Flexible and Foldable Fully-Printed Carbon Black Conductive

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Flexible and Foldable Fully-Printed Carbon Black Conductive Nanostructures on Paper for High-Performance Electronic, Electrochemical, and Wearable Devices Murilo Santhiago, Cátia C. Corrêa, Juliana S. Bernardes, Mariane P. Pereira, Letícia J. M. Oliveira, Mathias Strauss, and Carlos C. B. Bufon* Brazilian Nanotechnology National Laboratory (LNNano), Brazilian Center for Research in Energy and Materials (CNPEM), Campinas, Sao Paulo 13083-970, Brazil S Supporting Information *

ABSTRACT: In this work, we demonstrate the first example of fully printed carbon nanomaterials on paper with unique features, aiming the fabrication of functional electronic and electrochemical devices. Bare and modified inks were prepared by combining carbon black and cellulose acetate to achieve high-performance conductive tracks with low sheet resistance. The carbon black tracks withstand extremely high folding cycles (>20 000 cycles), a new record-high with a response loss of less than 10%. The conductive tracks can also be used as 3D paper-based electrochemical cells with high heterogeneous rate constants, a feature that opens a myriad of electrochemical applications. As a relevant demonstrator, the conductive ink modified with Prussian-blue was electrochemically characterized proving to be very promising toward the detection of hydrogen peroxide at very low potentials. Moreover, carbon black circuits can be fully crumpled with negligible change in their electrical response. Fully printed motion and wearable sensors are additional examples where bioinspired microcracks are created on the conductive track. The wearable devices are capable of efficiently monitoring extremely low bending angles including human motions, fingers, and forearm. Here, to the best of our knowledge, the mechanical, electronic, and electrochemical performance of the proposed devices surpasses the most recent advances in paper-based devices. KEYWORDS: carbon black, conductive ink, paper-based electronics, foldable devices, wearable sensors

1. INTRODUCTION

Carbon nanomaterials such as carbon nanotubes, graphene, and carbon black are highly desirable for the construction of flexible devices on paper due to their remarkable mechanical and electrical properties.9,20,21 In terms of fabrication, carbon nanomaterials are commonly patterned on paper by using wet transfer9,22,23 and printing technologies.20,24,25 In particular, printing is a straightforward method that allows large area, lowcost, and scalable fabrication processes.26 Recently, several routes have been used to create conductive pathways on paper targeting different applications such as digital microfluidics,24 optoelectronics,23 transistors,20 and electrochemical devices.25 Moreover, there is an increasing need for versatile electronic paper-based devices that can support important unmeet demands: (i) simple printing techniques of carbon nanomaterials on paper that can potentially form conductive pathways with low sheet resistance and high folding stability; (ii) carbon electrodes with high heterogeneous rate constants in flexible electrochemical cells; (iii) tunable electric properties to achieve high performance motion and wearable devices.27,28

Flexible and foldable electronic materials have attracted the attention in many fields due to their broad range of applications.1−3 When dealing with flexible devices, the substrate plays an important role since it hosts a variety of insulating, semiconducting, and conductive materials. In this context, paper is a very attractive substrate because of the myriad of possible relevant applications including energy storage elements,4,5 sensors,6,7 and electronic devices.8,9 Several of such applications became attractive by the fact that paper is lightweight, biodegradable, biocompatible, worldwide available, and foldable, apart from the low-cost production and manufacture.10−15 The folding capability of paper is also an interesting feature since it allows the fabrication of threedimensional (3D) electronic and electrochemical devices with unique properties. For instance, recently, two independent groups demonstrated that porous paper allows the fabrication of 3D conductive interconnects through the substrate using polypyrrole16 or PEDOT:PSS17 as conducting media. In addition to conducting polymers, metal nanostructures have been recently used for the fabrication of foldable and flexible devices on paper.8,18,19 © XXXX American Chemical Society

