Synthesis of single-crystalline perovskite nanorods composed of barium titanate and strontium titanate

Ferroelectric order in individual nanometre-scale crystals

Ferroelectricity in finite-dimensional systems continues to arouse interest, motivated by predictions of vortex polarization states and the utility of ferroelectric nanomaterials in memory devices, actuators and other applications. Critical to these areas of research are the nanoscale polarization structure and scaling limit of ferroelectric order, which are determined here in individual nanocrystals comprising a single ferroelectric domain. Maps of ferroelectric structural distortions obtained from aberration-corrected transmission electron microscopy, combined with holographic polarization imaging, indicate the persistence of a linearly ordered and monodomain polarization state at nanometre dimensions. Room-temperature polarization switching is demonstrated down to ~5 nm dimensions. Ferroelectric coherence is facilitated in part by control of particle morphology, which along with electrostatic boundary conditions is found to determine the spatial extent of cooperative ferroelectric distortions. This work points the way to multi-Tbit/in(2) memories and provides a glimpse of the structural and electrical manifestations of ferroelectricity down to its ultimate limits.

Self-templated synthesis and thermal conductivity investigation for ultrathin perovskite oxide nanowires

The large thermal conductivity of bulk complex metal oxides such as SrTiO(3), NaCo(2)O(4), and Ca(3)Co(4)O(9) has set a barrier for the improvement of thermoelectric figure of merit and the applications of these materials in high temperature (≥1000 K) thermoelectric energy harvesting and solid-state cooling. Here, we present a self-templated synthesis approach to grow ultrathin SrTiO(3) nanowires with an average diameter of 6 nm in large quantity. The thermal conductivity of the bulk pellet made by compressing nanowire powder using spark plasma sintering shows a 64% reduction in thermal conductivity at 1000 K, which agrees well with theoretical modeling.

One-dimensional nanostructures of ferroelectric perovskites

Nanorods, nanowires, and nanotubes of ferroelectric perovskites have recently been studied with increasing intensity due to their potential use in non-volatile ferroelectric random access memory, nano-electromechanical systems, energy-harvesting devices, advanced sensors, and in photocatalysis. This Review summarizes the current status of these 1D nanostructures and gives a critical overview of synthesis routes with emphasis on chemical methods. The ferroelectric and piezoelectric properties of the 1D nanostructures are discussed and possible applications are highlighted. Finally, prospects for future research within this field are outlined.

Direct synthesis of ultrafine tetragonal BaTiO3 nanoparticles at room temperature

A large quantity of ultrafine tetragonal barium titanate (BaTiO3) nanoparticles is directly synthesized at room temperature. The crystalline form and grain size are checked by both X-ray diffraction and transmission electron microscopy. The results revealed that the perovskite nanoparticles as fine as 7 nm have been synthesized. The phase transition of the as-prepared nanoparticles is investigated by the temperature-dependent Raman spectrum and shows the similar tendency to that of bulk BaTiO3 materials. It is confirmed that the nanoparticles have tetragonal phase at room temperature.

TiO2 nanotubes: synthesis and applications

TiO(2) is one of the most studied compounds in materials science. Owing to some outstanding properties it is used for instance in photocatalysis, dye-sensitized solar cells, and biomedical devices. In 1999, first reports showed the feasibility to grow highly ordered arrays of TiO(2) nanotubes by a simple but optimized electrochemical anodization of a titanium metal sheet. This finding stimulated intense research activities that focused on growth, modification, properties, and applications of these one-dimensional nanostructures. This review attempts to cover all these aspects, including underlying principles and key functional features of TiO(2), in a comprehensive way and also indicates potential future directions of the field.

The first ferroelectric templated borate: [Ni(en)2pip][B5O6(OH)4]2

The title compound (GDUT-4) is the first templated borate compound shown to have ferroelectric properties. Its structure consists of a metal complex and isolated polyanion. The chiral structure of the compound was induced by guest-complex cations according to host-guest symmetry through hydrogen bonds (H-bonds). The origins of ferroelectricity in GDUT-4 was shown to be dependent on proton transfer through H-bonds.

