Hematite Thin Films Research Articles

Structural, morphological and magnetic properties of iron oxide thin films obtained by atomic layer deposition as a function of their thickness

Hematite (α-Fe2O3) thin films with different thicknesses between 4 and 16 nm were synthesized by atomic layer deposition, employing 500 to 2000 cycles of FeCp2/O3. After a thermal reduction process, under a controlled atmosphere of hydrogen, it was possible to convert hematite to magnetite (Fe3O4). SEM images of the iron oxide thin films showed that they are macroscopically homogeneous, although the magnetic measurements by MOKE indicated that the samples should present holes that are responsible for pinning the domain walls, raising the coercivity to values ​​that vary between 0.3 and 0.7 kOe. XRD revealed the presence of the Fe3O4 phase by identifying the main peaks. The signal obtained by Raman spectroscopy was identified at 667 cm−1, which suggests the presence of the phonon mode A1g of Fe3O4 in the reduced sample. Controlling the thickness of Fe3O4 thin films allows adjusting their magnetic properties, coercivity and remanence, so that they can be used in potential technological applications.

Photoelectrochemical imaging system with high spatiotemporal resolution for visualizing dynamic cellular responses

Photoelectrochemical imaging has great potential in the label-free investigation of cellular processes. Herein, we report a new fast photoelectrochemical imaging system (PEIS) for DC photocurrent imaging of live cells, which combines high speed with excellent lateral resolution and high photocurrent stability, which are all crucial for studying dynamic cellular processes. An analog micromirror was adopted to raster the sensor substrate, enabling high-speed imaging. α-Fe2O3 (hematite) thin films synthesized via electrodeposition were used as a robust substrate with high photocurrent and good spatial resolution. The capabilities of this system were demonstrated by monitoring cell responses to permeabilization with Triton X-100. The ability to carry out dynamic functional imaging of multiple cells simultaneously provides improved confidence in the data than could be achieved with the slower electrochemical single-cell imaging techniques described previously. When monitoring pH changes, the PEIS can achieve frame rates of 8 frames per second.

Wasted photons: photogeneration yield and charge carrier collection efficiency of hematite photoanodes for photoelectrochemical water splitting

A comprehensive method for extraction of the wavelength-dependent fraction of absorbed photons that generate electron–hole pairs and the spatial charge carrier collection efficiency is presented and applied for a thin film hematite photoanode.

Improving light absorption and photoelectrochemical performance of thin-film photoelectrode with a reflective substrate.

The charge separation/transport efficiency is relatively high in thin-film hematite photoanodes in which the distance for charge transport is short, but simultaneously the high loss of light absorption due to transmission is confronted. To increase light absorption in thin-film Fe2O3:Ti, commercial substrates such as Cu foil, Ag foil, and a mirror are adopted acting as back-reflectors and individually integrated with the Fe2O3:Ti electrode. The promotion effect of the commercial back-reflectors on the light absorption efficiency and photoelectrochemical (PEC) performance of the hydrothermally prepared Fe2O3:Ti electrodes with a variety of film thicknesses is investigated. As a result, Ag foil and the mirror show favorable and equal efficacy while the promoting effect of Cu foil is limited. In addition, the photocurrent increment achieved by the Ag back-reflector decreases linearly along with the logarithmic of the film thickness and the optimized film thickness of the Fe2O3:Ti electrode is decreased from 520 to 290 nm. The high durability of Ag foil in the alkaline electrolyte during solar light irradiation is demonstrated. Furthermore, the reflective substrate also shows a promotion effect on the BiVO4 photoanode and CuBi2O4 photocathode, as well as the unbiased photocurrent from a tandem cell constituted by TiO2 and CuBi2O4.

An insulating doped antiferromagnet with low magnetic symmetry as a room temperature spin conduit

We report room-temperature long-distance spin transport of magnons in antiferromagnetic thin-film hematite doped with Zn. The additional dopants significantly alter the magnetic anisotropies, resulting in a complex equilibrium spin structure that is capable of efficiently transporting spin angular momentum at room temperature without the need for a well-defined, pure easy-axis or easy-plane anisotropy. We find intrinsic magnon spin-diffusion lengths of up to 1.5 μm, and magnetic domain governed decay lengths of 175 nm for the low-frequency magnons, through electrical transport measurements demonstrating that the introduction of nonmagnetic dopants does not strongly reduce the transport length scale, showing that the magnetic damping of hematite is not significantly increased. We observe a complex field dependence of the nonlocal signal independent of the magnetic state visible, in the local magnetoresistance and direct magnetic imaging of the antiferromagnetic domain structure. We explain our results in terms of a varying and applied field-dependent ellipticity of the magnon modes reaching the detector electrode allowing us to tune the spin transport.

