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Capasso, Andrea, Del Rio Castillo, Antonio Esau, Sun, H., Ansaldo, Al-berto, Pellegrini, Vittorio, & Bonaccorso, Francesco(2015)Ink-jet printing of graphene for flexible electronics: An environmentally-friendly approach.Solid State Communications, 224, pp. 53-63.

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https://doi.org/10.1016/j.ssc.2015.08.011

Corresponding author. Email address: francesco.bonaccorso@iit.it

Ink-jet printing of graphene for flexible electronics: towards an environmentally-

friendly approach

A. Capasso#, A.E. Del Rio Castillo

#, H. Sun, A. Ansaldo, V. Pellegrini, F. Bonaccorso*

Istituto Italiano di Tecnologia, Graphene Labs, I-16163 Genova, Italy

#These authors contributed equally to this work.

Abstract

Mechanical flexibility is considered an asset in consumer electronics and next-generation electronic

systems. Printed and flexible electronic devices could be embedded into clothing or other surfaces

at home or office or in many products such as low-cost sensors integrated in transparent and flexible

surfaces. In this context the inks based on graphene and related two-dimensional materials (2DMs)

are gaining increasing attention owing to graphene exceptional (opto)electronic, electrochemical

and mechanical properties. The current limitation relies on the use of solvents, providing stable

dispersions of graphene and 2DMs and fitting the proper fluidic requirements for printing, which

are in general not environmentally benign, and with high boiling point. Non-toxic and low boiling

point solvents do not possess the required rheological properties (i.e., surface tension, viscosity and

density) for the solution processing of graphene and 2DMs. Such solvents (e.g., water, alcohols)

require the addition of stabilizing agents like polymers or surfactants for the dispersion of graphene

and 2DMs, which however unavoidably corrupt their properties, thus preventing their use for the

target application. Here we demonstrate a viable strategy to tune the fluidic properties of

water/ethanol mixtures (low-boiling point solvents) to first effectively exfoliate graphite and then

disperse graphene flakes to formulate graphene-based inks. We demonstrate that such inks can be

used to print conductive stripes (sheet resistance of ~13 KΩ/) on flexible substrates (polyethylene

terephthalate), moving a step forward towards the realization of graphene-based printed electronic

devices.

Keywords: ink-jet printing, graphene ink, flexible electronics.

Highlights

Printing conductive stripes on polyethylene terephthalate (PET) exploiting graphene inks in

EtOH/H2O mixture (and NMP for comparison).

Flexible

Conductive

Inexpensive

Environmentally-friendly

Suitable for solar cells and flexible electronics

1. Introduction

Printed and flexible electronics is emerging as the next ubiquitous platform for the electronics

industry.[1] The realization of electronic devices with performances akin to that of rigid-based

platforms, but in lightweight, foldable, and flexible designs, would enable entirely new applications

such as conformal and transparent electronics.[1-4] Printing technologies can also play a key role

for the realization of rigid ultra-compact devices with tight assembly of components [5, 6]

conveying inherent advantages such as reduced cost and large electronic system integration by

using novel mass manufacturing approaches, unavailable from more traditional platforms.[7]

There is a clear market pull and flexible and printed electronics[8] bring advantages into devices

and components such as transistors,[4] solar cells,[9] organic light-emitting diodes,[10] and

sensors.[11] However, the real revolution is still to come due to a number of technological

challenges. In particular, commercial printed electronics should be electrically, optically and

mechanically robust, with materials and components meeting essential performance criteria, such as

low resistivity or transparency, under mechanical deformation.[12] Moreover, materials should be

environmentally-friendly. These requirements are unbearable to combine with any existing low-cost

mass-manufacturing approach. Graphene is expected to play a role here, and graphene-based

technology might deliver benefits in terms of both cost advantage and uniqueness of properties and

performance.[12]

Amongst printing and coating technologies,[13], such as spray or rod-coating, gravure,

flexographic, screen printing and laser patterning [14, 15], ink-jet printing[5] is a technique well-

suited for the direct deposition of novel nanomaterial-based inks. In an ink-jet process, it is

mandatory to obtain a regular jetting from the print-head nozzles and prevent printing instability,

such as satellite drops and jetting deflection.[16, 17] The realization of printable inks made of

nanomaterials is thus a very challenging task, since the various rheological properties such as

density (ρ), surface tension (γ), and viscosity (ν) have a strong effect on the printing process. [18]

These properties, along with the nozzle size, need to be carefully evaluated and tuned on-demand

for the proper formation and ejection of droplets from the nozzles.[19] The morphological

properties (the lateral size in particular for two-dimensional materials -2DMs-) of the

nanoparticles/nanotubes/flakes dispersed in the ink as well as the formation of aggregates in the ink

and their accumulation on the print-head can also contribute to printing instability. It has been found

[20] that limiting the lateral dimensions of the dispersed nanomaterials to ~1/50 of the nozzle

diameter can largely reduce these detrimental effects.

Several inks based on nanomaterials have been produced so far, ranging from organic

semiconductors[21] to metallic nanoparticles (MNPs)[22] and carbon nanotubes (CNTs).[23-26]

However, all these nanomaterials suffers several limitations. For example, organic

semiconductor[27], used mainly for the realization of thin film transistors (TFTs), have low

mobility (µ) of charge carriers (~ 1 cm2V

-1s

-1),[11] while metallic nanoparticles [28, 29] are mostly

based on costly materials (silver and copper) that are not stable in most common solvents (e.g.,

water, isopropyl alcohol, acetone, and many others), thus requiring stabilizing agents.[18, 30]

Moreover, once printed, MNPs-based inks tend to oxidize.[18, 30] Carbon nanotubes in theory have

the advantage, over other nanomaterial, of being electrically heterogeneous (can be both metallic

and semiconducting) in nature.[31] However, this in turn emerged as their main limitation for

practical applications, requiring a selective growth[32] and/or a sorting process for their separation

[33-37] to exploit in full the CNT electronic properties.

