Hematite Nanorods Research Articles

Optical and Nano structural properties of Hematite (α-Fe2O3) nanorods in interaction with Bovine Serum Albumin (BSA) Protein Solution

Hematite (α-Fe2O3) nanorods were synthesized by hydrothermal method using Cetyltrimethylammonium bromide (CTAB) as a surfactant agent. To study optical, nanostructural properties, and to control the morphology and shape of nanorods, 0.025 mol L-1, 0.05 mol L-1 and 0.1 mol L-1 concentration of CTAB were used. Moreover, the effect of interaction between bovine serum albumin (BSA) A9418-5G protein solution and hematite nanorods was investigated. Fourier transform infrared spectroscopy (FTIR), transmitting electron microscopy (TEM), X-Ray Diffraction (XRD), Energy Dispersive X-Ray Spectroscopy (EDS) and UV-vis spectroscopy were used to characterize α-Fe2O3 nanorods. The samples prepared by the interaction of BSA protein and hematite nanorods did not representa rod shape because the electrostatic interaction between CTAB and BSA would cause the nanorods to have a limited capacity for carrying protein. Hematite nanorods obtained from 0.025 mol L-1 of CTAB showed a maximum length of 25-30 nm. However, BSA protein solution falsification of rod shape particles increased. The results showed that BSA protein could affect the shape of hematite nanorods and their aggregation, and formspherical structures as well.

Sn Doping into Hematite Nanorods for High-Performance Photoelectrochemical Water Splitting

Photoelectrochemical water splitting is of great attention due to its environmental friendly generation of clean fuels. Hematite (α-Fe2O3) is considered one of the promising candidates due to its intrinsic properties for the high performance photoelectrochemical electrode such as favourable bandgap (2.0–2.2 eV), a suitable energy band position, non-toxicity, low cost, and excellent chemical stability. Herein, we report about Sn-doped hematite nanorods and their implementation as photoanodes for photoelectrochemical water splitting. We provide the simple but efficient route to incorporate the Sn into the hematite without structural damage in the nanostructure and scrutinize the effect of Sn dopant on the photoelectrochemical activity of the hematite. By the two-step heat-treatment process, Sn can be successfully incorporated into the hematite, which reveals the enhanced photoelectrochemical responses compared with undoped hematite. We elaborate the effect of Sn dopant in the hematite on the photoelectrochemical activities, thereby the optimum concentration of Sn dopant can be suggested. In addition, the catalyst layer of the cobalt phosphate is introduced to further increase the photoelectrochemical performance of Sn-doped hematite nanorods.

Anisotropy control in magnetic nanostructures through field-assisted chemical vapor deposition.

Chemical vapor deposition of iron pentacarbonyl (Fe(CO)5) in an external magnetic field (B = 1.00 T) was found to significantly affect the microstructure and anisotropy of as-deposited iron crystallites that could be transformed into anisotropic hematite (α-Fe2O3) nanorods by aerobic oxidation. The deterministic influence of external magnetic fields on CVD deposits was found to be substrate-independent as demonstrated by the growth of anisotropic α-Fe columns on FTO (F:SnO2) and Si (100), whereas the films deposited in the absence of the magnetic field were constituted by isotropic grains. TEM images revealed gradual increase in average crystallite size in correlation to the increasing field strength and orientation, which indicates the potential of magnetic field-assisted chemical vapor deposition (mfCVD) in controlling the texture of the CVD grown thin films. Given the facet-dependent activity of hematite in forming surface-oxygenated intermediates, exposure of crystalline facets and planes with high atomic density and electron mobilities is crucial for oxygen evolution reactions. The field-induced anisotropy in iron nanocolumns acting as templates for growing textured hematite pillars resulted in two-fold higher photoelectrochemical efficiency for hematite films grown under external magnetic fields (J = 0.050 mA cm−2), when compared to films grown in zero field (J = 0.027 mA cm−2). The dark current measurements indicated faster surface kinetics as the origin of the increased catalytic activity.

