Conducting Polymer Heterojunction Photoanodes for Efficient

Subscriber access provided by UNIV OF DURHAM

Article 3

WO/Conducting Polymer Heterojunction Photoanodes for Efficient and Stable Photoelectrochemical Water Splitting Dasom Jeon, Nayeong Kim, Sanghyun Bae, Yujin Han, and Jungki Ryu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19203 • Publication Date (Web): 20 Feb 2018 Downloaded from http://pubs.acs.org on February 20, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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

ACS Applied Materials & Interfaces

WO3/Conducting Polymer Heterojunction Photoanodes for Efficient and Stable Photoelectrochemical Water Splitting

Dasom Jeon, Nayeong Kim, Sanghyun Bae, Yujin Han, 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

*Email: [email protected] KEYWORDS: water-splitting, photocatalysis, energy conversion, polyoxometalates, organic-inorganic hybrid composites 1 ACS Paragon Plus Environment

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 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT

An efficient and stable heterojunction photoanode for solar water oxidation was fabricated by hybridization of WO3 and conducting polymers (CPs). Organic/inorganic hybrid photoanodes were readily prepared by the electropolymerization of various CPs and the co-deposition of tetraruthenium polyoxometalate (Ru4POM) water oxidation catalysts (WOCs) on the surface of WO3. The deposition of CPs, especially polypyrrole (PPy) doped with Ru4POM (PPy:Ru4POM) resulted in a remarkably improved photoelectrochemical performance by the formation of a WO3/PPy p-n heterojunction and the incorporation of efficient Ru4POM WOCs. In addition, there was also a significant improvement in the photostability of the WO3-based photoanode after the deposition of the PPy:Ru4POM layer due to the suppression of the formation of hydrogen peroxide, which was responsible for corrosion. We believe that this study provides insight into the design and fabrication of novel photosynthetic and photocatalytic systems with excellent performance and stability through the hybridization of organic and inorganic materials.

2 ACS Paragon Plus Environment

Page 2 of 31

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


INTRODUCTION

Tungsten oxide (WO3) is considered one of the most promising photoanode materials for solar water oxidation enabling efficient solar-to-chemical energy conversion, or artificial photosynthesis. Compared to other candidates, such as α-Fe2O3, BiVO4, and TiO2, it is not only composed of abundant elements but has excellent physicochemical properties, such as moderate bandgap (2.5–2.8 eV), excellent electrical properties (e.g., good conductivity and a long hole diffusion length), and high stability in acidic solutions.1–10 In this regard, numerous attempts have been made over the past decade to develop various synthesis methods (e.g., hydrothermal method,11 sol-gel reaction,12,13 chemical and physical vapor deposition,14,15 etc.) to explore its potential application in solar water oxidation.1,3–6 However, its inherently low photoelectrochemical (PEC) performance resulting from its small extinction coefficient, fast recombination of photogenerated excitons, and low catalytic activity1,3 has limited its practical application in artificial photosynthesis. In addition, WO3 has been frequently reported to be quickly deactivated under a certain condition due to the formation of reactive oxygen species, such as hydrogen peroxide, which leads to the dissolution or corrosion of WO3.16 To address such problems, research attempts have been made to control the morphology of WO3 and to modify its surface with functional materials. For example, nanostructured WO3 was found to suppress the recombination of photogenerated charge carriers.6,17,18 It has also been reported that the deposition of electrocatalysts provides high catalytic activity for water oxidation to WO3.19–21 Recently, the fabrication of a WO3-based heterojunction structure11,22–27 with inorganic semiconductors, such as BiVO4 and CuWO4, was found to be benesficial in improving the performance of WO3 photoanodes by improving light-harvesting performance, suppressing the exciton recombination, and increasing the 3 ACS Paragon Plus Environment


charge separation efficiency. However, these methods have only been partially successful and cannot fully address the intrinsic limitations of inorganic semiconductors, such as elemental abundance, low visible light activity, and instability in aqueous solutions. In addition, they often require special instruments and harsh processing conditions, such as high temperature and pressure,28,29 which limit their general applicability. To solve the aforementioned problems, in this study, we report for the first time the design and fabrication of WO3/conducting polymer heterojunction photoanodes for solar water oxidation. Various low-cost conducting polymers (CPs), such as polypyrrole (PPy), polyaniline (PANi), and 3,4-polyethylenedioxythiophene (PEDOT), were readily deposited on the surface of WO3 by oxidative electropolymerization. Polyoxometalates (POMs) with a tetraruthenium-active site (Ru4POMs) were deposited together as both a water oxidation catalyst (WOC)30–32 and a dopant33 for CPs. The deposition of CPs, especially PPy doped with Ru4POM WOCs, significantly improved the PEC performance of the WO3-based photoanode for solar water oxidation with a photocurrent density of 2.5 mA cm-2 at 1.23 V

vs. reversible hydrogen electrode (RHE). According to the electrochemical impedance spectroscopy (EIS) and the Mott-Schottky (M-S) analysis, the performance improvement was caused by (1) the increased light absorption by CPs having a high extinction efficient, (2) improved charge separation efficiency due to the formation of a WO3/PPy heterojunction structure, and (3) enhanced catalytic activity due to the incorporation of Ru4POM WOCs. Unexpectedly, the stability of the WO3-based photoanodes was also significantly improved after the deposition of CPs due to the suppression of the formation of corrosive hydrogen peroxide. We believe that this study paves the way for the realization of artificial photosynthesis by enabling the design and fabrication of unprecedented photosynthetic and


Page 4 of 31

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


photocatalytic systems with excellent performance and stability through the hybridization of organic and inorganic materials.

