One-pot aqueous solution syntheses of iron oxide nanostructures with controlled crystal phases through a microbial-mineralization-inspired approach

Iron-oxidizing bacteria produce trivalent iron oxides with the controlled crystal phases outside of their cells. Herein we have synthesized iron oxides with controlled oxidation states and crystal phases through a microbial-mineralization-inspired approach in an aqueous solution at low temperature. Trivalent iron oxides, such as lepidocrocite, ferrihydrite, goethite, and hematite, are selectively obtained from an aqueous solution containing divalent iron ions below 90 °C. The presence of a chelating agent facilitates the control of the oxidation states through the ligand-controlled approach because the precipitation of the divalent iron species is inhibited by the complexation between divalent iron ion and a chelating agent. The control of the crystal phases is achieved by the tuning of the synthetic conditions, such as the initial pH, the concentration of a chelating agent, and the reaction temperature. Furthermore, the resultant iron oxides have hierarchically organized structures consisting of nanoscale objects. The microbial-mineralization-inspired approach by using a chelating agent has potentials for the further morphological control of iron oxides and the further application to aqueous-solution syntheses of other metal oxides.     

Solution Combustion Synthesis: Towards a Sustainable Approach for Metal Oxides

Solution combustion synthesis (SCS) has been widely used to produce simple and complex oxides with a desired morphology (size and shape). SCS is valuable due to low cost, simplicity and energy efficient synthesis. To guarantee the best molecular-level mixing of reactants in an aqueous or solvent-based solution some parameters need to be controlled, such as fuel type, metal cations precursors, stoichiometry ratio (φ), pH effect, atmosphere and initiation type. These determine the final properties of the oxide materials, providing the potential to reach different morphologies, which are essential for their final applications. This Review article focuses on the crucial parameters in SCS and how these affect the overall materials properties from nanostructures to thin films. To finalize, special attention is given to the application of SCS to form metal oxide thin films at low temperature and their application in thin film transistors (TFTs).     

Bioinspired Peony‐Like β‐Ni(OH)2 Nanostructures with Enhanced Electrochemical Activity and Superhydrophobicity

Constructing complex nanostructures has become increasingly important in the development of hydrogen storage, self‐cleaning materials, and the formation of chiral branched nanowires. Several approaches have been developed to generate complex nanostructures, which have led to novel applications. Combining biology and nanotechnology through the utilization of biomolecules to chemically template the growth of complex nanostructures during synthesis has aroused great interest. Herein, we use a biomolecule‐assisted hydrothermal method to synthesize β‐phase Ni(OH)₂ peony‐like complex nanostructures with second‐order structure nanoplate structure. The novel β‐Ni(OH)₂ nanostructures exhibit high‐power Ni/MH battery performance, close to the theoretical capacity of Ni(OH)₂, as well as controlled wetting behavior. We demonstrate that this bioinspired route to generate a complex nanostructure has applications in environmental protection and green secondary cells. This approach opens up opportunities for the synthesis and potential applications of new kinds of nanostructures.     

A Large Family of Iron Ruddlesden‐Popper Relatives: from Oxides to Oxycarbonates and Oxyhydroxides

Iron oxides, oxyhydroxydes and oxycarbonates derived from the layered Ruddlesden‐Popper (RP) structure form a large family of layered compounds. Besides the classical RP oxides Sr_n+1Fe_nO_3n+1, single intergrowths with the generic formulation (A,Sr)_n+2Fe_nO_3n+2 and (A,Sr)_n+3Fe_nO_3n+3 (A = Tl, Pb, Bi…) can be generated by increasing the multiplicity of the rock salt layers, and multiple intergrowths of these single intergrowths can be synthesized. Starting from oxygen deficient RP oxides such as n = 3 member Sr₃NdFe₃O_9?δ, oxyhydroxydes hydrates and oxyhydroxydes such as Sr₃NdFe₃O_7.5(OH)₂·H₂O and Sr₃NdFe₃O_7.5(OH)₂ can be created topotactically. Carbonate groups can also replace FeO₆ octahedra in the n = 3 member Sr₄Fe₃O₁₀, leading to layered oxycarbonates Sr₄Fe_3?x(CO₃)_xO_10?4x with 0 < × ≤ 1. Shearing mechanism applied transversally to the layers allows collapsed structures to be generated such as the [Bi₂Sr₃Fe₂O₉]_n [Bi₄Sr₆Fe₂O₁₆] family and the ferrite Bi₁₃Ba₂Sr₂₅Fe₁₃O₆₆. Finally the replacement of rock salt SrO layers in the intergrowth Sr₂FeO₄ allows a new series of modulated structures [Sr₈Fe₁₂O₂₆]·[Sr₃Fe₂O₆]_n to be generated, built up of layers of FeO₅ bipyramids and tetragonal pyramids intergrown with perovskite layers.     

