X-ray diffraction studies on the thermal decomposition mechanism of nickel hydroxide

Article abstract of DOI:10.1021/jp906578u

Nickel hydroxide samples were prepared by using sodium hydroxide and ammonium hydroxide as precipitating agents. The powder X-ray diffraction pattern shows that the degrees of crystallinity in these samples are quite different. The thermal decomposition m

Full text of DOI:10.1021/jp906578u

13014
J. Phys. Chem. B 2009, 113, 13014–13017
X-ray Diffraction Studies on the Thermal Decomposition Mechanism of Nickel Hydroxide
Thimmasandra Narayan Ramesh*
Department of Chemistry, Central College, Bangalore UniVersity, Bangalore 560 001, India
ReceiVed: July 12, 2009; ReVised Manuscript ReceiVed: August 15, 2009
Nickel hydroxide samples were prepared by using sodium hydroxide and ammonium hydroxide as precipitating
agents. The powder X-ray diffraction pattern shows that the degrees of crystallinity in these samples are
quite different. The thermal decomposition mechanism of these two nickel hydroxide samples has been
determined using powder X-ray diffraction and thermogravimetric analysis. We observe that the transformation
of nickel hydroxide to nickel oxide in the crystalline sample is via a two-phase mixture, whereas in a poorly
ordered sample, it is through a single phase. This indicates that the decomposition mechanism mainly depends
on the preparative conditions and the nature of the sample.
Introduction
hydroxide). Nickel hydroxide-2 was obtained by the addition
of nickel nitrate (1M, 50 mL) solution to a strong alkali, that
is, sodium hydroxide (2M, 100 mL) solution, and the precipitate
obtained was aged for 30 min in the mother liquor. The product
was designated as SH-nickel hydroxide (SH- represents sodium
hydroxide).The obtained slurries were filtered, washed with
distilled water, and dried at 65 °C until a constant weight was
attained.
Nickel hydroxide is most widely used as a positive electrode
material in all Ni-based alkaline secondary cells.1 It also finds
application as an active material in electrochromic devices,
supercapacitors, and as a precursor for catalysts.2-4 Divalent
metal hydroxides derive their structure from brucite, a mineral
form of Mg(OH)2.5 The structure of nickel hydroxide comprises
a stacking of hydroxyl layers having the composition [M(OH)2].
A large matrix of preparative conditions has been explored to
generate a wide range of nickel hydroxide samples, of which
the common procedure to prepare nickel hydroxide is a
precipitation method. The precipitation conditions, such as (a)
rate of addition, (b) pH at precipitation, (c) temperature of
precipitation, (d) choice of aging conditions, (e) choice of
appropriate salt and alkali, and (f) drying temperature, affect
the outcome of the product.6 The thermal decomposition of
nickel hydroxide to synthesize nickel oxide has been widely
studied.7-9 The role of preparative conditions on the thermal
decomposition and its influence on the changes on the powder
X-ray diffraction pattern have been reported.10,11 During the
thermal decomposition of nickel hydroxide, an endothermic peak
in the differential thermal analysis of nickel hydroxide has been
observed. This has been attributed to the release of water during
the decomposition of nickel hydroxide.12 To the best of our
knowledge, there are not many reports on the thermal decom-
position mechanistic studies of nickel hydroxide. The aim of
this article is to examine the evolution of nickel oxide from
nickel hydroxide during thermal treatment of the sample at
different temperatures using powder X-ray diffraction patterns.
We have prepared nickel hydroxide samples with different
crystallinity and investigated its role on the decomposition
mechanism.
The samples were characterized by powder X-ray diffraction
(PXRD) using a Bruker D-8 Advance X-ray powder diffracto-
meter (Cu KR source, λ )1.5418 Å). Data were collected at a
scan rate of 4° 2θ min-1 with 2θ steps of 0.05°. The
thermogravimetric studies was carried out using a Mettler
Toledo model 851e TG/SDTA system (heating rate ) 10°
min-1). The wet chemical analysis of the samples was carried
out following the procedure described elsewhere.13 The chemical
composition and the weight loss of these samples are given in
Table 1. Isothermal heating of nickel hydroxide samples was
carried out at different intervals of temperature (RT and 100,
120, 140, 160, 175, 200, 225, 250, 300, 325, and 400 °C) for
2 h each and then cooled to room temperature. The powder
X-ray diffraction patterns of these samples were measured, and
the crystallite size was estimated using the Scherrer formula.
In Tables 2 and 3 are given the values of crystallite size along
selective directions of nickel hydroxide and nickel oxide.
Results and Discussion
The choice of precipitating agents, such as sodium hydroxide
and nickel hydroxide, to prepare nickel hydroxide was made
from the information based on the kinetics and thermodynamics
that interplays and controls the product workup.6 Highly ordered
nickel hydroxide can be obtained directly when ammonia is used
as a precipitating agent. Ammonia is bound to form a weak
complex with transition-metal ions. Therefore, addition of metal
nitrate to ammonia results in the formation of a deep blue nickel
ammine complex that is not stable at room temperature for a
long duration and undergoes hydrolysis to generate highly
ordered nickel hydroxide. A strong alkali, such as sodium
hydroxide, tends to generate a highly disordered phase of nickel
hydroxide at room temperature. If we need to prepare highly
crystalline nickel hydroxide using a strong alkali, such as sodium
hydroxide, the sample should be made at high temperatures and
Experimental Section
Nickel hydroxide samples were prepared as follows.13,14
Nickel hydroxide-1 was prepared by the addition of nickel nitrate
solution (1M) to a weak base, such as ammonium hydroxide
(2M, 200 mL), at room temperature, and the precipitate formed
was aged in the mother liquor for 2 h. The product was
designated as AH-nickel hydroxide (AH- represents ammonium
* To whom correspondence should be addressed. E-mail:
adityaramesh77@yahoo.com.
10.1021/jp906578u CCC: $40.75  2009 American Chemical Society
Published on Web 09/08/2009

