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

2

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

3

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.

4

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

5

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.

6

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

7

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

8

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.

9

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

10

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

11

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

12

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 -- -- -- --

13

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.

14

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

15

Figure 9. SEM image of 80 nm AgCl ink spin coated on a glass slide before exposure to UV

light

16

Figure 10. SEM image of 80 nm AgCl ink spin coated on glass slide after one hour of UV

exposure

17

Figure 11. Absorption spectrum suggesting small presence of small silver nanoparticles

18

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

20

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

21

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

22

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

23

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

24

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.

25

References

1. Homepage | International AntiCounterfeiting Coalition. (n.d.). Homepage | International

AntiCounterfeiting Coalition. Retrieved July 28, 2014, from http://www.iacc.org/

2. Song, J., Roh, J., Lee, I., & Jang, J. Low temperature aqueous phase synthesis of

silver/silver chloride plasmonic nanoparticles as visible light photocatalysts. Dalton

Transactions, 42, 13897.

3. Wang, P., Huang, B., Qin, X., Zhang, X., Dai, Y., Wei, J., & Whangbo, M. Ag@AgCl: A

Highly Efficient and Stable Photocatalyst Active under Visible Light. Angewandte

Chemie International Edition, 47, 7931-7933.

4. Hasiego, C. (2010, November 8). Chemistry of Photography - Other Topics - Articles -

Chemical Engineering - Frontpage - Cheresources.com. Cheresources.com Community.

Retrieved June 27, 2014, from http://www.cheresources.com/content/articles/other-

topics/chemistry-of-photography

5. Numan-Al-Mobin, A., Meruga, J., Kellar, J., Cross, W., & Anagnostou, D. QR code

antenna for wireless and security applications. Antennas and Propagation Society

International Symposium (APSURSI), 2013 IEEE, 1728-1729.

6. Kim, S., Chung, H., Kwon, J. H., Yoon, H. G., & Kim, W. Facile Synthesis of Silver

Chloride Nanocubes and Their Derivatives. Bulletin of the Korean Chemical Society, 31,

2918-2922.

7. Hansen, C. M. (2007). Hansen solubility parameters: a user's handbook (2nd ed.). Boca

Raton: CRC Press.

8. Peng, S., & Sun, Y. Ripening of bimodally distributed AgCl nanoparticles. Journal of

Materials Chemistry, 21, 11644.

9. Ankireddy, K., Vunnam, S., Kellar, J., & Cross, W. Highly conductive short chain

carboxylic acid encapsulated silver nanoparticle based inks for direct write technology

applications. J. Mater. Chem. C, 2013, 1, 572.

10. Fechine, G., Barros, J., & Catalani, L. Poly(N-vinyl-2-pyrrolidone) hydrogel production

by ultraviolet radiation: new methodologies to accelerate crosslinking. Polymer, 45,

4705-4709.

26

11. Tavlarakis, P., Urban, J. J., & Snow, N. Determination of Total Polyvinylpyrrolidone

(PVP) in Ophthalmic Solutions by Size Exclusion Chromatography with Ultraviolet-

visible Detection. Journal of Chromatographic Science, 49, 457-462.

27

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.