Simultaneous reduction of surface, bulk, and interface recombination for Au nanoparticle-embedded hematite nanorod photoanodes toward efficient water splitting

A novel method to reduce surface, bulk, and interface recombination simultaneously.

Activating the surface and bulk of hematite photoanodes to improve solar water splitting.

A simple electrochemical activation treatment is proposed to improve effectively the photoelectrochemical performance of Nb,Sn co-doped hematite nanorods. The activation process involves an initial thrice cathodic scanning (reduction) and a subsequent thrice anodic scanning (oxidation), which modifies both the surface and bulk properties of the Nb,Sn:Fe2O3 photoanode. First, it selectively removes the surface components to different extents endowing the hematite surface with fewer defects and richer Nb-O and Sn-O bonds and thus passivates the surface trap states. The surface passivation effect also enhances the photoelectrochemical stability of the photoanode. Finally, more Fe2+ ions or oxygen vacancies are generated in the bulk of hematite to enhance its conductivity. As a result, the photocurrent density is increased by 62.3% from 1.88 to 3.05 mA cm-2 at 1.23 VRHE, the photocurrent onset potential shifts cathodically by ∼70 mV, and photoelectrochemical stability improves remarkably relative to the pristine photoanode under simulated sunlight (100 mW cm-2).

Hematite nanorod arrays top-decorated with an MIL-101 layer for photoelectrochemical water oxidation.

The construction of a nano-heterojunction photoanode is an attractive strategy for accelerated charge separation. Here, α-Fe2O3 nanorod arrays (Fe2O3 NAs) top-decorated with an MIL-101 layer are achieved via the CVD method for constructing one intimate contact between two layers. The systematical analysis in time-dependent deposition shows that the photoelectric response capability can be modulated by controlling the thickness of the MIL-101 layer as a function of CVD duration. An optimized thickness of 4-6 nm MIL-101 in the Fe2O3/MIL-101 photoanode presents the best PEC performance with the IPCE of 25% and a photocurrent density of 1 mA cm-2 at 1.3 V vs. RHE, about 2.5 times higher than that of pristine Fe2O3 NAs. The exciton lifetime and the electrochemical active surface area of the composited electrode are greatly improved by 14 and 1.3 times respectively compared to that of pristine Fe2O3. The notable advancement can be attributed to the fact that the top-decorated MOF layer can promote charge separation efficiency led by the built-in electric field and form a tight interface between Fe2O3 and MIL-101 with a favorable diffusion length for holes, as well as the fact that it can reduce surface defects and provide more reaction sites.

Protected Hematite Nanorod Arrays with Molecular Complex Co‐Catalyst for Efficient and Stable Photoelectrochemical Water Oxidation

Herein, a hybrid structure of hematite (α‐Fe2O3) nanorod arrays is designed, with surface catalyzed by Co molecular complex (Co(dca)2, dca: dicyanamide) and then protected by TiO2 thin overlayer, for efficient and stable photoelectrochemical (PEC) water oxidation. The obtained α‐Fe2O3/Co(dca)2/TiO2 hybrid nanorod arrays show much improved and stabilized PEC performance as compared to the pristine, and even the Co(dca)2 or TiO2 modified α‐Fe2O3, with a photocurrent density of 0.35 mA cm–2 obtained at 1.23 V vs. reversible hydrogen electrode (RHE), and an incident photon‐to‐current conversion efficiency (IPCE) reaching 16 % at 400 nm at 1.6 V vs. RHE. It has been demonstrated that the adsorbed Co(dca)2 molecular complex could effectively promote the interface charge transfer process and accelerate the water oxidation reaction kinetics, meanwhile the atomic layer deposited TiO2 overlayer could passivate the surface defects of α‐Fe2O3 and suppress the surface charge carrier recombination. Moreover, the TiO2 overlayer could effectively protect Co(dca)2 from detaching from the α‐Fe2O3 surface and thus stabilize the PEC activity for water oxidation reaction. The present study provides some available thoughts and methods for rational design of highly efficient photoelectrodes for water splitting from the perspective of the surface and interface engineered charge carrier transfer and water oxidation processes.

