Alkali Treatment for Enhanced Photoelectrochemical Water Oxidation

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Alkali Treatment for Enhanced Photoelectrochemical Water Oxidation on Hematite Photoanode Xueliang Zhang, Xin Wang, Xinli Yi, Jinhua Ye, and Defa Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06465 • Publication Date (Web): 01 Feb 2019 Downloaded from http://pubs.acs.org on February 5, 2019

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ACS Sustainable Chemistry & Engineering

Alkali Treatment for Enhanced Photoelectrochemical Water Oxidation on Hematite Photoanode Xueliang Zhang,†, ‡ Xin Wang,†, ‡ Xinli Yi,†, ‡ Jinhua Ye*, †, ‡, §, and Defa Wang, †, ‡ †TJUNIMS

International Collaboration laboratory, Key Lab of Advanced Ceramics and Machining

Technology (Ministry of Education), Tianjin Key Lab of Composite and Functional Materials, School of Materials Science and Engineering, Tianjin University, 92 Weijin Road, Tianjin 300072, China ‡Collaborative

Innovation Center of Chemical Science and Engineering (Tianjin), 92 Weijin Road, Tianjin

300072, China §International

Center of Materials Nanoarchitectonics (WPIMANA), National Institute for Materials

Science (NIMS), 1-1Namiki, Tsukuba, Ibaraki 305-0044, Japan



Corresponding authors. E-mail: [email protected] (D. Wang); [email protected] (J. Ye). 1

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ABSTRACT As one of the most popular photoanode materials for photoelectrochemical (PEC) water splitting, hematite (-Fe2O3) is suffered from the low conductivity, severe electronhole recombination, and sluggish water oxidation kinetics unfortunately. Herein, we report an alkali-treatment method to effectively accelerate the water oxidation kinetics of -Fe2O3 and titanium doped -Fe2O3 (Ti:-Fe2O3) nanorod array photoanodes. The purpose of Ti-doping is to increase the conductivity of -Fe2O3. The photocurrent densities increased 3- and 2-times for -Fe2O3 and Ti:-Fe2O3 photoanodes after KOH treatment, respectively. Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) analyses demonstrated that a conformal thin layer grafted with hydroxyl (-OH) groups was formed on the hematite surface. Linear sweep voltammetry (LSV) curves under light irradiation and in dark indicated that the thin OH-grafted overlayer behaved like an electrocatalyst to accelerate the water oxidation kinetics on hematite photoanodes. Moreover, XPS valence band (XPS-VB) spectra, Mott-Schottky analysis and electrochemical impedance spectroscopy (EIS) revealed that a type II heterojunction was in situ formed by the OH-grafted overlayer on the hematite nanorod surface, which substantially enhanced the surface charge separation efficiency. The improved PEC performance could be attributed to the accelerated water oxidation kinetics and enhanced surface charge transfer.

KEYWORDS: Alkali treatment, Hematite, Photoanode, Water oxidation kinetics, Surface charge separation, Type II heterojunction

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INTRODUCTION As one of the most promising approaches for direct solar-to-chemical energy conversion and storage,1-5 photoelectrochemical (PEC) water splitting has been paid extensive attention.6 However, lacking of low-cost and robust photoanode materials limits its broad application.7, 8 On the other hand, water oxidation on the photoanodes is regarded as the bottleneck reaction in PEC water splitting because it involves a multi-step proton-coupled electron transfer,9 and needs a large overpotential. Therefore, it is of great significance to develop efficient and stable photoanode materials for PEC water splitting. A number of metal oxide semiconductors such as TiO2,1012 WO3,1315 BiVO4,16, 17 and Fe2O3,18, 19

have been extensively explored as photoanodes for PEC water oxidation. Among them, the

hematite (-Fe2O3) has been attracting much attention due to its chemical stability,20,

21

environmental benign,22 earth abundance,23 and especially the suitable band gap (2.1 eV) for visible-light-absorption.3 However, the short lifetime of photo-excited charge carrier,24 moderate hole diffusion length (24 nm),25,

