Charge Trapping in Semiconductor Photocatalysts: A Time- and Space-Domain Perspective.

Harnessing solar energy to produce value-added fuels and chemicals through photocatalysis techniques holds promise for establishing a sustainable and environmentally friendly energy economy. The intricate dynamics of photogenerated charge carriers lies at the core of the photocatalysis. The balance between charge trapping and band-edge recombination has a crucial influence on the activity of semiconductor photocatalysts. Consequently, the regulation of traps in photocatalysts becomes the key to optimizing their activities. Nevertheless, our comprehension of charge trapping, compared to that of well-studied charge recombination, remains somewhat limited. This limitation stems from the inherently heterogeneous nature of traps at both temporal and spatial scales, which renders the characterization of charge trapping a formidable challenge. Fortunately, recent advancements in both time-resolved spectroscopy and space-resolved microscopy have paved the way for considerable progress in the investigation and manipulation of charge trapping. In this Perspective, we focus on charge trapping in photocatalysts with the aim of establishing a direct link to their photocatalytic activities. To achieve this, we begin by elucidating the principles of advanced time-resolved spectroscopic techniques such as femtosecond time-resolved transient absorption spectroscopy and space-resolved microscopic methods, such as single-molecule fluorescence microscopy and surface photovoltage microscopy. Additionally, we provide an overview of noteworthy research endeavors dedicated to probing charge trapping using time- and space-resolved techniques. Our attention is then directed toward recent achievements in the manipulation of charge trapping in photocatalysts through defect engineering. Finally, we summarize this Perspective and discuss the future challenges and opportunities that lie ahead in the field.

Intricate Reaction Pathways on CH3NH3PbI3 Photocatalysts in Aqueous Solution Unraveled by Single-Particle Spectroscopy.

Organic-inorganic hybrid perovskites such as MAPbI3 (MA+ = CH3NH3+) have emerged as promising materials for solar cells and light-emitting devices. Despite their poor stability against moisture, perovskites work as hydrogen-producing photocatalysts or photosensitizers in perovskite-saturated aqueous solutions. However, the fundamental understanding of how chemical species or support materials in the solution affect the dynamics of the photogenerated charges in perovskites is still insufficient. In this study, we investigated the photoluminescence (PL) properties of MAPbI3 nanoparticles in aqueous media at the single-particle level. A remarkable PL blinking phenomenon, along with significant decreases in the PL intensity and lifetime compared to those in ambient air, suggested temporal fluctuations in the trapping rates of photogenerated holes by chemical species (I- and H3PO2) in the solution. Moreover, electron transfer from the excited MAPbI3 to Pt-modified TiO2 proceeds in a concerted fashion for photocatalytic hydrogen evolution under the dynamic solid-solution equilibrium condition.

Size-Controlled Synthesis of Luminescent Few-Atom Silver Clusters via Electron Transfer in Isostructural Redox-Active Porous Ionic Crystals.

Ag clusters with a controlled number of atoms have received significant interest because they show size-dependent catalytic, optical, electronic, or magnetic properties. However, the synthesis of size-controlled, ligand-free, and air-stable Ag clusters with high yields has not been well-established. Herein, it is shown that isostructural porous ionic crystals (PICs) with redox-active polyoxometalates (POMs) can be used to synthesize Ag clusters via electron transfer from POMs to Ag+ . Ag clusters with average numbers of three, four, or six atoms emitting blue, green, or red colors, respectively, are formed and stabilized in the PICs under ambient conditions without any protecting ligands. The cluster size solely correlates with the degree of electron transfer, which is controlled by the reduction time and types of ions or elements of the PICs. Thus, advantages have been taken of POMs as electron sources and PICs as scaffolds to demonstrate a convenient method to obtain few-atom Ag clusters.

Extension of the mechanoresponsive luminescence shift via formation of a doped organic crystal.

Herein, we propose a new strategy to tune the magnitude of the mechanoresponsive shift of the maximum emission wavelength (Δλem). The Δλem of thienylbenzothiadiazole crystals has been extended to 69 nm by doping with a trace amount of dithienylbenzothiadiazole, whereas the pure crystal of thienylbenzothiadiazole exhibited a Δλem of 10 nm. This doping strategy should accelerate the development of advanced mechanosensing materials composed of organic crystals.

