1D Cu(OH)2 Nanomaterial synthesis templated in water microdroplets

Article abstract of DOI:10.1021/ja101579v

For many applications, micro- and nanostructured materials show a strong correlation between their geometry and their function. We report here the interfacial precipitation of a copper/alkylamine complex to form Cu(OH) ₂ nanofibers in a two-phase system (H₂O/CH ₂Cl₂). Their aggregation results in porous microbeads. This mesoscale aggregation is due to the formation of a water-in-oil (W/O) emulsion. The fibers formed at the H₂O/CH₂Cl₂ interface adsorb on the water droplet surface leading to spherical networks of Cu(OH)₂ fibers. Our preparation technique is rapid (less than 1 h) and benefits from the simplicity and the tunability of emulsions.To our knowledge, this is the first demonstration of the in situ synthesis of 1D nanostructures that self-assemble at both the surface and the inside of emulsion droplets. We report the successful control over the chemical nature of the synthesized material, its size, and morphology at both the mesoscale (completely hollow versus porous) and the nanoscale (nanoribbons versus nanofibers) by the addition of a short chain alcohol. The transformation of these materials into porous CuO spheres has several potential applications, including a demonstrated sensitive response to visible light (measured photocurrent/dark current ratio of 2.22).

Full text of DOI:10.1021/ja101579v

Published on Web 04/22/2010
1D Cu(OH)2 Nanomaterial Synthesis Templated in Water Microdroplets
Gilles R. Bourret and R. Bruce Lennox*
Department of Chemistry, McGill UniVersity and Center for Self-Assembled Chemical Structure (CSACS),
801 Sherbrooke Street West, Montreal, Quebec, H3A 2K6, Canada
Received February 23, 2010; E-mail: bruce.lennox@mcgill.ca
Scheme 1. CuCl2-Butylamine Complex Precipitation at the Water
Droplet Surface and Anisotropic Growth of Cu(OH)2
The hierarchical assembly of nanomaterials creates many op-
portunities for the spontaneous formation of structures.1 For many
applications, micro- and nanostructured materials show a strong
correlation between their geometry and their function.2 Catalytic,
sensing, or photovoltaic activity often depends on the effective
surface area, making porous and hollow materials excellent
candidates for these applications. Controlled hollow material
synthesis strategies include hard templating, sacrificial templating,
soft templating, and template-free methods.1b,2 Soft templating is
a particularly attractive technique due its modulability, flexibility,
and simplicity. Emulsions are a good example of soft templates
and have been shown to be efficient in polymer particle2,3 and
porous material4 fabrication. They have, for example, been used
to make TiO2 and SiO2 hollow structures through sol-gel processes.
However, their application has often been restricted to the use of
reactive metal alkoxides.5
Mesoscale organization of metal oxide materials has been of
considerable interest given the current understanding of nonclassical
crystallization processes. For example, the aqueous assembly of
CuO nanoribbons into “dandelions” due to geometrical constraints,
has recently been reported.6 Similarly the crystallization of ZnO
via a local dipolar electric field has been recently demonstrated.7
Among the large number of metal oxide nanostructures that can
be synthesized, copper oxide is particularly important given its low
cost and its use as a p-type semiconductor in lithium-ion batteries,8
and photovoltaic,9 photocatalytic,10 and sensing11 applications. Template-
free syntheses of Cu(OH)2 and CuO of various shapes (microparticles,
nanoribbons, nanowires) have been reported,12,13 while copper oxalate
precipitation was recently templated using a microemulsion to produce
solid CuO microspheres via thermolysis.14
In this Communication we report the interfacial synthesis of
copper hydroxide nanofibers (NF) in a two-phase system (H2O/
CH2Cl2) that subsequently self-assemble to form porous microbeads.
We show that this mesoscale aggregation process is driven by the
water-in-oil (W/O) emulsion templating the formation of the NF.
The NF formed at the H2O/CH2Cl2 interface adsorb on the water
droplet surface leading to spherical networks of copper hydroxide
NF (Scheme 1). Thermolysis leads to novel photoresponsive porous
CuO beads.
Reactions were conducted in 8 dram vials (Fisherbrand) using a
1 cm stirring bar (Fisherbrand) under vigorous stirring (900 rpm).
