Ultrafast dynamics in polymeric carbon nitride thin films probed by time-resolved EUV photoemission and UV-Vis transient absorption spectroscopy

The ground- and excited-state electronic structures of four polymeric carbon nitride (PCN) materials have been investigated using a combination of photoemission and optical absorption spectroscopy. To establish the driving forces for photocatalytic water-splitting reactions, the ground-state data was used to produce a band diagram of the PCN materials and the triethanolamine electron scavenger, commonly implemented in water-splitting devices. The ultrafast charge-carrier dynamics of the same PCN materials were also investigated using two femtosecond-time-resolved pump-probe techniques: extreme-ultraviolet (EUV) photoemission and ultraviolet-visible (UV-Vis) transient absorption spectroscopy. The complementary combination of these surface- and bulk-sensitive methods facilitated photoinduced kinetic measurements spanning the sub-picosecond to few nanosecond time range. The results show that 400 nm (3.1 eV) excitation sequentially populates a pair of short-lived transient species, which subsequently produce two different long-lived excited states on a sub-picosecond time scale. Based on the spectro-temporal characteristics of the long-lived signals, they are assigned to singlet-exciton and charge-transfer states. The associated charge-separation efficiency was inferred to be between 65% and 78% for the different studied materials. A comparison of results from differently synthesized PCNs revealed that the early-time processes do not differ qualitatively between sample batches, but that materials of more voluminous character tend to have higher charge separation efficiencies, compared to exfoliated colloidal materials. This finding was corroborated via a series of experiments that revealed an absence of any pump-fluence dependence of the initial excited-state decay kinetics and characteristic carrier-concentration effects that emerge beyond few-picosecond timescales. The initial dynamics of the photoinduced charge carriers in the PCNs are correspondingly determined to be spatially localised in the immediate vicinity of the lattice-constituting motif, while the long-time behaviour is dominated by charge-transport and recombination processes. Suppressing the latter by confining excited species within nanoscale volumes should therefore affect the usability of PCN materials in photocatalytic devices.

Photophysics of nanographenes: from polycyclic aromatic hydrocarbons to graphene nanoribbons

Graphene quantum dots (GQDs) and nanoribbons (GNRs) are classes of nanographene molecules that exhibit highly tunable photophysical properties. There have been great strides in recent years to advance our understanding of nanographene photophysics and develop their use in light-harvesting systems, such as artificial photosynthesis. Here, we review the latest studies of GQDs and GNRs which have shed new light onto their photophysical underpinnings through computational and advanced spectroscopic techniques. We discuss how the size, symmetry, and shape of nanographenes influence their molecular orbital structures and, consequentially, their spectroscopic signatures. The scope of this review is to comprehensively lay out the general photophysics of nanographenes starting with benzene and building up to larger polycyclic aromatic hydrocarbons, GQDs, and GNRs. We also explore a collection of publications from recent years that build upon the current understanding of nanographene photophysics and their potential application in light-driven processes from display, lasing, and sensing technology to photocatalytic water splitting.

Elucidating the Mechanistic Origins of Photocatalytic Hydrogen Evolution Mediated by MoS2/CdS Quantum-Dot Heterostructures

Solar fuel generation mediated by semiconductor heterostructures represents a promising strategy for sustainable energy conversion and storage. The design of semiconductor heterostructures for photocatalytic energy conversion requires the separation of photogenerated charge carriers in real space and their delivery to active catalytic sites at the appropriate overpotentials to initiate redox reactions. Operation of the desired sequence of light harvesting, charge separation, and charge transport events within heterostructures is governed by the thermodynamic energy offsets of the two components and their photoexcited charge-transfer reactivity, which determine the extent to which desirable processes can outcompete unproductive recombination channels. Here, we map energetic offsets and track the dynamics of electron transfer in MoS₂/CdS architectures, prepared by interfacing two-dimensional MoS₂ nanosheets with CdS quantum dots (QDs), and correlate the observed charge separation to photocatalytic activity in the hydrogen evolution reaction. The energetic offsets between MoS₂ and CdS have been determined using hard and soft X-ray photoemission spectroscopy (XPS) in conjunction with density functional theory. A staggered type-II interface is observed, which facilitates electron and hole separation across the interface. Transient absorption spectroscopy measurements demonstrate ultrafast electron injection occurring within sub-5 ps from CdS QDs to MoS₂, allowing for creation of a long-lived charge-separated state. The increase of electron concentration in MoS₂ is evidenced with the aid of spectroelectrochemical measurements and by identifying the distinctive signatures of electron-phonon scattering in picosecond-resolution transient absorption spectra. Ultrafast charge separation across the type-II interface of MoS₂/CdS heterostructures enables a high Faradaic efficiency of ∼99.4 ± 1.2% to be achieved in the hydrogen evolution reaction (HER) and provides a 40-fold increase in the photocatalytic activity of dispersed photocatalysts for H₂ generation. The accurate mapping of thermodynamic driving forces and dynamics of charge transfer in these heterostructures suggests a means of engineering ultrafast electron transfer and effective charge separation to design viable photocatalytic architectures.

