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J. Phys. Chem. C 2010, 114, 16430–16438
Periodic DFT Study of Radical Species on Crystalline Silica Surfaces Federico Musso,†,‡ Piero Ugliengo,*,‡ Xavier Solans-Monfort,† and Mariona Sodupe*,† Departament de Quimica, UniVersitat Auto`noma Barcelona, Bellaterra 08193, Spain, and Dipartimento di Chimica IFM, UniVersita` degli Studi di Torino and NIS, Nanostructured Interfaces and Surfaces, Centre of Excellence, Via P. Giuria 7, 10125 Torino, Italy ReceiVed: April 14, 2010; ReVised Manuscript ReceiVed: July 29, 2010
Periodic DFT (BLYP, B3LYP, and BHandHLYP) calculations have been used to study the properties of SiO• radical defect on quartz, cristobalite, tridymite, and amorphous surface models. Crystalline orbitals are constructed by an expansion of Gaussian type orbitals in which all atoms are represented with a double-ζ plus polarization quality basis set. Starting from fully hydroxylated 2D slab models, the radical defect is constructed by removing a hydrogen atom from a silanol of the surface while conserving the other features of the crystalline polymorph. Among the different functionals used, the hybrid BHandHLYP is the one that better compares to the experimental EPR data and provides reaction energies in better agreement with CCSD(T). The GGA BLYP functional, however, tends to delocalize the spin density, which can have important consequences on the H-bonding at the surface, especially when it exhibits geminal silanols. Comparison between the hydroxylated and the radical slab shows that the radical defect at the surface does not induce significant tension at the crystalline structure, the Si-O bond distance associated to the radical defect being the only geometrical parameter that varies significantly (∼0.04 Å). The spin density analysis shows that, regardless of the surface, the unpaired electron is mainly localized on the radical defect and, thus, does not provide a clue to understanding the different behavior between crystalline and amorphous silica on the tSiO• + H2O f tSiOH + OH• reaction. Instead, the ability of the surfaces to establish new H-bonds with the recovered silanol appears to be a relevant feature on the process that triggers OH• formation from SiO•. Introduction Inhalation of pulverized crystalline silica polymorphs, in particular those of highest density (quartz, cristobalite, and tridymite), causes serious lung diseases.1–4 In contrast, glass and silicic acid, are considered to be nontoxic. Since surfaces are the contact interface between silica dust and the pulmonary tissue, the knowledge of their structural properties is important to provide some understanding on the processes that produce highly toxic species, such as reactive oxygen and nitrogen species (ROS and RNS)5 and free radicals. Grinding silica minerals implies breaking strong covalent Si-O bonds and forming either radical species (Si• and Si-O•) or ions (Si+ and Si-O-) in the new surface. Radicals and ions are both highly unstable species and tend to react with molecules present in the surrounding environment. Because of their reactivity and the water rich environment, silanol (Si-OH) becomes the most common functionality present at a silica surface, due to the reaction of radical and ionic sites with water molecules. Nevertheless, it is well established that few radical species may still persist even in aged dust particles.4,6–9 In fact, many defective centers have been detected, one of the most investigated ones being the SiO• center, also called nonbridging oxygen (NBO).6,8,9 The reaction between surface radicals Si-O• and Si• with water to lead to the formation of silanol groups and free hydroxyl radicals in freshly formed crystalline dust is of high interest,10–12 since it is thought to be one of the potentially relevant processes associated with the toxicity of * Corresponding authors. E-mail:
[email protected]; Piero.
[email protected]. † Universitat Auto`noma Barcelona. ‡ Universita` degli Studi di Torino and NIS.
