First-Principles Study of Interface Structures and Charge

Article pubs.acs.org/JPCC

Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

First-Principles Study of Interface Structures and Charge Rearrangement at the Aluminosilicate/Ru(0001) Heterojunction Mengen Wang,†,‡ Jian-Qiang Zhong,† Dario J. Stacchiola,† J. Anibal Boscoboinik,*,† and Deyu Lu*,† †

Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States Materials Science and Chemical Engineering Department, Stony Brook University, Stony Brook, New York 11790, United States

J. Phys. Chem. C Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 10/06/18. For personal use only.

‡

ABSTRACT: Aluminosilicate bilayer films consisting of corner-sharing [SiO4] and [AlO4] tetrahedra on a metal substrate are important model systems to study zeolite chemistry in heterogeneous catalysis. Understanding the interfacial electronic properties of the aluminosilicate/metal heterojunction is a fundamental step to rationalize the structure−property−function relationship essential to the catalytic activities of the model zeolite. In this work, we use density functional theory (DFT) to investigate the charge rearrangement at the aluminosilicate/Ru(0001) interface, which is attributed to hybridizations between the O pz and Ru dz2 and s orbitals and the subsequent electron redistribution. We found that the energy level alignment at the aluminosilicate/Ru(0001) heterojunction is determined by the surface and interface dipole moments resulting from the charge rearrangements and that the magnitude of these dipole moments can be modified by the aluminum concentration and the surface O coverage on Ru(0001).

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INTRODUCTION

providing opportunities for detailed surface science studies of reactions catalyzed by Brønsted acid sites.11,12 Knowledge of the atomic structures of the (alumino)silicate/Ru interface can provide insights into the permeation of small molecules through the silicate film.13 More importantly, the electronic structure of the interface affects energy levels of the catalyst, which in turn influence the chemistry at the confined interspace and at the surface. For example, the surface potential barrier of vanadyl pyrophosphate caused by the charge transfer from the bulk to the surface has a significant impact on the catalytic selectivity of the n-butane oxidation reactions.14 It is well established that charge transfer at the metal oxides (MOx)/metal interfaces plays an important role in surface chemistry, and the electron transfer can go from metal to MOx or vice versa depending on the nature of MOx.15−17

Zeolites are important industrial catalysts with active sites located inside the nanosized pores.1 In order to make the pore region readily accessible to surface science characterization techniques, two-dimensional (2D) monolayer silica films were synthesized on Mo(112),2 Ni(111),3 and Pd(100)4 as zeolite model systems to study their catalytically relevant physical and chemical processes using surface science approaches. The monolayer films have the stoichiometry of SiO2.5 with strong Si−O−metal bonds.5 To reduce the strong metal−silica interaction and maintain the right SiO2 stoichiometry, weakly interacting 2D bilayer silica films were synthesized on Pt(111)6 and Ru(0001).7,8 The building units of the bilayer films are hexagonal prisms consisting of [SiO4] corner-sharing tetrahedra.9 The success in synthesizing 2D silica films makes it feasible to fine-tune the structure and chemistry of the model systems to mimic real catalysts. For example, Au and Pd10 atoms were deposited on the silica film in an attempt to model the catalyst. The aluminosilicate films were further synthesized on Ru(0001) via the substitution of Si with Al atoms and the hydroxylation of aluminosilicate films resulted in the formation of acidic bridging hydroxyl groups exposed on the surface,7 © XXXX American Chemical Society

Special Issue: Hans-Joachim Freund and Joachim Sauer Festschrift Received: June 19, 2018 Revised: August 10, 2018 Published: September 9, 2018 A

