Fe2O3 Photoanode for Enhanced

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Photocharged FeTiO/FeO Photoanode for Enhanced Photoelectrochemical Water Oxidation Jiujun Deng, Xiaoxin Lv, and Jun Zhong J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08826 • Publication Date (Web): 06 Dec 2018 Downloaded from http://pubs.acs.org on December 9, 2018

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The Journal of Physical Chemistry

Photocharged Fe2TiO5/Fe2O3 Photoanode for Enhanced Photoelectrochemical Water Oxidation Jiujun Deng1, Xiaoxin Lv2, Jun Zhong3* 1

Institute for Energy Research, Jiangsu University, 301 Xuefu Road, Zhenjiang 212013, China

2

Automotive Engineering Research Institute, Jiangsu University, 301 Xuefu Road, Zhenjiang 212013, China

3

Institute of Functional Nano and Soft Materials Laboratory (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou 215123, China

ABSTRACT Photocharging is a novel and effective photoanode treatment to improve the photoelectrochemical water splitting performance. Here a photocharged Fe2TiO5/Fe2O3 photoanode was prepared with a significantly increased photocurrent density. Furthermore, the photocharging effect could also be deactivated via a discharging treatment under dark conditions. By using X-ray absorption spectroscopy and electrochemical impedance spectroscopy, the photocharging effect was revealed to be the formation of Fe(IV) intermediates at the interface to accelerate the water oxidation kinetics. The results offer a

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detailed understanding on the mechanism of the photocharging effect in hematite, which may help for the design of highly efficient water splitting catalysts. INTRODUCTION Among various semiconductor candidates, hematite (α-Fe2O3) is a promising material for photoelectrochemical (PEC) water splitting due to its abundance, low cost, extraordinary chemical stability in aqueous solution, and a favorable optical band gap of 2.0-2.2 eV accounting for a theoretical solar-to-hydrogen (STH) efficiency of 16.8%.1-5 However, the PEC water splitting performance of pristine hematite photoanode is limited by a number of intrinsic drawbacks, such as poor carrier conduction, low absorption coefficient, short hole diffusion length (2-4 nm), and slow oxygen evolution reaction (OER) kinetics.6-10 Extensive effective efforts have been devoted to address the above-mentioned problems, including morphological control, elemental doping, heterostructure formation, surface passivation and cocatalysts deposition.3-10 For instance, Ti-based modification has been widely used to significantly improve the solar water splitting efficiency of hematite photoanode by enhancing the conductivity via the introduction of Ti atoms (Ti doping)6,11 or reducing the photogenerated

carriers

accumulation

through

the

formation

of

Fe2TiO5/Fe2O3

heterostructure.7-15 Recently, a novel and effective photoanode treatment technique called ‘photocharging’ was reported on BiVO4 photoelectrode, which demonstrated that the PEC performance of BiVO4 photoanodes could be dramatically improved through the prolonged light exposure procedure.16-18 Meanwhile, the similar phenomenon was also observed on a Ti modified hematite photoanode in a recent report.15 Xie et al. found that the PEC activity of Ti doped hematite (Ti:Fe2O3) photoanode could be significantly self-improved by long-term (122


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70 h) working in the strong alkali electrolyte solution under illumination.15 Detailed characterizations revealed that a FeOOH layer was formed at the interface of Ti:Fe2O3/electrolyte and it could serve as a cocatalyst to suppress the recombination of photogenerated carriers.15 While this understanding is reasonable, the detailed information for the photocharging itself in hematite photoanode is still lacking. To further understand the detailed mechanism of photocharging for the enhanced PEC performance, here a photocharged Fe2TiO5/Fe2O3 photoanode was tested by prolonged exposing to AM 1.5G illumination. The photocurrent density of Fe2TiO5/Fe2O3 at 1.23 V vs. RHE significantly increased from 1.61 mA cm-2 to 2.01 mA cm-2 after irradiation for 150 min. Moreover, this promoted PEC activity could be deactivated through a discharging treatment under dark conditions. Based on the X-ray absorption spectra (XAS) and EIS characterizations, the photocharging effect can be attributed to the accelerated water oxidation kinetics with the formation of Fe(IV) intermediates at the photoanode/electrolyte interface. Meanwhile, the discharging effect could be explained by the decomposition of Fe(IV) intermediates. This result offers a novel and effective approach to improve the PEC performance of hematite. EXPERIMENTAL SECTION Synthesis of photoanodes: The synthesis procedure of Fe2TiO5/Fe2O3 photoanode has been reported in our previous work.12 Briefly, the cleaned FTO substrate (50 mm × 30 mm × 2 mm) was firstly immersed into TiCl4 aqueous solution (4 mM, 100 mL) and kept at 75°C for 30 min. Subsequently, a thin TiO2 underlayer was formed on the surface of FTO substrate after annealed at 180°C for 15 min in air. The Ti-treated FTO glass was then put into an autoclave containing 15 mL aqueous solution of 75 mM of FeCl3·6H2O and 0.1 M of NaNO3. The 3



