Direct visualization of Oxygen Reaction with Paired Hydroxyl on TiO2

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Direct Visualization of Oxygen Reaction with Paired Hydroxyl on TiO2(110) Surface at 78 K by Atomic Force Microscopy Huan Fei Wen, Quanzhen Zhang, Yuuki Adachi, Masato Miyazaki, Yoshitaka Naitoh, Yan Jun Li,* and Yasuhiro Sugawara

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Department of Applied Physics, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan ABSTRACT: The hydroxyl defects on rutile TiO2 surface play an important role in surface chemical reactions and understanding the catalytic reactions due to its excess electrons. In this work, the reaction behavior of oxygen molecule with paired hydroxyl defects on a rutile TiO2(110)-(1 × 1) surface was observed at 78 K with the atomic resolution by atomic force microscopy. The high reactivity of paired OH with O2 is attributed to the asymmetric spatial distribution of its excess electrons, and the reaction sites occupy a high distribution of excess electrons. The effect of tip to the surface reactions is discussed, and atomic contrast of their reaction product 2OHt is associated with the tip− sample distance. The present study is expected to provide some insights into catalytic reactions on oxide surface.

1. INTRODUCTION Rutile TiO2(110) has been extensively studied as a model catalyst for transition oxide and the supporting substrate for noble metal atoms to investigate the catalytic mechanism.1−5 Paired hydroxyl (OH) on TiO2(110) surface is formed by the dissociation of water molecule at an oxygen vacancy (Ov) site on the bridging 2-fold coordinated oxygen (O2c) rows in ultrahigh vacuum (UHV) conditions and is proposed to act as a charge donor site in surface chemical reactions.6,7 In addition, it was found that hydrogenated TiO2 can improve the photoelectrochemical conversion efficiency and enhance the reducing power of TiO2 in photocatalytic reactions.8,9 Furthermore, OH defects act as intermediates in the reaction cycle and can convert into water by reacting with O adatoms on the 5-fold coordinated Ti (Ti5c) sites.10 It should be noted that avoiding of the water molecule is difficult in real catalytic reaction systems under high-pressure conditions. Researches on hydroxylated TiO 2 (110) surface have yielded the progressive results and are continuing. Therefore, it is highly significant to explore the characterization of OH defects on TiO2(110) surface. Oxygen is a simple and common reactant in many surface catalytic reactions, and a number of researches on oxygen interaction on TiO2(110) surface have been conducted. O2 can dissociate at Ov and Ti5c sites at room temperature and low temperature (LT), and consecutive reaction steps were observed by scanning tunneling microscopy (STM).11,12 The initial, intermediate, and final states of O2 reactions with OH defects from 300 to 165 K have also been researched.10,13 The possible structures in the reaction processes were analyzed, and different intermediates were identified, such as HO2, H2O2, or hydroxyl (donated OHt) on the Ti5c rows. Single OH can adsorb two or more O2 molecules at LT. However, O2 does © XXXX American Chemical Society

not react with single OH at 80 K even at a higher bias voltage induction. The formed OH−O2 complex shows similar electronic states as those of single OH and could be a precursor of the HO2 species.14 Nevertheless, it has been speculated that paired OH can react with O2 to produce OHt on Ti5c rows at 90 K.9 On the basis of these researches, the reaction of O2 with OH will be understood at the atomic scale if the reaction of paired OH with O2 is verified with atomic resolution. However, up to now, only a few studies have focused on the effect of paired OH and direct experimental evidence is not available for O2 reaction with paired OH on TiO2(110) surface at LT. Atomic force microscopy (AFM) is a powerful tool for exploring surface properties at an atomic scale and has been applied in the investigation of gas/organic molecular adsorption processes.15−17 In the present study, we experimentally studied the reaction behaviors of O2 with paired OH on a TiO2(110) surface at an atomic scale at 78 K by AFM. The effect of tip−sample distance was studied and the reaction mechanism has been discussed combining the excess electron distribution. The results presented here would provide important insight into the characteristics of surface defects on TiO2(110) and would be useful for the design of the surface reaction path for future research.

2. EXPERIMENTAL DETAILS Experiments were performed in ultrahigh vacuum (UHV) (pressure: 3 × 10−11 Torr) at a low temperature (78 K) using noncontact AFM apparatus equipped with an optical beam deflection system. The deflection noise density was estimated Received: July 2, 2018 Published: July 11, 2018 A

