Hematite Thin Films Research Articles

Laser-Induced One-Pot Deposition of Medium-Entropy Cocatalysts on Hematite Thin Films for Improved Photoelectrochemical Water Splitting

Laser-Induced One-Pot Deposition of Medium-Entropy Cocatalysts on Hematite Thin Films for Improved Photoelectrochemical Water Splitting

Properties of nanostructured hematite prepared by various coating techniques for potential PEC water splitting applications

In this study, hematite (α-Fe2O3) thin films were synthesized using dip coating and spray pyrolysis techniques on fluorine-doped tin oxide substrates, and the morphological, optical, and structural properties of the hematite were studied. Six peaks were identified for both techniques from X-ray diffraction measurements. The (104) and (110) phases which are the main peaks describe the corundum structure of α-Fe2O3 while other peaks further confirm the formation of high purity α-Fe2O3. Raman spectroscopy revealed seven vibrational modes (two A1g and five Eg modes) of α-Fe2O3 were observed within the first Brillouin zone. Field emission scanning electron microscopy revealed mesoporous hematite nanospheres. The grain sizes were determined by average grain intercept and estimated as 45.8 ± 0.5 and 50.0 ± 0.5 nm respectively for films prepared by spray pyrolysis and dip-coating. Ultraviolet–visible spectroscopy results showed good absorbance for both techniques with the absorbance onset at 597 nm for dip coated films as compared to onset at 609 nm for spray pyrolysis films. From this work it was determined that coating techniques can affect the grain sizes of hematite films. This could potentially contribute to improved absorption of light for photoelectrochemical water splitting applications.

Optical, structural, morphological, and gas sensing properties of Mg-doped α-Fe2O3 thin films deposited by RF and DC magnetron Co-sputtering technique

In this research, Mg-doped hematite (α-Fe2O3) thin films with different Mg-doping concentrations were synthesized on glass substrates using the direct current (DC) and radio frequency (RF) magnetron co-sputtering technique, and the changes of some physical properties due to the concentration of the dopant were investigated. The optical, structural, morphological, and elemental properties of the obtained Mg-doped α-Fe2O3 thin films were determined by X-ray diffraction (XRD) analysis, UV–Vis spectroscopy, Raman spectroscopy, scanning electron microscopy (SEM), dispersive X-ray spectroscopy and atomic force microscopy (AFM). XRD analysis revealed that the investigated thin films have a rhombohedral crystal structure. The deposition of films at different DC sputtering voltages caused significant variations in stoichiometry and nanostructure. The thin films' band gap energy values were estimated based on absorption measurements, and results ranged from 2.15 eV to 2.69 eV. Raman peaks were observed between 218 cm−1 and 1305 cm−1. The thin films exhibit uniform surface morphology throughout the substrate according to the SEM images. The low RMS roughness values obtained from AFM images showed that the surfaces of Mg:Fe2O3 thin films are smooth. The Mg-doped α-Fe2O3 thin film doped by 150 W DC voltage exhibited a good gas-sensing response at 300 °C. A remarkably quick response/recovery time was achieved.

Enhanced Broadband Light Harvesting in Ultrathin Absorbers Enabled by Epitaxial Stabilization of Silver Thin Film Mirrors.

Silver thin film mirrors are attractive candidates for use as specular back reflectors to enhance broadband light absorption via strong optical interference in ultrathin film semiconductor photoabsorbers. However, deposition of metal-oxide absorbers often requires exposure to high temperature in an oxygen atmosphere, conditions that cause thermal etching and degrade the specular reflectance of silver films. Here, we overcome this challenge and demonstrate that epitaxial growth of silver mitigates thermal etching under the high-temperature oxygen-containing environments that cause polycrystalline films to degrade. The degree of thermal etching resistance is related to the epitaxial film structure, where high-quality films completely prevent thermal etching, allowing for direct deposition of metal-oxide thin film photoabsorbers at elevated temperatures without any degradation of the optical properties of the silver layer. As a proof of concept for device applications, a metal-oxide photoanode for photoelectrochemical water splitting is fabricated by directly growing epitaxial SnO2 and Ti-doped α-Fe2O3 (hematite) thin films onto stabilized silver reflectors by pulsed laser deposition. The photoanode displays enhanced broadband light absorption due to strong interference effects enabled by the highly reflective silver film and demonstrates stable operation in a photoelectrochemical cell under conditions of water photo-oxidation in alkaline electrolyte.

