Solvothermal Pathways to Transition Metal Oxides
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Solvothermal Pathways to Transition Metal Oxides Alexej Michailovski and Greta R. Patzke Laboratory of Inorganic Chemistry, ETH Zürich ETH Hönggerberg, Wolfgang-Pauli-Strasse 10, CH-8093 Zürich, Switzerland ABSTRACT A straightforward solvothermal pathway towards anisotropic nanoscale molybdenum, vanadium and tungsten oxides has been established. They are formed quantitatively from one-step procedures within a few days or hours of autoclave treatment in the temperature range between 100 and 220 °C. The addition of straightforward ionic additives (e.g. alkali halides) leads to a versatile interplay between the formation of novel polymolybdates(VI) and the production of oxidic nanoparticles. Key solvothermal features (role of the precursor, solvothermal parameter window, influence of ionic additives) of the individual transition metal oxides are investigated with respect to the development of general synthetic guidelines and predictive concepts. INTRODUCTION Solvothermal reactions are a key preparative method in modern inorganic and materials chemistry [1, 2]. They combine a wide range of experimental parameters and setups [3, 4] with the advantages of “chimie douce” approaches [5, 6]. Nevertheless, they are still difficult to systemise in the sense of a predictive preparative approach, because their theoretical and mechanistic foundations remain to be fully elucidated [7, 8]. In recent years, the potential of solvothermal reactions for the production of nanomaterials has been explored [9, 10]. For this purpose, the development of more general synthetic guidelines would be highly desirable in order to access tailor-made morphologies for nanoscale applications [11]. We have focused on the solvothermal synthesis of transition metal oxides [12]. This class of materials combines key applications and properties with an exceptionally rich structural chemistry. In this way, our experimental work crosses the border between the synthesis of new crystal structures and the development of solvothermal pathways towards new nanomaterials. Our primary targets are molybdenum-, tungsten- and vanadium oxides. They are well known for their excellent catalytic properties [13] – especially when combined into ternary oxides – and they provide a wide spectrum of other features, such as sensing properties [14], ion intercalation processes and photoluminescence [15]. In order to investigate the key parameters in solvothermal oxide synthesis, we pursue two complementary approaches. The first option is a “bottom up” strategy that is focused on the mechanistic investigation of selected solvothermal processes [16] with in situ-diffraction techniques (preferably EXAFS and EDXRD) [17, 18]. Parallel to these fundamental studies, we employ a “top down” strategy that is based on deriving synthetic rules from the summarization of solvothermal parameter fields. In this context, the combination of transition metal oxide-based precursors with straightforward ionic additives, such as alkali halides, has proven effective to cover a wide range of prod
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