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Enhanced Photoelectrochemical Water Oxidation by Fabrication of p‑LaFeO3/n-Fe2O3 Heterojunction on Hematite Nanorods Qi Peng,† Jun Wang,‡ Zijian Feng,‡ Chun Du,‡ Yanwei Wen,*,‡ Bin Shan,‡ and Rong Chen*,†,§ †

State Key Laboratory of Digital Manufacturing Equipment and Technology, School of Mechanical Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, Hubei, People’s Republic of China ‡ State Key Laboratory of Materials Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, Hubei, People’s Republic of China § School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China S Supporting Information *

ABSTRACT: The LaFeO3 film exhibits interesting p-type behavior and stable photocatalytic hydrogen production in aqueous solution, and we combine it with α-Fe2O3 nanorods to form a p-LaFeO3/n-Fe2O3 heterojunction to improve the photoeletrochemical (PEC) water oxidation performance of hematite. An atomic layer deposition (ALD) technique is adopted to deposit La2O3 controllably on β-FeOOH nanorods, and the p-LaFeO3/n-Fe2O3 heterojunction is achieved by postthermal treatment, which is evidenced by the XRD, XPS, and HRTEM images. Due to the well-matched band levels of LaFeO3 and α-Fe2O3, the onset potential for photocurrent is negatively shifted by ∼50 mV. Meanwhile, the photocurrent density is promoted from 0.37 to 0.58 mA/cm2 at 1.23 V versus RHE owing to the accelerated charge separation within the space depletion layer induced by the build-in potential. Furthermore, the heterojunction is further modified by CoOx cocatalyst to improve the surface water oxidation kinetics, and the photocurrent density is promoted to 1.12 mA/cm2 at 1.23 V versus RHE. As a result, the incident photon-to-current conversion efficiency is further promoted to 25.13% at 400 nm. Our work demonstrates ALD with a prominent advantage in fabrication of the heterojunction with controllable film thickness, which plays an important role in the PEC water splitting application.

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carrier recombination,9 and mismatch between the band edges and the water splitting potentials. To ameliorate the slow reaction kinetics, surface modifications of IrO2 and Co-Pi catalysts have been widely used.10−12 While the severe recombination of photoexcited charge carriers is mainly limited by the short diffusion length of the hole (2−4 nm), the thickness of the α-Fe2O3 film is desired to be thin to suppress such recombination. However, the light absorption would be dramatically reduced if the films thickness is quite thin. Doping, thickness control, and nanostructure fabrication have been introduced to find compromise between the light adsorption and carrier recombination.13−15 Combining hematite with a p-type semiconductor to form an n−p heterojunction is another efficient way to modify the band levels and improve the charge carrier separation since band bending is induced by the internal space depletion layer.16−21 Wang et al. fabricated the Mg-doped hematite film with p-type characteristics by atomic layer deposition (ALD) and

INTRODUCTION Due to the huge consumption of fossil fuel and emission of carbon dioxide, it is urgent to develop green technologies to solve the energy crisis in a renewable and friendly way. Solar energy is one of the most promising candidates to be the energy source with abundant energy densities and is friendly to the environment. However, the intermittence of solar light suppresses its unrestricted utilization in many fields. Among the solar energy technologies, solar water splitting is a promising approach for solar energy conversion to chemical fuels, such as hydrogen.1−3 On the basis of previous reports on the various photoelectrochemical (PEC) water splitting photoanode semiconductors,4−6 the light absorption flux would be suppressed if the band gap is too wide like TiO2, ZnO, etc.,4,5 and the water splitting reaction is hardly driven if the band gap is relatively narrow such that the semiconductor PEC performance is dramatically affected by the electronic band structure. Among lots of metal oxides, hematite (α-Fe2O3) is one of the most promising photoanode materials due to the excellent chemical stability, earth abundance, and suitable band gap for the water splitting reaction.6,7 However, the PEC efficiency of α-Fe2O3 still remains low owing to the slow reaction kinetics,8 severe © 2017 American Chemical Society

