Nanoconfined Water within Graphene Slit Pores Adopt Distinct

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Nanoconfined Water within Graphene Slit Pores Adopt Distinct Confinement–Dependent Regimes Sergi Ruiz-Barragan, Daniel Muñoz-Santiburcio, and Dominik Marx J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b03530 • Publication Date (Web): 20 Dec 2018 Downloaded from http://pubs.acs.org on December 23, 2018

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The Journal of Physical Chemistry Letters

Nanoconfined Water within Graphene Slit Pores Adopts Distinct Confinement–dependent Regimes Sergi Ruiz-Barragan,∗,† Daniel Muñoz-Santiburcio,†,‡ and Dominik Marx† Lehrstuhl für Theoretische Chemie Ruhr-Universität Bochum, 44780 Bochum, Germany, and CIC nanoGUNE, Tolosa Hiribidea 76, San Sebastián, Spain E-mail: [email protected]

∗

To whom correspondence should be addressed Lehrstuhl für Theoretische Chemie Ruhr-Universität Bochum, 44780 Bochum, Germany ‡ CIC nanoGUNE, Tolosa Hiribidea 76, San Sebastián, Spain †

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ACS Paragon Plus Environment

The Journal of Physical Chemistry Letters 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

Abstract In view of the increasing importance of nanoconfined aqueous solutions for various technological applications, it has become necessary to understand how strong confinement affects the properties of water at the level of molecular and even electronic structure. By performing extensive ab initio simulations of two–dimensionally nanoconfined water lamellae between graphene sheets subject to different interlayer spacings, we find new regimes at interlayer distances of 10 Å and less where water can neither be described to behave like interfacial water nor to be bulk–like at the level of its H–bonding characteristics and electronic structure properties. It is expected that this finding will offer new opportunities to tune both, diffusive and reactive processes taking place in aqueous environments that are strongly confined by chemically inert hard walls.

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Water-filled nanometric structures offering different dimensionalities of the confinement, such as carbon nanotubes, 1,2 graphene and graphene oxide slit pores 3–6 or metal-organic framework cages, 7 currently attract immense scientific interest not only for academic reasons but also in view of their broad technological applications. Too many studies to be comprehensively addressed here have disclosed a plethora of remarkable and surprising properties of nanoconfined w ater. L et u s h ighlight j ust a f ew o ut o f m any e xamples: Nanoconfined water shows almost frictionless flow w ithin g raphene s heets a nd c arbon n anotubes 8–11 but not within boron nitride nanotubes; 9 its phase behavior depends on both, the spatial dimensionality of the confinement a nd i ts s trength a s s hown b y u sing c arbon n anotubes of different diameter 12 as well as slit pores with different spacings, 13–15 and also depends on corrugation; 16 last but not least, nanoconfinement h as b een f ound t o e ven change chemical equilibria in opposite ways with respect to homogeneous bulk water depending on the dimensionality of the confinement a s d iscovered f or t he s elf-dissociation r eaction o f w ater in two-dimensional 17 versus one-dimensional confinement. 18 S uch e xamples m ake i t very clear that water responds extremely sensitively to the specific c onfinement co nditions. 19 In this respect, it must be noted that a significant p art o f o ur p resent k nowledge a bout w ater under confinement has been obtained from force field simulations 10,11,13–15,20 in order to access long times and large length scales, while molecular-level studies of nanoconfined liquid water within graphene slit pores employing first principles simulations are surprisingly scarce, 21,22 in contrast to other studies devoted to confined ice within graphene sheets of which there are more examples such as Refs. 23–25. This fact made us wonder whether crucial molecular effects taking place at the level of the hydrogen-bonding and even electronic structure of strongly confined liquid water are yet unknown and thus to be discovered. In this Letter, we unravel hitherto unknown astonishing properties of water confined within graphene sheets in different regimes from weak to extreme confinement a ccording to extensive ab initio molecular dynamics (AIMD) simulations. 26 Anticipating our key results, we will demonstrate that the dramatically changing water properties depending on the con-

