Hematite Photoanodes for Water Oxidation: Electronic Transitions, Carrier Dynamics, and Surface Energetics

We review the current understanding of charge carriers in model hematite photoanodes at different stages. The origin of charge carriers is discussed based on the electronic structure and absorption features, highlighting the controversial assignment of the electronic transitions near the absorption edge. Next, the dynamic evolution of charge carriers is analyzed both on the ultrafast and on the surface reaction timescales, with special emphasis on the arguable spectroscopic assignment of electrons/holes and their kinetics. Further, the competitive charge transfer centers at the solid-liquid interface are reviewed, and the chemical nature of relevant surface states is updated. Finally, an overview on the function of widely employed surface cocatalysts is given to illustrate the complex influence of physiochemical modifications on the charge carrier dynamics. The understanding of charge carriers from their origin all the way to their interfacial transfer is vital for the future of photoanode design.

Far-field polarization signatures of surface optical nonlinearity in noncentrosymmetric semiconductors

We analyse possibilities to quantitatively evaluate the surface second-order optical nonlinearity in noncentrosymmetric materials based on polarization-resolved analysis of far-field radiation patterns of second-harmonic generation. We analytically demonstrate that for plane-wave illumination the contribution to the second-harmonic signal from the surface of a nonlinear medium exhibits different polarization properties and angular dependencies compared to the contribution from the bulk. In view of this, we optimize the illumination geometry in order to enable the most efficient separation and comparison of both nonlinearities. Furthermore, we consider the illumination of an AlGaAs slab by a tightly-focused linearly-polarized Gaussian beam as an alternative measurement geometry. It is found that the reliable separation of the surface nonlinearity contribution as well as a wide range of detectable values can be achieved with this geometry as well.

Surface states mediated charge transfer in redox behavior of hemin at GaAs(100) electrodes

EIS and XPS investigations on the interaction of hemin with p- and n-doped GaAs(100) electrodes in PBS solution revealed significant differences concerning both the adsorbed species and the mechanism of the redox process caused by dopant nature. XPS data show that hemin is adsorbed on p-GaAs(100) by its carboxyl groups and adopts a vertical position favorable to a polymeric film formation whereas on n-GaAs(100), the adsorbed hemin is monomeric and has a rather planar configuration involving mainly the OH groups of the organic molecule. Hemin gives rise to a reversible redox process at the p-GaAs(100) electrode whereas at n-GaAs(100), there is only one reduction wave of a considerably lower current density appearing at a more negative potential. The effects of the applied potential on the phase angle measured at p-GaAs(100) point out major changes not only in the insulating properties of the adsorbed layer, as found at n-GaAs(100), but also in the electronic properties of the semiconductor triggered by the hemin redox process. Analysis of the experimental data points to a mechanism of charge transfer through surface states, the observed differences being related to the location of the surface states with respect to the formal potential of the hemin redox couple.

Paper brings evidence that surface states are responsible for different redox processes of hemin, reversible at p-GaAs(100) and ireversible at n-GaAs(100).