SnO2 Inverse Opal Heterostructure For Efficient Photoelectrochemical Water Splitting

Accepted Manuscript Title: Au Nanoparticles coupled Three-dimensional Macroporous BiVO4 /SnO2 Inverse Opal Heterostructure For Efficient Photoelectrochemical Water Splitting Authors: Shujie Zhou, Rui Tang, Luyuan Zhang, Longwei Yin PII: DOI: Reference:

S0013-4686(17)31476-7 http://dx.doi.org/doi:10.1016/j.electacta.2017.07.058 EA 29875

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Electrochimica Acta

Received date: Revised date: Accepted date:

22-1-2017 2-7-2017 9-7-2017

Please cite this article as: Shujie Zhou, Rui Tang, Luyuan Zhang, Longwei Yin, Au Nanoparticles coupled Three-dimensional Macroporous BiVO4/SnO2 Inverse Opal Heterostructure For Efficient Photoelectrochemical Water Splitting, Electrochimica Actahttp://dx.doi.org/10.1016/j.electacta.2017.07.058 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Au Nanoparticles coupled Three-dimensional Macroporous BiVO4/SnO2 Inverse Opal Heterostructure For Efficient Photoelectrochemical Water Splitting Shujie Zhou, Rui Tang, Luyuan Zhang*, Longwei Yin* Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials, Ministry of Education, School of Materials Science and Engineering, Shandong University, Jinan 250061, P. R. China *To whom correspondence should be addressed. Tel.: + 86 531 88396970. Fax: + 86 531 88396970. E-mail: [email protected]

Abstract: We demonstrated a strategy for coupling Au nanoparticles with ordered inverse opal heterostructure (AuBiVO4/SnO2 IO) via a novel template-assisted, swell-to-shrink, hydrothermal method. The synergistic function of surface plasmon resonance (SPR) effect of Au nanoparticles and slow photon effect of inverse opal structure on photoelectrochemical water splitting performance of Au-BiVO4/SnO2 IO is investigated. The Au-BiVO4/SnO2 IO shows enhanced light absorption ability in visible region and suppressed recombination of photogenerated electron– hole pairs. The inverse opal structure enables light to bounce and scatter between the highly ordered tunnels, leading to efficient light harvesting. By overlapping the slow photon effect of inverse opal, the SPR effect is effectively amplified, leading to a more striking enhancement in light utilization. The Au-BiVO4/SnO2 IO displays a photocurrent density of 3.83 mA cm-2 and an IPCE of 70.8% at 1.23V vs. RHE, more than 3 times higher than planar BiVO4. The improved photoelectrochemical performance is attributed to two aspects: one is the enhanced light harvesting from the coupling between the slow light and SPR effect, the other is the efficient separation of charge carriers owing to the Schottky barriers. The work provides promising strategies for designing plasmon coupled inverse opal semiconductor photoelectrodes to synergistically enhance PEC performance. Keywords: surface plasmon resonance; Au-BiVO4/SnO2; inverse opal; heterostructure; photoelectrochemical water splitting

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1. Introduction Photoelectrochemical (PEC) water splitting is regarded as one of the most promising strategies to solar energy conversion [1-5]. Since Fujishima and Honda reported solar water splitting performance from TiO2 photoanodes in 1972, much attention has been paid on this system. Among all the semiconductors, bismuth vanadate (BiVO4) has been considered to be an efficient photocatalyst for water splitting, owing to its suitable band gap of 2.4 eV, proper band location [6, 7], a dominated carrier lifetime and environmentally friendliness. However, the performance improvement of PEC water splitting based on BiVO4 electrodes suffers from poor charge transport performance [8, 9], and easy recombination of electron/hole pairs [8, 10, 11]. From this point, various methods have been applied to overcome these shortcomings, such as elemental doping [12-14], surface modification [15, 16], quantum dot sensitization [17, 18] and heterojunction construction [19-21]. Notably, the construction of heterojunction not only efficiently promotes the electron/hole separation by band structure engineering, but also integrates the merits of the optical response properties of the components [22, 23]. Recently, attempts have been made to construct heterostructures between BiVO4 and other semiconductors. Su et al. fabricated nanostructured WO3/BiVO4 heterojunctions for efficient photoelectrochemical water splitting[24]. Kim et al. reported that CaFe2O4/BiVO4 heterojunction photoanode exhibits improved photoelectrochemical activity by reduced surface recombination center in solar water oxidation [25]. BiVO4/SnO2 heterojunction has been considered as an efficient heterostructure for the charge transfer owing to matched band structure between BiVO4 and SnO2 [26]. Also, the very positive valence band position of SnO2 can prevent hole back-transfer through the SnO2 layer and forms a ‘hole mirror’ [27]. It is reported that SnO2 exhibits superior electron mobility (240 cm2 V-1S-1), which is 2−3 orders of magnitude higher than that of BiVO4 [28]. However, to date, the PEC water splitting performance based on BiVO4 photoanodes is still poor in general, because of high recombination charge rates, limited light harvesting light ability and limited active sites at the interface between electrolyte and electrodes. Therefore, it is of high importance and challenge to develop BiVO4-based photoanodes with rationally porous structure, which can enhance light harvesting ability and enhance charge transfer kinetics, provide enough electrolyte/photoanode interfacial reaction sites. Recently, inverse opal (IO) structure, as a typical three dimensional ordered periodically macroporous structure, is arousing increased interest as a promising candidate for potential photoanodes for PEC water splitting because the periodically macroporous IO structures provide enough electrolyte infiltration tunnels for the interfacial carrier transport [29]. The large surface area porous structure of IO offers large amount PEC reactive sites between the electrode material and electrolyte [30, 31]. Furthermore, inverse opal structure exhibits distinctive slow-photoneffect, which would slow down the group velocity of photons at a frequency near the photonic-band-gap (PBG) and lead to enhanced light storage and utilization performance. More importantly, the PBG of the inverse opal can be tuned by altering the size of the structural repeat unit according to the Bragg’s law [32, 33]. Hence, it is of great interest to enhance the light harvesting capability and PEC reaction activity of the photoanodes by incorporating the optical interaction into the inverse opal structure. Furthermore, as is well known, Au nanoparticles (NPs) exhibit strong size-tunable surface plasmon resonance (SPR) in the visible light region, providing a novel method to break through the limits in PEC water splitting [34, 35]. SPR effect caused by Au NPs can induce an enhanced light absorption and hot electrons transfer process [36-38], together with accelerating the generation and separation rates of electron-hole pairs in the semiconductor [39, 40]. It has been proved that, by coupling BiVO4 system with Au NPs, the powered electrons generated by SPR effects can be efficiently injected into the conduction band of BiVO4 [41]. In addition to the favorable SPR effects, due to the Schottky barriers formed between Au and semiconductors, Au NPs also act as the reservoir under incident light for photo-generated charge and prolong the lifetime of electron/hole pairs [42, 43]. It should be a crucial strategy to strike a balance between the plasmonic metallic nanostructures with SPR effect and heterostructure of ordered macroporous inverse opal nanostructure with tuned photonic band gap in order to achieve synergic effects on improving the PEC water splitting performance. However, the synergistic effect of constructing heterojunction of inverse opal nanostructures with a tuned PBG and coupling plasmonic 2

