Controlled synthesis of vertically aligned SnO2 nanograss-structured

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Controlled synthesis of vertically aligned SnO2 nanograssstructured thin films for SnO2/BiVO4 core-shell heterostructures with highly enhanced photoelectrochemical properties Susanta Bera, Sol A Lee, Chang-Min Kim, Hasmat Khan, Ho Won Jang, and Se-Hun Kwon Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b03179 • Publication Date (Web): 07 Nov 2018 Downloaded from http://pubs.acs.org on November 7, 2018

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Chemistry of Materials

Controlled synthesis of vertically aligned SnO2 nanograss-structured thin films for SnO2/BiVO4 core-shell heterostructures with highly enhanced photoelectrochemical properties Susanta Beraa,b,c, Sol A Leed, Chang-Min Kima, Hasmat Khane, Ho Won Jangd,* and Se-Hun Kwona,b,c,* aSchool

of Materials Science and Engineering, Pusan National University, Busan 46241, Republic of Korea Frontier R&D Center for Hybrid Interface Materials, Pusan National University, Busan 46241, Republic of Korea cInstitute of Materials Technology, Pusan National University, Busan 46241, Republic of Korea dDepartment of Materials Science and Engineering, Research Institute for Advanced Materials, Seoul National University, Seoul 08826, Republic of Korea eSol−Gel Division, CSIR-Central Glass and Ceramic Research Institute, 196 Raja S.C. Mullick Road, Kolkata 700032, West Bengal, India bGlobal

ABSTRACT: Fabrication of semiconductor thin films with uniform and vertically aligned one dimensional nanostructures is an active area of research. We report the synthesis of vertically aligned nanograss-structured SnO2 thin films on a wide range of substrates with a vapor-solid deposition process. In this process, some chemical and physical parameters such as chemical composition, deposition height from the precursor mixture, deposition temperature and substrate roughness are found to play key roles during the growth of SnO2 nanograsses (SNG). The effects of density change and cross-section dimension (width) of the nanograsses (NGs) on surface area improvement of the thin films have been examined by varying the respective parameters. BiVO4 (BV) solution layers were coated onto SNG, forming core-shell type-II heterojunction thin films (SNG-BV). The thickness of the drop-casted BiVO4 solution layers onto the NGs was controlled by the number density of the NGs per unit area. Light absorption efficiency (ηabs) of the core-shell SNG-BV films has been optimized by controlling quasi-arranged periodicity of the core NGs and accessible shell thickness of BiVO4 layers. The charge separation efficiency (ηsep) of SNG-BV films strongly depends on the thickness of the BiVO4 layers onto NGs. Thin layers of BiVO4 coating along the axial direction of thinner SnO2 NGs (25-50 nm) shows enhanced ηsep but lower ηabs due to poor light absorption. On the other hand, the thicker core NGs (40-200 nm) with low surface area provide thick layers of BiVO4 which drives strong light absorption but suffers from efficient ηsep. However, intermediate layers of BiVO4 onto uniformly arranged SnO2 NGs with 30-70 nm width shows enhanced ηabs as well as efficient ηsep compared to other SNG-BV samples. This result demonstrates that control over the horizontal dimension of the core materials in the core-shell heterojunction (keeping vertical restriction) is a viable approach for optimizing the photoelectrochemical efficiency. █ INTRODUCTION Micro/nanostructured metal oxide semiconductor (MOS) thin films are one of the most promising photoelectrodes for converting solar energy to chemical energy via photoelectrochemical (PEC) water splitting.1-8 In the PEC process, intense effort has been focused on the development of MOS with narrow band gap for better utilization of visible light harvesting, efficient charge separation, and transfer or injection properties.3-8 Among the various semiconductors, ternary oxide BiVO4 has been regarded as the best performing photoanode due to its moderate band gap for efficient light absorption and good photoelectrochemical stability.3 The maximum achievable photocurrent of BiVO4 for water oxidation (Jmax) under Air-Mass 1.5 Global (AM 1.5 G) solar illumination is ~7.5 mA/cm2.9 Practically, the water oxidation photocurrent in BiVO4 is substantially lower due to its poor charge separation and transport. The diffusion length is also shorter than the film thickness, which limits light absorption.3,10,11 The PEC efficiency can be expressed as the product of light absorption efficiency (ηabs) for generation of photoexcited charges, charge separation efficiency (ηsep) for separation and transport of photoexcited electrons and holes within the semiconductor materials, and charge transfer efficiency (ηtrans) at the semiconductor/electrolyte interface.12 However, ηtrans in BiVO4 has been improved by coating different oxygen evolution electrocatalysts on the surface of the material,3,13-15 but increasing ηabs × ηsep remains a challenge. To address the issue, an effective approach is to construct a highly

