- Email: [email protected]
Surface modification of hematite photoanode by NiFe layered double hydroxide for boosting photoelectrocatalytic water oxidation
Journal of Alloys and Compounds 764 (2018) 341e346
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Surface modification of hematite photoanode by NiFe layered double hydroxide for boosting photoelectrocatalytic water oxidation Yukun Zhu a, b, Xiaoliang Zhao b, **, Junzhi Li a, Huawei Zhang c, Shuai Chen d, Wei Han a, e, *, Dongjiang Yang b a
Key Laboratory of Physics and Technology for Advanced Batteries (Ministry of Education), College of Physics, Jilin University, Changchun 130012, China Collaborative Innovation Center for Marine Biomass Fibers Materials and Textiles of Shandong Province, School of Environmental Science and Engineering, Qingdao University, Qingdao 266071, China c College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, China d State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Science, Taiyuan 030001, China e International Center of Future Science, Jilin University, Changchun 130012, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 April 2018 Received in revised form 5 June 2018 Accepted 6 June 2018
The photoelectrocatalytic (PEC) oxygen evolution reaction (OER) is one of the most efficient ways for utilizing solar energy for water electrolysis. Nevertheless, up to now, the PEC conversion efficiency of the established photoanode is still low. In this work, a-Fe2O3/NiFe layered double hydroxide (LDH) photoanodes were synthesized by electrodeposition of LDH on a-Fe2O3. Compared with bare a-Fe2O3, the aFe2O3/Ni0.5Fe0.5-LDH photoanode displays about 3 times photocurrent enhancement and excellent longterm stability. The enhanced PEC activity of a-Fe2O3/NiFe-LDH is ascribed to the interface between aFe2O3 and NiFe-LDH which can facilitate charge transfer and improve carrier density. Simultaneously, NiFe-LDH as a co-catalyst can accelerate the surface OER kinetics. © 2018 Published by Elsevier B.V.
Keywords: Hematite Layered double hydroxide Electrodeposition Photoelectrocatalytic Oxygen evolution reaction
1. Introduction Converting and utilizing solar energy into the useable energy through photoelectrocatalytic (PEC) water splitting is one of the most challenges nowadays [1,2]. The crucial step in PEC water splitting is oxygen evolution reaction (OER) occurred at the photoanode, but it is restricted by a multistep four electrons transfer that requiring large overpotentials [3]. For decades, great efforts have been focused on the transition metal oxides as promising photoanode materials, such as TiO2 [4], WO3 [5], a-Fe2O3 [6], and BiVO4 [7]. Among them, hematite (a-Fe2O3) was extensively studied and has shown interesting OER performance due to the favourable optical bandgap and appropriate valence band position. However, several inherent drawbacks impede their PEC efficiency,
* Corresponding author. Key Laboratory of Physics and Technology for Advanced Batteries (Ministry of Education), College of Physics, Jilin University, Changchun 130012, China. ** Corresponding author. School of Environmental Science and Engineering, Qingdao University, Qingdao 266071, China. E-mail addresses: [email protected] (X. Zhao), [email protected] (W. Han). https://doi.org/10.1016/j.jallcom.2018.06.064 0925-8388/© 2018 Published by Elsevier B.V.
such as poor electrical conductivity, short hole diffusion lengths and slow surface OER kinetics [8]. Therefore, many efforts have been devoted to overcome these limitations by morphology engineering [9], facet engineering [10] and surface modification. Generally, surface modification of a-Fe2O3 with oxygen evolution co-catalyst (OEC) (e.g. IrO2/RuO2 [11], Ni(OH)2 [12], NiOOH [13] and Co-Pi [14]) can improve its PEC activities by accelerating the occurrence of surface reaction. Recently, layered double hydroxide (LDH) materials that are permeable to electrolyte have been introduced on photoelectrode as promising OEC showing greatly enhanced photoelectrocatalytic OER performances, such as ZnO@CoNi-LDH [15], TiO2/ZnFe-LDH [16], WO3@NiFe-LDH [17], aFe2O3/ZnCo-LDH [18] and BiVO4/Fe based-LDH [19] photoelectrode. Thus, it may be an effective strategy to improve the PEC activity through decorating a-Fe2O3 photoanode with NiFe-LDH as a kind of OEC. Herein, we develop a facile electrodeposition synthesis method to fabricate NiFe-LDH modified a-Fe2O3 photoanodes with enhanced PEC OER performance. The a-Fe2O3/Ni0.5Fe0.5-LDH electrode shows 3-fold higher photocurrent densities at 1.23 V versus reversible hydrogen electrode (RHE) than bare a-Fe2O3, and a very
342
Y. Zhu et al. / Journal of Alloys and Compounds 764 (2018) 341e346
long-term durability. Mott-Schottky (MS) and Electrochemical impedance spectroscopy (EIS) measurements reveal that the greatly enhanced PEC performances of a-Fe2O3/NiFe-LDH come from the reduced charge transfer resistance and improved carrier density. 2. Material and methods 2.1. Synthesis of a-Fe2O3 photoanode The a-Fe2O3 crystals were synthesized by a hydrothermal method [20,21]. 1.5 mmol KF$2H2O and 1.5 mmol FeCl3$6H2O were dissolved in 60 mL of deionized water under stirring. A clear solution was obtained and transferred to a Teflon-lined stainless steel autoclave and reacted at 220 C for 24 h. After the hydrothermal reaction, the precipitation was filtered, washed with distilled water and dried at 60 C. Before the Ti foil was used as substrate to deposit a-Fe2O3 powder. It was immersed in concentrated HCl solution (37%) and etched for 30 min at 90 C. Then, a-Fe2O3 photoanodes were prepared by electrophoretic deposition on the pre-treated Ti foil (Fig. S1) [22]. Firstly, 50 mg iodine and 20 mg a-Fe2O3 powder were dispersed in 30 mL acetone by sonication. The Ti foil and Pt foil were immersed in the above solution, and 50 V of bias potential was applied between them for 40 s using a potentiostat (ITECH IT6720). Lastly, the a-Fe2O3 coated Ti foil substrate was washed by ethanol and calcined at 400 C for 3 h in air. 2.2. Synthesis of a-Fe2O3/NiFe-LDH nanostructure The a-Fe2O3/NiFe-LDH nanostructures were prepared via a fast electrodeposition method [19,23]. Typically, the a-Fe2O3 photoelectrode, saturated calomel electrode (SCE) and Pt foil were used as the working, reference and counter electrodes, respectively. Ni(NO3)2$6H2O (0.10 M) and FeSO4$7H2O (0.10 M) were dissolved in 30 mL of water which purged with N2 to make the electrolyte for the electrosynthesis of Ni0.5Fe0.5-LDH. The electrodeposition was carried out at 1.00 V vs. SCE for 50 s. To optimize the composition