In a typical synthesis, 0.4 mM of CuCl2 ·2H2O (Sigma, 99%) is
dissolved in 5 mL of CH2Cl2 with 5 equiv of n-butylamine (Sigma,
99.5%), forming a blue copper-amine complex. Alkylamines
solubilize metal salts in organic solvents through coordination with
the metal cations15 and can form CuX2[alkylamine]2 complexes with
copper(II) salts.16 Upon addition of 5 mL of Milli-Q water, the
organic phase immediately becomes cloudy blue-green. The pH of
the aqueous phase increases to 10 due to the high concentration of
butylamine (butylamine pKa ) 10.6),17 causing the copper-amine
complex to rapidly precipitate at the H2O/CH2Cl2 interface. Copper
Figure 1. (A) TEM image of the Cu(OH)2 NF 1b present at the early
stage of the reaction. (B,C) SEM and (D) TEM of the resulting microspheres
2a composed of 1b.
hydroxide NF form in both phases (Figure 1, Supporting Informa-
tion(SI)) at this stage (referred to as 1a for the H2O phase and to
1b for the cloudy CH2Cl2 phase). Rapid isolation and filtration
(within 10 min) of both phases after 5 min of reaction through a
polyvinylidene fluoride membrane (0.2 µm, Pall FP Vericel 200)
and washing (1a with H2O and 1b with CH2Cl2) lead to isolation
of pure Cu(OH)2 NF. Fibers formed in water are 12.0 ( 3.7 nm
wide on average and >1 µm long. Those present in the organic
phase are shorter (average width of 14.5 ( 4.2 nm and length of
500 ( 217 nm) and have a pronounced curved shape. When the
reaction is allowed to proceed to completion (35 min), the organic
phase settles in 1 h in a separatory funnel and is isolated. Careful
successive washing/precipitation steps (using CH2Cl2) were con-
ducted over a period of 24 h followed by membrane filtration and
vacuum drying. A powdery blue material, revealed by electron
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C O M M U N I C A T I O N S
Scheme 2. Schematic Sectional View of the Different Morphologies
and Sizes As a Function of the Butanol Volume Used in the
Synthesisa
microscopy to be porous microspheres (named 2a) composed of
1b, is isolated. As seen in Figure 1B, the vast majority of the NF
are present as spherical aggregates. Copper,18a zinc,18b and
cadmium18c salts are known to form hydroxide NF in aqueous
solutions under specific conditions. Recently Cd(OH)2 nanowires
were found to slowly grow at a quiescent toluene-water
interface.18d These precedents suggest that similar processes occur
in this Cu(OH)2 system and are consistent with the interfacial
precipitation of the copper-butylamine complex as Cu(OH)2 NF.
Optical microscopy (SI) suggests that a microemulsion rapidly
forms in the CH2Cl2 phase (within 30 s after the water addition).
The microemulsion does not form if CuCl2 ·2H2O is absent.
Although a Pickering emulsion19 (i.e., an emulsion stabilized by
solid particles) might have formed, the absence of spheres at the
early stages of the reaction suggests that this is not the case. After
30 s, only a very small quantity of NF is apparent. It is likely that
the copper butylamine complex stabilizes the water droplet by acting
as a surfactant. An emulsion in fact occurs when the butylamine is
present in as low as 2 equiv. The curved shape of the fibers (Figure
1A) formed in the opaque CH2Cl2 phase after 5 min of stirring
(1b) is consistent with copper precipitation at the water droplet
surface. Geometric analysis of the fiber curvature (SI) yields a mean
diameter of curvature of 1.61 µm (standard error 0.15 µm),
consistent with the range of the H2O droplet diameters measured
with optical microscopy (1.8 µm after 5 min of stirring). The
smoothness and the evident filling of the spheres (Figure 1C and
1D) are further evidence that the precipitation at the microdroplet
surface determines the resulting material morphology. The late
appearance of the spheres in the reaction mixture results from the
slow growth of the NF from the water droplet surface into its
interior, which is limited by the generation and the diffusion of the
reactants (Cu2+, OH-, and Cl-) into the aqueous phase. This growth
process is clearly different from the classically described precipita-
tion at the droplet surface, as per that involved in emulsion-based
hollow materials.1b,2
a Blue implies Cu(OH)2, and green implies Cu2(OH)3Cl.
