Nucleation and growth of microdroplets of ionic liquids deposited by physical vapor method onto different surfaces

Applied Surface Science 428 (2018) 242–249

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Nucleation and growth of microdroplets of ionic liquids deposited by physical vapor method onto different surfaces José C.S. Costa a,b,∗ , Ana F.S.M.G. Coelho a , Adélio Mendes b , Luís M.N.B.F. Santos a,∗∗ a CIQUP–Centro de Investigac¸ão em Química, Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 687, P-4169-007 Porto, Portugal b LEPABE–Laboratory for Process Engineering, Environment, Biotechnology and Energy, Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, P-4200-465 Porto, Portugal

a r t i c l e

i n f o

Article history: Received 31 July 2017 Accepted 16 September 2017 Keywords: Ionic liquids Vacuum deposition Droplets Thin films Nucleation Droplets coalescence

a b s t r a c t Nanoscience and technology has generated an important area of research in the field of properties and functionality of ionic liquids (ILs) based materials and their thin films. This work explores the deposition process of ILs droplets as precursors for the fabrication of thin films, by means of physical vapor deposition (PVD). It was found that the deposition (by PVD on glass, indium tin oxide, graphene/nickel and goldcoated quartz crystal surfaces) of imidazolium [C4 mim][NTf2 ] and pyrrolidinium [C4 C1 Pyrr][NTf2 ] based ILs generates micro/nanodroplets with a shape, size distribution and surface coverage that could be controlled by the evaporation flow rate and deposition time. No indication of the formation of a wettinglayer prior to the island growth was found. Based on the time-dependent morphological analysis of the micro/nanodroplets, a simple model for the description of the nucleation process and growth of ILs droplets is presented. The proposed model is based on three main steps: minimum free area to promote nucleation; first order coalescence; second order coalescence. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Ionic liquids (ILs) are an important field of research of the past years, converging on the ultimate purpose of large scale industrial applications [1–3]. They are made of a cation and an anion held together by a wide range of intermolecular forces from weak and isotropic to strong and highly directional ones. ILs are commonly defined as organic or inorganic salts with melting temperatures below 373 K. Due to the several combinations of ion pairs, there are innumerous ionic liquids with different physical and chemical behaviors which have been deeply studied by different research groups [4–8]. Generally, ILs share mutual features, such as the ionic conductivity, high thermal stability and viscosity and low vapor pressure at room temperature [9,10]. Nowadays, the model that explains ionic liquids bulk properties is based on the formation of a network made of polar and nonpolar domains giving rise to a nanostructured molecular arrangement

∗ Corresponding author at: CIQUP–Centro de Investigac¸ão em Química, Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 687, P-4169-007 Porto, Portugal. ∗∗ Corresponding author. E-mail addresses: [email protected] (J.C.S. Costa), [email protected] (L.M.N.B.F. Santos). http://dx.doi.org/10.1016/j.apsusc.2017.09.137 0169-4332/© 2017 Elsevier B.V. All rights reserved.

[11]. Nevertheless, this behavior is a trend that these liquids follow with the increasing of the alkyl chains which after a specific number of carbons segregate into solvophobic regions [11,12]. Although the bulk properties of ILs have been advantageously utilized in many fields ranging from, green energy and environment [13] to cellulose processing [14], their interfacial behaviour is considered as very promising in energy storage [15], electrochemistry [16], crude oil industry [17], pesticides industry [18], and particle selfassembly [19], among others. This emerging subject focuses on the research of the surface layer and the near surface structure when in presence of a solid, liquid or gaseous interface. Considering the nanostructuration referred before it is expected that the addition of a macroscopic interface leads to the formation of a different, but related, molecular arrangement. Molecular level approaches to this behavior have been made mainly through computational methods namely Pensado by et al. who studied, using molecular simulations, the effect of the length of imidazolium cation’s alkyl side chain and the presence of an end hydroxyl group at its end at IL/vapor interface concluding that the alcohol groups led to a lowering of the organization of the liquid phase [20] and Kirchner et al. who observed at charged surfaces a multilayered to a monolayered transition of the electrical double layer using a coarse grained model [21]. Experimentally, the surface tension, contact angles and wettability of ILs in polar and nonpolar polar substrates has been

