Surface State Mediated NIR Two‐Photon Fluorescence of Iron Oxides for Nonlinear Optical Microscopy

The development of fluorescent iron oxide nanomaterials is highly desired for multimodal molecular imaging. Instead of incorporating fluorescent dyes on the surface of iron oxides, a ligand‐assisted synthesis approach is developed to allow near‐infrared (NIR) fluorescence in Fe₃O₄ nanostructures. Using a trimesic acid (TMA)/citrate‐mediated synthesis, fabricated Fe₃O₄ nanostructures can generate a NIR two‐photon florescence (TPF) peak around 700 nm under the excitation by a 1230‐nm femtosecond laser. By tailoring the absorption of Fe₃O₄ nanostructures toward NIR band, the NIR‐TPF efficiency can be greatly increased. Through internal etching, surface peeling, and ligand replacement, spectroscopic results validated that such resonantly enhanced NIR‐TPF is mediated by surface states with strong NIR‐IR absorption. This TPF signal evolution can be generalized to other iron oxide nanomaterials like magnetite nanoparticles and α‐Fe₂O₃ nanoplates. Using the developed fluorescent Fe₃O₄ nanostructures, it is demonstrated that their TPF and third harmonic generation (THG) contrast in the nonlinear optical microscopy of live cells. It is anticipated that the synthesized NIR photofunctional Fe₃O₄ will serve as a versatile platform for dual‐modality magnetic resonance imaging (MRI) as well as a magnet‐guided theranostic agent.     

Supercritical‐Fluid‐Assisted One‐Pot Synthesis of Biocompatible Core(γ‐Fe2O3)/Shell(SiO2) Nanoparticles as High Relaxivity T2‐Contrast Agents for Magnetic Resonance Imaging

Monodisperse iron oxide/microporous silica core/shell composite nanoparticles, core(γ‐Fe₂O₃)/shell(SiO₂), with a diameter of approximately 100 nm and a high magnetization are synthesized by combining sol–gel chemistry and supercritical fluid technology. This one‐step processing method, which is easily scalable, allows quick fabrication of materials with controlled properties and in high yield. The particles have a specific magnetic moment (per kg of iron) comparable to that of the bulk maghemite and show superparamagnetic behavior at room temperature. The nanocomposites are proven to be useful as T₂ MRI imaging agent. They also have potential to be used in NMR proximity sensing, theranostic drug delivery, and bioseparation.     

Simple Patterning via Adhesion between a Buffered‐Oxide Etchant‐Treated PDMS Stamp and a SiO2 Substrate

A very simple polydimethylsiloxane (PDMS) pattern‐transfer method is devised, called buffered‐oxide etchant (BOE) printing. The mechanism of pattern transfer is investigated, by considering the strong adhesion between the BOE‐treated PDMS and the SiO₂ substrate. PDMS patterns from a few micrometers to sub‐micrometer size are transferred to the SiO₂ substrate by just pressing a stamp that has been immersed in BOE solution for a few minutes. The patterned PDMS layers work as perfect physical and chemical passivation layers in the fabrication of metal electrodes and V₂O₅ nanowire channels, respectively. Interestingly, a second stamping of the BOE‐treated PDMS on the SiO₂ substrate pre‐patterned with metal as well as PDMS results in a selective transfer of the PDMS patterns only to the bare SiO₂. In this way, the fabrication of a device structure consisting of two Au electrodes and V₂O₅ nanowire network channels is possible; non‐ohmic semiconducting I–V characteristics, which can be modeled by serially connected percolation, are observed.     

Local structure around iron at the SiO2/Si interface

We have investigated the local structure around iron at the SiO₂/Si interface by the total-reflection fluorescence x-ray absorption fine structure technique, in conjunction with measurements of the angular dependence of x-ray fluorescence intensity. The Fe-O, Fe-Si and Fe-Fe bondings were observed around iron, in the layer formed at the SiO₂/Si interface. These results show the formation of iron silicate, consisting of iron, oxygen and silicon elements. The chemical state of iron was determined from the Fe-O bond-lengths. The Fe-valence is a mixture of Fe³⁺ and Fe²⁺, mostly Fe³⁺. These results indicate that the layer formed at the SiO₂/Si interface is iron silicate, in which a portion of Fe³⁺ ions were reduced.     

