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First-Principles Calculations of Sarin Adsorption on Anatase Surfaces Nam Q. Le, Chinedu E. Ekuma, Brett I Dunlap, and Daniel Gunlycke J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11509 • Publication Date (Web): 09 Jan 2018 Downloaded from http://pubs.acs.org on January 13, 2018

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The Journal of Physical Chemistry

First-Principles Calculations of Sarin Adsorption on Anatase Surfaces Nam Q. Le,∗,† Chinedu E. Ekuma,† Brett I. Dunlap,‡ and Daniel Gunlycke‡ †NRC Postdoctoral Associate ‡U.S. Naval Research Laboratory, Code 6189, Washington D.C. 20375 E-mail: [email protected]

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Abstract We report density functional theory calculations investigating the adsorption of the organophosphate nerve agent sarin (GB) on clean (101), (001)-(1×4), and (001)-(1×1) surfaces of anatase titanium dioxide (TiO2 ). Our calculations show that GB chemisorbs on all three surfaces by the formation of a dative bond between the phosphoryl oxygen and a five-coordinated titanium atom in the surface. The adsorption of GB on the (001)-(1 × 4) and (001)-(1 × 1) surfaces (−45.1 kcal mol−1 and −34.8 kcal mol−1 ) is substantially stronger than on the (101) surface (−18.2 kcal mol−1 ). This could be a result of reactive surface states observed within the TiO2 band gap at the (001) surfaces but not the (101) surface. Our calculations show that the GB adsorption passivates these surface states. GB adsorption also breaks a bridging oxygen bond on both (001) surfaces, leading to a titanyl group that is also predicted to occur in adsorption of the simulant dimethyl methylphosphonate (DMMP) on anatase (001). The ordering of the three anatase surfaces by strength of GB adsorption is the same as predicted for DMMP, while the GB adsorption is predicted to be weaker than DMMP adsorption by 8 kcal mol−1 on the (001)-(1 × 4) surface and by 3 kcal mol−1 on the (101) and (001)-(1 × 1) surfaces.

Introduction The controlled destruction of existing stockpiles of chemical warfare agents (CWAs), the detection of CWAs to prevent or respond to their deployment, and the decontamination of surfaces following deployment are important challenges motivating active research. 1,2 Among CWAs, organophosphate nerve agents are particularly potent. 1 One promising candidate material for decomposing CWAs by heterogeneous catalysis is TiO2 , given its high photoactivity, high stability, and low cost. 1,3 General trends have been identified in the interactions between organophosphates and metal oxides, often in experiments involving the adsorption and decomposition of dimethyl methylphosphonate (DMMP), 4–12 which is widely used as a simulant. There are fewer experimental studies of the adsorption and decomposition of the live nerve agents themselves, such as GB 11,13–16 and 2


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GD. 16–18 Experiments do suggest commonalities, such as a preference for adsorption between the phosphoryl (P=O) oxygen and either undercoordinated metal ions or surface hydroxyl groups. 1,2 The primary evidence is the red shift of the GB P=O stretching mode from 1255–1317 cm−1 in the gas and liquid phases to, e.g., around 1210–1234 cm−1 when adsorbed on nanocrystalline TiO2 . 8,13,19 Experiments also show, however, that details of the adsorption and decomposition processes differ among CWAs and their simulants despite their similar structure. In one case, GB was found to adsorb at a higher concentration and decompose at a higher rate than DMMP on the same TiO2 photocatalyst. 11 On amorphous SiO2 , however, DMMP has been found to adsorb more strongly than GB in both temperature-programmed desorption (TPD) 12,15 and inverse gas chromatography (IGC) experiments. 20 The atomic-scale characteristics of the adsorbate and adsorbent that are responsible for these macroscopic differences are not clear. Theoretical methods are well suited to evaluate possible explanations, especially given the difficulty of performing experiments involving CWAs. We are not aware of any computational work investigating the adsorption of GB on anatase surfaces. There are several papers reporting calculations of GB interacting with other molecules or finite clusters: solvation in H2 O 21,22 and adsorption to clusters representing surfaces of alumina, 23,24 silica, 15 Al-/Si-based clays, 25 and two-dimensional surfaces of graphene, CaO, and MgO. 26 Finite cluster models, however, do not reproduce distinct bulk states and surface states that would be present in crystallites with sizes on the order of tens of nanometers in typical nanocrystalline TiO2 for catalytic applications. 27 Calculations involving extended surfaces enable insight into interactions between bulk and surface electronic states that accompany adsorption. In this work, we used periodic density functional theory (DFT) calculations to calculate geometries and energies of GB adsorption on the dominant surfaces of anatase. First, we obtained optimized models of the isolated GB molecule and anatase (101), (001)-(1 × 4), and (001)-(1 × 1) surfaces and verified that their predicted structural and electronic properties were consistent with prior experimental and theoretical work. These models were then used to perform optimization

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calculations for the adsorbed GB/(101), GB/(001)-(1 × 4), and GB/(001)-(1 × 1) systems, producing geometries and energies of adsorption. As expected, the weakest adsorption was observed at the (101) surface, which is most stable. We calculated stronger adsorption on the (001) surfaces, and furthermore, stronger adsorption on the reconstructed (001)-(1 × 4) surface than on the bulkterminated (001)-(1 × 1) surface. In light of the common use of DMMP as a nerve agent simulant, we also present calculations of DMMP adsorption on the same three surfaces for direct comparison. To help understand the trends in adsorption energies in terms of underlying electronic effects, we compared the projected densities of states of the isolated and bound systems. The (101) surface exhibits no surface states in the gap, while both the reconstructed and unreconstructed (001) surfaces exhibit surface states that become passivated upon adsorption of GB, which was correlated with much stronger adsorption.

