Effect of Oxalic Acid Treatment on Conductive Coatings Formed by Ni@Ag Core-Shell Nanoparticles

Abstract

Low-cost metallic nanoink based on nickel-silver core-shell nanoparticles (Ni@Ag NPs) was used for the formation of conductive metallic coatings with low sintering temperature, which can be successfully applied for replacement of currently used silver-based nanoinks in printed electronics. The effect of oxalic acid (OA) on the sintering temperature and conductivity of coatings formed by Ni@Ag NPs was evaluated. It was found that the addition of OA to the ink formulation and post-printing treatment of deposited films with this acid provided a noticeable decrease in the sintering temperature required for obtaining conductive patterns that is especially important for utilizing the polymeric substrates. The obtained resistivity of metallic coatings after sintering at temperature as low as 100 °C was found to be 30 µΩ·cm, only ~4 times higher compared to the resistivity of bulk Ni that is promising for future application of such materials for fabrication of low-cost flexible printed patterns.

Keywords: conductive coatings; conductivity; nickel–silver core–shell nanoparticles; oxalic acid; sintering.

Figure 1

Scheme of the preparation of the conductive coatings based on Ni@Ag NPs: (1) with addition of oxalic acid to ink formulation; (2) after post-printing treatment with OA.

Figure 2

Dependence of size (A) and zeta potential (B) of Ni@Ag NPs on the storage time measured by DLS and by microelectrophoretic method, respectively.

Figure 3

Resistivity of coatings formed by Ni@Ag NPs inks containing 0.25 and 0.5 wt% OA as a function of sintering temperature in the range of 40–200 °C (heating duration 30 min). Black bar represents resistivity of ink coatings without OA.

Figure 4

Examples of optical microscopy images of ink coatings after drying (15 min, 40 °C) before (A) and after (B) treatment with 1% OA.

Figure 5

Dependence of the resistivity of the coatings based on Ni@Ag NPs on the concentration of OA (0–2%) after drying (40 °C, 15 min) and sintering (80–200 °C, heating duration 30 min). The inset graph presents the data in the resistivity range of 0–180 µΩ·cm.

Figure 6

Dependence of the resistivity of Ni@Ag NP coatings treated with 1% OA on sintering duration (15–75 min) at sintering temperature 100 °C.

Figure 7

XPS of Ni@Ag NP coatings (after sintering at 100 °C for 30 min) without OA treatment (A) and after treatment with 1 wt% OA (B) (metallic Ni—black curves, NiO—green curves, Ni²⁺—red curves, satellites—blue curves.

Figure 8

SEM images of Ni@Ag coatings after drying (A); after drying, dipping in OA (1 wt%) and drying (40 °C) (B); after drying, dipping in OA (1%) followed by drying and sintering (100 °C, 30 min) (C).