Efficient visible light-driven water oxidation and proton reduction by an ordered covalent triazine-based framework

Abstract

aWater oxidation is a rate-determining step in solar driven H 2 fuel synthesis and is technically challenging to promote. Despite decades of effort, only a few inorganic catalysts are effective and even fewer are effective under visible light. Recently, attention has been paid to synthetic semiconducting polymers, mainly on graphitic C 3 N 4 , with encouraging hydrogen evolution performance but lower activity for water oxidation. Here, a highly ordered covalent triazine-based framework, CTF-1 (C 8 N 2 H 4 ), is synthesised by a very mild microwave-assisted polymerisation approach. It demonstrates extremely high activity for oxygen evolution under visible light irradiation, leading to an apparent quantum efficiency (AQE) of nearly 4% at 420 nm. Furthermore, the polymer can also efficiently evolve H 2 from water. A high AQE of 6% at 420 nm for H 2 production has also been achieved. The polymer holds great potential for overall water splitting. This exceptional performance is attributed to its well-defined and ordered structure, low carbonisation, and superior band positions. Broader contextSplitting water by sunlight is an attractive renewable approach to generate clean hydrogen for producing chemicals or powering vehicles. This ultra-pure hydrogen also avoids the dreaded catalyst poisoning, which occurs even with very low levels of CO residuals from fossil-fuel generated hydrogen. The key challenge to sustain continued hydrogen generation from this artificial photosynthesis process is to speed up the water oxidation reaction, which is hard to proceed due to multiple electron transfer steps. Oxidation catalysts based on polymeric semiconductors are particularly promising for this purpose because of their abundance leading to low cost, tunable band structure to match the solar spectrum and variable degree of conjugation to impact on the p-p* excitation for efficient electron transfer. With a moderate microwave assisted strategy, we are able to control the degree of conjugation and minimise the undesirable structural carbonisation of a new type of polymeric photocatalyst-covalent triazine-based framework. The optimised catalyst shows advantageous band positions, to capture a wide spectrum of visible light and enhance the charge separation efficiency, leading to very high water oxidation and hydrogen evolution capabilities. The overall discovery paves the way for the development of efficient and continuous clean hydrogen production from the renewable visible-light water splitting process.

Bandgap Engineering of Organic Semiconductors for Highly Efficient Photocatalytic Water Splitting

“…For instance, the doping of cations (e.g., Rh) into SrTiO 3 introduced new energy levels and narrowed its wide bandgap (3.1 eV) to the visible light region (2.4 eV). [11][12][13][14][15][16][17] Currently, the majority of organic/polymeric photocatalysts still suffer from an intrinsic wide bandgap (e.g., ≈2.7 eV for g-C 3 N 4 ) and only responds to a limited region of the solar spectrum (<460 nm), not matching with the strongest portion of 450-700 nm in sunlight (2.7-1.8 eV photons). [10] Compared with the progress in bandgap engineering of inorganic photocatalysts, there have been limited reports of the emerging organic photocatalysts (e.g., heptazine-based polymers and covalent organic frameworks), although they are known for their suitable band positions for water splitting, low-cost, chemical stability, and good tunability of their framework and electronic structures.…”

Strong‐Base‐Assisted Synthesis of a Crystalline Covalent Triazine Framework with High Hydrophilicity via Benzylamine Monomer for Photocatalytic Water Splitting

“…It is known that the acids or bases adopted in the synthesis of Schiff‐ base linked COFs can have prominent influence on the crystallinity because it adjusts the reversibility of the imine bond formation . Thus, the strength of the base reagent may also affect the crystallinity of the CTFs like other ways, such as temperature, microwave power, feeding rate, and acidity –. As guided by this principle, various base reagents, that is, Cs 2 CO 3 (previous work), K 2 CO 3 ( p K a : 10.3), KOH ( p K a : 13.5), EtOK ( p K a : 16.5), and t BuOK ( p K a : 17.5) are screened .…”

“…The highly robust structures deriving from an aromatic triazine linkage and fully conjugated structures enriched by aromatic nitrogen atoms of CTFs make them appealing in various applications, such as heterogeneous catalysis and photocatalysis –. As for the synthesis of CTFs, some typical synthetic methods have been reported –. However, most of the CTFs synthesized from these methods are amorphous.…”

Recent Advances in Visible‐Light‐Driven Hydrogen Evolution from Water using Polymer Photocatalysts

“…Tang et al synthesized crystalline CTF photocatalysts using catalytic amounts of TfOH with microwave-assisted thermal methods; during this reaction, crystallinity was maintained and carbonization was minimal. [81] Notably, CTF-1-100 W is an efficient photocatalyst with an HER of 5,500 μmol h À 1 g À 1 and a high AQY of 6 % at 420 nm. Subsequently, they developed a novel approach to control the band gaps of organic photocatalysts, which could directly affect the charge transfer behavior, driving force, and harvest of visible-light photons.…”

“…Tang et al. synthesized crystalline CTF photocatalysts using catalytic amounts of TfOH with microwave‐assisted thermal methods; during this reaction, crystallinity was maintained and carbonization was minimal . Notably, CTF‐1–100 W is an efficient photocatalyst with an HER of 5,500 μmol h −1 g −1 and a high AQY of 6 % at 420 nm.…”