Molecular-Level Insights into Size-Dependent Stabilization

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Molecular-Level Insights Into Size-Dependent Stabilization Mechanism of Au Nanoparticles in 1-Butyl-3-Methylimidazolium Tetrafluoroborate Ionic Liquid Fangjia Fu, Yunzhi Li, Zhen Yang, Guobing Zhou, Yiping Huang, Zheng Wan, Xiang-Shu Chen, Na Hu, Wei Li, and Liangliang Huang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11043 • Publication Date (Web): 21 Dec 2016 Downloaded from http://pubs.acs.org on December 23, 2016

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The Journal of Physical Chemistry

Molecular-Level Insights into Size-Dependent Stabilization Mechanism of Au Nanoparticles in 1-Butyl-3-methylimidazolium Tetrafluoroborate Ionic Liquid

Fangjia Fu,1 Yunzhi Li,1 Zhen Yang,*,1 Guobing Zhou,1 Yiping Huang,1 Zheng Wan,1 Xiangshu Chen,*,1 Na Hu,1 Wei Li,2 Liangliang Huang*,3

1

College of Chemistry and Chemical Engineering, Jiangxi Inorganic Membrane Materials Engineering Research Center, Jiangxi Normal University, Nanchang 330022, People’s Republic of China 2

School of Chemistry and Chemical Engineering, Key Laboratory of Mesoscopic Chemistry of the Ministry of Education, Institute of Theoretical and Computational Chemistry, Nanjing University, Nanjing 210093, People's Republic of China 3

School of Chemical, Biological and Materials Engineering, University of Oklahoma, Norman, Oklahoma 73019, United States

1

ACS Paragon Plus Environment


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ABSTRACT Here we report a series of classical molecular dynamics simulations for the icosahedral Au nanoparticles with four different diameters of 1.0, 1.4, 1.8, and 2.3 nm in 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) room-temperature ionic liquid (RTIL). Our simulation results reveal for the first time a size-dependent stabilization mechanism of the Au nanoparticles in the [bmim][BF4] RTIL, which may help to clarify the relevant debate on the stabilization mechanism from various experimental observations. By comparison, the alkyl chains in the [bmim]+ cations are found to dominate the stabilization of the smallest Au13 nanoparticle in the RTIL while the imidazolium rings should be mainly responsible for the stabilization of other larger nanoparticles in the RTIL. Compared to the [bmim]+ cations, the [BF4]- anions are found to have an indirect influence on stabilizing the Au nanoparticles in the RTIL because of the weak interaction between the Au nanoparticles and the anions. However, such differences in the stabilization mechanism between the small and the large Au nanoparticles can be attributed to the unique hydrogen bond (HB) network between the cations and the anions in the first solvation shell. Meanwhile, increasing the particle size can lead to the enhanced HBs on the surface of Au nanoparticles, so that slower rotational motions and more pronounced orientation distribution of cations can be observed around the larger nanoparticles. Our simulation results in this work provide a molecular-level understanding of the unique size-dependent stabilization mechanism of the Au nanoparticles in the imidazolium-based RTILs.

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1．．INTRODUCTION In the past decades, metal nanoparticles (MNPs) have attracted intense attention in a broad range of fields, including catalysis, chemical sensors, electronic devices, optical properties and so on.1–4 However, it’s still a great challenge for the fabrication of the MNPs due to their highly unstable and aggregation-prone characteristics in traditional aqueous and organic solvents.5 To prevent such aggregations between the MNPs, various stabilizing agents, such as surfactants, polymers, and DNAs, have been widely used with the formation of self-assembled monolayers (SAMs) on the surface of MNPs.6–8 Nevertheless, the steric hindrance of SAMs can also hamper the target compounds to approach the active sites of MNPs, which has a negative effect on their catalysis and other applications. Alternatively, room-temperature ionic liquids (RTILs), especially the imidazolium-based RTILs, have been regarded as a promising solvent candidate for the stabilization of soluble MNPs without the need of additional ligands.9–13 Despite enormous efforts made by scientists, however, no consensus about the stabilization (or solvation) mechanism of the MNPs in the imidazolium-based RTILs has been obtained up to now. At present, different kinds of stabilization mechanism for the MNPs in the imidazolium-based RTILs have been proposed experimentally in terms of which segment in RTILs is the main contribution, including the imidazolium rings of cations,14–17 the alkyl chains of cations,18,19 the anions,20–23 and the earlier Deryagin-Landau-Verwey-Overbeek (DLVO) model.24–26 In terms of the DLVO model,24–26 the primary factor for the stabilization of metal nanoparticles in 3