Effect of surface modification on the dielectric properties of BaTiO3 nanocrystals

We present the first direct comparison of the dielectric properties of organically modified BaTiO(3) nanocrystals with unmodified BaTiO(3) nanocrystals. Well-defined 6 nm BaTiO(3) nanocrystals were functionalized with n-hexylphosphonic acid (HPA) to give a surface coverage of 2.4 phosphonate groups/nm(2). Chemisorption of HPA to the oxide surface occurs mainly via tridentate bonding of the deprotonated phosphonate, as determined by (31)P MAS NMR, FT-IR, and XPS spectroscopies. The HPA-modified BaTiO(3) (HPA-BaTiO(3)) nanocrystals possess improved dielectric properties, as demonstrated by decreased sensitivity to temperature and frequency for both the dielectric constant and dielectric loss. HPA-BaTiO(3) had a much lower dielectric loss than unmodified BaTiO(3), which also indicates an improvement in the dielectric quality of the material. Such improvements are of potential importance for the fabrication of high energy density nanocomposites.

Large-scale fabrication of titanium-rich perovskite PZT submicro/nano wires and their electromechanical properties

We report a 2-step approach to prepare tetragonal perovskite PbZr(0.1)Ti(0.9)O(3) submicro/nano wires in gram scale and with over 95% wire content. Non-perovskite precursor wires were first fabricated by hydrothermal processing. A subsequent annealing in a PbO atmosphere at 600 degrees C converted these wires into perovskite structures which retain the one-dimensional shape. Binding of the perovksite nanowires to a conductive substrate could be achieved by a similar heat treatment of the non-perovskite precursor wires on a flat Pt coated substrate. Taking advantage of the strong mechanical attachment and good electrical contact between the wires and the metallic layer, piezoresponse force microscopy (PFM) was used to measure the local piezoelectric and ferroelectric properties of the individual wires. Enhanced piezoelectric response relative to sputtered epitaxial PbZr(0.2)Ti(0.8)O(3) film and squared hysterisis loop with sharp switching indicate pronounced electro-mechanical and ferroelectric behavior. The 90 degree domain structure of the as-prepared perovskite PZT wires was confirmed by both PFM and transmission electron microscopy investigations.

Large Scale Synthesis of BaTiO3 Nanorods by a Template Way

Large scale BaTiO3 nanorods were successfully synthesized by a template method based on a precipitation process. The templates used in our method are H2Ti8O17 nanorods, which can be synthesized from K2Ti4O9 fibers. The unique process of the synthesis is BaC2O4•0.5H2O shell was coated on the 1-dimensional H2Ti8O17 nanorods (the core). The as-prepared products were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that the BaTiO3 nanorods are ~100-300 nm in diameter and ~2-10 m in length. The process described provides a general route to fabricate this kind of perovskite 1-dimensional nanostructures, such as SrTiO3 and PbTiO3.

Room-temperature synthetic pathways to barium titanate nanocrystals

Novel room-temperature pathways to BaTiO(3) nanocrystals have been recently developed, which stand in contrast to traditional high-temperature methods. Peptide-assisted, bio-facilitated routes have been developed for low-temperature nanocrystal growth, in addition to two low-temperature routes completely independent of biomolecules. These innovative methods lay the groundwork for the facile production of nanoscale BaTiO(3) in economical and energy-efficient ways.

Ferroelectric hollow particles obtained by solid-state reaction

Hollow particles of barium titanate were obtained by a two-step process combining colloidal chemistry and solid-state reaction. BaCO(3) crystals (size ≈1 µm) suspended in a peroxy-Ti(IV) aqueous solution were coated with an amorphous TiO(2) shell using a precipitation process. Calcination of the BaCO(3)@TiO(2) core-shell particles at 700 °C resulted in the formation of BaTiO(3) hollow particles (shell thickness of ≈70 nm) which retain the morphology of the BaCO(3) crystals. Formation of the cavity occurs because out-diffusion of the core phase is much faster than in-diffusion of the shell material. X-ray diffraction (XRD) and Raman spectroscopy indicate that the hollow particles possess a tetragonal ferroelectric structure with axial ratio c/a = 1.005. Piezoresponse force microscopy has shown strong piezoactivity and 180° ferroelectric domains. The process described provides a general route to fabricate hollow ferroelectric structures of several compounds.

