Adsorption of oxygen onPt3Sn(111)studied by scanning tunneling microscopy and x-ray photoelectron diffraction

Abstract

The adsorption of oxygen on the ͑111͒ surface of the Pt 3 Sn alloy was studied by means of scanning tunneling microscopy ͑STM͒ and x-ray photoelectron diffraction ͑XPD͒. As Auger electron spectroscopy, low-energy ion scattering, and x-ray photoelectron spectroscopy indicate, upon oxygen exposure ͑range 10000 L, with the sample at circa 770 K͒ Sn segregates to the surface and forms a two-dimensional tin-oxygen layer.In the x-ray photoemission spectra no pronounced chemical shift is visible that would indicate a thicker tin oxide layer. After exposure to 10000 L O 2 at 770 K low-energy electron diffraction shows a streaky 2ϫ2 pattern with additional spots. Scanning tunneling microscopy images show a surface with a 2ϫ2 periodicity characterized by an high defect density. The corrugation of this surface is substantially higher than that of the clean surface. After annealing in vacuum at temperatures ranging from 600 to 800 K, a sharp 4ϫ4 low-energy electron diffraction pattern can be observed. STM then reveals a superlattice of depressions, the remaining protrusions are slightly laterally displaced from their 2ϫ2 positions. X-ray photoelectron diffraction intensities of the 4ϫ4 phase show hardly any change for Pt 4 f , whereas Sn 3d azimuthal curves measured at higher polar angles are substantially modified after oxygen exposure. In order to understand the nature of the features observed in the STM images, the experimental XPD curves were compared with single and multiple scattering cluster calculations performed for various structural models. On the basis of these results we propose a model involving the reconstruction of the substrate surface.

Surface alloys and alloy surfaces: the platinum-tin system

“…8% can diffuse back into the bulk of the crystal but are stranded on the surface. The identification of the clusters as Sn is supported by recent O adsorption experiments [36]. Upon oxygen adsorption the clusters disappear and become part of the Sn-O overlayer structure formed.…”

Adsorption of oxygen onPt3Sn(110)studied by STM and LEED

“…No inner structure was observable, in all STM images the protrusions appear as uniform entities. In all known studies of the oxidation of Pt-Sn surfaces afterwards the outermost surface layer͑s͒ consisted merely of Sn and O, [5][6][7][8]10,11,[13][14][15][16][17] in agreement with the strong affinity of Sn toward O. Taking the large corrugation of Ϸ0.7 Å and diameter of Ͼ6.0 Å of the protrusions into account, they probably do not correspond to single atoms.…”

“…18,23,24 In this study the Pt 3 Sn͑110͒ alloy surface was oxidized following the same procedure as in the study of the ͑111͒ surface. 11 LEED afterwards showed an c͑4 ϫ 2͒ pattern, i.e., an additional c͑2 ϫ 2͒ superstructure with respect to the p͑2 ϫ 1͒ of Pt 3 Sn͑110͒. STM images of the oxidized surface readily elucidate this structure.…”

“…Recently the oxidation of Pt 3 Sn͑111͒ alloy surface 11,12 and of Sn-Pt͑111͒ surface alloy [13][14][15][16][17] single crystals has been studied. Both systems behave similar upon oxygen exposure: a very thin Sn-O layer is formed that is comprised of only a few atomic layers.…”

Disordered surface structure of an ultra-thin tin oxide film on Rh(100)

“…Since the tin-oxide/metal and tin/metal interfaces exhibit enhanced catalytic activity, 6-8 ultra-thin tin oxide films have been intensively investigated on several single crystals such as Pt(111), [9][10][11] Pd(111), 12 Au(111), [13][14][15][16] Pt 3 Sn(111), 10,17,18 and Rh(111). 19 The structure of the tin oxide film on Rh(111) was confirmed by STM, Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), LEED, and DFT calculations.…”