Doping of Pt into Anodic TiO2 Nanotubes for Water Oxidation

Article pubs.acs.org/JPCC

Doping of Pt into Anodic TiO2 Nanotubes for Water Oxidation: Underpotential Shock Method in Cl− Solution Sunkyu Kim, Hyeonseok Yoo, Oonhee Rhee, and Jinsub Choi* Department of Chemistry and Chemical Engineering, Center for Design and Applications of Molecular Catalysts, Inha University, Incheon 402-751, Korea

Downloaded by CENTRAL MICHIGAN UNIV on September 14, 2015 | http://pubs.acs.org Publication Date (Web): September 4, 2015 | doi: 10.1021/acs.jpcc.5b05790

S Supporting Information *

ABSTRACT: For the preparation of a highly stable and active electrode for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), Pt was doped into TiO2 nanotubes by means of an underpotential shock method, in which positively biased voltage lower than the anodizing voltage was applied to anodic TiO2 for short seconds immediately after anodization, to reduce the overpotential of the catalyzed hydrogen and oxygen evolution reactions of water splitting. Because aqueous acidic H2PtCl6 solution was used as a doping precursor, Cl− ions were generated during imposing high positive voltage (so-called potential shock), which would aggressively damage the anodic oxide if a normal potential shock were applied. Instead, the novel underpotential shock method allowed Pt to be homogeneously doped into TiO2 films without destroying the structures. Pt could not thoroughly penetrate the barrier oxide into the interface between the oxide and Ti metal because the voltage of the potential shock was lower than the anodizing voltage, which determines the thickness of barrier oxide. Thus, a high amount of Pt was doped in the outer region of the barrier oxide. In the water splitting application, both the HER and OER were greatly enhanced when catalyzed by TiO2 that was doped with Pt at 10 V in 2 mM H2PtCl6 for 10 s. charged ions such as RuO4−.23,24 However, these processes cannot be carried out simultaneously in an electrolyte containing a self-reduced precursor such as MnO4−. For example, MnO4− is reduced to Mn4+ by F− in the electrolyte, resulting in the formation of Mn film on the cathode during anodization.25 Very recently, we have reported that anodic potential shock, as an alternative to the cathodic reduction of metal, is a very useful method to homogeneously dope foreign elements into TiO2 by means of a very brief treatment.25,26 We have already demonstrated that high amounts of RuO2 and MnO2 can be doped into high-aspect-ratio nanotubular TiO2 and that the RuO2 (or MnO2)-doped TiO2 could be employed to reduce the overpotential of the oxygen evolution reaction (OER). In the present work, Pt was doped into TiO2 nanotubes by potential shock in an electrolyte containing H2PtCl6 (chloroplatinic acid). Because the electrolyte included chloride ions, which are well-known to be very aggressive, anodic TiO2 films would be damaged under a normal potential shock in this electrolyte. To avoid such damage, we applied a low-potential shock (hereafter termed underpotential shock) to carry out the doping. Compared to cathodic electrodeposition, the novel method yielded homogeneous doping of Pt into high-aspectratio TiO2 without damaging its structure.

1. INTRODUCTION Many TiO2 structures have been developed that offer catalytic properties and chemical stability; such structures have attracted great interest because of their potential use in water splitting,1−3 photocatalysis,4−6 and biomedical devices,7,8 as well as in other applications.9,10 Especially, high-aspect-ratio tubular structures of TiO2 can self-assemble when a TiO2 film is directly grown on a Ti substrate by means of electrochemical anodization in an electrolyte containing fluoride ions.11−14 Such structures can be directly applied as effective electrodes without a significant loss of conductivity at the interface between the current collector of Ti and the active component of TiO2 because polymer binders are not required in this system. However, because of the intrinsically low conductivity and relatively high band gap (typically 3.0−3.2 eV) of TiO2 itself, doping of foreign elements into TiO2 nanostructures has been investigated to tailor the band gap and/or to enhance the (photo)electrochemical catalytic properties of TiO2.15−18 Noble and non-noble metals such as Mn, Ru, and Pt have already been successfully introduced to anodic TiO2 nanostructures by means of several novel methods, which usually require complicated processes or facilities. For example, an incipient wetness method, sputter deposition, postannealing under doping gas, and a photoreduction process have been reported.19−22 Recently, we reported that the formation of anodic structures and homogeneous doping can be simultaneously achieved if anodization is carried out in an electrolyte containing negatively © XXXX American Chemical Society

