Dynamical and Structural Characterization of the Adsorption of

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Dynamical and Structural Characterization of the Adsorption of Fluorinated Alkane Chains onto CeO

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Giovanni Barcaro, Luca Sementa, Susanna Monti, Vincenzo Carravetta, Peter Broqvist, Jolla Kullgren, and Kersti Hermansson J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05554 • Publication Date (Web): 25 Sep 2018 Downloaded from http://pubs.acs.org on September 29, 2018

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The Journal of Physical Chemistry

Dynamical and Structural Characterization of the Adsorption of Fluorinated Alkane Chains onto CeO2 Giovanni Barcaro,∗,† Luca Sementa,† Susanna Monti,‡ Vincenzo Carravetta,† Peter Broqvist,¶ Jolla Kullgren,¶ and Kersti Hermansson¶ †CNR-IPCF, Institute of Chemical and Physical Processes, via G. Moruzzi 1, I–56124 Pisa, Italy ‡CNR-ICCOM , Institute of Chemistry of Organometallic Compounds, via G. Moruzzi 1, I–56124 Pisa, Italy ¶Department of Chemistry–˚ Angström, Uppsala University, Box 538, S–751 21, Uppsala, Sweden E-mail: [email protected]

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Abstract The widespread use of ceria–based materials and the need to design suitable strategies to prepare eco–friendly CeO2 supports for effective catalytic screening induced us to extend our computational multi–scale protocol to the modeling of the hybrid organic/oxide interface between prototypical fluorinated linear alkane chains (polyethylene– like oligomers) and low index ceria surfaces. The combination of quantum chemistry calculations and classical Reactive Molecular Dynamics simulations provides a comprehensive picture of the interface and discloses, at the atomic level, the main causes of typical adsorption modes. The data show that at room temperature, a moderate percentage of fluorine atoms (around 25%) can enhance the interaction of the organic chains by anchoring strongly pivotal fluorines to the channels of the underneath ceria (100) surface, whereas an excessive content can remarkably reduce this interaction because of the repulsion between fluorine and the negatively–charged oxygen of the surface.


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Introduction Cerium dioxide (CeO2 or ceria) is a very versatile material used for technologically relevant applications in the academia and industrial sectors. 1 These include, among the others, catalysis, 2–5 solid oxide fuel cells, 6,7 gas sensors, 8 antioxidant agents in biological systems, 9,10 biomedicine 11 coatings for UV filtering. 12,13 Being a prototypical reducible oxide with a smooth redox cycling between the Ce(III) and Ce(IV) states, ceria can be used for oxygen storage and release. This capability is a direct consequence of the low energy of formation of oxygen vacancies 1,14 and it is at the base of all the activities of this material. Size reduction of the oxide to the nanometer scale, in the form of ceria nanoparticles (NPs), revealed to be an appropriate methodology for boosting material properties. This is partially related to the decreased formation energy of the oxygen vacancies in confined systems. 1 Furthermore, nanocomposites are much more efficient in their catalytic action in comparison with their bulk analogues and tailored shapes (size and morphology) have emerged as equally important aspects. 15,16 As a matter of fact, control over morphology, size and faceting, through the functionalization of ceria NPs with different types of molecules, prevents coalescence and enhances compatibility with specific environments. As a consequence, the engineered NPs can be more easily internalized into the target regions where they induce catabolic pathways that regulate characteristic phenomena. However, more tuned and selective coatings are desired to reduce side effects and enhance the signals of these vehicles. The success of the combination strategy which incorporates polymers in metal oxide NPs to generate polymer/inorganic hybrid materials with very high potential in catalysis, was demonstrated in several investigations (for example, see Refs 17–20 ) but only a few focused on polymer/ceria complexes in various combinations. 21–24 In these last studies, the main concern was to produce film coatings, membranes, highly dispersed materials with extended surface area through the adsorption of polymer layers on the metal oxide or, inversely, through the formation of the oxide on the