efficient type II heterojunction of BiVO4 (acting as a guest or shell) with various MOS (acting as a host or core), such as TiO2/BiVO4, WO3/BiVO4, or SnO2/BiVO4 heterojunctions.16-22 It is found that SnO2 acting as a host material in the heterojunction is a promising candidate for solar to chemical energy conversion. Compared with WO3, SnO2 prevents uncontrolled doping of cations into BiVO4, which leads to decreased separation efficiency.16 SnO2 is also a wide band gap (~3.6 eV) semiconductor with higher electron mobility than anatase TiO2 and a more negative conduction band minimum which can facilitate electron transfer from low-band gap sensitizers.23 The value of ηabs greatly decreases due to optical losses resulting from increased transmission and reflection.24 One efficient approach is to structure thin layers of absorbing materials (shell) into vertically aligned one-dimensional (1D) nanostructures (core), such as nanorods, nanotubes, nanowires, and nanograss.16-18,25,26 This structuring addresses these issues by increasing light coupling and enhancing light absorption via light trapping through the 1D core-shell nanoarrays (NAs) which provide an elongated optical path for photon absorption.24,26 In this regard, Zhou et al.16 synthesized BiVO4coated Sb-doped SnO2 nanorod arrays to obtain an efficient SnO2/BiVO4 heterojunction. They found enhanced absorption efficiency due to the long optical path length in the BiVO 4 layer along the axial direction of an Sb:SnO2 nanorod and efficient separation efficiency due to facile transportation of photoexcited electrons from BiVO4 to the Sb:SnO2 core material.

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Figure 1. (a-c) FESEM images of SNG (SNG2) thin film with different magnifications, confirm the formation of nanograsses, (d) cross-sectional view of SNG on FTO-coated glass. (e) TEM image of one nanograss and (f,g) HRTEM images of nanograss measured from (e). However, 10 hydrothermal autoclaving steps were required to form the SnO2 with ~1 µm thickness, which is a very time consuming and complex process. The photoelectrochemical performance (ηabs × ηsep) of a cell strongly depends on the optical and electrical properties of the 1D core-shell NAs.24 Although, the core-shell NAs offer elongated optical path of the coated shell but precisely controlling the geometric parameters (radius/width, lattice periodicity, thickness, and volume fraction) of the core NAs can improve light scattering and absorption from the core-shell NAs.19,24,26,28 Also, it can strongly influence separation and transportation of photo-generated charge carriers at the core-shell interface.19,25 Therefore, ηabs × ηsep of the core-shell cell can be optimized through controlling the geometric parameters of the core. 24,26-28 Morphological and geometric studies on 1D NAs have investigated the irregularity and surface roughness of the substrates.29-31 The substrate roughness can act as a nucleation center for improving the subsequent oriented growth via reducing the nucleation energy barrier.30,31 Therefore, it is crucial to seek favorable substrate roughness for facile synthesis of 1D SnO2 core NAs, tailoring different geometrical morphologies of SnO2 core for SnO2/BiVO4 heterojunction, and optimize the photoelectrochemical performance of the heterojunction. Herein, we report a controlled single step vapor-solid deposition process for growth of high density vertically aligned uniform SnO2 nanograsses (SNG) on various substrates with different surface roughness. The width and number density of the nanograsses were controlled by changing some chemical and physical parameters, such as chemical composition, deposition height, deposition temperature and substrate roughness. BiVO4 solution layers were coated onto nanograsses to prepare a coreshell SnO2/BiVO4 heterojunction. Finally, optimization of the photoelectrochemical properties in core-shell SnO2/BiVO4 with different core SnO2 morphologies has been studied systematically.