Simple solvothermal approach to highly nanostructured hematite thin films

In this work, we present a solvothermal method for the synthesis of hematite thin films on fluorine-doped tin oxide substrates. This simple method uses a precursor solution of iron(III) 2,4-pentanedionate dissolved in ethanol with a microliter-scale amount of water and yields hematite ∼500 nm thick films after annealing. The synthesized films were characterized using an array of methods, including scanning electron microscopy, energy-dispersive X-ray spectroscopy, diffuse reflectance, and powder X-ray diffraction. Incorporating water into the precursor solution provides nucleation sites for the reaction and results show that by altering the amount of water used in the synthesis, it is possible to generate nanocrystalline films of different morphologies, nanocrystal size distributions, and surface areas. This synthetic procedure therefore provides control over the films’ physical and electrochemical characteristics. Doping of hematite thin films is also possible using this synthesis, as exemplified by doping with tin by adding tin(II) 2,4-pentanedionate to the precursor solution. To demonstrate utility, we build prototype photoelectrochemical cells using the synthesized hematite as the photoanode.

Role of morphology, composition, and structure on the optical response of nanostructured hematite thin films

Nanostructured hematite films are promising for photoactive materials applications. One of the main challenges in this matter is to improve the understanding about the behavior of the optical properties affected by the nanostructure. This work presents a robust ellipsometric analysis (supported by AFM, XRD, and XPS characterization) of the optical response of nanostructured hematite films synthesized by an alternative methodology based on chemical bath deposition. The good match between the measured and calculated Δ and Ψ angles after the fitting processes is an evidence of the reliability of the ellipsometric model constructed here and leads to a well-agreement between the thicknesses obtained from ellipsometry and those from AFM and profilometry. The analysis of the fitted model parameters made possible to determine the existence of two types of porosity in the films. On a microscopic scale, cracks or gross porosity were quantified directly from the v% of air in an effective medium layer of the model. A fine porosity with nanometric size persisting among the solid-phase that constitutes the nanostructured hematite films, was inferred from their lower refractive index with respect to bulk hematite. The extinction coefficient reveled a strong electronic transition centered at ~550 nm associated with the optical bandgap of the films, determined in approx. 2 eV (620 nm), which correspond with bulk hematite. All results shown here reveal a complex relationship between the morphology and the optical response, which introduce relevant aspects about the nanostructured hematite films, like density and porosity, that could be used to propose performance improvements in their potential applications as photoactive material. • Nanostructured hematite thin films synthesized by a chemical-bath based procedure. • Morphological, structural, and chemical composition supports the optical analysis. • Ellipsometry describes complex correlation between morphology and optical response. • Development of an alternative route toward viable photocatalytic films systems.

Operando X-ray Absorption Spectroscopy (XAS) Observation of Photoinduced Oxidation in FeNi (Oxy)hydroxide Overlayers on Hematite (α-Fe2O3) Photoanodes for Solar Water Splitting.

An FeNi (oxy)hydroxide cocatalyst overlayer was photoelectrochemically deposited on a thin-film hematite (α-Fe2O3) photoanode, leading to a cathodic shift of ∼100 mV in the photocurrent onset potential. Operando X-ray absorption spectroscopy (XAS) at the Fe and Ni K-edges was used to study the changes in the overlayer with potential in the dark and under illumination conditions. Potential or illumination only had a minor effect on the Fe oxidation state, suggesting that Fe atoms do not accumulate significant amount of charge over the whole potential range. In contrast, the Ni K-edge spectra showed pronounced dependence on potential in the dark and under illumination. The effect of illumination is to shift the onset for the Ni oxidation because of the generated photovoltage and suggests that holes that are photogenerated in hematite are transferred mainly to the Ni atoms in the overlayer. The increase in the oxidation state of Ni proceeds at potentials corresponding to the redox wave of Ni, which occurs immediately prior to the onset of the oxygen evolution reaction (OER). Linear combination fitting analysis of the obtained spectra suggests that the overlayer does not have to be fully oxidized to promote oxygen evolution. Cathodic discharge measurements show that the photogenerated charge is stored almost exclusively in the Ni atoms within the volume of the overlayer.

