Homogeneous precipitation from solution by urea hydrolysis: a ...

Homogeneous precipitation from solution by urea hydrolysis: a novel chemical route
to the a-hydroxides of nickel and cobalt
Mridula Dixit," Gonur N. Subbanna' and P. Vishnu Kamath""
"Department of Chemistry, Central College, Bangalore University, Bangalore 560 001, India
bMaterials Research Centre, Indian Institute of Science, Bangalore 560 01 2, India
Homogeneous precipitation from solution by hydrolysis of urea at elevated temperatures (T= 120 "C) yields novel ammoniaintercalated
a-type hydroxide phases of the formula M(OH), (NH,),,, (H,O),( N003), -x where x = 2, y = 0.68 for M = Ni and
x = 1.85, y = 0 for M = Co. These triple-layered hexagonal phases (a = 3.08 & 0.01 A, c = 21.7 & 0.05 A) are more crystalline than
similar phases obtained by chemical precipitation or electrosynthesis. This method can be adapted as a convenient chemical route
to the bulk synthesis of a-hydroxides.
The hydroxides of nickel and cobalt find applications as
electrode materials for alkaline secondary These
hydroxides have a hexagonal layered structure and exist in
two polymorphic forms, a and p.3 The a-hydroxifles of nickel
and cobalt have a. large interlayer spacing (> 7 A) compared
to the p form (4.6 A) and are known to exhibit a higher charge
capacity and better discharge efficiency. However, the a-
hydroxides are metastable and are difficult to synthesize as
they age rapidly to the p form during synthesis or on storage
in strong alkali. LeBihan et uL4 reported the first chemical
synthesis of a-nickel hydroxide by precipitation with liquid
NH, followed by rapid centrifugation. Genin et aL5 followed
a similar procedure, but in both studies, the material produced
was poorly ordered and displayed broad features in the X-ray
diffraction patterns. The most reliable synthetic route to a-
nickel hydroxide is the electrosynthetic route by the cathodic
reduction of nickel nitrate solutions.6 However, this reaction
is current density dependent and the a-hydroxide forms only
at low current densities, thereby making electrosynthesis a
very slow and inappropriate route to the bulk synthesis of a-
nickel hydroxide. There has been, therefore, a considerable
degree of interest in evolving an accelerated chemical route to
bulk a-hydroxides of nickel and cobalt. Organic additives such
as glucose, fructose, lactose and citric acid were found to
mediate the precipitation of the a-hydroxides of nickel and
cobalt.',' However, the products were found to have very poor
crystallinity.
Of late, homogeneous precipitation from solution by
hydrolysis of urea has been increasingly employed to synthesize
novel hydroxide phases' and fine-particle
There
are two reports in the literature of the use of this method for
the synthesis of nickel hydroxide. Maruthiprasad et report
the synthesis of nickel trihydroxyisocyanate, while Avena
et characterize their product as a Pb,-type nickel hydroxide,
where Pbc stands for a poorly crystallised p phase. While the
diffraction and spectral data presented by both groups are
similar, the inconsistency in their conclusions prompted us to
reinvestigate this simple method of preparation, especially as
the published data appear to point towards the formation of
a-type hydroxides.
Our investigations reveal that the product is a novel NH,
intercalated a-type hydroxide. We have extended this method
to the synthesis of a-cobalt hydroxide, a phase first synthesized
in our laboratory.' In addition, urea hydrolysis at elevated
temperatures (12O"C), as we have achieved, yields a highly
ordered product with very low reaction times (100 min). We
account for all the observed structural, compositional and
morphological features of the a-type hydroxides.
Experimental
Synthesis
To 50ml of the metal nitrate solution (0.5mol dm-3) was
added 12 g of urea. Homogeneous precipitation was affected
by placing this solution in a glass container in a stainless-steel
domestic pressure cooker (100 min, 120 "C, 1 kg ern-,). The
resulting precipitates were filtered, washed free of unreacted
urea and dried to constant mass at 65°C.
Wet chemical analysis
The metal content of the hydroxide samples was estimated
gravimetrically. The total base content was estimated by
dissolving a known quantity of the sample in an excess of
hydrochloric acid (0.4 mol drn-,) and back-titrating the excess
acid against standard sodium hydroxide using a pH meter.
Intercalated NH3 was estimated by the microkjeldah procedure.
A known quantity of the sample was suspended in
10 ml of 40 massoh sodium hydroxide solution. Ammonia was
liberated by steam distillation and absorbed in an excess of
2% boric acid solution. This was titrated against standard
sulfuric acid (0.025 mol drn-,) using a mixed (1 : 3) methylene
blue-methylene red indicator. The microkjeldah procedure was
standardized using a standard ammonium sulfate solution. All
chemical titrations were performed at least three times to get
concordant results.
