Reactive silver inks for patterning high-conductivity features at mild temperatures

Article abstract of DOI:10.1021/ja209267c

Reactive silver inks for printing highly conductive features (>10 ⁴ S/cm) at room temperature have been created. These inks are stable, particle-free, and suitable for a wide range of patterning techniques. Upon annealing at 90 °C, the printed electrodes exhibit an electrical conductivity equivalent to that of bulk silver.

Full text of DOI:10.1021/ja209267c

Communication
pubs.acs.org/JACS
Reactive Silver Inks for Patterning High-Conductivity Features at
Mild Temperatures
S. Brett Walker and Jennifer A. Lewis*
Department of Materials Science and Engineering and the Frederick Seitz Materials Research Laboratory, University of Illinois at
Urbana-Champaign, Urbana, Illinois 61801, United States
S
* Supporting Information
Scheme 1. Key Constituents in the Initial Solution, Ink, and
Printed Features
ABSTRACT: Reactive silver inks for printing highly
conductive features (>104 S/cm) at room temperature
have been created. These inks are stable, particle-free, and
suitable for a wide range of patterning techniques. Upon
annealing at 90 °C, the printed electrodes exhibit an
electrical conductivity equivalent to that of bulk silver.
(middle), and the patterned silver features (right). The
chemical reactions occurring at each step are shown in the
Supporting Information (SI). Silver particles formed during
initial mixing and were removed by sedimentation over a period
of 12 h, yielding a stable, clear supernatant constituting the
reactive silver ink.
rinted electronics constitute an emerging class of materials
P
with potential applications in photovoltaics,1 transistors,2,3
displays,4 batteries,5 antennas,6 and sensors.7 Recent efforts
have focused on the design of conductive inks for integration
on plastic,2,8 textile,9 and paper substrates.10,12 To date,
conductive polymer,2 carbon,5,9 and metallic nanoparticle
inks11,12 have been demonstrated. However, organic inks
typically exhibit low conductivity, whereas metallic nanoparticle
inks often require high annealing temperatures (>200 °C) to
decompose stabilizing agents and other polymeric additives that
inhibit electrical conductivity.13
This modified Tollens’ process has several inherent
advantages. First, by using silver acetate in place of silver
nitrate, we are able to create a stable, nonexplosive silver
precursor ink. Second, the use of formic acid results in the
formation of carbon dioxide and water, leaving no residual
reducing agent.18 Third, by using excess ammonia in water, we
directly create the diamminesilver(I) complex without the need
for a silver oxide intermediate. The excess ammonia in solution
complexed preferentially with formic acid, leading to the in situ
synthesis of ammonium formate (see the SI). The resulting ink
is composed of 22 wt % silver, which is comparable to other
silver-precursor-based inks.14 When stored in a sealed vial, this
solution is stable for months under ambient conditions without
further precipitation. However, upon ink patterning and
evaporation, silver particles rapidly form.
The reactive silver ink is highly transparent and can be
printed through highly flexible, ultrafine nozzles (100 nm
diameter) via direct ink writing (Figure 1a,b; also see the movie
in the SI). This ink can also be inkjet-printed and airbrush-
sprayed upon the addition of 2,3-butanediol (10% by volume)
as a humectant and viscosifying aid (see the SI). The UV−vis
absorption spectrum showed a lack of absorption in the 400−
425 nm range typically associated with the presence of silver
particles (Figure 1c), thus confirming the ink to be particle-
free.19 Thermogravimetric analysis (TGA) indicated that the
ink contains a solids loading of 26 wt % when dried at 23 °C
and a final silver content of 22 wt % after annealing at 90 °C for
15 min (Figure 1d). The maximum silver content is limited by
the solubility of the diamminesilver(I) cation in water.20 Under
Recent exploration into silver precursor inks has yielded
promising results. For example, silver compounds with
carbamate or other relatively low molecular weight ligands
(compared to polymer stabilizers) have been synthesized that
decompose at temperatures near 150 °C, yielding electrical
conductivities approaching that of bulk silver.14 Unfortunately,
even these temperatures render the ink incompatible with many
plastic15 and paper12,16 substrates used in flexible electronic and
biomedical devices. Hence, there is a need to develop particle-
free silver inks that possess high conductivity under ambient
conditions.
An optimal ink design would meet the following require-
ments. First, the ink synthesis procedure should be both simple
and high-yielding. Second, the ink should possess low viscosity
to make it compatible with a broad range of patterning
techniques, including direct ink writing, inkjet printing, and
airbrush spraying. Third, the patterned features should be
highly conductive at room temperature and achieve bulk
conductivity upon annealing at mild temperatures (<100 °C).
Finally, the ink should remain stable at room temperature for
months without particle precipitation.
To meet the above criteria, we synthesized a reactive silver
ink by a modified Tollens’ process.17 Specifically, we first
dissolved silver acetate in aqueous ammonium hydroxide.
Formic acid was then titrated into the solution, which was
mixed thoroughly. Scheme 1 shows the key constituents
present in the initial solution (left), the reactive silver ink
Received: October 1, 2011
Published: January 5, 2012
© 2012 American Chemical Society
1419
dx.doi.org/10.1021/ja209267c | J. Am. Chem.Soc. 2012, 134, 1419−1421

