α-Fe2O3 Nanosheets Research Articles

Chemical Vapor Deposition of Two-Dimensional Magnetite Nanosheets and Raman Study of Heat-Induced Oxidation Reaction

Two-dimensional magnetic materials exhibiting antiferromagnetic properties at room temperature offer significant potential for developing next generation spintronic nanodevices. This work presents controlled synthesis of two-dimensional magnetite (Fe3O4) nanosheets utilizing lost cost and convenient chemical vapor deposition (CVD) technique, Raman spectroscopy study of laser-heating and annealing induced oxidation reaction. X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscopy, and Raman spectroscopy characterizations demonstrated that Fe3O4 nanosheets down to about 10 nm with good crystalline quality, surface uniformity, and minimal defects can be obtained by optimizing the growth parameters in our CVD synthesis. Both laser-heating and annealing studies of Fe3O4 nanosheets indicated that ferrimagnetic Fe3O4 nanosheets gradually change to antiferromagnetic α-Fe2O3 nanosheets at about 400 °C. Our study showed that Raman spectroscopy provides a convenient and powerful method for estimating heat-induced temperature and investigating oxidation reaction.

Atomic Valence Reversal-Induced Polarization Resonance Spurs Highly Efficient Electromagnetic Wave Absorption in α-Fe2O3@Carbon Microtubes.

Variegation and complexity of polarization relaxation loss in many heterostructured materials provide available mechanisms to seek a strong electromagnetic wave (EMW) absorption performance. Here we construct a unique heterostructured compound that bonds α-Fe2O3 nanosheets of the (110) plane on carbon microtubes (CMTs). Through effective alignment between the Fermi energy level of CMTs and the conduction band position of α-Fe2O3 nanosheets at the interface, we attain substantial polarization relaxation loss via novel atomic valence reversal between Fe(III) ↔ Fe(III-) induced with periodic electron injection from conductive CMTs under EMW irradiation to give α-Fe2O3 nanosheets. Such heterostructured materials possess currently reported minimum reflection loss of -84.01 dB centered at 10.99 GHz at a thickness of 3.19 mm and an effective absorption bandwidth (reflection loss ≤ -10 dB) of 7.17 GHz (10.83-18 GHz) at 2.65 mm. This work provides an effective strategy for designing strong EMW absorbers by combining highly efficient electron injection and atomic valence reversal.

α-Fe2O3 nanosheets as Electro-Fenton catalyst for highly efficient phenol degradation

A two-dimensional material, α-Fe2O3 nanosheets, was prepared and used as heterogeneous Electro-Fenton catalysts in phenol degradation. The morphology and electrochemical characterization of the catalysts were carried out. Morphological and structural analysis showed that the prepared product was a hexagonal α-Fe2O3 nanosheet with, a thickness of about 10 nm, and a width of 200 nm. Electrochemical tests show that the catalysts have good catalytic performance. The electrochemical test results of α-Fe2O3 with different morphologies show that the two-dimensional nanosheet morphology improves the catalytic performance of α-Fe2O3. The effects of different factors (current density, aeration rate, pH, catalyst dosage, and electrolyte concentration) were discussed. Free radical quenching experiments demonstrated that OH and SO4− are the main active species. Furthermore, the catalyst was recycled 7 times, and the phenol removal rate did not decrease significantly.

Single Crystalline α-Fe2O3 Nanosheets with Improved PEC Performance for Water Splitting.

We report the photoelectrochemical (PEC) performance of a densely grown single crystalline hematite (α-Fe2O3) nanosheet photoanode for water splitting. Unlike expensive ITO/FTO substrates, the sheets were grown on a piece of pure Fe through controlled thermal oxidation, which is a facile low cost and one-step synthesis route. The sheets grow with a widest surface parallel to basal plane (0001). Iron oxide formed on Fe consisting of layer structure α-Fe2O3-Fe3O4-Fe is elucidated from GIXRD and correlated to spectral features observed in Raman and UV-vis spectroscopy. The top α-Fe2O3 nanosheet layer serves as a photoanode, whereas the conducting Fe3O4 layer serves to transport photogenerated electrons to the counter electrode through its back contact. Time-resolved photoluminescence (TRPL) measurements revealed significantly prolonged carrier lifetime compared to that of bulk. Compared to the thin film of α-Fe2O3 grown on the FTO substrate, ∼3 times higher photocurrent density (0.33 mA cm-2 at 1.23 VRHE) was achieved in the nanosheet sample under solar simulated AM 1.5 G illumination. The sample shows a bandgap of 2.1 eV and n-type conductivity with carrier density 9.59 × 1017 cm-3. Electrochemical impedance spectroscopy (EIS) measurements reveal enhanced charge transport properties. The results suggest that nanosheets synthesized by the simple method yield far better PEC performance than the thin film on the FTO substrate. The anodic shifts of flat band potential, delayed electron-hole recombination, and growth direction parallel to the highly conducting basal plane (0001) being some of the contributing factors to the higher photocurrent observed in the NS photoanode are discussed. Characterizations carried out before and after the PEC reaction show excellent stability of the nanosheets in an alkaline electrochemical environment.