polymer core. It was observed that at low polymer concentration, most of the polymer chains were accommodated at the NPs interface 25 where they acted as structure modifiers. The adsorbate layer was also responsible for the slower dynamics and improved thermal stability of the nanocarriers. 26 Essentially, the coating determined NPs hydrophilicity–hydrophobicity, their compatibility with other molecules, adsorption properties and reactivity. Given these premises it is well apparent that the choice of the polymer is fundamental to impart long/short–term stability, solubility and other characteristics to the supports. Although polyethylene (PE), polystyrene (PS), poly(methyl–methacrylate) (PMMA), poly(ethylene– glycol) (PEG), poly(dimethylsiloxane) (PDMS), starch, dextran, chitosan, to cite just a few, are prevalently employed in metal/metal oxide surface decoration, a new class of competitive materials with outstanding properties, such as thermal stability, chemical inertness, low refractive index, permittivity, durability, and resistance to oxidation, is emerging. This is the fluorinated polymer family. It was observed that the replacement of the hydrogens of the traditional hydrocarbons with fluorine greatly affects the properties of the molecules by increasing the volume of the chains (larger occupation of the –CF2 – groups) and reducing their flexibility. 27 This is reflected in the different phase behavior of the two species. Essentially, the fluorinated polymers are more likely to form occasional intermediate phases due to their lower interfacial curvature, they easily self-assemble and are more stable from a thermal point of view 27–29 than the respective hydrocarbons. Regarding the vast field of the fabrication of nanocomposite organic/inorganic materials, 30 just to give some examples of applications of fluorinated polymers, synthetic routes involving the initiation and propagation of the polymerization process at the surface of a properly fluorine-functionalized oxide (silica) particle have been used to achieve the optimization of a material with enhanced thermostability and hydrophobicity; 31 highly-porous nanofiber membranes prepared by electrospinning from poly(vinylidene fluoride) (PVDF)SiO2 blend solutions led to the significant improvement of mechanical properties and of the ionic conductivity. 32 4 ACS Paragon Plus Environment

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Considering the importance of ceria NPs on one side and of these new fluorinated stabilizers on the other side, we have chosen to investigate the interface compatibility of these two systems to generate a new hybrid material characterized by the interaction of a polyethilene-like chain where the fluorinated groups are located on the polymer backbone, onto low-index ceria surfaces. Given the scarceness of experimental data on this class of systems and the inherent difficulties to obtain detailed atomistic information on the structure and dynamics at the polymer/oxide interface – in comparison with the behavior of other more common species – we have employed a computational multi–scale approach to disclose at the atomic level the behavior of the organic/oxide interface where simultaneous physisorption and chemisorption events can, in principle, take place. According to a wider perspective, the data derived by our simulations will depict the paradigm of a new computational protocol able to furnish a smart strategy for designing new coating agents that can be strongly adsorbed on the inorganic supports.

Computational Details The multi-scale protocol that we employed in the present investigation was articulated in the following steps: (i) study at the first-principles (DFT) level of the adsorption mode on the oxide surfaces of quite small organic chains containing a variable content of fluorine (F) atoms in order to derive a rich database of both structures and energies; (ii) derivation and validation of a reactive force field (ReaxFF) describing the organic/oxide interaction; (iii) use of the derived ReaxFF to study organic/oxide interfaces untreatable at the first-principles level. These three steps will be described and discussed in the next sections.



Ab-initio Calculations Density Functional (DFT) calculations were performed using the Quantum Espresso computational code, 33 employing a basis set of plane waves, PAW pseudopotentials 34 and an exchange-correlation (xc) functional that takes into account self–consistently dispersion (van der Waals) interactions. 35 Energy cut-offs of 40 Ry and 400 Ry were chosen for the plane waves and electronic density, respectively. In a previous paper, 36 some of us investigated the structure of CeO2 bulk oxide in its fluorite–like crystal structure by employing a DFT+U approach 37 and we obtained an accurate estimate of the bulk lattice constant of 3.89 ˚ A, which satisfactorily agree with the experimental estimate of 3.91 ˚ A. This was used to build our low–index surface models, namely (1 1 1), (1 1 0) and (1 0 0); more details on the building procedure will be given in the next section. Linear alkane chains containing a number of carbon atoms from one to seven were adsorbed on the oxide surfaces by choosing properly the dimension of the unit cell (i.e. to reduce the interaction with replica images). Along the z axis (i.e. the direction perpendicular to the ceria surfaces) a minimum empty space of 15 ˚ A was created. A dipole correction 38 was applied to cancel spurious coulombic interactions among the replicated images and the electronic levels were broadened with a Gaussian smearing of about 0.6 kcal/mol. All the calculations were performed considering spin–unpolarized systems and by setting Hubbard U on Ce atoms to zero, after verifying that this choice did not affect the interaction energies of the oxide surfaces with the alkane chains.

