Metal Ions Induce Growth and Magnetism Alternation of α‐Fe2O3 Crystals Bound by High‐Index Facets

In this study, quasi‐cubic and hexagonal bipyramid α‐Fe₂O₃ polyhedrons with high‐index facets exposed were controllably synthesized by applying metal ions Zn²⁺ or Cu²⁺ as structure‐directing agents. The growth of the α‐Fe₂O₃ nanostructures with high‐index facets were induced by metal ions without the addition of any other surfactants. The quasi‐cubic form controlled by Zn²⁺ looks like a cube but has an angle of approximately 86° bound by (012), (10‐2), and (1‐12) facets, whereas the hexagonal bipyramid form controlled by Cu²⁺ has a sixfold axis bound by {012} facets. Magnetic measurements confirm that these two kinds of nanocrystals display shape‐ and surface‐dependent magnetic behaviors. The hexagonal bipyramid iron oxide nanocrystals show a lower Morin transition temperature of 240 K and might be spin‐canted ferromagnetically controlled at room temperature, and the ferromagnetism disappears at low temperature. The quasi‐cubic nanocrystals have a splitting between FC curve and ZFC curve from the highest experimental temperature and no Morin transformation occurs; this indicates that they would be defect ferromagnetically controlled at low temperature. The reported metal‐ion‐directing technique could provide a universal method for shape‐ and surface‐controlled synthesis of nanocrystals with high‐index facets exposed.     

Metal ions induce growth and magnetism alternation of α-Fe2O3 crystals bound by high-index facets

In this study, quasi-cubic and hexagonal bipyramid α-Fe(2)O(3) polyhedrons with high-index facets exposed were controllably synthesized by applying metal ions Zn(2+) or Cu(2+) as structure-directing agents. The growth of the α-Fe(2)O(3) nanostructures with high-index facets were induced by metal ions without the addition of any other surfactants. The quasi-cubic form controlled by Zn(2+) looks like a cube but has an angle of approximately 86° bound by (012), (10-2), and (1-12) facets, whereas the hexagonal bipyramid form controlled by Cu(2+) has a sixfold axis bound by {012} facets. Magnetic measurements confirm that these two kinds of nanocrystals display shape- and surface-dependent magnetic behaviors. The hexagonal bipyramid iron oxide nanocrystals show a lower Morin transition temperature of 240?K and might be spin-canted ferromagnetically controlled at room temperature, and the ferromagnetism disappears at low temperature. The quasi-cubic nanocrystals have a splitting between FC curve and ZFC curve from the highest experimental temperature and no Morin transformation occurs; this indicates that they would be defect ferromagnetically controlled at low temperature. The reported metal-ion-directing technique could provide a universal method for shape- and surface-controlled synthesis of nanocrystals with high-index facets exposed.     

Synthesis of porous hematite nanorods loaded with CuO nanocrystals as catalysts for CO oxidation

Porous hematite (α-Fe2O3) nanorods with the diameter of 20-40 nm and the length of 80-300 nm were synthesized by a simple surfactant-assisted method in the presence of cetyltrimethylammonium bromide (CTAB).The α-Fe2O3 nanorods possess a mesostructure with a pore size distribution in the range of 5-12 nm and high surface area,exhibiting high catalytic activity for CO oxidation.CuO nanocrystals were loaded on the surface of porous α-Fe2O3 nanorods by a deposition-precipitation method,and the catalysts exhibited superior activity for catalytic oxidation of CO,as compared with commercial α-Fe2O3 powders supported CuO catalyst.The enhanced catalytic activity was attributed to the strong interaction between the CuO nanocrystals and the support of porous α-Fe2O3 nanorods.     

