Origin of the Hydrophobic Behaviour of Hydrophilic CeO 2

: The nature of the hydrophobicity found in rare-earth oxides is intriguing. The CeO 2 (100) surface, despite its strongly hydrophilic nature, exhibits hydrophobic behaviour when immersed in water. In order to understand this puzzling and counter-intuitive effect we performed a detailed analysis of the confined water structure and dynamics. We report here an ab-initio molecular dynamics simulation (AIMD) study which demonstrates that the first adsorbed water layer, in immediate contact with the hydroxylated CeO 2 surface, generates a hydrophobic interface with respect to the rest of the liquid water. The hydrophobicity is manifested in several ways: a considerable diffusion enhancement of the confined liquid water as compared with bulk water at the same thermodynamic condition, a weak adhesion energy and few H-bonds above the hydrophobic water layer, which may also sustain a water droplet. These findings introduce a new concept in water/rare-earth oxide interfaces: hydrophobicity mediated by specific water patterns on a hydrophilic surface.

Rare-earth oxides hold a special place among the metal oxides as they have been found experimentally to exhibit particularly high degrees of hydrophobicity. [1]Normally, metal oxide surfaces are hydrophilic due to the exposed under-coordinated metal and oxygen atoms, which create local dipoles where water molecules adsorb strongly. [2,3] he origin of the apparently intrinsic and unexpected hydrophobic behaviour of rare-earth oxides is unclear and many different and contradicting scenarios have been proposed in the literature.Azimi et al. [1] proposed that the inaccessibility of the 4f electrons of the metal ions for interacting with the adsorbing water oxygen could explain an intrinsic hydrophobicity.Carchini et al. [4] instead proposed that the mismatch between the rare-earths lattice oxygens and the adsorbed water network would overrule a natural hydrophilic character of these metal oxide surfaces making them hydrophobic.They also proposed that a change in the protonation state (induced by oxygen vacancies, surface reduction and/or the degree of water splitting) of a rareearth oxide surface could switch its hydrophobic character to becoming a completely wet surface, in accordance with previous studies [5] that also discussed the role of surface hydroxylation.
Among the rare-earth oxides, ceria (CeO 2 ) has been seen as a reference material for the study of wetting properties due to its catalytic power for water splitting [6] and its high degree of apparent hydrophobicity, with water contact angles between 90°and 120°[ 1,7,8] (note: a large water contact angle is the definition of a hydrophobic surface).[11][12] The latter experimental results originate from X-Ray Photoelectron Spectroscopy (XPS) analysis of the ceria surfaces upon removal of water excess in order to measure the presence of adsorbed carbon species.Furthermore, the same studies reported a rapid increase of the water contact angle immediately after exposure of the ceria surface to the atmospheric moisture.Hence, a direct in situ measurement of the hydrophilic-hydrophobic transition at operando conditions during water exposure is missing, which makes it difficult to decipher the microscopic-level reasons behind this phenomenon.
All in all, it is fair to conclude that the current understanding of, and information about, the hydrophobicity / hydrophilicity of ceria is ambiguous.One can further note that the degree of CeO 2 hydrophobicity may depend on the specific crystal facets exposed (and their degree of hydroxylation) as suggested by density functional theory (DFT) based theoretical estimates of contact angles by Fronzi et al.; [13] in fact only the (100) facet was so far experimentally demonstrated to behave hydrophobically. [14,15]  this Communication we report a first-principles Molecular Dynamics (AIMD) simulation addressing the origin of hydrophobicity of a hydrophilic CeO 2 surface.We demonstrate that the investigated (100) facet of CeO 2 , although intrinsically hydrophilic, exhibits an effective hydrophobicity which is induced by the first adsorbed water layer rather than by the ceria surface itself.This waterinduced hydrophobicity is tested in an AIMD droplet-on-afixed-layer simulation, which is found to retain a sustainable non-wetting character.Finally the molecular diffusion rate of water confined between the CeO 2 surfaces is found to be enhanced with respect to that in the bulk water at the same thermodynamic condition.18][19][20][21] Theoretical studies of the clean CeO 2 (111) surface [4] and later also of clean (110) and (100) surfaces [13] have reported these to be hydrophobic.This conclusion was derived by estimating the water contact angle from expressions involving the interaction energy of a double ice layer with the CeO 2 surfaces.Such a computational procedure does not consider the contribution of liquid bulk water and disregards the water interaction beyond the double ice layer.Fully hydrated CeO 2 facets have indeed been investigated by computations, for example in the AIMD simulations by Camellone et al. [22] and by Ren et al.; [23] 1).This structure was suggested by Kropp et al. [24] and it was later confirmed experimentally by a Nuclear Magnetic Resonance (NMR) study showing that the (100) surface is dominated by the existence of two OH populations with different chemical environments, one weakly interacting and one strongly interacting. [15]In our study, the vacuum region above the surface (about 37 Å thick) was filled with H 2 O molecules corresponding to the water density at 1 atm and 310 K (see methods for further details).
