Hexamethylenetetramine directed synthesis and properties of a new family of α-nickel hydroxide organic-inorganic hybrid materials with high chemical stability

Article abstract of DOI:10.1021/jp055970t

A new family of organic-inorganic hybrid material of α-nickel hydroxide formulated as Ni(OH)_2-x(A^n-)_x/n- (C₆H₁₂N₄)_y·zH₂O (A = Cl^-, CH₃COO^-, SO₄^2-, NO^3-; x = 0.05-0.18,3; = 0.09-0.11, z = 0.36-0.43) with high stability and adjustable interlayer spacing ranging from 7.21 to 15.12 A has been successfully prepared by a simple hydrothermal method. The effects of various anions and hexamethylenetetramine (HMT) on the d values of α-nickel hydroxide have been systematically investigated. This family of hybrid materials is of such high stability that they can stand more than 40 days in 6 M KOH. The product with a formula Ni(OH)_1.95(C ₆H₁₂N₄)_0.11(Cl^-) _0.05(H₂O)_0.36 has a high surface area of about 299.26 m²/g and an average pore diameter of about 45.1 A. The coercivity (Hc) value is ca. 2000 Oe for the sample with a d spacing of 13.14 A. Moreover, the prepared α-Ni(OH)₂ in our experiment is of high stability in strong alkali solution. Such high stability could be derived from strong chelating interactions between the Ni ions and HMT molecules with the interlayers. This high chemical stability could make this material more suitable for the applications.

Full text of DOI:10.1021/jp055970t

J. Phys. Chem. B 2006, 110, 4039-4046
4039
Hexamethylenetetramine Directed Synthesis and Properties of a New Family of r-Nickel
Hydroxide Organic-Inorganic Hybrid Materials with High Chemical Stability
Bian-Hua Liu, Shu-Hong Yu,* Shao-Feng Chen, and Chun-Yan Wu
DiVision of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale,
Structure Research Laboratory of CAS, School of Chemistry and Materials, UniVersity of Science and
Technology of China, Hefei 230026, P. R. China
ReceiVed: October 19, 2005; In Final Form: December 13, 2005
n-
A new family of organic-inorganic hybrid material of R-nickel hydroxide formulated as Ni(OH)2-x(A )x/n
-
-
-
2-
-
(C
6
H
12
N
4
)
y
‚zH
2
O (A ) Cl , CH
3
COO , SO
4
, NO
3
; x ) 0.05-0.18, y ) 0.09-0.11, z ) 0.36-0.43) with
high stability and adjustable interlayer spacing ranging from 7.21 to 15.12 Å has been successfully prepared
by a simple hydrothermal method. The effects of various anions and hexamethylenetetramine (HMT) on the
d values of R-nickel hydroxide have been systematically investigated. This family of hybrid materials is of
such high stability that they can stand more than 40 days in 6 M KOH. The product with a formula
-
2
Ni(OH)1.95(C
6
H
12
N
4
)
0.11(Cl )0.05(H
2
O)0.36 has a high surface area of about 299.26 m /g and an average pore
diameter of about 45.1 Å. The coercivity (Hc) value is ca. 2000 Oe for the sample with a d spacing of 13.14
Å. Moreover, the prepared R-Ni(OH) in our experiment is of high stability in strong alkali solution. Such
2
high stability could be derived from strong chelating interactions between the Ni ions and HMT molecules
with the interlayers. This high chemical stability could make this material more suitable for the applications.
Introduction
isomorph of LDHs, R-nickel hydroxide is also isostructural with
1
3
hydrotalcite-like (HT-like) compounds. However, â-Ni(OH)2
is of brucite-like (Mg(OH)2) structure but without intercalated
species. Nevertheless, the Ni(OH)2 layer of two polymorphs
consists of a hexagonal planar arrangement of octahedrally
oxygen-coordinated Ni(II) ions in the same way.
