J. Phys. Chem. C 2010, 114, 10917–10921
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Density Functional Theory Study of Ag Adsorption on SrTiO3 (001) Surface Wei Wei, Ying Dai,* Meng Guo, Yingtao Zhu, and Baibiao Huang School of Physics, State Key Laboratory of Crystal Materials, Shandong UniVersity, Jinan 250100, People’s Republic of China ReceiVed: March 30, 2010; ReVised Manuscript ReceiVed: May 11, 2010
Employing the first-principles density functional theory approach, we evaluated the absorption properties of Ag on both SrO- and TiO2-terminated SrTiO3 (001) surfaces. Calculated grand thermodynamic potentials illustrate that SrO-terminated SrTiO3 (001) surfaces have a comparable range of thermodynamic stability with TiO2-terminated one. For a single Ag atom adsorption, bonding between Ag and O atoms indicates different mechanisms on different terminations. On SrO-termination, the Ag-O bond shows an ionic character while covalent-like upon TiO2-termination. Ag adsorption introduces formation of dipole moment layer at the interface and change in work function. With silver coverage increasing, it respectively indicates attractive and repulsive interaction between adatoms on the (001) surface with SrO- and TiO2-termination. The results for different size clusters (Ag4, Ag8) adsorption indicate that Ag cluster interacts with the substrate via its two Ag atoms bonding to two surface O atoms on the (001) surface. In addition, Ag clusters adsorption demonstrates obviously lower adsorption energies than a single Ag atom adsorption. 1. Introduction It has been identified that perovskite SrTiO3 exhibits metallicity, magnetism, high permittivity and ferroelectricity, hightransition temperature superconductivity, and giant magnetoresistance.1 Moreover, SrTiO3 is also extensively used as a substrate for the epitaxial growth of technologically important perovskite oxide thin films, which show unique physical properties. In addition, in the recent past SrTiO3 has been attracting more and more attention as a promising photocatalyst capable of decomposing organic compounds and overall water splitting for H2/O2 evolution.2-6 However, because of its wide band gap (about 3.2 eV) SrTiO3 can only absorb a small portion of the solar spectrum in the ultraviolet region, which greatly restricts its photocatalytic efficiency. To improve the photocatalytic activity, one of the most effective methods that depositing metal particles on the photocatalyst surface has been proposed. Hereinto, Ag is one of the popular candidates used in the quest of high-photocatalytic efficiency and it has been demonstrated that it plays an important role in photochemical or photoelectrochemical waste oxidation and also overall watersplitting experiments.7-9 For instance, Ag/TiO2, Ag/AgCl, Ag/ AgBr, and so forth have a much higher photoactivity compared to single semiconductor system.10-12 It was observed that Ag/ SrTiO3 possessed the most promising catalytic activity toward dye degradation under UV-vis light.13 The very recent experiment commented that silver nanoparticles deposited on a SrTiO3 surface could obviously improve the separation and migration mobility of photogenerated charge carriers.14 In principle, metals contacted with a semiconductor surface may give rise to a Schottky barrier at the metal/semiconductor interface, which plays a role as efficient electron trapping center and prevents the electron-hole from recombination in a photocatalytic process.15 Since the work function of metal is higher than that of semiconductor, photoinduced electrons in the conduction band of the semiconductor are believed to readily transfer to the metal until the Fermi levels of them are aligned. Furthermore, it should * To whom correspondence should be addressed.
be noticed that a shift of Fermi level can improve the energetics of the metal/semiconductor composite system and the efficiency of interfacial electron transfer process.15,16 Consequently, it is of fundamental interest in obtaining insights into the adsorption properties of Ag on SrTiO3 surface. To the best of our knowledge, except the scanning tunneling microscopy study of the structure and morphology of selfassembled silver nanocrystals supported on SrTiO3 (001) substrate,17 there have been no theoretical studies reported on the absorption properties of Ag on the SrTiO3 surface. In the present work, we examined the adsorption properties of Ag on the extensively studied SrO- and TiO2-terminated SrTiO3 (001) surfaces18-23 through the first-principles electronic structure calculations based on density functional theory (DFT). Geometric and electronic properties of silver adsorption on the SrTiO3 (001) surface were investigated. Moreover, adsorption properties of small cluster Agn (n ) 4, 8) on the (001) surfaces were also examined. 2. Computational Details The DFT calculations were performed by the Vienna ab initio Simulation Package (VASP)24,25 with projected augmented wave (PAW)26 pseudopotential. Generalized gradient approximation (GGA) in the scheme of Perdew-Bueke-Ernzerhof (PBE) was used for the exchange correlation functional.27,28 The wave functions were expanded into a basis set of plane waves with a kinetic energy cutoff of 400 eV. Ionic relaxations were carried out until the atomic forces were converged to 10-3 eV/Å and the convergence threshold for self-consistence-field iteration was set at 10-4 eV. A 4 × 4 × 1 grid of k points was used to sample the Brillouin zone integration in the total energy calculations. SrTiO3 (001) surfaces with both SrO- and TiO2-termination were simulated using a slab model with 2 × 2 surface unit cell periodicity and seven atomic layers. It should be noticed that the consideration of systems with seven atomic planes was sufficient for the consideration of calculated surface properties.29 In an experiment for the SrTiO3 (001) surface, (2 × 1), c(4 × 2), and c(4 × 4) reconstructions were presented through the
10.1021/jp102865r 2010 American Chemical Society Published on Web 05/27/2010
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Wei et al.
