Twisted Nanostructures of 2H-MoS2 Slabs

Twisted Nanostructures of MoS2 Slabs

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2009 MRS Fall Meeting 1217-Y06-02.R1 Symposium Y 04-Jan-2010 Ramos, Manuel; University of Texas at El Paso, MRTI Ferrer, Domingo; University of Texas at Austin, Microelectronics Research Center José-Yacamán, Miguel; University of Texas at San Antonio, Physics Berhault, Gilles; University of Lyon, France, IRCELYON, CNRS Torres, Brenda; University of Texas at El Paso, MRTI Chianelli, Russell; UTEP, MRTI; none simulation, transmission electron microscopy (TEM), catalytic

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Twisted Nanostructures of 2H-MoS2 Slabs Manuel A. Ramos1, Domingo A. Ferrer2, Miguel José-Yacamán3, Gilles Berhault4, Brenda Torres1, Russell R. Chianelli1 1 Materials Research and Technology Institute, University of Texas at El Paso, El Paso, TX. 79968, U.S.A 2 Microelectronics Research Center, University of Texas at Austin, Austin, TX. 78751, U.S.A. 3 Department of Physics & Astronomy, University of Texas at San Antonio, San Antonio, TX. 78249, U.S.A 4 IRCELYON, CNRS – University of Lyon, Villeurbanne, 69100, France. ABSTRACT In the last decades, HRTEM approach has been quite fruitful to study structural characteristics of layered transition metal sulfide (LTMS) catalytic materials since providing direct local information about the structural organization of this quite important class of catalysts at the nanoscale level. However, up to now, HRTEM observations of some common localized structural organization like honeycomb-like structures have remained unexplained. In the present study, a structural model corresponding to stacked 2H-MoS2 slabs twisted along their basal direction is proposed to explain honeycomb like-structures observed by HRTEM. This model is based on a comparison between experimental and simulated images of 2H-MoS2 catalysts promoted with cobalt. The resulting Density of States (DOS) of the twisted structure was then calculated.

INTRODUCTION Molybdenum sulfide catalysts promoted with nickel or cobalt has been extensively studied in the last two decades [1] due to its quite important role in hydrotreating catalysts for the removal of sulfur-containing molecules contained in crude oil [2]. Many aspects in terms of atomistic structure and textural have been well defined in the literature using modern characterization tools such as X-ray diffraction, Mössbauer spectroscopy, and High Resolution Transmission Electron Microscopy. More recently, the application of quantum computational modeling to investigate the electronic structure of these solids has triggered a new interest in determining the local structure of 2H-MoS2 slabs at the atomic scale. Such calculations can be done using Ab initio and Monte Carlo techniques or Density Functional Theory methods which nowadays are available in commercial softwares i.e. Cerius2 or Material Studio [4,5,6]. One of the most fast and relevant characterization techniques is HRTEM since providing information about several physical properties like d-spacing, folding, surface contact, dislocations. However, even if 2H-MoS2 presents a very distinguishable layered structure characterized by “fringes” on HRTEM pictures, some features like honeycomb-like structure were never satisfactorily explained [7]. The objective of the present study was to address this problem by confronting experimental and theoretical HRTEM results.

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EXPERIMENT HRTEM experiments were done using a JOEL 4000-EX TEM with a 0.17 nm of resolution equipped with a charge couple device camera. We present here a HRTEM simulation for two typical 2H-MoS2 slabs (“seeds”) models using simulaTEM contained in Cerius2 molecular software; the computational TEM simulator performs a full dynamical calculation by multi-slice method using projected potential f (U ) = ∑n ai e( −ibU ) , where U represents coordinates in 2

