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Multichannel Charge Transport of a BiVO/(RGO/WO)/W O ThreeStory Anode for Greatly Enhanced Photoelectrochemical Efficiency Zhuo Zhang, Bin Chen, Minki Baek, and Kijung Yong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15275 • Publication Date (Web): 29 Jan 2018 Downloaded from http://pubs.acs.org on January 30, 2018

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ACS Applied Materials & Interfaces

Multichannel Charge Transport of a BiVO4/(RGO/WO3)/W18O49 Three-Story Anode for Greatly Enhanced Photoelectrochemical Efficiency Zhuo Zhang, Bin Chen, Minki Baek, and Kijung Yong* Surface Chemistry Laboratory of Electronic Materials, Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea.

ABSTRACT: Photoelectrochemical (PEC) solar conversion is a green strategy for addressing the energy crisis. In this study, a three-story nanostructure BiVO4/(RGO/WO3)/W18O49 was fabricated as a PEC photoanode and demonstrated a highly enhanced PEC efficiency. The top and middle stories are a reduced graphene oxide (RGO) layer and WO3 nanorods (NRs) decorated with BiVO4 nanoparticles (NPs), respectively. The bottom story is the W18O49 film grown on a pure W substrate. In this novel design, experiments and modeling together demonstrated that the RGO layer and WO3 NRs with a fast carrier mobility can serve as multichannel pathways sharing and facilitating electron transport from the BiVO4 NPs to the W18O49 film. The high conductivity of W18O49 can further enhance the charge transfer and retard electron-hole recombination, leading to a highly improved PEC efficiency of the BiVO4/WO3 heterojunction. As a result, the as-fabricated three-story photoanode covered with FeOOH/NiOOH achieves an attractive PEC photocurrent density of 4.66 mA/cm2 at 1.5 V versus Ag/AgCl, which illustrates the promising potential of the three-story hetero-nanostructure in future photoconversion applications.

KEYWORDS: BiVO4, WO3, RGO, three-layer hetero-nanostructure, multichannel, charge transport, photoelectrochemical enhancement

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INTRODUCTION

As a smart approach for solar-energy conversion, photoelectrochemical (PEC) hydrogen (H2) generation has been recognized as an efficient, green, low-cost and thereby highly promising strategy to overcome the worsening energy and environmental crises.1-5 It is well known that the efficiency of a PEC system can be considerably enhanced by metal introduction,6, 7 using a hetero-junctional electrode with a type-II,8-12 or Z-scheme

13

band alignment. As for the

type-II band alignment of electrode, a narrow-band-gap semiconductor material should be coupled to another wide-band-gap material with both lower conduction and valence band edges. Using a photoanode constructed by n-type semiconductors as example, electrons migrate from the photoexcited narrow- to wide-band-gap material under illumination for the H2 evolution reaction (HER), while holes accumulate at the interface between the electrolyte and narrow-band-gap material for the oxygen evolution reaction (OER), leading to efficient charge separation, retarded charge recombination and thus improved photoconversion efficiency.14, 15 Among various n-type semiconductors, BiVO4 and WO3 have rapidly emerged as the star photoanode materials and attracted significant interest in recent years. They are prominently stable in PEC electrolyte over a wide pH range and can absorb high-energy visible photons due to their band gaps of ~2.4 and ~2.7 eV.16, 17 Thus, the BiVO4/WO3 heterojunction with a type-II band alignment is uniquely suitable for efficient OER photoanode application under visible light irradiation. However, when they are employed in PEC system for the full solar water splitting, an additional external bias should be applied because the conduction band edge of BiVO4 is lower than the potential for hydrogen reduction reaction.21 To date, innumerable BiVO4/WO3 hetero-nanostructures with various morphologies have appeared as building blocks for a photoanode, such as 0-dimensional (0-D) core-shell nanoparticles,22 1-D hetero-nanowires,23-25 2-D multilayer films,26, 27 and 3-D sponge-like structures.28 All of them have demonstrated appreciable PEC efficiencies, yet slow carrier transport in BiVO4 is the main weakness that holds charge separation and migration back. To conquer this issue, the groups of Craig A. Grimes,29 Xiaolin Zheng,30 and Yuriy Pihosh31 coated WO3 nanorods (NRs) with a BiVO4 thin layer using different synthetic strategies. In addition to 1-D hetero-NRs,


