Ti-doped Hematite Research Articles

Coupling Ti-doping and oxygen vacancies in hematite nanostructures for solar water oxidation with high efficiency

By coupling Ti-doping and oxygen vacancies in hematite nanostructures, an efficient photoelectrode for solar water oxidation was prepared which showed a high photocurrent of 2.25 mA cm−2 at 1.23 V vs. RHE and a remarkable maximum value of 4.56 mA cm−2 at 1.6 V vs. RHE at a relatively low activation temperature of 550 °C. In addition, the partial oxygen pressure range suitable to produce oxygen vacancies in Ti-doped hematite could be expanded to a wide region compared to that in undoped hematite, which was critical to the photoelectrode production in practical applications. The facile way by coupling independently developed methods with the cumulative effect stands for an effective strategy for efficient solar water oxidation.

Nanostructured Ti-doped hematite (α-Fe2O3) photoanodes for efficient photoelectrochemical water oxidation

We present a form of hematite (α-Fe2O3) nanostructured architecture suitable for photoelectrochemical water oxidation that is easily synthesized by a pulsed laser deposition (PLD) method. The architecture is a column-like porous nanostructure consisting of nanoparticles 30–50 nm in size with open channels of pores between the columns. This nanostructured film is generated by controlling the kinetic energy of the ablated species during the pulsed laser deposition process. In a comparison with the nanostructured film, hematite thin film was also synthesized by PLD. All of the developed films were successfully doped with 1.0 at% of titanium. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM) and UV–visible spectroscopy were used to characterize the films. To fabricate the photoelectrochemical (PEC) cell, Ti-doped hematite films were used as the working electrode, Ag/AgCl as the reference electrode, platinum wire as the counter electrode and an aqueous solution of 1 M NaOH as the electrolyte. The photovoltaic characteristics of all cells were investigated under AM 1.5G sunlight illumination of 100 mW/cm2. The photocurrent density was enhanced by approximately 220% using nanostructured film at 0.7 V versus Ag/AgCl compared to hematite thin film, and the highest photocurrent density of 2.1 mA/cm2 at 0.7 V/Ag/AgCl was obtained from the 1.0 at% Ti-doped hematite nanostructured film. The enhanced photocurrent density is attributed to its effective charge collection due to its unique column-like architecture with a large surface area.

Photocurrent Enhancement for Ti-Doped Fe2O3 Thin Film Photoanodes by an In Situ Solid-State Reaction Method

In this work, a higher concentration of Ti ions are incorporated into hydrothermally grown Ti-doped (2.2% by atomic ratio) micro-nanostructured hematite films by an in situ solid-state reaction method. The doping concentration is improved from 2.2% to 19.7% after the in situ solid-state reaction. X-ray absorption analysis indicates the substitution of Fe ions by Ti ions, without the generation of Fe²⁺ defects. Photoelectrochemical impedance spectroscopy reveals the dramatic improvement of the electrical conductivity of the hematite film after the in situ solid-state reaction. As a consequence, the photocurrent density increases 8-fold (from 0.15 mA/cm² to 1.2 mA/cm²), and it further increases up to ∼1.5 mA/cm² with the adsorption of Co ions. Our findings demonstrate that the in situ solid-state reaction is an effective method to increase the doping level of Ti ions in hematite films with the retention of the micro-nanostructure of the films and enhance the photocurrent.

Highly photoactive Ti-doped α-Fe2O3thin film electrodes: resurrection of the dead layer

Uniform thin films of hematite and Ti-doped hematite (α-Fe2O3) were deposited on transparent conductive substrates using atomic layer deposition (ALD). ALD's epitaxial growth mechanism allowed the control of the morphology and thickness of the hematite films as well as the concentration and distribution of Ti atoms. The photoelectrochemical performances of Ti-doped and undoped hematite electrodes were examined and compared under water oxidation conditions. The incorporation of Ti atoms into hematite electrodes was found to dramatically enhance the water oxidation performance, with much greater enhancement found for the thinnest films. An optimum concentration ∼3 atomic% of Ti atoms was also determined. A series of electrochemical, photoelectrochemical and impedance spectroscopy measurements were employed to elucidate the cause of the improved photoactivity of the Ti-doped hematite thin films. This performance enhancement was a combination of improved bulk properties (hole collection length) and surface properties (water oxidation efficiency). The improvement in both bulk and surface properties is attributed to the resurrection of a dead layer by the Ti dopant atoms.

