Fabrication and Kinetic Study of a Ferrihydrite-Modified BiVO4

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Fabrication and Kinetic Study of Ferrihydrite Modified BiVO4 Photoanode Fengshou Yu, Fei Li, Tingting Yao, Jian Du, Yongqi Liang, Yong Wang, Hongxian Han, and Licheng Sun ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03483 • Publication Date (Web): 24 Jan 2017 Downloaded from http://pubs.acs.org on January 28, 2017

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Fabrication and Kinetic Study of Ferrihydrite Modified BiVO4Photoanode †

†

‡

†

†

†

‡

Fengshou Yu, Fei Li, ,*Tingting Yao , Jian Du, Yongqi Liang, Yong Wang, Hongxian Han ,and †§

Licheng Sun , †

State Key Laboratory of Fine Chemicals, DUT-KTH Joint Education and Research Center on

Molecular Devices, Dalian University of Technology (DUT), Dalian 116024, China. ‡

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, ChineseAcademy of

Sciences, Dalian National Laboratory for Clean Energy, Dalian 116023, China. §

Department of Chemistry, School of Chemical Science and Engineering, KTH Royal Institute of

Technology, 10044 Stockholm, Sweden.

ABSTRACT: In spite of great progress on surface modification of semiconductorphotoelectrode, the role of metal oxide cocatalyst on photoelectrochemical (PEC) performance is still not well understood. In this study, ferrihydrite (Fh) as a novel cocatalyst was decorated on a worm-like nanoporous BiVO4photoanode. Surface kinetics study of Fh/BiVO4by intensity modulated photocurrent spectroscopy (IMPS) evidences the primary role of Fh on PEC performance enhancement varying with the loading of Fh. It was found that dispersed Fh nanoparticles

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accelerate hole transfer for water oxidation, but the resulted photoanode suffers from poor stability. The thick layers of Fh address the stability of electrode by suppressing surface charge recombination, but result in reduced hole transfer rates. Modification of BiVO4 film with optimally thick layers of discrete nanoflakes effectively reduce charge recombination without compromising stability, leading to a high AM 1.5 G photocurrent of 4.78 mA/cm2 at 1.23 V versus the reversible hydrogen electrode and an applied bias photon-to-current efficiency of 1.81% at 0.61 V. These values are comparable to the best results reported for undoped BiVO4.

KEYWORDS: bismuth vanadate, water splitting, photoanode, passivation, ferrihydrite

Introduction Splitting water by photoelectrochemical (PEC)cells is an ideal way for sustainable fuel supply.1-6 To construct an efficient PEC device, metal oxide semiconductors capable of absorbing sunlight broadly, transferring energy to excited states at high efficiency and efficiently catalyzing chemical reactions are desired.7 In this regard, bismuth vanadate (BiVO4) is one of the highest performing, visible light-active n-type photoanodes. It shows a direct band-gap of approximately 2.4 eV with an appropriate valence band position for O2 evolution as well as a favorable conduction band (CB) edge position near the thermodynamic H2 evolution potential.8,9 As a result, complete water splitting with BiVO4 requires only a small external bias. However, BiVO4 suffers from severe surface recombination, poor charge transport, and sluggish water oxidation kinetics, which limit its practical application.9To alleviate these limitations, various efforts such as element doping,10-14 construction of heterojunctions and surface modification have been made.15-19


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Decoration of metal oxides on photoanode is a promising approach to improve photoelectrochemical water oxidation.9,

11, 20-21

Despiteof a number of metal oxide cocatalysts

reported, their exact role in an integrated system is still controversial. Given that cocatalysts could improve charge transfer (e.g. water oxidation kinetics) or interfacialrecombination, which are the primary factors determining photoanodic performance under fixed light density and bulk charge separation22, understanding their detailed mechanism on water splitting becomes an urgent task. Recent studies on Co-Pi modified hematite suggest the primary contribution of CoPi is the passivation of surface states instead of accelerating hole transfer for water oxidation.23 Other metal oxides such as FeNiOx and IrO2 on hematite were also found to reduce surface recombination other than promote water oxidation.24In addition, the passivation effect on hematite photoanodewas regulated byapplied bias and cocatalyst deposition methodology.25 For example, Naldoniand co-workers found electrodeposited NiOxinduces only apartial passivation of surface defects, while the NiOxphotodeposition is able to completely modify thehematite defectivesites.25bHowever, the quantification of the impact of charge transfer and surface recombination has not yet been applied to BiVO4 system. It is interesting to know if the same mechanism also governs the water oxidation by metal oxide modified on BiVO4. Recently, ferrihydrite(Fh) was employed as a hole-storage layer to protect unstable Ta3N5photoanode against photocorrosion in solar water splitting.26-27 In particular, a Ni(OH)X/Fh/Ta3N5 film decorated with molecular cocatalysts exhibited high photocurrent density approaching the theoretical value for Ta3N5 under sunlight irradiation.27 In view of the advantage of this material on surface modification, we report here the decoration of Fh nanoparticles on a nanoporous BiVO4photoanode. By changing the loading of Fh, size and morphology effects on PEC properties were observed. We were able to relate these effects to the