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Suk Joon Hong

32

and Manjeet Chhetri 33 also fabricated very thin BiVO4/WO3 hetero-layer

nanofilms. The very thin thicknesses of BiVO4 layers are matched with the diffusion length of photogenerated electrons, thereby the electron lifetime is prolonged. It is undeniable that the PEC efficiency can be enhanced by limiting the size of BiVO4 in the abovementioned structures. However, the quantity of photogenerated electrons cannot be ignored, which relies on the total amount of BiVO4 irradiated by solar light. Here, the fast transport of electrons seems to be contrary to the enlarged volume of BiVO4 with a fixed WO3 skeleton. To solve this contradiction, and thereby optimize the charge transport and light harvesting together, a smart solution is to enhance the loading amounts of BiVO4 in the form of NPs with small sizes and then combine these NPs with the WO3 skeleton using a conductive and transparent material. Based on this idea, a notable three-story BiVO4/(RGO/WO3)/W18O49 hetero-nanostructure (HNS) is designed and synthesized as the photoanode for PEC H2 generation. The three stories from top to bottom are the RGO layer as the skeleton with highly loaded BiVO4 NPs, arrays of WO3 NRs decorated with BiVO4 NPs, and the W18O49 film grown on a pure W substrate. Our study demonstrates that the RGO layer and WO3 NRs can form multichannel pathways to facilitate electron transport from countless BiVO4 NPs to the W18O49 film. The fast carrier mobility of the WO3 NRs and the high conductivity of W18O49 can further enhance the charge transfer and retard electron-hole recombination, leading to highly improved OER kinetics for the BiVO4/WO3 heterojunction.

EXPERIMENTAL SECTION

Fabrication. 1) WO3 NRs with W18O49 bottom layer: Arrays of WO3 NRs with W18O49 bottom layer were grown on tungsten (W) wafer by using a two-step thermal vapor deposition method.34 First, W wafers were cleaned ultrasonically with acetone for 20 min and dried with flowing N2 gas before loading into the furnace for WO3 growth. The cleaned W wafers as the substrates were placed on the top of an alumina boat containing enough WO3 source powder of purity 99.99%. Then the alumina boat with the W substrates was kept in the uniform-temperature region of the tube furnace. After the furnace was evacuated to a vacuum



of 50 mTorr, the temperature of the furnace was gradually increased from the room temperature to the growth temperature (such as 1050 ◦C) at a rate of 15 ◦C min−1. After maintaining that temperature for 1 h, the furnace was allowed to cool naturally to room temperature. A deep blue color product (W18O49) was found on the surface of the W substrate. Then the obtained samples were annealed in air ambient at 400 ◦C for 60 min to get fully oxidized WO3 NRs (greenish yellow in color) samples. 2) BiVO4/WO3 HNRs: BiVO4/WO3 HNRs were prepared by a modified metal-organic decomposition (MOD) method.35 Bi(NO3)3·5H2O (Sigma-Aldrich Co.) in acetic acid (0.2 M) and vanadium(IV) (oxy) acetylacetonate (Sigma-Aldrich Co.) in acetylacetone (0.03 M) were mixed together by 1:1 stoichiometric ratio of Bi to V. The mixed solution was coated onto WO3 NRs by spin coating with speed of 1000 rpm and duration of 10 s. And then, the samples were annealed at 400 °C in air for 2 min. The spin coating and annealing were repeated 6 times. Finally, the samples should be further annealed at 400 °C in air for 30 min. 3) BiVO4/(RGO/WO3) HNS: First, a GO solution was chemically reduced by the reported procedure using NaBH4 (Sigma-Aldrich, 10 mM) as a reducing agent.36 Then, BiVO4/(RGO/WO3) HNS were fabricated by using the above MOD method. The ready-prepared solution containing Bi and V was mixed with the RGO solution by using the volume ratio 5:1. Next, the steps of spin coating and annealing were same with the above. 4) FeOOH/NiOOH catalyst: First, an FeOOH layer was deposited under illumination of 1 sun with AM~1.5 G in 0.1 M FeSO4 (99%, Sigma-Aldrich) solution for 13 min. During the deposition, an external bias of 0.2V vs Ag/AgCl was applied on the substrate. Next, the sample was placed into 0.1 M NiSO4 (99%, Sigma-Aldrich) under illumination with a bias of 0.11 V for 6 min. Finally, the bias was increased to 1.2 V for 90 s to make the NiOOH fully cover the surface of the anode.37 Characterization. The morphologies of the prepared nanostructures were confirmed on a field-emission scanning electron microscope (FE-SEM, XL30S, Philips) operated with a 5.0 kV beam energy and on a high-resolution scanning transmission electron microscope (HR-STEM; JEM-2200FS with Image Cs-corrector; JEOL) operated with a 200-kV beam energy.

