Charge Separation Research Articles

Graphitic carbon nitride sheets sandwiched metal oxides: A novel platform for S-Scheme heterojunction generation for efficient photodegradation of volatile organics

This study reports the preparation of new kind of S-Scheme heterojunction (SSHJ) photocatalyst by distributing two semiconducting metal oxides (titanium dioxide and ZnO) onto the surface of graphitic carbon nitride (G-CN) two dimensional (2D) sheets. The composite photocatalysts, designated as T/Z@ GCN SSHJ, were prepared with different mass ratios of T,Z and G-CN.X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM), field emission scanning electron microscopy, X-ray photoelectron spectroscopy (XPS), UV-Vis diffuse reflectance spectroscopy (DRS), photoluminescence (PL) spectroscopy were used to characterize the microstructure, morphology, chemical composition, electronic states, and optical properties of the T/Z@ GCN SSHJ composite. TEM image of the typical T/Z@ GCN SSHJ composite shows the presence of distribution of particles with sizes in the range 40–80 nm on the lamellar surface of G-CN inferring the formation of a heterostructure Z(0 D) /G-CN(2D)/T(0D) SSHJ composite. The comparison of the core level XPS spectrum of Ti 2p, Zn 2p, C1s, O1s and N1s of T/Z@ GCNSSHJ composite with the binary composites (T+G-CN and Z+G-CN) and pristine components clearly suggests that that Z, T and G-CN are kept together through van der Walls interactions. The T/Z@ GCN SSHJ composite photocatalyst with T: Z: G-CN mass ratio of 1.0: 0.80: 0.12 exhibited enhanced photocatalytic degradation (%) over binary and pristine single components for all the tested volatile organic compounds (VOCs), namely, acetaldehyde (ACD), ethyl acetate (EA), toluene (T) and benzaldehyde (BA). On comparing the % removal of VOCs after 20 minute of light irradiation, BA showed the highest removal % (94.3 %) and the others take the following order: ACD (70.9 %) >EA (56.45 %)> TL (40.12 %). The study explains the enhanced performance of T/Z@ G-CN HJ composite towards VOC photodegradation in terms of S-Scheme based charge transfer and internal electric field driven efficient charge separation. The charge transfer process of T/Z@ GCN HJ composite photocatalysts is discussed through S-Scheme HJ formation and supported with electronic states and band energy levels of the constituent components derived through the results of XPS and DRS analysis. This study gives insight into the design for regulating the hetero-interfaces for enhancing the photocatalysis and expected to further understand into charge transfer mechanism at multi-junction-based photocatalysts.

Programming Tetrathiafulvalene‐Based Covalent Organic Frameworks for Promoted Photoinduced Molecular Oxygen Activation

AbstractDespite the pivotal role of molecular oxygen (O2) activation in artificial photosynthesis, the activation efficiency is often restricted by sluggish exciton dissociation and charge transfer kinetics within polymer photocatalysts. Herein, we propose two tetrathiafulvalene (TTF)‐based imine‐linked covalent organic frameworks (COFs) with tailored donor‐acceptor (D–A) structures, TTF‐PDI‐COF and TTF‐TFPP‐COF, to promote O2 activation. Because of enhanced electron push‐pull interactions that facilitated charge separation and transfer behavior, TTF‐PDI‐COF exhibited superior photocatalytic activity in electron‐induced O2 activation reactions over TTF‐TFPP‐COF under visible light irradiation, including the photosynthesis of (E)‐3‐amino‐2‐thiocyano‐α,β‐unsaturated compounds and H2O2. These findings highlight the significant potential of the rational design of COFs with D–A configurations as suitable candidates for advanced photocatalytic applications.

The Ferroelectric Effects of Rhombohedral and Tetragonal BiFeO3 in Photoelectrochemical Water Splitting.

