Adjusting the Crystallinity of Mesoporous Spinel CoGa2O4 for Efficient

May 4, 2016 - Recent developments in complex metal oxide photoelectrodes. Fatwa F Abdi , Sean P Berglund. Journal of Physics D: Applied Physics 2017 5...
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Adjusting the Crystallinity of Mesoporous Spinel CoGa2O4 for Efficient Water Oxidation Zhe Xu,† Shi-cheng Yan,*,† Zhan Shi,‡ Ying-fang Yao,‡ Peng Zhou,‡ Hao-yu Wang,‡ and Zhi-gang Zou†,‡ †

National Laboratory of Solid State Microstructures, Collaborative Innovation Center of Advanced Microstructures, Jiangsu Provincial Key Laboratory for Nanotechnology, Eco-Materials and Renewable Energy Research Center (ERERC), College of Engineering and Applied Sciences, and ‡School of Physics, Nanjing University, Nanjing, Jiangsu 210093, China S Supporting Information *

ABSTRACT: Effective and stable electrocatalysts (ECs) are of great importance for the modification of semiconductor (SC) photoanodes, to achieve efficient photoelectrochemical (PEC) water splitting. Herein we demonstrate that the low-crystallinity mesoporous spinel CoGa2O4 oxygen evolution catalyst (OEC), exhibiting excellent bulk electrocatalytic stability and activity for oxygen-evolving reaction (OER), obviously improved water oxidization on a-Fe2O3 photoanode. Low crystallinity not only balances the stability and activity for ECs themselves but facilitates formation of adjustable Schottky junctions between ECs and SCs. Those would contribute to surface state passivation and photogenerated hole extraction, leading to lower onset potential and larger photocurrent. Thus, our finding suggests that low crystallinity could serve as a beneficial feature of ECs to achieve efficient PEC water splitting, owing to its preponderant tendency for the improvement of interface reaction kinetics. KEYWORDS: low-crystallinity electrocatalyst, water oxidation reaction, photoelectrocatalysis, electrocatalysis, photoanode modification

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INTRODUCTION An inspiring approach to capture and store abundant solar energy for a clean-energy future is the photoelectrolysis of water into oxygen and hydrogen fuels.1,2 However, the water oxidation half reaction is considered to be the rate-determining step for overall water splitting, owing to the four-electron process requiring a large overpotential.3,4 To cope with this kinetic challenge, adopting appropriate electrocatalysts (ECs) that modify light-absorbing semiconductors (SCs) is an ingenious strategy to obtain high-performance photoanodes for promoting the oxygen-evolving reaction (OER). Although broad research efforts on developing effective ECs have been made to conquer this specific problem, only expensive Ir- and Ru-based OER catalysts are worldwide commercially available so far.5 Recently, a series of earth-abundant metal oxides have been investigated to minimize the operating cost for industrial use,6−8 among which cobalt-based materials are promising candidates with high OER performance.9−11 In principle, OER by electrocatalysis is achieved via the redox cycle of active species. In the case of Co-based oxygen evolution catalysts (OECs), the resting state is Co2+/Co3+. It is oxidized to the high-valence state (Co4+) at high overpotential, acting as an active site, which reacts with OH− to produce O2.12 Therefore, an ideal OEC should offer an appropriate ligand environment with active ions to balance the competition between catalyst stability, requiring strong ion-constraint ability, © 2016 American Chemical Society

and high activity, claiming high-activity ions in a weak constraint situation. Commonly, compact catalysts with high crystallinity, i.e., strong crystal lattice constraint, promote stable catalytic activity during operation. However, such material can only perform “surface catalysis” of water oxidation on the crystal surface with no ions penetrating the bulk, leading to low catalytic efficiency. Efforts to construct porous structures for large reactive surface areas inside the crystals might relatively improve OER efficiency, which nevertheless cannot yet change the inherent obstacles, such as high material resistance and high overpotential, for the redox cycle of active catalytic species.13,14 On the contrary, amorphous catalysts with a weak ligand environment facilitate valence state change of catalytic species with low overpotential, leading to high activity.15 Yet, their stability was seldom satisfying, because the high chemical activity originates from the weak ligand environment. Recently developed amorphous Co−Pi demonstrated good stability from its self-repair mechanism and high activity from the shortrange-order Co−O cubane units.16,17 This result indicates that a catalyst with high stability via the self-repair mechanism should be ion permeable, which allows the ions diffusing into the bulk to stabilize the catalytic species. Porous materials with Received: April 1, 2016 Accepted: May 3, 2016 Published: May 4, 2016 12887

