Electrochemistry Communications 98 (2019) 6–9 Contents lists available at ScienceDirect Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom Production and printing of graphene oxide foam ink for electrocatalytic T applications Luis Baptista-Piresa, Alfredo de la Escosura-Muñiza, Marc Balsellsa, Julio C. Zuaznabar-Gardonab, Arben Merkoçia,c, ⁎ a Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Spain b Nanobiotechnology & Bioanalysis Group, Departament d' Enginyeria Química, Universitat Rovira i Virgili, Avinguda Països Catalans 26, 43007 Tarragona, Spain c ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain ARTICLE INFO ABSTRACT Keywords: A graphene-based ink printed as a foam-like structure with open pores is reported. The production of the ink is Graphene easier and faster than using existing methods and the obtained product is stable in water suspension. Ink Electrocatalytic applications of 3D structured electrodes printed onto plastic substrates were explored. Foam Print Electrocatalysis Sensors 1. Introduction reduction. Briefly, 50 mL of 2 g/L GO aqueous dispersion is sealed in a Teflon-lined autoclave and maintained at 150 °C for 5 h; the autoclave Graphene-based materials exist in a variety of forms that include is then cooled to room temperature, resulting in a three-dimensional 0D, 1D, planar 2D and 3D structures [1]. 3D graphene structures such (3D) RGO hydrogel [5]. In a different approach aimed at producing 3D as graphene-based foams have been much studied in recent years due to cellular films for supercapacitor applications, the methodology consists the remarkable electrical and mechanical properties associated with of pre-reduction of GO sheets, then filtering using vacuum filtration, their 3D structure, which has a large surface area. Graphene oxide (GO) followed by dropping the film into liquid nitrogen to solidify, causing is a material known for its high yield production process, defective the GO sheets to form a 3D structure [6]. Another approach using va- structure and insulating electronic properties due to the presence of cuum filtration for the production of thin films through direct laser large numbers of oxygen functional groups. These groups can be re- writing has been reported [7,8]. GO-based foam composites have also moved using reducing agents and the resulting material, reduced gra- been tested for use as a fire retardant [9]. Herein we propose a novel phene oxide (RGO), is conductive with semi-metallic-like behavior. methodology using a short (45 min) autoclave-based procedure for the RGO-based foams can be produced using innovative reducing strate- production of RGO foams; these are then suspended in water and so- gies. Z. Niu et al. have obtained RGO foams by simple vacuum filtration nicated. The resulting foam-like matrix can be printed over plastics followed by reduction of GO in hydrazine solution by autoclaving at using wax-printed membrane based technology. This material should be 90 °C for 10 h, resulting in a horizontal porous network from sub mi- of great interest for electronic applications in energy and (bio)sensor crometer to micrometer scales that is cross linked without a complete research due to its large surface area, electrochemical behavior, flex- separation between the different layers. This foam has a low electrical ibility and transparency. resistance of less than 100 Ω/square [2]. In a different approach, Pirkle et al. mixed 1 M KOH into a GO suspension and heated at 100 °C under 2. Experimental constant stirring until a paste was obtained. This paste can be simply filtered in vacuum and reduced further. The resulting porous film is The nitrocellulose membranes used are hydrophilic, with a pore size flexible, highly conductive and can be cut into desired shapes. The KOH of 0.025 μm and a diameter of 47 mm. The wax was printed with a concentration seems to be a key factor in obtaining homogeneous films Xerox ColorQube 8570 printer using the high resolution printing mode. [3,4]. Another way of producing RGO foams is through hydrothermal The electrode patterns were created using Corel Draw. ⁎ Corresponding author at: Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Spain. E-mail address: arben.merkoci@icn2.cat (A. Merkoçi). https://doi.org/10.1016/j.elecom.2018.11.001 Received 28 September 2018; Received in revised form 1 November 2018; Accepted 1 November 2018 Available online 10 November 2018 1388-2481/ © 2018 Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/). L. Baptista-Pires et al. Electrochemistry Communications 98 (2019) 6–9 Commercially available GO solution with a concentration of 5 mg/ Table 1 mL was obtained from Angstrom Materials (N002-PS-0.5) and diluted in C/O ratio calculated from C1s XPS measurements for water to a concentration of 1 mg/mL. To prepare the RGO foam and RGO-based foams obtained after autoclaving. The top transfer it onto a plastic substrate, 100 mL of a 1 mg/mL GO solution part of the table gives results obtained with a constant temperature of 125 °C for different