Fe2O3 Heterojunction Decorated

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An integrating photoanode of WO3/Fe2O3 heterojunction decorated with NiFe-LDH to improve PEC water splitting efficiency Shouli Bai, Xiaojun Yang, Chengyao Liu, Xu Xiang, Ruixian Luo, Jing He, and Aifan Chen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02267 • Publication Date (Web): 24 Aug 2018 Downloaded from http://pubs.acs.org on August 25, 2018

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ACS Sustainable Chemistry & Engineering

An integrating photoanode of WO3/Fe2O3 heterojunction decorated with NiFe-LDH to improve PEC water splitting efficiency Shouli Baia, Xiaojun Yanga, Chengyao Liua, Xu Xianga, *, Ruixian Luoa, b, Jing Hea, *, Aifan Chena a.

State Key Laboratory of Chemical Resource Engineering, Beijing Key

Laboratory of Environmentally Harmful Chemicals Analysis, Beijing University of Chemical Technology, Beijing 100029, China. b.

Guangxi Key Laboratory of Petrochemical Resource Processing and Process

Intensification Technology, School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China.

Corresponding authors Tell: +86 010 64438182 E-mail: [email protected]; [email protected] Street address: Beijing University of Chemical Technology No.15, Bei San Huan East Road, Chaoyang District, Beijing, CN 100029 1

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Combining semiconductor heterojunction and cocatalyst is an important strategy to improve photoelectrochemical (PEC) water splitting efficiency. Here, a photoanode of WO3/Fe2O3 heterojunction decorated by NiFe-layered double hydroxide (LDH) was fabricated by two-step hydrothermal methods. As expected, the photocurrent density of the ternary photoanode reaches up to 3.0 mA·cm-2, which respectively are 5 times and 7 times of WO3 and α-Fe2O3. The improvement benefits from the extending absorption of visible light, the effective separation of photogenerated charge carriers, and acceleration of water oxidizing reaction, which is caused by narrowing band gap and electron directionally flowing of heterojunction as well as catalyst timely consuming of holes accumulated at the electrode surface. The electron lifetime and the steady-state carrier density for four photoanodes were estimated from electrochemical impedance spectra (EIS) and were further confirmed by the intensity modulated photocurrent spectra (IMPS). The work provides a demonstration to develop a high efficiency and low-cost photoanode for application in solar energy PEC water splitting. KEYWORDS: photoelectrochemical water splitting, integrating photoanode, semiconductor heterojunction, cocatalyst, NiFe-layered double hydroxide.

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INTRODUCTION The energy demand and environmental pollution have caused great attention. PEC water splitting of solar energy has widely been recognized as an effective pathway to resolve both above mentioned.1-2 The key is to explore and investigate the visible light driving photoelectrode materials. The narrow band-gap semiconductors such as WO3 (~2.7 eV), BiVO4 (2.4 eV), and α-Fe2O3 (~2.2 eV) as the photoanode materials have received a lot of attention.3-4 The α-Fe2O3 is a suitable photoanode material for PEC water oxidation due to response to visible light, non-toxic, abundance, and good PEC stability in water and electrolyte.5-6 However, the single α-Fe2O3 is a poor conductor with a short holes diffusion length (2–4 nm), thus α-Fe2O3 is easy to suffer recombination of electron-hole pairs in the bulk and poor surface reaction dynamics.7-9 Tungsten trioxide (WO3), similar to α-Fe2O3, has caused wide attention as a photoanode material for PEC water splitting, which is attributed to sufficiently positive VB position than the water oxidation potential, a longer holes diffusion length than α-Fe2O3, high electron mobility, and low cost. However, a single metal oxide semiconductor can generally not meet the requirement of PEC water splitting. Many strategies have been attempted, such as heterojunction constructing,10 morphology controlling,5 doping,11 and incorporating of oxygen evolution cocatalysts.12 These trial results demonstrate that structuring heterostructure by introducing a second semiconductor with matching band gaps is a good strategy.13-14 Mao et al. reported the α-Fe2O3 modified WO3 heterojunctions for PEC water splitting, and the obtained composite showed visible light harvesting and reached the highest 3



