Abstract
Here, we show that metal oxide surfaces catalyze the formation of intermediate defluorinated tetrafluoroethylene (TFE) radicals, resulting in enhanced binding on the corresponding metal oxide surfaces. We attribute the preferential adsorption and radical formation of TFE on Cr2O3(0001) relative to TiO2(110) to the low oxygen coordination of Cr surface atoms. This hints at a possible dependence of the TFE binding strength to the surface stoichiometry of metal-oxide surfaces.
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Introduction
Being able to join dissimilar materials (cf., e.g., Refs.1,2,3,4 and references therein) is a key enabling technology to innovative and sustainable materials design for industrial applications. Some notable examples include: polymer-metal composites for bio-prosthetics and medical tools1,2; polymer-functionalized metal oxide surfaces for specialized applications3,4,5,6,7; polymers passivating metal oxide defects to increase carrier efficiency for better optoelectronic materials8,9; and polytetrafluoroethylene (PTFE) used as fluorine sources to form oxyfluoride surfaces and functionalize metal-oxides towards the realization of superconductors10. All of these applications fundamentally start with polymer adhesion on metal surfaces.
Two of the most commonly used metals for industrial applications are titanium and stainless steel, due to their notable physical properties, e.g., being lightweight and less susceptible to corrosion. In actual applications, these metals are exposed to oxidizing agents in the environment such as O2 or water vapor, hence, they still manifest a thin layer of metal oxide surface. For example, on stainless steel surfaces, a layer of Cr2O3 forms as a protective coating against further oxidation11. Similarly, TiO2 thin layers form on the surface of titanium, enhancing its biocompatibility for medical purposes12. Studies also show that the formation of thin metal oxide surfaces enhances binding to other metals and insulating polymers through welding or irradiation of the surface11,12,13,14,15. Here, we show the role of the reactivity of these thin metal oxide films to chemically bind with TFE.
In the following, we present results of our study on the adsorption of tetrafluoroethylene (TFE) on TiO2(110) and Cr2O3(0001). We found TiO2(110) inert and Cr2O3(0001) active to TFE (molecular) adsorption. This can be attributed to the nature of the surface metal atoms and the corresponding oxygen coordination. Furthermore, we found that defluorination of TFE promotes adsorption on both TiO2(110) and Cr2O3(0001). These results indicate the role of the surface as a catalyst to form intermediate TFE radicals and promote adsorption on metal-oxide surfaces. Thus, the possibility of joining dissimilar materials (in this case polymer and metal-oxide surface).
Results and discussions
Molecular Adsorption of TFE on TiO2(110) and Cr2O3(0001)
In Fig. 1, we see weak (ca. − 0.07 eV, Configuration 1) molecular adsorption of TFE monomer on TiO2(110) and strong (ca. − 1.38 eV, Configuration 1) adsorption on Cr2O3(0001). We find the adsorbed TFE retaining its planar structure, negligibly modified by TiO2(110). These results and observations could be compared with previous studies showing an inert TiO2 towards fluorination from PTFE forming surface oxyfluorides10. On the other hand, we find a relatively stronger binding for TFE adsorbed on Cr-terminated Cr2O3(0001), with the molecular plane tilted relative to the surface axis. We found that molecular adsorption of TFE on both metal oxide surfaces does not result in any significant relaxation of the surface. However, the difference in the adsorption energy could be attributed to the difference in the surface oxygen (O)-coordination of the surface metal atoms (Ti and Cr).
In Fig. 2, by inspection, we see that the surface Ti on TiO2(110) have higher O-coordination than the surface Cr on Cr2O3(0001). We attribute the difference in surface reactivity, i.e., adsorption preference, to the difference in surface metal–oxygen ratio. We define this ratio as the number of low coordinated surface metal ions to the fractional number of oxygen atoms bound to it, i.e., 3:7 for TiO2(110) and 1:1 for Cr2O3(0001). To verify this, we have added an additional Cr termination on the surface of Cr2O3(0001) (4:3 Cr to O ratio) and found a stronger adsorption of TFE with a pronounced non-planar geometry. As expected, we can enhance TFE adsorption on TiO2(110) by introducing oxygen vacancies (cf., e.g., Refs.16,17,18, and references therein).