RESULTS AND DISCUSSION WO3-based photoanodes for PEC water oxidation were prepared by the simultaneous electrodeposition of CPs and POMs on the WO3 surface. We prepared an n-type WO3 photoanode using the hydrothermal method according to the literature11 and subsequently deposited Ru4POM-doped CPs to form a heterojunction structure (Figure 1 and Figure S1). The CPs tested in this study include PPy, PANi, and PEDOT. They are known to be p-type semiconductors34 and thus have been used as hole-transporting materials for optoelectronic devices.35–37 Compared to inorganic semiconductors, they can be readily synthesized through electrochemical oxidative polymerization38–41 at ambient temperature and pressure, have a higher extinction coefficient in the UV and visible light region (Table S1 and Figure S2), and are highly stable in aqueous solutions, especially at acidic42–45 pH values. Ru4POM, with a molecular formula of [Ru4O4(OH)2(H2O)4(γ-SiW10O36)2]10-, was simultaneously deposited with CPs as both an acid dopant for CPs25 and as a molecular WOC30–32. In addition to its catalytic activity, Ru4POM is known to be highly stable at acidic pH values32,40,46 and exhibits fast hole-scavenging activity32. Thus, we hypothesized that the deposition of p-type CPs doped with Ru4POM on WO3 significantly improves the performance of the WO3 photoanode for solar water oxidation for the following reasons: (1) the increased efficiency of the photogeneration of charge carriers due to the broader and stronger light-absorption capability of CPs, (2) the improved charge separation efficiency due to the formation of the



p-n heterojunction structure, and (3) the enhanced catalytic activity for water oxidation due to the incorporation of Ru4POM WOC. The formation of CP-based organic/inorganic hybrid photoanodes was investigated using electron microscopy and water contact angle (WCA) measurement. The deposition of CPs on WO3 (WO3/CP) resulted in a significant morphological change (Figure 2a and Figure S3, S4). For example, while the bare WO3 photoanode was composed of aggregates of smooth nanowires (Figure 2a), the deposition of PPy (WO3/PPy) led to the formation of a particulate coating layer on the nanowire surface (Figure S3 and S4). It could be readily distinguished by a contrast difference in the transmission electron microscopy (TEM) images (Figure 2a): dark WO3 due to heavy elements, such as W, and bright PPy due to elements, such as C and N. When the electrodeposition was carried out in the presence of PPy monomers and Ru4POM, a thick and dense coating layer was formed on the nanowire surface, indicating the co-deposition of PPy and Ru4POM on WO3 (WO3/PPy:Ru4POM). The thickness of the coating layers for WO3/PPy and WO3/PPy:Ru4POM was 20 (± 1.5) and 27 (± 3.2) nm, respectively, when they were deposited with a charge density of 50 mC cm-2. The elemental mapping analysis by scanning TEM (STEM) confirmed the formation of an organic/inorganic hybrid coating layer and even distribution of Ru4POM inside it. The deposition of the Ru4POM-doped CP film could also be identified by measuring WCA (Figure S3a-c, inset). For example, while the WO3/PPy photoanode was slightly hydrophobic with a WCA of 46.9˚, the bare WO3 and WO3/PPy:Ru4POM were found to be very hydrophilic with WCAs of ~ 0 and 8.4˚, respectively. The deposition of CP and Ru4POM on the WO3 surface was also confirmed by measuring diffuse reflectance, Fourier transform infrared (FT-IR), and X-ray photoelectron spectra (XPS). There was a clear difference in the colors of WO3 (yellowish green), WO3/PPy 6 ACS Paragon Plus Environment

Page 6 of 31

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


(dark gray), and WO3/PPy:Ru4POM (black). Their optical bandgaps were calculated to be 2.5, 2.0, and 1.8 eV, respectively (Figure 2b and Figure S5). It is noteworthy here that the absorbance of the WO3-based photoanode significantly increased in the entire visible light region, even with a very thin layer of CPs due to their high extinction coefficient. The FT-IR spectra showed characteristic vibrational peaks of PPy, PANi, and PEDOT, confirming the successful deposition of the respective polymer with Ru4POM via electrodeposition (Figure 2c). XPS analysis of the WO3/CP photoanodes exhibited the characteristic peaks of Ru 3d and C 1s, demonstrating the co-deposition of CPs and Ru4POMs again (Figure 2d and Figure S4). We also investigated the crystalline structure of the WO3 photoanode before and after the deposition of the CP and CP:Ru4POM film and found that a monoclinic WO3 structure was maintained throughout the experiment (Figure S6). Based on these results, we studied the influence of types of CPs and the co-deposition of CP and Ru4POM on the PEC performance of the WO3 photoanode for visible light-driven water oxidation (Figure 3). The PEC performance of various photoanodes was evaluated using chronoamperometry and linear sweep voltammetry (LSV) under back-side illumination (substrate–electrode side) unless stated otherwise. It was found that even the deposition of CP alone led to a slight increase of photocurrent density, possibly due to the increased light absorption by CPs and more efficient charge separation at the n-WO3/p-CP interface. The performance improvement became more pronounced when Ru4POM WOCs were codeposited (Figure 3a). Among the three types of CPs tested, PPy doped with Ru4POM exhibited the best performance (Figure S7). For example, the photocurrent density at an applied bias of 1.4 V vs. RHE for WO3/PPy:Ru4POM was five to ten times higher than those for

bare

(WO3)

and

other

WO3-based

electrodes

(WO3/PANi:Ru4POM

and

WO3/PEDOT:Ru4POM). The onset potential of water oxidation for the WO3/PPy:Ru4POM 7 ACS Paragon Plus Environment


