Toward Eco-Friendly and Highly Efficient Solar Water Splitting Using

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Towards Eco-Friendly and Highly Efficient Solar Water Splitting using InS/Anatase/Rutile TiO Dual-StaggeredHeterojunction Nanodendrite Array Photoanode 2

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Jih-Sheng Yang, and Jih-Jen Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19139 • Publication Date (Web): 04 Jan 2018 Downloaded from http://pubs.acs.org on January 4, 2018

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ACS Applied Materials & Interfaces

Towards Eco-Friendly and Highly Efficient Solar Water Splitting using In2S3/Anatase/Rutile TiO2 Dual-Staggered-Heterojunction Nanodendrite Array Photoanode Jih-Sheng Yang, 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; Hierarchical TiO2 nanodendrite array; Neutral electrolyte; Seawater; Charge separation

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ABSTRACT

The TiO2-based heterojunction nanodendrite (ND) array composed of anatase NPs (ANP) on the surface of rutile ND (RND) array is selected as the model photoanode to demonstrate the strategies towards eco-friendly and efficient solar water splitting using neutral electrolyte and seawater. Compared to the performances in alkaline electrolyte, non-negligible potential drop across the electrolyte as well as impeded charge injection and charge separation are monitored in the ANP/RND array photoanode with neutral electrolyte, which are respectively ascribed to the series resistance of neutral electrolyte, the fundamentally pH-dependent water oxidation mechanism on TiO2 surface, as well as the less band bending at the interface of TiO2 and neutral electrolyte. Accordingly, a TiO2-based dual-staggered heterojunction ND array photoanode is further designed in this work to overcome the issue of less band bending with neutral electrolyte. The improvement of charge separation efficiency is realized by the deposition of a transparent In2S3 layer on the ANP/RND array photoanode for constructing additional staggered heterojunction. Under illumination of AM 1.5G (100 mWcm−2), the improved photocurrent densities acquired both in neutral electrolyte and seawater at 1.23 V vs. RHE, which approach to the theoretical value for rutile TiO2, are demonstrated in the dual-staggered-heterojunction ND array photoanode. Faradaic efficiencies of ~95% and ~32% for solar water oxidation in neutral electrolyte and solar seawater oxidation for 2h are acquired at 1.23 V vs. RHE, respectively.


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Introduction Photoelectrochemical (PEC) water splitting, which harvests energy directly from sunlight to form hydrogen, is a potential strategy for renewable energy production with no reliance on fossil fuels and no emission of greenhouse gases.1,2 A sequence of physicochemical processes occur in the photoelectrodes of PEC cells during solar water splitting, including light harvesting, charge generation, charge separation, charge transport, and charge injection. On the other hand, charge recombination which reduces the numbers of photocarriers in the photoelectrodes takes place along with these processes. The charge separation in photoelectrodes and the redox reaction on the surface must proceed within the lifetimes of photocarriers for successful water splitting. In reality, however, recombination is the main process of photocarriers undergoing, which results in the low efficiency of PEC water splitting. Water oxidation conducted at the photoanode is a crucial reaction for PEC water splitting to provide the necessary electrons for proton reduction. TiO2 has been one of the most studied photoanodes since Honda and Fujishima successfully demonstrated a PEC cell for water splitting using TiO2 photoanode.3 It is mainly due to its nontoxicity, stability, and low cost despite TiO2 only absorbs the UV part of solar irradiation. Using TiO2 as the model photoanode, we have reported the strategies of interfacial energetics and morphology controls for three-dimensional (3D) nanostructured heterojunction array for efficient PEC water splitting.4 The 3D heterojunction photoanode possesses a core-shell structure where the anatase TiO2 nanoparticles (NPs) were formed on the surface of rutile hierarchical TiO2 nanodendrite (ND) array grown on fluorine-doped tin oxide (FTO) substrate. The anatase NPs (ANPs) constructed on the surface of the rutile ND array (RND) enhance charge separation and suppress charge recombination in the photoanode due to the formation of staggered heterojunction of anatase/rutile TiO2. Moreover,


