Three-Dimensional Undoped Crystalline SnO2 Nanodendrite Arrays

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Three-Dimensional Undoped Crystalline SnO Nanodendrite Arrays Enable Efficient Charge Separation in BiVO/SnO Heterojunction Photoanodes for Photoelectrochemical Water Splitting 4

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Shih-Yu Chen, Jih-Sheng Yang, and Jih-Jen Wu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00203 • Publication Date (Web): 19 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018

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ACS Applied Energy Materials

Three-Dimensional Undoped Crystalline SnO2 Nanodendrite Arrays Enable Efficient Charge Separation in BiVO4/SnO2 Heterojunction Photoanodes for Photoelectrochemical Water Splitting Shih-Yu Chen, Jih-Sheng Yang and Jih-Jen Wu* Department of Chemical Engineering, National Cheng Kung University, Tainan 701, Taiwan * Corresponding author. E-mail address: [email protected]

KEYWORDS: Photoelectrochemical water splitting; Undoped SnO2 Nanodendrite array; Visible-light-driven photoanode; Heterojunction; Charge separation

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ABSTRACT

Instead of using the doped SnO2 nanostructured scaffolds as transparent conducting electrodes, in this work, a three-dimensional (3D) undoped crystalline SnO2 nanodendrite (ND) array is developed on fluorine-doped tin oxide (FTO) substrate to be the scaffold of the visible-lightdriven photoanodes for photoelectrochemical (PEC) water splitting. The performances of the undoped SnO2 nanorod (NR) and ND arrays are investigated by the formation of staggered (typeII) heterojunction photoanodes using BiVO4 as a model photocatalyst. The hole-scavengerassisted PEC measurements indicate that the charge separation efficiencies of 88% and 55% are respectively obtained in the 3D BiVO4/SnO2 ND array and one-dimensional BiVO4/SnO2 NR array photoanodes at 1.2 V vs. the reversible hydrogen electrode (RHE) under front illumination (through electrolyte to photoanode). We suggest that the presence of SnO2 branches in the BiVO4/SnO2 ND array increases the volumes of the depletion regions in both BiVO4/SnO2 heterojunction and BiVO4/electrolyte heterojunction compared to the BiVO4/SnO2 NR array, resulting in the enhanced charge separation efficiency and photocurrent density in the BiVO4/SnO2 ND array photoanode. The results demonstrate that the 3D undoped crystalline SnO2 ND array is a promising semiconductor core scaffold to couple with the visible-lightdriven photocatalyst shell for the formation of the type-II heterojunction photoanode with superior charge separation efficiency.


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Introduction Water oxidation providing required electrons for proton and CO2 reductions is a crucial step in photocatalytic water splitting and CO2 conversion, respectively, which produce solar fuels to store solar energy in the form of chemical bonds.1-5 Visible-light-driven photocatalysts with suitable valence band edge position for oxygen evolution, such as BiVO4 and α-Fe2O3, usually show rather poor solar-to-hydrogen (STH) conversion efficiency, which is mainly due to severe charge

recombination

occurring

in

the

photocatalysts.6,7

The

construction

of

photoelectrochemical (PEC) cells for water splitting under minimal applied potentials on photoelectrodes is helpful to reduce the charge recombination effect.2,3,7 Nevertheless, the STH efficiency is still too low to develop a commercial PEC cells for water splitting.6,7 Enormous research efforts have been put into the suppression of charge recombination in photoelectrodes during PEC water oxidation, including formation of heterojunction for improving charge separation, construction of nanostructured photoelectrode for increasing the surface area and shortening the hole diffusion path, as well as impurity doping for increasing electrical conductivity.1-5 One-dimensional (1D) nanostrcutured heterojunction array photoanodes, composed of high electron conductivity semiconductor core (array scaffold) and visible-light-driven photocatalyst shell, have been developed to enhance the PEC water oxidation efficiency by improving charge separation in the bulk and hole collection on the surface of photoanodes.8-11 The array scaffold plays crucial roles both in morphology and interfacial energetic controls for achieving efficient photoanodes. The requirements on the array scaffold for the purposes include appropriate band structure with shell for the formation of staggered (type II) heterojunction, fast electron transport rate, complementary solar light harvesting to the shell, etc. WO3,8,9 ZnO,10 and TiO2,11 nanorod