DOI: 10.1021/acs.jpcc.8b05853 J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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On the other hand, silicate bilayer films were believed to interact weakly with the substrate primarily via van der Waals (vdW) forces.9 However, recent studies on the silica bilayer film adsorbed on the Ru(0001) surface revealed that there is a charge transfer from the thin film to the substrate18 and the interfacial electronic structures can be tuned via oxidation/ reduction of the Ru(0001) surface.19 Moreover, the oxidation of the aluminosilicate/Ru(0001) interface is greatly suppressed, exhibiting an interesting protective property.20,21 A likely explanation is that the substitution of ∼16% of silicon by aluminum atoms causes a charge imbalance, which is compensated by either the protonation of the oxygen atoms or a charge transfer from the substrate to the film, and therefore results in a shorter film−substrate distance due to the electrostatic attraction between the aluminosilicate film and the substrate. As the interfacial space between the film and substrate becomes too small for oxygen to bind to Ru, the aluminosilicate film can protect the metal substrate from oxidation. Understanding the nature of the aluminosilicate/Ru(0001) interaction is an important step toward mechanistic understanding of the 2D zeolite models in catalytic reactions. In our previous work, the energy levels at the (alumino)silica/ Ru(0001) heterojunctions have been investigated by XPS during the oxidation and reduction of the Ru(0001) surface, where the O 1s core level binding energy in the aluminosilicate is lower than that in the silica film and shifts to lower binding energies by 0.6 eV during the oxidation of the Ru(0001) surface.18 In this study, we focus on the atomic structures and charge rearrangements at the aluminosilicate/Ru(0001) interface, which have not been systematically investigated before. We carried out density functional theory (DFT) studies to investigate the nature of the charge rearrangement at the (alumino)silicate/Ru(0001) interfaces. We found that the O− Ru hybridization at the silica/Ru and (alumino)silicate/Ru interface results in a charge rearrangement in both the surface normal and lateral directions. As the O coverage on the Ru(0001) surface is gradually increased, changes in dipole moments at the surface and interface can modify the work function and core-level binding energies of the (alumino)silicate film.

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RESULTS AND DISCUSSIONS Electronic Structure of the Silica/Ru(0001) Interface. Our previous calculations revealed that the O 1s core-level

Figure 1. (a) Side view of the structure of (SiO2)8/Ru. Color code: Ru (white), Si (yellow), and O (red). (b) Isosurface plot of the charge density difference upon the adsorption of SiO2 on Ru atoms. Isodensity value is 0.005 e/Å3. Red, electron accumulation regions; blue, electron depletion regions. (c) Projected density of states (PDOS) of px, py, and pz orbitals of Obot from the freestanding SiO2 film. Shaded gray area represents the DOS of the SiO2 film. (d) PDOS of dz2, s, and dxz + dyz orbitals of Ru under Obot (RuO) from the clean Ru atom layer. Shaded gray area represents the DOS of Ru atoms. (e) PDOS of px, py, and pz orbitals of Obot and dz2, s, and dxz + dyz orbitals of RuO in the combined SiO2/Ru system. The Fermi level (EF) is set to 0. Vacuum level of the freestanding silica film (c) is aligned with the adsorbed system (e). (f) The molecular orbital diagram illustrating the hybridization and charge transfer between Ru and Obot.

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COMPUTATIONAL METHODS Spin-polarized DFT calculations were performed using projector augmented-wave method implemented in the Vienna Ab initio simulation package (VASP).22,23 The optB86-vdw functional24,25 was used to treat nonlocal vdW interactions in the (alumino)silicate/Ru(0001) heterojunction. A kinetic energy cutoff of 800 eV was used, and the Brillouin zone was sampled with an 8 × 4 × 1 k-point grid. The substrate was modeled with five layers of Ru in a unit cell of a = 5.392 Å, b = 9.339 Å and c = 27 Å. The top two layers of the substrate and the aluminosilicate film were relaxed during the structure optimization until the interatomic forces were smaller than 0.01 eV/Å. The dipole correction method26 has been applied to remove the spurious electrostatic interactions with image cells. Core-level binding energies (EBE) were calculated using the transition state model27,28 of the excited systems and extrapolated to infinite supercell size limit as described in our previous work.18

binding energy shift in the silica film is caused by the charge rearrangement at the O/Ru(0001) surface and at the silica/ Ru(0001) interface, which modifies the total dipole moments of the system.18 Upon the preparation of the 2D silica film on Ru(0001), there is a charge transfer from the silica film to the Ru substrate despite the relatively large interface distance (Ru−Obot) of ∼3 Å.18 However, details of the charge transfer mechanism between the top layer Ru atoms and the silica film are obscured by the protruding background of the Ru(0001) substrate d band in the projected density of states (PDOS). In order to understand the charge transfer mechanism at the interface, we built a simplified model consisting of the silica bilayer film and only the top layer of the Ru substrate (Figure B


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Figure 2. (a) Isosurface plot of the charge density difference upon the adsorption of HAl3Si5O16 on the Ru(0001) surface. Isodensity value is 0.04 e/Å3. Red, electron accumulation regions; blue, electron depletion regions. (b) The number of transferred electrons (Δq) with respect to the interface separation distance d(Ru−OSi). Solid squares represent physical systems while hollow squares represent structures where d(Ru−OSi) in silica/Ru is stretched from the equilibrium distance of 2.84−3.84 Å. (c) PDOS of px, py, and pz orbitals of Obot from the freestanding HAl3Si5O16 film. Shaded gray area represents the DOS of the HAl3Si5O16 film. (d) PDOS of px, py, and pz orbitals of Obot from the adsorbed HAl3Si5O16 film. Shaded gray area represents the PDOS of the HAl3Si5O16 film. PDOS of dz2 (e) and s (f) of Ru atoms under Obot (RuO) from the clean Ru(0001) surface (black) and from HAl3Si5O16/Ru(0001) (red). Vacuum level of the freestanding aluminosilicate film (c) is aligned with the adsorbed system (d).