of the Ni2þ and Fe2þ, the total moles of them in the electrolyte were maintained at 0.20 M. 2.3. Characterization X-ray diffraction (XRD) was carried out with DX2700 operating at 40 kV and 30 mA equipped with Cu Ka radiation (l ¼ 1.5418 Å). The morphology of the samples were characterized by using a JSM6390LV scanning electron microscope (SEM). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were obtained using a JEM-2100F electron microscope. The X-ray photoelectron spectroscopy (XPS) were determined on a RBD 147 upgraded PHI 5000C ESCA system equipped with an Mg Ka anode. 2.4. PEC measurement The PEC performance was evaluated using CHI760E Electrochemical Workstation in a three-electrode cell with 0.5 M Na2SO4 solution as the electrolyte. The simulated solar illumination (100 mW/cm2) was provided by a PLS-SXE 300C Xe arc lamp (Perfect Light) with an AM 1.5G filter (Fig. S2). I-t curves were measured at 1.23 V vs. RHE and linear sweep voltammetry (LSV) curves were recorded with a scan rate of 10 mV/s. EIS was carried out at the open circuit potential with the frequency range of 105 to 102 Hz and a 5 mV amplitude. The MS measurements were performed at a fixed frequency of 500 Hz with 10 mV amplitude. The measured potentials vs. Ag/AgCl were converted to the RHE with the Nernst equation (ERHE ¼ EAg/AgCl þ 0.059 pH þ 0.1976). 3. Results and discussion The a-Fe2O3/NiFe-LDH films were fabricated via a three-step process. Firstly, the polyhedral a-Fe2O3 crystals were synthesized by a hydrothermal process, with a particle size of 0.5e2 mm (Fig. 1a). Subsequently, the a-Fe2O3 powders were assembled on Ti foil by an electrophoretic deposition method. Lastly, a fast electrodeposition method was adopted in aqueous solution with Ni2þ and Fe2þ
Fig. 1. SEM images of (a) bare a-Fe2O3, (b) a-Fe2O3/Ni0.8Fe0.2-LDH, (c) a-Fe2O3/Ni0.5Fe0.5-LDH and (d) a-Fe2O3/Ni0.2Fe0.8-LDH film.
Y. Zhu et al. / Journal of Alloys and Compounds 764 (2018) 341e346
Fig. 2. XRD patterns of a-Fe2O3 and a-Fe2O3/NiFe-LDH film.
cations to deposit NiFe-LDH arrays on a-Fe2O3 crystals. Fig. S3 shows the photographs of a-Fe2O3 and a-Fe2O3/NiFe-LDH films, the sample colour changes from brownish to yellowish after electrodeposition process, indicating the successful electrodeposition of LDH on a-Fe2O3 surface. As shown in Fig. 1bed, the NiFe LDH nanoarrays with a thickness of ca. 20e30 nm can be deposited on the a-Fe2O3 crystals surface uniformly. From the X-ray diffraction (XRD) patterns in Fig. 2a, the sharp diffraction peaks at 24.2 , 33.1, 35.7, 40.9 , 49.5 , 54.0 , 62.4 and 64.0 were all indexed to pure a-Fe2O3 (JCPDS 33-0664), indicating that the a-Fe2O3 polyhedron particles with high crystallinity. And the diffraction peaks at 35.2 , 38.5 , 40.4 , 53.3 and 63.2 were indexed to the Ti foil substrate. However, the diffraction peaks of NiFe-LDH could not be observed owing to the poor crystallization and low amount of the LDH layer [19]. According to the X-ray
343
photoelectron spectroscopy (XPS) results, the existence of Ni and Fe in the a-Fe2O3/Ni0.5Fe0.5-LDH film can be further confirmed (Fig. 3a). From Fig. 3b, the O 1s spectra of the a-Fe2O3/Ni0.5Fe0.5LDH was fitted into two peaks at 530.2 eV and 531.4 eV, corresponding to lattice oxygen species of a-Fe2O3 and surface hydroxyl group in LDH, respectively. Moreover, the binding energy of 855.4 eV and 873.2 eV indicates the Ni 2p3/2 and Ni 2p1/2 peaks revealing that the oxidation state of Ni is Ni2þ (Fig. 3c) [24]. And the Fe 2p1/2 (724.6 eV) and Fe 2p3/2 (711.2 eV) reveals the Fe3þ oxidation state in a-Fe2O3 and LDH (Fig. 3d). As illustrated in the transmission electron microscopy (TEM) image (Fig. 4a), a-Fe2O3 polyhedrons have a length of approximately 1e2 mm. Fig. 4b shows the high-resolution TEM (HRTEM) image of the a-Fe2O3, the clear lattice fringes spacing of 0.27 nm and 0.36 nm corresponding to the (104) and (012) lattice planes of a-Fe2O3, respectively. And the corresponding selected area electron diffraction (SAED) image (inset of Fig. 4b) showing the sharp diffraction spots demonstrates the single crystalline nature of the a-Fe2O3 polyhedron. From Fig. 4c and d, the Ni0.5Fe0.5-LDH arrays with a height of ca. 100e200 nm anchored onto the a-Fe2O3 surface tightly. The HRTEM image of Ni0.5Fe0.5-LDH which taken on the edge of a-Fe2O3/Ni0.5Fe0.5-LDH composite (inset of Fig. 4d) indicates the poor crystallization nature of NiFe-LDH, being in consistent with the XRD results (Fig. 2). Fig. 5a shows the transient photocurrent measured under chopped simulated sunlight illumination (AM 1.5G, 100 mW/cm2). A prompt and reproducible photocurrent response with a steady state plateau was obtained in ten light on-off cycles. When the electrode is subjected to a sudden illumination by chopping the light, a spike of initial photocurrent appeared to represent the immediate separation of photogenerated electron-hole pairs. Immediately following a gradual decay of photocurrent with time until a steady state is achieved. This photocurrent decay is a result of charge recombination processes [25e27]. For pure a-Fe2O3
Fig. 3. (a) XPS survey and high resolution XPS spectra of (b) O 1s, (c) Ni 2p, and (d) Fe 2p in the a-Fe2O3/Ni0.5Fe0.5-LDH film.
344
Y. Zhu et al. / Journal of Alloys and Compounds 764 (2018) 341e346
Fig. 4. (a) TEM and (b) HRTEM images of a-Fe2O3. Inset is the corresponding SAED pattern. (c) TEM and (d) HRTEM images of a-Fe2O3/Ni0.5Fe0.5-LDH. Inset is the HRTEM image of LDH.