Understanding the role of uniformly coated carbon overlayers in hematite nanorod photoandes

As one of the most promising photoanode materials, the performance of hematite has been greatly hampered by poor conductivity and severe charge recombination. Here, we report a simple synthesis of N-doped carbon layer coated hematite via low temperature chemical vapor deposition method, where hydroxytyramine hydrochloride (abbreviated as DA) was used as the carbon precursor and hematite thin films as the substrate. The thickness of carbon layers can be adjusted by the amount of DA. The photocurrent densities of the carbon coated hematite nanorods reached up to 1.63 mA cm−2. Based on TEM, XPS, Mott-Schottky and EIS measurement, it was found that the carbon overlayers could simultaneously improve the conductivity in the bulk and passivate the surface state. The correlation between the thickness of carbon layers and the PEC properties were also discussed in detail.

A highly transparent thin film hematite with multi-element dopability for an efficient unassisted water splitting system

A tandem system consisting of a combined hematite photoanode/perovskite solar cell is a novel configuration enabling unbiased solar-driven water splitting in a cost-efficient way. However, similar light absorption spectrums between hematite and perovskite, causing significant optical loss of the solar cell located behind the photoelectrochemical cell, have been a critical issue, resulting in a substantial reduction in the overall photoelectrochemical performance in the bias-free tandem system. Herein, we report an efficient thin film hematite/perovskite tandem cell optimized with consideration of both quantum efficiency of the hematite photoanode and optical loss of the solar cell. Ti,Si co-doping with controlled dopability and deposition of transparent NiFeOx co-catalyst on hematite (NiFeOx@Ti:Si–Fe2O3(t)) allowed for highly improved surface reaction kinetics and charge transport behaviors, endowing our thin film with state-of-the-art performance among thin film hematite photoanodes reported to date, with a photocurrent density of 2.62mAcm−2 at 1.23VRHE and an onset potential of 0.71V. The perovskite solar cell in the thin hematite/solar cell exhibited an anodic shift of Voc from 1.08V to 1.16V and a three-fold increased photocurrent density (15.5mAcm−2) compared to the solar cell of the conventional thick hematite/solar cell system. The assembled dual photoanodes (dual NiFeOx@Ti:Si–Fe2O3) reached a maximum photocurrent density of 4.0mAcm−2 at 1.23VRHE. Our tandem photoelectrochemical system comprising assembled dual thin film NiFeOx@Ti:Si–Fe2O3/solar cell showed outstanding performance of an unbiased solar-to-hydrogen (STH) conversion efficiency of 4.49%, which is the highest recorded value in the hematite/perovskite tandem cell systems. This work provides a great potential for further development of an unassisted solar water splitting system with the thin film hematite.

Modified annealing approach for preparing multi-layered hematite thin films for photoelectrochemical water splitting

Multi-layered hematite (α-Fe2O3) films were prepared on fluorine-doped tin oxide (FTO) using the dip coating method. The first three layers of the films were annealed at 500 °C and fourth layers at 500, 600, 700, 750 and 800 °C respectively, and their photoelectrochemical (PEC) performance was investigated. Films annealed at 750 °C recorded the best performance, producing 0.19 mA/cm2 photocurrent at 1.23 V vs reversible hydrogen electrode (RHE); 5.3 times more than what was recorded for films sintered at 500 °C, and the onset potential yielded a cathodic shift of 300 mV. The enhanced performance was linked to improved crystallization, absorption coefficient, lowered flat band potential, increased charge carrier density, decreased charge transfer resistance at the solid/liquid interface and increased surface states capacitance for films annealed at 750 °C. The PEC performance of multi-layered α-Fe2O3 films could be improved by annealing the last layers at elevated temperatures without damaging the conducting substrates.

