Surface Properties of Ionic Liquid Adsorbate Layer Are Influenced by

May 22, 2012 - Eric A. Muller , Matthew L. Strader , James E. Johns , Aram Yang , Benjamin W. Caplins , Alex J. Shearer , David E. Suich , and Charles...
0 downloads 0 Views 3MB Size
Download PDF
Article pubs.acs.org/Langmuir

Surface Properties of Ionic Liquid Adsorbate Layer Are Influenced by the Dipole of the Underneath Substrate Xiaoning Zhang, Lingbo Lu, and Yuguang Cai* Department of Chemistry, University of Kentucky, 505 Rose Street, Lexington, Kentucky 40506, United States S Supporting Information *

ABSTRACT: Ionic liquids (ILs) form nonfluidic layers at the solid−liquid interface. The properties of the IL interfacial layer play important roles in IL-based applications. Since the liquid-phase IL directly contacts and interacts with the IL interfacial layer rather than the underneath substrate, the surface properties of the interfacial layer could influence how the IL behaves on a solid surface. We used scanning probe microscopy (SPM) and force spectroscopy to investigate how chemical patterns with different dipoles reacted with ionic liquids. We find that even without direct contact on chemical patterns, the IL can form an adsorbate layer on chemical patterns via vapor-phase condensation. The dipole of the chemical pattern can direct the adsorption and assembly of the IL adsorbate. The surface properties of the IL adsorbate layer depend on the dipole of the underneath chemical patterns. Our results indicate that the interfacial IL layer may exist before the IL contacts a solid surface. The charge and dipole of the substrate can influence the structures and properties of the IL interfacial layer. Characterization and measurements of the IL interfacial properties must be conducted under the pretext that the charge/dipole of the substrate is known.

■

INTRODUCTION Ionic liquids (ILs) are low-temperature molten salts. ILs have been widely used as lubricants, extracting agents, a key component in novel batteries, supercapacitors, and e-ink displays. A main role of ILs in these applications is to improve the liquid−solid interfacial properties.1−12 Studies have revealed the existence of IL layers at the liquid−solid interface,13−16 which suggest that the improvements of interface properties are related to the IL interfacial layers. For instance, certain solidlike IL interfacial layers have been used as the key components in field effect transistors because of their ultrahigh capacitance.17,18 Presently, the properties of the IL interfacial layer are not fully understood yet. As revealed by earlier AFM and X-ray studies, the IL layer on the solid surface has its own structure and properties, which can be understood neither through extrapolating data from liquid-phase IL nor from the structures of the underneath solid surface. Nevertheless, further developments of IL-based applications need the detailed understanding of the IL interfacial layers, which is an important task that needs to be addressed. In this report, we studied the properties of the 1-butyl-3methylimidazolium chloride ([Bmim]Cl) interfacial layer formed on chemical patterns. [Bmim]Cl belongs to imidazolium family ILs, which have been widely used and thoroughly studied. Among imidazolium family ILs, [Bmim]Cl has characteristic asymmetric L-shaped cations, where the butyl chain is not in the same plane of the imidazolium ring.19,20 Computational study suggests that [Bmim]Cl has a heat of vaporization of 202 kJ/mol at 348 K, which indicates that [Bmim]Cl should have a typical vapor pressure among imidazolium family ILs.21 These structural characteristics and thermodynamic property determine that [Bmim]Cl could serve as a representative model IL in our study on the properties of © 2012 American Chemical Society

IL interfacial layers on different chemical patterns under IL vapor, which would provide us a framework to delineate the behaviors and properties of interfacial layers of other imidazolium family ILs over surfaces. The study would be the first step in understanding how the interfacial IL would affect the IL liquid−solid interaction. Specifically, for imidazolium family ILs, the positive charge is concentrated on the imidazolium part of the cation, which makes the charge distribution over the cation anisotropic. Hence, on charged surfaces or surfaces with a large dipole, the IL cation would have a preferential adsorption orientation. Different cation orientations lead to different assembly structures in IL film or IL crystals, which in turn correspond to different surface properties.22−26 On the surface, the charge/ dipole-induced alignment of the cation would affect the surface structure and properties of the IL layer adsorbed on the interface as well. Recent simulation studies pointed out that ions could form dense layers over a charged surface and the properties of the ion layers such as capacitance depend on the surface charge polarities.27−30 In particular, the density functional theory (DFT) simulation result from the Jiang group predicted that the IL could form alternating cation and anion layers on a charged surface, which hints that the IL adsorbate layer properties might be different for cation or anion layers.31 Here, we investigated the IL interface properties on surfaces with charge/dipole using experiments. We used scanning probe microscopy (SPM) to characterize IL layers adsorbed on surface patterns with different surface dipoles. We discovered Received: April 9, 2012 Revised: May 21, 2012 Published: May 22, 2012 9593

dx.doi.org/10.1021/la301452d | Langmuir 2012, 28, 9593−9600

Langmuir

Article

Scheme 1. Preparation of Different Chemical Patterns on the OTS surface and Subsequent Vapor-Phase IL Adsorption on the Chemical Patternsa

(a) Fabrication OTSpd discs (blue discs) on the OTS film using AFM deep oxidation lithography. (b) Incubating the carboxylic acid-terminated OTSpd discs into a 10 mM ZnCl2 solution. After removing the ZnCl2 solution from the sample surface, the chemical pattern was converted into the Zn2+-bound pattern (OTSpd-Zn, pink discs in the scheme), which has a different surface dipole from the OTSpd disc. (c) AFM deep oxidation lithography was used again to fabricate another OTSpd disc pattern near the OTSpd−Zn disc patterns. On the sample surface, both OTSpd and OTSpd−Zn discs co-existed. (d) IL was incubated together with the sample. (e) IL vapor adsorbed on high-energy OTSpd and OTSpd−Zn discs, forming adsorbate layers on these two chemical patterns. Nevertheless, the adsorbate layer on OTSpd (orange discs, OTSpd-IL) and adsorbate layer on OTSpd−Zn (purple discs, OTSpd-Zn-IL) are different. a