Layered double hydroxides (LDHs) are important lamellar
1
materials, which can be structurally characterized as containing
brucite-like (magnesium hydroxide like) layers in which some
divalent metal cations are substituted by trivalent ions to form
positively charged sheets. The metal cations occupy the centers
of octahedra whose vertexes contain hydroxide ion. By sharing
octahedral edges, these octahedra are connected to each other
to form an infinite sheet. Also, the residual positive charge in
2
For the â-Ni(OH)2, layers are perfectly stacked along the
c-axis with an interlamellar distance of 4.6 Å; while for the
R-Ni(OH)2, layers are completely misoriented relative to each
other, corresponding to a so-called turbostratic phase with the
presence of water molecules and anionic species in the van der
the layers is compensated for by the presence of hydrated anions
between the stacked sheets.3 On the basis of their wide
1
4
Waals gap. The interlamellar distance of the layers of R-Ni-
(OH)2 is about 7.5 Å. However, d spacing of the interlayers
applications in the production of catalyst, sorbents, ionic
exchangers, ionic conductors and electrochemical materials,
4
2
,10
can be adjusted from 7.5 to 31.7 Å
while long carbon chain
layered hydroxide materials have attracted a lot of interest in
-
recent years.5
organic molecules (CnH2n+1SO3 , n ) 10, 12, 14, 18) were
imposed into the interlayer of R-Ni(OH)2 to form Ni-
Nickel hydroxide (Ni(OH)2) as a typical LDH, has important
-
(
CnH2n+1SO3 )x(OH)2-x‚yH2O. Comparing with â-Ni(OH)2,
applications in alkaline rechargeable batteries such as Ni-Cd,
R-Ni(OH)2 has better electrochemical properties but less stabil-
6
Ni-Zn, Ni-Fe, and Ni-MH storage batteries, and also as a
15,16
ity.
metal cations (Al ,
to form a hydrotalcite-like compound formulated as Ni1-xAlx-
To improve its stability, researchers have mingled some
precursor for catalysts and NiO.7 There exist two polymorphs
,8
3
+ 17,18
2+ 19
3+ 20
Zn , and Co
) into the R-phase
9
of nickel hydroxide, R- and â-phases. Both forms crystallize
in the hexagonal system with the brucite-type structure by
stacking Ni(OH)2 layers along the c-axis. The main difference
between R- and â-Ni(OH)2 phase is if there exist other ions or
molecules between the stacking layers along the c-axis. R-Nickel
2
-
12b,21
(
OH)2(CO3 )2/x‚nH2O.
In this report, we present a convenient method to obtain new
and stable turbostratic R-Ni(OH)2 organic-inorganic hybrid
material with adjustable d spacing by introducing different
anions and hexamethyltetramine (HMT). The interlayer spacing
of the R-Ni(OH)2 varies, depending on the inserted anion species
and the amount of HMT. The molecular structure of the R-nickel
hydroxide consists of stacked Ni(OH)2-x layers intercalated with
various anions10 (e.g., carbonate, nitrate, etc.) or water molecules
1
1
in the interlayer space to maintain charge neutrality totally.
Hydrotalcite is a natural mineral Mg6Al2(OH)16CO3‚4H2O,
n-
2
+
3+
hydroxide can be formulated as Ni(OH)2-x(A )x/n-
which both Mg and Al are located in brucite (Mg(OH)2)
-
-
2-
-
type sheets while CO3 anions occupy the intersheet space to
-
(C6H12N4)y‚zH2O (A ) Cl , CH3COO , SO4 , NO3 ; x )
0.05-0.18, y ) 0.09-0.11, z ) 0.36-0.43) with different
interlayer d spacing ranging from 7.21 to 15.12 Å. Importantly,
as-synthesized R- Ni(OH)2 is highly stable in strong alkali or
acidic solution compared with that reported previously. After
heat treatment at 600 °C in the air atmosphere, the R-nickel
3
+
balance extra positive charges carried by trivalent Al cations
2
+ 12
(compared to divalent Mg ). In fact, besides being an
*
To whom correspondence should be addressed. Fax: + 86 551
3
603040, E-mail: shyu@ustc.edu.cn.
1
0.1021/jp055970t CCC: $33.50 © 2006 American Chemical Society
Published on Web 02/14/2006

4040 J. Phys. Chem. B, Vol. 110, No. 9, 2006
Liu et al.
