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J. Phys. Chem. C 2009, 113, 14356–14362

STM Study of Surface Species Formed by Methanol Adsorption on Stoichiometric and Reduced ZnO(101j0) Surfaces Xiang Shao,†,§ Ken-ichi Fukui,‡ Hiroshi Kondoh,¶ Mitsuhiko Shionoya,† and Yasuhiro Iwasawa*,† Department of Chemistry, Graduate School of Science, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan, Center for Nanobio Integration, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka UniVersity, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan, and Department of Chemistry, Faculty of Science and Technology, Keio UniVersity, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan ReceiVed: March 13, 2009; ReVised Manuscript ReceiVed: June 15, 2009

Methanol adsorption on the stoichiometric and slightly reduced ZnO(101j0) surfaces has been studied by scanning tunneling microscopy (STM). On the stoichiometric surface it was found that adsorbed methanol formed two types of ordered structures by dissociative adsorption in different modes, one of which was a dominant two-dimensional island structure residing on surface zinc atom rows, while the other was a linear chain structure residing between two surface zinc rows. In addition, a mobile phase of molecularly adsorbed methanol was deduced occupying the areas aside the ordered domains and keeping in equilibrium with the island structure. In contrast to the stoichiometric surface, on the slightly reduced surface, only the linear chain structure was observed as an ordered phase, accompanied by an unusual type of line defects. The STM observations indicated that the drastic change of surface properties occurred on the whole surface not at the specific sites such as point vacancies. 1. Introduction ZnO has widely been used as a component in a variety of functional materials, such as medicines, cosmetics, pigments, catalysts, rubbers, etc.1-3 Recent nanomaterials science has also found its capability of forming a variety of well-defined nanostructures, which may provide various advantageous properties and far-going future applications of ZnO.4 The exposure of ZnO surface to chemicals has also been studied in relation to the efficient design and utilization of ZnO-based devices,5-7 which exhibit high activity and sensitivity to the adsorption of various molecules.8 Synthesized ZnO microsingle crystals mostly expose the low-energy faces, such as (101j0), (0001)Zn, and (0001j)-O faces, among which the prismatic (101j0) face is major as characterized by scanning electron microscopy (SEM).9 The interactions of adsorbed chemicals with the prismatic surface would therefore be expected to play a quite important or even decisive role in the applications of the nanostructured ZnO materials, particularly those nanorods and nanowires that have long axes and dominating (101j0) faces. Among these applications, ZnO and metal/ZnO have often been used as catalysts for many chemical processes, such as dehydration, dehydrogenation, water-gas shift reaction, methanol synthesis, etc.10-13 Despite the success of utilization of ZnO in the catalytic processes, the key role of ZnO in good catalytic performances is still controversial and debated.14-16 Methanol synthesis from syngas (CO/H2) has received much attention from * To whom correspondence should be addressed. E-mail: iwasawa@ chem.s.u-tokyo.ac.jp. † Department of Chemistry, Graduate School of Science, The University of Tokyo. § Center for Nanobio Integration, The University of Tokyo. ‡ Osaka University. ¶ Keio University.

both fundamental and industrial interests.10-21 According to difficulties in the studies starting from the syngas and also to people’s interests in methanol decomposition to produce H2 for the fuel cell system, explorations about methanol adsorption on powders, thin films, and single crystals of ZnO in various conditions have been triggered by the microscopic reversible principle.22-30 Kung and co-workers have conducted systematic temperature programmed desorption (TPD) studies of adsorbed methanol in ultrahigh vacuum (UHV) and reported molecular desorption around 420 K and decomposition into CO, H2, CO2, H2O, etc. around 550 K, with rather low production.28-32 Hirschwald and co-workers33 used a combination of ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) and Solomon and co-workers34 used a combination of XPS, near-edge X-ray absorption fine structure (NEXAFS), and variable-energy photoelectron spectroscopy (PES) to find evidence for the kind of adsorbed species. They showed major formation of methoxy species and minor formation of methyl species on ZnO surfaces by the dissociative adsorption of methanol. These pieces of information are somehow macroscopic rather than microscopic. For the question how and where these species bound to the surface, and how they are converted to decomposition intermediates and final products, a clear microscopic picture is still unknown. In this respect, the microscopic structural information is very important for understanding oxide catalysis because usually the local structures at oxide surfaces account for their catalytic properties. Scanning tunneling microscopy (STM) is a powerful technique and has succeeded in solving various surface-related problems. This has been proved by extensive STM studies on TiO2 surfaces, providing invaluable information on the details of physical and chemical processes at the surfaces.35-47 For the ZnO materials, a few STM works have been reported mostly

