Precisely-controlled, a few layers of iron titanate inverse opal structure for enhanced photoelectrochemical water splitting

Accepted Manuscript Precisely-controlled, a few layers of iron titanate inverse opal structure for enhanced photoelectrochemical water splitting Hemin Zhang, Sung O. Park, Se Hun Joo, Jin Hyun Kim, Sang Kyu Kwak, Jae Sung Lee PII:

S2211-2855(19)30431-8

DOI:

https://doi.org/10.1016/j.nanoen.2019.05.025

Reference:

NANOEN 3739

To appear in:

Nano Energy

Received Date: 18 January 2019 Revised Date:

8 May 2019

Accepted Date: 8 May 2019

Please cite this article as: H. Zhang, S.O. Park, S.H. Joo, J.H. Kim, S.K. Kwak, J.S. Lee, Preciselycontrolled, a few layers of iron titanate inverse opal structure for enhanced photoelectrochemical water splitting, Nano Energy (2019), doi: https://doi.org/10.1016/j.nanoen.2019.05.025. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Precisely-controlled, a few layers of iron titanate inverse opal structure for

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enhanced photoelectrochemical water splitting

Hemin Zhang, Sung O Park, Se Hun Joo, Jin Hyun Kim, Sang Kyu Kwak* and Jae Sung

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Lee*

Department of Energy Engineering, School of Energy and Chemical Engineering, Ulsan

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National Institute of Science & Technology (UNIST), Ulsan, 44919 Republic of Korea.

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E-mail: [email protected]; [email protected]

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ABSTRACT Iron titanate (Fe2TiO5) is a promising photoanode material due to a narrow band gap, appropriate band edges, robustness and abundance. However, its performance is limited

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because of its low conductivity and short hole diffusion length. Precisely controlled, a few Fe2TiO5 layers of inverse opal structure (IOS) is fabricated via a layer-by-layer selfassembly and then treated by hybrid microwave annealing to produce a highly crystalline,

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yet IOS morphology-preserved Fe2TiO5 photoanode film for solar water splitting. The highly transparent Fe2TiO5 IOS film shows a greatly enhanced visible light harvesting,

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higher density of catalytically more active crystal planes, and many single crystalline nanoplates grown on the IOS architecture, relative to a reference planar film prepared under similar conditions. As a result, the optimized ‘exactly’ three Fe2TiO5 layers IOS electrode with a sacrificial gallium oxide underlayer and a ternary (Ni2CoFe)OOH co-

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catalyst records 2.08 mA cm−2 at 1.23 VRHE under 1 sun (100 mW cm−2) irradiation, which is the highest photocurrent density produced by Fe2TiO5 photoanode up to date. 1. Introduction

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Photoelectrochemical (PEC) water splitting is of great practical interest for the renewable solar energy conversion to clean and storable hydrogen fuel. The high solar-to-hydrogen

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(STH) conversion efficiency is the most critical requirement to make the technology viable as it directly impacts the land area to be covered with the facility and capital investment/cost of the system [1-2]. A photoelectrode (PE)-photovoltaic (PV) tandem device with two light absorbers could achieve high efficiencies of unassisted solar water splitting, if they have complementary band gaps, connected in series, and arranged in an optical stack configuration [3]. Lewis and coworkers reported detailed balance

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calculations for the Shockley˗Queisser limit of each absorber [4]. They found that the maximum STH efficiency for a water splitting tandem cell with a photoanode (Eg = 1.60 eV)/photocathode (Eg = 0.95 eV) could be up to 29.7 %. It is crucial that the top absorber

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should be responsive to high-energy photons but also transparent enough to allow lowerenergy photons to illuminate the bottom absorber. It is a great synthetic challenge to fabricate photoelectrodes of good transparency as well as high efficiency, because too

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thin a film is beneficial for transparency, but not effective for enough light attenuation.

Nearly all simple binary metal oxide semiconductors of photocatalytic properties have

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been studied as a photoelectrode for water splitting, but only a few are attractive that could give photocurrents larger than 3 mA cm−2 at 1.23 VRHE under 1 sun (100 mW cm−2) irradiation such as Fe2O3, WO3, and Cu2O [5-7]. As a complex metal oxide, BiVO4 has achieved a remarkable success producing high photocurrents of over 5 mA cm−2 or STH

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efficiency higher than 5 % as demonstrated by several research groups [8-10]. However, BiVO4 has an intrinsic limitation of a relatively large band gap (indirect 2.4−2.5 eV, and direct ~2.6 eV) that allows a small theoretical STH efficiency of only ~9 % [11]. There

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are more than 8,000 and 700,000 possible combinations for ternary and quaternary metal oxides, which provide numerous opportunities to find an ideal photocatalytic material.

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One promising candidate is pseudobrookite iron titanate (Fe2TiO5), which has an orthorhombic structure, with the cations located in two different octahedral sites (Fig. S1 of Supplementary Information, SI). Interestingly, Fe2TiO5 is an exact intermediate of stoichiometric Fe2O3 and TiO2, which inherits the well-adapted band gap from Fe2O3 and the good electronic properties from TiO2 [12]. As an n-type semiconductor, Fe2TiO5 has a narrow band gap of ~2.1 eV, enabling a wide absorption of visible light [13]. Besides

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good stability and earth-abundance, it has distinctive band edges straddling the water redox potentials, holding more negative conduction and valence band edges compared to Fe2O3 [14], which enables it to be widely used to form heterojunctions with Fe2O3 [15-17]

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and TiO2 [13, 18-19] in the past few years. However, only a few studies reported Fe2TiO5 as a photoanode material by itself [20-22] probably because of its low conductivity and short hole diffusion length [21].

