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Article 2+

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Enhancing Charge Transfer and Photoconversion of a (Mn -FeO)/RGO/ (Fe -WO) Photoelectrochemical Anode via Band-Structure Modulation 3+

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Zhuo Zhang, Lunyong Zhang, Bin Chen, Minki Baek, and Kijung Yong ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03368 • Publication Date (Web): 23 Aug 2018 Downloaded from http://pubs.acs.org on August 28, 2018

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ACS Sustainable Chemistry & Engineering

Efficient Photoconversion and Charge Separation of a (Mn2+-Fe2O3)/RGO/(Fe3+-WO3) Photoelectrochemical Anode via Band-Structure Modulation Zhuo Zhang,† Lunyong Zhang,‡,§ Bin Chen,† Minki Baek,† and Kijung Yong*,† †

Surface Chemistry Laboratory of Electronic Materials, Department of Chemical Engineering, Pohang

University of Science and Technology (POSTECH), 77 Cheongam-Ro, Nam-Gu, Pohang, Gyeongbuk 790-784, Korea. ‡

Max Plank POSTECH Center for Complex Phase Materials, Max Planck POSTECH/Korea Research

Initiative, 77 Cheongam-Ro, Nam-Gu, Pohang, Gyeongbuk 790-784, Korea. §

Max Planck Institute for Chemical Physics of Solids, Dresden 01187, Germany.

*E-mail: [email protected]; Fax: +82-54-279-8298; Tel.: +82-54-279-2278.

ABSTRACT: We report on a (Mn2+-Fe2O3)/RGO/(Fe3+-WO3) hetero-nanostructure (HNS) as a building block for photoelectrochemical (PEC) anodes: an array of Fe3+-doped WO3 nanorods (NRs) were covered with RGO, and both the NRs and RGO were decorated with Mn2+-doped α-Fe2O3 nanoparticles (NPs). Efficient electron-hole separation and carrier migration are ascribed to mid-gap states (MGSs) obtained via doping, type-II band alignment of WO3 and α-Fe2O3, and highly conductive RGO. In particular, the PEC efficiency enhances at first and then decays with increasing Mn2+ doping concentration. The optimum Mn2+ concentrations of 1% via experiment and 2% though density functional theory (DFT) are confirmed. DFT calculations reveal that the band structure of α-Fe2O3 can be modulated via tuning the Mn2+ concentration. With increasing Mn2+ concentration, the bandgap gradually narrows, and the MGSs gradually approach then merge into the valence band (VB) due to hybridization interactions between Mn2+, O2- and Fe3+ ions. Overall, we anticipate that this kind of HNS with modulated band structure can supply inspiration to the design and development of semiconductor materials for photoconversion applications.

KEYWORDS: hematite, photoelectrochemical, manganese doping, mid-gap states

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INTRODUCTION

Photoelectrochemical (PEC) hydrogen (H2) generation has been considered widely as an ecofriendly and promising strategy to overcome the crisis of tight energy resources and a worsening ecological environment. As the typical n-type transition metal oxides, hematite (α-Fe2O3) and tungsten oxide (WO3) have long been studied as photoanode materials owing to their low cost, earth abundance, nontoxicity and commendable visible-light absorption.1-10 The approximately 2.1 eV bandgap of α-Fe2O3

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is narrower than the 2.6-2.8 eV of WO3 12,

so various α-Fe2O3/WO3 hetero-nanostructures (HNSs) with a cascade band alignment have been constructed to facilitate electron-hole separation and migration.13-18 Additionally, α-Fe2O3 and WO3 of the HNSs are complementary in PEC carrier transfer. For example, the reported photoconversion efficiency (PCE) of α-Fe2O3 is far below the theoretical idealized efficiency of approximately 13 %

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and is mainly ascribed to photo-generated carriers with a

transient lifetime of less than 10 ps,20 and a short hole-diffusion length of approximately 2-4 nm.21 Comparatively, the transient lifetime and diffusion length of WO3 are 36.9 ps 0.15-5 µm

22, 23

17

and

, respectively. Although the conduction band positions of both α-Fe2O3 and

WO3 are lower than the potential of a normal hydrogen electrode (NHE), thereby requiring an external bias for H2 generation, the α-Fe2O3/WO3 HNS as an efficient and powerful combination is still a very promising building block for photoanodes in PEC system. It’s well known that BiVO4 is another very promising PEC materials due to the bandgap around 2.4 eV, broad visible light absorption and photocurrent higher than 4.5 mA/cm2. 24 The position of the bottom of conduction band of BiVO4 is close to the thermodynamic H2 evolution potential, which is better than that of WO3. However, considering slower charge transfer and faster recombination of BiVO4, WO3 with faster charge transfer is employed in this work. To further improve the light harvesting and charge separation in photoanodes, additional electronic states in the mid-gap region of the anode material could be created via doping.25-32 Among various elements, the transition metals Mn and Fe possess irreplaceable natural advantages as the dopants of α-Fe2O3 and WO3, respectively. For Mn-doped α-Fe2O3, Mn is next to Fe on the periodic table of elements, and they have similar atomic sizes and physical properties, thus avoiding significant crystalline distortions and defects. Gurudayal and et al.


