Published on the web August 5, 2010
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Supercritical Hydrothermal Synthesis of Carboxylic
Acid-surface-functionalized TiO Nanocrystals:
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pH Sensitive Dispersion and Hybridization with Organic Compounds
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Toshihiko Arita,* Ken-ichi Moriya, Kimitaka Minami, Takashi Naka, and Tadafumi Adschiri*
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Institute of Multidisciplinary Research for Advanced Materials, Tohoku University,
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-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577
Department of Chemistry, Graduate School of Science, Tohoku University,
-3 Aramakiaoba, Aoba-ku, Sendai, Miyagi 980-8578
WPI Research Center: Advanced Institute for Materials Research, Tohoku University,
-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577
National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047
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Received June 23, 2010; CL-100581; E-mail: tarita@tagen.tohoku.ac.jp, ajiri@tagen.tohoku.ac.jp)
Surface-functionalized TiO2 (anatase) nanocrystals (NCs)
avg. d = 6.7 « 1.6 nm) were synthesized by a supercritical
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hydrothermal method. A phosphonic acid derivative was used
as surface modifier of TiO2 NCs. It was verified that the
phosphonic acids were bound to the surface of TiO2 with very
strong binding energy. The TiO NCs also exhibited extraordi-
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narily high chemical stability. Such a strong binding between the
phosphonic acid modifier and TiO2 had not been obtained by
any other methods. The TiO2 NC showed sharp pH sensitive
dispersion. Because of the functional as well as stable surface
modifier layer, further manipulations on the surface using
chemical reagents could be possible.
Scheme 1. Schematic description for the synthesis of surface-
functionalized TiO (anatase) nanocrystal.
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that they can bind to the surface of TiO2 with much stronger
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interaction. Applying phosphonic acids as an anchor to TiO2
and carboxylic acid as a linker to the other materials, we
developed a highly functional TiO2 NC (Scheme 1). The
carboxylic acid linker is convenient for the hybridization of
TiO2 NC because it allows us to graft other NPs, polymers, and
compounds easily by esterification. In addition, because sc-
hydrothermal reaction is very fast, it is suited for large-scale
production of NPs. This is a huge advantage for industrial
production of NPs for hybrid materials.
Fabrication of nanocrystals (NCs) which have well-defined
crystal structure, morphology, purity, and surface structure has
keen importance in nanotechnology, since these factors can
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affect the properties and functionalities of nanosized devises.
Among these principal factors of NCs, surface treatment would
be the key technology for application of NCs into functional
devices because surface control could lead to increased handling
and functionalities of NCs. For instance, the hybridization of
Strong binding between surface modifier and TiO2 is
absolutely essential to synthesize surface-functionalized TiO2
NCs. We chose phosphonic acid derivatives as the modifiers
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NCs with other functional materials, e.g., peptides, amino
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acids, polymers, other NCs, and so on, would be one of the
most promising methods to fabricate new devices, e.g., medical
because phosphoric acid derivatives are well known to strongly
probes, organicinorganic composites,8 and metamaterials.
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adsorb to TiO surfaces. However, alkyl phosphates are easily
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Therefore, the development of a joining technique between
NCs and other materials is quite important. Among other
challenges and efforts for joining them, we have focused on one-
pot synthesis to introduce functional substituents on a NC.
It goes without saying that TiO2 is a quite well-known
functional semiconductor which is used as, for example,
photocatalyst, pigment, and high-refractive index material.
Furthermore, because it is not so poisonous to humans, e.g., it
can be used as sun blocks, enormous studies have been done to
hydrolyzed in super- and subcritical water so that they cannot be
used. Phosphonic acids have a strong PC bond instead of PO
C bonds, therefore they are not hydrolyzed in sc-water.
We synthesized a variety of phosphonic acid derivatives for
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the surface modifiers. In this study, ethyl 11-diethoxyphos-
phorylundecanoate (EDPU) was selected as the modifier for
surface functionalization. Water-soluble Ti/glyceric acid com-
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plex (TiGly21, [Ti]:[glyceric acid] = 2:1) was synthesized and
employed as Ti source to obtain homogeneous solution. The
homogeneity of the reaction system is quite important, espe-
cially for obtaining uniform reaction and products in accordance
with crystal growth theory. Our TiGly21 has another advantage
for sc-hydrothermal synthesis because it has quite low carbon
content. Since sc-hydrothermal reaction was conducted at
400 °C, where most organic compounds are not stable for long
periods, the low carbon content of TiGly21 will result in
reduction of impurities in the product.
synthesize, characterize, functionalize, and assemble TiO NCs.
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Functionalization by adequate surface modification is a great
breakthrough point for use of TiO2 nanoparticles (NPs). In order
to obtain both chemically and thermally stable and highly
functional surfaces, we have used a water-soluble Ti/glyceric
acid complex as Ti source and a phosphonic acid derivative as
modifier for supercritical (sc-) hydrothermal reaction. Phos-
phonic acids have smaller pKa values than carboxylic acids so
Chem. Lett. 2010, 39, 961963
© 2010 The Chemical Society of Japan