Bi-functional Fe2ZrO5 modified hematite photoanode for efficient solar water splitting

Journal Pre-proof Bi-functional Fe2 ZrO5 modified hematite photoanode for efficient solar water splitting Tingting Jiao, Cheng Lu, Duo Zhang, Kun Feng, Shuao Wang, Zhenhui Kang (Resources)Writing - Review and Editing) (Supervision), Jun Zhong (Resources)Writing - Review and Editing) (Supervision)

PII:

S0926-3373(20)30183-1

DOI:

https://doi.org/10.1016/j.apcatb.2020.118768

Reference:

APCATB 118768

To appear in:

Applied Catalysis B: Environmental

Received Date:

29 December 2019

Revised Date:

10 February 2020

Accepted Date:

14 February 2020

Please cite this article as: Jiao T, Lu C, Zhang D, Feng K, Wang S, Kang Z, Zhong J, Bi-functional Fe2 ZrO5 modified hematite photoanode for efficient solar water splitting, Applied Catalysis B: Environmental (2020), doi: https://doi.org/10.1016/j.apcatb.2020.118768

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Bi-functional Fe2ZrO5 modified hematite photoanode for efficient solar water splitting

Tingting Jiao1, Cheng Lu1, Duo Zhang2*, Kun Feng1, Shuao Wang2, Zhenhui Kang1*

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and Jun Zhong1*

Institute of Functional Nano and Soft Materials Laboratory (FUNSOM), Jiangsu

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Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow

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University, Suzhou 215123, China

State Key Laboratory of Radiation Medicine and Protection, School of Radiation

*Address

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Medicine and Protection, Suzhou 215123, China

correspondence

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[email protected];

[email protected];

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[email protected]

Graphical abstract

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A Fe2ZrO5 layer was firstly decorated on hematite with both enhanced photocurrent

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and lowered onset potential up to 180 mV.

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Highlights

A Fe2ZrO5 layer was firstly decorated on hematite by using Zr-MOFs as precursor

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The layer can finally improve the photocurrent to 2.88 mA cm-2 at 1.23 V vs. RHE

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The layer also leads to a large cathodic shift of the onset potential up to 180 mV

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The layer is a bi-functional material as a passivation layer and the Zr-source for

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doping

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

Abstract

Surface modification on hematite is widely used to improve its photoelectrochemical (PEC) performance for water splitting. Here by using Zr-based metal-organic frameworks as precursor, a thin Fe2ZrO5 layer has been created on hematite surface, 2

which can both significantly improve the photocurrent and reduce the onset potential with a prominent value of 180 mV. Moreover, by coupling with Ti-based modification and Co-Pi cocatalyst, the hematite photoanode decorated with Fe2ZrO5 layer can achieve an excellent photocurrent of 2.88 mA cm-2 at 1.23 V vs. RHE, which is more than 3 times higher than that of the original hematite. To the best of our knowledge, it is the first report for the decoration of Fe2ZrO5 on hematite to improve

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the water splitting efficiency. The Fe2ZrO5 layer acts as a bi-functional material to enhance the PEC performance, which can form a perfect passivation layer to suppress

the charge recombination and provide Zr for Zr-doping in hematite to highly increase

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the carrier density.

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Keywords: Hematite; Fe2ZrO5; Zr-doping; low onset potential; solar water splitting.

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1. Introduction Nowadays, over 80% of the world’s energy requirements are supplied by fossil fuels and it will result in predictable depletion of limited fossil fuels with serious environmental problems [1]. It is thus a general trend to find renewable energy sources that can replace fossil fuels. In recent years, as a clean, cheap and sustainable

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energy source, solar energy has attracted wide attention. Solar energy can be converted into electricity and can also provide chemical energy stored in fuels. If solar energy can be collected effectively, it will provide adequate power for all future

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energy demands [2]. Photoelectrochemical (PEC) cells can effectively convert solar

energy to chemical energy by the water splitting [3,4]. Hematite is an ideal candidate

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catalyst for PEC water oxidation with the earth-abundance, stability, low cost and

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suitable band gap (2.1-2.2 eV) [4-8]. According to the calculation, hematite can exhibit an excellent PEC photocurrent up to 12.6 mA cm-2 and a high

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solar-to-hydrogen efficiency (16.8%) [4-8]. However, by now the reported hematite performance was much lower than the ideal one due to several factors such as short

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hole life time, low conductivity and slow oxygen evolution reaction (OER) kinetics

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[4-8]. Nevertheless, strong recombination of photo-induced holes and electrons seriously inhibits the properties of Fe2O3 for water oxidation.

