Hole Transfer Pathways Research Articles

Effect of Halide Ions on the TiO2-mediated Photocatalysis of Carbendazim in Aqueous Medium Under Near-ultraviolet Light Irradiation

The degradation of carbendazim (CBZ) through TiO2 photocatalysis, in the presence of halide ions and under near-UV light irradiation, was investigated. HPLC–MS technique was used to characterize the photoproducts. Spectrophotometric analysis showed that CBZ degraded slowly in TiO2 aqueous dispersions containing no salt (CBZ conversion of 6% after ca. 5 h of irradiation). The photodegradation efficiency increased particularly by addition of bromide salts. Indeed, CBZ reached complete degradation after ca. 30 min at the maximum concentration of NaBr used (0.05 M). Two significant aspects have emerged from the data analysis: the bromide role is to cause inhibition of the electron–hole recombination, a reaction known to be competitive with the reactive process; CBZ photodegradation is especially initiated by direct hole transfer pathway, whereas the OH• role is crucial in the catalyst regeneration process. Degradation attempts under sunlight appeared promising for a more sustainable photocatalytic process.

Atomistic understanding of enhanced selectivity in photocatalytic oxidation of benzyl alcohol to benzaldehyde using graphitic carbon nitride loaded with single copper atoms.

The loading of graphitic carbon nitride (gCN) with transition metals has received significant attention for efficient light-driven catalysis. However, the contribution of the loaded metals to enhanced performance remains unclear. In this study, Cu is loaded onto gCN to understand how photocatalytic activity is regulated by the loaded metals. Loading gCN with 3 wt% of Cu increases the electron population by 8.1 and 4.6 times under UV (λ < 370 nm) and visible light (390 < λ < 740 nm), respectively. This sample shows nearly 100% selectivity for oxidizing benzyl alcohol to benzaldehyde and a high yield-to-power ratio, reaching 0.35 mmol g-1 h-1 W-1. The loaded Cu species exist as single atoms with a +1-oxidation state. Each Cu+ cation is coordinated to two (at 3 wt% Cu) or four (at 6 wt% Cu) N atoms within the cavity of the gCN framework. Doubling the Cu loading results in a smaller electron population and coordinatively more saturated Cu+ cations, making it catalytically less reactive. Ab initio molecular dynamics simulations show that Cu+ cations produce filled mid-gap states above the valence band, which function as hole traps and hence oxidation centers. The Cu+ cation and the neighboring N atoms are electron-depletion and electron-accumulation sites due to Cu → N electron transfer, making it highly reactive for oxidative transformations via the hole transfer pathway. The role of Cu as a hole-transfer site updates the received understanding that surface-loaded Cu serves as an electron-accumulation site. A strong correlation is observed between the electron population at steady-state and the product yield, indicating that it could serve as a promising performance indicator for the design of future photocatalysts.

Surface Termination Tuning Photoexcited-Carrier Dynamics and Carrier Transfer Pathway on Silicon for High-Performance Photocatalytic H2 Evolution from Pure Water

Tuning surface termination can improve the carrier dynamics and photoredox performance of photocatalysts by modifying the surface physicochemical properties. Herein, we prepared a Si photocatalyst by magnesium-thermal reduction and then treated it with trifluoroacetic acid (TFA). The as-obtained TFA-terminated Si photocatalyst (TFA-Si) improves double-layer capacitance and electrochemical/Brunauer–Emmet–Teller surface area, rendering more active sites. Moreover, the TFA modification causes an increase in wettability, charge carrier density, and photocurrent and a reduction in electrochemical impedance and overpotential, which enhances the adsorption/reduction of water molecules and the photoexcited-carrier dynamics for photocatalysis. TFA-Si shows high photocatalytic H2 evolution activity (1827 μmol·g–1·h–1) in pure water under visible light. The AQY of TFA-Si reached 6.1% at 400 nm. Interestingly, TFA molecules promote the oxidation capacity of Si photocatalysts for water oxidation through a hole transfer pathway, boosting its activity and stability. This work broadens the applications of Si-based materials for use in photocatalysis.

