Novel Synthesis Of Stable Polypyrrole Nanospheres Biology Essay

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In this study, a novel and exceedingly simple method for the aqueous synthesis of polypyrrole nanospheres was investigated. The method is template and surfactant free using only pyrrole monomer, water, and ozone. By varying the monomer concentration, exposure time to ozone, and the temperature, it was determined through particle size and transmission electron microscope (TEM) measurements that temperature was the critical factor controlling particle size. From the particle size measurements, a particle size distribution with a number weighted mean diameter of 73 nm and a standard deviation of 18 nm was achieved. The particles were also investigated using zeta potential measurements, UV-Vis spectroscopy, FTIR spectroscopy, and thermal gravimetric analysis in an effort to determine the identity of the nanoparticles as well as the mechanism by which the nanoparticles are formed and stabilized.

Due to their unique properties, there is currently great interest in polypyrrole nanoparticles and other carbon based nanoparticles in various applications such as chemical & biological sensors, optically transparent conducting materials, electrochromic devices, actuators, supercapacitor, photovoltaic cell, transistor and data storage and surface protection.[1-5] Due to the environmental stability and high conductivity of polypyrrole, it has become one of the most widely studied conducting polymers.[6-14] The challenge when synthesizing nanosized particles is to prevent agglomeration. This process is more pronounced with nanosized particles because of their increased surface area and therefore van der wall interactions. Many attempts have been made to synthesize polypyrrole nanoparticles using a large number of different synthesis conditions.[15-20] From these studies, two main strategies have emerged to synthesize non-aggregated polypyrrole. These strategies are known as hard and soft template processes.[19, 21-24]

Hard template processes use pores in "hard" materials such as anodized aluminum oxide to direct the growth of polypyrrole particles and wires and keep them from agglomerating.[25-27] After the synthesis the templates must then be etched away usually with strong acids. Another variation of the hard template approach is the coating of colloidal particles, such as silver or silica, with polypyrrole.[28-32] Soft template methods typically use surfactant micelles as a template for the formation of nanoparticles.[33-37] Microemulsion polymerization and reversed-microemulsion polymerization are examples of soft template based methods, wherein, structure and concentration of surfactant and monomers are critical factors for controlling morphological parameters of products.[21] The problem with hard template processes is that they are often complicated and therefore expensive, involving the very carefully controlled growth of oxide layers or colloidal particles to give precise and uniform pore sizes or particle sizes. After synthesis the templates must then be etched away which usually involves the use of harsh chemicals.[38] Soft template processes typically produce products in which the soft template can never be fully removed which will affect the properties of the product and large amounts of surfactant are typically required which adds to the expense of the process.

To circumvent these issues, there have been some processes developed which either do not use traditional template systems or use a template which double as a dopant for the polypyrrole. One such process developed by Kim et. al. uses a surfactant free emulsion system using water droplets in octanol as a soft template and FeCl3 as the oxidant.[39] Li et. al. employ a process that uses a dilute solution of rhodamine B as a molecular template that also acts as a dopant for the polypyrrole.[40] Henry et. al. presents production of polypyrrole nanofibers using bipyrrole in oxidative polymerization of pyrrole.[41] Catherine Debiemme-Chouvy has obtained nanostructures with diameters in the range of 40-120 nm by electrodepositing polypyrrole in the presence of jointly non-acidic and weak-acidic anions.[42] In this study, an even simpler method for the synthesis of unagglomerated polypyrrole nanospheres of a controlled size is outlined that only uses water, pyrrole, and ozone in a one pot, one step, synthesis.