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reported the PEC photocurrent-density of α-Fe2O3 could have a significant 3-fold improvement by doping with 5 mol % Mn.33 They ascribed the PEC enhancement to an increased charge carrier density, the suppression of electron-hole recombination and a reduction in the barrier for hole transport. Then, with respect to Fe-doped WO3, many studies have confirmed that the similar ionic radii (approximately 0.64 for Fe3+ 34 and 0.62 Å for W6+ 35

) could avoid a serious lattice mismatch, and that the narrow bandgap of Fe2O3 could reduce the

bandgap of WO3 and broaden light absorption. Apart from the above advantages, when spin-polarized Mn2+ or Fe3+ are doped into a host, a strong exchange coupling between the s-p electrons of the host and the d electrons of the dopant occurs.36, 37 This coupling splits one d-orbital into double levels of a higher 4T1 and a lower 6A1, then the forbidden d-d transition becomes partially unblocked. An electron transition can be generated from 4T1 to 6A1 leading to a photoluminescence (PL) emission;38 however, the reverse transition from 6A1 to 4T1 is prevented by the nature of spin.39 S. Joicy attributed the enhanced photocatalytic efficiency of Mn-doped ZnS to the fast charge transfer assisted by 4T1 and 6A1 levels inside the host.40 Therefore, it is essential to prompt an upward transition from lower to higher energy levels with the aid of 6A1 or 4T1 and other mid-gap states (MGSs), and at the same time, suppress the downward transition to further improve PEC efficiency. To prompt such an upward transition, taking Mn2+ as example, many studies have found the light harvesting and photoconversion of hosts could be optimized via tuning the Mn2+ doping concentration.41-43 Q. R. Deng ascribed the initially increasing and then decreasing photocatalytic efficiency with increasing Mn doping concentration to a gradually increased defect density.44 However, deeper studies on the relationships between doping concentration, band structure, PL properties and PEC performance are rare. For suppressing the downward transition, studies have confirmed that reduced graphene oxide (RGO) could clearly prolong carrier lifetime 45-47 and depress PL due to the high conductivity.48 In this work, Fe3+-doped WO3 nanorods (NRs) were draped with RGO layers and decorated with Mn2+-doped α-Fe2O3 nanoparticles (NPs). This HNS is named (Mn2+-Fe2O3)/RGO/(Fe3+

-WO3) and simplified as Mn-F/R/W. We tuned the Mn2+ concentration to optimize the band

structure and PEC performance of the HNS. Experimental and density functional theory (DFT) studies reveal that 1) hybridization interactions and d-orbital splitting are influenced by the



dopants Mn2+ and Fe3+, resulting in MGSs and a narrowed bandgap, and eventually broadening visible-light absorption and PL emission from 4T1 to 6A1; 2) with the increasing Mn2+ concentration, the MGSs increase in density, gradually approaching and then merging into a valence band (VB); 3) without the aid of RGO, the intensity of PL induced by MGSs reaches maximum values simultaneously when the doping concentration of Mn2+ is 1% via experiment and 2% via DFT calculations; and 4) RGO clearly depresses the PL intensity and facilitates the carrier transfer, resulting in the long lifetime of carriers and a highly improved PEC efficiency.

