Highly crystalline graphene formation from graphene oxides by ultrahigh temperature process using solar furnace
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Highly crystalline graphene formation from graphene oxides by ultrahigh temperature process using solar furnace Yoshihiro Kobayashi1, Takashi Ishida1, Yuichiro Miyata1, and Yoshihiko Shinoda2 1 Department of Applied Physics, Osaka University, Suita, Osaka 565-0871, Japan 2 Wakasawan Energy Research Center, Tsuruga, Fukui 914-0192, Japan ABSTRACT This work reports the efficient structural restoration of defective graphene oxide (GO) to a crystalline graphene by an ultrahigh temperature process at around 1800 ºC achieved by a solar furnace. The GO samples were treated at high temperature by irradiating concentrated sunlight and focusing it on the sample under an inert nitrogen environment at atmospheric and reduced pressure. The structural restoration of GO was analyzed by Raman spectra, and the features of their D- and 2D-bands were remarkably improved at ultrahigh temperatures. The restoration was induced not by a photochemical reaction but dominantly by a thermally stimulated reaction. The process under reduced pressure gives rise to significantly better features in the Raman spectra than that of the atmospheric condition. This tendency shows that a trace amount of impurities contained in pure nitrogen gas are not negligible and attack the GO surfaces to induce considerable defects. These results indicate the superiority of the ultrahigh temperature process at reduced pressure for efficient GO restoration and the formation of highly crystalline graphene. INTRODUCTION Graphene oxide (GO) is a promising material for large-scale applications of graphene because of their mass-production features due to the efficient chemical exfoliation of graphite [1, 2]. However, defects such as the adduct of oxygen-containing groups and lattice vacancies in graphene sheets are formed during their synthesis process and significantly degrade the superior properties of graphene. Therefore, the restoration of the defects is crucial for their practical use as graphene [3]. It has been reported that GO restoration effectively proceeds at high temperature under vacuum, inert gas [4-9], and reactive environments including such carbon-containing gas as ethanol [10-14]. But even if GO is annealed at 1100 ºC, which is the maximum temperature often used in the typical apparatus of chemical vapor deposition (CVD) processes, the crystallinity of the obtained GO is insufficient for device applications. Hence, processes at much higher temperatures are required for practical applications. Recently, several works examined the ultrahigh temperature treatments of GO at around 2000 ºC or above, and reported remarkably improved crystalline quality and the physical properties of the processed GO [15-18]. In these works, however, the process conditions were restricted to vacuum or inert environments at atmospheric pressure probably because of instrumental limitations. Investigation under much wider conditions such as vacuum environments at various pressures or reactive environments including ethanol-surrounding is desirable for producing highly crystalline
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