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1.
Artigo em Inglês | MEDLINE | ID: mdl-32810402

RESUMO

Built-in electric fields at semiconductor junctions are vital for optoelectronic and photocatalytic applications since they govern the movement of photogenerated charge carriers near critical surfaces and interfaces. Here, we exploit transient photoreflectance (TPR) spectroscopy to probe the dynamical evolution of the built-in field for n-GaAs photoelectrodes upon photoexcitation. The transient fields are modeled in order to quantitatively describe the surface carrier dynamics that influence those fields. The photoinduced surface field at different types of junctions between n-GaAs and n-TiO2, Pt, electrolyte and p-NiO are examined, and the results reveal that surface Fermi-level pinning, ubiquitous for many GaAs surfaces, can have beneficial consequences that impact photoelectrochemical applications. That is, Fermi-level pinning results in the primary surface carrier dynamics being invariant to the contacting layer and promotes beneficial carrier separation. For example, when p-NiO is deposited there is no Fermi-level equilibration that modifies the surface field, but photogenerated holes are promoted to the n-GaAs/p-NiO interface and can transfer into defect midgap states within the p-NiO resulting in an elongated charge separation time and those transferred holes can participate in chemical reactions. In contrast, when the Fermi-level is unpinned via molecular surface functionalization on p-GaAs, the carriers undergo surface recombination faster due to a smaller built-in field, thus potentially degrading their photochemical performance.

2.
Sci Adv ; 5(12): eaay0571, 2019 12.
Artigo em Inglês | MEDLINE | ID: mdl-31840072

RESUMO

Chiral-induced spin selectivity (CISS) occurs when the chirality of the transporting medium selects one of the two spin ½ states to transport through the media while blocking the other. Monolayers of chiral organic molecules demonstrate CISS but are limited in their efficiency and utility by the requirement of a monolayer to preserve the spin selectivity. We demonstrate CISS in a system that integrates an inorganic framework with a chiral organic sublattice inducing chirality to the hybrid system. Using magnetic conductive-probe atomic force microscopy, we find that oriented chiral 2D-layered Pb-iodide organic/inorganic hybrid perovskite systems exhibit CISS. Electron transport through the perovskite films depends on the magnetization of the probe tip and the handedness of the chiral molecule. The films achieve a highest spin-polarization transport of up to 86%. Magnetoresistance studies in modified spin-valve devices having only one ferromagnet electrode confirm the occurrence of spin-dependent charge transport through the organic/inorganic layers.

3.
J Chem Phys ; 151(20): 204305, 2019 Nov 28.
Artigo em Inglês | MEDLINE | ID: mdl-31779317

RESUMO

Quantitative colloidal ligand exchange on lead-halide perovskite nanocrystals (NCs) has remained a challenge due to the dynamic passivation of amines and carboxylic acids and the instability of core lead-halide perovskite systems. Here, we present a facile colloidal ligand exchange process using cinnamate acid ligands to quantitatively displace native oleate ligands on CsPbBr3 NCs. The short cinnamate ligands lead to a 23-fold enhancement of the electron-donating ability of the CsPbBr3 NCs when benzoquinone is used as an electron acceptor. A significantly increased photoredox activity is also observed in a complete photocatalytic reaction: the α-alkylation of aldehydes. Our results provide a new strategy to tune the photoredox activity of halide perovskite NCs as well as the exploration of NC-ligand interactions.

4.
Nat Commun ; 10(1): 4498, 2019 10 03.
Artigo em Inglês | MEDLINE | ID: mdl-31582749

RESUMO

Developing multijunction perovskite solar cells (PSCs) is an attractive route to boost PSC efficiencies to above the single-junction Shockley-Queisser limit. However, commonly used tin-based narrow-bandgap perovskites have shorter carrier diffusion lengths and lower absorption coefficient than lead-based perovskites, limiting the efficiency of perovskite-perovskite tandem solar cells. In this work, we discover that the charge collection efficiency in tin-based PSCs is limited by a short diffusion length of electrons. Adding 0.03 molar percent of cadmium ions into tin-perovskite precursors reduce the background free hole concentration and electron trap density, yielding a long electron diffusion length of 2.72 ± 0.15 µm. It increases the optimized thickness of narrow-bandgap perovskite films to 1000 nm, yielding exceptional stabilized efficiencies of 20.2 and 22.7% for single junction narrow-bandgap PSCs and monolithic perovskite-perovskite tandem cells, respectively. This work provides a promising method to enhance the optoelectronic properties of narrow-bandgap perovskites and unleash the potential of perovskite-perovskite tandem solar cells.