Thanks to their exceptional and complementary properties, graphene[38] and other 2DMs[15,

39-41] are being exploited as functional materials for ink formulation.[16, 42-45] The flakes of

these 2DMs can be dispersed in various solvents, both aqueous [46] and organic [47] by liquid

phase exfoliation (LPE) as a first step to produce printable inks.[16, 45, 48]. In particular, thanks to

their versatility, graphene-based inks have been exploited also for 3D printing, which holds great

potential for the fabrication of fully-customized, on-demand designs.[49] First example of

extrusion-based 3D printing of graphene-based structures with sub-micron diameter were recently

reported: Kwon’s group fabricated freestanding nanowires made of reduced graphene oxide (RGO)

[50], while Hersam’s group printed a composite of graphene and polylactide-co-glycolide (a

biocompatible elastomer) with potential applications in electronic, biological, and medical and

devices. [51]

The most-effective solvents for producing graphene inks, such as N-Methyl-2-pyrrolidone

(NMP), Dimethylformamide (DMF), Dimethyl sulfoxide (DMSO), just to cite a few, are not

environmentally benign [52] and this is posing a severe limitation for the development of a

graphene-based printing technology. Moreover, all the aforementioned solvents have high boiling

point (>170 °C).[53] Non-toxic and low boiling point solvents such as water and alcohols that

would be crucial to develop a fully environmentally-compatible ink-jet printing process, however,

require the addition of stabilizing agents like polymers or surfactants for the dispersion of graphene

and 2DMs. Nevertheless, the addition of such stabilizers compromises the graphene and 2DMs

(opto)electronic properties once printed on the target substrate.

In this work, after a brief overview on background and state-of-the-art of graphene inks

formulation and ink-jet printing parameters optimization, we demonstrate the printability of

graphene inks in environmentally-friendly, i.e., ethanol/water (EtOH/H2O) mixture, solvent on

flexible substrate such as polyethylene terephthalate (PET). The printed features do not need any

post-process treatment (as in case of graphene oxide (GO)[54] and RGO[55]), and, contrarily to

NMP inks, the ink-jet printing process can be carried out at low temperature (up to 60 °C). Further

optimization will enable the scaling up of production/formulation of environmentally-friendly

graphene-based EtOH/H2O inks and their deposition on flexible substrates.

2. Background on printable inks of pristine graphene

The first step in the production of graphene inks is the dispersion of the flakes in solvents to

create homogeneous, in term of lateral size and thickness, and stable dispersions.[56, 57] Solvents

such as N-methyl-2-pyrrolidone (NMP) and Dimethylformamide (DMF) are mostly used for the

production and processing of graphene dispersions as well as for their inks formulation, as they

possess minimal interfacial tension with the graphitic flakes,[58] a condition that eases the

dispersion of graphitic flakes. However, NMP and DMF are far from being optimal solvents for

fabricating devices by ink-jet printing owing to the limited sustainability of the process, since NMP

and DMF are included in the candidate list of substances of very high concern[52] and may have

teratogenic effects.[53],[59]

,[60] Moreover, the high boiling point (>170 °C)[53] of such solvents

makes difficult their removal after the coating/printing process, especially for plastic substrates due

to the limited resistance to evaporation process requiring high temperature (>150 °C). This, in turn,

influences both the morphology of the printed pattern and the optical/electrical performances (i.e.,

sheet resistance -Rs- and transmittance -Tr-).

The exploitation of low boiling-point solvents,[47] such as acetone, chloroform, isopropanol,

etc. can be an alternative. However, the γ of these solvents is too low (∼25 mN·m-1

) for the

exfoliation, making the yield (i.e., percentage of SLG) as well as the concentration of the dispersed

flakes in the solvent by far too low[47] compared to the ones achieved with, for example, NMP.[16,

56, 61] Water has a γ ∼72 mN·m-1

,[62] too high (∼30 mN·m-1

higher than NMP) for the dispersion

of graphitic flakes.[63] In this case, the exfoliated flakes can be stabilized against re-aggregation by

Coulomb repulsion using surfactants (e.g., sodium dodecylbenzenesulfonate,[57] sodium

cholate[64] and sodium deoxycholate,[58, 65] and/or polymers (e.g., pluronic [46]), etc. However,

having the dispersant molecules wrapped around the graphene/graphitic flakes [34] is not the best

option in view of the realization of conductive channels since the presence of these molecules can

decrease the inter-flake connectivity and consequently the electrical conductivity of the printed

channels.[15, 66]

As a consequence, there is a clear need for alternative solvents with appropriate rheological

properties for safety and sustainability of the printing process of graphene and 2DMs. Finding

environmentally-friendly, inexpensive and low-boiling point solvents would also make the up-

scaling of the process viable towards industrial production, where safety considerations related to

toxicity generally translate into cumbersome and costly countermeasures (e.g., safety equipment,

fume hoods, exhausts, etc.). An approach often used to avoid the use of unwanted solvents involves

the production of inks based on GO rather than pristine graphene, since GO can be easily dispersed

in safe and clean solvents such as water.[55, 67] However, GO is an insulating material and needs a

thermal or chemical treatment for its reduction.[68-70] Such post-processing treatments, usually,

require harmful chemicals, such as hydrazine,[71] or experimental conditions that are incompatible

with the fabrication of electronic devices.[72] Moreover, RGO does not fully regain the pristine

graphene electrical conductivity,[15, 73] thus limiting the possible applications of such printable

inks in flexible electronics. [1]

