Comparative study of Pt, Au and Ag growth on Fe3O4(0 0 1) surface

The epitaxial growth of Pt, Au and Ag layers on Fe₃O₄(0 0 1) as a function of temperature and thickness have been studied. The layers were deposited by sputtering in an ultra high vacuum chamber and the structural properties were investigated by Reflection High Energy Electron Diffraction, X-ray reflectivity and diffraction, High Resolution Transmission Electron Microscopy and Atomic Force Microscopy. Our studies give evidence for three different growth behaviours depending both on the nature of the metals and the temperature. Comparison between the growth modes of the three metals will be discussed in relation with surface and interfaces energies.     

Influence of substrate temperature in BiFeO3-CoFe2O4 nanocomposites deposited on SrTiO3 (0 0 1)

BiFeO₃-CoFe₂O₄ epitaxial nanocomposites have been deposited on SrTiO₃ (0 0 1) substrates by pulsed laser deposition. We present here a study of the influence of the deposition temperature (T_S), in the 550-800 °C range, on the film composition, morphology and microstructure. Electron-probe microanalysis shows strong reduction of the Bi content in the films when increasing T_S. Films prepared at T_S=750 °C and above are virtually Bi-free. X-ray diffraction (XRD) data show that, due to the volatility of Bi, there is a progressive reduction in the amount of BiFeO₃. The deposition temperature and the concomitant presence of Fe_xO_y spurious phases in the nanocomposites grown at high temperature promote radical changes in film morphology and magnetization. It thus follows that a temperature range suitable for controlled modification of nanocomposites morphology would be extremely narrow.     

Characterization of single-crystalline In2O3 films deposited on Y-stabilized ZrO2 (1 0 0) substrates by MOCVD

Epitaxial In₂O₃ films have been deposited on Y-stabilized ZrO₂ (YSZ) (1 0 0) substrates by metalorganic chemical vapor deposition (MOCVD). The films were deposited at different substrate temperatures (450-750 °C). The film deposited at 650 °C has the best crystalline quality, and observation of the interface area shows a clear cube-on-cube epitaxial relationship of In₂O₃(1 0 0)||YSZ(1 0 0) with In₂O₃[0 0 1]||YSZ[0 0 1]. The Hall mobility of the single-crystalline In₂O₃ film deposited at 650 °C is as high as 66.5 cm² V⁻¹ s⁻¹ with carrier concentration of 1.5 × 10¹⁹ cm⁻³ and resistivity of 6.3 × 10⁻³ Ω cm. The absolute average transmittance of the obtained films in the visible range exceeds 95%.     

Silane treatment of Fe3O4 and its effect on the magnetic and wear properties of Fe3O4/epoxy nanocomposites

In this study, the effect of silane treatment of Fe₃O₄ on the magnetic and wear properties of Fe₃O₄/epoxy nanocomposites was investigated. Fe₃O₄ nanopowders were prepared by coprecipitation of iron(II) chloride tetrahydrate with iron(III) chloride hexahydrate, and the surfaces of Fe₃O₄ were modified with 3-aminopropyltriethoxysilane. The magnetic properties of the powders were measured on unmodified and surface-modified Fe₃O₄/epoxy nanocomposites using SQUID magnetometer. Wear tests were performed on unmodified and surface-modified Fe₃O₄/epoxy nanocomposites under the same conditions (sliding speed: 0.18 m/s, load: 20 N).The results showed that the saturation magnetization (M_s) of surface-modified Fe₃O₄/epoxy nanocomposites was approximately 110% greater than that of unmodified Fe₃O₄/epoxy nanocomposites. This showed that the specific wear rate of surface-modified Fe₃O₄/epoxy nanocomposites was lower than that of unmodified Fe₃O₄/epoxy nanocomposites. The decrease in wear rate and the increase in magnetic properties of surface-modified Fe₃O₄/epoxy nanocomposites occurred due to the improved dispersion of Fe₃O₄ into the epoxy matrix.     