EXPERIMENTAL

Pyrrole was obtained from Sigma Aldrich and was freshly distilled before use. Millipore 18.2 MΩ water was the solvent used for the reaction. Ozone was obtained by running pure dried oxygen supplied by Air Gas through a model ATLAS 30 C ozone generator supplied by Absolute Ozone. Particle size measurements were carried out using a NICOMP 380 submicron particle sizer supplied by Particle Sizing Systems. A Gaussian analysis was applied to all data. For the TEM images, 300 mesh Formvar/carbon coated grids were dipped into the solution containing the particles and immediately wicked off using filter paper. After allowing the grid to dry, images were obtained using a JEOL JEM-100CX II Transmission Electron Microscope at 80 keV. UV Vis spectra of the polypyrrole nanoparticles in water were recorded on Varian-5000 UV-Vis-NIR Spectrophotometer. Thermal degradation analysis of the samples was performed using a thermogravimetric analysis instrument TGA Q 500 supplied by TA Instruments. The samples were heated from room temperature up to 800˚ C at a heating rate of 20˚ C/min. The obtained results were analyzed using the software, Universal Analysis 2000. A Nicolet FT-IR spectrometer was used for the FT-IR characterization. A Veeco Dimension 3100 atomic force microscope (AFM) with contact mode and current sensing probe was used for conductive AFM (C-AFM) measurements to characterize pressed pellets of the nanoparticles for surface morphology and conductivity. Zeta potential was measured using a Zetasizer (Malvern Instruments, Worcestershire, U.K.). All measurements were recorded at 25oC. The reactions that were carried out can be seen in Table 1.

Table 1. A summary of the reactions that were carried out.

Reaction

Pyrrole (M)

Ozone Exposure (S)

Temperature (°C)

1

0.17

60

23

2

0.17

60

4

3

0.17

30

23

4

0.085

60

23

The synthesis procedure for the first set of reactions was as follows: 100 ml of Millipore water was placed in a 125 ml Erlenmeyer flask followed by 1.35g of pyrrole. In the case of reaction 4, 0.675g of pyrrole was added. The mixture was then stirred until all of the pyrrole was dissolved. Oxygen was provided to the ozone generator at a pressure of 20 psi and a flow rate of 0.2 liters/minute. According to the literature provided by Absolute Ozone, this flow rate should be producing an oxygen/ozone mixture that is 18% ozone by weight. For reactions 1 and 4, the oxygen/ozone mixture was bubbled through the pyrrole solution for 60 seconds. For reaction 3 the oxygen/ozone mixture was bubbled through the pyrrole solution for 30 seconds. After the ozone exposure was complete, any remaining ozone in the flask was removed with a gently blowing stream of air from a compressed air line. The flasks were then sealed with a rubber stopper and allowed to sit for 4 days.

The procedure for reaction 2 was similar except that prior to the pyrrole being added to the Millipore water, the flask was placed in an ice bath until the water reached a temperature of 4° C. Pyrrole was then added and stirred until it dissolved. The oxygen/ozone mixture was then bubbled through the pyrrole solution while it remained in the ice bath. After ozone exposure, remaining ozone was removed with a stream of compressed air and the flask was sealed with a stopper and placed in a refrigerator at a temperature of 4° C for 4 days. The same reaction procedure was followed for all the reactions in this work; the variants include temperature, molar concentration and ozone exposure time. After the results from the first set of reactions were obtained, a larger set of reactions was carried out in an effort to determine factors affecting the stability and formation of the nanoparticles. These reactions can be seen in Table 2. The reactions in Table 2 were carried out using the same pressure and flow rate as the previous reactions from Table 1. All reactions in Table 2 were carried out at room temperature.

Table 2: Reactions for studying zeta potential measurement and particle size behavior as a function of monomer concentration and ozone exposure.

