Hybrid CopperâSilverâGraphene Nanoplatelet Conductive Inks on

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Hybrid Copper-Silver-Graphene Nanoplatelet Conductive Inks on PDMS for Oxidation Resistance under Intensive Pulsed Light Changyong Yim, Zachary Kockerbeck, Sae Byeok Jo, and Simon S. Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10748 • Publication Date (Web): 05 Oct 2017 Downloaded from http://pubs.acs.org on October 6, 2017

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ACS Applied Materials & Interfaces

Hybrid Copper-Silver-Graphene Nanoplatelet Conductive Inks on PDMS for Oxidation Resistance under Intensive Pulsed Light Changyong Yim1*, Zachary A. Kockerbeck1, Sae Byeok Jo2 and Simon S. Park1* 1

Department of Mechanical and Manufacturing Engineering, University of Calgary, Calgary, Alberta T2N 1N4, Canada 2

Department of Materials Science and Engineering, University of Washington, Seattle, WA 98195-2120, USA * Authors to whom correspondence should be addressed. E-mail: [email protected], [email protected] Abstract A simple, low-cost, and reliable process of production for conductive tracks and their transfer to polydimethylsiloxane (PDMS) substrate has been proposed. Flexible electrodes were fabricated using conductive nanoparticulates under intensive pulsed light which were then transferred on to a PDMS substrate via a pouring, curing, and peeling process. The combination of copper-silver nitrate-graphene nanoplatelets (GnPs) provided multiple benefits to the conductive tracks such as oxidation-resistance and increased durability on PDMS. The addition of silver nitrate reduced the speed of oxidation during the curing process of PDMS in the presence of heat and air. The addition of GnPs then increased the stability of conductive tracks on PDMS while the films without GnPs were not conductive on PDMS due to mechanical cracks. The copper-silver-GnP (CSG) electrodes on PDMS were successfully demonstrated as flexible electrodes and reveal the enhancement of oxidation-resistance during thermal oxidation for Joule heater application.

Keywords: Flash light, nanoparticle, conductive inks, PDMS, transfer, copper, silver, graphene nanoplatelet

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1. Introduction Printed electronics have drawn considerable attention due to new technologies such as flexible displays, organic transistors, organic light emitting diodes, etc.1 The key technique in printed electronics is the production of conductive tracks on polymeric substrates while maintaining low-cost and high electrical conductivity. Conventional sintering methods of metal nanoparticles require temperatures ranging from 150 to 300˚C under inert gas environments.2,3 In addition, the preparation of such temperature and gas environments takes time and requires expensive equipment along with temperature-resistant substrates. These conditions can make conventional sintering methods not ideal for mass production. To address the challenges such as process sintering time and the limitation on substrate materials, the intensive pulsed light (IPL) sintering technique has attracted a lot of attention due to its unique property in terms of exposing high power within milliseconds time scale to target materials. For metal nanoparticle sintering in flexible electronics, the IPL technique is not only ideal due to its short millisecond exposure time under high power of light but also the cost-effectiveness of the process.4-6 The IPL technique has a unique property with respect to its kilowatt power (~4 kW) for milliseconds (~6 ms) light exposure time. The short exposure time will locally heat the metal nanoparticles up to 300˚C in ambient air while the temperature of the substrate remains relatively low at around 150˚C (low thermal conductivity). The results show minimal damage to a wide variety of substrates using the IPL.6,7 Thus, the IPL technique can facilitate the manufacturing of conductive tracks in ambient air with a short process time. Unfortunately, even with its milliseconds exposure time, the IPL technique cannot avoid the wrinkle problem on low melting temperature polymeric substrates, such as PDMS. This results in flexible substrates being limited to


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polyethylene terephthalate (PET) film (Tm: 260 ˚C) for nanowire and polyimide (PI) film (decomposed at 560 ˚C) for nanoparticles.

8,9

Since nanowire is typically used for

transparent electrodes with a thickness of a few nanometers, it requires less flash power than the nanoparticles to be sintered. Consequently, the IPL technique is limited in potential applications since it cannot be applied to polymeric substrates with low Young modulus’ values due to the low melting temperature. To circumvent wrinkle problems in the PDMS, a transfer method is considered. In this paper, the combination of copper-silver-graphene nanoplatelets (GnPs) were used as conductive inks. Copper nanoparticles were chosen due to their exceptionally high electrical conductivity and relatively low cost when compared to other nanoparticles such as silver or gold. A major downside to using copper is the tendency to be oxidized in the presence of air.5,10,11 To prevent the thermal oxidation of copper, silver nitrate (AgNO3) salts were mixed with the copper nanoparticles to exchange the surface of copper to silver via galvanic replacement in the presence of formic acid. In addition, graphene nanoplatelets (GnP) were associated not only to connect the voids between metal nanoparticles, but also to better suppress thermal oxidation than carbon nanotubes (CNT).12 The hybrid conductive inks were placed onto a slide glass with a mask and then formed into a film using the doctor blade method. The film was not conductive before the IPL treatment. The conductive film was transferred by directly curing the PDMS onto the film on glass slide and then peeling it off. The AgNO3 acted as an oxidation protection, while the GnP acted as additional oxidation protection and a link between metal nanoparticles so that it could maintain its high electrical conductivity even after transferred to PDMS.