The first important result is the formation of an almost rigid, well structured, flat layer on top of the hydroxylated CeO 2 (100) surface.This water layer is visible in Figure 1 (upper panel) as the peak labeled L 1 in the water oxygen density profile at 4 Å and by the representative snapshot displayed (Figure 1 lower panel and Figure 2a).The L 1 layer interacts with the surface hydroxyl groups (which we label L 0 ).After room-temperature MD relaxation, in the absence of the water slab above them, the hydroxide ions in L 0 exhibit two conformations, orthogonal and parallel to the surface (OH ?and OH k ).These OH configurations are maintained in the presence of liquid water and can be identified by the hydrogen density peaks at 1.5 and 2.4 Å in Figure 1.Our resulting MD structure, which features an L 0 layer interacting strongly with a water layer (L 1 ) above it, is consistent with the structures obtained by Kropp et al. [24] in their static DFT structure optimizations.
Also in our MD simulations, L 1 displays quite a rigid structure which is maintained along the full MD trajectory (see Figure S1 in Supporting Information), suggesting significant adhesion to L 0 .This is also confirmed by a few separate single-point calculations from the AIMD trajectory where one single isolated water molecule from L 1 was left to interact with L 0 (all other water molecules were removed).This was done for two typical structures: the L 1 water donating a hydrogen bond to an OH k in L 0 , or the L 1 water accepting a hydrogen bond from OH ? in L 0 .The former has an interaction energy of 0.5 eV and the latter of 0.3 eV.
We furthermore computed the interaction energy (adhesion energy) between the layers L 1 and L 0 (see Table 1 and Supp.Info.), expressed per L 1 water molecule.The resulting energy is 0.53 eV per water molecule in L 1 .This value is larger than our interaction energy per water molecule in bulk water, 0.45 eV (see Table 1).The extra energy contribution is due to the strong interaction with the hydroxylated CeO 2 (100) surface, confirming its hydrophilic nature.In the same way we computed the adhesion energy between L 1 and L 2 , which instead resulted in 0.28 eV per water molecule, considerably lower that the respective value in bulk water.This implies a loss of energy for a water molecule in L 2 interacting with L 1 (which instead prefers the liquid water surroundings), which suggests an hydrophobic behaviour.
Returning now to the structure, inspection of the second water layer (L 2 , peak maxima at 6.8 Å in the density profile) is also informative.We note in particular the low H density between the first and second layers (i.e. between L 1 and L 2 ).The hydrogen atoms belonging to L 1 water molecules lie almost entirely within this layer, or they point towards the CeO 2 surface (peak at 3.1 Å).Just a minority of the H atoms are oriented towards L 2 .At the same time, the H atoms of the L 2 water molecules either lie completely in L 2 or they are, on average, slightly pointing away from L 1 .This suggests only weak interaction between the L 1 and L 2 layers. [25]ogether, L 0 and L 1 form a bi-layer with a distorted square lattice.A top view snapshot of L 1 is presented in Figure 2a.Classical MD studies [26][27][28] have demonstrated that distorted and ordered FCC and hexagonal patterns of water molecules might exhibit hydrophobic character.Interesting cases are the bi-layer hexagonal ice (BHI) structure observed experimentally for water on graphene at low temperature, [29] water within hydrophobic nanopores, [30] water on clay mineral surfaces [31] and water on gold surfaces [32][33][34] at low temperature.The BHI pattern has been hypothesized to behave like a hydrophobic surface due to the locking pattern of hydrogen bonds which impedes the interaction with the surrounding water molecules.This was recently confirmed experimentally for polytetrafluoroethylene surfaces where the first hexagonal adsorbed water layer was shown to behave hydrophobically. [35]The square lattice of the water bi-layer that we observe in our simulation resemble the hydrogen-bond-locking pattern observed in the BHI structure.
