Catalytic Multilayers for Efficient Solar Water Oxidation through

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Catalytic Multilayers for Efficient Solar Water Oxidation through Catalyst Loading and Surface-State Passivation of BiVO Photoanodes 4

Sanghyun Bae, Hyunwoo Kim, Dasom Jeon, and Jungki Ryu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20785 • Publication Date (Web): 13 Feb 2019 Downloaded from http://pubs.acs.org on February 16, 2019

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ACS Applied Materials & Interfaces

Catalytic Multilayers for Efficient Solar Water Oxidation through Catalyst Loading and Surface-State Passivation of BiVO4 Photoanodes Sanghyun Bae, Hyunwoo Kim, Dasom Jeon, and Jungki Ryu*

Department of Energy Engineering, School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST) Ulsan 44919, Republic of Korea

*To whom correspondence should be addressed: [email protected]

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ABSTRACT We studied the kinetics of photoelectrochemical (PEC) water oxidation using a model photoanode BiVO4 modified with various water oxidation catalysts (WOCs) by electrochemical impedance spectroscopy (EIS). In particular, we prepared BiVO4 photoanodes with catalytic multilayers (CMs), where cationic polyelectrolytes and anionic polyoxometalate (POM) WOCs were assembled in a desired amount at a nanoscale precision, and compared their performance with those with well-known WOCs such as cobalt phosphate (CoPi) and NiOOH. Our comparative kinetics analysis suggested that the deposition of the CMs improved the kinetics of both the photogenerated charge carrier separation/transport in bulk BiVO4 due to passivation of surface recombination centers and water oxidation at the electrode/electrolyte interface due to deposition of efficient molecular WOCs. On the contrary, the conventional WOCs were mostly effective in the former and less effective in the latter, which is consistent with previous reports. These findings explain why the CMs exhibit an outstanding performance. We also found that separated charge carriers can be efficiently transported to POM WOCs via a hopping mechanism due to the delicate architecture of the CMs, which is reminiscent of natural photosynthetic systems. We believe that this study can not only broaden our understanding on the underlying mechanism of PEC water oxidation, but also provide insights for the design and fabrication of novel electrochemical and PEC devices, including efficient water oxidation photoanodes. Keywords: electrochemical impedance spectroscopy, solar water oxidation, water oxidation kinetics, photoanodes, water oxidation catalysts


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INTRODUCTION Development of an efficient water oxidation photoanode is the key to the realization of artificial photosynthesis since photoelectrochemical (PEC) water oxidation can provide a clean and cheap source of electrons for the sustainable production of various chemicals.1,2 To date, various semiconducting materials have been explored as promising photoanodes such as Fe2O3,3-6 BiVO4,7-10 and WO3.11,12 However, they generally suffer from low efficiency especially due to the fast recombination of photogenerated charge carriers13-15 and sluggish water oxidation kinetics.6-8 To address these issues, respectively, engineering their bulk and interfacial properties using the passivation of surface recombination centers3,16 and decoration with water oxidation catalysts (WOCs)10, 17-24 has been attempted. In particular, heterogeneous WOCs such as cobalt phosphate (CoPi)19-21 and NiOOH10,17,18 have been suggested as a cheap and efficient WOC for various photoanodes. Contradictory to the term WOCs, however, recent kinetics studies suggested that they can improve the PEC performance of photoanodes by suppressing of surface recombination through the passivation of surface states, rather than the enhancement of water oxidation kinetics.4,5,9,16,25 Alternatively, immobilizing homogeneous molecular WOCs with an outstanding catalytic activity has been attempted.26-28 However, they require complex synthetic procedures and surface modification of photoanodes for their immobilization26-28 and suffer from stability issues26-27, often resulting in rather poor performance than expected. To address these issues, we recently suggested a simple method by which to deposit organic/inorganic hybrid multilayer films that have high catalytic activity and stability, which are called catalytic multilayers (CMs), on various photoanodes.29-32 Specifically, anionic polyoxometalates (POMs) with an oxo-bridged tetranuclear active site were employed as an efficient and robust molecular WOC and were precisely assembled with cationic 3 ACS Paragon Plus Environment


