Tunable Mesoporous Structure of Crystalline WO3 Photoanode toward

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Tunable Mesoporous Structure of Crystalline WO3 Photoanode towards Efficient Visible-Light-Driven Water Oxidation Debraj Chandra, Kenji Saito, Tatsuto Yui, and Masayuki Yagi ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04166 • Publication Date (Web): 25 Oct 2018 Downloaded from http://pubs.acs.org on October 28, 2018

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ACS Sustainable Chemistry & Engineering

Tunable Mesoporous Structure of Crystalline WO3 Photoanode towards Efficient Visible-Light-Driven Water Oxidation Debraj Chandra,* Kenji Saito, Tatsuto Yui, and Masayuki Yagi* Department of Materials Science and Technology, Faculty of Engineering, Niigata University, 8050 Ikarashi-2, Niigata 950-2181, Japan *Author to whom correspondence should be addressed. (D.C) E-mail: [email protected] (M.Y) E-mail: [email protected]; Tel and Fax, +81-25-262-6790.

KEYWORDS: mesoporous material, surfactant template, tungsten oxide, water oxidation, photoelectrocatalysis

ABSTRACT: Tunable mesoporous crystalline tungsten trioxide (WO 3) was synthesized by in situ surfactant-thermal-carbonization method using the series of 2-(alkylaminomethyl)pyridine (PAL2-n, n = 8, 12 and 16) surfactant templates with varied alkyl chain length. The d-spacing,

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corresponding to the pore center-to-center distance, for mesoporous WO3 annealed at 550 C decreased from 3.8 to 3.1 nm with shortened chain length of PAL2-n from n = 16 to 12 in response to changes in the smaller size of micellar self-assemblies of PAL2-n templates. The mesoporous structure was partially collapsed for the PAL2-8 template during the crystallization of WO3 framework, when annealed at 550 C. The longer alkyl chain of PAL2-n yielded the thicker porewalls in WO3/PAL2-n mesocomposites, affording better thermal stabilization of the organized mesoporous structure of WO3 along with the formation of higher content of carbonaceous species in mesopores through surfactant-carbonization, which act as a protective support during growth of WO3 nanocrystals by annealing. The degree of organization in mesoporous structures revealed direct impact on the visible-light-driven water oxidation performance. The incident photon-tocurrent conversion efficiency (IPCE) of mesoporous WO3 based on water oxidation increased from 24 to 36% at 420 nm and 0.5 V vs Ag/AgCl with the increase of the alkyl chain length of PAL2-n from n = 8 to 16, and the IPCE values of mesoporous samples are ca. 5 ~ 7 times higher than that (5%) of untemplated WO3.

Introduction Photoelectrochemical (PEC) water splitting into H 2 and O2 using nanomaterials has recently been a cutting-edge research area due to their growing expectation for clean and direct production of H 2 from water using abundant solar light.1-10 Photoanode materials for PEC water splitting have been extensively studied owing to the energy demanding bottleneck of a water oxidation reaction. 4-9, 1113,14 15-20

Since the critical limitation of extensively-studied TiO2 photoelectrodes that absorb only

UV light due to its wide electronic band gap (3.0 - 3.2 eV),3, 21 the materials with a relatively


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narrow bandgap (WO3, Fe2O3, Ta3N5, TaON, etc.) are currently being nurtured to expand light absorption in a visible region for abundant solar light.4, 6-8, 11-12, 17-20, 22-26 WO3 has attracted immense attention as an n-type semiconductor photoanode material for PEC water oxidation because of its visible light response (band gap energy, Eg = 2.6 - 2.8 eV), a valence band edge position thermodynamically possible for water oxidation (ca. 3 V versus the normal hydrogen electrode), and good photochemical stability under the acidic conditions. 4, 8, 19, 22-23, 27-37

Nanostructure control of WO3 is a promising approach to offer superior performance in

PEC water oxidation to those with unspecified WO3 structures due to an increase of active sites at a WO3-electrolyte interface and other structural benefits of nanostructures such as shorter chargecarrier diffusion length, unidirectional electron transport etc.8,

23, 30, 33-34, 38-45

A variety of

nanostructures of WO3 photoanodes such as mesoporous,8, 30 nanoplatelets,32 nanoparticles,39, 46 nanoflakes,47-49 nanotubes,50 nanobelts,51 and nanorods45,