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measured, however, only general trends have been identified so far [15,22,23]. Independently of the polarity of the substrates, ionic liquids with maximum and minimum surface tension had higher and lower contact angles, respectively, and their anions play a relevant role on their wettability on the contrary of the alkyl side chain length and cations which seem to have a smaller impact. Furthermore, X-ray photoelectron spectroscopy (XPS) has given insights about ILs structure when at the interface with vacuum showing an enrichment of the alkyl side chains outwards the bulk [24,25]. Moreover, due the negligible vapor-pressure of ionic liquids, a new field of study dedicated to thin films of ionic liquids and their application in nano-surface science and technology has been generated. Gusain et al. have shown the great potential of ILs to reduce the friction in micro/nano electromechanical systems by preparing an IL thin film on a silicon substrate with a self-assembly approach via covalent linkage [26]; Wang and Priest studied the wetting behavior of a family of imidazolium ILs as precursor thin films in mica using atomic force microscopy (AFM) having concluded that the presence of an agent that covers the surface completely or partially prevents the growth of the IL film and slows the contact angle relaxation [27]; Maruyama et al. have developed a new method for the synthesis of “nano-IL” through molecular beam deposition of the IL in ultrahigh vacuum [28]; finally, Cremer et al. reported the preparation and XPS characterization of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)-imide [C2 mim][NTf2 ] thin film on a glass substrate using an in situ thermal-evaporation/condensation process under ultrahigh vacuum conditions [29]. Our work aims for the production of micro/nanodroplets as precursor thin films of ionic liquids, by mean of physical vapor deposition (PVD), and its morphological study by scanning electron microscopy (SEM). In a previous work, the morphology of droplets and thin films of a series of imidazolium based ILs prepared through PVD on indium-tin-oxide-coated glass (ITO) was presented and the results showed the nanostructuration of ionic liquids [30]. In this study we have performed the vapor deposition of 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [C4 mim][NTf2 ] and 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)-imide [C4 C1 Pyrr][NTf2 ] on chemically different substrates – glass, ITO, graphene and gold. A model for the nucleation and growth mechanism of micro/nanostructures of these materials is proposed. The formation of thin films is highly regulated by kinetic as any complex transport property, involving phase transitions, interface potentials, molecular diffusion at the surface leading to a complex landscape of metastable local minimums of potential that in the limit of uniform film reaches the thermodynamic equilibrium. Nucleation processes involves the creation of a high are/volume proportion implying the existence of a critical nucleus radius, the minimum size that must be formed by atoms or molecules clustering. The Gibbs energy associated to the formation of a solid and spherical nucleus can be derived by Eq. (1) [31,32]. G = a3 r 3 GV + a1 r 2 l−v + a2 r 2 l−s − a2 r 2 s−v

(1)

The magnitude of G is dependent of the volume free energy associated to the condensation of vapor, GV , of the interfacial tensions () of the triphasic system created at the surface (l is the liquid, v is the vacuum and s is the solid) and of the geometric components of the nucleus formed (a1 r2 is the area of the curved surface, a2 r2 is the circular area projected on the substrate and a3 r2 is the nucleus volume). The critical nucleus radius (rcr ) can be calculated by Eq. (2), considering the thermodynamic equilibrium: d(G/dr) = 0. rcr =

−2(a1 l−v + a2 l−s − a2 s−v ) 3a3 rGV

(2)

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Fig. 1. Schematic representation of the change in Gibbs energy of a system for the process of nucleation and growth of thin films.

The Gibbs energy of a system when r = rcr (Gcr ) is given by Eq. (3). Gcr =

4(a1 l−v + a2 l−s − a2 s−v )3 27a3 2 GV 3

(3)