polyelectrolytes through electrostatic interactions on various photoanodes. Interestingly, the PEC performance of photoanodes and their stability were greatly enhanced in terms of both photocurrent density (jph) and onset potential (Eon) after the deposition of the CMs, even incorporating electrically and electrochemically inactive polyelectrolytes. Such improvements were observed, regardless of types of photoanodes. In particular, the deposition of the CMs resulted in a record-high shift of Eon for Fe2O3 and BiVO4 photoanodes. As a result, the CMs exhibited a superior performance even compared to conventional WOCs such as CoPi and NiOOH.30 To date, however, their underlying mechanism has not yet been explored. To elucidate the underlying mechanism for the outstanding performance of the CMs, in this study, we investigated the kinetics of PEC water oxidation with BiVO4 photoanodes modified with the CMs using electrochemical impedance spectroscopy (EIS) and various PEC characterization methods. BiVO4 was used as a model photoanode due to its excellent physical properties such as a moderate bandgap of 2.4 to 2.5 eV and a relatively light effective mass of holes and it was readily modified with the CMs. We found that the precise assembly of [Co4(H2O)2(VW9O34)2]10- (POM) WOCs and poly(diallydimethylammonium chloride) (PDDA) polyelectrolytes on a nm scale allows electrical conduction to the POM WOCs via hopping and imparts a high catalytic activity to the CMs. Furthermore, our analyses suggested that, contrary to the conventional WOCs, the CMs significantly improved both the water oxidation kinetics at the photoanode/electrolyte interface and the charge separation/transport kinetics in the BiVO4 photoanode due to the high catalytic activity of POM WOCs and passivation of surface recombination centers by the CMs, respectively. These results explain why the CMs exhibited a superior performance, even compared to the conventional WOCs. Based on our understanding of the underlying mechanism of efficient PEC water oxidation using


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photoanodes with the CMs, we could not only improve the PEC performance of photoanodes, but also design novel electrochemical and PEC devices.

RESULTS & DISCUSSION Molecular CMs were precisely assembled on BiVO4 photoanodes by the sequential deposition of polyelectrolytes and molecular WOCs through electrostatic interactions (Figure 1a). [Co4(H2O)2(VW9O34)2]10- POM was selected as an anionic molecular WOC due to its high catalytic activity (Figure S1) and robust stability,33 and PDDA was selected as a cationic polyelectrolyte due to its electrochemical oxidative stability (Figure S2). According to scanning and transmission electron microscopy (SEM and TEM, respectively), combined with elemental mapping analysis, the molecular CMs were uniformly and conformally coated, even on the entire surface of nanoporous BiVO4 photoanodes, with a simple solution process (Figure 1b-d and Figure S3). The conformal and uniform deposition of the CMs was also confirmed by surface-sensitive X-ray photoelectron spectroscopy (XPS), which showed the appearance of peaks corresponding to P and W of POM and N of PDDA, as well as the disappearance of Bi peaks due to burial of the underlying BiVO4 photoanode after the deposition (Figure S4). The deposited amount of the CMs and POM WOCs could be precisely controlled by varying the number (n) of the deposition cycles—i.e., the bilayers (BLs) of POM and PDDA—and determined using quartz crystal microbalance (data not shown). The deposited amount of POM WOCs was proportional to n with the areal POM density (𝜌𝑃𝑂𝑀) of 0.144 g cm-2 per BL or 1.36 × 1013 POMs cm-2 per BL. The thickness of the 10BL CMs was approximately 16.3±3.21 nm, suggesting that each BL thickness is comparable to the size of POM WOCs. Furthermore, the CMs were almost transparent compared to the underlying BiVO4 photoanode in the entire visible light region, implying that they would have negligible effect on the photogeneration of 5 ACS Paragon Plus Environment


charge carriers (Figure S5). The PEC performance of the BiVO4 photoanodes with the molecular CMs was evaluated using linear sweep voltammetry (LSV) and was compared to that of conventional WOCs. The PEC test was carried out in a 0.5 M phosphate buffer (pH 7.0) under visible light. The deposition of the CMs resulted in a significant improvement in the PEC performance of the underlying BiVO4 photoanodes (Figure 1e and Figure S6). The jph values increased with n up to 10 and slightly decreased thereafter, which is consistent with our previous report.29,30 The jph values of the BiVO4 without and with the 10BL CMs were 0.92 and 2.1 mA cm-2, respectively, at 1.23 V vs. reversible hydrogen electrode (RHE). After the deposition of the CMs, there was a shift of Eon from 0.65 to 0.23 V vs. RHE, which is one of the largest shifts reported to date for BiVO4.10,30 There was a negligible change in Eon values upon further deposition of BLs. It is noteworthy here that the measured jph is impressive, considering that we used BiVO4 without any doping and heterojunction structure. The incident photon-tocurrent conversion efficiency (IPCE) spectra of the CM-modified BiVO4 photoanode exhibited a similar spectral shape but with a much higher efficiency than the bare counterpart, indicating that the CMs had a negligible effect on the photogeneration of excitons (Figure S7). Oxygen evolution from the BiVO4 photoanode, with and without 10 BL of the CMs, was analyzed using gas chromatography (Figure 1f) to determine the Faradaic efficiency. When modified with the 10 BL CMs, the measured hydrogen to oxygen ratio was close to the ideal value of 2 : 1 and the Faradaic efficiency was significantly increased (90.1%). On the contrary, the bare counterpart had a non-stoichiometric hydrogen-to-oxygen ratio with a much lower Faradaic efficiency of 40.9%, suggesting the presence of side reactions. For the direct performance comparison under the same conditions, we also prepared BiVO4 photoanodes modified with well-known WOCs, such as CoPi21 and NiOOH10 (Figure S8). As shown in Figure 1g-h, the CMs exhibit superior performance compared to such WOCs in terms of both jph and Eon, 6 ACS Paragon Plus Environment