Computational Methods Periodic DFT calculations were performed using Quantum Espresso (QE) 28 with the Perdew, Burke, and Ernzerhof (PBE) functional 29 within the generalized gradient approximation. Valence– core electron interactions were modeled using ultrasoft pseudopotentials. 30 A kinetic energy cutoff of 60 Ry was used for the plane wave expansion of wavefunctions, and a cutoff of 600 Ry was used for the electronic density. Properties of bulk anatase were found to be converged in calculations using the conventional tetragonal unit cell with a Γ-centered, uniform 12 × 12 × 12 grid of k-points sampling the first Brillouin zone. The Broyden–Fletcher–Goldfarb–Shanno algorithm was used to optimize nuclear coordinates until all force components converged to within 10−3 atomic units and the total energy converged to within 10−4 atomic units. The optimized bulk crystal has lattice ˚ and c = 9.690 A ˚ and an indirect band gap of 2.1 eV, in agreement with parameters of a = 3.799 A prior calculations using PBE. 31,32 This underestimates the experimental band gap of about 3.2 eV owing to the well-known self-interaction error in DFT. 33 This error, however, has only a small effect on geometries and energies. 32,33

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The bulk crystal parameters were used to construct models of the most commonly observed anatase surfaces: (101), (001)-(1 × 4), and (001)-(1 × 1), in order of decreasing stability. The (101) surface of anatase is highly stable and is not known experimentally or predicted theoretically to reconstruct. 3,33 The (001) surface exhibits a (1 × 4) reconstruction when annealed 34 or grown at high temperature, 35 and may occur in nanocrystalline TiO2 samples for catalysis, which are generally heat treated at 400 ◦ C and above. 36 The unreconstructed (001)-(1 × 1) surface is also relevant for catalytic applications because it is suspected to be more reactive than the more stable surfaces, and techniques to stabilize it are under development. 37,38 Therefore, herein, we model adsorption on all three surfaces. The slab models were developed by fixing the cell dimensions in the plane of the surface to ˚ of vacuum in the z dibe consistent with the optimized bulk lattice parameters and adding 20 A rection. Cell dimensions were held fixed while the atomic coordinates were optimized, leading to surface relaxations consistent with prior modeling of the clean surfaces using PBE. 32 Sampling of the Brillouin zone of the supercells maintained a density of k points in the plane of the surface consistent with the converged bulk calculations and only the Γ point normal to the surface (the z direction). For example, calculations of a 4 × 4 × 1 supercell model of the anatase (001) surfaces used a 3 × 3 × 1 k-point sampling. The resulting surface energies, σ , were calculated as the difference between the total energy of the relaxed slab per unit area and the total energy of an equivalent amount of the optimized bulk anatase crystal. Calculations were performed to evaluate the effects of slab thickness on both the surface energy and the electronic structure. Based on the results of these tests, GB adsorption on the (101) and (001) anatase surfaces was modeled in this work using slabs comprising 4 × 2 × 2 and 4 × 4 × 1 unit cells, respectively, each slab being four O–Ti–O trilayers thick. Each unreconstructed model thus included 192 TiO2 atoms, while the (001)-(1 × 4) model included 204 TiO2 atoms due to ˚ × the added row of TiO2 . 39 The dimensions of the (101) and (001) supercells were (15.192 A ˚ × 27.074 A) ˚ and (15.192 A ˚ × 15.192 A ˚ × 29.688 A), ˚ respectively, including 20 A ˚ of 20.816 A vacuum in the z-direction. A dipole correction was used to decouple interactions between periodic

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images of the slabs. The corresponding surface concentrations of adsorbate are 0.53 µmol m−2 and 0.72 µmol m−2 . To mitigate size effects related to the finite thickness of the slabs, we incorporated 32 pseudohydrogen atoms (PHs) on the bottom surface of each slab to produce bulk-like coordination of the Ti and O atoms. The PH terminations were developed following a procedure used previously for ˚ below the rutile (110) surface. 40 PHs of +2/3 and +4/3 nuclear charge were initially placed 1 A two-coordinated O atoms (O2c ) and five-coordinated Ti atoms (Ti5c ), respectively. Pseudopotentials for the PHs were generated using the LD1 code in the QE software suite. Their positions were first optimized while all TiO2 atoms were fixed in their bulk positions. The positions of the PHs and the bottom trilayer of TiO2 atoms were then fixed in all subsequent optimizations. Examining the electronic structure of the final relaxed slabs confirmed the passivation of states at with the bottom surface as demonstrated previously for rutile (110), 40 which was found necessary to avoid coupling between the top and bottom surface states that could inadvertently affect the adsorption process. Optimized geometries of the isolated GB (C4 H10 FO2 P) and DMMP (C3 H9 O3 P) molecules and of the bound systems with the anatase surfaces were subsequently calculated using the same cell dimensions as the slab models. The adsorption energy of each bound system was calculated as Eads = Emol/surf − (Emol + Esurf ), where Emol/surf is the total energy of the bound system and Emol and Esurf are the respective energies of the isolated molecule and the clean surface. From the self-consistent field calculations for each optimized system, we calculated the density of states (DOS) by applying a Gaussian broadening of half width 0.05 eV to the Kohn–Sham eigenvalues. The DOS were compared to gain further insight into the differences between the geometries and energies of adsorption among the three anatase surfaces.

