Surface Engineering of Hematite To Synergistically

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Combining bulk/surface engineering of hematite to synergistically improve its photoelectrochemical water splitting performance Yufei Yuan, Jiuwang Gu, Kaihang Ye, Zhisheng Chai, Xiang Yu, Xiaobo Chen, Chuanxi Zhao, Yuan-Ming Zhang, and Wenjie Mai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04142 • Publication Date (Web): 08 Jun 2016 Downloaded from http://pubs.acs.org on June 14, 2016

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ACS Applied Materials & Interfaces

Combining Bulk/Surface Engineering of Hematite to Synergistically Improve its Photoelectrochemical Water Splitting Performance Yufei Yuan1, Jiuwang Gu1, Kaihang Ye2, Zhisheng Chai1, Xiang Yu2, Xiaobo Chen1, Chuanxi Zhao1,*, Yuanming Zhang2 and Wenjie Mai 1,* 1

Siyuan Laboratory, Guangzhou Key Laboratory of Vacuum Coating Technologies and New

Energy Materials, Department of Physics, Jinan University, Guangzhou, Guangdong 510632, China 2

Department of Chemistry, Jinan University, Guangzhou, Guangdong 510632, China

ACS Paragon Plus Environment

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KEYWORDS: Photoelectrochemical water splitting; hematite; Ti dopant; surface engineering; synergistic effect

ABSTRACT: One of the most promising candidates for photoelectrochemical (PEC) water splitting photoanode is hematite (α-Fe2O3) due to its narrow bandgap and chemical stability. However, the poor bulk/surface kinetics of hematite limits its PEC performance. Herein, a facile two-step approach is reported to synergistically improve the PEC performance of Fe2O3. First, through bulk engineering of Ti doping, the photocurrent density of Ti-Fe2O3 photoanode (1.68 mA cm-2 at 1.23 VRHE) shows a 3-fold increase compared with that of pure Fe2O3 photoanode (0.50 mA cm-2 at 1.23 VRHE). Second, the photocurrent density of Ti-Fe2O3 photoanode could be further enhanced to 2.31 mA cm-2 by surface engineering of FeOOH. The enhanced PEC water splitting performance is proposed to be the synergistic effect of bulk and surface engineering, which can be mainly attributed to the great increase of charge separation efficiency and surface transfer efficiency.


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1. INTRODUCTION For photoelectrochemical (PEC) water splitting, various photoanodes have been studied including hematite (α-Fe2O3), TiO2, ZnO, WO3 and BiVO4.1-8 Obviously, hematite has several promising properties, including narrow bandgap (1.9~2.2 eV) that maximizes absorption of the visible solar spectra,9 chemical stability, earth abundance and low cost.10 Thus, Fe2O3 is considered as one of the most promising candidates for photoanode materials. However, the PEC performance of Fe2O3 is still limited by several issues which include short hole diffusion length (2-4 nm),11 short excited-state lifetime,12 poor conductivity and sluggish oxygen evolution reaction (OER) kinetics13. The sluggish bulk kinetics of Fe2O3 owing to its poor majority carrier conductivity,4 and meager surface kinetics caused by its poor OER kinetics,14 limits its PEC performance. Generally, the maximum theoretical water oxidation photocurrent density (Jmax) for Fe2O3 is 12.6 mA cm-2 under AM 1.5 G solar illumination.15 Although great efforts have been devoted to improve the PEC performance of Fe2O3, the practical water oxidation photocurrent density is still far lower than Jmax. There are two predominant factors limit the practical PEC performance of Fe2O3, including poor charge separation efficiency (ηsep) and sluggish charge transfer efficiency (ηtrans). Great efforts have been devoted to improve the charge separation efficiency of Fe2O3. In this regard, diverse element dopant including Si, Ti, Sn, Zr, Nb, Mn, Ag, Pt,14-23 creating oxygen vacancy,24 formation of heterojunctions,25 graphene composites and introducing electron collecting scaffolds26-28 are versatile strategies to increase charge separation efficiency. Many attempts have also been made to enhance the charge transfer efficiency of Fe2O3. These include development of oxygen evolution catalysts (OEC) and formation of hole-storage layer22. For instance, Lee et al. developed a nanostructured Ti-doped Fe2O3 photoanode for PEC water


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splitting and generated a photocurrent density of 0.15 mA cm-2 at 1.23 VRHE.23 In addition, Cho et al. developed three processes to enhance PEC performance of Fe2O3, which includes Ti doping, dense layer for suppressing the back-injection of electron to FTO, and surface treatment, and consequently, all these processes promise a reduction of photocurrent onset potential of 380 mV.4 Despite all these achievements, the facile fabrication of prominent PEC water splitting photoanode still remains a challenge. Notably, a more efficient approach would be combining bulk/surface engineering to synergistically improve its PEC water splitting performance. Herein, a facile and efficient two-step approach is proposed to improve the PEC performance of Fe2O3 by combining bulk and surface engineering synergistically. In the first step, the introduction of Ti dopant into Fe2O3 crystal structure can lead to the enhanced charge separation efficiency and bulk kinetics. Namely, Ti doping increases the charge carrier density and electrical conductivity efficiently, resulting in the enhancement of charge separation efficiency. In the second step, FeOOH is deposited onto the Ti doped Fe2O3 to enhance its surface kinetics, which corresponds to the improved charge transfer efficiency. In this case, the modification of FeOOH is devoted to reducing the poor OER kinetics of Fe2O3. More importantly, integrating the first and second step simultaneously will synergistically improve the charge separation and transfer efficiency. It is worth noting that Ti-Fe2O3-FeOOH photoanode presents a synergistic effect, in which case the increase of its photocurrent density is much larger than the simple sum of the increments from individual contributions. The cooperation between Ti and FeOOH synergistically boosts the PEC performance of Ti-Fe2O3-FeOOH photoanode. 2. EXPERIMENTAL DETAIL 2.1. Synthesis of Fe2O3 and Ti-doped Fe2O3 nanorod arrays


