Supporting Online Material for

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1 Supporting Online Material for Enhanced Bonding of Gold Nanoparticles on Oxidized TiO 2 (110) D. Matthey, J. G. Wang, S. Wendt, J. Matthiesen, R. Schaub, E. Lægsgaard, B. Hammer, F. Besenbacher* *To whom correspondence should be addressed. fbe@inano.dk This PDF file includes: Materials and Methods Figs. S1 to S7 References Published 23 March 2007, Science 315, 1692 (2007) DOI: /science

2 Materials and Methods The experiments were carried out in an ultrahigh vacuum (UHV) chamber with a base pressure of Torr equipped with a homebuilt, temperature variable STM (1) and standard facilities for sample preparation and analysis (2). The sample temperature could be varied from 100 K (by cooling with liquid N 2 ) to 1200 K by radiative heating and electron bombardment from the back side of the sample; the temperature was measured by a K-type thermocouple spot welded on the sample plate very close to the crystal. Three TiO 2 (110) (1 1) samples were used for these STM studies and cleaned by cycles of Ar + bombardment at room temperature (RT) and vacuum annealing at 800 to 950 K. The density of bridging oxygen (O br ) vacancies was in the range of 5 8% ML, with 1 ML (monolayer) being the density of the (1 1) units, / cm 2. Clean TiO 2 (110) surfaces with O br vacancies (r- TiO 2 (110)) were prepared by applying of short flashes to 600 K when the samples reached RT after vacuum annealing (3). Subsequently, samples were transferred to the pre-cooled STM. Hydrated TiO 2 (110) surfaces (h-tio 2 (110)) were prepared by water exposure at 120 K to a reduced crystal, followed by slowly warming the crystal up to 400 K. When this recipe is applied, all O br vacancies can be converted into hydroxyl groups in the rows of O br atoms (OH br ) through water dissociation in the vacancies (3). Oxidized TiO 2 (110) surfaces (o-tio 2 (110)) were prepared via O 2 exposure (1.5 6 L with 1 L (Langmuir) = Torr s) at 120 K, followed by a short flash up to RT. The flash ensures that no molecular oxygen is left in the vicinity of the o-tio 2 (110) surface after O 2 exposure (4). The O 2 exposure was chosen as small as possible to avoid the formation of suboxides on the o-tio 2 (110) surface (5, 6, 7). Gold was evaporated onto the three TiO 2 (110) surfaces of interest 1

3 using an e-beam evaporator (Oxford Instruments) that had an angle of incidence of ca. 45º with respect to the crystal normal. During Au exposure, the background pressure was better than Torr even for the longest exposure time (80 sec). The Au flux was calibrated based on STM measurements on Au exposed Ni(111) substrates (8). The Au coverage is reported in monolayers (ML), where one ML equals atoms per cm 2 corresponding to Au(111). The given values of the Au coverages on the three TiO 2 (110) surfaces are upper limits, as the sticking coefficient s of Au on a flat oxide surface at RT is likely to be smaller than unity. For the Au coverages and the TiO 2 (110) crystals studied, the Au cluster morphologies were found to be independent of the defect densities. After Au deposition at RT, the TiO 2 (110) samples were quenched and transferred to the cooled STM. In this way, contamination by water was minimized, because the STM block at ca. 110 K and the liquid nitrogen Dewar of the manipulator act as efficient water traps. The STM images presented in this work were taken in the constant current mode using a tunneling voltage (V T ) of V and a tunneling current (I T ) of ~0.1 na. A special preamplifier was used, allowing us to work with this low tunnel current and with high feedback gains. All the STM images were acquired with the sample at K. Scanning with a high voltage (up to +10 V) was done to prepare the STM tip (9). For the deduction of the Au cluster height histograms from the STM data, total areas of ca nm 2 on the Au exposed TiO 2 (110) surfaces were scanned and analyzed. Thereby a threshold of 1.2 Å above the terrace was chosen when counting of the Au clusters. Control experiments delivered identical height histograms, corroborating that height histograms relying on total areas of ca nm 2 (more than 1000 Au clusters) are representative. 2