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The nanostructured electrode design plays a prominent role in addressing poor charge transport and slow reaction kinetics for photocatalytic semiconductor materials. As such a

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nanostructure, an ordered inverse opal structure (IOS) contains two pore structures involving continuous periodic macroporous architectures and mesoporous voids on the wall of IOS [23]. Coincidently, the macropores are advantageous for charge transport through its long range-ordered and interconnected architectures, while the mesopores can

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further enlarge the interfacial contact of electrode/electrolyte and electrode/current collector [24]. Especially, IOS could provide additional benefits for solar light harvesting by diffuse scattering and coherent multiple internal scattering [25]. Usually, IOS is

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fabricated by a vertical deposition method (Fig. S2) widely in the field of energy conversion and storage [26-27]. However, the uniformity and thickness cannot be

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controlled on a large scale, thereby limiting its applications. Here we report a new synthetic route to prepare precisely-controlled, a few layers of

IOS on a large scale via a layer-by-layer self-assembly. Compared with a usual planar film as a reference, IOS Fe2TiO5 electrode greatly improves visible light absorption that extends wavelength up to 650 nm. Highly crystalline Fe2TiO5 with the IOS morphology well-preserved is synthesized by hybrid microwave annealing (HMA) of the IOS

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precursor. An optimized, highly transparent photoanode made of ‘exactly’ three Fe2TiO5 layers, a gallium oxide as a sacrificial underlayer and a ternary (Ni2CoFe)OOH cocatalyst records a photocurrent of 2.08 mA cm−2 at 1.23 VRHE under 1 sun irradiation,

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which represents the highest value recorded by Fe2TiO5 photoanodes reported so far. In addition, the photocurrent onset potential shifts cathodically by ~150 mV relative to

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Fe2TiO5 photoanode without additional modifications.

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2. Experimental section 2.1. Fabrication of PS monolayer on FTO

The desired layers of Fe2TiO5 IOS on FTO was fabricated by spin-coating and layer-bylayer self-assembly route with PS as sacrificial templates modified from that introduced previously [28]. In brief, a conductive FTO (25 mm×50 mm) was ultrasonically cleaned

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by cleaning agent (deconex® 11 UNIVERSAL), ethanol, and acetone and then dried with N2 gas. After this thorough cleaning, the surface of the FTO becomes sufficiently hydrophilic. Monodispersed PS suspension (500 nm in diameter, 2.5 % in water) was

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obtained from Polyscience, Inc. The suspension was mixed with equal volume of ethanol

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and ultrasonicated for absolute uniformity. Then, a hydrophilic FTO was placed on a table with 3o tilted and deionized water was dropped on it to make a water film covering the entire surface of the FTO. The ethanol-diluted mixed PS suspension was taken with a micropipet and slowly injected into water at the interface of water and FTO surface. Most of water below the self-assembly monolayer PS was drawn out slowly by a syringe with a thin needle in order to reduce the time of evaporation. Finally, a large-area monolayer PS on FTO was obtained. 5

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2.2. Preparation of Fe2TiO5 precusor solution The Fe2TiO5 precursor sulution for spin-coating was produced as reported previously.[27]

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Briefly, a solution of 0.8 M Fe(NO3)3·9H2O (Sigma-Aldrich 99.8 %) was added dropwise into the solution of 0.4 M titanium(IV) propoxide (Aldrich 97 %) with equal volume, then the deionized water as a catalyst was dropped into the mixed solution (volume ratio, 10:0.8) to increase the rate of reaction, and kept stirring for 4 h. A long time aging (>36 h)

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at 60 oC is an effective way to obtain a homogeneous Fe2TiO5 precursor solution without

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any precipitation.

For hybrid microwave annealing (HMA), the Fe2TiO5 IOS on FTO was treated in a laboratory microwave oven (2.45 GHz, 1000 W) for 2 min in a Pyrex beaker filled with graphite powder as a susceptor.

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2.3. Modifications of Fe2TiO5 IOS

To obtain a homogeneous Ga2O3 underlayer, the clean FTO substrates were immersed into a solution containing 0.02 M gallium nitrate and 1 M urea, then kept for different

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times (10−40 min) at 75 oC. Finally, the FTO with amorphous Ga2O3 layer was treated in a furnace at 450 oC for 2 h. To load a ternary cocatalyst, a 2-methoxyethanol soution

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containing nickel, cobalt and iron nitrates (with optimized 2:1:1 atomic ratio) was dropped on the surface of the prepared photoanodes and dried in the air naturally, then kept for 5 min in a 60 oC oven. Finally, the photoanode was dipped into NH4OH solution (28−30 %) for several minitues. 2.4. Physical characterization

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The morphology and corresponding energy-dispersive X-ray spectroscopy (EDX) of the samples were obtained using a field-emission scanning electron microscope (FESEMS4800, HITACHI, operated at 5 and 15 keV for morphology and EDX, respectively).

