Letter www.acsami.org
Combustion-Assisted Photonic Annealing of Printable Graphene Inks via Exothermic Binders Ethan B. Secor,†,‡ Theodore Z. Gao,†,‡,§ Manuel H. Dos Santos,‡ Shay G. Wallace,‡ Karl W. Putz,‡ and Mark C. Hersam*,‡,⊥ ‡
Department of Materials Science and Engineering and ⊥Department of Chemistry, Department of Electrical Engineering and Computer Science, Northwestern University, Evanston, Illinois 60208, United States S Supporting Information *
ABSTRACT: High-throughput and low-temperature processing of high-performance nanomaterial inks is an important technical challenge for large-area, flexible printed electronics. In this report, we demonstrate nitrocellulose as an exothermic binder for photonic annealing of conductive graphene inks, leveraging the rapid decomposition kinetics and built-in energy of nitrocellulose to enable versatile process integration. This strategy results in superlative electrical properties that are comparable to extended thermal annealing at 350 °C, using a pulsed light process that is compatible with thermally sensitive substrates. The resulting porous microstructure and broad liquid-phase patterning compatibility are exploited for printed graphene microsupercapacitors on paper-based substrates.
KEYWORDS: carbon nanomaterials, conductive inks, energetic materials, intense pulsed light annealing, printed electronics
A
substrates, with straightforward integration in roll-to-roll processing.13−18 This method employs a broadband pulsed light source, typically a xenon arc lamp, to produce highintensity (∼1−5 kW/cm2) light pulses with durations on the order of milliseconds. Preferential optical absorption within the active film leads to rapid local heating that limits thermal damage to passive components such as the underlying substrate. Although the initial results on photonic annealing of printable graphene inks have been promising, optimal utilization of this technique requires complementary ink properties tailored to the annealing method. Toward that end, here we demonstrate a graphene ink designed for pulsed light annealing based on the polymer binder nitrocellulose, which exhibits rapid and exothermic decomposition. This approach enables highly conductive and porous graphene patterns with broad process compatibility and superlative performance metrics in addition to establishing a compelling ink design strategy for broader application in photonic annealing. The design of graphene inks for photonic annealing draws inspiration from literature precedent in the fields of thin film metal oxide electronics and ceramic synthesis. In this context, combustion processing of metal oxide thin-film transistors has been demonstrated by careful selection of exothermic precursors that lower the energy barrier for annealing and
dditive manufacturing of electronically functional materials offers compelling benefits for printed electronics, with applications in energy technologies, biomedical diagnostics, and the Internet of Things.1,2 With advanced direct-write capabilities, complicated digital designs can be fabricated with low cost, minimal waste, and high resolution.3,4 A key obstacle to the widespread adoption of this technology is the development of functional liquid inks with compatible printing and process integration. Among conductive inks, graphene has attracted considerable interest as a result of its high electrical and thermal conductivity, mechanical flexibility, and chemical and environmental stability.5,6 Pristine graphene can be exfoliated from flake graphite using scalable, liquid-phase processing methods, such as shear mixing, coupled with engineered solvent and dispersant systems.7,8 The use of polymer dispersants is particularly promising for downstream integration in printing processes, because the polymer offers high colloidal stability and an adjustable parameter to tailor fluid properties for specific printing methods.9 In this way, versatile and high-performance graphene inks have been designed for a range of liquid-phase printing methods.10−12 Despite the aforementioned advantages of polymer-stabilized graphene inks, the resulting printed structures require annealing to decompose the dispersant, which can present process integration limitations. In particular, traditional thermal processing limits process speed and compatibility with temperature-sensitive materials. As a result, a pulsed photonic annealing method (i.e., intense pulsed light annealing), has been demonstrated for rapid processing on thermally sensitive © 2017 American Chemical Society
Received: May 21, 2017 Accepted: August 18, 2017 Published: August 18, 2017 29418
DOI: 10.1021/acsami.7b07189 ACS Appl. Mater. Interfaces 2017, 9, 29418−29423
Letter
ACS Applied Materials & Interfaces
Figure 1. Process overview and proof of concept. (a) Chemical structure of nitrocellulose (left) and graphene (right). (b) Differential scanning calorimetry of graphene/nitrocellulose showing the large exothermic reaction of nitrocellulose at ∼200 °C. (c) Schematic illustration of photonic annealing, which initiates a nitrocellulose propagating reaction. Cross-sectional scanning electron microscopy reveals the resulting porous graphene microstructure even on a thermally conductive silicon substrate.