10.1021/jp9022597 CCC: $40.75  2009 American Chemical Society Published on Web 07/22/2009

STM Study of Methanol Adsorption on ZnO(101j0) for basic understanding of the crystal itself,48-51 while the adsorption of only hydrogen and water on ZnO surfaces have been examined by STM, showing important issues.52-55 To the best of our knowledge, no STM work on the methanol adsorption, which is a key issue for methanol synthesis and decomposition on ZnO surfaces, has been reported. Thus the aim of this study is to illustrate and document the adsorption of methanol as well as the local structure and dynamic behavior of surface species on a ZnO(101j0) surface by STM. In this paper we have exploited the adsorption structure of methanol on both a stoichiometric and a slightly reduced ZnO(101j0) surface at room temperature by STM. Our highly resolved STM images have identified the different surface species resulting from distinct adsorption reactions of methanol. Also the STM observations revealed that the methanol adsorption on the slightly reduced ZnO(101j0) surface presented rather different morphologies from that on the stoichiometric surface, which provided a clear clue of the surface reduction effect on chemical processes on metal oxides. 2. Experimental Section STM images were obtained by using either a UHV JAFM4500XT system (JEOL) in an STM mode or a UHV JSTM4500XT system (JEOL). Both systems had a base pressure of 1.0 × 10-8 Pa or slightly less after prolonged careful baking. They were equipped with an ion sputtering gun, a low-energy electron diffraction (LEED) optics, and a quadrupole mass spectrometer. ZnO(101j0) single crystals (6.5 × 1.0 × 0.5 mm3) (Surface Preparation Laboratory (SPL)) were first treated in an ultrasonic acetone bath and then annealed at 900 K for 2 h in air. The samples were heated in the STM systems by the electric current through a deposited Ni film or an attached thin Ta foil at the backside of the samples. The ZnO(101j0) surface was cleaned by cycles of Ar+ sputtering at 3 keV and annealing at 900-1000 K for 3 min in the sample preparation chamber of each system. The sample temperature was monitored by an infrared radiation thermometer. Methanol (>99.8%, WAKO) was further purified by freeze-pump-thaw cycles and introduced into the STM chamber through a variable leak valve immediately after the clean surface was cooled to near room temperature (typically 1 h after the annealing). Electrochemically etched polycrystalline tungsten tips were used for STM measurements in the constant current mode at positive sample bias voltage (Vs) around 1.5-3.0 V. Thus, STM images reflect the unoccupied states of the sample. 3. Results 3.1. As-Prepared Clean Surface. Figure 1a shows an STM image of a clean ZnO(101j0) surface prepared by a few cycles of Ar+ sputtering and annealing. The surface consists of flat terraces separated by monatomic-height steps running along the two stable crystalline orientations, [0001] and [12j10], forming near-rectangular rafts and pits. On the flat terrace, bright lines along the [12j10] direction were observed (Figure 1b) and sometimes the lines were resolved as bright spots matched with a rectangular unit cell of 0.33 × 0.52 nm2 (Figure 1c) as were reported in previous STM studies.49-53 The bright spots or lines at positive sample bias voltages can be assigned to Zn atom rows because Zn 4s-derived states contribute to the valence band minimum.56,57 The issue of the surface stoichiometry of ZnO(101j0) is not easy to address merely by STM characterization. Go¨pel et al. reported the change of valence band photoemission spectra by high-temperature annealing in vacuum and attributed it to the

