1474
J. Phys. Chem. C 2009, 113, 1474–1485
Quantum-Chemical Comprehensive Study of the Organophosphorus Compounds Adsorption on Zinc Oxide Surfaces Y. Paukku, A. Michalkova, and J. Leszczynski* Computational Center of Molecular Structure and Interactions, Department of Chemistry, Jackson State UniVersity, 1400 J. R. Lynch Street, P.O. Box 17910, Jackson, Mississippi 39217 ReceiVed: August 31, 2008; ReVised Manuscript ReceiVed: October 16, 2008
Ab initio calculations at the density functional theory and the second-order Møller-Plesset perturbation theory levels, and using the n-layered integrated molecular orbital and molecular mechanics (ONIOM) method, have been performed for the adsorption of dimethyl methylphosphonate (DMMP) and tabun (GA) on (ZnO)n (where n ) 4, 18, or 24 molecular clusters of (1010) and (0001) zinc oxide surfaces). Different adsorption sites and DMMP orientations were considered. Calculations include the evaluation of the optimized geometries, atomic charges, interaction energies, various methods applied, and different sizes and surface types of the ZnO fragments. On both surfaces, the molecular adsorption proceeds as chemisorption via the formation of a Zn · · · O chemical bond in the case of the DMMP adsorption complex and a P · · · O covalent bond or a Zn · · · N chemical bond for GA adsorption complexes. The type of surface greatly affected the strength of the intermolecular interactions and the interaction energies. The results indicate that the adsorption of DMMP and GA is energetically more preferable on the nonpolar (1010) ZnO surface. GA was determined to be bound more tightly to the ZnO surface than DMMP, but the adsorption energies were approximately twice as low as the values revealed for the adsorption of GA and DMMP on the CaO surface. [Paukku, Y.; Michalkova, A; Leszczynski, J. Struct. Chem. 2008, 19 (2), 307; Michalkova, A; Paukku, Y.; Majumdar, D.; Leszczynski, J. Chem. Phys. Lett. 2007, 438, 72.] Therefore, it can be concluded that the decomposition of nerve agents and their simulants will be easier on CaO, whereas ZnO should make an efficient sensor for the detection of such compounds. 1. Introduction Organophosphorus compounds (phosphorus-containing organic chemicals) are often used as pesticides and warfare agents. Because of their high toxicity, there is a serious demand for more-sophisticated and useful techniques for the detection and decomposition of these substances. Among the most dangerous organophosphorus compounds are the nerve agents. They disrupt the mechanism by which nerves transfer messages to organs. The disruption is caused by blocking acetylcholinesterase, which is an enzyme that normally relaxes the activity of acetylcholine, which is a neurotransmitter. One of two main classes of nerve agents is represented by the G-series, which includes sarin (GB), soman (GD), and tabun (GA). Nerve agents cause behavioral and psychological changes in humans; these changes include irritability, nervousness, fatigue, insomnia, memory loss, impaired judgment, slurred speech, and depression.1 Tabun, like all nerve agents, is toxic even in minute doses and is among the most commonly used agents. The GA dosage that is fatal to humans has been reported to be 0.01 mg/kg. The toxicity of GA, in terms of the measured LD50, is 14-21 mg/kg body weight (dermal) and 0.014 mg/kg (intravenous).2 Because of this extreme toxicity, the chemical agents are often substituted in the experimental studies with organophosphorus simulants, such as dimethyl methylphosphonate (DMMP) and trimethylphosphate (TMP). DMMP is a widely used simulant for organophosphorus compounds, because it is nontoxic and possesses the necessary elemental composition to mimic nerve agents.3 * Author to whom correspondence should be addressed. Phone: 001601-9793482. Fax: 001-601-9797823. E-mail address:
[email protected].