Several recent research efforts have been focussed on the development of printable inks of

pristine graphene flakes. One promising approach entails the exploitation of co-solvency effect to

increase the affinity between solvent and pristine graphene, as well as other 2DMs, by using a

mixture of solvents,[74] e.g., water/isopropyl alcohol,[75] water/ethanol,[74, 75] etc. By adjusting

the relative concentration of the co-solvents it is possible to tune the rheological properties (i.e., γ, ν

and ρ)[76] of the mixture “on-demand”. However, the concentration of the graphitic flakes

dispersed as well as the percentage of single layer graphene (SLG), obtained by the exfoliation

process of graphite in such co-solvent mixtures is, up to date,[74, 75] [0.39 mg mL-1

(for 2-

propanol/1,2-dichlorobenzene][77] lower than the ones achieved in NMP[16] [up to 1 mg mL-1

][78]

and water-surfactant dispersions [1 mg mL-1

].[57, 65, 79] Moreover, the stability of co-solvent

mixtures [74-76] mostly based on water and alcohols, is an issue. Indeed, γ changes exponentially

after the addition of alcohols to water[76] and is very sensitive to solvent evaporation.[75]

Moreover, all the rheological properties of alcohol-based co-solvents are very temperature-

sensitive.[76] This is a problem both during processing (the ultrasonication causes a local

temperature increase of the dispersion even if the process is thermalized) and for the shelf-life of the

dispersions/inks.

For ink-jet printing of graphene flakes, the challenge is represented by the requirements

concerning the choice of solvents able to disperse the flakes in addition to the aforementioned

constrains related to the ink printability (e.g., a printable ink should have a γ between 28-33 mN·m-

1[80]), thus limiting the selection/choice of suitable solvents. Table 1 reports the Rs and Tr literature

values obtained for stripes printed with graphene-based ink in different solvents and printing/post

processing conditions. The earliest attempts [16, 45] exploited graphene inks prepared in NMP and

DMF (subsequently exchanged to terpineol), respectively, to print conductive stripes reaching

Rs=30 KΩ/ on glass slides. Later on, graphene conductive stripes with Rs<15 KΩ/ were

reported, at the best of our knowledge, four times in literature [81] [44] [43] [48]. In Refs. [48, 81]

the authors printed graphene ink on rigid substrates (SiO2 [81] and glass [48]) previously treated

with hexamethyldisilazane, to prevent undesired coffee-ring effect of the printed features. In both

cases the as-printed graphene stripes were thermally post-annealed at temperatures higher than 250

°C, achieving Rs in the 1-3 KΩ/ range [48, 81]. Ref [44] exploited graphene ink in ethylene glycol

mixed with a copolymer of N-vinyl-2-pyrrolidone and vinyl acetate to print on “FS3” papers (a

glossy, polymer-coated paper specifically to increase the wettability), achieving Rs~1-2 KΩ/. Ref.

[43] used a graphene ink in NMP to print conductive stripes on PET foils coated with aluminium

oxide and polyvinyl alcohol (such coating reduces substrate-related drying problems [43], reaching

Rs=2 KΩ/.

Table 1. Printing tests with inks made of LPE graphene in various solvents.

Authors,

year Rs [KΩ/]

Tr [%] or

thickness

[nm]

Ink type Substrate Post-

treatment

Torrisi et

al.,

2012[16] 30 80% LPE graphite in NMP.

Si/SiO2 and glass with

hexamethyldisilazane

(HMDS) and O2 treatment.

170°C for 5

min.

Secor et

al.,

2012[81]

~3 140 nm

LPE graphite in ethanol and ethyl

cellulose mixed in

cyclohexanone/terpineol.

HMDS-treated SiO2. 250°C for

30 min.

Li et al.,

2013[45] 30 80%

LPE graphite in DMF, exchanged

to terpineol. Glass slides.

375-400°C

for 30-60

min.

Arapov et

al.,

2014[44]

1–2 800 nm

Ethylene glycol + Plasdone S-630

(copolymer of N-vinyl-2-

pyrrolidone and vinyl acetate ink).

FS3 paper (glossy, polymer-

coated paper from Felix

Schoeller) and LumiForte

paper(rough paper without

coating from Stora Enso).

None.

Finn et al.,

2014[43] ~2 160 nm LPE graphite in NMP.

PET coated with aluminium

oxide and polyvinyl alcohol. None.

Gao et al.,

2014[48] ~1 60%

LPE graphite mixed with ethyl

cellulose and cyclohexanone. HMDS-treated glass slides. None.

Building on the existing literature, we explore the use of low boiling point and environmentally-

friendly co-solvents (ethanol/water mixture), for the printing of electrically conductive graphene

stripes on PET without any pre- or post-treatments and at low temperature processing.

3. Experimental

3.1 Materials

Graphite flakes (+100 mesh, ≥75% min), NMP (99.5% purity) and ethanol (absolute alcohol,

without additive, ≥99.8%) were purchased by Sigma-Aldrich and used without further purification.

