Synthesis and photoelectrochemical water oxidation of (Y, Cu) codoped α-Fe2O3 nanostructure photoanode

Journal of Alloys and Compounds 814 (2020) 152349

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Synthesis and photoelectrochemical water oxidation of (Y, Cu) codoped a-Fe2O3 nanostructure photoanode Ch Venkata Reddy a, 1, I. Neelakanta Reddy a, 1, Bhargav Akkinepally a, Kakarla Raghava Reddy b, Jaesool Shim a, * a b

School of Mechanical Engineering, Yeungnam University, Gyeongsan, 712749, South Korea School of Chemical and Biomolecular Engineering, The University of Sydney, NSW, 2006, Australia

a r t i c l e i n f o

a b s t r a c t

Article history: Received 30 July 2019 Received in revised form 16 September 2019 Accepted 18 September 2019 Available online 19 September 2019

Hematite (a-Fe2O3) is one of the best capable photoanode material for water oxidation under visible light. The role of synergetic effect between two metal dopants for enhanced water oxidation is not adequately studied. The mono-doping (Y) and co-doping (Cu, Y) effect on the crystal structure, morphology, optical properties and their influence as photoanodes for energy harvesting applications are systematically explored. In this study, we have synthesized the pure, mono-doped (Y), and co-doped (Y and Cu) hematite nanostructures for enhanced photoelectrochemical (PEC) performance using a simple template free hydrothermal synthesis technique. The optimized photoelectrode showed a substantial improvement (~36-times) in the PEC photocurrent density over pristine, mono-doped and co-doped photoanodes. Electrochemical impedance spectroscopy analysis confirmed that the co-dopant enhanced the charge carrier density of hematite and it acts as an electron donor. Moreover, it is demonstrated that the photocurrent density increases after mono-doping from 0.012 mAcm
2 to 0.020 mAcm
2 and further improved to 0.439 mAcm
2 with co-dopant at 1.23 V vs. RHE. The considerably enhanced PEC activity is ascribed to the higher conductivity, enhanced interfacial charge transfer at the surface of hematite and the synergistic effect between two metal dopants. © 2019 Elsevier B.V. All rights reserved.

Keywords: Hematite Co-doping Photoelectrochemistry Water oxidation Photocurrent

1. Introduction The growth and use of renewable energy assets have been involved profusely of research considerations nowadays owing to the extremely usage of petroleum and coal [1]. The usage of solar energy is a perfect renewable energy source due to it is vast, unpolluted and eco-friendly. The splitting of water into hydrogen energy using a solar energy gives a possible solution for large scale and long run applications [2e4]. Among the present available artificial photosynthesis responses, photoelectrochemical (PEC) water splitting signifies one of the greatest auspicious and effective procedures to change solar light into hydrogen energy [5,6]. Among the encouraging photoanode materials, hematite (aFe2O3) has involved much consideration for PEC water splitting owing to its extensive capability of light adsorption, outstanding constancy under corrosive operative circumstances, less band gap

* Corresponding author. E-mail address: [email protected] (J. Shim). 1 These authors contributed equally. https://doi.org/10.1016/j.jallcom.2019.152349 0925-8388/© 2019 Elsevier B.V. All rights reserved.

(1.9e2.2 eV), ecologically benign influence. Moreover, it has 16.8% of the theoretical solar-to-hydrogen conversion efficacy, high photocurrent as 12.6 mAcm
2, and appropriate valence band energy position to oxidize water [7e9]. On the other hand, hematite limited its overall PEC water splitting performance due to its short lifetime of its photo-excited charge carriers (<10 ps), short hole diffusion length (2e20 nm) and slow oxygen generation reaction kinetics [10,11]. In order to overcome the above mentioned concerns, doping of hetero-elements can considerably improves the electrical conductivity, hence extend the holes diffusion length and also can increase the photocurrent in the hematite. Up to now, many elements have been successfully incorporated into a-Fe2O3, such as Ge, Ti, Cr, Mn, Sn, Si, S, and Se [12e19]. Among them, yttrium (Y) and copper (Cu) are considered to be one of the most accomplished dopants because of its selective photocatalytic reduction, better separation recombination rate of charge carriers and diminished the band gap for light harvesting [20]. Hence, we chooses the Cu and Y as a dopants into the hematite to improve the absorption of incident light and to improve the PEC water splitting efficacy. However, the