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Detailed microscopic structure and corresponding EDX data were observed using Cscorrected high-resolution scanning transmission electron microscope (JEOL, JEM-2100F, 200 kV). X-ray diffraction (XRD) spectra were measured by PW3040/60 X’per PRO,

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PANalytical, using Cu-Kα (λ= 1.54056 Å) radiation, an accelerating voltage of 40 kV and an emission current of 30 mA. Ultraviolet-visible absorbance was carried out with a

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UV-Vis spectrometer (UV-2401PC, Shimadzu). The surface atomic compositions and depth profiling characteristics were conducted by X-ray photoelectron spectroscopy (XPS, Thermo-Fisher, Kα). Time-of-flight secondary ion mass spectrometry (TOF-SIMS) was conducted on a TOF-SIMS V instrument (ION-TOF GmbH, Germany) using a 10 keV

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Bi+ primary ion beam source. Inductively coupled plasma optical emission spectrometry (ICP-OES) was measured on a Varian machine (700-ES). 2.5. Measurements of PEC performance

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All the PEC measurements were conducted on a potentiostat (IviumStat, Ivium

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Technologies) in a three electrode cell with Fe2TiO5 photoanode, Ag/AgCl (3 M NaCl), and Pt mesh as working, reference, and counter electrode, respectively, in 1 M NaOH electrolyte under the one-sun condition (100 mW cm−2) by a solar simulator (91160, Oriel) with an AM 1.5 G filter. The measured potentials vs. Ag/AgCl reference electrode were converted to the potentials vs. reversible hydrogen electrode (RHE) according to the Nernst equation: ERHE = EAg/AgCl + 0.059 pH +

/

(

/

= 0.1976 at 25 oC). The

potential was swept in the range of 0.4−1.8 VRHE with a sweep rate of 20 mV s−1. The 7

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EIS was carried out at 1.23 VRHE and the open˗circuit voltage under simulated one-sun condition with a frequency range of 0.1 Hz−100 kHz. The IPCE was carried out using a Xe lamp (300 W, Oriel) and a monochromator with a bandwidth of 5 nm at 1.23 VRHE in

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the same electrolyte. The Mott-Schottky plot was measured by sweeping 0.1−1.0 VRHE range with AC frequency of 1000 Hz under the dark condition. The Faraday efficiency

chromatography in a closed circulation system.

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2.6. DFT calculation

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was calculated from the measurements of H2 and O2 evolution by a HP 6890 gas

Spin-polarized density functional theory (DFT) calculations were performed using the CASTEP program [29-30]. The generalized gradient approximation (GGA) with PerdewBurke-Ernzerhof (PBE) functional was used for the exchange-correlation potential of electrons [31]. The norm-conserving pseudopotential was employed to describe electron-

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ion interactions. The 3s, 3p, 3d, and 4s states of Fe and Ti, and 2s, and 2p states of O were treated as valence states. The Hubbard U correction was applied to the 3d electrons of Fe. The value of Ueff was set to 3.3 eV, which provided the calculated band gap of 2.0

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eV comparable to the experimental value of 2.0-2.1 eV. The van der Waals interactions

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were corrected using Grimme’s D2 method [32]. The Brillouin zone was sampled using k-point sets with Monkhorst-Pack scheme [33]: 6 × 6 × 2 for bulk, 2 × 5 × 1 for (200) plane, 5 × 2 × 1 for (220) plane, and 5 × 1 × 1 for (230) plane, respectively. The wave functions were expanded using a plane-wave basis set with an energy cutoff of 900 eV. The self-consistent electronic minimization was performed with smearing of 0.1 eV, until the convergence criterion of 1.0 × 10-6 eV/atom was satisfied. The convergence criteria for geometry optimization were set to 1.0 × 10-5 eV/atom for the maximum energy 8

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change, 0.03 eV/Å for the maximum force, 0.05 GPa for the maximum stress, and 0.001 Å for the maximum displacement. To investigate the atomic structures and electronic properties of (200), (220), and (230)

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crystal planes of Fe2TiO5, we constructed symmetric and stoichiometric surface slab models using the optimized orthorhombic unit cell of Fe2TiO5 with the space group Bbmm (bulk Fe2TiO5 structures in Fig. S15). For each crystal plane, the two different

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surface terminations could be generated. Periodic boundary conditions were applied along two dimensions (i.e., in-plane directions), while a vacuum region of 20 Å was

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added perpendicular to the surface to avoid undesirable self-interactions of slabs. The surface energy (γ) for each model was calculated using the equation, −

γ = (1/2 ) ∙ (

)

(1)

where A, Eslab, n, and Ebulk are the surface area of the surface unit cell, the energy of the

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Fe2TiO5 slab, the number of the bulk Fe2TiO5 unit in the slab, and the energy of the bulk Fe2TiO5 unit cell, respectively. According to the results of the calculated surface energy (Table S1), the thermodynamically stable slab model was adopted for the analysis of the

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atomic structures and electronic properties for each plane.

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3. Results and discussion

3.1. Fabrication of Fe2TiO5 film on FTO with precisely-controlled number of layers An IOS film with a precisely-controlled number of Fe2TiO5 layers was fabricated using a unique layer-by-layer self-assembly route depicted in Fig. 1. First, a large scale polystyrene (PS) monolayer on FTO could be obtained by self-assembly at the interface of solid/liquid/air on slightly tilted (~30) FTO surface as depicted in Fig. S3. The formed 9

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large area PS monolayer on FTO is shown in Fig. S4, which exhibits three different colors under flash light reflecting its periodic architecture. This PS monolayer is briefly heated at 110 oC for 30 s in order to increase the structural stability and make PS adhere

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to the surface of FTO [34]. Subsequently, the Fe2TiO5 precursor solution was spin-coated and heated at 100 oC for 8 min to completely evaporate the solvent (see Fig. S5 for a PS monolayer on FTO before and after the spin-coating). The PS layer is removed by

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burning at 450 oC for 2 h to obtain an amorphous monolayer Fe2TiO5 IOS, which is

according to path 1 of Fig. 1.