Table 1. Surface Properties of OTSpd and OTSpd−Zn Patterns Before and After IL Vapor Adsorption before vapor adsorption surface potential (mV, with respect to OTS) Lennard−Jones potential

value (1 × 10−18 J) N

OTS

OTSpd

OTSpd−Zn

OTSpd−IL

OTSpd−Zn−IL

37.3 ± 3.5 80

−53 ± 14 18.7 ± 1.0 60

62 ± 8 10.3 ± 0.9 80

7 ± 11 7.7 ± 0.5 80

167 ± 16 5.8 ± 0.9 70

pattern is a hydrophilic high-energy surface. We dipped the OTSpd surface into a 10 mM ZnCl2 solution and converted the chemical pattern to Zn-bound OTSpd (OTSpd−Zn, pink discs in Scheme 1b) pattern. Since Zn2+ is divalent, when Zn2+ ions were bound on the carboxylic acid-terminated OTSpd surface, the surface charges and dipole direction were changed. Similar approaches have been used to convert a negatively charge mica surface into a positively charge surface in order to capture negatively charged DNA strands on a flat mica surface.34 Next, AFM deep oxidation lithography was used again on the same sample to fabricate additional OTSpd discs near the OTSpd−Zn patterns (Scheme 1c). After this step, the OTSpd and OTSpd−Zn patterns coexisted on the sample with different dipole directions. Next, the sample with both OTSpd and OTSpd−Zn patterns was incubated in a sealed vial with 2 g of [Bmim]Cl placed on the bottom, as shown in Scheme 1d. The vial was heated to 60 °C for 20 min. After heating, the adsorbate layers on OTSpd discs and OTSpd−Zn discs (Scheme 1e) were characterized using different SPM methods. Surface Characterization. The IL-coated chemical patterns were characterized using ac mode. Force−distance spectroscopy was measured using the MikroMasch CSC-17 tip in contact mode. An OTS-coated CSC-17 tip was used for friction measurement. The spring constant of the AFM tips was measured using Sader’s method.35,36 Our AFM cantilever has an 8° tilting angle. The sample’s tilting angle was carefully adjusted to match the tiling of the cantilever in order to ensure that the applied loading force is perpendicular to the sample. Before measurement, pure nitrogen was used to purge the AFM chamber at 50 mL/s for 1 h, and subsequent AFM force− distance measurements ensured that no water film existed on the sample surface. All measurements were conducted under a nitrogen environment.

that the IL interfacial layer formed from the IL’s own vapor, and surface properties for the IL layer depend on the surface dipole of the underneath substrate, which is consistent with simulation studies and could provide experimental data for further theoretical studies.31

■

after vapor adsorption

EXPERIMENTAL SECTION

Chemicals and Materials. Octadecyltrichlorosilane (OTS, 97%) was purchased from Gelest. [Bmim]Cl was purchased from SigmaAldrich. Silicon (100) wafers (Virginia Semiconductors, Nitrogen doped, 30 Ω·cm resistivity) were polished to ultraflat grade with a root-mean-square (rms) roughness smaller than 5 Å. Instruments. The surface patterns were fabricated using an Agilent PicoPlus 3000 Atomic Force Microscope (AFM) in an environmental chamber. MikroMasch CSC-17/Ti−Pt tips were employed for pattern fabrication. MikroMasch NSC-14 tips (150kHz resonant frequency) were used in sample characterization in ac mode. MikroMasch CSC-17 tips were employed in the friction and force spectroscopy measurements on the sample. All AFM images were processed and rendered using WSxM.32 Procedures. Silicon(100) wafers were cut into 1 × 1 cm2 pieces and cleaned using the piranha solution. After rinsing, samples were incubated in 5 mM OTS toluene solution overnight. Chemical patterns on the OTS samples were fabricated using deep oxidation lithography. Briefly, as shown in Scheme 1, during fabrication a Pt−Ticoated conducting AFM tip was used to contact the surface in contact mode AFM. In a 100% relative humidity (at 25 °C) environment, a +10 V bias was applied to the sample. As a result, the OTS surface near the tip was oxidized into a partially degraded, carboxylic acidterminated chemical pattern (OTSpd pattern, blue disk in Scheme 1a). Detailed setup, procedure, and characterization of the OTSpd surface have been documented in detail elsewhere.33 The methyl-terminated OTS film is hydrophobic and has a low surface energy. In contrast, the carboxylic acid-terminated OTSpd 9594

dx.doi.org/10.1021/la301452d | Langmuir 2012, 28, 9593−9600

Langmuir

■

Article

RESULTS IL Adsorbed on Chemical Patterns via Vapor Phase. We employed Kelvin probe force microscopy (KPFM) and force−distance spectroscopy to compare the surface properties of the same OTSpd and OTSpd−Zn discs before (Scheme 1c) and af ter (Scheme 1e) these patterns were incubated in IL vapor. Both AFM topography characterization and force− distance spectroscopy reveal that IL vapor did not adsorb on the low-energy OTS surface. Thus, we used the surface properties of OTS as internal references for comparison of the surface properties of different chemical patterns before and after vapor incubation. The results are summarized in Table 1. KPFM maps the surface potential distribution over the surface. The surface potential is a direct indicator of the surface dipole.37 Figure 1 reveals that the surface potential of clean

surfaces we always used the same tip. Therefore, the Lennard− Jones potentials obtained from the AFM force−distance curves correspond to the substrate surface property. A representative force−distance curve on the OTSpd−Zn disk after IL vapor incubation (denoted as OTSpd−Zn−IL) is plotted in Figure 2.

