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Cyclic production of biocompatible few-layer graphene ink with in-line shear-mixing for inkjet-printed electrodes and Li-ion energy storage

Cyclic production of biocompatible few-layer graphene ink with in-line shear-mixing for... www.nature.com/npj2dmaterials ARTICLE OPEN Cyclic production of biocompatible few-layer graphene ink with in-line shear-mixing for inkjet-printed electrodes and Li-ion energy storage 1,2 3 2 4 1 2 4 Tian Carey , Abdelnour Alhourani , Ruiyuan Tian , Shayan Seyedin , Adrees Arbab , Jack Maughan , Lidija Šiller , 2 2 2 2 1 3 Dominik Horvath , Adam Kelly , Harneet Kaur , Eoin Caffrey , Jong M. Kim , Hanne R. Hagland and Jonathan N. Coleman The scalable production of two-dimensional (2D) materials is needed to accelerate their adoption to industry. In this work, we present a low-cost in-line and enclosed process of exfoliation based on high-shear mixing to create aqueous dispersions of few- −1 layer graphene, on a large scale with a Y ~ 100% yield by weight and throughput of ϕ ~ 8.3 g h . The in-line process minimises basal plane defects compared to traditional beaker-based shear mixing which we attribute to a reduced Reynolds number, Re ~ 5 4 −1 10 . We demonstrate highly conductive graphene material with conductivities as high as σ ∼ 1.5 × 10 Sm leading to sheet- −1 resistances as low as R ∼ 2.6 Ω□ (t ∼ 25 μm). The process is ideal for formulating non-toxic, biocompatible and highly −1 concentrated (c ∼ 100 mg ml ) inks. We utilise the graphene inks for inkjet printable conductive interconnects and lithium-ion −1 battery anode composites that demonstrate a low-rate lithium storage capability of 370 mAh g , close to the theoretical capacity of graphite. Finally, we demonstrate the biocompatibility of the graphene inks with human colon cells and human umbilical vein −1 endothelial cells at high c ∼ 1mgml facilitating a route for the use of the graphene inks in applications that require biocompatibility at high c such as electronic textiles. npj 2D Materials and Applications (2022) 6:3 ; https://doi.org/10.1038/s41699-021-00279-0 INTRODUCTION advantages when applied, such as flexibility, biocompatibility, environmental stability or weight reduction. The Hummers Graphene and other 2D materials are expected to find major method emerged as one of the first methods to produce commercial applications in the coming years . By utilising the graphene in the form of graphene oxide (GO) flakes . However, unique electrical, optical, mechanical, chemical and thermal the GO flakes differ from pristine graphene, containing a properties of 2D materials, additional functionality or improved population of functional groups that disrupt the sp structure of performance can be added to many applications. Graphene could graphite . Liquid phase exfoliation (LPE) is established as the be useful in over 40 major application areas such as composites, primary method to produce dispersions of pristine graphene energy storage, thermal management, sensors and coatings . For flakes at scale and low cost . The most common techniques to example, graphene can be used as a barrier material for anti- 16 17 3 4 undertake exfoliation in liquid are ultrasonication , ball milling , corrosion , an additive for mechanical reinforcement in polymers , 18 19 20 shear-mixing , electrochemical exfoliation , wet-jet milling and or as a conductive material in sensors . Many of these applications microfluidization . To evaluate each technique’s effectiveness, will require few-layer graphene flakes (<10 layers) in large multi- figures-of-merit (FOM) are frequently used to characterise the tonne quantities if successfully commercialised . For example, if resulting dispersions. The yield by weight (Y ) is the ratio between graphene flakes are used at low loading (~1%) in the 300 million w the weight of the final graphene material and the starting graphite tonnes per year global plastic industry , or used to replace 6 7 flakes , while the throughput (ϕ)is defined as the mass of graphite (~10 tonnes per year) in Li-ion batteries for the electric graphene obtained per hour. In the literature, Y and ϕ are vehicle market it would create a demand of >1 kilotonne, far w commonly used as FOM to assess the production process. exceeding the global graphene supply . Few-layer graphene flakes Electrochemical exfoliation involves intercalating a 2D material could also be utilised as a cost-reduction replacement material for such as graphite with an ionic species that expands and exfoliates metal components in applications such as interconnects or 19,23,24 the bulk material into flakes . The process has high Y ~ electrodes, particularly where form factor is important such as 9 −1 10–75% with ϕ ~ 0.3 g h when post-processing (e.g. material electronic textiles . 19,23,24 washing) is considered . Ultrasonication is the most widely Currently, printable metal inks (e.g. silver and gold) are studied technique and typically involves using a bath sonicator , commonly used as interconnect or electrode materials and are which has been used to exfoliate graphene in toxic solvents (e.g. made of precious metals costing ~£1000 per litre on average and 10 11 12 16 n-methyl-2-pyrrolidone, NMP) and non-toxic (e.g. water) sol- can have oxidation issues , toxicity or nanoparticle migration , 25 −1 vents . However, ink concentrations, c up to ~ 1 mg ml in <24 degrading device performance. Graphene inks could be poten- 22 −1 tially produced for as little as ~£20 per litre once scaled due to h equating to ϕ ~1gh and Y ~3–5% has limited carbon’s elemental abundance while retaining several functional ultrasonication to lab-scale studies. Ball milling involves mixing 1 2 Cambridge Graphene Center, Department of Engineering, University of Cambridge, Cambridge, UK. CRANN and AMBER Research Centres, Trinity College Dublin, Dublin 2, 3 4 Ireland. Department of Chemistry, Bioscience and Environmental Engineering, University of Stavanger, Stavanger, Norway. School of Engineering, Newcastle University, Newcastle upon Tyne, UK. email: tian.carey@cantab.net; colemaj@tcd.ie Published in partnership with FCT NOVA with the support of E-MRS 1234567890():,; T. Carey et al. graphite powder, solvent and zirconia/metal balls together in a rotatory mill. The friction and shear forces with the balls enable 17 −1 the exfoliation of graphite . The ϕ of the process is ~ 0.2 g h with a Y ~ 12% . Microchannel-based techniques such as microfluidization and wet-jet milling have also been used in cyclic processes to exfoliate graphene nanoplatelets and 2D materials 20,21 with Y ~ 100% . Microfluidization involves using high pressure (~250 MPa) to push a liquid through an interaction chamber with several micron-sized (~87 μm) channels generating high shear 6 −1 21 (>10 s ) . The liquid is cycled back through the interaction −1 21 chamber over several hours to reach ϕ ~ 9.3 g h . Wet-jet milling is a similar technique which uses a hydraulic mechanism Fig. 1 Schematic of the in-line shear mixing process. Graphite, and piston at high pressure (~250 MPa) to push liquid through 20 deionised water and SDC stabilisation agent are added to the perforated (~100 μm) discs . The process has reached ϕ ~23g reservoir. The shear rotor head pushes the material around the −1 20 h . However, in practice, the interaction chamber in these system while generating shear force, enabling the exfoliation of microchannel-based techniques can frequently become blocked graphite into graphene. by graphite. Unless addressed, this would decrease ϕ in a commercial environment. Moreover, microfluidization systems We use graphite flakes as a starting material for the inks. We mix are currently expensive (>£40,000) and can overheat easily (even −1 the flakes (100 mg ml ) with sodium deoxycholate (SDC, with active cooling) risking system damage. Processes such as −1 5mgml ) (Sigma-Aldrich) in deionised water (~1 L) before ultrasonication, high-shear mixing and electrochemical exfoliation adding to the in-line system. The temperature of the system provide cheaper alternatives for 2D material production however increases over time once material passes through the rotor head. −1 their low ϕ ~ 0.2–5.3 g h has made high-volume production of Therefore, an ice bath is used to keep the temperature at ∼30 °C. 2D materials difficult. Furthermore, ultrasonication and high-shear Without cooling, the deionised water will evaporate, increase the mixing have had challenges to scale as the hydrodynamic liquid viscosity, decrease the liquid flow rate, and potentially processes can damage the graphene basal plane after 2 h of 26 damage the motor of the in-line shear-mixer. We define one cycle processing . Therefore, there is a need for a graphene production as the complete passthrough of the starting liquid volume process which is reliable, produces pristine flakes free of basal (~1000 ml) through the rotor head. Each cycle takes approximately plane defects and at a high Y and ϕ. High-shear mixing could 10.8 s to complete at 8000 rpm (i.e. 1000 ml passes through the provide a route forward as it currently has minimal maintenance system in 10.8 s, see Methods). We process the graphite and SDC requirements and high ϕ. Shear-mixing involves using a rotor dispersion for multiple cycle sets (2000, 4000 and 6000 cycles). The (rotating blades) and stator (stationary screen) to generate shear 18 process yields litre-scale dispersions of graphene ink with a high c forces to exfoliate material . The shear-mixing process is typically −1 of 100 mg ml (measured on a microbalance, see Methods), Y ~ undertaken in an open beaker (<1 L) or industrial-sized (>300 L) −1 18 −1 100% and ϕ ~ 8.3 g h . container , and has demonstrated ϕ ~ 5.3 g h but with a low Y ~ 0.001%. Therefore, the graphene production process is still in Investigating flake lateral size, thickness and quality need of significant improvement to fulfil the potential industrial scale demand. Furthermore, the low electrical conductivities of the After depositing the graphene ink on a silicon/silicon oxide (Si/ −1 resultant films (σ ~ 400 S m ) need to be improved for practical SiO ) substrate, we use scanning electron microscopy (SEM) in applications . The engineering of conductive ink with a c >1mg Fig. 2 to determine the lateral size <L> distribution of the graphite −1 3 −1 −1 ml , σ >10 Sm , ϕ >5gh , defect-free and in a non-toxic starting material (Fig. 2a) and its change in <L> as a function of solvent is highly desirable. In this work, we adapt the shear mixing the number of processing cycles: 2000 (Fig. 2b), 4000 (Fig. 2c) and −1/2 process to recycle unexfoliated material to maximise ϕ,c and Y . 6000 (Fig. 2d). We define the lateral size as <L> = (xy) , where x While minimising Re <10 in our system reduces basal plane and y are the length and width of the flake . For each processing defects and maximises σ which enables the creation of applica- cycle, we measure 50 flakes. The average lateral size is 4.8 ± tions such as inkjet printed interconnects and Li-ion batteries. 1.72 μm, 2.69 ± 1.89 μm, 1.47 ± 1.14 μm and 0.65 ± 0.48 μm for the dispersions of graphite, 2000, 4000 and 6000 cycles respectively, indicating a decreasing <L > with processing (Fig. 2e). The size RESULTS distribution characterised by standard deviation (σ ) changes sd with processing cycles. The graphite’s σ before processing is In-line shear-mixing process sd 1.72 μm which decreases to 0.48 μm after 6000 cycles. It might be We used an in-line shear mixing system (Silverson) to produce possible to narrow the lateral size distribution further by graphene flake dispersions (Fig. 1). In-line shear mixing is an increasing the processing cycles. However, it might not be enclosed, cyclic process where the rotor head’s high-speed desirable as further processing will likely damage the basal plane generates suction and drives the dispersion upwards into the of the graphene (see Raman spectroscopy section). We use atomic rotor head. Centrifugal force pushes the graphite towards the force microscopy (AFM) to further characterise the flakes and rotor head’s periphery and between the rotor (i.e. rotating blades) determine their apparent thickness and confirm <L>. In Fig. 2f, and the stator (i.e. a metal screen). The graphite experiences AFM micrograph of a typical flake reveals thickness of 8 nm and hydrodynamic stress which exfoliates the layered material and <L> ~ 270 nm. We undertake AFM statistics in Fig. 2g and find the pushes it out and away from the rotor and stator. The system is <L>of30 flakes after 6000 cycles. The flakes have a log-normal self-pumping, and fresh material continually is drawn into the distribution which peaks at 386 nm and has an average <L> rotor head, which results in a cyclic flow of material through the ~ 492 nm, consistent with the SEM measurements. The flakes’ system. Using this method, unexfoliated graphite is recycled by apparent thickness is measured in Fig. 2h and shows a peak flake undergoing repeated hydraulic shearing. An ice bath or chiller can thickness of 6.7 nm, indicating that we have made few-layer be used on