DOI: 10.1021/acs.jpcc.8b06289 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C as 9.8 fm/√Hz,18 and the AFM was operated in the frequency modulation mode. All images presented here were recorded in the constant height mode to eliminate the topographic crosstalk. A commercially available Ir-coated cantilever (Nano sensors SD-T10L100, f 0 ∼ 800 kHz) was employed as an AFM sensor, which was oscillated at its resonant frequency keeping the oscillation amplitude constant. In our previous experiment, we found that the imaging contrast on the TiO2(110) was easily changed by using an Si tip,19 because the Si tip apex is easily terminated by the surface species such as O, OH, or Ti during the imaging process. It was found that a stable contrast mode can be obtained using a metal-coated tip such as W and Ir.20,21 Therefore, metal-coated tips are widely applied to explore the surface properties. Before AFM imaging, the tip was annealed at 600 K and then cleaned by Ar-ion sputtering to remove the contaminants. As a sample, the TiO2(110) surface (provided by Furuuchi Chemical Corporation) was prepared by several cycles of Arion sputtering, followed by annealing. The details of sample preparation can be found in a previous report.22 After cleaning, the freshly prepared surface cooled to room temperature is exposed to O2 and then the sample was transferred to the observation chamber. AFM observation was performed after the sample temperature decreased to 78 K.

in the AFM images.26−28 In this study, the images were recorded in hole mode with the positive tip apex termination. Next, we experimentally demonstrate the reaction steps recorded in the constant height mode. In the frequency shift (|Δf |) images shown in Figure 2, the fast scanning direction is from left to right. The bright and dark row are the O2c and Ti rows, respectively, and the dark spot is the paired OH. The bright donut-shaped spots on the Ti5c rows are the product of O2 and paired OH reaction. Seven reactive adsorption behaviors around the paired OH are observed, as shown in Figure 2a−e. The black/blue dotted squares marked by black/ blue numbers represent the chemical reaction before/after O2 adsorption, respectively. Note that the sample was kept for 3 h in the observation chamber to attain a stable temperature of 78 K. Therefore, the atomic image in Figure 2a already included the generated product. In addition, as shown in Figure 2a (black dotted circle), some surface defects around the O reaction sites did not disappear. Next, we discuss the reaction processes on the basis of the AFM images before/after O2 adsorption and the corresponding structural models of TiO2(110)-(1 × 1) surface shown in Figure 3. In Figure 3a, it was clearly observed that the size of the dark spot that paired OH (dashed dark circle) occupied a two-lattice spacing by comparing with neighboring bright O2c rows and their contrasts (lower spot is darker than the upper spot) are different owing to the direction of hydrogen. After reaction, the dark spot is replaced by two bright atoms that are assigned to O2c atoms and the donut shape appeared at the same time. The whole reaction process is modeled as shown in Figure 3b and is given by

3. RESULTS AND DISCUSSION For convenience of analysis and discussion, we first display the ball model of rutile TiO2(110)-(1 × 1) surface, shown in Figure 1. It consists of alternating Ti5c rows (orange ball) and

2OH + O2 → 2OH t + 2O2c

(1)

2OHt is imaged as a donut shape in the AFM image owing to Pauli repulsion at a small tip−sample distance. 2OHt was very stable during the experiments, as shown in Figure 2. This is the first observation of OHt by AFM. In addition, we found an interesting phenomenon that all reactions occurred on the same side of the paired OH along [11̅0] direction, which demonstrated that the reaction sites are not casual around paired OH, as explained below. We now discuss the reason for the high reactivity of O2 toward the paired OH. It is believed that the reaction originates from the spatial asymmetric distribution of excess electrons associated with paired OH. On TiO2(110) surface, excess electrons of single OH on a O 2c row were experimentally observed as a symmetric four-labeled shape located on the Ti5c site at 78 K29 and O2 does not react with single OH at 78 K even under high tip tunneling current.14 According to previous studies, the reaction activation energy of paired OH should be lower than that of single OH. The spatial distribution of excess electrons associated with paired OH was simulated by theoretical calculation,9 whereas it was not observed experimentally. It was predicted that one of the excess electrons locates on the Ti site of the first subsurface under the H atom and the other electrons asymmetrically distribute on the Ti5c site near the paired OH.9 The local contact potential difference measured by Kelvin probe force microscopy demonstrates that surface potential distribution is asymmetric around the paired OH, which will be discussed in our other paper. In addition, the asymmetric spatial distribution of excess electrons associated with paired Ov has been reported at both room temperature and low temperature30,31 and the reaction activation energy of paired Ov is

Figure 1. Ball model of rutile TiO2(110)-(1 × 1) surface. (a) Top view and (b) side view.