Figure 2. Representative AFM force vs distance curve for the OTSpd− Zn−IL surface. (Black line) Curve was acquired when the tip was approaching the substrate surface. Lennard−Jones potential curve (red line) is calculated from the AFM force vs distance curve. Calculated Lennard−Jones potential listed in Table 1 corresponds to the area of the blue-shaded region on the force curve or the depth of the well on the red curve. Data were obtained using a calibrated MikroMasch CSC-17 tip, which has a SiO2 surface and a radius of 4.7 nm.

The purple-shaded area corresponds to the Lennard−Jones potential when the two surfaces touched with no force acted on the cantilever. Under our experimental condition, the AFM tip and surface interaction can be modeled as a hemispherical surface approaching a flat surface. Then the Lennard−Jones potential can be expressed according to formula 138−40

Figure 1. Surface potentials of the same clean OTSpd and OTSpd−Zn patterns before and after incubation in IL vapor. (a) Surface potentials of clean OTSpd and OTSpd−Zn pattern. (b) Surface potentials of the same patterns after incubation in IL vapor. Background is the OTS film. Two KPFM images are rendered in the same surface potential scale.

VvdW = − OTSpd and OTSpd−Zn patterns changed after they were incubated in IL vapor. The clean OTSpd and OTSpd−Zn pattern have a surface potential of −53 ± 14 and 62 ± 8 mV with respect to the background OTS film, respectively. After incubation in IL’s vapor, the surface potentials over the same OTSpd and OTSpd−Zn patterns changed to 7 ± 11 and 167 ± 16 mV with respect to OTS background, respectively. The change in surface potential reveals that additional material adsorbed on the clean chemical patterns. A control experiment was conducted in order to investigate other potential factors that could lead to the change in surface potential for the same experimental procedures. The clean OTSpd and OTSpd−Zn patterns were incubated in the sealed vial under the same condition without the presence of IL. Subsequent KPFM characterization revealed that the surface potential did not change. Therefore, the observed surface potential change was due to the IL’s vapor adsorption on the high-energy chemical patterns. As a complementary method, we acquired AFM force− distance spectra over the OTSpd and OTSpd−Zn patterns before and after IL vapor incubation using a well-calibrated MikroMasch CSC 17 SiO2 tip (4.7 nm in radius with a spring constant of 0.182 N/m). From the force−distance spectra we calculated the Lennard−Jones potential between the tip and the surface by integrating the attractive van der Waals force over the tip’s displacement. During our measurement over different

HR 6D

(1)

where R is the radius of the tip, D is the tip−surface distance, and H is the Hamaker constant. As Table 1 shows, the Lennard−Jones potential for the tip− OTS surface was 37.5 ± 3.5 × 10−18 J, which did not change before and after the IL vapor incubation. On clean OTSpd and OTSpd−Zn chemical patterns the Lennard−Jones potentials were 18.7 ± 1.0 × 10−18 and 10.3 ± 0.9 × 10−18 J, respectively. After the patterns were incubated in IL vapor, the Lennard− Jones potentials over OTSpd and OTSpd−Zn chemical patterns changed to 7.7 ± 0.5 × 10−18 and 5.8 ± 0.9 × 10−18 J, respectively. In contrast, a control experiment shows that the Lennard−Jones potential over the OTSpd patterns did not change after incubating the sample in the sealed vial without IL vapor. On the basis of the Lennard−Jones potential data, we conclude that IL adsorbed on the OTSpd and OTSpd−Zn chemical patterns, which is in agreement with the conclusion from AFM surface potential characterization. In summary, our results reveal that the vapor of the IL can adsorb on chemical patterns with high surface energy and form an adsorbate layer with distinct surface properties. Because of its very low vapor pressure, the role of IL’s vapor has been generally ignored during the studies of IL and solid interface interactions. Nevertheless, the vapor of ILs has its role in certain applications. For example, certain imidazolium ILs can be distilled, which makes recycling and purifying these IL 9595

dx.doi.org/10.1021/la301452d | Langmuir 2012, 28, 9593−9600

Langmuir

Article

possible.41 Our results indicate that for some popular ILs (such as imidazolium family ILs) vapor can adsorb on charged surfaces and form a stable layer. Since vapor always coexists with liquid and ions in the vapor travel faster than liquid’s diffusion and spreading, the stable adsorbate layer originated from the vapor would exist preceding any direct liquid−solid interaction. As a result, the liquid−solid interface properties such as wetting, spreading, and charge density would be influenced by the structure and properties of this interfacial layer. Surface Properties of IL Adsorbate Layers Depend on the Surface Dipole of the Underneath Chemical Pattern. The clean OTSpd and OTSpd−Zn patterns have different surface potentials, as shown in Table 1. The surface potentials were measured with the same tip under the same conditions, and the OTSpd/OTSpd−Zn patterns were on the same sample. Hence, the difference in surface potential indicates that they have different surface dipoles.37 The ions of ILs also have their distinct charges and dipoles. Upon adsorption, the charge/ dipole of the ion would interact with the dipole of a different chemical pattern. The charge−dipole and dipole−dipole interactions are directional. Therefore, different chemical patterns on the surface would induce different adsorption orientations of the IL ions. We fabricated IL layers adsorbed on OTSpd and OTSpd−Zn chemical patterns according to Scheme 1.Then we measured the surface properties of OTSpd−IL (orange discs in Scheme 1e) and OTSpd−Zn−IL (purple discs in Scheme 1e) in the following five aspects: surface potential, Lennard−Jones potential, energy dissipation during AFM tip tapping, tribological behavior, and response toward further IL vapor condensation. These two surfaces demonstrated differences in all five aspects. Difference in Surface Potential. As shown in Figure 1b and Table 1, using the surface potential of OTS film as an internal reference, the surface potentials over OTSpd−IL and OTSpd− Zn−IL adsorbate layers are 7 ± 11 and 167 ± 16 mV, respectively. In contrast, before IL adsorbate layer formed, the clean OTSpd and OTSpd−Zn patterns have surface potentials of −53 ± 14 and 62 ± 8 mV, respectively. Hence, formation of the IL adsorbate layer caused a 60 mV increase over the OTSpd pattern and a 105 mV increase over the OTSpd−Zn pattern. The difference in the surface potential increase indicates that the IL adsorbate layers over the OTSpd surface and OTSpd− Zn surface are different. They have different dipoles. Difference in Lennard−Jones Potential. Both OTSpd−IL and OTSpd−Zn−IL surfaces were prepared by incubating the clean OTSpd and OTSpd−Zn surface under IL vapor together. The data in Table 1 reveal that the Lennard−Jones potentials for OTSpd−IL and OTSpd−Zn−IL are different. In this case, the force−distance curves were measured with the same tip and the Lennard−Jones potentials listed in Table 1 correspond to potentials when the tip and sample surface just touched with no loading force. On the basis of formula 1, the Lennard−Jones potential is a function of the contact area (tip radius R in our case), tip−surface distance (D), and Hamaker constant. Since both R and D are the same during the force curve measurements over the OTSpd−IL and OTSpd−Zn patterns, the Lennard−Jones potential difference for OTSpd−IL (7.7 ± 0.5 × 10−18 J) and OTSpd−Zn−IL (5.8 ± 0.9 × 10−18 J) surfaces reflects that these two surfaces have different Hamaker constants. The Hamaker constant is determined by the surface properties of the substrate surface and the tip surface. Since the

