Monolayer Solids of Kr on Graphitized Carbon Black Surfaces and in Graphitic Hexagonal Pores

Abstract

To examine the effects of the inherent heterogeneity in graphitic hexagonal pores on Kr adsorption in the monolayer (ML) regime, we measured the powder X-ray diffraction (XRD) patterns of Kr adsorbed on the walls of the graphitic hexagonal pores, as well as the flat surfaces of graphitized carbon black, as a function of surface coverage at several different temperatures. On the flat surfaces of the graphitized carbon black, Kr atoms formed a hexagonally close-packed monolayer incommensurate with the graphite surface in the coverage range of 0.6–1.5 ML and the temperature range of 77–90 K, although the monolayer may coexist with a bilayer at high coverage, and the layers melted between 90 and 120 K. For the flat surfaces of the graphitized carbon black, the diffraction pattern for coverages less than ∼0.6 ML was very broad and very weak in intensity, indicating that two-dimensional (2D) clusters of Kr atoms formed are very small in size or highly defective. On the other hand, for the walls of the graphitic hexagonal pores, Kr atoms formed a hexagonally close-packed monolayer incommensurate with the graphite surface in the coverage range of 0.2–1.5 ML and the temperature range of 60–110 K, although the monolayer may coexist with a bilayer at high coverage, and the layers melted between 110 and 120 K. Formation of 2D crystallites at a low coverage of ∼0.2 ML indicates that a large number of nucleation sites for the 2D clusters exist on the walls of the graphitic hexagonal pores. The inherent heterogeneity due to pore shape in the graphitic hexagonal pores alters the growth mode of an adsorbed layer on the graphite surfaces. At a monolayer coverage, the 2D crystal size of Kr adsorbed on the walls of the graphitic hexagonal pores was smaller than that on the flat surfaces of the graphitized carbon black because the domain size of the carbon layer of the pore walls is smaller than that of the graphitized carbon black.