Various methods have been developed, which aim to tackle the aforementioned

drawbacks, including surface modification, elemental doping, morphology control, and surface co-catalysts [4-14]. For example, surface passivation layers such as 4

amorphous FeOOH, or Al2O3 were applied on hematite to effectively decrease the electron-hole recombination [9,12-15]. Ti-based treatments such as Ti-doping or forming Fe2TiO5 in hematite were also widely reported to enhance the performance [6,7,12]. Unfortunately, Ti-based treatments typically increase the photocurrent but lead to a worse onset potential [6,7,12]. In the periodic table Zr and Ti are in the same column and both of them have a high valence state of 4+. Thus Zr can also be

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expected to show excellent properties to enhance the performance of hematite. Although Zr-based treatments with ZrO2 or Zr-doping in hematite were also reported

[8,16,17], compared to the tremendous reports for Ti-treatments in the literatures,

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Zr-based modifications on hematite were still very limited and the achieved

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performance was much lower than that for Ti-based treatments [8,16,17].

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Here we report a facile hydrothermal method to prepare Zr-based passivation layer on hematite by using Zr-based metal-organic frameworks (MOFs) (UiO-66-(COOH)2)

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as the precursor. MOFs are emerging porous inorganic-organic hybrid materials with organic ligands and metal clusters [18-21]. In this work, the MOF-derived thin layer

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on hematite has been proved to be Fe2ZrO5 based on the synchrotron radiation X-ray

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absorption spectroscopy (XAS). Although Zr-based composites such as ZrO2 have been reported to modify hematite in the literatures, to the best of our knowledge, it should be the first report of Fe2ZrO5 as an effective material for the modification of hematite [8,16,17]. Different from Ti-based treatments [6,7,12], the Fe2ZrO5 as an effective passivation layer can both enhance the photocurrent and significantly reduce the onset potential with suppressed surface charge recombination. Moreover, a 5

diffusion of Zr from the surface Fe2ZrO5 layer to the bulk hematite may also occur, which can obviously improve the carrier density of hematite to increase the performance. Combining with the effects of surface passivation and Zr-diffusion, the MOF-derived Fe2ZrO5 could greatly improve the hematite performance with an obvious cathodic shift of the onset potential as large as 180 mV, and the photoanode could finally obtain an excellent photocurrent of 2.88 mA cm-2 at 1.23 V vs. RHE

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with the association of Ti-based modification and Co-Pi cocatalyst.

2. Experimental section

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2.1. Sample Preparation

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2.1.1.Synthesis of Fe2O3 photoanode and Ti-Fe2O3 photoanode

Fe2O3 and Ti-treated Fe2O3 (Ti-Fe2O3) photoanodes were prepared by a modified

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hydrothermal method reported in the literatures [10,22,23]. Briefly, FeCl3·6H2O was used as the precursor in hydrothermal reaction at 95 oC for 4 h, in which FeOOH was

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grown on the FTO substrate. After annealing at 550 oC for 2 h and at 750 oC for

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another 15 min in air, Fe2O3 was then prepared on FTO as the photoanode (see Supplementary Figure S1). For Ti-Fe2O3, similar processes were used but the FTO