Inorganic metal oxides are well-known for their use in chemical industry as adsorbents, sensors, catalyst, etc. Because of their unique morphological features and high surface area, nanocrystals of metal oxides were used as adsorbents for decomposition or detection of variety of pollutants and harmful substances, including organophosphorus compounds.4 Recently, zinc oxide (ZnO) nanoparticles have received much attention, because of various applications such as ultraviolet (UV) absorption, decomposition, deodorization, and antibacterial treatment. It is a well-known catalyst, adsorbent, toxic gas sensor, etc.5-8 ZnO has attracted research interest for its unique properties and applications in electronics, cosmetics, food, pharmaceutical, and chemical industries.9 ZnO crystallizes in three different forms: (a) wurtzite (hexagonal) structure, (b) zincblende structure, and (c) rock salt (NaCl) structure. Wurtzite is thermodynamically the most stable state under ambient conditions, although the other two structures also appear under specific conditions. In the wurtzite structure, each O2- anion is surrounded by tetrahedron of Zn2+ cations and vice versa.10-12 The tetrahedral coordination of the Zn2+ and O2- ions is not perfect but slightly distorted (the distances to the nearest neighbors are not all equal). Each Zn2+ cation, for example, is surrounded by three O2- anions at distances of 1.973 Å and by one O2- anion at a distance of 1.992 Å.11 Among the unique chemical properties of ZnO is that it lies on the borderline between ionic and covalent solids.13,14 A detailed theoretical description9,15-19 of ZnO, as well as its practical synthesis, has been reported in the literature.20 Four low-index surfaces dominate the morphology of ZnO, namely, the nonpolar (1010), which is the most stable surface of ZnO solid, and (1120), along
10.1021/jp807744a CCC: $40.75 2009 American Chemical Society Published on Web 01/07/2009
Organophosphorus Compounds Adsorption on ZnO with the polar (0001)-Zn and (0001)-O surfaces. These ZnO surfaces have been studied experimentally.14,21 Only a few experimental studies have been reported on the adsorption of toxic chemicals or simulants of the chemical agents on ZnO.8,22-26 The gas-sensing characteristics of ZnO-doped SnO2 sensors for simulants of the chemical agents were studied using the X-ray diffraction (XRD) analysis by Kwang-Hyun et al.8 ZnO nanoparticles were among the preferred metal oxides that have been considered as destructive adsorbents for biological and chemical contamination by Koper and co-workers22,26 and Klabunde.23 Their features were demonstrated under the laboratory conditions for DMMP and some nerve agents. The interaction of DMMP on ZnO, MgO, TiO2, Nb/TiO2, Al2O3, and WO3 surfaces were studied via Fourier transform infrared (FTIR) spectroscopy.24 DMMP was determined to be bound on the surface through the PdO bond. In addition to the aforementioned papers, several experimental works regarding the interactions of small inorganic substances with ZnO have also been published.27-32 Because this is not the focus of this work, we will just mention a few of them. Because of the fact that the ZnO surface has very large capacity for adsorption, it is often used as a sensor for different gases, such as atomic hydrogen.29 The mentioned study suggests that hydrogen adsorption leads to the formation of hydroxyl species via the interaction of H atoms with O atoms of the ZnO surface. The investigation of ammonia (NH3) adsorption on the nonpolar ZnO surface31 reveals that ammonia adsorbs on the surface molecularly at room temperature via the formation of hydrogen bonds between the surface O atoms and the H atoms of NH3. Annealing the ammonia-covered surface induced the partially decomposed NHx (x ) 1, 2) species. Halevi et al.33 performed a study of the reaction of CH3CH2SH and (CH3CH2)2S2 on the surface of ZnO using temperature-programmed desorption (TPD). The interaction of these molecules with ZnO was determined to be structure-sensitive. Both the thiol and disulfide were determined to be dissociatively adsorbed on the ZnO(0001) surface, forming adsorbed ethylthiolate intermediates. According to our best knowledge, there have not been published any theoretical works that have studied the adsorption of nerve agents or their simulants on ZnO. However, there are several computational studies on the adsorption of small organic molecules on the ZnO surface.10,34-38 Rodriguez and Maiti34 investigated the adsorption and decomposition of H2S on the surfaces of magnesium oxide, nickel-doped magnesium oxide, and zinc oxide, to compare the adsorption capacity of these materials. This study indicates that the bonding of H2S and its sulfur-containing dissociated species is substantially stronger on ZnO(0001) than on MgO. Casarin et al. reported the chemisorption of hydrogen and carbon monoxide on the ZnO surface.35 Reactions of molecular oxygen on ZnO were studied by Yan et al.36 Other computational studies have investigated formic acid adsorption on the ZnO surface.39,40 These studies suggest that the adsorption of HCOOH proceeds with the formation of unidentate complexes on a coordinatively unsaturated Zn2+ site and a six-membered structure on a next-neighbor pair of Zn2+-O2- acid-base center. The formation of a bidentate and bridge structures are also possible. The interaction of CO, H2, H2O, NH3, and CO2 with oxide surfaces and, in particular, with the ZnO surfaces is of great interest, because of numerous applications such as the catalytic synthesis of methanol and the hydrogenation of unsaturated hydrocarbons.31,41 As an adsorbent, ZnO is highly efficient for the adsorption of H2S, SO2, disulfides, and mercaptans.27,34 ZnO shows very strong sensitivity toward toxic substances such as halogens, sulfur, and