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effectiveness of a codoped (Cu, Y) photoelectrode for water splitting has not been reported in the literature. Herein, we have synthesized pristine, yttrium (mono-doped) and co-doped (Y, Cu) a-Fe2O3 nanostructures using a template free hydrothermal technique, studied the doping and co-doping effect on water splitting activity under visible light and method to increase the photocurrent and prevent the anodic shift of Von in hematite photoanode discussed simultaneously. Moreover, the significant reasons for generation of charge carriers, separation efficiency, and charge carrying mechanism are thoroughly described by a PEC water splitting analysis. 2. Experimental procedure The stoichiometric ratio of Y(NO3)3.6H2O, FeCl2$4H2O and CuCl2$4H2O were mixed separately in 40 ml of DI water under continuous (10 min) stirring. Then, the solutions were mixed together and NH4OH solution was slowly added to maintain a pH 10. Before transferred to this solution in to 150 ml autoclave, the entire solution was stirred for 30 min and heated at 180  C for 4 h. The final obtained solution was cleaned several times with DI water and ethanol and centrifugation, dried at 80  C for 12 h in a vacuum oven and further the samples were post-heated at 500  C for 2 h and named as pure sample. Using the aforementioned procedure, yttrium doped (0.1 mol %) a-Fe2O3 sample was prepared and hereafter the sample named as mono-doped (YFe) sample. And, series of different copper co-doping samples were prepared by maintaining a constant yttrium loading (0.1 mol% with respect to hematite) and varying the loading of Cu (0.1, 0.3, 0.5 mol% with respect to hematite). These samples are hereafter names as YCuFe1, YCuFe-3 and YCuFe-5, respectively.

Fig. 1. XRD patterns of all synthesized samples.

The prepared samples were characterized by the following techniques: XRD, FE-SEM, HR-TEM, UVeVis absorption spectroscopic technique, Raman spectroscopy, Photoluminescence (PL) spectroscopy, and XPS, respectively. In order to prepare the photoelectrodes, pure and doped a-Fe2O3 nanopowders were homogeneously distributed in ethanol (3 ml) by an ultrasonicator for 60 min. The details of the instrumentations and photoelectrode preparation are given elsewhere [21]. The measured potentials vs. Ag/AgCl (0.1 KOH) were converted to the reversible hydrogen electrode (RHE) scale according to the Nernst equation, ERHE ¼ EAg/ o AgCl þ 0.059 PH þ E Ag/AgCl, where ERHE is the converted potential vs RHE, E0 Ag/AgCl ¼ 0.1976 V at 25  C, and E Ag/AgCl is the experimentally measured potential against the Ag/AgCl reference.

concentration. Conversely, due to the Cu enters into host lattice as a codopant, the low doping concentration (0.1 and 0.3 mol %) samples did not show any major diffraction lines associated to Cu. It can be concluded the lower angle shifting of diffraction revealed the substitution of Cu ions can occupy in Fe2O3 as both substitution/ interstitial sites. Hence, there is no new phase of Cu was observed in YCuFe-1 and YCuFe-3 samples [22,23]. But, further increasing Cu loading (YCuFe-5 sample), the diffraction pattern shifting was identified clearly towards higher angles along with secondary phase formation of CuO. The peaks observed at 58.4 , 66.3 and 67.9 corresponding to the (202), (
311) and (113) planes of monoclinic CuO (JCPDS File no: 45e0937). The presence of the diffraction pattern of CuO was owing to the development of copper clusters with the host lattice [24,25]. These prospects specify the changes in host lattice is owing to the existence of copper ions and also one can expect a good interaction between host lattice and copper [26]. The average crystallite size was evaluated by the classical Scherrer's formula. The calculated average crystallite sizes are 27.8, 24.3, 20.3, 18.4 and 19.5 nm for pure, YFe, YCuFe-1 and YCuFe-3 and YCuFe-5 samples, respectively. The results remarkably display that as the dopant concentration increased with the decreased crystallite size up to 0.3 mol % of Cu dopant, and further increased the Cu content (0.5 mol%) the average crystalline size is reduced. Due to an interstitial replacement of yttrium and copper ions into Fe sites, introduced the d-spacing decrease in the host lattice structure, and hence reduced the crystallite size [21].