1 F ) [Eslab - NTiO2(µTiO2 + ETiO2) - NSrO(µSrO + ESrO)] 2
Figure 1. Calculated grand thermodynamic potential F as a function of the chemical potential of TiO2, µTiO2, for SrTiO3 (001) surface with different terminations.
scanning tunneling microscope method.30 However, because it indicates qualitatively credible results with respect to silver adsorption on the (001) surface based on the adopted 2 × 2 periodicity of surface unit cell, we will not discuss the effects of surface reconstruction on the adsorption properties in the present work. The relaxed lattice parameters for the (001) surface unit cell were a ) b ) c ) 3.927 Å and they were used throughout the present work and maintained fixed during the atomic position optimizations. The slab model was separated by a vacuum spacing of 21 Å to wipe out the interaction between periodic images. The three bottom layers of the slab were maintained in their bulk parameters and the upper four layers with the adsorbate were fully relaxed in our calculations. Total energy calculations for a Ag atom and Agn clusters were performed within a cubic box of side length 15 Å. The silver adsorptioninduced dipole moment was taken into account using a dipole correction in our calculations.31 In our test calculations, we addressed that spin-polarization correction had negligible influences on total energy and discussions of adsorption properties. As a consequence, we employed the nonspin-polarized calculation in the present work. 3. Results and Discussion To study the relative stability of the SrO- and TiO2-terminated SrTiO3 (001) surfaces, we calculated the grand thermodynamic potential F as a function of the TiO2 chemical potential, µTiO2, following the approach developed by Padilla et al32
where ESrO and ETiO2 are the total energies of bulk crystals per formula unit, which are respectively calculated from rock salt SrO and rutile TiO2. µSrO is defined as the chemical potential of SrO. Eslab is the energy of the relaxed slab and the factor of 1/2 indicates the fact that the slab contains two surfaces. We consider the fact that the slab is independently constituted by SrO and TiO2 units. For instance, the SrO-terminated (001) surface adopted in the present work corresponds to the respective unit numbers of NSrO ) 4 and NTiO2 ) 3. For TiO2-terminated SrTiO3 (001) surface, it indicates NSrO ) 3 and NTiO2 ) 4, individually. The calculated bulk energies of SrO and TiO2 are -12.10 and -26.43 eV, which is in excellent agreement with the previous DFT result.1 The grand thermodynamic potential as a function of µTiO2is depicted in Figure 1. As can be seen, SrO-terminated (001) surface has a comparable range of thermodynamic stability with TiO2-termination, which indicates that either surface can be formed depending on whether growth occurs in Sr-rich or Ti-rich condition. The adsorption energy, Eads, is calculated based on the following equation
Eads ) E(slab+adsorbate) - Eslab - Eadsorbate where E(slab+adsorbate) and Eadsorbate are the calculated total energies of slab with Ag adsorbate on it and Ag adsorbate in gas-phase, respectively. The relative lower value in adsorption energy indicates the preference of Ag adsorption on the surface. First, we consider a simple situation of a single Ag atom adsorption on the SrTiO3 (001) surface, which corresponds to coverage of 0.25 monolayer (ML). We checked a series of potential adsorption sites on both terminations, and the energetically favorable adsorption configurations are respectively shown in Figure 2a,d. On SrO-terminated (001) surface, the Ag atom converges to a O-top site with the Ag-O bond length of 2.240 Å. For Ag on the SrTiO3 (001) surface with TiO2-termination, the energetically favorable configuration indicates that the Ag atom prefers to locate at a 4-fold hollow site with the average Ag-O bond length of 2.407 Å. The adsorption energies are respectively predicted to -0.72 and -1.39 eV on SrO- and TiO2-terminated (001) surface. The more negative adsorption energy is in relation to the higher Ag coordination number with respect to surface O atoms.
