What's the Key Factor to Ensure the Photoactivity Enhancement of

College of Chemistry and Chemical Engineering, Yantai University, Yantai, Shandong 264005, China. J. Phys. Chem. C , 2017, 121 (45), pp 25364–25371...
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What’s the Key Factor to Ensure the Photoactivity Enhancement of Fe2O3 Films with Ni(OH)2 Loading: Clues from a Structural Modification with Flagella Nanowires Tao He,* Haihua Wang, Libo Wang, Yuhua Zhao, Dong Han, Peiqing Zhang, Feng Luan, Jiazhu Li, and Jie Liu College of Chemistry and Chemical Engineering, Yantai University, Yantai, Shandong 264005, China S Supporting Information *

ABSTRACT: A much higher photoactivity enhancement of Fe2O3 photoanode films was achieved by loading flagellananowire-modified Ni(OH)2 than by loading pure Ni(OH)2. Cyclic voltammetry curves and coupled i−t/potential step chronoamperometry measurements under super band gap irradiation revealed a much heavier hole accumulation in a pure Ni(OH)2 layer. Electrochemical impedance and coupled i−t/open circuit potential transient measurements were applied to explore the dynamics of hole transfer through the Fe2O3|Ni(OH)2|electrolyte multiple interface systems, finding that the structural modification of Ni(OH)2 with flagella nanowires can speed up the charge transfer at both the Fe2O3| Ni(OH)2 and Ni(OH)2|electrolyte interfaces. Based on a recent discovery that the ion-permeable Ni(OH)2 electrocatalyst acts as a surface-attached redox system, a theoretical model was proposed to explain the influence of hole accumulation in Ni(OH)2 layer on the photoactivity of Fe2O3 films. The outcome of this work implies that the key factor guaranteeing the enhancement effect is that hole transfer rate at the Ni(OH)2|electrolyte interface should be higher than that at the Fe2O3|Ni(OH)2 interface.

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INTRODUCTION Solar energy photoelectrochemical (PEC) water splitting is a promising route to green and renewable H2 fuel.1 α-Fe2O3 is a notable photoanode material for solar water oxidation due to its intrinsic stability, favorable band gap energy (∼2.2 eV), and harmlessness.2 However, its low electron mobility and short hole diffusion length (2−4 nm) are the key drawbacks limiting its PEC activity. Loading inexpensive and earth abundant Ni-based (oxy) hydroxide electrocatalyst (EC) is an effective way to reduce oxygen evolution reaction (OER) overpotential and enlarge photocurrents of α-Fe2O3 photoanodes.3−9 This performance enhancement was roughly attributed to improved separation of photogenerated electrons and holes in Fe2O3.4−8 However, more deep understanding about this enhancement is lacking, because it is still a challenge to directly measure the interface behaviors where carriers’ separation is realized through charge interface transfer.10 Recently, the Ni(OH)2-loading induced photocurrent enlargement was found to be partly related to hole accumulation in Ni(OH)2.9 The accumulation is attributed to much faster oxidation of Ni2+ to Ni3+ and relatively slower oxidation of Ni3+ to Ni4+, which was regarded as an active site of water oxidation reactions. These mechanism investigations indicate that the dynamics of charge transfer through Fe2O3| Ni(OH)2|electrolyte multiple interfaces are crucial to the © XXXX American Chemical Society

performance enhancements of Fe2O3 photoanodes induced by loading Ni(OH)2. In this work, flagella nanowires were used as biotemplate to modify the structure of Ni(OH)2, leading to a remarkable photoactivity improvement of Fe2O3 film. In order to make clear the enhancement mechanisms, comparative investigations on the PEC activity and charge interface transfer dynamics of different Ni(OH)2/ Fe2O3 films were carried out with cyclic voltammerty (CV), chronoamperometry (i−t), electrochemical impedance spectrum (EIS), coupled i−t/open circuit potential transient (i−t/Voc) and i−t/potential step chronoamperometry (PSCA) measurement techniques. This work provides a more deep understanding of the photoactivity enhancement of Fe2O3 films induced by loading Ni(OH)2.

