Photocatalytic water splitting with particulate semiconductors offers a scalable route to solar-to-hydrogen (STH) conversion, yet efficiency is limited by optical losses and carrier recombination. We present a predictive framework that integrates first-principles optical spectra, carrier diffusion, multilayer optics, and effective-medium theory. Applied to the visible-light-responsive photocatalyst Gd2Ti2O5S2, the model quantitatively reproduces apparent quantum efficiency (AQE) spectra and enables extraction of carrier diffusion lengths. The calculated absorptance is consistent with Kubelka-Munk theory and its diffuse-reflectance assumptions. Notably, particles larger than the diffusion length can still enhance STH performance via improved light harvesting. However, radiative photon escape, especially near shallow absorption edges, can outweigh recombination losses even in stacked configurations. These results underscore the need for simultaneous optimization of optical and electronic transport properties and challenge the assumption that maximizing photocatalyst loading alone ensures optimal efficiency once particle size is tuned. We also apply the framework to explain the gradual decline in AQE with increasing wavelength observed for particulate SrTiO3:Al photocatalysts that exhibit near-unity AQE.

Perovskite-type tantalum-based oxynitride photocatalysts are promising candidates for water splitting due to their suitable band positions and extended light absorption beyond 600 nm. However, their associated photocatalytic activities and quantum yields remain relatively low. Here, we show that a nano-sized single-crystalline BaxSr1-xTaO2N solid-solution perovskite photocatalyst exhibits state-of-the-art activity in separate oxygen and hydrogen evolution half-reactions. The improved performance is attributed to the nanoscale particle sizes, as well as the reduced defect densities achieved by using a mixed precursor comprising TaS2 and Ta3N5. The half-reaction activities can be modulated by applying a post-synthetic high-temperature treatment. Assessments of charge carrier dynamics, in conjunction with a mechanistic kinetic model, reveal that exponential-tail trap states are formed during this post-treatment. Such trap states, present on the photocatalyst surface, facilitate participation of holes during the oxygen evolution reaction. The development of such solid-solution photocatalysts broadens the range of potential materials for solar-driven hydrogen production. In addition, the present findings are expected to enable the selective tuning of bifunctional photocatalysts for either the hydrogen or oxygen evolution reaction.

Photocatalytic performance is dictated by particle size, carrier transport, and surface properties. We propose a diffusion-based framework that explicitly incorporates the minority-carrier diffusion length relative to particle size. Unlike prior studies emphasizing specific surface area and majority-carrier lifetime, our analysis identifies the diffusion length as a critical factor governing product formation. Fitting experimental data yields both the diffusion length and effective minority-carrier-extraction velocity, allowing separation of transport- and surface-limited contributions. The framework reveals a universal transition from diffusion- to reaction-controlled regimes with increasing particle size. Variations in minority-carrier-extraction velocity with specific surface area (S BET) reflect intrinsic surface properties independent of total surface area. This approach offers design principles for efficient photocatalysts by distinguishing general size effects from specific surface contributions.

Temperature gradients drive asymmetric ion distributions via thermodiffusion (the Soret effect), leading to deviations from the classical Debye-Huckel potential. We introduce the Eastman entropy of transfer, S-+/- (<^>)= alpha (+/-) k(B) for cations and anions, respectively, where k(B) is the Boltzmann constant, and analyze non-isothermal electric double layers in terms of the dimensionless Soret coefficients alpha(+/-). Analytical solutions of the generalized Debye-Huckel equation show that, for alpha (+) = alpha (-), the potential is exactly described by a modified Bessel function, while the marginal case alpha(+/-). = 1 exhibits algebraic decay. An effective screening length, lambda(eff, ) characterizes the near-electrode potential and increases with temperature, resulting in weaker screening on the hot side and stronger screening on the cold side for alpha(+/-). > - 1. The differential capacitance is controlled by alpha(+/-).via lambda(eff,) with its minimum coinciding with the potential of zero charge (PZC) even in the presence of a temperature gradient. These findings highlight the fundamental coupling between electrostatics and thermodiffusion in non-isothermal electrolytes.

