Structure and Electronic Properties of Small SilverâGold Clusters on

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Structure and Electronic Properties of Small Silver-Gold Clusters on Titania Photocatalysts for HO Production: An Investigation with Density Functional Theory 2

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Elham Moharramzadeh Goliaei, and Nicola Seriani J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09300 • Publication Date (Web): 14 Jan 2019 Downloaded from http://pubs.acs.org on January 15, 2019

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The Journal of Physical Chemistry

Structure and Electronic Properties of Small Silver-Gold Clusters on Titania Photocatalysts for H2O2 Production: an Investigation with Density Functional Theory

Elham Moharramzadeh Goliaeia, Nicola Serianib* a

Young Researcher and Elite Club, Bandar Anzali Branch, Islamic Azad University, P. O. Box 43111, Bandar Anzali, Iran b The Abdus Salam ICTP, Strada Costiera 11, 34151 Trieste, Italy

Abstract

Ag4-n–Aun (n = 1, 2 and 3) clusters deposited on the surface of TiO2(101) are more efficient than titania as photocatalysts. The mechanisms of this enhancement are however unclear. In this study, density functional theory was applied with the Hubbard correction to investigate the atomic and electronic structure of Ag4-n–Aun (n = 1, 2 and 3)@TiO2(101). The band gap of TiO2 in the presence of Ag3Au, Ag2Au2 and AgAu3 decreased to 1.00, 1.24 and 1.40 eV, respectively. The band edges of all systems satisfy the requirement for H2O2 production. H2O2 adsorption energies and O−O bond lengths have shown that Ag3Au@TiO2(101) and Ag2Au2@TiO2(101) systems are good candidates for H2O2 production without poisoning of the catalyst surface and H2O2 dissociation. The results show that Ag3Au and Ag2Au2 can produce midgap states at the top of the valence band from the direct Au−Ti bonds. The midgap state in Ag2Au2@TiO2(101) has a suitable position for H2O2 formation, thereby boosting the photoresponse of the system in visible and infrared light by improving the charge separation.

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1. Introduction H2O2 as a carbon−free energy carrier and green oxidants plays an important role in fuel cells.1-7 The electroreduction process of H2O2 dramatically affects fuel cell performance. Pd−based catalysts show high activity for H2O2 synthesis, although H2O2 production is limited as a result of (1) parallel water−forming reaction, (2) consecutive H2O2 to H2O destruction, and (3) explosive hydrogen/oxygen gas mixture.8-10 Attention has been attracted toward titania for its technological potential, which includes its use in heterogeneous catalysis, solar cells, gas sensors, waste remediation, biocompatible materials and photocatalysis. However, the photocatalytic efficiency of pure TiO2 is low because of the rapid recombination of its holes and electrons, and because its wide band gap (~ 3.0 eV) only allows for absorption of the ultraviolet spectrum. The approaches adopted to avoid this include the use of noble metal nanoparticles such as gold, silver and platinum.11-14 Deposition of metal nanoparticle on oxides can happen via different methods.15,16 The use of two noble metals simultaneously is necessary to reach maximum visible light absorption17 and the photocatalytic activity of TiO2 (or another suitable semiconductor) under visible light irradiation has been predicted to increase with a bimetallic system. Of the bimetallic systems, Ag–Au nanoparticles exhibit a wide range of visible light absorption.18 Furthermore, TiO2 as support promotes noble metal catalytic activity.19 On the other hand, a review of literature shows that the catalytic activity of supported silver and gold clusters20 dramatically depends on the sub-nanometer cluster size.21-25 In recent years, theoretical studies have presented valuable insight into electronic and structural behaviours of clusters including TaBn0/- (n = 10–20)26, NanQ (n = 10–25; Q = 0, – 1)27, NbSinQ (n = 2–20; Q = 0, ±1)28, neutral and charged Agn (n ≤ 16), AgnB (n ≤ 15) , AgnB2 (n ≤ 14)29 and PdnQ (n = 2–20; Q = 0, ±1).30 Despite the role of Ag–Au on photoabsorption ability of TiO2, no theoretical reports are available. However, in two recent decades, Ag@TiO231-38 and Au@TiO239-49 have been widely studied theoretically. Several studies have been conducted to shed light on the catalytic activity of Ag@TiO2 and Au@TiO2 in CO oxidation reaction.33,50-53 The latest study investigated CO oxidation on Au@anatase (101) and their results have shown that the presence of gold atoms decreases oxygen vacancy formation energy at Au@anatase (101), resulting in a low barrier energy for oxygen vacancy formation, which is the rate−determining step in CO2 reduction.54 Recent studies have revealed an increase in the photocatalytic activity of Ag–Au@TiO2 over that of Ag@TiO2 and Au@TiO2 systems55,56 in the water splitting reaction, oxidation of methanol to methyl formate, as well as oxidation of 2-propanol to acetone and CO2.57-59 Jorda and coworkers60 reported that the interaction between TiO2 and aqueous H2O2 results in hydroxyl radical generation and plays an active role in the epoxidation of cyclohexene. Several attempts have been made to computationally study H2O2 production on Au(211), Au(111),61,62 gold 2