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(NR) or nanowire (NW) array scaffolds have been employed to couple with visible-light-driven photocatalyst shell for the formation of heterojunction photoanodes. Three-dimensional (3D) TiO2 and ZnO nanodendrite (ND) array scaffolds have been demonstrated to further provide larger surface area for shell deposition as well.12-14 SnO2, which exhibits high chemical stability in a wide range of pH15 and high electron mobility,16 is suitable to be the array scaffold of the photoanodes. Successful improvements of the PEC performances have been reported by using porous Nb:SnO2 film,15 Sb:SnO2 colloid film,17 Sb:SnO2 NR array,18,19 and Sb:SnO2 nanotube array20 as the scaffolds of the photoanodes. The concept of transparent conducting oxides (TCOs) was taken in these investigations, in which impurity of Nb or Sb was doped into the SnO2 lattice to construct the conductive scaffolds.15,17-19 In this regard, however, the impurity concentration needs to be fine-tuned and the impurity distribution in the SnO2 lattice has to be well controlled for attaining highly conductive SnO2 scaffold.15,18 Unlike TiO2 and ZnO, the conduction band edge of SnO2 is more positive than those of BiVO4 and Fe2O3 before contact formation.2 Integrating with BiVO4 or Fe2O3, there is no need to solve the issue of band structural misalignment for the formation of undoped SnO2based staggered (type II) heterojunction PEC photoanodes.14,21,22 BiVO4-undoped SnO2 heterostructures have been investigated for PEC water splitting.23,24 Undoped-SnO2 thin films have been employed to be the hole-blocking layer in the BiVO4-based photoanodes.23 SnO2 inverse opals have been constructed as the electron transporter for the BiVO4-based photoanodes.24 However, a template-assisted method was needed for the formation of the SnO2 inverse opals.24 Zhou et al.19 demonstrated the formation of the BiVO4/Sb:SnO2 NR array photoanode for PEC water oxidation. The BiVO4/Sb:SnO2 NR array photoanode possessed the charge separation


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efficiencies (ηsep) of 92.4% and 61.6% acquired at 1.23 V vs. RHE under back (through back electrode to photoelectrode) and front (through electrolyte to photoelectrode) illuminations, respectively.19 The products of light harvesting efficiency (ηabs) and ηsep for the photoanode were estimated to be 66.5% and 44.9% at 1.23 V vs. RHE under back and front illuminations, respectively.19 They further prepared the ZnO-templated Sb:SnO2 nanotube arrays with longer thickness and lower packing density for BiVO4 coating.20 At 1.23 V vs. RHE, the optimized BiVO4/Sb:SnO2 nanotube array photoanode exhibited the ηabs ⋅ηsep products of 68.9% and 65.2% with the ηsep of~100% and ~90% under back and front illuminations, respectively.20 The enhancement was explained by the larger interstices of the Sb:SnO2 nanotube array compared to those of the previous Sb:SnO2 NR array,19 which prevented the formation of the BiVO4 film on the tops of the scaffolds.20 Nevertheless, a template-assisted method was needed for the formation of the Sb:SnO2 nanotube array.20 Rather than using the doped SnO2 NR array as transparent conducting electrode,15,17-19 in this work, a 3D undoped crystalline SnO2 ND array is designed to be the semiconductor core scaffold for the formation of type II heterojunction with visible-light-driven photocatalyst shell on the basis of the considerations of morphology and interfacial energetics for PEC water splitting. The 3D configuration of ND array not only decouples the light absorption path and hole transport path but also provides larger interfacial area with electrolyte.12-14 A wet-chemical template-free route to the 3D undoped crystalline SnO2 ND array on the fluorine-doped tin oxide (FTO) substrate is developed in this work. The undoped SnO2 NR array is first grown on the FTO substrate using hydrothermal method.18,19 The undoped SnO2 branches are then constructed on the surface of NRs by combing chemical bath deposition and hydrothermal methods in the absence of an organic structure-directing agent. BiVO4 is selected as a model photocatalyst to be


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formed on the undoped SnO2 nanostructured array for PEC water splitting. The hole-scavengerassisted PEC performances of the BiVO4/SnO2 NR array and BiVO4/SnO2 ND array photoanodes are examined in the present work. The charge separation efficiency of the 3D BiVO4/SnO2 ND array photoanode is significantly enhanced compared to that of the 1D BiVO4/SnO2 NR array photoanode.