Manipulation of charge carrier flow in Bi4NbO8Cl nanoplate photocatalyst with metal loading.

Separation of photoexcited charge carriers in semiconductors is important for efficient solar energy conversion and yet the control strategies and underlying mechanisms are not fully established. Although layered compounds have been widely studied as photocatalysts, spatial separation between oxidation and reduction reaction sites is a challenging issue due to the parallel flow of photoexcited carriers along the layers. Here we demonstrate orthogonal carrier flow in layered Bi4NbO8Cl by depositing a Rh cocatalyst at the edges of nanoplates, resulting in spatial charge separation and significant enhancement of the photocatalytic activity. Combined experimental and theoretical studies revealed that lighter photogenerated electrons, due to a greater in-plane dispersion of the conduction band (vs. valence band), can travel along the plane and are readily trapped by the cocatalyst, whereas the remaining holes hop perpendicular to the plane because of the anisotropic crystal geometry. Our results propose manipulating carrier flow via cocatalyst deposition to achieve desirable carrier dynamics for photocatalytic reactions in layered compounds.

Binary dopant segregation enables hematite-based heterostructures for highly efficient solar H2O2 synthesis.

Dopant segregation, frequently observed in ionic oxides, is useful for engineering materials and devices. However, due to the poor driving force for ion migration and/or the presence of substantial grain boundaries, dopants are mostly confined within a nanoscale region. Herein, we demonstrate that core-shell heterostructures are formed by oriented self-segregation using one-step thermal annealing of metal-doped hematite mesocrystals at relatively low temperatures in air. The sintering of highly ordered interfaces between the nanocrystal subunits inside the mesocrystal eliminates grain boundaries, leaving numerous oxygen vacancies in the bulk. This results in the efficient segregation of dopants (~90%) on the external surface, which forms their oxide overlayers. The optimized photoanode based on hematite mesocrystals with oxide overlayers containing Sn and Ti dopants realises high activity (~0.8 µmol min-1 cm-2) and selectivity (~90%) for photoelectrochemical H2O2 production, which provides a wide range of application for the proposed concept.

Mechanochromic Luminescence (MCL) of Purely Organic Two-Component Dyes: Wide-Range MCL over 300†nm and Two-Step MCL by Charge-Transfer Complexation.

Despite recent extensive studies on mechanochromic luminescence (MCL), rational control over the magnitude of the emission-wavelength shift in response to mechanical stimuli remains challenging. In the present study, a two-component donor-acceptor approach has been applied to create a variety of organic MCL composites that exhibit remarkable emission-wavelength switching. Dibenzofuran-based bis(1-pyrenylmethyl)diamine and typical organic fluorophores have been employed as donor and acceptor dyes, respectively. Outstanding wide-range MCL with an emission-wavelength shift of over 300 nm has been achieved by mixing the diamine with 3,4,9,10-perylenetetracarboxylic diimide. Unprecedented two-step MCL in response to mechanical stimuli of different intensity has also been realized for a two-component mixture with 9,10-anthraquinone. Fluorescence microscopy observations at the single-particle level revealed that the segregation and mixing of the two-component dyes contribute to the stimuli-responsive emission-color switching of the MCL composites.

In Situ Exploration of Stimulus-Induced Emission Changes in Mechanochromic Dyes.

The development of in situ analysis techniques for visualizing and linking macro- and nanoscopic features of external stimulus-responsive materials is crucial for their rational design and applications. Herein, we investigate the mechanical stress-induced emission changes in electron donor-acceptor type organic dye molecules in solid states through in situ single-particle fluorescence spectroscopy combined with macroscopic and nanoscopic stimulation systems. The change in emission color from green to yellow was attributed to repeated rubbing or scratching of the crystal surface, and not to simple cutting. This change was due to partial amorphization, which changed the intra- and intermolecular charge-transfer interactions of stacked molecules near the surface. We believe that this study will facilitate the efficient design of mechano-responsive materials with finely controlled and responsive properties.