The thermodynamic state of the nucleation process, associated with the formation of stable nucleus with a minimum radius (rcr ), is reached when the volumic component is higher than surface energy, as schematized by Fig. 1. The nucleation and growth of thin films can be described by three distinct models: Frank-van der Merwe growth (FM/2D growth); Volmer-Weber growth (VW/3D growth); StranskiKrastanov growth (SK growth). The 2D growth (layer by layer) is favored when a2 r2  s-v ≥ a1 r2  l-v + a2 r2  l-s . This layer-by-layer growth indicates that complete films form prior to growth of subsequent layers. In opposition, the 3D growth (island formation) is favored when a2 r2  s-v < a1 r2  l-v + a2 r2  l-s whereby thin films grow epitaxially at a solid surface or interface. The SK growth is a combination of 2D and 3D growth mechanisms [31,32]. 2. Experimental section 2.1. Reagents The ionic liquids used in this work, 1-butyl-3bis(trifluoromethanesulfonyl)-imide methylimidazolium [C4 mim][NTf2 ] and 1-butyl-1-methylpyrrolidinium [C4 C1 Pyrr][NTf2 ], were bis(trifluoromethanesulfonyl)-imide purchased from IOLITEC with a state purity of >99%. Before each vapor deposition, the ILs were dried under low pressure (<10 Pa) and stirred continually for a minimum of 48 h at 373 K to reduce the presence of water and volatile contents. Before the deposition experiments, the low content of water (<500 ppm) was measured and confirmed by Karl-Fischer. Additionally, the water content was highly reduced inside the vacuum chamber. 2.2. Substrates ILs micro/nanostructures were prepared on the surface of chemically different substrates: glass, indium tin oxide (ITO), graphene/nickel (G/Ni) and gold-coated quartz crystal. All surfaces were carefully cleaned first with acetone followed by isopropyl alcohol in an ultrasonic bath and dried with nitrogen. Surfaces and samples were stored in vacuum to avoid contamination. 2.3. Physical vapor deposition (PVD) The PVD methods describe a diversity of vacuum deposition techniques that are usually more environmental friendly that traditional coating methods in solution. Droplets of ionic liquids were

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Fig. 2. Schematic representation of the physical vapor deposition methodology: (a) ThinFilmVD apparatus (1–cooling system, 2–instrumentation box, 3–vacuum chamber, 4–N2 (l) metallic trap, 5–vacuum pumping system); (b) schematic detail of the PVD of ionic liquids.

prepared by physical vapor deposition using a customized thin film vapor deposition apparatus (ThinFilmVD) developed and tested in our laboratory [33]. A simple scheme of the apparatus (a) and the schematic process of the vapor deposition of ionic liquids (b) are presented in Fig. 2. The PVD system is based in a multi cell Knudsen effusion vaporization system that delivers a very precise mass/molar flow control allowing to assemble well-defined thin films of organic/inorganic materials based on their volatility. At temperature T, the amount n of the sample evaporated from the Knudsen cell, during the period of time t, is related to the vapor pressure p of the compound according to Eq. (4), where M is the molar mass of the vapor, R is the gas constant, Ao is the area of the effusion orifice and wo is the transmission probability factor. ϕ=

pwo n = Ao t (2RTM)1/2

(4)

2.4. Fabrication of micro/nanodroplets Due to the typical low vapor pressure of the ILs, [C4 mim][NTf2 ] and [C4 C1 Pyrr][NTf2 ] were vaporized at a reasonable high temperature of T = 505 K and T = 560 K, respectively. In situ mass flow rates (7–10 ng cm−2 s−1 ) were measured by quartz crystal microbalance (QCM) in real time for both compounds (molar flow rates of ≈ 0.02 nmol cm−2 s−1 ). The deposition of [C4 mim][NTf2 ] and [C4 C1 Pyrr][NTf2 ] and droplet formation was made in different surfaces (glass, ITO, graphene and gold), thermally controlled at T = 293 K, using the same mass flow rate and different times of deposition. 2.5. High resolution scanning electron microscopy (SEM) Topographic images of the micro/nanodroplets of ionic liquids obtained by vapor deposition onto different surfaces and using different times of deposition, were obtained by highresolution scanning electron microscopy with a FEI Quanta 400 FEG ESEM/EDAX Genesis X4 M apparatus at the CEMUP (Centro de Materiais da Universidade do Porto) facilities. Images were acquired using secondary (SE) and backscattered electrons (BSE) detectors. The acceleration voltage was 5 keV, while an in-lens detector was employed with a working distance of about 10 mm. 3. Results and discussion In our previous work, micro/nanodroplets of the extended [Cn mim][NTf2 ] series (n = 1–8) were deposited by a PVD methodology [30,33]. According to the topographic SEM images, the shape, size distribution and the surface coverage of the ILs on the ITO surface was found to be dependent of the chain length of the imidazolium cation. Also, concerning each sample of IL, higher deposition