photoanode was 0.46 V vs. Ag/AgCl (0.72 V vs. RHE) (Figure 3a). The outstanding performance of WO3/PPy:Ru4POM is thought to be closely related to the band-edge position of PPy. As shown in Figure 1a, among the three CPs, only PPy has a suitable highest occupied molecular orbital (HOMO) level or a valence band edge position for water oxidation (i.e., more positive than the standard reduction potential of oxygen evolution reaction [OER]). Based on these findings, we focused on PPy in the following studies. To study the effect of light absorption by PPy, we investigated the dependence of the PEC performance of the WO3 photoanode on the illumination direction, the deposited amount of PPy:Ru4POM film, and the illumination wavelength. As shown in Figure S8a, back-side (substrate–electrode side) illumination resulted in a significantly higher photocurrent density than front-side (electrolyte-electrode side) illumination. WO3 photoanodes with a different amount of the PPy:Ru4POM film were prepared by varying the electropolymerization charge density: 10, 50, and 100 mC cm-2. The photocurrent density increased with a charge density of up to 50 mC cm-2 and then decreased beyond it (Figure S8b). These findings are attributed to the tradeoff relationship between stronger light absorption and the relatively poor charge transport properties of PPy compared to that of WO3. As expected from the absorbance spectra and optical bandgap of WO3 (2.5 eV) and WO3/PPy:Ru4POM (1.8 eV), the deposition of PPy:Ru4POM led to a broader and stronger absorption of visible light. Considering a smaller relative permittivity leading to a stronger exciton-binding energy and a shorter diffusion length of excitons in the PPy film on a few tens of nm scale47, however, one can expect that the probability of exciton recombination in the PPy:Ru4POM film will be much higher than in WO3 and increase with the film thickness. Thus, it is thought that the observation resulted from the compromise between the increase in the number of photogenerated excitons and the decrease in the probability of exciton dissociation upon the 8 ACS Paragon Plus Environment

Page 8 of 31

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


deposition of thicker film. Indeed, the thickness of the PPy:Ru4POM film exhibiting the best performance (27 nm, as shown in Figure 2a) was similar to the diffusion length of excitons in the CP (a few tens of nm). The illumination direction-dependent performance of the WO3/PPy:Ru4POM photoanode can be explained in the same way. The incident photon-tocurrent conversion efficiency (IPCE) was measured to investigate the wavelength-dependent response of the respective photoanodes (Figure 3b). The bare WO3 exhibited an IPCE less than 1 % only at wavelengths below its bandgap wavelength (~ 448 nm). The deposition of PPy resulted in a significant increase in both IPCE and absorbance spectra in the entire spectral range (400-650 nm), indicating its huge contribution to light absorption. As expected, the doping of PPy with Ru4POM led to an even further increase in the IPCE due to the catalytic activity of Ru4POM. The overall results clearly demonstrate the role of PPy in the photogeneration of charge carriers. To elucidate the influence of the heterojunction structure on the improved performance of WO3-based photoanodes, EIS and M-S analyses were carried out. All the impedance spectra in the form of a Nyquist plot consisted of one semicircle (Figure 3c) and were well fitted to a 1-RC circuit model (Table 1). The charge transfer resistances (Rct) of WO3, WO3/PPy, and WO3/PPy:Ru4POM films were approximately 15.4, 4.9, and 1.7 kohms, respectively, indicating the higher catalytic activity of the PPy:Ru4POM film. It was found that even the deposition of PPy alone resulted in a significant decrease of Rct. Considering the low catalytic activity of PPy for water oxidation,48 the decrease of Rct could have partially resulted from the efficient dissociation of charge carriers or the delivery of holes by the formation of the heterojunction structure. Interestingly, the constant phase element exponent (CPE-P) for WO3/PPy:Ru4POM was found to be very close to the unity (Table 1), indicating near-ideal capacitor behavior.49 The capacitor-like behavior of the WO3/PPy:Ru4POM was 9 ACS Paragon Plus Environment


ascribed to the fast hole scavenging and storing ability of Ru4POM.32,46 We also carried out an M-S analysis to determine charge carrier density for each sample and to demonstrate the benefits of the heterojunction structure (Figure 3d). The positive slope of the M-S plot for pristine WO3 suggests that it is an n-type semiconductor. Further deposition of PPy or PPy:Ru4POM resulted in a significant reduction in the slope of the M-S plot, indicating a significant increase of the donor or charge carrier density (ND). Note that ND is inversely proportional to the slope of the M-S plot according to the following equation:4

1 2 ݇ܶ = ൬‫ ܧ‬− ‫ܧ‬ி஻ − ൰ ଶ ‫ܥ‬ ߳߳଴ ݁଴ ܰ஽ ݁଴ where C is the space charge capacitance, ε and ε0 are the electrode and free space permittivity, e0 is the elementary electric charge, E is the applied bias, EFB is the flat band potential, k is the Boltzmann constant, and T is the absolute temperature. The charge carrier densities for pristine WO3, WO3/PPy, and WO3/PPy:Ru4POM were found to be 2.4×1017, 1.65×1018, and 1.7×1020 cm-3, respectively. The significant increase in charge carrier density upon deposition of PPy or PPy:Ru4POM was attributed to the improved light harvesting and charge separation efficiency by the formation of the p-n heterojunction structure. Oxygen evolution by WO3-based photoanodes under visible light irradiation was confirmed by gas chromatography to calculate their Faradaic efficiency (Figure S9 and S10). As expected from the LSV and photocurrent measurement, the WO3/PPy:Ru4POM heterojunction photoanode generated a much larger amount of oxygen under the same condition: an applied bias of 1.4 V vs. RHE, back-side illumination, and 0.1 M HCl solution. For both the bare WO3 and WO3/PPy:Ru4POM, the amount of evolved hydrogen gas continuously increased upon measurement. However, the oxygen evolution profile was quite different. While a gradual and continuous increase of oxygen concentration occurred for the 10 ACS Paragon Plus Environment

Page 10 of 31

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


WO3/PPy:Ru4POM photoanode, saturation after 30 min of the test occurred for the bare WO3 photoanode, implying a higher stability of the WO3/PPy:Ru4POM photoanode than the bare WO3. The Faradaic efficiency for water oxidation by pristine WO3 and WO3/PPy:Ru4POM was calculated to be 21% and 56%, respectively. Considering that the oxidation of chloride ions to chlorine gas can compete with OER, the Faradaic efficiency of the WO3/PPy:Ru4POM photoanode is very impressive compared to previous reports.3 Note that the standard reduction potential for chlorine evolution is 1.36 V vs. RHE.50 To study the effect of the PPy:Ru4POM film on the stability of the WO3 photoanode, we observed the morphological and structural changes of the WO3 photoanode upon solar water oxidation in the presence and absence of the film using X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses. They were investigated before and after the PEC water oxidation test for 1.5 hours under visible light irradiation in 0.1 M HCl. While there was a negligible change in both the morphology and structure of the WO3/PPy:Ru4POM photoanode, there were significant changes in those of the bare WO3 photoanode (Figure 4ac). The size of WO3 crystals was remarkably decreased, and the underlying fluorine doped tin oxide (FTO) substrate was partially exposed in the absence of the polymeric heterojunction film. In addition, the bare WO3 photoanode underwent a phase transition from the most stable monoclinic (JCPDS 43-1035) to a less-stable hexagonal (JCPDS 75-2187) phase51. These results clearly demonstrate the role of the PPy:Ru4POM film as a protective film against photocorrosion and deactivation. We hypothesized that the deposition of the PPy:Ru4POM film suppresses the formation of hydrogen peroxide under illumination, protecting the WO3 photoanode against photocorrosion. According to the literature,52 the formation of reactive oxygen species such as hydrogen peroxide (a two-electron oxidation reaction) under visible light irradiation can be 11 ACS Paragon Plus Environment