Experimental Section Undoped SnO2 nanostructured arrays were grown on FTO substrates using wet chemical routes. SnO2 NR arrays were grown on seeded FTO substrates using two-batch solvothermal method. The seed layer was deposited by spin-coating an aqueous solution of 0.1 M SnCl4 onto the FTO substrate followed by heat treatment at 500 oC for 1 h. A solution composed of 0.12 M SnCl4 and 0.75 M NaBr in a ternary solvent system of 33 mL acetic acid, 5 mL DI water, and 2 mL ethanol was employed for the growth of the SnO2 NR array on the seeded substrate at 200 oC for 30 h. Additional batch of the solvothermal process with the ternary-solvent solution of 0.14 M SnCl4 and 1 M NaBr was conducted at 200 oC for 36 h. To develop the branches from the SnO2 NRs, chemical bath deposition of the sprout-like nanostructures on the SnO2 NR was first carried out in an aqueous solution of 0.35 M HCl and 1 mL tin isopropoxide at 95 oC for 4 h. Another solvothermal process to elongate the sprouts was then conducted at 200 oC for 4 h with the solution composed of 0.09 M SnCl4 and 1.25 M NaBr in a ternary solvent system of 32 mL acetic acid, 6 mL DI water, and 2 mL ethanol for the construction of SnO2 ND array. To construct the BiVO4/SnO2 nanostructured arrays, BiVO4 shells were deposited on the undoped SnO2 nanostructured arrays using metal organic decomposition method.14,21 A precursor solution was prepared by adding 0.05 M Bi(NO3)3 and 0.05 M vanadyl acetylacetonate into 5 mL 2-


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methoxyethanol followed by ultrasonication for 3 min. The BiVO4 layer was prepared by repeating the procedure of spin coating the precursor solution onto the SnO2 nanostructured arrays and subsequent annealing at 500 oC for 10 min for twice (2-layer deposition). The BiVO4/SnO2 nanostructured arrays were annealing at 500 oC for 2h afterwards. By examining at least 10 samples individually, the reproducibilities for the formations of the aforementioned nanostructured arrays are all good through well controlling the synthetic procedures. Morphologies of the nanostructured arrays were examined by field-emission scanning electron microscopy (FESEM, ZEISS AURIGA). Structures of the nanostructured arrays were characterized using Raman scattering spectrometer (integrated by Protrustech Corporation Limited) at excitation wavelength of 532 nm, x ray diffractometer (XRD, Rigaku D/MAX-2000), and transmission electron microscopy (TEM, FEI E.O Tecnai F20 G2 MAT S-TWIN). Optical absorptions of these nanostructured arrays were measured by a UV-vis-IR spectrophotometer (JASCO V-670). PEC performances of the BiVO4/SnO2 nanostructured array photoanodes were examined with a three-electrode system under AM 1.5G simulated sunlight at 100 mWcm-2 (100 W, Model 94011A, Oriel). The photoanode, Pt foil and an Ag/AgCl electrode were used as the working, counter and reference electrodes, respectively. The supporting electrolyte was prepared from 0.3 M Na2SO4 with potassium phosphate solution buffered at pH=7.5. For hole-scavengerassisted PEC performances, 0.3 M Na2SO3 was then added in the electrolyte as the hole scavenger. The PEC measurements were performed under front (through electrolyte to photoelectrode) illumination. Incident-photon-to-current efficiency (IPCE) spectra of the photoanodes were acquired at 1.23 V vs. the reversible hydrogen electrode (RHE) using 500 W xenon light source (Oriel) and a monochromator (Oriel Cornerstone) equipped with Si detector (Model 71640, Oriel).


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Results and Discussion Figures 1a and 1b respectively show the top-view and cross-sectional SEM images of the undoped SnO2 NR array grown on the seeded FTO substrates using solvothermal method. The diameters of the NRs are in the range of 20-50 nm and the average length of the NRs is 700 ±20 nm. The density of the undoped SnO2 NR array on FTO is ~2 x 1010 cm-2. A wet chemical rout, combining the chemical bath deposition of SnO2 sprouts on NRs and the solvothermal growth of 1D undoped SnO2 nanostructures from the sprouts, were further conducted to develop the branches on the NRs for the formation of the undoped SnO2 ND array. In the absence of an

Figure 1. (a) Top-view and (b) cross-sectional SEM images of undoped SnO2 NR array on FTO substrate. (c) Top-view and (d) cross-sectional SEM images of undoped SnO2 ND array on FTO substrate.