26

and sluggish oxygen-evolution kinetics27 have hindered its

further application. A great deal of efforts, such as element doping,7, 28, 29 nano-structuring,3032 and adding of electrocatalytic surface layers,3335 have been dedicated to increasing photocurrent density of the -Fe2O3 photoanodes by enhancing the optical absorption, electrical conductivity, charge collection ability, and water oxidation kinetics.3638 Previous studies have shown that doping of various elements such as Ti,7, 39 Si,40 Sn,41 Al,42 Cr,43 and Pt,44 could effectively improve the PEC efficiency of -Fe2O3 by enhancing its electrical conductivity. On the other hand, decorating -Fe2O3 with electrocatalysts (ECs) has been approved as a useful method to decrease the onset potential and to increase the photocurrent density.4547 However, decoration of ECs on the photoanode surface would create an extra interface between semiconductors (SCs) and ECs, which actually behaved as the charge carrier recombination center. Carroll et al. reported that a thin CoPi layer of 2.5 nm could improve the performance of mesostructured hematite photoanodes, while a 3



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thick layer led to worse performance due to the increased interfacial recombination.48 In fact, the precise regulation of ECs thickness on the photoanodes is fairly critical and intractable. In addition, the strong light absorption of the ECs, especially for Co- and/or Fe-based materials, severely block the incident light to the underneath semiconductors. Moreover, most ECs developed so far were suffered from the poor stability in electrolyte in such ways as peeling off from semiconductors and/or dissolving into the electrolyte solution, which undoubtedly affected their performance stability during PEC water splitting.4951 Surface engineering has been demonstrated an effective way to accelerate water oxidation kinetics and meanwhile avoid the interfacial recombination.52,

53

For example, Shen et al.

functionalized hematite nanorod with an amorphous TiO2 overlayer by surface-engineering, and achieved nearly 400% increase in the maximum incident photon-to-current efficiency (IPCE) at 350 nm due to the enhanced oxygen electrocatalysis.54 Lan et al. used a boron-termination method to passivate the surface Fe(IV) defects in -Fe2O3, which exhibited a large cathodic shift of the onset potential and a significantly improved PEC performance due to the suppression of surface charge recombination.19 Hu et al. developed a covalent fixation strategy to suppress the surface state of hematite photoanode, the onset potential of corresponding photoanode negatively shifted, and the photocurrent density was greatly increased.55 The above mentioned results have demonstrated the important role of surface engineering in ameliorating the surface state and lowering the onset potential of -Fe2O3. In terms of the significance of surface engineering strategies in enhancing the PEC performance of a semiconductor, herein, we propose to adopt the alkali treatment to change the surface band structure of -Fe2O3, as the band edge potentials of a semiconductor are closely pH-value-dependent.56 We used a facile alkali-treatment method to form a thin conformal hydroxyl-grafted overlayer on the surface of one-dimensional (1-D) -Fe2O3 nanorod arrays. As a comparison, Ti-doped -Fe2O3 (Ti:-Fe2O3) nanorod arrays was also treated by alkali, aiming to increase the conductivity. PEC water oxidation showed a pronounced photocurrent enhancement in 4


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both -Fe2O3 and Ti:-Fe2O3 photoanodes by alkali-treatment. On one hand, the in situ formed hydroxy-rich overlayer acted like ECs on the surface of hematite photoanodes, which substantially accelerated the water oxidation kinetics. On the other hand, the cathodically shifted conduction band (CB) and valence band (VB) potentials of the hydroxy-rich overlayer on the hematite surface rendered construction of a type II heterojunction, which favored the photogenerated charge carrier separation. More importantly, in comparison with the samples modified with ECs, the alkali-treatment significantly improved the photostability of hematite photoanode in electrolyte. This work has proved that alkali-treatment is a facile surface engineering strategy for effectively enhancing the PEC performance of hematite photoanodes by the improvement of both water oxidation kinetics and surface charge transfer. EXPERIMENTAL SECTION Chemicals. Potassium hydroxide (KOH) was purchased from Shanghai Aladdin Chemical Reagent Co. Ltd. Titanium tetrachloride (TiCl4), iron chloride (FeCl36H2O), sodium nitrate (NaNO3), sodium hydroxide (NaOH), potassium chloride (KCl) and absolute ethanol (99.9%) were purchased from Sinopharm Chemical Reagent Co. Ltd. All the reagents were used directly without further purification. Deionized water was used throughout the experiments. Preparation of -Fe2O3 and Ti:-Fe2O3 nanorod array photoanodes. Hematite photoanodes were prepared according to the previously reported method with little modification.57 Typically, 15 mmol FeCl36H2O and 15 mmol NaNO3 was dissolved into 50 mL deionized water under constant stirring at room temperature. The obtained transparent yellow precursor was transferred into a 50 mL Teflon-lined stainless-steel autoclave with 80% capacity, which contained a piece of FTO glass leaning against the wall of autoclave with the conducting side facing down. The FTO glass was ultrasonically cleaned in advance by alcohol, acetone, and alcohol in sequence and dried under high-pressure nitrogen stream. The autoclave was sealed and placed in an oven, and the hydrothermal reaction was carried out at 100 C for 5 h. After the autoclave was cooled down 5



