Nanocrystalline Pt thin films prepared by electrostatic spray deposition for automotive exhaust gas treatment

Solid State Ionics 177 (2007) 3491 – 3499 www.elsevier.com/locate/ssi

Nanocrystalline Pt thin films prepared by electrostatic spray deposition for automotive exhaust gas treatment A. Lintanf, R. Neagu, E. Djurado ⁎ Laboratoire d'Electrochimie et de Physico-Chimie des Matériaux et des Interfaces, Institut National Polytechnique de Grenoble, Domaine Universitaire, BP 75, 1130 rue de la Piscine, 38402 St Martin d'Hères Cedex, France Received 13 June 2006; received in revised form 16 October 2006; accepted 24 October 2006

Abstract The deposition of a platinum electrode on 8 mol% Y2O3-stabilized ZrO2 (YSZ) using the electrostatic spray deposition (ESD) technique was investigated. Films with various morphologies were obtained starting from different precursor solutions based on Pt(NH3)4(OH)2(H2O), PtCl4 and Pt(acac)2 dissolved in different solvents. The surface morphology was strongly influenced by the composition of the precursor solution, the deposition temperature, the nozzle to substrate distance, the precursor solution flow rate and the deposition time. The processes involved in the film formation were discussed. Nanocrystalline cubic Pt single-phase was present in the raw film as shown by X-ray diffraction. © 2006 Elsevier B.V. All rights reserved. Keywords: Electrostatic spray deposition; Nanocrystalline platinum; Yttria stabilized zirconia; Thin films; Electrocatalyst

1. Introduction Metallic platinum thin films present considerable interest for their wide range of applications including contacts in microelectronic devices [1–4], buffer layers [5] in the fabrication of high-Tc superconducting tapes, electrochemical and catalytic applications [6]. Pt is widely used as electrode material and catalyst in high temperature electrochemical devices such as oxygen sensors and oxygen pumps or NOx conversion. Supported platinum catalysts allow high and durable NOx conversion in real diesel exhaust, especially in the low temperature domain from 200 to 350 °C [7], corresponding typically to the working temperature of an engine in urban cycle. Moreover, they are resistant to poisoning by steam or SO2 present in the exhaust stream. However, the major drawback of platinumbased catalysts is that the majority of NO is reduced to N2O, a greenhouse effect gas, rather than being reduced to harmless N2. In order to selectively reduce nitrogen oxides in lean-burn engine exhausts, porous Pt deposited on yttria stabilized zirconia (YSZ) is also extensively investigated for the electrochem-

⁎ Corresponding author. Tel.: +33 4 7682 6684; fax: +33 4 7682 6777. E-mail address: [email protected] (E. Djurado). 0167-2738/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2006.10.019

ical promotion of catalysis (EPOC) [8]. This innovative concept is based on the enhancement of the catalytic function by electrochemically pumping O2− ions between a solid electrolyte, generally Y2O3-stabilized ZrO2 (YSZ), and the surface of a

Fig. 1. Experimental ESD set-up.

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Table 1 Different precursor solutions Precursor

Pt(NH3)4(OH)2(H2O)

PtCl4

Pt(acac)2

Solvents

H2O/EtOH (80:20, vol.%) H2O/BC (80:20, vol.%)

H2O/EtOH (80:20, vol.%) H2O/BC (33:67, vol.%)

H2O/BC (33:67, vol.%)

Solvents

porous catalyst. Therefore, the activity of the catalyst and its selectivity to N2 can be controlled in situ by an overpotential application. In addition, an increased catalytic activity requires a smaller quantity of catalyst. At the moment, most of the platinum films are obtained from platinum paste annealed at high temperature (800 °C) using a large quantity of platinum and leading to a bad dispersion of particles. Much work tried to improve the platinum selectivity to N2 by changing the nature of the support [7], the platinum dispersion [7], the preparation method of Pt film such as sol–gel method [9] and by using various hydrocarbons as reaction gases in catalytic tests [10]. Up to now, there is no practical effective solution. The morphology of the gas-exposed catalyst surface remains an influent parameter on the catalytic performances. Our long-term project will investigate the enhancement of catalytic activity and selectivity to N2 versus the microstructure of a platinum electrocatalyst prepared by Electrostatic Spray Deposition (ESD) and using the EPOC concept. The present work is focused on the preparation of platinum electrocatalyst thin films deposited on 8 mol% Y2O3-stabilized ZrO2 (YSZ) controlling their microstructure using the ESD process. In this process, an electrostatic field is set-up across the nozzle and the grounded substrate. This field penetrates the liquid surface, acts on the ions in the solution and induces the surface charge that causes an outward electrostatic pressure on the solution. This pressure is opposite to the inward directed pressure caused by surface tension. This leads to surface insta-