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The FeOOH and Ti doped FeOOH with the diverse Ti/Fe atomic ratio nanorod arrays were prepared by hydrothermal method. A more detailed description could be found elsewhere.17 Next, the as-grown FeOOH converted to Fe2O3 via two-step annealing, initially at 550 oC for 2 h then followed by 20 min short annealing time at 700 oC. 2.2. Photo-assisted electrodeposition of FeOOH catalyst For the process of photo-assisted electrodeposition of FeOOH catalyst on the as-prepared Tidoped Fe2O3 electrode, which was carried out in a 0.1 M FeSO4 solution with gently stirring. Before the deposition, the solution was purged with nitrogen gas for 1 h. A three-electrode PEC cell was assembled with the Fe2O3 work electrode, Pt counter electrode, and saturated Ag/AgCl reference electrode. A 300 W Xe arc lamp with an AM 1.5 G filter, neutral density filters, and a water filter (IR filter) was used as the light source. The light was illuminated through the FTO contact (back-side illumination), during which an external bias of 0.5 V vs. Ag/AgCl was applied. Finally, FeOOH was electrodeposited by applying 1.2 V vs. Ag/AgCl. A total charge of ~45 mC cm-2 was passed through for 1 min for the final electrodeposition. 2.3. Structural and composition characterization The phase purity of the as-prepared samples was characterized by X-ray diffraction (XRD) on the Rigaku MiniFlex 600 X-ray diffractometer using Cu Kα radiation (λ=0.15406 nm). Morphological and structural characteristics of the samples were observed by field emission scanning electron microscope (FE-SEM, ZEISS ULTRA 55), X-ray photoelectron spectroscopy (XPS, ESCALab250), and transmission electron microscope (TEM, JEOL 2100F, 200 kV) equipped with an energy-dispersive X-ray spectroscopy (EDS) detector. The transmittance spectra and absorption spectra of Fe2O3 films were measured on Enlitech’s QE-R system with an integrating sphere and UV-visible spectra photometer (Shimadzu UV-3600) respectively.


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2.4. Photoelectrochemical measurements The PEC and electrochemical measurements were conducted in a typical three electrode configuration with Ag/AgCl reference electrode, Pt counter electrode, and the Fe2O3 photoanode as working electrode. The linear sweep voltammograms (LSV) curves of samples were carried out in 1 M KOH (pH 13.6) electrolyte. A 300 W xenon arc lamp (6255, Newport) coupled with AM 1.5 G filter was used as light source. Its intensity was calibrated at 100 mW cm-2 (1 sun illumination) using silicon diode (Newport). IPCE were collected by a Solartron 1280B electrochemical station with a solar simulator (Newport 69920, 1000 W xenon lamp), coupled with an infrared water filter (Oriel 6127) and aligned monochromator (Oriel Cornerstone 130 1/8 m) in 1 M KOH solution (pH =13.6). The electrochemical impedance spectroscopy (EIS) was performed using an electrochemical workstation (Princeton Applied Research) with AC amplitude of 10 mV and frequency range between 100 kHz to 0.1 Hz. The measured EIS data was obtained at an applied bias of 0.9 VRHE at 25 ˚C. All other electrochemical measurements were performed on a CHI 760D electrochemical workstation (CHI, Shanghai). 3. RESULTS AND DISCUSSION 3.1 Structural and Composition Characterizations The aligned and ordered Fe2O3 nanorods (NRs) were synthesized by a facile hydrothermal method and a post-annealing process. The morphology of the Fe2O3 NRs was investigated on SEM. Figure 1 (a-d) shows that all the Fe2O3 NRs own similar morphology, suggesting that Ti doping has no significant effect on the shape and morphology. The obtained Fe2O3 NRs are vertically grown on FTO and about 400 nm in length (Figure 1d). Figure 2a shows a typical scanning transmission electron microscopy (STEM) image of an individual Ti-Fe2O3 nanorod, which indicates that the diameter of Ti-Fe2O3 nanorod is about 100


