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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

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Origin of H Formation on Perfect SrTiO (001) Surface: A First-Principles Study Yi Yang, Chen-Sheng Lin, Wendan Cheng, Chao Liu, and Tongxiang Liang J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 4, 2018

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The Journal of Physical Chemistry

Origin of H2 Formation on Perfect SrTiO3 (001) Surface: A First-principles Study Yi Yang1, Chen-Sheng Lin2, and Wen-Dan Cheng2,* , Chao Liu1, Tong-Xiang Liang1,* 1

School of Materials Science and Engineering, Jiangxi University of Science and

Technology, Ganzhou, 341000, China 2

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the

Structure of Matter, Chinese Academy of Sciences, Fuzhou, 350002, China

Corresponding Authors *W.D.C E-mail: [email protected] *T.X.L. E-mail: [email protected]

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract We systematically investigate saturated adsorption of H atoms on the SrTiO3 (001) surface to reveal the origin of H2 molecule formation using density functional theory methods. We find that H atoms prefer to adsorb on O sites at low coverage, while adsorbing on Ti sites at higher coverage. Interestingly, H on O sites and H on neighboring Ti sites (HTi) dimerize to form H2 molecules provided that enough electrons are doped in the conduction bands of SrTiO3. Bader charges and electronic structures show that negatively charged HTi plays a decisive role in the formation of H2. Both calculated hydrogen coverage and electron density at saturated adsorption are in good agreement with experimental values. Our results provide an illuminating instance of H2 formation on perfect oxide surface, which shall pave the way to further understanding the detailed mechanism of H2 evolution on more complex oxide surfaces.


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1. INTRODUCTION SrTiO3 (STO), one of most widely studied oxides, has attracted intense research interests due to its plenty of remarkable properties such as high photoactivity for water splitting1-5 and hydrogen evolution

6-8

under visible/ultraviolet light, and two

dimensional conductivity at its surfaces or interfaces with polar oxides like LaAlO39-11. In many of these applications, adsorptions of simple atoms (molecules) such as H and H2O on STO surfaces are very fundamental processes. The most prominent example is STO catalyzing water splitting under ultraviolet light into O2 and H2 gas on STO surface5. However, mechanisms underlying the surface reaction of water splitting are not well understood.2,12 Understanding the adsorption of H atoms and the detailed mechanism of H2 formation on oxide surface is prerequisite for further revealing the reaction mechanisms of photocatalytic water splitting and H2 evolution at atomic-level. As the simplest element, behavior of H atoms on oxide surfaces such as STO has attracted special scientific interests. For example, previous experiments have demonstrated that H atoms adsorption on the STO surface imparts conducting properties to original insulating STO bulk and induces two dimensional electron gas on the surface.13-15 H atoms adsorb the oxygen sites and form the O-H bonds if TiO2-terminated STO (001) surface is exposed to abundant H.14-16 The saturated coverage of H adsorption observed in experiments is 0.47±0.12 H per surface unit cell area (u.c. area),16 which is much lower than the full coverage of 3.0 H per u.c. area (a Ti site and two O sites) as expected and also suggested by a previous density functional theory (DFT) study.13 The origin of discrepancy in saturated H coverage between experiment and theory remains unclear. Therefore, there is a lack of systematic understanding of H adsorption behavior on STO surface. In this work, we systematically investigate saturated adsorption of hydrogen atoms on STO (001) surface to reveal the origin of H2 molecule formation using density functional theory methods. We find that H on O sites and H on neighboring Ti sites (HTi) dimerize to form H2 molecule provided that enough electrons exist in 3 ACS Paragon Plus Environment


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conduction bands of STO. Calculated Bader charges and electronic structures demonstrate that negatively charged HTi plays a decisive role in the formation of H2. The calculated H coverage agrees well with experimental observations. Our results provide a deep understanding of adsorption behavior of H atoms on STO surface and also provide an explanation to saturated H coverage found in experiments.