time led to a decrease in the number of droplets per unit area and the size of each droplet was found to increase due to coalescence. To assess these phenomena in other surfaces, PVD of [C4 mim][NTf2 ] was performed on the surface of glass, ITO, graphene/nickel and gold-coated quartz crystal. The topography of the nanostructures deposited with large deposition time (10 h of PVD) is presented in Fig. 3. As evidenced, on both surfaces, microdroplets of [C4 mim][NTf2 ] were fabricated. Keeping the deposition time and molar flow rate the size and surface coverage of the micro/nanodroplets are distinct for each substrate (topography of each substrate is presented as supporting information). The experiments are consistent with previous literature works concerning the formation of micro/nanodroplets of ionic liquids as thin film precursors [28–30,34–38]. Regarding some representative works, Beattie et al. investigated the spreading of molecularly precursor films emerging from microdroplets of ionic liquids that in part wet smooth mica surfaces [34]; Deyko et al. deposited ultrathin films/nanodroplets of two imidazolium ionic liquids by PVD on mica surfaces [35]. Concerning our PVD methodology, in all cases, deposition of ionic liquids on different surfaces leads to the formation of droplets instead of compact and flat thin films. Moreover, even using large deposition times (10 h), the formation of a thin film with high surface coverage was not observed. Concerning the thermal evaporation of ionic liquids under ultra-high vacuum conditions, depending on chemical properties of the material itself and the support/substrate used, a layer-by-layer growth, wetting-layer followed by island growth, and pure island-growth models can be observed for IL nanostructures [28,30–32,34–38]. For instance, Rietzler et al. investigated the initial adsorption behavior and growth of thin films of [C1 mim][NTf2 ] on different surfaces (pyrolytic graphite and graphene layer on nickel) and they concluded that the IL nucleation and growth is strongly dependent on the underlying substrate [38]. In addition, according to Deyko et al., the carbon contamination of the surface has also a strong influence on the growth mode of ILs nanostructures [35]. Maruyama et al. successful deposited nanoscale ionic liquids as droplets and thin films; the size of the IL nanodroplets was controlled by changing the deposition time and for the formation of a thin IL film they used a 5 nm of C60 seeding layer; the wettability of IL on the substrate was much improved compared to the case without the seeding layer, and therefore a ultrathin film of IL was obtained [28]. Recently, we found that the use of in − vacuum energetic particles (plasma) can be a pathway to the fabrication of a thin IL films [30]. The same phenomena were recently presented by Dai et al., who have characterized nanoscale grain coalescence induced in a plasma etching system [39]. The nucleation and growth mechanism and the wetting characteristics of the ionic liquid droplets/thin films in contact with solid substrates should be fully understood in order to progress on the potential applications of ILs as electrolytes for several energy appli-

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Fig. 3. SEM images (top view, 100 × ) of micro/nanodroplets of [C4 mim][NTf2 ] prepared by vapor deposition (10 h of PVD using molar flow rates of ≈ 0.02 nmol cm−2 s−1 ) on the surfaces of glass (a), indium tin oxide (ITO, b), graphene/nickel (G/Ni, c) and gold-coated quartz crystal (gold/QC, d). Images acquired by using SE detector.

cations. For this context, a comprehensive analysis of the process of adsorption/nucleation and growth of ionic liquid droplets should be addressed. Following this reasoning, PVD of two different ionic liquids, [C4 mim][NTf2 ] (Im) and [C4 C1 Pyrr][NTf2 ] (Pyrr), was explored for low deposition times. Micrographs of micro/nanodroplets of both ILs prepared on the surface of ITO and G/Ni are depicted in Fig. 4 (complementary figures are presented as Supporting information). These SEM images were acquired using backscattered electrons detector (BSE) in order to better define the area of the surface covered by the IL. Hence, the brightest areas of the micrographs represent the heavier elements belonging to the substrate (ITO or graphene) and where droplets of IL were not deposited. These experiments highlighted the formation of droplets of different families of ILs deposited on the surface of distinct materials. Also, it can be easily noted that the number and the size of the micro/nanodroplets is clearly changed with the deposition time. As perceived, for all samples (Im/ITO, Im/G, Pyrr/ITO, and Pyrr/G) the number of micro/nanodroplets is clearly larger for short deposition times. This evidence is in contrast with the surface coverage that is increased with the deposition time; for 10 min of PVD, the coverage is lower than 40% and with 20 min it is about 50% in all cases. These experimental results highlight the mechanisms of coalescence between clusters of ionic liquids. Morphologically, independently of the surface composition or the nature of the ionic liquid, a formation of microdroplets with areas lower than 20 ␮m2 was observed. Supporting the investigation of the deposition pattern, Fig. 5 presents the distribution of the area of droplets as a function of the

