Enhanced Photoelectrochemical Water Oxidation Performance of

Aug 9, 2017 - As a promising candidate for photoelectrochemical (PEC) water oxidation, the photoelectrochemical water splitting efficiency of hematite...
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Enhanced Photoelectrochemical Water Oxidation Performance of Fe2O3 Nanorods Array by S Doping Rong Zhang,† Yiyu Fang,‡ Tao Chen,† Fengli Qu,§ Zhiang Liu,§ Gu Du,∥ Abdullah M. Asiri,⊥ Tao Gao,*,‡ and Xuping Sun*,† †

College of Chemistry and ‡Institute of Atomic and Molecular Physics, Sichuan University, Chengdu 610064, China Chemistry and Chemical Engineering, Qufu Normal University, Qufu 273165, China ∥ Chengdu Institute of Geology and Mineral Resources, Chengdu 610064, China ⊥ Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia §

S Supporting Information *

ABSTRACT: As a promising candidate for photoelectrochemical (PEC) water oxidation, the photoelectrochemical water splitting efficiency of hematite (Fe2O3) is limited by its low electron mobility. In this work, we report a new strategy for great enhancement of PEC water oxidation activity on Fe2O3. The S-doped Fe2O3 (S:Fe2O3) nanorods array on a Ti plate shows a substantially increased photocurrent density of 1.42 mA cm−2 at 1.23 V vs RHE in 1.0 M NaOH under simulated sunlight irradiation (AM 1.5G, 100 mW cm−2), 2.45 times that of Fe2O3 counterpart (0.58 mA cm−2). Both density functional theory calculations and experimental measurements verify that the superior activity is contributed to the enhanced electron mobility after S doping. This study offers an attractive photoanode in water-splitting devices for solar hydrogen production application. KEYWORDS: Hematite, Photoelectrochemical water oxidation, S doping, Water-splitting devices

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INTRODUCTION Photoelectrochemical (PEC) water splitting represents an attractive approach for conversion and storage of solar energy into hydrogen.1,2 Since the first report of water photolysis using TiO2 as photoanode under solar illumination by Fujishima and Honda in 1972,3 worldwide research has been focused on semiconductors (such as hematite,4,5 ZnO, 6,7 WO 3 , 8,9 Ta3N5,10,11 and BiVO412,13) for PEC water splitting. Among various explored candidates, hematite has received increasing attention as a promising photoanode with the merits of favorable optical band gap (1.9−2.2 eV), remarkable chemical stability, natural abundance, and nontoxicity.14−17 However, most reported hematite photoanodes only offer much lower photocurrent density and solar-to-hydrogen efficiency compared with the maximum theoretical values of 12.6 mA cm−2 and 16.8% at 1.23 V vs RHE, respectively, under simulated sunlight irradiation (AM 1.5G 100 mW cm−2).18−22 Owing to the intrinsically heavy electron effective mass,23 the particularly low electron mobility of hematite (10−2 cm2 V−1 s−1) remains a major drawback that limits its PEC efficiency. It is reported that elements doping is an effective approach for enhancing the catalytic activity of materials.24,25 Metallic elements (Co,26 Ti,18,27 Mn,28 Zn,29 Pt,30 and Ru31) have been used as dopants to improve the PEC water oxidation activity of hematite by remarkably improving the electron mobility. Recent studies also demonstrated that nonmetallic elements like Si21,32 and P33,34 are effective dopants to enhance the © 2017 American Chemical Society

electron mobility and consequently the PEC performance of hematite. For example, Zhang et al. doped P into hematite bulk structure.34 Such photoanode shows superior PEC performance due to that the strong covalent interaction between P and O provides numerous electron carriers and thus greatly enhances the electron mobility. S has more valence electrons compared with P and the S−O bond is more covalent than P−O, promising its application as a dopant for hematite to improve PEC efficiencies. Although recent work has shown that S is effective to dope TiO2,35 ZnO,36 and g-C3N4/BiVO437 photoanodes, its use for hematite doping, however, has not been explored before. In this study, we report our recent finding that S-doped hematite (S:Fe2O3) nanorods array on Ti plate shows superior PEC performance for water oxidation, delivering a photocurrent density of 1.42 mA cm−2 at 1.23 V vs RHE in 1.0 M NaOH under standard AM 1.5G illumination, 2.45 times that of pristine Fe2O3 counterpart. Experimental and theoretical results demonstrate that the enhancement is ascribed to the increased electron carrier concentration and enhanced electron mobility as a result of S doping. Received: June 7, 2017 Revised: July 17, 2017 Published: August 9, 2017 7502

