Balancing Catalytic Activity and Interface Energetics of Electrocatalyst

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Balancing Catalytic Activity and Interface Energetics of Electrocatalyst Coated Photoanodes for Photoelectrochemical Water Splitting Zhe Xu, Haoyu Wang, Yunzhou Wen, Wenchao Li, Chuyu Sun, Yuting He, Zhan Shi, Lang Pei, Yongda Chen, Shicheng Yan, and Zhigang Zou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17348 • Publication Date (Web): 08 Jan 2018 Downloaded from http://pubs.acs.org on January 8, 2018

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ACS Applied Materials & Interfaces

Balancing Catalytic Activity and Interface Energetics of Electrocatalyst Coated Photoanodes for Photoelectrochemical Water Splitting ‡

⊥

Zhe Xu,† Haoyu Wang, Yunzhou Wen,§ Wenchao Li,†Chuyu Sun,† Yuting He,† Zhan Shi,‡ Lang Pei,† Yongda Chen, Shicheng Yan, †,* and Zhigang Zou†,‡ †

National Laboratory of Solid State Microstructures, Collaborative Innovation Center of Advanced Microstructures, Eco-Materials and Renewable Energy Research Center (ERERC), College of Engineering and Applied Sciences, Nanjing University, Nanjing, Jiangsu 210093, P. R. China ‡ Jiangsu Province Key Laboratory for Nanotechnology, School of Physics, Nanjing University, Nanjing, Jiangsu 210093, P. R. China § State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Laboratory of Advanced Materials, Fudan University, Shanghai 200438, P. R. China ⊥ National Laboratory of Solid State Microstructures, Jiangsu Provincial Key Laboratory of Advanced Photonic and Electronic Materials, School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, P.R. China KEYWORDS: interface energetics, electrocatalytic activity, electrochemical deposition, temperature regulation, Fe2O3 photoanode

ABSTRACT: For photoelectrochemical (PEC) water splitting, the interface interactions among semiconductor/electrocatalyst/electrolyte affect the charge separation and catalysis in turn. Here, by changing the bath temperature, Co-based oxygen evolution catalysts (OEC) with different crystallinities were electrochemically deposited on Ti-doped Fe2O3 (Ti-Fe2O3) photoanodes. We found: (1) the OEC with low crystallinity is highly ion-permeable, decreasing the interactions between OEC and photoanode due to the intimate interaction between semiconductor and electrolyte. (2) the OEC with high crystallinity is nearly ion-impermeable, is beneficial to form a constant buried junction with semiconductor and exhibits the low OEC catalytic activity. (3) the OEC with moderate crystallinity is partially electrolytescreening, thus contributing to the formation of ideal band bending underneath surface of semiconductor for charge separation and the highly electrocatalytic activity of OEC for lowering overpotentials of water oxidation. Our results demonstrate that to balance the water oxidation activity of OEC and OEC-semiconductor interface energetics is crucial for highly-efficient solar energy conversion, particularly the energy transducer is a semiconductor with shallow or moderate valance-band level.

■ INTRODUCTION Inspired by photosynthesis in nature,1-2 hydrogen fuel production from photoelectrochemical (PEC) water splitting on the semiconductor photoelectrodes provides one route to convert and store solar energy.3-5 To apply this technique to a large-scale practice, a key challenge is to accelerate oxygen-evolving reaction (OER), a rate-determining step for overall water splitting due to the four-electron process requiring high overpotentials.6-8 The oxygen evolution electrocatalysts (OECs) were routinely deposited onto n-type semiconductors (SCs) to promote the sluggish PEC water oxidation.9-10 Fe2O3 has generally emerged as a prototype photoanode for interface investigation,11 because this earth-abundant SC with favorable theoretical solar-to-hydrogen efficiency (~16.8 %) suffers from its poor intrinsic surface properties.12-13 To achieve improved PEC performance, the OER-active Co-based OECs,14-15 such as cobalt oxides and hydroxides, are being applied to modify Fe2O3 photoanodes for reducing the onset potential and/or enlarging the photocurrent.16-18 Although the PEC performance enhancements are widely confirmed, the charger extraction and transfer at the interfaces among SC|OEC|solution are far from fully understood.19 The increased performance is probably attributed to accelerating the sluggish OER kinetics, increasing band bending or preventing surface state recombination. 20-23 However, the IrOx nanoparticles with well-known excellent OER activity did not effectively enhance the PEC water oxidation on Fe2O3 photoanodes,24 meaning that there is no guarantee that the best electrocatalysts will produce the high PEC performance of photoelectrodes. The interfacial charge transfer processes, which depend on the interface energetics among the SC|OEC|solution junctions, are evidently more complicated.25-26 Experiments demonstrated that the SC|OEC|solution interface energetics is sensitive to the surface states of semiconductor photoanodes, such as TiO2, Fe2O3 and BiVO4,27-29 because the surface states within the SC band gap would give rise to surface

