Interaction of nanodiamond with in situ generated sp-carbon chains probed by Raman spectroscopy

Abstract

A carbon allotrope containing solely linear sp-carbon chains (carbyne) [1] has not yet been demonstrated convincingly, but composite carbonaceous structures containing sp/sp carbon atoms attract increasing attention. Among various challenges, the possible occurrence of spcarbon chain in the interior of carbon nanotube [2] and its formation during heat treatment of double wall carbon nanotubes [3] have been discussed recently. In 1994, Heimann [4] suggested that sp-carbon chains may play a role as intermediates during shock-induced transformation of graphite into diamond. Indeed, diamond was found in the products of shock compression of carbynoid material grown by dehydrohalogenation of poly(vinylidene halide) [1,5]. An opposite effect, i.e., formation of carbyne via shock-compression of diamond was observed, too [6]. Interestingly, carbyne was found in a diamond mine in China [7], but geochemical aspects of the carbyne/diamond conversion are unknown. Sun et al. [8] prepared diamond by pyrolysis (600 C) of poly(phenylcarbyne), (PhC)n made by total dechlorination of a,a,a-trichlorotoluene [9]. This reaction is an explicit demonstration of terminological inconsistency of carbyne: the (PhC)n is solely sp -bonded polymer, in which the name ‘‘carbyne’’ is correctly used in terms of the IUPAC-codified name for a triply bonded carbon radical. Curiously, various sp-bonded carbon allotropes (also termed ‘‘carbyne’’ in historical sense) were found in the products of pyrolysis (1000–1600 C) of poly(phenylcarbyne), (PhC)n along with cubic and hexagonal diamond [9]. Kudryavtsev and Evsyukov [10] reported on diamond growth in dehydrohalogenated poly(vinylidene chloride) mixed with a catalytic amount (1 wt.%) of ultra-dispersed diamond. The growth took place at 360–370 C and atmospheric pressure, but the mechanism for catalytic diamond growth remained unknown. Here we report on a variant of this synthesis, in which the sp-chains were generated in situ from a mixture of poly(tetrafluoroethylene) PTFE with nanodiamond (nD) by the action of gaseous potassium. This study has been motivated by the fact that the ex-PTFE sp-chains are of exceptional quality [1,11] compared to other chemically synthesized analogues, and, secondly, the reaction can be carried out with a complete exclusion of air oxygen and humidity. As the sp-carbon chains are extremely reactive, the absence of any undesired reactant is a prerequisite for detailed studies of their reactions and structural modifications. Also the nanodiamond used by Kudryavtsev and Evsyukov [10] has been revealed to be composed of submicron-sized aggregates of primary particles [12]. Hence, it would be challenging to use the primary particles of nD with 4–5 nm in diameter [13] instead of their aggregates. The sample of primary nD particles used in this work was prepared by disintegrating of the commercial ultradispersed diamond, which in turn was isolated as grey powder by oxidizing detonation soot from TNT–RDX explosion with hot nitric acid, washing with water and drying at 120 C for 12 h [14]. Disintegration of aggregates was