Paving the road toward the use of β-Fe₂O₃ in solar water splitting: Raman identification, phase transformation and strategies for phase stabilization

Abstract

Although β-Fe₂O₃ has a high theoretical solar-to-hydrogen efficiency because of its narrow band gap, the study of β-Fe₂O₃ photoanodes for water splitting is elusive as a result of their metastable nature. Raman identification of β-Fe₂O₃ is theoretically and experimentally investigated in this study for the first time, thus clarifying the debate about its Raman spectrum in the literature. Phase transformation of β-Fe₂O₃ to α-Fe₂O₃ was found to potentially take place under laser and electron irradiation as well as annealing. Herein, phase transformation of β-Fe₂O₃ to α-Fe₂O₃ was inhibited by introduction of Zr doping, and β-Fe₂O₃ was found to withstand a higher annealing temperature without any phase transformation. The solar water splitting photocurrent of the Zr-doped β-Fe₂O₃ photoanode was increased by 500% compared to that of the pure β-Fe₂O₃ photoanode. Additionally, Zr-doped β-Fe₂O₃ exhibited very good stability during the process of solar water splitting. These results indicate that by improving its thermal stability, metastable β-Fe₂O₃ film is a promising photoanode for solar water splitting.

The use of element doping to prevent the phase transition of metastable semiconductors has been approved for the application of photoelectrochemcial hydrogen production via water splitting for the first time.

INTRODUCTION

Since the concept of a hydrogen economy was introduced, solar hydrogen production using photoelectrochemical (PEC) or photocatalytic water splitting has occupied a very important position in the artificial utilization of inexhaustible solar energy [1–6]. Solar water splitting for hydrogen production has thereby attracted intensive and ever-increasing interest from many researchers [7–15]. The solar-to-hydrogen efficiency of PEC water splitting reached over 19% using III-V multi-junction semiconductors (including In and Ga) prepared by molecular beam epitaxial growth [16]. Unfortunately, there is a lack of low-cost, environmentally friendly, efficient and stable photoelectrodes for PEC water splitting [17–22], thus hindering application of PEC cells for hydrogen production.

In recent decades, iron-based semiconductors with narrow band gaps (e.g. α-Fe₂O₃) have been considered as promising photoanode materials with respect to nontoxicity, cost, stability and theoretical solar-to-hydrogen efficiency [23–34]. Recently, a β-Fe₂O₃ semiconductor with a direct band gap of 1.9 eV was reported by the present authors as a water splitting photoanode under Air Mass 1.5 Global spectrum (AM 1.5 G, 100 mW cm^–2) illumination [35]. Theoretically, a β-Fe₂O₃ photoanode for solar water splitting may exhibit a solar-to-hydrogen efficiency of 20.9% because of its narrow band gap of 1.9 eV, whereas its solar photocurrent density is still very low [35]. The phase transformation of metastable β-Fe₂O₃ to mature α-Fe₂O₃ may occur initially at the surface or interface during fabrication of photoanode films because metastable β-Fe₂O₃ cannot withstand the high temperature of annealing.

Phase identification of β-Fe₂O₃ is very important to avoid the effects of impurities (e.g. α-Fe₂O₃) during the process of solar water splitting. Phase characterization of inorganic materials is frequently achieved with such methods as X-ray diffraction (XRD), neutron diffraction, electron diffraction, Raman scattering spectroscopy and infrared absorption spectroscopy. Among these methods, Raman spectroscopy is fast and highly sensitive toward material surface structural information. However, Raman spectra of β-Fe₂O₃ (including Raman peak position and shape) reported in the literature are often contradictory [36,37]. Thus, this study aims to clarify the debates surrounding the Raman spectra of β-Fe₂O₃.

The wavelength and power of the excitation laser should be chosen carefully during the process of Raman detection. An appropriate wavelength of laser should be selected to enhance the Raman sensibility and minimize the fluorescence emission of the samples. It is equally important that the power of the laser be examined cautiously to avoid the influences of thermal effects caused by the laser, especially for those materials with poor thermal stability. For example, phase transformation of ϵ-Fe₂O₃ and γ-Fe₂O₃ may take place under laser irradiation [38,39], and β-Fe₂O₃ may be converted into α-Fe₂O₃ upon heat treatment [35,40]. Therefore, low-power laser irradiation is required to avoid phase transformation of the β-Fe₂O₃ samples.

