J. Phys. Chem. C 2007, 111, 6763-6771
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Morphological Changes at a Silver Surface Resulting from Exposure to Hyperthermal Atomic Oxygen Long Li* and Judith C. Yang Department of Mechanical Engineering and Materials Science, UniVersity of Pittsburgh, 3700 O’Hara Street, Pittsburgh, PennsylVania 15261
Timothy K. Minton Department of Chemistry and Biochemistry, Montana State UniVersity, 108 Gaines Hall, Bozeman, Montana 59717 ReceiVed: September 5, 2006; In Final Form: February 21, 2007
Bulk single crystals of Ag(100) and Ag(111) at a sample temperature of 220 °C were exposed to a beam containing hyperthermal atomic oxygen with a nominal translational energy of 5.2 eV. The resultant oxide scales and interface structures were characterized by cross-sectional (scanning) transmission electron microscopy ((S)TEM) and scanning electron microscopy (SEM). The oxide scales formed on Ag(100) and Ag(111) were more than 10 microns thick and contained numerous micropores, microchannels, grain boundaries, and defects (e.g., twins), which can provide pathways for the rapid transport of oxygen atoms through the thick oxide scales to the oxide/Ag interface. Energy dispersive X-ray spectroscopy (EDS) and nanoarea electron diffraction pattern (NA-EDP) investigations revealed that the oxide scales were predominately polycrystalline silver, containing only ∼3-5 atom % of oxygen, which is mainly in the form of Ag2O. Additional low-fluence exposures of thin foils of single-crystal Ag followed by analysis with TEM and NA-EDP identified crystalline grains of Ag, with dimensions of 500-800 nm, imbedded in a porous region of nanometer-sized Ag2O grains during the initial stages of surface transformation by hyperthermal atomic oxygen. A mechanism based on the rapid oxidation of Ag by atomic oxygen followed by thermal reduction of the Ag2O at 220 °C is proposed to explain the formation of the thick and unique polycrystalline silver “oxide” scales that were observed.
I. Introduction Space vehicles in low Earth orbit (LEO, altitude from ∼200 to ∼700 km) experience a harsh oxidizing environment, where atomic oxygen is the primary oxidizing species.1,2 Oxygen atoms are generated through the dissociation of molecular oxygen by intense solar ultraviolet radiation in the LEO environment. The O-atom flux on the ram surfaces of spacecraft is approximately 1015 atoms cm-2 s-1.3 The relative velocity between space vehicles and the ambient atomic oxygen is 7.4 km s-1, which corresponds to O atoms with roughly 4.5 eV of translational energy striking the ram surfaces. Oxygen atoms are particularly reactive with materials at these high (hyperthermal) collision energies; therefore, erosion and oxidation of materials on spacecraft in LEO are believed to be caused mainly by interactions involving O atoms. Because high collision energies may facilitate large energy transfers and the crossing of high reaction barriers, these hyperthermal O-atom collisions are expected to proceed through mechanisms that are quite different from those involving molecular oxygen at thermal velocities. The desire to understand the unique chemistry of hyperthermal O-atom-surface reactions has motivated many studies which have employed various atomic-oxygen sources, including plasma ashers, high-temperature RF/DC discharges, laser breakdown, etc.4-9 The hyperthermal source that generates neutral O atoms through breakdown of O2 by a CO2 laser is relatively well characterized and induces effects on materials that are similar to those observed in LEO exposures.9,10 * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: +01 412 624 9753. Fax: +01 412 624 9753.
Silver has a wide range of applications in LEO, such as an electron conductive material,11 a reflective medium on a solar concentrator,12,13 and a sensor material for calibrating atomicoxygen exposures.14-16 Despite the many uses of silver in environments where it is exposed to hyperthermal oxygen atoms, very little is known about the mechanisms by which hyperthermal O atoms react with a silver surface. Even the morphological changes that occur have not been investigated in detail at the microscopic level. The focus of this paper is on the microscopic changes that occur at single-crystal silver surfaces, Ag(100) and Ag(111), that are exposed to hyperthermal atomic oxygen. Although investigations of the oxidation of silver by hyperthermal oxygen atoms have been limited, a large body of literature exists on silver oxidation under a wide variety of conditions. Dissimilar oxide scales, and hence different mechanisms of silver oxidation, are noted when silver surfaces are exposed to different oxygen species (e.g., molecular oxygen, ozone, and atomic oxygen) at different surface temperatures and oxygen pressures. Silver tarnishes at room temperature in air, but it was found that the tarnish is not an oxide but a sulfide.17,18 Silver is not thermodynamically driven to oxidize by molecular oxygen at temperatures below 350 °C and at oxygen pressures below 10 Torr, and no evidence for the oxidation of silver was found at room temperature when the O2 pressure was 1 atm.19-23 When silver is exposed to a molecular oxygen atmosphere, temperature-dependent adsorption and dissociation of O2 typically occur on the surface. At low temperatures, below 40 K, oxygen molecules are physically adsorbed on the silver surface.24,25 Chemical adsorption dominates at temperatures be-
10.1021/jp0657843 CCC: $37.00 © 2007 American Chemical Society Published on Web 04/13/2007
