Fe(III)-mediated self-sustaining photo-Fenton system on metal-free pyridine-COF: Interfacial electron transfer for water purification

The reorganization energy (λ) for light initiated interfacial electron transfer (ET) and proton-coupled electron transfer (PCET) from a transparent conducting oxide to molecular acceptors has been quantified as a function of the thermodynamic driving force (-ΔGo ) and analyzed by Marcus-Gerischer theory [1]. The tin-doped indium oxide (ITO) utilized has a significantly high dopant concentration to raise the Fermi level, EF, into the conduction band providing metallic like properties while enabling time resolved spectroscopic characterization in a transmission mode. In this presentation molecular sensitizers, anchored to ITO, are photoexcited and undergo excited-state injection to the ITO to generate an interfacial charge separated state, ITO(e-)|-S+. The kinetics for ET, ITO(e-)|-S+ → ITO|-S, (and PCET, ITO(e-)-S+-OH + H+ → ITO|S-OH2) were then quantified spectroscopically. In the framework of Marcus-Gerischer analysis, EF was controlled with an applied potential (Eapp). Thus, the driving force for ET was systematically tuned, -ΔGo = e(Eo' - EF ) where Eo' is the formal reduction potential of the sensitizer. The driving force for proton transfer (PT) was controlled with pH. Subsequent analysis of the free energy dependent kinetic data allowed extraction of the reorganization energy and electronic coupling matrix element for elementary ET and PCET reactions occurring at conductive oxide electrolyte interfaces.After a discussion of the underlying Marcus-Gerischer framework for analysis of interfacial charge transfer kinetics, two specific studies will be discussed: 1) Interfacial electron transfer to molecular acceptors positioned within the aqueous electric double layer [2]. These data revealed a negligibly small within the Helmholtz planes consistent with negligible solvent reorganization. Importantly, increased to bulk values within the diffuse layer; 2) Proton coupled electron transfer (PCET) to surface anchored molecular water oxidation catalysts. Collectively, this data indicates that proton transfer rate constants can be tuned with pH while ET rate constants can be independently tuned with an applied potential [3]. The data revealed a stepwise ET-PT mechanism with evidence for concerted pathways at high driving forces and in the presence of specific buffers [4].[1] Perspectives in Dye Sensitization of Nanocrystalline Mesoporous Thin Films. Hu, K.; Sampaio, R.N.; Schneider, J.; Troian-Gautier, L.; Meyer, G.J. J. Am. Chem. Soc. 2020, 142, 16099-16116.[2] Kinetic Evidence that the Solvent Barrier for Electron Transfer is Absent in the Electric Double Layer. Bangle, R.E.; Schneider, J.; Conroy, D.T.; Aramburu-Troselj, B.M.; Meyer, G.J. J. Am. Chem. Soc. 2020, 142, 14940-14946.[3] Electronic Coupling and Reorganization Energies for Interfacial Proton-Coupled Electron Transfer to a Water Oxidation Catalyst . Kessinger, M.; Soudackov, A.; Schneider, J.; Bangle, R.E.; Hammes-Schiffer, S.; Meyer, G.J. J. Am. Chem. Soc. 2022, 144, 20514-20524. [4] Direct Evidence for a Sequential Electron Transfer – Proton Transfer Mechanism in the PCET Reduction of a Metal Hydroxide Catalyst. Kessinger, M.C.; Xu, J.; Cui, K.; Loague, Q.; Soudackov, A.V.; Hammes-Schiffer, S.; Meyer, G.J. J. Am. Chem. Soc. 2024, 146, 1742-1747.