Figure 3. Side (left) and top (right) view of the relaxed structures of (a) Si8O16/Ru(0001), (b) Al2Si6O16/Ru(0001), (c) HAl3Si5O16/ Ru(0001), (d) HAl3Si5O16/2O/Ru(0001), (e) HAl3Si5O16/4O/ Ru(0001) (conf. 1) where all of the ORu are located under the pores. (f) HAl3Si5O16/4O/Ru(0001) (conf. 2) where half of the ORu are located under the pores and the other half under Si/Al atoms, (g) (SiO2)8/4O/Ru(0001) (conf. 1) where all of the ORu are located under the pores and (h) (SiO2)8/4O/Ru(0001) (conf. 2) where half of the ORu are located under the pores and the other half under Si atoms. Black arrows represent the ORu under Si/Al atoms. Black rectangle represents the unit cell. Color code: Ru (white), Si (yellow), Al (tan), O from films (red), O chemisorbed on Ru(0001) (pink), and H (small white). Distances on the left represent the average film thickness (dz(O−O)) and the interface distance (dz(Ru−Obot)). dz(O−O) ranges from 3.21 to 4.82 Å in (e) due to the distortion of the HAl3Si5O16 lattice.

1a). The redistribution of the charge upon silica (SiO2) adsorption is quantified by the charge density difference defined Δρ = ρSiO2/Ru − (ρSio2 + ρRu) as where the charge density is calculated for the combined system (SiO2/Ru) and subsystems (SiO2 and Ru atoms) at the optimized structures of SiO2/Ru(0001).18 The isosurface plot of Δρ is shown in Figure 1b. The consistency in the isosurface plot of Δρ between the simplified model (Figure 1b) and full system of the silica/Ru(0001) interface (Figure 5 in ref 18) confirms that the simplified model with a single Ru layer can capture the trend of the charge rearrangement at the silica/Ru(0001) interface. While there is only one type of oxygen atoms (Obot) at the bottom layer of the SiO2 film, two types of Ru atoms are present: Ru atoms under O atoms of the SiO2 film (RuO) and under the pores (Rupore); orbital hybridization at the interface can only happen between Obot and RuO. Intuitively, one would

expect the charge transfer mostly involves the Obot pz orbitals and RuO dz2 and s orbitals. However, both Rupore and RuO are involved but behave differently in the charge rearrangement. It is clearly shown in Figure 1b that there is an electron depletion in pz orbitals of Obot and dz2 orbitals of RuO (regions in blue), accompanied by an electron accumulation in dxz and dyz orbitals of RuO and dz2 orbitals of Rupore (regions in red). The integrated amount of charge transfer across the interface Δq ∼ 0.2 e/unit cell is consistent with full system of the silica/ Ru(0001) interface.18 The nature of the charge transfer from C


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Figure 4. Plane-averaged charge density difference at the interface (Δρinter) for (a) Al2Si6O16/Ru(0001), (b) HAl3Si5O16/Ru(0001), (c) HAl3Si5O16/2O/Ru(0001), and (d) HAl3Si5O16/4O/Ru(0001). Plane-averaged charge density difference at the surface (Δρsur) for (e) 2O/ Ru(0001) and (f) 4O/Ru(0001). Δq is the net charge transfer per unit cell from Ru to the aluminosilicate film or from Ru to ORu based on Bader charge analysis. p is the interface/surface dipole moments caused by the charge rearrangements.

Table 1. Dipole Moment (p in eÅ) and the Work Function (Φ in eV) of nO/Ru(0001), (H)AlxSi8−xO16 (bilayer) and (H)AlxSi8−xO16/nO/Ru(0001) (total) for n = 0, 2, and 4 dipole moment (p in eÅ) Al2Si6O16/Ru(0001) HAl3Si5O16/Ru(0001) HAl3Si5O16/2O/Ru(0001) HAl3Si5O16/4O/Ru(0001)

work function (Φ in eV)

nO/Ru

bilayer

interface

total

nO/Ru

bilayer

total

0.02 0.03 0.01 −0.10

0.21 0.42 0.41 0.46

−0.28 −0.25 −0.40 −0.74

−0.05 0.20 0.02 −0.38

5.15 5.15 5.24 5.66

8.10 7.39 7.42 7.11

5.50 4.61 5.24 6.69

Figure 5. Plane-averaged electrostatic potential of (SiO2)8/Ru(0001), Al2Si6O16/Ru(0001), HAl3Si5O16/Ru(0001), HAl3Si5O16/2O/Ru(0001), and HAl3Si5O16/4O/Ru(0001). Red lines indicate EF and the blue lines represent VBM.