Received: February 24, 2017 Revised: June 2, 2017 Published: June 5, 2017 12991

DOI: 10.1021/acs.jpcc.7b01817 J. Phys. Chem. C 2017, 121, 12991−12998

Article

The Journal of Physical Chemistry C

technique before the thermal treatment. La2O3 was grown on a commercial ALD system SUNALE R200 reactor. La2O3 was deposited using alternating precursor of La(thd)3 (Aldrich, >98%) and O3.32 La(thd)3 was held at 227 °C, and the chamber was maintained at 300 °C. The ALD timing sequences were set to be 1 s (La(thd)3)−8 s (N2)−2 s (O3)−8 s (N2), and the general growth rate of La2O3 was calculated as ∼0.24 Å/cycle at 300 °C. After La2O3 deposited, the specimen was thermally treated at 800 °C for 3 min. The p-LaFeO 3 /n-Fe 2 O 3 heterojunction with 500 La2O3 ALD cycles is abbreviated to be LF-A in the following discussion. The β-FeOOH nanorods (hydrothermal treated for 5 h) were placed in aqueous solution 2 (0.15 M FeCl3 + 0.15 M La(NO3)3 + 1 M NaNO3 at pH ∼ 1.5) and held at 100 °C for 1 h. After the La aqueous reaction treatment, the specimen was thermally treated at 800 °C for 3 min, and the product is abbreviated to be LF-C. The CoOx modified p-LaFeO3/n-Fe2O3 heterojunction was fabricated by immersing the heterojunction photoanode into the Co2+ containing solution for 30 min and then held at 100 °C for 1−2 h. The Co2+ containing solution was obtained by adding 0.01 M NaOH into 0.01 M Co(NO)3 aqueous solution drop by drop just as the floccule appeared. The Co2+ containing solution remained pink during the whole immersion period. We abbreviated the CoOx modified p-LaFeO3/n-Fe2O3 heterojunction to be LF-A/CoOx. Characterizations of Materials. The grazing angle X-ray diffraction (XRD) patterns were recorded on an analytical X’pert PRO diffractometer operating at 40 kV and 40 mA, using Cu Kα radiation with λ = 1.5406 Å in the 2θ range from 20° to 60°. Scanning electron microscope (SEM) images and energy dispersive X-ray analysis (EDX) data were obtained by using a Helios G3 CX microscope operated at 20 kV. Transmission electron microscope and high-resolution transmission electron microscope (JEOL-2010FEF) operated at an acceleration voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) studies were performed on VG a Multilab 2000 X-ray photoelectron spectroscope with an Al Kα radiation. The starting angle of the photoelectron was set at 90°. The thickness of the La2O3 and Fe2O3 deposited on a Si wafer by ALD was measured by spectroscopic ellipsometer (SE, M2000) purchased from J. A. Woollam Co. Electrochemical and Photoelectrochemical Measurements. The bare α-Fe2O3, LF-C, LF-A, and LF-A/CoOx photoanodes were fabricated by soldering a copper wire onto a bare part of the FTO substrate. The substrate edges and the metal contact region were sealed with insulating epoxy resin. Unless otherwise stated, all experiments were performed at ambient temperature (20 ± 1 °C) and with continuous N2 flowing. The photocurrent densities were evaluated by measuring the electrochemical performance of current density via linear sweep voltammetry (LSV) under simulated solar illumination. The electrochemical signal was recorded using an Autolab electrochemical workstation with a standard threeelectrode electrochemical system. The photoanode acted as the working electrode, with Pt as the counter electrode and saturated Ag/AgCl as the reference electrode. A 1 M NaOH solution (pH ∼ 13.6) was used as the electrolyte. The reference potential was changed to be versus a reversible hydrogen electrode (RHE) according to eq 1. The light was obtained by full-spectra simulated sunlight from an AAB Solar Simulator (Perfect Light, PL-MS300). The illumination intensity was calibrated to be 100 mW/cm2 by a Sanpometer SM206 solar power meter. The area of exposed FTO was set to be 1 cm2,

