Nanorods of MoO 3 are accessible in gram quantities from MoO 3 ‚2H 2 O via a flexible onestep solvothermal reaction. Several hours of treatment in water at 180 °C are sufficient to convert the starting material quantitatively into rods with diameters around 100 nm and microscale lengths. The formation of MoO 3 rods proceeds in both neutral ionic and acidic media within a wide parameter window encompassing reaction temperatures between 90 and 180 °C and time scales ranging from several hours to 7 d. The rod morphology can be tuned by selecting a proper additive, and the nanorods withstand heating to 400 °C. Furthermore, the reaction pathways in various solvothermal media were investigated and both intermediate molybdic acids and the bulk nanorod products were characterized by means of EXAFS spectroscopy.

The reliable and straightforward fabrication of nanoparticles is essential for developing future nanotechnology. [1][2][3][4][5] Nanoscale transition-metal oxides provide a wide spectrum of important properties, [6,7] and their functionalization and alignment is greatly facilitated by anisotropic morphologies. [8] Among the transition-metal oxides, MoO 3 is the focus of much attention owing to its numerous applications, for example, in catalysis [9][10][11] or sensor technology. [12,13] We have recently developed a solvothermal procedure that provides a convenient access to highly anisotropic nanoscale MoO 3 fibers. [14,15] The solvothermal process [16] is famous for its unique control facilities regarding the particle size of the material produced. [17,18] Its major drawback, however, is that the predictive and rational preparation of a desired solid remains a major preparative challenge-especially if the morphology of the product is to be tuned as well. This problem is because of the tremendous impact of the synthetic parameters on the course of the reaction. Thus, the development of solvothermal nanomaterials syntheses may require considerable "trial and error" work. The elucidation of solvothermal reaction mechanisms is therefore vital for devising more predictive strategies so that the resulting materials can be optimized for commercial purposes. As solvothermal reactions are usually performed in thick-walled reaction containers, sophisticated in situ techniques employing high-intensity synchrotron radiation are required for their direct monitoring. [19][20][21][22] Generally, the development of high-pressure, in situ spectroscopic approaches is a challenging topic of chemistry and materials science. [23] Orientating kinetic studies may also be performed by quenching hydrothermal reactions, but this information is only valid if the recovered material is not affected by any irreversible changes upon cooling and isolation. Among the multitude of solvothermally generated materials, special emphasis has been placed upon the in situ study of zeolites, [24][25][26] open-framework compounds, [27,28] silicate minerals, [29,30] and sulfide-based systems. [22,31] For the transitionmetal oxides, the main focus has been on ternary compounds of industrial interest (e. g. bismuth molybdates [32] or BaTiO 3[33] ), whereas considerably fewer binary oxides (such as ZrO 2 [34] ) have been investigated by solvothermal in situ methods. Very little mechanistic information is available on their solvothermal transformation into nanomaterials.Herein, we report the first comprehensive in situ study on the growth of MoO 3 fibers by complementary EDXRD (energy dispersive X-ray diffraction) and XANES/EXAFS (X-ray absorption near edge structure/extended X-ray absorption fine structure) techniques. While XRD allows the long-range order to be monitored, XANES/EXAFS provides information on the short-range order. [35][36][37] This information is indispensable for describing the solvothermal crystallization of nanostructured matter, because ...

A high pressure in situ x-ray absorption spectroscopy cell with two different path lengths and path positions is presented for studying element-specifically both the liquid phase and the solid/liquid interface at pressures up to 250 bar and temperatures up to 220 °C. For this purpose, one x-ray path probes the bottom, while the other x-ray path penetrates through the middle of the in situ cell. The basic design of the cell resembles a 10 ml volume batch reactor, which is equipped with in- and outlet lines to dose compressed gases and liquids as well as a stirrer for good mixing. Due to the use of a polyetheretherketone inset it is also suitable for measurements under corrosive conditions. The characteristic features of the cell are illustrated using case studies from catalysis and solid state chemistry: (a) the ruthenium-catalyzed formylation of an amine in “supercritical” carbon dioxide in the presence of hydrogen; (b) the cycloaddition of carbon dioxide to propylene oxide in the presence of a solid Zn-based catalyst, and (c) the solvothermal synthesis of MoO3 nanorods from MoO3∙2H2O.

