# Characterization of  FeOOH Nanoparticles and Amorphous Silica Matrix in an  FeOOH-SiO2  Nanocomposite

Authors: Guido Ennas; Maria F. Casula; Sergio Marras; Gabriele Navarra; Alessandra Scano; Giaime Marongiu
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

A nanocomposite with an  FeOOH/SiO2 ratio equal to 17.7 wt% and the pertinent matrix, obtained by etching away the nanoparticles through reaction with hydrochloric acid, were investigated by XRD, TGA-DTA, heliostereopicnometry, BET, and TEM techniques. The study shows the presence in the nanocomposite of ferrihydrite nanoparticles phase with average dimensions around 4 nm. The FeOOH nanoparticles structure was analyzed by synchrotron X-ray diffraction data using the distribution difference curve method. The porous structure of the matrix resulting by etching away the nanoparticles differs significantly from that of a pure SiO2 sample obtained by hydrolysis of TEOS under the same operative conditions followed in the nanocomposite preparation.


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## Body

## 1. Introduction

Nanocomposite
materials, composed of nanometric metal or metal-oxide particles embedded in
amorphous matrices, present a variety of interesting magnetic, electric, and catalytic
properties that are strongly dependent on nanoparticles size and on their
distribution within the matrix [1, 2].
During recent years, several metal and metal oxide-silica
nanocomposites have been prepared in our laboratories by sol-gel method and
characterized by a multitechnique approach [3–5]. These highly porous materials obtained by
drying the gels either at ambient conditions or by supercritical solvent
evaporation receive increasing attention in physics, chemistry, and medicine.
One of the most intriguing questions which is still open is to identify the
steps which lead to nanoparticles formation and to clarify how their
interaction with the matrix influences its texture and structure and, from the
other point of view, how matrix texture and structure influence particle
formation and its size and distribution within the matrix.In the preparation route toγ-Fe2O3–SiO2 nanocomposite, it has been observed that around 350°C an intermediate iron
hydroxyoxide phase, FeOOH, is present which eventually gives rise to γ-Fe2O3 by further thermal treatment [6].
A sample of nanocomposite with an
FeOOH/SiO2 ratio equal to 17.7 wt% and one of the pertinent
matrix, obtained by etching away the nanoparticles through reaction with
hydrochloric acid, were investigated by TGA-DTA, heliostereopicnometry, BET,
XRD, and TEM techniques. The structure of FeOOH nanoparticles was investigated by the
analysis of synchrotron X-ray diffraction data, using the difference method
which was developed and tested in the analysis of several glasses [7, 8].


## 2. Experimental Section

The
nanocomposite samples were prepared by a sol-gel technique mixing an ethanolic
solution of tetraethoxysilane (TEOS, Aldrich 98%; Ethanol, EtOH, Carlo Erba
95%) with an ethanolic solution
of Fe(NO3)3·9H2O (Aldrich 98%). The hydrolysis-condensation reactions were promoted by
adding few drops of nitric acid (Carlo Erba 70%) in order to keep
pH value close to 0.6. The metal solution concentration was selected in order to obtain nanocomposite samples with
FeOOH/SiO2 weight ratio equal to 17.7%. Under these conditions,
the molar ratios of TEOS/EtOH/water are 0.02, 0.83, and 0.15, respectively.
After stirring for one hour, the clear sol was poured in a teflon becker and allowed to gel in air at
room temperature (RT). The fresh alcogel was powdered and heat treated at 350°C
in order to obtain the xerogel sample. During the first hour, calcination was
performed under argon atmosphere in order to avoid organics combustion and
consequent sample overheating; treatment was then continued in air for one more hour in order to remove the
residual water and organics. The xerogel, in the form of a brown-orange powder,
was then transferred into a desiccator and cooled down to RT. The nanocomposite xerogel was divided into two
portions: one portion was submitted to textural and structural investigations
while the other was dispersed in an aqueous hydrochloric
acid solution (≈6 M) and stirred for one hour at 75°C. The
resulting white powder, repeatedly washed with distilled water until complete
elimination of chloride ions, was then dried in an oven at 200°C for twelve hours
and then transferred into a desiccator
and cooled down to RT. As discussed later, this procedure is effective in removing
the dispersed phase from the nanocomposite sample, and therefore, the sample
obtained will be referred to as matrix. As a reference, a pure silica sample
was also prepared by adopting the sol-gel procedure followed in the preparation
of the nanocomposite xerogels.Thermal
gravimetric analysis (TG) and simultaneous differential thermal analysis (DTA)
were carried out on a Mettler-Toledo TGA/SDTA 851 apparatus. Thermal analysis
data were collected in the 25–1000°C range under oxygen flow (heating rate =
10°C·min−1; flow rate = 50 mL·min−1).X-ray diffraction data were recorded at room
temperature using MoKα radiation (λ = 0.7107 Å) on a D500 θ-2θ
Siemens diffractometer equipped with a graphite monochromator on the diffracted
beam. The scans were performed within the angular region 4° ≤ 2θ ≤ 130°; at least 100 000 counts were collected
at each point.Synchrotron
X-ray diffraction data were collected on beamline ID15 B at the ESRF (Grenoble, France).
The energy of the photons was set at 88925.2 eV corresponding to a wavelength
of 0.1395 Å. The sample was held in the beam using a 1 mm thick aluminum plate.
The sample was packed into a 5 × 5 mm cavity in the centre of the plate, which
was then covered on either side with kapton film to hold the powder in the
place. The X-ray scattering patterns were recorded using a 2-dimensional MAR
345 image detector. Details on experimental data treatment and correction are
given in [9].TEM bright
field (BF) and dark field (DF) images and selected-area electron diffraction
(SAED) patterns were obtained on a JEOL 200CX microscope equipped with a
tungsten cathode operating at 200 kV. The samples were dispersed in n-octane
and dropped on a holey carbon-coated copper grid.Textural
analysis was carried out on a Sorptomatic 1990 system (Fisons Instruments) by
determining the nitrogen adsorption/desorption isotherms at 77 K. Before analysis,
the samples were heated up to 200°C at a rate of 1°C·min−1 under
vacuum. The specific surface area (S) and the pore size range were assessed by
the Barrett-Emmett-Teller (BET) and the Barrett-Joyner-Halenda (BJH) methods,
respectively, [10, 11].Density measurements were carried out at room temperature by heliostereopicnometry on a
Quantochrome mod SPY-2 apparatus.