Reactive Force Field Fitting and Molecular Dynamics Simulations Force field optimization was carried out by means of the sequential algorithm available in the stand–alone version of the ReaxFF program provided by van Duin. 39,40 The interaction between the alkane chains and the oxide surfaces was parametrized by tuning the bonding


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parameters and the off-diagonal terms 40 between the terminal atoms of the alkane chains (H and F) and the atoms of the oxide surface (Ce and O). Reactive Molecular Dynamics simulations (RMD) were carried out through the LAMMPS code 41 considering a linear alkane chain made of 36 C atoms and a variable number of F substituents over the total possible 74 lateral sites. These oligomers were selected as a prototype to simulate the behavior of a fragment of linear poly–ethylene chain interacting with the ceria (1 0 0) surface. Energies and forces between all the components were calculated with: a force field derived to describe CeOx bulk and surfaces, 42 the original parametrization describing alkane chains, 40 and the new parametrization performed in this work to describe alkane/oxide composites. Both pure and mixed chains were considered: for the mixed chains, five different F contents were considered by varying the number of F atoms between 12% (9 F atoms per chain) and 60% (45 F atoms per chain); the mixed chains were generated by randomly choosing the sites of the chain where to replace a H with a F atom. For statistical purposes, for each stoichiometry, 5 different structures were randomly generated and their adsorption behavior investigated. The unit cell for this series of simulations was built by means of a (8×8) 4–layer thick CeO2 slab (with a side lengths of 31.0 ˚ A) exposing (1 0 0) surfaces and consisting of 256 Ce and 512 O atoms. The simulations were carried out in the NVT canonical ensemble with a time step of 0.25 fs. The Nosè–Hoover thermostat 43–45 with a relaxation (damping factor) time of 100 fs was employed. The analysis, both in terms of the alkane/oxide structure and interaction, was performed every 25 fs. More details on the time lengths and temperatures used in the simulations will be provided in the discussion of the results.



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Results and Discussion Ab–initio Calculations The first analysis consists in comparing the interaction of the CH4 and CH3 F molecules with the lowest-energy surfaces of CeO2 , namely (1 1 1), (1 1 0) and oxygen–terminated (1 0 0) (modelled according to Ref. 46 ) (Figure 1). These molecules were used as probes to explore the Potential Energy Surface (PES) by pointing one H or F atom towards the oxide keeping the corresponding C–H or C–F bond perpendicular to the surfaces. The other three H atoms were located far from the surface (Figure 2A). The alkane/oxide interaction energy was calculated as:

Eint = Etot − Eoxide − Echain

(1)

Where Etot corresponds to the total energy of the system and Eoxide and Echain are the energies of the two separate components frozen in their interacting configuration. For the (1 1 1) surface, we built a (2×2) rectangular unit cell made of two ceria layers (corresponding to 6 atomic layers in O–Ce–O stacking) and we used a 2×2×1 k–point grid (Figure 1A). For the (1 1 0) surface, we built a (2×2) rectangular unit cell made of 4 ceria layers and we used a 3×2×1 k–point grid (Figure 1B). For the (1 0 0) surface, we built a (2×2) square unit cell made of 2 ceria layers (corresponding to 5 atomic layers in O–Ce stacking) and we used a 2×2×1 k–point grid (Figure 1C). To accurately explore the PES of the three surfaces, we performed several constrained relaxations with the probe on–top of different characteristic sites, which are labelled in Figure 1: more specifically, relaxations were performed by fixing the xy position of the probe H or F atom on–top of each specific site and relaxing all the other coordinates of both the organic and the oxide part of the system. The derived interaction energies, reported in Table 1, suggests that the interaction of CH4 with the oxide is very


