Experimental and Computational Investigation of the Optical

Article pubs.acs.org/JPCC

Experimental and Computational Investigation of the Optical, Electronic, and Electrochemical Properties of Hydrogenated α‑Fe2O3 Yichun Yin,† Xiwang Zhang,‡ Li Li,§ Leone Spiccia,† and Chenghua Sun*,∥ †

School of Chemistry, Monash University, Clayton, Australia Department of Chemical Engineering, Monash University, Clayton, Australia § Australian Institute for Bioengineering & Nanotechnology, University of Queensland, St Lucia, Australia ∥ Department of Chemistry and Biotechnology, Faculty of Science, Engineering and Technology, Swinburne University of Technology, Hawthorn, VIC 3122, Australia ‡

ABSTRACT: With the combination of experimental and computational approaches, the impact of hydrogenation on the optical, electronic and electrochemical properties of Fe2O3 for water splitting have been studied. Under high pressure, hydrogen incorporation into the Fe2O3 lattice has been achieved, and hydrogen dopants can be released with heat treatment. With H-doping, magnetite Fe3O4 has been found to form at the edge of Fe2O3 nanoparticles. H-incorporation can narrow the band gap by ∼0.4 eV and slightly reduce the overpotential (by 140 mV) for oxygen evolution reaction, but due to the localization of hydrogen (bonded with oxygen), a slightly lower charge carrier density resulted. Computational studies reveal that all of the above changes are essentially related to the local states brought by H-dopants. This study provides an in-depth understanding of the optical, electronic, and catalytic properties of hydrogenated Fe2O3.

1. INTRODUCTION Photocatalytic water splitting has been viewed as a green approach to produce hydrogen, with semiconductors as absorption and reaction centers. To achieve high efficiency, semiconductors should have as small band gaps as possible so that most of the sunlight can be harvested, but to satisfy the thermodynamics of water splitting, the minimum band gap is 1.23 eV. Given that water splitting is a four-electron process, additional energy, namely overpotential (η), is required to get over the barriers associated with the elementary reactions. As a result, the band gap of photocatalysts for oxygen evolution should be (1.23 + η) eV. Semiconductors with a η of larger than 0.6 eV and a band gap in the range of 1.8−2.6 eV are often attractive for photocatalytic water splitting.1 Hematite (αFe2O3) is such a semiconductor, with band gap of 2.0−2.2 eV, which can absorb most of the visible light and offer a theoretical conversion efficiency of 12%, in addition to other attractive features, like low-cost, high stability, and nontoxicity.2,3 Therefore, α-Fe2O3 has been extensively studied as a photocatalyst with the capacity to work under visible light. For water splitting, however, pristine α-Fe2O3 cannot offer high photocatalytic performance because of its poor conductivity, high electron−hole recombination rates and large overpotential for oxygen evolution reaction (OER). Currently, extensive studies have been attracted on tuning its electronic structure to improve its photocatalytic activity on water splitting. Among various approaches, doping with metal4−9 and nonmetal10−12 impurities is very popular. For instance, incorporating transition metals into the lattice has been © XXXX American Chemical Society

regarded as an effective approach to enhance the photocatalytic activity of α-Fe2O3,13,14 which can affect the electronic and photoelectrochemical properties by increasing the charge carrier density and thus the conductivity. Typically, dopants replace lattice atoms (Fe or O), namely substitutional doping, leading to the shift of valence band maximum (VBM) or/and conduction band minimum (CBM). In recent years, another doping technology, hydrogenation, has been successfully developed and employed to tune the electronics of some semiconductors, particularly TiO2, in which H atoms diffuse into the interstitial positions and bond with oxygen, remarkably narrowing the band gap.15,16 Compared with TiO2, the hydrogenation of Fe2O3 has been rarely studied, which is probably attributed to the fact that Fe2O3 is already an excellent absorber of visible light, essentially different from TiO2. However, hydrogenation might still be useful for Fe2O3, such as improving the electrical conductivity which facilitates charge carrier transfer. According to a recent study, H-doped TiO2 shows higher conductivity through the electron injection to the system with the formation of O−H bonds.17 The CBM of Fe2O3 is lower than that of H+/H2 potential, as a result, it is not suitable for hydrogen evolution reaction (HER). Therefore, Fe2O3 is often combined with other semiconductors toward full water-splitting, like Z-scheme photocatalysts18 (e.g., Fe2O3/ WO3, etc.), in which case further narrowing the band gap by Received: January 18, 2017 Revised: July 2, 2017 Published: July 13, 2017 A