Enhancing Solar‐Driven Water Splitting with Surface‐Engineered Nanostructures

Functional nanoscale interfaces that promote the transport of photoexcited charge carriers are fundamental to efficient hydrogen production during photoelectrochemical (PEC) splitting of water. Here, the realization of a functional one‐dimensional nanostructure achieved through surface engineering of hematite (α‐Fe2O3) nanorods with a TiO2 overlayer is reported. The surface‐engineered hematite nanostructure exhibits significantly improved PEC performance as compared to untreated α‐Fe2O3, with an increase in the maximum incident photon‐to‐current efficiency (IPCE) of nearly 400% at 350 nm. While addition of the TiO2 overlayer did not alter the lifetime of photoexcited charge carriers, as evidenced from transient absorption spectroscopy, it is found that the presence of TiO2 could enhance oxygen electrocatalysis by interfacial electron enrichment, largely attributed to enhanced O(2p)−Fe(3d) hybridization. Moreover, the interfacial electronic structure revealed from XANES measurements of the α‐Fe2O3/TiO2 nanorods suggests that photoexcited holes in α‐Fe2O3 may efficiently transfer through the TiO2 overlayer to the electrolyte while electrons migrate to the external circuit along the one‐dimensional nanorods, thereby promoting charge separation and enhancing PEC splitting of water.

Synthesis, structural, thermal, optical and dielectric properties of chitosan biopolymer; influence of PVP and α-Fe2O3 Nanorods

The present work reports the influence of Polyvinylpyrrolidone (PVP) and hematite (α-Fe2O3) nanorods (NRs) on the physicochemical properties of chitosan (Cs), as an approach to broaden its medical and technological applications. Hematite NRs of 11.4 nm diameter and 87.9 nm crystallite size were prepared by a free-template chemical method. Cs, PVP/Cs and blend loaded with hematite NRs were prepared by solution casting. Significant changes in the films’ surface were clarified using the scanning electron microscope (SEM). Fourier transformation infrared spectroscopy (FT-IR) confirmed the interaction between the NRs and the NH2 and OH functional groups of Cs. DSC measurements showed one endothermic peak assigned to the water elimination, and an exothermic one, in the range 268–287 °C, attributed to the decomposition of saccharine structure in Cs. The swelling properties of the films were sensitive to the pH of the solution. PVP/Cs film showed ~ 85% transmittance in the visible region and its optical band gap narrowed from 5.4 eV to 4.05 eV after loading with 2.0 wt.% hematite. The influence of NRs content on the optical constants of the films is discussed. The dielectric properties depend on the film’ structure. The large Polaron tunneling (LPT) model is the best suitable mechanism for the electric conduction. Due to their high thermal stability and decomposition temperature, transmittance and high conductivity, the prepared films are a candidate for the packaging industry, for use in some medical applications such as treating some chronic wounds, and optical windows and fibers.

Charge carrier dynamics of Earth abundant co-catalyst (Ni) modified nanorod arrays for enhanced water splitting

The vertically oriented Ni (Ni0) decorated hematite nanorod array have been synthesized. The optimal Ni loading enhances photocurrent density > 4 folds at 1.23 V vs RHE and lowers the overpotential for driving the water oxidation, by ~70 mV. The Ni decorated hematite photoelectrode shows a longer decay time (~195 ps) and higher Stern-Volmer constant (5.25 M−1) than that of bare hematite photoelectrode (~156 ps and 3.89 M−1, respectively), implying an efficient charge separation and transfer across the hematite nanorod-Ni nanocrystal heterointerface. Such drastic improvement in the activity is due to the unified effects of enhanced charge separation and charge carrier densities, faster charge transfer kinetics, decreased width of depletion layer, and enhanced band bending – lead by the unison of the nanorod arrays morphology and Ni modification. These results suggest potential candidature of Earth abundant metallic Ni as a co-catalysts (-alternative to noble metals) for designing new photocatalysts.