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Results and Discussion Isolated GB and anatase surfaces. Structural and electronic properties of the isolated GB molecule were reproduced first. The optimized geometry of the GB molecule is shown in Fig. 1(a) and agrees closely with that obtained recently using MP2 with an aug-cc-pVTZ basis set. 19 The P atom is a chiral center, and the R enantiomer is shown. Because all of the anatase surfaces investigated in this work exhibit at least one (010) mirror plane, even when relaxed and reconstructed, 32 they are achiral, and hence the adsorption calculations using only the R enantiomer are sufficient to represent equivalent interactions with the S enantiomer as well. 41,42 The calculated density of states (DOS) of the optimized molecule is shown in gray in Fig. 1(b). The partial DOS (PDOS) from individual atomic sites were also analyzed. The PDOS of the phosphoryl oxygen is shown in Fig. 1(b) in orange, and that of the isopropoxy (P–O–C) oxygen is shown in magenta. We performed a natural bond orbital analysis, which showed that contributions from these two oxygen atoms dominate the frontier orbitals; the HOMO is a phosphoryl oxygen lone pair, and the LUMO is an isopropoxy C–O antibonding state (not shown). The predicted HOMO–LUMO gap is 6.1 eV, much larger than the predicted band gap in TiO2 . These observations are consistent with the expectation from experiments and theory that adsorption of GB on surfaces occurs preferentially through the phosphoryl group. The optimized slab models of the (101), (001)-(1 × 4), and (001)-(1 × 1) surfaces with PHs are shown in Fig. 1(c)–(e). The corresponding computed surface energies are σ(101) = 0.45 J m−2 , σ(001)-(1 × 4) = 0.52 J m−2 , and σ(001)-(1 × 1) = 0.91 J m−2 , which are quantitatively consistent with prior theoretical predictions 31,32,39 and qualitatively consistent with experimental observations that the (101) surface is more stable than (001). 3 All surfaces exhibit O2c and Ti5c sites, and the reconstructed (001)-(1 × 4) surface also exhibits Ti4c sites along the crest of the added row; 39 these undercoordinated sites are critical in adsorption. The surface relaxations of the three slab models agree closely with those predicted in past

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work, 31,32 and selected bond lengths and interlayer distances are shown in Fig. 1(c)–(e). Examples of apical (equatorial) Ti–O bonds are encircled by blue (green) dashed lines, whose lengths in ˚ (2.00 A). ˚ The (101) and (001)-(1 × 4) surfaces feature the optimized bulk structure are 1.95 A ˚ corresponding to a contraction symmetrical bridging Ti–O–Ti bonds with lengths of 1.83–1.86 A, compared to the bulk. In contrast, the Ti–O–Ti bonds are strongly asymmetrical along the raised ridge of the (001)-(1 × 4) surface and on the unreconstructed (001)-(1 × 1) surface. Within each ˚ and the other is contracted (1.74–1.82 A) ˚ compared pair, one bond is elongated (2.15–2.23 A) ˚ The weaker, elongated bonds at the (001) surfaces are those to bulk bond lengths (1.95–2.00 A). observed to break in the adsorbed structures. The electronic structure of the clean surfaces was analyzed after optimization and compared with the bulk. In all cases, the highest occupied eigenstates are dominated by O 2p character, while the lowest unoccupied eigenstates are dominated by Ti 3d character, as expected for TiO2 . 3 The DOS of the relaxed (101) slab differs only slightly from that of the bulk and exhibits no states in the gap. In contrast, the relaxed (001) slabs both exhibit surface states in the gap just above the valence band maximum (VBM). The presence of states just above the VBM at the (001) surface but not the (101) surface is consistent with prior theoretical work. 32 To examine the surface states more closely, the band structure of a 1 × 1 × 4 (labeled z = 4) slab model of the (001)-(1 × 1) surface with no PHs is shown in Fig. 2(a), at which point the electronic structure is converged with respect to the slab thickness. However, a 4 × 4 × 4 slab would be too large to perform optimization calculations with the present methods. Therefore, it serves as a reference for the thinner 4 × 4 × 1 slab with PHs that was used for the adsorption calculations, for which the band structure is shown in Fig. 2(b). In both slab models, bands corresponding to surface states are clearly visible in addition to the bulk-like states. Furthermore, the bands in the converged z = 4 slab are reasonably reproduced in the z = 1 slab, particularly the bands corresponding to the surface states. The bulk states near the Γ point are most poorly reproduced by the thin slab model. The total DOS of the two slabs are compared to the bulk in Fig. 2(c), showing the surface states extending about 0.3 eV above the VBM. These states are dominated by 2p character from the

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surface bridging O2c atoms. Overall, we conclude that the slab models with PHs in Fig. 1(c)–(e) reasonably reproduce both the structural and electronic properties of the anatase surfaces.