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organic structure-directing agent, as shown in Figures 1c and 1d, the ND array with 3D feature is successfully constructed on the FTO substrate. The diameters of the branches are less than 10 nm and the average length of the branches is 50±5 nm.

Figure 2. (a) Raman spectra of SnO2 NR array/FTO, SnO2 ND array/FTO, and FTO substrate. (b) XRD patterns of SnO2 ND array/FTO and FTO substrate. The crystal structures of the nanostructured arrays on FTO substrates were first examined using Raman spectroscopy. The Raman spectra of the NR and ND arrays are shown in Figure 2a. For comparison, the Raman spectrum of the FTO substrate is also displayed in this Figure. Two broad Raman scattering bands centered at 560 cm-1 and 633 cm-1 are obtained in the spectrum of the fluorine-doped SnO2 substrate. The bands centered at 476 cm-1, 633 cm-1, and 774 cm-1


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appear in the Raman spectra of the NR and ND arrays, which are pertaining to Eg, A1g and B2g active vibrations of the tetragonal rutile SnO2 structure, respectively.25-27 In addition, a significant Raman scattering band at 560 cm-1 is also present in both spectra. It was assigned to be surface vibration mode of SnO2 nanostructures, which intensity strengthens with the decrease of nanostructure size.26,27 The Raman characterizations reveal the formations of the tetragonal rutile SnO2 nanostructures on FTO substrates. Further structural characterization of the SnO2 ND array was conducted using XRD. Figure 2b shows the XRD patterns of the FTO substrate and ND array/FTO. The diffraction peaks of the FTO and ND array can be indexed as those of the tetragonal rutile SnO2 structure according to ICDD-PDF No. 00-041-1445 which also illustrated in this figure for comparison. Preferredorientation growth of the FTO layer is revealed by comparing the intensity ratios of (200)/(110) and (211)/(110) in the XRD pattern of FTO with those in ICDD-PDF No. 00-041-1445. On the other hand, the (002) diffraction peak, which is absent in the pattern of FTO, appears in the XRD pattern of the SnO2 ND/FTO. In addition to the higher intensity ratios of (200)/(110) and (211)/(110) pertaining to the FTO substrate, the higher intensity ratio of (002)/(110) is obtained in the XRD pattern of the SnO2 ND/FTO. It suggests that the SnO2 NDs are preferentially oriented in the c-axis direction on the FTO substrate. TEM characterizations of the SnO2 NR and ND are shown in Figure 3. Figure 3a displays the high-resolution (HR) TEM image of the tip region of a SnO2 NR scratched from the FTO substrate (inset). It reveals the single crystalline structure of the SnO2 NR. Moreover, the radial and longitudinal directions of the tetragonal NR are [110] and [001], respectively, as denoted in Figure 3a. Figure 3b reveals the HRTEM image of the interfacial region of the trunk and branch of the SnO2 ND. The angle between the trunk and branch is ~56o. The fast Fourier transform


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(FFT) diffraction patterns taken from the trunk (region A in b) and branch (region B in b) are shown in Figures 3c and 3d, respectively. They indicate that both tetragonal SnO2 trunk and branch possess the single-crystal structure with the radial direction of [110]. The HRTEM image and FFT diffraction patterns in Figures 3b-d demonstrate the successful formation of the quasisingle crystalline SnO2 ND using the wet chemical route developed in this work. The TEM characterizations confirm the XRD result that the SnO2 trunks (NRs) are preferentially grown along the c-axis direction.

Figure 3. (a) HR TEM image of the tip region of SnO2 NR in the inset. (b) HRTEM image of interfacial region of trunk and branch of SnO2 ND. (c), (d) FFT diffraction patterns of regions A and B denoted in (b). To investigate the performance of the undoped crystalline SnO2 ND as the core scaffold, in this work, BiVO4 is selected as a model photocatalyst for the formation of a type-II heterojunction with the undoped SnO2 nanostructured array for PEC water splitting. BiVO4 layers were deposited on the SnO2 NR and ND arrays by metal organic decomposition method.14,21 The precursor solution was spin-coated on the arrays followed by heat treatment to form the


