Vicinal Rutile TiO2 Surfaces and Their Interactions with O2 - American

Article pubs.acs.org/JPCC

Vicinal Rutile TiO2 Surfaces and Their Interactions with O2 Felix Rieboldt, Ralf Bechstein, Flemming Besenbacher, and Stefan Wendt* Interdisciplinary Nanoscience Center (iNANO), Department of Physics and Astronomy, Aarhus University, DK-8000 Aarhus C, Denmark S Supporting Information *

ABSTRACT: Slightly miscut TiO2(110) surfaces with high densities of step edges were studied by scanning tunneling microscopy (STM), temperature-programmed desorption (TPD), and ultraviolet photoemission spectroscopy (UPS). STM measurements provided information on the surface morphology and the density of defects and adstructures, whereas UPS measurements revealed information on the electronic structure and the surface reduction state before and after the conduction of O2 TPD experiments. It was found that the presence of step edges and adstructures has a strong influence on the O2−TiO2 interaction. The growth of TiOx islands occurred in the same way on stepped surfaces as on flat TiO2(110) surfaces, but the island densities were smaller. TPD measurements revealed that significantly less O2 desorbed between 300 and 410 K from stepped surfaces than from surfaces with large terraces. Importantly, the stepped TiO2 surfaces were characterized by clearly lower surface reduction states than flat TiO2(110) surfaces.

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INTRODUCTION Because of its wide range of applications, titania (TiO2) has inspired numerous research efforts.1−8 In fact, TiO2 is used in gas sensors, solar cells, heterogeneous catalysis, and photocatalysis, and it is now probably the most studied metal oxide. In most applications of TiO2, oxygen plays a crucial role. For instance, in many photocatalytic oxidation reactions, O2 is used as a scavenger of the photoexcited electrons to prevent negative charge accumulation on the surface.1,6 Despite its importance, the interaction of O2 with TiO2 is still not fully explored. Recently, quite a number of studies have appeared addressing this crucial topic, applying the “surface science approach”.5,7,9−19 It is generally accepted that O2 adsorption on TiO2 surfaces is associated with the withdrawal of negative charge from the surface.1,5−7,18,20 This charge is available on the surfaces of reduced TiO2 crystals because of various defects such as O vacancies and Ti interstitials.5,7,9,12,13,21−23 In the TiO2 valence band, the Ti3+ excess charge leads to the population of the defect state in the ∼3.1 eV wide band gap [in the following simply denoted as the gap state (GS)].9,20,23 On the wellstudied rutile TiO2(110)−(1 × 1) surface, the adsorption of O2 at low temperatures of 100 K < T < 150 K is known to occur both molecularly and dissociatively.11−15,24 In temperatureprogrammed desorption (TPD) experiments, O2 desorbs in a narrow peak between 360 and 410 K.11,13,24,25 Previously, it was reported by some of the present authors that this O2 desorption peak is caused by a depletion of excess charge in the nearsurface region as a result of a reaction between oxygen species and out-diffusing Ti interstitials.11 Thus, an ionosorption model is invoked to explain the O2 desorption peak between 360 and 410 K.11 Furthermore, the O2 desorption peak between 360 © 2014 American Chemical Society

and 410 K was found to depend strongly on the crystal reduction state.12,13 In most previous studies addressing rutile TiO2(110), the influence of step edges has often been ignored. Instead, the focus was on the prevailing point defects on the terraces and in the bulk, such as bridging oxygen (Obr) vacancies and Ti interstitials, respectively.1,5,7−9,13,23,26,27 However, several studies on a broad range of materials showed that extended defects such as step edges can be of high significance in surface reactions.28−32 Regarding rutile TiO2(110)−(1 × 1), two recent studies from the Aarhus group revealed that water and ethanol molecules adsorb dissociatively at the [11̅1] step edges, evidencing the existence of O vacancies along these steps.33,34 In the present work, vicinal TiO2(110) surfaces with different densities of step edges and their interactions with O2 were studied by means of scanning tunneling microscopy (STM), TPD, and ultraviolet photoemission spectroscopy (UPS) measurements. To guarantee identical bulk reduction states, the three samples were prepared identically. Thus, it was possible to directly compare these surfaces with intrinsically different morphologies but identical bulk reductions. It was found that the presence of step edges and adstructures has a strong influence on how O2 interacts with rutile TiO2. The growth of TiOx islands as a result of dosing O2 at 100 K and subsequent annealing to 500 K occurred in the same way on the stepped surfaces, but their densities were smaller than on flat TiO2(110). TPD measurements revealed that significantly less O2 desorbed at ∼400 K from stepped surfaces than from surfaces with extended terraces. Additionally, it was found that Received: November 18, 2013 Revised: January 17, 2014 Published: January 28, 2014 3620

