Reactive Inkjet Printing of Graphene Based Flexible Circuits and Radio Frequency Antennas

Graphene-based materials show great promise in wearable electronics due to their remarkable properties such as excellent electrical conductivity, high flexibility and light weight. Various techniques have been used to fabricate graphene-based electronics, such as lithography, template-assisted synthesis, and chemical vapor deposition. However, these patterning methods normally involve complex procedures, toxic solvents and extra machinery, which are highly challenging for large-scale industrial production. Herein, we developed an in-situ approach to fabricate reduced graphene oxide (rGO) conductive patterns on flexible substrates by reactive inkjet printing without any post-treatment. Electronic circuits and WIFI antennas consisting of conductive rGO lines with a minimum width of 100 μm and remarkable mechanical durability were successfully fabricated. The highest electrical conductivity of the printed rGO lines was 2.69 × 10 4 S m −1 using optimised printing conditions. The rGO based radio frequency antenna demonstrated transmission with a measured Domain Name System (DNS) delay of 243 ms. When accessed via a 100 Mbps router, the network speed reached up to 4.64 Mbps, which is comparable to the current commercial mobile phone antenna (DNS delay 237 ms, network speed 4.73 Mbps). This demonstrates the potential of reactive inkjet printing for the industrialisation of graphene-based wearable electronics.

High-resolution patterning is one of the key processes for fabricating flexible electronics. [12][13][14][15] However, several commonly used techniques for device fabrication including lithography, 16 template-assisted synthesis, 17 and chemical vapor deposition (CVD), 18 usually require complex multi-step procedures that are often incompatible with the fabrication of antennas that have dimensions much larger than that of present day CMOS devices. This usually results in the integration of the antenna either in the package or as a separate component mounted via PCB. By contrast, inkjet printing shows advantages of fast deposition, high resolution, and precise control of deposited materials in terms of quantity and location, simplified operating procedures and non-contact direct printing; therefore, it has gained popularity in flexible applications of electronic devices. 5,14,[19][20][21][22][23][24][25] Metallic nanoparticles such as gold, silver and copper are widely used as inkjet inks mainly owing to their paramount electrical conductivity. However, there are several disadvantages of using these inks to fabricate flexible electronics: (1) stabilising agents are normally required to avoid agglomeration of metal nanoparticles dispersed in inks, which is detrimental to electrical conductivity of printed features; (2) the post-annealing step commonly requires a high temperature which is harmful to most wearable substrates such as textiles; (3) gold and silver nanoparticles are expensive; (4) copper is easily oxidised when exposed to ambient environment, which results in short life-time of the fabricated electronics. [26][27][28] Graphene and its derivatives, such as graphene oxide (GO) and reduced graphene oxide (rGO) show great promise in fabricating flexible electronics on account of their excellent chemical and environmental stability, mechanical flexibility and electrical conductivity. 6,[29][30][31][32][33] It has been recently shown that the lateral confinement of electrons in semi-finite-size graphene nanoribbons (GNRs) enhances the material conductivity even up to the terahertz band. 34, 35 Moreover, compared with other conductive materials (such as metal nanowires, metallic nanoparticles), graphene materials can overcome the skin effect of existing materials. 6 However, most of current graphene patterning is which show exceptional conductivity. 44,45 Despite GO suspension being readily patternable, which makes it an ideal water-based printing ink, there are few reports on highly conductive graphene using GO-based inkjet printing. 46 The main obstacle is an insufficient reduction process after the GO printing. 30,[47][48][49][50][51][52][53] In order to improve the overall degree of reduction of the printed material, Su et al.
used a "weak oxidation-vigorous exfoliation" strategy to reduce the defects of the GO and thus to reduce the difficulty of subsequent reduction. However, an inherent problem with this approach is that weakly oxidized GO may not be efficiently dispersed and assembled. 53 Our previous work demonstrated that in situ formation of unprintable materials through reactive inkjet printing (RIJ) were achieved by alternative printing of two reactive components on top of each other to trigger the reaction and form the product. 54,55 Herein, we developed an encouraging in-situ approach combines the advantages of both reactive inkjet printing and graphene-based materials, and facilitates the fast and cost-effective fabrication of antennas. In addition, it provides an on-demand facile integration of graphene into various flexible substrates and electronic devices.