Electron Transfer Kinetics at Graphene Quantum Dot Assembly Electrodes

Electrochemical performance of nanostructured carbon electrodes was evaluated using cyclic voltammetry and a simple simulation model. The electrodes were prepared from soluble precursors by anodic electrodeposition of two sizes of graphene quantum dot assemblies (hexabenzocoronene (HBC) and carbon quantum dot (CQD)) onto a conductive support. Experimental and simulated voltammograms enabled the extraction of the following electrode parameters: conductivity of the electrodes (a combination of ionic and electronic contributions), density of available electrode states at different potentials, and tunneling rate constant (Marcus-Gerischer model) for interfacial charge transfer to ferrocene/ferrocenium (Fc/Fc⁺) couple. The parameters indicate that HBC and CQD have significant density of electronic states at potentials more positive than -0.5 V versus Ag/Ag⁺. Enabled by these large densities, the electron transfer rates at the Fc/Fc⁺ thermodynamic potential are several orders of magnitude slower than those commonly observed on other carbon electrodes. This study is expected to accelerate the discovery of improved synthetic carbon electrodes by providing fast screening methodology of their electrochemical behavior.

Proton-Coupled Electron Transfer from Water to a Model Heptazine-Based Molecular Photocatalyst

To gain mechanistic understanding of heptazine-based photochemistry, we synthesized and studied 2,5,8-tris(4-methoxyphenyl)-1,3,4,6,7,9,9b-heptaazaphenalene (TAHz), a model molecular photocatalyst chemically related to carbon nitride. On the basis of time-resolved photoluminescence (TR-PL) spectroscopy, we kinetically reveal a new feature that emerges in aqueous dispersions of TAHz. Using global target analysis, we spectrally and kinetically resolve the new emission feature to be blue shifted from the steady-state luminescence, and observe a fast decay component exhibiting a kinetic isotope effect (KIE) of 2.9 in H₂O versus D₂O, not observed in the steady-state PL. From ab initio electronic-structure calculations, we attribute this new PL peak to the fluorescence of an upper excited state of mixed nπ*/ππ* character. In water, the KIE suggests the excited state is quenched by proton-coupled electron transfer, liberating hydroxyl radicals that we detect using terephthalic acid. Our findings are consistent with recent theoretical predictions that heptazine-based photocatalysts can participate in proton-coupled electron transfer with H₂O.

Mechanism of photocatalytic water splitting with triazine-based carbon nitrides: insights from ab initio calculations for the triazine–water complex

Valuable theoretical insights into the mechanism of photocatalytic water-splitting using triazine as a model system for carbon-nitride materials.

A light-driven modulation of electric conductance through the adsorption of azobenzene onto silicon-doped- and pyridine-like N3-vacancy graphene

The ability to modulate the conductance of an electronic device under light irradiation is crucial to the practical applications of nanoscale electronics. Density functional theory calculations predict that the conductance of the photo-responsive graphene-based nanocomposites can be tuned through the noncovalent adsorption of an azobenzene (AB) derivative onto pristine, Si-doped, and pyridine-like N₃-vacancy graphene. AB@graphene systems were found to exhibit a visible-light response within the low-frequency region, rendering the trans-to-cis isomerizations of these nanocomposites under the irradiation of solar light. The excellent solar light absorption performances of these hybrids can then be used to modulate the conductance of both N₃-vacancy- and Si-doped-graphene AB hybrids effectively through the reversible change of the effective conjugate length of the AB molecule in the photoisomerization. In addition, the solar thermal energy up to 1.53 eV per AB molecule can be stored in the designed nanocomposites with the doped graphene. These findings provide clues for making multifunctional materials with potential applications as both optically controlled nanoelectronics and solar energy storage devices.