Well-designed α-Fe2O3/ZnFe2O4 heterojunction for photocatalytic CO2 reduction to fuels

Spinel ZnFe2O4 photocatalyst has been widely applied in CO2 reduction due to its negative enough conduction band edge and relatively narrow band gap, which enable the generation of strong reduction electrons in the presence of light irradiation. Unfortunately, the catalytic performance of pure ZnFe2O4 was seriously hindered because of inefficient charge transfer. Here, Z-scheme α-Fe2O3/ZnFe2O4 heterojunctions were synthesized by coupling α-Fe2O3 nanosheets with spinel ZnFe2O4 nanoparticles and applied to the photoreduction of CO2. The α-Fe2O3/ZnFe2O4 heterojunctions realized high charge separation efficiency and strong redox ability. As a result, the CH4 generation rate of α-Fe2O3/ZnFe2O4 heterojunctions was 2.6 times higher than that of ZnFe2O4. This work exhibits a new strategy to design ZnFe2O4-based Z-scheme heterojunction for converting CO2 to hydrocarbon fuels.

Facet-dependent reactivity of α-Fe2O3 nanosheet on reactive oxygen species generation in Fenton-like process

Differences in physical structure (e.g., size, morphology, and exposed facets) will lead to divergence in the understanding of Fenton-like mechanisms. Herein, an iron oxide nanosheet predominated by (3 1 1) facet was prepared by the ultrasonic-assisted method to reveal the single factor of the catalytic descriptors. The adsorption energy of H2O2 on the (3 1 1) facet was only −0.99 eV, which was lower than that on other facets such as (4 0 0) and (2 2 0). Moreover, the energy barriers for the H2O2 activation on (4 0 0), (2 2 0) and (3 1 1) facets were 0.82, 0.64 and 0.30 eV, respectively. This will facilitate the adsorption and activation of H2O2 on the (3 1 1) facet during hydroxyl radicals (HO•) formation. Furthermore, intermediate O2•- adsorbed on the (3 1 1) facet will act as a precursor to form the first-excited-state singlet oxygen (1O2) for advanced oxidation of pollutants. Additionally, the nanosheet structure and good electrochemical properties of the Fe2O3-sheet will also facilitate electron transfer in Fe2O3-sheet/H2O2 system. Overall, this study provides a comprehensive view on the relationship between exposed facets and the efficiency of H2O2 activation, and optimizes the design of catalysts with high Fenton-like activity in the deep purification of wastewater.

Hierarchical hollow α-Fe2O3/ZnFe2O4/Mn2O3 Janus micromotors as dynamic and efficient microcleaners for enhanced photo-Fenton elimination of organic pollutants

Micro/nanomotors that can promote mass transport have attracted more and more research concern in the photocatalysis field. Here we first report a newly-designed hierarchical α-Fe2O3/ZnFe2O4/Mn2O3 magnetic micromotor as a heterogeneous photocatalyst for the degradation of cationic dye methylene blue (MB) from wastewater. The resulting three-dimensional (3D) flower-like hollow Janus micromotors are fabricated through a green and scalable strategy, in which each component has different functions. ZnFe2O4 microspheres serve as a magnetic scaffold for the nucleation and growth of α-Fe2O3 nanosheets and for the recycling of the micromachine. α-Fe2O3 nanosheets have shown great potential as an ideal semiconductor material for the photocatalytic decontamination of pollutants. Mn2O3 nanoparticles are mainly utilized as a catalyst to produce O2 bubbles to propel the autonomic movement of the micromotors in the presence of H2O2 fuel and also as a Fenton-like catalyst to decompose H2O2 to generate reactive oxygen species. Furthermore, the resultant micromotors exhibited linear-like motion form with an average speed of 189.1 μm s−1 in 5 wt% H2O2 solution. Moreover, the self-driven micromotors exhibited a superior catalytic degradation property toward MB, which was attributed to the synergistic effect of heterogeneous photocatalyst and the boosted micro-mixing and mass transfer caused by the vigorous motion of the micro-actuator. The possible degradation intermediates and passways of MB by α-Fe2O3/ZnFe2O4/Mn2O3 micromotor were identified with time of flight mass spectroscopy (TOF-MS). The 3D Janus micromotors have the potential to be used as a high-efficiency and active heterogeneous photocatalyst for the degradation of organic pollutants.

