Direct Visualization of Au Atoms Bound to TiO2(110) O-Vacancies.

Au nanoparticles supported on reducible metal oxide surfaces are known to be active catalysts for a number of reactions including CO oxidation and hydrogen production. The exact choice of a metal oxide support has been shown to have a marked impact on activity, suggesting that interactions between Au and the support play a key role in catalysis. For TiO2, a model substrate for Au catalysis, it had been thought that bridging oxygen vacancies are involved in binding Au atoms to the (110) surface based on indirect evidence. However, a recent scanning transmission electron microscopy study of single Pt atoms on TiO2(110) suggests that subsurface vacancies are more important. To clarify the role of bridging or subsurface vacancies we employ scanning tunneling microscopy to determine the bonding site of single Au atoms on TiO2(110). Using in situ deposition as well as a manipulation method, we provide definitive evidence that the bonding site is atop surface oxygen vacancies.

Introduction

The nature of the interaction of metal nanoparticles and their metal oxide supports remains a key area of research in catalysis.[1−9] In particular, gold nanoparticles supported on TiO2 have received considerable attention[2] following the discovery that they are an effective low-temperature oxidation catalyst.[3] As a model catalyst, Au adsorption on rutile TiO2(110) has been extensively studied because this substrate is the most well-characterized metal oxide substrate.[2,4−9] When reduced, TiO2 (110) contains point defects in the form of oxygen vacancies (Ob-vacs) as well as Ti interstitial atoms in the bulk.[5] Ob-vacs, in particular, are a widely studied aspect of TiO2 fundamental catalysis because they are reactive sites on the surface and in many cases they act as bonding sites for surface adsorbates.[5]

A key feature of the Au/TiO2 system that remains controversial is the bonding site of individual Au atoms on the surface (Au1). This is potentially important in connection to single atom catalysis, which has been demonstrated for a number of systems,[10] most recently for Au atoms on carbon.[11] On TiO2(110), Au1 has been shown to be stable up to at least 600 K.[6] After soft landing Au+, Tong et al.[6] observed bonding of Au atoms to the bridging oxygen row using scanning tunneling microscopy (STM) and assigned the adsorption site to a bridging Ob-vac. On cooling to 300 K they observed a reversible shift of adsorption site to the Ti row, assigning the adsorption site as atop a five-fold coordinated Ti site (Ti5c). This was interpreted as a displacement of Au1 by interaction with adventitiously coadsorbed water at the lower temperature.[6] This is in line with previous spectroscopic work[7] and STM measurements.[8]

Although the STM results referenced above clearly observe Au1 associated with the bridging O row, there is no direct evidence of bonding to Ob-vac. On the contrary, several density functional theory (DFT) calculations suggest that Ob-vac is the preferred site.[8,9] More recent results have challenged the assignment of Ob-vac to the binding site of noble metals. In particular, aberration-corrected scanning transmission electron microscopy (STEM) has been used to image Pt1 on TiO2(110). In their paper, the authors conclude that Pt1 occupies in-plane O vacancies.[12] Theory predicts that the lowest energy site of Pt1 atoms on reduced TiO2(110) is the same as for Au1, that is Ob-vac.[13] Hence the implication of the STEM work is that Au1 will also occupy in-plane O vacancies. If this was correct, then the discrepancy between DFT and experiment could arise from a dominance of kinetic effects. This has recently been demonstrated for a related system, Au-CeO2(111), where kinetic effects prevent the occupation of oxygen vacancies by Au1.[14] Here we demonstrate that this is not the case for Au1 on a reduced rutile TiO2(110) surface (r-TiO2). We present direct evidence that single Au atoms bind at Ob-vacs through a comparison of STM images of the same area on the surface before and after dosing Au1. We have also employed a method to selectively displace Au atoms using voltage pulses from an STM tip, thereby exposing the underlying Ob-vac.