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the configuration of 3D nanostructured array not only decouples the light absorption path and hole transport path but also provides larger interfacial area with electrolyte. Accordingly, efficient hole transport toward the interface for further oxidizing water molecule is achieved in the hierarchical anatase/rutile TiO2 ND array with a high light-harvesting efficiency in the uv region.4 Since TiO2 is chemically stable under basic condition, most previous works on the PEC water splitting using TiO2 photoanodes were conducted in strong alkaline electrolytes5,6 which could facilitate photocatalytic oxidation of water.7–9 The benefits for PEC water oxidation in alkaline solution include the reduction of potential barrier,7 the improvement of the charge transfer from photoanode to electrolyte,8,9 and the inhibition of photocorrosion process for TiO2 photoanode.10 However, along with the strong basic electrolyte, which is hazardous and corrosive, the applications of PEC photoanodes will be significantly restricted. Instead of an alkaline solution, a neutral pH solution as the PEC electrolyte is desired for the promising designs towards ecofriendly solar fuels production, such as the tandem configurations of the photoanodes with the photocathodes and with the photovoltaic cells.11 Moreover, to provide the majority of the energy needs of human society from solar energy in the future, a solar plant has been proposed for largescale solar hydrogen production using the most earth-abundant water resource, i.e., seawater.12 However, the redox reactions occurring in PEC seawater splitting are more complicated because of the existences of Cl‒, Mg2+, and Ca2+ ions in seawater.13,14 Due to its excellent PEC water splitting performance in alkaline electrolyte,4 in this work, the 3D ANP/RND array photoanode was selected as the model photoanode to investigate the key issues for the degraded PEC performance in the neutral electrolyte. In addition to non-negligible voltage drop across the electrolyte, impeded charge separation and charge injection were


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monitored in the ANP/RND array photoanode with the neutral electrolyte. Therefore, a TiO2based dual-staggered heterojunction ND array photoanode was further designed for eco-friendly and efficient solar water splitting using neutral electrolyte and seawater. In2S3 and cobalt phosphate were sequentially deposited on the surface of the ANP/RND array photoanode to improve the charge separation and charge injection efficiencies, respectively. The improved PEC water oxidation in the neutral electrolyte, which approaches the theoretical performance of TiO2 water oxidation, is demonstrated in the dual-staggered In2S3 modified ANP/RND array photoanode. Moreover, the dual-staggered In2S3/ANP/RND array photoanode also exhibits a superior PEC seawater splitting performances in terms of not only the photocurrent density but also the O2 evolution efficiency.

Experimental Section Rutile TiO2 ND array was grown on a seeded FTO substrates using a wet chemical route.4,15 The FTO substrate was immersed in 0.05 M aqueous solution of TiCl4 at 50 °C for 1 h. The seeded layer was thus formed on FTO after a heat treatment at 450 °C for 30 min. A two-batch hydrothermal process was employed to prepare the well-aligned rutile TiO2 NW array on the seeded FTO, which was first conducted in a solution of 40 mL of 8 M HCl in DI water and 0.75 mL of titanium tert-n-butoxide (TnBT) (solution I) at 150 °C for 3.5 h followed by the second batch in a solution of 40 mL of 8 M HCl in saturated NaCl solution and 0.75 mL of TnBT at 150 °C for 18 h. The development of branches from the TiO2 NWs was carried out by the chemical bath deposition of sprout-like nanostructures on the surface of NWs in a solution of 40 mL of 0.25 M HCl and 1 mL of titanium tert-isopropoxide at 95 °C for 4 h followed by the 1.5 h hydrothermal process in solution I at 150 °C for the elongation of sprouts. The ANPs were


