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Insight into charge separation in WO3/ BiVO4 heterojunction for solar water splitting Sang Youn Chae, Chang Soo Lee, Hyejin Jung, Oh-Shim Joo, Byoung Koun Min, Jong Hak Kim, and Yun Jeong Hwang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 22 May 2017 Downloaded from http://pubs.acs.org on May 23, 2017

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ACS Applied Materials & Interfaces

Insight into charge separation in WO3/BiVO4 heterojunction for solar water splitting Sang Youn Chae†,§,‡, Chang Soo Lee∥,‡, Hyejin Jung†,#, Oh-Shim Joo†, #, Byoung Koun Min†,├, Jong Hak Kim∥,*, Yun Jeong Hwang†, #* †

Clean Energy Research Center, Korea Institute of Science and Technology, Seoul 02792,

Republic of Korea §

Department of Chemistry, College of Science, Korea University, Seoul 02841, Republic of

Korea ∥

Department of Chemical and Biomolecular Engineering, Yonsei University, Seodaemun-gu,

Seoul 03722, Republic of Korea #

Korea University of Science and Technology, Daejeon 34113, Republic of Korea

├

Green School, Korea University, Seoul, 02841, Korea

KEYWORDS Solar water splitting, Polymer-assisted synthesis, Mesoporous WO3, WO3/BiVO4 heterojunction, Charge separation.

ACS Paragon Plus Environment

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ABSTRACT

The WO3/BiVO4 heterojunction has shown promising photoelectrochemical (PEC) water splitting activity based on its charge transfer and light absorption capability, and notable enhancement of the photocurrent has been achieved via morphological modification of WO3. We developed a graft copolymer-assisted protocol for the synthesis of WO3 mesoporous thin films on a transparent conducting electrode, wherein the particle size, particle shape, and thickness of the WO3 layer were controlled by tuning the interactions in the polymer/sol-gel hybrid. The PEC performance of the WO3 mesoporous photoanodes with various morphologies and the individual heterojunctions with BiVO4 (WO3/BiVO4) were characterized by measuring the photocurrents in the absence/presence of hole scavengers using light absorption spectroscopy and intensitymodulated photocurrent spectroscopy. The morphology of the WO3 photoanode directly influenced the charge separation efficiency within the WO3 layer and concomitant charge collection efficiency in the WO3/BiVO4 heterojunction, showing smaller sized nanosphere WO3 layer showed higher values than plate-like or rod-like one. Notably, we observed that photocurrent density of WO3/BiVO4 was not dependent on the thickness of WO3 film or its charge collection time implying slow charge flow from BiVO4 to WO3 can be a crucial issue in determining the photocurrent, rather than the charge separation within the nanosphere WO3 layer.

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1. Introduction As a mimic of the natural photosynthesis system, the photoelectrochemical (PEC) cell comprising a semiconductor photocatalytic electrode has been proposed for the production of hydrogen from water and sunlight. To achieve complete solar water splitting, the PEC cell must perform multiple functions, namely, light-harvesting, charge transfer, and chemical redox reactions. The natural photosynthesis system has separate components, i.e. antenna complexes of photosystems I and II, an electron transport chain, and enzymatic reaction centers for the respective roles.1 Therefore, various strategies have been proposed for the development of semiconductor photoelectrodes in order to improve the individual functional activities, which are closely connected and affect each other. For example, the development of synthetic methods, band-gap narrowing, dual light absorber configuration, nano-structuring, doping, surface passivation, cocatalyst decoration, etc., can enhance the optical, electronic, and chemical properties of semiconductor electrodes.2 Further, researchers optimistically expect that combination of these strategies should result in synergetic effects to improve the solar-tohydrogen (STH) conversion efficiency. However, a simple combination of these approaches does not always result in higher PEC performance, and in certain cases, has in fact caused a decrease in the activity.3 Understanding the overall charge flow processes and circumventing bottleneck processes is a reasonable methodology for improving the water splitting activity while departing from trial and error. BiVO4 is one of the most promising photoanode materials at present, exhibiting visible light absorption capability and a high photocurrent of up to 4.5 mA·cm-2.4 Because of the poor charge transfer and fast recombination issues associated with BiVO4, this material is often coupled with other semiconductor materials; WO3/BiVO4 is currently the most successful heterojunction with


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an external quantum efficiency (EQE) of over 90%.5,6 Since Su et al.7 and Hong et al.8 proposed the WO3/BiVO4 photoanode, in which BiVO4 mainly plays the role of a visible light absorber and WO3 as an electron transfer agent, numerous studies of this system have been carried out to maximize the respective roles. Monoclinic, single-crystalline WO3 nanostructures demonstrate enhanced charge transport properties, resulting in photocurrent improvement because the fast charge transport in WO3 facilitates electron extraction from BiVO4.9 In the case of BiVO4, a porous structure is favorable for increasing the light absorption efficacy and surface area.10 Furthermore, effective light scattering within the WO3 helical nanostructure reportedly enhanced the light absorption by BiVO4, and the large WO3/BiVO4 interfacial area contributed to efficient charge separation.5 As shown in previous reports, the morphology is one of the most popular and effective variables that can be manipulated to increase the photoelectrochemical (PEC) activity of WO3/BiVO4. A simple and effective method for controlling the morphology of WO3 is based on microphase separation of the copolymer and its selective interaction with a hydrophilic metal oxide precursor. Amphiphilic block copolymers have been conventionally used to construct organized mesoporous morphologies via selective interaction between the WO3 precursor (i.e. tungsten(IV) chloride (WCl6), peroxotungstic acid (PTA)), and the hydrophilic domain of the copolymer. For instance, P123 and F127, composed of hydrophilic poly(ethylene oxide) and hydrophobic poly(propylene oxide), are the prevalent block copolymer templates used to fabricate the mesoporous WO3 nanostructure by tuning the phase separation of the copolymer.11-15 Owing to the large surface area and good crystallinity of WO3, excellent electrochemical properties were obtained, resulting in enhanced overall device efficiency.12,16 However, the block copolymer template has some disadvantages, such as high cost, and requires a complicated synthesis