Experimental Methods

STM measurements employed two UHV Omicron instruments. One is a low-temperature bath cryostat machine (LT-STM) operated at 78 K with a base pressure of 3 × 10–11 mbar. The second is a variable temperature microscope (VT-STM) operated at 300 K with a base pressure 5 × 10–11 mbar. A rutile TiO2 (110) (1 × 1) single-crystal (MaTecK) sample was prepared by successive cycles of argon ion sputtering and annealing to 1000 K. The sample cleanliness and long-range order were confirmed using X-ray photoelectron spectroscopy (XPS) or Auger electron spectroscopy (AES) and low-energy electron diffraction (LEED), respectively. Surfaces prepared in this manner typically have a coverage of ∼5% ML of Ob-vacs (where 1 monolayer is the number of surface unit cells).

Au was deposited in situ onto an as-prepared TiO2(110) surface present in the sample stage of the VT-STM at room temperature. Deposition was performed using a line-of-sight electron beam evaporator (Omicron EFM). The STM tip was retracted prior to deposition. In the LT-STM, Au was deposited onto as-prepared TiO2 (110) at room temperature using a Au rod wrapped around a tungsten filament, which was resistively heated to induce Au sublimation. The presence of Au on the TiO2 substrate was confirmed using XPS. In both systems, STM measurements were carried out in constant current mode using an electrochemically etched W tip conditioned by degassing at 500 K and voltage pulses in STM. There were no significant differences in Au coverage or dispersion seen in STM images recorded at 78 and 300 K.

Results and Discussion

Figure a shows an STM image of the as-prepared TiO2 (110) surface recorded after deposition of 0.08 monolayer (ML) of Au. Au coverage was determined by measuring the average density of Au atoms on the surface via STM and dividing by the density of a single (111) layer of bulk gold, that is, 1 ML = 1.387 × 1015 Au atoms·cm–2.

Figure 1

(a) STM image (VS = +1.3 V, IT = 50 pA) of r-TiO2 (110) at 78 K after deposition of 0.08 ML of Au at room temperature. The yellow solid circle marks an Ob-vac, whereas a blue circle and green ellipse mark single and paired hydroxyls (OHb), respectively. White solid and dashed circles mark Au1 and Au3 species, respectively. (b) On-top view structural model of TiO2(110) illustrating Ob-vac and single (s-) and double (d-) OH groups. These are highlighted in the same colored circles as used in panel a. Ti atoms are depicted in brown, with O atoms in red and blue, the latter representing bridging Ob atoms.

In STM, the TiO2 (110) surface is characterized by bright rows of five-fold coordinated Ti4+ (Ti5c) and dark rows of two-fold bridging O2– (Ob) running along the [001] direction.[5] Five distinct types of features appear evenly distributed over the surface. The three smallest features are all centered over the Ob rows. The first appears as a small horizontal line (yellow circle), whereas the two larger features are more circular (blue circle and green ellipse). Such features are well described in the literature and are attributed to Ob-vacs and OHb, respectively.[5] A model showing these features is shown in Figure b. The two remaining unidentified features in Figure a are not typical of STM images from r-TiO2 (110) and can be attributed to deposited Au species. The first (highlighted as white solid circles in Figure a) appears centered over the Ob rows on the surface with an average height of 193 ± 60 pm. The second (highlighted as white dashed circles in Figure a) is larger, spanning multiple Ob rows in the [110] direction with an average height of 233 ± 10 pm. The larger Au species are found centered over either Ob or Ti5c rows. Using STM in conjunction with DFT calculations, Matthey et al. identified the smallest Au species present on an Au-deposited r-TiO2 (110) to be Au1.[8] The second smallest species they assign as Au trimers (Au3) that may adsorb in one of two geometries centered either over Ob or Ti5c rows.[8] By direct comparison of our data with the previous work,[8] the smallest Au species observed in Figure a are assigned as Au1 and the larger ones to Au3.

