Synthesis of nanostructured nickel oxide

Article abstract of DOI:10.1134/S1070363209030049

The process of formation of nickel oxide nanostructured powders by annealing nickel hydroxide in the temperature range 200-700°C was studied. Nickel hydroxide was prepared by precipitation with alkali from nickel nitrate solutions. The annealing process w

Full text of DOI:10.1134/S1070363209030049

ISSN 1070-3632, Russian Journal of General Chemistry, 2009, Vol. 79, No. 3, pp. 356–361. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © V.V. Bakovets, L.N. Trushnikova, I.V. Korol’kov, V.V. Sokolov, I.P. Dolgovesova, T.D. Pivovarova, 2009, published in Zhurnal
Obshchei Khimii, 2009, Vol. 79, No. 3, pp. 366–371.
Synthesis of Nanostructured Nickel Oxide
V. V. Bakovets, L. N. Trushnikova, I. V. Korol’kov,
V. V. Sokolov, I. P. Dolgovesova, and T. D. Pivovarova
Nikolaev Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences,
pr. Akad. Lavrentyeva 3, Novosibirsk, 630090 Russia
e-mail:becambе@che.nsk.su
Received July 18, 2008
Abstract―The process of formation of nickel oxide nanostructured powders by annealing nickel hydroxide in
the temperature range 200–700°C was studied. Nickel hydroxide was prepared by precipitation with alkali from
nickel nitrate solutions. The annealing process was shown to be multi-step. In the first stage the hydrogel Ni
(
2 2
OH) ·nН О decomposes and partial dehydration of hydroxide occurs. Sizes of the formed particles decrease.
At the temperatures above 230°C, further hydrogel decomposition and coalescence of NiO particles proceed. In
view of the structural rearrangement of powder at the high temperatures 400–700°C , dehydration process is
monitored by the decrease of NiO particles surface area at their coalescence. According to the change in the
dehydration mechanism, the hierarchically nanostructured material forms, whose particle sizes are in the range
4
–5, 9–12, and 18–40 nm.
DOI: 10.1134/S1070363209030049
The nanostructured particles of metal oxides in pure
diethylamine [9] are used as precipitating agents.
Alcohols, in which metal salts and organic precipitants
are readily soluble, can be used as solvents. Organic
precipitants undergo hydrolysis with water contained
in the initial metal salt. This allows minimizing the use
of water, which is responsible for particles aggrega-
tion. A number of methods developed for luminophors
preparation, e.g., Y O :Eu, in nanocrystalline state can
be applied to obtain the nanostructured oxides of
others metals. The procedures include the spray-
pyrolysis [10] and sol–gel methods [7], allowing to
obtain spherical particles. Sometimes the use of the
latter procedure helps to avoid the particle aggregation.
The formation of superdispersed metal oxides through
hydroxides is characterized by two processes:
dehydration followed by transformation of hydroxide
into oxide and by sintering. However, a consistent
study of the correlation between these processes and
the oxide dispersity was not performed.
2+
state as well as doped with various cations (Ca ,
2+
3+
4+
Mg , Y , Zr , etc.) can be widely used, in particular,
in oxide combustion cells which are suggested to be
prospective energy sources of XXI century since they
are of high efficiency and, besides, are environ-
mentally friendly. From the multitude of oxides, NiO
can be singled out since it is a very interesting material
by its structure as well as by the electroconductivity
mechanism governed by this structure. The con-
ductivity of the nanostructured NiO was found to
increase by 6-8 orders of magnitude in comparison
with the monocrystals of this compound [1].
2
3
Nickel oxide, among the other oxides, is used in
nanocomposites, which are applied as the material for
gas sensors. They improve selectivity of high energy-
gap semiconductor oxides (SnO , ZnO, In O , WO ),
2
2
3
3
forming solid solutions on the matrix of SnO , in
2
particular, due to the close value of their ionic radii
2+
4+
The purpose of this work is the study of dehydra-
tion mechanism of nickel hydroxide and its connection
with the process of formation of the nanostructured
NiO powder obtained by nickel hydroxide precipita-
tion from nickel nitrate solutions with sodium
hydroxide followed by annealing the washed and dried
precipitate. This method is characterized by simplicity,
(
0.70 Å for Ni and 0.69 Å for Sn ) [2].
A great number of the methods for obtaining the
nanostructured oxide materials are known. Ammonum
and alkali metals hydroxides or carbonates or
hydrazine hydrate [3–5], urea [6–8] and others organic
compounds such as hexamethylene tetramine [3] and
356