4 The first-principles Density Functional Theory (DFT) calculations were performed using the DACAPO package (10, 11) with a plane-wave basis set (E cut = 25 Ry) and ultrasoft pseudopotentials. The generalized gradient approximation (GGA) with the revised Perdew-Burke-Ernzerhof (RPBE) functional (12) was used to describe the exchange-correlation effects. The TiO 2 (110) surface was modelled using periodic slabs of three tri-layers, the first tri-layer being fully relaxed. A (4 2) TiO 2 (110) surface unit cell and the theoretically derived lattice constants (a = 4.69 Å, a = 2.99 Å, and u = 0.305) were used. The Au n adhesion potential energies (APEs) on TiO 2 (110) with 1 n 5 and n = 7 (Table 1 and Fig. 5E) were calculated according to APE n = E tot (Au n + support) E tot (Au n ) E tot (support), with E tot (Au n + support), E tot (Au n ), and E tot (support) being the total energies of the combined systems, the most stable gas phase Au n 2D clusters, and the TiO 2 (110) surface in a certain oxidation state, respectively. The different oxidation states of the TiO 2 (110) surfaces were modelled via introduction of one O br vacancy and addition of one capping H ad-atom (OH br ), respectively, in the (4 2) surface unit cell. Accordingly, the oxidized state of the TiO 2 (110) surface was modelled by addition of one oxygen ad-atom on an in-plane O site in one Ti trough, as this is the most stable site for O adsorption on stoichiometric TiO 2 (110) (s-tio 2 (110)) (13). Once Au n is adsorbed it is more favourable for the oxygen ad-atom to move to the position atop the 5f-Ti. The nudged elastic band method (NEB) was used to find the most favourable diffusion pathways and the energy barriers for Au 1 diffusion on the three TiO 2 (110) surfaces with point defects (14). For reference, we also considered the s- TiO 2 (110) surface both for the Au 1 diffusion as well as when searching for stable 3

5 larger Au n clusters. The charge density changes induced upon Au 3 and Au 4 adsorption on r- and o-tio 2 (110), respectively (cf. Fig. 5A and C), were calculated according to ρ(r) = ρ Au/support (r) ρ Au (r) ρ support (r). 4

6 References of Materials and Methods 1. J. V. Lauritsen, F. Besenbacher, Adv. Catal. 50, 97 (2006). 2. E. Lægsgaard, L. Österlund, P. Thostrup, P. B. Rasmussen, I. Stensgaard, F. Besenbacher, Rev. Sci. Instrum. 72, 3537 (2001). 3. S. Wendt, R. Schaub, J. Matthiesen, E. K. Vestergaard, E. Wahlström, M. D. Rasmussen, P. Thostrup, L. M. Molina, E. Lægsgaard, I. Stensgaard, B. Hammer, F. Besenbacher, Surf. Sci. 598, 226 (2005). 4. M. A. Henderson, W. S. Epling, C. L. Perkins, C. H. F. Peden, U. Diebold, J. Phys. Chem. B 103, 5328 (1999). 5. M. Li, W. Hebenstreit, L. Gross, U. Diebold, M. A. Henderson, D. R. Jennison, P. A. Schultz, and M. P. Sears, Surf. Sci. 434, 173 (1999). 6. M. Valden, X. Lai, K. Luo, Q. Guo, D. W. Goodman, Science 281, 1647 (1998). 7. K. T. Park, M. H. Pan, V. Meunier, E. W. Plummer, Phys. Rev. Lett. 96, (2006). 8. J. Jacobsen, L. Pleth Nielsen, F. Besenbacher, I. Stensgaard, E. Lægsgaard, T. Rasmussen, K. W. Jacobsen, and J. K. Nørskov, Phys. Rev. Lett. 75, 489 (1995). 9. U. Diebold, J. Lehman, T. Mahmoud, M. Kuhn, G. Leonardelli, W. Hebenstreit, M. Schmid, and P. Varga, Surf. Sci. 411, 137 (1998). 10. The DACAPO code is available at S. R. Bahn and K. W. Jacobsen, Comput. Sci. Eng. 4, 56 (2002). 12. B. Hammer, L. B. Hansen, J. K. Nørskov, Phys. Rev. B 59, 7413 (1999). 13. Z.-w. Qu and G.-J. Kroes, J. Phys. Chem. B 110, (2006). 14. G. Henkelman, B. P. Uberuaga, H. Jόnsson, J. Chem. Phys. 113, 9901 (2000). 5

7 (Å) STM height Ti O Ti O Ti O Ti Ti O Ti O Ti O Ti Length along [110] (Å) 50 Figure S1: STM height-profiles of typical Au n clusters found on the Au exposed r-tio 2 (110) surface. The three clusters are ascribed to Au n bound to O br vacancies with n being 1 (red line) and 3, respectively (black and blue lines). The difference between the two elongated clusters corresponding to the profiles shown in black and blue is explained by symmetrical and unsymmetrical attachements of Au 3 clusters to isolated O br vacancies (cf. Figs. 4B,C).