Photocatalytic water splitting is an emerging renewable technology for producing green hydrogen fuel from sunlight and water on a large scale. Identifying charge-carrier transport properties is critical for establishing a design pathway for exciting visible-light-absorbing oxysulfide-based photocatalysts. Herein, the dynamics of distinct charge carriers in the Gd2Ti2O5S2 (GTOS) photocatalyst is revealed by transient optical spectroscopies (transient diffuse reflectance (TDR) and transient photoluminescence (TPL) spectroscopies) and theoretical modeling. We demonstrate that TDR and TPL signals can probe the evolution of photoexcited mobile electrons and holes separately for GTOS. The decay of optical signals primarily originates from bimolecular recombination of mobile electrons with detrapped holes from shallow trap states close to the valence band. Using different estimated parameters, the effects of the size reduction and charge carrier extraction rate ke (surface to electrolyte) on the internal quantum efficiency (IQE) are determined. Our results indicate that the IQE can be tremendously improved by simultaneously reducing particle size and increasing ke. After particle size reduction, we show that the high apparent quantum yield (similar to 30%) GTOS was achieved by improving ke (from surface treatment and optimizing the cocatalyst loading method) as compared to Y2Ti2O5S2 (0.7%). Our work presents a comprehensive methodology that identifies the critical photophysical properties of visible-light-absorbing photocatalysts for efficient and scalable particulate photocatalyst-based solar water splitting systems.

Recently, orders-of-magnitude modulation of mobility has been reported for chemical doping by photoinduced charge-transfer reactions; the carrier density and mobility can be controlled over more than 3 orders of magnitude by varying the photoirradiation time. The mobility changes are attributed to the change in accessible hole trap sites caused by varying the carrier density. However, the dependence of the mobility on the doping density has not yet been fully explored theoretically for chemical doping. Here we present theories to study mobility as a function of doping density for chemical doping. We consider the formation of ion-induced trap states expressed by few energy levels or a Gaussian density of states. We also use band-tail states expressed by an exponential density of states to consider the possibility that structural disorder is caused by chemical doping. The aforementioned theoretical results-one for the ion-induced trap states and the other for the band-tail trap states-were compared with experimental results showing a relatively slow increase in mobility with increasing doping density followed by a sharp increase in mobility toward saturation. The S-shaped dependence of mobility on doping density was more consistent with the model with the ion-induced trap states than the model with the band-tail states near the mobile states caused by structural disorder. These theoretical results indicate that the two types of trap states can be differentiated, which can be further examined using spectroscopic methods. The obtained semianalytical expressions can also be used to estimate the density of states as well as the energy difference between the trapped states and detrapped states from the mobility measurements. The proposed theory is further extended to include band-filling effects relevant to interdomain charge transport.

Recently, ionic thermoelectric supercapacitors have gained attention because of their high open circuit voltages, even for ions that are redox inactive. As a source of open circuit voltage (electromotive force), an asymmetry in electric double layers developed by the adsorption of ions at the electrode surfaces kept at different temperatures has previously been proposed. As another source, the Eastman entropy of transfer, which is related to the Soret coefficient, has been considered. Herein, we theoretically estimated the open circuit voltages generated in the Stern layer, the diffuse layer and by the Eastman entropy of transfer. The Grahame equation has been generalized to consider the temperature gradient in the diffuse layer. The ion coverage difference between the hot and cold electrodes and the open circuit voltage are obtained by solving self-consistent equations using the adsorption isotherm. The results are compared with experimental results using a metal electrode and a conductive polymer-based electrode. We show the possible origin of the high ionic Seebeck effect caused by the asymmetry in the coverages of adsorbed ions in terms of the various types of interface capacitance factor at the hot and cold electrodes.