In this study, the influence of laser wavelength and irradiation power on phase transformation of β-Fe₂O₃ to α-Fe₂O₃ was investigated, and the Raman vibrational spectrum of β-Fe₂O₃ was clarified. After being doped with Zr, the particle-assembled β-Fe₂O₃ photoanodes can withstand higher annealing temperatures during the post-treatment process. Therefore, the solar water splitting photocurrent density for the particle-assembled Zr-doped β-Fe₂O₃ photoanodes was improved significantly to 1.2 mA cm^–2, which is five times greater than that of pure β-Fe₂O₃ photoanodes. Thus, this study suggests metastable β-Fe₂O₃ films as promising photoanode materials for solar water splitting.

RESULTS AND DISCUSSION

β-Fe₂O₃ belongs to the space group , T_h⁷. Irreducible representations of the Γ-point phonon modes for β-Fe₂O₃ are shown in Supplementary Table S1. Factor group analysis of β-Fe₂O₃ predicts 49 phonon modes, 22 and 17 of which are Raman active modes and infrared active modes (Supplementary Table S2), respectively. Some Raman vibrational modes of β-Fe₂O₃ were observed in previous studies, whereas Rahman and coworkers reported different Raman spectra for β-Fe₂O₃ [36,37]. To clarify the Raman vibrational modes of β-Fe₂O₃, the Raman scatter spectrum of β-Fe₂O₃ was verified, as shown in Supplementary Fig. S1. Twelve clear vibrational peaks of β-Fe₂O₃ were observed at 158, 169, 234, 258, 274, 314, 328, 368, 386, 397, 522 and 635 cm^–1 using the 785 nm laser of 0.4 W. These peaks can clearly be distinguished from the Raman vibration peaks of α-Fe₂O₃ (227, 246, 293, 300, 411, 499, and 613 cm^–1). Compared to previous results from Liang and van de Krol [36], four new vibrational peaks at 258, 397, 522 and 635 cm^–1 were observed, whereas the vibrational peak at 208 cm^–1 was not observed.

The theoretical calculations and experimental values of Raman vibrational peaks are listed in Supplementary Table S3. Among the 22 vibrational modes predicted by the theoretical calculations, only 12 significant peaks were observed in the experiments. Three weak vibrational modes are marked with asterisks. The M10 (383 cm^–1) and M11 (389 cm^–1) modes merge into one peak (386 cm^–1) in the experimental spectrum. As given in Supplementary Fig. S1, typical peaks corresponding to vibrational modes 158 cm^–1 (T_g), 274 cm^–1 (T_g), 368 cm^–1 (T_g) and 386 cm^–1 (E_g+T_g) can be used as a characteristic spectrum for identifying the phase of β-Fe₂O₃. In addition, the theoretically calculated infrared vibrational peaks and the experimental infrared absorption spectrum are also given in Supplementary Table S4 and Supplementary Fig. S2, respectively.

The metastable β-Fe₂O₃ phase may be converted to the mature α-Fe₂O₃ phase upon laser irradiation of different wavelengths and power. Figure 1 shows the effects of laser wavelength on the Raman spectrum of β-Fe₂O₃. The Raman peaks at 227, 293, 411, 499 and 613 cm^–1 belonging to α-Fe₂O₃ appeared after irradiation of the β-Fe₂O₃ samples with a 532 nm or 633 nm laser. In the case of 785 nm laser irradiation, the experimental and theoretical Raman peaks for β-Fe₂O₃ match in shape but exhibit a slight shift. These results suggest that a 785 nm laser is ideal for detecting the Raman spectra of metastable β-Fe₂O₃.

Figure 1.

Theoretical calculation and experimental measurement of Raman spectra of β-Fe₂O₃ samples excited by a 0.4 mW laser at 785, 633 and 532 nm. The Raman spectrum of α-Fe₂O₃ is listed as a reference.