Reaction

0.17 Molar

R - 1

30 Seconds Ozone Exposure

R - 2

60 Seconds Ozone Exposure

R - 3

120 Seconds Ozone Exposure

0.34 Molar

R - 4

30 Seconds Ozone Exposure

R - 5

60 Seconds Ozone Exposure

R - 6

120 Seconds Ozone Exposure

0.51 Molar

R - 7

30 Seconds Ozone Exposure

R - 8

60 Seconds Ozone Exposure

R - 9

120 Seconds Ozone Exposure

0.68 Molar

R - 10

30 Seconds Ozone Exposure

R - 11

60 Seconds Ozone Exposure

R - 12

120 Seconds Ozone Exposure

0.85 Molar

R - 13

30 Seconds Ozone Exposure

R - 14

60 Seconds Ozone Exposure

R - 15

120 Seconds Ozone Exposure

Another set of reactions was performed to investigate the effect of pH in conjunction with ozone exposure on the synthesis reaction. The conditions used for these reactions can be seen in Table 3.

Table 3. Reactions studying the effect of pH and ozone exposure on the synthesis of PPY nanoparticles.

Reaction

Molar Concentration

pH

Ozone Exposure Time (s)

Temperature (ËšC)

R-16

0.17

2

60

23

R-17

0.17

4

60

23

R-18

0.17

6

60

23

R-19

0.17

8

60

23

R-20

0.17

10

60

23

R-21

0.17

12

60

23

R-22

0.17

2

60

0

R-23

0.17

1.8

60

0

R-24

0.17

1.65

60

0

R-25

0.17

1.5

60

0

R-26

0.17

1.3

60

0

R-27

0.17

1.8

60

0

R-28

0.17

1.6

60

0

R-29

0.17

1.4

60

0

R-30

0.17

1.8

120

0

R-31

0.17

1.6

120

0

R-32

0.17

1.4

120

0

R-33

0.17

1.8

240

0

R-34

0.17

1.6

240

0

R-35

0.17

1.4

240

0

R-36

0.17

1

480

0

A dradendroff reagent was used to test for the presence of a nonionic surfactant which could account for steric stabilization. The Dragendroff reagent was supplied by M/s Sigma-Aldrich. The test was performed in a small centrifuge tube. One milliliter sample solution is diluted to a 4-5ml with 20% acetic acid, combined with an equal volume of test solution, and shaken vigorously. The present of nonionic surfactant is shown by precipitation.[43]

RESULTS AND DISCUSSION

The first group of reactions carried out in this study was a survey to try to determine what factors may have an effect on the particle size of the products of the reaction. The results from the Gaussian analysis of the particle size distributions from the various reactions can be seen in Table 3. It is evident that the duration of ozone exposure has a moderate effect on the mean particle size reducing it by 58 nm. However, with this moderate decrease in the mean diameter of the particles there is a large increase in the standard deviation and therefore a decrease in the uniformity of the measured particle diameter. It is also evident that temperature has a large effect on the particle size. With a number weighted mean particle diameter of 73 nm and for reaction 2, a drop in temperature of 19Ëš C resulted in a drop in mean particle size of 251 nm and reduced the particle size to a value below 100 nm. The curves of the data from which these numbers were calculated can be seen in Figures 1-4.

Table 3. The number and volume weighted mean particle diameter and their standard deviations for reactions 1-4.

Reaction

Mean Weighting

Mean Diameter (nm)

Standard Deviation (nm)

1

Number

324

16

Volume

325

16

2

Number

73

18

Volume

88

22

3

Number

266

78

Volume

373

109

4

Number

288

32

Volume

301

34

Figure 1. The number weighted particle size distribution for the product of reaction 1.

Figure 2. The number weighted particle size distribution for the product of reaction 2.

Figure 3. The number weighted particle size distribution for the product of reaction 3.

Figure 4. The number weighted particle size distribution for the product of reaction 4.

Figures 1-4 show the particle size distribution for the products of reactions 1-4. It is evident from these plots that particle distributions achieved from these reactions all approximate Gausian behavior. The fact that these peaks are all Gaussian in nature indicates that there are likely not any large mechanistic differences between the products of the different reactions. A bimodal distribution, for example, would indicate that some effect that was causing a drastic difference in the products was influencing the products of the reactions. It is also evident that the decrease in temperature in reaction narrowed the distribution as well as shifted it to smaller diameters.