dx.doi.org/10.1021/jp411324u | J. Phys. Chem. C 2014, 118, 3620−3628

The Journal of Physical Chemistry C

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Figure 1. (a−c) STM (32 nm × 32 nm) images of the three clean TiO2 surfaces (110), (870), and (771), respectively. (d−f) Corresponding STM images after exposure of the surfaces to 200 L of O2 at 100 K followed by ramping of the temperature to 500 K (O2 TPD experiment). In images d− f, black circles mark newly formed TiOx (x ≈ 2) islands. The STM images were obtained at 90−110 K (∼1.2 V, 0.1 nA). A periodic color scale was used with 0.33-nm periodicity [one atomic layer on TiO2(110)].

annealing at (910 ± 10) K. During the sample preparation, heating and cooling of the samples were performed throughout using ramps of 1 K/s. The temperature was measured using Ktype thermocouples and double checked with a pyrometer. Typically, after the completion of each second preparation cycle, the same set of experiments was performed on all three samples. STM was carried out on both clean and O2-exposed samples at 90−110 K in the constant-current mode (bias voltage ≈ 1.2 V, tunneling current ≈ 0.1 nA). In the TPD experiments, O2 exposures were performed using the directional doser with the surface at 100 K (∼200 L, 120 s at 2 × 10−8 mbar background pressure; the pressure at the sample surface was estimated to be about 100 times higher). The samples were then ramped to 500 K at a heating rate of 2 K/s while desorbing O2 molecules were detected and then held at 500 K for 1 min. The measured quantities of desorbing O2 molecules are given in percentages of a monolayer, with 1 ML being the density of the (1 × 1) unit cells on the TiO2(110) surface, namely, 5.2 × 1014 cm−2. O2 TPD spectra acquired on Pt(111) were used to calibrate the quantity of desorbing O2 molecules. Valence-band (VB) spectra were obtained using He I irradiation (21.21 eV) at 100−120 K directly after sample preparation (clean surface) and after the acquisition of the O2 TPD spectra. The VB spectra were calibrated such that the UPS intensities at ∼10 eV binding energy (BE) were identical in all spectra. This allowed a direct comparison of the VB spectra and the integrated areas of the GS. To account for spontaneous changes of the UV light intensity, the integrated areas of the GS

stepped TiO2 surfaces are characterized by smaller GSs than flat TiO2(110) surfaces and their valence-band shapes differ from that known for TiO2(110) because of the presence of many step edges and adstructures on the surface.

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EXPERIMENTAL METHODS All experiments were carried out in the same ultra-high-vacuum (UHV) system (SPECS), which consisted of a preparation chamber and an analysis chamber. These two chambers could be separated from each other by a gate valve. In both chambers, the base pressure was lower than 1 × 10−10 mbar. The UHV system was equipped with an Aarhus-150 scanning tunneling microscope (SPECS), a mass spectrometer for TPD experiments (Hiden), a helium discharge lamp (Thermo Fischer), an X-ray source (SPECS), and a hemispherical energy analyzer (SPECS, PHOIBOS). Furthermore, this UHV system was equipped with a directional doser containing a 10-μm glass capillary array disk of ∼8-mm diameter (for O2 exposures), as well as standard facilities for sample cleaning and preparation. A flat rutile TiO2(110) sample and two TiO2(110) samples that were miscut by ∼3° either along or perpendicular to the [001] direction were investigated, all of which originated from the same batch (SurfaceNet). Through the 3° miscut, vicinal TiO2(110) surfaces were obtained: (i) TiO2(771) with a high density of [111̅ ] and [111̅ ]̅ step edges and (ii) TiO2(870) with a high density of [001] step edges. All three TiO2 single crystals were identically prepared by continuous cycles of Ar+ sputtering (10 min, 5 × 10−7 mbar, 1.5 keV, 10 mA) and 15 min of 3621



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needed to be ion-sputtered and then subjected to vacuum annealing. As mentioned above, for the three samples compared in Figure 1, mostly two cycles of sputtering/ annealing were conducted before the next set of experiments was initiated. TPD Studies. Even though one cannot determine directly from the presented STM data how much of the adsorbed O2 molecules had dissociated and further reacted with the three TiO2 surfaces considered, one can infer from our O2 TPD experiments (cf. Figure 2) how much O2 did not react with the

were normalized by dividing by the corresponding VB area. A GS area of 100% is defined as the highest measured GS area obtained on the TiO2(110) surface after ∼80 preparation cycles.

