Hematite Films Research Articles

Plasma‐assisted deposition of iron oxide thin films for photoelectrochemical water splitting

AbstractIron oxide thin films for photoelectrochemical (PEC) water splitting were deposited by radiofrequency sputtering of an iron target in argon/oxygen plasma mixtures, followed by thermal annealing. The chemical composition and structure of deposited film can be tuned by controlling the gas feed composition and the annealing temperature. The thermal treatment extensively improves the PEC water splitting performances of the films deposited at the lowest O2 percentages (0–1%), allowing to obtain photocurrent densities up to 1.20 mA/cm2 at 1.23 VRHE. Increasing the oxygen percentage in the plasma feed allows the direct growth of photoactive films; the best result is found for the hematite film produced at 50% O2, characterized by a photocurrent density of 0.21 at 1.23 VRHE.

Effect of Lactic Acid on the Photoelectrocatalytic Water Splitting of Hematite Prepared by Hydrothermal Method

The photoelectrocatalytic water splitting performance of the pristine hematite is limited because of its poor bulk charge separation efficiency. In this study, the lactic acid modified hematite films were grown in situ on Fluorine-doped tin oxide substrates by hydrothermal method and then annealed in nitrogen atmosphere. At 1.5 V versus RHE, the photocurrent density of the optimal lactic acid modified hematite photoanode (0.075LA-Fe2O3) was 1.5 mA cm− 2, which was three times that of the based hematite (0.5 mA cm− 2). The enhanced photoelectrocatalytic performance of the lactic acid modified hematite was attributed to its increased bulk charge separation efficiency caused by the oxygen vacancies and island-like pattern, as well as its improved surface charge injection efficiency. The effect of lactic acid on the morphology, crystalline structure and photoelectrocatalytic performance of the hematite photoanode are studied by systematical characterization.

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.

Tuning of oxygen vacancy-induced electrical conductivity in Ti-doped hematite films and its impact on photoelectrochemical water splitting

Titanium (Ti)-doped hematite (α-Fe2O3) films were grown in oxygen-depleted condition by using the spray pyrolysis technique. The impact of post-deposition annealing in oxygen-rich condition on both the conductivity and water splitting efficiency was investigated. The X-ray diffraction pattern revealed that the films are of rhombohedral α-Fe2O3 structure and dominantly directed along (012). The as-grown films were found to be highly conductive with electrons as the majority charge carriers (n-type), a carrier concentration of 1.09×1020 cm−3, and a resistivity of 5.9×10−2 Ω-cm. The conductivity of the films were reduced upon post-deposition annealing. The origin of the conductivity was attributed firstly to Ti4+ substituting Fe3+ and secondly to the ionized oxygen vacancies (VO) in the crystal lattice of hematite. Upon annealing the samples in oxygen-rich condition, VO slowly depleted and the conductivity reduced. The photocurrent of the as-grown samples was found to be 3.4 mA/cm−2 at 1.23 V vs. RHE. The solar-to-hydrogen efficiency for the as-grown sample was calculated to be 4.18% at 1.23 V vs. RHE. The photocurrents were found to be significantly stable in aqueous environment. A linear relationship between conductivity and water-splitting efficiency was established.

Identifying Structural Defects in Hematite Photoanodes through Structure-Property Analysis

Hematite photoanodes continue to exhibit highly variable photoelectrocatalytic water oxidation performance across the literature despite over two decades of intensive research. Studies have advanced the understanding of fundamental photophysical behavior of key electron transfer and enriched material design in hematite photoanodes, but poor photoelectrocatalytic performance and high variability in reported behavior indicate missing information. We believe that a lack of the chemical nature of structural defects in hematite and their specific impacts on material properties and photoelectrocatalytic water oxidation is a key problem. We target this issue with a structure-property analysis using photoelectrochemical, X-ray diffraction, Raman and UV-visible spectroscopic data on a series of hematite photoanodes. The analysis reveals a formally Raman inactive vibrational mode in hematite films prepared by annealing lepidocrocite films whose intensity varies with annealing protocols. Correlations between the intensity of this feature in the Raman spectrum with photocurrent density, semiconductor band structure, and the onset of photoelectrocatalysis signifies systematic changes in the magnitude of a crystal lattice distortion. Analysis of the nature of these key Raman vibrations and the synthetic conditions lead us to conclude that the observed defects are iron vacancies induced by trapped protons within the crystal lattice. This finding provides a means to rapidly diagnose a specific structural defect and will aid in the optimization of fabrication protocols for hematite photoanodes. Figure 1

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.