J. Phys. Chem. C, Vol. 113, No. 4, 2009 1475 volatile organic compounds.42-44 The sensing behaviors of doped ZnO elements were studied for the possible detection of organophosphorus compounds by Oh et al.44 The study has shown a promising sensing behavior of the oxide surface to DMMP. In the present work, we have studied the adsorption of a nerve agent (Tabun) and a simulant of organophosphorus compounds (DMMP) on polar and nonpolar ZnO surfaces, using the cluster model approach. Different cluster sizes and adsorption sites of ZnO are considered, and the influence of different computational methods on the nature of interactions of these molecules with the crystal surface of ZnO was investigated. 2. Computational Details The ZnO structure was generated using the experimental bulk structure of ZnO (the lattice parameters a, b, and c are 3.2494, 3.2494, and 5.2038 Å, respectively, and the R, β, and γ angles are 90°, 90°, and 120°, respectively; the space group for ZnO is P6_3mc).45 The more complicated the metal oxide surface is, the more rigorous the modeling technique that is required. In the cluster modeling of metal oxides, a few principles can be proposed.46 According to them, cluster should be cut out to be neutral and stoichiometric, with a minimal amount of dangling bonds. Therefore, two different strategies have been considered here in modeling the ZnO surface. First of all, it is well-known that defects often dominate the electronic and chemical properties and adsorption behaviors of metal oxide surfaces.47 On a perfect ZnO surface, only three-coordinated zinc and oxygen sites are present, whereas at the edge and the corner of the ZnO surface, which are highly reactive, the ion pairs that have a low coordination number are present. To mimic these sites of the ZnO surface, a small cluster that contains four O atoms and four Zn atoms with a chemical formula of Zn4O4 (the A model in Figure 1a) was used. Moreover, the Zn-O pairs at the corner site can also be viewed as sites of the polar (0001) and (0001) surface. This ZnO model is electroneutral. The adsorption of DMMP on this type of model is investigated to evaluate chemical reactivity of small crystallites. Also, such a calculation can clarify the effects of the next-neighbor pair of a two-coordinated acid and base center on geometry and energetics. On the other hand, a large cluster model containing 18 Zn atoms and 18 O atoms was prepared to study the adsorption on a regular ZnO surface and the effect of the cluster size. Such a surface contains stabilized reactive atoms. This surface was modeled using a slab containing two layers of Zn atoms and two layers of O atoms, arranged in two different ways, as shown in Figures 1b and 1c (the A1-L and A2-L models). To examine how the adsorption is site-dependent, two types of large cluster models were simulated that represent nonpolar (1010) and polar (0001) surfaces. Such models allow one to analyze the participation of low-coordinated acid-base centers in the adsorption. Both large ZnO models are also electroneutral. DMMP contains three different groups that can be involved in the intermolecular interactions with the active sites of the ZnO surface: PdO, O-CH3, and CH3. Therefore, several different initial orientations of DMMP toward the oxide cluster were tested. Application of the density functional theory (DFT)48 method, to study the CO adsorption on ZnO surfaces, has already been validated.49 It was found that the DFT reproduces MC-CEPA quantum-chemical calculations very well50 and, overall, is wellsuited for the study of adsorption on the ZnO surfaces. Therefore, in our study, the calculations of adsorption of DMMP and GA on the model surfaces of ZnO were conducted at the
1476 J. Phys. Chem. C, Vol. 113, No. 4, 2009
Paukku et al.
Figure 2. Zn60O60 cluster model of the ZnO surface used for the ONIOM calculations. High-layer atoms are shown as balls and sticks.
Figure 1. Models of the zinc oxide (ZnO) surface: (a) the A (Zn4O4) model, (b) the A1-L (Zn18O18) model, and (c) the A2-L (Zn18O18) model.
DFT level of theory,48 in conjunction with the B3LYP functional51 and the LanL2DZ basis set,52-54 as implemented in the Gaussian 03 program package.55 To test the influence of the method on the intermolecular interactions, DMMP and GA systems with small ZnO clusters were studied also at the MP2/ LanL2DZ//B3LYP/LanL2DZ level of theory.56-59 The geometries of the target molecules were fully optimized while the ZnO fragment was kept frozen, in a manner similar to that which was done in a previous paper by Martins et al.,37 in which the adsorption of small organic molecules on the ZnO clusters was studied. To estimate an error that occurs in our calculation due to keeping the ZnO fragment frozen, we have performed the test calculations of the A-DM and A1-DM-L systems of ZnO, so that these systems were fully optimized. The geometrical parameters and atomic charges obtained by applying the Mulliken population analysis60 were investigated. The existence of intermolecular interactions and their character is determined by applying the Atoms in Molecules (AIM) theory of Bader.61-63 This theory is based on the analysis of the total electron density (F(r)), in terms of topological features of its spatial distribution. Each atom acts as a local attractor of the electron density, and, therefore, it can be defined in terms of the local curvatures of the electron density. Two atoms are bonded if their atomic volumes share a common interatomic surface, and there is a (3, -1) critical point on this surface. A critical point is defined as a point in space where the gradient is zero (3F(r) ) 0). A (3,