DOI: 10.1021/acsami.6b03890 ACS Appl. Mater. Interfaces 2016, 8, 12887−12893

Research Article

ACS Applied Materials & Interfaces

Continuous-wave X-band electron paramagnetic resonance (EPR) spectra for Co-RT OEC after OER were collected under slow passage conditions (16.2 mT/s scan rate) using a Bruker EMXplus 10/12 spectrometer equipped with an ER4119 High-Q cylindrical cavity. Cryogenic temperatures were achieved using an Oxford Instruments ESR910 liquid helium continuous flow cryostat and set using an Oxford Instruments ITC503 temperature controller. Microwave frequency = 9.4 GHz; modulation amplitude = 8 G; modulation frequency = 100 kHz; nonsaturating microwave power = 1.02 mW; temperature = 5.7 K. Thirty scans were included for each spectrum. The mass of the EPR sample was determined by weighing. Electrochemical/Photoelectrochemical Measurements. The electrochemical/photoelectrochemical (PEC) properties of the films were tested in a three-electrode cell using an electrochemical analyzer (CHI-660D, Shanghai Chenhua, China) and rotating disk electrode (RRDE-3A, ALS Co., Japan). The electrolyte was aqueous 1 M NaOH solution (pH 13.6). The Co-based OEC-coated GC-RDE, FTO, or Fe2O3 photoanodes were used as a working electrode. A Pt foil and saturated calomel electrode (SCE) were used as a counter and reference electrode, respectively. 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 curves were measured using an electrochemical analyzer (2273, Princeton Applied Research, AMETEK). All the potentials described in this work refer to the reversible hydrogen electrode (RHE) potential, which was calculated following the formula: 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 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 5 mV s−1. To evaluate the Faradaic efficiency of the electrochemical OER, a gas chromatograph (GC-8A, Shimadzu, Japan) was used for the quantitative detection of O2. The experiment was performed in a gas-tight three-electrode electrochemical cell. To remove air before electrocatalysis, the cell was degassed by bubbling with high purity Ar for 2 h. Electrolysis with O2 detection was conducted at a constant current of 2 mA cm−2 for 5 h. For the PEC measurement, the film area exposed to the light was 0.28 cm2. The light source was AM 1.5 G simulated sunlight (100 mW cm−2) for illumination from the FTO side. Linear sweep voltammetry (LSV) was performed at a scan rate of 10 mV s−1.

low crystallinity might be a potential candidate to design a stable ion-permeable catalyst, which exhibits a weak constraint of the crystal field for the catalytic species and allows ions such as OH− to permeate the bulk by the complex effect. Indeed, reports showed that high-activity OER catalysts could be hydrous transition metal oxides, hydroxides, or oxyhydroxides, for example, Co-Pi, CoOOH, and so forth.8,16,18,19 These catalysts often exhibit broad redox waves associated with bulk redox processes and free movement of ions throughout the catalyst. Contrary to “surface catalysis”, the ion-permeable catalytic process was named as “bulk catalysis”. Here we discovered that low-crystallinity Co-based OEC, CoGa2O4, can achieve a stable and efficient bulk electrocatalysis for OER, because the low crystallinity helps to offer the appropriate ligand environment that allows self-healing of the catalyst and facilitates the redox cycle of active species. The bulk OEC-modifying photoanodes greatly contribute to forming adjustable Schottky junctions with semiconductors, surface state passivation, and photogenerated hole extraction, leading to lower onset potential and larger photocurrent from water oxidation. The bulk catalysis achieved by adjusting the material crystallinity offers a new strategy for the rational design of OECs applied to photoanodes.