Oxygen vacancies boosted photoelectrochemical performance of α-Fe2O3 photoanode via butane flame annealing

In this study, oxygen vacancies were introduced into the hematite (α-Fe2O3) photoanode via a simple and rapid butane flame annealing method. The presence of oxygen vacancies were demonstrated by high-resolution transmission electron microscopy (HRTEM), electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS). The photocurrent density of optimum α-Fe2O3 electrodes was 0.67 mA cm-2 at 1.23 VRHE, about 2.7 times higher than that of N2 annealed electrode. Our study showed that butane flame annealing process can introduce oxygen vacancies defects in Fe2O3 electrodes, which would improve carrier transfer efficiency, increase electron density, enhance water oxidation activity, and narrow the light optical bandgap. The use of butane flame annealing is a simple and effective strategy to enhance the PEC performance of hematite thin films.

Sn-Doped Hematite Films as Photoanodes for Photoelectrochemical Alcohol Oxidation

Here, the modification of semiconductor thin film hematite photoanode by doping with Sn ions is reported. Undoped and Sn-doped hematite films are fabricated by the electrochemical deposition of FeOOH from aqueous alkaline electrolyte, followed by calcination in air. The photoanodes were tested in photoelectrocatalytic oxidation of water, methanol, ethylene glycol, and glycerol. It is shown that modification by tin dramatically increased the activity of hematite in the photoelectrochemical oxidation of alcohols upon visible light irradiation. The photoelectrocatalytic activity of Sn-modified hematite increased in the sequence of: H2O < MeOH < C2H2(OH)2 < C3H5(OH)3. The quantum yield of photocurrent in the oxidation of alcohols reached 10%. The relatively low photocurrent yield was ascribed to the recombination of photoexcited holes within the hematite layer and on surface states located at the hematite/electrolyte interface. Intensity-modulated photocurrent spectroscopy (IMPS) was used to quantify the recombination losses of holes via surface states. The IMPS results suggested that the hole acceptor in the electrolyte (alcohol) influences photocurrent both by changing the charge transfer rate in the photoelectrooxidation process and by the efficient suppression of the surface recombination of generated holes. Thin-film Sn-modified hematite photoanodes are promising instruments for the photoelectrochemical degradation of organic pollutants.

Ultrathin Hematite‐Hercynite Films for Future Unassisted Solar Water Splitting

AbstractPhotoelectrochemical (PEC) water splitting requires stable, efficient, and cost‐effective photoelectrodes to enable future large‐scale solar hydrogen production. Ultrathin hematite‐hercynite photoanodes that meet all these criteria in an excellent way is presented here. Hematite‐hercynite photoelectrodes are synthesized in a self‐forming manner by thermal oxidation of iron–aluminum alloy films and characterized with regard to water splitting applications. Photoanodes fabricated from 17 wt.% Al at 493 °C for 8 h and 685 °C for 5 min exhibit, for instance, a photocurrent density of 1.24 and 1.53 mA cm−2 at 1.23 V versus RHE, respectively, as well as superior light absorption in the visible range of the solar spectrum. The PEC performance improvement in comparison to pure hematite thin film electrodes is first achieved by adjusting the aluminum concentration with an optimum range of 12–17 wt.% and second by optimizing the annealing conditions. The resulting photocurrent densities are about a factor of three higher than those obtained from electrodes synthesized from pure iron thin films using the same synthesis conditions. Finally, it is shown that ultrathin hematite‐hercynite photoelectrodes enable even unassisted solar water splitting in a NaOH (1 m) electrolyte with a maximum solar‐to‐hydrogen conversion efficiency of 0.78%.

