Highly Stable Graphene Inks Based on Organic Binary Solvents

Formulating highly stable graphene‐based conductive inks with consistency in electrical properties over the storage period has remained a significant challenge in the development of wearable electronics. Two highly stable graphene‐based inks (Cyclohexanone:Ethylene glycol (CEG) ink and Cyclohexanone:Terpineol (CT) ink) are prepared by using two different organic binary solvents, for the first time, without using solvent exchange methods. Both the inks display remarkably high stability (stable even after two months) with negligible variability in electrical properties. Here, it is demonstrated how such inks can be utilized to coat flexible substrates to create wearable e‐textiles. Both the inks coated e‐textiles show significantly low sheet resistance (≈209.1 Ω □−1 for CEG ink and ≈322.4 Ω □−1 CT ink) that show less than a 15% increase in electrical resistance over two months. Therefore, these inks offer high productivity and reproducibility and can be one of the most effective methods for formulating graphene‐based inks.


Introduction
The printed version of textile-based electronics provides an outstanding substitute to traditional technologies used in e-textiles by offering cost-effectiveness, large surface area, and adaptable devices [1,2] that are ornamented with the potentiality for application in various fields like energy storage, [3][4][5] food security, [6] electronic monitoring, [6,7] health diagnostics, [3] etc. In recent years, graphene has become the most promising material for formulating electroconductive ink for flexible electronics due to its supreme chemical and thermal stability, excellent stretchability, intrinsic flexibility, and superior charge carrier mobility. [8] Generally, graphene-based inks are prepared by composing graphene and/or its derivatives (graphene oxides and reduced graphene oxides and/or polymer composites, surfactants, and organic solvents). Graphene's characteristics determine the As a result, there is an obvious need for alternative solvents with good rheological properties for the safety and sustainability of graphene printing. One interesting technique is employing a cosolvent formulation to boost the affinity between the solvent and pristine graphene flakes by using a mixture of solvents, such as water/isopropyl alcohol, water/ethanol, and so on. [33,34] Changing the rheological characteristics (i.e., viscosity and surface tension) of the combination "on-demand" is feasible by adjusting the relative concentration of the cosolvents. [25] However, the stability of cosolvent combinations, mostly based on water and alcohols, is a concern. Indeed, adding alcohol to water changes dramatically and is extremely sensitive to solvent evaporation. [33,34] Furthermore, alcohol-based cosolvents' rheological properties are temperature-dependent. [34] This is a concern for both processing (ultrasonication induces a local temperature increase in the dispersion even when the process is thermalized) and dispersion of the ink's shelf life.
Building on the existing literatures, [25,32,35] we have formulated two highly stable binary solvents which facilitate the required solubility and mixing parameters for graphene. We have formulated two highly stable graphene-based inks with fluidic characteristics suitable for applications in e-textiles.