In Situ Exploration of the Structural Transition during Morphology- and Efficiency-Conserving Halide Exchange on a Single Perovskite Nanocrystal.

Controlled fabrication of semiconductor nanostructures with unique physicochemical properties is vital for future technologies. In this study, transformation from red-emitting metal halide perovskite CH3 NH3 PbI3 nanocrystals (NCs) to green-emitting CH3 NH3 PbBr3 NCs was achieved without significant morphological changes and loss of photoluminescence (PL) efficiency via a controlled halide exchange reaction. In situ single-particle PL imaging along with detailed structural and elemental characterizations revealed that sudden cooperative transitions between two light-emitting states via intermediate dark states with >100 s durations during halide exchange originate from two distinct defect-mediated reconstruction processes with different activation energies (0.072 and 0.40 eV), leading to an isokinetic temperature of ca. 314 K, across a solid-state miscibility gap between the I- and Br-rich phases inside a single NC.

TiO2 superstructures with oriented nanospaces: a strategy for efficient and selective photocatalysis.

Highly ordered superstructures of semiconductor nanocrystals contain abundant nanometer-scale pores between the crystals; however, there have been difficulties in controlling the size and orientation of these nanospaces without the use of a template or a capping reagent. This constraint has affected their development and applications in potential fields including catalysis and optoelectronics adversely. In this study, we synthesized a rod-shaped TiO2 mesocrystal (TMC) having a length of a few hundreds of micrometers and comprising regularly ordered anatase TiO2 nanocrystals that form oriented nanospaces by exposed {001} facets. Finite-difference time-domain (FDTD) calculations of electric fields and in situ fluorescence imaging with a polarization sensitive dye on a single mesocrystal were performed to reveal anisotropic adsorption and excitation of the dyes. Furthermore, the photodegradation of the dyes was found to be more facilitated in nanospaces formed by the specific facets, as compared with the dyes randomly adsorbed on the outer surfaces. Consequently, the selectivity of photocatalytic reactions based on the molecular size and redox was enhanced by introducing the concept of oriented nanospace.

Ultra-Narrow Depletion Layers in a Hematite Mesocrystal-Based Photoanode for Boosting Multihole Water Oxidation.

Significant charge recombination that is difficult to suppress limits the practical applications of hematite (α-Fe2 O3 ) for photoelectrochemical water splitting. In this study, Ti-modified hematite mesocrystal superstructures assembled from highly oriented tiny nanoparticle (NP) subunits with sizes of ca. 5 nm were developed to achieve the highest photocurrent density (4.3 mA cm-2 at 1.23 V vs. RHE) ever reported for hematite-based photoanodes under back illumination. Owing to rich interfacial oxygen vacancies yielding an exceedingly high carrier density of 4.1×1021  cm-3 for super bulk conductivity in the electrode and a large proportion of ultra-narrow depletion layers (<1 nm) inside the mesoporous film for significantly improved hole collection efficiency, a boosting of multihole water oxidation with very low activation energy (Ea =44 meV) was realized.

Structural Dynamics of Lipid Bilayer Membranes Explored by Magnetic Field Effect Based Fluorescence Microscopy.

Lipid bilayer membranes are known to exist as heterogeneous and dynamic structures where the molecules are always moving and fluctuating under physiological conditions. Magnetic field effects (MFEs) studied herein are phenomena in which the exciplex emission from an electron donor-acceptor dyad increases or decreases by applying an external magnetic field. The characteristic dependence of MFEs on the viscosity and polarity of the surrounding medium has been applied to investigate the local environments around the probe molecule. In this study, a novel MFE-based fluorescence microscopy technique was developed to explore the structural dynamics of lipid bilayer membranes. The vesicle formation during the membrane deformation was selectively visualized through the MFEs, thus allowing the extraction of information on the cellular dynamics at high temporal and spatial resolutions. This highly versatile and powerful technique is applicable to a wide range of areas, such as biology and material science.

Interfacial oxygen vacancies yielding long-lived holes in hematite mesocrystal-based photoanodes.