高比表面钛铌复合氧化物的制备

通过先用长链有机胺预支撑层状钛铌酸, 然后与含Ti4+的聚合阳离子进行交换反应, 再经高温焙烧后制备出高比表面的钛铌复合氧化物, 其比表面积达80～100 m2*g-1. 孔径分布测试结果表明, 这是一类以中孔为主的新型多孔材料.     

Core–Shell Hollow Microspheres of Magnetic Iron Oxide@Amorphous Calcium Phosphate: Synthesis Using Adenosine 5′‐Triphosphate and Application in pH‐Responsive Drug Delivery

Drug nanocarriers with magnetic targeting and pH‐responsive drug‐release behavior are promising for applications in controlled drug delivery. Magnetic iron oxides show excellent magnetism, but their application in drug delivery is limited by low drug‐loading capacity and poor control over drug release. Herein, core–shell hollow microspheres of magnetic iron oxide@amorphous calcium phosphate (MIO@ACP) were prepared and investigated as magnetic, pH‐responsive drug nanocarriers. Hollow microspheres of magnetic iron oxide (HMIOs) were prepared by etching solid MIO microspheres in hydrochloric acid/ethanol solution. After loading a drug into the HMIOs, the drug‐loaded HMIOs were coated with a protective layer of ACP by using adenosine 5′‐triphosphate (ATP) disodium salt (Na₂ATP) as stabilizer, and drug‐loaded core–shell hollow microspheres of MIO@ACP (HMIOs/drug/ACP) were obtained. The as‐prepared HMIOs/drug/ACP drug‐delivery system exhibits superparamagnetism and pH‐responsive drug‐release behavior. In a medium with pH 7.4, drug release was slow, but it was significantly accelerated at pH 4.5 due to dissolution of the ACP shell. Docetaxel‐loaded core–shell hollow microspheres of MIO@ACP exhibited high anticancer activity.     

Large‐Area Plasmonic Substrate of Silver‐Coated Iron Oxide Nanorod Arrays for Plasmon‐Enhanced Spectroscopy

One‐dimensional iron oxide materials fabricated on conducting glass substrates and their unique properties make these nanostructures promising candidates for a wide range of applications. Herein, vertically oriented α‐Fe₂O₃ nanorod arrays synthesized under hydrothermal conditions over a large area are described, as an active platform for surface‐enhanced resonance Raman scattering (SERRS) and surface‐enhanced fluorescence (SEF). From scanning electron microscopy images the formation of a homogeneous distribution of vertically oriented rods in a large area is confirmed. For activating the localized surface plasmon resonances, which are responsible for SERRS and SEF, a 6 nm layer of Ag is deposited onto the α‐Fe₂O₃ nanorod arrays by physical vapor deposition to form Ag islands.     

Thin Films that Consist of CuO Mesocrystal Nanosheets: An Application of Microbial‐Mineralization‐Inspired Approaches to Thin‐Film Formation

Thin films of copper oxides can be synthesized on substrates by using approaches that are inspired by microbial mineralization processes. In nature, precipitates of manganese and iron oxides with controlled oxidation states and crystal phases are produced through biomineralization by microorganisms. We have previously reported microbial‐mineralization‐inspired approaches that are comprised of direct and intermediate routes for the controlled syntheses of transition‐metal oxides. Herein, these approaches are applied to the thin‐film formation and coating of copper oxides and a related compound with controlled crystal phases and morphologies. Thin films of CuO, Cu₂O, and Cu₂(OH)₃Cl were selectively synthesized by using direct or intermediate routes. Notably, CuO mesocrystal nanosheets formed a thin film over the whole of the substrate. The resultant CuO mesocrystal nanosheets showed enhanced properties for the electrochemical detection of dopamine. This study shows the potential applicability of microbial‐mineralization‐inspired approaches to thin‐film coatings.     

Biomimetic Approach to the Formation of Titanium Dioxide Thin Films by Using Poly(2‐(dimethylamino)ethyl methacrylate)

We demonstrate that the biomimetic method—which has been used for the formation of silica thin films—also could be applied directly to the formation of titanium dioxide (TiO₂) thin films, which are technologically important materials because of their applications to photocatalytic purifiers, photochemical solar cells, and others. After generation of poly(2‐(dimethylamino)ethyl methacrylate) (PDMAEMA) films on gold surfaces by surface‐initiated polymerization, titanium bis(ammonium lactato)dihydroxide was used as a precursor of TiO₂. The TiO₂/PDMAEMA films were successfully formed on the surfaces in aqueous solution at neutral pH (pH 6.7) and room temperature, and were characterized by X‐ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, atomic force microscopy, scanning electron microscopy, and X‐ray diffractometry. The formed TiO₂ films have an amorphous nature and large area uniformity in thickness. The degree of crystallization was controlled by annealing. We also investigated the pH effect and the phosphate incorporation in the films by using phosphate‐buffered solutions. The TiO₂ films were formed in all the employed pH values in the range of 2 to 12, but phosphate anions were found to be incorporated into the films facilely only at low pH.     