PXRD Studies on the Thermal Decomposition of Ni(OH)2
J. Phys. Chem. B, Vol. 113, No. 39, 2009 13015
TABLE 1: Wet Chemical Analysis of Nickel Hydroxide Samples
weight percentage
sample
Ni2+
OH-
H2O
total weight loss (%)a
approximate formula
AH-nickel hydroxide
SH-nickel hydroxide
61.97
57.78
35.6
33.3
1.7
7
19.4 (21.45)
26.47 (27)
Ni(OH)2 ·0.1H2O
Ni(OH)2 ·0.5H2O
a Values in the parentheses are based on the approximate formula.
TABLE 2
expectation, SH-nickel hydroxide should be highly disordered.
Among the disorders reported by us is (i) interstratification,
which involves a nonintegral repeat unit along the stacking
direction. In ordered ꢀ-Ni(OH)2, the composition of the first
coordination shell is Ni(OH)6, whereas in the interstratified
(a) crystallite size (Å) in the selective directions of AH-nickel oxide
temperature (°C) (001) (100) (101) (102) (110) (111)
RT
155
155
158
158
232
1204
2063
2166
1083
2888
361
275
274
322
623
246
246
288
235
313
707
310
308
860
741
430
477
314
183
81
100
225
240
250
motifs, it can be represented as
a combination of
[Ni(OH)6-x(NO3)x] or [Ni(OH)6-x(H2O)x]. Another disorder is
(ii) cation vacancies, in which case the composition changes to
[Ni1-x0x(OH)6-2x(H2O)2x]. SH-nickel hydroxide is a combination
of interstratification and cation vacancies and is, therefore,
expected to be the least stable phase.15 The factors that contribute
to the broadening of reflections in nickel hydroxide have been
investigated by us in our earlier report.16 DIFFaX code enables
us to simulate the PXRD patterns by incorporating structural
disorders into the lattice. On the basis of DIFFaX simulation
studies, we had shown that the broadening of non-(hk0)
reflections can arise due to the presence of structural disorder.
The powder X-ray diffraction pattern of SH-nickel hydroxide
is similar to that of the NH65 nickel hydroxide sample reported
in ref 15. This clearly indicates that the SH-nickel hydroxide is
a structurally disordered material consisting of interstratification,
cation vacancies, and stacking faults. AH-nickel hydroxide is
highly ordered, indicating that the sample has very small
percentages of these disorder components. We have shown in
our earlier report that highly ordered nickel hydroxide requires
a trace amount of stacking defects for structural stability.17
(b) crystallite size (Å) in the selective directions of AH-nickel oxide
temperature (°C)
(200)
70
78
196
125
(111)
65
73
205
1203
(220)
64
70
166
109
300
350
400
450
TABLE 3
(a) crystallite size (Å) in the selective directions of SH-nickel
hydroxide
temperature (°C) (001) (100) (101) (102) (110) (111)
RT
20
13
13
13
10
10
92
79
79
79
97
97
25
25
26
26
40
38
18
16
19
18
69
18
96
126
114
84
66
97
58
48
54
36
69
49
100
200
225
240
250
(b) crystallite size (Å) in the selective directions of SH-nickel oxide
The decomposition mechanism of magnesium hydroxide
based on the kinetics and the surface properties has been
investigated using X-ray diffraction and electron microscopic
studies.18,19 Thermal degradation studies based on electron
microscopy and X-ray diffraction studies on nickel hydroxysul-
phate have been investigated.20,21 The general scheme of
decomposition of nickel hydroxide is Ni(OH)2 f NiO + H2O.
The decomposition of nickel hydroxide has been extensively
studied using electron microscopy on R- and ꢀ-nickel hydroxide
samples by Figlarz and co-workers.22 However, the study of
crystallinity in relation to the thermal decomposition process is
reported elsewhere. They show that the decomposition temper-
ature decreases for the poorly crystalline sample.11 There is also
one report on the thermodynamical size effect on the decom-
position temperature of nickel hydroxide.10 A systematic
investigation of nickel hydroxide samples with different degrees
of crystallinity has not been investigated. We were, therefore,
interested in investigating the decomposition mechanism of these
two nickel hydroxide samples using X-ray diffraction.
temperature (°C)
(200)
18
18
22
26
(111)
17
19
21
23
(220)
48
50
50
51
262
290
350
450
subject to hydrothermal conditions. Hence, we chose these two
methods to prepare samples having different degrees of
crystallinity.
The PXRD pattern of nickel hydroxide samples obtained at
room temperature using ammonia and sodium hydroxide as
precipitating agents is shown in Figures 1 and 2, respectively.
The reflections in the powder X-ray diffraction patterns of these
samples match with the ꢀ-phase of nickel hydroxide.
We observe nonuniform broadening of reflections for SH-
nickel hydroxide in contrast to AH-nickel hydroxide. Addition
of nickel nitrate to sodium hydroxide solution results in the
precipitation of nickel hydroxide as the solubility product of
nickel hydroxide exceeds in strong alkaline solution. Nickel
hydroxide crystallizes in two different polymorphic modifica-
tions, that is, R and ꢀ phase.1 The R phase of nickel hydroxide
is metastable and hydrated with higher d-spacing (7.6 Å),
initially forms at the time of precipitation of nuclei in the
solution, and immediately transforms to ꢀ-nickel hydroxide.
During the transformation stage, an intermediate phase desig-
nated as ꢀbc-nickel hydroxide is formed.13 This is an intergrowth
of the R and ꢀ phases. According to Ostwald’s law, the
thermodynamically least stable phase is the first to be formed
during crystallization from a solution. In keeping with this
Figure 3 shows the thermograms of SH-nickel hydroxide and
AH-nickel hydroxide. AH-nickel hydroxide loses weight in a
single step with the weight loss of 19.4%. On heating the
sample, we expect to observe changes in the crystal structure.
The AH-nickel hydroxide was heated isothermally from room
temperature (RT, 25 °C) to 240 °C at several steps separately.
The PXRD pattern remains unaffected (see Figure 1). The AH-
nickel hydroxide loses only 1% weight loss in the range of
25-240 °C. At 250 °C, we observe broadening of (101) and
(111) reflections with extra reflections emerging at 37.1° in 2θ.
This indicates that the powder X-ray diffraction pattern on the