TiO2 nanoparticles as a soft X-ray molecular probe

This communication reports the development of a TiO2-streptavidin nanoconjugate as a new biological label for X-ray bio-imaging applications; this new probe, used in conjunction with the nanogold probe, will make it possible to obtain quantitative, high-resolution information about the location of proteins using X-ray microscopy.

Open-Bench Method for the Preparation of BaTiO3, SrTiO3, and BaxSr1-xTiO3 nanocrystals at 80 degrees C

In this letter, we report the first facile open-bench synthesis of BaTiO3, SrTiO3 nanocrystals, and their nanosolid solutions BaxSr1-xTiO3 (BST) at 80 degrees C. The size of the BST nanoparticles was readily tuned from approximately 50 to approximately 10 nm with achievable giant dielectric constants. The process yielded these important perovskite mixed-metal oxide crystals of high quality on the nanometer scale without a history of thermal stress. This new synthesis system involves inorganic starting materials without organic components and does not require demanding conditions such as an inert environment, high pressure, and high temperature.

The Synthesis and Characterisation of Ferromagnetic CaMn2O4 Nanowires

The synthesis of marokite CaMn(2)O(4) nanowires using a hydrothermal method is reported. Transmission electron microscopy and electron diffraction measurements show that the nanowires are polycrystalline in nature with diameters between 10 and 20 nm and lengths ranging from approximately 100 to 500 nm. Most interestingly, in contrast with the bulk material, magnetization measurements show that these nanowires exhibit ferromagnetic ordering with a Curie temperature (T(C)) of approximately 40 K.

Surfactant-free hydrothermal synthesis of highly tetragonal barium titanate nanowires: a structural investigation

Barium titanate nanowires synthesized with a surfactant-free hydrothermal method have been characterized by various techniques such as transmission electron microscopy (TEM), scanning electron microscopy (SEM), synchrotron X-ray diffraction, X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. The TEM and SEM analyses show the uniform cylindrical nanowires. The Rietveld refinement with synchrotron X-ray powder diffraction showed that the lattice parameters of cubic and tetragonal phases were a (= b = c) = 4.0134 A and a (= b) = 3.9998 A, c = 4.0303 A, respectively. The final weighted R-factor, R(wp), was 6.75% and the goodness of fit indicator was 1.30. The mass fraction of tetragonal and cubic phases based on the refined scale factor for the two phases were 98.4% and 1.6%, respectively, which clearly show the nanowires are tetragonal. The XPS analysis has shown that as-obtained BaTiO3 nanowires were phase pure. The Raman spectra confirm the tetragonal phase of the BaTiO3 nanowires. The dielectric constant measurement shows the shift in the transition temperature (Tc = 105 degrees C) compared to the bulk transition temperature (Tc = 132 degrees C). The dielectric constant at Tc was 174 measured at 1 kHz frequency.

Shape control of semiconductor and metal oxide nanocrystals through nonhydrolytic colloidal routes

Inorganic nanocrystals with tailored geometries exhibit unique shape-dependent phenomena and subsequent utilization of them as building blocks for the fabrication of nanodevices is of significant interest. Herein, we review the recent developments in the shape control of colloidal nanocrystals with a focus on the scientifically and technologically important semiconductor and metal oxide nanocrystals obtained by nonhydrolytic synthetic methods. Many structurally unprecedented motifs have been discovered including polyhedrons, rods and wires, plates and prisms, and other advanced shapes such as branched rods, stars, inorganic dendrites, and dumbbells. The currently proposed shape-guiding mechanisms are presented and the important pioneering studies on the assembly of shape-controlled nanocrystals into ordered superlattices and the fabrication of prototype advanced nanodevices are discussed.

