The role of tunnel K⁺ ions on the growth and stability of tunnel-structured cryptomelane-type MnO₂ nanofibers (denoted as cryptomelane nanofibers hereafter) has been discussed by means of X-ray diffraction and electron microscopy. Cryptomelane nanofibers with typical diameters of 20–80 nm and lengths of 1–6 μm have been synthesized by means of a simple hydrothermal reaction of KMnO₄ and MnSO₄ aqueous solutions at 140 °C. The growth of cryptomelane nanofibers under hydrothermal conditions follows a dissolution–recrystallization process and involves a morphological transformation from a layered precursor to the tunnel-structured cryptomelane, in which the K⁺ ions play important roles in templating and stabilizing the tunneled framework. The presence of tunnel K⁺ ions also enhances the frame stability of the cryptomelane nanofibers at elevated temperatures. The formation of a layered K_xMn₂O₄ (x ≈ 0.26) with a hexagonal phase structure has been observed at about 900 °C. The transformation from tunneled cryptomelane to layered K_xMn₂O₄ also follows the dissolution–recrystallization growth mechanism, in which the diffusion of K⁺ ions at high temperatures represents a critical process. The topological correlation between the tunneled and layered MnO₂ materials might provide useful information for the synthesis of MnO₂ nanomaterials with controlled microstructures for different applications.

A simple, mild, and effective template approach has been used to produce hollow silica nanospheres with controlled sizes ranging from 40 to 150 nanometers. The obtained powders showed systematic variations in measured thermal conductivity, with values down to 0.024 W/(mK) so far, with en expressed goal to reach below 0.020 W/(mK). Surface hydrophobization was successfully performed. Thus, hollow silica nanospheres are considered to be promising building blocks for new hydrophobic, superinsulating materials.

Single-crystalline sodium tungsten bronze (Na-WO₃) nanorods with typical diameters of 10−200 nm and lengths of several micrometers were prepared via hydrothermal synthesis. The as-prepared Na-WO₃ nanorods crystallized in a hexagonal structure (space group P6/mmm) with unit cell parameters a=7.3166(8) Å and c= 3.8990(8) Å and elongated along the ⟨001⟩direction.

Chemical analyses indicated a stoichiometry of Na_0.18WO_3.09·0.5H₂O, revealing the existence of tunnel Na⁺ ions and water molecules in the structure, as confirmed also by the vibrational spectroscopic study. The as-prepared Na-WO₃ nanorods exhibited a direct-allowed electronic transition with band-gap energy of about 2.5 eV, which allows a visible-light-driven photochromism related to photogenerated carriers and a proton−electron double injection process. The proposed photochromism was discussed in detail by means of Fourier transform infrared spectroscopy. The involved local structural evolutions such as water decomposition and ion intercalation during the photochromic process were identified.

Single-crystalline sodium tungsten bronze (Na-WO₃) nanorods with typical diameters of 10–200 nm and lengths of several of microns were prepared via hydrothermal synthesis. X-ray diffraction data showed that the as-prepared Na-WO₃ nanorods crystallize in a hexagonal structure (space group P6 / mmm) with unit cell parameters a = 7.3166(8) Å and c = 3.8990(8) Å, and elongate along the <001> direction. The Na-WO₃ nanorods had a mean chemical composition of Na_0.18WO_3.09·0.5H₂O. The Na-WO₃ nanorods exhibited a typical cathodic coloration related to proton insertion, indicating their potentials in electrochromic smart window applications.