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naturally, the film was taken out and washed with deionized water and alcohol. The as-prepared -FeOOH film was dried at 60 C and then annealed in air at 550 C for 2 hours, and subsequently annealed in a preheated oven at 800 C for 15 minutes. As for the fabrication of Ti:-Fe2O3 nanorod arrays, a certain amount of 0.2 M TiCl4 ethanol solution was added into the precursors used for synthesizing -Fe2O3, and then the hydrothermal reaction was conducted under the conditions as mentioned above. Alkali treatment of -Fe2O3 and Ti:-Fe2O3 photoanodes. The fabrication process of KOH-treated Fe2O3 photoanodes is illustrated in Scheme 1. Briefly, the pre-synthesized Fe2O3 nanorod arrays were deposited on FTO substrate by a facile hydrothermal method and followed by subsequent annealing. Certain amount of KOH solution was spin-coated on the as-prepared photoanodes, first dried at 70 C for 12 hours, and then annealed at 400 C for 2 hours in air. The obtained KOH-treated -Fe2O3 photoanode sample was labelled as -Fe2O3-OH. For the treatment of Ti:-Fe2O3 by KOH, the procedure was same as that for -Fe2O3 as mentioned above, and the as-obtained sample was labelled as Ti:-Fe2O3-OH.

Scheme 1. Schematic illustration of KOH-treated -Fe2O3 nanorod arrays.

Preparation of -Fe2O3 and Ti:-Fe2O3 thin film photoanodes. The thin film photoanodes were fabricated by spin-coating KOH-treated -Fe2O3 and Ti:-Fe2O3 nanoparticles on conductive FTO substrate and subsequent annealing at 400 C in air. The -Fe2O3 and Ti:-Fe2O3 nanoparticles before and after KOH treatment were prepared by using the hydrothermal method and annealing process similar to that for preparation of -Fe2O3 and Ti:-Fe2O3 thin film photoanodes.

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Materials Characterization. Crystal structure was determined using a powder X-ray diffractometer (XRD; D/MAX-2500, Rigaku, Japan). Morphology and microstructure were observed on a field emission scanning electron microscope (FESEM; S4800, Hitachi, Japan) and a transmission electron microscope (TEM; FEI Tecnai G2 F20, USA), and each was equipped with an energy dispersive X-ray spectrometer (EDX). High angle annular dark field (HAADF) image and energy dispersive X-ray spectroscopic (EDX) elemental mapping analysis were performed on a Cs-corrected scanning transmission electron microscope (STEM, JEM-ARM200, operated at 200 kV). UVVis diffuse reflectance spectra were recorded on a spectrophotometer (UV-1800, Shimadzu, Japan) with an integrating sphere attachment using BaSO4 as the reference. X-ray photoelectron spectroscopy (XPS) measurements were performed on an Escalab 250 (Thermo Scientific, USA) using monochromated Al K radiation and C 1s peak (284.8 eV) as the reference. Infrared transmission spectra were collected using a Fourier transform-infrared (FT-IR) spectrophotometer (Nicolet 6700, Thermo SCIENTIFIC). PEC Property Evaluation. PEC performance was tested on a standard three-electrode electrochemical workstation (CHI 660E Instruments) using platinum wire, Ag/AgCl in 3 M KCl solution, and the prepared photoanode as the counter electrode, reference electrode, and working electrode, respectively. 1 M NaOH solution (pH = 13.6) was used as the electrolyte after saturation with Ar gas for 30 min unless otherwise stated. 1 M Na2SO4 (pH = 7.0) solution saturated with Ar gas was also used as neutral electrolyte for comparison. A 500 W xenon lamp with an AM 1.5G filter was used for the simulated solar illumination. The power intensity of incident light was adjusted to be 100 mW/cm2 by a spectroradiometer (Avantes AvaSpec-ULS2048). The measured potential vs. Ag/AgCl was converted to that vs. RHE using the Nernst equation: ERHE = EAg/AgCl + 0.059pH + E0

