One-pot preparation of water-soluble blue luminescent silica flakes via microwave heating

Article abstract of DOI:10.1246/cl.2010.370

Photoluminescent silica flakes were newly synthesized from silicon tetrachloride (SiCl₄) by reflux in N,N-dimethylformamide (DMF) without the use of any surfactants or other additives. Photolurninescence (PL) intensity was remarkably enhanced w

Full text of DOI:10.1246/cl.2010.370

doi:10.1246/cl.2010.370
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Published on the web March 6, 2010
One-pot Preparation of Water-soluble Blue Luminescent Silica Flakes via Microwave Heating
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Yasuhiko Iwasaki,* Yasuhisa Shibata, Akihiko Watanabe, Mitsuru Inada, Hideya Kawasaki, and Takashi Uchino
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Faculty of Chemistry, Materials and Bioengineering, Kansai University, 3-3-35 Yamate-cho, Suita 564-8680
Institute of Biomaterials and Bioengineering, Tokyo Medical and Dental University, Chiyoda-ku, Tokyo 101-0062
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Faculty of Engineering Science, Kansai University, 3-3-35 Yamate-cho, Suita 564-8680
Graduate School of Science, Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe 657-8501
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(
Received January 20, 2010; CL-100063; E-mail: yasu.bmt@kansai-u.ac.jp)
Photoluminescent silica flakes were newly synthesized from
ogy of the flakes. No critical lattice fringes were observed in the
flakes, indicating that the flakes were mostly formed from
amorphous silica. Microwave treatment did not influence the
size of the prepared flakes.
silicon tetrachloride (SiCl ) by reflux in N,N-dimethylformamide
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(
DMF) without the use of any surfactants or other additives.
Photoluminescence (PL) intensity was remarkably enhanced
with microwave treatment. The morphology of dispersible
solutes was 144 « 16 nm flakes. It was also clarified that DMF
was coordinated on the surface of the flakes and that it influences
the dispersion of flakes in both aqueous and organic solvents.
Figure 1b shows attenuated total reflectance (ATR)-FTIR
spectra of DMF and silica flakes. A very strong absorption at
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1655 cm , mostly due to ¯(C=O), is the most sensitive among
the DMF bands with respect to the interaction. Indeed, the band
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shifts toward lower wavenumbers (from 1650 to 1600 cm ) and
broadens for the DMF-protected silica flakes, suggesting
interaction of the C=O group in DMF with the silica flakes. A
Photoluminescent nanoparticles have recently been candi-
date nanoscopic assembled materials for numerous applications
due to the unique electronic and optical properties that result
from their nanometer size and the low size distribution in which
they can be produced. Particularly, in the biological areas,
imaging, labeling, and sensing are recent trends foreseen in the
application of nanoparticles.1
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new, strong absorption at 1700 cm for the silica flakes might
be assigned to the ¯(C=O) vibration mode of the carboxylic acid
group. Synthesis of the DMF-protected silica flakes seems to be
accompanied by the by-product of the oxidation of DMF. The
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1384 cm band for DMF solvent is a coupled vibration with
major contributions from CH3 deformation and C­N stretching
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Alternatively, silicon is of interest as a novel photolumi-
nescent element because of its lower toxicity.3 The photo-
luminescence of the nanoparticles is generated from quantum
confinement effects and defects located at the Si­SiO2 interface.
Various synthetic procedures of photoluminescent nanomaterials
containing silicon or silica have been proposed.4 However,
these procedures require specific chemicals and purification of
the materials is needed.
modes. It was found to shift to 1403 cm for DMF-protected
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silica flakes. We also see peaks at 1165, 1054, and 1022 cm
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which are attributed to the asymmetric stretching vibration of the
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Si­O­Si bonding, indicating that silica-related species are
indeed formed after microwave treatment. The absorption bands
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at 1090 and 1063 cm for DMF solvent are assigned to methyl
rocking vibrations coupled with a C­N characteristic, and this
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0
band is sensitive to the formation of complexes. The methyl
rocking vibration bands for DMF solvent may shift and overlap
In this study, we have explored novel synthetic methods of
photoluminescent silica flakes under ambient conditions. SiCl4
was used as a raw material and injected in boiling N,N-
dimethylformamide (DMF). In addition, the effect of microwave
heating on the formation of flakes was also clarified.
Figure 1a is a TEM picture and dynamic light-scattering
DLS) data of silica flakes prepared with microwave treatment
for 8 h. The size of the flakes was 144 « 16 nm with narrow
dispersion. The DLS data corresponded with the TEM morphol-
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the absorption of silica from 1000 to 1070 cm
.
We found that, after microwave treatment, the SiCl4/DMF
mixture solution tends to exhibit a blue PL emission peaking at
ca. 450 nm. Figure 2a shows the dependence of the PL peak
intensity of the mixture solution on reaction time under the
excitation of a 370-nm light. The PL intensity of the solution
increased with an increase in reaction time. Specifically, the PL
intensity was remarkably enhanced by microwave treatment.
(
a)
b)
a)
b)
Figure 2. a) Time dependence of the maximum PL intensity
of reacting solutions emitted at 370 nm. : Microwave treat-
ment; : no microwave treatment (only heat treatment). b)
Typical absorption and emission spectra of flakes (8-h reaction)
resuspended in equivalent water. Black line: microwave treat-
ment; gray line: no microwave treatment (only heat treatment).
Figure 1. a) TEM image and DLS data of silica flakes
prepared via microwave treatment for 8 h. b) ATR-FT-IR spectra
of DMF ( ) and silica flakes ( ).
Chem. Lett. 2010, 39, 370­371
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