13016 J. Phys. Chem. B, Vol. 113, No. 39, 2009
Ramesh
Figure 3. Thermograms of AH-nickel hydroxide and SH-nickel
hydroxide.
Figure 1. PXRD patterns of AH-nickel hydroxide as a function of
temperature. * is due to the sample holder.
broad. As the reaction proceeds, the dehydration interface moves
from the outside to the center of each crystallite. When the
sample is heated at 450 °C, the reflections of nickel oxide are
narrow and the peak width has decreased, indicating that the
sample is crystallized, and it is evident from the changes in the
crystallite size (see Table 2b).
Figure 3 shows the thermogram of SH-nickel hydroxide and
displays the weight loss in two steps. The total weight loss for
SH-nickel hydroxide was 28%. We have reported in an earlier
paper that the first step weight loss of SH-nickel hydroxide arises
due to the presence of reversible moisture content. The reversible
moisture content is associated with the presence of structural
disorder. The thermogram of SH-nickel hydroxide shows that
the dehydration starts at 70-90 °C and proceeds until comple-
tion up to 200 °C. This results in the elimination of a water
molecule associated with nickel hydroxide (see Table 1). The
crystallite size decreases in all the directions up to 225 °C
without significant changes in the peak positions of nickel
hydroxide. The dehydration of SH-nickel hydroxide results in
the broadening of reflections [selectively (001), (102), and (111)]
at 100 °C (see Figure 2 and Table 3a).
Figure 2. PXRD patterns of SH-nickel hydroxide as a function of
The crystallite size decreases in all the directions from room
temperature to above 250 °C. At 262 °C, pure nickel oxide peaks
are observed with complete absence of reflections for nickel
hydroxide. This indicates that the cation vacancies and inter-
stratified phases are eliminated. No information about the
dimensions of the c axis was observed in the case of SH-nickel
hydroxide at temperatures greater than 262 °C. Decomposition
results in a gradual decrease in intensity of the reflections until
they finally disappear, leaving only the reflections expected for
nickel oxide. This change may be interpreted as a gradual
rearrangement of the (001) or (111) planes until ordering takes
place between them required for nickel oxide is formed. On
heating the samples to higher temperatures, i.e, 350 to 450 °C,
we observe the narrowing of the peak widths indicating the
growth of crystallite size of nickel oxide particles (see Figure
2 and Table 3b). It has been mentioned earlier that SH-nickel
hydroxide is a highly disordered material comprising of inter-
stratification, cation vacancies and stacking faults. These
disorders favor the decomposition to take place at lower
temperatures compared to AH-nickel hydroxide sample via
disruption of interlayer water molecules leading to an amorphous
phase formation and then towards crystallization of nickel oxide.
The transformation of amorphous to crystallite transition takes
place through a topotactic reaction mechanism.
temperature. * is due to the sample holder.
thermal decomposition of AH-nickel hydroxide shows distinct
reaction stages (see Figure 1). The crystallite size of AH-nickel
hydroxide decreases continuously from room temperature to
250 °C along the (111) direction (from 430 to 81 Å) (see Table
2a). In the 250-350 °C range, the relative intensities of the
nickel hydroxide phase decrease and intensities of nickel oxide
peaks increase. At 350 °C, nickel hydroxide peaks completely
disappear and peaks due to pure nickel oxide phase are observed.
At the initial stages of the reaction, germination takes place on
the edges of the platelets and propagates to the interior regions.
The propagation requires sufficient thermal energy in the form
of heat. The nickel oxide is in a definite and reproducible
orientation relative to the nickel hydroxide, that is, (001)Ni(OH)2//
(111)NiO and (110)Ni(OH)2//(110)NiO.20 The conversion re-
quires that half the hydroxyl groups between the nickel
hydroxide sheets combine with hydrogen atoms on neighboring
hydroxyl groups and water molecules are eliminated. During
the decomposition process, we observe a decrease in the Ni-Ni
bond distance from 3.12 to 2.98 Å, exactly that required in the
(111) plane of nickel oxide. The interlayer spacing changes from
4.74 to 2.42 Å. The diffraction patterns of AH-nickel hydroxide
show that these changes do not occur simultaneously. Figure 1
clearly shows that the reflections related to nickel hydroxide
are very sharp and the reflections corresponding to NiO are
Gregg et al. had proposed the thermal decomposition mech-
anism of magnesium hydroxide.23 In general, the decomposition