-1) critical point is defined as a critical point at which two of the eigenvalues of the Hessian matrix are negative, while the other eigenvalue is positive. A bond path defines the line along which the electron density is a maximum, with respect to a neighboring line. The interatomic bonds are classified as either closed shell or shared, depending if the Laplacian of the electron density (32F) at the critical point is positive or negative, respectively. The interaction energies, corrected by the basis set superposition error (BSSE), were also calculated. To test the effect of the surrounding Zn and O atoms and additional layers on the adsorption of the target molecule, we have performed the ONIOM (n-layered integrated molecular orbital and molecular mechanics) calculations. The ONIOM method,64-66 which was developed by Morokuma and coworkers, is a more general hybrid method that can combine various numbers of molecular orbitals as well as different molecular mechanics methods. Moreover, it is reported to be an efficient tool for the accurate calculation of weak chemical interactions67 in large systems and in the calculation of large molecular systems with the accuracy afforded for smaller molecular systems.65,68-70 In this procedure, the molecular system being studied is divided into two layers, which are treated at different levels of theory. The adsorption system is called the real system. The most important part of the system (the chemically active part, the reaction center) is called the model system and is described at the highest level of theory, whereas neighbors’ effect is described by lower-level methods. The twolayered total energy expression for the ONIOM2 scheme is defined as E(ONIOM2) ) E(High, Model) + E(Low, Real)-E(Low, Model) ) E(High, Model) + ∆E(Low, Real r Model)
where
∆E(Low, Real r Model) ) E(Low, Real) - E(Low, Model) The terms “High” and “Low” refer to the high and low levels of approximation. E(Low, Real) is the total energy of the real system using the low-level method. E(High, Model) and E(Low, Model) define the total energies of the model part calculated with high- and low-level methods, respectively. The ONIOM scheme has been implemented for the extended A2-ON cluster having a chemical formula of Zn60O60. The A2-ON model was prepared by extending the A2-L model by 42 Zn and 42 O atoms (A2-L was chosen because it represents the most stable surface of ZnO). The A2-ON model was divided into two layers, as illustrated in Figure 2. The high-level model system (inner)
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J. Phys. Chem. C, Vol. 113, No. 4, 2009 1477
Figure 3. Optimized geometry of DMMP adsorbed on the Zn4O4 surface (the A-DM system) obtained at the B3LYP/LanL2DZ level of theory.
includes 7 Zn and 7 O atoms of the surface (the Zn7O7 fragment) and is marked with balls and sticks in Figure 2. This part is treated at the higher level of theory (B3LYP/LanL2DZ). The rest of A2-ON represents the lower-level layer (outer part), which is calculated using the semiempirical PM3 method.71 Analysis of the interaction of small molecules with the ZnO surfaces using the ONIOM method offers an opportunity to probe the validity of this methodology, as well as a better comprehension of the electronic and structural properties of the adsorption of DMMP and GA on this type of surface. 3. Results and Discussion 3.1. DMMP Adsorption on ZnO. The optimized structure of DMMP (B3LYP/LanL2DZ) adsorbed on the Zn4O4 fragment (A-DM) is illustrated in Figure 3. The main reason for the calculation of such model is to evaluate the interactions of the target molecule with the corner and defect sites and to compare the effect of the cluster size on the adsorption. Such a small model also was used in the study of the adsorption of HCOOH on ZnO,39 to test the influence of model size on the energetics. Because this model does not represent the regular ZnO surface, the main discussion focuses on the results obtained using larger ZnO models. The calculated interaction distances (r) (between DMMP and the ZnO fragment) with the corresponding electron density (F(r)) and Laplacian of the electron density (32F(r)) values are shown in Table 1. The DMMP adsorption occurs due to the formation of a chemical bond between the O atom of DMMP and the Zn atom of the ZnO fragment (it corresponds to the interaction of a lone electron pair of the O atom of DMMP and a vacant orbital of surface acid site - Zn2+ cation). It differs from the adsorption of CO on the Zn4O4 cluster with adsorbed H atoms,49 where the most stable orientation of CO toward the surface was determined to be via the C atom. The Zn · · · O distance is 1.97 Å in A-DM, and this bond is created with a coordinatively unsaturated Zn2+ acid site (two-coordinated). A-DM is additionally stabilized by the formation of two hydrogen bonding interactions of the C-H · · · O type. They are formed between the molecular methyl groups and the same O atom (A-DM) of the ZnO fragment. These hydrogen bonds possess the H · · · O distances that range from 2.03 Å to 2.07 Å, and the values of the electron density range from 0.024 e/au3 to 0.026 e/au3 (see Table 1). The values of the electron density
Figure 4. Optimized geometry of DMMP adsorbed on the Zn18O18 surface obtained at the B3LYP/LanL2DZ level of theory: (a) A1-DM-L and (b) A2-DM-L.