Understanding the physical and chemical factors that control the kinetics of interfacial electron-transfer (ET) reactions is important for a large number of technological applications. The present article describes electrochemical kinetic studies of these factors, in which standard interfacial ET rate constants (k(0)(l)) have been measured for ET between substrate Au electrodes and various redox couples attached to the electrode surfaces by variable lengths (l) of oligomethylene (OM), oligophenylenevinylene (OPV) and oligophenyleneethynylene (OPE) bridges, which were constituents of mixed self-assembled monolayers (SAMs). The k(0)(l) measurements employed the indirect laser-induced temperature jump (ILIT) technique, which permits the measurement of interfacial ET rates that are orders of magnitude faster than those measurable by conventional techniques using the macroelectrodes that are the most convenient substrates for the mixed SAMs. The robustness of the measured rate constants (k(0)(l)), together with the Arrhenius activation energies (E(a)(l)) and preexponential factors (A(l)), is demonstrated by their invariance with respect to several experimental system parameters (including the chemical nature and length of the diluent component of the mixed SAM). Analysis of the kinetic results demonstrates that all of the observed interfacial ET processes proceed through a common type of transition state (predominantly associated with solvent reorganization around the redox moiety) and that the actual ET step involves direct electronic tunnelling between the Au electrode and the redox moiety. However, for the full range of l investigated, a global exponential decay of A(l) is not found for any of the three types of bridges. Possible reasons for this behavior, including the role of rate determining steps associated with adiabatic mechanisms within or beyond the transition state theory framework, are discussed, and comparisons with related conductance measurements are presented.

Electrochemistry and bioelectrochemistry towards the single-molecule level: Theoretical notions and systems

Mixed-dimensional hybrid heterostructures have attracted a lot of experimental attention because they can provide an ideal charge-separated interface for optoelectronic and photonic applications. In this Letter, we have employed first-principles DFT calculations and nonadiabatic dynamics simulations to explore photoinduced interfacial electron and hole transfer processes in two PTB7- nL@MoS2 models ( n = 1 and 5). The interfacial electron transfer is found to be ultrafast and completes within ca. 10 fs in both PTB7-1L@MoS2 and PTB7-5L@MoS2 models, which demonstrates that the electron transfer is not sensitive to the thickness of the PTB7 polymer. Differently, the interfacial hole transfer is sensitive to the thickness of the PTB7 polymer. The transfer time is estimated to be ca. 70 ps in PTB7-1L@MoS2, while it is significantly accelerated to ca. 1 ps in PTB7-5L@MoS2. Finally, we have found that the electron transfer is mainly controlled by adiabatic electron evolution, whereas in the hole transfer, nonadiabatic hoppings play a dominant role. These findings are useful for the design of excellent charge-separated interfaces of mixed-dimensional TMD-based heterojunctions for a variety of optoelectronic applications.

Fe-bearing clay minerals are widely distributed in soils, sediments, and rocks, representing a significant Fe pool in the Earth’s crust. The electron transfer (ET) from/to structural Fe in clay minerals is a crucial electron and energy flux in the natural environment, which drives numerous biogeochemical processes and contaminant transformation. Depending on the types and properties of both clay minerals and exogenous reactants as well as aqueous chemistry, the ET processes could involve interfacial ET through edge/basal planes and interior ET inside clay minerals. This paper reviews the important ET reactions between Fe-bearing clay minerals and various reactants, including Fe-cycling microbes, redox-active organic compounds, and heavy metals. Moreover, we discuss the physical-chemical mechanisms of interfacial and interior ET processes and develop models to illustrate the thermodynamic and kinetic constraints on the ET rate and extent. On this basis, we emphasize the environmental implications of ET associated with clay minerals, such as their roles in serving as biogeobatteries for biogeochemical processes and contaminant transformation, coevolution with microbes, and regulation of greenhouse gas formation. Finally, research needs are proposed to advance our molecular-scale understanding of ET processes and utilize them for environmental mitigation and human health.

Interfacial electron transfer between an inorganic semiconductor and a metal-organic framework (MOF) is the key to photocatalysis in a composite photocatalytic system. The construction of structural defects in a single semiconductor or MOF has been regarded as an effective method for enhancing its photocatalytic performance. However, how the microenvironment modulation of photocatalytic sites between defective semiconductors and defective MOFs (quasi-MOFs) affects photocatalytic disinfection activity is worth studying. Herein, the integration of MOFs and semiconductors and their crystal defect construction is achieved by directly dropping ZIF-8 suspension onto the bismuth vanadate (BiVO4) nanoarray and subsequently activating it through low-temperature (300 °C) calcination under an N2 atmosphere. Compared with the original BiVO4 nanoarray, defective BiVO4 nanoarray (BiVO4-N2), and BiVO4-ZIF-8 nanoarray, (BiVO4-ZIF-8)-N2 exhibits enhanced photocatalytic disinfection activity (6.63 log10 CFU mL-1 after 4 h of simulated sunlight irradiation) due to its improved sluggish kinetics of electron transfer. In situ irradiated operando near-ambient pressure XPS (NAP-XPS) confirms its interfacial electron transfer, which can greatly modulate the microenvironment of the photocatalytic site and thus lead to efficient photocatalysis. This work presents a simple strategy to adjust the microenvironment of the photocatalytic sites between ZIF-8 and semiconductors to optimize photocatalytic bactericidal performance.

