Vertically Aligned Core-Shell PbTiO3@TiO2 Heterojunction

Article pubs.acs.org/JPCC

Vertically Aligned Core−Shell PbTiO3@TiO2 Heterojunction Nanotube Array for Photoelectrochemical and Photocatalytic Applications Jum Suk Jang,∇,† Chang Won Ahn,∇,‡ Sung Sik Won,‡ Ju Hun Kim,§ Wonyong Choi,∥ Byoung-Seob Lee,⊥ Jang-Hee Yoon,⊥ Hyun Gyu Kim,*,⊥ and Jae Sung Lee*,§ †

Environmental and Bioresource Sciences, Chonbuk National University, Iksan, 570-752 Republic of Korea Department of Physics, University of Ulsan, Ulsan, 680-749 Republic of Korea § School of Energy and Chemical Engineering, Ulsan National Institute of Science & Technology (UNIST), Ulsan, 689-798 Republic of Korea ∥ School of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang, 790-784, Republic of Korea ⊥ Busan Center, Korea Basic Science Institute, Busan, 609-735 Republic of Korea ‡

S Supporting Information *

ABSTRACT: Vertically aligned core−shell PbTiO3@TiO2 heterojunction nanotube arrays are fabricated on F:SnO2 (FTO) glass substrate via a unique three-step process: an electron beam vapor deposition of Ti thin film on FTO, anodic oxidation of the Ti film to TiO2 nanotubes, and finally formation of PbTiO3 perovskite layer at the inner wall of the TiO2 nanotubes. The PbTiO3@TiO2 nanotube array exhibits dramatically improved photoactivity relative to TiO2 nanotubes or PbTiO3/TiO2 composite powders in photoelectrochemical water splitting and photocatalytic isopropyl alcohol decomposition under visible light irradiation (>420 nm). In the core−shell heterojunction electrodes, PbTiO3 serves as a visible light responsive inorganic photosensitizer with its small band gap and forms a heterojunction with TiO2 for effective charge separation.

1. INTRODUCTION In solar energy conversion with small band gap semiconductors, fabrication of vertically aligned crystalline nanostructures is an effective method to develop highly efficient energy conversion materials. For example, an aligned onedimensional (1-D) nanostructure allows a short diffusion length for holes in radial direction, whereas the long axial direction of the structure becomes the preferred electron path that provides enough length of light attenuation as well.1−10 There have been numerous reports that aligned 1-D nanostructures provide superior photovoltaic, photocatalytic, and photoelectrochemical (PEC) properties relative to random-shaped particles.11−16 Another fruitful approach to improved photoactivity is to form a heterojunction structure between different semiconductors of a staggering (type II) band alignment.17−22 We fabricated various kinds of heterojunctions, such as WO3/BiVO4,17 CaFe 2 O 4 /PbBi 2 Nb 2 O 9 , 23 WO 3 /W/PbBi 2 Nb 2 O 9 , 24 and CaFe2O4/MgFe2O9,25 which showed much higher photocatalytic activity than the activity sum of the two semiconductor components under visible light. Lead-containing transition-metal oxides of Nb, Ta, and Ti can absorb visible light of the solar light spectrum because Pb 6s + O 2p hybridization in their valence bands makes smaller © 2017 American Chemical Society

band gap energies than usual metal oxide semiconductors, in which only O 2p orbital makes the deep valence bands.26,27 Among those materials, PbTiO3 is a good candidate for visible light active photocatalytic materials because of a band gap energy of 2.75 eV and suitable band positions for H2 and O2 production.28−30 In particular, PbTiO3 forms a staggering type II band alignment with TiO2 required for effective heterojunction formation. In contrast, more common visible light active oxide semiconductors such as WO3 and Fe2O3 have straddling type I band alignments with TiO2, and thus TiO2/ WO3 and TiO2/Fe2O3 heterojunctions are ineffective for charge separation. In the past, PbTiO3 was synthesized by traditional solid-state reactions at high temperatures, where the control of particle sizes or tuning surface characteristics was difficult. As an alternative method, formation of TiO2 nanotubes by anodization and their complete transformation to PbTiO3 nanotubes by a hydrothermal method were reported.29 However, in the report, TiO2 nanotubes were fabricated by anodization of Ti foil as the substrate, and the opaque photoanode cannot be Received: March 31, 2017 Revised: June 24, 2017 Published: June 27, 2017 15063