for the chemical bonds are almost three times larger than that for the hydrogen-bond interactions in these systems (0.0704 e/au3). Therefore, it can be concluded that DMMP is mainly stabilized by the formation of a chemical Zn · · · O bond while dipole-dipole and ion-dipole interactions play an insignificant role in the adsorption. A similar conclusion was derived by Rodriguez and Maiti34 for the adsorption of H2S on the (0001) surface of ZnO. They determined that (i) the reactivity of an oxide is mainly dependent on how well its electron bands mix with the orbitals of H2S and (ii) the electrostatic interactions with the dipole moment of H2S only play a secondary role in the binding. Figures 4a and 4b display the optimized structure of DMMP adsorbed on the large clusters of ZnO with a Zn18O18 chemical formula. Two different surfaces of this large clusters were tested (the Zn-terminated polar (0001) and the nonpolar (1010) surface). Three different high-symmetry adsorption sites with a 3-fold symmetry are present at the ZnO surface: on the top position, a hollow-site position above atoms in the second surface layer, and a hollow site with no atoms beneath. DMMP interacts preferably with the top Zn atoms and with the corner O atoms of the second layer. The O atoms of the second layer of the ideal (0001) ZnO surface were determined to play an active role also in the adsorption of CH3OH.72,73 The DMMP
(2.189)
1.674 (2.776)
2.229 (2.891) 114.8
1.965 (3.033) 2.676 (3.289) 162.7
0.027185 (0.100915) 0.006144 (0.024527) 2.136 (3.083) (2.176) 142.6
2.065 (2.990)
2.027 (3.060) 154.4
C1-H · · · Oa C1-H · · · Ob (L) P · · · Oa (GA) C4-H · · · Oa (A-GA-L) C2-H · · · Oa 0.026179 N1 · · · Zn2 (A-GA-L) (0.090301) C2-H · · · Ob O2 · · · Zn2 (A1DM-L) C1-H · · · Oc (A2DM-L)
139.2
0.018883 (0.073876) 0.044336 (0.194929)
2.091 (2.933) 131.3 2.298 (3.140) 132.0
0.017985 (0.080266)
116.8
(1.875)
0.044942 (0.165523)
166.9
(2.076)
0.052889 (0.261168) 0.051637 (0.177921) (2.151)
0.049115 (0.198364) 0.091233 (-0.205401) (1.611)
0.061245 (0.326628) 0.021736 (0.089722) (2.123)
0.049768 (0.225060) 0.015347 (0.058441) (1.974)
0.070367 (0.379798) 0.023908 (0.087174) O1 · · · Zn1
H· · ·Y ∠X-H-Y (X · · · Y) bond/characteristic
F (32 F)
H· · ·Y ∠X-H-Y (X · · · Y)
F (32 F)
H· · ·Y ∠X-H-Y (X · · · Y)
F (32 F)
H· · ·Y ∠X-H-Y (X · · · Y)
F (32 F)
H· · ·Y ∠X-H-Y (X · · · Y)
F (32 F)
A-GA-L A-GA A2-DM-L A1-DM-L A-DM
TABLE 1: Electron Density G (e/au3) (and the Laplacian of Electron Density, 32G (e/au5)), H · · · Y and X · · · Y (Given in Parentheses) Distances (Å), and ∠X-H-Y Bond Angles (deg) of Formed Bonds for the ZnO-DMMP and ZnO-GA Systems Calculated at the B3LYP/LanL2DZ Level of Theory
1478 J. Phys. Chem. C, Vol. 113, No. 4, 2009
Paukku et al. adsorption is similar for both surfaces. The main stabilization comes from the formation of a Zn1 · · · O1 chemical bond in which the PdO group of DMMP is involved. Such a result is in agreement with the conclusion of the experimental study of DMMP interaction with several metal oxides, among which also was ZnO.24 The authors have observed the shift of the PdO stretch to lower energy, indicating the P · · · O · · · M interaction (where M represents a surface atom). In A1-DM-L (representing the (0001) surface), this chemical bond is formed with a corner Zn2+ acid site, whereas in A2-DM-L (it represents the (1010) surface), such a bond is created with a surface Zn2+ cation placed approximately in the middle of the cluster. We would like just to mention that the Zn1 · · · O1 bond in A2-DM-L seems to be stronger than that in A-DM (the Zn · · · O distance is shortened by ∼0.36 Å) and even stronger than that in A1-DM-L (it is characterized by a much shorter distance; the difference is ∼0.6 Å). It is caused by the fact that, in A1-DM-L, two Zn · · · O bonds are formed (a bridge structure; two O atoms of DMMP interact with a next-neighbor surface Zn atom), which are characterized by similar distances and similar topological characteristics. In A2-DM-L, the Zn · · · O bond occurs as a unidentate (the O atom of the PdO group directly coordinates to the surface Zn atom). This means that the strength is divided almost equally between two bonds in A1-DM-L instead of being