Free Energy and Electronic Properties of Water Adsorption on the

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Free Energy and Electronic Properties of Water Adsorption on the SnO2(110) Surface Gianluca Santarossa,† Konstanze Hahn,‡ and Alfons Baiker*,†,§ †

Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, Hönggerberg, HCI, 8093 Zurich, Switzerland ‡ Institute of Physical Chemistry, University of Zurich, 8057 Zurich, Switzerland § Chemistry Department, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia ABSTRACT: A molecular understanding of the adsorption of water on SnO2 surfaces is crucial for several applications of this metal oxide, including catalysis and gas sensing. We have investigated water adsorption on the SnO2(110) surface using a combination of dynamic and static calculations to gain fundamental insight into the reaction mechanism at room temperature. The reaction dynamics are studied by following water adsorption and dissociation on the SnO2 surface with metadynamics calculations at low and high coverage. The electronic structure in the relevant isolated minima is investigated through Mulliken charge analysis and projected density of states analysis. Surface bridging oxygen (Obr) is found to play a decisive role in water adsorption forming rooted hydroxyl groups with the water H atoms. Bond formation with H significantly changes the electronic configuration of Obr and presumably leads to reduced band bending at the SnO2 surface. The free-energy estimation indicates that on a clean SnO2(110) surface at room temperature both associative and dissociative adsorption occur, with the latter being thermodynamically favored. Oxygen coverage strongly affects the ratio between associatively and dissociatively adsorbed H2O, favoring associative adsorption at high oxygen coverage (oxidized surface) and dissociative adsorption at low oxygen coverage (reduced surface). Electronic analyses of isolated surface minima show the existence of two different electron-transfer phenomena occurring at the surface, depending on the water adsorption mechanism. The relevance of these findings in explaining the changes in electric conductivity occurring in SnO2-based gas sensors upon water adsorption is discussed. Whereas associative adsorption leads to electron enrichment of the metal oxide surface, dissociative adsorption induces surface electron depletion. Both mechanisms are consistent with the electrical conductivity changes occurring upon interaction of SnO2 with water, causing cross sensitivity to the latter. The theoretical results form the basis for correlating the existing atomistic models with the experimental data and offer a coherent description of the reaction events on the surface at room temperature.

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INTRODUCTION

Controlling the effect of water on SnO2-based materials is of major importance in optimizing their reactivity in several industrial applications.1 In the gas-sensing industry, a common approach to increasing gas sensor performance with respect to cross sensitivity consists of altering the structure of the oxide by deliberately introducing impurities that would affect the chemical−physical properties of the material.11−13 For SnO2, it has been shown that the addition of Ti results in a significant stabilization of the sensor response in the presence of varying water vapor content.12 By doping SnO2 nanoparticles with a low Ti content (4.6 atom %), the cross sensitivity to relative humidity remarkably decreases while maintaining a high sensitivity toward the tested analyte (ethanol).12 Similar results have been obtained by doping SnO2 with transition metals such as Ru and Pt.14−17

Tin dioxide (SnO2) is a wide-band-gap n-type semiconducting metal oxide that is widely applied in several industrial sectors. Important applications of SnO2 include chemoresistive gas sensors,1,2 catalysts,1,3 glass coatings, and transparent conductors.1 The ability of SnO2 to change its conductivity when exposed to reacting gases makes it particularly interesting for the gas sensor industry, where it is the most widely utilized key component for analyte detection.2 As a result of their considerable reactivity, SnO2-based gas sensors also exhibit high sensitivity toward water,1,2 leading to serious limitations in their industrial application.2 In fact, the reaction between water vapor and the SnO2 surface causes remarkable changes in the oxide conductivity and consequently in the sensor’s response.4−10 This phenomenon, known as cross sensitivity to humidity, is the major drawback of SnO2 application for the detection of several relevant analytes such as CO, NOx, H2, and ethanol in air.2 © 2013 American Chemical Society

Received: January 24, 2013 Revised: April 6, 2013 Published: April 8, 2013 5487

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mechanisms is predominant is still debated4,40 and requires further theoretical and experimental studies.41 In this study, we present a dynamic investigation of the reaction mechanism of H2O on the SnO2 surface at room temperature. The adsorption mechanisms of H2O on the clean SnO2(110) surface are simulated at low (1/12 ML) and high (1 ML) coverage using a combination of first-principles dynamic and static calculations. Enhanced dynamics simulations, based on the metadynamics strategy,42−45 are used to identify the free-energy surface (FES) of adsorption and dissociation at room temperature. Static calculations of the identified minima are used to shed some light on the geometrical, energetic, and electronic properties of the surface.