52-53

have been developed so far to

improve performances in PEC water oxidation. Mesoporous metal oxides including WO3 have attracted tremendous interest in energy conversion and storage applications because of their high surface area and tunable design of porous architectures.8-10, 23, 54-59 A surfactant-templated strategy is one of the effective ways for fabricating organized mesoporous metal oxides, having highly interconnected and adjustable mesopores in addition to high surface areas.56, 60-64 Particularly in case of WO3 as a photoanode, high crystallinity is required to gain high performances for PEC water oxidation.34, 39 However, high crystallinity has been rarely achieved in an organized mesoporous system (pore diameter, ≤ 10 nm) of WO 3 due to the architectural susceptibility of the pore walls by growth of nanocrystals, leading to collapse of the mesoporous structure.8 Therefore, interparticle mesoporous system (particles spacing, > 10 nm) with a moderate surface area and an ill-defined porous structure were


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predominantly employed for WO3 photoanodes so far.23, 34, 39, 44 We recently reported the only example of surfactant-template approach for a small mesopore system (pore diameter, ca. 2-3 nm) of a novel crystalline WO3 photoanode; it is particularly appealing for a large surface area to increase the number of water oxidation sites on the WO3 surface and nanosized pore-walls permitting a shorter carrier diffusion length for highly efficient PEC water oxidation.8 An organic amphiphilic molecule, 2-(hexadecylaminomethyl)pyridine (PAL2-16)65-67 used as a surfactant template enabled thermal crystallization of nanosized pore-walls at high temperature by providing a carbon support inside pores68 through carbonization under N2 (in situ surfactant-thermalcarbonization method). In general, the surface area is possible to be further increased with shortened chain length of surfactant owing to the decreasing pore size in an organized mesoporous system.66,

69

However, the effect of the surfactant template on the formation of organized

mesoporous structures in crystalline WO3 and the PEC properties is still unclear. Nanostructural, surface and electronic properties of a mesoporous system vary significantly depending on the chain length of surfactant template molecules 61, 70-74 to provide a critical influence over PEC performances of WO 3 photoanodes. In this study, we have employed surfactant templates of the series of 2-(alkylaminomethyl)pyridine (PAL2-n, n = 8, 12 and 16) with varied chain length to investigate their impact on the mesopore formation and the PEC properties of the WO3 photoanode. Herein, we first report the tunable pore structure in an organized mesoporous system (pore diameter, < 5 nm) of crystalline WO3 photoanodes. The nanostructural properties and parameters for the formation of organized mesopores in crystalline WO3 are characterized to indicate that the longer alkyl chain of the PAL2-16 template affords better thermal stabilization of the mesoporous structure in crystalline WO3 photoanodes. The highly organized mesoporous structure is responsible for the higher performance and stabilization for PEC water


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oxidation yielded at WO3 photoanodes formed by the PAL2-16 template, compared with PAL2-n (n = 8 and 12) and untemplated one. Results and discussion Preparation and Physicochemical Characterization The as-prepared WO3/PAL2-n composites were synthesized by condensation/dissociation of anionic peroxo-tungstic acid (PA) precursors to amorphous WO3 in presence of PAL2-n (n = 8, 12 and 16) in ethanol. PAL2-n templates with longer chain length of n ≥ 18 were poorly soluble and separated out from the mixture solution under the optimal synthesis conditions, which led to ill organized WO3/PAL2-n (n ≥ 18) composites. The TEM images of as-prepared WO3/PAL2-n (n = 8, 12 and 16) composites with the different alkyl chain lengths of PAL2-n are shown in Figure 1. The WO3/PAL2-16 composite shows a highly ordered 2D hexagonal mesostructure (p6mm symmetry) of ca. 3 nm pores (Figure 1a), being in agreement with our previous observation. 8 However, only a locally ordered and completely disordered mesostructures were observed for WO3/PAL2-12 (Figure 1b) and WO3/PAL2-8 with the shorter alkyl chain lengths of PAL2-n (Figure 1c), respectively. A close look into these images also revealed that the average dimension of pores has been decreased with shortened alkyl chain length of PAL2-n templates. The amorphous WO3/PAL2-n composites were simultaneously carbonized and crystallized at 550 ºC under N2 according to the in situ surfactant-thermal-carbonization method,8 and then carbon residues were burned out by switching to O2 atmosphere at the same temperature. The complete removal of amorphous carbon was confirmed by the Raman spectroscopic analysis (Figure S1) in the resultant crystalline WO3/PAL2-n samples. The crystallization of nanosized pore-walls of WO3 is illustrated by a representative HRTEM images of WO3/PAL2-n (Figure 2a and S2) annealed at 550 ºC. The HRTEM images of WO3/PAL2-12 (Figure 2a) and WO3/PAL2-