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charge transport kinetics at semiconductor/electrolyte interface, the variation of charge transfer and surface recombination rate constants with the amount of Fh coating was highlighted. In brief, the presence of dispersedFhnanoparticles could efficiently improve hole transfer, while high amount of Fh forms a compact hole-storage layer which reduces both the interfacial electronhole recombination and charge transfer. At an optimal loading, a thin layer coating of Fh on BiVO4effectively suppresses charge recombination by surface passivation when maintaining the rates of hole transfer, leading to significantly improved PEC activity, which is among the highest performance for undoped BiVO4-based photoanodes.10, 16, 19, 28

Experimental Section BiVO4synthesis The BiVO4films were prepared by modification of a published procedure.28 Firstly, a BiOI film was electrodeposited on an indium tin oxide (FTO) substrate by the following procedure: 2 mmol Bi(NO3)3 was dissolved in 50 mL pH 1.7 HNO3 solution. After stirring for 10 min, 20 mmol KI was added to the solution at room temperature and the mixture was stirred for another 10 min. This solution was mixed with 20 mL of absolute ethanol containing 4.6 mmolp-benzoquinone, and was vigorously stirred for a few minutes. Electrodeposition was carried out in a typical three-electrode system at -0.1 V vs. Ag/AgCl for 5 min to obtain the BiOI electrode, which was rinsed with deionized water and dried in the air. BiVO4 electrodes were prepared by placing 30 µL of a dimethyl sulfoxide (DMSO) solution containing 0.2 M vanadylacetylacetonate (VO(acac)2) on the BiOI electrodes, followed by heating in a muffle furnace at 450oC(ramping rate 2 oC /min) for 2 h. After cooling to room


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temperature the electrodes were soaked in 0.5 M NaOH solution for 30 min to remove the excess V2O5. The obtained pure BiVO4 electrodes were rinsed with deionized water and dried in air. Photoanodes fabrication The deposition of Fh on BiVO4 and FTO substrates was conducted via hydrothermal reaction.29 5mmol ferric nitrate and 37.5 mmol sodium nitrate were dissolved in 100 mL water. The solution was poured into a Teflon-lined stainless steel autoclave until 80% of the volume of the autoclave was occupied and the substrate was immersed in it. After leaving the autoclave in a 373 K oven for a desired time period, it was cooled down to room temperature. The thin layer formed on substrate was thoroughly washed with water before oxygen evolution testing. In order to compare the photoelectrochemical water oxidation activity of FeOOH/BiVO4 with that of Fh/BiVO4, FeOOH was photo-deposited on BiVO4 from a 0.1 M FeSO4 solution at 0.25 V vs. Ag/AgCl by passing 17.6mC/cm2charges to ensure the same amount of Fe loading with 30Fh/BiVO4.28 The deposited film was washed with water before use.To explore the electrochemical property of FeOOH, it was electrodeposited on FTO from a 0.1 M FeSO4 solution at 1.2 V vs. Ag/AgCl at 70oC by passing 0.12 C/cm2 while gentle stirring.28 Photoelectrochemical and electrochemical measurements Photoelectrochemical and electrochemical performances of as-prepared anodes (BiVO4, Fh/BiVO4, FeOOH/BiVO4, FeOOH/FTO and Fh/FTO) were collected in a standard three electrode system, with the anodes as the working electrode, a platinum wire as the counter electrode, and Ag/AgCl as the reference electrode controlled by a CHI 760Epotentiostat. The simulated solar illumination was obtained by passing light from a 300 W Xenon lamp equipped