crystalline silica. However, many other processes can also take place, and in fact, the formation of free hydroxyl radicals from silica dust is still unclear.3,5 In this way, insights in the reactivity of different detected defective sites and specially that of SiO• is a fundamental step for the understanding of the global process. The structural properties of nondefective and defective SiO2 surfaces have been widely investigated in both crystalline and amorphous solids with experimental techniques.6–9,13–22 In particular, the NBO center (SiO•) has been characterized by photoluminescence23–28 and EPR spectroscopy.8,18,29,30 Using the latter technique, D. L. Griscom and co-workers concluded that the defect should be described as “a hole trapped in a pure 2pπ orbital of a single oxygen bonded to a single silicon in the glass network”.8 More recently, theoretical calculations on this defective center, using embedded cluster models and the B3LYP functional.31,32 have concluded that the singly occupied orbital is mainly centered in one of the 2p orbitals of oxygen31 and that theory is able to reproduce the EPR experimental data.32 In addition, the interaction and reactivity of nondefective surface with adsorbed external molecules has also been largely studied either experimentally and theoretically, considering SiOH group as the active functionality.14–16,33–51 It is worth mentioning that most of these models consider a fully hydroxylated surface. In contrast, the interaction and reactivity of defective silica surfaces with adsorbed molecules is much less studied, and only a few works treat the reactivity of radical defects at the surfaces.52–55 In this way, Narayanasamy and Kubicki53 investigated the OH• generation on radical surface sites of silica using a DFT56,57 approach based on cluster models. The authors consider two types of radical defects on silica (Si-O• and Si•) and their reaction with water to form Si-OH with the generation of OH• and H• radicals. In both cases they find fast kinetics and negative
10.1021/jp103342b 2010 American Chemical Society Published on Web 09/15/2010
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values for the ∆G of the reaction. Nevertheless, this study with cluster models could not take into account the specific properties of each different crystalline polymorph nor the long-range interactions on the surfaces, like H-bond chains that can be found on crystalline systems, due to the effect of the translational symmetry. In the present work we focus our attention on the superficial Si-O• defect and study the formation reaction of free radical OH•
tSiO• + H2O f tSiOH + OH• within a periodic DFT approach. Although one could think on different reacting pathways for the SiO• radical and different routes for generating OH• radicals, we have considered the SiO• defect as starting point. The previous study of Narayanasamy and Kubicki showed that reactions involving SiO• are faster than with Si• and other potential processes would imply more than one single step for generating the OH• radical. We consider different surfaces of the three polymorphs that show the highest toxicity: quartz, cristobalite, and tridymite. Periodic calculations on these systems will allow us to take into account the specific properties of each different crystalline polymorph and, in particular, to analyze the influence of surface hydrogen bonding. Our aim is to establish a relationship between the reaction energy associated to the formation of OH• and the structural features of the different crystalline surfaces. Computational Details Models. The surface selection was made as follows. The hydroxylated models were obtained by cutting a slab of thickness T (Å) from the bulk structure of quartz, cristobalite, and tridymite and adding H atoms to the Si-O dangling bonds. For quartz, the crystallographic planes were chosen according to crystallographic information on the natural habit of the crystal structure. In the case of cristobalite and tridymite, however, due to the lack of precise experimental data on crystal habit, the studied planes correspond to those with lower Miller indexes, since the corresponding surfaces should, in principle, be the most developed ones due to their slower rate growth. At this point it is worth mentioning that quartz, cristobalite, and tridymite are the three high density polymorphs of silica that present the highest thermodynamical stability and that show the highest pathogenic effect on lungs.1–4 Radical defects were constructed starting from the different, previously reported,46 fully hydroxylated 2D surface models of quartz, cristobalite, and tridymite. We considered the five surface models represented in Figure 1. Three of them are taken from quartz polymorph, one from cristobalite and one from tridymite. Models are built to take into account the most relevant properties of crystalline silica surfaces, i.e., the most common surface silanols: isolated, geminals, and vicinals (see Figure 2), and the most relevant types of H-bond interactions (no hydrogen bonding, isolated H-bonds, and infinite H-bond chains).46 Every surface model is indicated by a capital letter that specifies the polymorph and the triplet of Miller indexes that determines the surface plane. Using this nomenclature, the five models considered are Q(001), Q(100), and Q(011) for quartz, C(101) for cristobalite, and T(001) for tridymite. The SiO• radical defect is obtained by removing one H• atom on the top surface only. All nonequivalent silanols have been considered. Thus, for Q(001), Q(100), Q(011), and C(101) hydroxylated slabs, we constructed two different radical slabs, whereas for
Figure 1. Top view of the hydroxylated (S) and radical (R1 and R2) slabs for quartz Q(001), Q(100), and Q(011) surfaces, cristobalite C(101) surface, and tridymite T(001) surface.
Figure 2. Different types of surface silanols exhibited by crystalline silica surfaces.