The difference between the total base and the intercalated
NH, yields the hydroxy ion content of the sample. The net
difference in the positive (Ni2+) and negative charges (OH-)
was compensated by the inclusion of nitrate ions and finally
the unaccounted mass was assigned to the intercalated water
con tent.
Characterisation
Powder X-ray diffraction patterns (XRD) were recor9ed on a
Philips X-rayo diffractometer using Cu-Ka (A = 1.541 A) or Co-
Ka (1 = 1.79 A) radiation. The X-ray data were not of a quality
that permitted least-squares refinement of peak positions and
the patterns were indexed by a trial and error method starting
with the lattice parameters of the model compounds reported
previ~usly.'~ IR spectra were recorded using a Nicolet Impact
400D FTIR spectrometer using the KBr pellet method at a
resolution of 3 cm- '. Thermogravimetric studies were carried
out on a home-made system at a heating rate of 2.5 "C min-'.
Isothermal losses were recorded after holding the sample at
the desired temperature for 2 h. Electron microscopy was
J. Muter. Chem., 1996, 6(8), 1429-1432 1429

~~ ~
carried out using a JEOL 200 CX electron microscope Finely
powdered samples were dispersed onto carbon mesh (size 200)
for microscopic investigations
Results
In Fig 1A and B are shown powder XRD patterns of the
samples obtained by homogeneous precipitation from nickel
nitrate and cobalt nitrate solutions, respectively The prominent
d spacings are listed in Table 1 together with those of a-nickel
hydroxide obtained from the literature It is clear that the
hydroxides obtained from homogeneous precipitation are a-
type phases which c!n be indexed on triple-layered hexagonal
cell (a= 3 08 +O 01 A, c=21 7_+005 A) In contrast, the more
stable p-hydroxides obtained from conventionF1 chemica! synthesis
have a much smaller unit cell (a= 3 10 A, c=4 76 A)
As there has been considerable discussion in the literature
on the presence of intercalated anions in the a-type hydrox-
ides16 and especially among the products of urea hydrolysis
reactions,13 l4 a detailed chemical analysis of the samples was
carried out, the results of which are summarised in Table2
Estimation of the metal content clearly showed that the
samples were anything but stoichiometric divalent hydroxides
The observed metal content was much lower than what would
be expected of a stoichiometric divalent hydroxide (63 3%),
throwing open the possibility of the presence of several intercalated
species The a-hydroxides generally contained significant
amounts of intercalated anions with a matching deficiency in
the hydroxy ion contentI6 The total base contents of the
samples were therefore estimated using chemical titrations
The results were quite surprising The hydroxides of both
cobalt and nickel revealed a total base content in excess of
what would be expected from the metal content As the excess
base has to be of the neutral variety, the presence of intercalated
NH,, a product of urea hydrolysis, was surmised Accordingly,
microkjeldah estimations of ammoniacal nitrogen revealed the
presence of up to 04 moles of NH, in both the hydroxides
The difference in the chemical and microkjeldah estimations,
which corresponds to the hydroxy ion content, was found to
be stoichiometric for nickel hydroxide and deficient for cobalt
Table2 Results of wet chemical analysis of the hydroxides obtained
by homogeneous precipitation (hp)
hp nickel
hydroxide
hp cobalt
hydroxide
e[ I I I
lu
Y
15 35 55
metal content (%) 51 9 55 1
total base (Yo) 36 9 36 3
NH, content (YO) 64 68
net hydroxy content (YO) 30 6 29 6
anion content (YO) 0 87
water content (%) 109 0
net mass loss (%) 32 8 (33 2) 24 8 (24 9)
Values in parentheses are calculated on the basis of the proposed
formula (see text)
Table 3 Isothermal mass losses observed at different temperatures for
the products of homogeneous precipitation
hp nickel hydroxide
hp cobalt hydroxide
I I I I I 1
15 35 55
2Bldegrees
Fig. 1 A, Powder X-ray diffraction patterns of homogeneously precipitated
nickel hydroxide (a) and nickel oxide, Ni0 (b) obtained by
thermal dccomposition of the hydroxide Data recorded using Cu-Ka
(A= 1 541 A) radiation B, Powder X-ray diffraction patterns of