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with an AM 1.5 filter, and the power intensity of the incident light was calibrated to 100 mW/cm2. The 0.4 M sodium borate buffer solution (pH 9) wasused as the electrolyte. The photocurrent was measured by linear sweep voltammetry with a scan rate of 10 mV/s.The data were collected by back illumination. Intensity modulated photocurrent spectroscopy (IMPS) measurements were conducted by apotentiostat (IM6ex, Zahner Company) controlled by ZAHNER IMPS electrochemical workstation. Intensity modulated light was provided by a light-emitting diode (LED) that allowed superimposition of sinusoidal modulation (~10%) on a d.c. illuminationlevel. The wavelength of light is 430 nm with an average intensity of 1.5 W m-2, and the modulation amplitude of lamp voltage is 10 mV. The photocurrent as a function of frequency (from 10 KHz to 100 mHz) after turning on the light was recorded at different potentials. The Faradic efficiency was measured in a single gas-tight cell. Prior to measurement, the solution was degassed by bubbling Ar for 0.5 h. The produced gases were analyzed by gas chromatograph (GC). The applied bias photon-to-current efficiency (ABPE) was calculated from the J-V curve using the following equation, where J is the photocurrent density, Vbias is the applied bias, and Pin is the incident illumination power density (AM 1.5G, 100 mW/cm2).  J (mA / cm 2 ) × (1.23 − Vbias )(V )  ABPE (%) =  × 100  Pin (mW / cm 2 )   AM 1.5G

Results and Discussion Fabrication and characterization of photoanodes


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Porous BiVO4 film has been demonstrated to facilitate electron-hole separation and suppress bulk carrier recombination in semiconductor.9, 28 Thus a BiVO4 thin film featuring nanoporous topology was prepared by modification of a protocol reported by Choi’s group (see experimental section for details).28 In the first step, BiOInanosheets were grown on FTO by a reaction between Bi(NO3)3 and KI. Compared with the original method, the two reactants were added in a reversed sequence, this modificaiton resulted in the generation of a more densely packed BiOInanosheets (Figure S1). During the subsequent annealing process in the presence of VO(acac)2, the dense BiOInanosheets are prone to fuse together and form BiVO4 film composed of nanoworm-shaped particles with the relevant particle length scale of 200-500 nm. A similar worm-shaped film with smaller particle size has been reported by Domen’s group using a different synthetic method.30It should be noted that our BiVO4 film is lack of macropores that observed in Choi’s BiVO4 film. The fluffy packing of BiVO4nanoworms mainly forms micropores and mesopores on the surface as revealed by N2adsorption-desorptionisotherms (Figure S2). In addition, the nanoworm film has a more planar surface than Choi’s film (Figure S3).The loading of ferrihydrite was carried out via a hydrothermal reaction (see experimental section for details). The amount of Fh on nanoporous BiVO4 were controlled by varying the hydrothermal deposition period (15, 30 and 60 min) and the resulting composite electrodes are denoted as 15-, 30-, 60-Fh/BiVO4, respectively. The amounts of Feon electrode were quantified by inductive coupled plasmamass spectrometry(ICP-MS) to be 5.9×10-8mol/cm2 for 15-Fh/BiVO4, 1.8×10-7mol/cm2 for 30Fh/BiVO4, and 2.0×10-6mol/cm2for 60-Fh/BiVO4, respectively. Morphology changes on these electrodes were observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). For example, the SEM image of 15-Fh/BiVO4 in Figure 1a reveals a similar


