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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 Bi2Sr2Can-1CunO2n+4 which contains a van der Waals gap between adjacent bismuth-oxide layers.

The compound used in these studies, Bi2Sr1.8Ca1.2Cu2O8+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 150oC for 3-10 days. Mass uptake experiments confirm that iodine is inserted at a ratio of one iodine atom per Bi2Sr1.8Ca1.2Cu2O8+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 . Pyrrole monomer was purified with activated alumina. The I-Bi2Sr1.8Ca1.2Cu2O8+x sample was then vacuum sealed with pyrrole and heated to 100-150oC 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 Bi2Sr1.8Ca1.2Cu2O8+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 Bi2Sr1.8Ca1.2Cu2O8+x material. This oxidation is important because the pyrrole monomer is subsequently oxidized as it is inserted into the Bi2Sr1.8Ca1.2Cu2O8+x lattice. The oxidation of pyrrole initiates polymerization of the monomer species.

X-ray powder diffraction of the (ppy)yI1-2y-Bi2Sr1.8Ca1.2Cu2O8+x system reveals that the unit cell of Bi2Sr1.8Ca1.2Cu2O8+x has a spacing along the c-axis of 15.4 (x2) as shown in Figure 5.1(a). Iodine insertion increases this distance to 18.1 , Figure 5.1(b). After insertion of polypyrrole, this distance drops to 17.9 , Figure 5.1(c), giving a total c-axis expansion of ~3.5 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 Bi2Sr2CaCu2O8+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 Bi2Sr2CaCu2O8+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)xI1-2y-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 , rather than pyrrole positioned perpendicular to the bismuth-oxide sheets with a spacing of ~7 .

X-ray powder diffraction (XRPD) of the three stages of intercalation: a) Bi2Sr1.8Ca1.2Cu2O8+x starting material, b) iodine intercalated Bi2Sr1.8Ca1.2Cu2O8+x, and c) Bi2Sr1.8Ca1.2Cu2O8+x intercalated with polypyrrole.

Figure 5.1: X-ray powder diffraction of the three stages of intercalation: a) Bi2Sr1.8Ca1.2Cu2O8+x starting material, b) iodine intercalated Bi2Sr1.8Ca1.2Cu2O8+x, and c) Bi2Sr1.8Ca1.2Cu2O8+x intercalated with polypyrrole.

Schematic diagram of a) Bi2Sr1.8Ca1.2Cu2O8+x, b) iodine intercalated Bi2Sr1.8Ca1.2Cu2O8+x, and c) Bi2Sr1.8Ca1.2Cu2O8+x intercalated with polypyrrole.

Illustration 5.3: Schematic diagram of a) Bi2Sr1.8Ca1.2Cu2O8+x, b) iodine intercalated Bi2Sr1.8Ca1.2Cu2O8+x, and c) Bi2Sr1.8Ca1.2Cu2O8+x intercalated with polypyrrole.

Crystal packing motif for polypyrrole intercalated Bi2SrrCaCu2O8±x where the layers contain a) calcium, b) copper-oxide, c) strontium-oxide, d) bismuth-oxide, and e) polypyrrole.

Illustration 5.4: Crystal packing motif for polypyrrole intercalated Bi2SrrCaCu2O8x where the layers contain a) calcium, b) copper-oxide, c) strontium-oxide, d) bismuth-oxide, and e) polypyrrole.

Sample

C (%)

N (%)

H (%)

I-BSCCO (%)

x*

(A)

1.38

0.53

<0.5

96.66

0.26

(B)

<0.5

<0.5

<0.5

>98.5

0

(C)

16.13

0.71

5.42

77.74

4.4

* Calculated from carbon values.

Table 5.3: Elemental analysis for C, N, and H in a) polypyrrole intercalated Bi2Sr2CaCu2O8+x b) I-Bi2Sr2CaCu2O8+x and c) polypyrrole extracted from polypyrrole intercalated Bi2Sr2CaCu2O8+x. Here, the x value refers to the amount of pyrrole units.

