Various types of iron oxide and their physical properties (
Abstract
Epitaxial films of Rh-substituted α-Fe2O3 were fabricated by a pulsed laser deposition technique, and their photoelectrochemical characteristics were investigated for the development of visible light-responsive photoanodes for water splitting. The photocurrent in the films upon irradiation in the visible region was significantly enhanced after Rh substitution. Moreover, a near-infrared photocurrent was clearly observed for Rh:Fe2O3 photoanodes, whereas no photoresponse could be detected for the α-Fe2O3 films. These improved photoelectrochemical properties are attributed to the increased light absorption due to the hybridization of Rh-4d states and O-2p states at the valence band maximum. Moreover, Rh substitution also strongly influences the photocarrier transport properties of the films. The electrical conductivity of Rh:Fe2O3 is higher than that for α-Fe2O3 by two orders of magnitude, which is possibly due to the extended 4d orbitals of the Rh3+ ions. Thus, the improved electrical properties may lead to an increased photocurrent by lowering the recombination rate of photogenerated carriers.
Keywords
- solar water splitting
- pulsed laser deposition
- photoelectrochemical cell
- iron oxides
- bandgap engineering
1. Introduction
Iron oxides are well known to have various physical properties depending on their composition and crystal structures (see Table 1). They have been the subject of extensive investigation over the past decades from both fundamental and practical perspectives. For example, magnetite (Fe3O4) has been one of the most widely investigated oxides in various research fields owing to its high magnetic transition temperature (~585°C) and high spin polarization of carriers [1, 2, 3]. Numerous Fe3O4-based ferromagnetic semiconductors and related spintronics devices have been reported. Another simple iron oxide, wüstite (FeO) has attracted much attention in various fields such as Earth sciences, oxide electronics, spintronics, and chemical engineering [4, 5, 6]. Moreover, multifunctional bismuth ferrite (BiFeO3, BFO) has been of great interest owing to its potential applications in numerous room temperature multiferroic devices [7, 8, 9]. BFO is also considered to be a good candidate for use in solar energy conversion systems because of its electrical polarization-induced photovoltaic effects [10]. The triangular antiferromagnet RFe2O4 (R = Ho, Y, Yb, Lu, and In) is a multilayered oxide and was discovered in the 1970s by Kimizuka et al. [11]. RFe2O4 is composed of alternating hexagonal Fe─O and R─O layers stacked along the
2. Experimental procedures
The FRO films were grown using a PLD technique with an argon fluoride (ArF) excimer laser (λ = 193 nm). The laser pulse frequency was 5 Hz. The fluence remained constant at 1.1 J/cm2. The typical growth rate of the films was 0.5 nm/min. After deposition, the FRO films were annealed in air at 700°C to improve their crystallinity. The author employed two types of bottom electrodes, viz., TTO deposited onto an α-Al2O3 (110) substrate and polycrystalline fluorine-doped SnO2 (FTO) formed on a soda-lime glass substrate. An Fe2-
3. Crystal structures
The XRD 2 theta-omega scan of the FeRhO3 films is shown in Figure 4(a). For the as-deposited sample, broad peaks are observed at 35 and 75°, which are ascribed to the (110) and (220) reflections of corundum-type FRO, respectively. This indicates that the films grown along [110] despite their low crystalline quality. Sharp peaks appear after thermal annealing, suggesting an improvement in the crystallinity. The in-plane epitaxial relationship was evaluated to be TTO [010]//FRO [001] by in-plane XRD measurements. This result agrees with the atomic configurations in Figure 3(a) [30]. The lattice constants obey Vegard’s law, implying that Fe had been appropriately substituted with Rh. In contrast to the films deposited onto the sapphire substrates, the films deposited on the glass substrates are polycrystalline in nature, as shown in Figure 4(b) and (c).
4. Optical properties
Figure 5(a) shows the light absorption spectra of the films. The fundamental absorption edge of α-Fe2O3 is related to charge transfer from O 2
5. XPS spectroscopy
The results of XPS are presented in Figure 6. In the spectra of Fe 2
6. Photoelectrochemical properties
The current-potential curves of the films are shown in Figure 7(a) and (b). For α-Fe2O3, the photocurrent is 2.87 μA/cm2 at 0.5 V under irradiation with VIS light (λ = 400–700 nm). As shown in Figure 7(a), the VIS photocurrent is remarkably increased after Rh substitution (17.3 μA/cm2 at 0.5 V for
where
7. Summary
The Rh-substituted α-Fe2O3 photoelectrodes were successfully fabricated using a pulsed laser deposition method. Their bandgap narrowed as the Rh content increased. XPS analyses revealed that the bandgap narrowing is brought by the hybridization of the Rh 4
Acknowledgments
This work was supported by JSPS Core-to-Core Program, A. Advanced Research Networks, and JSPS KAKENHI grant numbers JP16K14226 and JP15H03563. The author would like to thank Prof. H. Tabata, Prof, H. Matsui, and Dr. H. Yamahara for their support and helpful discussion.
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