EXPERIMENTAL SECTION AND DFT CALCULATION SETUP

Fabrication. 1) WO3 NRs: Arrays of WO3 NRs were grown on tungsten (W) wafer by using previous method.49 In brief, first, the cleaned W wafers were placed on the top of an alumina boat containing enough WO3 powder. Then alumina boat was kept in the tube furnace with a vacuum of 50 mTorr. The temperature was gradually increased to 1050 ◦C, then maintained for 1 h, and subsequently the furnace was cooled naturally to room temperature. Finally, the obtained samples were annealed in air ambient at 400 ◦C for 60 min to get fully oxidized WO3 NRs samples with greenish yellow color. 2) Mn-F/R/W HNSs: First, a GO solution was chemically reduced by the reported procedure using NaBH4 (Sigma-Aldrich, 10 mM) as a reducing agent to prepare 0.1 M RGO solution. Then, FeCl3, MnCl2 and 100 mL RGO solution water were mixed together with the Fe3+ concentration of 0.2 M and Mn2+ concentration based on doping adjustment. The mixed solution was coated onto WO3 NRs by spin coating with speed of 1000 rpm and duration of 10 s. And then, the samples were annealed at 400 °C in air for 2 min. The spin coating and annealing were repeated 6 times. Finally, the samples should be further annealed at 400 °C in air for 30 min. Characterization. The morphologies of our nanostructures were checked on a field-emission scanning electron microscope (FE-SEM, XL30S, Philips) with aid of a 5.0 kV beam energy and on a scanning transmission electron microscope (STEM; JEM-2200FS with Image Cs-corrector; JEOL) with a 200-kV beam energy. Measurements. 1) X-ray diffraction (XRD) (X’Pert Pro MPD) was used to confirm


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crystalline information of our samples. 2) X-ray photoelectron spectroscopy (XPS) was performed by using an ESCALAB250 instrument (VG Scientific Company, USA) with Al Kα radiation as the excitation source. 3) A UV2501PC (Shimadzu) spectrometer was employed to obtain optical absorbance of the samples with an ISR-2200 integrating sphere attachment for diffuse reflection measurements. 4) Photoluminescence (PL) spectra were measured using a spectrofluorometer (HORIBA FluoroMax-4). 5) Photocurrent-voltage (J-V) measurements, EIS and electron lifetime were explored by using a typical three-electrode system (model 263A, EG&G Princeton Applied Research) with a saturated Ag/AgCl reference electrode. A Pt wire was used as the counter electrode. an aqueous solution of 0.5 M Na2SO4 with pH~6.8 was employed as the electrolyte. This three-electrode system was illuminated by a solar simulator (AM 1.5 G filtered, 100 mW/cm2, 91160, Oriel). DFT calculation. Due to the spin-polarized properties of transition metals, our DFT calculations were performed with plane-wave pseudopotentials as implemented in the Cambridge Serial Total Energy Package (CASTEP) program. Referenced from Yang’s setup,50 the exchange-correlation was treated using the local density approximation (LDA)+U method.

Within

the

generalized

gradient

approximation

(GGA),

we

use

the

Perdew-Burke-Ernzerhof (PBE) parametrization to calculate the band structure and density of state. A cutoff energy of 720 eV was used according to the Monkhorst-Pack grid for Brillouin zone sampling. The U in the computation is set as 4.3 eV. In the optimization process, the energy changes as well as maximum tolerances of the force, stress, and displacement were set as 5×10−6 eV/atom, 0.01 eV/Å, 0.02 GPa and 5×10−4 Å, respectively.

RESULTS AND DISCUSSION

Morphology and Structure Analyses Figure 1a-c presents structural sketches of the M-F/R/W HNS. The Fe3+-doped WO3 NRs are vertically aligned on the W substrate (Figure 1a) and covered with RGO layers (Figure 1b). Mn2+-doped α-Fe2O3 (M/F) NPs are evenly distributed on the surface of the M-F/R/W HNS (Figure 1c). The scanning electron microscope (SEM) image in Figure 1d reveals the height of the M-F/R/W HNS as approximately 8-9 µm. Taken from the red square in Figure 1a, the