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EXPERIMENTAL SECTION

Fabrication of CoGa2O4 with Different Crystallinity and CoGa2O4-Modified (Photo)electrodes. A porous low-crystallinity Co-based OEC was synthesized by the room-temperature ion exchange method. NaGaO2 solid powders were first prepared by heating a stoichiometric mixture of Na2CO3 and Ga2O3 at 850 °C for 12 h. After that, the preparation procedure for porous low-crystallinity CoGa2O4 was performed as follows: NaGaO2 colloidal suspension (0.2 mol L−1, 10 mL) was added to an aqueous solution of CoCl2 (0.05 mol L−1, 20 mL) and stirred for 3 h at room temperature to form porous CoGa2O4 with low crystallinity, which was separated by centrifugation and dried at 60 °C for 2 h (denoted as Co-RT). Porous high-crystallinity CoGa2O4 was synthesized by hydrothermal treatment of NaGaO2 (0.2 mol L−1, 10 mL) and CoCl2 (0.05 mol L−1, 20 mL) in a Teflon-lined hydrothermal autoclave at 180 °C for 3 h to form the porous CoGa2O4 with high crystallinity (denoted as Co-HT). Co-based OECs (∼0.3 mg cm−2) were loaded on a glassy carbon rotating disk electrode (GC-RDE) to observe their intrinsic electrocatalysis. Nafion (3% v/v) solution was added to enhance the adherence of OECs to the GC-RDE. As OECs were ultimately applied to the surface modification of photoanodes, electrocatalysis on planeconductive substrates should also be analyzed. Therefore, Co-based OECs were deposited on the fluorine-doped tin oxide (FTO) conductive substrate using the electrophoretic deposition (EPD) technique. Typically, the deposition bath consisted of Co-based OECs powders (∼30 mg) and iodine (∼5 mg) in acetone (25 mL) with the assistance of sonication. The EPD process was conducted between two parallel FTO electrodes immersed in the as-prepared suspensions. The Co-based OECs were electrodeposited under 80 V on FTO. Almost the same mass of Co-RT or Co-HT (∼0.5 mg cm−2) was loaded on FTO substrate, and the OEC loading mass was determined from the statistical average mass of several electrodes using a weighting method. Characterizations. The crystallinity of these as-prepared products was determined by powder X-ray diffraction (XRD, Rigaku Ultima III, Cu Kα radiation). 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 line at 284.8 eV. The morphology for the samples was observed with a transmission electron microscope (TEM, FEI Tecnai G2 F30 S-Twin, Hillsboro, OR).

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RESULTS AND DISCUSSION As shown in Figure 1, porous CoGa2O4 with low and high crystallinity was respectively synthesized by a room-temperature (Co-RT) and hydrothermal (Co-HT) ion exchange route based on the NaGaO2 porous colloid template.20 XRD patterns showed that the Co-RT sample exhibited two broadened peaks that can be indexed to (311) and (440) reflections of CoGa2O4, indicating the formation of low-crystallinity materials (Figure 2a), while all the diffraction peaks of the Co-HT sample were indexed as highly crystalline cubic CoGa2O4 (JCPDS no. 110698). TEM observation showed that the NaGaO2 colloid particles (Figure 2b) possess mesoporous structure, because the NaGaO2 colloidal particles tend to form the mesoporous NaGaO2 colloid via flocculation due to weak repulsion between the colloidal particles. After addition of Co2+ to the NaGaO2 colloid solution, the ion-exchange process between Co2+ and Na+, based on the NaGaO2 mesoporous framework, formed mesoporous CoGa2O4. Indeed, TEM observations confirmed two as-prepared CoGa2O4 samples with porous microstructures (Figure 2c and 2d); nitrogen adsorption−desorption curves presented a type-IV isotherm (Figure S1), typical for 12888