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substrate was firstly treated by TiCl4 solution before the hydrothermal reaction (see Supplementary Figure S2) [23]. 2.1.2. Synthesis of UiO-66-(COOH)2 powder UiO-66-(COOH)2 powder was prepared along with the method reported in the literatures [24,25]. Briefly, 1,2,4,5-benzenetetracarboxylic acid (BTEC), ZrCl4, and 6

acetic acid were dissolved in DMF and heated to 120 oC. Then the product was separated, washed and dried as UiO-66-(COOH)2 powder. 2.1.3. Preparation of Fe2ZrO5-Fe2O3 photoanode and Fe2ZrO5-Ti-Fe2O3 photoanode The decoration of UiO-66(COOH)2 powder on Fe2O3 photoanode or Ti-Fe2O3 photoanode was conducted through a hydrothermal method. The photoanodes were placed in a 100 mL Teflon-lined autoclave reactor with 70 mL UiO-66-(COOH)2

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solution at different concentrations. The reactor was heated to 95 °C for 4 h and then

cooled down naturally. The obtained samples were washed and then dried at 50 °C for 10 min. Finally the samples were labeled as Fe2ZrO5-Fe2O3 and Fe2ZrO5-Ti-Fe2O3,

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respectively. A photoassisted electro-deposition was performed to decorate Cobalt

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phosphate (Co-Pi) catalyst on the photoanode [3,23].

2.2. Structural characterization and PEC measurement

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XAS experiments were conducted at the Shanghai Synchrotron Radiation Facility

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(SSRF, 14W and 08U), the Taiwan Light Source (TLS, 17C1) and the National Synchrotron Radiation Laboratory (NSRL, XMCD beamline). The photoanodes were

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tested by using a typical way as shown in our previous work [7,16,23]. An electrochemical workstation (CHI 660D), a Xenon High Brightness Cold Light (XD-300)

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Source

with

an

AM

1.5

filter

and

a

Xenon

lamp

(CEL-HXF300/CEL-HXBF300) with a monochromator (Omni-λ3005) were used. 3. Results and discussion 3.1. Characterization of catalysts

7

Zr-MOFs have been added in a hydrothermal method to modify the surface of hematite. The detailed treatments for Fe2O3 and Ti-Fe2O3 can be found in the Experimental Section or in the Supplementary Figure S1 and S2, respectively. The treated samples are labeled as Fe2ZrO5-Fe2O3 and Fe2ZrO5-Ti-Fe2O3, respectively. SEM image and high resolution TEM (HRTEM) image of Fe2ZrO5-Fe2O3 are shown in Figure1. From the SEM image (Figure 1b), the Fe2ZrO5-Fe2O3 sample exhibits a nanorod

morphology,

similar

to

the

sample

before

treatment

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worm-like

(Supplementary Figure S3) [23]. XRD data in Figure 1a also reveals that after Zr-treatment the Fe2ZrO5-Fe2O3 sample can still maintain the hematite structure

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(JCPDS 33-0664) without any new XRD features [23]. However, HRTEM images in

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Figure 1d and 1e clearly reveal the different morphology between the hematite photoanodes before and after the Zr-treatment. Clear crystal structure can be observed

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for Fe2O3 while for Fe2ZrO5-Fe2O3, a thin coating layer can also be observed on the

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smooth surface of Fe2O3 (Figure 1e). The HRTEM image clearly confirms the presence of a coating layer on hematite after the treatment by using Zr-MOFs. The

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layer comes from Zr-treatment and the average layer thickness is about 3 nm. The elemental mappings of the Fe2ZrO5-Fe2O3 sample are also shown in Figure 1c,

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revealing that Zr element is successfully incorporated in the sample. The XRD data and TEM image of the Zr-MOFs have been shown in the Supplementary Figure S4. No precursor contamination can be found in the MOF-treated sample according to the XRD spectra in Figure S4. The results suggest that the Zr-based layer on hematite could be some MOF derivatives after the hydrothermal treatment. 8

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Figure 1. (a) XRD spectra of FTO, Fe2O3, and Fe2ZrO5-Fe2O3. (b) SEM image and (c)

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Dark field TEM image and the corresponding elemental mappings of Fe2ZrO5-Fe2O3.

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(d) and (e): HRTEM images of Fe2O3 (d) and Fe2ZrO5-Fe2O3 (e).

XPS and XAS are performed to characterize the exact contents of the coating layer.

