1.5 times smaller than the particles obtained without a surfactant. The effective influence of surfactant on the particle size begins in the temperature range of its decomposition and evaporation equal 350-400°C.

Full text of DOI:10.1134/S1070363210060137

ISSN 1070-3632, Russian Journal of General Chemistry, 2010, Vol. 80, No. 6, pp. 1115–1121. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © V.V. Bakovets, S.A. Savintseva, L.N. Trushnikova, P.E. Plyusnin, I.P. Dolgovesova, T.D. Pivovarova, I.V. Korol’kov, 2010,
published in Zhurnal Obshchei Khimii, 2010, Vol. 80, No. 6, pp. 945–951.
On the Nature of Decrease in the Particles Size
of Nanostructured Nickel Oxide Obtained
by the Sol-Gel Method in the Presence of Surfactants
V. V. Bakovets, S. A. Savintseva, L. N. Trushnikova, P. E. Plyusnin,
I. P. Dolgovesova, T. D. Pivovarova, and I. V. Korol’kov
Nikolaev Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences,
pr. Akademika Lavrent’eva 3, Novosibirsk, 630090 Russia
e-mall: becambe@che.nsk.su
Received January 14, 2010
Abstract—A processes of formation of nanostructured powders of nickel oxide by annealing in the
temperature range of 200–700°C of the nickel hydroxide obtained by the sol–gel method at 80°C from
solutions of nickel nitrate by precipitation with alkali in the presence of surfactant AF-12 (polyethylene oxide
alkylphenyl ether) was investigated. The formation of nanostructured powders of nickel oxide in the presence
of a surfactant reduces the size of nanoparticles to 20–25 nm, which is 1.5 times smaller than the particles
obtained without a surfactant. The effective influence of surfactant on the particle size begins in the
temperature range of its decomposition and evaporation equal 350–400°C.
DOI: 10.1134/S1070363210060137
Synthesis and study of the properties of metal
oxides nanoparticles is at present a promising area of
science and industry. The transition metal oxides of
nanoscale size can find wide practical application.
Among them the nickel oxide should be emphasized,
since this compound is interesting due to its physico-
chemical properties [1]. The nickel oxide is widely
used as catalyst [2]. Among the approaches developed
in recent times to obtaining catalysts with high activity
there is the use of thin films with particles of a metal or
its oxide deposited on them [3].
the nanoparticles are synthesized in solution by a sol–
gel method that includes exchange and hydrolysis
reactions. The ultrafine products obtained in such
reactions have a highly developed specific surface and,
respectively, their surface energy is high. Therefore the
ultrafine systems in most cases are thermodynamically
non-equilibrium and the resulting particles tend to
aggregate [6, 7]. To prevent the aggregation of fine
particles organic surfactants are commonly used
(surface active compounds, SAC) [8–10]. In [11] as
the surfactants alcohols with various length of
hydrocarbon chains are used: ethanol, propanol, and
butanol. It is known that the formation of metal oxides
by the sol–gel method proceeds consecutively through
formation of hydroxide sol, then hydroxide gel, and
the subsequent dehydration of the latter followed by
high-temperature annealing of the intermediate formed
to the metal oxide powder. It was found in [11] that at
the temperature of the annealing up to about 400°C the
density of cerium oxide powder pre-calcined under a
pressure does not depend on the type of alcohol, and
only at annealing above this temperature the oxide
derived from an alcohol with a long hydrocarbon chain
has slightly higher density of the cake. However, to
date the nature of the influence of surfactants on the
The particles of nanometer-sized metal oxides,
including nickel oxide, may be used as the material for
electrodes in lithium power sources, whose electric
capacity reaches 700 mA h g–1 [4]. Using NiO nano-
powder it is possible to reduce the proportion of metal
in the cermet anodes of fuel cells to 40–50 wt % with
simultaneous reducing the resistivity to (4–13)×
10–4 Ω cm, which is impossible in the case of the
nickel–cermet anodes obtained by conventional methods
[5]. The nickel oxide is used also as a magnetic
material. Therefore the development of simple and
effective methods of synthesis of the oxides in
nanostructured state is relevant. The most frequently
1115

1116
BAKOVETS et al.
