Intercalation Compounds: Dichalcogenides

Intercalation Compounds: Dichalcogenides Prof. Antonella Glisenti -Dip. ... The lattices consist of two close-packed chalcogen layers between...

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Laurea Magistrale in Scienza dei Materiali

Materiali Inorganici Funzionali

Intercalation Compounds: Dichalcogenides

Prof. Antonella Glisenti - Dip. Scienze Chimiche - Università degli Studi di Padova

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

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

MX2 (M=Ti, Zr, Hf, V; X = S, Se, Te)

Octahedral

2Ha

BaB/CaC

MX2 (M=Ta, Nb; X = S, Se)

Trigonal prismatic

2Hb

BcB/CbC

TaSe2 , NbSe2

Trigonal prismatic

4Hb

aB/CaC/BaC/BaB/Ca

TaSe2 , TaS2

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 unit cell.  2Ha (frequently referred to as 2H): the metal atoms lie directly above each other.  2Hc (frequently referred as 2H MoS2): the metal atoms are staggered.  4Hb = mixed octahedral/trigonal prismatic structure

(110) Projections of layered TM dichalcogenides

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 TaS2, 2H NbS2, 1T TiS2; 2H NbSe2 does not form organic IC compounds with the exception of ethylendiamine. Generic class

Examples

Amines Phosphines Amides Amine oxides Phosphine oxides N-heterocycles Isocyanides

RNH2, R2N, R3N, H2N(CH2)nNH2 R3P RCONH2, CO(NH2)2 Pyridine N-oxide R3PO Pyridine, substituted pyridines RNC

Organic molecules that form IC compounds

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 (200-300°C).

n-Alkylamines: CnH2n+1NH2 2H TaS2  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 = A2/3TaS2 (A = amine) NH2groups adjacent to the layers to interact with the Ta through the nitrogen lone pair. Schematic representation of the structure of (octadecylamine)2/3TaS2

n-Alkylamines: CnH2n+1NH2 2H TaS2 Increase in the interlayer spacing (triangles) and the onset temperature for superconduttivity (circles) as a function of n in CnH2n+1NH2 for the n-alkylamine intercalation compounds of TaS2.

n-Alkylamines: CnH2n+1NH2 TaS2, TiS2, NbS2  n ≤ 9,10

 Direct reaction 150°200°C for several days;  Indirect reaction: dichalcogenide preintercalated 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 = dhost + 2[(n-1)1.26 + 1.25 + 1.5 + 2.0)]Å CH2

C-N

NH2

CH3

n-Alkylamines: CnH2n+1NH2 Groups VI dichalcogenides

 IC

compounds can be prepared by ion-exchange reactions of the hydrated sodium intercalation compound Na0.1(H2O)0.6MoS2  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.

Organic guests: Pyridine  The reactivity of pyridine is closely analogous to that exhibited by

ammonia. Direct reaction of 2H-TaS2 with pyridine leads to the formation of the first stage phase with limiting composition The reaction proceeds until the limiting first stage composition TaS2(py)0.5.

Three models for the orientation of pyridine molecules between dichalcogenide layers

Organic guests: Pyridine 

Neutron diffraction studies on TaS2(py-d5)0.5 have determined that the nitrogen lone pair is directed parallel to the layers.

 Pyridine sublattice is ordered at RT in both (py)1/2TaS2 (py)1/2NbS2: rectangular superlattice 2a√3 x 13a.

and

Schematic representation of the packing and orientation of the guests in TaS2(Py)0.5

Bonding in organic intercalation compounds  Correlation

was observed between pyridine basicity and intercalation capability.  Difficulty with the lone pair donor model because NH3 and py IC compounds have the nitrogen lone pair midway between and parallel to the layers precluding a direct interaction with the dz2 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+ + 2 ex py + xH+  xpyH+ xpyH+ + (0.5 – x)py + xe- + TaS2  (pyH+)x(py)0.5-xTaS2

Metal Ion Guests 1959 – Rudorff and Sick Alkali metals in liquid ammonia + TiS2

1965 - Rudorff Alkaline earth ions, Eu2+, Yb2+ + TiS2 Whittingham and Gamble - 1975 Rouxel et al. 1979 Hydrated metal intercalates: AxMX2(H2O)y

Synthesis 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.  

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: NaxTiS2 Method

Stage 3

Stage 2

Stage 1 (trigonal prismatic)

Stage 1 (trigonal antiprismatic)

b

0.17 < x < 0.33

0.38 < x < 0.68

0.79 < x ≤ 1

b

? < x < 0.18

0.35 < x < 0.58

0.68 < x ≤ 1

c

?

?

0.46 < x < 0.70

0.80 < x ≤ 1

c

? < x < 0.11

0.12 < x < 0.25

0.50 < 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 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).

Selected examples of intercalation compounds formed by the metal disulphides with different guest ions and molecules

CRD

Li+

K+

4

73

151

6

90

152 Ionic radii

Cs+

181

Lithium intercalated lamellar metal dichalcogenides Host = TiS2  A single homogeneous phase has been found for the entire stoichiometry range LixTiS2 (0 ≤ x ≤1)  Lithium occupies the octahedral interlayer sites and the final product, LiTiS2, is isostructural with LiVS2 and LiCrS2.

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 octahedral or trigonal prismatic interlayer sites.

Phase relations for the alkali metal intercalates of TiS2 and ZrS2. 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.

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 MX2NH3 is achieved.  readily loses NH3 to go to the second stage material MX2(NH3)0.5. (1+x/3) NH3 + TaS2 x/6 N2 + (NH4+)x(NH3)1-xTaS2  These materials contain NH4+ solvated by neutral molecules.  Ammonia orientation seems to be determined by the ion-dipole interactions with the NH4+ cations

Schematic representation of the packing and orientation of the guests in TaS2(NH3)

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 MS2{Co(Cp)2}x

(M = Zr, Sn, Ta; x = 0.25-0.30):

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.

Macromolecular guests  Intercalation of poly(ethyleneoxide) – PEO – with an average

molecular weight of ca. 105 Daltons into MS2 (M = Mo, Ti) by means of two synthetic approaches: 



Delamination of the metal sulphides in aqueous suspension with an acetonitrile solution of PEO/LiClO4 followed by reconstitution of the lamellar structure upon drying. Treatment of the lithium intercalate LiMS2 with an aqueous solution of PEO and LiClO4.

 These materials behave as semiconductors with reduced band gaps.

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 TX2 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

PROFOUND CHANGES IN THE ELECTRONIC PROPERTIES OF THE HOST Host

Host Properties

Intercalate Intercalate Properties

1T-HfS2

wide band gap semiconductor

KxHfS2

metal

2H-NbSe2 metal superconductor

KxNbSe2

poor metal (x = 1) expect a semiconductor

2H-MoS2

KxMoS2

superconductor

diamagnetic semiconductor