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2007, 111, 12161-12164 Published on Web 08/01/2007
Well-Ordered Structure at Ionic Liquid/Rutile (110) Interface Lei Liu,† Shu Li,† Zhen Cao,† Yuxing Peng,† Guoran Li,† Tianying Yan,*,†,‡ and Xue-Ping Gao† Institute of New Energy Material Chemistry and Department of Material Chemistry, Nankai UniVersity, Tianjin 300071, China, and Institute of Scientific Computing, Nankai UniVersity, Tianjin 300071, China ReceiVed: July 12, 2007
The interface between the ionic liquid (IL) 1-butyl-3-methyl-imidazolium nitrate [BMIM+][NO3-] and the titanium dioxide rutile (110) surface is studied by classical molecular dynamics simulation. The simulation shows that the NO3- anions segregate on the surface and organize themselves into a highly ordered manner. The BMIM+ cations are found to occupy the region next to the absorbed NO3- layer, and the imidazolium (im) ring of the cation has a large tendency to slant with the im ring normal nearly perpendicular to the rutile (110) surface normal. The interfacial structure found in the current study may provide new insight to understand the double layer structure at the IL/solid interface.
The unique ionic liquid (IL)/solid interfacial properties have stimulated a large number of applications, such as synthesis of TiO2 nanoparticles in the IL environment,1-3 coating of an ultrathin film of IL as a lubricant,4 silica gel supported scandium/ IL catalyst for organic reactions,5 and electrodeposition of metals in ILs.6 The desired properties of ILs are also currently being explored as an electrolyte in dye-sensitized solar cells with TiO2 as the photoanode both experimentally7,8 and computationally.9,10 Recently, some distinct structures of the IL/solid interface have been observed experimentally; for example, the self-assembly of thiol-terminated ILs on a gold surface,11 the aggregation of a nonpolar surfactant mediated by an IL on a graphite surface,12 the coexistence of liquid and solid phases of ILs on a mica surface,13 and the crystallization of an IL in the confined space of a carbon nanotube.14 At this point, a deep understanding of the interfacial properties is of fundamental interest in order to explore the variety of applications of ILs at electrified interfaces.15 Sum-frequency generation (SFG) spectroscopy has been applied to IL/SiO216-18 and IL/platinum electrode interfaces,19-21 from which the orientations of the ions at the interfacial region are deduced. Despite the current level of research activity, many IL properties remain to be elucidated.22 In this letter, we report a well-ordered structure observed at the interface of the room-temperature ionic liquid (IL) 1-butyl3-methyl-imidazolium nitrate [BMIM+][NO3-] and titanium dioxide (TiO2) rutile (110), based on the classical molecular dynamics (MD) simulation. Among titania polymorphs, rutile is the thermodynamically stable form and is the well-known mineral in nature. In rutile, the (110) surface is the most stable crystalline face,23 and the polished rutile (110) surface can be obtained in the lab though an experimental method.24,25 To simulate the IL [BMIM+][NO3-]/TiO2 rutile (110) interface, we construct a slab of rutile crystal with the nonhydroxylated surface cleaved on the (110) surface, which contains * To whom correspondence should be addressed. E-mail: tyan@ nankai.edu.cn. † Institute of New Energy Material Chemistry, Nankai University. ‡ Institute of Scientific Computing, Nankai University.