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a)
b)
characterized by such electron-trapping sites, explaining the
observed stretched exponential decay profiles.
A structural origin of the PL from silica-based materials has
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been proposed by Uchino and co-workers. The dehydroxyla-
tion reaction of surface hydroxy groups results in the formation
of a defect pair consisting of a dioxasilirane, =Si(O2), and a
silylene, =Si:, center. One possible mechanism of PL generated
from silica flakes is the defect pair produced at boiling
temperature of DMF. Another possibility is the formation of
defects located at Si­SiO interface via the reduction of SiCl
Figure 3. a) Steady-state photoluminescence spectra and b) PL
decay curves for silica flakes measured at different temperatures.
The sharp peak in a) is the second harmonic (532 nm) of the
Nd:YAG laser contaminated in the incident laser beam. The
solid lines in b) indicate the results of fitting to the stretched
exponential function, eq 1.
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with pyrolytically generated carbon monoxide from DMF. The
structure of the flakes consists of a small quantity of reduction
species; that is, Si­Si bonds formed into a silica matrix was
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observed by X-ray photoelectron spectroscopy.
The significant point is that the silica flakes produced in
DMF were also well dispersed in water, chloroform, acetone,
ethanol, etc. without any surface modification.
The change in absorbance of the shoulder peaks at 315 nm
corresponded with the manner of PL intensity; that is, the
formation of the silica flakes was accelerated by microwave
treatment.
In summary, we explored a new process for making blue
luminescent silica flakes in DMF solution. The process does
not require any surfactants or other additives. With microwave
treatment, the formation of silica flakes was accelerated. The
silica flakes spontaneously dispersed in aqueous medium and
Figure 2b shows typical absorption and emission spectra of
silica flakes prepared with or without microwave treatment. In
both cases, the reaction time was 8 h and nanoparticles were
resuspended in water. The UV­visible absorption spectra of
silicon flakes in water increased toward the shorter wavelengths
from around 400 nm. The absorption at 240 nm is due to
unreacted DMF molecules coordinated on the silica flakes, and
the broad absorption band ranging from 300 to 380 nm probably
results from the vibrations of Si­O-related emission centers in
the silica flakes. Previously, a similar blue PL band peaking at
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display stable blue PL. Further surface modifications of silica
flakes for biorelated applications are probable because only
DMF was used during the processing.
The authors acknowledge Kansai University Special Re-
search Fund (#120135).
References and Notes
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450 nm was observed from nanometer-sized silica particles.
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A. P. Alivisatos, Science 1996, 271, 933.