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demonstrating the outstanding catalytic performance of the CMs. Prior to the detailed analysis of the underlying mechanism for the superior performance of the CMs, we made an equivalent circuit model (Figure 2a-b) and checked its validity for further in-depth analysis with EIS. The equivalent circuit consists of two RC components including the space charge capacitance of a bulk semiconductor (i.e., BiVO4) (CSC), the charge transport resistance from BiVO4 to catalytic active sites (RBulk), the surface states’ capacitance resulting from the surface structure of BiVO4 or from the deposition of WOCs (CSS), and the charge transfer resistance related to electrocatalytic reactions at the electrode/electrolyte interface (RCT). The EIS spectra of the BiVO4 photoanodes both with and without the CMs were well-fitted when using the suggested model (Figure S9). Our fitting results revealed that the CSS and RCT values were closely related to the PEC performance of the BiVO4 photoanodes, regardless of the presence of the CMs (Figure 2c-d). There was a rise and fall of RCT values around the respective Eon, followed by those of CSS at higher potentials than RCT. Our results are consistent with previous EIS studies for various photoanodes, such as BiVO434,35 and Fe2O3,5,36-38 suggesting the validity of the proposed equivalent circuit model for this study. For comparison, constant-phase elements (CPEs) were also employed, instead of standard capacitors (Cs), and resulted in a good fitting with a potential-dependent behavior similar to that employing Cs (Figure S9 and S10). According to literature,39-40 CPEs are often preferred for fitting Nyquist plots with asymmetric semicircles, an indication of non-ideal capacitors. However, fitting with CPEs may result in complexities of fitting due to the presence of extra parameters and may thus cause severe difficulties in the interpretation of potentialdependent EIS spectra.5,36-39 Since fitting with both Cs and CPEs led to the similar results and interpretations, we used the equivalent circuit employing C for further studies. We first investigated the effect of the CMs on the catalytic charge transfer kinetics at the electrode/electrolyte interface by monitoring the RCT and CSS values while varying the 7 ACS Paragon Plus Environment


applied bias (Eapp) under light (Figure 3). The deposition of the CMs resulted in a significant change of the RCT and CSS values (Figure 3a-b), regardless of n. In each sample, they exhibited an abrupt change near their respective Eon values. The RCT values of the CM-modified BiVO4 photoanodes was much smaller by several orders of magnitude than those of the bare counterpart and significantly decreased with Eapp at low and intermediate Eapp, due to high catalytic activity of the CMs. At high Eapp, the RCT values of the bare BiVO4 significantly decreased due to efficient water oxidation by the underlying BiVO4 itself and became comparable to those of the CM-modified BiVO4, which slightly increased—possibly due to the low electrical conductivity of the CMs. In the case of CSS values in each sample, they increased near Eon and decreased at intermediate Eapp, indicating the competition between the charge storage and catalytic reaction in the CMs. At high Eapp, there was a negligible difference in the CSS values between all the tested samples, again due to efficient water oxidation by BiVO4. As a result, the RC time constants for the interfacial catalytic charge transfer (CT) were monotonically decreased with Eapp, and those of the CM-modified BiVO4 photoanodes was much shorter by a few orders of magnitude than those of the bare counterpart, especially at intermediate Eapp (Figure 3c). Based on these results, the effect of n on the catalytic charge transfer kinetics was investigated for BiVO4 photoanodes with the CMs (where n ≥ 1). While CSS values monotonically increased with n, RCT reached the minimum value at 10 BL and then increased at low and intermediate Eapp. The increase of CSS with n can be attributed to the deposition of more POM WOCs, which can also act as a hole scavenging and storing molecule.33, 41-42 In general, the order of the overall PEC performance of the CM-modified BiVO4 photoanodes with different n in terms of jph (10 BL > 15 BL > 1 BL) was similar to that of RCT values (10 BL < 15 BL < 1 BL), suggesting that the former is closely related to the latter. However, CT, which is also related to the catalytic activity of the CMs, was monotonically increased with n 8 ACS Paragon Plus Environment