Analysis of reactions 1 and 2 via a transmission electron microscope yielded the images in Figure 5. It is clear in these images that both reactions yielded particles that were spherical in nature with small amounts of agglomeration. The particle size difference seen in Figures 1 and 2 are also apparent in these images with the particles produced from reaction 2 being much smaller. It is also important to note that the particles seen in Figure 1 seem to have a large amount of electron density which could be an indication that there is a significant amount of conjugation within the particles. This is evident because the particles are completely opaque under the TEM. Most nanosized polymer particles such as poly methyl methacrylate appear to be somewhat translucent under the TEM because they do not have a high density of electrons.[44] In contrast to this, conducting polymers have a high degree of unsaturation leading to much higher electron densities which make conducting polymer nanoparticles appear opaque. The results obtained in this analysis are in good agreement with other TEM results for conducting polymer nanoparticles.[45, 46]

c

d

b

a

Figure 5. The TEM images of the particles produced from reaction 1 (a and b) and reaction 2 (c and d).

After promising results were obtained from reactions 1-4, further study was carried out to study correlation between the monomer concentration, ozone exposure and particle size at room temperature. These reactions can be seen in Table 2. The TGA results for reactions 1-3 can be seen in Figure 6 below. It can be seen that greater amounts of ozone exposure increase the thermal stability of the product. This behavior could be indicative of a higher molecular weight product indicating that the ozone is initiating cationic radical polymerization of the polypyrrole similar to other initiators that act as oxidizing agents. TEM results from these reactions are shown in Figure 7. It is evident from these results that there is a layer that has formed on the outside of the particles despite the absence of a surfactant in the synthesis reaction. It is thought that this layer may be responsible for the stabilization of the particle dispersion. It may be possible that this outer layer is composed of more polar overoxidized polypyrrole which could act as a steric stabilizer. More evidence and discussion on this hypothesis is shown below.

Figure 6. The plot of the isothermal TGA data collected from reactions R-1 through R-3.

Figure 7: TEM Images of the particles (a) & (b): a layer can be seen around the nanoparticles, (c) & (d): ideal behavior of stable nanoparticles in water, (e) & (f): Agglomerated particles because of high ozone exposure and very low pH (Below pH of 1.65)

The effect of monomer concentration and ozone exposure can be seen in Figure 7. It is evident from this plot that increased ozone exposure and higher monomer concentration results in larger particle sizes at room temperature. It is also apparent that the amount of ozone exposure has a larger affect on particle size at higher monomer concentrations. These results may indicate that it is necessary to oxidize a certain percentage of the monomer in order for the particles sizes to be kept small. Once that percentage goes down in the case of the higher monomer concentrations it seems that the particle size increases.

Figure 8: Molar concentration vs. mean particle diameter for reactions R-1 through R-15.

In order to help determine the mechanism by which the nanoparticles may be stabilized and whether the layer on the outside of the particles may be involved, zeta potential measurements were performed on the dispersions. The zeta potential results of all the reactions from Table 2 can be seen in Table 3. For all of the measured reactions, the zeta potential was very near 0 V which indicates that there is not enough electrostatic activity in the dispersion to keep it stabilized. Therefore it is likely that the stability of the particles could be attributed to steric stabilization. To help confirm this, a Dragendroff reagent was added to a sample of the dispersion in a centrifuge tube.[43] In this case precipitation was observed after vigorously shaking the centrifuge tube for 15-20 minutes indicating that manner in which the nanoparticles were being stabilized was similar to a nonionic surfactant. According to Cataldo et al., ozone slowly forms amide ketones on the α carbons and hydroxide groups on the β carbons of a polypyrrole chain with some ring scission.[47] This may be another explanation for the inherent stability that seems to be imparted to the nanoparticles.

Table 3. Zeta potential and yield data for reactions R-1 through R-15.

