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Theoretical Study of Pt Cocatalyst Loading on Anatase TiO (101) Surface: From Surface Doping to Interface Forming Zongyan Zhao

J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp508074e • Publication Date (Web): 30 Sep 2014 Downloaded from http://pubs.acs.org on October 6, 2014

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The Journal of Physical Chemistry

Theoretical Study of Pt Cocatalyst Loading on Anatase TiO2 (101) Surface: From Surface Doping to Interface Forming Zong-Yan Zhao* Faculty of Materials Science and Engineering, Key Laboratory of Advanced Materials of Yunnan Province, Kunming University of Science and Technology, Kunming 650093, People’s Republic of China * Corresponding author, E-mail: [email protected]. Tel: +86-871-65109952, Fax: +86-871-65107922.

ABSTRACT: For photocatalytic water splitting, the cocatalyst of noble metal particles loading is a very crucial step. It can not only speed up the reaction rate, or even change the reaction pathway. However, for the modification mechanism of Pt cocatalyst, the consistent conclusions and effective theoretical support are not yet completely established at present, especially the growth mechanism and microstructure of interface. For this purpose, the overall evolutionary process of Pt cocatalyst loading on anatase TiO2 (101) surface, and the related microstructure and properties were systematically investigated, using combination computational method of density functional theory calculations and Monte Carlo simulations. With the increase of loading amount, the growth of Pt cocatalyst sequentially experiences the following stages: surface doping, cluster nucleating, cluster loading, one-dimensional nanowire loading, two-dimensional nanowire grid loading, ultrathin film ripening, and film forming via layer-by-layer mode. Finally, the microstructure of interface constructed by Pt (111) surface and anatase TiO2 (101) surface is determined by the structural characteristics and lattice match. The interfacial properties, such as, built-in electric field, band bending, Schottky barrier, and so on, were further estimated and analyzed. Based on the calculated results, it can be drawn that Pt cocatalyst loading can not only suppress the recombination of photo-generated electron-hole pairs, but also promote to absorb the visible-light due to the surface Plasmon resonance effect. This detailed study may provide further insight into the mechanism of Pt cocatalyst and elucidate the reactions that occur on the surfaces of Pt/TiO2 composite photocatalyst.

KEYWORDS Photocatalysis; Cocatalyst; Interfacial structure; DFT calculations

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1. INTRODUCTION After years of research and development, semiconductor photocatalysis technology has aroused widespread concern in many fields, such as solar energy applications, environmental cleanup, medical health, and so on, and has been obtained many important achievements both in the fundamental research and practical applications. At present, the development of semiconductor photocatalysis technology encountered two main urgent challenges: many semiconductor photocatalysts can only be driven by UV-light, and their solar quantum conversion efficiency is very low.1 For the technology of hydrogen generation from photocatalytic water splitting, as one of the most promising and potential renewable energy technologies, the latter challenge is even more prominent and challenging. In order to promote the photocatalytic water splitting experiments carried out, researchers attempted to load cocatalysts onto photocatalyst surface.2-11 For example, Sato et al. observed the reactivity of hydrogen and oxygen production is enhanced using Pt/TiO2 composite photocatalyst;9 Sclafani et al. had loaded Ag particles on TiO2 surface to improve the hydrogen evolution from the decomposition of alcohol, due to Ag particles capture the photo-generated electrons.10 Using pure semiconductor as a photocatalyst, the reactivity is very limited, due to their low quantum conversion efficiency. Furthermore, selectivity of photocatalytic reaction cannot be controlled. If trace cocatalyst is loaded onto photocatalyst surfaces, the photo-generated electron-hole pairs could be spatially separated, because electrons and holes could be respectively localized onto the surfaces of photocatalyst and cocatalyst. Consequently, the recombination of photo-generated electron-hole pairs could be inhibited, as well as the oxidation reaction and reduction reaction occur at different surface positions, thus greatly improving the photocatalytic activity and selectivity. In the present photocatalytic water splitting systems, the cocatalyst (e.g.: noble metals, transition metals and their oxides, etc.) loading become an indispensable process for photocatalytic water splitting reaction. The most outstanding example is the hydrogen generation efficiency of NaTaO3:La photocatalyst could be enhanced by 44 times through NiO cocatalyst loading.12 Recently, noble metal cocatalyst loading has been reported to be a suitable cocatalyst, because it can improve the solar quantum conversion efficiency and enhance the light-absorption efficiency. Compared with transition metal cocatalysts, noble metal cocatalysts show more stable, and have more obvious modification effects, such as surface Plasmon resonance (SPR),13-23 strong metal-support interaction (SMSI),24-26 and so on. By certain synthesis methods (for example: atomic layer deposition, impregnation reduction, surface sputtering), the noble metal could deposit onto photocatalyst surfaces, forming nano clusters or ultrathin films. The common noble metals used as cocatalyst in photocatalysis include Ag, Pt, Pd, Au, Ru.27-29 Maeda et al. believed that the noble metal cocatalysts provide reaction sites and decrease the activation energy for gas evolution, thus loading nanoparticulate cocatalysts onto a photocatalyst significantly improves the water-splitting rate.30 Furthermore, Tada et al. proposed the concept of “reasonable delivery photocatalytic reaction systems (RDPRSs)” based on the developments of noble metal nanoparticle-loaded TiO2 for highly efficient photocatalytic reactions.31 Although the roles and effects of noble metal cocatalyst can be speculated, but there is no direct evidence to demonstrate how they affect the process of water adsorption and decomposition on the modified surface of photocatalysts. So, it is necessary to further, systematic study the mechanism of noble metal loading for the photocatalytic water splitting. Pt/TiO2 composite photocatalyst is the most common system in this field. And, most previous works have confirmed that photo-generated electrons could transfer to Pt nanoparticles. Compared with that of pure TiO2 photocatalyst, Pt/TiO2 composite photocatalyst shows stronger photo-reduction ability. At the 2