enlarged view in Figure 1e reveals that the RGO layer is supported by Fe3+-doped WO3 NRs with a diameter of approximately 0.6-0.8 µm. Taken from Figure 1e, the close-up in Figure 1f demonstrates that the size of the M/F NPs is approximately 20-30 nm. To explore the crystalline structure of each component and the composition of the entire structure, transmission electron microscope (TEM) observations, X-ray diffraction (XRD) analysis and X-ray photoelectron spectroscopy (XPS) measurements were subsequently performed. Figure 2a shows a typical TEM image of the M-F/R/W HNS. Taken from Figure 2a, the HRTEM image in Figure 2b and c and the corresponding fast Fourier transform (FFT) patterns reveal the highly crystalline Fe-doped WO3 and Mn-doped Fe2O3 with lattice spacings of 0.37, 0.27 and 0.37 nm, corresponding to the (020) plane of WO3,51 and the (01-4) and (012) planes of α-Fe2O3.52 The zone axis of WO3 and α-Fe2O3 are [101] and [100], respectively. Taken from Figure 2a, the spatial elemental analysis using electron energy loss spectroscopy (EELS) in Figure 2d-h confirms that W (d), Mn (e), Fe (f), C (g) and O (h) are distributed evenly in their located positions. The XRD patterns of bare WO3 NRs, WO3 NRs doped with Fe3+ and decorated with pure α-Fe2O3 NPs (F/W HNRs), and M-F/R/W HNS with 2θ ranging from 20 to 70 degrees are presented in the Supporting Information, SI part 1. There is no apparent difference in the XRD patterns of the three samples due to the low concentration of the Mn2+ doping and the similar peak positions of WO3 and α-Fe2O3. Sectional XRD patterns from 2θ=33.00° to 34.50° of the three samples are shown in Figure 2i. For bare WO3 NRs, the peaks located at 33.43° and 34.29° correspond to the (002) and (220) planes of monoclinic WO3. Comparatively, the diffraction patterns of the F/W HNRs and M-F/R/W HNS exhibit a slight shift to smaller angles. Because Fe3+ (0.64 Å) is larger than W6+ (0.62 Å), according to Vegard’s Law, the size of the WO3 lattice increases after doping with Fe3+, and thus, the peaks in the XRD pattern shift to smaller angles.53 To further investigate the chemical binding states of the M-F/R/W HNS with a Mn2+ doping concentration of 1%, XPS analysis was carried out. Figure 3a demonstrates that the Fe 2p spectra consist of two groups of sub-peaks due to the spin-orbit coupling of the 2p1/2 and 2p3/2 states.54 For each group, there exist three sub-peaks corresponding to Fe3+ and Fe2+ states, as well as a satellite peak. As the binding energies of Fe3+ and Fe2+ for 2p1/2 are very close, no deconvolution was performed here. The black curve is the Fe 2p spectrum of the F/W HNRs


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without Mn2+ doping. It can be seen that no clear difference exists upon comparison with the spectrum of the M-F/R/W HNS, ascribed to the low Mn2+ concentration of approximately 1%. Similar to the Fe 2p, Figure 3b reveals that the Mn 2p spectra also include two sub-peaks due to the spin-orbit coupling of the 2p1/2 and 2p3/2 states.55 Figure 3c demonstrates that only the W-O chemical bonds of WO3 are observed by checking the binding energies of the W 4f peaks. The W 4f spectrum consists of two sub-peaks due to the spin-orbit coupling of the 4f5/2 and 4f7/2 states.56 Furthermore, the very weak W 5p peak is also observed at approximately 41.6 eV. By comparing this W 4f spectrum with that of the pure WO3 NRs marked by the black curve, we find that the spectrum of the M-F/R/W HNS shifts toward lower binding energy by 0.085 eV, indicating a larger number of oxygen vacancies are due to the Fe3+-doped WO3 NRs in M-F/R/W HNS.57 Finally, the C 1s spectrum in Figure 3d indicates that low-intensity peaks of C-O, C=O and O-C=O bonds can be observed compared with those of C-C and C=C bonds. The C-O peak originates from the C-OH and C-O-C binding states of RGO.58

Spectroscopic Characterization The whole PEC process involves light harvesting, charge separation and transport, and finally, the chemical reaction. To evaluate light harvesting and charge separation of the bare WO3 NRs, F/W HNRs, Mn2+-doped F/W HNRs and M-F/R/W HNS, the light absorption and PL spectra of these four structures were characterized. Here, Mn2+-doped F/W HNRs are simplified as M-F/W HNRs. In Figure 4a, the light absorbance of the bare WO3 NRs presents an onset wavelength of approximately 475 nm corresponding to a bandgap of approximately 2.7 eV.59 After being doped with Fe3+ and decorated with α-Fe2O3 NPs, the F/W HNRs have dramatically redshifted onset values due to MGSs by doping and the narrower bandgap of α-Fe2O3. Comparatively, the M-F/W HNRs and M-F/R/W HNS demonstrate further redshifted onset wavelengths after Mn2+ doping. The redshift of the onsets is ascribed to valence of Mn2+ being lower than that of Fe3+, resulting in large number of oxygen vacancies (Ovac) in α-Fe2O3 NPs, which can form the defect mid-gap states and intrinsically broaden the visible-light absorption. Thus, M-F/R/W HNSs with Mn2+ doping concentrations of 0.5%, 1%, 1.5% and 2% are fabricated subsequently. It can be seen from Figure 4b that the onset