Fig. 5. (a) I-t curves of a-Fe2O3/NiFe-LDH measured under chopped AM 1.5G (100 mW/cm2) illumination. (b) LSV curves of a-Fe2O3/NiFe-LDH measured under AM 1.5G (100 mW/ cm2) illumination.
photoanode, the photocurrent density of 47 mA/cm2 at 1.23 V vs. RHE was obtained. An initial increase with a decreasing molar ratio of Ni/Fe followed by a decrease with a further decreasing ratio beyond 1. In particular, the a-Fe2O3/Ni0.5Fe0.5-LDH photoelectrode exhibits the highest photocurrent densities of 141 mA/cm2. As shown in Fig. 5b, in the measured potential range, all the a-Fe2O3/ NiFe-LDH photoelectrodes exhibit entire higher photocurrent in the whole test potential range than pristine a-Fe2O3. Which is in agreement with I-t curves (Fig. 5a), the photocurrent densities vary with the amount of Fe3þ in the NiFe-LDH. This reveals the photoinduced hole-electron pairs can separate effectively after creating the a-Fe2O3/NiFe-LDH heterostructure. Moreover, from Fig. 5b, the a-Fe2O3/NiFe-LDH photoanodes exhibit a negative shift of onset potential compared with pure a-Fe2O3, suggesting that the OER kinetics are boosted. More importantly, as shown in Fig. S4a, the photocurrent density shows no obvious decrease after continuous illumination for 25 h. From the SEM image and XRD pattern
(Figs. S4b and c), the a-Fe2O3/Ni0.5Fe0.5-LDH photoelectrode had no obvious changes after illumination, revealing the good durability of these photoanodes. Nyquist plots of the electrochemical impedance spectroscopy (EIS) were used to evaluate the charge transfer at the interface of the photoelectrode/electrolyte. As shown in Fig. 6a, the measured data are the dot with different symbols, and the solid lines are the fitted curves using the equivalent circuit provided in Fig. 6b. The Nyquist plots reveal the occurrence of a well-defined semicircle. This arc corresponds to the charge transfer process at the photoanode/electrolyte interface [28]. The values of electron transfer resistance (Rct,trap) obtained from the equivalent circuit are shown in Fig. 6c. As illustrated in Fig. 6c, the a-Fe2O3/Ni0.5Fe0.5-LDH photoanode shows the smallest Rct,trap (10.6 kU cm2) among these four samples, while the pristine a-Fe2O3 photoanode displays the largest Rct,trap (30.2 kU cm2). This reveals the fast charge transfer kinetics occurred after creating interface between LDH and a-
Y. Zhu et al. / Journal of Alloys and Compounds 764 (2018) 341e346
345
Fig. 6. (a) Nyquist plots of EIS measured under AM 1.5G (100 mW/cm2) illumination. (b) The equivalent circuit for fitting the EIS results. (c) Fitted parameters of the EIS plots.
NiFe-LDH nanoarrays were successfully synthesized by a simple electrodeposition process. A remarkable enhancement of the PEC properties is observed over the a-Fe2O3/NiFe-LDH interface heterostructure in comparison to pure a-Fe2O3 under AM 1.5G illumination. This improvement is attributed to the interface mediated decreased charge transfer resistance and increased carrier density, and improved OER kinetics by LDH. This work open a simple way for the loading more efficient OEC on the photoelectrodes for water PEC splitting. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21501105, 51473081, 51672143, and 21571080). Fig. 7. Mott-Schottky curves of a-Fe2O3 and a-Fe2O3/Ni0.5Fe0.5-LDH electrode measured under dark condition.
Fe2O3. From Fig. 7, the ND (carrier density) and Efb (flat band potential) of the photoanodes can be estimated by the Mott-Schottky (MS) equation: 1=C 2 ¼ ð2=qεε0 ND A2 Þ$ðE Efb kT=qÞ, where C is the capacitance of space charge region, q is the electron charge (1.602 1019 C), ε is the dielectric constant of hematite (~80), ε0 is the permittivity of vacuum (8.85 1012 F/m), ND is the carrier density, A is the active area, E is applied bias potential, Efb is the flatband potential, k is the Boltzmann constant (1.38 1023 J/K) and T is the absolute temperature [29e31]. The Efb could be obtained from the x-intercepts of the linear region, which were found at 0.54 V and 0.50 V (vs. RHE) for a-Fe2O3 and a-Fe2O3/Ni0.5Fe0.5LDH photoanode, respectively. The negative shift of the Efb of aFe2O3/Ni0.5Fe0.5-LDH electrode as compared with bare a-Fe2O3 electrode is in consistent with the onset potential results (Fig. 5b). The ND was calculated from the slope of the linear region of the photoanode to be 0.21 1019 m3 and 0.36 1019 m3 for the bare a-Fe2O3 and a-Fe2O3/Ni0.5Fe0.5-LDH photoanode, respectively. The above results demonstrate that the creation of a-Fe2O3/NiFe-LDH interface can promote the charge transport process effectively and allow more carrier to take part in the water oxidation, moreover, the LDH acts as OEC can facilitate the surface OER kinetics. 4. Conclusions In conclusion, a-Fe2O3 crystals coated with good uniformity
Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.jallcom.2018.06.064. References [1] C. Liu, N.P. Dasgupta, P.D. Yang, Semiconductor nanowires for artificial photosynthesis, Chem. Mater. 26 (2014) 415e422. [2] S.S. Chen, T. Takata, K. Domen, Particulate photocatalysts for overall water splitting, Nat. Rev. Mater. 2 (2017) 17050. €gl, P. Strasser, Electrocatalytic oxygen [3] T. Reier, H.N. Nong, D. Teschner, R. Schlo evolution reaction in acidic environments-reaction mechanisms and catalysts, Adv. Energy Mater 7 (2017), 1601275. mez, The [4] T. Berger, D. Monllor-Satoca, M. Jankulovska, T. Lana-Villarreal, R. Go electrochemistry of nanostructured titanium dioxide electrodes, ChemPhysChem 13 (2012) 2824e2875. [5] S.C. Wang, H.J. Chen, G.P. Gao, T. Butburee, M.Q. Lyu, S. Thaweesak, J.H. Yun, A.J. Du, G. Liu, L.Z. Wang, Synergistic crystal facet engineering and structural control of WO3 films exhibiting unprecedented photoelectrochemical performance, Nano Energy 24 (2016) 94e102. [6] J.S. Kang, Y. Noh, J. Kim, H. Choi, T.H. Jeon, D. Ahn, J.Y. Kim, S.H. Yu, H. Park, J.H. Yum, W. Choi, D.C. Dunand, H. Choe, Y.E. Sung, Iron oxide photoelectrode with multidimensional architecture for highly efficient photoelectrochemical water splitting, Angew. Chem. Int. Ed. 56 (2017) 6583e6588. [7] Z.F. Huang, L. Pan, J.J. Zou, X.W. Zhang, L. Wang, Nanostructured bismuth vanadate-based materials for solar-energy-driven water oxidation: a review on recent progress, Nanoscale 6 (2014) 14044e14063. [8] K. Sivula, F.L. Formal, M. Gr€ atzel, Solar water splitting: progress using hematite (a-Fe2O3) photoelectrodes, ChemSusChem 4 (2011) 432e449. [9] C.W. Wang, S. Yang, H.B. Jiang, H.G. Yang, Chemical vapor deposition of FeOCl nanosheet arrays and their conversion to porous a-Fe2O3 photoanodes for photoelectrochemical water splitting, Chem. Eur J. 21 (2015) 18024e18028. [10] B.L. Lv, Z.Y. Liu, H. Tian, Y. Xu, D. Wu, Y.H. Sun, Single-crystalline dodecahedral and octodecahedral a-Fe2O3 particles synthesized by a fluoride anion-assisted hydrothermal method, Adv. Funct. Mater. 20 (2010) 3987e3996. [11] P. Dias, L. Andrade, A. Mendes, Hematite-based photoelectrode for solar water splitting with very high photovoltage, Nano Energy 38 (2017) 218e231.