DOI: 10.1021/acs.jpcc.7b03081 J. Phys. Chem. C 2017, 121, 15063−15070

Article

The Journal of Physical Chemistry C

Figure 1. FESEM and TEM images for top and cross-sectional views of (a) TiO2 nanotube-500 °C and (b) PbTiO3@TiO2-550 °C heterostructured nanotube arrays on Pt/FTO glass substrate.

Figure 2. TEM image and EDS line scan analysis of a PbTiO3@TiO2-550 °C nanotube. Note that Pb signal peaks at the inner surface of the TiO2 nanotube. The structural model is shown in the inset.

of the heterojunctions reported so far have focused on enhancing the charge separation,17−23 and only a limited number of studies demonstrated an extended range of solar light absorption using a material as an inorganic photosensitizer.17,18,21

used in a tandem cell with multiple light absorbers connected in series. In the present work, we successfully grow the TiO 2 nanotubes directly on the platinum-coated fluorine-doped tin oxide (Pt/FTO) glass substrate by anodic oxidation of Ti thin film deposited on FTO by electron beam vapor deposition. Then, PbTiO3 perovskite layer is formed at the inner wall of the TiO2 nanotubes to produce a PbTiO3@TiO2 core−shell heterojunction nanotube array photoelectrode on transparent FTO glass. We demonstrate the effect of PbTiO3 working as visible light responsive inorganic photosensitizer in the PbTiO3@TiO2 photoelectrode for PEC water splitting as well as photocatalytic oxidation of isopropyl alcohol. The stable photoactivity of the PbTiO3@TiO2 photoelectrode is demonstrated under visible light. The 1-D nanotube array and core− shell PbTiO3@TiO2 heterojunction structure reduce the chance of charge recombination during the charge transfer and enhance the efficiency as a photoanode for PEC water splitting and a photocatalyst for isopropyl alcohol decomposition. Most

2. RESULTS AND DISCUSSION 2.1. Fabrication of Vertically Aligned Core−Shell PbTiO3@TiO2 Nanotube Array on FTO. As schematically shown in Figure S1, the PbTiO3@TiO2 photoelectrode was fabricated via a unique three-step process: an electron beam vapor deposition of Ti thin film on FTO, anodic oxidation of the Ti film to TiO2 nanotubes, and finally formation of PbTiO3 perovskite layer at the inner wall of the TiO2 nanotubes. By making all steps taking place on FTO in situ, it is expected to give better contact between the semiconductor and FTO than the usual indirect method of separate nanotube synthesis and transfer to FTO. Figure 1a presents top-surface and crosssectional images of field emission scanning electron microscopy 15064


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Figure 3. (a) UV−vis absorbance spectra of TiO2 nanotube and PbTiO3@TiO2-550 °C heterojunction nanotube array. (b) Mott−Schottky plots. (c) Alignment of band structures of PbTiO3 and TiO2.

tube positions. There are small Pb signals outside of the tube indicating that a small amount of the lead precursor fills the void space between TiO2 nanotubes during spin coating. But its intensity is much smaller than that inside of the tube. Thus, the majority of Pb precursor solution seems to fill the inner tube preferentially because there is much more space inside the TiO2 nanotubes than between nanotubes. Thus, PbTiO3 is formed mainly at the inner wall of the TiO2 nanotube forming the PbTiO3@TiO2 core/shell heterostructure. In Figure S3, the X-ray diffraction (XRD) pattern of asfabricated TiO2 nanotube represents pure anatase phase and the heterojunction nanotubes synthesized by spin coating and heat treatment with Pb precursor show both PbTiO3 and TiO2 phases. The tetragonal PbTiO3 structure became more crystalline by calcination at higher temperatures showing dominant (002) and (200) peaks probably due to the 1-D nanotube morphology. The phase of TiO2 evolved with the calcination temperature, with only anatase phase at 550 °C, but with mixtures of anatase and rutile phases at 600 and 650 °C. Figure 3a shows the UV−vis diffuse reflectance spectra of TiO2 nanotube and PbTiO3@TiO2 heterostructured nanotube electrodes. As expected, TiO2 nanotube exhibits little absorbance of light at wavelengths longer than 400 nm. But, specifying a value for the band gap energy (Eg) of PbTiO3 is complicated by its anomalous absorption edge and a wide range of values between 2.78 and 3.6 eV. The absorbance spectra show that PbTiO3@TiO2 nanotube array exhibits two absorbance edges (ca. 460 and 760 nm). The absorbance edge at 460 nm or a band gap of 2.78 eV is in good agreement with the reported value,14 and much smaller than that of TiO2 (3.2 eV). The second edge comes from defects in PbTiO3 crystal structure formed during the synthesis process. Thus,