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photocurrent density of 0.91 mA/cm2 that is 9 times of pure WO3 in sodium sulfate electrolyte.15

Likewise,

Sivula

et

al.

successfully

synthesized

WO3/Fe2O3

heterojunctions and the photocurrent density increased 20% compared to pure WO3.16 Herein, α-Fe2O3 was used as the primary light absorber and WO3 as an electron acceptor to structure a n-n heterojunction of WO3/α-Fe2O3 for creating a high driving force of photogenerated carriers separation and transfer, which in turn results in the enhancement of PEC water splitting efficiency.17 Nevertheless, the water oxidation kinetics is still a bottleneck of water oxidation, thus various catalysts were introduced, such as IrO2, RuO2 have shown surprisingly high OER performances, but their rarity limits their practical use.18-19 LDHs are a category of inexpensive, high-active catalysts toward PEC water oxidation, and they can effectively integrate with semiconductors to accelerate the surface reaction kinetics of PEC water oxidation. Recently reported ZnO/LDH, TiO2/LDH, and BiVO4/LDH photoanodes have shown high catalytic activity for water oxidation.20-22 Among them, although Ni is an active component of catalytic cycle in the Ni and Fe coexistence LDH cocatalyst, Fe provides active sites for Ni species, both of which are indispensable to achieve a high active catalysis.23-24 In this work, an integrated photoanode consisting of WO3/Fe2O3 heterojunction and cocatalyst of NiFe-LDH was reported. The ternary photoanode exhibits a significant improvement to PEC properties compared to WO3, α-Fe2O3, and WO3/Fe2O3 photoanodes, which is attributed to the double effect of heterostructure and cocatalyst. The corresponding mechanism is also discussed in detail. 4


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EXPERIMENT SECTION Reagents and materials FTO substrate with the resistance of 14 Ω/square and thickness of glass of 2.2 mm was bought from Wuhan teaching instruments co., Ltd.; Ammonium oxalate ((NH4)2C2O4·H2O), sodium tungstate (Na2WO4·2H2O), and hydrochloric acid (HCl) were gained from Sinopharm Chemical Reagent (Beijing Co., Ltd.). Sodium nitrate (NaNO3·6H2O), ferric trichloride (FeCl3), nickel nitrate (Ni(NO3)2·6H2O), ferric nitrate (Fe(NO3)3·9H2O) and urea were purchased from Xilong Chemical Co., Ltd. Synthesis of WO3 films WO3 films were synthesized by hydrothermal method. Typically, 0.1237 g sodium tungsten dehydrates (Na2WO4·2H2O) was dissolved in 15 ml of distilled water as solution A, 0.1172 g ammonium oxalate ((NH4)2C2O4·H2O) was added into 15 ml of distilled water as a solution B. 5 ml of HCl solution (3 M) was slowly dropped into solution A. After stirring few minutes at room temperature, the white suspension was obtained. The solution B was then added into suspension. When the suspension became clear, the mixture was removed into a 50 ml of the stainless autoclave, the FTO glass was placed vertically to the bottom of the Teflon-lined stainless-autoclave and side facing down was immersed, then sealed and thermally treated at 140 oC for 6 h. Afterward, the obtained samples were washed thoroughly and dried in air overnight at 60 oC. The products were calcined at 500 oC for 2.5 h with 5 oC·min-1 rate to obtain the WO3 films. Preparation of WO3/Fe2O3 heterojunction 5



