Undoped α-Fe2O3 Research Articles

Highly oriented Ge-doped hematite nanosheet arrays for photoelectrochemical water oxidation

We report a simultaneous strategy of impurity doping and crystal growth for building highly oriented Ge-doped α-Fe2O3 nanosheet arrays vertically aligned on fluorine-doped tin oxide (FTO) glass substrates in hydrothermal environments, by using β-FeOOH nanorod arrays as sacrificial templates and highly reactive Ge colloidal solutions as dopant source. Microstructure characterization and elemental analysis reveal the preferential growth orientation, distribution of dopants, and doping level of Ge-doped α-Fe2O3 nanosheet arrays. Based on the doping of Ge atoms, proper feature size of nanosheets (with thickness <10nm) and preferential growth orientation within (001) basal plane of perpendicular nanosheet arrays, Ge-doped α-Fe2O3 nanosheet arrays show a photocurrent density of 1.4mAcm−2 at 1.23V vs. RHE, which is more than 50 times of the undoped α-Fe2O3 nanorod arrays. Moreover, we find that the annealing temperature remarkably affects the majority carrier density and optical absorption efficiency, which enabled the determination of photocurrent density of hematite nanostructure arrays.

Highly photoactive Ti-doped α-Fe2O3 nanorod arrays photoanode prepared by a hydrothermal method for photoelectrochemical water splitting

In this work, we report the Ti-doped α-Fe2O3 nanorod arrays prepared by a facile approach as well as their implementation as photoanodes for photoelectrochemical water splitting. Impedance measurements show that the Ti dopant serves as electron donor and increases the donor density of Ti-doped α-Fe2O3 nanorod arrays to 2.4×1019cm−3, which is approximately 4 times higher than that of undoped α-Fe2O3 nanorod arrays. Photoelectrochemical impedance spectroscopy shows that photogenerated holes could be transferred from valence band of hematite to electrolyte more efficiently in Ti-doped α-Fe2O3 nanorod arrays. With the optimal Ti/Fe atomic ratio, Ti-doped α-Fe2O3 nanorod arrays exhibit a remarkable photocurrent density of 0.72mA/cm2 at 0.23V vs. Ag/AgCl, which is 2 order of magnitude higher than that of undoped α-Fe2O3 nanorod arrays. The enhancive photoelectrochemical activity of Ti-doped α-Fe2O3 nanorod arrays is attributed to the improved donor density and reduced recombination of the electron-hole pairs.

Electrodeposited nanostructured α-Fe2O3 thin films for solar water splitting: Influence of Pt doping on photoelectrochemical performance

Electrochemically deposited α-Fe2O3 thin films, whose composition was tuned by Pt doping, were investigated as photoanode for photoelectrochemical water splitting. Morphological and structural characteristics of the nanostructured α-Fe2O3 thin films were studied by scanning electron microscopy and X-ray diffraction techniques. The films were characterized by Raman spectroscopy and X-ray photoelectron spectroscopy to determine the effect of Pt doping on the α-Fe2O3 structure. The photoelectrochemical performance of the films was examined by linear sweep voltammetry and electrochemical impedance spectroscopy. Results of these studies showed that Pt doping increased the density of small-sized nanoparticles in α-Fe2O3 thin films. The Pt doped films exhibited higher photoelectrochemical activity by a factor of 1.4 over un-doped α-Fe2O3 films. The highest photocurrent density of 0.56 mA cm−2 was registered for 3% pt doped film at 0.4 V versus Ag/AgCl in 1 M NaOH electrolyte and under standard illumination conditions (AM 1.5 G, 100 mW cm−2). This high photoactivity can be attributed to the high active surface area and increased donor density caused by Pt doping in the film. Electrochemical impedance analysis also revealed significantly low charge transfer resistance of Pt doped films, indicating its superior electrocatalytic activity for water splitting reaction compared to un-doped α-Fe2O3 thin films.

