Accepted Manuscript

Adsorption and diffusion of a silver atom and its cation on α-SiO2 (001): com‐

parison of a pure surface with a surface containing an Al defect

Nikita I. Vakula, Gulnara M. Kuramshina, Leonid G. Gorb, Frances Hill, Jerzy

Leszczynski

PII: S0009-2614(13)00291-1

DOI: http://dx.doi.org/10.1016/j.cplett.2013.02.067

Reference: CPLETT 31057

To appear in: Chemical Physics Letters

Received Date: 10 December 2012

Accepted Date: 26 February 2013

Please cite this article as: , Chemical Physics Letters (2013), doi: http://dx.doi.org/10.1016/j.cplett.2013.02.067

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Adsorption and diffusion of a silver atom and its cation on α-SiO2 (001): comparison of a pure

surface with a surface containing an Al defect

Nikita I. Vakulaa, Gulnara M. Kuramshinaa, Leonid G. Gorbb, Frances Hillc, Jerzy Leszczynskid

aDepartment of physical chemistry, Faculty of Chemistry, M.V. Lomonosov Moscow State University,

Moscow, 119991, Russia

bBadger Technical Services, Vicksburg, MS USA

cUS Army ERDC, Vicksburg, MS USA

dICNANOTOX, Department of Chemistry, Jackson State University, Jackson, Mississippi, 39217, USA

Abstract

By employing DFT approaches we studied the adsorption of a single Ag atom and its cation on

different sites of a pure α-quartz (001) surface and on an α-quartz (001) surface containing an Al defect.

The energetically different adsorption sites of Ag and Ag+ were revealed and the profiles of diffusion

were calculated. Diffusion of an Ag atom through the pure α-quartz surface is predicted to have a much

lower barrier than an Ag cation. However in case of diffusion through the α-quartz surface with an Al

defect the barriers are almost the same for both cation and neutral Ag.

Introduction

The interactions of transition metals with oxide surfaces and formations of the metal/oxide interfaces

have attracted considerable interest of researchers in the last decades due to broad applications of such

interfaces in the different areas of modern technology including heterogeneous catalysis, thin film

technology, gas sensors, etc. [1, 2]. It was discovered that metal thin films of dozens of nanometers

mostly tend to form as island structures with very specific and interesting physical properties. In

particular, vapor-deposited metal island films (MIFs) on different substrates play a vital role in optics.

The absorption behavior of metal islands can be influenced by the dielectric function of the metal, the

size and the shape of clusters and the properties of embedding medium [3]. MIFs demonstrate unique

optical properties due to participation in the electromagnetic enhancement of surfaces through the

propagation of local surface plasmon resonance by the free electrons in the islands.

By varying the conditions of deposition and simultaneously applying an intense electric field at an

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elevated temperature one can obtain 3D photonic structures based on the electric field assisted

dissolution (EFAD) of metal clusters [4]. It has been shown that application of a static electric field at

moderately elevated temperatures induces dissolution of metallic nanoparticles embedded in a glass

matrix [5, 6] or silica film [7]. The most important effect of simultaneously applying temperature and

electric voltage is the disappearing color of the sample (deposited silver films on the substrate), i.e. the

sample becomes completely transparent. The coloring of the sample is explained by the presence of

silver particles, and the bleaching of the glass in the treated area can be obviously explained by the

absence of the nanoparticles as a result of their dissolution. It has been suggested that this process of

dissolution followed from the ionization of metal clusters and latter ejection of ions from these clusters

[8].

One of the important characteristics of such a process is the magnitude of the adsorption energy of

metal clusters sorbed to the substrate. The adsorption energies of metal clusters sorbed to such surfaces

as rutile [9, 10] are strongly dependent on the nature of the adsorbing material and of the substrate. The

comparative investigation of the interactions between Au and Pt clusters and the (110) surface of rutile

(TiO2) has demonstrated the low values of adsorption energy of gold clusters in comparison with the

adsorption energy of Pt [9]. Nudged Elastic Band methods (NEB) and DFT calculations have been

successfully applied to study diffusion paths of carbon on iron surfaces [11]. Some aspects of applying

the NEB method to study the diffusion and migration paths in different systems have been analyzed by

means of quantum mechanical methods [11-14].

