Charge carrier dynamics of photoexcited Co3O4 in methanol: extending high harmonic transient absorption spectroscopy to liquid environments

The Role of Photo in Oxygen Evolution Reaction: A Review

Photo enhanced oxygen evolution reaction has recently emerged as an advanced strategy with great application prospects for highly efficient energy conversion and storage. In the course of photo enhanced oxygen evolution reactions, the other works focus has predominantly centered on catalysts while inadvertently overlooking the pivotal role of photo. Consequently, this manuscript embarks upon a comprehensive review of recent advancements in photo-driven, aiming to illuminate this critical dimension. A detailed introduction to the photothermal effect, photoelectronic effect, photon-induced surface plasmon resonance, photo and heterojunction, photo-induced reversible geometric conversion, photo-induced energy barrier reduction, photo-induced chemical effect, photo-charging, and the synthesis of laser/photo-assisted catalysts, offering prospects for the development of each case is provided. A detailed introduction to the photothermal effect, photoelectronic effect, photon-induced surface plasmon resonance, photo and heterojunction, photo-induced reversible geometric conversion, photo-induced energy barrier reduction, photo-induced chemical effect, photo-charging, and the synthesis of laser/photo-assisted catalysts is provided. At the same time, the overpotential and Tafel slope of some catalysts mentioned above at 10 mA cm-2 is collected, and calculated the lifting efficiency of light on them, offering prospects for the development of each case.

Solvent-induced local environment effect in plasmonic catalysis

Solvents are known to affect the local surface plasmon resonance of metal nanoparticles; however, how solvents can be used to manipulate the interfacial charge and energy transfer in plasmonic catalysis remains to be explored. Here, using NH3 decomposition on a Ru-doped Cu surface as an example, we report density functional theory (DFT) and delta self-consistent field (SCF) calculations, through which we investigate the effect of different protic solvent molecules on interfacial charge transfer by calculating excitation energy of an electronic transition between the metal and the molecular reactant. We find that the H-bonds between water and NH3 can alter the direct interfacial charge transfer due to the shift of the molecular frontier orbitals with respect to the metal Fermi level. These effects are also observed when the H-bonds are formed between methanol (or phenol) and ammonia. We show that the solvent possessing stronger basicity induces a more pronounced effect on the excitation energy. This work thus provides valuable insights for tuning the excitation energy and controlling different routes to channel the photon energy into plasmonic catalysis.

Unveiling the Origin of Co3 O4 Quantum Dots for Photocatalytic Overall Water Splitting

Spinel cobalt oxide displays excellent photocatalytic performance, especially in solar driven water oxidation. However, the process of water reduction to hydrogen is considered as the Achilles' heel of solar water splitting over Co₃ O₄ owing to its low conduction band. Enhancement of the water splitting efficiency using Co₃ O₄ requires deeper insights of the carrier dynamics during water splitting process. Herein, the carrier dynamic kinetics of colloidal Co₃ O₄ quantum dots-Pt hetero-junctions is studied, which mimics the hydrogen reduction process during water splitting. It is showed that the quantum confinement effect induced by the small QD size raised the conduction band edge position of Co₃ O₄ QDs, so that the ligand-to-metal charge transfer from 2p state of oxygen to 3d state of Co²⁺ occurs, which is necessary for overall water splitting and cannot be achieved in Co₃ O₄ bulk crystals. The findings in this work provide insights of the photocatalytic mechanism of Co₃ O₄ catalysts and benefit rational design of Co₃ O₄ -based photocatalytic systems.

Barrierless Self-Trapping of Photocarriers in Co3O4

The self-trapping of a free carrier in transition-metal oxides can lead to a small polaron, which is responsible for the inadequate performance of the oxide-based optoelectronic applications. Thus, fundamental understanding of the self-trapping mechanism is of key importance for improving the performance of these applications. Herein, the self-trapping in Co₃O₄ epitaxial monocrystalline films is investigated primarily by transient absorption spectroscopy. The spectral evolution corresponding to the ultrafast transition from free carriers to small polarons is identified, which allows us to extract the self-trapping kinetics. The relationship between the self-trapping rate and temperature suggests a lack of thermal activation energy. A barrierless self-trapping mechanism derived from the small polaron framework is then proposed, which can successfully describe the observation that self-trapping rate decreases linearly with increasing temperature. Given that small polarons are ubiquitous in transition-metal oxides, this self-trapping mechanism is potentially a general phenomenon in these materials.

