**4. Optical properties**

bottom electrodes, viz., TTO deposited onto an α-Al<sup>2</sup>

fluorine-doped SnO<sup>2</sup>

260 Iron Ores and Iron Oxide Materials

in the FRO films.

**3. Crystal structures**

The XRD 2 theta-omega scan of the FeRhO<sup>3</sup>

**Figure 4.** (a) XRD 2θ/ω scan of the FRO film (*x* = 1.0) grown on TTO/*α*-Al<sup>2</sup>

Japan Society of Applied Physics].

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(FTO) formed on a soda-lime glass substrate. An Fe2-*<sup>x</sup>*

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)

sample, broad peaks are observed at 35 and 75°, which are ascribed to the (110) and (220)

(110) substrate and polycrystalline

films is shown in **Figure 4(a)**. For the as-deposited

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(*x* = 0.2) grown on the FTO substrate. (c) Magnified image of the XRD pattern of (b). [Copyright (2012 and 2014), The

(110). (b) XRD pattern of the FRO film

Rh*<sup>x</sup>* O3

(*x* = 0.0–2.0)

**Figure 5(a)** shows the light absorption spectra of the films. The fundamental absorption edge of α-Fe<sup>2</sup> O3 is related to charge transfer from O 2*p* states to the upper Hubbard band promoted by photons [denoted by *T*CT in **Figure 3(b)**]. For films with a higher Rh content, a broadband appears at 1.5–4.5 eV that is possibly related to α-Rh<sup>2</sup> O3 . The optical transition in α-Rh<sup>2</sup> O3 is unclear; its absorption edge is considered to be associated with the *d*-*d* transition of Rh3+, judging from the bandgap structure [31, 39]. **Figure 5(b)** shows the values of an indirect bandgap (*E*<sup>g</sup> ), which were derived from the Tauc relation, *αhν* ∝ (*hν* − *E*<sup>g</sup> )2 (*α*: optical absorption coefficient and *hν*: photon energy). *E*<sup>g</sup> decreases as the content of Rh in the films increases according to the above discussion. Value of *E*<sup>g</sup> of 2.1 and 1.2 eV were obtained for α-Fe<sup>2</sup> O3 and α-Rh<sup>2</sup> O3 , respectively. These values are almost identical to those reported for polycrystalline films [40].

**Figure 5.** (a) Optical absorption coefficients as a function of the wavelength for FRO films on α-Al<sup>2</sup> O3 (110) substrate at 298 K. For clarity, each spectrum is offset, with a spacing scaled to the composition. The peaks of α-Fe<sup>2</sup> O3 (*x* = 0.0) are assigned according to Ref. 19. (b) Compositional dependence of the indirect bandgap energy *E*<sup>g</sup> and the absorption coefficients *α* at *λ* = 500 and 800 nm. The photographs of α-Fe<sup>2</sup> O3 (*x* = 0.0) and FRO (x = 0.2) films are inset. [Copyright (2012), The Japan Society of Applied Physics].

### **5. XPS spectroscopy**

The results of XPS are presented in **Figure 6**. In the spectra of Fe 2*p* core level (**Figure 6(a)**), main peaks are at around 710 and 723 eV and are assigned to Fe 2*p*2/3 and 2*p*1/3 orbitals of α-Fe<sup>2</sup> O3 , respectively [41–43]. These core level peaks become weaker as the Rh content increases. In turn, new distinct peaks appears at approximately 310 and 315 eV, which are assigned to Rh 3*d*3/2 and 3*d*5/2 orbitals, respectively [44, 45]. As seen in **Figure 6(c)**, the VBM of α-Fe<sup>2</sup> O3 is estimated to be 0.65 eV. In contrast, the VBM of α-Rh<sup>2</sup> O3 is located near the Fermi level (~0.0 eV). Three distinct peaks are observed in the VB spectrum of the films. The bands centered at 1, 2, and 3 eV in the VB spectrum of α-Fe<sup>2</sup> O3 are assigned to the Fe 3*e*<sup>g</sup> , 2*t*<sup>g</sup> , and 2*e*<sup>g</sup> orbitals, respectively [46]. The crystal field splitting energy between the Fe 3*e*<sup>g</sup> and 2*t*<sup>g</sup> orbitals was estimated to be 2.5 eV in a previous study [47], which agrees with the experimental value (2.4 eV) well. In the VB spectrum of α-Rh<sup>2</sup> O3 , three distinct peaks similarly appear. Unfortunately, there are hardly any reports of VB spectra for α-Rh<sup>2</sup>