Green synthesis and characterization of superparamagnetic Fe3O4 nanoparticles

In this paper, we have first demonstrated a facile and green synthetic approach for preparing superparamagnetic Fe₃O₄ nanoparticles using α-d-glucose as the reducing agent and gluconic acid (the oxidative product of glucose) as stabilizer and dispersant. The X-ray powder diffraction (XRD), X-ray photoelectron spectrometry (XPS), and selected area electron diffraction (SAED) results showed that the inverse spinel structure pure phase polycrystalline Fe₃O₄ was obtained. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) results exhibited that Fe₃O₄ nanoparticles were roughly spherical shape and its average size was about 12.5 nm. The high-resolution TEM (HRTEM) result proved that the nanoparticles were structurally uniform with a lattice fringe spacing about 0.25 nm, which corresponded well with the values of 0.253 nm of the (3 1 1) lattice plane of the inverse spinel Fe₃O₄ obtained from the JCPDS database. The superconducting quantum interference device (SQUID) results revealed that the blocking temperature (T_b) was 190 K, and that the magnetic hysteresis loop at 300 K showed a saturation magnetization of 60.5 emu/g, and the absence of coercivity and remanence indicated that the as-synthesized Fe₃O₄ nanoparticles had superparamagnetic properties. Fourier transform infrared spectroscopy (FT-IR) spectrum displayed that the characteristic band of Fe-O at 569 cm⁻¹ was indicative of Fe₃O₄. This method might provide a new, mild, green, and economical concept for the synthesis of other nanomaterials.     

Structures of Co and Pt nanoclusters on a thin film of Al2O3/NiAl(1 0 0) from reflection high-energy electron diffraction and scanning-tunnelling microscopy

With reflection high-energy electron diffraction (RHEED) and scanning-tunnelling microscopy (STM), we made measurements on Co and Pt nanoclusters grown by vapour deposition on a thin film of Al₂O₃/NiAl(1 0 0). The results show that the annealed Co nanoclusters (with mean diameters 2.5, 3.4, 5.8 nm and heights 0.7, 1.5, 1.5 nm, respectively) and Pt nanoclusters (with mean diameter 2.25 nm and height 0.4 nm) are highly crystalline and that their structures are significantly affected by the oxide substrate. Structural analysis based on the RHEED patterns indicates that both Co and Pt clusters have a fcc phase and grow with their (0 0 1) facets parallel to the θ-Al₂O₃(1 0 0) surfaces, and with their [1 1 0] and [−1 1 0] axes along the [0 1 0] and [0 0 1] directions of the oxide surface, respectively, so (Co(0 0 1)[1 1 0]∥Al₂O₃(1 0 0)[0 1 0] and Pt(0 0 1)[1 1 0]∥Al₂O₃(1 0 0)[0 1 0]). This growth is optimal as the Co and Pt fcc (0 0 1) facets match well with the oxygen mesh. To minimize the lattice mismatch, the lattice parameter of the Co clusters expands 4-5% relative to fcc Co bulk, whereas the lattice parameter of the Pt clusters remains near the bulk value, as the Pt fcc (0 0 1) plane has a close lattice match with the oxide surface.     

Green synthesis of soya bean sprouts-mediated superparamagnetic Fe3O4 nanoparticles

Superparamagnetic Fe₃O₄ nanoparticles were first synthesized via soya bean sprouts (SBS) templates under ambient temperature and normal atmosphere. The reaction process was simple, eco-friendly, and convenient to handle. The morphology and crystalline phase of the nanoparticles were determined from scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and X-ray diffraction (XRD) spectra. The effect of SBS template on the formation of Fe₃O₄ nanoparticles was investigated using X-ray photoemission spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FT-IR). The results indicate that spherical Fe₃O₄ nanoparticles with an average diameter of 8 nm simultaneously formed on the epidermal surface and the interior stem wall of SBS. The SBS are responsible for size and morphology control during the whole formation of Fe₃O₄ nanoparticles. In addition, the superconducting quantum interference device (SQUID) results indicate the products are superparamagnetic at room temperature, with blocking temperature (T_B) of 150 K and saturation magnetization of 37.1 emu/g.     