346
Y. Zhu et al. / Journal of Alloys and Compounds 764 (2018) 341e346
[12] Q. Liu, F.R. Cao, F.L. Wu, H. Lu, L. Li, Ultrathin amorphous Ni(OH)2 nanosheets on ultrathin a-Fe2O3 films for improved photoelectrochemical water oxidation, Adv. Mater. Interfaces (2016), 1600256. [13] F. Malara, A. Minguzzi, M. Marelli, S. Morandi, R. Psaro, V. Dal Santo, A. Naldoni, a-Fe2O3/NiOOH: an effective heterostructure for photoelectrochemical water oxidation, ACS Catal. 5 (2015) 5292e5300. [14] L. Fu, H.M. Yu, C.K. Zhang, Z.G. Shao, B.L. Yi, Cobalt phosphate group modified hematite nanorod array as photoanode for efficient solar water splitting, Electrochim. Acta 136 (2014) 363e369. [15] M.F. Shao, F.Y. Ning, M. Wei, D.G. Evans, X. Duan, Hierarchical nanowire arrays based on ZnO core-layered double hydroxide shell for largely enhanced photoelectrochemical water splitting, Adv. Funct. Mater. 24 (2014) 580e586. [16] R.K. Zhang, M.F. Shao, S.M. Xu, F.Y. Ning, L. Zhou, M. Wei, Photo-assisted synthesis of zinc-iron layered double hydroxides/TiO2 nanoarrays toward highly-efficient photoelectrochemical water splitting, Nano Energy 33 (2017) 21e28. [17] X.L. Fan, B. Gao, T. Wang, X.L. Huang, H. Gong, H.R. Xue, H. Guo, L. Song, W. Xia, J.P. He, Layered double hydroxide modified WO3 nanorod arrays for enhanced photoelectrochemical water splitting, Appl. Catal. Gen. 528 (2016) 52e58. [18] D.Y. Xu, Y.C. Rui, Y.G. Li, Q.H. Zhang, H.Z. Wang, Zn-Co layered double hydroxide modified hematite photoanode for enhanced photoelectrochemical water splitting, Appl. Surf. Sci. 358 (2015) 436e442. [19] Y.K. Zhu, J. Ren, X.F. Yang, G.J. Chang, Y.U. Bu, G.D. Wei, W. Han, D.J. Yang, Interface engineering of 3D BiVO4/Fe-based layered double hydroxide core/ shell nanostructures for boosting photoelectrochemical water oxidation, J. Mater. Chem. A 5 (2017) 9952e9959. [20] W. Jiao, J.R. Wu, S.W. Cui, N. Wei, Z.U. Rahman, M.Y. Yu, S.G. Chen, Y.T. Zhou, D.A. Wang, Hollow hematite single crystals deposited with ultra-thin Al2O3 by atom layer deposition for improved photoelectrochemical performance, Dalton Trans. 46 (2017) 10635e10640. [21] B. Xu, B.B. Huang, H.F. Cheng, Z.Y. Wang, X.Y. Qin, X.Y. Zhang, Y. Dai, a-Fe2O3 hollow structures: formation of single crystalline thin shells, Chem. Commun.
48 (2012) 6529e6531. [22] K. Maeda, M. Higashi, B. Siritanaratkul, R. Abe, K. Domen, SrNbO2N as a watersplitting photoanode with a wide visible-light absorption band, J. Am. Chem. Soc. 133 (2011) 12334e12337. [23] Z.H. Li, M.F. Shao, H.L. An, Z.X. Wang, S.M. Xu, M. Wei, D.G. Evans, X. Duan, Fast electrosynthesis of Fe-containing layered double hydroxide arrays toward highly efficient electrocatalytic oxidation reactions, Chem. Sci. 6 (2015) 6624e6631. [24] X.Y. Lu, C. Zhao, Electrodeposition of hierarchically structured threedimensional nickel-iron electrodes for efficient oxygen evolution at high current densities, Nat. Commun. 6 (2015) 6616. [25] Y.H. Ng, A. Iwase, A. Kudo, R. Amal, Reducing graphene oxide on a visible-light BiVO4 photocatalyst for an enhanced photoelectrochemical water splitting, J. Phys. Chem. Lett. 1 (2010) 2607e2612. [26] A. Kargar, Y. Jing, S.J. Kim, C.T. Riley, X.Q. Pan, D.L. Wang, ZnO/CuO heterojunction branched nanowires for photoelectrochemical hydrogen generation, ACS Nano 7 (2013) 11112e11120. [27] C.Y. Cummings, F. Marken, L.M. Peter, A.A. Tahir, K.G.U. Wijayantha, Kinetics and mechanism of light-driven oxygen evolution at thin film a-Fe2O3 electrodes, Chem. Commun. 48 (2012) 2027e2029. [28] G. Liu, Y. Zhao, K.F. Wang, D.Y. He, R. Yao, J.P. Li, Ultrasmall NiFe-phosphate nanoparticles incorporated a-Fe2O3 nanoarrays photoanode realizing high efficient solar water splitting, ACS Sustain. Chem. Eng. 6 (2018) 2353e2361. [29] C.C. Li, A. Li, Z.B. Luo, J.J. Zhang, X.X. Chang, Z.Q. Huang, T. Wang, J.L. Gong, Surviving high-temperature calcination: ZrO2-induced hematite nanotubes for photoelectrochemical water oxidation, Angew. Chem. 129 (2017) 1e7. [30] M. Ji, J.G. Cai, Y.R. Ma, L.M. Qi, Controlled growth of ferrihydrite branched nanosheet arrays and their transformation to hematite nanosheet arrays for photoelectrochemical water splitting, ACS Appl. Mater. Interfaces 8 (2016) 3651e3660. [31] K. Gelderman, L. Lee, S.W. Donne, Flat-band potential of a semiconductor: using the Mott-Schottky equation, J. Chem. Educ. 84 (2007) 685e688.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Surface modification of hematite photoanode by NiFe layered double hydroxide for boosting photoelectrocatalytic water oxidation Yukun Zhu a, b, Xiaoliang Zhao b, **, Junzhi Li a, Huawei Zhang c, Shuai Chen d, Wei Han a, e, *, Dongjiang Yang b a
Key Laboratory of Physics and Technology for Advanced Batteries (Ministry of Education), College of Physics, Jilin University, Changchun 130012, China Collaborative Innovation Center for Marine Biomass Fibers Materials and Textiles of Shandong Province, School of Environmental Science and Engineering, Qingdao University, Qingdao 266071, China c College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, China d State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Science, Taiyuan 030001, China e International Center of Future Science, Jilin University, Changchun 130012, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 April 2018 Received in revised form 5 June 2018 Accepted 6 June 2018
The photoelectrocatalytic (PEC) oxygen evolution reaction (OER) is one of the most efficient ways for utilizing solar energy for water electrolysis. Nevertheless, up to now, the PEC conversion efficiency of the established photoanode is still low. In this work, a-Fe2O3/NiFe layered double hydroxide (LDH) photoanodes were synthesized by electrodeposition of LDH on a-Fe2O3. Compared with bare a-Fe2O3, the aFe2O3/Ni0.5Fe0.5-LDH photoanode displays about 3 times photocurrent enhancement and excellent longterm stability. The enhanced PEC activity of a-Fe2O3/NiFe-LDH is ascribed to the interface between aFe2O3 and NiFe-LDH which can facilitate charge transfer and improve carrier density. Simultaneously, NiFe-LDH as a co-catalyst can accelerate the surface OER kinetics. © 2018 Published by Elsevier B.V.
