Indium and indium-oxide nanoparticle or nanorod formation within functionalised ordered mesoporous silica

Article abstract of DOI:10.1039/b302867b

Organized mesoporous silica materials containing phosphonate groups have been used for controlling the growth of indium(0) nanoparticles or nanorods, which are converted to indium-oxide without modification of their size and shape.

Full text of DOI:10.1039/b302867b

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Indium and indium-oxide nanoparticle or nanorod formation within
functionalised ordered mesoporous silicay
a
Yannick Guari, Katerina Soulantica, Karine Philippot, Chloe Thieuleux, Ahmad Mehdi,
b
b
a
a
´
a
Catherine Reye, Bruno Chaudret* and Robert J. P. Corriu*
b
a
´
a
L e t t e r
´
Laboratoire de Chimie Moleculaire et Organisation du Solide (CNRS UMR 5637), Universite
des Sciences et Techniques du Languedoc, Universite de Montpellier II, Place E. Bataillon,
´
´
F-34095, Montpellier cedex 5, France. Fax: +33 4 67 14 38 52E-mail: guari@univ-montp2.fr
Laboratoire de Chimie de Coordination du CNRS, 205, route de Narbonne, F-31077, Toulouse
cedex 04, France. Fax: +33 5 61 55 30 03E-mail: chaudret@lcc-toulouse.fr
b
Received (in Montpellier, France) 10th March 2003, Accepted 12th May 2003
First published as an Advance Article on the web 9th June 2003
We consider here the preparation of indium nanoparticles
and nanorods. Indium is a low melting point metal, which
can easily be oxidised into indium oxide (In2O3). Thin films
of this transparent n-type semiconducting oxide with a band
gap of around 3.6 eV are known to exhibit luminescence18
and gas sensing19 properties. In addition, indium oxide nano-
fibers20 and nanosized indium oxide particles dispersed within
pores of non-organised mesoporous silica21,22 have been
reported to give photoluminescent materials. Here, we present
the formation of indium nanoparticles within the pores of
phosphonate-functionalised SBA-15 and MCM-41 type mate-
rials and their coalescence to indium nanorods. Oxidation
of the as-obtained indium(0) nanometer scale objects gives
indium oxide without modification of the initial size and shape.
Mesoporous silica containing phosphonic acid diethyl ester
groups (EtO)2P(O)(CH2)3SiO1.5/9ꢀSiO2 A1 (SBA-15 type mate-
rial)12 and (EtO)2P(O)(CH2)3SiO1.5/28ꢀSiO2 B1 (MCM-41 type
material) were prepared by the co-condensation route of tri-
methoxysilylpropyldiethylphosphonate with tetraethylortho-
silicate (TEOS) in the presence of the structure-directing
agents, respectively [poly(ethylene oxide)]20[poly(propylene
oxide)]70[poly(ethylene oxide)]20 (Pluronic 123) in acidic media
or hexadecyltrimethylammonium bromide (CTAB) in basic
media. Using such a method, the lipophilic part of trimethoxy-
silylpropyldiethylphosphonate (i.e., propyldiethylphospho-
nate) will be located in the lipophilic core of the formed
micelles during the sol-gel process. As a result, the phospho-
nate groups will be located exclusively within the pore channels
of the as-obtained materials. The solids are named xNy where x
indicates the conditions for the synthesis of the solid, namely A
in acidic media and B in basic media, N denotes the number
given to the materials: 1 for materials not containing any
indium, 2 for composite materials accommodating indium(0)
nanoparticles and 3 for materials accommodating nanorods,
while y indicates the reaction conditions used for the impreg-
nation of indium, namely: a corresponding to a theoretical