Enhanced photoelectrochemical oxidation of water over undoped and Ti-doped α-Fe2O3 electrodes by electrochemical reduction pretreatment

We demonstrate that the photoelectrochemical oxidation of water over undoped and Ti-doped α-Fe2O3 electrodes could be highly enhanced by in situ electrochemical reduction pretreatment at −1.2 ≤ E < −0.6 V versus a saturated calomel electrode in the dark in aqueous 1 M NaOH electrolyte for tens of minutes. This enhancement is mainly due to a significant decrease of charge recombination and a favourable photohole transfer and is physically associated with the partial transformation of Fe2O3 to Fe3O4/FeOOH and Fe by electrochemical reduction.

Influence of the Potential on the Charge-Transfer Rate Constant of Photooxidation of Water over &lt;em&gt;&amp;alpha;&lt;/em&gt;-Fe&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; and Ti-Doped &lt;em&gt;&amp;alpha;&lt;/em&gt;-Fe&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;

It has been reported that applying a certain external anodic potential over un-doped (α-Fe2O3) and Ti-doped α-Fe2O3 (Ti-Fe2O3) electrodes can improve the photocurrent or the photoelectrochemical oxidation rate of water. However, it is assumed in the literature that the potential drops completely across the side of the solid semiconductor (band edge pinning), and the influence of the potential on the interfacial chargetransfer rate constant is rarely reported. In this article, the impact of the applied potential on the interfacial chargetransfer rate constant during photoelectrochemical oxidation of water over the two electrodes was investigated by electrochemical impedance spectroscopy. The results showed that by increasing the applied anodic potential, the interfacial charge-transfer rate constants for both electrodes were increased. The smaller increase in the magnitude of the rate constant than determined by theory indicates that not all of the applied potential drops across the Helmholtz layer, but takes place in both the space charge and Helmholtz layers simultaneously (Fermi level pinning). The results of the surface-state capacitance measurements suggested that the photo-generated charge can be accumulated in the surface states, resulting in the re-distribution of the potential at the interface and an improvement in the rate constant. Under the same applied potential, the higher the light intensity is, the more the photogenerated holes accumulated in the surface states. This causes an increase in the potential drop across the Helmholtz layer and consequently increases the charge-transfer rate constant. Compared with the α-Fe2O3, the improvement of the charge-transfer rate constant by the anodic potential is more obvious. 1954 上官鹏鹏等:电位对α-Fe2O3和掺钛α-Fe2O3光氧化水电荷转移速率常数的影响 No.9

Facile preparation of nanostructured α-Fe2O3 thin films with enhanced photoelectrochemical water splitting activity

We report on the use of a facile electrospray technique for the synthesis of α-Fe2O3 thin films on a FTO substrate for photoelectrochemical (PEC) water splitting. The effect of synthesis parameters such as substrate temperature, discharge potential and post-heat treatment on morphology, particle size and PEC performance of α-Fe2O3 films were investigated. With an increase in substrate temperature, the surface morphology of the α-Fe2O3 film was altered from a packed worm-like surface to highly porous nanostructures. XRD analysis revealed that the (110) grain orientation of the film was transformed to the (104) grain orientation at 300 °C, due to the oxidation of the precursor at the surface of the substrate. Raman spectroscopy and XPS analysis indicated the presence of highly pure α-Fe2O3 in the film. By changing the discharge potential, the size of particles in the film was reduced to a minimum of 23 nm. Under optimized conditions the nanostructured α-Fe2O3 films showed a water splitting photocurrent of ∼0.6 mA cm−2 at 1.23 V versus RHE under standard illumination conditions (AM 1.5 G 100 mW cm−2), and an incident photon to current efficiency (IPCE) of 13% at 350 nm (at 1.4 V versus RHE) which are among the best results obtained for undoped α-Fe2O3 photoanodes. This enhanced PEC performance can be attributed to the efficient charge separation at the α-Fe2O3-electrolyte interface due to the larger interfacial area of small-sized particles in the film. This study thus provides a simple route for the synthesis of highly active α-Fe2O3 thin films that can be extended to metal doped films such as Ti-doped α-Fe2O3.