Doped Hematite Research Articles

Integration of Layered Double Hydroxide to Enhance Charge Carrier Utilization of Hematite Photoanodes for Water Splitting

Photoelectrochemical (PEC) water splitting presents an effective approach to meet the ever increasing energy crisis by conversion of the inexhaustible solar energy to hydrogen. From practical application viewpoint, hematite is a promising candidate material due to its high theoretical solar to hydrogen (STH) efficiency of ~ 15%. However, it is still a challenge to increase the charge carrier utilization of hematite. Herein, we report the enhancement of carrier utilization of hematite by nanostructuring and addition of thin film of cocatalyst overlayer. Based on the characterizations and analysis of X-ray diffraction and SEM with EDX, it was found that a crystalline NiFe phase for composite electrodes was formed after treatment. Integrating NiFe layered double hydroxide (LDH) to doped hematite photoanodes resulted in an enhanced photocurrent density of 2.5 mA/cm2 at 0.8 V vs Ag/AgCl, due to the efficient hole transfer to the surface leading to better carrier utilization. Electrochemical Impedance Spectroscopy analysis further shows that addition of cocatalyst overlayer helps in improving the surface reaction kinetics and reducing the barrier of oxygen evolution reaction of the developed photoanode.

Effects of Ti Doping on Hematite Photoanodes: More Surface States.

Ti doped hematite photoanodes have been intensively investigated due to their excellent activity for photoelectrochemical water oxidation. However, little attention has been paid to the doping effect on the photocurrent onset potential of hematite and the underlying mechanism. In this paper, Ti doped hematite nanorod arrays were successfully prepared through a facile treatment of hematite with TiCl₃ solution. The photocurrent of the Ti doped hematite photoanode increases by three times, and its onset potential shifts more positively as compared with that of the undoped one. Electrochemical analyses were employed to unravel the mechanism of anodic shift of the onset potential. Cyclic voltammograms and electrochemical impedance spectra confirmed that more surface states were formed in Ti doped hematite than the undoped one. As a result, lower activity towards oxygen evolution reaction (OER) and increased electron-hole recombination after light on/off in low potential region were observed in Ti doped hematite. It is concluded that these doping induced surface states may be a hindrance to charge transfer and the onset potential of Ti doped hematite shifts anodically.

Effect of plasmonic Ag nanowires on the photocatalytic activity of Cu doped Fe2O3 nanostructures photoanodes for superior photoelectrochemical water splitting applications

The Present study focuses on the synthesis and analysis of Cu doped hematite (α-Fe2O3) nanostructures for effectively enhancing the optical properties as well as their implementation as photoelectrodes for energy-harvesting applications. In addition to this, the influence of noble metal plasmonic layer of Ag nanowires as a bottom layer for undoped and doped Fe2O3 photoanodes has been investigated. Herein, we studied the influence of dopant on morphology, structural, and optical properties of Fe2O3. X-ray diffraction technique and X-ray photoelectron spectroscopy analysis were confirmed Cu ion substitution into host Fe2O3 nanostructures. The optical band gap decreases from ~ 1.95eV to ~ 1.38eV with increasing of Cu dopant concentration. Impedance analysis reveals that the Cu dopant works as an electron donor and improves the Fe2O3 charge carrier density. The photoelectrochemical water splitting studies reveals that the photoanodes without plasmonic layer was shown improved photocurrents compare to the undoped sample, thus improving the absorption of the incident light. Significantly, the optimized 0.2mol% Cu-doped α-Fe2O3 photoelectrodes without Ag layer reached the maximum photocurrent density of ~0.31mA/cm2, ~ 28-fold that of pure Fe2O3 (0.011mA/cm2). Further, the same photoanode with plasmonic Ag nanowires showed a significantly improved photocurrent density of 1.48mA/cm2, which is ~ 135-fold that of pure Fe2O3 and ~ 5-folds that of 0.2mol% Cu doped α-Fe2O3 photoelectrodes without plasmonic nanowire layer. The superior photocurrent is ascribed to the enhanced electron donor density and reduced charge recombination rate, as an outcome of optimized Cu doping and Ag nanowires.