The use of density functional theory (DFT) to calculate the optical properties of materials frequently leads to a predicted energy bandgap that is narrower than that experimentally determined. When the energy bandgap is incorrectly evaluated by DFT, the resulting absorption coefficient must be adjusted to give the correct value, in particular in the vicinity of the energy bandgap. Recently, a method has been developed whereby the dielectric coefficient for a material calculated using DFT is blueshifted and its amplitude is scaled such that the scaled function satisfies the same moment sum rule as the unshifted dielectric coefficient. However, while the moment sum rule is a necessary condition for correctly predicting the optical properties, it is not a sufficient condition. In the present work, it is shown that this method of scaling the imaginary part of the dielectric coefficient is based on the fact that the optical conductivity obtained using the fluctuation–dissipation (FD) theorem can be shifted to adjust the energy bandgap. This assumes that the energy dispersion is accurately calculated using DFT, although the energy bandgap is shifted. This shift is taken into account simply by modifying the energy associated with inter-band transitions in an expression for the electron conductivity obtained using the FD theorem within an independent particle approximation. The effectiveness of this method is illustrated by predicting the optical properties of particulate oxysulfide photocatalysts recently shown to promote visible-light-driven overall water splitting.

Ion loss to space has played an important role in atmospheric escape and climate change on Mars because of intense solar activity during a younger, more active phase of the Sun. Although the existence of an intrinsic magnetic field on ancient Mars is also a key factor in ion loss, its effect remains unclear. Based on multispecies magnetohydrodynamics (MHD) simulations, we investigated processes and rates of ion loss from Mars under extreme solar conditions and the existence of a dipole field with different strengths. The effects of a dipole field on ion loss depend on whether the dipolar magnetic pressure is strong enough to sustain the solar wind dynamic pressure. When the dipole field is existent but weak, it facilitates the cusp outflow and increases the loss rates of molecular ions (O 2 + and CO 2 + ) by a factor of 6 through the high‐latitude magnetotail. When the dipole field is strong enough, the loss rates of molecular ions are decreased by 2 orders of magnitude, and peaks of the escape flux are located near the equatorial plane due to the magnetic reconnection in the northern‐dusk or southern‐dawn lobe regions. The pickup process on the extended oxygen corona created by the strong EUV flux contributes to the total O + loss. Therefore, the effects of the dipole field are less pronounced for O + . Under more moderate solar EUV conditions, the effects on O + loss can be stronger and thus contribute to climate change.

We analyzed time-of-flight (TOF) data from the Arase satellite to investigate temporal variations of O2+, NO+, and N2+ at 19.2 keV/q in the inner magnetosphere for 6.5 years from the solar declining to rising phases. Molecular ion counts were estimated by subtracting the background contamination of oxygen counts. While the number of clear molecular events was small, the estimated molecular ion counts exhibited good correlation with the solar wind dynamic pressure and SYM-H index. Long-term variations of molecular ions were different from that of oxygen ions. Additionally, we discuss the importance of the solar wind dynamic pressure in causing the escape of molecular ions into the magnetosphere through an increase in the convection electric field, which causes different evolutions of oxygen ions and molecular ions.

For the clarification of dynamics of photogenerated carriers in practical organic solar cell devices, we have developed a methodology to simultaneously acquire reflection-mode transient optical absorption (Delta A) and transient electric current (Delta i) signals. For a typical polythiophene:fullerene bulk heterojunction solar cell device, both the Delta A and Delta i signals due to the photogenerated carriers are characterized by the power-law decays of proportional to t(-alpha), which are interpreted by detrapping-limited recombination at earlier times than similar to 1 mu s and trap-free diffusion/drift at later times. Furthermore, we have succeeded in observing switching of the power index alpha for Delta A signals as well as for Delta i signals; the time at which switching occurs indicates the extraction of carriers by electrodes (transit times). From the transit times for Delta A and Delta i, transit mobilities mu(tr Delta A) and mu(tr Delta i) are obtained, which are on the same order. It has been found from the comparison of the cell parameters among several devices fabricated under similar conditions that the device-to-device variation of photon energy conversion efficiency (0.5%-2%) is strongly correlated with the ratio mu(tr Delta A)/mu(tr Delta i). It is considered that the charge accumulation at the active layer/electrode interfaces induces a delay between the carrier transport and electrode collection, which significantly lowers the power conversion efficiency. Our simultaneous optical and electrical detection thus allows us to diagnose carrier dynamics in individual devices that affect the solar cell performance.