(Ag/AgCl),

where EAg/AgCl is the experimentally measured potential and E0

(Ag/AgCl)

=

0.209 V at 25 °C for an Ag/AgCl electrode in 3 M KCl.33 Currentvoltage (JV) characteristics were measured by linear sweep voltammetry (LSV) at a scan rate of 10 mV/s in a quartz reactor under un-chopped or chopped illumination on an area of 1 7



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cm2. All the PEC performances were conducted by front illumination. Amperometric I–t curves were recorded at a constant bias of 1.6 V vs. RHE. The incident photon-to-current conversion efficiency (IPCE) was measured at the applied bias of 1.6 V vs. RHE using a 300 W xenon lamp coupled with an aligned monochromator in the range of 400700 nm. IPCE can be defined as a function of wavelength by the equation (1) as follows: (1) where I is the photocurrent density,  represents the incident light wavelength, and Jlight is the measured power density of incident light.33 Electrochemical impedance spectroscopy (EIS) was carried out in potentiostatic mode with the AC amplitude of 10 mV in the frequency range of 0.1 Hz1 MHz at 1.6 V vs. RHE under AM1.5G illumination. The surface charge transfer efficiency (ηsurf), which is the proportion of the holes that injected into the electrolyte to oxidize the water among those that have reached the interface of electrode/electrolyte.34 RESULTS AND DISCUSSION The morphology of the obtained photoanodes was first observed by SEM and TEM. Figures 1a and b are the typical SEM images of -Fe2O3 and Ti:-Fe2O3 photoanodes, both showing the aligned nanorod-like structure with the average diameter of 50 nm (insets in the Figures 1a and b). The length of Ti:-Fe2O3 nanorods was smaller than that of -Fe2O3, whereas the radical width of Ti:-Fe2O3 was larger than that of -Fe2O3. The Ti:-Fe2O3 nanorod arrays tended to evolve to nanosheet arrays due to the preferential crystal growth of Ti:-Fe2O3, which will be discussed below in along with the XRD patterns. High-resolution TEM (HRTEM) images (see Figures 1c and d) show the clear lattice fringes and smooth surfaces of -Fe2O3 and Ti:-Fe2O3, indicating their fine crystallinity. While no obvious change was observed in the SEM images of photoanodes after KOH treatment (Figure S1, SI), a thin conformal OH-grafted overlayer on the surface of -Fe2O3 8


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and Ti:-Fe2O3 photoanodes after KOH treatment could be clearly seen in the HRTEM images (see Figures 1e and f). The thickness of overlayer was measured to be 2 nm, which was suitable for both hole-collection and light transmission. Furthermore, we performed the EDX elemental mapping of Ti:-Fe2O3-OH on a spherical aberration corrected TEM. The bright field HRTEM image showed clear lattice fringes of Ti:-Fe2O3 and amorphous OH-grafted overlayer. In the element mapping images of the suspected OH-grafted overlayer, we only observed the signals of large amount of O and small amount of K, the latter of which might be adsorbed on the OH-grafted overlayer surface; whereas, alomost no signals of Fe and Ti were observed (Figure S2, SI). Considering that the element H was not able to detect by EDX, we could thus reasonably verify the formation of OH-grafted overlayer.

Figure 1. SEM images of (a) -Fe2O3 and (b) Ti:-Fe2O3; HRTEM images of (c) -Fe2O3, (d) Ti:-Fe2O3, (e), -Fe2O3-OH and (f) Ti:-Fe2O3-OH photoanodes.

Figure 2a shows the XRD patterns of the hematite films before and after KOH treatment. The hematite peaks are labelled with solid round dot, and marked the corresponding crystal face, the solid rhombus represents SnO2 from FTO substrate. Compared with the pristine -Fe2O3, the Ti:-Fe2O3 sample show preferential growth of some facets, including {012}, {104}, and {113}, 9



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tending to evolve to nanosheet arrays as described in the SEM image (Figure 1d). No other impurity phase was detected in Ti:-Fe2O3. For the -Fe2O3-OH photoanodes, no new phase was formed and no change in the crystal structure was observed. The optical absorption spectra of the hematite films before and after alkali treatment are shown in Figure 2b. We can see that the Ti-doping led to a slight red-shift of the absorption edge from 580 nm (2.19 eV, -Fe2O3) to 600 nm (2.15 eV, Ti:-Fe2O3) (Figure S3, SI). Moreover, an obvious decrease of the absorbance was observed from 650 to 800 nm, probably due to the surface state change by Ti-doping. The alkali-treatment did not change the main absorption edges of -Fe2O3 and Ti:-Fe2O3 essentially.

