Studying the Solvothermal Formation of MoO₃ Fibers by Complementary In Situ EXAFS/EDXRD Techniques

Abstract

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 ...

Hydrothermal synthesis of anisotropic alkali and alkaline earth vanadates

“…Unlike their lithium-and sodium-based analogues, the MX/V 2 O 5 (M ‫ס‬ Rb, Cs; X ‫ס‬ OH, F) systems exhibit a low tendency toward vanadate fiber growth, and microcrystalline MV 3 Among the various types of lithium vanadate fibers resulting from the hydrothermal reaction of V 2 O 5 in LiX (X ‫ס‬ OH, F − I) solutions, the products grown in the presence of LiOH exhibit the highest degree of crystallinity and anisotropy. The lithium vanadate rods exhibit average diameters in the range of 400-600 nm and lengths extending up to several micrometers (cf.…”

“…X-ray powder analysis was conducted on a STOE STADI-P2 diffractometer (STOE & Cie., Darmstadt, Germany) in transmission mode (flat sample holders, Ge-monochromated CuK ␣1 radiation) equipped with a position sensitive detector (resolution ∼0.01°in 2 ). The synchrotron x-ray powder diffraction data for Mg 7-x (VO 4 ) 4 (OH) 3 (cf. Fig.…”

“…has been carried out in the space group P6 3 mc, allowing for all the unit cell constants, necessary profile parameters, and all the atomic positional and thermal parameters to vary. One of the two different magnesium positions is certainly incompletely occupied, thus leading to the chemical formula Mg 7-x (VO 4 ) 4 (OH) 3 so that the formal electro-neutrality of the chemical formula is not obeyed. However, the difference Fourier maps did not show any additional maxima that were not accounted for in the model, thus proving the original structure motif to be correct [ Fig.…”

“…No morphological features have been reported for the zinc sulfate vanadate. 64 The synthesis of Mg 7-x (VO 4 ) 4 (OH) 3 , however, provides both a high yield (∼95%) and an interesting morphology.…”

“…Their preparative convenience has recently been demonstrated for the formation of molybdenum oxide fibers, 3,4 and the present study is focused on a hydrothermal pathway to anisotropic vanadates. The hydrothermal preparation and the sol-gel chemistry of vanadium oxides has been in the focus of research interest over the past decades.…”