(FESEM) for anodized TiO 2 nanotubes. This simple anodization process produced well-grown TiO2 nanotubes on Pt/FTO glass substrate as shown in Figure 1a with an average tube diameter of about 54 nm, wall thickness of 13 nm, and length of about 1 μm. This is a typical feature of anodic TiO2 nanotubes fabricated by the usual single-step anodic oxidation of a Ti film. Figure 1 displays top-surface and cross-sectional FESEM images and transmission electron microscopic (TEM) images of TiO2 nanotubes and PbTiO3@TiO2 core−shell heterostructured nanotubes which were synthesized by spin coating on TiO2 nanotubes with Pb(C2H3O2)2·3H2O (0.2 M) followed by calcination at 550 °C for 1 h. For PbTiO3@TiO2 nanotube array, the spacing between nanotubes is no longer apparent, and the nanotube wall and inner diameter become thicker and smaller, respectively. This change in surface morphology indicates that PbTiO3 was formed on the inner wall of TiO2 nanotubes while filling their inner pores. Interestingly, calcination at 650 °C induced clustering of 6− 10 nanotubes and partial collapse of the nanotube array as shown in Figure S2. The detailed morphologies observed by TEM are well in agreement with the FESEM images showing well-aligned nanotube arrays. In particular, the sample calcined at 650 °C shows that its nanotube array morphology is collapsed. At the high temperature, an excessive expansionforce difference could be present depending on whether it is directed at the contact point or empty space of the clustered tubes as reported previously.31 We additionally measured a high-resolution TEM image and an energy dispersive spectroscopic (EDS) line scan of PbTiO3@TiO2 heterojunction nanotube (calcined at 550 °C) to confirm the core/shell structure. As shown in Figure 2, Pb concentration peaks at inner tube positions away from Ti concentration peaks at outer 15065


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Figure 4. (a) Photocurrent densities (J)−applied potential (V) and (b) photocurrent−time transient response of TiO2 nanotube, PbTiO3@TiO2550 °C, PbTiO3@TiO2-600 °C, and PbTiO3@TiO2-650 °C nanotube array photoelectrodes measured under visible light (>420 nm) in 0.1 M KOH aqueous solution. Light source, 300 W Xe lamp; applied potential, 1.0 VRHE. (c) IPCE for PbTiO3@TiO2-550 °C electrode. (d) Time course of H2 production under visible light irradiation (λ > 420 nm). The reaction was continued for 8 h with a N2 purging in the middle.

heterojunction formation, in which the photoelectrons generated in PbTiO3 could trickle down from its conduction band to that of TiO2 by potential difference promoting the efficient separation of light-induced electron−hole pairs. 2.2. Photoelectrochemical Water Oxidation and Decomposition of Isopropyl Alcohol. The fabricated PbTiO3@TiO2 heterostructured nanotube was tested for PEC water oxidation in a 0.1 M KOH aqueous solution under visible light irradiation (λ ≥ 420 nm) in a typical three-electrode cell with PbTiO3@TiO2 nanotubes as a working electrode, a platinum wire as a counter electrode, and a saturated Ag/AgCl reference electrode. As shown by the photocurrent (J)−applied voltage (V) curves in Figure 4a, all PbTiO3@TiO2 nanotube electrodes show visible light PEC activity, but the reference TiO2 nanotubes do not. The photocurrents generated from the nanotube photoanodes were PbTiO 3 @TiO 2 -550 °C > PbTiO3@TiO2-600 °C > PbTiO3@TiO2-650 °C and showed an onset potential of ca. 0.3 VRHE. The observed difference in the photocurrents indicates that the well-defined nanotube structure calcined at the lower temperature is more important than improved crystallinity obtained at the higher calcination temperatures. The degradation of FTO at the higher temper-