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at low and intermediate Eapp (1 BL < 10 BL < 15 BL), causing a question about its interpretation to remain. To understand the origin of such observations, we investigated the Bode plots and turnover frequencies (TOFs) of POMs for various BiVO4 photoanodes. At 0.25 V vs. RHE, all the Bode plots displayed a typical bimodal distribution18,43―high- and low-frequency peaks corresponding to the fast charge transport process in the bulk semiconductor and slow catalytic reactions at the electrode/electrolyte interface, respectively―except for that of the bare one, which has a higher Eon (Figure 3d). After the deposition of the CMs, the phase angle in the lowand mid-frequency range was significantly decreased, indicating the transition from capacitorlike to resistor-like behavior.44 The deposition of the 1BL CMs accompanied a shift of the lowfrequency peak to a higher frequency. These results indicate facilitation of electrochemical reactions via the deposition of efficient POM WOCs. However, further deposition of BLs led to a shift of the low-frequency peak in the opposite direction, contradictory to the increase of jph of up to 10BL, while there was no change in the position of the high-frequency peak. To address this contradiction, we determined the average TOF of POMs in the overall CMs (kn,avg) and the effective TOF of POMs in the outermost BL of the corresponding CMs (kn,outer) based on the results of LSV and QCM analyses. It is assumed that outer layers do not affect the catalytic activity of inner layers; thus, each kn,outer value is independent of the CMs with a different n. As shown in Figure 3e, both kn,avg and kn,outer exponentially decreased with n. Considering that

CT, the position of the low-frequency peak in the Bode plots, 𝑘𝑛,𝑎𝑣𝑔, and

𝑘𝑛,𝑜𝑢𝑡𝑒𝑟 values have the same unit of s-1 and show a monotonic change with n, we conclude that they are closely related to each other and reflect the average catalytic activity of POM WOCs. One can anticipate that electrical conduction occurs through the CMs by hopping (Figure S11), which also shows the exponential decay of hopping probability () with



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distance,45,46 since PDDA is electrically and electrochemically inactive. Considering the similar dimensions of POM WOCs and each BL on a scale of few nm, indeed, the separation between the underlying BiVO4 photoanode and POM WOCs might be on a sub-nm scale, where hopping can effectively occur. Using the relationship between 𝑘𝑛,𝑎𝑣𝑔 and 𝑘𝑛,𝑜𝑢𝑡𝑒𝑟, 𝑛

𝑘𝑛,𝑎𝑣𝑔 =

∫1𝑘𝑥,𝑜𝑢𝑡𝑒𝑟𝑑𝑥 𝑛

(1)

,

the overall performance of the CMs in terms of jph can be determined by the following equation: 𝑛

𝑗𝑝ℎ = 4𝑒𝑛𝜌𝑃𝑂𝑀𝑘𝑛,𝑎𝑣𝑔 = 4𝑒𝜌𝑃𝑂𝑀∫1𝑘𝑥,𝑜𝑢𝑡𝑒𝑟𝑑𝑥,

(2)

where e is the elementary charge of electrons. According to this equation, jph should increase with n and eventually be saturated. However, jph increased with n up to 10 and then decreased, suggesting that our initial assumption is not valid beyond a certain n value. It is thought that the deposition of thicker CMs can lead to less wetting of the inner components (i.e., insufficient supply of water molecules for electrolysis), a reduction in effective catalytic activity of the inner POMs by outer BLs, and degradation of the overall performance beyond a certain n (Figure 3f). Next, we studied the effect of the CMs on the charge transport kinetics in the bulk semiconductor by measuring RBulk and CSC values at various Eapp under light (Figure 4a-c). After the deposition of the CMs, the RBulk and CSC values were simultaneously and significantly reduced at high Eapp, except for the RBulk values of the BiVO4 photoanode with the 15 BL CMs. This exception can be attributed to the low conductivity of the relatively thick CMs composed of POM and PDDA. Accordingly, the RC time constants for the charge transport in the bulk semiconductor (Bulk) became much smaller by at least one order of magnitude in the presence of the CMs, implying the improved charge separation/transport kinetics in the BiVO4 photoanodes by the deposition of the CMs. In each sample, RBulk values displayed the maximum value at around Eon and gradually decreased beyond it due to the expedited water oxidation 10 ACS Paragon Plus Environment

