Intercalation Compounds: Dichalcogenides

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1 Laurea Magistrale in Scienza dei Materiali Materiali Inorganici Funzionali Intercalation Compounds: Dichalcogenides Prof. Antonella Glisenti - Dip. Scienze Chimiche - Università degli Studi di Padova

2 Metal dichalcogenides TM dichalcogenides often possess layered structures (a). The lattices consist of two close-packed chalcogen layers between which reside the metal ions. metal ions can be found in sites of trigonal prismatic (b) or octahedral (c) symmetry. Intralayer bond: strong and largely ionic Interlayer bond: van der Waals

3 Metal dichalcogenides The ability of the metal atom to adopt octahedral and trigonal prismatic coordination and for the X-M-X units to stack in different sequences gives rise to a wide variety of polymorphic and polytypic forms Brown and Beernsten notation Polytype designation Stacking sequence Examples Metal coordination 1T ab/ca MX 2 (M=Ti, Zr, Hf, V; X = S, Se, Te) Octahedral 2Ha BaB/CaC MX 2 (M=Ta, Nb; X = S, Se) Trigonal prismatic 2Hb BcB/CbC TaSe 2, NbSe 2 Trigonal prismatic 4Hb ab/cac/bac/bab/ca TaSe 2, TaS 2 Octahedral, Trigonal prismatic Chalcogen layer illustrating the stacking sequence notation A, B, C = different anions in the layer; a, b, c = different metal sites; [a], [b], [c] = intercalated guest ions.

Metal dichalcogenides 1T = the simplest structure: all octahedral metals and one X-T-X slab per unit cell; 2Ha and 2Hc = the two most common polytypes of the all-prismatic structures; two layers per

4 Metal dichalcogenides 1T = the simplest structure: all octahedral metals and one X-T-X slab per unit cell; 2Ha and 2Hc = the two most common polytypes of the all-prismatic structures; two layers per unit cell. 2Ha (frequently referred to as 2H): the metal atoms lie directly above each other. 2Hc (frequently referred as 2H MoS 2 ): the metal atoms are staggered. 4Hb = mixed octahedral/trigonal prismatic structure (110) Projections of layered TM dichalcogenides

5 Organic intercalation compounds A wide range of organic molecules form intercalation compounds. All the reactions are characterized by an expansion of the crystal lattice along the c direction to an extent that may be correlated with the molecular dimensions of the guest and the stoichiometry. Stabilities vary and depend on the nature of guest and host; highest stabilities = 2H TaS 2, 2H NbS 2, 1T TiS 2 ; 2H NbSe 2 does not form organic IC compounds with the exception of ethylendiamine. Generic class Amines Phosphines Amides Amine oxides Phosphine oxides N-heterocycles Isocyanides Examples RNH 2, R 2 N, R 3 N, H 2 N(CH 2 ) n NH 2 R 3 P RCONH 2, CO(NH 2 ) 2 Pyridine N-oxide R 3 PO Pyridine, substituted pyridines RNC Organic molecules that form IC compounds

6 Organic intercalation compounds: synthesis Intercalation reactions with organic compounds are usually carried out by direct reaction of the dichalcogenide in powder form with the organic compound or with a benzene or toluene solution for high molecular weight systems. In some cases reactions are facilitated by pretreatment of the dichalcogenide with ammonia or hydrazine. The progress of the reaction can be followed (qualitatively) by observing the volume expansion of the solid phase or (quantitatively) by means of XRD. The host lattice may be recovered unchanged by thermal deintercalation of the organic molecules at temperature higher than the initial reaction T ( C).

7 n-alkylamines: C n H 2n+1 NH 2 2H TaS 2 A complete series of samples for n = 1 to 18 was prepared by direct reaction with the amine or amine in benzene solution for n > 12 at 25 C for 30 days. n 4: hydrocarbon chains parallel to the dichalcogenide layers; 5 n 11:?; n 12: perpendicular orientation; composition = A 2/3 TaS 2 (A = amine) NH 2 - groups adjacent to the layers to interact with the Ta through the nitrogen lone pair. Schematic representation of the structure of (octadecylamine) 2/3 TaS 2

8 n-alkylamines: C n H 2n+1 NH 2 2H TaS 2 Increase in the interlayer spacing (triangles) and the onset temperature for superconduttivity (circles) as a function of n in C n H 2n+1 NH 2 for the n-alkylamine intercalation compounds of TaS 2.