Although these attempts have led to relevant improvements in the sensitivity of SnO2-based sensors, their empirical nature still does not allow a controlled, systematic development of the materials’ properties on the industrial level. Defining new strategies to limit the cross sensitivity requires further understanding of the underlying reaction mechanism of water on SnO2 surfaces.18−21 Water adsorption on SnO2 was studied by transmission infrared spectroscopy (IR),22,23 temperatureprogrammed desorption (TPD) in He carrier gas,10,24−26 and conductivity measurements.10,27 The experimental studies revealed the coexistence of molecular water and hydroxyl groups on the metal oxide surface up to 320 K.23 At lower temperatures, TPD and IR features were assigned to the desorption of molecular water.10,24,26 At temperatures higher than 435 K, desorption peaks were assumed to originate from the desorption,23 recombination,10,26 or disproportionation24 of hydroxyl groups. In recent years, particular attention has been paid to the theoretical investigation of the reaction mechanism of H2O on pure SnO2 surfaces.28−33 On SnO2(110) surfaces, H2O tends to dissociate spontaneously at low, half, and full coverages, indicating that complete dissociation is the energetically most favored adsorption mechanism.30,32 Still, at full coverage, half dissociated/half molecularly adsorbed water was found to exhibit an only slightly lower adsorption energy compared to that of fully dissociated water.28,32 Dynamics studies of water proton transfer on SnO2 suggest that the dissociative and associative states are in dynamic equilibrium because of the formation of strong hydrogen bonds between water and the surface bridging oxygen (Obr, oxygen atom located on the surface layer).29 The response of SnO2 to water has been associated with the presence of OH groups on the metal oxide surface.4,6,10,34−36 Yamazoe et al. correlated the high-temperature desorption peak in TPD with decreases in conductivity and consequently concluded that the change in conductivity upon water adsorption on SnO2 is caused by hydroxyl groups.10 Theoretically, three main mechanisms have been suggested to describe the increase in the conductivity of SnO2 upon H2O adsorption: (i) simple H2O dissociation, (ii) the formation of oxygen vacancies, and (iii) indirect paths with preadsorbed species.4,37 For all of the models, it is suggested that H2O dissociates into a terminal OH group (OHT) binding to a surface metal atom and a H atom (HR) binding to a neighboring bridging oxygen atom according to

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COMPUTATIONAL METHODS

The dynamic and static simulations are based on density functional theory (DFT) as implemented in the CP2K program package, a suite of programs aimed at performing efficient electronic structure calculations and molecular dynamics at different levels of theory.46 The electronic structure calculations employ the Gaussian and plane wave (GPW) formalism.47,48 The interaction of the valence electrons with frozen atomic cores is described by Goedecker−Teter−Hutter norm-conserving, dual-space-type pseudopotentials,49 and the 2s and 2p electrons of oxygen and the 5s and 5p electrons of Sn have been explicitly considered in the valence shell. A double-ζ valence plus polarization (DZVP) basis set, optimized according to the Mol-Opt method, has been adopted.50 In the auxiliary PW expansion of the charge density, the energy cutoff has been set at 400 Ry. The exchange-correlation term was modeled using the Perdew−Burke− Ernzerhof (PBE) functional.51 Grimme’s empirical potential has been added to the simulations to include the effects of dispersion forces.52,53 The optimized bulk lattice constants of the unit cell are a = 4.856 Å, c = 3.278 Å, and u = 0.3069 Å.54 The metal oxide surface is simulated through a supercell consisting of 720 atoms exposing the (110) crystallographic plane.54 This supercell consists of the explicit (4 × 3) repetition of the unit cell in the xy plane, exposing 12 surface sites to the interaction with reactants and 10 repetitions in the z direction, perpendicular to the surface. The resulting supercell has sizes of 20.60 and 13.11 Å in the x and y directions, respectively. In the z direction, 20 Å of free space is used in order to avoid any spurious interaction between the slab and its periodic images. In the low-coverage simulation, a single water molecule is added to a completely oxidized surface. At high coverage, the SnO2 surface is chemically reduced by the inclusion of 12 dissociated water molecules filling up all of the adsorption sites. All of the simulations are performed with periodic boundary conditions. Molecular Dynamics and Metadynamics Calculations. In the dynamics simulations, the first three surface layers are left free to relax whereas the atoms of the other seven layers are kept fixed to mimic the bulk behavior. A time step of 0.5 fs and a wave function convergence of 10−6 have been chosen as a compromise between energy conservation and time efficiency for the MD simulations. The atoms are coupled to a CSVR thermostat to keep the temperature close to the chosen value of 300 K.55 The acceleration of the reaction events is performed by means of metadynamics simulations. The method allows the reconstruction of the free-energy surface with respect to a set of chosen collective variables (CVs) σα describing the reaction events to be simulated.42−45 For this investigation, the collective variables σα have the form of coordination numbers

H 2O(g) + Sn* + O*br → (Sn δ +−OH δT−) + OH+R + ne− (1)

where the asterisks indicate the active sites of the metal oxide surface, the (Snδ+−OHTδ−) species indicates the isolated terminal hydroxyl group, and OH+R indicates the newly formed rooted OH group with Obr. The three mechanisms differ in the process of electron delivery through the metal oxide. In the water dissociation mechanism (i), the ionization of the rooted OH is sufficient to induce an electron donation to the conduction band. According to the oxygen vacancy mechanism (ii), the rooted OH+R group evolves binding to a neighboring Sn atom, producing an oxygen vacancy.37 This vacancy donates two electrons to the conduction band of the semiconductor through an ionization process. The third mechanism (iii) implies an indirect process where preadsorbed species are changing the electron affinity of the surface. For instance, the terminal and rooted hydroxyl groups could react with acidic or basic groups present on the surface.38,39 Which one of these

⎡ ⎢ Nb 1 − = ∑ ⎢∑ ⎢ i=1 j=1 1 − ⎢⎣ j ≠ i Na

Ca,b

rij

p

⎤

( ) ⎥⎥, q > p ( ) ⎥⎥⎦ R a,b rij

R a,b

q

(2)

where Ca,b is an average measure of the number of atoms of type a that are neighbors of atoms of type b. Na and Nb are the numbers of atoms 5488