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grown on the rutile TiO2 ND array by a solvothermal method using 1 mL of TnBT mixed with 40 mL of 1-butanol solution of 3.5 M acetic acid at 200 °C for 12 h.4 In2S3 NPs were grown on the ANP/TiO2 ND array via a solvothermal method at 160 °C for 1 h in a solution of 34.6 mL of diethylene glycol and 5.4 mL of DI water containing 0.6 g of In(NO3)3 and 1.52 g of thiourea.16 Electrodeposition of the Co-Pi layer on the photoanodes was carried out in a three-electrode cell with a electrolyte composes of 0.5 mM cobalt nitrate in 0.1 M potassium phosphate solution buffered at pH 7. A Pt foil and a saturated calomel electrode (SCE) were employed as the counter and reference electrodes, respectively. The deposition was conducted at 1 V vs. SCE for 300 s. The morphologies of the photoanodes were examined by field-emission scanning electron microscopy (FESEM, JEOL JSM-6700). The elemental distributions of the In2S3 and Co-Pi on the ND array were examined using energy dispersive spectroscopy (EDS, X-maxN, Hobria) equipped on SEM (SU8010, Hitachi). The chemical states and structure of the In2S3 deposit were examined by x-ray photoelectron spectroscopy (XPS, PHI 5000 VersaProbe) and x ray diffractometer (XRD, Rigaku D/MAX-2000), respectively. Structural characterization of the In2S3/ANP/TiO2 nanostructure was conducted using transmission electron microscopy (TEM, FEI E.O Tecnai F20 G2MAT S-TWIN Field Emission Gun Transmission Electron Microscope). Optical absorptions of the photoanodes were acquired using an UV−vis−IR spectrophotometer (JASCO V-670). PEC performances of the photoanodes were characterized under AM 1.5G simulated sunlight at 100 mWcm‒2 (100 W, Model 94011A, Oriel) in a three-electrode system where the photoanodes with a well-defined area of 0.44 cm2, a Pt foil, and a Ag/AgCl electrode were respectively employed as the working, counter, and reference electrodes. Three electrolytes were


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employed in this work. The alkaline and neutral electrolytes were prepared from 1 M KOH solution (pH=13.6) and 0.3 M Na2SO4 with potassium phosphate solution buffered at pH=7.5, respectively. The artificial seawater was prepared by dissolving 33.4 g of red sea salt (Red Sea) in 1 l of water to form a solution containing ca 550 mM of NaCl.17 Hole scavenger-assisted PEC performances of the photoanodes were measured by added 0.3 M sodium sulfite in the electrolytes.18,19 Incident-photon-to-current efficiency (IPCE) spectra of the photoanodes were acquired at 1.23 V vs. reversible hydrogen electrode (RHE) using 500 W xenon light source (Oriel) and a monochromator (Oriel Cornerstone) equipped with Si detector (Model 71640, Oriel). To estimate the amount of O2 evolved during PEC water splitting through the photoanodes, O2 partial pressures in the headspace of the anodic compartment of the three-electrode PEC cell were acquired using an oxygen fluorescence probe (FOXY-R, Ocean Optics).20 The measurements were carried out under AM 1.5G simulated sunlight at 100 mW cm−2 (100 W, Model 94011A, Oriel) by applying a potential of 1.23 V vs. RHE at the photoanodes. The total amount of O2 evolved was estimated from the volume of O2 acquired in the headspace using the ideal gas law.

Results and Discussion Top-view and cross-sectional SEM images of the ANP/RND array on FTO substrate, which was employed as the model photoanode in this work, are shown in Figures 1a and 1b, respectively. The thickness of the ND array is 2 µm. The PEC performances of the ANP/RND array photoanode were examined in a three-electrode electrochemical system under the irradiation of AM 1.5G (100 mWcm‒2) simulated sunlight. The photocurrent density (J)-potential


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(V) curves acquired by linear sweep photovoltammetry measurement individually in alkaline and neutral electrolytes are shown in Figure 1c. The ANP/RND array photoanode demonstrates superior PEC water oxidation performances with the alkaline electrolyte. The onset potential at 0.05 V versus RHE is observed in the J-V curve, indicating a very low overpotential of the ANP/RND array photoanode for water oxidation with the alkaline electrolyte. Moreover, a steep increase in photocurrent with respect to potential is monitored in the J-V curve of the ANP/RND array photoanode with the alkaline electrolyte. A photocurrent density of 1.45 mAcm‒2 is swiftly attained at 0.3 V vs. RHE. At 1.23 V vs. RHE, the photocurrent density reaches 1.7 mAcm‒2 which is approximately close to the theoretical value of 1.8 mAcm‒2 for rutile TiO2 (denoted by

Figure 1. (a) Top-view and (b) cross-sectional SEM images of ANP/RND array. (c) J-V curves of ANP/RND photoanodes measured in alkaline and neutral electrolytes with and without Na2SO3. (d) Charge separation efficiencies and (e) charge injection efficiencies of ANP/RND photoanodes with alkaline and neutral electrolytes.