Sub-nanometer,Ultrafine α-Fe2O3 Sheets Realized by Controlled Crystallization Kinetics for Stable,High-Performance Energy Storage

The development of energy devices based on iron oxides/hydroxides is largely hindered by their poor conductivity and large volume changes, especially with regard to specific capacitance and cycle stability. Herein, superior capacitance (1575 F g⁻¹ at 1.25 A g⁻¹) and high rate performance (955 F g⁻¹ at 25 A g⁻¹) were realized by synthesizing sub-nanometer, ultrafine α-Fe₂O₃ sheets loaded on graphene (SU-Fe₂O₃-rGO). An assembled asymmetric supercapacitor showed outstanding cycle stability (106 % retention after 30 000 cycles). This excellent performance arises from the unique structural characteristics of the α-Fe₂O₃ sheets, which not only enrich electrochemically reactive sites, but also largely eliminate the volume changes after long-term charge/discharge cycling. The synthesis of SU-Fe₂O₃-rGO critically depends on control of the crystallization kinetics during growth. A controlled heterogeneous nucleation mechanism results in the formation of atomically thin α-Fe₂O₃ sheets on graphene rather than large particles in solvent, as clarified by theoretical calculations. This strategy paves a new way to synthesizing atomically thin transition metal oxide sheets and low-cost, eco-friendly iron-based energy storage.     

Binary α-Fe2O3–Co3O4 nanostructures for advanced oxidation process: Role of synergy for enhanced catalysis

Iron and its binary oxides are meticulously exploited for environmental remediations. However, only limited studies have been carried out on the degradation of industrial organics by advanced oxidation process. In this study, iron oxide, cobalt oxide, and iron–cobalt binary oxides were synthesized by a modified hydrothermal method as heterogeneous Fenton-like catalysts for the removal of methylene blue (MB) from wastewaters. The oxide nanostructures were characterized by different analytical techniques. Studying the effects of various parameters such as catalyst dose, MB concentration, and H₂O₂ concentration, the reaction conditions were optimized to enhance the removal of MB dye. The results revealed that α-Fe₂O₃–Co₃O₄ shows much higher activity than both Co₃O₄ and α-Fe₂O₃ for the degradation of MB at room temperature and beyond. The binary α-Fe₂O₃–Co₃O₄ shows degradation efficiency of 96.4% at 65 °C within 60 min. Furthermore, the binary α-Fe₂O₃–Co₃O₄ catalyst retains its activity for up to four successive cycles. A probable mechanism is also proposed, involving the generation of ‧OH radical as well as Fe²⁺/Fe³⁺ or Co²⁺/Co³⁺ redox couple of the binary α-Fe₂O₃–Co₃O₄ catalyst.     

Activation of α-Fe2O3 for Photoelectrochemical Water Splitting Strongly Enhanced by Low Temperature Annealing in Low Oxygen Containing Ambient

Photoelectrochemical (PEC) water splitting is a promising method for the conversion of solar energy into chemical energy stored in the form of hydrogen. Nanostructured hematite (α-Fe₂O₃) is one of the most attractive materials for a highly efficient charge carrier generation and collection due to its large specific surface area and the short minority carrier diffusion length. In the present work, the PEC water splitting performance of nanostructured α-Fe₂O₃ is investigated which was prepared by anodization followed by annealing in a low oxygen ambient (0.03 % O₂ in Ar). It was found that low oxygen annealing can activate a significant PEC response of α-Fe₂O₃ even at a low temperature of 400 °C and provide an excellent PEC performance compared with classic air annealing. The photocurrent of the α-Fe₂O₃ annealed in the low oxygen at 1.5 V vs. RHE results as 0.5 mA cm⁻², being 20 times higher than that of annealing in air. The obtained results show that the α-Fe₂O₃ annealed in low oxygen contains beneficial defects and promotes the transport of holes; it can be attributed to the improvement of conductivity due to the introduction of suitable oxygen vacancies in the α-Fe₂O₃. Additionally, we demonstrate the photocurrent of α-Fe₂O₃ annealed in low oxygen ambient can be further enhanced by Zn-Co LDH, which is a co-catalyst of oxygen evolution reaction. This indicates low oxygen annealing generates a promising method to obtain an excellent PEC water splitting performance from α-Fe₂O₃ photoanodes.     