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weak and is characterized by a quite flat PES on all of the three surfaces. Replacing the apex H with F does not substantially change the interacting scenario in the case of (1 1 1) and (1 1 0) and only a slight decrease in the interaction values is observed. On the contrary, a remarkable enhancement in the interaction is apparent in the case of the (1 0 0) surface, especially when the CH3 F molecule points to the middle of a channel (O1 site – Figure 2A). This peculiar behavior can be understood by looking at the side-views of the three investigated surfaces in Figure 1: the (1 1 1) surface appears quite flat, whereas both the (1 1 0) and the (1 0 0) surfaces present surface channels that can be used as pinning centers towards adsorbed molecules: nevertheless, the (1 1 0) channels result quite tight, about 2.9 ˚ A wide, whereas the channels of the (1 0 0) surface, about 4.1 ˚ A wide, are sufficiently large to accommodate the pivotal F atom when the CH3 F molecule is adsorbed. From the calculation of the interaction energy of the CH3 F molecule in correspondence of the Ce and O1 site (by using a pure GGA–PBE approach) we obtained energies of –4.4 and –12.0 kcal/mol, indicating that this effect is already qualitatively described at the pure GGA level and quantitatively enhanced when dispersion interactions are taken into account. The contribution of this chemical bond which is not due to dispersion forces is probably a mixture of covalent and ionic components. In fact, it is apparent in Figure 2A that fluorine is in close contact with two neighboring (under–coordinated) Ce atoms at the border of the channels. Furthermore, it is located in the site of a missing oxygen and hence can be stabilized by the Madelung (ionic) potential of the surface. The tendency of fluorine to interact with cerium oxide and to create cerium oxy-fluorite was observed in other investigations that demonstrated that the changes of the chemical properties of the oxide were due to the incorporation of F atoms into the oxide lattice by substitution of its oxygens or accumulation near the surface. 47–51 In order to characterize more deeply this peculiar effects, we extended our study to the interaction of slightly longer alkane chains, containing 4 and 7 carbon atoms, with the (1 1 9 ACS Paragon Plus Environment


Figure 1: Top (left) and perspective side (right) views of the (A) (1 1 1) (B) (1 1 0) and (C) (1 0 0) ceria surfaces, where the investigated adsorption sites are indicated by the same notation used in Table 1. O and Ce atoms are red and grey spheres, respectively.


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Table 1: DFT interaction energies (in kcal/mol) of CH4 and CH3 F with low– index ceria surfaces. Nomenclature of the sites is the same used in Figure 1.

(1 1 1) CH4 CH3 F (1 1 0) CH4 CH3 F (1 0 0) CH4 CH3 F

B site Ce site -2.3 -2.8 -3.9 -6.1 Ce site H1 site -3.2 -3.2 -6.7 -4.1 Ce site H site -2.3 -2.8 -7.1 -5.3

H site -2.3 -3.9 H2 site -4.1 -1.8 O1 site -3.9 -18.9

O site -2.3 -3.0 H3 site -4.6 -3.5 O2 site -2.3 -4.4

O site -3.7 -2.8

1) and the (1 0 0) surfaces, by varying the content of F atoms. The unit cells were extended to (2×4) and the k-grid was reduced to 2×1×1, in order to further reduce the interactions between replicated images in the xy plane. The optimized structures of the adsorbates are displayed in Figure 2B-H. Focusing on the adsorption of the C4 F2 H8 molecule on the (1 0 0) face (optimized structure in Figure 2B), it is apparent that the preferential adsorption configuration corresponds to the linear alignment of the polymer chain with two F atoms anchored to the underneath channel of the oxide surface. The two F atoms have the clear tendency to adsorb on-top of two O1 sites, as evident in the side -view of the molecule, where the two F atoms are located approximately at half-way between two O atoms of the oxide ridge. In this case the interaction energy is around –44.3 kcal/mol. For the corresponding not fluorinated species, the C4 H10 (optimized structure in Figure 2D) molecule, the interaction results drastically weakened (–15.7 kcal/mol), in perfect line with the data of the smaller CH4 species. A complete substitution of all the H atoms of C4 H10 with F atoms (C4 F10 , optimized structure in Figure 2C) also causes a weakening of the total interaction with the interface (–18.5 kcal/mol). This latter result can be probably ascribed to the repulsion between the negatively charged oxygens of the surface and those F atoms of the molecule that do not enter into the channel and hence remain located too close to the surface. As a confirmation of this



