Facile route to nanoporous NiO structures from the α-Ni(OH) 2/EG precursor and application in water treatment

Article abstract of DOI:10.1111/j.1551-2916.2008.02762.x

Nanoporous actinia-shaped NiO structures have been synthesized by thermal decomposition of an actinia-shaped precursor that consists of the coalescence of α-Ni(OH)₂/EG nanosheets achieved by the hydrothermal method. A possible formation mechanism is proposed to explain the formation of nanoporous actinia-shaped NiO structures. After calcination at different temperatures of 320° and 400°C, the actinia-like precursor completely transformed into nanoporous actinia-shaped NiO structures, which possessed a high specific surface and high nanoporous structures. As the calcination temperature increased, the BET surface area and the average pore diameter of products decreased, indicative of partial collapse of the nanoporous structures. The as-obtained nanoporous actinia-shaped NiO structures show an effective photocatalytic degradation of acid red 1 as well as a significant adsorption capacity of toxic chromium in water.

Full text of DOI:10.1111/j.1551-2916.2008.02762.x

J. Am. Ceram. Soc., 91 [12] 4105–4108 (2008)
DOI: 10.1111/j.1551-2916.2008.02762.x
r 2008 The American Ceramic Society
ournal
J
Facile Route to Nanoporous NiO Structures from the a-Ni(OH)2/EG
Precursor and Application in Water Treatment
Xuefeng Song and Lian Gao*,w
State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics,
Chinese Academy of Sciences, Shanghai 200050, China
Nanoporous actinia-shaped NiO structures have been synthe-
sized by thermal decomposition of an actinia-shaped precursor
that consists of the coalescence of a-Ni(OH)2/EG nanosheets
achieved by the hydrothermal method. A possible formation
mechanism is proposed to explain the formation of nanoporous
actinia-shaped NiO structures. After calcination at different
temperatures of 3201 and 4001C, the actinia-like precursor
completely transformed into nanoporous actinia-shaped NiO
structures, which possessed a high specific surface and high
nanoporous structures. As the calcination temperature in-
creased, the BET surface area and the average pore diameter
of products decreased, indicative of partial collapse of the nano-
porous structures. The as-obtained nanoporous actinia-shaped
NiO structures show an effective photocatalytic degradation of
acid red 1 as well as a significant adsorption capacity of toxic
chromium in water.
tional properties superior to counterpart bulk materials. A large
surface area and a nanoscale active pore wall make nanoporous
materials interact actively with atoms, ions, and molecules, and
so they easily capture liquid and gas molecules, and solid par-
ticles, which is beneficial for their application in catalysis, water
cleaning, and sensing.10 Therefore, developing a facile and tem-
plate-free method to prepare nanoporous NiO structures is of
scientific and practical importance.
II. Experimental Procedures
This work presents a simple template-free strategy to synthesize
nanoporous NiO structures with an actinia shape using the sol-
vothermal method, together with a subsequent calcination pro-
cess. All the chemicals were purchased from Shanghai Chemical
Reagent Co. (Shanghai, China) and used as received without
any purification. In a typical experiment, 0.28 g of NiCl2 Á 6H2O
was dissolved in 10 mL of ethylene glycol (EG) in a warm-water
bath at 801C, and then 0.7 g of sodium acetate (NaAc) was
added the mixture under vigorous stirring. The as-formed green
transparent solution was transferred into a 50 mL teflon-lined
autoclave, and maintained at 2001C for 14 h. After cooling to
room temperature, the gray product was separated by cen-
trifugation and washed several times by ethanol and deionized
water, respectively, and then dried in a dynamic vacuum at
601C. These nickel oxide precursors were calcined at 3201 and
4001C for 1 h to obtain nanoporous cubic NiO, which were,
respectively, denoted as T320 and T400.
The structure of the product was characterized by X-ray
diffraction (XRD) (CuKa, l 5 0.15418 nm, Rigaku D/max
2550V, Tokyo, Japan). The morphologies of the samples were
observed using a transmission electron microscope (TEM)
(JEM200CX, JEOL, Tokyo, Japan) and scanning electron
microscopy (SEM) (JSM 6700F, Tokyo, Japan). A Fourier
transform infrared (FT-IR) spectrometer was used to collect
the spectra of the samples (NEXUS, Nicolet, Madison, WI).
Nitrogen adsorption/desorption isotherms were obtained at 77
K with a Micromeritics ASAP 2010 micropore analysis system
(Micromeritics, Norcross, GA). Surface areas were determined
by the BET method, and average pore diameters were deter-
mined using the Barrett–Joyner–Halenda (BJH) method.