DOI: 10.1021/acsami.6b03890 ACS Appl. Mater. Interfaces 2016, 8, 12887−12893

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Figure 1. (a) Porous NaGaO2 colloid obtained by dispersing NaGaO2 into water. (b) Porous CoGa2O4 was prepared by room-temperature ion exchange reaction between porous NaGaO2 and Co2+ ions and then dried at 60 °C for 2 h to obtain the low-crystallinity electrocatalyst (denoted as Co-RT). (c) Hydrothermal treatment of CoGa2O4 colloid leads to the high-crystallinity CoGa2O4 (denoted as Co-HT), which exhibits surface catalysis for OER due to the ionimpermeable characteristic. (d) Low-crystallinity CoGa2O4 exhibits bulk catalysis for OER due to the ion-permeable properties.

Figure 3. (a) Linear sweep voltammetry (LSV) of Co-RT and Co-HT on GC-RDE. The scan was conducted at 5 mV s−1 with IR compensation in 1 M NaOH solution. (b) The 40 h stability evaluation for Co-RT on FTO at overpotential η = 0.47 V. Inset shows the O2 production at constant current of 2 mA cm−2 detected by GC (red dot) and the theoretical amount of O2 generation (dark line), assuming a Faradaic efficiency of 100%.

higher specific surface area than high-crystallinity Co-HT (86 cm2g−1), which would contribute to the low onset potential and low overpotential at 10 mA cm−2. The electrochemical OER performance of as-prepared OECs was then characterized with the cyclic voltammetry (CV) method. Two large oxidation waves at around 1.13 and 1.46 V could be observed in Co-RT, which should be assigned to the Co2+/3+ and Co3+/4+ redox species, respectively.12 Corresponding reduction peaks at around 1.06 and 1.39 V were also shown in the reversed scan, indicating two single-electron reversible processes of the bulk redox reactions for oxidizing Co2+ to Co4+ and possibly high stability during operation for Co-RT catalysts, owing to the facilitating valency change from the weak constraint of the crystal field. By contrast, the high-crystallinity CoGa2O4 at the same electrochemical conditions exhibits a typical surface catalysis; that is, only one strong catalytic wave resulting from the OER was observed in the cyclic voltammagram. Compared to the commercial CoO and Co3O4, the Co-RT and Co-HT respectively showed a higher and lower potential requirement in both current onset and current density at 10 mA cm−2, further confirming that the Co-RT has a higher performance in the OER (Figure S2a). No visible performance degradation was observed for the Co-RT catalyst at overpotential η = 0.47 V during a 40 h OER (Figure 3b). The bubbled oxygen gas release was densely uniform without accumulation, suggesting that the porous structure might be beneficial to the mass transport. The Faradaic efficiency for the OER was calculated based on a 5 h constant current electrolysis at 2 mA cm−2 to be 98% (inset in Figure 3b), demonstrating the high selectivity of

Figure 2. (a) XRD patterns for the Co-RT (red curve) and Co-HT (black curve). (b) TEM image shows the porous structure of NaGaO2 colloid particles. (c, d) TEM image respectively shows the porous structure of Co-HT and Co-RT.

mesoporous materials with pore diameters of 4−10 nm resulting from the aggregation of nanoparticles. To probe the intrinsic electrocatalytic properties of Co-RT and Co-HT, linear sweep voltammetry (LSV) was conducted on a glassy carbon rotating disk electrode (GC-RDE) in 1 M NaOH aqueous solution at a scan rate of 5 mV s−1 with IR compensation. This method can effectively reduce the diffusion limitation of the reagent during the catalytic reaction. As shown in Figure 3a, the onset potential cathodically shifted ∼100 mV from 1.63 V for Co-HT to 1.53 V for Co-RT, and the overpotential at 10 mA cm−2 (η10 mA) was about 70 mV lower for Co-RT (380 mV) than for Co-HT (450 mV). The lowcrystallinity Co-RT (252 cm2g−1) exhibited about 3 times 12889