3. Results and discussions

3.2. Surface analysis

3.1. Structural analysis

Fig. 2 shows the surface morphology images of the samples. SEM investigation of the synthesized samples reveal particle, flacks, and cacao bean-like morphology. Pure sample (Fig. 2 (a)) shows the particle-like morphology. In case of mono-doped sample (Fig. 2(b)), the surface morphology changes from particles to uniform cacao bean-like shapes. Furthermore, in co-doped samples (YCuFe-1 and YCuFe-3), the morphology changes from particles to mixtures of both cacao bean and flacks shapes noticed as shown in Fig. 2 (c, d). For further increased co-dopant Cu content (YCuFe-5) sample, comprehensive demolition of cacao bean and flacks shapes are noticed as shown in Fig. 2(e). Due to the presence of charged ions in the preparation process, the morphological changes may happened. Moreover, the molar concentration of the dopant and co-dopant also influence the morphology of the surface [27]. TEM and HR-TEM analysis of pristine and optimally doped (i.e. YCuFe-1)

2.1. Characterizations and preparation of photoelectrodes

XRD technique was employed in order to study the crystallite structures of the samples. Fig. 1 shows the XRD patterns of pure, mono-doped, and co-doped samples, respectively. From the figure, all the observed patterns displayed the distinctive peaks of a-Fe2O3 (JCPDS file: 33e0664). Furthermore, compared to the undoped sample peak shift (lower angle) is noticed in the case of monodoped and co-doped samples, indicate the successful incorporation of dopant ions into the hematite. When Cu co-dopant concentration (0.5 mol %) further increased, a higher angle shift was observed. The observed shift is may be due to the difference in ionic radii. The observed peaks for all the samples are sharp, signifying that they are highly crystalline. Moreover, initially the peak intensity decreased and then increased with the dopant

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Fig. 2. SEM images (a) pure, (b) YFe, (c) YCuFe-1, (d) YCuFe-3, and (e) YCuFe-5 of prepared samples.

sample is shown in Fig. 3. Fig. 3(aec) & (d-f) shows the TEM, HRTEM pictures and SAED patterns of pristine and YCuFe-1 samples. As a consequence, the samples have shown nanoparticles, and mixed microstructure like structures and 0.27 nm lattice fringes distance was observed for both the samples. The SAED patterns noticeably classify crystallinity nature, well agreement with the XRD examination. The TEM investigation outcomes are in well agreed with the outcomes of surface morphology examination. 3.3. Optical properties Fig. 4(a) displays the optical absorption spectra of the prepared samples. The visible region absorption edge was noticed in all the samples. From the figure, it is noticed that the mono-doped sample showed the blue shift absorption edge. However, in case of codoped samples, the red shift absorption edge was noticed with increasing the codopant (Cu) content [28]. Hence, the co-existence of codopants (Y, Cu) can change the light absorption range in host sites, leading to enhanced photocatalytic activity under solar radiation. The identified blue shift towards lower absorption edge for the mono-doped sample is due to yttrium-electron-donor in mono-doping and a red shift for co-doped samples signifying a

decrease in the band-gap energy owing to Cu-electron acceptor [29]. Moreover, a peak was identified at 663 nm, which is assigned to the electronic transition of (6A1g (S) / 4T2g (G)) Fe and is credited to the Fe3þ ions located in tetrahedral symmetry. Furthermore, using the 1240/wavelength (nm) relationship, the optical band gaps were calculated [30]. The estimated optical band gap energies of pure, mono-doped, and co-doped samples are shown in Fig. 4(b). The band gap values are found to be 1.89, 1.85, 1.53, 1.45, and 1.46 eV, respectively. The optical analysis specify that as dopant concentration increases up to 0.3 mol% of Cu, band gap decreases and band gap increases further increase the Cu content (0.5 mol %). Welderfael et al., and Ganesh et al., also observed the similar band gap phenomena [31,32]. In general, owing to the dopant into the host lattice, the extra impurity energy levels could be presented between the valence and conduction band, and also, many-body interaction might happen with dopants, leading to ionized impurities and free charge carriers, thus resulting in a reduced optical band gap. 3.4. Photoluminescence analysis Fig. 5 displays the PL spectra of all prepared samples. Two

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Fig. 3. TEM, HR-TEM images and SAED patterns of (aec) pure and (def) YCuFe-1 samples.