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2. COMPUTATIONAL METHODS We perform DFT calculations with the projector augmented-wave (PAW) method implemented in the Vienna ab initio simulation package (VASP).17-19 We utilize PBEsol within generalized gradient approximation (GGA) as exchange and correlation functional.20 Kinetic energy cutoff of 400 eV is used in all calculations. Semicore states of Ti and Sr are treated as valence electrons. Lattice constant of cubic STO calculated by PBEsol is 3.899 Å, which is in good agreement with experimental constant of 3.900 Å.21 We use STO slab with 2 × 2 in-plane cells and three unit cells thick to model the TiO2-terminated surface, as shown in Figure 1. All coordinates of ions in upper two unit cells are relaxed, while those in bottom unit cell are fixed as those in bulk. Relaxations are done until all atomic forces are less than 0.015 (eV/Å). A 4 × 4 ×1 Monkhorst-Pack mesh22 is used for structural relaxations of slabs and a denser 7 × 7 ×1mesh is used for electronic structure calculations. Slab is separated from its repeated units with a vacuum layer of 18 Å along c direction within periodic boundary condition. Dipole correction23 are used to correct the effect of electric dipole presented on surface. Bader charges are calculated to show charge transfer and bonding interactions. 24-25.


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Figure 1. (a) side view and (b) top view of STO slab used in simulating TiO2-terminated surface. Single H atom adsorbs at O site and forms O-H bond on the surface.

3. RESULTS AND DISCUSSION We simulate multi-H atoms adsorption, i.e. from low coverage to saturated coverage of H atoms, on TiO2-terminated STO (001) surface. Previous studies demonstrated that H atom prefers to sit on O atoms forming O−H bond which inclines about 75° with respect to the surface normal at low coverage (single H atom on 2 × 2 surface, denoted as STO-H, see Figure 1).13,

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This configuration has been

demonstrated to be about 0.4 eV more stable than the structure with O−H bond along the surface normal.26 Based on low coverage state of H adsorption, we consider higher coverage, i.e. multi-H on the 2 × 2 surface. Specifically, we begin with two H atoms on 2 × 2 surface, and then gradually add one more H until reaching saturated coverage of H. For example, for two H on 2 × 2 surface (denoted as STO-2H), we search the most favorable site for the second H by examining the H adsorbed configuration with lowest total energy. Based on the most stable configuration of STO-2H, we continue the third H adsorption. We repeat this process until the saturation coverage is reached. For exceptional case of STO-2H, two configurations with close total energies (energy difference is 0.03 eV) are identified as ground state structures and thus both 5 ACS Paragon Plus Environment


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are used for the third H adsorption. The structure with n H atoms on STO 2 × 2 surface is denoted as STO-nH. In the following sections, to describe the coverage of H, we use the notion of n per surface 2 × 2 cell area or n/4 per u.c. area for convenience of discussion. n H adsorption energy ( Eads ) for the nth H at favorable site is calculated by:

n Eads = ESTO-(n-1)H + EH − ESTO− nH

(1)

where ESTO-(n-1)H , EH and ESTO−nH are the total energies of STO-(n-1)H slab, H atom, n the corresponding STO-nH slab, respectively. The positive Eads indicates that the

adsorption of nth H is exothermic.

Table 1. Adsorption energies (Eads, in unit of eV) of the nth H at the most favorable site (site atom indicated in parentheses) on TiO2-terminated STO surface (see configurations in Figure 2). For exceptional case of STO-2H, two configurations with close total energy (energy difference is 0.03 eV) are identified as ground state structures, i.e. 2H and 2H#, and thus both are used for the third H adsorption. The most stable 3H configuration is based on 2H# structure (Figure 2c). nth H

H (O)

2H (O)

2H# (O)

3H (O)

4H (Ti)

5H (Ti)

6H (O)

Eads

2.89

2.38

2.35

2.04

2.13

2.35

2.99

Calculated adsorption energies for varied coverage of H at their most favorable sites are listed in Table 1. It shows that H atoms prefer to adsorb on O site for low coverage, while they would rather adsorb on Ti site for high coverage. Surprisingly, we find that at high coverage states such as STO-4H, HO on O (HO) and HTi on neighboring Ti tend to form a dimer HO—HTi (Figure 2d and right part of Figure 2e) Furthermore, with the fifth H adsorbed on Ti, the dimer HO—HTi further dimerizes into a H2 molecule (Figure 2e). Interestingly, another pair of HO and HTi also dimerizes to form a second H2 on the surface when the sixth H atom absorbs on another O site (Figure 2f, 2g and 2h). This H2 molecule can be easily formed when 6 ACS Paragon Plus Environment