Fermi level pinning and surface recombination.30-31 In particular, coating electrocatalytic species on surface of photoanode, the surface states may not be completely passivated due to poor SC|OEC interface interaction failing to prevent the intimate contact between photoanode and electrolyte.32-33 Indeed, it has well demonstrated that the electrolyte-screening capability of OECs directly tune the barrier height of photoelectrode-electrocatalyst junction.34-35 Thus, for developing high-performance PEC devices with efficiently charge extraction, transfer and subsequent catalysis of OER, it is an urgent task to understand the interface energetics among SC|OEC|solution. Here, we constructed three kinds of SC|OEC|solution interfaces by depositing Cobalt-based oxyhydroxide OECs (i.e. CoOOH) with different crystallinity on the Ti-doped Fe2O3 (TiFe2O3) photoanodes via modulating the bath temperature during electrodeposition. Varying the crystallinity of Co-based OECs from low, moderate to high gives rise to different ion permeability and electrochemical performances. Our results demonstrated that modifying low-crystalline OEC with high OER activity facilitates the photogenerated holes to participate into water oxidation, rather than promotes the charge separation in depletion region. For modification of high-crystalline OEC, the buried junction brings about the limited photovoltage contributing to charge separation, but the configuration is subjected to poor OER activity. The OEC with moderate crystallinity is partially electrolyte-screening, thus contributing to reducing interface recombination, enlarging band bending for charge extraction and maintaining approximate OER activity for charge injection. Our findings are beneficial to understand how to construct the high-quality SC|OEC|solution interfaces for achieving efficient PEC water splitting.

■ EXPERIMENTAL SECTION Preparation of Fe2O3 photoanodes. Ti-doped Fe2O3 (Ti-Fe2O3) photoanodes were fabricated by hydrothermal growth with a subsequent annealing treatment.36 The fluorine-doped tin oxide (FTO) conductive substrate was cleaned with acetone, ethanol and deionized (DI) water (18.25

/ 11Environment ACS Paragon 1Plus


Electrochemical/Photoelectrochemical measurements. The electro/photoelectrochemical (PEC) properties of samples were tested in a three-electrode configuration with an electrochemical analyzer (CHI660D, Shanghai Chenhua, China). The electrolyte was 1 M NaOH aqueous solution (pH 13.6). The Co-based OEC-coated FTO and Ti-Fe2O3 photoanodes were used as the working electrode. The Pt foil and SCE were used as counter and reference electrode, respectively. For electrochemical measurement, the film area exposed to electrolyte was about 1 cm2 geometric surface area (GSA). Cyclic voltammetry (CV) was performed at a scan rate of 10 mV s−1. All the potentials described in this work were converted to the reversible hydrogen electrode (RHE) potential according to Nernst equation of VRHE = VSCE + 0.241 + 0.059pH (where VRHE and VSCE represent the reversible hydrogen electrode potential and saturated calomel electrode potential, respectively, and pH is the pH value of electrolyte). For the PEC measurements, the film area exposed to the light was 0.28 cm2. The photoelectrodes were illuminated from the FTO side with AM 1.5 G simulated sunlight (100 mW cm−2) from a Newport Sol3A Class AAA simulator. Linear sweep voltammetry (LSV) was performed at a scan rate of 10 mV s−1. The electrochemical impedance spectra (EIS) were measured using an electrochemical analyzer (Solartron 1260 + 1287, AMETEK, Berwyn, PA) with a 10 mV amplitude perturbation and frequencies between 100 kHz and 0.01 Hz. The Mott−Schottky plots were measured using an electrochemical analyzer (2273, Princeton Applied Research, AMETEK) in 1M NaOH and the solution containing 0.05M K3Fe(CN)6, 0.35M K4(CN)6 and 1M KCl. The charge separation efficiency (Φsep) and injection efficiency (Φinj) were obtained at 1.23 and 1.60 VRHE. Since the addition of Na2SO3 into solution would approximately promote the injection efficiency of pho-

φinj =

J Na2 SO3

(1)

J abs

J H 2O

(2)

J Na2 SO3

where J is photocurrent measured in 1 M NaOH solution, J is the H O Na SO photocurrent obtained in 1 M NaOH electrolyte with 0.5 M Na2SO3 and Jabs is the photocurrent density obtained by integrating the solar spectrum in the light absorption region of photoanodes. 2

2

3

■ RESULTS AND DISCUSSION As we have previously reported, after hydrothermal growth with subsequent annealing treatment, XRD pattern of the asprepared electrode was assigned to the single-phase rhombohedral α-Fe2O3 (JCPSD No. 87-1166) (Figure S1). The top and crosssectional SEM morphologies (Figure S2) showed that Fe2O3 film composed of the 200 nm sintering particles with a film thickness of about 500 nm. The Fe2O3 film has relatively smooth surface with few cracks, originating from the volume shrinkage during phase tranformation from FeOOH to Fe2O3. Usually, Ti4+, as an electron donor, was introduced to replace the Fe3+ of Fe2O3 to improve its electrical conductivity. Here, the slight decrease in binding energy of Ti 2p (458.0 eV) core-level XPS spectrum compared to that in pure TiO2 (458.5 eV) confirmed that the Ti was incorporated into the lattice of Fe2O3 (Figure S3).38-39 And the actual doping amount of Ti was determined by XPS to be ~0.4 at%.