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involved, rij is the interatomic distance, Ra,b is the reference distance between a and b atoms, and exponents p and q determine the decay of the function. Two coordination numbers, CSn,O and CO,H, have been used as CVs to describe the adsorption and the dissociation events, respectively. CSn,O describes the adsorption event as the coordination of a water oxygen atom to any 5-fold Sn (Sn5c) surface atom. This CV varies between the values of 0.0 (no adsorption) and 1.0 (adsorption to a single Sn atom). The equilibrium distance RSn,O is set equal to 2.3 Å. p and q are set to 10 and 16, respectively. CO,H describes the dissociation event as the coordination of a water oxygen atom with any hydrogen atom available at the surface. The collective variable values range from 2 (molecular water) to 1 (dissociated water). The equilibrium distance ROH is set to 1.5 Å, and the values of p and q are 14 and 24, respectively. The sharp decay of the CO,H CV with these parameters has been chosen to avoid an overestimation of the O−H coordination number, in particular, in the high-coverage study. In the extended Lagrangian formulation of metadynamics, a coarsegrained MD trajectory is generated in the subspace of the σα variables by following the evolution of an equal number of auxiliary variables sα. A time-dependent potential, V(t, s), is constructed by superimposing a series of Gaussian beads centered at si = {sα(ti)} and added at discrete intervals Δt

⎡ (s − si)2 ⎤ V (t , s) = W ∑ exp⎢ − ⎥ ⎣ 2δ s2 ⎦ t 1.80), indicating the constant presence of molecular water. The large, shallow valley of B2 accounts for ca. 22% of the entire conformational space. The subsequent dissociation of the water molecule leads to two hydroxyl species on the surface, derived by the transfer of a hydrogen atom from the molecular water to a neighboring Obr. The hydroxyl anchored to Sn5c is designated as terminal OH (OHT), and the one including Obr is designated as rooted OH (OHR). The water dissociation transition barrier at CO,H = 1.80 has a small energy barrier of ca. 0.10 eV. The dissociative adsorption basin (B3) is located in the bottom part of Figure 1. The area corresponds to the lowest free energy (CSn,O = 0.90, CO,H = 1.00). B3 extends through the largest area of the FES (CO,H < 1.80). In this basin, the hydroxyl groups oscillate largely because of thermal energy, but they cannot desorb from the surface. The size and depth of B3 make it the largest FE basin, corresponding to ca. 78% of the total population. Representative Conformations at Low Coverage. To compare the free-energy results of Figure 1 to the potential energy from static calculations, representative geometries (conformations C1−C3.2) belonging to each of the low-

integrating the free energy of each basin in the space of the collective variables. It is worth noting that the adsorption and dissociation events describing the process are distinct both in time and in the space of the collective variables: initially the desorbed water molecule (B1) adsorbs to the oxide surface to form a stable molecularly bound species (B2). Afterward, the dissociation occurs on the surface leading to the most stable adsorption mode (B3). The free-energy minima of the identified basins are reported in Table 1. The desorbed water state (B1) is located at the top left of Figure 1. B1 is a small area centered at CSn,O = 0.20 and CO,H = 1.95. Note that because of the characteristic behavior of the CSn,O CV, which becomes zero as soon as the water leaves the surface, the free-energy value for this basin could not be calculated accurately. In the CSn,O direction, the basin is limited by the adsorption transition barrier (CSn,O = 0.35), which is a saddle point with a free energy of ca. 0.50 eV. The second free-energy minimum (B2) corresponds to the molecular adsorption in which water anchors to a Sn5c atom. At its minimum (CSn,O = 0.80, CO,H = 1.95), the FE value is ca. 0.30 eV. In the CSn,O direction, describing the adsorption process, B2 is a shallow, large basin (CSn,O > 0.35), whereas in 5490

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Table 2. Mulliken Charges on H2O and the Interacting Surface Oxygen Atom (Obr)a conformation

q(HT)

q(HR)

C1 C2.1 C2.2 C3.1 C3.2

desorbed associative associative dissociative dissociative

0.14 0.20 0.19 0.20 0.18

0.14 0.20 0.19 0.16 0.17

C4 C5 C6 C7 C8 C8

desorbed associative dissociative dissociative associative dissociative

0.14 0.18 0.17 0.20 0.19 0.17

0.14 0.18 0.17 0.19 0.18 0.17

q(OT)

q(Obr)

Low Coverage −0.27 −0.64 −0.19 −0.69 −0.23 −0.71 −0.51 −0.47 −0.48 −0.48 High Coverage −0.28 −0.62 −0.21 −0.68 −0.47 −0.47 −0.56 −0.44 −0.24 −0.69 −0.54 −0.46

q(Sn5c)

q(H2O)

1.46 1.40 1.41 1.41 1.43

0.01 0.21 0.15

1.44 1.38 1.38 1.51 1.37 1.39

0.00 0.15

q(OHT)

q(OHR)

−0.31 −0.30

−0.31 −0.31

−0.30 −0.36

−0.30 −0.25

−0.37

−0.29

0.13

a

HT and OT belong to the terminal OH (OHT), whereas Obr and HR generate the rooted OH (OHR). In C1 and C4, q(Obr) signifies the charge on a noninteracting Obr atom, in C2.1 it indicates the average charge of the two closest Obr atoms interacting with the two H atoms of the molecule. The charges in conformations C7 and C8 refer to the average charge of all adsorbed H2O species.

Figure 3. PDOS of stable conformations C1−C3.2 at low coverage projected onto the atoms of the water molecule (HT, HR, and OT, continuous lines) and the interacting surface atoms (Sn5c and Obr, dotted lines).