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2. Experimental 2.1. Chemicals For the preparation of the CSG inks, commercial copper nanoparticles (average diameter of 100 nm) were purchased from Tekna (Quebec, Canada). Silver nitrate (AgNO3), poly (N-vinylpyrrolidone) (PVP, MW: 40,000 g mol-1), diethylene glycol (DEG), and formic acid (HCOOH) were obtained from Sigma-Aldrich. Graphene nanoplatelets (GnP) with 25 µm in length were purchased from XGnP (Vermont, USA). Multi-wall carbon nanotube (MWCNT) with 30–50 nm in outer diameter and 10-20µm in length were purchased from Cheaptube. The PDMS monomer and curing agent (Sylgard 184) were obtained from Ellsworth (Canada).

2.2. Film Preparation and Film transfer using IPL and PDMS CSG inks were prepared with various concentrations of copper, silver, and graphene nanoplatelets (GnP) as shown in Table 1. DEG was used as a basic solvent for CSG ink and formic acid was used not only to remove copper oxide layer from copper nanoparticles but also to functionalize GnP becoming hydrophilic property by acid treatment. By using CSG inks, films were formed via the doctor blade method on glass slides with a patterned mask, and then dried on hotplate at 120 ˚C for 10 mins to evaporate residual solvents. It is worth to note that PVP was able to act as an adhesive between nanoparticulates whereas the film becomes brittle without the assistant of PVP. The films were then subjected to a xenon flash light for sintering the metal nanoparticles to induce conductivity. The glass slides with conductive films were then placed in 4-inch petri dishes and a 5 g : 1 g ratio of PDMS monomer to curing agent were poured onto the glass slides. The PDMS-glass petri dishes were vacuumed for 1 hr to degas any bubbles from the PDMS and then treated in an electrical oven at 85 ˚C


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for 1 hr. After that, the PDMS films were carefully peeled off from glass slides to obtain the transferred conductive tracks on PDMS. The overall process is shown in Figure 1. The detailed experimental conditions and mechanism of flash light sintering of metal nanoparticles can be found elsewhere.5 Table 1. CSG inks composition CuNP (g)

AgNO3 (g)

GnP (g)

PVP (g) HCOOH (mL) DEG (mL)

F-C

1

0

0

0.06

3.5

2

F-CG

1

0

0.1

0.06

3.5

2

F-CSG

1

0.3

0.1

0.06

3.5

2

Figure 1. Schematic of flash light system and PDMS transfer method. (Note: The size of GnP (25 µm) is bigger than the size of CuNPs (100 nm). Also, after flashlight the CuNPs were described as being overlapped before flashlight. This schematic is for guiding readers to understand PDMS transfer process)

2.3. Characterization The morphology and crystalline structure of the CSG films were confirmed through scanning electron microscopy (SEM) and X-ray diffraction (XRD), 5 Environment ACS Paragon Plus


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respectively. The resistivity of the conductive films was measured using a four-probe station connected with a multimeter (34460A, Keysight). For Raman analysis for GnP, Witec Raman/AFM (Germany) was used. For the Joule heater test, the two ends of the conductive film on PDMS were fixed in electrical wires to apply various voltages with measuring temperature changes by IR camera (FLIR-E63900, T198547, FLIR) 3. Results and discussion The solutions of GnP in formic acid and water, and corresponding films before and after flash exposure were prepared to compare the dispersion in solutions. Also, electrical resistances were measured before and after IPL treatment. Non-functionalized GnP was used to compare the difference in dispersion with and without formic acid treatment. In water, GnP was difficult to uniformly disperse while in formic acid, GnP could be evenly dispersed in water for more than after sonicating 1hr. This is theorized to be due to the presence of defects created in GnP, such as hydroxyl and carboxyl group, formed from formic acid treatment (Figure 2a). The electrical resistance of formic acid and water treated GnP revealed the effects of formic acid. The 10 timesaveraged initial electrical resistance (unflashed) of water treated GnP and formic acid treated GnP were 8.29 ±1.98 W/sq and 3.57 ±1.61 kW/sq, respectively (Figure 2b). The difference in electrical resistance regarding the type of solvent used supports the idea that formic acid generates defects in GnP, and that the defect hinders the electrical pathway resulting in high electrical resistance in kilo-ohm range. An interesting phenomenon was observed when the samples were subjected to the xenon flash light in terms of its effect on electrical conductivity. The electrical resistance of formic acid treated GnP was changed from kilo-ohm range (unflashed) to several ohms’ range (flashed, 5.65 ±2.61 W/sq), while the electrical resistance in flashed water treated GnP slightly increased to 13.33 ±4.76 W/sq (due to the PVP hindered the electrical pathway).