Support Morphology Effect on Selective Hydrogenation of 3-Nitrostyrene to 3-Vinylaniline over Pt/α-Fe2O3 Catalysts

Selective hydrogenation of substituted nitroaromatic compounds is an extremely important and challenging reaction. Supported metal catalysts attract much attention in this reaction because the properties of metal nanoparticles (NPs) can be modified by the nature of the support. Herein, the support morphology on the catalytic performance of selective hydrogenation of 3-nitrostyrene to 3-vinylaniline was investigated. Pt NPs supported on octadecahedral α-Fe₂O₃ supports with a truncated hexagonal bipyramid shape (Pt/α-Fe₂O₃-O) and rod-shaped α-Fe₂O₃ supports (Pt/α-Fe₂O₃-R) were prepared by glycol reduction method. Detailed characterizations reveal that the electronic structure and dispersion of Pt NPs can be modified by the supports. The Pt/α-Fe₂O₃-O catalyst exhibited superior catalytic performance for hydrogenation of 3-nitrostyrene because of its low coordinated Pt sites and the small Pt NPs size, which is benefit from the high-index exposed surfaces of truncated hexagonal bipyramid-shaped α-Fe₂O₃ support. The structural evolution during the catalytic reaction was investigated in detail by identical location transmission electron microscopy (IL-TEM) method, which found that the high cycling activity of Pt/α-Fe₂O₃-O catalyst during the cycle experiment results from the stability of Pt NPs.     

高指数晶面纳米催化剂的电化学制备及应用

催化剂的性能与其表面结构及组成密切相关,高指数晶面纳米晶的表面含有高密度的台阶原子等活性位点而表现出较高的催化活性. 本文综述了电化学方波电位方法用于Pt、Pd、Rh等贵金属高指数晶面结构纳米晶催化剂的制备、形成机理及其电催化性能的研究. 针对贵金属利用率问题,还着重介绍了具有较高质量活性的小粒径Pt二十四面体的制备. 在此基础上,还介绍了电化学方波电位方法用于低共熔溶剂中制备高指数晶面纳米晶,以及高指数晶面纳米催化剂的表面修饰及应用;最后对高指数晶面纳米催化剂的发展做出了展望.     

Ultrafine α-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> nanocrystals anchored on N-doped graphene: a nanomaterial with long hole diffusion length and efficient visible light-excited charge separation for use in photoelectrochemical sensing

The authors have coupled ultrafine α-Fe₂O₃ nanocrystals to N-doped graphene (NG) to obtain a novel material for use in a photoelectrode. The presence of NG is shown to strongly affect the morphology and size of the α-Fe₂O₃ nanocrystals formed on the NG sheets, and to improve their photoelectrochemical (PEC) activity. Interestingly, the PEC performance of the nanocomposite is closely correlated to the size of the α-Fe₂O₃ nanocrystals in that small nanocrystals display better PEC properties. The photocurrent of α-Fe₂O₃-NG is nearly 3.3-fold stronger than that of α-Fe₂O₃ nanocrystals. Based on the remarkable PEC performance of this nanocomposite, a PEC sensor was constructed for the sensitive determination of 1,4-dihydroxybenzene (HQ). Its photocurrent increases with the HQ concentration in the range from 3.0 nM to 3.3 μM, and the detection limit is 1.0 nM (at an S/N ratio of 3). In our perception, the study presented here can serve as a basis for a better understanding of the relationship between morphologies and PEC performance of such nanomaterials. Conceivably, it may be extended to other PEC sensing system and to other fields associated with nanotechnology.

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Graphical abstract Ultrafine α-Fe₂O₃ nanocrystals were prepared via coupling with N-doped graphene (NG) substances (α-Fe₂O₃-NG). They exhibit superior photoelectrochemical (PEC) performance and have been successfully utilized for PEC-based sensing.

     

Morphology-controlled synthesis of Ti-doped α-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> nanorod arrays as an efficient photoanode for photoelectrochemical applications

Few reports have been published on the optimization of nanostructures while doping with the Ti (Ti³⁺/Ti⁴⁺) elemental. Here, Ti-doped α-Fe₂O₃ nanorod arrays prepared via the hydrothermal method with the addition of TiCl₃ as the Ti source and urea as the morphological regulator were used as photoanodes in photoelectrochemical cells. In the process of a hydrothermal reaction, Ti elemental was incorporated into α-Fe₂O₃ photoanodes using TiCl₃ as precursor and urea was used as the morphological regulator to assist α-Fe₂O₃ to form nanorod arrays. The photoelectrochemical performance of the as-prepared Ti-doped α-Fe₂O₃ nanorod array (TF1) photoanodes exhibited a remarkable photocurrent of 0.22 mA cm^?2 (275 times higher than that of the undoped α-Fe₂O₃ nanorod arrays) at 1.23 V (vs. RHE) and a 150-mV cathodic shift of photocurrent onset potential. The enhanced photoelectrochemical performance was ascribed to the synergistic effect of the one-dimensional nanoarray structure and the Ti elemental doping, which increased donor density and reduced photogenerated electron–hole recombination.     

