Improving PPy Adhesion by Surface Roughening Yingkai Liu, Qi Gan, Shermeen Baig, and Elisabeth Smela* Mechanical Engineering, UniVersity of Maryland, College Park, Maryland 20742 ReceiVed: March 7, 2007; In Final Form: May 17, 2007 Conjugated polymers have found applications as “artificial muscles” because they undergo significant volume change upon electrochemical cycling. However, this large repeated strain also leads them to delaminate from the underlying electrode. Two methods of improving the adhesion of polypyrrole films to Au electrodes during extended electrochemical cycling were quantitatively characterized. Both involved roughening the Au surface, the first by electroplating and the second by etching. The extent of delamination was quantified using a tape test at regular intervals during switching between the fully oxidized and reduced states in an aqueous electrolyte, and the surfaces were characterized by scanning electron microscopy. Untreated smooth control surfaces were simultaneously monitored the same way. Without surface treatment, 3000 Å thick polypyrrole films doped with dodecylbenzenesulfonate, PPy(DBS), delaminated unpredictably between 1 cycle and 20 000 cycles, with almost half the samples failing before 3000 cycles. Electroplating even a thin layer of 1000 Å of Au in a low-concentration plating bath consistently improved lifetimes to at least 10 000 cycles. Thicker plating produced even better adhesion, and a 1 μm thick layer of electroplated Au virtually eliminated delamination, extending the lifetime beyond 60 000 cycles. However, this Au roughness is thicker than the PPy film itself, and is not useful for microactuator work. Etching also improved adhesion, with the best results obtained for the shortest etch times. With a roughness of only 700 Å, an undercut surface morphology was produced that allowed good mechanical interlocking of the PPy. This process is appropriate for use with thin films; however, it will be sensitive to the grain structure of the original Au film. On surfaces with no delamination, it was possible to measure the oxidative degradation of the PPy(DBS) in the aqueous electrolyte. The electroactivity decreased steadily over time, with 20% loss after 10 000 cycles, 50% at 35 000, and virtually no electroactivity by 60 000 cycles. 1 Introduction 1.1. Motivation. Conjugated polymer electrochromic win- dows, actuators, supercapacitors, and batteries undergo repeated electrochemical switching between oxidized and reduced states during operation. However, the very expansion and contraction during switching that is exploited for actuation also results in high shear stresses at the polymer/electrode interfaces in the device, 1 which can cause the polymer to delaminate, leading to reduced performance, slow degradation in performance, or catastrophic device failure. Figure 1 shows a film of PPy (black in the figure) that has delaminated from the Au electrode leads (white) to bilayer microactuators on the surface of a silicon wafer (gray). 2,3 As in the figure, delamination typically begins at corners and edges. Delamination can occur in as soon as a single cycle, or after several thousand cycles, depending on variables that are not fully understood. It is therefore critical to understand and improve adhesion between conjugated polymers and electrodes. 1.2. Prior Work on Improving Adhesion/Preventing Delamination. Previous work on improving adhesion can be divided into two approaches: mechanical interlocking and chemical bonding. The object of the former is the creation of protrusions or crevices around or into which the polymer is deposited, producing a mechanical bonding akin to the joining of puzzle pieces. The latter seeks to covalently join the polymer to the metal through an intermediating molecule with moieties that can bond with both, or by copolymerizing with a second polymer that can be grafted to the metal surface. 4 This paper builds upon earlier work on surface roughening to increase mechanical interlocking between a gold electrode and a conjugated polymer film. 5-7 The adhesion of electrode- posited PPy films was improved by first electroplating a rough layer of gold over an originally smooth gold film, producing a “fuzzy” surface that resulted in greater strains and improved * Corresponding author. E-mail: smela@eng.umd.edu. Tel: 301-405- 5265. Figure 1. Delamination of PPy from underlying Au electrode going to bilayer microactuators (bent at an angle of approximately 20° from the surface). 11329 J. Phys. Chem. C 2007, 111, 11329-11338 10.1021/jp071871z CCC: $37.00 © 2007 American Chemical Society Published on Web 07/03/2007