**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 (Fe<sup>3</sup> O4 ) 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 [1–3]. Numerous Fe<sup>3</sup> O4 -based ferromagnetic semiconductors and related spintronics

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toelectrochemical (PEC) water splitting [21–23]. A schematic of a PEC cell is shown in **Figure 1**. They consist of a photoactive electrode and a metal counter electrode immersed in a suitable electrolyte solution. The photogenerated electron-hole pairs are split by the electric field in the space-charge region at the surface of photoelectrodes. Since Honda and Fujishima's

wide research focused on the solar generation of hydrogen as a renewable and clean energy

as photoelectrodes. However, most of them are wide-gap semiconductors, and only a small fraction of the solar spectrum can be utilized by the PEC cells based on these materials. A high PEC responsivity to visible (VIS) and near-infrared (near-IR) light is required to harvest the

attention because of its promising properties for application as a photoanode in a solar water

tion of up to 40% of solar spectrum. However, the reported efficiencies for PEC water splitting

**Figure 1.** (a) Schematic of the photoelectrochemical (PEC) cell for solar water splitting and (b) electronic band structure of the PEC cell. Adapted by permission from Springer: Correlated Functional Oxides edited by H. Nishikawa, N. Iwata,


lower energy region of the solar spectrum. From this viewpoint, α-Fe<sup>2</sup>

photoelectrode [24], there has been world-

Bandgap-Engineered Iron Oxides for Solar Energy Harvesting

http://dx.doi.org/10.5772/intechopen.73227

have been investigated for their application

O3

) of 2.1 eV that allows for the absorp-

has attracted much

O3

257

pioneered work on PEC water splitting with a TiO<sup>2</sup>

source. Many kinds of materials including TiO<sup>2</sup>

splitting cell. It possesses a narrow bandgap energy (*E*<sup>g</sup>

using α-Fe<sup>2</sup>

O3

T. Endo, Y. Takamura, G. Lee, and P. Mele (2017).

**Table 1.** Various types of iron oxide and their physical properties (*ρ*: electrical resistivity, *T*N: Néel temperature, and *E*g : bandgap energy).

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 [4–6]. Moreover, multifunctional bismuth ferrite (BiFeO<sup>3</sup> , BFO) has been of great interest owing to its potential applications in numerous room temperature multiferroic devices [7–9]. 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 RFe<sup>2</sup> O4 (R = Ho, Y, Yb, Lu, and In) is a multilayered oxide and was discovered in the 1970s by Kimizuka et al. [11]. RFe<sup>2</sup> O4 is composed of alternating hexagonal Fe─O and R─O layers stacked along the *c*-axis, and Fe2+/Fe3+ charge order occurs in the Fe─O layers below 320 K, which is followed by magnetic ordering below ~240 K [12]. Recently, a number of studies on RFe<sup>2</sup> O4 have been stimulated by the discovery of the giant magnetoelectric response in LuFe<sup>2</sup> O4 and its application to multiferroic devices is currently the subject of extensive investigations [13, 14]. A great number of investigations on the magneto-optical (MO) properties of garnet-type ferrites (R<sup>3</sup> Fe5 O12) have been carried out for applications in the field of optical communications. They are currently recognized as the most promising materials in magnonics and related areas. Especially, they are widely used in ferromagnetic resonance experiments and magnon-based Bose-Einstein-condensates owing to their extremely low damping [15–18]. Furthermore, there has been much interest in hexaferrites, MFe12O19 (M = Ba and Sr) [19, 20], which are commonly applied in a wide variety of data storage and recording devices. One of the most favorable characteristics of the above iron oxides is their chemical stability, and they are also nontoxic. Moreover, iron and oxygen are abundant in the Earth. These features imply that iron oxides are favorable materials for applications in environmentally friendly electronics, spintronics, and magnonics. The author focuses on α-Fe<sup>2</sup> O3 commonly referred as a hematite, which is known as a promising candidate for semiconductor photoanodes for pho