SYNTHESIS OF NANOSTRUCTURED NICKEL OXIDE
357
1
00
9
9
9
9
8
8
8
8
8
7
7
6
4
2
0
8
6
4
2
0
8
6
3
2
0.5
.0
9
9
8
6
0
–
–
0.5
1.0
94
9
9
8
2
0
8
–1.5
–2.0
–2.5
86
84
82
80
78
1
–
–
3.0
3.5
2
00 250 300 350 400 450 500 T, °C
7
7
6
4
0
100 200 300 400 500 600 700 800 T, °C
Fig. 1. Thermal analysis results: (1) TG, %; (2) DTG, %;
and (3) DTA.
Fig. 2. Dependence of the sample weight (wt %) on annealing
temperature
but its essential drawback consists in agglomeration of
the obtained superdispersed particles. The precipitation
was carried out in the presence of isopropanol for the
particles disaggregation.
the sample dehydration. These facts attest that me-
chanism of the sample dehydration process has changed.
Evidently, up to 230°C the gel decomposition occurs,
and over this temperature Ni(OH) begins to transform
2
into NiO. Actually, DT curve 3 (Fig. 1) indicates that
up to 230°C the thermal effects are low that is charac-
teristic of the gel decomposition process at dehydration
The obtained nanostructured NiO powders were
studied by various methods (see Exprimental). The
temperature dependences of specific surface area of
[
12]. From 230 to 300°C the strong thermal effect is
the obtained powders (S ), of the X-ray diffraction
sp
observed that is characteristic of Ni(OH) dehydration
2
reflex broadening (2θ), and of the samples density (ρ)
shown in figures describe qualitatively such depen-
dences for all the samples examined.
to NiO [11].
Also, X-ray phase analysis data (Fig. 3) show that
during the process of heating at 200°C the phase NiO
According to derivatogram (Fig. 1), at 230°C the
calculated samples composition corresponds to NiO·
already appears along with the Ni(OH) phase. The
2
scope of the data on derivatography, gravimetric and
phase analyses is at variance with the published data
(
0.94±0.02)·H O, i.e. at this temperature the samples
2
are chemically close to Ni(OH) . The calculation of the
2
on the temperature of Ni(OH) → NiO transformation
initial samples composition with respect to dry NiO
residue gives excessive water content over stoichio-
2
(
230°C ) [13]. Perhaps, this divergence is due to the
dehydration features of nanostructured disperse
systems, to which the considered system belongs. For
metry of Ni(OH) by (0.45±0.15) mol. Since the
2
samples were dried after synthesis at 50°C for 15–20 h,
the found values of excess water content can be
assigned to the gel formation in accordance with the
common rule of transition metal hydroxides precipita-
tion [11]. The presented derivatogram was obtained at
instance, the stoichiometry of Ni(OH) composition
2
found by derivatograms at 230°C can be also related to
a two-phase system (xerogel) containing NiO and
hydrogel on the basis of Ni(OH) , what may be
2
–
1
presented as NiO + Ni(OH) · H O.
gradual heating with the rate of 10 deg min . Note that
in the samples annealed at 200, 230, 300, 400, 500,
2
2
The results of the study of the specific surface area
Ssp of disperse samples obtained in the course of the
gravimetric analysis showed (Fig. 4) that in the range
of annealing temperatures 50–230°Cthe specific surface
6
00, and 700°C for 2 h the water content in disperse
phase at each annealing temperature (gravimetric
analysis) is essentially less (Fig. 2) than for the cor-
responding temperatures in the derivatograms.
2
–1
increases to 250 ± 12 m g (curve 1, piece a–b). At
the temperature increase above 230–250°C (piece b–
d), the specific surface of samples sharply decreases.
However, at 300°C an inflection point is observed on
Temperature 230°C was found to correspond to the
area where the weight loss rate increases (Fig. 1) and
to the point of alteration of the curvature sign on the
weight loss curve (gravimetric analysis, Fig. 2) upon
the descending curve S . Hence this branch of the
sp
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 79 No. 3 2009