10.1021/jp075444x CCC: $37.00
the 2-fold coordinated bridging oxygen atoms protruding out of the surface layer and the 5-fold titanium atoms beneath, as compared to the bulk crystal with 3-fold O atoms and 6-fold Ti atoms. The rutile slab dimensions are 51.975 Å, 51.975 Å, and 50.303 Å, respectively, by repeating 8, 8, and 17 times of the rutile unit cell along the [1h10], [110], and [001] directions. The so-constructed rutile (110) slab was held rigid in the MD simulation. In the MD simulation of water and the TiO2 rutile (110) interface by Cummings et al.,26 it was found that flexible and rigid TiO2 surfaces gave similar results for the interfacial properties. The PBC box was then elongated in the rutile [110] direction up to 131.975 Å with 714 [BMIM+][NO3-] pairs put inside, corresponding to a density of 1.14 g/cm3, obtained by an independent NPT simulation at 350 K and 1 atm with Melchionna27 modification of the Hoover method. Figure 1 shows the model system used in the current MD simulation. Periodic boundary conditions were applied in all three directions with a total of 34 284 atoms in the PBC box. The van der Waals and real space electrostatic interaction cutoff distance is set to be 12 Å, and the electrostatic interactions are handled with conventional 3-D Ewald summation.28 The intergration time step is 2 fs with the Vetlet algorithm.28 After the initial annealing from 1000 K down to 350 K slowly and then NVT equilibrium for 1 ns at 350 K coupled to a Nose´-Hoover thermostat,29,30 the MD production run was performed with an NVE ensemble at 350 K for 2.5 ns using the DL_POLY package.31 A nonpolarizable force field is adopted in the simulation, and the force field parameters are provided in the Supporting Information. Our previous study showed that the polarizable model and the nonpoalrizable model gave very similar interfacial structural properties.32 Figure 2 shows a snapshot taken at the end of the MD simulation. A distinct feature from this figure is the segregation of the NO3- anions, which are templated by the rutile (110) surface and arrange themselves in a highly ordered manner. Further analysis shows that two oxygen atoms of the absorbed NO3- bind to two neighboring 5-coordinated Ti atoms, separated © 2007 American Chemical Society
12162 J. Phys. Chem. C, Vol. 111, No. 33, 2007
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Figure 1. The PBC box used in the MD simulation. The rutile slab is shown in green (Ti) and orange (O) and 714 [BMIM+][NO3-] IL pairs (yellow, imidazolium ring; purple, methyl group; cyan, butyl group; blue, N-atom on NO3-; and red, O-atom on NO3-) occupy 80 Å along the [110] direction in the PBC box, encapsulated by two nonhydroxylated rutile (110) surface planes. Periodic boundary conditions are applied on the three directions. The red arrow on the left corner shows [1h10], [110], and [001] directions.
Figure 2. A snapshot taken from the MD simulation at the IL/rutile (110) interfacial region: green, Ti-atom; orange, O-atom of TiO2; blue, N-atom of NO3-; red, O-atom of NO3-; yellow, imidazolium (im) ring; purple, methyl group; cyan, butyl group.
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Figure 3. (a) Number density profile of the COM of NO3- (black), geometrical center of the im ring (red), C6-atom, and C10-atom. (b) Orientational ordering parameter, P2 (θ) ) 〈(1/2) (3 cos2 θ - 1)〉, of the NO3- plane normal (black), im ring normal (red), and NN vector (blue), where θ is the angle between a direction vector (inset) and the surface normal, [110] direction (black arrow). The error bars are obtained with 95% confidence interval.
by 2.96 Å along the [001] direction, with the NO3- plane perpendicular to the TiO2 surface. Such ordered interfacial structure is repeated on the rutile (110) surface, with most of the N-N distances about 8.83 Å along the [001] direction for the two neighboring absorbed NO3- anions. This interfacial structure of the absorbed NO3- layer is stable over the 2.5 ns in the MD simulation. Figure 3a shows the number density profiles of the centerof-mass (COM) of the NO3- anions, as well as some distinguishable groups of the BMIM+ cations. The strong peak of NO3- at ∼2.7 Å to the rutile (110) surface shows clearly the surface aggregation of the NO3- anions, with the local density about 30 times higher than the average density of the bulk region. Apart from that, the density profiles of the im geometrical center, C6 atom, and C10 atom exhibit multiple peaks in the interfacial region within ∼15 Å of the surface. Thus, the cations have several orientations at the interfacial region. The strong peak of the C6 atom at ∼3.8 Å to the surface indicates that the preferred cation orientation is with the methyl group pointing toward the surface, and the im rings prefer to slant on the interfacial region next to the segregated NO3- layer, with the butyl group pointing toward the bulk liquid. The double peak at ∼5 and ∼6.4 Å for the im geometrical center, accompanied by the C10 atom peak at ∼3.6 Å, shows that there are also some BMIM+ cations with opposite orientation, that is, the butyl group pointing toward the surface and the methyl group pointing toward bulk liquid. Figure 3b shows the orientational ordering parameter for several direction vectors. It can be seen clearly that the absorbed NO3- anions stand on the surface with the NO3- plane normal nearly perpendicular to the surface normal, [110] direction. There are also small number of BMIM+ cations that lie flat on the surface, with the im ring normal nearly parallel to the surface normal and the NN vector perpendicular to it. In the interfacial region where the im geometrical center exhibits a double peak, the most
Figure 4. Vibrational DOS, projected along the [1h10], [110], and [001] directions of the COM of the absorbed NO3- anions. The inset shows the DOS of the bulk NO3- anions along the three directions.