hydrothermal reaction was conducted at 95 °C for 4 h. After sintering in air at 550 °C for 2 h and at 750 °C for additional 15 min, the Fe2TiO5/Fe2O3 photoanode was finally obtained. Structural characterization: The morphology images of all the samples were collected by High-resolution Transmission Electron Microscope (HRTEM, FEI/Philips Techai 12 BioTWIN). X-ray Photoelectron Spectrometry (XPS, VG Escalab 220iXL equipped with an Al Kα source) and X-ray absorption spectra were used to study the electronic structure and chemical composition of all samples, respectively. X-ray absorption spectra were collected at the National Synchrotron Radiation Laboratory (NSRL, XMCD beamline) and the Beijing Synchrotron Radiation Facility (BSRF, Soft X-ray beamline). The photoanodes were directly used as samples for XAS experiments. Samples were loaded in an ultra-high vacuum (UHV) chamber with a base pressure of ~8 x 10-9 torr and data acquisition pressure of ~1x10-8 torr. All spectra were recorded at room temperature using the total electron yield (TEY) mode, a surface-sensitive detection method with a typical probing depth of a few nm, in the energy range from 450 to 730 eV and with an experimental resolution of 0.2 eV. The incident beam was set at the angle about 45° relative to the sample. The XAS spectra have been processed using pre- and post-edge normalization routines. PEC Measurements. All PEC measurements of the photoanodes were carried out by a CHI 660D electrochemical workstation in a three-electrode configuration with 1.0 M NaOH (pH 13.6) aqueous solution as an electrolyte, Fe2TiO5/Fe2O3 photoanode covered by nonconductive hysol epoxy as a working electrode, a platinum wire as a counter electrode and Ag/AgCl (3 M KCl) electrode as a reference electrode. The J-V curves of all samples were obtained by using linear sweep voltammetry (LSV) method. For the photocharging process, 4


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the Fe2TiO5/Fe2O3 photoanode was exposed to 1 sun illumination for various time at 1.23 V vs. RHE. For the discharging process, the photocharged photoanode was subsequently left in the PEC cell for 10 min under dark conditions without any external bias. High Brightness Cold Light Source (XD-300) equipped with AM 1.5G filter was used as the light source and the power density is 100 mW cm-2. Electrochemical impedance spectra (EIS) under illumination (AM 1.5G, 100 mW cm-2) were recorded using the CHI 660D electrochemical workstation. A sinusoidal voltage pulse with the amplitude of 5 mV was applied at 1.23 V vs. RHE, with a frequency that ranged from 100 kHz to 1 Hz.

RESULTS AND DISCUSSION

Figure 1. (a) J-V plots of the Fe2TiO5/Fe2O3 photoanode with various light expose time. (b) Photocurrent- time curve of the Fe2TiO5/Fe2O3 photoanode at 1.23 V vs. RHE under illumination and in the dark. Figure 1a shows the photocurrent density versus applied potential (J-V) curves of Fe2TiO5/Fe2O3 photoanode with various expose time to the light. Before irradiation, the pristine Fe2TiO5/Fe2O3 photoanode shows a photocurrent density of 1.62 mA cm-2 at 1.23 V vs. RHE, 5



which is similar to that of previous reports.12 However, with the extended irradiation time, a progressive increase in photocurrent density can be observed (Figure 1b). For instance, a higher photocurrent density of 1.90 mA cm-2 was obtained after working for 60 min and it was further increased to 2.02 mA cm-2 when the expose time was extended to 150 min (Figure 1a). A drop can also be observed in Figure 1b, which can be attributed to the dark situation when the light is switched off. The photocharged effect can keep for a long time in dark with the applied bias and then continue to increase when the light is on. Previous studies also showed that the photocharging treatment could lead to a greatly enhanced PEC performance but the catalysts were mainly based on BiVO4. There are only few reports to investigate the photocharging effect on hematite-based photoanode.15 Herein, through long-term working in 1 M NaOH electrolyte under 1 sun illumination, a photocharged Fe2TiO5/Fe2O3 photoanode with improved photocurrent was prepared.