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the dash line in the figure). However, in comparison with the one measured in the alkaline electrolyte, an anodic-shift onset potential to 0.26 V vs. RHE and a gently increased photocurrent density to 1.33 mAcm‒2 at 1.23 V vs. RHE are shown in the J-V curve of the ANP/RND array photoanode obtained in neutral electrolyte. This photocurrent density is rather high even in comparison with those of nanostructured TiO2 photoanodes acquired in the alkaline electrolytes.6,21,22 Nevertheless, the result of inferior PEC water oxidation performance of the identical TiO2 photoanode obtained in neutral electrolyte compared to that in alkaline electrolyte is consistent with what previously reported in the literature.23,24 For comparison, the J-V curves of the model photoanode obtained in these two electrolytes with the addition of Na2SO3 as hole scavenger are also displayed in Figure 1c. With the addition of the hole scavenger, almost identical J-V curve is acquired in the alkaline electrolyte except a slightly cathodic shift of the onset potential to -0.07 V vs. RHE. In the case of the neutral electrolyte, however, a cathodic-shift onset potential and significant increase in photocurrent densities are observed in the J-V curve of the ANP/RND array photoanode as adding the hole scavenger in the electrolyte. The PEC current density is determined by three factors of light harvesting, charge separation and charge injection efficiencies.25 It is worth to compare the J-V curves of the ANP/RND array photoanode measured as the hole scavengers are added in both electrolytes, in which 100% hole injection efficiency are expected. With identical light harvesting efficiency of the photoanode in both circumstances, the lower saturation photocurrent density indicates that charge separation in the photoanode is essentially impeded as the PEC water oxidation is conducted in the neutral electrolyte. On the other hand, the rather low photocurrent densities are acquired at potentials of 0-0.3 V vs RHE in the absence of the hole


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injection resistance, suggesting that the series resistance of the neutral electrolyte contributes to a potential loss of the PEC water splitting.26 The charge separation and injection efficiencies of the ANP/RND array photoanode in alkaline and neutral electrolytes are respectively estimated from the ratio of the photocurrent density measured with hole scavenger to the photon absorption rate as well as the ratio of the photocurrent densities measured without and with hole scavenger, as shown in Figures 1d and 1e, which are based on the assumption of a 100% injection efficiency of the photoanode with the additions of hole scavengers in both electrolytes.18,19 In the alkaline electrolyte, over 85% charge separation efficiency and nearly 100% charge injection efficiency of the photoanode are attained at the potentials larger than 0.3 V vs. RHE. On the other hand, inferior charge separation and inject efficiencies of the photoanode with the neutral electrolyte are observed when the PEC water oxidation is carried out at low potentials. Although the charge separation and inject efficiencies of the photoanode respectively reach 82% and 90% in the neutral electrolyte at 1.23 V vs. RHE, they are still lower than those near 100% achieved in the alkaline electrolyte. Hole-scavenger-assisted PEC measurements indicate that both charge separation and injection efficiencies of the ANP/RND array photoanode are degraded in the neutral electrolyte compared to those in the alkaline electrolyte. The poorer charge injection efficiency in the neutral electrolyte, as shown in Figure 1e, can be well explained by the pH-dependent water oxidation mechanisms.8,9 Biswas et al.8 proposed a pH-dependent hole-trapping mechanism to explicate the fact of rapider oxygen evolution at higher pH, which was based on the main absorbed species of hydroxide ions and water molecules on TiO2 surface for oxygen evolution in alkaline and neutral electrolytes, respectively. They suggested that hydroxide ions trap holes more efficient than water molecules due to the electrostatic interaction. On the other hand, the water