TABLE 1: Summary of the Main Results on the d Spacings
of the Products Obtained under Different Reactant
Concentrations, Where d
and d
t
Is the Typical Diffraction Peak
f
Is the First Diffraction Peak
salts
dosage
nickel
source
no. (1 mmol)
dosage
of HMT
(mmol)
species
(mmol)
d
t
(Å)
f
d (Å)
A1 Ni(Ac)
A2 Ni(Ac)
A3 Ni(Ac)
A4 Ni(Ac)
2
2
2
2
10
10
20
20
10
10
20
20
10
10
20
20
10
10
20
20
NaAc
NaAc
NaAc
NaAc
NaCl
NaCl
NaCl
NaCl
NaNO
NaNO
NaNO
NaNO
10
20
10
20
10
20
10
20
10
20
10
20
10
20
10
20
8.14
8.31
8.10
8.31
8.39
8.70
8.02
8.77
7.21
8.48
7.25
8.10
8.94
9.02
8.89
9.04
12.88
14.20
15.12
14.67
13.26
12.99
12.48
12.91
13.71
14.48
13.22
13.35
13.88
13.50
13.70
13.30
B1 NiCl
B2 NiCl
B3 NiCl
B4 NiCl
2
2
2
2
C1 Ni(NO
C2 Ni(NO
C3 Ni(NO
C4 Ni(NO
3
3
3
3
)
2
2
2
2
3
3
3
3
)
)
)
D1 NiSO
D2 NiSO
D3 NiSO
D4 NiSO
4
4
4
4
Na
Na
Na
Na
2
2
2
2
SO
4
4
4
4
SO
SO
SO
Figure 1. XRD patterns of R-Ni(OH)
2
samples obtained at 140 °C
for 10 h when Ni(Ac) ‚4H O was selected as nickel source. The initial
2
2
amount of reactants for the samples are respectively: sample A1, 1
hydroxide can completely transform into nickel oxide (NiO).
The magnetic properties and the porosities of the samples have
been also investigated.
mmol Ni(Ac) ‚4H O + 10 mmol HMT + 10 mmol NaAc; sample A2,
2
2
1 mmol Ni(Ac)
2
‚4H O + 10 mmol HMT + 20 mmol NaAc; sample
2
A3, 1 mmol Ni(Ac)
A4, 1 mmol Ni(Ac)
2
‚4H
‚4H
details in Table 1). The peaks labeled with an asterisk can be indexed
to the typical diffraction of R-Ni(OH) . The peaks labeled with b and
4 may be indexed to the new series of d spacings between interlayers
of R-Ni(OH) (see details in text).
2
O + 20 mmol HMT + 10 mmol NaAc; sample
2
2
O + 20 mmol HMT + 20 mmol NaAc (see
Experimental Section
2
Chemicals. All chemicals were analytical grade and pur-
chased from Shanghai Chemical Reagents Company. These
chemicals are hexamethylenetetramine (HMT, C6H12N4, Mw )
2
ing voltage of 200 kV and a JEOL-2010 high-resolution
transmission electron microscopy (HRTEM), also at 200 kV,
respectively. SEM images were taken on a JEOL JSM-6700F
scanning electron microscope; the infrared spectrum was
obtained on a Fourier transform infrared spectrometer (FRIR,
Magna-IR-750); thermogravimetric analysis (TGA) was carried
out on a TGA-50 thermal analyzer (Shimadzu Corporation) with
1
40.19), nickel acetate (Ni(AC)2‚4H2O, Mw ) 248.86), nickel
chloride (NiCl2‚6H2O, Mw ) 237.69), nickel nitrate (Ni(NO3)2‚
6
2
H2O, Mw ) 290.80), nickel sulfate (NiSO4‚6H2O, Mw )
62.83), sodium acetate (NaAc, Mw ) 82.03), sodium chloride
(NaCl, Mw ) 58.44), sodium nitrate (NaNO3, Mw ) 84.89),
and sodium sulfate (Na2SO4, Mw ) 142.42). All reagents were
used without further purification.
-
1
a heating rate of 10 °C min in flowing air. Element analysis
was carried out on an INCA300 X-ray Energy Spectrum
instrument (OXFORD). N2 adsorption was determined by BET
measurements using an ASAP-2000 surface area analyzer. The
magnetic measurements on powdered samples enclosed in a
medical cap were carried out at 4 K using a commercial SQUID
magnetometer (MPMS-XL) from Quantum Design Corp.
Synthesis Procedures. In a typical experiment, 1 mmol of
nickel acetate (Ni(AC)2‚4H2O), 10 mmol of HMT, and 10 mmol
of sodium acetate were added into 40 mL of deionized water
with stirring until the solution was clear. Then, the solution was
put into a 50 mL capacity Teflon-lined autoclave. After being
sealed, the autoclave was heated at 140 °C for 10 h. After
hydrothermal treatment, green or pale blue precipitate was
separated from the solution by centrifugation, washed with
deionized water and absolute ethanol for several times, and dried
at 60 °C for 4 h in a vacuum. To understand the effects of anions
on the formation of R-nickel hydroxide, nickel chloride (with
sodium chloride), nickel sulfate (with sodium sulfate), and nickel
nitrate (with sodium nitrate) were used in equal molar amounts
to substitute for the nickel acetate (see details in Table 1).
Chemical Stability. The chemical stability of the synthesized
R-nickel hydroxide samples was examined by immersing the
powders in 6 M HNO3 and 6 M and 12 M KOH solutions for
a long time. After immersion, the products were collected by
centrifugation, then washed by deioned water and absolute
ethanol, and finally, dried at 60 °C for 4 h in a vacuum for
powder X-ray diffraction measurement.