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EXPERIMENTAL SECTION 1. Preparation of Escherichia coli Flagella Solution. E. coli culture is a conventional microbiological experiment technique. The bacteria were cultured in sterile Luria broth (containing 1 wt % NaCl, 1 wt % Tryptone, and 0.5 wt % Yeast extract, pH ∼ 6.8) and incubated at 37 °C with constant shaking at 140 rpm for 36 h. The isolation and purification of

Received: September 14, 2017 Revised: October 22, 2017 Published: October 26, 2017 A

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Figure 1. (A) TEM image of flagella, and SEM images of (B) Fe2O3, (C) Ni(OH)2, and (D) flagella/Ni(OH)2 layers loaded in the surface of Fe2O3 films.

flagella from the cultured E. coli cell are carried out according to the reported method.11,12 In order to obtain a dispersion of flagella in pure water, this procedure is modified. In brief, 2 L culture of E. coli (1.0 × 108 CFU/ml) were centrifuged at 7000g for 15 min and the pellet was resuspended in 400 mL phosphate-buffered saline (PBS) buffer. After repeating the above procedure three times, the PBS buffer was replaced with 400 mL sterile pure water to repeat the procedure another two times. The pellet was resuspended in 30 mL sterile pure water and vortexed for 2 min, and the dispersion was centrifuged at 12070 g for 15 min. The pellet was discarded; the supernatant was freezing dried to give powder, which was redissolved in 5 mL sterile pure water. Weighting a certain amount of the flagella solution before and after water is volatized entirely at 120 °C to estimate the mass percent concentration of flagella. For this purpose, an electronic analytic balance with an accuracy of 0.00001 g (Mettler Toledo MS105) is used, and a cleaned vial (5 mL) is used to hold the flagella solution. The weighting process is repeated three times, and the estimated concentration is ca.4.2 mg/mL. 2. Preparation of α-Fe2O3, Ni(OH)2/α-Fe2O3, Ni(OH)2/ Flagella/α-Fe2O3 (F−Ni(OH)2/α-Fe2O3) and Ni(OH)2/Glutamic Acid/α-Fe2O3 (G-Ni(OH)2/α-Fe2O3) Films. Electrochemical deposition with an electrophoresis apparatus power (DYY-10C, Beijing Liuyi Biological Technology Co., Ltd., China) was carried out to prepare these films in doubleelectrode system. In order to eliminate oxide film, Ti foils (0.01 × 2 × 2.8 cm, 99.9%, Hongyada-Ferrous Metal Co., LTD, Baoji, China) were fully polished with sandpaper (1500 mesh, 3 M) and washed supersonically in ultrapure water and then in absolute ethanol each for 5 min. After drying naturally at room temperature, these Ti foils were used as both the cathode and anode. For preparation of Fe2O3 films, a freshly prepared solution of 0.25 mM Fe(NO3)3·9H2O (AR, Sinopharm Chemical Reagent

Co., Ltd.) dissolved in absolute ethanol (AR, Sinopharm Chemical Reagent Co., Ltd.) was used as deposition solution. Intermediate Fe(OH)3 films were deposited on the surface of the cathode Ti foils. During the deposition, a 20 V dc bias was applied and gave a current density of ca. 12.5 mA/cm2, the deposition duration was 60 s. The as-deposited films were rinsed with absolute ethanol to eliminate free Fe3+ ions and then naturally dried in air. After that, the films were sintered in an oven at 500 °C for 4 h. Reddish films were obtained after naturally cooled to room temperature. Static photocurrent measurements were applied to evaluate the PEC activity of these films (see below for details), and films with nearly the same PEC activity were selected for the following experiments. For preparing F−Ni(OH)2/Fe2O3 films, 60 μL flagella solution was transferred with a pipet on the surface of the Fe2O3 film. A cleaned glass rod was used to make the flagella solution spread evenly on film surface, and then the films were dried naturally at room temperature. Electrochemical deposition with constant current mode (2.5 mA/cm2) was applied to deposit Ni(OH)2 on the surface of the flagella/Fe2O3 film, which acted as the cathode during the deposition. The deposition solution was a 10 mM solution of Ni(NO3)2· 6H2O (AR, Sinopharm Chemical Reagent Co., Ltd.) dissolved in ultrapure water (18.2 MΩ·cm), and the deposition time was 25 s. The as-deposited films were rinsed with ultrapure water to eliminate free Ni2+ and dried naturally at room temperature. After that, the films were heated at 100 °C for 1 h. For the preparation of G-Ni(OH)2/Fe2O3 films, 12.5 mg/mL glutamic acid solution was first prepared by dissolving glutamic acid (AR, Sinopharm Chemical Reagent Co., Ltd.) at 90 °C. The glutamic acid solution was added into the nickel nitrate deposition solution to make the molar ratio of glutamic acid vs Ni2+ to be 1:5. Ni(OH)2/α-Fe2O3 films were prepared following the above procedure without flagella or glutamic acid. 3. Sample Characterization. The nanowire morphology of flagella was characterized with transmission electron B