DOI: 10.1021/acs.jpcc.7b00593 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 1. Structural characterizations. (a) HRTEM image of H−Fe2O3 (inset is fast Fourier transform patterns of the dotted yellow square area); (b) line analysis corresponding to red and blue lines in Figure 1(a); (c) XRD patterns of Fe2O3, H−Fe2O3, and Fe2O3−HT 300 °C. 0 ERHE = EAg / AgCl + 0.059 pH + EAg / AgCl

shifting the CBM to lower energy is helpful to utilize more sunlight. Since β and ε phases of Fe2O3 are testified to be active for hydrogen production,19 the improvement of oxygen evolution activity on α-Fe2O3 is demanding. This may open up opportunity for overall water splitting on iron oxide phase junction. In addition, it is not clear whether hydrogenation can affect the OER performance of Fe2O3, especially the overpotential (η), which is high for pristine Fe2O3. As identified above, hydrogenation could be an effective approach to tune the properties of Fe2O3 as OER catalyst. This study aims to provide a comprehensive investigation of hydrogenated Fe2O3, focusing on its optical, electronic and electrochemical properties, based on experimental and computational studies.

Here E0Ag/AgCl is 0.1976 V at 25 °C, and EAg/AgCl is the experimentally measured potential vs. the saturated Ag/AgCl electrode. 2.4. AC Impedance Measurement. The Mott−Schottky analysis was accomplished to measure the charge transfer properties of hydrogenated Fe2O3. The AC impedance measurements were carried out by conducting an ImpedancePotential spectroscopy at 500 Hz by scanning the potential from −1.0 to 0 V in a step of 10 mV/s. The electrolyte were aqueous solutions of potassium phosphate buffer (pH = 6, 7, 8), 0.01 M NaOH (pH = 11.6) and 1 M NaOH (pH = 13.6). Charge carrier density (Nd) and flat band potential (Efb) were calculated using the following relation:

2. EXPERIMENTAL AND COMPUTATIONAL APPROACHES 2.1. Preparation of Hydrogenated Fe2O3. Iron(III) oxide (Fe2O3, Sigma-Aldrich, < 50 nm, ≥ 99.9%) nanoparticles were first maintained in vacuum for 1 h after being placed in the sample chamber of a high-pressure hydrogen system and then hydrogenated in a 70.0-bar H2 at 300 °C for 2 h. For comparison, the hydrogenated sample was annealed at 300 °C in an open-air box furnace for 1 h with a heating rate of 5 °C/ min. 2.2. Characterizations of the Structure and Optical Properties. A FEI Tecnai G2 F20 transmission electron microscopy (TEM) operating at 200 kV was used to collect TEM images for the samples. The X-ray diffraction (XRD) was recorded on a Rigaku diffractometer using Cu Kα irradiation. The UV−vis spectra were recorded using a UV−vis scanning spectrophotometer (Shimadazu, UV-2600, Japan). 2.3. Electrochemical Performance Measurements. Electrochemical measurements were carried out using a CHI 660E workstation in a three-electrode electrochemical cell, where Fe2O3 nanoparticles coated FTO electrodes, a Pt plate and a saturated Ag/AgCl as working, counter, and reference electrodes, respectively. The working electrodes were prepared by dispersing 5 mg iron oxide in 1 mL 0.1% Nafion (ethanol) thoroughly. Then 30 ul of the obtained mixed slurry was coated onto FTO glass by drop casting method and then the electrode was dried at 60 °C for 1 h. The liner sweep voltammetry test (LSV) was conducted at a scan rate of 50 mV/s in 1 M NaOH (pH = 13.6) purged with high purity N2 gas. The current densities acquired from the LSV were normalized by the geometric surface area. The potential referenced to saturated Ag/AgCl reference electrode was converted into a reversible hydrogen electrode (RHE) potential using the formula:

1 CSC

2

=

2 ⎛ kT ⎞ ⎜E − E − ⎟ fb ⎝ εε0eNd e ⎠

Here Csc is the space charge region capacitance per unit area, ε is the dielectric constant of iron oxide (ε = 14.2 F/m in this study), εo is the vacuum permittivity, e is the electron charge, Nd is the charge carrier density, k is Boltzmann’s constant, and T is the temperature (K). The position of conductive band (CB) could be approximated by the flat band potential (Efb).20 Distilled water (DI) was used for preparing the solution. All of the measurements were performed at room temperature (25 ± 2 °C). 2.5. Computational Parameters. To study the doping effect on the electronics, the bulk primary cell of Fe2O3 was employed, and single hydrogen atom was incorporated to the cell at the interstitial space, followed by full relaxation. H incorporation lead to bond break and formation; therefore, all calculations are performed under the scheme of spin-polarized density functional theory (DFT), together with the Perdew− Burke−Ernzerhof (PBE) functional21 and Projected Augmented Wave (PAW) pseudopotentials,22,23 which has been embedded in the Vienna ab initio simulation package (VASP).23 Reciprocal space is spanned with a plane-wave basis, with a cutoff energy of 400 eV, and K-space is sampled with Monkhorst Pack of 5 × 5 × 3 grid and γ point for bulk and slab models, respectively. In terms of the effective U-value, 4.0 and 0.0 eV are used for Fe and O, respectively, which is based on an extensively tested literature.14,24,25

3. RESULTS AND DISCUSSION 3.1. Phase, Structure, Composition, and Morphology. From the HRTEM image of H−Fe2O3 (Figure 1a), clearly resolved and well-defined lattice fringes of the nanocrystal were B


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Figure 2. Optical absorption. (a) UV−vis absorption profiles of original, H-doped, and annealed Fe2O3 (inset are appearance of original and H− Fe2O3); data fitting to generate (b) indirect band gap and (c) direct band gap.

Table 1. Experimental Overpotential η, Band Gap Eg, Charge Carrier Density Nd, and Flat Band Potential Efb

observed, suggesting that H−Fe2O3 is completely crystalline. Fast Fourier transform (FFT) of the selected area (yellow dotted square) (inserted in Figure 1a) indicated two sets of diffraction patterns. The marked red circle attributes to magnetite Fe3O4 (220), and the blue circle to α-Fe2O3 (104). The line analyze (Figure 1(b)) of the outer edge (red line) and inner region (blue line) demonstrated the distances between lattice planes are 0.302 and 0.267 nm, corresponding to Fe3O4 (220) and Fe2O3 (104) respectively, in agreement with the fast Fourier transform results. Figure 1(c) shows the XRD patterns of Fe2O3, H−Fe2O3, and H−Fe2O3 after 300 °C heat treatment (Fe2O3−HT). XRD patterns of the Fe2O3 sample showed the typical XRD patterns of highly crystallized hematite α-Fe2O3 (space group: R3c ̅ (167), a = 0.5036, b = 0.5036, c = 1.3749, JCPDS 33-0664). After hydrogenation, XRD patterns of H− Fe2O3 exhibited new peaks at 30°, 37°, 43°, 53°, and 57° compared to Fe2O3 samples, indicating new crystal phase formed after hydrogenation. These new XRD patterns can be attributed to magnetite Fe3O4 (space group: Pd-3m (227), a = b = c = 0.8396, JCPDS 19-0629), which are consistent with HRTEM results. The magnetite Fe3O4 formation in H−Fe2O3 suggested that hydrogen atom may be inserted in the interstitial positions of Fe2O3 to form Fe3O4. However, Fe2O3−HT obtained from H−Fe2O3 exposed at 300 °C in open air showed the same XRD patterns as Fe2O3, suggesting the hydrogen has been released. Thus, XRD result indicates that hydrogen has been successfully incorporated in the case of H−Fe2O3, forming the new phase Fe3O4, but the H-dopants can be released rapidly through heat treatment. 3.2. Optical Absorption. Figure 2a shows the UV−vis absorption spectra for Fe2O3, H−Fe2O3, and Fe2O3−HT 300 °C, according to which, H-dopants can remarkably increase the absorption of visible light in the range of 550−800 nm. Inset images show the appearance of Fe2O3 before and after hydrogenation, as dark red and black powders. The color change indicates increase of light absorption capacity and decrease of band gaps, which are examined below. As indicated by the color change in Figure 2a, hydrogenation can change the absorption edges. After annealing at 300 °C, such absorption almost disappears, confirming that the improved absorption is exactly resulted by the hydrogenated structures. To derive the band gap, Tauc plots of both indirect and direct transitions are shown in Figure 2, parts b and c, respectively, with the values of band gap energy listed in Table 1. Accordingly, pristine Fe2O3 shows an indirect bandgap of 2.06 eV, matching well with literature values (1.9−2.2 eV26,27). In the case of H−Fe2O3, it is down to 1.58 eV, resulting in the black color since almost all visible light can be absorbed. With annealing at 300 °C, the

bandgap (eV)

sample Fe2O3 H− Fe2O3 300 °C

overpotential (V) vs RHE exptl, at 0.5 mA/cm2

indirect

direct

Nd (cm )