XPS survey scans of the pristine Fe2O3 and Fe2ZrO5-Fe2O3 samples have been shown in Figure S5a. Both samples display strong Fe and O signals. In the survey scans C and Sn peaks also exist due to the carbon contamination and the Sn diffusion from 9

FTO [10]. High-resolution Zr 3d XPS spectrum of Fe2ZrO5-Fe2O3 is shown in Figure S5c, which confirms the existence of Zr. However, the XPS signal for Zr is too weak to clearly identify its chemical composition. To clearly elucidate the electronic structure of Zr-based material in the MOF-treated sample, Zr K-edge XAS spectrum of Fe2ZrO5-Fe2O3 with a comparison to the reference samples of ZrO2 and Zr-MOFs (UiO-66-(COOH)2) is shown in Figure 2a.

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XAS is very sensitive to the chemical environment. Here the XAS spectrum for ZrO2

shows a sharp white line peak A around 18022 eV with a broad peak C, in good agreement with the literatures [26-28]. The Zr-MOFs (UiO-66-(COOH)2) as the

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precursor also shows similar XAS spectrum as that of ZrO2, indicating a similar

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chemical environment. However, after the MOF-treatment, the Zr composition in Fe2ZrO5-Fe2O3 is obviously different from the precursor and the ZrO2 reference. The

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intensity of peak A for Fe2ZrO5-Fe2O3 significantly decreases compared to that of ZrO2. Moreover, a strong new peak B at around 18030 eV can also be observed with a

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slight shift of peak C, strongly indicating the formation of ZrO32- according to the

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literatures [26-28]. Although both ZrO2 and ZrO32- show a valence state of Zr4+, the local chemical environments of Zr can be significantly different and can be clearly

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detected by XAS as reported in the literatures [26-28]. Thus after the hydrothermal treatment by Zr-MOFs, here the Zr-based thin layer on hematite is Fe2ZrO5 instead of ZrO2. To the best of our knowledge, it is the first report of a Fe2ZrO5 coating layer on hematite to enhance the PEC performance. Fe L-edge XAS (Figure 2b) and XPS (Supplementary Figure S5b) spectra of Fe2O3 and Fe2ZrO5-Fe2O3 are also shown, 10

which exhibit almost no difference due to the similar Fe environments. The Extended X-ray

Absorption

Fine

Structure

(EXAFS)

data

of

ZrO2,

Zr-MOFs

(UiO-66-(COOH)2) and Fe2ZrO5-Fe2O3 are also shown in Figure S6, in which

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Fe2ZrO5-Fe2O3 exhibits clear Zr-O bonds confirming the Fe2ZrO5 structure.

Figure 2. (a) XAS spectra of ZrO2, UiO-66-(COOH)2 and Fe2ZrO5-Fe2O3 at Zr

3.2. PEC performance

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K-edge. (b) XAS spectra of Fe2O3 and Fe2ZrO5-Fe2O3 at Fe L-edge.

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Figure 3 exhibits the PEC performance of Fe2O3 and Fe2ZrO5-Fe2O3. Figure 3a exhibits the photocurrent density versus applied potential (J–V) plots of Fe2O3 and

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Fe2ZrO5-Fe2O3. The pristine Fe2O3 shows a photocurrent of 0.85 mA cm-2 at 1.23 V

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vs. RHE [23,29]. However, with the deposition of Fe2ZrO5 on Fe2O3, the sample shows a prominently enhanced performance with the photocurrent of 1.65 mA cm-2 at 1.23 V vs. RHE. It is about two times of the value for pristine Fe2O3 photoanode. Furthermore, the onset potential shows a large cathodic shift with a value up to 180 mV (from 0.95 to 0.77 V as shown in Figure 3a). Zr and Ti are the same column elements in the periodic table. Ti-based modification on hematite with the formation 11

of Fe2TiO5 was widely reported, which could facilitate the charge separation and then enhance the photocurrent [23,29]. However, for the Ti-treated hematite, the onset potential is typically worse than the pristine sample as reported in the literatures [23,29]. Interestingly, here we find that the Fe2ZrO5 layer can both improve the photocurrent and obviously reduce the onset potential with a value up to 180 mV, which might be a better choice than Ti for the modification of hematite in the future.