Figure 2. (a) XRD patterns ( -Fe2O3,  FTO) and (b) UVVis spectra of -Fe2O3 and Ti:-Fe2O3 photoanodes before and after KOH treatment; XPS spectra of O1s for (c) -Fe2O3 and (d) Ti:-Fe2O3 photoanodes before and after KOH treatment.

The XPS spectra of O1s for -Fe2O3 and Ti:-Fe2O3 before and after KOH treatment were measured to investigate the variation of hydroxyl group content. As shown in Figure 2c and d, the O1s peak could be fitted into two component peaks at 529.8 eV and 531.9 eV, which were attributed to the O2 species from FeOFe bond and surface labile oxygen from FeOH bond, 10


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respectively. It is interesting to note the slight shift of the O1s peak at 529.8 eV to lower binding energy and the increased intensity of the peak at 531.9 eV for both -Fe2O3-OH and Ti:-Fe2O3-OH, which could be ascribed to the change in the electron cloud density of O1s caused by chemical surrounding variation from the hydroxy group grafted on hematite surface. In addition, the high-resolution XPS survey spectra of -Fe2O3 and Ti:-Fe2O3 photoanodes confirmed the doping of Ti into -Fe2O3. We need to note that while the EDX element mapping images of -Fe2O3-OH showed the signal of small amount of K (Figure S2, SI), no obvious K2p signal (292.4 eV and 295.2 eV) was detected in the XPS survey spectra (Figure S5, SI). We think that the signal of K observed in the STEM-EDX mapping (Figure S2 SI) might be from the adsorbed K on the sample surface and highly likely, K was not doped into α-Fe2O3 and Ti: -Fe2O3 because of the low annealing temperature (400 C). FT-IR spectra of -Fe2O3 and Ti:-Fe2O3 before and after KOH treatment were also measured to confirm the grafting of hydroxyl groups over the samples (Figure S4, SI). Five extra bands at 673, 845, 1374, 1450, and 1634 cm1, which could be assigned to FeO vibrational mode from FeOOH appeared in the KOH-treated -Fe2O3, while the intensities of characteristic FeO bands at 453 and 533 cm1 from α-Fe2O3 remained unchanged.58,

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Similar change could also be observed in

Ti:-Fe2O3 before and after KOH treatment. PEC performances of hematite photoanodes doped with different amounts of Ti showed that with increasing the addition of Ti precursor, the photocurrent density increased and reached a maximum when 1 wt% of Ti precursor was added (Figure S6, SI). So, the doping content of Ti was 1 wt% for the Ti:-Fe2O3 samples in this work. The LSV curves of KOH-treated Ti-Fe2O3 photoanodes annealed at different temperatures were also measured (Figure S7, SI), giving rise to the optimal temperature 400 C, which was adopted hereinafter for the annealing treatment of Ti:-Fe2O3. The effect of KOH treatment on the PEC performances of -Fe2O3 and Ti:-Fe2O3 photoanodes was investigated by measuring their LSV curves. Figures 3a and b display the LSV 11