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kinetics. However, it is more difficult to find the relationship of Eon with RBulk and CSC values than RCT and CSS values because the interfacial catalytic charge transfer process is the ratedetermining step (i.e., CT >> Bulk). The improved charge separation/transport kinetics can be confirmed by measuring the charge separation efficiency (Φsep). Furthermore, Φsep was determined by comparing jph in the presence and absence of sacrificial electron donors, such as Na2SO3 in electrolytes (Figure S12). As shown in Figure 4d, the deposition of the CMs significantly improved Φsep. Since the CMs might have more influence on the interfacial properties of the underlying BiVO4 photoanodes than on the bulk ones, we hypothesized that the CMs suppress surface recombination via the passivation of surface states, leading to the improved charge separation/transport kinetics. To support our hypothesis, we measured the transient time constant of the photogenerated charge carriers (c) and the open circuit potentials (OCPs) of BiVO4 photoanodes in the presence and absence of the CMs. According to the literature,6-8 holes accumulated or trapped at the surface can recombine with photogenerated electrons in n-type semiconductors, such as BiVO4, leading to the generation of an anodic transient spike current immediately after the exposure to light (jin). Due to the competition between the photogeneration and recombination of charge carriers, the photocurrent (jt) then gradually decreases with time t until a steady state current (jst) is obtained, as shown in Figure S12. Furthermore c can be determined using the following equation:

(

𝑡

)

𝑗𝑡 ― 𝑗𝑠𝑡

(3)

𝐷 = exp ― 𝜏𝑐 = 𝑗𝑖𝑛 ― 𝑗𝑠𝑡

where c is a specific t where ln 𝐷 = ―1 and can be considered a carrier life time. Additionally,

c of the BiVO4 photoanodes with and without the 10 BL CMs were calculated to be 0.39 and 5.75 s, respectively (Figure 4e), implying the suppression of surface recombination by the deposition of the CMs. To demonstrate this, we measured OCPs of BiVO4 photoanodes with 11 ACS Paragon Plus Environment


and without the CMs. According to the literature,47,48 OCP values measured under dark and light conditions represent the Fermi level of holes at equilibrium state (Ef,p) and the quasi-Fermi level of electrons (Ef,n), respectively (Figure 4f). After the deposition of the 1 BL CMs, there was a negative shift of Ef,n from 0.326 to 0.254 V vs. RHE, indicating the effective passivation of surface states.47,49 Upon further deposition of BLs, Ef,n remained unchanged. On the contrary, there was a negligible change of Ef,p, suggesting the negligible effect of the CMs on band bending of BiVO4 photoanodes, except for the 15BL CMs.47,49 As a result, photovoltage (Vph) determined to be the difference between Ef,n and Ef,p ― a driving force for charge separation16,47,49―increased after the deposition of the CMs. A negative shift of Ef,p for the 15BL CMs can be attributed to the formation of a weak junction between the BiVO4 semiconductor and electrolyte due to burial inside the CMs. Thus, smaller band bending for BiVO4 with the 15BL CMs can result in less efficient charge separation.47,49 Overall, these results strongly support our hypothesis that the CMs can improve charge separation/transport kinetics by suppressing surface recombination through surface state passivation. Based on these results, EIS spectra of the BiVO4 photoanodes with conventional WOCs, such as CoPi and NiOOH, were analyzed and compared with those with the CMs to explain the outstanding performance of the CM-modified BiVO4 photoanodes. Our analysis of the charge transfer kinetics at the electrode/electrolyte interface (Figure 5a-c) showed that, similar to the CMs, the deposition of either CoPi or NiOOH resulted in a decrease of RCT values and an increase of CSS values. Unlike the CMs, however, their CSS values significantly increased by several orders of magnitude, leading to an increase of CT that was longer than that of the bare BiVO4. On the other hand, the deposition of all the tested WOCs led to a decrease of RBulk, CSC, and Bulk (Figure 5d-e). Both the Φsep and c values that were determined using chronoamperograms measured under intermittent light conditions significantly increased after 12 ACS Paragon Plus Environment