the rubber tubing to maintain the system graphene. temperature. Furthermore, unlike standard shear mixing, the We use Raman spectroscopy to identify the flakes’ quality as a liquid is entirely enclosed. function of the number of processing cycles. Figure 3a shows the npj 2D Materials and Applications (2022) 3 Published in partnership with FCT NOVA with the support of E-MRS 1234567890():,; T. Carey et al. a b 0 2000 4000 6000 Number of Cycles 10 9 f 14 -2 0 100 200 300 8 Distance (nm) 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 0 5 10 15 20 25 30 35 40 Lateral Size (μm) Thickness (nm) Fig. 2 Examination of flake lateral size and thickness. a SEM images of graphite. b 2000 cycles. c 4000 cycles. d 6000 cycles. e The lateral size distribution is plotted as a function of processing cycles, error is calculated as the standard deviation (n = 50). f An AFM micrograph displaying a typical profile of a 6000 cycles graphene flake with a thickness of 8 nm and lateral size of 270 nm. The scale bar is 230 nm. g Lateral size and h thickness of the 6000 cycles graphene ink obtained from AFM analyses. Fig. 3 Examination of flake quality. a Raman spectroscopy for graphite (black), 2000 (red), 4000 (blue) and 6000 (green) processing cycles. b The I(D)/I(G) ratio plotted as a function of the Disp(G) for 6000 processing cycles. c FWHM(G) plotted as a function of the I(D)/I(G) ratio. spectra of the graphene with 2000 cycles (red curve), 4000 cycles narrows to a single Lorentzian for single-layer graphene. For 2000 −1 (blue curve), 6000 cycles (green curve) and graphite (black curve). cycles, we observe a shoulder on the 2D peak (~ 2690 cm ) −1 In the graphene spectra, the G peak located at ~1581 cm , indicating the presence of multilayer graphene. However, after 29 −1 corresponds to the E phonon . The D peak at ~1350 cm is 4000 and 6000 cycles, the 2D peak is a single Lorentzian fit, 2g due to the breathing modes of six-atom rings and requires a indicating that even if the flakes are multilayers, they are defect for its activation . Typically in few-layer graphene electronically decoupled and behave as a collection of single dispersions, the D peak corresponds to flake edges rather than layers . In disordered carbon, the position of the G peak, Pos (G) 31,32 defects within the flakes’ basal plane . The 2D peak located at increases when the excitation wavelength, λ decreases from the −1 33 30,34 ~2700 cm is the second-order resonance of the D peak . In the IR to UV . Additionally, the dispersion of the G peak, Disp(G) = graphite sample (black curve), the 2D peak split into several ΔPos(G)/Δλ and the FWHM of the G peak, FWHM(G), increase with 35,36 components. As the number of layers decreases, the 2D peak disorder . Combining the intensity ratio of the D and G peaks, Published in partnership with FCT NOVA with the support of E-MRS npj 2D Materials and Applications (2022) 3 Thickness (nm) Number of Flakes Lateral Size (μm) Number of Flakes T. Carey et al. Fig. 4 Chemical, electrical and thermal analysis of graphene inks. a XPS C 1s spectra of the 6000 cycles graphene ink. b A plot of R as a function of the graphene and graphite film thickness for the 6000 cycles graphene ink with different post-processing conditions such as centrifugation and annealing. Error is calculated by standard deviation of mean (SDOM, n = 3). c A graph of the σ as a function of thickness for the 6000 cycles graphene ink with post-processing. d Thermogravimetric analysis is used to find thermograms of the graphene powder and SDC surfactant. The weight change is plotted as a function of temperature. I(D)/I(G), with FWHM(G) and Disp(G) can discriminate between mixing process (Re ~6 × 10 ), where ρ is the liquid density, ν is inline disorder localised at the edges and disorder in the basal plane of the liquid velocity, L is the characteristic length and η is the liquid the flakes. If the disorder is in the graphene basal plane, I(D)/I(G) viscosity. The reduced Re is attributed to the decreased L of inline will increase with FWHM(G) and Disp(G). In the former case, I(D)/I the in-line shear mixing system L ≃ 0.46L (Supplementary inline beaker (G) will be uncorrelated with FWHM(G) and Disp(G). In Fig. 3bwe Note 2) and suggests that reducing the turbulence of the system find that I(D)/I(G) is uncorrelated with Disp(G). Therefore, the D reduces the defects present in graphene flakes produced by shear peak originates from the flake edges rather than the basal plane. mixing. Therefore, in-line shear mixing could offer a route to −1 −1 Furthermore, the average Disp(G) ~ 0.019 cm nm , which is manufacture higher quality graphene material than is traditionally significantly lower than the Disp(G) for disordered carbons produced by shear mixing. −1 −1 34,37 (>0.1 cm nm ) . Therefore, the in-line shear mixing method produces pristine basal plane graphene flakes. In Fig. 3c, I(D)/I(G) is Investigating flake chemical, electrical and thermal properties plotted as a function of FWHM(G). We find the FWHM is 20 ± −1 2 We use X-ray photoelectron spectroscopy (XPS) analysis to 1cm for each sample, indicating that the sp grain size (L )~ investigate the oxygen and sodium content in the 6000 cycles 40 nm remains constant before and after exfoliation and that the 35,37 graphene ink. The XPS survey spectra (Supplementary Fig. 3a) cyclic process does not significantly damage the graphene . shows very high carbon content (92.5 ± 0.1 at.%) and low oxygen Since the D peak originates from the edge of the flakes and FWHM content (5.7 ± 0.1 at.%), attributed primarily to graphene flakes (G) remains constant in our samples, the I(D)/I(G) ratio can be used and sodium content (~1.8 at.%), which we attribute to residual to determine <L>, where I(D)/I(G) increases as a function of inverse 18,38 surfactant. The relatively low oxygen content and the position of <L> . In Fig. 3c, the I(D)/I(G) ratio increases with processing the main peak in O 1 s spectra at ~533 eV (Supplementary Fig. 3b) time from I(D)/I(G) ~ 0.1 for bulk graphite (black), I(D)/I(G) ~ 0.25 suggest C-O component dominance over C=O. Careful investiga- after 2000 cycles (red) and up to I(D)/I(G) ~ 0.5 after 4000 (blue) tion of the high-resolution XPS spectra (Fig. 4a) for carbon 1s and 6000 (green) processing cycles. We attribute the increase in I reveals a peak at ~285 eV attributed to the sp carbon component (D)/I(G) with processing time to a decrease in flake lateral size. We and indictive of the hexagonal lattice structure, also the satellite can estimate <L> using the formula I(D)/I(G) ≈ (I(D)/I(G)) + k/ graphite −1 peak (π–π*) contribution, corresponding to the broad peak at <L>, where k is the slope of I(D)/I(G) as a function of <L> 2 39 ~290 eV, is a characteristic feature of sp hybridisation of carbon . measured by transmission electron microscopy Using I(D)/I(G) ~ The π–π* transition occurs due to the delocalisation of the π- 0.5 for 4000 and 6000 cycles, average I(D)/I(G) ~ 0.11 for our electrons in the carbon aromatic ring . We observe that the π–π* graphite and a slope of k = 0.17 we can estimate that <L> ~ peak is more pronounced in the 6000 cycles graphene ink than 0.43 μm which is consistent with the SEM and AFM data .In conventional shear mixing in a beaker (Supplementary Fig. 3), contrast, a correlation between I(D)/I(G) and FWHM (G) is observed with a graphene ink made by conventional shear mixing in a suggesting that 6000 cycles graphene ink should have reduced beaker under similar exfoliation parameters (Supplementary Note basal plane defects. In general, the more pronounced π–π* 2 41 1) indicating that in-plane defects are present where L ~ 20 nm, satellite peak, the greater the degree of sp bonding . which will decrease graphene conductivity. We attribute the To examine the graphene inks’ electrical properties, we create introduction of in-plane defects to an increased Reynolds number films of the 6000 cycles graphene ink and graphite flakes by drop- −1 6 (Re = ρνL η ~ 1.3 × 10 ) compared to our in-line shear casting onto glass slides with a spacer to control the wet film beaker npj 2D Materials and Applications (2022) 3 Published in partnership with FCT NOVA with the support of E-MRS T. Carey et al. Fig. 5 Inkjet printed interconnects and electrodes. a Ink viscosity as a function of applied shear rate, demonstrating non-Newtonian shear thinning behaviour. b Pendant drop of the ink used to calculate the surface tension. c Literature review of other inkjet printable conductive inks comparing their σ with the ink c. d Jetting of the ink observed with a stroboscopic camera. e Optical image of the inkjet-printed transistor contacts (left, dark field) and interconnect (right, bright field). The scale bar is 100 μm (left) and 1 cm (right). thickness. The films are dried at ∼100 °C on a hotplate to remove air atmosphere. We plot the weight change as a function of excess water. The dry film thickness (t) depends on the c of the ink temperature, which shows that SDC significantly decomposes at and wet film thickness. Therefore, we vary t by increasing the temperatures >380 °C. The graphene powder has a carbon number of droplets cast (e.g. 5 to 50) from a syringe. We find the combustion temperature between 570 and 670 °C (maximum sheet resistance (R ) of each sample using a four-point probe mass decomposition rate ≈ 627 °C), which is associated with the (Jandel Probe) in three different locations to obtain an average. carbon decomposition temperature of few-layer graphene . We use a profilometer to determine t for each sample and measure three areas to obtain an average. In Fig. 4b, we find a R Inkjet printable interconnects −1 of ∼ 324 Ω□ for the graphite film at t ∼ 27 μm. Consequently, To make an inkjet printable ink requires the optimisation of c, −1 the conductivity (σ) can be estimated using σ = R t, equating to −1 surface tension, γ and η to ensure satellite droplet free ejection, σ ∼ 100 S m . Without any post-processing or centrifugation, we −1 and morphologically uniform (i.e. roughness minimisation) printed find that the 6000 cycles graphene ink has an R of ∼ 37 Ω□ at t 43,47 3 −1 films . We engineer the η of the graphene ink to ensure it is ∼ 24 μm, corresponding to σ ∼ 1.1 × 10 Sm , one order of within an optimal 1–10 mPa s range required for inkjet printing . magnitude higher than σ of graphite, indicating graphite’s We measure η with a parallel plate rotational rheometer (see exfoliation to few-layer graphene. Recently, Fernandes et. al has Methods). A known shear rate is applied, and the resultant torque determined through semi-automated AFM measurements that or shear stress is measured. The shear stress divided by the shear uncentrifuged graphene LPE dispersions could have a small rate calculates the liquid viscosity shown in Fig. 5a .We find number (∼5%) of graphite flakes, and the majority mass (∼90%) pseudoplastic behaviour with an infinite-rate η of ~ 1 mPa s within would be attributed to the thicker (>40 nm) material . Therefore, the optimal η range (~ 1–10 mPa) . The decrease in viscosity is we centrifuge the material at low speed (1k rpm, g ~ 125) to attributed to the reduction in the Brownian motion of the remove the small number of thicker flakes (>40 nm), and the film 3 −1 graphene flakes . In Fig. 5b, we determine the ink γ using the σ increases to ∼ 5× 10 Sm . We show the σ as a function of t in pendant drop method. The pendant drop method involves using a Fig. 4c. At t <4 μm, the σ is thickness dependant, and at t ∼ 4 μm, camera to visualise the shadow image of a droplet dispensed from the flakes form a percolative network, and the bulk σ is reached. a needle. The droplet’s radius of curvature is found from the Therefore, for electronic devices requiring high conductivity 3 −1 shadow images that can be used with the Young–Laplace >10 Sm a t of at least 4 μm should be used. The residual 49,50 equation to determine the liquid surface tension .We find a surfactant can degrade the electrical properties of a deposited −1 graphene ink γ of ~ 63 mN m , lower than the γ of water network of flakes . Therefore, we then anneal the samples at −1 ~72mNm , which we attribute to the presence of SDC 400 °C for 30 min to combust the SDC used in the graphene ink. 4 −1 surfactant. High c is needed to minimise the number of printing We find σ increases even further to ∼ 1.5 × 10 Sm and −1 layers required to reach a thick network of flakes, thus increasing demonstrates R ∼ 2.6 Ω□ (t ∼ 25 μm). The σ of the film is 20,44,45 comparable to state-of-the-art printable graphene inks .To the ϕ of a printing process to create devices. In contrast, high σ is investigate the removal of residual SDC surfactant after annealing, desirable to improve the performances of devices that require 51 52 we investigate the SDC decomposition temperature with thermo- high σ, such as electrodes for micro-supercapacitors , antennas gravimetric analysis (TGA) in Fig. 4d. The 6000 cycles graphene ink or transistor electrodes . Pristine inkjet printable graphene inks −1 is subjected to a freeze-drying process to obtain a graphene have commonly had low c (< 3 mg ml ), to help minimise nozzle