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16 (Figure S2a) exhibited that walls around the small mesopores are composed of randomly oriented tiny crystalline particles of an estimated size of ca. 4~8 nm. The crystalline lattice fringes were observed with interplaner distances of 3.82 Å and 3.74 Å, which closely correspond to the crystal planes of (002) and (020) of monoclinic WO 3 structure in accordance with our previous report.8 However, crystallization of the pore walls by annealing led to less ordered porous structure in WO3/PAL2-8 (Figure S2b) compared to those in WO3/PAL2-12 and WO3/PAL2-16, which is supported by the more broadening of the small-angle XRD pattern after annealing for the WO3/PAL2-8 (Figure 3, vide infra). The selected area electron diffraction (SAED) patterns (Figure 2b) of the WO3/PAL2-12 annealed at 550 ºC were also investigated in which the (020), (022) and (411) crystal planes corresponding to the interplaner distance of 3.75 Å, 2.73 Å and 1.71 Å were observed. The small-angle XRD data also revealed four reflections of 2D hexagonal mesophase for as-prepared WO3/PAL2-16 composites and single diffraction peak for both WO3/PAL2-12 and WO3/PAL2-8 composites showing no long range mesoscopic ordering, which is consistent with TEM observations (Figure 3A). The d-spacing values calculated from the XRD maxima for WO3/PAL2-n composites (Table 1) gradually decreased from 4.5 to 2.9 nm with shortened alkyl chain length from n = 16 to 8, being also consistent with the pore center-to-center correlation lengths of respective TEM images. The as-prepared WO3/untemplate composite shows no detectable XRD peak, signifying any mesostructure was not yielded without the PAL2-n templates in the precursor solution. These results suggest that the mesostructure formation of WO 3/PAL2-n composites have been guided by the micellar self-assemblies of protonated PAL2-n during condensation/dissociation of the PA precursor around it. All the annealed samples showed only a single diffraction peak with gradual broadening (Figure 3B). However, more prominent collapse


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of the mesostructure were observed using the PAL2-n template with the shorter alkyl chain. The collapse of the mesostructure could be traced back to the formation of dense nanocrystals by crystallization of the framework during annealing. The PAL2-n template with the shorter alkyl chain forms micellar self-assemblies of smaller dimension to produce WO3/PAL2-n composites with slimmer pore-walls, which is significantly affected by the growth of WO3 nanocrystals during annealing. The wide angle XRD patterns (Figure 3C) also revealed that framework of all the WO3/PAL2-n samples annealed at 550 ºC were highly crystallized as phase-pure monoclinic WO3 (JCPDS number: 43-1305). Average crystallite diameters estimated according to Scherrer equation using (002) reflections were 7.6, 8.1 and 11.2 nm for WO3/PAL2-16, WO3/PAL2-12 and WO3/PAL2-8 samples annealed at 550 ºC (Table 1), respectively, which suggests that formation of larger WO3 nanocrystals in the pore-walls is responsible for the partial collapse of the mesostructure observed using PAL2-8. N2 sorption isotherms of all the WO3/PAL2-n samples (Figure 4A) annealed at 550 C show representative type-IV curves, being characteristic of mesoporous materials. 8,

75-76

The

isotherms depict the gradual increase of the adsorption amount above relative pressures (P/P0) ≈ 0.15 due to N2 uptake in the mesopores through a capillary condensation step and a H2-type hysteresis loops (P/P0) ≈ 0.5 - 0.7 possibly due to the roughness of the surfaces of pores and particles.77 Also a sharp N2 uptake around P/P0 > 0.8 indicates the existence of secondary interparticle porosity.8 Although the isotherm patterns due to the mesostructure are maintained for all the WO3/PAL2-n samples, the hysteresis loops were gradually widened with shortened alkyl chain length of PAL2-n, suggesting the increased roughness of the surface due to the partial collapse of mesostructure. A narrow pore size distribution (Figure 4B) with the peak pore width at 3.5 and 2.9 nm was observed for the organized small mesopore structures in WO 3/PAL2-16 and