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the deposition of all the tested WOCs in the following order: 10 BL CM > CoPi, NiOOH > bare (Figure S13). These results suggest that, unlike the CMs leading to the improved PEC performance by the facilitation of both the charge separation/transport kinetics and interfacial catalytic reactions, CoPi and NiOOH improve the PEC performance of BiVO4 photoanodes by facilitating the former, not the latter, process (Figure 6). Our analysis results are consistent with recent studies reporting that these WOCs can improve the PEC performance of BiVO4 via the passivation of surface recombination centers, rather than by surface catalysis.9 It is worth noting here that the CMs displayed outstanding performance in every aspect we examined, explaining why the CMs exhibits a superior PEC performance, even compared to well-known WOCs. In this study, we investigated the kinetics of PEC water oxidation by BiVO4 photoanodes modified with the CMs. The CMs were precisely assembled from anionic molecular POM WOCs and cationic PDDA polyelectrolytes on a nm scale and exhibited superior performance, even compared to well-known WOCs, such as CoPi and NiOOH. The outstanding performance of the CM-modified BiVO4 photoanodes can be explained by the following: (1) improved charge separation/transport in the bulk BiVO4 due to the passivation of surface recombination centers by the CMs, (2) efficient charge transport from the underlying BiVO4 photoanode to POM WOCs via hopping conduction due to nanoscale organization of PDDA and POM WOCs, and (3) outstanding catalytic activity of the CMs incorporating efficient molecular WOCs. It is noteworthy here that the BiVO4 photoanodes with the CMs for solar water oxidation are reminiscent of the natural photosynthetic system, which can efficiently harvest sunlight and utilize photogenerated charge carriers, even with nonconducting components.45,46 We believe that there is still room for performance improvement using the CMs because the highest TOF of POM WOCs measured in this study (1.11 × 102 s1)

is still much lower than the maximum one reported by Craig Hill group (~ 1000 s-1),33,50 and 13 ACS Paragon Plus Environment


we can explore various polyelectrolytes, ensuring more efficient charge transport to the POM WOCs. Last, we would like to point out that the utilization of the CMs can be readily combined with conventional approaches, such as doping and foring heterojunction, to achieve state-ofthe-art performance.

CONCLUSION To summarize, we investigated the underlying mechanism for PEC water oxidation by BiVO4 photoanodes modified with the CMs. The CMs with precisely assembled POM WOCs, and even those with non-conducting PDDA polyelectrolytes on BiVO4 photoanodes, exhibited a superior performance, even compared to conventional WOCs such as CoPi and NiOOH. EIS analysis combined with various PEC characterization methods showed that the CMs improve both water oxidation kinetics at the photoanode/electrolyte interface via catalyst-loading effects and charge separation/transport kinetics in the BiVO4 photoanode via the suppression of surface recombination centers. On the contrary, the conventional WOCs were found to be effective, mostly in the latter and less effective in the former, explaining why the CMs exhibit an outstanding PEC performance. We believe that this study not only improves our understanding of the underlying mechanism for the PEC water oxidation by the CM-modified photoanodes, but also provides insights for the design and fabrication of various forms of electrochemical and PEC devices.

EXPERIMENTAL METHODS Materials. Chemicals were purchased from Sigma Aldrich (St Louis, MO, USA), unless stated otherwise. 14 ACS Paragon Plus Environment

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Potassium iodide was purchased from Acros Organics (NJ, USA).

Fabrication of BiVO4 Photoanodes. Nanoporous BiVO4 photoanodes were fabricated by electrodeposition of BiOI followed by annealing with acetylacetonate (VO(acac)2), according to the literature.10 A precursor solution for BiOI electrodeposition was prepared by mixing a 0.04 M Bi(NO3)3 solution in 0.4 M potassium iodide solution (pH 1.7) with a 0.23 M p-benzoquinone solution in absolute ethanol at a 2.5:1 volume ratio. The BiOI electrode was electrodeposited on a clean fluorine-doped tin oxide (FTO) substrate at an applied bias of -0.1 V vs. Ag/AgCl for 5 min. The electrodeposited BiOI was then covered with a 0.2 M VO(acac)2 solution and annealed at 450 °C for 2 h with a ramping rate of 2 K min-1 for the conversion of BiOI to BiVO4. After the annealing, the resulting BiVO4 photoanodes were soaked in a 1 M NaOH solution to remove excess V2O5 and then rinsed with deionized (DI) water.

