Ordered Mesoporous LiFe₅O₈ Thin‐Film Photoanodes for Water Splitting

Abstract

The optical and photoelectrochemical properties of ordered mesoporous a-LiFe 5 O 8 thin films have been investigated. These solÀgel thin films exhibit a complex band structure, indicating a band gap of 2.45 eV, with additional absorption in the visible region up to l = 600 nm. MottÀSchottky plots reveal their applicability as photoanodes for water splitting (as they are ntype semiconductors) and photoelectrochemical tests show good performance for water oxidation.Spinel ferrites have gained tremendous interest in the last decade as low-cost and earth-abundant electrodes for magnetic, charge storage, and photoelectrochemical applications. Regarding photoelectrochemical water splitting, spinel ferrites are particularly investigated as alternative photoanode materials to a-Fe 2 O 3 , looking for better charge diffusion properties. [1] Spinel ferrites of composition MFe 2 O 4 (M=Cu, Co, Mg, Ni, Zn) have been intensively investigated concerning their applicability in water oxidation, and many different preparation techniques have been presented, recently especially for highly considered ZnFe 2 O 4 , including CVD, [2] ALD, [3] and solutionprocessing routes. [4] When not employed as single-phase electrodes, spinel ferrites are also often used in the form of composites as a kind of sensitizer for better charge separation, e. g. in combinations like ZnFe 2 O 4 /a-Fe 2 O 3 [5] or NiFe 2 O 4 /TiO 2 . [6] Recently, it has been shown that the photoelectrochemical properties of ZnFe 2 O 4 can be tailored by mesostructural engineering. [7] To this end, large-pore mesoporous ZnFe 2 O 4 thin films, prepared using an evaporation-induced self-assembly (EISA) process along with different polymer structure-directing agents, were applied.The EISA process was introduced in the late 1990s, [8] and many important thin-film materials with ordered mesoporous structures have been reported since then, including ferrites with garnet [9] and spinel [10] structures. For example, an interesting room-temperature magnetic spinel ferrite, namely LiFe 5 O 8 , with large mesopores and nanocrystalline walls was reported, showing promise for use in micromagnetic actuation. [11] LiFe 5 O 8 exhibits promising photocatalytic properties, but in the application as pure photoelectrode for water oxidation, the material showed hardly any performance with addition of reduced graphene oxide, below 1 mA/cm 2 . [12] Here, we present investigations into the optical and photoelectrochemical properties of mesoporous LiFe 5 O 8 thin-film photoanodes. Both SEM and TEM reveal highly porous and phase-pure material with nanocrystalline walls. MottÀSchottky analysis indicates n-type behavior and flat-band potential of the photoelectrodes below the hydrogen production potential, making mesoporous LiFe 5 O 8 photoanodes interesting for oxygen generation. Photoelectrochemical studies show good performance under visible light irradiation; no photocurrent transients were observed, thereby demonstrating low surface recombination losses when using a hole scavenger. IPCE measurem...

First‐Principles Molecular Dynamics Study of the Threshold Displacement Energy in LiFe5O8

“…The conduction band minimum for minority‐spin states is lower in energy than that of majority‐spin states, while the valence band maximum is formed by a majority spin state at the Γ point. The calculated bandgap of LFO is 2.24 eV, which agrees with the experimental value of 2.45 eV [ 34 ] and other calculated value of 2.3 eV. [ 10 ] Figure 1c shows the total and atom resolved projected density of states (PDOS) of LFO.…”

Thermoanalytical, optical, and magnetic investigations on nanocrystalline Li0.5Fe2.5O4 and resulting ceramics prepared by a starch-based soft-chemistry synthesis

“…Diffuse reflectance spectra of calcined and sintered Li 0.5 Fe 2.5 O 4 samples were recorded to determine the optical band gap using the Kubelka−Munk theory [61,62] in which the optical band gap can be expressed by Eq. 2 [63]: assuming direct allowed as well as indirect allowed transition mechanisms [8,9,27,64]. To verify the type of transition, the exponent n can be estimated by linearization of Eq.…”

Thermoanalytical, optical, and magnetic investigations on nanocrystalline Li0.5Fe2.5O4 and resulting ceramics prepared by a starch-based soft-chemistry synthesis

“…13). Reported optical band gaps for Li 0.5 Fe 2.5 O 4 are between 1.4 and 2.2 eV assuming direct allowed as well as indirect allowed transition mechanisms [8,9,27,64]. To verify the type of transition, the exponent n can be estimated by linearization of Eq.…”