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adsorption configuration was optimized. Formation of H2 molecule makes the adsorption energy of the sixth H very large with the value of 2.99 eV, which is even larger than that of the first H adsorption with the value of 2.89 eV. For six H absorbing on the surface, release of two H2 molecules results in a STO-2H structure, which is just the ground state structure of STO-2H (The structures in Figure 2a and 2g are the same if H2 molecule is released in latter case). The STO-2H structure could repeat these two processes, i.e. adsorption of H atoms and desorption of H2 molecules, provided abundant H atoms presented on the surface, which is the condition used in experiments.14-16 This provides a vivid picture of H adsorption process on STO surfaces. Note that adsorption energies show minimum for STO-3H, indicating that once three H atoms adsorb on STO surface, subsequent adsorption processes (fourth to sixth) are more favorable than STO-3H case, which finally leads to a STO-6H structure (or STO-2H structure with two H2 molecules released). Therefore, if TiO2-terminated STO surface is exposed with abundant H atoms, the most favorable H-adsorbed structure is STO-2H, i.e. the saturated coverage is 0.5 H per u.c. area. This coverage is in good agreement with experimental value of 0.47±0.12 H per u.c. area evaluated from nuclear reaction analysis,16 but much lower than the full coverage state of 3.0 H per u.c. area as proposed previously based on DFT calculations.13 This is a very surprising result considering that full coverage of H is expected for oxide surfaces at low temperature, e.g. H atoms sit on both O and Zn sites of ZnO ( 1010 ) surface to reach a full coverage at 200 K.27



Figure 2. Relaxed configurations of different coverage of H adsorption on TiO2-terminated STO (001) surface: (a) STO-2H, (b) STO-2H#, (c) STO-3H, (d) STO-4H, (f) STO-5H, (g) STO-6H (one H2 molecule have been deleted). Atoms in rectangles of STO-4H, 5H, and 6H cases are also shown in (e) and (h) for a better view of bond lengths and connections marked by numbers (in unit of Å). Besides, (e) and (h) also show the dimerization process of HO and HTi to form H2 molecules after one more H is added to surface. Dashed lines connecting atoms show the distance between atoms. Only the surface TiO2 layer of the slab is displayed for simplicity and full structure of STO slab is presented in Figure 1. Asterisk indicates the position where one more H is added. To better understand HO-HTi dimer, we analyze in details the bond lengths and Bader charges of absorbed species on the surface of STO-nH for n = 4 and 5. We first examine the ground state structure of STO-4H. Bond lengths of O-HO, Ti-HTi, and HO-HTi are listed in Table 2 (also presented in Figure 2e). We can see that O-HO and Ti-HTi bonds are elongated in comparison with normal bond lengths without the tendency to form H-H dimer. In contrast, the distance between HO and HTi decreases to 1.26 Å. These results indicate that bond strength of Ti-H and O-H is weakened, 8 ACS Paragon Plus Environment

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while new HO-HTi bond starts to form. We note that our predicted most stable STO-4H configuration differs from a previous DFT prediction where each two H sit on adjacent O and Ti atoms13. We calculate the total energy of this adsorption configuration and find that this structure is 0.73eV in energy higher than our proposed ground state structure for STO-4H. Table 2. Bond lengths (Å) of O-HO, Ti-HTi and HO-HTi dimer; Bader charges (e) of HO and HTi for STO-4H and STO-5H structures. Benchmark column lists the normal values adopted from 2H# and 5H (Ti with asterisk) structures without the tendency of forming HO-HTi dimer for a comparison. STO-4H

STO-5H

Benchmark

O-HO(Å)

1.07

2.49

0.97 (2H#)

Ti-HTi(Å)

1.85

2.30

1.76 (5H)

HO-HTi(Å)

1.26

0.77

0.74

HO (e)

0.54

0.05

HTi (e)

-0.45

-0.03

Based on STO-4H structure, the most favorable site for the adsorption of the fifth H is another Ti site, leading to the ground state structure of STO-5H (Figure 2f). Interestingly, with the fifth H adsorbed, HO-HTi dimer tends to form a H2 molecule on STO-5H surface. Formation of H2 molecule can be characterized by the bond lengths (Table 2 and Figure 2e). On the one hand, bond lengths of Ti-HTi and O-HO bonds are 2.30 Å and 2.49 Å, respectively, which are significantly elongated compared with normal bond lengths. On the other hand, bond length of HO-HTi is 0.77Å, which is close to the length of 0.74 Å for molecule H2 in gas phase. Moreover, weak interaction between HO (HTi) and O (Ti) and formation of H2 molecules can also be identified using Bader charge. As shown in Table 2, Bader charges of HO and HTi are 0.05 and -0.03 e, respectively, both of which are close to zero. These nearly zero charge states are consistent with non-polar character of H2 molecule. Besides, nearly zero Bader charges of HO(Ti) also suggest that the charge transfer and then the bonding 9 ACS Paragon Plus Environment


