Synthesis of biomorphous nickel oxide from a pinewood template and investigation on a hierarchical porous structure

Article abstract of DOI:10.1111/j.1551-2916.2005.00741.x

The hydrothermal synthesis of biomorphous nickel oxide (NiO) with pine template and nickel nitrate precursor is reported here. The morphology, porosity and connectivity of porous products in different length scales were characterized by field emission scanning electron microscopy, X-ray diffraction and nitrogen adsorption measurements. Their porous structures were found to be hierarchical from 1 up to 25 μm (in micrometer scale) and from 2 nm to 60 nm (in nanometer scale). Furthermore, depending on the heat-treatment temperatures, the porosity of the pine-templated NiO can be designed.

Full text of DOI:10.1111/j.1551-2916.2005.00741.x

J. Am. Ceram. Soc., 89 [2] 662–665 (2006)
DOI: 10.1111/j.1551-2916.2005.00741.x
r 2005 The American Ceramic Society
ournal
J
Synthesis of Biomorphous Nickel Oxide from a Pinewood Template and
Investigation on a Hierarchical Porous Structure
Zhaoting Liu, Tongxiang Fan, and Di Zhangw
State Key Lab of Metal Matrix Composites, Shanghai Jiaotong University, Shanghai 200030, China
The hydrothermal synthesis of biomorphous nickel oxide (NiO)
with pine template and nickel nitrate precursor is reported here.
The morphology, porosity and connectivity of porous products in
different length scales were characterized by field emission scan-
ning electron microscopy, X-ray diffraction and nitrogen ad-
sorption measurements. Their porous structures were found to
be hierarchical from 1 up to 25 lm (in micrometer scale) and
from 2 nm to 60 nm (in nanometer scale). Furthermore, depend-
ing on the heat-treatment temperatures, the porosity of the pine-
templated NiO can be designed.
much simpler and a more uniform structure that favors infiltra-
tion of precursor solution. Over 90% of cells in softwood run
vertically and resemble long, straight tubes, which are tracheids.
The tracheids are characterized by prominent bordered pits
along their walls through which materials can flow from or
into the neighboring cells. In addition, resin canals and rays are
present in softwood perpendicular to the tracheids.
The knowledge of porous structure is an important step in
characterizing materials and predicting their behavior. Because
of this reason, the porosity of pine-templated NiO was analyzed
in detail to investigate the relationship between the preparation
factor and the porosity. Four calcination temperatures (6001,
8001, 10001, and 12001C) were utilized for this study. The results
suggest not only the inheritance of hierarchical porous struc-
tures from pine template but also the presence of designable
porous structure of a biomorphous oxide by heat treatment at
both micrometer and nanometer scales.
I. Introduction
HE process of in situ modification with biotemplates1–5 has
attracted considerable attention because of the structural
T
and compositional hierarchical order and its unique perform-
ance in the templates different from man-made materials. Wood
is a remarkable example of a biotemplate, because of its hier-
archical porous structure and consequent combination proper-
ties of high strength, stiffness, toughness, and low density. Their
porous structures range from the millimeter scale (growth ring
and vessel pore), via the micrometer scale (longitudinal tracheid,
fiber and transverse ray parenchyma), to the nanometer scale
(molecular fiber and cell membrane).6 Previously, some groups
have made use of wood templates and fabricated wood-tem-
plated carbide ceramics such as SiC7–11 and TiC,12,13 oxide ce-
ramics such as SiO2,14 Al2O3,15 and TiO2,16–17 and composite
ceramics such as SiOC/C,18 SiSiC,19 and YSZ.20
II. Experimental Procedure
(1) Materials and Synthesis
