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Self-Improvement of Ti:FeO Photoanodes: Photoelectrocatalysis Improvement after Long-Term Stability Testing in Alkaline Electrolyte Jiale Xie, Pingping Yang, Xiaorong Liang, and Jinyun Xiong ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00445 • Publication Date (Web): 22 May 2018 Downloaded from http://pubs.acs.org on May 25, 2018
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ACS Applied Energy Materials
Self-Improvement of Ti:Fe2O3 Photoanodes: Photoelectrocatalysis Improvement after Long-Term Stability Testing in Alkaline Electrolyte Jiale Xie,*,†,‡,┴ Pingping Yang, †,‡,┴ Xiaorong Liang,‡ and Jinyun Xiong ‡ † Institute of Materials Science and Devices, Suzhou University of Science and Technology, Kerui Road, Suzhou, 215009, People’s Republic of China. ‡ Institute for Clean Energy & Advanced Materials, Faculty of Materials and Energy, Southwest University, Tiansheng Road, Chongqing 400715, People’s Republic of China.
Corresponding Author *J.L. Xie, E-mail:
[email protected] 1 ACS Paragon Plus Environment
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KEYWORDS:
photoelectrocatalysis,
hematite,
stability,
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alkaline
electrolyte,
self-improvement ABSTRACT: Hematite photoanode is a widely accepted stable photoelectrode in a strong alkali solution, such as NaOH aqueous solution. However, no one systematically investigates the photoelectrochemical stability of hematite-based photoanodes. More importantly, there are some contradictory results about the stability of hematite photoanodes in the literature. Herein we investigate the long-term stability of Ti doped hematite (Ti:Fe2O3) photoanode in 1.0 M NaOH under visible light. Ti:Fe2O3 photoanode exhibits the significant photocurrent enhancement and the cathodic shift of the onset potential after long-term working in a strong alkali solution. Detailed characterizations reveal that a FeOOH layer, which serves as co-catalyst for self-improvement of Ti:Fe2O3 photoanodes, is formed at the interface of Ti:Fe2O3/electrolyte junction.
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1. INTRODUCTION Hematite
(Fe2O3)
has
been
extensively
investigated
as
photoanode
for
photoelectrochemical (PEC) water splitting.1-3 This should thank to its low bandgap of ~2.0 eV and its abundant reserves.4-5 More importantly, it is believed that hematite is of excellent (photo)electrochemical stability in a pH range from 3 to 14.6 In thermodynamics, hematite is the most stable form of iron oxide.7 Meanwhile, several spectroscopic techniques clearly confirm the existence of a surface state at the hematite/electrolyte interface where holes accumulate.8-12 This surface state will usually induce the charge recombination, the photocorrosion and the decay of photocurrent after long-term stability testing. Therefore, some ultrathin passivating layers, such as TiO2 and Al2O3,13-15 are applied to eliminate the risk of charge recombination at electrode/electrolyte interface. Indeed, the improved photocurrent and the more cathodic onset potential are observed by the passivation of surface states. However, there is increasing evidence that a surface state-mediated charge transfer process exists at the hematite/electrolyte interface.9, 16-17 Density functional theory (DFT) calculations indicate that the surface states act as recombination center at low bias, the surface state-mediated charge transfer happens at higher bias, and, at more anodic bias, the water oxidation proceeds via direct transfer of holes from the valence band.8 Despite the role of surface states, how about the long-term (>12 h) stability of hematite photoanodes. Typically, bare Fe2O3 photoanode shows ~29.23% decay after 3.3 h stability testing at 1.23 V (vs. RHE) in 1.0 M KOH.18 However, another work suggests that ~9.84% enhancement is achieved by bare Fe2O3 photoanode after 2.5 h stability testing at 1.23 V (vs. RHE) in 1.0 M NaOH.19 Recently, Dias et al. observed an extremely stability of 19 nm 3 ACS Paragon Plus Environment