Study of the lattice structure and magnetic spin order modification in chemical routed α-Fe1.4Cr0.6O3 oxide as an effect of heat treatment

Abstract The Cr doped hematite (α-Fe1.4Cr0.6O3) system has been prepared by chemical route. The chemical routed sample has been thermally annealed at different temperatures in the range 800–1000 °C to study modified lattice structure and magnetic spin order. The present Cr doped hematite samples do not exhibit Morin transition, a typical temperature that marks spin orientation from in-plane spin to out of plane directions in hematite system. A distinct change in magnetic properties has been seen by increasing the annealing temperature. The sample prepared by annealing at 800 °C showed superparamagnetic blocking of particles below 38 K. Ac susceptibility data have been used to study the mechanism of spin dynamics around the blocking temperature. Such blocking behaviour of particles is not observed in the samples annealed at 900–1000 °C. The modified spin order in the samples has been understood by using core-shell model of antiferromagnetic particles. Mossbauer spectra of the samples have confirmed a transformation of the spin order with the increase of annealing temperature and support the features observed from dc magnetic measurements.

Effect of tetravalent ions dopants and CoO surface modification on hematite nanorod array for photoelectrochemical degradation of Orange-II dye

Abstract The surface modified tetravalent ions doped hematite nanorod photoanodes has been synthesized by successive hydrothermal and spin coating approaches. The effect of tetravalent ion (Sn, Ti, and Zr) doping and high temperature annealing on to the morphological, structural and photoelectrochemical properties has been investigated. Photoelectrochemical analyses indicate tetravalent doping (Sn, Ti, and Zr) and high-temperature annealing (800 °C) showed higher photocurrent and improved activity towards Orange-II dye degradation, compared to the pristine hematite film. Amongst tetravalent dopants, Zr4+ doped hematite can significantly enhance the photocurrent density (1.37 mA cm−2 at 1.23 V vs. RHE) as well as the Orange-II dye degradation activity (93% under one sun illumination in 270 min). Kinetic parameters are also investigated by first-order rate equation. Further, Zr–Fe2O3 photoanode modified with the appropriate composition of CoOx and specific structural features demonstrated improvement in photocurrent from 1.6 to 1.78 mA cm−2 (at 1.4 V vs. RHE). The efficient charge carrier separation and generated hydroxyl radicals led to enhancements of the Orange-II dye degradation efficiency up to 97% within 270 min. The significant decrease in chemical oxygen demand values suggests the removal of Orange-II dye. This work demonstrates that enhancement in photoelectrochemical activity is due to the combined effect of passivation of surface states and the formation of CoOx/Zr–Fe2O3 heterojunction.

Synthesis of U-Pb doped hematite using a hydrated ferric oxide approach

Hematite (α-Fe2O3) has been successfully co-doped with uranium and lead using hydrated ferric oxide (HFO) as an intermediary, precipitated by ammonia from a ferric nitrate solution. A novel synthesis approach has been developed, involving a wet stage of doping colloidal HFO by absorption from nitrate solution containing U and Pb nitrates, followed by drying and heating to 700 °C to convert HFO to Fe2O3. The crystal phase present in the sample was confirmed as hematite by X-ray powder diffraction. Scanning electron microscopy was used to determine sample morphology and U/Pb isotope homogeneity was assessed using laser-ablation inductively-coupled-plasma mass spectrometry (LA-ICP-MS). Although reasonably homogeneous at the scale of a LA-ICP-MS ablation analysis spot, conspicuous domains are recognized that appear ‘bright’ or ‘dark’ on backscatter electron images. These domains display distinctly different element concentrations and U/Pb ratios on LA-ICP-MS isotope maps. Nevertheless, this synthetic U-Pb-doped hematite represents a potential reference material for use in microbeam U-Pb geochronology and the results show that the preparation methods and doping conditions are effective. The observed chemical and isotopic heterogeneity between bright and dark domains may, however, inhibit widespread use of the synthesized hematite as a reference material. U/Pb heterogeneity may be controlled by the heating and cooling regimes of the method. Assuming the ‘bright and dark domains’ can be analytically or mechanically separated through refinement of the preparation method, then a suitable reference material may be produced. Subject to validation via high-precision chemical analysis, this reference material can be used to date uranium-bearing hematite from various types of deposits. The high speed and precision of LA-ICP-MS analyses will allow measurement of geological ages in a very common mineral, providing new insights for mineral exploration worldwide.