Fig. 4. (a) optical absorption spectra and (b) band gap energies of prepared samples.

emission peaks are observed at 585 nm 711 nm in all samples, and a peak is identified at 467 nm for mono-doped and co-doped samples only. Because of the electronic movement within the Fe state related emission peak was observed at 467 nm [21,33]. Furthermore, owing to the recombination rate of photo excited charge carriers and oxygen vacancies defects associated emission peaks are identified at 584 and 711 nm [34]. Usually, hematite did not spectacle any emission because of the forbidden local ded transition and energy movement between the cations [35]. But, in current study, emission peaks are risen owing to enrichment of the neighboring magnetic coupling in the Fe3þ ions. Moreover, a reduced PL intensity was noticed with increasing the dopant concentration, it is well supported by the optical absorption analysis. Similarly, a reduced PL intensity revealed the progress in the efficiency of charge separation [36]. In the present study, the YCuFe-1 sample displayed a diminish emission intensity compared to other samples. Therefore, the PL analysis demonstrated the recombination rate of Fe2O3 greatly reduced due to dopants.

Fig. 5. Photoluminescence spectra of all prepared samples.

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3.5. XPS analysis XPS spectra of synthesized samples are displayed in Fig. 6. The observed binding energies (BE) are given in Table 1. The survey spectra (Fig. 6 (a)) show the Fe, O, Y, and Cu as a major elements in the samples. The XPS spectra of Fe is shown in Fig. 6 (b), Fe 2p3/2 and Fe 2p1/2 related peaks are identified at the BE of 710.5 ± 0.1 eV and 723.7 ± 0.1 eV, and a shakeup satellite peak is identified at the BE of 718.8 eV, representing the presence of Fe 3þ in the sample [37]. Moreover, the nonexistence of Fe2þ satellite peaks at 730 or 715 eV, specified that Fe 2þ does not occur at the surface. Fig. 6(c) illustrate XPS spectra of O1s state. The lattice oxygen, and surface hydroxyl groups associated peaks are noticed at BE of 529.2 eV and 531.1 eV [38]. The XPS spectra of Y 3d (Fig. 6(d)) displayed the BE at

5

159.2 eV (3d3/2), and 157.7eV (3d5/2), specifies þ3 oxidation state of yttrium. Fig. 6(e) shows the Cu XPS spectra. The BE of Cu 2p3/2 and Cu 2p1/2 states were noticed at 932.8 eV, and 952.6 eV, indicate þ2 oxidation state for copper. Moreover, the two extra shake-up satellite peaks were noticed at BE of 941.2 and 953.5 eV, specify the presence of Cu2þ state [39]. Furthermore, peak shifting was noticed in all doped samples over pure sample (Table .1). The shift is may be owed to the electronic structure disorder initiated by the dopant ions into hematite. 3.6. Photoelectrochemical analysis EIS measurement was executed to elucidate the separation of charge at interfacial and charge transmission procedure for the

Fig. 6. XPS (a) Survey, (b) Fe, (c) O, (d) Y and (e) Cu spectra of prepared samples.

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Table 1 Binding energies obtained from the XPS core level spectra's for pure and doped Fe2O3 nanostructures. Sample

Pure YFe YCuFe-1 YCuFe-3 YCuFe-5

Fe

Y

Y

Cu

Cu

2p3/2 (eV)

satellite (eV)

2p1/2 (eV)

satellite (eV)

FeeO (eV)

O1s FeeOH (eV)

3d5/2 (eV)

3d3/2 (eV)

2P3/2 (eV)

2P1/2 (eV)