Received: May 10, 2017 Accepted: June 26, 2017 Published: June 26, 2017 A

DOI: 10.1021/acsami.7b06598 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

the reagents, and the values of the pH of all the solutions were determined by an pHmeter from Marconi MA522, SP, Brazil. All the characterizations were performed on freshly prepared samples. 2.2. Preparation of the Conductive Ink. Carbon black ink was prepared as follows: 2.85 g of a mixture of cyclohexanone and acetone (1:1)33 was added to 0.150 g of cellulose acetate. The mixture was vigorously stirred for 2 h. Next, 0.1 g of carbon black was added to 1.0 g of the solution containing cellulose acetate, and the mixture was stirred for 2 min. 2.3. Preparation of the Conductive Ink Containing Prussian Blue (CB-PB). To prepare the modified ink, 20 mL of FeCl3 (0.16 mg mL−1) was added dropwise into 20 mL of deionized water containing 250 mg of carbon black and 8.5 mg of Fe(CN)64−. The resulting volume (40 mL) was separated into two tubes of 20 mL each and centrifuged (Sorvall, RC6+, Thermo Scientific) at 15 000 rpm for 30 min. The supernatant was removed and 20 mL of deionized water further added to each tube. This process (centrifuge/wash) was repeated four times. Before use, the resulting material (CB-PB) was kept in a vacuum oven at 45 °C for 18 h. The Prussian blue modified ink was prepared by replacing the bare carbon black by the CB-PB in the preparation described above. 2.4. Patterning CB Ink on Paper. Conventional copy paper was selected for the fabrication of all the devices. Black patterns (20.5 × 27 mm2) were printed on paper using a XEROX printer model ColorQube 8570. The wax patterned paper received a thermal treatment on a hot plate (Tecnal, SP, Brazil) at 120 °C during 5 min. Following the thermal treatment, the paper was allowed to cool at room temperature for 5 min before the application of the ink. We have added adhesive layers from 3 M to delimitate the area of the conductive pathways. Typically, we have created patterns of 0.5 × 8.0 cm2. To fabricate line widths of 1 mm or less, we have used a Silhouette knife plotter model Curio. After creating the appropriate patterns with the adhesive layers, the CB ink was spread over the surface with a squeegee, as illustrated in Figure 1a. The CB patterned paper was kept inside a fume hood for 1 h, and after this period the adhesive layers were carefully removed. Finally, the CB patterned paper was kept in the oven at 60 °C for 15 min. After this process, the CB conductive tracks are ready to use. 2.5. Construction of the 3D Paper-Based Electrochemical Cell. We have fabricated a paper-based electrochemical cell with three electrodes starting from a single CB track. First, on the CB containing face, silver ink was used to create contact pads for the working electrode (WE) and counter electrode (CE). Next, patterned tapes containing holes of ϕ = 1.0 mm and ϕ = 4.0 mm were added on the WE and CE electrodes, respectively. A hole of ϕ = 1 mm was punched on the center of the counter electrode. The reference electrode (RE) was fabricated on the back face of the device. We patterned the electrical contact pads of the RE with silver ink as well. To fabricate the AgCl layer, 50 μL of sodium hypochlorite was added on the center of the silver layer. After 10 min of reaction, the sodium hypochlorite was removed by washing the region with copious amount of water. This redox reaction formed AgCl (s) on the top of the silver layer. After drying, we punched a hole of ϕ = 1.0 mm on the center of the AgCl layer. After this process, the paper-based device was cut. Both RE/CE and WE were assembled together in a sandwich-type configuration using a paper-based spacer. The spacer consisted of a wax-treated piece of paper containing double-sided tape on both sides with a punched hole of ϕ = 4 mm. The vertical attachment of the electrodes was very similar to the case previous reported studies.6,12 2.6. Electrochemical Measurements. All the electrochemical measurements were conducted using the three-dimensional paperbased electrochemical cell. The cyclic voltammetric experiments were performed using a PGSTAT-204 model from AUTOLAB (Eco Chemie, Netherlands) interfaced by a computer and controlled by the NOVA 2.0 software. Typically, 50 μL of solution was added to the electrochemical cells analytical spot. 2.7. Fabrication of the Wearable Sensors. To monitor changes in resistance of the devices, we attached copper wires at both extreme ends of the conductive carbon black tracks. The copper wires were attached to the CB track using silver ink and epoxy resin to ensure

