silver chloride ink formulation for combined sensor...
TRANSCRIPT
Silver Chloride Ink Formulation for Combined Sensor-Antenna Applications
Prepared by:
Kennedy Southwick
Faculty Advisors:
Dr. Jon Kellar
Department of Materials and Metallurgical Engineering
Dr. Grant Crawford
REU Site Director, Department of Material and Metallurgical Engineering
Dr. William Cross
Department of Materials and Metallurgical Engineering
Dr. Alfred Boysen
Professor, Department of Humanities
Program Information:
National Science Foundation
Grant NSF #EEC-1263343
Research Experience for Undergraduates
Summer 2014
South Dakota School of Mines and Technology
501 E Saint Joseph Street
Rapid City, SD 57701
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Table of Contents
Abstract ........................................................................................................................................... 3
Introduction ..................................................................................................................................... 4
Broader Impact................................................................................................................................ 5
Procedure ........................................................................................................................................ 6
Materials ...................................................................................................................................... 6
Equipment ................................................................................................................................... 6
AgCl Nanoparticle Syntheses ..................................................................................................... 6
Ink Formulation ........................................................................................................................... 7
Spin Coating and Resistance Testing .......................................................................................... 8
Results ............................................................................................................................................. 8
Discussion ..................................................................................................................................... 21
Conclusion .................................................................................................................................... 23
References ..................................................................................................................................... 25
Acknowledgments......................................................................................................................... 27
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Abstract
The goal of this research is to formulate silver chloride (AgCl) nanoparticle ink for
antenna sensing applications in security antennas. AgCl undergoes a chemical decomposition in
the presence of ultraviolet (UV) light, causing the formation of silver, which could be used to
alter the resonant frequency of an antenna. Proof of concept was performed by ink formulation
and conductivity testing of AgCl reduced to Ag. The synthesis of AgCl nanoparticles of different
size and shape was possible by changing the reaction temperature. It was suspected that the
adsorption of the capping agent to the particles had decreased as a result of decreasing the
temperature leading to agglomeration of the nanoparticles and preventing the formulation of a
viable printing ink. The inks in this research were formulated by nanoparticle dispersion and
were deposited on glass slides by spin coating. After UV curing, it was found that AgCl did not
reduce to a continuous, conductive silver path under the conditions studied here, making the ink
unsuitable for the target application. However, a noticeable color change was observed when the
ink was cured with UV light, which suggests that the ink could potentially be used as an optically
variable ink.
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Introduction
An estimated $1.77 trillion is the anticipated value of global trade in counterfeit and
pirated goods in 2015. In 2013, Department of Homeland Security seized counterfeit goods
valued at over $1.7 billion at U.S. borders. The impact of counterfeiting spans across multiple
industries including apparel, accessories, software, medications, electronics, and automobile
parts [1]. Counterfeiting methods often involve the manipulation of legitimate packaging
materials to conceal counterfeit goods. The development of anti-counterfeiting and tampering
technology can reduce the impact of goods being counterfeited and tampered.
The focus of this research was to develop technology for the security printing and sensing
industry to control tampering utilizing silver chloride. Silver chloride (AgCl) is a chemical
compound well known for being photosensitive. Due to unique surface plasmon resonance
properties, noble-metal nanoparticles such as silver (Ag) can strongly absorb visible light [2, 3].
When visible light of a certain wavelength is exposed to AgCl particles, a series of reactions
begin producing Ag on the AgCl particles [4]. AgCl has a strong absorption in the visible region
which is almost as strong as that in the ultraviolet (UV) region [3]. Utilizing the UV region and
exposure of AgCl to produce tamper-resistant technology was explored as part of this research.
Industry often has issues with packaging being opened and the goods inside are altered or
replaced with products of lesser value. To prevent this tampering a UV-sensitive ink was
developed, composed of AgCl. The concept was to utilize the ink in a previously developed
Quick Response (QR) code antenna [5]. Specifically, the ink would utilize AgCl properties, and
act as a UV sensitive switch in a Radio Frequency (RF) antenna inserted in a QR code (Figure
1). RF antennas function based on an externally applied oscillating electric field. If the AgCl ink
infused RF antenna was to be used, it could help determine if tampering has occurred. If
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tampering occurred that would expose the AgCl particles to UV light, the QR code would change
frequency due to the curing of the AgCl block infused within the RF antenna to Ag.
Figure 1. QR code with AgCl “switch”
Broader Impact
As mentioned in the introduction, AgCl has properties that include decomposition to Ag
once exposed to UV light making what was once extremely resistant, conductive. By developing
an ink that utilizes AgCl properties, a QR code could be printed on the inside packaging of goods
and activated once exposed to ambient light. To date AgCl has not been utilized in security
printing. AgCl inks could also be used as an optical or light sensing ink. For the ink presented in
this research it appears that the optically variable application would be a better use.