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imidazolium-based RTILs was assumed to be an electrostatic double layer on the nanoparticle surface, where the first solvation shell consists of anions and the second shell is composed of cations. However, the D labeling and 2H NMR proposed by Finke and co-workers,14 where deuterium incorporation is apparent at the 2-H position of the imidazolium cation, has revealed that the [bmim]+ cations can form the N-heterocyclic carbine on the surface of 2.1 ± 0.6 nm Ir nanoparticle for three different [bmim]+-based RTILs, suggesting that the imidazolium rings of the [bmim]+ cations should be mainly responsible for the stability of the MNPs in the imidazolium-based RTILs. Further, the surface-enhanced Raman scattering (SERS) of Rubim et al.17 showed that the [bmim]+ cations first form a positively charged layer and the [BF4]- anions form a second negatively charged layer around the Ag nanoparticle with the diameter of 6 nm. Meanwhile, Raman signals were found to be enhanced for the imidazolium rings of the [bmim]+ cations due to the chemical and electromagnetic mechanism while the signal of [BF4]- anions is not shifted.17 These experimental phenomena mean that the imidazolium rings dominate the stability of MNPs in the imidazolium-based RTILs rather than the alkyl chains (or the anions). On the other hand, the recent in situ labelling and spectroscopic experiments of Campbell et al.19 demonstrated that the Ru nanoparticle are surrounded by the non-polar alkyl chains instead of the imidazolium rings, and the crystal growth of Ru nanoparticles with the diameter 1.1 ± 0.2 nm can be controlled by changing the group of alkyl chains. Additionally, Janiak and co-workers20 have successfully prepared various Ag nanoparticles with the median diameter from 2.8 to 26.1 nm by increasing 4


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the molecular volume of anions in RTILs, and Taubert and co-workers22 have also found that the formation and stabilization of Au nanoparticles in the [emim]+-based RTILs are strongly dependent on the nature of different anions. Above different stabilization mechanism of the MNP/RTIL suspensions may result from various systems investigated in these experiments, such as different sizes of MNPs and different concentrations of MNPs in RTILs. Furthermore, the interactions between the MNPs and the cations (or anions) are justly indirectly deduced from collective experimental observations. As pointed out by Schatz27, experimental observations of the objects on a nanoscale are often fraught with enormous difficulty. Therefore, it is critical to provide a molecular-level understanding of the stabilization mechanism of the MNPs in RTILs, which is necessary for the design and synthesis of the MNPs with tailored properties from RTILs. As a powerful analysis tool, molecular dynamics (MD) simulation can offer a molecular-level insight into various fundamental properties of the MNPs in various solvents.28–33 Recent MD simulations proposed by Padua and co-workers31 have been used to investigate the solvation and stabilization of Ru nanoparticle in [C1CnIm]+[NTf2]- (n = 2–10) RTILs. Their simulation results showed that the structural matrix from the RTILs should be responsible for such stabilization, rather than the steric effects of alkyl chains and the electrostatic double layer. Subsequently, they employed both experiments and simulation to explore the interaction energies between the Ru MNPs in different RTILs.32 By comparison, they further found that both the van der Waals interaction from the alkyl chains and the hydrogen-bond (HB) 5



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interaction between cations and anions play an important role in the interactions between the MNPs, while no significant effect of the anion structure was observed.32 However, it should be noted that one particle size of 2 nm in diameter (or 323 Ru atoms) was only chosen for the Ru nanoparticle in the MD simulation of Padua and co-workers.31,32 Actually, previous theoretical studies34–37 have revealed that the solvation free energies increase linearly with solute volume for smaller solutes while linearly with surface area for larger solutes. In other words, the solvation mechanism may be dependent on the nanoparticle size. Naturally, it is very interesting to evaluate the effect of particle sizes on the stabilization mechanism of MNPs in RTILs. To this end, a series of MD simulations have been carried out here to investigate the size-dependent solvation behavior of the icosahedral Au MNPs in the 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) RTIL, as described by the recent experiments of Mertens and co-workers,38 where the approximately 1.1 nm diameter Au MNP were directly generated without additional ligands in the [bmim][BF4] RTIL. The primary purpose of this work is to demonstrate how the solvation structures and dynamics of the [bmim][BF4] RTIL around the Au MNPs change with the particle sizes, including four different sizes: Au13, Au55, Au147, and Au309. Accordingly, their diameters are 1.0, 1.4, 1.8, and 2.3 nm, respectively. Meanwhile, the relevant HB network and dynamics between the [bmim]+ cations and the [BF4]- anions at the interfaces, as well as their relationship with the solvation structures and dynamics, have also been investigated and discussed in detail. In particular, this simulation will provide a novel explanation for the function of 6


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interfacial HBs in the stabilization mechanism of the Au MNPs in the [bmim][BF4] RTIL, which is of great help for us to better understand the size-dependent solvation mechanism of the MNPs in imidazolium-based RTILs at a molecular level.