Structural Engineering of BiVO4/CoFe MOF Heterostructures Boosting Charge Transfer for Efficient Photoelectrochemical Water Splitting

Boosting charge separation and transfer of photoanodes is crucial for providing high viability of photoelectrochemical hydrogen (H2 ) generation. Here, a structural engineering strategy is designed and synthesized for uniformly coating an ultrathin CoFe bimetal-organic framework (CoFe MOF) layer over a BiVO4 photoanode for boosted charge separation and transfer. The photocurrent density of the optimized BiVO4 /CoFe MOF(NA) photoanode reaches a value of 3.92mA cm-2 at 1.23V versus reversible hydrogen electrode (RHE), up to 6.03 times that of pristine BiVO4 , due to the greatly increased efficiency of charge transfer and separation. In addition, this photoanode records one onset potential that is considerably shifted negatively when compared to BiVO4 . Transient absorption spectroscopy reveals that the CoFe MOF(NA) prolongs charge recombination lifetime by blocking the hole-transfer pathway from the BiVO4 to its surface trap states. This work sheds light on boosting charge separation and transfer through structural engineering to enhance the photocurrent of photoanodes for solar H2 production.

Ultrasonic passivated hematite photoanode with efficient hole transfer pathway for enhanced photoelectrochemical water oxidation

Ultrasonic passivated hematite photoanode with efficient hole transfer pathway for enhanced photoelectrochemical water oxidation

Reaction Pathway of Photocatalytic Water Splitting on Two-Dimensional TiO2 Nanosheets

Using density functional theory and many-body Green’s function theory, we examine the mechanism of the water splitting reaction photocatalyzed by two-dimensional TiO2 nanosheets, which has gained attention recently in the TiO2 community. We reveal the barriers and the hole-transfer pathway in the reaction. Compared with its bulk isomer phases, such as the rutile TiO2, the advantage of the TiO2 nanosheet in this reaction is that the key reaction intermediate Ti-OH has good hole-trapping capacity, which is extremely beneficial for the oxidation half-reaction. However, the poor stability of Ti-OH relative to its precursor is a negative factor for the reaction on the TiO2 nanosheet and might have a huge effect on the photocatalytic efficiency of the nanosheet. We find that introducing Ti vacancies into the nanosheet to convert the TiO2 material into a p-type semiconductor can make bridging oxygen atoms become favorable active sites for water splitting and hence may be an effective way to increase the photocatalytic efficiency.

Efficient Hole Transfer via CsPbBr3 Quantum Dots Doping toward High‐Performance Organic Solar Cells

Among the emerging photovoltaic technologies, organic and perovskite quantum dots (PQDs) solar cells have thrived on low‐cost processing and extraordinary optoelectronic properties. Herein, CsPbBr3 PQDs are incorporated into PM6:Y6‐BO organic solar cell (OSC) to enhance device efficiency without scarifying the device stability. While the incorporation of PQDs has no impact on the molecular packing and phase separation of organic semiconductors, their presence enhances light absorption due to the Rayleigh scattering effect, promotes exciton dissociation in the Y6‐BO phase, and forms an efficient hole transfer pathway from Y6‐BO to PQDs and then to PM6 to improve hole transport. These contribute to increased short‐circuit current density (JSC) and fill factor (FF) of OSCs with constant VOC. With the presence of 1 wt% CsPbBr3 PQDs doping, the highest power conversion efficiency (PCE) of the corresponding PM6:Y6‐BO OSC is improved from 16.4% to 17.1%, where the device stability has not been affected due to the better phase stability of CsPbBr3 PQDs than CsPbI3 PQDs. This work unravels a new approach to enhance the efficiency of OSCs by applying PQDs doping to manipulate the photon‐to‐electricity conversion process.

An interesting possibility of forming special hole stepping stones with high-stacking aromatic rings in proteins: three-π five-electron and four-π seven-electron resonance bindings.