3.2 Preparation of the inks

We exploited liquid phase exfoliation of graphite[56] to produce the graphene inks[16] in NMP

and in the mixture EtOH/H2O. For the NMP-based ink, 1 g of graphite flakes (Sigma Aldrich) was

dispersed in 100 mL of NMP and ultrasonicated (Branson ® 5800) for 6 hours. The obtained

dispersion was then ultracentrifuged at ~16000 g (in a Beckman Coulter Optima™ XE-90 with a

SW41Ti rotor) for 30 mins at 15 °C, exploiting sedimentation-based separation (SBS) to remove

thick flakes and un-exfoliated graphite.[82, 83] After the ultracentrifugation process, we collected

the supernatant by pipetting. The optimization of ink-jet printing ideally requires highly

concentrated inks.[16, 45, 48] In order to achieve such a target, the supernatant extracted after the

first ultracentrifugation process was further ultracentrifuged at ~200000g for 60 mins at 15 °C. The

high g force value promotes the sedimentation of the graphene flakes at the bottom of the

ultracentrifuge tubes taking advantage of the higher density of the graphene flakes (~2.1g/cm3)[84]

in comparison with the solvent (ρNMP = 1.03 g/cm3).[85] The pellet (sedimented graphene flakes) is

collected and the supernatant is discarded. The pellet was re-suspended in 3 mL of pure NMP using

an ultrasonic bath for 10 min. This time was sufficient to re-disperse the graphene flakes, thus

obtaining a stable (for more than 2 months) ink.

The EtOH/H2O ink was then prepared as follows. 1 g of graphite was dispersed in an EtOH/H2O

mixture [1:1 in volume] by ultrasonication for 6 hours (Branson® 5800). The mixture was

centrifuged at 670g for 10 min (in a Beckman Coulter Optima™ XE-90 with a SW41Ti rotor),

longer centrifugation time or higher speed endorses the precipitation of the flakes in dispersion

together with non-exfoliated material. After the centrifugation process, the supernatant was

collected by pipetting. In order to get a concentrated graphene ink, the supernatant was ultra-

centrifuged at ~16000g for 15 min at 15 °C. The supernatant was discarded and the pellets were re-

suspended in 3 mL of pure EtOH/H2O [1:1] mixture, using an ultrasonic bath for 10 min, to re-

disperse the graphene flakes, thus obtaining the final ink. After 1 week, there is the formation of

sediments that are however easily re-dispersed in the same solvent by manual shaking of the bottle

containing the ink.

3.3 Characterization of the inks

3.3.1 Electron microscopy

As-prepared inks (both in NMP and EtOH/H2O [1:1] solutions) were characterized

morphologically (i.e., lateral size and thickness) by a by transmission electron microscopy (TEM) (

JOEL JEM 1011). The as-prepared inks (diluted 1:100 in NMP and EtOH/H2O [1:1]) were dropped

with a pipette on holey carbon 200 mesh grids and dried under vacuum overnight. The acceleration

voltage used for the measurements was 100kV.

3.3.2 Optical spectroscopy

3.3.2.1 Optical absorption spectroscopy

Optical absorption spectroscopy (OAS) of the graphene inks were performed in the range 300-

1200 nm with a Cary Varian 6000i UVvis-NIR spectrometer. The absorption spectra were acquired

using a 1 mL quartz glass cuvette. The inks were diluted to 1:10 for EtOH/H2O and to 1:100 for

NMP, to avoid scattering losses at higher concentrations. The corresponding solvent baseline was

subtracted to each spectrum.

The concentration of graphitic flakes is determined from the optical absorption coefficient at

660 nm, using A = αlc where l [m] is the light path length, c [gL−1

] is the concentration of dispersed

graphitic material, and α [Lg−1

m−1

] is the absorption coefficient, with α ~1390 Lg−1

m−1

at 660 nm.

[57, 58]

3.3.2.2 Raman characterization

The as-prepared NMP- and EtOH/H2O-based graphene inks were drop-cast onto a Si wafer

with 300 nm thermally grown SiO2 (LDB Technologies Ltd.) and dried under vacuum. Raman

measurements on both the graphene inks were collect by a Renishaw inVia confocal Raman

microscope using an excitation line of 532 nm (2.33 eV) with a 50X objective lens, and an incident

power of ~1 mW on the samples. We used Lorentzian functions to fit the peaks. For each sample

we collected more than 20 spectra.

3.3.3 Rheological measurement

The viscosity of the inks was measured with a Discovery HR-2 Hybrid Rheometer (TA

instruments), using a double-wall concentric cylinders geometry (inner diameter of 32 mm and

outer diameter of 35 mm), designed for low-viscosity fluids. The temperatures of the inks were set

and maintained at 25 º C throughout all the measurements.

3.4 Deposition of the inks on flexible substrates

The graphene inks (both in NMP and EtOH/H2O) were ink-jet printed on PET with a Fujifilm

Dimatix 2800 printer. After the preparation and characterization phase, 2 mL of ink was loaded into

the cartridge reservoir (fluid bag) via a syringe with a needle. The filled cartridge was then loaded

in the printer.

3.5 Characterization of printed patterns

3.5.1 Electron microscopy

The printed patterns were imaged by loading the printed PET samples in a field-emission

scanning electron microscope (FE-SEM) (Joel JSM-7500 FA) without any pre-treatment.

3.5.2 Optical absorption spectroscopy

Transmittance spectra of the printed patterns were performed in the range 300-1200 nm with a

Cary Varian 6000i UVvis-NIR spectrometer, using a 1mm pinhole holder. The pristine PET

substrate was used as a baseline. Each sample was measured 5 times and the averages value were

reported.

3.5.3 Raman spectroscopy

Raman measurements on the printed stripes (both from NMP- and EtOH/H2O based inks) are

collected by a Renishaw inVia confocal Raman microscope using an excitation line of 532 nm (2.33

eV) with a 50X objective lens, and an incident power of ~1 mW on the samples. We used

Lorentzian functions to fit the peaks. For each sample we collected 20 spectra.