(a)

30 Co-L

20 15

Co-M

Co-H

600

25

80

540

60

480

40

420

20

10 360

5

0 Co-L

Co-M

Coulombic efficiency (%)

Characterizations. The morphology of the samples was observed with a transmission electron microscope (TEM, JEOL JEM-200CX) and a fieldemission scanning electron microscope (FE-SEM, JEOL JSM-6700F). The crystalline phases were identified by powder X-ray diffraction (XRD) with a D8/ Advance diffractometer using Cu Ka radiation (λ= 0.15418 nm) at 40 kV and 40 mA in the 2θ range 20~80 oC. The UV-Vis absorption spectra were obtained by using an Ultraviolet-visible spectrophotometer (UV, Shimadzu UV-2550). Raman spectra (LabRAM, Horiba, Japan) were collected at the excitation wavelength of 516 nm with the intensity of 0.6 mW cm-2. X-ray photoelectron spectroscopy (XPS) was performed on a PHI5000 Versa Probe (ULVAC-PHI, Japan) with monochromatized Al Kα X-ray radiation (1486.6 eV). The energy resolution of the electrons analyzed by the hemispherical mirror analyzer is about 0.2 eV. The binding energy was determined in reference to the C 1s (284.6 eV).

φsep =

η @10 mA (mV)

Modification of Co-based OECs. The Co-based OECs were electrochemically deposited on FTO and Ti-Fe2O3 photoanodes via varying bath temperatures from 25 to 85 oC. The deposition electrolyte is a mixed solution of 0.05 M Co(NO3)2•6H2O (purity 99.9%, Sinopharm Chemical Reagent Co., Ltd) and 0.5M NaNO3 (purity 99%, Sinopharm Chemical Reagent Co., Ltd). The pH was adjusted at ~7.4 by addition of 1 M NaOH. The solution prior to electrodeposition was purged with N2 for about 1 h to prevent the atmospheric oxidation of Co2+, and the temperature of electrochemical cell was maintained with a water bath heating apparatus during deposition. The Co-based OECs were deposited in the water bath at low (25 oC), moderate (45 oC) and high (85 oC) temperatures, which were denoted as Co-L, Co-M and Co-H, respectively. The work electrode is FTO or Ti-Fe2O3 photoanode, while the counter electrode was a platinum foil and the reference electrode was saturated calomel electrode (SCE). The chronopotentiometry was conducted at the current density of 0.1 mA cm-2. At different deposition temperatures, the deposition period was tuned to achieve similar loading amount of Co-based OEC since the Faradaic efficiency is sensitive to environmental temperature.

toanodes to be ~100%, the charge separation efficiency (Φsep) and injection efficiency (Φinj) were calculated by the following equations37:

Current density (mA cm-2)

MΩ cm), and placed on the bottom of a 25 mL Teflon-lined stainless steel autoclave. An aqueous solution (20 mL) containing FeCl3 (purity 99%, Alfa Aesar, 0.15 M) and TiCl3 (30%, Aladdin) with a 0.5 at% atomic ratio of Ti:Fe wasr transferred into this autoclave, then sealed and heated at 100 o C for 4 h. After the eaction, the FeOOH formed on the FTO substrate was cleaned with DI water and annealed at 550 oC for 2 h for obtaining αFe2O3 film.

Co-L

Co-H

Co-H 0 1.0

(b) Current density (mA cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.2 1.4 1.6 Potential (V, vs. RHE)

1.8

3 Ti-Fe2O3 Ti-Fe2O3/Co-L 2

Ti-Fe2O3/Co-M Ti-Fe2O3/Co-H

1

0 0.6

0.8


1.6

Figure 1. CV curves on FTO (a) and chopped light photocurrent-potential (J-V) plots on Ti-Fe2O3 with modifications of Co-L (blue), Co-M (green), or Co-H (red) (b). Inset in Figure 1a shows the overpotential at 10 mA cm-2 (η@10 mA) (open diamond) and the coulombic efficiency of Co-based OECs deposition on FTO substrates (solid circle).


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Next, the Co-based OECs were deposited on FTO substrates and Ti-Fe2O3 plate electrodes at various temperatures. The coulombic efficiencies (inset in Figure 1a) during depositing OECs were determined by inductively coupled plasma mass spectrometry (ICP-MS) to be ~2.9% for Co-L (obtained at 25oC), ~5.1% for Co-M (obtained at 45oC) and ~44.3% for Co-H (obtained at 85oC), respectively. Experimental evidence confirmed that there is a liner relationship between deposition time and Co-based OEC deposition amount at a given temperature.40 The depositing temperatures and loading amounts of Co-based OECs on Ti-Fe2O3 were first optimized according to PEC performances of resulting photoelectrodes (Figure S4 and S5). We accordingly adjusted the deposition time and achieved a constant OEC deposition amount, ~10 mC cm-2 on Ti-Fe2O3 electrode, independent on the deposition temperatures. To explore the electrochemical properties of Cobased electrocatalysts, the increased depositing amount, ~200 mC cm-2 on FTO, was sued in this study. Cyclic voltammetry (CV) behaviors of Co-based OECs on FTO were recorded in 1M NaOH aqueous solution (pH 13.6) at a scan rate of 10 mV s-1 (Figure 1a). At potentials above 1.6VRHE, the current density of Co-based electrodes decreased with increasing the deposition temperatures, although their onset potentials seem indistinctive. The OER overpotentials at 10 mA cm-2 (η@10 mA) are in order of Co-H (560 mV) > Co-M (530 mV) > Co-L (490 mV). The difference in OER activities probably means that the deposition temperatures affect the material crystallinity, hence varying the catalytic performances, since the crystallinity of electrocatalyst is demonstrated to be a strong function of deposition temperature.40-41 Depositing the Co-based OECs, Co-L, Co-M, and Co-H, on TiFe2O3 photoanodes (denoted as Ti-Fe2O3/Co-L, Ti-Fe2O3/Co-M, and Ti-Fe2O3/Co-H, respectively) actually reduces the onset potential and enhances the photocurrent density (Figure 1b). Compared with bare Ti-Fe2O3 (onset potential at about 1.05 VRHE), the onset potentials of Ti-Fe2O3/Co-L and Ti-Fe2O3/Co-M exhibited the similar cathodic shift nearly 300 mV (at 0.75 VRHE), are 200