Keywords: Hematite Layered double hydroxide Electrodeposition Photoelectrocatalytic Oxygen evolution reaction
1. Introduction Converting and utilizing solar energy into the useable energy through photoelectrocatalytic (PEC) water splitting is one of the most challenges nowadays [1,2]. The crucial step in PEC water splitting is oxygen evolution reaction (OER) occurred at the photoanode, but it is restricted by a multistep four electrons transfer that requiring large overpotentials [3]. For decades, great efforts have been focused on the transition metal oxides as promising photoanode materials, such as TiO2 [4], WO3 [5], a-Fe2O3 [6], and BiVO4 [7]. Among them, hematite (a-Fe2O3) was extensively studied and has shown interesting OER performance due to the favourable optical bandgap and appropriate valence band position. However, several inherent drawbacks impede their PEC efficiency,
* Corresponding author. Key Laboratory of Physics and Technology for Advanced Batteries (Ministry of Education), College of Physics, Jilin University, Changchun 130012, China. ** Corresponding author. School of Environmental Science and Engineering, Qingdao University, Qingdao 266071, China. E-mail addresses: [email protected] (X. Zhao), [email protected] (W. Han). https://doi.org/10.1016/j.jallcom.2018.06.064 0925-8388/© 2018 Published by Elsevier B.V.
such as poor electrical conductivity, short hole diffusion lengths and slow surface OER kinetics [8]. Therefore, many efforts have been devoted to overcome these limitations by morphology engineering [9], facet engineering [10] and surface modification. Generally, surface modification of a-Fe2O3 with oxygen evolution co-catalyst (OEC) (e.g. IrO2/RuO2 [11], Ni(OH)2 [12], NiOOH [13] and Co-Pi [14]) can improve its PEC activities by accelerating the occurrence of surface reaction. Recently, layered double hydroxide (LDH) materials that are permeable to electrolyte have been introduced on photoelectrode as promising OEC showing greatly enhanced photoelectrocatalytic OER performances, such as ZnO@CoNi-LDH [15], TiO2/ZnFe-LDH [16], WO3@NiFe-LDH [17], aFe2O3/ZnCo-LDH [18] and BiVO4/Fe based-LDH [19] photoelectrode. Thus, it may be an effective strategy to improve the PEC activity through decorating a-Fe2O3 photoanode with NiFe-LDH as a kind of OEC. Herein, we develop a facile electrodeposition synthesis method to fabricate NiFe-LDH modified a-Fe2O3 photoanodes with enhanced PEC OER performance. The a-Fe2O3/Ni0.5Fe0.5-LDH electrode shows 3-fold higher photocurrent densities at 1.23 V versus reversible hydrogen electrode (RHE) than bare a-Fe2O3, and a very
342
Y. Zhu et al. / Journal of Alloys and Compounds 764 (2018) 341e346
long-term durability. Mott-Schottky (MS) and Electrochemical impedance spectroscopy (EIS) measurements reveal that the greatly enhanced PEC performances of a-Fe2O3/NiFe-LDH come from the reduced charge transfer resistance and improved carrier density. 2. Material and methods 2.1. Synthesis of a-Fe2O3 photoanode The a-Fe2O3 crystals were synthesized by a hydrothermal method [20,21]. 1.5 mmol KF$2H2O and 1.5 mmol FeCl3$6H2O were dissolved in 60 mL of deionized water under stirring. A clear solution was obtained and transferred to a Teflon-lined stainless steel autoclave and reacted at 220 C for 24 h. After the hydrothermal reaction, the precipitation was filtered, washed with distilled water and dried at 60 C. Before the Ti foil was used as substrate to deposit a-Fe2O3 powder. It was immersed in concentrated HCl solution (37%) and etched for 30 min at 90 C. Then, a-Fe2O3 photoanodes were prepared by electrophoretic deposition on the pre-treated Ti foil (Fig. S1) [22]. Firstly, 50 mg iodine and 20 mg a-Fe2O3 powder were dispersed in 30 mL acetone by sonication. The Ti foil and Pt foil were immersed in the above solution, and 50 V of bias potential was applied between them for 40 s using a potentiostat (ITECH IT6720). Lastly, the a-Fe2O3 coated Ti foil substrate was washed by ethanol and calcined at 400 C for 3 h in air. 2.2. Synthesis of a-Fe2O3/NiFe-LDH nanostructure The a-Fe2O3/NiFe-LDH nanostructures were prepared via a fast electrodeposition method [19,23]. Typically, the a-Fe2O3 photoelectrode, saturated calomel electrode (SCE) and Pt foil were used as the working, reference and counter electrodes, respectively. Ni(NO3)2$6H2O (0.10 M) and FeSO4$7H2O (0.10 M) were dissolved in 30 mL of water which purged with N2 to make the electrolyte for the electrosynthesis of Ni0.5Fe0.5-LDH. The electrodeposition was carried out at 1.00 V vs. SCE for 50 s. To optimize the composition