Fe2O3 films have been prepared by hydrothermal method. Specifically, 0.5099 g sodium nitrate (NaNO3·6H2O) was slowly dissolved in 30 ml of distilled water, 3 M of HCl solution was slowly dropped into the solution with constant stirring at room temperature to adjust pH to 1.4-1.5. 0.87 g ferric trichloride (FeCl3) was then added into the mixture. After a few minutes of stirring the mixture was placed into a 50 ml stainless autoclave in which immersed FTO substrate with WO3. The autoclave was maintained at 100 oC for 6 h. After the reaction, the obtained samples were washed thoroughly using distilled water and absolute alcohol followed by drying at 60 oC in the air. Finally, the obtained products were calcined at 600 oC for 2 h at 5 oC·min-1 heating rate. Preparation of WO3/Fe2O3/NiFe-LDH ternary photoanode The LDH film was hydrothermally on the WO3/Fe2O3 heterojunction. Typically, a 15 ml of the solution including 14 mM of Ni(NO3)2·6H2O, 14 mM of Fe(NO3)3·9H2O, and 140 mM urea were placed in a 25 ml of autoclave containing the WO3/Fe2O3 heterojunction film. After reacted for 6 h at 100 oC the NiFe-LDH film has been deposited on the surface of the WO3/Fe2O3 heterojunctions to form an integrating photoanode through rinsed with copious distilled water and dried at 80 oC for 2 h. The synthetic schematic diagram of WO3/Fe2O3/LDH photoanode is shown in Figure 1.

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Figure 1. Synthetic schematic diagram of WO3/Fe2O3/NiFe-LDH photoanode. Characterization of material structure and properties The sample morphologies were observed by field emission scanning electron microscope (FESEM, Zeiss Supra 55, on 20.0 kV). The information of sample crystal phases and structures was identified by X-ray diffraction (XRD) analysis (Rigaku D/MAX-2500 diffractometer) employing Cu Ka radiation (λ = 0.154 nm). The image of the high-resolution TEM (HR-TEM) was obtained at 200 kV of accelerating voltage using a microscope (JEOL JEM-2010). X-ray photoelectron spectroscopy (XPS) analysis was recorded on a spectrometer (VG ESCALAB-MK). The data were calibrated by the binding energy of C 1s. The ultraviolet-visible diffuse reflectance spectra of the prepared photoanodes were performed by a UV–Vis spectrophotometer (Shimadzu UV-2550, Japan). PEC performance measurements of photoanode All PEC properties of the photoanode were examined in 1 M of NaOH electrolyte solution (13.6 pH) using a typical three-electrode configuration by using the prepared photoanode as the working electrode, saturated Ag/AgCl electrode as a reference 7



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electrode, and a Pt wire as counter electrode. A xenon lamp with 300 W equipped with an AM 1.5 G of the filter was used as the light source, the illumination intensity and illumination area of the work electrodes were 100 mW/cm2 and 2 cm2, respectively.

An

electrochemical

workstation

(CHI

660E)

was

used

for

current−voltage measurement with a scan rate of 15 mV·s-1. The measured potential of the working electrode is converted into the potential of reversible hydrogen electrode (RHE) in terms of the equation below:

= ⁄ + ⁄ . + 0.059 In which, the value of EAg/AgCl

vs. NHE

(1)

is 0.197 V at the room temperature.

Electrochemical impedance spectroscopies (EIS) were carried out in a frequency range from 100 kHz to 0.1 Hz and at 10 mV amplitude of perturbation. RESULTS AND DISCUSSION Morphology and structure of materials The scanning electron microscope (SEM) images are shown in Figure 2 to observe the morphologies of WO3, WO3/Fe2O3 and WO3/Fe2O3/LDH films. The corresponding HRTEM images were investigated as shown in Figure 2(e) and 2(f). Figure 2(a) shows WO3 plates grown on the FTO substrate with high density and uniform vertical alignment. The surface of WO3 is smooth, the edge length ranges from 600 nm to 700 nm and the thickness is about 100 nm. As shown in Figure 2(b), it can be seen that the α-Fe2O3 nanorods cover the WO3 film and have a mean length of more than 1 µm. The SEM images of the WO3/Fe2O3/LDH film are shown in Figure 2(c) and 2(d), demonstrating the surface of the WO3/Fe2O3 has been covered 8