Facile preparation and electrochemical properties of cubic-phase Li4Mn5O12 nanowires

Single crystalline Li4Mn5O12 nanowires with cubic phase were prepared in a large scale by a simple molten salt route without using any surfactant as template; the nanowires exhibited high storage capacity and coulombic efficiency as cathode materials for lithium-ion batteries.

Catalyst-Assisted Solution−Liquid−Solid Synthesis of CdS/CdSe Nanorod Heterostructures

We report the synthesis and characterization of axial nanorod heterostructures composed of cadmium selenide (CdSe) and cadmium sulfide (CdS). The synthesis employs a solution-liquid-solid (SLS) mechanism with the assistance of bismuth nanocrystals adhered to a substrate (silicon or a III-V semiconductor). Transmission electron microscopy (TEM) and diffraction studies show that CdSe and CdS segments exhibit the wurtzite (hexagonal) crystal structure with <5% stacking faults. Both of these segments grow along the [002] direction with an epitaxial interface between them. Energy-dispersive X-ray (EDX) spectrometry using a high-resolution TEM operating in scanning mode confirms the alloy-free composition modulation in the nanorod heterostructures, showing that Se and S are localized in the CdSe and CdS portions of the nanorod heterostructures, respectively. This study demonstrates that SLS synthesis provides an alternate route to prepare axial nanorod heterostructures that have been difficult to generate using either vapor-liquid-solid growth or catalyst-free solution-phase synthesis.

Synthesis of Hexagonal BaTa2O6 Nanorods and Influence of Defects on the Photocatalytic Activity

Hexagonal barium tantalate (BaTa2O6) nanorods were synthesized by a hydrothermal method based on the reaction of concentrated Ba(OH)2 solution and Ta2O5. BaTa2O6 samples show a uniform cylindrical structure with diameters of 5-30 nm and the lengths of 50-200 nm. The formation of BaTa2O6 nanorods follows a dissolution-recrystallization mechanism and is governed by hydrothermal temperature and time. BaTa2O6 nanorod samples prepared at 270 degrees C for 72 h have exhibited the highest photocatalytic activity in the degradation of Rhodamine B (RhB) in aqueous solution under UV radiation. Hydrogen-related defects were detected in BaTa2O6 nanorods, which originate from the oxygen octahedron. The number of defects was dependent on the hydrothermal temperature, and the photocatalytic activities of BaTa2O6 nanorods increase with the decrease of defect amounts. On the basis of the experiment results, the difference in photocatalytic activities for samples is mainly caused by lattice defects, which can act as inactivation centers.

Composite-hydroxide-mediated approach for the synthesis of nanostructures of complex functional-oxides

We demonstrate a generic approach for the synthesis of single-crystal complex oxide nanostructures of various structure types, such as perovskites, spinels, monoclinic, corundum, CaF(2) structured, tetragonal, and even metal hydroxides. The method is based on a reaction between a metallic salt and a metallic oxide in a solution of composite-hydroxide eutectic at approximately 200 degrees C and normal atmosphere without using an organic dispersant or capping agent. The synthesis technique is cost-effective, one-step, easy to control, and is performed at low temperature and normal atomospheric pressure. The technique can be expanded to many material systems, and it provides a general, simple, convenient, and innovative strategy for the synthesis of nanostructures of complex oxides with important scientific and technological applications in ferroelectricity, ferromagnetism, colossal magnetoresistance, fuel cell, optics, and more.

Size-Manipulable Synthesis of Single-Crystalline BaMnO3and BaTi1/2Mn1/2O3Nanorods/Nanowires

We report a size-manipulable synthesis of single-crystalline nanorods/nanowires of barium manganite (BaMnO(3)) and barium titanium manganite (BaTi(1/2)Mn(1/2)O(3)) by using the composite-hydroxide-mediated approach. The synthesis cleanly yields nanorods with a hexagonal perovskite structure. Typical nanorods have widths ranging between 50 and 100 nm, and the lengths can be easily controlled by time and temperature or by adding a small amount of water during the synthesis process. Resistance measurement shows that a phase transition happened at 58 K on BaMnO(3). The photoluminescence spectrum of BaTi(1/2)Mn(1/2)O(3) presents two emission peaks at wavelengths of 465 and 593 nm, corresponding to blue and green fluorescence. The ability to synthesize nanorod manganites of a desired length should enable detailed investigations of the size-dependent evolution of magnetism, magnetoresistance, nanoscale phase separation, and realization of a nanodevice of magnetic sensors.