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Figure 2. (A) The first cycle of CV curves measured under band gap excitation with 420 nm LED lamp, (B) the enlarged part of (A), the initial potential of the CV scan is −1.0 V and the scan rate is 10 mV/s. (C) i−t curves measured with a 0.6 V (vsAg/AgCl) bias. The legends of panels A, B, and C are the same and shown only in panel A. (D) Negative current transient curves from the PSCA measurements, which are carried out immediately after an i−t process. Inset in (D) is the time scheme of the potential step.

are rotated to provide propulsion for bacterial. During the past decades, it has been used as a biotemplate to prepare inorganic nanostructures.12−15 The as-prepared flagella aqueous dispersion is transparent and gives Tyndall effect, indicating its high dispersibility in water. After being negatively stained with uranyl acetate, they show the characteristic nanowire morphology in TEM images and are self-assembled into bundles in the TEM sample grid (Figure1A). The Fe2O3 film exhibits irregular porous structure (Figure 1B), but the porous structure cannot be detected any longer after Ni(OH)2 is further deposited (Figure 1C). The upper Ni(OH)2 layer is more compact and constructed through random aggregation of fine nanoparticles (Figure1C). The introduction of flagella leads to no remarkable change in the morphology of Ni(OH)2 layer, except for some cracks (Figure 1D). Considering the soft hollow structure of flagella, the cracks may come from volume contraction of the film during drying at 100 °C. The EDS spectra indicate the elements in these films are mainly Fe, O, in the Fe2O3 film and Fe, O, and Ni in both the Ni(OH)2/Fe2O3 and F−Ni(OH)2/Fe2O3 films (Figure S1A−D). XRD patterns prove that the Fe2O3 film is of pure hematite structure and the Ni(OH)2 film is amorphous (Figure S1E,F). X-ray photoelectron spectroscopy (XPS) was used to further identify the electrodeposited Ni(OH)2 film (Figure S1G,H). The Ni 2p spectra show two major peaks whose binding energies are centered at 857.2 and 874.8 eV. The two peaks correspond to Ni 2p3/2 and Ni 2p1/2, respectively, and give a spin-energy separation of 17.6 eV characteristic of amorphous Ni(OH)2 phase.16 In addition, the O 1s spectrum with a strong peak centered at 532.3 eV is attributed to bound hydroxide groups (OH−).17 IR spectroscopy is a powerful technique for identification of microorganisms.18 FT-IR spectra reveal that there are direct chemical interactions between flagella and Ni(OH)2, probably through coordination of surface carboxyl and C−OH groups to Ni cations (Figure S2). The CV curves measured under band gap excitation are shown in Figure 2A. It can be seen that loading Ni(OH)2 or F− Ni(OH)2 makes a negative shift of the onset potential of anodic

microscope (TEM, JEOL, JEM-1400, Japan). The structure and morphology of the film electrodes were characterized with field emission scanning electron microscope (FE-SEM, Hitachi S4800, Japan) and X-ray diffraction (XRD, Shimadzu-6100 diffractometer with a Cu Kα radiation). Fourier transform infrared (FT-IR) spectra were used to illustrate the chemical interaction between flagella and Ni(OH)2. The IR spectra were obtained with Fourier transform infrared spectroscopy (Shimadzu IRAffinity-1) and the KBrtablet technique. For IR measurements, a 10 min deposition were applied to form much thicker Ni(OH)2 film directly on the surface of Ti substrate, and the deposited Ni(OH)2 powders were scraped out before and after electrochemical conditioning and used as the IR samples. 4. Electrochemical and Photoelectrochemical Measurements. The electrochemical and photoelectrochemical experiments were executed by using a CHI electrochemical analyzer (CHI660E) with a standard three-electrode system. A probe of Ag/AgCl, KCl saturated (218; Shanghai Leici Inc.) was used as the reference electrode; a 2 cm2 Pt sheet was used as the counter electrode. 0.5 M NaOH solution was used as the electrolyte. Potential values given in this work were referenced to the Ag/AgCl electrode. No IR correction was done for the electrochemical data reported in this work. CV and i−t measurements under super band gap irradiation were carried out by using a 420 nm LED lamp (LHFC084−10, 0.6 W·cm−2, Shenzhen Lamplic Science Co., Ltd.) connected to a controller (UVEC-4II, Shenzhen Lamplic Science Co., Ltd.). For electrochemical impedance spectra (EIS) measurements, samples were prepared through a 25-s electrochemical deposition with constant current mode (2.5 mA/cm2) to load Ni(OH)2 or flagella/Ni(OH)2 in surface of both Fe2O3 films and Ti foils. The frequency range was 5 × 105 ∼ 0.02 Hz and amplitude was 20 mV.

