Surface collapse of fine powders ground from HKUST-1 big crystals investigated by positron annihilation lifetime spectroscopy

Big crystals of HKUST-1 were synthesized under solvothermal conditions, and HKUST-1 fine powders were also ground from them and directly synthesized. The specific surface areas of the powders were found to be much smaller than that of the big crystals, indicative of micropore collapse in the powders. Interestingly, positron annihilation lifetime spectroscopy (PALS) showed two long ortho-positronium (o-Ps) lifetimes for the powders representing o-Ps annihilation in two types of inherent pores of HKUST-1, whereas only one long lifetime in the relatively larger pores could be derived for the big crystals. Meanwhile, continuous positron lifetime analysis showed two well-decomposed o-Ps lifetime distributions accompanied by relatively higher total o-Ps intensities in the powders. Due to the nature of o-Ps atoms in porous materials, they may diffuse in well-interconnected pores, and preferentially be localized and merely annihilate in larger pores in the big crystals of HKUST-1 with intact frameworks, resulting in only one o-Ps lifetime in them. The abnormal PALS results can be well explained for the collapse of large surface pores and the formation of a dense layer on the surfaces of fine powders of HKUST-1, demonstrating PALS is a useful method for studying the pore structures of metal-organic frameworks (MOFs).