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The performance of Fe2ZrO5-Fe2O3 has been optimized by controlling the concentrations of UiO-66-(COOH)2 solutions and the temperatures, which are shown

in the Supplementary Figure S7 with the best condition of 0.2 g L-1 UiO-66-(COOH)2

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at 95 oC for 4 h. ZrCl4 was also used instead of Zr-MOFs to modify the Fe2O3

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photoanode (labeled as Zr-Fe2O3) but its performance is much lower than the MOF-treated sample (Supplementary Figure S8).

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Figure 3b exhibits the UV-visible absorption curves of the pristine Fe2O3 and

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Fe2ZrO5-Fe2O3, which are very similar to each other indicating no significant changes of the light adsorption capability. The incident photon-to-current conversion

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efficiencies (IPCE) values for the different electrodes at 1.23 V vs. RHE are illustrated in Figure 3c. The Fe2ZrO5-Fe2O3 photoanode shows a high IPCE value

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over the whole range of 350-650 nm, which is consistent with the J-V scans. In particular, the IPCE value for Fe2ZrO5-Fe2O3 at 370 nm reaches 31.5%, which is 1.7 times higher than that of the pristine Fe2O3 (18.7%). For both samples the IPCE values are very low in the long-wavelength range [30,31]. Typically, the photon with higher energy has better charge separation efficiency. Then it will show higher 12

quantum efficiency at the short-wavelength range. The absorption spectra also show an absorption peak around 370 nm, thus the IPCE at 370 nm shows the highest IPCE value. Both the IPCE and J-V curves suggest that after the deposition of Fe2ZrO5 thin layer on hematite, the PEC performance can be significantly enhanced. The photochemical stability curve of Fe2ZrO5-Fe2O3 is also shown in Figure 3d. The photoanode keeps its initial photocurrent density without obvious decay during 3 h,

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suggesting good stability.

Figure 3. (a) J-V curves, (b) UV-visible absorption spectra and (c) IPCE spectra of Fe2O3 and Fe2ZrO5-Fe2O3. (d) Photochemical stability curves of Fe2ZrO5-Fe2O3 measured at 1.23 V vs. RHE.

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3.3. Research on catalytic mechanism The catalytic mechanism of Fe2ZrO5 layer to enhance the PEC performance has been carefully explored. Typically, a thin passivation layer can suppress the electron-hole recombination and then facilitate the surface OER reaction [14,29]. The charge separation efficiency at the solid-liquid interface can thus be significantly enhanced. To verify this point, we measured the J-V scans of different hematite

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samples by using H2O2 (0.5 M) as a hole scavenger [14,32]. By comparing to the curve with H2O2 as a reference of 100 % efficiency, the surface charge separation

efficiency (ηsurf) can thus be calculated [14,32]. It can be illustrated by the formula:

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ηsurf = JH2O/JH2O2 (J is the photocurrent) [14]. Figure 4a and 4b compare the J-V scans

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of the pristine Fe2O3 and Fe2ZrO5-Fe2O3 with (blue) and without (red) H2O2. Figure 4c presents the corresponding surface charge separation efficiencies. It is clear that

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Fe2ZrO5-Fe2O3 shows highly improved surface charge separation efficiency when

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compared to that for Fe2O3, confirming that the MOF-derived Fe2ZrO5 can act as an effective surface passivation layer to enhance the eletron-hole separation efficiency,

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and then reduce the onset potential for better PEC performance.

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Figure 4. J-V curves with (blue) and without (red) H2O2 (0.5 M) for Fe2O3 (a) and

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Fe2ZrO5-Fe2O3 (b). Charge separation efficiencies (ηsurf) (c) and Mott-Schottky plots

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(d) of Fe2O3 and Fe2ZrO5-Fe2O3.