nanoparticles has not been reported yet, and the reaction mechanism of this system is of great importance but is not clear by now. Herein, we demonstrate a three-dimensional periodically ordered macroporous BiVO4/SnO2 inverse opal heterostructure (BiVO4/SnO2 IO) by template-assisted, swell-to-shrink method. Plasmonic Au NPs are homogeneously loaded on the scaffolds of BiVO4/SnO2 inverse opal (Au-BiVO4/SnO2 IO). By adjusting the size of the polystyrene (PS) template spheres, the photonic band gap can be rationally tuned, and the slow photon effect region can overlap with the SPR region of the Au NPs, which leads to a striking enhancement in incident light utilization. The obtained Au-BiVO4/SnO2 IO induces facilitated electron separation and transfer performance, which greatly suppresses the intrinsic carrier recombination in BiVO4. The application of three dimensional ordered periodically macroporous inverse opal structure further optimizes the PEC performance by offering large surface area for providing increased active sites between electrode and electrolyte. Also, this structure enables light to bounce and scatter between the highly ordered tunnels, leading to more efficient light harvesting. The synergistic effect leads to a greatly enhanced photoelectrochemical performance, reaching 3.83 mA cm-2 at 1.23V vs RHE, which is more than 3 times higher than that of the planar BiVO4. We believe that the Au-BiVO4/SnO2 inverse opal heterostructured photoanode can be a potential candidate in the application of BiVO4 based PEC devices. 2. Experimental section 2.1 Materials: All chemicals were purchased and used without any further purification. Tin(IV) Chloride Pentahydrate (SnCl4·5H2O; 99.0%), ammonium metavanadate (NH4VO3; 99.0%), bismuth nitrate pentahydrate (Bi(NO3)3·5H2O; 99.0%) and chloroauric acid tetrahydrate (HAuCl4·4H2O ) were purchased from Sinopharm Chemical Reagent Co. Ltd. Fluorine doped Tin oxide (FTO) substrate was purchased from OPV·Tech Co., Ltd. 2.2 Preparation of Au-BiVO4/SnO2 inverse opal The polystyrene (PS) colloids with diameters of around 200, 300 and 400 nm were prepared via the emulsifierfree emulsion polymerization process, which is described in the supporting information. The precursor solution of SnO2 was obtained by dissolving 0.01 mol of SnCl4·5H2O in 20 ml ethanol under stirring to form a transparent solution. The BiVO4 precursor solution was composed of 0.005 mol of Bi(NO3)3, 0.005 mol of NH4VO3 and 0.01 mol of citric acid dissolving in 15 mL of 23.3% HNO3 aqueous solution. The PS template was pre-treated by heating at 100 °C for 5 min to fix the PS spheres on the FTO substrate. The PS template was firstly immersed into methanol for 1h to get a swelled template [44]. After that, the PS template was immersed vertically into the SnO2 precursor solution and kept for 5 min. Then, it was pulled out slowly and dried on the heating plate at 70 °C for 10 min. Then the PS template was immersed vertically into the BiVO4 precursor solution for another 5 min and pulled out slowly and dried on hot plate for 10 min. Both two procedures were repeated for 3 times. After that, the treated PS templates were annealed at 500°C for 3 h to remove the template, with a heating rate of 0.5 °C min-1 and the BiVO4/SnO2 inverse opal was then obtained. To synthesize planar BiVO4 photoanode, the precursor solution of BiVO4 is deposited onto FTO substrates via the spin coating method at a speed of 1000 rpm for 10 seconds and repeated for 3 times. Then the FTO substrates were annealed at 350 °C for 4h. Au NPs were prepared by a hydrothermal method. An aqueous solution of HAuCl4·3H2O (0.1 wt%, 10 ml) was added to deionized water (100 ml) and heated to boiling point. A sodium citrate solution (1 wt%, 2 ml) was then added and the resulting mixture was kept for 15 min under stirring. Upon cooling down to room temperature, the Au NPs were obtained in the solution. The obtained BiVO4/SnO2 inverse opals were carefully immersed into the Au NPs suspension and kept at 100°C for 30 min, followed by washing with DI water and ethanol, respectively. After dried in room temperature, the Au-BiVO4/SnO2 IO photoanodes were obtained. 2.3 Microstructure Characterization and photoelectrochemical (PEC) measurements The phase structure was characterized by XRD (Cu-Kα, 40 kV, 30 mA) in a 2θ range of 10-90o. The morphologies were characterized by field-emission scanning electron microscopy (FESEM, SU-70) equipped with energydispersive X-ray spectroscopy (EDS), high-resolution transmission electron microscopy (TEM, Tecnai 20U-Twin). 3