Figure 2. Large scale SEM images of the microspheres obtained when
butanol is added to the reaction mixture. (A) 3, 2 vol%; (B) 4, 4.5 vol%;
(C) 5, 9 vol%; (D) 6a, 16.5 vol%.
Both the size (Figure 2) and the morphology (Figure 3) of the
resulting structures were successfully altered by adding a short chain
alcohol (n-butanol, Sigma, HPLC grade) to the CH2Cl2 phase prior
to the water addition (Scheme 2). At 2 vol% and 4.5 vol% butanol,
porous spheres made of long fibers and short ribbons are obtained.
The sphere size (ca. 2.5 µm without butanol) increases to 3 µm (3,
2 vol% butanol) and to 3.8 µm (4, 4.5 vol% butanol). At 9 and
16.5 butanol vol%, completely hollow spheres made of self-
assembled short nanoribbons form (referred to as 5 and 6a). The
sphere diameter increases to 4.5 and 5 µm, respectively, in these
cases. The color of the different products depends on their
morphology. Blue products (attributed to Cu(OH)2) arise in the
absence of butanol and become green jade (characteristic of copper
hydroxychloride) as butanol is added. Products were characterized
with powder X-ray diffraction (XRD), Fourier transform infrared
(FTIR), thermogravimetric (TGA), energy-dispersive X-ray (EDS),
and UV-vis diffuse reflectance analyses. XRD and FTIR analyses
confirm that 2a, 3, and 4 are composed of Cu(OH)2 (PDF 13-0420)
and copper hydroxychloride (mixture of two polymorphs: atacamite,
PDF 78-0372 and clinoatacamite),20 while the samples made via a
greater butanol content (5 and 6a) are pure copper hydroxychloride.
FTIR spectra show the progressive disappearance of the intense
Figure 3. SEM and TEM images of the internal morphologies of the
spheres formed, as a function of the amount of butanol added to the reaction.
(A) SEM and (B) TEM images of 4 (4.5 butanol vol%). (C) SEM and (D)
TEM images of 5 (9 butanol vol%).
at the water droplet surface. Since Cu(OH)2 is known to spontane-
ously form 1D nanostructures in water,18a we propose that butanol
selectively alters the interfacial reaction conditions by preferentially
driving the precipitation of Cu2(OH)3Cl over Cu(OH)2 in the vicinity
of the water droplet surface. The precipitation of these two copper
hydroxide salts depends on the relative concentration of Cl- and
OH-, and added butanol will modify this equilibrium. We
investigated the influence of OH- concentration by using a basic
aqueous stock solution (0.01 M NaOH) in the reaction instead of
neutral pure water without butanol (2b) and with 16.5 butanol vol%
21
band around 700 cm-1 characteristic of pure Cu(OH)2 as the
butanol content increases (SI). The NF formed at the beginning of
the reaction without butanol (1a and 1b) are composed of pure
Cu(OH)2 (SI). Butanol allows for a fine-tuning of the size and the
morphology at both the meso- and the nanoscale by suppressing
the anisotropic precipitation of the copper-butylamine complex
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C O M M U N I C A T I O N S
over the structure size (from 2.5 to 5 µm) and morphology is
achieved at both the mesoscale (completely hollow versus porous)
and the nanoscale (nanoribbons versus nanofibers) by the addition
of a short chain alcohol. The selective precipitation of Cu2(OH)3Cl
over Cu(OH)2 by changing the solvent composition opens new paths
for emulsion-based syntheses and interfacial reactions. Finally, the
transformation of these materials into porous CuO spheres has
several potential applications, including demonstrated intense
response to visible light. The high surface area of these hollow
microstructures makes them also particularly good candidates for
use in catalytic and photocatalytic processes. We expect that this
self-assembly strategy will have a strong impact on the manipulation
of 1D nanostructures and the fabrication of hollow metal oxide
materials.
Figure 4. (A) Time evolution of the current measured at +10 V and -10
V applied bias (left axis) and the corresponding temperature of the sample
holder (right axis). We verified that the temperature changes were not
causing the large current variations observed (SI). Lines are a guide to the
eyes. (B) SEM image of 2a thermolyzed at 500 °C for 3 h.
Acknowledgment. The authors thank the Natural Sciences and
Engineering Research Council of Canada and the Center for Self-
Assembled Chemical Structures for their financial support.