First-principle calculations have been applied to the investigation of adsorption properties of different

metal atoms (Fe, Co, Ni, In, Ga) on a hydroxylated (111) surface of β-cristobalite SiO2 [15]. Binding

energies of different atoms to the surface were found to vary in a large region, e.g. In and Ga were

weakly bound while Fe, Co and Ni were bound to both Si and O atoms with strong chemical bonds. It

was found that some atoms (In, Ga) can diffuse on the surface because of the rather small diffusion

barrier (within 0.1-0.3 eV).

Investigations of optical properties of Ag island films deposited on a glass substrate have been carried

out in [16]. They have demonstrated very different optical properties of Ag island films obtained under

different conditions of deposition and very large sensitivity of properties to metal island structure.

Quantum mechanical calculations of adsorption of small silver clusters on stoichiometric rutile TiO2

(110) surface have been reported in [17]. A new mathematical algorithm for the estimation of thin films

optical properties (refractive index dispersion of silver island films embedded between silica layers)

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based on experimental data has been proposed in [18] for reliable determination of wavelength and

thickness dependencies of optical constants of thin metal films. Many publications related to

investigation of Ag adsorption on different substrates, e.g. on CeO2 [19], TiO2 [20], TiC (001) [21]

have been published in the last years. Though there are no reports on investigations from first

principles of silver MIFs formation on α-quartz nevertheless systems based on Ag/SiO2 are widely used

in different optical applications. Knowledge of the surface diffusion profiles for MIFs is very important

in order to understand the growth mode and evolution of the metal particles on these surfaces while the

values of the energy barriers should be useful for understanding of the occurrence of strong metal-

support interactions.

In this work we present the first comparative ab initio calculations for the adsorption and diffusion of

a single Ag and its cation Ag+ on a pure (001) α-SiO2 surface and for a surface with an Al impurity.

The adsorption of a single atom on a surface can be considered as the initial step of the EFAD

process and MIFs formation. Knowledge of adsorption geometry of Ag-quartz or Ag+-quartz systems

is crucial in order to understand the details of such a process.

This paper reports the results of the quantum chemical simulations of the initial process of Ag and Ag+

diffusion on (001) α-SiO2, based on the Density Functional Theory (DFT). α-quartz has been chosen as

the most stable crystalline phase of silica over a broad range of temperatures, pressures and ambient

conditions and its (001) surface has been used to model the adsorption. Aluminum is by far the most

common trace element in all varieties of natural quartz and is always present in synthetic counterparts

[22, 23]. It is well-known that aluminum plays an important role in determining physical and chemical

properties of this mineral [24] and in particular impurities of Al have influences on the interaction

between a quartz surface and silver. [AlO4]0 defect was chosen in this work as far as the samples in the

EFAD procedure were irradiated and it was found that irradiated samples of quartz crystals do have

[AlO4]0 defect [25]. This defect was first discovered in a smoky quartz [26] and confirmed to exist

experimentally using EPR spectroscopy [25].

The approaches that have been developed within a DFT approach are known to be successful for such

calculations and the calculations based on DFT method, pseudopotential and plane wave functions have

been effectively used for material simulations in recent years. However, to the best of our knowledge,

DFT methods have not been applied to the study of EFAD processes and no detailed quantum

mechanical calculations of EFAD have been carried out.

By performing DFT total energy calculations, the adsorption structures and the possible diffusion

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pathways and the most preferable locations for incorporation of silver ad-atoms on the silica (001)

surface have been determined. The theoretical results have been used for comparison with the

experimental data when appropriate.

Computational details

The following two-step strategy was used to predict the structure, energetic parameters and diffusion

barrier of silver atom interacting with an α-quartz surface. At first, calculations of the structures at the

DFT level implementing the periodic boundary conditions (PBC) were performed. Secondly, the

optimized structures obtained at the DFT(PBC) level were used to construct adsorption complexes to

calculate the adsorption energies and energy barriers using a cluster approximation. All units of energy

has been expressed in kcal/mol.