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clusters containing 2−5, 12−14, 25, 29 and 55 atoms61,63-68 and mixed gold-palladium systems.6973 In contrast to Au(111) surfaces, small gold clusters have a low potential for the production of hydrogen peroxide. Moreover, the photocatalytic production of H2O2 on Au@TiO2 has been studied.74,75 Tsukamoto et al. argue that two-electron reduction of O2 to H2O2 is accelerated on the gold particles; however, H2O2 decomposition also occurs on Au@TiO2. In addition, these authors have investigated the H2O2 production on Ag–Au@TiO2. Their results revealed that the presence of Ag atoms decreases H2O2 adsorption energy on Ag–Au@TiO2 system, therefore, the problem of H2O2 decomposition on Au@TiO2 system is solved. In this way, the Ag–Au@TiO2 system acts as a better photocatalyst for H2O2 production compared to the Au@TiO2 system. In this scope, determination of the structure under which hydrogen peroxide dissociates or is adsorbed may help in the design of photocatalyst systems for H2O2 production. Notwithstanding the strong evidence for an increased photocatalytic activity through addition of gold-silver clusters, the mechanisms of this enhancement are not understood. To this aim, in the present work, we investigate the structure and electronic properties of Ag4-n–Aun@TiO2 (n = 1, 2 and 3) systems, band edges and H2O2 adsorption energies on Ag4-n–Aun@TiO2 (n = 1, 2 and 3) by utilizing density functional theory (DFT) plus the Hubbard correction (DFT+U).

2. Computational methods Spin-polarized DFT calculations were performed using the Perdew-Burke-Ernzerhof (PBE)76 functional at the DFT+U level to account for the effect of the correlated 3d orbital of Ti atoms. A value of U = 4.2 eV for Ti atoms was used, as obtained from the literature.77,78 The Quantum Espresso package79 was utilized to make all calculations. The Vanderbilt ultrasoft (US) pseudopotentials80,81 with cut-off energy of 544 eV was used and for H2O2 adsorption energies calculations, cut-off energies increased to approximately 748 eV. The TiO2 (101) slab was created by a rectangular surface cell having 72 atoms. The thickness of the TiO2(101) slab was approximately 10 Å and the vacuum space between slabs was set to approximately 15 Å. All structures were fully relaxed until the atomic forces fell below 0.05 eV/Å in all systems and the total energy convergence reached 5×10–4 a.u. for all systems. Geometry relaxations were performed using Γ-point to represent the Brillouin zone and electron structure calculations were performed using 1×2×1 k-point grids. Molecular structure figures were created by XCrysDen.82 The following equation was utilized to calculate adsorption energy between H2O2 and Ag4-n– Aun@TiO2 (n = 1, 2 and 3) systems, Eads = E(Ag4-n–Aun – H2O2@TiO2 (n = 1, 2 and 3)) – E(Ag4-n–Aun @TiO2 (n = 1, 2 and 3)) – E(H2O2) (1)

