Morphology, structural and optical properties of iron oxide thin film photoanodes in photoelectrochemical cell: Effect of electrochemical oxidation

Hematite (alpha-Fe2O3) is a promising semiconductor as photoanode in solar hydrogen production from photoelectrolysis of water due to its appropriate band gap, low cost and high electrochemical stability in aqueous caustic electrolytes. Operation of such photoanode in a biased photoelectrochemical cell constitutes an anodization with consequent redox reactions at the electrode surface. alpha-Fe2O3 thin film photoanodes were prepared by simple and inexpensive dip coating method on fluorine doped tin oxide (FTO) glass substrate, annealed in air at 500 degrees C for 2 h, then electrochemically oxidized (anodized) in 1 M KOH at 500 mV for 1 min in dark and light conditions. Changes in structural properties and morphology of alpha-Fe2O3 nanoparticles films were investigated by XRD, Raman spectroscopy and a high resolution FESEM. The average grain size was observed to increase from similar to 57 nm for pristine samples to 73 and 77 nm for anodized samples in dark and light respectively. Broadening and red shift in Raman spectra in anodized samples may be attributed to lattice expansion upon oxidation. The UV-visible measurements revealed enhanced absorption in the photoanodes after the treatment. The findings suggest that the anodization of the photoelectrode in a biased cell causes not only changes of the molecular structure at the surface, but also changes in the crystallographic structure which can be detected with x-ray diffractometry. (C) 2015 Elsevier B.V. All rights reserved.

Raman Spectroscopy of GaAs Nanowires

Bernt Ketterer

Semiconductor nanowires offer a wide range of opportunities for newgenerations of nanoscale electronic and optic devices. For these applications to become reality, deeper understanding of the fundamental properties of the nanowires is required. In this thesis, Raman spectroscopy has been applied to examine the characteristics of GaAs nanowires facing the following challenges in current nanowire research: (i) understanding of the doping mechanisms in catalyst-free GaAs nanowires grown byMBE, (ii) examination of the electronic band structure of the wurtzite polytype of GaAs, and (iii) probing the properties of free carrier systems in nanowire based quantum-heterostructures. The Si-doping of GaAs nanowires was studied in the first part of the thesis. The investigation of the local vibrational modes of the silicon dopants in the GaAs host lattice allowed to identify the incorporation pathways the dopants take during the nanowire growth and to determine the limitations of the doping process due to compensation effects. Important information on the concentration and mobility of the free carriers in the doped material has been obtained by analyzing the coupled longitudinal optical phonon-plasmon modes. The second part of the thesis focused on the electronic band structure in wurtzite GaAs. Here, resonant Raman scattering from first and second order longitudinal optical phonons was used in oder to determine the band-gap, the position of the light-hole band as well as the temperature dependence of the crystal-field split-off band. As important parameters the spin-orbit coupling and crystal-field splitting have been obtained. The photoexcited electron-hole plasma and the confined optical phonons in nanowire-based GaAs/AlAs multi-quantum-well structures were studied in the last part of the thesis. Structural parameters could be deduced from the position of the confined phononmodes. The coupling of the plasmon to the phonon gives important information on the system of photogenerated carriers within the quantum structure.

EPFL2011

Raman Spectroscopy of GaAs Nanowires

Bernt Ketterer

EPFL2011