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subjected to HMA to obtain a highly crystalline monolayer Fe2TiO5 IOS (Fig. S6)

To obtain a two-layer film (path 2), self-assembly of the second PS monolayer is made on the precursor-coated first monolayer that has been treated with oxygen plasma (1−2 min) to make a hydrophilic surface. Then second spin-coating and heat treatment (110 oC,

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8 min) are carried out. Finally, Fe2TiO5 IOS of two layers is acquired by furnace annealing at 450 oC for 2 h, followed by HMA. Repeating this cycle provides additional layers of Fe2TiO5 IOS. It should be noted that

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the heating step of 110 oC for 30 s also leads to the deformation of neighboring PS beads and interconnection between them. After Fe2TiO5 precursor infiltration and removal of

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PS template, these interconnected parts generate window pores on the wall of final IOS. The power of the layer-by-layer self-assembly strategy in tuning the architecture of the photoelectrode film is demonstrated in Fig. S7-S8. Thus, the diameter of window pores is tuned easily by controlling the degree of PS deformation through different heating times at 110 oC. In the present case, the standard heating condition of 30 s at 110 oC gives window pores less than 100 nm. The PS size or even the nature of active materials in

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each layer could be varied as well. This precise control the IOS film fabrication is not possible with the usual vertical deposition method, which produces intrinsically nonuniform film thickness.

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The morphology of the as-prepared samples was characterized by a scanning electron microscopy (SEM). The top-view SEM images of single Fe2TiO5 layer IOS (Fig. 2a, b) show periodic hexagonal spherical pores in ordered IOS. The average pore size is ~420

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nm, which shows only 16 % shrinkage relative to the size of PS template, and the wall thickness is 100-200 nm. Upon HMA, however, the amorphous Fe2TiO5 changes into

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highly crystalline one with the wall thickness of ~50 nm, but still maintains the IOS morphology Fig. 2c). From the cross section SEM image (Fig. 2d), the window pores in IOS can be clearly observed with the size of 50−80 nm. The top view SEM images (Fig. 2e, f) of 2-layers IOS also show highly ordered spherical pores and three smaller pores

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(about 80 nm) below the top layer. After HMA, some nanoparticles appear on the IOS architecture, which would greatly increase the surface area. From the cross section SEM image (Fig. 2h), 2-layers IOS on FTO is clearly seen even though the top layer structure

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is partly broken. Similar morphology is observed for the 3-layers film in Fig. 2i−l. As a reference photoelectrode, the morphology and particle size distribution of a 3-cycles (of

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spin coating) planar film is shown in Fig. S9. The optical images of 1-4 layers of IOS and 1-3 cycles planar films are compared in Fig. S10, which indicates that IOS of 1-3 layers are transparent and homogeneous like corresponding planar film, but 4-layers IOS becomes opaque showing that 3-layers IOS is the optimum number of layers. After HMA, some nanoparticles appear on the IOS architecture, which would greatly increase the surface area. From the cross section SEM image (Fig. 2h), 2-layers IOS on FTO is clearly

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seen even though the top layer structure is partly broken. Similar morphology is observed for the 3-layers film in Fig. 2i−l. The microstructure of Fe2TiO5 IOS was investigated by transmission electron

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microscopy (TEM). As-synthesized Fe2TiO5 IOS before annealing is composed of numerous small crystallites with low crystallinity (Fig. S11). After annealing, highly crystalline Fe2TiO5 with 3-layers IOS is formed as in Fig. 3a. The TEM image of a single

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IOS in Fig. 3b shows that many big nanoplates grow on the architecture, which are evolved from the small crystallites in Fig. S11. During this transformation, so-called

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‘oriented-attachment growth mechanism’ plays an important role, which is a special case of aggregation that describes the spontaneous self-assembly of adjacent particles so that they share a common crystallographic orientation, followed by joining these particles forming a planar interface [35]. Fig. 3c is the local magnified TEM image of the circled

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area in Fig. 3b, which shows a typical big single-crystalline nanoplate formed by oriented-attachment growth. Fig. 3d is the HRTEM image of the square area in Fig. 3c, which shows d-spacing of 0.348 nm assigned to (220) planes of orthorhombic Fe2TiO5.

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More HRTEM images of big single-crystalline nanoplates are shown in Fig. S12. Energy dispersive X-ray element mapping by scanning TEM in Fig. 3e−h shows spatially

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homogeneous distribution of Fe, Ti, and O elements in the whole structure. 3.2. Light harvesting and structural properties of Fe2TiO5 IOS films In order to evaluate the solar light harvesting capability, the absorption spectra of all samples (IOS of 1-3 layers and planar films of 1-3 cycles) were collected by UV-vis spectrophotometer. Fig. 4a exhibits absorption spectra of HMA-treated, highly crystalline Fe2TiO5 IOS with different layers, showing that the absorption is greatly improved with 12

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increased number of layers as expected. The areas of absorption spectra of 2 and 3-layers IOS increase by 60.4 % and 123.2 % relative to that of 1-layer IOS, respectively. More importantly, HMA-treated, highly crystalline IOS films show much larger absorption for

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all layers than corresponding planar films by 100.1 %, 77.6 %, and 96.3 % for 1-3 layers, respectively (Fig. 4b-d). For comparison with the same amount of Fe2TiO5 material (detail in Fig. S13), the 1-, 2-, and 3-layers IOS films also show enhanced light

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harvesting relative to corresponding planar films by 74.9 %, 38.5 % and 23.5 %, respectively. Corresponding transmission spectra for IOS and planar films in amorphous

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and highly crystalline states are compared in Fig. S14, which shows that IOS films in both states give much lower transmission than corresponding planar films. This is owing to the stronger diffuse scattering and coherent multiple internal scattering in the architecture of IOS [36]. Thus, the first advantage of IOS is to enhance light harvesting

planar films.