Materials and methods
2.1 Materials. All reagents were analytical grade from commercial sources and used without further purification. GO was obtained from The Sixth Element (Changzhou) Materials Technology Co., Ltd.
China. Hydroiodic acid (HI, 45%) and anhydrous ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd. Polyethylene terephthalate (PET) films were used as substrates for inkjet printing.
Deionized (DI) water (Elga LabWater Chorus Complete) with a resistivity of 18.2 MΩ was used in all experiments.

Ink preparation.
GO solution was diluted to a concentration of 1 % using DI water. A 10 μm nylon filter membrane was used in a Brinell funnel to filter the solution thrice. The filtered solution was sonicated in a water bath (RT) for 2 hours to obtain GO ink (ink-A). The reductant ink (ink-B) was prepared with diluted hydroiodic acid.

Plasma treatment of PET substrate.
In order to improve the wettability of the substrate, the PET films were firstly cleaned with anhydrous ethanol followed by oxygen plasma treatment (CIF CPC-B-13.56) at 80 W for 90 seconds.

Reactive inkjet printing process.
Printheads with 80 μm nozzle diameter were used for printing.
Reactive inkjet printing was performed by a piezoelectric drop-on-demand (DOD) inkjet printer (Jetlab4xl-A, MicroFab) with a resolution of 5 μm in the XY directions. The volume of ejected droplets was ranged between 50-150 pL. Ink-A (GO suspension) and Ink-B (HI solution) were alternatively printed on the PET substrate, which was heated to 50 or 70 °C to accelerate the evaporation of the water. Except for the humidity test, all electrical performance and mechanical performance measurements were performed at 50% humidity.

Fabrication of RFID antenna and WIFI antenna. The dipole antenna and 2.4 G WIFI antenna
were designed by using CAD drawing software. The fabrication processes were as follows: 20 layers of GO ink was printed according to the designed antenna pattern followed by the printing of 3 layers of 45 % HI after the completely drying of the printed GO pattern. During the printing, the platform was heated to 70 °C to ensure the full reduction of GO. The antenna was dried overnight at room temperature in a vacuum oven.  of GO inks increased after 3 months of storage, they still showed excellent printability ( Figure S1).
Wettability of the substrate affects the printing resolution via droplet spreading on substrates. In order to ensure the consistent wettability of the substrate, the PET film was treated with oxygen plasma to improve its hydrophilicity. The contact angles of GO ink on the untreated and treated substrates were 55.2° and 26.0°, respectively ( Figure 2c). The contact angles of HI ink on the untreated and treated substrates were 29.7° and 16.5°, respectively ( Figure 2c). Therefore, the decrease in the contact angle indicates the improvement in the wettability of the substrate. In inkjet printing, surface tension and viscosity play a crucial role in the performance of the ejected droplet, which significantly affects the printability of an ink. Normally, inks with surface tension range from 28 mN/m to 350 mN/m and viscosity < 20 cP are accepted as printable inks. Highly viscose inks cannot be ejected to form a stable droplet stream for printing. The dimensionless number, i.e., Z number which is derived from surface tension and viscosity is commonly used to predict droplet formation. It has been reported that inks with 1 < Z < 10 are suitable for inkjet printing (Equation 1). 56