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nm. Figure 2b exhibits the enlarged high-resolution transmission electron microscopy (HRTEM) image of the corresponding selected area of Figure 2a, the ( 10 1 0 ), ( 2 2 02 ) atomic plane spacing is determined to be 0.25 nm and 0.21 nm, respectively. The enlarged HRTEM image and the corresponding Fast Fourier Transform pattern (inset of Figure 2b) suggest the single-crystal structure of the Ti-Fe2O3 NRs. The lattice distortion has not been observed due to the low concentration of Ti atomic (inset of Figure 2a) also suggests the single-crystal structure of the TiFe2O3 NRs. Figure 2c-f shows the TEM image and corresponding Energy-dispersive X-ray spectrometry (EDS) elemental mapping of single Ti-Fe2O3 nanorod. The clear edge region of EDS mapping and the EDS spectrum of the Ti-Fe2O3 nanorod (Figure S1) confirmed the incorporation of Ti ions into the Fe2O3 NRs. X-ray analysis was performed to study the crystal structure of the samples. Figure 3 shows the XRD patterns of FTO glass substrate, FeOOH, pure Fe2O3 and Ti-Fe2O3 on the FTO substrate. Evidently, the diffraction peaks of the FeOOH can be index well to the β-FeOOH (JCPDS No.34-1226). Two obvious diffraction peaks of (110) and (211) facets are observed. After the annealing, all the β-FeOOH diffraction peaks are vanished and new peaks emerge. These new diffraction peaks correspond well to the α-Fe2O3 (JCPDS No.33-0664), indicating that the complete conversion of β-FeOOH to α-Fe2O3. Therefore, it is reasonable to confirm that the αFe2O3 NRs were successfully obtained through the present facile hydrothermal method and a post-annealing process. There are no other secondary diffraction peaks observed from Ti-Fe2O3 sample due to the low concentrate of Ti ions. In addition, the characteristic diffraction signals of (110) and (300) facets are unambiguously observed in Fe2O3 XRD pattern, from which we can verify that the pure Fe2O3 crystal structure of such a film without any other secondary phase except the SnO2 peaks from FTO substrate.


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As shown in Figure 4, the XPS survey spectra are collected from Ti-Fe2O3-FeOOH NRs. Figure 4b shows that Fe 2p XPS spectrum, and the peaks centered at 710.7 and 724.1 eV are associated with Fe 2p3/2 and Fe 2p1/2 respectively, which are in accordance with the typical values of Fe3+ in Fe2O3.29 The absence of Fe2+ satellite peaks indicates that there is no Fe2+ on the surface of the sample.30 The XPS spectra of O 1s peaks (Figure 4c) can be disassembled into two peaks locates at 530.1 eV (Fe-O) and 531.6 eV (Fe-OH),31 which are associated with Fe2O3 and FeOOH, respectively. In Figure 4d, the Ti 2p3/2 and Ti 2p1/2 peaks located at 458.0 eV and 463.8 eV, while the binding energy of Ti 2p3/2 (458.0 eV) locates at the range of the typical values of TiO2 (458.8 eV) and metallic Ti (454.1 eV), which are in good agreement with the typical values of Ti4+ and also provide additional evidence of the introduction of Ti ions into the Fe2O3.32 3.2 PEC performance of Fe2O3 based photoanode Figure 5a shows the photocurrent density–potential (J-V) cures of pure Fe2O3, Ti-Fe2O3, Fe2O3-FeOOH and Ti-Fe2O3-FeOOH photoanode. The measured potentials referring to the Ag/AgCl electrode were converted to the reversible hydrogen electrode (RHE) scale according to the Nernst equation:33

ERHE = EAg / AgCl + 0.059 pH + 0.1976V where ERHE is the converted potential referred to the RHE and EAg/AgCl is the experimentally measured potential against the Ag/AgCl reference electrode. Figure S2 shows the typical J–V curves of Ti-Fe2O3 with different atomic percentage of Ti, which clearly indicates the optimal atomic percentage of Ti is 0.05 % for the highest photocurrent density. The pure Fe2O3 displays a low photocurrent density of 0.51 mA cm-2 at 1.23 VRHE, due to its poor electrical conductivity. However, a significant 3-fold (1.68 mA cm-2 at 1.23 VRHE) increase of photocurrent density is obtained from the Ti-Fe2O3 photoanode when compared with that of pure Fe2O3 photoanode.


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This improvement is attributed to the enhanced bulk kinetics by Ti doping resulting from improved electrical conductivity. Besides the bulk engineering, the surface kinetics also exhibit key role in the PEC performance for the active-site limited effect. Further improvement of photocurrent density could be obtained by surface photo-assisted electrodeposition of FeOOH on Ti-Fe2O3 photoanode (2.31 mA cm-2 at 1.23 VRHE). In order to distinguish the effect of bulksurface engineering effect on the PEC performance, parallel experiments with single modification have been carried out. As shown in Figure 5a, the Fe2O3-FeOOH photoanode only generated a photocurrent density of 0.89 mA cm-2 at 1.23 VRHE. Evidently, photocurrent density through co-modified by Ti doping and FeOOH is dramatically higher than those of individual modified photoanode. The charge carrier density has been greatly enhanced after Ti doping, resulting in enhanced charge separation and electrical conductivity. In addition, the large PEC water splitting overpotential of Fe2O3 caused by its poor OER kinetics can be reduced by FeOOH surface modification efficiently. The enhanced OER kinetics leads to the increase of charge transfer efficiency. In brief, the increase of electrical conductivity after Ti doping causes dramatic enhancement of photocurrent density of Fe2O3. After cooperating with FeOOH, the photocurrent density has exhibited a further improvement. Therefore, combining Ti dopant and FeOOH modification can effectively improve the charge separation efficiency and transfer efficiency simultaneously, resulting in dramatic photocurrent density of Ti-Fe2O3-FeOOH photoanode. It is worth noting that Ti-Fe2O3-FeOOH photoanode present a synergistic effect, in which case the increase of its photocurrent density is much larger than the simple sum of the increments from individual contributions. The cooperation between Ti and FeOOH synergistically boosts the PEC performance. Recently, nanostructured Ti-doped Fe2O3 photoanodes for PEC water splitting was reported by Lee et al. and generated a photocurrent