Ssp, m2 g–1
Δ2θ, deg
6
250
200
150
100
NiO
Ni(OH)2
4
2
0
2
1
4
4
3
50
0
3
2
0 100 200 300 400 500 600 700 800
1
T, °C
20
30
40
50
60
70
Fig. 2. Dependence of specific surface on temperature of
the samples (1) obtained without surfactant AF-12 and
(2) with surfactant AF-12; (3) broadening of the reflex
(111) on half-height 2θ degrees for the samples without
surfactant AF-12; and (4) the same in the presence of sur-
factant AF-12.
2θ, deg
Fig. 1. Diffraction patterns of samples (with AF-12)
obtained at different temperatures of dehydration: (1) 50°C,
(2) 200°C, (3) 300°C, and (4) 400°C.
gelation and the formation of nanostructured metal
oxide powders at annealing in air at elevated tem-
peratures remains unexplored. Yet more natural is an
assumption that low molecular weight alcohols
evaporate even at low annealing temperatures due to
the high pressure of their vapors; this also concerns pos-
sible products of decomposition of metal alcoxides [12].
phase analysis of the samples synthesized using AF-
12. Diffraction patterns of the samples obtained with
surfactant AF-12 and without it are identical, as can be
seen by comparing the diffraction patterns we
registered with those published in [13] for the process
without surfactant AF-12. The results indicate that the
products of the synthesis dried at 50°C are mixtures of
an amorphous phase with a polycrystalline nickel
hydroxide. However, annealing even at the tempera-
ture 200°C affords a phase of nickel oxide. The large
reflection width points to nanocrystallite state of the
product. Increase in the annealing temperature leads to
the increase in crystallinity and in the nickel oxide
fraction. Finally, in the temperature range 300–400°C
the phase of nanocrystalline hydroxide disappears and
remains only a phase of nickel oxide, whose crystal-
linity increases with further increase in temperature.
The research performed earlier [13] consisted in the
study of the formation mechanism of nanodispersed
nickel oxide at the dehydration of nickel hydroxide
obtained by the precipitation with alkali from aqueous
solutions of nickel nitrate, with iso-propanol as a
surfactant, and by subsequent annealing of washed and
dried precipitate. Obviously, in the initial stages of
formation of sol and gel alcohol is evaporated from the
system, therefore to investigate the effect of surfactant
on the processes and on the subsequent formation of
nanostructured nickel oxide it is necessary to use
additionally a surfactant with a much lower vapor
pressure. Such requirements are consistent with
surfactants with longer hydrocarbon chains.
Analysis of the data of measuring the specific
surface area of these samples shows that the chosen
method of synthesis allows to obtain nanoporous
hydrolysis products, which upon subsequent annealing
are transformed into nanostructured powders. Figure 2
shows the dependence of specific surface of the
samples obtained in the presence of AF-12 and without
(curves 1 and 2) on the annealing temperature, and the
broadening of the reflections (111) of Δ2θ diffraction
patterns at their half-heights (curves 3 and 4)
characterizing the change in size of pores (for gels),
and particles (for powders) of crystallites, respectively.
The specific surface area of dispersed samples
increases in the temperature range 50–230°C. This
The aim of this work was to study the effect of
long-chain nonionic surfactant AF-12, alkylphenyl
ether of polyethylene oxide, C9H19(C6H4)(CH2OCH2)12OH,
introduced into the reaction zone, on the dispersion of
nano-structured products formed sequentially in the
specified stages of the dehydration of nickel hydroxide
and nickel oxide formation process.
The nanostructured nickel oxide powders obtained
were studied by various methods described in the
experimental section. Fig. 1 shows the results of X-ray
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ON THE NATURE OF DECREASE IN THE PARTICLES SIZE
1117
stage is characterized by the formation of xerogels
(a)
[13], with an increase in porosity associated with the
destruction of the hydrogel. The presence of surfactant
AF-12 additives shows no effect at this stage. Further
increase in annealing temperature leads to a sharp
decrease in specific surface area of samples associated
with the processes of coalescence of the particles of
nickel oxide. In the region of medium temperatures of
annealing, the values of specific surface area of
powders obtained with surfactant AF-12 and without it
remain similar, and only at temperatures above 400°C
the value of specific surface area of samples prepared
with surfactant AF-12 becomes larger. This suggests
that in systems with AF-12 the coalescence of particles
at temperatures above 400°C is slightly lower than in
the samples without surfactant. The characteristic
feature of the process is the weak influence of
surfactant AF-12 on the size of crystallites in the
whole temperature region of annealing, which follows
from the proximity of the values of X-ray coherent
scattering in the two processes (Fig. 2, curves 3, 4).
