Omar Kassem

Mohamed Saadaoui

Mathilde Rieu
email: rieu@emse.fr

Sergio Sao

Jean Paul

Synthesis and inkjet printing of sol-gel derived tin oxide ink for flexible gas sensing application

Keywords: SnO2, inkjet printing, printed electronics, flexible substrate, gas sensor

The fabrication of printed electronic devices based on metal oxide inks requires the formulation of stable suspensions with specific fluidic properties. In our work, a tin oxide based solution was synthetized by aqueous sol-gel method, and transformed into an ink with appropriate viscosity and surface tension to be inkjet-printed on polyimide foil and sintered at relatively low temperature. Thermal analysis by TGA/DSC and microstructural analysis by XRD of synthetized sol show that a crystallized structure of SnO2 could be obtained at 350°C, which is lower than crystallization temperatures of SnO2 previously reported in the literature, and entirely consistent with the use of polyimide foil. The stability and the rheological properties of the ink were studied to ensure the jettability criteria of the inkjet-printer. Electrical measurements of the printed sensing films were performed to characterize the response to CO gas in different concentrations, at working temperature of 300°C.

Introduction

Flexible electronics holds a prominent position and is expected to meet emerging technological demands and unprecedented applications compared to rigid electronics devices [START_REF] Khan | Technologies for Printing Sensors and Electronics Over Large Flexible Substrates: A Review[END_REF][START_REF] Yan | An Intrinsically Stretchable Nanowire Photodetector with a Fully Embedded Structure[END_REF]. Particularly, the manufacture of flexible gas sensors based on metal oxide thin film presents an interesting perspective for progress in many fields such as the monitoring of atmospheric environment, the detection of hazardous gases [START_REF] Acuautla | Ozone flexible sensors fabricated by photolithography and laser ablation processes based on ZnO nanoparticles[END_REF][START_REF] Uddin | A novel flexible acetylene gas sensor based on PI/PTFE-supported Ag-loaded vertical ZnO nanorods array[END_REF] and smart food packaging [START_REF] Matindoust | Food quality and safety monitoring using gas sensor array in intelligent packaging[END_REF]. However, it is subject to restrictions requiring high operating temperatures which are not compatible with plastic foils.

The use of metal oxides as gas-sensitive material has been widely investigated because of its remarkable sensing properties and the simple sensing mechanism based on changes of conductance in contact with some gases [START_REF] Wang | Metal Oxide Gas Sensors: Sensitivity and Influencing Factors[END_REF]. Metal oxide gas sensors were introduced in the year 1960 by two Japanese researchers: Seiyama who studied the electrical resistance variation of ZnO for the detection of reducing gases and Taguchi who put in place the first gas sensor based on a SnO2 as sensitive material [START_REF] Neri | First fifty years of chemoresistive gas sensors[END_REF]. Among the various types of metal oxide materials, tin dioxide (SnO2) has attracted considerable attention, widely used as gas sensor [START_REF] Choi | Novel fabrication of an SnO 2 nanowire gas sensor with high sensitivity[END_REF][START_REF] Li | SnO2 Highly Sensitive CO Gas Sensor Based on Quasi-Molecular-Imprinting Mechanism Design[END_REF] because of its high chemical stability, low cost, rapid response and ease of integration during sensor design. SnO2 is an n-type semiconductor with a wide band-gap (3.6eV). The gas sensing mechanism mainly originates from the under-stoichiometric tin oxide state (SnO2-x) when exposed to air [START_REF] Jimenez | Interface effects for metal oxide thin films deposited on another metal oxide II. SnO2 deposited on SiO2[END_REF] due to oxygen vacancies in the crystal structure.

Metal oxide could be synthesized by different methods including chemical precipitation [START_REF] Fu | Effect of La doping on microstructure of SnO2 nanopowders prepared by co-precipitation method[END_REF],