2．．MODELS AND SIMULATION DETAILS First, the icosahedral structure composed of twelve (111) facets, was chosen for the Au13, the Au55, the Au147, and the Au309 MNPs. Unlike bulk materials, nanoparticles can exhibit various structural motifs, which include hexagonal close-packed (hcp), cuboctahedral, icosahedral, truncated decahedral, and amorphous structures.39 For the Au MNPs, Baletto et al.40 have found that icosahedra are dominated at small sizes, decahedra at intermediate sizes, and truncated octahedra at large sizes. In our previous work,41,42 we also used the manybody Sutton-Chen (SC) potential43 to confirm that the lowest energies are the icosahedral structure for the Aun (n = 13–561) nanoparticles compared to the decahedral and the truncated octahedral structures. The geometry of each Au MNP was determined by using a low storage Broyden-Fletcher-Goldfarb-Shanno nonlinear optimization44 with the many-body SC potential, as shown in our previous work.41,42 After the optimization, each Au MNP was treated as rigid during the MD simulation. In this work, the [bmim][BF4] RTIL was modeled by using a refined all-atom force field proposed by Wang and co-workers,45 which can reproduce various properties of this ionic liquid from experimental observations more satisfactorily than the AMBER force field, such as, density, vibrational frequencies, dielectric constant, and so on.45,46 And the Au atoms 7



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in neutral nanoparticles were treated as uncharged particles interacting through the Lennard-Jones (L-J) potential which has been widely used to describe the properties of Au nanoparticles in various solvents.47–49 Then, the mixed L-J parameters were derived from self-parameters using the Lorenz-Berthelot mixing rules. Despite its simplicity, the L-J potential for Au atoms permits one to reproduce many properties on Au surfaces reasonably and qualitatively. To the best of our knowledge, most of current MD simulations (up to time scale of nanosecond scales) for large and complex systems (such proteins and dendrimers) on Au surfaces are only with the L-J parameters for Au atoms.50–53 To further test the validity of the L-J parameters of Au atoms used in this work, we have used high-level density functional theory (DFT) with M06-2X functional to calculate the interaction energies between the icosahedral Au13 MNP and the [bmim][BF4] ion pair (including one cation and one anion), since this functional have taken into account the dispersion interactions at an intermediate range and been widely applied to study various systems of ionic liquids.54,55 And the 6-31G** basis set was used for the ionic liquid cluster and the LANL2DZ basis set was used for the Au MNP. It should be noted that the orientation and structure of ion pair was always fixed when it approaches the surface of Au13 MNP. Then, the interaction energies were calculated with the correction of basis set superposition error (BSSE) at the same level. All DFT calculations in this work were performed with the GAUSSIAN 09 program.56 As shown in Figure 1, the results from our force field used in this work are satisfactorily consistent with those from the corresponding DFT calculations. Therefore, the L-J parameters of Au atoms should be 8


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(at least qualitatively) reasonable for the Au MNP/[bmim][BF4] system in this work. Next, a bulk [bmim][BF4] RTIL consisting of 1287 pairs of the [bmim]+ cations and the [BF4]- anions, was arranged within a cubic simulation cell of 77.1 × 77.1 × 77.1 Å3 containing a small cubic void of 25.0 × 25.0 × 25.0 Å3 to accommodate the above Au MNPs with various particle sizes. For each initial structure composed of one Au MNP and 1287 ion pairs, a NPT MD of 5 ns was first carried out at T = 423.0 K and P = 1.0 atm, and a following NVT MD of 10 ns (T = 423.0 K) was performed for equilibration, and then the next NVT MD of 30 ns was performed for data analysis with the trajectories stored every 100 fs. After equilibration, the dimension of simulation box for each direction is in the range from 75.7 to 75.9 Å and typical equilibrium snapshots were shown in Figure S1 of the Supporting Information. During all MD simulations, the bond length of cations and anions was fixed through the RATTLE algorithm for the reason of computational economy. Then, the Newton’s equations of motion were integrated by using the velocity-Verlet algorithm with a time step of 2 fs, and the periodic boundary condition was used in all three directions. The cutoff distance of nonbonded interactions was set to 10 Å, and the long-range electrostatic interactions were calculated by using the particle-mesh Ewald (PME) method.57 Both the temperature and pressure were controlled by using the Berendsen algorithm with coupling times of 0.8 and 4.0 ps, respectively. In addition, another NVT MD simulation of 1 ns following the above final configuration was performed to calculate the continuous HB dynamics. The trajectories stored every 10 fs instead of 100 fs, which is short enough to accurately calculate continuous HB dynamics. In this 9



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work, all MD simulations were performed by using the modified Tinker 6.1 code.58