DOI: 10.1021/acssuschemeng.7b01799 ACS Sustainable Chem. Eng. 2017, 5, 7502−7506

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ACS Sustainable Chemistry & Engineering

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additional peaks are ascribed to S2−. XPS valence band spectra of Fe2O3 and S:Fe2O3 are presented in Figure S2. Compared to the undoped Fe2O3, the valence band edges of the S:Fe2O3 are shifted by 0.13 eV toward the higher binding energy side, whereas both samples exhibit the same band gap, as determined by the Tauc plots in Figure S3. Figure S4 presents the energy diagram of S:Fe2O3. We studied the PEC performance of Fe2O3 (loading: 0.77 mg cm−2) and S:Fe2O3 (loading: 0.83 mg cm−2) on Ti plate as photoanodes for water oxidation in 1.0 M NaOH with a scan rate of 5 mV s−1 using a typical three-electrode configuration. Figure 2a shows the linear sweep voltammetry (LSV) curves

RESULTS AND DISCUSSION S:Fe2O3 grown on Ti plate was derived from Fe2O3 by a lowtemperature chemical vapor deposition process (see SI for preparation details). Figure 1a shows the scanning electron

Figure 1. SEM images of (a) Fe2O3 and (b) S:Fe2O3 on Ti plate. (c) SEM image and corresponding EDX elemental mapping images of Fe, O, and S for S:Fe2O3 on Ti plate. (d) XRD patterns of Fe2O3 and S:Fe2O3. HRTEM images of (e) Fe2O3 and (f) S:Fe2O3. XPS spectra for S:Fe2O3 in the (g) Fe 2p, (h) O 1s, and (i) S 2p regions.

microscopy (SEM) image of Fe2O3 on Ti plate, indicating the Ti plate is fully coated with Fe2O3 nanorods array. After S doping, the product still preserves its nanoarray feature (Figure 1b). Both Fe2O3 and S:Fe2O3 nanorods are about 200 nm in height. The SEM image and corresponding energy-dispersive X-ray (EDX) elemental mapping images show the uniform distribution of Fe, O, and S elements in S:Fe2O3, as shown in Figure 1c. Inductively coupled plasma mass spectrometry (ICPMS) analysis concludes that the percentage of S is 6.7%. Figure 1d shows the X-ray diffraction (XRD) patterns of Fe2O3 and S:Fe2O3. Both samples present well-defined diffraction peaks characteristics of α-Fe2O3 (JCPDS 33-0664). High-resolution transmission electron microscopy (HRTEM) images (Figure 1e,f) taken from Fe2O3 and S:Fe2O3 nanorods reveal wellresolved lattice fringes with the same interplanar distance of 2.50 nm, corresponding to the (110) plane of Fe2O3. All observations reveal that S doping has no influence on the morphology and crystal phase of Fe2O3 nanoarray. Figure 1g shows the core-level X-ray photoelectron spectroscopy (XPS) spectrum in the Fe 2p region of S:Fe2O3. The Fe 2p3/2 and Fe 2p1/2 peaks are located at binding energies (BEs) of 711.6 and 726.1 eV, respectively, corresponding to Fe3+ in S:Fe2O3.18,22 A satellite peak of the Fe 2p3/2 at 719 eV also indicates the presence of a Fe3+ species.38 Compared with the XPS spectrum of Fe2O3 in the Fe 2p region (Figure S2), an obvious satellite peak at 716 eV in the Fe 2p region of S:Fe2O3 suggests that Fe2+ sites were created during the annealing process.39 Figure 1h shows the core-level O 1s XPS spectrum of S:Fe2O3. One peak at 533.0 eV can be ascribed to OH− surface group of S:Fe2O340 and another peak at 532.1 eV can be assigned to the S−O bond on the surface of S:Fe2O3.41,42 The core-level S 2p XPS spectrum for S:Fe2O3 (Figure 1i) shows two peaks at 168.9 and 170.0 eV for S 2p3/2 and S 2p1/2 respectively, fully consistent with typical values for S6+ species.41 Note that no