hydrothermal method [START_REF] Supothina | Effect of synthesis condition on morphology and yield of hydrothermally grown SnO2 nanorod clusters[END_REF], sonochemical process [START_REF] Sedghi | Low temperature CO and CH4 dual selective gas sensor using SnO2 quantum dots prepared by sonochemical method[END_REF], surfactant mediated method [START_REF] Ma | Preparation and characterization of SnO2 nanoparticles with a surfactant-mediated method[END_REF], radical oxidation [START_REF] Zang | Single cuprous oxide films synthesized by radical oxidation at low temperature for PV application[END_REF] and double-phase method [START_REF] Zang | Enhanced fluorescence imaging performance of hydrophobic colloidal ZnO nanoparticles by a facile method[END_REF]. However, in comparison with other chemical routes, the sol-gel technique is relatively simple and inexpensive, and it was widely used for preparation of functional inks [START_REF] Vidmar | Inkjet printing of sol-gel derived tungsten oxide inks[END_REF][START_REF] Rieu | Fully inkjet printed SnO 2 gas sensor on plastic substrate[END_REF]. This technique allows easy control of synthetized oxides and the elaboration of materials in the most varied forms (Thin films, fine powders, fibers, etc). Moreover, it can be operated under low temperatures [START_REF] Brinker | Sol-Gel Science The physics and chemistry of sol-gel processing[END_REF].

Once tin oxide based sol is chemically synthetized, it must be deposited onto the foil in order to make a thin film. Many traditional coating techniques have been implemented to prepare sensitive material as thin film including aerosol assisted CVD [START_REF] Vallejos | Microfabrication of flexible gas sensing devices based on nanostructured semiconducting metal oxides[END_REF], screen printing [START_REF] Dubourg | Fabrication and Characterization of Flexible and Miniaturized Humidity Sensors Using Screen-Printed TiO2 Nanoparticles as Sensitive Layer[END_REF], spray coating [START_REF] Yu | Flexible spray-coated TIPS-pentacene organic thin-film transistors as ammonia gas sensors[END_REF] and inkjet printing [START_REF] Rieu | Fully inkjet printed SnO 2 gas sensor on plastic substrate[END_REF]. In particular, inkjet printing technology is currently attracting a special attention for the manufacture of electronic components [START_REF] Kumashiro | Novel materials for electronic device fabrication using ink-jet printing technology[END_REF][START_REF] Azzellino | Fully Inkjet-Printed Organic Photodetectors with High Quantum Yield[END_REF][START_REF] Eggenhuisen | High efficiency, fully inkjet printed organic solar cells with freedom of design[END_REF][START_REF] Singh | Inkjet Printing-Process and Its Applications[END_REF] because this additive technique has the advantage of being fast, accurate and inexpensive, with its compatibility with various substrate and availability of contactless and no-mask patterning [START_REF] Huang | Graphene-based conducting inks for direct inkjet printing of flexible conductive patterns and their applications in electric circuits and chemical sensors[END_REF]. The principle of inkjet printing consists in the formation and the ejection of drops by propagating acoustic pressure waves in the fluid held in a chamber behind the printing nozzle [START_REF] Derby | Inkjet Printing of Functional and Structural Materials: Fluid Property Requirements, Feature Stability, Resolution[END_REF].

Nevertheless, challenges remain associated with the control and stability of the jet itself. Undesirable artifacts, such as deviated jets or unwanted generation of additional droplets, also known as satellite drops, can indeed occur and strongly influence the uniformity and the performance of printed components.

Therefore, it is crucial to study the stability of the ink and work on the adjustment of its rheological properties to be well printed with high resolution.

In this paper, inkjet was adopted as a deposition technique in order to print a flexible tin oxide gas sensing films. A stable sol was synthetized by sol-gel process, and was used as precursor ink with appropriate viscosity and surface tension. After deposition and curing, a SnO2 film was obtained and its electrical properties as a gas-sensing layer were tested under carbon monoxide injections.

Materials and methods

Synthesis of SnO2 based sol by aqueous sol-gel route

The SnO2 sol was prepared using aqueous sol-gel process as described in the literature for the preparation of SnO2 nanopowders [START_REF] Köse | Structural properties of size-controlled SnO2 nanopowders produced by sol-gel method[END_REF]. The process consists in dissolving 18 mmol of tin chloride (SnCl2 Sigma-Aldrich) in 100 ml of distilled water. Then, a 25% ammonia solution (NH3 Sigma-Aldrich) was added for complete removal of chloride ions. The addition of NH3 is stopped when the pH is 9. A precipitate of tin hydroxide was obtained (Sn(OH)n , n=2 or 4) and separated from the solvent by passing the solution through filter paper on a Büchner funnel and washed several times with diluted ammonia solution. The precipitate was dissolved in glacial acetic acid (Sigma-Aldrich) and ethylene glycol (Sigma-Aldrich) was added in order to form a tin complex. The mixture was heated at 90° C and a pale yellow sol containing a complex of tin cations was obtained after 1h (Fig. 1). Tin concentration in the sol was calculated to be 0.13 M.

