Understanding the Intrinsic Chemical Activity of Anatase TiO2(001)-(1

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Understanding the Intrinsic Chemical Activity of Anatase TiO(001)-(1×4) Surface 2

Haoqi Tang, Zhengwang Cheng, Shihui Dong, Xuefeng Cui, Hao Feng, Xiaochuan Ma, Bin Luo, Aidi Zhao, Jin Zhao, and Bing Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12917 • Publication Date (Web): 27 Dec 2016 Downloaded from http://pubs.acs.org on January 6, 2017

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The Journal of Physical Chemistry

Understanding the Intrinsic Chemical Activity of Anatase TiO2(001)-(1× ×4) Surface

Haoqi Tang, Zhengwang Cheng, Shihui Dong, Xuefeng Cui*, Hao Feng, Xiaochuan Ma, Bin Luo, Aidi Zhao, Jin Zhao, Bing Wang*

Hefei National Laboratory for Physical Sciences at the Microscale and Synergetic Innovation Center of Quantum Information & Quantum Physics, Key Laboratory of Strong-Coupled Quantum Matter Physics (CAS), University of Science and Technology of China, Hefei, Anhui 230026, China

*Correspondence should be addressed to [email protected] and [email protected]

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ABSTRACT: We report our investigation on the intrinsic chemical activity of the anatase TiO2(001)-(1×4) reconstructed surface, using epitaxially grown anatase TiO2(001) thin films and using methanol molecules as a probe, characterized by combining scanning tunneling microscopy and temperature programmed desorption. Our results provide direct evidence that the perfect (1×4) lattice sites of the surface are intrinsically quite inert for the reaction of methanol. We obtain that the activation energy for desorption of molecular methanol is about 0.55-0.64 eV, which is in good agreement with our first-principle calculations based on the structural model with five-fold coordinated Ti atoms at the ridges of (1×4) reconstruction. We find that two types of defect sites, i.e., reduced Ti pairs and partially oxidized Ti pairs, are responsible for the chemical activity of the surface, evidenced by the desorption of water due to the dehydrogenation of methanol at the defect sites. The methoxy left at the reduced Ti-pair sites further produce CH3 radical, and the methoxy near the partially oxidized Ti-pair sites produce formaldehyde and methanol through disproportionation reaction. The determination of these intrinsic properties can be important to understand the conflicting results from this surface in literatures and thus to reveal the actual reaction mechanisms.

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1. INTRODUCTION

TiO2 has been the most investigated catalyst or photocatalyst for its potential applications in water splitting and degradation of organic pollutants.1-7 Theoretical calculations suggested that the anatase (001) surface should be the most reactive one among various surfaces in different TiO2 polymorphs.8-13 For the fundamental understanding of the activity, the anatase TiO2(001) surface from mineral crystals,14-16 thin films,17-24 and chemically synthesized nanoparticles25-31 have been experimentally studied. Even with these efforts, the activity of the anatase (001) surface is still under debate due to the obviously conflicting experimental results.16,18, 21,25-28 Some recent studies also showed that some other factors should be considered, such as the transport properties of charge carriers in bulk24 and the difference in the electronic states of different surfaces.32 As for the (001) surface itself, the situations could be caused by the different samples used in the experiments, where the sample surfaces and/or the bulk could already be different. For instance, impurity elements in a natural mineral anatase crystal33 can be an imaginable factor to cause the samples deviating from the intrinsic one. To clearly understand the intrinsic activity of the (001) surface, a clean and well-controlled sample is needed. Epitaxial growth can be an ideal way to grow high quality and nearly contamination free single-crystal anatase TiO2 (001) films,17-24 which should be an appropriate sample to investigate the intrinsic activity of the surface, and more, to identify its active sites. Because of its high surface energy, the (001) surface favors a (1×4) reconstruction.2,14-24,30 Several models have been proposed to describe the structure of the (1×4) reconstruction, which are consistent with the images from scanning tunneling microscopy (STM)8,15,17,18,21 and transmission electron microscopy.30 However, it is still unsettled which structural model can satisfactorily be correlated to the intrinsic chemical activity of the surface, 3



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because the intrinsic properties of the surface are not yet well understood. Among the different models, the “ad-molecule” model (ADM) was suggested to be highly reactive for water and formic acid from density functional theory (DFT) calculations.8,13 In contrast, some recent experimental results indicated that no spontaneous dissociation of water was observed on the reconstructed anatase (001) surface,21 and the photocatalytic activity of anatase (001) surface is not prior to other rutile and anatase surfaces.18,25,27 To explain the experimental observations, we have proposed an ad-oxygen model (AOM) by modifying the ADM model,21 where the Ti atoms are five-fold coordinated (Ti5c) at the ridges, instead of the four-fold coordinated Ti atoms (Ti4c) in the ADM model. However, from the DFT calculations, it is argued that the AOM model is energetically less favored than the ADM model under the typical parameters of temperature and O2 pressure used in the experiment.30 The correctness of these models needs to be judged on the basis of a better understanding of surface properties, especially its intrinsic chemical activity. Therefore, it is of importance to well determine the intrinsic chemical activity of the reconstructed anatase TiO2(001)-(1×4) surface. In this work, using methanol molecules as a probe, we study the activity of the anatase TiO2(001)-(1×4) surfaces subject to different treatments and characterized by means of scanning tunneling microscopy (STM) and temperature programmed desorption (TPD). We find that the perfect (001)-(1×4) surface (as-grown sample) is quite inert for methanol, as well as for water. The activity can be enhanced after the surface is reduced and/or re-oxidized. Two types of active sites are identified. One is the characterized Ti-rich defects, i.e., Ti pairs,21 and the other is ascribed to O-bridged Ti pairs due to the partial oxidation of the paired Ti atoms. Our results show that the methanol molecules can thermally produce methyl radical or formaldehyde at these two types of defects, respectively. 4