The phase of BiFeO3 (BFO) as well as its domain configuration can be tuned by strain engineering. Phase change may greatly influence the properties of the polarization field and hence charge separation. However, the photoelectrochemical properties of different BFO phases have rarely been addressed. Here, the photoelectrochemical study of tetragonal (T-) and rhombohedral (R-) phase BFO films was conducted under visible light illumination. The photocurrent density of R-BFO is 5 times that of T-BFO. A ferroelectric domain study shows that T-BFO features single domain structure in contrast to the polydomain structure of R-BFO. Higher charge separation efficiency is achieved in R-BFO, dominated by the domain walls as conducting pathways for efficient charge separation and transfer. This work provides a fundamental understanding of the photoelectrochemical properties of T- and R-BFO, offering valuable insights for the development of BFO-based materials for solar energy conversion.

Construction of a Schottky junction for enhanced Cr(VI) photoreduction through partial derivation of NiB from 2D Ni-MOF

Incorporating cocatalysts to form Schottky junctions offers a promising strategy for enhancing photocatalytic performance, particularly in reducing environmental pollutants. This study introduces a novel method utilizing a two-dimensional nickel-based metal-organic framework (Ni-MOF), synthesized via a solvothermal process and partially derived for the decoration of nickel boride (NiB), establishing a Schottky junction photocatalyst. The hierarchically structured Ni-MOF/NiB was synthesized under ambient air conditions with varying molar ratios of NaBH4 (0.8, 1, and 1.2) and employed for Cr(VI) photocatalytic reduction. Among these ratios, the optimal Ni-MOF/NiB-1 exhibited a remarkable photocatalytic efficiency of 96% at pH = 2 under visible light within 75 min. This enhanced performance is attributed to the formation of the Schottky junction between NiB and Ni-MOF, facilitating effective charge carrier migration and separation, as well as the high surface area, numerous active sites, improved visible light absorption, and reduced recombination rate. This effective approach of anchoring a noble metal-free cocatalyst on a porous metal−organic framework creates an economical and sustainable photocatalyst to reduce Cr(VI).

S-scheme heterojunction phthalocyanine/TiO2 photoelectrochemical sensor for innovative glutathione detection.

A novel photoelectrochemical sensor, employing an S-scheme heterojunction of phthalocyanine and TiO2 nanoparticles, has been developed to enable highly sensitive determination of glutathione. By integrating the favorable stability, environmental benignity, and electronic properties of the TiO2 matrix with the unique photoactivity of phthalocyanine species, the designed sensor presents a substantial linear dynamic range and a low detection limit for the quantification of glutathione. The sensitivity is attributed to efficient charge transfer and separation across the staggered heterojunction energy levels, which generates measurable photocurrent signals. Systematic variation of phthalocyanine content reveals an optimal composition that balances light harvesting capacity and electron-hole recombination rates. The incorporation of phosphotungstic acid (PTA) in sample preparation effectively minimizes interference from compounds like L-cysteine and others. Consequently, this leads to an improvement in accuracy through the reduction of impurity levels. Appreciable photocurrent enhancements are observed upon introduction of both oxidized and reduced glutathione at the optimized composite photoanode. Coupled with advantageous features of photoelectrochemical transduction such as simplicity, cost-effectiveness, and resistance to fouling, this sensor holds great promise for practical applications in complex biological media.

Synthesis of polyaniline modified FeWO4 nanoparticles for superior visible-light photocatalysis

This communication reports the development of a new high-efficiency visible-light photocatalyst by coupling p-type FeWO4 with n-type polyaniline (PANI). PANI/FeWO4 nanocomposite was synthesized through a facile composition-controllable solid liquid mixing-evaporation of solvent method. The composition and structure of the synthesized PANI/FeWO4 nanocomposite were characterized by X-ray diffraction, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy. The photoabsorption and photogenerated charge separation and transfer performance of PANI/FeWO4 nanocomposite were compared with those of FeWO4 nanoparticles. The photocatalytic properties of PANI/FeWO4 nanocomposite and FeWO4 nanoparticles were tested in the reduction of aqueous Cr(VI) under visible-light (λ > 420 nm). The results showed that the photocatalytic activity of PANI/FeWO4 was significantly elevated compared to FeWO4. Coupling with PANI can increase the adsorption of Cr(VI), enhance the absorption of visible-light, and promote the separation and transfer of photogenerated charges of FeWO4, thus contributing to the better photocatalytic activity of PANI/FeWO4.