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photooxidation mechanism on TiO2 proposed by Nosaka et al.9 describes that the O2 evolution in alkaline solution is more favorable due to the suppression of OH radical formation. It also reflects the fact of more efficient hole injection of the TiO2 photoanode to alkaline electrolyte. That is, the charge injection efficiency of the TiO2 photoanode with the neutral electrolyte is essentially inferior due to the fundamental water oxidation mechanism. Sato et al.7 investigated the dependence of the band bending of TiO2 electrode on pH by measuring the pH dependencies of the flatband potentials, the rest photopotentials, and the rest dark potentials. The rest dark potentials of TiO2 electrode were almost constant with pH whereas the flatband potential showed the 60 mV/pH dependence.7 The results indicated that the band bending of TiO2 increases with an increase in pH of electrolyte under the rest dark potential state.7 It was concluded that the enhancement of the PEC water splitting on TiO2 electrode with alkaline electrolyte is attributed to the large band bending at interface of TiO2 and electrolyte.7 Accordingly, the inferior charge separation efficiency of the ANP/RND array photoanode in the neutral electrolyte compared to that in the alkaline solution, as shown in Figure 1d, is attributed to the less band bending established at interface of TiO2 and the neutral electrolyte. As aforementioned, the relatively poor charge injection efficiency of the ANP/RND array with the neutral electrolyte is an inherent issue due to the fundamental water oxidation mechanism on TiO2 surface. Nevertheless, the PEC water oxidation performance of the ANP/RND array with the neutral electrolyte is improvable by challenging the drawback of the inferior charge separation which is corresponding to the less band bending at the TiO2-neutral electrolyte interface, as illustrated in Figure 2a. To further enhance the charge separation of the ANP/RND array photoanode in the neutral electrolyte, in this work, In2S3 was deposited on the surface of this photoanode for the construction of the additional staggered heterojunction, as illustrated in


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Figure 2. (a) Schematic band diagrams of identical photoanode in alkaline and neutral electrolytes. (b) Schematic of In2S3/ANP/RND array photoanode.

Figure 3. (a) Top-view and (b) cross-sectional SEM images of In2S3/ANP/RND array. c) HRTEM image of In2S3/ANP/RND nanostructure denoted in the inset.


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Figure 2b. It should be addressed that in order to evidently verify the strategy of interfacial energetic control for enlarging the band bending in the neutral electrolyte, the sole function of the In2S3 deposits is for forming staggered heterojunction with TiO2, not for harvesting extra visible light. Therefore, the deposition of In2S3 layer should be well-controlled to keep identical light harvesting efficiencies of the ANP/RND array photoanodes with and without the In2S3 deposition. The growth of In2S3 on the ANP/RND array was conducted using a solvothermal method, which is optimized for enhancing the PEC performance of ANP/RND array photoanode without contributing to additional visible light harvesting. Figures 3a and 3b show the top-view and cross-sectional SEM images of the optimized In2S3/ANP/RND array, respectively. They reveal that the 3D morphology of the photoanode was preserved after the In2S3 deposition. The elemental distributions of the In2S3/ANP/RND array were examined using EDS. As shown in Figure S1, In and S are well-distributed throughout the surface of the ND array. Successful growth of a thick β-In2S3 film on the FTO substrate using the same solvothermal process was examined by x-ray diffraction as shown in Figure S2. However, the diffraction peak of In2S3 is not present in the XRD pattern of In2S3/ANP/RND array since a thin In2S3 layer was intentionally deposited on the surface of ANP/RND array. Formation of indium sulfide on the surface of ANP/RND array was confirmed by x-ray photoelectron spectroscopy analyses, as shown in Figure S3. Structural characterizations of the In2S3/ANP/RND array were further conducted using TEM. A TEM image of the top portion of the array photoanode is shown in the inset of Figure 3c. It demonstrates a core-shell structure of the trunk. Figure 3c shows the highresolution (HR) image of the trunk tip denoted in the inset. The single-crystalline rutile TiO2 trunk with the growth direction of [001] is shown in this figure. The trunk is shielded by NPs