T(001) we constructed only one. Hereafter, the hydroxylated model will be referred as S whereas the radical models will be denoted as R. When different radical species are possible, as in the case of Q(001), Q(100), Q(011), and C(101), radical models will be referred to R1 and R2. Level of Theory. All periodic calculations have been performed with the ab initio CRYSTAL06 code.58 This code implements the Hartree-Fock and Kohn-Sham self-consistent field method for the study of periodic systems,59 and in the
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TABLE 1: BHandHLYP, B3LYP, and BLYP Relevant Structural Data (lengths in Å) of the Crystalline Cells of Hydroxylated and Radical Slabs Q(001)
BHandHLYP
B3LYP
BLYP
a
Si-O• (Si-O•)a Si-O• (Si-O•)a Si-O• (Si-O•)a
Q(100)
Q(011)
C(101)
T(001)
S
R1
R2
S
R1
R2
S
R1
R2
S
R1
R2
S
R
1.620 0.966
1.620 0.964 1.675 +0.049 1.634 0.978 1.678 +0.036 1.648 0.992 1.666 +0.007
1.620 0.964 1.677 +0.041 1.634 0.978 1.680 +0.027 1.648 0.998 1.655 -0.013
1.620 0.967
1.620 0.965 1.676 +0.037 1.634 0.978 1.683 +0.027 1.644 0.990 1.668 -0.005
1.620 0.964 1.669 +0.048 1.634 0.978 1.674 +0.038 1.649 0.988 1.665 +0.015
1.620 0.955
1.619 0.956 1.676 +0.034 1.634 0.969 1.684 +0.025 1.648 0.982 1.672 -0.005
1.620 0.956 1.677 +0.061 1.634 0.968 1.685 +0.055 1.649 0.980 1.673 +0.029
1.619 0.961
1.619 0.960 1.676 +0.041 1.634 0.973 1.683 +0.032 1.648 0.985 1.675 +0.009
1.619 0.960 1.671 +0.041 1.634 0.973 1.680 +0.033 1.648 0.986 1.665 0.000
1.615 0.952
1.615 0.952 1.657 +0.038 1.629 0.964 1.664 +0.029 1.644 0.974 1.659 +0.009
1.633 0.980 1.648 0.992
1.633 0.980 1.648 0.992
1.635 0.968 1.648 0.979
1.633 0.974 1.649 0.986
1.629 0.964 1.643 0.974
Difference between Si-OH and Si-O• bond length in Å.
present version, it allows to perform full geometry optimization, both internal coordinates and cell size,60 as well as to compute the phononspectrum at Γ point of molecules, slabs, and crystals.61 It is worth noting that CRYSTAL06 models are true 2D systems at variance with the approach followed by plane wave based codes, in which the slab is artificially replicated through infinity also in the direction perpendicular to the slab by including a large amount of empty space. Three functionals have been chosen to perform all the periodic calculations: the pure GGA BLYP functional62,63 the hybrid B3LYP functional63,64 which includes 20% of the HartreeFock (HF) exchange and the hybrid BHandHLYP functional63,65 including 50% of HF exchange.66 The latter functional has been shown to better describe radicals and electron-delocalized systems.67–71 The multielectron wave function is described by linear combination of crystalline orbitals (CO) which, in turn, are expanded in terms of Gaussian type basis sets. The GTO basis set (BSPC) is equal to that used previously with success in many theoretical studies in silica and silica-based materials43,46,49,51,72–79 and it consists on a 66-21G(d) (Rsp ) 0.13, Rd ) 0.5), 6-31G(d) (Rsp ) 0.2742, Rd ) 0.538), and 6-31G(p) (Rs ) 0.1613, Rp ) 1.1) for Si, O, and H, respectively (outer shell exponent in bohr-2). All of the periodic radical slabs have been studied using an unrestricted open shell scheme. The DFT implementation is such that the exchange-correlation contribution is the result of a numerical integration of the electron density and its gradient, performed over a grid of points. In CRYSTAL the Gauss-Legendre quadrature and Lebedev schemes are used to generate angular and radial points of the grid, respectively. A good compromise between accuracy and cost of the calculation for geometry optimization and vibrational frequencies has been shown by using a pruned grid consisting of 75 radial points and 1 subinterval with 974 angular points (75, 974).58,80 Default values of the tolerances that control the Coulomb (7 7 7 7) and exchange (14) series have been adopted. This means that when the overlap between two atomic orbitals is smaller than 10-7 (for Coulomb) or 10-14 (for exchange) the integral is either approximated or disregarded.58 The Hamiltonian matrix has been diagonalized81 in 10 reciprocal lattice points (k points), corresponding to a shrinking factor of 4.58 All geometry optimizations have been performed in P1 layer group symmetry (no symmetry), in order to ensure the maximum degrees of freedom during the optimization. Both lattice constants and internal coordinates have been simultaneously optimized within the same run, using analytical gradients and upgrading the numerical Hessian with the Broyden-FletcherGoldfarb-Shanno algorithm.82–85 Convergence criteria are par-