An optically variable ink could be designed based on the color change that occurs when
AgCl reduces to Ag. This process could then be reversed if the particles were contained in a
polymer. The device that was made would then change colors outdoors or when exposed to a UV
light source, and change back to the initial color when out of UV exposure.
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Procedure
Materials
AgNO3 (99.9%), HCl (37.3%), ethylene glycol (99.0+%), NaCl (99.9%),
polyvinylpyrrolidone MW=55,000 (PVP), and polyvinyl alcohol (PVA) were purchased from
Sigma-Aldrich. Methanol (Laboratory Grade) was purchased from Fischer Scientific.
Equipment
UV exposure was done using a UVP MRL-58 Multiple Ray Lamp with a wavelength of
365nm. Field-emission scanning electron microscopy (FE-SEM) images were obtained using a
Zeiss Supra 40VP. Optical microscopy images were taken using the Zeiss Stemi 2000-c.
Separation of the particles from solvents was done by using the VWR Clinical 200 Centrifuge.
Sonication was done by using a 1510 Branson Sonicator and a 750 Watt Ultrasonic Processor
VCX. The Spin Coater Model WS-650SZ-6NPP/LITE was used to spin coat glass slides with
AgCl ink. Resistance testing was done using the Signatone 1160 Series Probe Station.
AgCl Nanoparticle Synthesis
AgCl nanocubes were synthesized via the reaction of AgNO3 with HCl in ethylene glycol
in the presence of the capping agent PVP. 425 mg of AgNO3 and 415 mg of PVP were mixed in
50 mL of ethylene glycol with a magnetic stirrer until dissolved. 200 µL of HCl was added to the
solution to cause complete dissolution; 15 mL of HCl was then added to the solution. The
reaction was heated to 150⁰C and held for 20 minutes. The solution was cooled to room
temperature. 100 mL of acetone and 150 mL of deionized (DI) water were added and centrifuged
at 4000 rpm for 20 minutes. The resulting precipitates were a white powder. The remaining
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ethylene glycol was disposed of and the precipitate was cleaned with the acetone and DI water an
additional three times [6]. The precipitate was then transferred to a vial for later use. The
particles were examined in the scanning electron microscope, showing that this synthesis
produced nanocubes with a mean side length of approximately 500 nm. In order to produce a
higher yield, this synthesis was multiplied by eight.
The above reaction was performed an additional time, changing the temperature at which
the reaction was held. The reaction was held at room temperature, and centrifuged four times in
DI water at 6000 rpm for 45 minutes. By performing the reaction at room temperature, this
mean side length was reduced to approximately 80 nm, observed in the SEM.
The reaction of AgNO3 and NaCl in water was also performed with the capping agent
PVA. 10 mg of AgNO3 and 500 mg of PVA were mixed in 50 mL of DI water. A solution of 344
g of NaCl and 10 mL was mixed in a separate container and 1 mL of this solution was added
drop wise into the AgNO3 and PVA solution. This reaction was held at room temperature and
was washed and centrifuged in methanol at 6000 rpm for 30 minutes. Centrifuging and washing
was done a total of four times [2]. Results were nanoparticles of approximately 60 nm in
diameter. This reaction was multiplied by 10 to increase yield.
Ink Formulation
The Hansen solubility parameters [7] of the 500 nm and 80 nm particles were found by
dispersion of the nanoparticles in the following 11 solvents: methyl benzoate, acetonitrile,
diethylene glycol hexyl ether, diethylene glycol, diethylene glycol monobutyl ether, ethylene
glycol monobutyl ether, 1-pentanol, ethanol, methanol, ethylene glycol, and water. 3 mL of each
solvent was place in separate vials. A small amount of the particles was placed in each solvent
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and sonicated for approximately 30 seconds. The behavior of the particles in each solvent was
observed and used to determine the solvents used for an ink formulation.
Spin Coating and Resistance Testing
The 80 nm ink was spin coated on a glass slide at 500 rpm for one minute. After which
the slide was placed under the four-point probe to measure resistance of the slide before
exposure to UV light. Resistance was also tested under 365 nm light in intervals of two minutes
up to 20 minutes.
An additional spin coat experiment was performed. Six layers of the ink were coated on a
single glass slide at 500 rpm. The resistance was tested after the same UV light exposure of one
hour.
Results
The PVP capped particles resulted in forming nanocubes with a mean side length of 500
nm, these particles are shown in Figure 2. This same synthesis held at room temperature resulted
in particles with a mean side length of 80 nm, and these particles are presented in Figure 3. The
PVA capped particles produced a mean diameter of 60 nm (Figure 4). This synthesis produced
an extremely small yield, which caused the synthesis to be discontinued from any further
experiments.
