Pt atoms adsorbed on ${\mathrm{TiO}}_{2}(110)\text{\ensuremath{-}}(1\ifmmode\times\else\texttimes\fi{}1)$ studied with noncontact atomic force microscopy and first-principles simulations

Pt atoms adsorbed on ${TiO}_{2} (110) − (1 \times 1)$ studied with noncontact atomic force microscopy and first-principles simulations

Delia Fernández-Torre, Ayhan Yurtsever, Jo Onoda, Masayuki Abe, Seizo Morita, Yoshiaki Sugimoto, and Rubén Pérez

Phys. Rev. B 91, 075401 – Published 2 February 2015

Abstract

We have studied the local properties of single Pt atoms adsorbed on hydroxylated ${TiO}_{2} (110)$ -( $1 \times 1$ ) by combining noncontact atomic force microscopy (nc-AFM) and first-principles calculations. Room-temperature high-resolution nc-AFM images for the most frequently observed contrast modes reveal bright and elongated protrusions that can be traced back to the Pt atoms, and that are centered on the fivefold coordinated titanium rows, confined between two bridging oxygen rows. These observations are in line with the theoretical results, as the lowest energy sites for the Pt atom on the ${TiO}_{2} (110)$ surface are in the neighborhood of the titanium rows, and high energy barriers have to be overcome to displace the Pt atom over the bridging oxygen rows. Single Pt atoms can be distinguished from H adsorbates (OH defects) due to their characteristic shape and binding site and, because they appear as the brightest surface features in all of the contrast modes. Force spectroscopy data over the protrusion and hole imaging modes and the corresponding tip-sample forces, simulated with O and OH terminated ${TiO}_{2}$ nanoclusters, provide an explanation for this puzzling result in terms of the intrinsic strength of the interaction with the Pt adatom and the adatom and tip apex relaxations induced by the tip-sample interaction. These imaging mechanisms can be extended to other electropositive metal dopants and support the use of nc-AFM not only to characterize their adsorption structure but also to directly probe their chemical reactivity.

Received 17 November 2014
Revised 15 January 2015

DOI:https://doi.org/10.1103/PhysRevB.91.075401

Authors & Affiliations

Delia Fernández-Torre^1,2, Ayhan Yurtsever^3,4,*, Jo Onoda³, Masayuki Abe⁵, Seizo Morita^3,4, Yoshiaki Sugimoto³, and Rubén Pérez^1,6,†

¹Departamento de Física Teórica de la Materia Condensada, Universidad Autónoma de Madrid, E-28049 Madrid, Spain
²Instituto de Estructura de la Materia, CSIC, Serrano 121, 28006 Madrid, Spain
³Graduate School of Engineering, Osaka University, 2-1 Yamada Oka, Suita, Osaka 565-0871, Japan
⁴The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan
⁵Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan
⁶Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, E-28049 Madrid, Spain

^*ayhan@afm.eei.eng.osaka-u.ac.jp
^†ruben.perez@uam.es

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Vol. 91, Iss. 7 — 15 February 2015