Long-range hole transfer of proteins plays an important role in many biological processes of living organisms. Therefore, it is highly useful to examine the possible hole stepping stones, which can facilitate hole transfer in proteins. However, the structures of stepping stones are diverse because of the complexity of the protein structures. In the present work, we proposed a series of special stepping stones, which are instantaneously formed by three and four packing aromatic side chains of amino acids to capture a hole, corresponding to three-π five-electron (π:π∴π↔π∴π:π) and four-π seven-electron (π:π∴π:π↔π:π:π∴π) resonance bindings with appropriate binding energies. The aromatic amino acids include histidine (His), phenylalanine (Phe), tyrosine (Tyr) and tryptophan (Trp). The formations of these special stepping stones can effectively reduce the local ionization potential of the high π-stacking region to efficiently capture the migration hole. The quick formations and separations of them promote the efficient hole transfer in proteins. More interestingly, we revealed that a hole cannot delocalize over infinite aromatic rings along the high π–π packing structure at the same time and the micro-surroundings of proteins can modulate the formations of π:π∴π↔π∴π:π and π:π∴π:π↔π:π:π∴π bindings. These results may contribute a new avenue to better understand the potential hole transfer pathway in proteins.

Uncovering the Role of Hole Traps in Promoting Hole Transfer from Multiexcitonic Quantum Dots to Molecular Acceptors.

Understanding electronic dynamics in multiexcitonic quantum dots (QDs) is important for designing efficient systems useful in high power scenarios, such as solar concentrators and multielectron charge transfer. The multiple charge carriers within a QD can undergo undesired Auger recombination events, which rapidly annihilate carriers on picosecond time scales and generate heat from absorbed photons instead of useful work. Compared to the transfer of multiple electrons, the transfer of multiple holes has proven to be more difficult due to slower hole transfer rates. To probe the competition between Auger recombination and hole transfer in CdSe, CdS, and CdSe/CdS QDs of varying sizes, we synthesized a phenothiazine derivative with optimized functionalities for binding to QDs as a hole accepting ligand and for spectroscopic observation of hole transfer. Transient absorption spectroscopy was used to monitor the photoinduced absorption features from both trapped holes and oxidized ligands under excitation fluences where the averaged initial number of excitons in a QD ranged from ∼1 to 19. We observed fluence-dependent hole transfer kinetics that last around 100 ps longer than the predicted Auger recombination lifetimes, and the transfer of up to 3 holes per QD. Theoretical modeling of the kinetics suggests that binding of hole acceptors introduces trapping states significantly different from those in native QDs passivated with oleate ligands. Holes in these modified trap states have prolonged lifetimes, which promotes the hole transfer efficiency. These results highlight the beneficial role of hole-trapping states in devising hole transfer pathways in QD-based systems under multiexcitonic conditions.

Anomalous Transition of Hole Transfer Pathways in Gold Nanocluster-Sensitized TiO2 Photoelectrodes

Metal nanoclusters (NCs) are a recent addition to the family of traditional photosensitizers (e.g., dyes and quantum dots) for TiO2-based photoelectrodes. Despite the resemblance between NC-based and traditional sensitized photoelectrodes in their design architecture, recent studies have revealed that the photoelectrochemical behavior of NC–TiO2 photoelectrodes differs from that of their traditional counterparts. Here, we use impedance spectroscopy to shed light on another different behavior of NC–TiO2 photoelectrodes. Unlike dye- or quantum-dot-sensitized TiO2 photoanodes, in which surface states do not participate in the hole transfer process, hole transfer can occur through surface trap states and depends on the NC size. This unprecedented transition in hole transfer pathways is anomalous because it occurs even with the use of an efficient hole scavenger that typically activates direct hole transfer from a photoelectrode to an electrolyte. Along with evidence, the implications of this behavior on photoelectrochemical performance are discussed.

One Way Traffic: Base-to-Backbone Hole Transfer in Nucleoside Phosphorodithioate.