kinetically favored over oxygen evolution (a four-electron oxidation reaction) and can cause the corrosion of WO3. To support our hypothesis, we tested the stability of the bare WO3 photoanode in the presence of a concentrated hydrogen peroxide solution (35% in 0.1 M HCl). After incubating overnight at room temperature (RT) and 50 ˚C, the morphology of the WO3 photoanode was studied using SEM. As shown in Figure S11, the WO3 photoanode underwent severe corrosion, even under dark conditions, especially at a higher temperature. Based on this finding, we attempted to measure the concentration of hydrogen peroxide formed during the PEC test from the WO3 photoanode with and without the (WO3/)PPy:Ru4POM film by using a commercially available colorimetric hydrogen peroxide detection kit. After testing for 1.5 hours, the concentration of hydrogen peroxide was determined to be approximately 3.0 and 1.8 µM for the WO3 and the WO3/PPy:Ru4POM photoanodes, respectively (Figure 4d). Considering that a large amount of hydrogen peroxide might already be consumed by dissolving WO3, it is thought that more hydrogen peroxide was formed from the bare WO3 photoanode than was measured. The suppression of hydrogen peroxide formation can be explained by considering the energy level alignment and the standard redox potentials for OER and hydrogen peroxide formation reactions. As shown in Figure 4e, the valence band edge of WO3 is more positive than the standard redox potentials for both reactions, and thus both reactions are competed under visible light illumination. On the contrary, the HOMO levels of PPy and redox potentials of Ru4POM WOCs only allow the evolution of oxygen by water oxidation, which results in the suppression of hydrogen peroxide formation. Compared to conventional approaches relying on mostly inorganic materials, our approach, which is based on the hybridization of organic and inorganic materials has many advantages to fabricating efficient and stable photosynthetic and photocatalytic systems. 12 ACS Paragon Plus Environment

Page 12 of 31

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


Although we only demonstrated the fabrication of an efficient and stable photoanode for solar water oxidation, our approach can be applied to the fabrication of various materials and systems for solar-to-chemical energy conversion because it does not require any complex fabrication processes, expensive instruments, or harsh processing conditions. In addition, we demonstrated that our approach can significantly improve the performance and stability of photoelectrodes by using organic or polymeric materials even at a low pH and a low ionic strength (Table 2). These materials have rarely been utilized in the fabrication of photosynthetic and photocatalytic systems due to concerns about their relatively low electrical properties and stability compared to inorganic materials. However, we found that the PEC performance and stability of the WO3 photoanode can be significantly improved by the deposition of an organic/inorganic hybrid (thin) film, especially PPy doped with Ru4POM due to its multifunctional roles. These include (1) improving light-harvesting properties, (2) suppressing the charge carrier recombination and facilitating efficient charge separation, (3) enhancing catalytic efficiency, and (4) preventing the formation of corrosive hydrogen peroxide.

CONCLUSIONS To summarize, we have for the first time successfully fabricated an efficient and stable WO3 and CP-based heterojunction photoanode for solar water oxidation. The deposition of CP, especially PPy doped with Ru4POM, significantly improved the performance of the WO3based photoanode due to its suitable band edge position for water oxidation and doping with Ru4POM WOCs. A detailed EIS and M-S analysis revealed that the improved performance resulted from the improved light-harvesting and charge separation efficiencies through 13 ACS Paragon Plus Environment


WO3/PPy heterojunction formation and enhanced catalytic activity through Ru4POM doping. Unexpectedly, the WO3/PPy:Ru4POM heterojunction photoanode exhibited a remarkably improved photostability compared to the bare WO3 due to the suppression of the formation of corrosive hydrogen peroxide. We believe that this study will provide insight into the design and fabrication of various solar energy conversion devices.

EXPERIMENTAL METHODS Materials. All chemicals were purchased from Sigma Aldrich (St. Louis, MO, USA), unless stated otherwise. Hydrogen peroxide was produced by Junsei (Tokyo, Japan). Sodium tungstate dehydrate was purchased from Alfa Aesar (Ward Hill, MA, USA). Preparation of Ru4POM WOCs and WO3 Photoanodes. Detailed procedures for the synthesis of Ru4POM WOCs30 and WO3 photoanodes11 can be found elsewhere. WO3 was deposited on fluorine-doped tin oxide (FTO) substrate. Electrodeposition of CPs on WO3. Various CPs such as PPy, PANi, and PEDOT were readily deposited on the surface of the WO3 photoanode by oxidative electropolymerization using a WMPG1000 multichannel potentiostat/galvanostat (WonA Tech Co. Ltd., Korea). Precursor solutions were prepared by dissolving CPs with and without Ru4POM in distilled water. The concentrations of Ru4POM and monomers such as pyrrole for PPy and aniline for PANi were kept constant at 1 and 100 mM, respectively. Electrodeposition of PPy or PANi was carried out under the following conditions: WO3-coated FTO as a working electrode (WE), Ag/AgCl as a reference electrode (RE), Pt wire as a counter electrode (CE), and constant applied bias of 0.6 V vs. Ag/AgCl. In the case of PEDOT, the precursor solution was prepared by dissolving 20 mM 3,4-ethylenedioxynthiophene (EDOT), 50 mM of KCl, and 1 14 ACS Paragon Plus Environment

