Vanadium Oxide Nanolayers

In the bulk form, vanadium oxides display different oxidation states and V—O coordination spheres and exhibit a broad variety of electronic, magnetic, and structural properties [96, 97], which make these materials attractive for many industrial applications. Prominent examples range from the area of catalysis, where V-oxides are used as components of important industrial catalysts for oxidation reactions [98] and environment pollution control [99], to optoelectronics, for the construction of light-induced electrical switching devices [100] and smart thermo-chromic windows. In view of the importance of vanadium oxides in different technological applications, the fabrication of this material in nanostructured form is a particularly attractive goal. 

We refrain here from giving an extensive overview of studies on the surface structure of vanadium oxide nanolayers, as this has already been done for up to year 2003 in our recent review [97]. Instead, we would like to focus on prototypical examples, selected from the V-oxide-Rh(l 1 1) phase diagram, which demonstrate the power of STM measurements, when combined with state-of-the-art DFT calculations, to resolve complex oxide nanostructures. Other examples will highlight the usefulness of combining STM and STS data on a local scale, as well as data from STM measurements, and sample area-averaging spectroscopic techniques, such as XPS and NEXAFS, to derive as complete a picture as possible of the investigated system. 

The surface phase diagram of vanadium oxides on Rh(l 11) has been investigated in a series of papers of our group [4, 18, 19, 90, 101-103]. It is characterized by pronounced polymorphism and many different oxide structures have been detected as a function of coverage and growth temperature. The vanadium oxide structures for coverages up to the completion of the first monolayer formed on Rh(l 11) under the different preparation conditions may be subdivided into highly oxidized phases 

Within the inverse model catalyst approach, the y/7-V309-Rh(l 11) nanostructures have been used to visualize surface processes in the STM with atomic-level precision [104]. The promoting effect of the V-oxide boundary regions on the oxidation of CO on Rh(l 1 1) has been established by STM and XPS by comparing the reaction on two differently prepared y/7-V309-Rh(l 11) inverse catalyst surfaces, which consist of large and small two-dimensional oxide islands and bare Rh areas in between [105]. A reduction of the V-oxide islands at their perimeter by CO has been observed, which has been suggested to be the reason for the promotion of the CO oxidation near the metal-oxide phase boundary. 

1 nA) (b) DFT model of the (9 x 9) V36O54 phase. The (9 x 9) and (2 x 2) unit cells are indicated with solid and dashed lines, respectively (V green, O red, Rh gray). The inset shows a DFT-simulated image, (c) STM images 

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Catalysis in internal nanospaces of liquid catalysis. A catalysis of this sort probably occurs in molten salts carried in pores of inert porous carriers, to increase gas-liquid contact surface areas — vanadium pentoxide catalysts for the oxidation of SO2 to SO3 [5]. Reactants diffuse into the nanolayer below the catalysts surface and the product countercurrently diffuses out. 

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