8634
Langmuir 2004, 20, 8634-8640
Analysis of Corrosion Processes at the Surface of Diamond-Like Carbon Protected Zinc Selenide Waveguides Markus Janotta,† Douglas Rudolph,† Angelika Kueng,† Christine Kranz,† Hannes-Stefan Voraberger,‡ Wolfgang Waldhauser,§ and Boris Mizaikoff*,† School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, Institute of Chemical Process Development and Control, Joanneum Research, Steyrergasse 17, A-8010 Graz, Austria, and Laser Center Leoben, Joanneum Research, Leobner Strasse 94, A-8712 Niklasdorf, Austria Received April 15, 2004. In Final Form: July 5, 2004 A detailed surface analytical study on the corrosion behavior of unprotected and diamond-like carbon (DLC)-coated mid-infrared (MIR) waveguides used in remote sensing applications at strongly oxidizing conditions is presented. High-quality DLC films, with a thickness of 100 nm serving as MIR-transparent corrosion barrier, have been produced at the surface of zinc selenide (ZnSe) attenuated total reflection waveguides via pulsed laser deposition techniques. IR microscopy and atomic force microscopy are applied to investigate the chemical inertness of DLC-based membranes against aqueous solutions of hydrogen peroxide. These stability studies show that uncoated ZnSe waveguides are subject to severe chemical surface modifications, while DLC-protected waveguides maintain their optical properties and chemical integrity. In situ studies on the corrosion behavior by a recently developed approach combining scanning electrochemical microscopy (SECM) with Au/Hg amalgam ultramicroelectrodes in a scanning stripping voltammetry experiment provides additional insight into the mechanisms of the corrosion process. It is demonstrated that the combination of surface analytical techniques and, in particular, the innovative application of SECM with amalgam electrodes provides superior information on corrosion processes at the surface of optical waveguides. This detailed study confirms the efficiency of protective DLC coatings deposited onto IR-transparent optical waveguides, rendering this novel concept ideal for sensing applications in harsh environments.
Introduction Increasing interest in chemical sensing systems is triggered to a significant extent by the demand for in situ and on-line operating analytical techniques. Environmental analysis and industrial process monitoring are excellent examples substantiating the need for rapidly responding, selective, and reliable sensing devices enabling cost-effective continuous analysis. However, harsh measurement conditions in many measurement situations, such as wastewater treatment or the sterilization industry, still represent a serious challenge for reliable device performance. Recently, a new approach for the direct analysis of oxidizing agents in aqueous solution has been developed taking advantage of diamond-like carbon (DLC)-protected mid-infrared transparent waveguides in combination with attenuated total reflectance (ATR) spectroscopy.1 High-quality thin DLC films have been coated via pulsed laser deposition (PLD) onto the surface of ZnSe ATR crystals with thicknesses