Results and Discussion
Cyclohexanone is an organic compound with the formula (CH 2 ) 5 CO. The compound is a six-carbon cyclic molecule with a functional group called a ketone. This clear, viscous liquid smells acetone-like and is colorless. [36] Ethylene glycol is an organic compound with the formula (CH 2 OH) 2 . Terpineol, also known as terpineol or p-menth-1-en-8-ol, belongs to the class of organic compounds known as menthane monoterpenoids. These are monoterpenoids containing o-, m-, or p-menthane backbones as the basis for their structure. The cyclohexane ring, which makes up p-menthane, has methyl and isopropyl groups at positions 1 and 4, respectively. For organic solvents, both ethylene glycol and terpineol are miscible in cyclohexanone. [37,38] Dispersion and stability are critical in the formulation of conductive inks. At the very least, the conductive ink should remain stable during the printing process. Ink should be preserved for several months in the best-case scenario for printed electronics applications. [11,39] Hansen solubility parameters (HPs) dictate the graphene inks' solubility and stability formulated in this work. According to Hansen's Handbook, [40] the distance between two points in the Hansen space is known as Hansen distance (R a ), expressed in MPa 1/2 . [40] Thus, an ideal solvent for the dispersion of a molecule or material is the solvent with the HPs located in the Hansen space closer to the coordinates of the molecule or material. [40] Consequently, the shorter the R a between the Hansen coordinates of the solvent and the Hansen coordinates of the material or molecule, the stronger the solvent/material interaction. [40] Contrary, if the R a is large, the solvent is not adequate to disperse or stabilize the material. According to the literature, to achieve a stable graphene flake dispersion, the required HPs of a solvent are δ D ≈ 18 MPa 1/2 , δ P ≈ 10 MPa 1/2 , δ H ≈ 7 MPa 1/2 . [13,35,[41][42][43][44]   graphene and Cyclohexanone:Terpineol (CT) based binary solvent is 2.44 MPa 1/2 . The value of R a is lower for both solvents, indicating strong graphene and solvents interaction. Therefore, the two inks inherit high solubility and stability. Viscosity is a crucial parameter for determining the rheological properties of graphene conductive inks. A previous study [41] reported that inkjet printing requires less viscous inks and a range of viscosity between 8 and 15 mPa s. The conductive ink flowing through the nozzle is enhanced for this viscosity range without issues bordering on leaking, drying out, or coagulation. The viscosity values of the CEG and CT-based graphene conductive inks just after formulation and after storage for two months are presented in Table 3. This table shows that the viscosity of both the conductive inks shows a minimal decrease throughout storage time. However, the values are still in the range of 9 to 10 mPa s, indicating that all the inks are compatible with inkjet printing. Concerning our inks' viscosity, Figure 1 depicts the inks' viscosity under shear rate (data at a constant shear rate of 10 s −1 are shown as an inset). In the shear rate range of 0 to 50 s −1 , for both formulated inks, the viscosity rises as the shear rate increases, which indicates shear thickening. In the shear rate range of 50-200 s −1 , the ink is nearly independent of the shear rate, which is a characteristic of a Newtonian fluid. The ink's rheological behavior can be explained as ordering the fluid molecules at a microscopic level. The molecules are organized and accessible in the presence of weak shear pressures. The arrangement of the graphene flakes prevents them from sliding over one another, increasing viscosity. The data indicate that there are adequate mixing and dispersion.
The zeta potential values on the colloidal particles can be used to verify the dispersion system's stability. One of the methods by which the dispersion stability is determined is through the magnitude of the zeta potential. If the particles' zeta potential values have large positive or negative magnitudes, they will repel each other. Zeta potential values between 0 and 10 mV are generally considered unstable, whereas those between 10 and 30 mV are mild to moderately stable. Values between 30 and 60 mV, on the other hand, suggest good stability, while values >60 mV indicate excellent stability. [45] Table 3 presents the zeta potential values of CEG and CT graphene electroconductive inks that were newly prepared and stored for two months. In both cases, the zeta potential is negative, which can be attributed to the Lewis charge transfer of interfacial solvent molecules and the graphene. According to Lewis's concept, the electron donor number reveals a molecule's propensity to exchange electrons with molecules acting as electron donors and acceptors. A solvent can have negatively charged graphene if the donor number is higher than 18 kcal mol −1 and acceptor number is in the range of 10-20 kcal mol −1 , and their viscosities are in the range of 0.8-2.0 mPa s. [46] According to Liu et al. cyclohexanone fulfills the mentioned criteria, and both the inks exhibit negative zeta potential. The literature shows, or electroconductive ink-based e-textiles, the stability of the inks over a reasonably long period is crucial. [11] As presented in Table 3, the change in the zeta potential values over two months for both CEG and CT based graphene electroconductive inks are negligible, and the final zeta potential values indicate that the inks are in the good stability range.
