A facile synthesis of NiO nanosheet with high-energy (111) surface

Article abstract of DOI:10.1246/cl.2011.156

NiO nanosheet with high-energy (111) polar surface was prepared in high yield under mild conditions using nickel acetate tetrahydrate as starting material via thermal decomposition of intermediate Ni(OH)(OCH₃). The NiO(111) nanosheet is far more active than conventional NiO for ethylbenzene dehydrogenation.

Full text of DOI:10.1246/cl.2011.156

doi:10.1246/cl.2011.156
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Published on the web January 15, 2011
A Facile Synthesis of NiO Nanosheet with High-energy (111) Surface
1
2
3
Kake Zhu,* Weiming Hua, and Xingyi Wang
1
UNILAB, Department of Chemical Engineering, East China University of Science and Technology,
Shanghai 200237, P. R. China
2
Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Department of Chemistry,
Fudan University, Shanghai 200433, P. R. China
3
Advanced Material Lab, Research Institute of Industrial Catalysis,
East China University of Science and Technology, Shanghai 200237, P. R. China
(
Received October 29, 2010; CL-100916; E-mail: kakezhu@ecust.edu.cn)
Ni2 and anionic O2¹ that binds them together also gives the
material a high ionic character and melting point (1955 °C). NiO
is an antiferromagnetic semiconductor with a wide band gap
+
NiO nanosheet with high-energy (111) polar surface was
prepared in high yield under mild conditions using nickel acetate
tetrahydrate as starting material via thermal decomposition of
intermediate Ni(OH)(OCH3). The NiO(111) nanosheet is far more
active than conventional NiO for ethylbenzene dehydrogenation.
1
2
of ca. 3.6 eV and is finding applications in heterogeneous
1
3
14
15
catalysis, electrochromic films, optical fibers, and Li-ion
batteries.
1
6,17
In the rock salt structure, both the (100) and (110)
The activity and selectivity of a catalyst is in principle
dictated by the electronic structure of the solid surface, which can
be tuned by composition or physical structure at the outermost
surface are composed of well-balanced nickel cations and
9
oxygen anions and are nonpolar Tasker I type surfaces. The
1
8
(111) surface, however, is a Tasker type III surface. The
NiO(111) nanosheet has exhibited superior surface activity to
1
layer. Surface structure control has proven to be an effective way
7
,19
to promote catalytic activities. It has been shown both exper-
imentally and theoretically that metal surfaces rich in steps and
low-coordinated sites with dangling bonds are active sites in
methanol activation and dye adsorption.
From literature survey, it is found that NiO nanosheet
structures in various forms have recently been reported. For
1
­3
20
catalysis, while on metal oxides the high-energy surfaces are
instance, Zhu and co-workers have reported ¢-Ni(OH) with
2
4­8
more favorable for heterogeneous catalysis. Surface orienta-
tion control can endow metal oxides of earth-abundant elements
with new or elevated catalytic activity. Unfortunately, high-
energy surfaces are not so stable during preparation and are more
likely to diminish during crystal growth, and thereby most of the
exposed surfaces of conventionally prepared oxides are domi-
nated by less reactive sites. As a result, studies on high-energy
surfaces are limited to single-crystal surfaces under ultrahigh
vacuum (UHV) conditions or thin films, which is particularly true
nanosheet structure that was prepared in mixed solvent media
at 200 °C. Thermal decomposition of ¢-Ni(OH)2 produces NiO
with sheet-like morphology, but the crystalline orientation of
2
1
NiO nanosheet is not preferential. Liu and co-workers have
prepared NiO nanosheet by means of solvent thermal synthesis
under autogenic pressure at 150 °C in the presence of anionic
surfactant, with Ni2CO3(OH)2 as an intermediate. The NiO sheet
preferentially grows along the nonpolar (200) and (220) facet
7
directions. Some of us as well as Richards have developed a
9
for the Tasker III type surfaces of metal oxides. To bridge up the
method to prepare NiO(111) nanosheet in supercritical methanol
(>245 °C), which entails high-temperature/pressure manipula-
tions. Herein, we report a facile synthesis of NiO(111) nanosheet
with Tasker III type surface under mild conditions via the
formation of an intermediate Ni(OH)(OCH3) under methanol
thermal conditions. Easily available nickel acetate tetrahydrate
was used as a precursor, and the productivity was improved to
grams level in a 130-mL autoclave due to the relatively high-
concentration solution used.
pressure gap and the material gap in these studies, it is desirable
to develop scalable methods to prepare metal oxides exposing
mainly such high-energy surfaces.
A Tasker III surface is a surface composed of alternating
positively charged anions and negatively charged cations, and a
polarity is built up perpendicular to the surface. Interest in this
type of surface has stemmed from the stabilization mechanism
and later to their interactions with guest molecules. Attempts have
been made to meet the formidable challenge to produce powder-
form metal oxides exposing Tasker III type surfaces, and a few
To prepare NiO(111) nanosheet, 12.44 g of nickel acetate
tetrahydrate (Sinopharmacy, ²98%) was dissolved in 60.0 mL
of anhydrous methanol (Sinopharmacy, ²99.5%) and 10.82 g of
benzyl alcohol (Lingfeng Chemical, ²99%) under stirring. The
resultant mixture was further stirred for 1 h to form a clear
solution. The blue solution was transferred into a 130-mL
Telfon-lined autoclave, which was subsequently heated in an
oven at 180 °C for 36 h. A blue powder was recovered by
filtration and methanol washing and air-dried overnight. Yield
of the as-prepared sample is ca. 95% on a metal basis according
successful examples have gained attention for their novel catalytic
or adsorptive properties.6
­8,10
For instance, hexagonal ZnO(0001)
platelet structure with polar surface is five times more active than
the rod-like ZnO with nonpolar surface in photocatalytic decom-
8
position of Methylene Blue. MgO nanosheet with (111) as major
surface has exhibited superior catalytic activity in transesterifica-
tion of sunflower or rapeseed oil to produce biofuels.6 In these
the Claisen­Schmidt condensation and transesterification have
highlighted the importance of preparation methods to tailor
oxides with Tasker III type high-energy surfaces, and it is still
challenging to find facile methods in tailoring such surfaces.
NiO is a typical ionic oxide that possesses a rock salt
structure, and the strong Coulomb attraction between cationic
,11
2
2
to the composition reported. The as-prepared powder was
¹
1
calcined in air at 550 °C for 4 h through a ramp of 3 °C min to
afford NiO(111) sheet. For comparison, NiO was also prepared
via thermal decomposition of nickel nitrate under the same
heating conditions.
Chem. Lett. 2011, 40, 156­158
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