Pinewood pieces with a dimension of 20 mmꢀ 20 mm ꢀ 10 mm
were cut in transverse sections. Extractives such as resins, salts,
and glues in the wood were removed using boiling 5% ammonia
solution for 6 h before precursor infiltration. Extracted speci-
mens were washed with deionized water and dried at 801C for 24
h. The precursor solution for NiO was prepared with 30 mmol
Ni(NO3)2 dissolved in a mixture solvent of 10 mL ethanol and 5
mL H2O. A high volume ratio of ethanol could provide enough
OHꢁ for the hydrolysis of nitrate. Later, the samples were
dipped in the precursor solution at 601C for 72 h in a glass con-
tainer. New precursor solution was supplemented, as liquid
evaporates, to keep the specimens submerged. Finally, the sam-
ples were taken out from the solution and air dried at 601C for
24 h, calcined at 6001, 8001, 10001, and 12001C for 3 h in the
atmosphere and air cooled to room temperature. The weight
changes of dry specimens in the preparation process are listed in
Table I. After all the steps, the pine specimen showed a weight
change of 53 wt%.
Numerous efforts have also been focused on the synthesis of
nickel oxide (NiO) because of its diverse applications in electri-
cal, optical, and magnetic fields.21–26 However, in the fabrication
of conventionally produced NiO ceramics with a porous struc-
ture and a high surface area it is still a challenge to enhance their
properties as catalysts,21 gas sensors,22,24 electrodes,22 electro-
chemical capacitors,23 etc. The incorporation of porosity is the
most plausible way of reducing the dielectric constant for inte-
grated circuits and photonic band gap materials, while large
surface area and a high pore connectivity are essential electrical
properties for large capacitance because of the decrease of an
effective current density and the effect on energy storage and
conversion. Therefore, biotemplates are exploited to obtain a
new route for the preparation of porous NiO.
(2) Characterization
Pine-templated bulk NiO samples were observed through a field
emission scanning electron microscope (FESEM) by an ELIO-
NIX ERA-8800FE analyzer (Elionix Inc., Tokyo, Japan). The
samples were pulverized with an agate mortar for X-ray diffrac-
tion (XRD) and nitrogen adsorption measurements. XRD pat-
terns were recorded on a JEOL JDX-3530 X-ray diffractometer
system (Jeol Ltd., Tokyo, Japan) with Cu Ka radiation and were
used to estimate the grain size by the Scherrer Equation. Nitro-
gen adsorption isotherms were measured with a Micromeritics
ASAP 2010 adsorption analyzer (Micromeritics Instrument
Corp., Norcross, GA) at 77 K. The pore size distribution
(PSD) curves were analyzed with the Barrett, Joyner, and Ha-
lenda (BJH) algorithm on the basis of the relationship between
the pore size and capillary condensation pressure.27,28 The sur-
face area was calculated using the Brunauer, Emmett, and Teller
In this work, we have applied pinewood as a template for the
hydrothermal synthesis of hierarchical porous NiO by sequen-
tial impregnation/calcination. In contrast to hardwood, pine-
wood (a softwood) is more suitable as a template because of a
W. M. Mullins—contributing editor
Manuscript No. 20714. Received June 28, 2005; approved August 18, 2005.
Supported by the National Natural Science Foundation of China (No. 50371055), Ex-
cellent Young Teachers Program of MOE of China, Key Project on Basic Research of
Shanghai (No. 03JC14044), Major Project on Basic research of Shanghai (No. 04DZ14002),
Fok Ying Tung Education Fund (No. 94010), Program for New Century Excellent Talents
in University, Nano-research Program of Shanghai (No. 0452nm045), Basic Research Pro-
gram of China (No. 2006CB601200) and Research Program of Shanghai (No. 045211059).
wAuthor to whom correspondence should be addressed. e-mail zhangdi@sjtu.edu.cn
662

