Unique Structures and Vibrational Spectra of Protic Ionic Liquids

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Unique Structures and Vibrational Spectra of Protic Ionic Liquids Confined in TiO2 Slits: The Role of Interfacial Hydrogen Bonds Zhongyang Dai, Lili Shi, Linghong Lu,* Yunhao Sun, and Xiaohua Lu* College of Chemical Engineering, State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University, 5 Xinmofan Road, Nanjing 210009, P. R. China

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S Supporting Information *

ABSTRACT: The ionic liquid (IL)/titanium dioxide (TiO2) interface exists in many application systems, such as nanomaterial synthesis, catalysis, and electrochemistry systems. The nanoscale interfacial properties in the above systems are a common issue. However, directly detecting the interfacial properties of nanoconfined ILs by experimental methods is still challenging. To help better learn about the interfacial issue, molecular dynamics simulations have been performed to explore the structures, vibration spectra, and hydrogen bond (HB) properties at the IL/TiO2 interface. Ethylammonium nitrate (EAN) ILs confined in TiO2 slit pores with different pore widths were studied. A unique vibrational spectrum appeared for EAN ILs confined in a 0.7 nm TiO2 slit, and this phenomenon is related to interfacial hydrogen bonds (HBs). An analysis of the HB types indicated that the interfacial NH3+ group of the cations was in an asymmetric HB environment in the 0.7 nm TiO2 slit, which led to the disappearance of the symmetric N−H stretching mode. In addition, the significant increase in the HB strength between NH3+ groups and the TiO2 surface slowed down the stretching vibration of the N−H bond, resulting in one peak in the vibrational spectra at a lower frequency. For the first time, our simulation work establishes a molecular-level relationship between the vibrational spectrum and the local HB environment of nanoconfined ILs at the IL/TiO2 interface, and this relationship is helpful for interface design in related systems. rutile (110) surface and the first water layer in the slit has a consistent orientation configuration with the oxygen atom facing the rutile (110) surface. A similar layered water structure has been observed on the anatase (101) surface, but the oxygen atom of water in the first layer prefers to orient away from the surface because of the different surface oxygen atom structure of anatase (101).17 Lu et al.18 performed MD simulations and reported that the number of water layers depends on the pore size of the TiO2 slit and decreases for small TiO2 slits because of a stronger confinement effect. Furthermore, scientists have attempted to reveal the mechanism of the characteristic structures of water molecules confined in TiO2 slits. As a typical metal oxide, TiO2 has a strong interaction with water molecules, and this interaction is crucial to determine the interfacial structure and other properties. Sofo and co-workers19 divided interfacial hydrogen bond (HB) interactions into two parts, i.e., HBs between exposed oxygen atoms of the TiO2 surface and water molecules and HBs between interfacial water molecules. The HB interaction between TiO2 and water molecules is stronger than the latter. This strong HB interaction is regarded as the key factor in forming a stable, ordered, layered structure near

1. INTRODUCTION Interfacial fluids under nanoconfinement have aroused widespread interest over the past several decades due to their unconventional properties at an interface, which are totally distinct from the properties of the bulk counterpart. The ionic liquid (IL)/titanium dioxide (TiO2) interface is a typical solid/ liquid interface that exists in many application systems, e.g., TiO2 nanomaterial synthesis, solar cell, photocatalysis, lubricant, and supercapacitor systems.1−7 Generally, experimental observations have shown that the unique properties of interfacial ILs lead to excellent performance, such as high ionic conductivity and low friction.8 Therefore, it is important to gain further fundamental insight into the structural and dynamic properties of IL at the IL/TiO2 interface. However, based on the current state of the art, using an experimental approach to directly detect the interfacial properties of ILs in various confinement conditions, e.g., TiO2 nanotubes and slits, is still a challenge.9,10 Molecular dynamics (MD) simulation, one of the most effective theoretical tools, can provide a detailed description about the microstructure and dynamic properties of fluids under nanometer-scale confinement. Recently, some simulation works were committed to study the behaviors of different fluids, especially for the water molecule, confined in the TiO2 slit.11−15 For example, Chen et al.16 found that confined water can form oriented and layered structures on the © 2018 American Chemical Society

Received: July 26, 2018 Revised: October 11, 2018 Published: October 12, 2018 13449

DOI: 10.1021/acs.langmuir.8b02527 Langmuir 2018, 34, 13449−13458

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cations and NO3− anions,38 is capable of building a threedimensional HB network. We aim to obtain fundamental information on the confined structure configurations, vibrational spectra, and HBs of EAN in TiO2 slits with different widths. In addition, a relationship between the HBs and spectral information is established to understand the confined IL behavior. The rest of this paper is organized as follows. Section 2 presents the MD simulation details. In Section 3, we show the detailed simulation results and relevant discussion, including the confined IL density profile, orientation of IL ions, vibrational spectra of EAN ion pairs, and structural and dynamic properties of the HBs. Finally, summary and concluding remarks are provided in Section 4.

