Enhancement of Fe2TiO5 Photoanode through Surface Al3+

Letter pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Enhancement of Fe2TiO5 Photoanode through Surface Al3+ Treatment and FeOOH Modification Siliang Kuang,‡ Meng Wang,‡ Zhibin Geng, Xiaofeng Wu, Yu Sun, Wei Ma, Deshun Chen, Jinghai Liu, Shouhua Feng, and Keke Huang* State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, No. 2699, Qianjin Street, Changchun 130012, People’s Republic of China

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S Supporting Information *

ABSTRACT: Fe2TiO5 is recognized as a novel and promising photoanode material for solar water splitting. Here, nanostructured Fe2TiO5 was fabricated on a fluorine-doped tin oxide substrate by an electrospray deposition technique. We utilized surface Al3+ treatment and FeOOH modification to improve performance of the Fe2TiO5 photoanode. After this two-step enhancement, the photocurrent density of the final Fe2TiO5 photoanode is 0.52 mA cm−2 at 1.23 VRHE which is 2.8 times that of the pristine one, and the onset potential is 200 mV lower than before. The enhanced performance can be attributed to a synergetic effect of surface Al3+ treatment and FeOOH modification, since the surface Al3+ treatment accelerates charge transport while the FeOOH layer improves catalytic activity. This strategy of surface modification provides an effective pathway for rational designing of original photoanodes with high practical performance. KEYWORDS: Fe2TiO5, Electrospray technique, Nanostructure, Water splitting, Surface modification

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INTRODUCTION

complex processes of preparing the electrodes would limit further development of Fe2TiO5 photoanodes. Currently, significant achievement has been made for better photocatalytic properties through nanostructure construction,7−11 impurity doping,12−14 heterojunction formation3,15,16 and surface modification.17−19 Especially, surface modification is a facile method to promote the separation of electron−hole pairs and improve reactivity of oxygen evolution reaction for better photocatalytic performance. Depositing a cocatalyst with high water oxidation activity onto an electrode surface, which can enhance the oxygen evolution reaction kinetics, has proved to be a feasible way to improve photoelectrochemical (PEC) activity.17 In a recent report, the BiVO4 photoanode modified with FeOOH through a facile solution method exhibited a remarkable photocurrent density which is higher than the conventional electrodeposition method.20 In addition, the practical performance of photoanode is greatly limited by inevitable surface defects like oxygen vacancies and crystalline disorder that can give rise to surface recombination.21 Previous research showed surface Al3+ treatment can offer an effective method to passivate these surface states.22 In that work, Tidoped Fe2O3 with Al3+ surface treatment exhibited an enhanced PEC property. By means of electrochemical and

Photoelectrochemical (PEC) water splitting is a promising method to take advantage of solar energy to produce hydrogen energy for a clean energy future. Although photoanode materials like Fe2O3, WO3, TiO2 and BiVO4 have been widely investigated,1 insufficient light absorption and poor charge transport ability still limit the practical application of these photocatalytic materials.2 Fe2TiO5 is a photoanode material with proper band gap (bandgap ≈ 2.2 eV) and sufficient transfer of charge, which is commonly used to form heterojunctions with conventional photoanode materials to improve their photocatalytic properties.3−5 But the research on Fe2TiO5 photoanode alone is still scarce due to difficulty to synthesize pure monophasic Fe2TiO5 with enough crystallinity and specific surface area.2 In our previous research, an electrospray technique has been proved to be an effective method to fabricate Fe2TiO5 photoanodes on a fluorine-doped tin oxide (FTO) substrate.6 In addition, Jae Sung Lee’s group has prepared highly crystalline Fe2TiO5 nanotube array photoanodes through an anodized Al2O3 template method and hybrid microwave annealing.2 Subsequently, three strategies including TiO 2 underlayer, H 2 treatment and FeNiO x cocatalyst are utilized to improve the photoelectrochemical performance of the photoanodes. Finally, the photocurrent at 1.23 VRHE was enhanced from 0.34 to 0.93 mA cm−2, the record of Fe2TiO5 photoanodes at present. However, the © XXXX American Chemical Society

Received: June 18, 2019 Revised: August 1, 2019 Published: August 14, 2019 A

DOI: 10.1021/acssuschemeng.9b03425 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Letter

ACS Sustainable Chemistry & Engineering

Figure 1. (a) Schematic illustration of making the FeOOH/Al-Fe2TiO5 nanostructure. (b) XRD patterns of Fe2TiO5, Al-Fe2TiO5 and FeOOH/AlFe2TiO5.

