Hydrothermal Growth of Manganese Dioxide into Three‐Dimensional Hierarchical Nanoarchitectures

Novel three‐dimensional (3D) hierarchical nanoarchitectures of ?‐MnO₂ have been synthesized by a simple chemical route without the addition of any surfactants or organic templates. The self‐organized crystals consist of a major ?‐MnO₂ dipyramidal single crystal axis and six secondary branches, which are arrays of single‐crystal ?‐MnO₂ nanobelts. The growth directions of the nanobelts are perpendicular to the central dipyramidal axis, which shows sixfold symmetry. The shape of the ?‐MnO₂ assembly can be controlled by the reaction temperature. The morphology of ?‐MnO₂ changes from a six‐branched star‐like shape to a hexagonal dipyramidal morphology when the temperature is increased from 160 to 180 °C. A possible growth mechanism is proposed. The synthesized ?‐MnO₂ shows both semiconducting and magnetic properties. These materials exhibit ferromagnetic behavior below 25 K and paramagnetic behavior above 25 K. The ?‐MnO₂ system may have potential applications in areas such as fabrication of nanoscale spintronic materials, catalysis, and sensors.     

Synthesis of a Stable Metallic Niobium Oxide Molecular Sieve and Subsequent Room Temperature Activation of Dinitrogen

Mesoporous niobium oxide was treated at room temperature with bis(toluene) niobium to make a new black material that exhibits metallic properties. The metallic behavior is attributed to low‐valent Nb^II in the walls of the porous structure and is fully supported by strong emissions at the Fermi level, variable temperature resistivity measurements, and temperature‐independent paramagnetism. These materials possess higher conductivities than reported for any molecular sieve and also represent the first example of a metallic oxide‐based molecular sieve. For comparison, the conductivity is almost 10 000 times greater than the highest value measured for an open structured mesoporous material. Treatment at room temperature with dinitrogen leads to formation of a thin nitride coat on the surface. Cleavage of dinitrogen is an extremely rare and important reaction that typically requires very forcing conditions in the solid state (high temperatures in excess of several hundred degrees, argon plasmas, etc.) or low‐valent coordinatively stressed metal centers in the homogeneous phase. This is the first example of a molecular sieve mediating this process and because of the controlled porosity and high surface areas of these materials they are ideal candidates for model studies for nitrogen reduction and catalytic nitrogen incorporation into organic compounds. Since porous materials are often the catalytic support of choice for industrial processes, these materials may lead to the development of commercial nitrogen incorporation processes.     

Large‐Scale Synthesis and Characterization of TiO2‐Based Nanostructures on Ti Substrates

Na₂Ti₆O₁₃ nanoplates, nanowires, and continuous nanowire network films are hydrothermally formed on a large scale directly on Ti substrates for the first time. The morphology of the formed Na₂Ti₆O₁₃ nanostructures can be easily tuned by varying the experimental parameters of temperature, reaction duration, and the NaOH concentration. Our study demonstrates that the synthesized Na₂Ti₆O₁₃ nanostructures are easily converted into H₂Ti₃O₇ nanostructures—a desirable precursor for the fabrication of various TiO₂‐based nanomaterials—with shape preservation, by an ion‐exchange process. Anatase, a mixture of anatase and rutile, and rutile TiO₂ nanowires are formed when the H₂Ti₃O₇ nanowires are annealed at 450, 600, and 750 °C, respectively. The optical properties and the photocatalytic activity of H₂Ti₃O₇ nanowires and of the TiO₂‐based nanomaterials are also addressed. The approach described in this study provides a simple and novel method for the large‐scale synthesis of various TiO₂‐based nanostructured materials that grow directly on Ti substrates and are ready for a wide range of practical applications, such as the photodegradation of wastewater.     