The attempt to meet all these demands by printing techniques is very challenging on paper. For instance, carbonbased materials printed on paper can be under certain conditions flexible and foldable. However, their electrochemical properties toward well-defined redox probes are very poor.25,29 In some cases, carbon tracks with high conductivity were even successfully prepared on papers, but a low roughness (∼350 nm) 20 must be ensured, which limits their potential applications in a variety of paper-based products. Moreover, motion and wearable sensors combining paper and carbonbased materials are at the early stage and very promising.30,31 However, the fully printed approach reported so far utilizes a water-based binder for the carbon particles,32 which is incompatible with wet surfaces and strongly limits the applications in the field of electrochemical sensors, for instance. Thus, a fully printed method that can potentially fabricate portable, wearable, flexible, foldable, and high-performance electrochemical and motion devices is an urgent demand. In this work, we report the fabrication of fully printed and highly electrical conductive carbon black (CB) tracks on paper that simultaneously meets all demands described above. We used sacrificial adhesive layers to pattern high loadings of ink in a single step. The patterning method is simple and fast. In addition, it does not create any damage to the paper and allows a pattern generation with resolution in width of ∼500 μm. The carbon black patterns were characterized by scanning electron microscopy, Raman spectroscopy, electrochemical experiments, atomic force microscopy, 3D-laser scanning microscopy, mechanical bending tests, and electrical measurements. The globular structures of the nanomaterial are well connected to each other, as demonstrated by the low sheet resistance (∼250 Ω sq−1). The conductive tracks are flexible and can be fully crumpled without losing the electrical performance. Furthermore, the devices can be bent more than 20 000 times, a new record-high for carbon based nanostructures on flexible substrates. To the best of our knowledge, the patterned nanostructures have the highest heterogeneous rate constant (1.0(±0.5) × 10−3 cm s−1) for fully printed carbon-based materials reported so far. Moreover, to demonstrate the versatility of the nanomaterial/patterning, the inks were modified with redox-active substances to detect hydrogen peroxide at low potentials. Finally, wearable devices that can detect human motions were created by locally applying a high pressure on the conductive tracks.

2. EXPERIMENTAL SECTION 2.1. Materials and Chemicals. All chemicals were of analytical grade. Conventional printing paper was acquired from Gimba, SP, Brazil. Potassium chloride, sodium phosphate dibasic, sodium phosphate monobasic, potassium ferricyanide, potassium ferrocyanide trihydrate, ferric chloride, cellulose acetate, and cyclohexanone were acquired from Sigma-Aldrich, SP, Brazil. Sodium hydroxide and acetone were acquired from Synth, SP, Brazil. Sodium hypochlorite was acquired from Bufalo, SP, Brazil. Hydrogen peroxide was acquired from Merck, Brazil. Carbon Black (CB) type XC-72R was acquired from Cabot Corporation. Additional information about the CB characteristics can be found in Tables S1 and S2. Silver ink from SPI supplies, PA, USA was used to construct conductive pads and reference electrodes on paper. Single and double-sided tapes from 3M, SP, Brazil were used to delimitate the area of the electrodes. Biopsy punches of 1 and 4 mm in diameter (ϕ) from Kolplast were used to pattern tapes and paper. All the solutions were prepared by using purified deionized water (18.2 MΩ cm) from Elga Veolia model Purelab Option-Q, UK. An analytical balance from Shimadzu AUW220D, SP, Brazil, with 5 decimals places was used to weigh all B


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Figure 1. (a) Schematic description of the fabrication process: (i) addition of sacrificial adhesive layers, (ii) deposition and spread of the carbon black ink, and (iii) the cure of the layers at 60 °C, followed by the removal of the adhesive layers. The 3D laser scanning microscope images (b) before and (c) after the removal of the sacrificial adhesive layer. The small red squares in panel a represent the position of the analyzed region. (d) Photograph of the flexible paper-based device with a CB track. (e) Picture of the carbon black conductive lines on wax-treated paper. (f) Stereomicroscope images of the conductive tracks with different widths.