AuNPs and CNTs embellished three-dimensional bloom-like α-Fe2O3 nanocomposites for highly sensitive electrochemical pesticides detection

In this experimental study, a highly sensitive electrochemical acetylcholinesterase (AChE) biosensor for the detection of organophosphorus pesticides (OPs) based on three-dimensional α-Fe2O3 nanoflowers decorated with Au nanoparticles (AuNPs) and carbon nanotube (CNTs) (α-Fe2O3 NFs-AuNPs-CNTs) was developed. With the blooms structure, the α-Fe2O3 NFs not only provided an excellent microenvironment for the loading of AChE, but also increased the contact specific surface area. Meanwhile, AuNPs dispersed uniformly on the α-Fe2O3 nanosheets and CNTs further enhanced the conductivity of the nanocomposites. Under the optimized conditions, the biosensor was made to exhibit favorable linear relationship with the logarithm of the concentration of omethoate from 1 × 10−13-1 × 10−7 M, with the detection limit of 1.9 × 10−15 M. In addition, the biosensor also possessed good repeatability, stability and practicality, proved to be a valuable tool for the detection of pesticides in environment.

Activating well-defined α-Fe2O3 nanocatalysts by near-surface Mn atom functionality for auto-exhaust soot purification

Herein, the atomically dispersed manganese (Mn) sites on well-defined α-Fe2O3 nanosheets (Mn1-Fe2O3) were fabricated by a ligand-assisted in-situ crystallization method. Single-step surface engineering facilitates the in-situ replacement of Fe atoms in near-surface [FeO6] octahedra with monodispersed Mn atoms, as well as the preservation of regularly exposed {001} surface. Under different NOx concentration (40, 500 and 2000 ppm), Mn1-Fe2O3-10 catalyst presented the superior catalytic performance, whose T50 values are 455, 388 and 340 °C, respectively. By combining in-situ dynamic characterizations and DFT calculations, the surface [MnO5] and adjacent [FeO5] octahedra cooperatively boost two crucial steps: the dissociation of adsorbed O2 and the desorption of molecular NO2. The greater the amount of molecular NO2 produced, the higher the efficiency of soot purification via the NO2-assisted oxidation mechanism. Insights into the near-surface modulation and structure-activity relationship provide a promising strategy to improve the availability of surface-active sites in future heterogeneous catalysis.

Construction of α-Fe2O3/Sulfur-Doped Polyimide Direct Z-Scheme Photocatalyst with Enhanced Solar Light Photocatalytic Activity.

A novel two-dimensional α-Fe2O3/sulfur-doped polyimide (FO/SPI) direct Z-scheme photocatalyst was successfully constructed by a facile thermal treatment method. The effects of α-Fe2O3 nanosheets on the morphology, chemical structure, and photoelectronic properties of FO/SPI composites were systematically characterized by different spectroscopic means. These methods include X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, transient fluorescence spectra, and so forth. It was confirmed that the small amounts of α-Fe2O3 can availably facilitate exfoliation of bulk SPI, resulting in a transformation of SPI from bulk to 2D layered composite that illustrates tight interface through the coordination Fe–N bond and an all-solid-state direct Z-scheme junction. Thus, the transfer and separation efficiency of photogenerated electron/hole pairs were significantly enhanced, which greatly promoted improvement of the photocatalytic activity of the FO/SPI composite for methyl orange degradation under solar light. This work provides a new approach to constructing efficient inorganic–organic Z-scheme photocatalyst based on strong interface interaction.

Vertical interface coupling between crystalline α-Fe2O3 and carbon nitride nanosheets for efficient degradation of organic pollutants

The interface coupling of crystalline catalytic materials exhibits unique functions in removing pollutants and purifying water resources; however, little attention has been paid to the effect of different crystal surface coupling on photogenerated charge separation. In this study, different coupling facets of α-Fe2O3 were constructed on cyclized carbon nitride (ACN) by impregnation hydrothermal method, and the coupling mechanism of α-Fe2O3 crystal facets on ACN was explained. The α-Fe2O3 nanosheets can be assembled parallelly or vertically on the surface of ACN due to the surface energy of α-Fe2O3 crystal facets and the different surface charges of ACN. The Z-scheme heterojunction formed by different interface coupling between α-Fe2O3 and ACN improved the separation efficiencies of photogenerated carriers. The photodegradation of organic pollutants and antibiotics showed that the photoactivity of Fe110/ACN+ was much higher than that of ACN, ACN+ and Fe001/ACN. The maximum removal rate of oxytetracycline by Fe110/ACN+ composites was 88.6%, and it had excellent adsorption and catalytic effect on organic dye Congo red within 120 min. Overall, the work may provide a new perspective for designing structures of unique crystal surface coupling and constructing heterojunctions with high catalytic performance.