I. L. Medintz, H. T. Uyeda, E. R. Goldman, H. Mattoussi,
Nat. Mater. 2005, 4, 435.
The intensity of the blue PL band of silica flakes prepared by
microwave treatment was significantly higher than those
prepared by only heat treatment.
3
J. D. Holmes, K. J. Ziegler, R. C. Doty, L. E. Pell, K. P.
Johnston, B. A. Korgel, J. Am. Chem. Soc. 2001, 123, 3743.
R. D. Tilley, K. Yamamoto, Adv. Mater. 2006, 18, 2053.
J.-H. Park, L. Gu, G. v. Maltzahn, E. Ruoslahti, S. N. Bhatia,
M. J. Sailor, Nat. Mater. 2009, 8, 331.
Figure 3a shows the PL peak energy of the silica flakes as a
function of measuring temperature. We see that the PL peak
position was almost unchanged even though the measuring
temperature changes from 77 to 350 K. This result indicates that
the observed PL emission does not result from the quantum
confinement of silicon nanoparticles rather the PL signal is
4
5
6
7
S.-W. Lin, D.-H. Chen, Small 2009, 5, 72.
Y. He, Y. Su, X. Yang, Z. Kang, T. Xu, R. Zhang, C. Fan,
S.-T. Lee, J. Am. Chem. Soc. 2009, 131, 4434.
A. Nishimura, N. Sagawa, T. Uchino, J. Phys. Chem. C
2009, 113, 4260.
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ascribed to the silica-related defect centers in the silica flakes.
As shown in Figure 3b, the PL decay profile was influenced by
the measuring temperature, indicating that the observed emission
processes compete with the thermally activated nonradiative
recombination process. We found that the PL decay profile is
highly nonexponential and can be fitted with the following
stretched exponential function
8
9
J. Lin, K. Baerner, Mater. Lett. 2000, 46, 86.
10 G. Durgaprasad, D. N. Sathyanarayana, C. C. Patel, Bull.
Chem. Soc. Jpn. 1971, 44, 316.
11 T. Uchino, N. Kurumoto, N. Sagawa, Phys. Rev. B 2006, 73,
2
33203.
¢
IðtÞ ¼ I0 exp½ꢀðt=¸Þꢁ
ð1Þ
1
2 D. Gautam, E. Koyanagi, T. Uchino, J. Appl. Phys. 2009,
105, 073517.
13 H. Scher, M. F. Shlesinger, J. T. Bendler, Phys. Today 1991,
44, 26.
14 L. Pavesi, M. Ceschini, Phys. Rev. B 1993, 48, 17625.
15 Y. Nakazaki, K. Fujita, K. Tanaka, T. Uchino, J. Phys. Chem.
C 2008, 112, 10878.
where I0 is the PL intensity at ¸ = 0, t is a characteristic decay
time, and ¢ is the stretching parameter that represents the degree
of deviation from a pure exponential decay. The stretched
exponential decay can generally be found in disordered systems
in which the dispersive diffusion of photoexcited electrons is
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possible.
It has previously been demonstrated that a number
of electron-trapping sites exist on silica surfaces, resulting in
highly nonexponential decay behavior.15 Indeed, a stretched
exponential PL decay in a timescale of nanoseconds was
reported from luminescent silica nanoparticles as well. Thus,
we believe that the surface of the present silica flakes is also
16 I. Pastoriza-Santos, L. M. Liz-Marzán, Adv. Funct. Mater.
2009, 19, 679.
17 Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
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Chem. Lett. 2010, 39, 370­371
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/