D


ÄÅ É ÅÄ ÑÉÅÄ ÑÉ ÅÅ E1 t ÑÑÑÅÄÅÅ c1 ÑÉÑÑ ÅÅ ÑÑÅÅ ÑÑ = EÅÅÅ1 s ÑÑÑÅÅÅÅ c1 ÑÑÑÑ ÅÅ Ñ ÅÅ ÑÑÅÅ c ÑÑ ÅÅÇ s 1ÑÑÑÖÅÅÅÇ c 2 ÑÑÑÖ ÅÅ t E2 ÑÑÅÇ 2 ÑÖ ÅÇ ÑÖ


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(1)

where E1 = −0.2 eV is the RuO dz2 and s peak position from the clean Ru(0001) surface shown in Figure 1d, and E2 = −6.7 eV is the Obot pz peak position from the freestanding silica film shown in Figure 1c. The bonding and antibonding states are located at −6.8 and 0.05 eV as shown in Figure 1e. The secular equation yields t = −1.3 eV and s = 0.071, and the eigenstates are given by |σ⟩ = 0.12|φdz2 + s⟩ + 0.99|φpz⟩ and |σ*⟩ = 0.98|φdz2 + s⟩ − 0.19|φpz⟩. The results are consistent with the partial charge density plot in Figure 1f, where the major contribution to the σ* orbital is the RuO dz2 and s orbital and the major contribution to the σ orbital is the Obot pz orbital. Overall the charge redistribution from the tight-binding model can be attributed to two effects. First, oxygen lone pair forms donor−acceptor bond (σ) with RuO dz2 states, which can be seen from Figure 1b and 1f. We note that the σ orbital involves a very weak mixing of the Obot pz and RuO dz2 and s orbitals, as evidenced by the small energy shift (−0.1 eV) from the Obot pz orbital energy in the freestanding film. Second, due to the Pauli repulsion, the oxygen lone pair pushes the RuO dz2 orbital to the σ* orbital with a higher energy. Consequently, electrons in the RuO dz2 orbital are partially redistributed into the RuO dxz and dyz as well as Rupore dz2 orbitals. A similar charge rearrangement behavior was also observed at the H2O/Ru interface, where electrons were transferred from the antibonding orbitals formed by Ru and O into the lower-lying energy orbitals.31 Charge Compensation at the Aluminosilicate/ Ru(0001) Interface. In zeolite chemistry, the substitution of Si4+ with Al3+ results in negatively charged [AlO4]− centers that are locally compensated by a proton or another cation. However, we want to emphasize here that the synthesis of the 2D aluminosilicate film is very different from the standard synthesis method used for zeolite, for example, using hydrothermal methods.32 For context, these 2D materials are synthesized in an ultrahigh vacuum (UHV) chamber with background pressures in the order of 10−13 Atm,9 so there is not an abundance of molecules that could provide a source of proton to compensate the charge immediately upon synthesis. Briefly on the film synthesis, Si and Al atoms were thermally evaporated onto the O/Ru(0001) surface at room temperature −7 under 2 × 10 mbar of O2.18,33 This was followed by a crystallization step at 1200 K,34 so even if there was a source for a proton, the bridging hydroxyl groups do not survive these conditions. Previous studies have shown that the bridging hydroxyl group in these 2D materials is stable only up to 500 K in UHV conditions.35 so an unprotonated system is potentially viable, either in the neutral state or anionic state with the ruthenium support providing the charge compensation. Note that the neutral [AlO4]0 center has been reported before in the zeolite research. From the zeolite literature, the [AlO4]0 center

Figure 6. Simulated (dot) and XPS (triangle) core-level binding energy (EBE) for OSi−Al (red) and OSi−Si (black). Lines in XPS represent error bars from experiments. EBE are given relative to the core-level binding energy of ORu in n = 4. EBE of XPS measurements are taken from ref 18.