Self-sustained Bio-Fenton system driven by Shewanella oneidensis for efficient degradation of persistent organic pollutants under oxygen-limited conditions.

The electron transfer (ET) dynamics from core/multi-shell (CdSe/CdS(3ML)ZnCdS(2ML)ZnS(2ML)) quantum dots (QDs) to adsorbed Fluorescein (F27) molecules have been studied by single particle spectroscopy to probe the relationship between single QD interfacial electron transfer and blinking dynamics. Electron transfer from the QD to F27 and the subsequent recombination were directly observed by ensemble-averaged transient absorption spectroscopy. Single QD-F27 complexes show correlated fluctuation of fluorescence intensity and lifetime, similar to those observed in free QDs. With increasing ET rate (controlled by F27-to-QD ratio), the lifetime of on states decreases and relative contribution of off states increases. It was shown that ET is active for QDs in on states, the excited state lifetime of which reflects the ET rate, whereas in the off state QD excitons decay by Auger relaxation and ET is not a competitive quenching pathway. Thus, the blinking dynamics of single QDs modulate their interfacial ET activity. Furthermore, interfacial ET provides an additional pathway for generating off states, leading to correlated single QD interfacial ET and blinking dynamics in QD-acceptor complexes. Because blinking is a general phenomenon of single QDs, it appears that the correlated interfacial ET and blinking and the resulting intermittent ET activity are general phenomena for single QDs.

It is well established that some reduced fermentation products or microbially reduced artificial mediators can abiotically react with electrodes to yield a small electrical current. This type of metabolism does not typically result in an efficient conversion of organic compounds to electricity because only some metabolic end products will react with electrodes, and the microorganisms only incompletely oxidize their organic fuels. A new form of microbial respiration has recently been discovered in which microorganisms conserve energy to support growth by oxidizing organic compounds to carbon dioxide with direct quantitative electron transfer to electrodes. These organisms, termed electricigens, offer the possibility of efficiently converting organic compounds into electricity in self-sustaining systems with long-term stability.

Recent advances in the improvement of bi-directional electron transfer between abiotic/biotic interfaces in electron-assisted biosynthesis system

There has been also growing recent interest in room-temperature ionic liquids, especially in those with 1,3-dialkylimidazolium cations due to their such important features as negligible vapor pressure, high ionic conductivity and thermal stability, fairly wide electrochemical window, and ability to dissolve organic and inorganic solutes. Such electrolytes are also of interest to electrochemical energy systems including dye sensitized solar cells (DSCs). The triiodide/iodide redox system has so far been the most commonly and most successfully used as a charge relay (mediator) in DSCs. The redox electrolyte plays a very important role in the DSC performance, and its usefulness largely depends on dynamics of both interfacial electron transfers and bulk charge propagation within the system. Obviously, the triiodide/iodide redox couple has been considered together with ionic liquids. The resulting redox-conducting electrolytes have several advantages: high conductivity, low vapor pressure, high iodide concentration and good electrochemical stability. Among disadvantages is their high viscosity that certainly contributes to the low mass transport coefficient of the triiodide/iodide redox couple, not only if the charge transport mechanism is predominantly physical at low concentrations but also when the Grotthus exchange mechanism is operative at high concentrations of the redox system. In general, both interfacial and bulk (self-exchange) electron transfers involving the triiodide/iodide redox system are somewhat complicated and appear slower than one would expect. Among kinetic limitations there is a need to break the I-I bond in the I3 - or I2 molecule; it has also been well-established that platinum (e.g. when deposited on the counter electrode) induces electron transfers within the iodine/iodide redox system. Strong interactions of Pt with iodide or iodine were reported and described. It is noteworthy that values of the iodine covalent radius (0.133 nm) and the Pt atomic radius (0.135 nm) are comparable. Formation of the monolayer type coverages of strongly adsorbed monoatomic iodine together with weakly bound electroactive iodine/iodide was also postulated. In the present work, we explore the interfacial (electrocatalytic) phenomena of nanostructured platinum or palladium (here Pt or Pd nanoparticles that are three-dimensionally distributed in the electrolyte phase at 2% weight level), and we utilize them to enhance triiodide/iodide electron transfers to develop more efficient charge relays for DSCs. Finally, to make the electrolyte more solid (non-fluid) and to improve the overall electron distribution within the redox-conducting electrolyte, we also introduce into our nanocomposite system multi-walled carbon nanotubes (CNTs) or reduced graphene oxide flakes, namely at 10% weight level, either themselves or as supports for dispersed iodine-modified Pt or Pd nanoparticles. Using the microelectrode-based and sandwich-type electroanalytical methodologies of solid-state electrochemistry, we address here the charge transport dynamics within the semi-solid triiodide/iodide ionic-liquid electrolyte admixed with noble metal or carbon nanostructures. We will comment on the charge propagation enhancement effects as well as on reasonably high power conversion efficiencies of DSCs utilizing such electrolytes. We acknowledge collaboration with Prof. Michael Graetzel, Dr. Shaik M. Zakeeruddin and Dr. Magdalena Marszalek of EPFL, Lausanne, Switzerland.