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which lattice spacing of 0.352 nm is coherent with that of (101) anatase TiO2. The outer layer covering the hierarchical TiO2 nanostructure is composed of the NPs with a lattice spacing of 0.328 nm, as shown in Figure 3c, which is consistent with the d spacing of (110) crystal planes of rhombohedral β-In2S3. On the other hand, TEM characterizations reveal that the conformality of In2S3 layer on the hierarchical TiO2 nanostructure is not perfect. The bottom portion of the ANP/rutile nanostructured array is not well shielded by the In2S3 NPs. Nevertheless, the PEC water oxidation performance of the In2S3/ANP/RND array is superior to that of the ANP/RND array photoanode and the details of which will be discussed later. Figure 4a shows the absorption spectra (100%−total reflectance (R%)−total transmittance (T%)) of the ANP/RND arrays with and without In2S3 deposition. Almost identical light harvesting efficiencies of the two photoanodes are exhibited. The absorption of the In2S3/ANP/RND array in the wavelengths longer than 420 nm is negligible. The J-V curve of the In2S3/ANP/RND array photoanode acquired in the neutral electrolyte is shown in Figure 4b. For comparison, the J-V curve of the ANP/RND array photoanode is reploted in this figure. With the same light harvesting ability, the photocurrent densities are significant improved with the deposition of In2S3 on the photoanode whereas the onset potentials of both electrodes are identical in the neutral electrolyte. The photocurrent density of the In2S3/ANP/RND array photoanode reaches 1.53 mAcm‒2 at 1.23 V vs. RHE, which is approaching to that of ANP/RND array photoanode obtained at 1.23 V vs. RHE in the alkaline electrolyte. The IPCE spectra of the ANP/RND arrays with and without In2S3 deposition in the wavelengths ranging from 300 to 460 nm, which are obtained at 1.23 V vs. RHE in the neutral electrolyte, are shown in Figure 4c. The thresholds of IPCE spectra of the two photoanodes are at the same wavelength, confirming


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Figure 4. (a) Absorption spectra, (b) J-V curves, (c) IPCE spectra, and (d) photocurrent transient responses of ANP/RND array and In2S3/ANP/RND array photoanodes acquired in neutral electrolyte.

that the deposition of In2S3 doesn’t attribute to additional visible light harvesting. Compare to the ANP/RND array photoanode, the higher IPCE values of the In2S3/ANP/RND array photoanode acquired in the uv region are consistent with the increase in photocurrent densities. The photocurrent transient behaviors of the ANP/RND array photoanodes with and without In2S3 deposition are shown in Figure 4d. A slight photocurrent density decay, as abruptly switching on the irradiation of AM 1.5G simulated sunlight, was monitored in the J-t curve of the ANP/RND array photoanode. The absence of photocurrent density decay was obtained in the J-t curve of the In2S3/ANP/RND array photoanode, indicating negligible surface recombination occurring in the


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In2S3-modified photoanode.27 It could be due to the fast charge separation and less surface trap states of the photoanode as the In2S3 is deposited on the surface. The hole-scavenger-assisted PEC measurements of the In2S3/ANP/RND array photoanode in the neutral electrolyte were further carried out in this work. The results are shown in Figure S4. The charge separation and injection efficiencies of the In2S3/ANP/RND array photoanode with the neutral electrolyte, which are estimated from hole-scavenger assisted PEC measurements, are shown in Figures 5a and 5b, respectively. Those of ANP/RND array photoanode are also reploted here for comparison. Figure 5a reveals that the charge separation efficiencies of the In2S3-modified photoanode are superior to those of the ANP/RND array photoanode. The efficiency is improved from 82% to 94% at 1.23 V vs. RHE in the neutral electrolyte. This result confirms the formation of dual staggered heterojunctions in the In2S3/ANP/RND array photoanode. On the other hand, the charge injection efficiencies are not significantly changed with the deposition of In2S3 on the surface of ANP/RND array photoanode, as shown in Figure 5b. Moreover, similar to the ANP/RND array photoanode, the injection efficiency of the In2S3/ANP/RND array photoanode is not monotonically increased with the applied potential. Inflection points at 0.6 V vs. RHE are obtained in both curves of Figure 5b, which is corresponding to the transitional current observed in their J-V curves as shown in Figure 4b. Interestingly, the transitional current is absent in the J-V curves with the addition of hole scavenger in the electrolyte, as shown in Figure S3. Since the TiO2 surface is not well covered by In2S3 in the In2S3/ANP/RND array photoanode, the transitional current which is absent as adding hole scavenger may be ascribed to the deprotonation of adsorbed water (Ti-OH2+) to Ti-OH on the TiO2 surface.9,28 It is reasonable to suggest that the transitional current can be suppressed by further improving the hole injection efficiency of the photoanode through the modified the TiO2