the TiO2 surface.20,21 In addition, similar layered structures and interfacial HB behaviors have been reported for other liquids, e.g., organic solvents and deep eutectic solvents, confined in TiO2 slits.22−24 ILs are some of the most promising solvents for many applications that involve in IL/TiO2 systems because of their excellent properties, such as chemical and thermal stability, designable structure, etc.25−27 The specific HB interaction is an important factor in determining the structure and properties of ILs.28−30 On the other hand, the presence of TiO2 disrupts the intrinsic HB environment of ILs and creates a new type of HB network. Kirchner et al.31 investigated the HB interaction between imidazolium cations and the surface oxygen atoms of anatase (101) and found that the HB between cations and TiO2 resulted in a parallel distribution of cation rings on the TiO2 surface. A similar parallel orientation induced by interfacial HB interactions was also observed for 1-methyl-3butylimidazolium cations on a rutile (110) surface.32,33 In addition, interfacial HBs result in many of the peculiar properties of ILs confined in TiO2 slits. For example, Malali and Foroutan34 found that the HB-induced structures of the first layer of ILs on rutile (110) surfaces resulted in the upper IL layers showing excellent nonwetting properties. Hung et al.32 studied the diffusion behaviors of ILs confined in different regions of TiO2 slits and determined that the diffusion coefficient of IL molecules in the first layer is far lower than that in the center region due to the strong HB interaction between the ILs and TiO2 surface. Although simulation studies have made considerable progress, it is still difficult to completely relate the simulation results with those from experiments because the properties calculated from simulations, such as interfacial HBs, are often difficult to observe by experimental methods. Experimentally, vibrational spectroscopy is always considered to be an effective technique to reveal the structure of nanoconfined fluid because of the vibrational signal sensitivity to changes in local environments. Understanding the relationship between calculated properties and vibrational spectra may help experimental scientists further understand interfacial fluid behavior. In recent years, scientists have made some progress in understanding the IL/carbon nanotube system using experiments together with theoretical simulations. Zhou et al.35 studied the vibrational spectra of protic ILs surrounding single-walled carbon nanotubes (SWNTs) and found that the red-shift phenomenon of interfacial anions was induced by the IL/SWNT interaction. Similar vibrational spectra were observed in the Fourier transform infrared experiments on [C6mim] [PF6] dispersed on the surface of SWNTs.36 Yang et al. constructed a structure−spectrum relationship for ILs confined in SWNTs via systematic MD simulations.37 In contrast to hydrophobic carbon materials, TiO2 is hydrophilic, and a fluid can form HB with the TiO2 surface, which is crucial for confined fluid properties. For example, previous work showed that interfacial HBs have an effect on the vibrational band of confined water molecules.15,22 Despite this progress, little related work has been reported for ILs confined in TiO2 slits. Hence, a direct correlation between spectral information and interfacial HBs for ILs confined in TiO2 slits is still needed. In this paper, ionic structures, vibrational spectrum, and HB properties of ethylammonium nitrate (EAN) IL confined in TiO2 slits have been systematically investigated. Two parallel rutile slabs with various slit widths were used to construct the host TiO2 material. EAN IL, composed of C2H5NH3+ (EA+)

2. SIMULATION DETAILS In the present work, four slit pores with widths of 0.7, 1.4, 2.1, and 2.8 nm were constructed with two rutile (100) TiO2 walls parallel to the x−y plane and kept frozen in all simulated systems. Meanwhile, the periodic boundary condition was considered in the x and y directions to eliminate the effects of edges.34 The box size for the x−y plane was up to 50.534 × 50.303 A2, and such simulation dimension can ensure enough ion pairs to be filled inside the TiO2 slit pore to obtain reliable simulation results. According to previous studies,39−43 the confined densities of EAN ILs inside the four TiO2 slit pores were comparable to the bulk EAN value and were estimated via an effective volume V = S(d − δo), where S is the crosssectional area in the x−y plane, d denotes the width of the TiO2 slit pore, and δo corresponds to the atomic size of oxygen atoms exposed on the rutile (100) surface. Accordingly, 128, 256, 384, and 512 EAN ion pairs were randomly placed inside TiO2 slit pores with widths of 0.7, 1.4, 2.1, and 2.8 nm, respectively. In the current work, the EAN IL was described by the all-atom force field developed by Acevedo and TiradoRives44,45 and is suitable for the interface between ILs and solid materials.34,35 The force-field parameters for TiO2 were taken from refs 46, 47. The nonbonded interactions, including van der Waals (vdW) and electrostatic interactions, between different species were described by Lennard-Jones (L-J) 12−6 and Coulombic potentials. All of the parameters used here are summarized in Table S1 of the Supporting Information (SI). The Lorenz−Berthelot mixing rules were employed to deal with the vdW interactions among different kinds of atoms. All production runs were conducted with the canonical (NVT) ensemble, in which the temperature was kept constant (353.0 K) via a Nosé−Hoover thermostat48 with the damping parameter of 100.0 fs. The motion of atoms was described by the classical Newton’s equation, which was solved using the velocity-Verlet algorithm49 with 1.0 fs time step. The cutoff distance of the nonbonded interactions was set to 1.0 nm, and the particle−particle particle−mesh method50 was performed to calculate the long-range electrostatic interactions. Each simulation was run for a total time of 30.0 ns, first 10 ns for relaxation and equilibration and the last 20.0 ns for collecting data with the trajectory being stored every 100.0 fs. The corresponding equilibrium configurations and two-dimensional density distribution contours are displayed in Figures S1 and S2 of the SI. In addition, total interaction energy curves after equilibration are shown in Figure S3. After the 30 ns calculations, we carried out another two successive NVT calculations to analyze the HB dynamics and vibrational spectra, respectively. For the HB dynamics calculations, each calculation was run for 500 ps with the coordinates being 13450