DOI: 10.1021/acsami.6b03890 ACS Appl. Mater. Interfaces 2016, 8, 12887−12893

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was about 10 times lower than that of Co-HT, which is the more direct evidence to confirm the excellent conductive behavior of the bulk Co-RT electrocatalysis. The chargetransfer resistance for the combined steps of the OER in CoRT is much lower than that for the dense Co-HT. Given this evidence, the ion-permeable bulk OEC would induce a short charge transport distance and enhance water oxidation by the weak-constraint Co species resulting from low crystallinity, thus improving the OER kinetics. The galvanostatic charge− discharge processes that measure the accumulative charge in the bulk OEC could be another rational explanation for the high OER kinetics, indicating that the Co-RT has much better ability for charge storage than Co-HT (Figure 4b). Indeed, the storage capacity of charges for Co-RT is about 3.5 times higher than that for Co-HT. However, the specific surface area of CoRT is only 3 times larger than that of Co-HT. This demonstrates that the excess accumulative charge would originate from the bulk active species in Co-RT, further confirming that the Co-RT is a bulk OEC. Furthermore, the turnover frequency (TOF) of Co-RT was calculated based on the total Co mole number on GC-RDE.21 The TOF value at an overpotential of 350 mV is about ∼8 × 10−3 s−1, in good agreement with the recently reported TOF value for Cocontaining bulk catalyst.22 The excellent electrochemical stability and efficient OER performance prompted us to explore the catalytic mechanism of the bulk catalyst. XPS analysis was applied to determine the species on the surface of Co-RT and Co-HT after an 8 h i−t test (Figure S5). The Co 2p3/2 binding energy and Co 2p1/2-Co 2p3/2 splitting shifted from 781.9, 16 eV, for CoGa2O4 to 780.1, 15.1 eV, for Co-RT. The Co 2p3/2 band at 780.1 eV and Co 2p1/2-Co 2p3/2 splitting of 15.1 eV located between 780.0, 15.0 eV, for CoOOH and 781.0, 16 eV, for Co(OH)2, confirming the formation of Co(OH)2/CoOOH on the surface.23 The molar ratio of Co(OH)2 to CoOOH in Co-RT was estimated according to their peak area of Co 2p1/2 spectra to be about 1:1.33 (Figure S6), in good agreement with the calculated results.24 Note that the spinel CoGa2O4 has a 0.575 inversion parameter,25 and the ratio of Co2+ to Co3+ is 1:1.35 in IV (Co0.425Ga0.575)VI[Co0.575Ga1.425]O4. This ratio of Co2+/Co3+ in Co-RT is very close to that in CoGa2O4, indicating that it was thermodynamically favorable for keeping the Co2+ and Co3+ of Co-RT by capturing OH− during the Ga leaching from the surface of the catalyst. For Co-HT after an 8 h i−t test, the Co 2p3/2 binding energy shifted from 781.9 to 780.1 eV with a Co 2p1/2-Co 2p3/2 splitting of 15.1 eV, the same as that for the Co-RT. This means that both catalysts followed the same catalytic mechanism during water oxidation, and thus they could be used as good models to distinguish surface catalysis and bulk catalysis. The low-temperature continuous-wave X-band electron paramagnetic resonance (EPR) technique was used to discover the catalytic species for the Co-RT OEC. After the i−t test for 8 h, an EPR resonance at g = 4.2 was observed (Figure 5a), corresponding to the octahedral Co2+ surrounded by hydroxide ions in β-Co(OH)2. No EPR signal of Co3+ in CoOOH was detected because the low-spin Co3+ is diamagnetic. An axial EPR signal with g⊥= 2.33 and g∥ = 2.06 was attributed to the diagnostic of the S = 1/2 system arising from the low-spin Co4+ center.26 The EPR signal of Co4+ species is consistent with the previously observed result from Co4O4 with one Co4+ and three Co3+ centers in a unimolecular Co-containing cubane, [Co4O4(C5H5N)4(CH3CO2)4](ClO4)(1).27 A qualitative EPR

the OER occurring on the Co-RT anode. The Co-HT also exhibited good stability after the sluggish activation process (Figure S2b). The Co-RT and Co-HT OECs were tested by XRD after 8 h potentiostatic electrolysis (i−t test) (Figure S3). The characteristic XRD peaks remained, indicating the great stability of these as-prepared electrocatalysts. Electrochemical impedance spectroscopy (EIS) and galvanostatic charge−discharge measurement were performed to further understand the difference in electrochemical behavior between Co-RT and Co-HT. The Nyquist plots of Co-RT and Co-HT are shown in Figures 4a and S4a,b and then fitted to the

