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Aug 7, 2015 - the C 1s peak of 284.6 eV. The Journal of Physical Chemistry C. Article. DOI: 10.1021/acs.jpcc.5b04892. J. Phys. Chem. C 2015, 119, 1960...
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Synergetic Effect of Conjugated Ni(OH)2/IrO2 Cocatalyst on TitaniumDoped Hematite Photoanode for Solar Water Splitting Zhiliang Wang,†,‡ Guiji Liu,†,‡ Chunmei Ding,†,‡ Zheng Chen,†,‡ Fuxiang Zhang,† Jingying Shi,† and Can Li*,† †

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Dalian National Laboratory for Clean Energy, Chinese Academy of Sciences, Dalian, 116023, China ‡ University of Chinese Academy of Sciences, Beijing, China S Supporting Information *

ABSTRACT: Hematite is a promising photoanode material for renewable solar fuel production via photoelectrochemical (PEC) water splitting. However, the fast electron−hole recombination and sluggish surface reaction retard it from getting satisfied performance. Herein, hematite nanorod arrays doped with titanium (Ti−Fe2O3) on the surface were prepared by a solutionbased process. Because of one-dimension anisotropy and improved charge transfer property, the photocurrent density is doubled compared to pure Fe2O3 at 1.50 V vs RHE under simulated sunlight (AM 1.5 G) irradiation. Loading conjugated Ni(OH)2/IrO2 cocatalyst further leads to about 200 mV negative shift of the onset potential and dramatic increase of the applied bias photon-to-current efficiency (ABPE). We find that Ni(OH)2 can efficiently capture the photogenerated holes from hematite as a hole-storage layer (HSL) to improve the charge transfer process across the interface of hematite and IrO2 electrocatalyst. Furthermore, the stored photogenerated holes in Ni(OH)2 can be utilized by IrO2 for water oxidation more facilely. This synergetic effect along with the efficient surface doping are proposed to be responsible for the enhanced performance.

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INTRODUCTION Photoelectrochemical (PEC) water splitting for hydrogen production offers the capability of harvesting solar energy in the form of chemical bonds directly.1 Among various material candidates for photoanodes, Fe2O3 (α phase, hematite) has aroused widespread research interest as it is nontoxic, abundant and possesses a suitable bandgap of about 2.0 eV with the theoretical solar-to-hydrogen (STH) efficiency up to 16.5%.2,3 However, several inherent drawbacks impede the practical application of hematite, such as (i) short diffusion length (LD ∼ 2−4 nm); (ii) low light absorption (α−1 ∼ 118 at 550 nm);4 (iii) low conductivity; (iv) sluggish surface water oxidation reaction.5 In principle, one-dimensional (1D) nanorod structure has special advantages for alleviating the first two problems, as the quantum confined charge transfer can facilitate the charge separation. Meanwhile, it will also improve light harvesting efficiency by extending the light absorption depth. The poor electron transport property can be improved by bulk doping with Si,6 Ti,7,8 Sn,9,10 Pt,11,12 etc., since it can change the carrier concentration,11 mobility, surface electronic structure13 and/or crystal size14 of hematite. As the separation and diffusion of electron−hole pairs occur in the depletion layer of the semiconductor, it is also expected to change the PEC performance by surface doping. Takei et al.15 found that the surface treatment with potassium on the III−V semiconductor (InAs) nanostructure shows similar effect as bulk doping. It is also reported that the gradient FeOx(PO4)y structure on the top © XXXX American Chemical Society

layer can improve PEC water oxidation performance of hematite.16 Nevertheless, the influence of cation doping on the surface layer of hematite is still unclear. To tackle with the issue of sluggish water oxidation,17 previous reports mainly focused on developing efficient oxygen evolution catalysts (OEC) to improve the semiconductor/ electrolyte (SC/E) interface to promote water oxidation.18,19 For instance, cobalt- and nickel-based OECs have been demonstrated to be effective for enhancing PEC performance of hematite.20−22 However, the OEC will bring dramatic influence on the band alignment in the double-layer and hence different performance.23,24 IrO2 is much more active in OER, but IrO 2 -modified Fe 2 O 3 electrode has been seldom reported.25,26 It may be resulted from the mismatch of crystal lattice or energy band level between the light harvesting semiconductor and cocatalyst which will lead to ineffective charge transfer through the SC/OEC junction as observed in the system of BiVO4/IrO2.27−29 Recently, we demonstrated that a hole-storage layer (HSL) can result in efficient hole extraction from semiconductor for water oxidation.30,31 Inspired by this new finding, we aimed to employing a HSL for tuning the SC/OEC interface of hematite to facilitate the charge transfer. Received: May 21, 2015 Revised: August 6, 2015