Optimization of Hematite to Improve PEC Performance in Water Splitting

Thus far, semiconductor materials explored to achieve theoretical solar to hydrogen efficiency (STH), included cadmium selenide (CdSe), zinc oxide (ZnO), copper(I)oxide (Cu2O), tungsten trioxide (WO3) and hematite (α-Fe2O3). Of these materials, hematite has received much attention for photoelectrochemical water splitting attributed to its stability in aqueous solution, a small band gap of 1.90 eV-2.20 eV, non-toxicity and abundance. However, it has associated limitations such as high electron-hole recombination rate, short hole diffusion length (2-4 nm) accompanied by short excited lifetime of 10 ps, and poor minority charge carrier mobility of 0.1 cm2 V-1s-1, leading to low photocurrent during water splitting. Furthermore, hematite promises a maximum theoretical STH efficiency of ~16.8 %, with photocurrent densities above 12 mAcm-2, but the reported findings are far below 10 mAcm-2. Using interfacial layers , annealing and doping, we have prolonged lifetimes and increased the photocurrent. In this work, hematite thin films and nanostructures were synthesized using dip coating and spray pyrolysis.

Reduction of recombination rates due to volume increasing, annealing, and tetraethoxysilicate treatment in hematite thin films

We report on the properties of hematite thin films prepared by spray pyrolysis on fluorine-doped tin oxide (FTO)-coated glass substrates and investigated the effect of the spray volume, tetraethoxysilicate treatment of the hematite, and post-annealing at 500 °C for 2 h with 10 °C/min ramping. Raman spectroscopy confirmed the characteristic Raman spectrum of all the films, while high-resolution confocal Raman microscopy showed a uniform intensity, suggesting a homogeneous coating of the hematite films on the FTO substrates. Ultrafast transient absorption spectroscopy indicates that all three experimental parameters—a larger spray volume, tetraethoxysilicate treatment, and annealing—slowed down electron–hole recombination. Global analysis of the difference absorption data resolved the spectra and associated decay lifetimes of three distinct processes, operating on the ultrafast, tens of picoseconds, and hundreds of picoseconds timescales.

Recovering Indium Tin Oxide Electrodes for the Fabrication of Hematite Photoelectrodes

Indium tin oxide (ITO) electrodes are widely used in basic and applied research as well as in commercial electronic devices. After a primary use, ITO electrodes are commonly wasted without further application. Herein, we recycled ITO electrodes previously used as anodes in organic photovoltaic devices in our group. These recycled electrodes were treated with piranha solution to obtain recovered ITO electrodes (ITOPS). Chemical and structural changes of ITOPS electrodes were evaluated using XPS, EDS, UV/VIS, XRD, SEM and cathodic polarization studies pointing out for a complete leach of indium from the structure leaving a conductive tin oxide-based electrode. Subsequently, these electrodes were used in the fabrication of hematite photoelectrodes by a two-step process of electrodeposition and annealing. Structural characterization of the deposited thin film was performed before and after annealing treatment with XDR, Raman spectroscopy and XPS. A hematite thin film deposited on an ITOPS substrate was successfully obtained from the thermal dehydration of akaganeite. SEM micrographs showed a homogeneous distribution of the hematite deposit. Under a 470 nm LED illumination, hematite photoelectrode achieved a photocurrent density of 223 μA cm−2 at 0.8 V vs SCE showing a potential second life for recycled ITO electrodes (ITOPS) as substrates for photoelectrocatalytic processes.

Synergies of co-doping in ultra-thin hematite photoanodes for solar water oxidation: In and Ti as representative case.

Solar energy induced water splitting in photoelectrochemical (PEC) cells is one of the most sustainable ways of hydrogen production. The challenge is to develop corrosion resistant and chemically stable semiconductors that absorb sunlight in the visible region and, at the same time, have the band edges matching with the redox level of water. In this work, hematite (α-Fe2O3) thin films were prepared onto an indium-doped tin oxide (ITO; In:SnO2) substrate by e-beam evaporation of Fe, followed by air annealing at two different temperatures: 350 and 500 °C. The samples annealed at 500 °C show an in situ diffusion of indium from the ITO substrate to the surface of α-Fe2O3, where it acts as a dopant and enhances the photoelectrochemical properties of hematite. Structural, optical, chemical and photoelectrochemical analysis reveal that the diffusion of In at 500 °C enhances the optical absorption, increases the electrode–electrolyte contact area by changing the surface topology, improves the carrier concentration and shifts the flat band potential in the cathodic direction. Further enhancement in photocurrent density was observed by ex situ diffusion of Ti, deposited in the form of nanodisks, from the top surface to the bulk. The in situ In diffused α-Fe2O3 photoanode exhibits an improved photoelectrochemical performance, with a photocurrent density of 145 μA cm−2 at 1.23 VRHE, compared to 37 μA cm−2 for the photoanode prepared at 350 °C; it also decreases the photocurrent onset potential from 1.13 V to 1.09 V. However, the In/Ti co-doped sample exhibits an even higher photocurrent density of 290 μA cm−2 at 1.23 VRHE and the photocurrent onset potential decreases to 0.93 VRHE, which is attributed to the additional doping and to the surface becoming more favorable to charge separation.