(h)

Co 2p 2p 1/2 Co-H

mV lower than that of Ti-Fe2O3/Co-H (at 0.95 VRHE). The photocurrent at 1.23 VRHE (J1.23V), the thermodynamic equilibrium potential for water oxidation, is remarkably increased from 0.5 mA cm-2 on bare Ti-Fe2O3, to 1.1 mA cm-2 on Ti-Fe2O3/Co-H, to 1.4 mA cm-2 on Ti-Fe2O3/Co-L, and to1.8 mA cm-2 on Ti-Fe2O3/Co-M. These evidences indicated that the PEC performance enhancement of Co-based OECs modified Ti-Fe2O3 photoanodes cannot be completely attributed to the difference in OER activity of electrocatalytic materials. The SEM, TEM and XPS analyses were carried out to know the microstructure, phase and composition of as-prepared Cobased OECs (Figure 2). The SEM observations indicated that the nanoparticles tend to grow and form a plate structure at high temperatures (Figure 2a-c). The high-resolution TEM observations revealed that the Co-L obtained at 25 oC composed of 2-3 nm nanocrystals with unclear lattice fringe, indicative of the low crystallinity (Figure 2d). Increasing the heating temperature, the nanocrystals grew up to 5-10 nm at 45 oC and above 20 nm at 85 o C(Figure 2e and f). The TEM images with a large scale also reveal that Co-H owns the relatively compact structure (Figure S6). Indeed, selected area electron diffraction (SAED) patterns gradually tend to present diffraction pattern of a single crystal particle with increasing the heating temperatures, further confirming the grain growth and improvement of crystallinity. All the samples exhibited a lattice spacing of 0.238 nm in high-resolution TEM images, was assigned to (101) plane of CoOOH, meaning that the CoOOH is the mainly deposited product. The Raman spectra of these electrocatalysts presented the enhanced peak intensity at 503 cm-1 with the depositing temperature increased (Figure S7), which is assigned to the crystalline CoOOH.42 XPS analysis was applied to determine the valence states of Cocontaining species (Figure 2h). The Co 2p3/2 binding energy at 780 eV and Co 2p1/2-Co 2p3/2 splitting of 15 eV are well assigned to the Co3+ species, confirming

(g) Intensity (a. u.)

Intensity (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2p 3/2

Co-M

O 1s O2OHCo-H Co-M

Co-L

Co-L 810

805

800 795 790 785 780 Binding energy (eV)

775

538

536

534 532 530 528 Binding energy (eV)

526

Figure 2. The SEM and HR-TEM images of Co-L (a, d), Co-M (b, e) and Co-H (c, f), and the corresponding XPS spectra of Co 2p (g) and O1s (h). The dot line regions in Figure 2d-f show the HR-TEM lattice spacing for a whole crystal.



(a)

700 Co-L Co-M Co-H

600

-2

Z`` (Ω Ω cm )

500 400 300

Rs

200 100 0 0

(b)

Rbulk

Rct

Cbulk

CEE

100 200 300 400 500 600 700 800 Z` (Ω Ω cm-2)

1.8

Co-L Co-M Co-H

1.5 1/C2 ( 1010 F-2 cm4)

1.2 0.9 0.6 0.3 0.0 0.6

(c)

0.2

0.8


4

charge storage (charge-discharge)