A

DOI: 10.1021/acs.jpcc.5b04892 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C In this work, hematite electrodes with nanorod structure were fabricated by chemical bath method. And the conducting property of hematite electrode was improved by doping of cations on the surface. Subsequently, we inserted a Ni(OH)2 layer between hematite and IrO2 catalyst to assemble a Ti− Fe2O3/Ni(OH)2/IrO2 electrode. The roles of Ni(OH)2 and IrO2 were studied to clarify the interaction between semiconductor and oxygen evolution catalyst.

ABPE =

j (mA·cm−2) × (1.23 − V ) (V vs. RHE) 100 mW· cm−2

× 100%

where j is the photocurrent at the potential of V under simulated light (100 mW/cm2) The induced-photon-to-current efficiency (IPCE) was measured on a monochromatic light irradiation provided by a tungsten lamp equipped with monochromator (CROWNTECH QEM24-D 1/4 m Double).

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EXPERIMENTAL SECTION Preparation of Ti-Doped Hematite Photoelectrode. The Ti-doped hematite photoelectrodes were prepared in two steps. First, FeOOH nanorod arrays were synthesized by chemical bath method on FTO sheet (see Supporting Information for detail). It was then immersed in 0.1 M tetrabutyl titanate (TBT) solution for 10 min followed by sweeping under compressed air and calcinating at 500 °C for 3 h. The as-prepared electrode was then treated at 750 °C for 10 min, taken as Ti−Fe2O3. Loading of Ni(OH)2/IrO2 Conjugated Cocatalysts. The cocatalyst was loaded by successive ion layer adsorption (SILA) method. For the loading of Ni(OH)2, the Ni(NO3)2 (0.05 M) precursor solution was dropped on the surface of the electrode and then placed for 10 min. The excess Ni precursor was swept away by compressed air followed by dropping NaOH (1 M) solution onto the surface. And 5 min later, the excess NaOH was removed (taken as Ti−Fe2O3/Ni(OH)2). Similarly, IrO2 was loaded by dropping IrO2 colloid (See Supporting Information for the preparation) onto the electrodes (Ti− Fe2O3 and Ti−Fe2O3/Ni(OH)2) and kept horizontal for 1−4 min before sweeping the excess colloid by compressed air. The electrodes were taken as Ti−Fe2O3/IrO2 and Ti−Fe2O3/ Ni(OH)2/IrO2. Characterization. The absorption spectra were acquired with Cary 5000 UV−vis-NIR spectrophotometer (Varian) from 300 to 800 nm. X-ray diffraction (XRD) measurement was conducted on Rigaku D/Max-2500/PC powder diffractometer operating at 40 kV and 200 mA with Cu Kα radiation (λ = 0.154 nm). The scanning rate was 5 degree/min. The morphology of the electrodes was imaged by Quanta 200 FEG scanning electron microscope (SEM) and HRSEM. Highresolution transmission electron microscopy (HRTEM) images were obtained on Tecnai G2 F30 S-Twin (FEI Company) with an accelerating voltage of 300 kV. X-ray photoelectron spectroscopy (XPS) was recorded on VG ESCALAB MK2 spectrometer with a monochromatic Al Kα radiation (12.0 kV, 240 W). Photoelectrochemcial Testing. The photoelectrochemical performance was conducted in a 3-electrode system with a potentiostat (Iviumstat, Ivium Technologies) in 1 M NaOH aqueous (pH 13.6) under AM 1.5G (100 mW/cm2, Newport Sol 3A, Class AAA Solar simulator). And a Pt foil (2 cm ×2 cm) was used as the counter electrode and the saturated calomel electrode (SCE) as the reference electrode. The potential referred to the SCE can be transferred to the reversed hydrogen electrode (RHE) scale as follow:

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RESULTS AND DISSCUSSION The XRD patterns in Figure 1a show that the raw electrodes are α-Fe2O3 (PDF #33-0664). No additional diffraction peaks

Figure 1. (a) XRD patterns of pure Fe2O3 (red), Ti−Fe2O3 (blue), and Ti−Fe2O3/Ni(OH)2/IrO2 (dark) and the standard PDF cards of SnO2 and hematite. Fine XPS spectrum of (b) Ti 2p, (c) Ni 2p, and (d) Ir 4f of Ti−Fe2O3/Ni(OH)2/IrO2. All the spectra are corrected by the C 1s peak of 284.6 eV.