Spectromicroscopic study of the transformation with low energy ions of a hematite thin film into a magnetite/hematite epitaxial bilayer

The search of new properties in novel oxide heterostructures requires the exploration of new fabrication methods and the study, at the microscopic level, of the processes involved during the synthesis. We present a synchrotron-based spectromicroscopic investigation of a magnetite/hematite bilayer on Pt(111) grown in a two-step process by thermal evaporation and Low Energy Ion Bombardment (LEIB). The characterization includes the study of structural, electronic, chemical, and magnetic properties using X-ray Absorption Spectroscopy (XAS), Low Energy Electron Microscopy (LEEM), Photoemission Electron Microscopy (PEEM), or X-ray Magnetic Circular Dichroism (XMCD). The aim is to obtain microscopic information of the thin film before, during, and after the ion bombardment. Ion bombardment gradually transforms the topmost layers of the hematite thin film into a defective sub-oxide, where magnetite nuclei grow and coalesce with increasing ion doses. Two rotational domains of magnetite coexist, which are typically a few tens of nanometres large and do not grow significantly with temperature annealings. The incoherent growth of the magnetite nuclei favours the formation of stable twin boundaries (TBs) and antiphase boundaries (APBs). Dichroic spectra show the characteristics of the ferrimagnetic (FiM) order of magnetite, and the spatial distribution of magnetic domains shows no apparent correlation with the structural image, displaying smooth domains separated by diffuse frontiers. These findings illustrate the importance of a spectromicroscopic characterization of novel oxide heterostructures for potential future applications.

Post-annealing of hematite films: The changes in surface chemistry, lattice dynamics, and photoelectrochemical properties

This research deals with tracking changes in the surface chemistry and lattice dynamics of α-Fe2O3 (hematite) thin films upon post-annealing and its correlation with the photoelectrochemical (PEC) properties. The hematite thin films were prepared by the electric field-assisted liquid phase deposition (EA-LPD) method. The as-prepared thin films were heated at three different heating rates: 10, 22, and 80 ℃/min to 350 ℃; and kept for 15 min and then quenched in the air. The structural characterizations showed that the post-annealing process induces oxygen vacancies and the formation of hydroxyl groups on the surface. The extent of the changes in the surface chemistry strongly depends on the heating rate. Such changes in the surface chemistry possess a synergic effect on the PEC properties. Here the post-annealed photoanodes with an 80 ℃/min heating rate showed significantly improved PEC activity and exhibited a maximum photocurrent density ⁓ 0.73 mA.cm−2 at 1.23 V vs. RHE, 2.5 times higher than that for pristine sample. The lattice dynamics investigations suggested that the as-prepared photoanodes are composed of α-FeOOH. Upon post-annealing, α-FeOOH was partially converted into a proto-hematite intermediate phase. The longitudinal optical (LO) modes for the post-annealed samples enhanced as an indicator for the development of lattice disorder, as the results of induced oxygen vacancy defects and hydroxyl groups on the surface.

Optical, Dielectric and Optoelectronic Properties of Spray Deposited Cu-doped Fe2O3 Thin Films