Flower-like Z-scheme heterostructure of cobalt porphyrin/ZnIn2S4 for boosting photocatalytic solar fuel production

Molecular catalysts have attracted significant attention in photocatalytic CO2 reduction (PCR) due to well-defined structure and exceptional catalytic selectivity. However, they are still limited by recycling challenges and inherent instability. Forming an organic-inorganic composite can address these issues and enhance PCR performance simultaneously. Herein, we developed a flower-like Z-scheme heterojunction of cobalt porphyrin ([meso-tetra(4-sulfonatophenyl)porphyrin], CoTPPS) and ZnIn2S4 (ZIS). The optimized ZIS@CoTPPS exhibited a superior CO evolution rate of 388.26 μmol g−1 h−1, which is 8.96 times of pristine ZIS (43.32 μmol g−1 h−1). The superior PCR activity is contributed by the enhanced light absorption, and photoinduced charge separation facilitated by the Z-scheme structure and the cobalt active sites. Moreover, theoretical calculations confirm the heterojunction relies on a substantial interfacial coupling between the Zn atoms of ZIS and the sulfonic acid groups of CoTPPS through coordination assembly. This investigation affords a useful inspiration for consciously constructing novel ZIS-incorporated atomistic Z-scheme photocatalysts.

Nuclear Quantum Effects Accelerate Charge Separation and Recombination in g-C3N4/TiO2 Heterojunctions.

We combined ring-polymer molecular dynamics (MD) and ab initio MD with nonadiabatic MD to study the effects of nuclear quantum effects (NQEs) on interlayer electron transfer and electron-hole recombination at the g-C3N4/TiO2 interface. Our simulations indicate that NQEs significantly affect electron transfer and electron-hole recombination dynamics, accelerating both processes. NQEs deform the g-C3N4 layer and expedite the movement of carbon and nitrogen atoms, thus, enhancing charge delocalization and interlayer coupling. This improved overlap between electronic state wave functions enhances nonadiabatic couplings, facilitating electron transfer and recombination. In addition to the enhanced nonadiabatic couplings accelerating electron transfer, the presence of NQEs narrows the energy gap and delays decoherence by mitigating overall fluctuations, because of restricted TiO2 movements overwhelming enhanced g-C3N4 fluctuations, thereby making the recombination faster. This work provides valuable insights into NQEs in light-element systems and contributes to guiding the development of highly efficient photocatalysts.

Bi2O3/MoO3 composites: A synergistic approach towards photocatalytic degradation and enhanced antimicrobial activity

The escalating concerns over the water containments and public health pose the great threat to sustainable environment. In this quest there is strong need of advance materials with sustainable solutions that provides the innovative methods for wastewater treatment. This article explores an in-depth study of synergistic effect of composites for photo degradation of methyl orange (MO) and tetracycline (TC) along with the antimicrobial activity. The composites were synthesized through facile hydrothermal method with the weight concentration of 1%, 3%, 5%, 7% of MoO3 represented by BOMO-1, BOMO-3, BOMO-5 and BOMO-7 respectively. Enhanced photocatalytic reported attributed to efficient charge separation and active sites formation of composites. Moreover significantly improved antibacterial activity of nanostructures occurs because of hetrojunction formation. The synergistic potential of Bi2O3/MoO3 composites resulted the generation of Reactive oxygen species (ROS) which performs the redox reaction to improved their activities. Additionally the fabricated nanomaterials evaluated by different characterization to check their optical, structural, morphological and electronic properties. The dual functionality of Bi2O3/MoO3 makes it a promising material for wastewater treatment as well as environmental remediation. These results predict its widely use for advance catalysis and hydrogen evolution procedure in near future.