GB and DMMP adsorption. The most stable atomic structures for GB adsorbed on the (101), (001)-(1 × 4), and (001)-(1 × 1) anatase surfaces are shown in Fig. 3(a)–(c). Of the three corresponding calculations with DMMP, the optimized adsorption on the (001)-(1 × 1) surface is shown in Fig. 3(d). Atomic coordinates for all six bound systems are provided in the Supporting Information. Limited by the computational expense of these calculations, initial geometries were constrained to those in which the P=O group is directed toward the anatase surface, which has been observed experimentally 4,6,8 and predicted theoretically 43–46 to be preferred for the adsorption of organophosphates on metal oxides. Within this constraint, optimizations were tested based on initial geometries with the adsorbate molecule ˚ to 3.7 A ˚ above the site. above either a surface Ti or O site and with the phosphoryl O from 2.7 A For all surfaces, the strongest interaction occurs by the formation of a dative bond between the phosphoryl O, which carries a lone pair of electrons and acts as a donor, and a Ti5c in the surface that acts as an electron acceptor. The bound structures are also stabilized by weak hydrogen bonds ˚ to 2.8 A, ˚ which are consistent with expectations for hydrogen with lengths in the range of 2.2 A bonding involving methyl groups. 47 In comparing the bound structures, we observe two primary features. First, the length of the (P=O)–Ti dative bond is significantly shorter in the GB/(001)-(1 × 4) and GB/(001)-(1 × 1) struc˚ compared to 2.17 A), ˚ consistent with a stronger tures than the GB/(101) structure (1.98 and 2.02 A interaction. The differences in dative bond strength appear to dominate smaller differences in hydrogen bond strength, in which we do not observe systematic patterns among the surfaces. Variations in the adsorption energy due to changes in hydrogen bonding were found previously to be within 2 kcal mol−1 in DMMP/anatase binding. 44 Second, at the (001) surfaces, which both exhibit strongly asymmetrical bridging Ti5c –O2c bond lengths when isolated, we also observe the breaking of a long Ti–O bridging bond and the contraction of an adjacent short Ti–O bond from 9



˚ to 1.64–1.65 A. ˚ The dangling oxygen is then stabilized by hydrogen bonding with GB; 1.74–1.82 A DMMP also forms a very similar structure. This corroborates the structure reported by Bermudez modeling DMMP adsorption on the anatase (001) surface using the B3LYP functional, 44 referring to the shortened bond as a double-bonded titanyl group (Ti=O). The adsorption energies for GB are reported in the first three rows Table 1, and they correlate well with the structural differences observed among the optimized geometries. The magnitudes of all calculated adsorption energies are characteristic of chemisorption. The GB adsorption is weakest on the (101) surface (−18.2 kcal mol−1 ), which is consistent with its being the most stable surface. Adsorption is much stronger on both the reconstructed (001)-(1 × 4) surface (−45.1 kcal mol−1 ) and the unreconstructed (001) surface (−34.8 kcal mol−1 ). Although the reconstructed (001)-(1 × 4) surface is more stable, it also exhibits the stronger adsorption, which we attribute in terms of atomic structure to the presence of reactive Ti4c sites along the crest of the added row. Following the energies for GB adsorption in Table 1, we also report the calculated energies for DMMP adsorption and compare them with calculations by Bermudez 43,44 that used different theoretical methods. The M06-2X//PBE adsorption energy reported for the adsorption of DMMP on (001)-(1 × 4) appears to be anomalously weak, possibly owing to the used of different functionals for the geometry optimization and energy calculations. Otherwise, the present PBE//PBE calculations agree with the B3LYP//B3LYP results within 4.6 kcal mol−1 or better, significantly less than the predicted differences in adsorption energy among the three surfaces. The agreement lends confidence to comparisons between the calculated adsorption energies for GB and DMMP. We observe that the anatase surfaces exhibit the same ordering with respect to adsorption energy of GB and DMMP, while GB adsorbs more weakly than does DMMP to all three anatase surfaces. The difference is greatest for the (001)-(1 × 4) surface, at which the GB adsorption is weaker by about 8 kcal mol−1 . For the other two surfaces, the GB adsorption was found to be weaker by about 3 kcal mol−1 . To our knowledge, the experimental measurements available for the closest comparison are

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Table 1: Calculated adsorption energies Eads of GB and DMMP bound to anatase surfaces with comparisons to theoretical and experimental results for similar systems. All values are in kcal mol−1 .

GB / anatase (101) GB / anatase (001)-(1 × 4) GB / anatase (001)-(1 × 1) DMMP / anatase (101) ” DMMP / anatase (001)-(1 × 4) ” ” DMMP / anatase (001)-(1 × 1) ” GB / AlO(OH)3 cluster ” GB / (Al2 O3 )n clusters ” GB / (SiO2 /SiOH) DMMP / (SiO2 /SiOH) GB / (SiO2 /SiOH) DMMP / (SiO2 /SiOH)

Eads −18.2 −45.1 −34.8 −21.5 −20.0 −53.9 −58.4 −34.5 −37.9 −42.5 −48.5 −53.0 −51.5 −49.0 −12.0 ± 0.1 −13.0 ± 0.1 −18.8 ± 5.5 −26.1 ± 1.5

Ref. This work ” ” This work [43] This work [44] ” This work [44] [25] ” [23] ” [15] [12] [20] [20]

Comment PBE//PBE (theoretical) ” ” PBE//PBE PBE//PBE PBE//PBE B3LYP//B3LYP M06-2X//PBE PBE//PBE B3LYP//B3LYP MP2//B3LYP B3LYP//B3LYP PBE//LDA; n = 4 B3LYP//B3LYP; n = 10 TPD (experimental) TPD IGC IGC

those comparing GB and DMMP adsorption on hydroxylated silica. DMMP has been found to bind more strongly than GB by about 1.0 kcal mol−1 using TPD 12,15 and by about 7.3 kcal mol−1 using IGC. 20 Although a quantitative comparison with experiment requires accounting for additional contributions to enthalpy, we observe empirically that predictions with 20 and without 46 these corrections have produced the same qualitative conclusion for SiO2 , and our results suggest that DMMP similarly binds more strongly than GB on anatase.