significantly reduced the onset potential for photocurrent, which is attributed to the band match of hematite modification with n−p heterojunction formation.22 Ahmed et al. fabricated the p-CaFe2O4/n-Fe2O3 nanorods’ heterojunction photoanode by chemical bath deposition (CBD) and obtained the photocurrent density of 0.53 mA/cm2 at 1.23 V versus RHE. With modification by Co-Pi cocatalyst, the PEC performance is further promoted, and the incident photon-to-current conversion efficiency (IPCE) at 420 nm reaches ∼20%.19 The Fe2O3/Fe2TiO5 heterojunction is prepared by the ALD technique and exhibits a photocurrent density of 1.63 mA cm−2 at 1.23 V versus RHE due to the increased charge separation efficiency. With decoration of FeNiOx, the onset potential negatively shifts by ∼150 mV, and the photocurrent is further improved to 2.7 mA cm−2.38 The perovskite LaFeO3 with band gap of 2.07 eV is one of the promising p-type candidates which exhibit high hydrogen and oxygen evolution rates under visible light illumination.23−30 Parida et al. give a comprehensive review of the progress of perovskite and layered perovskite related composites in photocatalytic applications.21 Yu28 et al. prepare the durable photocells with a p-type LaFeO3 thin film as photocathode and n-type α-Fe2O3 as photoanode by pulse laser deposition, which achieves long-life stability (>120 h) and high rate of water splitting (H2: 11.5l mol/h, O2: 5.7l mol/h). Our previous works indicate that the PEC performance of pristine LaFeO3 can be improved by doping and surface modifications.29,30 In this paper, we demonstrate the fabrication of the pLaFeO3/n-Fe2O3 heterojunction photoanode with controllable film growth by ALD for the first time. Compared with that of the bare α-Fe2O3 nanorods photoanode, the photocurrent density is increased 2-fold at 1.23 V versus RHE on the heterojunction photoanode. Such promotion of the photocurrent density is attributed to the accelerated charge transfer within the space depletion layer. The surface water oxidation kinetics of the heterojunction is improved by CoOx coating as a cocatalyst, and the photocurrent density is further promoted to 1.12 mA/cm2 at 1.23 V versus RHE. As a result, the IPCE is promoted to 25.13% at 400 nm on the CoOx modified pLaFeO3/n-Fe2O3 heterojunction photoanode. Our work indicates ALD is a powerful technique to construct the conformal and controllable nanostructures, which is potentially used in PEC water splitting.

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EXPERIMENTAL SECTION Bare n-Fe2O3 and p-LaFeO3/n-Fe2O3 Heterojunction Photoanode Preparation. The α-Fe2O3 nanorods arrays were prepared via a synthetic route according to the work of Vayssieres31 by controlled aqueous hydrothermal treatment. In our whole experiment, all of the chemical reactants are commercially available (analytical grade) without further purification. A 10 mL portion of aqueous solution 1 (0.15 M FeCl3 + 1 M NaNO3 at pH ∼ 1.5) was added to the Teflon lining. After stirring was complete, the FTO glass was vertically placed in the Teflon lining with the conductive film upward. Before that, the FTO glasses were cleaned with ultrasonic treatment in a sequence of DI water (∼15 MΩ), ethanol, and acetone for 10 min for each process. Then, the Teflon lining in the sealed reactor was set and held at 100 °C for 6 h. The βFeOOH nanorods were obtained and could converted to αFe2O3 nanorods with thermal treatment at 800 °C for 3 min. The p-LaFeO3/n-Fe2O3 heterojunction was fabricated with La2O3 atomically grown on the β-FeOOH nanorods by ALD 12992


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Figure 1. Schematic diagram of ALD experiment design and photogenerated electron/hole separation and transfer process within the p-LaFeO3/nFe2O3 heterojunction.

and the rest part was blocked by insulation tape. The IPCE was carried out on the CrownTech QTest Station500AD in the range 400−650 nm in the same standard three-electrode electrochemical system.

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E RHE = EAg/AgCl + 0.058 × pH + 0.198

(1)

RESULTS AND DISCUSSION According to a previous report, the conductive band and valence band of LaFeO3 are located at 0.03 and 2.12 eV,33 and

Figure 2. XRD characterization of bare α-Fe2O3 and LF-A photoanodes. Figure 3. XPS characterizations of LF-A photoanode. (a) The spectra of La and (b) the spectra of Fe.

those of α-Fe2O3 are at 0.20 and 2.30 eV.19 The band alignment of LaFeO3 and α-Fe2O3 is schematically shown in Figure 1. When LaFeO3 and α-Fe2O3 form a p−n heterojunction, the conduction and covalence bands will be bent due to the Fermi levels of LaFeO3 and α-Fe2O3 matching at the interface. Upon illumination, the photogenerated holes in α-Fe2O3 will be injected into the valence band of LaFeO3 while the photogenerated electron is extracted from the conductive band of LaFeO3 to that of α-Fe2O3 and finally transfers to the counter electrode. Therefore, the photogenerated electrons and holes are expected to be effectively separated within the space charge layer due to the formation of a p−n heterojunction, which would benefit their PEC water splitting performance.

