The N 2 adsorption isotherm measurements were performed at 77 K using a BELSORP Mini (BEL Japan, Inc.). Prior to measurements, samples were activated at 180 °C for 12 hours under dynamic vacuum. The N 2 adsorption isotherm in the p/p0 range 0.06 -0.25 was fitted to the Brunauer-Emmett-Teller (BET) equation to estimate the surface area of the samples.