█ EXPERIMENTAL SECTION All chemical reagents were used as received without further purification. Growth of SnO2 NGs (SNG) was performed from a mixture of SnCl2 (SC, Sigma-Aldrich) and zinc chloride (ZC, SigmaAldrich) with different mole ratio (SC/ZC = 0.04, 0.03, 0.02 designated as SNG1, SNG2, SNG3, respectively). These two chemicals were ground and placed together at the bottom of a ceramic crucible. Then, a FTO-coated glass substrate (procured from Sigma-Aldrich) with 2 cm x 2 cm area was cleaned with acetone, isopropanol and water in an ultrasonication bath and placed at different heights (1.5, 2, and 2.5 cm) from the source mixture. This setup was inserted into a box furnace in air atmosphere. The system was heated up to 400, 500, 550, or 600 °C at a rate of 10 °C per min. After the oven reached its target temperature, the setup was held in the oven for 1 min. After the deposition process, each film was dipped in 0.1 M HCl for 10 min to remove any ZnO subproducts. SNG thin films with SNG2 composition (SC/ZC = 0.03) were also deposited at 550 °C at a height of 2 cm from the source chemicals on ITO coated glass, back side of FTO glass, quartz glass wafer, and Si wafer while keeping the other parameters constant. A precursor solution for BiVO4 (SBV) thin film formation was made from Bi(NO3)3·5H2O (98%, Sigma-Aldrich) and VO(C5H7O2)2 (98%, Sigma-Aldrich) in a solvent mixture of 2methoxy ethanol (4 ml) and acetic acid (1 ml) in the presence of 0.25 ml acetylacetone (Sigma-Aldrich) as a solution stabilizer while stirring for 2 h. To hybridize BiVO4 with SNG, the SBV solution was drop-casted on SnO2 nanostructured thin films. Each SNG thin film with 2 cm x 1.4 cm area was coated with 3 drops of 5 µl SBV solution and dried on hot plate at 350 °C for 10 min. This SBV coating process on an SNG thin film was repeated four times. Finally, the coated films were cured at 550 °C for 2 h in an electrical furnace under air atmosphere and the cured film was designated as SNG-BV. In this regard, the SBV solution-coated SNG1, SNG2, and SNG3 films followed by heating, labelled

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as SNG-BV1, SNG-BV2, and SNG-BV3, respectively. To study the controlled amount of BiVO4 on SNG2 sample, 3, 5 and 6 times along with 4 times of BiVO4 solution coatings (labelled as 3 layers, 4 layers, 5 layers and 6 layers, respectively) were performed onto the SNG2 samples (see supporting information). █ CHARACTERIZATION Surface morphology and elemental contents of SNG and SNG-BV films have been characterized by field emission scanning electron microscope (FESEM, S-4800, Hitachi) and FESEM-energy dispersive X-ray spectroscopy (FESEM-EDS) study. Moreover, the cross-sectional view of the films has been analyzed from the cross-section FESEM study. Crystallinity of the samples was analyzed using X-ray diffractometer (XRD, D8 ADVANCE, Bruker) using Cu-Kα1 at a wavelength of 1.5418 Å . Surface roughness of the respective substrates was analyzed with the help of atomic force microscope (AFM, Nanosurf Easy scan 2, Switzerland). Line scan profiles of the surfaces were determined from their corresponding AFM images. In addition, the root-mean-square (RMS) surface roughness of the substrates was calculated from the AFM images with the help of WSxM 5.0 Develop 7.0-Image Browser software. Transmission electron microscopy (TEM) study has been performed using TALOS F200X (FEI), operating

at 200 kV. To prepare the specimen, the SNG-BV2 film matrix was scratched off and dispersed in methanol, followed by ultrasonication for ~2 h. Then, the dispersed solution was drop casted on a carbon coated 300 mesh Cu grid and dried in air to evaporate methanol. TEM and high resolution TEM (HRTEM) have been performed for analyses of width of SnO2 NG, particle size of BiVO4, crystal structure of metal oxides etc. The transmission and reflection spectra of the respective samples were measured with an integrating sphere and UV vis spectrophotometer (UV-3600, Shimadzu Corp.). Photoelectrochemical measurement of BiVO4 and SNGBV films was carried out with the help of Ivium Technologies, Nstat. A three-electrode system with a Ag/AgCl reference electrode and a Pt plate counter electrode was used in a 0.5 M Na2SO4 medium with or without 0.5 M Na2SO3 solution. A Xe arc lamp coupled with an AM 1.5G filter was used and calibrated to an output of 1 sun (100mW/cm2) using reference photodiode. The light was illuminated from the back side of the photoelectrodes. A scan rate of 10 mV s-1 was used for linear sweep scan. The incident photon to current efficiency (IPCE) was measured with a light source and a monochromator at 1.23 V vs. RHE.