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WO3/PAL2-12 samples, respectively, whereas a broad pore size distribution (imprecise peak pore width at 2.6 nm) was seen for the WO3/PAL2-8 sample due to more collapse of mesopores. However, the mesopore volumes of WO3/PAL2-n samples (Table 1) did not differ significantly because of the contribution from secondary interparticle porosity. The BET surface areas of the WO3/PAL2-n samples are noticeably high (Table 1); the decreasing trend of the BET surface areas from WO3/PAL2-16 (154 m2 g-1) towards WO3/PAL2-12 (131 m2 g-1) and WO3/PAL2-8 (108 m2 g-1) was also in accordance with more collapse of mesostructure. The increased surface area WO3/PAL2-n with longer chain length of PAL2-n is caused by the more organized mesopores in WO3/PAL2-n, which is evident from their narrower pore size distribution (Figure 4B) and more prominent XRD pattern (Figure 3B). In contrast, WO3/untemplate exhibited a type-II isotherm and significantly low adsorption amount up to P/P0 ≈ 0.8, which resulted in a very low BET surface area of 8 m2 g-1. In view of the high density of crystalline WO3 (7.16 g cm-1) compared with silica, the surface areas of the WO3/PAL2-n samples are comparable with ordered mesoporous silica. 7576

To investigate the origin of better thermal stabilization of the mesoporous structure by the longer alkyl chain of the PAL2-n, thermogravimetric (TG) analyses of different as-prepared WO3/PAL2-n composites were conducted under N2 (Figure 5A) and air (Figure 5B) atmosphere. The difference in weight loss observed at 550 C under N2 and air was ca. 12, 10 and 7 wt% for WO3/PAL2-16, WO3/PAL2-12 and WO3/PAL2-8 composites, respectively, indicating that corresponding amounts of amorphous carbonaceous species 8, 68, 78 (as confirmed from Raman spectroscopic analysis; Figure S1) remain in the mesopores for respective samples during crystallization of WO3 framework at 550 C under N2 in the in situ surfactant-thermalcarbonization method. The formation of the higher content of carbonaceous species for PAL2-n


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with the longer alkyl chain provided a more substantial protective support inside the mesopores to prohibit the undesired collapse of the mesostructure by suppressing the growth of WO3 nanocrystals (as depicted by crystallite diameters estimated from wide-angle XRD patterns), affording better thermal stabilization of the mesoporous structure along with the thicker pore-walls formed by PAL2-n with the longer alkyl chain.8, 68 Thus, the above results indicate that the alkyl chain length of PAL2-n has a strong impact on the formation of the mesoporous structure of crystalline WO3 and their surface properties. The absence of PAL2-n can only produce a nonporous structure of large WO3 crystallites in the WO3/untemplate sample (Table 1). Visible-light-response properties and performance of PEC water oxidation The top-view SEM images of all the WO3/PAL2-n layers on a FTO electrode annealed at 550 ºC exhibited a uniform surface formed by WO3 particles (ca. 30 ~ 50 nm) without any crack formation (Figure S3). No significant difference in particle morphologies have been observed among the WO3/PAL2-n layers. The appearance of void spaces between WO 3 particles was in accordance with the secondary interparticle porosity, as depicted by N 2 sorption analysis. The film thickness for all the WO3/PAL2-n layers was measured to be ca. 8 μm from the cross-sectional SEM image (Figure S3). The WO3/untemplate layer showed nearly the same film thickness as those of the WO3/PAL2-n layers. Figure 6 shows the Tauc plots of different WO3/PAL2-n electrodes based on the UV/Vis diffuse reflectance spectra (DRS; Figure S4) and the estimated bandgap energies are listed in Table 1. WO3/PAL2-8 provided bandgap energies of 2.67 eV which is identical to that of WO3/untemplate in agreement with previous reports of nanostructured WO3.30, 32, 45 However, the bandgap energies slightly shifted from 2.67 to 2.60 with lengthened alkyl chain from n = 8 to 16. The red-shift of absorption has been also observed for other porous and nanocrystalline metal