Two independent minima mapping within basin B2 have been isolated (C2.1 and C2.2). Figure 2b shows the perpendicular molecular adsorption mode (C2.1), which was previously reported in the literature as the most stable molecular adsorption mode at low coverage.28 In this conformation, the water molecule lies on top of a Sn5c surface atom at a Sn−O distance of 2.4 Å from the metal and 3.1 Å from the neighboring Obr atom. The adsorption energy is 0.57 eV, in agreement with the results reported in the literature.28 The C2.2 minimum shown in Figure 2c is representative of a molecular adsorption mode in which water interacts with the SnO2 surface in an asymmetric conformation. In this geometry, the water molecule binding to the Sn5c atom bends toward one neighboring bridging oxygen, forming a new stabilizing H-bond interaction. The water molecule lies at a distance of 2.4 Å from the surface and at 1.8 Å from the neighboring Obr. C2.2 has a potential energy of 0.27 eV. This conformation is similar to that

coverage FES basins have been selected from the MD trajectory and successively optimized. The representative configurations are points from the metadynamics trajectory, having CSn,O and CO,H values mapping within each of the FE basins. The potential energy (PE) values are calculated using eq 6 and are collected in Table 1, and the corresponding geometries are shown in Figure 2. Figure 2a shows a representative conformation (C1) for the desorbed case of basin B1. Here, the water molecule is at a distance of 14.0 Å from the surface (CSn,O = 0.00, CO,H = 1.97), and the corresponding potential energy is 1.61 eV. A systematic search at 0 K indicates that the potential activation energy barrier is located at a Sn−O distance of 5.0 Å. For Sn−O distances shorter than 5.0 Å, the water spontaneously adsorbs in a molecular configuration. As shown in Figure 2a, both the water molecule and the SnO2 surface stay in their relaxed geometry. 5491

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H2O molecule is bent toward the Obr atom, an additional perturbation of the 1b2 orbital by Obr is observed (Figure 3c). Here, the orbitals of the H atom are polarized by the Obr to which they are pointing. This indicates a strong H-bond interaction between the two atoms and is in agreement with the lower potential energy (stronger bond) of minimum C2.2. Only marginal differences are observed between the two dissociated conformations C3.1 and C3.2 (Figure 3d,e). Upon dissociation, the separated HR atom clearly forms a covalent bond with Obr. The corresponding orbital is ca. 2 eV lower in energy than that of the terminal OH group, indicating stronger OH bond formation compared to that of the associative adsorption mode C2.2. Figure 4 shows the PDOS on the Obr orbitals of conformations C2.2, C3.1, and C3.2. In the molecular

of the most stable molecular adsorption reported at high coverage.31 Two conformations (C3.1 and C3.2) have been isolated for the dissociative adsorption belonging to B3 as well. Figure 2d shows a symmetric dissociative adsorption (C3.1).28 The main characteristic of this geometry is that the two hydroxyl groups lie in the same plane perpendicular to the metal oxide surface. In C3.1, the terminal hydroxyl group lies on top of the Sn5c atom at a distance of 2.3 Å from it and at an OT−HR distance of 2.4 Å from the rooted OH. The energy of this conformation is 0.10 eV, consistent with the values reported in the literature.28 The most stable isolated dissociative adsorption state from B3 is C3.2 (Figure 2e). The conformation is characterized by the asymmetric orientation of the two neighboring hydroxyl groups, which in this case do not lie in the same plane. The terminal OH lies 2.3 Å from the metal surface and 1.9 Å from the rooted hydroxyl. The two hydroxyl groups form an angle of 118.5° between them that resembles a molecular water arrangement. Because of the favorable hydroxyl arrangement, C3.2 is the most stable geometry minimum, and its potential energy is set to 0.00 eV. Lindan et al. reported a similar conformation at half coverage having comparable energy.32 Electronic Properties at Low Coverage. The local minima geometries (C1−C3.2) are further investigated using the Mulliken charge distribution and the density of states analysis. The charges of the water molecule and Obr atoms are reported in Table 2. The desorbed H2O molecule in C1 is uncharged (+0.01), with the H atoms carrying a positive charge (+0.14) and the O atom carrying a negative charge (−0.27). Upon adsorption on the SnO2 surface, an electron donation from the H2O molecule to the surface occurs. The symmetrically associatively adsorbed H2O species (C2.1) carries a positive charge of 0.21. The charge shift is distributed equally among the atoms of the molecule. Compared to the desorbed species, the H and O atoms lose 0.06 and 0.08 electrons, respectively. In C2.2, where the molecule is tilted toward an Obr atom, this effect is less pronounced. In this case, the H2O molecule holds a positive charge of 0.15. Upon dissociation, electron donation from Obr to OT is observed. The overall charge on the OH groups is negative (−0.31 for each hydroxyl). In the dissociated configurations, the charge shift is mostly transferred back to the water oxygen atom (OT), holding charges of −0.51 (C3.1) and −0.48 (C3.2). The electronic structures of stable conformations C1−C3.2 have been further analyzed through the projected density of states (PDOS). In Figure 3, the orbitals initially belonging to water (HT, HR, and OT), Obr and Sn5c, are explicitly shown. In molecular configurations C1 and C2.1, the PDOS of the two H atoms are identical, thus only the PDOS of HT is visible (Figure 3a,b). In the desorbed conformation (C1), H2O is far enough from the surface to avoid electronic interactions with the surface, and no perturbation of its orbitals is observed (Figure 3a). In C2.1, where H2O is adsorbed associatively on a Sn5c site, the 2a1 and 1b2 orbitals remain unperturbed. They are found 21 and 19 eV, respectively, below the highest occupied state in the valence band (simply referred to as “the valence band” in the following text). However, the high-energy 1b1 (HOMO) and 3a1 (3 eV below the valence band) orbitals hybridize with the Sn5c orbitals, indicating covalent bond formation (Figure 3b), in agreement with Kumar et al.29 In conformation C2.2, where the