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same time, the photo-generated holes can freely diffuse at the TiO2 surfaces, and further photo-oxidize the absorbed organic compounds on the TiO2 surfaces.32 Facchin et al. loaded Pt particles on TiO2 surface by Sol-gel method, and observed a kind of oxidation of functional groups on the sample surfaces, which enhance the photocatalytic activity of TiO2.33 Previous works published in literature show that the effects of Pt cocatalyst loading depend on the existence of cocatalyst, Ptn cluster size, microstructure and interaction of Pt/TiO2 interface. In other words, tunable photocatalytic activity of Pt cocatalyst loading on TiO2 surface can be achieved by these factors. In addition to cluster loading, surface doping and interface forming are also the existences of Pt cocatalyst on TiO2 surfaces. The incorporation of Pt atoms onto TiO2 substrate can be interstitial or substitutional doping as well as being adsorption. Indeed, experiments showed that Pt atoms can thermally diffuse into TiO2 lattice under oxidizing atmosphere, and be oxidized to Pt2+ to substitute for Ti4+ or form the interstitial ions.34 Some reports have suggested that Pt clusters act as an electron-hole separation center and therefore inhibit electron-hole recombination at low Pt loadings, but that at high loading of Pt acts as electron-hole recombination center.28, 35, 36 These works suggest that the existence of an optimal loading amount of Pt for the performance of TiO2.37-39 Moreover, metal-semiconductor interface between Pt and TiO2 will be formed at higher Pt loading. Lee et al. deposited ultrathin Pt films by atomic layer deposition, and found that Pt cluster nucleating is significantly affected by the microstructure of TiO2.40 In this case, the charge transfer, microstructure, and interfacial states between Pt and TiO2 are also very important in photocatalytic reactions. Therefore, the growth process of Pt from surface doping, to cluster loading, and to interface forming, determine the photocatalytic performance of Pt/TiO2 composite, as well as the shape and size of Pt cluster. Unfortunately, these suggestions have not been verified or explained systematically, leaving the fields without proven, fundamental explanation for the mechanism by which Pt affect the photocatalytic activity of TiO2. Compared with experimental study, the electronic-, atomic-, and molecular-level theoretical and simulation can overcome the influence of anthropic factor, and is favorable to direct analysis the in-depth mechanism of noble metal loading. So, theory and computation have played an important role in understanding and predicting structure and chemical reactivity on oxide surfaces and nanoparticles by providing detailed mechanistic insights, interpretations of experimental phenomena, and even prediction of improved photocatalysts. In recent years, some research groups that engage in theoretical calculations began to concern on the modification effects of noble metal loading on photocatalyst surfaces. For example, Mete et al. had systematically investigation Pt and Au particles loading on anatase TiO2 (001) surface by first-principles calculations.41, 42 They found that noble metal particles loading can upper-shift the valence band of TiO2, resulting response the visible-light. And, if the Au atom replaces the Ti atom on the surface, the Fermi level of Au/TiO2 composite will be decreased to closer the valence band. Other researchers also found that Au particles loading on rutile TiO2 (110) surface causes some electrons to transfer toward the surface, and the band edge will be downward-bend of about 0.6 eV.43 Using ab-initio molecular dynamics simulations, San-Miguel et al. had to study the temperature effect on the structure of Pd clusters loading on rutile TiO2 (110) surface.44 They found that the microstructure of Pd clusters is mainly determined by the atomic structure of substrate at low temperature, and the morphology and electronic structure of Pd cluster will be obviously changed up to 800 K. These results obtained from theoretical calculation and simulation can help us to better understand the role and micro-mechanism of noble metal loading on photocatalyst. Although the research about Pt cocatalyst onto TiO2 surfaces have been carried out for many years, and has made considerable progress, but the consistent conclusions and effective theoretical support about the modification mechanism are not yet established at present, especially the growth mechanism and

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microstructure of the interface. In order to further investigate these issues, the interaction of Pt with anatase TiO2 (101) surface was systematically investigated by using a combination method of density functional theory (DFT) calculations and Monte Carlo simulations, to understand and predict the overall evolutionary process and regular pattern of Pt interact with TiO2 support as increasing of Pt loading amount. The growth behaviors of Pt dopants, clusters, or ultrathin films help to get more insight into the early state cluster nucleating of the Pt nanoparticles dispersed on the TiO2 surface. These findings are crucial for further research on these noble metal-TiO2 composite photocatalyst for practical applications.

2. COMPUTATIONAL MODELS AND METHODS The TiO2 (101) surface was simulated by a (2×3) periodic slab of 12 O-Ti-O trilayers (72 TiO2 units, 216 atoms). And, the slab model was separated by a 20-Å-thick vacuum layer. The lengths of the model along [101] , [010] and [101] directions were 11.0092, 11.3235, and 65 Å, respectively. Thus, all the lengths of the model were larger than 10 Å, which were enough to avoid the self interaction effects of the periodic boundary conditions. The bottom half of the slab was fixed to mimic the bulk effects. At certain adsorption coverage, the simulated annealing method was firstly adopted to find the possible stable adsorption configurations in the configurational space. Then, some possible adsorption configurations, which have the lowest system energy, have been optimized by DFT method. Based on this combined computational method, one could quickly find the stable adsorption configurations for Pt atoms on TiO2 (101) surface. In the present study, all of DFT calculations have been carried out by using Cambridge Serial Total Energy Package (CASTEP) codes, employing the ultrasoft pseudopotential.45 Exchange and correlation effects were described by the revised Perdew-Burke-Ernzerhof for solid (PBEsol) of generalized gradient approximation (GGA).46 An energy cutoff of 340 eV has been used for expanding the Kohn-Sham wave functions. The minimization algorithm has been chosen Broyden-Fletcher-Goldfarb-Shanno (BFGS) scheme.47 The K-points grid sampling of Monkhorst-Pack scheme was set as 1×1×1 in the irreducible Brillouin zone, and the fast Fourier transform grid was set as 60×60×360. To get accurate results, we optimized atomic coordinates, which obtained by minimizing the total energy and atomic forces. This was done by performing an iterative process in which the coordinates of the atoms are adjusted so that the total energy of the structure is minimized. The relaxation run was considered converged when the force on the atomic nuclei was less than 0.03 eV/Å, the stress on the atomic nuclei was less than 0.05 GPa, the displacement of the nuclei was less than 1×10-3 Å, and the energy change per atom was less than 1×10-5 eV. In order to improve accuracy of calculated adsorption energies for Pt atoms on TiO2 (101) surface, the dipole corrections were utilized for all models, which can be essential in eliminating nonphysical electrostatic interaction between periodic images.48 Using above periodic slab model and self-consistent dipole correction, the averaging electrostatic potential in the planes perpendicular to the slab normal could be obtained. Thus, the change in electrostatic potential through the slab could be plotted. Furthermore, the plot of electrostatic potential also contains the value of work function calculated as a difference between the potential level in a vacuum and the Fermi energy. Using the above calculation method, the bulk crystal structure of anatase TiO2 was firstly optimized, obtaining the following lattice constants: a = b = 3.7747 Å, c = 9.6289 Å, dap = 1.9898 Å, deq = 1.9329 Å, 2θ = 155.058°. This calculation results is well consistent with the experimental measurements:49 a = b = 3.7848 Å, c = 9.5124 Å, dap = 1.9799 Å, deq = 1.9338 Å, 2θ = 156.230°. Then, the structure of perfect TiO2 4