SnO2 ink formulation for inkjet

We use a commercial lab-scale Dimatix inkjet-printer (DMP-2800, FujiFilm) equipped with a 10 pL cartridge. This printer requires an ink with surface tension in the range of 28-33 mN/m, viscosity in the range of 10-12 mPa.s, and pH between 4 and 9. Finally, the ink formulation requires particles with diameter smaller than 1μm to avoid nozzles clogging [START_REF] Hoeng | Inkjet printing of nanocellulose-silver ink onto nanocellulose coated cardboard[END_REF].

For ink preparation, solvents mixture was added to the sol in order to adjust its rheology. For homogeneous and stable ink preparation, we used solvents with high boiling temperature (Tb) and low vapor pressure (Pvap) as ethylene glycol (Tb= 197.3°C, Pvap= 8 Pa) and glycerol (Tb= 290°C, Pvap= 0.4 Pa). Ethanol was then added for adjusting ink surface tension and viscosity. Finally, 2-Isopropoxyethanol surfactant was used for fine-tuning of the surface tension.

We use the dimensionless parameter Z to quantify the behavior of the ink in the inkjet printer. This dimensionless number is the inverse of the Ohnesorge number Oh, which can be used to measure the ratio between surface tension and viscous forces:

Z = 𝑂h -1 = √ρ. γ. a η
Where ρ, γ and η the density, surface tension and viscosity of the fluid, and "a" is a characteristic dimension, i.e., the diameter of the nozzle (DMC-11610 with a nozzle diameter of 21.5 µm were used)

The Z parameter is calculated during ink preparation and it should be between 1 and 10 for successful inkjet printing [START_REF] Derby | Inkjet Printing of Functional and Structural Materials: Fluid Property Requirements, Feature Stability, Resolution[END_REF].

Contact electrodes on foil

A 50µm thick polyimide foil was selected as a substrate (Upilex® S-Goodfellow) because of its high thermal and chemical resistance, and its excellent surface smoothness (Rz < 60 nm). Prior to processing, the substrate was cleaned by acetone, isopropanol and deionized water then it was dried in an oven at 120°C for 30 minutes.

In order to perform electrical measurements, gold electrodes have to be deposited. A 100 nm thick gold layer was e-beam evaporated onto polyimide (Boc Edwards -Auto 500) with four sources heated by the Joule effect. A 3 nm thick chromium layer was evaporated prior to gold to act as an adhesion layer.

A 1064 nm wavelength, pico-second pulsed laser source was used to pattern gold onto foil (Protolaser R, LPKF). Electrodes have width of 1 mm and space of 500 µm and 200 µm. The laser frequency and power were optimized to 75 KHz and 0.8 W in order to reduce heat diffusion and to avoid damages at the interface between gold and polyimide. The gold electrodes on foil are presented in Fig. 2.

Characterization techniques

The prepared sol was characterized by dynamic light scattering (DLS) using photocorrelation spectrometer, to estimate the average hydrodynamic size and size distribution of the particles (Malvern Instruments, Malvern, U.K.). For calculation, the refractive index of SnO2 and the room temperature viscosity of the sol were taken at 2.006 and 5 mPa.s, respectively.

Simultaneous thermogravimetric analyses (TGA) and differential scanning calorimetry (DSC) of the SnO2 xerogel (dried sol) was carried out in a Mettler-Toledo TGA/DSC STARe System under 5 L/h air flow at 10°C/min. Prior to characterization, the sol was heated at 80°C for few hours in standard oven to obtain a xerogel, and then it was placed in open alumina pan crucible.

Surface tension and contact angle measurements were characterized using a goniometer (Apollo Instrument OCA200, Dataphysics). We use Owens-Wendt-Rabel-Kaelble (OWRK) method to determine the surface energy of substrates [START_REF] Kaelble | Dispersion-Polar Surface Tension Properties of Organic Solids[END_REF]. This method is based on contact angle measurements of droplets of at least three reference liquids landing on the surface. The surface energy of the solid γ 𝑆 is additively made up of dispersive and polar parts:

γ 𝑆 = γ 𝑆 𝑑 + γ 𝑆 𝑝
Where γ 𝑆 𝑑 and γ 𝑆 𝑝 are the dispersive and polar part of the surface energy, respectively.