Combined effect of nano-structured NiCo2S4 coated hematite photoanodes for efficient photoelectrochemical water oxidation

• Thiospinel NiCo 2 S 4 electrocatalyst coated on 1-D hematite nanorods for low bias PEC water oxidation. • The intrinsic effects of NiCo 2 S 4 on PEC water oxidation is discussed in detail. • NiCo 2 S 4 facilitated to reveal the intrinsic photovoltage of hematite, and hastened the water oxidation kinetics. A major challenge in photo electrochemical (PEC) water oxidation is the generation of hydrogen at a low external bias. Herein, thiospinel NiCo 2 S 4 particles were grown on Sn-doped hematite 1D nanorods for low bias PEC water oxidation via a simple hydrothermal method. Upon incorporation of NiCo 2 S 4 over hematite, the photocurrent increased from 0.85 to 1.51 mA cm –2 with a cathodic/negative shift in the onset potential of ∼330 mV. In addition, the intrinsic photovoltage of hematite increased from 180 to 290 mV upon illumination. These collective improvements originated from the inherent electrocatalytic activity of NiCo 2 S 4 in the water oxidation reaction. The core level electrochemical analyses confirmed that the inclusion of NiCo 2 S 4 hastened the charge injection at the electrode/electrolyte interface by suppressing the charge accumulation and recombination, eventually reducing the kinetic over potential.

Sb-Doped SnO2 Nanorods Underlayer Effect to the α-Fe2 O3 Nanorods Sheathed with TiO2 for Enhanced Photoelectrochemical Water Splitting.

Here, a Sb-doped SnO2 (ATO) nanorod underneath an α-Fe2 O3 nanorod sheathed with TiO2 for photoelectrochemical (PEC) water splitting is reported. The experimental results, corroborated with theoretical analysis, demonstrate that the ATO nanorod underlayer effect on the α-Fe2 O3 nanorod sheathed with TiO2 enhances the PEC water splitting performance. The growth of the well-defined ATO nanorods is reported as a conductive underlayer to improve α-Fe2 O3 PEC water oxidation performance. The α-Fe2 O3 nanorods grown on the ATO nanorods exhibit improved performance for PEC water oxidation compared to α-Fe2 O3 grown on flat fluorine-doped tin oxide glass. Furthermore, a simple and facile TiCl4 chemical treatment further introduces TiO2 passivation layer formation on the α-Fe2 O3 to reduce surface recombination. As a result, these unique nanostructures show dramatically improved photocurrent density (139% higher than that of the pure hematite nanorods).

Dual Effects of Nanostructuring and Oxygen Vacancy on Photoelectrochemical Water Oxidation Activity of Superstructured and Defective Hematite Nanorods.

An Ar atmospheric treatment is rationally used to etch and activate hematite nanoflakes (NFs) as photoanodes toward enhanced photoelectrochemical water oxidation. The formation of a highly ordered hematite nanorods (NRs) array containing a high density of oxygen vacancy is successfully prepared through in situ reduction of NFs in Ar atmosphere. Furthermore, a hematite (104) plane and an iron suboxide layer at the absorber/back-contact interface are formed. The material defects produced by a thermal oxidation method can be critical for the morphology transformation from 2D NFs to 1D NRs. The resulting hematite NR photoanodes show high efficiency toward solar water splitting with improved light harvesting capabilities, leading to an enhanced photoresponse due to the artificially formed oxygen vacancies.

Solar water splitting with nanostructured hematite: The role of annealing-temperature

The vertically aligned hematite (Fe2O3) nanorods fabrication has been considered as an effective method for photoelectrochemical (PEC) water splitting. To improve the activity, two-step annealing treatment in the temperature range from 550 °C to 800 °C was proposed for the Fe2O3 nanorods synthesized by hydrothermal method. This study reveals that the OER (oxygen evolution reaction) performance increases with the annealing temperature, which yields photocurrent density of 1.036 mA cm−2 and ABPC (applied bias photon-to-current) efficiency of 0.86% under simulated solar illumination of 100 mW cm−2 (AM 1.5G) condition at 1.23 V vs. RHE (reversible hydrogen electrode) at 800 °C. The second-step short time high temperature annealing treatment has a primary contribution on the fine-tune crystallite size and the diffusion and incorporation of Sn atoms. In addition, examining the corresponding resistance properties reveal a substantial improvement in the conductivity within the hematite film and at the conducting interface, and a suppressed charge carrier recombination process.