Engineering Highly Ordered Iron Titanate Nanotube Array Photoanodes for Enhanced Solar Water Splitting Activity

Highly ordered iron titanate (Fe₂TiO₅) nanotube array photoanode is synthesized on F:SnO₂ glass with ultrathin anodized aluminum oxide as a hard template. Highly crystalline, yet the nanotube array morphology‐preserved Fe₂TiO₅ is fabricated by hybrid microwave annealing (HMA). The effects of the synthesis parameters on photoelectrochemical (PEC) water splitting activity under simulated sunlight are systematically studied including HMA time, pore size, wall thickness, and length of the nanotubes to optimize the nanotube array photoanode. In addition, triple modification strategies of TiO₂ underlayer, hydrogen treatment, and FeNiO_x cocatalyst loading effectively improve the PEC activity further. The systematically engineered nanotube array photoanode achieves a photocurrent density of 0.93 mA cm^?2 at 1.23 V_RHE under 1 sun (100 mW cm^?2) irradiation, which corresponds to 2.6 times that of the previous best Fe₂TiO₅ photoanode. In addition, the photocurrent onset potential shifts cathodically by ≈280 mV relative to the pristine nanotube array electrode.     

Core–Shelled Low‐Oxidation State Oxides@Reduced Graphene Oxides Cubes via Pressurized Reduction for Highly Stable Lithium Ion Storage

Metal oxides have been regarded as promising next‐generation anode materials for rechargeable lithium ion batteries; however, their poor stability, which is caused by large volume changes during repeated lithiation/delithiation, remains a challenge. Here, conformally encapsulated low‐oxidation state oxide cubes with reduced graphene oxide (RGO) obtained via a new pressurized reduction consisting of external mechanical compression and internal thermokinetic reduction from highly porous metal oxides/RGO aerogel (RGOA) are reported. Using single crystalline (SC) cobalt oxides and iron oxide cubes as model systems, the SC‐Co₃O₄ or Fe₂O₃ cube/RGOA are pressurized into compacted xerogel along with a uniform thermokinetic reduction, which result in topotactic transformation to core‐shelled CoO/RGO or Fe₃O₄@RGO cubes. The SC‐CoO and SC‐Fe₃O₄ cubes isolated perfectly in the RGO shells have dramatically improved their cycling stabilities for lithium ion storage to hundreds of times.     

One‐Step Synthesis of Highly Ordered Mesoporous Silica Monoliths with Metal Oxide Nanocrystals in their Channels

A simple, one‐step synthetic route to prepare ordered mesoporous silica monoliths with controllable quantities of metal oxide nanocrystals in their channels is presented. The method is based on the assisted assembly effect for mesostructure‐directing of the metal complexes formed by the interaction of metal ions with the –O– groups of copolymers. Highly ordered hexagonal silica monoliths, loaded with various metal oxide nanocrystals, including those of Cr₂O₃, MnO, Fe₂O₃, Co₃O₄, NiO, CuO, ZnO, CdO, SnO₂, and In₂O₃, can be obtained by this one‐step pathway. In the NiO/SiO₂ nanocomposite, nickel oxide nanorods with face‐centered cubic lattices are formed at low doping ratios, and they can be transformed into nanowires by increasing the quantities of the precursors. In the Fe₂O₃/SiO₂ nanocomposites, a one‐dimensional assembly of iron oxide nanoparticles is observed. In the In₂O₃/SiO₂ nanocomposites, single crystal nanowires with high aspect ratios are obtained. For the other metal oxide nanocomposites, including Cr₂O₃, MnO, Co₃O₄, CuO, ZnO, CdO, and SnO, only crystalline nanorods are obtained. N₂ sorption results of the metal oxide/SiO₂ mesostructured nanocomposites reveal that nanocrystals inside the pores do not severely decrease the pore volume or the Brunauer–Emmett–Teller (BET) surface area of the mesoporous silica host. The bandgaps of SnO₂ and In₂O₃ nanocrystals, calculated from UV‐vis spectra, are much larger than the corresponding bulk materials, implying the quantum confinement effect in the small particles. Co₃O₄/SiO₂ mesostructured nanocomposites catalyze the complete combustion of CH₄. These studies provide a new and simple method for templating synthesis of metal oxide nanostructures.     