The grazing angle XRD patterns of bare α-Fe2O3 and LF-A are obtained to examine the crystal structures in Figure 2. The formation of hematite is readily observed from the (110) diffraction peak at 35.7° both on bare α-Fe2O3 and LF-A photoanode (JCPDS 33-0664). However, the LaFeO3 film on an Fe2O3 nanorod is too thin to detect all the reflection peaks, and only one identified peak of LaFeO3 is observed at 32.2° which is indexed as the (002) diffraction peak of LaFeO3 (JCPDS 88-0641). Therefore, both the Fe2O3 hematite and LaFeO3 perovskite structures have been formed on the LF-A photoanode. Diffraction peaks of SnO2 (JCPDS 41-1445) come from the conductive fluorine-doped SnO2 layer. 12993


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Figure 4. SEM images of (a) α-Fe2O3 and (b) LF-A, with EDX results shown in insets. (c) TEM image and (d) HRTEM image of LF-A photoanode.

than 2 nm is observed between the separated space lattice, which directly demonstrates the formation of the p-LaFeO3/nFe2O3 heterojunction. Additionally, the p-type characteristics of LaFeO3 are directly evidenced by the cathodic photocurrent density and (see Figure S1 in SI) and negative slope from the Mott−Schottky plots (see Figure S2 in SI). With the characterizations and band alignment of α-Fe 2O 3 and LaFeO3, the p-LaFeO3/n-Fe2O3 heterojunction is successfully fabricated. We also attempt to fabricate the heterojunction via the controlling chemical bath deposition (CBD) by adding La into the β-FeOOH nanorod aqueous synthesis (sample LF-C). However, the XRD (Figure S3 in SI) and HRTEM (Figure S4 in SI) of the sample do not provide any indication of the presence of LaFeO3. The La is expected to be doped into the system rather than form a heterojunction with α-Fe2O3 by the CBD method. To investigate the PEC water oxidation performance of the bare α-Fe2O3 and LF-A photoanodes, the photocurrent densities with and without illumination have been measured via LSV under simulated solar illumination and are shown in Figure 5a. There is ∼50 mV negative shifting of the onset potential observed for the photocurrent of the LF-A photoanode as compared to that of the bare α-Fe2O3 photoanode, which can be attributed to the band bending in the p-LaFeO3/ n-Fe2O3 heterojunction as shown in Figure 1. As for LF-C, the shift of the onset potential for the photocurrent is negligible compared to that of the bare α-Fe2O3 photoanode. The chronoamperometric results of α-Fe2O3, LF-A, and LF-C at

XPS signals are recorded on LF-A and are shown in Figure 3 for further confirmation of the existence of La and the formation of LaFeO3 in the p-LaFeO3/n-Fe2O3 heterojunction. In Figure 3a, two pairs of XPS peaks are observed with 855.1 eV (851.0 eV) and 838.2 eV (834.1 eV). The two pairs of XPS peaks are identified as La 3d3/2 and 3d5/2 peak, respectively, which indicates the existence of La3+.34,35 The Fe 2p1/2 and 2p3/2 peaks at 724.6 and 711.2 eV, as well as 2p1/2 and 2p3/2 satellite peaks at 733.2 and 719.1 eV, demonstrate the existence of the Fe3+ state.36,37 SEM and TEM images of the bare α-Fe2O3 and LF-A photoanode are shown in Figure 4. The top view of the bare αFe2O3 photoanode in Figure 4a indicates the nanorods are grown perpendicular to the FTO substrate and the average diameter is around ∼65 nm. The morphology of the LF-A photoanode (Figure 4a) is similar to that of the bare α-Fe2O3 photoanode (Figure 4b). However, the cluster trend of the LFA nanorods is weaker than that of the bare α-Fe2O3 on the photoanode owing to the La2O3 thin film deposition before the thermal treatment. The corresponding EDX result in Figure 4b confirms the existence of La in LF-A samples. The typical TEM image and magnified HRTEM image of the individual nanorod on the LF-A photoanode are shown in Figure 4c,d, respectively. From Figure 4c, a homogeneous thin film with ∼3.74 nm is obviously observed to cover the nanorod surface. The lattice fingers are determined to be 2.78 and 2.52 Å in Figure 4d, which correspond to the (002) spacing of LaFeO3 and (110) spacing of α-Fe2O3, respectively. Moreover, a thin interface less 12994