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process. The utilization of a graft copolymer template was proposed for construction of the mesoporous structure of metal oxides due to the low cost, simple synthesis process, and capability to produce large pore sizes (>50 nm) compared to the block copolymer template.17-19 However, application of the low-cost graft copolymer-based approach for construction of the mesoporous WO3 nanostructure for water-splitting applications has not been documented thus far. Non-aqueous sol-gel chemistry has been evaluated for preparation of the WO3 precursor to achieve morphological control.20 However, the methodology for preparation of WO3 photoanodes generally requires a two-step calcination process involving: 1) synthesis of the WO3 nanoparticles (or thermal treatment, such as hydrothermal, solvothermal, etc.,) and 2) preparation of the photoanode by deposition on transparent conducting oxide (TCO) using the WO3 nanoparticles and a viscous binder. Herein, well-organized graft copolymer/sol-gel hybrids are employed in a single-step method to simultaneously synthesize the WO3 nanostructure and prepare the photoanode on TCO by exploiting the graft copolymer in the dual roles of the structure directing agent and the binder to prepare the viscous paste. The graft polymer-assisted synthesis approach developed herein is used to form mesoporous WO3 structures with a large surface area, excellent electrochemical properties, and that permit facile penetration of the gas, electrolyte, and water into the mesopores to enhance the catalytic activity.11, 17, 20, 21 The mesoporous WO3 film synthesized on fluorine-doped tin oxide (FTO) glass is coated with a BiVO4 shell for systematic characterization of the WO3/BiVO4 photoanodes by varying several parameters (i.e. the morphology, thickness, and n-n heterojunction). The WO3/BiVO4 photoanodes with various parameters are used as model systems to study the overall charge flow in the heterojunction and to identify the critical step for


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enhancing the PEC performance. Based on electrochemical/optical analysis of the PEC cell, experiments are designed to determine the important factors for the single junction (WO3) and heterojunction (WO3/BiVO4) photoanodes, respectively. The charge collection and separation properties are characterized by modulating the intensity of the incident photon and measuring the photocurrent with/without hole scavengers. It is demonstrated that the morphological changes of the mesoporous WO3 film can strongly influence the charge separation/collection efficiency. It is also shown that the photocurrents of the WO3/BiVO4 heterojunction are not always sensitive to the changes in the WO3 layer in particular, because charge flow at the WO3/BiVO4 interface or internal BiVO4 becomes a bottleneck step when the charge collection efficiency increases.

2. Experimental section 2.1. Materials Poly(vinyl chloride) (PVC, Mn ~55,000 g·mol-1), poly(oxyethylene methacrylate) (POEM, Mn ~500 g·mol-1), copper(I) chloride (CuCl, ≥99%), 1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA, 97%), tungsten(VI) chloride (WCl6, ≥99.9%), benzyl alcohol (99.8%, anhydrous), bismuth nitrate (Bi(NO3)3·5H2O, 99.99%), vanadium chloride (VCl3, 99%), and ammonium paratungstate ((NH4)10(H2W12O42)·5H2O, 99.999%) were purchased from Sigma-Aldrich and used without any purification. Ammonium molybdate (NH4)6Mo7O24·4H2O (Junsei, 81%) and ethylene glycol (99.5%) were purchased from Junsei and used without any purification. 2.2. Preparation of mesoporous WO3 photoanodes


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The poly(vinyl chloride)-g-poly(oxyethylene methacrylate) (PVC-g-POEM) graft copolymer template and the preformed WO3 species were respectively prepared to synthesize WO3 thin films with various shapes. PVC-g-POEM was synthesized through atom transfer radical polymerization (ATRP), according to our previously reported method.18, 22 Briefly, 6 g of PVC was dissolved in N-methyl-2-pyrrolidone (NMP, J.T. Baker, 99.5%) at 70 oC over the course of 3 h, followed by addition of 0.12 g CuCl, 0.23 ml HMTETA, and 12 ml POEM to the solution of well-dissolved PVC. The solution was purged with N2 for 30 min and reacted at 90 oC for 24 h. The product was precipitated by addition of excess methanol, washed three times, and dried at 50 o

C in a vacuum oven for 24 h.