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It is worthy to note that the flash light exposure can reduce the formic acid treated GnP and this can be explained by a photothermal heating mechanism.13,14

Figure 2. (a) Photograph of GnP in formic acid and water; (left) right after sonication and (right) after 1 hr, (b) Electrical resistance of formic acid and water treated GnP before and after flash. A Raman analysis was conducted on formic acid and water treated GnP samples before and after flash to verify the surface change on GnP during formic acid treatment. Three distinctive peaks were observed from the Raman data at 1350 cm-1, 1582 cm-1, and 2700 cm-1 which correspond to the D peak, G peak, and 2D peak, respectively (Figure 3a).15 The intensities and 2D/G ratio in Raman spectrum on the samples were plotted and processed as shown in Figure 3b. In macroscale, the thickness of GnP was over 100 µm due to the high concentration (0.06 g/ml) of GnP, and GnP itself had several layers which can be observed from 2D/G in microscale. The interconnectivity and overall thickness of the GnP were thick enough so that the each D peak from Raman data can be comparable. The D peak in formic acid treated GnP was remarkably changed before and after flash light exposure, which indicates that the number of sp3-hybridized carbons were decreased by the flash light. Ultimately this explains that the number of possible hydroxyl and carboxyl group must be reduced



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(Figure 3c).

13,14

The D peak in water treated GnP was slightly decreased compared to

formic acid treated GnP, which indicates the formic acid treated GnP had more sp3hybridized carbons in general. From Figure 2 and Figure 3, it is worth to note that formic acid can enhance the dispersion of GnP whereas GnP lost its electrical conductivity due to defects in hexagonal carbon structure. However, IPL technique could restore the GnP’s conductivity. Formic acid and IPL technique were able to compensate the GnP’s property in terms of dispersion and electrical conductivity.

Figure 3. (a) Raman spectrum of formic acid and water treated GnP before and after flash light exposure. (b) Intensity and ratio of D peak, G peak, and 2D peak from (a); “N” represents “unflashed” and “F” represents “flashed” sample. (c) Schematic illustration of the effect of flash light and formic acid on GnP. The various combinations of CSG inks were deposited onto glass slides by the doctor blade method using a square type mask. After drying the CSG inks, the films


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were subjected to a flash light before being transferred to the PDMS. It is noted that the unflashed films cannot be transferred to the PDMS due to PVP adhesion to the glass slide. Consequently, PVP is decomposed and metal nanoparticles are aggregated after flash light sintering so that the films could easily be peeled off from the glass slide by PDMS as shown in Figure 4a. However, flashed-CuNPs films without GnP were not conductive even with a complete transfer of the electrode to the PDMS. The magnified optical microscope images (Refer to Figure 4e) demonstrate the macro scale cracking in the F-C films due to the high degree of stretchability in PDMS. In contrast, CuNPGnP films were more connected than the CuNP films due to the presence of GnPs. To investigate this phenomenon, SEM analysis was conducted as seen in Figure 4b-e. The 25 µm diameter GnP particles filled the void and wrapping among the 100 nm copper nanoparticles (Figure 4b and Figure 4c). The PDMS transfer process was successfully conducted as revealed in Figure 4d. The morphology of Figure 4c and 4d were similar while CuNPs only on PDMS showed web-like structures as shown in Figure 4e. It is worth noting that the F-C samples were not conductive on PDMS while F-CG samples were conductive as depicted in Figure 4f. The SEM images and EDS analysis of F-CSG samples were provided in supporting materials (Figure S1). Before subjecting flashlight to F-CSG sample, high oxygen content was observed in unflashed F-CSG sample while the intensity of oxygen content was decreased due to the removal of copper salt so that copper, silver and GnP were remained after subjecting flashlight.



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Figure 4. Photograph of (a) F-CG film on slide glass and transferred F-CG film on PDMS. SEM images of the F-CG films (b) before flash on glass slide, (c) after flash on glass slide and (d) on PDMS after transfer. (e) F-C films on PDMS after transfer. The scale bar represents 10 µm. (f) Electrical resistance of F-C and F-CG films before and after PDMS transfer.