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experiments and simulations.46,47,54,55 Therefore, one can conclude that the hydrophilic nature of the TiO2 surface would result in the above preferential location of cation groups on the surface of TiO2 walls. In addition, a comparison between the NO3− anion and the NH3+ group of cation in the first layer illustrates that the NH3+ group peak is higher and closer to the TiO2 walls than the NO3− peak in the 0.7 nm TiO2 slit, implying that the NH3+ group of cation is more likely to adhere to the surface of the TiO2 walls than the NO3− anion. In contrast, for larger TiO2 slit sizes, i.e., 1.4, 2.1, and 2.8 nm, the peak height for both NH3+ groups of cations and NO3− anions in the first layers increases as the slit size increases, as shown in Figure 1. Meanwhile, these two peaks have almost an identical location, indicating that both the NO3− and NH3+ groups preferentially distribute near the TiO2 walls in large TiO2 slits. The results in Figure 1 shows that the peak height for the CH3+ groups of the cations confined in the 0.7 nm TiO2 slit is significantly different from that for CH3+ groups confined in larger TiO2 slits. For example, in the 0.7 nm TiO2 slit, the sole peak appears near the TiO2 walls and has a peak value of approximately 0.015/Å3, while in the 1.4, 2.1, and 2.8 nm TiO2 slits, the peak of CH3+ group weakens near the TiO2 walls and the corresponding peak values are approximately 0.0075, 0.006, and 0.007/Å3. Following the peaks near the walls, other larger CH3+ group peaks appear for the EAN IL confined in the 1.4, 2.1, and 2.8 nm TiO2 slits. This difference implies that the CH3+ groups in the 0.7 nm TiO2 slit have a more ordered arrangement than that of those in the larger slits due to the strong confinement effect in the 0.7 nm TiO2 slit. To better understand the structure of EAN IL molecules near the TiO2 walls, the orientation angle distribution for both cations and anions located in the first absorbed layers was explored. Figure 2 illustrates that for the cations the vector u1

stored every 5 time steps. However, for the vibrational spectral analysis, the NVT simulation was carried out for 200 ps with a smaller time step of 0.5 fs to capture the vibrational information precisely, and the velocity trajectory was updated every time step. In this study, all simulations were carried out using the large-scale atomic/molecular massively parallel simulator package (LAMMPs).51

3. RESULTS AND DISCUSSION 3.1. Structural Properties of Confined ILs. To reveal the structure of EAN ion pairs inside the TiO2 slit pores, we analyze the number density profiles of EAN IL along the direction perpendicular to the TiO2 wall (i.e., z-axis), displayed on Figure 1. The CH3 group, NH3+ group of cations, and

Figure 1. Number density profiles along the z-direction of the CH3 groups and the NH3+ groups in cations as well as the NO3− anions inside the TiO2 nanoslits: (a) 0.7 nm, (b) 1.4 nm, (c) 2.1 nm, and (d) 2.8 nm.