5.
Phys Chem Chem Phys ; 21(30): 16399-16407, 2019 Aug 14.
Artigo em Inglês | MEDLINE | ID: mdl-31298240

RESUMO

Ultrafast carrier dynamics at interfaces plays a major role in governing solar energy conversion efficiency in solar conversion systems, especially in solar cells and photoelectrochemical devices. In this perspective, we discuss recent advances in ultrafast spectroscopic probes to understand the dynamics at such interfaces. Transient reflectance (TR) enables monitoring of the surface carriers that are within ∼10 nm of the surface or interface of interest. Crucial parameters such as surface recombination velocity and carrier diffusion can be determined. When a strong surface or interfacial field is present the reflectance spectrum will exhibit Franz-Kelydesh oscillations. Monitoring the transient formation or decay of such oscillations informs about the interfacial field and is referred to as transient photoreflectance (TPR). Such fields are helpful in designing systems in order to move carriers away from surfaces where photocorrosion can occur and towards catalytic surfaces. Finally, we discuss transient infrared attenuated total reflection (TATR) to probe the ultrafast reaction intermediates in a photo-driven chemical reaction.

6.
Nat Commun ; 10(1): 2842, 2019 Jun 28.
Artigo em Inglês | MEDLINE | ID: mdl-31253800

RESUMO

Metal halide perovskite semiconductors possess outstanding characteristics for optoelectronic applications including but not limited to photovoltaics. Low-dimensional and nanostructured motifs impart added functionality which can be exploited further. Moreover, wider cation composition tunability and tunable surface ligand properties of colloidal quantum dot (QD) perovskites now enable unprecedented device architectures which differ from thin-film perovskites fabricated from solvated molecular precursors. Here, using layer-by-layer deposition of perovskite QDs, we demonstrate solar cells with abrupt compositional changes throughout the perovskite film. We utilize this ability to abruptly control composition to create an internal heterojunction that facilitates charge separation at the internal interface leading to improved photocarrier harvesting. We show how the photovoltaic performance depends upon the heterojunction position, as well as the composition of each component, and we describe an architecture that greatly improves the performance of perovskite QD photovoltaics.

7.
Angew Chem Int Ed Engl ; 58(34): 11737-11741, 2019 Aug 19.
Artigo em Inglês | MEDLINE | ID: mdl-31218795

RESUMO

Organic-inorganic hybrid two-dimensional (2D) perovskites (n≤5) have recently attracted significant attention because of their promising stability and optoelectronic properties. Normally, 2D perovskites contain a monocation [e.g., methylammonium (MA+ ) or formamidinium (FA+ )]. Reported here for the first time is the fabrication of 2D perovskites (n=5) with mixed cations of MA+ , FA+ , and cesium (Cs+ ). The use of these triple cations leads to the formation of a smooth, compact surface morphology with larger grain size and fewer grain boundaries compared to the conventional MA-based counterpart. The resulting perovskite also exhibits longer carrier lifetime and higher conductivity in triple cation 2D perovskite solar cells (PSCs). The power conversion efficiency (PCE) of 2D PSCs with triple cations was enhanced by more than 80 % (from 7.80 to 14.23 %) compared to PSCs fabricated with a monocation. The PCE is also higher than that of PSCs based on binary cation (MA+ -FA+ or MA+ -Cs+ ) 2D structures.

8.
Science ; 364(6439): 475-479, 2019 05 03.
Artigo em Inglês | MEDLINE | ID: mdl-31000592

RESUMO

All-perovskite-based polycrystalline thin-film tandem solar cells have the potential to deliver efficiencies of >30%. However, the performance of all-perovskite-based tandem devices has been limited by the lack of high-efficiency, low-band gap tin-lead (Sn-Pb) mixed-perovskite solar cells (PSCs). We found that the addition of guanidinium thiocyanate (GuaSCN) resulted in marked improvements in the structural and optoelectronic properties of Sn-Pb mixed, low-band gap (~1.25 electron volt) perovskite films. The films have defect densities that are lower by a factor of 10, leading to carrier lifetimes of greater than 1 microsecond and diffusion lengths of 2.5 micrometers. These improved properties enable our demonstration of >20% efficient low-band gap PSCs. When combined with wider-band gap PSCs, we achieve 25% efficient four-terminal and 23.1% efficient two-terminal all-perovskite-based polycrystalline thin-film tandem solar cells.