interaction between HO (HTi) and O (Ti) are negligible. Thus, bond lengths and Bader charges indicate the formation of H2 molecule from a pair of HO and HTi. Now, we turn to the interesting question, why HO on O and HTi on neighboring Ti tend to dimerize to form a H2 molecule, which thus results in a low saturated coverage of H on STO surface. We first note that two H of dimer in STO-4H, i.e. HO and HTi, display totally different charge states. HO is positively charged, indicated by the Bader charge of 0.54 e (Table 2). In contrast, HTi is negatively charged with Bader charge of -0.45 e. Thus, the formation of HO-HTi and further H2 molecule should originate from a nucleophilic attack onto electron-deficient HO from electron-rich HTi. Considering that charge state of H is usually positively charged, e.g. HO, then negatively charged HTi (or Ti-HTi) should play a crucial role in the formation of H2. Previous experimental work proposed that negatively charged H occupy the oxygen vacancy site on the oxygen-deficient SrTiO3−δ surface.16 DFT calculations also indicated that negatively charged H can be stably trapped at the oxygen vacancy in oxygen-deficient SrTiO3−δ bulk.28 Positively charged H (bonded to O atoms) have been found existing in SrTiO3 bulk which was annealed in H2 atmosphere at high temperature (e.g. 800 °C ).29 To further reveal the origin of dimerization, we analyze the electronic structures of STO-nH to elucidate the formation of Ti-H species. As aforementioned, H prefers to adsorb on Ti site if the coverage of H is relatively high, e.g. three H pre-adsorb on O sites. With H adsorbed on O sites and O-H bonds formed, electrons of H are transferred to the conduction bands of STO which are mainly contributed from Ti 3d orbitals. Therefore, with H adsorbed on O, Ti becomes electron-rich in comparison with empty occupation for no H adsorption. Note that the electronegativity of H (Pauling scale) is 2.2, which is larger than that of Ti with the value of 1.5. Then H attracts electrons from Ti if H bonds with Ti on the surface and excess electrons on Ti-3d orbitals could promote the formation of Ti-H bond. We verify this picture by calculating electronic structures of STO-3H and STO-4H.


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Figure 3. Total density of states (DOS) (black solid line) and projected density of states (PDOS) of Ti 3d (blue thick line) and O 2p orbitals (grey filled area with red line) for (a) STO-3H and (b) STO-4H ground state structure. Valence bands and conduction bands are denoted as VB and CB, respectively. Inset in (b) illustrates the bonding interaction of Ti-HTi, where PDOS of H 1s orbital is indicated by short dashed line. Vertical dash line indicates the Fermi level.

Figure 3a shows the total DOS and PDOS for STO-3H. It can be seen that excess electrons fill in bottom of conduction bands and results in metallic behavior of STO-3H film. These electrons mainly occupy Ti 3d orbitals. We calculate total excess electrons by summing all the occupied states in the conduction bands. The electrons in the conduction bands of STO-3H are 3.0 e. Therefore, each H on O site donates one electron to conduction bands. For STO-4H with one H on Ti, a new DOS peak appears near the top of valence bands. This peak is related to Ti-H bond, as indicated by the hybridization between Ti 3d and HTi 1s orbitals (inset of Figure 3b). It is also seen that hybridized orbitals mix with 2p orbitals of O. Total excess electrons in the conduction bands are calculated to be 2.0 e. Then each H on Ti withdraws one electron from conduction bands. These results demonstrate the formation of Ti-H species and the negatively charged character of HTi. In a recent study, hydride-type Ti-H species were identified as the photoactive 11 ACS Paragon Plus Environment