Three types of pre-WO3 precursors were prepared via the non-aqueous sol-gel process under different synthesis conditions, as summarized in Table 1. First, 1.5 g of WCl6 was dispersed in 10 ml of ethanol (for pre1-WO3) or toluene (for pre2-WO3 and pre3-WO3) under vigorous stirring at room temperature. Subsequently, 50 ml of benzyl alcohol was added to the WCl6 dispersion and further stirred at room temperature until the solution became blue-green. The mixture was reacted at 50 oC (for pre1-WO3), 70 oC (for pre2-WO3), or 90 oC (for pre3-WO3) for 15 h. The final product was collected via centrifugation and further dried at room temperature in a vacuum oven. The mesoporous WO3 (MW) photoanode was then prepared via a one-step calcination approach using PVC-g-POEM and pre-WO3. First, 0.22 g of PVC-g-POEM was dissolved in 2 ml of THF until the solution became transparent. Subsequently, 0.36 g of as-prepared pre-WO3 and 0.1 ml of specific additive were added to the solution, as specified in Table 1. The mixture was further stirred for 24 h and coated onto fluorine-doped tin oxide (FTO) glass via the doctor-blade


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method using 3M magic tape layers (thickness ~50 µm) as the spacer. The MW photoanodes were obtained after calcination at 500 oC for 1 h at a ramping rate of 4 oC·min-1. 2.3. Preparation of WO3/BiVO4 heterojunction photoanodes To prepare the WO3/BiVO4 photoanodes, W and a Mo co-doped BiVO4 layer were coated on the three different types of mesoporous WO3 films. The precursor solution was prepared by dissolution of 0.6403 g Bi(NO3)3·5H2O, 0.2359 g VCl3, 15.12 µg (NH4)10(H2W12O42)·5H2O, and 3.18 µg (NH4)6Mo7O24·4H2O in 20 mL ethylene glycol. The solution mixture was spin-coated at 2000 rpm, followed by annealing at 500 ˚C for 10 min; the spin coating and annealing processes were repeated thrice to obtain the desired thickness of the BiVO4 layer on the WO3 film. The WO3/BiVO4 heterojunction film was then annealed again at 500˚C for 2 h to achieve high PEC activity. 2.4. Photoelectrochemical (PEC) measurements PEC characterization of the WO3 or WO3/BiVO4 photoanode was carried out with a potentiostat (Iviumstat) in a three-electrode system composed of a Pt counter electrode and an Ag/AgCl reference electrode. A 0.1 M potassium phosphate buffer solution (pH 7.0) was used as the electrolyte. To measure the charge separation and injection efficiencies, 0.1 M Na2SO3 as a hole scavenger was added to the phosphate buffer solution (pH 7.18). All of the measured potentials were converted versus the reversible hydrogen electrode (RHE) using Equation (1). The measured pH of the electrolyte was considered. ERHE = EAg/AgCl + (0.0591·pH) + E0Ag/AgCl, E0Ag/AgCl (3 M NaCl) = 0.209 V at 25˚C

(1)


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The absorbed photon to current conversion efficiency (APCE) was determined from the incident photon to current conversion efficiency (IPCE) and absorbance data. A 1000 W Xe lamp mounted IR water liquid filter was coupled with a motorized monochromator (Newport, Cornerstone 130) for the IPCE measurement. The APCE values were determined by applying Equation (2). APCE % =

∙ ×. ∙ ! ×" ×#$%

()

× 100 = #$% × 100 %

(2)

Here, *+, is the measured photocurrent density under each incident light illumination condition, -.. is the power intensity of the calibrated and monochromated illumination, λ is the wavelength of the monochromic incident light, and 1239.8 3 ∙ 45 is a multiplication constant calculated from Planck’s constant and the speed of light. A is the absorbance of the sample, measured using UV-vis spectroscopy (Varian, Cary 5000). To understand the dynamics of the photogenerated charges, intensity modulated photocurrent spectroscopy (IMPS) analysis was performed with the same three-electrode system by using a potentiostat (Autolab 128N) coupled with a LED driver (Autolab LED driver, LDC470 – blue (λ = 470 nm, 700 mA). The modulation frequency of the incident light was varied from 100 kHz to 1 Hz, and the intensity of the incident light was 32.68 mW. A voltage of 1.2 V vs. Ag/AgCl was applied during the IMPS measurement in 0.1 M potassium phosphate buffer/0.1 M Na2SO3 electrolyte. 2.5. Characterization The chemical and physical properties of PVC-g-POEM and the PVC-g-POEM/pre-WO3 hybrid were analyzed by Fourier-transform infrared spectroscopy (FT-IR, Spectrum 100,


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PerkinElmer, USA), thermogravimetric analysis (TGA, Q-5000 IR, TA Instruments, USA), and differential scanning calorimetry (DSC, DSC 8000, Perkin Elmer, USA). The structures of the MW photoanodes were confirmed by field-emission scanning electron microscopy (FE-SEM, SUPRA 55VP, Carl Zeiss, Germany) and the WO3 particles in the photoanode were further characterized by energy-filtering transmission electron microscopy (EF-TEM, LIBRA 120, Carl Zeiss, Germany). An X-ray diffractometer (XRD, Dmax 2500 / PC, Rigaku, Japan) was used to confirm the crystal phase of the mesoporous WO3 photoanodes using Cu-Kα radiation (λ = 1.5406 Å) in the 2θ range of 20−70o. X-ray photoelectron spectroscopy (XPS, K-alpha, Thermo Scientific Inc., U.K.) was used to investigate the binding energy of the components of the WO3 particles in the mesoporous WO3 photoanodes by employing a sonication process.