The triiodide/iodide redox system has so far been the most commonly and most successfully used as a charge relay (mediator) in dye sensitized solar cells (DSCs). The redox electrolyte plays a very important role in the DSC performance, and its usefulness largely depends on dynamics of both interfacial electron transfers and bulk charge propagation within the system. There has been also growing recent interest in room-temperature ionic liquids, especially in those with 1,3-dialkylimidazolium cations due to their such important features as negligible vapor pressure, high ionic conductivity and thermal stability, fairly wide electrochemical window, and ability to dissolve organic and inorganic solutes. Obviously, the triiodide/iodide redox couple has been considered together with ionic liquids. The resulting redox-conducting electrolytes have several advantages: high conductivity, low vapor pressure, high iodide concentration and good electrochemical stability. Among disadvantages is their high viscosity that certainly contributes to the low mass transport coefficient of the triiodide/iodide redox couple, not only if the charge transport mechanism is predominantly physical at low concentrations but also when the Grotthus exchange mechanism is operative at high concentrations of the redox system. In general, both interfacial and bulk (self-exchange) electron transfers involving the triiodide/iodide redox system are somewhat complicated and appear slower than one would expect. Among kinetic limitations there is a need to break the I-I bond in the I3 - or I2 molecule; it has also been well-established that platinum (e.g. when deposited on the counter electrode) induces electron transfers within the iodine/iodide redox system. Strong interactions of Pt with iodide or iodine were reported and described. It is noteworthy that values of the iodine covalent radius (0.133 nm) and the Pt atomic radius (0.135 nm) are comparable. Formation of the monolayer type coverages of strongly adsorbed monoatomic iodine together with weakly bound electroactive iodine/iodide was also postulated. In the present work, we explore the interfacial (electrocatalytic) phenomena of nanostructured platinum (namely of Pt nanoparticles that are three-dimensionally distributed in the electrolyte phase at 2% weight level), and we utilize them to enhance triiodide/iodide electron transfers to develop more efficient charge relays for DSCs. Finally, to make the electrolyte more solid (non-fluid) and to improve the overall electron distribution within the redox-conducting electrolyte, we also introduce into our nanocomposite system multi-walled carbon nanotubes (CNTs), namely at 10% weight level, as supports for dispersed iodine-modified Pt nanoparticles. Using the microelectrode-based and sandwich-type electroanalytical methodologies of solid-state electrochemistry, we address here the charge transport dynamics within the semi-solid triiodide/iodide ionic-liquid electrolyte admixed with CNT-supported Pt nanostructures and comment on reasonably high power conversion efficiencies of DSCs utilizing such electrolytes.We acknowledge collaboration with M. Graetzel, S.M. Zakeeruddin and M. Marszalek of EPFL, Lausanne, Switzerland.

