Inorganic-organic chain assemblies as lamellar nanoreactors for growing one-dimensional Cu(OH)2 and CuO nanostructures

Article abstract of DOI:10.1039/c1cc14066c

One-dimensional Cu(OH)₂ or CuO nanostructures were fabricated using inorganic-organic chain assemblies, Cu(C_nH_2n+1X) ₂·nH₂O (X = CO₂, SO₄) as a lamellar nanoreactor, along with NaOH treatment. The shapes and aspect ratios of the Cu(OH)₂ or CuO nanostructures could be varied by adjusting the hydrophobicity of the lamellar nanoreactors.

Full text of DOI:10.1039/c1cc14066c

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Inorganic–organic chain assemblies as lamellar nanoreactors for growing
one-dimensional Cu(OH) and CuO nanostructuresw
2
a
b
b
Seong-Hun Park, Yong-Jung Lee and Young-Duk Huh*
Received 7th July 2011, Accepted 20th September 2011
DOI: 10.1039/c1cc14066c
One-dimensional Cu(OH) or CuO nanostructures were fabricated
2
necessary to explore molecular templating systems controlled
at the molecular level for the generation of inorganic 1-D
nanostructures.
using inorganic–organic chain assemblies, Cu(C H2n+1X) ÁnH O
n
2
2
2 4
(X = CO , SO ) as a lamellar nanoreactor, along with NaOH
treatment. The shapes and aspect ratios of the Cu(OH)
2
or CuO
Lipid-like inorganic–organic hybrids with long alkyl-chain
1
0
nanostructures could be varied by adjusting the hydrophobicity of
the lamellar nanoreactors.
derivatives, such as metal soaps, sulfonates and sulfates, have
not been used as templates for the synthesis of nanomaterials,
although they form well-characterized self-organized assemblies.
Due to their amphiphilic nature and the self-organization of the
alkyl chain assemblies, these hybrids form bilayer headgroup
structures. Their two-dimensional (2-D) structures provide
promising candidate lamellar nanoreactors by presenting distinct
chemical interface environments to polar guest species. Herein, we
report a template-directed synthetic strategy for preparing uniform
In general, the properties of an inorganic material depend
1
greatly on morphology, size, and crystallographic form.
A
challenge in material science is to develop synthetic strategies
for the controlled synthesis of nanostructured materials at the
mesoscopic level. Solution-phase synthetic approaches have
several advantages over conventional gas-phase synthetic
approaches in terms of cost, energy consumption, and large-scale
Cu(OH)
2
and CuO nanowires/nanorods using lamellar inorganic–
O upon
2
organic nanoreactors formed by Cu(C
n
H
2n+1X)
2
ÁnH
2
production capabilities. Various types of molecular nanoreactors,
À
introduction of a bridging OH ligand via NaOH treatment.
Our synthetic strategies are based on the confined crystallization
within the hydrophilic regions of lamellar nanoreactors.
including micelles, vesicles, microemulsion, polyelectrolyte capsules,
and liquid crystals, have been applied toward exploring molecular
3
templates as nanoreactors for the preparation of nanostructures.
In this work, we selected two typical inorganic–organic
However, harsh conditions or complicated procedures are generally
required to remove hard templates, and soft templates are
limited in their templating capabilities. An alternative approach
hybrids as lamellar nanoreactors: Cu
2
(C11
H
23CO
2
)
4
Á2H
2
O
11
(
for short, Cu-lau) and Cu(C H SO ) Á4H O (for short,
12 25
4 2
2
12
4
Cu-DS). Although the two typical chain assemblies displayed
lamellar structures with long-chain bilayer structures the local
copper(II) structures within the hydrophilic regions differed
dramatically (Fig. 1). The Cu-lau consisted of discrete paddle-
wheel Cu(II) dimers capped by four carboxylate chains and two
waters, whereas the Cu-DS exhibited discrete Cu(II) monomers
is provided by sacrificial or reactive template methods, which
provide both a template and a reactive precursor that takes
advantage of the highly anisotropic crystal structures of solids.
Over the past several decades, numerous works have
2
investigated the morphology of Cu(OH) materials and other
5
basic copper salts. These materials possess a well-characterized
layered structure that provides precursor materials to exploit
6
the unusual morphologies of CuO. Cu(OH)
2
nanowires may be
synthesized by three routes: precipitation of cupric salts, such as
CuX (X = NO , Cl), by mixing ammonia and NaOH in
2
3
7
8
solution, oxidation of copper foil in a basic ammonia solution,
and chemical transformation of basic copper(II) salts (Cu (OH) X)
2
3
9
as precursors by NaOH treatment via anion exchange. There
remains a considerable need to develop facile and mild methods for
generating Cu(OH) and CuO nanowires/nanorods. Therefore, it is
2
a
Department of Chemistry, University of Seoul, Seoul, 130-743,
Republic of Korea. E-mail: parksh@uos.ac.kr;
Fax: +82-2-2210-2319; Tel: +82-2-2210-5657
Department of Chemistry, Dankook University, Gyeonggi-Do,
448-701, Republic of Korea. E-mail: ydhuh@dankook.ac.kr;
Fax: +82-31-8005-3148; Tel: +82-31-8005-3154
Fig. 1 Schematic illustration of the crystal and molecular structures of
two compounds that behave as lamellar nanoreactors: (a) Cu-lau, displayed
paddle-wheel binuclear Cu(II) complexes capped by carboxylate and
dehydrate ligands; (b) Cu-DS, exhibited mononuclear Cu(II) complexes
with coordinated sulfate and tetrahydrate ligands.
b
w Electronic supplementary information (ESI) available: Experimental
details and characterization methods. See DOI: 10.1039/c1cc14066c
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 11763–11765 11763