Copper-doped hematite thin films were prepared by spray pyrolysis technique using a mixture of ethanol and distilled water precursors. Visual observations showed that aqua precursor produced films of less integrity compared with ethanol that produced thin, uniform and transparent yellowish-brown films that adhered well to the substrate. Composition and thickness measurements determined by RBS revealed that ethanol precursor produced thinner films of 94.45 and 51.77 nm while aqua precursor produced films of 1,370 and 1,120 nm for undoped and Cu-doped Fe2O3 respectively. This is an indication that ethanol solutions produced nano-thick films of high integrity. The composition revealed that only the Cu-doped Fe2O3 deposited by ethanol solution gave composition close to stoichiometric Fe2O3 while the others gave non-stoichiometric Fe(OH)3 . Optical characterization carried out using UV-visible spectrophotometer in transmittance mode indicated that the film thickness was directly proportional to the number of passes which is inversely proportional to the transmittance. Three bandgap determination methods namely; Tauc, Absorption Fitting Spectrum (AFS) and Davis-Mott were employed with the result that Tauc and AFS gave close direct and indirect bandgap energies (Eg) of 3.44 and 1.98 for AFS and 3.43 and 2.32 eV for Tauc respectively. The Urbach tail energy determined was 1,100 meV which is an indication of a broad onset of absorption. The steepness parameter (?) was found to be 7.83 while the electron-phonon (Eph ) coupling energy was found to be 0.85 eV. It was also observed that the refractive index (n) was about 15 times greater than the extinction coefficient (k). In the study of the dispersion parameters using single oscillator and Sellmier models, the values of the single oscillator energy (Eosc ), dispersion energy (Ed), zero frequency dielectric constant, zero frequency refractive index, the average oscillator strength (So), the average oscillator parameter and the dispersion parameters were determined. All the values of the parameters estimated are of the same order of magnitude with other semiconducting materials. The study showed that Cu-doped Fe2O3 could be employed as dielectric material as well as in optoelectronic devices.

Visible light driven CZTS QDs/α-Fe2O3-graphene p–n heterojunction for photoelectrochemical water splitting

This work investigates the implication of graphene and Cu2ZnSnS4 (CZTS) quantum dots (QDs) incorporation in the hematite thin film for its use in a photoelectrochemical cell. The thin film has been prepared by decorating the CZTS QDs over graphene-hematite composite by simple chemical approach. In Comparison to graphene modification and CZTS QDs modification separately over hematite thin film, the combination of both has produced more photocurrent. The photocurrent density obtained for CZTS QDs and graphene modified hematite thin film is 1.82 mA cm−2 at 1.23 V/RHE, which is 1.75 higher than pristine hematite. The presence of CZTS QDs over hematite-graphene composite enhances the absorption properties of composite along with creating the p–n junction heterostructure which aids the transportation of the charge carriers. The thin films have been characterized using x-ray diffractometer, Raman spectroscopy, field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy, and diffuse reflectance UV–vis spectroscopy for phase, morphology and optical properties analysis. The enhancement in photoresponse has been justified by Mott-Schottky and transient open circuit potential analysis.

Facile fabrication of P-type hematite thin film and application for enhanced photoelectrochemical oxygen evolution reaction efficiency

Facile fabrication of P-type hematite thin film and application for enhanced photoelectrochemical oxygen evolution reaction efficiency

α-Fe2O3 thin films deposited by a facile spray pyrolysis technique for enhanced ethanol sensing

In this research, α-Fe2O3 (hematite) thin films were deposited by spray pyrolysis on glass substrates at various growth temperatures of 350, 400, 450, and 500 °C. Samples were characterized using versatile techniques to study their morphological, structural, and optical properties. X-ray diffraction patterns and Raman spectra confirmed the formation of the hexagonal structure of hematite. Electron microscope images show the growth of cubic nanoparticles on the surface of all synthesized samples, the average size of cubes for the sample with the best sensing performance is about 56.2 nm. Ethanol sensing characteristics of samples were investigated at four elevated temperatures of 175, 200, 225, and 250 °C, exposed to different concentrations of ethanol. The film deposited at the substrate temperature of 350 °C reached the best responses of 30.2% toward 100 ppm and 51.4% toward 1000 ppm of ethanol at the optimum working temperature of 200 °C. The response and recovery times were recorded at 279 and 536 s, respectively, when this sensor was exposed to 100 ppm ethanol vapor at 200 °C. A possible mechanism of ethanol sensing is also discussed. This study optimized the growth conditions and sensing parameters of hematite thin films for ethanol sensing applications.