Figure 2: Adsorption configurations of (A) CH3 F and CH4 adsorbed on the O1 site of the (1 0 0) surface (side view); (B) C4 F2 H8 (side and top views), (C) C4 F10 (side view) and (D) C4 H10 (side view) adsorbed along the channel of the (1 0 0) surface; (E) C4 F2 H8 on–top of the (1 1 1) surface (side and top views); (F) C7 F3 H13 , (G) C7 F16 (side view) and (D) C7 H16 (side view) adsorbed along the channel of the (1 0 0) surface. O and Ce atoms are depicted as red and grey spheres, respectively; C, H and F atoms are depicted as yellow, white and green spheres, respectively. 12 ACS Paragon Plus Environment

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hypothesis, we have analyzed the bond distances between the fully fluorinated molecule and the oxide surface, finding that the minimum distance between F atoms (out of the channel) and O atoms of the ridges is about 3.4 ˚ A, to be compared to much smaller values (about 2.6 ˚ A) characterizing the partially hydrogenated chain (see distances reported in Figure 2). Interestingly, the distances between the F atoms in the channel and the nearer O atoms on the ridges are quite similar to the C4 F2 H8 molecule, further confirming that repulsion is located outside the channel. These results indicate that a moderate content of F may determine a remarkable enhancement of the organic/surface interaction. The adsorption of the same molecule on the (1 1 1) surface was also studied: in this case, in agreement with the results found for the CH3 F species, both C4 F10 (structure not shown) and C4 F2 H8 (optimized structure in Figure 2E) show similarly weak interaction energies, of –13.8 and – 15.4 kcal/mol, respectively. Considering these last data we could speculate that the observed F-driven interaction enhancement is peculiar of the (1 0 0) surface. A further confirmation comes from the examination of the seven–carbon chain, which was adsorbed on the (1 0 0) surface only. Again, the adsorption of the molecule with three F atoms in the channel, approximately located on-top of O1 sites (optimized structure in Figure 2F), is remarkably enhanced with respect to the species containing only F or H atoms (optimized structure in Figures 2G-H, respectively). The interaction energies of C7 F16, C7 F3 H13 and C7 H16 are in fact –21.4, –58.0 and –18.6 kcal/mol, respectively. Furthermore, the analysis of the most characteristic bond distances (see values reported in the panels of Figure 2) is in line with the considerations on the C4 family: the fully hydrogenated chain is not stabilized by the pivotal F atoms in the channel, although its atoms can form quite short (weak) bonds with the oxide surface; on the other side, in the fully fluorinated molecule, the stabilizing effect of the pivotal three F atoms in the channel is compensated by the repulsion between the ridge oxygen atoms and the F atoms out of the channel, as confirmed by the F–O elongated distances. 13 ACS Paragon Plus Environment


ReaxFF Parametrization and Validation After investigating the organic/oxide interface from first-principles, we used DFT to derive a rich database of structures to build an apt training set (TS) aiming at an affordable parametrization of the organic/oxide interaction (both in the absence and in the presence of F atoms). In particular, we selected the eight configurations relative to the adsorption of CH4 and CH3 F on the (100) surface together with the configurations from the C4 and the C7 families discussed in the previous section, and for each optimized configuration we built an interaction curve by translating the organic species inward and outward along a direction perpendicular to the oxide surface. In such way we built a TS made by 120 geometries, which were used to predict at the DFT level the adhesion at the interface by applying equation (1). Starting from the full set of ReaxFF parameters available in the literature for ceria 42 and from the original parametrization of van Duin 40 for the alkane chains, we optimized only the parameters that are more relevant to describe the hybrid connections: H– Oox bond (5 parameters); F–Oox bond (5 parameters); H–Ceox bond (5 parameters); F–Ceox bond (5 parameters); H–Oox off–diagonal terms (4 parameters); F–Oox off–diagonal terms (4 parameters); H–Ceox off–diagonal terms (4 parameters); F–Ceox off–diagonal (4 parameters). The performances of our as derived reactive force field (FF) on the minima of the 14 curves used for the parametrization are reported in the third column of Table 2. The optimized values of the parameters are reported in Table 3. From the data reported in Table 2, it appears that the derived FF overestimates the adhesion energy of the CH4 molecule with the (100) oxide surface. On the other side, in the case of the CH3 F species, the FF fairly reproduces the interaction enhancement taking place in correspondence of the O1 adsorption site. The good agreement between the adhesion energies of the C4 and C7 families suggests that the overestimation observed for the CH4 species does not influence the force field prediction of the interaction in the longer chains. In order to verify the transferability of the derived FF outside the systems contained in 14 ACS Paragon Plus Environment