of the Ni2þ and Fe2þ, the total moles of them in the electrolyte were maintained at 0.20 M. 2.3. Characterization X-ray diffraction (XRD) was carried out with DX2700 operating at 40 kV and 30 mA equipped with Cu Ka radiation (l ¼ 1.5418 Å). The morphology of the samples were characterized by using a JSM6390LV scanning electron microscope (SEM). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were obtained using a JEM-2100F electron microscope. The X-ray photoelectron spectroscopy (XPS) were determined on a RBD 147 upgraded PHI 5000C ESCA system equipped with an Mg Ka anode. 2.4. PEC measurement The PEC performance was evaluated using CHI760E Electrochemical Workstation in a three-electrode cell with 0.5 M Na2SO4 solution as the electrolyte. The simulated solar illumination (100 mW/cm2) was provided by a PLS-SXE 300C Xe arc lamp (Perfect Light) with an AM 1.5G filter (Fig. S2). I-t curves were measured at 1.23 V vs. RHE and linear sweep voltammetry (LSV) curves were recorded with a scan rate of 10 mV/s. EIS was carried out at the open circuit potential with the frequency range of 105 to 102 Hz and a 5 mV amplitude. The MS measurements were performed at a fixed frequency of 500 Hz with 10 mV amplitude. The measured potentials vs. Ag/AgCl were converted to the RHE with the Nernst equation (ERHE ¼ EAg/AgCl þ 0.059 pH þ 0.1976). 3. Results and discussion The a-Fe2O3/NiFe-LDH films were fabricated via a three-step process. Firstly, the polyhedral a-Fe2O3 crystals were synthesized by a hydrothermal process, with a particle size of 0.5e2 mm (Fig. 1a). Subsequently, the a-Fe2O3 powders were assembled on Ti foil by an electrophoretic deposition method. Lastly, a fast electrodeposition method was adopted in aqueous solution with Ni2þ and Fe2þ
Fig. 1. SEM images of (a) bare a-Fe2O3, (b) a-Fe2O3/Ni0.8Fe0.2-LDH, (c) a-Fe2O3/Ni0.5Fe0.5-LDH and (d) a-Fe2O3/Ni0.2Fe0.8-LDH film.
Y. Zhu et al. / Journal of Alloys and Compounds 764 (2018) 341e346
Fig. 2. XRD patterns of a-Fe2O3 and a-Fe2O3/NiFe-LDH film.
cations to deposit NiFe-LDH arrays on a-Fe2O3 crystals. Fig. S3 shows the photographs of a-Fe2O3 and a-Fe2O3/NiFe-LDH films, the sample colour changes from brownish to yellowish after electrodeposition process, indicating the successful electrodeposition of LDH on a-Fe2O3 surface. As shown in Fig. 1bed, the NiFe LDH nanoarrays with a thickness of ca. 20e30 nm can be deposited on the a-Fe2O3 crystals surface uniformly. From the X-ray diffraction (XRD) patterns in Fig. 2a, the sharp diffraction peaks at 24.2 , 33.1, 35.7, 40.9 , 49.5 , 54.0 , 62.4 and 64.0 were all indexed to pure a-Fe2O3 (JCPDS 33-0664), indicating that the a-Fe2O3 polyhedron particles with high crystallinity. And the diffraction peaks at 35.2 , 38.5 , 40.4 , 53.3 and 63.2 were indexed to the Ti foil substrate. However, the diffraction peaks of NiFe-LDH could not be observed owing to the poor crystallization and low amount of the LDH layer [19]. According to the X-ray
343
photoelectron spectroscopy (XPS) results, the existence of Ni and Fe in the a-Fe2O3/Ni0.5Fe0.5-LDH film can be further confirmed (Fig. 3a). From Fig. 3b, the O 1s spectra of the a-Fe2O3/Ni0.5Fe0.5LDH was fitted into two peaks at 530.2 eV and 531.4 eV, corresponding to lattice oxygen species of a-Fe2O3 and surface hydroxyl group in LDH, respectively. Moreover, the binding energy of 855.4 eV and 873.2 eV indicates the Ni 2p3/2 and Ni 2p1/2 peaks revealing that the oxidation state of Ni is Ni2þ (Fig. 3c) [24]. And the Fe 2p1/2 (724.6 eV) and Fe 2p3/2 (711.2 eV) reveals the Fe3þ oxidation state in a-Fe2O3 and LDH (Fig. 3d). As illustrated in the transmission electron microscopy (TEM) image (Fig. 4a), a-Fe2O3 polyhedrons have a length of approximately 1e2 mm. Fig. 4b shows the high-resolution TEM (HRTEM) image of the a-Fe2O3, the clear lattice fringes spacing of 0.27 nm and 0.36 nm corresponding to the (104) and (012) lattice planes of a-Fe2O3, respectively. And the corresponding selected area electron diffraction (SAED) image (inset of Fig. 4b) showing the sharp diffraction spots demonstrates the single crystalline nature of the a-Fe2O3 polyhedron. From Fig. 4c and d, the Ni0.5Fe0.5-LDH arrays with a height of ca. 100e200 nm anchored onto the a-Fe2O3 surface tightly. The HRTEM image of Ni0.5Fe0.5-LDH which taken on the edge of a-Fe2O3/Ni0.5Fe0.5-LDH composite (inset of Fig. 4d) indicates the poor crystallization nature of NiFe-LDH, being in consistent with the XRD results (Fig. 2). Fig. 5a shows the transient photocurrent measured under chopped simulated sunlight illumination (AM 1.5G, 100 mW/cm2). A prompt and reproducible photocurrent response with a steady state plateau was obtained in ten light on-off cycles. When the electrode is subjected to a sudden illumination by chopping the light, a spike of initial photocurrent appeared to represent the immediate separation of photogenerated electron-hole pairs. Immediately following a gradual decay of photocurrent with time until a steady state is achieved. This photocurrent decay is a result of charge recombination processes [25e27]. For pure a-Fe2O3
Fig. 3. (a) XPS survey and high resolution XPS spectra of (b) O 1s, (c) Ni 2p, and (d) Fe 2p in the a-Fe2O3/Ni0.5Fe0.5-LDH film.
344
Y. Zhu et al. / Journal of Alloys and Compounds 764 (2018) 341e346
Fig. 4. (a) TEM and (b) HRTEM images of a-Fe2O3. Inset is the corresponding SAED pattern. (c) TEM and (d) HRTEM images of a-Fe2O3/Ni0.5Fe0.5-LDH. Inset is the HRTEM image of LDH.