Journal of the American Chemical Society
Communication
Figure 2. (a) Log−log plot of ink viscosity as a function of shear rate
and (b) semilog plot of electrical conductivity as a function of
temperature for the reactive silver ink (blue), the silver particle ink
developed for pen-on-paper printing (green), and the silver nano-
particle ink developed for omnidirectional (3D) printing (red).
are considerably larger than the nozzle size (Figure 1b). By
comparison, silver particle inks developed for omnidirectional
printing11 and pen-on-paper writing12 have apparent viscosities
roughly 3−5 orders of magnitude higher at low shear (Figure
2a). Because of the presence of silver particles, those inks
cannot be printed through nozzles that are less than ∼30 μm
wide without intermittent clogging.
The electrical conductivity of printed features produced
using the reactive silver ink is shown in Figure 2b. For
comparison, data are also provided for electrodes produced
from silver particle inks developed for omnidirectional
printing11 and pen-on-paper writing.12 Upon drying at 23 °C,
the electrical conductivity of electrodes printed from the
reactive silver ink exceeded 104 S/cm, which is 8 orders of
magnitude higher than that of electrodes produced from silver
nanoparticle inks formulated for omnidirectional printing,
which contain a polymer capping agent and a humectant to
enable in- and out-of-plane printing of self-supporting
filaments.11 Moreover, their conductivity was an order of
magnitude greater than that observed for the silver particle ink
developed for pen-on-paper writing.12 Upon annealing of the
electrodes printed from this reactive silver ink at 90 °C for 15
min, their electrical conductivity was identical to that of bulk
silver (6.25 × 105 S/cm), while those produced from either of
the silver particle inks exhibited at least an order of magnitude
lower conductivity despite their substantially higher initial silver
contents (75 and 55 wt %, respectively). Importantly, the
reactive silver ink adheres very well to poly(ethylene
terephthalate) and polyimide substrates and moderately well
to other substrates such as glass and cellulose-based materials
(see the SI).
In conclusion, we have demonstrated a unique and facile
synthesis route for producing reactive silver inks that are
particle-free, low-viscosity, and highly conductive at room
temperature. These inks flow readily through ultrafine nozzles,
enabling patterning of conductive microelectrodes for flexible
electronics, displays, and photovoltaics. The ability to achieve
bulk silver conductivity upon annealing of the printed
electrodes at 90 °C for several minutes opens new avenues
for integrating printed electronic devices on low-cost substrates
such as plastic, paper, and textiles.
Figure 1. (a) Optical image of the reactive silver ink in a scintillation
vial. (b) 1D array of conductive silver lines (∼5 μm wide) printed on a
silicon substrate via direct-write assembly using a 100 nm nozzle. (c)
UV−vis spectrum of the reactive silver ink. (d) TGA curves for the
reactive silver ink held at 23 and 90 °C in air. (e) XRD patterns of
films patterned from the reactive silver ink dried at 23 °C for 24 h
(top) and annealed at 90 °C for 15 min (bottom). Asterisks denote
peaks corresponding to silver acetate.
ambient conditions, residual acetate groups are present, which
are removed upon heating to 90 °C. X-ray diffraction (XRD)
revealed the presence of both silver and silver acetate peaks in
drop-cast films produced from this ink and dried at 23 °C for
24 h, while only silver peaks were observed when these films
are heated to 90 °C for 15 min (Figure 1e). On the basis of
these data, we estimate that the dried ink is composed of
approximately 57 wt % silver and 43 wt % silver acetate.
Notably, diamminesilver(I) complexes exist only in solution,
reverting back to their constituent salts upon drying when the
complex is not reduced to metallic silver. Scanning electron
microscopy (SEM) images of the top surfaces of printed silver
electrodes dried at 23 °C and annealed at 90 °C for 15 min,
respectively, are provided in the SI. Unlike drop-cast films,
which required drying times of ∼24 h, the printed features dry
nearly instantaneously, yielding fine particles at 23 °C and after
annealing at 90 °C.
An important advantage of this reactive silver ink is that
particle formation occurs only after patterning, as evaporation
ensues. The ink possesses a low initial viscosity of 2 mPa·s
(Figure 2a), enabling it to flow through ultrafine nozzles.
However, it undergoes significant wetting and spreading on the
substrate, resulting in minimum feature widths (∼5 μm) that
1420
dx.doi.org/10.1021/ja209267c | J. Am. Chem.Soc. 2012, 134, 1419−1421