3
58
BAKOVETS et al.
2
600
–1
1
1
8
S, m g
Δ2θ, deg
0
3
00
2.5
200
0
Ni(OH)
2
(111)
b
250
2
1
.0
.5
000
a
5
0
c
2
1
1
00
50
00
50
0
2
00
30
50
00
00
00
00
00
NiO (111)
2
2
1
3
1.0
4
5
0
.5
6
2
d
7
0
5 10 15 20 25 30 35 40
0.0
800 T, °C
0
200
400
600
2
θ, deg
Fig. 4. The specific surface of (1) samples and (2)
broadening of peak 2θ, deg, as a function of the annealing
temperature.
Fig. 3. The variation in the diffraction pattern with
temperature.
curve is divided into two characteristic pieces: b–c and
c–d. Using the values of the found picnometric density
ρ of the samples (Fig. 5) and known equation of its
connection with particle size D [14], the average
particles size constituting the powder samples was
determined (Fig. 6, curves 2, 3).
The variation in width Δ2θ on the half-height
(coherent-scattering region) of reflexes from the plane
(111) correlates well with the features of specific
surface variation (Fig. 4, curve 2). In so doing the
width of α-Al O reference lines was taken into considera-
tion in the close angle range 2θ. The variation in
particle size is found from the change in X-ray reflexes
width using the known Sherrer equation (Fig. 6, curve
2
3
D = 6/ρSsp
.
(1)
1
). As seen from this figure, particle sizes are some-
As follows from Fig. 5, the density curve is also
changed at the point 230°C.
what larger than the values obtained with Eq. (1).
–3
ρ, g cm
D, nm
7
6
6
5
5
4
4
3
3
.0
d
50
.5
.0
.5
.0
.5
.0
.5
.0
d
4
3
2
1
0
0
0
0
0
1
2 3
a
c
b
b
0
200
400
600
800 T, °C
a
Fig. 6. Dependence of sample size on the annealing
temperature. (1) Sample obtained by precipitating at 60ºC,
particle size from X-ray data by Eq. (2); (2) sample
obtained by precipitating at 60ºC, particle size calculated
by Eq. (1); (3) sample obtained by precipitating at 80ºC,
particle size calculated by Eq. (1).
0
200
400
600
800 T, °C
Fig. 5. Dependence of sample density on the annealing
temperature.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 79 No. 3 2009

SYNTHESIS OF NANOSTRUCTURED NICKEL OXIDE
359
D = l/(βcos θ),
(2),
comes saturated, suggesting increase in the number of
contacts between particles of the samples annealed at
where λ is X-ray emission wavelength; β is line
broadening, rad; θ is the diffraction reflex position [15]
400°C [4]. Moreover, change in the adsorption–
desorption hysteresis character shows the increase in
mesoporosity (pore size 2–50 nm) with the rise in
temperature to 400°C and its disappearance at high
temperatures (curve 3). In accordance with the pore
size calculation from the adsorption isotherm [11] for
the samples annealed at 230°C, the pore size distribu-
tion maximum corresponds to 1.5 nm. There are two (2
and 6.5 nm) and three (1, 6, and 18 nm) pore size
distribution maxima for the samples annealed at 400°C
and 700°C respectively. In the case of spherical
approximation of the particle form their characteristic
diameters calculated by Eq. (1) are equal to 4–5 (230°C),
In general, it can be emphasized that the particle
size on the а–b piece decreases to 4–5 nm, and on the
b–c–d piece it increases. This is due to hydrogel
decomposition on the а–b piece and to xerogel
formation. With the growing annealing temperature the
decrease and disappearance of the Ni(OH) phase on
2
the b–d piece (Fig. 4) and NiO particles aggregation
through coalescence occur. Note that the latter process
as well as the rate of weight loss changes in the range
of 300–400°C (see Figs. 1 and 4). The presence of this
characteristic temperature range was also revealed by
calculating the ratio between the differences of the
samples weight loss in adjacent temperature points and
the corresponding temperature differences (Fig. 7,
slopes of pieces of the dependence in Fig. 2). Variation
of DМ/DT in the temperature intervals correlates with
DTG curve (Fig. 1), the annealing temperature
9
–12 (400°C), and 39 nm (700°C). As seen, the pore
size is less than the particle size approximately by half.
The spherical form of particles was confirmed by the
atomic-force microscopy of NiO patterned films
(
Fig. 9). The films have apparently a hierarchical
structure character. The big ellipsoid conglomerates of
the size ~200 nm (Fig. 6а) consist of spherical particles
of 18–40 nm size (Fig. 2b). Particles of the size of a
few nanometers are not resolvable due to the
insufficiently good structure of the obtained films
surface. Formation of ellipsoid conglomerates is
apparently connected with the structuring effect of
substrate during the film deposition by Langmuir–
Blodgett method. We established that aggregation of
the nanostructured species into the bigger particles was
observed at any annealing temperatures. To eliminate
this phenomenon the surfactant species blocking the
dependences of specific surface area S and particle
sp
sizes D (Fig. 3). All these facts imply an additional
change in the samples dehydration mechanism in the
range of 300–400°C .
For revealing the reasons of this effect nitrogen
adsorption and desorption isotherms were measured on
samples obtained by annealing at 230, 400 and 700°C
(
Fig. 8). From the characteristic features of isotherms
obtained, we can conclude that at the high nitrogen
partial pressure p/p the adsorption isotherm 2 be-
0
–1
q, ml g
2
50
00
0
0
0
0
0
0
0
0
0
.40
.35
.30
.25
.20
.15
.10
.05
.00
b
2
150
00
1
1
2
5
0
0
c
3
a
d
–
0.05
0
.0
0.2
0.4
0.6
0.8
1.0
p/p
0
100 200 300 400 500 600 700 800
T, °C
0
Fig. 8. Nitrogen sorption-desorption isotherms for samples
obtained by precipitating at 60ºC, annealing temperature,
ºC: (1) 230, (2) 400, and (3) 700.
Fig. 7. Variation of DМ/DT value with increasing annealing
temperature.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 79 No. 3 2009