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hematite film over 1000 h in 1.0 M NaOH at 1.45 V (vs. RHE).20 However, the onset potential exhibits negative shift as time increasing in the dark, but no cathodic onset potential shift under illumination. Therefore, these contradictory results inspire us to investigate the long-term stability of hematite-based photoanodes. As far as we know, no work clarifies this contradiction and tries to reveal the thing happened during the long-term testing. In order to overcome the poor conductivity limitation of Fe2O3, the foreign elements such as Sn, Si, Mo, Pt and Ti are broadly used to perform the n-type doping.21-25 Based on the previous reported results, Ti doped Fe2O3 photoanodes exhibit a highest photocurrent among the above doping elements.26 Thus, the Ti dopant is chosen to achieve a high photocurrent and make the phenomena more obvious. Herein, we investigate the long-term stability of hematite photoanodes with ~11.9% Ti doping and propose the surface changing mechanism on hematite photoanodes during this stability testing in alkaline electrolyte. Hematite film was prepared as modified chemical bath deposition method and followed by two-step annealing (i.e., 550 oC for 1 h, 800 oC for 20 min).13, 27 To achieve Ti doping, 60 µl titanium(IV) isopropoxide (TIP) ethanol solution (1/50 v/v) was dropped onto α-FeOOH nanorod arrays, and then the films were annealed as above conditions. To reduce the influence of temperature, the cold light source of light-emitting diode (LED) was used during PEC measurements. The light spectrum of LED is shown in Figure S1. 2. RESULTS AND DISCUSSION The microstructure of as-prepared films is shown in Figure S2. Mesoporous hematite film with around 100 nm necked particles can be observed. After Ti doping, Ti:Fe2O3 films keep 4 ACS Paragon Plus Environment
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the mesoporous structure, but not well in the degree of order. The thickness of Ti:Fe2O3 films is 675 nm. The Ti doping content of ~11.9% (molar ratio of Ti/(Ti+Fe)) is measured on 15 µm2 area via energy-dispersive spectroscopy (EDS). X-ray photoelectron spectroscopy (XPS) of Ti:Fe2O3 film further confirms the Ti doping (Figure S3). The crystallinity and phase purity of Ti:Fe2O3 films was confirmed by the X-ray diffraction (XRD) (Figure S4). The peaks of (110) and (300) just show some decreasing due to the Ti doping induced structure changing, but no additional peak from TiO2 is observed. This indicates the Ti atoms are uniformly embedded in the lattice of Fe2O3 due to relative long-term annealing at 800 oC. Transmission electron microscope (TEM) images also confirm this, but some crystal defects can be observed (Figure S5 c and d). The optical properties of fabricated films are characterized using UV-vis diffuse reflection spectroscopy (Figure S6). Hematite film shows a band gap of 2.03 eV, which is in agreement with the reported values.4 Ti:Fe2O3 film shows a lower band gap of 1.96 eV, but the absorbance value is lower in the range of 320-550 nm. The annealing temperature and the doping amount of Ti element with TiCl4 ethanol solution (1/50, v/v) were optimized firstly. As shown in Figure S7, the temperature of 800 oC and the volume of 30 µl are the most favorable experimental situation. However, we found that the synthesized Ti:Fe2O3 film is not homogeneous by using TiCl4. Therefore, the TIP ethanol solution is used as Ti source and this problem is well resolved. The volume of TIP ethanol solution used was further optimized and the volume of 60 µl is the best choice, equaling to 11.9% Ti doping (Figure S8). This Ti doping content is consistent with the reported optimal value of 10.9%.28
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Figure 1. (a) LSV curves of bare Fe2O3 and Ti doped Fe2O3 photoanodes at scan rate of 20 mV s-1. (b) Parameters extracted from IMPS spectra of Fe2O3 and Ti:Fe2O3 photoanodes. Transfer efficiency, η; Rate constant for charge recombination, krec; Rate constant for charge transfer, ktr. Figure 1 shows the Ti doping effects under 100 mW cm-2 visible light illumination in 1.0 M NaOH. Typically, Ti:Fe2O3 photoanode can obtain a higher photocurrent density by 3.2 times than that of bare Fe2O3 at 1.23 V. Both photoanodes show positive photocurrent and pronounced transients when the light is switched on. For Fe2O3 photoanode, the cathodic transients when the light is turned off can be observed visibly, indicating the accumulation of holes near the surface of Fe2O3. After Ti doping, the cathodic transients can only be observed 6 ACS Paragon Plus Environment
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in the low potential range of 0.9-1.1 V. The onset potential of both photoanodes under light illumination is almost equal to each other. No cathodic shift of the onset potential of Ti:Fe2O3 is also observed in the previous literature.29 This suggests Ti doping cannot significantly influence the catalytic activity of hematite photoanodes under light illumination. However, Ti doping can greatly increase the overpotential of water oxidation in the dark (Figure 1a). This should be attributed to the formation of Fe2TiO5 on the surface of hematite photoanodes, which is consistent with the reported results.28 Intensity-modulated photocurrent spectroscopy (IMPS) was further used to reveal the inherent mechanism behind the great enhancement.30-31 At low potential range (
