Monica Barroso

Dynamics of photogenerated holes in surface modified α-Fe2O3 photoanodes for solar water splitting

This paper addresses the origin of the decrease in the external electrical bias required for water photoelectrolysis with hematite photoanodes, observed following surface treatments of such electrodes. We consider two alternative surface modifications: a cobalt oxo/hydroxo-based (CoOx) overlayer, reported previously to function as an efficient water oxidation electrocatalyst, and a Ga2O3 overlayer, reported to passivate hematite surface states. Transient absorption studies of these composite electrodes under applied bias showed that the cathodic shift of the photocurrent onset observed after each of the surface modifications is accompanied by a similar cathodic shift of the appearance of long-lived hematite photoholes, due to a retardation of electron/hole recombination. The origin of the slower electron/hole recombination is assigned primarily to enhanced electron depletion in the Fe2O3 for a given applied bias.

Activation energies for the rate-limiting step in water photooxidation by nanostructured α-Fe2O3 and TiO2.

Competition between charge recombination and the forward reactions required for water splitting limits the efficiency of metal-oxide photocatalysts. A key requirement for the photochemical oxidation of water on both nanostructured α-Fe(2)O(3) and TiO(2) is the generation of photoholes with lifetimes on the order of milliseconds to seconds. Here we use transient absorption spectroscopy to directly probe the long-lived holes on both nc-TiO(2) and α-Fe(2)O(3) in complete PEC cells, and we investigate the factors controlling this slow hole decay, which can be described as the rate-limiting step in water oxidation. In both cases this rate-limiting step is tentatively assigned to the hole transfer from the metal oxide to a surface-bound water species. We demonstrate that one reason for the slow hole transfer on α-Fe(2)O(3) is the presence of a significant thermal barrier, the magnitude of which is found to be independent of the applied bias at the potentials examined. This is in contrast to nanocrystalline nc-TiO(2), where no distinct thermal barrier to hole transfer is observed.

Charge carrier trapping, recombination and transfer in hematite (α-Fe2O3) water splitting photoanodes

Hematite is currently considered one of the most promising materials for the conversion and storage of solar energy via the photoelectrolysis of water. Whilst there has been extensive research and much progress in the development of hematite structures with enhanced photoelectrochemical (PEC) activity, relatively limited information has been available until recently concerning the dynamics of photogenerated charge carriers in hematite and their impact upon the efficiency of water photoelectrolysis. In this perspective we present an overview of our recent studies of the dynamics of photoinduced charge carrier processes in hematite, derived primarily from transient absorption spectroscopy of nanostructured photoanodes. The relationship between PEC activity and transient measurements are discussed in terms of a phenomenological model which rationalizes the observations and in particular the impact of external potential bias on the relative rates of charge carrier trapping, recombination and interfacial transfer in hematite photoanodes for water oxidation.

Correlating long-lived photogenerated hole populations with photocurrent densities in hematite water oxidation photoanodes

Photogenerated charge carrier dynamics are investigated as a function of applied bias in a variety of different hematite photoanodes for solar water oxidation. Transient absorption spectroscopy is used to probe the photogenerated holes, while transient photocurrent measures electron extraction. We report a general quantitative correlation between the population of long-lived holes and the photocurrent amplitude. The yield of long-lived holes is shown to be determined by the kinetics of electron-hole recombination. These recombination kinetics are shown to be dependent upon applied bias, exhibiting decay lifetimes ranging from ca 5 μs to 3 ms (at −0.4 and +0.4 V versus Ag/AgCl, respectively). For Si-doped nanostructured hematite photoanodes, electron extraction and electron-hole recombination are complete within ∼20 ms, while water oxidation is observed to occur on a timescale of hundreds of milliseconds to seconds. The competition between electron extraction and electron-hole recombination is electron-density-dependent: the effect on recombination of applied bias and excitation intensity is discussed. The timescale of water oxidation is independent of the concentration of photogenerated holes, indicating that the mechanism of water oxidation on hematite is via a sequence of single-hole oxidation steps.

Intersecting-state model calculations on fast and ultrafast excited-state proton transfers in naphthols and substituted naphthols

Abstract The intersecting-state model (ISM) is applied to the calculation of absolute rate constants for proton-transfer reactions of naphthols and substituted naphthols in the first singlet state and for ground states. ISM incorporates quantum–mechanical tunnelling, zero-point energy corrections and an electrophilicity parameter to account for the lowering of the binding energy of transition states. Good agreement with experimental rates is observed over 12 orders of magnitude. The lower reactivity of the photoacids in alcohols when compared to the behaviour in water can be accounted for the differences in the potential energy curves of the OH bonds. Patterns of reactivity with respect to free-energy relations and kinetic isotope effects are also discussed.

Proton-Coupled Electron Transfer: A Carrefour of Chemical Reactivity Traditions

Proton-coupled electron transfer: introduction and state-of-the-art Application of the Marcus Cross Relation to proton-coupled electron transfer/hydrogen atom transfer reactions Theoretical and experimental criteria for proton-coupled electron transfer On the solvation of ionic systems Experimental approaches towards proton-coupled electron transfer reactions in biological redox systems Metal ion-coupled electron transfer Electrochemical concerted proton-electron transfers. Comparison with homogeneous processes

Molecular factor analysis in atom-transfer reactions

We present a systematic method to access molecular factors and model activation energies using partial least squares and a number of chosen descriptors. It is shown that activation energies for the set of atom-transfer reactions considered can be mostly described in terms of the reaction energy and Parrs electrophilicity index. It is also shown that the exact form in which both responses (the activation energy and several simple functional transformations) and predictors are expressed does not change the main results of the factor analysis.

Solar Energy Conversion

Photochemical conversion of solar photons is one of the most promising and sought after solutions to the current global energy problem. It combines the advantages of an abundant and widespread source of energy, the Sun, and Earth-abundant and environmentally benign materials, to produce other usable forms of energy such as electricity and fuels, without the negative impact of CO2 or other greenhouse gas release into the atmosphere. Dye-sensitised solar cells (DSSC) and organic bulk heterojunction (BHJ) solar cells are two examples of such systems, allowing the conversion of visible sunlight into electricity by inorganic or organic semiconductor materials, which are inexpensive and easy to process on a large scale. Photocatalytic (PC) and photoelectrochemical (PEC) water splitting systems offer a solution to the problem of diffuse and intermittent sunlight irradiation, by storing the energy of solar photons in the form of clean energy vectors such as H2. This chapter presents an overview of the technologies based on photochemical solar energy conversion and storage.

Chapter 5:Proton-Coupled Electron Transfer in Natural and Artificial Photosynthesis

Proton coupled electron transfer (PCET) is ubiquitous in redox chemistry, including important biological functions such as respiration, photosynthesis and nitrogen fixation. Understanding the role and mechanistic aspects of PCET in such contexts is crucial for the design and development of new biomimetic systems. In the context of solar energy conversion, oxygenic photosynthesis is an extraordinary example of how PCET can be used for an efficient proton management and redox levelling, to match one electron photochemistry with the challenging multi-electron/multi-proton catalysis of water oxidation. In this chapter, we will present an overview of the working principles of Photosystem II (PSII) and review the current understanding of the role of PCET in natural photosynthesis. These principles are applied to the design of functional models of PSII for the photochemical oxidation of water. Examples of catalyst structures and performances and photoelectrochemical device architectures are also discussed in the context of artificial photosynthesis for solar fuel production.