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morphology to that of bare BiVO4, suggesting tiny particles of Fh are dispersed on electrode surface.The sizesof these particles were determined to be 3-5 nm by TEM measurement (Figure 1e). For 30-Fh/BiVO4, small Fhnanoflakes (10-20 nm) are observed to constitute a discrete layer that provides an incomplete coverage of the underlying BiVO4 particles (Figure 1b and Figure 1c). TEM image shows the thickness of the discrete layer isless than 5 nm (Figure 1f) and displays a lattice spacing of 0.25 nm (Figure 1g), consistent with the d-spacing of the Fh (110) plane. For 60-Fh/BiVO4 (Figure 1d), the deposited Fh aggregatesto a compact film and individual large-sized islands on top of the film. The thickness of the compact coverage was more than 100 nm for 60-Fh/BiVO4 (Figure 1h).Moreover,the presence of Fhwas unambiguously confirmed by either energy-dispersive X-ray spectroscopy (EDS, Figure S4) or X-ray photoelectron spectroscopy (XPS, Figure S5). The amounts of Feare1.6%, 4.2% and 70%estimated by EDS and 3.8%, 9.2% and 90%estimated by XPS, for 15-, 30-, and 60Fh/BiVO4respectively. The X-ray diffraction (XRD) pattern of the as-preparedFh powder exhibits a poorly crystallized structure in agreement with the previous report (Figure S6).31 UVvis absorption spectra show essentially the same absorption edges for pure BiVO4 and 30Fh/BiVO4 (FigureS7), suggesting light absorption was less disturbed by the presence of Fh resulted from 30 min hydrothermal reaction. While a thicker layer of 60-Fh was found to compete with the underlying BiVO4 layer for light absorption (Figure S7).


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Figure 1.SEM images of 15-Fh/BiVO4 (a), 30-Fh/BiVO4 (b and c),and 60-Fh/BiVO4 (d)electrodes,TEM images of15-Fh/BiVO4 (e),30-Fh/BiVO4 (f) and 60-Fh/BiVO4 (h) electrodes, and HRTEM image of 30-Fh/BiVO4 electrode (g). Dependence of photocurrent on Fh loading The photoelectrochemical properties of photoanodes with different Fh loading were examined in 0.4 M pH 9 borate buffer solutions under AM 1.5G simulated sunlight (100 mW/cm2). The typical photocurrent-potential (J-V) plots for water oxidation in Figure 2a confirm that all Fh modified samples exhibit enhanced photocurrents than bare BiVO4. The highest performance was obtained by the 30-Fh/BiVO4samplewith high repeatability (Figure S8), showing a photocurrent onset potential at 0.25 V vs. RHE followed by rapid increase in photocurrent. A photocurrent density of 4.78 mA/cm2 was observed at 1.23 V, which is among the highest photocurrents achieved by undoped BiVO4.High current density was also obtained at the region of low bias region due to the high-fill factor. A current density as high as 2.86 mA/cm2 was achieved at half of the thermodynamic water oxidation potential of 0.6 V, which is considered to


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be favorable for practical use. By comparison, the J-V curve of 15-Fh/BiVO4 features a similar shape to that of 30-Fh/BiVO4 but with lower photocurrents. Rather poor activity was found for 60-Fh/BiVO4.

Figure 2.(a) J-V characteristics for bare BiVO4, 15-Fh/BiVO4, 30-Fh/BiVO4, and 60Fh/BiVO4photoanodes measured in 0.4 M borate buffer (pH 9) under illumination (AM 1.5G, 100 mW/cm2, scan rate = 10 mV/s). (b) J-t curves for bare BiVO4, 15-Fh/BiVO4, 30-Fh/BiVO4, and 60-Fh/BiVO4photoanodes measured at 1.23 V vs. RHE. Under chopped illumination, obvious photocurrent transient spikes were observed at the initial stage of each light on-off cycle for both bare BiVO4 (Figure S9a)and 15-Fh/BiVO4(Figure S9b), indicating that the photo-generated holes are partly lost to surface recombination.32 Under the same conditions, 30-Fh/BiVO4 exhibits large photocurrent density without transient spikes over a wide potential range, consistent with that obtained by linear sweep under continued irradiation (Figure S9c). It could be deduced that 30-Fh layer is able toefficiently reduce charge recombination. For 60-Fh/BiVO4, however, significantly reduced photocurrent and the appearance of reverse transient spikes indicate holes are stored in Fh layer (Figure S9d).26, 27The negative transient observed in chopped-light experiment following light switch-off has been