bilities. When the electrical field is strong enough, the electrostatically stressed liquid surface can be distorted into a stable conical shape (Taylor cone) [11]. The shape of the cone depends also on the conductivity of the solution. We have selected the ESD technique because it is particularly suitable in tailoring the film morphology of a large variety of ceramic thin films [12,13] and presents a good reproducibility. Moreover, it is an inexpensive manufacturing process for coating technology. In addition, the using of a minimal quantity of platinum consumed to produce films in the electrical field is an attractive feature for cost reduction. Very homogeneous layers with good adherence are expected. The influence of the ESD process parameters such as precursor solution, deposition temperature, nozzle to substrate distance, precursor solution flow rate and deposition time on the Pt coatings were studied in order to control the microstructure of the platinum coatings. A comparative study of the influence of different precursor solutions on the microstructure of platinum films was conducted as well. 2. Experimental section The platinum thin films were deposited on polished 8 mol% Y2O3-stabilized ZrO2 (YSZ) substrate, 20 mm in diameter and 1 mm in thickness, using a vertical ESD set-up in order to avoid the arrival of atypical droplets on the substrate (Fig. 1). The ESD technique uses an electrical field to atomize the precursor solution and transport the aerosol, towards a heated substrate, where a coating is formed. Different precursor solutions were prepared using ethanol EtOH (C2H5OH 99.9% from Prolabo) or butyl carbitol BC (diethylene glycol monobutyl ether, CH3(CH2)3(OC2H4)2OH, 99+% from Acros Organics) and distilled water as solvents. Platinum acetylacetonate (Pt(acac)2 98% from Acros Organics), platinum chloride (PtCl4 from Acros organics) and tetraammineplatinum (II) hydroxide hydrate Pt(NH3)4(OH)2(H2O), 59%

Fig. 2. DTA of the precursor solutions (heating rate: 1 °C/min).

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Fig. 3. DTA/TGA of the precursor (heating rate: 1 °C/min).

Pt, from Strem Chemicals) have been used as precursor salts. The different precursor solutions are presented in Table 1. The total concentration of the salt in the solution was constant and equal to 0.01 M. Solutions conductivities were measured with a CDRV 62 conductimeter. Thermal analyses were performed in air at a 1 °C/min heating rate using Netzsch Simultaneous Thermal Analyser STA 409 instrument. A syringe pump (Sage M361) was used to control the ESD solution flow rate in the range 0.67 to 1.67 mL/h. The precursor solution was atomized using a positive high voltage of 8 to 12 kV, connected to a stainless steel needle, 0.6 mm in diameter. The deposition temperature was ranging from 214° to 434 °C. The nozzle to substrate distance was varied from 27 to 47 mm. The time deposition was 5 to 60 min. Surface morphologies were analyzed using Scanning Electron Microscopy (SEM) (LEO S440). X-ray diffraction was carried out using a Siemens D500 θ/2θ diffractometer in the Bragg Brentano geometry (30° to 95° 2θ range, 0.04° 2θ step, 8 s counting time) with Fe cathode (λ = 0.1936 nm). Phases were identified using DIFFRAC-AT software (Socabim. Paris). The average crystallite size was calculated by applying Scherrer's formula (1) on the (111) XRD peak of Pt at 50°, D¼

