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Silver Ink

January 19, 2012

When I was a graduate student, I needed a very small amount of silver, just a few grams, as an alloy addition. Our supplies cabinet was bursting with bottles of metal powders, mostly iron and its neighbors in the periodic table, but there was no silver. I asked my thesis advisor if we could order some, but the minimum order was more than ten times what I needed.

Materials scientists come from varied backgrounds. I came from physics, but my advisor studied chemistry as an undergraduate. He sent me to a friend in the chemistry department to fetch two chemicals that are commonly used in undergraduate teaching laboratories. These are silver nitrate (AgNO3) and aqueous ammonia (ammonium hydroxide, NH4OH). He mixed the two, and some others that I can't remember, and magically precipitated pure silver from the solution.

The overall reaction produced ammonium nitrate (NH4NO3), which remained in solution, and silver. Reactions in which silver is produced are used also as an analytical test. Tollen's reagent, which is based on silver nitrate, along with ammonium hydroxide and other chemicals, is used as a test for a carbonyl group chemical. When an aldehyde is mixed with this reagent, silver will precipitate. Variants of this reaction are used to silver mirrors.

Scientists from the Department of Materials Science and Engineering and the Frederick Seitz Materials Research Laboratory at the University of Illinois at Urbana-Champaign have modified the Tollen's reaction as a way to write silver conductors, and they have published their research in a recent issue of the Journal of the American Chemical Society.[1-2] Their objective was the production of a silver ink that can be processed at temperatures low enough to write conductors on inexpensive and flexible plastic, or even paper.

Figure captionSilver ink airbrushed onto a plastic film to make a flexible silver electrode.

(Photo by S. Brett Walker. Used with permission.)

Their silver ink is actually a precursor for silver, and not an ink in the conventional sense. Inks are colloids of pigment particles in a fluid. The limitation of colloid ink is that the writing nozzle needs to be larger in diameter than the largest particle in solution. This places limits on the smallest linewidth that can be written with an ink, which is generally several tens of micrometers.[1]

Other silver precursor inks have been demonstrated to produce conductive traces with the same conductivity as bulk silver, but these require a decomposition reaction near 150 °C.[1] This is too high for most polymeric substrates, but not too high for polyimide. The University of Illinois Tollen's reaction is based on silver acetate, rather than silver nitrate. Their resulting ink has 22 wt-% silver.[1]

The reactive silver ink can be printed through nozzles of just 100 nm diameter, and it can be inkjet-printed and airbrush-sprayed if 10% by volume 2,3-butanediol is added to change the viscosity and maintain moisture content. Since the ink is a precursor, silver is formed only when the fluid evaporates and the material is heated. The dried solution contains diaminesilver (Ag(NH3)2CH3CO2), which decomposes to pure silver and volatile chemicals that are released at heating.[1]

Figure captionMicrostructure of the silver ink annealed at 90 °

(Photomicrograph courtesy of S. Brett Walker. Used with permission.)

Annealing the printed traces at 90 °C produces silver conductors with the same conductivity as bulk silver (>6.3 x 105 siemens/cm).[1] This is the highest reported conductivity in a printed ink. Says paper coauthor, S. Brett Walker,[2]
"For printed electronics applications, you need to be able to store the ink for several months because silver is expensive... Since silver particles don't actually form until the ink exits the nozzle and the ammonia evaporates, our ink remains stable for very long periods. For fine-scale nozzle printing, that's a rarity... We are now focused on patterning large-area transparent conductive surfaces using this reactive ink."

This research was supported by the National Science Foundation and the U.S. Department of Energy.[2]

References:

  1. S. Brett Walker and Jennifer A. Lewis, "Reactive Silver Inks for Patterning High-Conductivity Features at Mild Temperatures," J. Am. Chem. Soc., (Article ASAP, January 5, 2012), DOI: 10.1021/ja209267c.
  2. Liz Ahlberg, "Particle-free silver ink prints small, high-performance electronics," University of Illinois Press Release, January 12, 2012.