Biosynthesis and magnetic properties of mesoporous Fe3O4 composites

Magnetic Fe₃O₄ materials with mesoporous structure are synthesized by co-precipitation method using yeast cells as a template. The X-ray diffraction (XRD) pattern indicates that the as-synthesized mesoporous hybrid Fe₃O₄ is well crystallized. The Barrett-Joyner-Halenda (BJH) models reveal the existence of mesostructure in the dried sample which has a specific surface area of 96.31 m²/g and a pore size distribution of 8-14 nm. Transmission electron microscopy (TEM) measurements confirm the wormhole-like structure of the resulting samples. The composition and chemical bonds of the Fe₃O₄/cells composites are studied by Fourier transform infrared (FT-IR) spectroscopy. Preliminary magnetic properties of the mesoporous hybrid Fe₃O₄ are characterized by a vibrating sample magnetometer (VSM). The magnetic Fe₃O₄/cells composites with mesoporous structure have potential applications in biomedical areas, such as drug delivery.     

Synthesis and properties of Au-Fe3O4 and Ag-Fe3O4 heterodimeric nanoparticles

Monodisperse Au-Fe 3 O 4 heterodimeric nanoparticles (NPs) were prepared by injecting precursors into a hot reaction solution.The size of Au and Fe 3 O 4 particles can be controlled by changing the injection temperature.UV-Vis spectra show that the surface plasma resonance band of Au-Fe 3 O 4 heterodimeric NPs was evidently red-shifted compared with the resonance band of Au NPs of similar size.The as-prepared heterodimeric Au-Fe 3 O 4 NPs exhibited superparamagnetic properties at room temperature.The Ag-Fe 3 O 4 heterodimeric NPs were also prepared by this synthetic method simply using AgNO 3 as precursor instead of HAuCl 4.It is indicated that the reported method can be readily extended to the synthesis of other noble metal conjugated heterodimeric NPs.     

纳米Fe3 O4 颗粒的正电子湮没谱学研究

测量了磁性纳米Fe₃O₄颗粒的X射线衍射谱(XRD)、正电子湮没寿命谱(PALS)和符合多普勒展宽谱(CDBS),研究了不同压力和退火温度对磁性纳米Fe₃O₄颗粒物相、电子结构、缺陷及电子动量分布等的影响. XRD,PALS,CDBS测量结果表明:纳米Fe₃O₄颗粒的缺陷浓度随压力的增加而增大,但物相和缺陷类型并未发生变化;磁性纳米Fe₃O_{4< 关键词： 正电子 3}O₄')" href="#">Fe₃O₄ 寿命谱 多普勒展宽谱     

Strongly interacting superspins in Fe3O4 nanoparticles

In this paper we report structural and magnetic properties of Fe₃O₄ nanoparticles synthesized by thermal decomposition of ball milled iron nitrate and citric acid in N₂ and air ambient. The XRD pattern of samples which are prepared in air shows some impurity phases, while the samples synthesized in the N₂ atmosphere are almost pure Fe₃O₄ phase. The result shows that by increasing the particle size, the magnetization of the samples increases. The increase of magnetization by increasing the particle size could be attributed to the lower surface spin canting and surface spin disorder of the larger magnetic nanoparticles. The results of ac magnetic susceptibility measurements show that the susceptibility data are not in accordance with the Néel -Brown model for superparamagnetic relaxation, but fit well with conventional critical slowing down model which indicates that the dipole-dipole interactions are strong enough to cause superspin-glass like phase in these samples.     

Scanning tunneling microscopy study on a BC3 covered NbB2(0 0 0 1) surface

We have investigated a BC₃ covered NbB₂(0 0 0 1) surface using scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and low energy electron diffraction (LEED). The STM images reveal characteristic features of a Moiré pattern reflecting an incommensurate relation of the BC₃ sheet with the substrate: bright protrusions with the periodicity of the substrate lattice are modulated in intensity with the periodicity of the BC₃ lattice. As a result, the surface exhibits nm-scale patchy regions with either the √3 × √3 or the 1 × 1 structure of the substrate. The two-dimensional Fourier transformation pattern of the STM image is consistent with the LEED pattern proving the epitaxial and incommensurate relationship between BC₃ surface sheet and substrate. No feature of a predicted superconducting gap was found in STS spectra measured at 5 K.     