Figure 4. (a) Electrochemical impedance spectra (EIS) recorded at 1.44 V vs RHE are shown by Nyquist plots. Inset shows the transformation from the Warburg effect to the Faradaic reaction of CoRT in an intermediate frequency region with increasing applied voltages. All the above tests were conducted in aqueous 1 M NaOH solution. (b) Galvanostatic charge−discharge measurement performed for Co-RT and Co-HT at a constant current of 0.1 mA cm−2.

equivalent circuit models (Figure S4c,d). A semicircle is present in the intermediate frequency region (about 133 Hz) of the Nyquist plots in the inset of Figure S4b, which can be assigned to the grain boundary resistance (RGB) and grain boundary capacitance (CGB) of Co-HT, on account of the abundant grain boundaries in the ion-impermeable surface OEC, Co-HT. By contrast, Warburg’s diffusion process instead of the RGB and CGB was observed for Co-RT in the inset of Figure 4a, demonstrating the OH− ion diffusing into the ion-permeable bulk OEC, Co-RT, at the low applied voltage (1.44 V). When the applied voltage was increased to 1.84 V, the Warburg behavior converted to a Faradaic process, probably meaning that the OER can occur in the bulk owing to the low crystallinity. In addition, the total series resistance of Co-RT 12890

DOI: 10.1021/acsami.6b03890 ACS Appl. Mater. Interfaces 2016, 8, 12887−12893

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injecting electrons into high-valence Co species, transforming to low-valence Co species. Consequently, the healing step promoted the bulk OEC to participate in the next reaction cycle, which ensured the catalystic sustainability and stability. A low-crystallinity bulk 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. To confirm this fact, the Mott−Schottky (M-S) measurement was performed (Figure S7). In the linear range of the M-S relationship of the semiconductor depletion layer capacitance versus potential, the Co-HT presents a negative slope, which is a typical feature for a p-type semiconductor with a large flatband potential. For the Co-RT, first, it exhibits a positive slope at the range of low potential, indicative of an n-type semiconductor. Subsequently, the semiconductor capacitance is stabilized by charging the low-valence cobalt species (Co2+ in Co(OH)2). The redox energy levels of the cobalt species can be determined based on the derivative of the M-S relationship curve, which is gradually increased by capturing charges (Figure S8). After the charging is completed, a linear range of the M-S relationship with a negative slope was observed, which would be assigned to a characteristic of p-type semiconductor originating from high-valence cobalt species (Co3+ in CoOOH). This change of conductive behavior indicated that the Fermi level of the Co-based bulk electrocatalyst was dependent on the applied potentials which changed the average oxidation state of cobalt species. For PEC water splitting, the interface between lightabsorbing SC and EC is crucial for efficient charge transfer and enhancing the semiconductor’s photovoltage. The changes in the average oxidation state of the ion-permeable Co-RT bulk OEC are equivalent to changes in the Fermi level of OECs, thus providing an adjustable interface barrier between the SC photoanode and OEC, and the concomitant adjustability in photovoltage relative to dense OECs where the buried interface is difficult to be optimized for charge separation and photovoltage generation. To explore the availability of bulk OEC in the photoelectrochemical water oxidation, the Co-RT and Co-HT were respectively electrodeposited onto the Ti-doped Fe2O3 photoanode,33 and the photocurrent as a function of applied potential was obtained under AM 1.5 G irradiation (Figure 6a). Visible initial photocurrent of Co-RT/Ti-doped Fe2O3 is observed at an applied bias of 0.7 V, which is lower than 0.8 V for Co-HT/ Ti-doped Fe2O3 and 0.95 V for bare Ti-doped Fe2O3. The photocurrent of Co-RT/Ti-doped Fe2O3 increased slowly in the region of low applied bias, which can be attributed to the kinetic limitation of charge transfer through bulk electrocatalysis. A current spike of Co-RT/Ti-doped Fe2O3 during the steady-state photocurrent test at 0.9 V (Figure S9a) is larger than that of Co-HT/Ti-doped Fe2O3, and the current spike that followed multiexpotential decays to steady-state current needs a prolonged time, confirming the kinetic limitation of charge transfer as low bias. A similar phenomenon was also observed in Co-Pi bulk electrocatalyst-modified Fe2O3 photoanode.29 However, 1.45 mA cm−2 photocurrent of the Co-RT/Ti-doped Fe2O3 is achieved at 1.23 V, which is 0.8 and 0.45 times higher than that of bare Fe2O3 (0.8 mA cm−2) and Co-HT/Ti-doped Fe2O3 (1 mA cm−2), partly originating from the improvement in the kinetic limitation of charge transfer at a high bias of 1.23 V. EIS spectra at 0.9 V were recorded for understanding the nature of photocurrent enhancement of the Ti-doped Fe2O3