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RESULTS STM Studies. Figure 1 presents STM images of the two vicinal TiO2 samples, TiO2(870) and TiO2(771), together with STM images acquired on the flat TiO2(110) sample. STM images of the three clean TiO2 surfaces are shown in Figure 1a−c, whereas Figure 1d−f summarizes respective STM images obtained after O2 exposure at 100 K followed by heating to 500 K. A cyclic color scale with a periodicity of one atomic layer on the TiO2(110) surface (i.e., ∼0.33 nm) was used so that all terraces appeared with the same color gradient. On the terraces, alternating rows of 5-fold-coordinated Ti and bridging oxygen O (Obr) atoms in the [001] direction appear bright and dark, respectively.5,7,26 The bright spots within the Obr rows arise from Obr vacancies.26,35 Whereas the flat TiO2(110) surface is characterized by large terraces and only a few step edges (Figure 1a), the surfaces of the two miscut TiO2 crystals showed narrow terraces and high densities of step edges. Specifically, on the TiO2(870) surface, many [001] steps occurred (Figure 1b), whereas [11̅1] and [11̅1̅] steps dominated on the TiO2(771) surface (Figure 1c). Furthermore, in the [001] direction, elongated adstructures (strands) occurred on the TiO2(771) surface, as has been described previously.36 These strands are O-deficient and consist of an interconnection part between the strands and the [11̅1] and [11̅1̅] steps, a periodic center part, and the protruding end structure. The strands are a possibility to compensate for missing O atoms and, thus, are competitive to point defects on the TiO2(110) surface.36 After the reaction with O2 (Figure 1d−f), the abovedescribed morphologies of the three surfaces were still the same, namely, the TiO2(110) surface was characterized by extended terraces and a high step edge density, and narrow terraces dominated on the TiO2(870) and TiO2(771) surfaces. On the TiO2(771) surface, the strands appeared to be essentially unchanged through the O2 TPD experiments, that is, their average length and coverage did not change. The only apparent change after the O2 TPD experiments was that small islands appeared on the three surfaces. Because exclusively Ti and O peaks were evident in the corresponding photoelectron spectra (data not shown), these islands are assigned to TiOx, with x ≈ 2, which form upon reactions of out-diffusing Ti interstitials with oxygen species on the surface.4,8,9,11−13,27 The densities of the TiOx islands differed on the three surfaces, with the highest island density on the TiO2(110) surface, followed by the TiO2(870) surface and then the TiO2(771) surfaces. On the TiO2(771) surface, with the lowest density of TiOx islands, some islands appeared on top of the strands. As studied on another TiO2(110) crystal that was somewhat less reduced than the crystal used for our main studies (see Figures S1−S7, Supporting Information), annealing at temperatures higher than 500 K did not lead to the disappearance of the TiOx islands (see Figures S5−S7, Supporting Information), but the islands grew larger through agglomeration. At the same time, the island density decreased. Upon annealing at 520, 595, and 700 K, the distribution of the TiOx islands remained essentially unchanged, without any preference of the islands to be attached at the step edges. To restore a flat TiO2(110) surface without any TiOx islands, the TiO2(110) surface

Figure 2. (a) O2 TPD spectra obtained after exposing the three TiO2 surfaces to 200 L of O2 at 100 K. The spectra were acquired after the completion of ∼40 sample preparation cycles for each sample. (b) Integrated desorption peak areas (marked areas between 250 and 430 K in panel a) as a function of the number of preparation cycles. In panel b, note the break in the y axis.