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Figure 1. Large scale (a, 100 × 100 nm2), small scale (b, 20 × 20 nm2), and atomic resolution (c, 3 × 6 nm2) STM images of a clean ZnO(101j0) surface with near stoichiometric composition. (d) A schematic model for the surface, with larger red circles and smaller black circles for oxygen and zinc, respectively. Tunneling conditions were Vs ) 2.0 V and It ) 0.3 nA for panels a and b, and Vs ) 3.0 V and It ) 0.1 nA for panel c.

formation of a small portion of surface oxygen vacancies.58 Kung and co-workers estimated that the concentration of oxygen vacancies did not exceed 10-4 monolayer on the surface by hightemperature annealing.28 Recent theoretical calculations showed that missing zinc-oxygen dimers are most abundant defects on ZnO(101j0) under UHV or catalytic conditions for methanol synthesis.57,59 Actually a small number of shallow holes (about 0.09 nm deep, not shown here), which were identified as zinc-oxygen dimer vacancies in the literature,49,53 were also found, but no apparent oxygen vacancies were observed in our STM measurements. These results suggest that the surface composition was close to the stoichiometric one even after hightemperature annealing, with a small possibility for formation of oxygen defects by quenching of annealing. Therefore the asprepared ZnO(101j0) surface in the present study can be regarded as the stoichiometric surface. 3.2. Methanol Adsorption on the Stoichiometric Surface. After exposure to 10 L (Langmuir, 1 L ) 1.33 × 10-4 Pa · s) of methanol at room temperature, STM images showed two types of ordered phases on the surface (Figure 2): one is a twodimensional (2D) island structure (denoted as I hereinafter) covering around 30% of the surface area (see Figure S3 in the Supporting Information), and the other is a linear chain structure (denoted as L) with much less coverage around 3%. No preferential formation of these ordered phases at step edges was found. The island structure consisted of bright lines sitting on the Zn atom rows (Figure 2b) at a height of 0.10-0.15 nm. It was difficult to further resolve the bright lines, but sometimes vague contrast with 1 × 1 lattice periodicity was obtained on the island (see Figure S1 in the Supporting Information). In contrast, the bright lines forming the linear chain structure were located between the Zn atom rows (Figure 2b). At higher resolution image in Figure 2c, the bright line was resolved into bright spots at the separation of either 0.4 or 0.7 nm in the [12j10] direction, correlating with single or double the unit cell length (0.33 nm) on the substrate. We consider that both ordered structures consist of methanol-derived species. By comparing successively obtained STM images, it was found that the two ordered structures were not rigid but changed their shape even at room temperature. An example is shown in

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Figure 2. STM images after methanol adsorption on a stoichiometric ZnO(101j0) surface at room temperature. (a) A wide range image that shows dominant island structure (I) and minor linear chain structure (L). (b) A magnified STM image showing that the bright lines in I are in-phase and those in L are out-of-phase against the substrate Zn atom rows (dashed lines). (c) High-resolution STM image of the linear chain structure. The separations between the bright protrusions in a bright line are the multiple of the substrate unit vector. (d) Schematic models (top view and side view) for the two ordered structures. Tunneling conditions were Vs ) 2.0 V and It ) 0.3 nA.

Figure 3. Successive STM images obtained at room temperature starting from Figure 2a ((a) the same image as Figure 2a). The interval between panels a and b was 60 s. The blue ellipses highlight the areas where the island structure (I) grew (solid) and shrank (dashed). The white rectangle highlights the area where the linear chain structure (L) grew. Shrink of the linear chain structure was rarely observed. Tunneling conditions were Vs ) 2.0 V and It ) 0.3 nA. The horizontal line in panel b was caused by an accidental jump of the STM tip during scanning.

Figure 3 where two images were obtained at the same area with an interval of 60 s. In the region surrounded by a solid ellipse, the island structure grew by addition of several bright rows. In contrast, some bright rows in the island structure shrunk in length in the region of dashed ellipse. Between the two images in Figure 3, about 25 nm2 among the island structure region moved their position. This observation indicates a relatively low energy barrier for formation and decomposition of the island structure. In contrast, the linear chain structure was much more stable. We could hardly find the shrink of the linear chain structure, although new segments of the species were sometimes formed as shown in the region surrounded by a solid rectangle