Fig. 5. (a) I-t curves of a-Fe2O3/NiFe-LDH measured under chopped AM 1.5G (100 mW/cm2) illumination. (b) LSV curves of a-Fe2O3/NiFe-LDH measured under AM 1.5G (100 mW/ cm2) illumination.
photoanode, the photocurrent density of 47 mA/cm2 at 1.23 V vs. RHE was obtained. An initial increase with a decreasing molar ratio of Ni/Fe followed by a decrease with a further decreasing ratio beyond 1. In particular, the a-Fe2O3/Ni0.5Fe0.5-LDH photoelectrode exhibits the highest photocurrent densities of 141 mA/cm2. As shown in Fig. 5b, in the measured potential range, all the a-Fe2O3/ NiFe-LDH photoelectrodes exhibit entire higher photocurrent in the whole test potential range than pristine a-Fe2O3. Which is in agreement with I-t curves (Fig. 5a), the photocurrent densities vary with the amount of Fe3þ in the NiFe-LDH. This reveals the photoinduced hole-electron pairs can separate effectively after creating the a-Fe2O3/NiFe-LDH heterostructure. Moreover, from Fig. 5b, the a-Fe2O3/NiFe-LDH photoanodes exhibit a negative shift of onset potential compared with pure a-Fe2O3, suggesting that the OER kinetics are boosted. More importantly, as shown in Fig. S4a, the photocurrent density shows no obvious decrease after continuous illumination for 25 h. From the SEM image and XRD pattern
(Figs. S4b and c), the a-Fe2O3/Ni0.5Fe0.5-LDH photoelectrode had no obvious changes after illumination, revealing the good durability of these photoanodes. Nyquist plots of the electrochemical impedance spectroscopy (EIS) were used to evaluate the charge transfer at the interface of the photoelectrode/electrolyte. As shown in Fig. 6a, the measured data are the dot with different symbols, and the solid lines are the fitted curves using the equivalent circuit provided in Fig. 6b. The Nyquist plots reveal the occurrence of a well-defined semicircle. This arc corresponds to the charge transfer process at the photoanode/electrolyte interface [28]. The values of electron transfer resistance (Rct,trap) obtained from the equivalent circuit are shown in Fig. 6c. As illustrated in Fig. 6c, the a-Fe2O3/Ni0.5Fe0.5-LDH photoanode shows the smallest Rct,trap (10.6 kU cm2) among these four samples, while the pristine a-Fe2O3 photoanode displays the largest Rct,trap (30.2 kU cm2). This reveals the fast charge transfer kinetics occurred after creating interface between LDH and a-
Y. Zhu et al. / Journal of Alloys and Compounds 764 (2018) 341e346
345
Fig. 6. (a) Nyquist plots of EIS measured under AM 1.5G (100 mW/cm2) illumination. (b) The equivalent circuit for fitting the EIS results. (c) Fitted parameters of the EIS plots.
NiFe-LDH nanoarrays were successfully synthesized by a simple electrodeposition process. A remarkable enhancement of the PEC properties is observed over the a-Fe2O3/NiFe-LDH interface heterostructure in comparison to pure a-Fe2O3 under AM 1.5G illumination. This improvement is attributed to the interface mediated decreased charge transfer resistance and increased carrier density, and improved OER kinetics by LDH. This work open a simple way for the loading more efficient OEC on the photoelectrodes for water PEC splitting. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21501105, 51473081, 51672143, and 21571080). Fig. 7. Mott-Schottky curves of a-Fe2O3 and a-Fe2O3/Ni0.5Fe0.5-LDH electrode measured under dark condition.
Fe2O3. From Fig. 7, the ND (carrier density) and Efb (flat band potential) of the photoanodes can be estimated by the Mott-Schottky (MS) equation: 1=C 2 ¼ ð2=qεε0 ND A2 Þ$ðE Efb kT=qÞ, where C is the capacitance of space charge region, q is the electron charge (1.602 1019 C), ε is the dielectric constant of hematite (~80), ε0 is the permittivity of vacuum (8.85 1012 F/m), ND is the carrier density, A is the active area, E is applied bias potential, Efb is the flatband potential, k is the Boltzmann constant (1.38 1023 J/K) and T is the absolute temperature [29e31]. The Efb could be obtained from the x-intercepts of the linear region, which were found at 0.54 V and 0.50 V (vs. RHE) for a-Fe2O3 and a-Fe2O3/Ni0.5Fe0.5LDH photoanode, respectively. The negative shift of the Efb of aFe2O3/Ni0.5Fe0.5-LDH electrode as compared with bare a-Fe2O3 electrode is in consistent with the onset potential results (Fig. 5b). The ND was calculated from the slope of the linear region of the photoanode to be 0.21 1019 m3 and 0.36 1019 m3 for the bare a-Fe2O3 and a-Fe2O3/Ni0.5Fe0.5-LDH photoanode, respectively. The above results demonstrate that the creation of a-Fe2O3/NiFe-LDH interface can promote the charge transport process effectively and allow more carrier to take part in the water oxidation, moreover, the LDH acts as OEC can facilitate the surface OER kinetics. 4. Conclusions In conclusion, a-Fe2O3 crystals coated with good uniformity
Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.jallcom.2018.06.064. References [1] C. Liu, N.P. Dasgupta, P.D. Yang, Semiconductor nanowires for artificial photosynthesis, Chem. Mater. 26 (2014) 415e422. [2] S.S. Chen, T. Takata, K. Domen, Particulate photocatalysts for overall water splitting, Nat. Rev. Mater. 2 (2017) 17050. €gl, P. Strasser, Electrocatalytic oxygen [3] T. Reier, H.N. Nong, D. Teschner, R. Schlo evolution reaction in acidic environments-reaction mechanisms and catalysts, Adv. Energy Mater 7 (2017), 1601275. mez, The [4] T. Berger, D. Monllor-Satoca, M. Jankulovska, T. Lana-Villarreal, R. Go electrochemistry of nanostructured titanium dioxide electrodes, ChemPhysChem 13 (2012) 2824e2875. [5] S.C. Wang, H.J. Chen, G.P. Gao, T. Butburee, M.Q. Lyu, S. Thaweesak, J.H. Yun, A.J. Du, G. Liu, L.Z. Wang, Synergistic crystal facet engineering and structural control of WO3 films exhibiting unprecedented photoelectrochemical performance, Nano Energy 24 (2016) 94e102. [6] J.S. Kang, Y. Noh, J. Kim, H. Choi, T.H. Jeon, D. Ahn, J.Y. Kim, S.H. Yu, H. Park, J.H. Yum, W. Choi, D.C. Dunand, H. Choe, Y.E. Sung, Iron oxide photoelectrode with multidimensional architecture for highly efficient photoelectrochemical water splitting, Angew. Chem. Int. Ed. 56 (2017) 6583e6588. [7] Z.F. Huang, L. Pan, J.J. Zou, X.W. Zhang, L. Wang, Nanostructured bismuth vanadate-based materials for solar-energy-driven water oxidation: a review on recent progress, Nanoscale 6 (2014) 14044e14063. [8] K. Sivula, F.L. Formal, M. Gr€ atzel, Solar water splitting: progress using hematite (a-Fe2O3) photoelectrodes, ChemSusChem 4 (2011) 432e449. [9] C.W. Wang, S. Yang, H.B. Jiang, H.G. Yang, Chemical vapor deposition of FeOCl nanosheet arrays and their conversion to porous a-Fe2O3 photoanodes for photoelectrochemical water splitting, Chem. Eur J. 21 (2015) 18024e18028. [10] B.L. Lv, Z.Y. Liu, H. Tian, Y. Xu, D. Wu, Y.H. Sun, Single-crystalline dodecahedral and octodecahedral a-Fe2O3 particles synthesized by a fluoride anion-assisted hydrothermal method, Adv. Funct. Mater. 20 (2010) 3987e3996. [11] P. Dias, L. Andrade, A. Mendes, Hematite-based photoelectrode for solar water splitting with very high photovoltage, Nano Energy 38 (2017) 218e231.