surfaces. Figure 2a shows typical O2 TPD spectra acquired on the three surfaces after 200 L of O2 exposure at 100 K. Prior to the conduction of these O2 TPD experiments, the three TiO2 crystals had been sputtered and annealed ∼40 times. From the TiO2(110) surface, O2 desorbed at around 300−410 K (black curve), as reported previously.11,13,24,25 At the same temperatures, O2 molecules desorbed likewise from the two stepped surfaces (see the blue and green curves in Figure 2a). However, clearly less O2 desorbed from the two stepped surfaces than from the flat TiO2(110) surface. Compared to the TiO2(110) surface, the O2 peak areas on the TiO2(870) surface were ∼40%. On the TiO2(771) surface, even less O2 desorbed, namely, ∼10% compared to the integrated peak areas found on the (110) surface. Thus, the amount of O2 desorbing at around 300−410 K differed considerably for these samples with different surface morphologies. Whereas the O2 desorption peak at 300−410 K was particularly small on the TiO2(771) surface, a considerable amount of O2 molecules desorbed from this surface already between 100 and 300 K. As reported previously,11 the O2 molecules desorbing from flat TiO2(110) samples at these low temperatures cannot be ascribed as “background” O 2 desorption from the sample holder, but indeed originate from 3622



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Figure 3. UPS (He I) spectra of flat TiO2(110) and vicinal TiO2(870) and TiO2(771) surfaces acquired before (black, green, and blue curves) and after the O2 (red curves) TPD experiments. Spectra for different crystal reduction states are compared (see the numbers next to the spectra that indicate the numbers of preparation cycles). (a−c) Complete valence-band spectra. (d−f) Selected spectral regions of the TiO2 valence band at ∼5.5 eV (VB), the VBM, and the GS, respectively. Shifts of the VB and VBM of the (870) and (771) surfaces are highlighted by gray lines.

Figure 4. (a) UPS (He I) spectra of the three clean TiO2 surfaces (110), (870), and (771), respectively, acquired after the completion of ∼40 preparation cycles. The GS region (gray area) is enlarged by a factor of 50. Positions of the VBM and Fermi energy (EF) are indicated. (b) Integrated GS areas of the three clean TiO2 surfaces as a function of the number of preparation cycles (bulk reduction). (c) As in panel b but after the completion of the O2 TPD experiments. A GS area of 100% corresponds to the highest measured GS that was obtained on the TiO2(110) surface after the completion of ∼80 preparation cycles.

amounts of desorbed O2 remained nearly constant at low levels of ∼4% and ∼1% ML, respectively, irrespective of the sample history. Note that, in the case of the flat TiO2(110) surface with comparably high O2 peak areas of ∼8−13% ML, a maximum in O2 desorption appeared at ∼35−40 preparation cycles, in good agreement with previous results.13 UPS Studies. Selected UPS data (He I) acquired before (black, blue, and green curves) and after (red curves) the O2 TPD experiments as a function of the bulk reduction are summarized in Figure 3. Complete valence-band spectra

the sample surface. Accordingly, it is concluded that the O2 molecules desorbing between 100 and 300 K from the TiO2(771) surface also originated from the sample surface. On the TiO2(870) surface, the amount of O2 that desorbed between 100 and 300 K was smaller than that found on the TiO2(771) surface, followed by the flat TiO2(110) surface with only little O2 desorption at temperatures lower than 300 K. Figure 2b shows the total desorption peak areas as a function of the number of preparation cycles (integrated from 250 to 430 K). On the TiO2(870) and TiO2(771) surfaces, the 3623



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again showing that the GS is largest on rutile TiO2(110) with large terraces. The integrated GS areas are shown in Figure 4 as a function of the number of preparation cycles, both before (Figure 4b) and after (Figure 4c) the O2 TPD experiments were conducted. Compared to flat TiO2(110), the changes to the GS observed before and after the O2 TPD experiments were generally more pronounced on the two stepped TiO2 surfaces, particularly during the first ∼25 cycles of sputtering and annealing.