Electrodeposited chromium-doped α-Fe2O3 under various applied potential configurations for solar water splitting

In this work, high quality hematite (α-Fe2O3) Chromium (Cr)-doped thin films have been synthesized via electrodeposition technique, on fluorine-doped tin oxide-coated glass substrates, under various applied potential configurations [cyclic voltammetry (CV), linear sweep voltammetry (LSV) and −0.5 V]. Chromium was added to the electrolyte at such a proportion that the Cr/(Cr + Fe) ratio remained within 8%. The as-deposited films were subsequently annealed in air at 650 °C for 2 h. Our novel study highlights the effect of using variable potential approaches during the film preparation on the properties of Cr-αFe2O3 deposited films. The prepared thin films were analyzed by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, UV–Vis absorption and photoelectrochemical (PEC) analysis. XRD revealed that samples are crystallized in Cr-Fe2O3 cubic structure with a crystalline orientation in the plane (1 1 1) and a clear improvement of the crystallinity and size crystallite of the Cr-Fe2O3 deposited using CV process. SEM micrographs showed that the morphology grains were three-sided pyramid-shaped, expanding with increase of the crystallinity. The calculated band gap values are 2.18, 2.23 and 2.20 eV, respectively for −0.5 V, LSV, CV. The Cr-Fe2O3 films synthesized in this study showed high PEC activity with very low carrier density in comparison with the conventionally electrodeposited films. This Cr-doped hematite films ‘excellent photoelectrochemical performance was mainly attributed to improved charge carrier properties. Such high photoactivity was attributed to the large active surface area and increased donor density caused by increasing the Cr doping in the α-Fe2O3 films.

Influence of properties of hematite films on electrical characteristics of isotype heterojunctions Fe2O3/n-CdTe

Conditions for the production of straightening isotype heterostructures n-Fe2O3/n-CdTe by spray-pyrolysis of thin films of hematite n-Fe2O3 onto n-CdTe crystalline substrates were investigated. The presence of tunneling and space charge limits of the mechanisms of motion of electrons in the forward and reverse currents through the energy barrier of the heterojunction is established on the basis of the analysis of temperature dependences of I–V-characteristics. The role of energy states at the boundary of n-Fe2O3/n-CdTe in the formation of barrier parameters is determined. On the basis of the C–V-characteristics, the manifestation of the dielectric properties of the hematite film in a wide range of voltages is investigated. A model of the energy diagram of the heterostructure, which describes the experimental electrophysical phenomena well, has been proposed.

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.

Effects of L-arginine concentration on hematite nanostructures synthesized by spray pyrolysis and chemical bath deposition

Nanostructured hematite films were synthesized using spray pyrolysis and chemical bath deposition. X-ray diffraction measurements revealed sharp diffraction peaks of (012), (104) and (110), describing corundum hexagonal structure of hematite. Raman spectroscopy confirmed polycrystalline hematite symmetry with two Eg and five A1g vibrational phonon modes. Optical studies reported variation of absorbance as precursor concentration increased. The onset absorbance ranged from 595 nm to 650 nm. Scanning electron microscopy reported the shape transformation of nanoparticles to nanospheres. The average grain size obtained for the nanostructures ranged from 41.0 ± 0.5 nm for pristine film to 6.0 ± 0.5 nm for those synthesized with a FeCl3.6H2O:L-arginine concentration ratio of 1:3. A photocurrent density of 1.6 × 10−3 mA/cm2 for pristine hematite was produced while 2.7 × 10−3 mA/cm2, 5.4 × 10−3 mA/cm2, and, 9.8 × 10−3 mA/cm2 current densities were obtained for 1:1, 1:2 and 1:3 Fe3.6H2O:L-arginine respectively.

Electrodeposited Cr-Doped α-Fe2O3 thin films active for photoelectrochemical water splitting

Polycrystalline hematite (α-Fe2O3) Chromium (Cr)-doped thin films were electrodeposited on fluorine-doped tin oxide-coated glass substrates. The electrodeposition bath comprised an aqueous solution containing FeCl3·6H2O, NaCl, and H2O2.Chromium was added to the electrolyte at such a proportion that the Cr/(Cr + Fe) ratio remained within the 2–8 at. % range. The as-deposited films were subsequently annealed in air at 650 °C for 2 h. The structure and morphological characteristics of the undoped and Cr-doped α-Fe2O3 thin films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV–Vis spectroscopy. Cr doping led the main XRD lines to shift to lower angles, which mostly resulted from substituting Fe3+ for Cr4+ ions that leads to α-Fe2O3 lattice contraction. The SEM observations showed that the roughness and aspect of surfaces changed with the Cr doping level. The photoelectrochemical (PEC) performance of the α-Fe2O3 films was examined by chronoamperometry and linear sweep voltammetry techniques. The Cr-doped films exhibited greater photoelectrochemical activity than the undoped α-Fe2O3 thin films. The highest photocurrent density was obtained for the 8% Cr-doped α-Fe2O3 films in 1 M NaOH electrolyte. All the samples achieved their best IPCE at 400 nm. The IPCE values for the 8 at.% Cr-doped hematite films were 20-fold higher than that of the undoped sample.This Cr-doped hematite films ‘excellent photoelectrochemical performance was mainly attributed to improved charge carrier properties. Such high photoactivity was attributed to the large active surface area and increased donor density caused by increasing the Cr doping in the α-Fe2O3 films.