3
60
BAKOVETS et al.
(
a)
(b)
1
μm
2
00 nm
Fig. 9. Morphology of films by atomic-force microscopy at different expansion.
particle aggregation should be incorporated into the
system. The next report will be devoted to the study of
this phenomenon.
base component 99%), NaOH as a precipitating agent,
and chemically pure isopropanol. To obtain nickel
hydroxide, a solution with concentration ~0.25 М was
prepared as follows. In 150 ml of water 25.1 g of Ni
From the presented results the following sequence
can be assumed for the formation processes of the
nanostructured NiO powders from nickel hydroxide
obtained by precipitation with alkali from nitrate
solutions. In the range of annealing temperatures of
hydroxide 200–230°C the Ni(OH) ·1.3H O gel decom-
(
NO ) ·6H O was dissolved. This solution was diluted
3 2 2
with the same amount of water and 50 ml of
isopropanol was added to it. The precipitating agent
was prepared from 14.2 g of NaOH (twofold amount
over stoichiometry) and 350 ml of water. Both
solutions were placed into various vessels and added to
the heated precipitating agent solution (~10 ml) with
2
2
position occurs followed by the sample dehydration.
Xerogel formed contains NiO particles and residual
hydrogel. In the range of 230–300°C the final
transformation of residual hydrogel into NiO occurs;
therewith the particles coalesce, particles size
increases, and specific surface of the powder
decreases. At 300–400°C the rise of samples density
leads to the growing number of contacts between
particles and to increase in mesoporosity. At the
temperature above 400°C the rate of dehydration
undergoes changes. This is due to coalescence of the
remaining particles consisting of the NiO nucleus
covered with a hydrate layer [12]. In this case the
hydrated surface area decreases (effect of two particles
coalescence), and the rate of this process determines
the rate of the powder dehydration. As a result,
disperse NiO phase is formed with particle mean size
distribution with the maxima at 4–5, 9–12 and 18–40
nm, and their conglomerates.
–
1
rate of 100 drop min under the continuous stirring the
within 1.5–2.0 h. After precipitating, the stirring was
continued for 30 min. Temperature of nickel hydroxide
deposition was 60 and 80°C. The precipitates of hyd-
roxides were washed on a Buchner funnel with a large
water amount (~2 l) to neutral pH and dried at 50°C .
Some experiments were carried out as follows: version
1
at 60°C and version 2 at 80°. Study of dehydration
process mechanism shows that the dependences found
are qualitatively of the same type for all samples ob-
tained by both verions, though there are some
quantitative differences. These differences are shown
on the figures and are not of principal importance
when dehydration mechanism of the NiO nano-
structured particles is described.
The obtained nickel hydroxide samples dried at 50°C
were examined by thermal analysis method on a
Netzsch TG-209F1 device in a helium flow at a
–
1
heating rate of 10 deg min .
EXPERIMENTAL
The prepared samples were subjected to gravi-
metric analysis with thermal treatment at 200, 230,
250, 300, 400, 500, 600, and 700°C for 2 h under air.
In the work we used analytically pure grade nickel
nitrate hexahydrate Ni(NO ) ·6H O (content of the
3
2
2
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 79 No. 3 2009