wavelengths of these samples are narrowly divided and gradually redshifted with increasing doping concentration. To explore the mechanism, DFT calculations to determine the band structures of α-Fe2O3 with different Mn2+ doping concentrations were performed, and the corresponding analysis will be carried out in detail later. The effects of Mn2+ and RGO on the band structure and charge separation efficiencies can also be verified via photoluminescence (PL) spectroscopy at room temperature. Taking the PL intensity at 500 nm as the uniform reference point, the normalized PL spectra of the F/W HNRs and M-F/W HNRs with different Mn2+ concentrations, as well as the M-F/R/W HNS with a Mn2+ concentration of 1%, are shown in Figure 5a. In the PL spectra of all samples, the peak found at approximately 710 nm is ascribed to the 4T1 to 6A1 transition of the dopant Fe3+ in WO3.60 After doping with Mn2+, several clear peaks from 520 to 700 nm are observed in the PL spectra of the M-F/W HNRs and M-F/R/W HNS. The peak at approximately 580 nm is caused by the 4T1 to 6A1 transition of the dopant Mn2+.61 The other peaks are believed to originate from the mid-gap states, such as the Ovac or other defects of the composite structures. We still notice that with increasing Mn2+ doping concentration and without the aid of RGO, the PL intensity of the M-F/W HNRs increases at first and then decreases. Many previous studies have attributed the initially increasing PL intensity to the increased MGSs, and the subsequently decreasing PL to the resonance quenching of adjacent dopant atoms.62, 63 While for the M-F/R/W HNS, it is well known that the excellent conductivity of RGO is beneficial to the transmission of photogenerated electrons and holes. Thus, the M-F/R/W HNS demonstrates substantially more efficient PL quenching than the M-F/W HNRs. Subsequently, taking the PL intensity at approximately 580 nm as the reference point, the normalized PL spectra of the M-F/W HNRs with different Mn2+ concentrations are exhibited in Figure 5b. It can be seen that the peak positions of these samples are little redshifted with increasing Mn2+ concentration. Combining the data in Figure 5a and b, the relationships between Mn2+ concentration, PL intensity and peak position of the 4T1 to 6A1 transition of Mn2+ are shown intuitively in the inset of Figure 5b. To further explore the internal mechanisms that, with increasing Mn2+ concentration from 0.5% to 2%, drive the PL intensity to increase and decrease successively and result in the peak position corresponding to the transition from 4T1 to 6A1 to redshift slightly from 581 to 583 nm, as well as the light absorption to exhibit a


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redshift, it is necessary to perform DFT calculations on the band structures and density of states (DOS) of α-Fe2O3 with different Mn2+ doping concentrations.

DFT Calculation and Analysis α-Fe2O3 has a rhombohedral structure with R3c space group;64 thus, a supercell containing 12 Fe and 18 O atoms was constructed in our work for calculating the band structure and density of states (DOS) of pure α-Fe2O3. Similarly, another supercell including 96 Fe and 144 O atoms was built for Mn-doped α-Fe2O3 (Mn-αFe2O3). Here, one, two and three Fe atoms with random positions were replaced with Mn atoms in this supercell to stand for 1%, 2% and 3% doping concentrations. For example, Figure 6a shows α-Fe2O3 including 95 Fe and 1 Mn atoms and represents a Mn2+ concentration of 1%. For all of the 96 transition metal atoms, half are spin-up, and the others are spin-down. The calculated band structures of α-Fe2O3 with Mn2+ concentrations of 0%, 1%, 2% and 3% are shown in Figure 6b-e. The calculated bandgap of pure α-Fe2O3 in Figure 6b is approximately 2.18 eV, which is similar to the experimental value of 2.1 eV and the value calculated in other theoretical studies. Figure 6c-e demonstrate that α-Fe2O3 with Mn2+ doping concentrations of 1%, 2% and 3%, simplified as 1% Mn, 2% Mn and 3% Mn, have bandgap values of approximately 1.89, 1.81 and 1.58 eV, respectively. Figure 6f summarize that the bandgap of α-Fe2O3 becomes gradually narrower with increasing Mn2+ doping concentration. Here, the values of Mn doping-concentration and width of bandgap are set as x- and y-axis, respectively. The zero value of y-axis is in line with the top of valence band. We notice that the MGSs are present in only the spin-up band structure of 1% Mn, and in both spin-up and spin-down band structures of 2% Mn, while no MGSs persist in 3% Mn in Figure 6. To explore the underlying mechanism of this changing trend in bandgap and MGSs of α-Fe2O3 with different Mn2+ concentrations, the total DOS and partial DOS (PDOS) of α-Fe2O3 with Mn2+ concentrations from 0% to 3% are exhibited in Figure 7. The spin-up and spin-down channels are present by positive and negative values, respectively, and the Fermi level is set at the point of zero energy.50 Consistent with the band structures, the DOS in Figure 7a-d demonstrate a gradually narrowing bandgap of α-Fe2O3 with increasing Mn2+ concentration. Similar to previous computational observations, the PDOS in Figure 7e-h