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assigned to recombination of bulk BiVO4 electrons with the surface accumulated holes during irradiation, the integration of the negative transient current thus quantifies the stored charge associated with the back electron-hole recombination process.32-34Indeed, chronamperometry measurements showed the charge storage of BiVO4 is increased by 20-50 fold owing to the addition of Fh (Figure S10), further confirming 60-Fh serving as a hole-storage layer.26, 27 The photoanodes were imposed to a constant potential of 1.23 V for 300 s chronoamperometric J-t study (Figure 2b). All samples show satisfied stability except 15Fh/BiVO4. It produces a high photocurrent density in the early stage of photolysis, but the current decreases significantly within minutes, indicating that accumulationof photogenerated holes at the surface states of BiVO4results in severe anodic photocorrosion.20In contrast to 15Fh/BiVO4, the uniform coating of Fh in 60-Fh/BiVO4 sample can passivate the surface states on BiVO4 and the accumulated holes were stored in Fh layer, thus protecting BiVO4 against oxidative deactivation. As the best performed sample, a steady photocurrent of 5 mA/cm2 was obtained by 30-Fh/BiVO4, which is five times that of bare BiVO4 and three times that of 60Fh/BiVO4. Depending on these comparisons, it is clear that loading an optimal amount of Fh is essential to satisfy the demands for both high efficiency and high stability. PEC characteristics of 30-Fh/BiVO4 The wavelength dependence of the incident photon-to-current conversion efficiency (IPCE) for 30-Fh/BiVO4 was examined at 1.23 and 0.6 V using a monochrometer(Figure 3a). At 450 nm, the IPCE value reaches 88% at 1.23 V, which is superior to the previously reported undoped BiVO4 (60% @1.2 V by Domen and 60% @1.23 V by Lee).16,19 In terms of these measurements, photocurrent densities of 4.58 mA/cm2 at 1.23 V and 2.75 mA/cm2 at 0.6 V for 30-


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Fh/BiVO4were estimated by integrating the IPCE value over the AM 1.5G solar spectrum (Figure S11), these values agree well with our measured data using the solar simulator (4.78 mA/cm2 @1.23 V and 2.86 mA/cm2 @ 0.6 V).

Figure 3. (a) IPCE spectra of 30-Fh/BiVO4 at 1.23 and 0.6 V in 0.4 M pH 9 borate buffer with standard three-electrode system. (b) The applied bias photon-to-current efficiency (ABPE) for 30-Fh/BiVO4. (c) J-t curves of 30-Fh/BiVO4 (red) and bare BiVO4 (black) at 0.61 V vs. RHE for 50 h illumination withoutstirring. The maximum applied bias photon-to-current efficiency (ABPE) for 30-Fh/BiVO4 was determined to be 1.81% at 0.61 V in a standard three electrode system (Figure 3b). The ABPE obtained by us is comparable to that of NiOOH/FeOOH/BiVO4 reported by Choi (1.75%).28 At


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the potential attaining the maximum ABPE, a current density of 2.7 mA/cm2 maintained for at least 50 hours without obvious decay, demonstrating the excellent long-term stability (Figure 3c). During a 4 h photolysis, the Faradaic efficiency was determined to be 100% with a ratio of 2:1 for the produced oxygen and hydrogen (Figure S12). XPS spectra of photoanode show no apparent change after photolysis, indicative of the intrinsic robustness of Fh against photocorrosion (Figure S5). IMPS analysis of carrier kinetics In order to understand how Fh improves PEC water oxidation, the charge transfer and surface recombination kinetics were quantified by intensity modulated photocurrent spectroscopy (IMPS). In this measurement, an a.c. perturbation of the light intensity is superimposed on illumination of the electrode at a constant applied potential, and the periodic photocurrent response of the system is reported as a function of the modulation frequency. Figure 4a shows typical

IMPS

responses

of

bare

BiVO4,

15-Fh/BiVO4,

30-Fh/BiVO4,

and

60-

Fh/BiVO4photoanodes in the complex plane. The semicircle in the first quadrant is associated with the competition between interfacial minority carrier transfer and electron-hole recombination. The semicircle observed in the fourth quadrant provides information on the combination of charge transfer and relaxation inside the photoanode.33-35Accordingly, the frequency of the maximum imaginary corresponds to the sum of the charge transfer (ktrans) and recombination (krec) rate constants (ktrans + krec = 2πfmax). In addition, the ratio of ktrans/(krec + ktrans) could be given by comparing the steady state photocurrent and the instantaneous photocurrent observed upon light illumination.27,

36

The key parameters krec andktransare therefore readily

accessible.


