Tao Gao

A comparison study on Raman scattering properties of α- and β-MnO2

In this comment to a recent paper [Anal. Chim. Acta 585 (2007) 241-245], we report a comparison study on Mn oxide-related compounds with different crystallographic forms, which distinguish between beta-MnO(2) and alpha-MnO(2) type materials via Raman scattering (RS) spectroscopy. The tetragonal rutile-type beta-MnO(2) is characterized by a RS band at approximately 667cm(-1) of symmetry A(1g), whereas the alpha-MnO(2) type materials feature two main RS contributions at about 574 and 634cm(-1), belonging to A(g) spectroscopic species of a tetragonal hollandite-type framework. These data represent a clear signature for identifying beta-MnO(2) and alpha-MnO(2) type materials via RS spectroscopy.

Crystal Structures of Titanate Nanotubes: A Raman Scattering Study

Crystal structures of titanate nanotubes prepared from a NaOH treatment of TiO(2) with subsequent acid washing were discussed from a viewpoint of vibrational spectroscopy. The correlation between the vibrational feature and the polymerization nature of the TiO(6) octahedron was established by analyzing Raman scattering data of crystalline TiO(2) (anatase and rutile) and layered protonic titanates. Then, the polymerization nature of TiO(6) octahedra in the titanate nanotubes was identified by comparing their Raman scattering spectra with those of the crystalline TiO(2) and layered protonic titanates. It demonstrated that the titanate nanotubes consist of two-dimensional TiO(6) octahedral host layers with a lepidocrocite (gamma-FeOOH)-type layered structure. This conclusion was confirmed further by considering the Raman scattering properties of a restacked titanate prepared by assembling TiO(6) octahedral layers derived from the original scroll-like titanate nanotubes. Our findings offered a convenient approach to validate the crystal structures of the products from an alkaline treatment of TiO(2) under different experimental conditions.

Structural and morphological evolution of beta-MnO2 nanorods during hydrothermal synthesis

Beta-MnO(2) nanorods were synthesized via a redox reaction of (NH(4))(2)S(2)O(8) and MnSO(4) under hydrothermal conditions. In situ and ex situ x-ray diffraction and scanning electron microscopy were employed to follow the structural and morphological evolution during growth. It was found that the crystallization of beta-MnO(2) nanorods proceeds through two steps: gamma-MnO(2) nanorods form first via a dissolution-recrystallization process and then transform topologically into beta-MnO(2) with increasing temperature. The phase transformation was associated with a short-range rearrangement of MnO(6) octahedra. Vibrational spectroscopic studies showed that the beta-MnO(2) nanorods had four infrared absorptions at 726, 552, 462 and 418 cm(-1) and four Raman scattering bands at 759 (B(2g)), 662 (A(1g)), 576 (Ramsdellite impurity) and 537 (E(g)) cm(-1), which are in agreement with Mn-O lattice vibrations within a rutile-type MnO(6) octahedral matrix.

Raman Scattering Properties of a Protonic Titanate HxTi2-x/4◻x/4O4·H2O (◻, vacancy; x = 0.7) with Lepidocrocite-Type Layered Structure

Raman scattering spectroscopy is employed to characterize a layered titanate HxTi2-x/4[symbol: see text]x/4O4.H2O ([symbol: see text]: vacancy; x=0.7) with lepidocrocite (gamma-FeOOH)-type layered structure. Nine Raman lines corresponding to (3Ag+3B1g+3B3g) Raman-active modes expected from this orthorhombic structure (space group D2h25-Immm) are recorded at 183, 270, 387, 449, 558, 658, 704, 803, and 908 cm(-1), which are assigned to Ti-O lattice vibrations within the two-dimensional (2D) lepidocrocite-type TiO6 octahedral host layers. These intrinsic Raman bands present a clear signature that can be used for probing the protonic titanate HxTi2-x/4[symbol: see text]x/4O4.H2O and the 2D titanate nanosheets, as well as their corresponding derivates.

Microstructures, surface properties, and topotactic transitions of manganite nanorods.

Manganite (gamma-MnOOH) nanorods with typical diameters of 20-500 nm and lengths of several micrometers were prepared by reacting KMnO(4) and ethanol under hydrothermal conditions. Synchrotron X-ray diffraction (XRD) reveal that the gamma-MnOOH nanorods crystallize in the monoclinic space group P2(1)/c with unit cell dimensions a = 5.2983(3) A, b = 5.2782(2) A, c = 5.3067(3) A, and beta = 114.401(2) degrees . Transmission electron microscopy shows that the gamma-MnOOH nanorods are single crystalline and that lateral attachment occurs for primary rods elongated along 101. X-ray photoelectron spectroscopy studies indicate that the surfaces of the gamma-MnOOH nanorods are hydrogen deficient and compensated by surface complexation. The Raman scattering spectrum features five main contributions at 360, 389, 530, 558, and 623 cm(-1) along with four weak ones at 266, 453, 492, and 734 cm(-1), attributed to Mn-O vibrations within MnO(6) octahedral frameworks. The structural stability of the gamma-MnOOH nanorods was discussed by means of in situ time-resolved synchrotron XRD. The monoclinic gamma-MnOOH nanorods transform into tetragonal beta-MnO(2) upon heating in air at about 200 degrees C. The reaction is topotactic and shows distinctive differences from those seen for bulk counterparts. A metastable, intermediate phase is observed, possibly connected with hydrogen release via the interstitial (1 x 1) tunnels of the gamma-MnOOH nanorods.