(6b). Without butanol, the morphology does not change (SI). With
16.5 butanol vol%, the hollow spheres form from a mixture of fibers
and particles (SI), consistent with the anisotropic fibers being
composed of Cu(OH)2.
Supporting Information Available: TGA, IR, EDS, and XRD
analyses; SEM and TEM images; experimental protocols; size distribu-
tions; and summary tables. This material is available free of charge
via the Internet at http://pubs.acs.org.
Thermolysis of these microstructures (in ambient atmosphere)
leads to CuO22 porous microbeads (Figure 4) containing residual
Cu2O (3 h at 500 °C) and pure CuO (15 h at 600 °C). These
resulting CuO spheres are photoresponsive upon formation of a
metal/semiconductor/metal junction. CuO spheres dispersed in H2O
were drop-cast onto commercial gold microelectrodes (100 nm
thick, 15 µm wide and 15 µm spaced) deposited on glass (IAME
1504.3 from Abtech Scientific, Virginia) followed by overnight
drying in a vacuum oven. To avoid any contribution from oxygen23
we performed the electrical characterization of the device under
vacuum (10-6 mbar). The light source irradiance was 10.7
mW·cm-2, and the emission was restricted to the visible region
(350-750 nm). IV curves of the device (SI) show a significant
increase in the current upon light illumination due to the photo-
References
(1) (a) Mann, S. Nat. Mater. 2009, 8, 781–792. (b) Meldrum, F. C.; Colfen,
H. Chem. ReV. 2008, 108, 4332–4432. (c) Zhang, Q.; Liu, S.-J.; Yu, S.-H.
J. Mater. Chem. 2009, 19, 191–207.
(2) Lou, X. W.; Archer, L. A.; Yang, Z. AdV. Mater. 2008, 20, 3987–4019.
(3) Polymer Particles; Okubo, M., Ed.; Advances in Polymer Science Series
175; Springer: Berlin, 2005.
(4) Imhof, A.; Pine, D. J. Nature 1997, 389, 948–951.
(5) (a) Li, W.; Sha, X.; Dong, W.; Wang, Z. Chem. Commun. 2002, 2434–
2435. (b) Collins, A. M.; Spickermann, C.; Mann, S. J. Mater. Chem. 2003,
13, 1112–1114.
(6) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2004, 126, 8124–8125.
(7) Liu, Z.; Wen, X. D.; Wu, X. L.; Gao, Y. J.; Chen, H. T.; Zhu, J.; Chu,
P. K. J. Am. Chem. Soc. 2009, 131, 9405–9412.
(8) Park, J. C.; Kim, J.; Kwon, H.; Song, H. AdV. Mater. 2009, 21, 803–807.
(9) Mittiga, A.; Salza, E.; Sarto, F.; Tucci, M.; Vasanthi, R. Appl. Phys. Lett.
2006, 88, 163502.
generation of electrical carriers. The ratio (Ilight - Idark)/Idark ) Iphoto
/
Idark reaches 2.22 at +10 V bias (2.27 at -10 V) after 180 s of
illumination (Figure 4). We note a small irreversibility (10%) upon
recovery of the dark current, probably due to a local thermal
increase of the CuO spheres (SI). We are not aware of prior reports
regarding CuO nanostructure light sensors, and the high Iphoto/Idark
ratio of our device makes it competitive when compared with recent
results on Cu2O nanowires (Iphoto/Idark of ∼1 at +10 V bias).11c
To our knowledge, this is the first demonstration of the in situ
synthesis of 1D nanostructures that self-assemble at both the surface
and the inside of microemulsion droplets. Although anisotropic
particles have been used as efficient stabilizers for Pickering
emulsions,24 the relative surface tensions of the two liquid phases
and the particle defines the equilibrium location of the particle,
restricted to the droplet surface.19 Cross-linking of the resulting
structures often requires an additional polymerization step,25 that
might alter the catalytic and electronic properties of these materials.
Hollow ordered assemblies (spheres, tubes) have been prepared from
metal-polypyrrole nanorods that self-aggregate and form the
structure shells.26 This method however requires a complex process
(successive electrodeposition steps in a porous alumina template)
and highly monodisperse nanorods.