Surface modeling. Periodic boundary conditions. First-principles calculations have been performed

within the DFT approach by means of the Generalized Gradient Approximation (GGA), as proposed by

Perdew, Burke and Ernzerhof for exchange-correlation energy functional (PBE) [27]. The PBC

calculations of equilibrium geometry configurations and interaction energies have been performed with

the QUICKSTEP/CP2K program [28] which was developed specially for molecular simulations of

solid states. In this approach the Kohn-Sham orbitals have been expanded by linear combination of the

Gaussian-type orbitals. An efficient treatment of electrostatic interactions has been provided by using

the auxiliary plane wave basis set for the electronic charge density. The double-δ basis sets with

polarization functions (DZVP) and the Goedecker-Teter-Hutter pseudopotentials have been applied to

describe all atoms in the investigated systems [29]. These pseudopotentials take into account the

relativistic effects which are important for heavy elements, particularly Ag. DZVP basis set has been

chosen as it is the "biggest" basis set for silver available in CP2K. DZVP was also applied for the other

atoms in order to maintain consistency. References [30-32] showed that results obtained with DZVP

basis set were consistent with the results obtained using bigger basis sets (TZV2P). The following

valence configurations of atoms have been considered, namely, containing six valence electrons for

oxygen atoms (O(2s22p4)), four valence electrons for silicon atoms (Si(3s23p2)), three valence electrons

for aluminum atom (Al(3s23p1)), and 11 valence electrons for silver atom (Ag(4d105s1)). The charge

density was expanded in plane waves with a cutoff of 300 Ry. For the k point sampling of the Brillouin

zone (BZ), only the gamma (Γ) point has been used because of the large size of the supercell

considered here. Minima on the PBE potential surface were located with fully relaxed atomic positions.

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Energies were converged to 10-6 eV in all cases. The structures were optimized until the absolute

values of the Hellmann-Feynman forces acting on the atoms in the relaxed structures were less than

0.02 ev/Å.

A periodic model of α-SiO2 bulk was prepared using the experimental crystallographic data of α-quartz

[33] with lattice parameters a=b=4.921Å, c=5.416Å, α=β=90, γ=120º corresponding to the P3121 space

group (No. 152). The hexagonal unit cell has been transformed to the orthorhombic one and cell

parameters have been optimized as a=4.913Å, b=8.523Å, c=5.404Å, α=β=γ=90º. The α-SiO2 bulk

structure which consists of 4x2x3 orthorhombic supercell has been optimized and further the relaxed

crystal was cut to obtain (001) surface. The (001) α-quartz surface is the most stable one among other

quartz surfaces due to the relaxation of all surface silicon atoms. As a result Si atoms become

completely four-coordinated and form Si-O-Si bridges on the surface similar to the bulk structure [34,

35]. This periodic model corresponds to the orthorhombic supercell with the following parameters:

a=19.652Å, b=17.066Å, and c=40.0Å. The surfaces have been simulated using a repeated-slab model

replicating the central supercell in all three (x-, y-, z-) directions. A vacuum gap of 20 Å has been

inserted in the z-direction to prevent the interaction between images, i.e. for providing that the electron

density of the surface tends to zero in vacuum and the top layer of one slab has no effect on the bottom

layer of the next one.

Cluster approximation. Both charged and neutral systems have been studied in this work. When the

supercell is neutral it is sufficient to insert a large vacuum in the direction perpendicular to the surface

to avoid the interactions between periodic images. But the charged systems need to be immersed with

the neutralizing background to get convergence of Coulomb term in energy expression. Disappointedly,

the latter procedure gives rise to additional interactions between the system and background, and

energy of the supercell converges very slowly with respect to the size of the supercell making

calculations impractical [36].

Thus to study the charged systems and compare them with neutral ones we have used a cluster

approximation. The cluster Si17O46H24 was cut from the structures optimized by means of the periodic

calculations described above. The dangling bonds were saturated with hydrogen atoms. The cluster

calculations have been performed using two program packages: QUICKSTEP/CP2K [28] and Firefly

version 7.1G [37]. In CP2K/QUICKSTEP we used DFT/PBE functional with double-δ basis sets with

polarization functions (DZVP) and the Goedecker-Teter-Hutter pseudopotentials. Martyna-Tuckerman

decoupling scheme was used to treat the system as isolated [38]. The size of the box used has

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parameters a=b=30Å, c=20Å, and α=β=γ=90º. In Firefly calculations DFT/PBE0 functional and 6-

31+G(d,p) and LANL2DZ ECP mixed basis set have been used. Further in this paper we will refer to

properties of clusters calculated with CP2K and FIREFLY program packages as models A and B,

respectively.

To calculate interactions between silver atoms and surface, the geometry of the cluster was optimized

using the following procedure. Firstly, positions of all atoms except hydrogens were frozen and

positions of the hydrogens were optimized to reproduce electrostatic field of periodic structure. Then

the optimized positions of hydrogens were frozen and the rest of the system was relaxed.