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Figure 3. FESEM images of SNG grown on different substrates: (a) ITO coated glass, (b) back side of FTO, (c) quartz glass wafer and (d) Si wafer. AFM images of (e) FTO, (f) ITO, (g) back side of FTO, (h) quartz glass and (i) Si surfaces. The height profiles measured from the respective AFM images, as shown below of each AFM image. █ RESULTS AND DISCUSSION In the vapor-solid deposition process, SnO2 vapor is deposited onto FTO substrate through dispersion in a ZnCl2 vapor medium acting as an interspace separator for vertically oriented growth of SnO2 NGs. The morphology of the SNG2 (SC/ZC = 0.03) nanostructures grown on FTO-coated glass substrate with deposition height at 2 cm is shown in Figure 1. FESEM images (Fig. 1a-d) show the formation SnO2 NGs with length, width, and aspect ratio of ~1.8 µm, 30-70 nm and 25-60, respectively. NGs are grown upward with extending vertical orientation. The nanostructures were further studied by TEM analysis, and one single nanograss with width ranging from 42 to 52 nm is shown in Figure 1e. HRTEM images (Fig. 1f, g) of the nanograss with distinct lattice fringes with an inter-planar distance of 0.34 nm, corresponds to the (110) plane of tetragonal phase SnO2, supported by XRD analysis (Fig. S1). It is worth noting that some non-uniform NGs were found in a very small area in the center of the film. These NGs were not used for characterization to maintain a consistent comparison of optical and electrochemical properties. Growth of SnO2 NGs is found to be dependent on the deposition temperature, as revealed from XRD (Fig. S2a,) and FESEM studies (Fig. S2b-f). To confirm that the XRD signal comes from SnO2 NGs rather than FTO substrates, we performed the growth process on quartz glass wafer at different temperatures. No XRD peaks from the SnO2 thin film grown at 400 °C could be detected. However, FESEM images (Fig. S2b, c) of SnO2 on FTO glass grown at 400 °C show formation of small nanoparticles, which can be attributed to SnO2 nanoparticles deposited directly on the surface of the FTO substrates for the formation of SnO2 crystalline nuclei. These particles may act as seed crystals for subsequent growth of SnO2 NGs. It was observed that the vertical growth of SNG began at 500 °C (Fig. S2d), and high density

uniform NGs were formed at 550 °C (Fig. S2e). The growth process is schematically shown in Figure 2a. The grown NGs show increased mean width when the deposition temperature increased to 600 °C (Fig. S2f). This can probably reduce the surface area to volume ratio of the NGs. Chemical composition (SC to ZC mole ratio) and deposition height are also found to play key roles during growth of SnO2 NGs. It was already mentioned that ZC acts as an interspace separator for vertical SNG growth. Therefore, one can assume that the NG density per unit area and average NG width would vary with changes in the SC to ZC mole ratio. However, some thicker SnO2 NGs (40-220 nm) are observed when the SC/ZC mole ratio was = 0.04 (SNG1), as shown in Figure 2b, S3. This occurs due to a lower ZnCl2 content, which acts as an interspace separator. The lower ZnCl2 is insufficient for vertical growth of uniform NGs. High NG density and thinner width (25-50 nm) were found with higher ZnCl2 content (Fig. 2c, S4). Thus, the result clearly shows that with increasing ZnCl2 content, the density of NGs increased as the average width decreased (Fig. 2d). On the other hand, the concentration of SnO2 vapors became low with increasing deposition height, which can significantly impact the growth of NGs. Variations in nanograss density and average width were also observed as the deposition height from the precursor materials altered, as shown in Figure 2e-g. It is found that the width of the NGs became thicker (50-300 nm) when the film was deposited at a lower height (~1.5 cm) (Fig. 2d), while the NGs became thinner (15-50 nm) at higher deposition height (~2.5 cm) (Fig. 2e). The growth process of the NGs depends on the seed layer thickness of the SnO2 nanoparticles,32,33 which was deposited in situ during the V-S deposition process. Moreover, lower deposition height can lead to higher seed layer thickness of SnO2 nanoparticles, which can assist