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The carrier densities are also measured according to the slopes of the Mott-Schottky plots in Figure 4d. The donor density Nd can be calculated by the following formula

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[7]:

Nd = (2/e0εε0)[d(1/C2)/dV]-1

The Mott-Schottky slopes of Fe2O3 and Fe2ZrO5-Fe2O3 are positive confirming the n-type semiconductors. The calculated carrier densities of Fe2O3 and Fe2ZrO5-Fe2O3 are 7.01×1019 cm-3 and 6.70×1020 cm-3, respectively. The value for Fe2ZrO5-Fe2O3 is 15

around one order higher than that of Fe2O3, suggesting a great enhancement which could be attributed to the Zr-doping in hematite. In the synthesis process the Fe2ZrO5 layer can also act as a Zr source for the Zr-diffusion in hematite. Then the bulk conductivity of hematite can be significantly enhanced to facilitate the OER reaction. The bulk charge separation efficiency of Fe2ZrO5-Fe2O3 is also shown in Figure S9, which is obviously enhanced when compared to that of Fe2O3, suggesting the

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Zr-doping effect.

To probe the relationship between Fe2ZrO5 layer and the surface charge transfer,

electrochemical impedance spectroscopy (EIS) result is presented in the

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Supplementary Figure S10. Figure S10a exhibits the Nyquist plots at 1.23 V vs. RHE

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and the corresponding equivalent circuit (EC) model for simulation is shown in the inset [29]. Figure S10b exhibits the fitting parameters of EC, in which Rct represents

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the charge transfer resistance between the excited hole and the electrolyte [11,33,34].

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A smaller Rct for Fe2ZrO5-Fe2O3 has been observed when compared to that for Fe2O3, which means faster charge transfer at the liquid-solid interface. The Ctrap is also larger

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after the decoration of Fe2ZrO5 layer, suggesting more active sites for OER with the Fe2ZrO5 modification [34]. In Figure S10a Fe2ZrO5-Fe2O3 also shows a smaller

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diameter of the semicircle than that for the pristine sample, indicating the improved interfacial charge transport with the Fe2ZrO5 layer [34]. The EIS results clearly confirm that the Fe2ZrO5 layer can significantly facilitate the interface charge transfer, which will lead to the enhanced PEC performance. Considering the effect of Fe2ZrO5 layer as a passivation layer and Zr-doping source 16

to both enhance the photocurrent and reduce the onset potential, the performance of hematite can be further enhanced by coupling the Fe2ZrO5 layer with various treatments such as Ti-treatment and Co-Pi cocatalysts. Ti-treatment has been widely applied to enhance the hematite photocurrent [23,29]. By a hydrothermal method as shown in the literature [23], Ti-treated hematite (Ti-Fe2O3) was prepared for further modification. Fe2ZrO5-Ti-Fe2O3 has been synthesized by using a similar method as

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that for Fe2ZrO5-Fe2O3. Supplementary Figure S11 presents the SEM image and TEM image of Fe2ZrO5-Ti-Fe2O3, which reveal the similar nanorod morphology and the

existence of a coating layer. The elemental mappings in Figure S11 obviously suggest

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the presence of Ti and Zr in hematite. XPS data of Ti-Fe2O3 and Fe2ZrO5-Ti-Fe2O3

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are exhibited in the Supplementary Figure S12. Ti 2p signals have been clearly observed in both samples (Figure S12c). Fe2ZrO5-Ti-Fe2O3 also shows a very weak Zr

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signal as that for Fe2ZrO5-Fe2O3 (Figure S12d). According to the XAS spectra at Ti

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L-edge in Figure S13, both Ti-Fe2O3 and Fe2ZrO5-Ti-Fe2O3 show a Ti structure similar to that of Fe2TiO5, indicating the formation of Fe2TiO5 in hematite as reported

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in the literatures [23,29]. Supplementary Figure S14 also shows the Zr K-edge XAS spectrum of Fe2ZrO5-Ti-Fe2O3 and the corresponding EXAFS data, further

