Enhanced nucleation and decreased growth rates of Cu 2 O in Cu 0.5 Au 0.5 (001) thin films during in situ oxidation
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The initial oxidation behaviors of Cu–50 at.% Au (001) single-crystal thin film were studied by in situ ultra-high-vacuum transmission electron microscopy to model nano-oxidation of alloys with one oxidizing component and one inert component. The oxidation behaviors such as incubation time, oxide nucleation rate, oxide growth kinetics as well as nucleation activation energy were greatly changed by the addition of nonoxidizing Au. The reasons for these changes, such as Au segregation to the top surface, a decrease in Cu activity, and reduced lattice mismatch due to the addition of Au, were discussed, and a qualitative analysis of nucleation energetics was given.
I. INTRODUCTION
Elucidating the initial oxidation mechanisms of metals and alloys has great impact on many diverse fundamental and engineering problems, from corrosion, to passivation properties,1,2 and to the synthesis of self-assembled nano-oxide arrays for optical,3 magnetic,4 or catalytic applications.5,6 Classic investigations of the oxidation behavior of metals have been based mostly on thermogravimetric analysis (TGA), which measures weight gain and oxygen consumption. Classic models of oxidation assume uniform oxide film growth, and the growth of the oxide layer is facilitated either by anion or cation diffusion through the scale. TGA can provide important information about the dominant migrating defects such as metal cations, oxygen anion vacancies, and/or interstitials for the transport of the cations and anions. Yet, it is well known that many metals initially form oxide islands (e.g., Ni,7,8 Fe,3,4,9 Pb,10 and Ti11) that later coalesce into an oxide scale. Hence, it is more important in these cases to visualize the initial nucleation and growth of the oxide islands than just the weight changes. Many elegant surface science studies have been performed to reveal the interaction of oxygen gas with bare metal surfaces,10,12–14 but these studies only extend to a few monolayers. In situ ultra-high-vacuum (UHV) transmission electron microscopy (TEM) has been
a)
Address all correspondence to this author. e-mail: [email protected] b) This author was an editor of this focus issue during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www. mrs.org/publications/jmr/policy.html. DOI: 10.1557/JMR.2005.0237 1902
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 7, Jul 2005 Downloaded: 11 Mar 2015
demonstrated to be a powerful tool in visualizing the transient regime of pure metal oxidation from the nucleation of oxide to the formation of thermodynamically stable oxide. Information inaccessible to both surface science study and traditional oxidation methods can be ascertained. Previous works of Yang and colleagues15–18 and Zhou,11 using Cu as a model system, clearly demonstrated that heteroepitaxial concepts used for film growth also describe the nano-oxidation of metals surprisingly well, and a vast range of information is yielded regarding the dynamics of oxide f
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