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(101) surface using above calculation method was optimized, obtaining the surface configuration that is well consistent with previous reports.50, 51 On the other hand, the optimized lattice constants of fcc Pt metal is 4.0043 Å. These calculated results indicate that the calculation models and method in the present work are reasonable. The simulated annealing method identified possible adsorption configurations by carrying out Monte Carlo searches of the configurational space of Ptn/TiO2 (101) system as the temperature was slowly decreased. And this process was repeated to identify further local energy minima. During the course of simulation, Ptn atoms were randomly rotated and translated around TiO2 (101) surface. The configuration that results from one of these steps was accepted or rejected according to the selection rules of the Metropolis Monte Carlo method. In the computational parameters of Monte Carlo simulations were set as following: the COMPASS force field was utilized to deal the atomic interaction;52-54 the charges of atoms were assigned by force field; the summation method was chosen as Ewald method; the cutoff distance was chosen as 18.5 Å; the number of cycles was chosen as 10, with 105 steps per cycle; and the simulated annealing temperature cycle was determined automatically by the calculation package (maximum temperature is 105 K, final temperature is 100 K).

3. 3. RESULTS AND DISCUSSION 3.1 Overall evolutionary process When Pt cocatalyst deposited onto anatase TiO2 (101) surface, for either nanoparticles or nano ultrathin films, the most critical is to clearly understand the varying regularity and trend (namely growth mechanism or overall evolutionary process) of cocatalyst’s structure along with Pt loading amount. To achieve this purpose, a single Pt atom adsorption was considered as the starting point, and then successively increasing Pt loading amount (namely Pt coverage). At certain coverage, the most stable loading configuration was determined by the above computational method. In general, the interaction between Pt cocatalyst and TiO2 substrate is described by the concept of adsorption energy (Qi), However, in the process of Pt atoms deposit onto TiO2 surface, there is a competition relationship among the following interactions: the interaction between Pt cocatalyst and TiO2 substrate that is described by the interface formation energy (γinterface), Pt-Pt atomic interaction that is described by the cohesive energy (Ecoh) of Pt, and the surface deformation of anatase TiO2 (101) surface caused by Pt loading that is described by the surface deformation energy (∆γsurface). The definition of these concepts and the calculation formula of their corresponding energies could be found in our previously published article (Ref.55). Thus, the interaction between Pt cocatalyst and TiO2 substrate is very complicated and distinctive characteristics along with the Pt loading coverage as shown in Figure 1. The adsorption energy is increasing from 370.97 kJ/mol (= 3.85 eV at Θ = 1/16 ML) to 615.78 kJ/mol (= 6.38 eV at Θ = 5 ML) with the increase of Pt coverage, and finally its convergence is less than 4.16 kJ/mol (= 0.04 eV) when Θ > 4 ML. This result firstly suggests that the interaction of Pt cocatalyst with anatase TiO2 (101) surface is very strong, for example, the adsorption energy is as high as 370.97 kJ/mol at Θ = 1/16 ML, in which the Pt-Pt atomic interaction could be ignored. Moreover, in the supersaturated condition, the average release energy of Pt atoms condensed from the gas phase onto anatase TiO2 (101) surface will be constant. The overall evolutionary trend of cohesive energy of Pt atoms is similar with the trend of adsorption energy, and finally its convergence is less than 4.68 kJ/mol (= 0.05 eV) when Θ > 4 ML. 5



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At Θ = 5 ML, the cohesive energy of Pt atoms is 611.03 kJ/mol (= 6.33 eV), which is very close to the cohesive energy of bulk Pt with fcc structure (649.51 kJ/mol = 6.73 eV). And the cohesive energy and adsorption energy are in the same order of magnitude when Θ > 9/16 ML, indicating that the Pt-Pt atomic interaction is dominant among above mentioned interactions. The interface formation energy is collectively increased with the increase of Pt loading amount when Θ < 13/8 ML, and then is gradually decrease with the increase of Pt loading amount. Finally, the interface formation energy is converged at about 0.72 J/m2, with a convergence of 0.02 J/m2 when Θ > 4 ML. In the whole process, the range of surface deformation energy is very small, between 0.18 to 0.51 J/m2. It is converged at about 0.18 J/m2, with a convergence of 0.01 J/m2 when Θ > 4 ML. According to the relationship of “nQi = nEcoh + γinterface + ∆γsurface, where n is the number of loaded Pt atoms, and all energies are in units of eV” that is satisfied by these energy forms, one could found that the released energy of Pt/TiO2 composition system formation is mainly contributed by the cohesive energy of Pt atoms, and the deformation of anatase TiO2 (101) surface caused by Pt cocatalyst loading is relatively less prominent. To further comprehend Pt-TiO2 interaction and the entire growth process of Pt cocatalyst on anatase TiO2 (101) surface, the energetic trends as function of Pt coverage (or Pt loading amount) is divided into 8 stages as shown in Figure 1, combined with the variation of Ptn geometry configurations on anatase TiO2 (101) surface. Typical geometry configurations at every stage are shown in Figure 2~Figure 4, whose structural and electronic properties will be respectively presented and discussed in the following subsections. (1) In the first stage (at the lower coverage, Θ = 1/16 ML), the single Pt atom is adsorbed into the hollow site, forming a symmetrical configuration with neighboring surface atoms. The bond length of Pt atom with 2-coordinated bridge O atom (O2c), 5-coordingated Ti atom (Ti5c), and 6-coordingated Ti atom (Ti6c) is 2.041 Å, 2.748 Å, and 2.754 Å, respectively. For single Pt atom adsorbing, Qi (-3.85 eV) = Ecoh (0 eV) + γinterface (-5.21 eV) + ∆γsurface (1.36 eV), indicating that the strong interaction between Pt atom and TiO2 (101) surface. Because the surface deformation caused by a single Pt atom adsorbing is a local effect as shown in Figure 2(a), so the value of ∆γsurface is relatively small. Furthermore, the height of Pt atom is nearly equal to that of O2c atoms. Thus, single Pt atom adsorbing could be considered as surface doping for anatase TiO2 (101) surface. (2) In the second stage (Θ = 1/8 ML), the second Pt atom is bonded with the first Pt atom along with the [10 1] direction, with a bond length of 2.65 Å. The distance between the first Pt atom (PtL) and Ti6c atom is increased to 2.954 Å, so that the Pt-Ti6c bonds are broken. And the PtL-O2c bonds are slightly increased to 2.049 Å, while the PtL-O2c bonds are slightly decreased to 2.661 Å. The bond length of the second Pt atom (PtH) with Ti5c and Ti6c atoms are 2.692 Å and 2.627 Å, respectively. The height of PtL is not obviously changed, and the position of PtH atom is higher about 0.507 Å than that of PtL atom as shown in Figure 2(b). At this coverage, the stable configuration of Pt atoms is to form the adsorbed dimmer, which is agreement with previously published works.56 The cohesive energy at this coverage is 179.31 kJ/mol (= 1.86 eV). Meanwhile, compared with the situation of single Pt atom adsorbing, the values of Qi, γinterface, and ∆γsurface are obviously increased, owing to the Pt-Pt atomic interaction. This result indicates that the formation Pt-Pt bond provides extra stability and makes the additional Pt atom to form a dimmer on the surface. As this reason, this stage was seen as the cluster nucleating stage for the subsequent stages. (3) In the third stage (1/8 < Θ ≤ 7/8 ML), Ptn cluster configurations is formed on anatase TiO2 (101) surface due to the stronger Pt-Pt atomic interaction, if continuously increase Pt atoms based on adsorbed Pt2 dimer. At Θ = 3/16 ML, the nEcoh is reached to 8.91 eV, which is larger than the interaction of Pt atoms with