3

Ratio

the formation of CoOOH.43-44 Slight binding energy increase with elevating heating temperatures would indicate the improvement of material crystallinity. In addition, O 1s XPS spectra can be deconvoluted into two peaks: 531 eV for O from adsorbed hydroxyl or low-crystalline hydroxyl and 529 eV for crystal lattice oxygen (Figure 2g).45 Obviously, the content of crystal lattice oxygen gradually enlarged from Co-L, to Co-M, and to Co-H. This would indicate a crystal constraint enhancement from low-crystalline hydroxyl to high-crystalline hydroxyl, suggesting that the crystallinity of deposited product is sensitive to the change of temperatures. Further electrochemical measurements of Co-based OECs loaded on FTO were performed to understand the intrinsic physicochemical characteristics. The charge transfer in the Co-based OECs with different crystallinity was investigated by electrochemical impedance spectroscopy (EIS) at frequencies between 100 kHz and 0.01 Hz at 1.6 VRHE. Nyquist plots of EIS spectra were shown in Figure 3a and the classical Randles circuits were used to describe the charge transfer processes.46 The electrical components in EIS model consist of the series resistance (Rs), the bulk resistance (Rbulk) and capacitance (Cbulk), and the charge transfer resistance (Rct) and capacitance (CEE) at electrode-electrolyte interface. The small semicircles for all samples at high frequency region are associated with the parallel connection of Rbulk and Cbulk. In contrast, the large semicircles appeared at low frequency region were assigned to the electrode-electrolyte interface Rct and CEE. A similar equivalent radius of the semicircles at low frequency region was observed in both Co-L and Co-M, evidently small than that in Co-H. This means that the charge transfer in Co-H is more difficult than those in both Co-L and Co-M. Mott-Schottky (MS) measurement revealed that the MS curves for all the samples present the negative slope (Figure 3b), indicative of a p-type conductivity.47 The Fermi level of the Co-based bulk electrocatalyst was dependent on the applied potentials which changed the average oxidation state of cobalt species. The characteristic of p-type semiconductor starts at 1.0 VRHE for Co-H, obviously delayed than 0.9 VRHE for both Co-L and Co-M. A low-crystallinity OEC with moderate ion constraint in the crystal lattice would exhibit the adjustable Fermi level by changing the oxidation states of active species in bulk. In contrast, strong lattice constraint in highcrystallinity OEC causes the difficulty in changing valence states.48 As is well-known, the electrochemical properties of OEC affect the charge extraction, storage and injection at the SC|OEC interface. The electrochemically active surface area (ECSA) of electrocatalysts were approximately evaluated from capacitance measurements.49 The corresponding CV curves were displayed in Figure S8. And the CV curves depending on the scan rate are almost rectangular, indicating that the electrochemical behaviors originated from double-layer capacitance (Cdl) rather than pseudocapacitance. The Cdl can be calculated to describe the ECSA.50 As displayed in Figure 3c, the non-Faradaic currents were collected and fitted by the equation of Cdl=ic/υ, where ic is the charging current and υ is the scan rate. The Cdl of Co-L (10.2 mF cm-2) is much higher than those of Co-H (3.2 mF cm-2) and Co-M (4.6 mF cm-2). The comparable Cdl for Co-M and Co-H suggests their similar ECSA, suggesting the ECSA is not the main factor to affect the photocurrent. Besides, the charge storage of these Co-based OECs might be an important factor because the large charge storage capacity is

Current density (mA cm-2)

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1.6

Co-L Co-M Co-H

2 1 surface area

0.1

(ECSA)

Co-L

Co-M

Co-H

0.0 0

3

6 9 Scan rate (mV s-1)

12

Figure 3. EIS (a) and Mott-Schottky (b) for Co-L, Co-M, and Co-H. Inset in Figure 3a shows the equivalent circuit. ECSA (c) measurements for Co-L, CoM, and Co-H. Inset in Figure 3c shows that the ratio of charge storage capacity (solid symbols) and ECSA (open symbols) for Co-based OECs referring to the relevant values of Co-H.

beneficial to the extraction and storage of photogenerated holes from photoanode to electrocatalyst for water oxidation, suppressing surface recombination. As acommon approach to detect the capacity of charge storage,51 the galvanostatic charge-discharge process was conducted in the potential range from 1.0 to 1.4 VRHE (Figure S9). The potential range used ensures the occurrence of valence state transition of Co ions, and avoids the Faradaic reaction of water oxidation due to that the applied potentials are lower than OER onset potential. Therefore, the charge-discharge periods could reflect the charge storage process in electrocatalysts. Although the loading amounts of Co-based OECs are approximatively identical, the charge-discharge time of Co-L, Co-M and Co-H are 15s, 44s and 53s, respectively. To understand these behaviors, the ECSA and charge storage capacity for Co-based OECs were normalized by referring to the relevant values of Co-H. The Co-L owns the highest capacity of charge storage, which is 3 and 3.5 times as large as those of Co-M and Co-H, respectively. (Figure 3c inset). And the ECSA of Co-L and Co-M is 3.2 and 1.5 times as large as that of Co-L, respectively. It suggests that most Co ions


Figure 4.The HR-TEM and SEM images of Ti-Fe2O3 photoanodes modified with Co-L(a, d), Co-M (b, e) and Co-H (c, f).

in Co-L are exposed to electrolyte because of its low crystallinity, which allows the free movement of ions in its bulk. Only Co ions on the surface of Co-H, however, participate in the charge storage owing to its relatively high crystalline. Nevertheless, it is interesting that the Co-M shows similar charge storage to Co-L while its ECSA is only about 0.5 times higher than Co-H. The ECSA test mainly describes the capacitance from active species adsorbed on the surface active sites. And the galvanstatic charge-discharge reflects the charge storage capacity for the redox species in the electrode. Therefore, the difference between ECSA and galvanstatic charge-discharge process are dependent if the redox species originated from the surface or bulk of electrocatalyst. In our case, Co-H is ion-impermeable, Co-M is partial ion-permeable. Therefore, they exhibit the similar ECSA, but a large difference in charge storage. The moderate deposition temperature brings about the appropriate crystallinity for Co-M. As a result, the moderate lattice constraint makes the changing valence states of Co ions in the bulk of Co-M for charge storage easy. Modified with these Co-based OECs, the acquired samples exhibit similar SEM morphologies to as-prepared Ti-Fe2O3 photoanodes (Figure 4a-c), because the depositing amount is only ~10 mC cm-2. The HR-TEM images (Figure 4d-f) reveal that the thickness of electrocatalyst coating layer is 2~3 nm. As raising the heating temperatures, slight thinning of electrocatalyst layer would originate from the increased crystallinity inducing a volume shrink. Although the XRD patterns of these photoanodes modified with electrocatalysts just exhibit the diffraction peaks of Fe2O3 and FTO substrates (Figure S10), the energy dispersive spectroscopy (EDS) mapping indicated that Co-based OECs were uniformly dispersed on the photoanode surface (Figure S11). Moreover, the XPS spectra (Figure S12) show that the energy splitting of Co 2p3/2 and 2p1/2 maintains 15 eV for these Co-based species deposited on Ti-Fe2O3. That means the Co3+ species (i. e. CoOOH) were formed. In addition, the XPS signal of crystal lattice oxygen for these samples, mainly from the Ti-Fe2O3 substrate, are pronounced, confirming the thin Co-based OECs on surface of TiFe2O3.52 In contrast to EIS results of electrocatalysts on FTO just reflecting the OER process, the impedance behaviors of Ti-Fe2O3