However, a monotonic decrease in X-ray coherent
scattering at increase in temperature above 230°C
indicates agglomeration of crystallites. It is worth
noting that crystallite size determined from the data of
X-ray diffraction method (the broadening of reflexes,
Scherrer method), does not necessarily coincide with
the size of ultrafine particles, defined from the data on
the specific area of the sample surface measured by the
method of inert gas adsorption. The particles may
contain several crystallites, which have boundaries
between themselves and partly with the gas phase.
What hinders coalescence of particles of the products
of xerogel dehydration at temperatures above 400°C in
a system with AF-12?
100
95
2
1
2
4
A
90
85
80
75
70
0
B
C
D
–1
1
C1
3
–2
100 200 300 400 500 600 700 800
T, °C
(b)
100
95
4
3
1
2
3
90
85
80
75
70
4
2
1
0
100 200 300 400 500 600
T, °C
Fig. 3. (a) The curves of thermal analysis of samples in
helium: (1) TG without surfactant AF-12, (2) DTA without
surfactant AF-12, (3) TG with AF-12, (4) DTA with AF-
12; (b) The curves of thermal analysis of samples in an
argon–oxygen mixture: (1) TG without surfactant AF-12,
(2) DTA without surfactant AF-12, (3) TG with AF-12,
(4) DTA with AF-12.
The mass loss curves of the samples without AF-12
in He and Ar/O2 are identical, while the DTA curves
are identical only qualitatively (Fig. 3b) and indicate
the endo-effects associated with the samples
dehydration and evaporation of adsorbed isopropanol.
The curve of mass loss of the sample with surfactant
AF-12 (Fig. 3b) in the presence of oxygen is identical
to the curve for the sample without surfactant, but is
characterized by a lesser rate of mass loss. For the
sample with an AF-12 the endo-effect appears after a
significant exo-effect that can be associated only with
thermal transformation of the surfactant AF-12, and
threrafter the rate of dehydration of the sample with
decomposed AF-12 increases, and at 290°C the mass
loss of samples with AF-12 and without become
comparable.
It should be noted that the data presented here
indicate that the surfactant AF-12 is not washed out
completely at the stage of purification of precipitate.
The results of thermal analysis of the process of
annealing the samples with surfactant AF-12 and
without it in an atmosphere of He and Ar/O2 mixture
(ratio 4:1) are shown in Fig. 3. The mass loss up to
temperature 250°C is connected with the loss of free
water and adsorbed isopropanol. At this temperature
the product composition corresponds to the formula Ni
(OH)2 [13], although the phase analysis gives a
mixture of hydroxide and nickel oxide. The remaining
amount of OH groups corresponds to the formation of
the xerogel. Above this temperature begins the xerogel
dehydration [13].
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1118
BAKOVETS et al.
particle with the effective surfactant. It should be noted
(a)
that the concentration AF-12 in the initial solution
5.5×10–4 mol l–1 was chosen not to exceed the limit of
the critical concentration of the micelle formation.
Washing the product of the synthesis with water
reduces the content of AF-12 in the system
considerably. The found mass loss between points C
and D equals to 2.8 mg, and taking the initial sample
mass 56.7 mg as 100%, this corresponds to the amount
of water loss due to dehydration of the xerogel
~0.6 mg (C–C1) and to removal of ~2.2 mg of AF-12
from the system due to evaporation and decomposition
(C1–D). From these data and taking into account the
specific surface area of the product at 375°C according
to Fig. 2, we obtain the amount of adsorbed surfactant
AF-12 equal to 3×10–6 M. The obtained amount of
adsorbed surfactant corresponds to the degree of the
surface coverage of the dehydration products at the
point C equal to 10%. However, it should be taken into
account that the molecule of AF-12 contains 12 ad-
sorption-active CH2OCH2 groups that on adsorbtion on
the surface increase the degree of its coverage with the
surfactant. Thus, it is the presence of adsorbed
surfactant AF-12 that leads to a marked inhibition of
the dehydration of the products at the temperature
below 400°C. After evaporation of AF-12 at ~375°C
the process of dehydration of the xerogel is continued.