Intercalation of polypyrrole at higher temperatures appeared to increase the amount of polypyrrole in the ceramic sample. This is likely due to the fact that some polypyrrole is intercalated on the surface and between the grains. The amount of iodine also remains high as the sample temperature is increased. This is another indication that polypyrrole is not being intercalated. If it were, more iodine would leave the lattice to accommodate for the intercalated pyrrole species.

Polypyrrole is known to be a thermally stable material. Thermal gravimetric analysis of the polypyrrole intercalated Bi2Sr1.8Ca1.2Cu2O8+x shows no significant weight loss until ~200oC with little mass loss after 550oC. This behavior is quite similar to that of polypyrrole. Figure 5.2 shows the thermal gravimetric analysis of both materials. The similarities in these measurements and the fact that pyrrole itself has a boiling point of 130oC would suggest that the intercalated pyrrole has been polymerized.

Sample

Temp.

C (%)

N (%)

H (%)

I (%)

BSCCO (%)

y

z*

(A)

100

1.38

0.53

<0.5

0.10

96.66

0.10

0.26

(B)

150

<0.5

<0.5

<0.5

6.12

>98.5

0.46

0

(C)

---

16.13

0.71

5.42

<0.5

77.74

---

4.4

(D)

100

1.78

0.50

<0.5

3.21

94.51

0.24

0.35

(E)

100

2.11

0.82

<0.5

5.13

91.94

0.40

0.43

(F)

125

2.69

0.93

<0.5

5.45

90.93

0.43

0.55

(G)

150

2.72

1.05

<0.5

5.22

91.01

0.41

0.56

(H)

175

2.69

0.94

<0.5

5.68

90.69

0.45

0.60

(I)

150

7.55

1.33

0.81

---

>90.31

---

1.07

* Calculated from carbon values.

Table 5.4: Elemental analysis of intercalated Bi2Sr2CaCu2O8+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-Bi2Sr2CaCu2O8+x samples that were intercalated by vapor phase exposure to pyrrole at various temperatures. The exceptions are: b) I-Bi2Sr2CaCu2O8+x with no exposure to pyrrole c) ppy/I-Bi2Sr2CaCu2O8+x dissolved in HCl, and i) I-Bi2Sr2CaCu2O8+x exposed to 2,5-dimethylpyrrole instead of pyrrole. 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 Bi2Sr1.8Ca1.2Cu2O8+x (from x-ray diffraction data).

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 Bi2Sr1.8Ca1.2Cu2O8+x (from x-ray diffraction data).

Exposure of (ppy)yI1-2y-Bi2Sr2CaCu2O8+x to HCl for several hours reveals a resistance to the acid with part of the starting material not dissolving. Indeed, Bi2Sr2CaCu2O8+x and I-Bi2Sr2CaCu2O8+x dissolve within minutes. Unlike HNO3, HCl does not oxidize pyrrole and initiate polymerization of the monomer. Elemental analysis of the undissolved material reveals that C, N, and H are in the same proportions as polypyrrole and not pyrrole monomer. These facts point to pyrrole existing inside the matrix of the superconductor in its polymerized form.

The volume fraction of polypyrrole added to Bi2Sr2CaCu2O8+x is calculated from elemental analysis data to be ~20%. Scanning electron microscopy was performed to confirm that the polymerized pyrrole was not residing between the grains and on the surface on the superconductor. As shown in Illustration 5.6(a) and 5.6(c), there is little difference in the appearance of the surface morphology between the polypyrrole intercalated sample and the untreated sample. In contrast, Illustration 5.6(d) shows a Bi2Sr2CaCu2O8+x pellet with polypyrrole grown on the surface of the pellet. The intercalated sample has no polymer filling between the grains and no change in surface morphology. Illustration 5.7 shows scanning electron micrographs of polypyrrole intercalated into the Bi2Sr2Ca2Cu3O10+x phase with similar results.

Conductivity and magnetic studies reveal an increase in the resistivity and a loss of superconductivity after polypyrrole intercalation. This is likely due to the release of protons which is expected during the polymerization process according to Equation 5.1.

Thermal gravimetric analysis (TGA) of a) polypyrrole and b) polypyrrole intercalated into Bi2Sr2CaCu2O8+x.