Figure 4. Obr PDOS in stable low-coverage conformations C2.2, C3.1, and C3.2.

configuration (C2.2) of H2O adsorbed on SnO2, the orbitals of Obr at the upper edge of the valence band are slightly shifted to lower energies (by 0.4 eV) compared to the clean SnO2 surface. A similar shift is observed for the Obr s-type orbitals ca. 16 eV below the valence band. The lower energy of the states of surface O atoms indicates a stabilization of the surface. Significant changes in the PDOS of Obr are observed in dissociative configurations C3.1 and C3.2. Here, the s-type orbitals are located at even lower energies, ca. 19 eV below the valence band. Moreover, the highly localized p-type orbitals on the upper edge of the valence band of the clean slab delocalize upon dissociative H2O adsorption, and the valence band center shifts to lower energies. Both effects demonstrate a notable stabilization of the surface and justify the lower potential energy (stabilization) of dissociated H2O. Orbitals related to bulk O atoms are less localized than those of Obr atoms of the clean SnO2 surface and are found at lower energies, leading to band bending at the clean SnO2 surface. The reduction of differences between the bands of surface and bulk O atoms upon dissociative H2O adsorption suggests a decrease in band bending in this case. Effect of Oxygen Coverage. The effect of oxygen coverage has been investigated by simulating H2O adsorption on a partially reduced (low-oxygen-coverage) and partially oxidized (high-oxygen-coverage) SnO2 surface. In the reduced case, half of the Obr atoms are removed from the stoichiometric SnO2 surface. The reduction of the surface has only a marginal effect 5492

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Table 3. Mulliken Charges on H2O and the Interacting Surface Oxygen Atoms for H2O Adsorption on Reduced and Oxidized Surfacesa q(HT)

q(HR)

q(OT)

q(Obr)

q(Oot)

q(Sn5c)

q(H2O)

q(OHT)

q(OHR)

−0.37

−0.30

−0.16

−0.24

Reduced SnO2 clean H2O ass. H2O diss. clean H2O ass. H2O diss.

0.18 0.15

0.18 0.19

0.19 0.18

0.19 0.20

−0.23 −0.52

−0.64 −0.71 −0.48

1.34 1.34 1.36

0.14

−0.20 −0.35

Oxidized SnO2 −0.53 −0.32 −0.62 −0.42 −0.44 −0.32

1.47 (1.38) 1.40 1.38

0.18

a

For the clean oxidized or reduced surface, q(Obr) and q(Sn5c) refer to the average value of the corresponding surface atoms. When H2O is adsorbed, these values represent the charge on the corresponding atom with the H2O molecule. The value for q(Sn5c) given in parentheses for the oxidized surface refers to the charge on Sn surface atoms where the additional O atoms are adsorbed. On the oxidized surface, added O atoms are indicated with Oot. Again, the average value of all Oot species is given for the clean oxidized surface, and that of the Oot atom interacting with the H2O molecule is given for the adsorbed case. HT and OT belong to the terminal OH (OHT), and OR and HR generate the rooted OH (OHR).

Figure 5. Stable configurations of H2O adsorbed (a, c) associatively and (b, d) dissociatively on (a, b) a partially reduced SnO2 surface and (c, d) a partially oxidized SnO2 surface.

molecular species by 0.48 eV. Dissociatively adsorbed H2O, however, is stabilized only by 0.13 eV. Additional H-bond formation is observed on the stoichiometric surface between a terminal OH group and the nearest Obr atom when H2O is adsorbed dissociatively. Thus, the stabilization effect of preadsorbed Oot is reduced in this case compared to the case of associative H2O adsorption. On the oxidized SnO2 surface, additional charge transfer occurs from the surface to Oot atoms that is not present on the stoichiometric surface. This decreases the capability of electron donation from the surface to the O atom of the water molecule (OT) and causes OT to be less negative on the oxidized surface (−0.35, Table 3) than on the stoichiometric surface (−0.48, Table 2). The formation of additional H bonds leads to a change in the stability of molecular and dissociated H2O depending on the surface oxygen coverage. At low oxygen coverage (reduced surface), dissociated species were stabilized. However, at high

on the stability of associatively adsorbed H2O, which is 0.05 eV less stable than the most stable molecular configuration (C2.2) on the stoichiometric surface. In contrast, dissociatively adsorbed H2O is stabilized by 0.24 eV compared to the corresponding configuration (C3.2) on stoichiometric SnO2. The stabilization of dissociated H2O on reduced SnO2 results from an enhanced charge transfer between Sn5c and OT: the charges on Sn5c and OT are 1.36 and −0.52, respectively (Table 3). On the stoichiometric SnO2 surface, they have been found to be 1.41 and −0.48, respectively (Table 2). This indicates that under reducing conditions dissociative H2O adsorption is even more favored over associative one. The oxidized surface was simulated by 1/2 ML additionally adsorbed O atoms (Oot) on top of Sn5c sites. The presence of preadsorbed oxygen induces the formation of additional hydrogen bonds to the H atoms of the associatively adsorbed H2O molecule. This results in a remarkable stabilization of the 5493