9.
J Am Chem Soc ; 141(12): 4919-4927, 2019 Mar 27.
Artigo em Inglês | MEDLINE | ID: mdl-30821456

RESUMO

The marriage of colloidal semiconductor nanocrystals and functional organic molecules has brought unique opportunities in emerging photonic and optoelectronic applications. Traditional semiconductor nanocrystals have been widely demonstrated to initiate efficient triplet energy transfer at the nanocrystal-acene interface. Herein, we report that unlike conventional semiconductor nanocrystals, lead halide perovskite nanocrystals promote an efficient Dexter-like singlet energy transfer to surface-anchored pentacene molecules rather than triplet energy transfer. Subsequently, molecular pentacene triplets are efficiently generated via singlet fission on the nanocrystal surface. Our demonstrated strategy not only unveils the obscure energy dynamics between perovskite nanocrystal and acenes, but also brings important perspectives of utilizing singlet fission throughout the solar spectrum.

10.
Artigo em Inglês | MEDLINE | ID: mdl-30899758

RESUMO

Autotrophic hydrogenotrophic methanogens use H2/CO2 as sole carbon and energy source. In contrast to H2, CO2 is present in high concentrations in environments dominated by methanogens e.g., anaerobic digesters (AD), and is therefore rarely considered to be a limiting factor. Nonetheless, potential CO2 limitation can be relevant in the process of biomethanation, a power-to-gas technology, where biogas is upgraded by the addition of H2 and ideally reduce the CO2 concentration in the produced biogas to 0-6%. H2 is effectively utilized by methanogens even at very low concentrations, but little is known about the impact of low CO2 concentrations on methanogenic activity. In this study, CO2 consumption and CH4 production kinetics under low CO2 concentrations were studied, using a hydrogenotrophic methanogen, Methanobacterium congolense, as model organism. We found that both cellular growth and methane production were limited at low CO2 concentrations (here expressed as Dissolved Inorganic Carbon, DIC). Maximum rates (V max) were reached at [DIC] of 100 mM (extrapolated), with a CO2 consumption rate of 69.2 fmol cell-1 d-1 and a CH4 production rate of 48.8 fmol cell-1 d-1. In our experimental setup, 80% of V max was achieved at [DIC] >9 mM. DIC half-saturation concentrations (K m) was about 2.5 mM for CO2 consumption and 2.2 mM for CH4 production. No CH4 production could be detected below 44.4 µM [DIC]. These data revealed that the limiting concentration of DIC may be much higher than that of H2 for a hydrogenotrophic methanogen. However, DIC is not a limiting factor in ADs running under standard operating conditions. For biomethanation, the results are applicable for both in situ and ex situ biomethanation reactors and show that biogas can be upgraded to concentrations of 2% CO2 (98% CH4) while still retaining 80% V max at pH 7.5 evaluated from M. congolense. Since DIC concentration can vary significantly with pH and pCO2 during biomethanation, monitoring DIC concentration through pH and pCO2 is therefore important for keeping optimal operational conditions for the biomethanation process.

11.
J Am Chem Soc ; 141(14): 5972-5979, 2019 Apr 10.
Artigo em Inglês | MEDLINE | ID: mdl-30882210

RESUMO

Organic-inorganic halide perovskites incorporating two-dimensional (2D) structures have shown promise for enhancing the stability of perovskite solar cells (PSCs). However, the bulky spacer cations often limit charge transport. Here, we report on a simple approach based on molecular design of the organic spacer to improve the transport properties of 2D perovskites, and we use phenethylammonium (PEA) as an example. We demonstrate that by fluorine substitution on the para position in PEA to form 4-fluorophenethylammonium (F-PEA), the average phenyl ring centroid-centroid distances in the organic layer become shorter with better aligned stacking of perovskite sheets. The impact is enhanced orbital interactions and charge transport across adjacent inorganic layers as well as increased carrier lifetime and reduced trap density. Using a simple perovskite deposition at room temperature without using any additives, we obtained a power conversion efficiency of >13% for (F-PEA)2MA4Pb5I16-based PSCs. In addition, the thermal stability of 2D PSCs based on F-PEA is significantly enhanced compared to those based on PEA.

