Stacking textured films on lattice-mismatched transparent conducting oxides via matched Voronoi cell of oxygen sublattice.

Transparent conducting oxides are a critical component in modern (opto)electronic devices and solar energy conversion systems, and forming textured functional films on them is highly desirable for property manipulation and performance optimization. However, technologically important materials show varied crystal structures, making it difficult to establish coherent interfaces and consequently the oriented growth of these materials on transparent conducting oxides. Here, taking lattice-mismatched hexagonal α-Fe2O3 and tetragonal fluorine-doped tin oxide as the example, atomic-level investigations reveal that a coherent ordered structure forms at their interface, and via an oxygen-mediated dimensional and chemical-matching manner, that is, matched Voronoi cells of oxygen sublattices, [110]-oriented α-Fe2O3 films develop on fluorine-doped tin oxide. Further measurements of charge transport characteristics and photoelectronic effects highlight the importance and advantages of coherent interfaces and well-defined orientation in textured α-Fe2O3 films. Textured growth of lattice-mismatched oxides, including spinel Co3O4, fluorite CeO2, perovskite BiFeO3 and even halide perovskite Cs2AgBiBr6, on fluorine-doped tin oxide is also achieved, offering new opportunities to develop high-performance transparent-conducting-oxide-supported devices.

Narrowing the band gap and suppressing electron-hole recombination in ß-Fe2O3 by chlorine doping.

The effects of halogen (F, Cl, Br, I, and At) doping in the direct-band-gap ß-Fe2O3 semiconductor on its band structures and electron-hole recombination have been investigated by density functional theory. Doping Br, I, and At in ß-Fe2O3 leads to transformation from a direct-band-gap semiconductor to an indirect-band-gap semiconductor because their atomic radii are too large; however, F- and Cl-doped ß-Fe2O3 remain as direct-band-gap semiconductors. Due to the deep impurity states of the F dopant, this study focuses on the effects of the Cl dopant on the band structures of ß-Fe2O3. Two impurity levels are introduced when Cl is doped into ß-Fe2O3, which narrows the band gap by approximately 0.3 eV. After doping Cl, the light-absorption edge of ß-Fe2O3 redshifts from 650 to 776 nm, indicating that its theoretical solar to hydrogen efficiency for solar water splitting increases from 20.6% to 31.4%. In addition, the effective mass of the holes in halogen-doped ß-Fe2O3 becomes significantly larger than that in undoped ß-Fe2O3, which may suppress electron-hole recombination.

Effects of transition metal doping on electronic structure of metastable ß-Fe2O3 photocatalyst for solar-to-hydrogen conversion.

Metastable ß-Fe2O3 is a promising photocatalyst with a band gap of approximately 1.9 eV, while its intrinsic material properties remain rarely studied by theoretical calculations. Here, using density functional theory, we studied the electronic band structure and effective mass of carriers in Zr, Sn, and Ti doped ß-Fe2O3. The calculation results show that, through the doping of Zr, Sn, or Ti, the dipole moment of FeO6 octahedra in ß-Fe2O3 increases, which favors the separation of photo-excited electron-hole pairs. The electron and hole effective masses in the close-packed orientation [111] in cubic ß-Fe2O3 have the smallest absolute values. After doping with Zr, Sn, and Ti, the absolute values of electron and hole effective masses in the [111] orientation are further reduced. Furthermore, the relative ratio (D) mostly became larger after doping with Zr, Sn, and Ti, which indicates that the photoexcited carriers in the doped structure are effectively separated. Construction of Zr, Sn, and Ti doped ß-Fe2O3 in the [111] orientation may be effective to improve the photocatalytic efficiency.

Improved charge separation efficiency of hematite photoanodes by coating an ultrathin p-type LaFeO3 overlayer.