Size‐ and Shape‐Control of Crystalline Zinc Oxide Nanoparticles: A New Organometallic Synthetic Method

A novel organometallic synthetic method has been developed for the preparation of crystalline ZnO nanoparticles of controlled size and shape. Isotropic nanoparticles with a mean size between 3 and 6 nm and nanorods with a mean diameter of 3–4 nm and length up to 120 nm have been obtained in this way. This synthetic method takes advantage of the exothermic reaction of the precursor Zn(c‐C₆H₁₁)₂ ( 1 ) toward moisture and air and involves the presence of long‐alkyl‐chain amines as stabilizing ligands. The influence of the different experimental parameters (concentration, solvent, nature of the ligand, time, and temperature) on the size and shape of the ZnO nanoparticles has been studied, together with the mechanism of their formation, by NMR spectroscopy, transmission electron microscopy, and X‐ray diffraction techniques. The nanoparticles prepared in this way can be dissolved in most of the common organic solvents, forming colloidal solutions. The surface state of the nanoparticles as well as the possibility of forming luminescent solutions from which regular monolayers can be deposited are also reported.     

Composition‐ and Shape‐Controlled Synthesis and Optical Properties of ZnxCd1–xS Alloyed Nanocrystals

Composition‐tunable Zn_xCd_1–xS alloyed nanocrystals have been synthesized by a new approach consisting of thermolyzing a mixture of cadmium ethylxanthate (Cd(exan)₂) and zinc ethylxanthate (Zn(exan)₂) precursors in hot, coordinating solvents at relatively low temperatures (180–210 °C). The composition of the alloyed nanocrystals was accurately adjusted by controlling the molar ratio of Cd(exan)₂ to Zn(exan)₂ in the mixed reactants. The alloyed Zn_xCd_1–xS nanocrystals prepared in HDA/TOP (HDA: hexadecylamine; TOP: trioctylphosphine) solution exhibit composition‐dependent shape and phase structures as well as composition‐dependent optical properties. The shape of the Zn_xCd_1–xS nanocrystals changed from dot to single‐armed rod then to multi‐armed rod with a decrease of Zn content in the ternary nanoparticles. The alloying nature of the Zn_xCd_1–xS nanocrystals was consistently confirmed by the results of high‐resolution transmission electron microscopy (HRTEM), X‐ray diffraction (XRD), and UV‐vis absorption and photoluminescence (PL) spectroscopy. Further, the shape‐controlled synthesis of the ternary alloyed nanocrystals was realized by selecting appropriate solvents. Uniform nanodots in the whole composition range were obtained from TOPO/TOP solution, (TOPO: trioctylphosphine oxide) and uniform nanorods in the whole composition range were prepared from HDA/OA solution (OA: octylamine). The effect of the reaction conditions, such as solvent, reaction temperature, and reaction time, on the PL spectra of the alloyed Zn_xCd_1–xS nanocrystals was also systematically studied, and the reaction conditions were optimized for improving the PL properties of the nanocrystals.     

Shape‐Controlled Synthesis of Polyhedral Nanocrystals and Their Facet‐Dependent Properties

Growth of inorganic polyhedral nanocrystals with excellent morphology control presents significant synthetic challenges, especially when the development of synthetic schemes to make nanocrystals with systematic shape evolution is desired. Nanocrystals with fine size and shape control facilitate formation of their self‐assembled packing structures and offer opportunities for examination of their facet‐dependent physical and chemical properties. In this Feature Article, recent advances in the synthesis of nanocrystals with systematic shape evolution are highlighted. The reaction conditions used to achieve this morphology change offer insights into the growth mechanisms of nanocrystals. A novel class of polyhedral core–shell heterostructures fabricated using structurally well‐defined nanocrystal cores is also presented. Facet‐dependent photocatalytic activity, molecular adsorption, and catalytic and electrical properties of nanocrystals have been examined and are discussed. Nanomaterials with enhanced properties and functionality may be obtained through continuous efforts in the synthesis of nanocrystals with well‐defined structures and investigation of their plane‐selective properties.     