the silica bilayer film to the Ru(0001) substrate has been discussed in our previous paper.18 The nodal plane of the plane-averaged electron density difference is determined to be at 1.98 Å above the top layer of Ru and 0.86 Å below the bottom layer of the silica film (see Figure 5a of ref 18), which is an indication of the charge transfer mechanism. The integrated amount of charge transfer across the interface is 0.21 e/unit cell. It is not uncommon that charge transfer happens at an interface distance of ∼3 Å. For example, there is a charge transfer (Δq ∼ 0.01 electrons per carbon atom) from graphene to the metal substrate at the graphene−metal distance of ∼3 Å.29 Interface dipoles caused by charge transfer (Δq = 0.04 electrons per molecule) from graphene to copper phthalocyanine across a distance of 3.26 Å has been reported at the graphene/molecule interface.30 The origin of the charge transfer is revealed by comparing the PDOS of Obot of the freestanding SiO2 (Figure 1c), RuO of clean Ru layer (Figure 1d) and the combined system SiO2/Ru (Figure 1e). The major shifts of the peak positions happen in the dz2 and s orbitals of RuO. The new peak at −6.8 eV after adsorption corresponds to the bonding σ-state due to the mixing of the Obot pz orbital with RuO dz2 and s orbitals. The shape of the σ-orbital is plotted using the partial charge density at −6.8 eV at the Γ point (Figure 1f). In the clean Ru layer, RuO PDOS has a peak with both dz2 and s character in the spindown channel at −0.2 eV. This peak shifts to 0.05 eV after SiO2 adsorption, which results in a decrease in the occupation in dz2 and s states. The partial charge density in Figure 1f shows the antibonding σ* state from the hybridization of the Obot pz orbital with RuO dz2 and s orbital. Electrons in the σ* state are transferred to the RuO dxz and dyz orbitals as well as Rupore dz2, which are lower in energy. This charge rearrangement mechanism is summarized in the molecular orbital diagram in Figure 1f. We characterize the (dz2 + s) − pz hybridization with a tight-binding model containing the hopping integral t and the overlap integral s:

Table 2. Simulated and XPS Core−Level Binding Energies for Al2Si6O16/nO/Ru(0001) DFT (layer resolved) n=0 n=2 n=4

DFT (averaged)

XPS

Otop Si−Si

Omid Si−Si

Omid Si−Al

Obot Si−Al

OSi−Si

OSi−Al

OSi−Si

OSi−Al

2.08 1.50 −0.03

2.18 1.54 −0.15

1.08 0.45 −1.20

1.60 0.95 −0.27

2.11 1.51 −0.06

1.47 0.83 −0.50

2.20 1.90 1.60

1.20 0.90 0.60

E


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according to the separation between the (alumino)silicate film and Ru(0001). As the separation increases, there is an exponential decay of the amount of the transferred electron. In our previous work,18 we gradually increase d(Ru−OSi) in silica/Ru from the equilibrium distance of 2.84 Å to a larger separation of 3.84 Å, and the integrated amount of charge transfer (Δq) decays exponentially as a function of d(Ru−OSi) as shown in Figure 2b. The separation at the aluminosilicate/ Ru interface (2.23 Å) is much shorter than the silicate/Ru interface. As a result, the charge transfer at the aluminosilicate/ Ru interface (Δq = 1.27 e) is much more significant than that at the silica/Ru interface, which is close to the tail of the decay curve. Energy Level Shifts at the (H)AlxSi8−xO16/nO/Ru(0001) heterojunction. Based on the comparisons of the charge transfer behavior at silica/Ru and aluminosilicate/Ru interface, it is believed that the energy levels of the bilayer film are affected by Al concentrations. In order to investigate the influence of Al concentrations on the electronic structures of the aluminosilicate film, we studied three models: (SiO2)8/ Ru(0001) (Figure 3a), Al2Si6O16/Ru(0001) (Figure 3b), and HAl3Si5O16/Ru(0001) (Figure 3c), corresponding to SiO2, Al0.25Si0.75O2, and H0.125Al0.375Si0.625O2 films in the experiment. Then we gradually add surface O atoms onto the Ru(0001) surface of the HAl3Si5O16/nO/Ru(0001) models to study the influence of the surface oxygen on Ru(0001) on the properties of HAl3Si5O16, where n represents the number of O atoms (ORu) chemisorbed on the Ru(0001) surface. The effects of the ORu coverage on the O 1s core-level binding energies of silica have been studied in our previous work.18 Three systems with increasing oxygen coverage are investigated here: n = 0, 2, and 4 correspond to clean Ru(0001) [0 monolayer (ML)] (Figure 3c), p(2 × 2)-O/Ru(0001)47 (0.25 ML) (Figure 3d), and p(2 × 1)-O/Ru(0001)48 (0.5 ML) (Figure 3e,f). The film thickness (dz(O−O)) depends on the aluminum concentration. dz(O−O) for (SiO2)8/Ru(0001) is 4.25 Å. As the aluminum concentration increases, dz(O−O) increases to 4.43 Å in Al2Si6O16 and 4.52 Å in HAl3Si5O16. The increase in the film thickness explains the red shift of the vibrational mode of the vertical Si−O−Si linkage between the two layers in the infrared spectroscopy (IR) after the introduction of Al atoms.49 The interlayer distance (dz(Ru−Obot)) depends both on the aluminum concentration and the ORu coverage. In (SiO2)8/Ru(0001), dz(Ru−Obot) is 2.84 Å. It drastically decreases to 2.22 Å when 50% of the bottom layer Si atoms are substituted by Al atoms (Al2Si6O16/Ru(0001)) due to the charge rearrangement as discussed above. At n = 0, dz(Ru− Obot) is 2.23 Å in HAl3Si5O16/Ru(0001). As n increases, additional ORu atoms push the aluminosilicate film away from the Ru(0001) substrate, resulting in an increase of dz(Ru− Obot) to 2.61 Å in HAl3Si5O16/4O/Ru(0001). There is a similar trend in the silica/nO/Ru(0001) system where dz(Ru− Obot) increases by 1 Å as n increases from 0 to 4.18 At n = 4, the system is the most stable when all of the ORu are under the pores of the HAl3Si5O16 film. The relative position between HAl3Si5O16 and 4O/Ru(0001) is different from (SiO2)8/4O/Ru(0001), where half of the ORu are under Si atoms from the SiO2 film.9,18 This rearrangement of ORu allows stronger charge transfer at a closer interface distance. The HAl3Si5O16/4O/Ru(0001) system with all of the ORu located under the pores (conf. 1 in Figure 3e) is 2.45 eV lower in energy than that with half of the ORu under the pores (conf. 2 in Figure 3f), which is made possible by the distortion of the

