Adsorbed adatoms can influence surface structure and chemistry, including faceting of metallic and model bimetallic catalyst surfaces. The importance of bimetallic catalysts based on Pt-group metals has been increasingly recognised in recent decades, as they display important advantages over classical reforming catalysts. In particular, refractory metals (W, Mo, Re …) alloyed with Pt-group metals are active catalysts for hydrogenation and hydrogenolysis reactions. The tungsten (111) surface covered with monolayer-thick films of some metals (Pt, Pd, Rh, Ir, Au) and annealed at temperatures higher than 700 K undergoes a massive reconstruction to form three-sided pyramids of nanometre-scale dimensions with mainly {211} facets (Figure 88a). A minimum coverage of the metal (1.7 x 1015 atoms/cm2 in case of Pt) is necessary for the surface to become completely faceted. The pyramids are composed of tungsten, completely covered with the thin metal film. The driving force for faceting is attributed to the difference in surface energy of the different facets, but thermal annealing is needed to achieve sufficient surface atom mobility for mass transport [1].

This nanopatterned surface is of interest because it could be used as a template to grow ordered nanostructures, for instance with specific magnetic properties. This prompted us to undertake an X-ray study of the Pt/W(111) surface as a function of temperature to determine the appropriate conditions for ordering of small nanoparticles, and then to monitor the deposition of cobalt on this nanostructured surface. The experiments were carried out on the BM32 beamline, using a newly-developed setup allowing both grazing-incidence small-angle X-ray scattering (GISAXS) and grazing-incidence X-ray diffraction (GIXD) on the same sample, in situ, in ultra high vacuum (UHV), at different growth stages (here of Pt and Co) [2]. Morphological and structural features are derived in a combined way from grazing-incidence X-ray scattering at small and large emergence angles.

First, a 1.1 monolayer of Pt was deposited on the W(111) surface. The GIXD measurements indicated that the Pt was fully lattice-strained to the substrate parallel to the surface and that the deposit was two-dimensional. The GISAXS patterns (Figure 88b) were quantitatively analysed, which yielded the morphological parameters [3]. Then, the first onset of nanofaceting was achieved by thermal annealing at ~715 K with pyramids of approximately 5 nm in size and 6 nm in separation. Annealing above 715 K resulted in a continuous increase of all the characteristic sizes of the pyramids, while keeping a dense triangular packing.

Fig. 88: (a) Scanning-tunnelling microscopy of 1 monolayer Pt/W(111) annealed at 1100 K. (b) Experimental GISAXS pattern for 1.1 monolayer of Pt/W(111) annealed at 800 K with the scattering geometry (inset).


The nanopatterned surface with the smallest pyramids has been used as a template for the growth of ordered nanostructures. A 0.8 nm-thick cobalt layer was deposited at room temperature. The three-dimensional Co islands obtained were correlated with the Pt/W nanopyramids (Figure 89a) and Co was in a relaxed state on Pt/W (Figure 89b). At approximately 800 K, a CoPt alloy was formed and became more ordered as the annealing temperature increased. At 1100 K, both defaceting and phase separation began; the CoPt alloy segregated on the W(111) flat surface, while Co formed an epitaxial layer on the {211} facets. In addition, in the temperature range of 1100-1200 K, the great majority of {211} large facets coexisted with some {110} small facets. Finally, the surface became flat again at 1250 K.

Fig. 89: (a) Schematic representation of Pt/W pyramids covered with Co islands. (b) Measured intensity during radial scans along the (h h 0.0325) direction on the facetted Pt/W(111) surface before the Co deposit, after the 0.8 nm of Co at ambient temperature, and after successive thermal annealing.


To conclude, the nanofaceting of the W(111) surface into {211} three-sided Pt/W pyramids upon thermal annealing of a 1.1 monolayer Pt deposit was investigated in situ by a combination of GISAXS and GIXD. Co deposited at ambient temperature on the facetted surface formed three-dimensional islands correlated with the W nanopyramids.



[1] K. Pelhos, T.E. Madey, J.B. Hannon, G.L. Kellogg, Surf. Rev. Lett., 5, 767-774(1999).
[2] G. Renaud, R. Lazzari, C. Revenant , A. Barbier, M. Noblet, O. Ulrich, F. Leroy, J. Jupille, Y. Borensztein, C.R. Henry, J-P. Deville, F. Scheurer, J. Mane-Mane, O. Fruchart., Science 300, 1416-1419 (2003).
[3] C. Revenant, F. Leroy, R. Lazzari, G. Renaud, C.R. Henry, Phys. Rev. B 69, 035411-035428 (2004).

Principal publication and authors

C. Revenant (a), F. Leroy (a,b), G. Renaud (a), R. Lazzari (c), A. Létoublon (a,d), T. Madey (e), Surf. Sci. 601, 3431-3449 (2007).
(a) DRFMC, CEA, Grenoble (France)
(b) CRMCN-CNRS, Marseille (France)
(c) Institut des Nanosciences de Paris (France)
(d) INSA, Rennes (France)
(e) State University of New Jersey (USA)