same tip was used during the Lennard−Jones potential measurement, the different Hamaker constants indicate that the OTSpd−IL and OTSpd−Zn−IL surface are different. Difference in Energy Dissipation during Tip Tapping. We also characterized OTSpd−IL and OTSpd−Zn−IL patterns using ac mode AFM. Figure 3 shows the phase image of the

Figure 3. AFM phase image of two OTSpd−Zn−IL discs (c, top) and two OTSpd−IL discs (c, bottom) on OTS film background. (a) 2D zoom-in view of the upper right OTSpd−Zn−IL disc. (b) 2D zoom-in view of the lower right OTSpd−IL disc. All four discs received the same level of IL vapor exposure and were imaged by the same tip under the same instrumental setting. On the OTSpd−Zn−IL disc, additional IL existed as tiny droplets, which are highlighted by the green arrows. In contrast, there was no such droplet on OTSpd−IL disc in b. (Note: a and b are rendered in the gradient scale. c is rendered in an artificial multicolor gradient scale in order to illustrate the subtle phase signal difference between the OTSpd−IL and the OTSpd−Zn−IL surfaces.)

OTSpd−IL and OTSpd−Zn−IL surfaces. AFM phase signal reflects the tip’s energy dissipation on the surface during ac mode imaging. The two IL adsorbate layers have different colors in Figure 3, indicating they have different phase signals. Usually the phase signal is also influenced by the instrumental conditions and tip state and cannot be directly used to compare the properties of different surfaces. However, Figure 3 was obtained under the same instrumental settings with the same tip. Furthermore, the phase signal over OTS serves as an internal reference for comparison. Hence, the observed difference in the phase signals over OTSpd−IL and OTSpd− Zn−IL indicates that their surface properties are indeed different. The phase lag is larger when the tip tapped over the OTSpd−Zn−IL layer than on the OTSpd−IL layer. Therefore, the tip consumed more energy when it tapped the OTSpd−Zn−IL surface than the OTSpd−IL surface. Difference in Tribological Properties. The tribological properties of the OTSpd−Zn−IL and the OTSpd−IL surfaces are different too. On a sample with OTSpd−IL and OTSpd− Zn−IL layers (as shown in Scheme 1e), we measured the friction of the OTSpd−IL and OTSpd−Zn−IL as a function of the loading force using a well-calibrated OTS-coated tip. The result is plotted in Figure 4. The linear fitting of the friction vs loading curve yields that coefficient of friction (COF) for OTS is 0.032 ± 0.005, which is similar to published results.42 The black line and red line in Figure 4 show the friction responses of OTSpd−IL and OTSpd−Zn−IL surfaces, respectively. The friction vs loading curves for these two surfaces have a similar shape, but they are not identical. The linear fitting yields that the COF for OTSpd−Zn−IL and OTSpd−IL is 0.171 ± 0.015 and 0.106 ± 0.015, respectively. When the loading force is small, the friction on the OTSpd−Zn−IL surface is smaller than the OTSpd−IL surface. However, when the loading force is larger than 6 nN, the friction over the OTSpd−Zn−IL surface becomes higher than that of the OTSpd−IL surface. 9596

dx.doi.org/10.1021/la301452d | Langmuir 2012, 28, 9593−9600

Langmuir

Article

[Bmim][Tf2N], and 1-decyl-3-methylimidazolium chloride, [C10mim]Cl) were studied. These two ILs were selected because we want to investigate the impact of anion and cation on the interfacial properties, respectively. By comparing the surface potentials of the [Bmim]Cl adsorbate layer with that of [Bmim][Tf2N] we reveal the impact of the anion. Similarly, the difference between [Bmim]Cl and [C10mim]Cl revealed the impact of cation. KPFM characterization results are shown in the Supporting Information (Figures S1 and S2 and Table S1). KPFM characterization on these two ILs shows that the surface potential for the IL adsorbate layers on the OTSpd pattern was not the same as the adsorbate layer on the OTSpd−Zn pattern. The surface dipole of the chemical pattern determines the surface properties for these two ILs as well.

Figure 4. Friction vs loading curves for OTS surface (green), OTSpd− IL surface (black). and OTSpd−Zn−IL surface (red). These data are measured using an OTS-coated tip. Dashed lines are the linear fittings for friction vs loading curves. From the fitting, the coefficient of friction (COF) for OTS is 0.032 ± 0.005, COF for OTSpd−Zn−IL is 0.171 ± 0.015, and COF for OTSpd−IL is 0.106 ± 0.015.