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Figure 4.(a)Intensity modulated photocurrent spectroscopy (IMPS) responses ofbare BiVO4 (black), 15-Fh/BiVO4 (red), 30-Fh/BiVO4 (green), and 60-Fh/BiVO4 (dark cyan) at 1.0 V. (b) Chargetransfer rate constants (krec); (c) charge recombination rate constants (ktrans); (d) Charge transfer efficiencies of different anodes extracted from IMPS analysis.The data at 0.4 V is missing for 60-Fh/BiVO4 because no charge can reach the electrode surface. With bare BiVO4 as a reference, the higher ktrans values are obtained by 15-Fh/BiVO4and the lower ktrans values are obtained by 60-Fh/BiVO4 over the entire measured potential range. While 30-Fh/BiVO4shows comparable ktransvalues to those of bare BiVO4, especially at high bias (0.7 V < E < 1.0 V)(Figure 4b). The trend of ktransexhibited by Fh (15-Fh/BiVO4 > 30-Fh/BiVO4 > 60Fh/BiVO4) agrees with our general knowledge that catalyst particles with smaller size are more 14 Environment ACS Paragon Plus

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efficient oxygen evolution catalyst (OEC) to speed up the hole transfer from semiconductor to water. Figure 4c shows the surface recombination rate constants at different potentials. The kreccurve of 15-Fh/BiVO4 was found toessentially overlap with the one for bare BiVO4. In sharp contrast, significantly lower krecvalues are observed for 30-Fh/BiVO4 and 60-Fh/BiVO4 (Figure 4c). For instance, the value of krec is 10.2 s-1 for 15-Fh/BiVO4 at 0.6 V, which is 50 times larger than that of 30-Fh/BiVO4 (0.2 s-1) and 20 times larger than that of 60-Fh/BiVO4 (0.4 s-1). Given that the surface of photoanodeis largely covered by either 30-Fh or 60-Fh, the significantly reduced surface recombination was attributed to surface passivation by Fh layer. In such a case, the BiVO4/H2O interface is replaced by BiVO4/Fh/H2O, which greatly eliminates the Fermi-level pinning effect caused by surface states. In line with this assumption, the highly dispersed 15-Fh nanoparticles are inefficient to change the inherent BiVO4/H2O surface trap states due to an incomplete coverage. The overall effects of charge transfer and surface recombination are schematically summarized in Figure 5. The steady-state photocurrent densities exhibited by these photoanodes could be roughly evaluated by the charge transfer efficiency (TE) defined as ktrans/(krec+ ktrans).37 As shown in Figure 4d, across the entire measured potential range, high TE values are attained by 30-Fh/BiVO4 by collectively considering ktrans and krec. Although 15Fh/BiVO4 owns higherktransthan 30-Fh/BiVO4, its large krecfinally results in low TE. Even lower TEs are observed for 60-Fh/BiVO4 due to the detrimental effect of its low ktrans. The validity of this analysis is affirmed by the fact that the TE plots in Figure 4d mirror the trend of the photocurrents in Figure 2a.


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Figure 5.Schematic representativeof the working principles of 15-Fh/BiVO4, 30-Fh/BiVO4 and 60-Fh/BiVO4for PEC water oxidation. 2.5. PEIS analysis of carrier kinetics The photoelectrochemical impedance spectroscopy (PEIS) measurement was employed as a complementary technique to support the above analysis.37The typical PEIS responses of bare BiVO4, 15-Fh/BiVO4, 30-Fh/BiVO4, and 60-Fh/BiVO4 at 0.6 V under illumination (AM 1.5 G) were shown in Figure 6a. The complex plane impedance plot of Im(Z) vs. Re(Z) exhibits two semicircles,38 which could be fitted to an equivalent circuit where the series resistance (Rs) followed by two RC circuits (Figure 6c).39 The RC circuit with larger resistance (R2/CPE2) usually represents the electron transport at electrode/electrolyte interface, and R1/CPE1 is related to the electron transport in the bulk.40, 41, 42 The following information is obtained by fitting the experimental data. First, the values of R2 for Fh modified photoanodes, especially for 30-Fh/BiVO4, are significantly smaller than those of bare BiVO4 (Figure 6b), supporting the assumption that introduction of Fh results in better charge transfer efficiency. Second, the semicircle at high frequency region (R1/CPE1) is much larger for 60-Fh/BiVO4 (Figure 6a), suggesting that the bulk charge transfer in 60-Fh/BiVO4 is


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suppressed due to the thick heterogeneous film. Therefore, a portion of the applied potential will drop across this thick layer. This would ultimately lead to a lower apparent catalytic activity relative to a thin film. As a result, the large increase in the resistance of Fh layer significantly decreases the PEC performances. Of course, light adsorption by thick Fh layer was also suggested by UV-Vis spectroscopy to be detrimental to the overall performance. The results of PEIS measurements together with those of IMPS highlight the different working mechanisms for 15-Fh, 30-Fh and 60-Fh.