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2. EXPERIMENTAL SECTION

2.1 Sample Preparation. The anatase TiO2(001) thin films were grown in an ultrahigh-vacuum (UHV) multifunctional system, equipped with a pulsed laser deposition (PLD) chamber (1 × 10−10 mbar), a low temperature STM chamber (3 × 10−11 mbar, Omicron) and an x-ray/ultraviolet photoemission spectroscopy (XPS/UPS) analysis chamber (8 × 10−11 mbar, VG Scienta). A 0.7 wt% Nb-doped SrTiO3(001) substrate, with a typical size of 10 × 4 × 0.5 mm3, was used for epitaxial growth of the anatase TiO2(001) thin films by PLD with a KrF excimer laser (Coherent, 248 nm, operated at a repetition rate of 4 Hz and a pulse duration of 20 ns with an output power of ∼200 mJ/pulse for deposition). During the deposition of TiO2, the substrate temperature was kept at 920 K with an O2 pressure of 1.5 × 10−5 mbar, and slowly cooled to room temperature by keeping the O2 pressure.21,34 A lightly reduced sample was prepared by 1 keV Ar+ ion sputtering for 5 min, followed by annealing in vacuum (2 × 10−10 mbar) at 900 K for 20 min. A heavily reduced sample was prepared by about 10 cycles of Ar+ ion sputtering (1 keV, 5 min) and vacuum annealing (900 K, 20 min). A re-oxidized sample was further prepared from a heavily reduced sample by annealing it under O2 pressure of 1.5 × 10−5 mbar either at 700 or 1000 K for 20 min. In comparison, we also had an as-grown sample just annealed in vacuum at 900 K for about 9 h. 2.2 STM Characterization. The adsorption of methanol on the samples after different treatments was characterized using STM at 80 K, by transferring the sample to the STM chamber without breaking the vacuum. The STM images were all recorded in the constant-current mode using the sample bias with respect to the tip. During the dosing of methanol, the sample was kept on the cryostat of the microscope, and the STM tip was retracted about 10 µm from the surface to avoid shadowing 5



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effect. 2.3 TPD Measurements. We conducted the TPD measurements using a quadrupole mass spectrometer (QMS, MAX-120, Extrel) in an independent vacuum system with a base pressure of 1.8 × 10−11 mbar, equipped with low-energy electron diffraction and Auger electron spectrometer (LEED/AES, SPECTALEED, Omicron). The cleanness and the quality of the sample surface was examined by LEED/AES. The Auger spectra were recorded in dN/dE mode with primary electron energy of 3 keV and 1 Vp-p modulation voltage. For the TPD measurements, prior to the TiO2 deposition, a K-type thermal couple (0.12 mm in diameter) had been glued at the top surface of the SrTiO3(001) substrate within a spot size of about 1 mm2 using high temperature ceramic adhesive (Ceramabond 552, Aremco). The prepared anatase TiO2(001) thin films used in the TPD measurements had the same quality as the ones used in the STM measurements, subject to the same growth conditions and confirmed by the STM and the XPS/UPS characterizations before transferred for TPD measurements. During the transferring, the sample was kept in N2 atmosphere to minimize the contamination (Supporting Information Figure S1). In TPD measurements, the sample temperature was programmed with a constant rate of 1 K/s. The sample was heated by a Ta foil heater mounted behind the sample. An extended tube with a length of about 5 mm and an inner diameter of 3 mm was mounted in front of the QMS. During TPD measurements, the tube was perpendicular to the sample surface and the tube inlet was away from the surface by about 1 mm. Using this method, we could considerably enhance the signal to noise ratio in the TPD spectra. For methanol dosing, we used H2O, CH3OH (>99.9%, Riedel-de Haën), CH3OD and CD3OD (>99.5%, Sigma Aldrich). CH3OH was used in the STM measurements, and CH3OD and CD3OD were 6


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mainly used in the TPD measurements for a better tracking of the products. The methanol molecules were purified by several freeze−pump−thaw cycles to further remove dissolved gases, and were dosed via a leak valve connected with a dedicated tube that conducted the dosed methanol molecules to the sample surface as close as about 3 mm. For methanol dosing, we adjusted the valve to have the pressure increased by only 2 × 10−12 mbar (typical background pressure of 2~5 × 10−11 mbar). Noticed that this increased pressure roughly reflected the partial pressure of methanol in the vacuum, but could not give the actual amount of methanol exposure to the sample surfaces. The lasting time was in the range of 5 to 200 sec, to obtain various coverages. In this way, we benefited from the rapid recovering of the vacuum and keeping the pressure lower than 5.0 × 10−11 mbar even after many cycles of repeated methanol dosing in the TPD chamber. The coverages of methanol on the surface were counted by the STM images or estimated by integrating the TPD peaks. During methanol exposure, the samples were kept at 80 K (for STM) or 120 K (for TPD), respectively.