Structural and optical properties of hematite and L-arginine/hematite nanostructures prepared by thermal spray pyrolysis

A study of structural and optical properties of hematite and L-arginine/hematite films was carried out. Hematite films were deposited by thermal spray pyrolysis on fluorine-doped tin oxide at temperatures varying from 280 to 430 °C. X-ray diffraction results revealed (104) and (110) planes, describing the rhombohedral structure of hematite. Scanning electron microscopy on hematite films showed nanoparticles of grain sizes ranging between 40.0 ± 0.5 and 100.0 ± 0.5 nm. Hematite nanoparticles were transformed to nanospheres with uniform size of 35.0 ± 0.5 nm, using chemical bath deposition and L-arginine, a biomolecule as a structure-directing agent. Raman peaks revealed two A1g and five Eg phonon vibrational modes of hematite. Atomic force microscopy confirmed an increase in the surface root mean square roughness from 35 to 40 nm for hematite nanoparticles and L-arginine/hematite, respectively, which is desirable for charge transport. The films exhibited an indirect band gap varying from 2.08 to 2.30 eV and an onset absorbance that ranged between 525 and 560 nm. An improved current density was observed on l-arginine/hematite from pristine hematite nanostructures.

The effects of applied current density and bath concentration on the morphology, crystal structure and optical properties of electrodeposited hematite thin films

In this study, hematite thin films were electrodeposited galvanostatically on fluorine doped tin oxide substrates. The effects of the applied current density and bath concentration on the morphology, crystal structure and optical properties of the electrodeposited films were investigated using secondary electron microscopy, X-ray diffraction spectroscopy and UV–visible adsorption spectroscopy techniques. The results showed that the as-deposited films are hexagonal iron oxyhydroxide with very low crystallinity which need to be heat treated to be recrystallized as hematite films while the higher band gap values of the as deposited films, the higher that of the heat treated hematite films would be. Moreover, by increasing the applied current density during the electrodeposition process or using baths containing high amounts of Fe3+ ions, hematite films with fine morphology, small crystallite size and high band gap values will be electrodeposited.

Interfacial oxygen vacancies yielding long-lived holes in hematite mesocrystal-based photoanodes

Hematite (α-Fe2O3) is one of the most promising candidates as a photoanode materials for solar water splitting. Owing to the difficulty in suppressing the significant charge recombination, however, the photoelectrochemical (PEC) conversion efficiency of hematite is still far below the theoretical limit. Here we report thick hematite films (∼1500 nm) constructed by highly ordered and intimately attached hematite mesocrystals (MCs) for highly efficient PEC water oxidation. Due to the formation of abundant interfacial oxygen vacancies yielding a high carrier density of ∼1020 cm−3 and the resulting extremely large proportion of depletion regions with short depletion widths (<10 nm) in hierarchical structures, charge separation and collection efficiencies could be markedly improved. Moreover, it was found that long-lived charges are generated via excitation by shorter wavelength light (below ∼500 nm), thus enabling long-range hole transfer through the MC network to drive high efficiency of light-to-energy conversion under back illumination.

High-Throughput Screening and Surface Interrogation Studies of Au-Modified Hematite Photoanodes by Scanning Electrochemical Microscopy for Solar Water Splitting.

Au-modified hematite photoanode was screened for photoelectrochemical (PEC) water oxidation by the scanning electrochemical microscopy (SECM) technique with a scanning probe of the optical fiber for visible light irradiation of the photoanode substrate. The Au-modified hematite exhibited an enhancement in the photocurrent up to 3% (at. %), and the performance drop was observed with 4–10% (at. %) of Au modification. Subsequently, pristine and Au-modified hematite thin-film photoanodes were fabricated by the spin-coating method to confirm the results of SECM. The PEC response confirms that 3% (at. %) of Au is the optimum concentration to provide the best enhancement of PEC water oxidation with a ∼6-fold increase compared to the pristine hematite sample. Direct Au oxidation, charge recombination, and strong light absorption by Au are responsible for the decrease in PEC performance when the Au percentage is above 3%. The pristine and Au-modified hematite materials were also characterized by scanning electron microscopy and X-ray photoelectron spectroscopy. Au was found to exist in the form of embedded metallic nanoparticles in the modified hematite. Mott–Schottky analysis of the bulk samples confirms an improvement in charge carrier density for the Au-modified hematite. Additionally, there was little plasmonic enhancement as evidenced by UV–vis spectroscopy, with a minimal contribution toward photoactivity. Surface interrogation SECM quantitatively probed the reactive surface states (RSSs) such as OH• formed on hematite and Au-modified hematite surfaces during water oxidation. The coverage of RSSs was found to increase with the substrate potential. The interrogated charge under the dark condition for the 3% Au-modified hematite sample is higher than the pristine hematite sample because of the enhanced electronic conductivity of the hematite film.