The UV-visible diffuse-reflectance spectra were collected from UV-vis spectrophotometry (TU-1900). The photoluminescence (PL) spectra were recorded using a detection system of Hitachi U-4100 with an excitation wavelength of 400 nm. Photoelectrochemical characterization was carried out using a three-electrode configuration, with Pt and Ag/AgCl electrode as counter and reference electrodes. The electrolyte was 0.5 M aqueous Na2SO4 solution (pH=7). Measurements under illumination were performed using a Newport solar simulator (Class 3A, 94023A), simulating AM 1.5 solar illumination (100 mW cm-2). In all experiments, photoanodes were illuminated from the front-side. The electrochemical impedance spectra (EIS) were measured with a frequency range of 10 mHz to 100 kHz and the magnitude of the alternating potential was 5 mV. The Mott-Schottky plots were tested in dark condition at an AC frequency of 1.0 KHz. The incident photon-to-current efficiency (IPCE) data were obtained using from Newport equipment consisting of a 300 W Oriel Xenon lamp, a motorized monochromator and a Keithley 2400 digital source meter. 3. Results and discussion 3.1 Structure characterization We demonstrate a template-assisted, swell-to-shrink method to prepare the Au-BiVO4/SnO2 inverse opal nanostructures. To illustrate the design rationales, Scheme 1 outlines the typical procedure of the series synthesis process. Firstly, the colloidal templates are fabricated using polystyrene spheres (PS) via evaporation-induced, self-assembled method on the FTO substrate. Secondly, the PS templates are immersed into methanol in order to get a swelled template. Then the SnO2 precursor solution is infiltrated into the swelled PS template. After that, the swelled PS templates containing SnO2 precursor are heated to 100 °C to shrink the PS template, which enables the BiVO4 precursor to permeate into the space between the SnO2 skeleton and the PS template. After thermal calcination, joint connection between BiVO4 and SnO2 skeleton is formed, and three-dimensional periodically ordered macroporous BiVO4/SnO2 inverse opal heterostructure (BiVO4/SnO2 IO) is obtained. Finally, the synthesized gold nanoparticles (Au NPs) are incorporated onto the BiVO4/SnO2 inverse opal to obtain AuBiVO4/SnO2 IO heterostructures.

Scheme 1 Preparation process of Au-BiVO4/SnO2 inverse opals.

The crystallinity and phase components of the obtained planar BiVO4, BiVO4/SnO2 IO heterostructure and AuBiVO4/SnO2 IO are determined by XRD technique, as is shown in Fig. 1. For the planar BiVO 4 (black line), the 4

2θ diffraction peaks of 18.9, 28.9, 30.5,35.2, 39.5, 42.3, 45.6, 50.3, 53.3 and 58.5° can be respectively indexed to (011), (121), (040), (002), (141), (150), (231), (202), (161) and (321) planes of monoclinic BiVO4 (JCPDS. 14–88) [45]. For the SnO2/BiVO4 IO heterostructure (red line), except for the characteristic peaks of monoclinic BiVO4, additional peaks located at 26.6, 33.9, 37.9, 51.8, 61.9 and 65.9° can be indexed to (110), (101), (200), (211), (310) and (301) planes of tetragonal rutile-type SnO2 (JCPDS. 41-1445) [46], suggesting the formation of BiVO4/SnO2 heterostructure. For XRD pattern with Au NPs incorporated (blue line) heterostructures, an additional peak at 38.2° can be assigned to the (110) plane of Au, which overlaps with that of the (200) plane of SnO2, while another peak at 44.3° can be indexed as the (200) plane of Au, (JCPDS. 04-0784). The XRD results indicate that the AuBiVO4/SnO2 IO is successfully obtained.

Fig. 1 XRD patterns of BiVO4 IO, BiVO4/SnO2 IO and Au- BiVO4/SnO2 IO samples.