Efb (V) vs NHE

0.66 0.52

1.58 2.06

1.62 1.99

2.40 × 1020 1.26 × 1020

−0.67 −0.61

0.62

2.06

1.99

2.18 × 1020

−0.64

−3

band gap gets back to ∼2.0 eV because H-dopants have been released. 3.3. Electrochemical Measurements. To understand whether H-doping can improve the OER performance, the current generated under different bias has been collected, namely I−V curves, as shown in Figure 3a. The overpotential η for OER has been calculated using the current at I = 0.5 mA/ cm2 following early publications28 and listed in Table 1. For pristine Fe2O3, η is 0.66 V, which is high compared with those benchmark OER catalysts (e.g., IrOx, CoPi).29 Hydrogenation only slightly reduced η by ∼0.14 V, indicating that hydrogenation is not an effective approach to reducing the overpotential of Fe2O3, which is probably because H-dopants mainly bond with oxygen and hardly affect the electronic states of surface iron. To understand whether the density of charge carriers is affected by hydrogenation, the Mott−Schottky plots were collected in Figure 3b, and the donor densities can be derived through fitting the plots, with the data shown in Table 1, being ∼1020 cm−3. The positive slope indicates the n-type behavior of unmodified and hydrogenated iron oxide particles. It is noticeable that the charge carrier density decreased after hydrogen doping, but increased after annealing at 300 °C. According to the electrochemical results, it appears that hydrogen incorporation cannot bring additional free electrons, indicating that they are localized by chemical bonding, rather than as freely diffusing ions. In addition, heat treatment can release the hydrogen incorporated and might generate new defects in hematite by taking away part of oxygen atoms, which are harmful for the charge transport. For water-splitting photocatalysts, the VBM and CBM factors are critical, which can be experimentally studied by analyzing the flat band potential (Efb).20 In aqueous solutions, Efb monotonically decreased with a slope of 59 mV/pH; therefore, a linear relationship between Efb and pH can be established after measuring a couple of different pH points and the value of Efb can be extrapolated at pH = 0 V vs NHE. In our case, we measured Efb in potassium phosphate buffer (pH = 6, 7, 8) and 0.01 M NaOH (pH = 11.6) and 1 M NaOH (pH = C


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Figure 3. Electrochemical measurements. (a) I−V curves; (b) Mott−Schottky plot of iron oxide particles electrodes measured at 500 Hz, 1 M NaOH (pH = 13.6).

Figure 4. (a) Efb of undoped and H−Fe2O3 in dependence of pH; (b) comparative energy diagram of TiO2 and Fe2O3 reported (Eg = 3.0 and 2.3 eV, respectively) and hydrogenated Fe2O3 in aqueous solution at pH = 0. CB drawn assuming EF = Efb ≈ Ecb.

13.6), as shown in Figure 4a using pristine and H-doped Fe2O3 as examples. Here, Figure 4a shows slopes of ∼69 mV/pH, which should be a consequence of discreteness of charge effects in the Helmholtz double layer at the interface.30 To relate Efb to band edges, TiO2 has been employed as a reference given its VBM and CBM have been extensively studied. Combined with the band gap data, the generated band edges are shown in Figure 4b. CBM of pristine Fe2O3 is ∼0.3 eV as reported31 and hydrogenation shifted the CBM downward and the VBM upward, resulting in narrowed band gap. In other words, the reduction capacity of holes in VB will be reduced although higher absorption of visible light can be achieved with hydrogenation, which is harmful for OER catalyst. Overall, Hdoping is very efficient to improve the visible light absorption, but not a good approach to reduce the OER overpotential. 3.4. Computational Results. 3.4.1. Electronic Structures. From experiments, a typical feature brought by H-doping is the improvement of visible light absorption, which is one motivation to run the doping. To clarify this, the geometries and electronic structures of undoped and H-doped Fe2O3 have been calculated and shown in Figure 5 (a−b). From the relaxed geometries, it is clear that the H atom is chemically bonded with oxygen, with an O−H bond length of 1.00 Å, and the original O−Fe bond breaks, with an O−Fe distance of 3.55 Å, as indicated by the green dashed line. Other O−Fe bonds, however, remain, without remarkable changes, indicating the H-dopants are stabilized by the local O−H bond. To further examine the doping effect, density of states (DOS) of undoped and H-doped Fe2O3, with spin up and down, are calculated and shown in Figure 5. The calculated band gap of pristine Fe2O3 is ∼1.60 eV, which is close to the experimental value (2.0 eV). With H-doping, midband state (labeled as small red peak in Figure 5b) is observed, which essentially originates from O−H bonding. Such state is