homogeneously precipitated cobalt hydroxide (a) and cobalt oxide,
Co,O, (b) obtained by thermal Gecomposition of the hydroxide Data
recorded using Co-Ka (A = 1 79 A) radiation
T/”C mass loss (YO) T/”C mass loss (YO)
190
230
250
270
330
370
450
36 150 36
103 170 147
20 7 190 20 6
27 2 210 22 9
30 0 310 24 8
32 1
32 8
Table 1 X-ray powder diffraction data of the products of homogeneous
precipitation (hp)
loot
I
hp nickel hp cobalt
hkP a-~l(~~),b hydroxide‘ hydroxided
003 7 79 7 26 7 35
006 3 908 3 616 3 612
101 2 676 2 673 2 682
012 2 604 - 2 598
009 - 2 423 2 410
015 2 321 - 2 282
110 1541 1551 -
“The observed patterns were indexed according tq these hkl planes %n
a hexagonal cell bFrom ref 15 :a = 3 09 _+ 0 01 A, c = 21 77 f 0 05 A
da=308+001 A, ~=2166+005A
70t
I I , I I
0 100 200 300 400
TIOC
Fig. 2 TG data for homogeneously precipitated nickel hydroxide (a)
and cobalt hydroxide (b)
1430 J Muter Chem, 1996, 6(8), 1429-1432

hydroxide. The deficiency in the latter case was assumed to be
made up by the presence of nitrate ions to achieve charge
neutrality. The unaccounted-for mass was made up by the
inclusion of water molecules. The analysis yields approximate
molecular formulae Ni(OH)2(NH3)o 4(H20)0 68 and
Co(OH), 85(NH3)o 4(N03)0 15 for the products obtained from
nickel and cobalt nitrate solutions, respectively.
These formulae were verified by recording the isothermal
mass losses of the samples at various temperatures (see Table 3).
Both samples were decomposed completely by 400 "C, and the
mass losses after decomposition matched those expected from
the above formulae (nickel sample: obs. 32.8%, exptd 33.2%;
cobalt sample: obs. 24.8%, exptd. 24.9%). The expected mass
losses were calculated based on the observation of the formation
of Ni0 and Co304, respectively, as the products of
thermal decomposition (see Fig. 1 for XRD data). However,
these hydroxides showed a single-step dehydration and
decomposition in their thermograms (see Fig. 2).
In contrast, chemical analysis of a control sample of P-nickel
hydroxide yielded the formula Ni(OH),, with a net isothermal
mass loss of 18.1% (exptd. 19.4%), and a-nickel hydroxide
yielded the formula Ni(OH)2(H,0)o,, with a total mass loss
of 29% (exptd. 28.5%).
In Fig. 3 the IR spectra of the samples obtained from nickel
and cobalt nitrate solutions are shown. The spectra are typical
of the a-type hydroxides which are described in detail elsewhere.16
The only difference is the existence of an absorption
at 2200 cm-l, which other authors13 have attributed to the
existence of intercalated NCO- or CN- species in addition to
the peaks due to intercalated nitrate anions seen in the
1400-1000 cm-I region.
In Fig. 4 the results of electron microscopic investigations
1 I
4000 3500 3000 2500 2000 1500 1000 500
wavenumberkm
Fig. 3 IR spectra of homogeneously precipitated nickel hydroxide (a)
and cobalt hydroxide (b)
on the samples obtained from nickel and cobalt nitrate solutions
are presented. The nickel hydroxide sample shows a
fibrillar turbostratic morphology [Fig. 4(u)]. The ring electron
diffraction pattern taken from the same area at low magnification
is shown in Fig. 4(b). However, the cobalt hydroxide
particles are seen to have a needle-shaped morphology
[Fig. 4(c)], and the corresponding electron diffraction pattern
is sharp and spotty [Fig. 4(d)]. This is very much in contrast
to the hexagonal platelet morphology seen among the p-
Fig. 4 (a) Electron micrograph of homogeneously precipitated nickel hydroxide, (b) electron diffraction pattern of (a) (c) Electron micrograph of
homogeneously precipitated cobalt hydroxide, (d) electron diffraction pattern of (c)
J. Muter. Chern., 1996,6(8), 1429-1432 1431

hydroxides The turbostratic morphology is characteristic of
the a-hydroxides
Discussion
Based on the observation of a strong absorption at 2200 cm-l
in the IR spectra, Maruthiprasad et all3 characterised the
product of homogeneous precipitation from nickel nitrate
solution as nickel trihydroxyisocyanate [Ni2(0H),( NCO)]
However, the IR spectra published by them contain equally
strong evidence for the presence of intercalated nitrate ions, as
do ours If intercalated anions are indeed present, based on