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Also at high coverage three free-energy minima (B4−B6) are isolated, connected by a preferential pathway. B4 in the top left of Figure 6 corresponds to the desorbed state (CSn,O = 0.15, CO,H = 1.90). Basin B4 is limited by an activation barrier at CSn,O = 0.30. The molecular adsorption basin (B5) in the top right of Figure 6 has a large elliptical shape (CSn,O > 0.30, CO,H > 1.75). At the well minimum (CSn,O = 0.80, CO,H = 1.90), the free energy is 0.10 eV. B5 corresponds to ca. 33% of the total population. At CO,H = 1.75, the activation barrier for the desorption is found, with a dissociation energy barrier of 0.10 eV. The largest area of the FES is covered by the dissociated adsorption basin (B6). The basin extends from the dissociation activation barrier at CO,H = 1.75 to all of the dissociated conformations, with a minimum at CSn,O = 0.90, CO,H = 1.00. The large, deep B6 accounts for ca. 67% of the total population at high coverage. Representative Conformations at High Coverage. Representative geometries of the minima (B4−B6) at high coverage are shown in Figure 7, and their potential energies are collected in Table 1. Figure 7a shows an example of the desorbed water state (C4). The water molecule and the surface are 11.0 Å away from each other, and no interactions between the two can be observed. The PE of this conformation is 1.67 eV. The metal oxide surface is in a local minimum geometry with mixed molecular (2 molecules) and dissociated water (9 molecules) occupying the 12 available sites. To obtain consistent values for the potential energy of the other conformations, this surface arrangement is preserved in successive optimizations. At high coverage, only one stable molecular adsorption mode and one stable dissociative mode have been isolated. The molecular adsorption conformation at high coverage (C5) is shown in Figure 7b. The water molecule is bound to Sn5c at a distance of 2.3 Å. Similar to C2.2 at low coverage (Figure 2c), the molecule bends toward the neighboring Obr, creating a Hbond interaction with it at 1.5 Å. Additionally, the second water hydrogen is involved in a weak interaction with another neighboring OH group at a distance of 2.3 Å. The presence of

oxygen coverage (oxidized surface), associative adsorption is facilitated and was found to be the favored adsorption mechanism when 1/2 ML of O is preadsorbed on the SnO2 surface. High Coverage Water Adsorption. FES at High Coverage. The high-coverage water adsorption/dissociation on the SnO2(110) surface is investigated using a completely reduced oxide surface in which the 12 surface sites are entirely occupied by water molecules, corresponding to 1 ML coverage. In this case, all of the water molecules freely and continuously undergo dissociation−association processes on the surface, without desorbing from it. Only by applying an adequate potential to the Hamiltonian can the desorption event be observed. The FES as a function of CSn,O and CO,H is shown in Figure 6.

Figure 6. FES of water adsorption at high coverage on the SnO2(110) surface as a function of collective variables CSn,O and CO,H. The free energy is given in electronvolts with respect to the dissociative adsorption state. The free-energy basins (B4−B6) are described in the text. The adsorption (CSn,O = 0.30) and dissociation (CO,H = 1.75) transition barriers are indicated by dashed lines.

Figure 7. Geometry of representative conformations of water molecule adsorption on the SnO2(110) surface at high coverage. The three minima C4−C6 are isolated from the metadynamics simulation. C7 and C8 are stable configurations reported in the literature.28 C7 is representative of the completely dissociative H2O adsorption, and C8 is a mixed conformation where half of the species are adsorbed associatively and the other half are dissociated. Each conformation has been geometrically optimized to its closest minimum. Only the first oxide layer is shown for simplicity. 5494

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Figure 8. PDOS of stable conformations C4−C6 at high coverage. The orbitals belonging to the water atoms (HT, HR, and OT) and the surface (Sn5c and Obr) are explicitly shown.

this case, the charge effects of each of the two adsorption mechanisms can be observed simultaneously. The associatively adsorbed water molecules are positively charged, indicating the water-to-surface electron transfer also occurring at low coverage. The Obr atoms forming H bonds with associatively adsorbed H2O molecules carry a slightly less negative charge (−0.69) than those of the clean surface (−0.64). The average charges of the hydroxyl groups (−0.29, −0.37), however, indicate the presence of surface-to-water electron donation. The Obr charge is electron-depleted (−0.46), and the water oxygen carries a more negative charge (−0.54). Figure 8 shows the PDOS for the C4−C6 conformations. Similar to the Mulliken distribution of charges, no difference is seen in the PDOS of desorbed H2O above the clean oxidized surface (C1) and the water-occupied surface (C4, Figure 8a). When the H2O molecule is adsorbed associatively on the highcoverage surface (C5), the interaction between the closest Obr atom and the water H atoms leads to a polarization of their orbitals (at ca. 16 eV below the top of the valence band). At the same time, the 2a1 orbital is shifted to slightly lower energies compared to the desorbed H2O molecule (conformation C4). The electronic structure of C5 is comparable to that of C2.2, indicating that adsorbed H2O species on adjacent active sites hardly affect the binding mechanism of associatively adsorbed H2O (Figure 8b). In contrast, major differences arise in the PDOS of dissociated species (C6) when the surface is covered with water (Figure 8c). Additional perturbation of the orbitals related to the HT atom occurs, leading to the formation of states located at the 2a1 position of the rooted OH group (ca. 17.5 eV below the top of the valence band). This polarization of the H T orbitals results from H-bond formation with neighboring terminal OH groups that are not present at low coverage, leading to additional stabilization of the OH groups at high coverage.