Figure 2. (a, b) Scanning electron micrographs images of the carbon black track at low and high magnifications, respectively. (c) Resistance versus length plot. (d) dC/dz potential map. (e) Raman spectrum of CB track on paper. (f) Negative (−180°) folding test. (g) Positive (+180°) folding test. The carbon black ink was printed on an alumina substrate using the sacrificial adhesive layers. Prior to EFM (electrical force microcopy) imaging, the analyzed sample was fixed on a metal stub using adhesive tape. Scanning electron micrographs of the samples were acquired on a SEM-FEG FEI Inspect F50 microscope fitted with an Oxford System energy-dispersive X-ray (EDX) spectrometer and operating at 20 kV accelerating voltage and spot size 3.0. Samples were fixed with carbon tape on metal stubs and analyzed without metallization. Raman spectroscopy was obtained using a confocal Raman T64000 from Horiba Scientific with a 514 nm Argon laser and a 100× objective mounted on an Olympus optical microscope (spot size of 400 μm). The determination of the contact angle was performed at room temperature using a Tensiometer Optical Theta L with software Attension. The conductive tracks images were obtained using a

mechanical stability. The microcracks were locally created by folding the CB track and applying an external pressure of 4 MN m−2 on the folded region for 60 s. We have used commercially available adhesive bandages to attach the sensors to each finger of a human hand and arm. The resistance was recorded as a function of time using a Keithley 2000 digital meter. 2.8. Characterization. The capacitance gradient (dC/dz) images of the carbon black conductive tracks were acquired in a NanoSurf Flex atomic force microscope in tapping mode. By applying an appropriate bias to the force tip, the depletion induced capacitive variation, which was detected through the second harmonic of the AC signal, is proportional to the capacitance gradient (dC/dz) or capacitance coupling between tip and sample.34 A metalized cantilever with K = 2.8 N m−1 and resonance frequency within 75 kHz was used. C


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Figure 3. (a) Schematic description of the electrodes on paper. (b) Sandwich-type of configuration of the 3D electrochemical cell on paper. (c) Picture of the assembled electrochemical cell. (d) Cyclic voltammograms obtained in 0.5 M KCl solution containing 5 mM Fe(CN)63‑/4‑ at different scan rates. (e, f) Cyclic voltammograms obtained for the CB ink containing Prussian blue in 10 mM H2PO4−/HPO42− pH 7.4 + KCl 0.1 M. (e) Electrochemical fingerprint of the redox processes at 5 mV s−1. The vertical dotted lines shows the two redox couples. (f) Electrocatalytic process in the presence and absence of hydrogen peroxide evaluated at 5 mV s−1. Stereomicroscope Stemic 2000-C, Axiocam 105 color, Zeiss, Germany. A 3D Laser Scanning Confocal Microscope (LSCM) from Keyence model VK-X200 series, Osaka, Japan, was used to map the surface roughness of the paper-based devices. X-ray photoelectron spectroscopy (XPS) was performed with a Thermo Scientific K-Alpha spectrometer, UK.

