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 H₂W₂O₇· xH₂O (x = 3.49) with n-alkylamines (C_mH _2m+1NH₂, 4 ≤ m ≤ 14) in a system of heptane/n-alkylamine/H₂W₂O₇·xH ₂O (n-alkylamine:H₂W₂O₇· xH₂O 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 reversemicelle-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 ₂W₂O₇·xH₂O 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:H₂W₂O ₇·xH₂O 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 H₂W ₂O₇·xH₂O but also favor the formation of belt/tubelike morphology.