toelectrochemical (PEC) water splitting [21–23]. A schematic of a PEC cell is shown in **Figure 1**. They consist of a photoactive electrode and a metal counter electrode immersed in a suitable electrolyte solution. The photogenerated electron-hole pairs are split by the electric field in the space-charge region at the surface of photoelectrodes. Since Honda and Fujishima's pioneered work on PEC water splitting with a TiO<sup>2</sup> photoelectrode [24], there has been worldwide research focused on the solar generation of hydrogen as a renewable and clean energy source. Many kinds of materials including TiO<sup>2</sup> have been investigated for their application as photoelectrodes. However, most of them are wide-gap semiconductors, and only a small fraction of the solar spectrum can be utilized by the PEC cells based on these materials. A high PEC responsivity to visible (VIS) and near-infrared (near-IR) light is required to harvest the lower energy region of the solar spectrum. From this viewpoint, α-Fe<sup>2</sup> O3 has attracted much attention because of its promising properties for application as a photoanode in a solar water splitting cell. It possesses a narrow bandgap energy (*E*<sup>g</sup> ) of 2.1 eV that allows for the absorption of up to 40% of solar spectrum. However, the reported efficiencies for PEC water splitting using α-Fe<sup>2</sup> O3 -based photoelectrodes are significantly low. This poor PEC property of α–Fe<sup>2</sup> O3

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 chemi-

**Table 1.** Various types of iron oxide and their physical properties (*ρ*: electrical resistivity, *T*N: Néel temperature, and

great interest owing to its potential applications in numerous room temperature multiferroic devices [7–9]. 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

R─O layers stacked along the *c*-axis, and Fe2+/Fe3+ charge order occurs in the Fe─O layers below 320 K, which is followed by magnetic ordering below ~240 K [12]. Recently, a number of stud-

tigations [13, 14]. A great number of investigations on the magneto-optical (MO) properties

communications. They are currently recognized as the most promising materials in magnonics and related areas. Especially, they are widely used in ferromagnetic resonance experiments and magnon-based Bose-Einstein-condensates owing to their extremely low damping [15–18]. Furthermore, there has been much interest in hexaferrites, MFe12O19 (M = Ba and Sr) [19, 20], which are commonly applied in a wide variety of data storage and recording devices. One of the most favorable characteristics of the above iron oxides is their chemical stability, and they are also nontoxic. Moreover, iron and oxygen are abundant in the Earth. These features imply that iron oxides are favorable materials for applications in environmentally friendly

a hematite, which is known as a promising candidate for semiconductor photoanodes for pho

O4

(R = Ho, Y, Yb, Lu, and In) is a multilayered oxide and was discovered

O12) have been carried out for applications in the field of optical

have been stimulated by the discovery of the giant magnetoelectric response in

and its application to multiferroic devices is currently the subject of extensive inves-

is composed of alternating hexagonal Fe─O and

O3

commonly referred as

, BFO) has been of

cal engineering [4–6]. Moreover, multifunctional bismuth ferrite (BiFeO<sup>3</sup>

antiferromagnet RFe<sup>2</sup>

: bandgap energy).

256 Iron Ores and Iron Oxide Materials

O4

of garnet-type ferrites (R<sup>3</sup>

ies on RFe<sup>2</sup>

LuFe<sup>2</sup> O4

*E*g

O4

Fe5

electronics, spintronics, and magnonics. The author focuses on α-Fe<sup>2</sup>

in the 1970s by Kimizuka et al. [11]. RFe<sup>2</sup>

**Figure 1.** (a) Schematic of the photoelectrochemical (PEC) cell for solar water splitting and (b) electronic band structure of the PEC cell. Adapted by permission from Springer: Correlated Functional Oxides edited by H. Nishikawa, N. Iwata, T. Endo, Y. Takamura, G. Lee, and P. Mele (2017).

can be attributed to the short diffusion length of the photogenerated holes. For α–Fe<sup>2</sup> O3 -based PEC cells, only the holes generated near the electrolyte/photoanode interface can oxidize water [25, 26]. That is, most of the photogenerated electron-hole pairs recombine before reaching the photoelectrode surface. The hematite lattice is composed of an alternating stack of Fe bilayers and O layers along c-axis as illustrated in **Figure 2**. Spins of Fe3+ ions within each bilayer are parallel, whereas adjacent Fe bilayers have opposite spins. 3d electrons of Fe can move by hopping via the change in the Fe2+/Fe3+ valence within the Fe bilayers, whereas the exchange of electrons between neighboring Fe bilayers is spin forbidden [27–29]. Therefore, the orientation of a highly conducting (001) plane vertical to the substrate will facilitate the collection of photogenerated carriers and suppress their recombination. The author employed a Ta-doped SnO<sup>2</sup> (TTO) layer grown on α–Al<sup>2</sup> O3 (110) single-crystal substrates for the epitaxial growth of α–Fe<sup>2</sup> O3 films along the [110] direction. As shown in **Figure 3(a)**, the SnO<sup>2</sup> (101) plane matches the α–Fe<sup>2</sup> O3 (110) plane with a lattice mismatch of approximately 1.3%, which is favorable for the epitaxial growth of hematite along the [110] direction on the SnO<sup>2</sup> (101)/α–Al<sup>2</sup> O3 (101) substrate [30]. Another issue regarding α-Fe<sup>2</sup> O3 concerns its low responsivity to near-IR light. It is well known that the photocurrent in α-Fe<sup>2</sup> O3 is maximized at a wavelength (λ) of ~350 nm, exhibits a significant decrease with increasing λ, and approaches zero at approximately 600 nm, corresponding to its bandgap [31]. An improvement of the PEC responsivity in VIS and near-IR regions by controlling the bandgap would be useful for solar energy harvesting. Unfortunately, there exist few reports on such bandgap engineering in α-Fe<sup>2</sup> O3 . From this viewpoint, the author focused on Rh-substituted α-Fe<sup>2</sup> O3 (FRO). α-Rh<sup>2</sup> O3 has a bandgap *E*<sup>g</sup> of