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Where the first part is the total energy of Ag4-n–Aun – H2O2@TiO2 (n = 1, 2 and 3) complex, the second and third parts are total energies of Ag4-n–Aun @TiO2 (n = 1, 2 and 3) and H2O2 molecules, respectively. 3. Results and Discussion 3.1 Structural and electronic properties of Ag4-n–Aun @TiO2 (n = 1, 2 and 3) The anatase phase has very high activity compared to other titanium oxide phases. Here, the (101) TiO2 surface has been selected due to the density of its active sites.83 The adsorption of the Ag4-n–Aun (n = 1, 2 and 3) clusters on the surface was studied to analyze the effect of the TiO2 slab on structural features of these clusters. The planar structure of these clusters is known as the most stable in vacuum.84 Different forms of planar cluster adsorption were considered onto the slab. The most stable configurations, shown in Figures 1a, 1b and 1c, are those in which Ti atoms are bound to Au atoms and more O atoms bound to Ag atoms. Other structures are depicted in supporting information section. Interestingly, the intrinsic Ag3Au and Ag2Au2 planarity deforms to the shape of a chair. In addition, the TiO2 slab modifies the planarity of the AgAu3 cluster, in which the triangular pattern of the gold atoms changes to a linear structure. In a previous study, it was found that Ag4 cluster has a rhomboid geometry on TiO2(101)35 with an approximately flat structure, whereas the Au4 cluster tends to take a Y-shaped configuration35 similar to AgAu3 on TiO2(101). According to the literature, the rhomboid structure has been recognized as a stable isomer for Ag4 on anatase TiO2(100)85 and Au4 on rutile TiO2(110).86 In addition, the tetrahedral configuration of Ag4 has been reported on anatase TiO2 (100)85 and (101)87 and rutile TiO2 (110)86. These results confirm that not only the composition of the clusters, but also the symmetry of the anatase surface are important factors to determine the stable configuration.

Figure 1 The most stable structure of Ag4-n–Aun (n=1,2 and 3) clusters on the anatase TiO2(101); (a) Ag3Au@TiO2; (b) Ag2Au2@TiO2 and (c) AgAu3@TiO2. Dark violet balls: titanium; red balls: oxygen; light violet balls: silver; yellow balls: gold.

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In Figures 2a, 2b, 2c, 2d, 2e and 2f, density of states (DOS) and projected density of states (PDOS) are reported for Ag4-n–Aun (n = 1, 2 and 3)@TiO2. As shown in Figure 2a, in the presence of Ag3Au, the band gap of the TiO2 slab reaches to a minimum by 1.0 eV and the band gap increases by 1.24 and 1.40 eV for Ag2Au2 and AgAu3, respectively, as shown in Figures 2c and 2e. PDOS analysis shows that the tails of the 4d and 5d states of the Ag and Au atoms have reached the Fermi level for Ag3Au@TiO2 and AgAu3@TiO2; however, for Ag2Au2@TiO2, the highest peak of the binding energy is completely below the Fermi level. Moreover, the 3d state of the Ti atom for Ag2Au2@TiO2 is nearer to the Fermi level compared to other structures. Furthermore, there is a qualitative similarity between Ag3Au@TiO2 and Ag4@TiO2(101) PDOS,35 where Ag states are near to the Fermi level and O states are located away from Fermi level. Comparing the PDOS of AgAu3@TiO2 with previous publications suggests that it is similar to Au4@TiO2(101)35, where Au states are located near Fermi level and Ti states are 1 eV above. The presence of midgap states is an additional important feature of the PDOS of Figure 2. In the case of Ag3Au@TiO2, there is a transition from VB (midgap state of ~ 0 eV) to conduction band of 1 eV. At variance from Ag2Au2@TiO2, for Ag3Au@TiO2 the midgap states lie only 0.2 eV below the CB. As shown in Figures 2a, 2b, 2c and 2d, direct Au−Ti bonds and single Ag atom orbitals contribute together to the midgap states. According to the literature,35 both Ag4@TiO2 and Au4@TiO2 have midgap states in their PDOS which is originated from Ag and Au states, respectively. The important role of the midgap state in undoped mixed-phase TiO2 nanoparticles,35 Cu clusters supported on TiO288 and Ag cluster supported on TiO289 is wellestablished for their catalytic response to visible light. These states can be observed in Ag3Au@TiO2 and Ag2Au2@TiO2 structures; however, in the AgAu3@TiO2 nanostructure, a midgap state has not been found between the valence band (VB) and conduction band (CB), which can reduce its photocatalytic activity by improving charge recombination. It should however be mentioned that the position of midgap states is an important factor, in other words, only midgap states which are located in suitable positions can increase photocatalytic activity.90 Therefore, we assessed the position of the valence, conductivity bands and midgap states to determine Ag4-n–Aun (n = 1, 2 and 3)@TiO2 ability for H2O2 production. The positions can be evaluated according to the following equation:91 𝐸!"(!"#) = (𝜒!! 𝜒!! 𝜒!! 𝜒!! )

!

(!!!!!!!)

− 0.5𝐸! + 𝐸!

𝐸!"(!"#) = 𝐸!"(!"#) − 𝐸!

