Porphyrin-based photocatalysts for hydrogen production
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Porphyrin molecule-based photocatalysts
Solar-driven water splitting to produce clean and high-energy hydrogen fuel via a hot-carrier-mediated process is considered to be a promising route to reduce environmental pressures, caused primarily by the intensive consumption of nonrenewable fossil fuels. Developing efficient and robust catalysts to accelerate the water splitting speed and achieve high hydrogen production rates is a critical issue.1 Until now, various inorganic and organic photocatalysts have been investigated for hydrogen evolution.2–5 Porphyrins are molecules that exist widely in green plants and are one of the most important organic photocatalysts.6–11 Due to their unique macrocyclic structure (Figure 1), porphyrins show strong visible light absorption; they can also readily self-assemble into ordered nanostructures through noncovalent aggregation. Proper aggregation modes such as J-aggregation with head-to-tail stacking among porphyrins can broaden and red-shift absorption, which helps to stabilize the photoexcited electrons and holes. When modified with various functional groups, porphyrins can form metal–organic frameworks (MOFs), conjugated polymers, and even hybrids with other semiconductors to facilitate the separation of hotcharge carriers, which enhances the photocatalytic hydrogen production performance. In the following sections, we review recent advances in porphyrin-based photocatalysts for hydrogen production, focusing on the composition and structure of these materials.
In order to enhance photocatalytic hydrogen evolution using porphyrin molecules, much effort has been devoted to the chemical modification of the molecular structure to restrain the recombination of charge carriers.12–16 The most common strategy employed is to construct donor-bridge-acceptor type multistep electron transfer systems to induce intramolecular charge separation.12–14 For example, P. Yang and co-workers reported a triphenylamine-modified multibranched porphyrin molecule (5,10,15,20-tetrakis[4-(N,N-diphenylaminobenzoate)phenyl] porphyrin (TPPZ) and 5,10,15,20-tetrakis[4-(N,N-diphenylaminostyryl)phenyl] porphyrin (TPPX)),”13 in which porphyrins were covalently linked with triphenylamine (TPA) at the meso-position of the porphyrin ring by diverse covalent linkages (Figure 2a). They found that the porphyrin TPPZ with the ethylene bridge showed much higher photocatalytic hydrogen production activity than that of TPPZ with ester linkage, due to the faster intramolecular energy transfer from triphenylamine to porphyrin (Figure 2b). Similarly, the isomeric naphthalimide (NI) modified porphyrins with para-, meta-, and ortho-position on the phenyl as linker separately were also fabricated.14 They also showed comparable linkage dependent intramolecular energy-transfer efficiency and photocatalytic hydrogen production performance. Another strategy to restrain the recombination of charge carriers is to directly link the photosensitizer and the catalytic active unit together via covalent bonds, forming a sensitiz
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