8 A B a o-tio 2 (110) 3% ML Au o-tio 2 (110) 12% ML Au Density (10-4 nm -2 ) b c STM Height (Å) 8 10 Figure S2: (A) STM image of the o-tio 2 (110) after 12% ML Au exposure at RT ( Å 2 ). The STM heights of the Au clusters are given by contour lines at 1.2 Å, 3.2 Å and 5.2 Å above the terrace. (B) Height histograms obtained for 3% and 12% ML Au exposure onto o-tio 2 (110). The histograms rely on scanned areas of ca nm 2 each (ca Au clusters per histogram). Solid lines are Gaussian fits to the peaks in the histogram corresponding to the 12% Au exposure. The height differences between neighboring peaks a and b and peaks b and c of 2.6 Å and 2.1 Å, respectively, are close to the Au(100) (2.04 Å) and Au(111) (2.35 Å) interlayer spacings. Therefore, we ascribe peaks a, b, and c to Au clusters consisting of one, two and three Au layers, respectively.

9 Figure S3: Plots of the DFT based adhesion potential energies (APEs) of the most stable Au n clusters with 1 n 5 and n = 7 on r-, h-, s- and o-tio 2 (110) surfaces, respectively. The corresponding structure models are depicted in Fig. S4 to Fig. S7.

10 A B ev ev O br vacancy ev ev C D ev ev ev ev E F ev ev ev 0.27 ev Figure S4: Au n / r-tio 2 (110) configurations calculated using DFT: (A) Au 1, (B) Au 2, (C) Au 3, (D) Au 4, (E) Au 5 and (F) Au 7. Shown are side and top views of the 1 st (left panels) and 2 nd (right panels) most favorable structures found within a (4 2) TiO 2 (110) surface unit cell containing one O br vacancy as indicated (A). The adhesion potential energies (APEs) are given with respect to the corresponding most stable Au n cluster in the gas phase. Small, light red balls represent oxygen atoms, medium size, grey balls Ti atoms, and large yellow balls Au atoms.

11 A B ev ev ev 0.08 ev C D ev ev ev 0.12 ev E F ev ev ev 0.10 ev Figure S5: Au n / h-tio 2 (110) configurations calculated using DFT: (A) Au 1, (B) Au 2, (C) Au 3, (D) Au 4, (E) Au 5 and (F) Au. Presentation of the structures as described in 7 Fig. S4. Both the structure of the Au n cluster and the place of the capping H adatom within the unit cell were varied during the calculations. The APEs are given with respect to the corresponding most stable Au n cluster in the gas phase. Small, light red balls represent oxygen atoms, medium size, grey balls Ti atoms, and large yellow balls Au atoms. The H ad-atoms capping O br atoms are indicated by small white balls.

12 A B ev ev ev 0.13 ev C D ev ev ev ev E F ev ev ev ev Figure S6: (A) (F) Au n / s-tio 2 (110) configurations calculated using DFT: (A) Au 1, (B) Au 2, (C) Au 3, (D) Au 4, (E) Au 5 and (F) Au 7. Presentation of the structures as described in Fig. S4. The APEs are given with respect to the corresponding most stable Au n cluster in the gas phase. Small, light red balls represent oxygen atoms, medium size, grey balls Ti atoms, and large yellow balls Au atoms.

13 A B ev ev ev ev C D ev ev ev ev E F ev ev ev ev Figure S7: (A) - (F) Au n / o-tio 2 (110) configurations calculated using DFT: (A) Au 1, (B) Au 2, (C) Au 3, (D) Au 4, (E) Au 5 and (F) Au 7. Presentation of the structures as described in Fig. S4. The APEs are given with respect to the corresponding most stable Au n cluster in the gas phase. Small, light red balls represent oxygen atoms in the oxide lattice, medium size, grey balls Ti atoms, and large yellow balls Au atoms. The O ot atom in the Ti troughs (one per unit cell) are shown hatched.