Hematite (α-Fe2O3) is one of the most promising candidates as a photoanode materials for solar water splitting. Owing to the difficulty in suppressing the significant charge recombination, however, the photoelectrochemical (PEC) conversion efficiency of hematite is still far below the theoretical limit. Here we report thick hematite films (∼1500 nm) constructed by highly ordered and intimately attached hematite mesocrystals (MCs) for highly efficient PEC water oxidation. Due to the formation of abundant interfacial oxygen vacancies yielding a high carrier density of ∼1020 cm-3 and the resulting extremely large proportion of depletion regions with short depletion widths (<10 nm) in hierarchical structures, charge separation and collection efficiencies could be markedly improved. Moreover, it was found that long-lived charges are generated via excitation by shorter wavelength light (below ∼500 nm), thus enabling long-range hole transfer through the MC network to drive high efficiency of light-to-energy conversion under back illumination.

Rapid formation of small mixed-valence luminescent silver clusters via cation-coupled electron-transfer in a redox-active porous ionic crystal based on dodecamolybdophosphate.

Redox-active porous ionic crystals based on polyoxometalate (POM) were utilized to form and stabilize small mixed-valence luminescent silver clusters via cation-coupled electron-transfer (CCET) reactions. Reduction-induced ion-exchange between Cs+ and Ag+via CCET took less than 1 min to complete and consisted of two steps: electron transfer from reduced POM to Ag+ and the subsequent formation of a silver cluster, and diffusion of the silver cluster and exchange with Cs+. Notably, the simple ion-exchange took more than 24 h. The compound containing the silver cluster showed high affinity toward unsaturated hydrocarbon guests.

Several Orders of Magnitude Difference in Charge-Transfer Kinetics Induced by Localized Trapped Charges on Mixed-Halide Perovskites.

Partial halide substitution in organolead halide perovskites MAPbX3 (MA = CH3NH3+, X = Cl-, Br-, or I-) leads to semiconductor heterostructures with precisely tuned band-gap energies, which facilitates efficient charge extraction or separation for high-performance solar cells and optoelectronic devices. In this study, partially iodide-substituted MAPbBr3 perovskites were prepared through a halide-exchange reaction in the liquid phase, and in situ space- and time-resolved photoluminescence profiles were acquired by means of confocal microscopy. The rates of charge transfer from the bulk MAPbBr3 to the surface MAPbBr3- xI x domains, which are widely distributed over a single crystal, were found to greatly depend on the excitation-power density. In particular, an abnormally slow charge-transfer process, lasting a few nanoseconds, was observed at higher excitation density. To explain the dependence of this rate on the excitation density, and its correlation with the charge-trapping rate in the bulk MAPbBr3, we propose a plausible mechanism in which trap filling associated with surface-trapped holes induces band bending within the space charge region. This band bending modulates carrier dynamics near the surface, thereby leading to efficient charge extraction from the bulk. To validate the mechanism, the carrier dynamics was numerically simulated using a diffusion model that includes the effect of the localized electric field. Our findings provide significantly deeper insight into the carrier dynamics within heterostructured perovskites with nanoscale heterogeneities, and a robust route for manipulating the photogenerated charges in various types of perovskite devices.

Singlet-Fission-Born Quintet State: Sublevel Selections and Trapping by Multiexciton Thermodynamics.

Singlet fission (SF) is expected to exceed the theoretical limit of the solar cell efficiency. Quintet (Q) state generation in triplet-triplet pair is essential for preventing the unwanted loss of SF-born multiexciton through singlet channels, although little is known on the primary multiexciton spin dynamics following the intermolecular SF. In this study, time-resolved EPR revealed the intermolecular multiexciton dynamics, energetics and geometries in aggregated 6,13-bis(triisopropylsilylethynyl)pentacene and 2-phenyl-6,11-bis(triisopropylsilylethynyl)tetracene in diluted frozen solution. We have demonstrated sublevel selective generations of excited quintet states (|Q0⟩, |Q-1⟩ and |Q-2⟩) by singlet-quintet (SQ) mixings during triplet-exciton diffusions within geminate multiexcitons. The present fundamental characteristics of the quintet generations shows strong impact of coexistence of molecularly ordered "hot spot" and disordered regions for exergonic SQ mixings driven by entropy, thereby paving a new avenue for rational designs of organic devices with controlled multiexciton dynamics by optimizing film morphologies.