Silicalite‐1 Zeogrid: A New Silica Molecular Sieve with Super‐ and Ultra‐Micropores

Spherical, micrometer‐sized particles with a layered structure were obtained by precipitation of a Silicalite‐1 zeolite nanoslab suspension upon addition of cetyltrimethylammonium bromide (CTMABr) and subsequent calcination. The material had a specific micropore volume of 0.69 cm³ g^–1, distributed over super‐ and ultra‐micropores. The formation process of this peculiar microporous solid was studied using X‐ray diffraction (XRD), ²⁹Si MAS NMR spectroscopy, thermogravimetry (TG), and nitrogen adsorption. In the precipitate, the Silicalite‐1 nanoslabs were laterally fused into nanoplates and stapled into layers with intercalated surfactant molecules. Removal of the surfactant through calcination caused facial fusion, besides additional lateral fusion, of the nanoplates. Empty spaces left lying laterally between individual nanoplates were responsible for the super‐microporosity. The ultra‐micropores were zeolitic channels inside the fused nanoplates. The potential of these Silicalite‐1 zeogrids as molecular sieves was demonstrated with pulse gas‐chromatographic separation of alkane mixtures. The mass‐transfer resistance of a packed bed of zeogrid particles was considerably lower than of compacted zeolite powder.     

Growth of Bulk ZnO Single Crystals via a Novel Hydrothermal Oxidative Pressure‐Relief Route

A novel hydrothermal oxidative pressure‐relief (HOPR) route has been successfully developed for the growth of high‐quality bulk ZnO single crystals, using metallic zinc and H₂O₂ as the raw materials, at 400 °C for 20 h in an alkali solvent. X‐ray powder diffraction reveals the ZnO crystals have a wurtzite structure. Two typical morphologies of perfect hexagonal pyramidal and hexagonal prismatic ZnO single crystals, and bidirectional adhesive crystals, are identified by scanning electron microscopy analysis. The average size of the single crystals is ～ 1.0 mm in length and ～ 0.2 mm in diameter. Short hexagonal prismatic, novel polygonal layer‐like, and nanowire ZnO crystals are also obtained by altering the reaction conditions, such as the reaction time and the speed of pressure release. The growth mechanism is a spontaneous nucleation and self‐growth process. This novel HOPR route gives an alternative choice for obtaining well‐crystallized ZnO bulk crystals from solution.     

From Layered Basic Zinc Acetate Nanobelts to Hierarchical Zinc Oxide Nanostructures and Porous Zinc Oxide Nanobelts

Novel hierarchical ZnO nanostructures, porous ZnO nanobelts, and nanoparticle chains are prepared from a precursor of synthetic bilayered basic zinc acetate (BLBZA) nanobelts. BLBZA nanobelts are obtained by a simple synthetic route under mild conditions. X‐ray diffraction, scanning electron microscopy, transmission electron microscopy, infrared spectroscopy, and thermal analysis are used to characterize the BLBZA nanobelts and ZnO nanostructures. The obtained BLBZA precursor consists of a lamellar structure with two interlayer distances of 1.33 and 2.03 nm, exhibits a beltlike morphology, and has widths of 200 to 600 nm, thicknesses of 10 to 50 nm, and lengths of up to 50 μm. Refluxing an aqueous dispersion of BLBZA nanobelts at 120 °C for 12 h leads to the formation of well‐defined hierarchical ZnO nanostructures. The time‐dependent shape‐evolution process suggests that spindlelike ZnO particles form first, and then the ringlike nanosheets grow heterogeneously on the backbone of these spindles. In addition, calcination in air can remove ligand molecules and intercalated water molecules from BLBZA nanobelts, resulting in the formation of porous ZnO nanobelts and nanoparticle chains. The BLBZA nanobelts serve as templates during the transformation to form ZnO beltlike nanoparticle chains without morphological deformation. Photoluminescence results show that both the as‐synthesized hierarchical ZnO nanostructures and porous ZnO nanobelts show a narrow and sharp UV emission at 390 nm and a broad blue–green emission at above 466 nm when excited by UV light.     