Many metal-oxide candidates for photoelectrochemical water splitting exhibit localized small polaron carrier conduction. Especially hematite (α-Fe2O3) photoanodes often suffer from low carrier mobility, which causes the serious bulk electron-hole recombination and greatly limits their PEC performances. In this study, the charge separation efficiency of hematite was enhanced greatly by coating an ultrathin p-type LaFeO3 overlayer. Compared to the hematite photoanodes, the solar water splitting photocurrent of the Fe2O3/LaFeO3 n-p junction exhibits a 90% increase at 1.23 V versus the reversible hydrogen electrode, due to enlarging the band bending and expanding the depletion layer.

Long-term durability of metastable ß-Fe2O3 photoanodes in highly corrosive seawater.

Durability is one prerequisite for material application. Photoelectrochemical decomposition of seawater is a promising approach to produce clean hydrogen by using solar energy, but it always faces the problem of serious Cl- corrosion. We find that the main deactivation mechanism of the photoanode is oxide surface reconstruction accompanied by the coordination of Cl- during seawater splitting, and the stability of the photoanode can be effectively improved by enhancing the metal-oxygen interaction. Taking the metastable ß-Fe2O3 photoanode as an example, Sn added to the lattice can enhance the M-O bonding energy and hinder the transfer of protons to lattice oxygen, thereby inhibiting excessive surface hydration and Cl- coordination. Therefore, the bare Sn/ß-Fe2O3 photoanode delivers a record durability for photoelectrochemical seawater splitting over 3000 h.

Homogeneous solution assembled Turing structures with near zero strain semi-coherence interface.

Turing structures typically emerge in reaction-diffusion processes far from thermodynamic equilibrium, involving at least two chemicals with different diffusion coefficients (inhibitors and activators) in the classic Turing systems. Constructing a Turing structure in homogeneous solutions is a large challenge because of the similar diffusion coefficients of most small molecule weight species. In this work, we show that Turing structure with near zero strain semi-coherence interfaces is constructed in homogeneous solutions subject to the diffusion kinetics. Experimental results combined with molecular dynamics and numerical simulations confirm the Turing structure in the spinel ferrite films. Furthermore, using the hard-soft acid-base theory, the design of coordination binding can improve the diffusion motion of molecules in homogeneous solutions, increasing the library of Turing structure designs, which provides a greater potential to develop advanced materials.

Paving the road toward the use of ß-Fe2O3 in solar water splitting: Raman identification, phase transformation and strategies for phase stabilization.

Although ß-Fe2O3 has a high theoretical solar-to-hydrogen efficiency because of its narrow band gap, the study of ß-Fe2O3 photoanodes for water splitting is elusive as a result of their metastable nature. Raman identification of ß-Fe2O3 is theoretically and experimentally investigated in this study for the first time, thus clarifying the debate about its Raman spectrum in the literature. Phase transformation of ß-Fe2O3 to α-Fe2O3 was found to potentially take place under laser and electron irradiation as well as annealing. Herein, phase transformation of ß-Fe2O3 to α-Fe2O3 was inhibited by introduction of Zr doping, and ß-Fe2O3 was found to withstand a higher annealing temperature without any phase transformation. The solar water splitting photocurrent of the Zr-doped ß-Fe2O3 photoanode was increased by 500% compared to that of the pure ß-Fe2O3 photoanode. Additionally, Zr-doped ß-Fe2O3 exhibited very good stability during the process of solar water splitting. These results indicate that by improving its thermal stability, metastable ß-Fe2O3 film is a promising photoanode for solar water splitting.

A beta-Fe2O3 nanoparticle-assembled film for photoelectrochemical water splitting.

Since Fe2O3 is a promising photoanode material for water splitting, it has attracted much attention, while other phases of ferric oxide are ignored. Here, ß-Fe2O3 was used as a photoanode material for solar water splitting. The crystal structure and phase of ß-Fe2O3 were characterized by using X-ray diffraction, X-ray photoelectron spectroscopy, Raman scattering, Mössbauer spectra and a superconducting quantum interference device. The photocurrent density of the ß-Fe2O3 photoanode at 1.6 VRHE was 0.12 mA·cm-2 under the illumination of simulated sunlight (AM1.5G, 100 mW cm-2).