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curves of -Fe2O3 and Ti:-Fe2O3 photoanodes before and after KOH treatment under AM1.5G irradiation in 1M NaOH electrolyte. The pristine -Fe2O3 and Ti:-Fe2O3 photoanodes without KOH treatment exhibited a photocurrent density of 0.42 mAcm2 and 0.93 mAcm2 at 1.6 V vs. RHE, respectively, which were comparable to previously reported results.35 While after KOH treatment, the photocurrent density was dramatically increased to 1.20 mAcm2 and 2.14 mAcm2 for -Fe2O3-OH and Ti:-Fe2O3-OH photoanodes, respectively. The chopped I-t curves of -Fe2O3 and Ti:-Fe2O3 photoanodes with and without KOH treatment were measured at 1.6 V vs RHE, showing obviously a fast “photo-switching” response with the following order in terms of photocurrent: Ti:-Fe2O3-OH-Fe2O3-OHTi:-Fe2O3-Fe2O3 (Figure S8, SI). The LSV curves of planar films were also measured, and the results demonstrated the superiority of one-dimensional nanorod arrays of hematite (Figure S9, SI). Shown in Figures 3c and d are the current densities of as-prepared photoanodes measured in dark. As expected, the KOH-treated photoanodes exhibited enhanced current density since the OH-grafted overlayer accelerated the water oxidation kinetics. Interestingly, the LSV curves measured in dark demonstrated different electrochemical behaviors for -Fe2O3 and Ti:-Fe2O3. Although both -Fe2O3 and Ti:-Fe2O3 photoanodes showed increased current density for water oxidation after KOH treatment, only Ti:-Fe2O3 exhibited an obvious cathodic shift of onset potential. The performance of KCl treated photoanodes was also measured, and the results showed a slight increase of photocurrent density, which might be due to the increased conductivity by the possible K doping. Nonetheless, the photocurrent density of KCl-treated photoanodes was much lower than that of the photoanodes treated by KOH (Figure S10, SI). The above results revealed that it was mainly the hydroxy group rather than K ion that played a role in the increased PEC performance of KOH-treated photoanodes. The LSV curves of -Fe2O3 and Ti:-Fe2O3 photoanodes before and after KOH treatment were measured in a neutral electrolyte (1 M Na2SO4 solution), showing similar tendency to that obtained in alkali electrolyte (Figure S11, SI). But all 12


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the photoanodes showed lower photocurrent density and anodic shift of onset potential than that obtained in alkali electrolyte, being constent with previous reports.60, 61

Figure 3. LSV curves of (a) -Fe2O3 and -Fe2O3-OH; (b) Ti:-Fe2O3 and Ti:-Fe2O3-OH under AM1.5G irradiation (solution: 1 M NaOH, scan rate: 10 mV/s). LSV curves of (c) -Fe2O3 and -Fe2O3-OH; (d) and Ti:-Fe2O3 and Ti:-Fe2O3-OH in dark (solution: 1 M NaOH, scan rate: 10 mV/s). (e) It curves and (f) IPCE plots measured at 1.6V vs RHE for -Fe2O3 and Ti:-Fe2O3 photoanodes before and after KOH treatment. The incident light wavelength range for the IPCE measurement in (f) was from 400 to 700 nm.

The photostability was also tested at 1.6 V vs. RHE in 1 M NaOH electrolyte under light 13



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illumination. As shown in Figure 3e, no obvious decay of photocurrent density was observed in a run of 60 min, indicating the excellent photostability of Ti:-Fe2O3-OH photoanode. This should be particularly underlined because most previously reported surface-treatment methods suffered performance degradation, more or less, in the long-term measurement.6266 Using various monochromatic lights, the wavelength-dependent IPCEs were measured at 1.6 V vs RHE in 1 M NaOH electrolyte. As shown in Figure 3f, the IPCE values of -Fe2O3 and Ti:-Fe2O3 at 400 nm were 17% and 27%, and became zero at about 600 nm and 620 nm, respectively, being consistent with their absorption spectra. Compared with -Fe2O3, the Ti:-Fe2O3 showed apparently increased IPCE value in the whole absorption region, because the conductivity of Ti:-Fe2O3 was increased and hence the bulk charge carrier recombination was suppressed. Moreover, owing to the enhanced water oxidation kinetics, the KOH-treated photoanodes exhibited much higher IPCE values than the pristine ones in the whole absorption region, especially from 400 to 500 nm.

Figure 4. MottSchottky plots of (a) -Fe2O3 and -Fe2O3-OH; (b) Ti:-Fe2O3 and Ti:-Fe2O3-OH. 14


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XPS-VB spectra of (c) -Fe2O3 and -Fe2O3-OH; (d) i:-Fe2O3 and Ti:-Fe2O3-OH.