I. Introduction
ANOPOROUS materials have been extensively investigated for
their versatility in electrical, optical, and photochemical
N
properties, and thus they have been applied in many fields such
as catalysis,1 molecular separation,2 photonics,3 and gas sen-
sors.4 Many methods for creating nanopores have been re-
ported. Generally, templating-synthesis strategies are the most
popular for the preparation of novel nanoporous materials
including hard and soft templates. Zeolite, mesoporous silica,
and porous anodic alumina are commonly utilized in the hard
template method. The soft templates generally include surfact-
ant micelles, polymer, and organic-based molecules.5 Liu et al.6
used Ni2(CO3)(OH)2 as the precursor obtained by polyethylene
glycol (PEG) as the soft template to synthesize urchin-like nano-
porous NiO nanostructures with a higher specific capacitance
and good cycling performance. Chen et al.7 successfully synthe-
sized nanoporous hematite nanotubes with enhanced electro-
chemical activity in lithium-ion batteries and good sensitivity to
alcohol using porous anodic alumina as hard templates. Al-
though considerable progress has been made in the synthesis of
nanoporous metal oxide structures,6,8,9 there are few researches
on the synthesis of nanoporous NiO structures with an actinia
shape using the template-free method. Recently, the discovery of
nanoporous materials has provided a novel route to obtain
powerful functional materials because the large surface area and
nanosize active pore wall likely confer them with novel func-
III. Results and Discussion
Figures 1(a) and (b) depict the SEM image and TEM image of
the as-obtained precursor, respectively, showing that the gray
precursor is actinia shaped with diameter of about 620 nm.
Figure 1(b) indicates that the actinia-shaped architectures are
assembled by several nanosheets and the surfaces of the nano-
sheets are rather smooth. The XRD pattern (Fig. 1(c)) of the
resulting precursor shows that the diffraction peaks are in good
agreement with a-Ni(OH)2. In the FT-IR spectra (Fig. 1(d)) of a
rod-like precursor, vibrational bands of CH2 at 2946 and 2869
R. K. Sahu—contributing editor
Manuscript No. 24980. Received February 9, 2008; approved September 9, 2008.
This work was supported by the National Natural Science Foundation of China
(Nos.50572116, 5060249) and Shanghai Nanotechnology Promotion Center (No.
0652nm022), respectively.
*Member, The American Ceramic Society.
wAuthor to whom correspondence should be addressed. e-mail: liangaoc@online.sh.cn
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Vol. 91, No. 12
(c)
003
(d)
101
O-H
110
006
C-OH
1000
O-H
3500
Ni-OH
10
20
30
40
2θ/deg
50
60
70
3000
2500
Wavenumber (cm−1
2000
1500
500
)
Fig. 1. (a) Scanning electron microscopic (SEM) image, (b) transmission electron microscopic (TEM) image of the as-obtained actinia-shaped nickel
oxide precursor, (c) X-ray diffraction (XRD) pattern of the as-obtained nickel oxide precursor, and (d) fourier transform infrared (FT-IR) spectrum of
the nickel oxide precursor.
cmÀ1 as well as the stretching band of C–OH at 1061 cmÀ1 de-
rived from an EG unit were observed.11 The absorption at 650
cmÀ1 is due to the Ni–OH bending vibration. Combining the
results of FT-IR and XRD, we can conclude that the precursor
is an a-Ni(OH)2/EG composite and the EG may intercalate into
the interlayered place of a-Ni(OH)2.
BJH desorption average pore diameter reduces to ca. 6.8 nm,
indicating the partial collapse of the mesopore structure in the
as-synthesized materials as the annealing temperature increases.
Based on the above results and analysis, we can propose a
reasonable formation mechanism for the actinia-shaped nano-
porous NiO structures. First, the hydrolysis of the acetate group
results in the formation of OHÀ ions, which offer a weak basic
environment for the formation of nickel hydroxide. Second,
Ni21 ions react with OHÀ to form sheet-like a-Ni(OH)2 in the
EG solution. Sheet-like a-Ni(OH)2 as the dominant morphology
is due to its layered structure, while, EG may intercalate into
the interlayered space of the a-Ni(OH)2 sheet to form an
a-Ni(OH)2/EG composite. Third, sheet-like a-Ni(OH)2/EG
composites spontaneously attach together to form actinia-
shaped aggregates to minimize their surface energy by reducing
the exposed areas. Moreover, glycol helps to generate a highly
viscous and stable mixture solution, which favors the stable
growth of the formation of an actinia-shaped a-Ni(OH)2/EG
composite with nanosheets as building blocks.13 Finally, after
calcination, the actinia-shaped a-Ni(OH)2/EG composites trans-
fer into highly porous NiO structures, maintaining a precursor
morphology by virtue of the removal of organic species and
water in the precursor by pyrolysis.
The photocatalytic activity of the product was tested during
the degradation of aqueous acid red 1 solution (20 mg/L). The
suspension was irradiated with a 300 W medium-pressure mer-
cury lamp. The concentration of the photocatalyst was 0.6 g/L.