Candle flame-treatment as an effective strategy to enhance the photoelectrochemical properties of Ti-doped hematite thin films

Flame-treatment as a simple, low cost, and highly reliable strategy was applied to effectively enhance the photoelectrochemical (PEC) properties of liquid phase deposited Ti-doped hematite thin films.

Thermal Cloak with Switchable Manipulating Direction Based on Linear Transformation Thermotics

Thermal Cloak with Switchable Manipulating Direction Based on Linear Transformation Thermotics

Defect Segregation and Its Effect on the Photoelectrochemical Properties of Ti-Doped Hematite Photoanodes for Solar Water Splitting

Optimising the photoelectrochemical performance of hematite photoanodes for solar water splitting requires better understanding of the relationships between dopant distribution, structural defects and photoelectrochemical properties. Here, we use complementary characterisation techniques including electron microscopy, conductive atomic force microscopy (CAFM), Rutherford backscattering spectroscopy (RBS), atom probe tomography (APT) and intensity modulated photocurrent spectroscopy (IMPS) to study this correlation in Ti-doped (1 cat.%) hematite films deposited by pulsed laser deposition (PLD) on F:SnO2 (FTO) coated glass substrates. The deposition was carried out at 300 {\deg}C, followed by annealing at 500 deg C for 2 h. Upon annealing, Ti was observed by APT to segregate to the hematite/FTO interface and into some hematite grains. Since no other pronounced changes in microstructure and chemical composition were observed by electron microscopy and RBS after annealing, the non-uniform Ti redistribution seems to be the reason for a reduced interfacial recombination in the annealed films, as observed by IMPS. This results in a lower onset potential, higher photocurrent and larger fill factor with respect to the as-deposited state. This work provides atomic-scale insights into the microscopic inhomogeneity in Ti-doped hematite thin films and the role of defect segregation in their electrical and photoelectrochemical properties.

Sn-Doped Hematite for Photoelectrochemical Water Splitting: The Effect of Sn Concentration

Abstract Hematite-based photoanodes have been intensively studied for photoelectrochemical water oxidation. The n-type dopant Sn has been shown to benefit the activity of hematite anodes. We demonstrate in this study that Sn-doped hematite thin films grown by atomic layer deposition can achieve uniform doping across the film thickness up to at least 32 mol%, far exceeding the equilibrium solubility limit of less than 1 mol%. On the other hand, with the introduction of Sn doping, the hematite crystallite size decreases and many twin boundaries form in the film, which may contribute to the low photocurrent observed in these films. Density functional theory calculations with a Hubbard U term show that Sn doping has multiple effects on the hematite properties. With increasing Sn4+ content, the Fe2+ concentration increases, leading to a reduction of the band gap and finally to a metallic state. This goes hand in hand with an increase of the lattice constant.

An investigation on the phase purity of iron pyrite (FeS2) thin films obtained from the sulfurization of hematite (Fe2O3) thin films

Iron pyrite (FeS2) thin films were obtained by sulfurization of hematite (Fe2O3) thin films at different sulfurization temperatures (TS) and sulfurization time (tS). Phase purity of sulfurized films was investigated with X-ray diffraction, Raman Spectroscopy and Vibrating sample magnetometry (VSM). Sulfurization temperature and time were found to have a complementary effect on the phase purity of pyrite films. Secondary phase formation in the pyrite films decreased with increasing sulfurization temperature or time. Pyrite films confirmed to be phase pure with XRD and Raman spectroscopy showed the presence of other phase(s) when analyzed with VSM. Merely XRD measurements were not sufficient to claim the phase purity of pyrite films.