Regular Paper Microscope Observation of MoS2 Nanoparticles Synthesized on Rutile TiO2 Single Crystals

Nanometer-sized particles of MoS2 were synthesized on (100), (001), and (110) surfaces of rutile TiO2. Molybdenum oxide was deposited on the TiO2 substrates and then sulfided to simulate the preparation of industrial catalysts. The nanoparticles which appeared on the sulfided surfaces were assigned to MoS2 particles. The topography of the nanoparticles was observed by atomic force microscopy (AFM) and by transmission electron microscopy (TEM). The number density of the nanoparticles was related to the surface energy of the substrates through the population of nucleation centers. On the TiO2(110) surface, AFM and TEM images assignable to MoS2 crystals edge-bonded to the substrate were observed. The coincidental lattice spacings of MoS2 and this particular surface were related to the observed edge-bonded growth. [DOI: 10.1380/ejssnt.2004.32]


I. INTRODUCTION
MoS 2 -based catalysts have been intensely investigated for the hydrodesulfurization (HDS) reaction of petroleum [1]. It has been claimed that the aspect ratio (height vs. width) of MoS 2 nanoparticles supported on a substrate controls the selectivity of the reaction, i.e., the HDS or hydrogenation [2,3]. MoS 2 /Al 2 O 3 catalysts promoted with cobalt additives present more advanced catalytic performances. It also has been pointed out that on Co-promoted catalysts, the CoMoS-II phase cluster of a multilayered structure is more active than the CoMoS-I phase of a single-layer structure [4,5]. In addition, the role of MoS 2 crystals edge-bonded to the substrates has been debated [6][7][8]. Recently, edge-bonded particles have been found on γ-Al 2 O 3 [9,10] and anatase TiO 2 [11,12] substrates by transmission electron microscopy (TEM). These edge-bonded particles showed improved activity for hydrogenation [12]. Therefore, a method for making various figures of MoS 2 clusters is needed to control catalytic reactions. It is important to understand the 3-D topography of supported MoS 2 nanoparticles. Handling real catalysts is one of the advantages of TEM, while information on the cluster height is limited. Scanning probe microscopy (SPM) is a powerful method to deduce the 3-D topography of nanoparticles supported on a flat substrate. Macroscopic crystals of MoS 2 have been frequently observed by scanning tunneling microscopy (STM) as a substrate of compound semiconductors [13][14][15][16][17], as a conductive support of metal clusters [18], and as a model catalyst for the HDS reaction [19,20]. MoS 2 nanoparticles have been observed by STM on graphite [21,22] and on gold [23][24][25] substrates. Atomic-scale topography of triangular-shaped MoS 2 was resolved on the latter substrate. In the present study, we chose rutile TiO 2 as the substrate to construct a more realistic model of industrial MoS 2 catalysts supported on metal oxides. Nanoparticles of MoS 2 were synthesized on (100), (001), and (110) single-crystalline surfaces of rutile TiO 2 and observed by atomic force microscopy (AFM) and by TEM, as an extension of our earlier works [9,26]. Molybdenum oxide was deposited on the TiO 2 substrates and then sulfided to simulate the preparation of industrial catalysts.

II. EXPERIMENTAL
One-side polished single-crystalline substrates of rutile TiO 2 (Shinkosha, 10 × 10 × 0.5 mm 3 ) were used as substrates. The preparation method of the MoS 2 model catalyst on these substrates was described elsewhere [9,26]. Molybdenum oxide was vacuum evaporated from a Knudsen cell at 783 K to the substrate which was maintained at room temperature. The thickness of the deposited film was controlled to be 0.3 nm by referring to a quartz oscillator. The deposited samples were oxidized with O 2 gas (1.3 × 10 −3 Pa) at 673 K for 30 min and then sulfided with H 2 -balanced 5%-H 2 S gas (1.0 × 10 5 Pa) at 673 K for 30 min.
The topography of the sulfided samples was observed with an AFM (Jeol, JSPM-4200) under a vacuum (5 × 10 −4 Pa) at room temperature using a SiN 4 cantilever (Olympus, OMCL-TR400PSA-1). The 3-D scale of the microscope was calibrated on HOPG (Advanced Ceramics, STM-1) and MoS 2 wafers. Constant force topography determined in a contact mode is presented in a gray scale. TEM (Topcon EM-002B) observation was performed at 200 kV with a point resolution of 0.19 nm on the substrates, which were prepared by using a combination of punching, grinding, and ion-milling techniques [27]. Figure 1 shows 500-nm-square AFM images of the TiO 2 (100) sample following the fabrication steps. The surface as received (washed by supersonic wave in acetone before use) (a) was flat with vertical corrugations less than 0.6 nm as shown in cross section (d). Molybdenum oxide was thermally deposited on the surface of (a) and oxidized at 673 K. The topography (b) and cross section (e) of the oxidized surface remained flat, indicating the TiO 2 substrate was homogeneously coated with molybdenum oxide. Similarly, a flat topography of the MoO x layers was observed on TiO 2 (001) and (110) substrates (not shown). This is consistent with the epitaxial growth of MoO 2 found on these TiO 2 substrates [26]. The O/Mo atom ratio of the oxide-covered samples was determined to be 2 by Auger electron spectroscopy (AES). MoO 2 of a distorted rutile structure was proposed to epitaxially crystallize with Mo-O-Ti linkages along the following orientations, MoO 2 (010) // TiO 2 (100),