Received: June 17, 2015 Revised: July 27, 2015

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DOI: 10.1021/acs.jpcc.5b05790 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

2. EXPERIMENTAL SECTION Titanium foil (0.127 mm, 99.7% purity) was degreased and cleaned by sonication in acetone, ethanol, and deionized (DI) water for 15 min each, and then dried by a nitrogen stream. TiO2 nanotubes with approximately 1 μm length were prepared by anodization of titanium in an aqueous solution of 1 M H3PO4 and 1 M NaOH with 0.3 wt % HF. A two-electrode cell including titanium foil as the anode and Pt mesh as the cathode was used for the anodization, which was carried out at 20 V for 3 h by using a dc power supply (SourceMeter 2400, Keithley). Immediately after the anodization, a potential shock (5−50 V) was applied to the anodic TiO2 in aqueous H2PtCl6 (1−6 mM) solution for 10 s at room temperature. Thermal annealing treatment was performed in an air furnace at 450 °C for 1 h to crystallize the doped TiO2 nanotubes. The morphology of the samples was analyzed by means of scanning electron microscopy (SEM, S4300, Hitachi, Japan). Hydrogen evolution reaction (HER) and OER performance was measured by cyclic voltammetry (CV) carried out over the range from −2.0 to 2.0 V with the scan rate of 20 mV/s in 1 M KOH solution, with a standard three-electrode cell comprising Pt-doped anodic TiO2 as the working electrode, platinum mesh as the counter electrode, and Ag/AgCl 3 M KCl as the reference electrode. An X-ray photoelectron spectroscope (XPS, VGESCALAB 220i-XL spectrometer, Fisons) equipped with an Al Kα X-ray source was used to measure the surface chemical composition of the samples, and a time-of-fight secondary ion mass spectrometer (TOF-SIMS, TOF SIMS 5, ION TOF) equipped with a 30 keV Bi primary ion beam was used to analyze the depth profile of Pt-doped TiO2 nanotubes from the bottom surface. The loading quantity of Pt-doped in the TiO2 nanotubes was measured by means of inductively coupled plasma mass spectrometry (ICP-MS, ELAN 6100, PerkinElmer). The mechanism of charge transfer at the electrode/electrolyte interface was characterized based on electrochemical impedance spectroscopy (EIS) measurements acquired using a frequency response analyzer (Autolab, PGSTAT 302 N, Netherlands) under potentiostatic control over the frequency range between 100 kHz and 0.01 Hz with the amplitude of 10 mV.

Figure 1. (a) Schematic representation of cathodic electrodeposition onto TiO2 nanotubes. (b) SEM images of TiO2 nanotubes treated by electrodeposition in 2 mM H2PtCl6 aqueous solution at 10 V for 10 s. (c) Schematic representation of anodic potential shock for the doping of Pt into TiO2 nanotubes. (d) SEM images of TiO2 nanotubes treated by potential shock in 2 mM H2PtCl6 aqueous solution at 10 V for 10 s.

reaction at the interface between the H2PtCl6 electrolyte and the TiO2 oxide can be expressed as follows:29 PtCl 6 2 − + 2e− → PtCl4 2 − + 2Cl−

PtCl4 2 − + 2e− → Pt + 4Cl−

(1)

on TiO2

(2)