3. Results 3.1. Interactions and morphology of mesoporous WO3 films The amphiphilic graft copolymer, PVC-g-POEM, was composed of a hydrophobic PVC main chain and hydrophilic POEM side chains to facilitate partial embedding in the hydrophilically modified pre-WO3 in the POEM domain, as shown in Scheme 1. The morphology of the WO3 nanoparticles could be manipulated to generate plate-like (P-WO3), rod-like (R-WO3), and spherical WO3 (S-WO3) structures, depending on the additives in the PVC-g-POEM/pre-WO3 hybrid solution. The MW thin films were prepared via a one-step calcination process at 500 oC using the PVC-g-POEM graft copolymer with dual functionality, i.e. playing the roles of a viscous binder and structure directing agent for the WO3 nanoparticles with specific morphologies. It should again be noted that the PVC-g-POEM graft copolymer is an attractive


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material as alternative to conventional block copolymers owing to the simple synthesis and low cost. The chemical bonds of PVC-g-POEM and PVC-g-POEM/pre-WO3 were evaluated by FT-IR analysis of the functional groups, as shown in Figure 1. The PVC-g-POEM graft copolymer exhibited strong bonds of the ether (C-O-C) and carbonyl (C=O) stretching absorptions at 1099 and 1727 cm-1, indicating the successful polymerization of POEM on the PVC backbone; the representative chloride (C-Cl) band of PVC was observed at 610 cm-1. After the addition of preWO3 and specific additives to the PVC-g-POEM solution, the intensity of the absorption band in the wavenumber range of 500−800 cm-1 increased significantly due to the chemical bonds of the heavy W atom, rather than the organic molecules, according to Hooke’s law23 :

9

6 = 7 8:

(1)

where ν is the vibrational frequency, k is the force constant of the chemical bond, and µ is the reduced mass of the molecules comprising the chemical bonds. The ether (C-O-C) absorption band of PVC-g-POEM/pre-WO3 shifted slightly to lower wavenumber because the selective interaction of the hydrophilic pre-WO3 with the unoccupied electrons in the C-O-C bond decreased the strength of the C-O-C bonds.18 Notably, the ether band of the PVC-g-POEM/pre3WO3/H2O2 hybrid shifted to even lower wavenumber (1093 cm-1) compared to the other hybrids, indicating stronger interactions of pre3-WO3 with the PVC-g-POEM graft copolymer. The high temperature (90 oC) used for pre3-WO3 might have resulted in better dissolution of pre-WO3 in H2O2 and greater compatibility with PVC-g-POEM.24 Figure 2 shows FESEM images of the surface of the MW thin film photoanodes, synthesized using the PVC-g-POEM/pre-WO3 non-aqueous sol-gel hybrids with different additives. Semi-


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cubic and plate-like WO3 (P-WO3) species were deposited on the conductive FTO glass, and the average particle size was approximately 100 nm (denoted as MW1, Figure 2a). However, when HCl was used as the additive in the PVC-g-POEM/pre2-WO3 hybrid, rod-like WO3 (R-WO3) nanoparticles were formed without any significant cracks (denoted as MW2, Figure 2b). We postulate that formation of the R-WO3 structure was mainly triggered by the addition of HCl that converted the pre-WO3 into H2WO4 nanorods, based on previously reported data.25-27 Finally, the addition of H2O2 to the solution of PVC-g-POEM/pre3-WO3 generated a film of spherical WO3 (S-WO3) nanoparticles with an average particle diameter of 50 nm and interstitial spherical pore size of 50 nm (denoted as MW3, Figure 2c). The mesoporous structure with organized pores and good interconnectivity was developed because of the greater dissolution of pre3-WO3 in H2O2 to form peroxo-tungstic acid and selective interaction with the POEM side chain of the PVC-gPOEM micelle. Notably, the MW1, MW2, and MW3 photoanodes all exhibited interstitial pores between the nanoparticles. This is attributed to selective interaction of the hydrophilically modified pre-WO3 with the POEM side chains, generating mesopores from the hydrophobic PVC domain after calcination of the PVC-g-POEM template.28 In addition, depending on the synthesis conditions, a significant discrepancy in the transparency of the MW photoanodes was observed, as shown in the photographical images (Figure S1). The MW1 photoanode appeared opaque due to the large, irregular interstitial pores as well as the large particle size of P-WO3, where the particles assembled into a rod-like structure that scattered light back into the photoanode. MW2 appeared to be translucent, but was much more transparent than P-WO3. MW3 had the highest transparency due to the small particle size and organized mesoporous morphology.