Fig. 2a shows the SEM images of the obtained self-assembled PS spheres on FTO substrate. The PS spheres with uniform diameter of 300 nm orderly arrange on the FTO substrate with a face-centered cubic (FCC) structure. After heating at 500 °C for 3 h, the PS templates are removed, and the BiVO4/SnO2 IO exhibits highly ordered macroporous inverse opal structure with clear hexagon structural unit, indicating that the applied swell-to-shrink method and calcination process do not destroy the ordered macroporous structure produced by the PS template (Fig. 2b-2c). It is worth mentioning that, compared with the PS templates, the diameter of the macropores decreases due to the shrinkage caused by calcinations treatment. To confirm the composition of the as synthesized BiVO4/SnO2 IO, EDS mapping images are tested and shown in Fig. S1. It can be seen that the elements of Sn, Bi, V distribute homogeneously among the inverse opal structure. As is shown in Fig. 2d, after introducing Au NPs, the surface and boundaries of Au-BiVO4/SnO2 IO become rough with Au NPs dispersed among the skeletons of IO structure. It can be further revealed from the cross-sectional SEM image of Au-BiVO4/SnO2 IO in Fig. S2 that the well-defined ordered structure of inverse opal retains well after the formation of BiVO4/SnO2 heterostructure and deposition of Au NPs, showing a thin film thickness of about 2 μm. For comparison, planar BiVO4 is prepared and the SEM image of BiVO4 is shown in Fig. S3.

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Fig. 2 FESEM images of (a) PS templates, (b-c) BiVO4/SnO2 IO, (d) Au-BiVO4/SnO2 IO.

In order to verify the relationship between the slow-photon region of inverse opal structure and the SPR region of Au NPs, inverse opals with different sizes are prepared. Fig. S4a, 4c exhibit the PS template with diameter of 200 nm and 400 nm, which further proves a well-defined periodically macroporous inverse opal structure. After annealing, the BiVO4/SnO2 IO (200 nm) retains a pore size of about 150 nm (Fig. S4b), while BiVO 4/SnO2 IO (400 nm) retains a pore size of around 340 nm (Fig. S4d) owing to the shrinkage of calcination. From the TEM images of BiVO4/SnO2 IO (300 nm) in Fig. 3a-3b, it can be further verified that the BiVO4/SnO2 IO inherits the highly ordered macroporous structure of inverse opal, which is in good agreement of SEM results. Even after annealing and deposition of Au nanoparticles, the ordered inverse opal structure is not destroyed. Also, it can be seen clearly that Au NPs are mainly dispersed among the skeletons of BiVO4/SnO2 IO structure (Fig. 3c). HRTEM lattice image of Au-BiVO4/SnO2 IO in Fig. S5 demonstrates the coexistence of BiVO4, SnO2 and Au, and the diameter of Au nanoparticles is about 20 nm. Lattice spacing of 0.20 nm corresponds well to the (200) plane of cubic Au (JCPDS 04-0784), while lattice spacing of 0.48 nm and 0.23 nm match well the (100) plane of BiVO4 and (111) plane of and SnO2, respectively. The selected area electron diffraction (SAED) in Fig. 3d demonstrates the phase components of the AuBiVO4/SnO2 IO, where diffraction rings correspond well to that of (101), (221) and (220) planes of SnO2, and (121), (110), (112) and (231) planes of BiVO4, respectively. From the TEM EDS elemental mapping image analysis (Fig. S6), the composition of as synthesized Au-BiVO4/SnO2 IO can be further confirmed. It can be proved the elements of Bi, V, Sn, and Au are uniformly distributed among the inverse opal skeletons.

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Fig. 3 TEM images of (a-b) BiVO4/SnO2 IO, (c) Au-BiVO4/SnO2 IO. (d) Electron diffraction pattern of Au-BiVO4/SnO2 IO.

3.2 Optical property characterization To verify the influence of slow-photon effect caused by PBG effect of inverse opal structure and the SPR effect of Au nanoparticles, the optical absorption performance of the planar BiVO4, BiVO4 IO, BiVO4/SnO2 IO and AuBiVO4/SnO2 IO is comparatively investigated by UV-vis absorbance spectra (Fig. 4a). In order to provide a comparison on the water splitting performance, these electrodes are immersed in water for 10 s before optical measurements. According to the modified Bragg’s law, the stop-band of BiVO4 inverse opals is calculated using the following equation [47]: 2

2 2 (1 − 𝑓) − 𝑠𝑖𝑛2 𝜃 λ= 2√ 𝐷 √𝑛𝑓𝑖𝑙𝑙𝑒𝑟 𝑓 + 𝑛𝑣𝑜𝑖𝑑𝑠 3

(1)

where λ is the wavelength of the PBG, D is the pore diameter of the inverse opal structure, which is 220 nm for AuBiVO4/SnO2 IO, nfiller and nvoids refer to the refractive indices of the (n BiVO4 = 2.4, n SnO2= 2.2) and voids materials (n water = 1.3), respectively, f is the volume fraction in the inverse opal (f = 0.2), and 𝜃 is the incident angle of light (   0 ). It can be seen that as intrinsic BiVO4 shows a band gap value of 2.4 eV, corresponding to an optical absorbance wavelength edge around 520 nm. According to the pore size of IO structure, it can be estimated that the PBG of the obtained BiVO4/SnO2 IO heterostructure is centered at around 550 nm. Therefore, an additional peak centered at about 550 nm can be seen in all three IO samples (BiVO4 IO, BiVO4/SnO2 IO and Au-BiVO4/SnO2 IO), corresponding to the PBG of the IO structure.