powder (see Methods). The SDC and graphene powder are heated clogging. Identification of stabilising solvents by solubility −1 16 from ~ 25 °C up to 1000 °C ramped at a rate of ~ 10 °C min in an parameter-matching has proven to be a useful starting point Published in partnership with FCT NOVA with the support of E-MRS npj 2D Materials and Applications (2022) 3 T. Carey et al. for ink formulation. However, the resultant c can be low, < 0.2 mg (CV) data is measured for a graphene/SWCNT anode at a scanning −1 44 −1 st ml ; therefore, many (>50) printing passes are required to rate of 0.1 mV s , shown in Fig. 6a. During the 1 cycle, an build a percolating film of material. Other strategies have been irreversible cathodic peak at ~1 V is observed, which can be developed to improve ink’s c. For example, vacuum filtration can attributed to the formation of a solid electrolyte interface (SEI) and 51 53 be utilised with flocculation agents or a solvent exchange to the reaction of lithium ions with residual oxygen-containing −1 63–66 redisperse graphene at high c (> 10 mg ml ). However, the functional groups on the graphene flakes . In the range of −1 53 + 67 additional vacuum filtration step reduces ϕ <1 g h Alternately 0.01–0.25 V, we observe Li insertion into graphene flakes . In the solvent interface trapping has been used engineer high c (> second cycle, the cathodic peak at 1 V is not observed, indicating −1 100 mg ml ) ink by trapping graphene between a polar and non- the oxygen-containing functional groups on the graphene flakes polar solvents . GO powders also been used to directly disperse are completely reduced. After five cycles, the redox peaks −1 −1 material at a high (> 1 mg ml ) c but with low σ (< 1000 S m ) between 0.01 and 0.25 V display no apparent changes, illustrating 55,56 + due to the defective nature of the flakes . To increase the σ, reversible Li intercalation/deintercalation in the graphene layer. printed films are typically annealed at high temperatures Thus, the CV data confirms that the 6000 cycles graphene flakes 45,57 68 >300 °C . However, this process is incompatible with many store lithium effectively . textile (e.g. cotton) or polymer (e.g. polyethylene terephthalate, For cyclability measurement, the galvanostatic charge and discharge are performed initially with a 43-cycle of activation at PET) substrates which degrade at temperatures >100 °C . There- 3 −1 −1 −1 fore inkjet printable graphene inks with high σ (> 10 Sm ) and 30 mA g followed by 375 cycles at 100 mA g , as shown in Fig. −1 high c (> 10 mg ml ), which do not clog the inkjet nozzle have 6b. All data is normalised to the active mass of graphene (see −1 been desirable but difficult to achieve. The 6000 cycles graphene Methods). The first activation cycle (30 mA g ) specific capacity 3 −1 −1 −1 −1 ink achieves these requirements (σ >10 Sm , c ∼ 100 mg ml ), (C ) was ~646 mAh g for discharge and ~363 mAh g for sp however it is not compatible with most inkjet nozzles due to the charge, with an initial coulomb efficiency (CE) of ~56%. After the −1 flake <L >> 1 μm. Typically the flakes should be 1/50th of the activation cycles, the cells are tested for 375 cycles at 100 mA g . nozzle diameter (a = 21 μm) to minimise nozzle clogging (∼ The graphene/SWCNT anodes show a lithium storage capability of 44 −1 −1 420 nm) . Therefore we centrifuge the 6000 cycles graphene ink ~228 mAh g and ~243 mAh g for discharge and charge, with a −1 at low speed (1k rpm, g ~ 125) to create a graphene ink Gr CE of ~100% for the first cycle at ~100 mA g (Fig. 6b, inset). The inkjet −1 with c ~ 5.9 mg ml ,(Y ∼ 6%) which minimises the probability of charging and discharging capacity changed in the first 4 cycles 3 −1 −1 nozzle clogging and increases σ ∼ 5×10 Sm without before stabilising at a capacity of ~233 mAh g for both annealing. We compare the σ and c of Gr to others (Fig. 5c) discharge and charge with a CE of ~100% over 365 cycles. inkjet and find that both c and σ are higher than most inkjet printable The rate performance of our anodes is tested with voltage inks in the literature, where an optimal ink would be in the top profiles shown in Fig. 6c and rate dependent cycling data in Fig. −1 right corner of the graph, assuming that the ink would not cause 6d. The anodes exhibited an initial performance of 641 mAh g −1 −1 nozzle clogging issues or satellite droplets due to its concentra- for discharge and 376 mAh g for first cycle charge at 10 mA g 44,45,51,53,55–59 tion . We use a drop-on-demand inkjet printer to with an initial CE of ~58.6% that approaches ~99% for subsequent print the ink. We image the jetting of the ink with a stroboscopic cycles. The specific charge capacity decreased with increasing −1 camera (Fig. 5d), the black dot at 0 μm represents the inkjet specific currents whereby at 180 mA g , a specific charge −1 nozzle. The ink is observed to eject from the nozzle without capacity of ~107 mAh g was achieved. In contrast, the specific −1 satellite droplets, which is important to achieve well resolved discharge capacities fell off to 374, 287, 197 and 116 mAh g for printed films with resolution <100 μm. In Fig. 5e, we deposit the each rate’s first cycle, respectively. The low-rate capacity (20 mAh −1 −1 g ) was ~370 mAh g , very close to graphite’s theoretical ink onto a PET substrate coated with aluminium oxide nanopar- −1 69 ticles (Novele, Novacentrix) which have a low roughness (~18 nm). capacity (372 mAh g ) . The fact that we can reach close to We print a 2 cm layout of interconnect array. Through optical the theoretical value attests to the high quality of graphene flakes inspection, we observe morphological uniformity of the print and produced using in-line method while facilitated by the presence 60,62 evenly distributed flakes in the form of a film. We also inkjet print of SWCNT . transistor contacts Fig. 5e demonstrating a ~ 50 μm gap between two graphene electrodes. The resolution of ~ 50 μm is as expected Biocompatibility with human cells 47,53 and comparable to previous works on inkjet printing . These Understanding the biocompatibility of the graphene flakes with −1 results indicate that the high concentration (5.9 mg ml ) human cells is essential to utilise the material for applications that printable graphene inks can be produced using in-line shear come into contact with humans, such as electronic textiles, mixing and can be inkjet printed at high resolution (~ 50 μm) in 9,70,71 bioscaffolds or drug delivery . To test if there is any acute desirable patterns for flexible electronics. toxicity, we performed viability assays using increasing graphene flake concentrations (6000 cycles graphene ink) in Human Li-ion battery anode umbilical vein endothelial cell (HUVECs) and standard human Graphene is an attractive material to replace graphite in lithium- cancer cell line SW948. In Fig. 7a, the upper left quadrant ion battery anodes since Li-ion battery anodes need high in-plane represents acute toxicity by induction of cell necrosis stained by 3 −1 conductivity (>10 Sm ) to improve rate performance, and thus, 7-AAD. In contrast, the lower right quadrant shows cells under- 60,61 charging speed . Here, we demonstrate the potential of the going programmed cell death (Annexin V staining), i.e. apoptosis. graphene flakes produced using in-line shear mixing method as Cells stained by both dyes are in a transitional phase between Li-ion anodes. Rather than using a polymeric binder, we add a apoptosis and necrosis and fall in the right upper quadrant. small amount (15%) of single-walled carbon nanotubes (SWCNT) Repeated measurements show no acute toxicity found when −1 to the electrode. Using SWCNT is known to simultaneously using the highest concentration of graphene (1 mg ml )in48h maximise the electrode’s mechanical and electrical properties and cell culture treatments (N = 2 for SW948, N = 3 for HUVECs). This tends to yield electrodes that perform near their theoretical was comparable to a control, where no graphene was added. capability . The resultant graphene/SWCNT anodes have an To confirm our cellular apoptotic measurements, we used a cell −2 active mass loading of ~1 mg cm combined with a SWCNT counting kit-8 (CCK8) assay based on a tetrazolium salt reduced by −2 loading of ~0.18 mg cm and thickness of ~20 μm (see Methods). viable cells to a soluble formazan dye. The resulting dye absorbs To evaluate the electrochemical Li storage mechanism of light at 450 nm, which correlates with cellular viability (Fig. 7b). graphene produced using in-line method, cyclic voltammetry After 48 h of graphene treatment, the cell cultures did not show npj 2D Materials and Applications (2022) 3 Published in partnership with FCT NOVA with the support of E-MRS T. Carey et al. Fig. 6 Li-ion battery performance: performance of graphene flakes/SWCNT (85:15 wt%) as lithium-ion battery anodes. a Cyclic −1 voltammograms at a scan rate of 0.1 mV s for various cycles. b Cycling capacity versus cycle number for composite anodes based on −1 −1 graphene flakes cycled initially at 30 mA g and then at 100 mA g . Coulombic efficiency (CE) is shown in the inset. c Galvanostatic charge- discharge curves measured at different charging currents. d Specific capacity as a function of charging current. In all cases, C is normalised Sp to the graphene mass. 4 −1 any dose-dependent toxicity up to the highest concentration and σ ∼ 1.5 × 10 Sm after centrifugation and annealing, which −1 tested at 1 mg ml for SW948 and no significant toxicity up to is state of the art for graphene inks. We achieve high σ by −1 500 μgml for HUVECs. We compared our measurements against reducing Re < 10 which minimises basal plane topological a control of deionised water and SDC, which did not show any defects. Therefore, in-line shear mixing could offer a route to −1 dose-dependent toxicity up to 38.46 μgml of SDC (relative to manufacture higher quality graphene or potentially other 2D −1 the amounts contained in 1 mg ml graphene suspension, materials, than is traditionally produced by shear mixing. We Supplementary Fig. 4). To the best of our knowledge, this is one demonstrate the graphene inks’ versatility by making anode of the highest c that has demonstrated LPE graphene’s electrodes with SWCNTs for Li-ion energy storage, achieving a biocompatibility with human cells to date, which is essential for −1 low-rate capacity of ~370 mAh g . We show that highly applications in printed electronics that use c of at least 1 mg −1 concentrated (5.9 mg ml ) and inkjet printable inks can be −1 43 ml We attribute the biocompatibility on the ink to the lack of manufactured for use in inkjet-printed conductive interconnects oxygen functional groups (~5.7%, determined by XPS) as with a resolution ∼50 μm. As a final demonstration, we examine graphene oxide dispersions have previously demonstrated toxicity the toxicity of the graphene flakes with human colon cells and −1 72,73 to human cells as low as 1–10 µg ml attributed to its high HUVEC cells. We observe no dose-dependent toxicity up to 48 h oxygen content (~47%) that induces the formation of reactive 74 indicating the graphene flakes’ biocompatibility at high concen- oxygen species that can be toxic to cells . To detect any cell −1 trations ∼1mgml , which is essential to utilise graphene in deformities due to graphene treatment, we imaged the cells using textile electronics, composites and printed interconnects that laser scanning confocal microscopy with propidium iodide to stain could involve human contact with high concentrations of whole fixed cells with the nuclear Hoechst dye (Fig. 7c). graphene. Representative images show cells with similar morphology to control at high graphene treatments (Fig. 7c). METHODS DISCUSSION Inkjet printing We demonstrated an enclosed, cyclic exfoliation of graphene with We used a Fujifilm Dimatix DMP-2800 inkjet printer with a 21 µm diameter −1 in-line shear-mixing, which has a higher ϕ ∼ 8.3 g h than nozzle (Fujifilm DMC-11610) for the printing of interconnects. The inkjet previous graphene production techniques such as ultrasonication printing platen temperature was kept at ambient conditions (20 °C). An and ball milling. The Y ∼ 100% is several orders of magnitude w inter-drop spacing of 25 μm was used for the interconnects and an inter- increase compared to previous work on shear mixing , and can drop spacing of 40 μm for the transistor electrodes. We print 100 layers of be used to create dispersions with extremely high concentration c ink to construct the interconnect pattern and 10 layers of ink for the −1 3 −1 ∼ 100 mg ml , achieving σ ∼ 10 Sm without post-processing transistor electrodes. We used a maximum jetting frequency of 2 kHz. Published in partnership with FCT NOVA with the support of E-MRS npj 2D Materials and Applications (2022) 3 T. Carey et al. Fig. 7 Human cell viability study: biocompatibility with human umbilical vein endothelial cell (HUVECs) and SW948 human cells. a Muse ™ Apoptotic profile using Annexin V for apoptosis staining and 7-Aminoactinomycin D (7-AAD) fluorescence intensity for necrosis is shown in a flow scatter using same cut offs