As the band edge potentials of a semiconductor are closely pH-value-dependent,56 it is reasonable to expect a band structure change on the hematite surface by alkali treatment. Using the flat band potential (Efb) and the valence band top (Ev) position measured by electrochemical technique and XPSVB spectra, we obtained the Mott-Schottky plots of -Fe2O3 and Ti:-Fe2O3 electrodes before and after KOH treatment (see Figures 4a and b). The positive slopes of the MottSchottky plots indicated the nature of -Fe2O3 as an intrinsic n-type semiconductor with electrons being the majority carrier, and Ti-doping could increase its donor density.67 The flat band potential could be determined from the extrapolated x-intercept of the linear relationship of 1/C2 and applied potential based on the Mott-Schottky equation.68 In addition, previous reports demonstrated that the Fermi level was close to the flat band potential of a semiconductor in equilibrium with a redox couple.67 Therefore, the Fermi level of hematite films could be estimated according to the flat band potential calculated from Mott-Schottky equation. As shown in the Mott-Schottky plots, the Fermi levels of both -Fe2O3 and Ti:-Fe2O3 shifted anodically after KOH treatment, because the surface Fermi level of the acceptor-like OH-rich hematite surface was pinned near the VB edge. A similar phenomenon was also observed in KOH-treated GaN.69 Besides, the remarkably decreased slopes of the Mott-Schottky plots were observed in Ti:-Fe2O3, indicating an increased charge carrier density owing to the inverse proportion of plots and charge carrier density. Moreover, the plot slopes of KOH-treated -Fe2O3 and Ti:-Fe2O3 both decreased, suggesting that KOH treatment could also help increase the charge carrier density of hematite. Figures 4c and d represent the XPSVB spectra of -Fe2O3 and Ti:-Fe2O3 with and without alkali treatment. The VB top positions could be calculated as 1.17 eV and 1.39 eV for -Fe2O3 before and after KOH treatment, respectively. Considering the anodic shift of Fermi level, the VB top position of -Fe2O3-OH photoanode was calculated to move to a lower potential. The similar change of band edge was also observed in Ti:-Fe2O3. The CB potential (Ecb) could be calculated 15



Page 16 of 31

by the equation: Ecb = Evb  Eg, where Eg could be estimated from the Mubelka-Munk plots (Figure S3, SI). From the calculated flat band potentials, band gaps, VB and CB potentials (Table S1, SI), we schematically illustrated the band structures of -Fe2O3 and Ti:-Fe2O3 photoanodes before and after KOH treatment (Figure S8, SI), showing clearly a positive shift of band edges for the KOH-treated hematite irrespective of doping or not. Therefore, a type II heterojunction would be formed in situ in the surface of hematite photoanodes, accelerating the separation and transfer of photogenerated charge carriers and hence improving the PEC performances. In addition, the anodic shift of onset potential for Ti:-Fe2O3 might be due to the positive shift of CB position of Ti:-Fe2O3 in comparison with -Fe2O3. EIS measurement was performed to investigate the conductivity and interfacial charge transfer process. As shown in Figures 5a and b, the semicircles for KOH treated samples were much smaller than the pristine ones, indicating the important role of KOH treatment in increasing the electrical conductivity. The obtained EIS data was further fitted to an equivalent circuit containing one resistor (Rs) and two RC circuits (Rsc, Rct, Csc and CH). The parameter Rs represents a series of resistances including the substrate resistance, the electrolyte resistance and the external contact resistance, while the two elemental RC circuits can be assigned to the charge transfer in the bulk (Rsc and Csc) and on the surface (Rct and CH) of hematite, respectively. The calculated electrochemical parameters are listed in the Table 1. The fitted results showed that the charge transfer resistances in the bulk and surface decreased dramatically after KOH treatment for both -Fe2O3 and Ti:-Fe2O3 photoanodes, indicating a more efficient separation of photogenerated electrons and holes owing to the in situ formed type II heterojunction on the hematite surface.

16


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Figure 5. EIS spectra of (a) -Fe2O3 and (b) Ti:-Fe2O3 photoanodes before and after KOH treatment under AM 1.5G in 1 M NaOH electrolyte at 1.6 V vs RHE. The insets in (a) and (b) are the corresponding equivalent circuits, respectively. The calculated surface charge separation efficiencies of (c) -Fe2O3 and (d) Ti:-Fe2O3 photoanodes before and after KOH treatment by comparing the photocurrent densities for water oxidation and oxydol oxidation.