Exploring Burstein-Moss type effects in nickel doped hematite dendrite nanostructures for enhanced photo-electrochemical water splitting.

The Burstein-Moss (B-M) effect, which suggests that the optical band gap of degenerately doped semiconductors increases when all states close to the conduction band get populated due to shifting of an absorption edge to higher energy, is important, as it gives a chance to obtain different optical properties for the same material. Here, we report our observations of the similar shift in the optical band gap in NixFe2-xO3 nanocomposites as a function of composition with the help of cyclic voltammetry (CV) and XPS valence band (VB) position measurements. The conduction band edge (CBE) position of the NixFe2-xO3 nanocomposites as determined using CV was noted to move towards more negative potential with increasing Ni-concentration. A similar shift is also noted in the CBE estimated using XPS measurements (by subtracting the VB position from the optical band gap values). The observed shift in the optical band gap along with the CBE position gives the corresponding shift in the Fermi level, which is found to move closer to the CBE position, suggesting the observation of an effect similar to the B-M shift. Also, the extent of band bending estimated from the deviation of the CBE from the flat band potential (measured through Mott-Schottky plots) is found to increase with increasing Ni-concentration. Moreover, the Ni-composition has been observed to enhance the current density as well as to facilitate water splitting at a much lower onset potential compared to pure hematite. The NixFe2-xO3 nanocomposite with an 11 mol% Ni-composition shows the highest photo-electrochemical response with an almost ten times enhancement in the current density at 1.9 V vs. RHE in alkaline medium, as compared to the dark current. This enhanced performance is attributed to the improved charge separation and higher charge carrier density as a result of the higher extent of band bending in the NixFe2-xO3 nanocomposites.

Charge carrier dynamics in tantalum oxide overlayered and tantalum doped hematite photoanodes

The effects that Ta2O5-overlayer and Ta-doping have on the photoelectrochemical performance and surface state capacitance of hematite photoanodes.

Tubular morphology preservation and doping engineering of Sn/P-codoped hematite for photoelectrochemical water oxidation.

Tubular hematite with high-concentration, uniform doping is regarded as a promising material for photoelectrochemical water oxidation. However, the high-temperature annealing commonly used for activating doped hematite inevitably causes deformation of the tubular structure and an increase in the trap states. In the present work, Sn-doped tubular hematite on fluorine-doped tin oxide (FTO) is successfully obtained at 750 °C from a Sn-coated FeOOH tube precursor. Sn/P codoping, which is rarely considered for hematite, is also achieved via a gas phase reaction in phosphide atmosphere. The tubular morphology allows the dopant to diffuse from both the inner and outer surfaces, thus decreasing the doping profile in the radial direction. The even distribution of Sn and P synergetically increases the carrier density of hematite by one order of magnitude, which shortens the width of the depletion layer to ca. 2.3 nm (compared with 19.3 nm for the pristine sample) and leads to prolonged carrier lifetime and efficient charge separation. In addition, this codoping protocol does not introduce additional surface trap states, as evidenced by the increased charge injection efficiency and surface kinetic analysis using intensity modulated photocurrent spectroscopy (IMPS). As a result, the morphology- and doping-engineered hematite exhibits photocurrents of 0.9 mA cm-2 at 1.23 V and 3.8 mA cm-2 at 2.0 V vs. RHE under AM 1.5 G illumination (100 mW cm-2) in 1.0 M NaOH, representing 4.5-fold and 4.8-fold enhancements, respectively, compared with the photocurrents of undoped hematite. The present method is shown to be effective for preparing multi-element-doped hematite nanotubes and may find broad application in the development of other nanotubular photoelectrodes with or without doping for efficient and robust water oxidation.