Figure 6.(a) Photoelectrochemical impedance spectroscopy (PEIS) of bare BiVO4 (black), 15Fh/BiVO4 (red), 30-Fh/BiVO4 (green), and 60-Fh/BiVO4 (cyan) electrodes in Nyquist plots at 0.6 V with an a.c. potential frequency range from 100,000 to 0.01Hz. Inset shows the magnified viewof high frequency region. (b)The interfacial resistance (R2) of different anodes by fitting the PEIS data.(c) The equivalent circuit. 2.6. Comparison of Fh and FeOOH


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FeOOH has been reported to be an efficient cocatalyst when coupled with BiVO4 in phosphate buffer.20, 28To demonstrate the superiority of Fh over FeOOH in borate, the PEC performance of 30-Fh/BiVO4 was directly compared with that of FeOOH/BiVO4. The electrodeposition of FeOOH on porous BiVO4 was carried out based on a known method28and the thickness of FeOOH was adjusted to achieve comparable Fe loading to that of 30-Fh/BiVO4. It is found in Figure 7 that 30-Fh/BiVO4 produces much improved photocurrent than FeOOH/BiVO4 with an enhancement of ~ 1 mA/cm2 observed at 1.23 V. In addition, the decoration of Fh leads to a noticeable cathodic shift of 60 mV in onset potential. When Fh and FeOOH were deposited on FTO, electrochemical measurement in the dark reveals that the key difference between Fh and FeOOH lies in their water oxidation activity. Comparing with FeOOH, an earlier electrocatalytic onset shifted by 200 mV and a much steeper water oxidation current were observed for Fh (Figure S13), confirming Fh is a more active oxygen evolution catalyst with respect to FeOOH under the conditions for PEC measurement. Previously, NiOOH has been deposited over FeOOH/BiVO4 in order to compensate for the low kinetics of water oxidation.28In this study, a second OEC layer is not necessary, which is considered to bring substantial convenience to electrode fabrication.


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ACS Catalysis

Figure 7.J-V curves of FeOOH/BiVO4 and 30-Fh/BiVO4in 0.4 M borate buffer under AM 1.5 G light illumination.

Conclusion In summary, ferrihydrite as a novel cocatalyst has been decorated on a worm-like nanoporous BiVO4 by simple hydrothermal approach. Controlling the loading amount of Fh led to a thin layer of Fhnanoflakes covered on the surface of BiVO4. The Fh/BiVO4 exhibits a high photocurrent of 4.87 mA/cm2 at 1.23 V and an ABPE value of 1.81% at 0.61 V, comparable to the best performance attained by BiVO4-based photoanodes. More importantly, studies on Fh/BiVO4 surface kinetics by IMPS provide new insights into the mechanism of metal oxide improved photoanodic water oxidation. Effective suppression of surface recombinationwithout loweringwater oxidation kinetics were achieved by building a suitable Fh/BiVO4 interface, which is valuable for rational design of photoelectrodes towards efficient solar energy conversion.


ACS Catalysis

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ASSOCIATED CONTENT Supporting Information. Methods of characterization, additional J-V curves, SEM data,EDS data, EDX data, XRD data, UV-visable data, Faradaic efficiency data and other data as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ACKNOWLEDGMENT This work was supported by the National Natural Science foundation of China (21476043, 21120102036 and 21361130020), the National Basic Research Program of China (973 program, 2014CB239402), the Swedish Energy Agency, and the K & A Wallenberg Foundation. REFERENCES (1) Fujishima,A.; Honda, K. Nature 1972, 238, 37-38. (2) Grätzel,M. Nature2001, 414, 338-344. (3) Hisatomi, T.; Kubota, J.;Domen,K.Chem. Soc. Rev. 2014, 43, 7520-7535. (4) Walter, M. G.; Warren,E. L.;McKone, J. R.; Boettcher,S. W.;Mi, Q.; Santori,E. A.; Lewis,N. S. Chem. Rev. 2010, 110, 6446-6473.


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Fabrication and Kinetic Study of a Ferrihydrite-Modified BiVO4

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