Facile synthesis of tin oxide nanoparticles stabilized by dendritic polymers

Article abstract of DOI:10.1021/ja056902n

Tin(IV) oxide nanoparticles were synthesized via the reaction of carbon dioxide with stannate ions immobilized by dendritic polymers. For PAMAM and PPI dendrimer hosts, resultant nanoparticle diameters were 2.5-3 nm; 3-3.5 nm nanoparticles resulted from use of a poly(ethyleneimine) hyperbranched polymer. Our conditions represent the only precedent for SnO₂ nanoparticulate growth using dendritic architectures, as well as a novel application for CO₂ as a reactive gas for the controlled growth of metal oxide nanoparticles. Copyright

Full text of DOI:10.1021/ja056902n

Published on Web 12/21/2005
Facile Synthesis of Tin Oxide Nanoparticles Stabilized by Dendritic Polymers
Venkateswarlu Juttukonda, Robert L. Paddock,† Jeffery E. Raymond, Dan Denomme,
Andrew E. Richardson, Laura E. Slusher, and Bradley D. Fahlman*
Department of Chemistry, Central Michigan UniVersity, Mount Pleasant, Michigan 48859
Received October 9, 2005; E-mail: fahlm1b@cmich.edu
The size of semiconducting metal oxide nanoparticles has been
shown to drastically affect their properties,1,2 allowing these
materials to be used for a range of applications such as gas sensors3
and catalysis.4 An intriguing catalytic application of metal oxides
is the surface deactivation of organophosphorus-based chemical
warfare agents.5 The increased surface area of nanoparticulate
oxides has been reported to increase their activities.6
A high band-gap material, such as tin oxide, is especially
desirable for gas sensor applications, affording this material a high
sensitivity toward reducing gases.7 There are many routes that have
been used for SnO2 nanoparticle growth, with most reports
appearing in the foreign literature and patents. Techniques include
Figure 1. TEM images of SnO2 nanoparticles immobilized on (a) G4-
PAMAM-NH2 (scale bar ) 10 nm) and (b) a poly(ethyleneimine)
hyperbranched polymer (scale bar ) 20 nm).
thermolysis of organometallic precursors,8 sol-gel,9 oxidation of
SnCl2,10 sonochemistry,11 and hydrothermal synthesis.12 A few
recent reports have utilized surfactants to prevent agglomeration
of the nanoparticles.13 However, none of the microemulsion
methodologies have resulted in nanoparticles with diameters <10
nm under ambient reaction conditions.
The architecture of dendritic polymers is well-known to encap-
sulate a variety of species, such as metals,14 II-VI semiconductors
(e.g., CdS),15 and organics such as drug molecules.16 However, the
dendritic architecture has not been employed for the controlled
growth of semiconducting metal oxide nanoparticles. Herein, we
report the synthesis of tin oxide nanoparticles stabilized by a variety
of dendritic polymers. In contrast with other reports that utilize
high temperatures or moisture-sensitive reagents (e.g., tin chloride),
our methodology consists of the simple oxidation of an encapsulated
stannate salt via reaction with carbon dioxide under ambient
conditions.
For our studies, we employed two fourth-generation dendritic
architectures: poly(propyleneimine) (PPI) and poly(amidoamine)
(PAMAM), with diaminobutyl cores. For PAMAM, we used both
amine- and hydroxy-terminated analogues. As an inexpensive
alternative, we also utilized a poly(ethyleneimine) hyperbranched
polymer (BASF, FW ) 25 000). PAMAM and PPI dendrimers were
purchased from Dendritic Nanotechnologies, Inc. and Aldrich,
respectively. Deionized ultrafiltered water was obtained from Fisher,
and simple vaporization of dry ice was used as the carbon dioxide
source for our experiments.
For a typical experiment, an aqueous or ethanolic solution
containing sodium stannate and a polymeric host was first stirred
for a minimum of 2 h to ensure saturated host-guest chelation.
Gaseous carbon dioxide was then bubbled through the room-
temperature mixture with vigorous stirring for 30 min, which
afforded a light yellow solution. It should be noted that the
analogous reaction with sodium aluminate afforded nanoparticles
of Al2O3 (2-3 nm diameters using TEM analysis), with the
stoichiometry confirmed by EDS and elemental analysis.
The best dendrimer:sodium stannate molar ratio was 1:4. If more
stannate was introduced, a white cloudy precipitate formed upon
Figure 2. EDS spectrum of the as-grown SnO2@PAMAM nanoparticles.
