Carbon-Induced Changes in the Morphology and Wetting Behavior of Ionic Liquids on the Mesoscale

Thin films of ionic liquids (ILs) have gained significant attention due to their unique properties and broad applications. Extensive research has focused on studying the influence of ILs’ chemical composition and substrate characteristics on the structure and morphology of IL films at the nano- and mesoscopic scales. This study explores the impact of carbon-coated surfaces on the morphology and wetting behavior of a series of alkylimidazolium-based ILs. Specifically, this work investigates the effect of carbon coating on the morphology and wetting behavior of short-chain ([C2C1im][NTf2] and [C2C1im][OTf]) and long-chain ([C8C1im][NTf2] and [C8C1im][OTf]) ILs deposited on indium tin oxide (ITO), silver (Ag), and gold (Au) substrates. A reproducible vapor deposition methodology was utilized for the deposition process. High-resolution scanning electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy were used to analyze the morphological and structural characteristics of the substrates and obtained IL films. The experimental data revealed that the IL films deposited on carbon-coated Au substrates showed minor changes in their morphology compared to that of the films deposited on clean Au surfaces. However, the presence of carbon coatings on the ITO and Ag surfaces led to significant morphological alterations in the IL films. Specifically, for short-chain ILs, the carbon film surface induced 2D growth of the IL film, followed by subsequent island growth. In contrast, for long-chain ILs deposited on carbon surfaces, layer-by-layer growth occurred without island formation, resulting in highly uniform and coalesced IL films. The extent of morphological changes observed in the IL films was found to be influenced by two crucial factors: the thickness of the carbon film on the substrate surface and the amount of IL deposition.


Figures and schemes depicting the vapor deposition methodology (Figures
), molecular structure of the ionic liquids studied, thin-film architectures, and high-resolution micrographs of IL films deposited on different solid substrates used both with and without additional carbon coating (Figures S5-S13 and S23), XPS spectra of the substrates and the IL film surface (Figures S14-S22), as well as tables reporting physicochemical properties of ILs (Table S1), experimental conditions for the physical vapor deposition of each ionic liquid (Tables S2-S6) and experimental data derived from the XPS analysis (Table S7), are presented.*Corresponding author jose.costa@fc.up.pt

S19
Table S1.Molar mass, density, viscosity, melting temperature, glass transition temperature, and superficial tension values for the ionic liquids

S20
Table S2.Experimental conditions for the physical vapor deposition/thermal evaporation of each ionic liquid.Experimental variables related to the study of the influence of carbon on the morphology of different ionic liquids deposited at different amounts on the ITO/glass and C/ITO/glass surfaces.

S20
Table S3.Experimental conditions for the physical vapor deposition/thermal evaporation of each ionic liquid.Experimental variables related to the study of the influence of carbon on the morphology of different ionic liquids deposited on the ITO/glass and C/ITO/glass, Ag/ITO/glass and C/Ag/ITO/glass surfaces, and Au/ITO/glass and C/Au/ITO/glass surfaces.

S21
Table S4.Experimental conditions for the physical vapor deposition/thermal evaporation of [C2C1im][NTf2].Experimental variables related to the influence of carbon on the morphology of different ionic liquids deposited on the gold-coated quartz crystal (QC) and on the carbon-coated QC (C/QC).

S21
Table S5.Experimental conditions for the physical vapor deposition/thermal evaporation of each ionic liquid.Experimental variables related to the study of the influence of carbon thickness on the morphology of different ionic liquids deposited on ITO/glass substrates.

S22
Table S6.Experimental conditions for the physical vapor deposition/thermal evaporation of each ionic liquid.Experimental variables related to the time-dependent study on the morphology of different ionic liquids deposited on ITO/glass and C/ITO/glass substrates.

Figure S12 .
Figure S12.Thin-film architectures and detailed micrographs of thin films of [C2C1im][NTf2] (images a and b) and [C2C1im][OTf] (images c and d) deposited on Au/ITO/glass (first column) and carbon/Au/ITO/glass (second column) surfaces (carbon thickness ≈ 20 nm).Each IL was deposited on both surfaces with a thickness of 100 ML.Micrographs were acquired at a lateral view of 45° with a magnification of 20,000×, using a high-resolution scanning electron microscope and employing a secondary electron detector.

Figure S13 .
Figure S13.Thin-film architectures and detailed micrographs of [C2C1im][NTf2] thin films deposited on Au/QC (first column) and carbon/Au/QC (second column) surfaces (carbon thickness ≈ 20 nm).The IL was deposited on both surfaces with a thickness of 400 ML.Micrographs were acquired from a top view with magnifications of 5000× (images a and b) and 20,000× (images c and d), using a highresolution scanning electron microscope and employing a secondary electron detector.

Figure S14 .
Figure S14.XPS survey spectra of the ITO/glass surface.The two spectra correspond to data obtained from the analysis of two different areas, each measuring 300 µm × 700 µm.

Figure S15 .
Figure S15.XPS survey spectra of the Au/ITO/glass surface.The two spectra correspond to data obtained from the analysis of two different areas, each measuring 300 µm × 700 µm.