12.
J Am Chem Soc ; 140(42): 13753-13763, 2018 Oct 24.
Artigo em Inglês | MEDLINE | ID: mdl-30255707

RESUMO

We developed a postsynthetic treatment to produce impurity n-type doped PbSe QDs with In3+ as the substitutional dopant. Increasing the incorporated In content is accompanied by a gradual bleaching of the interband first-exciton transition and concurrently the appearance of a size-dependent, intraband absorption, suggesting the controlled introduction of delocalized electrons into the QD band edge states under equilibrium conditions. We compare the optical properties of our In-doped PbSe QDs to cobaltocene treated QDs, where the n-type dopant arises from remote reduction of the PbSe QDs and observe similar behavior. Spectroelectrochemical measurements also demonstrate characteristic n-type signatures, including both an induced absorption within the electrochemical bandgap and a shift of the Fermi-level toward the conduction band. Finally, we demonstrate that the In3+ dopants can be reversibly removed from the PbSe QDs, whereupon the first exciton bleach is recovered. Our results demonstrate that PbSe QDs can be controllably n-type doped via impurity aliovalent substitutional doping.

13.
J Phys Chem Lett ; 9(10): 2595-2603, 2018 May 17.
Artigo em Inglês | MEDLINE | ID: mdl-29714488

RESUMO

The exciton binding energy in methylammonium lead iodide (MAPbI3) is about 10 meV, around 1/3 of the available thermal energy ( kB T ∼ 26 meV) at room temperature. Thus, exciton populations are not stable at room temperature at moderate photoexcited carrier densities. However, excitonic resonances dominate the absorption onset. Furthermore, these resonances determine the transient absorbance and transient reflectance spectra. The exciton binding energy is a reflection of the Coulomb interaction energy between photoexcited electrons and holes. As such, it serves as a marker for the strength of electron/hole interactions and impacts a variety of phenomena, such as, absorption, radiative recombination, and Auger recombination. In this Perspective, we discuss the role of excitons and excitonic resonances in the optical properties of lead-halide perovskite semiconductors. Finally, we discuss how the strong light-matter interactions induce an optical stark effect splitting the doubly spin degenerate ground exciton states and are easily observed at room temperature.

14.
Appl Environ Microbiol ; 83(23)2017 Dec 01.
Artigo em Inglês | MEDLINE | ID: mdl-28939599

RESUMO

Most sulfate-reducing microorganisms (SRMs) present in subsurface marine sediments belong to uncultured groups only distantly related to known SRMs, and it remains unclear how changing geochemical zones and sediment depth influence their community structure. We mapped the community composition and abundance of SRMs by amplicon sequencing and quantifying the dsrB gene, which encodes dissimilatory sulfite reductase subunit beta, in sediment samples covering different vertical geochemical zones ranging from the surface sediment to the deep sulfate-depleted subsurface at four locations in Aarhus Bay, Denmark. SRMs were present in all geochemical zones, including sulfate-depleted methanogenic sediment. The biggest shift in SRM community composition and abundance occurred across the transition from bioturbated surface sediments to nonbioturbated sediments below, where redox fluctuations and the input of fresh organic matter due to macrofaunal activity are absent. SRM abundance correlated with sulfate reduction rates determined for the same sediments. Sulfate availability showed a weaker correlation with SRM abundances and no significant correlation with the composition of the SRM community. The overall SRM species diversity decreased with depth, yet we identified a subset of highly abundant community members that persists across all vertical geochemical zones of all stations. We conclude that subsurface SRM communities assemble by the persistence of members of the surface community and that the transition from the bioturbated surface sediment to the unmixed sediment below is a main site of assembly of the subsurface SRM community.IMPORTANCE Sulfate-reducing microorganisms (SRMs) are key players in the marine carbon and sulfur cycles, especially in coastal sediments, yet little is understood about the environmental factors controlling their depth distribution. Our results suggest that macrofaunal activity is a key driver of SRM abundance and community structure in marine sediments and that a small subset of SRM species of high relative abundance in the subsurface SRM community persists from the sulfate-rich surface sediment to sulfate-depleted methanogenic subsurface sediment. More generally, we conclude that SRM communities inhabiting the subsurface seabed assemble by the selective survival of members of the surface community.