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density of 0.15 mA cm-2 at 1.23 VRHE.23 In contrast, the current work employing Ti doping and FeOOH modification synergistically generates a much higher photocurrent density of Ti-Fe2O3FeOOH (2.31 mA cm-2 at 1.23 VRHE). Similar approach was reported to improve the PEC performance of Fe2O3 by Cho et al,4 which developed three processes to enhance PEC performance of Fe2O3, including Ti doping, dense layer for suppressing the back-injection of electron to FTO, and surface treatment, resulting in a low-bias performance of Fe2O3 and photocurrent density of 1.51 mA cm-2 at 1.23 VRHE. The effect of surface FeOOH could be distinguished from LSV experiment. Figure S3 shows the LSV curves measured in 1 M KOH aqueous solution containing 15 % H2O2 served as hole scavenger. The photocurrent density for water oxidation from the Fe2O3-FeOOH photoanode is higher than that of pure Fe2O3 photoanode, but the photocurrent density for H2O2 oxidation is still lower than that of pure photoanode. The above comparison result indicates that the interface states formed at the Fe2O3FeOOH junction can serve as recombination centers and cause surface recombination.33 To quantitatively investigate the PEC activity of Fe2O3 photoanode, as shown in Figure 5b, the wavelength dependence of the incident photon-to-current conversion efficiency (IPCE) is examined. The IPCE of each sample was measured in 1 M KOH at 1.23 VRHE under monochromatic illumination, which is calculated based on the equation:8

IPCE =

1240 I λ J light

where I is the photocurrent density, Jlight is the measured irradiance, and λ is the wavelength of the monochromatic light. As shown in Figure 5b, the IPCE spectra of all samples exhibit similar feature, and the photocurrent responses of Ti-Fe2O3-FeOOH and Ti-Fe2O3 photoanode are much higher than those of pure Fe2O3 and Fe2O3-FeOOH photoanode. The results are in accordance with Figure 5a. Figure S4 shows the UV-vis absorbance spectra of pure Fe2O3, Fe2O3-FeOOH,


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Ti-Fe2O3 and Ti-Fe2O3-FeOOH samples. The absorption edge of each sample shows no obvious change, which indicating that co-modified by Ti doping and FeOOH has no significant effect on the absorption of visible light. To further understand the bulk engineering, the charge separation efficiency (ηsep) is determined. The ηsep is calculated based on the equation:8, 34-36

J H2O = J max ×ηabs ×ηsep ×ηtrans where J H 2O is the measured photocurrent density, η abs is the light absorption efficiency and ηtrans is the surface transfer efficiency. With the presence of H2O2, assuming the oxidation kinetics of H2O2 is very fast, the ηtrans is taken as 100 %. As a consequence, the photocurrent density in presence of H2O2 is obtained by:

J H 2O2 = J max ×ηabs ×η sep Here J H 2O2 is the photocurrent density in the presence of H2O2. Hence, the separation efficiency could be determined by the following equation:

ηsep = J H O / ( J max ×ηabs ) 2 2

The ηsep are plotted in Figure 5c. The ηsep of Ti-Fe2O3 (31.2 % at 1.23 VRHE) is significantly higher than that of Fe2O3 (21.3 % at 1.23 VRHE), which means that Ti dopant can efficiently enhanced the charge separation efficiency. Likewise，the ηsep of Ti-Fe2O3-FeOOH (35.9 % at 1.23 VRHE) is also significantly higher than that of Fe2O3-FeOOH (17.5 % at 1.23 VRHE) as anticipated. The increase of charge separation efficiency after Ti-doping results from enhancement of electrical conductivity, as testified by the larger photocurrent density shown in Figure 5a. According to the work reported by Meng et al.37, Ti dopant has similar electronic structure but different orbital energy with Fe 3d orbital. The Fermi level of Fe2O3 will move


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from the low position of the band gap to high energy position after Ti doping and enter the extended conduction band, resulting in the improvement of electron concentration near the Fermi level. Therefore, an electrical conductivity transition will occur in Ti-Fe2O3 from an antiferromagnetic semiconductor to a conducting state, leading to the enhancement of electrical conductivity. The PEC reaction always has to compete with electron-hole recombination in the photoanode, besides the recombination determines the hole lifetime. Undoubtedly, the increase of charge separation efficiency after Ti doping results in reduction of electron-hole recombination, extending hole lifetime and promising the prominent PEC performance. In brief, enhanced electrical conductivity originates from the nonequivalent nature of Ti-doping, which causes the Fermi level move into the conduction band and results in an electric conductivity transition from the original semiconducting state to a metallic state. Such a transition is beneficial for photocatalytic applications because it can extend the lifetime of charge carriers and hinder the recombination of electron-hole pairs. Furthermore, the surface transfer efficiency (ηtrans) is also investigated to further understand the effect of surface engineering. The ηtrans are calculated by ηtrans = J H 2O / J H 2O2 , as plotted in Figure 5d. The result reveals that the ηtrans of Fe2O3-FeOOH photoanode (66.8 % at 1.23 VRHE) is significantly higher than that of pure Fe2O3 photoanode (31.5 % at 1.23 VRHE). It’s interesting to note that Ti-Fe2O3-FeOOH (81.9 % at 1.23 VRHE) shows the highest surface transfer efficiency, as well as better than that of Ti-Fe2O3 (70.6 % at 1.23 VRHE). Based on the above comparison, it is reasonable to confer that surface modified by FeOOH can be a beneficial method to enhance the surface transfer efficiency of hematite photoanode. The modification of FeOOH provides an alternative pathway for water oxidation that avoids hole accumulation at the Fe2O3 surface. Therefore, the large PEC water splitting overpotential of Fe2O3 caused by its poor OER kinetics