Nanostructured Vanadium Oxide Electrodes for Enhanced Lithium‐Ion Intercalation

This article summarizes our most recent studies on improved Li⁺‐intercalation properties in vanadium oxides by engineering the nanostructure and interlayer structure. The intercalation capacity and rate are enhanced by almost two orders of magnitude with appropriately fabricated nanostructures. Processing methods for single‐crystal V₂O₅ nanorod arrays, V₂O₅·n H₂O nanotube arrays, and Ni/V₂O₅·n H₂O core/shell nanocable arrays are presented; the morphologies, structures, and growth mechanisms of these nanostructures are discussed. Electrochemical analysis demonstrates that the intercalation properties of all three types of nanostructure exhibit significantly enhanced storage capacity and rate performance compared to the film electrode of vanadium pentoxide. Addition of TiO₂ to orthorhombic V₂O₅ is found to affect the crystallinity, microstructure, and possible interaction force between adjacent layers in V₂O₅, and subsequently leads to enhanced Li⁺‐intercalation properties in V₂O₅. The amount of water intercalated in V₂O₅ is found to have a significant influence on the interlayer spacing and electrochemical performance of V₂O₅·n H₂O. A systematic electrochemical study has demonstrated that the V₂O₅·0.3 H₂O film has the optimal water content and exhibits the best Li⁺‐intercalation performance.     

Hydrothermal Synthesis of Rare Earth (Tb,Y) Hydroxide and Oxide Nanotubes

In this paper, Tb(OH)₃ and Y(OH)₃ single‐crystalline nanotubes with outer diameters of 30–260 nm, inner diameters of 15–120 nm, and lengths of up to several micrometers were synthesized on a large scale by hydrothermal treatment of the corresponding oxides in the presence of alkali. In addition, Tb₄O₇ and Y₂O₃ nanotubes can be obtained by calcination of Tb(OH)₃ and Y(OH)₃ nanotubes at 450 °C. X‐ray diffraction (XRD), field‐emission scanning electron microscopy, transmission electron microscopy (TEM), electron diffraction (ED), energy‐dispersive X‐ray spectroscopy (EDS), thermogravimetry, and differential scanning calorimetry (DSC) have been employed to characterize these nanotube materials. The growth mechanism of rare earth hydroxide nanotubes can be explained well by the highly anisotropic crystal structure of rare earth hydroxides. These new types of rare earth compound nanotubes with open ends have uses in a variety of promising applications such as luminescent devices, magnets, catalysts, and other functional materials. Advantages of this method for easily realizing large‐scale production include that it is a simple and unique one‐pot synthetic process without the need for a catalysts or template, is low cost, has high yield, and the raw materials are readily available. The present study has enlarged the family of nanotubes available, and offers a possible new, general route to one‐dimensional single‐crystalline nanotubes of other materials.     

Shape‐Controlled Synthesis of Pd Nanocrystals in Aqueous Solutions

This article provides an overview of recent developments regarding synthesis of Pd nanocrystals with well‐controlled shapes in aqueous solutions. In a solution‐phase synthesis, the final shape taken by a nanocrystal is determined by the twin structures of seeds and the growth rates of different crystallographic facets. Here, the maneuvering of these factors in an aqueous system to achieve shape control for Pd nanocrystals is discussed. L ‐ascorbic acid, citric acid, and poly(vinyl pyrrolidone) are tested for manipulating the reduction kinetics, with citric acid and Br^– ions used as capping agents to selectively promote the formation of {111} and {100} facets, respectively. The distribution of single‐crystal versus multiple‐twinned seeds can be further manipulated by employing or blocking oxidative etching. The shapes obtained for the Pd nanocrystals include truncated octahedron, icosahedron, octahedron, decahedron, hexagonal and triangular plates, rectangular bar, and cube. The ability to control the shape of Pd nanocrystals provides a great opportunity to systematically investigate their catalytic, electrical, and plasmonic properties.     

Controlled Synthesis of Various Hollow Cu Nano/MicroStructures via a Novel Reduction Route

Hollow Cu nano/microstructures are prepared by reduction of CuSO₄ · 5 H₂O with glucose by using a mild hydrothermal process. The X‐ray powder diffraction and energy‐dispersive X‐ray analysis indicate that the products are pure Cu and of cubic phase. The morphology of the products can be controlled between nanotubes and microspheres assembled from hollow nanoparticles by adjusting the concentration of sodium dodecyl sulfate. A series of experiments confirm that the concentration of the glucose and NaOH also play important roles in the formation of the hollow Cu nano/microstructures.     