(2) (3)

Where in E!"(!"#) and E0 are the potential of the CB and scale factor equal to −4.5 eV relating the reference electrode’s redox potential to the absolute vacuum scale (AVS), and Eg and E!"(!"#) are the band gap and VB potentials, respectively. 𝜒! , 𝜒! , 𝜒! and 𝜒! are the absolute electronegativity of A, B, C and D atoms for an AaBbCcDd compound. Here, χ!" , χ! , χ!" and χ!" are equal to 3.45, 7.54, 5.77 and 4.44 eV, respectively.92 The Mulliken electronegativity of 5



Ag3Au@TiO2, Ag2Au2@TiO2 and AgAu3@TiO2 are equal to 5.75, 5.77 and 5.9 eV, respectively, and our pure TiO2 surface electronegativity is equal to 5.81 eV. Knowing that Eg values are obtained from DOS plots, the positions of valence and the conductivity band edge can be calculated. According to equation (2) and (3), E!" for Ag3Au@TiO2, Ag2Au2@TiO2 and AgAu3@TiO2 are equal to 0.12, 0.26 and 0.54 eV and E!" are equal to 2.37, 2.26 and 1.94 eV, respectively. It should be mentioned that our pure TiO2 surface has CB and VB by −0.32 and 2.93 eV, respectively. Here, we can compare our results with published findings on composites including TiO2, which have used the equations of (2) and (3) in their calculations. Song et al,93 added silver iodate to AgI/TiO2 (AIT), their results showed that the ECB order is equal to AgIO3 (0.46 eV) > TiO2 (–0.25 eV) > AgI (–0.34 eV), therefore electron transfer happens from AgI CB to TiO2 CB and then from CB TiO2 to AgIO3. This trend is in favor of electron–hole separation. The present CB and VB of AgAu3@TiO2 (0.54, 1.94 eV) are more positive and more negative compared to the AgIO3, TiO2 and AgI. In addition, the increased photocatalytic activity of SnO2/TiO2 compared to TiO2 has been reported.94 Ag4-n–Aun (n = 1, 2 and 3)@TiO2 CB and VB edges shows more positive and more negative values compared to reported for SnO2 and TiO2.The calculated data for the CB and VB of TiO2 are equal to −0.20 and 2.8 eV, respectively, and the CB and VB of SnO2 are equal to 0.0 and 3.5 eV. Hence, electrons can be transferred from the CB of TiO2 to the CB of SnO2 and holes move from the VB of SnO2 to the VB of TiO2. With this process, the electrons deposited on the CB SnO2 move from photoanode to cathode under the potential application, and this process effectively increases the electron transport rate and ultimately reduces the electron and hole recombination. Since the Ag–Au@TiO2 has photocatalytic ability to generate H2O2,75 we evaluate Ag4-n–Aun (n = 1, 2 and 3)@TiO2(101) systems performance in this reaction. Generally, according to previous studies, electron-hole recombination is slower in the anatase phase compared to the rutile phase95,96 and the presence of midgap states with proper position inhibit the recombination of charge carriers. As shown in Figure 3, all valence band edges locate below the oxidation levels. This means that all systems can be utilized for direct water splitting to produce O2 without considering the role of midgap states. As shown in Figure 3, the conductivity bands in all structures are more negative than the potential for H2O2 production, hence Ag4-n–Aun (n = 1, 2 and 3)@TiO2(101) systems can be ideal candidates for the efficient production of H2O2 under visible light. Although, we should not ignore the role of midgap states. As mentioned above, the position of midgap states can have a positive or negative effect on the photocatalytic activity. In fact, in Figure 3, midgap states should have at least lower than 1.23 eV (more positive potential) and at least higher than 0.69 eV (more negative potential) to ensure that their role in water splitting reaction and H2O2 production is positive. Here, the presence of midgap states decreases the ability of Ag3Au@TiO2 photocatalysts to produce oxygen from water splitting, as a result of the overlapping potential of the midgap state and the potential level of water oxidation. As explained in pervious study,97 more negative redox potential than the midgap state positions facilitate the oxidation of reductants. Therefore, just Ag3Au@TiO2 activity is insufficient for water oxidation and both Ag2Au2@TiO2 and AgAu3@TiO2 have the ability to act as 6