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oxides, most probably due to the quantum confinement effect79-80 and/or developed strains67, 81 in narrow nanocrystalline pore-walls of organized mesoporous network. The cyclic voltammograms (CVs) of WO 3/PAL2-n electrodes in a 0.1 M phosphate solution (pH ≈ 6.0) are shown in Figure 7. All WO 3/PAL2-n electrodes show markedly higher photocurrent than that (0.7 mA cm-2) of the WO3/untemplate electrode. Although the photoanodic current due to water oxidation was observed above 0.22 V upon visible-light irradiation for all the WO3/PAL2-n electrodes, the current at 1.5 V for the WO3/PAL2-16 (2.8 mA cm-2) electrode was 1.3 and 1.6 times higher than that of the WO3/PAL2-12 (2.1 mA cm-2) and WO3/PAL2-8 (1.7 mA cm-2) electrodes, respectively. The higher photocurrent of WO3/PAL2-16 is explained by the higher number of the active sites on the WO 3 surface due to the higher surface area. Figure 8A shows the action spectra of incident photon-to-current conversion efficiency (IPCE) at 0.5 V versus Ag/AgCl (pH ≈ 6.0, 1.05 V vs. RHE) for different WO3/PAL2-n electrodes. The photocurrents were generated below the onset wavelength of 480 nm for all the electrodes used, which is in agreement with band gap energies estimated from the DRS data of the WO3/PAL2-n electrodes, respectively (Figure 6 and Table 1). This indicates that the photocurrent is generated based on the band gap excitation of WO3. The IPCE values (IPCE420) at 420 nm for WO3/PAL2n increased from 24 to 36% with the lengthened alkyl chain from n = 8 to 16 in consistence with the CV data, and the IPCE values of mesoporous samples are ca. 5 ~ 7 times higher than that (5%) of the WO3/untemplate electrode. In Figure 8B, the IPCE420 values of different WO3 electrodes were compared with their respective specific surface areas. It has been clearly observed that the increased IPCE420 values of WO3/PAL2-n electrodes with the lengthened alkyl chain from n = 8 to 16 are direct outcome of their higher surface area. These results signify that the superior mesostructural properties of crystalline WO 3 obtained using WO3/PAL2-n with the longer alkyl


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chain are effective for high photocurrent generation. The IPCE420 values of the WO3/PAL2-n electrodes were compared with those of the hitherto-reported WO3 electrodes (Table S1). The IPCE420 values for WO3/PAL2-16 (36%) and WO3/PAL2-12 (32%) electrodes are mostly noticeably higher than those of other reported WO 3 electrodes as measured under the similar conditions (for example, 25.4% of IPCE420 at 0.5 V vs Ag/AgCl and pH = 6 in 0.1 M phosphate solution for a N2-intercalated mesoporous WO330; 18% of IPCE420 at 0.8 V vs SCE in 0.1 M Na2SO4 solution for a macroporous WO382; ~19% of IPCE420 at 1.23 V vs RHE and pH = 8 for a nanowire WO383; ~11% of IPCE420 at 0.8 V vs Ag/AgCl and pH = 7 for a pallet WO384) except in the case of recently reported N2-intercalated nanorods WO3 (~44% at 1.05 V vs RHE and pH = 6 in 0.1 M phosphate solution)45 and WO3 nanoplate electrode (65% at 1 V vs. RHE in 1 M HClO4).33 The bulk photoelectrolyses were performed under potentiostatic condition at 0.5 V vs. Ag/AgCl (1.05 V vs. RHE) in a 0.1 M Na2SO4 solution (pH ≈ 6.0) for 1 h using different WO3/PAL2-n electrodes. The current-time profiles during photoelectrolyses are displayed in Figure 9. After an initial spike in the photocurrent density (related to hole accumulation and the capacitance component at the electrode-liquid interface), the photoanodic currents due to water oxidation were observed for all the electrodes. The superior performance of the WO3/PAL2-16 electrode for PEC water oxidation was demonstrated by the higher charge amount passed (1.95 C) and amount (nO2) of O2 evolved (3.8 µmol, 75% Faradaic efficiency (FEO2)) than those of the WO3/PAL2-12 (charge amount 1.66 C; nO2 3.3 mol, FEO2 77%) and WO3/PAL2-8 (charged amount 1.44 C; nO2 2.8 mol, FEO2 76%) electrodes (Table 2). The enhancement effect (defined as Rmes = nO2 for WO3/PAL2-n / nO2 for WO3/untemplate) of the mesoporous structure on the PEC water oxidation were R mes = 6.3, 5.5 and 4.7 for WO3/PAL2-n electrode (n = 16, 12 and 8), respectively (Table 2), which signifies the importance of organized mesopores in crystalline WO 3