with an unpaired electron has actually been observed in γirradiated H-zeolites36 and quartz at low temperature37 when H is removed. The presence of [AlO4]0 in quartz is widely studied using electron paramagnetic resonance spectroscopy38−40 and DFT calculations.41−43 Here we built aluminosilicate model systems to study the nature of this charge compensation at the aluminosilicate/ Ru(0001) interface. The aluminosilicate film is composed of [SiO4] and [AlO4] tetrahedra. It has been established experimentally that Al atoms populate the bottom layer of the aluminosilicate film first.7 Moreover, Al−O−Al linkage in the zeolitic framework is forbidden according to Lowenstein’s rule.44 H atoms are only adsorbed on the top layer of the aluminosilicate, since the bottom layer charge is expected to be compensated by the Ru substrate. Therefore, the aluminosilicate/Ru(0001) heterojunction is modeled by HAl3Si5O16/ Ru(0001), which corresponds to H0.125Al0.375Si0.625O2 in the experiments. As shown in Figure 2a, 50% of the bottom Si atoms and 25% of the top Si atoms are substituted by Al atoms and H atoms are linked to O atoms in the top layer. The interlayer distance between the bottom layer O atoms in the bilayer film and the top layer Ru atoms (dz(Ru−Obot)) is 2.23 Å, as shown in Figure 2b. In the freestanding HAl3Si5O16 film, O atoms in the bottom layer (Obot) have unpaired p electrons, as shown in the PDOS in Figure 2c, which is consistent with previous Hartree−Fock and DFT calculations for quartz using cluster models.37,43,45 With two Al atoms at the bottom layer in the unit cell, we found the ferromagnetic configuration is 0.02 eV lower in energy than the antiferromagnetic configuration. Because of this near-degeneracy between the ferromagnetic and antiferromagnetic configurations, the ferromagnetic spin configuration is used in the rest of the paper to model the freestanding HAl3Si5O16 film without losing generality. After the HAl3Si5O16 film is adsorbed on the Ru(0001) surface, the magnetic moment (M) of the system vanishes, which indicates that the Ru substrate transfers electrons to the unsaturated Obot. The redistribution of charge upon adsorption is examined using the charge density difference defined as Δρ = ρHA13Si5O2/Ru - (ρHA13Si5O2 + ρRu). The isosurface plot of the charge density difference in Figure 2a shows that the major charge transfer arises from dz2 and s orbitals of RuO to px and py orbitals of Obot. From the PDOS of the adsorbed HAl3Si5O16 film, one can see that the unoccupied spin-down px and py states before adsorption are now occupied. The electrons are mostly transferred from dz2 and s orbitals of RuO near the Fermi level. The hybridization between RuO and Obot is also present as explained in the silica/Ru system. The new peak at −6.5 eV in Figure 2d−f after adsorption shows a bonding state due to the mixing of the Obot pz orbital with RuO dz2 and s orbital. Electrons in the antibonding states from the hybridization of the Obot pz orbital with RuO dz2 and s orbital are transferred to the Rupore dz2 and RuO dxz and dyz orbitals, which are lower in energy. The net charge transfer from Ru substrate to the HAl3Si5O16 film is 1.27 e− per unit cell in HAl3Si5O16/Ru(0001) as obtained from Bader charge analysis.46 In comparison, the charge is transferred from the silica film to the Ru substrate at the SiO2/Ru(0001) interface, which explains that the O 1s core-level binding energies of aluminosilicate films are ∼0.7 eV lower than silica films at the similar O coverage.18 The amount of charge transfer across the interface can be quantified F