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Figure 2. SEM image of the reaction of AgNO3 and HCl capped with PVP at 150⁰C resulted in a
mean side length of 500 nm particles
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Figure 3. SEM image of the reaction of AgNO3 and HCl capped with PVP at RT resulted in a
mean side length of 80 nm particles
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Figure 4. SEM image of the reaction of AgNO3 and NaCl capped with PVA at RT resulted in a
mean diameter of 60 nm particles
Hansen solubility parameters were found for the 500 nm and 80 nm particles and were observed
in order to make a viable ink for each particle size. The 500 nm particles stayed dispersed in 1-
pentanol the longest of the solvents, so 1-pentanol was chosen as a solvent for the ink. Shown in
Figure 5 are the 11 different solvents with the 80-nm particles dispersed within each. The 80 nm
particles did not disperse in any of the solvents easily, but methanol and water showed more
promise. Consequently, a solvent matrix of 70 vol. % methanol and 30 vol. % water was used for
the 80 nm particle ink. Both of the inks were made by 30 wt. % of AgCl particles and 2 mL of
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solvent. Hansen solubility parameters for the particles are shown in Table 1, along with a
solubility space diagram for each, shown in Figures 6 and 7.
Figure 5. HSP experiment for 80 nm particles, slight dispersion in vials 9 (methanol) and 11 (DI
water)
Table 1. Hansen solubility parameters for each synthesis
δd (MPa0.5) δp (MPa0.5) δh (MPa0.5) R0 (MPa0.5)
500 nm 15.98 7.03 12.49 1.9
80 nm 13.15 16.04 32.25 11.1
60 nm -- -- -- --
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Figure 6. Solubility space for 500 nm particles Figure 7. Solubility space for 80 nm particles
Spin coating was performed using the 80 nm particle ink; this resulted in a thin layer of AgCl ink
on a glass slide. Resistance testing was performed on the slide with a four point probe. After
exposure to UV light for approximately 20 minutes, Figure 9 shows the development of silver on
the slide. SEM images of the spin coated glass slide before and after exposure to one hour UV
light are presented in Figures 9 and 10.
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Figure 8. Optical Microscope image of 80 nm AgCl ink spin coated glass slide after exposure to
20 minutes UV light. AgCl reduction to Ag was observed by presence of large, dark spots
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Figure 9. SEM image of 80 nm AgCl ink spin coated on a glass slide before exposure to UV
light
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Figure 10. SEM image of 80 nm AgCl ink spin coated on glass slide after one hour of UV
exposure
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Figure 11. Absorption spectrum suggesting small presence of small silver nanoparticles
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Figure 12. SEM image of AgCl ink spin coated multiple layers on a glass slide after one hour of
UV exposure
Figure 13. EDS layered image of Ag and Cl on multiple layered AgCl spin coated glass slide
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Figure 14. Spectrum of Spectrum 2 point on multiple layered AgCl spin coated glass slide
Figure 15. Spectrum of entire area of multiple layered AgCl spin coated glass slide
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Discussion
Hansen solubility parameters were observed when developing an ink with the 500 nm
particles (Table 1). The 500 nm particles did not stay dispersed in many of the solvents. In the
best solvent, 1-pentanol, the particles remained dispersed for a duration of only 10 minutes. This
suggested that the particles were too large to be made into a viable ink. This was not detected
until printing issues were presented. A study showed that heating the AgCl nanoparticles at
160⁰C drove the nanoparticles to undergo a number of changes compared to the AgCl
nanoparticles at lower temperatures. The size of the particles increased and the morphology of
the particles were transformed when heated to 160⁰C [8]. This information led to altering the
synthesis to be performed at room temperature to develop smaller particle sizes. Figures 2 and 3
shows the PVP capped particles at the different temperatures performed. The temperature
altering did show decreased size in particle size, to 80 nm. The 80 nm particles were used for
further experimentation. The experimental determination of the Hansen solubility parameters is
shown in Figure 5 for the 80 nm particles. Once the 80 nm particles were submerged in each
solvent, the particles tended to agglomerate and did not disperse even after excessive sonication.
Figure 5 displays that only two of the solvents, methanol and DI water, were able to keep the
particles dispersed. This led to the ink formulation of 70 vol. % methanol and 30 vol. % DI
water.
Difficulties with the particles and their chosen solvents resulted in performing a proof of
concept to test the ink’s resistance. Spin coating on a glass slide was performed with the 80 nm
particle ink. The method was first done at higher rpm and acceleration, and therefore produced a
thin-coated layer of the AgCl ink. To increase the density of the ink on the slide, the rpm and
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acceleration were decreased. The layer of ink had remained thin, but resistance testing was still
performed.
