Effect of adsorbed polyaniline on the thermal stability of iron and arsenic oxides

Abstract

Iron and arsenic oxide grains are coated with the conducting organic polymer polyaniline. The obtained samples were characterized by infrared spectroscopy, SEM, conducting measurements and thermogravimetry. The thermal stability of both oxides are increased. For As2O3 the sublimation temperature is increased from 165ºC in the pure oxide to 206ºC in the polymer modified sample. The pure Fe3O4 sample exhibits sublimation at 780ºC whereas the polyaniline coated oxide is stable until at least 1000ºC.

polyaniline; thermal degradation; oxide

ARTIGO

Effect of adsorbed polyaniline on the thermal stability of iron and arsenic oxides

Robson Fernandes de Farias

Departamento de Química - Universidade Federal de Roraima - UFRR - 69310-270 - Boa Vista - RO

Recebido em 9/3/99; aceito em 21/10/99

e-mail: [e-mail]

Iron and arsenic oxide grains are coated with the conducting organic polymer polyaniline. The obtained samples were characterized by infrared spectroscopy, SEM, conducting measurements and thermogravimetry. The thermal stability of both oxides are increased. For As₂O₃ the sublimation temperature is increased from 165^oC in the pure oxide to 206^oC in the polymer modified sample. The pure Fe₃O₄ sample exhibits sublimation at 780^oC whereas the polyaniline coated oxide is stable until at least 1000^oC.

Keywords: polyaniline; thermal degradation; oxide.

INTRODUCTION

Polyaniline (PANI) is one of the most studied conducting polymers, not only due the facility in preparation procedures, but also related to high stability to the environmental exposition. The normal polimerization to prepare polyaniline has an enthalpy value of 439 kJ mol^-1 for the emeraldine form¹, and its protonation (doping)² and many other physical-chemical properties are connected to the presence of the -NH- group³.

A great number of polymers, such as those derivative of polyaniline or other having some electroactive properties can be inserted into inorganic 2D, 3D or porous host solids ^4-6 as phosphonates^7-8, phosphates^9-11, sulfides¹² and oxides^13-15, displaying the so called nanocomposites^16-21, which have enhanced their uncommon electrical properties.

Since polyaniline has characteristic photocatalytical properties²², having a conducting band in the visible region which is activated by solar radiation²³, some kind of synergic relationship could arise when PANI is used as part of a organic/inorganic mixed material. In this direction, inorganic matrices such many metal oxides, which exhibit a very wide range of electrical resistivity²⁴ with catalytical²⁵ or photocatalytical properties^26,27 have been explored. For this purpose, deposition of a layer, or layers of polyaniline on the oxide surface, which polymer is used as a contact interface between oxide and another chemical species and/or electrode surfaces, could exert interesting and important effects on the oxidation-reduction properties of the substrate.

Recently, polyaniline adsorbed on MoO₃ was used to electrocatalyse the oxidation of molybdate ion, enhancing several times the electrode responce, and so, can be used as a selective electrode for molybdenum²⁸. The effects of polyaniline on redox process on As₂O₃ were also studied²⁹.

The aim of this work is to study the effects of adsorbed polyanile on the thermal degradation process for iron and arsenic oxides in an attempting to stablish a experimental procedure that allows use this oxides at higher temperatures than the usual ones.

EXPERIMENTAL

Polyaniline was chemically obtained ³⁰, by using a 0.50 mol dm^-3 solution of bidistilled anyline (Aldrich) in 1.0 mol dm^-3 HCl (Aldrich) as starting materials. To this solution mantained at 278 K was added, dropwise for 2h, 100 cm³ of a 0.80 mol dm³ of (NH₄)₂S₂O₈ solution. After the complete addition of the oxidant solution, the mixture was stirred for 10 h, until finished the polymerization. The final polymer, was washed several times with distilled water and acetonitrile, to remove oligomers as well another possible impurities. The obtained polyaniline was then treated with a 1.0 mol dm³ of NH₄OH solution to promote deprotonation.

