Advanced Technology and Manufacturing Institute (ATAMI)

Novel donor-type graphite intercalation compounds (GICs) containing tetra-n-alkylammonium (TAA) cations have been synthesized by using both ion-exchange and electrochemical methods. Structural and compositional data of the resulting TAA-GICs are investigated by using powder X-ray diffraction (PXRD), one-dimensional electron density calculations, thermogravimetric and elemental analyses, and capillary zone electrophoresis (CZE). A new GIC with composition of [(C₄H₉)₄N]C₄₄ is prepared by intercalation of tetra-n-butylammonium cation, (C₄H₉)₄N⁺, via ion exchange from a [Na(en)₁.₀]C₁₅ GIC (en = ethylenediamine). The synthesis reaction proceeds at 60°C for 90 min in a N,N-dimethylformamide (DMF) solvent. A dull-black stage-1 [(C₄H₉)₄N]C₄₄ provides the gallery height of di = 0.802 nm, indicating a presence of flattened cation conformation. CZE data reveal that the Na(en)⁺ cationic complex is quantitatively displaced by (C₄H₉)₄N⁺ cations. A homologous series of TAA-GICs; i.e. TAA = symmetric (C[subscript n]H[subscript 2n+1])₄N⁺ (n = 3-8) and asymmetric (CH₃)₃(C₁₂H₂₅)N⁺, (CH₃)₃(C₁₈H₃₇)N⁺ and (CH₃)₂(C₁₈H₃₇)₂N⁺, are prepared using the similar ion-exchange procedure, albeit with shorter reaction time (10 min), in dimethylsulfoxide (DMSO). The obtained TAA-GICs contain either monolayer or bilayer arrangement of flattened TAA intercalates with significant co-intercalation of DMSO molecules in a bilayer arrangement. PXRD data suggest that the monolayer is also observed with small TAA intercalates such as (C₃H₇)₄N⁺ and (C₄H₉)₄N⁺ with di ~ 0.80 nm. On the other hand, larger symmetric TAA cations, (C[subscript n]H[subscript 2n+1])₄N⁺ (n = 5-8), and asymmetric TAA cations all form only the bilayer arrangement with d[subscript i] ~ 1.10 nm. Thermogravimetric analyses combined with mass spectrometry and elemental analyses show a presence of ~1-2 DMSO co-intercalates per bilayer cation. The generated electron density map is sufficient to confirm the existence of bilayer structures, including DMSO co-intercalates. These GICs have very low charge density on graphene sheets for stage-1 GICs, namely C₆₃⁻ for [(C7H₁₅)₄N]C₆₃⁻1.4DMSO, as confirmed by Raman peak shifts. In addition, TAA-GICs are also synthesized using the electrochemical reduction on a graphite electrode in TAABr/DMSO-based electrolytes. Similar to GIC products obtained from a chemical ion-exchange method, large TAA cations, (C[subscript n]H[subscript 2n+1])₄N⁺ (n = 5-8), form the bilayer arrangement with 0.7-1.2 DMSO co-intercalates per TAA cation and di ~ 0.11 nm. A mixed phase product, including a stable (C₄H₉)₄N⁺ monolayer arrangement (di = 0.815 nm), is observed in (C₄H₉)₄N⁺ intercalation with a little amount of DMSO. No stable and isolable GIC products are obtained in case of TAA cations smaller than (C₄H₉)₄N⁺ even though cyclic voltammograms show the characteristic features of reversible intercalation/de-intercalation for these cations. Therefore, a surface passivation model is proposed to describe the relative stabilities of GICs containing large TAA intercalates. The effect of surface passivation is further studied on the preparation of (C₂H₅)₄N-GIC. Large TAA cation such as (C₆H₁₃)₄N⁺, (C₇H₁₅)₄N⁺ or (C₈H₁₇)₄N⁺, is used to passivate the graphite surface of [Na(en)₁.₀]C₁₅, followed by ion exchange with (C₂H₅)₄N⁺ to obtain a (C₂H₅)₄N-GIC product. PXRD data suggest the formation of a stage-1 compound with di ~ 0.81 nm, indicating monolayer arrangement of intercalate. The GIC composition is found to be [(C₂H₅)₄N]C₅₇.0.5DMSO. Additionally, the hydrophobic nature of passivated GIC surfaces enhances the chemical stabilities in aqueous media and other protic solvents.