0:9k bcosh

(thermal behavior, electrical conductivity, surface tension, boiling point of the precursor solution). In a second part, the influence of the ESD deposition parameters (deposition temperature, nozzle to substrate distance, precursor solution flow rate, deposition time) on the microstructure of Pt coatings obtained from Pt(NH3)4(OH)2(H2O) was discussed. 3.1. Role of the precursor solution on the atomization of Pt(NH3)4(OH)2(H2O) The precursor solution is consisted of Pt(NH3)4(OH)2(H2O) dissolved in a mixture of H2O/alcohol (80: 20, vol.%) (ethanol or butyl carbitol). DTA analyses of this precursor solution without and with the salt such as tetraammineplatinum (II) hydroxide hydrate are shown in Fig. 2. If the solution contains 80 vol.% H2O and 20 vol.% ethanol, only one endothermic peak at 78 °C – which corresponds to the boiling point of the solution – was observed, due to the presence of an azeotrope point in this non-ideal solution (However, the boiling point usually extends from 83 °C to 94 °C for 80:20, vol.% in the H2O/EtOH mixture). When the precursor salt is incorporated in this solution, a slight boiling point increase of about 5 °C was measured as shown in Fig. 2 and may be the cause of different

ð1Þ

where D is the crystallite size (in nm), λ is the wavelength (in nm), β is the corrected FWHM (in radians) from high purity silicon and θ is the diffraction angle. 3. Results and discussion A part of the present work was focused on the influence of the composition of platinum precursor solution on the coating microstructure. The precursor solution based on Pt(NH3)4(OH)2 (H2O) was investigated, in terms of physico-chemical properties

Table 2 Physico-chemical characteristics of solvents Solvents

Boiling point Bp (°C) a

Conductivity σ (S/cm) b

Surface tension γ (N/m) c

Butyl carbitol Ethanol Water

230 78 100

85 × 10− 9 1.4 × 10− 9 0.5

0.030 0.022 0.073

a D.R. Lide. Handbook of Chemistry and Physics 75th edn. (CRC Press. London 1994). b Measured values with a CDRV 62 conductimeter. c J.A.Dean.Lange. Handbook of Chemistry 14th edn. (McGraw-Hill Inc. New York. 1992).

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Fig. 4. Measured conductivities of different precursor solutions.

surface microstructures obtained by ESD as shown in the following. In fact it has been reported that surface morphologies of ESD coatings are very sensitive to a deposition temperature shift of about 5 °C [15]. When butyl carbitol is used instead of ethanol, another small endothermic peak is observed at higher temperature (146 °C) as shown in Fig. 2, due to the separation of components in this ideal solution. In fact this endothermic accident is observed only at 138 °C with salt addition. This can be explained by DTA analysis of the precursor salt which shows an endothermic peak at a lower temperature from 120 °C to 130 °C (Fig. 3) assigned to the

loss of one H2O molecule. Moreover, the exothermic peak observed by DTA analysis at about 200 °C was related to the complete thermal decomposition of the precursor salt, since a loss of 4 NH3 and 1/2 O2 molecules were observed by TGA analysis simultaneously. In the ESD process, the conductivity of the solution and the surface tension are influent parameters on the cone shape. The Taylor cone will be obtained for a low conductivity (10− 11– 10− 6 S cm− 1) [11] while a solution with a very high conductivity (10− 3–10− 1 S cm− 1) leads to the formation of a multi-jet cone [11]. The influence of the surface tension γ and the

Fig. 5. SEM micrographs of films deposited on YSZ for 1 h from a precursor solution based on Pt(NH3)4(OH)2(H2O) dissolved in: (a) H2O/EtOH (80:20, vol.%) at 260 °C (b) H2O/EtOH (67:33, vol.%) at 260 °C (c) H2O/BC (80:20, vol.%) at 434 °C (d) H2O/BC (67:33, vol.%) at 434 °C. Flow rate: 1.67 mL h− 1. Nozzle to substrate distance: 47 mm.

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conductivity σ of the precursor solution on the droplet size is described by the following Kelvin's relation: sffiffiffiffiffiffiffiffi 2 3 ge 0 ð2Þ dc r2 q where ϵ0 is the electrical permittivity of vacuum, γ is the surface tension, σ is the conductivity and ρ is the density of the solution. Physico-chemical properties of the precursor solution, such as surface tension γ [14] and conductivity σ are reported in Table 2 and Fig. 4. Fig. 5a and b show porous microstructures with more or less dried particles as a function of solvent composition. When the

Fig. 7. SEM micrographs of films deposited at 260 °C on YSZ by spraying for 1 h a solution containing Pt(NH3)4(OH)2(H2O) in H2O/EtOH (80:20, vol.%) at different nozzle to substrate distances: (a) 27 mm, (b) 37 mm, (c) 47 mm. Flow rate: 0.67 mL h− 1.