Amine-assisted synthesis of concave polyhedral platinum nanocrystals having {411} high-index facets

High-index surfaces of a face-centered cubic metal (e.g., Pd, Pt) have a high density of low-coordinated surface atoms and therefore possess enhanced catalysis activity in comparison with low-index faces. However, because of their high surface energy, the challenge of chemically preparing metal nanocrystals having high-index facets remains. We demonstrate in this work that introducing amines as the surface controller allows concave Pt nanocrystals having {411} high-index facets to be prepared through a facile wet-chemical route. The as-prepared Pt nanocrystals display a unique octapod morphology with {411} facets. The presence of high-index {411} exposed facets endows the concave Pt nanocrystals with excellent electrocatalytic activity in the oxidation of both formic acid and ethanol.     

Synthesis of nanocrystals with variable high-index Pd facets through the controlled heteroepitaxial growth of trisoctahedral Au templates

The shape-controlled synthesis of noble metal nanocrystals (NCs) bounded by high-index facets is a current research interest because the products have the potential of significantly improving the catalytic performance of NCs in industrially important reactions. This study reports a versatile method for synthesizing polyhedral NCs enclosed by a variety of high-index Pd facets. The method is based on the heteroepitaxial growth of Pd layers on concave trisoctahedral (TOH) gold NC seeds under careful control of the growth kinetics. Polyhedral Au@Pd NCs with three different classes of high-index facets, including concave TOH NCs with {hhl} facets, concave hexoctahedral (HOH) NCs with {hkl} facets, and tetrahexahedral (THH) NCs with {hk0} facets, can be formed in high yield. The Miller indices of NCs are also modifiable, and we have used the THH NCs as a demonstrative example. The catalytic activities of these NCs were evaluated by the structure-sensitive reaction of formic acid electro-oxidation. The results showed that the high-index facets are generally more active than the low-index facets. In summary, a seeded growth process based on concave high-index faceted monometallic TOH NC templates and careful control of the growth kinetics is a simple and effective strategy for the synthesis of noble metal NCs with high-index facets. It also offers tailorability of the surface structure in shape-controlled synthesis.     

Influence of the preparation history of α-Fe2O3 on its reactivity for hydrogen reduction

TG experiments on the hydrogen reduction of α-Fe₂O₃ were carried out to elucidate the influence of the preparation history of the oxide on its reactivity. α-Fe₂O₃ samples were prepared by the thermal decomposition of seven iron salts in a stream of oxygen, air or nitrogen at temperatures of 500–1200°C for 1 h. Thirteen metal ions such as Cu²⁺, Ni²⁺, etc. were used as doping agents. The reactivity of the oxide was indicated by the initial reduction temperature (T_i. α-Fe₂O₃ prepared at lower temperatures showed lower T_i values and the reduction proceeded stepwise (Fe₂O₃ → Fe₃O₄ → Fe). T_i values increased with the rise in the preparation temperature of the oxide. The oxides prepared at higher temperatures showed that two reduction steps (Fe₂O₃ → Fe₃O₄ → Fe) proceed simultaneously. the preparation in oxygen gave higher T_i than that in air or nitrogen. The doping by metal ions, except Ti⁴⁺, lowered the T_i of α-Fe₂O₃. The Cu²⁺ ion showed the lowest T_i, while Ti⁴⁺ showed the highest T_i and the inhibition effect.The reduction process was expressed by two equations; Avrami—Erofeev's equation for α-Fe₂O₃ → Fe₃O₄ and Mampel's equation for Fe₃O₄ → Fe.     