The hydrothermal formation of mixed nanoscale W/Mo-oxides with the hexagonal tungsten bronze (HTB) structure has been investigated by in situ EDXRD (energy dispersive X-ray diffraction). Compared to the binary oxide systems, they display intermediate kinetics with a nucleation-controlled mechanism dominated by the slow growing tungsten component. Furthermore, the thermal stability of nanostructured W/Mo-HTB compounds has been monitored through combined in situ X-ray absorption spectroscopy (XAS) and XRD in reducing and oxidizing atmospheres. Their transformation into other mixed nanostructures was only observed above 300 °C in O 2 -and H 2 -containing atmospheres. In addition, the shape of nanoscale hexagonal W/Mo-oxides can be expanded into a variety of morphologies via the use of alkali chlorides as hydrothermal additives. The alkali cations exert a two-fold role as internal stabilizers and external shape control agents. Their mobility within the channels of the W/Mo-oxide host framework has been investigated by solid state NMR spectroscopy.

Two novel poly(oxofluoromolybdate) clusters, [Mo6O18F6]6- and [Mo7O22F3]5-, have emerged from systematic field studies on the hydrothermal fluorination of poly(oxometalates). They are accessible via the hydrothermal treatment of Mo(VI)-based precursors in the presence of MF additives (M = Li, Na, Cs, NMe4). The new fluorinated polyanions are stabilized by specific alkali-cation combinations, and their packing motifs can be tuned through the careful choice of cations and synthetic parameters. The [Mo6O18F6]6- anion can be described as a cored and fluorinated form of the Anderson-Evans cluster type. It is stabilized by the interaction of two alkali cations with the fluorinated faces of the ring-shaped anion. The partial replacement of these capping alkali cations by the bulky, organic NMe4(+) cation leads to the formation of [Mo7O22F3](5-)-based compounds. Thus, the extent of fluorination can be controlled through the polarizability of the cationic environment: in the [Mo7O22F3]5- anion, half of the fluoride atoms of the [Mo6O18F6]6- anion are replaced by a capping MoO4 tetrahedron, thereby rendering this anion a fluorinated, lacunar derivative of the alpha-[Mo8O26]4- octamolybdate anion. All compounds have been structurally characterized by single-crystal X-ray diffraction and solid-state NMR spectroscopy. The templating role of the alkali cations is discussed and explained with the help of electrostatic calculations.

Formation of nanostructured hexagonal alkali tungstates (HTBs) from ammonium metatungstate (AMT) and alkali chlorides has been investigated by a combined strategy encompassing in situ spectroscopic techniques and hydrothermal synthetic studies. They demonstrate how the morphology of the HTBs can be tuned by the straightforward hydrothermal reaction of ammonium metatungstate with the choice of appropriate alkali chlorides. The resulting particle shapes vary from highly anisotropic fibers to multilevel assemblies of hierarchically grown nanorods. Microwave-hydrothermal techniques have further expanded this morphological spectrum. Next, the growth process of alkali HTBs was explored for selected cases by combination of in situ EXAFS and EDXRD investigations. They both show that nucleation starts after a certain induction time and is strongly dependent on the concentration. Though the individual kinetics of alkali HTB formation differ slightly from each other, all alkali cations have a nucleation-controlled growth mechanism in common.

The preparative flexibility of hydrothermal syntheses needs to be systemised for exploring complex structure-synthesis relationships and morphology control options in materials chemistry. This is demonstrated for the targeted hydrothermal preparation of molybdenum oxide materials: firstly, in situ studies were employed for the efficient production of MoO(3) nanofibres. Furthermore, ionic substances as structure-directing tools brought forward a new class of fluorinated polyoxomolybdates.

The title compound (systematic name: 4,10-dinitro-2,6,8,12-tetraoxa-4,10-diazatetracyclo[5.5.0.0(3,11).0(5,9)]dodecane), C(6)H(6)N(4)O(8), exhibits the highest density among known N-nitramines, due to its close-packed crystal structure. It may be regarded as consisting of a distorted hexagonal close-packed lattice formed by the isowurtzitane cages, with the nitro groups occupying the free space between the cages.