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coordinated by two sulfate alkyl-chains and tetrahydrates. The
crystal structure revealed that the Cu(II) ions were more tightly
bound to the headgroups in Cu-lau than in Cu-DS, which
favored alignment of the alkyl chains normal to the basal plane.
According to the strength of binding interactions between
the interfaces, Cu-lau and Cu-DS are conveniently classified as
hard- and soft-template, respectively.
The two nanoreactors Cu-lau and Cu-DS were synthesized
1
0c
in an aqueous solution, as described previously.
The
compositions, crystal structures, and alkyl chain structures
of the nanoreactors Cu-lau and Cu-DS were analyzed by
elemental analysis, powder XRD, and FT-IR, respectively.w The
two nanoreactors, Cu-lau (and Cu-DS), were dispersed in water at
room temperature, giving immiscible shiny blue-colored dispersions
as a result of the particle hydrophobicity. After incubating in the
aqueous solutions, addition of NaOH produced translucent
solutions that formed brown-colored precipitates, corresponding
Fig. 3 FE-SEM images of (a) Cu-lau (hard-template) showing the thin
2
plate morphologies, (b) Cu(OH) nanowires, (c) high-magnification
view of the Cu(OH) nanowires, and (d) CuO nanorods.
2
2
to Cu(OH) . And two precursors were dispersed in water at 50 1C,
and addition of NaOH to dispersions yielded black-colored
precipitates, corresponding to CuO. The crystal structures and
morphologies of the as-prepared products obtained by NaOH
treatment were analyzed by powder XRD, FT-IR, and FE-SEM.w
Fig. 2(a) shows the powder XRD patterns and FT-IR
spectrum of the Cu-lau precursor. The higher order peaks
Fig. 3(a) shows that Cu-lau assumed a thin plate-like
morphology, as expected, indicative of a layered structure.
FE-SEM images (Fig. 3(b) and (c)) revealed that the Cu(OH)2
products consisted of uniform nanowire arrays 20 nm in
diameter and several microns in length. In addition, the
crystallinity of Cu(OH) nanowires was further examined by
2
(
00l) of the XRD patterns of Cu-lau indicated the presence of
HRTEM (Fig. S1, ESIw). The CuO products (Fig. 3(d)) also
revealed nanorod arrays on relatively short length scales.
Similar experiments were conducted using the nanoreactor
Cu-DS to investigate the effects of the binding strength of the
inorganic–organic nanoreactor. Fig. 4(a) shows the XRD
13
a lamellar structure with an interlayer spacing of d = 3.20 nm.
Fig. 2(b) and (c) show the XRD patterns and FT-IR spectra of
the Cu(OH) and CuO products. The diffraction peaks of the
2
two products corresponded to those of bulk orthorhombic
1
Cu(OH) (JCPDS 80-0656) or monoclinic CuO (JCPDS
2
4
patterns of Cu-DS, Cu(OH)
2
, and CuO, demonstrating a
and CuO
1
5
7
2
2-0629). Inspection of the XRD patterns of Cu(OH) and
complete transformation from Cu-DS to Cu(OH)
2
CuO revealed that reflections of the parent Cu-lau at low 2y
range disappeared, and peaks corresponding to the surfactant
were not observed. This result indicated that the starting
material, which acted as a template, had been expelled from
the as-prepared nanostructure after complete reaction. The
nanostructures. All XRD peaks of Cu-DS were consistent with
those reported previously. The interlayer spacing was found
12
to be d = 2.48 nm, which was smaller than that of Cu-lau.
FE-SEM images of Fig. 4(b)–(d) show that Cu-DS, Cu(OH)2,
and CuO exhibited microplates, nanorods, and nanoparticles,
respectively. In general, the XRD and FE-SEM results
revealed a close resemblance to Cu-lau, but their aspect-ratios
of the 1-D nanostructures obtained are shortened.
transformation from Cu-lau into Cu(OH) and CuO was also
2
observed in the FT-IR spectra (Fig. 2, right). The FT-IR
spectra of Cu-lau exhibited features over the regions
À1
À1
3
000–2800 cm and 1600–1300 cm , which corresponded
to the stretching vibrations of –CH – (or –CH ) and the
stretching vibrations of –CO , respectively. In the case of
Cu(OH) and CuO, the vibrations characteristic of Cu-lau or
2
3
1
6
2
2
other amorphous functionalized carboxylate surfactants were
not observed. The XRD and FT-IR spectra demonstrated that
NaOH treatment provided an effective method for removing
the hydrophobic alkyl-chains after finishing the reaction.
Fig. 4 (a) XRD patterns of the Cu-DS (soft-template), Cu(OH) , and
2
Fig. 2 XRD patterns (left) and FT-IR spectra (right) of (a) Cu-lau
CuO. FE-SEM images of (b) Cu-DS, (c) Cu(OH)
(d) CuO nanoparticles.
2
nanorods, and
2
(nanoreactor), (b) Cu(OH) nanowires, and (c) CuO nanorods.
1
1764 Chem. Commun., 2011, 47, 11763–11765
This journal is c The Royal Society of Chemistry 2011