Results and Discussion
Synthesis of r-Ni(OH)2 Organic)Inorganic Hybrid Ma-
terials. Previously, the XRD pattern of R-Ni(OH)21
presented a series of typical peaks with a low angle reflection
around 7.79 Å, secondary reflections at 3.90 and 1.54 Å, and a
broad asymmetric band at 2.2-2.7 Å. Figure 1 shows the XRD
patterns of as-prepared products with the same amount of nickel
acetate at different amount of HMT and sodium acetate via the
hydrothermal treatment at 140 °C for 10 h. All of the XRD
patterns (samples A1-A4) have the characteristic diffraction
peaks of R-Ni(OH)2 and show a diffraction peak at low angle
(2θ located at about 10°) around 8.10-8.30 Å and a broad
asymmetric band at about 2.66 Å. The asymmetric nature of
the reflection at about d ) 2.66 Å indicates the formation of a
turbostratic phase observed in most of the R-type hydroxides,
corresponding to the formation of R-form of nickel hydroxide.25
From Figure A1 (see the experiment details in Table 1), the
entire asterisk labeled peaks obviously display hydrotalcite-like
3,22-24
Characterization. The products were characterized by X-ray
diffraction pattern (XRD), recorded on a MAC Science Co. Ltd.
MXP 18 AHF X-ray diffractometer with monochromatized Cu
KR radiation (λ ) 1.54056 Å). Transmission electron micros-
copy (TEM) and high-resolution transmission electron micros-
copy (HRTEM) were performed on a Hitachi (Tokyo, Japan)
H-800 transmission electron microscope (TEM) at an accelerat-
2
6,27
features
and the d values at 8.14, 4.05, 2.66, and 1.54 Å
could also be assigned to (003), (006), (101), and (110) planes

Organic-Inorganic Hybrid Materials
J. Phys. Chem. B, Vol. 110, No. 9, 2006 4041
of R-Ni(OH)2. The characteristic XRD pattern of hydrotalcite-
like structures is that the d values are corresponding to
successive diffraction peaks by basal planes; i.e., d003 ≈2d006
≈
3d009. The value of d(00l) depends on the thickness of the
brucite-like layer and on the nature of the interlayer anions.
2
5
Furthermore, the d values of the samples corresponding to
successive diffractions by the basal planes agree well with each
2
6,28
other, i.e., d003 ≈ 2d006. According to the literature,
the cell
constant c is commonly calculated as c ) 3d003, assuming a 3R
polytipism for the hydrotalcite, while the value of cell constant
a is calculated as a ) 2d110.1 It has been also proposed that c
can be better determined by averaging the position to the (003),
2a
(
006), and (009) planes29 according to eqs 1 and 2:
c
1
/3
) / (d(003) + (2d(006)))
(1)
(2)
2
c
1
/3
) / (d(003) + (2d(006) + (3d(009))))
3
On the basis of the d spacing of the reflections in the XRD
pattern of the sample A1 in Figure 1, we can calculate the cell
parameters of the sample A1 as a ) 3.08 Å and c ) 24.91 Å,
and the calculated c value is larger than those reported earlier
for R-Ni(OH)2 particles precipitated from urea-containing solu-
30,31
tions.
The d003 spacing of the products obtained here is about
8.10 Å, which is larger than the d spacing of the literature value
7
.79 Å (JCPDS Card 38-0715). It is well-known that the main
feature of R-nickel hydroxide is that Ni(OH)2 layers could be
intercalated with various anions.10 So, such a spacing difference
may correspond to the difference in the thickness of a poorly
packed molecule layer separating the octahedral Ni(OH)2 sheets
-
or the amount of the anion(CH3COO ) intercalated to the Ni-
3
2,33
(OH)2 layers.
In fact, the previous literature reported that
21a
2
intercalation of a large amount of water and other anions to
the interlayers of R-Ni(OH)2 can also influence the value of
the d spacings.