Synthesis of Na10[Co4(H2O)2(VW9O34)2] WOCs. Na10[Co4(H2O)2(VW9O34)2 POM WOCs were synthesized according to literature procedures.33 Co(NO3)2∙6H2O (1.2 g) and Na2WO4∙2H2O (6.0 g) were dissolved in 0.5 M of sodium acetate buffer (120 mL, pH 4.8). After vigorously stirring for 5 min, 0.27 g of NaVO3 was added to the above solution. This solution mixture was refluxed at 80 °C for 2 h, cooled to room temperature, and filtered to remove any precipitates or impurities. The filtrate solution was stored in a refrigerator for one week to obtain dark brown crystals. POM WOCs were collected by filtration and dried in vacuum.



Modification of BiVO4 Photoanodes with CMs and Conventional WOCs. A cationic polyelectrolyte and an anionic POM WOC solution for the CM deposition were prepared by dissolving PDDA and POM WOCs in phosphate buffered saline (pH 7.4) at 3 mM in terms of monomer concentration, respectively. Each BL of the CMs was deposited using the following procedures: dipping in the PDDA and POM solutions for 20 min and 5 min, respectively, and washing with DI water for 30 s three times after each step. These processes were repeated for the desired number of times. For comparison, BiVO4 photoanodes were modified with conventional WOCs such as CoPi21 and NiOOH10, via photo-assisted electrodeposition methods.

Characterization. The morphological and elemental analyses of photoanodes were carried out using an S-4800 scanning electron microscope (Hitachi High-Technologies, Japan), a Tecnai transmission electron microscope (Thermo Fisher Scientific, MA, USA) and a K-alpha XPS (Thermo Fisher Scientific, MA, USA). The deposited amount of CMs was quantified by a QCM200 quartz crystal microbalance (Stanford Research Systems, Sunnyvale, CA, USA).

PEC Characterization. PEC tests were carried out in a 0.5 M phosphate buffer (pH 7) under back-side illumination. A 300 W Xe arc lamp equipped with a 400 nm cut-on filter and an infrared water filter was used as a light source throughout this study. PEC performance of photoanodes was evaluated by LSV, chronoamperometry, and EIS in a three-electrode configuration: a photoanode as a WE, a Ag/AgCl as a RE, a FTO coated with 100 nm-thick Pt as a CE, and light intensity of 100 mW 16 ACS Paragon Plus Environment

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cm-2. For the LSV measurements, potentials were regulated by a WMPG1000 multichannel potentiostat/galvanostat (WonA Tech Co. Ltd, Korea) under the following conditions: scan range, 0.00 to 1.23 V vs. RHE, and scan rate, 10 mV s-1. The charge separation efficiency (Φsep ) was calculated using the following equation: 𝑗𝐻2𝑂

Φsep = 𝑗𝑁𝑎 𝑆𝑂 2

(4) 3

where 𝑗𝐻2𝑂 and 𝑗𝑁𝑎2𝑆𝑂3 are photocurrent densities measured in the absence and presence of sacrificial electron donors, such as Na2SO3, respectively. The IPCE spectra were measured on a wavelength range from 300 to 550 nm with a 20 nm interval using a 300 W Xe arc lamp equipped with a CS130 monochromator (Newport Corporation, CA, USA). An EIS analysis was carried out under light using a SP-150 (Bio-Logic Science Instruments, France) under the following conditions: applied bias range from 0.15 ~ 1.23 V vs. RHE with a 0.1 V interval, a modulation amplitude of 20 mV, and a frequency range from 0.1 Hz to 100 kHz. Numerical fitting of EIS data was conducted using an EC-Lab software (Bio-Logic Science Instruments, France). Experimental methods to determine the transient time constant of photogenerated charge carriers are described in the Result and Discussion sections. Evolved oxygen and hydrogen gases were quantified using a GC 2010 Plus gas chromatograph (Shimadzu Co., Japan). OCPs were measured using an SP-150 (Bio-Logic Science Instruments, France) under dark and light conditions.

Calculation of TOFs. The average TOF of POMs in the overall CMs (kn,avg) ― the number of oxygen molecules produced by each POM for a given time―was calculated using the following equation: 17 ACS Paragon Plus Environment


𝑗𝑝ℎ

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(5)

𝑘𝑛,𝑎𝑣𝑔 = 4𝑒𝑛𝜌𝑃𝑂𝑀

where 𝑗𝑝ℎ is the measured photocurrent density, e is the elementary charge of electrons, n is the number of bilayers (BLs), and 𝜌𝑃𝑂𝑀 is the areal POM density per BL, which is determined by QCM analysis. The effective TOF of the POMs in the outermost BL of the corresponding CMs (kn,outer) was estimated using the following equation: 𝑛

𝑘𝑛,𝑎𝑣𝑔 =

∫1𝑘𝑥,𝑜𝑢𝑡𝑒𝑟𝑑𝑥

(6)

𝑛

which assumes that only under layers affect the performance of POMs and that further deposition of BLs does not influence kn,outer values.