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Interestingly, the X-ray diffraction (XRD) patterns of the MW photoanodes, especially the spherical WO3 (MW3), showed relatively different facet proportions. All three mesoporous WO3 photoanodes comprised monoclinic crystal-phase WO3 (JPCDS 04-007-2425; Figure 3a) based on the representative peaks at 2θ values of 23.1o, 23.6o, and 24.2o, associated with the (002), (020), and (200) planes, respectively. The peak intensity profiles of the MW1 and MW2 photoanodes were largely similar, indicating the (200) facet to be the most dominant facet. The relatively dominant growth of (200) facet would be mainly attributed to the plate-like morphological similarity of particle itself. In detail, the particle morphology of MW2 (nanorod) was similar to the linked plate-like WO3 nanoparticles. 29,30 For the MW3 photoanode, the peaks of the (002) and (020) facets were more intense than for the other two MW photoanodes. Interestingly, the spherical morphology of WO3 nanoparticles could have resulted in the similar crystal growth between (020) and (200) facets due to better solubility of WO3 precursor in H2O2.31 The X-ray photoelectron spectroscopy (XPS) survey data (Figure S2) showed characteristic peaks at 37.9 and 35.9 eV in the W 4f spectra, corresponding to the W 4f5/2 and W 4f7/2 states of W6+, respectively.32, 33 These peaks demonstrate that the mesoporous WO3 film was successfully prepared via the graft copolymer approach. 3.2. PEC characterization of WO3 photoanode First, the PEC performance of the three different MW photoanodes was compared by linear scan voltammetric (LSV) analysis of water oxidation (Figure 4a). All three WO3 photoanodes had a similar onset potential (0.6 V vs. RHE), but the photocurrent density was highly dependent on the morphology of the MW films, where the highest photocurrent was obtained with the large nanoparticles of MW1. The photocurrent densities were 0.39, 0.17, and 0.22 mA·cm-2 at 1.23 V vs. RHE for MW1, MW2, and MW3, respectively. When hole scavenging SO32- ions were added


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in the electrolyte, we can estimate the charge injection on the WO3 photoanodes surface is fast, and the obtained photocurrent is limited by charge separation and light absorption properties of the photoanodes resulting in the photocurrent densities of 0.29, 0.31, and 0.41 mA·cm-2 (at 1.0 V vs. RHE) for the MW1, MW2, and MW3 photoanodes, respectively (Figure 4b). It is also observed that the onset potentials of the photocurrent were cathodically shifted by ~ 200 mV from ~ 0.65 V vs. RHE to ~ 0.45 V when adding hole scavenger in the electrolyte, as the overpotential of scavenger oxidation reaction becomes small. Therefore, the onset potential became close to the theoretical flat band potential of the semiconductor when hole scavenger added. The charge separation efficiency (Φsep) and charge injection efficiency (Φinj) of the respective samples were compared (Figure 4c and d). Interestingly, the highest Φsep value was obtained with MW3, while MW1 gave rise to the highest Φinj, especially at high bias potential (> 1.0 V vs. RHE). 3.3. PEC characterization of WO3/BiVO4 photoanode with different WO3 morphologies A W, Mo-doped BiVO4 layer was coated on each MW photoanode (Figure 2g-i, and Figure S3) and the PEC water oxidation activities of the WO3/BiVO4 heterojunction photoanodes were compared to understand the charge flow and separation within the heterojunction (Figure 5). All three WO3/BiVO4 photoanodes showed enhanced photocurrents compared to that of the mesoporous WO3 films due to the visible light absorption capability associated with the smaller band-gap of BiVO4. Notably, however, the final photocurrent densities of the WO3/BiVO4 heterojunctions did not follow a trend consistent with the photocurrent of single-junction WO3. In contrast with MW1, the MW1/BiVO4 photoanode gave rise to the poorest photocurrent density, with only a ~1.2-fold enhancement in the photocurrent at 1.23 V vs. RHE, whereas


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MW2/BiVO4 and MW3/BiVO4 gave rise to an almost six-fold increase in the photocurrent compared to MW2 and MW3, respectively. In addition, Φsep and Φinj of WO3/BiVO4 were measured in the same manner as used for the WO3 single-junction photoanode (Figure 5b and c). The MW1 single-junction and MW1/BiVO4 heterojunction photoanodes gave rise to the lowest Φsep values; Figure 5c clearly shows that the WO3/BiVO4 photoanodes had similar Φinj values, independent of the MW morphology. In addition, the transience time of the electron (τd) was characterized by using IMPS (Figure 5d) with a 470 nm LED light source that selectively excites BiVO4 to generate an electron-hole pair. The MW1/BiVO4 photoanode showed the slowest transience time (62.2 ms), which was much slower than that of MW2/BiVO4 (3.71 ms) and MW3/BiVO4 (4.26 ms). In the ensuing experiments, the MW3 film was used as the WO3 layer unless otherwise noted because MW3/BiVO4 showed the highest photocurrent density, as shown in Figure 5a. 3.4. PEC characterization of WO3/BiVO4 photoanode with different WO3 thickness To examine the role of the WO3 layer in the heterojunction photoanodes, the thickness of the WO3 film was varied. A trade-off is expected because a thicker WO3 film may be favorable as a template layer for conferring a higher surface area for the BiVO4 coating but may be unfavorable for providing a long electron transfer pathway to the FTO back contact. In fact, the electron transience time almost tripled (from 2.31 ms to 7.50 ms) for the WO3/BiVO4 photoanodes as the thickness of the WO3 film increased from 490 nm to 1600 nm (Figure 6a). However, surprisingly, the photocurrents of the WO3/BiVO4 heterojunctions were found to be independent of the thickness of the WO3 thin film (Figure 6b). In other words, the variation in the transience time within these ranges was not sufficiently meaningful to influence the PEC activity of the