can be observed after Ti modification or Ni(OH)2/IrO2 loading. To confirm the surface species on the hematite, the X-ray photoelectron spectroscopy (XPS) was used to characterize the electrode of Ti−Fe2O3/Ni(OH)2/IrO2. Figure 1b shows that the 457.7 and 463.5 eV peaks are indexed to Ti 2p3/2 and Ti 2p1/2,8 respectively. The concentration of Ti atom in the Ti− Fe2O3 is about 3 at. % determined by XPS but is only about 0.1 at. % by inductive coupled plasma emission spectrometer (ICP) analysis, suggesting that the Ti element is mainly dispersed on the surface layer of hematite. And the element mapping (Figure S1) further confirms the uniform dispersion of Ti. The peaks of 873.2 and 855.5 eV in Figure 1c can be ascribed to Ni 2p. Moreover, a separation of 17.3 eV between the two peaks indicates that Ni presents as Ni2+.33 The binding energy of 62.0 and 64.9 eV (Figure 1d) attributed to Ir 4f5/2 and Ir 4f3/2 indicates the presence of IrO2.34 The morphology of the electrodes was imaged with SEM and TEM. Figure 2a shows that the electrodes have the nanorodlike morphology with the thickness of about 350 nm (Figure 2d). HRSEM images display that the surfaces of the hematite

E(RHE) = E(SCE) + 0.059pH + 0.2411

E(RHE) is the potential referred to the RHE and E(SCE) is the potential referred to the SCE. The applied bias photon-to-current efficiency (ABPE) was calculated from the linear sweep voltammetry data based on the following expression:32 B

DOI: 10.1021/acs.jpcc.5b04892 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 2. SEM images of the Ti−Fe2O3electrode: (a) top view and (d) cross-section view. HRSEM and HRTEM of (b, e) Ti−Fe2O3 and (c, f) Ti− Fe2O3/Ni(OH)2/IrO2.

electrode shows an ultimate photocurrent below 1.0 mA/cm2. After the Ti modification, the photocurrent is increased to 2.2 mA/cm2 where the dark current arises. However, this treatment leads to a positive shift of onset potential. This phenomenon was also found for Fe2O3 electrodes modified with other cations (Sn, Pt, Nb, Ta, Li), that is, they all show the improvement of the photocurrent at high bias together with positive shift of the onset potential (Figure S2). In order to clarify the role of Ti cation, the charge transfer property was probed by H2O2 molecular35 and Mott−Schottky analysis. Parts a and b of Figure S3 show that Ti has little influence on the light absorption, hence the similar integrated current. However, the charge separation efficiency has more than 2-fold increase together with 1 order of magnitude improvement of the carrier concentration after modification with Ti (Figure S3, parts c and d). Obviously, Ti doping is beneficial to improve the electrical conducting property of hematite so as to increase the photoresponse. At the same time, the flat band potential is positively shifted from 0.34 to 0.40 V vs RHE after Ti modification, which agrees with the positive shift observed in the onset potential. This result is possibly due to additional donor level formed by surface doping with Ti, which affects band bending at the SC/E interface. Theoretical research by Toroker36,37 also indicated that much larger overpotential was needed for water oxidation when Ti dopants existence on the surface of hematite. To address the issue on the onset potential, we further modified the Ti−Fe2O3 electrodes with Ni(OH)2/IrO2. The loading of Ni(OH)2 or IrO2 shows marginal effect on the onset potential (Figure S4, at 20 μA/cm2), while the codecoration of Ni(OH)2 and IrO2 leads to about 200 mV negative shift of the onset potential and the substantial photocurrent improvement in the low bias region (Figure 3a). Figure 3b shows that the pristine and Ti−Fe2O3 electrodes possess similar low ABPE value; while after a series loading of Ni(OH)2 and IrO2, the ABPE of Fe2O3 electrode increases by two folds. The IPCE

rods are smooth even after the modification with Ti (Figure 2, parts b and e), while after the loading of Ni(OH)2 layer, the surface of hematite appears fluffy and IrO2 nanoparticles are dispersed as islands on the Ni(OH)2 layer (Figure 2, parts c and f), in accordance with XPS results. Figure 3a shows the linear sweep voltammetry (LSV) curves of the electrodes for water oxidation. The pristine Fe2O3