for all measurements (N = 3 for HUVECs and N = 2 for SW948). b CCK8 viability normalised to 0 μg −1 −1 ml control over increasing concentrations of graphene (0–1000 μgml )(N = 9 from three independent experiments for HUVECs in blue and N = 6, from two independent experiments for SW948 in red). c Laser scanning confocal microscopy with the addition of propidium iodide (PI), used here to image whole cells, and image overlay with Hoechst cell nuclear staining with bright field transmittance of graphene pre- treated fixed cells at ×63 magnification, scale bar 50 μm for HUVECs and 25 μm for SW948 cells. X-ray diffraction The XPS measurement data were analysed using the CasaXPS software (Casa Software Ltd). XRD patterns were recorded with a D2 Phaser (Bruker) powder diffractometer equipped with LynxEye detector using Cu Kα radiation (λ = 1.54 Å) at a 2θ scan step of 0.03° and 1 s dwell time. XRD samples were prepared by drying the graphene dispersions under room condition and Profilometry transferring the powder obtained to the Bruker sample holder. We used a stylus profilometer (Bruker DektakXT) to measure the thickness of the deposited films on a quartz substrate. A stylus force of 3 mg was used with a tip of radius 12.5 μm over the sample. X-ray photoelectron spectroscopy Samples were prepared by depositing graphene dispersions on Si/SiO Thermogravimetric analysis substrates mounted on aluminium stubs using conductive silver paint. XPS measurements were conducted using a K-Alpha™ X-ray Photoelectron To prepare the graphene sample, we take 30 ml of the 6000 cycles Spectrometer (Thermo Scientific). High-resolution C1s, O 1s, and Na 1s graphene ink and froze it at −18 °C. The frozen sample was then placed spectra were collected at 40 eV pass energy with a 0.05 eV step size. inside a freeze dryer (Telstar LyoQuest) to remove ice crystal by Spectra were acquired using a monochromatic Al Kα X-ray (hν = 1486.6 eV) sublimation under vacuum overnight to make a graphene powder. We with an X-ray beam spot size of 400 μm, average of 3 points per sample then undertake thermal analysis of the graphene powder and SDC were taken. Any energy shifts were calibrated using gold foil attached to surfactant using a thermogravimetric analyser (TA Instruments Q50) in air −1 the films and measuring the Au 4f binding energy at 84 eV as a standard. with a ramping temperature of 10 °C min from ~25 °C up to 1000 °C. npj 2D Materials and Applications (2022) 3 Published in partnership with FCT NOVA with the support of E-MRS T. Carey et al. −2 Raman spectroscopy furnace to remove CMC. The mass loading of graphene was ~1 mg cm , with 15 wt.% SWCNT. The anode thickness was ~20 μm. We acquired the Raman spectra at 514.5 nm with a Reinshaw InVia Half-cell coin cells were assembled with Li-metal discs (diameter: 14 mm, equipped with a 50× objective. We kept the power on the samples below MTI Corp.) as counter/reference electrodes. The electrolyte used was 1.2 M ~1 mW to minimise thermal damage from the laser. About 20 spectra are LiPF in a mixture of ethylene carbonate/ethyl methyl carbonate (EC/EMC, taken for each map. All measurements are acquired after drop-casting 1:1 in v/v, BASF) with 10 wt% fluoroethylene carbonate (FEC). A Celgard sample solution on Si/SiO wafer. The resolution of our spectrometer is −1 2320 (thickness 20 μm) was used as the separator. The cells were ~1 cm and our spot size was ~2 μm. For measurement of the Disp(G) we assembled in a glovebox filled with highly pure argon gas (UNIlab Pro, acquired additional Raman spectra at 457 nm and 633 nm, respectively. Mbraun, with O and H O levels <0.1 ppm). 2 2 Shear-mixing and ink preparation Cyclic voltammetry We used a Silverson Model L5M, 250 W single phase motor high-shear Cyclic voltammetry of the cells was carried out using a laboratory mixer. An interchangeable in-line mixing assembly was used for galvanostat–potentiostat between 0.01 and 1.2 V vs. Li /Li at a scan rate in-line processing. It was equipped with a 4-blade rotor placed inside a −1 of 0.1 mV s for 10 cycles. For cycling capability tests, the cells were stator with a rotor-stator gap (ΔR) of 300 μm and a rotor diameter (D) of −1 performed at 100 mA g for 375 cycles after 3 cycles activation at 30 mA 31.1 mm. We use graphite flakes (Imerys Graphite) as a starting material for −1 −1 g . For rate capability measurement, cells were running at different the inks. We mix the flakes (100 mg ml ) with sodium deoxycholate (SDC, −1 −1 current rates of 10, 20, 45, 90, 180 mA g and then went back to 10 mA 5mgml ) (Sigma-Aldrich) in deionised water at 8000 rpm. The through- −1 −1 −1 g . There are only 4 cycles at 10 mA g , then there are 5 cycles for the put was calculated as 1000 ml of 100 mg ml ink (i.e. 100 g) processed for next steps at different current rates. 4000 cycles (i.e. 12 h). The volume of the ink reservoir can be increased −1 4 −1 (>1 L) if further scale is required. The shear rate γ ≈ πND ΔR ≈ 4×10 s , where N is the revolutions per second of the shear mixer (8000 rpm = Cell viability study −1 18 133 s ) . Optical inspection of the ink reveals a homogeneous stable SW948 cells were seeded in 96 well plates at 15k cells per well in dispersion of graphene flakes. Dulbecco’s Modified Eagle Medium (DMEM) media (5 mM glucose), with added 10% Foetal Bovine Serum (FBS), 5mM L-Glutamine (L-Glu), 5 mM Scanning electron microscopy penicillin (PC) and 5 mM streptomycin (SP), and left overnight in a humidified CO infused incubator at 37 °C. Graphene ink was added to the The ink was diluted 1:999 and drop-cast on 1 cm Si/SiO wafer. We 2 −1 −1 wells to reach c ranging between 7.8 μgml to 1000 μgml . After 48 h, acquired scanning electron microscopy images with a Magellan 400 L SEM. the cells were gently washed three times with PBS, where cells in one of We used an accelerating voltage of 5 kV and gun current of 25pA during the plates were resuspended in cell media (i.e. DMEM/glucose/FBS/L-Glu/ operation and obtained the images in secondary electron detection mode PC/SP), containing 10% of the viability reagent cell counting kit 8 (CCK-8, using an immersion lens and through-lens-detector. We measured <L>in Dojindo Molecular Technologies, Inc.). According to the assay protocol, one ImageJ. plate was incubated at 37 °C for 2 h before measuring absorbance at 450 nm using Spectramax Paradigm plate reader (Molecular Devices, Atomic force microscopy Sunnyvale, CA 95134). After measuring absorbance, these wells were The ink was diluted 1:999 and drop-cast onto clean 1 cm Si/SiO wafers. 2 rewashed with phosphate-buffered saline (PS) at 0.01 M in deionised We used a Bruker Dimension Icon in peakforce mode to scan a 20 μm× water, before another absorbance measurement was conducted at 450 nm 20 μm area of the wafer. We then find the <L> and thickness of 30 flakes to determine any background absorbance resulting from cellular by manual counting in NanoScope Analysis. internalised graphene, which was subsequently subtracted. An identical 96 well cell plate was fixed with 4% w/v paraformaldehyde (PFA) solution −1 containing 15 μgml Hoechst for 30 min, washed once with PBS, before Surface tension Hoechst fluorescence was measured at 360nm /500nm on a Excitation Emission We used a contact angle goniometer to measure the graphene ink’s Spectramax Paradigm plate reader (Molecular Devices, Sunnyvale, CA surface tension through the pendant drop method. A suspended droplet 95134). from a needle forms a pendant’s shape resulting from the downward force due to gravity and the upward force due to surface tension. We used drop- Cell microscopy shape analysis to calculate the surface tension from a shadow image of the droplet. Cells were imaged using Leica SP8 confocal microscope in epifluorescence mode for full well overviews and laser scanning confocal mode for high- resolution imaging. Rheometry We used a parallel-plate rotational rheometer (DHR Rheometer, TA instruments) to find the viscosity as a function of the shear rate. We use Muse™ flow cytometry viability and apoptosis assays a plate-to-plate distance of 500 μm and we loaded the ink between the SW948 cells were seeded in sterile 24 well plates at 45k cells per well in cell plates by capillary action at a gap of 550 μm. We find the infinite-rate culture media (complete DMEM), and left to attach in a humidified CO viscosity for the ink. infused incubator at 37 °C overnight. The next day, graphene ink was added to the cell growth media in each well to make a final c of 125, 250, −1 500, 1000 µg ml before being left for 48 h in the incubator as described Microbalance above. Following 48-h treatment with graphene inks, cells were washed To find the c of the ink we use a microbalance (Sartorius ME5), we placed with pre-heated phosphate-buffered saline (PBS), and detached from 1 ml of ink in a metal container and weighed the graphene flakes after culture wells using disodium ethylenediaminetetraacetic acid (EDTA) and evaporation of the solvent. 0.25% Trypsin (w/v%) (Thermo Fisher 25200056). 20 µL of cell suspension was added to 380 µL of count and viability Muse reagent and left for 5 min Electrochemical characterization before measuring 1000 cellular event using Muse™ cell analyser. For apoptosis 100 µL of cell suspension was added to 100 µL of Muse Annexin The 6000 cycles graphene ink was mixed with a SWCNT aqueous V & Dead Cell Reagent and left for 20 mins at room temperature before dispersion (0.4 wt% SWCNT in water, ~0.6 wt% carboxymethyl cellulose measuring 5000 cellular event using Muse™ cell analyser. (CMC) as a surfactant stabiliser, Tuball, OCSiAl), which increases conductivity and functions as a binder. A mortar and pestle ground the mixed solution to obtain a uniform slurry, and then the slurry was cast onto DATA AVAILABILITY copper foil using a doctor blade. The slurry cast anodes were slowly dried at 40 °C overnight in an oven to remove residual water. The films produced The authors declare that the data supporting the findings of this study are available were cut to the desired dimensions (area = 1.131 cm ) for electrochemical within the paper and its supplementary information files. Data are also available from testing. 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Biomimetic carbon fiber systems engineering: a modular design ADDITIONAL INFORMATION strategy to generate biofunctional composites from graphene and carbon Supplementary information The online version contains supplementary material nanofibers. ACS Appl. Mater. Interfaces 11, 5325–5335 (2019). available at https://doi.org/10.1038/s41699-021-00279-0. 71. Liu, J., Cui, L. & Losic, D. Graphene and graphene oxide as new nanocarriers for drug delivery applications. Acta Biomater. 9, 9243–9257 (2013). Correspondence and requests for materials should be addressed to Tian Carey or 72. Vranic, S. et al. Live imaging of label-free graphene oxide reveals critical factors Jonathan N. Coleman. causing oxidative-stress-mediated cellular responses. ACS Nano 12, 1373–1389 (2018). Reprints and permission information is available at http://www.nature.com/ 73. Hoyle, C. et al. Small, thin graphene oxide is anti-inflammatory activating nuclear reprints factor erythroid 2-related factor 2 via metabolic reprogramming. ACS Nano 12, 11949–11962 (2018). Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims 74. Das, S. et al. Oxygenated functional group density on graphene oxide: its effect in published maps and institutional affiliations. on cell toxicity. Part. Part. Syst. Charact. 30, 148–157 (2013). ACKNOWLEDGEMENTS Open Access This article is licensed under a Creative Commons XPS data were taken at NEXUS, Newcastle University, facility that was funded by UK Attribution 4.0 International License, which permits use, sharing, Engineering and Physical Sciences Research Council (EPSRC), grant number NS/ adaptation, distribution and reproduction in any medium or format, as long as you give A000015/1. We have also received support from the Science Foundation Ireland (SFI) appropriate credit to the original author(s) and the source, provide a link to the Creative funded centre AMBER (SFI/12/RC/2278_P2) and availed of the facilities of the SFI- Commons license, and indicate if changes were made. The images or other third party funded AML and ARM labs. J.M.K acknowledges funding from Smart Quantum Dot material in this article are included in the article’s Creative Commons license, unless Lighting (EPSRC, EP/P027628/1). T.C. acknowledges funding by a Marie Skłodowska- indicated otherwise in a credit line to the material. If material is not included in the Curie Action “MOVE” (Grant Number 101030735). article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons. AUTHOR CONTRIBUTIONS org/licenses/by/4.0/. T.C. conceived the experiments. T.C. and J.C. directed the research and designed experiments. T.C. manufactured the inks and undertook Raman Spectroscopy with © The Author(s) 2022 assistance from AD.A. and J.M.K. T.C. undertook Inkjet printing, Rheology, Surface Published in partnership with FCT NOVA with the support of E-MRS npj 2D Materials and Applications (2022) 3 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png npj 2D Materials and Applications Springer Journals

