Dynamics of photogenerated holes in surface modified alpha-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 (CoO(x)) overlayer, reported previously to function as an efficient water oxidation electrocatalyst, and a Ga(2)O(3) 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 Fe(2)O(3) for a given applied bias

The surface oxidation potential of human neuromelanin reveals a spherical architecture with a pheomelanin core and a eumelanin surface

Neuromelanin (NM) isolated from the substantia nigra region of the human brain was studied by scanning probe and photoelectron emission microscopies. Atomic force microscopy reveals that NM granules are comprised of spherical structures with a diameter of ≈30 nm, similar to that observed for Sepia cuttlefish, bovine eye, and human eye and hair melanosomes. Photoelectron microscopy images were collected at specific wavelengths of UV light between 248 and 413 nm, using the spontaneous-emission output from the Duke OK-4 free electron laser. Analysis of the data establishes a threshold photoionization potential for NM of 4.5 ± 0.2 eV, which corresponds to an oxidation potential of −0.1 ± 0.2 V vs. the normal hydrogen electrode (NHE). The oxidation potential of NM is within experimental error of the oxidation potential measured for human eumelanosomes (−0.2 ± 0.2 V vs. NHE), despite the presence of a significant fraction of the red pigment, pheomelanin, which is characterized by a higher oxidation potential (+0.5 ± 0.2 V vs. NHE). Published kinetic studies on the early chemical steps of melanogenesis show that in the case of pigments containing a mixture of pheomelanin and eumelanin, of which NM is an example, pheomelanin formation occurs first with eumelanin formation predominantly occurring only after cysteine levels are depleted. Such a kinetic model would predict a structural motif with pheomelanin at the core and eumelanin at the surface, which is consistent with the measured surface oxidation potential of the ≈30-nm constituents of NM granules