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blocking of the interfacial charge transfer by surface oxygen. As shown in Figure 3e, 33% of the Obot were lifted by ORu, which prevented the hybridization and charge transfer. Because ΔE decreases as the ORu coverage on the Ru(0001) surface increases, the core-level binding energies (EBE) of the bilayer are expected to follow the same trend. We calculated EBE of the O 1s core-level of Al2Si6O16/nO/Ru(0001) and compared with the XPS results for Al0.16Si0.84O2/nO/Ru(0001).18 The calculated and measured EBE are summarized in Figure 6. There are two types of O atoms in the aluminosilicate film: O bonded with two Si atoms (OSi−Si) and O bonded with a Si atom and an Al atom (OSi−Al). Since OSi−Al and OSi−Si have different chemical environment, EBE are calculated separately for OSi−Al and OSi−Si at the top and middle layers. O atoms at the bottom layer are all OSi−Al. EBE are extrapolated to infinite unit cell size and are given relative to that of the ORu in n = 4. The layer-resolved EBE are summarized in Table 2. As n increases from 0, 2, to 4, the average of EBE is 2.11, 1.51, and −0.06 eV for OSi−Si and 1.47, 0.83, and −0.50 eV for OSi−Al. EBE for OSi−Si is higher than OSi−Al, which is consistent with the peak split of 1 eV in the XPS measurement.18 The shift in EBE (ΔEBE) is −2.17 eV for OSi−Si and −1.97 eV for OSi−Al as n increases from 0 to 4. The shift in EBE follows the trend in ΔE and in the core-level XPS spectra for the Al0.16Si0.84O2 film, where ΔEBE is −0.6 eV in both OSi−Si and OSi−Al.18 This decreasing trend in EBE qualitatively agrees with the experiment results, while our calculations systematically overestimate ΔEBE as shown in Figure 6. There are multiple factors that may contribute to the difference between theory and experiment. Experimentally, there is an error bar for the estimate of ±0.05 eV for the core level binding energy and the estimate of the ORu coverage has an error of ±0.05 ML, which corresponds to an error of ±0.4 in n shown in Figure 6. In addition, the major shift of EBE originates from n = 4 where a distortion of the bilayer is observed (Figure 3e), which may also induce redistribution of the surface O atoms that affect EBE. On the other hand, approximations in the EBE calculations can introduce errors. For example, the final state of system was treated with neutral excitations in our model instead of charged excitations.18

