Orientational epitaxy of the hexagonally reconstructed Pt(001) surface

Abstract

We describe x-ray-scattering studies of the orientational epitaxy displayed by the hexagonally reconstructed Pt(OOl) surface. At 1685 K, the reconstructed surface layer undergoes a continuous rotational transformation with respect to the underlying bulk and exhibits mean-field behavior. A second, discontinuous rotational transformation is observed at 1580 K. These results are inconsistent with the predictions of current theories and lead us to propose a mean-field description of rotational transformations.PACS numbers: 68.35.Rh, 64.70.Dv An important aspect of the growth of one crystal upon another involves their relative orientation: How are the high-symmetry directions of the ad-lattice and of the substrate lattice aligned? Our present understanding of this question has been guided by the calculations of Novaco and McTague [1], who explicitly considered incommensurate monolayers of rare gases adsorbed on a rigid, basal plane substrate of graphite. They showed that, even though there is no translational registry at the interface, there may exist a preferred orientation. This is called orientational epitaxy. Their calculations further revealed that the high-symmetry directions of an incommensurate overlayer may be rotated away from the high-symmetry directions of the substrate and that the rotation angle is determined by the incommensurability of the overlayer and by its phonon dispersion relations. In many instances, most notably for the rare gases on graphite [2] and for alkali metals on transition-metal surfaces [3], the predictions of Novaco and McTague describe the orientational epitaxy of an adsorbed layer. Other systems for which the rotational epitaxy has been characterized include layers of alkali metals intercalated into graphite [4], in situ deposited monolayers of Pb on Ag(lll) and Au(lll) electrodes [5], crystallites of Pb deposited on Si(l 11) and Ge(lll) substrates [6], and clean Au (001) surfaces held in vacuum [7] and under electrochemical conditions [8].In this paper, we describe the rotational behavior exhibited by the clean (001) surface of Pt between 300 and 1820 K. We have found that between 7 = 1685 and 1820 K, the surface reconstructs to form a buckled, hexagonal monolayer [9] on top of the bulk planes of square symmetry lying immediately beneath. The top layer is incommensurate in both directions within the surface plane, and the average area per atom is contracted by 8% relative to its value in a bulk (111) plane. In this phase, a hexagonal high-symmetry direction is aligned with the cubic (1,1,0) direction. However, as the temperature is reduced below T c = 1685 K, there is a structural transformation in which domains of the hexagonal overlayer rotate continuously to angles ±0.9° away from the bulk (1,1,0) direction. Remarkably, the rotational transformation exhibits mean-field behavior: That is, the dependence of the rotation angle on temperature follows a onehalf power law versus reduced temperature e = (T c -T)/ T c . At 1580 K, there is a second, discontinuous transformation, lea...

Potential dependent surface relaxation of the Pt(001)/electrolyte interface

“…This indicates that the Pt(001) is rougher than the ideal interface. Furthermore, at no potentials between H2 evolution and oxide formation does the spec- ular CTR exhibit the characteristic shape associated with the reconstruction [2,4,5] and no in-plane peaks associated with the reconstruction were observed, conclusive evidence that the Pt surface is in the (lxl) state. The fits to the data shown were obtained using a model similar to the one described in Ref.…”

“…A precise description of the structure of a crystal surface can be obtained by measuring crystal truncation rods (CTR's), both specular and nonspecular [2,4,5,13]. Two CTR's for a Pt(001)/0.1JV KOH solution interface at 0.02 V and 0.42 V are shown in Fig.…”

“…In contrast, platinum surfaces have generally much higher interaction energies with most adsorbates than is the case for gold, and thus adsorption may have a more profound effect on the surface structure and issues of cleanliness are of crucial importance. X-ray scattering studies have shown that the vacuum interface of the Pt(001) surface at room temperature is reconstructed, with the top layer well represented by an incommensurate-distorted-hexagonal overlayer which is rotated approximately ±0.8° from the [110] direction [5]. The vacuum interface reconstruction is removed, however, on exposure to hydrogen [6], water vapor, and several other adsorbates [7].…”

Thermal roughness of a close-packed metal surface: Pt(001)

“…Our procedures yielded a clean surface which remained clean at elevated temperatures for extended periods [13]. The sample was a Pt disk with a mosaic of 0.01° full width at half maximum (FWHM) and surface orientation such that the macroscopic surface normal was inclined at an angle of 0.06° away from the crystallographic (001) direction towards (OlO).…”

A new (3 × 3)R40° surface reconstruction of the Pt(1 1 1) surface

“…It was first observed about 50 years ago in low-energy electron diffraction (LEED) studies of clean surfaces of silicon and germanium crystals [1]. Now it is known as a very common phenomenon for many surfaces of metals, for example, (1 0 0) surface of Ir [2], Au [3][4][5], Pt [6][7][8][9], (1 1 0) plane of Au [10][11][12][13] and Pt [14,15]. The study of the arrangement of atoms on the surface of Pt is of special interest because of the industrial importance of Pt as a catalyst, as it is known that the rate of chemical reaction can be very sensitive to its surface structure [16,17].…”