Figure 5. (a) CW X-band EPR spectrum for the Co-RT after OER. (b) Illustration of the catalytic OER mechanism and the self-healing process of Co-RT OEC, including intermediate states of Co-□, CoHO*, Co−O*, and Co-OOH*.

analysis through double integration of the peak relative to that of an S = 1/2 spin standard, CuSO4·5H2O powders, shows that 3% and 12% of all the Co centers in the Co-RT, respectively, were in the Co2+ and Co4+ oxidation state. The proportion of Co2+ in the Co-RT detected by EPR was lower than that obtained by XPS analysis, because the EPR signal detected was closer to the high-valence state of Co species during OER. According to the EPR results, we believe that the OER for the Co-RT is achieved by cycling among Co2+, Co3+, and Co4+ oxo oxidation states, as demonstrated in the molecular catalysis involving O2/H2O cycles at Co centers.28 This proposed catalytic mechanism could also be confirmed by CV (Figure S2a). In situ formation and long-term stability of the Co-RT implied a self-healing mechanism. As shown in Figure 5b, the complete catalysis cycle consists of catalytic OER reaction steps and the healing step of Co species capturing OH−, reducing the high-valence Co species. This catalytic reaction process allowed OH− ions to permeate the Co-RT bulk and leads to the OER emerging there. During the catalytic step, Co3+/Co4+ oxidized OH− to produce O2, resulting in the reduction of active Co species.29,30 Usually, a low-valence state of Co species is not stable and dissolves into the solution easily. Fortunately, the propensity of metal ion dissolution had been shown to correlate with ligand substitution.31 In our case, the Co-RT would exhibit a moderate constraint of the crystal lattice for Co species if compared to the amorphous materials. The healing step prevented the dissolution, of which the process was achieved by Co species recapturing OH−. Considering that Co3+ is substitutionally inert relative to Co2+, Co2+ is more active in capturing the OH− in solution or catalyst bulk. Hence, the coexistence of Co2+ and Co3+ is beneficial to stabilizing the redox cycle of cobalt species because the phase transformation between Co(OH)2 and CoOOH is kinetically facile in the dissolution/nucleation process.26,32 A dynamic equilibrium among Co-□, Co−OH*, Co−O*, and Co-OOH* would be established via oxidation of Co2+ or Co3+ with removal of protons. As a result, the oxygen−oxygen bond was formed by 12891

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driven OER photoanode such as Mo-doped BiVO 4 (BiV0.97Mo0.03O4)37 (Figure S11). Therefore, the low-crystallinity bulk electrocatalyst enhancement of the PEC performance of the Ti-doped Fe2O3 photoanode can be attributed to the surface state passivation from the strong interface interaction and the adjustable Schottky junctions inducing the efficient charge separation.