The directionality of the hole-transfer processes between DNA backbone and base was investigated by using phosphorodithioate [P(S- )=S] components. ESR spectroscopy in homogeneous frozen aqueous solutions and pulse radiolysis in aqueous solution at ambient temperature confirmed initial formation of G.+ -P(S- )=S. The ionization potential of G-P(S- )=S was calculated to be slightly lower than that of guanine in 5'-dGMP. Subsequent thermally activated hole transfer from G.+ to P(S- )=S led to dithiyl radical (P-2S. ) formation on the μs timescale. In parallel, ESR spectroscopy, pulse radiolysis, and density functional theory (DFT) calculations confirmed P-2S. formation in an abasic phosphorodithioate model compound. ESR investigations at low temperatures and higher G-P(S- )=S concentrations showed a bimolecular conversion of P-2S. to the σ2 -σ*1 -bonded dimer anion radical [-P-2S 2S-P-]- [ΔG (150 K, DFT)=-7.2 kcal mol-1 ]. However, [-P-2S 2S-P-]- formation was not observed by pulse radiolysis [ΔG° (298 K, DFT)=-1.4 kcal mol-1 ]. Neither P-2S. nor [-P-2S 2S-P-]- oxidized guanine base; only base-to-backbone hole transfer occurs in phosphorodithioate.

An efficient hole transfer pathway on hematite integrated by ultrathin Al2O3 interlayer and novel CuCoOx cocatalyst for efficient photoelectrochemical water oxidation

Highly photoelectrochemical water oxidation efficiency could be realized by means of efficient photogenerated holes transfer pathway and low-cost cocatalysts. Here, we have integrated an ultrathin Al2O3 layer and novel copper-cobalt oxide (CuCoOx) cocatalyst on α-Fe2O3 photoanode to construct unobstructed holes transfer pathway and improve PEC activity. The resultant α-Fe2O3/Al2O3/CuCoOx photoanode exhibits high photocurrent density of 2.23 mA/cm2 at 1.23 V versus the reversible hydrogen electrode (VRHE) and negative onset potential of 0.7 VRHE, which is about 17.6 times higher and 200 mV cathodic shift than bare α-Fe2O3 nanorod arrays. Detailed experimental results indicate the surface recombination passivation function of Al2O3 layer and improvement in charge separation and transfer kinetics of the role of low overpotential of CuCoOx with abundant oxygen vacancies for OER. The comparison discussion with CoOx and CuOx oxygen evolution catalysts (OECs) further illustrates the excellent catalytic performance of the CuCoOx. This work points to the effective utilization of novel but cheap CuCoOx OECs on photoanodes and provides a strategy for designing high activity photoanodes in photoelectrochemical water oxidation.

Competition of Dexter, Förster, and charge transfer pathways for quantum dot sensitized triplet generation.

Quantum dot (QD) sensitized triplet exciton generation has demonstrated promising applications in various fields such as photon up-conversion through triplet-triplet annihilation. However, how direct triplet energy transfer from the QD to the acceptor through Dexter energy transfer (DET) competes with other processes, including Förster resonance energy transfer (FRET) and charge transfer, remains poorly understood. Herein, the competition of these pathways for QD-sensitized triplet excited state generation in CdSe QD-modified boron dipyrromethene (BODIPY) complexes is studied using transient absorption spectroscopy. After excitation of the CdSe QD with 500 nm pulses, the BODIPY triplet excited state is generated through charge recombination in a charge separated intermediate state (QD-·-BODIPY+·). This intermediate state is populated either through FRET from the excited QD to BODIPY followed by electron transfer from the singlet excited state of BODIPY to the QD or through hole transfer from the excited QD to BODIPY. The triplet excited state generation efficiencies from the FRET and hole transfer pathways are estimated to be (6.18 ± 1.39)% and (13.5 ± 3.1)%, respectively. Compared to these indirect pathways, direct DET from the QD to the BODIPY triplet state is kinetically not competitive. These results demonstrate that sequential charge transfer can be an efficient pathway for triplet excited state generation in QD-acceptor complexes.