acquired on the three clean TiO2 surfaces are depicted in Figure 3a−c, whereas enlarged plots of the dominant valence-band peak (VB), valence-band maximum (VBM), and GS are shown in the corresponding panels at the bottom (see Figure 3d−f). The numbers next to the spectra indicate the numbers of preparation cycles conducted. The TiO2 valence band between the VBM at ∼3 eV and ∼9.5 eV BE is derived mainly from hybridization of O 2p and Ti 3d orbitals with peaks corresponding to different bonding and nonbonding states.20,37−41 Specifically, bonding states at rather high binding energies (peaks B and C in Figures 3 and 4) correspond to σ bonds, with the bonds being stronger the higher the BE. The peak at ∼5.5 eV (A) derives from π bonding, and electron emission from lower binding energies (∼3−4 eV, mostly seen as a shoulder at the dominating A peak) corresponds to nonbonding O electrons.37,39,40 The Ti 3d defect state at ∼0.9 eV BE originates from Ti3+ excess charge carriers in the near-surface region, and its area is a relative measure of the surface reduction state.9,13,20 The valence-band shapes of the three clean TiO2 surfaces considered were similar, but differences could be recognized. On the TiO2(110) surface (Figure 3a), a distinct peak A appeared at ∼5.5 eV BE, along with a smaller peak B at ∼7 eV. On peak B, a shoulder at ∼8 eV (C) can be seen. In the case of the TiO2(870) surface (Figure 3b), peak A is almost as distinct as on the (110) surface, but peak B appears less distinct because of the increased signal at C. In the case of the TiO2(771) surface (Figure 3c), peak A is less intense than on the other two TiO2 surfaces, and the signal at higher BE resembles the one for the (870) surface with two overlapping peaks B and C. The distinct GS on the (110) surface was larger than those on the two stepped surfaces. With ongoing sample preparation, the VB shape did not change considerably for any of the three samples considered. However, the GS increased continuously as a function of the increasing bulk reduction upon the application of cycles of sputtering and annealing. After the completion of the O2 TPD experiments, changes in the valence-band spectra with respect to the clean surfaces were recognized on the TiO2(870) and TiO2(771) surfaces (red curves in Figure 3b,c). First, for the first 25−30 cycles of sample preparation, the VB (including the VBM) shifted to lower BEs by ∼0.2 eV, whereas the valence-band spectra on the (110) surface did not shift at all (red curves in Figure 3a). Second, the GSs were quenched on the (870) and (771) surfaces after the completion of the O2 TPD experiments for low bulk reduction, which was not observed for the TiO2(110) surface. However, after the completion of more than 25−30 preparation cycles, the GSs were no longer quenched on the two vicinal surfaces. On the TiO2(110) surface, the O2 TPD experiments did not influence the GS in a measurable fashion. The shifts to lower BEs for the VBs observed simultaneously with the quenching of the GS on TiO2(771) and TiO2(870) can be explained by band bending. Generally, an O deficiency on surfaces leads to shifts of the valence bands to higher BEs,16,20 and hence, reoxidation of surfaces shifts the valence bands to lower BEs, as was observed on the TiO2(771) and TiO2(870) surfaces for low bulk reduction states. For direct comparison of the valence-band spectra, Figure 4a shows selected UPS spectra acquired on the three clean TiO2 surfaces after the completion of ∼40 cycles of sputtering and annealing. This direct comparison shows the above-described differences of the VB shapes on the compared surfaces even better. In addition, a direct comparison of the GSs is possible,

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DISCUSSION STM Data. It is well-known that O2 molecules adsorb on the reduced TiO2(110) surface both dissociatively and molecularly at 100 K and that Ti interstitials react with oxygen species with increasing temperature in TPD experiments.9,11−13 As a result, small TiOx islands formed on the terraces during the O2 TPD experiments, as can be seen in Figure 1d. On the (870) and (771) surfaces, the same kind of islands were found (Figure 1e,f), indicating that the O2−TiO2 interaction scheme on these two surfaces is similar to that on the well-studied, flat TiO2(110) surface. However, the densities of the TiOx islands on the stepped surfaces were smaller than that on the (110) surface, with the (771) surface exhibiting the smallest number of TiOx islands. Because more excess charge was available on the (110) surface than on the two stepped TiO2 surfaces (cf. Figures 3 and 4), it can be assumed that a larger quantity of oxygen species was adsorbed on the (110) surface, particularly on the large terraces. However, it can also be speculated that the diffusion of Ti interstitials to the surface is easier beneath flat terraces than in the proximity of step edges. Both possible reasons (or a combination of the two) would explain why the density of TiOx islands was largest on TiO2(110). Nevertheless, the fact that some TiOx islands also occurred on the strands on the (771) surface (cf. Figure 1f) indicates that oxygen species also adsorbed on the O-deficient strands. The adsorption of O2 molecules on the strands is a possible explanation for the large quantity of O2 molecules desorbing from TiO2(771) at temperatures lower than 300 K (cf. green curve in Figure 2). TPD Data. It is interesting that the TiO2(110) surface with the highest density of TiOx islands also showed the highest O2 desorption peak at ∼400 K (cf. Figure 2). The two stepped surfaces, however, showed significantly less O2 desorption at ∼400 K. The O2 desorption between 300 and 400 K was particularly small on the TiO2(771) surface. Because the peak integrals scale roughly with the observed areas of flat terraces on the three surfaces considered, it can be argued that the flat surface areas are decisive for the O2 desorption peak at ∼400 K. This interpretation implies that less O2 desorbed from the TiO2(870) and TiO2(771) surfaces because of the smaller amounts of available excess charge (cf. Figures 3 and 4). If this were indeed the case, it would mean that fewer oxygen species were adsorbed on the TiO2(870) and TiO2(771) surfaces than on the TiO2(110) surface. However, it is also possible that similar amounts of oxygen species were adsorbed on the three surfaces and that more of the adsorbed O2 molecules dissociated on the TiO2(870) and TiO2(771) surfaces and finally reacted with out-diffusing Ti interstitials. However, in that case, it is expected that more TiOx islands appear on the two stepped TiO2 surfaces than found by STM (cf. Figure 1e,f). Instead, fewer TiOx islands were found on the TiO2(870) and TiO2(771) surfaces than on the TiO2(110) surface, pointing to the former interpretation that less O2 desorbed from the vicinal TiO2 surfaces because of the smaller amounts of available 3624