Adsorption energies (ΔEads) were calculated as the differences between the total energies of the whole

system (silver atoms and quartz surface) and the total energies of the subsystems. Quantities obtained

with the Firefly program package were corrected for basis set superposition error (BSSE). The BSSE is

rather small (less than 10%) in all cases.

Diffusion profile calculations. CP2K calculations. The climbing image Nudged Elastic Band (CI-

NEB) [39] method has been applied to find the barrier of penetration. This method is an effective one

for finding saddle points and minimum energy paths between known reactants and products. The

method works by optimizing a number of intermediate images along the reaction path. Consequently,

we have applied this method to determine the minimum energy diffusion paths and the corresponding

barriers between given initial and final geometries (local minima) described above for diffusion of Ag

into the pure and an α-quartz surface with an Al defect. This procedure starts from a chain of

geometries (replicas) interpolating between the initial and final stable geometries and the rigorous

convergence to a saddle point is obtained. The climbing image moves up the potential energy surface

along the elastic band and down the potential surface perpendicular to the band. The other images in

the band serve the purpose of defining the one degree of freedom for which a maximization of energy

is carried out. As long as CI-NEB method converges, the climbing image will converge to a saddle

point. A spring constant of 0.03 a.u. has been used to describe the spring force between replicas. A

coupled steepest descent-DIIS optimization has been exploited to optimize the band.

Firefly calculations. To calculate the energy barrier with Firefly program package the following

procedure has been used: a series of calculations were carried out for the different fixed distances

between silver atom and surface, while the rest of the system was relaxed. Thus starting from the

optimized structure of silver on the quartz surface and changing the distance between silver and surface

minimal energy path was constructed.

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Results and discussion

Structure of adsorption complexes. First we compare results obtained from periodic calculations with

results from cluster model and then we discuss results for charged systems obtained from cluster

model.

The structure of α-SiO2 bulk and its surface was optimized as described above and the structure of the

crystal was cut to obtain (001) surface. This surface contains the topmost dangling oxygen atoms which

are under-coordinated and the topmost three-coordinated silicon atoms which during geometry

relaxation participate in formation of -O-Si-O- bridges regaining the bulk properties. The topmost -O-

Si-O- layer combines with the second layer forming six-membered rings. Such construction has

compensated free valences and has provided four-coordinated Si atoms and two-coordinated O atoms.

The resulting densities of states (DOS) of SiO2 bulk and its surface are depicted in the Fig. 1a and 1b,

respectively.

DOS associated with the surface is similar to DOS of the bulk. The main difference between them is

that for the surface the gap between the lower O 2p bands (σ-bands) and the upper, nonbonding O 2p

bands (lone pairs) disappear due to the strong hybridization between these states.

The surface cluster extracted from the periodic structure as described above is presented in Fig. 2. The

size of the cluster was chosen to be sufficiently large to better reproduce relaxation of the quartz

structure. The calculated DOS of this cluster (Fig. 1c) allows us to evaluate the quality of the chosen

cluster model. Comparison of the DOS for the cluster and periodic DFT calculations provides a very

good test of the cluster model. The width of the upper valence band as well as the gap between lower

and upper valence bands in the representative surface cluster are in a good agreement with similar data

for the periodic calculations. Therefore, the cluster model provides a consistent description of the

electronic structure of the α-SiO2 (001) surface.

The surface with an Al defect was constructed and relaxed by replacing one of the surface silicon

atoms with an aluminum atom (Fig. 2). A cluster of the same size as in a case of pure surface was

extracted from the periodic structure and optimized. To estimate the quality of the cluster model for

this surface with a defect, the plots of corresponding DOS for both periodic and cluster models were

calculated (Fig. 1d and 1e, respectively). It is shown that relative peaks positions along with band gaps

are well-correlated for both periodic and cluster models.