Raman spectra mapping images shown in Fig. 2 also indicate phase transformation of the particle-assembled β-Fe₂O₃ films without any necking treatment, in which the α-Fe₂O₃ phase appears when the β-Fe₂O₃ film samples are irradiated with a 10 mW laser of 532 and 633 nm. Scanning electron microscope (SEM) images also suggest this phase transformation. Even when β-Fe₂O₃ was irradiated with a laser of 785 nm with 10 mW power, slight damage of the β-Fe₂O₃ films was observed.

Figure 2.

Raman mapping and SEM images of particle-assembled β-Fe₂O₃ films excited by a 0.4 mW laser at 785 nm after irradiation by a 10 mW laser with various wavelengths: (a-c) 532 nm, (d-f) 633 nm and (g-i) 785 nm. (a, d, g) α-Fe₂O₃; (b, e, h) β-Fe₂O₃.

Additionally, the effects of laser power on the Raman spectrum of β-Fe₂O₃ were investigated. When the power of the 785 nm laser is increased, a phase change of β-Fe₂O₃ to α-Fe₂O₃ possibly occurs (Fig. 3). After β-Fe₂O₃ is irradiated by 0.4 mW 785 nm laser for an extended period of 500 s, there is no signal corresponding to α-Fe₂O₃. A significant phase transformation occurs with as little as 2 s irradiation with a 4 mW 785 nm laser. As the laser power increases and the exposure time is prolonged, all the β-Fe₂O₃ gradually converts to α-Fe₂O₃.

Figure 3.

Raman spectra of β-Fe₂O₃ under different power laser irradiations of 785 nm.

Figure 4 shows Raman spectra mapping and SEM images of the particle-assembled β-Fe₂O₃ films without any necking treatment after laser illumination of 785 nm. When the β-Fe₂O₃ films were irradiated with 785 nm laser at 0.4 W power, no α-Fe₂O₃ signal appeared. With the increasing power of the 785 nm laser, the signals of α-Fe₂O₃ were enhanced, suggesting laser-induced phase transformation of β-Fe₂O₃ to α-Fe₂O₃. The SEM images of Fig. 4 also corroborate this result. The phase transformation of β-Fe₂O₃ to α-Fe₂O₃ is initiated by laser irradiation, likely as a result of the thermal effects of the laser. Therefore, low-power laser irradiation is required to avoid the phase transformation of metastable materials during Raman characterization to minimize its thermal effects. Thus, a 0.4 mW 785 nm laser is ideal for measuring the Raman spectra of β-Fe₂O₃.

Figure 4.

Raman mapping and SEM images of particle-assembled β-Fe₂O₃ films excited by a 0.4 mW laser of 785 nm after the irradiation of 785 nm laser with various power: (a-c) 40 mW, (d-f) 20 mW, (g-i) 10 mW, (j-l) 4 mW and (m-o) 0.4 mW. (a, d, g, j, m) α-Fe₂O₃; (b, e, h, k, n) β-Fe₂O₃.

Figure 5 shows transmission electron microscope (TEM) and selected area electron diffraction (SAED) images of the β-Fe₂O₃ samples. A simulated SAED image of α-Fe₂O₃ is shown in Fig. 5 for comparison. The measured SAED image of β-Fe₂O₃ samples (Fig. 5a) is in good agreement with the simulated SAED image of α-Fe₂O₃ (Fig. 5d) rather than the simulated SAED image of β-Fe₂O₃ samples (Fig. 5c), suggesting that the phase transformation of β-Fe₂O₃ to α-Fe₂O₃ induced by electron irradiation was also observed in this study. Note that the use of SAED to detect β-Fe₂O₃ should be done with caution. The phase transformation of β-Fe₂O₃ to α-Fe₂O₃ may be driven by laser irradiation, electron irradiation and heating (Supplementary Figs S3 and S4).

Figure 5.

SAED and TEM images of the samples. (a) Measured SAED image of the β-Fe₂O₃ sample. (b) TEM image of the β-Fe₂O₃ sample. (c) Simulated SAED image of β-Fe₂O₃. (d) Simulated SAED image of α-Fe₂O₃.