Figure 3. (a) Current−voltage curves of the electrodes of pure Fe2O3 (red), Ti−Fe2O3 (blue), and Ti−Fe2O3/Ni(OH)2/IrO2 (blue). Corresponding (b) ABPE and (c) IPCE at 1.23 V vs RHE. (d) Injection efficiency of each electrode, including Ti−Fe2O3 (blue), Ni(OH)2 loaded (orange), IrO2 loaded (yellow), and Ni(OH)2/IrO2 loaded (black) Ti−Fe2O3 electrode. For the cocatalyst-loaded electrode, the linear sweep voltammertry was recorded below 1.3 V vs RHE since the dark current arose at about 1.2 V vs RHE in the NaOH/H2O2 electrolyte. C

DOI: 10.1021/acs.jpcc.5b04892 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C (Figure 3c) also confirms the promotion under the entire absorption upon Ni(OH)2/IrO2 loading. Figure 3d shows the injection efficiency35,38 of Ti−Fe2O3/ Ni(OH)2 is akin to that of Ti−Fe2O3. On the contrary, when IrO2 is further loaded, the charge injection efficiency is significantly improved. This result suggests that the IrO2 is an efficient catalyst that greatly accelerates the surface reaction on the electrode. However, the presence of Ni(OH)2 seems indispensable since the charge injection of Ti−Fe2O3/IrO2 is still poor, indicating that IrO2 is not efficient to draw holes from hematite directly. Fe2O3 is a kind of semiconductors with many surface states but low water oxidation activity. And under chopped light, the charging or discharging of these surface states will lead to the formation of the transient photocurrent (non-Faradaic current) which will be added to the steady water oxidation current (Faradaic current). But the non-Faradaic current decays quickly and sharp current peaks appear at the switching of light.38 By analyzing the peak under stepped potential chronoamperometry test, the charge accumulated on the surface states (SS) could be obtained.39 As shown in Figure 4a, the Ti−Fe2O3/

electrode. The poor charge injection (Figure 3d), together with greatly enhanced hole storage capacity of Ti−Fe2O3/ Ni(OH)2 electrode, suggest that Ni(OH)2 behaves as a typical HSL for Ti−Fe 2O3 electrode, in accordance with our previous results.30,31 Furthermore, the stored holes in the Ti−Fe2O3/ Ni(OH)2 electrode decreases owing to the addition of IrO2, confirming the role of IrO2 as a catalyst for water oxidation. Ni(OH)2 has the ability to store charges because of the variable oxidation states of Ni. The electrochemical characterization of Ni(OH)2 and IrO2 electrode (see Supporting Information for preparation) in Figure 4c shows that Ni(OH)2 has an oxidation peak at 1.42 V originated from the oxidation of Ni(OH)2 to NiO2.40 While after coupling IrO2 to Ni(OH)2, the oxidation peak shifts negatively to 1.39 V. Ni(OH)2 has been reported to be redox-active ion-permeable OEC41 which can change the oxidation level in situ during the reaction to adapt to the interface of SC/E. When it is inserted in the SC/OEC interface between Fe2O3 and IrO2, Ni(OH)2 may tune the interface by changing redox level of Ni species with photogenerated holes from hematite upon illumination. And severe charge recombination originally occurred at the Fe2O3 and IrO2 interface could be renovated owing to the inset of Ni(OH)2. For this reason, Ti−Fe2O3/IrO2 only exhibits improvement in PEC performance and charge injection efficiency with the Ni(OH)2 layer. On the other hand, the photogenerated holes might be captured through the oxidation of Ni(OH)2 to Ni4+. But the water oxidation is not fast enough,42 large amount of charges are accumulated in it. When the IrO2 was coupled, more holes are consumed for the water oxidation rather than stored in the oxidized Ni species. So it can protect the Ni(OH)2 from being oxidized to higher oxidation state by accelerating water oxidation (Figure 4d). As a result, the Ni(OH)2/IrO2 modified electrodes show higher steady photocurrent but lower charge accumulation. The open circuit potential (OCP) was measured to further understand the shift of the onset potential. The OCP difference between under dark and illumination (ΔVOC) is related to the photovoltage. It is found that the pure Fe2O3 electrode has a ΔVOC of 300 mV, in consistence with previous reports,22 while surface doping with Ti reduces the ΔVOC to only 100 mV (Figure S6). Interestingly, after loading Ni(OH)2/IrO2, it increases to the original ΔVOC, along with the change of the onset potential. Further research below confirms that the great change of ΔVOC is mainly caused by Ni(OH)2. The Ti−Fe2O3/ IrO2 electrode showed ΔVOC of only 140 mV, but 200 mV for the Ti−Fe2O3/Ni(OH)2 electrode. In principle, a larger ΔVOC means a larger photovoltage generated by the electrode and less bias is required. It has also been demonstrated that an enlarged photovoltage is amenable to induce a significant cathodic shift in hematite-based PEC water splitting.22 Therefore, it is concluded that the negatively shift of the onset potential is mainly attributed to Ni(OH)2. Electrochemical impedance spectra (EIS) were taken to get further insight into the role of Ni(OH)2/IrO2. An equivalent circuit43,44 (Figure 5a) is used to simulate the electrode and interfaces, in which Rs is serial resistance between the FTO and bulk hematite; Cbulk is the capacity of the space-charge layer; Rtrap is the bulk charge trapping resistance; Rct is the charge transfer resistance across the SC/E; Ctrap is the capacity associated with the surface trap states. The electrode shows much smaller radius of the semicircle after loading Ni(OH)2/ IrO2 (Figure 5b). And parts c and d of Figure 5 show that under the tested potential, the Ti−Fe2O3/Ni(OH)2/IrO2 has