Protonic titanate derived from CsxTi2−x/2Mgx/2O4 (x = 0.7) with lepidocrocite-type layered structure

A layered titanate CsxTi2−x/2Mgx/2O4 (x = 0.7) with lepidocrocite (γ-FeOOH)-type layered structure was prepared via solid-state reaction. Extraction of both Mg2+ ions in the host layers and interlayer Cs+ ions was achieved during an acid-exchange process, producing a new protonic titanate HxTi2−x/2O4−x/2·H2O. This phase was distinguished from isomorphous related compounds in terms of removable lattice Mg and O atoms. The protonic titanate HxTi2−x/2O4−x/2·H2O showed excellent exfoliation/delamination reactivity upon intercalating organic amine ions as well as the ability to produce single two-dimensional titanate nanosheets with small thickness of about 1 nm. These findings offered a new clue for understanding the physicochemical properties of lattice dopants in lepidocrocite titanates.

Syntheses, structures, and magnetic properties of nickel-doped lepidocrocite titanates.

Ni-doped titanate Cs(x)Ti(2-x/2)Ni(x/2)O(4) and its protonic derivative H(x)Ti(2-x/2)Ni(x/2)O(4) x xH(2)O (x = 0.7) were synthesized and characterized by means of synchrotron X-ray diffraction, Raman scattering, X-ray photoelectron spectroscopy (XPS), and magnetic measurements. Cs(x)Ti(2-x/2)Ni(x/2)O(4) crystallizes in an orthorhombic structure (space group Immm), consisting of infinite two-dimensional (2D) host layers of the lepidocrocite (gamma-FeOOH) type. The substitution of Ni atoms for Ti in the 2D octahedral layers results in negative charges that are compensated by interlayer Cs(+) ions. Raman scattering and XPS indicate that local structural perturbations are induced upon exchange of interlayer Cs ions with protons H(3)O(+). Magnetic measurements reveal typical paramagnetism induced by Ni substitution; the effective paramagnetic moment mu(eff) = 1.57(1) mu(B) and Curie-Weiss temperature -2.51(1) K are obtained for H(x)Ti(2-x/2)Ni(x/2)O(4) x xH(2)O. Ni- and Mg-codoped titanates Cs(x)Ti(2-x/2)(Ni(y)Mg(1-y))(x/2)O(4) (x = 0.7, 0 < or = y < or = 1) were also reported. The crystal structure, interlayer chemistry, and magnetic properties of the titanates depend on the Ni substitution levels, indicating opportunities for tuning of the properties by controlling the nature and level of lattice substitutions.

In situ studies of structural stability and proton conductivity of titanate nanotubes

Titanate nanotubes were prepared by hydrothermal treatment of anatase TiO2 with concentrated NaOH solution. In situsynchrotron X-ray diffraction studies revealed that the nanotubes are thermally unstable at temperatures above 360 °C and can transform directly to anatase via a dehydration and recrystallization process. This indicated that the titanate nanotubes possess an orthorhombic lepidocrocite (γ-FeOOH)-type layered structure. An aggregate of the as-prepared nanotubes showed an electric conductivity of about 1 × 10−6 S cm−1 at 50 °C in humid atmospheres. The conductivity depended on humidity of the atmosphere and decreased with increasing temperatures, suggesting that the proton conduction in the titanate nanotubes might correlate with the interlayer H3O+ ions.

Preparation of Nb-substituted titanates by a novel sol-gel assisted solid state reaction.

Single-phase layered Nb-substituted titanates, Na(2)Ti(3-x)Nb(x)O(7) (x = 0-0.06) and Cs(0.7)Ti(1.8-x)Nb(x)O(4) (x = 0-0.03), were for the first time synthesized by a novel sol-gel assisted solid state reaction (SASSR) route. Conventional solid state reactions as well as sol-gel synthesis did not succeed in producing phase pure Nb-substituted titanates. In the SASSR synthesis route we combine the advantages of traditional sol-gel technique (i.e., homogeneous products formed at low temperatures) and solid state reaction (i.e., formation of stable, crystalline phases) for preparing single-phase niobium-substituted layered titanates. The obtained products were characterized by X-ray powder diffraction, scanning electron microscopy, inductively coupled plasma-atomic emission spectrometry, Raman spectroscopy, and thermogravimetric analysis. Results indicate that the Ti(IV) in the host layer of the samples could be partially replaced by Nb(V) without structural deterioration. After proton-exchange, more water molecules were intercalated into the interlayer of H(0.7)Ti(1.8-x)Nb(x)O(4) x nH(2)O with increasing niobium content, whereas the interlayer distance of H(2)Ti(3-x)Nb(x)O(7) (x = 0-0.06) was unchanged.