Ultrasmall NiFe-Phosphate Nanoparticles Incorporated α-Fe2O3 Nanoarrays Photoanode Realizing High Efficient Solar Water Splitting

The practical application of hematite (α-Fe2O3) in solar water splitting is severely limited by the highly charge recombination rate though its abundant reserves and suitable bandgap of ∼2.1 eV. This work describes the synthesis of ultrasmall NiFe-phosphate (NFP) nanoparticles incorporated α-Fe2O3 nanoarrays photoanode via a facile dip-coating and annealing process to demonstrate combined effects on enhanced photoelectrochemical (PEC) water oxidation. The NFP uniformly decorating on the surface of hematite nanorods not only could improve water oxidation kinetics and charge separation efficiency, but also could suppress the charge recombination in company with the surface states passivation. Furthermore, the phosphate (P) in the NFP nanoparticles could also play a synergistic effect on promoting the multiproton-coupled electron transfer (PCET) process for the PEC water oxidation. All of these lead to ∼140 mV cathodic shift of onset potential, ∼2.3-fold enhancement of the photocurrent and excellent long-term stability at 1.23 VRHE in 0.1 M KOH solution for α-Fe2O3/NFP photoanode. Along with these advantages, the NFP nanoparticles may possess new opportunities for modulating PEC water oxidation performances in hematite and other metal oxide photoanodes.

Visible light-induced electronic structure modulation of Nb- and Ta-doped α-Fe2O3 nanorods for effective photoelectrochemical water splitting

The photoelectrochemical (PEC) water splitting activity of Nb and Ta-doped hematite (α-Fe2O3) nanorods was investigated with reference to electronic structures by in situ synchrotron x-ray absorption spectroscopy (XAS). Current density-potential measurements demonstrate that the PEC activity of α-Fe2O3 nanorods depends strongly on the species and concentrations of dopants. The doping of α-Fe2O3 nanorods with a low level of Nb or Ta can improve their electrical conductivity and thereby facilitate charge transport and reduced electron–hole recombination therein. The photoconversion effects of Nb and Ta-doped α-Fe2O3 by in situ XAS in the dark and under illumination revealed opposite evolutions of the spectral intensities of the Fe L-edge and Nb/Ta L-edge, indicating that charge transfer and a conduction pathway are involved in the photoconversion. Analytic in situ XAS results reveal that the α-Fe2O3 that is doped with a low level of Nb has a greater photoconversion efficiency than that doped with Ta because Nb sites are more active than Ta sites in α-Fe2O3. The correlation between PEC activity and the electronic structure of Nb/Ta-doped α-Fe2O3 is examined in detail using in situ XAS and helps to elucidate the mechanism of PEC water splitting in terms of the electronic structure.

Synthesis and Properties of Ni-doped Goethite and Ni-doped Hematite Nanorods

Ni-doped goethite (α-FeOOH) nanorods were synthesized from mixed Fe(III)-Ni(II) nitrate solutions with various Ni/(Ni+Fe) ratios (0, 5, 10, 20, 33 and 50 mol % Ni) by hydrothermal precipitation in a highly alkaline medium using the strong organic alkali, tetramethyl¬ammonium hydroxide (TMAH). Ni-doped hematite (α-Fe2O3) nanorods were obtained by calcination of Ni- doped goethite nanorods at 400 °C. The Ni2+- for-Fe3+ substitution in goethite and hematite was confirmed by determination of the unit cell expansion (due to the difference in the ionic radii of Fe3+ and Ni2+) using XRPD and determination of the reduction of a hyperfine magnetic field (due to the difference in magnetic moments of Fe3+ and Ni2+) using Mossbauer spectroscopy. Single- phase goethite nanorods were found in samples containing 0 or 5 mol % Ni. A higher Ni content in the precipitation system (10 mol % or more) resulted in a higher Ni2+-for-Fe3+ substitution in goethite, and larger Ni-doped goethite nanorods, though with the presence of low crystalline Ni-containing ferrihydrite and Ni ferrite (NiFe2O4) as additional phases. Significant changes in FT-IR and UV-Vis-NIR spectra of prepared samples were observed with increasing Ni content. Electrochemical measurements of samples showed a strong increase in oxygen evolution reaction (OER) electrocatalytic activity with increasing Ni content.