NO3− anions were represented by characteristic atoms C, N, and N, respectively. Here, the number density distribution was calculated through the ratio of local atom numbers, n(z), within an orthorhombic shell at position z to the corresponding volume, v (v = lx × ly × z, where lx and ly represent the size of the x-axis and y-axis of the box, respectively, and z is the unit size along the z-axis), i.e., ρ(z) = n(z)/v = n(z)/(lx × ly × z). As shown in Figure 1, one remarkable feature is that the TiO2 slits have an effect on the structure of the EAN IL. The formation of layered structures near the TiO2 walls can be clearly observed in the four systems, and the local density of the EAN IL in the layers is much higher than that in the bulk phase. Such well-defined layers have been reported previously for ILs under slitlike confinement conditions.32,52,53 A comparison of the peak locations of the different groups in Figure 1 shows that the NH3+ group of the cation and the NO3− anion tend to be closer to the TiO2 wall than the CH3 group. This means that the NH3+ group of cations tends to face toward TiO2, compared with the CH3 group. For the EAN IL at the liquid/vapor interface, the X-ray reflectivity and vibrational sum frequency spectroscopy experiments uncovered that the CH3 groups of EA+ cations prefer to orient toward the gas phase mainly because of their solvophobic characteristics. It is well accepted that TiO2 surfaces have a characteristic hydrophilicity based on both

Figure 2. Schematic illustrations for the definition of orientation vectors for both the cations and the anions; the z-axis is defined as the surface normal. Here, the cyan, white, red, blue and brown spheres represent the carbon, hydrogen, oxygen, nitrogen, and titanium atoms, respectively.

points from the N atom in NH3+ to the C atom in CH3, whereas the vector u2 points from the N atom to the C atom in CH2. In addition, the vector u3 for the NO3− anions represents the dipole vector of the anions. We analyze the normalized orientation distribution of the cation and anion in terms of α, β, and γ, which are defined as the angles between u1, u2, and u3 and the normal vector, respectively. Figure 3a,b shows that the distributions of α have one sharp peak at 80° for the 0.7 nm slit and display a narrow plateau ranging from 20 to 40° for the 13451

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orientation to the TiO2 surface but the CH3 groups orient in the opposite direction. This orientation configuration is in favor of the HBs formed between the NH3+ groups and the TiO2 surface under confinement. The results of Figure 3c suggest that the γ profiles have a peak at approximately 40° for all of the studied TiO2 slits, indicating that the NO3− anions have a tendentious orientation, i.e., anions lean along the TiO2 surface and the N atom faces the TiO2 walls. In the condition that the NH3+ groups point toward the TiO2 walls, such a configuration is beneficial to form HBs between EA+ cations and NO3− anions. 3.2. Vibrational Spectra of the First Layer ILs. From the experimental viewpoints, vibrational spectroscopy is always considered to be an effective technique to elucidate the interfacial properties of the fluids under the confinement conditions. In this work, we calculated the vibrational spectra of the EAN IL molecules in the first adsorbed layers of EAN IL in the TiO2 slits. To avoid the problems of characteristic peak overlap between the ion pairs, the vibrational spectra for EA+ and NO3− were separately analyzed. Here, we obtained the EAN IL vibrational spectral results through Fourier transformation (FT) of the velocity autocorrelation function (VACF). The normal VACF is defined as56,57 Cv(t ) =

⟨vi⃗(0)vi⃗(t )⟩ ⟨vi⃗(0)vi⃗(0)⟩

(1)

where vi⃗(t ) represents the instantaneous velocity at time t for a specific atom i in the EAN ion pairs. The sign of angular brackets represents a statistical average for all atoms under the distinct reference time. Then, FT is carried out for the obtained VACF to generate the vibrational density of states, namely, vibrational spectra S(ω)56,57

Figure 3. Angular distribution of α, β, and γ for both the cations and anions of the first adsorbed layer inside the TiO2 slits with various pore sizes. On the left, the schematic illustrations for the definitions of corresponding angles are shown; here, the yellow slab represents the TiO2 surface.

S(ω) =

other larger slits. All β profiles have two sharp peaks with different positions and intensities. Specifically, for the 0.7 nm slit, the two peaks are located at 20 and 60°, whereas for the larger slits, the corresponding peaks are at 20 and 70°. It should be noted that the angular distribution plots of the cations demonstrate some relatively sharp peaks for the 0.7 nm TiO2 slit. In particular, Figure 3 clearly shows that for cations inside the 0.7 nm TiO2 slit the distribution of P(α) and P(β) shows a maximum value up to 0.035 and 0.03, respectively, whereas these values in the other TiO2 slits are approximately 0.02 and 0.02, correspondingly. This indicates that the cations of EAN ILs can form a more ordered structure in the 0.7 nm TiO2 slit. With this orientation information, one can demonstrate that the cations confined inside the 0.7 nm TiO2 slit show a distinct orientation pattern, i.e., an “alkylhorizontal” orientation, corresponding to vector u1, u2 being nearly parallel to the TiO2 wall, i.e., the α and β peaks at 80 and 70°, respectively. Meanwhile, in larger TiO2 slits, the “alky1-horizontal” and “alky1-vertical” orientations are present. The alky1-vertical orientation corresponds to the vector u1, u2 being normal to the TiO2 wall, i.e., the α plateau from 20 to 40° and the β peak at 20°. For the alkyl-horizontal orientation, the EA+ cations tend to lie along the TiO2 walls, whereas for the alky1-vertical orientation, the cations are more likely to be upright on the TiO2 walls. These results can be attributed to the fact that in the small 0.7 nm TiO2 slit severe confinement effects induce the cations of EAN ILs to form more ordered structures. In addition, for both alky1-horizontal and alky1vertical, the NH3+ groups in cations show a preferred