Effects on electronic structure. In this section, we further investigate the differences in GB adsorption among the anatase surfaces in the context of their electronic structure. First, the DOS of the clean anatase (101) surface and the bound GB/(101) system are compared in Fig. 4. In each panel, the PDOS from selected groups 11



of atoms are shown in color. The total DOS of the clean (101) slab [Fig. 4(a), gray] exhibits the same band gap of 2.1 eV as the bulk material, indicating that no states are present in the gap. This is confirmed by examining the respective PDOS contributions from the 112 subsurface oxygens (red) with the contributions from the 16 bridging oxygens on the surface (cyan). Any surface states would be dominated by contributions from the bridging oxygens, 3 but the their PDOS is instead spread broadly in energy within the bulk-like valence band (VB). The PDOS of the titanium sites (blue) dominates the conduction band (CB), which is also essentially the same as in the bulk DOS. The total DOS of the bound GB/(101) system [Fig. 4(b), gray] exhibits the same features as the clean (101) surface. Based on the DOS of the isolated GB molecule [Fig. 1(b)], we expect the oxygen 2p states to interact most strongly with the electronic states of anatase. Examining contributions from all of the GB atoms confirms this. Among the molecular states, only the PDOS of the phosphoryl O (orange) and the isopropoxy O (magenta), marked by arrows, have an appreciable contribution near the VB and CB edges. They are strongly hybridized with the O 2p states that form the valence band of the anatase surface. The magnitude of the contributions from these single atoms is scaled in Fig. 4(b) to facilitate comparison with that of the 16 surface O atoms (cyan). The electronic states of the TiO2 slab still strongly resemble those of both the clean (101) surface and the bulk, except near the conduction band minimum (CBM), which exhibits a slight upward shift and a peak in the energy distribution. Examination of the PDOS reveals that the features near the CBM are contributed roughly uniformly from Ti sites throughout the slab (e.g., summed contributions from Ti sites in the upper and lower halves of the slab are shown in solid and dashed green). We therefore attribute the feature to the (101) slab being thinner, and hence exhibiting a less realistic strain response to adsorption, compared to the (001) slabs with equal numbers of monolayers, which do not exhibit this effect on the DOS upon adsorption. Overall, we infer that the relatively low adsorption energy of the GB/(101) system is directly related to the electronic stability of the (101) surface. We find more differences in comparing the DOS of the clean and GB-adsorbed anatase (001)(1 × 4) surfaces in Fig. 5. In the clean surface [Fig. 5(a)], the PDOS of the bridging oxygens along

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the raised ridge (green) is not aligned with that of the bulk-like subsurface oxygens (red). Instead, they exhibit energies in the gap, just above the VBM, reducing the effective gap to 1.9 eV from the bulk value of 2.1 eV. However, in the DOS of the bound GB/(001)-(1 × 4) system [Fig. 5(b)], the contribution from those oxygens is passivated. A significant difference is the presence of states near the VBM contributed from the titanyl O (black). Similar contributions were predicted for the titanyl resulting from DMMP adsorption on anatase. 44 The phosphoryl and isopropoxy O states are hybridized with the VB states of TiO2 as in the GB/(101) system, albeit much deeper in energy, consistent with the stronger binding. Adsorption on the unreconstructed (001)-(1 × 1) surface affects the DOS in a manner similar to the reconstructed surface. The DOS of the clean slab [Fig. 6(a)] exhibits surface states contributed from the bridging O (cyan), extending about 0.2 eV above the VBM. This confirms that the states within the gap in Fig. 2 are localized surface states. In the DOS of the bound system [Fig. 6(b)], the PDOS of the surface (cyan) and subsurface oxygens (red) of TiO2 are more closely aligned, while there is a large contribution from the titanyl O near the VBM (black). The highest occupied GB states hybridize with the TiO2 VB states at energies below the VBM similar to but slightly higher than those observed in the reconstructed surface, which correlates with the weaker calculated adsorption energy.

Conclusions We have calculated optimized atomic and electronic structures with corresponding adsorption energies for the chemisorption of GB on the (101), unreconstructed (001)-(1 × 1), and reconstructed (001)-(1 × 4) surfaces of anatase TiO2 , in order of increasing adsorption strength. We predict the same ordering for DMMP adsorption, and furthermore we find that GB adsorption is weaker on all three surfaces than DMMP, with the (001)-(1 × 4) surface exhibiting the largest difference. The stronger adsorption at both the reconstructed and unreconstructed (001) surfaces is associated with the passivation of electronic surface states, which are not present at the more stable (101) surface,

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and the concomitant hybridization of O 2p states in GB and in the valence band of anatase. These results provide atomic-level details of the interaction between GB and anatase-based catalysts, and they suggest the limits within which adsorption of the simulant DMMP may be used to inform development of such catalysts for applications with nerve agents, e.g., with respect to control of crystallite faceting. 37,38 This information regarding adsorption is also a critical starting point for detailed investigations of decomposition on the surface. It is not yet known whether the adsorption strengths calculated herein may be correlated with, e.g., activation energies for decomposition, for which the dominant mechanism is SN 2 hydrolysis. 48 We speculate, however, that stronger adsorption through the P=O moiety would be associated with higher susceptibility of the phosphorus to nucleophilic attack. If so, our results suggest that among the three selected anatase surfaces, adsorption on the (001)-(1 × 4) surface would be associated with the lowest barrier to hydrolysis. First-principles calculations have recently been demonstrated to calculate such barriers in the hydrolysis of DMMP on zirconium hydroxide 49 and could be used to evaluate this hypothesis.