PbTiO3 can absorb visible light photons in the solar radiation. The optical property of PbTiO3 could be understood with the band structure calculated by density functional theory in Figure S4. The conduction and valence bands of this compound consist mostly of empty Ti 3d and occupied O 2p orbitals, respectively. Even though the band structure of PbTiO3 is very similar to that of TiO2, PbTiO3 shows a smaller band gap because of higher energy Pb 6s orbitals that mix with the O 2p to form a hybridized valence band. To determine the band edge positions of PbTiO3, the reference PbTiO3 powder was prepared by a traditional solid state reaction and characterized by Mott−Schottky (M−S) analysis (Figure 3b, Figure S5). The values of the flat band potential (Vfb) of PbTiO3 and TiO2 calculated by extrapolating the linear portion in the M−S plots (1/C2 vs V) to the voltage axis were −0.7 and −0.55 V vs Ag/AgCl, respectively, as shown in Figure 3b. With determined Eg and Vfb, band diagrams were presented in Figure 3c. It is noteworthy that band positions of both materials straddle the reduction (0.0 V) and oxidation (1.23 V) potentials of water, showing the possibility of simultaneous H2 and O2 generation from water. The band alignment also represents type II suitable for effective 15066


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Now PbTiO3@TiO2 heterostructured nanotube was also tested for photocatalytic decomposition of a volatile organic compound. Figure 5 shows the time curves of CO2 evolution

atures may have also contributed to the difference to some extent. Under intermittent light irradiation, time-dependent photocurrent generation (chronoamperometry) is presented in Figure 4b. The photocurrents respond exactly to the presence of visible light as the cell operation is interrupted every 50 s (light on/off) under 1.0 VRHE reference potential, and the steady state behavior of each photoanode follows the same trend as that of the J−V curves. In addition, there are no appreciable dark currents, demonstrating the photochemical stability of all three photoanodes in the alkaline solution. The steady-state photocurrents do not show any significant degradation with time. Incident photon-to-current conversion efficiency (IPCE) measurements were carried out for PbTiO3@TiO2 nanotube photoanodes and another reference N-doped TiO2 (TiO2−xNx) as shown in Figure 4c. The IPCE curve of PbTiO3@TiO2 nanotubes closely follows its absorption spectrum in Figure 4c, indicating that photooxidation currents are derived from absorbed photons by the band-to-band transition. TiO2−xNx is a well-known visible light active photocatalyst fabricated by N-doping into TiO2 lattice.32 But its visible light IPCE above 400 nm is very small, appearing only as a shoulder. Thus, PbTiO3 plays the role of an effective inorganic photosensitizer in PbTiO3@TiO2 nanotubes to absorb visible light. The evolution of gas products (H2 from the cathode and O2 from the anode) was monitored by gas chromatographic analysis. The stoichiometric 2H2/O2 evolution was observed, and Figure 4d shows time curves of the PEC H2 production at 1.0 VRHE for 8 h with a brief N2 gas purging at 4 h (dotted line). There was no noticeable decrease of photoactivity (no slope change) for the whole run of 8 h. The PbTiO3@TiO2 nanotube photoanodes exhibited a higher rate of H2 production than those of two reference electrodes: PbBi2Nb2O9 nanorods and N-doped TiO2. The PbBi2Nb2O9 nanorod photoelectrode is an optimized photoanode for water splitting that we reported previously.14 The TiO2−xNx particulate film electrode produced only a trace amount of H2 gas. In order to see the effect of nanostructured morphology, PbTiO3/TiO2 nanocomposite powder was also prepared using a conventional synthetic method described in the Supporting Information. Its morphology was observed by SEM and TEM as shown in Figures S6 and S7. In PbTiO3/TiO2 powders, PbTiO3 particles of ca. 1 μm were decorated with TiO2 nanoparticles of ca. 20 nm in pure anatase phase as evident in Figure S8. Then, the PbTiO3/TiO2 nanocomposite powders and PbTiO3@TiO2 nanotubes were compared in PEC water splitting at 1 VRHE under visible light irradiation (λ ≥ 420 nm). As shown in Figure S9, PbTiO3/TiO2 nanocomposite powder shows much higher H2 production rate than N-doped TiO2 photocatalysts, reflecting the effects of extended light absorption and heterojunction formation. Yet, PbTiO3@TiO2 nanotubes show ca. 45 times higher photoactivity than PbTiO3/TiO2 powders due to their unique morphology of a well-aligned 1-D nanotube array. Thus, the electron percolation through particles in a powder-type film electrode is much slower due to a large number of interfaces as compared to the nanotube structure that favors the directional charge transport along a 1-D axis. This kind of axial electron transport path in core−shell nanotubes and additional heterojunction effects between PbTiO3 and TiO2 reduce the chance of charge recombination and enhance the efficiency of the PbTiO3@TiO2 photoanode for PEC water oxidation.