Resistance testing was performed at intervals of two minutes to approximately 20
minutes on one sample. The resistance remained high throughout the entire 20 minutes, which
revealed that the resistance was not changing and conductivity was nonexistent. This led to
examining the coated slide under the optical microscope. The decomposition of AgCl to Ag does
not produce a uniform layer of Ag, and therefore does not overcome the percolation threshold,
long-range connected network of conductive material, necessary to change the resistance (Figure
8). A continuous path of Ag was not achieved when exposed to UV light, and as a result the
conductivity was not increased. Exposure to UV light caused the slide to turn a purple color.
Increased UV exposure led to darker shades of purple, which suggested that the ink could be
used as an optically variable ink.
Under UV exposure of one hour, SEM images of the 80 nm ink coated glass slide showed
that the before and after exposure had little to no change. Seen in Figure 10 were larger particles
amongst the smaller AgCl particles, which could be small amounts of Ag particles. An
absorption spectrum with a peak of approximately 513 nm (Figure 11) suggests that a small
presence of Ag particles were formed after the sample was exposed to one hour of UV light [9].
In an attempt to achieve the percolation threshold, a multiple layered spin coat
experiment was performed, making the coat of AgCl much denser than before experiments
(Figure 12). The slide was exposed to one hour of UV light, and tested for conductivity. SEM
and energy-dispersion spectroscopy (EDS) was performed, and results are shown in Figures 13
through 15. The spectrum in Figure 13 was a result of scanning at the point Spectrum 2, while
Figure 14 was a scan of the entire area. A higher Ag peak was observed on the spectrum at the
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point Spectrum 2 than on the entire area spectrum. The larger particles were a higher
concentration of Ag.
The capping agent used on the 80 nm particles, PVP, was also determined as a potential
problem when formulating the ink. The long chains of PVP molecules increase the overall
viscosity of the reaction solution to further slowdown the precipitation reaction and assist the
formation of AgCl nanoparticles [8]. The article that gave the synthesis of the 500 nm particles
did not confirm a reason for choosing PVP as a capping agent for this reaction. UV induced cross
linking, densifying the PVP and may have been the reason for the AgCl particles lack of UV
absorption [10]. Although when further research was done, a UV-vis graph was found that
12.0% UV light was absorbed by the PVP, leaving a vast amount of light to be absorbed by the
AgCl [11]. Another observation was done during sonication; the particles did not like the
solvents in the ink which could mean that the PVP was not binding to the particles. PVP is a
water soluble capping agent and if the particles were completely covered by the PVP there
should be dispersion in the formulated ink. Many properties of the capping agent were
researched, and led to many possible issues.
Since the AgCl ink did not produce a continuous, conductive path of Ag, the QR code
application could not be achieved. In order for the QR code antenna to work the percolation
threshold had to be achieved. With the ink in the above procedure, this could not be achieved.
However, if Ag nanoparticles were mixed with the ink the percolation threshold may be met.
Conclusion
Neither of the AgCl inks formulated during this research were ideal for the QR code
antenna application. The frequency response of the QR code antenna was designed to change as
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the result of resistivity changes in the AgCl ink upon exposure to UV light, but while the AgCl
ink does decompose to Ag, this decomposition does not produce a corresponding change in
resistance. The AgCl reduces to a non-conductive purple film that may have alternative
applications, such as an optically-variable ink, but is not suitable for the QR code antenna
application.
For the future of the QR code antenna, the AgCl particles prepared during this research
could be mixed with Ag nanoparticles in order to reach the percolation threshold necessary to
cause conductivity when exposed to UV light. Another approach could be changing the capping
agent that was used to produce the 80 nm particles, a capping agent that would be more suitable
for the particles.
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References
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Highly Efficient and Stable Photocatalyst Active under Visible Light. Angewandte
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Raton: CRC Press.
8. Peng, S., & Sun, Y. Ripening of bimodally distributed AgCl nanoparticles. Journal of
Materials Chemistry, 21, 11644.
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carboxylic acid encapsulated silver nanoparticle based inks for direct write technology
applications. J. Mater. Chem. C, 2013, 1, 572.
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4705-4709.
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11. Tavlarakis, P., Urban, J. J., & Snow, N. Determination of Total Polyvinylpyrrolidone
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Acknowledgments
The author would like to acknowledge the National Science Foundation for the funding
of this research. Thanks to Dr. Grant Crawford, Dr. Jon Kellar, and Dr. William Cross for aiding
in direction and sharing their expertise. More thanks due to Jacob Peterson, Dr. Jeevan Meruga,
and Dr. Krishnamraju Ankireddy for the supervision and knowledge provided. A final thanks to
Dr. Alfred Boysen for the guidance in the writing of this paper and the rest of the REU staff for
their assistance.