6764 J. Phys. Chem. C, Vol. 111, No. 18, 2007 tween 50 and 150 K.19,24,26 At higher temperatures, above 170 K, chemisorbed molecular oxygen dissociates to oxygen atoms on the surface.19,26-30 If the temperature is high enough (greater than 420 K), the resulting O atoms may diffuse and dissolve into the Ag lattice near the surface,27,31,32 sometimes forming less than a monolayer of Ag-O within the first couple of monolayers on the Ag surface.30,32-38 It has been reported that O atoms may dissolve in bulk silver with a solubility of ∼0.0002 wt % at 200 °C.39 When Ag is exposed to ozone at 300 K, both AgO and Ag2O are formed, with molecular oxygen gas released to the environment.40,41 Because of the larger chemical potential of atomic oxygen relative to molecular oxygen42 and the consequent strong affinity of O atoms to Ag surfaces,43 atomic oxygen can react with Ag readily even at room temperature.11,14-16 The effects of atomic oxygen on Ag have been studied through experiment and modeling, using LEO exposures15,44,45 and laboratory exposures.11,43,46-49 Earlier experiments focused on the change of mass as measured with a quartz-crystal microbalance (QCM),43,45,47 electrical resistance as measured with Ag-coated actinometers,15,43,45,47,49 and surface chemistry/structure as measured by spectroscopic ellipsometry,12 Auger electron spectroscopy (AES),48 X-ray photoelectron spectroscopy (XPS),11,45,50 and X-ray diffraction (XRD).11,43,47 Below 110 °C, stable silver oxides, AgO and Ag2O, are formed on the polycrystalline silver surfaces, increasing the thickness of the Ag samples.11,43,47,50-52 When the temperature exceeds 110 °C at 1 atm of oxygen pressure, AgO is never stable and reduces to Ag2O.11,37,49,51,53,54 Several models suggest that the continuous oxidation of Ag by atomic oxygen at temperatures near 24 °C progresses through cracks in the oxide surface and spallation of the oxide scales.11,51 Some studies investigated the structure of the oxidized silver surface by scanning electron microscopy (SEM) and atomic force microscopy (AFM) and found that surface morphology changes, with increased sizes of surface grains and formation of cracks and flakes.11,50 All of these surface analysis techniques reveal only specific elements of the O-atom-induced surface changes; SEM and AFM can only investigate the morphology changes near the top of the scale, and XPS and XRD cannot reveal where in the scale Ag2O and AgO are formed. Knowledge of the entire structure and chemistry of the oxide scales is necessary for a fundamental understanding of the oxidation mechanisms of Ag by atomic oxygen. However, no direct structural observation across the whole thickness of the oxide scale has been reported previously. Cross-sectional transmission electron microscopy (TEM), with high spatial resolution, can provide essential structural and chemical information across the entire oxide scales, including the gas/oxide surface and the oxide/metal interface where O atoms react with the Ag substrate. Our initial work by cross-sectional TEM46,55 revealed that oxide scales, with thicknesses greater than 10 µm, formed on Ag(100) and Ag(111) as a result of exposure to hyperthermal atomic oxygen at 220 °C. As this temperature is slightly higher than the decomposition temperature of Ag2O (various reports place this temperature between 170 and 193 °C),20,37,54,56,57 our study provides an opportunity to study the relationship between thermodynamics and kinetics in the transformation of a singlecrystal Ag surface exposed to hyperthermal oxygen atoms. Herein, we present more detailed structural investigations of the oxide scale and interface using scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), a high-angle annular dark-field technique (HAADF), nanoarea electron diffraction patterns (NA-EDPs), and energy
Li et al. dispersive X-ray spectroscopy (EDS). These nanoprobes reveal a remarkable structural transformation of single-crystal Ag surfaces to complex polycrystalline Ag surfaces that are almost devoid of oxygen. II. Experimental Details Two experimental investigations of the transformation of single-crystal Ag surfaces under hyperthermal atomic-oxygen bombardment were performed. One experiment probed the chemistry and morphology of thick oxide scales that formed on Ag(100) and Ag(111) when these surfaces were exposed to a relatively high fluence of hyperthermal oxygen atoms, and the second experiment provided a view into the initial stages of the surface transformation after a single-crystal surface was exposed to a low fluence of hyperthermal oxygen atoms. Both experiments started with single-crystal silver samples, 10 mm in diameter by 1 mm thick, which were obtained from Accumet Materials Co. All sample surfaces were mechanically polished with 0.05 µm grit abrasive, followed by electropolishing. XRD and SEM measurements of the surfaces, as well as cross-sectional TEM observations, confirmed that the crystal faces were Ag(111) and Ag(100). A. Experiment 1. The samples, described above, were rinsed with a mixture of 25% ethanol and 75% trichloroethylene (by volume) and mounted in a vacuum chamber for exposure to atomic oxygen. Ag(100) and Ag(111) single crystals were exposed simultaneously to a hyperthermal beam containing atomic and molecular oxygen. The beam was created with a pulsed laser detonation source, and sample exposures were conducted similarly to earlier exposures in our laboratory.58,59 The average translational energies of the O and O2 components of the beam were 5.2 and 8.8 eV, respectively, with respective energy widths of 2.5 and 5 eV (full width at half-maximum). The fraction of atomic oxygen in the beam was ∼70%, with the balance being molecular oxygen. The samples were held at a temperature of 220 °C during exposure, and after exposure to the hyperthermal beam ceased, the samples were allowed to cool to room temperature in vacuum (