In our previous work, phase transformation of β-Fe₂O₃ to α-Fe₂O₃ occurred after annealing at 650°C [29]. Phase transformation of β-Fe₂O₃ was also proven by the appearance of the Raman peaks of α-Fe₂O₃ (Supplementary Fig. S3). In this study, Zr was introduced into the metastable β-Fe₂O₃ to inhibit phase transformation of β-Fe₂O₃ to α-Fe₂O₃. The formation energy of Zr_0.03Fe_1.97O₃ (Zr-doped β-Fe₂O₃) was calculated as −0.16 eV relative to pure β-Fe₂O₃, suggesting that Zr can be doped into the lattice of β-Fe₂O₃ to form a more stable structure. The density of states (DOS) for Zr_0.03Fe_1.97O₃ indicates that the band gap of β-Fe₂O₃ does not change obviously by doping Zr (Supplementary Fig. S5).

The XRD pattern of Zr-doped β-Fe₂O₃ (Supplementary Fig. S6) is in good agreement with β-Fe₂O₃ (JCPDS#39–0238), suggesting that there is no impurity phase after doping with Zr. The X-ray photoelectron spectroscopy in Supplementary Fig. S7 shows that Zr was doped into the sample. The absorption spectra of pure β-Fe₂O₃ and Zr-doped β-Fe₂O₃ are shown in Supplementary Fig. S8. After annealing, the band gap of pure β-Fe₂O₃ changed, whereas the band gap of Zr-doped β-Fe₂O₃ did not. In previous work, we have reported that the absorption spectrum of β-Fe₂O₃ varies after different calcining processes [35]. The change in band gap indicates that a phase transformation may occur. Indeed, after being annealed at 1023 K, the characteristic Raman peaks of α-Fe₂O₃ appeared in the Raman spectrum of the β-Fe₂O₃ sample (Supplementary Fig. S9a), indicating that there was a phase transformation. The Raman spectra of Zr-doped β-Fe₂O₃ did not change after being annealed at 1023 K (Supplementary Fig. S9b), indicating that there was no phase transformation. Note that the results of differential scanning calorimetry show that the phase transition temperature of Zr-doped β-Fe₂O₃ is clearly increased, as shown in Supplementary Fig. S10. The above results show that Zr doping effectively increases the phase transition temperature of metastable iron oxide.

SEM images of the particle-assembled β-Fe₂O₃ photoanodes with and without Zr doping are shown in Supplementary Fig. S11. Figure 6a shows a comparison of the PEC performances of the particle-assembled β-Fe₂O₃ photoanodes with and without Zr doping under AM 1.5 G irradiation (100 mW cm^–2).  The results show that the photocurrent of Zr-doped β-Fe₂O₃ photoanodes is much higher than that of pure β-Fe₂O₃ photoanodes. Zr-doped β-Fe₂O₃ photoanodes also exhibit much larger monochromatic incident photon-to-electron conversion efficiency (IPCE) than pure β-Fe₂O₃ photoanodes (Fig. 6b). Compared to the pure β-Fe₂O₃, the carrier density of Zr-doped β-Fe₂O₃ has been improved from 1.8 × 10²⁰ to 4.6 × 10²⁰ cm⁻³, which can be estimated from the Supplementary Fig. S12a. As shown in Supplementary Fig. S12b, the charge separation efficiency of Zr-doped β-Fe₂O₃ is improved by approximately four times at 1.6 V versus Reversible Hydrogen Electrode (RHE), compared to that of β-Fe₂O₃. The increase in the carrier concentration may favor the charge transport and contribute to the improvement in water splitting photocurrent of the Zr-doped β-Fe₂O₃ photoanode.

Figure 6.

(a) Photocurrent density, (b) IPCE, (c) photochemical stability at 1.6 V_RHE and (d) Faradic efficiencies of the particle-assembled β-Fe₂O₃ films with Zr doping under illumination of standard simulated sunlight (100 mW cm⁻²) in 1 M NaOH electrolyte (pH = 13.6). Reference sample: α-Fe₂O_3.

Under AM 1.5 G irradiation (100 mW cm^–2), both Zr-doped and pure β-Fe₂O₃ photoanodes exhibit very good photochemical stability for water splitting during the PEC reaction over 10 h, as shown in Fig. 6c. In the case of particle-assembled Zr-doped β-Fe₂O₃ photoanodes, the Faradaic efficiencies for hydrogen production and oxygen production are 85% and 80%, respectively (Fig. 6d).  This result suggests that the photocurrent of the particle-assembled β-Fe₂O₃ photoanodes is mainly attributed to water splitting.