Table 1. The calculated electrochemical parameters of as-prepared photoanodes. Samples

Rs

Rsc

Csc

Rct

CH

-Fe2O3

171.4

1356

2.82E-6

4019

1.26E-5

-Fe2O3-OH

34.5

56.61

5.10E-9

1025

1.1E-5

Ti:-Fe2O3

69.89

33.33

4.83E-6

1113

2.05E-5

Ti:-Fe2O3-OH

42.53

31.52

1.78E-6

189.6

5.37E-5

The surface charge separation efficiencies (surf) were also calculated by comparing the photocurrent densities of water (H2O) and oxydol (H2O2) oxidation. The photocurrent density of water oxidation (

,31 where Jabs is the photocurrent density

) can be described as

integrated with photon absorption rate, ηbulk presents the bulk charge separation efficiency. The 17



Page 18 of 31

surface charge separation efficiency of H2O2 oxidation is typically considered as 100% as the surface recombination for H2O2 oxidation is negligible.70 Accordingly, the photocurrent density for H2O2 oxidation (

) can be calculated by the equation:

. The bulk charge separation

efficiency (ηbulk) for water oxidation is equal to that for H2O2 oxidation,71 therefore, the surface charge separation efficiency can be described as

. From the JV curves of -Fe2O3

and Ti:-Fe2O3 before and after KOH treatment for H2O oxidation and H2O2 oxidation (Figure S9, SI), we obtained greatly increased surface charge separation efficiencies of both -Fe2O3 and Ti:-Fe2O3 photoanodes after KOH treatment (Figures 5c and d).

Figure 6. Schematic mechanism for charger carrier separation on the surface of as-prepared -Fe2O3 photoanodes (a) before and (b) after alkali treatment.

Based on the above results and discussion, the mechanism of charge carrier separation/transfer and subsequent PEC water splitting on alkali-treated -Fe2O3 was proposed in Figure. 6. Due to the low conductivity and sluggish water oxidation kinetics, the pristine -Fe2O3 photoanode shows a poor PEC performance. The alkali treatment creates an OH-grafted conformal overlayer on the surface of -Fe2O3 photoanode, accelerating the water oxidation kinetics greatly. Moreover, the OH-grafted conformal overlayer on the surface of -Fe2O3 constructs a type II heterojunction, which is beneficial for substantially suppressing the charge carrier combination. The increased water oxidation kinetics and surface charge carrier separation accounts for the enhanced PEC performance. 18


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CONCLUSIONS In summary, a novel alkali treatment method was developed to improve the PEC performance of -Fe2O3 and Ti:-Fe2O3 photoanodes for water oxidation. Significantly improved PEC performance has been achieved for KOH treated -Fe2O3 and Ti:-Fe2O3 photoanodes with photocurrent densities of 1.2 mA/cm2 and 2.1 mA/cm2 at 1.6 V vs RHE under AM1.5G illumination, which were 3- and 2-times higher than the pristine ones without KOH treatment. The enhanced PEC performance could be attributed to accelerated water oxidation kinetics and improved charge separation efficiency, which were benefited from the in situ formed type II heterojunction by the thin conformal OH-grafted overlayer on the surface of hematite nanorods. Our work demonstrates that alkali treatment is a promising strategy to construct efficient metal oxide photoanodes for PEC water splitting. ASSOCIATED CONTENTS Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: SEM and TEM images of -Fe2O3 and Ti:-Fe2O3 after KOH treatment; Mubelka-Munk plot, XPS spectra, and chopped I-t curves of -Fe2O3 and Ti:-Fe2O3 photoanodes before and after KOH treatment; LSV curves of Ti:-Fe2O3 with different Ti contents; LSV curves of -Fe2O3 treated by KOH at different temperatures; LSV curves of -Fe2O3 treated by KCl; schematic band structures of -Fe2O3 and Ti:-Fe2O3 photoanodes before and after KOH treatment. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (D. W.) 19



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*E-mail: [email protected] (J. Y.) ORCID Defa Wang: 0000-0001-7196-6898 Jinhua Ye: 0000-0002-8105-8903 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors thank Ms. Jinfeng Zhang and Ms. Huilin Hu for the help with STEM-HAADF and EDX measurements. Financial support from the National Natural Science Foundation of China (51572191, 21633004) is highly appreciated. REFERENCES

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28


in

Hematite

for

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TOC/Abstract Graphic

A type II heterojunction is in situ formed on the surface of alkali-treated α-Fe2O3 and Ti-dopedα-Fe2O3 photoanodes, showing enhanced water oxidation kinetics and charge separation efficiency for photoelectrochemical water oxidation with excellent stability.

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Alkali Treatment for Enhanced Photoelectrochemical Water Oxidation

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