February 2006
Weight (g)
Synthesis of Biomorphous Nickel Oxide from Pinewood Template
663
Table I. Weight Changes of Pine Specimens
(Fig. 1(b)) shows a well-arranged and connected porous struc-
ture. Four kinds of pores are marked (a, b, c, and d) in Fig. 1(b).
The largest a-type pores (about 20–25 mm) and b-type pores
(about 10–15 mm) were derived from dissimilar tracheid cells.
Because of the anisotropic thermal contraction on the process of
drying and calcination, some cracks were created on the cell
walls and were named c-type pores (about 5 mm) and d-type
pores (below 1 mm). Figure 1(c) shows the enlarged pits’ micro-
structure indicated by an arrow in Fig. 1(a) in the tangential
section. The cracks were also formed on the walls of the pits.
Various types of pores in the transverse section and pit pores in
the tangential section make the NiO microstructure hierarchical
and well connective.
Before
After
After
After calcination
treatment
extraction
infiltration
(8001C)
10.40
9.35
24.24
5.52
When the calcination temperature continued to increase, the
crystallinity of NiO increased, but the pine’s structure was still
preserved by NiO in the main, as shown in Fig. 2. Figure 2(a)
was taken from the tangential section of NiO calcined at
12001C, which reproduced the fiber’s surface, while Fig. 2(b)
was taken from the transverse section. More cracks were pro-
duced on the fiber walls by calcination at 12001C as shown in
Fig. 2(a). In comparison with the tracheid pores in Fig. 1(b), the
pores in Fig. 2(b) arose from the same original cells, tracheid,
but shrank from 20 to 25 mm (a-type pores) at 6001C down to
about 10 mm in diameter. The thermal contraction by high heat-
treatment temperature causes a large shrinkage of approximate-
ly 50%–60% and more cracks at the pore walls, which can in-
crease the connectivity of the NiO product. By altering the
calcination temperature, the transformation of NiO structure
suggests the presence of designable porous structure of a
biomorphous NiO at the micrometer scale.
Fig. 1. Field emission scanning electron microscope images of different
magnitudes taken from the pine-templated NiO calcined at 6001C.
(a) Transverse and tangential section; (b) transverse section; and (c) tan-
gential section.
(2) XRD
XRD analysis was used to validate the components of our pine-
templated pulverized samples from 6001 to 12001C as illustrated
in Fig. 3. All the XRD patterns show well-resolved reflections,
indicating a single phase of cubic NiO. A higher calcination
temperature caused higher and sharper peaks, which means larg-
er grain size of NiO according to the calculated data from the
Scherrer equation. The grain size changes from 20.7 nm (6001C)
to 37.4 nm (12001C) with temperature as listed in Table II.
(BET) equation.29 The connectivity Z was also determined from
nitrogen adsorption isotherms according to the percolation the-
ory established by Seaton et al.27–30
III. Results and Discussion
(1) FESEM
(3) Porosity
Figure 1 shows the microstructure characteristics with different
magnitudes taken from the pine-templated NiO calcined at
6001C. The original cellular anatomy of the pine template was
retained integrally in the NiO product. A NiO fiber assembly
can be observed in Fig. 1(a) along the tangential section, which
was derived from long and channel-like tracheid cells, with
functions of conducting water and supporting in pine. The
pits, through which the Ni-precursor penetrated efficiently,
were also preserved by NiO as indicated by the arrow in
Fig. 1(a). A higher magnification image in transverse section
The nitrogen adsorption analyses of porosity (pore size, volume,
and distribution etc.) and surface area of the samples calcined
from 6001 to 12001C are shown in Table II and Fig. 4. As com-
pared in Table II, with the increase of calcination temperature,
both BET surface area and porosity decrease obviously. This
suggests that the grain size of NiO can influence the porosity
directly: a larger grain causes a smaller void space (pore) at the
nanometer scale.
Nitrogen adsorption and desorption isotherms and corre-
sponding PSD curves are shown in Fig. 4, and the enlarged
Fig. 2. Field emission scanning electron microscope images of different magnitudes taken from the pine-templated NiO calcined at 12001C.