3. RESULTS AND DISCUSSION 3.1 TPD Spectra from the As-Grown, Reduced, and Re-Oxidized Surfaces. Figure 1 shows the TPD spectra of methanol (CH3OD) obtained from the anatase TiO2(001)-(1×4) surfaces subject to different treatments. We first consider the TPD signal of mass-to-charge ratio m/z = 33 (CH3OD+). It is seen that the spectra of signal 33 are basically very similar in these samples, except the much pronounced shoulder peak around 290 K in Figure 1d and 1e. The spectra in Figure 1b shows the results obtained from the as-grown sample that was just annealed at 900 K for 9 h (5 × 10−10 mbar) without Ar+ sputtering. It is seen that such a treatment does not obviously affect the spectra of signal 33. 7



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For comparison, we prepared a lightly reduced sample from an as-grown sample by a single cycle of Ar+ ion sputtering (1 keV for 5 min) and annealing (2 × 10−10 mbar, at 900 K for 20 min) and a heavily reduced sample by over 10 cycles of the sputtering and annealing treatments. The shoulder peak at around 290 K is visible in the lightly reduced sample (Figure 1c), and becomes much pronounced in the heavily reduced sample (Figure 1d). The re-oxidized samples were prepared from a heavily reduced sample by annealing under an O2 partial pressure of 1.5 × 10−5 mbar at 700 and 1000 K, respectively, for 20 min (Figure 1e and 1f). A shoulder peak around 290 K is still visible after annealing at 700 K (Figure 1e), but it almost disappears after annealing under O2 pressure at 1000 K (Figure 1f). In all of these samples, the peaks from monolayer adsorption shows a first-order desorption behavior. From the coverage-dependent peaks for the TPD signals of m/z = 33 varying from 200 to 230 K, we obtain the activation energy for methanol desorption in the range of 0.55-0.64 eV, estimated from the first-order desorption model.35 The experimental values are consistent with the calculated adsorption energy of 0.55 eV, which is obtained from our calculations for a methanol molecule at the Ti5c ridge site using a (4 × 4) unit cell based on the AOM slab model, similar to the calculations for water.21 It is noticed that the peak from monolayer adsorption of water in our TPD spectra centers at 180 K (Supporting Information Figure S2), corresponding to the activation energy of 0.49 eV for water desorption, which is also consistent with the calculated adsorption energy of 0.45 eV for water.21 These results strongly indicate that the AOM model can well describe the adsorption behaviors of methanol and water on the anatase TiO2(001)-(1×4) surface, in contrast to the unproved prediction of the spontaneously dissociative adsorption of water based on the ADM model.8,9 In addition to signal of m/z = 33, we simultaneously recorded other signals of m/z = 29, 20, and 15, 8


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which correspond to CHO+, D2O+, and CH3+, respectively. No obvious peak shows up above 300 K in these signals measured from the as-grown sample (Figure 1a). It means that methanol desorbs through the molecular form below 300 K in the as-grown sample, which is also confirmed by the measured relatively intensities of the various fragments in our experiment. However, obvious features appear in signals 29, 20, and 15 in the reduced and the re-oxidized surfaces. In signal 20, a tiny peak centered at 210 K is seen in the sample just by vacuum annealing at 900 K for 9 h (Figure 1b), while double peaks with varied relative heights can been seen in other samples (Figure 1c-f), where the peak at 240 K is typically more pronounced than that at 210 K. Some differences in signals 15 and 29 can also been seen between the reduced and the re-oxidized samples. In the reduced samples, a peak occurs in signal 15 in the range of 525-600 K, showing a certain dependence on the reduction degree (Figure 1c and 1d), while there is still no peak in signal 29. In the re-oxidized samples, a peak centered at 635 K occurs in both signals 15 and 29 (Figure 1e and 1f). It is noticed that in each panel these concerned peaks in signals 15, 20, and 29 are almost overlapped, respectively, in four measurements at various methanol coverages, reflecting their independence on the coverages of methanol. Such information strongly suggests that the products corresponding to these overlapped peaks are correlated to a certain well-defined number of active sites, which can be associated with the defect sites but not the perfect lattice sites, by considering the fact that the as-grown sample does not produce such species (Figure 1a). It is also noticed that the TPD signals do not show obvious dependence on the density of the step edges. In our experiment, the density of the step edges in the as-grown sample is typically higher than that in the reduced and the re-oxidized samples. Such observations indicate that the step edges should not be the active sites to cause the 9



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difference in the TPD spectra among the surfaces. In the reduced sample (Figure 1c and 1d), the peak in signal 15 can be assigned to CH3 radical, since no peak was observed in higher m/z signals in this temperature range, excluding the possibility that it may be from the fragments of other species. However, in the re-oxidized samples (Figure 1e and 1f), the peak at 635 K in signal 15 is related to the species of m/z = 29. As we will discuss below, the peaks at 635 K in signals 15 and 29 can be ascribed to the products of formaldehyde and/or methanol from the disproportionation reaction of methoxy, similar to the process in rutile TiO2(110) surface.41,42 The peaks in signal 20 can be assigned to water, whose peak positions are consistent with the TPD results from the water exposure on these surfaces (Supporting Information Figure S2).

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Figure 1. TPD spectra of m/z = 33 (CH3OD+) and 20 (D2O+) by using CH3OD from anatase TiO2(001)-(1×4) surfaces, (a) as-grown surface by degassing at 900 K for 1 h, (b) as-grown surface by annealing at 900 K for 9 h, (c) lightly reduced surface by a single cycle of Ar+ ion sputtering and annealing, (d) heavily reduced surface by over 10 cycles of Ar+ ion sputtering and annealing, (e) re-oxidized surface annealed under O2 at 700 K, (f) re-oxidized surface annealed under O2 at 1000 K. We define that the spectrum with a relatively symmetric peak in (b) corresponds to 1 monolayer (ML), and the spectrum is reproduced in each panel (dashed red curve) for comparison. The insets show the 11



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simultaneously acquired signals 33 (CH3OD+), 29 (CHO+), and 15 (CH3+) in 4 measurements with various coverages. In the TPD measurements, a rate of 1 K/sec was used.