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Figure 4. (a) Schematic illustration of BiVO4 nanoparticles coating onto the surface of SNG, forming SNG-BV. (b) Cross sectional view of SNG-BV2, inset shows the top view of the film and (c) magnified image of (b). TEM and HRTEM images of SNG-BV2 film: (d) BiVO4 coated NG, inset shows HRTEM image taken from (d); (e) BiVO4 coated NG; (f) magnified image of (e), inset shows HRTEM image of BiVO4 NPs taken from the position marked in (f). growth of low density NGs with thicker width.33 In contrast, higher deposition height leads to thinner NGs with high density due to thin seed layer deposition33 on the substrate and low concentration SnO2 vapor. Figure 2g shows that the width of the NGs gradually decreased with increasing deposition height from the precursor materials. Although a higher deposition height resulted in high density and thinner NGs, each NG was not wellseparated, which can lead to decreased surface area. Therefore, a deposition height of ~2 cm can result in optimum seed layer thickness to obtain high density and uniformly separated SnO2 NGs. The well-organized high density NGs can provide large surface area for homogeneous thin layers solution coating of BiVO4 sensitizer for core-shell SnO2/BiVO4. The core-shell morphology could be beneficial for enhanced light harvesting and can provide a direct conduction pathway for efficient charge separation and transport within the semiconductors. To study the influence of vertical growth of SnO2 NGs, a controlled experiment was performed using different substrates beside the FTO-coated glass, such as ITO coated glass, back side of FTO, quartz glass wafer, and Si wafer under similar composition (SNG2, SC/ZC = 0.03) and deposition conditions (deposition height, ~2 cm, temperature 550 °C). FESEM images of the NGs grown on different substrates are shown in Figure 3a-d. It is noted that randomly oriented SnO2 NGs were formed on an Si substrate. This result can be explained by the analysis of AFM surface roughness (Fig. 3e-i, Table S1). The vertical growth of the semiconductors by V-S approach depends on the irregularity and surface roughness of the substrates.29-31 In the V-S process, the substrate roughness can act as a nucleation center to improve the subsequent oriented growth by reducing the nucleation energy barrier.30,31 AFM results show that the smooth Si surface exhibited the minimum root-mean-square (RMS) surface roughness (Fig. 3i, Table S1) compared to the other substrates (Fig. 3e-h). However, the rough surface of FTO, ITO, back side of FTO, and quartz glass wafer can provide a large number

of nucleation centers for vertical growth of SNG, whereas NGs preferred to align parallel to the substrate plane on smooth Si substrate.30 Thus, the surface roughness of the substrates plays a crucial role for vertical growth of SNG. Similar results were also found for the growth of SbSI nanorods on anodic aluminum oxide,30 ZnO nanorods on chemically etched sapphire substrate,29 and V2O5 nanowires on chemically etched Si substrates.31 According to the AFM surface roughness analysis (Table S1), FTO-coated glass exhibited maximum RMS roughness compared to other substrates, which provides many binding sites for SnO2 nucleation. A continuous supply of incoming SnO2 vapor for further subsequent 1D growth results in the formation of uniform nanograss structures. However, successful growth of SnO2 NGs on different substrates can be more beneficial for various specific applications such as lithium ion battery, gas sensors, photovoltaics, and field-emitters. To demonstrate PEC water splitting, BiVO4 solution (SBV) was coated on the SNG films, resulting the formation of SNG-BV hybrid films. This constructs a nanostructured type-II heterojunction, which can promote PEC efficiency.16-20 The hostguest heterojunction structure is schematically shown in Figure 4a, in which dispersed BiVO4 nanoparticles coated onto SnO2 NGs act as a visible light sensitizer. X-ray diffraction results (Fig. S5) from the SNG-BV thin films shows the presence of crystalline monoclinic BiVO4 along with tetragonal SnO2. Cross-sectional (Fig. 4b,c) and top view FESEM (inset Fig. 4b) images, and FESEM-EDS mapping (Fig. S6, SI) further illustrate that BiVO4 nanoparticles uniformly coated the surface of SnO2 NGs. However, BiVO4 NPs with wide size variation (Fig. 4c) are formed during the solution coating process16. It is found that the larger BiVO4 particles are detrimental to improve ηsep due to their short electron diffusion length (˂ 100 nm).3,10,11 The identically prepared pure BiVO4 thin film on FTO glass shows the formation of larger NPs (Fig. S7), which could be attributed that high surface area SnO2 NGs are advantageous to prevent growth