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(
200)
(
a)
(b)
(003)
(111)
(
220)
NiO-nanosheet
(222)
311)
(
(
012)
(
015)
(
018) (0111)
110)
(1014)
113)
NiO
(
006)
(
(
101)
30
(107)
(
1010)
(
(
205)
10
20
40
50
60
70
80
10
20
30
40
50
60
70
80
2
θ
/°
2
θ
/°
(a)
(b)
(d)
Figure 1. XRD patterns of (a) as-prepared Ni(OH)(OCH3),
b) NiO(111) nanosheet and conventional NiO.
(
Powder X-ray diffraction (XRD) patterns of the as-prepared
and calcined samples are displayed in Figures 1a and 1b,
respectively. XRD pattern of conventional NiO from nitrate
thermal decomposition is also shown together with NiO nano-
sheet in Figure 1b. The as-prepared powder is identified as
Ni(OH)(OCH3), the indexation of which is according to
Le Bihan and co-workers22 in Figure 1a. Sharpness of peaks
indicates high crystallinity of Ni(OH)(OCH3). The selection rule
to the structure is ¹ h + k = 3n according to their hexagonal
indexation, a = 0.312 nm, c = 2.302 nm. The structure can also
be indexed to a brucite-like structure according to Kubo and co-
(
c)
Figure 2. TEM (a, b) and SEM (c, d) images of Ni(OH)(OCH3) with
sheet-like morphology.
2
3
workers, who reported the synthesis of Mg(OH)(OCH3) from
¹
¹
Mg(OH)2 and believed that OCH3 substitutes some OH groups
in the brucite structure and expands the lattice of crystalline
Mg(OH)2. Accordingly, they indexed the structure to a brucite
structure. At a matter of fact, there is still no consensus on the
definite structure of this metastable material, due to the small
particle size that challenges X-ray determination. The structure
of Mg(OH)(OCH3) is identical to that of Ni(OH)(OCH3) in
crystallinity and atomic radii,22 both are prepared via reaction
between turbostratic hydrated Mg/Ni hydroxide and methanol
under high temperature and pressures (for Mg, 245­315 °C and
220
0
2-2
202
(
a)
(b)
50 nm
5
1/nm
(c)
(
d)
240 bar; for Ni, 240 °C to get highly crystalline samples). From
Figure 1b, it is also seen that thermal decomposition of
Ni(OH)(OCH3) produces NiO with rock salt structure (JCPDS-
ICDD card No. 78-0643, space group: Fm3m [No. 225]), no
difference can be discerned by powder XRD between NiO
nanosheet and conventional NiO from nitrate decomposition.
From TEM images of Figures 2a and 2b, it is seen that
Figure 3. TEM images of NiO nanosheet (a, b) and its ED pattern
inset of b), SEM images of (c) NiO nanosheet and (d) conventional
NiO.
(
ꢀ
(OCH3), ranging from 50 nm to ca. 200 nm, as a result of the
lattice shrinkage or break of crystals during thermal decom-
position. Furthermore, the NiO sheet becomes more porous when
the organics are removed by combustion, which is similar to what
Ni(OH)(OCH ) exists in sheet-like morphologies. The sizes
3
of these sheet range from 100 to 300 nm, and the sample
unanimously has a sheet-like structure on the copper grid. In
some areas overlapping of sheet can occur. As most sheet-like
particles sit on the copper grid in almost all areas observed, it
is difficult to identify the thickness of such sheet in TEM
measurement. In field emission scanning electron microscopy
6
,7
was observed for supercritically prepared material. But the
holes on these sheets are irregular in their shapes. When we zoom
in each dispersed NiO sheet and do a electronic diffraction (ED)
of the crystal, a characteristic diffraction pattern perpendicular
to the (111) surface can be observed, as shown in the inset of
Figure 3b. Overlapping of NiO sheet-like structures is more
frequently observed than in Ni(OH)(OCH3), probably due to the
instability of the (111) polar surfaces. The holey NiO nanosheet
can also be identified from SEM images, as demonstrated in
Figure 3c. From the SEM image of Figure 3d, one can see that
conventional NiO has no preference in sizes and shapes, they are
normally aggregated particles having various exposing facets and
are much bigger in size than NiO nanosheet.
(SEM) images, sheet-like morphology is more clearly seen,
which is consistent with the TEM observations. The thickness of
these samples can be estimated to range from 20 to 40 nm,
mostly around 30 nm, as evidenced in Figures 2c and 2d. It is
also seen that particle size of sheet observed in SEM is bigger
than that measured by TEM, as a result of overlapping.
NiO nanosheet produced from thermal decomposition of
morphology-tailored Ni(OH)(OCH3) reserves the sheet-like
morphology of the as-prepared sample, as can be intuitively
identified in the TEM images in Figure 3. The overall particle
size of NiO is smaller than that of the intermediate Ni(OH)-
As far as the formation mechanism is concerned, the
morphology control of metastable intermediate Ni(OH)(OCH3)
Chem. Lett. 2011, 40, 156­158
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