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Figure 3: Top and side views of the configurations investigated to validate the ReaxFF outside the TS: (a) C10 F0 H22 ; (b) C10 F4 H18 with 4 F atoms inside the channels; (c) C10 F4 H18 with 2 F atoms inside the channels; (d) C10 F4 H18 with 0 F atoms inside the channels; (e) C10 F22 H0 . O and Ce atoms are depicted as red and grey spheres, respectively; C, H and F atoms are depicted as yellow, white and green spheres, respectively.

the TS, we investigated several adsorption configurations of a slightly longer carbon chain (formed by 10 C atoms) on the (100) oxide surface at three different stoichiometries: pure–H, pure–F and an F content corresponding to about 18 % (4 F atoms on a total 22 lateral sites). The 5 configurations correspond to chains adsorbed above two neighboring (100) channels of the underneath oxide support; the DFT-optimized geometries are depicted in Figure 3 and the corresponding adhesion energies (predicted at both DFT and FF levels) are reported in Table 2: as it can be seen, the FF is able in accurately reproduce (both qualitatively and quantitatively) the adhesion enhancement when passing from 0 F atoms in the channels (configurations A, D and E) to 2 F atoms in the channels (configuration C) to 4 F atoms in the channel (configuration B). This result in very important from our point of view, as it is a quite strong verification of the transferability of the FF outside the TS and legitimates its use on systems untreatable at the DFT level, as it will be illustrated in the next section.



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Table 2: Interaction energies (in kcal/mol) calculated at the DFT level (second column) and at the ReaxFF level (third column) for a selection of systems contained in the TS.

System (inside the TS) CH4 – 100 surf – Ce site CH4 – 100 surf – H site CH4 – 100 surf – O1 site CH4 – 100 surf – O2 site CH3 F – 100 surf – Ce site CH3 F – 100 surf – H site CH3 F – 100 surf – O1 site CH3 F – 100 surf – O2 site C4 F10 – 100 surf C4 F2 H8 – 100 surf C4 H10 – 100 surf C7 F16 – 100 surf C7 F3 H13 – 100 surf C7 H16 – 100 surf System (outside the TS) C10 F0 H22 – 100 surf C10 F4 H18 – 100 surf - 4 F in the channels C10 F4 H18 – 100 surf - 2 F in the channels C10 F4 H18 – 100 surf - 0 F in the channels C10 F22 H0 – 100 surf

DFT value -2.3 -2.8 -3.9 -2.3 -7.1 -5.3 -18.9 -4.4 -18.5 -44.3 -15.7 -21.4 -58.0 -18.6 DFT value -21.5 -75.9 -52.0 -17.9 -28.1


FF prediction -7.2 -7.4 -7.5 -7.4 -6.1 -2.4 -20.4 -0.8 -27.0 -44.5 -14.4 -20.0 -54.1 -21.4 FF prediction -26.9 -69.7 -48.4 -23.1 -23.0

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Table 3: ReaxFF parameters describing the organic/oxide interaction. For more details on the physical meaning of the parameters, refer to Ref. 40