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capability of the Fe2TiO5 film in the visible light range (380 nm <λ< 650 nm) relative to

The X-ray diffraction (XRD) patterns in Fig. 4e, f show three diffraction peaks at 2θ

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=18.1o, 25.5 o, and 32.5 o due to (200), (220), and (230) crystal planes of orthorhombic phase Fe2TiO5 (JCPDS no.89-8066), respectively. With the increase of thickness, the

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peak intensities of (220) and (230) planes are significantly enhanced while that of (200) plane show little variation. In contrast, XRD patterns of planar films show greatly enhanced (200) peaks with increasing thickness compared to (220) and (230) planes. This indicates that they have different preference in directional growth during the crystallization process because IOS provides 3-dimentional freedom for growth. Application of the Scherrer equation to the strongest (200) peak of 3-cycles planar film

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(Fig. S15A) gives an average grain size of ~25.4 nm in agreement with that observed in SEM images (Fig. S9). However, the strongest (220) peak of 3-layers IOS (Fig. S15B) yields a much bigger calculated size of ~41.5 nm due to the formed big nanoplates. Thus

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IOS facilitates preferential growth of (220) and (230) planes, which is not observed in the isotropic planar film.

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3.3. DFT Calculations

In order to elaborate the unique structural behavior of IOS films connected to reactivity,

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density functional theory (DFT) calculation was performed for (200), (220), and (230) crystal planes with model structures presented in Fig. S16. (See detailed DFT calculation procedure in Experimental Section). Fig. 5a shows the atomic arrangements of (200), (220), and (230) planes exposed on the Fe2TiO5 surfaces. The transition metal ions on the exposed crystal planes are traditionally called “coordinately unsaturated sites (CUS)”,

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which are known as preferred sites of many catalytic reactions. Thus, the crystal plane with more of CUS is considered having higher density of active sites. As a result, the (220) crystal plane has the highest CUS density of transition metal ions (0.077 Å‒2),

). In the (200) plane, the surface consists of two five-fold Fe3+, while the (220) plane

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2

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followed by the (230) plane (0.075 Å‒2), and the (200) plane shows the lowest (0.054 Å‒

consists of three five-fold Fe3+ and one five-fold Ti4+, and the (230) plane consists of three five-fold Fe3+ and two five-fold Ti4+. Obviously, the (220) and (230) crystal planes bear more catalytic active CUS sites compared to (200) crystal plane. In addition, the density-of-states (DOS) analysis revealed the electronic states originated from the different Fe2TiO5 surfaces. Thus, Fig. 5b shows DOS of the first and second layers for each exposed crystal plane. Compared to bulk Fe2TiO5, Fe3+ ions on the 14

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exposed (200) plane forms the surface states below the conduction band edge, implying that Fe3+ ions on the Fe2TiO5 surface could induce the unoccupied states trapping the photo-generated electrons. Interestingly, in the cases of the (220) and (230) planes,

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surface states were formed in both the valence and conduction band edges with larger amounts; the photo-generated holes and electrons could be localized to the (220) and (230) planes more than (200) plane. The surface states of the (220) plane were mainly

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located at the valence band edge (i.e., occupied states), while those of the (230) plane were mainly located at the conduction band edge (i.e., unoccupied states). Thus, the

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photo-generated holes and electrons can be separately trapped in the (220) and (230) planes, respectively. It implies that charge separation can be induced and recombination rate of electron-hole pairs can be lowered [37]. Hence, the (220) and (230) planes can play a significant role in PEC water splitting. Thus, compared to the planar film, PEC

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water splitting efficiency of Fe2TiO5 is expected to be higher on the IOS film, because the IOS film has the (220) and (230) planes dominantly over (200) plane as confirmed by XRD results.

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3.4. Photoelectrochemical water splitting under sunlight The PEC water splitting performance of as-fabricated Fe2TiO5 films were evaluated in a

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three-electrode cell with the photoanode, Ag/AgCl (3 M NaCl), and Pt mesh as working, reference, and counter electrodes, respectively, in a 1 M NaOH solution under AM 1.5G simulated sun light irradiation (100 mW cm−2). All the measured Fe2TiO5 films were in the highly crystalline state by the HMA treatment. Fig. 6a-c compare linear sweep curves of 1-3 layers IOS and the reference 1-3 cycles planar films. The 1-cycle planar film (0.09 mA cm−2 at 1.23 VRHE) shows higher current density than that of 1-layer IOS (0.07 mA

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cm−2) owing to the exposed naked FTO in the center of IOS (See Fig. 2c), which causes a short circuit at the FTO/electrolyte interface [35-36]. When the second layer IOS is constructed, the improved light absorption and the decreased area of naked FTO

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contribute to higher current density (0.20 mA cm−2) of the IOS film compared to that of 2-cycles planar film (0.16 mA cm−2). Fig. S17 illustrates how there still exists the naked FTO area for the 2-layers IOS. With 3-layer IOS, all the naked area disappears giving 2

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times higher current density (0.49 mA cm−2) relative to that of the 3-cycles planar film (0.18 mA cm−2), which also contains contribution from the greatly improved light

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absorption (Fig. 4d). The 4-layers IOS photoanode shows similar performance to that of 3-layers IOS (Fig. S18) but it becomes opaque (Fig. S10), leading us to determine that the 3-layers IOS is the optimum. Besides, Nyquist plots of 3-layers IOS film and 3-cycels planar film reference (Fig. S19) show that the 3-layers IOS film has a much smaller

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charge-transfer resistance across the electrode/electrolyte relative to that of 3-cycles planar film, which is consistent with their J–V curves. It should be noted that this kind of layer-resolved investigation is not possible with the usual vertical deposition method that

