Thermolytic Transformation of Organometallic
Polymers Containing the Cr(CO)5Precursor into
Nanostructured Chromium Oxide
Carlos Dı´az,1,2Paola Castillo,1and Maria Luisa Valenzuela1
Thermal treatment in air of the organometallic polymerf NPðO2C12H8Þ
NPðOC6H4CH2CN ? ½CrðCOÞ5?0:13Þ2
nanometer-size metal oxide particles. Cr particles in the 35–85 nm range, mostly
54 nm, immersed in an phosphorus oxides matrix were found. ATG studies in air
suggest that the formation of the nanostructures occurs in four steps, the first
involving loss of the carbonyl groups of the Cr(CO)5fragment. The following
steps involve the oxidation of the organic matter and finally the oxidation of the
chromium to give the pyrolytic product. The use of these kinds of organometallic
polymers as precursors for a general and potential new route to materials having
metal/metal oxide nanostructures is discussed.
0:18gn(1) results in the formation of
KEY WORDS: Metallic nanoclusters; organometallic polyphosphazenes; chro-
mium nanoparticles; pyrolysis.
The development of synthetic methods that allow fine control of solid-state
structures at the atomic level is a fundamental goal in materials science. In
this context there has been significant interest in routes to nanostructured
metal and metal oxide materials . Although chemical and physical
methods for preparing these kinds of materials from solution have been
widely used, solid state methods are scarce. In particular, the methods for
1Departamento de Quı´mica, Facultad de Ciencias, Universidad de Chile, 653, Casilla,
2To whom correspondence should be addressed. E-mail: email@example.com
synthesizing metal oxides are only slightly controllable and they are
generally physical, starting from the metal oxide prepared and ground to the
nano-level [1, 2]. An alternative approach for the preparation of nano-
structured metal oxides could be the use of polymeric organometallic
precursors . The advantages of the single polymeric precursor routes
include the efficient incorporation of metals and the in situ formation of the
corresponding oxides by oxidation in air. Burning of the organic matter may
produce ‘‘holes’’ which serve as templates for producing the aggregates. On
the other hand, the inorganic polymer backbone could serve as a matrix to
stabilize the metallic cluster.
Transition metal oxides form a large family of materials which find use
in areas of catalysis, superconductivity, large magnetoresistances, piezo-
electricity, etc. Their nanosize form differs greatly in many properties from
the bulk materials . The most common nanosize materials are nonopar-
ticles. Among the metal oxides, chromium oxide is used commercially as
catalyst in the polymerization of ethylene and in the dehydrogenation of
light paraffins . Nanostructured chromium oxide may exhibit enhanced
catalytic activity; however, to the best of our knowledge, few examples have
been reported. Most of them refer to chromium oxide nanoparticles
supported on another metal oxide (SiO2[4b], Al2O3[4a], or others [2a]). We
have already reported the preparation of the chromium organometallic
polymerf½NPðO2C12H8Þ?0:8½NPðOC6H4CH2CN ? ½CrðCOÞ5?0:13Þ2?0:18gn
and found high pyrolytic residues in TGA studies in N2. This encouraged
us to study the pyrolysis of organometallic polymer (1) in air and the
characterization of the pyrolytic residues. In this paper we report the
thermolytic conversion of (1) to nanoclusters of Cr2O3in a P4O7matrix.
Reagents and Methods
Reactions were performed under nitrogen. Organometallic polymer (1)
was prepared as described previously . Thermal treatment of polymer (1)
was carried out in a Lindberg Blue programmable furnace. The pyrolysis
experiments were performed by pouring a weighed portion (0.05–0.15 g) of
the organometallic polymers into an aluminium oxide boat that was placed
in a furnace under a flow of air using a temperature program. The pyrolytic
residues were characterized by IR, XRD, scanning electron microscopy, and
transmission electron microscopy. IR spectra were recorded on a Bruker
Vector 22 spectrophotometer. Thermogravimetric analyses were performed
on a Mettler TA 4000 instrument. The samples were heated at a rate of
10?C/min from ambient temperature to 800?C under a constant flow of air
(200 mL/min). X-ray diffraction (XRD) was made on a Siemens D-5000
diffractometer with h-2h geometry working at room temperature. The XRD
data were collected using Cu–ka radiation (40 kV and 30 mA) wave.
SEM micrographs were performed on a Philips EM 300 apparatus.