SO2 pollutant conversion to sulfuric acid inside a stand-alone photoelectrochemical reactor: A novel, green, and safe strategy for H2SO4 photosynthesis

Instead of performing the risky gas-phase oxidization of SO2, to produce sulfuric acid, this noxious gas can be first dissolved in water and then the resulting sulfite (SO32-) anions could be safely oxidized through a green photoelectrochemical route upon an environmentally-friendly, affordable photoanode. In this article, the authors deal with this environmentally/industrially important issue and conduct the sulfite-to-sulfate conversion process upon an n-type semiconducting hematite thin-film, deposited electrochemically on a commercially available tin-oxide substrate. The investigations revealed an excellent photoelectrochemical response and a fair oxidizing ability for the fabricated photodevice/reactor, to convert sulfite to sulfate in a two-electrodic standalone setup [no external potential was applied]. Furthermore, a superior activity was achieved for the production of sulfate upon the Co (II) impregnated photoanode. By drawing the system's energy diagram and noticing the reactions which occur during the photoconversion process, the phenomenon was also interpreted from a physicoelectrochemical mechanistic perspective.

Enhanced Oxygen Evolution Reaction Activity in Hematite Photoanodes: Effect of Sb-Li Co-Doping.

Co-doping represents a valid approach to maximize the performance of photocatalytic and photoelectrocatalytic semiconductors. Albeit theoretical predictions in hematite suggesting a bulk n-type doping and a surface p-type doping would deliver best results, hematite co-doping with coupled cations possessing low and high oxidation states has shown promising results. Herein, we report, for the first time, Sb and Li co-doping of hematite photoanodes. Particularly, this is also a seminal work for the introduction of the highly reactive Sb5+ directly into the hematite thin films. Upon co-doping, we have a synergistic effect on the current densities with a 67-fold improvement over the standard. Via a combined investigation with profuse photoelectrochemical measurements, X-ray diffraction, X-ray photoelectron spectroscopy, and Raman analyses, we confirm the two doping roles of Sb5+ and Li+ as the substitutional and interstitial dopant, respectively. The improvements are attributed to a higher charge carrier concentration along with a lower charge transfer resistance at the surface.

Selective placement of modifiers on hematite thin films for solar water splitting

A dual-modification strategy enabling the design of hematite with synergistic bulk and interfacial engineering for improved performance as photoanode.

Flow cell for operando X-ray photon-in-photon-out studies on photo-electrochemical thin film devices.

Background: Photo-electro-chemical (PEC) water splitting represents a promising technology towards an artificial photosynthetic device but many fundamental electronic processes, which govern long-term stability and energetics, are not yet fully understood. X-ray absorption spectroscopy (XAS), and particularly its high energy resolution fluorescence-detected (HERFD) mode, emerges as a powerful tool to study photo-excited charge carrier behavior under operating conditions. The established thin film device architecture of PEC cells provides a well-defined measurement geometry, but it puts many constraints on conducting operando XAS experiments. It remains a challenge to establish a standardized thin film exchange procedure and concurrently record high-quality photoelectrochemical and X‑ray absorption spectroscopy data that is unperturbed by bubble formation. Here we address and overcome these instrumental limitations for photoelectrochemical operando HERFD-XAS. Methods: We constructed a novel operando photo-electro-chemical cell by computer numerical control milling, guided by the materials' X‑ray and visible light absorption properties to optimize signal detection. To test the cell's functionality, semiconducting thin film photoelectrodes have been fabricated via solution deposition and their photoelectrochemical responses under simulated solar light were studied using a commercial potentiostat in a three-electrode configuration during HERFD-XAS experiments at a synchrotron. Results: We demonstrate the cell's capabilities to measure and control potentiostatically and in open‑circuit, to detect X‑ray signals unperturbed by bubbles and to fluently exchange different thin film samples by collecting high-resolution Fe K-edge spectra of hematite ( α -Fe 2O 3) and ferrite thin film ( MFe 2O 4, M= Zn, Ni) photoelectrodes during water oxidation. Conclusions: Our cell establishes a measurement routine that will provide experimental access of photo-electro-chemical operando HERFD-XAS experiments to a broader scientific community, particularly due to the ease of sample exchange. We believe to enable a broad range of experiments which acquired fundamental insights will spur further photoelectrochemical research and commercialization of water splitting technologies.

