Effects of molecular shapes, molecular weight, and types of edges on peak positions of C1s X-ray photoelectron spectra of graphene-related materials and model compounds

Nanocarbon materials such as graphene and graphene nanoribbons (GNRs) have been studied for various applications such as electrodes and catalysts. For precise analyses of nanocarbon materials, analyses other than conventionally used analyses such as Raman and infrared spectroscopies and microscopies are essential. C1s X-ray photoelectron spectroscopy (XPS) is one of the general techniques used to analyze the structures of carbon materials. However, XPS has not been commonly utilized to investigate the structures of carbon materials in detail. In this work, the relevance of molecular shapes, molecular sizes, and types of edges on peak positions of C1s XPS spectra were revealed. For example, adjusted peak positions of C1s spectra of rectangular graphene-like structures such as GNRs tended to be higher than those of triangle or hexagonal graphene-like structures at the same atomic ratio of H/C mainly because rectangular structures of GNRs enhanced the conjugation more than those of hexagonal and triangle structures of graphene-related materials. Besides, the structures of graphene-related materials, such as rectangular with either zigzag or armchair edges, triangle, hexagonal, and the other structures, can be estimated without using a microscope by measuring combustion elemental analysis, mass spectrometry, C1s XPS spectra, and the highest occupied molecular orbital and comparing them with the tendencies of H/C ratio and molecular weight versus adjusted peak positions of C1s XPS spectra. Moreover, the reasons for the shift and the broadening of experimental C1s XPS spectra have been revealed to be the charge-up effect caused by the large size of the powder of analyzed compounds.