Hydrothermal Synthesis of Polyaniline Mesostructures

Polyaniline (PAni) mesostructures have been synthesized under hydrothermal conditions. The mesostructures show different forms—fibers, dendrite fibers, textured plates, featureless plates, and spheres. The obtained morphologies are more sensitive to the concentration of the doping acid, HCl, than the reaction temperature. Similar morphological evolutions have also been observed in the cases of phosphoric (H₃PO₄) and fluoroboric (HBF₄) acids. The measured results of UV‐vis and Fourier transform IR spectroscopy, as well as X‐ray diffraction data, suggest that the morphology evolution is related to the charged property and reactivity of the semiquinone radical cation on the main chain of PAni, formed by polymer doping. Hydrothermal synthesis by tuning the concentration of the doping acid provides a platform for understanding the nature of the polymerization and the fabrication of 1‐ and 2D conjugated polymers.     

Non‐Hydrothermal Synthesis of 1D Nanostructured Manganese‐Based Oxides: Effect of Cation Substitution on the Electrochemical Performance of Nanowires

The synthesis of cation‐substituted 1D manganese oxide nanowires is achieved via a non‐hydrothermal oxidation reaction using a solid‐state precursor and conventional glassware. By changing the reaction conditions and precursor composition, it is possible to control the crystal structure and chemical composition of the resulting 1D nanostructured materials. The nanowires are found to exhibit promising electrochemical properties as cathode materials in lithium secondary batteries; moreover, the electrochemical properties of the nanowires can be improved by the partial replacement of Mn with Cr³⁺ cations. The measurements strongly suggest that the cation composition of the nanostructured metal oxides is very important for optimizing their electrode performance. The non‐hydrothermal solution synthesis method presented here provides an effective composition‐tailored approach for the fabrication of large quantities of 1D manganese oxide nanowires.     

Growth of Single‐Crystalline Wurtzite Aluminum Nitride Nanotips with a Self‐Selective Apex Angle

Single‐crystalline, hexagonal aluminum nitride nanotips are fabricated using a vapor‐transport and condensation process (VTCP) on silicon substrates with or without a catalyst layer. The resultant tips have very sharp nanoscale apexes (～1 nm), while their bases and lengths are up to hundreds of nanometers wide and several micrometers long, respectively. It has been demonstrated that the thickness of the gold‐catalyst layer plays a critical role in controlling the size of the tip; in addition, a catalyst‐free growth mode has been observed, which results in lesser control over the nanotip morphology. Nevertheless, a remarkably narrow distribution of the apex angle of the nanotips, regardless of whether or not a catalyst was used in the VTCP, has been obtained. Compared with the commonly observed ridge and pyramid structures, the nanotips produced by VTCP have higher angles (～81°) between the tilted (221) and the basal (001) planes that encase it. A mechanism for this self‐selective apex angle in aluminum nitride nanotip growth is proposed.     

Growth of Heteroepitaxial ZnO Thin Films on GaN‐Buffered Al2O3 (0001) Substrates by Low‐Temperature Hydrothermal Synthesis at 90 °C

Heteroepitaxial ZnO films are successfully grown on nondoped GaN‐buffered Al₂O₃ (0001) substrates in water at 90 °C using a two‐step process. In the first step, a discontinuous ZnO thin film (ca. 200 nm in thickness) consisting of hexagonal ZnO crystallites is grown in a solution containing Zn(NO₃)·6 H₂O and NH₄NO₃ at ca. pH 7.5 for 24 h. In the second step, a dense and continuous ZnO film (ca. 2.5 μm) is grown on the first ZnO thin film in a solution containing Zn(NO₃)·6 H₂O and sodium citrate at ca. pH 10.9 for 8 h. Scanning electron microscopy, X‐ray diffraction, UV‐vis absorption spectroscopy, photoluminescence spectroscopy, and Hall‐effect measurement are used to investigate the structural, optical, and electrical properties of the ZnO films. X‐ray diffraction analysis shows that ZnO is a monocrystalline wurtzite structure with an epitaxial orientation relationship of (0001)[11 0]_ZnO∥(0001)[11 0]_GaN. Optical transmission spectroscopy of the two‐step grown ZnO film shows a bandgap energy of 3.26 eV at room temperature. A room‐temperature photoluminescence spectrum of the ZnO film reveals only a main peak at ca. 380 nm without any significant defect‐related deep‐level emissions. The electrical property of ZnO film showed n‐type behavior with a carrier concentration of 3.5 × 10¹⁸ cm^–3 and a mobility of 10.3 cm² V^–1 s^–1.     