高比表面积Y-掺杂二氧化铈纳米颗粒和纳米棒的制备及电学性质

分别通过室温老化和高温沉降热处理成功的合成了大量均匀的Y-掺杂铈纳米颗粒和纳米棒,并对Y-掺杂铈粉末中Y的含量和形态对于固体溶液性质的影响进行了研究。结果表明,Y的掺杂大大提高了铈纳米颗粒和纳米棒的比表面积,其中30%的Y-掺杂纳米颗粒的比表面积最大,可达197 m2.g-1。此外,所制备的粉末可以很容易被烧结和硬化,而相应的电解质具有良好的电学性质,其中Y0.3Ce0.7O1.85纳米颗粒电解质在850℃时具有最大电导率(4.32 S.m-1)和最小表观活化能(0.81 eV)。     

High‐Performance Lithium‐Ion Anodes with Hierarchically Assembled Single‐Crystal SnO2 Nanoflake Spheres

A large‐scale hierarchical assembly route is reported for the formation of SnO₂ on the nanoscale that contains rigid and robust spheres with irregular channels for rapid access of Li ions into the hierarchically structured interiors. Large volume changes during the process of Li insertion and extraction are accommodated by the SnO₂ nanoflake spheres’ internal porosity. The hierarchical SnO₂ nanoflake spheres exhibit good lithium storage properties with high capacity and long‐lasting performance when used as lithium‐ion anodes. A reversible capacity of 517 mA h g^?1, still greater than the theoretical capacity of graphite (372 mA h g^?1), after 50 charge–discharge cycles is attained. Meanwhile, the synthesis process is simple, inexpensive, safe, and broadly applicable, providing new avenues for the rational engineering of electrode materials with enhanced conductivity and power.     

Controlled Orientation in a Bio‐Inspired Assembly of Ag/AgCl/ZnO Nanostructures Enables Enhancement in Visible‐Light‐Induced Photocatalytic Performance

In a bio‐inspired approach, polyamine‐mediated mineralization of ZnO was explored to develop an environmentally benign methodology for synthesizing Ag/AgCl/ZnO nanostructures. The assembling properties displayed by the polyamines to create composite structures was utilized to have the nanocomponents effectively interact with each other in a way that is desirable for the application envisaged. The polyamines, which act as a mineralizing agent for ZnO nanoparticles, also facilitate the formation of Ag/AgCl within ZnO under ambient conditions. Thus synthesized Ag/AgCl/ZnO nanostructures represent a multi‐heterojunction system in which the nanocomponents lead in a synergistic way to enhancement in the photocatalytic activity under visible‐light irradiation.     

Step‐by‐Step Growth of Complex Oxide Microstructures

The synthesis of complex and hybrid oxide microstructures is of fundamental interest and practical applications. However, the design and synthesis of such structures is a challenging task. A solution‐phase process to synthesize complex silica and silica–titania hybrid microstructures was developed by exploiting the emulsion‐droplet‐based step‐by‐step growth featuring shape control. The strategy is robust and can be extended to the preparation of complex hybrid structures consisting of two or more materials, with each having its own shape.     

Synthesis and Properties of Oval‐Shaped Iron Oxide/Ethylene Glycol Mesostructured Nanosheets

We have demonstrated a new and facile bottom‐up protocol for the effective synthesis of oval‐shaped iron oxide/ethylene glycol (FeO_x/EG) mesostructured nanosheets. Deprotonated ethylene glycol molecules are intercalated into iron oxide layers to form an interlayer distance of 10.6 Å. These materials display some peculiar magnetic properties, such as the low Morin temperature T_M and ferromagnetism below this T_M value. CdSe/ZnS nanoparticles can be loaded onto these mesostructured nanosheets to produce nanocomposites that combine both magnetic and fluorescence functions. In addition, iron oxide/propanediol (or butanediol) mesostructured materials with increased interlayer distances can also be synthesized. The developed synthetic strategy may be extended toward the creation of other ultrathin mesostructured nanosheets.     

Development of an Iron(II)‐Catalyzed Aerobic Catechol Cleavage and Biomimetic Synthesis of Betanidin

An aerobic iron(II)‐catalyzed cleavage of catechols was developed. This reaction allows for the preparation of 2‐methoxy‐2 H‐pyrans that can be employed as versatile building blocks for synthesis. The utility of this biomimetic oxidative cleavage is featured in the synthesis of betanidin, a natural colorant with antioxidant properties.