5.5 Polypyrrole Intercalation into the Bi-Sr-Ca-Cu-O System

Using the knowledge base described previously, this new research has been on hybrid structures composed of alternating layers of conducting polymer and high-temperature superconductor that could be utilized in the formation of nanocomposite assemblies that have the ability to have their electronic properties modified upon room temperature doping and undoping within the conducting polymer layers.  The first step toward this goal is the controlled intercalative insertion of a conducting polymer into a layered superconductor such as the Bi₂Sr₂Ca_n-1Cu_nO_2n+4 which contains a van der Waals gap between adjacent bismuth-oxide layers. 

The compound used in these studies, Bi₂Sr_1.8Ca_1.2Cu₂O_8+x, was prepared as described earlier by conventional solid-state reaction of stoichiometric amounts of Bi₂O₃, SrCO₃, CaCO₃, and CuO.  Next, the material was intercalated with iodine through vapor phase exposure to iodine at 150^oC for 3-10 days.  Mass uptake experiments confirm that iodine is inserted at a ratio of one iodine atom per Bi₂Sr_1.8Ca_1.2Cu₂O_8+x unit.  This step increases the oxidation state of copper and possibly bismuth.  The reaction also expands the gap between the bismuth-oxide layers by ~3.7 Å.  Pyrrole monomer was purified with activated alumina.  The I-Bi₂Sr_1.8Ca_1.2Cu₂O_8+x sample was then vacuum sealed with pyrrole and heated  to 100-150^oC for 3-10 days.  X-ray powder diffraction (XRPD) was done with a Philips x-ray powder diffractometer.  Elemental analysis was done by Galbraith Labs. The sample’s morphology was visualized with a JEOL 35C Scanning Electron Microscope (SEM).

Direct intercalation of pyrrole into the Bi₂Sr_1.8Ca_1.2Cu₂O_8+x phase proved unsuccessful without prior intercalation of iodine.  Iodine may be used as prop to hold the lattice open so that pyrrole units can be inserted.   In addition, iodine serves to further oxidize the Bi₂Sr_1.8Ca_1.2Cu₂O_8+x material.  This oxidation is important because the pyrrole monomer is subsequently oxidized as it is inserted into the Bi₂Sr_1.8Ca_1.2Cu₂O_8+x lattice.  The oxidation of pyrrole initiates polymerization of the monomer species.

X-ray powder diffraction of the (ppy)_yI_1-2y-Bi₂Sr_1.8Ca_1.2Cu₂O_8+x system reveals that the unit cell of Bi₂Sr_1.8Ca_1.2Cu₂O_8+x has a spacing along the c-axis of 15.4 Å (x2) as shown in Figure 5.1(a).  Iodine insertion increases this distance to 18.1 Å, Figure 5.1(b).  After insertion of polypyrrole, this distance drops to 17.9 Å, Figure 5.1(c), giving a total c-axis expansion of ~3.5 Å between each bismuth oxide layer.  This is consistent with polypyrrole in a planar (i.e., edge on) arrangement.  Illustration 5.3 shows a schematic diagram at each stage of the polymer insertion.  A representation of the possible crystal structure of polypyrrole intercalated Bi₂Sr₂CaCu₂O_8+x is shown in Illustration 5.4.

Elemental analysis reveals carbon and nitrogen are in a 1:4 ratio, consistent with polypyrrole.  Elemental analysis also shows that there is about 0.25 pyrrole units for every Bi₂Sr₂CaCu₂O_8+x as well as a significant loss of iodine after intercalation. 

Based on the packing models¹¹⁷ derived from electron diffraction data, planar polypyrrole is expected to have a maximum packing of (ppy)_xI_1-2y-Bi₂Sr₂CaCu₂O₈.  The packing¹¹⁷ of polypyrrole is expected to pack in a head-to-tail arrangement at seen in Illustration 5.5.  Although the exact arrangement of polypyrrole is unclear, it is believed that polypyrrole packs in a planar arrangement since the distance between the bismuth-oxide layers was increased by approximately the thickness of planar pyrrole, 3.5 Å, rather than pyrrole  positioned  perpendicular to the bismuth-oxide sheets with a spacing of ~7 Å.

Figure 5.1:     X-ray powder diffraction of the three stages of intercalation: a) Bi₂Sr_1.8Ca_1.2Cu₂O_8+x starting material, b) iodine intercalated Bi₂Sr_1.8Ca_1.2Cu₂O_8+x, and c) Bi₂Sr_1.8Ca_1.2Cu₂O_8+x intercalated with polypyrrole.

Illustration 5.3:     Schematic diagram of a) Bi₂Sr_1.8Ca_1.2Cu₂O_8+x, b) iodine intercalated Bi₂Sr_1.8Ca_1.2Cu₂O_8+x, and c) Bi₂Sr_1.8Ca_1.2Cu₂O_8+x intercalated with polypyrrole.

Illustration 5.4:     Crystal packing motif for polypyrrole intercalated Bi₂Sr_rCaCu₂O_8±x where the layers contain a) calcium, b) copper-oxide, c) strontium-oxide, d) bismuth-oxide, and e) polypyrrole.

Table 5.4:      Elemental analysis of intercalated Bi₂Sr₂CaCu₂O_8+x.  The x value refers to the amount of pyrrole units and y refers to the amount of iodine in the formulation (ppy)_zBi₂Sr₂CaCu₂O₈.  Samples (a,d-h) are of I-Bi₂Sr₂CaCu₂O_8+x samples that were intercalated by vapor phase exposure to pyrrole at various temperatures.  The exceptions are: b) I-Bi₂Sr₂CaCu₂O_8+x with no exposure to pyrrole c) ppy/I-Bi₂Sr₂CaCu₂O_8+x dissolved in HCl, and i) I-Bi₂Sr₂CaCu₂O_8+x exposed to 2,5-dimethylpyrrole instead of pyrrole.

Illustration 5.5:     Graphical illustration of the ordering of three pyrrole oligomers (from electron diffraction data¹¹⁷) when polypyrrole is in a planar arrangement.  The underlying framework shows location of alternating bismuth and oxygen atoms within the bismuth-oxide sheet of Bi₂Sr_1.8Ca_1.2Cu₂O_8+x (from x-ray diffraction data).