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Figure 2. SEM and HRTEM images of (a, c) the pristine and (b, d) photocharged Fe2TiO5/Fe2O3 photoanodes. SEM, HRTEM and XRD characterizations were performed to investigate the effect of photocharging treatment on the morphology and crystal structure of Fe2TiO5/Fe2O3 photoanode. As shown in the SEM images (Figure 2a and 2b), the pristine and photocharged Fe2TiO5/Fe2O3 (irradiation for 60 min) samples show similar nanorod-like morphology, implying that the photocharging treatment does not alter the morphology. The HRTEM images further confirm this conclusion. As observed in Figure 2c and 2d, there is no obvious difference between the pristine and photocharged Fe2TiO5/Fe2O3 (irradiation for 60 min) films. In addition, no cocatalyst overlayers can be observed on the surface of hematite nanostructures. It means that the morphology of Fe2TiO5/Fe2O3 photoanode is well retained after photocharging treatment. 7



Figure S1 shows the XRD data of the pristine and photocharged samples, which also reveal that the photocharged sample preserves the crystal structure of hematite (JCPDS 33-0664) without any additional XRD peaks. These results suggest that the enhanced photocurrent density of the photocharged Fe2TiO5/Fe2O3 is not originated from the morphology change or the formation of cocatalyst overlayer on the hematite surface, such as the FeOOH cocatalyst reported in previous study.15

Figure 3. (a) Electrochemical impedance spectra (EIS) measured at 1.23 V vs. RHE for Fe2TiO5/Fe2O3 photoanode with various light expose time. The inset shows the equivalent circuit. (b) Parameters of equivalent circuit elements. To further investigate the effect of photocharging on the charge transfer process of Fe2TiO5/Fe2O3 photoanode, EIS under light illumination were carried out. As a powerful tool, EIS has been widely used in exploring the working mechanism of hematite photoanodes.12,19,20 Figure 3a shows the Nyquist plots of photocharged Fe2TiO5/Fe2O3 photoanode with various light expose time at 1.23 V vs. RHE. All the hematite samples exhibit one semicircle curves. Generally, two semicircles can be observed in all types of photoanodes at a low applied bias. However, when the applied bias increases, the semicircle at the high-frequency is maintained but the semicircle at the low-frequency can reduce. At a more positive bias such as 1.23 V vs. 8


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RHE, the two semicircles can merge into a single semicircle as reported in previous studies.20,21 From Figure 3a, it can also be clearly observed that the semicircle diameter of photocharged Fe2TiO5/Fe2O3 photoanode gradually decreases with the increased expose time, indicating the promoted charge transfer kinetics during the photocharging process.12,19,20 Next, the EIS data were further studied by using an equivalent circuit (EC) model and the fitted results are shown in Figure 3b. As displayed in the inset of Figure 3a, the EC model comprises a series resistance of the PEC cell (Rs), a depletion layer capacitance (Cbulk), a trapping resistance of the charge transfer from the bulk Fe2TiO5/Fe2O3 to the traps (Rtrap), as well as a charge transfer resistance (Rct) and a Helmholtz capacitance (Ctrap) at the electrode/electrolyte interface.12,19,20 From the fitting results, a gradual decrease in Rct values from 3458 Ω to 1935 Ω can be observed in Figure 3b with the increased expose time to 150 min,

suggesting

the

better

transportation

of

photogenerated

holes

across

the

electrode/electrolyte interface.12,19,20 This result also reveals that the facilitated OER activity by photocharging treatment is responsible for the enhancement in photocurrent density of Fe2TiO5/Fe2O3 photoanode.

