  The quest for efficient and economically accessible cleaner methods to
develop sustainable carbon-free energy sources induced a keen interest in the
production of hydrogen fuel. This can be achieved via the water-splitting
process exploiting solar energy but requiring the use of adequate
photocatalysts. Covalent triazine-based frameworks (CTFs) are target
photocatalysts for water-splitting. Both electronic and structural
characteristics of CTFs, optical bandgaps and porosity, are directly relevant
for water-splitting. These can be engineered through chemical design. Porosity
can be beneficial to water-splitting by providing larger surface area for the
catalytic reactions. However, porosity can also affect both charge transport
within the photocatalyst and mass transfer of both reactants and products, thus
impacting the overall kinetics of the reaction. We focus on the link between
chemical design and water (reactants) mass transfer, playing a key role in the
water uptake process and the subsequent hydrogen generation. We use neutron
spectroscopy to study water mass transfer in two porous CTFs, CTF-CN and CTF-2,
that differ in the polarity of their struts. Quasi-elastic neutron scattering
(QENS) is used to quantify the amount of bound water and the translational
diffusion of water. Inelastic neutron scattering measurements complement QENS
and provides insights into the softness of the CTF structures and the changes
in librational degrees of freedom of water in CTFs. We show that CTF-CN
exhibits smaller surface area and water uptake due to a softer structure than
CTF-2. The current study leads to new insights into the
structure-dynamics-property relationship of CTF photo-catalysts that pave the
road for a better understanding of the guest-host interaction at the basis of
water splitting applications.