PXRD Studies on the Thermal Decomposition of Ni(OH)2
J. Phys. Chem. B, Vol. 113, No. 39, 2009 13017
reaction of a metal hydroxide is as follows: Solid A solid B +
gas, where each crystallite A decomposes to give a solid B,
and the atomic positions in solid B are retained as they were in
solid A during the thermal decomposition process. If this is true,
we should observe an intermediate structure during the trans-
formation process. In both the samples we do not observe such
an intermediate phase. Even if such phases are formed, then it
should be observed more likely in the SH-nickel hydroxide
sample. Ball and Taylor proposed an alternate inhomogeneous
decomposition for metal hydroxide that has donor and acceptor
sites. During the thermal degradation, the metal ions migrate
from the hydroxide to the oxide in the acceptor sites, while
protons migrate in the opposite direction to create water
molecules in the donor regions.24 It has been well-established
that the NiO prepared by decomposition of brucite will bear a
well-defined orientation relationship with the parent crystal; that
is, the decomposition is topotactic. On the basis of our results,
it indicates a trend that the inhomogeneous mechanism can be
a better explanation of the formation of nickel oxide from the
SH-nickel hydroxide sample. It is assumed that a crystallite-
amorphous-crystal transformation takes place in the SH-nickel
hydroxide sample. The results clearly show that these two nickel
hydroxide samples having different degrees of crystallinity
undergo decomposition using two distinct mechanisms. The
decomposition mechanism depends mainly on the crystallinity
and structural disorder associated with the sample.
recording X-ray diffraction patterns. The author also thanks
reviewers for their useful comments.
References and Notes
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Acknowledgment. T.N.R. thanks the Council of Scientific
and Industrial Research, GOI, for the award of a Research
Associate Fellowship. The author gratefully acknowledges Prof.
P. Vishnu Kamath for providing laboratory facilities to carry
out the research work and his encouragement and generous
support to publish the results. I also thank B. E. Prasad for
JP906578U