3.2 Surface Structures of the Samples after Different Treatments. Figure 2 shows the images of the bare surfaces subject to the different treatments corresponding to the conditions used in the TPD measurements. In Figure 2a and 2b and their insets, the dark spots and the bright spots in the as-grown and the reduced surfaces can be assigned to the defects of “TiO2” vacancies (dV) and reduced Ti-atom pairs (dR-Ti) in the ridges of the (1×4) reconstructed structure, which have been characterized in our previous work.21 After a heavily reduced surface was annealed under O2 pressure of 1.5 × 10−5 mbar at 1000 K for 20 min, the surface just leaves dark spots in the ridges imaged at a relatively high bias voltage of 1.6 V (Figure 2c). However, it is noticed that the dark spots in the re-oxidized surface could have a structure different from the one in the as-grown surface. As shown in Figure 2d and 2e acquired within the same area, while some of the dark spots do not vary with the change of bias voltage from 1.6 V to 1.2 V (marked by the white arrows), some other dark spots at 1.6 V turn out to be dim protrusions at 1.2 V. The unchanged dark spots can be assigned to the same defects of “TiO2” vacancies (dV), as those in the as-grown sample surface.21 While, considering the re-oxidized surface from the treatment of the reduced surface under a partial pressure of O2, we suggest that the voltage-dependent defects in the images could be from the oxidation of the Ti pairs, and we thus propose these partially oxidized defects as O-bridged Ti pairs (dO-Ti), as shown in Figure 2h. In the re-oxidized surfaces annealed under O2 pressure of 1.5 × 10−5 mbar at a lower temperature of 700 K, it is found that the reduced defects (dR-Ti in bright spots) may still coexist (Figure 2f and 2g). 12


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Figure 2. STM images of bare surfaces of anatase TiO2(001)-(1×4), (a) as-grown surface (70 × 70 nm2, and inset: 10.1 × 10.1 nm2), (b) reduced surface (41 × 41 nm2, and inset: 10.2 × 10.2 nm2), and (c) re-oxidized surface (50 × 50 nm2, and inset: 13.3 × 12.7 nm2). (d) and (e) Voltage-dependent images of the re-oxidized surface (13.3 × 12.7 nm2) annealed under O2 pressure of 1.5 × 10−5 mbar at 1000 K. (f) and (g) Voltage-dependent images of re-oxidized surface (18.1 ×15.8 nm2) annealed under O2 pressure of 1.5 × 10−5 mbar at 700 K. (h) Proposed structural model of the partially oxidized defect of dO-Ti (lower) from the reduced defect of dR-Ti (upper).

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3.3 Adsorption of Methanol on Defective Surfaces Characterized by STM.

Figure 3. (a) Images showing the adsorption of methanol at the dR-Ti sites in the reduced surface, image size: 6.3 × 4.5 nm2, imaged at 1.5 V and 10 pA; Upper panel: before methanol exposure, middle panel: after methanol exposure and followed by scanning at 2.0 V and 10 pA, and lower panel: schematically structural model of dissociative methanol at the dR-Ti site. The green arrows mark the methanol adsorption at the dR-Ti sites, and the white arrows mark the dV site. (b) Images showing the adsorption of methanol near the dO-Ti sites in the re-oxidized surface, image size: 5.8 × 4.1 nm2, imaged at 1.2 V and 10 pA; Upper panel: before methanol exposure, middle panel: after methanol exposure, and lower panel: schematically structural model of paired molecular methanol at the Ti5c sites in both sides of the dO-Ti site. (c) Line profiles corresponding to the lines 1-4 in (a) and (b). The measurements were performed at 80 K.

Figure 3 shows the typical adsorption behaviors with a low methanol coverage performed at 80 K. In the reduced sample (Figure 3a), the methanol molecules adsorb at the dR-Ti sites to form relatively dim spots imaged at a bias voltage of 1.5 V, and they can be dissociated to asymmetrically paired weak 14


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spots after scanning at a relatively high bias voltage of 2.0 V (marked by green arrows in the images). We assign the paired weak spots to the dissociative methanol at the dR-Ti site, that is, a pair of methoxy and hydroxyl, as shown by the structural model in the lower panel of Figure 3a. This behavior is quite similar to the one for water at this type of defects.21 At a low coverage, methanol does not show obvious adsorption at the dV sites (marked by the white arrows). In the re-oxidized sample (Figure 3b), the adsorbed methanol molecules tend to be in paired protrusions at both sides of the dO-Ti site (marked by blue arrows in the images). We here suggest that these paired protrusions are molecular methanol at 80 K, as schematically shown in the lower panel of Figure 3b. Singly adsorbed methanol can also be seen. The line profiles correspondingly show the heights of methanol adsorbed at the dR-Ti and dO-Ti sites, in comparison with the heights of defects before methanol adsorption (Figure 3c). Using the surface with co-existed dR-Ti and dO-Ti defects (prepared by sputtering and annealing under O2 pressure at 700 K), we directly compare the adsorption behaviors of methanol at these defects within the same frame, as shown in Figure 4a and 4b. At a higher coverage, accompanied by the adsorption of methanol in chains at the lattice Ti5c sites and as paired species near the dO-Ti sites, some dR-Ti sites become much more protruded (marked by red arrows) (Figure 4b), which is very different from the behavior at the lower coverage (Figure 3a). Figure 4c shows the profile of the much protruded species, in comparison with the profile of dR-Ti before methanol adsorption. Some more features can be seen at various coverages, as shown in the magnified images of a1, b1, and b2 in Figure 4. It is observed that methanol more preferentially adsorbs near the dO-Ti sites than at the dR-Ti sites at the low coverage (Figure 4b1). At the higher coverage (Figure 4b2), some dR-Ti sites either become dim (white circles) or show as single or paired protrusions (green circles), in addition to the much protruded species (large 15