ZnO Nanotetrapods: Controlled Vapor‐Phase Synthesis and Application for Humidity Sensing

A systematic study on controlled synthesis of ZnO nanotetrapods by combining metal‐vapor transport, oxidative nucleation/growth, fast‐flow quenching, and water‐assisted cleaning is reported. The technique developed in this work makes possible the fabrication of ZnO nanotetrapods with different morphologies, with arm diameters down to 17 nm, and with arm lengths ranging from 50 nm up to a few micrometers. The octa‐twin model is verified for the growth of the ZnO nanotetrapods. Photoluminescence (PL) studies indicate a higher level of surface and subsurface oxygen vacancies for smaller ZnO nanotetrapods. The ZnO nanotetrapods are first used for the fabrication of resistor‐type humidity sensors, which show high sensitivity, quick response/recovery, long lifetime, and a wide range of humidity response. These favorite characteristics of the humidity sensors are ascribed to the unique morphology of the nanotetrapods, which can create a film with faceted pores and large internal surfaces.     

Structure‐Properties Relationship in Iron Oxide‐Reduced Graphene Oxide Nanostructures for Li‐Ion Batteries

Non‐aqueous sol‐gel routes involving the reaction of metal oxide precursors in organic solvents (e.g., benzyl alcohol) at moderate temperature and pressure, offer advantages such as high purity, high reproducibility and the ability to control the crystal growth without the need of using additional ligands. In this paper, a study carried out on a series of iron oxide/reduced graphene oxide composites is presented to elucidate a structure‐properties relationship leading to an improved electrochemical performance of such composites. Moreover, it is demonstrated that the easy production of the composites in a variety of temperature and composition ranges, allows a fine control over the final particles size, density and distribution. The materials obtained are remarkable in terms of the particle's size homogeneity and dispersion onto the reduced graphene oxide surface. Moreover, the synthesis method used to obtain the graphene oxide clearly affects the performances of the final composites through the control of the restacking of the reduced graphene oxide sheets. It is shown that a homogeneous and less defective reduced graphene oxide enables good electrochemical performances even at high current densities (over 500 mAh/g delivered at current densities as high as 1600 mA/g). The electrochemical properties of improved samples reach the best compromise between specific capacity, rate capability and cycle stability reported so far.     

Controlled Growth and Properties of One‐Dimensional ZnO Nanostructures with Ce as Activator/Dopant

A large amount of one‐dimensional (1D) Ce‐doped ZnO nanostructures with different morphologies has been successfully synthesized by annealing a polymeric precursor at various temperatures. The evolution of the morphologies and microstructures was investigated by field‐emission scanning electron microscopy (FE‐SEM), transmission electron microscopy (TEM), and high‐resolution TEM (HRTEM). The results show that the morphologies vary drastically with increasing synthesis temperature and the photoluminescence (PL) of the products depends on both the synthesis and measurement temperatures. The CeO layer forms first and becomes a catalytic center for the ZnO growth. At a synthesis temperature lower than the boiling point of Zn, Zn and O atoms can stack epitaxially along the CeO catalytic layer and form a bicrystal nanobelt‐like structure with a trapezoid‐like end and a concave growth fault center. At a synthesis temperature higher than the boiling point of Zn, however, nanowires with an incommensurately modulated superstructure are obtained due to the high reaction rate and the formation of a periodic separation of the CeO layer. As for the room‐temperature PL of ZnO, the incorporation of donor Ce leads to the disappearance of the green band and the appearance of a purplish‐blue emission peak, whose position shifts towards the red and whose intensity decreases with increasing synthesis temperature. Analysis of this temperature‐dependent luminescence indicates that the purplish‐blue emission of nanobelts prepared at 850 °C originates from a donor‐bound exciton emission, and, contrary to the nanowires, it undergoes a change from an emission of the electron–hole plasma (EHP) to an emission of the donor‐bound exciton with decreasing measurement temperature.