Adsorption of Iodine on Pt(111) surface. Alexandre Tkachenko Marcelo Galván Nikola Batina

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1 Adsorption of Iodine on Pt(111) surface Alexandre Tkachenko Marcelo Galván Nikola Batina

2 Outline Motivation Experimental results Geometry Ab initio study Conclusions

3 Motivation Unusual structural richness from experimental studies (STM, LEED, etc) (*) Iodine protects the Pt(111) surface from contamination Presence of collective monolayer effects No previous periodic studies Distinct structural phases can be used for controlled molecule adsorption (*) Schardt et al., Science 243, 1989

4 Experimental Results Four different structural phases Very similar results for vacuum, air and solution studies (*) Convergent results for several experimental techniques, still for some coverages unresolved phases exist (*) (*) O. M. Magnussen, Chem. Rev. 2002, 102

5 Geometry The distinct phases of the same system can often behave quite differently, therefore it is necessary to propose the most likely candidates of all the possible arrangements of Iodine on Pt(111) surface It is not feasible to sample the geometrical parameters space based on ab initio approach. Molecular dynamics could be applied, but only a small fraction of space could be sampled

6 Geometry We suppose a hard-sphere model of the Iodine adsorption on Pt(111) surface, where the system is defined by the ratio between both species radii (R(Pt) = and R(I) = 2.06). This ratio can be modified to simulate the monolayer compression This is clearly a very crude approximation, but it will be further based on ab initio calculations

7 Geometry 1) Starting with a Pt(111) substrate and the Iodine monolayer R =1.388 R=2.06 2) Merging these two layers

8 Geometry Iodine is known to form perfect hexagonal arrangements on the Pt(111) surface, those only differ in their coverage (the ratio of I to Pt atoms in the unit cell)

9 Geometry The hexagonal arrangement is preserved if we apply one of the following transformations : Translation X Y - Compression - Rotation -Translation Rotation

10 Geometry A detailed analysis of each structural phase can be performed and important geometrical parameters extracted :

11 Geometry Dynamical analysis were performed for several kinds of transformations :

12 Geometry Based on the previous analysis, three distinct structures can be identified for the hard-sphere model which are labeled as : - (3x3)-sym - (3x3)-asym - (3x3)-linear The first two are known to exist experimentally, while the third one is new

13 Geometry One simple parameter which can be used for the present system and which seems to describe the geometry of the preferred structural phases is 'Average substrate-adsorbate distance'. The three phases can be arranged based on this parameter. Distance from the substrate normalized to the substrate atomic radius / A 25% (2.344) 75% 75% (2.379) 25% 50% (2.397) 50% atop site bridge site hollow site Average substrateadsorbate distance (3x3) sym (3x3) asym (3x3) lin

14 Geometry X and Y translations keeping the intersphere distance and rotation angle fixed gives : (3x3)-linear Y shift (3x3)- symmetric (3x3)-asym X shift

15 Complete coverage interval analysis Minimum average height between adsorbate and substrate spheres / relative units Adsorbate inter-sphere distance normalized to substrate radius/ relative units

16 Extending geometry We are not considering any interactions between Iodine atoms with the average distance parameter. This could be taken care of by introducing a more complex geometrical parameter : Density Vertical position Roughness

17 ALSA All of these analysis tools have evolved into a software product called ALSA (Atomic Level Surface Assembler) with the following characteristics : *) Design and analysis of the surface of crystalline material with the wellordered structure *) Atomic adlayer simulation in the static and dynamic modes for monolayers or unlimited number of adlayers *) Different modes of graphical presentation (two-dimensional, threedimensional, STM like image presentation (topographic mode), crosssection analysis, and many others. *) Possibility to work on different platforms: Windows, Linux or Unix. *) Open source code for easy upgrade and future development.