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Figure 5. (a) Charge separation efficiency and (b) charge injection efficiency of ANP/RND array and In2S3/ANP/RND array photoanodes acquired in neutral electrolyte.

surface. Therefore, the In2S3/ANP/RND array photoanode was further modified with co-catalyst cobalt phosphate (Co-Pi). Details in the structural characterizations and PEC performances of the Co-Pi/In2S3/ANP/RND array are described in Supporting Information (Figures S5-S9). As shown in Figure S8, significant increase in photocurrent density at ~0.6 V vs. RHE is observed in the J-V curve of the Co-Pi modified photoanode compared to the In2S3/ANP/RND array photoanode. It confirms that the deprotonation of Ti-OH2+, which results in the transitional current, can be suppressed by modifying the TiO2 surface. In this work, the PEC seawater splitting performances of the dual-straggered-heterojunction In2S3/ANP/RND array photoanode were also examined using the seawater prepared from red sea salt.17 The J-V curves of the ANP/RND array and In2S3/ANP/RND array photoanodes with seawater are shown in Figure 6. The onset potentials of the two photoanodes are both at 0.36 V vs RHE. Nevertheless, similar to those obtained in the neutral electrolyte, an increase in photocurrent density is achieved by using the dual-straggered-heterojunction In2S3/ANP/RND


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Figure 6. J-V curves of ANP/RND array and In2S3/ANP/RND array photoanodes measured in seawater.

array photoanode. Photocurrent densities of 1.42 mAcm‒2 and 1.57 mAcm‒2 were obtained in the ANP/RND array and In2S3/ANP/RND array photoanodes at 1.23 V vs. RHE, respectively. The results show that the PEC seawater splitting performance of the ANP/RND array photoanode is improved also by the enhancement of charge separation through forming additional staggered heterojunction on the surface. The photoresponses of the In2S3/ANP/RND array photoanode with the neutral electrolyte and with seawater as functions of time were measured at 1.23 V vs RHE under the irradiation of AM 1.5G simulated sunlight. As illustrated in Figure S10, constant current densities without decay are acquired for 4h, showing the high stability of the In2S3/ANP/RND array photoanode in the neutral electrolyte and seawater during PEC oxidation. Morphology and structural characterizations of the photoanode after PEC oxidation were also conducted in this work. The SEM images in Figure S11 reveal that the 3D morphology of the photoanode was conserved after PEC measurement for 4h. Moreover, XPS analyses show that


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Figure 7. O2 evolution of In2S3/ANP/RND photoanodes in neutral electrolyte and seawater operated at applied bias of 0.75 V vs. RHE in dark in first 30 min (control experiment), followed by 120 min water splitting under AM 1.5G (100 mW cm‒2) at applied voltage of 1.23 V vs. RHE.

the In2S3 layer on the ANP/RND array possesses the same chemical state before and after PEC oxidation as shown in Figures S3 and S12. The results indicate the superior structural stability of the In2S3/ANP/RND array photoanode. The oxygen evolutions during PEC water splitting in neutral electrolyte and during PEC seawater splitting through the In2S3/ANP/RND array photoanode were further monitored at 1.23 V vs. RHE under AM 1.5G (100 mWcm‒2) using oxygen fluorescence probe.20 In the case of PEC water splitting in neutral electrolyte, as shown in Figure 7, the O2 amount linearly increased during 2h irradiation, which is close to the theoretical amount of O2 with 100% Faradaic efficiency. Faradaic efficiencies are larger than 90% during 2h measurement, as illustrated in Figure 7, indicating the superior stability and high efficiency of the In2S3/ANP/RND array