Furthermore, we also found that new series peaks have
occurred in the XRD pattern for sample A1 in Figure 1. Here
the peaks labeled with dots correspond to d1 ) 12.8750 Å and
d2 ) 6.4025 Å which have a characteristic of d1 ≈ 2d2 (12.8750
Å ≈ 2 × 6.4025 Å), and the same is true for the peaks labeled
with triangles (10.6180 Å ≈ 2 × 5.3107 Å). From above
1
calculated c values (c ) 24.91 Å), it can be seen that d1 ≈ /2c,
1
d2 ≈ /4c. These two peaks can be indexed to the diffraction of
d(002) and d(004) of R-Ni(OH)2. The peaks labeled with
triangles (10.6180 Å ≈ 2 × 5.3107 Å) can also be treated as
the new diffraction of R-Ni(OH)2 due to the disorder property
2
Figure 2. XRD patterns of R-Ni(OH) samples obtained with different
nickel sources and corresponding sodium salts. The 2B sample series
B1-B4 correspond to the products obtained using nickel chloride as
nickel source; the 2C sample series correspond to the products obtained
using nickel nitrate as nickel source; the 2D sample series correspond
to the products obtained using nickel sulfate as nickel source. The
compositions of all these samples are shown in Table 1. The peaks
labeled with the dot line can be assigned as the typical reflection of
26,27
of the layered double hydroxide (hydrotalcite-like features).
When the amounts of HMT and sodium acetate are varied, the
XRD patterns of these samples also present typical peaks for
R-Ni(OH)2, but the d value of the first peak ranges from 12.87
to 15.12 Å. This phenomenon may be induced by the extent of
R-Ni(OH)
2
, labeled with an asterisk (/) can be indexed to the new series
intercalating organic molecules in the layered structure of the
of reflection peaks of the interlayer of the R-Ni(OH)
2
.
R-Ni(OH)2.2
2a,34
No peaks for â-Ni(OH)2 can be observed in
these XRD patterns shown in Figures 1 and 2.
be obtained: (i) The value of d003 ranges from 7.21 to 9.04 Å,
To study the effect of anions on the final products, acetate
anion was replaced by chloride, nitrate and sulfate anions,
respectively. The summary of experimental details is listed in
Table 1. The XRD patterns (Figure 2) presented a typical
hydrotalcite-like structure just like the patterns shown in Figure
and this may be related to the species and amount of anions
2
0,22a
intercalated in the interlayer of R-Ni(OH)2 too.
(ii) Similar
to what was presented in Figure 1, the d value of the first
diffraction peaks changes from 12.48 to 14.48 Å. which can be
regarded as the introduction of an organic molecule in the
reaction just like what happened in samples A1-A4.
Altering the reaction temperature did not remarkably affect
the d spacings of the end products. If no salts were added to
the experimental system, or ammonia or sodium hydroxide
solution was selected to replace HMT as the alkali source, no
1. Each XRD pattern shown in Figure 2 presents a series of
typical peaks and a broad of asymmetric band (labeled by dot
lines), which can be characterized as R-Ni(OH)2. Moreover,
other peaks labeled with asterisk also show the hydrotalcite-
like structure feature, which can be treated similarly as stated
above. From Table 1 and Figure 2, interesting information can

4042 J. Phys. Chem. B, Vol. 110, No. 9, 2006
Liu et al.
Figure 3. TEM image and electron diffraction pattern of the sample A2 (Ni(OH)1.70(C
H
6 12
4
N )
0.10(Ac-)0.30(H
2
O)0.95): (a) TEM image; (b) typical
electron diffraction pattern taken on on a piece of extended nanosheet of the nanoflower along the [001] direction.
Figure 4. HRTEM images taken on the very thin nanosheets of the flowers shown in Figure 3a after exposure on electron beam for a while: (a)
general view; (b) typical Ni nanoparticle formed due to the reduction by the electron beam.
R- nickel hydroxide can form. The effect of anions on the
formation of R-Ni(OH)2 is still uncertain at this stage.
Morphology of r-Ni(OH)2 Organic)Inorganic Hybrid
Materials. The transmission electron microscope (TEM) image
in Figure 3a shows that the sample A2 is composed of flowerlike
structures. The electron diffraction pattern taken on on a piece
of extended nanosheet of this flower like structure shows a
typical single crystalline nature (Figure 3b), which is similar to
that hexagonal R-Ni(OH)2 phase (JCPDS Card No. 38-0715).
However, the long time exposure of the sample under the
electron beam during TEM observation will destroy the
structures, accompanied by the formation of a lot of nanopar-
ticles with a size of 3-6 nm (Figure 4a). The lattice-resolved
high-resolution TEM (HRTEM) image in Figure 4b showed a
spacing of 2.04 Å, corresponding to the value for (111) planes
of cubic Ni phase (JCPDS Card No. 04-0850), underlying that
the very thin R-Ni(OH)2 nanosheets of the flower structure are
not stable under the electron beam and can be reduced into Ni
nanoparticles.