PPy decorated α-Fe2O3 nanosheets as flexible supercapacitor electrodes

PPy decorated α-Fe2O3 nanosheets as flexible supercapacitor electrodes

Mode-locked fiber laser based on [formula omitted]-Fe2O3 nanosheets as saturable absorbers

With broadband saturable absorption properties and a high third-order nonlinear optical susceptibility, α-Fe2O3 exhibits potential applications in nonlinear optics. In this study, α-Fe2O3 nanosheets are embedded into polyvinyl alcohol (PVA) films to fabricate thin-film saturable absorbers (SAs) that feature an uninterrupted broad absorption band spanning over 2000 nm and a modulation depth of 16.13%. In α-Fe2O3-PVA SA-based erbium-doped fiber lasers, two stable mode-locked operations are achieved: dark-bright pulse pairs and bright solitons. The results indicate that α-Fe2O3 nanosheets are excellent candidates for two-dimensional saturable absorption materials.

Ferromagnetism in 2D α-Fe2O3 nanosheets

In this report, hematene (2D α-Fe2O3 nanosheets) with an exceptionally high coercivity of up to 7.5 kOe has been synthesized via a soft-chemical exfoliation process. The high coercivity correlates with the surface magnetic anisotropy that originates from enhanced uncompensated spin canting as a result of the 2D morphology. This observation is different from the behavior of the bulk counterpart that exhibits collinear antiferromagnetic ordering with no net magnetization at low temperatures. In addition, our study shows a suppression of the Morin transition in 2D nanosheets, which further confirms that the surface spins deviate strongly from the collinear antiferromagnetic ordering. We also observed a spin-glass-like transition with a rapid increase in saturation magnetization and a decrease in anisotropy in the ultra-thin α-Fe2O3 nanosheets at temperatures below 48 K. The spin-glass behavior is correlated with the observed exchange bias and the magnetic field dependence of spin-glass freezing temperature.

Preparation of Si–α-Fe2O3/CdS composites with enhanced visible-light photocatalytic activity for p-nitrophenol degradation

Si–hematite (α-Fe2O3) nanosheets with highly exposed (110) facets were obtained first, and Si–α-Fe2O3/CdS composites were developed using a well-designed hydrothermal method. The prepared Si–α-Fe2O3/CdS composites showed enhanced visible-light photocatalytic activity compared with Si–α-Fe2O3 and CdS. The construction of the Si–α-Fe2O3/CdS heterojunction can inhibit the recombination of the photogenerated electron–hole pairs on the catalyst surface effectively. The synergistic effects of the photo-Fenton and the heterojunction promoted the production of the active species in the Si–α-Fe2O3/CdS system and enhanced the photocatalytic degradation activity under visible-light irradiation. The SFC-1:1 composite exhibited the highest photocatalytic activity toward p-nitrophenol degradation. The photocatalytic degradation mechanism was also investigated.

Nonlinear optical responses of α-Fe2O3 nanosheets and application as a saturable absorber in the wide near-infrared region

In the present work, we report a strategy to synthesize α-Fe2O3 nanoflakes as a nonlinear optical (NLO) material for the optical pulse generation in the near-infrared (NIR) band. With the excitation wavelength of 1 µm, the effective nonlinear absorption coefficients βeff was −0.501 cm/GW and the nonlinear refractive index n2 was 9.33 × 10-4 cm2/GW. While at 1.34 µm, βeff was −0.713 cm/GW and n2 was 5.01 × 10-4 cm2/GW. The two-photon absorption cross sections at 1 and 1.34 µm were as high as 9.6 × 105 and 5.7 × 105 GM, respectively, indicating that the hematite nanosheets could be used as the nonlinear optical devices. The third-order NLO susceptibilities were 6.3 × 10-11 and 3.75 × 10-11 esu at 1 and 1.3 µm, respectively. Using the ultrafast spectroscopy, the excited state lifetime was ~2.62 ps, suitable for the (ultra)short pulse generation. Based on the marvelous nonlinear absorption properties of the α-Fe2O3 nanoflakes saturable absorber, passively Q-switched Nd:GdVO4 lasers at 4F3/2 → 2I11/2 and 4F3/2 → 2I13/2 lasing transitions were demonstrated, producing the shortest pulse duration of 135 ns. The experimental results confirmed that α-Fe2O3 nanoflakes were an excellent candidate for the short NIR pulse generation in a broad band.