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can be reduced by FeOOH surface modification efficiently. Notably, Ti dopant and FeOOH modification can efficiently and synergistically improve the ηsep and ηtrans, which result in dramatic photocurrent density of Ti-Fe2O3-FeOOH photoanode. Next, the Mott-Schottky (M-S) analysis is employed to further determine the charge separation efficiency of the samples. The experiment is measured at a frequency of 1000 Hz in 1 M KOH aqueous solution under dark condition. The calculation is based on the equation:14

K T 1 2 = (V − V fb − B ) 2 2 C εε 0 A eN D e where C is the space charge capacitance of the semiconductor, ε is the dielectric constant of Fe2O3, ε0 is the permittivity of vacuum, A is the active area of the photoanode, e is the electronic charge, ND is the charge carrier concentration, V is the applied potential, Vfb is the flat band potential, kB is Boltzmann’s constant, and T is the absolute temperature. Moreover, the charge carrier density implies charge separation efficiency. The charge carrier density is calculated based on the equation:

 d (1/ C 2 )  ND = ( ) εε 0e  d (V )  2

−1

According to Figure 6a, all the slopes of the M-S curves are positive, indicating that all the samples are n-type semiconductors. In addition, the slopes of the M-S curves are corresponding to the charge carrier concentration in photoanode. There is no significant difference between the slope of the M-S curves collected from Ti-Fe2O3 and Ti-Fe2O3-FeOOH photoanode. Similarly, the slope of the M-S curves collected from Fe2O3 and Fe2O3-FeOOH photoanode are almost the same, which indicate that the FeOOH electrocatalyst deposition has no significant effect on charge carrier concentration. Nevertheless, the slope of the M-S curves collected from Ti-Fe2O3 (Ti-Fe2O3-FeOOH) is much smaller than that of Fe2O3 (Fe2O3-FeOOH), which suggests that Ti


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dopant increases the charge carrier concentration significantly. The above result also proves that the introduction Ti dopant can greatly enhance the charge separation efficiency. The charge transfer efficiency of the samples is further analyzed by the electrochemical impedance spectroscopy (EIS) analysis. As shown in Figure 6b, the Nyquist plots of all samples exhibit different radius of similar semicircle. The radius implies charge transfer resistance across the semiconductor/electrolyte interface. Fe2O3-FeOOH photoanode shows a much smaller radius than that of pure Fe2O3 photoanode indicating its smaller charge transfer resistance. The radius of Ti-Fe2O3-FeOOH is even smaller than that of Ti-Fe2O3 photoanode, which implies that the surface FeOOH layer can facilitate the charge transfer across the semiconductor/electrolyte interface. The modification of FeOOH provides an alternative pathway for water oxidation that avoids hole accumulation at the Fe2O3 surface. Hence, all these results demonstrate that the deposition of FeOOH electrocatalyst can significantly improve the transfer efficiency of TiFe2O3 photoanode. Moreover, the applied bias photon-to-current efficiency (ABPE) was also employed to quantitatively evaluate the PEC water splitting efficiency of the samples, which is based on the equation:8

ABPE =

J ph (1.23 − Vb ) Ptotal

where Vb is the applied bias vs. RHE, Jph is the photocurrent density at the measured potential, and Ptotal is the power density of incident light (100 mW cm-2). As shown in Figure 6c, the photoconversion efficiency of Ti-Fe2O3-FeOOH photoanode is calculated to be 0.18 %, which is much higher than that of pure Fe2O3 (0.04 %), Fe2O3-FeOOH (0.09 %) and Ti-Fe2O3 (0.11 %) photoanode. The results are in agreement with above Figure 5a. It is well-known that the stability of photoanode is another important property for the PEC water splitting. As shown in


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Figure 6d, the stability of pure Fe2O3, Fe2O3-FeOOH, Ti-Fe2O3 and Ti-Fe2O3-FeOOH photoanode is examined at 1.23 VRHE for a continuous 3.5 h irradiation without additional sacrificial reagent. As we can see, there is not obvious decay of photocurrent density for the samples, which indicates their excellent stability. 4. CONCLUSIONS In summary, an efficient bulk and surface engineering method is proposed to synergistically boost the PEC water splitting performance of Fe2O3 photoanode. For the bulk engineering approach, Ti dopant is intentionally introduced to increase the electrical conductivity of hematite based photoanode. The photocurrent density of Ti-Fe2O3 photoanode (1.68 mA cm-2 at 1.23 VRHE) shows a 3-fold increase when compared with that of pure Fe2O3 photoanode (0.50 mA cm-2 at 1.23 VRHE). Through surface engineering by FeOOH, Ti-Fe2O3 photoanode shows a further enhanced photocurrent density of 2.31 mA cm-2. It is worth noting that Ti-Fe2O3-FeOOH photoanode presents a synergistic effect, in which case the increase of its photocurrent density is much larger than the simple sum of the increments from individual contributions. On the one hand, Ti dopant can efficiently enhance the charge separation efficiency owing to the enhanced conductivity, and on the other hand, surface modified with FeOOH will further increase the charge transfer efficiency attributing to the enhanced OER kinetics. Combining Ti dopant and FeOOH modification can efficient improve the charge separation efficiency and transfer efficiency simultaneously. The synergistic effect could be realized by bulk and surface engineering, both of which account for the excellent PEC performance. All the prominent PEC response properties demonstrate that Ti-Fe2O3-FeOOH photoanode is one of the most promising candidates used for PEC water splitting. ASSOCIATED CONTENT