Measurements. 1) X-ray diffraction (XRD) (X’Pert Pro MPD) was employed to explore


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crystalline information of the samples. 2) The optical absorbance of the samples was analyzed using a UV2501PC (Shimadzu) spectrometer with an ISR-2200 integrating sphere attachment for diffuse reflection measurements. 3) X-ray photoelectron spectroscopy (XPS) was performed using an ESCALAB250 instrument (VG Scientific Company, USA) with Al Kα radiation as the excitation source. 4) Photoluminescence (PL) lifetime was measured using an inverted-type scanning confocal microscope (MicroTime-200, Picoquant, Germany) with a 40× (air) objective. The lifetime measurements were performed at the Korea Basic Science Institute (KBSI), Daegu Center, Korea. A single-mode pulsed diode laser (379 nm with ~30 ps pulse width and ~50 µW power) was used as an excitation source. A dichroic mirror (Z375RDC, AHF), a longpass filter (HQ405lp, AHF), a 100 µm pinhole, a short-pass filter (600 nm, Thorlabs), and an avalanche photodiode detector (PDM series, MPD) were used to collect emissions from the samples. Time-correlated single-photon counting (TCSPC) technique was used to count emission photons. Exponential fitting for the obtained photoluminescence decays was performed using the Symphotime-64 software. 5) Photocurrent-voltage (J-V) measurements, EIS, electron lifetime and PEC stability tests were performed using a typical three-electrode potentiostat system (potentiostat/galvanostat, model 263A, EG&G Princeton Applied Research) with a Pt counter electrode and a saturated Ag/AgCl reference electrode. The electrolyte was an aqueous solution of 0.5 M Na2SO4 with pH~6.8, through which nitrogen was bubbled. The working electrode was illuminated from the front with a solar simulator (AM 1.5 G filtered, 100 mW/cm2, 91160, Oriel). The H2 gas evolution measurement was performed by using two-electrode system, and other parameters are same with above measurement. 6) IPCE tests: Incident photon-to-current efficiency (IPCE) was measured using a 300 W Xe lamp (66 905, Oriel Instruments) with a monochromator (74-004, Oriel Cornerstone 130 1/8 m) from 300 to 600 nm at intervals of 20 nm.

RESULTS AND DISCUSSION

Morphology and Structure Analysis Figure 1a presents a sketch of the three-story BiVO4/(RGO/WO3)/W18O49 HNS, which is simplified as B/(R/W) in the following paragraphs. The top story is the RGO film decorated



with highly loaded BiVO4 NPs and simplified as B/R. It looks like a dry streambed (Figure 1b) due to surface stress. The enlarged view, shown in Figure 1c, demonstrates that the thickness of the layer is approximately 570 nm, and the diameter of the BiVO4 NPs is approximately 50 nm. Arrays of WO3 NRs decorated with BiVO4 NPs are the middle story and simplified as B/W HNRs. The W18O49 film is the bottom story and is grown on pure W substrate. Figure 1d clearly presents the side-view morphology of B/(R/W). The height of B/W HNRs and the thickness of W18O49 film are approximately ~15 and ~7 µm, respectively. Here, one thing should be noticed that the top story of B/(R/W) is constructed by many numbers of B/RGO pieces. Thus it does not form a continuous layer due to the limited sizes and areas of RGO. The sketch only reflects the morphology based on one piece of B/R structure. To deeply explore the material and crystalline structure of B/(R/W), an analytical strategy was performed. From a structural point of view, the WO3 NRs and the W18O49 bottom film together play an important skeletal role in supporting the three-story B/(R/W). Thereby, the formation and crystalline properties of the WO3 NRs and of the W18O49 bottom film were first explored. Supporting Information (SI) Part 1 reveals that the color of the as-grown NRs is slowly changed from deep dark blue to bluish yellow with increased annealing temperature. Concomitantly, the intrinsic light absorption gradually broadens from the UV to visible region. The XRD measurement in Figure S2 demonstrates that the as-grown NRs changed from W18O49 to WO3. For the WO3 NRs, the length and diameter are highly dependent on the growth temperature and duration. Figures S3-S6 in SI Part 2 confirm that both the length and diameter increase exponentially with the growth temperature. Comparatively, along with the growth duration, the diameter slowly enlarges, while the length increases linearly. The SEM images in SI Part 3 also demonstrate that the W18O49 film is constructed of NRs that are tightly contacted with each other and have larger diameters than those of WO3. After being covered by a layer of RGO with decorated BiVO4 NPs, B/(R/W) is formed on the skeleton of the WO3 NRs. TEM observations and XPS measurements were subsequently performed to explore the crystalline structures of each component and ingredients of the entire structure. Figure 2a shows a typical TEM image of B/W HNRs and evenly distributed BiVO4 NPs. Taken from Figure 2a, the HRTEM image in Figure 2b reveals the crystalline