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and (771) surfaces were characterized by about the same density of step edges, peak C was markedly more intense on the (771) surface. As mentioned above, strands on the (771) surface consist of additional undercoordinated, 4-fold Ti atoms.36 The Ti atoms in these strands might lead to an enhanced number of σ bonds with the O atoms on the surface than is the case on flat TiO2(110). Accordingly, it is proposed that peak C in the VB of the (771) surface originated from bonding orbitals at the strands. At the same time, the decreased intensity at 5.5 eV (peak A) can be explained by strands covering a relatively large portion of the (771) surface. UPS is a surface-sensitive technique42 with an inelastic mean free path for TiO2 of less than 1−2 nm.43 Thus, only electrons from the first three to five atomic layers were detected. Because the strands grew on top of the terraces, they “blocked” the terrace sites. Thus, the contribution of in-plane and bulk Ti atoms to the valence-band spectra of the (771) surface was smaller than that for the (870) surface. It appears that the shape of the valence band on the (771) surface is largely determined by the presence of strands rather than by the step edges. Whereas the shapes of the valence bands were rather invariant with increasing bulk reduction, the GS increased with ongoing preparation for all three samples (cf. Figure 4b,c). Interestingly, the GSs on the (870) and (771) surfaces were systematically smaller than on the (110) surface, disregarding the bulk reduction, which we believe was comparable for all three TiO2 crystals. Accordingly, a distinction between surface reduction and bulk reduction is necessary. Surprisingly, stepped surfaces are characterized by a smaller surface reduction throughout the experiments, even though they had a high density of step edges and, in the case of the (771) surface, a large amount of reduced adstructures (i.e., strands). It could be assumed that strands with undercoordinated atoms (4-foldcoordinated Ti atoms) and step edges donate additional excess charge, leading to an enhanced overall GS. However, the presence of step edges and strands did not lead to large GSs. In fact, the overall GSs on the (870) and (771) surfaces were always smaller than that found on the corresponding (110) surface. Thus, the contributions to the GS from the steps and strands must be smaller than that of the terraces and the nearsurface region beneath them. Accordingly, it is concluded that most of the Ti3+ excess charge originated from the terraces and the near-surface regions beneath them. This conclusion is consistent with previously published results from the Aarhus group9,11−13 and a recent electronic structure characterization of the rutile TiO2 (110)−(1 × 2) surface,44 wherein the contribution of the Ti atoms present at the surface reconstruction in the (1 × 2) surface to the GS was found to be much smaller than that originating from the near-surface region. Another interesting UPS result is that the GSs on the (870) and (771) surfaces were quenched during the first 25−30 preparation cycles after the completion of the O2 TPD experiments (see Figures 3e,f and 4b,c). By dosing of O2 at ∼100 K followed by annealing at 500 K, these surfaces could be reoxidized. All of the excess charge was used to form TiOx islands and adstructures, leading to almost completely stoichiometric surfaces, and brief annealing at 500 K was not sufficient to reduce the surfaces again. In contrast, on the flat TiO2(110) surface, the GS was completely restored after the completion of the O2 TPD experiments (Figure 3d). Probably, for the TiO2(110) surface, following the diffusion and reaction of the Ti interstitials within the O2 TPD experiments, new