this additional interaction stabilizes the molecular adsorption, reaching a PE of 0.39 eV. The dissociative case shown in Figure 7c (C6) resembles C3.2 at low coverage (Figure 2e). The terminal OH is 2.2 Å from the surface and interacts with a neighboring rooted OH group at a distance of 1.6 Å. Additionally, the terminal hydroxyl is involved in an interaction with a neighboring water molecule at a distance of 2.3 Å. The potential energy of this conformation is 0.00 eV. Electronic Properties at High Coverage. The Mulliken charge analysis in high-coverage minima C4−C6 is shown in Table 2. The charge distribution of the desorbed H2O molecule (C4) is identical to that at low coverage (C1). The total charge of the entire molecule is zero, with the H atoms carrying a positive charge (+0.14) and the O atom carrying a negative charge (−0.28). Approaching the surface occupied with water, H2O adsorbs in a molecular geometry (C5) similar to conformation C2.2 at low coverage. Electron donation takes place, leading to electron depletion of the H2O molecule. Both H and O atoms are more positively charged compared to the desorbed configuration. The dissociated configuration (C6) exhibits electron back donation from the Obr atom to the adsorbate, resulting in the formation of two identical hydroxyl groups with a total negative charge of −0.30. The charge is mostly transferred to the water O atom (−0.47), and the negative charge of Obr decreases from −0.68 to −0.47. As a further validation of these results, the charge distribution is investigated in other stable configurations at 1 ML as previously reported in the literature (Table 2).28 C7 represents pure dissociative adsorption of H2O species (Figure 7d). Charge values in C7 confirm the surface electron depletion due to water dissociation: the charge on Obr is reduced to −0.44, and additional electronic charges are found on the hydroxyl group originating from a water molecule (−0.36). C8 is a mixed conformation, where half of the species are adsorbed associatively and the other half are dissociated (Figure 7e). In 5495

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observed. These findings are consistent with theoretical29,32 and experimental results showing the coexistence of several species on the surface.10,24 In particular, the possibility of mixed molecularly dissociated pools almost as stable as completely dissociated species has been suggested by Lindan et al.32 and Kumar et al.29 The latter reported the formation of mixed molecular (40%) and dissociative (60%) adsorption states at dynamic equilibrium.29 According to UPS and TPD studies, dissociated species should account for only 35% of the entire surface population.24 The orthogonality of the adsorption and dissociation mechanisms shown in Figures 1 and 6 indicates that they can be separated in time. Initially, starting from desorbed states B1 and B4, the systems evolve only as a function of CSn,O toward the molecular adsorption state. Afterward, surface dissociation evolves as a function of CO,H. The activation energy barrier for the dissociation process is 0.10 eV at both low and high coverage. A major contribution of this investigation is the inclusion of temperature effects in the simulations. Figures 1 and 6 show that the molecular adsorption and dissociative adsorption basins are quite shallow and contain several nonminima conformations that are easily accessible at room temperature. The large shallow size of the adsorption and dissociation basins, in particular in the adsorption direction (CSn,O), indicates the presence of the relevant flexibility of molecular water or hydroxyl groups on the metal surface at room temperature. In the dissociated configuration, the variability depends partially on the oscillation of the hydroxyl groups on the surface and partially on the hydroxyl interactions. The ability to catch this flexibility, entirely as a result of the presence of thermal energy, is a major advantage of the use of MD. At high coverage, molecular and dissociative adsorption modes are more stabilized than at low coverage. The molecular adsorption free energy at high coverage (B5) is 0.20 eV more stable than that at low coverage (B2). For the desorbed-state free energy, the difference between high coverage (B4) and low coverage (B1) is 0.10 eV. It seems therefore that the presence of additional H2O molecules on the oxide surface favors the adsorption of additional molecules. The minima at low and high coverage show significant similarities. In the molecular adsorption mode, conformation C2.2 at low coverage is similar to conformation C5 at high coverage, while in the dissociative adsorption mode low coverage conformation C3.2 is similar to high coverage conformation C6. Our simulations confirm the findings of previous theoretical and experimental studies10,26 where dissociative H2O adsorption has been reported to be the favored mechanism at temperatures higher than 200 °C, implying the formation of hydroxyl groups on the SnO2 surface. Various mechanisms have been proposed to explain the increase in conductivity of SnO2 when hydroxyl groups are formed on the surface4 in which the hydroxyl either acts directly as an electron donor or induces the formation of oxygen vacancies. Our investigation of the charges of single atoms by Mulliken analysis suggests that electron transfer occurs from the rooted hydroxyl group to the terminal hydroxyl group rather than to the SnO2 surface. Thus, the transfer of electrons to the surface is more likely to be induced by the formation of oxygen vacancies. In this respect, further research could focus on the formation of vacancies on the SnO2 surface in the presence of hydroxyl groups. Significant changes in the PDOS on Obr are observed upon dissociation. p-type orbitals at the upper edge of the valence band delocalize and

To investigate further the effect of H2O adsorption on the electronic structure of the slab atoms, the PDOS of C7 and C8 Obr atoms is analyzed (Figure 9). On the clean SnO2 surface,

Figure 9. PDOS on the orbitals of Obr atoms at high H2O coverage in the pure dissociated configuration (C7) and in the mixed configuration (C8) where half of the species are dissociated and half are adsorbed associatively.

highly localized states of the Obr atoms exist at the upper edge of the valence band. These states delocalize and spread over the entire valence band upon adsorption of H2O (between −9 and 0 eV). Additional density differences are identified for the states located between 15 and 20 eV below the valence band. These are found ca. 16 eV below the valence band on the clean SnO2 slab. When all Obr atoms form a hydroxyl group with the H atoms of the H2O molecule (pure dissociative adsorption, C7), these states disappear whereas additional ones evolve at lower energies (ca. 18 eV below the valence band) belonging to the binding 2a1 orbital. In the mixed configuration (C8), both states are present.