A. AFM observation
on the basis of reflection high-energy electron diffraction (RHEED) [26]. Nanometer-sized particles appeared when the oxide-covered (100), (001), and (110) surfaces were sulfided. The topography of the sulfided (100) surface is shown in (c). These particles should have been assigned to the MoS 2 nanoparticles of composition, because the S/Mo atom ratio was determined at 1.9 on a similarly treated TiO 2 (100) substrate [26]. The size and number density of the MoS 2 nanoparticles were examined on wide-area scans and are described in the next paragraph.
Corrugations on the flat plane other than the nanoparticles remained less than 0.6 nm. The amount of deposited Mo (equivalent to an oxide thickness of 0.3-nm) was involved much more in the clusters as excess MoS 2 than the nanoparticles of MoS 2 composition. We thus assumed that the excess MoS 2 yielded clusters of sub-nanometer sizes and were spread over the plane among the nanoparticles. Those sub-nanometer clusters which directly interfaced with the TiO 2 substrate may have been incompletely sulfided [26]. Incompletely sulfided clusters which were anchored to TiO 2 substrates via Mo-O-Ti linkages were also observed by EXAFS [28] and XPS [29].  Table 1). The height of the particles was measured on cross sections, a fraction of which are presented in (d)-(f). The distribution of the particle height is summarized in the bar-graph of Fig. 3. On the (001) substrate more than 80% of the particles fall in the 2-nm segment. The average height is listed in Table 1.
The number density of the nanoparticles is related to the surface energy of the substrates. A substrate with a larger surface energy yields more particles. The particles should be nucleated on singularities (steps, vacancies, etc.) of the single crystalline substrates. It is expected that the creation of those singularities is enhanced on a surface of large surface energy. When the total MoS 2 quantity available for the particle formation is the same on the three substrates, the maximum particle size should be found on the surface with the least number of particles. This was the case on the (110) substrate in Table 1.
On the other hand, the sub-nanometer clusters assumed in the interpretation of Fig. 1 Figure 5 shows the TEM image of model catalysts prepared on (110) and (100) substrates sulfided at 773 K. Short, dark rows were observed on the (110) substrate in (a), whereas only a few were recognized on the (100) substrate in (b). Most particles were fully sulfided on TiO 2 (100) surface (Table 1). Basal-bonded MoS 2 clusters should be grown on TiO 2 (100) during the sulfidation. However, it is difficult to image basal-bonded clusters by TEM [8]. Typical examples of the rows are pointed by white arrows in (a). Similar dark rows have been observed on MoS 2 supported on γ-Al 2 O3 [7-9, 27] and anatase TiO 2 powders [11] and assigned to the side-viewed Mo planes of MoS 2 crystals edge-bonded to the substrates. As shown in the zoomed images of (c), (d), and (e), the crystal plane of the edge-bonded clusters was oriented parallel to the [001], [110], and [112] directions. The preferred directions observed by AFM (Fig. 4) and TEM (Fig. 5) were consistent. The length and layer-thickness of the edge-bonded cluster ranged from 1-3 nm and 1-3 layers in the TEM images, whereas its height ranged from 4-6 nm as shown in the AFM results.

B. TEM observation
On the basis of the atomically resolved TEM images of Fig. 5, Fig. 6     bonded growth.