2-
reaction with CO2 due to the presence of free SnO3 ions in
solution. In addition to affording control over nanoparticle growth,
our use of a deficient number of stannate ions results in the majority
of primary amines remaining unchelatedsavailable for subsequent
reaction/functionalization, if desired. Although we also performed
the analogous reactions with CO2 under supercritical conditions,
there were no further benefits from the introduction of high pressure.
However, this approach would likely be relevant for higher
generation dendrimers or other surface-densified macromolecular
hosts.
Figure 1 shows TEM images of representative SnO2 nanopar-
ticles. It is unlikely that the dendrimer hosts remain intact in the
TEM images due to their thermal sensitivities. For TEM analysis,
the suspended nanoparticles were aerosol-sprayed onto carbon-
coated Cu grids (Ted Pella). The grids were then dipped into water
to remove the adsorbed sodium carbonate byproduct and were air-
dried. A representative energy-dispersive X-ray spectrum is shown
in Figure 2, illustrating the high purity of the as-grown encapsulated
nanoparticles. The 2:1 O:Sn ratio was confirmed by elemental
analysis.
Blank experiments were performed in the absence of polymer.
In these solutions, a white precipitate formed immediately that was
an amorphous agglomeration of bulk SnO2, as confirmed by SEM/
EDS, elemental analysis, and powder XRD.
† Current address: Shell Chemical LP, Houston, Texas 77082.
9
420
J. AM. CHEM. SOC. 2006, 128, 420-421
10.1021/ja056902n CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1. Representation of Our General Methodology for
Nanoparticle Growth (shown is a fourth-generation,
amine-terminated PPI dendrimer)
Figure 3. HRTEM image of a SnO2@PAMAM nanocluster (6 nm
diameter), showing the lattice spacings (scale bar ) 20 Å).
The yield of nanoparticles was greater for amine-terminated
PAMAM/PPI dendrimers, relative to hydroxy-terminated analogues
that gave rise to significant amounts of large-diameter SnO2 powder.
Using TEM and DLS analyses, the average nanoparticle diameters
for -NH2- and -OH-terminated PAMAM dendrimers were 3 and
50+ nm, respectively. This is contrary to what is reported for metal
encapsulation. For amine-terminated dendrimers, the metal ion is
likely chelated to the peripheral groups where final reduction takes
place. Usually, the use of surface hydroxyl groups force the ions
further within the architecture, resulting in smaller-diameter nano-
particles.17 However, for our study, the relatively large SnO32- ions
may be sterically encumbered from entering the interior voids of
the dendritic backbone (Scheme 1). The chelation between the
Lewis acidic Sn4+ site and primary amines is also likely greater
than that with interior tertiary amine moieties. In accord with the
comparative diameters of PAMAM and PPI dendrimers, we found
a slightly smaller diameter for SnO2 nanoparticles using a PPI host
(2.5 nm).
Use of an amine-terminated hyperbranched PEI host also resulted
in SnO2 nanoparticles, with average diameters of 3-3.5 nm.
Dynamic light scattering shows a wider diameter distribution
relative to PAMAM/PPI, but much narrower than that reported for
metal encapsulation using amine-terminated PEI.18 In general, the
lack of extensive agglomeration from -NH2-terminated polymers
in our system is likely due to the “buffer” of oxygen atoms that
surround the immobilized Sn sites, preventing their cross-linking
with neighboring dendrimers.
conditions in our system make this methodology amenable for the
synthesis of other metal oxide nanoparticles, for a range of
intriguing sensor, nanoelectronic, nanomagnetic, and catalytic
applications.
Acknowledgment. This work is dedicated to the memory of
Richard E. Smalley (1943-2005) for his pioneering contributions
to nanotechnology. We gratefully acknowledge Dr. Donald Tomalia
for his continuing support of dendrimer-related research at CMU.
We also thank Drs. Steven McKnight and Lars Piehler at the Army
Research Laboratory for funding this and further work. We also
acknowledge Research Corporation for a Cottrell College Science
Award (CC6045) in support of our ongoing dendritic composite
research projects. HRTEM/EDS analysis was performed by Xudong
Fan at MSU.
Supporting Information Available: TEM/EDS image of Al2O3@
PAMAM, and dynamic light scattering (DLS) histograms, showing the
polydispersity of all as-grown nanoparticles. This material is available
free of charge via the Internet at http://pubs.acs.org.
References
(1) Ramgir, N. S.; Mulla, I. S.; Vijayamohanan, K. P. J. Phys. Chem. B 2005,
109, 12297.
(2) Diaz, R.; Arbiol, J.; Sanz, F.; Cornet, A.; Morante, J. R. Chem. Mater.