Oxygen was bubbled into the reaction suspension at a flow rate
of 100 mL/min in the entire experimental process. Samples were
taken out at regular time intervals, and the concentration of acid
red 1 in the aqueous solution was detected using the Lambda
950 UV-vis spectrophotometer by collecting the absorbance of
acid red 1 at 505 nm. From Fig. 4, it can be seen that the T320
sample has a higher photocatalytic activity than the T400 sam-
ple. The T320 sample shows a significant activity as high as 95%
after 200 min of UV-vis light irradiation. However, the
T400 sample presents a lower activity of ca. 70%. The effective
After calcination at 3201C for 1 h, the SEM image (Fig. 2(a))
and TEM image (Fig. 2(b)) of the product showed that the
smooth surfaces of nanosheets transformed into porous and
rough structures. The TEM image (Fig. 2(b)) further confirms
the nanoporous nature of the material with a pore size of several
nanometers. High-resolution TEM (HRTEM) image indicates
the polycrystalline characteristic of the material (Fig. 2(c)). The
interplanar distance corresponding to the spacing of the (111)
plane is 0.241 nm. The distance of the crystal plane having a
dihedral angle of 54.71 with the (111) plane is 0.206 nm, which is
consistent with the (200) plane of the NiO nanoparticle. Con-
tinuous sharp circles on the selected-area electron diffraction
(SAED) pattern (inset of Fig. 2(c)) further confirm the poly-
crystalline nature of nanoporous NiO structures. The XRD pat-
terns (Fig. 2(d)) show that the as-obtained sample is made of
cubic NiO (JCPDS 78–0643). The average crystal sizes of sam-
ples T320 and T400 were calculated to be 10.2 and 16.5 nm,
respectively, indicating that the average size of NiO nano-
particles increases as the calcination temperature increases.
Figure 3 gives the nitrogen adsorption/desorption isotherms
and pore size distribution of the samples annealed at 3201 and
4001C, respectively. The hysteresis loop (inset) appearing in the
relative pressure (p/p0) range of 0.45À1 indicates the presence of
mesopores in the as-obtained samples, which is associated with
the mesopore filling and the monolayer coverage of the meso-
pores.12 The BET surface area of the T320 sample is 167.7 m2/g,
and the pore-size distribution plot shows a bimodal mesopore
distribution, whose pore sizes are ca. 3.5 and 14.9 nm obtained
by the desorption branch. As the annealing temperature in-
creases to 4001C, the BET surface area of sample T400 decreases
to 93.2 m2/g, and the peak of pore size distribution bates and the

December 2008
Communications of the American Ceramic Society
4107
(a)
(b)
200
(c)
(d)
111
220
i
ii
10
20
30
40
2θ/deg
50
60
70
Fig. 2. (a) Scanning electron microscopic (SEM) image, (b) transmission electron microscopic (TEM) image, and (c) high-resolution transmission
electron microscopic (HRTEM) image, (with the selected-area electron diffraction (SAED) pattern in the inset) of the nanoporous actinia-shaped NiO
structures, (d) X-ray diffraction (XRD) patterns of as-annealed nanoporous NiO obtained at different temperatures: (i) 4001C, and (ii) 3201C.
photocatalytic activity of the T320 sample could be due to the
higher specific surface area and the higher photoadsorption effi-
ciency. T320 samples with large accessible surface areas and
more pore-wall structures are of particular interest, as they can
afford more active sites to adsorbate molecules and facilitate
diffusion of reactant during the reaction, which is beneficial to
the enhancement of catalytic activity.
A kinetic study was carried out to determine the rate of
Cr(VI) removal by nanoporous actinia-shaped NiO structures.
Several potassium dichromate (K2Cr2O7) solutions containing
the same initial concentration of Cr (54 mg/L) were prepared.
Then 0.1 g NiO powder was added to 50 mL of the above
solutions under stirring at room temperature, respectively.
The mixture was stirred at room temperature for different
time intervals and then filtered to remove the solid. The adsorp-
tion capacities of products for Cr(VI) were measured using the
Lambda 950 UV-vis spectrophotometer. Figure 5 shows the ad-
sorption isotherms of Cr(VI) on the T400 and T320 samples,
indicative of the higher removal capacity of the T320 sample for
Cr(VI) than the T400 sample. It can be seen from the curve of
the T320 sample that the adsorption is rapid initially, and
about 70% of Cr(VI) is already removed. A steady value is
reached up to 150 min; the removal capacity is measured to be
26.5 mg Á CrÁ gÀ1, at which Cr(VI) is almost completely re-
250
200
1.0
T400
T320
a
150
0.8
100
0.6
50
b
0
a
0.0
0.2
0.4
0.6
0.8
1.0
0.4
0.2
0.0
Relative Pressure (p/p )
0
b
0
10
20
Pore Diameter (nm)
30
40
0
50
100
150
200
Fig. 3. Barrett–Joyner–Halenda (BJH) pore-size distribution curves of
different products: (a) T320, (b) T400. The inset shows the N2 adsorp-
tion–desorption isotherms. The pore-size distribution curves are derived
from the desorption branch of the isotherms.