Page 14 of 31

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


mM of Ru4POM in distilled water. The pH of the precursor solution was then adjusted to 4.5. Cyclic voltammetry was applied for electropolymerization of PEDOT on WO3 under the following conditions: WO3 WE, Ag/AgCl RE, Pt wire CE, a scan rate of 100 mV/s, cyclic potential sweep from 0.0 to 1.5 V vs. Ag/AgCl.38 Characterizations. The morphology of samples was observed using an S-4800 SEM (Hitachi High-Technologies, Japan) and a JEM-2100 TEM (JEOL, Japan). Water contact angles were measured using a DSA100 contact angle analyzer (KRUSS, Germany). UV/visible absorbance spectra of WO3-based photoanodes were obtained with a Cary 5000 (Agilent, California, USA) in diffuse reflectance mode. Elemental analysis was carried out using a K-alpha XPS (ThermoFisher, Massachusetts, USA). Photoelectrochemical (PEC) Characterizations. A 300 W Xe lamp filtered with a 400 nm cut-on filter was used as a visible light source. The PEC performance of various photoanodes for visible-light-driven water oxidation was evaluated using linear sweep voltammetry (LSV) and photocurrent density measurement with and without visible light illumination (100 mW cm-2). They were carried out in a three-electrode configuration with Ag/AgCl RE, Pt wire CE, and 0.1 M of HCl (pH 1.0) using a WMPG1000 multichannel potentiostat/galvanostat (WonA Tech Co. Ltd., Korea). Quantification of oxygen and hydrogen gases was performed with a GC-2010 Plus gas chromatograph (Shimadzu Co., Japan) at an applied bias of 1.4 V vs. reversible hydrogen electrode (RHE) under visible light illumination. Electrochemical impedance spectra were obtained with a Solartron 1260 impedance analyzer under the following conditions: Ag/AgCl RE, Pt wire CE, applied bias of 0.6 vs. Ag/AgCl, amplitude of 20 mV, frequency scan from 100 kHz to 0.1 Hz. A Mott-Schottky analysis was carried out under the following conditions: a frequency of 10 Hz, AC amplitude of 2 mV, and an applied bias from -0.3 to 1.0 V vs. Ag/AgCl with a 0.1 V interval. 15 ACS Paragon Plus Environment


ASSOCIATED CONTENT Supporting Information. A listing of the contents of each file supplied as Supporting Information should be included. For instructions on what should be included in the Supporting Information as well as how to prepare this material for publications, refer to the journal’s Instructions for Authors.

ASSOCIATED CONTENT The Supporting Information of available free of charge on the ACS Publications website Molecular structure of CPs tested in this study; Optoelectronic properties of CPs; SEM images and XPS spectra of WO3-based photoanodes; X-ray diffraction patterns of WO3, Ru4POM, and WO3/PPy:Ru4POM; Photocurrent densities of various WO3-based photoanodes under different measurement conditions; Time profile of oxygen and hydrogen gas evolution from WO3-based photoanodes under visible light illumination; A chronoamperogram measured during the PEC test; Corrosion of WO3 in the presence of hydrogen peroxide..

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.


Page 16 of 31

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


ACKNOWLEDGMENT This

work

was

supported

by

Basic

Science

Research

Program

(NRF-

2015R1C1A1A02037698) and Nano-Material Technology Development Program (NRF2017M3A7B4052802) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT of Korea. This work was also supported by the 2018 Research Fund (1.180014.01) of UNIST (Ulsan National Institute of Science and Technology).



REFERENCES (1)

Huang, Z. F.; Song, J.; Pan, L.; Zhang, X.; Wang, L.; Zou, J. J. Tungsten Oxides for Photocatalysis, Electrochemistry, and Phototherapy. Adv. Mater. 2015, 27, 5309–5327.

(2)

Li, W.; Da, P.; Zhang, Y.; Wang, Y.; Lin, X.; Gong, X.; Zheng, G. WO3 Nanoflakes for Enhanced Photoelectrochemical Conversion. ACS Nano 2014, 8, 11770–11777.

(3)

Zhu, T.; Chong, M. N.; Chan, E. S. Nanostructured Tungsten Trioxide Thin Films Synthesized for Photoelectrocatalytic Water Oxidation: A Review. ChemSusChem. 2014, 7, 2974–2997.

(4)

Liu, C.; Yang, Z.; Li, Y. Preparation and Enhanced Visible-Light Photocatalytic Activity of Pancake Rocks-like WO3-x/C Nanocomposite. RSC Adv. 2016, 6, 32983– 32988.

(5)

Lin, J.; Hu, P.; Zhang, Y.; Fan, M.; He, Z.; Ngaw, C. K.; Loo, J. S. C.; Liao, D.; Tan, T. T. Y. Understanding the Photoelectrochemical Properties of a Reduced Graphene oxide–WO3 Heterojunction Photoanode for Efficient Solar-Light-Driven Overall Water Splitting. RSC Adv. 2013, 3, 9330–9336.

(6)

Kwong, W. L.; Nakaruk, A.; Koshy, P.; Sorrell, C. C. Photoelectrochemical Properties of WO3 Nanoparticulate Thin Films Prepared by Carboxylic Acid-Assisted Electrodeposition. Thin Solid Films 2013, 544, 191–196.

(7)

Hahn, N. T.; Ye, H.; Flaherty, D. W.; Bard, A. J.; Mullins, C. B. Reactive Ballistic Deposition of Alpha-Fe2O3 Thin Films for Photoelectrochemical Water Oxidation.

ACS Nano 2010, 4, 1977–1986. (8)

Ghayeb, Y.; Momeni, M. M. Solar Water-Splitting Using Palladium Modified Tungsten Trioxide-Titania Nanotube Photocatalysts. J. Mater. Sci. Mater. Electron. 2016, 27, 1805–1811. 18 ACS Paragon Plus Environment

Page 18 of 31

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


(9)

Momeni, M. M.; Ghayeb, Y.; Shafiei, M. Preparation and Characterization of CrFeWTiO2 Photoanodes and Their Photoelectrochemical Activities for Water Splitting. Dalt. Trans. 2017, 46, 12527–12536.