A theoretical frame for in situ electrochemical scanning tunneling microscopy (STM) of large adsorbed redox molecules is provided. The in situ STM process is viewed as two consecutive interfacial single-step electron transfer (ET) processes with full vibrational relaxation between the steps. The process is therefore a cycle of consecutive molecular reduction and reoxidation. This extends previous approaches where resonance tunneling, or coherent single-channel ET, were in focus. The dependence of the tunneling current on the bias voltage and overvoltage is calculated when both transitions are either fully adiabatic or fully diabatic, and when one transition is fully adiabatic and the other one fully diabatic. A particular feature of the fully adiabatic limit is that each oxidation−reduction cycle is composed of manifolds of individual interfacial ET events at both electrodes, enhancing electron tunneling significantly compared to single-ET. The voltage dependences show spectrocopy-like features. Particula...

Interfacial electron transfer between electroactive microorganisms (EAMs) and electrodes underlies a wide range of bio‐electrochemical systems with diverse applications. However, the electron transfer rate at the biotic‐electrode interface remains low due to high transmembrane and cell‐electrode interfacial electron transfer resistance. Herein, a modular engineering strategy is adopted to construct a Shewanella oneidensis‐carbon felt biohybrid electrode decorated with bacterial cellulose aerogel‐electropolymerized anthraquinone to boost cell‐electrode interfacial electron transfer. First, a heterologous riboflavin synthesis and secretion pathway is constructed to increase flavin‐mediated transmembrane electron transfer. Second, outer membrane c‐Cyts OmcF is screened and optimized via protein engineering strategy to accelerate contacted‐based transmembrane electron transfer. Third, a S. oneidensis‐carbon felt biohybrid electrode decorated with bacterial cellulose aerogel and electropolymerized anthraquinone is constructed to boost the interfacial electron transfer. As a result, the internal resistance decreased to 42 Ω, 480.8‐fold lower than that of the wild‐type (WT) S. oneidensis MR‐1. The maximum power density reached 4286.6 ± 202.1 mW m−2, 72.8‐fold higher than that of WT. Lastly, the engineered biohybrid electrode exhibited superior abilities for bioelectricity harvest, Cr6+ reduction, and CO2 reduction. This study showed that enhancing transmembrane and cell‐electrode interfacial electron transfer is a promising way to increase the extracellular electron transfer of EAMs.

Interfacial electron transfer (ET) between semiconductor nanomaterials and molecular adsorbates is an important fundamental process that is relevant to applications of these materials. Using femtosecond midinfrared spectroscopy, we have simultaneously measured the dynamics of injected electrons and adsorbates by directly monitoring the mid-IR absorption of electrons in the semiconductor and the change in adsorbate vibrational spectrum, respectively. We report on a series of studies designed to understand how the interfacial ET dynamics depends on the properties of the adsorbates, semiconductors, and their interaction. In Ru(dcbpy)2(SCN)2 (dcbpy = 2,2‘-bipyridine-4,4‘-dicarboxylate) sensitized TiO2 thin films, 400 nm excitation of the molecule promotes an electron to the metal-to-ligand charge transfer (MLCT) excited state, from which it is injected into TiO2. The injection process was characterized by a fast component, with a time constant of <100 fs, and a slower component that is sensitive to sample condition. Similar ultrafast electron injection times were measured in TiO2 films sensitized by Ru(dcbpy)2(X)2 (X2 = 2CN- and dcbpy). Electron injection in these systems was found to compete with the vibrational energy relaxation process within the excited state of the molecules, leading to an injection yield that depends on the excited-state redox potential of the adsorbate. The injection rate from Ru(dcbpy)2(SCN)2 to different semiconductors was found to obey the trend TiO2 > SnO2 > ZnO, indicating a strong dependence on the nature of the semiconductor. To understand these observations, various factors, such as electronic coupling, density of states, and driving force, that control the interfacial ET rate were examined separately. The effect of electronic coupling on the ET rate was studied in TiO2 sensitized by three adsorbates, Re(Ln)(CO)3Cl [Ln is a modified dcbpy ligand with n (=0, 1, 3) CH2 units between the bipyridine and carboxylate groups]. We found that the ET rate decreased with increasing number of CH2 units (or decreasing electronic coupling strength). The effect of driving force was investigated in Ru(dcbpy)2X2 (X2 = 2SCN-, 2CN-, and dcbpy) sensitized SnO2 thin films. In this case, we observed that the ET rate increased with the excited-state redox potential of the adsorbates, agreeing qualitatively with the theoretical prediction for a nonadiabatic interfacial ET process.