Irradation Time (min)
Fig. 4. Relationship plots of photodegradation rate of acid red 1 with
irradiation time by UV-vis light for different catalysts: T400 and T320.

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Vol. 91, No. 12
high-nanoporous structures. The as-obtained products are
promising catalyst and adsorption materials for photocatalytic
degradation of organic dyes and for reducing toxic chromium
pollution in water.
1.0
T400
T320
0.8
0.6
0.4
0.2
0.0
References
1X. He and D. Antoneili, ‘‘Recent Advances in Synthesis and Applications of
Transition Metal Containing Mesoporous Molecular Sieves,’’ Angew. Chem. Int.
Ed., 41, 214–7 (2001).
2J. M. Kisler, A. Dahler, G. W. Stevens, and J. O’Connor, ‘‘Separation of
¨
Biological Molecules Using Mesoporous Molecular Sieves,’’ Microporous Meso-
porous Mater., 44, 769–74 (2001).
3X. S. Zhao, F. B. Su, Q. F. Yan, W. P. Guo, X. Y. Bao, L. Lv, and Z. C. Zhou,
‘‘Templating Methods for Preparation of Porous Structures,’’ J. Mater. Chem., 16,
637–48 (2006).
4M. E. Davis, ‘‘Ordered Porous Materials for Emerging Applications,’’ Nature,
417, 813–9 (2002).
5Y. S. Hu, Y. G. Guo, W. Sigle, S. Hore, P. Balaya, and J. Maier, ‘‘Electro-
chemical Lithiation Synthesis of Nanoporous Materials with Superior Catalytic
and Capacitive Activity,’’ Nat. Mater., 5, 713–7 (2006).
0
50
100
150
200
Time (min)
6X. M. Liu, X. G. Zhang, and S. Y. Fu, ‘‘Preparation of Urchinlike NiO Nano-
structures and their Electrochemical Capacitive Behaviors,’’ Mater. Res. Bull., 41,
620–7 (2006).
Fig. 5. Adsorption rate curves of Cr (VI) on the T400 and T320 sam-
ples.
7J. Chen, L. N. Xu, W. Y. Li, and X. L. Gou, ‘‘a-Fe2O3 Nanotubes in Gas
Sensor and Lithium-Ion Battery Applications,’’ Adv. Mater., 17, 582–6 (2005).
8R. K. Sahu, A. K. Ray, S. K. Das, A. J. Kailath, and L. C. Pathak, ‘‘Micro-
wave-Assisted Combustion Synthesis of Ni Powder Using Urea,’’ J. Mater. Res.,
21, 1664–73 (2006).
moved. However, the removal capacity of the T400 sample is
only 18.4 Á mg Á Cr gÀ1. The mechanism for the removal of metal
cationic contaminant was proposed to be through the formation
of surface complexation and ion exchange between the oxide
surface and the toxic ions.11 Therefore, the T320 sample with
higher surface areas and more pore-wall structures is more ad-
vantageous to remove Cr(VI) ions from water.
9J. Liu and D. F. Xue, ‘‘Thermal Oxidation Strategy Towards Porous Metal
Oxide Hollow Architectures,’’ Adv. Mater., 20, 2622–7 (2008).
10X. S. Zhao, ‘‘Novel Porous Materials for Emerging Applications,’’ J. Mater.
Chem., 16, 623–5 (2006).
11L. S. Zhong, J. S. Hu, H. P. Liang, A. M. Cao, W. G. Song, and L. J. Wan,
‘‘Self-Assembled 3D Flowerlike Iron Oxide Nanostructures and their Application
in Water Treatment,’’ Adv. Mater., 18, 2426–30 (2006).
12J. H. Huang and L. Gao, ‘‘Synthesis and Characterization of Porous Single-
Crystal-Like In2O3 Nanostructures Via a Solvothermal-Annealing Route,’’ J. Am.
Ceram. Soc., 89, 724–7 (2006).
IV. Conclusions
In summary, nanoporous actinia-shaped NiO materials are
synthesized by a facile hydrothermal method combined with a
calcination process. They possess high specific surface areas and
13L. X. Yang, Y. J. Zhu, L. Li, L. Zhang, H. Tong, W. W. Wang, G. F. Cheng,
and J. F. Zhu, ‘‘A Facile Hydrothermal Route to Flower-Like Cobalt Hydroxide
and Oxide,’’ Eur. J. Inorg. Chem., 4787–90 (2006).
&