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Figure 5. Optical and photoelectrochemical (for sulfite oxidation) properties of BiVO4 and different SNG-BV films: (a) light harvesting efficiency (LHE), (b) photocurrent-potential (J-V) curves (dark and photo currents are shown as dotted and bold lines, respectively), (c) charge separation efficiency (ηsep), (d) incident photon-to-current conversion efficiency (IPCE). And, (e) photocurrent-time (J-t) curves of SNG-BV2 measured at 0.6 VRHE. of larger BiVO4 particles. High resolution TEM images (Fig. 4df) show the formation of BiVO4 NPs with sizes ranging from 5 to 25 nm, depending upon the width (30-70 nm) of the SnO2 NGs. The thinner NGs influence to grow smaller BiVO4 NPs (5-10 nm) (Fig. 4d, inset), whereas thicker NGs result in larger BiVO4 NPs (~24 nm) (Fig. 4e,f). HRTEM images (inset of Fig. 4d, f) confirm the formation of crystalline BiVO4 NPs. Therefore, randomly oriented BiVO4 NPs with sizes ranging from 5 to 25 nm could be favorable for enhancing the photoelectrochemical charge separation efficiency. The light harvesting efficiency (LHE) is defined as 100% - reflection (%) - transmission (%), indicating the optical absorption plus scattering.2 LHE was measured from the back side of the films using an integrating sphere.16 LHE spectra for different SNG-BV films and pure BiVO4 are shown in Figure 5a. Compared to a pure BiVO4 film, a significant enhancement of visible light harvesting in SNG-BV films was found for wavelengths ranging from ~300 to 450 nm. The SnO2 NGs show strong absorption below ~320 nm (Fig S8), indicating strong absorption coming from the BiVO4 shell. The LHE value of SNGBV3 having thinner SnO2 core is lower than the other SNG-BV samples. According to the fundamental property of waveguides, nanowire-arrays drive strong absorption peaks due to their supporting guided resonance modes, and number of bound modes increases with increasing radius or cross-section width of the nanowires (NWs).24,34,35 However, periodicity and volume fraction of the nanowires have also strong effect on the number of resonance modes because arranging of NWs in periodic lattice influences the periodicity and volume fraction of the