The absorbance of the solution at a given wavelength has a constant, linear functional relationship to the concentration of the absorbing species. At higher solute loadings, divergence from the normal absorbancy is often taken as evidence of aggregation. [47] Here, K/S values have been used, at the wavelength of maximum absorption, to gain a quantitative assessment of the amount of graphene aggregation in the binary solvents. Table 3 represents the K/S values for both CEG and CT graphene electroconductive inks. After two months of storage time, both the inks show a slight decrease in K/S value, which is also in line with the change in the zeta potential and viscosity for both inks. The minimal changes in K/S values determine that the inks exhibit very little or no aggregation.
Raman spectroscopy is a vibrational method that is particularly sensitive to a molecule's geometric structure and bonding. Even minor variations in geometric design can result in considerable differences in a molecule's reported Raman spectrum. [48][49][50] The normalized spectra curves of graphene nanoplatelets, CEG, and CT graphene electroconductive inks are shown in Figure 2a. The spectrum has a relatively straightforward structure, with two primary bands identified as the G and 2D bands (a third band, the D, is also apparent in graphene as defects are also present within the carbon lattice). The band placements and forms of the G bands and the 2D bands differ significantly, as does the relative intensity of these bands. It is worth mentioning that the band position can be influenced by temperature, doping, and even minute quantities of strain in the sample. When strain, temperature, and   The full width at half-maximum (FWHM) of the 2D band is also a quantitative guide to determining the layer number. There are consistent, substantial, and distinguishable ranges for single-, bi-, tri-, four-, and five-layer graphene at 27.5 ± 3.8, 51.7 ± 1.7, 56.2 ± 1.6, 63.1 ± 1.6, and 66.1 ± 1.4 cm −1 , respectively. [49] Figure 2c shows the FWHM(2D) of graphene nanoplatelets and different inks based on ten measurements for each sample. The FWHM(2D) ranges from 38.11 to 62.89 cm −1 for the graphene Nanoplatelets, 35.32-66.49 cm −1 for CEG based ink, 32.07-69.32 cm −1 for CT based ink. It is worth pointing out that high-quality graphene can also be identified by analyzing the peak intensity ratio of the 2D and G as well as D and G. The ratio I(2D)/I(G) of highquality defect-free) single-layer graphene is equal to 2. The ratio of ID and IG should be minimum for high-quality graphene inks. [48][49][50] Figure 2d,e illustrates the I(D)/I(G) and I(2D)/I(G), respectively. After analyzing the Raman spectrum data presented in Figure 2, it is evident that a slight improvement in the quality of graphene flakes occurs when graphene flakes are dispersed into both binary solvents. Based on the above explanation, it can also be seen that CEG-based ink contains more high-quality graphene flakes than the CT based ink, which shows a clear relationship between the quality of graphene inks and solvent parameters. Further, cyclohexanone:Ethylene glycol binary solvents facilitate better solubility and dispersion parameters than the CT binary solvent, and these parameters, along with the sonication, enhance the capability of the delamination, which in terms produces better quality graphene in ink. The performance of electrical conductivity is one of the significant properties essential for printed e-textiles. [52] Figure 3 shows the sheet resistance values of CEG and CT graphene electroconductive ink printed e-textiles (100% nonwoven polyester substrate of 43 GSM). Based on Figure 3, CEG-based ink shows lower sheet resistance values than CT-based ink. This result is attributed to the higher dispersion ability of cyclohexanone:Ethylene glycol binary solvents, as shown in Tables 2 and 3. In addition, the presence of fewer defects, as evidenced through Raman analysis, also contributes to the lower sheet resistance in the case of the CEG-based ink. Figure 3 also presents the change in sheet resistance for both inks with the storage time duration of one week, two weeks, one month, and two months. The sheet resistance of both the binary solvents-based graphene-coated e-textiles marginally increased after two months of storage. The value changed from 183.3 to 209.1 Ω □ −1 for CEG-based ink and from 289.4 to 322.4 Ω □ −1 for CT based ink. This increment in sheet resistance for both binary solvent-based inks has a clear relationship with the fluidic properties of the binary solvents and follows similar trends for both cases. The quality of graphene nanoplatelets-based ink heavily depends on the rheological properties. It is well known that the major influencing factors of rheological properties are viscosity, size distribution, interaction, dispersion, and maximum packaging fraction. [53] In addition, the degree of agglomeration affects both the mixing times and the final electromechanical properties of the ink. [54] On the other hand, the long sonication period can be inconvenient because it reduces the  sheet size. It can also introduce defects that affect graphene's characteristics. [55] In the formulation method, the amount of ultrasonication time is very less; therefore, both the inks exhibit good stability and have more consistent sheet resistance values.