ticularly stiff, the tolerance for the maximum allowed gradient and displacement have been set to 0.0003 hartree Bohr-1 and 0.0012 Bohr, respectively. EPR data have been computed with the procedure implemented in Gaussian 0386 using the cluster models illustrated in Figure S2. We have performed single point calculation at the optimized periodic geometries. Two different basis sets have been considered, the same used in the periodic calculations (BSPC) and another one BSEPR which has been constructed to reproduce accurately hyperfine coupling constants (hcc) and which consists on the EPR-III basis sets87 for H and O and the 6-311+G(2df) for Si.88 Results and Discussion Results are organized as follows. First, the effect of generating a SiO• radical on the fully hydroxylated 2D models is analyzed and related to the electronic structure of the defective surface. Attention is mainly paid to the variations induced on the local SiO environment and on the lattice parameters of the slab. Second, changes on hydrogen bonding are analyzed and finally, the energetics associated to OH• radical formation, through the tSiO• + H2O f tSiOH + OH• reaction, is reported. Structural Analysis. Table 1 shows the most relevant structural data of the hydroxylated and radical slabs obtained at different levels of theory. Cell parameters (a, b, γ), cell area (A), and the thickness (T) of each slab obtained with BHandHLYP, B3LYP, and BLYP functionals are given in Tables S1-S3, respectively. It can be noted that the cell vectors and the thickness of the slabs increase on going from BHandHLYP to B3LYP and BLYP (Tables S1-S3). Indeed, decreasing the percentage of exact exchange contribution in the functional provokes a slight expansion of the structure. Moreover, averaged Si-O and O-H bond distances of radical and hydroxylated models show negligible variations with all methods. Most of them change less than 1% and largest modifications remain always under 2%. Therefore, it can be concluded that the presence of a radical defect at the surface do not introduce any tension in the structure of the model. In contrast, generation of a SiO• radical significantly influences the Si-O• bond distance (Table 1) and the shape, number and strength of the hydrogen bonds present on the surface (Table 2). These variations are functional dependent. BHandHLYP reports a general elongation of the Si-O• bond length upon formation of the SiO• radical by an average of 0.043 Å. This increase is significantly higher (0.061 Å) for Q(011)R2, the only example where the defect is generated on a simply H-donor SiOH group. B3LYP also predicts an elongation of the Si-O• bond, specially for the
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TABLE 2: H-Bonds Lengths (in Å) on the Hydroxylated and Radical Slabsa Q(001) S
BHandHLYP B3LYP BLYP
Q(100) S
Q(011)
C(101)
R1
R2
R1
R2
S
R1
R2
H1
H2
H1
H2
H1
H2
H1
H2
H
H1
H2
H1
S H2
R1 H1
R2 H2
2.100 2.101 2.094
1.869 1.868 1.874
2.245 2.214 2.024
1.885 1.834 1.690
1.763 1.752 1.754
2.241 2.224 2.224
1.800 1.773 1.690
2.573 2.528 2.525
2.023 2.027 2.023
2.190 2.113 1.988
2.458 2.401 2.279
2.251 2.252 2.270
2.174 2.151 2.106
2.389 2.314 2.129
2.418 2.305 2.007
a H1 and H2 indicate the two different H-bonds in both the saturated (S) and radical (R1, R2) radical slabs. In the case of Q(011), there is only one H-bond on the hydroxylated structure, indicated as H.
TABLE 3: Spin Density on the Radical Oxygen in Crystalline Cellsa Q(001) BHandHLYP B3LYP BLYP a
Q(100)
Q(011)
C(101)
T(001)
R1
R2
R1
R2
R1
R2
R1
R2
R
0.99 0.94 0.81
0.99 0.94 0.74
0.99 0.96 0.87
0.98 0.93 0.81
0.99 0.96 0.84
0.99 0.96 0.85
0.99 0.96 0.84
0.99 0.96 0.84
0.99 0.96 0.89
The total spin density inside the crystalline cell is equal to 1.
Q(011)R2, but this elongation is less pronounced, the averaged value being 0.034 Å. Finally, BLYP functional suggests that the SiO• bond is almost not affected by the formation of the radical, the only exception being Q(011)R2 where the Si-O• bond elongates by about 0.03 Å. At this point it is worth mentioning that, regardless of the functional, the Si-O bond distance involved in the Q(011)R2 formation is the one that presents the larger lengthening upon radical formation. This seems to be associated with the fact that the formation of the Q(011)R2 defective surface requires the largest geometrical reorganization as the defect is generated in a SiOH H-bond donor which becomes H-acceptor upon defect formation. The Si-O• elongation is in agreement with the fact that the spin density is mainly centered on the radical oxygen of the surface (Table 3), particularly with BHandHLYP which provides values very close to 1 in every system. The description provided by B3LYP is close to that obtained with BHandHYLP, the value of the spin density being around 0.95 at the radical oxygen. However, BLYP provides a significantly more delocalized picture of the spin density, the computed values lying between 0.74-0.89 electron on the radical center and 0.24-0.11 electron at the three oxygen atoms of the tetrahedron that contains the radical O•. Thus, as noted previously for silica based radical systems,69–71 the spin density becomes more and more delocalized upon decreasing the amount of exact exchange in the functional. This larger spin delocalization obtained with BLYP, however, appears to have a small effect on the global structural relaxation of the model since, as found with BHandHLYP and B3LYP, structural differences between the radical and hydroxylated slabs are very