The discovery of carbon nanotubes in 1991 is a milestone in nanomaterials research. Since then, more and more anisotropic nanoparticles have been detected and characterized. The development of nanodevices might benefit from the distinct morphology and high aspect ratio of nanorods and nanotubes as these can be functionalized in unique ways such as incorporation of nanorods in nanotubes. Downscaling a broad range of materials to 1D nanoscopic structures is currently the focus of a rapidly growing scientific community. Developing general pathways to this goal would transfer a wide variety of properties to the nanoscale-a spectrum of phenomena so diverse that it would cover not only inorganic systems but all of materials science. Synthesis of real functional materials, however, always involves considerable synthetic ingenuity, interdisciplinary collaboration, as well as technological and economical realism. The major topic of this review is to provide a survey of recent progress in the synthesis of oxidic nanotubes and nanorods-with their non-oxidic counterparts briefly highlighted-and to outline the major synthetic routes leading to them. With the challenges of synthesizing bulk oxidic materials in mind, the establishment of trustworthy and uncomplicated ways of providing them as anisotropic nano-modules on an industrial scale appears to be more or less serendipity. Of the methods utilized in nanotube and nanorod synthesis solvothermal processes have emerged as powerful tools for generalizing and systematizing controlled syntheses of nano-morphologies. The flexibility and reliability of this synthetic approach is demonstrated here for the transformation of transition-metal oxides into high-quality anisotropic nanomaterials.

Colloidal lead halide perovskite nanocrystals (NCs) have recently emerged as a novel class of bright emitters with pure colors spanning the entire visible spectral range. Contrary to conventional quantum dots, such as CdSe and InP NCs, perovskite NCs feature unusual, defect-tolerant photophysics. Specifically, surface dangling bonds and intrinsic point defects such as vacancies do not form midgap states, known to trap carriers and thereby quench photoluminescence (PL). Accordingly, perovskite NCs need not be electronically surface-passivated (with, for instance, ligands and wider-gap materials) and do not noticeably suffer from photo-oxidation. Novel opportunities for their preparation therefore can be envisaged. Herein, we show that the infiltration of perovskite precursor solutions into the pores of mesoporous silica, followed by drying, leads to the template-assisted formation of perovskite NCs. The most striking outcome of this simple methodology is very bright PL with quantum efficiencies exceeding 50%. This facile strategy can be applied to a large variety of perovskite compounds, hybrid and fully inorganic, with the general formula APbX3, where A is cesium (Cs), methylammonium (MA), or formamidinium (FA), and X is Cl, Br, I or a mixture thereof. The luminescent properties of the resulting templated NCs can be tuned by both quantum size effects as well as composition. Also exhibiting intrinsic haze due to scattering within the composite, such materials may find applications as replacements for conventional phosphors in liquid-crystal television display technologies and in related luminescence down-conversion-based devices.

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Vanadium oxide nanotubes were obtained as the main product in a sol−gel reaction followed by hydrothermal treatment from vanadium(V) alkoxide precursors and primary amines (C n H2 n +1NH2 with 4 ≤ n ≤ 22) or α,ω-diamines (H2N[CH2] n NH2 with 14 ≤ n ≤ 20). The structure of the nanotubes has been characterized by transmission electron microscopy, X-ray powder diffraction, X-ray photoelectron spectroscopy, and magnetic measurements. The tubes are up to 15 μm long and have outer diameters ranging from 15 to 150 nm and inner diameters from 5 to 50 nm. The tube walls consist of 2−30 crystalline vanadium oxide layers with amine or diamine molecules intercalated in between. The distance between the layers (1.7−3.8 nm) is proportional to the length of the alkylamine, which acts as a structure-directing template. The structure within the layers has a square metric with a ≈ 0.61 nm. Cross-sectional TEM images demonstrate the predominance of serpentine-like scrolls rather than of concentric tubes. The intercalated templates can be easily substituted, e.g. by diamines, while the tubular morphology is preserved. This points to a highly flexible structure.

Cryptomelane-type manganese dioxide (K-MnO 2 ) nanofibers with typical diameters of 20-60 nm and lengths of 1-6 µm were prepared by reacting KMnO 4 with MnSO 4 under hydrothermal conditions. Rietveld refinement from synchrotron X-ray powder diffraction data showed that the K-MnO 2 nanofibers crystallize in a bodycentered tetragonal structure (space group I4/m) with unit cell parameters a ) 9.8241(5) Å and c ) 2.8523(1) Å and elongate along the <001> direction. The K-MnO 2 nanofibers had a mean chemical composition of K 0.11 (H 3 O) 0.05 MnO 2 . The optical band gap of the K-MnO 2 nanofibers was estimated to be 1.32 eV based on the UV-visible absorption. The K-MnO 2 nanofibers had four diagnostic infrared absorptions at 722, 593, 524, and 466 cm -l , which represents specific fingerprints of the vibrational features of MnO 2 materials containing (2 × 2) + (1 × 1) tunnel structures. The Raman scattering spectrum of the K-MnO 2 nanofibers had nine Raman bands with four main contributions at 183, 386, 574, and 634 cm -1 along with five weak ones at 286, 330, 470, 512, and 753 cm -1 , which are attributed to the Mn-O lattice vibrations within the MnO 6 octahedral frameworks. These intrinsic vibrational features can be conveniently used for online and/or in situ analyses of the K-MnO 2 nanofibers during electrochemical and/or ion-exchange reactions.