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finement strength yields three qualitatively distinct regimes not only in terms of structural and topological aspects such as hydrogen bonding patterns and density stratification, but also at the level of the electronic structure of nanoconfined water depending on the width of the slit pore. Computational approach.— We designed five models for nanoconfined water between graphene sheets with different interlayer spacings dint ranging from about 7 to 20 Å, which we refer to as the Extra-Small (XS: dint ≈ 6.9 Å), Small (S: 9.4 Å), Medium (M: 12.2 Å), Large (L: 14.1 Å) and Extra-Large (XL: 19.4 Å) slit pore systems (Fig. 1) following the procedure explained in Sec. I.A of the Supporting Information (SI). The AIMD simulations carried out at 300 K, each of 100 ps length after careful equilibration based on initial con-ditions obtained from extensive force field simulations (see SI) were performed using the CP2K simulation package 27,28 based on the RPBE–D3 density functional 29–31 together with GTH pseudopotentials 32–34 and the TZV2P(triple-ζ plus double polarization quality) Gaus-sian basis set. More specific details about the model systems and computational setup, including extensive validation of the RPBE–D3 functional to describe water/graphene sys-tems, can be found in the SI. Spatial and orientational distribution of nanoconfined water.— The density profiles ρ(z) reveal a pronounced stratification of the nanoconfined aqueous phase perpendicular to the graphene layers (Fig. 1). In all studied systems, water is found to form sharply defined ‘interfacial’ layers in immediate contact with graphene, while in those systems with enough interlayer space (namely M, L and XL) it additionally features smoother ‘intermediate’ layers in the central region of the lamellae. As we will see below, such ‘interfacial’ versus ‘intermediate’ (hereafter ‘IF’ and ‘IM’ for short) partitioning of the water lamellae is particularly useful when disclosing the different properties of water for each system and layer kind; the water-water radial distribution functions of IF versus IM water are analyzed in Sec. II.A of the SI and compared to bulk liquid water. This stratified structure is qualitatively consistent with what has been observed previ-

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Figure 1: Water density profiles and O/H number density profiles along the z–axis normal to the graphene sheets for systems XS to XL, see text, together with representative configuration snapshots at each confinement level; the graphene sheets are positioned exactly at the limits of the respectively shown z–ranges and all distributions have been symmetrized w.r.t. z = 0 Å. The vertical dashed lines in systems S,M, L and XL at the minima of the water density profile visualize the partitioning between the different water layers, which are labelled as either interfacial (IF) of intermediate (IM) as explained in the text. Note that in system XL we distiguish between the outermost and the innermost IM water layers, indicating the latter by IMi as opposed to plain IM for the former. ously, not only for graphene/water 21 but also for water at other hard interfaces. 35–37 However, a close comparison between our system L and the setup introduced in the pioneering AIMD study 21 (see Table 1 and Figure 1(b) therein), which is equivalent by construction (see details in the SI), reveal that our simulations predict a much stronger stratification. The density modulations of our system L are much sharper, up to the point that the very structure of the confined water layer seems different at the center of the lamella since our system L is characterized by two well–defined IM water layers as opposed to the much less defined structure in that region of the equivalent system in Ref. 21. As demonstrated in detail in Sec. I.B of the SI, the bare PBE functional used in 2008 to describe the water/graphene system 21 is found

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to significantly underestimate the strength of the H2 O· · · graphene interactions compared to high–quality coupled cluster CCSD(T) reference data, whereas the dispersion–corrected RPBE–D3 approach available now – one decade later – convincingly describes these crucial water/surface interactions. Regarding other structural features, the O–H bond orientation distributions (Fig. 2) in the IF water layers not only greatly differ from the isotropic limit, but sensitively depend on the confinement strength. In case of systems M, L and XL the OH bonds are mostly arranged either quasi-parallel to the graphene sheets or pointing toward the adjacent water layer, showing an obvious preference to form what one could call ‘in-layer’ and ‘out-oflayer’ hydrogen bonds (H–bonds), respectively. Importantly, only a small contribution of dangling OH groups (i.e. those sticking out of the water lamellae and pointing toward the graphene sheets) is found, which significantly agrees with recent experimental insights into the graphene/water interface. 38 In stark contrast, the orientational preferences of the more strongly confined S and XS systems are markedly different. While this might not be very surprising in case of the ultimatelely confined system XS, where a single water monolayer at the center of the slit pore forms thus enforcing a large number of dangling bonds, the behavior of system S seems puzzling. Phenomenologically speaking, it looks like a greatly enhanced version of the orientational modulations of systems M to XL: the vast majority of the OH bonds is either quasi-parallel or quasi-normal to the graphene sheets while dangling and obliquely-oriented OH groups contribute only negligibly, but why so? On the other hand, the orientation of the OH bonds in the IM water layers of systems M to XL is astonishingly close to the isotropic distribution, which is actually close to perfectly realized in the innermost water layer of the largest system, XL. The quantitative equivalence between systems M, L and XL together with the stark differences in systems XS and S compared to those hint at profound differences of strongly confined water in the latter case, as we will work out in the following. Hydrogen bonding and electronic structure of nanoconfined water.— The electronic struc-