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by NiFe-LDH sheets. The more detailed microscopic information shows in Figure 2(e) and 2(f), the HRTEM images show the lattice fringe spacing of 0.365 nm corresponding to the (200) plane of monoclinic WO3 and the lattice fringe spacing of 0.269 nm corresponding to the (104) plane of α-Fe2O3, the results indicate that the WO3/Fe2O3 composite with n–n heterojunction has been synthesized successfully. Besides, an apparent interface between the LDH and the WO3/Fe2O3 is observed as shown in Figure 2(f), indicating the NiFe-LDH has decorated on the surface of WO3/Fe2O3 heterojunctions.

Figure 2. Top-view SEM images of (a) pure WO3, (b) WO3/Fe2O3, (c) and (d) WO3/Fe2O3/LDH; HRTEM images of (e) WO3/Fe2O3 and (f) WO3/Fe2O3/LDH. The crystalline structures of samples were verified by X-ray diffraction (XRD) analyses. Figure 3 displays the characteristic peaks of WO3, WO3/Fe2O3, and WO3/Fe2O3/LDH photoanodes. The diffraction peak at 37.8o corresponds to the FTO substrate (JCPDS Card No. 46–1088). The WO3 several characteristic diffraction peaks of 23.1o, 23.5o, 24.3o, 26.2o, 34.2o corresponding to planes of (002), (020), 9



(200), (120), (202) can be observed, which demonstrates that the monoclinic WO3 (JCPDS No.72-0677) has been successfully synthesized. The diffraction peaks located at 33.2o and 35.6o corresponding to (104) and (110) planes are indexed to the diffraction peak position of monoclinic α-Fe2O3 (JCPDS No. 33-0664). The analysis result is consistent with the HR-TEM observation. The characteristic peak located at 10.7o corresponding to (003) plane in the XRD patterns indicates the successful growth of NiFe-LDH on the surface of α-Fe2O3 although other diffraction peaks corresponding to (006), (009), (015) planes appear weaker due to a shorter deposition time and the preferentially oriented growth of the LDH crystallites.

Figure 3. XRD patterns of WO3 (black), WO3/Fe2O3 (green), WO3/Fe2O3/LDH(blue). To confirm the existence of W, Fe, and Ni elements in the sample and their oxidation state for later analysis of PEC performance and mechanism the XPS 10


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spectrum of the WO3/Fe2O3/LDH film was performed as shown in Figure 4. The W 4f peaks at about 34.9 and 37.1 eV shown in Figure 4(a) can be assigned to W6+ 4f7/2 and 4f5/2, respectively. As shown in Figure 4(b), the XPS spectrum of O 1s can be used to study the surface oxygen species of the sample. In which, two peaks corresponding to the binding energy located at 531.1 eV and 529.6 eV were obviously observed, they respectively are assigned to the surface O2− species possessed in the oxide and the surface O-H (or C-O) bond.24 The XPS spectrum of Fe 2p is shown in Figure 4(c), it can be seen that two major peaks located at 711.2 eV and 725.0 eV were detected for Fe 2p3/2 and 2p1/2, they corresponding binding energies are good match with the characteristics of Fe3+.25 Therefore, it can be concluded that the oxidation state of Fe element existed in the α-Fe2O3 and the NiFe-LDH is 3+. In the XPS spectrum of Ni 2p, a small Ni 2p3/2 peak and an associated satellite peak were detected, and they correspond to binding energies of 855.2 eV and 863.3 eV as shown in Figure 4(d). The existence of two Ni 2p peaks confirms that the oxidation state of Ni existed in the NiFe-LDH cocatalyst is 2+.26 Therefore, the XPS results confirm that the Fe exists as Fe3+, while Ni exists as Ni2+ in LDH and the characteristic peaks of Ni2+ (Ni-O) and Fe3+ (Fe-O) in WO3/Fe2O3/LDH can be considered to be the successful decoration of the LDH on the WO3/Fe2O3 heterojunction.