Conclusions
In conclusion, we have successfully developed highly stable CEG and CT binary solvents-based graphene electroconductive inks that can be used for wearable printed electronics. The development was achieved by adjusting the solubility parameters, mixing parameters, and fluidic characteristics of the ink formulation to achieve two superior and stable dispersions suitable even for printing processes in addition to the dip coating. These graphene inks exhibit considerably higher stability than previously published. The new inks also offered very low sheet resistance (183.3 Ω □ −1 for CEG-based ink and 289.4 Ω □ −1 for CT-based ink) compared to other reported graphene-based inks (Table 1) for printing. A direct relationship has also been observed between binary solvents' electrical conductivity and fluidic properties. The stability and characterization results of the inks confirm the repeatability and high scalability of this method, signifying the viability of creating next-generation wearable e-textiles. Future works will involve detailed investigation, including the effect of different ratios of the solvents on the properties of graphene ink and electromechanical performances of the printed substrates under bending, strain, and compression.
All binary solvents were formulated using the Hansen Solubility Parameters (HPs). The HPs were computed using three different types of energy: dispersion forces between molecules (δD), dipolar intermolecular interactions between molecules (δP), and energy from the electron exchange parameter (δH). These three energy factors dictate the solubility and stability of the graphene inks generated in this work. These three characteristics can be considered coordinates for a point in three dimensions, commonly referred to as the Hansen space. [40] The Hansen solubility parameters (δD, δP, δH) of different solvents were calculated using Equations (1) -(3), and their respective R a was calculated using Equation (4 The subscripts G, S, S 1 , and S 2 , ∅ refer to graphene, the solvent mixture, solvent 1 in the binary solvents, solvent 2 in the binary solvent, and the volume fraction of solvents, respectively. The resultant dispersion was subjected to magnetic stirring for 60 min at room temperature, then sonication using a tip sonicator (UP50H, Ultrasonic processor, Hielscher) for 30 s. This magnetic stirring and sonication were sufficient to disperse the graphene flakes, thus obtaining stable inks.
Characterization of Inks: Raman Spectroscopy: Raman spectroscopy was used to characterize all of the formulated graphene inks. All graphene inks were drop-casted onto a silicon wafer and cured under vacuum using a 300 nm thermally generated SiO 2 . All graphene inks were analyzed using a Horiba LabRam microscope equipped with a 50× objective lens and an incident power of 1 mW on the samples. We obtained ten spectra for each graphene ink and fitted the peaks using Lorentzian functions.
Rheological Measurement: The viscosity of the inks was determined using an AR-G2 Hybrid Rheometer equipped with a double-wall concentric cylinder geometry, which is optimized for low-viscosity fluids. Throughout testing, the inks' temperature was maintained at 25 °C.
K/S Value Analysis: The color strength K/S of all the prepared samples was measured by DATACOLOR 500 spectrophotometer using the Kubelka Munk theory to determine the aggregation.
ZETA Potential Analysis: The Zeta potential of prepared inks was measured using a Zetasizer Nano ZSP from Malvern analytical, UK, in which Electrophoretic Light Scattering was used.
Imparting Ink onto E-Textiles: The "dip and dry" coating method deposited the inks on nonwoven textile substrates. 100% Polyester nonwoven fabrics of 43 GSM were (45 mm × 45 mm) dipped either into the CEG graphene ink or CT graphene ink. Each sample was coated one time with the inks for better comparison. After a certain number of dip and dry coating cycles, the graphene take-up percentage can be