Fig. 4 (a) UV-vis absorption spectra and (b) PL emission spectra measured with excitation wavelength of 400 nm of planar BiVO4， BiVO4 IO, BiVO4/SnO2 IO and Au-BiVO4/SnO2 IO.

Group velocity of light can be decreased when incident light wavelength approaches the PBG from the red side, and the path length of light increases, and thus the light utilization is strongly improved. Importantly, obviously enhanced light absorption intensity can be observed in the IO structured samples comparing with planar BiVO4, which should be attributed to the enhanced light interaction and multiple incident light scattering among the highly periodically 3D macroporous IO structures. For the obtained 20 nm Au nanoparticles, the SPR peak is centered at around 550 nm [48]. It is obvious that after coupling with Au nanoparticles, the absorption intensity increases in the whole absorption region, which can be ascribed to the light absorption facilitation caused by SPR effect of Au NPs. Additionally, the absorption peak centered at 550 nm is further enhanced in the Au-BiVO4/SnO2 IO due to the overlapping of the PBG of inverse opal structure and SPR effect region of Au NPs. It is worth mentioning that the absorbance performance of Au-BiVO4/SnO2 IO structures can be tuned by adjusting the size of PS template (Fig. S7). It is estimated that Au-BiVO4/SnO2 IO (200 nm) with a pore size of around 150 nm has a PBG centered at 370 nm, while the Au-BiVO4/SnO2 IO (400 nm) with a pore size of around 320 nm has a PBG wavelength around 790 nm. All the samples show similar absorbance wavelength edge around 7

520 nm due to the band gap of BiVO4 (2.4 eV). If the pore diameter of the inverse opal structure is tuned to be 220 nm, an additional peak around 550 nm corresponding to PBG wavelength can be observed for BiVO4/SnO2 IO. While for Au-BiVO4/SnO2 IO with pore size of 220 nm, it is obvious that the peak centered at 550 nm is sharper in Au-BiVO4/SnO2 IO than the other two samples, which can be ascribed to the overlap of the PBG of inverse opal structure and SPR region of Au nanoparticles. There remains a small heave around 380 nm in AuBiVO4/SnO2 IO (200 nm), which is in agreement of the estimated results. A little peak around 800 nm can also be observed in Au-BiVO4/SnO2 IO (400 nm), which is in accordance of the estimated results. It can be inferred from the absorbance results that the overlap of PBG and SPR region can integrate and amplify the slow-photon effect and SPR effect, which will lead to a great enhanced PEC performance. Photoluminescence (PL) emission property is tested to investigate the photo-generated carrier separation performance of the obtained photoanodes with excitation wavelength of 400 nm (Fig. 4b). In general, the PL intensity depends on the recombination of excited electrons and holes, which means that the lower PL intensity represents the more efficient electron/hole separation and lower carrier recombination rate. It can be seen from the PL spectra that pure BiVO4 (BiVO4 IO and planar BiVO4) exhibits a strong emission peak at 520 nm, which can be due to the intrinsic carrier recombination for BiVO4 [49]. It is shown that after the formation of BiVO4/SnO2 heterojunction, the PL intensity drastically decreases. This should be attributed to the uniquely matched energy state can induce a facilitated photo-induced exciton separation process. Notably, after the BiVO4/SnO2 IO is decorated with Au NPs, the PL intensity further decreases. It demonstrates Au NPs plays an important role in carrier transporting behavior, and the presence of Au NPs can effectively suppress the carrier recombination rate, which hence devotes to an enhanced PEC performance. 3.3 Photoelectrochemical (PEC) performance To evaluate the PEC performance of the obtained Au-BiVO4/SnO2，BiVO4/SnO2 and BiVO4 IO structured photoanodes, a set of PEC measurements are carried out. For a comparison, a planar BiVO4 photoanode is also investigated. A typical three-electrode electrochemical cell with a Pt mesh counter electrode and Ag/AgCl reference electrode are used in the experiments. 0.5 M Na2SO4 aqueous solution (pH = 7) is used as the electrolyte, which is bubbled with N2 for 30 minutes to remove the dissolved oxygen. Illumination is performed under a 300 W Xe lamp simulating AM 1.5 solar. The potentials are measured versus (vs.) the Ag/AgCl reference electrode, which can be converted into the reversible hydrogen electrode (RHE) scale via the Nernst equation: ERHE = EAg/AgCl + E0Ag/AgCl + 0.059pH (2) In this equation, ERHE refers to the converted potential vs. RHE, and EAg/AgCl is the external potential measured against the Ag/AgCl reference electrode. E0Ag/AgCl represents the standard potential of Ag/AgCl, with a value of 0.198 V at 25 °C. Electrons are generated under illumination, which transfer through the conduct band of BiVO4 and SnO2 and finally transfer to the counter electrode, where the electrons are caught by H+ in the electrolyte and reduced to H2. Meanwhile, the remaining holes are consumed by OH- to oxidize H2O to O2 at the surface of photoanode. During the PEC test, we can see bubbles near the photoanode (O2) and the Pt courter electrode (H2). Linear sweep voltammograms (LSV) of Au-BiVO4/SnO2 IO with different pore sizes are examined at a scan rate of 10 mV s-1, as is shown in Fig S8. It can be seen that the Au-BiVO4/SnO2 IO (300 nm) exhibits the highest LSV, comparing with the Au-BiVO4/SnO2 IO (200 nm) and Au-BiVO4/SnO2 IO (400 nm), which are in well correspondence with the absorbance results. LSV results indicate that Au-BiVO4/SnO2 IO (300 nm) displays the best PEC performance, owing to the overlap of slow-photon region of inverse opal structure and SPR region of Au NPs. In that case, we further investigate the synergistic effect of the localized surface plasmon resonance (SPR) of Au NPs, the formation of BiVO4/SnO2 heterojunction as well as the slow photon effect of the inverse opal structure with the sample Au-BiVO4/SnO2 IO (300 nm) on the PEC performance, which is simplified as AuBiVO4/SnO2 IO in the following discussions. Fig. 5a presents the linear sweep voltammograms (LSV) of the prepared photoanodes under illumination and in dark condition at a scan rate of 10 mV s-1. The dark current density is negligible (green line). However, under 8

