Invited Paper AS-WeM5
In-situ Surface Analysis by Optical Means

Wednesday, October 17, 2007, 9:20 am, Room 610

Understanding natural geochemical systems requires investigating fundamental reactions (adsorption, dissolution/growth, electron transfer, catalysis) at a variety of solid-solution interfaces.  The availability of techniques for characterizing solid-liquid interfaces in-situ has made this task simpler.  Such techniques include synchrotron-based X-ray absorption spectroscopies (e.g., EXAFS, XANES), scanning probe microscopes (STM, AFM, and their variations), and developments in Raman and IR specroscopies.  Here, we explore some perhaps less well-known techniques: Optical second harmonic generation (SHG), optical waveguide lightmode spectroscopy (OWLS), and photocurrent measurements coupled to impedance spectroscopy. SHG is a nonlinear optical technique.  Briefly, the interface represents a noncentrosymmetric setting between two centrosymmetric bulk phases.  Intense laser light impinging on an interface can produce a few photons of doubled-frequency (second harmonic) whose intensity and polarization can be related to the concentration - and possibly orientation - of adsorbed species.  We have used SHG to observe the adsorption of organic molecules to oxide surfaces and to study the structure of water near charged oxide surfaces. OWLS operates on the basis of small changes in the effective refractive index of a waveguide of sub-wavelength thickness as the result of molecular adsorption to the waveguide surface.  We are using OWLS to study the adsorption of proteins, particularly outer membrane cytochromes from iron-reducing bacteria, on oxide surfaces.  This technique has proven crucial in studying the adsorption of small amounts of protein.  Furthermore, comparison of quartz crystal microbalance (QCM) adsorption results (which includes associated water in the adsorbed mass) to OWLS results (which excludes associated water) shows that of the total adsorbed mass, only 27% is protein in these cytochromes.   Our work with semiconducting oxide electrodes necessitates electrode charcterization with regard to flatband potential, charge carrier density, and other properties.  In addition, these and photocurrent transient techniques allow us to locate electronic states both at the semiconductor surface and within its bulk bandgap.  This presentation will briefly show how such states may be located and studied using photocurrent transient spectroscopy and impedance spectroscopy in the case of iron oxide photocatalysts.