number of droplets per surface area (previously order by size) for different times of deposition ([C4 mim][NTf2 ] deposited by PVD on ITO). These data were explored by using image processing in the ImageJ software application by examining the SEM images of the samples Im/ITO prepared by 5, 10, 15, 20, 40 and 60 min of deposition. Details of the analysis can be found as Supporting information. As expected, the number of droplets formed per surface area is higher for low deposition times. Interesting, two distinct regions were observed: “Region A”; “Region B”. For shorter deposition times (between 2 and 10 min, “Region A”), we can observe a linear dependence (between the droplets area and number of droplets/␮m2 ) associated to a transition region tendentiously exponential that corresponds to a number of droplets per surface area approximately constant for the larger droplets. For larger deposition times (“Region B”) the profile of the graph tends for a linear dependence, wherein the number of droplets/␮m2 is nearly constant. “Region B” evidences heterogeneous populations of ILs microdroplets independently of the ionic liquid and the substrate. Furthermore, along the deposition time we can observe an island-growth mechanism, which is in nice agreement with previous reports [35–38]. Nevertheless, the formation of a firstly thin wetting-layer before the island growth was not observed by SEM. Taking into account the methodology of physical vapor deposition, the formation of thin films is regulated by thermodynamic and kinetic mechanisms. These mechanisms encompass processes of nucleation and growth: chemical interaction between the material and the surface and respective adsorption; surface diffusion and accommodation on the energetically favored sites; nucleation and

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Fig. 4. SEM images (top view, 5000 × ) of micro/nanodroplets of [C4 mim][NTf2 ] and [C4 C1 Pyrr][NTf2 ] prepared by vapor deposition (5, 10, 15 and 20 min of PVD using molar flow rates of ≈ 0.02 nmol cm−2 s−1 ) on the surfaces of indium tin oxide (ITO, a1–a4, c1–c4) and graphene/nickel (G/Ni, b1–b4, d1–d4). Images acquired by using BSE detector.

formation of stable clusters; structuration and growth of the film; post-nucleation processes including coalescence mechanisms. Due to the physical-chemical properties of ILs including their wetting behavior, the post-nucleation processes are very relevant for the process of growth of ionic liquid droplets. For a better view of the shape of the droplets, Fig. 6 presents micrographs with the oblique view (45◦ ) of micro/nanodroplets of [C4 mim][NTf2 ] and [C4 C1 Pyrr][NTf2 ] prepared on the surface of ITO and G/Ni by 1 and 2 h of PVD. Contrary to Fig. 4, these SEM images were acquired using secondary electrons detector (SE), thus, the edges of the material tend to be brighter than the surface, leading to images with a well-defined and three-dimensional appearance. Matching both depositions (1 and 2 h of PVD) it is clear that the number of microdroplets decreases, as expected. By these results we can also conclude that larger deposition times lead to a higher thickness of the droplets. Comparing the morphology of the droplets fabricated for the imidazolium (Im) and pyrrolidinium (Pyrr)-based ILs (Figs. 3 and 4) there is a shape differentiation: independently of the surface and deposition time the droplets of [C4 mim][NTf2 ] were found to be more spherical than droplets of [C4 C1 Pyrr][NTf2 ]. Considering the classic wetting model that envisages contact angles and wetting

behavior and taking into account the free energy of the surfaces and the substrate/droplet interface, liquid materials with lower surface tension usually have a propensity to wet solid surfaces better depending on the chemical properties of the substrate. According to the Young-Laplace law, liquid molecules at a surface are subject to an inward force that results in a minimum surface contact area, which in this work justify the spherical shape of ILs clusters. Analyzing carefully our results and taking into account the literature reports on this area, the process of nucleation and growth of ILs is dependent on several factors that difficult a rigorous theory for the formation of thin films of ionic liquids [35–38]. Herein, we present a simple and alternative model for the process of nucleation and growth of ILs droplets fabricated by physical vapor deposition. This model is based on three main processes as evidenced by the scheme of Fig. 7: minimum free area to promote nucleation; first order coalescence; second order coalescence. On the first seconds of deposition, molecules of IL condense on the substrate surface with a specific kinetic energy and through processes of thermal accommodation and surface diffusion, the clusters of ILs will be located at places energetically favored. For clustering, molecules of IL should exhibit a minimum size (critical nucleus radius). By the continuous vaporization and deposition of

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Fig. 5. Schematic representation of the area of droplets as a function of the number of droplets per surface area (previously order by size) for the deposition of [C4 mim][NTf2 ] on ITO surface by different times: 2 min (red); 5 min (blue); 10 min (yellow); 15 min (pink); 20 min (green); 40 min (black); 60 min (gray). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 8. SEM image of IL deposited on ITO surface showing the three main processes of nucleation and growth of ILs droplets fabricated by physical vapor deposition: minimum free area to promote nucleation (a); first order coalescence (b); second order coalescence (c).