illumination, the photocurrent increases significantly as a function of applied bias potential, indicating prepared photoanodes exhibit perfect PEC activities. In addition, it can be seen that inverse opal structured photoanodes exhibit obvious increase in photocurrent density compared with planar BiVO4, suggesting that 3D periodically ordered macroporous inverse opal structure can effectively improve the photoelectric chemical water splitting kinetics through enhancing the contact between electrode materials and electrolyte and the light harvesting efficiency (LHE) by improving multi-light scattering process. Furthermore, it should be noted that the photocurrent density of the BiVO4/SnO2 IO heterostructure further increases, reaching 2.92 mA cm-2 at 1.23 V vs. RHE. It demonstrates that constructed IO heterostructure makes it more efficient for photo-induced charge generation and separation. Also, we can clearly see that there is a considerable further boost in photocurrent density of the Au-BiVO4/SnO2 IO compared with three other samples, reaching a value of 3.83 mA cm-2 at 1.23 V vs. RHE, 3 times higher than that of planar BiVO4 (1.15 mA cm-2 at 1.23 V vs. RHE). To further investigate the photoelectrochemical activities of the inverse opal photoanode materials, the switched on/off LSV curves are also measured (Fig. S9). After the illumination is turned off, the photocurrent density decreases drastically, indicating the photoanode is of great sensitivity to the light illumination. The photocurrent density shows similar variation trend as LSV and the spikes are in relation to the generation and recombination of charges carriers, which will be further discussed in the EIS analysis. It is worth noting that this excellent enhancement is achieved without using any additional oxygen evolution catalyst such as Pt or Co-Pi. This remarkable improvement can be attributed to the synergistic function of SPR and PBG effect. Generally, the photo-conversion efficiency (η) of photoanodes with bias voltage, which is also called applied bias photon-to-current efficiency (ABPE), can be calculated according to the following equation [50, 51]: η = I (1.23-Vapp)/Plight (3) where Vapp is the applied bias vs. RHE, I is the photocurrent density at the measured potential, and Plight is the illumination intensity, which is 100 mW cm-2 (AM 1.5G). The calculated ABPE of the obtained photoanodes is shown in Fig. 5b as a function of biasing potential. It can be seen that the maximum ABPE of planar BiVO4 is 0.39% at 0.75 V vs. RHE comparing with Au-BiVO4/SnO2 IO of 1.13% at 0.72 V vs. RHE. The boosted ABPE of AuBiVO4/SnO2 IO photoanode further confirms that IO heterostructure with matched band gap energy structure and introduction of Au NPs with SPR effect can be effective approaches to enhance the photoelectrochemical performance.

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Fig. 5 (a) Linear sweep voltammograms measured under illumination and in dark condition, (b) Applied bias photon-to-current efficiency, (c) Photocurrent density-time curves at 1.23V vs. RHE, (d) Incident photon-to-electron conversion efficiency spectra measured at 1.23 V vs RHE for planar BiVO4, BiVO4 IO, BiVO4/SnO2 IO and Au- BiVO4/SnO2 IO, the inset is partial enlarged view of the IPCE of the wavelength from 475 nm to 625 nm.

The amperometric photocurrent density-time (I-t) curves recorded at 1.23V vs. RHE under chopped illumination exhibit similar photoelectrochemical performance trend compared to the LSV plots (shown in Fig. 5c). All samples show great sensitivity to the light illumination and excellent light response during several cycles of light on/off processes. Photocurrent density of BiVO4/SnO2 IO increases obviously compared with planar BiVO4 and BiVO4 IO, because the formed BiVO4/SnO2 IO heterojunction effectively induces enhanced carrier transport kinetics and suppresses the charge recombination. Decorated with Au NPs, Au-BiVO4/SnO2 IO exhibits further increased photocurrent density compared with three other samples, implying the SPR effect of Au NPs can effectively enhance the light utilization and charge separation processes. The remarkable enhancement of PEC performance can be attributed the synergistic effect of the SPR effect of Au NPs, the formation of BiVO4/SnO2 heterojunction as well as the uniform and periodically ordered macroporous inverse opal structure. In order to investigate the synergistic influences of SPR effect and the BiVO4/SnO2 IO structure on the enhancement of PEC performance, incident photon-to-current conversion efficiency (IPCE) measurements are performed at a bias potential of 1.23 V vs. RHE, as is shown in Fig. 5d. The IPCE values are calculated according to the following equation [52]: IPCE (%) =