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heterojunction. In addition, the dependence of the response to the wavelength of the incident light was measured by APCE (Figure 6c). The contribution of the thickness of the WO3 film the photocurrent differed in the short wavelength UV region (< ~400 nm) versus the visible light region (> ~400 nm). Figure 6d shows a comparison of the APCE values at λ = 360 nm (UV light region) and λ = 450 nm (visible light region) depending on the WO3 thickness. The APCE values decreased slightly at λ = 360 nm as the thickness of WO3 increased, whereas similar APCEs were obtained at λ = 450 nm. 4. Discussion 4.1. PEC characterization of WO3 photoanodes The size of the WO3 particles and the interstitial pores between the particles can be controlled by manipulating the specific interaction between the amphiphilic copolymers, additives, and tungsten precursors, which consequently changes the preferential facets and transparency of the WO3 photoanode films. These morphological changes are expected to critically influence the final photocurrent densities because the photo-excited electrons and holes must be separated and pass through the WO3 photoanode films. The photocurrent density of water spitting (JPEC) is expressed by Equation (2). JPEC(water) = Jlim × Φabs × Φsep × Φinj

(2)

where Jlim is the theoretical photocurrent limit under 1 sun illumination, estimated from the band-gap of the semiconductor, Φabs is the light absorption efficiency, Φsep is the charge separation efficiency, and Φinj is the charge injection efficiency at the semiconductor/electrolyte interface. Therefore, Jlim could be derived by integration of the 1 sun photon flux above the band-


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gap of the semiconductor, giving a value of 3.979 mA·cm-2 for WO3. If there is no loss in the charge separation and injection process (i.e. Φsep and Φinj are 100%), then all the absorbed photons are converted to photocurrent (Jabs), as simplified in Equation (3). Jabs = Jlim × Φabs

(3)

Additionally, Φinj can be assumed to be 100% when a hole scavenger is added to the electrolyte due to the fast oxidation kinetics. Here, the accumulated holes on the semiconductor surface can be immediately injected into the hole scavenger SO32- ion and oxidize it. In the presence of the scavengers, the charge separation of the internal semiconductor is the critical factor limiting the photocurrent, as represented in Equation (4).34 JPEC(hole scavenger) = Jlim × Φabs × Φsep = Jabs × Φsep= Jsep

(4)

Therefore, Φsep is obtained from Jsep divided by Jabs, and Φinj is obtained from JPEC(water) divided by Jsep. Φsep and Φinj provide information about the electron-hole pair behavior, which reflects the electron transport and electrochemical reaction kinetics on the photoelectrode surface. From UV-vis absorption spectroscopy and analysis of the photocurrent in the presence of the hole scavenger, we could characterize the charge separation and injection properties for solar water oxidation using the WO3 photoanodes with different morphologies. Increased charge separation efficiency was achieved with the small WO3 nanoparticles; the MW3 sample with the smallest particles and spherical morphology gave rise to the highest Φsep, followed by MW2 and MW1 in the inverse order of size. Notably, the charge separation of the MW3 sample was highly efficient compared to that of the other two samples, and it should be recalled that the peaks of the of (002) and (020) facets were relatively more intense for the MW3


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photoanode, whereas the distribution of the (002), (020), and (200) facets was almost equal for MW1 and MW2 (Figure 3b). This suggests that both the particle size and the dominance of the (002) and (020) facets contribute to the improved charge separation in the mesoporous WO3 photoanode. On the other hand, the highest injection efficiency was achieved with the plate-like morphology of the MW1 sample, where the injection efficiency is the major source of the highest PEC water oxidation activity exhibited by MW1 under high bias potentials, as shown in Figure 4a. Compared to MW1, MW2 and MW3 are composed of smaller nanoparticles that are expected to have higher electrode/electrolyte interface areas. However, the PEC measurements suggest that the shape of the WO3 nanoparticle is more critical for increasing the rate of charge injection into the electrolyte for water oxidation. Notably, almost identical injection efficiencies were obtained with the MW2 and MW3 photoanodes despite the difference in the (002), (020), and (200) facet distributions. We could conclude that these facet distributions do not critically influence the water oxidation kinetics, but influence charge transfer across the WO3 thin film photoanode. In addition, the morphology of the nanoparticles and the porosity of the film can affect the reflection of the incident light, although the band-gaps of the nanoparticles are the same. It was observed that the transparency of the three MW films decreased dramatically as the particle size increased (Figure S4(a)); thus, higher absorption efficiency was achieved with the larger particles based on the UV-vis data (Figure S4(b)), even for the same film thickness of 1.4 μm (Figure 2). Therefore, the highest Jabs was obtained with MW1; the Jabs of the respective WO3 films were calculated as 2.29, 2.04, and 1.76 mA·cm-2 for MW1, MW2, and MW3. The differences in the Jabs values of the three MW samples are relatively insignificant, and thus


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should have less influence on the photocurrents compared to the differences in Φsep and Φinj. Overall, even with a single junction, the PEC water oxidation activity did not follow a simple trend based on particle size, transparency, or facet dependence, because Φabs, Φsep, and Φinj followed distinct trends. This combination of factors resulted in the highest photocurrent being achieved with the MW1 photoanode, followed by MW3, and MW2. 4.2. PEC characterization of WO3/BiVO4 photoanodes with different WO3 morphologies We achieved a significant enhancement of the photocurrent after coating BiVO4 on the mesoporous WO3 photoanodes, but found that the extent of the enhancement varied based on the type MW thin film. UV-vis absorption spectroscopy (Figure S5) showed that the three different MW/BiVO4 photoanodes had similar light absorption features, as the BiVO4 layer is the main light absorber, indicating that Φabs is not a limiting factor for photocurrent enhancement. To understand the PEC behavior of the WO3/BiVO4 heterojunction, the charge separation and charge injection efficiencies of WO3/BiVO4 were compared as done for the WO3 single-junction photoanode. The data revealed that efficient charge separation at the WO3 single junction is the most critical factor for enhancing the water oxidation activity of the WO3/BiVO4 heterojunction. A high Φsep of the MW photoanode resulted in a high photocurrent for MW/BiVO4 (Figure 4(c) and Figure 5(a)). Figure 5(b) also shows that the Φsep values of the WO3 single-junction and WO3/BiVO4 heterojunction photoanodes are directly correlated, suggesting that charge carriers are mainly separated from BiVO4 through the WO3 layer in the heterojunction. When the photons are absorbed by the heterojunction, the photoexcited electrons are expected to be transferred toward the WO3 layer and reach the FTO back contact because the conduction band minimum (CBM) of WO3 is located at lower potential than that of BiVO4 (Scheme 2(b)). Therefore, fast electron transfer through the mesoporous WO3 film also enhances the internal


