Abstract
Covalent triazine-based frameworks (CTFs) are a subclass of conjugated microporous polymers (CMPs) that can be used as organic photocatalysts for photocatalytic hydrogen evolution from water. Seven materials with varied spacer units from phenylene to quarterphenylene were synthesized, either by trifluoromethanesulfonic acid (TfOH) catalysis from nitriles or by Suzuki-Miyaura polycondensation. The photocatalytic performance under visible light of all materials was systematically studied in the presence of a hole-scavenger, showing that both synthesis routes produce CTFs with similar hydrogen evolution rates (HER), but different optical properties. The highest hydrogen evolution rate in the cyclotrimerized series was found for CTF-2 with an apparent quantum yield of 1.6% at 420 nm in a mixture of water and triethanolamine with a platinum co-catalyst. Based on (TD-)DFT calculations, the highest performance was expected for CTF-1 and this discrepancy is explained by a trade-off between increased light absorption and decreased thermodynamic driving force.
Highlights
Until recently, direct photocatalytic hydrogen production using particulate photocatalysts has been dominated by inorganic materials [1e3], but there is a growing interest in organic and polymeric photocatalysts [4]
The photocatalytic hydrogen evolution rates of seven Covalent triazine-based frameworks (CTFs) using sacrificial hole-scavengers were studied and the results were compared with the predictions of (TD)-DFT calculations
CTF-2 showed the highest hydrogen evolution rate under visible light using triethanolamine as sacrificial hole-scavenger and this shows that microporosity does not preclude efficient photocatalytic water reduction
Summary
Direct photocatalytic hydrogen production using particulate photocatalysts has been dominated by inorganic materials [1e3], but there is a growing interest in organic and polymeric photocatalysts [4]. Materials are typically modified by varying the nitrogen rich starting materials [13,14] or by doping methods; for example, by adding sulfur [15] or potassium hydroxide [16] This difficulty in tailoring the photophysical properties of gC3N4 makes it attractive to study polymeric organic photocatalysts, which can be synthesized by coupling a diverse range of monomers at low temperatures, allowing fine synthetic control over the. We prepared a range of structurally related CTFs using low temperature synthesis routes and explored the effect of structure and synthesis conditions on photocatalytic hydrogen evolution under sacrificial conditions. The mixer mill was equipped with 10 mL stainless steel grinding jars that contained two stainless steel grinding balls with a diameter of 7 mm. (TD-)DFT calculations were performed using the Turbomole 6.6 code [39], the B3LYP [40,41] density functional and the COSMO solvation model [42], for more details see the supporting information
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