Enhanced catalytic performance with Fe@α-Fe2O3 thin nanosheets by synergistic effect of photocatalysis and Fenton-like process

Developing novel catalysts and catalysis processes are highly attractive in environmental protection and energy utilization. Photocatalysis-Fenton combinational process is an environmentally friendly technology and has broad application prospects. In this work, thin Fe@α-Fe2O3 nanosheets with thickness of 2–5 nm were designed by oxidation of Fe metal nanostructure synthesized using a modified solution reaction method. The nanosheet structure with thin thickness has short diffusion distance for the photogenerated carriers and prolongs the carriers' life. Simultaneously, the presence of Fe can reduce the resistance of system and further promote the transport and transfer of photogenerated carriers from the bulk to the surface, causing less carrier recombination and better photocatalytic performance. What's more, the Fe@α-Fe2O3 composite can induce the Fenton-like reaction to generate active •OH radicals, which significantly improve the degradation rate. The synergistic effect of photocatalysis and Fenton processes makes the system show an enhanced photodegradation performance: 96.5% of RhB is degraded after 60 min visible light irradiation, especially, 84.3% of RhB is degraded within the first 10 min, which are much higher than that of the pure α-Fe2O3 sample. This work demonstrates that the conjunction of photocatalysis and Fenton processes based on the Fe@α-Fe2O3 nanosheets provides a potential application for pollutants degradation.

Passively Q-switched Tm:CaLu0.1Gd0.9AlO4 laser at 2 µm with hematite nanosheets as the saturable absorber.

We report the first passive Q-switching operation at 1.95 µm utilizing the disordered Tm:CaLu0.1Gd0.9AlO4 (Tm:CLGA) crystal and the hematite (α-Fe2O3) nanosheets as the saturable absorber. The nonlinear saturable absorption properties of the hematite nanosheets were investigated by the conventional Z-scan technology. The modulation depth of 14.3% with the low saturation intensity of 205 kW/cm2 was obtained, indicating that the hematite could be a suitable saturable absorber for the mid-infrared pulse generation. Using the disordered Tm:CLGA crystal as the gain medium, the passive Q-switching operation could be realized with the hematite nanosheets as the saturable absorber, producing the shortest pulse duration of 402 ns with a repetition rate of 76 kHz. The experimental results convinced us that the hematite nanosheets could be of great interest in the optical pulse generation in the mid-infrared region.

Hematite (α-Fe2O3) nanosheets with enhanced photo-electrochemical ability fabricated via single step anodization

In the present study, synthesis of hematite nanosheets and their photocurrent response have been reported. The nanosheets are fabricated through a single step-anodization technique at room temperature. Light absorption study of nanosheets, using diffuse-reflectance-spectroscopy, depicts the band gap of ~2.16 eV. Photocurrent density of as-fabricated nanosheets is found to be ~0.72 mA cm−2. The enhanced photocurrent density as compared to iron oxide based photoelectrodes can be attributed to high surface area for light absorption and good crystallinity. These features improve the charge separation to overcome charge carrier recombination which favours their preference as a promising candidate for visible-light driven applications.

A Resource utilization method for volatile organic compounds emission from the semiconductor industry: Selective catalytic oxidation of isopropanol to acetone Over Au/α-Fe2O3 nanosheets

We prepare α-Fe2O3 nanosheet supported 0.38, 0.81, and 1.36 wt% Au (average particle size = 4.0 nm) nanocatalysts, and investigate their performance and mechanism for the selective catalytic oxidation of isopropanol to acetone. In the presence of 1.2 vol% isopropanol and 40 vol% O2, 1.36 wt% Au/α-Fe2O3 exhibits excellent catalytic performance, due to its moderate acidic sites and better redox properties, with acetone selectivity and yield being as high as 99% and 95% at 220 oC, respectively. In addition to acetone, little propylene, acetic acid, acetaldehyde, methyl vinyl ketone, 2-butanone, isopropyl ether, isopropyl acetate, 3-penten-2-one, isopropyl acrylate, isopropyl propionate, and 2, 4-dimethylfuran are detected. The possible reaction mechanism is proposed for the selective catalytic oxidation of isopropanol to acetone over the present catalysts. We believe the present selective catalytic oxidation method, rather than the traditional complete catalytic oxidation method, provides an alternative and economic method for VOCs emissions control.