Metallophthalocyanines possess unique physicochemical, electronic, and electrocatalytic properties, making them useful in various application fields. There is vast literature regarding their use as sensors [1–3] as their properties are readily modified by the presence of certain molecules. The possibility of depositing these phthalocyanine complexes as thin films compatible with microelectronic devices is another driving force for this purpose. Another use is as electrocatalysts in the reduction of oxygen as they can overcome the spin barrier and provide a low-energy route for the highly stable dioxygen to react, thanks to the redox potential of the metal in the phthalocyanine . These complexes have also been employed as oxidation catalysts owing to (1) the resemblance of their macrocyclic structure with that of porphyrins widely used by nature in the active sites of oxygenase enzymes; (2) their rather cheap and facile preparation on a large scale; and (3) their chemical and thermal stability .
There are various studies involving the fixation of phthalocyanine complexes onto different supports. The composites of metal phthalocyanines/carbon nanotubes [CNTs] have inspired considerable research interest because of their high quantum efficiency facilitated by the charge transfer between them and the complementary properties of the composites. The resulting metallophthalocyanine/CNT complexes possess the unique properties of phthalocyanine without any destruction of electronic properties and structures of CNTs.
An important aspect to be considered is the interaction between the metallophthalocyanine complex and the CNT. Several authors claim covalent bonding for certain metallophthalocyanines, while non-substituted complexes would be non-covalently adsorbed onto the carbon nanotubes via π-π interactions [6, 7]. In this work, we study the introduction of different surface groups onto CNT and their effect on the interaction between the carbon material and an ionic iron phthalocyanine.
CNTs were synthesized by the catalytic vapor decomposition method. The reaction setup and conditions are described elsewhere . The CNTs obtained were chemically treated in order to functionalize the surface. This consisted of a two-step procedure. Firstly, the originally prepared CNTs were oxidized with HNO3 (65 wt.%, 363 K, 72 h), thereby obtaining oxidized CNT, which was further treated with an amine (ethylenediamine in n-hexane, 343 K, 24 h) to give the aminated CNT [ACNT]. The as-synthesized and treated CNTs were reacted with a commercially available iron(III), phthalocyanine-4,4",4",4""-tetrasulfonic acid (FePcS), which is a hydrated monosodium salt compound that contains oxygen (Sigma-Aldrich, St. Louis, MO, USA), in order to obtain three composites, FePcS/CNTs (of 5 wt.% Fe). The procedure involved stirring 200 mg of carbon nanotubes in an aqueous solution of FePcS for 17 h at room temperature. After that, the solvent was evaporated and the solid dried, 373 K for 18 h.
Various analyses were carried out in order to fully characterize the prepared CNTs as well as the corresponding composites. Transmission electronic microscopy [TEM] was performed on synthesized CNTs employing a JOEL JEM 2000FX system. Surface area and pore size distribution were determined from N2 adsorption at 77 K (Micromeritics ASAP 2000 surface analyzer). Samples were previously degassed at 393 K for 5 h. Thermogravimetric analysis data were collected using a SDTQ600 5200 TA system. The samples were heated under an inert helium and air atmosphere (1,273 K, 10 K min-1). Temperature-programmed desorption [TPD] experiments were performed under vacuum in a quartz reactor coupled with a mass spectrometer (Baltzers, QMG 421, 1,100 K, 10 K min-1). The surface of the CNTs and composites was analyzed by X-ray photoelectron spectroscopy [XPS] with an Omicron spectrometer system equipped with a hemispherical electron analyzer operating in a constant pass energy using Mg Kα radiation (hν = 1,253.6 eV). C 1 s, O 1 s, N 1 s, Na 1 s, and Fe 2p3/2 individual high-resolution spectra were measured. All binding energies were referenced to C 1 s line at 284.6 eV.