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scale image in Figure 4b). The sites marked by the white circles could be attributed to the adsorption of molecular methanol at dR-Ti (Figure 4d), while the sites marked by the green circles could be due to the adsorption of molecular methanol at dR-Ti accompanied by two methanol at the adjacent Ti5c sites. The latter case should be much likely since there is enough space according to the model of the reduced defects (Figure 2h). It is also noticed that there are some species appeared as weak dips at the ridges (marked by the blue arrows in Figure 4a1 and 4b2). Such species are quite different from the adsorbed methanol in chain at the lattice sites (marked by the white rectangles). Since such species are not observed in the images of the as-grown surface, we tend to believe that they can be most likely attributed the hydrogen atoms, which are produced from the dissociative adsorption of methanol at the defect sites. The appearance of H atoms away from the defect sites could be mediated by the transient diffusion of methanol during exposure, similar to the H migration mediated by diffusive species in the rutile surface,36-40 which has been directly visualized in our recent work.36 These H atoms can thus be related to the desorption of water in the TPD spectra in the defective surfaces (Figure 1), which will be discuss below. The profile in Figure 4c1 shows the relative heights of these typical features. In comparison, in the re-oxidized sample annealed under O2 pressure at 1000 K, the much protruded feature is seldom observed after adsorption of methanol at a relatively high coverage (Figure 4e). In this case, some paired features can still be seen, but they are not very distinct from the methanol chains at the lattice sites.

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Figure 4. (a) and (b) Large scale images (32 × 32 nm2, 1.5 V and 10 pA) before and after methanol adsorption for the re-oxidized surface annealed under O2 pressure of 1.5 × 10−5 mbar at 700 K. The red arrows indicate the much protruded species at the dR-Ti sites. (c) Line profiles showing the height of the much protruded species at the dR-Ti site. Magnified images, (a1) bare surface, (b1) surface with a low methanol coverage, and (b2) surface with a high methanol coverage, within the same frame (16 × 16 nm2). The blue arrows indicate the sites with appeared weak dips after methanol adsorption at a relatively high coverage. (c1) Line profiles along the corresponding lines indicated in the magnified images. (d) Image of dR-Ti before and after methanol dosing, size: 5.7 × 4.3 nm2. (e) Large scale image (32 × 32 nm2, 1.5 V and 10 pA) with about one monolayer adsorption of methanol on the re-oxidized 17



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surface (annealed under O2 pressure of 1.5 × 10−5 mbar at 1000 K). The measurements were performed at 80 K.

We further characterize the adsorption of methanol using the reduced and the re-oxidized (annealed at 1000 K) samples at room temperature, as shown in Figure 5. Noticed that the similar experiment was also performed for the as-grown sample, but no obvious feature change was observed. The methanol exposure of about 100 L was conducted at room temperature. Much protruded species appear in the reduced sample (Figure 5a), and paired protrusions occur in the re-oxidized sample (Figure 5f). These protrusions are consistent with the observed features at the relatively high coverage imaged at 80 K. Considering our TPD results that the desorption of water at around 210-240 K in the reduced and the re-oxidized samples (Figure 1), we suggest that these species should be methoxy at the dR-Ti sites and at the both adjacent Ti5c sites close to each dO-Ti site, respectively, where the H atoms from the dissociative methanol have desorbed from the surfaces through producing water. The observed much protruded species at 80 K (Figure 4b) can thus be consistently assigned to methoxy at the dR-Ti sites, where the H atoms are separated from the methoxy mediated by transient diffusion of methanol during exposure, by considering that the H atoms may not desorb at 80 K. The much protruded species in the reduced surface can be desorbed by the tip at a relatively high bias voltage (Figure 5b and 5c). After the species were removed the sites present as a dim protrusion (marked by the red arrows in Figure 5c), which is quite similar to the image of the dO-Ti defects in the re-oxidized surface (Figure 2e and 2g). The typical line profiles before and after removal of the species are shown in Figure 5d. In the measurements, we located the tip over the protrusions at a setpoint of 1.5 V and 10 pA (feedback off), and had the bias voltage swept. Figure 5e shows a typical current-voltage 18


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(I/V) curve, where the current jump reflects the removal of the species at the corresponding voltage. A threshold voltage of about 2.1 V is obtained by measuring over 50 adsorbates (the inset in Figure 5e). This observation can be understood by considering the dissociation of methoxy from the dR-Ti sites through breaking the C-O bond induced by the injected electrons, where the O atoms are left on the surface to form dO-Ti defect and the fragments of CH3 radical depart from the surface. This result is consistent with the production of m/z = 15 from the reduced surface in the TPD signals (Figure 1c and 1d). Noticed that in the re-oxidized surface (Figure 5f and 5g), the apparent height of the paired protrusions is a little bit lower than that observed at 80 K (Figure 3b), which can be due to the height difference between the dissociative and the molecular methanol. The separation of the paired species is also consistent with the lattice distance between the Ti5c sites in both sides of dO-Ti. We also performed the tip manipulation on the paired species, and got a larger bias voltage of about 2.4 V to remove the paired species, which is also consistent with the TPD peak at a relatively higher temperature of 635 K. Since the tip-induced removal of species involves the injected tunneling electrons, its mechanism should be different from the thermally-induced processes in the TPD measurements. Nevertheless, the results obtained from the tip manipulations can basically support our proposed models and our assignments about the species. As a further confirmation, we measured the TPD spectra of the remained species on the surface after removing the molecular methanol. In the experiment, the samples were prepared by dosing methanol at 120 K but followed by pre-annealing to 500 K. Figure 5h shows the TPD spectra from the methanol-dosed surfaces that were pre-annealed to 500 K (red curves), in comparison with the ones from the samples without pre-annealing (black curves), measured using the reduced and the re-oxidized 19