Acknowledgments This work was supported by the Office of Naval Research (ONR) through the U.S. Naval Research Laboratory (NRL). N.Q.L. and C.E.E. acknowledge support from NRL through the National Research Council (NRC) Research Associateship Programs. The authors thank I. V. Schweigert, S. A. Fischer, J. J. Pietron, and P. A. DeSario for valuable discussions.

Supporting Information Optimized atomic coordinates of six systems: GB and DMMP bound to the (101), (001)-(1 × 4), (001)-(1 × 1), and (001)-(1 × 1) surfaces of anatase.

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References (1) Kim, K.; Tsay, O. G.; Atwood, D. A.; Churchill, D. G. Destruction and Detection of Chemical Warfare Agents. Chem. Rev. 2011, 111, 5345–5403. (2) Jang, Y. J.; Kim, K.; Tsay, O. G.; Atwood, D. A.; Churchill, D. G. Update 1 of: Destruction and Detection of Chemical Warfare Agents. Chem. Rev. 2015, 115, PR1–PR76. (3) Diebold, U. The Surface Science of Titanium Dioxide. Surf. Sci. Rep. 2003, 48, 53–229. (4) Rusu, C. N.; Yates, J. T. Adsorption and Decomposition of Dimethyl Methylphosphonate on TiO2 . J. Phys. Chem. B 2000, 104, 12292–12298. (5) Kim, C. S.; Lad, R. J.; Tripp, C. P. Interaction of Organophosphorous Compounds with TiO2 and WO3 Surfaces Probed by Vibrational Spectroscopy. Sens. Actuators, B 2001, 76, 442–448. (6) Trubitsyn, D. A.; Vorontsov, A. V. Experimental Study of Dimethyl Methylphosphonate Decomposition over Anatase TiO2 . J. Phys. Chem. B 2005, 109, 21884–21892. (7) Panayotov, D. A.; Morris, J. R. Catalytic Degradation of a Chemical Warfare Agent Simulant: Reaction Mechanisms on TiO2 -Supported Au Nanoparticles. J. Phys. Chem. C 2008, 112, 7496–7502. (8) Panayotov, D. A.; Morris, J. R. Uptake of a Chemical Warfare Agent Simulant (DMMP) on TiO2 : Reactive Adsorption and Active Site Poisoning. Langmuir 2009, 25, 3652–3658. (9) Panayotov, D. A.; Morris, J. R. Thermal Decomposition of a Chemical Warfare Agent Simulant (DMMP) on TiO2 : Adsorbate Reactions with Lattice Oxygen as Studied by Infrared Spectroscopy. J. Phys. Chem. C 2009, 113, 15684–15691. (10) Kozlova, E. A.; Lyubina, T. P.; Nasalevich, M. A.; Vorontsov, A. V.; Miller, A. V.; Kaichev, V. V.; Parmon, V. N. Influence of the Method of Platinum Deposition on Activity and Stability of Pt/TiO2 Photocatalysts in the Photocatalytic Oxidation of Dimethyl Methylphosphonate. Catal. Commun. 2011, 12, 597–601.

15



(11) Hirakawa, T.; Sato, K.; Komano, A.; Kishi, S.; Nishimoto, C. K.; Mera, N.; Kugishima, M.; Sano, T.; Negishi, N.; Ichinose, H. et al. Specific Properties on TiO2 Photocatalysis to Decompose Isopropyl Methylphosphonofluoridate and Dimethyl Methylphosphonate in Gas Phase. J. Photochem. Photobiol., A 2013, 264, 12–17. (12) Wilmsmeyer, A. R.; Gordon, W. O.; Davis, E. D.; Troya, D.; Mantooth, B. A.; Lalain, T. A.; Morris, J. R. Infrared Spectra and Binding Energies of Chemical Warfare Nerve Agent Simulants on the Surface of Amorphous Silica. J. Phys. Chem. C 2013, 117, 15685–15697. (13) Hirakawa, T.; Sato, K.; Komano, A.; Kishi, S.; Nishimoto, C. K.; Mera, N.; Kugishima, M.; Sano, T.; Ichinose, H.; Negishi, N. et al. Experimental Study on Adsorption and Photocatalytic Decomposition of Isopropyl Methylphosphonofluoridate at Surface of TiO2 Photocatalyst. J. Phys. Chem. C 2010, 114, 2305–2314. (14) Sato, K.; Hirakawa, T.; Komano, A.; Kishi, S.; Nishimoto, C. K.; Mera, N.; Kugishima, M.; Sano, T.; Ichinose, H.; Negishi, N. et al. Titanium Dioxide Photocatalysis to Decompose Isopropyl Methylphosphonofluoridate (GB) in Gas Phase. Appl. Catal., B 2011, 106, 316–322. (15) Davis, E. D.; Gordon, W. O.; Wilmsmeyer, A. R.; Troya, D.; Morris, J. R. Chemical Warfare Agent Surface Adsorption: Hydrogen Bonding of Sarin and Soman to Amorphous Silica. J. Phys. Chem. Lett. 2014, 5, 1393–1399. (16) Petrea, N.; Petre, R.; Epure, G.; S¸omoghi, V.; T˘anase, L. C.; Teodorescu, C. M.; Neat¸u, S¸. The Combined Action of Methanolysis and Heterogeneous Photocatalysis in the Decomposition of Chemical Warfare Agents. Chem. Commun. 2016, 52, 12956–12959. ˇ ˇ (17) Stengl, V.; Maˇr´ıková, M.; Bakardjieva, S.; Subrt, J.; Opluˇstil, F.; Olˇsanská, M. Reaction of Sulfur Mustard Gas, Soman and Agent VX with Nanosized Anatase TiO2 and Ferrihydrite. J. Chem. Technol. Biotechnol. 2005, 80, 754–758. (18) Wagner, G. W.; Chen, Q.; Wu, Y. Reactions of VX, GD, and HD with Nanotubular Titania. J. Phys. Chem. C 2008, 112, 11901–11906.