Figure 5. Calculated geometries and electronics for (a) undoped and (b) H-doped Fe2O3, with Fe, O and H labeled as purple, red, and white spheres. The energy in the DOS profile has been shifted with EF energy (dashed red line) as the energy reference. The interband state brought by H-doping is highlighted by the red peak.

separated from the inherent VB of Fe2O3, and slightly higher as an occupied state, which is a theoretical evidence that Hdoping can narrow the band gap. As determined by the localization feature, such band gap narrowing cannot bring the overall shift (“shoulder-b shoulder shift”) of the UV−vis absorption, but more as a wide and irregular absorption “hump”, and such feature has been also found in H-doped TiO2, in which H-doping shows similar local states as oxygen vacancies.16 Comparing the spin up and spin down, net spin D


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Figure 6. OER overpotential. (a) Experimental I−V curves for overpotential measurement and (b) DFT calculated free energy for OER elementary steps.

higher than 1.23 eV (minimum requirement) is helpful to drive the reaction, and for practical use an ideal energy input is in the range of 1.6−2.2 eV to achieve a good balance between light harvest and reaction kinetics given an overpotential is present. In other words, Fe3O4 does improve the optical absorption (together with H-dopants), however, most photons absorbed by Fe3O4 (small amount) cannot provide enough energy for water splitting. The last but not the least is the effect on OER overpotential. As the performance of pure Fe2O3 phase is dissatisfactory, and experimentally it is observed that H-doped Fe2O3 shows smaller overpotential (0.52 V), but such improvement is unstable when it is annealed (see Table 1). To examine whether such improvement fully or partially comes from Fe3O4, pure Fe3O4 nanoparticles have been synthesized and tested for comparison. From Figure 6a, we could see that the OER overpotential of Fe3O4 is slightly smaller than that of undoped Fe2O3, but still a bit larger compared with hydrogenated Fe2O3. Such a test agrees well with an early publication.33 We noticed that Fe3O4 generated after H-doping is dominated by (220), rather than widely observed low-indexed surfaces, like (111) or (001). Therefore, DFT calculations have been performed to examine the elementary reactions following an established strategy for the calculations of OER overpotential using standard hydrogen electrode (SHE) as a reference.34 For OER, four elementary reactions are considered, including:

around the Fermi level has been observed in the DOS profile (Figure 5(b)), indicating that H-doped Fe2O3 shows magnetic properties, which is consistent with our experiments. Using gasphase H2 as a reference, the incorporation energy for each hydrogen is 0.12 eV, suggesting that hydrogen favors to move out, which is why they can be easily released by heat treatment (e.g., 300 °C). From experimental UV−vis results, the absorption before and after 400 nm is clearly different, which can be also understood based on the DOS profile. In the UV region (λ < 400 nm), the absorption is contributed by the direct charge transfer from O2p-dominated VB to Fe3ddominated CB, as band-to-band excitation. For the visible light range, interband states associated with H-doping starts to dominate the absorption, but the mobility of electrons is lower due to the localized nature, therefore, the strength is not as strong as the band-to-band case, but more as a defect-like excitation, which is useful for light absorption, but not efficient for photocatalytic reactions, indicating that H-doped Fe2O3 is a good sunlight absorber, but not a good catalyst. Overall, Hdoping can generate local states introduced by H−O bonding, result in color change to black, which is helpful to narrow the band gap to 1.58 eV (see Figure 4). However, H-dopants prefer to be released, as observed in our heat treatment (300 °C), and the color changes back to be dark red. 3.4.2. Catalytic Properties. As identified by HRTEM and XRD, a magnetite Fe3O4 phase has been observed at the edges of H−Fe2O3 particles; therefore, a full discussion would be helpful to clarify the effects between H-dopant and the Fe3O4 edge. Magnetite Fe3O4 is a stable phase at room temperature, with an excellent conductivity as high as 2 × 102 S/cm, showing metallic features according to theoretical calculations.32 Such high conductivity is very helpful for electrochemical reactions, particularly when the conductivity of the main phase Fe2O3 is poor. As shown by theoretical calculations, H-doping can introduce local states on the VB top, but those states are associated with O−H bonding, meaning highly localized. So its contribution to the conductivity should not be significant as that by the presence of Fe3O4 phase. Therefore, one of the key roles by Fe3O4 is to improve the conductivity of the electrode. As indicated by the black color of Fe3O4, it offers excellent photo absorption, with a wide absorption capacity covering from UV to infrared light. Hence, it can strengthen the visible light absorption to a certain extent. Experimentally, annealing resulted in the increase of optical band gap from 1.58 eV (Hdoped Fe2O3) to ∼2.0 eV (after annealing at 300 °C), suggesting that the amount of Fe3O4 should be small, as a result the measured optical gap is still dominated by the main phase (Fe2O3). In the context of OER, only photons with an energy