i =1

reciprocal space (u, v, w) [14] and same microscope operational conditions as in reference [7]. The molecular model used here was made using crystal builder module contained in Materials Studio. This model corresponds to two typical 2H-MoS2 slabs [12, 15], rotated in the c-axis [(001) basal plane] by a 12o with respect the ab-plane which was placed in a supercell (space group P1, non-symmetry) with a = 16.0 Å, b = 17.0 Å, and c = 15.0 Å and α = β = 90o and γ = 120o. For further investigations, the supercell was subjected to band structure and density of states calculations using Cambridge Serial Total Energy Package (CASTEP) with a revised Perdew-Burke-Ernzerhof functional (general gradient approximation), with a cutoff energy of 290 eV in the reciprocal space for gamma point only and a self-consistent field (SCF) convergence threshold of 1x10-6 and ultra-soft pseudopotential of 290 eV for Mo and 220 eV for S. For the two 2H-MoS2 slabs rotated at certain 12o angle, the inter-layered (c-direction) distance of 3.484 Å for sulfur-sulfur (S-S) bonding and 6.403 Å for molybdenum-molybdenum (Mo-Mo) bonding in the c-axis direction was kept as reported in the literature [8]. For comparison similar calculations were done using two typical 2H-MoS2 slabs non-twisted structure with which was placed in a supercell arrangement (space group P1, non-symmetry) with a = 16.0 Å, b = 17.0 Å, and c = 15.0 Å and α = β = 90o and γ = 120o. DISCUSSION Figure 1 reports a typical Moiré pattern observed on commercial 2H-MoS2/Co catalysts in which honeycomb-like features can be seen [7]. This pattern was in fact observed for 2HMoS2 slabs seen along their basal plane [16]. Observation of molybdenum disulfide on high resolution transmission electron microscope (HRTEM) pictures is rarely obtained along the basal direction while “fringes” characteristic of slabs shown along their edges are more commonly detected. Molybdenum disulfide is a layered compound formed of more and less stacked slabs maintained by van der Waals forces [12, 16]. These slabs are in fact rarely stacked perfectly on commercial catalysts and they present generally some disorders resulting from a rotation of slabs from each other like a spread “deck of cards”. This rotational disorder or turbostraticity must be considered if one wants to precisely describe the nanoscale organization of this catalyst. Therefore, starting from a model “seeds” formed of two stacked 2H-MoS2 slabs, we have envisaged different degrees of rotation along the basal direction between these 2H-MoS2 layers. The optimal fit between simulated and experimental pictures (figure 1A and 1B) corresponds to a rotation of 12° in the c-axis between the two 2H-MoS2 slabs. Therefore, a certain degree of turbostraticity or rotational disorder is confirmed on real 2H-MoS2 catalysts. This disordered organization is not without consequences on the catalytic properties of molybdenum disulfide. Indeed, Daage and Chianelli [9] have proposed that two different types of sites, “rim” and “edge” sites are present on 2H-MoS2 slabs. These sites only differ by their location on stacked

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slabs. While “rim” sites able to perform hydrogenation and carbon-sulfur (C-S) rupture steps in the hydrodesulfurization of dibenzothiophene are located on exterior layers, “edge” sites able only to break C-S bonds are on interior layers. These different catalytic functionalities observed between “edge” and “rim” sites are a direct consequence of their location. Indeed, the prerequisite for hydrogenation of dibenzothiophene is a flat adsorption by the benzene ring. This mode of adsorption is sterically possible only on “rim” sites. The following result showing rotational disorder would propose an explanation about the absence of hydrogenation on “edge” sites: indeed, such a turbostratic disorder must increase the steric hindrance of “edge” sites and limits seriously the possibility of a flat adsorption of dibenzothiophene leading to hydrogenation. .

Figure 1: A) left: HRTEM image of superlattice corresponding to 2H-MoS2 slabs from [7]. B) Right: Bright field HRTEM simulation for two 2H-MoS2 slabs rotated by 12o which corresponds to superlattice model proposed in figure 3.

9.49 Å

9.49 Å

9.49 Å 9.49 Å

Figure 2: Left: Two slabs of 2H-MoS2 (“seeds”) without rotation (9.49 Å x 9.49 Å). Right: Two slabs of 2H-MoS2 (“seeds”) rotated by 12o (9.49 Å x 9.49 Å) used in the Band Structure and DOS calculations corresponding results are presented in figure 4 and 5.

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56.66 Å

12o

56.66 Å

Figure 3: A proposed supercell of two typical 2H-MoS2 slabs (50.66 Å x 50.66 Å) i.e. reference [12, 15] rotated by 12o showing the honeycomb like structure as it is observed in reference [7] and used for TEM simulation presented in figure 1B. Band structure calculations for both models composed of two slabs of 2H-MoS2 (un-twisted) or of two slabs of 2H-MoS2 (twisted by 12o) were carried out using DFT methods. The models are presented figure 2. Table 1 present results while figures 4 and 5 present the resulting total density of states (DOS) for untwisted and twisted typical 2H-2H-MoS2 slabs. Supercell of 2H-MoS2 Twisted (12o) Un-twisted

Band Structure Total Energy 831.81 eV/atom 830.99 eV/atom (E12o - E0o) = 0.82 eV/atom

Table 1: obtained DFT calculations for two slabs of 2H-MoS2 un-twisted and 12o twisted with respect of ab-plane. All calculations were done using CASTEP with a revised Perdew-BurkeErnzerhof functional. As shown figure 4, the density of states of the untwisted 2H-MoS2 model formed of finite stacked slabs presents a slight electronic density contribution at the Fermi level contrary to what is expected based on the bulk semiconductor properties of molybdenum disulfide. In fact, the finite size of these slabs leads to an increasing contribution of the electronic density of edge sites [10]. These sites were already observed to possess some metallic properties as reported by Raybaud et al. [10] for the 2H-MoS2 (010) surface. This metallic contribution is therefore expected to be significant on finite 2H-MoS2 slabs. This point is also reminiscent of the observation of metallic properties on the so-called “brim” sites located on edges by Lauritsen et al. [11]. The rotational disorder leads to an implication of this metallic character as observed figure 5 from the density of states for the twisted model.