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Isotropic XS S M L XL

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Figure 2: Distribution of the angle formed between the O–H vector and the xy–plane (parallel to the graphene sheets) of all systems split into interfacial (top) and intermediate (bottom) layer water together with the corresponding isotropic reference distribution, P (α) = 12 cos α. Angles close to −90◦ correspond to the OH bonds pointing towards the closest graphene sheet, while α ≈ +90◦ implies OH bonds pointing away from the closest graphene layer and 0◦ implies orientations parallel to the confining surfaces as schematically illustrated at the bottom. Note that by symmetry, α = −90◦ is equivalent to α = 90◦ in case of system XS, in the IM layer of M, and in the innermost IM layer of XL (denoted by XLi ). ture of the nanoconfined water molecules can be conveniently described via their maximally localized Wannier functions 39 by computing the distance distribution of the Wannier centers (X) with respect to the central O atom in each water molecule (Fig. 3). In addition, it

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Interfacial water P(dO−X )

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Figure 3: Normalized distribution functions of the distance from the Wannier centers (X) in each H2 O molecule to the respective oxygen position of all systems split into interfacial (top) and intermediate (bottom) layer water together with the corresponding reference distributions obtained from a bulk water simulation; XLi stands for the innermost of the IM water layers in system XL. The distributions are computed separately for those Wannier orbitals that correspond to the bonding electron pairs in O–H bonds (right panels) and those representing the lone electron pairs (left panels). is particularly illustrative to analyze the dO−X distributions in Fig. 3 side-by-side with the H–bond statistics in Fig. 4. In this way, we observe that both the dO−X distribution and the H–bond statistics for the IM water layers (bottom panels) of the M to XL systems are almost quantitatively identical to those in homogeneous bulk water. This might be surprising given the non-vanishing density modulations even for intermediate layer water seen in the corresponding panels of Fig. 1. On the other hand, very characteristic differences appear 8


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in the IF layers: for systems M, L and XL, also the IF water looks rather similar at the level of dO−X distributions and H–bond statistics, which show reduced H–bonding w.r.t. the bulk as expected, while system S surprisingly yields these properties in between those that characterize IF water under M, L and XL confinement and bulk water. Finally, system XS is vastly different from any of the previous cases, with a greatly reduced H–bonding propensity and very much distorted dO−X distributions compared to the bulk limit. These findings confirm previous work on water within graphene sheets, 21 which describe a lower average dipole moment for IF water compared to the innermost part of the confined lamella. Yet, the outstanding differences between IF water at extreme confinement and more moderately confined water that the present simulations unveil was so far unknown. Interfacial layers XS

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Figure 4: Hydrogen bond statistics of interfacial and intermediate water layers for systems XS to XL as well as for bulk water in terms of the percentage (%) of corresponding water molecules that accept and/or donate i = 2, 1 or 0 H–bonds denoted by Ai and Di , respectively. The background color goes from white (0 %) to purple (62 %) to visualize the qualitative differences and similarities. The fact that the stark differences in H–bonding and in the electronic structure depending on the confinement strength go hand in hand is not surprising. Lone electron pairs that do no accept any H–bond tend to contract towards the O nucleus (thus displacing dO−X towards smaller distances) which, therefore, allows the two bonding electron pairs to displace away 9