Ag-catalyzed strain engineering in ZnO for tailoring defects towards bacterial inactivation and removal of organic dyes for environmental sustainability

This study explored Ag-guided strain formation in ZnO that can manipulate defects in related properties, which are helpful in enhancing the photocatalytic performance of ZnO. Ag@ZnO nanosheets were synthesized with varying Ag concentrations (0–5 at%) through a cost-effective hydrothermal method and employed for the efficient removal of environmentally concerning dyes, namely methylene blue (MB), methyl orange (MO), and rhodamine B (RhB), under natural sunlight radiation. Microscopic images (SEM and TEM) revealed the nanoplates regardless of the Ag concentrations. The XRD pattern confirmed the pure phase of the hexagonal wurtzite structure of ZnO, while the crystallite size decreased from 25 to 20 nm, augmented with the bright field-TEM images. The energy bandgap, Eg, decreased from 3.24 to 3.18 eV, as Ag concentration increased from 0 to 5 at%. The highest photocatalytic efficiency of Ag@ZnO:5 at% was achieved to be ∼93 %, ∼97.5 %, and ∼98 % for MO, MB, and RhB, respectively. The enhanced photocatalytic activity of Ag@ZnO:5 at% was attributed to the high charge separation efficiency and many active sites due to the surface integration of Ag atoms and defects states. Ag@ZnO:5 at% nanoplates showed significant inactivation against environmentally transmitted pathogenic bacteria within 30 min. These findings underscore the significant potential of Ag@ZnO photocatalysts for sunlight-driven removal of organic pollutants, aligning with the sustainability goals advocated by the United Nations. The outcomes of this research contribute valuable insights into the development of efficient photocatalytic materials for environmental pollution remediation.

Hyper-Rayleigh Scattering and Third-Harmonic Scattering in Chiral Liquids: Basic Evidences and Differences with Linear Chiroptical Techniques.

Nonlinear chiroptical methods like hyper-Rayleigh optical activity (HROA) and third-harmonic optical activity (THOA) have a great potential in characterizing structure-chiroptical properties of molecular systems. Here, with these methods, we bring for the first time experimental and theoretical evidence of nonlinear chiroptical differences between enantiomers of simple chiral molecules, using exclusively linearly polarized incident light. The origin of these unexpected nonlinear chiroptical contributions comes from a new nonlinear source term of the form βOA(∇×μ(nω)), which is a bilinear coupling term including the linear chirality parameter, βOA, and the curl of the nonlinear induced electric dipole moment, μ(nω), both involving dipolar magnetic interactions. Through a simple nonlinear chiroptical model that we propose, we show that specific nonlinear chiroptical parameters can be quantified, thus bringing new insights into stereochemical and electronic structural features of molecular and supramolecular systems.

Modelling Photocatalytic N2 Reduction to Ammonia: Where We Stand and Where We Are Going.

Artificial ammonia synthesis via the Haber-Bosch process is environmentally problematic due to the high energy consumption and corresponding CO emissions, produced during the reaction and before hand in hydrogen production upon methane steam reforming. Photocatalytic nitrogen fixation as a greener alternative to the conventional Haber-Bosch process enables us to perform nitrogen reduction reaction (NRR) under mild conditions, harnessing light as the energy source. Herein, we systematically review first-principles calculations used to determine the electronic/optical properties of photocatalysts, N2 adsorption and to expound possible NRR mechanisms. The most commonly studied photocatalysts for nitrogen fixation are usually modified with dopants, defects, co-catalysts and Z-scheme heterojunctions to prevent charge carrier recombination, improve charge separation efficiency and adjust a band gap to for utilizing a broader light spectrum. Most studies at the atomistic level of modeling are grounded upon density functional theory (DFT) calculations, wholly foregoing excitation effects paramount in photocatalysis. Hence, there is a dire need to consider methods beyond DFT to study the excited state properties more accurately. Furthermore, a few studies have been examined, which include higher level kinetics and macroscale simulations. Ultimately, we show there is still ample room for improvement with regard to first principles calculations and their integration in multiscale models.