charge neutrality considerations a matching deficiency in the
hydroxy content can be expected However, chemical titrations
reveal an excess base content Urea hydrolysis results in the
formation of free NH,10-12 which under elevated pressures, as
are obtained at 120 "C, can be expected to become intercalated
within the hydroxide layers, leading to the ready formation of
a-type phases The mole percentage of intercalated NH, is
considerable, as revealed by the microkjeldah procedure While
the cobalt hydroxide does show a 75% deficiency in the
hydroxy content, the nickel hydroxide exhibits a stoichiometric
hydroxy ion content It is our suggestion that the strong
absorptions seen in the 1600-1000cm-1 region as well as at
2200 cm-l arise from both intercalated and adsorbed species
The X-ray diffraction data prove unequivocally that the
phases obtained by homogeneous precipitation are a-type
hydroxides The a-hydroxides are poorly ordered phases and
exhibit broad bands in their X-ray diffraction patterns As a
result, there is a considerable variation in the interlayer spacing
reported by different authors3 The a phases obtained by us
are rFasonably well ordered and the interlayer spacing observed
(73 A) is less than the range of reported values l7 The
presence of intercalated NH, in layered hydroxides is not
unknown In the present instance, intercalated NH, appears
to profoundly influence the thermal behaviour of the a-type
hydroxides While a-hydroxides of nickel and cobalt are known
to undergo multi-step mass losses,' l7 the samples obtained
from homogeneous precipitation undergo a single-step dehydrationdecomposition
reaction The observed mass loss agrees
well with what is expected from the formulae obtained from
chemical analysis
Electron microscopy results also show that the samples
obtained from homogeneous precipitation exhibit a morphology
more like the a-hydroxides, in contrast with the
hexagonal platelet morphology of the 0-type hydroxides
Conclusions
We report here a simple chemical route to the bulk synthesis
of a-hydroxides of nickel and cobalt by homogeneous precipitation
from solution by the hydrolysis of urea The phases
which contain intercalated NH, are stable during synthesis
and workup and this method can be adapted for the synthesis
of nickel hydroxide positive electrodes for alkaline secondary
battery applications
Two of us (M D and P V K ) thank the Department of Science
and Technology, Govt of India for financial support MD
thanks the Council for Scientific and Industrial Research for
the award of a Senior Research Fellowship
References
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
K Watanabe, T Kikuoka and N Kumagai, J Appl Electrochem,
1995,25,219
J Bauer, D H Buss, H J Harms and 0 Glemser, J Electrochem
SOC , 1990,137,173
P Oliva, J Leonardi, J F Laurent, C Delmas, J J Braconnier,
M Figlarz and F Fievet, J Power Sources, 1982,8,229
S LeBihan, J Guenot and M Figlarz, C R Acad Sci Ser C, 1970,
270,2131
P Genin, A Delahaye-Vidal, F Portemer, K Tekia-Elhsissen and
M Figlarz, Eur J Solid State Inorg Chem , 1991,28, 505
K C Ho and J Jorne, J Electrochem SOC , 1990,137,149
P V Kamath, J Ismail, M F Ahmed, G N Subbanna and
J Gopalakrishnan, J Muter Chem , 1993,3,1285
J Ismail, M F Ahmed, P V Kamath, G N Subbanna, S Uma
and J Gopalaknshnan, J Solid State Chem , 1995, 114, 550
R J Candal, A E Regazzoni and M A Blesa, J Muter Chem,
1992,2,657
J L Shi and J H Gao, J Muter Sci, 1995,30,793
K Kandori, M Toshioka, H Nakashima and T Ishikawa,
Langmuir, 1993,9, 1031
K Kandori, H Nakashima and T Ishikawa, J Colloid Interface
Sci , 1993,160,499
B S Maruthiprasad, M N Sastri, S Rajagopal, K Seshan,
K R Krishnamurthy and T S R P Rao, Proc Ind Acad Scz,
1988,100,459
M J Avena, M V Vazquez, R E Carbino, C P DePauli and
V A Macagno, J Appl Electrochem, 1994,24,256
J J Braconnier, C Delmas, C Fouassier, M Figlarz, B Beaudouin
and P Hagenmuller, Rev Chzm Mzner , 1984,21,496
F Portemer, A Delahaye-Vidal and M Figlarz, J Electrochem
Soc , 1992,139,671
B Mani and J P deNeufville, J Electrochem Soc , 1988,135,800
P Benard, J P Auffredic and D Louer, Thermochim Acta, 1994,
232,65
Paper 6/03062I, Received 30th April, 1996
1432 J Mater Chem , 1996, 6(8), 1429-1432