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photoanode in the neutral electrolyte for PEC water oxidation. On the other hand, the O2 amount obtained through the In2S3/ANP/RND array photoanode with seawater is significantly negative departure from the amount of O2 with 100% Faradaic efficiency estimated from the photocurrent density, as displayed in Figure 7. Faradaic efficiency decreases from 80% to 32% during 2h irradiation. Instead of O2 formation, the main photoanode reaction in seawater splitting is the formation of Cl2 because the presence of Cl‒ ions in seawater and its lower overpotential compared to O2 formation.13 Therefore, the O2 amount obtained during PEC seawater splitting is much lower than that acquired in the neutral electrolyte although the stability and photocurrent densities of the In2S3/ANP/RND array photoanode measured in the neutral electrolyte and in seawater at 1.23 V vs. RHE are comparable, as shown in Figure S10. Nevertheless, Faradaic efficiency of the seawater oxidation using In2S3/ANP/RND array photoanode is ~32% after 2h irradiation, which is higher than the yield of 20% obtained from the modified BiVO4 photoanode.13

Conclusions The ANP/RND heterojunction array, with excellent PEC water splitting performance in alkaline electrolyte, was selected as the model photoanode in this work to investigate the key issues for the degraded PEC performance in the neutral electrolyte. Apart from a potential loss of PEC water splitting due to the series resistance of neutral electrolyte, poor charge separation and charge injection efficiencies of the ANP/RND array photoanode were acquired in neutral electrolyte. The charge injection efficiency is restricted from the fundamentally pH-dependent water oxidation mechanism, however, charge separation efficiency is improvable by challenging the drawback of less band bending at the interface of TiO2 and neutral electrolyte. Accordingly, a


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thin In2S3 layer was further deposited on the surface of the photoanode, which sole function is for forming the additional staggered heterojunction with the hierarchical TiO2 ND array, not for harvesting extra visible light. A photocurrent density of 1.53 mA cm−2 at 1.23 V vs. RHE was attained using the dual-staggered-heterojunction In2S3/ANP/RND array photoanode with neutral electrolyte under illumination of AM 1.5 G (100 mWcm−2). The PEC seawater splitting performance of the In2S3/ANP/RND array photoanode was also examined in this work, which exhibited a photocurrent density of 1.57 mA cm−2 at 1.23 V vs. RHE. Through the strategy of interfacial energetic control, superior photocurrent densities of PEC water splitting in neutral electrolyte and PEC seawater splitting, which approach to the theoretical value of 1.8 mA cm−2 for rutile TiO2, are demonstrated in the TiO2-based dual-staggered-heterojunction ND array photoanode. Moreover, Faradaic efficiencies of ~95% and ~32% for solar water oxidation in neutral electrolyte and solar seawater oxidation for 2h were acquired through the TiO2-based dual-staggered-heterojunction ND array photoanode at 1.23 V vs. RHE under AM 1.5G (100 mWcm−2) irradiation, respectively. ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Elemental SEM-EDS mapping images for In2S3/ANP/RND and Co-Pi/In2S3/ANP/RND arrays, SEM image and XRD pattern of In2S3/FTO, XRD patterns of ANP/RND, In2S3/ANP/RND and Co-Pi/In2S3/ANP/RND arrays as well as FTO, XPS spectra of In2S3/ANP/RND and CoPi/In2S3/ANP/RND arrays, hole-scavenger-assisted PEC performances of In2S3/ANP/RND and ANP/RND photoanodes in neutral electrolyte, morphologies, light absorption, and PEC


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performance of Co-Pi/In2S3/ANP/RND array, photoresponses of the In2S3/ANP/RND array photoanode with neutral electrolyte and with seawater, photoresponses of the CoPi/In2S3/ANP/RND array photoanode with neutral electrolyte, SEM images of In2S3/ANP/RND array and Co-Pi/In2S3/ANP/RND array photoanodes after PEC oxidation, XRD pattern and XPS spectra of In2S3/ANP/RND array photoanode after PEC oxidation. (PDF)

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] 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. We thank Professor Chia-Yu Lin for technique support.

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Table of Contents Graphic

A TiO2-based dual-staggered-heterojunction nanodendrite array photoanode is demonstrated towards eco-friendly and efficient solar water splitting using neutral electrolyte and seawater. Photocurrent densities of the photoanode measured in neutral electrolyte and seawater at 1.23 VRHE approach to the theoretical value for rutile TiO2. Faradaic efficiency of ~95% is acquired for solar water oxidation in neutral electrolyte.


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Toward Eco-Friendly and Highly Efficient Solar Water Splitting Using

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