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WO3 NR and the marked BiVO4 NP with diameters of ~12 and ~11 nm, respectively. Here, it should be noted that the diameters of all the WO3 NRs in one sample have a wide distribution from ~10 to ~100 nm (see Figure S3 and S5 in SI Part 2). Thus, only the NRs with thinner diameters were selected for TEM observation. The enlarged views (left) and the corresponding fast Fourier transform (FFT) patterns (right) in Figure 2c and d (green and red squares in Figure 2b) demonstrate the highly crystalline structures of WO3 and BiVO4 with lattice spacings of 0.37 and 0.31 nm, corresponding to the (020) plane of WO3 and (121) plane of BiVO4, respectively.38, 39 Taken from Figure 2a, the spatial elemental analysis using electron energy loss spectroscopy (EELS) in Figure 2e confirms that the W (1), Bi (2), V (3) and O (4) elements are distributed evenly in their located positions. Figure 2f and its enlarged view in Figure 2g exhibit densely packed BiVO4 NPs in the B/R layer. The diameters of BiVO4 are approximately 10 nm. The HRTEM image (Figure 2h) and FFT pattern (Figure 2i) (red square in Figure 2g) together reveal the (011) and (121) planes of the crystalline BiVO4 NP with lattice spacings of 0.47 and 0.31 nm, respectively. Using EELS, the elemental distribution is confirmed in Figure 2j. It can be seen that some W is distributed on the B/R layer due to evaporation of the W wafer. Bi, V, C and O are obvious and distributed evenly in their located positions. To further investigate the chemical binding states of B/(R/W), X-ray photoelectron spectroscopy (XPS) analysis was carried out. Figure 3a demonstrates that only the W-O chemical bonds of WO3 are observed by checking the binding energy of the W 4f peaks. The figure shows that the W 4f spectrum consists of two sub-peaks due to spin-orbit coupling at 36.5 and 35.5 eV, corresponding to 4f5/2 and 4f7/2 states, respectively.40 Similarly, the sub-peaks at 164.6 and 159.3 eV in Figure 3b, as well as at 524.4 and 517 eV in Figure 3c, are identified and demonstrate the existence of Bi-O and V-O bonds in BiVO4.41 In addition to V5+, a weak peak of V4+ is found in Figure 3c because the V5+ could be reduced to lower oxidation state of V4+ by the oxygen vacancies.42, 43 Finally, the C 1s spectrum in Figure 3d demonstrates that lower-intensity peaks of C-O, C=O and O-C=O bonds can be observed compared with C-C and C=C bonds. The C-O bonds originate from the C-OH and C-O-C binding states of RGO.44 Here, RGO is not oxidized to GO after annealing in the air. Because



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the RGO is fully covered with BiVO4, it is believed to be effectively protected from air.