(10)

Weng, B.; Xiao, Z.; Meng, W.; Grice, C. R.; Poudel, T.; Deng, X.; Yan, Y. Bandgap Engineering of Barium Bismuth Niobate Double Perovskite for Photoelectrochemical Water Oxidation. Adv. Energy Mater. 2017, 7, 1602260.

(11)

Su, J.; Guo, L.; Bao, N.; Grimes, C. a. Nanostructured WO3/BiVO4 Heterojunction Films for Efficient Photoelectrochemical Water Splitting. Nano Lett. 2011, 11, 1928– 1933.

(12)

Ahn, H.-J.; Shim, H.-S.; Sung, Y.-E.; Seong, T.-Y.; Kim, W. B. Electrochromism of Sn-Modified WO3 Electrodes Prepared from Sol-Gel Method. Electrochem. Solid-

State Lett. 2007, 10, E27–E30. (13)

Wang, Z.; Chumanov, G. WO3 Sol-Gel Modified Ag Nanoparticle Arrays for Electrochemical Modulation of Surface Plasmon Resonance. Adv. Mater. 2003, 15, 1285–1289.

(14)

Palgrave, R. G.; Parkin, I. P. Chemical Vapour Deposition of Titanium Chalcogenides and Pnictides and Tungsten Oxide Thin Films. New J. Chem. 2006, 30, 505–514.

(15)

Losier, P.; Ashrit, P. V. Flash Evaporated Tungsten Oxide Thin Films for Electrochromic Applications. J. Mater. Sci. Lett. 2003, 22, 1095–1098.

(16)

Cui, X.; Zhang, H.; Dong, X.; Chen, H.; Zhang, L.; Guo, L.; Shi, J. Electrochemical Catalytic Activity for the Hydrogen Oxidation of Mesoporous WO3 and WO3/C Composites. J. Mater. Chem. 2008, 18, 3575–3580.

(17)

Yin, C.; Zhu, S.; Zhang, D. 3D Nanostructured WO3/BiVO4 Heterojunction Derived from Papilio Paris for Efficient Water Splitting. RSC Adv. 2017, 7, 27354–27360.



(18)

Song, K.; Gao, F.; Yang, W.; Wang, E.; Wang, Z.; Hou, H. WO3 Mesoporous Nanobelts towards Efficient Photoelectrocatalysts for Water Splitting.

ChemElectroChem 2018, 5, 322–327. (19)

Baeck, S. H.; Jaramillo, T.; McFarland, E. W. Influence of Composition and Morphology on Photo and Electrocatalytic Activity of Electrodeposited Pt/WO3. In

ACS Division of Fuel Chemistry, Preprints; 2002, 47, 567–569. (20)

Wicaksana, Y.; Liu, S.; Scott, J.; Amal, R. Tungsten Trioxide as a Visible Light Photocatalyst for Volatile Organic Carbon Removal. Molecules 2014, 19, 17747– 17762.

(21)

Nam, K. M.; Cheon, E. A.; Shin, W. J.; Bard, A. J. Improved Photoelectrochemical Water Oxidation by the WO3/CuWO4 Composite with a Manganese Phosphate Electrocatalyst. Langmuir 2015, 31, 10897–10903.

(22)

Chae, S. Y.; Lee, C. S.; Jung, H.; Joo, O.-S.; Min, B. K.; Kim, J. H.; Hwang, Y. J. Insight into Charge Separation in WO3/BiVO4 Heterojunction for Solar Water Splitting. ACS Appl. Mater. Interfaces 2017, 9, 19780–19790.

(23)

Rao, P. M.; Cai, L.; Liu, C.; Cho, I. S.; Lee, C. H.; Weisse, J. M.; Yang, P.; Zheng, X. Simultaneously Efficient Light Absorption and Charge Separation in WO3/BiVO4 Core/Shell Nanowire Photoanode for Photoelectrochemical Water Oxidation. Nano

Lett. 2014, 14, 1099–1105. (24)

Zhang, J.; Liu, Z.; Liu, Z. Novel WO3/Sb2S3 Heterojunction Photocatalyst Based on WO3 of Different Morphologies for Enhanced Efficiency in Photoelectrochemical Water Splitting. ACS Appl. Mater. Interfaces 2016, 8, 9684–9691.

(25)

Li, Y.; Zhang, L.; Liu, R.; Cao, Z.; Sun, X.; Liu, X.; Luo, J. WO3@α-Fe2O3 Heterojunction Arrays with Improved Photoelectrochemical Behavior for Neutral pH Water Splitting. ChemCatChem 2016, 8, 2765–2770. 20 ACS Paragon Plus Environment

Page 20 of 31

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


(26)

Jin, B.; Jung, E.; Ma, M.; Kim, S.; Zhang, K.; Kim, J. Il; Son, Y.; Park, J. H. SolutionProcessed Yolk-Shell-Shaped WO3/BiVO4 Heterojunction Photoelectrodes for Efficient Solar Water Splitting. J. Mater. Chem. A 2018, 6, 2585–2592.

(27)

Weng, B.; Grice, C. R.; Ge, J.; Poudel, T.; Deng, X.; Yan, Y. Barium Bismuth Niobate Double Perovskite/Tungsten Oxide Nanosheet Photoanode for High-Performance Photoelectrochemical Water Splitting. Adv. Energy Mater. 2018, DOI: 10.1002/aenm.201701655.

(28)

Chiarello, G. L.; Bernareggi, M.; Pedroni, M.; Magni, M.; Pietralunga, S. M.; Tagliaferri, A.; Vassallo, E.; Selli, E. Enhanced Photopromoted Electron Transfer over a Bilayer WO3 n–n Heterojunction Prepared by RF Diode Sputtering. J. Mater. Chem.

A 2017, 5, 12977–12989. (29)

Gondal, M. A.; Dastageer, M. A.; Oloore, L. E.; Baig, U. Laser Induced Selective Photo-Catalytic Reduction of CO2 into Methanol Using In2O3-WO3 Nano-Composite.

