Oxygen guest molecules in the nanopores of nanographite-network

Magnetic properties of nanographite-network-based nanoporous carbon having high surface area are investigated in the presence of magnetic oxygen guest molecules. The nanographite domains (stacked nanographene sheets) of 2-3 nm average in-plane size have localized spins of non-bonding π-electron state (edge-state) in the zigzag-shaped periphery of individual nanographene sheets. The electron paramagnetic resonance signal of edge-state spins is found to be highly sensitive to the presence of magnetic oxygen molecules. The line-width variation in the limit of lower oxygen pressure (1-20 Pa) shows that the magnetic interaction of oxygen can be divided into two temperature regimes, viz., the regime below 100 K, where chemisorption of oxygen is effective and the regime above 100 K, where only physisorption of oxygen takes place. Here, 100 K marks the energy of physisorption of oxygen molecules on nanographene, which is higher than the condensation energy of oxygen molecules represented by the boiling point of oxygen, 90 K, owing to the capillary effect. Above 100 K, magnetically active physisorbed oxygen molecules work to increase the line-width, which is governed by the dipolar field with oxygen molecules. The presence of a maximum in the line-width around 150 K, which is higher than 100 K, suggests the diffusion motion of oxygen molecules reduces the strength of exchange interaction between the edge-state spins and the oxygen spins as the temperature is elevated.