Figure 4. (a) J-V plots of the pristine, charged and discharged Fe2TiO5/Fe2O3 photoanodes. (b) 9



J-V plots of the recharged Fe2TiO5/Fe2O3 photoanodes with various repeat times. Subsequently, this enhancement of photocurrent for photocharged Fe2TiO5/Fe2O3 sample was found to be dynamically turned off by a discharging treatment in the dark conditions. As shown in Figure 4a, after ceasing the light source and left the photoanode in the PEC cell for 10 min, the re-measured LSV curve under illumination is almost the same as that for the pristine Fe2TiO5/Fe2O3 photoanode before photocharging. This shows that the discharging process facilitates the deactivation of improved PEC activity. However, the process of photocharging repeated later on (recharging process) is equally effective and results in a recovery of the improved PEC performance. As shown in Figure 4b, the photocurrent of recharged Fe2TiO5/Fe2O3 photoanode gradually fell back to previous photocharging level after the LSV measurements repeated for 3 times. This photocharging and discharging effect is also clearly observed at a different bias, for example 1.0 V vs. RHE (Figure S2). It is the first time to observe this effect in Ti-modified hematite.

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Figure 5. (a) Ti L-edge, (b) Fe L-edge and (c) O K-edge XAS spectra of the pristine Fe2TiO5/Fe2O3 film (black curve), a photocharged Fe2TiO5/Fe2O3 film for 60 min (red curve), and a discharged Fe2TiO5/Fe2O3 film for 10 min (blue curve), respectively. To explore the changes of electronic structure and chemical state of Fe2TiO5/Fe2O3 photoanode in photocharging/discharging process, XPS and synchrotron radiation based XAS measurements were carried out. XAS has been shown as a powerful and useful technique in probing the electronic structure of nanostructured hematite photoanode.7,12,22 Recently, Trześniewski et al. performed in-situ XAS experiments at V K-edge to probe the photocharging effect of BiVO4 photoanode but no obvious electronic structure changes could be observed.23 It could be attributed to the hard X-ray XAS experiments with a bulk sensitivity. In addition, 11



Braun et al. have also performed the operando XAS experiments of hematite photoanode and observed the electronic structure changes at the O K-edge.22 Thus here we use the surfacesensitive XAS in the soft X-ray range (O K-edge) to probe the electronic structure changes. As shown in Figure 5, the normalized Ti L-edge, Fe L-edge and O K-edge XAS spectra of the pristine, photocharged and discharged Fe2TiO5/Fe2O3 samples have been illustrated, respectively. Among them, the Ti L-edge spectra displayed in Figure 5a show similar features to those of Fe2TiO5 in previous reports, clearly revealing the formation of Fe2TiO5/Fe2O3 heterojunction photoanode in this study.7,12 The Fe L-edge spectrum of photocharged and discharged Fe2TiO5/Fe2O3 compared with the spectra of pristine Fe2TiO5/Fe2O3 sample are also shown in the Figure 5b. There is almost no difference in these three samples, indicating that photocharging/discharging process did not affect the main crystal structure of Fe2TiO5/Fe2O3 nanostructure. While, comparing with that of the pristine Fe2TiO5/Fe2O3 sample, an additional peak at 532.7 eV is observed in O K-edge XAS spectra of photocharged Fe2TiO5/Fe2O3 sample as shown in Figure 5c. According to the previous results,24, 25 this peak can be assigned to the formation of Fe intermediate species in higher oxidation state. i.e., Fe(IV) at the interface between hematite and electrolyte. Moreover, a clear shift of the O1s peak towards lower binding energies is also observed in XPS spectra of photocharged Fe2TiO5/Fe2O3 sample (Figure S3d), which further indicates the formation of higher oxidation state of Fe in the photocharging process. However, due to the limited sensitivity of XPS compared to XAS, it is difficult to clearly identify the Fe(IV) signal from the XPS spectra in this case. Based on the results of EIS and XAS characterizations, the process of photocharging and discharging treatments are illustrated in Figure 6. As shown in Figure 6a, when excited with a 12


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photon, an electron-hole pair is generated in bulk Fe2TiO5/Fe2O3 photoanode. Subsequently, the photo-generated holes are transferred to the interface and then stored in surfaceaccumulated Fe(IV) intermediates as described by equation (1): Fe(III) ― OH + h + →Fe(IV) = O + H + The presence of high-valet Fe state (such as Fe(IV)) was already reported in the literatures,2628 which

could act as an important intermediate to further oxidize water to produce O2.27 Fe(IV)