4
3
2
1
0
100
90
80
70
60
50
40
30
20
10
0
1
2
100
200
300
400
500
T, °C
(b)
15
13
11
9
100
90
80
70
60
50
40
30
20
10
1
7
5
3
2
1
0
100
200
300
T, °C
400
500
Fig. 4. (a) The curves of thermal analysis of AF-12 in
helium. (1) TG and (2) DTA. (b) The curves of thermal
analysis of AF-12 in the presence of oxygen: (1) TG and
(2) DTA.
At the thermal transformation of AF-12 surfactant
in the presence of oxygen (Fig. 4b) the mass loss of
AF-12 in the region of 250–375°C is accompanied by
a strong exo-effect. In the temperature range 370–430°C
the mass loss is slowing and is characterized by a
decrease in the exo-effect. But at the end of this stage
there is a strong two-step exo-effect. Such a trans-
formation of AF-12 in the presence of oxygen can be
attributed to its thermal decomposition in the first stage
and the formation of solid products of decomposition,
presumably resin or even graphite. In the second stage,
apparently, there is a burn-out of these products.
Similar effects are observed on the derivatograms of
dehydration of hydrogels in an atmosphere of Ar/O2
(Fig. 3b). The set of described transformations of
surfactant AF-12 results in the partial suppression of
coalescence of particles at the temperatures above 400°C,
which follows from Fig. 2 (curves 1 and 2).
In contrast to these transformations, the curve of
mass loss of the sample with surfactant AF-12 in an
inert atmosphere is characterized by two steps
(Fig. 3a). To the point C the mass loss curves are
almost identical. There is only a slight slowdown in the
relative loss of mass. After this point, there is an
additional rather sharp mass loss which may be
associated with the transformation of the surfactant
AF-12, as evidenced by the second endo-effect.
Figure 4 demonstrates the results of thermal
analysis of AF-12 in the atmosphere of He and Ar/O2,
which implies that in an inert atmosphere (Fig. 4a)
there is only a process of evaporation increasing
sharply at about 350°C. This indicates a low vapor
pressure of the AF-12 at the temperature of formation
and dehydration of hydrogel and xerogels below 300°C.
Thus, the deceleration in the mass loss before point C
can be explained only by blocking the surface of the
Using the found values of pycnometric density ρ
(Fig. 5) and the specific surface area Ssp (Fig. 2) of the
samples, we found the average diameters of particles
in the ash, hydrogel, xerogel, and nanostructured
nickel oxide in a spherical approximation, from the
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 80 No. 6 2010

ON THE NATURE OF DECREASE IN THE PARTICLES SIZE
1119
40
35
7
6
5
4
3
30
2
1
1
25
2
20
15
10
5
3
4
0
200
400
600
800
0
T, °C
0
200
400
600
800
T, °C
Fig. 6. Dependence on the temperature of dehydration of
particle sizes of samples synthesized at 80°C: (1, 3)
without surfactant AF-12, (2, 4) with surfactant AF-12; the
values are found by equations from [14] and [15], respectively.
Fig. 5. Density vs. change in temperature of dehydration of
samples obtained: (1) with surfactant AF-12 and (2) without
surfactant AF-12.
known equation connecting these values [14 ]. The
calculation results for the samples obtained with
surfactant AF-12 and without it, at different annealing
temperatures, are shown in Fig. 6, curves 1 and 2. As
can be seen, in both cases after the synthesis of nickel
hydroxide the particles with a diameter of about 10 nm
are formed. Analysis of changes in particle size during
annealing shows that at 230°C their smallest size is
reached, equal 4–5 nm, and at further increase in
temperature the enlargement of their size is observed.
No significant difference occurs between the sizes of
particles obtained with and without the addition of
surfactant AF-12 at annealing to ~400°C. Figure 6
demonstrates the changes in the crystallite size
obtained with the Scherrer equation [15]. As follows
from the results obtained, the particle sizes exeed 2–
3 times the size of the crystallites. This means that
each particle contains on the average 10 to 20
crystallites. The established facts indicate that the
coalescence of the crystallites occurs at their interface,
where the presence of the AF-12 surfactant is prac-
tically excluded.
From the figure it also follows that above 400°C the
NiO particle size in a system with AF-12 is less than in
the system without it, and in the final product annealed
in the 600–700°C the particle size is by 30–50%
smaller. Figure 7 shows photographs of nickel oxide
after annealing at 700°C confirming the accuracy of
the values of particle sizes found above by indirect
methods.