18 Geometry conclusions (3x3)-sym and (3x3)-asym are ''closer'' to the substrate being a local minima of the average substrate-adsorbate distance, these two structures were the only ones observed for a specific coverage interval for Iodine on Pt(111) (3x3)-lin is the farthest phase from the substrate, being a local maximum of the average substrateadsorbate distance Even quite ''naive'' parameters such as average distance could be used in the search for candidates for stable structural phases

19 DFT calculations Plane wave pseudopotential calculations(*) based on DFT were performed to elucidate the electronic structure of Iodine adsorption on Pt(111) surface A pseudopotential for Pt included relativistic effects A periodic slab model of Pt(111) from 3 to 5 layers was used (supercell). LDA VWN is used for geometry and GGA PBE for adsorption energy calculations An energy cutoff of 40 Ry was used for all the calculations (*) Payne, M. C.; Teter, M. P.; Allan, D. C.; Arias, T. A.; Joannopoulos, J. D. Rev. Mod. Phys. 1992, 64, 1045.

20 In all the calculations, the relaxation of the Iodine layer and the first two Platinum layers were performed using LDA, the energy calculations were done with a 5 Pt layers slab and Iodine adsorbed on both sides to cancel the dipole moment DFT calculations

21 Energy at equilibrium Z for atop-fcc trajectory Energy Displacement

22 Different sites Density difference Site Equilibrium Charge Exp. Eq. Z Z transfer fcc hcp bridge atop Fcc Atop

23 Charge transfer We use Hirshfeld method for charge partitioning(*), where an effective charge on the adsorbate is defined as an integral over _eff(i) : (*) F. L. Hirshfeld, Theoret. Chim. Acta 44, 1977

24 Structural phases The charge transfer per atom changes drastically as the Iodine monolayer coverage increases (from 0.14 on =0.25 to 0.1 on =0.333 to 0.04 on =0.43), which indicates the importance of the collective adsorbate effects. This simulation would require very large cluster model with the localized basis set approach Although a detailed analysis of bonding can not be made with plane waves, the density difference figures provide qualitative insight into its structure

25 Structural phases All the experimentally found phases were confirmed to be local energy minima by performing relaxation of the Iodine monolayer and the first two layers of the Pt(111) slab optimizing from many different starting points. Interestingly, several other phases were found to be local minima. Ab initio molecular dynamics for a few picoseconds at room temperature on several of these structures gave no indication of phase transitions

Energetically, this phase lies between the (3x3)-sym and (3x3)-asym structures, both of which were observed experimentally.

26 New Phases We have relaxed the (3x3)-linear phase and obtained a local minima which has never been observed before for Iodine on Pt(111). The new phase resembles (3x3)-linear very much, with two Iodine atoms per unit cell shifted into three-fold hollow sites. Energetically, this phase lies between the (3x3)-sym and (3x3)-asym structures, both of which were observed experimentally. This phase would have similar LEED pattern as the above two, so it could not be distinguished in some experiments Asym : 12.9 kcal/mol Zigzag : 4.5 kcal/mol Sym : 0 kcal/mol

27 DFT conclusions Collective monolayer effects are very important Many more structural phases may be stable than those found in experiments The structures generated with geometrical principles are good starting points for calculations The both approaches (Geometry and ab initio calculations) are complimentary

28 Conclusions Only the simplest structures appear to be visible in experiments (perfect hexagonal pattern). Some recent papers (1996-) reveal new complex structures for halogens on Pt(111) under different experimental conditions(*) Several (many?) other structures may exist for Iodine on Pt(111) surface Simple geometric parameters can be used to identify candidates for stable configurations, what could let design structures with defined geometrical properties. In turn, these could be analyzed using ab initio periodic supercell approach (*) J.M. Orts et al, J. Phys. Chem 1996, 100

29 Acknowledgments Prof. Dr. Nikola Batina Prof. Dr. Marcelo Gálvan Felipe Aparicio UAM Supercomputing Laboratory SNI/CONACYT for financial support

Unequal-Sphere Packing Model for Simulation of the Uniaxially Compressed Iodine Adlayer on Au(111)

21710 J. Phys. Chem. B 2005, 109, 21710-21715 Unequal-Sphere Packing Model for Simulation of the Uniaxially Compressed Iodine Adlayer on Au(111) Alexandre Tkatchenko and Nikola Batina* Laboratorio de Nanotecnologia