Photoelectrochemical (PEC) cells offer the ability to convert electromagnetic energy from our largest renewable source, the Sun, to stored chemical energy through the splitting of water into molecular oxygen and hydrogen. Hematite (α-Fe(2)O(3)) has emerged as a promising photo-electrode material due to its significant light absorption, chemical stability in aqueous environments, and ample abundance. However, its performance as a water-oxidizing photoanode has been crucially limited by poor optoelectronic properties that lead to both low light harvesting efficiencies and a large requisite overpotential for photoassisted water oxidation. Recently, the application of nanostructuring techniques and advanced interfacial engineering has afforded landmark improvements in the performance of hematite photoanodes. In this review, new insights into the basic material properties, the attractive aspects, and the challenges in using hematite for photoelectrochemical (PEC) water splitting are first examined. Next, recent progress enhancing the photocurrent by precise morphology control and reducing the overpotential with surface treatments are critically detailed and compared. The latest efforts using advanced characterization techniques, particularly electrochemical impedance spectroscopy, are finally presented. These methods help to define the obstacles that remain to be surmounted in order to fully exploit the potential of this promising material for solar energy conversion.

Solar Water Splitting: Progress Using Hematite (α‐Fe2O3) Photoelectrodes

Thin films of three iron oxide polymorphs, hematite, maghemite, and magnetite, were produced on KBr substrates using a conventional electron beam deposition technique coupled with thermal annealing. This method allowed for iron oxide thin films free from chemical precursor contaminants. The films were characterized using Fourier-transform infrared spectroscopy (FTIR), Raman microspectroscopy, and ellipsometry. These spectroscopic techniques allowed for a clear assignment of the phase of the iron oxide polymorph films produced along with an examination of the degree of crystallinity possessed by the films. The films produced were uniform in phase and exhibited decreasing crystallinity as the thickness increased from 40 to 250 nm.

Vibrational Spectroscopic Characterization of Hematite, Maghemite, and Magnetite Thin Films Produced by Vapor Deposition

We demonstrated in this paper the shape-controlled synthesis of hematite (alpha-Fe(2)O(3)) nanostructures with a gradient in the diameters (from less than 20 nm to larger than 300 nm) and surface areas (from 5.9 to 52.3 m(2)/g) through an improved synthetic strategy by adopting a high concentration of inorganic salts and high temperature in the synthesis systems to influence the final products of hematite nanostructures. The benefits of the present work also stem from the first report on the <20-nm-diameter and porous hematite nanorods, as well as a new facile strategy to the less-than-20-nm nanorods, because the less-than-20-nm diameter size meets the vital size domain for magnetization properties in hematite. Note that the porous and nonporous hematite one-dimensional nanostructures with diameter gradients give us the first opportunity to investigate the Morin temperature evolution of nanorod diameter and porosity. Evidently, the magnetic properties for nanorods exhibit differences compared with those for the spherical particle counterparts. Hematite nanorods are strongly dependent on their diameter size and porosity, where the magnetization is not sensitive to the size evolution from submicron particles to the 60-90 nm nanorods, while the magnetic properties change significantly in the case of <20 nm. In other words, for the magnetic properties of nanorods, in a comparable size range, the porous existence could also influence the magnetic behavior. Moreover, applications in formaldehyde (HCHO) gas sensors and lithium batteries for the hematite nanostructures with the diameter/surface area gradient reveal that the performance of electrochemical and gas-sensor properties strongly depends on the diameter size and Brunauer-Emmett-Teller (BET) surface areas, which is consistent with the crystalline point of view. Thus, this work not only provides the first example of the fabrication of hematite nanostructure sensors for detecting HCHO gas, but also reveals that the surface area or diameter size of hematite nanorods can also influence the lithium intercalation performances. These results give us a guideline for the study of the size-dependent properties for functional materials as well as further applications for magnetic materials, lithium-ion batteries, and gas sensors.