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photoanodes for efficient PEC water oxidation. The FEO2, lower than 100% could be possibly due to the parallel photo-oxidation of WO3 surface to form tungsten-peroxo adducts,19 which has been commonly observed for WO3-based photoanodes.8, 19, 30, 35 We have reported that Co2+ ions in aqueous electrolyte solutions act as catalysts (without deposition, as in a Co-Pi catalyst85) to facilitate water oxidation at the surface of mesoporous and nanoporous WO3.8, 30, 35 The bulk photoelectrolyses were conducted in the presence of 0.05 and 0.1 mM Co2+ ions. Both the charge amount and nO2 were higher by a factor of ca. 1.3 ~ 1.4 than those without Co2+ ions for the respective WO3/PAL2-n electrodes (Figure 9, Table 2). This corroborates that the addition of Co2+ ions prevails for the PEC water oxidation at the mesoporous WO3/PAL2-n electrodes. The moderate decrease in the catalytic current could involve influence of the decreased effective surface area caused by adherence of O2 bubbles86 and the decrease local pH close to the surface87 during photoelectrocatalysis. However, the degree of the current decrease considerably depended on the concentration of Co2+ ions (31~ 43% and 23~ 32% photocurrent decreases after 1 h in the presence of 0.05 and 0.1 mM Co2+ ions, respectively). This suggests the possibility to improve photostability of the mesostructured WO3 electrodes by the presence of Co2+ ions in electrolyte solution. The slow kinetics of the water oxidation results in hole accumulation at the surface and subsequent photo-oxidation of WO3 by holes to form inactive tungsten-peroxo adducts.19 Co2+ ions act as catalysts to facilitate PEC water oxidation at the surface of the WO3/PAL2-n electrodes, leading to attenuation of photo-oxidation of the WO3 surface by holes. The current decreases in the presence of 0.1 mM Co2+ ions for WO3/PAL2-16 and WO3/PAL2-12 (23 and 24% photocurrent decrease after 1 h, respectively) were lower compared to that of WO3/PAL2-8 (32% photocurrent decrease after 1 h). The higher surface-to-mass ratio of WO3/PAL2-16 and WO3/PAL2-12 could decrease the effective concentration of accumulated


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holes at per unit surface to suppress photo-oxidation of WO3 by holes. This is further supported by the 51% decrease (after 1 h) of photocurrent even in presence of 0.1 mM Co2+ ions (Figure S5) for WO3/untemplate electrode having very low surface-to-mass ratio.

Conclusions Tunable small mesopore systems (< 5 nm) of crystalline WO3 photoanodes have been achieved by controlling the alkyl chain length of surfactant templates (PAL2-n). The alkyl chain length of PAL2-n significantly influenced the mesostructural and PEC properties of the WO 3 photoanodes. The longer alkyl chain of PAL2-n yielded the thicker pore-walls in WO3/PAL2-n composites, affording better stabilization of the organized mesoporous structure of WO 3 along with the formation of the higher content of carbonaceous species as a protective support during WO3 framework crystallization at 550 C by the in situ surfactant-thermal-carbonization method. The incident photon-to-current conversion efficiency (IPCE) of mesoporous WO3/PAL2-n samples based on water oxidation increased from 24 to 36% at 420 nm and 0.5 V vs Ag/AgCl with lengthening the alkyl chain of PAL2-n from n = 8 to 16; and the IPCE of WO3/PAL2-n is ca. 5 ~ 7 times higher than that (5%) of WO3/untemplate. The performances of PEC water oxidation were 4.7 ~ 6.3 times enhanced by the mesoporous structure for WO 3/PAL2-n samples, compared to the WO3/untemplate samples. These crystalline small mesopore systems of WO3 are expected to be applied to a photoanode for a PEC water splitting cell as artificial photosynthesis to improve the solar energy conversion efficiency. FIGURES:


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Figure 1. TEM images of as-prepared (a) WO3/PAL2-16, (b) WO3/PAL2-12 and (c) WO3/PAL28 composites.

Figure 2. a) HRTEM image and b) SAED pattern of WO3/PAL2-12 sample after annealed at 550 C. A few pores are highlighted by white circles in image a.