Reaction

Zeta potential (mV)

Yield (gm)

0.17 Molar

R - 1

30 Seconds Ozone Exposure

0.0688

0.01

R - 2

60 Seconds Ozone Exposure

-0.075

0.03

R - 3

120 Seconds Ozone Exposure

0.5038

0.02

0.34 Molar

R - 4

30 Seconds Ozone Exposure

-0.0767

0.02

R - 5

60 Seconds Ozone Exposure

-0.128

0.03

R - 6

120 Seconds Ozone Exposure

0.27256

0.02

0.51 Molar

R - 7

30 Seconds Ozone Exposure

-0.07708

0.03

R - 8

60 Seconds Ozone Exposure

-0.00432

0.04

R - 9

120 Seconds Ozone Exposure

-0.23866

0.04

0.68 Molar

R - 10

30 Seconds Ozone Exposure

0.0602

0.03

R - 11

60 Seconds Ozone Exposure

0.045

0.03

R - 12

120 Seconds Ozone Exposure

-0.29964

0.04

0.85 Molar

R - 13

30 Seconds Ozone Exposure

0.105056

0.03

R - 14

60 Seconds Ozone Exposure

0.12702

0.02

R - 15

120 Seconds Ozone Exposure

-0.14228

0.03

If the layer surrounding the nanoparticles was made up of overoxidized polypyrrole, it could possibly act as a sterically stabilizing layer. The ketone and hydroxyl functional groups that would be added onto the polypyrrole chains as a result of overoxidation would make them more polar and therefore more hydrophilic. This hydrophilicity would allow the chains to relax in the surrounding water. Aggregation would then require these surrounding chains to take on less relaxed conformations which would result in a decrease in entropy. Aggregation would therefore be entropically unfavorable. This hypothesis is further supported by the presence of a carbonyl peak and a hydroxyl peak in the FTIR results in Figure 9.

Figure 9: FTIR Spectra: effect of increased reaction time and reduced pH. (R-24, R-34 and R-36)

FTIR spectra of reactions performed at 0˚ C with different ozone exposure times are shown in Figure 8. The bands at 1563 (2,5-substituted pyrrole) and 1436 cm-1 may be assigned to polypyrrole ring vibrations. [48-50] The bands at 1345, and 1074 cm-1 may be corresponding to =C-H in plane vibrations while the band at 885 cm-1 may correspond to out of plane vibrations indicating polymerization of pyrrole. [49, 50] The absorption peak at 1658 cm-1 likely correspond to a -C=O linkage. [51] The C=O structure at the β-C of pyrrole ring is typically due to the overoxidation of polypyrrole. [52]

The presence of overoxidized polypyrrole could also be an explanation for the results of conductivity measurements that were performed using the conductive AFM technique. In all of the fifteen reactions in Table 2, the synthesized product does not exhibit measurable conductivity. A typical set of images collected from each of the samples can be seen in Figure 10. It is the far right image that would show measurable current as white areas in the image. If overoxidized oligomers are forming an insulating layer around the particles with more conductive polypyrrole at the center, it could disrupt conduction through a pressed pellet of the product and therefore prevent conductivity in the sample. This insulating behavior is due to the presence of the Carbonyl functional groups which disrupt the conjugation of polypyrrole chains and therefore reduce the conductivity of polypyrrole.

Figure 10. C-AFM images collected from a pressed pellet of the PPY nanoparticles.