Contrastingly, it was previously proven that anodic potential shock is able to dope foreign elements such as RuO4− and MnO4− inside nanotubular TiO2.25,26 This occurs because, under anodic potential shock, the reaction occurs not at the interface between the electrolyte and the TiO2 oxide layer but at the interface between the TiO2 oxide layer and the Ti substrate. Thus, deep doping of foreign elements is possible depending on the voltage applied (Figure 1c). Basically, because the TiO2 nanotubes are prepared at constant voltage (herein, 20 V), which determines the thickness of the barrier oxide layer; potential shock voltage more than the oxide formation voltage of 20 V is required to allow the foreign elements to penetrate through the entire depth of the film. However, the most commonly used precursor of Pt contains Cl− ions, which are very aggressive ions that can break down the TiO2 film. Thus, it is strongly expected that doping of Pt by treatment with PtCl62− solution and high potential shock will lead to the breakdown of oxides. In this work, similar to underpotential deposition, we found that underpotential shock can dope Pt into TiO2 nanotubes in the presence of Cl− without damaging the nanotubes’ structure. Figure 1d shows TiO2 nanotubes doped with Pt by potential shock in 2 mM H2PtCl6 aqueous solution at 10 V for 10 s without morphological changes. The damaging effects of high shock voltages were demonstrated in the present work by using the conventional shock voltages of 30 and 50 V, under which Cl− attack partially broke down the oxides; contrastingly, no surface damage was observed when the underpotential shock of 10 V was applied (Figure 2). The doping of Pt in TiO2 was confirmed by XPS measurements; no Pt binding energy results were observed for the pristine TiO2 nanotubes, whereas Pt 4f5/2 and Pt 4f7/2 binding energies were clearly observed after the application of underpotential shock (Figure 3). The Pt XPS signal consisted

3. RESULTS AND DISCUSSION Parts a and c of Figure 1 schematically illustrate the doping of Pt into TiO2 by conventional cathodic reduction and anodic potential shock, respectively. In the case of cathodic reduction on high-aspect-ratio TiO2 nanotubular structures, it can be easily expected that nanoparticles will be deposited first on the mouths of TiO2 nanotubes, which are geometrically favorable for the reduction of Pt because Pt ions arrive at the surface first, without mass transfer hindrance. Once Pt is deposited, Pt nanoparticles become stacked on the mouths of nanotubes, blocking the mouths instead of doping inside the TiO2 nanotubes. As expected, the mouths of TiO2 nanotubes were completely blocked by Pt nanoparticles by electrochemical reduction in the electrolyte of 2 mM H2PtCl6 at 10 V for 10 s (Figure 1b). Of course, there are several methods to avoid the formation of Pt nanoparticles that will block the mouth, such as pulse deposition.27,28 However, because in principle the reduction of Pt occurs at the interface between the TiO2 and electrolyte, it is not possible to deeply dope Pt into the TiO2 oxide layer by cathodic reduction. The electrodeposition B


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Figure 3. X-ray photoelectron spectra of TiO2 nanotubes (a) before and (b) after potential shock treatment carried out in an electrolyte containing 2 mM H2PtCl6 at 10 V for 10 s.

Figure 2. SEM images of Pt-doped TiO2 nanotubes treated by potential shock in 2 mM H2PtCl6 for 10 s, using various voltages: (a) 10, (b) 30, and (c) 50 V.

of several doublets; the most intense doublet was assigned to metallic Pt (74.04 and 70.89 eV), and the second (75.49 eV) and third (72.06 eV) doublets were assigned to Pt(IV) and Pt(II) in the PtOx species of PtO2 and PtO, respectively. It was known that the metallic form of Pt shows higher electrochemical activity than oxidation state in the case of Pt-doped TiO2.30 The penetration depth of Pt ions resulting from the application of underpotential shock was investigated by means of depth profiling based on secondary ion mass spectrometry; for these measurements, Ar sputtering was applied from the bottom surface of the nanotube oxide (Figure 4). There was a

Figure 4. Depth profile of Pt concentration in TiO2 acquired by means of secondary ion mass spectrometry. Note that Ar sputtering for depth profiling was done from the bottom surface of TiO2 nanotubes (see the inset).

