**7. Summary**

(λ = 700–900 nm). As shown in **Figure 7(b)**, for *x* = 0.2, a near-IR photocurrent is evidently

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

are shown. The IPCE was estimated using the following relationship: *IPCE* (%) = 100 × [*hc*/*e*] × *I*/

the elementary charge, the photocurrent, and the power of the illumination per unit area, respec-

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

*d*-*d* transition in Rh3+ is significantly high [32, 51]. This nature impairs the PEC performance for

*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 photocur-

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* = [2/(*e*<sup>0</sup> *ε*<sup>0</sup> *ε*)] [*d*(1/*C*2)/*dV*]−<sup>1</sup> (1)

tivities, 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

tron donors, increases when Fe3+ is substituted with Ti4+ or Si4+ owing to the charge neutrality. In

O3

), and *P*(mW/cm<sup>2</sup>

other hand, the recombination probability of the carriers generated in α-Rh<sup>2</sup>

a higher Rh content. We note that that the rate of decrease in *E*<sup>g</sup>

rent of the single-crystal FRO grown on a TTO/Al<sup>2</sup>

represents an electron charge, and *ε*<sup>0</sup>

O3

tively [22]. The IPCE for *x* = 0.1 and 0.2 is much higher than that of α-Fe<sup>2</sup>

O3

[22, 38, 40]. In **Figure 7(c)**, the spectra of the incident photon-to-current efficiency (IPCE)

O3

) denote the Planck constant, the light velocity,

, the IPCE decreases to zero when the wavelength exceeds 610 nm,

diffuse through the bands related to the Fe 3*d* and O 2*p* states [22]. On the

O3

O3

is drastically decreased when

(110) substrate is higher than that of the

and *ε* are the vacuum and relative electric permit-

. It is considered that the number of Fe2+ ions, which acts as elec-

film. These improved PEC

in the 340–850 nm

following the

observed, whereas a photocurrent is hardly detected for the α-Fe<sup>2</sup>

α-Fe<sup>2</sup> O3

[*P* × *λ*], where *h*, *c*, *e*, *I*(mA/cm<sup>2</sup>

264 Iron Ores and Iron Oxide Materials

wavelength region. For α-Fe<sup>2</sup>

O3

carriers in α-Fe<sup>2</sup>

where *e*<sup>0</sup>

case for Ti- or Si-doped α-Fe<sup>2</sup>

The Rh-substituted α-Fe<sup>2</sup> O3 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*d* state with the O2*p*–Fe 3*d* states at the VBM of α-Fe<sup>2</sup> O3 . The photocurrent was significantly enhanced for 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 energy conversion devices based on iron oxides.