6-fold coordinated Ti (Ti6c) rows surrounded by in-plane 3fold coordinated oxygen (O3c) rows (blue ball) and bridging O2c rows (gray ball). As mentioned in the introduction, the surface defects mainly contain Ov (dashed dark circle) and OH defects (green small ball). Ov is usually a single state due to the repulsions between the paired Ov on the same O2c rows.23 The paired OH was formed by the dissociation of background residual water molecule on an Ov site, and O adatom (Oad: light green large ball) was generated by O2 dissociation on the Ov or Ti5c sites. OHt can been formed by the reaction of Oad and OH or water dissociation.10,24,25 Depending on the tip apex polarity, O2c rows were observed as bright or dark features B

DOI: 10.1021/acs.jpcc.8b06289 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 2. AFM images of TiO2(110) surface. Black/blue number: before/after chemical reaction. (f 0 = 799 kHz, Q = 24388, VDC = 0 V, A = 500 pm, Δf = −350 Hz, 10 × 10 nm2).

occurring was low in this study, as shown in Figure 2a. For two paired OH, the excess electron distribution should be asymmetric according to the occupied state of the Ov tetramer.31 The reactivity of two paired OH owing to more excess electrons is higher than that of single paired OH, and they preferentially react with O2. The detailed process was not observed in current experiments and is proposed as follows. One of the paired OH reacts with an O2 molecule, and the reactivity of the other paired OH becomes weak in the entire experiment due to the produced 2OHt that probably occupies its excess electrons and prevents further O2 adsorption. Such reactions constitute 9% of the surface reactions shown in Figure 2. We now discuss the tip effect on the reaction of paired OH with O2 and tip−sample distance-dependent AFM images. Figure 4 shows the four AFM topographic images of rutile TiO2(110)-(1 × 1) surface obtained with different tip−sample distances. As in the above introduction, the bridging O2c rows are bright, whereas the Ti5c rows and paired OH are dark. The bright spots and donut shape (marked by black circles) on the Ti5c rows are the 2OHt. STM tip-induced O2 dissociation has been observed at high bias voltage and tunneling current in the previous literature.12 To avoid tip-induced surface reactions in our experiment, the AFM images were obtained first with a large tip−sample distance at a low bias voltage; however, the surface reactions had already occurred, as shown in Figure 4a. Therefore, influence of the tip to the surface reactions of paired OH with O2 molecule can be neglected. At large tip−sample distance, 2OHt was imaged as slightly enlarged bright spots shown in Figure 4a. As shown in Figure 4b, the bright spots are observed as obvious elliptical shapes (marked by black elliptical circles) with decreasing tip−sample distance and are tilted toward right (with respect to [001]). With further decrease in tip−sample distance, the elliptical shape becomes donut-shaped, as shown in the Figure 4c, with two bright spots located at the upper-right and lower-left of the image. Moreover, the shapes are not influenced by tip movement, as observed by comparing the forward and

Figure 3. AFM images (a) before O2 adsorption, (b) after O2 adsorption, and (c, d) corresponding structural models on TiO2(110) surface.

lower than that of single Ov by 82 mV.30 The electronic structure of OH is similar to that of Ov, so the research of Ov indirectly supports our observations. Therefore, on the basis of our analysis, the reaction mechanism of O2 molecule with paired OH was explained in which the reactive sites have higher electron distribution; consequently, in the experiment, the reaction site around paired OH has higher electron distribution. Next, we discuss the phenomena of surface defects around the O2 reaction sites. Besides the predominant event of reaction of paired OH with O2 molecule, a rare event indicated by the dotted black circle in Figure 2a occurred. In this case, the surface defects before the reaction are attributed to two paired OH formed by the dissociation of two H2O molecules on paired Ov. Paired Ov can be produced during the processes of sample preparation. The probability of this situation C

DOI: 10.1021/acs.jpcc.8b06289 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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direction. Further experimental and theoretical research is needed to investigate the effect of paired OH groups with/ without Tiint contribution to the distribution of excess electrons. The results presented here are expected to explain certain phenomena, such as chemical reactivity and catalytic activity on the TiO2(110) surface and other oxide materials.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Huan Fei Wen: 0000-0002-2972-9669 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research from Japan Society for the Promotion of Science (JSPS) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) by MEXT/JSPS KAKENHI Grant Number (16H06327,16H06504 and 17H01061) and Osaka University’s International Joint Research Promotion Program (J171013014 and J171013007).

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REFERENCES

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Figure 4. AFM images (hole mode) of TiO2(110) surface in constant height mode. (a−c) Recorded with decreasing tip−sample distance and (d) with increasing tip−sample distance ( f 0 = 799 kHz, Q = 24388, VDC = 0 V, A = 500 pm, 10 × 4 nm2).

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4. CONCLUSIONS The reactive adsorption behaviors of O2 with paired OH were observed along [11̅0] direction on rutile TiO2(110) surface with atomic resolution at 78 K. The asymmetric spatial distribution of excess electrons associated with the paired OH was considered the main contributor to the reactive adsorption behaviors and determined the reactive sites. The tip−sample distance-dependent AFM images were clearly shown. In addition, the excess electrons from Tiint were a factor contributing to anisotropic adsorption along the [11̅ 0] D

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DOI: 10.1021/acs.jpcc.8b06289 J. Phys. Chem. C XXXX, XXX, XXX−XXX