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RESULTS AND DISCUSSION Flagella are helical protein nanowires whose diameter and length are usually 10−25 nm and 10−15 μm, respectively. They C

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Figure 3. (A) equivalent circuit of the Fe2O3|Ni(OH)2|electrolyte system, consisting of series resistance Rs, capacitances of bulk Fe2O3 (CFe2O3) and Ni(OH)2 layer (CNi(OH)2), charge transfer resistances at Fe2O3|Ni(OH)2 interface (Rct,Fe2O3|Ni(OH)2) and at Ni(OH)2|electrolyte interface (Rct,Ni(OH)2|electrolyte),24 potential-dependent evolution of (B) CNi(OH)2, (C) Rct,Ni(OH)2|electrolyte, and (D) Rct,Ni(OH)2|Fe2O3 derived from the EIS measurements under band gap excitation with 420 nm LED lamp, the Nyquist plots are given in Figure S4A,B.

the photocurrent ΔI (=I200S − I300S) increased monotonically. For the Ni(OH)2/Fe2O3 film, however, the i−t measurements in the potential range of Vonset ∼ Vonset + 0.15 V gave only static cathodic current, and ΔI was kept almost unchanged (Figure S3C). Therefore, the “Vonset” of the Ni(OH)2/Fe2O3 film shown in the CV curves (Figure 2A,B) is probably not its real Vonset. It is reasonable to think that loading Ni(OH)2 will greatly alter the interface transfer behaviors of photogenerated holes. For the Fe2O3|electrolyte system, hole interface transfer corresponds to water oxidation. While, the first step of hole interface transfer in the system of Fe2O3|Ni(OH)2|electrolyte relates mainly to the much easier and faster oxidation process of Ni2+→ Ni3+.4−8 The above discussion implies that at the beginning of photo excitation, Ni(OH)2 acts first as a hole collector, leading to a much higher separation efficiency of photogenerated electrons and holes in Fe2O3. Consequently, “anodic photocurrent” appears at more negative potentials. However, the static photocurrent measurements reveal that at potential bias near the “Vonset”, holes are just accumulated in the Ni(OH)2 layer, and almost no water oxidation reaction occurs (Figure S3B). It means that the anodic photocurrent near the “Vonset” in the CV curves of the Ni(OH)2/Fe2O3 film is of more capacitive nature (Figure 2A). The first evidence of hole accumulation in Ni(OH)2 can be found in Figure 2A, in which, when potential is scanned negatively, the Ni(OH)2/Fe2O3 film gives the remarkable cathodic current peak. Figure 2D shows the PSCA current transients of both the Fe2O3 and Ni(OH)2/Fe2O3 films from the coupled i−t/PSCA measurements. After a 100 s i−t process at a bias of 0.4 V, the 420 nm LED lamp is turned off, and the bias is shifted immediately from 0.4 V to −0.8 V, a negative current transient is recorded. The coupled i−t/PSCA measurements are controlled with the Macro command provided in the CHI 660E software, and there is no time interval between them (Please see the time scheme for the coupled measurements in Figure 2D). The −0.8 V bias is much higher than the energy

photocurrent (Vonset, marked with arrows in Figure 2B) by ca. 400 mV. When potential bias is scanned reversely from 0.5 to −1.0 V (Figure 2A), the Ni(OH)2/Fe2O3 film gives a large cathodic current peak in 0 ∼ −0.5 V (marked with a dotted circle). However, this phenomenon is not obvious for the F− Ni(OH)2/Fe2O3 film. The i−t measurements (Figure 2C) indicate the F−Ni(OH)2/Fe2O3 film gives the largest static photocurrent (0.89 mA/cm2 at 700 s) which is larger than that of pure Fe2O3 film (0.46 mA/cm2 at 700 s) by more than 2 times. It is usually difficult to make an objective comparison on the photoactivity with other reports, because of the great differences among these PEC systems, including the film structure, electrolyte, light source, and so on. However, it has been reported recently that a photodeposited amorphous NiOOH layer can improve the photocurrent of a coral-like nanostructured α-Fe2O3 film by 50%.19 Some other studies have also reported Ni(OH)2-loading induced photocurrent enhancements usually no larger than 50% based on their reported data.7,20 Therefore, herein, the 2-fold improvement of the photocurrent may illuminate the important role of flagella for the Ni(OH)2-loading-induced photoactivity enhancement. The photocurrent of the Ni(OH)2/Fe2O3 film within the initial 5 s is close to that of the F−Ni(OH)2/Fe2O3 film, but the photocurrent decays quickly until comparable to that of the Fe2O3 film after 100 s (Figure 2C). Both the negative shift of Vonset and the much higher photocurrent in the first CV cycle (Figure 2A) seem to support the general viewpoint, that is, loading Ni(OH)2 can improve the PEC activity of Fe2O3.3−9 However, the long-time i−t measurements (Figure 2C) prove that this enhancement is not obvious unless flagella is introduced. Thus, these experimental results may provide some new clue to explore the key factors, which ensure the photoactivity enhancement of Fe2O3 films with Ni(OH)2 loading. Static photocurrents at potential bias near the Vonset (Vonset, Vonset+0.05 V, Vonset + 0.1 V, Vonset + 0.15 V) were measured with the i−t technique (Figure S3A and B). For the Fe2O3 film, D

