Atomic Palladium
Authors: Gawron Erin L. , Hira Steven M. , Jonke Alex P , Josowicz Mira A , Janata Jiri 
Published: February 2020

Polyaniline (PANI) is a conducting polymer that has been extensively studied and characterized and has been used in sensors, catalysis and other electrochemical applications1. Incorporating metal clusters into the polymer matrix creates a composite material that has improved activity over the polymer itself. The properties of the metal clusters are size-dependent, where inert metals can even become active when they are scaled down from bulk to nanometer or sub-nanometer sizes2. An important factor with nanocluster creation is controlling the size of the final metal cluster, which is typically done synthetically with capping agents. However, taking advantage of the properties and structure of PANI can allow for electrochemically controlled growth of metal clusters at the imine sites while PANI is acting as an aggregation barrier, thus eliminating the need for capping agents3.

Cluster formation begins with metal anions, such as AuX4
- and PdX4
2- (X = halide), that have an affinity for protonated imine sites of the PANI; these anions can be spontaneously reduced to neutral metal atoms by the emeraldine salt form of PANI. Previous work used this process with an attempt to control cluster sizes in a “top-down” manner by controlling time and concentration variables4. In this work, a controlled deposition and “bottom-up” approach of one metal atom per imine site is explored. The controlled process is completed by holding the PANI at a potential where it is in the oxidized state until excess metal anions are rinsed away and the potential is dropped and the metal is reduced3. A flow-cell connected to a flow-injection system allows for uninterrupted solution and potential changes.

Our recent interest is on understanding the PANI-metal interaction and how to tune the interaction5 to build atomic-sized clusters. The most recent work involves the study of the intercalation of Pd in PANI. PdN (N=0-7) clusters are inserted into a polyaniline matrix through this controlled process using the PdCl4
2- anion. Our recent findings show that the speciation of the metal anion is of critical importance for a specific PANI-metal anion interaction. The mechanism is believed to be dependent on the Bronsted-like interactions and/or charge transfer Results have been confirmed using Raman spectroscopy, XPS, and electrochemical methods. In this presentation we will explain the fundamental steps of the formation of the atomic Pd clusters in PANI and how that Pd-PANI interaction is affected and/or tuned by controlling various conditions such as concentration, potential, and degree of PANI protonation.

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(3) Jonke, A. P.; Josowicz, M.; Janata, J.; Engelhard, M. H. J. Electrochem. Soc.
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(4) Saheb, A.; Smith, J. A.; Josowicz, M.; Janata, J.; Baer, D. R.; Engelhard, M. H. J. Electroanal. Chem.
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(5) Harbottle, A. M.; Hira, S. M.; Josowicz, M.; Janata, J. Langmuir
2016, 32 (33), 8315.