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does not allow this precise control in the IOS film fabrication. To take advantage of the good PEC performance of the optimized IOS Fe2TiO5

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photoanode, two modification strategies were adopted to boost the PEC performance further; the Ga2O3 underlayer and a cocatalyst. A Ga2O3 underlayer was prepared by a chemical bath deposition (CBD) at the optimized condition of 20 min at 70 oC as shown in Fig. S20-S21 in detail. From the J–V curves (Fig. 6d), the 3-layers IOS electrode with the optimized Ga2O3 underlayer shows more than 2 times higher current density (1.06

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mA cm−2) than that without the underlayer (0.49 mA cm−2). As shown in Fig. S22, the 3cycles planar film also shows the similar promotional effect by the Ga2O3 underlayer. The time-of-flight secondary ion mass spectrometry (TOF-SIMS) (Fig. S23) and X-ray

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photoelectron spectroscopy (XPS) depth profiles (Fig. S24) demonstrate that the Ga2O3 underlayer is the source of a sacrificial gallium doping for Fe2TiO5. As shown in Fig. S1, a pseudobrookite consists of two octahedral sites M1 and M2. One is larger and more

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distorted and connects to the other with edge/corner sharing. However, the cations only occupy 1 M1 and 2 M2, suggesting that the voids/large interstitial sites equal to M1 site

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are vacant [38]. For example, monophasic Fe2TiO5 is usually obtained by adding more titania instead of the exact Fe2TiO5 stoichiometry, indicative of an iron-deficient pseudobrookite [39]. During the process of HMA, the gallium atoms from the underlayer would fill into the cationic vacancies and/or substitute for iron atoms to form

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Ga2TiO5/GaFeTiO5 pseudobrookite structure, which would be beneficial for the bulk conductivity and stability of Fe2TiO5. For instance, Bursill and Stone showed that Al3+ substitution for Ga3+ stabilized the intergrowth of Ga4Ti3O12 [40]. Besides, the XPS data

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in Fig. S24 (at the depth of 0 nm) indicate that there exists high gallium atomic concentration and not fully oxidized state of gallium on the surface of Fe2TiO5, which

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would promote charge transfer to electrolyte because of the concomitant generation of oxygen vacancies (analogous to TiO2 and hematite photoanode) [41, 42]. The corresponding Mott−Schottky plots in Fig. S25 show that the carrier density increases significantly by the underlayer, which would improve charge transport. In addition, flat band potential of Fe2TiO5 is raised toward its conduction band, which will promote more band bending in the space charge layer, leading to better charge separation. As a result,

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the improved bulk conductivity and better band bending by gallium doping from the sacrificial underlayer lead to higher photocurrents and a negatively shifted onset potential by ~100 mV relative to pristine Fe2TiO5 IOS electrode (Fig. S26). More importantly, the

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measured charge separation efficiencies (Fig. S27) shows that the sacrificial underlayer of Ga2O3 can significantly improve the surface and bulk charge seperation efficiencies by 65.2 % and 39.8 %, respectively.

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As an additional modification strategy, the ternary Ni−Co−Fe cocatalyst was adopted instead of usual binary catalyst, where Fe can stabilize Ni in +2 state, while Co tends to

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facilitate Ni in +3 state [43]. The charge transfer effects by Co assist the formation of the conductive NiIIIOOH phase, thus activating the Fe sites, which cannot transfer electrons in the nonconductive NiII(OH)2 host lattice [44]. In Ni−Co−Fe systems in general, ternary cocatalyst have better performance than binary catalyst owing to its better conductivity

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and induced strain effects. The composition optimization of the ternary cocatalyst on the 3 layers IOS electrode in Fig. S28 shows that the concentration of 0.5×10−3 M of Ni2CoFe gives the highest current density of 2.08 mA cm−2 at 1.23 VRHE, which is almost

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2 times that with only Ga2O3 underlayer and 4 times that of the pristine Fe2TiO5 IOS electrode. In contrast, the 3-cycles planar film with the optimum underlayer and

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cocatalyst improves only 23 % relative to that with underlayer only and less than 2.5 times that of the pristine (Fig. S22). This is reasonable because Fe2TiO5 IOS electrode has a unique porous structure, high surface area and many nanoplates with singlecrystallinity. The porous structure and high surface area would extend its contact area with the electrolyte and single-crystalline nanoplates would facilitate charge separation and transport. Thus, the IOS electrode far outperforms the corresponding planar film.

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Simultaneously, the ternary cocatalyst effectively mitigated the trapping states at the electrode surface (Fig. S29) and also contributed to additional negative shift of onset potential (by ~50 mV) (Fig. S26).

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Incident photon-to-electron conversion efficiency (IPCE) spectra in Fig. 6e show improved efficiency of the 3-layers IOS electrode with introduction of the underlayer and cocatalyst that extends all the way to 650 nm. In contrast, the 3-cycles planar film with

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the same modifications displays low IPCE values and narrow photoresponse wavelength due to its poor light harvesting ability (Fig. S30). The doubly modified 3-layers IOS

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electrode exhibits no degradation of photocurrent in a stability test at 1.23 VRHE for 8 h (Fig. S31). However, pristine and only underlayer-modified electrodes suffer substantial losses of currents with time. Thus, the two modifications improve not only the activity but also stability of Fe2TiO5 IOS electrodes. As mentioned above, the gallium atoms from

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the sacrificial layer could fill cationic vacancies and partially substitute for Fe, leading to the configurational stabilization of pseudobrookite Fe2TiO5 structure. The cocatalyst assists oxygen evolution reaction, reducing the competitive photo-corrosion. In addition,