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The Journal of Physical Chemistry C level of the E c of α-Fe 2 O 3 , make the Fe 2 O 3 films conductive.21−23 During the coupled measurements, the electrolyte is continuously bubbled with N2 gas to eliminate O2 molecules, which usually act as electron extractors. On the other hand, the PSCA transient curves indicate that the electron injection processes are almost finished within the first second. Therefore, the negative current transients result mainly from electrons injecting into Fe2O3 film or into both Fe2O3 and Ni(OH)2 in the case of the Ni(OH)2/Fe2O3 film. The integral of the transient curves gives the amount of injected charge (shown in Figure 2D). It can be seen that loading Ni(OH)2 makes the injected charge increase 7.5-fold. Consistent with the large cathodic current peak given in the CV curve of the Ni(OH)2/Fe2O3 film (Figure 2A), Figure 2D is another evidence of the hole collector nature of the Ni(OH)2 layer. This hole collector effect originates from the much higher energy level of Ni2+/ Ni3+or Ni4+ than the valence band edge of Fe2O3 and, on the other hand, from intimate connection between Ni(OH)2 and Fe2O3. The second point can be supported by Ni(OH)2-loading-induced great reduction of the surface trap states of Fe2O3 (Figure S3D). For the Fe2O3|Ni(OH)2|electrolyte systems, PEC water splitting reactions relate to multiple interface transfer of photogenerated holes. Recently, EIS technique with an appropriate equivalent circuit (Figure 3A) was used to investigate charge transfer in the Fe2O3|EC (Co-pi)|electrolyte multiple-interface electrode system.24 Herein, the charge transfer dynamics were studied with the same equivalent circuit model. Figure 3B shows the plots of CNi(OH)2 versus applied potential, indicating that more serious charge accumulation occurs in the Ni(OH)2 layer when much larger anodic DC biases are applied. The monotonic increase of Rct,Fe2O3|Ni(OH)2 (Figure 3C) and decrease of Rct,Ni(OH)2|electrolyte (Figure 3D) along with the anodic shifts of DC bias are consistent with former reports.24 Importantly, Figure 3 indicates that introducing flagella nanowires into Ni(OH)2 will efficiently weaken hole accumulation in Ni(OH)2 (Figure 3B) and improve charge transfer efficiencies at both Fe2O3|Ni(OH)2 and Ni(OH)2|electrolyte interfaces (Figure 3C and D). In order to further investigate hole transfer dynamics at Ni(OH)2|electrolyte interface, a coupled i−t/Voc measurement strategy is developed herein. For the i−t/Voc measurements, the samples are Ni(OH)2 and F−N(OH)2 films, which were deposited directly on the surface of Ti foil through the same electrochemical deposition process (Experimental Section). For the i−t process, a 0.6 V (vs Ag/AgCl) bias that is positive enough to initiate OER is applied, and the duration of the i−t process is 600 s to make the current almost unchanged any longer. The system at this time is thus in a quasi static state, and the concentration of the hole accumulated in Ni(OH)2 (chemically Ni3+/Ni4+) is kept constant. The Voc transient is tracked at once after the i−t process. The coupled i−t and Voc measurements are also controlled with the Macro command provided in the CHI 660E software. During the V oc measurement, the accumulated holes are continuously consumed probably mainly through OER, that is, through oxidation of OH− in this 0.5 M NaOH electrolyte solution since there is no other species with much stronger reducibility than OH− in this system, causing Voc decay continuously. Therefore, the decay rate of Voc reflects the speed of charge transfer at Ni(OH)2|electrolyte interface, a faster Voc decay corresponds to a much quicker interface charge transfer. The

idea of the coupled i−t/Voc transient measurement is schematically illustrated in Figure 4.

