Tungstate-Based Inorganic−Organic Hybrid Nanobelts/Nanotubes with Lamellar Mesostructures: Synthesis, Characterization, and Formation Mechanism

The formation process of novel tungstate-based inorganic−organic hybrid nanobelts/nanotubes with lamellar mesostructures has been investigated, with an emphasis on monitoring the morphological and microstructural changes of the products during the reactions of H 2 W 2 O 7 · x H 2 O ( x = 3.49) with n -alkylamines (C m H 2 m +1 NH 2, 4 ≤ m ≤ 14) in a system of heptane/ n -alkylamine/H 2 W 2 O 7 · x H 2 O ( n -alkylamine:H 2 W 2 O 7 · x H 2 O molar ratio of about 30) under ambient conditions. The results indicate that normal intercalation occurrs in the early stage to form intercalation compounds with double-octahedral W−O layers, which are then dissolved in the highly alkaline aqueous solutions confined in the reverse-micelle-like media, where the dissolved species recrystallize to form hybrid nanobelts/nanotubes with single-octahedral W−O layers. Both the intercalation compounds obtained after a short reaction time (e.g., 30 min) and the hybrid nanobelts/nanotubers formed after a long reaction time (e.g., 5 days) possess a bilayered arrangement of n -alkyl chains, but their tilt angle in the intercalation compounds (42°) is much smaller than that in the hybrid nanobelts/nanotubes (71°). The interlayer water released from H 2 W 2 O 7 · x H 2 O upon intercalation of n -alkylamine reacts with excess n -alkylamine molecules to form highly alkaline aqueous solutions, which have vital effects on the subsequent dissolution of the double-octahedral W−O layers to be single-octahedral layers. In addition, the high molar n -alkylamine:H 2 W 2 O 7 · x H 2 O ratios (e.g., 30) are necessary to form tungstate-based inorganic−organic nanobelts/nanotubes, and the nonpolar solvents not only facilitate the reactions between n -alkylamines and H 2 W 2 O 7 · x H 2 O but also favor the formation of belt/tubelike morphology.