Elisa Jimenez-Izal, Jon M. Matxain, Mario Piris and Jesus M. Ugalde Kimika Fakultatea, Euskal Herriko Unibertsitatea and Donostia International Physics Center,

PK 1072, 20080 Donostia, Euskadi (Spain)elisa.jimenez@ehu.es

DESIGNING POROUS SOLIDS BASED ON ENDOHEDRALLY DOPED CdS NANOCLUSTERS

[1] E. Jimenez-Izal et al., J. Phys. Chem. C 114, 2476 (2010)[2] E. Jimenez-Izal et al., Phys. Chem. Chem. Phys. DOI: 10.1039/c2cp41273j[3] Handbook of Zeolite Science and Technology, ed. S. M. Auerbach et al., Marcel Dekker, New York, 2003[4] J. M. Soler et al., J. Phys.: Condens. Matter 14, 2745 (2002)[5] J. Perdew et al., Phys. Rev. Lett. 77, 3865 (1996)[6] B. Hammer et al., Phys. Rev. B: Condens. Matter 59, 7413 (1999)[7] N. Troullier et al., Phys. Rev. B: Condens. MatterPhys. Rev. B: Condens. Matter 43, 1991 (1993)

CdS clusters have been observed to trap alkali metals and halogen atoms. Furthermore, the electron affinities (EA) of halogen doped clusters and the ionization potentials (IE) of the alkali doped clusters have been predicted to be very similar [1]. This makes them suitable to form cluster-assembled porous solid materials. With this aim we have focused on the assembly of bare CdiSi and endohedral K@CdiSi - X@CdiSi (i= 12, 16, X= Cl, Br) clusters in order to obtain new solids with tailored semiconducting and structural properties [2]. Since these hollow nanoclusters possess square and hexagonal faces, three different orientations have to be considered, namely, edge-to-edge, square-to-square and hexagon-to-hexagon, which lead to distinct zeolite-like nanoporous bulk CdS solid phases denoted as SOD, LTA and FAU respectively [3]. The thermal stability of these new solids was confirmed by quantum molecular dynamics calculations.

INTRODUCTION METHODMolecular solids composed of both bare and endohedrally doped CdiSi clusters were explored by DFT calculations performed by using the SIESTA computer code [4]. Both the intra- and intercell parameters were optimized. Exchange and correlation effects were described by using the generalized gradient approximation (GGA), within the rPBE functional [5,6], and a basis set of double-z plus (DZP) polarization quality. Core electrons were replaced by Troullier-Martins norm-conserving pseudopotentials [7]. To further explore the thermal stability of these new solid phases, ab-initio thermal molecular dynamics simulations were carried out for 10ps, with a chosen time-step of 1 fs, at 300K.

BIBLIOGRAPHYACKNOWLEDGEMENTS

The Basque Government (SAIOTEK program), the SGI/IZO-SGIker UPV/EHU supported by Fondo Social Europeo and MCyT is gratefully acknowledged for generous allocation of computational resources.

The characterized new crystalline phases of CdS are shown above. These solids are low-density crystalline nanoporous materials that might be useful in a wide range of applications ranging from molecular sieves for hetereogeneous catalysis to gas storage templates. All the solids have been found to be thermodynamically stable with cohesive energies similar to many soft solids. Furthermore, the endohedral doping increases their stability. However, these phases are metastable, as it can be observed in the total energy vs cell volume graphic (bottom, left), where wurtzite is clearly the most stable phase. The analysis of the band gaps at the point Γshows that all the solids are predicted to be semiconductors. Moreover, endohedral doping decreases the band gaps. This can be understood looking at the band plots (bottom, in the middle): observe that some of the dopant-occupied bands are located between valence and conducting bands. In all cases, SOD structure is the phase with the smallest band-gap because it has the most compact crystalline structure. Regarding the molecular dynamics simulations, all the solids are predicted to be thermally stable. At the bottom right, the trajectory of the dopant atoms during the MD simulation is shown for K@Cd12S12-Br@Cd12S12 solid. Thus, we conclude that it should be possible to stabilize new nanoporous polymorphic solid phases of CdS, with much larger unit cells as comparing with the solid phases found in nature.

RESULTS AND DISCUSSION

SODLTAFAU

Cd12S12 K@Cd12S12-Cl@Cd12S12

FAU

Cd12S12 Cd16S16 K@Cd16S16-Br@Cd16S16