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Figure 5. (a) LSV of bare α-Fe2O3, LF-C, and LF-A photoanodes measured in 1 M NaOH (pH ∼ 13.6) with a scan rate of 10 mV/s. (b) Transient photocurrent response with 30 s cyclic illumination at 1.23 V vs RHE. (c) LSV of LF-A photoanodes with different La ALD growth cycles. (d) Transient photocurrent response with 30 s cyclic illumination at 1.23 V vs RHE of LF-A photoanodes with different La ALD growth cycles.

Fe2O3 nanorods, which implies the heterojunction is not achieved within such small ALD cycles. On the other hand, La2O3 deposited with 800 cycles is so thick (∼11.35 nm) that only the neighboring layer within a few nanometers could combine with Fe2O3 to form LaFeO3 after thermal treatment, and the excessive La2O3 covering on the Fe2O3 nanorods hinders the surface water oxidation (see Figure S9 in SI). We notice the photocurrent density of the heterojunction is 0.58 mA/cm2 at 1.23 V versus RHE, which is not so impressive. It is assumed that the slow reaction kinetics at the surface of LaFeO3 suppresses the PEC performance as indicated in a previous study.29 On the other hand, the valence band of LaFeO3 locates at 2.12 eV versus NHE,33 more negative than that of α-Fe2O3 (2.30 eV), which indicates the oxygen evolution driving force is reduced in the formed heterojunction. To solve these problems, CoOx cocatalyst is selected to modify the surface water oxidation kinetics of the p-LaFeO3/n-Fe2O3 heterojunction. The Co 2p XPS spectrum is shown in Figure S10 in SI and provides evidence for the existence of CoOx loading. The photocurrent densities for α-Fe2O3, LF-A, and LFA/CoOx are shown in Figure 6. It is observed that the onset potential of the photocurrent for LF-A/CoOx exhibits more than ∼100 mV negative shifting to the bare α-Fe 2 O 3 photoanode. At 1.23 V versus RHE, the LF-A/CoO x photoanode obtains a photocurrent of ∼1.12 mA/cm2, which is 3 times that of bare α-Fe2O3 photoanode. To investigate the effect of heterojunction and cocatalyst on the carrier densities and flat band, Mott−Schottky plots of αFe2O3, LF-A, and LF-A/CoOx photoanodes are shown in Figure 7. All of them exhibit positive slopes with the n-type characteristic. The carrier concentrations of the photoanodes could be calculated by the Mott−Schottky equation (eq 2 in

1.23 V versus RHE with 30 s cyclic illumination are shown in Figure 5b. The sensitive response of current densities with light on and off indicates the rapid photocurrent generation. The photocurrent density of bare α-Fe2O3 is 0.37 mA/cm2, and it increases to 0.42 mA/cm2 for LF-C and 0.58 mA/cm2 for LF-A at 1.23 V versus RHE. More than 50% promotion of the photocurrent density on the LF-A photoanode has been obtained, which is mainly attributed to the improved charge carrier separation as a result of the p-LaFeO3/n-Fe2O3 heterojunction formation. As for LF-C, the average rod diameter is around ∼40 nm (Figures S4 and S5 in SI), smaller than that of the bare α-Fe2O3 and LF-A photoanode. The higher aspect ratio may be responsible for the improved PEC performance (Figure S6 in SI). Moreover, the enhancement of the photocurrent density for LF-A is rather stable under the light on and off conditions without any decay under 1 h illumination (see Figure S7 in SI), which implies the superior PEC stability of the p-LaFeO3/n-Fe2O3 heterojunction. The photocurrent densities on the LF-A photoanodes with different La2O3 ALD deposition cycles (200, 500, and 800 cycles) have also been studied, and the results are shown in Figure 5c,d. Briefly, we note the photoanode with 200 and 800 La2O3 ALD deposition cycles as LF-A-200 and LF-800. In Figure 5c, it is found that all the photoanodes exhibit negative shifting of the onset potential for the photocurrent. The LF-A with 500 ALD cycles of La2O3 growth yields the largest negative shifting of the onset potential and highest photocurrent density at 1.23 V versus RHE (Figure 5d). For clarification regarding why the sample with 500 cycles of La2O3 ALD deposition exhibits the best PEC performance, HRTEM images are shown in Figure S8 in SI. When deposited with 200 cycles of La2O3, the coating film is discontinuous on the α12995