## 3. Results and Discussion

TEM dark field image of the nanocomposite,
reported in Figure1, shows a uniform dispersion within the silica matrix of
almost spherical nanoparticles with an average particle size around 4 nm. The
SAED pattern consists of two diffuse diffraction rings which correspond to
interplanar distances in agreement with the bump at 15° 2 ϑ and the small peak around 28° 2 ϑ in the XRD pattern of the
nanocomposite (Figure 2), these peaks are characteristics of the so-called
two-lines ferrihydrite phase [12, 13]. This
presence is the only appreciable difference between the XRD patterns of
nanocomposite and matrix samples, which are both dominated by the broad peak
centered at 10° 2 ϑ, characteristic of the amorphous
silica diffraction spectra [14].TEM dark field image and electron diffraction
pattern of the nanocomposite sample.
(a)(b)Figure 2
X-ray diffraction patterns of the
nanocomposite (thick line) and of the
matrix (thin line) obtained by laboratory apparatus (λ = MoKα).The most significant thermal features of
both nanocomposite and matrix happen below 200°C, as shown by the thermograms
reported in Figure 4. Endothermic DTA peaks and corresponding TGA steps are
ascribable to the loss of the water adsorbed by the samples within the desiccator during cooling
to room temperature, since the presence of silica in the desiccator does not avoid
humidity capture from the samples. The matrix weight loss is equal to 11%,
from which a silica content equal to 89% can be inferred. The nanocomposite
weight loss, equal to 9%, includes a partial contribution due to FeOOH
nanoparticles dehydration. According to data reported in the literature [12], the weight loss of FeOOH is in the
range of 13% to 22% when temperature
increases from RT up to 200°C. By taking into account this contribution,
the nanocomposite stoichiometry can be assumed as SiO2 79%, FeOOH
14%, H2O 7%.Density
values, determined at room temperature by heliostereopicnometry, are 2.15 g·cm−3 and
1.95 g·cm−3 for nanocomposite and matrix, respectively, which are in agreement with the
values calculated according to the assumed stoichiometries.Figures3(a) and 3(b) report the physisorption isotherms of the nanocomposite and matrix
samples, respectively. The nanocomposite exhibits a reversible isotherm which
can be considered as a type I type IV with a prevalent type I contribution,
compatible with a microporous structure. This is also in agreement with the
pore size distribution reported in the inset, which suggests the presence of
micropores. The isotherm of the matrix sample can still be considered as a type
I type IV composite isotherm; the contribution arising from mesoporosity,
however, is more relevant in this sample as suggested by the presence of a
small hysteresis loop. The pore size distribution shows a narrow peak due to
the presence of mesopores with average diameter of 3.8 nm. These results
indicate that the textural modification observed in the matrix with respect to
the nanocomposite is related to the appearance of voids as a consequence of
nanoparticles dissolution. This fact is confirmed by the average value of
mesopores diameter, about 3.8 nm, which is compared to the average diameter of
nanoparticles. Both isotherms are definitely different from that of the pure
SiO2 sample obtained by hydrolysis of TEOS under the same operative
conditions followed in the nanocomposite preparation. In fact, this sample is
wholly microporous and shows a type I isotherm (not shown). In order to exclude
the possible effect of hydrochloric acid on the silica texture, physisorption
isotherms were carried out on this sample after acid treatment. Their perfect
coincidence confirms that acid attack does not affect SiO2  texture.N2-physisorption curves recorded
at 77 K for nanocomposite (a) and pertinent matrix (b). Calculated pore size
distributions are reported in the inset.
(a)(b)(a), (c) TG and DTA curves for nanocomposite and (b), (d)
pertinent matrix samples.
(a)(b)(c)(d)The differences in the physisorption isotherms suggest
that SiO2  matrix adapts its texture to host the nanoparticles, which
can be easily etched away by chemical attack without any influence on the short
range order of SiO2. On this assumption, a detailed analysis of