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Different possible locations of a silver atom on the quartz surface were considered. A silver atom

located above the center of the six-membered -Si-O- ring appeared to be characterized by a stable

configuration. The optimized distance between neutral Ag atom and the ring surface is equal to 3.101 Å

and the adsorption energy is equal to 1.84 kcal/mol. Analogous values for a silver atom were obtained

within cluster approximation: 3.009 Å and 2.07 kcal/mol for model A and 3.085 Å and 1.38 kcal/mol

for model B. The values of adsorption energy calculated within cluster models are consistent with

results of periodic calculations. The very small value of adsorption energy for a neutral silver atom can

be explained by examining DOS (Fig. 3a for periodic model and 3c for cluster). The red line

corresponds to projected DOS (PDOS) of O atoms and blue line corresponds to PDOS of Ag atom.

Because the number of oxygen atoms in structures exceeds the number of silver atoms, the lines

corresponding to silver PDOS are not seen in a plot, the zoomed insets of Ag PDOS containing zones

were inserted in a plot. One can see (Fig. 3) that no overlapping between O and Ag PDOS is observed,

so only electrostatic interactions (no chemical bonding) between these atoms can exist and as a result

the adsorption energy is small. This result is well-correlated with results of [40] where X-ray

photoemission spectra have been used for investigation of the electronic structure of a single atom Ag

implanted on SiO2.

A second cluster model was used to study the charged system. A silver cation adsorbed on the α-quartz

surface has the same stable configuration as in the case of a neutral system. The optimized distance

between Ag+ and the surface is 1.809 Å and the adsorption energy is 53.59 kcal/mol. We have

calculated DOS to discern difference in energies of adsorption in charged and neutral systems (Fig. 3e).

This figure reflects the Fermi level shift due to the loss of an electron by the system and an overlapping

between PDOS of silver and oxygen atoms. This result indicates that the silver atom is bound to the

surface. The electron density also shows that for the neutral system there is no chemical bonding

between a silver atom and the surface, but such bond appears in the charged system. That is shown in

the Fig 4.

Similar to the case shown above for the pristine α-quartz surface, the preferable location for a single

silver atom is directly above the center of the six-membered -Si-O- ring.

The distance between a silver atom and the surface with an Al defect and the adsorption energy are

1.758 Å and 86.48 kcal/mol, respectively, for the periodic model, and 1.736 Å and 78.89 kcal/mol for

cluster model A, and 1.724 Å and 82.34 kcal/mol for cluster model B. The adsorption energies obtained

for both models are in a good agreement. DOS (Fig. 3b, 3d, 3f) shows an overlapping between PDOS

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of silver and oxygen atoms that corresponds to strong bonding between the surface with a defect and

silver atom. The theoretical adsorption energies presented here have not been corrected for zero-point

energy (ZPE) and thermal effects, but the estimation of these corrections has been investigated by

accounting for the phonon contribution to the adsorption energy. To estimate a role of ZPE and thermal

effects, normal modes and vibrational frequencies have been calculated within harmonic approximation

using finite difference displacements of 0.03 Å. The value of the correction has been evaluated at

temperature 573 K and for Ag/SiO2(pure) it is equal to 3.63 kcal/mol, for the systems Ag+/SiO2(pure)

and Ag0(+)/SiO2(Al-defect) it is equal to 6.99 kcal/mol.

Diffusion barriers. The next step of our study involves investigation of diffusion of a silver atom into

the surface of α-quartz by constructing the reaction path for penetration of the pure α-quartz (001)

surface and the surface with an Al defect by silver atoms. During the process of EFAD the electric field

is perpendicular with respect to the surface and Ag atom do penetrate to silica bulk, thus in this study

the silver diffusion in a perpendicular direction with respect to quartz surface has been considered. In

order to construct such a profile we needed to choose an appropriate reaction coordinate that would be

sufficient to describe the penetration. The distance between the silver atom and the center of six-

member -Si-O- ring was chosen as a process coordinate. We optimized the structure where an Ag atom

was placed under the surface layer of α-quartz to find the final state of the penetration. Intermediate

configurations were chosen for further construction of the path.

The converged reaction path of diffusion of an Ag atom calculated using cluster model A is depicted in

Fig.5. Graphs 5a and 5b represent the paths of diffusion of silver atom through the pure quartz surface

for neutral and charged systems, respectively. Graphs c and d in Figure 5 represent diffusion profiles of

neutral and charged systems in the case of a surface with an Al defect. The structures of initial (R),

transition (TS) and end (P) states are depicted in the upper model and the labeled points in the graphs

correspond to these three states.