Efficient Charge Generation via Hole Transfer in Dilute Organic Donor-Fullerene Blends.

Efficient organic photovoltaics (OPVs) require broadband charge photogeneration with near-unity quantum yield. This can only be achieved by exploiting all pathways that generate charge. Electron transfer from organic donors to acceptors has been well-studied and is considered the primary path to charge photogeneration in OPVs. In contrast, much less is known about the hole transfer pathway. Here we study charge photogeneration in an archetypal system comprising tetraphenyldibenzoperiflanthene:C70 blends using our recently developed multispectral two-dimensional electronic spectroscopy (M-2DES), supported by time-dependent density functional theory and fully quantum-mechanical Fermi's golden rule rate calculations. Our approach identifies in real time two rapid charge transfer pathways that are confirmed through computational analysis. Surprisingly, we find that both electron and hole transfer occur with comparable rates and efficiencies, facilitated by donor-acceptor electronic interactions. Our results highlight the importance of the hole transfer pathway for optimizing the efficiency of OPV devices employing small-molecule heterojunctions.

EMap: A Web Application for Identifying and Visualizing Electron or Hole Hopping Pathways in Proteins

eMap is a web-based platform for identifying and visualizing electron or hole transfer pathways in proteins based on their crystal structures. The underlying model can be viewed as a coarse-grained version of the Pathways model, where each tunneling step between hopping sites represented by electron transfer active (ETA) moieties is described with one effective decay parameter that describes protein-mediated tunneling. ETA moieties include aromatic amino acid residue side chains and aromatic fragments of cofactors that are automatically detected, and, in addition, electron/hole residing sites that can be specified by the users. The software searches for the shortest paths connecting the user-specified electron/hole source to either all surface-exposed ETA residues or the user-specified target. The identified pathways are ranked according to their effective length. The pathways are visualized in 2D as a graph, in which each node represents an ETA site, and in 3D using available protein visualization tools. Here, we present the capability and user interface of eMap 1.0, which is available at https://emap.bu.edu .

Synergistic effect of silane and graphene oxide for enhancing the photoelectrochemical water oxidation performance of WO3NS arrays

Photoelectrochemical (PEC) water splitting using semiconductor-based photoelectrodes is a promising approach for renewable solar fuel production. One of the biggest challenge is that semiconductor electrodes suffer from lower utilization of photogenerated-carriers and sluggish surface water oxidation reaction. Herein, we report that a novel composite photoelectrode based on silane molecule/graphene oxide being modified on the surface of well-aligned WO3 nanosheet arrays (WO3 NS/Silane/GO), efficiently harvests holes from the semiconductor and provides more reactive surface for water oxidation owing to the synergistic effect. The results demonstrate that 1.8 times photocurrent enhancement of WO3 NS/Silane/GO electrode (1.25 mA cm−2 at 1.23 V vs RHE) can be obtained compared to that of the pristine WO3 NS (0.69 mA cm−2) electrode, and it is even much higher than the photocurrent sum of WO3 NS/Silane and WO3 NS/GO electrodes. The remarkably enhanced PEC performance mainly stems from the induced external electric field (EEF) created by silane acting as hole-storage spots and the efficient carrier migration channel provided by GO, dramatically suppress the recombination of photogenerated carriers. As a result, the synergistic effect of silane and GO leads to a steady and continuous hole-transfer pathway, and promotes water oxidation kinetic by the increased reactive site quantity and activity. This study indicates that it can be a general design strategy for the preparation of efficient PEC electrodes in solar water splitting.

In situ fabrication of nanoporous BiVO4/Bi2S3 nanosheets for enhanced photoelectrochemical water splitting

Nanoporous BiVO4/Bi2S3 nanosheets have been firstly fabricated through a two-step annealing followed by hydrothermal method using BiOI two-dimensional nanosheets as precursors, which could greatly improve the photoelectrochemical performances of BiVO4 due to the enhanced charge separation efficiency and the enlarged absorption range. The optimal content of Bi2S3 in the BiVO4/Bi2S3 nanocomposites has been explored by tuning the reaction time. In addition, it is found that loading co-catalysts FeOOH could further optimize the hole transfer pathway and thus enhance the photoelectrochemical water splitting ability.