J. Photochem. Photobiol. A Chem. 2017, 343, 40–50. (30)

Geletii, Y. V.; Botar, B.; Kögerler, P.; Hillesheim, D. A.; Musaev, D. G.; Hill, C. L. An All-Inorganic, Stable, and Highly Active Tetraruthenium Homogeneous Catalyst for Water Oxidation. Angew. Chem. Int. Ed. 2008, 47, 3896–3899.

(31)

Sartorel, A.; Carraro, M.; Scorrano, G.; De Zorzi, R.; Geremia, S.; McDaniel, N. D.; Bernhard, S.; Bonchio, M. Polyoxometalate Embedding of a tetraruthenium(IV)-OxoCore by Template-Directed Metalation of [γ-SiW10O36]8-: A Totally Inorganic Oxygen-Evolving Catalyst. J. Am. Chem. Soc. 2008, 130, 5006–5007.

(32)

Orlandi, M.; Argazzi, R.; Sartorel, A.; Carraro, M.; Scorrano, G.; Bonchio, M.; Scandola, F. Ruthenium Polyoxometalate Water Splitting Catalyst: Very Fast Hole Scavenging from Photogenerated Oxidants. Chem. Commun. 2010, 46, 3152–3154.



(33)

Yang, H.; Song, T.; Liu, L.; Devadoss, A.; Xia, F.; Han, H.; Park, H.; Sigmund, W.; Kwon, K.; Paik, U. Polyaniline/polyoxometalate Hybrid Nanofibers as Cathode for Lithium Ion Batteries with Improved Lithium Storage Capacity. J. Phys. Chem. C 2013, 117, 17376–17381.

(34)

Chen, X. L.; Jenekhe, S. A. Bipolar Conducting Polymers: Blends of P-Type Polypyrrole and an N-Type Ladder Polymer. Macromolecules 1997, 30, 1728–1733.

(35)

Lattante, S. Electron and Hole Transport Layers: Their Use in Inverted Bulk Heterojunction Polymer Solar Cells. Electronics 2014, 3, 132–164.

(36)

Deng, X.-Y.; Xian-Yu. Light-Emitting Devices with Conjugated Polymers. Int. J. Mol.

Sci. 2011, 12, 1575–1594. (37)

Alias, A. N.; Zabidi, Z. M.; Ali, A. M. M.; Harun, M. K.; Yahya, M. Z. A. Optical Characterization and Properties of Polymeric Materials for Optoelectronic and Photonic Applications. Int. J. Appl. Sci. Technol. 2013, 3, 11–38.

(38)

Yuan, C.; Guo, S.; Wang, S.; Liu, L.; Chen, W.; Wang, E. Electropolymerization Polyoxometalate (POM)-Doped Pedot Film Electrodes with Mastoid Microstructure and Its Application in Dye-Sensitized Solar Cells (DSSCs). Ind. Eng. Chem. Res. 2013, 52, 6694–6703.

(39)

Kim, S.; Jang, L. K.; Park, H. S.; Lee, J. Y. Electrochemical Deposition of Conductive and Adhesive Polypyrrole-Dopamine Films. Sci. Rep. 2016, 6, 30475.

(40)

Anwar, N.; Sartorel, A.; Yaqub, M.; Wearen, K.; Laffir, F.; Armstrong, G.; Dickinson, C.; Bonchio, M.; McCormac, T. Surface Immobilization of a Tetra-Ruthenium Substituted Polyoxometalate Water Oxidation Catalyst through the Employment of Conducting Polypyrrole and the Layer-by-Layer (LBL) Technique. ACS Appl. Mater.

Interfaces 2014, 6, 8022–8031.


Page 22 of 31

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


(41)

Duong, T. T.; Tuan, T. Q.; Dung, D. V. A.; Van Quy, N.; Vu, D. L.; Nam, M. H.; Chien, N. D.; Yoon, S. G.; Le, A. T. Application of Polyaniline Nanowires Electrodeposited on the FTO Glass Substrate as a Counter Electrode for Low-Cost Dye-Sensitized Solar Cells. Curr. Appl. Phys. 2014, 14, 1607–1611.

(42)

Pei, Q.; Qian, R. Protonation and Deprotonation of Polypyrrole Chain in Aqueous Solutions. Synth. Met. 1991, 45, 35–48.

(43)

Li, Y.; Qian, R. Studies on the Chemical Compensation of Conducting Polypyrrole by NaOH Solution. Synth. Met. 1988, 26, 139–151.

(44)

Chen, S.; Song, L.; Tao, Z.; Shao, X.; Huang, Y.; Cui, Q.; Guo, X. Neutral-pH PEDOT:PSS as over-Coating Layer for Stable Silver Nanowire Flexible Transparent Conductive Films. Org. Electron. physics, Mater. Appl. 2014, 15, 3654–3659.

(45)

Korostynska, O.; Arshak, K.; Gill, E.; Arshak, A. Review on State-of-the-Art in Polymer Based pH Sensors. Comput. Eng. 2007, 7, 3027–3042.

(46)

Natali, M.; Orlandi, M.; Berardi, S.; Campagna, S.; Bonchio, M.; Sartorel, A.; Scandola, F. Photoinduced Water Oxidation by a Tetraruthenium Polyoxometalate Catalyst: Ion-Pairing and Primary Processes with Ru(bpy)32+ Photosensitizer. Inorg.

Chem. 2012, 51, 7324–7331. (47)

Kowalski, D.; Tighineanu, A.; Schmuki, P. Polymer Nanowires or Nanopores? Site Selective Filling of Titania Nanotubes with Polypyrrole. J. Mater. Chem. 2011, 21, 17909–17915.

(48)

Lee, D.-G.; Kim, S. H.; Joo, S. H.; Ji, H.-I.; Tavassol, H.; Jeon, Y.; Choi, S.; Lee, M.H.; Kim, C.; Kwak, S. K.; et al. Polypyrrole-Assisted Oxygen Electrocatalysis on Perovskite Oxides. Energy Environ. Sci. 2017, 10, 523–527.