The scanning electron microscope (SEM) images of the
obtained R-Ni(OH)2 samples are shown in Figure 5. Parts a-d
of Figure 5 correspond to the morphologies of the samples A2,
B2, C2, and D2 (see the experiment details in Table 1) while
the inserted photographs are the enlarged images of each sample.
Previous literature reported that R-Ni(OH)2 tended to form a
1
3,22a,35
filmlike morphology.
Recently, hollow spheres and thin
films of nickel hydroxide have been fabricated using poly-
(styrene-methyl acrylic acid) (PSA) latex particles as tem-
36
plate. Ribbon- and boardlike nanostructured nickel hydroxide
can be synthesized using the amorphous R-Ni(OH)2 as precur-
37
sor. In this work, flowerlike R-nickel hydroxide nanostructures
can be synthesized without using any template. Figure 5 presents
the images of various flowerlike morphologies with a diameter
ranging from 2 to 10 µm, which are organized by thin R-Ni-
(OH)2 films with a thickness of about 10 nm. The flower
structures shown in Figure 5b,c are more complex and compact
than those in Figures 5a,d, while the very thin nanosheets shown
in Figures 5a,d are more flexible than those in Figure 5b,c. On

Organic-Inorganic Hybrid Materials
J. Phys. Chem. B, Vol. 110, No. 9, 2006 4043
Figure 5. (a-d) SEM images of the R-Ni(OH)
2
samples obtained at 140 °C for 10 h for the samples A2, B2, C2, and D2, respectively. The
inserted images correspond to the magnified SEM images of these samples.
existed in R-Ni(OH)2 in the samples. A broad band at ca 3435
-1
cm is due to the VO-H vibration of H-bonded water molecules
located in the interlamellar space of turbostratic structure of
-
1
R-Ni(OH)2. The band at ca. 1630 cm is characterized to the
bending vibrational mode of the interlayer water molecules. The
-
1
peaks at ca. 2890 and 2885 cm are attributed to the -C-H
vibrational mode of -CH2-, respectively. The absorption bands
-
1
at 1020-1220 cm are attributed to the vibrational mode of
-
1
C-N. The band at ca. 615 cm is assigned to δO-H and the
-
1
band at ca. 430 cm can be assigned to metal-oxygen
vibrations and metal-oxygen-hydrogen bending vibrations2
that can be ascribed to Ni-OH in this system.
0,38
From the FT-IR spectra, we can see that all samples have
absorption bands for the -CH2- and C-N functional groups.
-
1
Furthermore, a weak band at 1400 cm in Figure 6a can be
-
indexed to the CdO vibration of CH3COO . The vibration
-1
bands at 1385 cm, 1340, and 830 cm in Figure 3c correspond
-
-
to the vibration of NO3 (i.e., an NO3 anion with D3h symmetry
Figure 6. FT-IR spectra of the obtained R-Ni(OH) samples: (a-d)
2
2
7
-1
samples A2, B2, C2, and D2, respectively.
in the interlayer space) and the vibration band at 990 cm in
2
- 39
Figure 6d can be assigned to the vibration of SO4
. Consider-
ing the initial materials we used in these experiments, it is
believed that a part of the precursor HMT and anions have been
intercalated into the R-Ni(OH)2 interlayer.
The element analysis and thermogravimetric analysis (TGA)
were used to confirm the above speculation. It is well-known
that the R-nickel hydroxide always undergoes multisteps mass
loss, and it has a larger mass loss (about 34%) than â-nickel
hydroxide (18%). The thermogravimetric curves of the R-Ni-
the basis of these experiments, we can conclude that the reason
for this phenomenon is due to various anions in this reaction
system, but a more detailed mechanism about the formation of
these morphologies is under further exploration.
FT-IR and Thermogravimetric Analysis of r-Ni(OH)2
Organic)Inorganic Hybrid Materials. The Fourier transfor-
mation infrared spectra (FT-IR) shown in Figure 6 for the
samples A2, B2, C2, and D2 have the same vibration bands at
ca. 3650, 3435, 2890, 2850, 1630, 1340, 1195, 1070, 615, and
(OH)2 (A2, B2, C2, and D2) samples with the temperature
-
1
4
30 cm which are labeled with the dotted lines. The narrow
ranging from 30 to 600 °C show two steps at ca. 100 °C and
ca. 350 °C with different net weight loss, that is 35.2% (A2),
34.1% (B2), 33.1% (C2), and 30.6% (D2), respectively (Figure
7). All the peaks in the XRD patterns in Figure 8 of the end
-
1
band at ca. 3650 cm is attributed to the VO-H stretching of
the geminal free hydroxyl groups of the brucite-like structure,
which can be attributed to the flexing vibration of free hydroxyl
13,22a

4044 J. Phys. Chem. B, Vol. 110, No. 9, 2006
Liu et al.