Figure 2. Energetically favorable Ag and Agn (n ) 4, 8) adsorption structures on SrTiO3 (001) surface. (a-c) The adsorption configurations for Ag, Ag4, and Ag8 on SrO-terminated surface, respectively. (d-f) Adsorption configurations for Ag, Ag4, and Ag8 on TiO2-terminated surface, respectively. Only the first slab layer is shown. The red, green, gray, and big light spheres respectively represent O, Sr, Ti, and Ag atoms.
DFT Study of Ag Adsorption on SrTiO3 (001) Surface
J. Phys. Chem. C, Vol. 114, No. 24, 2010 10919 of this Ag-O bond. Electron from Ag 5s states accumulates on the 2p states of the under-lying O. From the PDOS, it can be seen that the equilibrium position of Fermi level (EF) is close to the conduction band. If this Ag/semiconductor composite system is activated, excited electrons from the semiconductor surface will occupy the empty Ag 5s states above the EF. In addition, charging Ag 5s orbital will shift the EF to the conduction band direction, and this shift can improve the energetics of the composite system and efficiency of the interfacial electron transfer process.18,19 The EF directly relates to the number of accumulated electrons, as illustrated in the following equation15
EF ) ECB + kT ln
Figure 3. PDOS for a single Ag atom adsorption on (a) SrO-, and (b) TiO2-terminated SrTiO3 (001) surface. The dashed lines represent the Fermi level (EF).
Figure 4. Projective density of states (PDOS) for a single Ag atom adsorption on (a) SrO-, and (b) TiO2-terminated SrTiO3 (001) surface. The dashed lines represent the Fermi level (EF).
To get a more detailed description about the interaction between Ag and O, the projective density of states (PDOS) are calculated. For a comparison, PDOS of both the clean terminations of SrTiO3 (001) surface are also calculated (not shown in the present work). Band gaps for clean SrO- and TiO2termination are respectively 1.38 and 0.75 eV, which are obviously smaller than that of bulk crystal of 1.83 eV.33 This decreasing can be attributed to the surface states. For a Ag atom adsorbed on the SrO-terminated surface (as shown in Figure 2a), the PDOS is plotted in Figures 3a and 4a. Figure 3a shows the total density of states (TDOS), PDOS of the O 2p and Ti 3d states, and to highlight, Ag 4d, 5s, and 2p states of O atom bonding to the Ag atom are separately shown in Figure 4a. It can be seen that Ag 5s states dominantly locate in energy within the band gap, which is responsible for the smaller photon excitation energy between the valence band and gap states compared with band-band excitation. This may be one of the potential reasons for the photocatalytic activity of Ag/SrTiO3 composite system under visible light in experimental observation.13 As can be seen from Figure 4a, the O 2p states show two main regions, which is indicative of formation of bonding and antibonding states. Localized Ag 4d states and overlapping between Ag 5s and O 2p states reveal some ionic component
nc Nc
ECB is the conduction band energy, nc is the density of accumulated electrons, and Nc is the charge carrier density. When an acceptor is accreted, for this metal/semiconductor composite, on-surface Ag is assumed to play a mediating role in storing and shutting the photogenerated electrons from the semiconductor to acceptor in a photocatalytic process. For a Ag atom adsorption on the (001) surface with TiO2termination (as shown in Figure 2d), the PDOS plots are shown in Figures 3b and 4b. Different from Ag on SrO-termination, Ti 3d states are responsible for the bottom of the conduction band. As can be noticed, Ag 4d states are more delocalized in Figure 4b compared with that in Figure 4a, which indicates that bonding between Ag and O is covalent confirmed by the hybridization between Ag 4d and O 2p states. There are no silver related states around the EF, and the Ag 5s states are mainly unoccupied in the high-energy region in the conduction band. We also calculated the d band center of Ag atom adsorbed on both terminations. Comparing with Ag on the SrO-terminated (001) surface, the band center of 4d states of Ag on TiO2terminated (001) surface shifts 0.27 eV downward. It has been identified that upshift of d band center strengthens the interaction while downshift weakens the binding interaction.34 Consequently, Ag-O bond length in the case of Ag/SrO-terminated (001) surface is shorter than that in Ag/TiO2-terminated (001) surface. Figure 5 presents the average electrostatic potential in vacuum for both terminations with a Ag atom adsorbed in which the work function change (∆Φ, in eV) is defined. Figure 5 can be acquired via averaging the total electrostatic potential with respect to the SrTiO3 (001) surface along the normal direction to the (001) surface. Using Helmholtz equation, the surface dipole moment (in debye) is calculated in the present work35