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Figure 4. (a) Top-view and (b) cross-sectional SEM images of BiVO4/SnO2 NR array on FTO substrate. (c) Top-view and (d) cross-sectional SEM images of BiVO4/SnO2 ND array on FTO substrate. BiVO4/SnO2 heterojunction nanostructured arrays. The top-view and cross-sectional SEM images of the BiVO4/SnO2 NR and ND arrays where BiVO4 layers were prepared using the same process conditions are shown in Figure 4. The diameters of the heterojunction NR and ND increase and the lengths of the arrays remain to be ~700 nm after the depositions of the BiVO4 layers. The crystal structure of the BiVO4 layers was also characterized using Raman spectroscopy. Figure 5 illustrates the Raman spectra of the BiVO4/SnO2 NR and BiVO4/SnO2 ND arrays. The Raman scattering peaks at 324 cm-1, 366 cm-1, 710 cm-1, and 826 cm-1 are present in both spectra, which pertain to asymmetric (Bg) bending mode, symmetric (Ag) bending mode, as well as symmetric (Ag) V–O stretching mode of the longer and shorter V-O bond of


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monoclinic bismuth vanadate, respectively.28 It indicates the successful formation of the crystalline BiVO4 on the SnO2 NR and ND arrays.

Figure 5. Raman spectra of the BiVO4/SnO2 NR and BiVO4/SnO2 ND arrays. The absorptance (1-total transmittance (T)-total reflection (R)) spectra of the BiVO4/SnO2 NR array and BiVO4/SnO2 ND array photoanodes are shown in Figure 6a. The absorption spectrum of the SnO2 NR array is also displayed in this figure for comparison. Figure 6a reveals that the absorption edge shifts from ~350 nm to ~510 nm as the BiVO4 layer is formed on the SnO2 nanostructured arrays. Moreover, the light-harvesting ability of the BiVO4/SnO2 NR array photoanode is better than that of the BiVO4/SnO2 ND array photoanode, indicating that the BiVO4 layer on the ND array is thinner than that on the NR array. Although the process conditions for the BiVO4 layers on both SnO2 nanostructured arrays are identical, the thinner BiVO4 layer formed on the ND array is ascribed to the steric effect of the 3D ND array for infiltration of the precursor solution into the interstices of the array during spinning coating. The maximum photocurrent density (Jabs) of the photoanodes can be calculated by integrating the products of the wavelength-dependent intensities of solar spectrum and the corresponding light


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harvesting efficiencies of the photoanodes. Accordingly, the Jabs of the BiVO4/SnO2 NR array and BiVO4/SnO2 ND array photoanodes are 3.2 mAcm-2 and 2.2 mAcm-2, respectively.

Figure 6. (a) Absorptance spectra of SnO2 NR array, BiVO4/SnO2 NR array, and BiVO4/SnO2 ND array photoanodes. (b) J-V curves of BiVO4/SnO2 NR array and BiVO4/SnO2 ND array photoanodes. The PEC performances of the BiVO4/SnO2 NR array and BiVO4/SnO2 ND array photoanodes were examined in a three-electrode electrochemical system with 0.3 M Na2SO4 electrolyte at pH=7.5 under the irradiation of AM 1.5G (100 mWcm‒2) simulated sunlight on the front side of photoande (through electrolyte to photoelectrode). The photocurrent density (J)-potential (V) curves acquired by linear sweep photovoltammetry measurement are shown in Figure 6b.


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Compared to the BiVO4/SnO2 NR array photoanode, the onset potential of the BiVO4/SnO2 ND array photoanode shows a cathodic shift from 0.7 to 0.5 V vs. RHE. The BiVO4/SnO2 ND array photoanode possesses higher photocurrent densities than the BiVO4/SnO2 NR one does although the Jabs of the BiVO4/SnO2 NR array is superior to the BiVO4/SnO2 ND array photoanode. At 1.23 V vs. RHE, the photocurrent densities of the BiVO4/SnO2 NR array and BiVO4/SnO2 ND array photoanodes are 0.3 and 0.6 mAcm-2, respectively. Figure S1 shows the IPCE spectrum of the BiVO4/SnO2 ND array photoanode acquired at 1.23 V vs. RHE. The threshold of IPCE spectrum is at ~510 nm that is consistent with the absorption edge of the absorptance spectrum shown in Figure 6a. Figure S2 shows the absorptances and PEC performances of the BiVO4/SnO2 ND array photoanodes with 2-5 layers of BiVO4 deposited on the SnO2 ND arrays. As shown in Figure S2a, the absorptances of the photoanodes increase with the BiVO4 loadings. The BiVO4/SnO2 ND array photoanode with 5-layer-deposited BiVO4 exhibits the absorptances higher than 85% at wavelengths shorter than 510 nm. However, Figure S2b reveals that lower photocurrent densities of the photoanodes are acquired as the number of BiVO4 layers is increased. The BiVO4/SnO2 ND array photoanode with 2-layer deposited BiVO4 has the highest photocurrent densities among these photoanodes. Since the dimensions of the SnO2 ND arrays in those photoanodes are identical, the thickness of the BiVO4 on the SnO2 ND increases with the number of deposition layers, which may result in the inferior PEC performance. The photocurrent density of the photoelectrode is determined by the factors of Jabs, charge separation efficiency (ηsep), and charge injection efficiency (ηinject).13-14 Jabs is the product of the light absorption efficiency (ηabs) and the theoretical maximum photocurrent density based on band gap of the photoelectrode. It should be addressed that in the case of the 1D and 3D nanostructured array photoanodes, the light absorption path and minor carrier transport path are


