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Journal of the Chilean Chemical Society

versión On-line ISSN 0717-9707

J. Chil. Chem. Soc. v.50 n.1 Concepción mar. 2005

http://dx.doi.org/10.4067/S0717-97072005000100009 

 

J. Chil. Chem. Soc., 50, N 1 (2005)

Synthesis of nanostructured materials by a new solid state pyrolysis organometallic polymer method

 

CARLOS DÍAZ* and MARIA LUISA VALENZUELA

Departamento de Química, Facultad de Ciencias, Universidad de Chile, Casilla 653, Santiago,Chile. E-mail: cdiaz@uchile.cl


ABSTRACT

In this study, the preparation of nanostructured particles of metal/metal oxides by using a pyrolysis of organometallic polyphosphazenes having the metal anchored to the polymeric chain is described. The elaboration process is based on the thermal decomposition of organometallic derivatives of polyphosphazenes in air at temperature of 800 C°. As an example, the preparation of chromium, iron, ruthenium and manganese nanoclusters have been described, but the method is very general and a variety of meta/metal oxide materials can be obtained. Preliminary results on some special properties of the products are given and potential applications are discussed.

Keywords: nanomaterials, organometallic polymers, nanoparticles, solid state.


 

INTRODUCTION

Nowadays one of the main challenges in material science concerns the synthesis of nanomaterial since they exhibit interesting properties which can be different of those of bulk materials1,2. For instance optical, magnetic and electrical properties are sensitive to size effects. Furthermore nanosized particles are equally very efficient in the field of catalysis3,4 due to high ratio of surface to volume. Consequently numerous processes of nanomaterials synthesis have been investigated aiming to control their size, morphology, structure and chemical composition. A large numbers of studies concerning the production of nanoparticle have been published1,2. There exist two main routes of material elaboration: chemical methods using for instance the aqueous method or the sol-gel technique and physical methods using spray pyrolysis or vapor condensation method for instance.

In this context a pyrolytic method is an interesting alternative to elaborate nanomaterials.

This work refers to a new method to prepare structured nanomaterials using chemical transformation of an organometallic precursor. More precisely the organometallic polymer precursor is thermally decomposed to material containing metal which give access to the nucleation and growth of particles. Its well known that a good material -among other- must fulfill two main requisites i) to make a special property and ii) be procesable. Actually almost all the nanoparticle obtention methods are in solucion then they only fulfill the i) requisite but not the ii).

In this context a thermal method to obtain these type of material and operating at 800 °C could gives solid and most procesable materials, thus containing nanometallic structures.

In previous works5-10 we have reported the synthesis and characterization of a series of organometallic polymers of the type shown in scheme 1:


Scheme 1: General representation of organometallic polyphosphazenes structure.

and their thermal as well as electrical properties studied. Results from the termogravimetry measurements indicated that the pyrolysis on a nitrogen atmosphere afford high pyrolytic yields residues. This encourages us to study the pyrolysis of the organometallic polymer in air ambient.

In this paper we describe the solid state formation of metal containing nanomaterials from the pyrolysis of organometallic polymer having organometallic fragments anchored to the polymer backbone chain. In the present work a description of the experimental set-up and operating conditions, a brief presentation of the different types of nanostructured materials are described. Detailled results of the nature of each one of the differents products will be given in future papers.

EXPERIMENTAL

Synthesis
The polymers {[NP(O2C12H8)] x [NP(OC6H4X.MLn) 2]Y[NP(OC6H5 )(OC6H4X.MLn)] z}n

with X=CN, CH2CN, PPh2 and with OC6H4X= OC5H4N and MLn=CpFe(dppe), Cp*Fe(dppe), CpRu(PPh3)2, (CH3-C5H4)Mn(CO) 2 and M(CO)5 M= Cr and W, were prepared as recently reported5-10. Detailed formula of the polymer exhibited in this work is displayed in Table 1.


Table 1. Formulas and approximate composition for the precursors organometallic polymer of the nanostructured pyrolytic products.

Pyrolysis

The pyrolysis experiments were performed by pouring weighed portion (0.05 ­ 0.15g) of the organometallic polymer into an oxide aluminum boat which was placed into a box furnace (Lindberg Blue M) under a flow of air using a temperature program. The experimental conditions of some of the experiments are summarized in Table 2.


Table 2. Yields, morphology, nanosize and color of the pyrolytic products.

Characterization

IR spectra were recorded with FT ­ IR Perkin­Elmer 2000 Spectrophotometer.

Scanning electron microscopy performed on a JEOL 5410 scanning electron microscope. Elemental microanalysis was performed by energy dispersive X-ray analysis using a NORAN Instrument micro-probe attached to the scanning electron microscopy. TEM image were made on a JEOLSX100 Transmission microscope. The finally powered samples were dispersed in water and dropped on a conventional carbon ­ coated copper grid.