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Figure 4. STM images obtained before and after methanol adsorption on a slightly reduced ZnO(101j0) surface: (a) before adsorption (Vs ) 3.0 V and It ) 0.1 nA) and (b and c) after adsorption by exposure to 10 L of methanol at room temperature (Vs ) 2.0 V and It ) 0.08 nA for panel b and Vs ) 1.2 V and It ) 0.06 nA for panel c). (d) Line profiles obtained at the positions indicated in panel c.

in Figure 3. This indicates a relatively high energy barrier to decompose the linear chain structure. How are the components of the two structures transferred between the separate domains? We suppose that there are a certain number of molecularly adsorbed methanols that are mobile on the ZnO surface other than covered by the two ordered phases. These weakly bound methanol molecules migrate too fast to be observed by STM. They probably serve as an intermediate at an equilibrium with the ordered phases. We tried to collect I-V curves on each ordered phase but failed because of fluctuations of the ordered phases as well as thermal drift. However, the average I-V curve obtained on the methanol-adsorbed ZnO(101j0) surface showed a gapless feature in contrast to a wide gap at EF on the bare ZnO(101j0) surface (see Figure S2 in the Supporting Information). Measurements at low temperature are necessary to elucidate the structurespecific I-V characteristics and their possible relation with the gapless feature reported on hydrogen adsorbed on ZnO(101j0).60 3.3. Methanol Adsorption on the Slightly Reduced ZnO(101j0) Surface. Previous TPD experiments showed that methanol-adsorbed ZnO(101j0) surfaces evolved CO, CO2, H2, and H2O at around 650 K via surface formate intermediate.29,30 The formate intermediate was formed from the major surface species of methoxy by extracting an surface oxygen. Therefore, thermal decomposition of adsorbed methanol results in the reduction of the ZnO surface. Actually, we found that the cycles of methanol adsorption and thermal decomposition on a ZnO(101j0) surface affected the chemical properties of the surface. Panels b and c of Figure 4 show STM images of a ZnO(101j0) surface after 10 L of exposure to methanol at room temperature. The surface in Figure 4a had experienced six times of methanol adsorption and subsequent heating to 750 K to decompose adsorbed methanol. Between each adsorption experiment, the ZnO(101j0) substrate was cleaned by cycles of Ar+ sputtering at 3 keV and annealing at 900-1000 K for 3 min. If one compares panels b and c of Figure 4 with Figure 2

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Figure 5. Sequential STM images after methanol adsorption on a slightly reduced ZnO(101j0) surface (Vs ) 1.2 V and It ) 0.06 nA). The surface was exposed to 10 L of methanol at room temperature. The images were collected at an interval of 60 s. Red and blue arrows indicate the initial length of a line defect and the distance to a protruding chain located on the extended line of the line defect, respectively. They make references for the dimensional changes. The green dashed ellipse highlights the area where linear chain structures formed, grew, shrank, and diffused. The purple circle highlights the area where a line defect was extended and another line defect was formed. The scale bars inserted were 3 nm for all the images.

where methanol adsorbed on the stoichiometric ZnO(101j0), the most striking difference is the absence of the island structure. The island structure was the dominant ordered structure on the stoichiometric surface. Note that the surface structure before methanol adsorption (Figure 4a) showed no evident difference from the stoichiometric ZnO(101j0) (Figure 1). As a control experiment, adsorbed methanol was removed by Ar+ sputtering instead of thermal decomposition in the aforementioned processes. In this case, the island structure remained the dominant structure after methanol exposure. Therefore, we considered that the change in the surface chemical property was caused by the slight reduction on the surface by methanol decomposition. In addition to the absence of island structure, line defects appeared along the Zn atom rows in panels b and c of Figure 4. The line defect was 0.6, 0.09, and 3 nm in width, depth, and average length, respectively, as indicated by the line profile shown in Figure 4d. This type of line defect was never found on the stoichiometric ZnO(101j0) after methanol exposure. It was neither found on the slightly reduced ZnO(101j0) surface without methanol adsorption (Figure 4a). Considering the dimensions of the line defects, the line defect can be assigned to the missing row of zinc-oxygen dimers, which are formed by methanol decomposition at the Zn-O dimer defect. A proposed mechanism will be discussed later. Short protrusive chains along the Zn atom rows are also observed in panels b and c of Figure 4. The chain had a height of about 0.14 nm and an average length of around 3 nm (Figure 4d) and was located roughly in between the Zn atom rows. From the size and its location against the Zn atom row, we consider that the bright chains are regarded as the linear chain structure found on a stoichiometric surface by methanol adsorption (Figure 2). Although mostly they were observed as single chains on the slightly reduced surface as shown in Figure 4, sometimes double chains as those in Figure 2 were found. Such an example is found in Figure 5. Whole bright chains were observed as single chains from panel a to panel e in Figure 5, but suddenly