Figure 4. Schematic illustration of the Voc transient measurements after a 600 s static OER process. The ion-permeable Ni(OH)2 film acts as a redox pair during the OER process.10,25,26 The dotted blue lines mark the energy levels of the redox pair, which correspond to the Voc values at different time points.

Two points can be drawn from the i−t/Voc transient measurements. First, the Voc,Ni(OH)2 and Voc,F−Ni(OH)2 at the end of the i−t process are respectively 0.38 and 0.36 V(Figure 5A,B). As discussed above, under quasi static state, Voc is related

Figure 5. (A) Voc transient curves from the coupled i−t/Voc measurements, (B) the enlarged part of (A), potential-dependent evolution of (C) C and (D) Rct derived from EIS measurements at different DC bias. Before the EIS measurements, the films were subjected to a 35 cycles of CV precondition (in the potential range of −0.2−0.8 V and with a scan rate of 10 mV/s) to make the films in a stable state. The Nyquist plots are given in Figure S4C,D.

to the concentration of the accumulated hole: a much higher hole concentration will give a more positive Voc. The 20 mV difference indicates that more holes are accumulated in the pure Ni(OH)2 film. Second, the Voc transient curves can be divided into two time domains. During the first 30 s, there is a quick Voc decay for both the two films, and the two curves are nearly parallel (Figure 5B). The Voc transients in this time domain maybe attributed to water-oxidation-induced consuming of the Ni3+/Ni4+ just located in out surface of the films, and thus exhibit similar decay dynamics. After the first 30 s, the decay of Voc,Ni(OH)2 is still kept much slower while the decay of Voc,F−Ni(OH)2 goes faster. The decay values (ΔVoc) at 500 s are respectively 0.086 V for the former and 0.492 V for the latter, giving an average decay rate of the F−Ni(OH)2 film 5.7 E

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The Journal of Physical Chemistry C times larger than that of the Ni(OH)2 film. Therefore, the coupled i−t/Voc transient measurements demonstrate that the F−Ni(OH)2 film shows much faster charge interface transfer behavior. In order to further confirm the remarkable difference of the interface charge transfer rate, EIS measurements are carried out along with the proceeding of the oxygen evolution electrocatalysis reaction. The Nyquist plots were fitted with the Randles equivalent circuit to track potential dependent evolution of interface capacitance (C) and Rct (Figure 5C,D). In the potential range of 0.6−0.7 V vs Ag/AgCl, Ni(OH)2 is conductive like a metal because Ni2+ is fully oxidized. C thus can be approximated to be the double layer capacitance (Cdl). It has been pointed out that Cdl is a reliable indicator of the electrochemically active surface area (ECSA) when the EC film is highly conductive.27 Herein, the fact that Cdl shows a roughly linear relationship with electrochemical deposition time (loading) can support this point (Figure S5). Figure 5C implies that the ECSAs of Ni(OH)2 and F−Ni(OH)2 are very close. It means that flagella did not lead to remarkable improvement of ECSA. The relatively smaller ECSA of F− Ni(OH)2 is probably related to its much higher OER EC activity, that is, more O2 is formed and adsorbed in the surface of F−Ni(OH)2, causing a much larger reduction of ECSA.27 Consistent with Figure 3D, importantly, Figure 5D also shows that Rct, EC|electrolyte decays along with the anodic shift of DC bias, and the F−Ni(OH)2|electrolyte interface gives much smaller charge transfer resistance. Recently, it has been revealed that the ion-permeable Ni(oxy)hydroxide electrocatalytst (EC) layer in effect functions as a surface-attached redox system.10,25,26 For an ion-permeable EC/semiconductor photoanode, the EC has a dynamic Fermi level independent of the semiconductor’s band structure. Based on this discovery, a model is proposed herein to explain how the multiple-interface hole transfer exerts influence on the photoactivity of Fe2O3 (Figure 6). As holes are accumulated in the Ni(oxy)hydroxide layer, the Fermi level of the redox system (Eredox‑EC) is shifted toward the valence band top of Fe2O3. As a result, the driving force (Δη = Ef,p − Eredox‑EC) for hole transfer from Fe2O3 to Ni(oxy)hydroxide may be weakened (Figure