minor. That is, even with BLYP the SiO• radical remains essentially as a localized defect. The local nature of the SiO• defect was already suggested by the analysis of the EPR data8,18,29,30 and confirmed by the calculations of Pacchioni and co-workers, who showed that the computed B3LYP EPR data matched reasonably well with that coming from experiments.31,32 In this latter work they also pointed out that the presence of other SiOH groups in the vicinity of the defect alters slightly the g tensor. With the aim of analyzing how the degree of localization influences the EPR values and determining which functional describes more accurately the electronic distribution at the defect, we have computed both g-tensor and the isotropic and dipolar hyperfine coupling constants of the NBO defect at two surfaces, Q(001)R1 and T(001)R1 (See Figure 1). Results are summarized in Table 4, along with the computed values of ref 32 and the available
experimental data.18,29,30 Regardless the surface and level of theory, the g tensor presents three very distinct components in overall agreement with experiments, the g3 value being significantly larger than the other two. Moreover, this value significantly varies depending on the kind of structure, the surface with a nearby SiOH group showing a smaller g3 value than the surface model presenting isolated SiO defects. On the other hand, the computed g3 component is shown to be highly dependent on the functional used. BLYP functional always gives the lowest value while BHandHLYP leads to the highest one. Experimentally the g3 value is difficult to measure since it is characterized by a broad distribution with a weighted average at 2.078.32 Therefore, BHandHLYP values which range between 2.069 and 2.106 seem to be those closer to experiment. In contrast, BLYP functional which describe a more delocalized defect provides smaller values (2.040 and 2.034) in less agreement with the experimental data. Moreover, inclusion of exact exchange in the functional also strongly modifies the hyperfine coupling constants (hfcc), especially those of the unpaired electron wearing oxygen atom. That is, the highest the amount of exact exchange, the highest the absolute values of the isotropic and dipolar hyperfine coupling constants are. Unfortunately, the computed values are quite basis sets dependent and thus, comparison with experiment is not evident. When using BSEPR, the basis sets developed for reproducing hfcc values, both Aiso and Adip values computed with BHandHLYP functional are closer to the experimental values than those obtained with the other two functionals, suggesting again that BHandHLYP is the functional that provides the more accurate electronic structure description. Hydrogen Bonding on the Surfaces. All of the hydroxylated and radical slabs described in the present work present hydrogen bonds between the surface silanols, except T(001) and its radical that present isolated silanols (see Figure 1) and noninteracting radical defects, respectively. Table 2 contains the values of the corresponding H-bond in the fully hydroxylated and radical surfaces. In the case of the hydroxylated Q(001), Q(100), and C(101) models, the upper surface presents two nonequivalent hydroxyl groups, that form an infinitive chain of H-bonds. Since each of the two hydroxyl groups is a H-bond donor and a H-bond acceptor at the same time the construction of the radical defect starting from the hydroxylated surface has two effects on the H-bond properties: (i) it breaks the infinitive chain because it eliminates the H-bond in which the corresponding OH in the
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TABLE 4: Computed g Tensor and Isotropic and Dipolar Hyperfine Coupling Constants (in Gauss) for Q(001)R1 and T(100)R SiO• Radicalsa g tensor Q(001)R1
T(100)R
R-quartzc,d edingtonitec,f expt.g
BLYP/BSPC BLYP/BSEPR B3LYP/BSPC B3LYP/BSERP BHandHLYP/BSPC BHandHLYP/BSERP BLYP/BSPC BLYP/BSEPR B3LYP/BSPC B3LYP/BSERP BHandHLYP/BSPC BHandHLYP/BSERP B3LYPe B3LYPe
2.005 2.005 2.003 2.003 2.002 2.002 2.003 2.004 2.003 2.003 2.002 2.002 2.0025 2.0025 2.000
2.010 2.010 2.011 2.012 2.012 2.013 2.010 2.011 2.011 2.012 2.012 2.013 2.0102 2.0107 2.010
2.034 2.035 2.043 2.042 2.069 2.062 2.040 2.039 2.058 2.053 2.106 2.083 2.0380 2.0621 2.078
Aiso(17O),G
Adip(17O)
Aiso(29Si),G
spinb
-24.3 -10.3 -30.8 -14.4 -36.2 -21.1 -25.3 -10.1 -29.9 -13.6 -34.6 -19.9 -31 -32 26
38.7 40.5 42.2 44 43.3 48.4 35.0 43.7 41.7 44.3 43.3 45.1 -42 -43 42
11.5 13.6 11.8 14.3 11.5 14.6 12.2 14.4 11.9 14.7 11.8 15.1 -15 -14 14
0.78 0.77 0.94 0.91 0.99 0.96 0.87 0.84 0.95 0.93 0.99 0.97
a
Experimental values and those coming from previous calculations are added for comparison. b Spin density over the oxygen atom supporting the unpaired electron. c Taken from from ref 32. d The SiO defect at the R-quarz model presents a geminal SiOH group. e Calculations are performed using a valence double-ζ plus polarization basis set different to that used here. f Edingtonite model presents isolated SiO defects. g Values taken from ref 18.