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Figure 4. XPS spectra of (a) W 4f, (b) O 1s, (c) Fe 2p and (d) Ni 2p for WO3/Fe2O3/NiFe-LDH photoanode. To investigate the effect of heterostructure on the harvesting of visible light, UV-visible

absorption

spectroscopy

of

WO3,

α-Fe2O3,

WO3/Fe2O3,

and

WO3/Fe2O3/LDH were undertaken as illustrated in Figure 5(a). The absorption edge of WO3 and α-Fe2O3 are 340 nm and 560 nm, respectively, while the WO3/Fe2O3 heterojunction exhibits a slight shift towards longer wavelength due to the reduction of the band gap. Comparing with WO3 and α-Fe2O3, the light absorption intensity also increases, which arises from the heterojunction formation at the interface between α-Fe2O3 and WO3. After loading LDH, the absorption edge basically does not change because the NiFe-LDH has no effect on the band gap of the heterostructure composite. We use the Tauc equation to find the values of the optical band gap (Eg) for single and 12


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composites.27 Eg values of the WO3, α-Fe2O3, and WO3/Fe2O3 heterojunction were estimated by the plots as shown in Figure 5(b). The band gaps of WO3, α-Fe2O3, WO3/Fe2O3, and WO3/Fe2O3/LDH were calculated to be 2.65, 2.18, 2.02 and 2.02 eV, respectively. The decrease of Eg can widen the absorption spectrum range, which is beneficial to the enhancement of PEC property.

Figure 5. (a) UV−vis absorbance spectra and (b) the energy gap of WO3 (black), α-Fe2O3(red), WO3/Fe2O3 (green) and WO3/Fe2O3/LDH (blue). PEC performance of photoanode Photocurrent density is the most direct and distinct reflection to explore the influence of heterostructure and NiFe-LDH catalyst on the PEC performance of photoanode. The photocurrent density-voltage (I–V) curves of the photoanodes were measured under back-side illumination in a 1 M NaOH electrolytic solution (pH = 13.6). Figure 6(a) presents the measured photocurrent density-voltage (I–V) curves for bare oxides, heterojunction, and LDH decorated heterojunction photoanodes. The obtained current density of WO3/Fe2O3 heterojunction reaches up to 1.6 mA·cm-2, which is not only 2.3 times of WO3 (0.6 mA·cm-2) at 1.8 V vs. RHE but also higher 13



than that of WO3/Fe2O3 core-shell structures reported in the literature.28 The enhanced reason is attributed to the separating efficiency enhancement of photogenerated electron-hole caused by the electrons directionally flowing in heterostructures. After loading LDH, the photocurrent density reaches up to 3.0 mA·cm-2, which is 5 times of WO3 and 7 times of α-Fe2O3. Because the NiFe-LDH can timely consume holes accumulated at the electrode surface, which in turn accelerates the rate of water oxidation reaction and enhances the separation efficiency of charge carriers. The transient photocurrent curves of as-obtained photoanodes shown in Figure 6(b) indicate a rapid current response to visible light when the light turns on and off, which demonstrates the current response resulting from visible light illumination.29

Figure 6. (a) Photocurrent density-voltage curves and (b) transient photocurrent curves of the photoanodes. To explore the charge transport properties at the electrode/electrolyte interface, the electrochemical impedance spectroscopies (EIS) were measured for photoanodes. Figure 7(a) shows the EIS Nyquist plots of WO3, α-Fe2O3, WO3/Fe2O3, and WO3/Fe2O3/LDH photoanodes. To obtain the resistance (Rct), the measured 14