Parameter Value Parameter Value σ σ H–Oox De 3.4465 H–Ceox De 0.5466 H–Oox pbe1 0.4093 H–Ceox pbe1 0.6914 H–Oox pbe2 -0.0288 H–Ceox pbe2 1.0481 H–Oox pbo1 -0.4107 H–Ceox pbo1 -0.0324 H–Oox pbo2 4.7379 H–Ceox pbo2 2.9120 F–Oox Deσ 0.0380 F–Ceox Deσ 0.2344 F–Oox pbe1 0.1499 F–Ceox pbe1 0.0349 F–Oox pbe2 0.9594 F–Ceox pbe2 1.2205 F–Oox pbo1 -0.4031 F–Ceox pbo1 -0.0233 F–Oox pbo2 9.5093 F–Ceox pbo2 6.2714 H–Oox Ediss -0.1051 H–Ceox Ediss 1.6832 H–Oox Rvdw 1.1308 H–Ceox Rvdw 0.8506 H–Oox α 14.5156 H–Ceox α 8.3173 H–Oox σ rad 0.2287 H–Ceox σ rad 0.3481 F–Oox Ediss -0.1603 F–Ceox Ediss 7.6536 F–Oox Rvdw 1.6896 F–Ceox Rvdw 1.2627 F–Oox α 17.5281 F–Ceox α 11.7392 F–Oox σ rad 1.5415 F–Ceox σ rad 0.7054

Reactive MD The derived FF was then tested with a more complex system, consisting of a long single chain (C36 Fx H74−x ) adsorbed on the (1 0 0) surface. The adhesion energy between the substrate and the chain was calculated at each step by using equation (1). Oxide and chain energy were extracted from the geometry of each configuration by separating the fragments, but still keeping them in the same unit cell (in-cell approach). This operation was possible thanks to the large empty space perpendicular to the oxide surface (around 60 ˚ A). We performed two series of RMD simulations, corresponding to the two plots of Figure 4. The first series of simulations consisted in NVT dynamics carried out at very low temperature (about 10 K) by keeping the atoms of the oxide frozen in their crystalline positions (optimized in absence of adsorbate species). These simulations, see results in Figure 4-top, were performed in order to 17 ACS Paragon Plus Environment


Figure 4: RMD results reporting interaction energy as a function of simulation time (in picoseconds) for C36 Fx H74−x adsorbed on CeO2 (1 0 0) in the case of frozen support (upper panel) and without frozen support (lower panel).


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sample the minima of the organic/oxide Potential Energy Surface (PES) as a function of the chain stoichiometry; in practice, six different stoichiometries were considered by increasing the number of F atoms in the chain by groups of nine atoms: 0% corresponded to 0 F atoms, whereas 60% to 45 F atoms per chain. Pure F chain (100% F content) case was not considered as, when exceeding 60% stoichiometry, we observed a strong repulsion between the organic chains and the oxide surface. Moreover, for statistical purposes, for each stoichiometry considered, five different chains (both in terms of the location of the F–H substitution on the carbon skeleton and in the starting conformation of the overall chain) were built and investigated at the RMD level. As a consequence, each stoichiometry curve in both panels of Figure 4 has been obtained by averaging over five different interaction profiles. By looking at the results, we observe in Figure 4-top that at each stoichiometry a plateau region is reached: in the case of the pure H and mixed chains exceeding 48% in F content the interaction is weaker with respect to the low F content mixed chains, in agreement with the DFT trends observed for the smaller oligomers. Still, a difference between the pure H case and the other cases has to be highlighted: a pure polyethylene-like chain (total absence of F–substitution), even at very low temperature, is quite mobile on the surface, explores a variety of configurational states no atoms manage to be anchored to specific sites of the oxide channels (see Figure 5A). Instead, the mixed chains (not exceeding 60% in F content) approach the surface very quickly and remain entrapped into the channels of the (1 0 0) face via some pivotal F atoms as it can be noticed in Figure 5B, where it is displayed a configuration extracted from the simulation at 24% F content. After this quick entrapment, only small rearrangements of the adsorbate take place in the considered time scale. Due to this strong entrapment, the interaction energy remains constant at more negative values with respect to the pure H chain. The interaction is practically non-existent, or very small, in the case of F content exceeding 50%, while we observe a noticeable enhancement for a F content in the range 12–36%. As observed for the shorter oligomers by DFT calculations, 19 ACS Paragon Plus Environment