Equation 1
= Where ƞ, ρ, σ are viscosity, density, surface tension, and d is the diameter of the printhead nozzle.
The surface tension of the GO and HI inks were 58.5 and 76.9 mN/m, respectively. The viscosities of GO and HI inks were 13.5 and 2.8 cP. Therefore, the Z number of GO and HI inks were 7 and 3.6, respectively, indicating the excellent printability of both inks as shown in Figure S2.  3.3 Optimization of reactive inkjet printing conditions. The self-assembly performance and degree of reduction of GO is affected by whether a drying step is included between each printed layer or not.
The XRD results showed that peak width at half maximum of printing with drying intervals is smaller than that of printing without drying intervals, indicating the better self-assembly and uniformity of the former (Figure 3a). This may be due to the fact that evaporation of liquid (gradually reduced volume) facilitates the self-assembly of GO sheets. 42 Therefore, the resultant structure of printing with drying intervals has more self-assembled layers than that of printing without drying intervals.  Figure 3c. 58 Less branchy groups hinder the formation of ordered graphitic structure as shown in Figure 3d. It can be seen that the 2θ of rGO is 26.31° and the interlayer spacing is 3.67 Å, very close to that of natural graphite. On the other hand, the 2θ of GO is 11.85°, and the interlayer spacing is 7.34 Å. The reduced interlayer spacing is due to the removal of oxygen-containing functional groups after reduction. Figure 3e, 3f show the C1s XPS spectra of GO and rGO, indicating the noticeable enhancement of the C-C/C=C bond (aromatic rings) and a reduced intensity of oxygen-containing groups after reduction. 59 Figure S5 shows a significant decrease in the opacity of the rGO film compared to the GO film.
The morphology of printed GO and rGO lines is shown in Figure 3g, 3h. The surface of printed GO pattern has many "ridges". The ridges were produced by the evaporation of the solvents after GO droplets deposited on the substrate. During this process, GO sheets were carried by the droplet shrinking due to the solvent vaporizing, forming ridges. When GO was exposed to HI, a large number of oxygen-containing functional groups were removed by reduction, resulting in well-ordered rGO sheets, reflected by the flattening of the "ridges" surface, 60 which improved the electrical conductivity. Figure S6a represents the optical micrograph of printed GO droplets under an AFM probe, and Figure   S6b is an AFM image at the edge of a circular droplet. The cobwebs in this image are similar to the "ridges" in SEM. Due to the shear deformation during the preparation process of 20-layer rGO cross section, the 100-layer rGO cross section was used as the representative for characterization. Figure   3i, 3j showed the laminar structure of cross-sections of both GO and rGO patterns (100 layers). The SEM images show that the rGO films stack like paper sheets in a book, and the gap between the layers clearly testifies the remarkable well-ordered self-assembly of the rGO layers.
The degree of reduction of GO affects the electrical conductivity of final rGO patterns. Figure 4a shows that the conductivity of the rGO patterns increases with the number of layers for a fixed HI concentration, and the increase of HI concentration until 30 %. With an increase of printed GO layers or substrate temperature, their orientation improves, resulting in smaller ratio of the intensity of I D /I G bands, higher conductivity and darker films Figure S7, S8. To investigate the reason of the rapid increase of electrical conductivity of 20 ~ 30 layers, Raman and X-ray diffraction spectra were used to study the apparent in-plane crystallite size (La) and apparent crystallite size in the c direction (Lc) of the 20 and 30 layers, respectively. 61 As displayed in Table S1, with an increase of printed rGO layers, the Lc value increased from 6.36 nm to 10.41 nm. The increase of longitudinal grain size leads to a sharp increase in the electrical conductivity of rGO films. On the other hand, the conductivity of rGO patterns gradually increases with HI layers (Figure 4b). However, beyond 3 HI layers irregular HI residues are formed on the surface of the GO pattern. Although no significant influence of temperature on the reduction between 25 °C and 50 °C, beyond 70 °C, the conductivity is slightly elevated as shown in Figure 4c. In summary, with intermittent drying between each layer, 0.5 mg mL -1 GO ink, 45 % HI ink was used to print 30 layers of GO, 3 HI layers at a base plate temperature of 70 °C, to yield the maximum conductivity (2.69 × 10 4 S m -1 ) of rGO film. By studying the resistivity changes of rGO under different humidity conditions (The humidity conditions in this experiment ranged from 50 % to 100 %), the rGO can also be used as a humidity sensor (Figure 4d).