exhibit that the localized Fe 3d states are the main components of the conduction band (CB) and have higher densities than the O 2p. While in the VB, the O 2p electrons play a substantially more dominant role than those in Fe 3d. It can be seen from Figure 7f and g, and especially in the enlarged views of the insets, that all the MGSs are mainly generated from the Mn 3d states. Here, the MGSs also include weak Fe 3d states due to hybridization interactions between Mn and neighboring Fe ions.65 Additionally, 2% Mn shows MGSs in both spin-up and spin-down channels, demonstrating higher DOMGSs than that of 1% Mn. For the 3% Mn, the MGS is connected with the Mn 3d state and merged into a VB, leading to the vanishing of the MGS in the bandgap and the bandgap being further narrowed. In addition, the band structure and DOS of the Fe3+-doped WO3 (Fe-WO3) are also calculated. Figure 8a shows monoclinic WO3 with W, O and Fe atoms marked with blue, red and green callous. Compared with the calculated band structure of pure WO3 in SI part 2, that of the Fe3+-doped WO3 in Figure 8b shows a narrowed bandgap of approximately 2.55 eV, and MGSs are inserted in the bandgap. The DOS in Figure 8c demonstrate that the VB and CB are mainly dominated by O 2p and W 5d states, respectively. The two MGSs at approximately 1 and 1.5 eV show hybridizations of O-Fe and O-W, respectively. Based on the above theoretical results, the offsets of conduction band (∆Ec) and valence band (∆Ev) are calculated to estimate the band matching of the Mn-αFe2O3/Fe-WO3 hetero-junction. Firstly, Barroso et al. 66 and Bledowski et al. 67 confirmed that the electron affinities of α-Fe2O3 (χ1) and WO3 (χ2) are 4.9 and 4.74 eV below the vacuum level, respectively. So ∆Ec could be estimated as χ1-χ2=4.9-4.74=0.16 eV. And then, referenced from Figure 6 and 8, the bandgaps of Mn-αFe2O3 and Fe-WO3 are set as 1.81 and 2.55 eV, respectively. Thus, the difference of the bandgaps is 0.74 eV and ∆Ev=0.74-∆Ec=0.58 eV. Therefore, a schematic band structure with the energy levels of the Mn-αFe2O3/Fe-WO3 HNS is presented in Figure 8d. With the aid of MGSs and the split d-states of Fe or Mn, the electrons have more pathways for the upward transition from the VB to CB than do the electrons of materials without doping. Correspondingly, the PL emission also has significantly more probabilities through downward transitions, leading to carrier recombination, so the PL should be depressed to enhance electron-hole separation and prolong the electron lifetime of the Mn-αFe2O3/Fe-WO3 HNS.


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PEC Properties In the Mn-αFe2O3/Fe-WO3 HNS, RGO was inserted between Mn-αFe2O3 and Fe-WO3 to enhance the carrier separation and prolong their lifetime. Open-circuit voltage decay (OCVD) and electron lifetime measurements were performed to verify the above theoretical analysis, and especially the effect of doping and RGO on charge recombination kinetics. Figure 9a shows the OCVD curves of the F/W HNRs, M-F/W HNRs and M-F/R/W HNSs as a function of time. Here, the Mn2+ doping concentration is 1%. These samples were illuminated for 30 s to obtain an equal open-circuit voltage (Voc), and then, the Voc decay was measured without illumination. Compared with the F/W HNRs, the M-F/W HNRs exhibit a slower Voc decay due to the MGSs caused by Mn2+, and therefore, possess a faster electron-hole separation. The M-F/R/W HNSs demonstrate the slowest Voc decay because the electron-hole recombination and PL are efficiently retarded by RGO. Based on the Voc decay rate, the electron lifetime curves can be deduced from the equation, τ௘௟௘௖௧௥௢௡ = −

௄ಳ ் d௏ೀ಴ ିଵ 68 ቀ d௧ ቁ , ௘

where kBT is the

thermal energy, e is the positive elementary charge, and dVoc/dt denotes the derivative of the transient open-circuit voltage. Figure 9b shows the electron lifetime (τelectron) as a function of Voc. The M-F/R/W HNSs exhibits the longest lifetime because of the combined effects mentioned above. To further study the contribution of each part of the M-F/R/W HNSs to the overall PEC efficiency, photocurrent-potential (J-V) measurements and electrochemical impedance spectroscopy (EIS) were subsequently performed. The J-V measurements of the bare WO3 NRs, F/W HNRs, M-F/W HNRs and M-F/R/W HNSs were performed under white-light illumination. Figure 10a shows that the F/W HNRs have an enhanced current density of 1.30 mA/cm2 at a potential of 1.6 V vs. Ag/AgCl, which is higher than the current density of 0.68 mA/cm2 of the bare WO3 NRs. This result is ascribed to the enhanced light absorption and photoconversion of α-Fe2O3 and the efficient electron-hole separation via the cascade band alignment of WO3 and α-Fe2O3, as well as the MGSs caused by doping Fe3+ into WO3. The M-F/W HNRs demonstrates a further improved current density of approximately 1.45 mA/cm2 due to the MGSs caused by doping Mn2+ into α-Fe2O3 NPs. Finally, with the aid of RGO, the current density was dramatically increased to 2.55 mA/cm2, ascribed to the fast