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the gases evolved from the finally optimized photoanode were quantified by a gas chromatography (Fig. S32). The Faraday efficiency of H2 and O2 evolution reaction are

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close to unity producing gases in the stoichiometric amounts (H2:O2 = 2:1). To investigate the charge transport process in the 3-layers IOS electrodes, electrochemical impedance spectroscopy (EIS) was carried out at 1.23 VRHE and opencircuit voltage under 1 sun illumination. At 1.23 VRHE, the Nyquist plots at the frequency of 0.1 Hz−100 kHz in Fig. 6f were fitted to a two-RC circuit model (inset) and the fitting results are shown in Table S2. In the model, RS represents the series resistance of FTO

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substrate, external contact and the electrolyte. One RC circuit (R1, CPE1) is considered as internal resistance and capacitance of the depletion region in the bulk of Fe2TiO5. R2 and CPE2 represent the charge transfer resistance and double layer capacitance at the

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electrodeǁelectrolyte interface, respectively. Both RS and R1 for 3-layers IOS with underlayer decrease with respect to that of bare 3-layers IOS, indicating that a sacrificial Ga2O3 underlayer effectively mitigates the drift of Sn atoms from FTO substrate and Ga

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dopants perform a little better in Fe2TiO5 relative to Sn dopants. Cocatalyst modification does not vary the value of RS and R1. However, both underlayer and cocatalyst

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modifications markedly decrease the R2 value, suggesting that the charge transfer resistance at the electrode/electrolyte interface decreases owing to the not-fully oxidized state of gallium and active sites of co-catalyst on the surface of Fe2TiO5. At the open-circuit voltage, the Nyquist plots in Fig. S33A show that the underlayer

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and cocatalyst modifications decrease charge transfer resistance remarkably. Besides, the corresponding Bode phase diagrams in Fig. S33B show that the characteristic maximum frequency peaks (fmax) shift negatively with the underlayer and cocatalyst modifications.

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In fact, fmax is inversely associated with the lifetime of charge carriers (τn) according to an equation τn = 1/(2πσfmax), where σ is dependent on the kinds of material [45]. Thus fmax

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decreases from ~4.2 to ~3.1 and then to ~1.3 Hz, indicating that the carrier lifetime increases by 35.5 % and 2.2-fold relative to the pristine by the underlayer and cocatalyst/underlayer, respectively. Therefore, the increased carrier lifetime would be beneficial to charge separation and collection, eventually leading to enhanced PEC performance. 4. Conclusions

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In summary, we report a new route to fabricate the precisely-controlled layers of IOS, which enable Fe2TiO5 to greatly improve light harvesting due to the stronger diffuse scattering and coherent multiple internal scattering in the macropores of IOS. The IOS

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also facilitates preferential growth of (220) and (230) planes, which contain higher densities of active catalytic sites as elucidated by the layer-resolved DFT calculation. This layer-by-layer self-assembly strategy allows to tune the sizes of macropores and

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window pores in IOS in each layer. The hybrid microwave annealing offers high crystallinity with the IOS morphology well-preserved. The optimized 3-layers,

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transparent IOS electrode modified with a sacrificial underlayer and a ternary (Ni2CoFe)OOH cocatalyst records 2.08 mA cm−2 at 1.23 VRHE under 1 sun (100 mW cm−2) irradiation, which is the highest photocurrent density obtained with Fe2TiO5 photoanode up to date. The modifications also shift the photocurrent onset potential

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negatively by ~150 mV and improve the stability dramatically. The outcome provides an initial platform to circumvent the balance between transparency and efficiency, opening

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up opportunities to develop higher-performance devices.

Acknowledgements

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This research was supported by a Basic Science Grant (NRF-2018R1A2A1A05077909), the Climate Change Response Project (2015M1A2A2074663), Korea Center for Artificial Photosynthesis (KCAP, No. 2009-0093880), and Korea-China Key Joint Research Program (2017K2A9A2A11070341), Next Generation Carbon Upcycling Project (2017M1A2A2043138), which are funded by the Ministry of Science and ICT, Republic of Korea; and Project Nos. 10050509 and KIAT N0001754 of the Ministry of

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Trade, Industry and Energy, Republic of Korea. S.K.K. acknowledges the financial support from NRF-2014R1A5A1009799 and computational resource from UNIST-HPC.

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Appendix A. Supplementary data

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Fig. 1. Schematic procedure for the fabrication of precisely controlled layers of Fe2TiO5 IOS on FTO substrate. Path 1 is for fabrication of monolayer Fe2TiO5 IOS film on FTO and path 2 is for 2 layers of Fe2TiO5 IOS film. Repeating this cycle provides additional layers of Fe2TiO5 IOS. This layer-by-layer self-assembly strategy can also vary the PS

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size or the active material in each layer, rendering a powerful tool to tune the architecture

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of the photoelectrode film.

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Fig. 2. Morphology characterization of 1 (yellow, a-d), 2 (red, e-h), and 3 (green, i-l)-

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layers IOS. Top view (a-b, e-f, and i-j), after HMA (c, g, and k), and cross-section (d, h, and l) SEM images for 1-, 2-, and 3-layers of Fe2TiO5 IOS, respectively.

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Fig. 3. Detailed microstructure characterization. TEM images of 3-layers IOS (a), typical single IOS (b), single crystal nanoplate (c), HRTEM image (d), dark-field TEM (e), and

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corresponding EDX mapping (f-h) images for Fe2TiO5.