664
Journal of the American Ceramic Society—Liu et al.
Vol. 89, No. 2
(a)
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0.00
60
(b)
600°C
800°C
NiO
600°C
1000°C
1200°C
50
40
30
20
10
0
800°C
1000°C
1200°C
1200°C
1000°C
800°C
0.0 0.2 0.4 0.6 0.8 1.0
1
10
100
600°C
Pore Diameter nm
Relative Pressure P/P0
Fig. 4. (a) Nitrogen isotherms of pine-templated NiO calcined at 6001,
8001, 10001, and 12001C; and (b) corresponding pore size distribution
curves of these samples.
10
20
30
40
50
60
70
80
90
2-Theta-Scale
Fig. 3. X-ray diffraction patterns of pine-templated NiO calcined at
6001, 8001, 10001, and 12001C.
(40.5), no hysteresis loop results from the capillary condensa-
tion phenomenon in macropores.
On the other hand, according to the PSD curves as shown in
Figs. 4(b) and 5, the samples concentrate their pore sizes in 7
and 47 nm at 6001C, 45 nm at 8001C, and 2 and 60 nm at
10001C. When the temperature increased up to 10001C, the me-
sopores in 6001C shrank to micropores because of the grain
growth, accompanied by more macropores appearing because of
the anisotropic thermal contraction as mentioned in Fig. 2.
However, after the temperature was increased up to 12001C, the
monomodal PSD curve was substituted by a multimodal curve
in the whole measuring range. A high calcination temperature
shrinks the pore volumetrically but imparts higher connectivity,
thereby shifting it to the nanometer-scale hierarchy, owing to the
pore’s diversity.
Table II. Porosity and Grain Size of Samples
Calcination
Grain size
(nm) (200)
BET surface
area (m2/g)
Pore volume
(cm3/g)
Average pore
diameter (nm)
temperature (1C)
600
800
1000
1200
20.7
31.4
36.4
37.4
19.74
4.01
1.29
0.77
0.0895
0.0213
0.0037
0.0016
18.4
14.7
11.8
10.1
The grain size of pine-templated NiO powder samples was calculated from the
data of the XRD pattern. BET surface area, pore volume, and average pore di-
ameter were calculated from nitrogen adsorption isotherms. XRD, X-ray diffrac-
tion; BET, Brunauer, Emmett, and Teller
(4) Connectivity Z
Connectivity Z and average linear dimension L can be solved by
fitting a series of (F, f) data for the function h:
curves of the 10001 and 12001C samples are individually shown
in Fig. 5. With the increase in calcination temperature, the iso-
therms transformed from a IV-type with an H3-type hysteresis
loop for the 6001 and 8001C samples, to a II-type one for the
10001 and 12001C samples where the hysteresis loop almost dis-
appeared completely.The IV-type isotherm is caused by the
capillary condensation of mesopores, and a narrow H3-type
hysteresis loop is attributed to asymmetric slot-shaped meso-
pores and channels in coincidence with the tubular characteristic
pores in pine. This kind of isotherm indicates an interconnected
mesoporous system and a very high pore connectivity according to
the percolation theory. Different from the IV-type curve formed
by mesopores, the II-type isotherm is attributed to macro-
pores. At a low relative pressure (o0.5), the nitrogen-adsorbing
action of macropores was the same as the monolayer adsorption
of mesopores. However, above the medium relative pressure
Lb=gZF ¼ h½ðZf ꢁ 1:5ÞL1=g
ꢂ
where b and g are critical exponents that have the fixed values of
0.41 and 0.88, respectively. The function h has a certain shape
for all three-dimensional systems. The bond occupation proba-
bility f is obtained as a function of percolation probability F
from the adsorption and desorption isotherms, using the PSD
curves as an input.
From the fitting results shown in the histogram (Fig. 6), the
relationship between connectivity and calcination temperature
is evident. A higher temperature leads to a better connectivity,
consistent with the above analysis from isotherms. The dramatic
(a)
(b)
Pore Diameter (nm)
10
Pore Diameter (nm)
10
1
100
1.2
1
100
1000°C
0.0012
0.0010
0.0008
0.0006
0.0004
0.0002
2. 5
2.0
1.5
1.0
0.5
0.0035
0.0030
0.0025
0.0020
0.0015
0.0010
0.0005
1200°C
PSD
1.0
0.8
0.6
0.4
0.2
0.0
PSD
isotherm
0.8
isotherm
0.8
0.0
0.2
0.4
0.6
1.0
0.0
0.2
0.4
0.6
1.0
Relative Pressure P/P
Relative Pressure P/P
0
0
Fig. 5. The enlarged isotherms and pore size distribution curves of pine-templated NiO samples: (a) calcined at 10001C; and (b) calcined at 12001C.

February 2006
Synthesis of Biomorphous Nickel Oxide from Pinewood Template
665
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more macropores led to an excellent connectivity property.
IV. Conclusions
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Pine-templated NiO prepared via our simple and efficient hy-
drothermal route proved to retain the hierarchical porous struc-
ture from natural wood. Besides preserving the pores from
wood, our process of synthesis created new kinds of pores at
the cellular walls, making the NiO more connective. The mor-
phology and porosity of NiO samples were found to be depend-
ent on the heat-treatment conditions. By controlling the pore
size and distribution of biomorphous NiO, its applications in the
optical, electrical and magnetic fields are possible.
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