γ 𝑆 could be extracted from the equation:

γ 𝐿 (1 + cos 𝜃) = 2√γ 𝐿 𝑑 γ 𝑆 𝑑 + 2√γ 𝐿 𝑝 γ 𝑆 𝑝
Where θ is the contact angle between the surface of the substrate and the liquid, γ 𝐿 is the surface tension of the liquid, γ 𝐿 𝑑 and γ 𝐿 𝑝 are dispersive and polar surface tension of the liquid, respectively [START_REF] Owens | Estimation of the surface free energy of polymers[END_REF][START_REF] Rudawska | Analysis for determining surface free energy uncertainty by the Owen-Wendt method[END_REF].

During this work, we use deionized water, diiodomethane, ethylene glycol and a mixture of 40% ethylene glycol and 60% (Volume) water to determine the surface energy of substrates. Surface tension values of each liquid are summarized in table 1. the reference JCPDS (00-041-1445) and the crystallite size was obtained by the Scherrer formula from the broadening of X-ray diffraction peaks:

𝐷 = 0.9𝜆

𝛽𝐶𝑜𝑠𝜃

Where D is the crystallite size of crystalline structure, λ is the X-ray wavelength (1.5406 A°), β is the full width at half maximum (FWHM) and θ is the Bragg angle.

Atomic force microscopy (AFM) was performed to determine the microstructural properties of the printed film using atomic force microscopy Nanowizard III (JPK Instruments AG, Germany) in the tapping mode.

The image was obtained from a 1 µm 2 surface, scanning with 512 by 512 pixels.

Thickness of printed layers was determined by SEM microscopy (Carl Zeiss -Ultra 55). A cross-section of the SnO2 layer was prepared using Broad Ion Beam (BIB) equipment (Ilion Gatan), at a beam energy of 6 keV. The thickness was estimated using ImageJ software.

Finally, electrical characterization of the printed sensor was performed using CO gas, at 300°C in dry synthetic air. A cylinder of CO gas (300 ppm, diluted in nitrogen) purchased from Air Product and Chemicals Company was used. The sensor was placed on a heating element equipped with 2 metallic probes in a glass chamber (500mL) kept under continuous flowing air at a rate of 15 L.h -1 . The flexible SnO2 gas sensor was stabilized at the operating temperature for a duration between 6h and 12h before starting CO injection. A K type thermocouple (Chromel/Alumel ) was used to control the operating temperature of the sensor. Sensor response measurements, 20 min exposure to CO/air mixture and 35 min recovery in air, were recorded for different concentration from 20 ppm to 100 ppm of CO gas. Gas injections were done using flow meters, and data acquisition was controlled through LabView software.

Results and discussion

Sol characterization

The size distribution profile of the sol synthesized at 90°C for 1h was analyzed by dynamic light scattering (DLS). The average hydrodynamic diameter of the particles was found to be equal to 6 nm ±3 nm. This result demonstrates the complete peptization of the Sn(OH)n in acetic acid and ethylene glycol as no aggregation was detected. The presence of aggregation may lead to nozzles clogging during inkjet printing. In Fig. 4, X-ray diffraction patterns of the sol being annealed at temperatures up to 500°C is shown. We can notice that the samples annealed at 200°C and 300°C are slightly crystalline with an amorphous background. Crystallization of SnO2 occurs at temperature around 350°C, which is lower than crystallization temperatures of 400°C and 500°C reported in literature [START_REF] Rieu | Fully inkjet printed SnO 2 gas sensor on plastic substrate[END_REF][START_REF] Bagheri-Mohagheghi | The effect of the postannealing temperature on the nano-structure and energy band gap of SnO2 semiconducting oxide nano-particles synthesized by polymerizing-complexing sol-gel method[END_REF], making this SnO2 deposit compatible with plastic foil. XRD peaks become progressively narrower as the calcination temperature increases. On the other hand, no structural transformation occurred and diffraction peaks can be attributed to the rutile structure. The crystallite size determined by the Scherrer formula was estimated to be approximately 4.5 nm ± 0.1 nm at 350°C, 4.8 nm ± 0.1 at 400°C and 8.0 nm ± 0.1 nm at 500°C, respectively.