Cyclic production of biocompatible few-layer graphene ink with in-line shear-mixing for inkjet-printed electrodes and Li-ion energy storage

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www.nature.com/npj2dmaterials ARTICLE OPEN Cyclic production of biocompatible few-layer graphene ink with in-line shear-mixing for inkjet-printed electrodes and Li-ion energy storage 1,2 3 2 4 1 2 4 Tian Carey , Abdelnour Alhourani , Ruiyuan Tian , Shayan Seyedin , Adrees Arbab , Jack Maughan , Lidija Šiller , 2 2 2 2 1 3 Dominik Horvath , Adam Kelly , Harneet Kaur , Eoin Caffrey , Jong M. Kim , Hanne R. Hagland and Jonathan N. Coleman The scalable production of two-dimensional (2D) materials is needed to accelerate their adoption to industry. In this work, we present a low-cost in-line and enclosed process of exfoliation based on high-shear mixing to create aqueous dispersions of few- −1 layer graphene, on a large scale with a Y ~ 100% yield by weight and throughput of ϕ ~ 8.3 g h . The in-line process minimises basal plane defects compared to traditional beaker-based shear mixing which we attribute to a reduced Reynolds number, Re ~ 5 4 −1 10 . We demonstrate highly conductive graphene material with conductivities as high as σ ∼ 1.5 × 10 Sm leading to sheet- −1 resistances as low as R ∼ 2.6 Ω□ (t ∼ 25 μm). The process is ideal for formulating non-toxic, biocompatible and highly −1 concentrated (c ∼ 100 mg ml ) inks. We utilise the graphene inks for inkjet printable conductive interconnects and lithium-ion −1 battery anode composites that demonstrate a low-rate lithium storage capability of 370 mAh g , close to the theoretical capacity of graphite. Finally, we demonstrate the biocompatibility of the graphene inks with human colon cells and human umbilical vein −1 endothelial cells at high c ∼ 1mgml facilitating a route for the use of the graphene inks in applications that require biocompatibility at high c such as electronic textiles. npj 2D Materials and Applications (2022) 6:3 ; https://doi.org/10.1038/s41699-021-00279-0 INTRODUCTION advantages when applied, such as flexibility, biocompatibility, environmental stability or weight reduction. The Hummers Graphene and other 2D materials are expected to find major method emerged as one of the first methods to produce commercial applications in the coming years . By utilising the graphene in the form of graphene oxide (GO) flakes . However, unique electrical, optical, mechanical, chemical and thermal the GO flakes differ from pristine graphene, containing a properties of 2D materials, additional functionality or improved population of functional groups that disrupt the sp structure of performance can be added to many applications. Graphene could graphite . Liquid phase exfoliation (LPE) is established as the be useful in over 40 major application areas such as composites, primary method to produce dispersions of pristine graphene energy storage, thermal management, sensors and coatings . For flakes at scale and low cost . The most common techniques to example, graphene can be used as a barrier material for anti- 16 17 3 4 undertake exfoliation in liquid are ultrasonication , ball milling , corrosion , an additive for mechanical reinforcement in polymers , 18 19 20 shear-mixing , electrochemical exfoliation , wet-jet milling and or as a conductive material in sensors . Many of these applications microfluidization . To evaluate each technique’s effectiveness, will require few-layer graphene flakes (<10 layers) in large multi- figures-of-merit (FOM) are frequently used to characterise the tonne quantities if successfully commercialised . For example, if resulting dispersions. The yield by weight (Y ) is the ratio between graphene flakes are used at low loading (~1%) in the 300 million w the weight of the final graphene material and the starting graphite tonnes per year global plastic industry , or used to replace 6 7 flakes , while the throughput (ϕ)is defined as the mass of graphite (~10 tonnes per year) in Li-ion batteries for the electric graphene obtained per hour. In the literature, Y and ϕ are vehicle market it would create a demand of >1 kilotonne, far w commonly used as FOM to assess the production process. exceeding the global graphene supply . Few-layer graphene flakes Electrochemical exfoliation involves intercalating a 2D material could also be utilised as a cost-reduction replacement material for such as graphite with an ionic species that expands and exfoliates metal components in applications such as interconnects or 19,23,24 the bulk material into flakes . The process has high Y ~ electrodes, particularly where form factor is important such as 9 −1 10–75% with ϕ ~ 0.3 g h when post-processing (e.g. material electronic textiles . 19,23,24 washing) is considered . Ultrasonication is the most widely Currently, printable metal inks (e.g. silver and gold) are studied technique and typically involves using a bath sonicator , commonly used as interconnect or electrode materials and are which has been used to exfoliate graphene in toxic solvents (e.g. made of precious metals costing ~£1000 per litre on average and 10 11 12 16 n-methyl-2-pyrrolidone, NMP) and non-toxic (e.g. water) sol- can have oxidation issues , toxicity or nanoparticle migration , 25 −1 vents . However, ink concentrations, c up to ~ 1 mg ml in <24 degrading device performance. Graphene inks could be poten- 22 −1 tially produced for as little as ~£20 per litre once scaled due to h equating to ϕ ~1gh and Y ~3–5% has limited carbon’s elemental abundance while retaining several functional ultrasonication to lab-scale studies. Ball milling involves mixing 1 2 Cambridge Graphene Center, Department of Engineering, University of Cambridge, Cambridge, UK. CRANN and AMBER Research Centres, Trinity College Dublin, Dublin 2, 3 4 Ireland. Department of Chemistry, Bioscience and Environmental Engineering, University of Stavanger, Stavanger, Norway. School of Engineering, Newcastle University, Newcastle upon Tyne, UK. email: tian.carey@cantab.net; colemaj@tcd.ie Published in partnership with FCT NOVA with the support of E-MRS 1234567890():,; T. Carey et al. graphite powder, solvent and zirconia/metal balls together in a rotatory mill. The friction and shear forces with the balls enable 17 −1 the exfoliation of graphite . The ϕ of the process is ~ 0.2 g h with a Y ~ 12% . Microchannel-based techniques such as microfluidization and wet-jet milling have also been used in cyclic processes to exfoliate graphene nanoplatelets and 2D materials 20,21 with Y ~ 100% . Microfluidization involves using high pressure (~250 MPa) to push a liquid through an interaction chamber with several micron-sized (~87 μm) channels generating high shear 6 −1 21 (>10 s ) . The liquid is cycled back through the interaction −1 21 chamber over several hours to reach ϕ ~ 9.3 g h . Wet-jet milling is a similar technique which uses a hydraulic mechanism Fig. 1 Schematic of the in-line shear mixing process. Graphite, and piston at high pressure (~250 MPa) to push liquid through 20 deionised water and SDC stabilisation agent are added to the perforated (~100 μm) discs . The process has reached ϕ ~23g reservoir. The shear rotor head pushes the material around the −1 20 h . However, in practice, the interaction chamber in these system while generating shear force, enabling the exfoliation of microchannel-based techniques can frequently become blocked graphite into graphene. by graphite. Unless addressed, this would decrease ϕ in a commercial environment. Moreover, microfluidization systems We use graphite flakes as a starting material for the inks. We mix are currently expensive (>£40,000) and can overheat easily (even −1 the flakes (100 mg ml ) with sodium deoxycholate (SDC, with active cooling) risking system damage. Processes such as −1 5mgml ) (Sigma-Aldrich) in deionised water (~1 L) before ultrasonication, high-shear mixing and electrochemical exfoliation adding to the in-line system. The temperature of the system provide cheaper alternatives for 2D material production however increases over time once material passes through the rotor head. −1 their low ϕ ~ 0.2–5.3 g h has made high-volume production of Therefore, an ice bath is used to keep the temperature at ∼30 °C. 2D materials difficult. Furthermore, ultrasonication and high-shear Without cooling, the deionised water will evaporate, increase the mixing have had challenges to scale as the hydrodynamic liquid viscosity, decrease the liquid flow rate, and potentially processes can damage the graphene basal plane after 2 h of 26 damage the motor of the in-line shear-mixer. We define one cycle processing . Therefore, there is a need for a graphene production as the complete passthrough of the starting liquid volume process which is reliable, produces pristine flakes free of basal (~1000 ml) through the rotor head. Each cycle takes approximately plane defects and at a high Y and ϕ. High-shear mixing could 10.8 s to complete at 8000 rpm (i.e. 1000 ml passes through the provide a route forward as it currently has minimal maintenance system in 10.8 s, see Methods). We process the graphite and SDC requirements and high ϕ. Shear-mixing involves using a rotor dispersion for multiple cycle sets (2000, 4000 and 6000 cycles). The (rotating blades) and stator (stationary screen) to generate shear 18 process yields litre-scale dispersions of graphene ink with a high c forces to exfoliate material . The shear-mixing process is typically −1 of 100 mg ml (measured on a microbalance, see Methods), Y ~ undertaken in an open beaker (<1 L) or industrial-sized (>300 L) −1 18 −1 100% and ϕ ~ 8.3 g h . container , and has demonstrated ϕ ~ 5.3 g h but with a low Y ~ 0.001%. Therefore, the graphene production process is still in Investigating flake lateral size, thickness and quality need of significant improvement to fulfil the potential industrial scale demand. Furthermore, the low electrical conductivities of the After depositing the graphene ink on a silicon/silicon oxide (Si/ −1 resultant films (σ ~ 400 S m ) need to be improved for practical SiO ) substrate, we use scanning electron microscopy (SEM) in applications . The engineering of conductive ink with a c >1mg Fig. 2 to determine the lateral size <L> distribution of the graphite −1 3 −1 −1 ml , σ >10 Sm , ϕ >5gh , defect-free and in a non-toxic starting material (Fig. 2a) and its change in <L> as a function of solvent is highly desirable. In this work, we adapt the shear mixing the number of processing cycles: 2000 (Fig. 2b), 4000 (Fig. 2c) and −1/2 process to recycle unexfoliated material to maximise ϕ,c and Y . 6000 (Fig. 2d). We define the lateral size as <L> = (xy) , where x While minimising Re <10 in our system reduces basal plane and y are the length and width of the flake . For each processing defects and maximises σ which enables the creation of applica- cycle, we measure 50 flakes. The average lateral size is 4.8 ± tions such as inkjet printed interconnects and Li-ion batteries. 1.72 μm, 2.69 ± 1.89 μm, 1.47 ± 1.14 μm and 0.65 ± 0.48 μm for the dispersions of graphite, 2000, 4000 and 6000 cycles respectively, indicating a decreasing <L > with processing (Fig. 2e). The size RESULTS distribution characterised by standard deviation (σ ) changes sd with processing cycles. The graphite’s σ before processing is In-line shear-mixing process sd 1.72 μm which decreases to 0.48 μm after 6000 cycles. It might be We used an in-line shear mixing system (Silverson) to produce possible to narrow the lateral size distribution further by graphene flake dispersions (Fig. 1). In-line shear mixing is an increasing the processing cycles. However, it might not be enclosed, cyclic process where the rotor head’s high-speed desirable as further processing will likely damage the basal plane generates suction and drives the dispersion upwards into the of the graphene (see Raman spectroscopy section). We use atomic rotor head. Centrifugal force pushes the graphite towards the force microscopy (AFM) to further characterise the flakes and rotor head’s periphery and between the rotor (i.e. rotating blades) determine their apparent thickness and confirm <L>. In Fig. 2f, and the stator (i.e. a metal screen). The graphite experiences AFM micrograph of a typical flake reveals thickness of 8 nm and hydrodynamic stress which exfoliates the layered material and <L> ~ 270 nm. We undertake AFM statistics in Fig. 2g and find the pushes it out and away from the rotor and stator. The system is <L>of30 flakes after 6000 cycles. The flakes have a log-normal self-pumping, and fresh material continually is drawn into the distribution which peaks at 386 nm and has an average <L> rotor head, which results in a cyclic flow of material through the ~ 492 nm, consistent with the SEM measurements. The flakes’ system. Using this method, unexfoliated graphite is recycled by apparent thickness is measured in Fig. 2h and shows a peak flake undergoing repeated hydraulic shearing. An ice bath or chiller can thickness of 6.7 nm, indicating that we have made few-layer be used on the rubber tubing