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Supporting Information EDS spectrum, J–V curves, and UV-vis absorption spectra. This Supporting Information is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected]

*

E-mail: [email protected]

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We are grateful for the financial supports from the National Natural Science Foundation of China (Grant 21376104) and the Natural Science Foundation of Guangdong Province, China (Grants 2014A030306010 and 2014A030310302). REFERENCES 1. Liu, Z.-Q.; Xie, X.-H.; Xu, Q.-Z.; Guo, S.-H.; Li, N.; Chen, Y.-B.; Su, Y.-Z., Electrochemical Synthesis of ZnO/CdTe Core-shell Nanotube Arrays for Enhanced Photoelectrochemical Properties. Electrochim. Acta 2013, 98, 268-273. 2. Ibadurrohman, M.; Hellgardt, K., Morphological Modification of TiO2 Thin Films as Highly Efficient Photoanodes for Photoelectrochemical Water Splitting. ACS Appl. Mater. Interfaces 2015, 7 (17), 9088-9097. 3. Wheeler, D. A.; Wang, G.; Ling, Y.; Li, Y.; Zhang, J. Z., Nanostructured Hematite: Synthesis, Characterization, Charge Carrier Dynamics, and Photoelectrochemical Properties. Energy Environ. Sci. 2012, 5 (5), 6682-6702. 4. Cho, I. S.; Han, H. S.; Logar, M.; Park, J.; Zheng, X., Enhancing Low-Bias Performance of Hematite Photoanodes for Solar Water Splitting by Simultaneous Reduction of Bulk, Interface, and Surface Recombination Pathways. Adv. Energy Mater. 2016, 6 (4), 1501840. 5. Dotan, H.; Sivula, K.; Grätzel, M.; Rothschild, A.; Warren, S. C., Probing the Photoelectrochemical Properties of Hematite (α-Fe2O3) Electrodes Using Hydrogen Peroxide as a Hole Scavenger. Energy Environ. Sci. 2011, 4 (3), 958-964. 6. Kuang, P. Y.; Ran, J. R.; Liu, Z. Q.; Wang, H. J.; Li, N.; Su, Y. Z.; Jin, Y. G.; Qiao, S. Z., Enhanced Photoelectrocatalytic Activity of BiOI Nanoplate-Zinc Oxide Nanorod p-n Heterojunction. Chem. - Eur. J. 2015, 21 (43), 15360-15368.