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use of the intermediate shock voltage of 10 V was attributed to the breakdown of oxide at a high potential shock in the presence of Cl−, whereas low doping of Pt was achieved at the shock voltage of 5 V. For comparison, TiO2 was doped by Cl− by means of potential shock in NaCl and was tested as a possible catalyst; its lack of catalytic activity clearly demonstrated that the enhancement of catalytic properties of TiO2 by the potential shock treatment arose from Pt doping and not Cl− doping (Figure S2). To optimize the concentration of the H2PtCl6 doping precursor, underpotential shock was carried out by using different concentrations of H2PtCl6 at 10 V for 10 s. ICP-MS analysis showed that 2−4 mM H2PtCl6 was an adequate concentration range for the underpotential shock method; the loadings of Pt were dramatically diminished when the concentration of the electrolyte was increased up to 6 mM (Figure 6). The poorer loading at higher precursor


region in the middle of the oxide in which the concentration of Pt ions changed considerably, confirming that Pt ions cannot fully migrate to the Ti/TiO2 interface under a low potential shock. In fact, based on XPS measurements of the backside of the TiO2, we have no evidence of full penetration of Pt through the barrier oxide film (Figure S1). Electrochemical properties of HER and OER catalysis by Ptdoped TiO2 nanotubes, prepared by using various potential shock voltages, were measured by means of CV in 1 M KOH (Figure 5a). Pt-doped TiO2 nanotubes prepared by potential

Figure 6. ICP-MS analysis of Pt loading concentration in TiO2 nanotubes treated by potential shock at 10 V for 10 s in H2PtCl6 solutions of various concentrations (1−6 mM).

concentration probably arose from the higher concentration of Cl−, which prevented negatively charged Pt ions from doping TiO2 by means of anionic competition. The doping amount of Pt in TiO2 affects the resulting catalytic activity for the HER and OER. Figure 7 shows the electrochemical properties of samples that were prepared using different precursor concentrations in 1 M KOH. Samples prepared in 2−4 mM H2PtCl6 showed similar onset potential and current density for the HER and OER, whereas the highest onset potential and lowest current density at a fixed potential were exhibited when 6 mM H2PtCl6 was employed. Note that 2 mM H2PtCl6 showed the best performance for the HER and OER. Optimization of potential shock time is described in detail in Figure S3. Potential shock for shorter than 10 s might be too short to dope Pt into TiO2 nanotubes sufficiently. When potential shock was applied for a longer time (>10 s), the barrier layer of TiO2 nanotubes was thickened. A length of barrier layer of nanotubes prepared for 20 s of potential shock was approximately 90−134 nm, whereas 52−60 nm was formed for the potential shock time of 10 s. The annealing process was indispensable for maintaining the stability of the oxide before HER and OER measurements; in unannealed samples, the mouth of the oxide was damaged after a few CV test cycles (Figure S4). After annealing, the CV cyclability was greatly improved (Figure S5).

Figure 5. (a) Cyclic voltammograms of TiO2 nanotubes doped by Pt using 2 mM precursor for 10 s under various potential shock voltages from 5 to 50 V. (b) Maximum current density at the fixed potentials of −2.0 and 2.0 V for the HER and OER, respectively, for samples prepared under various potential shock voltages.

shock at 10 V showed the lowest onset potential for the HER and OER and the highest current density for both HER and OER at a fixed voltage. Figure 5b shows the maximum current density at a fixed voltage for the HER (−2.0 V) and OER (2.0 V), for nanotube catalysts prepared by using various potential shock voltages, indicating that Pt doping at the underpotential shock of 10 V was the optimum condition for the preparation of these catalysts. The optimal performance arising from the D


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Figure 8. Nyquist plots of EIS analyses of bare Ti, TiO2 nanotubes, and Pt-doped TiO2 nanotubes for (a) HER and (b) OER.