(49)

Shao, Y.; Wang, X.; Engelhard, M.; Wang, C.; Dai, S.; Liu, J.; Yang, Z.; Lin, Y. Nitrogen-Doped Mesoporous Carbon for Energy Storage in Vanadium Redox Flow Batteries. J. Power Sources 2010, 195, 4375–4379.

(50)

Karlsson, R. K. B.; Cornell, A. Selectivity between Oxygen and Chlorine Evolution in the Chlor-Alkali and Chlorate Processes. Chem. Rev. 2016, 116, 2982–3028.

(51)

Liu, X.; Wang, F.; Wang, Q. Nanostructure-Based WO3 Photoanodes for Photoelectrochemical Water Splitting. Phys. Chem. Chem. Phys. 2012, 14, 7894–7911.

(52)

Nosaka, Y.; Nosaka, A. Y. Generation and Detection of Reactive Oxygen Species in Photocatalysis. Chem. Rev. 2017, 117, 11302–11336.


Page 24 of 31

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


Figure 1. Experimental scheme for the fabrication of a WO3/CP:Ru4POM heterojunction photoanode for solar water oxidation. (a) Band-edge positions of the WO3 photoanode and various CPs tested in this study with respect to the standard reduction potentials of hydrogen (HER) and oxygen evolution reactions (OER). (b) Energy level diagram showing a series of PEC processes for bare and CP:Ru4POM-coated WO3 photoanodes: (i) exciton generation, (ii) exciton dissociation, (iii) charge transport, (iv) catalytic charge transfer by water oxidation, and (v) recombination. (c) Experimental procedures for the fabrication of WO3/CP:Ru4POM heterojunction photoanodes.



Figure 2. Deposition of CP and CP:Ru4POM on the surface of the WO3 photoanode. (a) TEM, STEM, and elemental mapping images showing morphological changes of WO3 upon the deposition of PPy alone or the co-deposition of PPy and Ru4POM. (b) UV/visible absorbance spectra of the bare WO3, WO3/PPy, and WO3/PPy:Ru4POM photoanodes. (c) FTIR spectra of various WO3/CP:Ru4POM photoanodes. (d) XPS C 1s and Ru 3d spectra of the WO3/PPy:Ru4POM photoanode.


Page 26 of 31

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


Figure 3. Improved performance of WO3-based photoanodes by the deposition of CPs and Ru4POM. (a) LSV curves of bare, PPy-coated, and PPy:Ru4POM-coated WO3 in the presence (solid line) and absence (dotted line) of light illumination. (b) Absorbance (solid lines) and IPCE (solid lines with symbols) spectra of the respective photoanodes. The inset shows a magnified IPCE spectrum for the bare WO3 photoanode. (c) A Nyquist plot showing the electrochemical impedance of respective photoanodes. The inset shows an equivalent circuit used for fitting. (d) An Mott-Schottky plot to study the effect of the PPy and PPy:Ru4POM films on the charge carrier density of WO3-based photoanodes. The inset shows a magnified plot for the WO3/PPy:Ru4POM photoanode.



Page 28 of 31

Table 1. Fitting results of electrochemical impedance spectra shown in Figure 3c. Rs, Rct, CPE1-T, and CPE1-P indicate bulk resistance, charge transfer resistance, constant phase element admittance, and constant phase element exponent, respectively. Obtained value (fitting error)

WO3

WO3/PPy

WO3/PPy:Ru4POM

Rs / Ω

Rct / Ω

CPE1-T / F

CPE1-P

20

15405

0.00033579

0.8768

(1.29 %)

(9.82 %)

(2.92 %)

(0.62 %)

12

4948

0.00357

0.7768

(0.70 %)

(7.82 %)

(1.03 %)

(0.51 %)

31

1717

0.0045358

1.00

(0.14 %)

(5.81 %)

(0.35 %)

(0.02 %)


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


Figure 4. Improved stability of the WO3 photoanode by the deposition of PPy:Ru4POM. (a, b) SEM images showing the morphologies of (a) WO3 and (b) WO3/PPy:Ru4POM before and after the PEC test for 90 min. (c) The XRD patterns of the respective photoanode before and after the PEC test. (d) The measured concentration of hydrogen peroxide produced from the respective photoanode during the PEC test. (e) The suggested mechanism for the improved stability of the WO3/PPy:Ru4POM photoanode.



Page 30 of 31

Table 2. Comparison of the PEC performance of WO3-based heterojunction photoanodes Heterojunction

Co-catalyst

Onset potential (V vs. RHE)

Current density (mA cm-2)a

Light source

Electrolytes

Ref.

WO3/PPy

Ru4POM

0.72

2.5

300 W Xe lamp 100 mW cm-2

0.1 M HCl pH 1.0

This study

WO3/BiVO4

-

N.A.

0.8b

AM 1.5G 100 mW cm-2

0.5 M Na2SO4

11

WO3/CuWO4

MnPOx

-0.15

> 0.6

150 W Xe lamp 100 mW cm-2

0.1 M phosphate pH 7.0

21

WO3/BiVO4

-

0.6

0.39

N.A.


22

WO3/BiVO4

-

0.34

3.1

AM 1.5G 100 mW cm-2


23

WO3/Sb2S3

-

N.A.

1.79c

Xe lamp 100 mW cm-2

1 M H2SO4

24

WO3/α-Fe2O3

-

~0.85

~2

AM 1.5G 100 mW cm-2

0.5 M Na2SO4 pH 7.0

25

WO3/BiVO4

FeOOH /NiOOH

N.A.

5.0

AM 1.5G 100 mW cm-2

0.5 M Na2SO4 pH 6.8

26

WO3/BBNOd

Co0.8Mn0.2Ox

N.A.

6.02

N.A.

0.5 M Na2SO4 pH 7.0

27

a

Current density measured at 1.23 V vs. RHE. A type of reference electrode used is unknown. c Current density measured at 0.8 V vs. RHE. d Barium bismuth niobate b


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


Table of Contents Graphic


Conducting Polymer Heterojunction Photoanodes for Efficient

Recommend Documents