Amorphous solid water prevails inside spherical nano-pores of MIL-101(Cr) framework below 200K: PALS, XRD and BDS studies

Abstract

The glass transition of nano-confined supercooled water is still a vivid topic of discussion. Previous calorimetric studies on nano-confined water have reported that no glass transition is visible for water confined in nano-pores having size less than 2nm due to lack of network formation. The literature report of a sharp increase in heat capacity, or a cross-over in relaxation of water inside the nano-pores (size~1-1.8nm) at temperature 210-180K were due to pre-melting of fine ice crystal, fragile to strong transition or the non-cooperative relaxation. In the present study we have carried out low temperature positron annihilation lifetime spectroscopy (PALS), synchrotron-based X-ray diffraction and broadband dielectric spectroscopy (BDS) on water confined in the nanopores (size ~0.7-1.8nm) of synthesized MIL-101(Cr) metal organic framework (MOF) to clearly understand low temperature phase transition of confined water. The PALS spectra have shown three different slopes at three different temperatures. At high temperature, the change in slope of ortho-positronium (o-Ps) pick-off lifetime (tau_o-Ps) profile near 280K corresponds to the structural re-arrangement of water molecules in liquid phase [4]. At around 230K, a slight change in slope of tau_o-Ps corresponds to partial crystallization of confined supercooled water. The XRD pattern showed evolution of short range crystalline peaks below 230K confirming partial crystallization of water under confinement. At around 200K, tau_o-Ps profile showed a sharp change in slope, and below 200K, tau_o-Ps remains almost constant. Interestingly, XRD peaks did not show any noticeable change at 200K, suggesting that the transition at 200K is not associated with any crystallization change. Rather, weak temperature dependence of tau_o-Ps (~0.5ns) below 200K corresponds to o-Ps annihilation in the free-volume associated with amorphous solid water (ASW) inside the MIL-101(Cr) nano-pores. The dielectric relaxation of confined water showed a cross-over from Vogel-Fulcher-Tammann (VFT) to Arrhenius dependence at around 190K, indicating a transition from fragile glass forming liquid to ASW like strong glassy phase inside the nano-pores. Hence, a combined study using XRD, PALS and BDS confirms that the supercooled liquid transforms into to ASW glassy phase at 200K-190K inside the nanopores of MIL-101(Cr) having pore size distribution around 0.7-1.8nm.