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Page 16 of 24

Page 17 of 24 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


(19) Shan, X.; Vincent, J. C.; Kirkpatrick, S.; Walker, M. D.; Sambrook, M. R.; Clary, D. C. A Combined Theoretical and Experimental Study of Sarin (GB) Decomposition at High Temperatures. J. Phys. Chem. A 2017, 121, 6200–6210. (20) Taylor, D. E.; Runge, K.; Cory, M. G.; Burns, D. S.; Vasey, J. L.; Hearn, J. D.; Griffith, K.; Henley, M. V. Surface Binding of Organophosphates on Silica: Comparing Experiment and Theory. J. Phys. Chem. C 2013, 117, 2699–2708. (21) Lee, M.-T.; Vishnyakov, A.; Gor, G. Y.; Neimark, A. V. Interactions of Phosphororganic Agents with Water and Components of Polyelectrolyte Membranes. J. Phys. Chem. B 2011, 115, 13617–13623. (22) Alam, T. M.; Pearce, C. J.; Jenkins, J. E. Ab Initio Investigation of Sarin Micro-Hydration. Comput. Theor. Chem. 2012, 995, 24–35. (23) Bermudez, V. M. Energy-Level Alignment in the Adsorption of Phosphonyl Reagents on γ-Al2 O3 . Surf. Sci. 2008, 602, 1938–1947. (24) Bermudez, V. M. Computational Study of Environmental Effects in the Adsorption of DMMP, Sarin, and VX on γ-Al2 O3 : Photolysis and Surface Hydroxylation. J. Phys. Chem. C 2009, 113, 1917–1930. (25) Michalkova, A.; Martinez, J.; Zhikol, O. A.; Gorb, L.; Shishkin, O. V.; Leszczynska, D.; Leszczynski, J. Theoretical Study of Adsorption of Sarin and Soman on Tetrahedral Edge Clay Mineral Fragments. J. Phys. Chem. B 2006, 110, 21175–21183. (26) Papas, B. N.; Petsalakis, I. D.; Theodorakopoulos, G.; Whitten, J. L. CI and DFT Studies of the Adsorption of the Nerve Agent Sarin on Surfaces. J. Phys. Chem. C 2014, 118, 23042–23048. (27) Chen, X.; Mao, S. S. Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications. Chem. Rev. 2007, 107, 2891–2959. (28) Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L.; Cococcioni, M.; Dabo, I. et al. QUANTUM ESPRESSO: A Modular and OpenSource Software Project for Quantum Simulations of Materials. J. Phys.: Condens. Matter 2009, 21, 395502.

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(29) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865–3868. (30) Vanderbilt, D. Soft Self-Consistent Pseudopotentials in a Generalized Eigenvalue Formalism. Phys. Rev. B 1990, 41, 7892–7895. (31) Lazzeri, M.; Vittadini, A.; Selloni, A. Structure and Energetics of Stoichiometric TiO2 Anatase Surfaces. Phys. Rev. B 2001, 63, 155409. (32) Zhao, Z.; Li, Z.; Zou, Z. Surface Properties and Electronic Structure of Low-Index Stoichiometric Anatase TiO2 Surfaces. J. Phys.: Condens. Matter 2010, 22, 175008. (33) De Angelis, F.; Di Valentin, C.; Fantacci, S.; Vittadini, A.; Selloni, A. Theoretical Studies on Anatase and Less Common TiO2 Phases: Bulk, Surfaces, and Nanomaterials. Chem. Rev. 2014, 114, 9708– 9753. (34) Hengerer, R.; Bolliger, B.; Erbudak, M.; Grätzel, M. Structure and Stability of the Anatase TiO2 (101) and (001) Surfaces. Surf. Sci. 2000, 460, 162–169. (35) Liang, Y.; Gan, S.; Chambers, S. A.; Altman, E. I. Surface Structure of Anatase TiO2 (001) : Reconstruction, Atomic Steps, and Domains. Phys. Rev. B 2001, 63, 235402. (36) DeSario, P. A.; Pietron, J. J.; Taffa, D. H.; Compton, R.; Schünemann, S.; Marschall, R.; Brintlinger, T. H.; Stroud, R. M.; Wark, M.; Owrutsky, J. C. et al. Correlating Changes in Electron Lifetime and Mobility on Photocatalytic Activity at Network-Modified TiO2 Aerogels. J. Phys. Chem. C 2015, 119, 17529–17538. (37) Liu, G.; Yang, H. G.; Pan, J.; Yang, Y. Q.; Lu, G. Q.; Cheng, H.-M. Titanium Dioxide Crystals With Tailored Facets. Chem. Rev. 2014, 114, 9559–9612. (38) Peng, Y.-K.; Hu, Y.; Chou, H.-L.; Fu, Y.; Teixeira, I. F.; Zhang, L.; He, H.; Tsang, S. C. E. Mapping Surface-Modified Titania Nanoparticles with Implications for Activity and Facet Control. Nat. Commun. 2017, 8, 675.

