ADDIDTIONAL INFORMATION
Tuning the stoichiometry of surface oxide phases by step morphology:
Ag(511) vs Ag(210)
L. Savio*1,2, C. Giallombardo1,3, L. Vattuone1,3, A. Kokalj4, and M. Rocca1,2
1
Dipartimento di Fisica, Università di Genova, 2IMEM/CNR and 3CNISM Genova,Via Dodecaneso
33, 16146 Genova (Italy)
Jožef Stefan Institute, Jamova 39, Ljubljana, (Slovenia)
4
*Corresponding Author. E-mail: savio@fisica.unige.it
The XPS measurements.
Still open questions about the work we have presented in the related letter are:
a) Why, while on Ag(210) the on-surface and sub-surface O species are clearly discernible by
XPS, on Ag(511) only one peak at binding energy Eb~530 eV is detected below room
temperature?
b) Which is the nature of the O species with Eb=528.1 eV forming on Ag(511) above room
temperature? In particular, why adsorption at the proposed h2 site would induce a shift of
the O1s binding energy to 528.1 eV, while all other O species have Eb~530 eV without
noticeable chemical shift?
We underline that a complete answer is not trivial and would require a further set of high resolution
XPS (and possibly XPD) experiments and/or the calculation of chemical shifts using ab-initio
methods. However the following discussion, which was omitted from the manuscript because of
length constraints, may be useful to the interested reader.
XPS measurements on Ag(210) were performed at the ELETTRA synchrotron radiation source,
with a high photon flux and a typical resolution better than 0.1 eV. Moreover, the binding energy of
the AgO-like phase was 529.6 eV, i.e. degenerate with the one of carbonates but easy to evaluate by
subtraction [1]. On the contrary, a conventional, non monochromatised X-ray source with a
resolution of 0.9 eV was employed for experiments on Ag(511). Such resolution, sufficient to
resolve a clearly separated oxide-related peak as the one detected on Ag(210), is not enough if the
oxide intensity is embedded in the major peak due to chemisorbed oxygen.
The information provided by XPS demonstrates that:
a) Chemisorbed atomic and molecular oxygen and subsurface oxygen (oxide-like phase) give rise to
a peak at an O1s binding energy Eb 530 eV. This is the only feature present in the O1s spectra
below RT and results, evidently, from the superposition of the contributions of the above mentioned
species. The difference in their Eb(O1s) must be smaller than experimental resolution. A summary
of the O1s binding energy measured on Ag surfaces is reported in [2,3].
b) There is a further, on-surface atomic oxygen species with Eb=528.1 eV. It is stable above RT and
strongly reactive towards CO. Since the XPS intensity at 528.1 eV grows in that temperature range
in which the oxide related vibration at 68 meV disappears, we suggest that thermal dissolution of
the oxide causes a, at least partial, segregation of oxygen atoms from the tetrahedral interstitials to
the closest on-surface location (the h2 site). The existence of O species characterised by different
O1s binding energy is not surprising [3]. In particular, on Ag(100) we reported both a covalent O
moiety with Eb(O1s)=530.3 eV and an oxidic one with Eb(O1s)=528.3 eV, depending on crystal
temperature [2]. The former corresponds to O-atoms sitting in the previous four-fold hollows of a
missing row reconstructed surface while the latter consists of adatoms in the four-fold hollow sites
of an unreconstructed Ag(100) layer. Now we note that, among all investigated sites on Ag(511),
h2 is the only one with a pure four-fold hollow geometry since all Ag atoms have coordination 8
(i.e. equal to or larger than the coordination number on Ag(100) terraces). All other on-surface sites
(h1, hcp and fcc) have at least two Ag atoms with reduced coordination. This may suggest a
difference also in the chemical state of chemisorbed oxygen and thus in its O1s binding energy.
Finally we mention that the investigation of Ag3d5/2 bands could not provide additional information
since the resolution was not sufficient to investigate possible core-level shifts; surface core-level
shifts are expected to be indeed quite small for Ag [4] and also in presence of a significant oxygen
coverage the shift is of only -0.4 eV with respect to the Ag(100) atoms not in contact with O [2]. In
our case the relative amount of the oxide-like phase with respect to chemisorbed oxygen is
reasonably small, so that also a possible core-level shift would be small and difficult to be detected.
[1] L. Savio et al., J. Phys. Chem. B 110, 942 (2006)
[2] M. Rocca et al., Phys. Rev. B 61, 213 (2000)
[3] A.I. Boronin, S.V. Koscheev and G.M. Zhidomirov, J. El. Spectr. and Rel. Phenomena 96, 43
(1998)
[4] G. Pirovano et al. Phys. Rev. B 54, 8892, (1996)