3. RESULTS AND DISCUSSIONS To explore the solvation structures of the [bmim][BF4] RTIL around the Au MNPs with different particle sizes, we first present the radial distribution functions (RDFs) g(r) of [bmim]+ cations (including both the geometric center of the imidazolium ring and the terminal C atom of the alkyl chain) and [BF4]- anions (B atom) with respect to the mass center of the Au MNP in Figure 2. For the alkyl chains, we can see clearly from Figure 2a that a well-defined solvation shell along with obvious density oscillations is formed on the surface of the Au MNP. Moreover, increasing particle size can lead to an obvious decrease of the height of the first RDF peak for the alkyl chains but an increase of the second peak height, indicating that the alkyl chains prefer to aggregate on the surface of smaller Au MNPs. For the imidazolium rings in the [bmim]+ cations, however, we can see from Figure 2b that the height of the first RDF peak initially increases and follows a slight decrease as the particle size increases from Au13 to Au309 MNPs, meaning that the imidazolium rings contribute more to the stabilization of larger Au MNPs in the [bmim][BF4] RTIL. As shown in Figure 2c, on the other hand, increasing particle size results in an obvious increase of the height of the first RDF peak for the [BF4]- anions, which is quite contrary to that for the alkyl chains (see Figure 2a). Such observation suggests that the stabilization contributions arising from the [BF4]- anions should be gradually enhanced with the particle size. 10


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Above these phenomena clearly demonstrate a size-dependent stabilization mechanism of the Au MNPs in the [bmim][BF4] RTIL. Specifically in the smallest Au13 MNP, as shown in Figure 2, the height of the first RDF peak for the alkyl chains is 2.6, which is much more than the corresponding values (both are about 2.0) of the imidazolium rings in the [bmim]+ cations and the [BF4]- anions. This indicates that the alkyl chains dominate the first solvation shell around the smallest Au13 MNP, which is similar to those observations from the recent in situ labelling and spectroscopic experiments for the Ru MNPs with the identical diameter of around 1 nm in RTILs.19 As the particle size increases, however, the imidazolium rings begin to replace the alkyl chains to predominantly stabilize the larger Au MNP in the [bmim][BF4] RTIL, which is well consistent with the previously experimental observations.14–17 In the larger Au55 MNP, the first RDF peak of the imidazolium rings is found to be up to 2.8, which is much more than the corresponding values of 2.4 and 2.0 for the alkyl chains and the anions, respectively. Then, the stabilization contributions arising from the imidazolium rings are further enhanced in the Au147 MNP. When the particle size increases up to the largest Au309, we can find from Figure 2 that the first peaks for both the imidazolium rings and the anions are almost identical to each other and their heights are much more than that of the alky chains. This means that the stabilization of the Au309 MNP in the [bmim][BF4] RTIL seems to be dominated by both the imidazolium rings and the anions rather than the alkyl chains in the smallest Au13 MNP. Therefore, current debate on the stabilization mechanism for the MNPs in the imidazolium-based RTILs may arise partly from different particle sizes in the 11



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previous experiments. Additionally for each Au MNP, we can observe from Figure 2 that all positions of the first RDF peaks are almost the same for the alkyl chains, the imidazolium rings, and the anions, corresponding to the presence of both the cations and the anions coinciding on the surface of Au MNP. Moreover, this is further confirmed by the radial charge density profiles of an electrostatic interface shell of around 5 Å, as shown in Figure 3. In the range of this electrostatic interface shell, a small positive charge shell is first the closest to the nanoparticle surface, after which one large negative shell and then one large positive shell appear in turn. Similar charge profiles of imidazolium-based RTILs can be also found on the surface of crystalline and amorphous Ru MNPs,31,32 as well as on the bulk Au(100) surface.59 Such radial charge density profiles is significantly different from the previous DLVO-type assumption,24–26 where the first charge layer consists of the anions rather than the cations. This is because the imidazolium cation are roughly regarded as only a spherical particle with positive charge in the DLVO model so that the steric effects from cations are often overestimated. Actually, the imidazolium cations can adopt the favorable orientations to decrease such steric effects. Furthermore, the DLVO model only combining the effects of the van der Waals attraction and the electrostatic repulsion,24 is not able to describe the HB interaction with localized and directional characters. In the following, our simulation results reveal that the HB interactions play an essential role in determining the interfacial properties of the [bmim][BF4] RTIL around the Au MNPs. Therefore, the DLVO model fails to describe these systems of 12