Thus, it is found that the formation of nano-
structured powders of nickel oxide by sol–gel method
from nitrate solutions by precipitation of nickel
(a)
(b)
100 nm
100 nm
Fig. 7. Photomicrographs of the particles of nickel oxide dehydrated at 700°C obtained using scanning electron microscopy:
(a) synthesized with surfactant AF-12 and (b) synthesized without surfactant.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 80 No. 6 2010

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BAKOVETS et al.
hydroxide with a solution of alkali in the presence of
AF-12 can reduce the size of particles 1.5-fold
compared with the particles obtained without it. Effect
of low molecular weight alcohols, for example,
isopropanol, on the particle size stops at the stage of
gelation due to the high vapor pressure of these
substances and, consequently, due to their evaporation
from the system. To reduce the rate of accretion of the
nickel oxide nanosize particles at temperatures above
400°C, it is useful to add into the system a high-
molecular surfactant of AF-12 type, which has low
vapor pressure and high temperature of decomposition
into volatile products. Reducing the rate of accretion of
particles in these systems is probably the result of the
blocking the surface with organic tar or graphite,
which eventually burn out.
A similar procedure of investigation was used at the
introduction to the system of a surfactant with a long
hydrocarbon chain. We used alkylphenyl ether of
polyethylene oxide with the number of ethoxy groups
12 (AF-12), which is chemically stable in alkaline and
acidic media [17]. The surfactant was added dropwise
in equal amounts in the initial nitrate and precipitant
solutions. The concentration of AF-12 in the reaction
medium was about 5.5×10–4 mol l–1, which cor-
responded to the critical concentration of micelle
formation. However, at subsequent washing of the
precipitated product an abundant water foaming was
observed and washing out of AF-12 till the final stage
of purification.
Yield of the final product with respect to nickel was
100% for both processes. Samples of nickel hydroxide
obtained after drying at 50°C were subjected to
gravimetric analysis with thermal treatment in air at
200, 230, 250, 300, 400, 500, 600, and 700°C, for 2 h
at each temperature. The phase composition of samples
(X-ray phase analysis) was studied on a DRON-3M
instrument, radiation CuKα, λ = 1.54178 Å. Thermal
analysis of the processes of dehydration of products
and transformation of AF-12 was performed using a
NETZSCH micro thermal balance TG 209 F1 (heating
rate 10 deg min–1, flow of inert gas 70 ml min–1 (He or
a mixture of Ar/O2 = 4/1). The specific surface of
samples Ssp was determined on an adsorption analyzer
Sorbtometr-M with a relative standard deviation of
±5%. The density of samples was determined by
pycnometric method with an uncertainty of ±(0.2–
0.5%), as a working fluid was used bidistilled water.
The morphology of nickel oxide particles was studied
with a scanning electron microscope EX-23000 BU.
EXPERIMENTAL
We used Ni (NO3)2·6H2O of analytical grade,
NaOH of analytical grade, isopropyl alcohol of
qualification OP-1 high purity. Solutions of nickel
nitrate of concentration ~0.25 M and sodium hyd-
roxide of 1.0 M, 350 ml each (to a solution of nickel
nitrate was added 50 ml of isopropanol) were placed in
different vessels and introduced into the reactor
containing 10 ml of hot solution of NaOH at a rate of
about 100 drops per minute at continuous stirring with
a mechanical stirrer within 1.5–2.0 h. Amount of the
added isopropanol was 0.06 g per one cm3 of solution,
which corresponded to the number of the alcohol
molecules 1017 per 1 cm2 of the surface of formed
nanostructured nickel hydroxide, as determined by the
results of the synthesis. It is known that for the
formation of an adsorbed monolayer it is enough to
have ~1014 molecules. After deposition of the pre-
cipitate, the stirring was continued for 30 min. The
temperature of deposition of nickel hydroxide was 80°C,
which was maintained during the experiment using a
water bath. The precipitated hydroxide was washed on
a Büchner funnel with a large amounts of water (~ 2 l)
to a neutral medium and dried at 50°C to a constant
weight, then it was ground in an agate mortar. The
achievement of permanent weight indicated that free
water and not chemisorbed isopropyl alcohol were
evaporated from the system. By analogy with the
known facts of the stability of chemisorbed monolayer
of alcohol on the surface of some transition metal
oxides (Cr, Fe, Mo) [16], in this case the existence of
an adsorbed monolayer of alcohol on the surface of
nickel hydroxide was accepted after washing with
water at room temperature and drying at 50°C.
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