(10) Hara, M.; Kondo, T.; Komoda, M.; Ikeda, S.; Shinohara, K.; Tanaka, A.;
Kondo, J. N.; Domen, K. Chem. Commun. 1998, 357–358.
(11) (a) Zhang, J.; Liu, J.; Peng, Q.; Wang, X.; Li, Y. Chem. Mater. 2006, 18,
867–871. (b) Zhang, H.; Zhu, Q.; Zhang, Y.; Wang, Y.; Zhao, L.; Yu, B.
AdV. Funct. Mater. 2007, 17, 2766–2771. (c) Sahoo, S.; Husale, S.; Colwill,
B.; Lu, T.-M.; Nayak, S.; Ajayan, P. M. ACS Nano 2009, 3, 3935–3944.
(12) Durand-Keklikian, L.; Matijevic, E. Colloid Polym. Sci. 1990, 268, 1151–
8.
(13) (a) Lu, C.; Qi, L.; Yang, J.; Zhang, D.; Wu, N.; Ma, J. J. Phys. Chem. B
2004, 108, 17825–17831. (b) Wen, X.; Xie, Y.; Choi Chun, L.; Wan Kwun,
C.; Li, X.-Y.; Yang, S. Langmuir 2005, 21, 4729–37. (c) Zhong, Z.; Ng,
V.; Luo, J.; Teh, S.-P.; Teo, J.; Gedanken, A. Langmuir 2007, 23, 5971–7.
(d) Wang, H.; Shen, Q.; Li, X.; Liu, F. Langmuir 2009, 25, 3152–3158.
(14) He, Y. Mater. Res. Bull. 2007, 42, 190–195.
(15) Yang, J.; Ying, J. Y. Nat. Mater. 2009, 8, 683–689.
(16) Busico, V.; Carfagna, C.; Salerno, V.; Vacatello, M. Thermochim. Acta
1980, 39, 1–5.
(17) Lide, D. R., Ed. CRC Handbook of Chemistry and Physics, 90th ed.; CRC
Press: 2010.
(18) (a) Luo, Y.-H.; Huang, J.; Jin, J.; Peng, X.; Schmitt, W.; Ichinose, I. Chem.
Mater. 2006, 18, 1795–1802. (b) Peng, X.; Jin, J.; Kobayashi, N.; Schmitt,
W.; Ichinose, I. Chem. Commun. 2008, 1904–1906. (c) Ichinose, I.;
Kurashima, K.; Kunitake, T. J. Am. Chem. Soc. 2004, 126, 7162–7163.
(d) Mlondo, S. N.; Andrews, E. M.; Thomas, P. J.; O’Brien, P. Chem.
Commun. 2008, 2768–2770.
(19) Hunter, T. N.; Pugh, R. J.; Franks, G. V.; Jameson, G. J. AdV. Colloid
Interface Sci. 2008, 137, 57–80.
(20) Jambor, J. L.; Dutrizac, J. E.; Roberts, A. C.; Grice, J. D.; Szymanski,
J. T. Can. Mineral. 1996, 34, 61–72.
(21) Cabannes-Ott, C. Comptes rendus 1956, 242, 2825–2827.
(22) (a) Sharkey, J. B.; Lewin, S. Z. Thermochim. Acta 1972, 3, 189–201. (b)
Tanaka, H.; Sadamoto, T. Thermochim. Acta 1982, 54, 273–280.
(23) Kolmakov, A.; Moskovits, M. Annu. ReV. Mater. Res. 2004, 34, 151–180.
(24) Madivala, B.; Vandebril, S.; Fransaer, J.; Vermant, J. Soft Matter 2009, 5,
1717–1727.
(25) Chen, T.; Colver, P. J.; Bon, S. A. F. AdV. Mater. 2007, 19, 2286–2289.
(26) Park, S.; Lim, J.-H.; Chung, S.-W.; Mirkin, C. A. Science 2004, 303,
348–341.
The preparation technique reported here is rapid (less than 1 h)
and benefits from the simplicity and the tunability of emulsions.
Preliminary experiments to make similar Zn(OH)2 and Cd(OH)2
structures, known to form comparable fibers,18b,c have not been
successful to date. However, alternative pathways are being
explored, and we anticipate being able to create new precursors of
these technologically important luminescent oxides. Facile control
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