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the Au MNPs in the [bmim][BF4] RTIL. Additionally, the [bmim][BF4] RTIL can be found to form one small negative charge shell first close to the uncharged graphite surface.60 Unlike the non-metal graphite surface, above radial charge density profiles on metal surfaces demonstrate that the interaction between the imidazolium rings and the metal surface is stronger than that between the anions and the metal surface, which is also confirmed by the enhanced [bmim]+ Raman signals around the Ag MNP in the observation of SERS spectra.17 To quantitatively evaluate the relevant interactions between the Au MNPs and the [bmim][BF4] RTIL, Figure 4 presents the average interaction energies per segment in the first solvation shell arising from the alkyl chains, the imidazolium rings, and the anions with the Au MNP, respectively. Based on the RDF curves shown in Figure 2, the thickness of the first solvation shell is found to be about 3 Å for all Au MNPs, corresponding to the distance between the particle surface (i.e., the maximum radial distance of g(r) = 0) and the first minimum valley. Then, the average number of the [bmim]+ cations in the first solvation shell is 6.7, 9.3, 15.1, and 19.8 and that of the [BF4]- anions is 6.1, 13.4, 19.5, and 31.1 as the particle size increases. As shown in Figure 4a, the average interaction between the Au13 MNP and each alkyl chain is the strongest compared to the imidazolium rings and the anions, which is well consistent with the corresponding RDF in the Au13 MNP. Nevertheless, it is unexpected from Figure 4 that the average interaction between the Au MNP and each [BF4]- anion is always the weakest in all cases, and the counterpart of the imidazolium rings is always the strongest except for the Au13 MNP. Especially in the largest Au309 MNP, as shown 13



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in Figure 4d, the interaction between the nanoparticle and each imidazolium ring (-6.0 kcal/mol) is nearly three times as much as that between the nanoparticle and each anion (-2.2 kcal/mol), although the heights of both the first RDF peaks are identical (see Figure 2). This means that the considerable aggregation of the anions around the Au MNPs should not be attributed to the interaction between the Au MNPs and the anions. In other words, the imidazolium rings in cations should be mainly responsible for the direct stabilization of the larger Au MNPs in the [bmim][BF4] RTIL, while the primary function of anions on the surface is to eliminate the electrostatic repulsion between the cations, rather than interact strongly with the nanoparticles. On the other hand, the non-polar alkyl chains have a predominant influence on stabilizing the smaller Au MNPs in the [bmim][BF4] RTIL instead of the imidazolium rings. Next, Figure 5 presents the orientational distribution of the cations in the first solvation shell to better elucidate the detailed solvation structure of the [bmim][BF4] RTIL around the Au MNPs. It should be emphasized that the orientational distribution of the [BF4]- anions is not given here because its tetrahedral structure is almost spherical. As shown in Figure 5a, two vectors of [bmim]+ cations are defined: one r1 is the normal vector of the imidazolium ring and the other r2 is from the geometric center of the imidazolium ring to the terminal C atom of the alkyl chain. Meanwhile, another vector r3 is from the geometric center of the imidazolium ring to the center of the Au MNP. Then, the orientation of the cations on the surface of the Au MNP can be characterized via the angle α between r1 and r3 and the angle β between r2 and r3. Therefore, it is see clearly from Figure 5b that the distribution of the angle α in the 14


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Au13 case displays a broad plateau from 17° to 90°, indicating that the orientation of the imidazolium rings is arbitrary on the Au13 surface. However, there is only a big peak at about 17° for other larger Au MNPs and the height of this peak increases with the particle size, suggesting that the cations prefer to align with their imidazolium rings mostly parallel to the nanoparticle surface with the particle size. On the other hand, Figure 5c indicates that all distribution of the angle β in all Au MNPs shows a big peak but the angle of this peak increases with the particle size, meaning that the terminal methyl group of alkyl chain is closer to the smaller nanoparticle surface. In addition, we can find from Figure 5c that the cations are less restricted in the Au13 case, since the height of the β peak is significantly less than those in other Au MNPs and there is no peak in the α distribution of the Au13 MNP. Besides the solvation structure, Figure 6 further shows that the rotational dynamics of the cations and the anions in the first solvation shell. Here, above normal vector of the imidazolium ring r1 is used to represent the orientation of cations and the vector r4 for the orientation of anions is from the B atom to one F atom. Then, the rotational dynamics of cations and anions can be calculated as the following time correlation functions (TCFs),61,62

C r (t ) =

1 Ni

Ni

∑u

j

(t )u j (0)

(1)

j =1

where Ni is the total number of ions (the cations or the anions) in the first solvation shell at time 0 and the uj(t) is the unit vector of the jth ion at time t. The angular bracket represents that the ensemble averaging is taken over all tagged ions at different reference initial times. For comparison, the rotational TCF curves of cations 15