1
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Table 1. Catalytic activity and surface area of NiO and NiO(111)
nanosheet in dehydrogenation of ethylbenzene
is crucial to the rock-salt-structured NiO(111) sheet. It seems that
in the presence of water and methanol in certain ratios, under
either solvent thermal or supercritical conditions, Ni(OH)(OCH3)
is more stable than the corresponding salt, hydroxide or
methoxide. When either a solid precursor Ni(OH)2 or a molecular
crystal water-containing solution of nickel nitrate or acetate is
Surface area Conversion Reaction rate
Catalyst
¹1 ¹2
2
¹1
/m g
/%
/¯mol h
m
NiO
5.62
6.19
5.2
12.0
24
51
NiO(111) nanosheet
adopted as precursor, Ni(OH)(OCH ) is inevitably produced. The
3
Ni(OH)2 route is a top-down method while the solution approach
7
,22
is bottom-up. In the top-down method, only under rather harsh
The method uses nickel acetate tetrahydrate as starting material,
conditions Ni(OH)(OCH ) can be produced. At 200 °C, only
via the formation of a sheet-like Ni(OH)(OCH ) as an
3
3
quasi-crystalline samples can be produced, while highly crystal-
line Ni(OH)(OCH3) was synthesized at 240 °C. Our recent
experiment has proven that it is possible to lower the temperature
in the bottom-up approach to tailor the morphology of Ni(OH)-
intermediate, and subsequent thermal decomposition produces
NiO that preserves the sheet morphology. The synthetic
conditions are relatively mild, and the method can produce
NiO in large quantities on a gram scale. TEM, SEM, and ED
analyses corroborate that the major surface is the polar (111)
surface unanimously for all sheet particles. Catalytic activity for
dehydrogenation of ethylbenzene on NiO(111) nanosheet is
compared with conventional NiO, and the former is 2.1 times
more active than the latter. Our results show that the reaction is
structure-sensitive and that Tasker III type surface orientated
(111) surface is far more reactive than other surfaces.
(
OCH3) to nanosheet. In a top-down route, the insertion of
¹
OCH3 ligand into the Ni(OH)2 slabs to expand the lattice
requires high reaction temperatures, while in a bottom-up
approach the molecular build-up of Ni(OH)(OCH3) does not
need such harsh conditions. The metastable M(OH)(OCH3),
M = Mg, Ni, Co, etc., has not received much attention in the past
for their instability; however, it can be used as an important
intermediate to prepare metal oxides that expose Tasker III
surfaces with sheet-like morphology.
The authors are grateful for the State Basic Research Project
of China (No. 2006CB806103) and National Science Founda-
tion of China (Nos. 21006024, 20773027, and 20633030) for
financial support.
To compare the catalytic performance of NiO(111) nano-
sheet with conventionally prepared NiO, the dehydrogenation of
ethylbenzene was used as a model reaction. Dehydrogenation of
ethylbenzene was carried out in a flow-type fixed-bed micro-
reactor at 550 °C under atmospheric pressure. To supply the
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In summary, a method to synthesize NiO nanosheet
exposing high-energy Tasker III type (111) surface is developed.
Chem. Lett. 2011, 40, 156­158
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/