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CONCLUSIONS To summarize, by using the low-crystallinity Co-RT as model material, we demonstrated that efficient and stable bulk OER electrocatalysis can be achieved by low-crystallinity materials. The ideal efficient and stable bulk OECs must have a high specific surface area to offer more catalytic sites, have appropriate crystallinity for minimizing the potential loss during the redox cycle of the active species and keeping the stability of the catalyst, and be ion permeable for efficient mass transport and the self-repair function. Perfect electrocatalytic activity and sustainability are crucial for hole extraction. The facile oxidation and ion permeability of OECs themselves convey a p-type characteristic, forming adjustable Schottky junctions for photogenerated hole−electron separation. The loose structure of low-cystallinity OECs leads to better attachment to the photoanodes, passivating the surface states. All of these features would effectively promote the OECs to enhance the kinetics of water oxidation on photoanodes. This new insight should thus facilitate the design of highperformance catalysts for renewable solar energy conversion.

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Figure 6. (a) Photocurrent density−voltage curves for Ti-doped Fe2O3 with a loading of Co-RT (red) or Co-HT (blue), compared with bare photoanode (black). (b) Open-circuit potential (OCP) for the Co-RT/Ti-doped Fe2O3 (red star ★), Co-HT/Ti-doped Fe2O3 (blue tilted square ◆), and bare Ti-doped Fe2O3 (black dot ●) in the dark. The open symbols, star ☆, tilted square ◇, and circle ○, indicate the OCP of Co-RT/Ti-doped Fe2O3, Co-HT/Ti-doped Fe2O3, and bare Ti-doped Fe2O3 under the AM 1.5 G illumination, respectively.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b03890. BET analysis, CV curves, potentiostatic plot, XRD patterns, EIS Nyquist plots, XPS spectra, M-S plots, and (steady-state) photocurrents of photoanodes (PDF)

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photoanode after modification of Co-based OECs. An equivalent circuit model is shown in Figure S10. Compared to the Co-HT/Ti-doped Fe2O3, the Co-RT/Ti-doped Fe2O3 has a lower hole transfer resistance (Rct,bulk/EC) at the interface between SC and OEC, meaning that the bulk OEC, Co-RT, exhibited a strong ability for hole extraction from SC. A lower charge transfer resistance (Rct,EC) at the interface between CoRT/Ti-doped Fe2O3 and electrolyte confirmed a faster OER reaction to proceed at 0.9 V bias if compared to Co-HT/Tidoped Fe2O3. To further observe the effectiveness of bulk OEC, we probed the open circuit potentials; that is, we measured the Fermi levels under equilibrium and quasiequilibrium conditions. Compared to bare Fe2O3, the Fermi levels of Fe2O3 photoanodes modified by Co-HT and Co-RT decreased 30 and 70 mV in the dark, respectively. A similar Fermi level shift is also observed in the illumination. The Fe2O3 photoanode usually exhibits an undesired surface Fermi level pinning effect due to the abundant surface states.34−36 Therefore, it is believed that the Fermi level shift probably resulted from surface state passivation by modification of the electrocatalysts. Obviously, low-crystallinity Co-RT is a better candidate for reducing the Fermi level pinning effects, probably because it is easy to form a strong interaction between Fe2O3 and low-crystallinity Co-RT with moderate ion-constraint ability. A similar improvement in PEC performance by modification of bulk Co-RT is also confirmed in another

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Corresponding Author

*E-mail: [email protected]. Funding

This work is supported by the National Basic Research Program of China (973 Program, 2013CB632404), the National Natural Science Foundation of China (no. 51572121), the Natural Science Foundation of Jiangsu Province (BK20151265), the State Key Laboratory of NBC Protection for Civilian (no. SKLNBC2014-09), and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the high magnetic field laboratory of Chinese Academy of Sciences for the ESR measurements performed by Dr. Tong Wei, Lu Yu, and Youming Zou.

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