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and anions in bulk [bmim][BF4] RTIL are also given in Figure 6. We can find from this figure that all TCF curves of the [BF4]- anions in the first solvation shell are almost identical to the bulk curve but those of the [bmim]+ cations decay much slower than the bulk curve, meaning that the Au MNPs restrict the rotational motions of cations considerably but there is little influence on the rotational motions of anions. Furthermore, increasing the particle size can enhance such restriction of the rotational dynamics. In addition, the rotational motions of anions are expected to be much faster than those of cations in bulk phase and at the interface because of their symmetrical structures. It is well-known that the rotational motions of cations do not change their positions but require the rearrangements of their neighboring hydrogen bonds (HBs). In other words, the rotational motions of cations should be dominated by the local and directional HBs on the surface of Au MNPs. To better understand the HB properties in the first solvation shell around the Au MNPs, the relevant HB dynamics are explored by both the continuous TCF SHB(t) and the intermittent TCF CHB(t), which are defined in terms as the following expressions:63–65 S HB (t ) =

h ( 0) H ( t ) h (0 ) h ( 0 )

(2)

and C HB (t ) =

h(0) h(t ) h ( 0) h ( 0)

(3)

where the variable H(t) is unity when the tagged HB in the first solvation shell remains continuously from time 0 to time t, and zero otherwise. On the other hand, the 16


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h(t) is unity as long as the tagged HB in the first solvation shell exists at time t, and zero otherwise. Hence, the continuous function provides a more accurate HB lifetime than the intermittent function, while the intermittent function allows the reformation of broken HBs in the interval of time t so that it can provide much information on the structural relaxation of HBs. In other words, the relaxation time of SHB(t) is usually called the average lifetime of HBs (τ SHB ) while the relaxation time of CHB(t) describes the structural relaxation time of HBs ( τ CHB ). The larger values of τ SHB and τ CHB mean more enhanced HB strength. It should be noted that the HB formation (C-H…F) in this work is defined in terms of the geometric criteria,66 as shown in Figures S3–S5 of the Supporting Information. Then, the two relaxation times of τ SHB and τ CHB are calculated by fitting the SHB(t) and the CHB(t) curves through three weighed exponentials, which can be expressed as61,67,68

R(t ) = A exp(− t τ a ) + B exp(− t τ b ) + C exp(− t τ c )

(4)

and then

τ R = Aτ a + Bτ b + Cτ c

(5)

where A, B, and C are the fitting parameters (A+B+C=1), as well as τa, τb and τc are the time constants. R(t) is the SHB(t) or the CHB(t), and τR is the τ SHB or the τ CHB . The calculated SHB(t) and CHB(t) curves in the first solvation shell around the Au MNPs and in bulk are shown in Figure 7. We can see clearly from this figure that the SHB(t) curves in the first solvation shell decay slower with the particle size, suggesting that the HBs between the cations and the anions can be further enhanced on the surface of larger Au MNPs. Accordingly, we can see from Table 1 that the calculated τ SHB 17



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values in the first solvation shell are 0.79 ± 0.04, 0.94 ± 0.03, 1.10 ± 0.04, and 1.33 ± 0.05 ps in turn from Au13 to Au309 MNPs, which are larger than the bulk value of 0.68 ± 0.01 ps. Similar phenomena can be observed from the CHB(t) curves and the τ CHB values, as shown in the inset of Figure 7 and Table 2. The enhanced HBs around the larger Au MNPs can significantly restrict the rotational motions of the cations as discussed in Figure 6 and result in more pronounced orientation distribution of the cations in the first solvation shells as discussed in Figure 5. However, it should be noted that the CHB(t) allows for long sojourns after the HB breaking so that the cations on the surface of Au MNPs can re-form HBs with its neighboring anions during the sojourn time. Therefore, the τ CHB values are found to be in the range from 457.8 ± 6.2 to 1923.8 ± 38.1 ps both in bulk phase and in the first solvation shells, which are almost three orders of magnitude more than the corresponding τ SHB values by comparisons with Tables 1 and 2. Figure 8 illustrates typical equilibrium snapshots and HB networks of the Au MNPs surrounded by the cations and the anions in the first solvation shell. As shown in Figure 8a, some alkyl chains are found to be close to the Au13 surface and some imidazolium rings are perpendicular to the Au13 surface. However in Figure 8b-d, most of alkyl chains are away from the surface of lager nanoparticles and most of imidazolium rings parallel to the surface. Above these phenomena shown in Figure 8 are well consistent with the orientational distribution of Figure 5. On the other hand, we can see intuitively from Figure 8 that a distorted HB network has been formed on the surface of Au MNPs, which is significantly different from the HB network of one 18


