**2. Experimental procedures**

can be attributed to the short diffusion length of the photogenerated holes. For α–Fe<sup>2</sup>

O3

the epitaxial growth of hematite along the [110] direction on the SnO<sup>2</sup>

films along the [110] direction. As shown in **Figure 3(a)**, the SnO<sup>2</sup>

ing. Unfortunately, there exist few reports on such bandgap engineering in α-Fe<sup>2</sup>

O3

O3

O3

O3

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 harvest-

SnO<sup>2</sup>

α–Fe<sup>2</sup> O3

the α–Fe<sup>2</sup>

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258 Iron Ores and Iron Oxide Materials

(TTO) layer grown on α–Al<sup>2</sup>

strate [30]. Another issue regarding α-Fe<sup>2</sup>

**Figure 2.** (a) Corundum-type crystal structure of *α*-Fe<sup>2</sup>

illustrated. (c) Antiferromagnetic spin coupling in α-Fe<sup>2</sup>

is well known that the photocurrent in α-Fe<sup>2</sup>

viewpoint, the author focused on Rh-substituted α-Fe<sup>2</sup>

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

(110) plane with a lattice mismatch of approximately 1.3%, which is favorable for

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(110) single-crystal substrates for the epitaxial growth of

(FRO). α-Rh<sup>2</sup>

concerns its low responsivity to near-IR light. It

is maximized at a wavelength (λ) of ~350 nm,

O3

. (b) The transport of electron in the Fe 3d band is schematically

. [Copyright (2014), The Japan Society of Applied Physics].

O3

(101) plane matches

(101) sub-

. From this

of

O3

O3

has a bandgap *E*<sup>g</sup>

(101)/α–Al<sup>2</sup>


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 bottom electrodes, viz., TTO deposited onto an α-Al<sup>2</sup> O3 (110) substrate and polycrystalline fluorine-doped SnO<sup>2</sup> (FTO) formed on a soda-lime glass substrate. An Fe2-*<sup>x</sup>* Rh*<sup>x</sup>* O3 (*x* = 0.0–2.0) pellet prepared by a solid-state reaction was used as a target for PLD. The growth temperature was kept at 700 and 800°C for the FRO and TTO films, respectively. The crystal structures of the samples were confirmed using X-ray diffraction (XRD). In the PEC measurement, the *I*-*V* properties were measured using an electrochemical analyzer under the illumination of Xe lamp (500 W). Optical measurements were conducted using a Vis-UV spectrometer. X-ray photoemission spectroscopy (XPS) was performed to evaluate the structure of the valence band (VB) in the FRO films.

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,

**Figure 5(a)** shows the light absorption spectra of the films. The fundamental absorption

promoted by photons [denoted by *T*CT in **Figure 3(b)**]. For films with a higher Rh content, a

tion of Rh3+, judging from the bandgap structure [31, 39]. **Figure 5(b)** shows the values of an

broadband appears at 1.5–4.5 eV that is possibly related to α-Rh<sup>2</sup>

**Figure 5.** (a) Optical absorption coefficients as a function of the wavelength for FRO films on α-Al<sup>2</sup>

assigned according to Ref. 19. (b) Compositional dependence of the indirect bandgap energy *E*<sup>g</sup>

298 K. For clarity, each spectrum is offset, with a spacing scaled to the composition. The peaks of α-Fe<sup>2</sup>

O3

absorption coefficient and *hν*: photon energy). *E*<sup>g</sup>

coefficients *α* at *λ* = 500 and 800 nm. The photographs of α-Fe<sup>2</sup>

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

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increases according to the above discussion. Value of *E*<sup>g</sup>

is related to charge transfer from O 2*p* states to the upper Hubbard band

, respectively. These values are almost identical to those reported for

is unclear; its absorption edge is considered to be associated with the *d*-*d* transi-

), which were derived from the Tauc relation, *αhν* ∝ (*hν* − *E*<sup>g</sup>

O3

Bandgap-Engineered Iron Oxides for Solar Energy Harvesting

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

decreases as the content of Rh in the films

of 2.1 and 1.2 eV were obtained for

O3

(*x* = 0.0) and FRO (x = 0.2) films are inset. [Copyright

O3

(110) substrate at

and the absorption

(*x* = 0.0) are

. The optical transition

)2

(*α*: optical

261

as shown in **Figure 4(b)** and **(c)**.

**4. Optical properties**

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edge of α-Fe<sup>2</sup>

O3

indirect bandgap (*E*<sup>g</sup>

and α-Rh<sup>2</sup>

polycrystalline films [40].

in α-Rh<sup>2</sup>

α-Fe<sup>2</sup> O3