710.5 710.2 710.3 710.4 710.8

718.7 718.5 718.7 718.8 718.9

723.7 723.7 724.1 724.0 724.4

732.5 732.3 732.4 732.4 732.5

529.2 529.1 529.7 529.2 529.9

531.1 531.1 531.8 531.3 531.6

e 157.7 159.1 157.4 158.0

e 159.2 161.2 159.4 159.8

e e 932.8 932.5 932.9

e e 952.6 952.2 952.8

prepared photoanodes. The EIS Nyquist plots of prepared photoanodes are shows in Fig. 7 and measured at a basis of 1.23 V vs RHE under visible light (l > 400 nm). The capacitive arc of the co-doped (YCuFe-1) photoanode reveal a progressively reduced radius than the pure, mono-doped, and co-doped (YCuFe-3, YCuFe-5) photoanodes, signifying that the YCuFe-1 photoanode can greatly increase the hematite electrical conductivity. Moreover, the YCuFe-1 photoanode displayed the considerably reduced resistances in the movement of charge and charge transfer procedures, compared to those of pure, mono-doped and co-doped (YCuFe-3, YCuFe-5) photoelectrodes. Likewise, it is specify that the optimized codoped (YCuFe-1) sample can considerably decrease the resistance of charge transfer in electrode for enhance the electrical conductivity. It is signifying that the photo-excited electrons quickly moved to the FTO substrate through the a-Fe2O3 nanostructures array and can competently encourage the separation of charge at the interface between electrode and electrolyte, and hence improve the dynamics of OER reactions. Therefore, the holes will quickly contribute in the water oxidation process. Fig. 8 show the cyclic voltammetry (CeV) examination of prepared photoelectrodes and the photocurrents were recorded in dark and light illumination. The difference between photocurrents (DJ) in the dark and with light illumination was carried. The detailed achieved photocurrents for all the photoelectrodes are given in Table 2. The co-doped electrodes displayed better photocurrent density over the pristine and mono-doped electrode. Furthermore, the optimized YCuFe-1 electrode exhibited the higher photocurrent density 0.510 mAcm
2 over the pure (0.066 mAcm
2), mono-doped (0.109 mAcm
2), YCuFe-3 (0.425 mAcm
2) and YCuFe-5 (0.263 mAcm
2) electrodes. The achieved photocurrent of the optimized electrode showed nearly ~8 times and ~1.7 times higher over the pristine and mono-doped electrodes. The reason for these improved photocurrents may be

Fig. 7. EIS spectra of all prepared samples.

Fig. 8. Cyclic voltammetry (CeV) curves of prepared all photoelectrodes.

owing to suitable dopant concentration, lesser resistance of charge transmission, suitable band gap, reduced charge carrier recombination rate and improved generation of charge carrier and transport. Likewise, the IeV examination of photoelectrodes are presented in Fig. 9. The photocurrents of all the electrodes originated at 1.2 V (vs RHE) and then gradually rises up to 1.8 V. The co-doped electrodes exhibited improved currents over other electrodes. Moreover, the photocurrents were enhanced with increasing the codopant content up to 0.1 mol%, and then decreased for further increased the co-dopant (0.3, 0.5 mol%) concentration. The obtained photocurrents for all samples are given in Table 2. The optimized sample showed the extreme photocurrent 0.439 mAcm
2 over the pure (0.012 mAcm
2) and mono-doped (0.020 mAcm
2) samples, which is 36 times and ~1.8 times greater than undoped and mono-doped electrodes. Furthermore, the co-doping samples also displays a substantial influence on the Von in the curve. The reason for this improved photocurrents can be owing to lesser crystalline size, the lesser size increase the speed charge movement to the interface and reduced the charge path distance between electrode/electrolyte and electrode interface and hence, increases the gathering of holes [40], which improves the visible light absorption capability, and active separation of photoexcited charge carriers. When a photoelectrode interacts with the electrolyte, a space charge layer [41] and an interface dipole is produced at the photoelectrode/electrolyte interface. Hence, substituting the surface dipole and band twisting at the photoelectrode surface. Moreover, the electric field in the space charge layer can efficiently advance the isolation of the active charge carriers associated with the diffusion region. For photoelectrodes, the charge pairs produced beyond the depletion layer can disappear by recombination due to the lower charge diffusion lengths [42]. Therefore, the depletion layer in optimized doped sample improves the photocurrent owing to the smaller particle sizes by reducing