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TiO2 surface (γinterface = 6.50 eV), so the stable absorbed configuration is adsorbed Pt3 trimer (equilateral triangle configuration inclined on the surface). And then, the polyhedral clusters, such as, deformed regular tetrahedron for Pt4 cluster (Θ = 1/4 ML), irregular four side cone for Pt5 cluster (Θ = 5/16 ML), etc., could be found on the anatase TiO2 (101) surface as shown in Figure 2(c), until the coverage reach to 7/8 ML. Thus, this stage was considered as the Ptn cluster loading stage in the present work. For these Ptn clusters, the most height away from the surface is about 5.019 Å at Θ = 13/16 ML, which is almost reached to the thickness of three Pt atomic layers. In this cluster growth process, the cohesive energy of Ptn cluster is continuously increasing. The adsorption position and formation of Pt atoms with the topmost O2c, O3c, Ti5c, and Ti6c atoms are uncertain in this process, so that the deformation energy of surface does not show uniform rules. At Θ = 11/16 and 7/8 ML, the atomic displacement of topmost atoms on the surface are most obvious. While the interaction of Ptn atoms with surface show the feature of oscillation change, and show the minimum value (0.642 J/m2) at Θ = 3/8 ML. These reasons lead to the adsorption energy (Qi) show some slight local mutation while maintaining the overall increase in this process. (4) In the fourth stage (7/8 < Θ ≤ 19/16 ML), if continuously increase the loading amount based on above Ptn clusters, the interaction of Ptn clusters along the surface normal direction is gradually weakened, while the interaction of Ptn clusters along the direction parallel to the surface gradually increase. Therefore, the isolated Ptn clusters (zero-dimensional island-like configurations) will be contacted with each other, and finally connected together. In the present work, we found that the connection of isolated Ptn clusters firstly occurs along the [1 1 1] direction on anatase TiO2 (101) surface, resulting in forming one-dimensional chain-like configurations as shown in Figure 3(a). Thus, this stage was seen as the Pt nanowire loading stage in the present work. In this process, the height of Pt nanowire away from the surface is maintaining about the thickness of two Pt atomic layers, and the values of Qi and Ecoh are also keeping the tendency of increasing. While the values of γinterface and ∆γsurface show the tendency of first decreasing and then increasing. (5) In the fifth stage (19/16 < Θ ≤ 13/8 ML), according to the same reason mentioned above, the isolated Pt nanowire will be contacted with each other through forming another one-dimensional chain-like configurations along the [010] direction. Thereby, Pt cocatalyst is formed two-dimensional grid-like configurations that are constructed by the intersecting Pt nanowires as shown in Figure 3(b). Thus, this stage was considered as the Pt nanowire grid loading stage in the present work. As the same situation as in the fourth stage, the height of Pt nanowire away from the surface is maintaining about the thickness of two Pt atomic layers, and the values of Qi, Ecoh, and ∆γsurface are also keeping the tendency of increasing. While the value of γinterface still show the tendency of first decreasing and then increasing. At Θ = 13/8 ML, the value of γinterface is reached to the maximum of 1.52 J/m2. (6) The sixth stage (13/8 < Θ ≤ 2 ML) is a transition stage. The added Pt atoms are randomly filled the hollow of two-dimensional grid-like configurations along the edge of nanowire, forming two-dimensional plane configurations that is completely covered TiO2 surface. In this process, the values of Qi, Ecoh, and ∆γsurface are also keeping the trend of increasing, while the value of γinterface shows the tendency of decreasing. In particularly, the two-dimensional plane configuration keeps the thickness of two Pt atomic layers. Finally, Pt cocatalys forms the ultrathin film containing two atomic layers. In the present work, one Pt atomic layer contains 16 atoms, so we defined the unit of coverage of ML as one monolayer equal 16 Pt atoms on anatase TiO2 (101)-(2×3) surface (1 ML ≈ 1.366×1019 atoms/m2) in the present work. Therefore, this stage could be seen as the formation stage of ultrathin film. (7) In the seventh stage (2 < Θ ≤ 3 ML), the values of Qi, and Ecoh are keeping the tendency of

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increasing, while the value of γinterface and ∆γsurface show the tendency of decreasing. From two Pt atomic layers adsorption to three Pt atomic layers adsorption, the structure of ultrathin films is gradually stabilized and fixed, and finally processes the structure parameters (such as bond length, bond angle, distance between atomic layers, and so on) that is very close to those of bulk fcc Pt metal as show in Figure 4(a)~(d). According to the basic theory of thin film growth, this stage could be considered as ripening stage. (8) In the eighth stage (Θ > 3 ML), the varying difference of Qi, Ecoh, γinterface, and ∆γsurface is very small, and all these energies finally respectively converges value. As show in Figure 4(d)~(h), the growth of Pt ultrathin films obviously show the feature of layer-by-layer growth mechanism of thin films. Furthermore, a clear interface between Pt atoms and TiO2 surface is formed in this stage. Therefore, up to this stage, it is can be safely considered that the Pt/TiO2 interface formation or Pt film loading on TiO2 surface is accomplished. Review the whole evolutionary process, one could find that Pt/TiO2 interface formation or Pt film loading on TiO2 surface has experienced the following steps: surface adsorption (in the first stage), surface cluster nucleating (in the second stage), nanocluster loading (in the third stage), nanowire loading (in the fourth and fifth stage), ultrathin film forming (in the sixth stage), film ripening (in the seven stage), and layer-by-layer growth (in the eighth stage). Thus, the growth of Pt film on anatase TiO2 (101) surface is obey the law of “island-layer” mixed growth mechanism.