Physical and photoelectrochemical characterization of Ti-doped hematite photoanodes prepared by solution growth

We present the fabrication and characterization of Ti-doped hematite (α-Fe2O3) films for application as photoanodes in photoelectrochemical (PEC) cells for water splitting. It is demonstrated that Ti doping significantly improves the PEC activity as the photocurrent at 1.0 V vs. Ag/AgCl electrode for a 400 nm thick Ti-doped film (0.66 mA cm−2) was found to be ∼14 times higher than that of an undoped film (0.045 mA cm−2). The films were characterized by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and ultrafast transient absorption spectroscopy to obtain information about their structural, electronic, and charge carrier dynamic properties. Based on characterization of the chemical states of the involved elements as well as the charge carrier dynamics of the films with Ti doping, it appears that the photocurrent enhancement is related to an increase in charge carrier density or reduced electron–hole recombination. The highest incident photon conversion efficiency (IPCE) measured for this system was 27.0% at 360 nm at a potential of 1.23 V vs. reversible hydrogen electrode (RHE), which was obtained on a 400 nm thick Ti-doped α-Fe2O3 film.

Ti-doped hematite nanostructures for solar water splitting with high efficiency

Ti-doped hematite nanostructures have been synthesized for efficient solar water splitting by adding TiCN as the Ti precursor in a hydrothermal method. Ti-doped hematite nanostructures show an urchin-like morphology with nano feature size, which increases the effective surface area compared to undoped nanostructures. A remarkable plateau photocurrent density value of 3.76 mA/cm2 has been observed for Ti-doped nanostructures under standard illumination conditions in 1 M NaOH electrolyte, which is 2.5 times higher than that for undoped nanostructures (1.48 mA/cm2). The photocurrent at 1.23 V vs. RHE (1.91 mA/cm2) is also enhanced to be over 2 times higher than that for undoped nanostructures (0.87 mA/cm2). X-ray photoelectron spectroscopy and x-ray absorption spectroscopy have been used to investigate the electronic structure of Ti-doped hematite, which suggest the increased donor density of hematite by Ti doping. The remarkable plateau current density in Ti-doped hematite nanostructures can be attributed to both the favorable urchin-like morphology and the Ti doping.

Ti-Doped Hematite Nanostructures for Solar Water Splitting with High Efficiency

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Enhanced photoelectrochemical performance of Ti-doped hematite thin films prepared by the sol–gel method

Ti-doped α-Fe2O3 thin films were successfully prepared on FTO substrates by the sol–gel route. Hematite film was characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and energy dispersive spectrometer (EDS). The XRD data showed α-Fe2O3 had a preferred (110) orientation which belonged to the rhombohedral system. Interestingly, the grains turned into worm-like shape after annealed at high temperature. The IPCE could reach 32.6% at 400nm without any additional potential vs. SCE. Titanium in the lattice can affect the photo electro chemical performance positively by increasing the conductivity of the thin film. So the excited electrons and holes could live longer, rather than recombining with each other rapidly as undoped hematite. And the efficient carrier density on the Ti-doped anode surface was higher than the undoped anode, which contribute to the well PEC performance.

X-ray microdiffraction from α-Ti 0.04Fe 1.96O 3 (0001) epitaxial film grown over α-Cr 2O 3 buffer layer boundary

Ti-doped hematite (α-Ti 0.04Fe 1.96O 3) film grown over patterned α-Cr 2O 3 buffer layer on α-Al 2O 3(0001) substrate was characterized with synchrotron X-ray microdiffraction. The film was grown by oxygen plasma assisted molecular beam epitaxy method. The film growth mode was correlated to buffer layer boundary and Ti concentration variation. Epitaxial α-Ti 0.04Fe 1.96O 3 film was formed on bare substrate adjacent to the buffer layer. The epitaxial film was connected laterally to a strain-relaxed epitaxial α-Ti 0.04Fe 1.96O 3 film grown on the buffer layer. On bare α-Al 2O 3 substrate with diminished Ti concentration only a small portion of α-Ti xFe 1 − x O 3 film was epitaxial either as coherent to the substrate or strain-relaxed form.