Nanoscale TiO2 Protection Layer Enhances the Built-In Field and Charge Separation Performance of GaP Photoelectrodes

Nanoscale oxide layer protected semiconductor photoelectrodes show enhanced stability and performance for solar fuels generation, although the mechanism for the performance enhancement remains unclear due to a lack of understanding of the microscopic interfacial field and its effects. Here, we directly probe the interfacial fields at p-GaP electrodes protected by n-TiO₂ and its effect on charge carriers by transient reflectance spectroscopy. Increasing the TiO₂ layer thickness from 0 to 35 nm increases the field in the GaP depletion region, enhancing the rate and efficiency of interfacial electron transfer from the GaP to TiO₂ on the ps time scale as well as retarding interfacial recombination on the microsecond time scale. This study demonstrates a general method for providing a microscopic view of the photogenerated charge carrier's pathway and loss mechanisms from the bulk of the electrode to the long-lived separated charge at the interface that ultimately drives the photoelectrochemical reactions.

Gas-propelled biosensors for quantitative analysis

Gas-propelled biosensors display a simple gas-based signal amplification with quantitative detection features based on the target recognition event in combination with gas propulsion. Due to the liquid-gas conversion, the gas not only pushes the ink bar forward in the microchannel, but also serves as the power to propel the micromotors in the liquid. Thus, this continuous motion leads to a shift in distances which is associated with the target amount. Therefore, gas-propelled biosensors provide a visual quantification based on distance or speed signals without the need for expensive instruments. In this review, we focus on current developments in gas-propelled biosensors for quantitative analysis. First, we list the types of gas utilized as actuators in biosensors. Second, we review the representative gas-propelled biosensors, including the propulsion mechanisms and fabrication methods. Moreover, gas-propelled quantification based on distance and speed is summarized. Finally, we cover applications and provide a future perspective of gas-propelled biosensors.

Mesoporous Octahedron-Shaped Tricobalt Tetroxide Nanoparticles for Photocatalytic Degradation of Toxic Dyes

The present article reports a facile approach to fabrication of mesoporous octahedron-shaped tricobalt tetroxide nanoparticles (Co₃O₄ NPs) with a very narrow size distribution for eco-friendly remediation of toxic dyes. Co₃O₄ NPs were fabricated by a sol-gel process using cobalt chloride hexahydrate (CoCl₂·6H₂O) and monosodium succinate (C₄H₅O₄Na) as a chelating/structure-directing agent and sodium dodecyl sulfate as a surfactant. Moreover, the phase structure, elemental composition, and thermal and morphological facets of Co₃O₄ NPs were investigated using XRD, FT-IR, EDS, Raman, XPS, TGA, SEM, and TEM techniques. The face-centered cubic spinel crystalline structure of the Co₃O₄ NPs was confirmed by XRD and SEM, and TEM analysis revealed their octahedron morphology with a smooth surface. Moreover, the narrow pore size distribution and the mesoporous nature of the Co₃O₄ NPs were confirmed by Brunauer-Emmett-Teller measurements. The photocatalytic activity of Co₃O₄ NPs for degradation of methyl red (MR), Eriochrome Black-T (EBT), bromophenol blue (BPB), and malachite green (MG) was examined under visible light irradiation, and the kinetics of the dye degradation was pseudo-zero-order with the rate constant in the order of MR > EBT > MG > BPB. Furthermore, the mechanism of photo-disintegration mechanism of the dye was examined by a scavenging test using liquid chromatography-mass chromatography, and its excellent photodegradation activities were attributed to the photogenerated holes (h⁺), superoxide (O₂ ^-) anions, and hydroxyl (^·OH) radicals. Finally, the synergistic effect of the nano-interconnected channels with octahedron geometry, mesoporous nature, and charge transfer properties along with photogenerated charge separations leads to an enhanced Co₃O₄ photocatalytic activity.