3.2 Electronic structures The electronic structure (local and partial density of states) of Pt/TiO2 composite photocatalyst at the typical lower Pt loading amounts is illustrated in Figure 5. For surface doping (Θ = 1/16 ML), the impurity energy bands that are mainly consisted by the hybridization between Pt-5d states and O-2p states are located at the top of valence band (VB). The Fermi energy level (EF) is located at the top of impurity energy band. Importantly, Pt surface doping could make TiO2 to realize visible-light absorption, owing to the matched energy bands. In other words, these Pt-5d states could act as an intermediate band, in which the lower energy photons could firstly excite electrons from valence band to it and subsequently to conduction band, resulting in visible-light absorption. In the case of cluster nucleating stage (Θ = 1/8 ML), the impurity energy band is also located at the top of VB, which is mainly consisted by PtH-5d states, and the hybridization of PtL-5d states with O2c-2p states. Similarly, the EF positions are relatively upward shifting. In the case of initial of cluster loading (Θ = 7/16 ML), the isolated energy band of Pt-5d states is reached to the bottom of conduction band (CB), and partially mixed with the main peak. At the same time, the main peak is much closed to EF. In the case of typical cluster loading (Θ = 9/16 ML), another isolated energy band appears at the bottom of VB, which is mainly consisted by the hybridization between Pt-5s states and O-2p states and different from the situation of surface doping. In the case of later cluster loading (Θ = 3/4 ML), the isolated energy band of Pt-5d states is almost completely mixed with the main peak that is much closed to the EF, and the energy band of Pt-5s is completely isolated from the bottom of VB. In the case of nanowire loading (Θ = 17/16, and 11/8 ML), the hybridization of Pt-5d states with O-2p states is more obvious, owing to the enhanced interaction between Pt atoms and surface atoms that are contacted with each other. Meanwhile, the delocalization feature of Pt-5d states that distribute in whole regime of VB and CB. For one-dimensional Pt nanowire loading, there is a distinct stepped surface between Pt nanowire and TiO2 surface. And, for two-dimensional Pt nanowire grid loading, there is a distinct kink surface between Pt nanowires and TiO2 surface, which have relatively high energy. Because the dangling bonds on the Pt nanowires also process catalytic activity, so two-dimensional Pt nanowire grid loading provide important active sites for surface photocatalytic reactions, as well as promote the photo-generated 8


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electron-hole pairs spatially separated. Biswas et al. proposed that the Pt particle size plays an important role in the photocatalystic activity of TiO2.57 The underlying mechanism of this view is that the position of the energy bands associated with Pt could be adjusted by the loading amount of cocatalyst. In the present work, it is could be found that the relative positions of EF and the impurity energy band are different for different doping models, in comparison with the electronic structure of pure TiO2. Electronic level alignment at the interface between adsorbed Pt cocatalyst and TiO2 substrate determines the photocatalytic activity and efficiency, which is can be determined by ultraviolet photoelectron spectroscopy, two-photon photoemission spectroscopy, or quasiparticle (QP G0W0) DFT techniques.58 However, the position of EF is fixed at 0 eV in standard DFT calculations. And, the quasiparticle technique is too expensive computing resources and time, making it cannot be used to large model. In order to solve this issue, we took the energy band of O-2s states that is located at -19~-17 eV as the baseline to determine the positions of other energy bands, because it is belong to inner electronic states and its position is relatively fixed in different models. Thus, it could be used to align the band positions for different Pt/TiO2 composite systems, using the procedure proposed in Ref.59, 60, as illustrated in Figure 6. In addition, another concept of d-band center was also utilized to further analyze this situation, which is important parameter to character the catalytic activity of metal.61 The position of the d-band relative to the Fermi level determine the reactivity of noble metal catalyst, because this determines both the size of the bonding and anti-bond energy shifts and the degree of filling of the anti-bonding states. The offset of EF compared with bulk TiO2 and d-band center of Pt in different models as function of Pt loading amount are shown in Figure 6. For Θ ≤ 3/16 ML, the upward shift of EF is relatively small, < 1.1 eV. In the late of third stage, the offset of EF exhibits a characteristic of oscillation. After the fourth state and the fifth stage, the offset of EF tends to be stable at ~1.9 eV. The variation tendency of d-band center is very similar: it is very small in the first two stages; it exhibits a characteristic of oscillation in the third stage; it is linear declining is the seventh stage; and then it tens to be stable at ~-0.45 eV in the eighth stage. Along with the increase of Pt loading amount, the EF is upward shifting and the d-band center of Pt is downward shifting. This result implies that the carrier transfer process between TiO2 substrate to Pt cocatalyst and the catalytic activity of Pt cocatalyst are very closely related to the Pt loading amount. Thus, the performance of Pt cocatalyst could be adjusted by the Pt loading amount, as following discussions. Take into account of band shifting, the re-aligned density of states of Pt-5d states is shown in Figure 7, in which the reference DOS of bulk TiO2 layer is took from the middle fixed trilayer. For Θ ≤ 3/16 ML, the DOS of Pt-5d states is relatively localized and isolated. Because its main peak is closed to the top of valence, resulting in the small offset of EF. These localized and isolated energy bands of Pt are very favorable to capture photo-generated holes. Moreover, their d-band centers are also much closed to EF, which is favorable for the catalytic activity of Pt cocatalyst. Therefore, low Pt amount loading is very favorable, whether for the photocatalytic activity of substrate, or for the catalytic activity of cocatalyst. For Ptn cluster loading (3/16 < Θ ≤ 7/8 ML), the offset of EF is around ~1.8 eV. At Θ = 9/16 ML, the offset of EF is again reached to the minimum, ~1.65 eV. When the coverage from 1/16 ML to 7/8 ML, the isolated energy band of Pt-5d states are gradually downward shifting from the upper valence band to the band gap, and finally completely mixed together with its main band. On the other hand, the d-band center is reached the local maximum at Θ = 7/16 ML and 3/4 ML.