It requires energy to break the C–F bond of TFE and, in Fig. 1, we see an endothermic dissociative adsorption of TFE (i.e., Configuration 2, with dissociated C–F bond) on both TiO2(110) and Cr2O3(0001), with respect to the molecular state (Configuration 1). However, upon surface relaxation, the total energy lowers, resulting in a rather exothermic adsorption for C2F3 + F on both oxide surfaces (cf., Fig. 1). As mentioned earlier, such surface relaxations are negligible in TFE molecular adsorption. The binding of C2F3 on surface O atom and the binding of F on surface metal atom (Ti and Cr) resulted in an upward (coordinate) shift of the interacting surface atoms. By comparison, we can see a greater upward shift of Cr and O towards the vacuum for Cr2O3, whereas a relatively smaller relaxation on TiO2 upon adsorption of the defluorinated TFE (cf., Fig. 3). (Note that the energies from Configuration 0 to 2 on both TiO2(110) and Cr2O3(0001) lowers after considering van der Waals correction (vdW-DFT-D2) in the calculation, as it is expected. Still, the energy trend remains (stronger binding on Cr2O3 than on TiO2).
The relative energy plots suggest that the presence of the metal oxide surfaces lowered the energy needed to break the TFE C–F bond. Note that it requires 5.3 eV to dissociate one F from TFE in vacuum. To explore the possibility of a lowered TFE C–F bond dissociation barrier in the presence of metal-oxides, we implemented a simple dissociation model of TFE using the molecular counterpart of the metal-oxide surfaces. In Fig. 4, we show the calculated potential barriers from the molecular TFE state to the dissociated TFE state on Cr2O3 (ca. 1.39 eV) and TiO2 (ca. 2.16 eV). It can be seen from the simple molecular model that C–F dissociation energy lowers in the presence of metal-oxides. These results indicate the role of the surface as a catalyst to form intermediate defluorinated TFE radicals. In the following, we focus on the adsorption of defluorinated radicals of TFE on Cr2O3 and TiO2 surfaces.
Adsorption of defluorinated radicals of TFE on TiO2(110) and Cr2O3(0001)
Upon defluorination (cf., e.g., Fig. 1, Configuration 2), the C2F3 creates a new bond with surface O atoms and the dissociated F atom adsorbs atop the adjacent transition metal atom. We also see a relatively more stable adsorption on Cr2O3(0001) than on TiO2(110). This results in a higher charge population around the carbon end of C2F3 on Cr2O3(0001) than on TiO2(110) (cf., Fig. 5). The relatively higher accumulation of charge from Cr2O3 (0.2 e higher) results in a longer C=C bond length (shown in Fig. 3) as compared to that on TiO2. From the corresponding charge density difference distribution (cf., Fig. 6) electron contribution comes from both surface (oxygen and metal) atoms. We see a more pronounced participation of Cr in TFE radical bonding as compared to Ti shown by the charge gain region (yellow region) between C and Cr surface atom. By plotting the projected density of states (PDOS), after TFE radical bonding, we show a strong hybridization of the C p states with the d electrons of Cr. This is less evident in the case of TiO2 where hybridization is mainly through the surface oxygen atom. As mentioned in the previous section, the surface metal–oxygen ratio influences metal-oxide surface reactivity towards TFE adsorption. From the TiO2(110) geometry, we find the first Ti layer completely enclosed by the octahedral cage of O, resulting in a low surface Ti–O ratio. This accounts for the weak interaction of surface Ti towards C2F3. Next, we show in Table 1 the corresponding adsorption energies of CF, CF2, CF3, CF4, C2F, C2F2, C2F3 on TiO2(110) and Cr2O3(0001). In general, defluorinated TFE radicals with intact C=C bond show stronger adsorption, and preference for adsorption on Cr2O3(0001). We also show that in most cases, radicals with low fluorine content manifest stronger binding on the oxide surfaces. These results indicate that chemical adsorption of the TFE monomer starts with defluorination and adsorption with an intact C=C.