Fig. 6. SEM micrographs of films deposited on YSZ for 1 h by spraying a solution containing Pt(NH3)4(OH)2(H2O) dissolved in H2O/EtOH (80:20, vol.%) for different deposition temperatures: (a) 260 °C (b)343 °C (c) 434 °C. Flow rate: 1.67 mL h− 1. Nozzle to substrate distance: 47 mm.

concentration in ethanol is increased from 20 to 33 vol.%, a decrease of 19 and 22% was found respectively for the surface tension [14] and the conductivity of the solution (Fig. 4). In fact, the effect of the surface tension is minor compared to the conductivity parameter. Indeed, the γ/σ2 ratio in the Kelvin's relation (Eq. (2)) increases leading to a slight increase of droplet size of about 10%. Indeed, larger particles are observed (Fig. 5b) for a larger ethanol concentration. Moreover, a decrease of the precursor solution conductivity of 18% was measured when ethanol has been replaced by butyl carbitol (Fig. 4). This corresponds to an increase of the droplet size of 15% as deduced from the Kelvin's relation (Eq. (2)). These larger droplets are at the origin of a larger spreading of liquid droplets which arrive on the substrate and can explain the formation of a dense layer when

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substrate heated at 260 °C. When the temperature is increased from 260 °C to 343 °C and 434 °C this layer becomes relatively flat and shows large pores. Contrary to the literature, where a coral-like microstructure is expected for the higher temperature, we have found a surface with incorporated particles on a dense sub-layer, this surface being characterized by a lower recovering content. We could interpret this thermal evolution as an effect of the precursor decomposition. Indeed, when the substrate temperature (343 and then 434 °C) exceeds the temperature of Pt(NH 3 ) 4(OH) 2(H 2O) decomposition (280 °C), the film is more porous as shown in Fig. 6b and c. At 434 °C the temperature near the substrate is so high that the precursor particles are decomposed during the flight and a flat dense layer of pure platinum is deposited directly from the vapour phase.

Fig. 8. SEM micrographs of films deposited on YSZ for 1 h by spraying a solution containing Pt(NH3)4(OH)2(H2O) in H2O/EtOH (80:20, vol.%) for different flow rates and nozzle to substrate distances: (a) 0.67 mL h− 1 and 27 mm (b)1.67 mL h− 1 and 27 mm (c) 1.67 mL h− 1 and 47 mm. Deposition temperature: 260 °C.

20 vol.% and 33 vol.% butyl carbitol are used as shown respectively in Fig. 5c and d. 3.2. Influence of deposition temperature The influence of deposition temperature was investigated on the microstructure of Pt coatings starting from Pt(NH3)4 (OH)2(H2O) in H2O/EtOH (80:20, vol.%). Fig. 6 shows different surface morphologies of Pt films deposited at 47 mm for 1 h with 1.67 mL h− 1 at different temperatures (260 °C, 343 °C and 434 °C). As shown in Fig. 6a, the coating at 260 °C presents small dried and agglomerated particles on a dense sub-layer (Fig. 9). This dense sub-layer is due to the presence of large droplets which arrive liquid and are spreading on the

Fig. 9. SEM micrographs of films deposited on YSZ by spraying a solution containing Pt(NH3)4 (OH)2 (H2O) in H2O/EtOH (80:20, vol.%) for (a) 5 (b) 10 (c) 30 min. Deposition temperature: 260 °C. Flow rate: 1.67 mL h− 1. Nozzle to substrate distance: 47 mm.

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Fig. 10. SEM micrographs of films deposited on YSZ for 1 h, starting from PtCl4 at (a) 343 °C and (b) 434 °C, starting from Pt(NH3)4 (OH)2(H2O) at (c) 343 °C and (d) 434 °C. Solvent composition: H2O/EtOH (80:20, vol.%). Flow rate: 1.67 mL h− 1. Nozzle to substrate distance: 47 mm.

