Stephanie R. Pendlebury

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.

Back Electron–Hole Recombination in Hematite Photoanodes for Water Splitting

The kinetic competition between electron-hole recombination and water oxidation is a key consideration for the development of efficient photoanodes for solar driven water splitting. In this study, we employed three complementary techniques, transient absorption spectroscopy (TAS), transient photocurrent spectroscopy (TPC), and electrochemical impedance spectroscopy (EIS), to address this issue for one of the most widely studied photoanode systems: nanostructured hematite thin films. For the first time, we show a quantitative agreement between all three techniques. In particular, all three methods show the presence of a recombination process on the 10 ms to 1 s time scale, with the time scale and yield of this loss process being dependent upon applied bias. From comparison of data between these techniques, we are able to assign this recombination phase to recombination of bulk hematite electrons with long-lived holes accumulated at the semiconductor/electrolyte interface. The data from all three techniques are shown to be consistent with a simple kinetic model based on competition between this, bias dependent, recombination pathway and water oxidation by these long-lived holes. Contrary to most existing models, this simple model does not require the consideration of surface states located energetically inside the band gap. These data suggest two distinct roles for the space charge layer developed at the semiconductor/electrolyte interface under anodic bias. Under modest anodic bias (just anodic of flatband), this space charge layer enables the spatial separation of initially generated electrons and holes following photon absorption, generating relatively long-lived holes (milliseconds) at the semiconductor surface. However, under such modest bias conditions, the energetic barrier generated by the space charge layer field is insufficient to prevent the subsequent recombination of these holes with electrons in the semiconductor bulk on a time scale faster than water oxidation. Preventing this back electron-hole recombination requires the application of stronger anodic bias, and is a key reason why the onset potential for photocurrent generation in hematite photoanodes is typically ~500 mV anodic of flat band and therefore needs to be accounted for in electrode design for PEC water splitting.

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.

Dynamics of photogenerated holes in undoped BiVO4 photoanodes for solar water oxidation

The dynamics of photogenerated holes in undoped BiVO4 photoanodes for water splitting were studied using transient absorption spectroscopy, correlated with photoelectrochemical and transient photocurrent data. Transient absorption signals of photogenerated holes were identified using electron/hole scavengers and applied electrical bias in a complete photoelectrochemical cell. The yield of long-lived (0.1–1 s) photogenerated holes is observed to correlate as a function of applied electrical bias with the width of the space charge layer, as determined by electrochemical impedance spectroscopy. The transient absorption decay time constant of these long-lived holes is also observed to be dependent upon the applied bias, assigned to kinetic competition between water oxidation and recombination of these surface accumulated holes with bulk electrons across the space charge layer. The time constant for this slow recombination measured with transient absorption spectroscopy is shown to match the time constant of back electron transfer from the external circuit determined from chopped light transient photocurrent measurements, thus providing strong evidence for these assignments. The yield of water oxidation determined from these measurements, including consideration of both the yield of long-lived holes, and the fraction of these holes which are lost due to back electron/hole recombination, is observed to be in good agreement with the photocurrent density measured for BiVO4 photoanodes as a function of bias under continuous irradiation. Overall these results indicate two distinct recombination processes which limit photocurrent generation in BiVO4 photoanodes: firstly rapid (≤microseconds) electron/hole recombination, and secondly recombination of surface-accumulated holes with bulk BiVO4 electrons. This second ‘back electron transfer’ recombination occurs on the milliseconds–seconds timescale, and is only avoided at strong anodic biases where the potential drop across the space charge layer provides a sufficiently large energetic barrier to prevent this recombination process.

Rate law analysis of water oxidation on a hematite surface.

Water oxidation is a key chemical reaction, central to both biological photosynthesis and artificial solar fuel synthesis strategies. Despite recent progress on the structure of the natural catalytic site, and on inorganic catalyst function, determining the mechanistic details of this multiredox reaction remains a significant challenge. We report herein a rate law analysis of the order of water oxidation as a function of surface hole density on a hematite photoanode employing photoinduced absorption spectroscopy. Our study reveals a transition from a slow, first order reaction at low accumulated hole density to a faster, third order mechanism once the surface hole density is sufficient to enable the oxidation of nearest neighbor metal atoms. This study thus provides direct evidence for the multihole catalysis of water oxidation by hematite, and demonstrates the hole accumulation level required to achieve this, leading to key insights both for reaction mechanism and strategies to enhance function.

Ultrafast Charge Carrier Recombination and Trapping in Hematite Photoanodes under Applied Bias

Transient absorption spectroscopy on subpicosecond to second time scales is used to investigate photogenerated charge carrier recombination in Si-doped nanostructured hematite (α-Fe2O3) photoanodes as a function of applied bias. For unbiased hematite, this recombination exhibits a 50% decay time of ∼6 ps, ∼103 times faster than that of TiO2 under comparable conditions. Anodic bias significantly retards hematite recombination dynamics, and causes the appearance of electron trapping on ps−μs time scales. These ultrafast recombination dynamics, their retardation by applied bias, and the associated electron trapping are discussed in terms of their implications for efficient water oxidation.

Where Do Photogenerated Holes Go in Anatase:Rutile TiO2? A Transient Absorption Spectroscopy Study of Charge Transfer and Lifetime

Anatase:rutile TiO2 junctions are often shown to be more photocatalytically active than anatase or rutile alone, but the underlying cause of this improvement is not fully understood. Herein, we employ transient absorption spectroscopy to study hole transfer across the anatase:rutile heterojunction in films as a function of phase composition. By exploiting the different signatures in the photoinduced absorption of trapped charges in anatase and rutile, we were able to separately track the yield and lifetime of holes in anatase and rutile sites within phase composites. Photogenerated holes transfer from rutile to anatase on submicrosecond time scales. This hole transfer can significantly increase the anatase hole yield, with a 20:80 anatase:rutile composite showing a 5-fold increase in anatase holes observed from the microsecond. Hole transfer does not result in an increase in charge-carrier lifetime, where an intermediate recombination dynamic between that of pure anatase (t1/2 ≈ 0.5 ms) and rutile (t1/2 ≈ 20 ms) is found in the anatase:rutile junction (t1/2 ≈ 4 ms). Irrespective of what the formal band energy alignment may be, we demonstrate the importance of trap-state energetics for determining the direction of photogenerated charge separation across heterojunctions and how transient absorption spectroscopy, a method that can specifically track the migration of trapped charges, is a useful tool for understanding this behavior.

Effect of Internal Electric Fields on Charge Carrier Dynamics in a Ferroelectric Material for Solar Energy Conversion

Spontaneous polarization is shown to enhance the lifetimes of photogenerated species in BaTiO3 . This is attributed to polarization-induced surface band bending acting as a thermal barrier to electron/hole recombination. The study indicates that the efficiencies of solar cells and solar fuels devices can be enhanced by the use of ferroelectric materials.