■

DISCUSSIONS AFM phase signal reflects the tip’s energy dissipation over surfaces. Figure 3 qualitatively demonstrates that the different dipoles of the OTSpd and OTSpd−Zn patterns directed the structures of IL adsorbate layers, which led to different energy dissipation upon AFM tip tapping. We further investigated the energy dissipation of IL adsorbate layers on different chemical patterns quantitatively. We obtained the mechanical properties and geometry of the tip at first. The MikroMasch NSC-14 tip we used has a spring constant of 15.5 N/m, natural frequency of 177.20 kHz, and quality factor of 276. The tip radius is 3.7 nm, which is obtained through scanning the tip over the monatomic step edge of a piece of a graphene sheet. Then we used ac mode to image the OTSpd−IL and OTSpd−Zn−IL layer using a different tapping intensity in a 100% nitrogen environment. With the tip’s mechanical parameters in hand, we got the tip’s energy dissipation in one tapping cycle over the OTSpd−IL and OTSpd−Zn−IL surfaces through the following equation43

Difference in Further Adsorption. The OTSpd−Zn−IL surface and OTSpd−IL surface also have a different adsorption affinity toward the IL vapor. Figure 3a and 3b shows the representative OTSpd−Zn−IL and OTSpd−IL patterns incubated together in IL vapor for 20 min incubation at 60 °C. The two patterns received the same level of vapor exposure. The IL adsorbed on OTSpd and OTSpd−Zn surfaces and formed flat interfacial layers on both surfaces. However, IL can also adsorb on the OTSpd−Zn−IL surface, appearing as white spots, as highlighted by the green arrows in Figure 3a. When imaged by ac mode AFM, the phase signals over these spots had huge spikes, indicating that the tip dissipated a large amount of energy. This characteristic reveals that these white spots shown in Figure 3a are in liquid phase. Hence, when a full OTSpd−Zn−IL layer had formed, IL could continue to adsorb on OTSpd−Zn−IL as droplets. In contrast, in Figure 3b, the surface of OTSpd−IL is clean. IL does not further adsorb on the OTSpd−IL surface once the adsorbate IL layer formed. As shown in Figure 3a, at an exposure level of 20 min, only a few droplets existed on top of the OTSpd−Zn−IL surface. Their size is about 10 nm. When we further increased the vapor exposure level, more IL was formed as droplets on the OTSpd−Zn−IL surface. Eventually these droplets fused into a large drop sitting on the center of the disk-shaped OTSpd− Zn−IL pattern, and the size of the drop increases with exposure level. In contrast, no droplet formed on the OTSpd pattern or OTS background. Furthermore, the surface properties of OTSpd−Zn−IL, OTSpd−IL, and OTS remain constant regardless of IL vapor exposure level, which suggests that OTSpd−IL and OTSpd−Zn−IL do not further grow in IL vapor once they formed a full layer. In summary, vapor of a representative IL ([Bmim]Cl) coexists with its liquid phase. The vapor can adsorb on highenergy chemical patterns and form distinctive layers. The important IL−solid interface properties are determined by the properties of the interfacial IL layer. Five independent experiments all confirmed that the surface properties of interfacial IL layers are determined by the surface dipole of the underneath chemical patterndifferent surface dipoles lead to different IL interfacial layers. We also investigated the interfacial properties of other ILs using the same KPFM experimental scheme. Two ILs (1-butyl3-methylimidazolium bis(trifluoromethylsulfonyl) imide,

E=

⎞ kzπA2 ⎛ A 0 ⎜ sin φ − 1⎟ ⎠ Q ⎝A

(2)

where kz is tip’s spring constant, Q is the tip’s quality factor, A0 is the tip’s free oscillation amplitude, A is the actual set oscillation amplitude, and φ is the phase shift angle. During the experiment, the tip’s free amplitude (A0) is 49.0 nm. The tapping intensity is defined as I ≡ A0 − A

(3)

The energy dissipation as function of tapping intensity of clean OTS surface, clean OTSpd, clean OTSpd−Zn, OTSpd−IL, and OTSpd−Zn−IL is plotted in Figure 5. During one tapping cycle the tip is subjected to three types of forces, all of which are functions of the tip−substrate distance z44 Ftip(z) = FvdW (z) + Felastic + inelestic(z) + Fcap(z)

(4)

where FvdW(z) is the van der Waals force, Felastic+inelestic(z) is the repulsive elastic and inelastic forces acting on the tip, which are from deformation of the surface, and Fcap is the capillary force from the water meniscus between the tip and the surface. Therefore, the energy dissipation of the tip during one cycle of tapping can be expressed by 9597

dx.doi.org/10.1021/la301452d | Langmuir 2012, 28, 9593−9600

Langmuir

Article

The IL layer adsorbed on OTSpd has a higher Young’s modulus than the IL layer adsorbed on OTSpd−Zn. For the same tapping intensity, the IL adsorbate layer on OTSpd deformed less than the IL adsorbate layer on OTSpd−Zn. The tip dissipated more energy to compress the IL layer adsorbed on OTSpd−Zn than IL layer on OTSpd−IL layer. This conclusion from the phase analysis can explain the observed tribological difference for OTSpd−IL and OTSpd− Zn−IL as well. As Figure 5 shows, below 6 nN loading force, the OTSpd−Zn−IL surface has a lower friction than the OTSpd−IL surface. Above 6 nN loading force, the OTSpd− Zn−IL surface has a higher friction than the OTSpd−IL surface. Therefore, the OTSpd−Zn−IL surface has a higher COF than OTSpd−IL surface. Such difference can be understood through the difference in contact area for these two surfaces under the same loading force. The tip’s contact area A on the surface can be estimated using the Hertzian model45

Figure 5. Energy dissipation of a NSC-14 tip during one cycle of tapping over different surfaces. Tapping intensity is defined as the difference between the tip’s free oscillation amplitude in air and the actual tapping amplitude on the surface.