photoanodes coated with Co-based OECs can be collected to survey the charge transfer of photogenerated holes during PEC water oxidation. Therefore, EIS tests were conducted in the same electrochemical cell under the illumination of AM 1.5G simulated sunlight in the frequency region from 100 kHz to 0.01 Hz. The applied potential was fixed at 1.0 VRHE, at which potential photoanode gives rise the feasible photocurrent to observe charge transfer and avoids the disturbance of drastic bubbles from water splitting. The Figure 5a shows the Nyquist plots and the corresponding equivalent circuit. Comparing with circuit configuration for electrocatalyst loaded on FTO, the photoanodes coated with electrocatalysts contain the similar electrical components to describe resistances and capacitances. The apparent semicircles arise in the high frequency region (left part in Nyquist plot) for all photoanodes, describing the resistance (Rbulk) and capacitance (Cbulk) of Ti-Fe2O3 photoanodes. Meanwhile, the low-frequency semicircles reflect the charge transfer resistance (Rct) and capacitance (CEE) at electrolyte-photoelectrode interface during water oxidation. The smallest Rct is achieved on Ti-Fe2O3/Co-M with the best PEC performance, implying that the improvement of PEC performance resulted from the enhancement in the transfer kinetics of photogenereted holes. In addition to the charge transfer at the electrolytephotoelectrode interface, the carrier dynamics was analyzed by the anodic transient photocurrent measurement. The anodic photocurrent spikes are associated with photogenerated hole accumulation on the photoelectrode surface, thus its decay behavior has been used to reveal the hole lifetime. As shown in Figure S13, the transient photocurrents of photoelectrodes with and without modification

(a)

Ti-Fe2O3

600

Ti-Fe2O3/Co-L

Rs

Ti-Fe2O3/Co-M

Z`` (Ω Ω cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Rct

Rbulk

Ti-Fe2O3/Co-H

CEE

400

Cbulk 200

0

(b)

0

200

400 600 Z` (Ω Ω cm2)

0

800

1000

Ti-Fe2O3 Ti-Fe2O3/Co-L Ti-Fe2O3/Co-M Ti-Fe2O3/Co-H

-1 lnD

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light off

-2

Jin

Jst light on

-3

0.0

0.2

0.4 Time (s)

0.6

0.8

Figure 5. The EIS under AM 1.5G simulated sunlight illumination (a) and anodic transient dynamics (b) for Ti-Fe2O3, Ti-Fe2O3/Co-L, Ti-Fe2O3/Co-M, and Ti-Fe2O3/Co-H photoanodes.



of OECs were collected at 1.23 VRHE in 1M NaOH aqueous solution. The anodic photocurrent spike arising upon illumination subsequently decays with time, corresponding to the recombination process of photogenerated carriers. A steady-state photocurrent then can be observed. The decay time can be approximately calculated by a logarithmic plot of parameter D,53-54 using the equation of D=(Jt-Jst)/(Jin-Jst), where Jt is the photocurrent at time t, Jst is the steady-state photocurrent, and Jin is the photocurrent spike (inset of Figure 5b). To get a qualitative comparison, the transient decay time, τ, can be defined as the time at which lnD = -1. Generally, a lower recombination rate could be reflected by longer transient decay times. The Figure 5b shows the plots of lnD versus decay time. The decay time curves did not show linear behavior, indicating that the decay mechanism is complex. All the electrocatalyst modifications appear to increase the decay time of TiFe2O3 photoanode (0.18 s). The Ti-Fe2O3/Co-M obtains a longer decay time (0.36 s) than those of Ti-Fe2O3/Co-L (0.29 s) and TiFe2O3/Co-H (0.28 s). It suggests the Co-M modification enlarges the hole lifetime, preventing hole-electron recombination. The retardation of carrier recombination is probably attributed to the interface energetics that were affected by the electrocatalyst modification. The open circuit potential (OCP) measurement is a typical approach to get the thermodynamic information with minimum interference since the open-circuit conditions have a zero net exchange current across the photoelectrode-electrolyte interface.55 As displayed in Figure 6a, the OCPs of photoanodes with and without electrocatalyst modification were obtained in the

(a) OCP (V vs. RHE)

0.7

AM 1.5G

0.8

0.9

1.0 dark 1.1

Ti-F e

2

(b)

O

3

Ti-F Ti-F Ti-F eO eO eO 2 2 2 3 /Co 3 /Co 3 /Co -H -L -M

100

30

20 60 40

Φ sep (%)

80

Φinj (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

10 20 0

Ti-F eO 2

3

Ti-F e

2

0 Ti-F Ti-F eO eO O /C 2 2 / / 3 3 Co 3 Co o-L -H -M

Figure 6. The OCPs under illumination and in the dark (a) and efficiency of charge separation (Φsep) and injection (Φinj) (b) for Ti-Fe2O3, Ti-Fe2O3/Co-L, Ti-Fe2O3/Co-M, and Ti-Fe2O3/Co-H photoanodes.

