Abstract-Relaxation dynamics of surface-adsorbed water molecules in nanoporous silica probed by terahertz spectroscopy

Yu-Ru Huang^1,2, Kao-Hsiang Liu³, Chung-Yuan Mou³ and Chi-Kuang Sun^1,2,4,a)

1 Department of Electrical Engineering and Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taipei 10617, Taiwan
2 Molecular Imaging Center, National Taiwan University, Taipei 10617, Taiwan
3 Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan
4 Research Center for Applied Sciences and Institute of Physics, Academia Sinica, Taipei 11529, Taiwan
a) E-mail: sun@ntu.edu.tw
http://scitation.aip.org/content/aip/journal/apl/107/8/10.1063/1.4929767
Appl. Phys. Lett. 107, 081607 (2015); http://dx.doi.org/10.1063/1.4929767

Relaxation dynamics of an exclusively adsorbed water molecule in mesoporous silica MCM-41-S was studied by using terahertz spectroscopy. With the temperature controlled from 0 to 50 °C, we observed strongly frequency- and temperature-dependent dielectric relaxation responses, implying that, unlike ice, surface-adsorbed water molecules retained flourishing picosecond dynamics. Based on the Debye relaxation model, a relaxation time constant was found to increase from 1.77 to 4.83 ps when the water molecule was cooled from 50 to 0 °C. An activation energy of ∼15 kJ/mol, which was in close agreement with a hydrogen-bonding energy, was further extracted from the Arrhenius analysis. Combined with previous molecular dynamics simulations, our results indicate that the reorientation relaxation originated from the “flip-flop” rotation of a three hydrogen-bonded surface-adsorbed water molecule.