excess charge. Nevertheless, for the stepped surfaces, the possibility cannot be completely excluded that the out-diffusing Ti interstitials reacted with oxygen species at the step edges, leading to new TiOx units directly attached at the step edges. In this case, the small O2 desorption peaks at ∼400 K on the TiO2(870) and TiO2(771) surfaces resulted because more of the adsorbed species had reacted off during the TPD experiment. All together, it can be concluded that the flat areas on the TiO2 surfaces are decisive for the occurrence of the O2 desorption peak at ∼400 K, allowing the adsorption of most oxygen species, because this interpretation is most consistent with the experimental data and most straightforward. Furthermore, this conclusion is supported by previous STM and TPD data acquired on differently prepared TiO2(110) surfaces that were characterized by different step densities.11 The important effects of the surface morphology on the O2 desorption peak at ∼400 K is also apparent from the data presented in Figures 2b and 4b. As is evident from Figure 2b, the amounts of desorbing O2 molecules differed considerably for the samples with different morphologies but did not depend much on the bulk reduction state. Because the defect state increased with increasing bulk reduction (Figure 4b), one could expect that the total amounts of desorbing O2 molecules would scale with the reduction state of the sample. However, this was barely the case. What counteracts the desorption of more O2 molecules from more strongly reduced TiO2 crystals is the enhanced reactivity of Ti interstitials with the oxygen species.12,13 Thus, even though more oxygen species can be stabilized on the surfaces of strongly reduced TiO2 crystals compared to those of less reduced crystals (at 100 K), this does not necessarily mean that more O2 molecules desorb at ∼400 K in the TPD experiments. In fact, a maximum of the O2 desorption peak area was evident on TiO2(110) at medium reduction states, namely, 35−40 preparation cycles. Apparently, an optimum reduction state exists at which the adsorption of oxygen species at 100 K is high but the reactivity of oxygen species with the Ti interstitials upon increasing temperature is not high enough to react off all of the additional O2 molecules. Such an optimum reduction state at which the O2 desorption at ∼400 K is largest has been observed previously.12,13 However, in the present study, the maximum in O2 desorption on TiO2(110) was less pronounced than in the previous studies by Lira et al. To understand this difference, it should be noted that the preparation procedures in the two considered experiments were different. Whereas Lira et al. increased the annealing temperature from 810 to 980 K for subsequent preparation cycles, the annealing temperature in the present study was kept constant at (910 ± 10) K. Because the TiO2(110) samples by Lira et al. were initially less reduced, the maximum appeared more pronounced than in the present experiments. However, in both set of experiments, the largest O2 desorption integrals around ∼400 K were found at medium bulk reduction corresponding to ∼40 preparation cycles. UPS Data. Considering that the atoms at step edges have bonding conditions clearly different from those atoms on the terraces, it is not surprising that the different surface morphologies led to different shapes of the UPS spectra (cf. Figures 3 and 4a). This seems to be the main reason for the increased emission at ∼8 eV (C peak) for the (870) and (771) surfaces compared to the (110) surface. The undercoordination of Ti atoms at step edges might lead to stronger bonding to O atoms compared to in-plane, 5-fold Ti atoms on the surface and 6-fold Ti atoms in the bulk. However, even though the (870) 3625



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surfaces compared to the flat TiO2(110) surface. Especially the TiO2(771) surface showed a very small O2 desorption peak. UPS measurements acquired before and after the O2 TPD experiments displayed that the GSs on the stepped TiO2(870) and TiO2(771) surfaces were significantly smaller than that on the flat TiO2(110) surface. Additionally, it was revealed that the two stepped TiO2 surfaces can be reoxidized for low bulk reduction states. The latter was not the case for the flat TiO2(110) surface. In the case of reoxidation, the valence bands shifted slightly to lower BEs as a result of band bending. After about 25 preparation cycles, this ability to reoxidize the TiO2(870) and TiO2(771) surfaces vanished, and the reduction state no longer changed through the O2 TPD experiments. These results underline the large contribution of Ti3+ excess charge from the near-surface region to the GS. Specifically, it appears that the presence of large terraces is clearly advantageous for a large GS on TiO2(110). Furthermore, the UPS data revealed that the presence of step edges and strands results in changes in the valence-band shapes by enhanced O 2p−Ti 3d bonding features. These features in the valence band became more dominant because of an increase of the number of undercoordinated Ti atoms at step edges and in strands. Clearly, the presence of step edges and adstructures such as strands influences the O2−TiO2 interaction strongly; however, it appears that step edges on rutile TiO2(110) are not beneficial for enhancing the O2−TiO2(110) interaction. This result can be understood considering the fact that stepped TiO2(110) surfaces are characterized by smaller GSs than flat ones.