346
Y. Zhu et al. / Journal of Alloys and Compounds 764 (2018) 341e346
[12] Q. Liu, F.R. Cao, F.L. Wu, H. Lu, L. Li, Ultrathin amorphous Ni(OH)2 nanosheets on ultrathin a-Fe2O3 films for improved photoelectrochemical water oxidation, Adv. Mater. Interfaces (2016), 1600256. [13] F. Malara, A. Minguzzi, M. Marelli, S. Morandi, R. Psaro, V. Dal Santo, A. Naldoni, a-Fe2O3/NiOOH: an effective heterostructure for photoelectrochemical water oxidation, ACS Catal. 5 (2015) 5292e5300. [14] L. Fu, H.M. Yu, C.K. Zhang, Z.G. Shao, B.L. Yi, Cobalt phosphate group modified hematite nanorod array as photoanode for efficient solar water splitting, Electrochim. Acta 136 (2014) 363e369. [15] M.F. Shao, F.Y. Ning, M. Wei, D.G. Evans, X. Duan, Hierarchical nanowire arrays based on ZnO core-layered double hydroxide shell for largely enhanced photoelectrochemical water splitting, Adv. Funct. Mater. 24 (2014) 580e586. [16] R.K. Zhang, M.F. Shao, S.M. Xu, F.Y. Ning, L. Zhou, M. Wei, Photo-assisted synthesis of zinc-iron layered double hydroxides/TiO2 nanoarrays toward highly-efficient photoelectrochemical water splitting, Nano Energy 33 (2017) 21e28. [17] X.L. Fan, B. Gao, T. Wang, X.L. Huang, H. Gong, H.R. Xue, H. Guo, L. Song, W. Xia, J.P. He, Layered double hydroxide modified WO3 nanorod arrays for enhanced photoelectrochemical water splitting, Appl. Catal. Gen. 528 (2016) 52e58. [18] D.Y. Xu, Y.C. Rui, Y.G. Li, Q.H. Zhang, H.Z. Wang, Zn-Co layered double hydroxide modified hematite photoanode for enhanced photoelectrochemical water splitting, Appl. Surf. Sci. 358 (2015) 436e442. [19] Y.K. Zhu, J. Ren, X.F. Yang, G.J. Chang, Y.U. Bu, G.D. Wei, W. Han, D.J. Yang, Interface engineering of 3D BiVO4/Fe-based layered double hydroxide core/ shell nanostructures for boosting photoelectrochemical water oxidation, J. Mater. Chem. A 5 (2017) 9952e9959. [20] W. Jiao, J.R. Wu, S.W. Cui, N. Wei, Z.U. Rahman, M.Y. Yu, S.G. Chen, Y.T. Zhou, D.A. Wang, Hollow hematite single crystals deposited with ultra-thin Al2O3 by atom layer deposition for improved photoelectrochemical performance, Dalton Trans. 46 (2017) 10635e10640. [21] B. Xu, B.B. Huang, H.F. Cheng, Z.Y. Wang, X.Y. Qin, X.Y. Zhang, Y. Dai, a-Fe2O3 hollow structures: formation of single crystalline thin shells, Chem. Commun.
48 (2012) 6529e6531. [22] K. Maeda, M. Higashi, B. Siritanaratkul, R. Abe, K. Domen, SrNbO2N as a watersplitting photoanode with a wide visible-light absorption band, J. Am. Chem. Soc. 133 (2011) 12334e12337. [23] Z.H. Li, M.F. Shao, H.L. An, Z.X. Wang, S.M. Xu, M. Wei, D.G. Evans, X. Duan, Fast electrosynthesis of Fe-containing layered double hydroxide arrays toward highly efficient electrocatalytic oxidation reactions, Chem. Sci. 6 (2015) 6624e6631. [24] X.Y. Lu, C. Zhao, Electrodeposition of hierarchically structured threedimensional nickel-iron electrodes for efficient oxygen evolution at high current densities, Nat. Commun. 6 (2015) 6616. [25] Y.H. Ng, A. Iwase, A. Kudo, R. Amal, Reducing graphene oxide on a visible-light BiVO4 photocatalyst for an enhanced photoelectrochemical water splitting, J. Phys. Chem. Lett. 1 (2010) 2607e2612. [26] A. Kargar, Y. Jing, S.J. Kim, C.T. Riley, X.Q. Pan, D.L. Wang, ZnO/CuO heterojunction branched nanowires for photoelectrochemical hydrogen generation, ACS Nano 7 (2013) 11112e11120. [27] C.Y. Cummings, F. Marken, L.M. Peter, A.A. Tahir, K.G.U. Wijayantha, Kinetics and mechanism of light-driven oxygen evolution at thin film a-Fe2O3 electrodes, Chem. Commun. 48 (2012) 2027e2029. [28] G. Liu, Y. Zhao, K.F. Wang, D.Y. He, R. Yao, J.P. Li, Ultrasmall NiFe-phosphate nanoparticles incorporated a-Fe2O3 nanoarrays photoanode realizing high efficient solar water splitting, ACS Sustain. Chem. Eng. 6 (2018) 2353e2361. [29] C.C. Li, A. Li, Z.B. Luo, J.J. Zhang, X.X. Chang, Z.Q. Huang, T. Wang, J.L. Gong, Surviving high-temperature calcination: ZrO2-induced hematite nanotubes for photoelectrochemical water oxidation, Angew. Chem. 129 (2017) 1e7. [30] M. Ji, J.G. Cai, Y.R. Ma, L.M. Qi, Controlled growth of ferrihydrite branched nanosheet arrays and their transformation to hematite nanosheet arrays for photoelectrochemical water splitting, ACS Appl. Mater. Interfaces 8 (2016) 3651e3660. [31] K. Gelderman, L. Lee, S.W. Donne, Flat-band potential of a semiconductor: using the Mott-Schottky equation, J. Chem. Educ. 84 (2007) 685e688.