Defluorination and adsorption of tetrafluoroethylene (TFE) on TiO2(110) and Cr2O3(0001)

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.


Scientific Reports
| (2021) 11:21551 | https://doi.org/10.1038/s41598-021-00952-w www.nature.com/scientificreports/ 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 TiO 2 (110) have higher O-coordination than the surface Cr on Cr 2 O 3 (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 TiO 2 (110) and 1:1 for Cr 2 O 3 (0001). To verify this, we have added an additional Cr termination on the surface of Cr 2 O 3 (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 TiO 2 (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 TiO 2 (110) and Cr 2 O 3 (0001), with respect to the molecular state (Configuration 1). However, upon surface relaxation, the total energy lowers, resulting in a rather exothermic adsorption for C 2 F 3 + F on both oxide surfaces (cf., Fig. 1). As mentioned earlier, such surface relaxations are negligible in TFE molecular adsorption. The binding of C 2 F 3 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 Cr 2 O 3 , whereas a relatively smaller relaxation on TiO 2 upon adsorption of the defluorinated TFE (cf., Fig. 3). (Note that the energies from Configuration 0 to 2 on both TiO 2 (110) and Cr 2 O 3 (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 Cr 2 O 3 than on TiO 2 ).
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 Cr 2 O 3 (ca. 1.39 eV) and TiO 2 (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 Cr 2 O 3 and TiO 2 surfaces.

Adsorption of defluorinated radicals of TFE on TiO 2 (110) and Cr 2 O 3 (0001). Upon defluorination
(cf., e.g., Fig. 1, Configuration 2), the C 2 F 3 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 Cr 2 O 3 (0001) than on TiO 2 (110). This results in a higher charge population around the carbon end of C 2 F 3 on Cr 2 O 3 (0001) than on TiO 2 (110) (cf., Fig. 5). The relatively higher accumulation of charge from Cr 2 O 3 (0.2 e higher) results in a longer C=C bond length (shown in Fig. 3) as compared to that on TiO 2 . 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 TiO 2 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 adsorp-    . 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 TiO 2 (110) and Cr 2 O 3 (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 Cr 2 O 3 (0001) has a lower fractional oxygen coordination as compared to the surface Ti on TiO 2 (110). As a result, we find stronger bonding of TFE on Cr 2 O 3 (0001) than on TiO 2 (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 27,28 . We adopt the Monkhorst and Pack method to perform the Brillouin zone integrations, with (9 × 9 × 1) special k-points 29 . To model TiO 2 (110) and Cr 2 O 3 (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 Cr 2 O 3 (0001) is 5.03 Å and the lattice constants for TiO 2 (110) are 2.97 Å and 6.59 Å. These structural geometries are in good agreement with experimental and theoretical studies [30][31][32] . Each slab consists of 2 layers (7 atomic planes) of O-Ti-O and Cr-O 3 -Cr. In the case of Cr 2 O 3 , we used a Cr terminated surface as it was found to be more stable than other terminations 30 . 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.   www.nature.com/scientificreports/ Publisher's note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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