Figure 5.2: Thermal gravimetric analysis of a) polypyrrole and b) polypyrrole intercalated into Bi2Sr2CaCu2O8+x.

Scanning electron micrographs (SEMs) of a) Bi2Sr1.8Ca1.2Cu2O8+x b) IBi2Sr1.8Ca1.2Cu2O8+x c) (ppy)0.25I0.5Bi2Sr1.8Ca1.2Cu2O8+x and d) polypyrrole coated IBi2Sr1.8Ca1.2Cu2O8+x surface.

Illustration 5.6: Scanning electron micrographs of a) Bi2Sr1.8Ca1.2Cu2O8+x b) IBi2Sr1.8Ca1.2Cu2O8+x c) (ppy)0.25I0.5Bi2Sr1.8Ca1.2Cu2O8+x and d) polypyrrole coated IBi2Sr1.8Ca1.2Cu2O8+x surface.

Scanning electron micrographs (SEMs) of a) Bi2Sr2Ca2Cu3O10 and b) polypyrrole intercalated Bi2Sr2Ca2Cu3O10.

Illustration 5.7: Scanning electron micrographs of a) Bi2Sr2Ca2Cu3O10 and b) polypyrrole intercalated Bi2Sr2Ca2Cu3O10.

Intercalating

Material

Host

Temp.

(OC)

XRPD

Observations

iodine

BSCCO

150

new pattern

more brittle

pyrrole

BSCCO

I-BSCCO

150

little change

new pattern

little change

little change

2,5 dimethyl-pyrrole

I-BSCCO

150

low intensity

---

aniline

BSCCO

150

minor shift

---

pyridine

BSCCO

150

no change

---

thiophene

BSCCO

150

no change

---

TCNQ solution

TCNQ solution

TCNQ / iodine

BSCCO

I-BSCCO

BSCCO

80

80

150

no change

no change

no ID*

---

---

yellow surface

TTF / iodine

BSCCO

150

no ID*

"flattened"

TMTSF / iodine

BSCCO

150

no ID*

---

ET

ET / iodine

BSCCO

BSCCO

150

150

no ID*

no ID*

deformed

deformed

*XRPD pattern is different from the starting material, but not yet identified.

Table 5.5: Table showing a variety of materials examined for possible intercalation.

Intercalating

Material

Host

Temp.

(OC)

XRPD

Observations

KI / iodine

BSCCO

190

both patterns overlapped

colored surface

cyclooctane

BSCCO

150

no change

no change

quinoline

BSCCO

150

no change

liquid was dark

bromine

BSCCO

200-225

no change

no change

Cl-Al-Pc

BSCCO

200

no ID*

gray

bromobenzene

BSCCO

I-BSCCO

150

150

no change

no ID*

no change

---

bromothiophene

BSCCO

I-BSCCO

150

150

no change

no ID*

no change

---

nitrobenzene

BSCCO

I-BSCCO

150

150

no change

no ID*

no change

---

perylene

BSCCO

300

noisy

white coating

AlCl3

BSCCO

300

no peaks

small dots of orange

BF3:etherate

BSCCO

25

no change

no change

BBr3

BSCCO

BSCCO

25

100

no change

inconclusive

no change

no change

*XRPD pattern is different from the starting material, but not yet identified.

Table 5.6: A second table showing a variety of materials examined for possible intercalation.

nC4NH5 => (C4NH3)n + 2nH+ + 2ne- Eq. 5.1

Reoxidation of the material through a re-doping process may possible restore its superconducting properties although experiments related to this were not investigated.

A large variety of materials were examined for possible intercalation. A summary of these trials can be seen in Tables 5.5 and 5.6. When heating the majority of samples in the presence of Bi2Sr2CaCu2O8+x, it was found that there was no effect. The samples that were first intercalated with iodine show more sign of chemical reaction, but lack evidence for ordered intercalation into the layered material. The intercalation of iodine into Bi2Sr2CaCu2O8+x and the intercalation of pyrrole into I-Bi2Sr2CaCu2O8+x show evidence for controlled insertion of molecules into the superconductor lattice without chemical degradation of the lattice structure.

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Updated on: April 15, 2010 8:26 PM