concentrated in one, as it is in the case of A2-DM-L. This also indicates that the electrostatic interactions between DMMP and the surface dipole moments have an insignificant role in the stabilization of the target molecule, compared to the effect of the Zn · · · O bond formation. It agrees with the finding for the DMMP adsorption on small Zn4O4 clusters and with the conclusion of Rodriguez and Maiti34 previously mentioned. Greater strength of the Zn · · · O bond in A2-DM-L than in A1-DM-L is accompanied by larger values of F and 32F (the difference in F is ∼0.02 e/au3, and the difference in 32F is ∼0.1 e/au5). For the adsorption of water and hydroxyl group on the (ZnO)4, (ZnO)5, and (ZnO)6 clusters having the (1010) and (0001) surfaces, the Zn · · · O distances were observed to be similar to the Zn · · · O distances in this paper (in the range of 1.83-2.33 Å).74 The difference between A1-DM-L and A2-DM-L is in the number of hydrogen bonds that are formed (in the A2-DM-L system, three C-H · · · O hydrogen bonds are created, whereas in A1-DM-L, only two such bonds are found). Different ionicities and amounts of the covalent character in the Zn-O interactions on polar (A1-L) and nonpolar (A2-L) ZnO surfaces result in lower strength of the C2-H · · · Oa and C1-H · · · Ob hydrogen bonds in A1-DM-L (the difference in F is ∼0.006-0.008 e/au3). The third muchweaker hydrogen bond created in A2-DM-L contributes only negligibly to the stabilization of DMMP on this type of ZnO surface. One can see that the type of the ZnO surface affects the character and strength of the intermolecular interactions with DMMP. If the surface is mostly formed from the Zn atoms, the molecule prefers to form chemical bonds with the Zn atoms (the (0001) polar surface represented by the A1-L model). On the other hand, if the surface is formed from the O and Zn atoms, the molecule is adsorbed more strongly, forming stronger interactions (the (1010) nonpolar surface represented by the A2-L model). The geometrical parameters of atoms of DMMP that are directly involved in the intermolecular interactions are affected by the adsorption, as one can see from Table 2. The experimental results also indicate that the DMMP molecule was labilized by the adsorption on various oxide surfaces, among which ZnO also was considered.24 This effect is proportional to the strength and character of the interactions. For example, the PdO1 bond,
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J. Phys. Chem. C, Vol. 113, No. 4, 2009 1479
TABLE 2: Geometric Parameters of Isolated DMMP and GA and Adsorbed on ZnO Obtained at the B3LYP/LanL2DZ Level of Theory system/feature
A1-DM-L
bond lengths (Å) D(P-O1) D(P-C1) D(P-O2) D(O2-C2) bond angles (deg) R(O1-P-C1) R(P-O2-C2) R(O1-P-O2) dihedral bond angles (deg) γ(O1-P-O2-C4) γ(O1-P-O2-C2) γ(O1-P-C1-O2)
A2-DM-L
1.612 1.838 1.719 1.508
1.611 1.839 1.690 1.496
A-DM
DMMP
1.634 1.838 1.688 1.497
1.592 1.840 1.718 1.470
117.8 124.8 106.4
119.6 125.5 107.5
115.7 124.7 113.0
121.3 121.4 113.5
173.0 121.6
140.3 128.9
62.0 131.3
26.3 126.0
A-GA
A-GA-L
1.660 1.942 1.760
GA
1.602 1.882 1.655
A-DM-ON
1.587 1.831 1.710
131.2
98.6
115.4
100.5
125.8
118.1
21.8
60.4
42.3
99.0
131.0
124.4
1.579 1.842 1.719 1.469 118.5 122.2 107.4 177.7 125.0
TABLE 3: Change in the Mulliken Atomic Charges (e) of X, H, and Y Atoms of X-H · · · Y and X · · · Y Bonds in DMMP-ZnO and GA-ZnO Systems Obtained Using the B3LYP/LanL2DZ Methoda Change in the Mulliken Atomic Charges, ∆q (e) A-DM system/bond
X
H
A1-DM-L Y
X
H
A2-DM-L Y
O1 · · · Zn1 0.099 -0.189 0.073 -0.166 C1-H · · · Oa 0.032 -0.029 0.154 0.013 -0.038 0.078 C1-H · · · Ob (L) P · · · Oa (GA) C4-H · · · Oa (A-GA-L) 0.038 -0.024 0.154 -0.011 -0.035 0.042 C2-H · · · Oa N1 · · · Zn2 (A-GA-L) C2-H · · · Ob 0.113 -0.028 O2 · · · Zn2 (A1DM-L) C1-H · · · Oc (A2DM-L) total charge
0.088
0.137
X
H
A-GA Y
X
H
0.118 -0.199 0.080 0.075 -0.044 0.111 -0.144 0.036 -0.033
0.083
0.075 -0.044
0.048
0.085
A-GA-L Y
X
H
Y
-0.045 0.111 -0.161 0.135 0.025 -0.083 0.136
0.013 0.006
-0.100
0.135 0.051
0.073
0.241
a Positive values indicate an increase in negative charge and a decrease in positive charge; negative values indicate an decrease in negative charge and a increase in positive charge.