Electrolyte effects on the alkaline hydrogen evolution reaction: A mean-field approach

This paper introduces the combination of an advanced double-layer model with electrochemical kinetics to explain electrolyte effects on the alkaline hydrogen evolution reaction. It is known from experimental studies that the alkaline hydrogen evolution current shows a strong dependence on the concentration and identity of cations in the electrolyte, but is independent of pH. To explain these effects, we formulate the faradaic current in terms of the electric potential in the double layer, which is calculated using a mean-field model that takes into account the cation and anion sizes as well as the electric dipole moment of water molecules. We propose that the Volmer step consists of two activated processes: a water reduction sub-step, and a sub-step in which OH− is transferred away from the reaction plane through the double layer. Either of these sub-steps may limit the rate. The proposed models for these sub-steps qualitatively explain experimental observations, including cation effects, pH-independence, and the trend reversal between gold and platinum electrodes. We also assess the quantitative accuracy of the water-reduction-limited current model; we suggest that the predicted functional relationship is valid as long as the hydrogen bonding structure of water near the electrode is sufficiently maintained.

Tuning Charge-Transfer States by Interface Electric Fields.

Intermolecular charge-transfer (CT) states are extended excitons with a charge separation on the nanometer scale. Through absorption and emission processes, they couple to the ground state. This property is employed both in light-emitting and light-absorbing devices. Their conception often relies on donor-acceptor (D-A) interfaces, so-called type-II heterojunctions, which usually generate significant electric fields. Several recent studies claim that these fields alter the energetic configuration of the CT states at the interface, an idea holding prospects like multicolor emission from a single emissive interface or shifting the absorption characteristics of a photodetector. Here, we test this hypothesis and contribute to the discussion by presenting a new model system. Through the fabrication of planar organic p-(i-)n junctions, we generate an ensemble of oriented CT states that allows the systematic assessment of electric field impacts. By increasing the thickness of the intrinsic layer at the D-A interface from 0 to 20 nm and by applying external voltages up to 6 V, we realize two different scenarios that controllably tune the intrinsic and extrinsic electric interface fields. By this, we obtain significant shifts of the CT-state peak emission of about 0.5 eV (170 nm from red to green color) from the same D-A material combination. This effect can be explained in a classical electrostatic picture, as the interface electric field alters the potential energy of the electric CT-state dipole. This study illustrates that CT-state energies can be tuned significantly if their electric dipoles are aligned to the interface electric field.

Molecular Photoelectrochemical Energy Storage Materials for Coupled Solar Batteries.