Understanding the role of nanostructuring in photoelectrode performance for light-driven water splitting

The analysis of capacitance data for regular nanostructured photoelectrodes is revisited using a hematite nanorod array as an example. The effects of the cylindrical nanorod geometry on the capacitance-voltage behaviour are outlined, and the limiting case of complete depletion is discussed in terms of the residual geometric capacity at the base of the nanorods. Since nanorod arrays generally leave areas of the substrate exposed, it is necessary to consider the parallel capacitance associated with the fraction of uncovered surface. The sensitivity of the capacitance fitting to parameter variation is explored. The enhancement of external quantum efficiency (EQE) by nanostructuring is also discussed using hematite nanorod arrays as experimental examples. It is shown that, although very substantial EQE enhancement should be achieved by simple geometric effects, the performance of nanostructured hematite electrodes in the visible region of the spectrum is considerably lower than predicted if all charge carriers generated in the space charge region (SCR) were collected. Further analysis reveals that the internal quantum efficiency increases with photon energy, suggesting that the probability of generating free, rather than bound, electron-hole pairs in hematite depends on the excess energy hν - Egap.

Synthesis and Tuning of Hematite (α-Fe2O3) Iron Oxide Nanorods with Magnetism

In recent years, hematite (α-Fe2O3) iron oxide nanorods have attracted the attention of many researchers due to their wide applications in nanobiosensors, lithium batteries and nanomagnetism. In the present work, hematite nanorods with magnetic and optical properties were synthesized in desirable qualities via a fast and low-cost method. Features of these nanorods were determined using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), ultraviolet–visible (UV–vis) spectroscopy and vibrating sample magnetometry (VSM). Uniform nanorods with a length of between 20 and 30 nm were fabricated in a 0.05:1:3.4 molar ratio of cetyltrimethylammonium bromide (CTAB)/FeCl3/NaOH. The VSM technique showed that the presence of the CTAB as a surfactant agent in 0.1, 0.05 and 0.025 mol L− 1 concentrations can reduce the magnetization of nanorods with an increase in surface layers. The optical absorbance properties of the hematite nanorods demonstrated that the absorption peaks are well tuned in wavelengths of about 250 and 400 nm. The hematite nanorods with the tuned structure in terms of magnetic and optical properties can be proposed as a desirable option in future applications.

Cobalt oxide and carbon modified hematite nanorod arrays for improved photoelectrochemical water splitting

Given the proper band gap, low cost and good stability, hematite (α-Fe2O3) has been considered as a promising candidate for photoelectrochemical (PEC) water splitting, however suffers from the sluggish surface water oxidation reaction kinetics. In this study, a simple dip-coating process was used to modify the surface of α-Fe2O3 nanorod arrays with cobalt oxide (CoOx) and carbon (C) for the improved PEC performance, with a photocurrent density at 1.6V (vs. reversible hydrogen electrode, RHE) increased from 0.10mA/cm2 for the pristine α-Fe2O3 to 0.37mA/cm2 for the CoOx/C modified α-Fe2O3 nanorods. As revealed by electrochemical analysis, thanks to the synergistic effect of CoOx and C, the PEC enhancement could be attributed to the enhanced charge transfer ability, decreased surface charge recombination, and accelerated water oxidation reaction kinetics. This study serves as a good example for improving PEC water splitting performance via a simple method.