IL, the minimization of the energy at the surface is favored by the agglomeration between molecules of ILs leading to the formation of clusters with increased volumes. The condensation of ILs molecules (ion pairs) at the surface will follow a mechanism of surface diffu-

sion before reaching the near cluster. The formation of a seeding cluster was found to be dependent of a minimum (characteristic or free) area to promote nucleation that was found to be above 5 ␮m2 (see schematic representation in Fig. 8). The consecutive process of

Fig. 6. SEM images (oblique view, 45◦ , 5000×) of micro/nanodroplets of [C4 mim][NTf2 ] and [C4 C1 Pyrr][NTf2 ] prepared by vapor deposition (1 and 2 h of PVD using molar flow rates of ≈0.02 nmol cm−2 s−1 ) on the surfaces of indium tin oxide (ITO, a1,a2,c1,c2) and graphene/nickel (G/Ni, b1,b2,d1,d2). Images acquired by using SE detector.

Fig. 7. Schematic representation of the mechanism of nucleation and growth of droplets of ionic liquids prepared by PVD.

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Fig. 9. SEM images (top view, 100× and 500×) of micro/nanodroplets of [C4 mim][NTf2 ] (a1, a2) and [C4 C1 Pyrr][NTf2 ] (b1, b2) prepared by vapor deposition (10 h of PVD using molar flow rates of ≈0.02 nmol cm−2 s−1 ) on the surface of glass. Images acquired by using BSE detector.

coalescence of native clusters will increase the size of stable clusters that we define as first order coalescence. The first order coalescence is a coalescence process between two or more native droplets, which leads directly to an increasing volume of the clusters. The second order coalescence is a process of coalescence between droplets that had already been coalesced. After the coalescence empty places are created, which are available for new processes of nucleation. As observed by Figs. 6 and 8 and schematized in Fig. 7, droplets with lower sizes are located near the higher clusters; this is an evidence of the continuously creation of small droplets that occurs in energetically favored and available areas (above 5 ␮m2 ) after the coalescence of precedent clusters. For larger times of deposition the empty places will be occupied by successive clusters that after thermal accommodation and successive mechanisms of first and second order coalescences will contribute for a large surface coverage and thin film structuration. This process can be observed by Fig. 9 that presents the SEM morphology (BSE images) of the imidazolium and pyrrolidinium-based ILs deposited by a large deposition time. According to these data, an almost completed surface coverage is observed. Boundaries between the film precursors (droplets) can be noted. As consequence of first and second order coalescences the sphericity of the droplets decreases along the time. As reported for other materials the thin film nucleation and growth is dependent of several experimental features [39–41]. Concerning the ILs nanostructures, all steps of the mechanism of nucleation and growth of droplets, schematized in Fig. 7, are quite dependent of several parameters such as the support/substrate temperature, the vaporization temperature and respective molar flow rate, the substrate rotation speed, the surface diffusion, among others.

4. Conclusion Controlling the molar flow rate and deposition time, micro/nanodroplets of imidazolium and pyrrolidinium–based ILs were formed onto surfaces of glass, indium tin oxide, graphene/nickel and gold-coated quartz crystal. The number of droplets was found to be larger for short deposition times, in contrast with the surface coverage that increased along the deposition. In all cases, an island-growth mechanism without the firstly thin wetting-layer was observed. Comparing the morphology of [C4 mim][NTf2 ] and [C4 C1 Pyrr][NTf2 ], droplets of imidazolium based-ILs have a tendency to be more spherical, which is the result of a differentiation on wetting properties. In addition, a simple model for the nucleation and growth of ILs droplets fabricated by PVD was proposed, which is based on a continuous nucleation and coalescences of first and second orders.

Conflict of interest The authors declare no competing financial interest.

Acknowledgements This work was financially supported by the projects: (i) POCI-01-0145-FEDER-006939 (Laboratory for Process Engineering, Environment, Biotechnology and Energy – UID/EQU/00511/2013) funded by the European Regional Development Fund (ERDF), through COMPETE2020 – Programa Operacional Competitividade e Internacionalizac¸ão (POCI) and by national funds, through FCT – Fundac¸a˜o para a Cieˆncia e a Tecnologia.

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