ConspectusSolar-to-electrochemical energy storage is one of the essential solar energy utilization pathways alongside solar-to-electricity and solar-to-chemical conversion. A coupled solar battery enables direct solar-to-electrochemical energy storage via photocoupled ion transfer using photoelectrochemical materials with light absorption/charge transfer and redox capabilities. Common photoelectrochemical materials face challenges due to insufficient solar spectrum utilization, which restricts their redox potential window and constrains energy conversion efficiency. In contrast, molecular photoelectrochemical energy storage materials are promising for their mechanism of exciton-involved redox reaction that allows for extra energy utilization from hot excitons generated by superbandgap excitation and localized heat after absorption of sub-bandgap photons. This enables more efficient redox reactions with a less restricted redox potentials window and, thus, better utilization of the full solar spectrum. Despite these advantages, practical application remains elusive due to the mismatch between the short lifetime of the charge separation state (<ns) and the slower redox reaction rate (>μs). This mismatch results in a significant portion of the photogenerated charges recombining before participating in desired electrochemical energy storage reactions, leading to diminished overall efficiency. It is therefore highly important to develop molecular materials with intrinsic prolonged charge separation state and extrinsic effective mass-electron transfer to enable efficient coupled solar batteries for practical applications.In this Account, we begin with an introduction of the general solar-to-electrochemical energy storage concept based on molecular photoelectrochemical energy storage materials, highlighting the advantages of periodic oxidative donor-reductive acceptor porous aggregate structures that have synergistic implications on charge separation state lifetime extension and mass-electron transfer. We then present our earliest trial on the design and application of molecular photoelectrochemical energy storage materials, which stimulated our subsequent studies on tuning electron donor and acceptor structures for enhanced charge separation and diverse photoelectrochemical redox reactions. Moreover, we introduce the best practices in the design and assembly of various coupled solar battery devices, along with our literature contributions and progresses in solar-to-electrochemical energy storage efficiency (ηSES) over nearly the past decade. Finally, we conclude by highlighting the universality of our strategies as essential design principles, spanning from regulating long-lived charge separation states and photocoupled ion transfer processes in molecular materials to the construction of efficient coupled solar batteries. We offer perspectives on the synergy between photovoltage and redox potentials and the practical significance of 3D printing, providing key evaluation indicators for large-scale application. This Account provides molecular level insights for the construction of high-efficiency photoelectrochemical energy storage materials and guidance for practical solar-to-electrochemical energy storage applications.

Novel high-valent copper species-involved photo-Fenton-like process boosted by nitrogen/oxygen dual defects modulation

High-valent metal species generated during peroxymonosulfate (PMS) activation have demonstrated significant potential in wastewater treatment. However, the rapid generation of high-valent metal species, especially high-valent copper (Cu(III)), has been challenging. Moreover, the mechanisms behind Cu(III) generation and its role in the PMS-based Fenton-like process have not been fully understood. Herein, a novel N, O dual defects co-modified 0D/2D hybrid of CuO nanodots (NDs)/g-C3N4 nanosheets was constructed as an integrated photocatalytic platform to efficiently activate PMS, which exhibited excellent catalytic activity and stability towards tetracycline degradation through consecutive generation of surface Cu(III) as the key active species. The N, O dual-defects and strong interfacial coupling of NDs/g-C3N4 markedly increased charge separation and induced electron-rich Cu sites. This led to a continuous and rapid Cu(II)/Cu(I) conversion, further triggering persistent Cu(III) generation through Cu(I)-oxidation with a two-electron transfer. The work provides new insights into exploring the properties of Cu(III) species during PMS activation and offers encouraging guidance for their application in pollutant removal.

Construction of an In2O3/Bi2S3 Z-Scheme Heterojunction for Enhanced Photocatalytic CO2 Reduction.

Photocatalytic conversion of CO2 to hydrocarbon fuel is a potential strategy to solve energy shortage and mitigate the greenhouse effect. Here, direct Z-scheme heterojunction photocatalysts (In2O3/Bi2S3) without an electron mediator are prepared by a simple hydrolysis method. The In2O3/Bi2S3 composite photocatalysts show greatly boosted photoactivity on CO2 conversion to CO compared with the pristine In2O3 and Bi2S3. The highest CO evolution rate of 2.67 μmol·g-1·h-1 is achieved by In2O3/Bi2S3-3, without any sacrificial agent or cocatalyst, which is about 3.87 times that of In2O3 (0.69 μmol·g-1·h-1). The boosted photocatalytic performance of In2O3/Bi2S3 composite catalysts can be ascribed to the establishment of a Z-scheme heterojunction, improving the photoabsorption and facilitating charge separation and transfer. This study provides a reference for designing and fabricating high-efficiency Z-scheme heterojunction photocatalysts for photocatalytic CO2 reduction.