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dark and under illumination. In the dark, the measured OCP reflects the position of Fermi level at the equilibrium state between photoelectrode and electrolyte. Under illumination, the quasiFermi level of holes is responsible for the photoelectrodeelectrolyte equilibrium state, while the quasi-Fermi level of electrons was detected as OCP. The difference between the OCPs measured in the dark and under illumination is regarded as photovoltage (Vph). The Ti-Fe2O3/Co-L and Ti-Fe2O3 show the similar Vph (~100 mV), because the low-crystalline Co-L cannot completely prevent the electrolyte infiltration into the surface of TiFe2O3, thus weakening the interactions between Co-L and TiFe2O3. It is possible that the Co-L modification mainly promotes the OER kinetics on photoanodes due to its relatively good electrocatalytic activity, thus negatively shifting the onset potential (Figure 1b). In contrast, the loading of Co-M or Co-H allows to improve the Vph. The Ti-Fe2O3/Co-M achieves larger Vph (~290 mV) than Ti-Fe2O3/Co-H (~160 mV). The difference in Vph can be attributed to the difference in junction barrier heights at the interface between Ti-Fe2O3 and electrocatalyst. Coating the ionimpermeable Co-H on Ti-Fe2O3 creates a buried junction with a constant barrier height, thus a constant Vph. As a contrary, an adjustable barrier formed at the interface between the partially ionpermeable Co-M and the Ti-Fe2O3. In this case, the oxidation of Co ions in the Co-M bulk shifts its Fermi level relative to TiFe2O3 band edges, increasing the barrier height. Thus, the efficient hole extraction and improved Vph can be realized simultaneously by Co-M modification. The relatively negative OCP of TiFe2O3/Co-M under illumination probably implies the effective suppression of Fermi level pinning, as the Co-M might affect the density and/or occupancy of surface states on Ti-Fe2O3. The completely ion-permeable Co-L cannot function as well as Co-M in Vph enhancement, probably because the weak interactions between Co-L and Ti-Fe2O3 did not contribute to the surface state passivation of Ti-Fe2O3, suppressing the charge extraction from semiconductor. This means that the partially ion-permeable properties of Co-M effectively create the high-quality interface by passivating surface states and the catalytic layer with moderate thickness, thus inducing the efficient charge extraction and lowering OER kinetics. Indeed, the slight difference among the OCPs of photoanodes in the dark would be a result of potential drop within the Helmholtz layer, depending on surface components.56 The interface energetics were further checked by MS tests.57 We performed the electrochemical impedance measurements to collect the MS plots for the samples immersed in Fe(CN)63/4- or 1M NaOH solution (Figure S14). Ti-Fe2O3 and Ti-Fe2O3/Co-L exhibit similar MS characteristics due to that highly ion-permeable Co-L could not prevent the intimate contact between photoanode and electrolyte. Consistent with previous report,17 the electrolytepermeable Co-based OEC modification contributes to negligible change of the flat band potential (Efb) of Ti-Fe2O3. The modification of Co-M leads to slightly negative shift of Efb, which would enlarge the Vph. On contrast, the Efb of Ti-Fe2O3 was positively shifted by Co-H modification, probably is attributed to formation of buried junction between the ion-impermeable Co-H and TiFe2O3, as well demonstrated on TiO2 photoanodes.26 Noting that the classical Mott-Schottky relationship was deduced on the plate electrode model. In our case, the roughness surface of Ti-Fe2O3 electrode may induce the slight offset in correlation of M-S measurement and Vph.58, 59 In addition, the MS tests determined the Efb when the charge space capacitance is zero, while the OCP under illumination reflects the photo-induced carriers driven equilibrium among photoanode, electrocatalyst, and electrolyte. Therefore, the