Part of this problem may be due to using water as a solvent. Peter Novak has suggested in his work that carbonyl linkages form due to the reaction of pyrrole with water and hydroxy radicals which are in abundance when ozone is dissolved in water.[53] This phenomenon is pH dependent however. Overoxidation is thermodynamically more favorable at basic pH's so, by adjust the pH with hydrochloric acid, it was thought that a conductive product could be obtained.[53] An additional benefit that lower pH levels could have on this reaction would be to decrease the rate of decomposition for ozone. Ozone dissociation in water is initiated by negatively charged OH ions, by decreasing the pH of the reaction solution, this reaction can be slowed which may result in less overoxidation of the polypyrrole as well.[54] Low temperatures during the reaction could also help in reducing the inner space of micelles by virtue of deactivating the chain mobility of the sterically stabilizing layers on the outside of the particles and by preventing overoxidation.[4]

Therefore it was necessary to investigate the effect that pH had on this synthesis reaction. The effect of pH was studied using HCl and NaOH to adjust the pH of the synthesis solutions prior to ozone exposure. The dispersions produced from these reactions were observed visually for colloidal stability and with UV Vis spectroscopy to detect the presence of bipolarons which would indicate conductivity. For this study we prepared solutions of pyrrole in water at different pH concentrations of, 2, 4, 6, 8, 10, and 12. It was observed in Figure 11 that increasing pH reduces stability of the nanoparticles; the nanoparticles were most stable at pH 2. By analyzing the UV Vis spectra of these reactions in Figure 12, it could be concluded from the peaks at approximately 294 nm that the dispersions of nano-particles contain large amounts of terpyrrole oligomers.[55] By reducing the Ph of the reaction, it could be observed that the peak for terpyrrole oligomers became less intense and it shifted towards larger wavelengths which may indicate an increase in the molecular weight of the polypyrrole.[55]

Figure 11: The stability of polypyrrole nanoparticles dispersions 20 days after synthesis. (R-22 to R - 26)

Figure 12: UV Vis spectroscopy of reactions R-24 and R-17 (pH of 4 and 1.6).

After the above results were observed for the reactions at different pH levels, five more reactions were carried out from a pH of 2 to a pH of 1.3 to determine if the stability or conductivity of the nanoparticles could be improved. The amount of ozone exposure was also increased for these reactions in an effort to increase the molecular weight of the polypyrrole. The images of the dispersions produced from these reactions can be seen in Figure 13. It was evident from visual assessment that the dispersions were not stable below a pH of 1.65. It may be beneficial to perform the synthesis reaction at the lowest pH possible as this would provide the largest number of chloride ions from the hydrochloric acid to act as dopant ions if bipolarons were formed on the polypyrrole chains. UV-Vis spectroscopy was used to determine if there was a difference between the different products of the reactions.

Figure 13: Optimizing stabilization for lowest pH (R-22 to R-26) (Observed that lowest possible pH we could go is 1.65)

Figure 14 shows effect of increased reaction time and reduced pH on the UV-Vis spectra of the products. It can be observed that with reduction in pH; the terpyrrole peaks at approximately 300nm were reduced which suggests a decrease in the amount of terpyrrole in the final product.[55] The broad band at 475 nm, which has been assigned to the pi-pi* transition of polypyrrole, indicates that higher molecular weight polypyrrole is produced.[55-57] The peaks at 475 nm also show bipolaron absorption due to Cl- ion doping.[58, 59] However, as earlier reported, the increase in reaction time is prone to producing agglomeration of nanoparticles.

Figure 14: UV-Vis spectra results at increased reaction time, and reduced pH. (R-33 to R-36)

CONCLUSION

This study investigated the template free synthesis of polypyrrole nanoparticles via chemical oxidative polymerization using ozone as the oxidizing agent. It was found that reaction temperature was the factor that had the largest effect on particle size with colder temperatures producing smaller particles. It was observed that there is layer surrounding the particles that may be contributing to the stability of the nanoparticles. FTIR and UV-Vis results indicated that this layer is likely to be overoxidized pyrrole monomers. Zeta potential measurements indicated that the mode by which the particles were stabilized was steric stabilization. Future work will investigate the use of colder temperatures by introducing co-solvents such as methanol into the synthesis reaction to keep the reaction from freezing. Additional oxidizing agents will also be investigated to increase the yield of the reaction.

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