Assuntos
Bactérias/isolamento & purificação , Bactérias/metabolismo , Sedimentos Geológicos/microbiologia , Sulfatos/metabolismo , Bactérias/classificação , Bactérias/genética , Baías/microbiologia , DNA Bacteriano/genética , Dinamarca , Oxirredução , Filogenia , RNA Ribossômico 16S/genética , Água do Mar/microbiologia
15.
Sci Rep ; 7(1): 2400, 2017 05 25.
Artigo em Inglês | MEDLINE | ID: mdl-28546547

RESUMO

The factors controlling the relative abundances of Archaea and Bacteria in marine sediments are poorly understood. We determined depth distributions of archaeal and bacterial 16S rRNA genes by quantitative PCR at eight stations in Aarhus Bay, Denmark. Bacterial outnumber archaeal genes 10-60-fold in uppermost sediments that are irrigated and mixed by macrofauna. This bioturbation is indicated by visual observations of sediment color and faunal tracks, by porewater profiles of dissolved inorganic carbon and sulfate, and by distributions of unsupported 210Pb and 137Cs. Below the depth of bioturbation, the relative abundances of archaeal genes increase, accounting for one third of 16S rRNA genes in the sulfate zone, and half of 16S rRNA genes in the sulfate-methane transition zone and methane zone. Phylogenetic analyses reveal a strong shift in bacterial and archaeal community structure from bioturbated sediments to underlying layers. Stable isotopic analyses on organic matter and porewater geochemical gradients suggest that macrofauna mediate bacterial dominance and affect microbial community structure in bioturbated sediment by introducing fresh organic matter and high-energy electron acceptors from overlying seawater. Below the zone of bioturbation, organic matter content and the presence of sulfate exert key influences on bacterial and archaeal abundances and overall microbial community structure.


Assuntos
Archaea/classificação , Bactérias/classificação , Biodiversidade , Sedimentos Geológicos/microbiologia , DNA Arqueal , DNA Bacteriano , Filogenia , Filogeografia , RNA Ribossômico 16S/genética
16.
Proc Natl Acad Sci U S A ; 114(11): 2940-2945, 2017 03 14.
Artigo em Inglês | MEDLINE | ID: mdl-28242677

RESUMO

Bacterial and archaeal communities inhabiting the subsurface seabed live under strong energy limitation and have growth rates that are orders of magnitude slower than laboratory-grown cultures. It is not understood how subsurface microbial communities are assembled and whether populations undergo adaptive evolution or accumulate mutations as a result of impaired DNA repair under such energy-limited conditions. Here we use amplicon sequencing to explore changes of microbial communities during burial and isolation from the surface to the >5,000-y-old subsurface of marine sediment and identify a small core set of mostly uncultured bacteria and archaea that is present throughout the sediment column. These persisting populations constitute a small fraction of the entire community at the surface but become predominant in the subsurface. We followed patterns of genome diversity with depth in four dominant lineages of the persisting populations by mapping metagenomic sequence reads onto single-cell genomes. Nucleotide sequence diversity was uniformly low and did not change with age and depth of the sediment. Likewise, there was no detectable change in mutation rates and efficacy of selection. Our results indicate that subsurface microbial communities predominantly assemble by selective survival of taxa able to persist under extreme energy limitation.


Assuntos
Evolução Biológica , Sedimentos Geológicos/microbiologia , Metagenômica , Microbiota , Archaea/classificação , Archaea/genética , Bactérias/classificação , Bactérias/genética , Biodiversidade , Biomassa , Variação Genética , Metagenômica/métodos , Mutação
17.
J Am Chem Soc ; 139(5): 1830-1841, 2017 02 08.
Artigo em Inglês | MEDLINE | ID: mdl-27997159

RESUMO

The initial step of photocatalytic water oxidation reaction at the metal oxide/aqueous interface involves intermediates formed by trapping photogenerated, valence band holes on different reactive sites of the oxide surface. In SrTiO3, these one-electron intermediates are radicals located in Ti-O• (oxyl) and Ti-O•-Ti (bridge) groups arranged perpendicular and parallel to the surface respectively, and form electronic states in the band gap of SrTiO3. Using an ultrafast sub band gap probe of 400 nm and white light, we excited transitions between these radical states and the conduction band. By measuring the time evolution of surface reflectivity following the pump pulse of 266 nm light, we determined an initial radical formation time of 1.3 ± 0.2 ps, which is identical to the time to populate the surface with titanium oxyl (Ti-O•) radicals. The oxyl was separately observed by a subsurface vibration near 800 cm-1 from Ti-O located in the plane right below Ti-O•. Second, a polarized transition optical dipole allows us to assign the 1.3 ps time constant to the production of both O-site radicals. After a 4.5 ps delay, another distinct surface species forms with a time constant of 36 ± 10 ps with a yet undetermined structure. As would be expected, the radicals' decay, specifically probed by the oxyl's subsurface vibration, parallels that of the photocurrent. Our results led us to propose a nonadiabatic kinetic mechanism for generating radicals of the type Ti-O• and Ti-O•-Ti from valence band holes based on their solvation at aqueous interfaces.