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Table 2 Photocurrent achieved from CeV and IeV for all the synthesized photoanodes. Sample Name

CeV

IeV

Photocurrents (mAcm
2)

Pure YFe YCuFe-1 YCuFe-3 YCuFe-5

Dark state

Light state

0.190 1.344 5.117 3.125 3.945

0.256 1.453 5.627 3.550 4.208

DJ (mAcm
2)

0.066 0.109 0.510 0.425 0.263

Fig. 9. IeV curves of all prepared photoelectrodes.

the charge recombination rate. A similar phenomenon has been reported in literature [43,44]. The amperometric (I-t) studies were examined for all photoelectrodes under visible light illumination and shown in Fig. 10. The co-doped samples exhibited the higher currents over pure and doped samples. The YCuFe-1 sample showed maximum currents over the other samples. The stability tests revealed that no decrease of photocurrent density was noticed even after 30 min of light illumination. The light on, off states of current of YCuFe-1 sample is shown in inset Fig. 10. Furthermore, no overshoot photocurrent of electrodes are identified, revealed that the photo-excited charges

Fig. 10. Amperometric (I-t) curves of prepared samples.

Photocurrents (mAcm
2) Dark state

Light state

0.200 1.395 4.485 2.376 3.524

0.212 1.415 4.924 2.581 3.697

DJ (mAcm
2)

0.012 0.020 0.439 0.205 0.173

are free from grain boundaries. The maximum photocurrent density was observed in optimized doped sample among the all other photoelectrodes. The following reasons were ascribed to the improved photo activity of the electrode. (i) The efficiently absorbed incident visible light, charge carrier separation, transport and generationeextraction and (ii) the loading of co-dopant into the host, further allowing electron transfer at the photoelectrode/ electrolyte interface and improving the water oxidation. The above analysis showed that the co-dopant in the host lattice is vital for efficient PEC water splitting. Hence, these results demonstrated that the optimal codopant is a best photocatalyst for greater PEC performance. Fig. 11 shows the proposed mechanism of PEC water oxidation of prepared photoanode. As it can be observed that after doping of Y and Cu into host lattice, enhanced the donor concentration efficiently. Furthermore, holes can move quickly to the surface of photoanode and contributed in oxygen evolution reaction due to the lesser space charge region. After light irradiation, a large portion of holes moved to the surface and subsequently greatly reduced the recombination rate of photo generated charge carriers. Also, due to the enhanced electron mobility, collected photoexcited electron more capably on the FTO surface before recombination. Hence, the photocurrent intensity increased, leads to enhanced PEC activity. Lastly, the electrons transferred to the cathode (Pt) respond with Hþ ions and generate H2 and while the holes present in the anode oxidize H2O and generate O2. 4. Conclusions In summary, pure, mono-doped and co-doped hematite photoanodes are successfully synthesized using a template-free hydrothermal method. The co-doped (Y, Cu) hematite photoanode

Fig. 11. PEC water oxidation mechanism of prepared photoanode.