* + H 2O → *OH + H+ + e−

(1)

*OH → *O + H+ + e−

(2)

*O + H 2O → *OOH + H+ + e−

(3)

*OOH + H 2O → O2 + H+ + e−

(4)

Here * represents the catalyst surface, and *O, *OH, and *OOH are the intermediate states associated with OER. On the basis of calculated energy profile, the energy input for each step can be derived. Given the minimum requirement for OER is 4.92 eV (four-electron process), each elementary step (associated with one-electron transfer) needs a minimum energy of 1.23 eV at the equilibrium case, and reaction energy higher than this value is referred as an overpotential. For simplicity, elementary steps are labeled as *, *O, *OH, and *OOH in the following discussion. In principle, (220) may be terminated with three- or four-coordinated Fe depending on the cleavage. Given adsorbed oxygen (O*) is one of the major intermediate states, it is reasonable to choose a slab with fourcoordinated case for the calculation. The calculated energy E



■

profile for OER is shown in Figure 6b, according to which an overpotential of 1.03 V is derived for Fe3O4 (220), notably higher than that of pure Fe2O3. Summarizing the above results, it is concluded that the observed improvement in terms of OER overpotential should be related to H-dopants, rather than Fe3O4. Overall, the Fe3O4 edge is helpful for the improvement of electrical conductivity and visible light absorption, but the contributing factor to overpotential reduction is hydrogen incorporation. To identify the potential structural origin of the lowered overpotential, two specific structures associated with H-doping have been investigated, namely H-dopant in the subsurface and Fe2O3 with oxygen vacancy (OV) in the surface. As shown in Figure 6b, both H-dopants and OV can offer relatively low overpotential, as 0.63 and 0.52 V, being close to experimental observation in this study. It is worth to mention that OV can be generated even for pure Fe2O3 under OER conditions through the reaction between lowly coordinated oxygen (Os) on the surface by Os + H2O → OsOH + (H+ + e−) → OV + O2 + 2(H+ + e−), for which the electron transfer is similar to OER steps (see reaction 1-4). In other words, OV is not necessarily generated through H-doping. It is also necessary to point out that both H-dopants and OV are not stable, especially under the annealing with air, which can explain the phenomenon that low overpotential is not approachable once the samples are annealed at 300 °C.

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4. CONCLUSIONS Combined with experimental and computational approaches, the effect of hydrogenation on the optical, electronic and electrochemical properties of Fe2O3 has been investigated, and the following points are found: (i) hydrogenation is an effective technology to reduce the band gap of Fe2O3, but H-dopants can be easily released by annealing; (ii) H−Fe2O3 have showed slightly reduced OER overpotential by 0.14 V, compared with the original one; (iii) hydrogenation has shifted CBM downward and VBM upward. As revealed by theoretical calculations, interstitial H-dopants are stabilized by O−H bonding, generating localized midband states, which plays a key role for the narrowed band gap, and lowered electron mobility in these states. Fe3O4 thin layer resulted on the edge of H− Fe2O3 particles is helpful to improve the light absorption and conductivity while hydrogen dopants are intrinsically beneficial to lower the overpotential.

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AUTHOR INFORMATION

Corresponding Author

*(C.S.) E-mail: [email protected]. ORCID

Chenghua Sun: 0000-0001-7654-669X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the Australian Research Council (ARC) for support through the Excellence for Electromaterials Science (ACES), Discovery Project (DP130100268), and Future Fellowship (C.S., FT130100076). We acknowledge the use of facilities within the Monash Centre for Electron Microscopy (MCEM) and ARC funding (LE110100223). We also thank the National Computational Infrastructure (NCI) for providing the computational resources. F


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Experimental and Computational Investigation of the Optical

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