in Figure 5f all of them changed to the double chains. This indicates the double chains are artifacts. The coverage of the linear chain structure was higher than that on the stoichiometric surface (0.03 at the maximum). It was close to 0.1 when the slightly reduced surface was exposed to 20 L of methanol. The increase of the coverage may originate from the decrease in the formation energy barrier on the slightly reduced surface. Sequential STM images in Figure 5 indicate that the structure of the methanol-adsorbed surface dynamically changes even at room temperature. Two distinct changes different from methanoladsorbed stoichiometric ZnO(101j0) were observed: one was high mobility of the linear chain structure and the other was extension and formation of the line defect. The area enveloped by a green dashed ellipse showed the growth, shrink, and sudden change in position for the linear chain structure. On the stoichiometric surface, only extension of the linear chain structure was observed as a minor process (Figure 3). In the area just below the blue arrow it was found that the linear chain structure was pinned when its end faced the line defect. Therefore such end-to-end connected pairs of the bright chains and line defects were often found on the surface as shown in Figure 4c. The purple circle at the bottom left in Figure 5 highlights the area where a line defect was extended and a new line defect appeared at its adjacent row. Removal of the surface atoms was probably induced by methanol decomposition, as will be discussed later. 4. Discussion 4.1. Methanol-Derived Ordered Structure on the Stoichiometric Surface. Previous TPD experiments28-32 and XPS and UPS measurements33 indicated that methanol dissociatively adsorbs on ZnO(101j0) with minor molecularly adsorbed species. Two dissociative adsorption pathways were proposed by C1s XPS spectra:33 (1) a major path forming methoxy species bound to surface Zn (CH3O-Zn(s)) and hydroxy (H-O(s)) species

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and (2) a minor path yielding methyl species bound to surface oxygen (CH3-O(s)) and hydroxy species bound to surface Zn (HO-Zn(s)). Pathways 1 and 2 represent the O-H bond breaking and the C-O bond breaking in CH3OH, respectively. Recent DFT calculations showed that energy gain by pathway 1 was 0.8 eV, which was twice that through pathway 2.61 Thus, CH3O-Zn(s) and H-O(s) are expected to be the major surface species formed by dissociative adsorption at room temperature. Our STM results after methanol adsorption on the stoichiometric ZnO(101j0) surface clearly showed that two ordered structures were formed on the surface (Figures 2 and 3). The island structure was the major one and the linear chain structure occupied the area at a tenth of the former one. Therefore it can be considered that the island structure consists of CH3O-Zn(s) and H-O(s) through pathway 1 and the linear chain structure consists of CH3-O(s) and HO-Zn(s) through pathway 2. The bright lines in the island structure were located on the Zn atom rows as shown in Figure 2b. The side view of the model for the island structure in Figure 2d was drawn referring to the stable structure for dissociated CH3O-Zn(s) and H-O(s) by a DFT calculation.61 One can expect that most protruding methoxy species effectively contribute to the electron tunneling forming bright lines on the Zn rows. The rather packed structure along the Zn row may be the reason for insufficient resolution along the bright line by STM. The bright lines in the linear chain structure, in contrast, located between the Zn atom rows. As noted above double chains found in Figures 2 and 3 may be artifacts of single chains. Although CH3 and HO are located on an O-Zn pair by the DFT calculation,61 CH3-O(s) and HO-Zn(s) are aligned to form a zigzag row in the model of the linear chain structure in Figure 2d. Periodicity along the row is twice that of the unit vector (0.66 nm) and it qualitatively reproduces the image feature in Figure 2c. It is not straightforward which part of the zigzag structure mainly contributes for electron tunneling. Thus theoretical calculations for STM simulation as well as actual energy for the two ordered structures are necessary to fully confirm the validity. Both the growth and the shrink of the island structure were observed even at room temperature (Figure 3). This indicates a low energy barrier for formation and decomposition of the island structure. We consider that weakly bound methanol molecules which migrate too fast to be observed by STM serve as an intermediate for transport on the area uncovered by ordered structures. The existence of molecularly adsorbed methanol was suggested by C1s photoemission from a methanol-adsorbed ZnO(101j0) surface at room temperature.33 That is, molecularly adsorbed methanol is in equilibrium with dissociative species of CH3O-Zn(s) and H-O(s) as in eq 1.