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Page 18 of 24

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(39) Lazzeri, M.; Selloni, A. Stress-Driven Reconstruction of an Oxide Surface: The Anatase TiO2 (001)(1×4) Surface. Phys. Rev. Lett. 2001, 87, 266105. (40) Kowalski, P. M.; Meyer, B.; Marx, D. Composition, Structure, and Stability of the Rutile TiO2 (110) Surface: Oxygen Depletion, Hydroxylation, Hydrogen Migration, and Water Adsorption. Phys. Rev. B 2009, 79, 115410. (41) Rampulla, D. M.; Gellman, A. J. In Dekker Encyclopedia of Nanoscience and Nanotechnology; Schwarz, J. A., Contescu, C. I., Putyera, K., Eds.; Marcel Dekker, 2004; Vol. 2; Chapter Enantioselectivity on Surfaces with Chiral Nanostructures, pp 1113–1123. (42) Zaera, F. Chirality in Adsorption on Solid Surfaces. Chem. Soc. Rev. 2017, 46, 7374–7398. (43) Bermudez, V. M. Ab Initio Study of the Interaction of Dimethyl Methylphosphonate with Rutile (110) and Anatase (101) TiO2 Surfaces. J. Phys. Chem. C 2010, 114, 3063–3074. (44) Bermudez, V. M. First-Principles Study of Adsorption of Dimethyl Methylphosphonate on the TiO2 Anatase (001) Surface: Formation of a Stable Titanyl (Ti=O) Site. J. Phys. Chem. C 2011, 115, 6741– 6747. (45) Trubitsyn, D. A.; Vorontsov, A. V. Molecular and Reactive Adsorption of Dimethyl Methylphosphonate over (001) and (100) Anatase Clusters. Comput. Theor. Chem. 2013, 1020, 63–71. (46) Troya, D.; Edwards, A. C.; Morris, J. R. Theoretical Study of the Adsorption of Organophosphorous Compounds to Models of a Silica Surface. J. Phys. Chem. C 2013, 117, 14625–14634. (47) Knak Jensen, S. J.; Tang, T.-H.; Csizmadia, I. G. Hydrogen-Bonding Ability of a Methyl Group. J. Phys. Chem. A 2003, 107, 8975–8979. (48) Kuo, I.-F. W.; Grant, C. D.; Gee, R. H.; Chinn, S. C.; Love, A. H. Determination of the Surface Effects on Sarin Degradation. J. Phys. Chem. C 2012, 116, 9631–9635. (49) Schweigert, I. V.; Gunlycke, D. Hydrolysis of Dimethyl Methylphosphonate by the Cyclic Tetramer of Zirconium Hydroxide. J. Phys. Chem. A 2017, 121, 7690–7696.

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TOC Graphic

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Page 20 of 24

Page 21 of 24

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(a) Sarin (GB)

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(c) Anatase (101) 1.86 Å

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3.51 Å [101]

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[101]

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[001] [100] [010]

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Figure 1: (a) Optimized geometry of GB. C in gray, H in white, O in red, P in orange, and F in green. (b) DOS of GB (gray) with PDOS from the phosphoryl (orange) and isopropoxy (magenta) oxygen sites. Zero energy denotes the highest occupied state. (c) Optimized model of the anatase (101) surface. Ti in light gray, PHs with +4/3 (+2/3) nuclear charge in black (purple). Apical (equatorial) Ti–O bonds are identified by blue (green) dashed lines. (d) Same for anatase (001)(1 × 4), with inset rotated view of the added-row ridge. (e) Same for unreconstructed anatase (001).

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Page 22 of 24

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Figure 2: Electronic band structure of the clean (001)-(1 × 1) surface as modeled by (a) a slab with thickness of 4 unit cells with no PHs (z = 4, gray) and (b) a z = 1 slab with PHs (red), each compared with the bulk anatase band structure (black). (c) The total DOS of the three respective systems. Zero energy denotes the lowest unoccupied state.

(a) GB / (101)

(b) GB / (001)-(1x4)

2.24 Å

1.50 Å 1.52 Å

2.41 Å 2.17 Å

2.45 Å

2.83 Å

2.69 Å

1.65 Å

1.97 Å

(c) GB / (001)-(1x1)

(d) DMMP / (001)-(1x1)

2.56 Å 1.51 Å 2.80 Å

2.02 Å

2.79 Å 2.65 Å 1.64 Å

2.56 Å 1.52 Å 2.46 Å 1.64 Å 1.99 Å

Figure 3: Optimized geometries of GB adsorbed on the anatase (a) (101), (b) (001)-(1 × 4), and (c) (001)-(1 × 1) surfaces. (d) Same for DMMP on anatase (001)-(1 × 1). Colors are as in Fig. 1.

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Figure 4: Density of states per slab supercell (DOS) in gray of (a) the clean anatase (101) surface and (b) GB adsorbed on anatase (101). Partial DOS are shown in color as described in insets and in the text. Zero energy denotes the highest occupied state.

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Figure 5: The same as Fig. 4 for the reconstructed anatase (001)-(1 × 4) surface, (a) clean and (b) with GB adsorbed. Zero energy denotes the lowest unoccupied state.

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Figure 6: The same as Fig. 4 for the unreconstructed anatase (001)-(1 × 1) surface, (a) clean and (b) with GB adsorbed. Zero energy denotes the lowest unoccupied state.

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First-Principles Calculations of Sarin Adsorption on ... - ACS Publications

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