which is involved in the formation of a Zn · · · O chemical bond with the surface, is enlarged and is determined to be weakening, compared to the isolated DMMP. This effect is the most significant in the case of the A-DM system (∼0.04 Å). The P-C1 bond distance is changed less significantly, because it participates only in weak hydrogen bonds. The P-O2 bond length is shortened the most significantly in A-DM and the O2-C2 bond length is enlarged the most in A1-DM-L (∼0.04 Å). Because the geometrical changes of DMMP in A-DM are partially caused also by the small size of the ZnO model, we can conclude that the adsorption on a polar ZnO surface (A1L) affects the conformation of DMMP much more than the interactions with a nonpolar ZnO surface (A2-L). A similar trend is revealed for the changes in bond and dihedral angles: O1-P-C1 shows the largest decrease for the A-DM system, and O1-P-O2 shows the largest decrease for the A1-DM-L (∼6°-7°). Only exception is the P-O2-C2 angle, because it is increased the most for the A2-DM-L system (∼4°). The O1-P-O2-C2 dihedral angle is changed the greatest in all DMMP systems, comparied to other dihedral angles. This change is the most significant for the A-DM system (∼36°). 3.1.1. Analysis of Atomic Charges. Table 3 presents the changes in the Mulliken atomic charges of the X, H, and Y atoms that are involved in the intermolecular interactions of the studied systems, relative to the isolated DMMP molecule (positive values denote an increase in negative charge and a decrease in positive charge, whereas negative values denote a decrease in negative charge and an increase in positive charge). According to these values, one can quantify the electron density transfer, which can be evaluated based on the surface reactivity. In the DFT calculations for large ZnO clusters, Mulliken charges
of 0.81-0.97 e have been reported for the Zn cations.75 In our study, the calculations predict Zn1 charges of 0.983, 0.828, and 1.061 e in the isolated ZnO, ZnO1-L, and ZnO2-L clusters. One can see that the larger the positive charge on the metal center, the larger its bonding ability. A comparison of the Zn1 and O1 charges in isolated and interacting ZnO and DMMP shows a charge transfer caused by the formation of a Zn · · · O chemical bond. The transfer is proportional to the strength of this bond, the values of the electron density, and the Laplacian of the electron density (see Table 1). The largest transfer was revealed for the A2-DM-L system: the positive charge of zinc is increased by ∼0.2 e and the negative charge of oxygen is increased by ∼0.12 e. In the case of hydrogen bonds, the bridging H atom becomes more positive, while the C1, C2, Oa, and Ob atoms become more negative (except for the C2 atom in A1-DM-L, which becomes less negative but the charge decreases only by 0.01 e). In all systems, the charge of the hydrogen H(-C1) atom is changed the most significantly among all H atoms involved in the formation of hydrogen bonds. We would like just to mention that the atomic charge of hydrogen (C3-H) of adsorbed DMMP is not modified, compared to the isolated DMMP, as a consequence of no participation in the hydrogen bonding. The change in the charge of the X and Y atoms involved in the hydrogen bonds is the most significant for the C1 atom (proton donor) in A2-DM-L (0.075 e) and for the Oa atom (proton acceptor) in A-DM (about 0.15 e), because these atoms form two hydrogen bonds. The charge increase of the H atoms results in a H atom that is more attracted toward the negatively charged C atom. The adsorbed DMMP carries a net positive charge of 0.085-0.137 e. This indicates an adsorbate f substrate electron transfer, which was determined to be the largest for the A1-
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TABLE 4: Interaction Energy Values (kcal/mol) of the ZnO-DMMP and ZnO-GA Systems Obtained at the B3LYP/ LANL2DZ and MP2/LANL2DZ//B3LYP/LANL2DZ Levels of Theory
interaction energy BSSEa value
A1DM-L
A2DM-L
-47.5 10.8
-54.1 12.0
A-DM
A-GA
A-GA-L
A-DM-ON
B3LYP -51.5 8.2
-52.8 10.6
-58.9 5.8
-46.3 28.0
MP2 -46.4 16.0
-47.2 24.1
interaction energy BSSEa value a
BSSE ) basis set superposition error.
DM-L complex and the smallest for the A2-DM-L complex. Such an electron donation of DMMP to the ZnO surface supports the Lewis acidity of the Zn sites, which can accept a pair of electrons. 3.1.2. Energetics. Table 4 presents the interaction energies corrected by the basis set superposition error (BSSE) for all studied systems. The interaction energy of A-DM was determined to be approximately -50 kcal/mol. This adsorption energy is much higher than the bonding energy predicted at the DFT level for H2S on ZnO clusters (-32 kcal/mol).76 Comparison of the energetic situation of A-DM with the DMMP systems that contain the large ZnO clusters shows that the extension of the ZnO cluster enhances its reactivity. This finding is supported by the results of a theoretical evaluation of the influence of the ZnO cluster size, which has shown that large ZnO clusters interact more strongly with formic acid than the Zn4O4 and Zn9O9 clusters.39,40 Our results also indicate that the adsorption strength and the target molecule stabilization are dependent on the type and polarity of the surface. DMMP is more strongly adsorbed on the A2-DM-L system (representing a nonpolar (1010) surface), because this type of the surface contains more active acidic and basic sites. This causes the formation of much stronger Zn · · · O chemical bonds and hydrogen bonds than those in A1-DM-L (representing a polar (0001) surface). The energy difference is ∼7 kcal/mol. This means that the formation of a unidentate Zn · · · O bond in the A2-DM-L complex is energetically more favorable than the formation of a