9 TaS 2, TiS 2, NbS 2 n 9,10 Direct reaction C for several days; Indirect reaction: dichalcogenide preintercalated n-alkylamines: C n H 2n+1 NH 2 with ammonia or hydrazine and then reacted for some hours at 100 C. n > 10 Displacement reactions of amine intercalation compound of lower C number. d = d host + 2[(n-1) )]Å CH 2 C-N NH 2 CH 3

10 Groups VI dichalcogenides n-alkylamines: C n H 2n+1 NH 2 IC compounds can be prepared by ion-exchange reactions of the hydrated sodium intercalation compound Na 0.1 (H 2 O) 0.6 MoS 2 1 n 5; c-spacing = constant; 6 n < 11: c increases linearly with Δd/n = 2.3 per C, implying a bilayer tilted at 68 ; n > 11 alkylammonium cations are perpendicular.

11 Organic guests: Pyridine The reactivity of pyridine is closely analogous to that exhibited by ammonia. Direct reaction of 2H-TaS 2 with pyridine leads to the formation of the first stage phase with limiting composition The reaction proceeds until the limiting first stage composition TaS 2 (py) 0.5. Three models for the orientation of pyridine molecules between dichalcogenide layers

$Organic guests: Pyridine Neutron diffraction studies on TaS 2 (py-d 5 ) 0.5 have determined that the nitrogen lone pair is directed parallel to the layers.$

12 Organic guests: Pyridine Neutron diffraction studies on TaS 2 (py-d 5 ) 0.5 have determined that the nitrogen lone pair is directed parallel to the layers. Pyridine sublattice is ordered at RT in both (py) 1/2 TaS 2 and (py) 1/2 NbS 2 : rectangular superlattice 2a 3 x 13a. Schematic representation of the packing and orientation of the guests in TaS 2 (Py) 0.5

13 Bonding in organic intercalation compounds Correlation was observed between pyridine basicity and intercalation capability. Difficulty with the lone pair donor model because NH 3 and py IC compounds have the nitrogen lone pair midway between and parallel to the layers precluding a direct interaction with the d z 2 orbital. Bonding is described as an electrostatic interaction between negatively charged layers and cations, analogous to alkali and organometallic intercalation compounds. 2 py bipy + 2H e - x py + xh+ xpyh + xpyh + + (0.5 x)py + xe - + TaS 2 (pyh + ) x (py) 0.5-x TaS 2

14 Metal Ion Guests 1959 Rudorff and Sick Alkali metals in liquid ammonia + TiS Rudorff Alkaline earth ions, Eu 2+, Yb 2+ + TiS 2 Whittingham and Gamble Rouxel et al Hydrated metal intercalates: A x MX 2 (H 2 O) y

15 a) High-temperature synthesis from the host material and the metal of the elements; b) Intercalation of the host material with a solution of the metal (alkali metal in liquid ammonia, butyllithium, sodium naphthalide); c) Electrochemical intercalation. Synthesis Cointercalation of the solvent is also possible with metods b) and c) (cointercalated ammonia, as an example, has to be removed by heat treatment); Methods b) and c) are RT methods so metastable phases may be produced and equlibration may take long time. Example: Na x TiS 2 Method Stage 3 Stage 2 Stage 1 (trigonal prismatic) Stage 1 (trigonal antiprismatic) b 0.17 < x < < x < < x 1 b? < x < < x < < x 1 c?? 0.46 < x < < x 1 c? < x < < x < < x <? 0.81 < x 1

$Ordering of the intercalate ions At low enough temperatures the ions will order on superlattices for certain fractional values of the composition; as the temperature increases, the disorder$

16 Ordering of the intercalate ions At low enough temperatures the ions will order on superlattices for certain fractional values of the composition; as the temperature increases, the disorder increases, and, at a critical temperature, le long-range order collapses and the system becomes disordered. At low temperatures these ordered phases will have a compositional range. The stoichiometry can be varied within certain limits by creating vacancies or adding interstitial atoms. Depending on the coordination of the intercalate ion, the sublattice in the van der Waals gap is (1) a honeycomb lattice (trigonal prismatic coordination, TP), (2) a triangular lattice (octahedral coordination, O), or (3) a puckered honeycomb lattice (tetrahedral coordination, T).