To unambiguously identify the binding site of Au1, we performed in situ deposition of Au and imaged the same region of the r-TiO2 surface before and after deposition. This was achieved by collecting an STM image of r-TiO2 in an Omicron VT-STM instrument. The STM tip was then retracted and Au was deposited onto the imaged region using an in situ electron-beam evaporator that points to the STM stage. After deposition, the STM tip was reapproached to exactly the same position. Both the Au deposition and STM measurements were carried out at 300 K. As shown in Figure , after deposition some Ob-vacs that were on the r-TiO2 are now filled by individual Au atoms (marked by circles in Figure a,b). Similar entities appear centered over Ti5c rows but are much less frequent (<10%).

Figure 2

Room-temperature STM images (VS = +1.3 V, IT = 20 pA) of r-TiO2(110) recorded (a) before and (b) after in situ deposition of <1% ML Au at room temperature. Circles mark the Ob-vacs on r-TiO2 that are filled by Au1 after deposition. The bright (Ti) rows run in the [001] direction.

In addition, low-temperature measurements were also performed using an Omicron LT-STM system. At 78 K no Ti5c-centered Au species were observed at all. On this basis, we conclude that Ob-vacs are the most stable binding sites for Au1 on the r-TiO2 surface.

To provide further evidence that Au1 binds to Ob-vacs, STM tip pulsing was used to laterally manipulate Au1 on r-TiO2. STM tip-induced manipulation has been widely employed to form nanostructures on planar surfaces, and this field is reviewed in ref (15). To ensure stability and reproducibility, pulsing was performed at 78 K. Figure shows atomically resolved STM images of Au1 on r-TiO2 (110) taken before (Figure a) and after (Figure b) application of a 100 ms tip pulse over the Au1 species at a sample bias of −2 V.

Figure 3

STM images (VS = +1.2 V, IT = 10 pA) of Au1 on r-TiO2(110) at 78 K showing its displacement from (a) one Ob-vac to (b) another Ob-vac site, induced by a 100 ms tip pulse over the Au1 species at a sample bias of −2 V. Arrows indicate the directions of Au1 displacement. Three spectator adsorbates are present in the images, namely, a single and two double OHb groups. The bright (Ti) rows run in the [001] direction.

By tracking its position before and after pulsing, the Au1 appears to move from its original site into an Ob-vac site on an adjacent Ob row (Au1-ovac). On inspection of the image in Figure b, the original position of the Au1, which is now uncovered, is identified as an Ob-vac. To eliminate the possibility that this Ob-vac was generated as a consequence of the applied tip pulse, further tip pulses were performed over bare Ob rows using the same parameters. No Ob-vacs could be created by pulsing the bare r-TiO2 (110). Minato et al. have shown that it is possible to desorb Ob atoms using −2 V tip pulses; however, in their work they use a pulse duration of 1 s compared with the 100 ms pulses applied here.[16] Hence the Au1 manipulation experiment also points to bonding with Ob-vac.

In addition to movement from one Ob-vac to another, Au1 could be manipulated from an Ob-vac onto a Ti5c site (Au1-Ti) following application of a tip pulse (see Figure ). The formation of these Au1-Ti species was observed in a minority of movement events. After several STM scans (∼20 min) the Au1-Ti were seen to spontaneously move onto nearby Ob-vacs sites becoming Au1-ovac. After moving from Au1-Ti to Au1-ovac, no subsequent changes were observed.

Figure 4

(a) STM images (VS = +1.2 V, IT = 10 pA) of Au1 on r-TiO2(110) at 78 K before (a) and after (b) STM tip-induced displacement of Au1 to a Ti5c row induced by a −2 V, 100 ms tip pulse centered above Au1. The TiO2(110) surface lattice has been overlaid on the image, where intersections in the grid indicate Ti5c positions. Arrows indicate the movement of Au1 from an Ob-vac to a Ti5c site. The bright (Ti) rows run in the [001] direction.