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Fig. 4. Comparison of light harvesting and preferential crystal growth between IOS and planar films. UV-vis absorption (a-d) spectra for 1, 2, and 3-layers IOS and 1, 2, and 3cycles planar films. XRD patterns of IOS (e) and planar films (f) treated by HMA. The

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strong diffraction peaks at 26.4 and 33.6 o are due to FTO substrate.

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Fig. 5. The atomic arrangements and the electronic properties of the Fe2TiO5 surfaces. (a)

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Top and front views of the optimized Fe2TiO5 surfaces. The blue, grey, and red spheres

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represent Fe, Ti, and O atoms, respectively, and the black spheres represent the fixed atoms during the geometry optimization. The green and magenta circles indicate 5-fold Fe3+ and 5-fold Ti4+, respectively. The green and blue square boxes refer to the 1st and 2nd layers for each surface, which was analyzed for DOS. (b) Layer-resolved total DOS of the (200), (220), and (230) planes. The layer-resolved DOS shows all contributions from s, p, and d orbitals for each layer.

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Fig. 6. Performance in PEC water splitting. J−V curves of 1-3 layers IOS and 1-3 cycles planar films (a-c). J−V curves (d), IPCE (e), and Nyquist plots (f) of the 3-layers IOS

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electrode, that with underlayer, and that with underlayer/cocatalyst.

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Precisely-controlled, a few layers of iron titanate inverse opal structure for

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enhanced photoelectrochemical water splitting

Hemin Zhang, Sung O Park, Se Hun Joo, Jin Hyun Kim, Sang Kyu Kwak* and Jae Sung

TOC Graphic

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Lee*

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Precisely-controlled Fe2TiO5 layers of inverse opal structure photoanode with enhanced light harvesting and high cystallinity is achieved by combination of layer-by-layer selfassembly and hybrid microwave annealing for efficient photoelctrochemical water

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splitting activity.

Keywords: iron titanate, inverse opal structure, layer-by-layer self-assembly, hybrid microwave annealing, photoelectrochemical water splitting

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Hemin Zhang received his Ph.D. degree in Materials Chemistry and Physics from Hefei Institutes of Physical Science, Chinese Academy of Sciences (CAS), China, in 2012. He

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worked as a postdoctoral researcher at National Institute of Materials Science (NIMS), Japan, from 2012‒2015. After that he joined Prof. Jae Sung Lee group (eco-friendly catalysis and

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energy laboratory) as a postdoctoral fellow at Ulsan National Institute of Science and Technology (UNIST), Republic of Korea, from 2016‒2018. He is now working as a

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Research Assistant Professor from 2019 at UNIST to develop abundant and efficient photoelectrodes for solar energy conversion and storage.

Sung O Park has received the B.S. degree in Chemical Engineering from Ulsan National Institute of Science and

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Technology (UNIST), South Korea in 2014. Currently, he is a graduate student in Chemical Engineering in UNIST and

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supervised by Prof. Sang Kyu Kwak. His research interest is in line with the computational study for electrocatalysts and

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electrode-electrolyte interface.

Se Hun Joo has received his B.S. degree in Chemical Engineering and Materials Science Engineering at Ulsan National Institute of Science and Technology (UNIST) in 2015. Currently, he is a graduate student in School of Energy and Chemical Engineering at UNIST and supervised by Prof. Sang

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Kyu Kwak. His research interests include battery materials, 2-dimensional materials, and polymers.

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Jin Hyun Kim received his undergraduate degree from the University of Seoul, Republic of Korea, in 2012. He joined Prof. Jae Sung Lee’s research group as an integrated MS + PhD

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candidate of Pohang University of Science and Technology (POSTECH), Republic of Korea. He received his PhD on the

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subject of semiconductor photoelectrode development for photoelectrochemical solar energy conversion in 2016. He is now working as a postdoctoral fellow in the same group in Ulsan National Institute of Science and Technology (UNIST) to undertake a project to develop metal oxide-based solar energy harvesting, conversion, and storage .

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Sang Kyu Kwak earned his Ph.D. in Chemical Engineering at State University of New York at Buffalo in 2005. He worked at Nanyang Technological University (NTU) in Singapore for 6

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years as assistant professor and moved to Ulsan National

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Institute of Science and Technology (UNIST) in Korea in 2012. He has published more than 140 peer-reviewed journals. His

expertise is specialized in multidimensional-simulation principles based on statistical thermodynamics, chemical physics, physical chemistry, and molecular physics. He is members of AIChE since 1998 and KJChE since 2012, respectively.

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Jae Sung Lee received his B.S. degree from Seoul National University in 1975, M.S. degree from Korea Advanced Institute of Science and Technology in 1977, Korea, and Ph.D. degree

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from Stanford University in 1984. After brief tenure at Catalytica Inc. as a research fellow, he became a professor of chemical engineering in Pohang University of Science and

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Technology (POSTECH). He and his group (eco-friendly catalysis and energy laboratory) moved to Ulsan National Institute of Science and

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Technology (UNIST) in 2013, whose research interests are placed on photocatalysis for energy applications, electrocatalysis for fuel cells, and heterogeneous catalysis of CO2

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utilization.

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Precisely-controlled, a few layers of iron titanate inverse opal structure for enhanced photoelectrochemical water splitting

Highlights

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Hemin Zhang, Sung O Park, Se Hun Joo, Jin Hyun Kim, Sang Kyu Kwak* and Jae Sung Lee*

Precisely-controlled Fe2TiO5 layers of IOS is fabricated via a layer-by-layer selfassembly.

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Hybrid microwave annealing yields highly transparent and crystalline Fe2TiO5 IOS photoelectrode

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IOS provides enhanced light harvesting and higher density of catalytically active crystal planes

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The modified IOS photoanode shows a high PEC water splitting activity.

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