∫0

∞

Cv(t ) cos ωt dt

(2)

Theoretically, the vibrational spectra for ion pairs of ILs can be separately calculated and in turn the respective vibrational information of cations or anions can be observed in their corresponding vibrational spectra. First, for the cations confined in four TiO2 slit pores, the calculated vibrational spectra are listed in Figure 4 and the counterpart for bulk cations is also shown here for comparison. Of the three characteristic peaks located at around 2923, 2957, and 3010 cm−1, the first one is assigned to the symmetric stretching vibration for C−H bonds, while the other two peaks correspond to the asymmetric stretching vibration for C−H bonds in methylene and methyl groups. In addition, the two peaks appearing at about 3250 and 3344 cm−1 are assigned to N−H vibrational modes with symmetric and asymmetric stretching, respectively. Figure 4 demonstrates that the vibration modes of C−H bonds in all systems behave quite similar to those in the bulk EAN IL. This similarity is because the alkyl groups tend to be far from the TiO2 surface and thus their vibrational behavior is minimally influenced by the surface. However, for the NH3+ groups, Figure 4 shows that for the 1.4, 2.1, and 2.8 nm TiO2 slits, both the intensities of N−H symmetric and asymmetric stretching vibrations reduce compared to the bulk counterpart. In addition, the confined cations have a shoulder before the conventional asymmetric N−H stretching vibrational peak at 3344 cm−1. Above vibrational spectra are different from those of other interfaces 13452

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Figure 5. Fourier transformation vibrational spectra in the range of 1200−1800 cm−1 for the anions of the first layer inside (a) 0.7 nm, (b) 1.4 nm, (c) 2.1 nm, and (d) 2.8 nm TiO2 slits. For comparison, the results for the bulk cations are also shown.

Figure 4. Fourier transformation vibrational spectra in the range of 2800−3400 cm−1 for the cations of the first layer inside (a) 0.7 nm, (b) 1.4 nm, (c) 2.1 nm, and (d) 2.8 nm TiO2 slits. For comparison, the results for the bulk cations are also shown.

confined in SWNTs. Their work indicated that ion pairs confined in SWNTs with different diameters have different HB strengths and result in different red shifts of anions. Accordingly, the red shift of NO3− anions confined in the TiO2 slits can be attributed to the HB interactions between cation and anion ion pairs. The proportion of HBs between the EAN ion pairs in the 0.7 nm TiO2 slit is less than that in the larger TiO2 slits (i.e., 1.4, 2.1, and 2.8 nm), which leads to a smaller red shift in the 0.7 nm TiO2 slit than in the larger TiO2 slits. 3.3. Hydrogen Bond Properties of the First Layer ILs. To verify our hypothesis about the vibrational spectra for EAN ion pairs inside TiO2 slits, related HB properties of EA+−TiO2 and EA+−NO3− in the first layers were calculated for the four studied systems, including the proportion distribution of the HB number, the HB continuous time correlation functions (TCFs), and the average HB lifetime. Generally, there are two geometric criteria for the HB formation, as given by64,65