Electrical conductivity and mechanical durability of rGO patterns.
The rGO line was printed on a plasma-treated PET film, then a conductive copper glue was used to connect the rGO line with a LED light strip (Figure 5a, 5b). Figure 5c demonstrates the conductivity of the printed rGO line. As aiming for flexible electronic devices, the whole conductive circuit was bent to test the flexibility of the printed rGO lines (Figure 5d). It was found that the brightness of the LED light strip was not affected by the bending of the substrate (Supporting Information Video 2). The mechanical durability was tested by a large number of cyclic mechanical deformations, i.e., inward and outward bending (Supporting Information Video 3). Figures 5e, 5g show that the absolute resistance slightly increases with cycle number. The mechanical durability of the printed rGO features in this study was compared with previously reported works (Table 1). 46,[62][63][64][65][66][67] After 2000 cycles of inward or outward bending, there is no measurable change in resistance (<3%). The change of relative resistance after 10000 cycles was up to 50 % and 30 % of inward and outward bending, respectively (Figure 5f, 5h). The SEM images show the surface morphology of the printed rGO lines before and after 10000 cycles of mechanical deformations (Figure 5i-l). In Figure S9 and Table S2, we conducted quantitative statistics on the crease density and width after bending. The results show that bending has no obvious effect on crease width, but the crease density increased significantly after 10000 inward bending. The main reason for the increase of resistance after bending may be the increase in crease density. The electrical conductivity of the substrate after bending can be improved by using substrate materials with better bending performance, such as PDMS, or by pre-stretching the patterned surface of the substrate. period is shown in the animation ( Figure S10). In addition, the magnitude of the current near the centre is stronger than that of two ends. The direction of the current keeps unchanged along the strip.
Therefore, it is a half-wavelength dipole antenna. The rGO line was printed on a dielectric laminate with a permittivity of 2.2 and a thickness of 1.6 mm. A 50-Ω coaxial cable was utilized to feed the antenna without using balanced designs. It is noted that in the applications where the antenna is used inside the circuits of a complete system, the need for the balun is automatically eliminated. Figure 6b compares the simulated and measured |S 11 |, showing a measured null at 864 MHz, which is only 3% higher than that of the simulation due to the fabrication tolerance. The measured gain of the antenna was 0.64 ~ 1.5 dB less than that of a traditional half-wavelength dipole antenna. It should be noted that the perfect conductor is assumed in the simulation. However, the conductivity of the printed rGO lines is 1.69 × 10 4 S m −1 (20-layers) which causes an Ohmic loss. The measured and simulated radiation pattern in the xoy-plane (E-plane) and yoz-plane (H-plane) are presented in Figure 6c&6d, which show good agreement between the simulation and measurement.
Moreover, the levels of the cross-polarization in the E-plane and H-plane are less than -20 dB and -10 dB, respectively. The antenna owes omnidirectional radiation, which make it suitable for a mobile communication system with randomly oriented terminals.
In order to verify that the rGO antenna is fully compatible and functional in the smart mobile phone,

Conclusion
In conclusion, we developed a simple and effective strategy to fabricate rGO patterns on flexible substrates through reactive inkjet printing. By controlling the reaction conditions and printing methods to control the degree of reduction and self-assembly of GO, highly conductive rGO patterns were obtained. Smooth conductive lines with a resolution of 100 μm were printed on the plasmatreated PET films. The highest electrical conductivity of the printed rGO lines was 2.69 × 10 4 S m −1 .
In addition, the printed rGO patterns possess excellent mechanical durability, suitable for flexible electronic devices. Furthermore, we applied this approach to fabricate a mobile phone WIFI antenna with a measured DNS delay of 243 ms after access to a 100 Mbps router enabling a measured network speed of 4.64 Mbps. The performance of the fabricated antenna is comparable to a commercial mobile phone antenna. It is worth noting that though reactive inkjet printing, any shaped electronic patterns (such as electronic circuits, antennas, RFID tags and sensors) can be fabricated for wireless wearables.
The current work demonstrates that reactive inkjet printing is a promising technology for the fabrication of graphene-based flexible electrics.

Declaration of competing interest
The authors declare that there are no competing interests.