Figure 2. (a)Ti 2p, (b) Fe 2p, (c) O 1s and (d) Al 2p XPS spectrum of Fe2TiO5, Al-Fe2TiO5 and FeOOH/Al-Fe2TiO5. homemade equipment, including injection pump (Kent Scientific Corporation, USA), high voltage source (Boher High Voltage Power Supplies Co., Ltd., China) and a heater. There are more details about the electrospray technique in Figure S1. Surface Modification. The nanostructured Fe2TiO5 photoelectrode was immersed in aqueous solution of aluminum nitrate and urea (0.01 g of aluminum nitrate nonahydrate, 0.162 g of urea, 100 mL of water), placed in a 75 °C oven for 1 h, and then rinsed off with deionized water. Then, it was placed in a tube furnace and annealed at 500 °C for 2 h in air atmosphere. The sample subjected to surface FeOOH modification was immersed in aqueous solution of ferric chloride (0.1351 g of ferric chloride hexahydrate, 50 mL of water) for 10 h, and then taken out and rinsed with deionized water. General Characterization. Powder X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM) and ultraviolet− visible (UV−vis) spectroscopy (HITACHIU-4100) techniques were employed to collect information.

photoelectrochemical measurement, it is proposed that surface Al3+ treatment can passivate the surface states and then decrease the accumulation of charge at the surface of a TiFe2O3 photoanode. On the basis of the above, we proposed an efficient strategy combining surface Al3+ treatment and FeOOH modification to improve the PEC performance of Fe2TiO5 photoanodes. Through simple treatments in Al3+ solution and Fe3+ solution, the photocurrent density at 1.23 VRHE of the final Fe2TiO5 photoanode was enhanced to 0.52 mA cm−2 which was about 2.8 times that of the pristine sample, and the final onset potential reduced 200 mV to 0.8 VRHE.

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EXPERIMENTAL SECTION

Synthesis of Fe2TiO5 Films. Nanostructured Fe2TiO5 photoelectrode was prepared by electrospray technique. The spray solution was prepared by dissolving 0.03 M iron acetylacetonate and 0.015 M tetrabutyl titanate in ethanol and then sonicated to make it homogeneous. The Fe2TiO5 thin films were deposited using a B


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modification due to the low amount of modified Al3+ which is covered up by the FeOOH nanolayer. SEM and TEM were applied to observe morphology of the samples as shown in Figure 3. The surface morphology of the

Photoelectrochemical Measurements. The photocurrent was measured by the Shanghai Chenhua model 660E electrochemical workstation, and a three-electrode system was used. The Fe2TiO5 film was used as the working electrode, the Ag/AgCl electrode was used as the reference electrode, and the platinum plate was used as the counter electrode. The fixed area exposed to the working electrode was about 0.2 cm2, and the electrolyte was a 1 M aqueous NaOH solution (pH = 13.6). The relationship between the potential of the test relative to Ag/AgCl and the potential of the reversible hydrogen electrode is as follows: E RHE = EAg/AgCl + 0.059pH + E0Ag/AgCl

(1)

Where EAg/AgCl is the measured potential relative to the Ag/AgCl electrode, and E0Ag/AgCl = 0.1976 V (at 25 °C). The linear sweep voltammetry (LSV) scan is a negative positive mode with a speed of 10 mV s−1. The light source we used was a solar simulator with a light intensity of 100 mA cm−2. Electrochemical impedance spectroscopy (EIS) was used to compare the conductivity of different Fe2TiO5 films with a voltage of 0.3 EAg/AgCl and a scan range of 0.1 Hz−1 MHz.