3.5.4 Electrical characterization

All the electrical measurements have been performed with a Keithley Model 2612A Dual-

channel System Source Meter in two wires configuration. The printed pattern (1 by 4 mm) was

placed crossing two parallel palladium wires spaced by 1 mm. The crossing, between the printed

pattern and the palladium wires, defines a 1 by 1 mm measuring area. A constant voltage of 1 V

was applied.

4. Results and discussion

The production of the environmentally-friendly graphene ink in ethanol and water mixture was

carefully designed to obtain a stable ink, having the required parameters in term of ρ, γ and ν for the

optimal ink-jet printing process.[19] In this context, graphene flakes are neither stable in ethanol nor

in water due to the mismatch between the γ of these two solvents (72 mN·m-1

for water)[62], and

24.5 mN·m-1

for ethanol[50] and the surface energy of graphite.[15]

According to Wang et al. in order to obtain stable dispersion of graphene flakes, γ should be

close to ~46.7 mN·m-1

[63] (for comparison γ of NMP is ~41 mN·m-1

).[56] Some groups[47, 86-89]

also exploited the Hansen solubility parameters (HPs)[90] to study the stability of graphene flakes

in diverse solvents. The HPs[90] are three: the energy from dispersion forces between molecules

(D), the energy from dipolar intermolecular force between molecules (P) and the energy from

hydrogen bonds between molecules or electron exchange parameter (H).[90] These parameters

generally describe the solubility of a molecule in a solvent.[90] In our case the HPs describe the

graphene dispersion stability in different solvents.[47, 86-88, 90] The required HPs of a solvent to

stabilize graphene flakes are: D ~18 MPa1/2

, P~10 MPa1/2

, H ~7 MPa1/2

,[47, 86-88] (for

comparison HPs of pure NMP are: D ~17.4 MPa1/2

, P~13.7 MPa1/2

, H ~11.3 MPa1/2

.[47, 86-88,

90] These three parameters can be treated as coordinates for a point in three dimensions also known

as the Hansen space.[90] According to Hansen’s Handbook,[90] the distance between two points in

the Hansen space is known as Hansen distance (Ra), expressed in MPa1/2

. Thus, an ideal solvent for

dispersion of a molecule or material is the solvent which has the HP located in the Hansen space

closer to the coordinates of the molecule or material.[90] Consequently, shorter is the Ra, between

the Hansen coordinates of the solvent and the Hansen coordinates of the material or molecule,

stronger will be the solvent/material interaction.[90] Contrary, if the Ra is large, the solvent is not

adequate to disperse or stabilize the material. In our particular case the Hansen distance can be

defined as: [90]

𝑅𝑎2 = (𝛿𝑃G − 𝛿𝑃S)2 + 4(𝛿𝐷G − 𝛿𝐷S)2 + (𝛿𝐻G − 𝛿𝐻S)2 Equation 1.

Where the subscripts G and S refers to graphene and the solvent mixture, respectively.

The HP values for the mixture EtOH/H2O are calculated as follows:[90]

𝛿𝑃S = 𝜏𝛿𝑃EtOH + (1 − 𝜏)𝛿𝑃H2O Equation 2.

𝛿𝐷S = 𝜏𝛿𝐷EtOH + (1 − 𝜏)𝛿𝐷H2O Equation 3.

𝛿𝐻S = 𝜏𝛿𝐻EtOH + (1 − 𝜏)𝛿𝐻H2O Equation 4.

Where 𝜏 is the volume fraction of ethanol.

Table 2 shows the γ adapted from ref. [91] and the calculated HPs for different ratios of EtOH/H2O

mixtures and their respective Ra to the graphene Hansen coordinates.

Table 2. γ values were taken and adapted from ref.[91]. The HPs for different EtOH:H2O volume ratios

were calculated.

% Vol γ Hansen Solubility Parameters (MPa1/2

)

EtOH H2O mN·m-1

D P H Ra

0 100 72.72

16.70 18.70 16.70 19.96

10 90 51.80

16.61 17.71 16.97 18.41

20 80 41.47

16.52 16.72 17.24 16.96

30 70 36.25

16.43 15.73 17.51 15.62

40 60 33.18

16.34 14.74 17.78 14.45

50 50 30.90

16.25 13.75 18.05 13.47

60 40 29.00

16.16 12.76 18.32 12.72

70 30 27.43

16.07 11.77 18.59 12.27

80 20 26.25

15.98 10.78 18.86 12.13

90 10 25.37

15.89 9.79 19.13 12.32

100 0 24.55

15.80 8.80 19.40 12.82

The calculated Ra corresponding to the best γ for graphene (~46.7 mN·m-1

[63]), reported in

Table 2, indicates that the EtOH/H2O (20:80) mixture provides the best condition to obtain stable

graphene dispersion. The calculated value agrees with results reported by other research groups.[47,

86-88, 90] (for comparison, the calculated Ra between NMP and graphene is 5.7 MPa1/2

). [86, 90] In

principle the EtOH/H2O (20:80) mixture should be the ideal combination to perform exfoliation and

dispersion of the graphene flakes. However, considering the fluid formulation guidelines for the

Fujifilm Dimatix 2800 printer, the optimal γ values should be in the range of 28-33 mN·m-1

.[80]

Values in this order can be met with a 50% volume ratio EtOH:H2O (30.90 mN·m-1

). As reported in

Table 2, the 50% EtOH:H2O volume ratio shows a good compromise between γ and HP, thus we

produced the ink starting from this volume ratio of the two solvents.

