Solar Energy Harvesting with Carbon Nitrides: Do We Understand the Mechanism?

Abstract

The photocatalytic splitting of water into molecular hydrogen and molecular oxygen with sunlight is the dream reaction for solar energy conversion. Since decades, transition-metaloxide semiconductors and supramolecular organometallic structures have been extensively explored as photocatalysts for solar water splitting. More recently, polymeric carbon nitride materials consisting of triazine or heptazine building blocks have attracted considerable attention as hydrogen-evolution photocatalysts. The mechanism of hydrogen evolution with polymeric carbon nitrides is discussed throughout the current literature in terms of the familiar concepts developed for photoelectrochemical water splitting with semiconductors since the 1970s. We discuss in this perspective an alternative mechanistic paradigm for photoinduced water splitting with carbon nitrides, which focusses on the specific features of the photochemistry of aromatic N-heterocycles in aqueous environments. It is shown that a water molecule which is hydrogen-bonded to an N-heterocycle can be decomposed into hydrogen and hydroxyl radicals by two simple sequential photochemical reactions. This concept is illustrated by first-principles calculations of excited-state reaction paths and their energy profiles for hydrogen-bonded complexes of pyridine, triazine and heptazine with a water molecule. It is shown that the excited-state hydrogen-transfer and hydrogen-detachment reactions are essentially barrierless, in sharp contrast to water oxidation in the electronic ground state, where high barriers prevail. We also discuss in some detail the products of possible reactions of the highly reactive hydroxyl radicals with the chromophores. We hypothesize that the challenge of efficient solar hydrogen generation with carbon-nitride materials is less the decomposition of water as such, but rather the controlled recombination of the photogenerated radicals to the closed-shell products H 2 and H 2 O 2 . * 4.75 0.002

Hydrogen evolution by polymer photocatalysts; a possible photocatalytic cycle

“…Finally, as already mentioned above, the mechanism proposed by Domcke and co-workers, [33][34][35][36][37] where the exciton on the polymer dissociates homolytically rather than heterolytically, i.e. by forming two neutral rather than two oppositely charged fragments, and a hydrogen atom is transferred between the molecule that gets oxidized and the polymer, is a special case of the mechanism discussed here.…”

“…Instead of sequential electron and proton transfer upon excitation of the polymer, concerted electron-proton transfer, and thus effectively hydrogen atom transfer, could take place, analogous to the mechanism discussed by Domcke and co-workers for water oxidation, [33][34][35][36][37] see Eq. 9 for an example involving PP n and TEA.…”

“…McCulloch and co-workers demonstrated that upon removal of palladium from polymers prepared via Suzuki coupling the HER rates drop to effectively zero 29,30 but other authors report HER by polymers that contain negligible noble metals and/or are prepared via a noble-metal free route. 31,32 Domcke, Sobolewski and co-workers have proposed an alternative micropscopic HER mechanism, based on abinitio calculations on hydrogen bonded pyridine, 33 acridine, 34 heptazine, 35 and triazine-water complexes, 36,37 which does not involve an heterolytic-type exciton dissociation, and thus the formation of an open-shell ion pair. Instead photon absorption induces an electron driven proton transfer, i.e.…”

Two-photon, visible light water splitting at a molecular ruthenium complex

“…Secondly, the two-photon nature of the water splitting reaction was elucidated using six different methods inspired by those previously used: [18][19][20][21][22][23][24] (1) A square relationship between initial rate of O 2 formation and photon flux (see Fig. 3A) indicates that two photons are involved in the reaction.…”

“…17 Related two-photon mechanisms have previously been reported in the context of photochromism, 18,19 organic photoredox reactions, 20,21 and OH generation. [22][23][24] Evidence for these mechanisms was garnered through square rate/photon flux relationships, 18,19,21 dual irradiation and wavelength dependency experiments, 18,19,21 independent generation of the second photon absorbing intermediate, [19][20][21][22][23] single and two pulse transient absorption spectroscopy, 18,19,[21][22][23] irradiation with ultrashort pulses, 18 kinetic modelling, 18,19,22,23 and quantum chemical studies. 19,24 Compared to water splitting, the previously studied reactions are relatively simple transformations (single bond breaking events).…”