3. RESULTS AND DISCUSSION 3.1. Patterning Carbon Black Nanostructures on Paper. To prepare the CB ink suitable for applications on wet surfaces, such as skin and buffer solutions, we have used cellulose acetate as a water-insoluble binder for the carbon material. The schematic of the fabrication process is shown in Figure 1. A fundamental part of the fabrication relies on a sacrificial adhesive layers (SAL). First, the adhesive layers (orange features) are attached to the wax treated paper, as illustrated in Figure 1a(i). The CB ink is then homogeneously spread on the surface by using a squeegee (Figure 1a(ii)). Next, the samples are cured for 15 min at 60 °C in a vacuum oven. Finally, the sacrificial adhesive layers are removed leaving the conductive CB track patterned on the paper (Figure 1a(iii)). Figure 1b and c show the images obtained using the 3D laser scanning microscope at the same region of the paper before and after removing the sacrificial adhesive layer. We ran a complete fabrication step to check surface properties. The surface roughness of the paper, monitored in six different regions before and after the fabrication step, was found 4.2 (±0.4) μm and 4.0 (±0.6) μm, respectively. Our results clearly show that the adhesive layers does not create any damage to the paper substrate. Figure 1d shows a picture of the flexible conductive tracks on paper. The SAL method provides a simple alternative to pattern high-loadings of conductive inks on flexible substrates in a single printing step. The method can be scaled

Figure 4. (a−e) Photographs of the CB electronic circuits on paper. The CB electronic circuit illustrated in panel a was (b) crumpled and (c) operated after unfolding. (d, e) Flexible linear arrays of LEDs are operated on the back of the hand.

D


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Figure 5. (a) Schematic process of the pressure induced fabrication of microcracks. We applied a pressure of 4 MN m−2 on the CB track for certain time. (b) Operation mechanism of the paper-based device. The scale bars are 100 μm long. (c) Paper-based device was attached inside of a book and the response was monitored at different angles. (d, e) Devices were attached on each finger of a human hand using adhesive bandages as support. (f) Electric response of each human finger during motion. (g) Device response before, during, and after the exercise. Inset photograph: wearable paperbased sensor attached on the arm.

resistance obtained here can mainly be attributed to the excellent electrical coupling between CB aggregates. The darker regions in the image show that the CB globular aggregates are mostly covered by the cellulose acetate. The Raman spectrum of the CB conductive tracks presents two distinct groups of peaks (Figure 2e). The peaks centered at 1329 and 1597 cm−1 (in red) are, respectively, related to the D and G bands of the conductive graphitic nanodomains of the CB particles.36,37 The intensity ratio of these bands (ID/IG) is around 1.7, which indicates that such graphitic domains have sizes (La) about 20 nm. In addition, ID/IG ≈ 1.7 suggests that a single CB particle is composed of multiple graphitic domains.37,38 The peaks at 1221, 1426, and 1551 cm−1 (in blue trace) are associated with amorphous carbon species, from mixed sp2-sp3 bonds or polyenics compounds,39 produced during the CB manufacturing. Next, we discuss the folding capability of CB conductive tracks. As observed in Figure 2f and g, the CB structures have two distinct response patterns that are intrinsically related to the folding angle as follows. (i) The resistance decreases as the paper-based device is folded at −180° (Figure 2f). In this case, the mechanical stress caused by the negative folding angle increases the number of contact points between CB particles. Consequently, the electrical linear resistance (ELR) decreases. On the other hand, (ii) when the CB track is folded +180° (Figure 2g), the ELR increases because the CB particles are forced to separate from each other in the region where maximum mechanical stress occurs. We have noticed that the cycling stability under folding tests, performed at +180°, is over 20 000 cycles (see Figure S2). Here, to the best of our knowledge, the present CB-based structure fabrication method

up to allow several structures in parallel, as illustrated in Figure 1e. The width of the conductive tracks can be tuned by patterning the adhesive layers with a knife plotter. The smallest reproducible CB line width was found around 650 μm (Figure 1f). 3.2. Morphological, Structural, and Electrical Characterization. The scanning electron microscopy (SEM) image exhibited in Figure 2a reveals randomly arranged fibers coated by CB ink. The surface roughness before and after the ink deposition remains the same, indicating that the coating is conformal to the paper topography (see Figure S2). High magnification SEM images of the CB ink on paper exhibit globular aggregates of approximately 30−75 nm in diameter, as depicted in Figure 2b. Such nanostructures are well-connected to each other, forming an electrical conductive film with long current percolation pathways and remarkable low sheet resistance (250 Ω sq−1), as can be observed in Figure 2c. The coefficient of variation of the prepared tracks was found 18% (n = 4). To further understand the electrical properties of the nanostructured CB ink, capacitance gradient imaging was carried out. Such evaluation consists of measuring the local electrostatic surface properties, using a modulated high frequency electrical field, to obtain the local capacitance of the surface. The representative micrograph is shown in Figure 2d, where the globular aggregates are clearly observed. By using the dC/dz information, the local distribution of CB and cellulose acetate can be evaluated. The capacitance contrast, illustrated in Figure 2d, reveals that the interfaces between aggregates are CB rich. Such contrast is evidenced by the high local capacitance (lighter areas).35 Thus, the low sheet E