CH3OH + Zn-O(s) h CH3O-Zn(s) + H-O(s)

(1) CH3OH + Zn-O(s) F CH3-O(s) + HO-Zn(s)

(2) In contrast, the linear chain structure happened to be extended but rarely shrunk. Thus it is a one-way reaction as in eq 2. As was noted, a DFT calculation showed that the stabilization energy was higher for eq 1.60 Then it is reasonable to consider that the different behaviors of the two reactions come from the different activation energy for each reaction when the molecularly adsorbed methanol attacks the edge of the two ordered

structures. It is partly understood from the configurations of each structure: it seems easier to recombine to the methanol molecule for the island structure (Figure 2d). Generally, recombinative desorption is often found from methoxy and hydrogen atom but not from methyl and OH on various metals and metal oxides. TPD results for methanol on some ZnO surfaces showed that the amount of dissociated methanol was higher for a stepped ZnO(505j1) surface than the flat ZnO(101j0) surface.28,30 A recent TPD study of CH3SH on ZnO surfaces also showed the same tendency against the surface structure.62 It was proposed that the step edges are the active sites for dissociation to provide CH3S species.62 Thus, such surface defects are proposed to be the site for O-H (or S-H) bond scission. In our STM observations, no apparent correlation was found between the location of the ordered structures from the methanol derived species and step edges. The step edges may contribute to the C-H bond cleavage necessary for formation of the formate intermediate or C-O bond scission for methane production during the heating process. 4.2. Adsorption Behavior on the Slightly Reduced Surface. Several unexpected features were observed for methanol adsorption on the slightly reduced ZnO(101j0) surface that was prepared by several cycles of methanol adsorption and decomposition. The surface structure after methanol adsorption at room temperature was quite different from that on the stoichiometric surface, although few noticeable differences were found on the surface structure before methanol adsorption (Figures 1b and 5a). We have only observed a trace of surface defects that could possibly be attributed to the surface oxygen vacancies on the slightly reduced surface (see Figure S4 in the Supporting Information). However, after methanol adsorption, three striking differences were observed: first the absence of the island structure, which was the dominant ordered structure on the stoichiometric surface, second the formation of line defects, and finally smaller coverage of the ordered structure. These things are probably correlated with each other as will be discussed below. The line defects emerged only after the slightly reduced surface was exposed to methanol vapor. They were extended even after methanol exposure was stopped, but neither shrank nor migrated on the surface. These results suggest that the structure is stable at room temperature if it is once formed. Thus the accumulation of migrated components of defects, whatever they are, is not plausible. As we have discussed above, molecularly adsorbed methanol migrates on the surface (too fast to be observed by STM) at least on the area where the structure remained the same as that before methanol adsorption. We considered that the reaction of methanol molecules induced the formation of the line defect. The line defect was 0.6 and 0.09 nm in width and depth, respectively (Figure 4d). The depth corresponds to the separation between the ZnO layers. On the basis of the apparent size by STM, we have tentatively assigned the line defect to the missing row of zinc-oxygen dimers. The Zn-O dimer defect was suggested to be the most abundant defect on ZnO(101j0) by calculation.57,58 We should consider other possibilities for the assignment. An example is the oxygen line vacancy, which is known as a stable surface vacancy on some oxides.63 But a theoretical calculation study showed that a pair of oxygen vacancies show a repulsive nature for ZnO, which has a covalent bonding nature.64 Besides the surface oxygen vacancy has been considered as the site of higher reactivity where a methanol molecule dissociatively adsorbed and pinned the structure, as illustrated in Figure 6a. Thus the oxygen line vacancy is not likely for the line defect. Adsorption