Mixed oxides of Rh-Cr (RhCrOx), containing Rh3+ and Cr3+ cations, are commonly used as cocatalysts for the hydrogen evolution reaction (HER) on particulate photocatalysts. The precise physicochemical mechanisms of the HER at the catalytic sites of these oxides are not well understood. In this study, model cocatalyst electrodes, composed of nanoparticulate RhCrOx, were fabricated to investigate the physicochemical mechanisms of the HER. Electroanalytical and X-ray photoelectron spectroscopic measurements revealed that nanoparticulate RhCrOx produces reduced Rh (Rh0) species by maintaining an electrode potential more negative than 0.03 V versus the reversible hydrogen electrode (VRHE). This results in significant enhancement of the HER activity. The catalytic activity for the HER stems from the reduced Rh species, and the inclusion of Cr3+ (CrOx) aided in the electron transfer process at the solid/liquid interface, resulting in a higher current density during the HER. To achieve a solar-to-hydrogen efficiency of over 3%, the conduction band minimum of the particulate photocatalyst should be positioned more negatively than -0.10 VRHE. Moreover, the formation of electron trap states at potentials more positive than 0.03 VRHE should be avoided. This study highlights the importance of understanding the catalytic sites on metal oxide cocatalysts. Moreover, it offers a design strategy for enhancing the efficiency of photocatalytic water splitting.

We recently presented a quantitative model to explain the particle-size dependence of photoluminescence (PL) quantum yields and revealed that exciton quenching is not diffusion controlled, but limited by surface reactions. However, the exciton decay kinetics has not been analyzed yet using our theoretical model. Here, we study kinetic aspects of the model and show that it should be extended to take into account subdiffusion rather than normal diffusion to maintain consistency with the observed complex decay kinetics; we also show that the PL decay kinetics is nonexponential even when the PL quenching is limited by surface reactions under subdiffusion. Our theoretical analysis of the PL quantum yield and the PL decay kinetics provides a comprehensive picture of mobile charge carriers, immobile polarons, and self-trapped excitons.

Boundaries between space plasmas occur in numerous contexts and scales, from astrophysical jets to planetary magnetospheres. Mass and momentum transport across boundaries poses a fundamental problem in magnetospheric physics. Kelvin–Helmholtz instability (KHI) is a promising mechanism to facilitate transport. Although previous studies have suggested KHI occurrence in various space plasmas, theory predicts that compressibility prevents KHI excitation at boundaries with large density gradients because of previously considered boundary structures where density varies with velocity. Based on the observations of a large density gradient boundary by MAVEN at Mars, where we can observe an extreme case, in this study, we show that it is the entropy, instead of the previously considered density, that varies with the velocity in the real velocity-sheared boundary. The entropy-based boundary structure places the velocity shear in a lower-density region than the traditional density-based structure and weakens the compressibility effect. This new boundary structure thus enables KHI excitation even at large density gradient boundaries, such as at the ionopause of unmagnetized planets and the plasmapause of magnetized planets. The result suggests the ubiquitous occurrence of KHI in the plasma universe and emphasizes its important role in planetary cold plasma escape from unmagnetized planets.

Triplet fusion (TF) has recently attracted attention because of its great potential of various applications. Since high reaction efficiency of TF is demanded in any applications, it is necessary to clarify the mechanism of triplet exciton dynamics that is an important elementary process to enhance the TF efficiency. Therefore, in this paper, we study the triplet exciton dynamics of 9,10-diphenylanthracene (DPA), which shows the TF-based fluorescence, in a solvent of tetraethylene glycol dimethyl ether and in polycrystal of DPA, in combination with the triplet sensitization using platinum octaethylporphyrin. The observed emission kinetics and their magnetoluminescence effect indicate that the rotational and the multitrapping diffusions of the triplet exciton play an inherent role respectively in the fluid solution and in the polycrystalline solid.