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our daily life in an enormous number of products. Next, we demonstrate a CB-based circuit prepared on standard office paper by the SAL method as an alternative for flexible and foldable electronics. As shown in Figure 4a−e, the circuit operates under extreme mechanical and electrical conditions. The circuit works continuously even after being fully crumpled (see Figure 4b,c). We also demonstrate the fabrication of linear interconnects for LED arrays that can be integrated and operated on curved surfaces (Figure 4d,e). Since paper is lightweight and foldable, an additional potential application can be envisioned as motion sensors in wearable and portable electronic devices. To fabricate vibration sensitive sensors as a demonstrator, we have used a biological system as a source of inspiration. Spiders, for instance, have outstanding vibration detection systems positioned at their leg joints.44 At the joints, a few microcracks are responsible to detect small vibrations coming from the spider surroundings.45 Here, we introduce for the first time, a fully printed approach of CB conductive structures on paper, as low cost and simple device that can be easily integrated to different interfaces. First, as previously described, the microcracks are created in the CB tracks by folding (+180°) and applying a constant pressure (4 M N/m2) in the device (Figure 5a). The mechanical stress at this region promotes the formation of microcracks localized at the folded region alone, analogous to what is observed in spider’s leg joint. Figure 5b shows an image of the CB tracks containing the microcracks. By comparing the images taken on flat and bended devices, we can precisely follow the structural changes in the CB film. When the CB track is bent, some regions are disconnected, and the resistance of the conductive track increases (see the bended device in Figure 5b). Thus, the operation mechanism of the motion sensor is related to changes in resistance caused by structural transformations during bending. It is important to highlight that this process is fully reversible; once the device is brought to its original flat state, the resistance recovers the initial value. The remarkable reversibility of the system is further applied in the demonstrators described below. One of the challenges faced by wearable and portable devices is the detection small bending angles (20 000 cycles), fast heterogeneous electron transfer when used as working electrode in a paper-based electrochemical cells, and the possibility of modification with redox active substances, such as Prussian blue, to detect hydrogen peroxide at very low potentials. In addition, we demonstrated that electronic circuits can be crumpled and operated with no significant loss of response and that highly sensitive wearable sensors, inspired in the spider sensory system, can be fully printed on paper. The latter was used to detect very small folding angles and precisely track human motions (finger and arm, for instance). Here, to the best of our knowledge, the mechanical, electronic, and electrochemical properties of our fully printed devices surpasses the most recent advances in paper-based devices. The present process is also simple, low-cost, and scalable. The method described here can be extended to other conductive materials and applications. Other possibilities that may take advantage from the proposed route may include energy harvesting systems, supercapacitors, and batteries. To further expand the use of wearable sensors, biocompatible and self-attaching materials would be excellent candidates. In this way, we believe that modified cellulose-based materials can play an important role in this field.

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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.7b06598. Additional experiments, figures, and tables (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone +55(19) 35175098. ORCID

Murilo Santhiago: 0000-0002-9146-9677 Mathias Strauss: 0000-0003-0296-0956 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. All authors contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge CNPq (Project 483550/2013-2) and FAPESP (Projects 2013/22127-2 and 2014/25979-2) for the financial support. We also thank Davi H. S. de Camargo (DSFLNNano), Leirson D. Palermo (DSF-LNNano), Evandro M. G


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Flexible and Foldable Fully-Printed Carbon Black Conductive

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