Characterization of fine particles of the α-Fe2O3-SnO2 system with residual SO2−4 ions on the surface

The α-Fe₂O₃-SnO₂ system in fine powder form prepared by thermal decomposition at 873 K of precursory oxide hydroxides and containing a few weight percent of residual SO²⁻₄ ions on the surface has been characterized by X-ray diffraction, by the⁵⁷Fe and¹¹⁹Sn Mo¨ssbauer effects, and by other techniques. A certain α-Fe₂O₃-rich composition of this system has been used practically as an excellent but cheap CH₄ sensor as a city gas-leak alarm. Fine particles of generally ≲10³ nm³ with intermediate compositions remain far from the equilibrium state in the aspect that the solubility is drastically widened for both α-Fe₂O₃ and SnO₂, especially for the latter: solid solutions (Fe₂O₃)_1−x(SnO₂)_x withx ≲ 0.2 crystallizing in the corundum-type structure and withx ≳ 0.7 crystallizing in the rutile-type structure seem to be formed. The SO²⁻₄ ions may be considered to form microscopic cages around fine particles which, although being thermodynamically unstable, are nevertheless practically stable for more than a few years at the operating temperature of 673 K as demonstrated by the practical use as a sensor. A rapid separation into and a rapid particle growth of α-Fe₂O₃ and SnO₂ are brought about by thermal release of the SO²⁻₄ ions at 1073 K.     

Self‐Template Synthesis of Hybrid Porous Co3O4–CeO2 Hollow Polyhedrons for High‐Performance Supercapacitors

In this work, hybrid porous Co₃O₄–CeO₂ hollow polyhedrons have been successfully obtained via a simple cation‐exchange route followed by heat treatment. In the synthesis process, ZIF‐67 polyhedron frameworks are firstly prepared, which not only serve as a host for the exchanged Ce3⁺ ions but also act as the template for the synthesis of hybrid porous Co₃O₄–CeO₂ hollow polyhedrons. When utilized as electrode materials for supercapacitors, the hybrid porous Co₃O₄–CeO₂ hollow polyhedrons delivered a large specific capacitance of 1288.3 F g^?1 at 2.5 A g^?1 and a remarkable long lifespan cycling stability (<3.3 % loss after 6000 cycles). Furthermore, an asymmetric supercapacitor (ASC) device based on hybrid porous Co₃O₄–CeO₂ hollow polyhedrons was assembled. The ASC device possesses an energy density of 54.9 W h kg^?1, which can be retained to 44.2 W h kg^?1 even at a power density of 5100 W kg^?1, indicating its promising application in electrochemical energy storage. More importantly, we believe that the present route is a simple and versatile strategy for the preparation of other hybrid metal oxides with desired structures, chemical compositions and applications.     

α-Fe2O3/SiO2纳米颗粒混合体系表面的酸碱性质及其对重金属离子的吸附行为

以Fe(NO₃)₃·9H₂O和正硅酸乙酯(TEOS)为原料, 通过溶胶-凝胶法和辅助模板法分别制备了纳米α-Fe₂O₃和SiO₂, 并对所合成样品进行了粉末X射线衍射(XRD)和BET表征. 使用自动电位滴定仪测定了α-Fe₂O₃/SiO₂纳米颗粒混合体系的表面酸碱性质. 研究了在不同pH下α-Fe₂O₃/SiO₂混合体系对Cu²⁺、Pb²⁺、Zn²⁺离子的吸附行为. 基于上述实验数据, 用WinSGW软件计算了α-Fe₂O₃/SiO₂混合体系表面酸碱配位常数, 并得出结论: α-Fe₂O₃/SiO₂混合体系表面反应为单一脱质子反应≡XOH ⇔ ≡XO^-+ H⁺(lg K = -8.19±0.15), 明显区别于同时具有加质子和脱质子反应的α-Fe₂O₃/SiO₂/γ-Al₂O₃, α-Fe₂O₃/γ-Al₂O₃和SiO₂/γ-Al₂O₃等纳米颗粒混合体系. 在此基础上拟合得到α-Fe₂O₃/SiO₂混合体系吸附重金属离子Cu²⁺、Pb²⁺、Zn²⁺的表面络合反应平衡常数分别为:
≡XOH + M²⁺ ⇔ ≡XOM⁺+ H⁺ [lg K = -3.1, -3.6, -3.8 (M = Cu, Pb, Zn)].
≡XOH＋M²⁺＋H₂O ⇔≡XOMOH＋2H⁺[lg K = -8.8, -8.0, -10.5 (M = Cu, Pb, Zn)]     

