Characterization of Mo Deposited on a TiO₂(110) Surface by Scanning Tunneling Microscopy and Auger Electron Spectroscopy

Abstract

The properties of Mo ultrathin films deposited on a TiO2(110) surface were investigated by scanning tunneling microscopy (STM) and spectroscopy (STS), as well as by Auger electron spectroscopy (AES). The substrate exhibited mainly large (1 x 1) terraces decorated by additional [001] rows (missing or added 1D structures) of reduced TiO(x) phases. Only a few percent of the surface exhibited a cross-linked (1 x 2) arrangement. The deposition of Mo layers at room temperature with a rate of approximately 0.4 monolayer/ min resulted in nanoclusters of 1-2 nm with a low-profile shape (2D-like). Preferential decoration of the atomic steps was not found; at the same time, the 1D defect sites of missing or added rows on the (110) terraces were characteristically decorated by larger Mo nanocrystallites. This behavior indicates that the mobility of Mo atoms is higher on the more reduced regions of the substrate. The high dispersion of the Mo adlayer changed only slightly on annealing up to 700 K; in the range of 900-1050 K, however, a significant increase of the particle size accompanied by splitting of the TiO2(110) terraces was observed. This behavior may be explained by the partial oxidation of the supported Mo (accompanied by the reduction of the substrate) into tetragonal crystallites oriented and slightly elongated in the [001] or [110] direction of the TiO2(110) support. STS measurements indicated that the crystallites or the support/crystallite interface formed above 900 K possesses a wide band gap. The annealing above 1050 K resulted in the disappearance of Mo from the TiO2(110) surface, which may be explained by the formation and sublimation of MoO3 species at the perimeter of the nanoparticles. The change of AES signal intensities for O(KLL) and Mo(MNN) as a function of the annealing temperature also supports this mechanism.

Low temperature CO oxidation on differently prepared TiO2(110) supported Au catalysts

“…All of the kinetics data are calculated for the particle distribution measured at the beginning of the reaction. 110) support and also about the thermal behaviour of the deposited Au on clean and Ar + treated surfaces can be found in a previous paper [25,36]. The main conclusion of these works was that the generation of oxygen defect sites both by Ar (Fig.…”

“…Potassium was deposited by a SAES alkali source. The coverage of K and Mo is expressed in equivalent monolayer (eqML) which corresponds to approximately 1.5 x 10 15 atom cm -2 and calibrated by the relative AES intensity (I K252 / I Ti385 and I Mo185 / I Ti385 ) measured after the deposition at room temperature [25,36]. The actual coverage of Au was calculated by the determination of the volume of the particles [25].…”

“…Both K and Mo have a larger affinity to the surface oxygen than the Ti atoms have, therefore, they are able to reduce the TiO 2 (110) surface. Additionally, the potassium oxides are more stable (have a larger band gap), and the formation of Mo oxides is rather sensitive for the local oxidation state of the TiO 2 substrate sublayers [36][37][38]. The STM images of 50 x 50 nm 2 in Fig 2A, D depict the surface after the deposition of 0.15 eqML of K and 0.5 eqML of Mo, respectively.…”

Enhanced dispersion and stability of gold nanoparticles on stoichiometric and reduced TiO2(1 1 0) surface in the presence of molybdenum

“…, 300 K, 2 h) and annealing at 900 K. It is known from our previous STM investigations that similar treatments result basically in (1 Â 1) bulk terminated registry, although the presence of 1-2% of defect sites (0D dots and 1D strings of Ti 2 O 3 stoichiometry) cannot be excluded [31,32]. The long range ordered (1 Â 2) reconstruction of TiO 2 (1 1 0) surface does not occur at these relatively low temperatures.…”

“…This definition is frequently used in the literature [9,36]. The surface concentrations of the deposited metals in the STM chamber were estimated from the volume of the nanoparticles (after a short annealing at 500 K for Au and at 900 K for Mo) [11,31]. In the XPS-LEIS chamber the coverage of gold was determined both by XPS and LEIS.…”