Tracking the Metal-Centered Triplet in Photoinduced Spin Crossover of Fe(phen)32+ with Tabletop Femtosecond M-Edge X-ray Absorption Near-Edge Structure Spectroscopy

Fe(II) coordination complexes are promising alternatives to Ru(II) and Ir(III) chromophores for photoredox chemistry and solar energy conversion, but rapid deactivation of the initial metal-to-ligand charge transfer (MLCT) state to low-lying (d,d) states limits their performance. Relaxation to a long-lived quintet state is postulated to occur via a metal-centered triplet state, but this mechanism remains controversial. We use femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy to measure the excited-state relaxation of Fe(phen)₃²⁺ and conclusively identify a ³T intermediate that forms in 170 fs and decays to a vibrationally hot ⁵T_2g state in 39 fs. A coherent vibrational wavepacket with a period of 249 fs and damping time of 0.63 ps is observed on the ⁵T_2g surface, and the spectrum of this oscillation serves as a fingerprint for the Fe-N symmetric stretch. The results show that the shape of the M_2,3-edge X-ray absorption near-edge structure (XANES) spectrum is sensitive to the electronic structure of the metal center, and the high-spin sensitivity, fast time resolution, and tabletop convenience of XUV transient absorption make it a powerful tool for studying the complex photophysics of transition metal complexes.

Photoinduced valence tautomerism of a cobalt-dioxolene complex revealed with femtosecond M-edge XANES

Cobalt complexes that undergo charge-transfer induced spin-transitions or valence tautomerism from low spin Co^III to high spin (HS) Co^II are potential candidates for magneto-optical switches. We use M_2,3-edge X-ray absorption near-edge structure (XANES) spectroscopy with 40 fs time resolution to measure the excited-state dynamics of Co^III(Cat-N-SQ)(Cat-N-BQ), where Cat-N-BQ and Cat-N-SQ are the singly and doubly reduced forms of the 2-(2-hydroxy-3,5-di-tert-butylphenyl-imino)-4,6-di-tert-butylcyclohexa-3,5-dienone ligand. The extreme ultraviolet probe pulses, produced using a tabletop high-harmonic generation light source, measure 3p → 3d transitions and are sensitive to the spin and oxidation state of the Co center. Photoexcitation at 525 nm produces a low-spin Co^II ligand-to-metal charge transfer state which undergoes intersystem crossing to high-spin Co^II in 67 fs. Vibrational cooling from this hot HS Co^II state competes on the hundreds-of-fs time scale with back-intersystem crossing to the ground state, with 60% of the population trapped in a cold HS Co^II state for 24 ps. Ligand field multiplet simulations accurately reproduce the ground-state spectra and support the excited-state assignments. This work demonstrates the ability of M_2,3-edge XANES to measure ultrafast photophysics of molecular Co complexes.

Ultrafast Dynamics of Charge Transfer and Photochemical Reactions in Solar Energy Conversion

For decades, ultrafast time-resolved spectroscopy has found its way into an increasing number of applications. It has become a vital technique to investigate energy conversion processes and charge transfer dynamics in optoelectronic systems such as solar cells and solar-driven photocatalytic applications. The understanding of charge transfer and photochemical reactions can help optimize and improve the performance of relevant devices with solar energy conversion processes. Here, the fundamental principles of photochemical and photophysical processes in photoinduced reactions, in which the fundamental charge carrier dynamic processes include interfacial electron transfer, singlet excitons, triplet excitons, excitons fission, and recombination, are reviewed. Transient absorption (TA) spectroscopy techniques provide a good understanding of the energy/electron transfer processes. These processes, including excited state generation and interfacial energy/electron transfer, are dominate constituents of solar energy conversion applications, for example, dye-sensitized solar cells and photocatalysis. An outlook for intrinsic electron/energy transfer dynamics via TA spectroscopic characterization is provided, establishing a foundation for the rational design of solar energy conversion devices.