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3.3 Interfacial structure After the fifth stage, the added Pt atoms are no longer attracted to the nanowires, but gradually filled the hollow between the nanowire grids. Then, the ultrathin film of adsorbed Pt bilayer is formed at Θ = 2 ML. After ripening stage (the seven stage), the structure features of this ultrathin film and the interface of Pt/TiO2 have been very clear and stable when Pt coverage is reached to 3 ML. In following, the case of Θ = 5 ML will be considered as example to describe the composition and structure of Pt/TiO2 interface. Starting from the interface layer, the atomic layer spacing of Pt is 2.201, 2.174, 2.169, and 2.194 Å. The bond lengths between Pt atoms are in the range of 2.74-2.85 Å, and the bond angles between Pt atoms is in the range of 100-115°. For the Pt (111) unrelaxed surface, the atomic layer spacing of is 2.312 Å, the bond length between Pt atoms is 2.831 Å, and the bond angles between Pt atoms is 120°. Based on these structural information, we can preliminary determine the crystal orientation of Pt ultrathin film is the [111] direction on anatase TiO2 (101) surface. Moreover, the important parameter is the lattice mismatch for hetero-junction or hetero-structure materials. The two lattice constants of these two surfaces are respectively listed as following: u = 5.5046 Å, v = 3.7747 Å, γ = 110.0508° for anatase TiO2 (101) surface; and u = v = 2.8315 Å, γ = 120° for fcc Pt (111) surface. Between anatase TiO2 (101)-(2×3) surface and fcc Pt (111)-(4×4) surface, the lattice mismatch is listed as following: ∆u = 2.878%, ∆v = 0.017%, and ∆γ = 9.041%. Owing to so small lattice mismatch, the hetero-junction that is consisted by these two surfaces has small interfacial stress, so it could be stably formed. Accordingly, it could be determined that the structure of final Pt cocatalyst on anatase TiO2 (101) surface is its (101) surface on high coverage. In other words, the final interface of composite material is the Pt-(111)/TiO2-(101) interface. This calculated result is well agreement with previously published experimental observations.62 In order to fully and accurately analyze the structure and properties of Pt-(111)/TiO2-(101) interface, an interfacial model containing 12 Pt atomic layers (Θ = 12 ML, 192 Pt atoms) was further constructed, as shown in Figure 9(a). The primary task is to determine the atomic positioning near the interface. In Figure 8, the top two TiO2 trilayer and the first Pt layer at the interface are illustrated. The most important feature is that a 2D primitive cell of TiO2 (101) surface is matched with two deformed 2D primitive cells of Pt (111) surface, which is illustrated by the dashed parallelogram in Figure 8. So, the four Pt atoms at vertices just adsorbed at the on-top sites between O2c atom and Ti5c atom. The corresponding bond length is 2.119 Å of Pt-O2c and 2.952 Å of Pt-Ti5c. And then, the rest Pt atoms are periodically arranged according to the deformed 2D lattice (u ≈ 2.758 Å, v ≈ 2.832 Å, γ ≈ 110°), implying the average Pt-Pt bond length is 2.758 Å along the [1 1 1] direction and 2.832 Å along the [010] direction. Based on the first Pt layer, the second Pt layer is arranged in accordance with the atomic arrangement of Pt (111) surface. In the same manner, the Pt-(111)/TiO2-(101) interface is eventually constructed. In the middle layers (for example: the fifth or sixed layer), the average Pt-Pt bond length is 2.775 Å along the [1 1 1] direction and 2.831 Å along the [010] direction. The shorter bond length implies the existence of interfacial stress induced by the compressed crystal lattice along the [1 1 1] direction. The latter is equal to the bond length of bulk fcc Pt, owing to the very small lattice mismatch along the [010] direction. The atomic layer spacing between TiO2 layer and Pt layer is 2.014 Å, which is shorter than that of Pt layers, indicating the stronger interaction between TiO2 layer and Pt layer. The atomic layer spacing between TiO2 layers or Pt layers were listed in Figure 9(a). Apart from some obvious difference near the interface, the layer spacing variation has similar feature in 10


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comparison with clean relaxed TiO2 (101) surface or Pt (111) surface. The electrostatic potential along the interfacial normal direction was illustrated in Figure 9(b), in compared with that of bulk TiO2 along the [101] direction or bulk Pt along the [111] direction. The obvious feature is the potential of TiO2 layer (with average potential of -22.294 eV) is lower than that of Pt layer (with average potential of -5.388 eV). Therefore, the built-in electric field from TiO2 layer to Pt layer under equilibrium, after the interface is formed and stable. Compared with bulk counterpart, the average potential of TiO2 layer is decreased about 1.479 eV, while that of Pt layer is increased about 0.701 eV. For the clean unrelaxed TiO2 (101) surface, the work function ( φTiO2 ) is 4.115 eV, which is lower than that of the clean unrelaxed Pt (111) surface ( φPt = 5.807 eV). So, when they contact together, the electrons will be transferred from TiO2 layer to Pt layer, resulting the existence of space charge region (or depletion layer). It is could be confirmed the average electron density difference along the interfacial normal direction as shown in Figure 9(c). The result of electron transfer is the Fermi energy ( EF ,TiO2 ) and the vacuum energy level ( EVac ,TiO2 ) of TiO2 is downwards shifting, until the EF ,TiO2 and the EF , Pt are aligned. Finally, the

EF , Pt /TiO2 is located at below the bottom of CB of TiO2 about 0.5 eV. From Figure 9(c), the thickness of space charge region could be estimated by about 3 Å. These features can be confirmed by the layer-localized DOS as shown in Figure 10. Compared with bulk TiO2, the energy bands is obviously downwards shifting, while this situation is not obvious for Pt. In addition, there are not obvious interfacial states in the band gap of TiO2. For the first two TiO2 or Pt layers, the features of DOS are obviously different from that of others, especially different from bulk counterpart. These interfacial states are ascribed from the stronger interaction of Pt atoms with its bonded surface atoms on TiO2 surface. Away from the interface, in the case of the 5th-7th TiO2 layers and the 4th-5th Pt layers, the features of DOS are very similar with their counterparts. In other words, the interfacial states are only localized near the interfacial region (from about -10 Å to about 9 Å). Based on above calculated data and discussion, we proposed the energy band diagram of Pt-(111)/TiO2-(101) interface as illustrated in Figure 11. Because of the existence of built-in electric field, the energy band edges of TiO2 are shifted upwards, which is called as band bending. The degree of energy band bending (VBB) is defined as the difference of work function between Pt (111) surface and TiO2 (101) surface,

