Effect of synthesis conditions on the performance of weakly agglomerated nanocrystalline NiO

Article abstract of DOI:10.1016/j.jallcom.2006.02.064

Weakly agglomerated nanocrystalline NiO was synthesized by a homogeneous precipitation method in the presence of urea as the precipitation agent. The reaction mechanism about the hydrolyzation of urea, the precipitate process of the as-synthesized precursor, and the decomposition of the precursor during heating treatment was discussed. The effects of the ions concentration and the calcinations temperature on the powder properties were investigated. The as-synthesized precursor was characterized as hydrated basic carbonate by IR spectrum and TG analysis. The results revealed that crystallization started at about 350 °C, and the nanoparticles with the average crystallite size of 5 nm were obtained. With increasing the molar ratio of urea to Ni²⁺ ion, the crystalline size showed a slight increase, and then decrease, however the pH and the yield indicated a pronounced increase. During heating, an enhanced crystallite growth was observed.

Full text of DOI:10.1016/j.jallcom.2006.02.064

Journal of Alloys and Compounds 427 (2007) 251–255
Effect of synthesis conditions on the performance of weakly
agglomerated nanocrystalline NiO
a
a
b
a
a
¨
Xianshuang Xin , Zhe Lu , Baibin Zhou , Xiqiang Huang , Ruibin Zhu ,
Xueqing Shaa, Yaohui Zhanga, Wenhui Sua,c,d,∗
a Center for Condensed Matter Science and Technology, Harbin Institute of Technology, Harbin 150001, PR China
b Department of Chemistry, Harbin Normal University, Harbin 150080, PR China
c Department of Condensed Matter Physics, Jilin University, Changchun 130023, PR China
d International Center for Materials Physics, Academia Sinica, Shenyang 110015, PR China
Received 13 January 2006; received in revised form 26 February 2006; accepted 28 February 2006
Available online 5 April 2006
Abstract
Weakly agglomerated nanocrystalline NiO was synthesized by a homogeneous precipitation method in the presence of urea as the precipitation
agent. The reaction mechanism about the hydrolyzation of urea, the precipitate process of the as-synthesized precursor, and the decomposition
of the precursor during heating treatment was discussed. The effects of the ions concentration and the calcinations temperature on the powder
properties were investigated. The as-synthesized precursor was characterized as hydrated basic carbonate by IR spectrum and TG analysis. The
results revealed that crystallization started at about 350 ◦C, and the nanoparticles with the average crystallite size of 5 nm were obtained. With
increasing the molar ratio of urea to Ni2+ ion, the crystalline size showed a slight increase, and then decrease, however the pH and the yield indicated
a pronounced increase. During heating, an enhanced crystallite growth was observed.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Nanostructured materials; Oxide materials; Chemical synthesis; Characterization
1. Introduction
Ultra fine NiO particles with a good dispersion are desirable
for many applications such as in the manufacture of elec-
trochromic films, magnetic materials, p-type transparent con-
ducting films, gas sensors, catalyst, alkaline batteries cathode
and solid oxide fuel cells (SOFCs) anode [8–12]. Many methods
have been used to synthesize NiO nanoparticles [13], nanorods
[14], nanoribbons [15] and nanowire [16]. To acquire better
applications about NiO, the synthesis conditions must be well
controlled to obtain ultra fine powders with a narrow particle size
distribution that enhance the performance of materials. Among
the diverse synthesis routes of the ultra fine powders, homoge-
neous precipitation method [17–19] is one of the economically
feasible processes to prepare monodisperse metal oxide parti-
cles of various shapes and sizes. If the precipitating ligand is
added directly by simply pouring one solution into another, then
there is little control of the chemistry reaction during precipi-
tation because of the large and inhomogeneous gradients in the
solution concentration. However, a better control of the chem-
ical and morphological characteristics can be achieved if the
precipitating ligand is generated simultaneously and uniformly
There has been an increasing interest in the applications of
nanomaterials during the last two decades. A reduction in par-
ticle size to the nanometer scale results in variously special
properties such as the quantum size effects, the high surface
area and the lower sintering temperature. Nanomaterials exhibit
significantly mechanical, electronic, magnetic, optical, thermal
and catalytic properties in comparison with their bulk coun-
terparts [1–7]. However, practical methods are still needed for
synthesizing high-quality and ultra fine powders with required
characteristics in terms of their size, morphology, microstruc-
ture, composition purity, crystallizability, etc. These character-
istics significantly affect the later-stage processing and sintering
properties, and eventually determine the microstructure and per-
formance of the final ceramic materials.
∗
Corresponding author. Tel.: +86 451 86418570; fax: +86 451 86412828.
E-mail addresses: xinxianshuang@sohu.com (X. Xin),
suwenhui@hit.edu.cn (W. Su).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2006.02.064