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Figure 6. Schematic model illustrating adsorption, migration, and reaction of methanol-derived species on a slightly reduced ZnO(101j0) surface. Dissociative adsorption of methanol (a) at the oxygen vacancy site and (b) at the edge of a line defect. (c) Diffusion of methyl (blue ellipse) and hydroxy (green ellipse) via a bridge intermediate. Large red circles represent oxygen atoms colored from faint to deep corresponding to atoms from bottom to outmost surface; middle size circles are zinc (black) and carbon (cyanic); small empty circles are hydrogen atoms. (d) Formation of an oxygen vacancy on the top layer at the edge of a line defect. The dashed line indicates the level of the second layer. The oxygen atom in the top layer is shown in red.

of some methanol-derived species on the Zn row, which diminish the local density of states at the unoccupied states of the Zn row, may be a possible reason for a slight depression. But it should not be so large to explain the depression of 0.09 nm. If missing Zn-O dimers are formed on the surface as the line defect, a possible reaction for the line defect extension is schematically shown in panels b and d of Figure 6. As shown in Figure 6b, one can suppose the equilibrium of eq 3 (like in eq 1) inside the line defect because unsaturated Zn and O present in the trough.

CH3OH + Zn-O (2nd layer) h CH3O-Zn (2nd layer) + H-O (2nd layer) (3) For the methoxy adsorbed on the second layer Zn at the end of the line defect, extraction of the H atom of the methoxy by the neighboring first layer O atom seems to be easy from the geometrical point of view. If two hydroxy species at the first layer and the second layer recombine to desorb as H2O, the first layer O atom with less coordination should be removed leaving an oxygen vacancy at the first layer. Desorption of formaldehyde from the formaldehyde-adsorbed ZnO(101j0) was observed around 400 K similar to the methanol desorption.30 The produced formaldehyde molecule freely migrates on the surface as methanol does. If such removal of the first layer atom by the reaction is assumed, the extension of the defect will be stopped when its end faces the saturated O atom. Actually, the extension was stopped when the line defect faced the bright row of the linear chain structure (Figure 4c). We should admit, however, we do not have a clear conclusion for the fate of the Zn atom, which should be removed at the line defect edge when the paired O atom has been removed. It is known that the Zn

cation can possess the interstitial position in ZnO bulk and the diffusion barrier between the interstitial sites was calculated to be 0.3-0.6 eV.65,66 Diffusion to the interstitial sites may be a possible way for the remaining Zn atom formed by the reaction. The decrease in apparent surface coverage observed by STM was unexpected because a previous TPD study indicated that a higher number of decomposed products were formed on the reduced ZnO(101j0) surface compared with that on the stoichiometric surface.29 It is noted that the amount of highly mobile species cannot be estimated by STM. Molecularly adsorbed methanol on the stoichiometric surface is an example. We do not have any direct evidence; however, an assumption is made to explain the discrepancy between STM and TPD results, that is, the diffusion barrier of adsorbates is lowered on the slightly reduced surface. As discussed already, the dynamic behavior of the island structure was attributed to the equilibrium of eq 1 mediated by highly mobile molecularly adsorbed methanol. If CH3O- and H- easily migrate along the Zn atom row and O atom row, respectively, they are attached more frequently by molecularly adsorbed methanol to be converted according to eq 1. In this case, the island structure does not show a rigid structure any more. The reverse reaction of eq 2 does not occur easily; therefore, the migration of CH3- and HO- themselves along the O atom row and Zn atom row, respectively (illustrated in Figure 6c), is necessary for the dynamic change observed in Figure 5 (in the green dashed ellipse). If the migration of the components is allowed, the shrink of the bright chain can be regarded as migration of the chain against the slow scanning direction (from up to down in Figure 5) without actual shrink. Many unknown factors still remain in the understanding of the strange results on the slightly reduced surface. Further systematic studies with a help of a method to accurately identify the surface composition as well as variable-temperature STM measurements

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