| AVS 58th Annual International Symposium and Exhibition | |
| In Situ Spectroscopy and Microscopy Focus Topic | Monday Sessions |
| Session IS+AS+SS-MoM |
| Session: | In Situ Studies of Catalysis and Gas-Solid Reactions |
| Presenter: | Tiffany Kaspar, Pacific Northwest National Laboratory |
| Authors: | T.C. Kaspar, Pacific Northwest National Laboratory T. Droubay, Pacific Northwest National Laboratory S.A. Chambers, Pacific Northwest National Laboratory P.S. Bagus, University of North Texas |
| Correspondent: | Click to Email |
For decades, it has been a goal to elucidate the mechanisms behind the unique chemistry of both oxygen-exposed silver metal and silver oxides. Silver compounds in bulk, thin film, and nanoparticle form are widely investigated for applications including industrially-relevant catalysis, electrochemistry, transparent conducting oxides, and antimicrobial coatings. Determining the chemical state of both silver and oxygen is critical to developing a mechanistic understanding of the remarkable properties of these materials. Ex situ x-ray photoelectron spectroscopy (XPS) has been applied, starting in the 1970’s, to determine the chemical state of Ag in various silver metal and silver oxide compounds. In contrast to most elements, Agx+ (x>0) cations exhibit a negative binding energy (BE) shift relative to metallic Ag(0); thus, the lowest XPS core level binding energy observed for the Ag 3d peak, 367.3 eV, has been assigned to Ag(III) in AgO [Ag(I)Ag(III)O2]. However, the XPS analysis has been hindered by the ease with which silver oxides form carbonate species upon atmospheric exposure, as well as the instability of silver oxides in vacuum. In this work, silver oxide films have been formed under very oxidizing conditions, by molecular beam epitaxy (MBE) deposition of silver metal in the presence of activated oxygen. In situ XPS was then collected in an appended chamber. For the most highly oxidizing deposition conditions, a substantially lower BE, 366.8 eV, was found for the Ag 3d peak, with an associated satellite located at 368.2 eV. This oxide species proved unstable in vacuum over several days, but could be recovered by further exposure to activated oxygen. Based on the decomposition behavior of the Ag 3d and O 1s spectra, the low BE species was assigned as Ag(III), while the previous peak position for Ag(III) was re-assigned as Ag(I). These assignments are supported in part by electronic structure calculations predicting the photoemission spectra of Ag(III). The combination of highly oxidizing deposition conditions and in situ characterization allowed identification of the true Ag(III) XPS spectrum for the first time.[1]
[1] T.C. Kaspar, T. Droubay, S.A. Chambers, and P.S. Bagus. J. Phys. Chem. C 114 21562 (2010).