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RESULTS AND DISCUSSION As shown in Figure 1a, a two-step modification of the Fe2TiO5 photoanode is developed. Before that, the nanostructured Fe2TiO5 thin film on FTO substrate was prepared by an electrospray technique. Subsequently, the Fe2TiO5 film was immersed in Al-based solution and annealed at 500 °C.22 Finally, FeOOH was anchored onto the surface of the Fe2TiO5 photoanode after soaking the sample in the Fe-based solution.20 The samples before and after modification were assigned as Fe2TiO5, Al-Fe2TiO5 and FeOOH/Al-Fe2TiO5. The crystal structure of the samples was investigated by Xray diffraction (XRD), as shown in Figure 1b. The pristine Fe2TiO5 sample shows only diffraction peaks from pseudobrookite Fe2TiO5 (JCPDS# 73-1631) and FTO, indicating that monophasic Fe2TiO5 was prepared on the FTO substrate. After surface Al3+ treatment, there is no change in the XRD patterns, indicating that Al3+ treatment does not change the crystal phase. However, no typical diffraction peaks of Fe-based oxides or hydroxides are observed in the XRD patterns of the FeOOH/Al-Fe2TiO5 sample due to the low amount and poor crystallization of FeOOH.23 And according to the results of UV absorbance spectrum, it indicates that the surface modification does not change the light absorption of the Fe2TiO5 film, as shown in Figure S2. X-ray photoelectron spectroscopy (XPS) was conducted to further study the effect of surface modification, and the results are shown in Figure 2. Figure 2a,b shows XPS spectra of Ti 2p and Fe 2p. For the pristine Fe2TiO5, the Ti 2p1/2 and Ti 2p3/2 peaks are centered at 463.8 and 458.0 eV and the Fe 2p1/2 and Fe 2p3/2 peaks centered at 724.4 and 710.8 eV. The binding energy is slightly lower compared with those of other reports on monophasic pseudobrookite Fe2TiO5.24,25 This phenomenon can be attributed to oxygen deficiency, which leads to the existence of Ti3+ and Fe2+ on the surface of Fe2TiO5. Moreover, after surface Al3+ treatment the Ti 2p and Fe 2p peaks shifted to higher binding energies. We suspect that a few Al3+ doped in the lattice and occupied the position of Ti4+, resulting in the valence increase of Ti and Fe to keep electric neutrality. The O 2p XPS spectra in Figure 2c shows that after surface Al3+ treatment, the mount of surface hydroxyl increased (∼531 eV). And after FeOOH modification, the mount of adsorbed oxygen increased (∼532 eV). In Figure 2d, the Al 2p peak can be observed after surface Al3+ treatment, which is located at 74.6 eV. But the Al 2p peak disappears after FeOOH

Figure 3. SEM images of (a) Fe2TiO5, (b) Al-Fe2TiO5, (c) FeOOH/ Al-Fe2TiO5 and (d) TEM image of FeOOH/Al-Fe2TiO5; the inset (e) image is a high-resolution TEM image of the edge of FeOOH/AlFe2TiO5.

Fe 2 TiO 5 electrode before and after modification was characterized by SEM. As is shown in Figure 3a, the pristine Fe2TiO5 photoanode prepared through the electrospray technique was aggregated by nanoparticles of 50∼200 nm size. Figure 3b shows there was no significant change in the electrode surface morphology after surface Al3+ treatment. But after FeOOH modification, shown in Figure 3c, the electrode surface became rougher, indicating that the FeOOH layer was deposited on the electrode surface. TEM was used to further study the composite structure of the electrode. After surface Al3+ treatment, there was no significant change at the boundary of the samples, shown in Figure S3a,b. As for the FeOOH/AlFe2TiO5 sample shown in Figure 3d,e, there is a thin layer around the nanoparticle, indicating that the FeOOH layer was deposited on the electrode surface after the sample soaking in the Fe-based solution. The thickness of FeOOH was about 5 nm. To further confirm the successful preparation of Fe2TiO5, the HRTEM images of pristine Fe2TiO5 are shown in Figure S3c. The lattice spacing of 0.35 nm determined from the HRTEM image corresponds to the (110) plane of pseudobrookite Fe2TiO5. The photocurrent densities of the samples before and after modification under sustained light are shown in Figure 4a. To study the effect of Al3+ treatment and FeOOH modification respectively, we prepared the Fe2TiO5 sample with FeOOH modification merely assigned as FeOOH/Fe2TiO5. As the curves show, the FeOOH/Al-Fe2TiO5 sample has the highest performance, suggesting the synergetic effect of surface Al3+ treatment and FeOOH modification. Moreover, samples with FeOOH modification (FeOOH/Fe2TiO5 and FeOOH/AlFe2TiO5) showed improved electrocatalytic activity at 1.6 VRHE, indicating that modified FeOOH can promote the water oxidation reaction. The stability curves in Figure 4b show the C


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Figure 4. (a) Photocurrent curves and (b) current density at 1.23 VRHE of Fe2TiO5, Al-Fe2TiO5, FeOOH/Fe2TiO5 and FeOOH/Al-Fe2TiO5. (c) Current curves under chopped light and (d) electrochemical impedance spectra of Fe2TiO5, Al-Fe2TiO5 and FeOOH/Al-Fe2TiO5.