Summary and conclusion
In summary, we have shown that defluorination is necessary to increase chemical bonding between tetrafluoroethylene (TFE) on TiO2(110) and Cr2O3(0001). The metal oxide surface catalyzes defluorination, resulting in the formation of intermediate radicals that bind strongly to the corresponding metal oxide surfaces. As expected, the reactivity of the corresponding metal oxide surfaces depends on the oxygen coordination of metal surface atoms. The surface Cr on Cr2O3(0001) has a lower fractional oxygen coordination as compared to the surface Ti on TiO2(110). As a result, we find stronger bonding of TFE on Cr2O3(0001) than on TiO2(110). This also indicates that introducing oxygen vacancies (cf., e.g., Ref.16,17,18, and reference therein), and non-ionizing radiations (cf., e.g., Ref.19 and references therein) to form intermediate radicals could promote binding of polymers to metals. These results should provide insights for better materials design, specifically towards polymer adhesion on metal-oxide surfaces.
Computational method
To study the adsorption of TFE and its fragments on TiO2(110) and Cr2O3(0001), we performed density functional theory20,21 (DFT)-based total energy calculations22,23,24,25,26 using projector augmented wave (PAW) formalism and plane wave basis set (cutoff energy of 550 eV), and Perdew–Burke–Enzerhof (PBE) generalized gradient (GGA) exchange correlation functionals27,28. We adopt the Monkhorst and Pack method to perform the Brillouin zone integrations, with (9 × 9 × 1) special k-points29. To model TiO2(110) and Cr2O3(0001), we used periodically repeated slabs of (2 × 1) and (1 × 1) surface unit cells, respectively, separated by 15 Å thick vacuum region along the surface normal. The lattice constant obtained upon structural optimization for Cr2O3(0001) is 5.03 Å and the lattice constants for TiO2(110) are 2.97 Å and 6.59 Å. These structural geometries are in good agreement with experimental and theoretical studies30,31,32. Each slab consists of 2 layers (7 atomic planes) of O-Ti–O and Cr-O3-Cr. In the case of Cr2O3, we used a Cr terminated surface as it was found to be more stable than other terminations30. We performed geometric optimization considering energy convergence of less than 10−5 eV and residual forces below 0.01 eV/Å. For the molecular and dissociated adsorption of TFE we implemented both frozen and relaxed surface calculations. We implemented van der Waals correction using DFT-D2 incorporated in the VASP code.
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Acknowledgements
This work is supported in part by MEXT Grants-in-Aid for Scientific Research (JP20H02638, JP20K21171, JP17H01057) and METI Strategic Basic Technology Support Project (2018). Some of the numerical calculations presented here were done using the computer facilities at the following institutes: CMC (Osaka University), ISSP, KEK, NIFS, and YITP. Structures are drawn using the VESTA package (ver.3.5.7)33.
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K.W., H.N., T.K., and W.A.D. conceived the calculations. J.S.G. and N.E.A.S. performed the DFT calculations. J.S.G. and W.A.D. drafted the manuscript. All Authors contributed to the analyses of results and review of the manuscript.
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Gueriba, J.S., Salehuddin, N.E.A., Diño, W.A. et al. Defluorination and adsorption of tetrafluoroethylene (TFE) on TiO2(110) and Cr2O3(0001). Sci Rep 11, 21551 (2021). https://doi.org/10.1038/s41598-021-00952-w
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DOI: https://doi.org/10.1038/s41598-021-00952-w
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