charge transfer through RGO. The PEC J-V curves of the M-F/R/W HNS with different Mn2+ doping concentrations under white-light illumination and dark conditions are shown in Figure 10b. The inset presents the current densities of the M-F/R/W HNS with different Mn2+ doping concentrations at a potential of 1.4 V vs Ag/AgCl. It can be seen that the PEC current density of the M-F/R/W HNS increases at first and then decreases with increasing Mn2+ concentration and that the optimized Mn2+ doping concentration is 1%. Although this experimentally optimized concentration differs a little from the theoretical value of 2%, the theoretical result in the previous section could confirm that the M-F/R/W HNS with a Mn2+ doping concentration of 1% has the highest density of MGSs, comparatively. EIS reveals the interfacial properties of the electrodes and electrolyte. The EIS plots of the bare WO3 NRs, F/W HNRs, M-F/W HNRs and M-F/R/W HNSs in a 0.5 M Na2SO4 solution under illumination are shown in SI part 3. The EIS plot of the M-F/R/W HNSs has a smaller diameter and starting point on the Z’ axis than those of the other structures, indicating a faster charge transfer at the anode/electrolyte interface and a lower intrinsic resistance due to RGO. These results are consistent with the OCVD measurement results mentioned above. Furthermore, to evaluate PEC performance of our M-F/R/W HNSs, comparison of the photocurrent with reported works is present in Table 1. It can be observed from this table that M-F/R/W HNSs as the building block of PEC anode possesses pretty high photocurrent and appears to be a promising candidate for next-generation photo-conversion material. Finite element modeling (FEM) was employed to analyze the difference in the charge transfers between the M-F/W HNR and M-F/R/W HNS. The simplified models of the M-F/W HNR and M-F/R/W HNS are shown in SI part 4. Orange, light green and yellow parts in Figure S5a represent the M/F NPs, WO3 NRs and RGO, respectively. Fig. S5b demonstrates that under illumination, RGO plays an important role in facilitating carrier transfers and prolonging their lifetimes, leading to the highly improved PEC efficiency.

CONCLUSIONS

A novel M-F/R/W HNS is demonstrated as an efficient PEC photoanode. The enhanced PEC efficiency of the M-F/R/W HNS can be ascribed to an efficient charge separation via the type-II cascade band alignment of α-Fe2O3 and WO3 and the fast charge transport through the


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RGO layer, as well as the MGSs induced by Mn2+ and Fe3+ doping. Moreover, experimental and DFT studies confirm that the band structure of α-Fe2O3 can be modulated via tuning the Mn2+ concentration. With increasing Mn2+ concentration, the bandgap gradually narrows, and the MGSs gradually approach then merge into the VB. The M-F/R/W HNS demonstrates a maximum PEC efficiency when the doping concentration of Mn2+ is 1% via experiment. The M-F/R/W HNS and DFT calculation proposed in the current study have promising potential in the design of future energy devices.

■ ASSOCIATED CONTENT Supporting Information Supporting Information is available free of charge on the ACS Publications website at DOI: XRD spectra, theoretical DFT calculation, finite elemental modelling and EIS measurement of the nanostructures.

■ AUTHOR INFORMATION Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS

This work was supported by (NRF-NRF-2016R1A2B2011416).

the

National

Research

Foundation

of

Korea

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Figure 1. a-c) Schematic process for the fabrication of the M-F/R/W HNS: (a) bare WO3 NRs, (b) covered with an RGO layer, and (c) decorated with Mn-Fe2O3 NPs; d) 45° tilted-view SEM image of the M-F/R/W HNS; e, f) enlarged views of the WO3 NRs covered with the M-F/R layer (e) and Mn-Fe2O3 NPs (f) taken from the red squares of figure 1d and e. The scale bars in figure 1d-f are 10 µm, 1 µm and 100 nm.



Figure 2. a) TEM image of M-F/R/W HNS; b, c) enlarged views taken from the green (b) and red (c) squares of figure 2a. The insets of figure 2b and c are their respective FFT patterns. d-h) EELS elemental maps taken from Figure 2a. i) Sectional XRD patterns of the WO3 NRs, F/W HNRs and M-F/R/W HNS. The smoothed curves are marked by red. The diffraction peaks of WO3 and Fe2O3 are labeled by green and blue arrows, respectively.