VBB = φPt − φTiO2 = 1.692 eV . And, the Schottky barrier ( φSB ) is estimated by

φSB = φPt − χTiO = 1.892 eV , in which χTiO is the electron affinity of TiO2.63 The presence of the space 2

2

charge region can prevent more electrons flow from TiO2 layer to Pt layer, and the presence of the Schottky barrier make the flow of electron to need more large energy. Thus, the photo-generated electron-hole pairs can be spatially separated by the Pt/TiO2 interface. Moreover, in the Pt layer, the valence electrons can be collectively excited by lower energy photon, which is also called a surface Plasmon resonance. These electrons only need to overcome the Schottky barrier, transferring to the conduction band of TiO2. On the other hand, the photo-generated holes in the valence band of TiO2 also can transfer to the valence band of Pt, driven by the built-in electric field. Eventually, the oxidation reaction may occur on the Pt surface with the separated holes, while the reduction reaction may occur on the TiO2 surface with the separated electrons.

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So, Pt cocatalyst loading can not only suppress the recombination of photo-generated electron-hole pairs, but also promote to absorb the visible-light due to the surface Plasmon resonance effect.

4. SUMMARY AND CONCLUSIONS When Pt cocatalyst loading onto anatase TiO2 (101) surface, its existing form is closely related to the coverage or loading amount. At the very lower coverage, Pt loading effects show more the characteristics of surface doping, in which the metal induced gap states (MIGS) is very localized and obvious. Because the MIGS overlapping with the top of VB in the case of Pt surface doping, this effect is very favorable for the photocatalysis. When more Pt atoms loading, they will tend to gather together, for example at Θ = 1/8 ML. So, this stage could be called cluster nucleating stage. If 1/8 < Θ ≤ 7/8 ML, Pt atoms are formed isolated cluster island on surface. With increasing cluster size, the interaction will gradually arise between the clusters, resulting in MIGS become delocalized states and distribute whole the range of VB and band gap. This interaction is further strengthened with the increase of Pt loading amount, the clusters will be interconnected each other along the [1 1 1] direction to form 1D nanowire. And then, these nanowirs will further be interconnected with each other along the [010] direction to form 2D nanowire grid. Base on this stage, the added Pt atoms will be filled the hollow in the nanowire grid to form ultrathin film. When Θ > 2 ML, stable Pt ultrathin film is growing through the layer-by-layer mode. In the whole evolutionary process, the Pt-Pt atomic interaction in general dominates the tendency of energy variation. However, in different stages, the energy of adsorption, Pt cohesive, Pt-TiO2 interaction, and surface deformation experienced different variations and had different competition relationship. At the same time, in this process, the Fermi energy level is upwards shifting and eventually converges to ~1.94 eV, while the d-band center of Pt-5d states are downwards shifting and converge to ~-0.45 eV below the top of VB. Based on the structural characteristics and lattice mismatch, it is could be determined that Pt cocatalyst loading onto anatase TiO2 (101) surface is formed Pt-(111)/TiO2-(101) interface. Owing to electron transfer, the built-in electric field is established at the interface from TiO2 layer to Pt layer. At the interface, the energy bands of TiO2 bend upward towards ~1.69 eV. The contact type of Pt cocatalyst on anatase TiO2 (101) surface belongs to Schottky contact, with a barrier of ~1.89 eV on the conduction band of TiO2. Therefore, Pt cocatalyst loading can not only suppress the recombination of photo-generated electron-hole pairs, but also promote to absorb the visible-light due to the surface Plasmon resonance effect.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. . Tel: +86-871-65109952, Fax: +86-871-65107922. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors would like to acknowledge financial support from the National Natural Science

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Foundation of China (Grant No.21263006).

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Nanoparticles on TiO2 Single Crystals. J. Am. Chem. Soc. 2012, 134, 11276-11281. (58) Migani, A.; Mowbray, D. J.; Zhao, J.; Petek, H.; Rubio, A., Quasiparticle Level Alignment for Photocatalytic Interfaces. J. Chem. Theory Comput. 2014, 10, 2103-2113. (59) Wei, S.-H.; Zunger, A., Band Offsets and Optical Bowings of Chalcopyrites and Zn-Based II-VI Alloys. J. Appl. Phys. 1995, 78, 3846-3856. (60) Chen, S.; Walsh, A.; Yang, J.-H.; Gong, X. G.; Sun, L.; Yang, P.-X.; Chu, J.-H.; Wei, S.-H., Compositional Dependence of Structural and Electronic Properties of Cu2ZnSn(S,Se)4 Alloys for Thin Film Solar Cells. Phys. Rev. B 2011, 83, 125201. (61) Hammer, B.; Nørskov, J. K., Electronic Factors Determining the Reactivity of Metal Surfaces. Surf. Sci. 1995, 343, 211-220. (62) Zou, J.-J.; Chen, C.; Liu, C.-J.; Zhang, Y.-P.; Han, Y.; Cui, L., Pt Nanoparticles on TiO2 with Novel Metal-Semiconductor Interface as Highly Efficient Photocatalyst. Mater. Lett. 2005, 59, 3437-3440. (63) Zhang, Z.; Yates, J. T., Band Bending in Semiconductors: Chemical and Physical Consequences at Surfaces and Interfaces. Chem. Rev. 2012, 112, 5520-5551.

