Structure of new carbonaceous materials: The role of vibrational spectroscopy

Abstract

The search for materials aimed at energy storage has prompted the synthesis of new materials which were obtained by pyrolysis of aromatic precursors under controlled conditions for the production of carbon based structurally disordered networks aimed at hydrogen or lithium storage. Obviously these materials consist of fully insoluble mixtures of different molecular species which escape the traditional physico-chemical techniques for structure determination. With the purpose of overcoming this difficulty we have developed and report here a systematic vibrational spectroscopic work which lays the basic concepts to be considered in the structural understanding of the molecules studied and can be extended to similar classes of complex carbonaceous materials.

The (partial) structure of these systems and some of the reaction pathways at the molecular level can be inferred from the spectroscopic signals presented and discussed to be taken as key features for structural analysis.

Introduction

In recent years Material Science and Soft Matter Physics have shifted their attention on carbonaceous materials ranging from amorphous carbon to fullerenes and nanotubes [1], [2], [3], [4], [5], [6], [7]. Each of this class of materials seems to become essential for the development of new and revolutionary technologies.

Numerous are the ways adopted by various groups for the preparation of carbon based materials and great difficulties have been met in finding suitable methods for their characterisation. More than once the properties of these substances have been strongly affected and altered by the large amount of impurities which were not properly removed [8]. Necessarily the preparation and purification processes need to be followed and supported by whichever technique of molecular and bulk characterisation is found suitable for the particular system considered. Obviously carbonaceous materials are black, amorphous and insoluble and any attempt of characterisation has to face such unpleasant reality.

We adopt the molecular approach to obtain a new class of carbon based materials, exploiting the pyrolysis of chosen precursor molecules, in order to obtain suitable and peculiar properties and features.

We had found earlier that poly phenyl precursors, when having the appropriate topology, can smoothly be transformed into flat polycyclic aromatic hydrocarbons by an intramolecular cyclodehydrogeneration. The simplest case is that of the propeller-shaped hexa(phenyl)benzene (I) which upon treatment with, e.g., iron trichloride produces hexa-peri-hexabenzocoronene (II) C₄₂H₁₈ (HBC) [9], [10].

HBC is a remarkable disc-type π-system because it has a D_6h symmetry and can be regarded as a super benzene (C₄₂) with three times the size of triphenylene (III). Hexa alkyl derivatives of HBC are soluble and meltable and due to their phase-forming properties play an important role as semiconductors compounds of electronics devices [10], [11], [12].

What prompted the present study was the expectation that pyrolysis of (I) might also lead to intramolecular dehydrogenation processes under formation of larger conjugated discs. Pyrolysis in the lamella, however, could also lead to intermolecular dehydrogenation giving rise to aryl–aryl-coupling processes. Inclusion of hexa(p-bromophenyl)benzene (IV) and its iodo analogue (V) seemed to have appropriate aryl-aryl bound cleavage, and thus rearrangements of phenyl groups might also occur at elevated temperatures. While this scenario might then appear quite complex it has the potential of forming a three-dimensional network containing more or less extended graphitic areas. Such a network appears particularly useful for the uptake of guest atoms or molecules such as occurring in the lithium storage of secondary battery elements.

In principle many are the reactions that might occur during the pyrolytic process and consequently many could be the synthesised structures; in order to investigate the main processes involved and the structures obtained we developed a method of spectroscopic characterisation aimed at discovering spectroscopic signals characteristic of specific structural features which may help in the characterisation of the material at the “molecular” scale. The interpretation of the Raman spectra required considerations of the electronic properties in the ground and excited states; these studies have been carried out with quantum chemical calculations which are reported independently elsewhere [6], [7]; analogously the precise dynamics of the simplest systems has been considered also from the quantum mechanical viewpoint [6], [7]. We discuss in this paper the dynamical problem in terms of group theory and of dynamics and propose spectroscopic correlations useful for the understanding of the chemical nature specifically of the disordered carbonaceous materials obtained from the pyrolysis of suitable precursors. We report here first the results obtained from the infrared spectra which may be of more immediate use for the characterisation of unknown materials.

The measured decrease of hydrogen content after pyrolysis is necessarily accounted for with the formation of intermolecular (a sort of polymerisation) or intramolecular (ring condensation) reactions that lead to the formation of CC bonds, respectively between or within, the precursor molecules; it follows that the extention of π electrons delocalisation in these materials is unknown and may be variable from structure to structure throughout the sample.

One can envisage: (i) the formation of mostly two-dimensional disordered (probably rippled) sheets of graphitic systems, (ii) the formation of a crosslinked dendrimeric three-dimensional polyphenylenic molecules, (iii) the formation of a three-dimensional disordered networks of bonded benzene rings connected in a complex structure where both dendrimeric and graphitic structures coexist.

We first identified the most meaningful spectroscopic signals of the precursor molecules, then, keeping in mind the three structural scenarios presented above we proceeded in a comparison of the spectra of many pyrolytic products obtained under different reaction conditions (time and temperature) (see Fig. 1).

In this paper we show that the products obtained upon pyrolysis consist of mostly two-dimensional polycyclic aromatic hydrocarbon (PAH) sheets with peculiar features (“holes”) strictly related to the “molecular approach” of the synthetic method. With respect to an ideal graphite plane those “holes” can be described as six missing carbon “replaced” by six hydrogens (see Fig. 2).

Necessarily the concept of “molecular” purity in these systems becomes very vague or even meaningless since they have to be considered complex mixtures of structures where carbon atoms are likely to be in sp² and sp³ hybridisation states linked to each other in a disordered network of C=C and C–C bonds where (full or partial) π electron delocalisation can also occur over topologically small or large domains. There is no doubt that these systems can be considered as molecularly heterogeneous structures and any description originated from any physical or chemical test must be taken as “average” over several structural situations. Moreover, each physical analytical technique focuses mostly on specific structural or chemical features. XPS may indicate the average amount of carbon atoms in sp, sp² and sp³ hybridisation, STM and AFM can provide an overall picture at the nanometer level, infrared or Raman may provide informations on more or less localised structures through a sort of group frequency correlations.