such as EAN−air, EAN−carbon nanotube, and water−Au nanoparticle.37,58−62 Some previous works35,37 have confirmed that the characteristic asymmetric stretching vibration peak of N−H at 3344 cm−1 is due to HBs between EAN ion pairs. For the EAN IL molecules confined inside the TiO2 slits, the EA+ cations can form two types of HBs: one with the TiO2 wall and the other with the NO3− anions. Formation of HBs between the EA+ cations and the TiO2 walls produces a slow asymmetric stretching vibration, corresponding to the shoulder before the asymmetric vibration peak at 3344 cm−1. Meanwhile, the HBs between the EA+ cations and the TiO2 walls make fewer NH3+ groups in the symmetric HB environment compared with that in the bulk phase, which leads to a decay of the symmetric vibration characteristic peak of N−H at 3250 cm−1, for the 1.4, 2.1, and 2.8 nm TiO2 slits. In particular, for the small 0.7 nm TiO2 slit, Figure 4a shows that the characteristic symmetric stretching vibration peak of N−H almost disappears, demonstrating that in the 0.7 nm TiO2 slit there are few free NH3+ groups, which form HBs neither with the anions nor with the TiO2 walls. On the other hand, we can observe in Figure 4a that there is a splitting phenomenon for the N−H asymmetric stretching vibration mode, with the appearance of one lower-frequency peak. This can be attributed to the strong HBs between the EA+ cations and the TiO2 walls, causing those N−H bonds vibrate slowly. Overall, both the symmetric and asymmetric vibration characteristic peaks of the NH3+ group decay relative to those of the bulk phase because the EA+ cations form HBs with the TiO2 surface. Then, the vibrational spectra of the first layer NO3− anions were investigated and are shown in Figure 5. The sole characteristic peaks appearing around 1550 cm−1 are assigned to the asymmetric stretching vibration for N−O bonds of NO3− anions. Figure 5 indicates that the asymmetric N−O stretching vibration characteristic peak of confined anions exhibits a small red shift compared to that of the bulk counterpart, and this red shift is further enhanced in the larger 1.4, 2.1, and 2.8 nm TiO2 slits. Eaton et al.63 have reported that enhanced HB interactions resulted in a red shift of Nmethylacetamide vibrational spectra. Zhou et al.37 used HB dynamics to explain the red shift of the anions of EAN ILs

RON < R cON and θ ONH < θcONH

(3)

where O is the HB acceptor from the TiO2 surface or NO3− anion, whereas N is the HB donator from the NH3+ cation. RON is the distance between the HB acceptor and donator, whereas θONH denotes the O···N−H angle. RcON/θcONH corresponds to the distance/angular criterion for HB formation, respectively. According to previous works,35 the values for RcON and θcONH were set to 3.7 Å and 30°, respectively. We calculated the average HB number between the EA+ cations and NO3− anions and between the EA+ cations and the TiO2 walls in the first layers of the four systems, as shown in the Table 1. We can see in this table that in the TiO2 slits the average HB number between the cations and anions decreases compared to that in the bulk phase. In addition, the EA+ cations in the 0.7 nm slit form more HBs with the TiO2 wall than those in the other three slit systems. To obtain details on the HBs, we calculated the proportion distribution of HB number for the EA+ cation and NO3− anion and the EA+ cation and the TiO2 wall in the first layers, respectively. The proportion distribution of HB number per EA+ cation is defined as the ratio of the number of NH3+ groups in cations with the same HB number to the total number of NH3+ groups 13453

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cations and the TiO2 walls in Figure 6b. The EA+ cation forms zero or one HB with the TiO2 wall, and almost 90% NH3+ groups in the EA+ cation form one HB with the TiO2 wall in the 0.7 nm TiO2 slit, which is approximately 1.5 times more than that in the larger TiO2 slits. Combining the results for the 0.7 nm slit in Figure 6a, it can be inferred that more than 80% NH3+ groups of the EA+ cation form two HBs with the NO3− anion. One can analyze that in the case of 0.7 nm TiO2 over 80% NH3+ in cations can form two HBs with the NO3− anions and one HB with the TiO2 wall, which indicates that in the small (i.e., 0.7 nm) TiO2 slit the three N−H per NH3+ group for most of the EA+ cations are in different stretching vibration environments. This results in the absence of the symmetric stretching vibration mode of the N−H bonds for the EA+ cation in the first layer of the 0.7 nm TiO2 slit. Finally, we investigated the relevant HB dynamics in the first layers of the four studied systems to probe the HB interactions. The HB continuous time correlation function (TCF) SHB(t) is defined in terms of the following expression64

Table 1. Average HB Numbers among EAN ILs, as Well as between EA+ Cations and the TiO2 Wall in the First Layers of 0.7, 1.4, 2.1, and 2.8 nm Slitsa slit width

HB number (EA+−NO3−)

HB number (EA+−TiO2)

bulk 0.7 nm 1.4 nm 2.1 nm 2.8 nm

2.60 1.98 2.24 2.26 2.27

0.88 0.56 0.54 0.52

a

The average HB numbers of EAN bulk are also shown for comparison.

that form HBs. As shown in Figure 6a, the proportion distributions of one, two, and three HBs between the EA+

SHB(t ) =

⟨h(0)h(t )⟩ ⟨h(0)h(0)⟩

(4)

in which the variable h(t) is unity when a targeted pair is hydrogen-bonded continuously from 0 to time t; otherwise, its value is zero. Figure 7 shows the SHB(t) counterparts of the

Figure 6. (a) Proportion distribution of the NH3+ groups of cations with one, two, and three HBs in the first layers, confined in 0.7, 1.4, 2.1, and 2.8 nm TiO2 slits. For comparison, the corresponding results in the bulk phase are also shown. Here, the HB refers to those formed between the cations and anions; (b) proportion distribution of the NH3+ groups of cations with zero or one HB in the first layers, confined in 0.7, 1.4, 2.1, and 2.8 nm TiO2 slits. Here, the HB refers to those formed between the cations and TiO2 wall.