autoclaving times. was mixed with 100 mL of 1 mg/mL of ascorbic acid (Sigma-Aldrich), The bottom part of the table gives results obtained with a followed by autoclaving at 125 °C for 45 min. Material was obtained constant time of 45 min for different autoclaving tem- from the remaining RGO agglomerated in water solution after autoclave peratures. treatment and transferred to a 50 mL plastic flask filled with 40 mL of water and sonicated for 15 min at 20 Hz using a probe sonicator Time (min) C/O ratio (Qsonica Q125). The resulting solution was left at room temperature for 1 3.69 ± 0.36 1 day in order to precipitate the unsuspended particles. The aqueous 3 3.83 ± 0.22 suspension of RGO was then filtered through wax-printed membranes 5 3.95 ± 0.44 using a 1 L vacuum-filtering flask, 300 mL glass filter holder and 47 mm 10 4.59 ± 0.42 15 5.52 ± 0.32 SS screen (1/Pk). The RGO-modified wax-printed membranes (WPMs) 25 6.32 ± 0.11 were transferred using the roller-to-roller system used for paper trans- 35 6.56 ± 0.21 port in the wax-based printer. 45 6.78 ± 0.17 The samples were electrochemically characterized with K3[Fe(CN)6] 60 6.82 ± 0.12 (Sigma-Aldrich) as redox probe in KNO3 (Sigma-Aldrich) as supporting electrolyte using an Autolab302 potentiostat/galvanostat/frequency- Temperature (°C) C/O ratio response analyzer PGST30, controlled by GPES/FRA Version 4.9. The X- ray photoelectron spectroscopy (XPS) measurements were performed 75 2.73 ± 0.29 with a Phoibos 150 analyzer (SPECS GmbH, Berlin, Germany) under 100 3.20 ± 0.31 ultrahigh vacuum conditions (base pressure 1 × 10−10 mbar) with a 125 6.78 ± 0.17 150 6.80 ± 0.13 monochromatic aluminum Kα X-ray source (1486.74 eV). The energy resolution (as measured by the fwhm of the Ag 3d5/2 peak for a sputtered silver foil) was 0.58 eV. Scanning electron microscopy (SEM) obtained material by C1s X-ray photoelectron spectroscopy (XPS). The was carried out on a FEI Quanta FEG (pressure, 70 Pa; HV, 20 kV; and data in Table 1 show that the C/O ratio increases with both time (at a spot, four). fixed temperature of 125 °C) and temperature (at a fixed time of 16 nm gold nanoparticles (AuNPs) were synthesized by reducing 45 min), reaching a plateau at a value of approximately 6.78 which is tetrachloroauric acid (Sigma-Aldrich) with trisodium citrate (Sigma- obtained by heating at 125 °C for 45 min. These were the conditions Aldrich), a method pioneered by Turkevich [10]. Electrochemical de- used for RGO reduction. The resulting material was characterized by tection of AuNPs was performed by placing 50 μL of the suspension of SEM and is composed of a network of RGO sheets that upon drying at AuNPs in 1 M HCl (Sigma-Aldrich) and applying a potential of +1.35 V room temperature form a crumpled RGO (see Fig. 2a). However, once for 1 min (electrochemical pretreatment) followed by a potential of freeze dried (which can indicate the form after thermal treatment) it −1.00 V for 120 s in chronoamperometric mode [11]. forms a cavity-like structure with stacked RGO layers (see Fig. 2b). The remaining RGO agglomerated in ascorbic acid solution after autoclave treatment was transferred to a 50 mL plastic flask filled with 40 mL of 3. Results and discussion water and sonicated for 15 min at 20 Hz using a probe sonicator. The resulting solution was left at room temperature for 1 day in order to The experimental procedure for preparing the RGO foam and precipitate the unsuspended particles. The homogeneous water–RGO transferring it onto a plastic substrate is illustrated in Fig. 1. This in- suspension was transferred to a different flask and remained stable for volves mixing 100 mL of a 1 mg/mL GO solution with 100 mL of 1 mg/ over a month. mL of ascorbic acid, followed by autoclaving. The autoclaving tem- It is important to note that in the presented methodology we avoid perature and time were optimized by evaluation of the C/O ratio of the Fig. 1. Scheme showing (a) the experimental procedure for preparation of the RGO foam suspension and (b) its transfer onto a plastic substrate. 7 L. Baptista-Pires et al. Electrochemistry Communications 98 (2019) 6–9 Fig. 2. SEM images of (a) the crumpled RGO formed upon drying at room temperature; and (b) the porous strucuture obtained after freeze drying. (c–f) SEM images (at different magnifications) of the open porous network obtained upon printing with the foam-based ink. the drying step after the autoclave treatment and maintain the bulk (with highly interconnected RGO flakes) and porous (due to the fil- foam form in water solution during the whole ink formulation process. tration channels). The separation of the sheets upon sonication offers an innovative ad- This material was also characterized electrochemically. Electrodes vantage, providing small clusters of RGO sheets approximately 200 nm prepared using the method described above were evaluated by cyclic in size suspended in water. voltammetry in 2 mM K3[Fe(CN)6]/0.2 M KNO3 using a three-electrode Printing using these foam-based inks was performed by adapting