Cluster-Dependent Charge-Transfer Dynamics in Iron-Sulfur Proteins.

Photoinduced charge-transfer dynamics and the influence of cluster size on the dynamics were investigated using five iron-sulfur clusters: the 1Fe-4S cluster in Pyrococcus furiosus rubredoxin, the 2Fe-2S cluster in Pseudomonas putida putidaredoxin, the 4Fe-4S cluster in nitrogenase iron protein, and the 8Fe-7S P-cluster and the 7Fe-9S-1Mo FeMo cofactor in nitrogenase MoFe protein. Laser excitation promotes the iron-sulfur clusters to excited electronic states that relax to lower states. The electronic relaxation lifetimes of the 1Fe-4S, 8Fe-7S, and 7Fe-9S-1Mo clusters are on the picosecond time scale, although the dynamics of the MoFe protein is a mixture of the dynamics of the latter two clusters. The lifetimes of the 2Fe-2S and 4Fe-4S clusters, however, extend to several nanoseconds. A competition between reorganization energies and the density of electronic states (thus electronic coupling between states) mediates the charge-transfer lifetimes, with the 2Fe-2S cluster of Pdx and the 4Fe-4S cluster of Fe protein lying at the optimum leading to them having significantly longer lifetimes. Their long lifetimes make them the optimal candidates for long-range electron transfer and as external photosensitizers for other photoactivated chemical reactions like solar hydrogen production. Potential electron-transfer and hole-transfer pathways that possibly facilitate these charge transfers are proposed.

Ultrafast Charge-Transfer Dynamics in the Iron-Sulfur Complex of Rhodobacter capsulatus Ferredoxin VI.

Iron-sulfur proteins play essential roles in various biological processes. Their electronic structure and vibrational dynamics are key to their rich chemistry but nontrivial to unravel. Here, the first ultrafast transient absorption and impulsive coherent vibrational spectroscopic (ICVS) studies on 2Fe-2S clusters in Rhodobacter capsulatus ferreodoxin VI are characterized. Photoexcitation initiated populations on multiple excited electronic states that evolve into each other in a long-lived charge-transfer state. This suggests a potential light-induced electron-transfer pathway as well as the possibility of using iron-sulfur proteins as photosensitizers for light-dependent enzymes. A tyrosine chain near the active site suggests potential hole-transfer pathways and affirms this electron-transfer pathway. The ICVS data revealed vibrational bands at 417 and 484 cm-1, with the latter attributed to an excited-state mode. The temperature dependence of the ICVS modes suggests that the temperature effect on protein structure or conformational heterogeneities needs to be considered during cryogenic temperature studies.

Catalytic routes and oxidation mechanisms in photoreforming of polyols

Photocatalytic reforming of biomass-derived oxygenates leads to H2 generation and evolution of CO2via parallel formation of organic intermediates through anodic oxidations on a Rh/TiO2 photocatalyst. The reaction pathways and kinetics in the photoreforming of C3–C6 polyols were explored. Polyols are converted via direct and indirect hole transfer pathways resulting in (i) oxidative rupture of C–C bonds, (ii) oxidation to α-oxygen functionalized aldoses and ketoses (carbonyl group formation) and (iii) light-driven dehydration. Direct hole transfer to chemisorbed oxygenates on terminal Ti(IV)-OH groups, generating alkoxy-radicals that undergo ß-C–C-cleavage, is proposed for the oxidative C–C rupture. Carbonyl group formation and dehydration are attributed to indirect hole transfer at surface lattice oxygen sites [Ti⋯O⋯Ti] followed by the generation of carbon-centered radicals. Polyol chain length impacts the contribution of the oxidation mechanisms favoring the C–C bond cleavage (internal preferred over terminal) as the dominant pathway with higher polyol carbon number.