SYNTHESIS OF NANOSTRUCTURED NICKEL OXIDE
361
The phase analysis was performed on a X8APEX
Bruker diffractometer (МоKα radiation, graphite
monochromator, CCD-detector with a resolution
of the film were obtained on an atomic-force micro-
scope HT-MDT Solver Pro.
1
024×1024 pixels, L = 50 mm) at room temperature.
REFERENCES
The samples were fine-dispersed powders deposited on
a glass stick (d = 0.2 mm) coated with epoxy resin. As
an external reference was used finely dispersed powder
1
2
. Biju, V. and Abdul Khadar, M., Mat. Res. Bull., 2001,
vol. 36, p. 21.
. Rumyantseva, M.N., Kovalenko, V.V., Gas’kov, А.М.,
and Pan’ye, T., Ros. Khim. Zh., 2007, vol. 50, no. 6, p. 61.
of α-Al O3 prepared similarly to the studied
2
compositions. The obtained powder patterns were
brought into three-dimensional type I(2θ) by means of
Fit2D program [16].
3. Chen Pei-Lin and Chen I-Wei, J. Am. Ceram. Soc., 1993,
vol. 76, p. 1577.
. Li, J.-G., Ikegami, T., Wang, Y., and Mori, T., J. Solid
State Chem., 2002, vol. 168, p. 52.
. Nakane, S., Tachi, T., Yoshinaka, M., Hirota K., and
Yamaguchi, O., J. Am. Ceram. Soc., 1997, vol. 80, p. 3221.
. Matijevic, E. and Hsu, W.P., J. Colloid and Interface
Sci., 1987, vol. 118, p. 506.
. Chu, X., Chung, W., and Schmidt, L.D., J. Am. Ceram.
Soc., 1993, vol. 76, p. 2115.
. Alken, B., Hsu, W.P., and Matijevic, E., J. Am. Ceram.
Soc., 1988, vol. 71, p. 845.
. Li, J.-G, Ikegami, T., Lee, J.-H., and Mori, T., Acta
Mater., 2001, vol. 49, p. 419.
4
5
6
7
8
9
The specific surface of sample was determined on a
specific surface adsorption analyser Sorbtometr-М
with relative standard divergence of ±5%. The sample
density was measured by picnometric method using
water as a reference liquid with relative error ±0.2–
0
.5%.
Nitrogen adsorption was carried out at –196°C and
desorption at –100°C . The adsorbed nitrogen amount
was determined by BET equation at relative pressure
p/p = 0.35. Specific surface values were calculated
0
1
0. Kang, Y.C, Park, S.B., Lenggoro, I.W., and Okuyama, K.,
from the adsorbed nitrogen amount taking into account
the specific surface value of the nitrogen molecule,
J. Mater. Res., 1999, vol. 14, no. 6, p. 2611.
2
11. Karnaukhov, A.P., Adsorbtsiya. Tekstura dispersnykh i
poristykh materialov (Adsorption. Texture of Disperse
and Porous Materials), Novosibirsk: Nauka, 1999, p. 468.
which is equal to 0.16 nm [11]. Moreover, the
nitrogen adsorption and desorption isotherms at –196°
C were obtained. They were used for analysis of the
sample porosity change due the temperature variation
at the thermal treatment of the initial Ni(OH)2.
1
2. Voyutskii, S.S., Kurs kolloidnoy khimii (Course in
Colloidal Chemistry), Moscow: Khimiya, 1964, p. 574.
1
3. Kratkaya khimicheskaya entsyklopediya (Concise
Chemical Encyclopedy), vol. 3, Мoscow: Sovetskaya
Entsyklopediya, 1964, p. 466.
4. Karagedov, G.R. and Lyakhov, N.Z., Khimiya v
interesakh ustojchivogo razvitiya (Chemistry in the
Interests of Sustainable Development), 1999, vol. 7, p. 229.
5. Kovba, L.M., Rentgenografiya v neorganicheskoj khimii
(Radiography in Inorganic Chemistry), Мoscow: Mosk.
Gos. Univ, 1991, p. 255.
16. Hammersley, A.P., Svensson, S.O., Hanfland, M.,
Fitch, A.N., and Hausermann, D., High Pressure
Research, 1996, vol. 14, p. 235.
For the study of the particle form and size, we
prepared the films of nanostructured NiO. The initial
sample powder was dispersed in toluene and then
treated in an ultrasonic disintegrator UD-11. The
prepared toluene dispersion was settled for some time
and afterwards it was transferred with a pipette to
produce an ultrafine suspension film on distilled water
surface by the modified Langmuir–Blodgett method
1
1
[
17]. Then the polished titanium plate was slowly
drawn from water through the densely packed film of
NiO particles. As a result, a film of NiO particles
forms on the surface this plate. The microphotographs
17. Sheludko, K., Kolloidnaya khimiya (Colloidal
Chemistry), Мoscow: Mir, 1984, p. 319.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 79 No. 3 2009