Magnetic Measurements

Magnetic Susceptibility measurements were performed with a SHE 906 SQUID (susceptometer quantum interface device) instrument in the range 5-300K. The applied field was 1Koe. Pascal´s constants were used to estimate the correction for the underlying diamagnetism of the sample.

RESULTS AND DISCUSSION

The results, chemical composition, morphology and approximate size of the pyrolytic products are given in Table 2. Figure 1 shows the TEM image of pyrolytic product from organometallic polymer (5) where a material with approximate spherical metallic nanostructures are observed. The EDAX shown in the inset of figure 1 clearly indicated the presence of chromium, phosphorus and oxygen.



Fig. 1: TEM image and EDAX(onset) of the pyrolysis product from the chromium polymer (5).

Morphology changes either before or after the pyrolysis were studied by SEM. In general different changes were observed depending on the metal as well as on the donor atom. For instance, for the chromium organometallic derivative (5) an enhance of the particle size of the powder was observed after pyrolysis. The polymer before calcinations is a porous material. After calcination the material become most rougose. For the manganese polymer (6) an enhance of the particle size maintaining the veterite shape was observed (see figure 2).



Fig. 2: SEM image of the veterite disc of the polymer (5): (a) before calcination and (b) after calcination. Magnification of both images is 1 10 5.

All the IR spectra of the pyrolyzed products exhibit a similar pattern absorption, i.e. strong bands around 1100 cm-1, 780 cm-1 and 500 cm-1, suggesting the formation of a similar material, probably phosphorus oxides and carbon. Consistently with this the pyrolytic products exhibit a prominent stretching mode around 1090 cm-1 and 1110 cm-1 in Raman, indicating the presence of P-O and P = O bonds arising from phosphorus oxides11. Other strong absorptions around 480 ­ 620 cm-1 and a weak bond around 790 cm-1 are in agreement with this. According to this, the pyrolysis products of a metal without polyphosphazene (which has carborane units (OCH2)2C2B10 H10 anchoring to polymer chain) given an IR spectrum12 similar to those containing pendent organometallic fragments. This is due to the vibration of the metal or metal oxides that appears normally in the low zone 700 cm-1, generally as weak bands. Thus, pyrolysis of polyphosphazenes without metal in absence of air, exhibit infrared spectra indicating the presence of phosphorus nitrides14.

With exception of the pyrolityc product from the ruthenium polymer, the nature of the formed material is no clear. FT- IR, EDAX, and magnetic data suggest the formation of a complex mixture of metal oxide and/or metal and/or metal phosphates immersed in a matrix of oxides phosphorus/carbon. In the case of the pyrolysis of the organometallic polymer (3) some amorphous and paramagnetic RuO2 form was obtained15.

Possible mechanism for the formation of preceramic materials from organometallic polymers.

Some insight of the possible mechanism of the metallic nanostructures from the polyphosphazenes containing the organometallic fragments anchored to the polymer chain, can be obtained from the following observations: i.- the polyphosphazene polymer backbone affords the matrix which maintains separate the metal centers. ii.- the calcinations of the organic matter produce holes in the polymeric matrix which allows an agglomeration of the metallic particles and then a grown to give the nanocluster structure. The grown of the particles could be stopped when begin the formation of the phosphorus oxides/carbon (in small amounts). iii.- the CO evolved during the calcination of the organic matter, probably reduces the M+n metal ion (arising from the organometallic fragment) to the M(0) specie which are known to generally react with oxygen at 800 °C to give the metal/metal oxide mixture.

The finding that incorporation of organometallic fragments to polyphosphazene polymers can afford nanostructured material can be useful in the field of the fabrication of microstructures and components for microlectromechanical systems with special properties (magnetic, electrical) using soft lithography techniques16.

On the other hand, this method show an alternative way to prepare materials having metallic and/or metallic oxide nanoclusters with a novel potential applications such as catalysis3,4, chemical sensors17, and single electron devices18. More significantly in the field of catalysis4, this method offer the potential possibility of tuning the metal/oxide metal nanostructure according to the desired catalytic process; gold for the oxidation of CO to CO2, vanadia for the dehydrogenation of alkanes to olefins, ruthenium for the Fisher-Tropsch synthesis, Pt in fuel combustion, etc. Possible special properties related to the nanostructures as magnetism (as the ruthenium product) and photoluminescence as preliminary results from the manganese pyrolytic material are underway. On the other hand, pyrolytic products are obtained at high temperature (800 °C) and they are potentially preceramic materials.

It is worthy to note that by introducing silicon fragments to the organometallic polymers, followed by pyrolysis, advanced ceramic materials containing metallic nanostructures could be obtained, with potential applications in modern technologies19. As mentioned previously detailed studies of the nature of the pyrolytic products, some of their special properties (already observed) as well as pyrolysis experiments with polyphosphazenes with another organometallic fragments are in progress and their results will be published in future papers.

The authors would like to thank FONDECYT (project 1030515) and to Conicyt Student fellowships (MLV) for financial support.

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