Probing the evolution and morphology of hard carbon spheres Vilas G. Pol a , Jianguo Wen b , Kah Chun Lau b , Samantha Callear c , Daniel T. Bowron c , Chi-Kai Lin a , Sanket A. Deshmukh e , Subramanian Sankaranarayanan e , Larry A. Curtiss b , William I.F. David c,d , Dean J. Miller b , Michael M. Thackeray a, * a Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, IL 60439, USA b Materials Science Division, Argonne National Laboratory, Lemont, IL 60439, USA c ISIS, Rutherford Appleton Laboratory, Didcot OX11 0QX, UK d Inorganic Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QR, UK e Center for Nanoscale Materials, Argonne National Laboratory, Lemont, IL 60439, USA ARTICLE INFO Article history: Received 11 July 2013 Accepted 21 October 2013 Available online 29 October 2013 ABSTRACT Monodispersed hard carbon spheres can be synthesized quickly and reproducibly by auto- genic reactions of hydrocarbon precursors, notably polyethylene (including plastic waste), at high temperature and pressure. The carbon microparticles formed by this reaction have a unique spherical architecture, with a dominant internal nanometer layered motif, and they exhibit diamond-like hardness and electrochemical properties similar to graphite. In the present study, in situ monitoring by X-ray diffraction along with electron microscopy, Raman spectroscopy, neutron pair-distribution function analysis, and computational mod- eling has been used to elucidate the morphology and evolution of the carbon spheres that form from the autogenic reaction of polyethylene at high temperature and pressure. A mechanism is proposed on how polyethylene evolves from a linear chain-based material to a layered carbon motif. Heating the spheres to 2400–2800 °C under inert conditions increases their graphitic character, particularly at the surface, which enhances their elec- trochemical and tribological properties. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Carbon exists in many allotropic forms, from familiar dia- mond and graphite through to fullerenes, graphene, and fam- ilies of ‘‘amorphous’’ and ‘‘hard’’ carbons [1], with extreme variations in physical and chemical behavior [2–5]. The prop- erties of carbon compounds are dictated by synthesis proto- cols and by their structure at the nanoscale [6]. Mesophase spherical carbon, formed by soaking or heat-treating pitch at approximately 400 °C under atmospheric pressure, was reported by Brooks and Taylor in the 1960s [7,8]. Of particular relevance to our studies is the related work of Inagaki and co- workers who have described, in detail, the radial arrangement of carbon spherules when synthesized by the decomposition of polyethylene and other organic precursors at high temper- ature (>600 °C) and by applying a surrounding high pressure (30 GPa) to counter the pressure increase in the sealed reac- tion vessel [9–15]. More recently, Titirici and Antonietti have reviewed the synthesis of functional carbonaceous materials from biomass derived sources via the hydrothermal 0008-6223/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.10.059 * Corresponding author: Fax: +1 630 252 4176. E-mail address: thackeray@anl.gov (M.M. Thackeray). CARBON 68 (2014) 104 – 111 Available at www.sciencedirect.com ScienceDirect journal homepage: www.elsevier.com/locate/carbon