TABLE 3: Interlayer Distances and Compositions of the
r-Ni(OH) Samples with Different Intercalated Anions
2
inter- interlayer
sample calated distance
no.
anion
(Å)
chemical composition
-
-
A2
B2
C2
D2
Ac
Cl
14.19
13.14
14.43
13.63
Ni(OH)1.87(C6H12N4)0.10(Ac )0.13(H2O)0.37
-
-
Ni(OH)1.95(C6H12N4)0.11(Cl )0.05(H2O)0.36
-
-
NO3
Ni(OH)1.82(C6H12N4)0.09(NO3 )0.18(H2O)0.40
SO42
-
Ni(OH)1.78(C6H12N4)0.09(SO4 )0.11(H2O)0.43
2-
-
-
2-
-
(
0
C6H12N4)y‚zH2O (A ) Cl , CH3COO , SO4 , NO3 ; x )
.05-0.18, y ) 0.09-0.11, z ) 0.36-0.43).
Chemical Stability of r-Ni(OH)2 Organic)Inorganic
Hybrid Materials. The stability of the synthesized R-Ni(OH)2
samples were investigated by immersing each powder in 6 M
HNO3 and 6 and 12 M KOH solutions for several hours to tens
of days. After immersion for a certain period, the products were
washed by deionized water and absolute ethanol and then dried
at 60 °C for 4 h in a vacuum for powder XRD measurement.
The XRD patterns in Figure 9 indicate these products after
treatment are also R-Ni(OH)2 which show characteristic dif-
fraction peaks at low angle (2θ is about 10°) around 8.25-
Figure 7. Thermogravimetric curve (TG) of the R-Ni(OH)
a-d) samples A2, B2, C2, and D2, respectively.
2
samples:
(
1
0.84 Å and a broad asymmetric band at about 2.66 and 1.54
Å. It was found that the as-synthesized R-Ni(OH)2 (the sample
A2) is stable for more than 40 days in 6 M KOH and for more
than 48 h in 12 M KOH solution at room temperature. Similar
results can be obtained from other samples (B2, C2, and D2)
in alkali solutions (data not given).
From the XRD pattern of Figure 9A, it was found that the
main diffraction peak for R-Ni(OH)2 samples (curves a-c)
which are treated by immersing in strong acid and alkali is
shifted to smaller scattering angles compared to that of the
primary sample obtained (curve d). On the basis of the reported
4
0
literature, it is believed that this shift may be induced by ions
-
exchanging; i.e., the hydroxyls have replaced the CH3COO
or more water molecules have intercalated into the R-Ni(OH)2
interlayers. This exchangeable progress resulted in enlarging
the d values, and the structure of R-Ni(OH)2 layered double
4
0,41
hydroxide may be reoriented.
Figure 8. XRD pattern of the NiO products after thermal decomposi-
tion of R-Ni(OH) samples A2, B2, C2, and D2, respectively.
To our best knowledge, the chemical stability of R-Ni(OH)2
the sample A2) in our experiments is better than those of R-Ni-
OH)2 materials synthesized by any other method.13 Interest-
2
(
(
TABLE 2: Element Analysis Data of Sample A2
ingly, we found that the R-Ni(OH)2 of the A2 sample can stand
erosion of strong oxidant acid (6 M HNO3) for more than 14 h,
but other samples (B2, C2, and D2) failed to tolerate this
element
weight %
atomic %
C
N
8.63
4.74
18.04
8.48
13
experimental condition. In comparison with a previous report,
O
30.75
47.56
8.32
48.20
20.32
4.96
we believe that this high stability may be attributed to the species
of intercalated organic precursor we used in this experiment.
Usually, HMT is a strong chelating ligand with the interaction
of transition metal ions; hence, it is reasonable to consider that
this high chemical stability is derived from such strong chelating
interaction between the Ni ions and HMT molecules. This high
stability could make this material more suitable for the applica-
tions.
Ni
Cu
totals
100.00
product after TGA measurement can be indexed to the pure
cubic NiO with a ) 2.412 Å (JCPDS 47-1049).