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samples, respectively. It is seen that the pre-annealing treatments do not obviously affect the products of m/z = 15 (CH3+) and m/z = 30 (CDO+), respectively, according to the almost overlapped peaks in the both samples. These results give a strong support that the species observed in the STM images at room temperature are responsible for the products occurred at the relatively high temperatures in the TPD spectra, which can well support our assignments of the species left at the dR-Ti and the dO-Ti sites at room temperature. Noticed that in the reduced surface the signals of 29 (CHO+) and 32 (CH3OH+) are almost negligible, but in the re-oxidized surface the intensity of signal 36 (CD3OD+) is about a quarter of the intensity of signal 30 (CDO+). These results also support that the left species at the dR-Ti sites only produce CH3 radical, while the paired species near the dO-Ti sites produce formaldehyde and methanol, as we will discuss in the next section (see below in Figure 6).

20


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Figure 5. (a) Large scale image (50 × 50 nm2, 1.5 V and 10 pA) of the reduced sample after methanol exposure of about 100 L at room temperature. (b) and (c) Magnified images (6.3 × 5.7 nm2), and (d) line profiles, showing the features of the sites before and after the species were removed by the STM tip. (e) Typical I-V curve recorded the removal of the species. The inset shows the distribution of the bias voltage needed to remove the species. (f) Image (13 × 13 nm2, 1.5 V and 10 pA) of the re-oxidized sample (annealed under O2 at 1000 K) after methanol exposure of about 100 L at room temperature, and (g) line profile of the paired species. The STM images were all acquired at room temperature. (h) TPD spectra from the reduced sample by dosing CH3OH (upper panel) and from the re-oxidized sample (annealed under O2 at 700 K) by dosing CD3OD (lower panel). Black curves: the spectra measured immediately after methanol exposure at 120 K, colored curves: the spectra measured after

21



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methanol exposure at 120 K and pre-annealing to 500 K. Insets: the signals are enlarged by a factor of 10 in the range of 400-750 K. 3.4 Reactions of Methanol at the Defect Sites. On the basis of our results shown above, it allows us to conclude that the adsorption of methanol at the dR-Ti sites can produce CH3 radical, and the adsorption of methanol near the dO-Ti sites can produce formaldehyde. The reactions at the dR-Ti sites in the reduced sample can be described by CH3OH → CH3O + Had,

(1)

CH3O → •CH3 + Oad,

(2)

and the reactions near the dO-Ti sites in the re-oxidized sample by 2CH3OH → 2CH3O + 2Had,

(3)

2CH3O → CH3OH + CH2O,

(4)

where the subscript ‘ad’ indicates the adatom. In both of the samples, the intermediate process through breaking the O-H (or O-D) bond (eq 1 or eq 3) should undergo at a relatively low temperature, since the desorption of water was observed at much lower temperatures of about 210-240 K, in contrast to the results without water production from the as-grown sample (Figure 1). The produced methoxy then undergoes the reaction either through breaking the C-O bond to produce methyl radical centered around 530-600 K (eq 2) in the reduced sample, or through the disproportionation reaction to produce methanol and formaldehyde at around 635 K (eq 4) in the re-oxidized sample. According to eqs 1 and 2 the reaction should cause the oxidation of the reduced surface by leaving the O adatoms. However, the TPD signals did not obviously change in the lightly reduced sample after repeated methanol dosing about tens of cycles, and the reduction degree of the much heavily reduced 22


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sample can be slightly decreased, judging from the change of TPD signals over 50 cycles (Supporting Information Figure S3). By considering the fact that the desorption of water need to take the O atoms from the substrate (Supporting Information Figure S4), the reduction degree of the surface can thus be compensated. In the re-oxidized sample, the reaction through eqs 3 and 4 can be highly repeated in our measurements over 50 cycles of methanol dosing, judging from the production of formaldehyde. However, it is observed that the peak of signal 15 (CH3 radical) becomes wider and stronger after repeated measurements. This means that the reduction degree of the re-oxidized surface is increased, according to the dependence of the TPD spectra of signal 15 on the reduction degree. The reduction of the surface is also caused by desorption of water through taking the O atoms from the substrate. Note that during TPD measurements by heating the sample to a high temperature up to 750 K, the O deficiency of the surface may lead to the formation of the reduced sites, as the process by annealing an Ar+-sputtered sample.

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Figure 6. (a) TPD spectra of m/z = 29 (CHO+), 30 (CH2O+), 31 (CHOD+ or CH3O+), 32 (CH2OD+), 33 (CH3OD+), and 15 (CH3+) by exposure of CH3OD (coverage: 1.1 ML). Inset: Magnified signals at the higher temperature region marked by the dashed rectangle. (b) and (c) Relative intensities of the peaks at 200 K and 635 K in (a), which are normalized according to the signal of m/z = 29. The data are from the set of simultaneously recorded TPD signals at the methanol coverage of 1.1 monolayer in Figure 1f, measured using the re-oxidized sample annealed under O2 pressure of 1.5 × 10−5 mbar at 1000 K. A rate of 1 K/sec was used in the TPD measurement. (d) and (e) Relative intensities of TPD signals of various fragments by using CH3OH and CH2O, respectively. The intensities of signals in (d) and (e) are normalized according to the intensity of m/z = 29, where the intensity of m/z = 29 is set to 0.5. (f) Summations over the corresponding signals in (d) and (e).