Experimental Analysis of PEC Enhancement The as-fabricated B/(R/W) is a complicated structure. To explore the contributions of each component on the enhanced PEC efficiency, four different samples were fabricated: bare WO3 NRs (Figure 4a), B/W HNRs (Figure 4b), and B/(R/W) with lower [Figure 4c, simplified as L-B/(R/W)] and higher loading amounts of BiVO4 NPs [Figure 4d, simplified as H-B/(R/W)]. It can be seen that H-B/(R/W) has a thicker B/R layer and much denser BiVO4 NPs than that of L-B/(R/W). In addition, their XRD spectra in Figure 5a show that obvious peaks of WO3 are marked by red spots and are present in all of the four structures, while BiVO4 peaks marked by blue spots can only be observed in B/W HNRs, L-B/(R/W) and H-B/(R/W). Due to the limitation of space in Figure 5a, only selected BiVO4 peaks are annotated, such as (121) and (211). All of the other peaks are annotated in SI Part 4. The complete PEC process includes light harvesting, charge separation and transport, and finally the chemical reaction. To evaluate light harvesting of these four structures, light absorptions of these four structures are selected and compared in Figure 5b. The light absorbance of the bare WO3 NRs exhibits an onset wavelength of approximately 475 nm corresponding to their band gap of ~2.7 eV. After being decorated with BiVO4 NPs, the onset value of the BiVO4 NPs is shifted to nearly 500 nm due to the smaller band gap of BiVO4 about 2.4 eV. Comparatively, the L-B/(R/W) and H-B/(R/W) HNSs demonstrate similar and redshifted onset wavelengths of approximately 520 nm. The redshift of the onset is ascribed to increased Ovac in BiVO4, which could form the defect midgap state in the band gap of BiVO4 and intrinsically improve the optical absorption performance.45 Regarding charge separation and transport, their efficiencies can be verified via steady-state

photoluminescence

(PL)

quenching

spectroscopy

and

time-resolved

photoluminescence (TRPL) measurement at room temperature. It is well known that the excellent conductivity of RGO is beneficial for the transmission of photogenerated electrons and holes. Thus, the PL spectra of B/W HNRs, L-B/(R/W) and H-B/(R/W) are collected in Figure 6a. In PL spectra, the peaks found around 420 and 482 nm are ascribed to the


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recombination of free excitons of WO3 and BiVO4, respectively, which are designated as near band-edge emissions.46, 47 The other peaks are believed to originate from the defects of the composite structures. Here, L-B/(R/W) and H-B/(R/W) demonstrate more efficient PL quenching than B/W HNRs, ascribing to their fast electron transport capability though RGO. H-B/(R/W) exhibits a little slower PL quenching than L-B/(R/W) due to a higher loading amount of BiVO4 NPs. The PL quenching can also be revealed by exploring charge carrier lifetime via the TRPL decay. Similar to the PL spectra, Figure 6b shows that L-B/(R/W) and H-B/(R/W) decay faster than B/W HNRs. L-B/(R/W) also decays slightly faster than H-B/(R/W). From the normalized curve of instrumental response function (IRF) of the TRPL setup, the full width at half-maximum (FWHM) of the IRF could be deduced as 92 ps. Here, to quantitatively compare their fluorescence lifetimes, the observed TRPL decay curves are fitted using = ∑ exp −/ ,48 where I(t) is the intensity at time t, A is a normalization term, τPL is the PL lifetime, and ordinal i=1, 2, 3. After calculation, τPL of the B/W HNRs, L-B/(R/W) and H-B/(R/W) HNSs are estimated to be 0.228, 0.213 and 0.218 ns, respectively. The shorter carrier lifetime of H-B/(R/W) and L-B/(R/W) than B/W is ascribed to that the charge carrier transfer of B/W could be facilitated at the interface between B/W and RGO due to the electron acceptor of RGO. Such interfacial charge transfer competed with the radiative charge recombination of B/W, leading to the reduced carrier lifetime observed for H-B/(R/W) and L-B/(R/W).49, 50 Therefore, to understand this make up, open-circuit voltage decay (OCVD) measurements were subsequently performed. Figure 7a shows the OCVD curves of the bare WO3 NRs, B/W HNRs, L-B/(R/W) and H-B/(R/W) as a function of time. These samples were illuminated for 30 s to obtain the same open-circuit voltage (Voc), and then, the Voc decay was measured without illumination. The bare WO3 NRs exhibit a faster Voc decay due to the single band structure and thereby fast electron-hole recombination. B/W HNRs demonstrate a slower Voc decay because the region of light absorption is broadened, and the electron-hole recombination is retarded via the type-II band alignment of WO3 and BiVO4. With the aid of the B/R layer, Voc decays of L-B/(R/W) and H-B/(R/W) are further moderated. Furthermore, H-B/(R/W) exhibits a more stable and slower Voc decay than L-B/(R/W). Here, a combined