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MS results would not be completely consistent with the OCP results. It is worth noting that the enlarged Vph would imply the improved charge separation in the depletion region after OECs modification. The photocurrent of photoanodes were collected in the electrolyte with the hole scavenger SO32- at 1.23 VRHE (Figure S13). And we measured the transmittance (T) and reflectance (R) curves of the Ti-Fe2O3 photoanode (Figure S15). The absorption spectrum (A) of Ti-Fe2O3 was calculated following the equation: A=1-T-R. Integrating the solar spectrum was conducted to obtain the theoretical photocurrent to calculate the charge separation efficiency Φsep and charge injection efficiency Φinj (details in experimental section) (Figure 6b). The Φinj is remarkably increased from 43% for Ti-Fe2O3 to 83% for Ti-Fe2O3/Co-L, confirming the excellent electrocatalytic activity of Co-L. However, the Φsep of Ti-Fe2O3/Co-L and Ti-Fe2O3 maintain the same to be about 11%, demonstrating that the weak interactions between Co-L and TiFe2O3. A slight increase in Φsep for Ti-Fe2O3/Co-H (13%) than TiFe2O3 (11%) is because the buried junction enhanced the band bending. The Φinj (61%) of Ti-Fe2O3/Co-H is not as high as that of Ti-Fe2O3/Co-L (83%) or Ti-Fe2O3/Co-M (80%), resulting from that the OER catalytic activity is limited by strong ion constraint in Co-H. According to mentioned-above dynamic and thermodynamic results, the interface energetics among Co-based OECs|TiFe2O3|electrolyte is illustrated in Figure 7. The charge exchange between Ti-Fe2O3 and electrocatalyst would affect the quasi-Fermi level of holes (EF,p) and electrons (EF,n), thus changing the Vph. The low-crystalline Co-L could not prevent the diffusion of ions to surface of Ti-Fe2O3, did not enhance the junction effect at their interface, and hence did not improve the Vph of Ti-Fe2O3 photoanode. Recently, interface investigation on Ni0.8Fe0.2Oxcatalyzed Fe2O3 also demonstrates that the ion-permeable electrocatalyst with high valence states is not completely beneficial to enlarge the photovoltage of Fe2O3 photoanode.60 In contrast to high-crystalline Co-H creating a buried junction on Ti-Fe2O3, the moderate-crystalline Co-M produces the largest Vph on Ti-Fe2O3. Moderate-crystalline Co-M is partially ion-permeable, which is beneficial to form a high-quality interface that contributes to an

EF,p kct

e-

kinj

TiTi-Fe2O3/Co/Co-L

EF,p kct

FTO

FTO

Co--M Co

EF,n Co--L Co

EF,n

e-

EF,n

EF,p kinj

TiTi-Fe2O3/Co/Co-M

kct

enlarged junction effect for promoting charge separation and extraction, and the bulk of OER catalytic layer for extracting charge and accelerating OER kinetics, also probably to suppressing surface recombination via passivating the surface states of Ti-Fe2O3. The enhanced built-in potential at the semiconductorelectrocatalyst interface to efficiently extract charge from the semiconductor has been demonstrated to be mainly responsible for PEC performance enhancement on TiO2 photoanode.61 Due to an enough deep valence level to oxidize water, the PEC performance of the wide-band TiO2 is mainly restricted by charge extraction. Different to the TiO2, the PEC performances for most narrowband semiconductors, such as Fe2O3, BiVO4 and Ta3N5, are still limited by both the charge extraction and sluggish OER kinetics.6265 In this case, it is strongly expected that the OEC functions as an efficient OER catalytst and a regulator of interface energetics.

■ CONCLUSIONS In this work, the Co-based electrocatalysts with different crystallinity were electrochemically deposited on Ti-Fe2O3 plate photoanodes to understand the influence of interface properties of electrocatalyst-coated photoelectrode on the PEC water splitting. The electrodeposition at room temperature produces the lowcrystalline Co-L, exhibiting the excellent electrocatalytic OER activity due to the weak lattice constraint. High-temperature electrodeposition fabricates the high-crystalline Co-H with low OER performance because of the limited active surface area and difficulty in active species transition. The electrodeposition at moderate temperature gives rise to moderate-crystalline Co-M, having the medium OER activity and favorable bulk behavior to store charge. Modifying Ti-Fe2O3 by Co-L, the high-activity Co-L accelerating OER kinetics facilitates the photogenerated holes to participate into water oxidation, but the weak junction effect at their interface is not useful to charge separation. For Ti-Fe2O3/CoH, the buried junction gives rise to the limited photovoltage increase promoting charge separation while the poor OER activity still hinders charge transfer. The moderate-crystalline Co-M is beneficial to form a high-quality interface that contributes to an adjustable junction effect for promoting charge transfer, and the sufficient OER catalytic species in its bulk for storing charge and accelerating OER kinetics. Our results imply that suitable electrocatalysts on photoanodes should balance the OER activity and the interface energetics to achieve favorable charge transfer and injection for efficient solar-driven water splitting.

ASSOCIATED CONTENT Co Co--H

e-

FTO

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


kinj

Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: XRD pattern, SEM image, TEM image, Raman spectrum, XPS spectrum, Mott-Schottky curves, CV plots, J-V curves under illumination and absorbance spectra.

TiTi-Fe2O3/Co/Co-H

Figure 7. Interface energetics of Co-based OEC coated Ti-Fe2O3 photoanodes under illumination. The photogenerated electrons (e-) are collected by the FTO substrate, and the holes should go through the charge transfer (kct) and injection into electrolyte (kinj) for water oxidation. The relevant photovoltage (Vph) is associated with the quasi-Fermi level of holes (EF,p) and electrons (EF,n). The wider arrow means the faster charge transfer rate.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]


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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported primarily by the National Basic Research Program of China (2013CB632404), the National Natural Science Foundation of China (51572121, 21603098 and 21633004), the State Key Laboratory of NBC Protection for Civilian (SKLNBC2014-09), the Natural Science Foundation of Jiangsu Province (BK20151265, BK20151383 and BK20150580), the Fundamental Research Funds for the Central Universities (021314380084), the Postdoctoral Science Foundation of China (2017M611784) and the program B for outstanding PhD candidate of Nanjing University (201702B084).

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