252
X. Xin et al. / Journal of Alloys and Compounds 427 (2007) 251–255
throughout the solution, i.e. a homogeneous precipitation pro-
cess.
The purpose of this study is to report the synthesis of NiO
nanoparticles by the homogeneous precipitation process in the
presence of urea. The reaction mechanism was discussed about
the process of the urea decomposition and the transformation
of the precipitate to nickel oxide under the calcinations con-
dition. The effects of various synthesis parameters such as the
concentration of ions and the calcinations temperature, on the
powder properties of morphology, crystallite size and degree of
agglomeration, were investigated.
2. Experimental
2.1. Preparation of NiO nanoparticles
Fig. 1. FT-IR spectra of the as-synthesized NiO precursor (a), and the calcined
powders at 350 ◦C (b) and 600 ◦C (c).
Ni(NO3)2·6H2O (99.0%) was dissolved into deionized water and mixed with
a urea (99.0%) solution, and then stirred uniformly. The stoichiometric solutions
were transferred to a Teflon cup, and then put into a stainless steel autoclave.
The autoclave was heated to 85 ◦C for 1 h to form a kind of light green sediment.
The precipitated powders were filtered, washed with deionized water to remove
the soluble ions and ethanol to reduce agglomeration, and dried in an oven at
85 ◦C for 2 h in air, finally calcined at different temperatures for 2 h.
banding vibration of NiO. The weak band near 3400 cm−1 and
the bands in the region of 831–1384 cm−1(Fig. 1b and c) are due
to the fact that ultra fine powers tend to physically absorb H2O
and CO2.
2.2. Characterization
3.2. Chemical considerations and TG analysis
Infrared (IR) spectra were obtained with an IFS 66 v/S Fourier transform
infrared (FT-IR) spectrometer. Samples were prepared by mixing the powders
withpotassiumbromide(1:300byweightratio)inasteeldie. Thermogravimetric
analysis (TGA) was measured between room temperature and 800 ◦C (heating
rate 10 ◦C min−1) by a TA SDT2960 (operated in air, flow rate 100 ml min−1).
The crystalline structure was identified by powder X-ray diffractometer
(Bede Scientific D1, operated at 40 kV, 45 mA), employing Cu K␣ radiation
In this study, heating the solutions of nickel salts in the pres-
ence of urea forms the precursor precipitate by the homogeneous
precipitation method. Urea produces ammonium and cyanate
ions:
k
H2N − CO − NH2−→ NH3 + HOCN ⇔ NH4+ + OCN−
˚
(λ = 1.5418 A). The average crystalline size was estimated according to the
Scherrer equation [20]. The grain size and morphology of the samples were
taken by transmission electron microscopy (TEM JEOL JEM-1200EX working
at 100 kV).
(1)
The rates of urea decomposition at elevated temperatures
(k = 3.3 × 10−6 and 1.1 × 10−5 s−1 at 80 and 90 ◦C, respec-
tively) are quite slow [17].
3. Results and discussion
In an acidic solution, cyanate ions react rapidly according to
+
OCN− + 2H+ + H2O → CO2 + NH4
(2)
3.1. FT-IR study
whereas, in neutral or basic solutions, carbonate and ammonium
ions or ammonia are formed
The as-synthesized precursor and the powders calcined at dif-
ferent temperatures were investigated by IR spectroscopy. The
IR spectrum of the as-synthesized precursor (Fig. 1a) exhibits
extensive absorption peaks. The broad absorption band centered
at 3400 cm−1 is attributable to the band O–H stretching vibra-
tions, and the band near 1650 cm−1 (H–O–H bending mode)
provides the evidence for the water of hydration in the structure.
The bands at 831, 912, 1157 and 1384 cm−1 are primarily due
to the banding vibration of ionic CO32−. The IR spectrum of the
as-synthesized precursor is characteristic of hydrated basic car-
bonates [17]. It is interesting to note that the bands near both 640
and 2250 cm−1 confirm the presence of the –NCO species pro-
duced by the urea decomposition. After the sample was heated to
350 ◦C (Fig. 1b), the broad OH− and CO32− bands are reduced,
+
OCN− + 2H2O → CO32− + NH4
(3)
(4)
OCN− + OH− + H2O → CO32− + NH3
The presence of metal salts can accelerate urea decomposi-
tion. The nickel ion, Ni2+, is slightly hydrolyzed in water. The
hydrated Ni2+ ions can form complexes with H2O molecules or
OH− ions
[Ni(H2O)n]2+ + H2O → [Ni(OH)(H2O)n−1]+ + H3O+ (5)
With increasing the number of nickel ions, the equilibrium shifts
to the right. The precipitation of the basic carbonates proceeds
then according to the overall mass balance equation:
2−
which implies a cleavage of the hydroxyl and CO3 ligands,
and the band of –NCO disappears. After the sample was heated
2−
2[Ni(OH)(H2O)n−1]+ + CO3
to 600 ◦C (Fig. 1c), the bands of CO3 are further reduced.
2−
The strong band at 440 cm−1 is observed, corresponding to the
→ [Ni2(CO3)(OH)2]·xH2O ↓ +(2n − x − 2)H2O
(6)