Journal of the American Chemical Society
Communication
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental methods for ink synthesis, assembly, and
characterization, a movie (MPG) showing direct ink writing,
and complete ref 15. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
jalewis@illinois.edu
■
ACKNOWLEDGMENTS
■
This material is based on work supported by the U.S.
Department of Energy, Materials Sciences and Engineering
Division, under Award DEFG-02-07ER46471 (ink synthesis)
and the NSF Center for Nanoscale Chemical-Electrical
Mechanical Manufacturing Systems (DMI-0328162) (ink
patterning). We gratefully acknowledge the Center for
Microanalysis of Materials in the Frederick Seitz Materials
Research Laboratory as well as Dr. Bok Ahn for experimental
assistance, Vania Petrova for SEM imaging, Scott McLaren for
AFM assistance, and the Murphy and Sottos groups for use of
their experimental facilities.
REFERENCES
■
(1) Li, L.; Coates, N.; Moses, D. J. Am. Chem. Soc. 2010, 132, 22−23.
(2) Sirringhaus, H.; Kawase, T.; Friend, R.; Shimoda, T.; Inbasekaran,
M.; Wu, W.; Woo, E. Science 2000, 290, 2123−2126.
(3) Okimoto, H.; Takenobu, T.; Yanagi, K.; Miyata, Y.; Shimotani,
H.; Kataura, H.; Iwasa, Y. Adv. Mater. 2010, 22, 3981−3986.
(4) Comiskey, B.; Albert, J. D.; Yoshizawa, H.; Jacobson, J. Nature
1998, 394, 253−255.
(5) Hu, L.; Choi, J.; Yang, Y.; Jeong, S.; Mantia, F.; Cui, L.; Cui, Y.
Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 21490−21494.
́
(6) Subramanian, V.; Frechet, J. M. J.; Chang, P. C.; Huang, D. C.;
Lee, J. B.; Molesa, S. E.; Murphy, A. R.; Redinger, D. R.; Volkman, S.
K. Proc. IEEE 2005, 93, 1330−1338.
(7) Lim, S.; Kemling, J.; Feng, L.; Suslick, K. Analyst 2009, 134,
2453−2457.
(8) Bok, A.; Lorang, D.; Lewis, J. Nanoscale 2011, 3, 2700−2702.
(9) Hu, L.; Pasta, M.; La Mantia, F.; Cui, L.; Jeong, S.; Deshazer, H.;
Choi, J.; Han, S.; Cui, Y. Nano Lett. 2010, 10, 708−714.
(10) Dennuelin, A.; Blayo, A.; Neuman, C.; Bras, J. J. Nano. Res.
2011, 13, 3815−3823.
(11) Ahn, B.; Duoss, E.; Motala, M.; Guo, X.; Park, S.; Xiong, Y.;
Yoon, J.; Nuzzo, R.; Rogers, J.; Lewis, J. Science 2009, 323, 1590−1593.
(12) Russo, A.; Ahn, B.; Adams, J.; Duoss, E.; Bernhard, J.; Lewis, J.
Adv. Mater. 2011, 23, 3426−3430.
(13) Pan, H.; Ko, S.; Grigoropoulos, C. Appl. Phys. Lett. 2008, 93,
No. 234104.
(14) U.S. Patent Application 20090120800A1.
(15) Kim, D.; et al. Nat. Mater. 2010, 9, 511−517.
(16) Martinez, A.; Phillips, S.; Butte, M.; Whitesides, G. Angew.
Chem., Int. Ed. 2007, 46, 1318−1320.
(17) Yin, Y.; Li, Z.; Zhong, Z.; Gates, B.; Xia, Y.; Venkateswaran, S. J.
Mater. Chem. 2002, 12, 522−527.
(18) Lawson, A. J. Catal. 1968, 11, 295−304.
(19) Kumbhar, A.; Kinnan, M.; Chumanov, G. J. Am. Chem. Soc.
2005, 127, 12444−12445.
(20) Fox, B.; Beyer, M.; Bondybey, V. J. Am. Chem. Soc. 2002, 124,
13613−13623.
1421
dx.doi.org/10.1021/ja209267c | J. Am. Chem.Soc. 2012, 134, 1419−1421