Figure 2. (a) LSV curves of Fe2O3 and S:Fe2O3 photoanodes under AM 1.5G illumination and in the dark in 1.0 M NaOH with a scan rate of 5 mV s−1. (b) Photoconversion efficiencies as a function of applied potential. (c) IPCE spectra measured in 1.0 M NaOH at 1.23 V vs RHE. (d,e) Amperometric transient photocurrent responses under chopped illumination at 1.23 V vs RHE. (f) Time-dependent current density curves at 1.23 V vs RHE under illumination.

under AM 1.5G illumination and in the dark. S:Fe2O3 gives considerably enhanced photocurrent density of 1.42 mA cm−2 at an applied potential of 1.23 V vs RHE, 2.45 times that of the pristine Fe2O3 (0.58 mA cm−2). Table S1 compares the PEC performance of S:Fe2O3 with reported Fe2O3 and doped Fe2O3. The applied bias photon-to-current efficiencies (ABPE) were calculated to quantitatively evaluate the PEC water oxidation efficiency of Fe2O3 and S:Fe2O3 (Figure 2b). The maximum photoconversion efficiency of 13.1% is achieved for S:Fe2O3 at 1.02 V, whereas it is only 4.9% for Fe2O3 at a higher applied potential of 1.06 V. The incident photon-to-current conversion efficiency (IPCE) values were measured as a function of wavelength under biases of 1.23 V vs RHE in 1.0 M NaOH. As shown in Figure 2c, S:Fe2O3 exhibits obvious higher IPCE values than Fe2O3 in the wavelength range of 350−600 nm, although the UV−visible absorption characteristics are not changed much by S doping (Figure S6). The typical transient 7503

DOI: 10.1021/acssuschemeng.7b01799 ACS Sustainable Chem. Eng. 2017, 5, 7502−7506

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effect of S incorporation on charge transfer dynamics of Fe2O3, we employed charge separation efficiency (ηseparation) and charge injection efficiency (ηinjection) by adding 0.5 M H2O2 as the hole scavenger into 1.0 M NaOH electrolyte.45 Figure S8a,b shows the photocurrent responses of Fe2O3 and S:Fe2O3 in 1.0 M NaOH with and without H2O2 under illumination, respectively. As shown in Figure S8c, the ηseparation of S:Fe2O3 is 30.0% at 1.23 V vs RHE, which is 2.31 times that of Fe2O3 (12.9% at 1.23 V vs RHE). Meanwhile Figure S8d reveals that the ηinjection of S:Fe2O3 (56.2% at 1.23 V vs RHE) is higher than that of untreated Fe2O3 (39.6% at 1.23 V vs RHE). These results suggest that S incorporation can efficiently improve the ηseparation and ηinjection of photoanode, resulting in dramatic photocurrent density of S:Fe2O3. Density functional theory (DFT) was further applied to understand the effect of S incorporation on electronic structure and charge transfer of Fe2O3 (see SI for calculation details). We calculated four substitution sites (marked as Fe1, Fe2, Fe3, and Fe4). The calculated band gap of pure Fe2O3 is 2.0 eV (Figure S9a), which is fully consistent with the reported values.17,32 The band gap of other S-doped Fe2O3 (Figure S9b−e) are almost unchanged, indicating that the S doping has no significant effect on the absorption of visible light. Figure S10 shows the band structures of pure Fe2O3 and S-doped Fe2O3. It can be clearly seen that the Fermi level of the S-doped Fe2O3 becomes much higher than that of pure Fe2O3 and is close to the bottom of the conduction band, demonstrating the n-type characteristic of S doping. The occupied states near the Fermi level are derived from the Fe 3d state, as shown by the partial density of states (DOS) as shown in Figure 4a. The S dopant can donate