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DISCUSSION The free-energy findings and the static results converge toward a consistent description of the adsorption/dissociation of water on SnO2. The FESs isolated in Figures 1 and 6 indicate that there are three main basins in the process, both at low and high coverage. These basins correspond to desorbed, molecularly adsorbed, and dissociatively adsorbed states. All of the minimum conformations previously reported in the literature28,30−33 have been sampled by MD and identified with static calculations. The free energy has been taken as a measure of the probability of any set of conformations and used to estimate the population distribution at equilibrium for each free-energy minimum. At low coverage, the ratio of the population density between molecular and dissociative adsorption is 1:3.5, whereas at high coverage this ratio is 1:2 (Table 1). As expected, dissociative adsorption is favored with respect to molecular adsorption at both low and high coverage. These results confirm that, in the free-energy space and at finite temperature, water preferentially dissociates on SnO2 surfaces.28,30−32 However, the population ratio indicates that both molecular and dissociated states coexist on the surface, although the dissociated state is more stable. This is confirmed by the MD simulations at high coverage, where the continuous dissociation and formation of molecular water on the surface is 5496

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low-energy conformations available at finite temperature. The minima predicted by static calculations have been explored naturally by the MD simulation along with several others. The free-energy surfaces at low and high coverage additionally indicate that adsorption and dissociation are phenomena occurring independently in time and that at high coverage water adsorption is stabilized through the interaction with other surface water. Static calculations performed on partially reduced and oxidized SnO2(110) indicate that oxygen coverage strongly affects the population density of associatively and dissociatively adsorbed water. The electronic charges and PDOS analyses suggest the coexistence of different sources of electronic charges on the metal oxide surface, depending on the water adsorption mechanism. In particular, surface electron enrichment is observed after the associative adsorption of water whereas depletion is found for dissociative adsorption. Both events can lead to the experimentally evidenced changes in SnO2 conductivity upon water adsorption.

shift to lower energies. This presumably leads to a decrease in band bending at the SnO2 surface and gives a possible explanation of the increase in conductivity in the presence of water vapor. Static calculations on nonstoichiometric SnO2(110) indicate a significant influence of oxygen coverage on the H2O adsorption mechanism. On a partially reduced surface, the dissociation of H2O is fortified, resulting from an enhanced charge transfer from Sn5c to the adsorbate. This indicates that the reduction of the surface promotes its role as an electron donor. At higher oxygen coverage, however, the electron transfer from the surface to the adsorbate is reduced, leading to a destabilization of dissociated H2O species. However, the presence of additional O atoms (Oot) on top of Sn5c atoms enables the formation of additional H-bonds between the associatively adsorbed H2O and Oot, leading to a stabilization of the adsorbate. As a result, associative H2O becomes favored over dissociative H2O on the oxidized SnO2 surface. Finally, it should be stressed that our findings rigorously hold only for the idealized stoichiometric, reduced, and oxidized (110) surfaces of SnO2. So far, very little is known about the structure sensitivity of the governing mechanism of water adsorption and the related sensor response. Although most experiments have been performed on (110) surfaces, a recent study proposes that the shape of SnO2 crystals (i.e., a different ratio of exposed (221) and (110) faces) influence the sensor response.62 However, considerable further experimental and theoretical work is necessary to establish a firm understanding of the structure sensitivity of the sensor response. Future research should therefore be directed toward the elucidation of the structure sensitivity and the control of the morphology of SnO2-based sensors, as advocated in a recent Feature Article.41 Additionally, it should be noted that in spite of its relevance as a crucial step in explaining the cross sensitivity of gas sensors to water adsorption, there are several other aspects that have to be considered in gaining a complete understanding of its effect on the sensor response, including, for example, the coadsorption of analyte, water, and oxygen; temperature; pressure; the effect of grain boundaries; and mass and charge transfer.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Thanks are due to Dr. Ari Paavo Seitsonen for his invaluable scientific and technical advice. Financial support by the Claude & Giuliana Foundation is kindly acknowledged. The Swiss National Supercomputing Centre (CSCS, Manno) and ETHZ are acknowledged for providing computing resources.

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REFERENCES

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CONCLUSIONS We have used MD simulations based on metadynamics to investigate water adsorption and dissociation on the SnO2(110) surface at finite temperature. This investigation is aimed not only at gaining some fundamental insight into water adsorption on SnO2 but also at providing a basis for understanding the surface processes that are relevant in explaining the effect of water adsorption on the response of gas sensors. An attempt is made to bridge the gap still existing between the available static theoretical calculations at 0 K and the experimental evidence collected at finite temperature. By introducing temperature effects into the simulations, an estimation of the free energy of reaction at room temperature is obtained that can be compared to static data. Through the integration of the FES in the space of the chosen collective variables, the population distribution at room temperature is estimated. The calculations confirm that the dissociative adsorption of water on stoichiometric SnO2(110) is favored over associative adsorption, but at finite temperature, a combination of associative and dissociative water adsorption is expected, in agreement with experimental results. The explicit inclusion of temperature into the MD simulations discloses the flexibility of associatively and dissociatively adsorbed water at room temperature by exploring several 5497

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