Figure 7. (a) Cyclic voltammograms of TiO2 nanotubes doped by Pt at 10 V for 10 s by using solutions of various H2PtCl6 concentrations ranging from 1 to 6 mM. (b) Maximum current density at the fixed potential of −2.0 V for the HER and 2.0 V for the OER for Pt-doped TiO2 nanotube catalysts prepared by using various precursor concentrations.

Samples were analyzed by means of EIS to investigate charge transfer during the HER (Figure 8a) and OER (Figure 8b); the Nyquist EIS semicircle was reduced when Pt was doped into the TiO2 nanotubes. This clearly demonstrated that charge transfer in the Pt-doped TiO2 for the HER and OER was much faster than in nondoped TiO2. The size of the semicircle clearly showed that the HER was much faster than the OER. Interestingly, TiO2 nanotubes showed faster and slower charge transfer for the HER and OER, respectively, than that of the Ti substrate itself. It is probably due to the affinity of evolved O2 to TiO2 being greater than that of evolved O2 to Ti and vice versa for H2 evolution. Pt-doped TiO2 that was prepared under the optimized conditions (10 V potential shock and 2 mM H2PtCl6) was tested by means of CV for the HER and OER in 1 M KOH and was compared to other electrodes (Figure 9). Among the samples tested, TiO2 nanotubes containing a small amount of Pt showed the greatest capacity to be used as alternative

Figure 9. Cyclic voltammograms of the HER and OER for various electrodes: titanium, TiO2 nanotubes, TiO2 nanotubes with cathodically electrodeposited Pt, and Pt-doped TiO2 nanotubes prepared by potential shock in 2 mM H2PtCl6 at 10 V for 10 s.