µ)
A∆Φ 12πΘ
where A is the area in Å2 per 1 × 1 surface unit cell and Θ is the coverage. As can be seen from Figure 5, dipole layer forms in both Ag/surface composite systems, which induces the change in work function of 2.21 and -0.64 eV. According to the Helmholtz equation, surface dipole moments of 3.62 and -1.04 D are correspondingly obtained for Ag/SrO- and Ag/TiO2terminated (001) interfaces. Although the overall charge transfer, the detailed charge redistributions are site dependence, which determines the absolute value and sign of the work function change as well as the dipole moment.36 The small absolute value of dipole moment of the later system is also in relation to the specific atomic geometry. Moreover, different surface characters,
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Wei et al. TABLE 1: Ag Coverage Θ (ML) on the Surface and the Calculated Adsorption Energies Eads (eV), Ag-O Bond Lengths dAg-O (Å),a Change in Work Function ∆Φ (eV) and Dipole Moment µ (D) for Ag Adsorption on the SrTiO3 (001) Surface with SrO- and TiO2-Terminations Θ(ML) Eads (eV) dAg-O (Å) ∆Φ (eV) µ (D) SrO-termination TiO2-termination
0.25 0.50 1.00 0.25 0.50 1.00
-0.72 -1.10 -1.22 -1.39 -1.13 -1.12
2.240 2.343 2.401 2.407 2.498 2.806
2.21 2.54 3.09 -0.64 -0.62 0
3.62 2.08 1.27 -1.04 -1.58 0
a For Ag adsorption on the TiO2-terminated SrTiO3 (001) surface, we summarized the average bond lengths.
Figure 5. Electrostatic potential averaged in plane parallel to the surface and bulk like oscillations are obtained in the middle of the slab. (a) Ag on SrO-terminated SrTiO3 (001) surface, and (b) Ag on TiO2-terminated SrTiO3 (001) surface. The dashed lines represent the Fermi level (EF).
Figure 6. Charge density differences for Ag adsorbed on (a) SrO- (b) TiO2-terminated SrTiO3 (001) surface at 0.006 (au) isosurface value. Charge density flows from the yellow to cyan region. The atom notation is the same as in Figure 2.
ionic SrO- and covalent TiO2-termination, also contribute to the differences in the calculated dipole moments. To give an intuitive description, the charge redistributions for Ag adsorption on both terminations of the (001) surface are shown in Figure 6. For TiO2-termiated (001) surface with Ag adsorbed on it, it can be seen that some O 2p electron back-donates to Ag, which is also confirmed by the PDOS plots shown in Figure 4b. From the PDOS of Ag 5s states shown in Figure 4b, it can be seen that there also some occupied 5s states in the valence band. This is the reason for the negative surface dipole moment. To check the coverage dependence of the adsorption properties, a single Ag atom adsorption on (001) surface with 1 × 1 and 2 × 1 periodicity of surface unit cell, which respectively corresponds to the coverage of 1 and 0.5 ML, is examined. To obtain more accurate results, for the former the k points were improved to 8 × 8 × 1 and 6 × 6 × 1 for the latter. For Ag adsorption on the SrO-termination at either coverage it shows an on-top adsorption in which the Ag directly locates above the O. Similarly, Ag on the TiO2-termination also indicates a 4-fold hollow site location. The calculated adsorption energies Eads, Ag-O bond lengths dAg-O, work function changes ∆Φ and surface dipole moments µ are summarized in Table 1. It is found that the Eads of Ag adsorption on SrO-termination decreases with coverage increasing, which reveals an attractive