system, with Ag/AgCl as the reference electrode and a platinum wire our previously reported technology using wax-printed membranes counter electrode. As shown in Fig. 3a, a number of different scan rates (Fig. 1b) [12]. Briefly, 5 mL of the ink were filtered and patterned using in the range 2–100 mV/s were evaluated, revealing well-resolved wax-printed membranes of the desired shape. The printing metho- anodic and cathodic peaks corresponding to the reversible [Fe dology consists of the filtration of a water suspension through the un- (CN)6]4− / [Fe(CN)6]3− oxidation/reduction system. As shown in clogged areas not covered by the wax. These areas enable the water to Fig. 3b, a straight line was observed on plotting the current intensity of be filtered out, leaving a RGO film on the top of the membrane. These both anodic and cathodic peaks against the square root of the scan rate, WPMs coated with RGO particles are placed over the target substrate which is consistent with a diffusion-controlled redox process. and the back of the WPM is dried rapidly (for 10 s) using compressed These data make it possible to estimate (using the well-known air. To transfer the filtered RGO material, pressure is applied using the Randles-Sevcik equation) that the electrochemically active area is roller-to-roller mechanism in the wax printer, leaving the patterned around 0.047 cm2. This effective area is considerably higher than that structures on top of the substrates. The mechanism of passing the calculated for standard screen-printed carbon electrodes (SPCEs) of the sample through the printer rollers from the manual feed tray could be same geometric size (around 0.016 cm2), probably due to the high substituted by a normal desktop laminator, as once demonstrated porosity and 3D structure of the foam electrodes. Additionally, the elsewhere by our group; no temperature is needed, only sufficient standard heterogeneous rate constant of electron transfer (k0) was es- pressure to make contact between the filtered layer and the target timated following the Nicholson method [15]. An average rate constant substrate – even simply using pressure from a spatula or a finger of 2.4*10−4 cm/s for the [Fe(CN)6]3−/4− redox pair was obtained. [12,13]. Unlike other reported strategies, the ink made by the process Such values of k0 were of the same order of magnitude as for tested reported here is stable in water for at least 1 month without the need for commercially available SPCEs (k0 ~5 ∗ 10−4 cm/s). These results il- surfactants or additional solvents; the shaping capability introduces a lustrate the possibility of using the RGO foam as an excellent substrate step forward for printing sensors where the geometric area is important; for monitoring electrochemical processes. and the roller-to-roller transfer process introduces the possibility of The advantages of such electrodes were also evaluated by studying mass production and industrialization of one-step printing of porous 3D the hydrogen evolution reaction (HER) catalyzed by gold nanoparticles conductive films [13,14]. (AuNPs). This method was developed and used extensively in our group When the resulting foam-based ink is used for printing, an open for the sensitive detection of AuNP tags in biosensing systems, using porous network with a highly active surface area (Fig. 2c–f) is obtained. SPCE electrodes [11]. Chronoamperograms recorded at –1 V in 1 M HCl This may be due to the combination of the stable microclusters of RGO showed increased catalytic currents for different AuNPs concentrations particles and the use of vacuum filtration technology. in the range 59–3000 pM, as shown in Fig. 3c. A logarithmic relation- The vacuum filtration process is faster for RGO than for GO as it ship between the current intensity and the AuNP concentration was does not clog the pores and there are open channels for the water to be found in this range (Fig. 3d), allowing a detection limit of around 0.5 filtered, resulting in shorter filtration times (compared with GO). This pM to be estimated. This is lower than that obtained experimentally suggests that filtration is a building block in this open network. In ad- with SPCEs (around 1 pM). The sensitivity of the detection system was dition, the 300 nm thick film does not have any room for shrinkage also substantially better than the corresponding values for SPCEs as the once it is attached to the substrate; this means the film is both compact slope of the curve is more than twice as large (Fig. 3d). These results are 8 L. Baptista-Pires et al. Electrochemistry Communications 98 (2019) 6–9 Fig. 3. Electrochemical characteriza- tion of the prepared RGO foam elec- trodes. (a) Cyclic voltammograms re- corded in 2 mM K3[Fe(CN)6] / 0.2 M KNO3 at different scan rates in the range 2–100 mV/s. (b) Plot of current intensity of anodic and cathodic peaks vs the square root of the scan rate. (c) Chronoamperograms recorded at −1 V in 1 M HCl for different concentrations of AuNPs. 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