3.3. Influence of the nozzle to substrate distance The influence of the nozzle to substrate distance was studied on the surface morphologies of Pt coatings starting from Pt (NH3)4(OH)2(H2O) in H2O/EtOH (80:20, vol.%). Fig. 7 shows the morphologies of films deposited at 260 °C for 1 h for nozzle to substrate distances of 27, 37 and 47 mm. We have selected a lower flow rate (0.67 mL h− 1), compared to the previous one, for a shorter nozzle to substrate distance in order to avoid the formation of cracks in the films. At short distances an accumulation of liquid on the substrate could lead to the film crack due to the fast drying step occurring at the end of the deposition process [16]. In Fig. 7a, corresponding to a short distance of 27 mm, we observe a deposit free of cracks which consists of particles wrapped in the layer. This microstructure is the result of a large spreading of the droplets still liquid at such a short nozzle to substrate distance. When the distance is increased from 27 to 37 and 47 mm (Fig. 7b and c) a rougher surface is obtained. Indeed, the increase of the distance leads to a larger evaporation of the solvents from the droplets during the transport. Therefore the droplets dry completely and the resulting smaller particles impact the substrate. 3.4. Influence of the solution flow rate The amount of precursor solution arriving onto the substrate is not only controlled by the nozzle to substrate distance but also

by the solution flow rate as shown by our previous work on the preparation of dense and thin doped zirconia films[17]. In this previous work, we have defined optimal ESD conditions investigating the correlations between parameters such as substrate temperature, nozzle to substrate distance and solution flow rate. All these parameters influence the equilibrium between the flux of incoming solution and the solvent evaporation at the substrate surface. The effect of a decrease of the nozzle to substrate distance in the ESD process was found similar to an increase of the solution flow rate. In the case of Pt films, Fig. 8a and b show the influence of the solution flow rate (0.67 and 1.67 mL h− 1) on the surface morphologies of Pt films deposited at 260 °C for 1 h at 27 mm. For a high solution flow rate of 1.67 mL h− 1, some cracks are observed (Fig. 8b), certainly due to an excess of liquid in droplets which is at the origin of large stresses during the final fast drying step. This liquid accumulation comes from an increase of about 35% of the droplet diameter related to the higher flow rate, as shown by the following relation [11]: d~e1=6  r

 1=3 Q j

ð3Þ

where Q is the solution flow rate, d is the diameter of droplets, εr the relative electrical permittivity of the solution and k is the electrical conductivity of the solution. When the solution flow rate is larger, the nozzle to substrate distance has to be increased in order to compensate the solvent excess in arriving droplets on

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3.5. Influence of the deposition time The influence of the deposition time was followed on the surface morphologies of Pt films starting from Pt(NH3)4(OH)2 (H2O) in H2O/EtOH (80:20, vol.%). Fig. 9 shows the evolution versus time – from 5 to 30 min – of the morphology of a layer deposited in the same conditions as previously, i.e. at 260 °C, at 47 mm with 1.67 mL h− 1. Under such conditions a microporous film is obtained after 1 h of deposition (Fig. 6a). A first thin (≈ 50 nm) and dense layer is observed after 5 min (Fig. 9a) when the first droplets are spreading on the substrate. Simultaneously, solid particles can be observed on the surface coating. The atomization in multi-jet mode leads to a broader particle size distribution. Moreover, droplets disruption during the aerosol transport occurs starting from volatile alcohol as solvent and for a long nozzle to substrate distance. Therefore, when time is increased (Fig. 9b and c), a porous film is growing up to 120 nm in thickness. This is due to a phenomenon usually called preferential landing — the first solid particles arrived on the substrate act as concentrators of the electrical field lines and the incoming particles and droplets are preferentially attracted on top of those already in the coating. This causes particle agglomeration and the roughness of the coating is increased [11]. 3.6. Comparative study of precursor solutions Fig. 11. SEM micrographs of films deposited on YSZ for 1 h from different precursor solutions containing (a) Pt(acac)2 (b) PtCl4, dissolved in H2O/BC (33:67, vol.%). Deposition temperature: 343 °C. Flow rate: 0.67 mL h− 1. Nozzle to substrate distance: 47 mm.

the substrate in order to prevent the formation of cracks. In our case the distance was increased from 27 to 47 mm – as shown in Fig. 8b and c – keeping constant the large 1.67 mL h− 1 flow rate. Cracks have disappeared since the evaporation of solvents occurred during the transport. Consequently the particles arrive dried on the substrate and lead to a microporous microstructure with a recovery similar to Fig. 8a.