NWs.24 In this context, we investigated LHE of different SNG samples to understand the effect of SnO2 core morphology toward light trapping property. Little improvement of LHE is found with increasing width of the NGs from SNG3 to SNG2, but eventually decrease in SNG1 despite its thicker width NGs (Fig. S9). This could be due to the lack of adjustment of the periodic lattice and the volume fraction of the NGs. These arise because, on increasing the width/radius of the NWs, we can either fix the volume fraction through adjusting the periodicity or fix the periodicity through adjusting volume fraction.24 However, strong light absorptions of SNG-BV samples are coming from the coated BiVO4 layers with large optical path length along the axial direction of NGs. Different thicknesses of the BiVO4 layers resulted depending upon the surface area of SNG (Fig. 4b,c and Fig. S10) as same amount of BiVO4 solution was drop-casted onto each SNG film. The coated thickness of BiVO4 strongly influence the LHE of the samples. A large number density of SnO2 NGs per unit area in SNG3 sample could allow a thinner BiVO4 coating along the axial surface of the NGs (Fig. S10c,d), resulting in decreased light absorption in SNG-BV3 compared to SNGBV1. On the other hand, enhanced LHE of SNG-BV2 sample could be due to the combined effect of the intermediate thickness of the BiVO4 layers onto the NGs and efficient light scattering from BiVO4 coated arranged NGs. The photoelectrochemical (PEC) properties of BiVO4 and SNG-BV samples with different morphologies have been investigated using three electrode systems in the presence of 0.5 M Na2SO3, which acted as a scavenger. Compared to water oxidation, sulfite oxidation is thermodynamically and kinetically more facile.3,16 Photocurrent of water oxidation current can be expressed as JH2O = Jmax × ηabs × ηsep × ηtrans, where Jmax is the maximum photocurrent density for photons with energy ≥ band gap energy.3 ηtrans ~100% for sulfite oxidation due to extremely fast oxidation kinetics and negligible recombination at the electrode/electrolyte interface, resulting in Jsulfite/Jmax ≈ ηabs × ηsep.3,16 We determined Jmax by integrating the photon flux using the National Renewable Energy Laboratory (NREL) reference solar spectral irradiance at AM 1.5G over 300 to 515 nm and multiplying by the electron charge.3,16 Using these calculations, we find that Jmax = 7.404 mA/cm2. ηabs for different films were also determined by dividing the absorption photocurrent by Jmax (Fig. S11). The absorption photocurrent was determined by integrating the LHE and photon flux over 300 to 515 nm for a given spectral irradiance.16,25 Typical photocurrent density-potential (J-V) curves toward sulfite oxidation of the respective samples are illustrated in Figure 5b. Among the photoanodes, SNG-BV2 showed highest photocurrent values at 0.6 VRHE and 1.23 VRHE toward sulfite oxidation (Table S2, supporting information) which are sufficiently higher than those other reported SnO2/BiVO4 films.21,36 Also, the values are comparable with the undoped SnO2/BiVO4 nanorods-arrays reported by Zhou et al.16 Increase in photocurrent density was found from SNG-BV1 to SNG-BV2 and then decreased in SNG-BV3. The same behavior corresponds to ηabs × ηsep (Fig. S12, Table S3, supporting information). However, the trend for ηsep was slightly changed because comparable ηsep values were found for SNG-BV2 and SNGBV3 (Fig. 5c, Table S4, supporting information). The thin BiVO4 layers onto the NGs in SNG-BV3 and SNG-BV2 (Fig. 4b,c and Fig. S10) can provide short charge diffusion length for facile transport of photoexcited electrons to the core NGs toward improving ηsep.16,17 Thus, although SNG-BV3 has a lower LHE value than that of SNG-BV1, it showed higher photocurrent density compared to SNG-BV1 due to its higher ηsep whereas lower photocurrent was found than that of SNG-BV2 due to its lower ηabs.

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Chemistry of Materials To understand the nonlinear changes of the photocurrents, relative electrochemical active surface area (ECSA) was estimated from the slope of the current density vs. scan rate curves (Fig. S13, supporting information).37,38 The estimated result shows that SNG-BV3 sample has ~1.7 and ~1.4 times larger ECSA than SNG-BV1, SNG-BV2, respectively, suggesting that the improvement of photocurrent is just not having higher surface area.37 The effect of the thickness of BiVO4 layers onto SNG toward efficient PEC performance was also optimized through adopting different BiVO4 layers coating (see supporting information, Fig. S14). The incident photon-to-current conversion efficiency (IPCE) of the respective samples for sulfite oxidation were measured at 1.23 VRHE (Fig. 5d). IPCE is a measure of external quantum efficiency (i.e., ηabs × ηsep), assuming again ηtrans ~100% for sulfite oxidation.2 IPCE values for the SNG-BV films at 420 nm are listed in Table S5 (supporting information). One can see that SNG-BV2 and SNG-BV3 show comparable efficiency, and both values are higher than SNG-BV1. This could be due to a poor ηsep value in SNG-BV1 compared to other films. It is noted that improvement of IPCE was observed at longer wavelengths. However, the LHE at the shorter (λ < 450 nm) and longer (λ > 450 nm) wavelengths as well as absorption efficiency (ηabs) of SNG-BV1 was higher than SNG-BV3 (Fig. 5a, S11). From the result, we can say that the enhancement of IPCE is not due to a change in ηabs. Also, Yang group2 found the same behavior for TiO2 nanowire arrays. So, it can be suggested that the increase in IPCE of SNG-BV2 and SNG-BV3 at longer wavelengths could be due to efficient ηsep. The photostability of the SNG-BV2 sample was measured at 0.6 VRHE toward sulfite oxidation, and significant photostability was observed after 3 h (Fig. 5e). The photocurrent in the SNG-BV2 sample for water oxidation in 0.5 M Na2SO4 without addition of sulfite is shown in Figure S15a (supporting information). The photocurrent is considerably lower than that of sulfite oxidation. This result indicates that a majority of holes are lost at surface/electrolyte recombination1,3. The estimated charge transfer efficiency (ηtrans) for water oxidation is 29% at 1.23 VRHE, which was calculated from the ratio of JH2O/Jsulfite (assuming 100% charge transfer efficiency for sulfite oxidation). To improve the catalytic kinetics of photo-water oxidation in SNG- BV2, we hydrothermally deposited ferrihydrite (Fh) as a cocatalyst according to a procedure in the literature.13 The amount of the catalyst was controlled by setting the deposition time on the electrodes to 15 min, 20 min, 25 min, and 30 min, which are labeled 15mFh, 20m-Fh, 25m-Fh, and 30m-Fh, respectively. 20 min deposition time was found to maximize the photocurrent at 1.23 VRHE (Fig. S15b), which was ~1.9 times higher than the photocurrent for bare SNG-BV2 but was still lower than the sulfite oxidation photocurrent. The charge transfer efficiency of a cocatalystcoated photoanode toward water oxidation was found to be ~55% at 1.23 VRHE, indicating that all surface holes were not engaged in oxygen evolution. Because the photoelectrode/cocatalyst junction may serve as a recombination center,3 resulting in surface recombination. Finally, we measured the stability of the film at 1.23 VRHE and found efficient photostability after 4 h (Fig. S15c). Further improvements in water oxidation kinetics due to the photoanode can be made by introducing an efficient oxygen evolution catalyst (such as NiFe-(oxy)hydroxide/borate, NiOOH, Co-Pi) and increasing the conductivity of the core SnO2 NGs by doping or other methods for improving the charge collection efficiency. █ CONCLUSION