Elemental analyses of the pyrolytic products were made by energy dispersive
X-ray analysis using a NORAN Instruments micro-probe attached to a
JEOL 5410 scanning electron microscope. Satisfactory data according to the
proposed structure for the pyrolytic residues were obtained. TEM images
were made on a JEOL SX 100 transmission electron Microscope. The finely
powdered samples were dispersed in water and dropped on a conventional
carbon-coated copper grid, Particle size distribution was obtained by
manual measurements of particle diameters from the bright-field TEM
RESULTS AND DISCUSSION
The solid state thermolytic conversion of (1) to nanostructured
materials in an air atmosphere was examined. At a preparative scale the
experiments were performed in the furnace in an air atmosphere (see
Experimental section) and details of the processes were obtained from a
thermogravimetric study. Pyrolysis of the polymer (1) affords a green
powder in ca. 11% yield. The material was characterized by IR, XRD,
SEM-EDAX and TEM microscopy (see below). The TGA of the sample
(see Fig. 1) in air shows a small initial weight loss that can be attributed to
the loss of four CO groups from the Cr(CO)5organometallic fragment
(calculated weight loss 5.59%, found 5.66%).
Fig. 1. TGA curve for polymer (1) in air.
The second sudden weight loss, starting at ca. 300?C, can be assigned to
the transformation of the carbon-bispiro groups to CO2(calculated weight
loss 43.6%, found 39.23%).
The final smooth decrease of the weight loss curve – which can be
divided into two steps – may be ascribed to the formation of CO2from the
carbon phenyl groups together with the formation of nitrogen oxides in the
first step (calculated weight loss 23.3%, found 22.7%). The second step can
be attributed to the formation of Cr2O3/P4O7, the final residue (calculated
weight loss for a mixture of 30.74% Cr2O3and 69.265% P4O7: 25.18, found:
These results are consistent with the formulation of the pyrolysis
product as nanostructured Cr2O3immersed in a P4O7matrix. On the other
hand, the SEM-EDAX analysis was consistent with a mixture of 30.7%
Cr2O3and 69% P4O7. These results suggest that the main Cr species present
in the pyrolysis is the thermodynamically favoured chromium oxide Cr2O3.
In agreement with this, the IR spectrum showed bands at 632, 562 and
483 cm)1, typical of Cr2O3[4b]. It is well known that the thermolysis in air
of substrates containing the Cr(CO)5moiety yields chromium oxides as
Cr2O3 [6, 7]. On the other hand, Intense IR bands around 1100 and
1000 cm)1, assigned to the m (P=O) vibrations of the phosphorus oxides ,
confirm their presence as a matrix in the pyrolysis product. The transmission
electron microscopy (TEM) image of a typical organometallic pyrolytic
residue is shown in Fig. 2.
These particles have a broad size distribution, as seen in the histogram
of Fig. 3. The size distribution ranges from 33 at 86 nm, with 54 nm being
the most abundant.
As shown in the inset of Fig. 1, EDAX analysis of the materials
confirmed the presence of chromium, phosphorus, oxygen, and traces of
grid. The darker circular region is made of chromium oxide nanoparticles. (b) EDAX analy-
sis of a portion of the material.
(a) TEM image of the product of the pyrolysis of the polymer (1) on a holes copper
carbon. EDAX analysis carried out on several zones of the material showed
that it was homogeneous.
The morphology observed on calcination of the sample was investi-
gated by scanning electron microscopy. The material both before and after
calcination is porous, with a evident 3D assemblies networks (after
pyrolysis) as shown in Fig. 4.
The X-Ray patterns of the pyrolytic material (see Fig. 5) exhibits the
diffraction peaks expected (W 15406) for P4O7at 2h = 18.62?, 18.93? and
19.34?. The signals for Cr2O3were not observed due to either broadening of
the expected signals [2a, 4a, b] or masking by the phosphorus oxide matrix.
The intense peak in the low-angle diffraction regime at 2.17? not observed in
the XRD patterns of P4O7 can due to a typical secondary reflections
presumably arising from the nanoparticles of Cr2O3.
The pyrolysis experiments showed that the product was etched on the
ceramic crucible, and therefore chemical treatment of the crucible with
concentrated acid (HNO3/H2SO4), alkali (NaOH) or an HCl/HNO3mixture
did not dissolve the chromium materials.
Possible Mechanism for the Formation of Cr2O3/P4O7from Polymer (1)
A mechanism for the formation of the nanostructurated Cr2O3
immersed in a matrix of phosphorus oxides can be proposed considering
the results discussed above. The polyphosphazene polymer backbone
affords the matrix that initially keeps the metal centres separated. Release
Fig. 3.Histogram of Cr particles from the pyrolytic residue.