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Figure 3. Fe 2p (a), Mn 2p (b), W 4f (c) and C 1 s (d) XPS spectra measured from the M-F/R/W HNS.



Figure 4. a) Light absorption curves of the M-F/R/W HNS (green), M-F/W HNRs (blue), F/W HNRs (red) and bare WO3 NRs (black). b) Light absorption curves of the M-F/R/W HNS with Mn doping concentrations of 2.0% (green), 1.5% (blue), 1.0% (red) and 0.5% (black).


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Figure 5. a) Using the PL intensity at a wavelength of 500 nm as the reference point, the normalized PL spectra of the M-F/W HNRs with Mn2+ concentrations of 0.5% (solid red), 1.0% (solid blue), 1.5% (solid green), and 2.0% (solid pink); the M-F/R/W HNSs (dashed blue); and the F/W HNRs (black). b) Using the maximum PL intensity as the reference point, the normalized PL spectra of the M-F/W HNRs with Mn2+ concentrations of 0.5% (red), 1.0% (blue), 1.5% (green) and 2.0% (pink). The inset is a 3D curve for the relationships among the Mn2+ concentration of the M-F/W HNRs, maximum PL intensity and the corresponding wavelength.



Figure 6. a) Crystal structure of Mn2+-doped α-Fe2O3. b-e) The calculated band structures of Mn2+-doped α-Fe2O3 with Mn2+ concentrations of 0% (b), 1% (c), 2% (d) and 3% (e); the left and right columns correspond to spin-up and spin-down, respectively. f) A comparison of the calculated bandgaps among Mn2+-doped α-Fe2O3 with different Mn2+ concentrations.


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Figure 7. DOS (a-d, upper row) and PDOS (e-h, bottom row) of Mn2+-doped α-Fe2O3 with Mn2+ concentrations of 0% (a, e), 1% (b, f), 2% (c, g) and 3% (d, h). The negative and positive curves stand for spin-up and spin-down, respectively.



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Figure 8. a-c) Crystal structure (a), calculated band structure (b) and DOS (c) of Fe3+-doped WO3. d) A schematic

of the energy levels and

(Mn2+-Fe2O3)/(Fe3+-WO3).


electron

transitions of

the

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Figure 9. a) OCVD curves of the photoanodes made from the M-F/R/W HNS (blue), M-F/W HNRs (red) and F/W HNRs (black). b) The electron lifetime as a function of the normalized Voc obtained from figure 9a.



Figure 10. a) PEC current density as a function of potential (Ag/AgCl) for the M-F/R/W HNS with 1% Mn2+ (dark green), M-F/W HNRs (blue), F/W HNRs (red) and bare WO3 NRs (black) in response to on-off cycles of illumination. b) PEC current densities as a function of potential (Ag/AgCl) for the M-F/R/W HNS with Mn doping concentrations of 2.0% (pink), 1.5% (dark green), 1.0% (blue), 0.5% (red) and 0% (black) under visible-light illumination and dark conditions. The inset shows the current densities of the M-F/R/W HNS with different Mn2+ doping concentrations at a potential of 1.4 V vs Ag/AgCl.


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Table 1. PEC photocurrents of M-F/R/W HNSs in this work and α-Fe2O3/WO3 HNSs in other reported works

Name

Morphology

Doping in Doping Third-party

Electrolyte/

Photocurrent

α-Fe2O3

in WO3

Material

Concentration (M)

(mA/cm2) *

Fe2O3/WO3[13]

3D Mesoporous

×

×

×

NaOH/0.1

0.9

Fe2O3/WO3

[69]

Hetero-nanorod

×

×

×

Na2SO4/0.1

1.3

Fe2O3/WO3

[14]

Hetero-nanoneedle

×

×

Co-Pi

KH2PO4/K2HPO4/0.1

1.9

Nanosheet

×

×

×

Na2SO4/0.5

2.9

WO3/Fe2O3

[70]

Heterojunction

×

×

CoPd

KPi buffer/pH=7

0.6

WO3/Fe2O3

[71]

Composite Film

×

×

×

Na2SO4/0.5

2.5

**

Hetero-nanorod

Mn

Fe

RGO

Na2SO4/0.5

2.5

WO3@Fe2O3[17]

M-F/R/W *

: At potential of 1.5 V vs RHE;

**

: This work.



TOC:

Synopsis: The as-prepared (Mn2+-Fe2O3)/RGO/(Fe3+-WO3) photoanode shows excellent photoelectrochemical efficiency.


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(Fe -WO3) - ACS Publications

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