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effect is confirmed that the charge transport is improved by RGO, and the total amount of photogenerated electrons is increased via the higher loading amount of the BiVO4 NPs, which leads to a highly enhanced photoconversion efficiency. Based on the Voc decay rate, the electron lifetime curves can be deduced from equation τ = −

d d

"#

! ,51 where

kBT is the thermal energy, e is the positive elementary charge, and dVoc/dt denotes the derivative of the open-circuit voltage transient. Figure 7b shows the electron lifetimes τelectron as a function of Voc. H-B/(R/W) exhibits the longest lifetime because of the combined effects mentioned above. To further study the contribution of B/(R/W) to the overall PEC efficiency, electrochemical impedance spectroscopy (EIS) and photocurrent-potential (J-V) were measured subsequently. EIS reveals the interfacial properties of electrodes. The diameter of the semicircle of the EIS plot depends on the electron transfer resistance. The EIS plots of the bare WO3 NRs, B/W HNRs, L-B/(R/W) and H-B/(R/W) in a 0.5 M Na2SO4 solution under illumination are shown in Figure 8a. The equivalent circuit is shown in the inset of Figure 8a, which consists of the solution resistance (Rs), space charge capacitance and resistance (Csc and Rsc), double layer capacitance (Cdl), charge transfer resistance (Rct) and bounded Warburg element (W). All of these fitting values of each component in the equivalent circuit are provided in SI part 5. The smaller radius of the EIS curve reveals the lower Rct with a faster migration of photogenerated charges. The EIS semicircle of H-B/(R/W) has a smaller diameter than those of the other structures, indicating a lower Rct and faster charge transfer at the anode/electrolyte interface.52 These results are consistent with the OCVD measurement results. The J-V measurements of the bare WO3 NRs, B/W HNRs, L-B/(R/W) and H-B/(R/W) were performed under white light illumination. Figure 8b shows that B/W HNRs have an enhanced current density of 1.59 mA/cm2 at a potential of 1.5 V vs. Ag/AgCl, which is higher than the current density of 0.58 mA/cm2 of the bare WO3 NRs. It is ascribed to the enhanced light absorption and photoconversion of BiVO4, as well as the efficient electron-hole separation via cascade band alignment of WO3 and BiVO4. L-B/(R/W) and H-B/(R/W) demonstrate more improved current densities of approximately 2.22 and 2.94 mA/cm2 due to the faster charge transfer through RGO and higher loading amount of the BiVO4 NPs. Finally, after decoration of the


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FeOOH/NiOOH catalyst material, the current density is further increased to 4.66 mA/cm2, and the onset potential is clearly decreased compared with those of the others. Furthermore, the monochromatic incident photon-to-electron conversion efficiency (IPCE), H2 gas evolution and PEC stability measurement of the bare WO3 NRs, B/W HNRs, H-B/(R/W) and FeOOH/NiOOH+B/(R/W) are performed successively. Firstly, the IPCE spectra were compared for these four samples as the electrodes at 1.5 V versus Ag/AgCl. The IPCE value at a given wavelength can be calculated via the following equation: IPCE% = )* + ,

#./0

× 12 × 100%,11 where Isc is the photocurrent, P is the power of the incident light of a

specific wavelength, and λ is the incident wavelength. It can be seen from Figure 9a that the observed photocurrents of all three samples are enhanced in the UV region, and that the photoactivity could be obviously increased from the UV to the visible region by loading BiVO4 and B/R layers. Moreover, it can be further enhanced from UV to visible light region through employing FeOOH/NiOOH catalysts. Secondly, to reinforce the significance of this work, the H2 gas evolution data as a function of duration and with a bias of 1.5 V vs Ag/AgCl are shown in Figure 9b. Linear gas evolution curves indicate stable gas harvesting. From the gradient of the gas evolution curve, the H-B/(R/W) and FeOOH/NiOOH+B/(R/W) demonstrate H2 evolution rate of 3.14 and 5.24 µmol/h, respectively. Finally, the PEC stability are tested for the photoanodes and the obtained results are shown in Figure 9c. The bare WO3 NRs, B/W HNRs, H-B/(R/W) and FeOOH/NiOOH+B/(R/W) samples exhibit highly stable PEC performances over 100 minutes under 1.5 V bias vs Ag/AgCl as seen the results.