As we mentioned, the existence of an ordered single water monolayer above the hydroxylated (100) ceria surface was predicted theoretically for a low water coverage scenario. [24]Here we confirm that such a structure (the L 1 layer) is stable also at room temperature and we furthermore explore the nature of its interactions with excess water added above it, which has not been discussed in the literature before.We propose that the locking mechanism and the interface structures may have significant consequences for the understanding of ceria hydrophobicity.This Table 1: Adhesion energies involving the L 1 water layer, expressed in energy per L 1 water molecule, and compared with the average interaction energy in bulk water.(1st row:) The label "L 1 -L 0 " stands for the adhesion energy between L 1 and the "L 0 + ceria" slab.(2nd row:) The label "L 1 -L 2 " here stands for the adhesion energy between it L 1 and the water film "above" it.(3rd row:) The intermolecular interaction energy per water molecule from a separate BLYP-D3 bulk simulation.The "L 1 -L 2 " interaction is seen to be considerably weaker than the other two cases.See Supp.Info.for the mathematical expressions used for the calculations.

Interaction type Energy [eV]
L 0 -L 1 0.53 L 1 -L 2 0.28 bulk water 0.45 might be the case also for the (111) facet of ceria, for which the formation of an ordered water layer was also predicted. [24]o corroborate our hypothesis regarding the hydrophobicity of L 1 , we performed an ab-initio MD simulation of a water droplet placed on the surface of an extended L 1 water layer (see methods).Figure 2b shows a characteristic simulation snapshot of the droplet interacting with the layer after 40 ps.It is clearly seen that the water droplet is not wetting the underlying water layer but forms a half-spherical shape typical of water droplets on hydrophobic surfaces.Although the simulation time might result too short for the characteristic relaxation time of the droplet interface, we note that in a simulation with the same droplet placed instead on the bare hydroxylated CeO 2 (100) surface, it completely spreads on the available surface after a few picoseconds.This supports the notion of the hydrophobic behaviour of L 1 compared to that of the bare hydroxylated CeO 2 (100) surface, in agreement with the interaction energy differences reported in Table 1.Similar computational experiments were performed using classical MD simulations for water droplets on TiO 2 , [36] on Al 2 O 3 [37] and on regular water patterns, [26] providing support for the notion that certain structures of water in the first adsorbed water layer can increase the water contact angle.
In the following section we present results for water dynamics above the layer L 1 in our system, which provide compelling evidence in support of our conjecture that the structure of the first adsorbed water layer plays a key role for the hydrophobicity of ceria (100).
The effects of the proximity of a confining wall on liquid dynamics have been explored in a number of studies. [16,39,40] Iwas found that a wide range of liquids demonstrate enhanced dynamics close to non-interactive walls.[44][45] On the other hand, a strong enhancement of translational diffusion was observed in a similar simulation of water confined between two flat walls of graphene, [18] which is well-known to exhibit a pronounced hydrophobicity.Thus, the comparison of the diffusion rate in water close to confining surfaces with that in the bulk water can be regarded as a robust test of the surface hydrophobicity.
Following these arguments, we investigated both the translational and rotational diffusion of the water in our CeO 2 model system in order to find supporting evidence for our conjecture that the water bilayer previously described behaves as a hydrophobic surface with respect to the rest of the water.Our first important observation is a significant enhancement of the translational diffusion in the water confined between the CeO 2 (100) surfaces.Figure 3a shows the three-dimensional (3D) Mean Square Displacement (MSD) of water molecules confined between CeO 2 (100) and TiO 2 anatase (101) surfaces as compared with the MSD calculated for the bulk water at the same thermodynamic conditions.It is evident that the confinement between the supposedly hydrophilic CeO 2 (100) surfaces enhances the water diffusion, while the opposite behaviour is observed for the TiO 2 case.
We also calculated the 2D MSD for L 1 and the water above L 1 .The results are presented in Figure 3b, where the lateral MSD parallel to the CeO 2 and TiO 2 surfaces are compared with the 2D MSD of bulk water at the same temperature and water density.Here we note that practically no diffusion is observed in L 1 for neither CeO 2 nor TiO 2 , confirming their rigid structures.The water above L 1 for CeO 2 instead displays significant lateral diffusion enhancement relative to the bulk water and the TiO 2 surface.[19] A similar effect was also reported for a water layer next to a repulsive model surface [20] and for a water layer on hydrophobic Lennard-Jones surfaces. [21]e note that Cicero et al., [18] using ab-initio MD simulations, demonstrated that the dipole-dipole correlation  [38] Angewandte Chemie Communications in water confined between hydrophobic graphene sheets decayed at the same rate as that of bulk water, when the graphene surface separation is 25 Å.In our simulation the dipole correlation function calculated for water between L 1 layers on CeO 2 separated by 28 Å (between z = 6 Å and z = 34 Å in Fig. Figure 1) closely agrees with the dipole correlation of bulk water, supporting the hydrophobic effect of L 1 (see Figure S2 in Supp.Info.).