Figure S23 .
Figure S23.Thin-film architectures and detailed micrographs of [C2C1im][OTf] (images a-d) and [C8C1im][OTf] (images e-h) films (100 deposited on ITO/glass (a and e) and carbon/ITO/glass (b,c, d, f, g, and h).Each IL was deposited on ITO surfaces coated with varying amounts of carbon: 0 nm (a and e); 10 nm (b and f); 20 nm (c and g); 30 nm (d and h).Micrographs were acquired at a lateral view of 45° with magnification of a 20,000× using a high-resolution scanning electron microscope and employing a secondary electron detector.
1 im / C (10 nm) / ITO C 2 C 1 im / C (20 nm) / ITO C 2 C 1 im / C (30 nm) / ITO C 8 C 1 im / ITO C 8 C 1 im / C (10 nm) / ITO C 8 C 1 im / C (20 nm) / ITO C 8 C 1 im / C (30 nm) / ITO Increasing the thickness of the carbon layer S20 Table EVP at each evaporation temperature was derived from literature data reporting volatility studies of the ILs: [C2C1im][NTf2] and [C8C1im][NTf2]. 18b) Accurate data for the EVP of [C2C1im][OTf] and [C8C1im][OTf] were not found elsewhere.Nevertheless, there are reports on the determination of vaporization enthalpies indicating the lower volatility of the [OTf]-based ILs in comparison to their congeners [NTf2]-based ILs.In fact, in this work, at the same evaporation temperature lower deposition rates were observed for [OTf]-based.The EVP of [C2C1im][OTf] and [C8C1im][OTf] at the studied evaporation temperatures are estimated to be within the interval between 0.01 and 1 Pa.In addition, their EVP may be lower than observed for [C2C1im][NTf2] and [C8C1im][NTf2].
a) storage time (days) before morphological characterization.b) The EVP at each evaporation temperature was derived from literature data reporting volatility studies of [C2C1im][NTf2]. 18c) Accurate data for the EVP of [C2C1im][OTf] were not found elsewhere.Nevertheless, there are reports on the determination of vaporization enthalpies indicating the lower volatility of the [OTf]-based ILs in comparison to their congeners [NTf2]based ILs.In fact, in this work, at the same evaporation temperature lower deposition rates were observed for [OTf]-based.The EVP of [C2C1im][OTf] at the studied evaporation temperatures is estimated to be within the interval between 0.01 and 1 Pa.In addition, their EVP may be lower than observed for [C2C1im][NTf2].

Table S2 .
Experimental conditions for the physical vapor deposition/thermal evaporation of each ionic liquid: effusion temperature (Teff.);substrate temperature (Tsubst.);equilibrium vapor pressure (EVP); orifice diameter of the Knudsen effusion cell; mass flow rate at the substrate surface [Φ (QCM)] and corresponding deposition rate in Å•s -1 ; deposition time.Experimental variables related to the study of the influence of carbon on the morphology of different ionic liquids deposited at different amounts (different monolayers, ML) on the ITO/glass and C/ITO/glass surfaces (C = carbon).

Table S3 .
Experimental conditions for the physical vapor deposition/thermal evaporation of each ionic liquid: effusion temperature (Teff.);substrate temperature (Tsubst.);equilibrium vapor pressure (EVP); orifice diameter of the Knudsen effusion cell; mass flow rate at the substrate surface [Φ (QCM)] and corresponding deposition rate in Å•s -1 ; deposition time.Experimental variables related to the study of the influence of carbon on the morphology of different ionic liquids deposited with 100 ML on the ITO/glass and C/ITO/glass, Ag/ITO/glass and C/Ag/ITO/glass surfaces, and Au/ITO/glass and C/Au/ITO/glass surfaces (C = carbon).

Table S4 .
Experimental conditions for the physical vapor deposition/thermal evaporation of [C2C1im][NTf2]: effusion temperature (Teff.);substrate temperature (Tsubst.);equilibrium vapor pressure (EVP); orifice diameter of the Knudsen effusion cell; mass flow rate at the substrate surface [Φ (QCM)] and corresponding deposition rate in Å•s -1 ; deposition time.Experimental variables related to the influence of carbon on the morphology of different ionic liquids deposited with 400 ML on the goldcoated quartz crystal (QC) and on the carbon-coated QC (C/QC) (C = carbon).

Table S5 .
Experimental conditions for the physical vapor deposition/thermal evaporation of each ionic liquid: effusion temperature (Teff.);substrate temperature (Tsubst.);equilibrium vapor pressure (EVP); orifice diameter of the Knudsen effusion cell; mass flow rate at the substrate surface [Φ (QCM)] and corresponding deposition rate in Å•s -1 ; deposition time.Experimental variables related to the study of the influence of carbon thickness on the morphology of different ionic liquids deposited with 100 ML on ITO/glass substrates (C = carbon).Accurate data for the EVP of [C2C1im][OTf] and [C8C1im][OTf] were not found elsewhere.Nevertheless, there are reports on the determination of vaporization enthalpies indicating the lower volatility of the [OTf]-based ILs in comparison to their congeners [NTf2]-based ILs.In fact, in this work, at the same evaporation temperature lower deposition rates were observed for [OTf]-based.The EVP of [C2C1im][OTf] and [C8C1im][OTf] at the studied evaporation temperatures are estimated to be within the interval between 0.01 and 1 Pa.In addition, their EVP may be lower than observed for [C2C1im][NTf2] and [C8C1im][NTf2].

Table S6 .
Experimental conditions for the physical vapor deposition/thermal evaporation of each ionic liquid: storage time before morphological characterization; effusion temperature (Teff.);substrate temperature (Tsubst.);equilibrium vapor pressure (EVP); orifice diameter of the Knudsen effusion cell; mass flow rate at the substrate surface [Φ (QCM)] and corresponding deposition rate in Å•s -1 ; deposition time.Experimental variables related to the time-dependent study on the morphology of different ionic liquids deposited with 50 and 150 ML on ITO/glass and C/ITO/glass substrates (C = carbon).