XAFS Study of MoS2 Intercalation Compounds Y. Zubavichus, A. Golub, Y. Novikov, Y. Slovokhotov, A. Nesmeyanov, P. Schilling, R. Tittsworth
To cite this version: Y. Zubavichus, A. Golub, Y. Novikov, Y. Slovokhotov, A. Nesmeyanov, et al.. XAFS Study of MoS2 Intercalation Compounds. Journal de Physique IV Colloque, 1997, 7 (C2), pp.C3-1057-C3-1059. <10.1051/jp4:19972136>.
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J PHYS IV FRANCE 7 (1 997) Colloque C2, SupplCment au Journal de Physique I11 d'avril 1997
XAFS Study of MoS2 Intercalation Compounds Y.V. Zubavichus, A.S. Golub, Y.N. Novikov, Y.L. Slovokhotov, A.N. Nesmeyanov, P.J. Schilling* and R.C. Tittsworth* Institute of Organoelement Compounds (INEOS RAS), 28 Vavilov St., Moscow 11 7334, Russia * Center for Advanced Microstructures and Devices (CAMD), Louisiana State University, 3990 West Lakeshore Drive, Baton Rouge, LA 70803, U.S.A.
Abstract: MO&intercalated with Mn, CO,Ni, and Ru species, prepared fiom single layer MO&dispersions in water, were studied by x-ray absorption spectroscopy. M K-edge EXAFS results (M = Mn, CO,Ni) are consistent with an alternating
MO&-M(OH),layered structure. For the Mn and COintercalates the MO&layers were fomd to be very similar to that of the parent crystalline MO&with higonal prismatic coordination of the MOatoms. For the Ni and Ru intercalates, MOK edge EXAFS revealed structural rearraagement of MO&layers similar to that observed for MO&single layers, with octahedral coordination of MO. S K edge XANES spectra reveal that the rearrangement is coupled with a change in the electronic structure of the S atoms in the matrix. 'Ihese changes can be associated with charge transfer between 'host' and 'guest' layers. h addition, SO4* and Mowwere detected in some of the intercalated materials, presumably due to exposure to air, suggesting that transition metal intercalation may increasethe susceptibility of the MO&layers to oxidation.
A number of important MO&-basedcatalysts exhibit enhanced activity and selectivity when doped with transition metals [l31. Intercalation of MO& is a convenient method for inclusion of transition metal atoms into the MO& matrix and the resultant compounds provide useful models for atomic level structural studies (EXAFS) of the MO&-M(OH), association. Furthennore, MO&-M(0m intercalation compounds may be of industrial signi6cance as key components of high-energy density batteries and lubricants. W-MO&,a semiconductorwith a hexagonal layered structure in which each MOatom is coordinated to six S atoms m a trigonal prismatic arrangement, is readily intercalated by lithium.  Upon intercalation, a crystallographic transformation occurs in which the MOcoordination changes from trigonal prismatic to octahedral, which has been explained in terms of charge transfer Erom the Li intercalant to the MO& host and the respective energy-band diagrams of octahedral W and prismatic 1T structures [S]. LilMo& can be exfoliated into monolayers by reaction with water 161. Structural studies of exfoliated MO&single layers and freshly restacked MO&films [7,8] have indicated that the single-layer MO& in suspension in water has a0 = 3.20 A and octahedral coordination of MO to S, with the structure strongly distorted, giving a [email protected] superlattice with MeMo closest distances of about 2.8 and 3.8 A. Upon restacking the octahedrally coordinated MO& appears to be metastable, and reverts to =MO& with heating or aging [S]. MO& samples intercalated with transition metals can be synthesized from single layer MO& dispersions by an ion exchange method, in which the cation neutralizes the charge of the MO& layers and the material flocculates with alternating layers ofUoS2 and M(O%. [9,10,1 l]. Previously conducted XRD and EXAPS studies have revealed a M(OI-&like structure for the intercalant . In this paper, we investigate the detailed structures of both 'host' and 'guest' layers in intercalated compounds prepared in this manner.
MoS2 samples intercalated with M = Mn, CO, and Ni were synthesized by ion exchange with single layer MO& water dispersions as outlined above [lO]. The scheme can be summarized as follows:
A sample of MO& intercalated by a Ru complex was synthesized similarly using (C~&)RU(H~O~* instead of M". The synthetic details and discussion of the nature of this intercalated species will be reported elsewhere . M K-edge XAFS data were collected on beamline 7.1 of the SRS at Daresbuty Laboratory in transmission mode using a &tuned Si(ll1) double crystal monochromator. MoK edge EXAFS data were collected at the CAMD light source at Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:19972136
JOURNAL DE PHYSIQUE IV
Figure 1. Fourier Transforms of EXAFS spectra of transition metal (M) intercalates (solid lines) and M(OH), (dots) taken at the correspondingM K-edge; M= Ni (l), CO (2) and Mn (3).