species on the rutile TiO2 (110) surface using various spectroscopic techniques and DFT calculations.30 DFT calculations showed that oxygen vacancies donate excess electrons on the surface and promote the formation of Ti-H species. A more recent study found that the surface Ti-H species can form on reduced reconstruction surface of BaTiO3 (001) from the dissociative adsorption of water on the surface.31 Excess electrons presented on this surface originate from reduced Ti species. Note that formation of Ti-H species in these two oxide surfaces was attributed to excess electrons on the surfaces, which is similar to STO-nH case here. We note that dimerization of HO and HTi does not occur in the STO-3H if two H adsorb on O sites and one H on Ti. In contrast, it does occur in the STO-4H with three H on O sites and one H on Ti site. This indicates that enough electrons on Ti sites are indispensable for dimerization since there is one more electron in conduction bands for the latter case. As aforementioned, the saturated coverage is 0.5 H per u.c area (STO-2H, 2H atoms on O sites). Then, electron density is 0.5 e per u.c. area since each H on O site donates one electron to conduction bands. This density is close to experimental estimation of 0.54 e per u.c. area under the condition of the surface exposing to abundant H atoms (4000 L H-dosage).15 Moreover, the STO-2H films always keep metallic state upon H adsorption. This is consistent with experimental findings that the electron transport of H-saturated SrTiO3 surface is in the metallic conduction regime.14

4. CONCLUSIONS In summary, we conducted DFT calculations to elucidate the origin of H2 formation through simulating the adsorption of H atoms on the TiO2-terminated SrTiO3 surface. We find that H prefers to adsorb on O sites at low coverage, while adsorbing at Ti site at higher coverage. Interestingly, HTi and HO tend to form a dimer and further dimerize into H2 molecule provided that Ti is electron-rich. Bader charges and electronic structures demonstrate that negatively charged HTi (or Ti-HTi) plays a 12 ACS Paragon Plus Environment

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crucial role in formation of H2. The dimerization originates from a nucleophilic attack onto electron-deficient HO from electron-rich HTi. As a result, saturated coverage of hydrogen is 0.5 H per u.c. area, which is in good agreement with experimental value of 0.47±0.12 H per u.c. area. Calculated electron density is 0.5 e per u.c. area, which is close to experimental value of 0.54 e per u.c. area. Understanding the detailed mechanism of H2 formation on oxide surface is of fundamental importance in further revealing the reaction mechanisms of photocatalytic water splitting and H2 evolution. Our results show that H2 molecule can be readily formed with the presence of adjacent O-H and Ti-H on STO (001) surface and excess electrons in conduction bands (or with reduced Ti). H2 evolution on such type of surface has been observed on STO (111) surface (no platinum or other co-catalysts present).2 Moreover, our results demonstrate that reduced Ti plays critical role in the formation of H2 molecules, which supports the experimental argument that reduced Ti may be involved in hydrogen production.2,32 Although the adjacent O-H and Ti-H species in our model are produced by H atoms adsorption on the surface, these adjacent two species can also be produced from the dissociative adsorption of water on the reduced reconstruction surface of BaTiO3 (001).31 Our results thus provide deep insights into understanding the mechanism of H2 formation on perfect oxide surface, which shall pave the way to further understanding the detailed mechanism of H2 evolution on more complex oxide surfaces, e.g. on reconstructed SrTiO3 (001) surfaces33-34. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China under Projects 21473203, 21271114 and Jiangxi University of Science and Technology Scientific Research Starting Foundation (jxxjbs17052). This work was performed using High-Performance Computing resources from Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences. REFERENCES (1) Wrighton, M. S.; Ellis, A. B.; Wolczanski, P. T.; Morse, D. L.; Abrahamson, H. B.; 13 ACS Paragon Plus Environment