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Figure 8. (a) EIS Nyquist plots of bare α-Fe2O3, LF-A, and LF-A/ CoOx photoanodes, and (b) the equivalent resistance occurring of the EIS Nyquist.

Figure 6. (a) LSV of bare α-Fe 2 O 3 , LF-A, and LF-A/CoOx photoanodes measured in 1 M NaOH (pH ∼ 13.6) with a scan rate of 10 mV/s. (b) Transient photocurrent response with 30 s cyclic illumination at 1.23 V vs RHE.

Table 1. Equivalent Circuit-Fitting Results of Bare α-Fe2O3, LF-A, and LF-A/CoOx Photoanodes Based on the Nyquist Plots

α-Fe2O3 LF-A LF-A/CoOx

Rs (kΩ)

Rct (kΩ)

Cct (×10−4 F)

Rce (kΩ)

Cce (×10−4 F)

0.632 0.621 0.617

31.82 21.01 17.40

2.21 5.13 8.12

213.25 143.28 75.08

4.17 8.16 11.56

Figure 7. Mott−Schottky plots of bare α-Fe2O3, LF-A, and LF-A/ CoOx photoanodes.

SI). The bare α-Fe2O3 photoanode exhibits the carrier concentration of ∼1.19 × 1021 cm−3. The carrier concentration for LF-A increases to ∼5.38 × 1021 cm−3 due to the formation of a p−n heterojunction. After CoOx modification, it boosted to 3.2 × 1022 cm−3. On the other hand, the flat band of the αFe2O3 photoanode derived from the Mott−Schottky plot is 0.42 V versus RHE, while for LF-A, the flat band exhibits negative shifting of ∼90 mV compared to that of the α-Fe2O3, indicating that a quasi-Fermi-level of the LF-A band bends upward due to the p-LaFeO 3 coating. Since p-LaFeO3 introduces a more negative band level than α-Fe2O3, the band match between them induces well a built-in potential at the p-LaFeO3/n-Fe2O3 interface, which benefits the carrier

Figure 9. IPCE of bare α-Fe2O3, LF-A, and LF-A/CoOx photoanodes measured at 1.23 V vs RHE.

transport in the photoanode. Upon CoOx modification, the flat band is derived to be 0.31 V versus RHE which is the same as that of LF-A (0.32 V vs RHE); thus, the contribution of the CoOx cocatalyst to PEC performance mainly comes from the promotion of surface reaction kinetics rather than the 12996


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mainly be attributed to the accelerated charge separation due to the favorable band matching for the build-in potential. We further modify the p-LaFeO3/n-Fe2O3 heterojunction by loading CoOx cocatalyst on the surface, and this increases the photocurrent density to 1.12 mA/cm2 at 1.23 V versus RHE. The IPCE of the CoO x modified p-LaFeO 3 /n-Fe 2 O 3 heterojunction is promoted to 25.13% at 400 nm, indicating that ALD is a promising technique for fabrication of nanostructures in PEC cell design for water splitting.