1240 I

 PLight

 100%

(4)

where I is the photocurrent density, Plight is the incident light irradiance, and  is the incident light wavelength. Comparing the planar BiVO4 photoanode, the IPCE value of BiVO4 IO is still relative low due to the poor intrinsic carrier transport performance of BiVO4. However, compared with the BiVO4 IO, there is an IPCE boost from 10.5% to 40.6% for BiVO4/SnO2 IO, which can be ascribed to the formation of heterojunction between BiVO4 and SnO2. In this system, BiVO4 exhibits excellent ability of light absorption owing to its proper energy band structure and 10

band location, while SnO2 plays the role of carrier transmitter for accepting the photogenerated electrons from the neighboring semiconductors and hinders the electron-hole recombination because of its lower-lying conduction and valence bands. The heterojunction between BiVO4 and SnO2 facilitates visible light absorption and the electronhole separation simultaneously. Additionally, after decorated with Au NPs, the Au-BiVO4/SnO2 IO photoanode achieves the highest IPCE of up to 70.8%, which is almost 5 times higher than that of planar BiVO 4. After introducing of Au NPs, the IPCE increases from 40.6% of BiVO4/SnO2 IO to 70.8%, indicating that SPR effect of Au NPs can greatly improve the light absorption in visible range, and the powered electrons bring about efficient photogenerated electrons injection. Furthermore, from the inset partial enlarged view of the IPCE of the wavelength from 475 to 625 nm (Fig. 5d), an additional heave can be observed in the Au-BiVO4/SnO2 IO sample, which is in well agreement with the light absorbance results, verifying the overlap of slow-photon region of inverse opal structure and SPR effect can integrate and synergistically magnify the enhancement. For the BiVO4/SnO2 IO and BiVO4 IO, IPCE in this region is relatively weaker than Au-BiVO4/SnO2 IO, which is consistent with the light absorbance results. The optimization between structure and component guarantees the enhancement of PEC performance and provides a reference for other material systems.

Fig. 6 EIS Nyquist plots of planar BiVO4, BiVO4, BiVO4/SnO2 and Au- BiVO4/SnO2 inverse opal samples in dark condition, (b) under illumination.

The electrochemical impedance spectroscopy (EIS) is performed to further investigate the charge transfer performance of obtained photoanodes under and without illumination (Fig. 6). The arc diameter in the Nyquist plot is related to the interfacial charge transfer kinetics. Generally, the smaller arc diameter in the Nyquist plot indicates superior generation ability of electrons and efficient charge transfer kinetics [53]. It is believed that the uniquely ordered porous structures of inverse opal photoanodes could enhance the charge transfer kinetics and decreases the charge transfer resistant. It can be revealed that the BiVO4 IO, BiVO4/SnO2 IO, and Au-BiVO4/SnO2 IO display smaller charge transfer resistant than planar BiVO4, indicating that macroporous inverse opal structure promotes the interfacial interaction and thus improves the PEC performance. Also, it is obvious that the charge transfer resistant of inverse opal structure containing BiVO4/SnO2 heterojunction diminishes further compared with BiVO4 IO, which confirms that the uniquely matched energy band position of BiVO4 and SnO2 efficiently suppresses the electron/hole recombination. In both illumination and dark condition, Au-BiVO4/SnO2 IO photoanode shows the smallest diameter among the four photoanodes, demonstrating that Au NPs can further facilitate carrier transport performance by powering the photogenerated electrons, and the Au NPs work synergistically with the opal structure of BiVO4/SnO2 to improve the PEC performance.

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Fig. 7 Mott-Schottky plot of planar BiVO4，BiVO4, BiVO4/SnO2 and Au- BiVO4/SnO2 inverse opal samples.

Mott-Schottky plots are used to study the relationship between the enhancement in the PEC performance and the IO architecture nanostructures. As is shown in Fig. 7, all the samples exhibit positive slopes, indicating that both BiVO4 and SnO2 are n-type semiconductors. The donor density (Nd) can be calculated by the slope of the MottSchottky plots via the equation [52]:

  dV  Nd = e0 0  d 1 C2  2

 

    

(5)

where e0 is the electronic charge,  is the dielectric constant of the sample,  0 is the permittivity of the vacuum, Nd is the donor density and V is the applied voltage. Generally, the smaller the slope is, the higher the carrier density is. Significant improvement of carrier density for Au-BiVO4/SnO2 IO can be further confirmed by the Mott-Schottky plots because Au-BiVO4/SnO2 IO shows the smallest slope among the four samples, implying the largest carrier concentration, which stands for more efficient electron/hole separation and thus an improved PEC performance. The flat band potential can be estimated by the following equation [54]: 1 2 kT      E  E fb  2 e0 0 Nd  e  C