Figure 4 XRD patterns of SnO2 films annealed at different temperatures for 1 hour

SnO2 ink formulation and inkjet printing

The synthesized tin oxide sol has to be formulated into an ink suitable for inkjet printing.

Step one involved the adjustment of the pH as the synthesized sol is acid with pH 2.4. This was achieved by stirring the sol at 80°C for a few hours to evaporate the majority of the acetic acid, thus increasing the pH up to 4-5. The heating temperature was chosen below the boiling temperature of acetic acid in order to avoid aggregation of nanoparticles in the solution.

At this step, the obtained solution is composed of 0.4 M SnO2 in ethylene glycol and residual amount of acetic acid, with a high surface tension of 40 mN/m and a viscosity of 14.1 mPa.s. First, we added ethanol gradually to adjust ink rheology and we found that ethanol decreases surface tension and viscosity of the ink down to 29 mN/m and 6.2 mPa.s, respectively. Then, we added glycerol to increase the ink viscosity up to 11 mPa.s but glycerol, which has a surface tension 64 mN/m, increased the surface tension of the mixture to 37 mN/m. Finally, the addition of 2-isopropoxyethanol allows fine-tuning of the surface tension to a target value of 32 mN/m. The dimensionless Z number of the ink is 2.7 indicating good printing quality (target range between 1 and 10). Table 2 summarized final proportions of added solvents and ink properties. The stability of this ink was maintained over several months if the ink is refrigerated and stored in a closed vessel.

Uniform droplets ejection was successfully achieved with an adjusted waveform using 17 V as firing voltage and 5 KHz as frequency. The gap between the nozzle and the substrate was maintained at 1 mm.

Very good ejection quality (Fig. 5) was then achieved with single spherical droplets well oriented vertically.

Figure 5 Strobe photographic images showing an ejected drop of SnO2 ink

After adjusting printing parameters, single droplets were printed onto Upilex in order to determine the best drop spacing. The diameter of these drops measured using optical microscope (Nikon Eclipse LV150) was 46 µm ± 1 µm (Fig. 6) and the best drop spacing that gives good surface coverage when printing rectangle was then fixed at 25 µm.

Substrates surface energy

For sensor fabrication, SnO2 ink will be printed onto polyimide and over gold electrodes. As the gold will be patterned locally, the surface energy of polyimide after laser treatment was also characterized.

Table 3 summarizes surface energy γ 𝑆 values of materials used in the present study. The surface energy is one of the fundamental parameters controlling the wettability of ink on substrate [START_REF] Ely | Patterning quality control of inkjet printed PEDOT:PSS films by wetting properties[END_REF]. The substrate should present a surface energy higher than the surface tension of the ink in order to allow the ink spreading. In our case, the surface energy of gold deposit by evaporation and the cleaned Upilex were measured at 38 mN/m and 47 mN/m, respectively. The surface energy of Upilex decreases down to 42 mN/m after laser patterning. The decrease of the surface tension of polyimide could be explained by surface roughness, and chemical modifications in polyimide surface after exposure to laser beam [START_REF] Bizi-Bandoki | Modifications of roughness and wettability properties of metals induced by femtosecond laser treatment[END_REF][START_REF] Lu | Polyimide surface modification by pulsed ultraviolet laser irradiation with low fluence[END_REF]. Surface energies of Upilex and gold are compatible with the surface tension of formulated ink of 32 mN/m. The surface energy of the substrate is great enough to wet and spread the ink without the need of any surface treatment.