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cation with three HBs in bulk RTILs. Meanwhile, we have calculated the total HB number of the cations in the first solvation shell, which is about 12.9, 17.4, 26.6, and 36.0 in turn from the Au13 to the Au309 MNPs. Then, the corresponding average HB number per cation can be up to around 1.92 ± 0.01, 1.85 ± 0.01, 1.76 ± 0.01, and 1.81 ± 0.02 in turn on the surface of Au MNPs, suggesting that the cations in the first solvation shell of all Au MNPs studied here almost have two HBs with the [BF4]anions. However in the Au13 MNP (marked by the arrows in Figure 8a), we can find that the imidazolium rings perpendicular to the particle surface have only one CH bond forming the enhanced HB with the [BF4]- anions in the first solvation shell. Different HB networks have been observed for the larger Au MNPs (marked by the circles in Figure 8b-d), where most of imidazolium rings parallel to the particle surface form two HBs with the surrounding anions. A further analysis demonstrates that the cations in the first solvation shell around the Au13 MNP have more than half of the HBs (about 57.3%) with the anions in the second solvation shell, while the corresponding proportions are only 32.8%, 30.5%, and 22.1% for the Au55, Au147, and Au309 MNPs, respectively. Such differences can be attributed to that the Au13 surface can’t provide enough space to accommodate the HB formations between cations and anions so that some imidazolium rings in the first solvation shell have to be perpendicular to the Au13 surface, forming the enhanced HBs with the anions in the second solvation shell. In this way, the residual surface space around the Au13 MNP is much larger than that of imidazolium rings parallel to the surface. Therefore, the alkyl chains can approach 19



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the Au13 surface more easily to dominate the stabilization of the Au13 MNP in the [bmim][BF4] RTIL compared to other larger nanoparticles. Accordingly, the first RDF peak of the alkyl chains is the highest while those of both the imidazolium rings in cations and the anions are lowest in the Au13 MNP compared to the larger Au MNPs, as shown in Figure 2.

4. CONCLUSIONS In this work, a size-dependent stabilization mechanism of the icosahedral Au MNPs with various particle sizes in the [bmim][BF4] RTIL has been revealed for the first time by using classical MD simulations, where the alkyl chains in the [bmim]+ cations are found to dominate the stabilization of the Au13 MNP in the [bmim][BF4] RTIL while their imidazolium rings should be mainly responsible for the stabilization of other larger Au MNPs in the RTIL. Meanwhile, our simulation results show that the [BF4]- anions only have an indirect influence on stabilizing the Au MNPs in the RTIL since the interaction between the Au MNPs and the [BF4]- anions is always the weakest compared to those from both the alkyl chains and the imidazolium rings for all Au MNP cases studied here. Furthermore, the stabilization mechanism of the Au MNPs in the RTIL is not in according to the DLVO theory through the analysis of the radial charge density profiles. Such size-dependent stabilization mechanism can be attributed to the unique interfacial HB network of the cations and the anions around the Au MNPs. In the Au13 MNP, the particle surface can’t provide enough space to accommodate the HB 20


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formations between cations and anions, so that the imidazolium rings of the cations in the first solvation shell have to be perpendicular to the particle surface, forming more than half of the HBs with the anions in the second solvation shell rather than in the first solvation shell. Therefore, the alkyl chains can approach the Au13 surface more easily to dominate the stabilization of the Au13 MNP in the [bmim][BF4] RTIL compared to other larger Au MNPs. In addition, the interfacial HBs between the cations and the anions in the first solvation shell are found to be enhanced as the particle size increases, which results in slower rotational motions and more pronounced orientation distribution of cations around the larger Au MNPs. The simulation results in this work provide some insights for experimental scientists into the unique size-dependent stabilization mechanism of the Au MNPs in the imidazolium-based RTILs and will be a bit favorable to clarifying the existing debate on the stabilization mechanism of various MNPs in the imidazolium-based RTILs.

ASSOCIATED CONTENT Supporting Information 1. Typical equilibrium snapshots; 2. Average interaction energies per unit surface area; 3. Definition of the HB formation. The information is available free of charge via the Internet at http://pubs.acs.org/.

AUTHOR INFORMATION Corresponding Author 21



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*E-mail: [email protected] (Z. Y.) *E-mail: [email protected] (X.S.C.) *E-mail: [email protected] (L.L.H.)

Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 21306070, 21463011, and 21476099), Natural Science Foundation of Jiangxi Province (No. 20151BAB203014), Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund (the second phase), the Sponsored Program for Cultivating Youths of Outstanding Ability in Jiangxi Normal University.

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Table 1. The fitting parameters and the average lifetime τSHB (ps) of the C-H…F HBs between the cations and the anions in the first solvation shell around the Au MNPs as well as those of the bulk RTIL SHB(t)

A

B

C

τa

τb

τc

τSHB

Bulk Au13

0.26 0.29

0.50 0.54

0.24 0.17

0.13 0.16

0.55 0.70

1.56 2.17

0.68 ± 0.01 0.79 ± 0.04

Au55 Au147

0.30 0.29

0.54 0.53

0.16 0.18

0.17 0.20

0.80 0.88

2.85 3.18

0.94 ± 0.03 1.10 ± 0.04

Au309

0.38

0.50

0.12

0.24

1.27

4.99

1.33 ± 0.05

Table 2. The fitting parameters and the structural relaxation time τCHB (ps) of the C-H…F HBs between the cations and the anions in the first solvation shell around the Au MNPs as well as those of the bulk RTIL CHB(t)