Introduction of oxygen vacancies into hematite in local reducing atmosphere for solar water oxidation

Sn Doping and creation of oxygen vacancies have been adopted universally to overcome the poor electric conductivity and unfavorable hole diffusion length of α-Fe2O3 photoanodes. Generally, Sn doping is realized via longitudinal migration of tin element from FTO (fluorine-doped tin oxide) substrates into α-Fe2O3 at high temperature. To introduce oxygen vacancies along with Sn into hematite for further promoting its electric conductivity, we have created a local reducing atmosphere via partial oxidation of graphite while doping hematite with Sn. The donor density of the resultant Fe2O3 photoanode annealed on graphite (G-Fe2O3) at 770 °C for 20 min is increased to ∼1.7 times that of the counterpart annealed on SiO2 powders (S-Fe2O3), indicating that the electric conductivity of hematite is improved after introduction of oxygen vacancies. Moreover, oxygen vacancies have been demonstrated to significantly reduce the charge transfer resistance of Sn doped hematite. Consequently, the photocurrent density of G-Fe2O3 is enhanced remarkably (∼70%) compared with S-Fe2O3. However, the improvement in photocurrent density due to oxygen vacancies becomes less significant when more Sn is doped into hematite. The strategy for creation of oxygen vacancies reported here can be extended to other photoanodes for better understanding the effect of oxygen vacancies on PEC performance.

Acid-treated Ti4+ doped hematite photoanode for efficient solar water oxidation—Insight into surface states and charge separation

Acid-treatment has been proved to be an efficient approach to improve the photoelectrochemical (PEC) performance of hematite. However, efforts to optimize hematite photoanode have been limited by an inadequate understanding of the semiconductor surface. Here we make efforts to understand the microscopic charge separation processes of Ti4+ doped Fe2O3 photoanode before and after acid-treatment. Surface photovoltage (SPV) transient and the work function measurements directly reveal that acid-treatment leads to passivation of the surface states. Surface photovoltage (SPV) spectroscopic studies coupled to open-circuit photovoltage (OPV) measurements indicate that the surface states of hematite photoanode before acid-treatment result in the pinning of the Fermi level, which reduce the intensity of interfacial electric field at the semiconductor-electrolyte interface.

Sn doped α-Fe2O3 (Sn=0,10,20,30 wt%) photoanodes for photoelectrochemical water splitting applications

Abstract One pot hydrothermal route was adapted to synthesis pristine and Sn doped α-Fe2O3 nanospheres successfully. Sharp high intense diffraction peaks obtained from XRD confirmed crystalline nature of rhombohedral hematite. The secondary SnO2 face formation was due to increasing Sn dopant concentration. Raman spectra confirmed intrinsic phonon vibration modes [Eg(1)+Eg(2)+Eu] of hematite nanospheres. 2P3/2(1) → 2P1/2 transition by emission peak at 549 nm confirmed hematite phase formation. Metal oxygen vibration (Fe O stretching) was confirmed by absorption band situated at 539 cm−1. The noticeable variation in band gap of pristine hematite nanospheres was due to tetravalent Sn4+ dopant concentration. The lowest band gap energy 1.90 eV was found for 10 wt% Sn4+ doped hematite. Highest photocurrent 2.34 mA/cm2 at 0.098 V V RHE was obtained for 10% Sn doped hematite nanospheres. The EIS exposed the charge transferring mechanism of synthesized pristine and Sn doped α-Fe2O3 nanospheres. M-S plot evidenced that the lower shift of flat band potential for 10 wt% Sn4+ doped hematite was as −0.35 V. CA study proved the good stability over 4 h of the best performed photoanodes. Sn4+ doping and its dopant concentration on pristine hematite had dominant effect on photocatalytic activity of hematite nanospheres.