The element analysis data of sample A2 (shown in Table 2)
agreed well with those of thermogravimetric analysis, indicating
that a small parts of the precursor HMT and anions have indeed
been intercalated to the interlayer of R-Ni(OH)2 to form the
R-Ni(OH)2 organic-inorganic hybrid materials. On the basis
of the results of thermogravimetric analysis and element
analysis, the chemical composition of the sample A2 can be
BET Analysis of r-Ni(OH)2 Organic)Inorganic Hybrid
Materials. All samples (A2, B2, C2, D2) are determined by
N2 adsorption by BET measurements using an ASAP-2000
surface area analyzer. Among these samples, the sample B2
showed the biggest BET (Brunauer-Emmett-Teller) surface
area. N2 adsorption-desorption isotherm and the pore size
distribution curve for the macroporous Ni(OH)2 (the sample B2)
are shown in Figure 10. The hysteresis of the desorption branch
is due to hindered desorption which is often observed in
-
formulated as Ni(OH)1.87(C6H12N4)0.10(Ac )0.13(H2O)0.37. The
results of other samples are similar to this and the molecular
formulas are listed in Table 3. These results have demonstrated
that a new series of R-nickel hydroxide with adjustable interlayer
spaces can be synthesized in the present system. The general
42
particulate samples. The pore size distribution calculated from
the adsorption branch of isotherm indicated that the Ni(OH)2
retained a relatively uniform framework with pore size distribu-
n-
molecular formula can be summarized as Ni(OH)2-x(A )x/n-

Organic-Inorganic Hybrid Materials
J. Phys. Chem. B, Vol. 110, No. 9, 2006 4045
Figure 9. Left (A): XRD patterns taken on the samples after immersing the hydrothermal treatment synthesized sample A2 into the alkali and
acidic solutions. Key: the XRD patterns of sample A2 immersed (a) in 6 M KOH for 40 days, (b) in 12 M KOH for 48 h, and (c) in 6 M HNO
for 14 h; (d) XRD pattern of the primary obtained Ni(OH) . (B) is the amplification parts of Figure 9A.
3
2
Figure 10. N adsorption-desorption isotherm curve and pore size distribution (inset) of the sample B2.
2
tion centered at around 50 Å. It is also found that there are
micropores existing in the nanosheets of the R-Ni(OH)2
organic-inorganic flowers based on the BET measurement. The
a and b of Figure 11 are the isothermal magnetization loops
measured at 4 K on a commercial SQUID magnetometer for
the samples B2, C2 with d spacings of 13.14 and 14.43 Å,
respectively. The hysteresis loops indicated that the samples
show a ferromaganetic behavior with saturation magnetization
(Ms), remanent magnetization (Mr), and coercivity (Hc) values.
The Hc values of the sample B2 [Ni(OH)1.95(C6H12N4)0.11-
2
micropore area is about 68.03 m /g. The BET (Brunauer-
Emmett-Teller) surface area, calculated from nitrogen adsorp-
tion isotherms, shows that the surface area of the R-Ni(OH)2
2
sample B2 is 299.26 m /g, and the BJH (Barret-Joyner-
-
Halenda) average pore diameter is 45.1 Å. The determined
(Cl )0.05(H2O)0.36] and the sample C2 [Ni(OH)1.82(C6H12N4)0.09-
2
-
surface area is much larger than that of 62 m /g recently reported
(NO3 )0.18(H2O)0.40] are 2000 and 1280 Oe, respectively. The
for the hollow Ni(OH)2 spheres.36 Similarly, the BET surface
different Hc for these samples may be ascribed to the different
2
2
2,44
areas of other samples are 136.40 m /g (A2), 71.22 m /g (D2),
d spacings and imperceptible structural difference.
2
and 47.08 m /g (C2), respectively. From the results of these
data, it can be concluded that all the R-Ni(OH)2 organic-
inorganic samples synthesized by this approach are of high BET
surface area.
Conclusions
In summary, a new family of organic-inorganic hybrid
material of R-nickel hydroxide formulated as Ni(OH)2-x(A )x/n-
(C6H12N4)y‚zH2O (A ) Cl , CH3COO , SO4 , NO3 , x )
0.05-0.18, y ) 0.09-0.11, z ) 0.36-0.43) with high chemical
stability and adjustable interlayer spacing ranging from 7.21 to
15.12 Å has been successfully prepared by a convenient
n-
Magnetic Properties of r-Ni(OH)2 Organic)Inorganic
Hybrid Materials. The magnetic properties of the R-Ni(OH)2
organic-inorganic hybrid materials have been believed to be
highly dependent on the shape, crystallinity, magnetization
-
-
2-
-
4
3,44
direction, and the d spacing of the sample and so on.
Parts

4046 J. Phys. Chem. B, Vol. 110, No. 9, 2006
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(
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Acknowledgment. S.H.Y. would like to express thanks the
funding support from the Centurial Program of the Chinese
Academy of Sciences, the National Science Foundation of China
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