Hydrogel composites based on chitosan and CuAuTiO2 photocatalysts for hydrogen production under simulated sunlight irradiation

This study explored the photocatalytic hydrogen evolution reaction (HER) using novel biohydrogel composites comprising chitosan, and a photocatalyst consisting in TiO2 P25 decorated with Au and/or Cu mono- and bimetallic nanoparticles (NPs) to boost its optical and catalytic properties. Low loads of Cu and Au (1 mol%) were incorporated onto TiO2 via a green photodeposition methodology. Characterization techniques confirmed the incorporation of decoration metals as well as improvements in the light absorption properties in the visible light interval (λ > 390 nm) and electron transfer capability of the semiconductors. Thereafter, Au and/or Cu NP-supported TiO2 were incorporated into chitosan-based physically crosslinked hydrogels revealing significant interactions between chitosan functional groups (hydroxyls, amines and amides) with the NPs to ensure its encapsulation. These materials were evaluated as photocatalysts for the HER using water and methanol mixtures under simulated sunlight and visible light irradiation. Sample CuAuTiO2/ChTPP exhibited a maximum hydrogen generation of 1790 μmol g−1 h−1 under simulated sunlight irradiation, almost 12-folds higher compared with TiO2/ChTPP. Also, the nanocomposites revealed a similar tendency under visible light with a maximum hydrogen production of 590 μmol g−1 h−1. These results agree with the efficiency of photoinduced charge separation revealed by transient photocurrent and EIS.

Molecular Triplet Generation Enabled by Adjacent Metal Nanoparticles.

High-efficiency generation of spin-triplet states in organic molecules is of great interest in diverse areas such as photocatalysis, photodynamic therapy, and upconversion photonics. Recent studies have introduced colloidal semiconductor nanocrystals as a new class of photosensitizers that can efficiently transfer their photoexcitation energy to molecular triplets. Here, we demonstrate that metallic Ag nanoparticles can also assist in the generation of molecular triplets in polycyclic aromatic hydrocarbons (PAHs), but not through a conventional sensitization mechanism. Instead, the triplet formation is mediated by charge-separated states resulting from hole transfer from photoexcited PAHs (anthracene and pyrene) to Ag nanoparticles, which is established through the rapid formation and subsequent decay of molecular anions revealed in our transient absorption measurements. The dominance of hole transfer over electron transfer, while both are energetically allowed, could be attributed to a Marcus inverted region of charge transfer. Owing to the rapid charge separation and the rapid spin-flip in metals, the triplet formation yields are remarkably high, as confirmed by their engagement in production of singlet oxygen with a quantum efficiency reaching 58.5%. This study not only uncovers the fundamental interaction mechanisms between metallic nanoparticles and organic molecules in both charge and spin degrees of freedom but also greatly expands the scope of triplet "sensitization" using inorganic nanomaterials for a variety of emerging applications.

New Insights into Photo-Fenton Chemistry: The Overlooked Role of Excited IronIII Species.

Direct photoreduction of FeIII is a widely recognized route for accelerating FeIII/FeII cycle in photo-Fenton chemistry. However, most of the wavelengths covering the full spectral range are insufficient to supply enough photon energy for the direct reduction process. Herein, the hitherto neglected mechanism of FeIII reduction that the FeIII indirect reduction pathway initiated by light energy-dependent reactivity variation and reactive excited state (ES) was explored. Evolution of excited-state FeIII species (*FeIII) resulting from metal-centered charge excitation (MCCE) of FeIII is experimentally verified using pulsed laser femtosecond transient absorption spectroscopy with UV-vis detection and theoretically verified by quantum chemical calculation. Intense photoinduced intravalence charge transition was observed at λ = 380 and 466 nm, revealing quartet 4MCCE and doublet 2MCCE and their exponential processes. Light energy-dependent variation of *FeIII reactivity was kinetically certified by fitting the apparent rate constant of the radical-chain sequence of photo-Fenton reactions. Covalency is found to compensate for the intravalence charge separation following photoexcitation of the metal center in the MCCE state of Fenton photosensitizer. The *FeIII is established as a model, demonstrating the intravalence hole delocalization in the ES can be leveraged for photo-Fenton reaction or other photocatalytic schemes based on electron transfer chemistry.