Figure 5. CO2 evolution from photooxidation of isopropyl alcohol under visible light (>420 nm) with different photocatalysts. □, PbTiO3@TiO2-550 °C; △, TiO2−xNx; ○, TiO2. Inset shows a proposed charge flow in PbTiO3@TiO2-550 °C heterojunction nanotube array on FTO.

from the photooxidation of isopropyl alcohol (IPA) under visible light (λ ≥ 420 nm). It clearly shows that PbTiO3@TiO2 heterostructured nanotube is more efficient than TiO2−xNx in this photocatalytic oxidation of IPA as well. The topology of PbTiO3@TiO2-550 °C heterostructured nanotube array on Pt/ FTO leads to efficient charge separation for photocatalytic oxidation in the same manner as in PEC water oxidation under visible light as shown in the inset of Figure 5. Time-resolved photoluminescence (TRPL) imaging was performed for PbTiO3@TiO2 heterostructured nanotubes and N-doped TiO2 nanoparticles as shown in Figure 6. The measured TRPL images clearly reveal that two samples possess vastly different PL lifetimes represented by vastly different colors. As listed in Table S1, the average lifetime of PbTiO3@ TiO2 heterojunction nanotubes is much longer than that of Ndoped TiO2 nanoparticles, indicating a prolonged recombination process due to more effective charge separation. This result is well corroborated with the PEC and photocatalytic activities. To summarize what we have observed, Figure 7 shows a schematic model that shows the charge transport in a PbTiO3@ TiO2 heterojunction nanotube and its TEM image. When the photoanode of the PEC cell is illuminated, the electrons generated from PbTiO3 are transferred from its conduction band to that of TiO2 by potential difference between the two semiconductors as is also shown in Figure 3c. The electrons make a directional transport through the axial direction of the nanotube for charge collection by FTO substrate. This 1-D electron flow is much faster than the electron percolation through particles as depicted schematically in Figure S10. Holes are collected in the valence band of PbTiO3 and diffuse to the photoanode/electrolyte interface to oxidize water to O2 gas and H+ ions. The morphology of the nanotube dictates that the hole diffusion length is very short through the thin wall of the nanotube in the radial direction. As a result, the light-induced electron−hole pairs can be efficiently separated, which reduces the chance of charge recombination. The collected photo15067


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Figure 6. Time-resolved photoluminescence (TRPL) images of (a) PbTiO3@TiO2 heterojunction nanotubes and (b) N-doped TiO2 samples.