photocurrent measurements on Fe2O3 and S:Fe2O3 were conducted under chopped illumination at 1.23 V vs RHE (Figure 2d). When light was turned on, the transient photoresponses display spikes for both samples, but the photocurrent quickly turns back to a steady state, suggesting superior photoresponses of Fe2O3 and S:Fe2O3. The area under the photocurrent spikes of S:Fe2O3 are much smaller than those of Fe2O3, indicating more effective suppression of the electron−hole recombination on S:Fe2O3 (Figure 2e). Given that long-term stability is another critical criterion for practical application of photoelectrodes, we performed continuous photoelectrolysis on Fe2O3 and S:Fe2O3 at a fixed potential of 1.23 V vs RHE for 10 h (Figure 2f). Both samples show no loss in photocurrent densities, exhibiting excellent stability of S:Fe2O3. All these results demonstrate that S doping significantly enhances the PEC performance of Fe2O3. We also prepared S-doped Fe2O3 with different doping degrees and found that the photocurrent reaches a maximal value with a moderate S content of 6.7% in Fe2O3 (Figure S6). Considering lots of research of Fe2O3 has been prepared on fluorine-doped tin oxide (FTO),43 we further prepared Fe2O3 and S:Fe2O3 on FTO. The PEC performance was studied in 1.0 M NaOH in the dark and under back-side and front-side illumination. As shown in Figure S7, Fe2O3 shows a higher photocurrent density of 0.86 mA cm−2 1.23 V vs RHE under back-side illumination than that of Fe2O3 (0.74 mA cm−2) under front-side illumination, which is reported in previous work.44 S-doped Fe2O3 gives considerably enhanced photocurrent density of 1.48 and 1.22 mA cm−2 at 1.23 V vs RHE under front- and back-side illumination, respectively. Mott−Schottky (M-S) measurements were conducted in 1.0 M NaOH at a fixed frequency of 1000 Hz under dark condition to determine the type of semiconductor and carrier density (ND) of Fe2O3 and S:Fe2O3. As shown in Figure 3a, both M-S

Figure 4. (a) DOS of Fe2O3 and S-doped Fe2O3. (b) Electron density distribution plot for S-doped Fe2O3 (110) surface. Figure 3. (a) M-S plots of Fe2O3 and S:Fe2O3. (b) EIS Nyquist plots of Fe2O3 and S:Fe2O3 under illumination.

electrons into the Fe 3d orbital of Fe3+ to form Fe2+, providing more electron carriers in hematite. Further investigation of the electron density distribution demonstrates the strong covalent interactions between S and O (Figure 4b and S11), indicating that the S 3p and O 2p states originate from the antibonding orbital of the S−O bond. We also calculated the electron effective mass in Fe2O3 and S incorporated Fe2O3 (Table S2). The electron effective mass in pure Fe2O3 is 6.5 me, which is close to the value of recent report.46 After S doping, the electron effective mass of Fe2O3 was obviously decreased, implying that S can greatly enhance the electron mobility of hematite materials.

plots of Fe2O3 and S:Fe2O3 show positive slopes, characteristic of an n-type semiconductor. The ND calculated from the slope of the M-S plots for Fe2O3 is 1.6 × 1019 cm−3, which is in good agreement with previous report.19,34 In sharp contrast, S:Fe2O3 has a much higher ND value of 7.8 × 1019 cm−3, 4.88 times that of pure Fe2O3. The noticeable increase of carrier density indicates that S doping serves as an electron donor that can apparently increase electrical conductivity and further improve the charge transport and photocurrent density. Electrochemical impedance spectroscopy (EIS) data were collected to further study the influence of S incorporation on Fe2O3. Figure 3b presents the EIS Nyquist plots of Fe2O3 and S:Fe2O3 measured at a basis of 1.23 V vs RHE under AM 1.5G illumination. The capacitive arc of the S:Fe2O3 has a much smaller radius than that of Fe2O3, suggesting that S incorporation can largely improve the electrical conductivity of hematite. To quantify the

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CONCLUSION In conclusion, S doping has been proven as an effective strategy to enhance the PEC water oxidation efficiency of Fe2O3 photoanode. Such S:Fe2O3 nanoarray shows a photocurrent density of 1.42 mA cm−2 at 1.23 V vs RHE, 2.45 times that of 7504

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untreated Fe2O3, with long-term photostability. The superior activity is due to the enhanced electron mobility, which is supported by experiments and DFT calculations. Our study is the first proof-of-concept demonstration of using S as a nonmetallic dopant for Fe2O3 semiconductor. Such S:Fe2O3 also avoids the involvement of more toxic Se as a dopant47 and thus may hold great promise as an attractive photoanode in water-splitting devices for solar hydrogen production application.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01799. Experimental section; XPS spectra; LSV curves; Tauc plots; energy diagram; UV−visible absorption spectra; DOS plots; band structure plots; electron density distribution plots (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (T.G.). *E-mail: [email protected] (X.S.). ORCID

Abdullah M. Asiri: 0000-0001-7905-3209 Xuping Sun: 0000-0001-5034-1135 Author Contributions

R.Z. and Y.F. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21575137). We also appreciate Hui Wang from the Analytical & Testing Center of Sichuan University for her help with SEM characterization.

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DOI: 10.1021/acssuschemeng.7b01799 ACS Sustainable Chem. Eng. 2017, 5, 7502−7506