Figure 4. (a) Chronoamperometry photocurrent of Ti−Fe2O3 (blue), Ti−Fe2O3/Ni(OH)2 (black), and Ti−Fe2O3/Ni(OH)2/IrO2 (black) under stepped potential (green, dash curve). (b) Accumulated charge in the spike under different potential. (c) Cycle voltammertry test of IrO2 (yellow), Ni(OH)2 (orange), and Ni(OH)2/IrO2 (black) on the FTO (purple) electrode under dark at the scan rate of 50 mV/s. (d) Scheme for the charge transfer from hematite to H2O through Ni(OH)2 and/or IrO2.

Ni(OH)2/IrO2 electrode shows almost the same transient photocurrent but higher steady photocurrent compared to the Ti−Fe2O3/Ni(OH)2 electrodes. For the accumulated charges (Figure 4b), when the driving force is large enough (>1.1 V vs, RHE) to get rid of the capture of the SS, some holes will be stored. But in the mediate region (0.7−1.1 V vs RHE), 1 order of magnitude increase of charges were accumulated as long as Ni(OH)2 was loaded. And the CV test for the photoanodes under light (Figure S5) also confirms that the Ni(OH)2 has the ability to store charges based on the facts that the charge accumulated by the pesudocapacitance of Ti−Fe2O3/Ni(OH)2 has about 6.6 folds increase compared with the Ti−Fe2O3 D

DOI: 10.1021/acs.jpcc.5b04892 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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charge separation efficiency calculation, Mott−Schottky plots, and cycle voltammertry and OCP of each electrode (PDF)

AUTHOR INFORMATION

Corresponding Author

*(C.L.). E-mail: [email protected]. Telephone: 86-41184379070. Fax: 86-411-84694447. Homepage: http://www. canli.dicp.ac.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Basic Research Program of the Ministry of Science and Technology, China (Grant 2009CB220010), National Basic Research Program of China (No. 2014CB239403), and National Natural Science Foundation of China (No. 21061140361, 21090340).

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Figure 5. (a) Equivalent circuit of the electrochemical impedance spectroscopy. (b) Tested (dot) and simulated (line) result of the electrodes of pure Fe2O3 (red), Ti−Fe2O3 (blue), and Ti−Fe2O3/ Ni(OH)2/IrO2 (black). Simulated result of (c) Rtrapping and (d) Rct based on the EC under a serials potential.