17 Selected examples of intercalation compounds formed by the metal disulphides with different guest ions and molecules CRD Li + K + Cs Ionic radii

18 Lithium intercalated lamellar metal dichalcogenides Host = TiS 2 A single homogeneous phase has been found for the entire stoichiometry range Li x TiS 2 (0 x 1) Lithium occupies the octahedral interlayer sites and the final product, LiTiS 2, is isostructural with LiVS 2 and LiCrS 2. only a small expansion along the c-axis is required to accomodate this cation.

Alkali metal intercalated lamellar dichalcogenides The structures adopted by intercalation compounds formed with other alkali metals are much more varied, as these larger ions can occupy either

19 Alkali metal intercalated lamellar dichalcogenides The structures adopted by intercalation compounds formed with other alkali metals are much more varied, as these larger ions can occupy either octahedral or trigonal prismatic interlayer sites. Phase relations for the alkali metal intercalates of TiS 2 and ZrS 2. I, II and IV indicate 1st, 2nd and 4th stage intercalates, respectively. At low alkali metal concentrations (except lithium) staging results in the formation of compounds with alternating sequences of filled and empty van der waals gaps.

20 Ammonia as a guest Anhydrous ammonia + layered metal dichalcogenide at 78 C followed by warming to RT leads to a rapid reaction. The onset of intercalation is marked by swelling of the sample and often a slight colouration of the solution. The reaction proceeds until the limiting first stage composition MX 2 NH 3 is achieved. readily loses NH 3 to go to the second stage material MX 2 (NH 3 ) 0.5. (1+x/3) NH 3 + TaS 2 x/6 N 2 + (NH 4+ ) x (NH 3 ) 1-x TaS 2 These materials contain NH 4+ solvated by neutral molecules. Ammonia orientation seems to be determined by the ion-dipole interactions with the NH 4+ cations Schematic representation of the packing and orientation of the guests in TaS 2 (NH 3 )

$These molecules have almost a spherical van der Waals surface Van der Waals dimensions of cobaltocene X-ray and neutron diffraction techniques applied to MS 2 {Co(Cp) 2 } x (M = Zr, Sn, Ta; x$

21 Organometallic guests and orientation The lattice expansion of ca 5.3 Å observed for all simple metallocene intercalates does not immediately reveal the orientation of the guest. These molecules have almost a spherical van der Waals surface Van der Waals dimensions of cobaltocene X-ray and neutron diffraction techniques applied to MS 2 {Co(Cp) 2 } x (M = Zr, Sn, Ta; x = ):

complexes always adopt a preferred orientation in which

22 Organometallic guests and orientation > Second ring size > lattice expansion In general organometallic sandwich complexes always adopt a preferred orientation in which their metal-to-ring centroid axes lie parallel to the host layer planes.

23 Macromolecular guests Intercalation of poly(ethyleneoxide) PEO with an average molecular weight of ca Daltons into MS 2 (M = Mo, Ti) by means of two synthetic approaches: Delamination of the metal sulphides in aqueous suspension with an acetonitrile solution of PEO/LiClO 4 followed by reconstitution of the lamellar structure upon drying. Treatment of the lithium intercalate LiMS 2 with an aqueous solution of PEO and LiClO 4. These materials behave as semiconductors with reduced band gaps.

24 Electronic Structure and Bonding Schematic representation of the band structures of the layered Group IVB, VB, and VIB TM dichalcogenides The valence electrons of the alkali atoms are transferred to the TX 2 sandwich filling the lowest unoccupied d-band levels. The dispersion and relative position of the d bands stay almost unchanged upon intercalation. The upper conduction and lower valence bands change considerably

25 PROFOUND CHANGES IN THE ELECTRONIC PROPERTIES OF THE HOST Host Host Properties Intercalate Intercalate Properties 1T-HfS 2 2H-NbSe 2 2H-MoS 2 wide band gap semiconductor metal superconductor diamagnetic semiconductor K x HfS 2 metal K x NbSe 2 poor metal (x = 1) expect a semiconductor K x MoS 2 superconductor

Solid-gas reactions. Direct reaction of between the elements

Solid-gas reactions. Direct reaction of between the elements Solid-gas reactions Overcoming the problems related to poor contact in solid-solid reactions Excess gas is often used in order to shift reaction towards products. Example: Si 3 N 4 α- and β-phase. (α-si