excess charge carriers diffused from the bulk to the near-surface region beneath the terraces, and thus the GS was restored. In contrast, on the TiO2(870) and TiO2(771) surfaces, the surface reduction might not be high enough to induce immediate diffusion of excess charge carriers to the near-surface region underneath step edges and terraces at temperatures lower than 500 K. After the completion of 25−30 preparation cycles, the GS was no longer quenched on the TiO2(870) and TiO2(771) surfaces as well. At this stage, the surface reduction on the TiO2(870) and TiO2(771) samples reached a level that was comparable to the one for the TiO2(110) surface after very few preparation cycles. Thus, it can be speculated that the diffusion of Ti interstitials from the bulk to the near-surface region is more facile on such surfaces that reached a certain level of reduction. Comparison with Other Molecules. From the presented data, it appears that step edges on rutile TiO2(110) are not advantageous for enhancing the O2−TiO2(110) interaction. This rather unexpected result is related to the fact that Ti3+ excess charge is required for the adsorption of oxygen species, and the stepped TiO2(870) and TiO2(771) surfaces are characterized by smaller GSs than the flat TiO2(110) surface. However, the O2 TPD data obtained on the TiO2(771) surface indicate that, most likely, O2 molecules were stabilized on the strands, because the desorption of O2 molecules at low temperatures (between 100 and 300 K) was clearly larger on this surface than on the TiO2(110) and TiO2(870) surfaces. Nevertheless, all together, it appears that step edges on TiO 2 (110) are not beneficial for the O 2 −TiO 2 (110) interaction. This is different from H2O−TiO2(110) and ethanol−TiO2(110) interactions, for which recent studies showed that step edges are important.33,34 Specifically, for water33 and ethanol,34 the [111̅ ] and [111̅ ]̅ step edges were found to be essential for a complete understanding of the interactions. This difference between the O2−TiO2(110) interaction and the H2O−TiO2(110) and ethanol−TiO2(110) interactions can be rationalized considering that the Ti3+ excess charge has a negligible influence on the adsorption of water and ethanol, whereas Ti3+ excess charge is essential for the adsorption of oxygen species.

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ASSOCIATED CONTENT

* Supporting Information S

Additional STM data showing the evolution of a mildly reduced TiO2(110) surface (Figure S1) upon O2 exposure at 108 K (Figure S2), followed by vacuum annealing at 393 K (Figure S3), 450 K (Figure S4), 520 K (Figure S5), 595 K (Figure S6), and 700 K (Figure S7). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: ++45/87156731.

CONCLUSIONS The interaction of O2 with vicinal TiO2(110) and flat TiO2(110) surfaces was studied by means of STM, TPD, and UPS. Intentionally miscut (by ∼3°) TiO2(110) surfaces were compared that were characterized by high densities of [001] step edges [TiO2(870) surface] and [11̅1] and [11̅1̅] step edges [TiO2(771) surface]. All three samples were prepared in exactly the same way to ensure identical bulk reductions. The STM studies revealed differences in the surface morphologies. Both the TiO2(870) and TiO2(771) surfaces were characterized by high step edge densities and narrow terraces compared to the TiO2(110) surface. Additionally, the TiO2(771) surface was characterized by a large number of reduced adstructures growing along the [001] direction (strands). As is known for flat TiO2(110) surfaces, TiOx islands grew as a result of O2 exposure at 100 K and subsequent annealing to 500 K. The growth of TiOx islands occurred on the TiO2(870) and TiO2(771) surfaces as well. However, fewer TiOx islands appeared on the two stepped TiO2 surfaces than on the TiO2(110) surface. The O2 TPD experiments revealed that significantly less O2 desorbed between 300 and 410 K from the TiO2(870) and TiO2(771)

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Danish Research Agency, the Strategic Research Council, the Villum Kahn Rasmussen Foundation, the Lundbeck Foundation, the Carlsberg Foundation, Haldor Topsøe, and the European Research Council through an Advanced ERC grant (F.B.).

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REFERENCES

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Vicinal Rutile TiO2 Surfaces and Their Interactions with O2 - American

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