1.2–1.4 eV [32] and the same corundum-type crystal structure as α-Fe<sup>2</sup>

O3

and SnO<sup>2</sup>

O3

O3

O3

a schematic of the band alignment of FRO [35, 36]. α-Fe<sup>2</sup>

2g) band in α-Rh<sup>2</sup>

gap of α-Fe<sup>2</sup>

O3

**Figure 3.** (a) Top: Crystal structures of α-Fe<sup>2</sup>

in-plane atomic configuration of α-Fe<sup>2</sup>

orbitals. The Rh 4*d* (*t*

state. In contrast, the bandgap of α-Rh<sup>2</sup>

with their electronic structures.

**2. Experimental procedures**

O3

Bandgap-Engineered Iron Oxides for Solar Energy Harvesting

http://dx.doi.org/10.5772/intechopen.73227

259

originates from the ligand field splitting of the Rh 4*d*

lies near the O 2*p* band, and they effectively hybridize

is a charge transfer-type insulator

(rutile type). Bottom: Schematic showing the

could be narrowed by Rh substitution in the films [33, 34]. **Figure 3(b)** shows

with a bandgap between the Fe 3d state (upper Hubbard band) and the fully occupied O 2*p*

(corundum type) and SnO<sup>2</sup>

at the valence band maximum (VBM) [32, 37, 38]. In this chapter, the PEC characteristics of FRO photoanodes fabricated using pulsed laser deposition (PLD) are discussed in association

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/cm<sup>2</sup>

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

O3

. [Copyright (2012), The Japan Society of Applied Physics].

. Therefore, the band-

.

**Figure 2.** (a) Corundum-type crystal structure of *α*-Fe<sup>2</sup> O3 . (b) The transport of electron in the Fe 3d band is schematically illustrated. (c) Antiferromagnetic spin coupling in α-Fe<sup>2</sup> O3 . [Copyright (2014), The Japan Society of Applied Physics].

Bandgap-Engineered Iron Oxides for Solar Energy Harvesting http://dx.doi.org/10.5772/intechopen.73227 259

**Figure 3.** (a) Top: Crystal structures of α-Fe<sup>2</sup> O3 (corundum type) and SnO<sup>2</sup> (rutile type). Bottom: Schematic showing the in-plane atomic configuration of α-Fe<sup>2</sup> O3 and SnO<sup>2</sup> . [Copyright (2012), The Japan Society of Applied Physics].

1.2–1.4 eV [32] and the same corundum-type crystal structure as α-Fe<sup>2</sup> O3 . Therefore, the bandgap of α-Fe<sup>2</sup> O3 could be narrowed by Rh substitution in the films [33, 34]. **Figure 3(b)** shows a schematic of the band alignment of FRO [35, 36]. α-Fe<sup>2</sup> O3 is a charge transfer-type insulator with a bandgap between the Fe 3d state (upper Hubbard band) and the fully occupied O 2*p* state. In contrast, the bandgap of α-Rh<sup>2</sup> O3 originates from the ligand field splitting of the Rh 4*d* orbitals. The Rh 4*d* (*t* 2g) band in α-Rh<sup>2</sup> O3 lies near the O 2*p* band, and they effectively hybridize at the valence band maximum (VBM) [32, 37, 38]. In this chapter, the PEC characteristics of FRO photoanodes fabricated using pulsed laser deposition (PLD) are discussed in association with their electronic structures.