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photocatalysts for water splitting. However, Ag2Au2@TiO2 has a great ability due to having midgap states which can trap holes and retard the recombination of photogenerated charges. In addition, in the presence of midgap states (for Ag2Au2@TiO2), we predict that the use of visible light and infrared can result in the effective production of H2O2 such as utilizing hydrogenated TiO2 for H2 production in the visible and infrared light regions.98 Figure 4a shows that the HOMO of Ag3Au@TiO2 is mainly populated over the gold and silver metals, while the HOMO of Ag2Au2@TiO2 and AgAu3@TiO2 are mainly originated from the gold atoms according to Figures 4b and 4c. In addition, the HOMO orbitals consist of silver and gold s and d states, and the LUMO orbitals in Ti atoms with characteristic d are consistent with DOS. It should be noted that oxygen atoms participate in HOMO. In other words, electrons would be excited from oxygen, silver and gold atoms under light irradiation. Upon photoexcitation, the electron density tends to shift from midgap states (mainly delocalized on the gold and silver atoms) to the LUMO (localized over Ti atoms) state. The next section evaluates the photocatalytic performance of Ag4-n–Aun (n = 1, 2 and 3)@TiO2 for the production of H2O2 using the energy of H2O2 adsorption.

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Figure 2 PDOS plots of the Ag and Au atoms with direct binding to O and Ti atoms, respectively in (a and b) Ag3Au@TiO2, (c and d) Ag2Au2@TiO2 and (e and f) AgAu3@TiO2. The calculated total DOS has been multiplied by a factor 0.02 to fit into the figures. The midgap states are shown with red arrows. The Fermi levels are situated at 0 eV, and are shown by red lines. 8


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Figure 3 Band edge potentials for Ag4-n–Aun (n = 1, 2 and 3)@TiO2 compared with water oxidation level O2/H2O (1.23 eV) and hydrogen peroxide production level O2/H2O2 (0.69 eV), midgap states are shown by rectangular boxes. VB and CB potentials are displayed by black horizontal lines, band gap values are presented by E! , and E!" presents the difference between the midgap states and the CB potentials, all potential values are vs. the potential value of normal hydrogen electrode (NHE). The yellow and gray colors match the color code utilized for the gold and silver atoms, respectively.

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Figure 4 Isodensity surface plots (0.002 au) for the HOMO (left side) and LUMO (right side) orbitals for (a) Ag3Au@TiO2, (b) Ag2Au2@TiO2, (c) AgAu3@TiO2. Dark violet balls: titanium; red balls: oxygen; light violet balls: silver; yellow balls: gold.

3.2 Adsorption of H2O2 on Ag4-n–Aun@TiO2 (n = 1, 2 and 3) Pioneering work has shown that the use of H2O2 has a positive effect on the catalytic activity of TiO2 as a photocatalyst for the oxidation reaction, which depends on its role as an electron 10


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acceptor. In fact, hydrogen peroxide prevents electron−hole recombination, thereby enhancing the productivity of photocatalytic action.99,100 Although, some studies have shown that at certain concentrations, hydrogen peroxide can have a negative effect on the efficiency of the photocatalytic process and hinder the formation of OH radicals.101,102 The results of the present study show that presence of midgap states Ag2Au2@TiO2 with a suitable position, which could act as separation centers, trapping the photoinduced electrons and hindering charge recombination,103 hence these systems hold a potential to serve as better photocatalysts compared to AgAu3@TiO2 and AgAu3@TiO2. According to study by Tsukamoto et al.75 the photocatalytic decomposition of hydrogen peroxide has decreased on alloy catalyst. Here, we intend to analyze the adsorption energy of H2O2 on Ag4-n–Aun@TiO2 (n = 1, 2 and 3). Since the photocatalytic reduction of H2O2 is mainly happened on negatively charged gold atoms, we considered the adsorption energies on gold atoms. Figure 5 shows the configuration of H2O2 on Ag4-n– Aun@TiO2 (n = 1, 2 and 3), where H2O2 adsorbed on gold atom of Ag3Au in Figure 5a, on gold atoms of Ag2Au2 in Figures 5b and 5c and on gold atoms of AgAu3 in Figures 5d, 5e and 5f. According to Figure 5, the overall processes of adsorption are exothermic, ranging from –0.14 to –0.28 eV, both of these adsorption energies are belong to AgAu3@TiO2 structures. In all cases, O–O bond lengths of H2O2 except for AgAu3@TiO2(A) (with 1.46 Å) reach 1.47 Å, which is slightly shorter than that in the gas phase (here it reaches 1.48 Å). A literature review shows that the O–O bond distance of H2O2 from 1.48 Å in the gas phase is reduced to 1.45 Å on (010)-2 BNb2O2 surface104, while increasing O–O bond lengths, reached to 1.49 on Pd(111)72 (GGA/PBE approximation and Ultrasoft pseudopotentials) and Pd(100)105 (GGA-PW91 approximation and Ultrasoft pseudopotentials) and 1.48 on Au@Pd(111)72 and Pd(111)105 (GGA-PW91 approximation and Ultrasoft pseudopotentials) are evident in many Pd−based catalyst. Therefore, the dissociation probability of the O–O bond of H2O2 becomes higher in Pd−based catalyst compared to non−Pd−based catalyst and alloy Pd−based catalyst. On the other hand, H2O2 adsorption energy ranges differ from –0.32 for Pd(111) and –0.36 eV for Pd(100)105, –0.4 eV and –0.67 eV for different sites of Pd12+12+7 cluster106 (B3LYP as the Hamiltonian and LANL2DZ as the basis), –1.1 and –1.24 eV for H0.33MoO3 (100)107 (B3LYP and 6-311++G(d,p) basis set), – 1.49 eV for (010)–2 B–Nb2O5104, to –0.17, –0.18, –0.21 and –0.24 eV for Pd monomer, dimer, trimer and pure Pd surrounded by Au atoms108 (GGA-PW91 approximation and PAW pseudopotentials), respectively. Therefore, poisoning of the catalyst surface by H2O2 is less significant in alloy Pd–based catalyst as a result of lower adsorption energies. Here, the adsorption energies of Ag3Au@TiO2 and Ag2Au2@TiO2(101) are close to the calculated adsorption energies of Pd−Au alloy. To sum up H2O2 adsorption on Ag4-n–Aun (n = 1, 2 and 3)@TiO2, it can be predicted that Ag3Au@TiO2 and Ag2Au2@TiO2(101) have a good potential for H2O2 production. Since the H2O2 dissociation is limited (O–O bond distance of 1.47 Å than 1.48 Å in gas phase) and poisoning of the catalyst surface does not occur (–0.16 to −0.23 eV for adsorption energies).