ASSOCIATED CONTENT Supporting Information. The supporting information is available free of charge on the xxx Publications website. CV and LSV curves showing catalytic activity of POMs; CV curves showing the oxidative stability of PDDA polyelectrolytes; SEM and XPS analysis confirming the deposition of various WOCs, such as CMs, CoPi, and NiOOH, on BiVO4 photoanodes; UV/visible absorbance spectra of the CMs assembled on FTO and BiVO4 electrodes; effect of the number of BLs on the PEC performance of BiVO4 photoanodes; IPCE spectra of BiVO4 photoanodes with and without the 10 BL CMs; Nyquist plots and fitting results for kinetics analysis of solar water oxidation by BiVO4 photoanodes; Energy level diagram describing the suggested charge transport mechanism in this study; photocurrent profiles of BiVO4 photoanodes with and without WOCs for the determination of the charge separation efficiency and transient time constant.

AUTHOR INFORMATION Corresponding Author 18 ACS Paragon Plus Environment

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*E-mail: [email protected] (J.R.)

ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program (2015R1C1A1A02037698 and 2018R1D1A1A02046918), the Nano-Material Technology Development Program (2017M3A7B4052802), and the Technology Development Program to Solve Climate Changes (2017M1A2A2087630) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT of Korea.



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Figure 1. (a) Schematic illustration for the preparation of the BiVO4 photoanodes modified with molecular catalytic multilayers. (b) SEM and (c, d) TEM mapping analysis of BiVO4 photoanodes with and without the 10 BL catalytic multilayers. The insets show corresponding TEM images. (e) Linear sweep voltammograms showing the n-dependent photoelectrochemical performance of BiVO4 photoanodes. (f) Quantification of evolved O2 and H2 gases upon photoelectrochemical test using BiVO4 photoanodes with and without the 10 BL catalytic multilayers for 90 min. (g) Linear sweep voltammograms with (dotted line) and without (solid lines) hole scavengers (Na2SO3) and (h) summarized results for comparison of the photoelectrochemical performance of BiVO4 photoanodes with various water oxidation catalysts. 25 ACS Paragon Plus Environment


Figure 2. (a, b) Graphical illustrations of BiVO4 photoanodes (a) before and (b) after the modification with the molecular CMs with corresponding equivalent circuits. (c, d) The CSS and RCT values estimated by fitting with the suggested equivalent circuit were plotted for each BiVO4 photoanode with a corresponding LSV curve.


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Figure 3. EIS analysis showing the effect of the CMs on the catalytic charge transfer kinetics at the electrode/electrolyte interface. Potential-dependence of the estimated values, such as (a) RCT, (b) CSS, and (c) CT. The inset shows a magnified view of the corresponding plot. (d, e) Bode plots and TOF plots showing the effect of n on the catalytic charge transfer kinetics. (f) Graphical illustration explaining the n-dependent PEC performance of the CMs where η is the hopping probability. The blue dotted and pink solid curves indicate the photocurrent densities (jph) estimated and experimentally measured.


ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Figure 4. Effect of the molecular CMs on the charge transport/separation kinetics in the bulk BiVO4. (a-c) EIS results were analyzed in terms of (a) RBulk, (b) CSC, and (c) Bulk. The inset shows a magnified view of the corresponding plot. (d-f) The improved charge transport/separation kinetics was confirmed by measuring (d) the charge separation efficiency (Φsep), (e) the transient time constant of photogenerated charge carriers, and (f) the open circuit potentials under light (open circles) and dark (closed circles).


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Figure 5. A comparison between BiVO4 photoanodes with different WOCs such as molecular CMs, NiOOH, and CoPi. The performance of each photoanode was evaluated in terms of (a) RCT, (b) CSS, (c) CT, (d) RBulk, (e) CSC, and (f) Bulk.


ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Figure 6. A suggested mechanism for the outstanding performance of the BiVO4 photoanode with molecular CMs. Simplified energy level diagrams showing the charge transfer processes of BiVO4 photoanodes (a) without any WOCs, (b) with conventional WOCs such as CoPi and NiOOH, and (c) with catalytic multilayers.


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A table of contents (TOC)


Catalytic Multilayers for Efficient Solar Water Oxidation through

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