Morphological and Kinetic Studies on Hexagonal Tungstates

Abstract

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.

Site‐Selective Deposition of Metal Nanoparticles on Aligned WO3 Nanotrees for Super‐Hydrophilic Thin Films

“…In addition, it is noted that the dense incubation layer of our films acts as a binder between the nanotree arrays and the substrate, leading to good adhesion to the substrates. Further, the addition of alkali salts in a hydrothermal solution was very important for the formation of nanostructure of WO 3 [ 19] They reported that the smaller cations led to fibrous structure with microscale length, while the larger cations yielded stacked nanorods with hierarchical patterns. In addition, Gu et al synthesized WO 3 nanostructure by adding metal salts in a hydrothermal solution, and Rb 2 SO 4 led to nanorod and nanoplate structures, whereas K 2 SO 4 led to ribbonlike structures.…”

Targeted Synthesis and Environmental Applications of Oxide Nanomaterials

“…[26] We have applied this strategy on the alkali chloride-assisted growth of hexagonal tungsten oxides. [16] Here, the particle morphology and degree of organization of the obtained alkali tungstate nanorods varies with the alkali cationalbeit not as pronounced as in the case of the mixed hexagonal W/Mo-oxides, e.g. their tungsten-based analogues do not display the organization into hierarchical spheres.…”

“…[15] Moreover, they can also exert a stabilizing effect via the incorporation into an otherwise unaltered open structural motif. [16] Oxides with such versatile structures are, for example, found among the W-, Mo-and V-containing transition metal oxides that also excel through their high application potential, e.g. in catalysis, sensor and battery technology.…”

“…[20] This synthetic route can subsequently be tuned by using alkali chlorides as additives in the above-mentioned hydrothermal procedure. [16,21] They exert a dual function: on the one hand, they stabilize the wide interior of the hexagonal channel system through intercalation and on the other hand, the morphology of the products can be steered with the alkali cation. The use of LiCl and NaCl gives rise to nanorod formation, whereas very small nanorods organize into cylinders that furthermore form spherical aggregates in the presence of KCl, RbCl and CsCl (Fig.…”

“…In the case of hexagonal tungstates, the alkali cations only influence the reaction kinetics and the morphology, but not the formation mechanism of the nanostructured oxides. [16] In situ EDXRD investigations on the hexagonal W/Mo-tungstates, however, revealed a different mechanistic scenario. The formation of nanorods in the presence of Li + and Na + follows complicated mechanistic sequences as indicated by the different subsequent slopes of the SH plots, whereas the spherical architectures obtained with K + /Rb + /Cs + -ions as additives all emerge from the same diffusion controlled mechanism so that their SH plots can be fitted with a straight line (Fig.…”