The rheological properties of the as-formulated inks were characterized by means of OAS and

viscosity measurements under shear conditions. The concentration of the two inks was calculated by

OAS. Fig 1a) shows the absorption spectra of NMP ink diluted 100 times and the EtOH/H2O ink

diluted 10 times. The peak at ~266 nm, for both inks, is a signature of the van Hove singularity in

the graphene density of states.[92] The asymmetry of the UV peak in both spectra, with a high-

wavelength tail, is attributed to excitonic effects.[93] The obtained concentrations were 3.32 mg

mL-1

for NMP ink and 0.62 mg mL-1

for EtOH/H2O ink.

Figure 1. a) Absorption spectra of graphene inks in EtOH/H2O (green curve) and NMP (orange

curve) diluted 1:10 and 1:100 respectively. The concentrations are 0.62 mg mL-1

for EtOH/H2O ink

and 3.32 mg mL-1

for NMP ink. b) ν of the produced inks vs shear rate for EtOH/H2O (green curve)

and NMP (orange curve) inks. In the inset-table the ν values of the inks and of the solvents are

reported, measured at 25 °C for a constant shear rate of 10 s-1

(each value was averaged over 100

measurements).

Concerning the ν of our inks, Figure 1b shows the ν of the inks under shear stress (the ν values

at the constant shear rate of 10 s-1

are reported as an inset). The NMP ink has ν = 3.14 mPa s which

is almost twice the one of NMP solvent (ν = 1.59 mPa s). By contrast, the EtOH/H2O ink has ν =

2.47 mPa s, which is only 8.5% higher than the EtOH/H2O solvent mixture’s one (2.26 mPa s).[94]

ν of the NMP-based ink decreases with increasing shear rates (Fig. 1b), showing shear thinning

properties, as expected for a structured fluid (i.e., a colloidal suspension).[95, 96] The ν of the

EtOH/H2O-based ink instead is instead almost independent of the shear rate, at least in the shear

rate range here investigated, a behaviour typical of Newtonian fluid (i.e., a fluid in which the

viscosity arising from its flow is linearly proportional to the strain rate, such as water).[97] These

different trends for the two inks are directly linked to their concentrations in the respective solvents,

being the NMP-based ink five times more concentrated than the EtOH/H2O ink. Indeed, diluting the

NMP ink (concentration value of ~0.6 mg mL-1

, i.e., comparable with the EtOH/H2O one) we

obtained ν values independent on shear rate. These results demonstrates that the inks’ viscosities are

within the range required for inkjet printing (up to 12 mPa s).[80]

We further carried out the characterization of the morphological properties of the flakes

dispersed in the two inks by Raman spectroscopy, which is a fast and non-destructive technique

widely used to identify number of layers, defects, doping, disorder and chemical modifications of

graphene.[98, 99] Figure 2a plots in black the typical spectrum, with excitation wavelength of 532

nm, of graphite deposited on Si/SiO2. Additionally, the Raman spectra of representative flakes of

NMP- and EtOH/H2O-based graphene inks are plotted in orange and green lines, respectively. The

G peak corresponds to the E2g phonon at the Brillouin zone centre.[100]

Figure 2. (a) Raman spectra of graphite and graphene inks in NMP (orange) and EtOH/H2O (green), (b)

Pos(2D) for graphene ink in NMP and EtOH/H2O, Distribution of (c) FWHM(2D), (d) I(2D)/I(D), (e) ratios

I(D)/I(G), (f) Pos(G), (g) FWHM(G) and (h) I(D)/I(G) ratios as a function of FWHM(G) .

The D peak is due to the breathing modes of sp2 rings and requires a defect for its activation by

double resonance.[98, 99, 101] The 2D peak is the second order of the D peak.[98] This is a single

peak in monolayer graphene, whereas it splits in multi-layer graphene, reflecting the evolution of

the band structure.[98] The 2D peak is always seen, even when no D peak is present, since no

defects are required for the activation of two phonons with the same momentum, one backscattered

from the other.[98] Double resonance can also happen as intra-valley process, i.e. connecting two

points belonging to the same cone around K or K’.[98] This process gives rise to the D’ peak. The

2D’ is the second order of the D’. From statistical analysis, based on 20 measurements for each

sample, we find the 2D peak position (pos(2D)) peaked at ~2695 cm-1

and ~2700 cm-1

for NMP-

and EtOH/H2O graphene inks, respectively. The full width at half maximum of 2D (FWHM(2D))

(Fig.2b) varies from 62 to 76 cm-1

and from 64 to 78 cm-1

NMP- and EtOH-based graphene inks,

respectively. The I(2D)/I(G) ranges from 0.45 to 0.7 for NMP-based ink and from 0.4 to 0.75 for

EtOH/H2O-based ink (Fig 2d). This is consistent with the samples being a combination of SLG and

few-layer graphene (FLG) flakes.[82] The Raman spectra show significant D and D’ peaks

intensity, with I(D)/I(G) ranging from 0.1 to 0.5 and from 0.4 to 1.6 for EtOH/H2O and NMP-based

inks, respectively (Fig. 2e). This is attributed to the edges of our sub-micrometre flakes[102] rather

than to the presence of a large amount of structural defects within the flakes. Indeed, if a large

amount of defects are present in the basal plane of graphene the G and D’ peak become thicker

merging into a broader band,[101] which is not the case for the spectra of our inks. Moreover, there

is not a clear correlation between I(D)/I(G) and FWHM(G) (see Fig. 2h), an indication that the

major contribution to the D peak comes from the sample edges, for both NMP- and EtOH/H2O-

based ink. We can attribute this to disorder in the graphene edges rather that defects within

flakes.[102] Indeed this information is reinforced analysing the Pos(G) and FWHM(G) (Fig. 2f and

2g respectively).