Synthesis on an ultra large scale of nearly monodispersed γ-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoparticles with La(III) doping through a sol–gel route assisted by propylene oxide

Nearly monodispersed La³⁺ doped γ-Fe₂O₃ nanoparticles were synthesized on an ultra-large scale of about 60 g in a single reaction by a low temperature sol–gel route. The nanoparticles were obtained by the reaction of FeCl₂ and La(NO₃)₃ in ethanol solution with propylene oxide to form the sol, followed by the boiling of the sol solution. The La³⁺ doping promotes the phase transformation temperature of γ-Fe₂O₃ nanoparticles from 350 to 650 °C by the La³⁺ doping induced enhancement of phase transformation activation energy. This large scale synthesis strategy offers important advantages over other conventional routes for the preparation of undoped and doped γ-Fe₂O₃ nanoparticles. These guarantee the promising application of this route in the industrial production.     

Amorphous/Crystalline Hetero-Phase TiO2-Coated α-Fe2O3 Core–Shell Nanospindles: A High-Performance Artificial Nitrogen Fixation Electrocatalyst

Electrochemical nitrogen fixation techniques have emerged as a promisingly sustainable approach to face the challenge associated with nitrogen activation of ammonia synthesis by the Haber–Bosch process under ambient conditions. Herein, the performance of electrocatalytic nitrogen reduction for the production of α-Fe₂O₃ nanospindles coated with mesoporous TiO₂ with different crystallinity [denoted as α-Fe₂O₃@mTiO₂-X (X=300, 400, and 500 °C)] were investigated. The as-prepared α-Fe₂O₃@mTiO₂-400 composite exhibits a large NH₃ yield (27.2 μg h⁻¹ mg_cat.⁻¹) at −0. 5 V vs. the reversible hydrogen electrode and a high Faradaic efficiency (13.3 %) in 0.1 m Na₂SO₄, with excellent electrochemical durability. This work presents a novel avenue for the rational design of efficient unique hetero-phase nanocatalysts toward sustainable electrocatalytic N₂ fixation.     

Growth of High Magnetic α-Fe2O3 and Fe3O4 Nanowires via an Oxide Assisted Vapor-Solid Process

We describes a controllable synthesis procedure for growing α-Fe₂O₃ and Fe₃O₄ nanowires. High magnetic hematite α-Fe₂O₃ nanowires are successfully grown on Fe_0.5Ni_0.5 alloy sub-strates via an oxide assisted vapor-solid process. Experimental results also indicate that previous immersion of the substrates in a solution of oxalic acid causes the grown nanowires to convert gradually into magnetite (Fe₃O₄) nanowires. Additionally, the saturated state of Fe₃O₄ nanowires is achieved as the oxalic acid concentration reaches 0.75 mol/L. The aver-age diameter and length of nanowires expands with an increasing operation temperature and the growth density of nanowires accumulates with an increasing gas flux in the vapor-solid process. The growth mechanism of α-Fe₂O₃ and Fe₃O₄ nanowires is also discussed. The results demonstrate that the entire synthesis of nanowires can be completed within 2 h.     

Calcination-temperature-dependent gas-sensing properties of mesoporous α-Fe2O3 nanowires as ethanol sensors

The mesoporous α-Fe₂O₃ nanowires (NWs) were successfully synthesized by changing the calcination temperature from 550 to 750 °C (marked NWs-550, NWs-650 and NWs-750) via using SBA-15 silica as the hard templates with the nanocasting method. The characterization results indicated that the bandgap of the as-prepared samples hardly changed and the high BET surface areas changed a little with the calcination temperature from 550 to 750 °C. Mesoporous α-Fe₂O₃ NWs had been found to possess the remarkable gas-sensing performance to ethanol gas. The gas-sensing behavior indicated that α-Fe₂O₃ NWs-650 exhibited the higher response than that of α-Fe₂O₃ NWs-550 and α-Fe₂O₃ NWs-750. The calcination-temperature-dependent gas-sensing properties were mainly attributed to the competition of surface defects and body defects by the crystallization temperature. The lower calcination temperature could create more surface defects to improve the gas-sensing response, while the higher temperature would reduce the body defect and make the charge carriers transport easily. As the result, the suitable calcination temperature was desired to optimize the defects of nanostructures to improve the gas sensitivity.