In the next stage, 0.40 g of the prepared polyaniline was dissolved in 45.0 cm³ of N,N' dimethylformamide (Merck). This solution was added to 2.0 g of As₂O₃ (99,9%, Carlo Erba) and 0.50 cm³ of H₃PO₄ to promote the doping². The mixture was aged for 48 h at room temperature, before withdrawing the excess of solution and dried at 363 K overnight. The green powder product containing polyaniline adsorbed on oxide As₂O₃ is named from here as As₂O₃.pani. A similar procedure was used to prepare a polyaniline coated Fe₃O₄ sample, producing Fe₃O₄.pani.

The pure and polymer coated samples were characterized by FR-IR in a Bomem apparatus, with pressed KBr pellets and 25 scans, TG in a Shimadzu TGA-50 under argon atmosphere with heating rate of 8.3 x 10^-2 Ks^-1. The SEM images were obtained in a Jeol, JSM T-300 apparatus, using an accelerating voltage of 15 kV. Conductance measurements were taken in a four probe apparatus, using pressed sample discs. No success was achieved in the attempting to obtain ¹³C CP-MAS spectra for oxide-polyaniline samples.

RESULTS AND DISCUSSION

In comparing the infrared spectra of PANI in Figure 1 with that for As₂O₃ and As₂O₃.pani as shown in Figures 3 (a) and (b), the spectra of As₂O₃.pani shows three bands, which could be assinged as derived from pani bands: at 579 cm^-1, 1146 cm^-1 and 1255 cm^-1. In an attempting to assign the last two bands can be associated to those located in free pani at 1140 cm^-1 and 1301 cm^-1,respectively, which are usually attributed to the doping process¹. However, the band at 579 cm^-1 has not a clear attribution. For Fe₃O₄ and Fe₃O₄.pani samples (Figure 2), a broad peak at 1088 cm^-1 which could be associated with the free band at 1140 cm^-1 in free polyaniline is observed. Emeraldine has two types of basic sites capable of coordinating Lewis acids: amine and imine nitrogens. In the polymer chain they are present in equal numbers. As observed for Lewis acid doped polyaniline³¹, both types of sites could be involved in the interaction (coordination) process.

The SEM micrographs for the pure and polymer oxide samples are shown in figures 4 and 5. As can be noted, mainly to arsenic oxide, the polymer are adsorbed on the grains surface.

The conductivity of pellets obtained by pressing powders are 3.4 x 10^-4W^-1 cm^-1 and 34.2 x 10^-4W^-1 cm^-1, for As₂O₃ and As₂O₃:pani, respectively. These values indicated that the material with polyaniline has a conductivity, which is about ten times larger than that of the pure oxide. This result coroborated that the polymer deposition occurred on the substrate surface.

The termogravimetric curve of As₂O₃ presents a total mass lost due to the sublimation of the oxide, whereas the TG curve of As₂O₃.pani shows a first mass loss step of 87% due to oxide sublimation and a second one of 13%, due to the thermal degradation of the organic moiety. The second mass loss step exhibits a profile consistent with the thermal degradation of emeraldine^32-33. Thermogravimetric technique can be considered as a reliable tool to characterize solid surfaces^34,35, and then, the fraction of 13% can be assumed as been the total amount of deposition of polyaniline on As₂O₃ surface.

For Fe₃O₄.pani a mass loss of 11% can be observed due to the release of polyaniline molecules. So, for both oxides the total amount of adsorbed polyanilne is almost the same.

For both oxides an increase in the thermal stability was observed since the sublimation temperature of arsenic oxide is about 165^oC for the pure oxide and 206^oC for the polymer modified one. The pure iron oxide sample used sublimates at 780^oC whereas the polyaniline modified sample do not sublimate until the maximum temperature used, 1000^oC.

The reasons to the observed phenomena could be atributed to the interaction of nitrogen atoms or p electrons of the aromatic rings of polyaniline with the acidic sites in the oxides surfaces, with polyaniline coating sheets acting as a "thermal barrier", since it absorbs the heat transfered to the polymer coated samples.

CONCLUSION

The obtained experimental results shows that polyaniline can be used to increase the thermal stability of iron and arsenic oxides. Since polyaniline, as well as many metal oxides exhibit electrical and catalytic properties, an increase in thermal stability could be an desirable improvement in such organic-inorganic materials.

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Publication Dates

• Publication in this collection
  08 Dec 2000
• Date of issue
  June 2000

History

• Accepted
  21 Oct 1999
• Received
  09 Mar 1999