In summary, we report controlled synthesis of uniform nanograss-structured SnO2 thin films by adopting a vapor-solid deposition process. The mole ratio of SnCl2 to ZnCl2, deposition temperature, deposition height from the precursor mixture, and substrate roughness were examined as key factors for growth of the nanograsses (NGs). The density, uniformity, and crosssection dimension (width) of the NGs can be tuned by controlling the synthesis parameters. Light absorption efficiency (ηabs) and charge separation efficiency (ηsep) of the SNG-BV films can be optimized by quasi-arranging periodicity of the core NGs and controlling the thickness of the drop-casted BiVO4 solution layers depending upon the number density of the NGs per unit area. Optimized BiVO4 thin layers onto uniformly arranged SnO2 NGs (width dimension, 30-70 nm) with relatively high density exhibited enhanced ηabs and efficient ηsep for superior photoelectrochemical performance. The optimized film also showed a higher value of ηabs × ηsep compared to other SNG-BV samples. These results show that the geometric property of the core material in the core-shell heterojunction must be considered to achieve high photoelectrochemical efficiency.

ASSOCIATED CONTENT Supporting Information. A brief statement in nonsentience format listing the contents of material supplied as Supporting Information, XRD of different samples, FESEM images with cross-sectional view of SNG and SNG-BV samples, FESEM-EDS mapping of SNG-BV, light harvesting efficiency of SNG, absorption efficiency, absorption efficiency × charge separation efficiency, BiVO4 layers dependent photocurrents, cyclic voltammetry of the photoanodes, photoelectrochemical activity toward water oxidation.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] * E-mail: [email protected]

Funding Sources This research was mainly supported by the Global Frontier R&D Program (2013M3A6B1078874) on Center for Hybrid Interface Materials (HIM) funded by the Ministry of Science, ICT & Future Planning, Republic of Korea, and partially supported by the Lab to Convergence Program funded by Busan Institute of S&T Evaluation and Planning (BISTEP).

Notes Any additional relevant notes should be placed here.

ACKNOWLEDGMENT The authors would like to thank Ms. Seong-Hee Jeong, and Ms. Jong-Ah Chae for the help in FESEM measurements, and TEM investigation, respectively.

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BiVO4/GaOxN1−x Photoanode for Solar Water Oxidation. Adv. Mater. Interfaces 2017, 4, 1700323.

Table of Content (TOC) 80 70 60 SNG1 BiVO4

SNG2

sep(%)

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

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50 40 30 20 10 0 0.0

SNG3

0.2

0.4

0.6

0.8

1.0

Potential (V vs. RHE)

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