Substrate effect on the structural phase formation and magnetic properties of α-Fe2O3 and Ti doped α-Fe2O3 thin films

We report the synthesis and magnetic properties of hematite (α-Fe2O3) and Ti doped hematite (α-Fe1.4Ti0.6O3) films. The films have been grown at 400 °C on different substrates by using Pulsed Laser Deposition technique. The as-grown film has been heated in the temperature range 600–730 °C either in air or vacuum. Microstructure of the films has been studied using X-ray diffraction pattern, Atomic force microscope, Field effect Scanning Electron Microscope, and magneto-optic Kerr effect (MOKE). Crystalline structure of the films has been stabilized either in rhombohedral phase or in cubic spinel phase depending on the type of substrates. The crystalline films have been found highly oriented on specific directions. Magnetic order of the films at room temperature has been studied using MOKE. The films on selected substrates have been used for a detailed study of low temperature magnetic properties. The temperature dependent magnetization of the films has exhibited bifurcation between zero field cooled and field cooled magnetic curves at all measurement temperatures (5–350 K). Magnetic spin order of the films has been affected by the nature of substrates. Some of the films with room temperature ferromagnetism or canted antiferromagnetism seem to be useful for spintronics applications.

Influence of coating techniques on the optical and structural properties of hematite thin films

Hematite (α-Fe2O3) thin films were deposited by dip, spin and combined coating techniques on fluorine-doped tin oxide (FTO)/glass substrates. Structural properties observed from X-ray Diffraction (XRD) and Raman spectra analysis suggest better crystallinity for films prepared by dip and combined coating techniques as compared to the ones prepared by spin coating. The Raman spectra of films prepared by spin coating and combined techniques showed red shifted and more broadened peaks relative to the ones prepared by dip coating. The red shifts were attributed to increased strain and defects in the films. Field emission scanning electron microscopy (FE-SEM) showed spherical nanoparticles with some agglomeration into small larvae-shape nanostructures for all samples. Cross-sectional images of the films revealed that samples prepared by dip and spin coating had the lowest and highest thicknesses of 452 ± 28 and 622 ± 30 nm respectively. Ultraviolet-visible (UV–Vis) spectroscopy studies revealed that the films absorb light in the visible region due to their bandgap of 1.98 ± 0.03 eV. However, films prepared using combined coating technique showed better light absorption than the ones produced by spin coating at wavelengths below 485 nm despite having less film thickness. This study showed that coating techniques influences crystallization and optical absorption behaviour of hematite films which may have impact on their performance when applied towards photoelectrochemical (PEC) water splitting.

Extended light absorption and enhanced photoelectrochemical activity of palladium‐decorated hematite nanotubes prepared by photodeposition method

Palladium particles were deposited on hematite nanotube surfaces via a facile photodeposition method. Anodization method was then used for direct vertical growth of hematite nanotube arrays (HNTs) on a pretreated iron substrate. Field emission scanning electron microscopy, X‐ray diffraction, diffraction reflection spectroscopy and X‐ray photoelectron spectroscopy analyses were then used to carry out morphological and structural investigations. Compared with the bare HNTs, Pd‐decorated hematite films (Pd/HNTs) exhibit intense visible light absorption and show a red shift of the band edge. Pd/HNTs are more photoelectrochemically active than the non‐decorated films, according to chronoamperometry measurements. Thus, a promising methodology has been developed for the design of a simple and effective photoelectrode for application in photoelectrochemical water splitting systems.

The effects of applied current density, bath concentration and temperature on the morphology, crystal structure and photoelectrochemical properties of electrodeposited hematite films

In this study, the effects of applied current density, bath concentration and temperature on the morphology, crystal structure, lattice defects and photoelectrochemical performance of the electrodeposited hematite films were investigated using scanning electron microscopy, x-ray diffractometry, Mott-Schottky analysis, photovoltage measurement, linear sweep voltammetry and electrochemical impedance spectroscopy techniques. The results showed that increase in applied current density, decrease in bath concentration and decrease in bath temperatures result in the formation of films with finer morphology, [001] crystallographic orientation, low donor density, high photovoltage/photocurrent values, low charge transfer resistance at the film/electrolyte interface and good photoactivity for water splitting in a photoelectrochemical cell. Moreover, uniform hematite films cannot be electrodeposited at very high applied current density and from very dilute baths at very low temperatures. The best electrodeposition parameters were found to be as the applied current density of 4 mA cm−2 and bath containing 10 mM/10 mM of FeCl3NaF at 50 °C.