Fabrication and Optical Characteristics of Position‐Controlled ZnO Nanotubes and ZnO/Zn0.8Mg0.2O Coaxial Nanotube Quantum Structure Arrays

The position‐controlled growth and structural and optical characteristics of ZnO nanotubes and their coaxial heterostructures are reported. To control both the shape and position of ZnO nanotubes, hole‐patterned SiO₂ growth‐mask layers on Si(111) substrates with GaN/AlN intermediate layers using conventional lithography are prepared. ZnO nanotubes are grown only on the hole patterns at 600 °C by catalyst‐free metal–organic vapor‐phase epitaxy. Furthermore, the position‐controlled nanotube growth method allows the fabrication of artificial arrays of ZnO‐based coaxial nanotube single‐quantum‐well structures (SQWs) on Si substrates. In situ heteroepitaxial growth of ZnO and Zn_0.8Mg_0.2O layers along the circumference of the ZnO nanotube enable an artificial formation of quantum‐well arrays in a designed fashion. The structural and optical characteristics of the ZnO nanotubes and SQW arrays are also investigated using synchrotron radiation X‐ray diffractometry and photoluminescence and cathodoluminescence spectroscopy.     

High-frequency reactive diode sputtering of magnetite films on the sapphire surface

The conditions (oxygen partial pressure and growth rate) for the deposition of magnetite (iron oxide) Fe₃O₄ on the r plane of the single-crystalline sapphire using the high-frequency reactive diode sputtering of the Fe target are determined. The resulting ferromagnetic layers exhibit polycrystalline structure with a typical block size of 100–200 nm. The X-ray analysis is used to demonstrate that the textured phase of magnetite that is normally oriented with respect to the substrate dominates in the blocks and Fe and Fe₂O₃ impurities are almost absent.     

Surface passivation of high‐efficiency silicon solar cells by atomic‐layer‐deposited Al2O3

Atomic‐layer‐deposited aluminium oxide (Al₂O₃) is applied as rear‐surface‐passivating dielectric layer to passivated emitter and rear cell (PERC)‐type crystalline silicon (c‐Si) solar cells. The excellent passivation of low‐resistivity p‐type silicon by the negative‐charge‐dielectric Al₂O₃ is confirmed on the device level by an independently confirmed energy conversion efficiency of 20·6%. The best results are obtained for a stack consisting of a 30 nm Al₂O₃ film covered by a 200 nm plasma‐enhanced‐chemical‐vapour‐deposited silicon oxide (SiO_x) layer, resulting in a rear surface recombination velocity (SRV) of 70 cm/s. Comparable results are obtained for a 130 nm single‐layer of Al₂O₃, resulting in a rear SRV of 90 cm/s. Copyright © 2008 John Wiley & Sons, Ltd.     