Synthesis of hematite (alpha-Fe2O3) nanorods: diameter-size and shape effects on their applications in magnetism, lithium ion battery, and gas sensors.

https://espace.library.uq.edu.au/view/UQ:113b151/UQ113b151_OA.pdf

Boosting the cycling stability of transition metal compounds-based supercapacitors

Doped and undoped hematite (α-Fe2O3) thin film electrodes for photoelectrochemical hydrogen production have been prepared using reactive magnetron sputtering. We present an extensive characterization of the photoelectrochemical, structural, electrical, and optical properties of the α-Fe2O3 films to assess the mechanisms by which the dopants influence photoelectrode performance. The Si-doped and Ti-doped α-Fe2O3 exhibited much higher photoelectrochemical activity than the undoped material. The Ti-doped α-Fe2O3 showed the largest photocurrent as well as the lowest onset potential, the highest electrical conductivity, the lowest activation energy, and the highest charge carrier density. The crystallographic orientation of the films does not appear to be a dominant factor affecting the photocurrent. The changes in conductivity with doping are insufficient to explain the observed changes in the photoelectrochemical activity. The proposed mechanism for the enhanced photocurrent is reduction of recombination due...

Enhancement of Photoelectrochemical Hydrogen Production from Hematite Thin Films by the Introduction of Ti and Si

By carefully controlling the reacting conditions, including atmosphere, temperature, and time, we directly acquire the nanowires of γ-Fe2O3 from the nanowires of α-Fe2O3 in a reduced atmosphere. X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectrum, and transmission electronic microscope analyses demonstrate that the nanowires are single-crystalline γ-Fe2O3. The nanowires have a diameter ranging from 50 to 90 nm and their typical lengths are in the range of 10∼20 μm. The optical property of the nanowires is observed by photoluminescence spectrum. The magnetic behavior of it is investigated by a magnetic property measurement system. The blocking temperature is found to be about 200 K. In addition, the mechanism of the transformation from α-Fe2O3 nanowires to γ-Fe2O3 nanowires is preliminarily studied by ab initio approach. It is found that the role of H2 is to change the Fe−O bonds in α-Fe2O3 nanowires, and then α-Fe2O3 is transformed into γ-Fe2O3.

Growth and properties of single-crystalline γ-Fe2O3 nanowires

By carefully controlling the reacting time, temperature, and humidity, we have prepared Co3O4 nanowires by heating a pure cobalt foil in atmosphere Scanning electronic microscopy demonstrates that the nanowires have a diameter ranging from 20 to 100 nm and their typical lengths are in the range of 10−20 μm X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectrum, and transmission electronic microscopy analyses demonstrate that the nanowires are Co3O4 The optical property of the nanowires is determined by photoluminescence spectrum The magnetic behavior of it is investigated by a magnetic property measurement system The nanowires exhibit some novel optical and magnetic properties, which are different from its bulk material

Synthesis and Physical Properties of Co3O4 Nanowires

Hollow Co2P nanoflowers (Co2P HNFs) were successfully prepared via a one-step, template-free method. Microstructure analysis reveals that Co2P HNFs are assembled from nanorods and possess abundant mesopores and an amorphous carbon shell. Density functional theory calculations and electrochemical measurements demonstrate the high electrical conductivity of Co2P. Benefiting from the unique nanostructures, when employed as an electrode material for supercapacitors, Co2P HNFs exhibit a high specific capacitance, an outstanding rate capability, and an ultralong cycling stability. Furthermore, the constructed Co2P HNF//AC ASC exhibits a high energy density of 30.5 W h kg−1 at a power density of 850 W kg−1, along with a superior cycling performance (108.0% specific capacitance retained after 10 000 cycles at 5 A g−1). These impressive results make Co2P HNFs a promising candidate for supercapacitor applications.

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Hollow Co2P nanoflowers assembled from nanorods for ultralong cycle-life supercapacitors

By carefully controlling the reacting conditions, including atmosphere, temperature, and time, α-Fe 2 O 3 nanoleaves have been synthesized by oxygenating pure iron. X-ray diffraction and transmission electronic microscope analyses demonstrate that the nanoleaves are single crystalline. The X-ray photoelectron spectrum, Raman spectrum, optical absorption spectrum, and photoluminescence spectrum are also carried out to characterize the nanoleaves. Magnetic measurement shows that the blocking temperature of the nanoleaves is about 120 K and the coercivity decreases as temperature increases. The more interesting is that a small exchange bias is found in 2 T field cooling hysteresis loops. This small exchange bias may possibly originate from a different magnetic order on the surface of the nanoleaves or the coexistence of a tiny amount of Fe 3 O 4 .

Yingying Xu

Papers

Growth and properties of single-crystalline γ-Fe2O3 nanowires

Synthesis and Physical Properties of Co3O4 Nanowires

Hollow Co2P nanoflowers assembled from nanorods for ultralong cycle-life supercapacitors

Synthesis and characterization of single-crystalline α-Fe2O3 nanoleaves

Magnetic properties of α-Fe2O3 nanowires