hydroxylated slab was involved as H-bond donor and (ii) it changes the properties of the H-bond in which the O• radical (OH in the hydroxylated) is involved as acceptor. As a consequence, H-bond distances in radical slabs are significantly different than in the hydroxylated surface. The observed changes, however, vary depending on the functional used. The BHandHLYP and B3LYP functionals describe, in almost any case, the OH · · · O• bond as larger and, thus, weaker than the corresponding OH · · · OH bond, in agreement with the smaller basicity of SiO•89 compared to that of SiOH. The only exception is observed for the radical Q(001)R2 at the B3LYP level, which shows a H-bond distance (1.834 Å) that is somewhat shorter than the corresponding one in the hydroxylated slab (1.868 Å). In contrast, the BLYP functional provides a different description of the H-bond changes. That is, excluding Q(100), the hydrogen bond distances decrease and thus, appears to strengthen upon introducing the radical defect in the slab. Particularly striking is the case of Q(001)R2 for which the H-bond distance changes from 1.874 to 1.690 Å. The fact that BLYP functional tends to shorten the H-bonds between an OH group and the radical oxygen O• can be related to the electronic properties provided by the BLYP functional. As previously shown, with BLYP the spin density on the radical oxygen is computed to be smaller than with the other functional; that is, the unpaired electron is more delocalized on the other oxygens inside the crystalline cell. This effect increases the basicity of the radical oxygen giving shorter H-bond interactions, as compared to nondelocalized situations. Moreover, in Q(001)R2 the higher basicity of the O• atom is accompanied with a higher acidity of the donor OH group (this group has most of the spin density that is not localized in the SiO• group) and thus the O-H · · · O• shortening is more pronounced. The case of Q(011) is particular because no infinite H-bond chains are present in this slab. Thus, the H-bond is expected to be maintained only if the radical is constructed on the acceptor OH, (Q(011)R1 in Figure 2) since introducing the radical defect at the OH donor implies eliminating the H atom involved in the H-bond interaction. Despite that, structural relaxation of Q(011)R2 leads to a hydrogen bonded slab in which the OH that was initially acting as proton acceptor rearranges to act as proton donor in a new H-bond with the radical oxygen. Differences between the H bonds on Q(011)R1 and Q(011)R2 result from the different structural environment. The same
behavior is observed with the three functionals showing that the formation of a new hydrogen (OH · · · O•) bond is always preferred over a 2-center/3-electron interaction90–94 between the unpaired radical electron and the lone pairs of the oxygen of the hydroxyl, which is normally overestimated with pure GGA functionals like the BLYP. OH• Formation Reaction Energy. In this section we discuss the energetics associated to the formation of the free hydroxyl radical by the reaction of the defective surface with H2O. This reaction can be written as
(slab)O• + H2O f (slab)OH + OH• where (slab)O• represents the radical slab and (slab)OH indicates the correspondent hydroxylated slab. The reaction energy is expressed as C C ∆E ) (E(slab)OH + EOH•) - (E(slab)O • + EH O) 2
where E C(slab)OH and E C(slab)O• are the cell energies of the hydroxylated and radical slabs, and E OH• and E H2O the energies of the hydroxyl radical and a molecule of water, respectively. The computed reaction energies for each radical slab with the three functionals are given in Table 5. Regardless of the considered surface and the level of theory used, the formation of the OH• radical is exoenergetic, the only exception being the B3LYP and the BLYP values of the Q(011) surface which predicts an almost isoenergetic process. Despite this equal global general trend, the computed reaction energies are also functional dependent. In general, the larger the amount of exact exchange in the functional, the more negative the reaction energy is. These variations are mainly due to the description of radical species by the different functionals, since pure GGA, as well as hybrid functionals with low percentages of exact exchange, tend to overstabilize situations with a too delocalized spin density67–71 (see above). Nevertheless, in order to validate the reliability of the functionals used in the present work (BHandHLYP, B3LYP, and BLYP) for studying this kind of systems, results for the same reaction but considering a simple H3SiO• radical cluster have been compared with those obtained
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TABLE 5: ∆E of the (slab)O• + H2O f (slab)OH + OH• Reaction in kcal mol-1 Q(001) BHandHLYP B3LYP BLYP