Voltammetric determination of mefenamic acid at lanthanum hydroxide nanowires modified carbon paste electrodes

Lanthanum hydroxide nanowires modified carbon paste electrode (LNW/CPE) exhibiting an electrocatalytic response toward the oxidation of mefenamic acid (MFA) is described. The catalytic action of the LNW/CPE on the oxidation of MFA via one-electron and one-proton transfer is attributed to the formation of the porous construction and the increase of efficient surface of the electrode due to the adulteration of LNW with carbon powders. Using the LNW/CPE, a linear sweep voltammetric method for the determination of MFA and other drugs with diphenylamine parent is proposed. A linear range of 2.0 x 10(-11) to 4.0 x 10(-9)mol L(-1) is obtained along with a detection limit of 6.0 x 10(-12)mol L(-1).

Germanium Telluride Nanowires and Nanohelices with Memory-Switching Behavior

We report the synthesis of single-crystalline GeTe nanowires (NWs) and nanohelices (NHs) using a vapor transport method assisted by metal catalysts. The NWs have typical diameters of 65 +/- 20 nm and lengths reaching up to 50 mum, while NHs have an average helix diameter of 135 +/- 30 nm, with widely varying pitches. Electron microscopy and diffraction measurements show that these NWs and NHs are single crystalline and exhibit a rhombohedral structure. The devices incorporating individual GeTe NWs exhibit nonvolatile resistance changes associated with voltage-driven crystalline-amorphous transitions, suggesting that these NWs can be the basis of an electrically driven nonvolatile memory.

Ferroelectric phase transition in individual single-crystalline BaTiO3 nanowires

We report scanned probe characterizations of the ferroelectric phase transition in individual barium titanate (BaTiO3) nanowires. Variable-temperature electrostatic force microscopy is used to manipulate, image, and evaluate the diameter-dependent stability of ferroelectric polarizations. These measurements show that the ferroelectric phase transition temperature (TC) is depressed as the nanowire diameter (dnw) decreases, following a 1/dnw scaling. The diameter at which TC falls below room temperature is determined to be approximately 3 nm, and extrapolation of the data indicates that nanowires with dnw as small as 0.8 nm can support ferroelectricity at lower temperatures. We also present density functional theory (DFT) calculations of bare and molecule-covered BaTiO3 surfaces. These calculations indicate that ferroelectricity in nanowires is stabilized by molecular adsorbates such as OH and carboxylates. These adsorbates are found to passivate polarization charge more effectively than metallic electrodes, explaining the observed stability of ferroelectricity in small-diameter BaTiO3 nanowires.

Hydrothermal Synthesis of Single-Crystalline Anatase TiO2Nanorods with Nanotubes as the Precursor

Preparation of anatase TiO2 nanorods from solutions in the absence of surfactants or templates has rarely been reported. The present work has found that hydrothermal treatment of titanate nanotube suspensions under an acidic environment resulted in the formation of single-crystalline anatase nanorods with a specific crystal-elongation direction. The nanotube suspensions were prepared by treatment of TiO2 in NaOH, followed by mixing with HNO3 to different pH values. The crystal size of the anatase nanoparticles obtained from the hydrothermal treatment increased with the pH of the suspensions, and nanorods with an aspect ratio up to 6 and a long axis along the anatase [001] were obtained at a pH slightly less than 7. A mechanism for the tube-to-rod transformation has been proposed on the basis of the crystalline structures of the tubes and rods. The local shrinkage of the tube walls to form anatase crystallites and the subsequent oriented attachment of the crystallites have been suggested to be the key steps involved in the nanorod formation.