Hematite (Fe2O3) is a promising earth-abundant, visible light absorber, and easily processable photocatalytic material. Understanding the dynamics of photogenerated electrons and holes in hematite and their optical signatures is crucial in designing hematite thin film devices such as photoanodes for water oxidation. We report carrier dynamics in hematite films as measured by ultrafast transient absorption spectroscopy (TA) with a pump pulse centered at 400 nm (3.1 eV) and a probe pulse spanning the visible range. We observe a small negative response for wavelengths shorter than 530 nm (2.34 eV) and a large positive response for longer wavelengths. We interpret the spectrally resolved TA data based on recent electronic band structure calculations, while accounting for excited state absorption, ground state bleach, and stimulated emission within the relevant bands. We propose that the origin of the positive TA response is absorption of the probe by photoexcited electrons within the conduction bands. This interpretation is consistent with features observed in the data, specifically an abrupt boundary near 530 nm, a diffuse edge at lower energy probes with a ∼ 250 fs decay time characteristic of carrier relaxation, and slower decay components of ∼5.7 and >670 ps characteristic of carrier recombination. We propose that the negative TA signal arises at short wavelengths where excited state absorption within the conduction bands is no longer possible and ground state bleach and stimulated emission dominate. This study will assist in understanding the origins of transient optical responses and their interpretation in hematite-based devices such as photoanodes.

Transducing light energy to changes in material properties is central to a large range of functional materials, including those used in light harvesting. In conventional semiconductors, photoconductivity arises due to generation of mobile electrons or holes with light. Here we demonstrate, to our knowledge for the first time, an analogue of this effect for protons in an organic polymer solution and in water. We show that when a material is doped with photoacids, light excitation generates extra mobile protons that change the low-frequency conductivity of the material. We measure such change both in poly(ethylene glycol) (PEG) and in water sandwiched between two transparent electrodes and doped with a well-known photoacid 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS). The complex impedance of the material is measured over a range of 0.1 Hz-1 MHz in both the presence and absence of light, and it is found that shining light changes the low frequency impedance significantly. We model the impedance spectra of the material with a minimal circuit composed of a diffusive impedance (Warburg element), a parallel capacitance, and a resistance. Fitting the light and dark impedance spectra to the model reveals that light reduces the low-frequency diffusive impedance of the material, which is consistent with generation of extra free carriers by light. We further suggest that the light-induced conductivity change arises mainly due to those photoreleased protons that manage to escape the zone of influence of the parent ion and avoid recapture. Such escape is more likely in materials with larger diffusion coefficient for protons and shorter electrostatic screening lengths for the parent ion. This explanation is consistent with our observed differences in the photoconductivity of solution of HPTS in water and in PEG. We anticipate that this scheme can be employed in protonic circuits where direct transduction of energy from light to protonic gradients or protonic currents is necessary. This work will also serve as a basis for using photoacids as optical handles for characterizing the molecular mechanisms of conductivity in proton conducting materials.

Understanding the mechanisms of multielectron and multiproton electrochemical reactions, particularly in the context of solar-to-fuel water splitting, is an outstanding challenge. Historically, Pourbaix diagrams are used to show the influence of potential and pH on the thermodynamic stability of electrode−electrolyte systems. These diagrams do not carry kinetic or mechanistic information, which often restricts their use to cases in which the thermodynamic limit can be assumed. We introduce and construct from experimental data two new types of diagrams that demonstrate the kinetic variations of electrochemical reactions as a function of pH and potential. These diagrams show the variation of the electron-transfer parameter (α) and the proton reaction order (ρ) in a wide range of potential and pH. We present α(pH, E) and ρ(pH, E) for water electrolysis on an iron oxide electrode in the range of pH 7 to 13. In these plots, regions of acidic and basic mechanisms, relationship to surface protonation equilibria, and switching between acidic and basic mechanisms due to electrochemical production of protons can be easily identified. The proton reaction order is zero in the acidic side, while it is nonzero in the basic limit. A larger empirical electron-transfer parameter is observed in the basic compared to the acidic region. These observations are related to the differences in oxidation mechanism between the two regions. We propose the use of such diagrams to gain an expanded and enhanced view of the kinetics of multielectron and multiproton electrochemical reactions.

The empirical electron-transfer coefficient α is a valuable electrochemical observable that bridges the thermodynamics and kinetics of redox reactions. For reactions that involve protons, the value of α is expected to be pH-dependent. However, even for the simplest redox processes, the nature of this dependency remains unclear. Toward clarifying this problem, we follow two goals. First, we calculate the electron-transfer coefficient α and its pH dependence based on a model 2D potential energy surface that has been investigated by Koper and Schmickler for proton reduction. According to the model, α is pH-independent for high-pH values and pH-dependent for low-pH values, with α increasing as the pH is lowered. Second, we report our experimentally measured α for hydrogen evolution on several electrode materials over a wide pH range. We observe that several features of the data show similarities to the predictions of the model. The data show different behavior in two distinct pH regions. In the acidic region, a linearly increasing α with decreasing pH and in the basic side a pH-independent α are observed for several electrodes. However, certain predictions of the model, in particular the transition pH between the two regions, do not seem consistent with the data, which we propose likely arises due to mass-transfer limitations of the rate. We hope that this work will help better understand the pH dependence of interfacial electron–proton transfer reactions and, in particular, inspire further work to isolate mass-transfer limitations from interfacial chemistry effects in measuring and interpreting the electron-transfer coefficient.

Emulating water oxidation catalyzed by the oxomanganese clusters in the photosynthetic apparatus of plants has been a long-standing scientific challenge. The use of manganese oxide films has been explored, but while they may be catalytically active on the surface, their poor conductivity hinders their overall performance. We have approached this problem by using manganese oxide nanoparticles with sizes of 4, 6 and 8 nm, produced in a sputter-gas-aggregation source and soft-landed onto conducting electrodes. The mass loading of these catalytic particles was kept constant and corresponded to 45%-80% of a monolayer coverage. Measurements of the water oxidation threshold revealed that the onset potential decreases significantly with decreasing particle size. The final stoichiometry of the catalytically active nanoparticles, after exposure to air, was identified as predominantly MnO. The ability of such a sub-monolayer film to lower the reaction threshold implies that the key role is played by intrinsic size effects, i.e., by changes in the electronic properties and surface fields of the nanoparticles with decreasing size. We anticipate that this work will serve to bridge the knowledge gap between bulk thick film electrocatalysts and natural photosynthetic molecular-cluster complexes.