to maintain the system graphene. temperature. Furthermore, unlike standard shear mixing, the We use Raman spectroscopy to identify the flakes’ quality as a liquid is entirely enclosed. function of the number of processing cycles. Figure 3a shows the npj 2D Materials and Applications (2022) 3 Published in partnership with FCT NOVA with the support of E-MRS 1234567890():,; T. Carey et al. a b 0 2000 4000 6000 Number of Cycles 10 9 f 14 -2 0 100 200 300 8 Distance (nm) 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 0 5 10 15 20 25 30 35 40 Lateral Size (μm) Thickness (nm) Fig. 2 Examination of flake lateral size and thickness. a SEM images of graphite. b 2000 cycles. c 4000 cycles. d 6000 cycles. e The lateral size distribution is plotted as a function of processing cycles, error is calculated as the standard deviation (n = 50). f An AFM micrograph displaying a typical profile of a 6000 cycles graphene flake with a thickness of 8 nm and lateral size of 270 nm. The scale bar is 230 nm. g Lateral size and h thickness of the 6000 cycles graphene ink obtained from AFM analyses. Fig. 3 Examination of flake quality. a Raman spectroscopy for graphite (black), 2000 (red), 4000 (blue) and 6000 (green) processing cycles. b The I(D)/I(G) ratio plotted as a function of the Disp(G) for 6000 processing cycles. c FWHM(G) plotted as a function of the I(D)/I(G) ratio. spectra of the graphene with 2000 cycles (red curve), 4000 cycles narrows to a single Lorentzian for single-layer graphene. For 2000 −1 (blue curve), 6000 cycles (green curve) and graphite (black curve). cycles, we observe a shoulder on the 2D peak (~ 2690 cm ) −1 In the graphene spectra, the G peak located at ~1581 cm , indicating the presence of multilayer graphene. However, after 29 −1 corresponds to the E phonon . The D peak at ~1350 cm is 4000 and 6000 cycles, the 2D peak is a single Lorentzian fit, 2g due to the breathing modes of six-atom rings and requires a indicating that even if the flakes are multilayers, they are defect for its activation . Typically in few-layer graphene electronically decoupled and behave as a collection of single dispersions, the D peak corresponds to flake edges rather than layers . In disordered carbon, the position of the G peak, Pos (G) 31,32 defects within the flakes’ basal plane . The 2D peak located at increases when the excitation wavelength, λ decreases from the −1 33 30,34 ~2700 cm is the second-order resonance of the D peak . In the IR to UV . Additionally, the dispersion of the G peak, Disp(G) = graphite sample (black curve), the 2D peak split into several ΔPos(G)/Δλ and the FWHM of the G peak, FWHM(G), increase with 35,36 components. As the number of layers decreases, the 2D peak disorder . Combining the intensity ratio of the D and G peaks, Published in partnership with FCT NOVA with the support of E-MRS npj 2D Materials and Applications (2022) 3 Thickness (nm) Number of Flakes Lateral Size (μm) Number of Flakes T. Carey et al. Fig. 4 Chemical, electrical and thermal analysis of graphene inks. a XPS C 1s spectra of the 6000 cycles graphene ink. b A plot of R as a function of the graphene and graphite film thickness for the 6000 cycles graphene ink with different post-processing conditions such as centrifugation and annealing. Error is calculated by standard deviation of mean (SDOM, n = 3). c A graph of the σ as a function of thickness for the 6000 cycles graphene ink with post-processing. d Thermogravimetric analysis is used to find thermograms of the graphene powder and SDC surfactant. The weight change is plotted as a function of temperature. I(D)/I(G), with FWHM(G) and Disp(G) can discriminate between mixing process (Re ~6 × 10 ), where ρ is the liquid density, ν is inline disorder localised at the edges and disorder in the basal plane of the liquid velocity, L is the characteristic length and η is the liquid the flakes. If the disorder is in the graphene basal plane, I(D)/I(G) viscosity. The reduced Re is attributed to the decreased L of inline will increase with FWHM(G) and Disp(G). In the former case, I(D)/I the in-line shear mixing system L ≃ 0.46L (Supplementary inline beaker (G) will be uncorrelated with FWHM(G) and Disp(G). In Fig. 3bwe Note 2) and suggests that reducing the turbulence of the system find that I(D)/I(G) is uncorrelated with Disp(G). Therefore, the D reduces the defects present in graphene flakes produced by shear peak originates from the flake edges rather than the basal plane. mixing. Therefore, in-line shear mixing could offer a route to −1 −1 Furthermore, the average Disp(G) ~ 0.019 cm nm , which is manufacture higher quality graphene material than is traditionally significantly lower than the Disp(G) for disordered carbons produced by shear mixing. −1 −1 34,37 (>0.1 cm nm ) . Therefore, the in-line shear mixing method produces pristine basal plane graphene flakes. In Fig. 3c, I(D)/I(G) is Investigating flake chemical, electrical and thermal properties plotted as a function of FWHM(G). We find the FWHM is 20 ± −1 2 We use X-ray photoelectron spectroscopy (XPS) analysis to 1cm for each sample, indicating that the sp grain size (L )~ investigate the oxygen and sodium content in the 6000 cycles 40 nm remains constant before and after exfoliation and that the 35,37 graphene ink. The XPS survey spectra (Supplementary Fig. 3a) cyclic process does not significantly damage the graphene . shows very high carbon content (92.5 ± 0.1 at.%) and low oxygen Since the D peak originates from the edge of the flakes and FWHM content (5.7 ± 0.1 at.%), attributed primarily to graphene flakes (G) remains constant in our samples, the I(D)/I(G) ratio can be used and sodium content (~1.8 at.%), which we attribute to residual to determine <L>, where I(D)/I(G) increases as a function of inverse 18,38 surfactant. The relatively low oxygen content and the position of <L> . In Fig. 3c, the I(D)/I(G) ratio increases with processing the main peak in O 1 s spectra at ~533 eV (Supplementary Fig. 3b) time from I(D)/I(G) ~ 0.1 for bulk graphite (black), I(D)/I(G) ~ 0.25 suggest C-O component dominance over C=O. Careful investiga- after 2000 cycles (red) and up to I(D)/I(G) ~ 0.5 after 4000 (blue) tion of the high-resolution XPS spectra (Fig. 4a) for carbon 1s and 6000 (green) processing cycles. We attribute the increase in I reveals a peak at ~285 eV attributed to the sp carbon component (D)/I(G) with processing time to a decrease in flake lateral size. We and indictive of the hexagonal lattice structure, also the satellite can estimate <L> using the formula I(D)/I(G) ≈ (I(D)/I(G)) + k/ graphite −1 peak (π–π*) contribution, corresponding to the broad peak at <L>, where k is the slope of I(D)/I(G) as a function of <L> 2 39 ~290 eV, is a characteristic feature of sp hybridisation of carbon . measured by transmission electron microscopy Using I(D)/I(G) ~ The π–π* transition occurs due to the delocalisation of the π- 0.5 for 4000 and 6000 cycles, average I(D)/I(G) ~ 0.11 for our electrons in the carbon aromatic ring . We observe that the π–π* graphite and a slope of k = 0.17 we can estimate that <L> ~ peak is more pronounced in the 6000 cycles graphene ink than 0.43 μm which is consistent with the SEM and AFM data .In conventional shear mixing in a beaker (Supplementary Fig. 3), contrast, a correlation between I(D)/I(G) and FWHM (G) is observed with a graphene ink made by conventional shear mixing in a suggesting that 6000 cycles graphene ink should have reduced beaker under similar exfoliation parameters (Supplementary Note basal plane defects. In general, the more pronounced π–π* 2 41 1) indicating that in-plane defects are present where L ~ 20 nm, satellite peak, the greater the degree of sp bonding . which will decrease graphene conductivity. We attribute the To examine the graphene inks’ electrical properties, we create introduction of in-plane defects to an increased Reynolds number films of the 6000 cycles graphene ink and graphite flakes by drop- −1 6 (Re = ρνL η ~ 1.3 × 10 ) compared to our in-line shear casting onto glass slides with a spacer to control the wet film beaker npj 2D Materials and Applications (2022) 3 Published in partnership with FCT NOVA with the support of E-MRS T. Carey et al. Fig. 5 Inkjet printed interconnects and electrodes. a Ink viscosity as a function of applied shear rate, demonstrating non-Newtonian shear thinning behaviour. b Pendant drop of the ink used to calculate the surface tension. c Literature review of other inkjet printable conductive inks comparing their σ with the ink c. d Jetting of the ink observed with a stroboscopic camera. e Optical image of the inkjet-printed transistor contacts (left, dark field) and interconnect (right, bright field). The scale bar is 100 μm (left) and 1 cm (right). thickness. The films are dried at ∼100 °C on a hotplate to remove air atmosphere. We plot the weight change as a function of excess water. The dry film thickness (t) depends on the c of the ink temperature, which shows that SDC significantly decomposes at and wet film thickness. Therefore, we vary t by increasing the temperatures >380 °C. The graphene powder has a carbon number of droplets cast (e.g. 5 to 50) from a syringe. We find the combustion temperature between 570 and 670 °C (maximum sheet resistance (R ) of each sample using a four-point probe mass decomposition rate ≈ 627 °C), which is associated with the (Jandel Probe) in three different locations to obtain an average. carbon decomposition temperature of few-layer graphene . We use a profilometer to determine t for each sample and measure three areas to obtain an average. In Fig. 4b, we find a R Inkjet printable interconnects −1 of ∼ 324 Ω□ for the graphite film at t ∼ 27 μm. Consequently, To make an inkjet printable ink requires the optimisation of c, −1 the conductivity (σ) can be estimated using σ = R t, equating to −1 surface tension, γ and η to ensure satellite droplet free ejection, σ ∼ 100 S m . Without any post-processing or centrifugation, we −1 and morphologically uniform (i.e. roughness minimisation) printed find that the 6000 cycles graphene ink has an R of ∼ 37 Ω□ at t 43,47 3 −1 films . We engineer the η of the graphene ink to ensure it is ∼ 24 μm, corresponding to σ ∼ 1.1 × 10 Sm , one order of within an optimal 1–10 mPa s range required for inkjet printing . magnitude higher than σ of graphite, indicating graphite’s We measure η with a parallel plate rotational rheometer (see exfoliation to few-layer graphene. Recently, Fernandes et. al has Methods). A known shear rate is applied, and the resultant torque determined through semi-automated AFM measurements that or shear stress is measured. The shear stress divided by the shear uncentrifuged graphene LPE dispersions could have a small rate calculates the liquid viscosity shown in Fig. 5a .We find number (∼5%) of graphite flakes, and the majority mass (∼90%) pseudoplastic behaviour with an infinite-rate η of ~ 1 mPa s within would be attributed to the thicker (>40 nm) material . Therefore, the optimal η range (~ 1–10 mPa) . The decrease in viscosity is we centrifuge the material at low speed (1k rpm, g ~ 125) to attributed to the reduction in the Brownian motion of the remove the small number of thicker flakes (>40 nm), and the film 3 −1 graphene flakes . In Fig. 5b, we determine the ink γ using the σ increases to ∼ 5× 10 Sm . We show the σ as a function of t in pendant drop method. The pendant drop method involves using a Fig. 4c. At t <4 μm, the σ is thickness dependant, and at t ∼ 4 μm, camera to visualise the shadow image of a droplet dispensed from the flakes form a percolative network, and the bulk σ is reached. a needle. The droplet’s radius of curvature is found from the Therefore, for electronic devices requiring high conductivity 3 −1 shadow images that can be used with the Young–Laplace >10 Sm a t of at least 4 μm should be used. The residual 49,50 equation to determine the liquid surface tension .We find a surfactant can degrade the electrical properties of a deposited −1 graphene ink γ of ~ 63 mN m , lower than the γ of water network of flakes . Therefore, we then anneal the samples at −1 ~72mNm , which we attribute to the presence of SDC 400 °C for 30 min to combust the SDC used in the graphene ink. 4 −1 surfactant. High c is needed to minimise the number of printing We find σ increases even further to ∼ 1.5 × 10 Sm and −1 layers required to reach a thick network of flakes, thus increasing demonstrates R ∼ 2.6 Ω□ (t ∼ 25 μm). The σ of the film is 20,44,45 comparable to state-of-the-art printable graphene inks .To the ϕ of a printing process to create devices. In contrast, high σ is investigate the removal of residual SDC surfactant after annealing, desirable to improve the performances of devices that require 51 52 we investigate the SDC decomposition temperature with thermo- high σ, such as electrodes for micro-supercapacitors , antennas gravimetric analysis (TGA) in Fig. 4d. The 6000 cycles graphene ink or transistor electrodes . Pristine inkjet printable graphene inks −1 is subjected to a freeze-drying process to obtain a graphene have commonly had low c (< 3 mg ml ), to help minimise nozzle powder (see Methods). The