Q(100)
Q(011)
C(101)
T(001)
R1
R2
R1
R2
R1
R2
R1
R2
R
-11.1 -9.0 -4.5
-10.6 -8.4 -3.4
-9.5 -7.4 -3.5
-12.7 -10.3 -5.9
-5.5 -3.5 0.3
-5.3 -3.2 0.4
-9.3 -7.1 -3.9
-8.6 -6.6 -3.0
-6.4 -4.3 -1.5
from single point energy CCSD(T) calculations, at the MP2 optimized structures (see Figure S1 of the Supporting Information for details). Results show that, as found previously,69 the functional that better reproduces the energetics obtained with CCSD(T) is the BHandHLYP one, and thus, hereafter, we will only refer to these values. It can be observed in Table 5 that the reaction energies for the radical slabs of Q(011) and T(001) surfaces are the less exoenergetic, whereas those of radical slabs of Q(001), Q(100), and C(101), with infinite chains of hydrogen bonds in the hydroxylated surface, are the more exoenergetic ones. Since electronic properties do not change significantly for the different radical slabs, with the unpaired electron mainly localized in the radical oxygen (see above), this aspect is remarkable because it establishes a relation between the ∆E and the hydrogen bonding occurring at the surface. Indeed, Q(011) and T(001) are the only two surfaces in which the reaction of the radical defect with H2O do not provide any new H-bond contact on the hydroxylated slab. In the case of T(001) the surface silanol reconstructed by the reaction is an isolated silanol and with both Q(011)R1 and Q(011)R2 radicals the OH group reconstructed in the product remains acting as an acceptor group in the same H-bond (same number of H-bonds). In contrast, for all radicals of Q(001), Q(100), and C(101), the reaction leads to the formation of a new hydrogen bond per crystalline cell. This new H-bond arises from the interaction of the silanol (SiOH) formed on the radical oxygen and reconstructs the H-bond chain broken with the formation of the radical defect. Therefore, the reaction energy associated to the formation of the free hydroxyl radical OH• becomes more exoenergetic. Furthermore, in the case of Q(001), Q(100), and C(101) surfaces the reaction energy appears to depend on the strength of the new H-bond formed, the shorter the hydrogen bond distance, the more exoenergetic the reaction is. Indeed, Figure 3 shows that there is a good correlation between the ∆E of the reaction, computed with BHandHLYP, and the H · · · O
Figure 3. BHandHLYP correlation between ∆E of reaction (kcal mol-1) and the length (Å) of the H-bond (H · · · O distance) formed as product in the case of Q(001), Q(100), and C(101) surfaces.
distance. It should be noted that the O-H · · · O• angle (around 165°) is almost equal for all systems. There are only two situations that do not follow this trend: Q(100)R1 and C(101)R1. In these cases, other effects such as the structural relaxation of the model can play a role. Nevertheless, such a good correlation for different surfaces indicates that the H-bond formation at the silica surface is probably the most relevant property on the considered process. At this point it is worth mentioning that water, which is usually present in the media, will probably act as another way of establishing hydrogen bonds, the reaction energy being dependent on the ability of the surface silanols to interact with water. Amorphous Silica. Since the amorphous silica dust does not show any toxic effect on lungs, we have also analyzed the properties of a single Si-O radical on an amorphous silica surface. The amorphous radical has been constructed starting from a hydroxylated amorphous slab proposed by some of us recently.43 The fundamental idea in the study of an amorphous structure with a periodic code is the construction of large disordered cell of silica that cannot maintain the characteristic high internal symmetry of the crystalline polymorphs of silica. The periodic cells of the radical and hydroxylated amorphous slabs contain 203 and 204 atoms, respectively, and present an hydroxyl density upon the surface close to 4.5 OH/nm2, a value which is consistent with an experimental silica AEROSIL sample pretreated at 423 K. The crystalline cell of the radical slab is described in Figure 4. The study of the amorphous radical has been performed only with the BHandHLYP functional, since is the one that better describes the chemical nature of the SiO• defect. As found for crystalline slabs, differences between the hydroxylated and the radical slab on the structural parameters are negligible (see Table S4 in the Supporting Information),
Figure 4. Top view of the radical amorphous silica surface. Radical oxygen indicated by the arrow.
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Figure 5. BHandHLYP structures and reaction energies of Q(001) and T(001) surfaces envisaging H2O and OH• H-bonded surfaces.