Sintering of SnO2 ink

The sintering temperature of the ink ([SnO2]ink = 0.09 M) was fixed at 350°C for 1 h (heat rate of 5°C/ min) in order to crystallize SnO2. The annealing was performed in Rapid Thermal Annealing (RTA) oven (JIPELEC, Jet First). It is well known in the literature that sol-gel solution deposited SnO2 films suffer from pronounced cracks because of capillary forces exerted by the evaporating solvents leading to shrinkage of pores [START_REF] Hench | The sol-gel process[END_REF][START_REF] Shiomi | Preparation of SnO2 monolithic gel by solgel method[END_REF]. In addition, the difference in coefficient of thermal expansion between plastic and SnO2 thin film may also enhance cracks within the film. Both phenomena maybe responsible of tensile stress that generates channeling cracks when its value exceeds the tensile strength of the film. In the literature, successful attempts were conducted to prevent cracks in sol-gel films by reducing the capillary force during drying, using solvents with low surface tension and low evaporation rate like glycerol, N,Ndimethylformamide (DMF) and formamide (FA), and using gels with larger pores [START_REF] Shiomi | Preparation of SnO2 monolithic gel by solgel method[END_REF][START_REF] Kadhim | Effects of glycerin volume ratios and annealing temperature on the characteristics of nanocrystalline tin dioxide thin films[END_REF]. However, this proposal is ineffective when using inkjet because of ink rheological constraints mentioned above.

The top surface views of the samples were examined by an optical microscope (Nikon Eclipse LV150) and a scanning electron microscope (SEM Ultra 55 -Carl Zeiss). Fig. 7 shows the top surface views of different SnO2 films obtained by different conditions: single pass printing, two layers and three layers printed by wet on wet and wet on dry method. The wet on wet method consist in printing a next layer directly after the first one, while in wet on dry method, the first layer is completely sintered at 350°C before printing the next one. Figure 8 shows clearly the formed cracks on the surface of the film printed two and three times by wet on wet method. Crack-free SnO2 films were achieved by using a single-print of the ink (Fig. 7a). Cracks number and width increase with the number of printed layers by wet on wet method (Fig. 7b, 7d and Figure 8). Printing more layers leads to larger amount of solvent in the film, and therefore, the capillary forces exerted by the evaporating solvents become more important which induce more retraction and strong cracks propagation [START_REF] Lee | Why Do Drying Films Crack?[END_REF]. These cracks may raise the resistance of films by altering the electronic conduction path. On the other hand, homogeneous and crack-free film could be obtained by introducing an annealing step at 350°C between every single print during multi-pass printing (wet on dry method, Fig. 7c,7e). In this case, a low heating ramp of 5°C/min was used in order to reduce film-drying speed which tends to reduce the capillary force that lead to crack-free films.

The microstructure and the morphology of printed sensing film were extracted from 1.0 μm × 1.0 μm AFM image. Figure 9 shows that the SnO2 printed film, made of nano-sized particles, with size range between 30 and 40 nm, is well uniform and dense without porosity. The surface roughness of the printed film is about 5.5 nm.

Figure 9: AFM images of the SnO2 film printed three times by wet on dry method on polyimide and cured at 350°C

A 5x5 mm² square shape of SnO2 was printed onto Upilex and over gold electrodes 3 times by wet on dry method, and sintered at 350°C for 1h with heating ramp of 5°/min. The sensor layout is presented in Fig. 10. We have fixed the electrodes spacing to 200µm and we have tested the sensors made with one and two printed layers (Fig. 14). The conductance of SnO2 increases with the increasing number of layers: the conductance of one, two and three layers of SnO2 when the sensor is exposed to air is about 1.3x10 -8 Ω -1 , 2.2x10 -8 Ω -1 and 4.2x10 -7 Ω -1 respectively. As in the case of three layers described previously, the response increases upon exposure to CO gas and returns to the original value upon exposure to air. Sensor conductance under CO exposure increases with the number of printed layers. As electrical conductance under air is increased by increasing the film thickness, the conductance measured under CO is also higher.

Figure 1

 1 Figure 1 Mixture of tin hydroxide with acetic acid and ethylene glycol before heating (left); after heating (right) at 90°C

Figure 2

 2 Figure 2 Gold electrodes patterned on plastic foil using laser, with spacing of 500 µm (left) and 200

Fig. 3

 3 Fig. 3 displays TGA/DSC curves of the SnO2 xerogel. A weight loss of about 88 % is clearly observed

Figure 3

 3 Figure 3 TGA/DSC thermograms of the SnO2 xerogel

Figure 6

 6 Figure 6 Optical micrograph of SnO2 ink drops printed on Upilex observed by Dimatix printer camera (left) and optical microscope (right)

Figure 7 :Figure 8 :

 78 Figure 7: Optical micrographs of SnO2 thin film obtained by printing (a) once (b) twice wet on wet (c) twice wet on dry (d) three times wet on wet (e) three times wet on dry