SDC and graphene powder are heated clogging. Identification of stabilising solvents by solubility −1 16 from ~ 25 °C up to 1000 °C ramped at a rate of ~ 10 °C min in an parameter-matching has proven to be a useful starting point Published in partnership with FCT NOVA with the support of E-MRS npj 2D Materials and Applications (2022) 3 T. Carey et al. for ink formulation. However, the resultant c can be low, < 0.2 mg (CV) data is measured for a graphene/SWCNT anode at a scanning −1 44 −1 st ml ; therefore, many (>50) printing passes are required to rate of 0.1 mV s , shown in Fig. 6a. During the 1 cycle, an build a percolating film of material. Other strategies have been irreversible cathodic peak at ~1 V is observed, which can be developed to improve ink’s c. For example, vacuum filtration can attributed to the formation of a solid electrolyte interface (SEI) and 51 53 be utilised with flocculation agents or a solvent exchange to the reaction of lithium ions with residual oxygen-containing −1 63–66 redisperse graphene at high c (> 10 mg ml ). However, the functional groups on the graphene flakes . In the range of −1 53 + 67 additional vacuum filtration step reduces ϕ <1 g h Alternately 0.01–0.25 V, we observe Li insertion into graphene flakes . In the solvent interface trapping has been used engineer high c (> second cycle, the cathodic peak at 1 V is not observed, indicating −1 100 mg ml ) ink by trapping graphene between a polar and non- the oxygen-containing functional groups on the graphene flakes polar solvents . GO powders also been used to directly disperse are completely reduced. After five cycles, the redox peaks −1 −1 material at a high (> 1 mg ml ) c but with low σ (< 1000 S m ) between 0.01 and 0.25 V display no apparent changes, illustrating 55,56 + due to the defective nature of the flakes . To increase the σ, reversible Li intercalation/deintercalation in the graphene layer. printed films are typically annealed at high temperatures Thus, the CV data confirms that the 6000 cycles graphene flakes 45,57 68 >300 °C . However, this process is incompatible with many store lithium effectively . textile (e.g. cotton) or polymer (e.g. polyethylene terephthalate, For cyclability measurement, the galvanostatic charge and discharge are performed initially with a 43-cycle of activation at PET) substrates which degrade at temperatures >100 °C . There- 3 −1 −1 −1 fore inkjet printable graphene inks with high σ (> 10 Sm ) and 30 mA g followed by 375 cycles at 100 mA g , as shown in Fig. −1 high c (> 10 mg ml ), which do not clog the inkjet nozzle have 6b. All data is normalised to the active mass of graphene (see −1 been desirable but difficult to achieve. The 6000 cycles graphene Methods). The first activation cycle (30 mA g ) specific capacity 3 −1 −1 −1 −1 ink achieves these requirements (σ >10 Sm , c ∼ 100 mg ml ), (C ) was ~646 mAh g for discharge and ~363 mAh g for sp however it is not compatible with most inkjet nozzles due to the charge, with an initial coulomb efficiency (CE) of ~56%. After the −1 flake <L >> 1 μm. Typically the flakes should be 1/50th of the activation cycles, the cells are tested for 375 cycles at 100 mA g . nozzle diameter (a = 21 μm) to minimise nozzle clogging (∼ The graphene/SWCNT anodes show a lithium storage capability of 44 −1 −1 420 nm) . Therefore we centrifuge the 6000 cycles graphene ink ~228 mAh g and ~243 mAh g for discharge and charge, with a −1 at low speed (1k rpm, g ~ 125) to create a graphene ink Gr CE of ~100% for the first cycle at ~100 mA g (Fig. 6b, inset). The inkjet −1 with c ~ 5.9 mg ml ,(Y ∼ 6%) which minimises the probability of charging and discharging capacity changed in the first 4 cycles 3 −1 −1 nozzle clogging and increases σ ∼ 5×10 Sm without before stabilising at a capacity of ~233 mAh g for both annealing. We compare the σ and c of Gr to others (Fig. 5c) discharge and charge with a CE of ~100% over 365 cycles. inkjet and find that both c and σ are higher than most inkjet printable The rate performance of our anodes is tested with voltage inks in the literature, where an optimal ink would be in the top profiles shown in Fig. 6c and rate dependent cycling data in Fig. −1 right corner of the graph, assuming that the ink would not cause 6d. The anodes exhibited an initial performance of 641 mAh g −1 −1 nozzle clogging issues or satellite droplets due to its concentra- for discharge and 376 mAh g for first cycle charge at 10 mA g 44,45,51,53,55–59 tion . We use a drop-on-demand inkjet printer to with an initial CE of ~58.6% that approaches ~99% for subsequent print the ink. We image the jetting of the ink with a stroboscopic cycles. The specific charge capacity decreased with increasing −1 camera (Fig. 5d), the black dot at 0 μm represents the inkjet specific currents whereby at 180 mA g , a specific charge −1 nozzle. The ink is observed to eject from the nozzle without capacity of ~107 mAh g was achieved. In contrast, the specific −1 satellite droplets, which is important to achieve well resolved discharge capacities fell off to 374, 287, 197 and 116 mAh g for printed films with resolution <100 μm. In Fig. 5e, we deposit the each rate’s first cycle, respectively. The low-rate capacity (20 mAh −1 −1 g ) was ~370 mAh g , very close to graphite’s theoretical ink onto a PET substrate coated with aluminium oxide nanopar- −1 69 ticles (Novele, Novacentrix) which have a low roughness (~18 nm). capacity (372 mAh g ) . The fact that we can reach close to We print a 2 cm layout of interconnect array. Through optical the theoretical value attests to the high quality of graphene flakes inspection, we observe morphological uniformity of the print and produced using in-line method while facilitated by the presence 60,62 evenly distributed flakes in the form of a film. We also inkjet print of SWCNT . transistor contacts Fig. 5e demonstrating a ~ 50 μm gap between two graphene electrodes. The resolution of ~ 50 μm is as expected Biocompatibility with human cells 47,53 and comparable to previous works on inkjet printing . These Understanding the biocompatibility of the graphene flakes with −1 results indicate that the high concentration (5.9 mg ml ) human cells is essential to utilise the material for applications that printable graphene inks can be produced using in-line shear come into contact with humans, such as electronic textiles, mixing and can be inkjet printed at high resolution (~ 50 μm) in 9,70,71 bioscaffolds or drug delivery . To test if there is any acute desirable patterns for flexible electronics. toxicity, we performed viability assays using increasing graphene flake concentrations (6000 cycles graphene ink) in Human Li-ion battery anode umbilical vein endothelial cell (HUVECs) and standard human Graphene is an attractive material to replace graphite in lithium- cancer cell line SW948. In Fig. 7a, the upper left quadrant ion battery anodes since Li-ion battery anodes need high in-plane represents acute toxicity by induction of cell necrosis stained by 3 −1 conductivity (>10 Sm ) to improve rate performance, and thus, 7-AAD. In contrast, the lower right quadrant shows cells under- 60,61 charging speed . Here, we demonstrate the potential of the going programmed cell death (Annexin V staining), i.e. apoptosis. graphene flakes produced using in-line shear mixing method as Cells stained by both dyes are in a transitional phase between Li-ion anodes. Rather than using a polymeric binder, we add a apoptosis and necrosis and fall in the right upper quadrant. small amount (15%) of single-walled carbon nanotubes (SWCNT) Repeated measurements show no acute toxicity found when −1 to the electrode. Using SWCNT is known to simultaneously using the highest concentration of graphene (1 mg ml )in48h maximise the electrode’s mechanical and electrical properties and cell culture treatments (N = 2 for SW948, N = 3 for HUVECs). This tends to yield electrodes that perform near their theoretical was comparable to a control, where no graphene was added. capability . The resultant graphene/SWCNT anodes have an To confirm our cellular apoptotic measurements, we used a cell −2 active mass loading of ~1 mg cm combined with a SWCNT counting kit-8 (CCK8) assay based on a tetrazolium salt reduced by −2 loading of ~0.18 mg cm and thickness of ~20 μm (see Methods). viable cells to a soluble formazan dye. The resulting dye absorbs To evaluate the electrochemical Li storage mechanism of light at 450 nm, which correlates with cellular viability (Fig. 7b). graphene produced using in-line method, cyclic voltammetry After 48 h of graphene treatment, the cell cultures did not show npj 2D Materials and Applications (2022) 3 Published in partnership with FCT NOVA with the support of E-MRS T. Carey et al. Fig. 6 Li-ion battery performance: performance of graphene flakes/SWCNT (85:15 wt%) as lithium-ion battery anodes. a Cyclic −1 voltammograms at a scan rate of 0.1 mV s for various cycles. b Cycling capacity versus cycle number for composite anodes based on −1 −1 graphene flakes cycled initially at 30 mA g and then at 100 mA g . Coulombic efficiency (CE) is shown in the inset. c Galvanostatic charge- discharge curves measured at different charging currents. d Specific capacity as a function of charging current. In all cases, C is normalised Sp to the graphene mass. 4 −1 any dose-dependent toxicity up to the highest concentration and σ ∼ 1.5 × 10 Sm after centrifugation and annealing, which −1 tested at 1 mg ml for SW948 and no significant toxicity up to is state of the art for graphene inks. We achieve high σ by −1 500 μgml for HUVECs. We compared our measurements against reducing Re < 10 which minimises basal plane topological a control of deionised water and SDC, which did not show any defects. Therefore, in-line shear mixing could offer a route to −1 dose-dependent toxicity up to 38.46 μgml of SDC (relative to manufacture higher quality graphene or potentially other 2D −1 the amounts contained in 1 mg ml graphene suspension, materials, than is traditionally produced by shear mixing. We Supplementary Fig. 4). To the best of our knowledge, this is one demonstrate the graphene inks’ versatility by making anode of the highest c that has demonstrated LPE graphene’s electrodes with SWCNTs for Li-ion energy storage, achieving a biocompatibility with human cells to date, which is essential for −1 low-rate capacity of ~370 mAh g . We show that highly applications in printed electronics that use c of at least 1 mg −1 concentrated (5.9 mg ml ) and inkjet printable inks can be −1 43 ml We attribute the biocompatibility on the ink to the lack of manufactured for use in inkjet-printed conductive interconnects oxygen functional groups (~5.7%, determined by XPS) as with a resolution ∼50 μm. As a final demonstration, we examine graphene oxide dispersions have previously demonstrated toxicity the toxicity of the graphene flakes with human colon cells and −1 72,73 to human cells as low as 1–10 µg ml attributed to its high HUVEC cells. We observe no dose-dependent toxicity up to 48 h oxygen content (~47%) that induces the formation of reactive 74 indicating the graphene flakes’ biocompatibility at high concen- oxygen species that can be toxic to cells . To detect any cell −1 trations ∼1mgml , which is essential to utilise graphene in deformities due to graphene treatment, we imaged the cells using textile electronics, composites and printed interconnects that laser scanning confocal microscopy with propidium iodide to stain could involve human contact with high concentrations of whole fixed cells with the nuclear Hoechst dye (Fig. 7c). graphene. Representative images show cells with similar morphology to control at high graphene treatments (Fig. 7c). METHODS DISCUSSION Inkjet printing We demonstrated an enclosed, cyclic exfoliation of graphene with We used a Fujifilm Dimatix DMP-2800 inkjet printer with a 21 µm diameter −1 in-line shear-mixing, which has a higher ϕ ∼ 8.3 g h than nozzle (Fujifilm DMC-11610) for the printing of interconnects. The inkjet previous graphene production techniques such as ultrasonication printing platen temperature was kept at ambient conditions (20 °C). An and ball milling. The Y ∼ 100% is several orders of magnitude w inter-drop spacing of 25 μm was used for the interconnects and an inter- increase compared to previous work on shear mixing , and can drop spacing of 40 μm for the transistor electrodes. We print 100 layers of be used to create dispersions with extremely high concentration c ink to construct the interconnect pattern and 10 layers of ink for the −1 3 −1 ∼ 100 mg ml , achieving σ ∼ 10 Sm without post-processing transistor electrodes. We used a maximum jetting frequency of 2 kHz. Published in partnership with FCT NOVA with the support of E-MRS npj 2D Materials and Applications (2022) 3 T. Carey et al. Fig. 7 Human cell viability study: biocompatibility with human umbilical vein endothelial cell (HUVECs) and SW948 human cells. a Muse ™ Apoptotic profile using Annexin V for apoptosis staining and 7-Aminoactinomycin D (7-AAD) fluorescence intensity for necrosis is shown in a flow scatter using same cut offs for all measurements (N = 3 for HUVECs and N = 2 for SW948). b CCK8 viability normalised to 0 μg −1 −1 ml control over increasing concentrations of graphene (0–1000 μgml )(N = 9 from three independent experiments for HUVECs in blue and N = 6, from two independent experiments for SW948 in red). c Laser scanning confocal microscopy with the addition of propidium iodide (PI), used here to image whole cells, and image overlay with Hoechst cell nuclear staining with bright field transmittance of graphene pre- treated fixed cells at ×63 magnification, scale bar 50 μm for HUVECs and 25 μm for SW948 cells. X-ray diffraction The XPS measurement data were analysed using the CasaXPS software (Casa Software Ltd). XRD patterns were recorded with a D2 Phaser (Bruker) powder diffractometer equipped with LynxEye detector using Cu Kα radiation (λ = 1.54 Å) at a 2θ scan step of 0.03° and 1 s dwell time. XRD samples were prepared by drying the graphene dispersions under room condition and Profilometry transferring the powder obtained to the Bruker sample holder. We used a stylus profilometer (Bruker DektakXT) to measure the thickness of the deposited films on a quartz substrate. A stylus force of 3 mg was used with a tip of radius 12.5 μm over the sample. X-ray photoelectron spectroscopy Samples were prepared by depositing graphene dispersions on Si/SiO Thermogravimetric analysis substrates mounted on aluminium stubs using conductive silver paint. XPS measurements were conducted using a K-Alpha™ X-ray Photoelectron To prepare the graphene sample, we take 30 ml of the 6000 cycles Spectrometer (Thermo Scientific). High-resolution C1s, O 1s, and Na 1s graphene ink and froze it at −18 °C. The frozen sample was then placed spectra were collected at 40 eV pass energy with a 0.05 eV step size. inside a freeze dryer (Telstar LyoQuest) to remove ice crystal by Spectra were acquired using a monochromatic Al Kα X-ray (hν = 1486.6 eV) sublimation under vacuum overnight to make a graphene powder. We with an X-ray beam spot size of 400 μm, average of 3 points per sample then undertake thermal analysis of the graphene powder and SDC were taken. Any energy shifts were calibrated using gold foil attached to surfactant using a thermogravimetric analyser (TA Instruments Q50) in air −1 the films and measuring the Au 4f binding energy at 84 eV as a standard. with a ramping temperature of 10 °C min from ~25 °C up to 1000 °C. npj 2D Materials and Applications (2022) 3 Published in partnership with FCT NOVA with the support of E-MRS T. Carey et al. −2 Raman spectroscopy furnace to remove CMC. The mass loading of graphene was ~1 mg cm , with 15 wt.% SWCNT. The anode thickness was ~20 μm. We acquired the Raman spectra at 514.5 nm with a Reinshaw InVia Half-cell coin cells were assembled with Li-metal discs (diameter: 14 mm, equipped with a 50× objective. We kept the power on the samples below MTI Corp.) as counter/reference electrodes. The electrolyte used was 1.2 M ~1 mW to minimise thermal damage from the laser. About 20 spectra are LiPF in a mixture of ethylene carbonate/ethyl methyl carbonate (EC/EMC, taken for each map. All measurements are acquired after drop-casting 1:1 in v/v, BASF) with 10 wt% fluoroethylene carbonate (FEC). A Celgard sample solution on Si/SiO wafer. The resolution of our spectrometer is −1 2320 (thickness 20 μm) was used as the separator. The cells were ~1 cm and our spot size was ~2 μm. For measurement of the Disp(G) we assembled in a glovebox filled with highly pure argon gas (UNIlab Pro, acquired additional Raman spectra at 457 nm and 633 nm, respectively. Mbraun, with O and H O levels <0.1 ppm). 2 2 Shear-mixing and ink preparation Cyclic voltammetry We used a Silverson Model L5M, 250 W single phase motor high-shear Cyclic voltammetry of the cells was carried out using a laboratory mixer. An interchangeable in-line mixing assembly was used for galvanostat–potentiostat between 0.01 and 1.2 V vs. Li /Li at a scan rate in-line processing. It was equipped with a 4-blade rotor placed inside a −1 of 0.1 mV s for 10 cycles. For cycling capability tests, the cells were stator with a rotor-stator gap (ΔR) of 300 μm and a rotor diameter (D) of −1 performed at 100 mA g for 375 cycles after 3 cycles activation at 30 mA 31.1 mm. We use graphite flakes (Imerys Graphite) as a starting material for −1 −1 g . For rate capability measurement, cells were running at different the inks. We mix the flakes (100 mg ml ) with sodium deoxycholate (SDC, −1 −1 current rates of 10, 20, 45, 90, 180 mA g and then went back to 10 mA 5mgml ) (Sigma-Aldrich) in deionised water at 8000 rpm. The through- −1 −1 −1 g . There are only 4 cycles at 10 mA g , then there are 5 cycles for the put was calculated as 1000 ml of 100 mg ml ink (i.e. 100 g) processed for next steps at different current rates. 4000 cycles (i.e. 12 h). The volume of the ink reservoir can be increased −1 4 −1 (>1 L) if further scale is required. The shear rate γ ≈ πND ΔR ≈ 4×10 s , where N is the revolutions per second of the shear mixer (8000 rpm = Cell viability study −1 18 133 s ) . Optical inspection of the ink reveals a homogeneous stable SW948 cells were seeded in 96 well plates at 15k cells per well in dispersion of graphene flakes. Dulbecco’s Modified Eagle Medium (DMEM) media (5 mM glucose), with added 10% Foetal Bovine Serum (FBS), 5mM L-Glutamine (L-Glu), 5 mM Scanning electron microscopy penicillin (PC) and 5 mM streptomycin (SP), and left overnight in a humidified CO infused incubator at 37 °C. Graphene ink was added to the The ink was diluted 1:999 and drop-cast on 1 cm Si/SiO wafer. We 2 −1 −1 wells to reach c ranging between 7.8 μgml to 1000 μgml . After 48 h, acquired scanning electron microscopy images with a Magellan 400 L SEM. the cells were gently washed three times with PBS, where cells in one of We used an accelerating voltage of 5 kV and gun current of 25pA during the plates were resuspended in cell media (i.e. DMEM/glucose/FBS/L-Glu/ operation and obtained the images in secondary electron detection mode PC/SP), containing 10% of the viability reagent cell counting kit 8 (CCK-8, using an immersion lens and through-lens-detector. We measured <L>in Dojindo Molecular Technologies, Inc.). According to the assay protocol, one ImageJ. plate was incubated at 37 °C for 2 h before measuring absorbance at 450 nm using Spectramax Paradigm plate reader (Molecular Devices, Atomic force microscopy Sunnyvale, CA 95134). After measuring absorbance, these wells were The ink was diluted 1:999 and drop-cast onto clean 1 cm Si/SiO wafers. 2 rewashed with phosphate-buffered saline (PS) at 0.01 M in deionised We used a Bruker Dimension Icon in peakforce mode to scan a 20 μm× water, before another absorbance measurement was conducted at 450 nm 20 μm area of the wafer. We then find the <L> and thickness of 30 flakes to determine any background absorbance resulting from cellular by manual counting in NanoScope Analysis. internalised graphene, which was subsequently subtracted. An identical 96 well cell plate was fixed with 4% w/v paraformaldehyde (PFA) solution −1 containing 15 μgml Hoechst for 30 min, washed once with PBS, before Surface tension Hoechst fluorescence was measured at 360nm /500nm on a Excitation Emission We used a contact angle goniometer to measure the graphene ink’s Spectramax Paradigm plate reader (Molecular Devices, Sunnyvale, CA surface tension through the pendant drop method. A suspended droplet 95134). from a needle forms a pendant’s shape resulting from the downward force due to gravity and the upward force due to surface tension. We used drop- Cell microscopy shape analysis to calculate the surface tension from a shadow image of the droplet. Cells were imaged using Leica SP8 confocal microscope in epifluorescence mode for full well overviews and laser scanning confocal mode for high- resolution imaging. Rheometry We used a parallel-plate rotational rheometer (DHR Rheometer, TA instruments) to find the viscosity as a function of the shear rate. We use Muse™ flow cytometry viability and apoptosis assays a plate-to-plate distance of 500 μm and we loaded the ink between the SW948 cells were seeded in sterile 24 well plates at 45k cells per well in cell plates by capillary action at a gap of 550 μm. We find the infinite-rate culture media (complete DMEM), and left to attach in a humidified CO viscosity for the ink. infused incubator at 37 °C overnight. The next day, graphene ink was added to the cell growth media in each well to make a final c of 125, 250, −1 500, 1000 µg ml before being left for 48 h in the incubator as described Microbalance above. Following 48-h treatment with graphene inks, cells were washed To find the c of the ink we use a microbalance (Sartorius ME5), we placed with pre-heated phosphate-buffered saline (PBS), and detached from 1 ml of ink in a metal container and weighed the graphene flakes after culture wells using disodium ethylenediaminetetraacetic acid (EDTA) and evaporation of the solvent. 0.25% Trypsin (w/v%) (Thermo Fisher 25200056). 20 µL of cell suspension was added to 380 µL of count and viability Muse reagent and left for 5 min Electrochemical characterization before measuring 1000 cellular event using Muse™ cell analyser. For apoptosis 100 µL of cell suspension was added to 100 µL of Muse Annexin The 6000 cycles graphene ink was mixed with a SWCNT aqueous V & Dead Cell Reagent and left for 20 mins at room temperature before dispersion (0.4 wt% SWCNT in water, ~0.6 wt% carboxymethyl cellulose measuring 5000 cellular event using Muse™ cell analyser. (CMC) as a surfactant stabiliser, Tuball, OCSiAl), which increases conductivity and functions as a binder. A mortar and pestle ground the mixed solution to obtain a uniform slurry, and then the slurry was cast onto DATA AVAILABILITY copper foil using a doctor blade. The slurry cast anodes were slowly dried at 40 °C overnight in an oven to remove residual water. The films produced The authors declare that the data supporting the findings of this study are available were cut to the desired dimensions (area = 1.131 cm ) for electrochemical within the paper and its supplementary information files. Data are also available from testing. 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Biomimetic carbon fiber systems engineering: a modular design ADDITIONAL INFORMATION strategy to generate biofunctional composites from graphene and carbon Supplementary information The online version contains supplementary material nanofibers. ACS Appl. Mater. Interfaces 11, 5325–5335 (2019). available at https://doi.org/10.1038/s41699-021-00279-0. 71. Liu, J., Cui, L. & Losic, D. Graphene and graphene oxide as new nanocarriers for drug delivery applications. Acta Biomater. 9, 9243–9257 (2013). Correspondence and requests for materials should be addressed to Tian Carey or 72. Vranic, S. et al. Live imaging of label-free graphene oxide reveals critical factors Jonathan N. Coleman. causing oxidative-stress-mediated cellular responses. ACS Nano 12, 1373–1389 (2018). Reprints and permission information is available at http://www.nature.com/ 73. Hoyle, C. et al. Small, thin graphene oxide is anti-inflammatory activating nuclear reprints factor erythroid 2-related factor 2 via metabolic reprogramming. ACS Nano 12, 11949–11962 (2018). Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims 74. Das, S. et al. Oxygenated functional group density on graphene oxide: its effect in published maps and institutional affiliations. on cell toxicity. Part. Part. Syst. Charact. 30, 148–157 (2013). ACKNOWLEDGEMENTS Open Access This article is licensed under a Creative Commons XPS data were taken at NEXUS, Newcastle University, facility that was funded by UK Attribution 4.0 International License, which permits use, sharing, Engineering and Physical Sciences Research Council (EPSRC), grant number NS/ adaptation, distribution and reproduction in any medium or format, as long as you give A000015/1. We have also received support from the Science Foundation Ireland (SFI) appropriate credit to the original author(s) and the source, provide a link to the Creative funded centre AMBER (SFI/12/RC/2278_P2) and availed of the facilities of the SFI- Commons license, and indicate if changes were made. The images or other third party funded AML and ARM labs. J.M.K acknowledges funding from Smart Quantum Dot material in this article are included in the article’s Creative Commons license, unless Lighting (EPSRC, EP/P027628/1). T.C. acknowledges funding by a Marie Skłodowska- indicated otherwise in a credit line to the material. If material is not included in the Curie Action “MOVE” (Grant Number 101030735). article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons. AUTHOR CONTRIBUTIONS org/licenses/by/4.0/. T.C. conceived the experiments. T.C. and J.C. directed the research and designed experiments. T.C. manufactured the inks and undertook Raman Spectroscopy with © The Author(s) 2022 assistance from AD.A. and J.M.K. T.C. undertook Inkjet printing, Rheology, Surface Published in partnership with FCT NOVA with the support of E-MRS npj 2D Materials and Applications (2022) 3

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