the reduction of the interaction at high F content is essentially due to an enhanced repulsive contribution between the negatively O ions of the surface and the F atoms of the chains. Moreover, in the investigated systems two peculiar features are commonly visible in the arrangement of the chains (Figure 5), namely the placement of several segments of the chain along the (1 0 0) channels of the oxide surface and the tendency of these conformations to strongly adhere to the surface. A second series of simulations for the same systems was performed by annealing the oligomers from 10 to 100 K still keeping the oxide frozen (in about 10 ps) and then leaving the oxide free to relax at a constant temperature of 100 K; the results of these RMD simulations are reported in the curves of the lower panel of Figure 4. When considering the pure H chain, relaxation of the substrate did not alter the landscape, due to the scarce organic/oxide interaction at this stoichiometry. On the other side, in the the case of 60% F content, the chain, heated by the higher temperature, was strongly repelled from the oxide and completely detached, resulting in practically zero interaction (black curve), as it happened also at very low temperatures for chains with a F content exceeding 60%. In the case of the intermediate stoichiometries, the results are not qualitative altered with respect to the simulations with a frozen oxide surface: we can observe an overall decrease of the interaction energy consequent to the relaxation at the interface (corresponding to an enhanced adhesion between the oxide and the organic chains) and still a minimum of the interaction energy in correspondence of a F content in the range 12–36%. The present analysis suggests an interaction mechanism which results from the competition between: (i) the stabilizing action played by the pivotal F atoms which succeed in penetrating in the channels present on the (100) surface; (ii) the repulsion between the F atoms which do not penetrate in the channels and the O atoms of the surface ridges. As already a low fraction of F atoms in wrong positions can produce a strong repulsive contribution to the interaction of a whole fragment of chain, the present results suggest that an enhanced interaction is 20 ACS Paragon Plus Environment

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realized by a moderate F content (around 25 %) and an appropriate arrangement of the F atoms on the chain backbone in order to match the structure of the (100) surface channels. By looking at the DFT results for the C7 F3 H13 chain, we observe that substitution of a F atom every 4 C atoms on the polymer backbone might favor the regular disposal of a linear chain fragment over the surface. Further computational investigations could be carried out in this sense in order to design an optimal configuration of the (H,F) mixed chains.

Conclusions The multi-scale protocol, combination of Density-Functional and Reactive Molecular Dynamics simulations, designed to study this hybrid organic/oxide system (fluorinated linear alkane chains and low index ceria surfaces) disclosed the most important features of the adsorption and the motion of the fluoro-polymer molecules on the selected ceria facets. It was found that, on the one hand, the pure hydrocarbon chains interact quite weakly with the oxide surface and are even slightly repelled in the case of total F substitutions; on the other hand, an intermediate content in F atoms (below 60%) is able to strongly anchor the chains to the surface by the F atoms displaced along the (100) surface channels. The largest interaction is found in correspondence to an F content of about 25%. These data suggest that the characteristics of ceria-based fluoro-polymer coatings, proposed as alternatives to epoxy-based commercial materials, can be effective modulated by tuning, for example, the fluorine content and also the chain design in terms of linear or branched structures. These studies could be extended to other more realistic and complex scenarios, where many other components, such as solvent and ionic species, could be taken into account, in order to predict how to improve the corrosion resistance of the coatings for selective applications. Considering the experimental difficulties to obtain detailed characterization of structure and dynamics at the interface between the two different materials investigated here, the mod-



Figure 5: Side views (left) and top views (right) of the adsorption configurations of (a) C36 H74 and (b) C36 F18 H56 adsorbed on the (1 0 0) surface. O and Ce atoms are red and grey spheres, respectively; C, H and F atoms are yellow, white and green spheres, respectively. The unit cell is a (8×8) model and only a portion of it is shown in these pictures.


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elling procedure becomes fundamental for a tuned design.

Acknowledgments S.M. is grateful to Adri C. T. van Duin for the stand-alone version of ReaxFF and his support. This work has been developed as part of the European project Nanodome that has received funding from the European Union’s Horizon 2020 Research and Innovation Programme, under Grant Agreement number 646121.



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Figure 6: TOC Graphic

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Dynamical and Structural Characterization of the Adsorption of

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