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(3) Shin, S.; Choi, Y. W.; Choi, J. Water Splitting by Dimensionally Stable Anode Prepared Through Micro-arc Oxidation. Mater. Lett. 2013, 105, 117−119. (4) Ding, Z.; Lu, G. Q.; Greenfield, P. F. Role of the Crystallite Phase of TiO2 in Heterogeneous Photocatalysis for Phenol Oxidation in Water. J. Phys. Chem. B 2000, 104, 4815−4820. (5) Wang, X.; Yu, J. C.; Ho, C.; Hou, Y.; Fu, X. Photocatalytic Activity of a Hierarchically Macro/Mesoporous Titania. Langmuir 2005, 21, 2552−2559. (6) Fujishima, A.; Zhang, X.; Tryk, D. A. TiO2 Photocatalysis and Related Surface Phenomena. Surf. Sci. Rep. 2008, 63, 515−582. (7) Unosson, E.; Welch, K.; Persson, C.; Engqvist, H. Stability and Prospect of UV/H2O2 Ativated Titania Filmsfor Biomedical Use. Appl. Surf. Sci. 2013, 285, 317−323. (8) Treccani, L.; Klein, T. Y.; Meder, F.; Pardun, K.; Rezwan, K. Functionalized Ceramics for Biomedical, Biotechnological and Environmental Applications. Acta Biomater. 2013, 9, 7115−7150. (9) Wang, M.; Guo, D.; Li, H. High Activity of Novel Pd/TiO2 Nanotube Catalysts for Methanol Electro-Oxidation. J. Solid State Chem. 2005, 178, 1996−2000. (10) Mirabolghasemi, H.; Liu, N.; Lee, K.; Schmuki, P. Formation of ‘single walled’ TiO2 Nanotubes with Significantly Enhanced Electronic Properties for Higher Efficiency Dye-Sensitized Solar Cells. Chem. Commun. 2013, 49, 2067−2069. (11) Yoriya, S.; Prakasam, H. E.; Varghese, O. K.; Shankar, K.; Paulose, M.; Mor, G. K.; Latempa, T. J.; Grimes, C. A. Initial Studies on the Hydrogen Gas Properties of Highly-Ordered High Aspect Ratio TiO2 Nanotube-Arrays 20 to 222 μm in Length. Sens. Lett. 2006, 4, 334−339. (12) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Use of Highly-Ordered TiO2 Nanotube Arrays in Dye-Sensitized Solar Cells. Nano Lett. 2006, 6, 215−218. (13) Regonini, D.; Satka, A.; Allsopp, D. W. E.; Jaroenworaluck, A.; Stevens, R.; Bowen, C. R. Anodised Titania Nanotubes Prepared in a Glycerol/NaF Electrolyte. J. Nanosci. Nanotechnol. 2009, 9, 4410− 4416. (14) Yoriya, S.; Bao, N.; Grimes, C. A. Titania Nanoporous/Tubular Structures via Electrochemical Anodization of Titanium: Effect of Electrolyte Conductivity and Anodization Voltage on Structural Order and Porosity. J. Mater. Chem. 2011, 21, 13909−13912. (15) Litter, M. I.; Navío, J. A. Photocatalytic Properties of IronDoped Titania Semiconductors. J. Photochem. Photobiol., A 1996, 98, 171−181. (16) Lin, M. Z.; Chen, H.; Chen, W. F.; Nakaruk, A.; Koshy, P.; Sorrell, C. C. Effect of Single-Cation Doping and Codoping with Mn and Fe on the Photocatalytic Performance of TiO2 Thin Films. Int. J. Hydrogen Energy 2014, 39, 21500−21511. (17) Alamgir; Khan, W.; Ahmad, S.; Mehedi Hassan, M.; Naqvi, A. H. Structural Phase Analysis, Band Gap Tuning and Fluorescence Properties of Co Doped TiO2 Nanoparticles. Opt. Mater. 2014, 38, 278−285. (18) Zhang, Y.; Creatore, M.; Ma, Q. B.; El Boukili, A.; Gao, L.; Verheijen, M. A.; Verhoeven, M. W. G. M.; Hensen, E. J. M. NitrogenDoping of Bulk and Nanotubular TiO2 Photocatalysts by PlasmaAssisted Atomic Layer Deposition. Appl. Surf. Sci. 2015, 330, 476−486. (19) Mohapatra, S. K.; Misra, M.; Mahajan, V. K.; Raja, K. S. Design of a Highly Efficient Photoelectrolytic Cell for Hydrogen Generation by Water Splitting: Application of TiO2‑xCx Nanotubes as a Photoanode and Pt/TiO2 Nanotubes as a Cathode. J. Phys. Chem. C 2007, 111, 8677−8685. (20) Chen, H.; Nambu, A.; Wen, W.; Graciani, J.; Zhong, Z.; Hanson, J. C.; Fujita, E.; Rodriguez, J. A. Reaction of NH3 with Titania: N-Doping of the Oxide and TiN Formation. J. Phys. Chem. C 2007, 111, 1366−1372. (21) Lai, Y.; Gong, J.; Lin, C. Self-Organized TiO2 Nanotube Arrays with Uniform Platinum Nanoparticles for Highly Efficient Water Splitting. Int. J. Hydrogen Energy 2012, 37, 6438−6446.


electrodes. Especially, compared to cathodic electrochemical deposits of Pt on TiO2 nanotubes, Pt-doped TiO2 prepared by anodic potential shock showed much higher current density for the HER and OER. Unlike other research works, which have been done for Pt doping by complicated process doping or have studied on Pt/C for HER, we successfully prepared highaspect-ratio TiO2 nanotubes with a doping of Pt by simple electrochemical methods. Furthermore, we demonstrated that Pt-doped TiO2 nanotubes prepared at the optimum condition show good electrochemical activity for not only HER but also OER.

4. CONCLUSIONS We demonstrated that Pt can be successfully doped into TiO2 nanotubes by applying an underpotential shock of short duration. After fabrication of TiO2 nanotubes by anodization at 20 V for 3 h, a positive charge was imposed on these highaspect-ratio structures for 10 s in an electrolyte containing the doping precursor H2PtCl6. We found that the concentration of 2 mM and the potential shock of 10 V was the optimum condition for doping of Pt into TiO2 nanotubes for use as a water splitting catalyst. As the applied potential and the precursor concentration was increased, the surfaces of the TiO2 nanotubes were increasingly damaged by the action of aggressive Cl− ions originating from the H2PtCl6. Pt-doped TiO2 nanotubes showed lower onset potentials and higher current densities at a fixed voltage for both the HER and OER, compared to undoped TiO2 nanotubes and TiO2 nanotubes with cathodic deposits of Pt; this was confirmed by EIS experiments.

