On the STM imaging contrast of graphite: towards a “true’' atomic resolution

Different phenomena observed in the high-resolution images of graphite by scanning tunneling microscopy (STM) or atomic force microscopy (AFM) such as the asymmetry in the charge density of neighboring carbon atoms in a hexagon, the high corrugation amplitudes and the apparent absence of point defects has led to a controversial discussion since the first published STM images of graphite. Different theoretical concepts and hypotheses have been developed to explain these phenomena. Despite these efforts a generally accepted interpretation is still lacking. In this paper we discuss a possible imaging mechanism based on mechanical considerations. Forces acting between tip and sample are taken into account to explain the image contrast. We present for the first time a direct atomic resolution of the graphite hexagonal structure by transmission electron microscopy (HRTEM), revealing the expected hexagonal array of atoms and the existence of several types of defects. We discuss the possibility that the STM image of graphite is a result of convolution of the electronic properties and the atomic hardness of graphite

Sulfated Zirconia Thin Films

Sulfated zirconia (SZ) has been found to be catalytically active for the isomerization of n-butane at room temperature [1]. This discovery has led to numerous investigations into the catalyst; however, no consistent theories have been devised to satisfactorily explain its structure, acidity and reactivity [2]. In order to improve surface characterization, a model system consisting of a nanocrystalline SZ thin film supported on a silicon wafer has previously been developed [3, 4]. The films are prepared using an aqueous route via a self-assembled monolayer (SAM), in which the film thickness is accurately controlled by deposition time. SZ thin films have been synthesized as described in [3, 4]. Successful formation of a SZ thin film has been verified by using XPS, SEM and HRTEM techniques. References [1] M. Hino, K. Arata., J. Am. Chem. Soc., 1979, 101, 6439-6441. [2] X. Song and A. Sayari, Catal. Rev. - Sci. Eng., 1996, 38, 329-412. [3] F.C. Jentoft, A. Fischer, G. Weinberg, U. Wild, R. Schlögl, Stud. Surf. Sci. Cat., 2000, 130, 209-214. [4] A. Fischer, Doctoral Thesis, Fritz Haber Institute, 2001

Oxygen Exchange on Vanadium Pentoxide

The isotopic exchange of 18O2 on polycrystalline V216O5 was studied by Raman spectroscopy at different temperatures between 300 and 580 °C and in the presence of different mixtures of oxygen with ethane, propane, or n-butane in the gas phase. Supported by DFT calculations, a method was developed to determine which of the three differently coordinated oxygen atoms in the crystal structure of V2O5 (vanadyl oxygen O1, 2-fold-coordinated oxygen O2, and three-coordinated oxygen O3) are involved in the exchange with 18O2 from the gas phase. Thus, it was found that the band at 994 cm–1, which is commonly exclusively assigned to a V═16O1 stretching (Ag) vibration, also contains contributions of an 16O1–V–16O2 stretching vibration (B2g). If only the O1 position is exchanged, the B2g component shifts to 964.2 cm–1, while if both O1 and O2 are exchanged, a shift to 953.4 cm–1 is expected. In contrast, the Ag component shifts only to 955 cm–1, regardless of whether only the O1 position or all three oxygen atoms are exchanged. On this basis, it was found that oxygen exchange at 573 °C in absence of an alkane involves O1 and O3 atoms, whereas in the presence of propane all three oxygen atoms are exchanged. In the latter case, the overall exchange rate appears to be limited by bulk diffusion. At typical reaction temperatures for the oxidative dehydrogenation of propane between 320 and 430 °C, no exchange occurs in pure oxygen. In presence of ethane or propane, only O1 is partly exchanged possibly at the surface and/or in a near-surface region. Under the typical reaction conditions of oxidative dehydrogenation of propane at 400 °C, there is hardly any variation in the spectra, and the small changes observed after long times on stream only affect O1, which, considering the sensitivity of the measurement method, leaves open whether the Mars–van Krevelen mechanism is indeed the predominant reaction mechanism under the conditions of oxidative dehydrogenation of alkanes on V2O5