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impedance is fitted to the equivalent circuit using the Zview software package. The derived Rct values for WO3, α-Fe2O3, and WO3/Fe2O3 respectively are 803 Ω, 2257 Ω, and 576 Ω. The Rct of WO3/Fe2O3/LDH with optimum deposition amounts was further reduced to 506 Ω. The above data also indicate that the WO3/Fe2O3 heterojunction possesses rapid electron transfer speed across the electrode/electrolyte interface due to the appropriate arrangement of band-gap structure, while the holes transferred by LDH on the interface will have a relatively long lifetime to carry out the water evolution oxygen reaction, leading to the enhancement of PEC water oxidation efficiency. The effective electron lifetime (τe) was calculated using the equation (2):30

=

(2)

In which, fmax is the frequency value of the highest point in the semicircle of the test. The τe values were estimated to be 2.06, 4.04, 5.40, 7.94 ms corresponding to α-Fe2O3, WO3, WO3/Fe2O3, and WO3/Fe2O3/LDH, respectively. The increase of the electron lifetime means the recombination rate decreases of photogenerated charges. And the steady-state electron density (ns) in the photoanode can be estimated from equation (3):30

= # $

×"

(3)

××%& '×())

In which Kb, T, q, L, and A represent Boltzmann constant, absolute temperature, proton charge, electrode thickness, and electrode area, respectively. The estimated ns for α-Fe2O3 and WO3 are about 1016 and the ns of WO3/Fe2O3 and WO3/Fe2O3/LDH photoanodes are ~1017. It is concluded that the ns values of heterojunction and 15



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integrating photoanode is much higher than pure oxide based photoanode, which is excellent agreement with the results obtained from EIS. Meanwhile, the WO3/Fe2O3/LDH photoanode with the highest ns also shows the best photocurrent density and the fastest water oxidation rate. The charge transport kinetics was further investigated by intensity modulated photocurrent spectroscopy (IMPS) and the IMPS plots of the photoanodes are shown in Figure 7(b). The response displays one semicircle, in which the apex frequency can be related with the transient time of the electron (τd) and can be estimated from the following equation (4):31

* = +2-./01 23

(4)

In which, fmin represents the characteristic frequency that located at the lowest point of the IMPS plot. According to Figure 7(b), τd values of WO3 and α-Fe2O3 were calculated to be 1.83 ms and 2.10 ms, respectively. The calculated τd values for WO3/Fe2O3 heterojunction and WO3/Fe2O3/LDH photoanode are 1.12 ms and 0.78 ms, respectively. The value of WO3/Fe2O3 heterojunction is much smaller than pure WO3 and α-Fe2O3. This result indicates that the WO3/Fe2O3 photoanode has a higher electron transfer rate than that of WO3 and α-Fe2O3 photoanodes, which results from band bending at the interface in heterojunction, leading to a directional flow of electron. The τd value of WO3/Fe2O3/LDH is 70% that of WO3/Fe2O3 heterojunction, indicating that the cocatalytic of LDH can further enhance carries transport for water oxidation, which supports the EIS results very well.

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Figure 7 (a) EIS spectra measured at the open-circuit potential under illumination and (b) IMPS response plot of WO3, α-Fe2O3, WO3/Fe2O3 and WO3/Fe2O3/LDH photoanodes. To understand enhanced PEC efficiency origin, Mott–Schottky (M-S) plots were measured in term of equation (5) for the prepared photoanodes at the frequency of 1000 Hz under dark conditions. 1/6 7 = +2/899: ;< =7 2 × > ? @ ? AB/8C

(5)