6B), causing the increase of recombination of photogenerated electrons and holes in Fe2O3. This model implies that the Ni(OH)2-loading-induced PEC activity enhancement of Fe2O3 should depend on the hole transfer rates at both the two interfaces. Only if hole transfer rate is much faster at Ni(OH)2| electrolyte than at Fe2O3|Ni(OH)2 can the enhancement be achieved. Otherwise, hole accumulation will occur and damage the enhancement effect eventually. This model can be well supported by the electrochemical measurement results obtained in this work. During the PEC process, the accumulation of holes in EC will shift their Fermi level downward and consequently make the water oxidation driving force (η = Eredox‑EC − EO2/OH−) gradually enlarged (Figure 6B), which can explain the gradual decrease of Rct, EC|electrolyte in Figures 3D and 5D. Moreover, more holes accumulating in EC will cause a much faster decay of Rct,EC|electrolyte, consistent with the situation of pure Ni(OH)2 shown in Figures 3D and 5D. Actually, an enough positive bias (for example, 0.7 V vs Ag/AgCl in Figure 5D) can make Rct, Ni(OH)2|electrolyte quite close to Rct,F−Ni(OH)2|electrolyte. The adaptive junction nature of ion-permeable EC|semiconductor system predicted that, for semiconductors with deep valence bands, like TiO2, the device performance should be independent of EC’s activity, while the EC’s activity is crucial to photoanodes with less positive valence-band positions.10,25,26 Considering the less positive Ev of Fe2O3, herein, the fact that compared with pure Ni(OH)2, F−Ni(OH)2 can induce much larger photoactivity enhancement seems to support this prediction. It is because the less positive Ev gives a more weakened driving force (η = Eredox‑EC − EO2/OH−) for OER at EC|electrolyte interface. For our systems herein, obviously, both the Fe2O3|Ni(OH)2|electrolyte and Fe2O3|F−Ni(OH)2| electrolyte systems have the same ηmaximum (when Eredox‑EC is aligned with Ev). In this situation, the EC’s activity, that is, the hole transfer efficiency at EC|electrolyte interface is paramount to avoid undue accumulation of hole in EC, which may cause the increase of recombination in Fe2O3, as will be discussed below. As has been revealed in Figures 2A,D and 3B, the Ni(OH)2/ Fe2O3 film has much heavier hole accumulation in Ni(OH)2 layer. As a result, along with the proceeding of PEC process, the Eredox‑EC will be gradually shift downward, and a much larger η will be reached in the end (corresponding to the situation shown in Figure 6B). Considering the usually high OER EC activity of Ni(OH)2, the relatively larger η would have exerted some positive effects on the photoactivty of the Ni(OH)2/ Fe2O3 film. However, Figure 2C indicates that both the Fe2O3 and Ni(OH)2/Fe2O3 films have nearly the same static photocurrent, indicating that the enhancement effect of the Ni(OH)2 layer prepared herein on the photoactivity of Fe2O3 is limited. Therefore, in addition to η, loading Ni(OH)2 induced photoactivity enhancement probably also relates to other factors. Based on the model proposed herein (Figure 6), we emphasize that the hole transfer rate at the Fe2O3|Ni(OH)2 interface should also be an important factor. The accumulation of hole in EC makes Eredox‑EC shift downward and Δη = Ef,p − Eredox‑EC is thus reduced (Figure 6B). On the other hand, for nanostructured Fe2O3 hole interface transfer is mainly through surface trap states.28 It means that photogenerated holes are first trapped in these surface states from which hole interface transfer occurs. Thus, the real driving force (Δη) for hole transfer at Fe2O3|Ni(OH)2 interface may not be Ef,p − Eredox‑EC

Figure 6. Band diagrams for semiconductor|ion-permeable EC interface, highlighting time-dependent evolution of the driving force for hole transfer at EC|electrolyte interface (η) and at semiconductor| EC interface (Δη). (A) corresponds to a situation at the initial stage of PEC process and (B) to a situation after a much longer PEC process. Neglecting the shift of hole quasi-Fermi level (Ef,p) considering the less positive valence-band position of Fe2O3. Ec, Ev and EO2/OH− are respectively the energy levels of the conduction band and valence band of Fe2O3 and the O2/OH− redox pair. F

