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Chemistry Letters Vol.36, No.1 (2007)
Exfoliation-free Nanosheet Synthesis of Transition-metal Hydroxynitrate
and Its Transformation to Oxide Particulate Nanosheet
Hongtao Cui, Marcos Zayat, and David Levyꢀ
Instituto de Ciencia de Materiales de Madrid, C.S.I.C 28049 Cantoblanco, Madrid, Spain
(Received October 18, 2006; CL-061232; E-mail: d.levy@icmm.csic.es)
A new strategy, epoxide-assisted precipitation route present-
ticles and then the morphologies of the oxide nanostructure after
calcination.
ed in this work, allows the exfoliation-free nanosheet synthesis
of Ni3(NO3)2(OH)4 by the control of dynamics of the precipita-
tion reaction. The mechanism of growth limitation in the third
direction can be explained by the bonding of the molecule result-
ing from the ring-opening reaction of the epoxide to the surface
of primary nanosheets, preventing the thickening of the layer.
In this work, an epoxide-assisted precipitation route was re-
ported for the one-step direct synthesis of nickel hydroxynitrate
nanosheets by the control of the dynamics of the precipitation re-
action. The nanosheets can be transformed to NiO particulate
nanosheets through the heat treatment. Bulk NiO is a two-sublat-
tice antiferromagnetic material with NaCl crystal structure and
´
Neel temperature of 523 K. However, when the particle size of
Layered metal hydroxides including layered double hydrox-
ides (LDHs) and layered basic metal salts (LBMSs) are a kind of
compounds composed of alternately stacked hydroxide and in-
tercalated species (anions or water) layers with 2D structure.
The exfoliation of the layers can be achieved by reducing the
interaction of layers, controlling the intercalation species to in-
crease the interlayer distance. Due to their unique microstruc-
ture, physical and chemical properties, layered metal hydroxides
have obtained a wide spectrum of useful properties including
high catalytic activity, absorbing efficiency, bioactivity as an in-
organic carrier, electroactivity and photoactivity.1 The recently
antiferromagnetic materials including NiO reaches the nanoscale
range, the particles show ferromagnetic properties.4,5 The espe-
cially interesting point is that the surface anisotropy of their
nanometric particles gives the main contribution to the effective
anisotropy, which may give rise to excellent magnetic proper-
ties.5 These new findings initiate a branch of investigation on
magnetism and make this kind of antiferromagnetic material a
promising candidate for applications.
The obtained precipitate precursor resulting from the
reaction between Ni(NO3)2 and propylene oxide was identified
by XRD as Ni3(NO3)2(OH)4 (JCPDS 22-0752). The structure
of the precursor remains unchanged until it is converted into
NiO (JCPDS 44-1159) during the calcination at 300 ꢂC. The
particles after the heat treatment at 400 ꢂC grow larger, as can
be concluded from the sharpening of the XRD diffraction lines.
A representative TEM image of the precursor Ni3(NO3)2(OH)4
is shown in Figure 1a. It is very interesting to note that the
entire sample consist of very thin nanosheets with undefined
width from several hundreds of nanometers to a few microns.
Ni3(NO3)2(OH)4 is a hydroxynitrate compound with layered
structure, previously prepared by the thermal decomposition of
reported magnetic properties of layered basic transition-metal
II
salts,2 M(OH)2ꢁnXn zH2O (M = Cu, Co, and Ni; X = inor-
.
ganic or organic anion), are one of the most fascinating findings
among the investigations on the magnetic materials due to their
tunable magnetic properties induced by their special 2D struc-
tures.
In order to maximize their usage or promote their perform-
ance in a variety of applications, it is necessary to minimize the
thickness of this kind of layered metal hydroxide into the nano-
metric range (nanosheet). However, their preparations by usual
routes do not produce nanosheets directly due to the difficulty
of control of their continue growth in the third dimension. The
only way is to exfoliate their layers by inserting other species
into the interlayer space to destroy the weak interaction of the
layers. The main disadvantage of exfoliation is the affinity be-
tween the layers of the compound, which is main impediment
for the delamination process. Therefore, the simplest way is to
synthesize the nanosheet directly avoiding the exfoliation step.
However, the realization of this idea is still a challenge, because
the 2D anisotropy growth of hydroxides remains a difficult issue
needing to be solved.
.
Ni(NO3)2 6H2O. From the structural point of view, the hydroxy-
nitrate is derived from the metal hydroxide having a layered
CdI2 structure. Part of the hydroxy groups are substituted by
the nitrate groups, which induce an increase of the interplanar
distance. This kind of layered hydroxy compounds, obtained
by traditional routes, usually shows irregular morphologies and
very thick thickness.6,7 To obtain the nanosheets from the thick
layered precursor, the exfoliation of the layers should be carried
out.7 However, it is very fascinating that very thin nanosheets of
Ni3(NO3)2(OH)4 can be obtained directly by the epoxide-assist-
ed precipitation.
An epoxide route for the synthesis of metal oxide offers an
alternative solution for the synthesis of hydroxides nanosheets
due to the unique chemistry of the epoxide-assisted precipitation
reaction. In this route the epoxide has been successfully used as a
gelation agent to prepare metal oxide nanoparticles and aero-
gels.3 Due to the low acidity of some metal ions in the solution,
the addition of the epoxide often results in the precipitation of
hydroxides after long reaction times, instead of the formation
of a gel. This issue, however, offers an opportunity to control
the structure and morphology of the precipitated hydroxide par-
Propylene oxide acts as an acid scavenger through the pro-
tonation of its oxygen and the subsequent ring opening by the
nucleophilic anionic conjugate base. It consumes protons from
the hydrated nickel ion complexes, which promotes the hydrol-
ysis of the complexes. In the solution, the acidity of the +2 metal
ion aquo complexes [Ni(H2O)6]2þ is much lower than that of
other ions with higher charge, which slows down the protonation
of the propylene oxide and then the hydrolysis rate of
[Ni(H2O)6]2þ. This slow hydrolysis process can not consume
the hydrated water in [Ni(H2O)6]2þ completely, which makes
Copyright Ó 2007 The Chemical Society of Japan
Cui, Hongtao