The low-resolution TEM bright field images (Fig. 3a and 3e for NMP- and EtOH/H2O-based

inks, respectively) ) show that graphene flakes in the two inks have markedly different lateral size

distributions, with an average lateral size peaked at ~200 nm for the NMP-based ink and a much

broader size distribution in the 100-2000 nm range for the EtOH/H2O-based ink (see histograms in

Fig 3b and 3f). The high-resolution images of isolated flakes in Fig. 3c (NMP) and Fig. 3g

(EtOH/H2O) and the corresponding electron diffraction patterns (Fig. 3d and Fig. 3h) demonstrate

that the flakes are crystalline. All the rings can be indexed as h,k,-h-k,0 reflections of an hexagonal

lattice with a=0.244(1)nm, in agreement with the graphene structure.[103]

Figure 3. TEM analysis of graphene flakes cast from NMP- (upper row) and EtOH/H2O-based

(lower row) inks: a) and e) low-magnification region with several flakes; b) and f) histograms of

statistical lateral size distribution; c) and g) high-magnification images of isolated flakes with d)

and h) selected area diffraction patterns.

Printing on PET

When using a new ink, the printer settings need in general to be adjusted in order to obtain a

regular and constant drop jetting.[18, 19] In our case, we ran preliminary tests with both NMP- and

EtOH/H2O-based ink, founding out a set of parameters for the two inks. Following this procedure,

we were able to make a comparison of the printed patterns made with the two inks in the same

printing conditions. Clearly, the printer parameters can be further optimized for each ink to improve

the process to a greater extent, a study that is beyond the scope of this work. In particular, the main

parameters to be set are the cartridge temperature, the nozzle’s voltage waveform, the drop spacing

and the platen temperature, as explained in details in the following. The cartridge temperature can

be controlled to regulate the working ν of the fluid, if needed. In our case it was set to 35 °C for all

the depositions. A voltage waveform controls the movements of the printhead nozzles. During the

printing process, the nozzles need to draw fluid into the pumping chamber, hold the fluid for a

certain time, and then eject the fluid out forming a drop.[19] These movements are driven and

controlled by piezoelectric elements.[19] The voltage applied to the piezoelectric elements has a

specific waveform that can be edited to regulate the drop ejection and speed. We designed a

waveform that triggered a regular ejection of drops for both the inks. A low amplitude pulse was

additionally given to the nozzles at a frequency of 2 KHz to prevent the accumulation of residual

flakes on the nozzle outlet. The drop spacing is the centre-to-centre distance in X and Y of the drops

that the printer deposits on the target substrate to create the pattern. The optimization of the drop

spacing is crucial to avoid possible “coffee ring” effects, which would limit the uniformity of the

flakes spreading.[104] The “coffee ring” effect takes place during the solvent evaporation and is

more or less marked on the account of the ink ν and the interface tension existing between the

substrate and the solvent.[20, 105] The “coffee ring” effect is responsible of an anisotropic

deposition of the material on the ring of the drops and might reduce the homogeneity of the

deposition.[104] After preliminary tests, the drop spacing was fixed to 25 µm, a value that limited

the “coffee ring” effect and provided a reasonable uniformity in the deposition with both the inks.

After tuning the printer settings, the ejected drops came out in phase amongst each other, all with a

regular round shape, as shown in Fig. 4 for the EtOH/H2O ink.

Figure 4. Drop formation of one single nozzle after the optimization of the ink-jet parameters for the

EtOH/H2O ink.

1 to 5 nozzles out of the 16 available on the printhead were used for printing. The use of more

than 1 nozzle reduces the time required for the process, which can be lengthy for a high number of

printing passes, but it deteriorates the reproducibility of the process, particularly relevant in the

optimization phase of the ink-jetting We found a good compromise for the rheological properties of

our inks with up to 5 nozzles. The PET substrate was kept at 60 °C (i.e., the maximum temperature

possible of the printer’s platen) to ease the solvent evaporation during the print. Rectangular stripes

(5 × 2 mm) were printed with a number of printing passes spanning from 3 to 25. We aimed at

printing continuous layers of interconnected graphene flakes without voids or agglomerates in order

to obtain an uniform coverage of the substrate within the whole pattern area. The continuity and

uniformity of the printed graphene stripes was desired to lower their Rs as much as possible,

keeping a constant Tr.

The morphology of the printed flakes as well as the coverage obtained on the substrate after

each print were characterized by SEM analysis. The SEM micrographs of the printed stripes in Fig.

5 highlight some differences in the deposition carried out with the two inks. After 3 printing passes,

the area printed with the NMP ink is completely covered with graphene flakes (Fig. 5a)), while with

the EtOH/H2O ink the deposition is less continuous with areas covered with graphene flakes

alternated with ones where the coating is not uniform (Fig. 5c). In these cases, such differences can

be mainly accounted for by the different concentrations of the two inks (3.32 mg mL-1

for NMP ink

and 0.62 mg mL-1

for EtOH/H2O ink). It is interesting to note, however, that the graphene flakes

from NMP ink appear all embedded in the printed stripe and do not show well-defined boundaries

between each other (identifiable instead in the EtOH/H2O printed flakes): This may be caused by

some NMP residues left after the printing process, considering the high boiling point of the solvent

(~202 °C).[58]

When increasing the number of printing passes to 12 or more, the stripes printed with both inks

appear more uniform with interconnected graphene flakes (see the SEM micrographs in Figs. 5b

and d), and therefore a continuous film is formed.