synchrotron X-ray diffraction data was carried out in order to better
investigate the nanoparticles structure. The structure functions S(Q) [15, 16]
of the two samples, shown in Figure 5(a), evidence a very close similarity at
high Q values, as a result of the preservation of short range order in the
silica network. According to the Faber and Ziman formalism [16], the structure function S(Q) is
obtained from the coherently scattered intensity Ic(Q) and the Laue
monotonic scattering term IL(Q),
(1)S(Q)=Ic(Q)−IL(Q)〈f2〉,
where
(2)〈f2〉=∑αcαfα2(Q),
and Q = 4πsin(θ)/λ is the modulus of the scattering vector; fα and cα are the atomic scattering factor and
the concentration of the α species, respectively.The total scattering structure functionsS(Q) of the nanocomposite (thick
line) and of the matrix (thin line)
(a), pair distribution functions g(r) of
the nanocomposite (thick line) and of
the matrix (thin line) (b).
(a)(b)At low values, and in particular atQ = 2.5 Å−1 and 4.5 Å−1,
the scattering contribution of nanoparticles is evident in the nanocomposite
pattern. This contribution is even more clear in the pair distribution
functions g(r) (see Figure 5(b)) calculated by Fourier transform of the pertinent
S(Q) and scaled to a stoichiometric unit containing one silicon atom. The nanocomposite
function shows at ≈2.1 Å and ≈3.1 Å the contribution of Fe-O and Fe-Fe
distances, respectively.Under the
assumption that the structure of the matrix remains substantially unaltered
when nanoparticles are removed, their contribution to the total experimental
structure function was evaluated by subtracting the experimental coherent
scattering intensity of the SiO2 matrix from that of the
nanocomposite, according to a procedure which was successfully adopted in the
structural analysis of glasses [7, 8]. In the calculated difference structure
function ΔS(Q) (see Figure 6(a)), the contributions
due to Si–O, O–O, and Si–Si pairs are practically cancelled, while those due to
Fe–O and Fe–Fe are definitely enhanced. This effect is markedly evident in the
difference pair distribution function Δg(r) (see Figure 6(b)) where the peaks at
1.6 Å and 2.6 Å practically disappear. Δg(r) is dominated by a peak at 2.0 Å,
which compares to Fe–O distances in FeO6 octahedral polyhedra, and by
overlapped peaks at 3.1 Å and 3.4 Å characteristics of Fe-Fe distances for linked
polyhedra. The results are in agreement with a recent study on ferrihydrite
based on a very accurate analysis of pair distribution function obtained by
high-energy X-ray total scattering [17]. It shows that the primary difference
between the so-called 2- and 6-line ferrihydrite is the limited size of the
coherent scattering domains rather than a fundamental difference in atomic
arrangement, confirming the power of pair distribution function analysis
described in detail in the book of Egami
and Billinge [15]. As a matter of fact, the Δg(r) function worked out by the X-ray
diffraction data collected with laboratory apparatus  (see Figures 1 and 2 in Supplementary Material available
online at doi:10.1155/2007/61816 does not differ significantly from that obtained with synchrotron
radiation data, suggesting the general feasibility of the analysis.The difference structure functionΔS(Q) (a) and the difference pair
distribution functions Δg(r) (b).
(a)(b)


## 4. Conclusions

The
investigation suggests that, in the nanocomposite sample, iron-oxyhydroxide
nanoparticles are embedded into the pores of the silica matrix without any
significant bonding interactions, and they can be easily etched away by the
action of hydrochloric acid. Electron microscopy and synchrotron X-ray
investigations confirm that 4 nm nanoparticles present in the sample treated at
350°C consist of ferrihydrite which eventually
transform to γ-Fe2O3 by
further increasing the sample temperature up to 700°C. We interpret this data
assuming that in the sol-gel
transformation hydrated ferric oxyhydroxide nanoparticles are formed
within the SiO2 matrix. During this stage, the matrix adapts its
texture to host the nanoparticles, and the tendency of the nanoparticles to
grow is counterbalanced by the increasing rigidity of the SiO2 network which limits the pore size.



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