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C

B

b)

2

4

6

2 /



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a) b)

c)

c)

d)

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

202 220

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Intensity (a.u.)

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

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Figure 3. (A and B) Small-angle and (C) Wide-angle XRD patterns of (A) as-prepared composites and (B and C) annealed samples at 550 C for (a) WO3/PAL2-16, (b) WO3/PAL2-12, (c) WO3/PAL2-8 and (d) WO3/untemplate.

B

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a) b) c) d)

0.006

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Dv(d) [cm Å g ]

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Volume of N2 adsorbed (cm g )

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80 40 0 0.0

0.004

0.002

0.000 0.2 0.4 0.6 0.8 1.0 Relative pressure (P/P0)

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4 6 8 Pore Diameter (nm)

10

Figure 4. (A) N2 sorption isotherms and (B) pore size distributions of (a) WO 3/PAL2-16, (b) WO3/PAL2-12, (c) WO3/PAL2-8 and (d) WO3/ untemplate samples after annealed at 550 C. For Clarity plots for N2 sorption isotherms (50 and 20 cm3 g-1 and for WO3/PAL2-16 and WO3/PAL212, respectively) and pore size distribution (0.004 and 0.001 cm3 Å-1 g-1 and for WO3/PAL2-16 and WO3/PAL2-12, respectively) are vertically shifted.


15


A

B

100

100

95 90 85

95 16% 17%

20%

Weight Loss %

Weight Loss %

c)

80

b)

75

a)

70 65

100 200 300 400 500 600 700 o Temperature/ C

90

23%

85

27%

80

32%

c)

75

b)

70

a)

65

100 200 300 400 500 600 700 o Temperature/ C

Figure 5. TG analysis curves under (A) N2 and (B) air started with as-prepared (a) WO3/PAL216, (b) WO3/PAL2-12 and (c) WO3/PAL2-8 composites.

8

a) b) c) d)

2

6 2

(h)  eV

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

Page 16 of 36

4

2

0

3.2

3.0

2.8 2.6 h  eV

2.4

Figure 6. Tauc plots based on UV/Vis diffuse reflectance spectra (DRS) of (a) WO3/PAL2-16, (b) WO3/PAL2-12, (c) WO3/PAL2-8 and (d) WO3/untemplate electrodes annealed at 550 C. The dotted lines show tangent line near the edge of the respective spectra.


16

Page 17 of 36

-2

3.0

Current density / mA cm

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


2.5 2.0

0.4

Potential / V vs RHE 0.8 1.2 1.6 2.0

a) b) c) d)

1.5 1.0 0.5

Dark Light

0.0 -0.5

0.0 0.4 0.8 1.2 Potential / V vs Ag/AgCl

1.6

Figure 7. Cyclic voltammograms (CVs) of (a) WO3/PAL2-16, (b) WO3/PAL2-12, (c) WO3/PAL28 and (d) WO3/untemplate electrodes annealed at 550 C, as measured in a 0.1 M phosphate buffer solution (pH ≈ 6.0). The solid and dashed lines are CVs recorded upon visible light irradiation (λ > 390 nm, 100mW cm−2) and in the dark, respectively.


17

350

400 450 Wavelength (nm)

-1

2 -1

20

10

0 300

500 0

120

120

10

40

80

0

30

40

80 40

160

-1

20

160

2

20

30

0

120

0

20

80

10

40

Figure 8. (A) Action spectra of IPCE of (a) WO3/PAL2-16, (b) WO3/PAL2-12, (c) WO3/PAL2-8 0

0

and (d) WO3/untemplate electrodes annealed at 550 C, as measured in a 0.1 M phosphate buffer solution (pH ≈ 6.0) at 0.5 V vs. Ag/AgCl (1.05 V vs. RHE). (B) Comparison of IPCE at 420 nm (IPCE420) with respective specific surface area of different WO3 samples.