O 2*p* valence band with the Rh *t*

tocurrent is 2.87 μA/cm<sup>2</sup>

(*λ* = 700–900 nm, 640 mW/cm<sup>2</sup>

*λ* = 600–900 nm (d) *I*-*V* curves of α-Fe<sup>2</sup>

1 eV is attributed to the *t*2g orbitals of the RhO<sup>6</sup>

**6. Photoelectrochemical properties**

by ultraviolet photoemission spectroscopy (UPS) for ZnRh<sup>2</sup>

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dependences on the Rh content, as shown in **Figure 6(d)**. In addition, the change in the VBM (0.7 eV) for *x* = 1.0 is close to the bandgap decrease (0.8 eV) for *x* = 1.0. From these results, we can conclude that the bandgap decrease by Rh substitution is due to the hybridization of the

are assigned to the Rh 4*d*, 5 *s*, and 5*p* mixed states [48]. The VBM and *E*<sup>g</sup>

2g band at the VBM.

The current-potential curves of the films are shown in **Figure 7(a)** and **(b)**. For α-Fe<sup>2</sup>

**Figure 7.** Chopped *I*-*V* curves under illumination with (a) VIS light (*λ* = 400–700 nm, 100 mW/cm<sup>2</sup>

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Ag/AgCl in an aqueous electrolytic solution containing 1.0 M NaOH. The inset shows the magnified IPCE spectra at

films deposited on TTO/α-Al<sup>2</sup>

(e) Mott-Schottky plots for FRO films. [Copyright (2012 and 2014), The Japan Society of Applied Physics].

**Figure 7(a)**, the VIS photocurrent is remarkably increased after Rh substitution (17.3 μA/cm<sup>2</sup>

0.5 V for *x* = 0.2). The effect of Rh substitution becomes more obvious with near-IR irradiation

. However, by comparing with results obtained

Bandgap-Engineered Iron Oxides for Solar Energy Harvesting

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[48, 49], the peak observed at

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

. The peaks at 2 and 3 eV

exhibit a similar

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, the pho-

) and (b) NIR light

(110) (blue) and FTO glass (black) substrates.

at

263

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octahedra in *α*-Rh<sup>2</sup>

at 0.5 V under irradiation with VIS light (λ = 400–700 nm). As shown in

) for *x* = 0.0 and 0.2. (c) IPCE as a function of wavelength for the FRO films at 0.55 V vs.

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**Figure 6.** XPS spectra of (a) Fe 2p and (b) Rh 3d core levels for FRO films. (c) Valence band XPS spectra of FRO films. The inset shows the enlarged VB spectra near the Fermi level. (d) Compositional dependence of VBM and bandgap energy. [Copyright (2012), The Japan Society of Applied Physics].

hardly any reports of VB spectra for α-Rh<sup>2</sup> O3 . However, by comparing with results obtained by ultraviolet photoemission spectroscopy (UPS) for ZnRh<sup>2</sup> O4 [48, 49], the peak observed at 1 eV is attributed to the *t*2g orbitals of the RhO<sup>6</sup> octahedra in *α*-Rh<sup>2</sup> O3 . The peaks at 2 and 3 eV are assigned to the Rh 4*d*, 5 *s*, and 5*p* mixed states [48]. The VBM and *E*<sup>g</sup> exhibit a similar dependences on the Rh content, as shown in **Figure 6(d)**. In addition, the change in the VBM (0.7 eV) for *x* = 1.0 is close to the bandgap decrease (0.8 eV) for *x* = 1.0. From these results, we can conclude that the bandgap decrease by Rh substitution is due to the hybridization of the O 2*p* valence band with the Rh *t* 2g band at the VBM.
