Efficient Passivation of Ag Nanowires with 11‐Mercaptoundecanoic Acid Probed Using In Situ Total‐Internal‐Reflection Surface‐Enhanced Raman Scattering Spectroscopy

Abstract

A promising passivation strategy of one class of Ag nanostructures of high interest for optoelectronics, namely Ag nanowires (AgNWs), with 11‐mercaptoundecanoic acid (MUA) is described. Combining XPS, electron microscopy and original in‐situ total‐internal‐reflection (TIR) surface‐enhanced Raman scattering (SERS) spectroscopic measurements, the substitution of polyvinylpyrrolidone (PVP) molecules on as‐synthesized PVP‐coated AgNWs by their MUA counterparts is clearly demonstrated. The relevance of the SERS approach to examine the anchoring of the MUA onto the silver surface of specific AgNWs with pentagonal cross‐sections is supported by FDTD simulations. In particular, the TIR‐SERS technique reveals that the PVP‐MUA substitution process can be divided into two main steps, namely (i) the desorption of PVP molecules associated with the anchoring of MUA molecules via the formation of Ag−OOC and Ag−S bonds (accomplished within 40 min), and (ii) the subsequent reorientation of MUA molecules to form only Ag−S bonds (finished in 90 min). The resulting passivation of AgNWs with MUA significantly improves their resistance to corrosion, which is crucial for future commercial applications.

Increasing Silver Nanowire Network Stability through Small Molecule Passivation

“…This strategy of passivating the nanowires after their formation into electrodes, rather than before, was necessary to achieve a uniform dispersion of nanowires in the network. When the NWs were passivated with MUA in suspension, such as done in our prior work [35], there was significant aggregation when they were subsequently deposited as a film due to the weak repulsion between MUA-capped AgNWs (Figure S1). The agglomeration affected the transparency of the NW electrodes passivated before their formation into networks (Table S1).…”

“…Due to the strong affinity between the thiol function and silver, MUA can easily replace PVP on the wire surface [35]. To perform this surface modification, the NWs on glass were immersed in an 18 mM solution of MUA in ethanol.…”

“…In this experimental condition, the MUA fully saturated the surface. Previous Raman investigations evidenced that a saturated layer was produced within 70 min, with a MUA solution of 5 mM [35].…”

“…The short molecule 11-mercaptoundecanoic acid (MUA) can easily attach to Ag through a thiolate bond and has been widely used to modify Ag and Au surfaces [31,32,33,34]. We recently showed using surface enhanced Raman spectroscopy that the PVP remaining on AgNWs, after their synthesis, could easily be replaced by MUA molecules by a simple immersion of the NWs in a solution of the thiolated compound [35]. A MUA monolayer on Ag forms a packing order due to the electrostatic and Van der Waals forces between the chains [33], which in turn can act as a barrier against corrosion.…”

“…In this work, we explore the effectiveness of MUA in reducing the corrosion tendency of AgNWs in air. In our previous paper, only a preliminary investigation of the effectiveness of MUA on the electrical stability of a Ag nanowire network was done [35]. Here, we conduct stability tests of MUA-coated AgNWs under both daylight and in the dark, compare its efficacy to a PPh 3 coating, report results on nanowires of different diameters, and determine the effect of MUA on the transparency and initial sheet resistance of the nanowire network.…”