∮ Ftip(z)dz

E =

A3/2 = A

0

=

∫A FvdW(z)dz + ∫0 FvdW(z)dz A 0 + ∫ Felastic + inelastic(z)dz + ∫ Fvcap(z)dz A 0

∮ Ftip(z)dz = ∫A

(5)

0

Felastic + inelastic(z)dz

(7)

where the R is the tip radius, L is the loading force, E1 and E2 are the Young’s moduli of the tip and the IL layer, and ν1 and ν2 are the Poisson numbers of the tip and IL layer, respectively. Since the IL layer on OTSpd−Zn has a lower Young’s modulus than the IL layer on the OTSpd surface, the tip’s contact area on the OTSpd−Zn−IL surface would be larger than that on the OTSpd−IL surface. A large tip−surface contact area yields a stronger van der Waals force, which corresponds to a larger friction when the tip moves across the surface.46 Therefore, due to different stiffness, under the same loading force, the friction force acted upon the AFM tip would be larger for the softer OTSpd−Zn−IL surface, which leads to a higher friction at high loading force. Previous studies on the tribological properties of imidazolium IL film on solid surfaces demonstrated that different anions and different IL film thickness yields different COFs.47,48 Here, our data indicate that the friction of an IL interface layer is also determined by its structure, adsorption orientation, and loading force. Finally, the lower Young’s modulus for the OTSpd−Zn−IL surface can also qualitatively explain that the OTSpd−Zn−IL surface has a better affinity toward further vapor-phase adsorption than that of the OTSpd−IL surface. Since adsorption of IL ions on the surface occurs through the ion− surface collision, the softer OTSpd−Zn−IL surface would provide a “soft landing” surface for the ion−surface collision, which dissipated more kinetic energy of the incident ions, and increase the sticking coefficient. An incident ion with lower kinetic energy adsorbs on the surface easier. In contrast, the “harder” OTSpd surface would let incident ions in the vapor phase be bounced off the surface, causing low affinity toward further adsorption.

where A is the amplitude of the tip’s oscillation. Since the van der Waals force is a conservative force, we have ∫ 0AFvdW(z)dz = −∫ A0 FvdW(z)dz. Then the van der Waals energy is not reflected in the tip’s energy dissipation. Therefore, from formula 5, we know that the total energy dissipation computed from the AFM phase shift is a summation of two parts: The first part is the repulsive energy, in which the tip dissipates energy to the surface, causing the molecules on the surface to have elastic change and inelastic permanent structural deformation. The second part is the attractive energy, in which the tip dissipates energy to overcome the capillary force between the tip and the surface when the tip moves away from the surface. Before ac mode imaging we purged the AFM environmental chamber with dry nitrogen for 1 h. At room temperature (25 °C), the relative humidity inside the chamber approached to 0%, which ensured that no water film existed on the hydrophobic OTS surface and no capillary condensation occurred. Therefore, there was no capillary force and the term ∫ A0 Fvcap(z)dz = 0. Then, formula 5 becomes E=

2 1 − ν22 ⎞ 3/2 3 ⎛ 1 − ν1 ⎟π RL ⎜ + 4 ⎝ E1 E2 ⎠

(6)

Therefore, under our experimental condition, the energy computed from the phase shift angle only reflects the tip’s energy dissipation during compression of the IL adsorbates. As shown in Figure 5, the tip always dissipated more energy on the OTSpd surface than on the OTSpd−Zn surface at the same tapping intensity. Hence, the OTSpd−Zn surface is stiffer than the OTSpd surface. In contrast, when IL layers adsorbed on these two chemical patterns, the tip consumed more energy on the OTSpd−Zn−IL layer than on OTSpd−IL layer, since the green curve is always above the red curve. Thus, as composite surface, the OTSpd−IL is stiffer than the OTSpd−Zn−IL layer. When cumulatively considering the change of the stiffness during IL adsorption on different chemical patterns, we conclude that the IL adsorbate layer on the OTSpd surface is stiffer than the IL layer adsorbed on the OTSpd−Zn surface.

■

CONCLUSION The properties of the ionic liquid−solid interface play a key role in most IL applications. The imidazolium ILs are the most widely used ILs. Because of their ionic nature, the role of their vapor has been largely ignored. In this study, we investigated the IL−solid interface formed by vapor adsorption. We found that the IL ions can adsorb on high-energy surfaces. The adsorbed IL forms a smooth layer with its own surface properties, which would affect the subsequent liquid-phase IL− 9598