Oriented Ti doped hematite thin film as active photoanodes synthesized by facile APCVD

To improve the optoelectronic properties of iron oxide as a photoelectrode, hematite (α-Fe2O3) thin films were doped with titanium using atmospheric pressure chemical vapor deposition (APCVD) for synthesis. The films were prepared by pyrolysis of Fe(CO)5 and TiCl4 precursors on fluorine-doped tin oxide (FTO) substrates and found to have a polycrystalline morphology with faceted particulates ∼20 to 50 nm in size with a preferred crystallographic growth along the [110] direction. The performance of the photoanodes was measured as a function of titanium concentration. A maximum efficiency was observed at ∼0.8 atom% Ti in hematite. The Incident Photon-to-current Conversion Efficiency (IPCE) to hydrogen was measured in alkaline electrolyte. Under an applied bias of 0.6 V vs.Ag/AgCl at 400 nm the IPCE for water splitting in alkaline solution was found to be 27.2%, the highest efficiency reported for Ti doped hematite photoanodes. The IPCEs of the photoanode thin films at lower applied bias were further increased by calcination at 500 °C and by use of glucose as an anolyte.

Ti-doped α-Fe 2O 3 by quantum-chemical modeling

Structure and electronic properties of Ti-doped hematite (α-Fe 2O 3) crystal have been studied using a quantum-chemical method based on the Hartree–Fock theory. A supercell model employing a system consisting of 120 atoms has been exploited throughout the investigation. The impurity presence produces defect-inward displacement for the majority of Fe atoms whereas the O atoms experience both defect-inward and defect-outward shifts depending on their original position in the crystal. A small reduction in the band-gap width due to the Ti incorporation is also observed which might lead to some increase in the electrical conductivity in concordance to the available experimental measurements. Two new phenomena are discovered according to results of our modeling, being those of electron density redistribution between different 3 d Atomic Orbitals (AOs) within the same Fe atom and extra valence electron escape from the Ti-surrounding region towards far-situated Fe atoms. The latter presumably could explain some contradictory results on saturation magnetization in Ti-doped α-Fe 2O 3 compounds.

NiFe-oxide electrocatalysts for the oxygen evolution reaction on Ti doped hematite photoelectrodes

Nickel iron binary oxide electrocatalysts prepared from different precursors were evaluated to facilitate the oxygen evolution reaction (OER) on semiconducting metal-oxide photoelectrodes. The electrocatalysts deposited from Ni(II) and Ni(II)Fe(II) precursors had the highest activity for OER, however, their presence on the surface of the Ti doped hematite photoelectrode decreased the incident photon-to-current efficiency (IPCE) of the photoelectrode for water splitting in an alkaline electrolyte. In contrast, the NiFe-oxide deposited from the Ni(II)–Fe(III) precursor which had a lower OER activity was found to increase the IPCE of the photoelectrode by as much as a factor of 5.

Low temperature transformation from γ-Fe2O3 to Ti doped α-Fe2O3 nanoparticles through an epoxide assisted sol–gel route

Maghemite and Ti doped hematite nanoparticles were synthesized through an epoxide assisted sol-gel route. Maghemite nanoparticles were prepared through the boiling of the resultant obtained by the gelation reaction of propylene oxide with FeCl2. The production of Ti doped hematite nanoparticles follows the same preparation procedure with one difference where TiCl3 was added to the FeCl2 solution before reaction. The unique chemistry of the route results in the low temperature preparation of monodisperse Maghemite nanoparticles. It also minimizes the huge difference of reactivity between Ti3+ and Fe2+ ions, which guarantees the synthesis of Ti doped hematite at low temperature, resulting in the small size of the particles.

Predicting the Band Structure of Mixed Transition Metal Oxides: Theory and Experiment

Fuel production from sunlight using mixed metal oxide photocatalysts is a promising route for harvesting solar energy. While photocatalytic processes can operate with high efficiency using UV light, it remains a challenge of paramount importance to drive them with visible light. Engineering the electronic energy band structure of mixed metal oxides through judicious control of atomic composition is a promising route to increasing visible light photoresponse. The goal of this paper is to develop a simple mixed metal oxide band structure (MMOBS) method to predict the electronic band structure of mixed metal oxides. Several materials in the Ti−Fe−O system that span the composition spectrum were considered in this study: anatase TiO2, Fe-doped anatase TiO2, ilmenite TiFeO3, Ti-doped hematite α-Fe2O3, and hematite α-Fe2O3. The predictions by the MMOBS method for the Ti−Fe−O system were tested and confirmed using first-principles density functional theory (DFT) calculations and experimental UV−visible absorptio...

Changes in Fe L23 White Lines Intensities in Ti Doped Hematite Thin Fims

Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.