18.
Nat Chem ; 8(6): 549-55, 2016 06.
Artigo em Inglês | MEDLINE | ID: mdl-27219698

RESUMO

Although the water oxidation cycle involves the critical step of O-O bond formation, the transition metal oxide radical thought to be the catalytic intermediate for this step has eluded direct observation. The radical represents the transformation of charge into a nascent catalytic intermediate, which lacks a newly formed bond and is therefore inherently difficult to detect. Here, using theoretical calculations and ultrafast in situ infrared spectroscopy of photocatalysis at an n-SrTiO3/aqueous interface, we reveal a subsurface vibration of the oxygen directly below, and uniquely generated by, the oxyl radical (Ti-O(•)). Intriguingly, this interfacial Ti-O stretch vibration, once decoupled from the lattice, couples to reactant dynamics (water librations). These experiments demonstrate subsurface vibrations and their coupling to solvent and electron dynamics to detect nascent catalytic intermediates at the solid-liquid interface at the molecular level. One can envision using the subsurface vibrations and their coupling across the interface to track and control catalysis dynamically.

19.
Biosens Bioelectron ; 63: 61-71, 2015 Jan 15.
Artigo em Inglês | MEDLINE | ID: mdl-25058940

RESUMO

Fluorescent carbon nanoparticles (F-CNPs) as a new kind of fluorescent nanoparticles, have recently attracted considerable research interest in a wide range of applications due to their low-cost and good biocompatibility. The fluorescent detection of metal ions is one of the most important applications. In this review, we first present the general detection mechanism of F-CNPs for the fluorescent detection of metal ions, including fluorescence turn-off, fluorescence turn-on, fluorescence resonance energy transfer (FRET) and ratiometric response. We then focus on the recent advances of F-CNPs in the fluorescent detection of metal ions, including Hg(2+), Cu(2+), Fe(3+), and other metal ions. Further, we discuss the research trends and future prospects of F-CNPs. We envision that more novel F-CNPs-based nanosensors with more accuracy and robustness will be widely used to assay and remove various metal ions, and there will be more practical applications in coming years.


Assuntos
Técnicas Biossensoriais , Cobre/isolamento & purificação , Íons/isolamento & purificação , Ferro/isolamento & purificação , Mercúrio/isolamento & purificação , Carbono/química , Cobre/química , Fluorescência , Transferência Ressonante de Energia de Fluorescência , Íons/química , Ferro/química , Mercúrio/química , Nanopartículas/química , Água
20.
J Am Chem Soc ; 136(30): 10632-9, 2014 Jul 30.
Artigo em Inglês | MEDLINE | ID: mdl-25029360

RESUMO

Interfacial hole transfer between n-SrTiO3 and OH(-) was investigated by surface sensitive transient optical spectroscopy of an in situ photoelectrochemical cell during water oxidation. The kinetics reveal a single rate constant with an exponential dependence on the surface hole potential, spanning time scales from 3 ns to 8 ps over a ≈1 V increase. A voltage- and laser illumination-induced process moves the valence band edge at the n-type semiconductor/water interface to continuously change the surface hole potential. This single step of the water oxidation reaction is assigned to the first hole transfer h(+) + OH(-) → OH(•). The kinetics quantify how much a change in the free energy difference driving this first hole transfer reduces the activation barrier. They are also used to extrapolate the kinetic rate due to the activation barrier when that free energy difference is zero, or the Nernstian potential. This is the first time transient spectroscopy has enabled the separation of the first hole transfer from the full four hole transfer cycle and a direct determination of these two quantities. The Nernstian potential for OH(-)/OH(•) is also suggested, in rough agreement with gas-phase studies. The observation of a distinct, much longer time scale upon picosecond hole transfer to OH(-) suggests that a dominant, more stable intermediate of the water oxidation reaction, possibly a surface bound oxo, may result.

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