During the process of solar water splitting, β-Fe₂O₃ as a photoanode material may be exposed to standard simulated sunlight for a long time. To assess the possibility for phase transformation of β-Fe₂O₃ to α-Fe₂O₃ under simulated sunlight, the Raman spectra of the samples were measured before and after the photochemical stability testing. As shown in Supplementary Fig. S13, the phase of the β-Fe₂O₃ photoanodes before and after the reaction of PEC water splitting remains unchanged. In a typical PEC reaction, the intensity of the light used is not sufficient to promote the phase transformation of β-Fe₂O₃ particle-assembled films. Therefore, β-Fe₂O₃ is a promising photoanode for water splitting. The PEC performance of β-Fe₂O₃ may be improved further by not only the rational design of electrocatalysts but also the optimization of preparation methods [41,42].

CONCLUSION

In conclusion, the Raman spectrum of β-Fe₂O₃ under excitation by a 785 nm laser with 0.4 W power shows 12 significant vibrational modes corresponding to β-Fe₂O₃. The phase transformation of metastable β-Fe₂O₃ to mature α-Fe₂O₃ may be observed under laser irradiation, electron irradiation and heating. Zr doping was introduced to particle-assembled β-Fe₂O₃ films, thus not only increasing the carrier concentration but also suppressing the phase transformation of β-Fe₂O₃. The PEC performance of the Zr-doped β-Fe₂O₃ photoanode was vastly boosted, in comparison with that of the pure β-Fe₂O₃ photoanode. This study demonstrates that metastable β-Fe₂O₃ remains stable during the PEC reaction and is a promising photoanode material for decomposing water, thus paving the road toward the use of β-Fe₂O₃ in solar water splitting (Supplementary Scheme S1).

METHODS

In this study, β-Fe₂O₃ powder was synthesized by calcining the mixture of NaFe(SO₄)₂ (or 1.5% ZrSO₄ doped NaFe(SO₄)₂) and NaCl in a muffle furnace at 450°C for 1 h [43]. As-prepared β-Fe₂O₃ powder was then deposited on fluorine-doped tin oxide (FTO) glass by electrophoretic deposition to prepare the particle-assembled films. In the case of necking treatment for the particle-assembled β-Fe₂O₃ films, 0.2 μmol of TiCl₄ in methanol solution was dropped on the particle-assembled β-Fe₂O₃ films. Afterward, the films were annealed in a muffle furnace at 600°C (or 650°C) for 1 h.

Raman spectra of the samples were characterized with a confocal laser Raman spectrometer (Japan, Horiba, LabRAM Aramis, calibrated with silicon). Except for Fig. 1, all of the Raman spectra data acquisition used a 785 nm laser as the excitation source. Except for Fig. 3, all of the Raman spectra data acquisition used a 0.4 mW power laser as the excitation source. Raman imaging was obtained using a 785 nm, 0.4 mW laser. All laser-induced phase-change samples were confirmed using a 785 nm laser as the excitation source before laser irradiation to ensure that the samples contained no α-Fe₂O₃. The phosphor spectrum calculations were calculated in Material Studio with the Local Density Approximate, using the norm-conserving situation.

Density functional theory calculations on Zr-doped β-Fe₂O₃ were implemented in the VASP (Vienna Ab-initio Simulation Package) with a projected-augmented-wave method in the scheme of generalized-gradient approximation, whereas the strong on-site Coulomb repulsion among the localized Fe 3d electrons was described with the generalized-gradient approximation +U approach (‘U’ is ‘the strength of the on-site Coulomb interaction’).

SEM images were obtained on an SEM (Germany, Zessis, Ultra 55) and SAED images of the samples were obtained on a TEM (Japan, JEOL, Ltd. JEM2100).

PEC water splitting of the β-Fe₂O₃ particle-assembled films was carried out in 1 M NaOH solution (pH = 13.6) under AM 1.5 G standard simulated sunlight (American, Newport, Oriel Sol3A, 100 mW cm⁻²).

FUNDING

This work was supported by the National Key Research and Development Program of China (2018YFA0209303), the National Natural Science Foundation of China (U1663228, 51902153 and 51972165) and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

Conflict of interest statement. None declared.