platinum-coated fluorine-doped tin oxide (Pt/FTO) glass substrate by an anodic oxidation process with Ti thin film. The Pt coating layer on FTO helps homogeneous formation of TiO2 nanotubes during the anodization process as demonstrated in Figure S11. Ti thin films with a thickness of 750 nm were deposited on the Pt/FTO glass substrates using electron beam vapor deposition. Prior to anodic oxidation, the Ti/Pt/ FTO glass samples were cleaned with ultrasonication in isopropyl alcohol and ethanol successively, and then rinsed with deionized (DI) water and dried in a N2 stream. The anodic oxidation was performed with a carbon plate as the cathode. A high-purity glycerol (99.5%) solution with 2.0 wt % NH4F and 2.5 vol % DI water was used as the electrolyte. Anodic oxidation was carried out using a potential sweep from 0 to 25 V with a sweep rate of 1 V/s, followed by holding at an optimized condition of 25 V for 3 h (Figure S12). The TiO2 nanotubes were annealed at 500 °C for 3 h in ambient air with a sweep rate of 1 °C/min. A 0.2 M Pb(C2H3O2)2·3H2O (99.99+%) precursor solution in 2-methoxyethanol was spincoated onto TiO2 nanotube/Pt/FTO glass substrates at 3000 rpm for 20 s. The wet film was dried in air at 150 °C for 5 min on a hotplate, and then annealed at 550, 600, and 650 °C for 1 h in air for crystallization of PbTiO3 formed on TiO 2 nanotubes. The two reference photocatalysts, PbBi2Nb2O9 nanorod and TiO2−xNx powder, were synthesized according to the reported procedure.14,32 4.2. Physical Characterization. The crystallinity of the annealed films was examined using glancing angle X-ray diffraction (XRD, Philips X’Pert). The microstructure of the films was investigated using a field emission scanning electron microscope (FE-SEM, JSM-6500F, Jeol). Transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDX) including EDX line scan were performed with JEM 2100F (Jeol). 4.3. Time-Resolved Photoluminescence Measurements. The steady-state photoluminescence (PL) spectrum was measured using a spectrophotometer (Hitachi F-7000). Time-resolved PL decays were measured using an inverted-type scanning confocal microscope (MicroTime-200, Picoquant, Germany) with a 40× (air) objective. A single-mode pulsed diode laser (379 nm with a pulse width of ∼30 ps and a laser power of ∼10 μW) was used as an excitation source. A dichroic mirror (Z375RDC, AHF), a long-pass filter (HQ405lp, AHF), a 75 μm pinhole, a band-pass filter, and an avalanche photodiode detector (PDM series, MPD) were used to collect emissions from the samples. Time-correlated single-photon counting technique was used to obtain fluorescence decay curves, as a function of time with a resolution of 16 ps. Exponential fittings for the obtained fluorescence decays were

Figure 7. (a) Proposed mechanism of charge separation in PbTiO3@ TiO2 heterojunction nanotube electrode for photoelectrochemical water splitting. (b) TEM image of PbTiO3@TiO2 core−shell nanotube.

electrons move through the external circuit to the Pt cathode to reduce H+ ions to H2 gas. Hence, this kind of charge separation by forming a heterojunction is another critical role of combining PbTiO3 and TiO2 to enhance the PEC and photocatalytic efficiency of the PbTiO3@TiO2 nanotubes.

3. SUMMARY A vertically aligned PbTiO3@TiO2 heterojunction nanotube on FTO glass substrate is synthesized by a three-step process: an electron beam vapor deposition of Ti thin film on FTO, anodic oxidation of the Ti film to TiO2 nanotubes, and finally formation of PbTiO3 perovskite layer into the inner wall of the TiO2 nanotubes. By making all steps taking place on FTO, it is expected to give better contact between the semiconductor and FTO than the usual indirect method of separate nanotube synthesis and transfer to FTO. In PEC water splitting and photocatalytic IPA decomposition under visible light irradiation, dramatically improved photoactivity was obtained for PbTiO3@TiO2 heterojunction nanotube photoelectrode. In the electrodes, PbTiO3 serves as a visible light responsive inorganic photosensitizer with its small band gap. In addition, it forms a heterojunction with a staggering (type II) band alignment sending electrons and holes to opposite directions. Therefore, fabrication of PbTiO3@TiO2 nanotubes is an effective method to develop a highly efficient photoelectrode for PEC water splitting and a photocatalyst for oxidation of isopropyl alcohol. 4. EXPERIMENTAL SECTION 4.1. Preparation of Vertically Aligned Core−Shell PbTiO3@TiO2 Nanotube Array on FTO and Characterization. The preparation procedure is schematically presented in Figure S1. The TiO2 nanotubes were directly grown on the 15068