The water desorption peak also shows a certain degree of dependence on the concentration of 24


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defects, as shown by signal 20 (D2O) in Figure 1. The desorption peak of water centers at about 210 K in the case of low defect concentrations (Figure 1b and 1c), and the peak moves to about 240 K in the case of higher defect concentrations (Figure 1d-f). The positions of the desorption peak are consistent with the TPD spectra of water on these surfaces (Supporting Information Figure S2). However, water desorption from methanol-adsorbed defective surfaces should involves a process of water formation. Our results strongly indicate that the hydrogen atoms are only from dissociative methanol at the sites of dR-Ti or dO-Ti. Therefore, the formation of water needs the migration of the hydrogen atoms. As indicated by the signatures by the dip sites (Figure 4b2), the migration of the hydrogen atoms may already happen during methanol dosing. It is also noticed that the temperatures of the water desorption peaks are slightly higher than that of the desorption peaks of monolayer methanol (Figure 1). During desorption of molecular methanol from some lattice sites, the H atoms may migrate, leading to formation and desorption of water. Considering that the H atoms themselves are difficult to migrate in the rutile surface,38 we tend to believe that the migration of H atoms is also mediated by the diffusive methanol on the anatase (001) surface, similar to the processes in rutile surface.36-40 In eq 4, the occurrence of the disproportionation reaction is consistent with the adsorption of paired methanol near the dO-Ti sites in the re-oxidized sample. To further confirm it, we make a quantitative analysis, as shown in Figure 6. We measured the relative intensities of different TPD signals at various conditions in the re-oxidized sample, using CH3OD, CH3OH, and CH2O. Figure 6a shows the signals from 29 to 33, and 15, and the inset shows the enlarged signals in the higher temperature region. The relative intensities of the peaks at 200 and 635 K are shown in Figure 6b and 6c, respectively, showing their quite different relative intensities. The absence of the peak for signal 33 (CH3OD+) at 635 K is 25



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consistent with the dehydrogenation of methanol near the dO-Ti sites and the desorption of water (D2O) at lower temperatures. In comparison, Figure 6d and 6e shows the measured relative intensities of the TPD signals by dosing CH3OH and CH2O, where the intensities are normalized according to the intensity of signal 29. By considering that the disproportionation reaction of methoxy (CH3O) through eq 4 should produce equivalent amounts of CH3OH and CH2O, the signals in Figure 6d and 6e are correspondingly summed, and the summations are given in Figure 6f. It is now seen that the relative intensities of signals from 29 to 32 can be well comparable to the ones in Figure 6c, which gives strong evidence that the peaks centered at 635 K are from the equivalent amounts of CH3OH and CH2O from the disproportionation reaction of methoxy (CH3O). The relatively higher intensity of signal 15 in Figure 6c can be understood by considering the fact that the coexisted dR-Ti sites may also contribute to signal 15. Even the proper treatments to produce the defect sites can somewhat enhance the activity of the surface, our results show that the reaction productions do not significantly enhanced even in the highly defective surfaces. Our experimental results agree with the AOM model,21 where the lattice sites are not active because of the five-fold coordinated Ti atoms at the ridges, obviously disagree with the expected active lattice sites due to the four-fold coordinated Ti atoms at the ridges in the ADM model.8 However, it is argued that the surface energy of the AOM model is much higher than that of the ADM model from DFT calculations.30 This situation needs to be further investigated. On the other hand, in a semiconductor, a very low concentration of doping elements may significantly vary the electronic property of the semiconductor. The impurity elements in mineral anatase crystal33 could play such a role. It implies that by properly introducing dopants the activity of the anatase surface can be modified 26


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by considering the reports that the relatively high activities were observed in mineral anatase TiO2 (001) surface.14-16 Fundamental understanding of the intrinsic chemical reactivity can be a key step to investigate the highly reactive materials based on anatase TiO2.

4. CONCLUSIONS

In summary, we investigated the activity of the anatase TiO2(001)-(1×4) surface. We find that the perfect (1×4) lattice sites are intrinsically inert for the reaction of methanol. However, by introducing two types of defects, that is, Ti pairs and O-bridged Ti pairs, through reduction or followed by re-oxidation treatments, the activity of the surface can be enhanced, and the thermal reactions of methanol are still dominant at the defect sites. We believe that many theoretical explanations for the reactions on the anatase (001) surface based on the ADM model should be reexamined by considering the fact that the perfect ridge sites are quite inert. Our findings shed light on the comprehensive understanding of this long-term puzzling anatase (001) surface.

Supporting Information Available: Figure S1: Auger spectra and LEED pattern of the TiO2 (001)-(1×4) surfaces. Figure S2: TPD spectra of water obtained from the surface under different treatments. Figure S3: TPD spectra of heavily reduced surface after many cycles of repeated methanol exposure. Figure S4: TPD spectra by dosing CH318OH, CH3OD, CH3OH, and CH2O, to show the formation of water through picking up lattice O atoms from the substrate for the methanol-dosed surfaces.

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Acknowledgment This work was financially supported by MSTC (Grant 2016YFA0200603, 2013CB834605), the “Strategic Priority Research Program” of CAS (grant XDB01020100), and NSFC (Grants 91321309, 91421313, 51132007, 21421063, 21573207). Author contributions H.Q.T. and Z.W.C. contributed equally to this work.