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The Journal of Physical Chemistry C but Etrap − Eredox‑EC. Apparently, it will make Δη further weakened. A smaller driving force leads to a higher Rct,Fe2O3|EC and consequently a higher recombination. Along with the proceeding of PEC process, hole accumulation in the pure Ni(OH)2 layer becomes more and more heavier (Figures 2A,D and 3B). Correspondingly, Δη will become smaller and smaller (Figure 6B), leading to a gradual rising of Rct, Fe2O3|Ni(OH)2 (Figure 3C). The above discussions imply that the recombination in the Ni(OH)2/Fe2O3 film is more serious. It is reasonable to think that the Ni(OH)2loading-induced photoactivity enhancement cannot appear any longer when the recombination is so serious that charge transfer at Fe2O3|Ni(OH)2 becomes the rate-controlling step. It is probably why both Ni(OH)2/Fe2O3 and Fe2O3 have almost the same static photocurrent (Figure 2C). For a given system, on the other hand, the Etrap is probably fixed, and the driving force Δη (Δη = Etrap − Eredox‑EC) thus relies mostly on Eredox‑EC. Considering the adaptive character of the Fe2O3|ion-permeable EC junction, a faster hole transfer at EC|electrolyte interface (a higher EC activity) results in less accumulation of holes in EC; consequently, a much higher Eredox‑EC level can be retained. It means that, for the F− Ni(OH)2/Fe2O3 system, an enough large Δη may be kept during PEC process. It can explain why the F−Ni(OH)2/Fe2O3 film shows relatively higher photoactivity than both the Fe2O3 and Ni(OH)2/Fe2O3 films. Glutamic acid-assisted electrochemical deposition was applied to prepare the G-Ni(OH)2 films, which show remarkably enhanced OER activity. Systematic photo/electrochemical measurements were also carried out with the GNi(OH)2/Fe2O3 films (Figure S6). The measurement results also support our model (Figure 6). Figure 6 implies that the adaptive junction may play a “double-edge sword” role in ion-permeable EC|semiconductor photoanode system. Hole-accumulation-induced shift of Eredox‑EC on one hand can increase the driving force of OER at EC|electrolyte interface (η), while on the other hand, will weaken the driving force of hole transfer at semiconductor|EC interface (Δη) (Figure 6B). This should be paid attention for semiconductors with less positive valence bands. Long time i−t measurements (Figure S7) and IR spectra analysis (Figure S2) prove that the photoelectrochemical response of the F−Ni(OH)2/Fe2O3 films is not related to oxidation of flagella protein. As has been revealed above, introducing flagella leads to no improvement of ECSA (Figure 5C). Therefore, there should be some other factors responsible for the efficient hole transfer dynamics at the F−Ni(OH)2| electrolyte interface. Considering the coordination interaction between Ni caitons and carboxyl group in flagella surface (Figure S2), it is reasonable to think that flagella nanowires probably act as nucleation sites of Ni(OH)2, exerting influence on the microstructure of catalytic active sites (M−O bond length, coordination number), layer spacing, and nanostructures. As a result, the OER kinetic overpotential and ion diffusion behavior through Ni(OH)2 may be modified due to the existence of flagella. These factors need to be further investigated.

proceeding of the PEC process, however, the ion-permeable character of Ni(OH)2 will shift Eredox‑EC downward and consequently weaken the driving force for hole transfer at the Fe2O3|Ni(OH)2 interface, that is, will raise Rct,Ni(OH)2|Fe2O3. Much heavier hole accumulation in Ni(OH)2 will take place when the charge transfer rate at Ni(OH)2|electrolyte is smaller than that at the Fe2O3|Ni(OH)2 interface, which further reduces the driving force for hole transfer at the Fe2O3| Ni(OH)2 interface and thus causes more serious recombination. Therefore, for ion-permeable EC/Fe2O3 photoelectrodes, a high enough charge transfer efficiency at EC|electrolyte interface is the key to guarantee the EC induced photoactivity enhancement of Fe2O3.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b09167. Additional data of characterization about the structure, composition, chemical state of the photoelectrode films and the chemical interactions between flagella and Ni(OH)2; Nyquist plots of the EIS spectra, CV and i− t data (1) to make clear the capacitive nature of the photocurrent at potentials near the Vonset, (2) to estimate the electrochemical active surface area, (3) to explore charge transfer dynamics in the G-Ni(OH)2/Fe2O3 film system, and (4) to confirm the enhancement of the photocurrent is not related to oxidation of flagella with holes (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tao He: 0000-0001-7055-746X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Grant No. 21171144, 21675138), the Shandong Province Natural Science Foundation (Grant No. ZR2015BQ012, ZR2016BQ15), and the Graduate Innovation Fund of Yantai University (Grant No.YDYB1714).

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REFERENCES

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CONCLUSIONS In summary, Ni(OH)2 acts first as hole collector in the system of Ni(OH)2/Fe2O3 photoanode, responsible for the high separation efficiency of photogenerated electron and hole in Fe2O3 at the initial stage of the PEC process. Along with G

DOI: 10.1021/acs.jpcc.7b09167 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.7b09167 J. Phys. Chem. C XXXX, XXX, XXX−XXX