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confirming the Fe2ZrO5 structure. The J-V curves of Ti-Fe2O3, Fe2ZrO5-Ti-Fe2O3 and Co-Pi-Fe2ZrO5-Ti-Fe2O3 are

presented in Figure 5a. According to the literatures, Fe2TiO5 can facilitate the charge separation and then enhance the photocurrent [23,29]. Thus in Figure 5a Ti-Fe2O3 shows an enhanced photocurrent at 1.23 V vs. RHE (1.63 mA cm-2 at 1.23 V vs. RHE) 17

compared to that of Fe2O3. However, its onset potential is even worse. The hematite sample after Ti-treatment can be well coupled with the Fe2ZrO5 layer. As a result, the Fe2ZrO5-Ti-Fe2O3 sample exhibits a much lower onset potential and an obviously enhanced photocurrent of 2.31 mA cm-2 at 1.23 V vs. RHE compared to the sample before Zr-treatment. The photocurrent is more than 2.5 times higher than that of the Fe2O3 photoanode and around 1.5 times higher than that of the Ti-Fe2O3 photoanode.

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Moreover, the performance has been further improved by depositing Co-Pi cocatalyst on the surface layer to accelerate the OER kinetics, which can finally achieve an

excellent photocurrent of 2.88 mA cm-2 at 1.23 V vs. RHE for the

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Co-Pi-Fe2ZrO5-Ti-Fe2O3 photoanode. The results suggest that the MOF-derived

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deposition of Fe2ZrO5 layer can significantly enhance the performance of hematite for

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PEC water oxidation.

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Figure 5. (a) J-V curves of Ti-Fe2O3, Fe2ZrO5-Ti-Fe2O3 and Co-Pi-Fe2ZrO5-Ti-Fe2O3.

photochemical

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(b) IPCE spectra (left and bottom) of Ti-Fe2O3 and Fe2ZrO5-Ti-Fe2O3 and stability curves

(right

and

top)

of

Fe2ZrO5-Ti-Fe2O3

and

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Co-Pi-Fe2ZrO5-Ti-Fe2O3. (c) Charge separation efficiencies (ηsurf) and (d)

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Mott-Schottky plots of Ti-Fe2O3 and Fe2ZrO5-Ti-Fe2O3.

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Figure 5b shows the IPCE values (left and bottom) and stability curves (right and top). Fe2ZrO5-Ti-Fe2O3 photoanode exhibits better IPCE value over the whole range when compared to that of Ti-Fe2O3. In particular, the IPCE for Fe2ZrO5-Ti-Fe2O3 at 370 nm achieves a high value of 51.4%, which is about 1.6 times higher than that of Ti-Fe2O3

(30.0%).

The

photocurrents

of

Fe2ZrO5-Ti-Fe2O3

and

Co-Pi-Fe2ZrO5-Ti-Fe2O3 are also very stable as shown in Figure 5b. The surface 19

charge separation efficiencies (ηsurf) of Ti-Fe2O3 and Fe2ZrO5-Ti-Fe2O3 are compared in Figure 5c (J-V curves with H2O2 are shown in the Supplementary Figure S15), which reveals the greatly enhanced efficiency for Fe2ZrO5-Ti-Fe2O3 with the Fe2ZrO5 layer. Bulk charge separation efficiencies (ηbulk) of Fe2O3, Ti-Fe2O3 and Fe2ZrO5-Ti-Fe2O3 are also shown in Supplementary Figure S16, in which Fe2ZrO5-Ti-Fe2O3 exhibits the highest efficiency (ηbulk) among the three samples.

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Mott-Schottky plots are also shown in Figure 5d to calculate the carrier densities [7], which are 1.26×1020 cm-3 for Ti-Fe2O3 and 2.65×1021 cm-3 for Fe2ZrO5-Ti-Fe2O3, respectively. The carrier density for Fe2ZrO5-Ti-Fe2O3 is also one order higher than

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that for the sample without Fe2ZrO5, clearly indicating the Zr-doping effect with the

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coating layer. Supplementary Figure S17 shows the EIS results for Fe2O3, Ti-Fe2O3 and Fe2ZrO5-Ti-Fe2O3. Ti-Fe2O3 shows a smaller semicircle diameter and Rct than

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that of Fe2O3, while Fe2ZrO5-Ti-Fe2O3 shows the smallest semicircle diameter and Rct.