Figure 7. Continuous TCF SHB(t) for both EA+−NO3− HBs and EA+−TiO2 HBs of the first layers inside (a) 0.7 nm, (b) 1.4 nm, (c) 2.1 nm, and (d) 2.8 nm TiO2 slits. For comparison, the results for the bulk EAN IL are also shown.

EA+−NO3− HBs and EA+−TiO2 HBs in the first layers of the four studied systems and the SHB(t) counterparts of bulk ion pairs for comparison. We can find that for all confined systems the SHB(t) profiles for the EA+−NO3− HBs show a slightly slower decay when compared with the bulk counterpart, whereas the SHB(t) curves of the EA+−TiO2 HBs show a much distinct slow decay. Such a difference reveals that the HB strengths inside the TiO2 slits follow the following order: EA+−TiO2 ≫ EA+−NO3− (confined) > EA+−NO3− (bulk). Slit confinement leads to strong hydrogen bonding interactions, which is further verified in the Supporting Information. For a better comparison, we calculated the average lifetime of the HBs (τHB s ) by fitting the SHB(t) curves with three weighted exponentials, given as follows66−68

−

cation and the NO3 anion are almost identical in the first layers of the 1.4, 2.1, and 2.8 nm TiO2 slits and the proportion distributions of three HBs that offer the same N−H stretching vibration environment for the NH3+ group show a decrease of approximately 50% compared to that of the bulk EAN IL. Thus, we attribute the decrease in the N−H symmetric vibration intensity of the EA+ cation in the larger TiO2 slits to the reduction in the three HB numbers between the EA+ cations and NO3− anions. Moreover, in the smaller TiO2 slit (i.e., 0.7 nm), the proportion of the three HBs is greatly reduced with a value lower than 0.1, indicating that few NH3+ groups form three HBs with the NO3− anions. We also show the proportion distribution of HB numbers between the EA+ 13454

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the orientation of the cations, meanwhile the orientation of the NO3− anions is represented by the vector from N atom to one O atom. Then, we have calculated the rotational dynamics for both cations and anions based on the time correlation functions (TCFs) given by70

SHB(t ) = A exp( −t /τa) + B exp(−t /τb) + C exp(−t /τc) (5)

and then τsHB = Aτa + Bτb + Cτc

(6)

in which A, B, and C represent the fitting parameters (A + B + C = 1) and τa, τb, and τc are the time constants, and we have listed the corresponding values in Tables S2 and S3 of the Supporting Information. The average HB lifetimes for EA+− TiO2 and EA+−NO3− in the first layers are shown in Figure 8.

Cr(t ) =

1 Ni

Ni

∑ uj(t )uj(0) j=1

where Ni corresponds to the total number of cations or anions in the first layer at time 0 and uj(t) denotes the unit vector of the jth ion at time t. The angular bracket is indicative of the ensemble average that is performed for all tagged ions under distinct reference time. The calculated TCF curves for all systems studied here are displayed in Figure 9, and the

Figure 8. Average lifetime τHB (ps) for both EA+−NO3− HBs and s + EA −TiO2 HBs of the first layers inside the TiO2 slits with various sizes. The dashed line represents the results of the bulk EAN IL for comparison.

indicate a greater HB strength. The Larger values of τHB s average lifetimes of the EA+−TiO2 HBs are obviously larger than those in the bulk, which means that the HB strength is greatly enhanced, leading to a red shift in the N−H asymmetric stretching vibration. Moreover, the lifetime of the EA+−TiO2 HBs in the 0.7 nm TiO2 slit is about 12 ps, which is approximately 2 times larger than that in the larger TiO2 slits; thus, a larger red shift in the N−H asymmetric stretching vibration is observed. Combining the analyzed results from Figure 6b, it can be inferred that the proportion of the EA+−TiO2 HBs in the first layer of the 0.7 nm TiO2 slit is approximately 1.5 times greater than that in the larger TiO2 slits. The differences between the 0.7 nm slits and larger ones lead to a split in the asymmetric N−H stretching vibrational peak; i.e., one peak at a lower frequency for the 0.7 nm TiO2 slit, whereas a shoulder for the larger ones, as shown in Figure 4. On the other hand, the lifetime of the EA+−NO3− HBs in the four studied systems is about 1.5 ps, which is slightly larger than the value of bulk ion pairs and leads to a minor red shift in the N−O stretching vibration of NO3− anions confined in the TiO2 slits, as shown in Figure 5. In addition, as mentioned before, more EA+ cations form HBs with the TiO2 walls in the 0.7 nm TiO2 slit than in the larger ones due to confinement, meaning that the HB proportion of EA+−NO3− in the 0.7 nm TiO2 slit is minimal for all studied systems, which in turn contributes to the fact that the red shift of the N−O stretching vibration for the 0.7 nm slits is not obvious compared that for the larger ones (see Figure 5). As reported in previous studies,69 an increase in the HB lifetime can be related to the slow ion rotation in the ILs. Hence, the rotational dynamics of cations and anions in the first layer was explored here to explain the enhancement of HB strength as mentioned above. Here, the vector u2 is denoted as