dx.doi.org/10.1021/la301452d | Langmuir 2012, 28, 9593−9600

Langmuir

Article

(6) Zhu, S. D.; Wu, Y. X.; Chen, Q. M.; Yu, Z. N.; Wang, C. W.; Jin, S. W.; Ding, Y. G.; Wu, G. Dissolution of cellulose with ionic liquids and its application: A mini-review. Green Chem. 2006, 8, 325−327. (7) Lee, Y. G.; Chou, T. C. Ionic liquid ethanol sensor. Biosens. Bioelectron. 2004, 20, 33−40. (8) Buzzeo, M. C.; Hardacre, C.; Compton, R. G. Use of room temperature ionic liquids in gas sensor design. Anal. Chem. 2004, 76, 4583−4588. (9) Jin, X. X.; Yu, L.; Garcia, D.; Ren, R. X.; Zeng, X. Q. Ionic liquid high-temperature gas sensor array. Anal. Chem. 2006, 78, 6980−6989. (10) Sun, W.; Yang, M. X.; Jiao, K. Electrocatalytic oxidation of dopamine at an ionic liquid modified carbon paste electrode and its analytical application. Anal. Bioanal. Chem. 2007, 389, 1283−1291. (11) You, H.; Steckl, A. J. Three-color electrowetting display device for electronic paper. Appl. Phys. Lett. 2010, 97, 023514. (12) Nanayakkara, Y. S.; Moon, H.; Armstrong, D. W. A tunable ionic liquid based rc filter using electrowetting: A new concept. ACS Appl. Mater. Interfaces 2010, 2, 1785−1787. (13) Tamam, L.; Ocko, B. M.; Reichert, H.; Deutsch, M. Checkerboard self-patterning of an ionic liquid film on mercury. Phys. Rev. Lett. 2011, 106, 197801. (14) Mezger, M.; Schramm, S.; Schroeder, H.; Reichert, H.; Deutsch, M.; De Souza, E. J.; Okasinski, J. S.; Ocko, B. M.; Honkimaki, V.; Dosch, H. Layering of [Bmim]+-based ionic liquids at a charged sapphire interface. J. Chem. Phys. 2009, 131, 094701. (15) Atkin, R.; Warr, G. G. Structure in confined room-temperature ionic liquids. J. Phys. Chem. C 2007, 111, 5162−5168. (16) Yokota, Y.; Harada, T.; Fukui, K. I. Direct observation of layered structures at ionic liquid/solid interfaces by using frequencymodulation atomic force microscopy. Chem. Commun. 2010, 46, 8627−8629. (17) Quinn, A.; Sedev, R.; Ralston, J. Influence of the electrical double layer in electrowetting. J. Phys. Chem. B 2003, 107, 1163−1169. (18) Ye, J. T.; Inoue, S.; Kobayashi, K.; Kasahara, Y.; Yuan, H. T.; Shimotani, H.; Iwasa, Y. Liquid-gated interface superconductivity on an atomically flat film. Nat. Mater. 2009, 9, 125−128. (19) Saha, S.; Hayashi, S.; Kobayashi, A.; Hamaguchi, H. Crystal structure of 1-butyl-3-methylimidazolium chloride. A clue to the elucidation of the ionic liquid structure. Chem. Lett. 2003, 32, 740− 741. (20) Holbrey, J. D.; Reichert, W. M.; Nieuwenhuyzen, M.; Johnson, S.; Seddon, K. R.; Rogers, R. D. Crystal polymorphism in 1-butyl-3methylimidazolium halides: Supporting ionic liquid formation by inhibition of crystallization. Chem. Commun. 2003, 14, 1636−1637. (21) Raabe, G.; Koehler, J. Thermodynamical and structural properties of imidazolium based ionic liquids from molecular simulation. J. Chem. Phys. 2008, 128, 154509. (22) Law, G.; Watson, P. R. Surface orientation in ionic liquids. Chem. Phys. Lett. 2001, 345, 1−4. (23) Carmichael, A. J.; Hardacre, C.; Holbrey, J. D.; Nieuwenhuyzen, M.; Seddon, K. R. Molecular layering and local order in thin films of 1alkyl-3-methylimidazolium ionic liquids using x-ray reflectivity. Mol. Phys. 2001, 99, 795−800. (24) Binnemans, K. Ionic liquid crystals. Chem. Rev. 2005, 105, 4148−4204. (25) Hardacre, C.; Holbrey, J. D.; McMath, S. E. J.; Bowron, D. T.; Soper, A. K. Structure of molten 1,3-dimethylimidazolium chloride using neutron diffraction. J. Chem. Phys. 2003, 118, 273−278. (26) Li, L. B.; Groenewold, J.; Picken, S. J. Transient phase-induced nucleation in ionic liquid crystals and size-frustrated thickening. Chem. Mater. 2005, 17, 250−257. (27) Reed, S. K.; Lanning, O. J.; Madden, P. A. Electrochemical interface between an ionic liquid and a model metallic electrode. J. Chem. Phys. 2007, 126. (28) Kislenko, S. A.; Samoylov, I. S.; Amirov, R. H. Molecular dynamics simulation of the electrochemical interface between a graphite surface and the ionic liquid [Bmim][PF6]. Phys. Chem. Chem. Phys. 2009, 11, 5584−5590.

solid interaction. Therefore, for applications involving the IL’s wetting, dewetting, spreading, extraction on surface, and applications involving repetitive liquid−solid contact/disengagement, our study suggests that the role of vapor-phase adsorption has to be considered, even if the vapor pressure of IL is small. Furthermore, our study reveals that different surface chemistry will affect the orientation of the ions adsorbed on the surface because the ions of IL are usually large and have a directional charge/dipole distribution. We reveal that for the representative [Bmim]Cl, even under the same vapor exposure level, the IL adsorbate layer on the OTSpd−Zn pattern is softer, has a higher friction under low loading force, and has better affinity toward further vapor adsorption than the OTSpd chemical pattern. In summary, not only does the role of vapor need to be considered but also the surface dipole and possible surface−ion interaction shall be considered in the development of IL-based application, especially those applications involving liquid−solid interfaces. Characterization and measurements of the IL interfacial properties must be conducted under the pretext that the charge/dipole of the substrate is known.

■

ASSOCIATED CONTENT

S Supporting Information *

KPFM images of other two ILs (1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide, [Bmim][Tf2N], and 1decyl-3-methylimidazolium chloride, [C10mim]Cl) on OTSpd and OTSpd−Zn surfaces and corresponding analyses of the images. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research was partially supported by the University of Kentucky faculty start-up fund. Y.C. thanks Dr. De-en Jiang of Oak Ridge National Laboratory for providing [Bmim][Tf2N] as a gift for this research. X.Z. is thankful for the support from the Chinese Government Scholarship and the Kentucky Opportunity Fellowship.