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The Journal of Physical Chemistry C performed by iterative least-squares deconvolution fitting using the Symphotime software (version 5.3). 4.4. Electrochemical Measurements. All measurements were carried out on a potentiostat (DY2300, Digi-Ivy) in a standard three-electrode configuration with a platinum wire counter electrode, a saturated Ag/AgCl reference electrode, and PbTiO3@TiO2 nanotubes as a working electrode. The photocurrent−potential curves were recorded under λ > 420 nm irradiation obtained by using a 300 W Xe lamp along with an optical filter. An electrolyte solution of 0.1 M KOH was used for all the electrochemical measurements. Mott−Schottky measurements were performed under dark condition with a dc applied potential window of −0.9 to 0.1 V vs Ag/AgCl at 0.5 kHz ac frequency with the amplitude of ac voltage of 10 mV. For measuring the incident photon-to-current conversion efficiencies (IPCEs), a 300 W Xe lamp (Newport, 9225) was coupled to a grating monochromator (Newport, 74125) operating in a wavelength range from 340 to 540 nm, and the incident light intensity was measured with a UV silicon detector (Newport, 71675). The photoelectrode was biased at 1.0 VRHE during all IPCE measurements. 4.5. Photoelectrochemical Water Splitting and Photocatalytic Degradation of Isopropyl Alcohol. The rates of gas evolution by PEC water splitting were measured under chronoamperometry conditions using a sealed two-electrode cell with photoelectrode (working electrode) and a platinum foil (counter electrodes) biased at 1.0 VRHE. The concentrations of H2 and O2 were determined by using a gas chromatograph equipped with a thermal conductivity detector (a molecular sieve 5 Å column and Ar carrier). The photocatalytic degradation of isopropyl alcohol (IPA) was studied to evaluate the photocatalytic activities of PbTiO3@TiO2 nanotube and TiO2−xNx film samples on Pt/FTO under visible-light irradiation (λ ≥ 420 nm). TiO2−xNx and TiO2 film electrodes were fabricated by a doctor blade casting method on Pt/FTO using TiO2−xNx and TiO2 powders, respectively. About 200 ppm gaseous IPA was injected into a 500 mL Pyrex reaction cell filled with air and 0.7 mm × 0.7 mm size of the PbTiO3@ TiO2 nanotube and the TiO2−xNx film. The concentration of the reaction products (CO2) was determined by using a gas chromatograph equipped with a CTR 1 packed column.

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Jae Sung Lee: 0000-0001-6432-9073 Author Contributions ∇

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. J.S.J. and C.W.A.: These authors contributed equally.

Funding

This work was supported by Korea Basic Science Institute grants (T35434, T34439), Basic Science Research Programs (2015R1D1A3A01019470), Climate Change Response projects (2015M1A2A2074663, 2015M1A2A2056824), Basic Science Grant (NRF-2015R1A2A1A10054346), Korean Center for Artificial Photosynthesis (NRF-2011-C1AAA0001-20110030278) funded by MSIP, and Project No. 10050509 and KIAT N0001754 funded by MOTIE of Republic of Korea. Notes

The authors declare no competing financial interest.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b03081. Time-resolved PL data, schematic illustration of sample preparation, XRD results, total and partial DOS of PbTiO3, M−S plot of TiO2 nanotube, SEM and TEM images of TiO2/PbTiO3 composite powder, photoelectrochemical H2 production, FESEM of TiO2 nanotube grown on FTO and Pt/FTO, and SEM images of TiO2 nanotubes grown at different anodic oxidation potentials (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.G.K.). *E-mail: [email protected] (J.S.L.). ORCID

Jum Suk Jang: 0000-0001-6874-8216 Wonyong Choi: 0000-0003-1801-9386 15069


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