Figure 9. Rotational TCFs of anions in the first slayer of different TiO2 slits. For comparison, the corresponding results of the anions in the bulk EAN ILs are also shown. The inset shows rotational TCFs of the cations in the first layer.

counterpart in the bulk phase is also shown here for comparison. Figure 9 shows, for the anions in the first layer, that all four TCF profiles decay more slowly compared to those in the bulk curve, in good agreement with the longer lifetime for EA+−NO3− HB in TiO2 slits compared to that in the bulk. The inset shows the rotational TCF curves of the cations in the first layer, showing that the TiO2 surface considerably restricts the rotational motion of cations, which leads to strong HBs between the cations and the TiO2 surface. In addition, the rotational motions in the first layer of the 0.7 nm TiO2 slit are much slower than those of the cations in the other large slits, leading to the strongest HB strength between the cations and the TiO2 surface in the 0.7 nm slit.

4. CONCLUSIONS In this work, the structures, vibrational spectra, and HBs of EAN IL molecules confined in TiO2 slits with different widths were systematically explored by classical MD simulations. The microstructure of the confined EAN IL molecules was analyzed in detail through density and angular distribution profiles. The vibrational spectra of interfacial ILs showed a unique local environment compared to that of the bulk. In addition, we investigated the HB interactions, especially the HBs between IL molecules and the TiO2 surface, to clarify the underlying mechanisms of the anomalous structure and vibrational spectra at the molecular level. The density profile perpendicular to the TiO2 wall showed that both cations and anions inside the TiO2 slits can form a 13455

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Langmuir well-organized layered structure and the NH3+ groups of cations are close to the TiO2 wall. The slit width has a significant effect on the microstructure of the EAN IL. In the 0.7 nm slit, the EA+ cations showed an ordered distribution with the cations lying down along the TiO2 walls. This cation configuration is beneficial for forming HBs with the TiO2 surface. With an increase in the slit width, both the HB proportion and HB strength between the EAN IL molecules and TiO2 surface decrease, which diversifies the orientation of interfacial ILs. We also found that the anomalous vibrational signals of cations and anions in the TiO2 slits were related to the corresponding HBs. The NH3+ group of cations can form HBs with the TiO2 surface. This behavior is obviously different from that of the bulk IL or ILs confined in carbon-based materials, which form HBs only between cations and anions. The HB strength between the EA+ cation and the TiO2 wall was significantly higher than that between the EA+ cation and the NO3− anion in the bulk, which lowered the stretching frequency of the N−H mode. In particular, the proportion of HBs formed with the TiO2 walls with a slit width of 0.7 nm was 1.5 times greater than that with the larger TiO2 slits, which led to splitting of the asymmetric N−H stretching mode into two characteristic peaks. Similarly, in larger TiO2 slits, the asymmetric N−O stretching mode red-shifted with respect to that of the bulk due to the enhancement of the HB strength. Our results in this work provided a molecular-level understanding of the correlation between anomalous vibrational spectra and HBs as well as confined structures of nanoconfined ILs in TiO2 slits, which is significantly beneficial for experimental scientists to have deep insights into the unique structures and properties of confined ILs in TiO2 slits.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the National Natural Science Foundation of China Grants (Grant Nos. 21676137, 91334202, and 21490584), the National Basic Research Program of China (Grant No. 2015CB655301), and the Project of Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). We are grateful to the High Performance Computing Center of Nanjing Tech University for supporting the computational resources.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.8b02527.

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Article

Related force-field parameters; fitting parameters and the average lifetime of HBs between cations and anions; fitting parameters and the average lifetime of HBs between cations and the TiO2 surface; equilibrium configurations for studied systems; density contours for the CH3 groups of cations for four EAN/TiO2 systems; total interaction energy after equilibration for four EAN/ TiO2 systems; proportion distribution of HBs in the first layers for the confined system and free system; HB continuous TCFs in the first layers of the confined system and free system; schematic illustrations for the TiO2/EAN interfacial HB network structure (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (L.L.). *E-mail: [email protected] (X.L.). ORCID

Linghong Lu: 0000-0003-2963-8270 Xiaohua Lu: 0000-0001-9244-6808 Notes

The authors declare no competing financial interest. 13456

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