(1) Bard, A. J.; Fox, M. A. Artificial Photosynthesis: Solar Splitting of Water to Hydrogen and Oxygen. Acc. Chem. Res. 1995, 28, 141−145. (2) Chen, Z.; Jaramillo, T. F.; Deutsch, T. G.; Kleiman-Shwarsctein, A.; Forman, A. J.; Gaillard, N.; Garland, R.; Takanabe, K.; Heske, C.; Sunkara, M. Accelerating Materials Development for Photoelectrochemical Hydrogen Production: Standards for Methods, Definitions, and Reporting Protocols. J. Mater. Res. 2010, 25, 3−16. (3) Kennedy, J. H.; Frese, K. W. Photooxidation of Water at α-Fe2O3 Electrodes. J. Electrochem. Soc. 1978, 125, 709−714. (4) Schrebler, R.; Ballesteros, L. A.; Gómez, H.; Grez, P.; Córdova, R.; Muñoz, E.; Schrebler, R.; Ramos-Barrado, J.; Dalchiele, E. A. Electrochemically Grown Self-Organized Hematite Nanotube Arrays for Photoelectrochemical Water Splitting. J. Electrochem. Soc. 2014, 161, H903−H908. (5) Sivula, K.; Le Formal, F.; Grätzel, M. Solar Water Splitting: Progress Using Hematite (α-Fe2O3) Photoelectrodes. ChemSusChem 2011, 4, 432−449. (6) Cesar, I.; Kay, A.; Gonzalez Martinez, J. A.; Grätzel, M. Translucent Thin Film Fe2O3 Photoanodes for Efficient Water Splitting by Sunlight: Nanostructure-Directing Effect of Si-Doping. J. Am. Chem. Soc. 2006, 128, 4582−4583. (7) Glasscock, J. A.; Barnes, P. R. F.; Plumb, I. C.; Savvides, N. Enhancement of Photoelectrochemical Hydrogen Production from Hematite Thin Films by the Introduction of Ti and Si. J. Phys. Chem. C 2007, 111, 16477−16488. (8) Deng, J.; Zhong, J.; Pu, A.; Zhang, D.; Li, M.; Sun, X.; Lee, S. T. Ti-Doped Hematite Nanostructures for Solar Water Splitting with High Efficiency. J. Appl. Phys. 2012, 112, 084312−084312−6. (9) Ling, Y.; Wang, G.; Wheeler, D. A.; Zhang, J. Z.; Li, Y. Sn-Doped Hematite Nanostructures for Photoelectrochemical Water Splitting. Nano Lett. 2011, 11, 2119−2125. (10) Xi, L.; Chiam, S. Y.; Mak, W. F.; Tran, P. D.; Barber, J.; Loo, S. C. J.; Wong, L. H. A Novel Strategy for Surface Treatment on Hematite Photoanode for Efficient Water Oxidation. Chem. Sci. 2013, 4, 164−169. (11) Hu, Y. S.; Kleiman-Shwarsctein, A.; Forman, A. J.; Hazen, D.; Park, J. N.; McFarland, E. W. Pt-Doped α-Fe2O3 Thin Films Active for Photoelectrochemical Water Splitting. Chem. Mater. 2008, 20, 3803− 3805. (12) Kim, J. Y.; Magesh, G.; Youn, D. H.; Jang, J. W.; Kubota, J.; Domen, K.; Lee, J. S. Single-Crystalline, Wormlike Hematite Photoanodes for Efficient Solar Water Splitting. Sci. Rep. 2013, 3 (2681), 1−8. (13) Liao, P.; Toroker, M. C.; Carter, E. A. Electron Transport in Pure and Doped Hematite. Nano Lett. 2011, 11, 1775−1781.

much lower Rtrap and larger Ctrap because of the effective surface doping with Ti and cocatalyst loading. The increased Ctrap can be explained by the existence of Ni(OH)2 which could capture and store the photogenerated holes as the result indicated by the stepped potential chronoamperometry tests. The decrease of the charge transfer resistance (Rct) after loading Ni(OH)2/ IrO2 indicates the acceleration of surface water oxidation reaction by the presence of IrO2 which act as an efficient water oxidation catalyst.

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CONCLUSIONS In this work, hematite electrode doped with Ti cations on the surface was found to have one fold increase in photocurrent under a large bias. We find that Ti modification leads to one order magnification increase of the carrier concentration. The further loading of Ni(OH)2/IrO2 conjugated cocatalyst on the Ti−Fe2O3 electrode can facilitate the water oxidation and decrease the onset potential by about 200 mV. Synergetic effect was found for the photocurrent and charge injection after the coloading of the Ni(OH)2 and IrO2. The synergetic effect is originated from the cooperation between Ni(OH)2 and IrO2 where the Ni(OH)2 layer works as a hole storage layer whereby to accumulate the excess photogenerated holes and IrO2 acts as the catalyst to drift the stored hole to water and protect the Ni(OH)2 from being oxidized. The increased carrier concentration of surface doping and synergetic effect of Ni(OH)2/ IrO2 leads to 2.2 times of ABPE compared to the pristine Fe2O3 electrode.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b04892. Details of experimental methods, LSV curves for hematite doped with other cations, theoritical Jabs and E

DOI: 10.1021/acs.jpcc.5b04892 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.jpcc.5b04892 J. Phys. Chem. C XXXX, XXX, XXX−XXX