The value of the barrier for the diffusion of an Ag atom obtained within cluster model A, 45.54

kcal/mol, is well-correlated with similar value in periodic model, 48.99 kcal/mol. For the charged

system the value of the energy barrier is almost two times less than for the neutral case, being equal to

22.54 kcal/mol. However, when we consider Al defects on the surface, the barriers are almost the same

for neutral (26.91 kcal/mol – periodic model, 19.55 kcal/mol – cluster model A, Fig. 5c) and charged

(20.93 kcal/mol – cluster model A (Fig. 5d)) systems.

The barrier energies have not been corrected with ZPE and thermal effects, but estimation of these

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corrections has been investigated and this value has been found to be 5.55 kcal/mol.

These results can be summarized as follows: for the pure surface a mostly electrostatic interaction

between a neutral atom and the surface is observed and there is no bonding interaction. Thus, to

penetrate through the surface, energy applied to the system should be sufficiently large to overcome an

electrostatic repulsion. But for the case of a silver cation there is a bonding interaction between Ag+

and the surface and as a result the electrostatic repulsion is decreased in comparison with neutral

system. For the surface with an Al defect, both charged and neutral species take part in bonding

interactions with Ag, thus the barriers are almost the same.

Conclusions

In summary we have revealed details of the very initial stages of growth of silver atom on α-quartz

(001) surface by carrying out ab initio total-energy calculations. The cluster model was applied to the

considered systems, that correctly and efficiently describe charged species. Our results demonstrate that

the adsorption of an Ag atom and its cation Ag+ on a pure α-quartz (001) surface is achieved on similar

sites, but the energetic characteristics of the two processes are quite different due to the different type

of interactions: only electrostatic interactions are involved for the neutral atom case, while a charged

silver atom is chemically bound to the surface. For a single Ag adsorption, the Ag ad-atom was found

to preferably sit at the hollow site of the SiO2 (001) surface with adsorption energy equal to 1.84 and

2.07 kcal/mol in periodic and cluster models, correspondingly. Ag+ also binds to the same location with

adsorption energy equal to 53.59 kcal/mol. For the case of adsorption of Ag on the surface with an Al

defect, the values of adsorption energy are equal to 78.89 and 86.48 kcal/mol for cluster and periodic

models, respectively.

The values of the energy barrier for silver atom penetration through the surface calculated within both

periodic and clusters models are in a very good agreement and allow us to conclude that the presence of

Al impurities on quartz surface can lower this barrier. This result is in good correspondence with

experimental observations that the process of electric field assisted dissolution of metal clusters was

followed from the ionization of clusters.

Acknowledgments

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This work was partly supported by the RFBR grant No 11-03-00040a and by the National Science

Foundation grant (NSF/CREST HRD-0833178).

We are grateful to the Research Computing Center of Lomonosov Moscow State University for the

computational facilities and the use of the SKIF MSU "Chebyshev" Supercomputer and to the

Mississippi Center for Supercomputing Research (MCSR) for generous computational grant.

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Figure captions

FIG. 1. Density of states of alpha-quartz (a - bulk material, b - pure surface (periodic), c - pure surface (cluster), d - Al-defected surface (periodic), e - Al-defected surface (cluster))

FIG. 2. Alpha-quartz surface cluster (in case of a pure surface the blue atom is Si and in case of Al-defected the blue atom is Al)

FIG. 3. Represents the density of states (DOS) for: (a) Ag adsorbed on pure alpha-SiO2 (periodic), (b) Ag adsorbed on pure alpha-SiO2 (cluster), (c) Ag+ adsorbed on pure alpha-SiO2 (cluster), (d) Ag adsorbed on Al-defected alpha-SiO2 (periodic), (e) Ag adsorbed on Al-defected alpha-SiO2 (cluster), (f) Ag+ adsorbed on Al-defected alpha-SiO2 (cluster)

FIG. 4. Electron density, on the left side – Ag+ on pure alpha-quartz surface; on the right side - neutral Ag atom on alpha-quartz surface.

FIG. 5. Converged reaction path of Ag diffusion through alpha-quartz surface cluster (a and b - diffusion through a pure surface when system is neutral and charged, respectively, c and d - diffusion through Al-defected surface when system is neutral and charged, respectively)

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Highlights

1. Adsorption of Ag atom and cation on α-quartz (001) surfaces were modeled. 2. The influence of Al-defect on the Ag adsorption was investigated. 3. Cluster and periodic models were applied in this study. 4. The energy barriers for Ag diffusion were calculated.

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