amount of 12.95 wt % of indium in a one-step impregnation
treatment, b corresponding to a theoretical amount of 42.65
wt % of indium in a five-step impregnation treatment, and c
corresponding to a theoretical amount of 42.65 wt % of indium
in a one-step impregnation.
Organized mesoporous silica materials containing phosphonate
groups have been used for controlling the growth of indium(0)
nanoparticles or nanorods, which are converted to indium-oxide
without modification of their size and shape.
Quantum size and surface effect properties of nanometer-
scaled semiconductor and metal particles have been of consid-
erable interest during the past two decades.1 However, the pre-
paration of such nanoparticles with a uniform size and shape
and, moreover, their organization in 2 or 3 dimensions remain
very challenging problems.2,3 Both requirements can be met by
synthesizing monodisperse particles capped with ligands that
allow them to self-organize in 2 or 3 dimensions. This method
has recently been used for the preparation of quantum dot
nanoparticles and, by some of us, for the prepartion of nano-
particles of main group elements.4,5 The drawback of this
method concerns the stability of the nanomaterial, the self-
organization being easily lost. Another attractive method is
the growth of metal and semi-conductor nanoparticles within
the pores of a regular mesoporous material.6 The use of the
regular channels of ordered mesoporous silica as matrices for
the controlled growth of mono-,7 bimetallic8 or oxide9 nano-
particles and nanorods10 has been reported. In this case, the
inorganic matrix acts as a template for the nanoparticles.
For this purpose, it is necessary to control the growth within
and not outside the pores of the matrix. Inclusion of ligands,
which will selectively coordinate to the precursors and further
to the nanoparticles, is an attractive way to control nanoparti-
cle growth exclusively within the pores. Some of us have
demonstrated the possibility to selectively functionalise the
pores of regularly organised mesoporous materials.11,12 We
have previously shown that the hexagonal channels of thiol-
functionalised mesoporous silica were effective hosts for the
growth of gold nanoparticles.13 Our strategy is based on the
chemical complexation of previously reported organometallic
precursors.14,15 In this case, reduction under mild conditions
affords gold(0) nanoparticles with a narrow size distribution
directly correlated to the pore size distribution. Other research
groups have also explored a similar approach using functiona-
lised silica for nanoparticles16 or nanorod17 formation.
The organometallic precursor chosen, InCp,23 decomposes
spontaneously at room temperature in the presence of materi-
y Electronic supplementary information (ESI) available: high angle
powder X-ray diffraction pattern of A2b/O2 , TEM images of A2a ,
A2b and A2c and correlation between the pore diameter and the
indium(0) nanoparticle size for B2a . See: http://www.rsc.org/supp-
data/nj/b3/b302867b/
A
als A1 orB1 to give materials named 2a,b,c and B2a . When the
decomposition of InCp is performed in toluene under reflux
instead of room temperature, nanorod formation takes place
within the channels of the host material. Material A3c is
DOI: 10.1039/b302867b
New J. Chem., 2003, 27, 1029–1031
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This journal is # The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2003