from this oxygen toward the two H nuclei in that water molecule (i.e. shifting dO−X towards larger values), making them less positively charged and, in turn, less hydrophilic and less prone to donate H–bonds. The surprising feature is the fact that the M, L and XL systems are identical in the sense that their stratification subregions (both the IF and IM water layers) behave essentially identically, IF water layers of the most confined systems, both XS and S, are qualitatively different from each other and moreover distinct from the IF layers in the three less strongly confined systems. This finding is supported by most carefully analyzing the H–bonding topology of IF water in systems S to XL (see Sec. II.B in the SI where we resolve in SI Fig. 3 the H–bond statistics w.r.t. the location of the donor/acceptor molecules within the slit pores). Moreover, it is very clear that the IM layers in M, L and XL are indistinguishable from regular bulk water at both, the electronic structure and H– bonding levels, which is rather surprising in view of the marked density stratification at these three confinement levels and especially in system M, being the most confined one that still features non–interfacial water. Taken together, we conclude at this stage that increasing the confinement width even more until effectively reaching the interface/bulk limit will not much change the features of the water-graphene interface as disclosed here for the M, L and XL setups. According to all the data laid out, three qualitatively different regimes for nanoconfined water can be distinguished: (i) water in the extreme confinement / monolayer limit (system XS), (ii) water in the strong confinement / bilayer limit (system S), and (iii) water in moderate to weak confinement (systems M, L and XL). The key question as to what governs water’s behavior in nanoconfinement is what kind of cross-talk can be established between the different subregions in the system as determined by the density stratification (see vertical lines in Fig. 1). These three observed regimes correspond to the following three possibilities: (i) one isolated interfacial water layer, (ii) two interfacial water layers in mutual contact, and (iii) two interfacial water layers separated by intermediate water (which has been shown to be essentially bulk-like considering its H–bonding and electronic structure properties).

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This cross-talk modulates the behavior of water such that it presents qualitatively different properties at the very molecular level, therefore being a different medium in each of the three regimes. While for weakly to moderately confined systems the overall properties of the confined water lamella arise from the fact that it is composed of ‘interfacial’ and ‘bulk-like’ water in different proportions, strongly confined water, which is stratified in terms of a bilayer structure, can not be simply understood as standard interfacial water, but instead must be recognized as a very special H–bonded liquid. In other words: Water confined within two parallel graphene sheets behaves like interfacial water (when in direct contact with one of the graphene layers) plus bulk-like water (in the center region) from large down to moderate interlayer distances of ≈ 12 Å, whereas it changes its properties significantly when forming a bilayer structure at strong confinement of close to 10 Å. In summary, our ab initio simulations of two-dimensionally nanoconfined water between graphene sheets reveal a peculiar behavior of water in between two planar hydrophobic hard interfaces, disclosing the existence of three qualitatively different regimes for nanoconfined water depending both, sensitively and critically on the width of the slit pore. In all cases, the nanoconfined aqueous phase is strongly stratified in terms of density modulations normal to the confinement. From weak to moderate confinement, corresponding to roughly 20 and 12 Å interlayer distances, we can distinguish a smoothly modulated central region that is sandwiched between two sharply defined interfacial water layers that are in direct contact with the confining sheets. Concerning their H–bonding and electronic properties, the water molecules in these two regions are not different from standard homongeneous bulk water and from usual interfacial water close to wet hydrophobic surfaces, respectively, in this confinement regime independently from the precise interlayer distance. On the other hand, upon confining water strongly (10 Å) and even extremely (7 Å), respectively leading to the formation of a water bilayer and monolayer structure, the properties of this associated liquid have been found to greatly change at the level of both, H–bonding and electronic structure. Although first examples of astonishing changes of chemical reactivity 17,40 and

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charge migration 36,41 in the strong confinement r egime a s r ealized b y w eakly interacting hard–walled slit nanopores are available, we expect that new chemistry might be accessible in aqueous phases that are squeezed to the bilayer or even monolayer limit.

Acknowledgement We are grateful to Roland Netz and Stephan Gekle for inspiring discussions and acknowledge partial financial s upport v ia t he C luster o f E xcellence “ RESOLV” ( EXC 2 033) f unded by the Deutsche Forschungsgemeinschaft (DFG) as well as computing resources provided by HPC@ZEMOS, HPC–RESOLV, BOVILAB@RUB, and RV-NRW.

Supporting Information Available Computational Approach, Model Systems, Ab Initio Molecular Dynamics Settings and Validation of the RPBE–D3 Approach, Extended Structural and Hydrogen Bonding Analysis. This material is available free of charge via the Internet at http://pubs.acs.org/.

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Nanoconfined Water within Graphene Slit Pores Adopt Distinct

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