The roles of buckled geometry and water environment in the excitonic properties of graphitic C3N4

Photooxidation of water with heptazine-based molecular photocatalysts: Insights from spectroscopy and computational chemistry

We present a conspectus of recent joint spectroscopic and computational studies that provided novel insight into the photochemistry of hydrogen-bonded complexes of the heptazine (Hz) chromophore with hydroxylic substrate molecules (water and phenol). It was found that a functionalized derivative of Hz, tri-anisole-heptazine (TAHz), can photooxidize water and phenol in a homogeneous photochemical reaction. This allows the exploration of the basic mechanisms of the proton-coupled electron-transfer (PCET) process involved in the water photooxidation reaction in well-defined complexes of chemically tunable molecular chromophores with chemically tunable substrate molecules. The unique properties of the excited electronic states of the Hz molecule and derivatives thereof are highlighted. The potential energy landscape relevant for the PCET reaction has been characterized by judicious computational studies. These data provided the basis for the demonstration of rational laser control of PCET reactions in TAHz-phenol complexes by pump-push-probe spectroscopy, which sheds light on the branching mechanisms occurring by the interaction of nonreactive locally excited states of the chromophore with reactive intermolecular charge-transfer states. Extrapolating from these results, we propose a general scenario that unravels the complex photoinduced water-splitting reaction into simple sequential light-driven one-electron redox reactions followed by simple dark radical-radical recombination reactions.

Low-Dimensional Semiconductors in Artificial Photosynthesis: An Outlook for the Interactions between Particles/Quasiparticles

By virtue of their intriguing electronic structures and excellent surface properties, low-dimensional semiconductors hold great promise in the field of solar-driven artificial photosynthesis. However, owing to promoted structural confinement and reduced Coulomb screening, remarkable interactions between particles/quasiparticles, including electrons, holes, phonons, and excitons, can be expected in low-dimensional semiconductors, which endow the systems with distinctive excited-state properties that are distinctly different from those in the bulk counterparts. Consequently, these interactions determine not only the mechanisms but also quantum yields of photosynthetic energy utilization. In this Outlook, we review recent advances in studying the unique interactions in low-dimensional semiconductor-based photocatalysts. By highlighting the relevance of different interactions to excited-state properties, we describe the impacts of the interactions on photosynthetic energy conversion. Furthermore, we summarize the regulation of these interactions for gaining optimized photosynthetic behaviors, where the relationships between these interactions and structural factors/external fields are elaborated. Additionally, the challenges and opportunities in studying the interaction-related photosynthesis are discussed.

Heterojunctions of halogen-doped carbon nitride nanosheets and BiOI for sunlight-driven water-splitting

A fluorine-doped, chlorine-intercalated carbon nitride (CNF-Cl) photocatalyst has been synthesized for simultaneous improvements in light harvesting capability along with suppression of charge recombination in bulk g-C3N4. The formation of heterojunctions of these CNF-Cl nanosheets with low bandgap, earth abundant bismuth oxyiodide (BiOI) was achieved, and the synthesized heterojunctions were tested as active photoanodes in photoelectrochemical water splitting experiments. BiOI/CNF-Cl heterojunctions exhibited extended light harvesting with a band-edge of 680 nm and generated photocurrent densities approaching 1.3 mA cm-2 under AM1.5 G one sun illumination. Scanning Kelvin probe force microscopy under optical bias showed a surface potential of 207 mV for the 50% BiOI/CNF-Cl nanocomposite, while pristine CNF-Cl and BiOI had surface photopotential values of 83 mV and 98 mV, respectively, which in turn, provided direct evidence of superior charge separation in the heterojunction blends. Enhanced charge carrier separation and improved light harvesting capability in BiOI/CNF-Cl hybrids were found to be the dominant factors in increased photocurrent, compared to the pristine constituent materials.

Where do photogenerated holes at the g-C3N4/water interface go for water splitting: H2O or OH-?

Graphitic carbon nitride (g-C3N4), a metal-free two-dimensional photocatalyst, has drawn increasing attention due to its application in photocatalytic water splitting. However, its quantum efficiency is limited by the poor performance of the oxygen evolution reaction (OER). Therefore, it is important to clarify the behavior of photogenerated holes in the OER. In this work, we investigate the energy level alignment using the GW method and the exciton properties using the Bethe-Salpeter equation within the ab initio many-body Green's function theory at the g-C3N4/water interface. We found that the g-C3N4 substrate can elevate energy levels of OH- and H2O molecules at the interface by up to 0.6 eV. This effect can make the electronic levels of OH- surpass the valence band maximum (VBM) of g-C3N4. However, orbital energies of H2O molecules remain far below the VBM of g-C3N4. This indicates that a photogenerated hole after exciting g-C3N4 can relax to OH- instead of neutral H2O. Moreover, OH- could be directly oxidized through electron transfer from OH- to g-C3N4 by light near the optical absorption edge of g-C3N4, which is beneficial for efficient carrier separation at the interface.

Mechanism of Photocatalytic Water Oxidation by Graphitic Carbon Nitride

Carbon nitride materials are of great interest for photocatalytic water splitting. Herein, we report results from first-principles simulations of the specific electron- and proton-transfer processes that are involved in the photochemical oxidation of liquid water with heptazine-based molecular photocatalysts. The heptazine chromophore and the solvent molecules have been described strictly at the same level of electronic structure theory. We demonstrate the critical role of solvent molecules for the absorption properties of the chromophore and the overall photocatalytic cycle. A simple model is developed to describe the photochemical water oxidation mechanism. Our results reveal that heptazine possesses energy levels that are suitable for the water oxidation reaction. We suggest design principles for molecular photocatalysts which can be used as descriptors in future experimental and computational screening studies.