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TOC GRAPHIC

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Figure 1. TPD spectra of m/z = 33 (CH3OD+) and 20 (D2O+) by using CH3OD from anatase TiO2(001)-(1×4) surfaces, (a) as-grown surface by degassing at 900 K for 1 h, (b) as-grown surface by annealing at 900 K for 9 h, (c) lightly reduced surface by a single cycle of Ar+ ion sputtering and annealing, (d) heavily reduced surface by over 10 cycles of Ar+ ion sputtering and annealing, (e) re-oxidized surface annealed under O2 at 700 K, (f) re-oxidized surface annealed under O2 at 1000 K. We define that the spectrum with a relatively symmetric peak in (b) corresponds to 1 monolayer (ML), and the spectrum is reproduced in each panel (dashed red curve) for comparison. The insets show the simultaneously acquired signals 33 (CH3OD+), 29 (CHO+), and 15 (CH3+) in 4 measurements with various coverages. In the TPD measurements, a rate of 1 K/sec was used. 127x149mm (300 x 300 DPI)



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Figure 2. STM images of bare surfaces of anatase TiO2(001)-(1×4), (a) as-grown surface (70×70 nm2, and inset: 10.1×10.1 nm2), (b) reduced surface (41×41 nm2, and inset: 10.2×10.2 nm2), and (c) re-oxidized surface (50×50 nm2, and inset: 13.3×12.7 nm2). (d) and (e) Voltage-dependent images of the re-oxidized surface (13.3×12.7 nm2) annealed under O2 pressure of 1.5×10-5 mbar at 1000 K. (f) and (g) Voltagedependent images of re-oxidized surface (18.1×15.8 nm2) annealed under O2 pressure of 1.5×10-5 mbar at 700 K. (h) Proposed structural model of the partially oxidized defect of dO-Ti (lower) from the reduced defect of dR-Ti (upper). 153x164mm (300 x 300 DPI)


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Figure 3. (a) Images showing the adsorption of methanol at the dR-Ti sites in the reduced surface, image size: 6.3×4.5 nm2, imaged at 1.5 V and 10 pA; Upper panel: before methanol exposure, middle panel: after methanol exposure and followed by scanning at 2.0 V and 10 pA, and lower panel: schematically structural model of dissociative methanol at the dR-Ti site. The green arrows mark the methanol adsorption at the dR-Ti sites, and the white arrows mark the dV site. (b) Images showing the adsorption of methanol near the dO-Ti sites in the re-oxidized surface, image size: 5.8×4.1 nm2, imaged at 1.2 V and 10 pA; Upper panel: before methanol exposure, middle panel: after methanol exposure, and lower panel: schematically structural model of paired molecular methanol at the Ti5c sites in both sides of the dO-Ti site. (c) Line profiles corresponding to the lines 1-4 in (a) and (b). The measurements were performed at 80 K. 82x61mm (300 x 300 DPI)



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Figure 4. (a) and (b) Large scale images (32×32 nm2, 1.5 V and 10 pA) before and after methanol adsorption for the re-oxidized surface annealed under O2 pressure of 1.5×10-5 mbar at 700 K. The red arrows indicate the much protruded species at the dR-Ti sites. (c) Line profiles showing the height of the much protruded species at the dR-Ti site. Magnified images, (a1) bare surface, (b1) surface with a low methanol coverage, and (b2) surface with a high methanol coverage, within the same frame (16×16 nm2). The blue arrows indicate the sites with appeared weak dips after methanol adsorption at a relatively high coverage. (c1) Line profiles along the corresponding lines indicated in the magnified images. (d) Image of dR2 2 Ti before and after methanol dosing, size: 5.7×4.3 nm . (e) Large scale image (32×32 nm , 1.5 V and 10 pA) with about one monolayer adsorption of methanol on the re-oxidized surface (annealed under O2 pressure of 1.5×10-5 mbar at 1000 K). The measurements were performed at 80 K. 129x136mm (300 x 300 DPI)


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Figure 5. (a) Large scale image (50×50 nm2, 1.5 V and 10 pA) of the reduced sample after methanol exposure of about 100 L at room temperature. (b) and (c) Magnified images (6.3×5.7 nm2), and (d) line profiles, showing the features of the sites before and after the species were removed by the STM tip. (e) Typical I-V curve recorded the removal of the species. The inset shows the distribution of the bias voltage needed to remove the species. (f) Image (13×13 nm2, 1.5 V and 10 pA) of the re-oxidized sample (annealed under O2 at 1000 K) after methanol exposure of about 100 L at room temperature, and (g) line profile of the paired species. The STM images were all acquired at room temperature. (h) TPD spectra from the reduced sample by dosing CH3OH (upper panel) and from the re-oxidized sample (annealed under O2 at 700 K) by dosing CD3OD (lower panel). Black curves: the spectra measured immediately after methanol exposure at 120 K, colored curves: the spectra measured after methanol exposure at 120 K and preannealing to 500 K. Insets: the signals are enlarged by a factor of 10 in the range of 400-750 K. 126x123mm (300 x 300 DPI)



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Figure 6. (a) TPD spectra of m/z = 29 (CHO+), 30 (CH2O+), 31 (CHOD+ or CH3O+), 32 (CH2OD+), 33 (CH3OD+), and 15 (CH3+) by exposure of CH3OD (coverage: 1.1 ML). Inset: Magnified signals at the higher temperature region marked by the dashed rectangle. (b) and (c) Relative intensities of the peaks at 200 K and 635 K in (a), which are normalized according to the signal of m/z = 29. The data are from the set of simultaneously recorded TPD signals at the methanol coverage of 1.1 monolayer in Figure 1f, measured using the re-oxidized sample annealed under O2 pressure of 1.5×10-5 mbar at 1000 K. A rate of 1 K/sec was used in the TPD measurement. (d) and (e) Relative intensities of TPD signals of various fragments by using CH3OH and CH2O, respectively. The intensities of signals in (d) and (e) are normalized according to the intensity of m/z = 29, where the intensity of m/z = 29 is set to 0.5. (f) Summations over the corresponding signals in (d) and (e). 136x98mm (300 x 300 DPI)


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Understanding the Intrinsic Chemical Activity of Anatase TiO2(001)-(1

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