bridge structure between the surface Zn atoms and molecular O atoms. The formation of a second Zn · · · O interaction and two weak C-H · · · O hydrogen bonds in the A1DM-L system contributes to the adsorption strength less significantly than the formation of three stronger hydrogen bonds in combination with stronger Zn · · · O interaction. The A2-DM-L system was found to be the most stable among all studied systems with the interaction energy equals to -54 kcal/mol. The interaction energy value is very similar to that observed for the adsorption of NH3 on an unrelaxed (0001) ZnO surface (-51 kcal/mol).77 It can be concluded that the Zn-terminated polar (0001) surface of ZnO is less active for the DMMP molecular adsorption than the nonpolar (1010) surface. The binding energies of water and hydroxyl group adsorbed on the ZnO clusters (among which also was Zn4O4) model were also determined to vary according to the surface type and ZnO cluster size.74 DMMP was found to be much better stabilized on the nonhydroxylated CaO surface,78 which was characterized by a much larger interaction energy (approximately -90.0 kcal/mol) than that on the ZnO surface. This can be explained by the larger degree of ionicity in CaO than in ZnO and by the higher activity of the Ca cations toward the DMMP adsorption. The acidity, electronegativity, and metal-oxygen formation energy of metal oxides were determined to affect the labilization of the PdO bond of DMMP during its adsorption.24 The authors have found a correlation between changes in the PdO force constant and
the isoelectric point, electronegativity of the metal, and heat of the M-O bond (where M stands for a surface atom) formation of metal oxide. The main stabilizing interaction in CaO-DMMP (in A1-DM, it is the P · · · O bond) possesses a length of 1.733 Å versus that of the Zn · · · O bond in ZnO-DMMP (1.974 Å). Moreover, longer elongation of the molecular P-O bond lengths (0.2 Å versus 0.04 Å) is found for the CaO-DMMP system than for the ZnO-DMMP system. These results indicate that the decomposition of DMMP would be easier on the CaO surface than on ZnO. The experimental study of DMMP adsorption on MgO, ZnO, Al2O3, TiO2, and WO3 has concluded that these metal oxides are good for the concentration stage of the decontamination process.24 One of the reasons to use Zn4O4 was to test different orientations and locations of DMMP toward the surface, to determine the most stable situation. As an example, we would like to briefly discuss a different structure of the DMMP-Zn4O4 complex (A′-DM), which is much less stable than the A-DM system. These systems differ in the character of the intermolecular interactions and in the adsorption strength. DMMP is still adsorbed through a Zn · · · O chemical bond but the O · · · Zn bond distance is longer (2.044 Å) than that observed for A-DM (1.97 Å). This shows that this interaction is weaker in a new A′-DM complex, because of the different character of the interacting Zn site (two-coordinated in A-DM but threecoordinated in A′-DM). Hydrogen bonds in this complex, which are formed between the C-H groups and two different surface O atoms (not with the same O atom as that in A-DM), are also weaker and contribute less to the DMMP stabilization. The values of the electron density for the chemical bond (0.058 e/au3 for A′-DM and 0.0704 e/au3 for A-DM) are ∼2-3 times larger than that for the hydrogen-bond interactions in these two systems. The interaction energy of A′-DM is approximately -30 kcal/mol, whereas the interaction energy of A-DM is approximately -50 kcal/mol. The adsorption energy of A′-DM is similar to the bonding energy predicted at the DFT level for H2S on ZnO clusters (-32 kcal/mol).76 It can be concluded that the character of the ZnO adsorption sites has a crucial role in the adsorption strength of DMMP on such an oxide. 3.1.3. Full Optimization. We have performed the test calculations to address the issue of frozen geometry of the A and A1-L clusters of ZnO during the DMMP adsorption. The full optimization of a model containing small Zn4O4 (A) clusters leads to a complete distortion of the ZnO cluster. In the case of systems that contain the large ZnO model (A1-L), the crystal structure was completely changed and did not correspond to the experimental crystal structure of wurtzite, which was used to prepare the ZnO clusters. To estimate the error that occurs in our calculation, which is due to keeping the ZnO fragment frozen, we have compared the adsorption of DMMP, by applying partial and full optimization. In the fully optimized Zn4O4-DMMP system, DMMP remains to be chemisorbed through the formation of two Zn1 · · · O1 and Zn2 · · · O2 chemical
Organophosphorus Compounds Adsorption on ZnO
Figure 5. Optimized geometry of GA adsorbed on the Zn4O4 surface (A-GA) and on the Zn18O18 surface (A-GA-L) obtained at the B3LYP/ LanL2DZ level of theory: (a) A-GA and (b) A-GA-L.
bonds (instead of one, as in A-DM,) which are slightly weaker (∼0.01 Å) than those in A-DM. In the case of the fully optimized A1-DM-L model, DMMP is adsorbed in the same way on the ZnO surface as in partially optimized A1-DM-L (through two Zn · · · O chemical bonds and two C-H · · · O hydrogen bonds). Such adsorption differs only slightly in the strength of intermolecular interactions (∼0.01 Å for chemical bonds and ∼0.02 and 0.05 Å for hydrogen bonds). The corrected interaction energy value of fully optimized A-DM is -51.5 kcal/ mol, whereas partial optimization gives a value of -52.8 kcal/ mol. For fully optimized A1-DM-L, this value amounts to -47.5 kcal/mol, whereas partial optimization gives -49.6 kcal/mol. One can see that full optimization of the DMMP-ZnO clusters leads to negligible changes in strength of the intermolecular interactions (