Ginley, D. S. Strontium titanate photoelectrodes. Efficient photoassisted electrolysis of water at zero applied potential. J. Am. Chem. Soc. 1976, 98, 2774-2779. (2) Wagner, F. T.; Somorjai, G. A. Photocatalytic hydrogen production from water on Pt-free SrTiO3 in alkali hydroxide solutions. Nature 1980, 285, 559-560. (3)Kudo, A.; Miseki, Y. Heterogeneous Photocatalyst Materials for Water Splitting. Chem. Soc. Rev. 2009, 38, 253-278. (4) Zhong, Y.; Ueno, K.; Mori, Y.; Shi, X.; Oshikiri, T.; Murakoshi, K.; Inoue, H.; Misawa, H. Plasmon-Assisted Water Splitting Using Two Sides of the Same SrTiO3 Single-Crystal Substrate: Conversion of Visible Light to Chemical Energy. Angew. Chem. Int. Ed. 2014, 53, 10350-10354. (5) Zhang, P.; Ochi, T.; Fujitsuka, M.; Kobori, Y.; Majima, T.; Tachikawa, T. Topotactic Epitaxy of SrTiO3 Mesocrystal Superstructures with Anisotropic Construction for Efficient Overall Water Splitting. Angew. Chem. Int. Ed. 2017, 56, 5299-5303. (6) Kato, H.; Kudo, A. Visible-Light-Response and Photocatalytic Activities of TiO2 and SrTiO3 Photocatalysts Codoped with Antimony and Chromium. J. Phys. Chem. B 2002, 106, 5029-5034. (7) Wang, D.; Ye, J.; Kako, T.; Kimura, T. Photophysical and Photocatalytic Properties of SrTiO3 Doped with Cr Cations on Different Sites. J. Phys. Chem. B 2006, 110, 15824-15830. (8) Ouyang, S.; Tong, H.; Umezawa, N.; Cao, J.; Li, P.; Bi, Y.; Zhang, Y.; Ye, J. Surface-Alkalinization-Induced Enhancement of Photocatalytic H2 Evolution over SrTiO3-Based Photocatalysts. J. Am. Chem. Soc. 2012, 134, 1974-1977. (9) Ohtomo, A.; Hwang, H. Y. A Migh-mobility Electron Gas at the LaAlO3/SrTiO3 Heterointerface. Nature 2004, 427, 423-426. (10) Meevasana, W.; King, P. D.; He, R. H.; Mo, S. K.; Hashimoto, M.; Tamai, A.; Songsiriritthigul, P.; Baumberger, F.; Shen, Z. X. Creation and Control of a Two-dimensional Electron Liquid at the Bare SrTiO3 Surface. Nature Mater. 2011, 10, 114-118. (11) Di Capua, R.; Radovic, M.; De Luca, G. M.; Maggio-Aprile, I.; Miletto Granozio, F.; Plumb, N. C.; Ristic, Z.; Scotti di Uccio, U.; Vaglio, R.; Salluzzo, M. Observation of a Two-dimensional Electron Gas at the Surface of Annealed SrTiO3 Single Crystals by Scanning Tunneling Spectroscopy. Phys. Rev. B 2012, 86, 155425. (12) Plaza, M.; Huang, X.; Ko, J. Y. P.; Shen, M.; Simpson, B. H.; Rodríguez-López, J.; Ritzert, N. L.; Letchworth-Weaver, K.; Gunceler, D.; Schlom, D. G.; et al. Structure of the Photo-catalytically Active Surface of SrTiO3. J. Am. Chem. Soc. 2016, 138, 7816-7819. (13) Lin, F.; Wang, S.; Zheng, F.; Zhou, G.; Wu, J.; Gu, B.-L.; Duan, W. Hydrogen-induced Metallicity of SrTiO3 (001) Surfaces: A Density Functional Theory Study. Phys. Rev. B 2009, 79, 035311. (14) D'Angelo, M.; Yukawa, R.; Ozawa, K.; Yamamoto, S.; Hirahara, T.; Hasegawa, S.; Silly, M. G.; Sirotti, F.; Matsuda, I. Hydrogen-induced Surface Metallization of SrTiO3(001). Phys. Rev. Lett. 2012, 108, 116802. (15) Yukawa, R.; Yamamoto, S.; Ozawa, K.; D’Angelo, M.; Ogawa, M.; Silly, M. G.; 14 ACS Paragon Plus Environment

