**6. Photoelectrochemical properties**

**5. XPS spectroscopy**

262 Iron Ores and Iron Oxide Materials

mated to be 0.65 eV. In contrast, the VBM of α-Rh<sup>2</sup>

and 3 eV in the VB spectrum of α-Fe<sup>2</sup>

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

In the VB spectrum of α-Rh<sup>2</sup>

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>

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

Three distinct peaks are observed in the VB spectrum of the films. The bands centered at 1, 2,

to be 2.5 eV in a previous study [47], which agrees with the experimental value (2.4 eV) well.

**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.

O3

are assigned to the Fe 3*e*<sup>g</sup>

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

tively [46]. The crystal field splitting energy between the Fe 3*e*<sup>g</sup>

O3

O3 ,

O3 is esti-

orbitals, respec-

orbitals was estimated

is located near the Fermi level (~0.0 eV).

, and 2*e*<sup>g</sup>

, 2*t*<sup>g</sup>

and 2*t*<sup>g</sup>

, three distinct peaks similarly appear. Unfortunately, there are

The current-potential curves of the films are shown in **Figure 7(a)** and **(b)**. For α-Fe<sup>2</sup> O3 , the photocurrent is 2.87 μA/cm<sup>2</sup> 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/cm<sup>2</sup> at 0.5 V for *x* = 0.2). The effect of Rh substitution becomes more obvious with near-IR irradiation

**Figure 7.** Chopped *I*-*V* curves under illumination with (a) VIS light (*λ* = 400–700 nm, 100 mW/cm<sup>2</sup> ) and (b) NIR light (*λ* = 700–900 nm, 640 mW/cm<sup>2</sup> ) for *x* = 0.0 and 0.2. (c) IPCE as a function of wavelength for the FRO films at 0.55 V vs. Ag/AgCl in an aqueous electrolytic solution containing 1.0 M NaOH. The inset shows the magnified IPCE spectra at *λ* = 600–900 nm (d) *I*-*V* curves of α-Fe<sup>2</sup> O3 films deposited on TTO/α-Al<sup>2</sup> O3 (110) (blue) and FTO glass (black) substrates. (e) Mott-Schottky plots for FRO films. [Copyright (2012 and 2014), The Japan Society of Applied Physics].

(λ = 700–900 nm). As shown in **Figure 7(b)**, for *x* = 0.2, a near-IR photocurrent is evidently observed, whereas a photocurrent is hardly detected for the α-Fe<sup>2</sup> O3 film. These improved PEC properties of the FRO films may be caused by the increased light absorption in these wavelength regions. Furthermore, the electrical properties of the films also affect their PEC performance. The electrical conductivity for *x* = 0.2 (*σ* = 3.8 × 10−4 Ω−1 cm−1 at 300 K) is significantly larger than that for *x* = 0.0 (*σ* = 2.6 × 10−6 Ω−1 cm−1 at 300 K). This is possibly due to the extended profile of the Rh 4*d* state [50]. Thus, the improved electrical conductivity possibly causes an increased photocurrent by lowering the recombination rate of the photocarriers as in the case for Ti- or Si-doped α-Fe<sup>2</sup> O3 [22, 38, 40]. In **Figure 7(c)**, the spectra of the incident photon-to-current efficiency (IPCE) are shown. The IPCE was estimated using the following relationship: *IPCE* (%) = 100 × [*hc*/*e*] × *I*/ [*P* × *λ*], where *h*, *c*, *e*, *I*(mA/cm<sup>2</sup> ), and *P*(mW/cm<sup>2</sup> ) denote the Planck constant, the light velocity, the elementary charge, the photocurrent, and the power of the illumination per unit area, respectively [22]. The IPCE for *x* = 0.1 and 0.2 is much higher than that of α-Fe<sup>2</sup> O3 in the 340–850 nm wavelength region. For α-Fe<sup>2</sup> O3 , the IPCE decreases to zero when the wavelength exceeds 610 nm, corresponding to its bandgap. On the other hand, for *x* = 0.2, the IPCE is 2.35% at 610 nm and gradually decreased to 0.11% at 850 nm as shown in the inset of **Figure 7(c)**. The IPCE decreases drastically when *x* exceeds 0.2 as shown in **Figure 7(c)**, and a photocurrent is hardly detected for *x* ≥ 0.75 in the 340–900 nm wavelength region. These results possibly reflect the drastic change in the optical transition process caused by Rh substitution. On the one hand, the photogenerated carriers in α-Fe<sup>2</sup> O3 diffuse through the bands related to the Fe 3*d* and O 2*p* states [22]. On the other hand, the recombination probability of the carriers generated in α-Rh<sup>2</sup> O3 following the *d*-*d* transition in Rh3+ is significantly high [32, 51]. This nature impairs the PEC performance for a higher Rh content. We note that that the rate of decrease in *E*<sup>g</sup> is drastically decreased when *x* exceeds 0.2. This result suggests that the band-edge electronic structure is not strongly influenced by Rh substitution; therefore, the *d*-*d* transition predominantly occurs for *x* > 0.2. The IPCE peak wavelength shifts from 350 to 430 nm as Rh content increases from *x* = 0.0 to 0.2. This is a desirable feature for energy harvesting, because the peak of the solar spectrum is at ~475 nm. The PEC properties of the polycrystalline film (*x* = 0.2) are shown in **Figure 7(d)**. The photocurrent of the single-crystal FRO grown on a TTO/Al<sup>2</sup> O3 (110) substrate is higher than that of the polycrystalline FRO grown on FTO glass. This result can be explained by the electronic transport properties of the films. The conductivity of the (110)-oriented single-crystalline film along the out-of-plane direction (*σ* = 3.4 × 10−4 Ω−1 cm−1) is much larger than that of the polycrystalline film (*σ* = 8.8 × 10−6 Ω−1 cm−1). The improvement in the electrical conductivity in the single-crystal films may accelerate the collection of photocarriers, resulting in their enhanced PEC properties. In **Figure 7(e)**, the Mott-Schottky plots are shown. The density of donors *N* is expressed as follows:

$$N = \left[ \Im / (e\_0 \varepsilon\_o \varepsilon) \right] \left[ d(1/\mathbb{C}^2) / dV \right]^{-1} \tag{1}$$

contrast, in the FRO films, the valence of the Rh ions is +3, and hence, the content of Fe2+ does not increase after Rh substitution. Nevertheless, the conductivity for *x* = 0.2 is two orders of magnitude larger than that for *x* = 0.0. Therefore, it is considered that the carrier mobility is remarkably increased after Rh substitution probably owing to the extended nature of the Rh 4*d* states.

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*d* state with

lower Rh contents. Moreover, the PEC properties were improved by the control of crystal growth condition. These results will be utilized in the development of high-efficiency solar

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.

Center for Spintronics Research Network (CSRN), Graduate School of Engineering,

[1] Bibes M, Berthelemy A. Oxide Spintronics. IEEE Transactions on Electron Devices.

[2] Zhang Z, Satpathy S. Electron states, magnetism, and the Verwey transition in magnetite. Physical Review B. 1991;**44**:13319-13331. DOI: 10.1103/PhysRevB.44.13319

[3] Seki M, Hossain AKM, Kawai T, Tabata H. High-temperature cluster glass state and pho-

O4

films. Journal of Applied Physics. 2005;

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photoelectrodes were successfully fabricated using a pulsed

. The photocurrent was significantly enhanced for

Bandgap-Engineered Iron Oxides for Solar Energy Harvesting

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

265

**7. Summary**

The Rh-substituted α-Fe<sup>2</sup>

**Acknowledgements**

**Author details**

Munetoshi Seki

**References**

O3

the O2*p*–Fe 3*d* states at the VBM of α-Fe<sup>2</sup>

energy conversion devices based on iron oxides.

Address all correspondence to: m-seki@ee.t.u-tokyo.ac.jp

2007;**54**:1003-1023. DOI: 10.1109/TED.2007.894366

tomagnetism in Zn and Ti-substituted NiFe<sup>2</sup>

**97**:083541(1)-(6). DOI: 10.1063/1.1863422

The University of Tokyo, Tokyo, Japan

where *e*<sup>0</sup> represents an electron charge, and *ε*<sup>0</sup> and *ε* are the vacuum and relative electric permittivities, respectively. By employing the reported value of ε = 80 for hematite, the donor densities are calculated to be 4.2 × 10<sup>17</sup> (*x* = 0.2) and 5.3 × 10<sup>17</sup> cm−3 for *x* = 0.0 (hematite) and 0.2, respectively. Thus, the donor density does not significantly change after Rh substitution, in contrast to the case for Ti- or Si-doped α-Fe<sup>2</sup> O3 . It is considered that the number of Fe2+ ions, which acts as electron donors, increases when Fe3+ is substituted with Ti4+ or Si4+ owing to the charge neutrality. In contrast, in the FRO films, the valence of the Rh ions is +3, and hence, the content of Fe2+ does not increase after Rh substitution. Nevertheless, the conductivity for *x* = 0.2 is two orders of magnitude larger than that for *x* = 0.0. Therefore, it is considered that the carrier mobility is remarkably increased after Rh substitution probably owing to the extended nature of the Rh 4*d* states.
