In Situ Electrochemical Approach to Reproducible Percolation Networks for Chemiresistors
Authors: Li Weishuo , Lefferts Merel J. , Lister Abigail Mary , Armitage Ben I. , Castell Martin R. 
Published: July 2021

Detecting low concentrations of gases is critical in various fields, including daily safety for households or industry, systematic surveillance of processes or environment, and so forth. Compared to most of gas detection devices, chemiresistors offer the potential to achieve easy-processing and low-cost devices with rapid responses. Conducting polymers have been widely used as the sensing layers in chemiresistors owing to their unique advantages. Nowadays, one of the biggest challenges is to fabricate polymer-based chemiresistors exhibiting high sensitivity. Increasing the surface-to-volume ratios of sensing layers has been considered as a mainstream way to overcome this challenge by preparing different forms of nanoscale conducting polymers.1 Recently, a novel concept based on electrical percolation has been introduced and it has been proved that percolation networks of conducting polymers can improve device sensitivity.2, 3 However, it is still challenging to achieve reproducible percolation networks due to variations in initial resistance of chemiresistors obtained by simple three-electrode electropolymerization. This may be attributed to the random growth of conducting polymers and their extremely high sensitivity to deposition conditions in the percolation region. In this study, the in situ electrochemical-conductance method has been exploited to investigate the electrical percolation of polypyrrole (PPy) networks during electropolymerization. PPy networks have been deposited onto interdigitated Pt electrodes (IDEs) using chronoamperometry while a series of potential bias has been applied between the branches of the IDEs. The drain current between these branches can directly reflect the conductance change of PPy networks in real-time, starting in the insulating region and increasing in conductance into the thin-film region (Fig 1a). Relevant factors that can affect percolation thresholds have been studied, including monomer concentrations, polymerization potentials, and applied potentials within IDEs. Higher concentrations and potentials may result in lower thresholds owing to faster electropolymerization. A current-controlled method has been introduced to prepare chemiresistors with similar resistances by stopping electropolymerization at certain current changes between IDE branches (Fig 1b). This current-controlled method can achieve smaller variations in initial resistance of PPy-based chemiresistors than the traditional polymerization controlled by deposition time, providing more reproducible percolation networks. Based on the current-controlled method, a series of polymer-based chemiresistors have been prepared by chronoamperometry and the relationships between device resistances and sensing performances have been studied. Furthermore, chemiresistors with similar resistances have been fabricated near the percolation threshold and the reproducibility of their sensing responses has been investigated.

Fig 1. (a) Drain current of PPy networks between IDE branches applied with different potentials. (b) Conductance (in black) and its semi-logarithmic plot (in orange) of PPy-based chemiresistors prepared at different current changes. Results of conductance obtained at around 10, 20, 50, 100, 400, and 700 μA are grouped into G10, G20, G50, G100, G400, and G700. Percolation networks were obtained after the drain current increased by 20 μA, and thin films were formed when the drain current increased by 700 μA.

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Figure 1