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Figure 5 Optimized structures of H2O2 on (a) Ag3Au@TiO2, (b) Ag2Au2@TiO2(A), (c) Ag2Au2@TiO2(B), (d) AgAu3@TiO2(A), (e) AgAu3@TiO2(B), (f) AgAu3@TiO2(C), corresponding adsorption energies (eV) and O–O bond lengths (Å) after adsorption. Dark violet balls: titanium; red balls: oxygen; light violet balls: silver; yellow balls: gold; small Aqua balls: hydrogen.

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4. Conclusion In summary, the structural and electronic properties of Ag4-n–Aun (n = 1, 2 and 3) clusters deposited on the TiO2(101) surface were investigated using DFT+U. The main conclusions have been listed as follow: a) The presence of TiO2 as a support leads to distortion of the symmetry of Ag3Au and Ag2Au2 and strong distortion of the shape of AgAu3 from their vacuum geometry. b) The TiO2 surface in the presence of Ag3Au has a smaller band gap by 1.00 eV compared to Ag2Au2 and AgAu3, and band gaps by 1.24 and 1.40 eV, respectively. These results show that HOMO orbitals have been located on Ag and Au atoms and LUMO orbitals have been located on Ti atoms, as a result, it helps to obtain an efficient charge separation. c) There are midgap states above the top of the valence band of Ag3Au@TiO2 (101) and Ag2Au2@TiO2(101), but there is no midgap state in AgAu3@TiO2(101). d) Ag4-n–Aun (n = 1, 2 and 3)@TiO2(101) systems have the ability to oxidize H2O and produce H2O2 according to calculated band edges, although the midgap state of Ag3Au@TiO2(101) has been located near the water oxidation level which reduces Ag3Au@TiO2(101) activity for water oxidation. e) H2O2 adsorption energies of Ag3Au@TiO2(101) and Ag2Au2@TiO2(101) (−0.16 and −0.23 eV) and O−O bond lengths (1.47 Å) are in good ranges for H2O2 production. These results are compared with H2O2 adsorption on Pd−Au alloy catalysts. f) Having midgap state with suitable position for H2O2 production, makes Ag2Au2@TiO2(101) a good photocatalyst candidate that can enhance the photoresponse of TiO2 in visible light. Corresponding Author *E-mail: [email protected]

Supporting Information. Figure S1, other energetically stable adsorption configurations of Ag4-n–

Aun (n = 1, 2 and 3) on TiO2(101).

Acknowledgement EMG acknowledges financial support from the Islamic Azad University, NS acknowledges financial support from ICTP. 13



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