Abstract
Hydrogen fuel can contribute as a masterpiece in conceiving a robust carbon-free economic puzzle if cleaner methods to produce hydrogen become technically efficient and economically viable. Organic photocatalytic materials such as conjugated microporous materials (CMPs) are potential attractive candidates for water splitting as their energy levels and optical band gap as well as porosity are tunable through chemical synthesis. The performances of CMPs depend also on the mass transfer of reactants, intermediates, and products. Here, we study the mass transfer of water (H2O and D2O) and of triethylamine, which is used as a hole scavenger for hydrogen evolution, by means of neutron spectroscopy. We find that the stiffness of the nodes of the CMPs is correlated with an increase in trapped water, reflected by motions too slow to be quantified by quasi-elastic neutron scattering (QENS). Our study highlights that the addition of the polar sulfone group results in additional interactions between water and the CMP, as evidenced by inelastic neutron scattering (INS), leading to changes in the translational diffusion of water, as determined from the QENS measurements. No changes in triethylamine motions could be observed within the CMPs from the present investigations.
Highlights
The need for a renewable energy carrier has resulted in intense research over the last decades on the generation of hydrogen from water via water splitting
Most of the photocatalysts studied are inorganic,[1,2] but, since the first report on carbon nitrides as potential photocatalyst in 2009,3 organic polymer photocatalysts have been studied intensively.[4−6] Initially, carbon nitrides[3,7] were the main focus, but in recent years, conjugated microporous polymer networks (CMPs),[8−10] linear conjugated polymers,[11−17] triazine-based frameworks,[18−21] covalent organic frameworks (COFs),[22−24] and molecular compounds[25,26] have been proposed for sacrificial proton reduction half-reaction. Activities that rival those obtained with inorganic systems have been achieved in some cases.[27−29] The interest in organic photocatalysts arises from the ease of synthesis of polymer photocatalysts via lowtemperature routes that allow for precise control over the polymer sequence, allowing for tailoring of their functionalities.[5,30]
These studies have led to an understanding of the importance of several factors that result in high activity in polymer photocatalysts, such as light absorption,[8,31,32] driving force for proton reduction and scavenger oxidation,[31] exciton separation,[16,33] and crystallinity.[34−36] Due to the hydrophobic nature of most polymeric photocatalyst surface, wetting seems to be important.[37−39] Several studies have shown that the introduction of polar groups results in materials with higher photocatalytic activities.[12,39−41] Large surface area to maximize the exposed surface to water can be beneficial
Summary
The need for a renewable energy carrier has resulted in intense research over the last decades on the generation of hydrogen from water via water splitting. Most of the photocatalysts studied are inorganic,[1,2] but, since the first report on carbon nitrides as potential photocatalyst in 2009,3 organic polymer photocatalysts have been studied intensively.[4−6] Initially, carbon nitrides[3,7] were the main focus, but in recent years, conjugated microporous polymer networks (CMPs),[8−10] linear conjugated polymers,[11−17] triazine-based frameworks,[18−21] covalent organic frameworks (COFs),[22−24] and molecular compounds[25,26] have been proposed for sacrificial proton reduction half-reaction Activities that rival those obtained with inorganic systems have been achieved in some cases.[27−29] The interest in organic photocatalysts arises from the ease of synthesis of polymer photocatalysts via lowtemperature routes that allow for precise control over the polymer sequence, allowing for tailoring of their functionalities.[5,30]. In this work, using the doubly focused Cu(220) monochromator setting, the incident energy was ca. 210−3500 cm−1, leading after subtraction of the fixed final energy value (4.5 meV) to an accessible energy transfer range of ca. 180− 3500 cm−1, covering the full molecular vibrational frequencies
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