X. Xin et al. / Journal of Alloys and Compounds 427 (2007) 251–255
253
Table 1
Effects of urea and Ni2+ concentrations on the pH of the solutions, the average
crystalline size, and the yields (the samples calcined at 350 ◦C)
[Urea]
(mol dm−3
[Ni2+
]
[Urea]/[Ni2+
]
pH
Crystalline
size (nm)
Yield
(%)
)
(mol dm−3
)
0.4
0.8
1.6
3.2
3.2
0.2
0.2
0.2
0.2
0.1
2
4
8
16
32
6.4
7.0
7.2
8.0
8.5
6.8
6.7
6.3
7.7
5.8
41.8
45.0
69.2
88.4
84.4
of all the samples. The average crystallite sizes were calculated
from the full-width at half maximum (FWHM) of the reflections
using the Scherrer equation:
Fig. 2. TG curves for the as-synthesized NiO precursor.
0.89λ
D =
(8)
Thermogravimetric (TG) analysis was conducted for the as-
synthesized precursor to examine the conversion process during
calcinations. Fig. 2 shows the thermal decomposition result of
the precursor precipitate from room temperature to 800 ◦C in
air atmosphere. The weight loss (24.0%) before 200 ◦C may be
attributed to the thermal dehydration and the evaporation of the
physically absorbed matter including absorbed water. There is a
weight loss of 29.2% between 200 and 350 ◦C, which is very
close to the theoretically stoichiometric weight loss, 29.3%,
calculated from reaction (7). This can be connected with the
decomposition of nickel basic carbonates, corroborating the
results of the IR spectrum about the as-synthesized precursor
being the hydrated basic carbonates. So the following decompo-
sition reaction can be deduced from the weight losses evaluated
from the TG curve:
β cos θ
where D is the average crystalline size, β the full-width at half-
height of the peak, λ the wavelength of X-ray and θ is the
diffraction angle of the peak.
Table 1 indicates that at a constant concentration of Ni2+, with
increasing the urea concentration, the average crystalline size
slightly decreases and then increases. However, at a constant
concentration of urea, compared with the molar ratios of 16, the
solution of rather high molar ratios of [Urea]/[Ni2+] (∼32) indi-
cate the minimum of the crystalline size. This result suggests the
significance of the molar ratio of [Urea]/[Ni2+] on the generation
of the resulting nanoparticles. Fig. 4 shows the TEM micrograph
of the as-synthesized precursor (prepared from the solutions
of the molar ratios [Urea]/[Ni2+] = 32). The micrograph shows
◦
◦
[Ni2(CO3)(OH) ] · xH2O + absorbed matter−24.0%−(3→5−200 C)Ni2(CO3)(OH)
−→
−29.2%(200−350 C)2NiO + CO2 ↑ +H2O ↑
(7)
2
2
3.3. Effect of the concentration
similarity to the other as-synthesized precursor samples from
the different molar ratios. Namely there are no regular shape and
size, and there seems to be a flocculent structure observed of all
the samples. As analyzed from Eq. (1) and the IR result, OCN−
or HNCO species exists at the as-synthesized precursor. The
absorbed matter as Eq. (7) may include not only the absorbed
water, but also the species of HNCO or OCN− which is easy
to form a complex compound to Ni2+, so the organic network
whose structure is flocculent may be formed.
Fig. 3 shows the XRD patterns of NiO samples (calcined
at 350 ◦C) synthesized from the different molar ratios of urea
to Ni2+ ion, [Urea]/[Ni2+]. Good crystalline states are observed
The precipitation process requires, first of all, the formation
of stable nuclei, after which particle growth can take place. The
controlled release of CO3 and OH− ions by urea decompo-
2−
sition causes precipitation once the critical supersaturation in
terms of reaction components is achieved. At lower urea con-
tents ([Urea]/[Ni2+] < 8) the precipitation process is slow, or
even incomplete, as shown by the lower pH from the mother
liquor of the corresponding precipitation (Table 1). Under such
conditions, small crystalline sizes are initially formed, and
the low conversion ratios appear. The yields increase with
increasing the urea concentration. At the larger urea contents
([Urea]/[Ni2+] ∼ 16) the process is faster, resulting in large parti-
cles at the same reaction time. However, the urea itself appears to
affect the number of original nuclei, and the number increases in
Fig. 3. XRD patterns of nanocrystalline NiO (calcined at 350 ◦C) synthesized
from the different molar ratios of urea to Ni2+ ion: (a) 2, (b) 4, (c) 8, (d) 16 and
(e) 32.