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7. Kuang, P.-Y.; Su, Y.-Z.; Xiao, K.; Liu, Z.-Q.; Li, N.; Wang, H.-J.; Zhang, J., DoubleShelled CdS- and CdSe-Cosensitized ZnO Porous Nanotube Arrays for Superior Photoelectrocatalytic Applications. ACS Appl. Mater. Interfaces 2015, 7 (30), 16387-16394. 8. Ye, K.-H.; Chai, Z.; Gu, J.; Yu, X.; Zhao, C.; Zhang, Y.; Mai, W., BiOI–BiVO4 Photoanodes with Significantly Improved Solar Water Splitting Capability: p–n Junction to Expand Solar Adsorption Range and Facilitate Charge Carrier Dynamics. Nano Energy 2015, 18, 222-231. 9. Ling, Y.; Wang, G.; Wheeler, D. A.; Zhang, J. Z.; Li, Y., Sn-Doped Hematite Nanostructures for Photoelectrochemical Water Splitting. Nano Lett. 2011, 11 (5), 2119-2125. 10. Hamann, T. W., Splitting Water with Rust: Hematite Photoelectrochemistry. Dalton Trans. 2012, 41 (26), 7830-7834. 11. Kennedy, J. H., Flatband Potentials and Donor Densities of Polycrystalline α-Fe2O3 Determined from Mott-Schottky Plots. J. Electrochem. Soc 1978, 125 (5), 723. 12. Cherepy, N. J.; Liston, D. B.; Lovejoy, J. A.; Deng, H.; Zhang, J. Z., Ultrafast Studies of Photoexcited Electron Dynamics in γ- and α-Fe2O3 Semiconductor Nanoparticles. J. Phys. Chem. B 1998, 102 (5), 770-776. 13. Dare-Edwards, M. P.; Goodenough, J. B.; Hamnett, A.; Trevellick, P. R., Electrochemistry and Photoelectrochemistry of Iron(III) Oxide. J. Chem. Soc., Faraday Trans. 1 1983, 79 (9), 2027-2041. 14. Tamirat, A. G.; Su, W.-N.; Dubale, A. A.; Chen, H.-M.; Hwang, B.-J., Photoelectrochemical Water Splitting at Low Applied Potential Using a NiOOH Coated Codoped (Sn, Zr) α-Fe2O3 Photoanode. J. Mater. Chem. A 2015, 3 (11), 5949-5961. 15. Kim, J. Y.; Magesh, G.; Youn, D. H.; Jang, J. W.; Kubota, J.; Domen, K.; Lee, J. S., Single-Crystalline, Wormlike Hematite Photoanodes for Efficient Solar Water Splitting. Sci. Rep. 2013, 3, 2681. 16. Glasscock, J. A.; Barnes, P. R. F.; Plumb, I. C.; Savvides, N., Enhancement of Photoelectrochemical Hydrogen Production from Hematite Thin Films by the Introduction of Ti and Si. J. Phys. Chem. C 2007, 111 (44), 16477-16488. 17. Fu, Z.; Jiang, T.; Liu, Z.; Wang, D.; Wang, L.; Xie, T., Highly Photoactive Ti-Doped αFe2O3 Nanorod Arrays Photoanode Prepared by a Hydrothermal Method for Photoelectrochemical Water Splitting. Electrochim. Acta 2014, 129 (0), 358-363. 18. Wang, G.; Ling, Y.; Wheeler, D. A.; George, K. E.; Horsley, K.; Heske, C.; Zhang, J. Z.; Li, Y., Facile Synthesis of Highly Photoactive α-Fe2O3-Based Films for Water Oxidation. Nano Lett. 2011, 11 (8), 3503-3509. 19. Xi, L.; Chiam, S. Y.; Mak, W. F.; Tran, P. D.; Barber, J.; Loo, S. C. J.; Wong, L. H., A Novel Strategy for Surface Treatment on Hematite Photoanode for Efficient Water Oxidation. Chem. Sci. 2013, 4 (1), 164-169. 20. Gurudayal; Chiam, S. Y.; Kumar, M. H.; Bassi, P. S.; Seng, H. L.; Barber, J.; Wong, L. H., Improving the Efficiency of Hematite Nanorods for Photoelectrochemical Water Splitting by Doping with Manganese. ACS Appl. Mater. Interfaces 2014, 6 (8), 5852-5859. 21. Shen, S.; Zhou, J.; Dong, C. L.; Hu, Y.; Tseng, E. N.; Guo, P.; Guo, L.; Mao, S. S., Surface Engineered Doping of Hematite Nanorod Arrays for Improved Photoelectrochemical Water Splitting. Sci. Rep. 2014, 4, 6627. 22. Wang, Z.; Liu, G.; Ding, C.; Chen, Z.; Zhang, F.; Shi, J.; Li, C., Synergetic Effect of Conjugated Ni(OH)2/IrO2 Cocatalyst on Titanium-Doped Hematite Photoanode for Solar Water Splitting. J. Phys. Chem. C 2015, 119 (34), 19607-19612.


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23. Lee, M. H.; Park, J. H.; Han, H. S.; Song, H. J.; Cho, I. S.; Noh, J. H.; Hong, K. S., Nanostructured Ti-Doped Hematite (α-Fe2O3) Photoanodes for Efficient Photoelectrochemical Water Oxidation. Int. J. Hydrogen Energy 2014, 39 (30), 17501-17507. 24. Ling, Y.; Wang, G.; Reddy, J.; Wang, C.; Zhang, J. Z.; Li, Y., The Influence of Oxygen Content on the Thermal Activation of Hematite Nanowires. Angew. Chem., Int. Ed. 2012, 51 (17), 4074-4079. 25. Hou, Y.; Zuo, F.; Dagg, A.; Feng, P., Visible Light-Driven α-Fe2O3 Nanorod/Graphene/BiV(1-x)MoxO4 Core/Shell Heterojunction Array for Efficient Photoelectrochemical Water Splitting. Nano Lett. 2012, 12 (12), 6464-6473. 26. Agegnehu, A. K.; Pan, C.-J.; Rick, J.; Lee, J.-F.; Su, W.-N.; Hwang, B.-J., Enhanced Hydrogen Generation by Cocatalytic Ni and NiO Nanoparticles Loaded on Graphene Oxide Sheets. J. Mater. Chem. 2012, 22 (27), 13849-13854. 27. Dubale, A. A.; Su, W.-N.; Tamirat, A. G.; Pan, C.-J.; Aragaw, B. A.; Chen, H.-M.; Chen, C.-H.; Hwang, B.-J., The Synergetic effect of Graphene on Cu2O Nanowire Arrays as a Highly Efficient Hydrogen Evolution Photocathode in Water Splitting. J. Mater. Chem. A 2014, 2 (43), 18383-18397. 28. Lin, Y.; Zhou, S.; Sheehan, S. W.; Wang, D., Nanonet-Based Hematite Heteronanostructures for Efficient Solar Water Splitting. J. Am. Chem. Soc. 2011, 133 (8), 23982401. 29. Liu, Y.; Yu, Y.-X.; Zhang, W.-D., Photoelectrochemical Properties of Ni-Doped Fe2O3 Thin Films Prepared by Electrodeposition. Electrochim. Acta 2012, 59 (0), 121-127. 30. Miao, C.; Ji, S.; Xu, G.; Liu, G.; Zhang, L.; Ye, C., Micro-Nano-Structured Fe2O3:Ti/ZnFe2O4 Heterojunction Films for Water Oxidation. ACS Appl. Mater. Interfaces 2012, 4 (8), 4428-4433. 31. Yang, P.; Ding, Y.; Lin, Z.; Chen, Z.; Li, Y.; Qiang, P.; Ebrahimi, M.; Mai, W.; Wong, C. P.; Wang, Z. L., Low-Cost High-Performance Solid-State Asymmetric Supercapacitors Based on MnO2 Nanowires and Fe2O3 Nanotubes. Nano Lett. 2014, 14 (2), 731-736. 32. Magnan, H.; Stanescu, D.; Rioult, M.; Fonda, E.; Barbier, A., Enhanced Photoanode Properties of Epitaxial Ti Doped α-Fe2O3 (0001) Thin Films. Appl. Phys. Lett. 2012, 101 (13), 133908. 33. Kim, T. W.; Choi, K. S., Nanoporous BiVO4 Photoanodes with Dual-Layer Oxygen Evolution Catalysts for Solar Water Splitting. Science 2014, 343 (6174), 990-994. 34. Zhong, D. K.; Choi, S.; Gamelin, D. R., Near-Complete Suppression of Surface Recombination in Solar Photoelectrolysis by “Co-Pi” Catalyst-Modified W:BiVO4. J. Am. Chem. Soc. 2011, 133 (45), 18370-18377. 35. Rao, P. M.; Cai, L.; Liu, C.; Cho, I. S.; Lee, C. H.; Weisse, J. M.; Yang, P.; Zheng, X., Simultaneously Efficient Light Absorption and Charge Separation in WO3/BiVO4 Core/Shell Nanowire Photoanode for Photoelectrochemical Water Oxidation. Nano Lett. 2014, 14 (2), 10991105. 36. Abdi, F. F.; Han, L.; Smets, A. H. M.; Zeman, M.; Dam, B.; van de Krol, R., Efficient Solar Water Splitting by Enhanced Charge Separation in a Bismuth Vanadate-Silicon Tandem Photoelectrode. Nat. Commun. 2013, 4. 37. Meng, X. Y.; Qin, G. W.; Li, S.; Wen, X. H.; Ren, Y. P.; Pei, W. L.; Zuo, L., Enhanced Photoelectrochemical Activity for Cu and Ti Doped Hematite: The First Principles Calculations. Appl. Phys. Lett. 2011, 98 (11), 112104.