To verify the above chemical reactions, XRD patterns of F-CG films with various conditions such as before and after flash and PDMS transfer were collected (Figure 5). The F-CG sample before the IPL treatment shows several peaks at 43.2°, 50.4°, and 74.1° corresponding to the (111), (200), and (220) peaks of the CuNPs, per JCPDC number 040836. This sample also shows small peaks in the range from 20˚ < 2q < 55˚ which correspond to copper formate since some of these peaks disappeared after flash light sintering while the copper peaks slightly increased.5 This implies that the copper formate structures were converted to copper by the aforementioned chemical reaction of copper formate to copper. The peaks near 20˚ would then correlate to the


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peaks from the GnPs, which were maintained even after flash light sintering. The interesting point is that GnP peaks were decreased after the PDMS transfer. Further detailed analysis will be conducted with Raman spectrometer in the future to investigate this phenomenon. It can be concluded that the flash light sintered films were successfully transferred from glass slide to PDMS as demonstrated by both SEM and XRD analysis.

Figure 5. X-ray diffraction patterns (XRD) of the F-CG films with various conditions (Black: before flash on slide glass, red: after flash on slide glass, and blue: PDMS transferred); f: copper formate, GnP: graphene nanoplatelet, and Cu: copper.

To evaluate the thermal oxidation resistance of F-CSG inks, the electrical resistance changes of F-C, F-CG, F-CSG and F-C with CNT were measured after heat treatment in air at 180 ˚C (Figure 6). The F-C sample was easily oxidized in the air at the elevated temperature so that the electrical resistance rapidly changed in sub megaohms range. The interesting point is the electrical resistances were suppressed by adding either CNT and GnP. Kim et al. reported that CNT could suppress the thermal oxidation, however, it is revealed that GnP can further enhance the thermal oxidation



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resistance from Figure 6b. This is because GnP can wrap copper nanoparticles due to its 2D sheet structure so that it can prevent the access of oxygen while CNT is a 1D rod structure. As a result, F-CSG sample showed the best anti-oxidation property as expected due to the deposition of silver on copper nanoparticles.

Figure 6. (a) Electrical resistance of F-C (black, square), F-C with CNT (blue, inverted triangle), F-CG (red, circle), and F-CSG (green, triangle) samples after heat treatment at 180 ˚C. (b) magnified graph of (a).

To determine the effectiveness of proposed flexible copper electrodes as a Joule heater application, the electrical resistance and temperature changes were determined with various applied voltages (Figure 7). As the voltage increased, the temperature of F-CSG film was increased up to 190 oC at 3V and maintain the electrical resistance in few ohm ranges as shown in Figure 7a. For repeatability test, consecutive 1 V was applied to both F-CSG and F-C sample in Figure 7b and Figure 7c, respectively. As expected from Figure 7a, F-CSG sample showed reproducible changes of temperature and maintain its electrical resistance (Figure 7b) while F-C sample showed temperature changes only at initial voltage cycle. In addition, the temperature changes were failed by thermal oxidation of film as further voltage applied and electrical resistance was increased up to killo-ohms. It is revealed that F-CSG sample


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enabled to maintain its performance in terms of temperature changes and electrical resistance by preventing thermal oxidation of conductive tracks. The F-CSG sample can be applied as a flexible heater on glass vial as shown in Figure 7d. This F-CSG sample on PDMS can be useful for curved surface as portable and wearable heater.

Figure 7. Temperature and electrical resistance changes of (a) F-CSG film with various applied voltage from 0V-3.0V (b) F-CSG film with consecutive 1 V and (c) F-C film with consecutive 1 V. (d) optical and IR camera image of F-CSG film on PDMS for Joule heater application.

4. Conclusions We found an effective way to formulate copper-silver-graphene nanoplatelet hybrid nanoparticle inks for conductive tracks on PDMS substrate. The conductive films have maintained electrical property by GnP and have oxidation-resistant properties at high temperatures. The formic acid enabled GnP to become hydrophilic so that GnP was able to be dispersed in the conductive inks, however electrical conductivity of GnP was lost. To restore the electrical conductivity, GnP was subjected



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to flashlight. By combining formic acid treatment and IPL technique, we could successfully disperse GnP and keep its electrical property. In addition, the oxidation resistance of GnP was compared to CNT. GnP showed better thermal oxidation resistance than CNT by wrapping copper nanoparticles due to its two-dimensional sheet shape. For application, we applied F-CSG film on PDMS as Joule heater. After several consecutive voltage cycles, the copper-silver-graphene nanoplatelet hybrid inks on PDMS show great potentials in terms of flexible Joule heater and cost-effectiveness. For future research, the hybrid copper-silver-graphene nanoplatelet inks will be applied for wearable heater in clothes so that they can be used in real life application.

Acknowledgement The authors acknowledge the support of the Natural Sciences and Engineering Research Council of Canada (NSERC), Alberta Innovates Technology Future (AITF), This research was also supported by an Eyes High research fellowship from the University of Calgary. The authors are grateful to Julie Kim (Karnegie Mellon University) and Harry Liu (University of Calgary) for assisting with the conductive track oxidation experiments.

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