Our objective is the preparation of coatings with different microstructures in order to be applied as new electrocatalysts for automotive exhaust gas treatment. Up to now, we have studied the influence of ESD parameters such as deposition temperature, solution flow rate and nozzle to substrate distance on the microstructure of Pt films for one precursor solution. In the following, the ESD microstructural evolution of Pt films will be investigated as a function of the substrate temperature and conductivity of different precursor solutions based on the same solvent composition. The main criterion of choice of solvents was the solubility of precursors.

Fig. 12. XRD pattern of Pt film deposited on YSZ from precursor solution containing Pt(NH3)4(OH)2(H2O) for 1 h at 260 °C with 1.67 mL h− 1 flow rate. Nozzle to substrate distance: 47 mm.

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3.6.1. Starting from Pt(NH3)4(OH)2(H2O) and PtCl4 in H2O/ EtOH (80:20, vol.%) Similar porous microstructures were obtained starting from PtCl4 (Fig. 10a and b) or Pt(NH3)4(OH)2(H2O)(Fig. 10c and d) when the solution was atomized for 1 h at 47 mm with 1.67 mL h− 1 at 343 °C (Fig. 10a and c) and 434 °C (Fig. 10b and d). At these temperatures, Pt(NH3)4(OH)2(H2O) is fully decomposed while PtCl4 decomposition – which occurs at 575 °C – is incomplete and leads to the presence of residues as shown previously in this paper. Moreover, when Pt(NH3)4(OH)2(H2O) is replaced by PtCl4, the conductivity increases by 330% as shown in Fig. 4. Consequently, particles size was estimated to decrease of 62% from the Kelvin's equation (Eq. (2)), keeping constant surface tension, permittivity and density for the same precursor solution. During aerosol transport from the nozzle to the substrate, smaller droplets will evaporate faster leading to drier particles which impact the substrate. A “cauliflower” type morphology was obtained (Fig. 10a and b).

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reported in this present work. Chloride residues were fully eliminated after annealing at 600 °C and no microstructural modification was detected after thermal treatment. 4. Conclusion To conclude, we have successfully deposited different microstructures of nanocrystalline cubic Pt coatings starting from different precursor solutions and investigating the optimal ESD process parameters such as deposition temperature, nozzle to substrate distance, precursor solution flow rate and deposition time. When Pt(NH3)4(OH)2(H2O) and PtCl4 are used as precursors in appropriated solvents, microporous morphology was usually obtained while Pt(acac)2 precursor solution led to dense films. Catalytic measurements are currently in progress in order to investigate the role of the microstructural properties of platinum films on the N2 selectivity and the propene to CO2 conversion. Acknowledgment

3.6.2. Starting from PtCl4 and Pt(acac)2 in H2O/BC (33:67, vol%) Two different precursor solutions using PtCl4 and Pt(acac)2 in a different mixture of solvents based on water and butyl carbitol instead of a mixture of water and ethanol were investigated. This choice is required by the insolubility of Pt(acac)2 in ethanol. The deposition has been carried out at 47 mm for 1 h at 343 °C with a low flow rate of 0.67 mL h− 1. Two types of microstructures were obtained: a very dense coating (Fig. 11a) starting from Pt(acac)2 as precursor and a dense coating with incorporated particles starting from PtCl4 (Fig. 11b). In the latter case the 30 times higher conductivity solution (0.27 × 10− 3 S.cm− 1 compared to 8.8 × 10− 5 S.cm− 1 for Pt(acac)2) yields smaller droplets (52% smaller) that dry during transport and are incorporated as particles in the layer (Fig. 11b).

The authors would like to thank S. Brice-Profeta for DTA measurements.

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3.7. Structural characterization of Pt films It is of great importance to characterize the structure of Pt films by X-ray diffraction and to determine the grain size in order to be used as new electrocatalysts for automotive exhaust gas treatment. Fig. 12 shows the XRD diagram of microporous film deposited for 1 h with 1.67 mL h− 1 at 260 °C and 47 mm. A single-phase of pure crystalline cubic platinum was detected. Extra peaks correspond to YSZ cubic phase, the substrate. The grain size measured from the XRD peak broadening using Scherrer's formula was of about 42 nm and 29 nm in average when Pt(NH3)4(OH)2(H2O) and PtCl4 were used as precursors respectively. These results are coherent since the estimated droplet size decreased by 62% when the salt precursor was changed from Pt(NH3)4(OH)2(H2O) to PtCl4, as previously

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