We studied the photoluminescence decay kinetics of three nanosized anatase TiO2 photocatalysts (particle diameter: 7, 25, or 200 nm) at the pico- and nanosecond timescales for elucidating the origin of the luminescence. Luminescence spectra from these photocatalysts obtained under steady-state excitation conditions comprised green luminescence that decayed on the picosecond timescale and red luminescence that persisted at the nanosecond timescale. Among the photocatalysts with different sizes, there were marked differences in the rate of luminescence decay at the picosecond timescale (<600 ps), although the spectral shapes were comparable. The similarity in the spectral shape indicated that self-trapped excitons (STEs) directly populated in the bulk of the particle by light excitation emit the luminescence in a picosecond timescale, and the difference in the rate of luminescence decay originated from the quenching at the particle surface. Furthermore, we theoretically considered excitation light intensity dependence on the quantum yield of the luminescence and found that the quenching reaction was not limited by the diffusion of the STEs but by the reaction at the particle surface. Both the spectral shape and time-evolution of the red luminescence from the deep trapped excitons in the nanosecond timescale varied among the photocatalysts, suggesting that the trap sites in different photocatalysts have different characteristics with respect to luminescence. Therefore, the relation between trap states and photocatalytic activity will be elucidated from the red luminescence study.

Thermogalvanic cells have emerged as promising devices for direct heat-to-electricity conversion. In this study, we investigate the utilization of heavy water (D2O) as a solvent in thermogalvanic cells, with a focus on two systems: K-3[Fe(CN)(6)]/K-4[Fe(CN)(6)] and guanidinium chloride (GdmCl) added K-3[Fe(CN)(6)]/K-4[Fe(CN)(6)]. For the K-3[Fe(CN)(6)]/K-4[Fe(CN)(6)] system, D2O consistently exhibits a higher Seebeck coefficient compared to regular water (H2O) across a wide range of concentration levels, which could be attributed to the dielectric constant difference of D2O. However, the higher internal resistance resulting from D2O's higher viscosity offsets the improved Seebeck coefficient, leading to a diminished power output. For the GdmCl added K-3[Fe(CN)(6)]/K-4[Fe(CN)(6)] system, the cell utilizing D2O not only shows a higher Seebeck coefficient but also demonstrates a higher power output, unlike the system without GdmCl addition. UV-Vis measurements and quantification of precipitated crystals provide evidence of a stronger interaction between Gdm(+) and Fe(CN)(6)(4-) in D2O. The higher reaction equilibrium constant between Gdm(+) and Fe(CN)(6)(4-) in D2O supports the observed enhancement in Se, consistent with our previous studies. This investigation highlights the promising potential of D2O as a solvent in thermogalvanic cells, offering valuable insights into the intricate interplay between solvent properties, ion interactions, and thermoelectric performance.

High-performance solar-water-splitting technologies are of paramount interest for the cost-effective generation of hydrogen fuel; however, their realization is majorly limited by the poor solar light absorption and charge separation inside photoanode semiconductors. Herein, we develop photoanodes made from polycrystalline tantalum nitride nanorods (Ta3N5 NRs) to overcome the above-mentioned challenges. The morphology and crystalline properties of Ta3N5 NRs are optimized by tuning essential parameters of glancing angle deposition and nitridation techniques, respectively. Under a simulated AM1.5G solar spectrum, the photoanodes demonstrate a tremendous gain in photocurrent from 1.54 mA cm(-2) to 10.96 mA cm(-2) at 1.23 V versus reversible hydrogen electrode for water oxidation activity. Photoluminescence, transient diffuse reflectance spectroscopy, and theoretical analyses identify prominent factors (like charge carrier lifetime, diffusion length, etc.) responsible for the enhanced performance. Our work presents the significance of designing the narrow-energy band-gap photoanodes with broad implications toward efficient solar-water-splitting devices for green hydrogen production.

An isotropic thermo-electrochemical cell is introduced with a high Seebeck coefficient (S e) of 3.3 mV K-1 that uses a ferricyanide/ferrocyanide/guanidinium-based agar-gelated electrolyte. A power density of about 20 µW cm-2 is achieved at a temperature difference of about 10 K, regardless of whether the heat source is on the top or bottom section of the cell. This behavior is very different from that of cells with liquid electrolytes, which exhibit high anisotropy, and for which high S e values are achieved only by heating the bottom electrode. The guanidinium-containing gelatinized cell does not exhibit steady-state operation, but its performance recovers when disconnected from the external load, suggesting that the observed power drop under load conditions is not due to device degeneration. The large S e value and isotropic properties can mean that the novel system represents a major advancement from the standpoint of harvesting of low-temperature heat, such as body heat and solar thermal heat.