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charge separation efficiency of the heterojunction. On the other hand, the Φinj values of the WO3/BiVO4 photoanodes were almost the same, despite the variations in the morphology of the mesoporous WO3 films (Figure 5(c)), which confirmed that hole injection to the electrolyte occurs on the surface of BiVO4. The similarity of the injection efficiencies implies that direct hole transfer from the bottom WO3 layer to the electrolyte is negligible in the present heterojunction. For more detailed characterization of the electron transport in the WO3/BiVO4 heterojunction, the intensity of the incident light was modulated with various frequencies in aqueous medium (Figure 5(d)). The response of the photocurrent to modulation of the frequency of the illumination intensity provides the transience time (τd) of the electron, which is the time required to reach the back contact (FTO) substrate. The response appears as a semicircle, and τd can be calculated from τd = (2πfmin)-1, where fmin is the frequency at the minimum imaginary photocurrent in the IMPS response.35 Here, the IMPS signal originates from photo-absorption of BiVO4 at λ= 470 nm. The IMPS measurement showed that the τd values of the three MW/BiVO4 photoanodes are inversely proportional to Φsep (Figure S6), suggesting that electron transfer to the back contact is a limiting factor for achieving efficient charge separation at the WO3/BiVO4 interface. The MW1/BiVO4 photoanode showed the lowest Φsep and had the most sluggish transience time, which was almost 20 times slower than those of the MW2/BiVO4 and MW3/BiVO4 congeners. Therefore, we can conclude that the present strategy involving morphological control of the mesoporous WO3 film is reasonable for enhancing internal charge transport and thus improves the PEC water oxidation activity of WO3/BiVO4 without affecting the light absorption and charge injection efficiencies. 4.3. PEC characterization of WO3/BiVO4 photoanode with different WO3 thickness


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The PEC activity of the WO3/BiVO4 photoanode is also affected by the film thickness because of the previously mentioned trade-off effect between the light absorption and charge transfer. As expected, the IMPS measurement showed that the thicker WO3 film led to a longer τd value for the WO3/BiVO4 heterojunction, which can be understood because the electrons have to travel a long distance across the WO3 layer before being collected by the back contact (Figure 6(a)). Interestingly, even though the thickness of the MW3 film increased from 490 nm to 1600 nm, the τd value only increased by ~5 ms. This is a much smaller difference considering that the τd values varied by ~60 ms for MW1, MW2, and MW3 with variation in the morphology of the WO3 film (Figure 5(d)). Moreover, the photocurrent was almost the same, irrespective of the thickness of the MW3/BiVO4 photoanodes (Figure 6(b)). For detailed evaluation of the thickness effect, τd and the internal quantum efficiency (i.e. APCE) values in the visible region (λ > 400 nm) were compared. In addition, there were no noticeable changes in the APCE below λ > 400 nm, even though the charge transience time (τd) was varied. BiVO4 mainly absorbs visible light due to the smaller band-gap of 2.4 eV, and the photogenerated electrons in the BiVO4 layer have to move to the bottom WO3 layer to complete the charge collection process. These APCE results show that the PEC activity of the WO3/BiVO4 heterojunction could not be enhanced once τd exceeded certain level (τd≈3 ms), indicating that charge transfer across the WO3 layer is no longer the limiting factor for charge separation in BiVO4. This is valid if the charge separation at the WO3/BiVO4 interface or internal BiVO4 layer is poorer than the charge transfer within the WO3 layer. This can also explain why MW2/BiVO4 and MW3/BiVO4 had similar charge separation efficiencies and consequently similar photocurrents although MW2 had a faster τd than MW3. In the present series of heterojunction