species can contribute to an efficient OER activity as reported in some recent studies.29,30 Thus, the kinetic process of water oxidation on hematite can be accelerated to enhance the performance. Actually, the important intermediate Fe(IV)=O groups were also directly observed by in-situ infrared spectroscopy in a recent report.26 Thus, the increased photocurrent density of photocharged Fe2TiO5/Fe2O3 photoanode could be attributed to the formation and accumulation of Fe(IV) species at the interface to promote the OER activity. Interestingly, after discharging under dark conditions for 10 min, the peak in O K-edge XAS spectra for the photocharged Fe2TiO5/Fe2O3 sample at 532.7 eV disappears as shown in Figure 5c. This could be assigned to the decomposition reaction of Fe(IV) surface intermediate shown in Figure 6b and described by the equation (2)28: 2Fe(IV) = O→2Fe(III) + O2 All the results confirm our hypothesis that the photocharging effect can be attributed to the formation of Fe(IV) surface intermediate, which can be used to effectively enhance the performance of hematite photoanode for water splitting.

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Figure 6. Illustration of (a) photocharging and (b) discharging of Fe2TiO5/Fe2O3 photoanode.

CONCLUSIONS In summary, a photocharging modified Fe2TiO5/Fe2O3 photoanode with enhanced PEC activity was prepared through prolonged exposure to AM 1.5G illumination. We found that such photocharging treatment could be related to a facilitated charge transfer from photoanode to electrolyte solution. Furthermore, the photocharging effect could be deactivated via a discharging treatment under dark conditions. Based on the XAS and EIS data, the photocharging effect could be attributed to the formation and accumulation of Fe(IV) intermediates at the interface of photoelectrode/electrolyte, which could significantly promote the OER activity of Fe2TiO5/Fe2O3 photoanode. The results offer a detailed understanding for the photocharging effect in hematite to improve the PEC efficiency.

ASSOCIATED CONTENT Supporting Information. J-V plots (at 1.0 V vs. RHE), XRD and XPS of pristine and photocharged Fe2TiO5/Fe2O3 photoanode. This material is available free of charge via the Internet at http://pubs.acs.org. 14


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ACKNOWLEDGMENT We acknowledge the support from NSRL and BSRF for the XAS experiments, the highperformance computing platform of Jiangsu University. This work was financially supported by the Jiangsu University Foundation (18JDG019), the National Natural Science Foundation of China (21808090, U1732110). This is also a project supported by Collaborative Innovation Center of Suzhou Nano Science & Technology, Soochow University-Western University Centre for Synchrotron Radiation Research, and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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(24)Bora, D. K.; Braun, A.; Erat, S.; Ariﬃn, A. K.; Löhnert, R.; Sivula, K.; Töpfer, J.; Grätzel, M.; Manzke, R.; Graule, T.; et al. Evolution of An Oxygen Near-Edge X-ray Absorption Fine Structure Transition in the Upper Hubbard Band in α-Fe2O3 upon Electrochemical Oxidation. J. Phys. Chem. C 2011, 115, 5619-5625. (25)Braun, A.; Hu, Y. L.; Boudoire, F.; Bora, D. K.; Sarma, D. D.; Grätzel, M.; Eggleston, C. M. The Electronic, Chemical and Electrocatalytic Processes and Intermediates on Iron Oxide Surfaces during Photoelectrochemical Water Splitting. Catal. Today 2016, 260, 72-81. (26)Zandi, O.; Hamann, T. W. Determination of Photoelectrochemical Water Oxidation Intermediates on Haematite Electrode Surfaces Using Operando Infrared Spectroscopy. Nat. Chem. 2016, 8, 778-783. (27)Wijayantha, K. G. U.; Saremi-Yarahmadi, S.; Peter. L. M. Kinetics of Oxygen Evolution at α-Fe2O3 Photoanodes: A Study by Photoelectrochemical Impedance Spectroscopy. Phys. Chem. Chem. Phys. 2011, 13, 5264-5270. (28)Klahr, B.; Gimenez, S.; Fabregat-Santiago, F.; Bisquert, J.; Hamann, T. W. Electrochemical and Photoelectrochemical Investigation of Water Oxidation with Hematite Electrodes. Energy Environ. Sci. 2012, 5, 7626-7636. (29)Takashima, T.; Ishikawa, K.; Irie, H. Detection of Intermediate Species in Oxygen Evolution on Hematite Electrodes Using Spectroelectrochemical Measurements. J. Phys. Chem. C 2016, 120, 24827-24834.

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Fe2O3 Photoanode for Enhanced

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