Improved charge transfer multiplet method to simulate M- and L-edge X-ray absorption spectra of metal-centered excited states

Charge transfer multiplet (CTM) theory is a computationally undemanding and highly mature method for simulating the soft X-ray spectra of first-row transition metal complexes. However, CTM theory has seldom been applied to the simulation of excited-state spectra. In this article, the CTM4XAS software package is extended to simulate M_2,3- and L_2,3-edge spectra for the excited states of first-row transition metals and also interpret CTM eigenfunctions in terms of Russell-Saunders term symbols. These new programs are used to reinterpret the recently reported excited-state M_2,3-edge difference spectra of photogenerated ferrocenium cations and to propose alternative assignments for the electronic state of these cations responsible for the spectroscopic features. These new programs were also used to model the L_2,3-edge spectra of Fe^II compounds during nuclear relaxation following photoinduced spin crossover and to propose spectroscopic signatures for their vibrationally hot states.

Elucidating ultrafast electron dynamics at surfaces using extreme ultraviolet (XUV) reflection–absorption spectroscopy

Time-resolved XUV reflection–absorption spectroscopy probes core-to-valence transitions to reveal state-specific electron dynamics at surfaces.

Gas-generating reactions for point-of-care testing

Gas generation-based measurement is an attractive alternative approach for POC (Point-of-care) testing, which relies on the amount of generated gas to detect the corresponding target concentrations.

A sealable ultrathin window sample cell for the study of liquids by means of soft X-ray spectroscopy

A new sample cell concept for the analysis of liquids or solid-liquid interfaces using soft X-ray spectroscopy is presented, which enables the complete sealing of the cell as well as the transport into vacuum via, for example, a load-lock system. The cell uses pressure monitoring and active as well as passive pressure regulation systems, thereby facilitating the full control over the pressure during filling, sealing, evacuation, and measurement. The cell design and sample preparation as well as the crucial sealing procedure are explained in detail. As a first proof-of-principle experiment, successful nitrogen K-edge fluorescence yield near-edge X-ray absorption fine structure experiments of a biomolecular solution are presented. For this purpose, it is shown that the careful evaluation of all involved parameters, such as window type or photon flux, is desirable for optimizing the experimental result.

Phase matching of noncollinear sum and difference frequency high harmonic generation above and below the critical ionization level

We investigate the macroscopic physics of noncollinear high harmonic generation (HHG) at high pressures. We make the first experimental demonstration of phase matching of noncollinear high-order-difference-frequency generation at ionization fractions above the critical ionization level, which normally sets an upper limit on the achievable cutoff photon energies. Additionally, we show that noncollinear high-order-sum-frequency generation requires much higher pressures for phase matching than single-beam HHG does, which mitigates the short interaction region in this geometry. We also dramatically increase the experimentally realized cutoff energy of noncollinear circularly polarized HHG, reaching photon energies of 90 eV. Finally, we achieve complete angular separation of high harmonic orders without the use of a spectrometer.

Frontiers of water oxidation: the quest for true catalysts

Development of advanced analytical techniques is essential for the identification of water oxidation catalysts together with mechanistic studies.

Shrinking the Synchrotron: Tabletop Extreme Ultraviolet Absorption of Transition-Metal Complexes

We show that the electronic structure of molecular first-row transition-metal complexes can be reliably measured using tabletop high-harmonic XANES at the metal M2,3 edge. Extreme ultraviolet photons in the 50-70 eV energy range probe 3p → 3d transitions, with the same selection rules as soft X-ray L2,3-edge absorption (2p → 3d excitation). Absorption spectra of model complexes are sensitive to the electronic structure of the metal center, and ligand field multiplet simulations match the shapes and peak-to-peak spacings of the experimental spectra. This work establishes high-harmonic spectroscopy as a powerful tool for studying the electronic structure of molecular inorganic, bioinorganic, and organometallic compounds.

High gas-sensor and supercapacitor performance of porous Co3O4 ultrathin nanosheets

Porous Co₃O₄ ultrathin nanosheets exhibited good sensitivity and low concentration selectivity to ethanol.