Modeling Analysis of PEC Enhancement The experimental data demonstrate the highly improved PEC efficiency of B/(R/W). To deeply explore the contribution of the B/R layer to the PEC enhancement, finite element (FE) modeling was carried out for a simplified HNS structure. As shown in Figure 10a, three WO3 NRs are vertically erected on the W substrate. A layer of RGO is covered on the top tips of the WO3 NRs. The top surface of RGO is decorated with BiVO4 NPs, which are employed as the electron source. The electrical conductivity of RGO in this model is defined as σRGO = 45.0 S/m.53 It can be seen that the distances from the BiVO4 NP to the top tips of the three



WO3 NRs are different. However, the modeling result in Figure 10b demonstrates that the current densities of the three WO3 NRs are equal. This confirms that all the WO3 NRs can serve as multichannel pathways to afford and share the charge transport equally, which leads to highly enhanced PEC efficiency. Here, the premise that three WO3 NRs can act as multichannel pathways to equally share charge transport is due to the high conductivity of RGO. Although the value of σRGO =45.0 S/m is lower than that of a good conductor, such as Au or Cu, it cannot serve as an obvious resistance to electron transfer. Only if the conductivity of RGO is decreased to one ten-thousandth, a clear difference between the current densities through the three WO3 NRs can be exhibited in our model (Figure 10c). To further verify this, the re-defined σ’RGO applied in our model is gradually changed from σRGO×1 to σRGO/10000, and Figure S9 in SI Part 6 reveals that there is only little difference between the current densities through all the WO3 NRs even when σ’RGO=σRGO/5000. However, in reality, RGO is unlikely to have such low electrical conductivity. Therefore, the result shown in Figure 10b is reliable. Furthermore, our designed three-story structure has another advantage. Specifically, although some WO3 NRs are broken, the charge transfer through these NRs will not be blocked. Figure 10d shows the model of a simplified B/(R/W) HNS. The BiVO4 NPs are decorated on the RGO layer, WO3 NRs, and broken NRs. The result in Figure 9e demonstrates that electrons in the broken NRs can still migrate to the W wafer via detoured pathways through the RGO layer and other NRs. The direction of electron migration in the broken NR is marked by a red arrow. In addition, all other intact NRs together form the pathways to share and facilitate charge transfer. Based on the abovementioned experimental and modeling results, Figure 11 exhibits a scheme of the band structure and PEC process of B/(R/W). Overall, a cascade band alignment is constructed by BiVO4 and WO3 with the band gaps of 2.4 and 2.7 eV, respectively. This facilitates the transport of electrons and holes and prevents their recombination. Under illumination, the holes migrate successively through the valence bands of WO3 and BiVO4, and then accumulate on the surface of BiVO4 to oxidize water to O2. Meanwhile, photogenerated electrons in the conduction band of BiVO4 then inject into WO3. After passing


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through the W18O49 layer and W substrate, the electrons finally arrive at Pt cathode to participate the reaction for H2 generation. For the B/R layer, the large surface area of the BiVO4 NPs can absorb more light and provide plenty of interfacial area with the electrolyte for the PEC reaction. Most importantly, arrays of WO3 NRs are connected by RGO and can act as multichannel and shared pathways to facilitate charge transfer. As a result, the PEC efficiency and electron lifetime can be clearly enhanced by B/(R/W).

CONCLUSIONS

A novel B/(R/W) HNS is demonstrated as efficient photoanodes for PEC H2 generation. The enhanced PEC efficiency of B/(R/W) can be ascribed to the increased amount of photoelectrons via the high loading of BVO4 NPs on the RGO layer, the efficient charge separation via the type-II cascade band alignment of BiVO4 and WO3, and the fast charge transport through the RGO layer and W18O49 film. Moreover, the electron lifetime and charge transfer of B/(R/W) can also be highly improved via multichannel and shared charge-transport pathways constructed using RGO and the WO3 NRs. The product presented in the current study has promising potential in future PEC anode materials and other applications in various energy devices.

ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:. Experimental details about morphology, structure, optical and XRD spectroscopy of the prepared HNSs.

AUTHOR INFORMATION

Corresponding Authors *E-mail: [email protected]; Fax: +82-54-279-8298; Tel.: +82-54-279-2278. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS ACS Paragon Plus Environment


This

work

was

supported

by

the

National

Research

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Foundation

of

Korea

(NRF-NRF-2016R1A2B2011416).

REFERENCES

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Figure 1 Sketch (a) and SEM images (b-d) of the B/(R/W) three-story HNS. Figure 1b is a close-up image taken from the top layer of Figure 1d, and Figure 1c is an enlarged view of the red square in Figure 1b. The scale bars in Figure 1b, c and d are 1 µm, 500 nm, and 5 µm, respectively.


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Figure 2 a, b) TEM (a) and HRTEM (b) images of B/W NRs. c, d) Left column: enlarged views taken from the green (c) and red (d) squares of Figure 2b. Right column: the corresponding FFT patterns. e) EELS elemental mapping taken from Figure 2a. f, g) TEM (f) and HRTEM (g) images of the B/R layer. h) Enlarged view taken from the red square of Figure 2g. i) the corresponding FFT pattern of Figure 2h. j) EELS elemental mapping taken from Figure 2f. The scale bars in Figure 2a, b, f and g are 20, 2, 50 and 5 nm, respectively.



Figure 3 a-d) W 4f (a), Bi 4f (b), V 2p (c) and C 1 s (d) XPS spectra measured from the B/(R/W) HNS.


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Figure 4 a, b) Enlarged views of the bare WO3 NRs (a) and B/W HNRs (b). c, d) Top-views of the L-B/(R/W) HNS and H-B/(R/W) HNS. The scale bars in Figure 4a, b, c and d are 200 nm, 200 nm, 2 µm and 2 µm, respectively.



Figure 5 XRD spectra (a) and light absorption curves (b) of the bare WO3 NRs, B/W HNRs, L-B/(R/W) HNS and H-B/(R/W) HNS. The peaks of WO3 and BiVO4 are marked by red and blue spots, respectively, in Figure 5a.


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Figure 6 Time-steady (a) and time-resolved (b) PL spectra of B/W HNRs, L-B/(R/W) HNS and H-B/(R/W) HNS with an excitation wavelength of 395 nm at room temperature. The IRF of the TRPL setup is marked as the black dot-curve and filled with gray color.



Figure 7 (a) OCVD curves of the photoanodes made of the bare WO3 NRs (black), B/W HNRs (green), L-B/(R/W) HNS (blue) and H-B/(R/W) HNS (red). b) Electron lifetime as a function of the normalized Voc obtained from Figure 7a.


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Figure 8 a) EIS plots with the equivalent electric circuit of the bare WO3 NRs (black), B/W HNRs (green), L-B/(R/W) HNS (blue) and H-B/(R/W) HNS (red) under white light (AM 1.5G, 100 mW/cm2) illumination. b) PEC current densities as the function of potential (upper RHE; bottom Ag/AgCl) of the bare WO3 NRs (black), B/W HNRs (green), L-B/(R/W) HNS (blue), H-B/(R/W) HNS (red) and H-B/(R/W)+FeOOH/NiOOH HNS (gray).



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Figure 9 a-c) IPCE plots (a), H2 evolution measurements (b) and PEC stabilities (c) of the bare

WO3

NRs

(black),

B/W

HNRs

(green),

H-B/(R/W)

FeOOH/NiOOH+B/(R/W) HNS (gray), respectively.


HNS

(red)

and

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Figure 10 a) Structure for the FE modeling of the simplified B/(R/W) HNS. b, c) Pathways and density of the photogenerated current through the model of the simplified B/(R/W) HNS with σRGO= 45.0 S/m (b) and σ’RGO=σRGO/10000 (c). d) Simplified B/(R/W) HNS with highly loaded BiVO4 NPs and numerous WO3 NRs. One broken NR is marked by a red circle. e) Pathways and current density through the B/(R/W) HNS in Figure 9d.



Figure 11 Schematic of the band alignment of WO3 and BiVO4 and the PEC process of the B/(R/W) HNS, as well as the multichannel charge transport.


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