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photoanodes, charge separation from BiVO4 to WO3 could be similarly low because the same preparation method was used for BiVO4 coating in all samples. Further improving the photocurrent of the heterojunction by modifying the WO3 layer may be limited because interfacial charge transfer from BiVO4 to WO3 is the most critical step. On the other hand, in UV region, the APCE values of the WO3/BiVO4 photoanodes decreased with the thicker WO3 films because both WO3 and BiVO4 competitively absorb these short wavelength photons, and the thicker WO3 film would exhibit greater absorption. When WO3 absorbs light, the photogenerated holes must move from the bottom WO3 layer to the upper BiVO4 surface to be utilized for water oxidation, and the electrons must move within the WO3 layer toward the back contact (Scheme 2 (c)). Thus, a slow τd will decrease the charge separation efficiency and decrease the internal quantum efficiency of the heterojunction. In addition, the holes must pass through the WO3/BiVO4 interface, which may reduce the charge separation. 5. Conclusions The particle size and morphology of WO3 mesoporous nanostructures were successfully varied by tuning the interaction of the polymer/sol-gel hybrids to prepare mesoporous WO3 photoanodes. When WO3 only is used for the PEC reaction, the morphological changes directly affect the light absorption, charge separation efficiency, and injection efficiency. Notably, large WO3 nanoplates show higher injection efficiency, whereas the smaller nanospherical congeners show higher charge separation efficiency. However, in the WO3/BiVO4 heterojunction photoanodes, these charge carrier movements interfere with each other. The injection efficiency of the top layer BiVO4 and the charge separation efficiency of the bottom WO3 layer are the main parameters, which well supports the separation of the photogenerated carriers across the


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interfaces. However, notably, we found that further increasing the photocurrent by varying the thickness of the WO3 layer is limited when the charge collection is fast enough, implying that slow charge flow from BiVO4 to the WO3 layer becomes the critical cause of recombination loss. This suggests that WO3/BiVO4 interface engineering is the preferential choice for improving the overall PEC activity, although previous enhancement of the activity has mainly been achieved by engineering the WO3 layer. This characterization of the bottleneck factor is a useful approach for the prospective design of photoelectrodes and ultimately to achieve high STH conversion efficiency.


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FIGURES

Figure 1. FT-IR spectra of PVC-g-POEM graft copolymer and PVC-g-POEM/pre-WO3 prepared under different conditions with various additives.


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Figure 2. FESEM images of mesoporous WO3 thin films on the FTO substrate (a) 70˚C, ethanolbased (denoted as MW1), (b) 70˚C, toluene-based (denoted as MW2), (c) 90˚C, toluene-based (denoted as MW3); (d), (e), and (f) show cross-sectional images of (a), (b), and (c), respectively; (g), (h), and (i) show the surface morphology of WO3/BiVO4, prepared by BiVO4 coating on (a), (b), and (c) respectively.


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Figure 3. (a) XRD data of respective WO3 thin films and (b) fitting results for three main peaks of WO3. Numbers indicate ratio of facets in the three peaks.


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Figure 4. I-V curves of respective WO3 photoanodes (a) measured in 0.1 M potassium phosphate electrolyte for water oxidation, (b) measured in the presence of hole scavenger (0.1 M Na2SO3/0.1 M potassium phosphate); (c) charge separation efficiency and (d) injection efficiency of respective WO3 thin film photoanodes.


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Figure 5. (a) I-V curves of respective WO3/BiVO4 photoanodes, measured in 0.1 M potassium phosphate electrolyte. (b) Charge separation efficiency and (c) injection efficiency of WO3/BiVO4. (d) IMPS spectra of WO3/BiVO4 measured at 1.2 V vs. Ag/AgCl.


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Figure 6. (a) IMPS spectra of WO3/BiVO4 with variation in WO3 thickness. (b) I-V curves of WO3/BiVO4 photoanodes measured in 0.1 M phosphate buffer electrolyte. (c) APCE spectra of WO3/BiVO4 measured at 1.2 V vs. Ag/AgCl. (d) Comparison of APCE at 1.2 V vs. Ag/AgCl under illumination at different wavelengths.


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Scheme 1. Schematic illustration of synthesis of MW photoanodes using PVC-g-POEM amphiphilic graft copolymer with different additives and band-gap of WO3/BiVO4 core/shell structure.

Scheme 2. Photoelectrochemical reaction diagram for (a) WO3 single-junction photoanode and (b) WO3/BiVO4 heterojunction photoanode, and (c) charge flow in the WO3/BiVO4 heterojunction photoanode under irradiation.


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Table 1. Conditions for polymer-assisted synthesis of pre-WO3

Precursor

WCl6

Solvent

Benzyl alcohol

Reaction temperature

Pre1-WO3

1.5 g

10 mL (ethanol)

50 mL

50 °C

Pre2-WO3

1.5 g

10 mL (toluene)

50 mL

70 °C

Pre3-WO3

1.5 g

10 mL (toluene)

50 mL

90 °C


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ASSOCIATED CONTENT Supporting Information. Photographic image, XPS data, absorbance and transmittance data of WO3, absorbance of WO3/BiVO4 heterojunctions, and relationship between separation efficiency and inversion of transient time in WO3/BiVO4 are included in the Supporting Information. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Phone: +82-2-958-5227. Fax: +82-2-958-5809. *E-mail: [email protected] Phone: Tel: +82-2-2123-5757

Fax: +82-2-312-6401

Author Contributions Sang Youn Chae and Chang Soo Lee performed detailed experiment, analysis and wrote the manuscript. Hyejin Jung help material synthesis. All authors approved the final version of the manuscript. Yun Jeong Hwang, Jong Hak Kim, Oh-Shim Joo, and Byoung Koun Min planned the overall contents of project. ‡These authors contributed equally. Funding Sources This work was supported by the Korea Institute of Science and Technology (KIST) and by the Korea Center for Artificial Photosynthesis (KCAP) through the National Research Foundation of Korea (No. 2014M1A2A2070004), and partially supported by a National Research Foundation (NRF) grant through the Center for Advanced Meta-Materials (CAMM) (2014M3A6B3063716).


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