These results show that translational and rotational dynamics of water confined between the CeO 2 (100) surfaces match the behaviour of water confined between hydrophobic graphene surfaces.On the other hand, the observed dynamics is quite different from that in the water confined between TiO 2 surfaces.We regard this observation as a strong evidence of the hydrophobic nature of the L 1 layer.
The main result of this work is our deciphering of the hydrophobic origin of the hydrophilic hydroxylated ceria (100) facet.A first-principles AIMD simulation of liquid water "on top of" this facet was performed and the following key observations and conclusions were made.(1) An ordered water layer was found to form on the hydroxylated surface, the two together (L 0 + L 1 ) forming an H-bonded key-lock pattern which persists at room temperature.(2) The ordered water layer (L 1 ) interacts only weakly with the water film above it (L 2 ), as evidenced both by the scarcity of H-bonds between the layer and the film above and by the modest interaction energy between them.(3) The ordered water layer formed on the ceria (100) facet thus makes it hydrophobic.This observation is consistent with the results of a separate AIMD simulation for a water droplet deposited on the ordered water layer.(4) The hydrophobicity of such an ordered layer manifests itself in the enhancement of water diffusion as compared to bulk water.18][19][20][21] Altogether our results reveal an atomistic mechanism which explains the origin of the natural hydrophobicity of the hydrophilic ceria (100) and possibly of other rare-earth oxides as well: hydrophobicity triggered by specific water patterns on hydrophilic surfaces.
Figure1).This structure was suggested by Kropp et al.[24] and it was later confirmed experimentally by a Nuclear Magnetic Resonance (NMR) study showing that the (100) surface is dominated by the existence of two OH populations with different chemical environments, one weakly interacting and one strongly interacting.[15]In our study, the vacuum region above the surface (about 37 Å thick) was filled with H 2 O molecules corresponding to the water density at 1 atm and 310 K (see methods for further details).The first important result is the formation of an almost rigid, well structured, flat layer on top of the hydroxylated CeO 2 (100) surface.This water layer is visible in Figure1(upper panel) as the peak labeled L 1 in the water oxygen density profile at 4 Å and by the representative snapshot displayed (Figure1lower panel and Figure2a).The L 1 layer interacts with the surface hydroxyl groups (which we label L 0 ).After room-temperature MD relaxation, in the absence of the water slab above them, the hydroxide ions in L 0 exhibit two conformations, orthogonal and parallel to the surface (OH ?and OH k ).These OH configurations are maintained in the presence of liquid water and can be identified by the hydrogen density peaks at 1.5 and 2.4 Å in Figure1.Our resulting MD structure, which features an L 0 layer interacting strongly with a water layer (L 1 ) above it, is

Figure 1 .
Figure 1.Contents of our periodic AIMD simulation box for a hydroxylated CeO 2 (100) surface in liquid water.The water oxygen and hydrogen density profiles (along the direction perpendicular to the surface) averaged along the MD trajectory are shown above the box.The CeO 2 (100) surface is stabilized by hydroxyl groups (layer L 0 ) which arrange themselves alternatively parallel and perpendicular to the ceria surface and form an intricate key-lock pattern with the first water layer (L 1 ).

Figure 2 .
Figure 2. a) A snapshot from the AIMD simulation of the system in Figure 1, but here only the L 1 water layer is displayed.For visualization purposes it has been expanded periodically 3 × 3 times in the xy plane.b) Simulation of a water droplet (at 310 K) on a fixed L 1 structured water layer formed on the hydroxylated CeO 2 (100) surface.The droplet persists with a contact angle of circa 90°after 40 ps.

Figure 3 .
Figure 3. MSD of water confined between CeO 2 (100) surfaces and between TiO 2 anatase (101) surfaces, respectively, compared with bulk water at the same temperature and density.a) The total 3D MSD, and b) the lateral 2D MSD.The overall 3D diffusion enhancement is mainly attributed to the lateral component.The water data on the TiO 2 anatase (101) surface are taken from Ref.[38]