B

0.8 0.6 0.4 0.2 0.0 0

light on 10

20 30 40 Time / min

50

60

1.0

a) b) c)

0.8 0.6 0.4 0.2 0.0 0

light on 10

20 30 40 Time / min

50

60

-2

a) b) c)

C Current density / mA cm

-2

1.0


-2

A

1.0

a) b) c)

0.8 0.6 0.4 0.2 0.0 0

Surface Area (m g )

420 nm30

160

2

40

40

Surface Area (m g )

40

IPCE420 (%)

IPCE (%)

60

B

a) b) c) d)

IPCE420 (%)

80

IPCE420 (%)

A


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

Page 18 of 36

Surface Area (m g )


light on 10

20

30 40 Time / min

50

60

Figure 9. Current-time profiles of (A) WO3/PAL2-16, (B) WO3/PAL2-12 and (C) WO3/PAL2-8 electrodes annealed at 550 C in (a) absence and (b and c) presence of (b) 0.05 and (c) 0.1 mM Co2+ ions in a 0.1 M Na2SO4 solution (pH ≈ 6) at 0.5 V versus Ag/AgCl (1.05 V vs. RHE) upon visible light irradiation (λ> 390 nm, 100 mW cm−2).


18

Page 19 of 36 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


TABLES Table 1. Physicochemical properties of different WO3/PAL2-n samples annealed at 550 C.

d-spacinga (as prepared)

d-spacinga (annealed)

Crystallite diameterb

[nm]

[nm]

[nm]

WO3/PAL2-16

4.5

3.8

WO3/PAL2-12

3.8

WO3/PAL2-8

Sample name

WO3/untemplate a

Surface area

Pore volume

Pore size

Band gap

[m2 g−1]

[cm3 g−1]

[nm]

[eV]

7.6

154

0.16

3.5

2.60

3.1

8.1

131

0.16

2.9

2.65

2.9

–

11.2

108

0.19

2.6

2.67

–

–

17.4

8

–

–

2.67

d-sapcings are calculated form the maxima of the small angle XRD patterns; b Crystallite

diameters are based on D002 obtained from [002] peak using Scherrer equation.


19


Page 20 of 36

Table 2. Summary of data on photoelectrocatalytic water oxidation in a 0.1 M Na2SO4 solution (pH ≈ 6.0) at 0.5 V vs Ag/AgCl (1.05 V vs. RHE) for 1h using different WO3/PAL2-n electrodes annealed at 550 C.

Sample name

[Co2+] / mM

Charge / C

nO2 [mol]

FEO2a) [%]

nH2b) [mol]

FEH2c) [%]

Rmesd)

WO3/PAL2-16

0

1.95

3.8

75

9.8

98

6.3

WO3/PAL2-16

0.05

2.37

4.7

77

11.7

95

WO3/PAL2-16

0.1

2.51

5.0

78

12.4

96

WO3/PAL2-12

0

1.66

3.3

77

8.1

95

WO3/PAL2-12

0.05

2.16

4.4

78

10.6

94

WO3/PAL2-12

0.1

2.38

4.6

75

11.6

94

WO3/PAL2-8

0

1.44

2.8

76

7.0

94

WO3/PAL2-8

0.05

1.71

3.4

77

8.4

95

WO3/PAL2-8

0.1

2.01

4.0

76

9.8

95

WO3/Untemplate

0

0.34

0.6

71

1.6

91

WO3/Untemplate

0.1

0.45

0.9

75

2.3

97

a)

5.5

4.7

FEO2 is Faraday efficiency for O2 evolution; b) nH2 is the amount of H2 evolved at the Pt counter

electrode compartment; c)FEH2 is Faraday efficiency for H2 evolution; d)Rmes was defined as ratio of the amounts (nO2) of O2 evolved for WO3/PAL2-n and WO3/untemplate


20

Page 21 of 36 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


ASSOCIATED CONTENT Supporting Information. The supporting information is available free of charge via the Internet at http://pubs.acs.org. Experimental detail, Table for IPCE data, Resonance Raman spectra, HRTEM and SEM images, DRS spectra, Current-time plot (PDF) AUTHOR INFORMATION Corresponding Author *(D.C) E-mail: [email protected] *(M.Y) E-mail: [email protected]; Tel and Fax, +81-25-262-6790. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI Grant Number JP17H06439, 18H02071 in Scientific Research on Innovative Areas “Innovations for Light-Energy Conversion (I4LEC)”. DC thanks JSPS for providing postdoctoral fellowship. REFERENCES 1.

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For Table of Content Use Only

Synopsis: Tunable mesoporous crystalline WO3 photoanodes are developed using templates with varied alkyl chain length and applied towards efficient solar fuel devices.


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Tunable Mesoporous Structure of Crystalline WO3 Photoanode toward

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