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decoupled to different directions.14 The ηabs of the 1D and 3D nanostructured array photoanodes can be increased by elongating the length of the array photoanode without sacrificing charge separation efficiency. Accordingly, rather than comparing the product of ηabs and ηsep,19,20 it is meaningful to compare the ηsep of the optimized BiVO4/ SnO2 NR array and BiVO4/ SnO2 ND array photoanodes. To examine the charge separation efficiencies of the two BiVO4/SnO2 nanostructured array photoanodes, in this work, the hole-scavenger-assisted PEC measurements were further conducted with the addition of Na2SO3 in the electrolyte as the hole scavenger during PEC measurements. By assuming a 100% injection efficiency of the photoanode with the addition of Na2SO3 in the electrolyte, the charge separation efficiency can be estimated using the ratio of photocurrent density measured with Na2SO3 addition in the electrolyte to the Jabs.29 The results are also compared to those reported in Ref. 19 in which the SnO2 NR arrays exhibited the comparable dimensions to those prepared in this work. Figure 7a show the J-V curves of the BiVO4/SnO2 NR array and BiVO4/SnO2 ND array photoanodes acquired with the addition of 0.3 M Na2SO3 in the electrolyte. The photocurrent densities of the BiVO4/SnO2 ND array photoanode are significantly higher than those of the BiVO4/SnO2 NR array photoanode as shown in Figure 7a. Moreover, the photocurrent density of 2.0 mAcm-2 was acquired from the BiVO4/SnO2 ND array photoanode at 1.23 V vs. RHE, which approaches to the Jabs of 2.2 mAcm-2 estimated from the light harvesting efficiencies shown in Figure 6a. In the case of the BiVO4/SnO2 NR array photoanode, however, the photocurrent density of 1.5 mAcm-2 at 1.23 V vs RHE is significantly lower than the corresponding Jabs of 3.2 mAcm-2.


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Figure 7. (a) J-V curves of BiVO4/SnO2 NR array and BiVO4/SnO2 ND array photoanodes with Na2SO3 added in the electrolyte. (b) Charge separation efficiencies of BiVO4/SnO2 NR array and BiVO4/SnO2 ND array photoanodes. Figure 7b shows the charge separation efficiencies of the BiVO4/SnO2 nanostructured array photoanodes, which are calculated from the ratios of photocurrent densities to Jabs. Under front illumination, the BiVO4/SnO2 NR array photoanode possesses the charge separation efficiency of 55.3% at 1.2 V vs RHE. With the similar photoanode structure, it is slightly lower than that of 61.6% referring to the reported BiVO4/Sb:SnO2 NR array.19 Nevertheless, the charge separation efficiencies of 87.8% and 94% are respectively obtained in the BiVO4/SnO2 ND array photoanode at 1.2 V and 1.8 V vs. RHE under front illumination. It indicates that the charge