Figure 2. Chemical characterization of photonically annealed graphene/nitrocellulose films on glass. (a) FTIR spectra for graphene/nitrocellulose films following different photonic annealing conditions. (b) Evolution of the FTIR peak intensity with photonic pulse energy, showing a sharp threshold. (c) Elemental composition of the films as determined by XPS, which has a typical uncertainty of ∼1% and selectively analyzes the sample surface (∼10 nm depth).
generality. Moreover, the evolution of volatile gaseous species upon NC decomposition leads to a highly porous microstructure, offering advantages for applications such as energy storage, catalysis, and sensing.25,26 The composite graphene/NC powder is produced by high shear mixing, centrifugation, and flocculation steps as previously reported (detailed experimental methods are provided in the Supporting Information).22 This procedure yields a powder containing ∼35 wt % graphene flakes with a typical thickness of ∼2 nm and lateral size of ∼300 nm (Figure S1a). During photonic annealing, the strongly absorbing graphene film will undergo rapid photothermal heating primarily near the surface, and this heat will dissipate through the bulk of the film. For the suggested propagating reaction to occur, the energy released upon NC decomposition must be sufficiently greater than the energy required to initiate the decomposition reaction. Differential scanning calorimetry (DSC) results shown in Figure 1b indicate that NC decomposes exothermically at ∼200 °C, releasing ∼3× more
densification, ultimately yielding desirable performance with reduced processing temperatures.19 In ceramic materials, the technique of self-propagating high-temperature synthesis (SHS) similarly uses a locally initiated combustion wave to achieve thermal processing in a rapid and efficient manner.20 Because the decomposition of reactive precursors leads to gas evolution, this approach is particularly effective for producing porous, high-surface-area morphologies.21 To use photonic annealing to initiate combustion reactions in graphene inks, a reactive binder is required that undergoes rapid, exothermic decomposition. A promising candidate is nitrocellulose (NC), which has recently been shown to effectively disperse graphene, providing stable inks with graphene concentrations up to 10% w/v.22 In addition, the oxidizing nitrate groups of NC afford high chemical reactivity that favors exothermic decomposition with rapid kinetics that are consistent with photonic annealing (Figure 1a).23,24 Because of its rapid and exothermic decomposition, NC acts as an intrinsic energy source to fuel a propagating reaction, with positive implications for process 29419
DOI: 10.1021/acsami.7b07189 ACS Appl. Mater. Interfaces 2017, 9, 29418−29423
Letter
ACS Applied Materials & Interfaces
Figure 3. Electrical characterization of photonically annealed graphene/nitrocellulose films. (a) Sheet resistance as a function of photonic annealing pulse energy on glass; each data point shows the average and standard deviation for three samples. (b) Normalized sheet resistance relative to thermal annealing for graphene/nitrocellulose films on various substrates. (c) Sheet resistance for graphene/nitrocellulose films as a function of thickness following thermal and photonic annealing on glass. The reported thickness corresponds to the thickness prior to annealing (photonic annealing leads to thicker films with the same amount of material due to the resulting porous microstructure).