Figure.2. Fourier Transforms of MO K-edge spectra of crystalline MO&(1) and transition metal (M) intercalates; M = CO(2), M n (3), Ni (4), and Ru (5).
Louisiana State University in transmission mode using the LNLS double crystal monochromator [l41 with Si(220) crystals. Fluorescence and total electron yield XANES data from the MOLm and S K edges were also collected at the CAMDLNLS DCM beamline, using detuned Si(1l l) crystals. Data analysis of EXAFS spectra was performed using the UWXAFS curvefitting software [l51 with FEFF [l61 phase and amplitude functions. 3. RESULTS AND DISCUSSION
As with earlier XRD and EXAFS data, [l21 M K-edge EXAFS results are consistent with the presence of 2-dimensional hexagonal sheets containing M (Mn, CO,Ni) atoms which are octahedrally coordinated to 0 atoms. These sheets represent slightly distorted layers of the corresponding crystalline M(Ow (Fig. 1). Best-fit interatomic distances M-0 and M...M in the intercalates display systematic shortening(by 0.02-0.03 A) as compared to crystallineM(0X-Q. For the Mn and COintercalates, MOK-edge EXAFS data show no s i d c a n t changes as compared to the parent MO& (where MOatoms are in ttigonal prismatic coordination), with only slight disordering of the matrix (Fig. 2). In contrast, substantial structural rearrangement is observed for the Ni and Ru intercalates. The peaks in the FT of the MO K-edge spectra corresponding to MO...MOinteractions (3.16 A) are drastically diminished, splitting into 2 peaks at ca 2.76 A and 3.82 A (labeled with arrows in Fig. 2). Similar rearrangement of the MO environment was reported for single layer suspensions of MO& , which exhibited the distorted octahedral layer structure discussed earlier [S]. Thus, for the samples studied here, the trigonal prismatic bulk structure has been successfully restored in the M n and COintercalated compounds, but in the Ni and Ru samples, the single layer structure with octahedral coordination of MOto S is probably maintained. S K edge XANES spectra of the Ni and Ru intercalated compounds exhibit a distinct shoulder at the low energy side of the major K-edge resonance (Fig. 3). This feature was observed in' spectra recorded in both fluorescence and total electron yield. The app.?arance of this resonance is indicative of a change in the electronic structure of S, which can be related to an increase of local electron density on S and the structural rearrangement associated with electron transfer to the MO& layers. A similar but less pronounced feature was observed in the MO, L XANES spectra, suggestingthat the MO&matrix in the Ni and Ru intercalates is negatively charged. The presence of the single layer type structure is evidently associated with a charge separation between 'host' and 'guest' layers. In addition, S K and MO Lm edge XANES spectra reveal the presence of SO-: and MO& in the Mn, CO, and Ni intercalates (Fig. 4). Thus oxidation is observed in examples of intercalates with both the bulk trigonal prismatic layer structure (Mn and CO compounds) and the octahedral single layer structure (Ni compound). A comparison of the surface sensitive TEY data to the bulk fluorescence data suggests that the SO: results fiom oxidation at the surface. While this interpretation is not conclusive due to possible orientation effects in the fluorescence data , XPS data (MO3d and S 2p core levels) support the conjecture. SO-: has been detected previously in MO$ intercalation compounds [IS].
ENERGY, eV Ffgare 3. S K edge XANES spectra (fluorescence yield) for crystalline MO% (l), and transition metal (M) intercalates; M = CO(2), Mn (3), Ni (4), and Ru (5).
ENERGY, eV Fignre 4. Pmiiles of X-ray absorption in the region of S K and MO L, edges (total electron yield) for crystalline MoS2 (l), and transition metal (M) intercalates; M = CO (2), Mn (3), Ni (4), and Ru (5).
The XAFS data reported are consistent with the formation of the MO$-M(Ointercalation compounds with alternating MoS2 and M ( O a layers. MO K-edge EXAFS analysis of the Mn and CO intercalates suggests the presence of MoS2 layers with the same trigonal prismatic coordination as in the parent W-MO$. In contrast, a distorted MO$ structure like that 0 b S e ~ e din single layer dispersions of MO$ (with octahedral coordination of MO), was found in the Ni and Ru intercalates. The apparent presence of this structure, combined with the S K and MOL= XANES results, suggests that a charge separation between 'host' and 'guest' layers is maintained in the Ni and Ru intercalated solids. Furthermore, the observation of ~ 0 4 "and MO& in some of the intercalates suggests that, unlike bulk MO&, MO% intercalated by transition metals may undergo oxidation in air.
This work was partially supported by the International Science Foundation (grant MNC300) and Russian Foundation for Basic Research (grant 96-03-32684).Y.V.Z.is grateful to the CAMD administration and personally to Volker Saile for financial support during his work at CAMD, LSU.
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