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separation efficiency in the BiVO4/SnO2 ND array is significantly higher than those in the BiVO4/SnO2 NR array and BiVO4/Sb:SnO2 NR array19 acquired under front illumination. Moreover, the charge separation efficiencies acquired in the BiVO4/SnO2 ND array photoanode under front illumination is comparable to that of 92.4% obtained in the reported BiVO4/Sb:SnO2 NR array at 1.2 V vs. RHE under back illumination.19 The low photocurrent densities acquired from the BiVO4/SnO2 ND array photoanode, as shown in Figure 6b, are due to inferior light harvesting and charge injection efficiencies. The charge injection efficiency can be improved by surface modification of the BiVO4/SnO2 ND array with co-catalysts. Moreover, the light harvesting efficiencies of the BiVO4/SnO2 ND array can be further enhanced by increasing the length of the SnO2 ND array for BiVO4 deposition because the light harvesting path and minor carrier transport path are decoupled to different directions in the nanostructured array photoelectrodes.14 Considering the illuminated BiVO4/SnO2 nanostructured heterojunction, photoelectrons which are created within the effective photoelectron generation region, i.e., the depletion region and an electron diffusion length of the depletion region on BiVO4 side, can effectually transport into SnO2 nanostructure followed by transporting to the FTO electrode. We suggest that the presence of SnO2 branches in the BiVO4/SnO2 ND array could increase the volumes of the depletion regions in both BiVO4/SnO2 heterojunction and BiVO4/electrolyte heterojunction compared to the BiVO4/SnO2 NR array.30 The numbers of photocarriers generated within the effective photocarrier generation region in the BiVO4/SnO2 ND array are therefore increased, resulting in the enhanced charge separation efficiency and photocurrent density in the BiVO4/SnO2 ND array photoanode.


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It has been reported that photocurrent densities measured under front illumination are usually inferior to those measured under back illumination unless the photoanode possesses superior charge separation/transport properties.8 Under back illumination, the excess carriers are mainly photogenerated in the region close to the FTO back electrode, where the necessary path for photoelectron transport to FTO is shorter compared to that under front illumination. For the photoanode with poor electron transport property, the higher charge separation efficiency and therefore the improved photocurrent density can be obtained under back illumination due to the reduction of charge recombination before charge collection. As shown in Figure 7, the BiVO4/SnO2 ND array photoanode demonstrates superior intrinsic characteristics under front illumination that the charge separation efficiencies are as high as ~87-94% at 1.2 -1.8 V vs. RHE. It indicates that the 3D undoped crystalline SnO2 ND array is a promising scaffold for enhancing the PEC performance of the visible-light-driven photoanodes. Instead of employing the doped SnO2 nanostructures as TCO scaffolds,15,17-20 in this work, the undoped crystalline SnO2 ND array scaffold has been successfully developed to couple with the visible-light-driven photocatalyst shell for the formation of the type-II heterojunction photoanode with superior charge separation efficiency.

Conclusions In this work, a 3D undoped crystalline SnO2 ND array is constructed to be the semiconductor core scaffold for the formation of type II visible-light-driven heterojunction array photoanodes for PEC water splitting. A wet-chemical route to the undoped SnO2 ND array on the FTO substrate is first developed. XRD and TEM characterizations show that the SnO2 trunks are preferentially grown along the c-axis direction. Moreover, TEM characterizations reveal that


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both trunk and branch possess the single-crystal tetragonal SnO2 structure, indicating the successful formation of the quasi-single-crystalline SnO2 ND. To examine the core-scaffold characteristics of the undoped crystalline SnO2 ND array, BiVO4 is selected as a model photocatalyst for the formation of the type-II heterojunction with the undoped SnO2 nanostructured array for PEC water splitting. According to the hole-scavenger-assisted PEC measurements, the charge separation efficiencies of 87.8% and 94% are respectively obtained in the 3D BiVO4/SnO2 ND array photoanode at 1.2 V and 1.8 V vs. RHE under front illumination, which are significantly enhanced compared to the 1D BiVO4/SnO2 NR array photoanode investigated in this work. Rather than using the doped SnO2 NR array as a TCO electrode, the 3D undoped crystalline SnO2 ND array is a promising semiconductor scaffold to couple with the visible-light-driven photocatalyst shell for the formation of the type-II heterojunction photoanode with superior charge separation efficiency.

ASSOCIATED CONTENT Supporting Information. The following file is available free of charge. IPCE spectrum of BiVO4/SnO2 ND array photoanode as well as absorptance spectra and J-V curves of BiVO4/SnO2 ND array photoanodes with various BiVO4 loadings. (PDF) AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This research is supported by the Ministry of Science and Technology in Taiwan under Contracts MOST 105−2221-E-006−251-MY3 and MOST 106−2221-E-006−202-MY3.

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Table of Contents Graphic


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Three-Dimensional Undoped Crystalline SnO2 Nanodendrite Arrays

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