In which C, e, ε, ε0, k, and T are the capacitance at the semiconductor/electrolyte interface (F·cm-2), the elementary charge (1.60×10-19 C), the relative dielectric constant (F·m-1), the permittivity of the vacuum (8.85×10-12 F·m-1), Boltzmann's constant (1.38×10-23 F·m-1), and the absolute temperature (K), respectively. As shown in Figure 8(a), the M-S plots of the semiconductors and their junction all show positive slopes corresponding to their plots, which means that the materials constituted photoanode are n-type semiconductors. Moreover, the flat band potential (EFB) is a useful parameter for the electron flow direction between semiconductors in heterojunction and they can be derived from the intercept of MS plots to be −0.33, −0.42, and −0.37 vs. Ag/AgCl, respectively for WO3, α-Fe2O3, and WO3/Fe2O3 films. 17



The results indicate that the flowing direction of the photogenerated electron from α-Fe2O3 to WO3 due to the CB edge of α-Fe2O3 higher than WO3 (i.e., more negative than WO3) as shown in Figure 8(b). Hence, when the heterojunction-based photoanode was radiated by visible light, the excited electrons in α-Fe2O3 will transfer to the CB of WO3 followed by transfer to FTO due to the formation of potential gradient caused by the band bending at the interface between two semiconductors. Meanwhile, the holes in the VB of WO3 will transfer to the VB of α-Fe2O3 and move to the electrode/electrolyte interface to oxidize water evolution oxygen, leading to the rapid separation of photogenerated charge carriers. Consequently, their overall PEC efficiencies are improved compared to their single component. From EFB values, the flat band potential of WO3/Fe2O3 heterojunction exhibits a positive shift for α-Fe2O3 and negative shift for WO3, which also implies the directional flow of electrons in WO3/Fe2O3 heterojunctions, thus inhibiting the recombination of photoexcited electron-hole and the improvement of PEC performance.

Figure 8. (a) MS plots of WO3, α-Fe2O3, WO3/Fe2O3, and WO3/Fe2O3/LDH in 1M NaOH without illumination of 1 kHz; (b) Energy band diagram of the WO3/Fe2O3. Mechanism of LDH cocatalyst enhanced water oxidation at photoanode 18


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The schematic illustration of WO3/Fe2O3/LDH photoanode combined with a Pt counter electrode for the PEC water oxidation mechanism is shown in Figure 9. When α-Fe2O3 (as the primary light-absorber) forms a heterojunction with WO3 (as an electron acceptor), the band bending formed at the interface between α-Fe2O3 and WO3 impels the carriers to diffuse in opposite direction until their Fermi levels reach equivalence. Upon irradiation, the photogenerated electrons and holes appear in the conduction bands and the valence bands of α-Fe2O3 and WO3, respectively. The electrons and holes transfer rapidly in the opposite direction due to the heterojunction-generated inner electric field. The electrons of the conduction band in the α-Fe2O3 flow to the WO3 conduction band, whereas the holes flow to the valence band of α-Fe2O3 from WO3 for an efficient oxygen evolution reaction. When loading the LDH catalyst on the surface of the heterojunction, it can timely consume the holes at the electrode surface and suppress the recombination of electron-hole pairs. Because the NiFe-LDH has the pseudocapacitance property, the photogenerated holes can be easily transferred from the VB of α-Fe2O3 to the cocatalyst layer to oxide Ni2+ to Ni3+ and Ni4+ species in NiFe-LDH. The Ni species with high valence transfer the hole to oxidize the water for releasing oxygen, simultaneously the high valence of Ni species are reduced into the low valence species (Ni2+), the process completes a catalytic cycle. Thus, the NiFe-LDH cocatalyst effectively separates the photogenerated electron−hole pairs and accelerates the water oxidation kinetics, which is due to LDH rapidly extracting the holes at the surface of photoanode.