Insight into the influence of high temperature annealing on the onset potential of Ti-doped hematite photoanodes for solar water splitting

For Ti-doped hematite photoanodes, high temperature annealing drastically increases the water oxidation plateau photocurrent, but also induces an anodic shift of onset potential by about 100mV, thus hindering the performance under low applied bias. To the best of our knowledge, the effects of high temperature annealing on the onset potential have been rarely studied. Herein, both X-ray photoelectron spectroscopy (XPS) measurements and theoretical calculations indicated that the increase of surface Ti/Fe atomic ratio after high temperature annealing decreased the adsorption capacity of hydroxide ions on the hematite surface. Subsequently, the flatband potential (i.e., the theoretical onset potential) of Ti doped hematite photoanodes positively shifted, which was supported by the Mott-Schottky measurements.

Probing the dynamics of photogenerated holes in doped hematite photoanodes for solar water splitting using transient absorption spectroscopy.

Hematite (α-Fe2O3) has been extensively studied as a promising candidate for photoelectrochemical water splitting; however its overall efficiency is still relatively low. Doping is believed to be efficient in enhancing the photoactivity, while direct evidence for the promoted charge carrier dynamics is very limited. Herein, transient absorption spectroscopy was used to directly investigate the yield and decay dynamics of the photogenerated holes in Sn and/or Ti doped α-Fe2O3. Sn or Ti doping was observed to have different origins to the enhanced water oxidation photocurrent: Sn doping retarded the electron-hole recombination, while Ti doping mainly increased the photogenerated charge carrier density. Our results also demonstrated that co-doping may combine both advantages to enhance the overall photoactivity of α-Fe2O3.

Perovskite solar cell for photocatalytic water splitting with a TiO2/Co-doped hematite electron transport bilayer.

Hydrogen production using a photoelectrochemical (PEC) route promises to be a clean and efficient way of storing solar energy for use in hydrogen-powered fuel cells. Iron oxide (α-Fe2O3) is best suited to be used as a photoelectrode in PEC cells for solar hydrogen production due to its favorable band gap of ∼2.2 eV. Herein, chemical solution deposition was used for the preparation of a series of Co-doped Fe2O3 thin films on a titania buffer layer at different doping concentrations (3.0, 7.0 and 10.0 at%). The maximum anodic photocurrent reached up to 3.04 mA cm−2 by optimizing the balance between the doping concentrations, enhanced donor density, light absorbance, and surface roughness. The optical properties show that the light absorbance tendency switches to the higher wavelength with the further increment of Co beyond 3.0%. Finally, synthesized photosensitive perovskite CH3NH3PbI3 materials were added as a surface treatment agent on the photoelectrode to enhance the photocurrent absolute value. This inorganic nanostructured perovskite CH3NH3PbI3 (MAPbI3) coated on the Co-doped hematite photoanode achieved an overall solar-to-hydrogen conversion efficiency of 2.46%. Due to its low temperature processing, stability, and enhance efficiency, this perovskite coated TiO2/Co-doped hematite multilayer thin film solar cell has high potential to be applied in industry for hydrogen production.