Similar characteristics are also observed with X-ray photoelectron spectroscopy (XPS, Figure 2c and Figure S4a), which is used to track elemental composition. At low pulse energies, the films show similar composition to the as-cast film, with an abrupt transition to fully annealed films after a threshold pulse energy is exceeded. In addition, Raman spectroscopy (Figures S4b-c) shows the prominent D and G peaks of graphene, with a slight reduction in the ID/IG ratio from 0.58 to 0.50 upon annealing, which is consistent with a decrease in sp3 bonding character in the film.16 Importantly, thermally annealed graphene/NC films show FTIR and Raman peaks corresponding to amorphous carbon residue from NC, which are not evident following photonic annealing (Supporting Information, Figure S5). Altogether, this chemical characterization suggests a sharp threshold for annealing after which the propagating, selfsustaining nature of the reaction leads to nearly complete decomposition with minimal evidence of residual NC. Electrical characterization of the graphene films is provided in Figure 3. As the annealing energy is increased, a sharp drop in sheet resistance is observed (Figure 3a) in a manner consistent with the preceding chemical characterization. In contrast, polymer binders with endothermic decomposition can be partially annealed by low energy pulses, which manifests itself as a gradual decrease in sheet resistance as a function of annealing energy (Figure S6). Despite its increased porosity, the graphene/NC films maintain a low sheet resistance similar to that of thermally annealed films, thus presenting a desirable combination of high surface area and efficient charge transport. Furthermore, photonic annealing with NC allows the formation of conductive graphene films on a variety of substrates as shown in Figure 3b. These results show that photonic annealing is effective for graphene/NC films on PET and PEN, substrates incompatible with temperatures greater than 200 °C,15 along with low-cost, paper-based substrates such as glassine and standard office paper.27 The somewhat higher (∼2−4×) sheet resistance measured on these paper-based substrates is likely a result of the increased surface roughness relative to the thermally annealed control on low-roughness glass. Nevertheless, the realization of conductive graphene films on paperbased substrates offers compelling prospects for printed electronics because the low cost, sustainable production, and recyclability of paper make it an attractive substrate for largevolume, disposable devices.28,29
energy than the amount required to heat the material to the decomposition temperature, which suggests that the NC combustion will likely propagate once locally initiated. In contrast, a control sample of graphene/ethyl cellulose (EC), a common dispersant for graphene inks,9 exhibits endothermic decomposition characteristics (Figure S1b). The differences in reaction kinetics and thermodynamics between NC and EC are further demonstrated by a photonic annealing test on a silicon substrate. Importantly, photonic annealing is traditionally limited to substrates that act as poor heat sinks, such as paper and plastic. Substrates with higher thermal mass and thermal conductivity, such as silicon, extract heat out of the active film rapidly to quench the process, limiting its applicability.15,13 However, the high exothermicity of NC combustion enables photonic annealing on silicon, wherein the rapid release of volatile decomposition products leads to a porous microstructure, as shown in the cross-sectional scanning electron microscopy (SEM) image in Figure 1c. Although the bottom portion of the film (90% capacitance retention following 1000 cycles (Figure 4f). In summary, we have demonstrated rapid photonic annealing of graphene/NC films, in which the exothermic NC binder acts as a built-in energy source to assist the annealing reaction. Chemical and electrical characterization reveals effective annealing with a single millisecond light pulse, resulting in properties comparable to extended thermal annealing at 350 °C. Moreover, the rapid volatilization of the NC binder leads to a porous microstructure, providing enhanced surface area for applications such as energy storage, sensing, and catalysis. The favorable process compatibility of this method is leveraged in combination with the versatile liquid-phase processing platform for graphene inks, enabling direct ink writing of printed solidstate graphene microsupercapacitors on flexible substrates such as glassine paper. While particularly useful for graphene inks, the attributes of NC as a synergistic binder for photonic annealing can likely be generalized to other material systems of interest to additive manufacturing.
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Mark C. Hersam: 0000-0003-4120-1426 Present Address §
T.Z.G. is currently at the Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305-4034, United States Author Contributions †
E.B.S. and T.Z.G. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Air Force Research Laboratory under agreement number FA8650-15-2-5518. EBS was further supported by the Department of Defense (DoD) through the National Defense Science and Engineering Graduate (NDSEG) Fellowship Program, the Ryan Fellowship administered through the Northwestern University International Institute for Nanotechnology, and the Cabell Terminal Year Fellowship. This work made use of the EPIC and Keck-II facilities within the Northwestern University NUANCE Center, which has received support from the NSF Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (ECCS1542205); the NSF MRSEC program (DMR-1121262); the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois. TGA and DSC were performed in the Materials Characterization and Imaging facility at Northwestern University, which receives support from the NSF MRSEC (DMR-1121262). The U.S. Government is authorized to reproduce and distribute reprints for Governmental purposes notwithstanding any copyright notation thereon. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the sponsors.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b07189. Detailed experimental procedure; characterization of control graphene/EC samples with DSC, SEM, and electrical measurements; and additional thermal, chemical, rheological, and electrochemical characterization (PDF)
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REFERENCES
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DOI: 10.1021/acsami.7b07189 ACS Appl. Mater. Interfaces 2017, 9, 29418−29423
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