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Table 1 Some relevant characteristics for the mesoporous materials
Organic
Th./exp.a
Wall
thicknessc /
contenta / indium /
SBET
/
Dp /
Vp/cm3
b
Sample mmol gꢁ1 wt%
m2 gꢁ1
A
gꢁ1
A
˚
˚
d100/A
˚
A1
0.96
0.85
0.63
0.79
0.73
0.51
0.54
0/0
671
70/62 0.99
103
–
49/57
A2a
A2b
A2c
A3c
B1
12.95/11.63 429
42.65/28.72 250
42.65/25.65 231
42.65/31.75 235
–
–
–
0.65
0.41
0.40
0.38
–
–
–
–
–
0/0
1006
23/20 0.85
– 0.51
39
–
22/25
–
B2a
12.95/11.50 977
a a
Calculated from elemental analysis. b b Calculated from adsorption/desorption
c c
branches.
Calculated from a0 ꢁ Dp [a0 ¼ 2d100/3(1/2)].
prepared in the latter conditions from material A1 using a
one-step impregnation treatment. The preparation of materi-
alsA2a,b,c ,B2a and A3c is described in the experimental section.
The amount of indium in the resulting materials A2a or B2a
was established by elemental analysis (Table 1). In the case
of material A2b(Fig. 1), only 28.72% of indium was found
while the expected quantity was of 42.65%. This is in agree-
ment with the observation of some residual InCp precursor
in the filtrate from the fourth and fifth impregnation treat-
ments. The presence of residual organometallic precursor in
the filtrate was also observed for the preparation of materials
A2c and A3c .
Pore-filling of the host material was monitored both by the
decrease in BET surface area and pore volume when com-
paring materials 1 and 2 or 1 and 3 (Table 1). As observed
previously,13 a decrease in the intensity of the reflections is
obtained for the powder X-ray diffraction patterns of materials
2 and 3 compared to materials 1. Such an observation can be
explained both by the similar contrasts induced by silica and
indium24 and by the random distribution of nanoparticles or
nanorods, which would lower the periodicity.25 TEM studies
showed a narrow nanoparticle size distribution for materials
2 centred at 5.3 nm for A2a and 3.5 nm for B2a , in agreement
with the pore size of the host material, respectively 7.0/6.2
nm (adsorption/desorption branch) for A1 and 2.3/2.0 nm
(adsorption/desorption branch) for B1. It is worth noting that
egg-shaped nanoparticles are obtained for material B2a while
in all other cases the nanoparticles obtained are spherical.
The value of 3.5 nm corresponds to the smaller section of these
particles. We propose that this shape results both from the
relatively high concentration of indium present inside the
channels and from the constraint induced by the pore size.
In the case of material A1, the channels are larger, which
explains the more isotropic shape of the nanoparticles.
Fig. 2 TEM image of material A3c. Scale bar ¼ 100 nm.
Small amounts of outer pore growth defects are visible for
A2b and A2c , leading to a size tail, which is substantially higher
in the latter case. As observed by TEM studies for material A3c
(Fig. 2), formation of nanoparticles and nanorods with lengths
from 20 to 110 nm is observed within the pores, together with
out-of-pore nanocrystallites of large size with a distribution
centred at 285 nm. All our attempts to avoid crystallite growth
outside the pores were unsuccessful. It has been previously
proposed that the growth mechanism of nanorods could be
related to the preferred coordination of the ligand along a
face of a growing crystal.26,27 In the present case, the coordina-
tion of the ligand to the crystal is dictated by the channel geo-
metry of the matrix. This organisation leads to the presence of
ligand-free crystal surfaces following the axis of the channel
and allowing, under the conditions used, nanorod formation
by the seed-mediated growth mechanism or, more likely, coa-
lescence of nanoparticles in close proximity. The careful obser-
vation of Fig. 2 shows that indeed uniform nanorods form.
However, in some cases, we can observe nanoparticles in close
contact, which suggests that the coalescence mechanism is
taking place as it was previously observed for the formation
of indium4 or ruthenium28 nanowires.
When materials 2 and 3 are exposed to air, partial oxidation
of the indium(0) nanoparticles or nanorods occurs as can be
concluded from the powder XRD diffraction patterns. For
material A2b , the CP MAS 31P NMR spectra display an
enlarged signal at 33 ppm compared to the sharp signal
observed for the free phosphonic acid diethylester group. This
can be explained by the weak coordination of the organic func-
tions to the nanoparticles. In order to confirm the coordinating
role of the phosphonate groups, we performed the same
experiments starting from non-functionalised SBA-15 meso-
porous silica29 instead of A1 or B1. For a material containing
amounts of indium comparable to the indium weight within
materials A2a orB2a as given by elemental analysis, we observed
the growth of the indium(0) nanoparticles exclusively within
the pores. However, for higher loadings, crystallites resulting
from out-of-pore growth with a very large distribution centred
at 180 nm were obtained, even with a five-step impregnation
procedure as followed for A2b , thus demonstrating the benefit
of using functionalised silica.
Subsequently, materials 2 and 3 were oxidised under a con-
stant dioxygen flow of 50 mL minꢁ11. The temperature was
increased from room temperature to 353 K at a rate of 3 K
minꢁ1 and maintained for 1.5 h. Then, the temperature was
increased to 473 K at a rate of 1 K minꢁ1. The resultant sam-
ples are designated 2/O2 and 3/O2 . The powder XRD diffrac-
tion pattern of the resulting material A2b/O2 shows diffraction
peaks indexed as (222), (400), (440) and (622) reflections of the
Fig. 1 TEM image of material A2b . Scale bar ¼ 20 nm.
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New J. Chem., 2003, 27, 1029–1031

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body-centred cubic structure of indium oxide.30 TEM studies
carried out on materials 2/O2 and 3/O2 clearly show that the
size and shape of the nanometer scale objects are not affected
by the thermal treatment under dioxygen atmosphere.
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The authors thank the CNRS and the Universite de Montpel-
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