i.e. the presence of a radical defect on the surface does not introduce any structural tension. Moreover, the electron properties are exactly the same as those computed for the crystalline structures, with the spin density being mainly located on the O• (0.99). The reaction energy (∆E ) -5.3 kcal/mol) is very similar to that found for the T(001) (∆E ) -6.4 kcal/mol), with a noninteracting silanol. These results confirm thus, that the reaction energy is mainly driven by hydrogen bonding, which in the absence of water takes place between the SiOH groups of the surface, and not by the electronic properties of the radical species in the different crystalline cells, which are similar in all cases. Improvement of our model would require the addition of water molecules usually present in surfaces. Nevertheless, inclusion of physisorbed water in our modeling is really complex, since the number of water molecules that should be added for realistic simulations would lead to a large number of potential configurations that would require the use of ab initio molecular dynamics simulations, which is out of the scope of the present work. However, with the aim of exploring the role of water on the surface we have added one water molecule to the system in two representative cases: Q(001)R1 and T(001)R, the first one leading to infinite H-bond chains and the second one to isolated silanols when reacting with water. Structural and energetic details are provided in Figure 5. In both Q(001)R1/ H2O and T(001)R/H2O, the adsorbed water molecule establishes two hydrogen bonds between the Si-O• center and the closest SiOH group. After the reaction, the resulting OH• radical also interacts with the silanol groups of the surface through hydrogen bonding and acting as hydrogen bond donor. Overall, the
presence of one water molecule changes the H-bond pattern, the reaction energy differences being reduced because now both systems are partially stabilized. Nevertheless, trends are maintained, the hydroxilated system presenting the larger number of hydrogen bonds leading to the more favorable reaction energy. In conclusion, the different behavior between amorphous and crystalline systems as far as OH• radical formation from SiO• is concerned has to be found in the formation of hydrogen bonds once the SiOH group is recovered and not on the electronic structure of the radical defect which is found to be similar in all surfaces. This H-bonding can occur between SiOH groups of the surface, leading to H-bond infinite chains, or between SiOH groups of the surface and physisorbed water molecules. Summary and Conclusions Radical defects on quartz, cristobalite, and tridymite surfaces have been described within a density functional approach with three common functionalssBHandHLYP, B3LYP, and BLYPsand a double-ζ polarized Gaussian basis set. Starting from fully hydroxylated 2D slab models, the radical models have been constructed by removing a hydrogen atom from a silanol on the top surface of the slabs and considering the different features of the crystalline surfaces. From a methodological point of view, results show that BLYP tends to delocalize the spin density, as compared to hybrid functionals and particularly to BHandHLYP, for which the unpaired electron is completely localized at the radical defect. This delocalization has an important effect on the variations of
Radical Species on Crystalline Silica Surfaces the Si-O• bond length upon radical formation and on the computed EPR data. Even more important, it can have dramatic consequences on the H-bonding at the surface, especially when the surface exhibits geminal silanols, as different functionals may even predict opposite changes upon introducing a radical defect at the surface. As found previously for other radical systems, reaction energies computed at the CCSD(T) level with a small cluster that represents tSiO• have shown that BHandHLYP is the functional that provides the best agreement with the highly correlated CCSD(T) method. Moreover, BHandHLYP is the functional that provides EPR data in best agreement with experimental data, which points out the importance of using hybrid functionals for describing the electronic properties of these defects. Therefore, GGA functionals, widely used in solid state calculations because of the low computational cost, do not seem suitable for the treatment of radical species of this type. On the other hand, the present study shows that a single radical defect on the surface does not introduce any particular tension on the crystalline structures. Moreover, the spin density analysis shows that the unpaired electron is always mainly localized on the radical defect regardless of the considered surface and thus, does not provide a clue to understand the behavior of different silica polymorphs with respect to their toxicity. The surface features that change the most by the presence of a radical defect are the H-bonds, which tend to be weaker in the radical surfaces compared to the hydroxylated ones. Consequently, the tSiO• + H2O f tSiOH + OH• reaction energies for the different surfaces are in general exoenergetic, those surfaces recovering a larger number of hydrogen bonds presenting more negative values. Indeed we stress that the ability of the surfaces to establish new H-bonds with the recovered silanol (either with other surface silanols or with the water present in the media) may be a relevant feature on the process that triggers OH• formation from SiO•. Since the H-bond pattern on the crystalline silica surfaces depends on the polymorph and on the (hkl) plane, this might have some influence in the different behavior of crystalline surfaces as far as OH• formation from SiO• is concerned. Acknowledgment. Financial support from MICINN (CTQ2008-06381) and BSC-MN for generous allowance of computing time (QCM-2008-1-0012) are gratefully acknowledged. F.M. thanks the Ministerio de Ciencia e Innovacio´n for a FPU fellowship. X.S.M. acknowledges the MICINN for the Ramo´n y Cajal position. P.U. acknowledges CINECA supercomputing centre for allowance of computer time. Supporting Information Available: Calibration of the method, including potential energy profiles (Figure S1) and cluster models (Figure S2). Tables S1-S3 containing BH and HLYP, B3LYP and BLYP cell parameters, cell areas, and slab thicknesss for crystalline systems; Table S4 reporting BH and HLYP structural data for amorphous slabs. Additional references are also included. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Fenoglio, I.; Martra, G.; Coluccia, S.; Fubini, B. Chem. Res. Toxicol. 2000, 13, 971–975. (2) Fenoglio, I.; Ghiazza, M.; Ceschino, R.; Gillio, F.; Martra, G.; Fubini, B. In Surface Chemistry in Biomedical and EnVironmental Science; Blitz, J. P., Gun’ko, V. M., Eds.; Springer: New York, 2006; Vol. 228, p 287. (3) Fubini, B. In The Surface Properties of Silicas; Legrand, A. P., Ed.; John Wiley & Sons: West Sussex, 1998; p 415-458. (4) Fubini, B.; Arean, C. O. Chem. Soc. ReV. 1999, 28, 373–381.
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