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showed 36 and 1.8 fold greater photocurrent density at 1.23 V vs. RHE over pristine and mono-doped photoanode. Complete structural and PEC analysis reveal that the improved PEC water oxidation activity can be credited to the synergistic effect between two metal dopants. Moreover, the codopant could not only act passivation effect to improve the photo-excited voltage and lesser the onset potential by reduce the recombination rate of the charge carriers, but also increase the surface PEC kinetics to enhance the photocurrent by rushing the separation of the surface charge and utilization. The extraordinarily enhanced electrical conductivity (as evidenced from electrochemical impedance spectroscopy) of the co-doped (Y, Cu) hematite photoanode highlight the significance of co-doping. Acknowledgments This work was supported by the National Research Foundation of Korea and funded by the Ministry of Science of the Korean government (MEST) (NRF-2017R1A4A1015581 &-2018R1D1A1B07048307). References [1] M.G. Walter, E.L. Warren, J.R. Mckone, Solar water splitting cells, Chem. Rev. 110 (2010) 6446e6473. [2] Anqi Wang, Kang Hu, Yuqian Liu, Ruiqi Li, Chenlu Ye, Zixiao Yi, Kai Yan, Flower-like MoS2 with stepped edge structure efficient for electrocatalysis of hydrogen and oxygen evolution, Int. J. Hydrogen Energy 44 (2019) 6573e6581. [3] Kang Hu, Jiahui Zhou, Zixiao Yi, Chenlu Ye, Hanying Dong, Kai Yan, Facile synthesis of mesoporous WS2 for water oxidation, Appl. Surf. Sci. 465 (2019) 351e356. [4] Yujie Wang, Hao Deng, Chenlu Ye, Kang Hu, Kai Yan, Facile synthesis of mesoporous TiC-C nanocomposite microsphere efficient for hydrogen evolution, J. Alloy. Comp. 775 (2019) 348e352. [5] A.G. Tamirat, J. Rick, A.A. Dubale, Using hematite for photoelectrochemical water splitting: a review of current progress and challenges, Nanoscale Horiz. 1 (2016) 243e267. [6] A. Eftekhari, V.J. Babu, S. Ramakrishna, Photoelectrode nanomaterials for photoelectrochemical water splitting, Int. J. Hydrogen Energy 42 (2017) 11078e11109. € nüllü, [7] Kaouk Ali, Tero-Petri Ruoko, Myeongwhun Pyeon, Yakup Go Kimmo Kaunisto, Helge Lemmetyinen, Sanjay Mathur, High water-splitting efficiency through intentional in and Sn codoping in hematite photoanodes, J. Phys. Chem. C 120 (2016) 28345e28353. [8] Min Wang, Hongyan Wang, Quanping Wu, Chuangli Zhang, Song Xue, Morphology regulation and surface modification of hematite nanorods by aging in phosphate solutions for efficient PEC water splitting, Int. J. Hydrogen Energy 41 (2016) 6211e6219. [9] A.B. Murphy, P.R.F. Barnes, L.K. Randeniya, Efficiency of solar water splitting using semiconductor electrodes, Int. J. Hydrogen Energy 31 (2006) 1999e2017. [10] R.F.G. Gardner, F. Sweett, D.W. Tanner, The electrical properties of alpha ferric oxide-II.: ferric oxide of high purity, J. Phys. Chem. Solids 24 (1963) 1183e1196. [11] K. Sivula, F. Le Formal, M. Graetzel, Solar water splitting: progress using hematite (alpha-Fe2O3) photoelectrodes, Chemsuschem 4 (2011) 432e449. [12] P. Zhang, A. Kleiman-Shwarsctein, Y.S. Hu, J. Lefton, S. Sharma, A.J. Forman, E. McFarland, Oriented Ti doped hematite thin film as active photoanodes synthesized by facile APCVD, Energy Environ. Sci. 4 (2011) 1020e1028. [13] S. Shen, J. Jiang, P. Guo, C.X. Kronawitter, S.S. Mao, L. Guo, Effect of Cr doping on the photoelectrochemical performance of hematite nanorod photoanodes, Nanomater. Energy 1 (2012) 732e741. [14] J. Liu, Y.Y. Cai, Z.F. Tian, G.S. Ruan, Y.X. Ye, C.H. Liang, G.S. Shao, Highly oriented Ge-doped hematite nanosheet arrays for photoelectrochemical water oxidation, Nanomater. Energy 9 (2014) 282e290. [15] S.Y. Gurudayal, M.H. Chiam, P.S. Kumar, H.L. Bassi, J. Seng, L.H. Barber Wong, Improving the efficiency of hematite nanorods for photoelectrochemical water splitting by doping with manganese, ACS Appl. Mater. Interfaces 6 (2014) 5852e5859. [16] Y. Ling, G. Wang, D.A. Wheeler, J.Z. Zhang, Y. Li, Sn-doped hematite nanostructures for photoelectrochemical water splitting, Nano Lett. 11 (2011) 2119e2125. [17] R. Zhang, Y. Fang, T. Chen, F. Qu, Z. Liu, G. Du, A.M. Asiri, T. Gao, X. Sun, Enhanced photoelectrochemical water oxidation performance of Fe2O3 nanorods array by S Doping, ACS Sustain. Chem. Eng. 5 (2017) 7502e7506.

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