(6)

the flat band potential (Efb) is determined by taking the x intercept of a linear fit to the Mott-Schottky plot, 1/C2, as a function of applied potential (E). It has been reported that more negatively shifted flat band potential can provide higher quasi-Fermi levels of electrons (EFn) that drives a greater separation with the quasi-Fermi level of holes (EFp). It can also be seen from the Mott-Schottky plot that the Efb of planar BiVO4 is -0.29 V, while the Efb of AuBiVO4/SnO2 IO is -0.38 V, which exhibits a negative shift of nearly 0.1 eV. This shifting of flat band potential suggests a higher concentration of carriers, and further confirms the SPR of Au NRs and PBG effect of the inverse opal structure leads to a decreased charge recombination rate, which is favor of the enhancement of the PEC performance. We have tested the stability property of the Au-BiVO4/SnO2 IO by long-term time-dependent photocurrent density at a bias potential of 1.23V vs. RHE, as is shown in Fig. S10a. It can be seen that the obtained photoanode exhibits excellent stability with less than 20% photocurrent decrease under high bias potential. In addition, the morphology of the photoanode was tested after the long-term I-t measurement (Fig. S10 b), which maintains the inverse opal structure on the whole despite some slight breakage occurred among the macroporous frame works. Therefore, it indicates that the Au-BiVO4/SnO2 IO presents excellent PEC performance and satisfactory stability, which should be a promising material in the application of photoelectrochemical water splitting. 12

Fig. S11 shows a schematic energy diagram of the plasmonic enhanced Au-BiVO4/SnO2 IO photoanode. The electrons are generated by the SPR excitation of Au nanoparticles by visible light and become “hot electrons”, which which will be excited to the CB of BiVO4. Then, the electrons will immediately transfer from the CB of BiVO4 to the CB of SnO2 under illumination [55]. The formation of BiVO4/SnO2 heterojunction with matched band structure can facilitates the electron/hole separation while the resulting energy barrier would suppress back electron transfer. The photogenerated electrons finally transfer to the counter electrode and the remaining holes are consumed to oxidized H2O to O2. To verify the observed bubbles are H2 and O2 originated from water splitting instead of any other reactions, gas evolution tests of Au-BiVO4/SnO2 IO photoanode was performed at a bias of 0.65 V vs RHE using a gas chromatograph and the gas evolution and corresponding photocurrent response are provided in Fig.S12. The PEC performance of Au-BiVO4/SnO2 inverse opal can be comparable with or even superior to the previous works and we have made a table of some comparison of the earlier literature based on BiVO4 electrode, which is shown in Table.S1. For example, the fabricated BiOI/BiVO4 p-n junction photoanodes can achieve a photocurrent density of 3.27 mA cm-2 at 1.23 V vs. RHE and an IPCE of 60.8% at 475 nm at 1.23 V vs. RHE [56]. According to Ma’s work, a photocurrent density of 3.3 mA cm-2 at 1.23 V vs. RHE can be achieved from double-deck BiVO4/WO3 inverse opal photoanodes [44]. Zhao reported the fabrication of ultrathin BiVO4 photoanode film on textured poly dimethylsiloxane (PDMS) substrates, showing a photocurrent density of 1.37 mA cm-2 at 1.23 V vs. RHE on patterned PDMS substrates [57]. We believe that we have achieved a competitive PEC performance based on the Au NRs coupled BiVO4/SnO2 IO photoanodes. On the basis of the above experimental results, the great enhancement in the PEC performance of Au-BiVO4/SnO2 IO compared with planar BiVO4 can be attributed to the following reasons. Firstly, the construction of 3D ordered macroporous inverse opal nanostructure with large specific surface area and periodically porous structure to provide an effective strategy to tune photo band gap energy, thus to greatly improve light absorption ability. Secondly, the formation of BiVO4/SnO2 heterojunction efficiently suppresses the electron/hole recombination and facilitates charge transfer. Thirdly, the coupling with Au NPs, not only further enhances the light absorption ability, but also induces the charge separation, which is benefiting from the slow-photon effect of inverse opal and SPR effect of Au NRs. Fourthly, by regulating the size of Au NPs and tuning the pore size of the inverse opal, the SPR region of Au NPs can overlap with the PBG of the inverse opal structure, which integrates the SPR effect and the slow photon effect, resulting in a further improved PEC performance. 4. Conclusions In summary, we demonstrated a template-assisted, swell-to-shrink hydrothermal route to prepare the ordered three-dimensional (3D) Au-BiVO4/SnO2 inverse opal structures. The 3D ordered macroporous inverse opal structure with large surface area, and the BiVO4/SnO2 heterojunction with a matched band gap energy structure is conducive to facilitate charge separation. By altering the size of the polystyrene (PS) template spheres and introduction of Au NPs, the slow photon effect region could overlap with the SPR region of the Au NPs, resulting in a striking enhancement of incident light utilization. Applied as the photoanode for photoelectrochemical water splitting, the Au-BiVO4/SnO2 IO displays an enhanced photoelectrochemical performance, reaching a photocurrent of 3.83 mA cm-2 and an IPCE of 70.8% at 1.23V vs RHE, more than 3 times higher than that of the planar BiVO 4. The great enhancement can be ascribed to the synergistic effect of the inverse opal nanostructure, the construction of IO heterostructure and the decoration of plasmonic Au nanoparticles. The work reported here provides promising strategies for designing complex plasmon coupled semiconductor photoelectrodes to synergistically enhance PEC performance.

Acknowledgements 13

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