A

B

C

τa

τb

τc

τCHB

Bulk Au13 Au55 Au147 Au309

0.55 0.44 0.43 0.45 0.40

0.32 0.20 0.32 0.48 0.31

0.13 0.36 0.25 0.07 0.29

2.3 1.8 2.1 2.5 2.3

413.0 205.3 726.1 1479.8 1089.5

2495.7 1447.2 2748.7 8795.4 5466.0

457.8 ± 6.2 562.8 ± 24.2 920.4 ± 21.3 1327.1 ± 27.4 1923.8 ± 38.1

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Figure Captions Figure 1. Comparisons of the interaction energies between the Au13 MNP and the [bmim][BF4] ion pair with two orientations along the distance of their centers of mass.

Figure 2. Radial distribution functions (RDFs) of the [bmim][BF4] RTIL around the Au MNPs: (a) the terminal C atom of the alkyl chain, (b) the geometric center of the imidazolium ring, and (c) the B atom of the anion.

Figure 3. Radial charge density distribution of the [bmim][BF4] RTIL around the Au MNPs with respect to the corresponding particle surfaces. To enhance visual clarity, the curves of Au55, Au147, and Au309 are shifted upward by 0.002, 0.004, and 0.006 e/Å3, respectively.

Figure 4. Average interaction energies per segment between the Au MNPs and different segments of the [bmim][BF4] RTIL in the first solvation shell around the Au MNPs: (a) Au13, (b) Au55, (c) Au147, and (d) Au309.

Figure 5. (a) Schematic illustrations for the definitions of vectors and angles, and angle distribution of (b) α and (c) β for the [bmim]+ cations in the first solvation shell around the Au MNPs with different particle sizes.

Figure 6. Rotational TCFs of the anions in the first solvation shell around the Au MNPs with different particle sizes. The inset shows rotational TCFs of the cations in the first shell. For comparison, the corresponding results of the anions and the cations in the bulk [bmim][BF4] RTIL are also shown.

Figure 7. Continuous TCF SHB(t) for the C-H…F HBs between the cations and the anions in the first solvation shell around the Au MNPs as well as in the bulk [bmim][BF4] RTIL. The inset shows the corresponding intermittent TCF CHB(t).

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Figure 8. Typical equilibrium snapshots and HB networks of the Au MNPs surrounded by the cations and the anions in the first solvation shell: (a) Au13, (b) Au55, (c) Au147, and (d) Au309. The dash lines denote the HB formation between cations and anions. The arrows indicate a cation forming one HB with the surrounding anions. The circles show a cation forming two HBs with the surrounding anions.

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Figure 1. Comparisons of the interaction energies between the Au13 MNP and the [bmim][BF4] ion pair with two orientations along the distance of their centers of mass.

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Figure 2. Radial distribution functions (RDFs) of the [bmim][BF4] RTIL around the Au MNPs: (a) the terminal C atom of the alkyl chain, (b) the geometric center of the imidazolium ring, and (c) the B atom of the anion.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. Radial charge density distribution of the [bmim][BF4] RTIL around the Au MNPs with respect to the corresponding particle surfaces. To enhance visual clarity, the curves of Au55, Au147, and Au309 are shifted upward by 0.002, 0.004, and 0.006 e/Å3, respectively.

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Figure 4. Average interaction energies per segment between the Au MNPs and different segments of the [bmim][BF4] RTIL in the first solvation shell around the Au MNPs: (a) Au13, (b) Au55, (c) Au147, and (d) Au309.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5. (a) Schematic illustrations for the definitions of vectors and angles, and angle distribution of (b) α and (c) β for the [bmim]+ cations in the first solvation shell around the Au MNPs with different particle sizes.

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Figure 6. Rotational TCFs of the anions in the first solvation shell around the Au MNPs with different particle sizes. The inset shows rotational TCFs of the cations in the first shell. For comparison, the corresponding results of the anions and the cations in the bulk [bmim][BF4] RTIL are also shown.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 7. Continuous TCF SHB(t) for the C-H…F HBs between the cations and the anions in the first solvation shell around the Au MNPs as well as in the bulk [bmim][BF4] RTIL. The inset shows the corresponding intermittent TCF CHB(t).

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Figure 8. Typical equilibrium snapshots and HB networks of the Au MNPs surrounded by the cations and the anions in the first solvation shell: (a) Au13, (b) Au55, (c) Au147, and (d) Au309. The dash lines denote the HB formation between cations and anions. The arrows indicate a cation forming one HB with the surrounding anions. The circles show a cation forming two HBs with the surrounding anions.

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Table of Content

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Molecular-Level Insights into Size-Dependent Stabilization

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