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Figure 1
Ball-and-stick structure of the hydroxylated ${TiO}_{2} (110)$ surface, highlighting the important $O_{b}, {Ti}_{5 f}$ , and $O_{in}$ sites and the common hydroxyl (OH) defects. O (Ti) atoms are displayed as red (gray) balls; H atoms as white.
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Figure 2
Room-temperature nc-AFM topography images showing the appearance of the Pt atoms deposited on $h − {TiO}_{2} (110)$ for the most common image contrasts. (a)–(c) Protrusion mode image ( $8.25 \times 8.25 {nm}^{2}$ ), hole mode image ( $7.25 \times 7.25 {nm}^{2}$ ), and neutral mode image ( $4.75 \times 4.75 {nm}^{2}$ ) for the $h − {TiO}_{2} (110)$ , respectively. (d)–(f) Protrusion mode image ( $15 \times 15 {nm}^{2}$ ), hole mode image ( $20 \times 20 {nm}^{2}$ ), and neutral mode image ( $12.75 \times 12.75 {nm}^{2}$ ) of the Pt/ $h − {TiO}_{2} (110)$ surface, respectively. OH defects are highlighted by square boxes. The arrows indicate single Pt atoms. The acquisition parameters were as follows: (a) $f_{0} = 163.014$ kHz, $A = 4.8$ nm, $Δ f = - 22.9$ Hz; (b) $f_{0} = 166.55$ kHz, $A = 15$ nm, $Δ f = - 0.95$ Hz; (c) $f_{0} = 167.44$ kHz, $A = 11.5$ nm, $Δ f = - 0.73$ Hz; (d) $f_{0} = 166.42$ kHz, $A = 6.4$ nm, $Δ f = - 14.42$ Hz, $V_{s} = 0.6$ V; (e) $f_{0} = 159.54$ kHz, $A = 14.6$ nm, $Δ f = - 4.6$ Hz, $V_{s} = 0.95$ V; (f) $f_{0} = 167.91$ kHz, $A = 11.5$ nm, $Δ f = - 2.12$ Hz, $V_{s} = 0.45$ V.
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Figure 3
Ball-and-stick structures for Pt atoms adsorbed on ${TiO}_{2} (110)$ . The same color code of Fig. 1 is used, with Pt atoms represented by green balls. (a) Pt adsorbed on a hollow (H) position, between two $O_{in}$ atoms, with the Pt atom bound to one ${Ti}_{5 f}$ atom and one $O_{b}$ atom. (b) Pt adsorbed on top of an $O_{in}$ atom, forming bonds with two ${Ti}_{5 f}$ atoms and one $O_{in}$ atom. (c) Top view of the ${TiO}_{2} (110)$ surface indicating the different atomic sites and the easy diffusion paths for Pt.
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Figure 4
Tip-sample forces for the protrusion mode imaging contrast of ${Pt / TiO}_{2} (110)$ . (a) Experimental short-range forces over Pt (green line) and OH (black line). Inset: nc-AFM image of the surface region where the forces were measured. The green and black arrows indicate the Pt and OH sites where measurements were taken. The acquisition parameters were as follows: $f_{0} = 160.17$ kHz, $A = 12.3$ nm, $Δ f = - 1.61$ Hz. KPFM parameters: $U_{ac} = 0.5$ V, $f_{ac} = 500$ Hz. Image size: $3.5 \times 3.5$ nm $^{2}$ . The tip-sample distance in the experimental curves has been shifted to align the attractive force minima of Pt atom with the theoretical calculations. (b) Calculated forces over Pt and OH, with the same color code. Inset: tip-sample structure over the Pt site at $d = 4.25 Å$ . In our calculations, the Pt is adsorbed on the most stable site [hollow; see Figs. 3 and 3].
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Figure 5
Tip-sample relaxations induced by the interaction with an O-terminated tip. The structures over the Pt site at $d = 4.25$ and 4.5 Å illustrate the large upward displacement of the Pt ( $\sim 1 Å$ over the $O_{b}$ surface atoms) associated with the strong chemical interaction with the O atom at the tip apex. This relaxation explains the relative position of the force minima on the Pt and OH sites.
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Figure 6
Tip-sample forces for the hole imaging mode of ${Pt / TiO}_{2} (110)$ surface. (a) Experimental short-range forces over Pt (green line) and $O_{b}$ (blue line). Inset: nc-AFM image of the surface region where the forces were measured, with the green and blue arrows indicating the Pt and $O_{b}$ sites where the forces were measured. The tip-sample distance in the experimental curves has been shifted to align the attractive force minima of Pt atom with the theoretical calculations. The acquisition parameters were as follows: $f_{0} = 158.33$ kHz, $A = 16.9$ nm, $Δ f = - 2.12$ Hz, $V_{s} = 1.0$ V. Image size: $5.5 \times 5.5 {nm}^{2}$ . (b) Calculated forces over Pt and $O_{b}$ , with the same color code. Inset: tip-sample structure over the Pt site at $d = 3.0 Å$ . The Pt atom is adsorbed on a hollow site (as in Fig. 4).
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Physical Review B

covering condensed matter and materials physics

Pt atoms adsorbed on ${TiO}_{2} (110) − (1 \times 1)$ studied with noncontact atomic force microscopy and first-principles simulations

Delia Fernández-Torre, Ayhan Yurtsever, Jo Onoda, Masayuki Abe, Seizo Morita, Yoshiaki Sugimoto, and Rubén Pérez

Phys. Rev. B 91, 075401 – Published 2 February 2015

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Physical Review B

covering condensed matter and materials physics

Pt atoms adsorbed on TiO2(110)−(1×1) studied with noncontact atomic force microscopy and first-principles simulations

Delia Fernández-Torre, Ayhan Yurtsever, Jo Onoda, Masayuki Abe, Seizo Morita, Yoshiaki Sugimoto, and Rubén Pérez

Phys. Rev. B 91, 075401 – Published 2 February 2015

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Pt atoms adsorbed on ${TiO}_{2} (110) − (1 \times 1)$ studied with noncontact atomic force microscopy and first-principles simulations