Tuning the Conductivity of Polyaniline through Doping by Means of Single Precursor Vapor Phase Infiltration

Significance Statement

Conductive polymers have been a topic of discussion in the last few years. Enhancement of conductivity and thermal or chemical resistance of these polymers will allow for substitution of inorganic materials currently used in a number of electronic devices such as light emitting diodes, batteries, thin-film transistors, solar cells, and super capacitors. Polyaniline is one of the most prominent organic semiconductors which functions as a model for other conductive polymers.

Polyaniline level of conductivity is of ultimate importance when it comes to most applications and depends largely on the switching between the various polymer states: leucoemeraldine, emeraldine, and pernigraniline states, which occur as a response to an electrical or chemical trigger. Its conductivity can be altered by doping with inorganic protonic acids, alkali metal salts, transition metal salts, and lewis and organic acids.

However, doping, which depends on wet chemistry, introduces some impurities to the polyaniline by inclusion of additives, and affects the morphology as well as structure of polyaniline. Fortunately, vacuum-based processing appears to be an easy way out to avoid the negative influences from solvents. A team of researchers led by professor Mato Knez from the CIC nanoGUNE in Spain presented an infiltration approach which can be used for polyaniline doping.  This infiltration process was originally developed as bioinspired strategy to mechanically reinforce soft matter. The group demonstrated greatly enhanced mechanical properties of a variety of natural and synthetic polymers in the past years. Now the infiltration process was adapted to alter electronic properties of suitable polymers. As a result, the approach allowed for controlled polymer conductivity through a few infiltration methods. Professor Mato KnezHe said “The possibility to apply the infiltration technology to modify electronic properties of polymers is an amazing perspective for this new way of materials fabrication. Given the vast number of materials that can be infiltrated and the even larger number of polymers that can be synthesized, optimizing the processes and materials can easily keep us busy for the next decades.”. The research work is now published Advanced Materials Interfaces.

The authors prepared polyaniline nanofibers through a rapid mixing polymerization. They prepared aqueous solutions of aniline and ammonium peroxydisulfate and mixed them at room temperature. The solution was then shaken well for homogenous mixing. This was followed by 12 h polymerization. The resulting HCl doped polyaniline was collected through filtration and washed with hydrochloric acid. The researchers then obtained dedoped polyaniline through treatment of the HCl doped polyaniline with ammonium hydroxide solution.

Glass substrates were cleaned and dried in an oven. Then, the glass substrates were coated with polyaniline and polyaniline doped with HCl and the samples dried in an oven. The doped polyaniline films deposited on the glass substrates were exposed to molybdenum chloride and tin chloride in a pulse-exposure-purge sequence with a varying number of repetition cycles.

The authors observed that polyaniline doped in the proposed way was not significantly affected by thermal treatment in a vacuum at 150 °C. However, the conductivity of the polyaniline doped with 1 Molar Hydrochloric acid was almost lost. The conductivity loss was attributed to deprotonation of the doped polymer as well as HCl evaporation. This resulted in the recovery of the nonconductive emeraldine based on the polyaniline.

Doping with molybdenum chloride and tin chloride led to oxidation of the polyaniline and in complexation of the chlorides with the polyaniline nitrogen. As a consequence, the electron mobility across the polyaniline chains was improved and the structure was stabilized even at high temperatures.

“The outcomes of the study are not only focal for the vapor phase infiltration doping technique and obtaining enhanced thermal stability of the doped polymer, but also for the possible top-down doping of the synthesized polyaniline. This will allow for better shaping of the developed materials, therefore, more efficient fabrication. This will pave way for similar infiltration processes for other conductive polymers such as polypyrrole and polythiophene. In fact, a follow-up paper of the group showed similarly efficient doping of P3HT, a polythiophene based conductive polymer” said professor Mato Knez.

Tuning Conductivity of Polyaniline through Doping by Means of Single Precursor Vapor Phase Infiltration

About The Author

Weike Wang received the M.S. degree in Physical Chemistry from the Taiyuan University of Technology, Taiyuan, China in 2013. He is currently pursuing a doctoral degree in Physics of Nanostructures and Advanced Materials at CIC nanoGUNE in San Sebastián, Spain.

His research interest relates to exploitation of the atomic layer deposition-based infiltration process, the vapor phase infiltration (VPI), as an alternative strategy for top-down doping of various conducting polymers.

About The Author

Mato Knez studied chemistry at the University of Ulm (Germany) and finished his doctoral thesis in natural sciences at the Max-Planck Institute of Solid State Research in Stuttgart (Germany) in 2003.  During his postdoctoral stay at the Max-Planck Institute MSP in Halle (Germany), he received the Nanofutur Award of the German Ministry of Education and Research in 2006 and grew a junior research group with focus on investigating functional materials grown by atomic layer deposition (ALD).

Since 2012 he is Ikerbasque research professor and group leader at the research center CIC nanoGUNE in San Sebastián (Spain). In 2012 he received the Gaede Prize of the German Vacuum Society.

REFERENCE

Weike Wang, Fan Yang, Chaoqiu Chen, Lianbing Zhang, Yong Qin, and Mato Knez. Tuning the Conductivity of Polyaniline through Doping by Means of Single Precursor Vapor Phase Infiltration. Advanced Material Interfaces 2017, 4, 1600806.

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