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Caption of Figures: Figure 1. The adsorption energy (Qi), cohesive energy (Ecoh) of Pt on anatase TiO2 (101) surface, the formation energy (γinterface) of Pt/TiO2 interface, and the deformation energy (∆γsurface) of anatase TiO2 (101) surface induced by Pt adsorption, as function of Pt coverage. Figure 2. Top view (upper plane) and side view (lower plane) of models: (a) Single Pt atom adsorption; (b) Double Pt atoms adsorption; (c) Typical Ptn cluster loading. The red balls represent oxygen atoms; the grey balls represent titanium atoms; the navy balls represent platinum atoms. The top view only shows the top two O-Ti-O trilayers. Figure 3. Top view of models: (a) typical 1D Pt narowire loading; (b) typical 2D Pt nanowire grid loading. Figure 4. Side view of models of Pt ultrathin film loading on anatase TiO2 (101) surface, and the formation of Pt/TiO2 interface, at typical Pt coverage. Figure 5. The local and partial density of states of Pt/TiO2 composite system at different lower typical coverages. Figure 6. The shifting of Fermi energy level of anatase TiO2 (101) surface induced by Pt loading and the d-band center of Pt atoms as function of Pt coverage. Figure 7. The partial density of states of Pt on anatase TiO2 (101) surface at different coverages, which are re-aligned by the Fermi energy shifting. Figure 8. The top view of the first two TiO2 trilayers and the first Pt layer of Pt-(111)/TiO2-(101) interface model, the straight lines indicate the size of model in the present work, and the dashed lines indicate the bonding Pt atoms on anatase TiO2 (101) surface. Figure 9. (a) The side view of Pt-(111)/TiO2-(101) interface model, the layer spacing is listed, in compared with the clean relaxed anatase TiO2 (101) surface or Pt (111) surface, whose layer spacing is listed at the top line with red color; (b) the average electrostatic potential and (c) the average electron density difference along the interfacial normal direction. Figure 10. The local density of states of Pt-(111)/TiO2-(101) interface model, in compared with bulk anatase TiO2 or bulk Pt. Figure 11. The energy band diagram of Pt-(111)/TiO2-(101) interface model.

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Figure 1. The adsorption energy (Qi), cohesive energy (Ecoh) of Pt on anatase TiO2 (101) surface, the formation energy (γinterface) of Pt/TiO2 interface, and the deformation energy (∆γsurface) of anatase TiO2 (101) surface induced by Pt adsorption, as function of Pt coverage

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Figure 2. Top view (upper plane) and side view (lower plane) of models: (a) Single Pt atom adsorption; (b) Double Pt atoms adsorption; (c) Typical Ptn cluster loading. The red balls represent oxygen atoms; the grey balls represent titanium atoms; the navy balls represent platinum atoms. The top view only show the top two O-Ti-O trilayers.

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Figure 3. Top view of models: (a) typical 1D Pt narowire loading; (b) typical 2D Pt nanowire grid loading

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Figure 4. Side view of models of Pt ultrathin film loading on anatase TiO2 (101) surface, and the formation of Pt/TiO2 interface, at typical Pt coverage.

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Figure 5. The local and partial density of states of Pt/TiO2 composite system at different lower typical coverages.

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Figure 6. The shifting of Fermi energy level of anatase TiO2 (101) surface induced by Pt loading and the d-band center of Pt atoms as function of Pt coverage.

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Figure 7. The partial density of states of Pt on anatase TiO2 (101) surface at different coverages, which are re-aligned by the Fermi energy shifting.

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Figure 8. The top view of the first two TiO2 trilayers and the first Pt layer of Pt-(111)/TiO2-(101) interface model, the straight lines indicate the size of model in the present work, and the dashed lines indicate the bonding Pt atoms on anatase TiO2 (101) surface.

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Figure 9. (a) The side view of Pt-(111)/TiO2-(101) interface model, the layer spacing is listed, in compared with the clean relaxed anatase TiO2 (101) surface or Pt (111) surface, whose layer spacing is listed at the top line with red color; (b) the average electrostatic potential and (c) the average electron density difference along the interfacial normal direction.

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Figure 10. The local density of states of Pt-(111)/TiO2-(101) interface model, in compared with bulk anatase TiO2 or bulk Pt.

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Figure 11. The energy band diagram of Pt-(111)/TiO2-(101) interface model.

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Figure 1. The adsorption energy (Qi), cohesive energy (Ecoh) of Pt on anatase TiO2 (101) surface, the formation energy (γinterface) of Pt/TiO2 interface, and the deformation energy (∆γsurface) of anatase TiO2 (101) surface induced by Pt adsorption, as function of Pt coverage. 209x148mm (300 x 300 DPI)


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Figure 2. Top view (upper plane) and side view (lower plane) of models: (a) Single Pt atom adsorption; (b) Double Pt atoms adsorption; (c) Typical Ptn cluster loading. The red balls represent oxygen atoms; the grey balls represent titanium atoms; the navy balls represent platinum atoms. The top view only shows the top two O-Ti-O trylayers. 1886x1999mm (72 x 72 DPI)



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Figure 3. Top view of models: (a) typical 1D Pt narowire loading; (b) typical 2D Pt nanowire grid loading. 2559x1608mm (72 x 72 DPI)


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Figure 4. Side view of models of Pt ultrathin film loading on anatase TiO2 (101) surface, and the formation of Pt/TiO2 interface, at typical Pt coverage. 1879x1999mm (72 x 72 DPI)



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Figure 5. The local and partial density of states of Pt/TiO2 composite system at different lower typical coverages. 648x218mm (300 x 300 DPI)


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Figure 6. The shifting of Fermi energy level of anatase TiO2 (101) surface induced by Pt loading and the dband center of Pt atoms as function of Pt coverage. 209x148mm (300 x 300 DPI)



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Figure 7. The partial density of states of Pt on anatase TiO2 (101) surface at different coverages, which are re-aligned by the Fermi energy shifting. 209x227mm (300 x 300 DPI)


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Figure 8. The top view of the first two TiO2 trilayers and the first Pt layer of Pt-(111)/TiO2-(101) interface model, the straight lines indicate the size of model in the present work, and the dashed lines indicate the bonding Pt atoms on anatase TiO2 (101) surface. 1383x1636mm (72 x 72 DPI)



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Figure 9. (a) The side view of Pt-(111)/TiO2-(101) interface model, the layer spacing is listed, in compared with the clean relaxed anatase TiO2 (101) surface or Pt (111) surface, whose layer spacing is listed at the top line with red color; (b) the average electrostatic potential and (c) the average electron density difference along the interfacial normal direction. 2567x1799mm (72 x 72 DPI)


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Figure 10. The local density of states of Pt-(111)/TiO2-(101) interface model, in compared with bulk anatase TiO2 or bulk Pt. 209x234mm (300 x 300 DPI)



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Figure 11. The energy band diagram of Pt-(111)/TiO2-(101) interface model. 193x118mm (300 x 300 DPI)


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From Surface Doping to Interface Forming - American Chemical Society

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