Electron Spectrometer: HREELS, UHV Surface Chemistry
Contact:
Henderson,Michael A
The Ultra-high Vacuum (UHV) Surface Chemistry—High-Resolution Electron Energy Loss Spectroscopy (HREELS) System—located in the Surface Science and Catalysis Laboratory of EMSL's Interfacial and Nanoscale Science Facility—is designed to study the molecular-level chemistry of adsorbates on metal oxide surfaces. This system is equipped with several spectroscopic tools that follow changes in adsorbate chemistry, including HREELS, secondary ion mass spectrometry, and ultraviolet photoemission and (electronic) electron energy loss spectroscopies. This system also contains an ion gun for sample cleaning, and an Auger electron spectrometer and low-energy electron diffraction system for characterizing sample surface composition and structure, respectively.
Electron-stimulated and temperature-programmed desorption studies are routinely performed using this system. During temperature-programmed desorption studies, the researcher can obtain typical information such as quantity and nature (intact or dissociated molecule) of an adsorbed gas. In addition, they can also estimate the sticking coefficient and activation energy for desorption and/or reaction of the adsorbed molecule.
EMSL researchers have successfully used this system to characterize the surface chemistry of a variety of adsorbates (e.g., water, formic acid, carbon dioxide, methanol, oxygen, chromyl chloride) on TiO2 and Fe2O3 single crystal substrates. The following information briefly describes the instrument configuration and provides an operational overview to facilitate user planning.
System Configuration and Operational Overview
Sample Preparation and Handling
While experiments on realistic metal, oxide (ceramic and glass), and polymer materials can be performed using this system, the UHV Surface Chemistry—HREELS System is primarily used to study model (often single-crystal) materials. EMSL researchers who are responsible for this equipment are experienced in mounting conductive metal and semiconductive and insulating oxide samples, and in attaching thermocouples to these types of samples for temperature measurements. Typically, square and (nearly) round samples with areas of about 1 cm2 and a thickness of 1 to 2 mm or more are used.
All work in the Surface Science and Catalysis Laboratory and with the UHV Surface Chemistry—HREELS System must be performed in compliance with EMSL practices and permits.
Sample Manipulator
Samples are mounted on a Vacuum Generators x,y,z, manipulator. Heating is accomplished by passing dc current through metallic backing plates used to mount the crystals. Cooling to below 100 Kelvin is accomplished through a gravity-fed liquid nitrogen dewar.
Surface Analysis Chamber
Several techniques typically conducted in the system's surface analysis chamber were chosen due to the system being designed to study molecular-level chemistry of adsorbates on metal oxide surfaces. With HREELS, surface vibrational and electronic dipoles are probed with an electron beam of 3 mV to 5 mV in resolution. For example, the adsorbed state of water (molecular versus dissociative) has been determined on TiO2 single crystal surfaces based on the vibrational features observed in the electron energy loss spectrum.
Other techniques used in this chamber that complement HREELS and provide molecular-level information include
- secondary ion mass spectrometry
- electron-stimulated desorption
- temperature-programmed desorption.
All of these techniques use an Extrel quadrupole mass spectrometer; a Princeton Scientific reverse-view low-energy electron diffraction apparatus; and a PHI double-pass cylindrical mirror analyzer for Auger electron spectrometer, ultraviolet photoemission, and work function change measurements. In addition, ion and electron beam studies are performed with guns obtained from Kimball Physics, and the light source for ultraviolet photoemission studies is a VSW helium lamp.
The chamber is pumped by a 270 L/s ion pump and a 230 L/s turbomolecular pump, and routinely achieves pressures in the low 10-10 Torr range. Molecules are dosed onto well-characterized surfaces either by backfilling the chamber or through effusive beam sources.
Researchers external to EMSL may use this instrument for their research. Collaborations among EMSL and external researchers using this instrument include the following:
- A researcher from Tulane University visited EMSL to study the effect of vacuum reduction and oxidation on the surface structure and chemistry of the (110) surface of Ti02 rutile. (Li et al. 2000; Li et al. 1999a; Li et al. 1999b; Henderson et al. 1999; Epling et al. 1998)
- A researcher from the University of Texas visited EMSL to examine photochemical phenomena on the Ti02(110) surface. (White et al. in press)
References
Epling WS, CHF Peden, MA Henderson, and U Diebold. 1998. "Evidence for Oxygen Adatoms on TiO2(110) Resulting from O2 Dissociation at Vacancy Sites." Surface Science 412-413:333-343.
Henderson MA, WS Epling, CL Perkins, CHF Peden, and U Diebold. 1999. "Interaction of Molecular Oxygen with the Vacuum Annealed TiO2(110) Surface: Molecular and Dissociative Channels." Journal of Physical Chemistry B 103(25):5328-5337.
Li, M, W Hebenstreit, U Diebold, AM Tyryshkin, MK Bowman, GG Dunham, and MA Henderson. 2000. "The Influence of the Bulk Reduction State on the Surface Structure and Morphology of Rutile TiO2(110) Single Crystals." Journal of Physical Chemistry B 104(20):4944-4950.
M Li, W Hebenstreit, U Diebold, MA Henderson, and DR Jennison. 1999a. "Oxygen-Induced Restructuring of Rutile TiO2(110): Formation Mechanism, Atomic Models, and Influence on Surface Chemistry." Faraday Discussions 114:245-258.
M Li, W Hebenstreit, L Gross, U Diebold, MA Henderson, DR Jennison, PA Schultz, and MP Sears. 1999b. "Oxygen Induced Restructuring of the TiO2(110) Surface: a Comprehensive Study." Surface Science 437(1-2):173-190.
White, JM, J Szanyi, and MA Henderson. 2003. "The Photon-Driven Hydrophilicity of Titania: A Model Study using TiO2(110) and Adsorbed Trimethylacetate." Journal of Physical Chemistry B (in press).