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inorganic–organic chain assemblies via NaOH treatment.
2
The morphologies of the Cu(OH) and CuO nanostructures
obtained from Cu-lau and Cu-DS were almost identical, that
is, independent of the nanoreactor, but the aspect ratios of the
nanostructures depended on the hydrophobic strength of the
inorganic–organic chain assembly precursors.
In this work, we demonstrated that inorganic–organic chain
assemblies acted as lamellar nanoreactors for the formation of
1
-D nanostructures. This synthetic strategy using lamellar
nanoreactors provides a general method for tuning the shape
and aspect ratio of 1-D nanostructures by controlling the alkyl
chain length and the binding affinity of the hydrophilic groups.
This work was supported by the National Research
Foundation of Korea (KRF-2011-0005120).
Scheme 1 Schematic diagram of the synthetic strategy for preparing
-D nanostructured materials using a lamellar nanoreactor, which acts
1
as both a metal ion supplier and a structural template. The selective
À
penetration of OH ions into the hydrophilic sheets yielded the
2
Cu(OH) and CuO nanostructures.
An important feature of the Cu(OH)
2
or CuO products is that
Notes and references
the nanowires were uniformly aligned in a particular orientation
as bundles, irrespective of lamellar nanoreactors. The uniform
1
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À
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´
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2
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2
7
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reaction temperatures, CuO nanorods with small aspect ratios were
obtained, ascribed to the reduced template effect and dehydration
1
7
2
of Cu(OH) . As the reaction temperature was increased, lamellar
8
structures became unstable due to breaking of the interchain
interactions, including the hydrogen-bonding and hydrophobic
À
interactions. The bridging OH ligands penetrated the copper ions
in the hydrophilic sheets, yielding intermediate Cu(OH) nanowires
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1
1
It is worth noting that the shapes and aspect ratios of the
2
Cu(OH) or CuO nanostructures were varied by choosing
Cu-lau or Cu-DS as nanoreactors. Although the alkyl chains
were similar in size, the interlayer d-spacings of the chain
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1
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2
.48 nm, respectively. The differences in the d-spacings and the
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1
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À
chains normal to the basal plane. Penetration of the OH ions
into the lamellar nanoreactors, which could be controlled via
the hydrophobicity, played an important role in the selectivity
of the lamellar nanoreactors.
14 J. R. Gunter and H. R. Ostwald, J. Appl. Crystallogr., 1970, 3, 21.
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In summary, we successfully fabricated uniform, 1-D
Cu(OH)
2
and CuO nanostructures from self-assembled
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 11763–11765 11765