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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.5b05790. SEM images, XPS measurements, cyclic voltammograms of Cl−- and Pt-doped TiO2 nanotubes, optimization of potential shock time, effects of the annealing process, and HER and OER cyclability tests (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +82-32-860-7476/82-32-860-7471. Fax: +82-32-8660587. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Grant number: 2015042434).

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REFERENCES

(1) Mohapatra, S. K.; Misra, M.; Mahajan, V. K.; Raja, K. S. A Novel Method for the Synthesis of Titania Nanotubes Using Sonoelectrochemical Method and Its Application for Photoelectrochemical Splitting of Water. J. Catal. 2007, 246, 362−369. (2) Yu, J.; Qi, L.; Jaroniec, M. Hydrogen Production by Photocatalytic Water Splitting over Pt/TiO2 Nanosheets with Exposed (001) Facets. J. Phys. Chem. C 2010, 114, 13118−13125. F


Article


The Journal of Physical Chemistry C (22) Nguyen, N. T.; Yoo, J. E.; Altomare, M.; Schmuki, P. ‘‘Suspended’’ Pt Nanoparticles over TiO2 Nanotubes for Enhanced Photocatalytic H2 Evolution. Chem. Commun. 2014, 50, 9653−9656. (23) Shin, S.; Kim, K.; Choi, J. Fabrication of Ruthenium-Doped TiO2 Electrodes by One-Step Anodization for Electrolysis Applications. Electrochem. Commun. 2013, 36, 88−91. (24) Yoo, H.; Choi, Y. W.; Choi, J. TiO2 Nanotubes with a Doping of Ruthenium Oxide by Single-Step Anodization for Water Oxidation Applications. ChemCatChem 2015, 7, 643−647. (25) Seong, M.; Kim, S.; Yoo, H.; Choi, J. Anodic Nanotubular TiO2 Electrodes Containing MnO2 Catalysts for Water Oxidation. Catal. Today 2015, DOI: 10.1016/j.cattod.2015.06.006. (26) Gim, Y.; Seong, M.; Choi, Y. W.; Choi, J. RuO2-Doping into High-Aspect-Ratio Anodic TiO2 Nanotubes by Electrochemical Potential Shock for Water Oxidation. Electrochem. Commun. 2015, 52, 37−40. (27) He, H. C.; Xiao, P.; Zhou, M.; Zhang, Y. H.; Jia, Y. C.; Yu, S. J. Preparation of Well-Distributed Pt-Ni Nanoparticles on/into TiO(2) NTs by Pulse Electrodeposition for Methanol Photoelectro-Oxidation. Catal. Commun. 2011, 16, 140−143. (28) Li, H.; Wang, J.; Liu, M.; Wang, H.; Su, P.; Wu, J.; Li, J. A Nanoporous Oxide Interlayer Makes Better Pt Catalyst on Metallic Substrate: Nanoflowers on Nanotube Bed. Nano Res. 2014, 7, 1007− 1017. (29) Whalen, J. J.; Weiland, J. D.; Searson, P. C. Electrochemical Deposition of Platinum from Aqueous Ammonium Hexachloroplatinate Solution. J. Electrochem. Soc. 2005, 152, C738−C743. (30) Lee, J. S.; Choi, W. Y. Photocatalytic Reactivity of Surface Platinized TiO2: Substrate Specificity and the Effect of Pt Oxidation State. J. Phys. Chem. B 2005, 109, 7399−7406.

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Doping of Pt into Anodic TiO2 Nanotubes for Water Oxidation

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