Photoelectrochemical Detection of Calcium Ions Based on Hematite Nanorod Sensors.

α-Fe2O3 (hematite) thin films have been shown to be a robust sensor substrate for photoelectrochemical imaging with good stability and high spatial resolution. Herein, one-dimensional (1D) hematite nanorods (NRs) synthesized via a simple hydrothermal method are proposed as a substrate which provides nanostructured surfaces with enhanced photocurrent responses compared to previously described hematite films, good stability, and excellent spatial resolution for potential imaging applications. The photoelectrochemical sensing capability of hematite NRs was demonstrated by a high pH sensitivity without modification. The modification of the hematite NRs with a thin poly(vinyl chloride) (PVC)-based ion-selective film allowed highly reversible amperometric detection of calcium ions with sensor materials traditionally employed in potentiometric devices.

Synthesis and characterization of porous TiO2 film decorated with bilayer hematite thin film for effective photocatalytic activity

• Fe 2 O 3 was successfully deposited onto porous TiO 2 by simple and developed electrochemicaltechnique. • TiO 2 /bilayer hematite film has good stability and the high specific surface area. • The as-prepared porous photocatalysts Fe 2 O 3 /TiO 2 show high visible light photoactivities for the degradation of the methylene blue dye. In this work, porous TiO 2 film decorated with bilayer hematite (nanoparticles/nanorods) were synthesized via the electrodeposition technique followed by hydrothermal treatment. Structural and morphological properties of the TiO 2 /bilayer hematite films were determined by X-ray diffraction (XRD), Also with energy dispersive X-ray spectral (EDS), and scanning electron microscopy (SEM). Due to its good stability and the high specific surface area, TiO 2 /bilayer hematite film, as a potential absorber film, can be applied to enhance organic dye degradation. The photodegradation of the porous, thin film trijunction was remarkable under visible light and normal pH.

Vertically Aligned CdO-Decked α-Fe2O3 Nanorod Arrays by a Radio Frequency Sputtering Method for Enhanced Photocatalytic Applications.

Green hydrogen production is one of the most desirable sustainable goals of the United Nations. Thus, for that purpose, we developed hematite (α-Fe2O3), an n-type semiconductor, a desirable candidate for photoelectrochemical (PEC) water splitting, enabling hydrogen evolution. High recombination losses, low efficiency, and large-scale production hinder its potential. To address these issues, we have fabricated optimized bare and cadmium oxide (CdO)-decorated hematite thin film nanorod arrays using a throughput radio frequency (RF) sputtering with efficient water splitting behavior. To the best of our knowledge, no work has been done so far on the synthesis of CdO/α-Fe2O3 via RF sputtering for PEC application. Bare α-Fe2O3 samples, with a morphology of vertically aligned nanorods, were fabricated with optimized parameters such as as-deposited 70 nm of Fe, an angle of deposition of 70°, and an annealing temperature of 600 °C, which showed a photocurrent density of 0.38 mA/cm2 at 1.65 V vs reversible hydrogen electrode (RHE). Characterizations depicted that this unique morphology with high crystallinity directly enhanced the performance of hematite photoanodes. Further, deposition of 30 nm of cadmium (CdO) on the α-Fe2O3 nanorods produced a corn-like morphology with CdO nanoparticles (∼2 nm), resulting in 4-times enhancement of the PEC performance (1.2 mA/cm2 at 1.65 V vs RHE). CdO acted as a co-catalyst, responsible for satisfactory suppression of recombination and facilitating the hole transfer, directly enhancing the overall photocurrent density. This photoanode showed an extremely stable behavior over a period of 26 h when kept under constant illumination. Furthermore, the CdO-modified photoanode showed a better dye degradation (98% in 40 min) than the bare hematite (60% in 40 min), proving to be an efficient photoanode.