Figure 10 :

 10 Figure 10: Sensor Layout observed under binocular microscope

Figure 11 :

 11 Figure 11: Cross -sectional image of triple layer of wet on dry SnO2

Figure 12 :Figure 13 :Figure 14 :

 121314 Figure 12: The change of conductance of SnO2 thin film printed three times (wet on dry method), with electrodes spacing of 500 µm and 200 µm, upon exposure to different concentrations of CO gas at 300°C

Table 1 Properties of liquids used for contact angle measurements

 1

		γ 𝐿 𝑝 (mN/m)	γ 𝐿 𝑑 (mN/m)	γ 𝐿 (mN/m)
	Water	51	21.8	72.8
	Diiodomethane	0.38	50.42	50.8
	Ethylene Glycol	21.3	26.4	47.7
	Ethylene Glycol 40%	40.04	20.25	60.29

Structural evolution of SnO2 with temperature was studied using X-ray diffraction recorded on Siemens D5000 at 40 mA and 40 kV with Cu-Kα radiation (1.5406 A°). Droplets of SnO2 sol were deposited on (111) silicon wafer and then annealed at different temperatures. Diffraction patterns were compared with

Table 2 proportions of SnO2 ink solvents with rheological properties given at 20°C

 2

			Ethylene
								Surface
			Glycol +	Ethanol	Glycerol	2isopropoxyethanol	Viscosity		Z
		[SnO2]ink						tension
			Acetic acid	% vol.	% vol.	% vol.	(mPa.s)		number
								(mN/m)
			% vol.
	Ink SnO2	0.09 M	57	29	5	9	10	32	2.7

Table 3 total surface energy of Upilex and Gold substrate

 3

	Substrate

Gas sensing application

It should be noted that sensors analyzed in this part are printed by wet on dry method to avoid cracks formation.

The influence of CO gas on the sensor response was studied at 300°C. We tested two different sensors containing three layers, with electrodes spacing of 200 µm and 500 µm. These layers were printed annealed at 350°C with a low heating ramp as described previously. Fig. 12 shows the response of SnO2.

We notice that the conductance of SnO2 increases upon exposure to different concentration of CO gas (20-100 ppm), and it drops rapidly and returns to its original state when the injection of CO gas is stopped in the testing atmosphere in each cycle, indicating that the gas sensor has reversible response for different CO concentrations. The change of conductance is mainly due to the interactions between the CO gas and the surface of the semiconductor sensor. At 300°C, CO gas reacts with the oxygen ions chemisorbed on the surface of SnO2 to form CO2 and release an electron, which leads to an increase of conductance [START_REF] Jin | Application of nano-crystalline porous tin oxide thin film for CO sensing[END_REF][START_REF] Tiemann | Porous Metal Oxides as Gas Sensors[END_REF]. Once the CO injection is interrupted, the reaction between CO and the oxygen ions is stopped, and the conductance returns to its initial state. Consequently, this result demonstrates that the deposited SnO2 layers by inkjet have adequate properties for gas sensing [START_REF] Li | SnO2 Highly Sensitive CO Gas Sensor Based on Quasi-Molecular-Imprinting Mechanism Design[END_REF].

We notice that the conductance under air increases by decreasing the electrodes spacing. The electrical conductance under air of the sensor with electrodes spacing of 200 µm was 4.2x10 -7 Ω -1 , while it was 2.6x10 -7 Ω -1 for the sensor with electrodes spacing of 500 µm. In order to compare the response of these two sensors, we define the normalized sensor response as G/G0 where G and G0 are the electrical sensor conductance when exposed to CO gas and air, respectively. Fig. 13 shows the normalized sensor response as function of CO concentration (in ppm) when the SnO2 thin film was printed three times, with electrodes space of 500µm and 200µm, at an operating temperature of 300°C. We observe that the response to CO increases by decreasing the electrodes spacing. With electrodes space of 500µm, the response to 40 ppm CO is 6.3 while it is 7.5 with electrodes space of 200µm. This correlation, between the electrodes geometry and the gas sensor performance, is compatible with the results discussed in the literature [START_REF] Tamaki | Effect of micro-gap electrode on detection of dilute NO2 using WO3 thin film microsensors[END_REF][START_REF] Lee | Electrodes for Semiconductor Gas Sensors[END_REF].