Figure 5. FE-SEM images of the surface of the patterns ink jet printed on PET with the two different

graphene inks while varying the number of passes. NMP ink: (a) 3 and (b) 12 printing passes. EtOH-H2O

ink: (c) 3 and (d) 15 printing passes.

Figure 6 shows the Raman spectra of graphene patterns printed on PET while varying the

number of passes. Up to 12 printing passes, the graphene bands are superimposed to those Raman

peaks of the PET. With the increase of the number of passes only the graphene peaks are seen.

Moreover, the D peaks and 2D peaks of graphene show no significant changes in shape or intensity

ratios for printing passes higher than 18. The analysis of the Raman spectra in Figs. 6c-f highlights

that by increasing the number of printed passes, the deposited material shows Raman features very

close to those of the corresponding graphene inks. In particular, the 2D peak still displays a

FWHM(2D) distinctly different from that of graphite. This implies that the flakes are electronically

decoupled, behaving as a collection of either SLG or FLG. Moreover, Raman analysis, and in

particular the I(D)/I(G) ratio as function of the FWHM(G), does not reveal structural defects on the

deposited flakes for both inks, see Fig. 6.

Figure 6. Raman spectra of bare PET foil and patterns printed with inks in (a) NMP and (b) EtOH/H2O.

Peak analysis of 12 and 25 printed passes, (c) FWHM(G), (d) I(D)/I(G), (e) Pos(2D), (f) FWHM(2D), (g)

I(2D)/I(G) and (h) I(D)/I(G) vs FWHM(G).

The Rs of the printed stripes vs the Tr at 550 nm is reported in Fig. 7. The graphene stripes

printed from NMP have a Rs ranging from 22 to 173 KΩ/, with a corresponding Tr ranging from 5

to 37%. The Rs of the printed graphene films from EtOH/H2O ranges from 13 KΩ/ to more than

10 MΩ/, with a corresponding Tr of 22 to 70%, respectively. Due to the higher concentrations of

the NMP ink, the NMP films are on average less transparent than the EtOH/H2O ones for same

numbers of printing passes.

Figure 7. Tr vs Rs for 6, 12, 18 and 25 printing passes. Inset: optical image of printed stripes for NMP (left)

and EtOH/H2O (right) inks after 6 and 25 printing passes on PET substrates, respectively.

The electrical measurements confirm that a good substrate coverage was obtained after 3

printing passes only with the NMP ink, while almost only isolated flakes were obtained with the

EtOH/H2O in the same printing conditions.

With 6 printing passes, the EtOH/H2O stripe has a Tr=70% but the Rs is higher than 10 MΩ/,

meaning that the printed graphene flakes form a rather discontinuous percolation path for the charge

carriers (as observed by SEM in Fig. 5 for 3 printing passes). With the same printing passes (6), the

NMP stripe instead has Rs=173 KΩ/ (with Tr=37%), as result of a better electrical percolation

path. By increasing the number of printing passes to 12, the EtOH/H2O stripes have lower Tr (55%

vs 22%), but similar Rs value (~75 KΩ/). At 18 printing passes and beyond, the stripes printed

from EtOH/H2O have at the same time higher Tr and lower Rs than the NMP ones.

These trends are remarkable as they demonstrate that by printing on PET at low temperature and

without any post-annealing treatments, higher values of conductivity can be reached with an

EtOH/H2O ink by depositing a smaller amount of material (the same number of printing passes, 18

or more, but with an ink ~5.3 times less concentrated), keeping at the same time a higher Tr. The

conductivity of the NMP stripes could most probably be increased by a post-annealing treatment at

high temperatures (as did in ref. [16] at 170°C) to further remove solvent residual on and between

the graphene flakes, but such process would be of course destructive for PET. Different ranges of

annealing temperatures in inert atmosphere could be explored to improve the NMP stripes

conductivity/transmittance ratio without compromise the substrate, but such optimization is beyond

the scope of this work and, however, still require post-processing treatments that we have

demonstrated here to be overcome by the use of EtOH/H2O-based inks.

5. Conclusion.

We demonstrated the use of low boiling point and environmentally-friendly solvents, such as

ethanol and water to produce graphene- and few-layer graphene-based inks suitable for ink-jet

printing processes. A volume ratio of 1:1 between these solvents gives a surface tension of 30.9

mN·m-1

, which is in the required range for ink-jet printing (28-33 mN·m

-1). We were able to obtain

inks of graphene and few-layer graphene in EtOH/H2O, reaching concentrations of ~0.6 mg·mL-1

.

These inks were used to print conductive stripes with sheet resistance of ~13 KΩ/. Our work is the

first attempt in the graphene printing technology that is at the same time 1) based on

environmentally-friendly solvents, 2) carried out on flexible substrates, 3) does not require any pre-

or post-treatments. Remarkably, the sheet resistance values obtained for our printed stripes compare

with those reported by other groups exploiting not environmentally-friendly solvents and with post-

processing treatments.

Currently, there is much room for improvement and many challenges have to be met to optimize

the stability of the ink as well as the printing process. These inks have potential to be scaled up to

the industrial level and exploited in the production of printed batteries, solar cells, super-capacitors

and conductive flexible patterns, just to cite a few applications.

Acknowledgements

The research leading to these results has received funding from the European Union Seventh

Framework Programme under grant agreement n°604391 Graphene Flagship.

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