introduction of the band bending at the surface. The individual Mott−Schottky plot of the Fe2O3 nanorod and LaFeO3 nanoscale thin film are measured, respectively, in Figure S2. Fermi levels of the Fe2O3 nanorod and LaFeO3 nanoscale thin film are 0.42 and 0.85 eV, respectively. Thus, the Fermi level alignment is about 0.43 eV, which provides the internal electrical field for efficient charge transfer and separation. The Nyquist plots of the three photoanodes during the PEC process are shown in Figure 8 by EIS measurement with the frequency ranging from 0.01 Hz to 100 kHz. It is observed that the radius of the bare α-Fe2O3 is the largest, and LF-A/CoOx exhibits the smallest one in Figure 8a. For the heterojunction, the charge transport resistance decreases in comparison with bare α-Fe2O3 due to improved electron and hole separation within the space depletion layer induced by the built-in potential. Upon CoOx coating, the resistance of the LF-A/ CoOx photoanode is further reduced due to the accelerated carrier transport by promoted surface reaction kinetics. The Nyquist plots are fitted into an equivalent resistance occurring in Figure 8b. Rce/Cce and Rct/Cct are charge transport resistance occurring in bulk and at the semiconductor/electrolyte interface. The fitting data of the bare α-Fe2O3, LF-A, and LFA/CoOx photoanodes are listed in Table 1. The Rct and Rce values for LF-A and LF-A/CoOx are smaller than those of bare α-Fe2O3. The order of the resistance values is in good agreement with that of the arc radius. We notice the order is in accordance with the photocurrent densities of the three photoanodes, which implies the heterojunction and cocatalyst play important roles in modifying the electronic behaviors as well as the PEC performance. IPCE values of the bare α-Fe2O3, LF-A, and LF-A/CoOx photoanodes are measured at 1.23 V versus RHE in 1 M NaOH. In Figure 9a, bare α-Fe2O3 demonstrates an IPCE value of 10.11% at 400 nm, and it diminishes to zero at ∼600 nm, which is in accordance with the band gap of α-Fe2O3. The IPCE of LF-A is increased to 15.47% at 400 nm. As for LF-A/ CoOx, a significant promotion of the IPCE is observed (25.13%) at 400 nm, and we notice that the promotion trend extends to the region up to 600 nm. The α-Fe2O3 nanorod and LF-A heterojunction exhibit similarly extending visible light absorption in wavelength in Figure S12a, and the optical band gaps are calculated to be 2.05 and 2.06 eV, respectively, which implies a very slight change of band gap upon formation of the heterojunction. Moreover, the IPCE curves at different external bias are investigated, and IPCE values tend to be reduced with the decrease of the external bias (see Figure S11 in SI). It is reasonable since the driving force of the photogenerated carrier transport becomes weaker when lower bias is applied.

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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/acs.jpcc.7b01817. I−V plots and Mott−Schottky plots of LaFeO 3 synthesized by ALD, structure and morphology characterization and optimized PEC performance of LF-C, HRTEM of LF-A with different La2O3 ALD cycles, PEC stability measurementm characterization of the CoOx decorated on LF-A, and UV−vis characterization of αFe2O3 and LF-A (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

Bin Shan: 0000-0001-7800-0762 Rong Chen: 0000-0003-0210-2433 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Basic Research Program of China (2013CB934800) and the National Natural Science Foundation of China (51575217 and 51572097), as well as the China Postdoctoral Science Foundation (2017M612451). R.C. acknowledges the Thousand Young Talents Plan, the Recruitment Program of Global Experts, the Hubei Province Funds for Distinguished Young Scientists (2015CFA034), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT13017). The authors would also like to acknowledge the technology support by the Texas Advanced Computing Center (TACC) at the University of Texas at Austin for providing grid resources that have contributed to the research results reported within this paper, and the technology support by the Analytic Testing Center and the Flexible Electronics Research Center of the HUST

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CONCLUSION The p-LaFeO3/n-Fe2O3 heterojunction has been successfully fabricated by ALD techniques that deposited La2O3 on βFeOOH nanorods followed by a thermal treatment at 800°. The formation of a heterojunction is evidenced by XRD, XPS, and HRTEM. The photocurrent density of the heterojunction increases to 0.58 from 0.37 mA/cm2 for bare α-Fe2O3 at 1.23 V versus RHE, and the onset potential of the photocurrent exhibits a negative shift of ∼50 mV. For comparison, the samples fabricated by the CBD method followed by thermal treatment demonstrates La doping into the structure rather than forming a heterojunction with α-Fe2O3 nanorods. It is found that the heterojunction formed via ALD exhibits superior PEC performance as compared to that of CBD, which could

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REFERENCES

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