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

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 within nanopores having size less than 2 nm due to a lack of network formation [M. Oguni <i>et al, J.Phys. Chem. B</i>, 2011, <b>115</b>, 14023-14029]. According to literature reports, a sharp increase in heat capacity [G. P. Johari, <i>J. Chem. Phys.</i>, 2009, <b>130</b>, 124518] or a cross-over in relaxation [J. Swenson <i>et al.</i>, <i>Phys Rev. Letts</i> 2006, <b>96</b>, 247802] of water inside the nanopores (size ∼1-1.8 nm) in the temperature range of 210-180 K was due to pre-melting of fine ice crystals, fragile to strong transitions or 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 within the nanopores (size ∼0.7-1.8 nm) of the synthesized MIL-101(Cr) metal organic framework (MOF) to clearly understand the 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 the slope of the <i>ortho</i>-positronium (<i>o</i>-Ps) pick-off lifetime (<i>τ</i>_<i>o</i>-Ps) profile near 280 K corresponds to the structural rearrangement of water molecules in the liquid phase. At around 230 K, a slight change in the slope of the <i>τ</i>_<i>o</i>-Ps profile corresponds to partial crystallization of confined supercooled water. The XRD pattern showed the evolution of short range crystalline peaks below 230 K, confirming partial crystallization of water under confinement. At around 200 K, the <i>τ</i>_<i>o</i>-Ps profile showed a sharp change in the slope, and below 200 K, <i>τ</i>_<i>o</i>-Ps remains almost constant. Interestingly, XRD peaks did not show any noticeable change at 200 K, suggesting that the transition at 200 K is not associated with any crystallization change. Rather, the weak temperature dependence of <i>τ</i>_<i>o</i>-Ps (∼0.5 ns) below 200 K corresponds to <i>o</i>-Ps annihilation in the free volume associated with amorphous solid water (ASW) inside the MIL-101(Cr) nanopores. The dielectric relaxation of confined water showed a cross-over from Vogel-Fulcher-Tammann (VFT) to Arrhenius dependence at around 190 K, indicating a transition from a fragile glass forming liquid to an ASW like strong glassy phase inside the nanopores. Hence, a combined study using XRD, PALS and BDS confirms that the supercooled liquid transforms into to an ASW glassy phase at 200-190 K inside the nanopores of MIL-101(Cr) having pore size distribution around 0.7-1.8 nm.