Trimetallic Mn‐Fe‐Ni Oxide Nanoparticles Supported on Multi‐Walled Carbon Nanotubes as High‐Performance Bifunctional ORR/OER Electrocatalyst in Alkaline Media

Discovering precious metal‐free electrocatalysts exhibiting high activity and stability toward both the oxygen reduction (ORR) and the oxygen evolution (OER) reactions remains one of the main challenges for the development of reversible oxygen electrodes in rechargeable metal–air batteries and reversible electrolyzer/fuel cell systems. Herein, a highly active OER catalyst, Fe_0.3Ni_0.7O_X supported on oxygen‐functionalized multi‐walled carbon nanotubes, is substantially activated into a bifunctional ORR/OER catalyst by means of additional incorporation of MnO_X. The carbon nanotube‐supported trimetallic (Mn‐Ni‐Fe) oxide catalyst achieves remarkably low ORR and OER overpotentials with a low reversible ORR/OER overvoltage of only 0.73 V, as well as selective reduction of O₂ predominantly to OH^?. It is shown by means of rotating disk electrode and rotating ring disk electrode voltammetry that the combination of earth‐abundant transition metal oxides leads to strong synergistic interactions modulating catalytic activity. The applicability of the prepared catalyst for reversible ORR/OER electrocatalysis is evaluated by means of a four‐electrode configuration cell assembly comprising an integrated two‐layer bifunctional ORR/OER electrode system with the individual layers dedicated for the ORR and the OER to prevent deactivation of the ORR activity as commonly observed in single‐layer bifunctional ORR/OER electrodes after OER polarization.     

Layer‐By‐Layer Dendritic Growth of Hyperbranched Thin Films for Surface Sol–Gel Syntheses of Conformal,Functional, Nanocrystalline Oxide Coatings on Complex 3D (Bio)silica Templates

Here, a straightforward and general method for the rapid dendritic amplification of accessible surface functional groups on hydroxylated surfaces is described, with focus on its application to 3D biomineral surfaces. Reaction of hydroxyl‐bearing silica surfaces with an aminosilane, followed by alternating exposure to a dipentaerythritol‐derived polyacrylate solution and a polyamine solution, allows the rapid, layer‐by‐layer (LBL) build‐up of hyperbranched polyamine/polyacrylate thin films. Characterization of such LBL‐grown thin films by AFM, ellipsometry, XPS, and contact angle analyses reveals a stepwise and spatially homogeneous increase in film thickness with the number of applied layers. UV–Vis absorption analyses after fluorescein isothiocyanate labeling indicate that significant amine amplification is achieved after the deposition of only 2 layers with saturation achieved after 3–5 layers. Use of this thin‐film surface amplification technique for hydroxyl‐enrichment of biosilica templates facilitates the conformal surface sol–gel deposition of iron oxide that, upon controlled thermal treatment, is converted into a nanocrystalline (～9.5 nm) magnetite (Fe₃O₄) coating. The specific adsorption of arsenic onto such magnetite‐coated frustules from flowing, arsenic‐bearing aqueous solutions is significantly higher than for commercial magnetite nanoparticles (≤50 nm in diameter).     

p‐Si/SnO2/Fe2O3 Core/Shell/Shell Nanowire Photocathodes for Neutral pH Water Splitting

Silicon is one of the promising materials for solar water splitting and hydrogen production; however, it suffers from two key factors, including the large external potential required to drive water splitting reactions at its surface and its instability in the electrolyte. In this study, a successful fabrication of novel p‐Si/n‐SnO₂/n‐Fe₂O₃ core/shell/shell nanowire (css‐NW) arrays, consisting of vertical Si NW cores coated with a thin SnO₂ layer and a dense Fe₂O₃ nanocrystals (NCs) shell, and their application for significantly enhanced solar water reduction in a neutral medium is reported. The p‐Si/n‐SnO₂/n‐Fe₂O₃ css‐NW structure is characterized in detail using scanning, transmission, and scanning transmission electron microscopes. The p‐Si/n‐SnO₂/n‐Fe₂O₃ css‐NWs show considerably improved photocathodic performances, including higher photocurrent and lower photocathodic turn‐on potential, compared to the bare p‐Si NWs or p‐Si/n‐SnO₂ core/shell NWs (cs‐NWs), due to increased optical absorption, enhanced charge separation, and improved gas evolution. As a result, photoactivity at 0 V versus reversible hydrogen electrode and a low onset potential in the neutral solution are achieved. Moreover, p‐Si/n‐SnO₂/n‐Fe₂O₃ css‐NWs exhibit long‐term photoelectrochemical stability due to the Fe₂O₃ NCs shell well protection. These results reveal promising css‐NW photoelectrodes from cost‐effective materials by facile fabrication with simultaneously improved photocathodic performance and stability.     