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Figure 9. The catalytic cycle of water oxidation at the integrating photoanode. CONCLUSION The present study demonstrates that the combinatory of NiFe-LDH cocatalyst and the WO3/Fe2O3 heterojunction is an effective strategy for improvement of the light absorption, separation of photogenerated charge carriers, and acceleration of the surface reaction. The photocurrent density of WO3/Fe2O3/LDH reaches up to 3.0 mA·cm-2 at 1.8 V vs. RHE, which is significantly higher than the pristine both oxides. Furthermore, the integrating photoanode also improves the electron transfer speed at the interface electrode/electrolyte and effective electron lifetime. Such good PEC properties result from the formation of semiconductor heterojunction and the cocatalytic effect of LDH because the dual effects overcome electron-hole recombination and enhance holes mobility for accelerating the water oxidation. AUTHOR INFORMATION Corresponding authors Tell: +86 010 64438182 E-mail: [email protected]; [email protected] 20


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ORCID Shouli Bai: 0000-0001-6069-0590 Xiaojun Yang: 0000-0002-6045-339X Chengyao Liu: 0000-0003-1008-0334 Xu Xiang: 0000-0003-1089-6210 Ruixian Luo: 0000-0002-7754-0772 Jing He: 0000-0002-2940-6675 Aifan Chen: 0000-0001-7509-373X Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by State Key Laboratory of Chemical Resource Engineering and the National Natural Science Foundation of China (Grant Nos. 51772015 and 21576016). REFERENCES (1) Zhen, C.; Chen, R.; Wang, L.; Liu, G.; Cheng, H. M. Tantalum (Oxy)nitride Based Photoanodes for Solar-Driven Water Oxidation. J. Mater. Chem. 2016, 4 (8), 2783-2800, DOI 10.1039/c5ta07057k. (2) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238 (5358), 37-38, DOI 10.1038/238037a0. (3) Kronawitter, C. X.; Vayssieres, L.; Shen, S.; Guo, L.; Wheeler, D. A.; Zhang, J. Z.; Antoun, B. R.; Mao, S. S. A Perspective on Solar-Driven Water Splitting with 21



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For Table of Contents Use Only

The WO3/Fe2O3/NiFe-LDH photoanode combining heterojunction and cocatalyst to improve the PEC water splitting efficiency is related to the sustainable development of clean energy.

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Figure 1. Synthetic schematic diagram of WO3/Fe2O3/NiFe-LDH photoanode. 107x48mm (300 x 300 DPI)


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Figure 2. Top-view SEM images of (a) pure WO3, (b) WO3/Fe2O3, (c) and (d) WO3/Fe2O3/LDH; HRTEM images of (e) WO3/Fe2O3 and (f) WO3/Fe2O3/LDH. 106x50mm (300 x 300 DPI)



Figure 3. XRD patterns of WO3 (black), WO3/Fe2O3 (green), WO3/Fe2O3/LDH(blue). 94x72mm (300 x 300 DPI)


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Figure 4. XPS spectra of (a) W 4f, (b) O 1s, (c) Fe 2p and (d) Ni 2p for WO3/Fe2O3/NiFe-LDH photoanode. 93x73mm (300 x 300 DPI)



Figure 5. (a) UV−vis absorbance spectra and (b) the energy gap of WO3 (black), α-Fe2O3(red), WO3/Fe2O3 (green) and WO3/Fe2O3/LDH (blue). 108x43mm (300 x 300 DPI)


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Figure 6. (a) Photocurrent density-voltage curves and (b) transient photocurrent curves of the photoanodes. 89x37mm (300 x 300 DPI)



Figure 7 (a) EIS spectra measured at the open-circuit potential under illumination and (b) IMPS response plot of WO3, α-Fe2O3, WO3/Fe2O3 and WO3/Fe2O3/LDH photoanodes. 107x39mm (300 x 300 DPI)


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Figure 8. (a) MS plots of WO3, α-Fe2O3, WO3/Fe2O3, and WO3/Fe2O3/LDH in 1M NaOH without illumination of 1 kHz; (b) Energy band diagram of the WO3/Fe2O3. 94x36mm (300 x 300 DPI)



Figure 9. The catalytic cycle of water oxidation at the integrating photoanode. 66x40mm (300 x 300 DPI)


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Fe2O3 Heterojunction Decorated

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