Charge-Transfer Character Drives Möbius Antiaromaticity in the Excited Triplet State of Twisted [28]Hexaphyrin.

Möbius aromatic molecules have attracted great attention as new functional materials because of their π-orbital cyclic conjugations lying along the twisted Möbius topology. To elucidate the electronic character of the lowest excited triplet (T1) state of a Möbius aromatic [28]hexaphyrin, we employed a time-resolved electron paramagnetic resonance (TREPR) method with applied magnetophotoselection measurements at 77 K. Analyses of the EPR parameters have revealed that the T1 state possesses intramolecular charge-transfer (CT) character together with local excitation character residing at one side in the Möbius strip ring. We have also demonstrated that the CT character between orthogonal unpaired orbitals triggers quick triplet deactivation by spin-orbit coupling. This deactivation can be an important barometer to represent the "antiaromaticity" because of a connection between the orthogonal CT character and instability by a weakened spin-spin exchange coupling in the T1 state.

The Development of Functional Mesocrystals for Energy Harvesting, Storage, and Conversion.

Higher-ordered semiconductors have attracted extensive research interest as an adopted engineering for active solar energy harvesting, storage, and conversion. It is well-known that the effective separation and anisotropic migration of photogenerated charges are the basic driven force required for superior efficiency. However, the morphology and stoichiometric variation of these semiconductors play essential roles in their physicochemical properties of bulk and surface, especially for efficient interparticle or interfacial charge transfer. To this point, the strategy of controlling the topotactic transformation toward superstructures with optimized functionality is preferable for a wide range of optoelectronic and catalytic engineering applications. In this Minireview, we provide an overview of the crystal orientation, synthetic engineering, functional applications, and spatial and temporal charge dynamics in TiO2 mesocrystals and others. The viewpoint of in-depth understanding of the structure-related kinetics would offer an opportunity for design of versatile mesocrystal semiconductors sought-after for potential applications.

Direct Observation of Charge Collection at Nanometer-Scale Iodide-Rich Perovskites during Halide Exchange Reaction on CH3NH3PbBr3.

Organolead halide perovskites MAPbX3 (MA = CH3NH3+, X = Cl-, Br-, or I-) are known to undergo reversible halide exchange reactions, enabling bandgap tuning over the visible light region. Using single-particle photoluminescence (PL) imaging for in situ observation, we have studied the structure-dependent charge dynamics during halide exchange with iodide ions on an MAPbBr3 crystal. In particular, we optically detected nanometer-scale iodide-rich domains (i.e., MAPbBrI2) and found that their lifetimes of several tens of milliseconds are limited by reaction with diffusing vacancies. Furthermore, it was discovered that these domains effectively collect the charge carriers from the bulk crystal, thus resulting in amplified spontaneous emission (ASE) under continuous-wave laser irradiation. Our findings will provide direction for development of perovskite heterostructures with enhanced charge utilization.

Topotactic Epitaxy of SrTiO3 Mesocrystal Superstructures with Anisotropic Construction for Efficient Overall Water Splitting.

The higher-order structures of semiconductor-based photocatalysts play crucial roles in their physicochemical properties for efficient light-to-energy conversion. A novel perovskite SrTiO3 mesocrystal superstructure with well-defined orientation of assembled cubic nanocrystals was synthesized by topotactic epitaxy from TiO2 mesocrystals through a facile hydrothermal treatment. The SrTiO3 mesocrystal exhibits three times the efficiency for the hydrogen evolution of conventional disordered systems in alkaline aqueous solution. It also exhibits a high quantum yield of 6.7 % at 360 nm in overall water splitting and even good durability up to 1 day. Temporal and spatial spectroscopic observations revealed that the synergy of the efficient electron flow along the internal nanocube network and efficient collection at the larger external cubes produces remarkably long-lived charges for enhanced photocatalysis.