Tuning orientation of doped hematite photoanodes for enhanced photoelectrochemical water oxidation

The limited conductivity in hematite has been currently considered as a critical bottleneck in improving the efficiency of solar-driven water oxidation. Here we shed light on the improvement of carrier conductivity by combing orientation control and transition metal doping. To rule out the influence of surface facets of hematite when adjusting the film orientation, the hematite photoanode films were fabricated by assembling the spherical particles with single crystalline nature. The doped hematite particles were aligned with (001) plane normal to the substrate by a magnetic field (MF) during the drop-casting process. A considerable improvement in photocurrent density was resulted from the orientation control, which is due to the increased carrier density from Sn doping and efficient charge transportation in the (001) plane observed through electrochemical impedance spectra.

Residual Energy Harvesting from Light Transients Using Hematite as an Intrinsic Photocapacitor in a Symmetrical Cell

Hematite as a sustainable photoabsorber material offers a band gap close to 2 eV and photoanode characteristics, but usually requires additional catalysts to enhance surface redox chemistry during steady state light energy harvesting for water splitting. Here, for a highly doped hematite film, sufficient intrinsic photocapacitor behavior is reported for the conversion of light transients into energy. Residual energy is harvested in a symmetric architecture with two opposing mesoporous hematite films on conductive glass. Transient light energy harvesting is shown to occur without the need for water splitting.

Direct Mapping of Band Positions in Doped and Undoped Hematite during Photoelectrochemical Water Splitting.

Photoelectrochemical water splitting is a promising pathway for the direct conversion of renewable solar energy to easy to store and use chemical energy. The performance of a photoelectrochemical device is determined in large part by the heterogeneous interface between the photoanode and the electrolyte, which we here characterize directly under operating conditions using interface-specific probes. Utilizing X-ray photoelectron spectroscopy as a noncontact probe of local electrical potentials, we demonstrate direct measurements of the band alignment at the semiconductor/electrolyte interface of an operating hematite/KOH photoelectrochemical cell as a function of solar illumination, applied potential, and doping. We provide evidence for the absence of in-gap states in this system, which is contrary to previous measurements using indirect methods, and give a comprehensive description of shifts in the band positions and limiting processes during the photoelectrochemical reaction.

Self-surface-passivation of titanium doped hematite photoanode for efficient solar water and formaldehyde oxidation

Herein, we present a new and available Ti doping and surface self-passivation strategy to significantly boost the PEC performance of the Fe2O3 nanorods for both solar water splitting and formaldehyde (HCHO) oxidation. Upon Ti doping and surface passivation with KOH treatment, the self-passived Ti4+ doped Fe2O3 nanorods exhibit substantially higher PEC performance compared to the pristine Fe2O3 and Ti4+ doped Fe2O3 nanorods, achieving a remarkable photocurrent of 3.7mAcm−2 at 1.5V vs. RHE. Furthermore, these self-passived Ti4+ doped Fe2O3 nanorods also show a 100mV cathodic shift in onset potential than that of Ti4+ doped Fe2O3 nanorods and excellent durability. Such the substantially improved photoelectrochemical performance is ascribed to the dramatically increased donor densities and reduced electron-hole recommendation as a result of Ti doping and surface passivation layer that formed after KOH treatment.

Synthesis of Ru doped hematite nanorods for application as photo-anode material in a photoelectrochemical cell (PEC)

Nanostructured thin films of hematite doped with different concentrations of ruthenium were grown on fluorine doped tin oxide glass substrates using the aqueous chemical growth method. On further heat treatment at 500 ∘C the structures morphed into hematite nanorods (NRs). The Ru concentration in the NRs was controlled by varying the Ru concentration in the RuCl 3⋅H2O precursors. Scanning Electron Microscopy confirmed the formation of the hematite nanorods, while. X-ray diffraction and Mossbauer spectroscopy (MS) data provided clear evidence of the crystallinity of the nanorods and incorporation of ruthenium in the hematite nanorod structure. The band gap of the Ru-doped hematite NRs, estimated from UV-Vis optical absorption intensity vs photon energy curves, were found to be directly related to the Ru concentration. For concentrations in the range 6–30 mg the band gaps are in the range well suited to drive the water splitting process in a photoelectrochemical cell without application of an external bias.