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Sirotti, F.; Matsuda, I. Electronic Structure of the Hydrogen-adsorbed SrTiO3 (001) Surface Studied by Polarization-dependent Photoemission Spectroscopy. Phys. Rev. B 2013, 87, 115314. (16) Takeyasu, K.; Fukada, K.; Ogura, S.; Matsumoto, M.; Fukutani, K. Two Charged States of Hydrogen on the SrTiO3(001) Surface. J. Chem. Phys. 2014, 140, 084703. (17) Blöchl, P. E. Projector Augmented-wave Method. Phys. Rev. B 1994, 50, 17953-17979. (18) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-energy Calculations Using a Plane-wave Basis Set. Phys. Rev. B 1996, 54, 11169-11186. (19) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-wave Method. Phys. Rev. B 1999, 59, 1758-1775. (20) Perdew, J. P.; Ruzsinszky, A.; Csonka, G. I.; Vydrov, O. A.; Scuseria, G. E.; Constantin, L. A.; Zhou, X.; Burke, K. Restoring the Density-Gradient Expansion for Exchange in Solids and Surfaces. Phys. Rev. Lett. 2008, 100, 136406. (21) Cao, L.; Sozontov, E.; Zegenhagen, J. Cubic to Tetragonal Phase Transition of SrTiO3 under Epitaxial Stress: an X-ray Backscattering Study. phys. stat. sol. (a) 2000, 181, 387-404. (22) Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-zone Integrations. Phys. Rev. B 1976, 13, 5188-5192. (23) Neugebauer, J.; Scheffler, M. Adsorbate-substrate and Adsorbate-adsorbate Interactions of Na and K Adlayers on Al(111). Phys. Rev. B 1992, 46, 16067-16080. (24) Bader, R. F. W. Atoms in Molecules: A Quantum Theory; Clarendon Press: Oxford, UK, 1994. (25) Tang, W.; Sanville, E.; Henkelman, G. A Grid-based Bader Analysis Algorithm without Lattice Bias. J. Phys.: Condens. Matter. 2009, 21, 084204. (26) Yang, Y.; Lin, C.-S.; Cheng, W.-D. Hydrogen Adsorption Induced Antiferrodistortive Distortion and Metallization at the (001) Surface of SrTiO3. J. Appl. Phys. 2015, 118, 105303. (27) Wang, Y.; Meyer, B.; Yin, X.; Kunat, M.; Langenberg, D.; Traeger, F.; Birkner, A.; Wöll, C. Hydrogen Induced Metallicity on the ZnO ( 1010 ) Surface. Phys. Rev. Lett. 2005, 95, 266104. (28) Iwazaki, Y.; Gohda, Y.; Tsuneyuki, S. Diversity of hydrogen configuration and its roles in SrTiO3−δ. APL Mater. 2014, 2, 012103. (29) Tarun, M. C.; McCluskey, M. D. Infrared absorption of hydrogen-related defects in strontium titanate. J. Appl. Phys. 2011, 109, 063706. (30) Wu, Z.; Zhang, W.; Xiong, F.; Yuan, Q.; Jin, Y.; Yang, J.; Huang, W. Active Hydrogen Species on TiO2 for Photocatalytic H2 Production. Phys. Chem. Chem. Phys. 2014, 16, 7051-7057. (31) Koocher, N. Z.; Martirez, J. M. P.; Rappe, A. M. Theoretical Model of Oxidative Adsorption of Water on a Highly Reduced Reconstructed Oxide Surface. J. Phys. Chem. Lett. 2014, 5, 3408-3414. (32) Wagner, F. T.; Ferrer, S.; Somorjai, G. A. Photocatalytic hydrogen production from water over SrTiO3 crystal surfaces, electron spectroscopy studies of adsorbed H2, 15 ACS Paragon Plus Environment


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TOC Graphic



Figure 1. (a) side view and (b) top view of STO slab used in simulating TiO2-terminated surface. Single H atom adsorbs at O site and forms O-H bond on the surface. 61x31mm (300 x 300 DPI)

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Figure 2. Relaxed configurations of different coverage of H adsorption on TiO2-terminated STO (001) surface: (a) STO-2H, (b) STO-2H#, (c) STO-3H, (d) STO-4H, (f) STO-5H, (g) STO-6H (one H2 molecule have been deleted). Atoms in rectangles of STO-4H, 5H, and 6H cases are also shown in (e) and (h) for a better view of bond lengths and connections marked by numbers (in unit of Å). Besides, (e) and (h) also show the dimerization process of HO and HTi to form H2 molecules after one more H is added to surface. Dashed lines connecting atoms show the distance between atoms. Only the surface TiO2 layer of the slab is displayed for simplicity and full structure of STO slab is presented in Figure 1. Asterisk indicates the position where one more H is added. 94x110mm (300 x 300 DPI)



Figure 3. Total density of states (DOS) (black solid line) and projected density of states (PDOS) of Ti 3d (blue thick line) and O 2p orbitals (grey filled area with red line) for (a) STO-3H and (b) STO-4H ground state structure. Valence bands and conduction bands are denoted as VB and CB, respectively. Inset in (b) illustrates the bonding interaction of Ti-HTi, where PDOS of H 1s orbital is indicated by short dashed line. Vertical dash line indicates the Fermi level. 61x46mm (600 x 600 DPI)


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Surface

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