254
X. Xin et al. / Journal of Alloys and Compounds 427 (2007) 251–255
Fig. 5. XRD patterns of the as-synthesized NiO precursor and the powders
calcined at different temperatures.
powders were calcined at 300 ◦C, indicating that the reaction as
Eq. (7) starts but it is not completed to form good crystalline
state. A well-crystallized state is observed at 350 ◦C. During
heating, a simultaneous growing and narrowing of the peaks
is observed due to the enhanced crystalline growth. The crys-
talline growth is the process that the average crystalline size
of the nanomaterial increases continuously during heating treat-
ment in order to minimize interfacial surface energy [21]. All the
diffraction peaks can be perfectly indexed to the cubic structure
NiO.
The evolution of the average crystallite sizes (Fig. 6) was
investigated during heating treatment. The results show that the
average crystalline size is about 5 nm at 350 ◦C. The crystalline
size increases with increasing the calcinations temperature, and
increases to an average crystalline size of 38 nm at 650 ◦C.
Smaller crystalline sizes result from the powders synthesized in
the presence of the concentrated precipitation agent solutions.
The crystalline sizes of NiO are affected, in fact, by the nucle-
ation rate, which is in turn proportional to the concentration of
the precipitation agent. Thus, the higher the precipitation agent
concentration reaches, the higher the nucleation rate will be, and
then the smaller particle sizes result.
Fig. 4. TEM micrograph of the as-synthesized NiO precursor.
the presence of a still larger excess of urea ([Urea]/[Ni2+] ∼ 32).
This process gives a single burst of nuclei with consequent dis-
persions of narrow size distribution, and represents a higher
nuclei concentration, resulting again in smaller final crystalline
size. During a homogenous precipitation only a short time inter-
val passes during which the concentration is high enough to
nucleate particles. Further material precipitates by enlarging the
existing nuclei. This works because the precipitation agent will
be produced only slowly by the precursor, in this case urea.
Increasing the urea concentration protracts the nucleation inter-
val. Much more particles will be nucleated which finally will
have a smaller grain size, despite of the higher yield.
The reaction temperature of ∼85 ◦C, which was found to be
critical, is determined by the necessity not only to achieve a
single burst of nuclei but also to control the uniform growth
of particles as studied in literature [17]. At the reaction tem-
peratures >100 ◦C, the rate of the urea decomposition is rather
fast so as to produce broad size distributions. At temperatures
<70 ◦C the urea decomposition is exceedingly slow and almost
no precipitation occurs.
3.4. Effect of the calcinations temperature
The XRD patterns of the as-synthesized precursor (prepared
from the solutions of the molar ratios [Urea]/[Ni2+] = 32), as
well as of the nickel oxide powders produced by heating the
precursor at different temperatures are shown in Fig. 5. The
as-synthesized precursor and the powders calcined at 200 ◦C
show no detectable X-ray diffraction peak, indicating the amor-
phous nature of the powders. The weak peaks appear after the
Fig. 6. Effectofthecalcinationstemperatureonthecrystallinesizesofnanocrys-
talline NiO.

X. Xin et al. / Journal of Alloys and Compounds 427 (2007) 251–255
255
Fig. 7. TEM micrographs (the magnificated micrograph of the corresponding sample as an inset) of nanocrystalline NiO at different calcined temperatures.
3.5. TEM results
represented the minimum of the crystalline size, indicating that
the nucleation rate increases more rapidly than the crystalline
growth rate does.
TEM analysis provides information on the size and mor-
phology of primary particles and their state of agglomer-
ation. The particles obtained by homogeneous precipitation
([Urea]/[Ni2+] = 32) clearly exhibit well-crystallized nanocrys-
tallineofhomogeneousandweaklyagglomeratedassemblywith
the primary particle size of about 5 nm (Fig. 6 350 ◦C). This is in
good agreement with the powder X-ray diffraction result. The
results also suggest that the crystallites have nearly spherical
morphology.
Acknowledgement
The authors gratefully acknowledge financial supports from
the Ministry of Science and Technology of China under contract
NO. 2001AA323090.
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than the nucleation rate does. Therefore, the average size of
NiO nanoparticles increases with the increase of the calcinations
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of the calcinations temperature, a simultaneous growing and nar-
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