This supports the view that Au1 binds more strongly to Ob-vacs than to Ti5c sites.[8,9] In this case, the observed Au1-Ti complex is likely to have been stabilized by the cryogenic conditions used (78 K).

To determine the underlying mechanism of Au1 displacement, a detailed statistical analysis of pulsing events was performed. This follows a methodology used previously to investigate tip-induced H desorption from Si(100)-2x1:H[17] and TiO2(110)–OH.[18] During a tip pulsing experiment the STM tip is first centered over an Au1 species. By monitoring the tunneling current (IT) for the duration of each pulse, a plot of IT(t) is created (Figure inset). For each pulse an IT set point is reached and held constant by the feedback loop of the STM. During manipulation, a peak in the IT(t) plot can be seen. This peak corresponds to the sudden displacement of the Au1 beneath the tip. The increase in tip–sample distance caused by the now absent Au1 decreases IT sharply before the feedback loop corrects for the change by bringing IT back to the initial set point. The pulse duration required to induce movement can be determined from the position of the excursion in IT. This process was repeated multiple times (∼60) for each set of pulsing parameters used in this work. After plotting the resulting data in histogram form, an exponential decay was fit to the data to yield the average value of τ for each pulsing parameter, an example being shown in Figure . For each plotted histogram, bin widths were varied, within a reasonable range, such that the resulting change in τ could be factored into the error of each average value determined.

Figure 5

Histogram showing the distribution of Au1 displacement times for tip pulses of VS = −1.8 V and I = 10 pA. The red line represents an exponential fit to the distribution of statistically independent events. The inset is an example of an individual I(t) trace used to compile the histogram where It is the set point of the pulse (10 pA) and τ is the pulse duration prior to Au displacement.

To confirm that the measured distribution of Au1 displacement times does indeed follow an exponential decay, a semilog plot of ln N(t) was created for each data set (Figure ).

Figure 6

Plot of ln N(t) versus time for the data shown in Figure . The red line shows a linear fit to the data. The last eight points of the plot have been attributed to random noise and are omitted from the fitting. From the slope of the linear fit, τ was determined to be 17.4 ms. The fitting from this plot was used to refine the exponential fitting to the histogram shown in Figure and hence the value of τ.

The displacement yield (events per electron), Y, was determined using the average value of τ measured for each pulsing parameter. Y values were calculated for a range of pulsing currents and voltages and were plotted as a function of IT. The relationship between Y and IT was then used to determine the displacement reaction order, that is, the number of principal electrons involved in the dissociation of a single Au1 from an Ob vac (Figure ). Because for an n-electron process Y ∝ IT(,[18] the gradients observed in Figure , which are approximately unity, indicate a two-electron vibrational ladder-climbing process. Such a mechanism is typical for adsorbates on semiconductors[15] and has been proposed previously for STM tip-induced dissociation of H from TiO2(110)–OH.[18]

Figure 7

Double-logarithmic plot of displacement yield, Y, as a function of tunneling current, IT. Solid circles, squares, triangles, and rhombi represent the measured yields of Au1 displacements at different pulse voltages (labeled). Lines show power-law fits to the experimental data, with exponents (α) given for each pulse voltage.

Conclusions

The direct evidence provided here for bonding of gold atoms to bridging oxygen vacancies on TiO2(110) is clearly in accord with the predictions of DFT calculations.[8,9] There is, however, a marked disagreement between this work and the conclusions of a scanning transmission electron microscopy study of platinum atoms on TiO2(110).[12] In this previous work, bond sites atop subsurface vacancies were proposed, with no observed bonding to bridging oxygen vacancies. A likely cause of this difference is that the surface in the previous work was hydroxylated, a state that is known to promote occupation of Ti5c sites.[6,8] Hence, our result removes an important discrepancy in the literature between theory and experiment and reinstates the importance of bridging oxygen vacancies in the catalytically related chemistry of TiO2.