海藻酸改性的空心Fe3O4纳米颗粒的制备与应用

不使用任何模板一步制得空心Fe₃O₄纳米颗粒,然后将海藻酸钠嫁接在氨基化的空心Fe₃O₄表面,再利用海藻酸盐与钙离子的作用,在空心Fe₃O₄表面形成一个凝胶化层,制得海藻酸盐凝胶化的空心Fe₃O₄纳米颗粒,粒径约为400～500 nm．采用TEM、XRD、XPS、VSM等手段对纳米微球进行表征．VSM表征结果表明在室温下样品磁性材料为超顺磁性．改性Fe₃O₄纳米颗粒成功地用于柔红霉素的载负和缓释,最大载负率和载药量分别为28.4%和14.2%．缓释结果表明,海藻酸盐凝胶化层的存在,能更有效控制柔红霉素缓慢地释放．     

Positive and negative magnetoresistance in Fe3O4-based heterostructures

Fe₃O₄-based heterostructures, including Fe₃O₄/MgO/Fe₃O₄, Fe₃O₄/MgO/Si and Fe₃O₄/SiO₂/Si, were fabricated by magnetron sputtering to investigate the perpendicular-to-plane magneto-transport properties. In the Fe₃O₄/MgO/Fe₃O₄ and Fe₃O₄/MgO/Si heterostructures, the typical magneto-transport properties of single Fe₃O₄ films, such as negative magnetoresistance (MR) and extreme values of MR−T curves at 120 K, were observed, suggesting that the spin polarization of conducting electrons conserves through MgO barrier. MR in the Fe₃O₄/MgO/Fe₃O₄ heterostructure is larger than that in the Fe₃O₄/MgO/Si heterostructure, because the spin of electrons is disturbed in the depletion layer of Si and the SiO₂ layer introduced by Fe₃O₄/MgO growth. The Fe₃O₄/SiO₂/Si heterostructure has a positive MR of 2% at 120 K, which may originate from the scattering of conducting electrons in amorphous SiO₂ and the spin polarization reversal at the Fe₃O₄/SiO₂ interface.     

Synthesis and magnetic properties of hard magnetic (CoFe2O4)-soft magnetic (Fe3O4) nano-composite ceramics by SPS technology

CoFe₂O₄/Fe₃O₄ nano-composite ceramics were synthesized by Spark Plasma Sintering. The X-ray diffraction patterns show that all samples are composed of CoFe₂O₄ and Fe₃O₄ phases when the sintering temperature is below 900 °C. It is found that the magnetic properties strongly depend on the sintering temperature. The two-step hysteresis loops for samples sintered below 500 °C are observed, but when sintering temperature reaches 500 °C, the step disappears, which indicates that the CoFe₂O₄ and Fe₃O₄ are well exchange coupled. As the sintering temperature increases from 500 to 800 °C, the results of X-ray diffractometer indicate the constriction of crystalline regions due to the ion diffusion at the interfaces of CoFe₂O₄/Fe₃O₄ phases, which have great impact on the magnetic properties.     

Relationship between surface structure and morphology of Fe3O4/silica composite nanospheres and nucleic acid extraction

Fe₃O₄/silica composite nanospheres with different surface structure and morphology were synthesized by changing reaction conditions. As-synthesized nanospheres were characterized by high performance particle sizer (HPPS), transmission electron microscopy (TEM), nitrogen adsorption and thermogravimetry (TGA). Besides thoroughly characterization, the particles were used for DNA extraction. We found that the particle surface structure and morphology affected the nucleic acid extraction efficiency. When comparing different samples with the same silanol density (10¹⁸), the one with a surface area of 60.37 m²/g extracted DNA most effectively. Also, with increasing silanol density per surface area, DNA extraction efficiency increased.     