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This increased metallic character resulting from a higher rotational disorder is once again important from a catalytic point of view since it would not only hinder the hydrogenating character of “edge” sites according to the model of Daage and Chianelli [9] but also it would enhance the hydrogenating character of “rim” sites leading to a quite distinct two-site kinetic comportment in hydrodesulfurization.

Figure 4: Calculated density of states using CASTEP for 2H-MoS2 slabs (“seeds”) without rotation corresponding to figure 2-Left, in agreement with reference [13].

Figure 5: Calculated density of states using CASTEP for two typical 2H-MoS2 slabs (“seeds”) rotated by 12o corresponding to figure 2-Right.

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CONCLUSIONS Computational modeling for observed honey-comb structure in HRTEM techniques is presented here. Modeled HRTEM by using SimulaTEM contained in Cerius2 of honeycomb-like pictures of 2H-MoS2 catalysts showed the presence of a certain degree of rotational disorder of stacked molybdenum disulfide slabs in real commercial catalytic systems. The presence of such a type of disordered organization would have important consequences on a catalytic point of view for hydrodesulfurization purposes, which in principle could be an optimal structure to have strong promotion activity, as observed in DOS results. Indeed, this rotational disorder provides a basis for explaining the different abilities to hydrogenate between sites located on exterior (“rim”) or interior (“edge”) layers. This disordered organization also seems to increase the hydrogenating character of “rim” sites. ACKNOWLEDGMENTS The authors want to thank Consejo Nacional de Ciencia y Tecnología, Mexico, Departamento de Educación y Cultura del Gobierno del Estado de Chihuahua, MRTI and Department of Metallurgy and Materials at the University of Texas at El Paso. REFERENCES 1. O. Weisser, S. Landa, Sulphide Catalysis, Their properties and Applications, Pergamon Press, N.Y. (1973). 2. H. Topsøe, B.S. Clausen, F.E. Massoth, in: J.R. Anderson, M. Boudard (Eds.), Catalysis, Science and Technology, 11, Springer, Berlin, (1996). 3. H. Topsøe, B.S. Clausen, R. Candia, C. Wivel, S. Morup, Journal of Catalysis 68, 433 (1981). 4. F. Pedraza, J. Cruz-Reyes, D. Acosta, M.J. Yañez, M. Avalos-Borja, S. Fuentes, Journal of Physics Condensed Matter 5, 219 (1993) 5. R.R. Chianelli, G. Berhault, P. Raybaud, S. Kasztelan, J. Hafner, H. Toulhoat, Applied Catalysis, A, 227, 83 (2002). 6. L.S. Byskov, J.K. Nørskov, B.S. Clausen, H. Topsøe, Journal of Catalysis 187, 109 (1999) 7. R. R. Chianelli, A.F. Ruppert, M. José-Yacamán, A. Vázquez-Zavala, Catalysis Today 23, 269 (1995). 8. L.S. Byskov, B. Hammer, J.K. Nørskov, B.S. Clausen, H. Topsøe, Catalysis Letters 47, 177 (1997) 9. M. Daage, R.R. Chianelli, Journal of Catalysis 149, 414 (1994). 10. P. Raybaud, J. Hafner, G. Kresse, H. Toulhoat, Physical Review Letters 80, 1481 (1998). 11. J. V. Lauritsen, S. Helveg, E. Lægsgaard, I. Stensgaard, B. S. Clausen, H. Topsøe, F. Besenbacher, Journal of Catalysis 197, 1 (2001). 12. J. A. Spirko, M. L. Neiman, A. M. Oelker, K. Klier, Surface Science 542 (2003) 192–204. 13. D.W. Bullet, J. Phys. C 11 (1978) 4501. 14. A. Gómez-Rodríguez, L. M. Beltrán del Río, R. Herrera-Becerra, Ultramicroscopy (In press, Accepted Sept. 2009). 15. Jellinek, F., Brauer, G., Müller, H.: Nature (London) 185 (1960) 376. 16. P. Santiago, J.A. Ascencio, D. Mendoza, M . Pérez-Alvarez, A. Espinosa, C. Reza-Sangermán, P. Schabes-Retchkiman, G.A. Camacho-Bragado, M. José-Yacamán, Appl. Phys. A 78, 513–518 (2004)