probable BMIM+ cation position, the im rings tend to slant with the ring normal nearly perpendicular to the [110] direction, and the NN vector aligns with it. Figure 4 shows the vibrational density of state (DOS), obtained by Fourier transform of the COM velocity autocorrelation function of the NO3- anions that absorbed on the rutile (110) surface. A distinct feature is that the DOS of the absorbed anions is highly anisotropic along different crystal directions. Comparing with the DOS of the bulk liquid (inset of Figure 4), the peaks of the absorbed NO3- anions shift to higher frequencies, with the peak of the [110] direction appearing at the highest frequency. Thus, the motion is more confined for the absorbed NO3- anions. The lack of the diffusive band, especially in the DOS projected along the [110] direction, indicates that the
12164 J. Phys. Chem. C, Vol. 111, No. 33, 2007 adsorbed anions bind tightly on the rutile (110) surface. In contrast, the projected DOSs along different directions are indistinguishable for the NO3- anions in the bulk liquid. Therefore, the ILs transform to the isotropic bulk liquid beyond the absorbed layer. From the above analyses, we can draw a relatively clear picture of the detailed structure of the IL [BMIM+][NO3-]/rutile (110) interface. In the interfacial region, the NO3- anions segregate with the NO3- plane nearly perpendicular to the surface, whereas the BMIM+ cations likely occupy the region next to the absorbed NO3- layer. The hierarchical organization of ILs on the surface found here is in good agreement with a recent simulation study of the interface between the IL and NaCl crystal surface.33 In the simulation of the interface between the IL [DMIM+][Cl-] and a charged surface, it was also found that the Cl- anions segregate on the surface.10 For im-based ILs, the charges carried by the cation are largely delocalized on the im ring. In addition, the bulky size of the cations makes them not easily rearranged in an orderly manner due to the spatially steric hindrance. On the other hand, the segregation of the smaller size anions, templated by the specific surface plane, leads to a rapid screening of the electric field from the surface charges. Thus, the surface segregation of an anion is more energetically favorable. In the region beyond the absorbed NO3layer, BMIM+ tends to slant with the im ring normal nearly perpendicular to the [110] direction, the methyl group pointing toward the surface, and the butyl group pointing toward the bulk liquid. Such orientation of the cations is in good agreement with the SFG-deduced IL/SiO2 interfacial structures.16-21 A combined experiment and Monte Carlo study of the adsorption of the neutral molecule n-pentane on the rutile surface also demonstrated the template effect on the rutile (110) surface.34 Furthermore, the surface-mediated separation of the polar (im ring) and the nonpolar (butyl) groups found here may be analogous to what was found in the pure IL. Due to the unique structure of the im-based ILs with the polar im ring and the nonpolar side chain, the heterogeneous organization in the pure IL system was observed from both computer simulations35-38 and experiments.39,40 In summary, we have observed the detailed structure of the IL [BMIM+][NO3-]/rutile (110) interface, obtained from computer simulation. The NO3- anions are found to segregate and tightly bind on the surface. Thus, the interfacial region is highly corrugated and intersected by the BMIM+ cations, which have a large tendency to slant on the interfacial region. This interfacial structure may be common at the IL/polar solid interface, where the orientation of the cations is modulated by the absorbed anions, whereas the latter are templated by the surface plane. We hope that the current simulation result contributes to the understanding of the double layer structure at the IL/solid interface. In addition, the result of this simulation may provide new insight into the design and manipulate the nanoscale materials for the selective deposition of certain ions. Acknowledgment. This research is supported by the NSFC (No. 20503013) and the 973 Program (2002CB211800) of China. T.Y.’s participation in the project was also supported in part by Renshi Chu startup funding of Nankai University. The allocation of computer time from Institute of Scientific Comput-
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