Kong, Dan; (2019) Investigation into covalent triazine frameworks for high efficiency visible-light driven water splitting. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Efficient utilisation of solar energy could alleviate major energy and the related environmental issues. The conversion of solar energy into chemical fuels by artificial photosynthesis has thus received much attention, e.g. production of renewable hydrogen from water. Since the first photoelectrode titanium dioxide was found for photoelectrochemical water splitting in 1972, substantial progress on semiconducting materials for photocatalytic water splitting has been made. Specifically, to utilise sunlight efficiently, developing visible-light-responsive photocatalysts is indispensable to realise the application of solar-to-chemical energy conversion in practice. Considering few overall water splitting systems reported, investigation on the oxidative and the reductive half reactions separately is significant for fundamental understanding, optimisations and finally complete water-splitting cycles. Among these photocatalysts, inorganic photocatalysts have been widely explored for the hydrogen evolution reaction. However, most of them are either only active under UV light irradiation or their efficiency is moderate, due to either large band gap energy or fast charge recombination. In the past few years, the increasing interest in a class of metal-free organic photocatalysts for water splitting has been raised, as these organic polymers feature twodimensional (2D) conjugated structures, high chemical stability, ease of modification to achieve suitable thermodynamical potentials to overall water splitting. The most common metal-free organic photocatalyst is melon-based graphitic carbon nitride (for simplicity, usually denoted gC3N4). Moreover, a series of covalent triazine frameworks (CTFs) were synthesised recently. These materials were formed by the ionothermal trimerization of aromatic nitriles in molten ZnCl2 and built up by alternating triazine and phenyl building blocks. Because of the covalent triazine-based structure, CTFs possess excellent thermal and chemical stability, beneficial as new catalysts in liquid phase reactions. CTFs with the π-stacked aromatic units would also be expected to promote exciton separation and charge transportation, promising for photocatalytic lightdriven water splitting. As such, the research project targets on visible light-driven CTF photocatalysts for pure water splitting. Firstly, the photooxidation of water using oxygen doped CTF-1(OCT) was investigated. The OCT was created by a simple dynamic trimerization reaction of the precursor 1,4-dicyanobenzene in ionothermal conditions, that is, in molten zinc chloride at high temperature. It was found that due to the oxygen-containing reaction atmosphere, some oxygen was doped in the crystals to modify the structure, optical, and electrical properties of the materials, resulting in the much better operation window (from UV to NIR) than the benchmark photooxidation catalyst BiVO4 (only active from UV to 500 nm). The external quantum efficiency of OCT was determined to be 2.6% at the wavelength of 400 nm, 1.5% at 500 nm, even ~0.2% at wavelength as long as 800 nm. Structure optimization, thermodynamic calculation and electronic structure analysis of OCT calculated by density functional theory (DFT) were carried out to illustrate the mechanism of the increasing photooxidation yield successfully, which could be applied to improve other semiconductors. Furthermore, hydrogen and oxygen evolutions from water were carried out by another polymer photocatalyst CTF-0, which is one member of the CTFs group and based on 1,3,5-tricyanobenzene as monomer under ionothermal conditions. Compared with OCT, CTF-0 crystals have higher nitrogen ratio and smaller pore size. Herein, two different synthesis ways of a novel photocatalyst covalent triazine framework CTF-0 were utilised and tested for photocatalytic H2 and O2 evolution under visible light irradiation. The CTF-0-M2 produced by a microwave method shows an almost 7 times higher photocatalytic activity of hydrogen evolution (up to 701 µmol/h) than the CTF-0-I produced by an ionothermal trimerization method under similar photocatalytic conditions, which leads to an extremely high turnover number (TON) of 726 over a platinum cocatalyst after seven circles. This can be attributed to the narrow band gap and the rapid photogenerated charge separation and transportation. Whereas, CTF-0-I has produced rough 6 times higher oxygen of 22.6 µmol in the first hour than CTF-0-M2 under the same experimental condition. A high apparent quantum efficiency (AQY) of 7.2% at 420 nm for oxygen production was obtained from aqueous AgNO3 solution without any cocatalysts, exceeding most of the reported CTFs, due to the large driving force of water oxidation and the large number of active sites. Finally, considering that the CTF-0 has a wide bandgap, which could produce both hydrogen and oxygen theoretically, decorating the different co-catalysts on the CTF-0 was explored for the entire water splitting to produce hydrogen and oxygen. The presence of the cocatalyst Pt and Co3O4 promotes the H2 and O2 evolution on the surface of the photocatalysts simultaneously, due to enhanced separation of photogenerated charge carriers, more active sites for catalytic H2 and O2 evolution and the improved stability by suppressing photo-corrosion. Loading different ratio of cobalt cocatalysts on CTF-0 has been explored for overall water splitting. And it is found that water splitting rates are influenced by the concentration of the cocatalyst. 6 wt% Co3O4 and 3wt% Pt-deposited CTF-0 shows the best photocatalytic performances of 0.82 μmol/h H2 and 0.42 μmol/h O2, nearly close to the stoichiometric H2/O2 ratio of 2:1. Whereas, the system didn’t work under the visible light but UV light irradiation, which might be because of the limitation of light absorption range and the efficiency of the charged carriers. Further work is required to confirm the factors and mechanism of the pure water splitting of CTF-0s.

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