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Since Ti-treatment can also improve the charge separation [32,33], the diameter and Rct value for Fe2ZrO5-Ti-Fe2O3 are even smaller than that for Fe2ZrO5-Fe2O3,

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suggesting a synergistic effect of Fe2ZrO5 modification and Ti-treatment to enhance

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the performance [33].

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Figure 6. Working mechanism illustration of Fe2ZrO5-Fe2O3.

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Figure 6 demonstrates the working mechanism of Fe2ZrO5 layer on hematite. A thin

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layer of Fe2ZrO5 has been created on the surface of hematite, which can act as an effective passivation layer to block the surface defects and then suppress the charge

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recombination. The layer can also accelerate the charge transport at the solid-liquid

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interface, which can enhance the OER efficiency. Moreover, the Fe2ZrO5 layer can act as the Zr source for Zr-doping in hematite through diffusion in the synthesis

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process, which can significantly enhance the donor density and improve the conductivity of hematite. Combining all the benefits, the Fe2ZrO5 layer can both

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improve the photocurrent and reduce the onset potential of hematite for efficient PEC water splitting, which could act as an effective modification method on hematite and might be coupled with other treatments to achieve the practical applications.

4. Conclusion 21

In summary, by using Zr-MOFs as the precursor we have firstly reported the deposition of Fe2ZrO5 layer on hematite to improve the PEC water splitting efficiency. The Fe2ZrO5 modified hematite shows an enhanced photocurrent around two times higher than that of the pristine sample, reaching to 1.65 mA cm-2 at 1.23 V vs. RHE. Especially, a large cathodic shift of the onset potential up to 180 mV can also be observed. By coupling with Ti-based treatment and Co-Pi cocatalyst, the hematite

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photoanode decorated with Fe2ZrO5 layer can finally obtain an excellent photocurrent

of 2.88 mA cm-2 at 1.23 V vs. RHE, which is about 3 times higher than that of the pristine Fe2O3. The Fe2ZrO5 layer can act as an effective surface passivation layer,

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which will passivate the surface defects and facilitate the charge separation to

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improve the performance. Moreover, the Fe2ZrO5 layer can also provide Zr for Zr-doping in hematite, which will significantly enhance the donor density and then

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improve the conductivity for better performance.

Credit Author Statement

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Tingting Jiao: Experiments, Measurement.

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Cheng Lu: Measurement.

Duo Zhang: Measurement. Kun Feng: Measurement. Shuao Wang: Measurement. 22

Zhenhui Kang: Resources, Writing - Review & Editing, Supervision. Jun Zhong: Resources, Writing - Review & Editing, Supervision.

Declaration of Interest Statement We would like to submit the enclosed manuscript, entitled “Bi-functional Fe2ZrO5 modified hematite photoanode for efficient solar water splitting” by T. T. Jiao et al. to Applied Catalysis B: Environmental for publishing consideration. With the submission of this manuscript I would like to undertake that the

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above-mentioned manuscript has not been published elsewhere, accepted for publication elsewhere or under editorial review for publication elsewhere.

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The authors declare no competing financial interest.

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ASSOCIATED CONTENT

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Supporting Information.

Experimental illustration, XRD, J-V scans for various hematite samples, TEM image

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of Zr-MOFs (UiO-66-(COOH)2). Acknowledgement

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We acknowledge the support from NSRL, SSRF, and TLS for the XAS experiments.

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We acknowledge the National Natural Science Foundation of China (U1932211, U1732110, 11805139, 51821002, 51725204, 21771132, 51972216), Natural Science Foundation of Jiangsu Province (BK20190041, BK20190828), and Key-Area Research and Development Program of GuangDong Province (2019B010933001). We acknowledge the support from Users with Excellence Program of Hefei Science Center CAS (2019HSC-UE002). This is also a project supported by the Collaborative 23

Innovation Center of Suzhou Nano Science & Technology, the Collaborative Innovation Center of Radiological Medicine of Jiangsu Higher Education Institutions, the Soochow University-Western University Centre for Synchrotron Radiation Research, the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and the Fund for Innovative Research Teams of Jiangsu Higher Education Institutions.

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