Chemical quenching of positronium in Fe2O3/Al2O3 catalysts

Fe₂O₃/Al₂O₃ catalysts were prepared by solid state reaction method using α-Fe₂O₃ and γ-Al₂O₃ nano powders. The microstructure and surface properties of the catalyst were studied using positron lifetime and coincidence Doppler broadening annihilation radiation measurements. The positron lifetime spectrum shows four components. The two long lifetimes τ₃ and τ₄ are attributed to positronium annihilation in two types of pores distributed inside Al₂O₃ grain and between the grains, respectively. With increasing Fe₂O₃ content from 3 wt% to 40 wt%, the lifetime τ₃ keeps nearly unchanged, while the longest lifetime τ₄ shows decrease from 96 ns to 64 ns. Its intensity decreases drastically from 24% to less than 8%. The Doppler broadening S parameter shows also a continuous decrease. Further analysis of the Doppler broadening spectra reveals a decrease in the p-Ps intensity with increasing Fe₂O₃ content, which rules out the possibility of spin-conversion of positronium. Therefore the decrease of τ₄ is most probably due to the chemical quenching reaction of positronium with Fe ions on the surface of the large pores.     

Autocatalytic partial reduction of FeO(1 1 1) and Fe3O4(1 1 1) films by atomic hydrogen

The interaction of atomic hydrogen with thin epitaxial FeO(1 1 1) and Fe₃O₄(1 1 1) films was studied by TDS, XPS and LEED. On the thin, one Fe-O bilayer thick FeO film, partial reduction occurs in two steps during exposure. It ends after removal of 1/4 monolayer (ML) of oxygen with a 2 × 2 pattern appearing in LEED. This FeO_0.75 film is passive against further reduction. The first reduction step saturates after removal of ∼0.2 ML and shows autocatalytic kinetics with the oxygen vacancies formed during reduction causing acceleration. The second step is also autocatalytic and is related with reduction to the final composition and an improvement of the 2 × 2 order. A structure model explaining the two-step reduction is proposed. On the thick Fe₃O₄ film, irregular desorption bursts of H₂O and H₂ were observed during exposure. Their occurrence appears to depend on the film quality and thus on surface order. Because of the healing of reduction-induced oxygen vacancies by exchange of oxygen or iron with the bulk, a change of the surface composition was not visible. The existence of partially reduced oxide phases resistant even to atomic hydrogen is relevant to the mechanism of dehydrogenation reactions using iron oxides as catalysts.     

Molecular dynamics study on surface structure and surface energy of rutile TiO2 (1 1 0)

The formula for surface energy was modified in accordance with the slab model of molecular dynamics (MDs) simulations, and MD simulations were performed to investigate the relaxed structure and surface energy of perfect and pit rutile TiO₂(1 1 0). Simulation results indicate that the slab with a surface more than four layers away from the fixed layer expresses well the surface characteristics of rutile TiO₂ (1 1 0) surface; and the surface energy of perfect rutile TiO₂ (1 1 0) surface converges to 1.801±0.001 J m⁻². The study on perfect and pit slab models proves the effectiveness of the modified formula for surface energy. Moreover, the surface energy of pit surface is higher than that of perfect surface and exhibits an upper-concave parabolic increase and a step-like increase with increasing the number of units deleted along [0 0 1] and [1 1 0], respectively. Therefore, in order to obtain a higher surface energy, the direction along which atoms are cut out should be chosen in accordance with the pit sizes: [] direction for a small pit size and [0 0 1] direction for a big pit size; or alternatively the odd units of atoms along [1 1 0] direction are removed.     

Synthesis and microwave absorption properties of graphene-oxide(GO)/polyaniline nanocomposite with Fe3O4 particles

In order to investigate the impedance matching properties of microwave absorbers,the ternary nanocomposites of GO/PANI/Fe₃O₄（GPF） are prepared via a two-step method,GO/PANI composites are synthesized by dilute polymerization in the presence of aniline monomer and GO,and GO/PANI/Fe₃O₄ is prepared via a co-precipitation method.The obtained nanocomposites are characterized by scanning electron microscopy（SEM）,X-ray diffraction（XRD）,and Fourier transform infrared spectroscopy（FTIR）,respectively.The microwave absorbability reveals enhanced microwave absorption properties compared with GO,PANI,and GO/PANI.The maximum reflection loss of GO/PANI/Fe₃O₄ is up to-27 dB at 14 GHz with its thickness being 2 mm,and its absorption bandwidths exceeding-10 dB are more than 11.2 GHz with its thickness values being in the range from 1.5 mm-4 mm.It provides that GO/PANI/Fe₃O₄ can be used as an attractive candidate for microwave absorbers.