Effects of resistive switching in Au/FeO<Subscript>x</Subscript>/Pt structures

The dynamics of forming and resistive switching in Au/iron oxide/Pt structures based on magnetite (Fe₃O₄) is investigated. It has been revealed that these processes have an electrochemical nature and are accompanied by formation of low-resistance conductive filaments formed by inclusions of magnetite in the matrix of high-resistance maghemite (γ-Fe₂O₃). It has been found that the electric field stimulates reversible redox reactions in iron oxide. The direction and rate of these reactions are determined by the polarity and the amplitude of voltage pulses. The possibility of setting the resistance of the structure by changing the amplitude of the “record” voltage pulses opens prospects for the development of multilevel memory elements.     

A Facile Synthesis and Characterization of Monodisperse Spherical Pigment Particles with a Core/Shell Structure

In this paper, a facile sol–gel process for producing monodisperse, spherical, and nonaggregated pigment particles with a core/shell structure is reported. Spherical silica particles (245 and 385 nm in diameter) and Cr₂O₃, α‐Fe₂O₃, ZnCo₂O₄, CuFeCrO₄, MgFe₂O₄, and CoAl₂O₄ pigments are selected as cores and shells, respectively. The obtained core/shell‐structured pigment samples, denoted as SiO₂@Cr₂O₃ (green), SiO₂@α‐Fe₂O₃ (red), SiO₂@MgFe₂O₄ (brown), SiO₂@ZnCo₂O₄ (dark green), SiO₂@CoAl₂O₄ (blue), and SiO₂@CuFeCrO₄ (black), are well characterized by using X‐ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and UV‐vis diffuse reflection, as well as by investigating the magnetic properties. The results of XRD and high‐resolution TEM (HRTEM) demonstrate that the pigment shells crystallize well on the surface of SiO₂ particles. The thickness of the pigment shell can be tuned by the number of coatings, to some extent. These pigment particles can be well dispersed in some solvents (such as glycol) to form relatively more stable suspensions than the commercial products. Apart from the color characteristics, some of pigments like SiO₂@Cr₂O₃, SiO₂@MgFe₂O₄, and SiO₂@CuFeCrO₄ also show magnetic properties with coercivities of 1098 Oe (5 K), 648 Oe (5 K), and 91 Oe (298 K), respectively.     

Enhanced formaldehyde-sensing properties of mixed Fe2O3–In2O3 nanotubes

Pure In₂O₃ and mixed Fe₂O₃–In₂O₃ nanotubes were prepared by simple electrospinning and subsequent calcination. The as-prepared nanotubes were characterized by scanning electron microscopy, powder X-ray diffraction, and energy-dispersive X-ray spectrometry. Gas sensors were fabricated to investigate the gas-sensing properties of In₂O₃ and Fe₂O₃–In₂O₃ nanotubes. Compared to pure In₂O₃, Fe₂O₃–In₂O₃ nanotubes exhibited better gas-sensing properties for formaldehyde at 250 °C. The response of the Fe₂O₃–In₂O₃ nanotube gas sensor to 100 ppm formaldehyde was approximately 33, which is approximately double the response of the pure In₂O₃ nanotube gas sensor. In both cases the response time was ~5 s and the recovery time was ~25 s.     

Magnetics and magnetoresistance in epitaxial magnetite heterostructures

We have synthesized and characterized epitaxial magnetite (Fe₃O₄) heterostructures that have been grown by pulsed laser deposition. Trilayers of magnetite, separated by a paramagnetic insulating CoCr₂O₄ layer, exhibit two distinct coercive fields corresponding to the two magnetite layers. However, detailed magnetic measurements indicate that the two magnetite layers are exchange coupled across CoCr₂O₄ layers as thick as 10 nm. Current-voltage characteristics in a range of temperatures show nonlinear behavior reminiscent of junction behavior. Magnetoresistance and its temperature dependence are characteristic of a single magnetic layer.     