interaction between the adatoms.36 This is also confirmed by the decreased surface dipole moments with coverage increasing. When the coverage Θ < 0.50 ML for Ag adsorption on TiO2termination, there is a clear repulsion between the adatoms. On the other hand, when Θ > 0.50 ML the adsorption energies are almost the same. For a Ag on both terminations, the Ag-O bond lengths reveal an increasing tendency. Particularly for Ag on TiO2-termination with 1.00 ML, the average Ag-O bond length of 2.806 Å indicates that the Ag-O bond is so long that may result in the zero dipole moment. Silver particles on the surface with different sizes should present different chemical and physical properties. We further consider the adsorption properties of Ag4 and Ag8 clusters on SrTiO3 (001) surface. The energetically favorable adsorption structures are shown in Figure 2b,c,e,f, respectively. As can be seen, Agn clusters bind with the surfaces via two Ag atoms bonding to surface O atoms. For Ag4 and Ag8 on the SrOterminated (001) surface, the adsorption energies are predicted to be -1.67 and -3.37 eV, respectively. For Ag4 and Ag8 on the (001) surface with TiO2-termination, the adsorption energies are calculated to be -1.75 and -2.93 eV, respectively. It can be noticed that Ag cluster adsorption energies are obviously lower than that of a single Ag atom adsorption. As n increases, the adsorption energy decreases for Ag on both terminations. From the PDOS plots (not shown in the present work), it can be found that Ag 4d states are delocalized for Ag4 and Ag8 adsorption on both terminations, which indicates covalent Ag-O bond. 4. Conclusions In summary, we calculated the energetics of the SrTiO3 (001) surface with different terminations and the silver adsorption properties via the first-principles DFT approach. The following conclusions are remarked: (1) Both SrO- and TiO2-terminated SrTiO3 (001) surfaces have a comparable range of thermodynamic stability. This is an indication that either surface can be formed depending on whether growth occurs in Sr-rich or Ti-rich conditions. (2) At Ag coverage of Θ ) 0.25, adsorption energy of Ag on TiO2-termianted (001) surface is obviously lower than that on SrO-terminated surface because of the higher Ag coordination numbers with respect to surface O atoms. The shift of Ag 4d band center is responsible for the difference in Ag-O bond lengths. With Ag coverage increasing, it indicates an attractive interaction between adatoms on the SrO-termination. There is an obvious repulsive interaction between the adatoms for Ag adsorption on the SrTiO3 (001) surface with TiO2-termination when the Ag coverage Θ < 0.50 ML.
DFT Study of Ag Adsorption on SrTiO3 (001) Surface (3) At Ag coverage of Θ ) 0.25, bonding mechanisms between Ag and O on (001) surfaces with different terminations are different. Upon SrO-termination, Ag 4d states are strongly localized and Ag 5s states are partially occupied, which reveals an ionic like Ag-O bond and charge separation. On the TiO2termination, Ag 4d states sufficiently hybridize with O 2p states, which indicates a covalent Ag-O bond. (4) Because of the different electronegativities of Ag and O, a dipole moment layer forms and then results in the change in work function. Differences in dipole moments can be ascribed to the atomic geometry and the detailed charge redistributions at the interface. The formation of a dipole moment can reduce the recombination rate of photogenerated electron-hole pairs. (5) Cluster adsorption on the SrTiO3 (001) surface is lower in adsorption energies than a single Ag atom adsorption. Ag-O bonds reveal covalent character for Ag4 and Ag8 clusters adsorption on both terminations. Acknowledgment. This work is supported by the National Basic Research Program of China (973 program, Grant 2007CB613302), National Natural Science Foundation of China under Grants 10774091 and 20973102, and Natural Science Foundation of Shandong Province under Grant Y2007A18. References and Notes (1) Wang, J.; Fu, M.; Wu, X. S.; Bai, D. J. Appl. Phys. 2009, 105, 083526. (2) Miyauchi, M.; Takashio, M.; Tobimatsu, H. Langmuir 2004, 20, 232. (3) Matsumoto, Y.; Koinuma, H.; Ohsawa, T. J. Phys. Chem. C 2007, 111, 10523. (4) Puangpetcha, T.; Sreethawonga, T.; Yoshikawab, S.; Chavadej, S. J. Mol. Catal A: Chem. 2008, 287, 70. (5) Chen, L.; Zhang, S.; Wang, L.; Xue, D.; Yin, S. J. Cryst. Growth 2009, 311, 735. (6) Liu, Y.; Xie, L.; Li, Y.; Yang, R.; Qu, J.; Li, Y.; Li, X. J. Power Sources 2008, 183, 701. (7) Schurch, D.; Currao, A.; Sarkar, S.; Hodes, G.; Calzaferri, G. J. Phys. Chem. B 2002, 106, 12764. (8) Glaus, S.; Calzaferri, G.; Hoffmann, R. Chem.-Eur. J. 2002, 8, 1785.
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