HAl3Si5O16 lattice that induces a smaller interface distance. Therefore, HAl3Si5O16/4O/Ru(0001) with all of the ORu located under the pores (Figure 3e) is used in the following discussion. In comparison, the two ORu arrangements in (SiO2)8/4O/Ru(0001) are similar in energy (conf. 1 in Figure 3g and conf. 2 in Figure 3h) with a 0.034 eV difference in energy. The charge redistribution at the aluminosilicate/Ru(0001) heterojunction strongly depends on the aluminum concentration and ORu coverage. We evaluated the charge density difference using the Bader charge analysis46 and the planeaveraged charge density difference (Figure 4) at the interface (Δρinter = ρbilayer/nO/Ru − (ρbilayer + ρnO/Ru)) and the surface (Δρsur = ρnO/Ru − (ρnO + ρRu)). The dipole moment (p) and work function (Φ) of the combined system and subsystems are summarized in Table 1. Φ is measured as the energy difference between EF and the vacuum level (Evac). The interface dipole moment (pinter) caused by Δρinter is calculated from pinter = pbilayer/nO/Ru − (pbilayer + pnO/Ru), while the surface dipole moment (psur) due to Δρsur is calculated from psurf = pnO/Ru − (pnO + pRu). The total dipole moment of the system (ptot) is mainly determined by pinter, psur, and the dipole moment of the aluminosilicate film (pbilayer). Δρinter in Al2Si6O16/Ru(0001) behaves similarly (Figure 4a) to that of the HAl3Si5O16/Ru(0001) interface (Figure 4b), where Δρinter spreads over the interface in an oscillating fashion due to the charge rearrangement discussed above. Based on the Bader charge analysis, the net charge transfer (Δq) from Ru to the aluminosilicate film is 1.25 e− per unit cell in Al2Si6O16/Ru(0001) and 1.27 e− in HAl3Si5O16/Ru(0001), which results in pinter (Al2Si6O16/Ru(0001)) = −0.28 eÅ and pinter (HAl3Si5O16/Ru(0001)) = −0.25 eÅ. However, pbilayer (HAl3Si5O16) is 0.21 eÅ larger than pbilayer (Al2Si6O16), which makes ptot (HAl3Si5O16/Ru(0001)) larger than ptot (Al2Si6O16/ Ru(0001)) and Φ (HAl3Si5O16/Ru(0001)) 0.89 eV smaller than Φ (Al2Si6O16/Ru(0001)). As n increases from 0 (Figure 4b), 2 (Figure 4c), to 4 (Figure 4d), there is an increase in dz(Ru−Obot) that results in a decrease in the interface charge transfer. pinter decreases from −0.25 to −0.74 eÅ mainly due to the increase in dz(Ru−Obot). On the other hand, ORu draw 1.74 e− from Ru atoms at n = 2 that leads to a negative surface dipole moment psur = −0.01 eÅ (Figure 4e). As n increases to 4, ORu draw 3.27 e− from Ru atoms, which yields psur = −0.10 eÅ (Figure 4f). The dipole moment and work function of the subsystems are summarized in Table 1. Considering pbilayer (HAl3Si5O16) is ∼0.4 eÅ, ptot changes from positive (0.20 eÅ) to negative (−0.38 eÅ) as n increases from 0 to 4 as a result of both pinter, and psur which leads to an increase in the work function from 4.61 to 6.69 eV. As n increases from 0 to 4 in HAl3Si5O16/nO/Ru(0001), the decrease in the total dipole moment of the system leads to electrostatic potential shifts of (H)AlxSi8−xO16/nO/Ru(0001) as shown in Figure 5. The Fermi level (EF) are aligned with (SiO2)8/Ru(0001). Because of the charge transfer from the Ru substrate to the aluminosilicate film, Φ of aluminosilicate/nO/ Ru(0001) films are higher than that of (SiO2)8/Ru(0001). The decrease in ptot also moves the valence band maximum (VBM) closer to EF, which is characterized as the energy separation between EF and VBM (ΔE). Si substitution with Al atoms causes an increase in Φ and a decrease in ΔE. As n increases from 0, 2, to 4, ΔE decreases from 2.80, 2.14, to 0.30 eV, which follows the trend of increasing Φ. ΔE for n = 4 is close to 0 eV, which partially results from the unsaturated Obot due to the

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CONCLUSIONS The mechanisms of the charge rearrangement and energy level alignments at the (alumino)silicate/Ru(0001) interfaces are investigated with DFT calculations. We found that the charge rearrangement at the interface takes place in both the surface normal and lateral directions, resulting from the electron transfer from the substrate to the (alumino)silicate film through hybridizations between the O pz and Ru dz2 and s orbitals, and the subsequent electron redistribution among Ru d orbitals. With the increasing Al doping concentrations, the degree of interfacial charge transfer is enhanced, which in turn increases the work function and lowers the O 1s core-level binding energy shift in the bilayer film. Chemisorbed O atoms on Ru(0001) can block the charge transfer at the interface and decrease the total dipole moment of the system due to the buildup of the negative surface and interface dipole moment, which lowers the core-level binding energies of the bilayer. The trend of our core-level binding energy results is in good agreement with X-ray photoelectron spectroscopy (XPS) measurement. Our study of the (alumino)silicate/Ru(0001) systems provides physical insights into the energy level G


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alignment of the zeolite models, which is of great importance to the mechanistic understanding of their catalytic properties.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Mengen Wang: 0000-0003-4575-9080 Jian-Qiang Zhong: 0000-0003-2351-4381 Dario J. Stacchiola: 0000-0001-5494-3205 J. Anibal Boscoboinik: 0000-0002-5090-7079 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Research was carried out at the Center for Functional Nanomaterials and the Scientific Data and Computing Center, a component of the Computational Science Initiative and the IOS beamline of the National Synchrotron Light Source II at Brookhaven National Laboratory, which are supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-SC0012704. This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. J.Q.Z. and M.W. are supported by BNL LDRD Project No. 15-010.

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First-Principles Study of Interface Structures and Charge

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