Synthesis and Characterization of Iron/Iron Oxide Core/Shell Nanocubes

A simple method for the preparation of iron/iron oxide nanoparticles with core/shell cubic morphology is presented. The synthesis of the nanocubes was carried out through decomposition of a preformed iron oleate complex at high temperature. Although this procedure has been shown previously to produce monodisperse magnetite spheres,^[1] the use of squalene as a solvent and the presence of sodium oleate was found to induce cube formation. A detailed high‐resolution transmission electron microscopy (HRTEM) analysis of the nanocubes was performed for structural characterization. The core/shell structure, an iron core surrounded by magnetite (Fe₃O₄) shell, was confirmed by fast Fourier transform (FFT) filtering analysis. The results obtained by HRTEM analysis are in agreement with X‐ray Photoelectron Spectroscopy (XPS) and magnetic analysis. The Fe nanocubes are superparamagnetic at room temperature with a saturation magnetization M_S = 101 A m² kg^–1 and magnetic anisotropy density K_eff = 1.6 × 10⁵ J m^–3 at low temperatures.     

Fe3O4 Nanoparticles Confined in Mesocellular Carbon Foam for High Performance Anode Materials for Lithium‐Ion Batteries

Fe₃O₄ nanocrystals confined in mesocellular carbon foam (MSU‐F‐C) are synthesized by a “ host–guest ” approach and tested as an anode material for lithium‐ion batteries (LIBs). Briefly, an iron oxide precursor, Fe(NO₃)₃·9H₂O, is impregnated in MSU‐F‐C having uniform cellular pores ～30 nm in dia­meter, followed by heat‐treatment at 400 °C for 4 h under Ar. Magnetite Fe₃O₄ nanocrystals with sizes between 13–27 nm are then successfully fabricated inside the pores of the MSU‐F‐C, as confirmed by transmission electron microscopy (TEM), dark‐field scanning transmission electron microscopy (STEM), energy dispersive X‐ray spectroscopy (EDS), X‐ray diffraction (XRD), and nitrogen sorption isotherms. The presence of the carbon most likely allows for reduction of some of the Fe³⁺ ions to Fe²⁺ ions via a carbothermoreduction process. A Fe₃O₄/MSU‐F‐C nanocomposite with 45 wt% Fe₃O₄ exhibited a first charge capacity of 1007 mA h g^?1 (Li⁺ extraction) at 0.1 A g^?1 (～0.1 C rate) with 111% capacity retention at the 150^th cycle, and retained 37% capacity at 7 A g^?1 (～7 C rate). Because the three dimensionally interconnected open pores are larger than the average nanosized Fe₃O₄ particles, the large volume expansion of Fe₃O₄ upon Li‐insertion is easily accommodated inside the pores, resulting in excellent electrochemical performance as a LIB anode. Furthermore, when an ultrathin Al₂O₃ layer (<4 Å) was deposited on the composite anode using atomic layer deposition (ALD), the durability, rate capability and undesirable side reactions are significantly improved.     

A Universal Approach to Fabricate Various Nanoring Arrays Based on a Colloidal‐Crystal‐Assisted‐Lithography Strategy

In this paper a convenient and universal strategy for preparing nanoring arrays of different compositions based on a colloidal‐crystal‐template strategy is reported. Large‐area arrays of polystyrene, magnetite, Au, Si, magnetite nanoparticle/polystyrene and Au/polystyrene double‐layer composite nanorings are prepared. Many kinds of nanoring structures, including Fe₃O₄ nanoparticle/polystyrene and Au/polystyrene double‐layer nanorings, can be released from the substrates, resulting in free‐standing composite nanorings, which might be used as self‐assembly building blocks and ultrasensitive bio‐ and chemical sensors.