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Figure Caption Figure 1. SEM image of (a) pure Fe2O3, (b) Ti-Fe2O3, (c) Ti-Fe2O3-FeOOH, (d) Cross-sectional SEM image of Ti-Fe2O3 Figure 2. (a) STEM image of Ti-Fe2O3 NR.(b) HRTEM image of Ti-Fe2O3 NR, an enlarged area in (a) (inset is Fast Fourier Transform pattern). (c) STEM image of a select area. (d-f) EDS element mapping images from the same area as in (c). Figure 3. XRD patterns collected for FTO, FeOOH and Fe2O3 samples. Figure 4. (a) XPS survey spectra. XPS spectra of (b) Fe 2p, (c) O 1s and (d) Ti 2p collected from the Ti-Fe2O3-FeOOH nanorod arrays. Figure 5. (a) J–V curves recorded under AM 1.5 G irradiation. (b) IPCE spectra collected at the incident wavelength range from 400 to 700 nm at 1.23 VRHE in potassium hydroxide electrolyte (pH 13.6). (c) Bulk charge separation efficiency (ηsep) of all the photoanode. (d) Surface charge transfer efficiency (ηtrans) of all the photoanode. Figure 6. (a) Mott–Schottky plots measured in 1 M KOH solution at 1 kHz Frequency. (b) Electrochemical impedance spectra (EIS) measured at 0.9 VRHE AM 1.5 G irradiation. (c) Photoconversion efficiency. (d) The Chronoamperometry (i–t) curves of all the photoanode collected under AM 1.5 G irradiation.


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Figure 1. SEM image of (a) pure Fe2O3, (b) Ti-Fe2O3, (c) Ti-Fe2O3-FeOOH, (d) Cross-sectional SEM image of Ti-Fe2O3


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Figure 2. (a) STEM image of Ti-Fe2O3 NR.(b) HRTEM image of Ti-Fe2O3 NR, an enlarged area in (a) (inset is Fast Fourier Transform pattern). (c) STEM image of a select area. (d-f) EDS element mapping images from the same area as in (c).


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Figure 3. XRD patterns collected for FTO, FeOOH and Fe2O3 samples.


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Figure 4. (a) XPS survey spectra. XPS spectra of (b) Fe 2p, (c) O 1s and (d) Ti 2p collected from the Ti-Fe2O3-FeOOH nanorod arrays.


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Figure 5. (a) J–V curves recorded under AM 1.5 G irradiation. (b) IPCE spectra collected at the incident wavelength range from 400 to 700 nm at 1.23 VRHE in potassium hydroxide electrolyte (pH 13.6). (c) Bulk charge separation efficiency (ηsep) of all the photoanode. (d) Surface charge transfer efficiency (ηtrans) of all the photoanode.


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Figure 6. (a) Mott–Schottky plots measured in 1 M KOH solution at 1 kHz Frequency. (b) Electrochemical impedance spectra (EIS) measured at 0.9 VRHE AM 1.5 G irradiation. (c) Photoconversion efficiency. (d) The Chronoamperometry (i–t) curves of all the photoanode collected under AM 1.5 G irradiation.


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TOC


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Surface Engineering of Hematite To Synergistically

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