2002, 14, 3277.
(3) Chaudhary, V. A.; Mulla, I. S.; Vijayamohanan, K.; Hegde, S. G.; Srinivas,
D. J. Phys. Chem. B 2001, 105, 2565.
(4) Harrison, P. G.; Bailey, C.; Bowering, N. Chem. Mater. 2003, 15, 979.
The experimentally determined hydrodynamic diameter of the
G4-PAMAM dendrimer is 4-5 nm.19 Hence, the larger 6 nm SnO2
nanoparticle shown in Figure 3 is likely a composite of more than
one individual dendrimer unit. This is confirmed by examining the
lattice spacings of the nanoparticle, where two separate domains
are visible, each with spacings of 3.35 Å, matching the {110} planes
of SnO2.
(5) Ekerdt, J. G.; Klabunde, K. J.; Shapley, J. R.; White, J. M.; Yates, J. T.
J. Phys. Chem. 1988, 92, 6182.
(6) Mitchell, M. B.; Sheinker, V. N.; Cox, W. W.; Gatimu, E. N.; Tesfam-
ichael, A. B. J. Phys. Chem. B 2004, 108, 1634.
(7) Crooks, R. M.; Zhao, M.; Sun, L.; Chechik, V.; Yeung, L. K. Acc. Chem.
Res. 2001, 34, 181.
(8) Nakamoto, M.; Yamamoto, M. Kagaku to Kogyo 2004, 78, 503.
(9) Briois, V.; Belin, S.; Chalaca, M. Z.; Santos, R. H. A.; Santilli, C. V.;
Pulcinelli, S. H. Chem. Mater. 2004, 16, 3885.
There are many precedents for the pH-induced variability of
metal ion chelation with a dendritic polymer.20 We performed a
similar investigation with the amine-terminated PAMAM den-
drimer. It has been shown that for copper encapsulation, as the pH
is lowered, the Cu2+ ions are moved further within the dendritic
architecture rather than bound to the surface primary amines.14
(10) Jiang, L.; Sun, G.; Zhou, Z.; Sun, S.; Wang, Q.; Yan, S.; Li, H.; Tian, J.;
Guo, J.; Zhou, B.; Xin, Q. J. Phys. Chem. B 2005, 109, 8774.
(11) Zhu, J.; Lu, Z.; Aruna, S. T.; Aurbach, D.; Gedanken, A. Chem. Mater.
2000, 12, 2557.
(12) Shen, E.; Wang, C.; Wang, E.; Kang, Z.; Gao, L.; Hu, C.; Xu, L. Mater.
Lett. 2004, 58, 3761.
(13) Chen, D.; Gao, L. J. Colloid Interface Sci. 2004, 279, 137.
(14) For example, see: Balogh, L.; Tomalia, D. A. J. Am. Chem. Soc. 1998,
2-
However, due to the relatively large size of the SnO3 ion, a
120, 7355.
decrease in pH causes the stannate ions to remain unbound, resulting
in a white precipitate of bulk SnO2 upon reaction with CO2.
In summary, we have successfully synthesized nanoparticles of
SnO2, using a series of dendritic hosts. This work represents the
first use of CO2 as a co-reactant for metal oxide nanoparticulate
growth using a dendritic host. Further, we show that controlled
growth of small-diameter metal oxide nanoparticles is possible from
both structurally precise PAMAM/PPI dendrimers, as well as
hyperbranched PEI hosts. The ambient temperature and pressure
(15) Lemon, B. I.; Crooks, R. M. J. Am. Chem. Soc. 2000, 122, 12886.
(16) Haag, R. Angew. Chem., Int. Ed. 2003, 43, 278.
(17) Garcia-Martinez, J. C.; Lezutekong, R.; Crooks, R. M. J. Am. Chem. Soc.
2005, 127, 5097.
(18) Kramer, M.; Perignon, N.; Haag, R.; Marty, J.-D.; Thomann, R.; Viguerie,
N. L.; Mingotaud, C. Macromolecules 2005, 38, 8308.
(19) Prosa, T. J.; Bauer, B. J.; Amis, E. J.; Tomalia, D. A.; Scherrenberg, R.
J. Polym. Sci., Part B: Polym. Phys. 1997, 35, 2913.
(20) For example, see: Zheng, J.; Stevenson, M. S.; Hikida, R. S.; Van Patten,
P. G. J. Phys. Chem. B 2002, 106, 1252.
JA056902N
9
J. AM. CHEM. SOC. VOL. 128, NO. 2, 2006 421