■

REFERENCES

(1) Liu, H. T.; Liu, Y.; Li, J. H. Ionic liquids in surface electrochemistry. Phys. Chem. Chem. Phys. 2010, 12, 1685−1697. (2) Sugimoto, T.; Atsumi, Y.; Kono, M.; Kikuta, M.; Ishiko, E.; Yamagata, M.; Ishikawa, M. Application of bis(fluorosulfonyl)imidebased ionic liquid electrolyte to silicon-nickel-carbon composite anode for lithium-ion batteries. J. Power Sources 2010, 195, 6153−6156. (3) Ishikawa, M.; Sugimoto, T.; Kikuta, M.; Ishiko, E.; Kono, M. Pure ionic liquid electrolytes compatible with a graphitized carbon negative electrode in rechargeable lithium-ion batteries. J. Power Sources 2006, 162, 658−662. (4) Sato, T.; Masuda, G.; Takagi, K. Electrochemical properties of novel ionic liquids for electric double layer capacitor applications. Electrochim. Acta 2004, 49, 3603−3611. (5) Ue, M.; Takeda, M.; Toriumi, A.; Kominato, A.; Hagiwara, R.; Ito, Y. Application of low-viscosity ionic liquid to the electrolyte of double-layer capacitors. J. Electrochem. Soc. 2003, 150, A499−A502. 9599

dx.doi.org/10.1021/la301452d | Langmuir 2012, 28, 9593−9600

Langmuir

Article

(29) Ghatee, M. H.; Zolghadr, A. R.; Moosavi, F.; Ansari, Y. Studies of structural, dynamical, and interfacial properties of 1-alkyl-3methylimidazolium iodide ionic liquids by molecular dynamics simulation. J. Chem. Phys. 2012, 136. (30) Vatamanu, J.; Borodin, O.; Bedrov, D.; Smith, G. D. Molecular dynamics simulation study of the interfacial structure and differential capacitance of alkylimidazolium bis(trifluoromethanesulfonyl)imide [Cnmim][TFSI] ionic liquids at graphite electrodes. J. Phys. Chem. C 2012, 116, 7940−7951. (31) Wu, J. Z.; Jiang, T.; Jiang, D. E.; Jin, Z. H.; Henderson, D. A classical density functional theory for interfacial layering of ionic liquids. Soft Matter 2011, 7, 11222−11231. (32) Horcas, I.; Fernandez, R.; Gomez-Rodriguez, J. M.; Colchero, J.; Gomez-Herrero, J.; Baro, A. M. Wsxm: A software for scanning probe microscopy and a tool for nanotechnology. Rev. Sci. Instrum. 2007, 78, 013705. (33) Cai, Y. G. The partially degraded hydrophilic silane pattern and its application in studying the structures of long chain alkane films. Langmuir 2009, 25, 5594−5601. (34) Hansma, H. G.; Revenko, I.; Kim, K.; Laney, D. E. Atomic Force Microscopy of long and short double-stranded, single-stranded and triple-stranded nucleic acids. Nucleic Acids Res. 1996, 24, 713−720. (35) Green, C. P.; Lioe, H.; Cleveland, J. P.; Proksch, R.; Mulvaney, P.; Sader, J. E. Normal and torsional spring constants of Atomic Force Microscope cantilevers. Rev. Sci. Instrum. 2004, 75, 1988−1996. (36) Green, C. P.; Sader, J. E. Torsional frequency response of cantilever beams immersed in viscous fluids with applications to the Atomic Force Microscope. J. Appl. Phys. 2002, 92, 6262−6274. (37) Lu, J.; Delamarche, E.; Eng, L.; Bennewitz, R.; Meyer, E.; Guntherodt, H. J. Kelvin Probe Force Microscopy on surfaces: Investigation of the surface potential of self-assembled monolayers on gold. Langmuir 1999, 15, 8184−8188. (38) Hamaker, H. C. The london - van der Waals attraction between spherical particles. Physica 1937, 4, 1058−1072. (39) Dzyaloshinskii, I. E.; Lifshitz, E. M.; Pitaevskii, L. P. The general theory of van der Waals forces. Adv. Phys. 1961, 10, 165−209. (40) Lee, S. W.; Sigmund, W. M. Afm study of repulsive van der Waals forces between Teflon AFtm thin film and silica or alumina. Colloids Surf. A 2002, 204, 43−50. (41) Earle, M. J.; Esperanca, J.; Gilea, M. A.; Lopes, J. N. C.; Rebelo, L. P. N.; Magee, J. W.; Seddon, K. R.; Widegren, J. A. The distillation and volatility of ionic liquids. Nature 2006, 439, 831−834. (42) Flater, E. E.; Ashurst, W. R.; Carpick, R. W. Nanotribology of octadecyltrichlorosilane monolayers and silicon: Self-mated versus unmated interfaces and local packing density effects. Langmuir 2007, 23, 9242−9252. (43) Cleveland, J. P.; Anczykowski, B.; Schmid, A. E.; Elings, V. B. Energy dissipation in tapping-mode Atomic Force Microscopy. Appl. Phys. Lett. 1998, 72, 2613−2615. (44) Zitzler, L.; Herminghaus, S.; Mugele, F. Capillary forces in tapping mode Atomic Force Microscopy. Phys. Rev. B 2002, 66, 155436. (45) Carpick, R. W.; Agrait, N.; Ogletree, D. F.; Salmeron, M. Variation of the interfacial shear strength and adhesion of a nanometer-sized contact. Langmuir 1996, 12, 3334−3340. (46) Mazyar, O. A.; Jennings, G. K.; McCabe, C. Frictional dynamics of alkylsilane monolayers on SiO2: Effect of 1-n-butyl-3-methylimidazolium nitrate as a lubricant. Langmuir 2009, 25, 5103−5110. (47) Zhao, W. J.; Zhu, M.; Mo, Y. F.; Bai, M. W. Effect of anion on micro/nano-tribological properties of ultra-thin imidazolium ionic liquid films on silicon wafer. Colloids Surf. A 2009, 332, 78−83. (48) Perkin, S.; Albrecht, T.; Klein, J. Layering and shear properties of an ionic liquid, 1-ethyl-3-methylimidazolium ethylsulfate, confined to nano-films between mica surfaces. Phys. Chem. Chem. Phys. 2010, 12, 1243−1247.

9600

dx.doi.org/10.1021/la301452d | Langmuir 2012, 28, 9593−9600