Figure 5. J−V curves of (a) Fe2TiO5 and (b) Al-Fe2TiO5 and (c) surface charge separation efficiencies of Fe2TiO5 and Al-Fe2TiO5.

modified sample can keep a high photocurrent density at 1.23 VRHE for a few hours. The little break in the FeOOH/Al-

Fe2TiO5 curve is attributed to bubbles releasing. Figure 4c shows the curves of photocurrent density under chopped light, D



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CONCLUSIONS In summary, an efficient strategy was developed for better photoelectrochemical performance of the Fe2TiO5 photoanode. First, the performance of the Fe2TiO5 photoanode was improved through surface Al3+ treatment. Considering the XPS spectrum, this improvement is due to the passivated surface states and reduced surface defects. Subsequently, the photocurrent of the electrode was further enhanced after the deposition of FeOOH, which can improve the sluggish kinetics of the water oxidation. Based on the photocurrent curves and EIS results, the surface Al 3+ treatment and FeOOH modification have shown a synergetic and collective effect to enhance the performance of Fe2TiO5 photoanodes for PEC water splitting. The photocurrent density was finally increased to 0.52 mA cm−2 at 1.23 VRHE. This was 2.8 times higher than that of the original photocurrent. And the on-set potential also reduced 200 mV to 0.8 VRHE after the surface modification. This strategy of surface modification would provide useful guideline to design novel photoanodes effectively.

exhibiting a two-step enhancement of the Fe2TiO5 photoanode through surface Al3+ treatment and FeOOH modification. After surface Al3+ treatment, the photocurrent density was increased to 0.31 mA cm−2 from 0.18 mA cm−2 at 1.23 VRHE. And the photocurrent was further improved to 0.52 mA cm−2 at 1.23 VRHE after deposition of FeOOH. This was a 2.8 times higher photocurrent density than that of the original sample. In addition, the on-set potential was also reduced by about 200 mV after the deposition of FeOOH. This indicates that surface Al3+ treatment and FeOOH modification can effectively improve the catalytic performance of Fe2TiO5. EIS was conducted to confirm the working mechanism of the modified sample, and the results are shown in Figure 4d. The inset in Figure 4d shows the equivalent circuit (EC) to simulate the Nyquist plot and Table S1 shows the fitting parameters of EC elements in EIS. Compared to the pristine Fe2TiO5, the Al-Fe2TiO5 exhibits a smaller diameter, indicating faster charge transfer kinetics at the electrode interface.22 This phenomenon can be attributed to reduced surface states, since the decrease of surface defects is beneficial to charge separation and transport. The curve diameter for the FeOOH/Al-Fe2TiO5 sample is even smaller, suggesting an even faster charge transfer with the surface FeOOH accelerating the transport of holes.20 There are more information from the fitting parameters of EC elements in Table S1: RS is the series resistor, including substrate resistance, electrolyte resistance and external circuit contact resistance. Series resistance plays a small role in charge transport and separation. RSC is the internal resistance of the semiconductor electrode and can reflect the transport properties of charge carriers inside the electrode. RSC decreases significantly from Fe2TiO5 to AlFe2TiO5, confirming the faster charge transfer in Al-Fe2TiO5 and it increases from Al-Fe2TiO5 to FeOOH/Al-Fe2TiO5 due to the poor conductivity of FeOOH. The RCT for the interfacial charge transfer decreases sharply from Fe2TiO5 to Al-Fe2TiO5 and then FeOOH/Al-Fe2TiO5, confirming the faster charge transfer at the electrode interface. Compared to the Fe2TiO5 and Al-Fe2TiO5 samples, FeOOH/Al-Fe2TiO5 shows a larger CH, indicating a higher charge separation efficiency which strongly confirms the effective surface FeOOH modification to improve the performance. Overall, the surface modifications not only accelerate the transport and separation of carriers in bulk of the sample but also promote the transfer of carriers between the sample and the electrolyte. For a more quantitative analysis of the enhanced charge separation ability after surface Al3+ treatment, we measured surface charge separation efficiencies (ηsurf) of Fe2TiO5 and AlFe2TiO5 samples using H2O2 as a hole scavenger.26,27 The J−V curves of Fe2TiO5 and Al-Fe2TiO5 with and without H2O2 (0.5 M) are shown in Figure 5a,b. Figure 5c shows surface charge separation efficiencies calculated by the following equations: ηsurf = JphotoH O /JphotoH O

(2)

Jphoto = Jlight − Jdark

(3)

2

2

2

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b03425. Fitting parameters of EC elements in EIS, schematic diagram of homemade electrospray equipment, UV−vis absorption spectra and TEM images of Fe2TiO5 and AlFe2TiO5 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*K. Huang. E-mail: [email protected]. ORCID

Keke Huang: 0000-0002-8995-2176 Author Contributions ‡

These authors contributed equally.

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 (Grants 21427802, 21671076, 21831003 and 21621001).

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REFERENCES

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As shown in Figure 5c, Al-Fe2TiO5 has greater surface charge separation efficiency than pristine Fe2TiO5. This result indicates that surface Al3+ treatment can significantly improve surface charge separation through surface passivation, which is corresponding to the EIS curves. E


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Enhancement of Fe2TiO5 Photoanode through Surface Al3+

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