830
Chemistry Letters 2000
Monodispersed Microporous Polyoxotungstate with Regular Shape
Takeru Ito,* Kei Inumaru,# and Makoto Misono##
Department of Applied Chemistry, Graduate School of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656
(Received March 29, 2000; CL-000295)
Monodispersed fine particles of (NH4)3PW12O40 having
regular shape were successfully prepared as precipitates by the
titration of NH4HCO3 solution to H3PW12O40 solution. The par-
ticles were ca. 1 µm in size and microporous with 151 m2 g–1 of
BET surface area. AFM image demonstrated that these particles
were aggregates of nanocrystallites of ca. 15 nm in size.
d(BET) and L(XRD), respectively.
Figure 1a shows SEM image of particles of A, which indi-
cates sharp size distribution. They were dodecahedra with
slightly round edges, and all particles observed were ca. 1 µm
in size. When a drop of n-hexane in which A was dispersed
was deposited on a slide glass and allowed to dryness in air, the
particles self-assembled into two-dimensional closely packed
colloidal array as shown in Figure 1b. This demonstrates that
the size of these particles was uniform and the shape was regu-
lar. In this respect, these particles may be regarded monodis-
persed (note that B did not show sharp size distribution (0.3 – 1
µm)8).
Monodispersed colloidal particles are promising materials
for electronics devices, recording materials, pigments and cata-
lysts,1 and at the same time interesting subjects of theoretical
research.2 Under certain conditions, they self-assemble into a
three-dimensional lattice known as colloidal crystals.3 In order
to prepare such monodispersed particles, a method using
reverse micells4 or precipitation from homogeneous solutions5
has been devised. Simple methods such as titration method
have scarcely been employed.6 In the case of microporous par-
ticles of µm-order, an elaborate method is usually required.7
Here we report a very simple method for the preparation of µm-
order monodispersed particles of polyoxotungstate,
(NH4)3PW12O40, which were precipitated by titration of
NH4HCO3 solution to an aqueous solution of H3PW12O40.
They had microporosity as well as high surface area.
H3PW12O40·6H2O was first prepared by evacuation at 323
K. Then, stoichiometric amount of aqueous solution of
NH4HCO3 (typically 0.45 mmol, 3 mL of 0.15 M solution) was
added dropwise using a Pasteur pipette to aqueous solution of
H3PW12O40·6H2O (typically 0.15 mmol, 60 mL of 0.0025 M
solution) with vigorous stirring. Typically, 0.25 mL of
NH4HCO3 solution (ten drops) was added 12 times at 1 min
interval to obtain monodispersed particles. These are designat-
ed as A. The concentrations of NH4HCO3 (0.0075–0.75 M)
and H3PW12O40 (0.0025–0.25 M) solutions as well as the rate
of the addition of NH4HCO3 solution were changed in some
cases. The other samples denoted by B were prepared by the
previous method8 in which aqueous solution of NH4HCO3
(0.055 M) was added dropwise to aqueous solution of
H3PW12O40·6H2O (0.025 M) with the rate of 0.40 mL min–1. In
each case, the temperature of H3PW12O40 solution was kept at
368 K during the titration. After aged for 30 min at 368 K and
cooled to room temperature, white suspensions resulted and
they were dried up at 328 K with a vacuum rotary evaporator to
obtain white powder of (NH4)3PW12O40. The yield of the pow-
der was above 80%. The samples were characterized by scan-
ning electron microscopy (SEM) and atomic force microscopy
(AFM). Nitrogen adsorption isotherms were measured at 77 K.
Powder X-ray diffraction patterns showed cubic structure with
1.17 nm of lattice constant. The average sizes of the nanocrys-
tallites were calculated from the BET surface areas assuming
spherical shape. The lengths of ordered crystal structure were
estimated by the Scherrer equation from the (222) linewidth of
XRD as in the previous study.8 These values are denoted by
The monodispersed particles were formed more readily
when the concentration of H3PW12O40 solution was low. There
are proper combinations of the rate of titration and the concen-
tration of NH4HCO3 in order to prepare monodispersed parti-
cles like those shown in Figure 1. The size of monodispersed
particles decreased when the rate of titration was low (one drop
of NH4HCO3 solution was added at 1 min interval). The parti-
cles were not monodispersed when the concentration of
NH4HCO3 was 0.375 M (2.5 × standard concentration (= 0.15
M)).
Figure 2a shows the N2 adsorption isotherm of A. The
isotherm of B is given for comparison (Figure 2b). Both
isotherms were of Type I, typical of microporous materials.9
The larger initial uptake observed for the isotherm of A indi-
cates that it has more micropores than B. Actually, total pore
volume of A, 0.075 cm3 g–1, was twice as large as that of B,
0.036 cm3 g–1. Besides, A had a quite large BET surface area
of 151 m2 g–1 compared with B (65 m2 g–1). It is worthy to
comment that colloidal particles of µm-order rarely have micro-
porosity or high surface area.7,10
A was characterized also by AFM. Figure 3 shows the sur-
face image. Nanocrystallites of ca. 15 nm in size were
observed on the surface, which are considered to be the primary
particles. Similar images of nanocrystallites were obtained by
SEM, as well. Therefore, A is aggregates of fine nanocrystal-
Copyright © 2000 The Chemical Society of Japan
Ito, Takeru
